Synthesis method for a hydrogen-gas-driven deposition of iridium on platinum surfaces, iridium-coated platinum nanoparticles, and use thereof in electrochemical devices

Abstract

The invention relates to iridium-coated platinum nanoparticles having an iridium content of 1 to 40 wt.%, based on the total weight of the iridium-coated platinum nanoparticles, to metal oxide-platinum-iridium core-shell-shell particles having an iridium content of 1 to 50 wt.%, based on the total weight of the particles, and to a method for the hydrogen-driven deposition of iridium on platinum surfaces, in particular platinum nanoparticles, characterized in that the surface of platinum nanoparticles is brought into contact with Ir cations in an acidic liquid reaction medium in the presence of H2 gas. The invention also relates to the use of Ir-coated Pt nanoparticles and non-platinum support materials with iridium-coated Pt surfaces in electrochemical devices.

Description

[0001] Research Center Jülich GmbH

[0002] H70452WO

[0003] Synthesis methods for hydrogen gas-driven deposition of iridium on platinum surfaces, iridium-coated platinum nanoparticles, and their use in electrochemical devices

[0004] The invention relates to iridium-coated platinum nanoparticles and materials consisting of a non-platinum support material with iridium-coated platinum surfaces, a synthesis method for hydrogen gas-driven deposition of iridium on platinum surfaces, in particular on platinum nanoparticles, the use of materials consisting of a non-platinum support material with iridium-coated platinum surfaces and iridium-coated platinum nanoparticles in electrochemical devices, and electrochemical devices produced using the same, such as electrolyzers and fuel cells.The invention further relates to a process for the oxidation of iridium-coated platinum nanoparticles and materials with iridium-coated platinum surfaces, oxidized iridium-coated platinum nanoparticles and materials with oxidized platinum surfaces, as well as their use in electrochemical devices and electrochemical devices produced using them, such as electrolyzers and fuel cells. The invention also relates to a manufacturing process for providing metal oxide platinum core-shell particles with platinum shells that are more than 70% closed, preferably completely closed, and which are used as substrates for the hydrogen gas-driven deposition of iridium on platinum surfaces for catalyst production, as well as the metal oxide platinum core-shell particles obtained by the process with platinum shells that are more than 70% closed, preferably completely closed.

[0005] Description

[0006] The climate crisis poses an ever-greater challenge to humanity. Its negative consequences can only be mitigated by generating energy from sources other than fossil fuels, which necessitates an effective transition to renewable energy sources. This requires the development of an energy storage infrastructure. Current research suggests that this can be achieved through the parallel deployment of various technologies, such as batteries, e-fuels, and hydrogen-based technologies. It is estimated that approximately 50% of energy storage should be covered by hydrogen-based technologies. The development of new catalysts with low precious metal content and the development of new technologies for using hydrogen as an energy carrier are therefore of central importance.Hydrogen is also an important basic chemical, of which 90 million tons are currently produced per year by reforming fossil methane, which represents a significant environmental burden.

[0007] An alternative method for producing hydrogen as an energy carrier or basic chemical is proton exchange membrane water electrolysis (PEMWE). In this process, water is split into its components, hydrogen and oxygen, in an electrolyzer, from which their molecular gases are then produced. The catalyst material for the oxygen evolution reaction (OER) plays a key role. Iridium-based catalysts have proven particularly suitable for the oxygen evolution reaction in PEMWE, as well as in fuel cells.

[0008] One of the biggest challenges of these electrochemical reactions, especially PEMWE, is the high demand for iridium, coupled with low global production of approximately 7-8 tons annually. Industrial PEMWE applications typically use catalyst powders containing over 75 wt% iridium. Current research therefore focuses on reducing the amount of iridium in the catalyst layer to lower costs and utilize available quantities as efficiently as possible. This often involves the use of support materials (e.g., TiÜ2) to increase the active surface area of ​​the catalyst. For PEMWE hydrogen production to become widely applicable, the amount of iridium used must be significantly reduced.

[0009] Currently available commercial catalysts typically use inorganic oxides (e.g., TiÜ2) as supports to increase the catalytically active surface area. These oxides have very low electrical conductivity, making iridium responsible for electrical percolation. Reducing the iridium content can lead to some iridium fragments losing electrical contact. Consequently, some of the valuable iridium does not participate in the reaction, and the catalyst utilization drops drastically. Typically, the iridium content in catalysts for these applications is at least 30 wt%, and often higher, at 75–100 wt%, in industrially / commercially available catalysts. Reducing the amount of catalyst in the catalyst layer to lower the iridium content can result in inhomogeneous or excessively thin layers, which in turn can prevent some iridium fragments from participating in the reaction.

[0010] WO 2013 / 092566 A1 discloses iridium-based catalyst materials for use as anode catalysts in PEM water electrolysis. These iridium oxide-containing catalyst materials incorporate an inorganic oxide with low electrical conductivity, such as Ti2 or Al2O3, as a support material. The catalyst material is obtained by precipitation of iridium oxide in a suspension of the inorganic support material and subsequent calcination of the resulting material at temperatures above 300 °C.

[0011] In M. Bernt et al., Journal of the Electrochemical Society, 2018, 165, F305, it was shown for the commercially available catalyst "Elyst 75" from Umicore, which is lrÜ2 supported on TiÜ2 (with 75 wt% Ir), that the limit for reducing the amount of catalyst in the catalyst layer for this catalyst at an iridium loading of 0.5 mg Ir em -2If the iridium loading in the layer is further reduced, this results in a significantly higher cell voltage and thus significantly higher losses when using electrical energy for hydrogen production.

[0012] In PA Loichet Torres et al., Journal of the Electrochemical Society, 2021, 168, 52508, a process was disclosed that utilizes the high reactivity of platinum with H₂ to reduce dissolved Cu metal ions and deposit them on a platinum surface. The process was successfully carried out with Cu on a platinum disk electrode and carbon-supported platinum particles. The deposition of platinum group metals (PGMs), in particular iridium, as well as the deposition of a metal on unsupported platinum nanoparticles, is not disclosed therein.

[0013] In Lim et al., Applied Catalysis B: Environmental, 2020, 272, 118955, a platinum-supported iridium-based catalyst and its application in PEM water electrolysis and in renewable fuel cells are disclosed. Here, iridium is deposited by electrodeposition either on a titanium fiber felt or on a titanium felt previously coated with hemispherical platinum particles of approximately 0.5–1.5 m in diameter, obtained by electrolytic deposition. Targeted and exclusive deposition of iridium on the platinum particles deposited on the titanium support material is not possible.

[0014] In da Silva et al., ACS Catalysis, 2018, 8, 2081-2092, a bifunctional catalyst based on platinum and iridium is disclosed, along with its use in PEM water electrolysis and renewable fuel cells. Platinum particles and iridium oxide particles exist as separate phases, and both particles are synthesized simultaneously via a hydrothermal process. Since the iridium is present not as amorphous monolayers but as crystallites, the iridium used cannot be completely available for the catalytic reaction at the material's surface, resulting in a less than optimal mass-specific activity of the iridium.

[0015] In AL Strickler et al., ACS Energy Lett., 2017, 2, 244-249, a platinum-iridium-based bimetallic catalyst with a core-shell structure is disclosed, along with its application in a proton exchange membrane (PEM) fuel cell. Here, iridium forms the core and platinum the shell of the catalyst particles. The particles are formed by first creating monodisperse iridium nanoparticles through the reduction of a precursor compound in ethylene glycol, and then reducing a platinum precursor compound onto the existing iridium cores to form the core-shell nanoparticles. While the particles exhibit higher conductivity compared to metal oxide-supported catalyst materials, little or no iridium is available for reaction catalysis at the particle surface.

[0016] In T. loroi et al., Journal of Applied Electrochemistry 31, 2001, 1179, an IrO2 / Pt catalyst for use in regenerative fuel cells is disclosed. In the catalyst production process, IrO2 particles are deposited onto platinum particles, which are then calcined at 400 °C to form rutile-structured IrO2 particles on platinum particle clusters. Due to the formation of crystalline iridium particles, the mass-specific activity of the iridium is not optimal compared to amorphous iridium layers on the catalyst surface, meaning that higher amounts of iridium must be used.

[0017] Regmi et al., ACS Catalysis 2020, 10, 13125-13135, describe the fabrication of iridium-supported catalysts in which a layer of platinum nanoparticles is deposited as a conductive layer on a TiCh support, onto which iridium is then deposited by an incipient wetness impregnation method. While the described methods also aim to reduce the precious metal content of water oxidation catalysts for water electrolysis, the iridium in the disclosed catalysts is present in crystalline form. This means that the iridium used is not completely available for the catalytic reaction at the surface of the material, resulting in suboptimal mass-specific activity of the iridium. Furthermore, the conductive Pt layer is not in the form of a continuous layer but rather as platinum nanoparticles, which does not guarantee optimal conductivity.

[0018] US2022 / 0275525 discloses a catalyst for use in an ammonia decomposition device, comprising cubical Pt nanoparticles and Ir atoms on the surface of the Pt nanoparticles, wherein the cubical Pt nanoparticles are located on the surface of a carbon support. The Ir atoms are individually scattered on the surface of the Pt nanoparticles. The use of carbon as a support for the Pt nanoparticles with iridium atoms on the surface is not suitable for use in devices operated at high voltages, such as PEM water electrolyzers, because corrosion of the carbon support occurs under these conditions.

[0019] CN114752947 discloses a manufacturing process that produces iridium-coated platinum nanoparticles on a Ti3C2-MXene support material, which can be used for water electrolysis. In this catalyst, the iridium particles are isolated on the surface of the platinum nanoparticles. The MXene support material used is unsuitable for use in PEM water electrolyzers because it corrodes oxidatively during operation of the electrolyzers.

[0020] LIS2015 / 0368817 A1 discloses the deposition of iridium particles on metal oxide supports by precipitation in ethylene glycol. The material obtained in this way is used as an anode catalyst layer in PEM-based electrolysis cells. Here, too, the deposited iridium is present on the support in the form of crystalline particles, so that the amount of iridium used is not completely available for catalyzing a reaction.

[0021] Against this background, the object of the invention is to provide an efficient catalyst with minimal iridium loading for use in electrochemical devices, in particular in PEMWE, as well as a simple, environmentally friendly and cost-effective method for coating the platinum surface of substrates, in particular platinum nanoparticles, with iridium.

[0022] One specific objective is the provision of iridium-coated platinum substrates, particularly iridium-platinum nanoparticles, in which a maximum specific iridium surface area is available for the reaction with minimal iridium usage, and in which high electrical conductivity is present throughout the catalyst material, as well as methods for their production. This objective of the invention is achieved by the materials and methods according to the invention, which are described in detail below.

[0023] In the materials with iridium-coated platinum surfaces and the iridium-coated platinum nanoparticles obtained by the synthesis process according to the invention, the iridium is deposited in atomic form onto the platinum surfaces. In contrast to conventional catalysts, the material on which the iridium is deposited in the platinum nanoparticles according to the invention does not consist of inorganic oxides, but exclusively of platinum, which has very high electrical conductivity. Although it is a precious metal, platinum (Pt) is about 70 times more abundant than iridium (Ir) and currently costs one-fifth of the price of iridium. The iridium-coated platinum nanoparticles obtained in this way therefore exhibit high electrical conductivity. A minimal amount of iridium is deposited amorphously on the platinum surfaces, for example in the form of one or more monolayers.Thus, with minimal iridium usage, a maximum iridium surface area is available for reactions, resulting in high mass-specific activity of the iridium. Furthermore, high electrical conductivity is present throughout the entire material. In the materials according to the invention, which consist of a non-platinum support material with iridium-coated platinum surfaces, the amorphous iridium coating is selectively located on the platinum surfaces. This allows the amount of iridium used to be reduced without impairing the efficiency of the catalysts, due to the high mass-specific activity of iridium. Furthermore, the positive effects exhibited by an iridium layer deposited directly on a platinum surface or by iridium atoms in a catalyst according to T. Reier et al., J. Electrochem. Soc. 2014, 161, F876-F882, can be utilized.Since in the synthesis processes according to the invention the iridium is preferably deposited on platinum (iridium-platinum deposition) and iridium-iridium deposition is disadvantageous, the growth of iridium particles in which the iridium is present in crystalline form and is not optimally available for catalyzing reactions is prevented.

[0024] The iridium-coated platinum nanoparticles and support materials with iridium-coated platinum surfaces, which can be used in electrochemical devices, particularly electrolyzers, are characterized by a high specific mass activity of iridium. This allows for a reduction in the required amount of iridium compared to conventional catalyst materials. Simple and safe processes are provided for the production of such materials and particles, utilizing cost-effective and commercially available starting materials, apart from the inherently expensive precious metal compounds and precursors.The process provides iridium-coated platinum surfaces, in particular iridium-coated platinum nanoparticles, which do not contain iridium in the form of crystalline iridium particles, but rather in amorphous form, especially as amorphous monolayers deposited on platinum surfaces. This increases the specific mass activity of iridium in electrochemical applications and reduces the iridium requirement for the production of efficient catalyst materials. The provision of core-shell-shell particles, whose core consists of metal oxides and has a platinum shell that is more than 70% closed, preferably completely closed, on which iridium is deposited, further reduces the platinum requirement for the production of efficient catalysts. Neither the process itself nor the post-treatment of the resulting iridium-coated materials requires a high-temperature step, such as calcination.

[0025] Summary of the invention

[0026] The present invention relates to iridium-coated platinum nanoparticles, characterized in that the iridium content of the iridium-coated platinum nanoparticles, based on the total weight of the iridium-coated platinum nanoparticles, is 1 to 40 wt%, preferably 2 to 30 wt%, more preferably 4 to 25 wt%, even more preferably 7 to 20 wt%, and most preferably 12 to 18 wt%, as well as to metal oxide-platinum-iridium core-shell-shell particles, characterized in that the particles consist of a metal oxide core covered with a platinum shell that is more than 70% closed, preferably completely closed, on which an outer layer of iridium is deposited, and which are micro- or nanoparticles, and in which the iridium content of the metal oxide-platinum-iridium core-shell-shell particles, based on the total weight of the The metal oxide-platinum-iridium core-shell-shell particle content is 1 to 50% by weight.

[0027] The invention relates to the use of iridium-coated platinum nanoparticles or metal oxide platinum iridium core-shell-shell particles in electrochemical devices, and electrochemical devices comprising these iridium-coated platinum nanoparticles or metal oxide platinum iridium core-shell-shell particles.

[0028] The invention further relates to a method for producing the above-mentioned iridium-coated platinum nanoparticles by hydrogen-driven deposition of iridium onto platinum nanoparticles, characterized in that the surface of platinum nanoparticles is brought into contact with Ir cations in an acidic liquid reaction medium in the presence of H2 gas, iridium-coated platinum nanoparticles obtainable by the method, characterized in that the iridium content of the iridium-coated platinum nanoparticles is 1 to 40 wt% based on the total weight of the iridium-coated platinum nanoparticles, and their use in electrochemical devices, and electrochemical devices comprising these iridium-coated platinum nanoparticles.

[0029] The invention further relates to a method for hydrogen-driven deposition of iridium on one or more platinum surfaces located on a non-platinum support material, characterized in that the support material with the platinum surfaces is brought into contact with Ir cations in an acidic liquid reaction medium in the presence of H2 gas, as well as a material obtainable by this method comprising a non-platinum support material with iridium-coated platinum surfaces, the use of the material in electrochemical devices, and electrochemical devices comprising this material.

[0030] Finally, the invention relates to a method for the oxidation of iridium-coated platinum nanoparticles or of non-platinum support materials with iridium-coated platinum surfaces, characterized in that the iridium coating of the platinum nanoparticles or of the non-platinum support materials with iridium-coated platinum surfaces is at least partially converted into iridium oxide, the oxidized iridium-coated platinum nanoparticles and non-platinum support materials with oxidized iridium-coated surfaces obtainable by the method, the use of the oxidized iridium-coated platinum nanoparticles or non-platinum support materials with oxidized iridium-coated surfaces in electrochemical devices, and electrochemical devices.which comprise these oxidized iridium-coated platinum nanoparticles or non-platinum support materials with oxidized iridium-coated surfaces. Finally, the invention relates to a method for producing micro- or nano-metal oxide platinum core-shell particles covered with a platinum shell that is more than 70% closed, preferably completely closed, and which serve as starting material for producing the metal oxide platinum iridium core-shell-shell particles, which consist of a metal oxide core covered with a platinum shell that is more than 70% closed, preferably completely closed, on which an outer layer of iridium is deposited.

[0031] Detailed description of the invention

[0032] The first aspect of the invention comprises iridium-coated platinum nanoparticles, characterized in that the iridium content of the iridium-coated platinum nanoparticles, based on the total weight of the iridium-coated platinum nanoparticles, is 1 to 40 wt%, preferably 2 to 30 wt%, more preferably 4 to 25 wt%, even more preferably 7 to 20 wt%, and most preferably 12 to 18 wt%.

[0033] For the purposes of the present invention, the term "platinum nanoparticles" means particles having an average particle size of 0.5 to 1000 nm, more preferably 0.8 to 250 nm, more preferably 1 to 100 nm, and more preferably 2 to 50 nm, and consisting (apart from technically unavoidable impurities) exclusively of platinum, i.e., consisting of 99 wt% or more, more preferably 99.9 wt% or more, more preferably 99.99 wt% or more, and more preferably 99.999 wt% or more of Pt.

[0034] The platinum content, i.e., the purity of the platinum nanoparticles, can be determined by X-ray fluorescence analysis (XRF analysis). For X-ray fluorescence analysis, a sample is preferably measured in a Bruker M4 Tornado apparatus using a tungsten X-ray source and applying a voltage of 50 kV. The platinum nanoparticles are placed in the measuring instrument in powder form for measurement.

[0035] Another way to determine purity is to dissolve the particles and then analyze them by inductively coupled plasma mass spectrometry (ICP-MS).

[0036] The mean particle size of platinum nanoparticles, which according to the invention is the average particle diameter of the platinum nanoparticles, can be determined by transmission electron microscopy (TEM), dynamic light scattering (DLS), laser diffraction, X-ray diffraction (XRD), and N₂ adsorption / desorption methods for determining the particle size or particle size distribution of metallic nanoparticles, wherein the mean particle size is preferably determined by TEM according to the invention. The mean particle size of metal oxide particles, platinum-coated metal oxide particles, and metal oxide-platinum-iridium core-shell-shell particles are also preferably determined by TEM. According to the invention, the TEM measurement is preferably carried out with a Talos F200i (Thermo Fisher Scientific) equipped with a Schottky field emission gun (X-FEG) and a dual Bruker XFIash 6 | 100 EDXS detector.Imaging was performed using a Thermo Fisher Scientific double-tilt holder with low background and high visibility, employing a molybdenum (Mo) clamp. A primary electron energy of 200 keV was used for spectral imaging and scanning transmission electron microscopy (STEM). For STEM, the electron sample was set to a beam current of 41 pA with a convergence angle of 10.5 mrad. Elastically scattered electrons were collected using a HAADF detector in an angular range of 58–200 mrad. Catalyst samples were deposited on conventional Lacey carbon-Cu gratings and plasma-cleaned prior to imaging using a Tergeo-EM plasma cleaner (PIE Scientific).

[0037] To determine the mean particle size from a TEM measurement of a sample of micro- or nanoparticles, three image sections are opened using the program "ImageJ" (version 1.54g). The pixel size is determined using the program's "Analyze -> Set Scale" command and the scale bar in the image. Subsequently, 30 particles in the image section are measured using the program's "Analyze -> Measure" command. The average diameter of the measured particles in the three image sections is calculated by averaging the particle diameters displayed in the program's results table.

[0038] The term "average value" therefore refers to the arithmetic mean obtained by summing the diameters of the 90 measured particles as described and then dividing by 90. This value is the mean particle size of the platinum nanoparticles, the iridium-coated platinum nanoparticles, the metal oxide particles, the metal oxide-platinum core-shell particles, and the metal oxide-platinum-iridium core-shell particles referenced in the description.

[0039] For the production of commercially available platinum nanoparticles, a wide range of literature procedures are available to those skilled in the art. Generally, platinum nanoparticles are produced by the reduction of platinum ion precursors such as potassium hexachloroplatinate or platinum(II) chloride, or by the decomposition of Pt(O) complexes in solution, often aqueous solution, in the presence of a stabilizing or masking agent. Common reducing agents include hydrogen gas, sodium borohydride, and ethylene glycol. Stabilizing or masking agents used include, for example, sodium polyacrylic acid or sodium citrate to stabilize the surfaces of the nanoparticles in solution and prevent aggregation.The size and shape of the platinum nanoparticles can be influenced by the choice of reaction conditions, in particular the precursor compound, the ratio of precursor compound to stabilizing agent, the temperature, and the addition of seed crystals. The platinum nanoparticles can have various shapes and exist as spheres, rods, cubes, or tetrahedra.

[0040] In the definition of the mean particle size according to the present invention, which is based on the evaluation of TEM images, a spherical shape is assumed for the platinum nanoparticles used.

[0041] Accordingly, for the purposes of the present invention, the term "iridium-coated platinum nanoparticles" refers to the aforementioned platinum nanoparticles on whose surface iridium atoms are present in the form of individual iridium atoms, in the form of one or more clusters of iridium atoms, or as at least a single layer of iridium atoms. These iridium atoms constitute the iridium coating of the platinum nanoparticles.

[0042] It is preferred that the iridium atoms form at least a single layer of atoms, also known as a monolayer.

[0043] According to the present invention, based on TEM images, it is assumed that the layer thickness of a monolayer of iridium atoms on platinum nanoparticles, on iridium atoms or on both surfaces side by side is approximately 0.26 nm.

[0044] To specify the degree of coverage of the platinum nanoparticles with iridium atoms, i.e., the thickness of the iridium coating, the term "mean layer thickness" is used. This is calculated based on the mean particle size of the platinum nanoparticles or the mean radius of the platinum nanoparticles, the value of which is half the mean particle size (the average diameter) according to the invention, and the iridium content of the iridium-coated platinum nanoparticles, i.e., the mass of the iridium coating.

[0045] A mean layer thickness of 0.26 nm corresponds to a monolayer; a mean layer thickness of 0.39 nm indicates a monolayer that is approximately half-covered with additional iridium atoms; and a mean layer thickness of 0.10 nm indicates that a platinum nanoparticle is less than half-covered with iridium atoms in a single layer. According to the present invention, the mean layer thickness, also referred to as shell height or shell thickness, is calculated using the following formula: where hschaie represents the average layer thickness in nm, rpt represents the average radius of the platinum nanoparticles in nm, and ppt represents the density of platinum in g / cm³. 3 represents

[0046] Pir the density of iridium in g / cm³ 3 represents, and

[0047] X is the mass ratio of Pt to Ir in the iridium-coated platinum nanoparticles, calculated according to the formula

[0048] X = m(Pt) / m(lr), represents.

[0049] Preferably, the ratio of the mass of Pt m(Pt) contained in the iridium-coated platinum nanoparticles to the mass of Ir m(lr) contained in the iridium-coated platinum nanoparticles is determined by X-ray fluorescence (XRF) analysis of the iridium-coated platinum nanoparticles.

[0050] While the platinum atoms in the platinum nanoparticles and the iridium-coated platinum nanoparticles are in crystalline form, i.e., they possess short- and long-range order, the iridium atoms of the iridium coating are not crystalline but amorphous, and preferably do not exhibit long-range order or crystallinity that can be detected analytically by XRD.

[0051] Since both the platinum nanoparticles and the iridium coating of the iridium-coated platinum nanoparticles according to the invention are present in elemental form, the iridium-coated platinum nanoparticles consist (apart from technically unavoidable impurities) of Pt and Ir. Accordingly, the combined content of Ir and Pt, i.e., their combined proportion in the total weight of the iridium-coated platinum nanoparticles, is 99 wt% or more, preferably 99.9 wt% or more, more preferably 99.99 wt% and even more preferably 99.999 wt% or more.

[0052] The platinum and iridium content can be determined by X-ray fluorescence analysis (XRF). It is also possible to dissolve the iridium-coated nanoparticles and determine the proportion of iridium, platinum, and any impurities present in the resulting solution using ICP-MS.

[0053] The iridium-coated platinum nanoparticles according to the invention have an iridium content of at least 1 wt.% and a maximum of 40 wt.% based on their total weight.

[0054] The iridium content in wt.% is therefore calculated using the formula 100 where m(lr) is the mass of the iridium contained in the iridium-coated platinum nanoparticles, i.e., the iridium coating, and m(Pt) is the mass of the platinum contained in the iridium-coated platinum nanoparticles, i.e., the platinum nanoparticles.

[0055] The iridium content of existing iridium-coated platinum nanoparticles can be determined by X-ray fluorescence analysis (XRF).

[0056] For X-ray fluorescence analysis, a sample is preferably measured in a Bruker M4 Tornado device using a tungsten X-ray source and applying a voltage of 50 kV. The iridium-coated platinum nanoparticles are placed in the measuring instrument in powder form for measurement.

[0057] During the production of iridium-coated particles, the iridium content of the particles can be determined by gravimetrically measuring the weight of the platinum nanoparticles and the weight increase of the platinum nanoparticles during Ir deposition. Alternatively, the iridium-coated platinum nanoparticles can be dissolved and the iridium content determined by ICP-MS.

[0058] Below an iridium content of 1 wt% of the iridium-coated platinum nanoparticles, based on the total weight of the iridium-coated platinum nanoparticles, the coverage of the surface of the platinum nanoparticles with iridium atoms may be too low to achieve a sufficient catalytic effect with a small amount of catalyst.

[0059] Above an iridium content of 40 wt.%, the mass-specific activity of the iridium can be reduced because the iridium atoms are arranged in multiple layers, meaning that some of the iridium atoms are not available for catalysis on the surface of the iridium-coated platinum nanoparticles. Preferably, the iridium content of the iridium-coated platinum nanoparticles according to the invention is 2 to 30 wt.%, more preferably 4 to 25 wt.%, even more preferably 7 to 20 wt.%, and most preferably 12 to 18 wt.%.

[0060] The iridium-coated platinum nanoparticles have a core-shell structure, wherein the platinum nanoparticle constitutes the core and the iridium coating on the surface of the platinum nanoparticle constitutes the shell. According to the present invention, a particle with a core-shell structure is fundamentally a particle with spherical, elliptical, or cylindrical symmetry, in which the properties of the interior, the core, differ from those of the outer surface, the shell. In the platinum nanoparticles with iridium coating described here, the core consists of platinum, and the shell consists of the iridium layer or iridium coating. Also according to the present invention, the outer shell of a core-shell particle can also be referred to as a layer or coating.Preferred are iridium-coated platinum nanoparticles with a mean particle size of 2 to 20 nm for the core platinum nanoparticles and a mean layer thickness of 0.13 nm to 0.39 nm. Due to the relatively small size of the core, i.e., the platinum nanoparticle, such iridium-coated platinum nanoparticles exhibit a large total surface area relative to the weight of the platinum nanoparticles. Furthermore, by covering the core with a shell consisting of approximately half to one and a half monolayers, the amount of iridium used is effectively available for catalysis at the surface.

[0061] The terms “iridium coating” and “iridium layer” may be used interchangeably in the following with regard to the iridium-coated platinum nanoparticles.

[0062] In a preferred embodiment of the invention, the particle size of the platinum nanoparticles is in the range of 0.5 to 150 nm, preferably in the range of 1 to 100 nm, more preferably in the range of 2 to 50 nm, even more preferably in the range of 3 to 35 nm, even more preferably in the range of 4 to 20 nm, and most preferably in the range of 5 to 10 nm.

[0063] The term particle size here refers to the mean particle size, which is determined by one of the methods mentioned above for determining the particle size of platinum nanoparticles, preferably by TEM. Particles with a mean particle size below the range according to the embodiment are difficult to produce and therefore very expensive. Larger particles lose specific surface area relative to the weight of the nanoparticles and thus actively usable surface area for reactions.

[0064] In a further preferred embodiment of the invention, the iridium layer of the iridium-coated platinum nanoparticles forms a closed layer on the surface of the platinum nanoparticle.

[0065] The closed layer consists of at least one monolayer and is characterized by the fact that the surface of the iridium-coated platinum nanoparticles is completely covered with iridium atoms.

[0066] In the context of the invention, a monolayer is understood to be a layer of iridium atoms with a thickness of one atom. As explained above, a monolayer has an average layer thickness of approximately 0.26 nm, according to the definition above.

[0067] A closed layer can consist of a monolayer, a monolayer on which further individual iridium atoms or clusters of iridium atoms are located, or several superimposed monolayers. Therefore, the presence of a closed layer within the meaning of the invention means that the average thickness of the iridium coating must be > 0.26 nm. In the embodiment according to the invention, the iridium is in amorphous form, even if the iridium coating consists of several superimposed monolayers. The iridium-coated platinum nanoparticles of this embodiment do not have any platinum atoms on their surface.

[0068] Due to the presence of a closed layer of iridium atoms, iridium atoms are available for catalysis on the entire surface of the iridium-coated platinum nanoparticles as designed, which allows for high catalytic activity relative to the weight of the iridium-coated platinum nanoparticles.

[0069] In the production of iridium-coated platinum nanoparticles according to the invention by hydrogen gas-driven deposition of Ir ions onto platinum nanoparticles, the formation of a closed layer can be indirectly determined by monitoring the amount of iridium deposited over time. Since hydrogen-driven deposition of Ir on Pt is significantly more favorable than deposition of Ir on Ir, the rate of Ir deposition decreases sharply as soon as no more Pt is available on the surface of the particles, because a closed layer of Ir atoms has formed.

[0070] Figure 4 shows a graph plotting the iridium content of iridium-coated platinum nanoparticles against time during the deposition reaction. A distinct and abrupt slowdown in deposition is evident at a reaction time of 4 h, suggesting the formation of a continuous layer of iridium atoms that significantly slows down further coating of the surface of the iridium-coated platinum nanoparticles. Since the amount of Ir cations available in the reaction mixture at the start of the reaction would be sufficient to form more than three monolayers on the platinum nanoparticles, it can be ruled out that the significant slowdown is primarily due to a reduced amount of Ir cations in the reaction solution.

[0071] Due to the significantly more favorable deposition rates on platinum surfaces compared to iridium, it can be assumed that a continuous layer is formed once an average layer thickness of 0.26 nm is reached. This is because iridium atoms are only deposited onto already deposited iridium atoms after the platinum surface of the platinum nanoparticle is completely covered with iridium atoms. The average layer thickness and the presence of a continuous iridium layer can be directly determined by TEM analysis.

[0072] In yet another preferred embodiment of the invention, the thickness of the iridium layer is 0.10 nm to 5 nm, preferably 0.14 nm to 4 nm, more preferably 0.18 nm to 3 nm, even more preferably 0.22 nm to 2.0 nm, and most preferably 0.26 nm to 1.8 nm.

[0073] According to the invention, the thickness of the iridium layer is understood to be the layer thickness of iridium atoms on the platinum nanoparticle. The values ​​given refer to the "mean layer thickness" of the iridium coating already defined above. The thickness of the iridium layer in iridium-coated platinum nanoparticles can be determined using the formula above for calculating the layer thickness from the mean particle size of the platinum nanoparticles and the iridium content of the iridium-coated platinum nanoparticles. This calculation assumes that the platinum nanoparticles and the iridium-coated platinum nanoparticles are spherical.In another preferred embodiment of the invention, the particle size of the iridium-coated platinum nanoparticles is in the range of 1 to 200 nm, preferably in the range of 2 to 150 nm, more preferably in the range of 3 to 100 nm, even more preferably in the range of 4 to 50 nm, even more preferably in the range of 4 to 30 nm, and most preferably in the range of 5 to 15 nm.

[0074] The term particle size here refers to the mean particle size in the sense of the average particle diameter, which is determined by one of the methods mentioned above for determining the particle size of platinum nanoparticles, preferably by TEM. Preferably, the method described above for determining the average particle diameter of platinum nanoparticles by evaluating TEM images is used to determine the particle size of iridium-coated platinum nanoparticles.

[0075] In another preferred embodiment of the invention, no iridium peaks are present in the XRD diffractogram of the iridium-coated platinum nanoparticles.

[0076] XRD analysis is X-ray diffraction, an analytical method suitable for the structural investigation of crystalline materials. While the inability to identify amorphous materials is generally considered a disadvantage of this analytical method, this property of X-ray diffraction by XRD can be used to demonstrate that the iridium content of the iridium-coated platinum nanoparticles of this embodiment according to the invention is present exclusively in amorphous form.

[0077] This represents an advantage over iridium coatings that are in crystalline form, since in iridium crystallites some of the iridium atoms cannot be available at the surface to catalyze reactions.

[0078] For XRD analysis of iridium-coated platinum nanoparticles or of material consisting of a non-platinum support material with iridium-coated platinum surfaces, such as metal oxide-platinum-iridium core-shell particles, an XRD instrument Rigaku Smartlab SE can be used; preferably, an XRD diffractogram obtained with the instrument Rigaku Smartlab SE under the conditions Cu Ka (1.54 Ω) source (40 kV, 50 mA) measured between 5-90° with a step size of 0.01° shows no iridium peak.

[0079] In another preferred embodiment of the invention, the iridium layer of the iridium-coated platinum nanoparticles consists of one or more iridium monolayers, preferably a single iridium monolayer. With one or more iridium monolayers present, the surface of the platinum nanoparticles is completely covered with iridium atoms, so that the entire surface of the iridium-coated platinum nanoparticles can exhibit activity in iridium-catalyzed reactions. The iridium is present in the amorphous form of one or more monolayers. Whether one or more monolayers are present can be determined based on the particle size or particle radius of the platinum nanoparticles and the weight fraction of the iridium coating, using the formula for determining the average layer thickness mentioned above.

[0080] According to the embodiment, a monolayer is assumed for a mean layer thickness of 0.22 to 0.30 nm according to the specified formula. The iridium-coated platinum nanoparticles according to the embodiment exhibit high mass-specific iridium activity and high stability.

[0081] In yet another preferred embodiment of the invention, the iridium-coated platinum nanoparticles exhibit a mass-specific activity of Ir of at least 100 A gi r ' 1 , preferably at least 500 A gi r ' 1 , even more preferably at least 1000 A gir' 1 , preferably at least 3500 A gi r ' 1 on.

[0082] The mass-specific activity of iridium (Ir) is a measure of the efficiency with which the Ir used is available for catalysis when iridium-coated platinum nanoparticles are used as a catalyst. To determine the mass-specific activity, a half-cell measurement using a rotating disk electrode (RDE) serves as a model reaction setup; preferably, the setup described in Example 2 is used for activity determination by half-cell measurement using the RDE.

[0083] In a particularly preferred embodiment of the invention, the iridium content of the iridium-coated platinum nanoparticles is 2 to 30 wt% and the particle size of the platinum nanoparticles is in the range of 1 nm to 100 nm; preferably, the iridium content is 2 to 30 wt% and the particle size of the platinum nanoparticles is in the range of 2 nm to 50 nm; more preferably, the iridium content is 4 to 25 wt% and the particle size of the platinum nanoparticles is in the range of 3 nm to 35 nm; even more preferably, the iridium content is 7 to 20 wt% and the particle size of the platinum nanoparticles is in the range of 4 nm to 20 nm; and most preferably, the iridium content is 12 to 18 wt% and the particle size of the platinum nanoparticles is in the range of 5 nm to 10 nm, wherein the iridium content is based on the total weight of the iridium-coated platinum nanoparticles. refers to.

[0084] Iridium-coated platinum nanoparticles, which have platinum nanoparticle cores that, due to their size, ensure a large total surface area of ​​the platinum nanoparticles relative to the weight of the platinum nanoparticles, and an iridium content that ensures sufficient coating of the surface of the platinum nanoparticle with iridium, are preferred.

[0085] In a particularly preferred embodiment of the invention, the iridium content of the iridium-coated platinum nanoparticles is 2 to 30 wt% and the particle size of the iridium-coated platinum nanoparticles is in the range of 2 nm to 150 nm; preferably, the iridium content is 2 to 30 wt% and the particle size of the iridium-coated platinum nanoparticles is in the range of 3 nm to 100 nm; more preferably, the iridium content is 4 to 25 wt% and the particle size of the iridium-coated platinum nanoparticles is in the range of 4 nm to 50 nm; even more preferably, the iridium content is 7 to 20 wt% and the particle size of the iridium-coated platinum nanoparticles is in the range of 4 nm to 30 nm; and most preferably, the iridium content is 12 to 18 wt% and the particle size of the iridium-coated platinum nanoparticles is in the range of 5 nm to 15 nm,where the iridium content refers to the total weight of the iridium-coated platinum nanoparticles.

[0086] The iridium-coated platinum nanoparticles according to the embodiment have a size that ensures a sufficiently large total surface area relative to the weight of the particles, and an iridium content that ensures sufficient coating of the surface with iridium atoms.

[0087] In a further particularly preferred embodiment of the invention, the particle size of the platinum nanoparticles is in the range of 1 to 100 nm and the thickness of the iridium layer is 0.10 nm to 5 nm; preferably, the particle size of the platinum nanoparticles is in the range of 2 to 50 nm and the thickness of the iridium layer is 0.14 nm to 4 nm; more preferably, the particle size of the platinum nanoparticles is in the range of 3 to 35 nm and the thickness of the iridium layer is 0.18 nm to 3 nm; even more preferably, the particle size of the platinum nanoparticles is in the range of 4 to 20 nm and the thickness of the iridium layer is 0.22 nm to 2 nm; and most preferably, the particle size of the platinum nanoparticles is in the range of 5 to 10 nm and the thickness of the iridium layer is 0.26 nm to 1.8 nm. The thickness of the iridium layer is that described above.

[0088] Formula determines average layer thickness.

[0089] In a particularly preferred embodiment of the invention, the particle size of the iridium-coated platinum nanoparticles is in the range of 2 to 150 nm and the thickness of the iridium layer is 0.10 nm to 5 nm; preferably, the particle size of the iridium-coated platinum nanoparticles is in the range of 3 to 100 nm and the thickness of the iridium layer is 0.14 nm to 4 nm; more preferably, the particle size of the iridium-coated platinum nanoparticles is in the range of 4 to 50 nm and the thickness of the iridium layer is 0.18 nm to 3 nm; even more preferably, the particle size of the iridium-coated platinum nanoparticles is in the range of 4 to 30 nm and the thickness of the iridium layer is 0.22 nm to 2 nm; and most preferably, the particle size of the iridium-coated platinum nanoparticles is in the range of 5 to 15 nm and the thickness of the iridium layer is 0.26 nm to 1.8 nm.

[0090] In a further preferred embodiment of the invention, the iridium atoms in the iridium layer are present in an amorphous form. This can be demonstrated by TEM images and by the absence of Ir peaks in an XRD diffractogram of the iridium-coated platinum nanoparticles.

[0091] Preferably, the iridium atoms are in amorphous form and the mean thickness of the iridium layer as described above is 0.26 nm or more, which suggests the presence of at least one closed monolayer of iridium atoms on the surface of the platinum nanoparticle.

[0092] In a further embodiment, the iridium layer of the previously described iridium-coated platinum nanoparticles was oxidized by heat treatment or electrochemical treatment and / or at least partially converted into crystalline form.

[0093] Another aspect of the present invention relates to metal oxide-platinum-iridium core-shell-shell particles, characterized in that the particles consist of a metal oxide core covered with a platinum shell that is more than 70% closed, preferably completely closed, on which an outer layer of iridium is deposited, that they are micro- or nanoparticles, and that the iridium content of the metal oxide-platinum-iridium core-shell-shell particles is 1 to 50% by weight, based on the total weight of the metal oxide-platinum-iridium core-shell-shell particles.

[0094] For the purposes of the present invention, the term "micro- or nanoparticles" refers to particles having a particle size of 1 nm to 50 .m, preferably 2 nm to 10 .m, more preferably 3 nm to 1 .m, and even more preferably 5 nm to 500 nm.

[0095] Unlike the particles described in the previous aspect, these particles are not core-shell particles, where the core consists exclusively of platinum, but rather core-shell-shell particles. According to the present invention, a core-shell-shell particle is fundamentally a particle with spherical, elliptical, or cylindrical symmetry, in which the properties of the interior, the core, differ from those of a layer on the core surface, the inner shell, and from those of a further, differently structured layer, the outer shell. In the present aspect of the invention, the core of the core-shell-shell particles consists of a metal oxide covered with an inner shell of platinum that is more than 70% closed, preferably completely closed.This inner platinum shell in turn is coated with iridium, which in the sense of the present invention means that iridium atoms are present on the surface of the platinum shell, which is more than 70% closed, preferably completely closed, in the form of individual iridium atoms, in the form of one or more assemblies of iridium atoms or as at least a single layer of iridium atoms, and thus form the outer shell.

[0096] Preferably, the iridium atoms form at least one single layer of atoms, also called a monolayer, as the outer shell.

[0097] Instead of the term "metal oxide platinum iridium core shell shell particle", the term "MO@Pt@lr" or "MO@Pt@lr particle" can also be used, where "MO" stands for "metal oxide".

[0098] The structural difference compared to the nanoparticles of the previous aspect is that the iridium atoms in this aspect are deposited on the platinum surface of the platinum shell, which is more than 70% closed, preferably completely closed, and which fully covers a metal oxide core, instead of on the platinum surface of the platinum core of the nanoparticles of the previous aspect. The advantage of the particles of this aspect is that the proportion of platinum required can be reduced by using the metal oxide core, while the catalytic properties of the particles are comparable due to the presence of the conductive, more than 70% closed, preferably completely closed, platinum layer.

[0099] According to the invention, a platinum layer with a coverage of more than 70% is a layer in which more than 70% of the surface of the metal oxide core is covered by platinum atoms. The degree of coverage is preferably determined by TEM analysis of the platinum layer on the metal oxide cores. Depending on the choice of metal oxide, the particles based on metal oxide cores are not only more cost-effective due to the reduction in platinum requirements, but also exhibit high mechanical and chemical stability.

[0100] The choice of metal oxide core is not particularly restricted, as long as the metal oxide is not water-soluble or does not react with water to form water-soluble metal hydroxides. Therefore, metal oxides of alkali metals and alkaline earth metals are excluded. Examples of suitable metal oxides include SiO₂, TiO₂, ZnO, iron oxides FeO₃ and FeO₂, CeO₂, AlO₂, ZrO₂, CuO, and NiO, whose micro- and nanoparticles are known from the literature and commercially available. Preferred metal oxide cores consist of titanium oxide, niobium oxide, cerium oxide, tantalum oxide, and zirconium oxide, or alloys or mixed oxides of these metal oxides.

[0101] The purity of the metal oxides is at least 95% by weight based on the total weight of the metal oxide cores, preferably at least 99% by weight, more preferably at least 99.9% by weight, more preferably at 99.99% by weight, and most preferably at least 99.999% by weight.

[0102] The purity of the metal oxide cores can be determined by XRF analysis or ICP-MS, as described above for the Pt nanoparticles.

[0103] The particle size can also be determined using the methods TEM, DLS and XRD described above for Pt nanoparticles, with the mean particle size preferably being determined by TEM, as described above.

[0104] A key feature of the metal oxide platinum iridium core shell-shell particles according to the invention is the presence of an inner platinum shell that is more than 70% closed, preferably completely closed.

[0105] Various technical processes are available to those skilled in the art for coating substrates, for example micro- or nanoparticles, with platinum, such as impregnation processes in which substrate particles are impregnated with Pt ion solutions containing platinum ions in PtO under contact with air and thermal conditions. xThey are converted and then transformed into metallic platinum using a reducing agent, such as H2. Other methods include colloidal processes for immobilizing Pt nanoparticles on the support surface, deposition precipitation processes, sputtering, or ALD processes.

[0106] Preferably, the platinum coating of the metal oxide cores of the particles of this aspect is carried out by the process for producing micro- or nano-metal oxide-platinum core-shell particles covered with a platinum shell that is more than 70% closed, preferably completely closed, according to the present invention, which is a subsequently described aspect of the invention. The platinum shell, which is more than 70% closed, preferably completely closed, is characterized in that the surface of the metal oxide core is covered by at least one monolayer to more than 70%, and preferably completely covered by at least one continuous monolayer of platinum atoms. Since the metal oxide cores are non-conductive, the covering of the metal oxide cores with platinum forms an electrical percolation path.A preferred method for determining the presence of a platinum shell that is more than 70% closed, preferably completely closed, is TEM analysis.

[0107] The thickness of the platinum shell layer is not specifically limited, except that it must be at least a monolayer covering more than 70%, preferably completely, of the metal oxide core. The average layer thickness is preferably determined by TEM analysis.

[0108] The iridium content of the metal oxide-platinum-iridium core-shell particles, based on the total weight of the particles, is at least 1% by weight and at most 40% by weight. The content of platinum, iridium, and the metal of the metal oxide core can be determined by X-ray fluorescence analysis (XRF).

[0109] It is also possible to dissolve the iridium-coated particles and determine the proportion of iridium, platinum, and any impurities present in the resulting solution using ICP-MS. Determination by XRF is preferred.

[0110] In one embodiment of the invention, the metal oxide core consists of one of the following metal oxides or alloys of these metal oxides: titanium oxide, niobium oxide, cerium oxide, tantalum oxide, and zirconium oxide.

[0111] Micro- or nanoparticles of these metal oxides are readily available, exhibit good chemical and mechanical resistance, and are therefore well suited for the production of the metal oxide-platinum-iridium core-shell-shell particles. In a further embodiment of the invention, the metal oxide core comprises metal oxide particles with a mean particle size of 10 nm to 5 µm, preferably 50 nm to 1 µm, most preferably 100 nm to 500 µm.

[0112] The mean particle size is a value determined by one of the methods mentioned above for determining the particle size of the platinum nanoparticles, preferably by TEM.

[0113] In a further embodiment of the invention, the mean layer thickness of the Pt shell of the metal oxide platinum iridium core-shell-shell particles is 1 nm to 50 nm, preferably 2 nm to 10 nm.

[0114] A greater layer thickness would necessitate the unnecessary use of platinum or platinum precursor compounds in the fabrication of the metal oxide-platinum-iridium core-shell-shell particles, while a smaller layer thickness would lead to reduced conductivity of the platinum layer and less shielding of the metal oxide core. The mean thickness of the platinum layer can be determined analogously to the equation shown above for calculating the mean thickness of iridium on platinum nanoparticles, based on the weight fraction of platinum (as determined by XRF) relative to the total particle weight, the weight fraction of the metal oxide, and the particle size of the metal oxide core. Preferably, the layer thickness is determined by TEM analysis.

[0115] In yet another embodiment of the invention, the metal oxide platinum iridium core-shell-shell particles have a platinum content of 5 to 70 wt.%, preferably 7 to 50 wt.%, more preferably 10 to 40 wt.%, most preferably 12 to 35 wt.%, based on the total weight of the core-shell-shell particles.

[0116] The platinum content according to the embodiment enables good conductivity of the inner Pt shell without having to use unnecessarily large amounts of platinum.

[0117] The weight fraction of platinum in the inner shell, which is more than 70% closed, preferably completely closed, relative to the total weight of the core-shell-shell particles can be determined by XRF analysis.

[0118] In a further embodiment of the invention, the mean thickness of the Ir layer is from 0.10 nm to 20 nm, preferably from 0.2 nm to 15 nm, and even more preferably from 0.3 nm to 10 nm. As already explained analogously with regard to the iridium-coated platinum nanoparticles, according to the invention, the thickness of the iridium atoms refers to the thickness of the layer of iridium atoms on the platinum surface, in this case the thickness of the layer on the inner platinum shell, which is considered the outer iridium shell. The values ​​given refer to the "mean thickness" of the iridium coating already defined above. The thickness of the iridium layer in the iridium-coated platinum nanoparticles can be determined using the formula above for calculating the layer thickness from the mean particle size of the platinum nanoparticles and the iridium content of the iridium-coated platinum nanoparticles.This assumes the presence of a spherical shape for the platinum nanoparticles and the iridium-coated platinum nanoparticles.

[0119] The determination of the mean thickness of the iridium layer by TEM analysis is preferred.

[0120] In yet another embodiment, the outer iridium layer was oxidized and / or partially converted into crystalline form by heat treatment or electrochemical treatment.

[0121] Under oxidative conditions, for example at elevated temperatures, especially in ambient air or under an oxygen atmosphere, or through electrochemical treatment, atoms of the iridium layer can be oxidized and / or at least partially converted into a crystalline form, for example in the application of the metal oxide platinum iridium core-shell-shell particles in water electrolysis.

[0122] In a preferred embodiment of the invention, no Ir peaks are present in the XRD diffractogram of the metal oxide-platinum-iridium core-shell-shell particles. This confirms the absence of iridium atoms present in a crystalline structure within the iridium layer.

[0123] In a further preferred embodiment, the iridium atoms in the iridium layer are present in amorphous form. According to this embodiment, all iridium atoms are present in amorphous form, preferably in the form of at least one continuous monolayer, and accordingly, the XRD diffractogram of the metal oxide-platinum-iridium core-shell-shell particles of this embodiment exhibits no Ir peaks. The formation of a layer of amorphous iridium atoms on the inner Pt shell can be ensured by applying the inventive method for hydrogen gas-driven Ir deposition on Pt surfaces.

[0124] In a preferred embodiment, the iridium content of the metal oxide-platinum-iridium core-shell-shell particles, based on the total weight of the metal oxide-platinum-iridium core-shell-shell particles, is 2 to 30 wt%, preferably 4 to 25 wt%, more preferably 7 to 20 wt%, and most preferably 12 to 18 wt%.

[0125] Another aspect of the invention relates to the use of the iridium-coated platinum nanoparticles or the metal oxide platinum-iridium core-shell-shell particles described above in electrochemical devices, in particular in electrolyzers and fuel cells.

[0126] An electrochemical device according to the present invention is a device that either directly converts chemical energy into electrical energy, for example in a fuel cell, or that uses electrical energy to trigger a chemical reaction, for example in an electrolyzer in which water is split into hydrogen and oxygen by electrical energy.

[0127] Various reactions can be used in a fuel cell, for example the oxidation of hydrogen, methanol or methane with oxygen, with the reaction of hydrogen with oxygen being particularly preferred.

[0128] The previously described iridium-coated platinum nanoparticles or metal oxide platinum-iridium core-shell-shell particles can be advantageously used as catalysts in electrochemical devices, especially in electrolyzers and fuel cells.

[0129] Here, iridium-coated platinum nanoparticles or metal oxide-platinum-iridium core-shell-shell particles are primarily used for the production of electrodes and electrode surfaces for use in electrolyzers or fuel cells, particularly as catalyst layers. To produce the electrodes or electrode surfaces, a catalyst ink is often first created by dispersing the iridium-coated platinum nanoparticles or metal oxide-platinum-iridium core-shell-shell particles in a solvent that may contain one or more ionomers or binders. This catalyst ink is then applied to a conductive material, usually a metal, especially a metal with a positive standard potential relative to a hydrogen electrode, such as gold, silver, copper, and preferably titanium and nickel.After the catalyst ink is applied, the electrodes are usually dried at elevated temperature to fix the catalyst material.

[0130] The iridium-coated platinum nanoparticles or the metal oxide platinum iridium core-shell-shell particles can be used in the manufacture of membrane electrode assemblies, especially PEM water electrolyzers.

[0131] The iridium-coated platinum nanoparticles or the metal oxide-platinum-iridium core-shell-shell particles can be used, in particular, for the production of anode catalysts in electrolyzers. For use in electrolyzers, the iridium-coated platinum nanoparticles or metal oxide-platinum-iridium core-shell-shell particles according to the invention are applied to electrodes, preferably made of titanium and nickel, for example, by first preparing a catalyst tincture by suspension in a liquid medium, e.g., an alcohol, which is then applied and dried.

[0132] When used in membrane electrode assemblies of PEM water electrolyzers, the iridium-coated platinum nanoparticles or the metal oxide-platinum-iridium core-shell-shell particles according to the invention can be applied as a thin film to an inert foil after the preparation of a catalyst ink by dispersion in a solvent. The foil is then dried. A durable proton exchange membrane (PEM) is placed between the anode thus prepared and a thin film prepared analogously with a suitable catalyst powder, which serves as the cathode. The electrodes and the membrane are then pressed together under elevated temperature and pressure to obtain a membrane electrode assembly after removal of the inert foil.

[0133] When used in fuel cells, iridium-coated platinum nanoparticles or metal oxide-platinum-iridium core-shell-shell particles are employed as catalytically active electrode material. For this purpose, the particles are preferably used as a catalyst layer on one or both sides of a PEM membrane, for example, on the cathode side and / or the anode side. The use of iridium-coated platinum nanoparticles or metal oxide-platinum-iridium core-shell-shell particles as catalytically active electrode material in reversible fuel cells is also preferred.

[0134] Electrochemical devices comprising the iridium-coated platinum nanoparticles or metal oxide platinum-iridium core-shell particles described above according to the invention, such as electrolyzers and fuel cells, also represent an aspect of the invention.

[0135] The electrochemical devices comprising the iridium-coated platinum nanoparticles or metal oxide-platinum-iridium core-shell-shell particles can be, in particular, electrolyzers and fuel cells. Preferably, the electrochemical devices, especially electrolyzers and fuel cells, contain the platinum nanoparticles or metal oxide-platinum-iridium core-shell-shell particles according to the invention as electrode material, preferably in an electrode coating. Specific electrochemical devices are proton exchange membrane (PEM) fuel cells, in particular electrodes or electrode sheets of PEM fuel cells, PEM water electrolyzers, in particular electrodes or electrode sheets of PEM water electrolyzers, and membrane electrode assemblies, as well as metal electrodes coated with the iridium-coated platinum nanoparticles or metal oxide-platinum-iridium core-shell-shell particles.

[0136] Another aspect of the invention relates to a method for producing the above-described iridium-coated platinum nanoparticles by hydrogen-driven deposition of iridium onto platinum nanoparticles, wherein the method is characterized in that the surface of platinum nanoparticles is brought into contact with Ir cations in an acidic liquid reaction medium in the presence of H2 gas.

[0137] The synthesis process according to the invention is based on H₂-gas-promoted deposition of iridium, in which the high reactivity of platinum with hydrogen is used to reduce dissolved iridium ions and deposit them on the platinum surface. According to the present invention, the term deposition refers to the partial or complete coating of the surface of a platinum nanoparticle with iridium atoms, and, in the case of a closed layer of iridium atoms on a platinum nanoparticle, to the further coating of the same with iridium atoms.

[0138] In this synthesis method, platinum nanoparticle surfaces are brought into contact with dissolved iridium ions in an acidic liquid reaction medium in the presence of hydrogen gas. The hydrogen dissolved in the reaction medium adsorbs onto the platinum surfaces and reduces their surface potential (H₂@Pt = 0 V vs. Reversible Hydrogen Electrode (RHE)). This low surface potential allows the dissolved iridium ions to reduce and deposit onto the surfaces. Since this crucial process would not occur in the absence of hydrogen in the acidic reaction medium, the deposition is referred to as "hydrogen-driven." For a net reaction to occur, the hydrogen oxidation reaction (HÖR) must take place on the platinum surfaces.This deposition has the advantage that it preferentially takes place on the platinum surface, since the HOR activity of Pt is higher than the activity of iridium for HOR (which is ~ 3x lower).

[0139] Using this simple method, iridium can be deposited in minimal and defined quantities onto platinum nanoparticles, controlled by the amount of dissolved iridium ions, the reaction time and the amount of hydrogen present in the reaction medium, and catalysts with very high iridium-specific activity and very high electrical conductivity can be produced.

[0140] Fig. 1 schematically illustrates the synthesis process using a platinum nanoparticle:

[0141] Platinum nanoparticles (1) adsorb hydrogen (3) in an acidic electrolyte in which lr 3+Iridium ions (2) are present. Due to the high activity of platinum towards hydrogen, the surface potential of the platinum nanoparticle is lowered sufficiently to allow adsorbed hydrogen to be oxidized and the iridium ions (2) on the surface to be reduced. Thus, iridium is preferentially deposited in minimal amounts on the platinum nanoparticle. With a sufficient quantity of Ir cations (2) in the reaction medium, sufficient H₂ supply, and sufficient reaction time, a continuous layer of iridium atoms (2) is formed on the platinum nanoparticle (1).

[0142] The platinum nanoparticles used in the process are characterized in that they have a mean particle size of 0.5 to 1000 nm, preferably 0.8 to 250 nm, more preferably 1 to 100 nm, and even more preferably 2 to 50 nm, and (apart from technically unavoidable impurities) consist exclusively of platinum, i.e., 99 wt% or more, more preferably 99.9 wt% or more, more preferably 99.99 wt% or more.

[0143] The acidic liquid reaction medium comprises a solvent and, if required, one or more Brønsted acids, and is not specifically limited except that it must be liquid at the temperature at which the process is carried out and exhibit acidic properties. In the acidic liquid reaction medium, the platinum nanoparticles are brought into contact with Ir cations in the presence of H₂ gas. Therefore, the solvent used should preferably have good solubility for Ir salts and hydrogen. Preferably, the solvent used is liquid in the range of -20 to 120 °C, more preferably in the range of 0 °C to 100 °C, even more preferably in the range of 5 °C to 70 °C, and still more preferably in the range of 10 °C to 50 °C.

[0144] The solvent comprising the reaction medium, which preferably constitutes more than 80 vol%, more preferably more than 90 vol%, even more preferably more than 95 vol%, and even more preferably more than 98 vol% of the total volume of the reaction medium, is not specifically limited. It may be, for example, water, organic solvents such as ethers, alcohols, esters, amides, carboxylic acids, sulfoxides, phosphate esters, or ionic liquids, with water being preferred.

[0145] In principle, water and homogeneous mixtures of organic solvents with water are preferred solvents; water is particularly preferred. For the purposes of this invention, the term "homogeneous mixtures" means that water and the organic solvents used are completely soluble in each other in the quantities used, and that a single phase is present.

[0146] For the purposes of the present invention, "acidic" with respect to the reaction medium means that it has a pH value less than 7.0. If the reaction medium does not contain water and a direct pH determination is not possible, the minimum concentration of dissociated protons in the reaction medium must be at least 10⁻⁵. 7 mmol H + / L.

[0147] The presence of H₂ gas is achieved by bringing the reaction medium into contact with H₂ gas, causing the hydrogen to dissolve in the reaction medium and be adsorbed by the platinum nanoparticles. According to the present invention, the H₂ gas is hydrogen or a hydrogen-containing gas mixture, preferably pure hydrogen or a mixture of hydrogen with an inert gas. According to the invention, an inert gas is understood to be a gas that does not react with any of the compounds present in the process, for example, argon. The manner in which the H₂ gas is brought into contact with the reaction medium for the purpose of dissolving hydrogen in the reaction medium is not particularly restricted. For example, the reaction medium can be shaken or stirred in an atmosphere of H₂ gas at normal or elevated pressure, or the H₂ gas can be introduced into the reaction medium, the latter being preferred.

[0148] The choice of Ir cations is not particularly restricted, except that they must have a positive charge. Preferably, Ir cations are used. 3+ -cations. In order to bring the Ir cations into contact with the platinum nanoparticles, one or more Ir salts are dissolved in the reaction medium or pre-dissolved in a solvent or the reaction medium and then added to the reaction medium containing the platinum nanoparticles.

[0149] The ir-salt used is not particularly restricted as long as it contains ir- cations; preferably it is an ir- ion. 3+ -Salt, for example IrCh.

[0150] In the context of the present invention, bringing the platinum nanoparticles into contact with the Ir cations means that they are both present in the acidic liquid reaction medium. The interaction of the Ir cations and platinum nanoparticles is preferably facilitated by agitation of the reaction medium, for example, by stirring or shaking. The choice of the reaction vessel in which the process is carried out is not particularly restricted. Examples of suitable reaction vessels include glass flasks and glass tubes, such as multi-necked flasks and Schlenk flasks with volumes in the milliliter or liter range, steel autoclaves, pressure reactors, stirred tank reactors, mixers, and tank reactors. The reaction vessels can be made of glass, metal alloys, plastics, or any other suitable material and preferably have a mixing device, such as a stirrer, for example, a propeller stirrer or an anchor stirrer.

[0151] In a preferred embodiment of the above-mentioned process according to the invention, the acidic liquid reaction medium is an aqueous solution. The term aqueous solution here encompasses a solution of Ir cations containing water or a mixture of water with an organic solvent that is completely miscible with water in the selected mixing ratio under the reaction conditions. Due to the pyrophoric properties of platinum nanoparticles and, in particular, metal nanoparticles that have hydrogen adsorbed on their surface, the use of water as the solvent in the reaction medium is strongly preferred for safety reasons. Furthermore, water is preferred because of the good solubility of iridium salts and Brønsted acids in water.Another advantage of water is the good dispersibility of platinum nanoparticles in water, as well as the good separability of the iridium-coated platinum nanoparticles from water-based reaction media after iridium deposition. Possible mixtures of water and organic solvents that can be used as reaction media include, but are not limited to, mixtures of water with an organic solvent selected from the group consisting of alcohols, e.g., methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, sec-butanol, tert-butanol, ethanediol, propane-1,2-diol, propane-1,3-diol, or glycerol; ethers, e.g., THF, dioxanes; polyethers, such as polyethylene glycols or polypropylene glycols; carboxylic acids, e.g., acetic acid; amides, e.g., dimethylformamide or dimethylacetamide; and sulfoxides, e.g., dimethyl sulfoxide.Preferably, mixtures of water with monohydric or dihydric alcohols are used, preferably with methanol, ethanol, propanol, ethanediol, particularly preferably with methanol and ethanol.

[0152] In a further preferred embodiment of the method according to the invention, the method is characterized in that the acidic liquid reaction medium has a pH value in the range of 6 to 0, preferably in the range of 4 to 0.5, more preferably in the range of 3 to 0.75, even more preferably in the range of 2.5 to 1, and most preferably in the range of 2 to 1.25.

[0153] The pH value is the opposite of the decadic logarithm of the hydrogen ion activity and thus represents a measure of the acidic or basic character of an aqueous solution. Determining the pH value therefore requires the presence of water in the reaction medium. According to the present invention, the pH value of the reaction medium is determined potentiometrically, e.g., with a pH meter with a glass electrode at 20 °C. The reaction proceeds under acidic conditions, i.e., at a high concentration of H₂. + The reaction favors the formation of iridium ions. The hydrogen oxidation reaction is the electron source for the reaction of iridium cations. This reaction does not occur quickly enough in the neutral or basic pH range, or it follows a different mechanism, so the desired overall reaction does not take place. Therefore, an acidic medium is required to carry out the process.

[0154] In a further preferred embodiment of the inventive process for producing the iridium-coated platinum nanoparticles, the acidic liquid reaction medium is a solution of one or more Brønsted acids. Preferably, it is an aqueous solution of one or more Brønsted acids. According to the present invention and the generally accepted definition, a Brønsted acid consists of particles that accept protons (H₂). + -ions) can be transferred to a second reaction partner, a base. The acid strength of Brønsted acids is usually given by the acid constant Ks, which is the equilibrium constant of the reaction of the Brønsted acid with a protonable solvent, especially water, or its negative decadic logarithm pKs. According to the embodiment, Brønsted acids with a pKs relative to water are preferred. s-Value less than 5, preferably less than 4, even more preferably less than 1 and even more preferably less than -2.

[0155] The Brønsted acid according to the present embodiment can be, but is not limited to, an inorganic acid, such as one from the group consisting of phosphoric acid (H3PO4), nitric acid (HNO3), sulfuric acid (H2SO4), hydrochloric acid (HCl), perchloric acid (HClIO4), sulfurous acid (H2SO3), nitrous acid (HNO2), and boric acid (H3BO3). It can also be an organic acid, such as sulfonic acids, for example, toluenesulfonic acid, or mono-, di-, or tricarboxylic acids, such as formic acid, acetic acid, oxalic acid, or citric acid. Preferred Brønsted acids according to the present embodiment are perchloric acid and sulfuric acid, with sulfuric acid being particularly preferred.

[0156] Preferably, the liquid acidic reaction medium is a solution comprising water, sulfuric acid and Ir^'ions, and more preferably consists of water, sulfuric acid and a dissolved Ir(III) compound.

[0157] In a particularly preferred embodiment of the process according to the invention, the Brønsted acid is perchloric acid or sulfuric acid. Sulfuric acid is particularly preferred because it is readily available, inexpensive, and environmentally friendly to dispose of. The sulfuric acid anions formed during the dissociation of protons in the reaction medium do not impair the course of the reaction.

[0158] In a further particularly preferred embodiment of the process according to the invention, the concentration of Brønsted acid in the acidic liquid reaction medium is 10 to 5000 mmol / L, preferably 25 to 3000 mmol / L, more preferably 50 to 2000 mmol / L, more preferably 75 to 1500 mmol / L, and most preferably 100 to 1000 mmol / L. The presence of dissociated protons in the reaction medium in which the hydrogen-driven iridium deposition takes place is, as described above, necessary for the process to proceed, since otherwise the hydrogen oxidation reaction does not occur and the desired overall reaction cannot take place.

[0159] Particularly preferably, the Brönsted acid is perchloric acid or sulfuric acid, especially sulfuric acid, which is present as a liquid reaction medium at a concentration of 100 to 1000 mmol / L in water or a water / solvent mixture, preferably water.

[0160] In a further embodiment of the method according to the invention, the Ir cations originate from Ir 3+ -compounds, preferably selected from the group consisting of IrCh, IrAca, lr(acac)3, IrBrs and lridium(lll)2,4-pentanedionate, most preferably from IrCh.

[0161] I⁻ compounds are readily accessible and storage-stable iridium compounds. The term I⁻ compounds includes not only the respective salts, but also the Ir⁻ group. 3 * as a cation, also hydrates and complexes of such salts, which are sometimes more or faster soluble in water and other solvents than the anhydrous or uncomplexed salts. The mention of specific lr 3+- According to the invention, compounds always include their hydrates “XH₂O”, for example, “IrCh” also includes iridium trichloride hydrate “IrCh XH₂O”. The preferred compounds IrCh, IrAcs, IrBrs and iridium(III)2,4-pentanedioate are commercially available and readily accessible. IrCh, which is frequently used as a starting compound for the synthesis of iridium complexes, is a source of Ir 3+ -cations are particularly preferred, with the IrCh hydrate being even more preferred due to its better solubility in water and other solvents.

[0162] In another preferred embodiment, the concentration of Ir cations in the acidic liquid reaction medium at the beginning of the deposition is 0.01 to 1500 mmol / L, preferably 0.5 to 750 mmol / L, more preferably 1.0 to 350 mmol / L, even more preferably 2.0 to 200 mmol / L, and most preferably 5.0 to 100 mmol / L.

[0163] The concentration of Ir cations in the acidic liquid reaction medium that can be brought into contact with the platinum surfaces allows control over the iridium content of the iridium-coated platinum nanoparticles produced in the process and the deposition rate. The selected concentration depends on the particle size of the platinum nanoparticles used and the desired iridium content relative to the total weight of the iridium-coated platinum nanoparticles or the desired thickness of the iridium layer.

[0164] In a further preferred embodiment of the process according to the invention, the amount of substance of the Ir cations used is 0.01 to 10 times, preferably 0.02 to 5 times, more preferably 0.05 to 2 times, even more preferably 0.1 to 1 times, most preferably 0.2 to 0.5 times the amount of substance of the platinum of the platinum nanoparticles present in the reaction medium [mmol / mmol].

[0165] The ratio of the amounts of substance is determined by weighing the platinum nanoparticles and the iridium compound used. The amount of iridium compound used, relative to the amount of platinum nanoparticles, determines the maximum amount of iridium that can be deposited as an iridium coating on the surface of the platinum nanoparticles, influences the rate at which the deposition can be carried out, and affects the average thickness of the deposited iridium coating.

[0166] In yet another preferred embodiment of the inventive method, the amount of substance of the Ir cations used is 1*10 -7 mmol to 1*10 -3 mmol, preferably 5*10' 7 mmol to 5*10' 4 mmol, preferably 1*10 -6 mmol to 1*10 -5 mmol, or even more preferably 2*10' 6 mmol to 2*10' 5 mmol, preferably 3*10' 6mmol to 5*10' 5 mmol per cm 2 the surface area of ​​the platinum nanoparticles determined by BET analysis.

[0167] The BET surface area of ​​the platinum nanoparticles is measured before the hydrogen-driven iridium deposition process, preferably according to DIN 66132. Determining the BET surface area allows calculation of the amount of iridium required to achieve a desired coating level on the platinum nanoparticles, for example, to form a monolayer. To form, for instance, a monolayer of iridium atoms covering the entire surface of the platinum nanoparticles, approximately 3 × 10⁻⁶ µm² of iridium atoms are needed. 6 mmol iridium cations per cm 2the surface area determined by BET analysis is required, whereby typically an excess of the iridium compound is used compared to the amount of iridium atoms to be deposited. Preferably, at least 1.5 times, more preferably at least 2.5 times, even more preferably at least 4 times, and most preferably at least 6 times the amount of iridium atoms [mmol] of Ir cations [mmol] to be deposited, for example as Ir 3+ -Salt, used to avoid long reaction times to achieve the desired iridium coating and to enable a rapid execution of the process.

[0168] In another preferred embodiment of the method according to the invention, the H2 gas is pure H2 or a mixture of H2 and an inert gas selected from nitrogen, argon, neon, krypton, and xenon, helium, radon, preferably argon, wherein the H2 content is preferably in the range of 0.0001 to 100 vol.%, preferably 0.01 to 50 vol.%, more preferably 0.1 to 10 vol.%, most preferably 0.3 to 1 vol.%.

[0169] The concentration of H₂ in the H₂ gas present during the process allows for control of the iridium deposition rate on the platinum surface, as the hydrogen adsorbed onto platinum is required as a reducing agent for the reduction of Ir cations from the reaction medium. To enable sufficient H₂ adsorption onto the surface of the platinum nanoparticles, hydrogen must be dissolved in the reaction medium. Pure H₂ and mixtures of H₂ and an inert gas with a high hydrogen concentration are advantageous when the process is carried out in an atmosphere of H₂ gas at normal pressure or, preferably, overpressure, without a continuous supply of H₂ to the reaction vessel. In contrast, when the H₂ gas is introduced into the acidic liquid reaction medium, a lower concentration of H₂ in the H₂ gas is sufficient.The rate at which hydrogen is supplied to the reaction medium can also be controlled by the introduction rate of the hydrogen gas.

[0170] In a particularly preferred embodiment of the method according to the invention, the H2 gas is supplied to the platinum surfaces of the platinum nanoparticles by introducing a stream of H2 gas into the reaction medium.

[0171] By introducing the H2 gas stream into the liquid acidic reaction medium, for example through a gas introduction tube, a gas introduction tube with a glass filter plate (frit), a gas introduction tube with a gas frit made of polytetrafluoroethylene or any other device immersed in the reaction medium, which ensures a uniform and fine bubbling of the introduced H2 gas, a rapid establishment of the saturation equilibrium of the reaction medium with the H2 gas and thus a rapid absorption of H2 on the platinum surface is ensured.

[0172] In a further preferred embodiment of the method according to the invention, the contact of the Pt surfaces with Ir cations is carried out in the presence of H2 gas at a temperature in the range of 10 to 75 °C, preferably from 15 to 50 °C, more preferably from 17 to 40 °C, even more preferably from 20 to 35 °C, and most preferably from 25 to 30 °C.

[0173] The temperature at which the process according to the invention can be carried out is not specifically limited. According to the embodiment, a temperature range is preferred in which the reaction media used can be handled safely, no energy-intensive heating of the reaction vessel is required, and the deposition of iridium can proceed at a sufficiently high rate. This temperature range is particularly advantageous when water is used as the solvent in the reaction medium and sulfuric acid as the Brønsted acid to acidify the reaction medium.

[0174] In a preferred embodiment of the method according to the invention, the method comprises a further step of removing the reaction medium, washing and drying.

[0175] To ensure the storage of the iridium-coated platinum nanoparticles obtained by the inventive process without altering the catalyst material and its properties, it is preferred to remove the iridium-coated platinum nanoparticles from the acidic liquid reaction medium after completion of the hydrogen-driven iridium deposition, wash them, and dry them. Various methods are practical for separating the iridium-coated platinum nanoparticles, for example, filtration, centrifugation, and / or decantation, the method chosen depending on the quantity of iridium-coated platinum nanoparticles to be separated. To prevent oxidation of the platinum or iridium surface of the iridium-coated platinum nanoparticles, removal from the acidic liquid reaction medium can be carried out by separation under a protective gas, for example, argon.The washing step can be performed during separation or in a separate step, for example, by rinsing the filter cake obtained by filtration, or by slurrying with a suitable washing liquid and re-centrifuging the iridium-coated platinum nanoparticles separated by centrifugation and decantation. In principle, any solvent suitable for washing the iridium-coated platinum nanoparticles is also suitable as a solvent in the acidic liquid reaction medium of the process, i.e., one that exhibits sufficiently high solubility of the Ir salts and acids used. Water and mixtures of water and organic solvents are preferred. Particularly preferred are water and sufficiently polar organic solvents with a low boiling point, such as methanol and ethanol, since these can be easily removed by evaporation.Washing of the iridium-coated platinum nanoparticles can be carried out under a protective gas atmosphere. Drying of the iridium-coated platinum nanoparticles is not particularly restricted and can be carried out by ventilation, heating, and / or under reduced pressure. Drying can be performed under a protective gas atmosphere to prevent oxidation of the particle surface.

[0176] In a particularly preferred embodiment of the method according to the invention, the method comprises the following steps: i) Preparation of a solution of an Ir 3+-compound, preferably IrCh, by dissolving the I^-compound in a liquid reaction medium, preferably water; ii) preparation of a solution of a Brønsted acid, preferably sulfuric acid, in a liquid reaction medium, preferably water; iii) preparation of a dispersion of platinum nanoparticles in the solution prepared in step ii); iv) combining the solution from step i) and the dispersion from step iii) to form a reaction solution and contacting the reaction solution with H2 or a H2-containing gas mixture, preferably by introducing a hydrogen / argon gas mixture and / or stirring the reaction mixture; v) separation of the iridium-coated platinum nanoparticles, preferably by filtration, decantation or centrifugation; vi) optionally washing and / or drying the iridium-coated platinum nanoparticles obtained in step v).

[0177] More preferably, the process comprises the following steps: i) preparing a solution by dissolving IrCh in water or a mixture of water and another solvent; ii) preparing a solution of sulfuric acid in water or a mixture of water and another solvent; iii) preparing a dispersion of platinum nanoparticles in the solution prepared in step ii); iv) combining the solution from step i) and the dispersion from step iii) to form a reaction solution and contacting the reaction solution with a hydrogen-containing gas mixture, preferably by introducing a hydrogen / argon gas mixture and / or stirring the reaction mixture; v) separating the iridium-coated platinum nanoparticles, preferably by filtration, decantation, or centrifugation; vi) optionally washing and / or drying the iridium-coated platinum nanoparticles obtained in step v).

[0178] It is preferred that step 1 is carried out in water, the concentration of sulfuric acid in the reaction mixture in step iv) is 50 to 2000 mmol / L, the temperature in step iv) is between 10 and 50 °C, and the amount of IrCh in step i) is 0.1-1 times the amount of platinum nanoparticles dispersed in step iii).

[0179] In another aspect, the invention relates to iridium-coated platinum nanoparticles obtainable by the inventive method described in the preceding embodiments and further characterized in that the iridium content of the iridium-coated platinum nanoparticles, based on the total weight of the iridium-coated platinum nanoparticles, is 1 to 40 wt%, preferably 2 to 30 wt%, more preferably 4 to 25 wt%, more preferably 7 to 20 wt%, and most preferably 12 to 18 wt%.

[0180] The method described under the previous aspect of the invention yields iridium-coated platinum nanoparticles which can exhibit all the properties described under the first aspect of the invention.All properties, features and preferred areas described in the embodiments of the first aspect, for example the mean particle size of the platinum nanoparticles and the iridium-coated platinum nanoparticles, the mean layer thickness of the iridium layer and the presence of a closed iridium layer on the surface of the platinum nanoparticles, the absence of iridium peaks in the XRD diffractogram of the iridium-coated platinum nanoparticles can be achieved by carrying out the inventive process for producing iridium-coated platinum nanoparticles by hydrogen-driven deposition of iridium on platinum nanoparticles, and the iridium-coated platinum nanoparticles produced by the process are the subject of the invention.

[0181] An important advantage of the process according to the invention is that, during hydrogen-driven deposition of iridium on the platinum nanoparticles, the iridium atoms are deposited exclusively in amorphous form. Thus, even when the platinum surface of the platinum nanoparticles is completely covered with a monolayer of iridium atoms, further iridium atoms are deposited in amorphous form as single atoms, in aggregates of atoms, and finally as amorphous layers, without the formation of crystalline iridium structures. This property allows the process according to the invention to produce iridium-coated platinum nanoparticles in which the iridium exhibits a particularly high mass-specific activity. The iridium-coated platinum nanoparticles obtained by the process according to the invention, in whose XRD diffractogram no iridium peaks are present, are therefore particularly preferred.

[0182] Another aspect of the invention relates to the use of iridium-coated platinum nanoparticles according to the above aspect in electrochemical devices, in particular in electrolyzers and fuel cells, as well as electrochemical devices, in particular electrodes and catalyst layers, comprising iridium-coated platinum nanoparticles according to the aspect described above.

[0183] The iridium-coated platinum particles obtained by the described inventive method for hydrogen-driven deposition of iridium on platinum nanoparticles are used in electrochemical devices and encompassed by such electrochemical devices in the same way as has already been described for the inventive iridium-coated platinum nanoparticles of the first aspect of the invention.

[0184] Finally, one aspect of the invention relates to a method for hydrogen-driven deposition of iridium on one or more platinum surfaces located on a non-platinum support material, characterized in that the support material with the platinum surfaces is brought into contact with Ir cations in an acidic liquid reaction medium in the presence of H2 gas.

[0185] The process corresponds to the hydrogen-driven deposition of iridium on platinum nanoparticles described above, except that the deposition does not take place on the surface of platinum nanoparticles, but rather on platinum surfaces located on a non-platinum support material. The process is otherwise carried out in the same manner as the process for producing iridium-coated platinum nanoparticles by hydrogen-driven deposition of iridium on platinum nanoparticles, and features the same preferred embodiments and preferred ranges of material and reaction parameters.

[0186] The synthesis process according to the invention is therefore also based on hydrogen-gas-promoted deposition of iridium, in which the high reactivity of platinum with hydrogen is used to reduce dissolved iridium ions and deposit them on the platinum surface. In an acidic liquid reaction medium, platinum surfaces of materials consisting of non-platinum support materials on which platinum surfaces are located are brought into contact with dissolved iridium ions in the presence of hydrogen gas. The hydrogen dissolved in the reaction medium adsorbs onto the platinum surfaces and reduces the surface potential of the surfaces (H₂@Pt = 0 V vs. reversible hydrogen electrode (RHE)). The low surface potential of the platinum surfaces can reduce the dissolved iridium ions and deposit them on the surfaces.

[0187] For a net reaction to occur, the hydrogen oxidation reaction (HOR) must take place on the platinum surfaces. This deposition has the advantage of preferentially occurring on the platinum surface because the HOR activity of platinum is higher than that of iridium for HOR (which is approximately three times lower). Using this simple method, iridium can be deposited on the platinum surfaces in minimal and precisely defined amounts, and catalysts with very high iridium-specific activity can be produced. The size of the platinum surfaces is not particularly limited in this process.The platinum surfaces on the substrate materials can be the surface of platinum particles or platinum layers deposited onto the substrate by electroplating, thermal deposition, chemical vapor deposition, or physical vapor deposition (sputtering), or by sintering the substrate materials with platinum particles. The substrate material is not specifically restricted, except that it must not be platinum. Preferably, the substrate material consists of carbon, for example, carbon black, an inorganic oxide, for example, silicon dioxide, titanium dioxide, aluminum oxide, or zirconia, or an elemental metal, for example, nickel or titanium.

[0188] The support material is preferably in the form of fibers or small particles, so that the BET surface area of ​​the support materials and the support materials on which platinum surfaces are located is in the range of 10 to 250 m². 2 / g, preferably in the range of 30 to 150 m 2 The surface area of ​​the BET is determined according to DIN 66132. It is further preferred that the support material is in the form of small particles having an average particle size of 1–200 pm, preferably 2–100 pm, more preferably 5–80 pm, and even more preferably 10–50 pm. The term “average particle size” is defined, with respect to the support material and the support material on which platinum surfaces are located, as the D50 value of the particle size distribution. The particle size distribution is determined by laser diffraction particle size analysis. “D50 value of the particle size distribution” means that 50% (based on volume) of the particles have a particle size below the D50 value expressed in pm.

[0189] These values ​​of the particle size distribution can, in principle, be determined by analytical laser diffraction particle analysis.

[0190] The particle size distribution values ​​of the invention are generally obtained by analytical laser diffraction techniques (see, for example, http: / / pharmazie-lehrbuch.de / kapitel / 3-1.pdf). Preferably, the particle size distributions according to the present invention are obtained using an LS 13 320

[0191] The particle size distribution was obtained using a Beckman Coulter laser diffraction particle size analyzer, following the instructions “LS 13 320 Laser Diffraction Particle Size Analyzer Instructions For Use PN B05577AB (October 2011)” and employing complete Mie theory. This form of laser diffraction yields volume-weighted distributions, where the contribution of each particle to the distribution is proportional to its volume (which, assuming uniform density, corresponds to its mass), meaning the relative contribution is proportional to its size. The particle size distribution according to the present invention is particularly preferably obtained using a 20 g sample of particles measured with a Beckman Coulter LS laser particle size analyzer equipped with a dry powder system. A run length of approximately 13 inches and an attenuation of 4% are used.The particle size distribution is calculated using a computer program based on the cumulative percentage subsize distribution.

[0192] The process for hydrogen-driven deposition of iridium on one or more platinum surfaces located on a non-platinum support material is carried out analogously to the process described above for hydrogen-driven iridium deposition on platinum nanoparticles, except that the support materials bearing platinum surfaces are used instead of the platinum nanoparticles. The same specifications apply and the same embodiments are preferred as described above for the deposition of iridium on platinum nanoparticles with regard to the selection of the acidic liquid reaction medium, the selection and quantity of any Brønsted acids present, the presence of H₂ gas, the selection and quantity of Ir compounds, the reaction temperature, and the other reaction conditions and additional reaction steps.In this aspect of the process for hydrogen gas-driven deposition of iridium on one or more platinum surfaces located on a non-platinum support material, it is therefore preferred if the acidic liquid reaction medium is an aqueous solution; and / or the acidic liquid reaction medium has a pH value in the range of 6 to 0, preferably in the range of 4 to 0.5, more preferably in the range of 3 to 0.75, even more preferably in the range of 2.5 to 1, and most preferably in the range of 2 to 1.25; and / or the acidic liquid reaction medium is a solution of one or more Brønsted acids; and / or one of the Brønsted acids is perchloric acid or sulfuric acid; and / or.

[0193] - the concentration of Brønsted acid in the acidic liquid reaction medium is 10 to 5000 mmol / L, preferably 25 to 3000 mmol / L, more preferably 50 to 2000 mmol / L, more preferably 75 to 1500 mmol / L, and most preferably 100 to 1000 mmol / L; and / or

[0194] - the Ir cations are derived from I ^ compounds, preferably selected from the group consisting of IrCh, IrAca, lr(acac)a, IrBrs and lridium(III)2,4-pentanedionate, most preferably from IrCh; and / or

[0195] - the concentration of Ir cations in the acidic liquid reaction medium at the beginning of the deposition is 0.01 to 1500 mmol / L, preferably 0.5 to 750 mmol / L, more preferably 1.0 to 350 mmol / L, even more preferably 2.0 to 200 mmol / L, and most preferably 5.0 to 100 mmol / L; and / or

[0196] - the amount of substance of the Ir cations used is 0.01 to 10 times, preferably 0.02 to 5 times, more preferably 0.05 to 2 times, even more preferably 0.1 to 1 time, most preferably 0.2 to 0.5 times the amount of substance of platinum on the platinum surfaces present in the reaction medium, which are located on the non-platinum support material, [mmol / mmol]; and / or

[0197] - the amount of substance of the Ir cations used 1*10 -7 mmol to 1*10 -3 mmol, preferably 5*10- 7 mmol to 5*10' 4 mmol, preferably 1*10 -6 mmol to 1*10 -5 mmol, or even more preferably 2*10' 6 mmol to 2*10' 5 mmol, preferably 3*10' 6 mmol to 5*10' 5 mmol per cm 2 the surface area of ​​the platinum surfaces located on the non-platinum support material, as determined by BET analysis; and / or

[0198] - the H2 gas is pure H2 or a mixture of H2 and an inert gas selected from nitrogen, argon, neon, krypton, and xenon, helium, radon, preferably argon, wherein the H2 content is preferably in the range of 0.0001 to 100 vol.%, preferably 0.01 to 50 vol.%, more preferably 0.1 to 10 vol.%, most preferably 0.3 to 1 vol.%; and / or

[0199] - the H2 gas is supplied to the platinum surfaces by introducing a stream of H2 gas into the reaction medium; and / or

[0200] - the contact of the Pt surfaces with Ir cations is carried out in the presence of H2 gas at a temperature in the range of 10 to 75 °C, preferably from 15 to 50 °C, more preferably from 17 to 40 °C, even more preferably from 20 to 35 °C, and most preferably from 25 to 30 °C; and / or

[0201] - the process includes a further step of removing the reaction medium, washing and drying; and / or

[0202] - the procedure includes the following steps: i) Preparation of a solution of an Ir 3+ -compound, preferably IrCh, by dissolving the I^-compound in a liquid reaction medium, preferably water; ii) preparation of a solution of a Brønsted acid, preferably sulfuric acid, in a liquid reaction medium, preferably water; iii) preparation of a dispersion, suspension or immersion of the support material with the platinum surfaces in the solution prepared in step ii); iv) combining the solution from step i) and the dispersion, suspension or

[0203] Immersion from step iii) to a reaction solution and contact of the reaction solution with H2 or a H2-containing gas mixture, preferably by introducing a hydrogen / argon gas mixture and / or stirring the reaction mixture; v) separation of the non-platinum support material with iridium-coated platinum surfaces, preferably by filtration, decantation or centrifugation; optionally washing and / or drying of the non-platinum support material with iridium-coated platinum surfaces obtained in step v).

[0204] A further advantage of the described method for depositing iridium on platinum surfaces mounted on a non-platinum substrate is that the deposition occurs selectively on the platinum surfaces. This selectivity allows the amount of iridium used to be kept low. This is a significant advantage over methods described in the literature, where iridium is deposited not only on the platinum surfaces but also in an uncontrolled manner on the substrate, often in crystalline form.

[0205] In a preferred embodiment of the method, the non-platinum support material with one or more platinum surfaces comprises metal oxide platinum core-shell particles covered with a platinum shell that is more than 70% closed, preferably completely closed, and which are micro- or nanoparticles.

[0206] According to this embodiment, the process particularly preferably comprises the following additional steps for producing the metal oxide core particles, which are covered with a platinum shell that is more than 70% closed, preferably completely closed: a) Production of an aqueous solution containing Pt 4+_- Ions, preferably PtC ions; containing, by dissolving one or more platinum precursor compounds, such as H2PtCl or other water-soluble platinum salts, in particular sodium or potassium hexachloroplatinate, in water; b) Production of a dispersion of the metal oxide core particles, in particular titanium oxide, niobium oxide, cerium oxide, tantalum oxide or zirconium oxide core particles, in water and adjusting the pH to less than 6, preferably less than 5, more preferably less than 4.5, even more preferably less than 4, by adding an acid, preferably an inorganic acid, more preferably hydrochloric acid, nitric acid, sulfuric acid or perchloric acid, for example by adding aqueous HNO3, and subsequent dispersion of the metal oxide core particles, for example by ultrasonic dispersion;c) Combining the solution prepared in step a) and the dispersion prepared in step b), preferably by adding the solution prepared in step a) to the dispersion prepared in step b), optionally adding a hole catcher, for example isopropanol, and preferably passing N2 through to remove O2 from the solution; d) Deposition of metallic platinum on the surface of the core particles by irradiating the reaction mixture prepared in step c) with UV radiation, preferably with a wavelength in the range of 200 nm to 280 nm, preferably UV radiation with a wavelength of 254 nm, for example from a mercury vapor lamp; e) optionally filtration, washing and drying of the powder prepared in step d) to obtain metal oxide-platinum core-shell particles.

[0207] It is preferred that, after carrying out step d), a reduction of the remaining Pt precursor compounds is carried out as an additional synthesis step, for example by heating the reaction mixture, so that a hole catcher contained in the reaction mixture, for example isopropanol, acts as a reducing agent.

[0208] Platinum precursor compounds are defined here as all platinum-containing compounds that can be used in the process for depositing metallic platinum on the surface of the core particles. These can be salts with platinum cations or platinum complex compounds, with Pt4+ salts, such as the salts of hexachloroplatinic acid, especially the sodium and potassium salts, hexachloroplatinic acid itself, and platinum(IV) halides, being preferred. The salts can also exist as hydrates.

[0209] Another aspect concerns the material, which consists of a non-platinum support material with iridium-coated platinum surfaces and is obtained by the process described in the preceding section. The resulting materials are characterized by the presence of amorphous iridium atoms deposited on the platinum surfaces, but not on the surfaces of the support material. The iridium-coated platinum surfaces of the non-platinum support materials exhibit amorphously deposited iridium, which can be present as individual atoms, clusters of atoms, or closed layers of iridium atoms. The iridium used can be efficiently utilized for catalysis; furthermore, the XRD diffractograms of these materials show no Ir peak, as no crystalline iridium is present.

[0210] In a preferred embodiment of the material according to the invention, the carrier material, which does not consist of platinum, is a nickel, titanium or titanium oxide carrier, preferably titanium or titanium oxide fibers.

[0211] Nickel, titanium, or titanium oxide supports consist of nickel or titanium or titanium dioxide, respectively, and their shape is not particularly restricted. However, they can preferably be in the form of fibers or small particles with an average particle size of 1–200 pm, determined by laser diffraction spectrometry, as described above. Nickel and titanium supports are distinguished from supports consisting of inorganic oxides such as SiO₂ by their high conductivity. Nickel, titanium, or titanium oxide supports consisting of fibers of the metals or metal oxide with diameters ranging from 10 pm to 500 pm are particularly preferred. For the purposes of this invention, metal or metal oxide fibers are linear structures consisting of a metal or metal oxide with a diameter of 500 pm to 0.5 pm.The definition also includes short fibers where the length-to-diameter ratio is less than 100, whereas for the other fibers it is 100 or more. The method of providing the platinum surfaces of the material according to the embodiment is not specifically restricted, but can, for example, be achieved by electrodeposition of platinum particles onto the Ni or Ti support.

[0212] In a further preferred embodiment, the material obtained comprises metal oxide-platinum-iridium core-shell-shell particles, consisting of a metal oxide core having a platinum shell that is more than 70% closed, preferably completely closed, on which an outer iridium layer is deposited, and which are micro- or nanoparticles. In this embodiment, it is preferred that the iridium content of the metal oxide-platinum-iridium core-shell-shell particles, based on the total weight of the metal oxide-platinum-iridium core-shell-shell particles, is 1 to 40 wt%, preferably 2 to 30 wt%, more preferably 4 to 25 wt%, even more preferably 7 to 20 wt%, and most preferably 12 to 18 wt%.

[0213] It is also preferred that the metal oxide core consists of one of the following metal oxides or alloys of these metal oxides: titanium oxide, niobium oxide, cerium oxide, tantalum oxide, and zirconium oxide.

[0214] Furthermore, the obtained metal oxide-platinum-iridium core-shell-shell particles according to the embodiment are preferably characterized in that the metal oxide core is a metal oxide particle with a mean particle size of 10 nm to 5 .m, preferably 50 nm to 1 .m, most preferably 100 nm to 500 nm.

[0215] The average particle size of these particles is preferably determined by TEM analysis.

[0216] Preferably, the mean layer thickness of the Pt shell is from 1 nm to 50 nm, preferably from 2 nm to 10 nm.

[0217] The mean layer thickness of the Pt shell is preferably determined by TEM analysis.

[0218] Finally, according to the embodiment, it is preferred that the Pt content of the obtained metal oxide-platinum-iridium core-shell-shell particles is 5 to 70 wt%, preferably 7 to 50 wt%, more preferably 10 to 40 wt%, most preferably 12 to 35 wt%, based on the total weight of the core-shell-shell particles, and it is preferred that the mean layer thickness of the iridium layer is from 0.1 nm to 20 nm, more preferably from 0.2 nm to 15 nm, and even more preferably from 0.3 nm to 10 nm. It is also particularly preferred that in the obtained metal oxide-platinum-iridium core-shell-shell particles the iridium layer forms a closed layer on the surface of the platinum shell, and / or that no Ir peaks are present in the XRD diffractogram of the metal oxide-platinum-iridium core-shell-shell particles, and / or that the iridium atoms in the iridium layer are in amorphous form.

[0219] In a further preferred embodiment, the iridium layer of the obtained material was oxidized and / or at least partially converted into crystalline form by heat treatment or electrochemical treatment. A further aspect of the invention relates to the use of the material described above in electrochemical devices, in particular in electrolyzers and fuel cells, as well as electrochemical devices, in particular electrodes and catalyst layers, comprising such a material.

[0220] The material, which consists of a non-platinum support material with iridium-coated platinum surfaces, is used in electrochemical devices as previously described for the iridium-coated platinum nanoparticles and the metal oxide platinum iridium core-shell-shell particles.

[0221] Another aspect of the invention relates to a method for the oxidation of iridium-coated platinum nanoparticles or of the material consisting of a non-platinum support material with iridium-coated platinum surfaces, such as those according to the aspects described above, characterized in that the iridium coating of the platinum nanoparticles or the platinum surfaces of said material is at least partially converted into iridium oxide.

[0222] The iridium-coated platinum nanoparticles described as the first aspect of the invention, the particles obtained by the inventive process for depositing iridium onto platinum nanoparticles, and the non-platinum support materials with iridium-coated platinum surfaces, such as the metal oxide-platinum-iridium core-shell-shell particles, which are micro- or nanoparticles, can be subjected to a process by which the iridium coating is partially or completely converted into iridium oxide. The iridium oxide is primarily lrÜ2, i.e., lr(IV) oxide. However, since non-stoichiometric iridium oxides can also be formed during the oxidation of iridium, the term also includes other potentially present oxygen-containing oxidation products of iridium in which iridium is in a positive oxidation state. Preferably, the iridium oxide is lrÜ2.The procedure can be carried out in different ways.

[0223] A preferred embodiment involves bringing the iridium-coated platinum nanoparticles or the materials with iridium-coated platinum surfaces, especially the metal oxide-platinum-iridium core-shell-shell particles according to the invention, into contact with oxygen or an oxygen-containing atmosphere, for example ambient air (room air), preferably at an elevated temperature. By bringing the material into contact with oxygen, the iridium of the surface coating is converted into iridium oxide by oxidation.

[0224] In a further preferred embodiment of the aspect, the oxidation of the iridium coating is carried out in an electrochemical process by oxidizing the iridium-coated platinum nanoparticles used as anode material or non-platinum support materials with iridium-coated platinum surfaces, such as the metal oxide-platinum-iridium core-shell-shell particles, which are micro- or nanoparticles.

[0225] By using them as an anode material in an electrochemical process, preferably in an electrolyzer for the electrolysis of water, the iridium coating of the iridium-coated platinum nanoparticles or non-platinum support materials with iridium-coated platinum surfaces is converted into iridium oxide. The resulting oxidized iridium-coated platinum nanoparticles or non-platinum support materials with oxidized iridium-coated platinum surfaces are further used as anode material, as they represent effective catalyst materials for use in electrochemical devices.

[0226] Another aspect of the invention relates to oxidized iridium-coated platinum nanoparticles or non-platinum support materials with oxidized iridium-coated platinum surfaces, such as the metal oxide platinum iridium core-shell-shell particles, which are micro- or nanoparticles obtainable by the above-described process for the oxidation of iridium-coated platinum nanoparticles.

[0227] In the oxidized iridium-coated platinum nanoparticles or non-platinum support materials with oxidized iridium-coated platinum surfaces according to the invention, such as metal oxide-platinum-iridium core-shell-shell particles, which are micro- or nanoparticles, at least part of the iridium coating is in the form of iridium oxide.

[0228] In yet another aspect, the invention relates to the use of the oxidized iridium-coated platinum nanoparticles or non-platinum support materials with oxidized iridium-coated platinum surfaces according to the above aspect in electrochemical devices, in particular in electrolyzers and fuel cells, as well as the electrochemical devices, in particular electrodes and catalyst layers, which comprise the oxidized iridium-coated platinum nanoparticles or non-platinum support materials with oxidized iridium-coated platinum surfaces according to the above aspect.

[0229] The oxidized iridium-coated platinum nanoparticles or materials with oxidized iridium-coated platinum surfaces fulfill the same requirements as catalyst materials as the previously described iridium-coated platinum nanoparticles or materials with iridium-coated platinum surfaces according to the invention, and are formed from these by oxidation when used in electrochemical devices. Accordingly, the oxidized iridium-coated platinum nanoparticles or materials with oxidized iridium-coated platinum surfaces are used in the previously described electrochemical devices and applications, such as fuel cells and electrolyzers, particularly as electrodes or electrode materials, just like the previously described iridium-coated platinum nanoparticles or materials with iridium-coated platinum surfaces.

[0230] A final aspect of the invention relates to a method for producing micro- or nano-metal oxide platinum core-shell particles covered with a platinum shell that is more than 70% closed, preferably completely closed.

[0231] These particles are used as starting materials in the hydrogen gas-driven process according to the invention for the deposition of iridium on one or more platinum surfaces located on a non-platinum support material, in order to produce the metal oxide-platinum-iridium core-shell-shell particles according to the invention, which consist of a metal oxide core covered with a platinum shell that is more than 70% closed, preferably completely closed, on which an outer layer of iridium is deposited.

[0232] In this process, a more than 70% closed, preferably completely closed, layer of metallic platinum is deposited on a metal oxide core consisting of a metal oxide micro- or nanoparticle by preparing an acidic, Pt cation-containing dispersion of metal oxide cores, and reducing the platinum cations by irradiation with UV light and depositing them in the form of elemental platinum on the surface of the metal oxide cores.

[0233] Optionally, a hole catcher can be added to the Pt cation-containing dispersion of metal oxide nuclei. In the photochemical reduction of metals, the excitation of electrons from the valence band to the conduction band creates free, high-energy electrons in the conduction band and positive charge gaps, "electron holes," in the valence band. The presence of a hole catcher—that is, a compound that rapidly captures positively charged holes by being oxidized itself—prevents the recombination of electron holes and free electrons, thus making the electrons available for the reduction of the metal ions. The presence of a hole catcher can significantly increase the efficiency of photochemical metal reduction.The most common hole catchers are organic alcohols, such as methanol, ethanol, isopropanol, organic acids, such as formic acid or oxalic acid, or inorganic ions, such as iodide ions or sulfite ions, with alcohols being preferred in the present process.

[0234] The process for producing micro- or nano-metal oxide platinum core-shell particles covered with a platinum shell that is more than 70% closed, preferably completely closed, preferably comprises the following steps: a) Preparation of an aqueous solution containing Pt 4+-ions, preferably containing PtC'-ions, by dissolving platinum precursor compounds, such as H2PtCl or other water-soluble platinum salts, in particular sodium or potassium hexachloroplatinate, in water; b) producing a dispersion of the metal oxide core particles, in particular titanium oxide, niobium oxide, cerium oxide, tantalum oxide or zirconium oxide core particles, in water and adjusting the pH to less than 6, preferably less than 5, more preferably less than 4.5, even more preferably less than 4, by adding an acid, preferably an inorganic acid, more preferably hydrochloric acid, nitric acid, sulfuric acid or perchloric acid, for example by adding aqueous HNO3, and subsequently dispersing the metal oxide core particles, for example by ultrasonic dispersion;c) Combining the solution prepared in step a) and the dispersion prepared in step b), preferably by adding the solution prepared in step a) to the dispersion prepared in step b), optionally adding a hole catcher, for example isopropanol, and preferably passing N2 through to remove O2 from the solution; d) Deposition of metallic platinum on the surface of the core particles by irradiating the reaction mixture prepared in step c) with UV radiation, preferably with a wavelength in the range of 200 nm to 280 nm, preferably UV radiation with a wavelength of 254 nm, for example from a mercury vapor lamp; e) optionally filtration, washing and drying of the powder prepared in step d) to obtain metal oxide-platinum core-shell particles.

[0235] In a preferred embodiment, the metal oxide core particles are titanium oxide, niobium oxide, cerium oxide, tantalum oxide or zirconium oxide core particles.

[0236] In a further preferred embodiment of the manufacturing process, the mean particle size of the metal oxide particles is in the range of 10 nm to 5 .m, preferably 50 nm to 1 .m, most preferably 100 nm to 500 nm.

[0237] In yet another preferred embodiment of the process, after carrying out the above-mentioned process step d), a reduction of the remaining Pt precursor compounds is carried out as an additional synthesis step, for example by heating the reaction mixture, so that a hole catcher contained in the reaction mixture, preferably isopropanol, acts as a reducing agent.

[0238] The additional reduction step of this embodiment is not carried out photochemically. In the presence of a suitable oxidizing agent, for example a C1-C6 alkyl alcohol, in particular methanol, ethanol, or isopropanol, or a C1-C6 mono-, di-, or tricarboxylic acid, such as formic acid, acetic acid, oxalic acid, or citric acid, which can act as an oxidizing agent, any unreacted Pt cations remaining in the reaction mixture can be wholly or partially reduced. This reduction of the Pt cations takes place particularly at elevated temperatures, for example at 40 °C or more, preferably at 60 °C or more, and even more preferably at 75 °C or more.

[0239] Finally, the invention relates to micro- or nano-metal oxide platinum core-shell particles covered with a platinum shell that is more than 70% closed, preferably completely closed, and obtainable by the manufacturing process described above. These particles preferably have the features described above for the metal oxide core and the platinum shell, which is more than 70% closed, preferably completely closed, of the metal oxide platinum iridium core-shell particles according to the invention.

[0240] Summary of embodiments of the invention

[0241] The embodiments of the invention are summarized below:

[0242] 1. Iridium-coated platinum nanoparticles, characterized in that the iridium content of the iridium-coated platinum nanoparticles, based on the total weight of the iridium-coated platinum nanoparticles, is 1 to 40 wt%, preferably 2 to 30 wt%, more preferably 4 to 25 wt%, even more preferably 7 to 20 wt%, and most preferably 12 to 18 wt%.

[0243] 2. Iridium-coated platinum nanoparticles according to embodiment 1, characterized in that the particle size of the platinum nanoparticles is in the range of 0.5 to 150 nm, preferably in the range of 1 to 100 nm, more preferably in the range of 2 to 50 nm, even more preferably in the range of 3 to 35 nm, even more preferably in the range of 4 to 20 nm, and most preferably in the range of 5 to 10 nm.

[0244] 3. Iridium-coated platinum nanoparticles according to one of embodiments 1 or 2, characterized in that the iridium layer forms a closed layer on the surface of the platinum nanoparticle.

[0245] 4. Iridium-coated platinum nanoparticles according to any of the preceding embodiments 1 to 3, characterized in that the thickness of the iridium layer is 0.10 nm to 5 nm, preferably 0.14 nm to 4 nm, more preferably 0.18 nm to 3 nm, more preferably 1.22 nm to 2.0 nm, and most preferably 0.26 nm to 1.8 nm.

[0246] 5. Iridium-coated platinum nanoparticles according to any of the preceding embodiments 1 to 4, characterized in that the particle size of the iridium-coated platinum nanoparticles is in the range of 1 to 200 nm, preferably in the range of 2 to 150 nm, more preferably in the range of 3 to 100 nm, more preferably in the range of 4 to 50 nm, more preferably in the range of 4 to 30 nm, and most preferably in the range of 5 to 15 nm.

[0247] 6. Iridium-coated platinum nanoparticles according to any of the preceding embodiments 1 to 5, characterized in that no Ir peaks are present in the XRD diffractogram of the iridium-coated platinum nanoparticles.

[0248] 7. Iridium-coated platinum nanoparticles according to any one of the preceding embodiments 1 to 6, characterized in that the iridium layer consists of one or more iridium monolayers, preferably one iridium monolayer. 8. Iridium-coated platinum nanoparticles according to any one of the preceding embodiments 1 to 7, characterized in that they have a mass-specific activity of Ir of at least 100 Ai. r ' 1 , preferably at least 500 A gir' 1 , even more preferably at least 1000 A gir' 1 , preferably at least 3500 A gir' 1 exhibit.

[0249] 9. Iridium-coated platinum nanoparticles according to any one of the preceding embodiments 1 to 8, characterized in that the iridium content of the iridium-coated platinum nanoparticles is 2 to 30 wt% and the particle size of the platinum nanoparticles is in the range of 1 nm to 100 nm, preferably the iridium content is 2 to 30 wt% and the particle size of the platinum nanoparticles is in the range of 2 nm to 50 nm, more preferably the iridium content is 4 to 25 wt% and the particle size of the platinum nanoparticles is in the range of 3 nm to 35 nm, even more preferably the iridium content is 7 to 20 wt% and the particle size of the platinum nanoparticles is in the range of 4 nm to 20 nm, and most preferably the iridium content is 12 to 18 wt% and the particle size of the platinum nanoparticles is in the range of 5 nm to 10 nm, wherein The iridium content refers to the total weight of the iridium-coated platinum nanoparticles.

[0250] 10. Iridium-coated platinum nanoparticles according to any of the preceding embodiments 1 to 9, characterized in that the particle size of the platinum nanoparticles is in the range of 1 to 100 nm and the thickness of the iridium layer is 0.10 nm to 5 nm, preferably the particle size of the platinum nanoparticles is in the range of 2 to 50 nm and the thickness of the iridium layer is 0.14 nm to 4 nm, more preferably the particle size of the platinum nanoparticles is in the range of 3 to 35 nm and the thickness of the iridium layer is 0.18 nm to 3 nm, even more preferably the particle size of the platinum nanoparticles is in the range of 4 to 20 nm and the thickness of the iridium layer is 0.22 nm to 2 nm, and most preferably the particle size of the platinum nanoparticles is in the range of 5 to 10 nm and the thickness of the iridium layer is 0.26 nm to 1.8 nm.

[0251] 11. Iridium-coated platinum nanoparticles according to any one of the preceding embodiments 1 to 10, characterized in that the iridium atoms in the iridium layer are in amorphous form.

[0252] 12. Metal oxide-platinum-iridium core-shell-shell particles, characterized in that the particles consist of a metal oxide core covered with a platinum shell that is more than 70% closed, preferably completely closed, on which an outer layer of iridium is deposited, that they are micro- or nanoparticles, and that the iridium content of the metal oxide-platinum-iridium core-shell-shell particles is 1 to 50% by weight based on the total weight of the metal oxide-platinum-iridium core-shell-shell particles.

[0253] 13. Metal oxide platinum iridium core-shell-shell particle according to embodiment 12, further characterized in that the metal oxide core consists of one of the following metal oxides or alloys of these metal oxides: titanium oxide, niobium oxide, cerium oxide, tantalum oxide, and zirconium oxide.

[0254] 14. Metal oxide platinum iridium core-shell-shell particle according to one of embodiments 12 or 13, further characterized in that the metal oxide core is a metal oxide particle with a mean particle size of 10 nm to 5 .m, preferably 50 nm to 1 .m, most preferably 100 nm to 500 nm.

[0255] 15. Metal oxide platinum iridium core shell-shell particle according to one of embodiments 12 to 14, further characterized in that the mean layer thickness of the Pt shell is from 1 nm to 50 nm, preferably 2 nm to 10 nm.

[0256] 16. Metal oxide platinum iridium core-shell-shell particles according to one of embodiments 12 to 15, further characterized in that the Pt content of the particles is 5 to 70 wt%, preferably 7 to 50 wt%, more preferably 10 to 45 wt%, most preferably 12 to 40 wt%, based on the total weight of the core-shell-shell particles.

[0257] 17. Metal oxide platinum iridium core-shell-shell particle according to one of embodiments 12 to 16, further characterized in that the mean layer thickness of the Ir layer is from 0.26 nm to 20 nm, preferably from 0.3 nm to 15 nm, more preferably from 0.4 nm to 10 nm.

[0258] 18. Metal oxide platinum iridium core-shell-shell particle according to one of embodiments 12 to 17, further characterized in that the outer iridium layer has been oxidized by heat treatment or electrochemical treatment and / or at least partially converted into crystalline form.

[0259] 19. Metal oxide platinum iridium core-shell-shell particle according to one of embodiments 12 to 18, further characterized in that the iridium layer forms a closed layer on the surface of the platinum shell.

[0260] 20. Metal oxide-platinum-iridium core-shell-shell particles according to one of embodiments 12 to 19, further characterized in that no Ir peaks are present in the XRD diffractogram of the metal oxide-platinum-iridium core-shell-shell particles. 21. Metal oxide-platinum-iridium core-shell-shell particles according to one of embodiments 12 to 20, further characterized in that the iridium atoms in the iridium layer are in amorphous form.

[0261] 22. Metal oxide-platinum-iridium core-shell-shell particles according to one of embodiments 12 to 21, further characterized in that the iridium content of the metal oxide-platinum-iridium core-shell-shell particles, based on the total weight of the metal oxide-platinum-iridium core-shell-shell particles, is 2 to 40 wt%, preferably 4 to 30 wt%, more preferably 5 to 25 wt%, most preferably 7 to 20 wt%.

[0262] 23. Use of the iridium-coated platinum nanoparticles according to any of the preceding embodiments 1 to 11 or the metal oxide platinum iridium core-shell-shell particles according to any of the preceding embodiments 12 to 22 in electrochemical devices, in particular in electrolyzers and fuel cells.

[0263] 24. Electrochemical devices, in particular electrodes and catalyst layers, comprising the iridium-coated platinum nanoparticles according to any one of the preceding embodiments 1 to 11 or the metal oxide platinum iridium core-shell-shell particles according to any one of the preceding embodiments 12 to 22.

[0264] 25. Method for producing the iridium-coated platinum nanoparticles according to any one of embodiments 1 to 11 by hydrogen-driven deposition of iridium onto platinum nanoparticles, characterized in that the surface of platinum nanoparticles is brought into contact with Ir cations in an acidic liquid reaction medium in the presence of H2 gas.

[0265] 26. The method according to embodiment 25, characterized in that the acidic liquid reaction medium is an aqueous solution.

[0266] 27. The method according to any of the preceding embodiments 25 or 26, characterized in that the acidic liquid reaction medium has a pH value in the range of 6 to 0, preferably in the range of 4 to 0.5, more preferably in the range of 3 to 0.75, even more preferably in the range of 2.5 to 1, and most preferably in the range of 2 to 1.25.

[0267] 28. The process according to any of the preceding embodiments 25 to 27, characterized in that the acidic liquid reaction medium is a solution of one or more Brønsted acids. 29. The process according to the preceding embodiment 28, characterized in that one of the Brønsted acids is perchloric acid or sulfuric acid.

[0268] 30. The method according to one of embodiments 28 and 29, characterized in that the concentration of Brønsted acid in the acidic liquid reaction medium is 10 to 5000 mmol / L, preferably 25 to 3000 mmol / L, more preferably 50 to 2000 mmol / L, more preferably 75 to 1500 mmol / L, and most preferably 100 to 1000 mmol / L.

[0269] 31. The method according to any of the preceding embodiments 25 to 30, characterized in that the Ir cations are obtained from lr 3+ - Compounds are preferably selected from the group consisting of IrCh, IrAca, lr(acac)3, IrBrs and lridium(lll)2,4-pentanedionate, most preferably from lrCI3.

[0270] 32. The method according to any of the preceding embodiments 25 to 31, characterized in that the concentration of the Ir- cations in the acidic liquid reaction medium at the beginning of the deposition is 0.01 to 1500 mmol / L, preferably 0.5 to 750 mmol / L, more preferably 1.0 to 350 mmol / L, more preferably 2.0 to 200 mmol / L, and most preferably 5.0 to 100 mmol / L.

[0271] 33. The method according to any of the preceding embodiments 25 to 32, characterized in that the amount of substance of the Ir cations used is 0.01 to 10 times, preferably 0.02 to 5 times, more preferably 0.05 to 2 times, even more preferably 0.1 to 1 times, most preferably 0.2 to 0.5 times the amount of substance of the platinum of the platinum nanoparticles present in the reaction medium [mmol / mmol].

[0272] 34. The method according to any of the preceding embodiments 25 to 33, characterized in that the amount of substance of the Ir cations used is 1*10 -7 mmol to 1*10 -3 mmol, preferably 5*1 O' 7 mmol up to 5*10 -4 mmol, preferably 1*10 -6 mmol to 1*10 -5 mmol, or even more preferably 2*10' 6 mmol to 2*10' 5 mmol, preferably 3*10' 6 mmol to 5*10' 5 mmol per cm 2 the surface area of ​​the platinum nanoparticles determined by BET analysis.

[0273] 35. The method according to any one of the preceding embodiments 25 to 34, characterized in that the hL gas is pure H2 or a mixture of H2 and an inert gas selected from nitrogen, argon, neon, krypton, xenon, helium, radon, preferably argon, wherein the H2 content is preferably in the range of 0.0001 to 100 vol.%, more preferably 0.01 to 50 vol.%, more preferably 0.1 to 10 vol.%, and most preferably 0.3 to 1 vol.%. 36. The method according to any one of the preceding embodiments 25 to 35, characterized in that the H2 gas is supplied to the platinum surfaces by introducing a stream of H2 gas into the reaction medium.

[0274] 37. The method according to any of the preceding embodiments 25 to 36, characterized in that the contacting of the Pt surfaces with Ir cations is carried out in the presence of the H2 gas at a temperature in the range of 10 to 75 °C, preferably from 15 to 50 °C, more preferably from 17 to 40 °C, even more preferably from 20 to 35 °C, and most preferably from 25 to 30 °C.

[0275] 38. The method according to any of the preceding embodiments 25 to 37, characterized in that the method comprises a further step of removing the reaction medium, washing and drying.

[0276] 39. The method according to any of the preceding embodiments 25 to 38, characterized in that the method comprises the following steps: i) Preparation of a solution of an Ir 3+-compound, preferably IrCh, by dissolving the I^-compound in a liquid reaction medium, preferably water; ii) preparation of a solution of a Brønsted acid, preferably sulfuric acid, in a liquid reaction medium, preferably water; iii) preparation of a dispersion of platinum nanoparticles in the solution prepared in step ii); iv) combining the solution from step i) and the dispersion from step iii) to form a

[0277] v) Preparation of the reaction solution and contact of the reaction solution with H2 or an H2-containing gas mixture, preferably by introducing a hydrogen / argon gas mixture and / or stirring the reaction mixture; v) Separation of the iridium-coated platinum nanoparticles, preferably by

[0278] Filtration, decanting, or centrifugation; vi) optionally washing and / or drying the iridium-coated platinum nanoparticles obtained in step v). 40. Iridium-coated platinum nanoparticles obtainable by the process described in embodiments 25 to 39, further characterized in that the iridium content of the iridium-coated platinum nanoparticles, based on the total weight of the iridium-coated platinum nanoparticles, is 1 to 40 wt%, preferably 2 to 30 wt%, more preferably 4 to 25 wt%, even more preferably 7 to 20 wt%, and most preferably 12 to 18 wt%.

[0279] 41. Use of the iridium-coated platinum nanoparticles according to embodiment 40 in electrochemical devices, in particular in electrolyzers and fuel cells.

[0280] 42. Electrochemical devices, in particular electrodes and catalyst layers comprising iridium-coated platinum nanoparticles according to embodiment 40.

[0281] 43. A process for hydrogen-driven deposition of iridium on one or more platinum surfaces located on a non-platinum support material, characterized in that the support material with the platinum surfaces is brought into contact with an acidic liquid reaction medium containing Ir cations in the presence of H₂ gas, wherein the acidic liquid reaction medium is optionally an aqueous solution; and / or the acidic liquid reaction medium has a pH value in the range of 6 to 0, preferably in the range of 4 to 0.5, more preferably in the range of 3 to 0.75, even more preferably in the range of 2.5 to 1, and most preferably in the range of 2 to 1.25; and / or the acidic liquid reaction medium is a solution of one or more Brønsted acids; and / or one of the Brønsted acids is perchloric acid or sulfuric acid;and / or the concentration of Brønsted acid in the acidic liquid reaction medium is 10 to 5000 mmol / L, preferably 25 to 3000 mmol / L, more preferably 50 to 2000 mmol / L, more preferably 75 to 1500 mmol / L, and most preferably 100 to 1000 mmol / L; and / or the Ir cations are derived from I⁻ compounds, preferably selected from the group consisting of IrCh, IrAca, Ir(acac)a, IrBrs and Iridium(III)2,4-pentanedionate, most preferably IrCh; and / or the concentration of Ir cations in the acidic liquid reaction medium at the beginning of the deposition is 0.01 to 1500 mmol / L, preferably 0.5 to 750 mmol / L, more preferably 1.0 to 350 mmol / L, even more preferably 2.0 to 200 mmol / L, and most preferably 5.0 to 100 mmol / L;and / or the amount of substance of the Ir cations used is 0.01 to 10 times, preferably 0.02 to 5 times, more preferably 0.05 to 2 times, even more preferably 0.1 to 1 time, most preferably 0.2 to 0.5 times the amount of substance of the platinum of the platinum surfaces present in the reaction medium, which are located on the non-platinum support material, [mmol / mmol]; and / or the amount of substance of the Ir cations used is 1*10; -7 mmol to 1*10 -3 mmol, preferably 5*10 -7 mmol up to 5*10 -4 mmol, preferably 1*10 -6 mmol to 1*10 -5 mmol, or even more preferably 2*10' 6 mmol to 2*10' 5 mmol, preferably 3*10' 6 mmol to 5*10' 5 mmol per cm 2the surface area of ​​the platinum surfaces located on the non-platinum support material, as determined by BET analysis; and / or the hL gas is pure H2 or a mixture of H2 and an inert gas selected from nitrogen, argon, neon, krypton, and xenon, helium, radon, preferably argon, wherein the H2 content is preferably in the range of 0.0001 to 100 vol.%, more preferably 0.01 to 50 vol.%, more preferably 0.1 to 10 vol.%, most preferably 0.3 to 1 vol.%.-%; and / or the H2 gas is supplied to the platinum surfaces by introducing a stream of H2 gas into the reaction medium; and / or the contact of the Pt surfaces with Ir cations is carried out in the presence of the H2 gas at a temperature in the range of 10 to 75 °C, preferably from 15 to 50 °C, more preferably from 17 to 40 °C, even more preferably from 20 to 35 °C, and most preferably from 25 to 30 °C; and / or the process comprises a further step of removing the reaction medium, washing and drying; and / or the process comprises the following steps: i) Preparation of a solution of an Ir. 3+-compound, preferably IrCh, by dissolving the I^-compound in a liquid reaction medium, preferably water; ii) preparation of a solution of a Brønsted acid, preferably sulfuric acid, in a liquid reaction medium, preferably water; iii) preparation of a dispersion, suspension or immersion of the support material with the platinum surfaces in the solution prepared in step ii); iv) combining the solution from step i) and the dispersion, suspension or

[0282] Immersion from step iii) to a reaction solution and contact of the reaction solution with H2 or an H2-containing gas mixture, preferably by introducing a hydrogen / argon gas mixture and / or stirring the reaction mixture; v) separation of the non-platinum support material with iridium-coated platinum surfaces, preferably by filtration, decantation or centrifugation; vi) optionally washing and / or drying of the non-platinum support material obtained in step v).

[0283] Platinum existing substrate material with iridium-coated platinum surfaces.

[0284] 44. Method according to embodiment 43, characterized in that the non-platinum support material with one or more platinum surfaces is a metal oxide platinum core shell particle covered with a platinum shell that is more than 70% closed, preferably completely closed, and which is a micro- or nanoparticle.

[0285] 45. A method according to claim 44, characterized in that the method additionally comprises the following steps for producing the metal oxide core particles covered with a platinum shell that is more than 70% closed, preferably completely closed: a) Preparation of an aqueous solution containing Pt 4+ -ions, preferably PtCk 2'Irons contained, by dissolving one or more platinum precursor compounds, such as H2PtCl or other water-soluble platinum salts, in particular sodium or potassium hexachloroplatinate, in water; b) Production of a dispersion of the metal oxide core particles, in particular titanium oxide, niobium oxide, cerium oxide, tantalum oxide or zirconium oxide core particles, in water and adjusting the pH to less than 6, preferably less than 5, more preferably less than 4.5, even more preferably less than 4, by adding an acid, preferably an inorganic acid, more preferably hydrochloric acid, nitric acid, sulfuric acid or perchloric acid, for example by adding aqueous HNO3, and subsequent dispersion of the metal oxide core particles, for example by ultrasonic dispersion;c) Combining the solution prepared in step a) and the dispersion prepared in step b), preferably by adding the solution prepared in step a) to the dispersion prepared in step b), optionally adding a hole catcher, for example isopropanol, and preferably passing N2 through to remove O2 from the solution; d) Deposition of metallic platinum on the surface of the core particles by irradiating the reaction mixture prepared in step c) with UV radiation, preferably with a wavelength in the range of 200 nm to 280 nm, preferably UV radiation with a wavelength of 254 nm, for example from a mercury vapor lamp; e) optionally filtration, washing and drying of the powder prepared in step d) to obtain metal oxide-platinum core-shell particles.

[0286] 46. ​​Method according to embodiment 45, characterized in that, after carrying out step d), a reduction of the remaining Pt precursor compounds is carried out as an additional synthesis step, for example by heating the reaction mixture, so that a hole catcher contained in the reaction mixture, for example isopropanol, acts as a reducing agent.

[0287] 47. Material comprising a non-platinum support material with iridium-coated platinum surfaces, obtainable by the method described in the preceding embodiments 43 to 46.

[0288] 48. Material according to embodiment 47, characterized in that the support material is a nickel, titanium or titanium oxide support, preferably titanium or titanium oxide fibers.

[0289] 49. Material according to embodiment 47, further characterized in that it is a metal oxide platinum iridium core-shell-shell particle consisting of a metal oxide core having a platinum shell more than 70% closed, preferably completely closed, on which an outer iridium layer is deposited, and that it is a micro- or nanoparticle.

[0290] 50. Material according to embodiment 49, further characterized in that the iridium content of the metal oxide-platinum-iridium core-shell-shell particles, based on the total weight of the metal oxide-platinum-iridium core-shell-shell particles, is 1 to 50 wt%, preferably 2 to 40 wt%, more preferably 4 to 30 wt%, even more preferably 5 to 25 wt%, most preferably 7 to 20 wt%.

[0291] 51. Material according to one of embodiments 49 and 50, characterized in that the metal oxide core consists of one of the following metal oxides or alloys of these metal oxides: titanium oxide, niobium oxide, cerium oxide, tantalum oxide, and zirconium oxide.

[0292] 52. Material according to one of the embodiments 49 to 51, characterized in that the metal oxide core consists of metal oxide particles with a mean particle size of 10 nm to 5 .m, preferably 50 nm to 1 .m, most preferably 100 nm to 500 nm.

[0293] 53. Material according to one of the embodiments 49 to 52, characterized in that the mean layer thickness of the Pt shell is from 1 nm to 50 nm, preferably from 2 nm to 10 nm.

[0294] 54. Material according to one of embodiments 49 to 53, characterized in that the Pt content is 5 to 70 wt%, preferably 7 to 50 wt%, more preferably 10 to 45 wt%, most preferably 12 to 40 wt%, based on the total weight of the core-shell-shell particles.

[0295] 55. Material according to one of the embodiments 49 to 54, characterized in that the mean layer thickness of the iridium layer is from 0.1 nm to 20 nm, preferably from 0.2 nm to 15 nm, more preferably from 0.3 nm to 10 nm.

[0296] 56. Material according to one of embodiments 49 to 55, characterized in that the iridium layer forms a closed layer on the surface of the platinum shell, and / or that no Ir peaks are present in the XRD diffractogram of the metal oxide-platinum-iridium core-shell-shell particles, and / or that the iridium atoms in the iridium layer are in amorphous form.

[0297] 57. Material according to one of embodiments 47 to 56, characterized in that the iridium layer has been oxidized and / or at least partially converted into crystalline form by heat treatment or electrochemical treatment.

[0298] 58. Use of the material according to any embodiment 47 to 57 in electrochemical devices, in particular in electrolyzers and fuel cells. 59. Electrochemical devices, in particular electrodes and catalyst layers, comprising a material according to any embodiment 47 to 57.

[0299] 60. Method for the oxidation of iridium-coated platinum nanoparticles or of non-platinum support materials with iridium-coated platinum surfaces according to one of embodiments 1-22, 40 or 47-57, characterized in that the iridium coating of the platinum nanoparticles or the iridium-coated platinum surfaces is at least partially converted into iridium oxide.

[0300] 61. Method according to the preceding embodiment 60, characterized in that the oxidation of the iridium coating is carried out in an electrochemical process by oxidizing the iridium-coated platinum nanoparticles used as anode material or the non-platinum support materials with iridium-coated platinum surfaces.

[0301] 62. Oxidized iridium-coated platinum nanoparticles or non-platinum support materials with oxidized iridium-coated surfaces, obtainable by the method described in embodiments 60 and 61.

[0302] 63. Use of the oxidized iridium-coated platinum nanoparticles or non-platinum support materials with oxidized iridium-coated surfaces according to embodiment 62 in electrochemical devices, in particular in electrolyzers and fuel cells.

[0303] 64. Electrochemical devices, in particular electrodes and catalyst layers, comprising oxidized iridium-coated platinum nanoparticles or non-platinum support materials with oxidized iridium-coated surfaces according to embodiment 62.

[0304] 65. A method for producing micro- or nano-metal oxide platinum core-shell particles covered with a platinum shell that is more than 70% closed, preferably completely closed, characterized in that it comprises the following steps for producing the metal oxide platinum core-shell particles: a) Preparation of an aqueous solution containing Pt 4+ -ions, preferably PtCk 2' contains ions, by dissolving one or more platinum precursor compounds, such as H2PtCl or other water-soluble platinum salts, in particular sodium or potassium hexachloroplatinate, in water; b) producing a dispersion of the metal oxide core particles, in particular titanium oxide, niobium oxide, cerium oxide, tantalum oxide or zirconium oxide core particles, in water and adjusting the pH to less than 6, preferably less than 5, more preferably less than 4.5, even more preferably less than 4, by adding an acid, preferably an inorganic acid, more preferably hydrochloric acid, nitric acid, sulfuric acid or perchloric acid, for example by adding aqueous HNO3, and subsequently dispersing the metal oxide core particles, for example by ultrasonic dispersion;c) Combining the solution prepared in step a) and the dispersion prepared in step b), preferably by adding the solution prepared in step a) to the dispersion prepared in step b), optionally adding a hole catcher, for example isopropanol, and preferably passing N2 through to remove O2 from the solution; d) Deposition of metallic platinum on the surface of the core particles by irradiating the reaction mixture prepared in step c) with UV radiation, preferably with a wavelength in the range of 200 nm to 280 nm, preferably UV radiation with a wavelength of 254 nm, for example from a mercury vapor lamp; e) optionally filtration, washing and drying of the powder prepared in step d) to obtain metal oxide-platinum core-shell particles.

[0305] 66. Method according to embodiment 65, characterized in that the metal oxide core particles are titanium oxide, niobium oxide, cerium oxide, tantalum oxide or zirconium oxide core particles.

[0306] 67. Method according to one of embodiments 65 or 66, characterized in that the mean particle size of the metal oxide particles is in the range of 10 nm to 5 .m.

[0307] 68. Method according to one of embodiments 65 to 67, characterized in that, after carrying out step d), a reduction of the remaining Pt precursor compounds is carried out as an additional synthesis step, for example by heating the reaction mixture, so that a hole catcher contained in the reaction mixture, for example isopropanol, acts as a reducing agent.

[0308] 69. Micro or nano metal oxide platinum core shell particles covered with a platinum shell more than 70% closed, preferably completely closed, obtainable by the method described in the preceding embodiments 65 to 68.

[0309] Examples

[0310] Materials used: Pt nanoparticles: “High surface area Pt black” acquired from “Fuel Cell Store” with a mean particle size of 5.0–7.5 nm (manufacturer's specification) were used as precursors in the synthesis of the Pt@lr nanoparticles and as a Pt reference material; lrChxH2O: “IrCh hydrate 54% Ir” from Sigma Aldrich was used;

[0311] Sulfuric acid stock solution: An H2SO4 stock solution was prepared by mixing 972.1 mL of H2O (MilliQ, Merck Millipore) with 27.9 mL of H2SO4 (Suprapur 96%, Merck KGaA) to obtain a 0.5 M electrolyte solution for the following experiments;

[0312] H2, Ar and O2 were used as supplied by Air Liquide Deutschland GmbH (purity: 99.999 mol-%);

[0313] 1-Propanol: 1-Propanol was used as supplied by Sigma Aldrich (99.9%); Iridium nanoparticles: Ir Black (Umicore, purity 99.9 1t. manufacturer)

[0314] Analytical methods and equipment:

[0315] X-ray fluorescence spectroscopy (XRF) was used to determine the amount of iridium deposited on the platinum nanoparticles. The sample was measured in a Bruker M4 Tornado instrument using a tungsten X-ray source and a voltage of 50 kV. The results from over 100 measurement points were averaged. This method is also suitable for determining the weight fractions or ratios of metal oxide, platinum, and iridium in metal oxide-platinum-iridium core-shell particles, and for determining the weight fractions or ratios of metal oxide and platinum in metal oxide-platinum core-shell particles.

[0316] TEM images were acquired using a Talos F200i (Thermo Fisher Scientific) equipped with a Schottky field emission gun (X-FEG) and a dual Bruker XFIash 6|100 EDXS detector. Imaging was performed with a Thermo Fisher Scientific double-tilt holder with low background and high visibility using a molybdenum (Mo) clamp. A primary electron energy of 200 keV was used for spectral imaging and scanning transmission electron microscopy (STEM). For STEM, the electron sample was set to a beam current of 41 pA with a convergence angle of 10.5 mrad. Elastically scattered electrons were collected with a HAADF detector in an angular range of 58–200 mrad. The catalyst samples were applied to conventional Lacey carbon-Cu grids and plasma-cleaned with a Tergeo-EM plasma cleaner prior to imaging (PIE Scientific).To determine the mean particle size from a TEM measurement of a sample of micro- or nanoparticles, three image sections are opened using the program "ImageJ" (version 1.54g). The pixel size is determined using the program's "Analyze -> Set Scale" command and the scale bar in the image. Subsequently, 30 particles in the image section are measured using the program's "Analyze -> Measure" command. The average diameter of the measured particles in the three image sections is calculated by averaging the particle diameters displayed in the program's results table.

[0317] The term "average value" therefore refers to the arithmetic mean obtained by summing the diameters of the 90 measured particles as described and then dividing by 90. This value is the mean particle size of the platinum nanoparticles, the iridium-coated platinum nanoparticles, the metal oxide particles, the metal oxide-platinum core-shell particles, and the metal oxide-platinum-iridium core-shell particles referenced in the description.

[0318] The TEM method (specifically: STEM method) and the statistical evaluation method described above are also suitable for determining the mean thickness of the iridium layer on platinum nanoparticles, the mean thickness of the iridium layer forming the outer layer or shell of metal oxide-platinum-iridium core-shell particles, and the mean thickness of the closed platinum shell of metal oxide-platinum core-shell particles. Here, the layer thickness is determined analogously to the determination of the particle size by analyzing the layer thickness in the image sections. According to the invention, determining the aforementioned parameters by STEM measurement as described is preferred.

[0319] To determine the presence of a closed layer of iridium on a platinum surface, the following procedure is preferably used:

[0320] The presence of a continuous Ir layer can be determined by electrochemical cyclic voltammetry (CV) using a rotating disk electrode (RDE). The CV curve of the Pt surface exhibits a distinct feature with two peaks at 0.125 and 0.25 V compared to the reversible hydrogen electrode (RHE), as shown in Fig. 5. In contrast, the CV curve of the Ir surface does not exhibit this two-peak feature in the same potential range. CV measurements of the Pt particles or MO@Pt particles are performed prior to Ir deposition to determine the coverage of the Pt surface with Ir atoms. If the intensity of the Pt peaks at 0.125 and 0.25 V RHE decreases by > 90%, continuous Ir coverage of the Pt surface is achieved. For all RDE experiments, the catalyst ink was prepared by dispersing 3.92 mg of the catalyst powder in 5 ml of 1-propanol.The ink was treated with ultrasound for 40 minutes, and 10 pl of the ink were dropped onto a polished Au-RDE (Pine Research) tip.

[0321] The RDE tip was then inserted into a commercially available RDE setup from Pine Research, and a Biology SMP 300 potentiostat was used. A graphite rod was used as the counter electrode and an RHE (HydroFlex) as the reference electrode for all experiments. The experiments were performed in a 150 ml glass cell from Pine Research, and argon gas was purged by a 160 ml / min flow for 30 minutes before and during each experiment. -1 Degassed. CV curves were recorded between 0.8 and 0.0 V vus RHE at a scan rate of 100 mV / s.

[0322] This determination is supplemented by TEM and STEM analyses, in which the Ir layer covering the Pt surface is observable.

[0323] To determine the presence of a closed Pt layer on the metal oxide core of a metal oxide platinum core-shell particle, the following procedure is preferably used:

[0324] The continuous Pt layer on the MO particles forms an electrically conductive path for the particles, since the MO particles themselves are non-conductive. Therefore, powder conductivity measurements are performed to determine the Pt coverage of the MO particles. A powder conductivity of > 10 S / cm indicates that the Pt coverage of the MO particles forms an electrical percolation path.

[0325] Powder conductivity measurements were performed in a 3D-printed setup. 20 mg of catalyst powder were compressed to 3 MPa. Compression was precisely controlled by a tensile tester in compression mode (Shimadzu EZ-SX), which simultaneously allowed for thickness measurement of the compressed powder. Using a Van der Pauw configuration, four embedded copper electrodes were employed to determine the powder conductivity. Current and voltage were measured using the integrated source measurement unit of a four-point probe (Ossila BV, four-point probe T2001A3).

[0326] In addition, the determination of a more than 70% closed, preferably completely closed, Pt layer is supplemented by performing energy-dispersive X-ray spectroscopy (EDX) imaging and the STEM method. With a significantly higher atomic mass of Pt compared to transition metals such as titanium oxide, niobium oxide, cerium oxide, tantalum oxide, or zirconium oxide, STEM imaging shows a clear contrast between the Pt layer and the MO layer, making the Pt layer coverage observable. Furthermore, TEM (EDX) elemental imaging is used to visualize the Pt layer coverage on MO particles.

[0327] It should be noted that the presence of a Pt shell that is more than 70% closed, preferably completely closed, can be reliably determined by STEM analysis using optical inspection of the STEM images obtained by the method described above. Therefore, determining the presence of a Pt shell that is more than 70% closed, preferably completely closed, by STEM is preferred.

[0328] Example 1: Synthesis of an iridium-coated platinum nanoparticle (Pt@lr catalyst)

[0329] In a 250 mL round-bottom flask, 0.269 g of hydrated iridium chloride (IrChxFLO) was dissolved in 50 mL of deionized water to obtain an aqueous solution with an iridium ion concentration of 15 mM / L. The solution was stirred for 24 hours. In another 250 mL quartz round-bottom flask, 20 mg of Pt nanoparticles were dissolved in 150 mL of 0.5 M sulfuric acid (aq) and dispersed in an ultrasonic bath for 20 minutes. The solution was then degassed for a further 10 minutes by introducing argon. Subsequently, 5 mL of the IrCh solution was added to the quartz round-bottom flask containing the Pt nanoparticle dispersion (0.5 mM IrChxFLO). 3 * in reaction solution) and hydrogen were introduced. The gas flows were controlled by mass flow controllers to 2 l / min. -1 Ar and 5 ml min -1H₂ was adjusted to obtain an H₂ concentration of 0.25 vol% in the gas stream. The reaction mixture was stirred for 120 min, after which the solid was separated by centrifugation and washed three times with 100 mL of deionized water each time. The catalyst particles were dried at 70°C for 24 h. 23 mg of iridium-coated platinum nanoparticles were obtained.

[0330] XRF analysis of the iridium-coated platinum nanoparticles showed that the iridium content after synthesis was 15.9 wt% iridium based on the total weight of the iridium-coated platinum nanoparticles.

[0331] Transmission electron microscopy (TEM) images of the obtained iridium-coated platinum nanoparticles show that no two phases (iridium and platinum) are formed during the synthesis, thus also suggesting preferential deposition of iridium on platinum. The TEM images are shown in Fig. 2. Furthermore, the TEM-EDX measurements show that Pt and iridium are very homogeneously distributed in the catalyst powder. Even at the highest resolution, both elements can be clearly visualized. However, the particles, at 5–7 nm, are too small to definitively demonstrate a core-shell structure.

[0332] Example 2: Use of iridium-coated platinum nanoparticles in a three-electrode setup

[0333] The catalysts were characterized in half-cell measurements using the so-called rotating disk electrode (RDE). For this purpose, an ink was first prepared from the catalyst particles from Example 1. To do this, 3.92 mg of the catalyst powder from Example 1 were mixed with 5 ml of 1-propanol and dispersed by sonication in an ultrasonic bath for 10 minutes.

[0334] Coating the RDE electrode (gold, diameter s mm, Pine Research) with 10 pl ink resulted in a catalyst loading of 40 pg catalyst-cm. -2The test was performed in 0.5 M H₂O₄, which was degassed with argon for 30 minutes prior to the measurement. A graphite rod served as the counter electrode and a reversible hydrogen electrode (RHE, Hydroflex) as the reference electrode. The RDE measurement was carried out at 1600 rpm. The experiment was performed in a glass cell acquired from Pine Research, with continuous argon degassing at 150 mL / min throughout the experiment.

[0335] The Pt@lr catalyst with an iridium content of only 15.9 wt% from Example 1 showed the same activity compared to pure, commercially available iridium powder (Iridium Black, Umicore). Fig. 3 shows the mass-specific activity of iridium in the two catalysts. The Pt@lr catalyst showed a mass-specific activity of 236.8 A glr 1 At 1.55 V vs. RHE, the commercial Ir reference catalyst showed a mass-specific activity of 48.7 A glr 1at 1.55 V vs. RHE. This represents an 8-fold increase in lr-specific activity.

[0336] Example 3: Production of TiO2@.Pt@lr core-shell particles a) TiO2©)Pt core-shell particles

[0337] To prepare an aqueous solution 1 of a platinum precursor compound, 512.8 mg of H₂PtCl₆*6H₂O (0.99 mmol) were dissolved in 100 mL of water in a 250 mL volumetric flask by mechanical stirring with a magnetic stir bar for 30 minutes. A dispersion 2 of TC₂-Parti₆ was prepared by dissolving 200 mg of TiO₂ particles (rutile particles, size < 5 µm, Sigma Aldrich) in 100 mL of water in a second 250 mL volumetric flask, then adding 4 mL of 0.1 N HNO₃ (aq), adjusting the pH to approximately 3, followed by 30 minutes of ultrasonic treatment in an ultrasonic bath.

[0338] Next, solution 1 and dispersion 2 were mixed in the dispersion 2 flask, followed by the addition of 40 mL of isopropanol as a hole catcher to the mixture. The mixture was then purged with nitrogen for 40 minutes before the reaction was started. Under continuous stirring for 6 hours in a nitrogen atmosphere, platinum was deposited onto the surface of the TiO₂ core particles by UV illumination (excitation wavelength X = 254 nm). In the next step, the reaction mixture was filtered through 0.54 µm nylon filter paper and washed with 500 mL of water. Finally, the TiO₂@Pt core-shell particle powder was dried overnight at 70 °C in an air-flow oven. 400 mg of TiO2@Pt core-shell particles with a weight fraction of 50 wt% platinum based on the total weight of the particles, as determined by XRF analysis, were obtained. b) TiO2( Pt( Jr core-shell-shell particles

[0339] The deposition of the Ir shell onto the TiO2@Pt core-shell particles to form the TiO2@Pt@lr core-shell particles was carried out using the hydrogen gas-driven Ir deposition method described in Example 1.

[0340] The only difference was that instead of Pt nanoparticles, TiO2@Pt core-shell particles, the preparation of which was described in Example 3a), were used.

[0341] To carry out the synthesis on a 200 mg scale, 200 mg of the TiO₂@Pt particles from Example 3a) were dispersed in 500 mL of 0.5 M H₂SO₄ in a 1000 mL round-bottom flask, and the dispersion was degassed. Subsequently, 10.44 mL of a previously prepared 15 mM IrCh solution were added, and a H₂ gas stream (0.9 vol%) was introduced into the flask. The gas flows were controlled to 2 l / min using mass flow controllers. -1 Ar and 18 ml min -1H₂ was adjusted to obtain an H₂ concentration of 0.9 vol% in the gas stream. The reaction was carried out by stirring the reaction solution for 240 minutes. The resulting catalyst powder was then washed five times with 100 mL of deionized water each time, using centrifugation until neutrality was achieved. The powder thus obtained was dried overnight at 80 °C. 216 mg of TiO₂@Pt@lr core-shell-shell particles were obtained, containing 44 wt% Pt and 8 wt% Ir, based on the total weight of the TiO₂@Pt@lr core-shell-shell particles, as determined by XRF analysis. Example 4: Use of TiO₂@Pt@lr core-shell-shell particles in a three-electrode setup.

[0342] The catalytic activity of the TiO2@Pt@lr core-shell-shell particles in the oxygen evolution reaction (OER) was tested according to the same procedure as in Example 2. The results are shown in Figure 3, where the results from

[0343] Example 2 and a comparison experiment with "Ir black" under identical conditions as a reference are shown. The TiO2@Pt@lr catalyst showed a mass-specific activity of 163.5 A / glr, thus significantly exceeding the activity of the commercially available reference catalyst ("Ir black") of 48.7 A / glr at 1.55 V vs. RHE.

Claims

Jülich Research Centre GmbH H70452WO Patent claims 1. Iridium-coated platinum nanoparticles, characterized in that the iridium content of the iridium-coated platinum nanoparticles, based on the total weight of the iridium-coated platinum nanoparticles, is 1 to 40 wt%, preferably 2 to 30 wt%, more preferably 4 to 25 wt%, even more preferably 7 to 20 wt%, most preferably 12 to 18 wt%, wherein the iridium layer is a closed layer on the surface of the platinum nanoparticle.

2. Iridium-coated platinum nanoparticles according to claim 1, characterized in that the particle size of the platinum nanoparticles is in the range of 0.5 to 150 nm, preferably in the range of 1 to 100 nm, more preferably in the range of 2 to 50 nm, even more preferably in the range of 3 to 35 nm, even more preferably in the range of 4 to 20 nm, and most preferably in the range of 5 to 10 nm.

3. Iridium-coated platinum nanoparticles according to any one of the preceding claims 1 and 2, characterized in that the particle size of the iridium-coated platinum nanoparticles is in the range of 1 to 200 nm, preferably in the range of 2 to 150 nm, more preferably in the range of 3 to 100 nm, more preferably in the range of 4 to 50 nm, more preferably in the range of 4 to 30 nm, and most preferably in the range of 5 to 15 nm.

4. Iridium-coated platinum nanoparticles according to any one of the preceding claims 1 to 3, characterized in that no Ir peaks are present in the XRD diffractogram of the iridium-coated platinum nanoparticles.

5. Iridium-coated platinum nanoparticles according to any one of the preceding claims 1 to 4, characterized in that the iridium content of the iridium-coated platinum nanoparticles is 2 to 30 wt% and the particle size of the platinum nanoparticles is in the range of 1 nm to 100 nm, preferably the iridium content is 2 to 30 wt% and the particle size of the platinum nanoparticles is in the range of 2 nm to 50 nm, more preferably the iridium content is 4 to 25 wt% and the particle size of the platinum nanoparticles is in the range of 3 nm to 35 nm, even more preferably the iridium content is 7 to 20 wt% and the particle size of the platinum nanoparticles is in the range of 4 nm to 20 nm, and most preferably the iridium content is 12 to 18 wt% and the particle size of the platinum nanoparticles is in the range of 5 nm to 10 nm, the iridium content referring to the total weight of the iridium-coated platinum nanoparticles.

6. Metal oxide-platinum-iridium core-shell-shell particles, characterized in that the particles consist of a metal oxide core covered with a platinum shell that is more than 70% closed, preferably completely closed, on which an outer layer of iridium is deposited, further characterized in that they are micro- or nanoparticles, and the iridium content of the metal oxide-platinum-iridium core-shell-shell particles is 1 to 50% by weight based on the total weight of the metal oxide-platinum-iridium core-shell-shell particles.

7. Metal oxide platinum iridium core-shell-shell particle according to claim 6, further characterized in that the metal oxide core consists of one of the following metal oxides or alloys of these metal oxides: titanium oxide, niobium oxide, cerium oxide, tantalum oxide, and zirconium oxide.

8. Metal oxide platinum iridium core-shell-shell particle according to claim 6 or 7, further characterized in that the iridium layer forms a closed layer on the surface of the platinum shell.

9. Metal oxide platinum iridium core-shell-shell particles according to one of claims 6 to 8, further characterized in that no Ir peaks are present in the XRD diffractogram of the metal oxide platinum iridium core-shell-shell particles.

10. Use of the iridium-coated platinum nanoparticles according to any one of the preceding claims 1 to 5 or the metal oxide platinum iridium core-shell-shell particles according to any one of the preceding claims 6 to 9 in electrochemical devices, in particular in electrolyzers and fuel cells.

11. Method for producing the iridium-coated platinum nanoparticles according to any one of claims 1 to 5 by hydrogen-driven deposition of iridium on platinum nanoparticles, characterized in that the surface of platinum nanoparticles is brought into contact with Ir cations in an acidic liquid reaction medium in the presence of H2 gas, wherein the acidic liquid reaction medium is preferably an aqueous solution.

12. The method according to the preceding claim 11, characterized in that the acidic liquid reaction medium is a solution of one or more Brønsted acids, wherein one of the Brønsted acids is preferably perchloric acid or sulfuric acid, and / or wherein the concentration of the Brønsted acid in the acidic liquid reaction medium is preferably 10 to 5000 mmol / L, more preferably 25 to 3000 mmol / L, more preferably 50 to 2000 mmol / L, more preferably 75 to 1500 mmol / L, and most preferably 100 to 1000 mmol / L.

13. The method according to one of the preceding claims 11 or 12, characterized in that the amount of substance of the Ir cations used is 0.01 to 10 times, preferably 0.02 to 5 times, more preferably 0.05 to 2 times, even more preferably 0.1 to 1 times, most preferably 0.2 to 0.5 times the amount of substance of the platinum of the platinum nanoparticles present in the reaction medium [mmol / mmol], and / or characterized in that the amount of substance of the Ir cations used is 1*10 -7 mmol to 1*10 -3 mmol, preferably 5*1 O' 7 mmol to 5*1 O' 4 mmol, preferably 1*10 -6 mmol to 1*10 -5 mmol, preferably 2*10 -6 mmol up to 2*10 -5 mmol, preferably 3*10' 6 mmol to 5*10' 5 mmol per cm 2 the surface area of ​​the platinum nanoparticles determined by BET analysis.

14. The method according to any one of the preceding claims 11 to 13, characterized in that the method comprises the following steps: i) Preparation of a solution of an Ir 3+ -compound, preferably IrCh, by dissolving the I^-compound in a liquid reaction medium, preferably water; ii) preparation of a solution of a Brønsted acid, preferably sulfuric acid, in a liquid reaction medium, preferably water; iii) preparation of a dispersion of platinum nanoparticles in the solution prepared in step ii); iv) combining the solution from step i) and the dispersion from step iii) to form a reaction solution and bringing the reaction solution into contact with H2 or a H2-containing gas mixture, preferably by introducing a hydrogen / argon gas mixture and / or stirring the reaction mixture; v) separation of the iridium-coated platinum nanoparticles, preferably by filtration, decantation or centrifugation; vi) optionally washing and / or drying of the iridium-coated platinum nanoparticles obtained in step v).

15. Iridium-coated platinum nanoparticles obtainable by the method claimed in claims 11 to 14, further characterized in that the iridium content of the iridium-coated platinum nanoparticles, based on the total weight of the iridium-coated platinum nanoparticles, is 1 to 40 wt%, preferably 2 to 30 wt%, more preferably 4 to 25 wt%, more preferably 7 to 20 wt%, and most preferably 12 to 18 wt%, wherein the iridium layer is a closed layer on the surface of the platinum nanoparticle.

16. Use of the iridium-coated platinum nanoparticles according to claim 15 in electrochemical devices, in particular in electrolyzers and fuel cells.

17. A method for hydrogen-driven deposition of iridium on one or more platinum surfaces located on a non-platinum support material, characterized in that the support material with the platinum surfaces is brought into contact with Ir cations in an acidic liquid reaction medium in the presence of H2 gas.

18. Method according to claim 17, characterized in that the non-platinum support material with one or more platinum surfaces is a metal oxide platinum core shell particle covered with a platinum shell that is more than 70% closed, preferably completely closed, and which is a micro- or nanoparticle.

19. Method according to claim 18, characterized in that the method comprises the following steps for producing the metal oxide core particles which are covered with a platinum shell that is more than 70% closed, preferably completely closed: a) Preparation of an aqueous solution containing Pt 4+a) containing ions, by dissolving one or more platinum precursor compounds, such as H2PtCl or other water-soluble platinum salts, in particular sodium or potassium hexachloroplatinate, in water; b) producing a dispersion of the metal oxide core particles, in particular titanium oxide, niobium oxide, cerium oxide, tantalum oxide or zirconium oxide core particles, in water and adjusting the pH to less than 6, preferably less than 5, more preferably less than 4.5, even more preferably less than 4, by adding an acid, preferably an inorganic acid, more preferably hydrochloric acid, nitric acid, sulfuric acid or perchloric acid, for example by adding aqueous HNO3, and subsequently dispersing the metal oxide core particles, for example by ultrasonic dispersion;c) Combining the solution prepared in step a) and the dispersion prepared in step b), preferably by adding the solution prepared in step a) to the dispersion prepared in step b), optionally adding a hole catcher, for example isopropanol, and preferably passing N2 through to remove O2 from the solution; d) Deposition of metallic platinum on the surface of the core particles by irradiating the reaction mixture prepared in step c) with UV radiation, preferably with a wavelength in the range of 200 nm to 280 nm, preferably UV radiation with a wavelength of 254 nm, for example from a mercury vapor lamp; e) optionally filtration, washing and drying of the powder prepared in step d) to obtain metal oxide-platinum core-shell particles.

20. Material comprising a non-platinum support material with iridium-coated platinum surfaces, obtainable by the method claimed in any one of the preceding claims 17 to 19.

21. Material according to claim 20, characterized in that the support material is a nickel, titanium or titanium oxide support, preferably titanium or titanium oxide fibers.

22. Material according to claim 20, further characterized in that it comprises metal oxide platinum iridium core-shell-shell particles consisting of a metal oxide core having a platinum shell more than 70% closed, preferably completely closed, on which an outer iridium layer is deposited, and that it comprises micro- or nanoparticles, wherein the iridium content of the metal oxide platinum iridium core-shell-shell particles is 1 to 50 wt% based on the total weight of the metal oxide platinum iridium core-shell-shell particles.

23. Use of the material according to any one of claims 20 to 22 in electrochemical devices, in particular in electrolyzers and fuel cells.

24. A method for the oxidation of iridium-coated platinum nanoparticles or of non-platinum support materials with iridium-coated platinum surfaces according to one of claims 1 to 9, 15, or 20 to 22, characterized in that the iridium coating of the platinum nanoparticles or the iridium-coated platinum surfaces is at least partially converted into iridium oxide, wherein the oxidation is preferably carried out in an electrochemical process by oxidizing the iridium-coated platinum nanoparticles used as anode material or the non-platinum support materials with iridium-coated platinum surfaces.

25. Oxidized iridium-coated platinum nanoparticles or non-platinum support materials with oxidized iridium-coated surfaces, obtainable by the method claimed in claim 24.

26. Use of the oxidized iridium-coated platinum nanoparticles or non-platinum support materials with oxidized iridium-coated surfaces according to claim 25 in electrochemical devices, in particular in electrolyzers and fuel cells.

27. Electrochemical devices, in particular electrodes and catalyst layers, comprising iridium-coated platinum nanoparticles according to any one of claims 1 to 9 or 15, or material consisting of a non-platinum material existing support material with iridium-coated platinum surfaces according to one of claims 20 to 22, or oxidized iridium-coated platinum nanoparticles or non-platinum support materials with oxidized iridium-coated surfaces according to claim 25.

28. A method for producing micro- or nano-metal oxide platinum core-shell particles covered with a platinum shell that is more than 70% closed, preferably completely closed, characterized in that it comprises the following steps for producing the metal oxide platinum core-shell particles: a) Preparation of an aqueous solution containing Pt 4+a) containing ions, by dissolving one or more platinum precursor compounds, such as H2PtCl or other water-soluble platinum salts, in particular sodium or potassium hexachloroplatinate, in water; b) producing a dispersion of the metal oxide core particles, in particular titanium oxide, niobium oxide, cerium oxide, tantalum oxide or zirconium oxide core particles, in water and adjusting the pH to less than 6, preferably less than 5, more preferably less than 4.5, even more preferably less than 4, by adding an acid, preferably an inorganic acid, more preferably hydrochloric acid, nitric acid, sulfuric acid or perchloric acid, for example by adding aqueous HNO3, and subsequently dispersing the metal oxide core particles, for example by ultrasonic dispersion;c) Combining the solution prepared in step a) and the dispersion prepared in step b), preferably by adding the solution prepared in step a) to the dispersion prepared in step b), optionally adding a hole catcher, for example isopropanol, and preferably passing N2 through to remove O2 from the solution; d) Deposition of metallic platinum on the surface of the core particles by irradiating the reaction mixture prepared in step c) with UV radiation, preferably with a wavelength in the range of 200 nm to 280 nm, preferably UV radiation with a wavelength of 254 nm, for example from a mercury vapor lamp; e) optionally filtration, washing and drying of the powder prepared in step d) to obtain metal oxide-platinum core-shell particles.

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