Bimetallic alloy-based heterogeneous catalysts and a photoactive semiconductor medium and their use in safe hydrogen storage

Abstract

The present invention concerns the preparation of heterogeneous catalysts by reductive photodeposition of a palladium-silver alloy as active phase on a support of titania or tungsten trioxide. The catalyst is used for storing hydrogen by means of a formate / bicarbonate cycle.

Description

[0001] BIMETALLIC ALLOY-BASED HETEROGENEOUS CATALYSTS AND A PHOTOACTIVE SEMICONDUCTOR MEDIUM AND THEIR USE IN SAFE HYDROGEN STORAGE

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to bimetallic alloy-based heterogeneous catalysts and a photoactive semiconductor medium, their preparation and their use in a hydrogen storage process.

[0004] The present invention has its origin in the sector of chemical systems for “materials- based” hydrogen storage.

[0005] In particular, the invention relates to a one-pot process of producing heterogeneous catalysts which comprise a selected bimetallic alloy as an active catalytic phase deposited on a photoactive semiconductor medium, and a chemical process for hydrogen storage that avails of an organic substrate in liquid form based on an aqueous solution of formic acid / bicarbonate.

[0006] BACKGROUND ART

[0007] Hydrogen is considered an energy vector and a particularly promising fuel in a sustainable energy economy. There is therefore a growing need for technologies that can ensure simple and safe hydrogen storage, in industrial amounts. Notwithstanding recent progress in this sector of technology, at present no practical solutions exist that meet the requirements of industry and which at the same time ensure a high safety profile.

[0008] Two principal types of techniques are known for storing hydrogen, the first of which is “physics-based” storage, and the second of which is “materials-based” storage.

[0009] Currently, physics-based hydrogen storage comprises the following three principal technologies: compressed hydrogen, liquid hydrogen, and cryo-compressed hydrogen.

[0010] In technology for storing gaseous hydrogen by means of compression, the gas is compressed at high pressures, typically in the range 35-70 MPa.

[0011] This technology involves a high gravimetric and volumetric density and rapid kinetics of hydrogen storage and release, and for these reasons it is one of the most widely-used techniques for storing hydrogen. However, this technology presents a certain risk of explosion. To reduce these risks, very large tanks are used, made of advanced and expensive materials such as carbon fiber composites, one of the very few materials capable of combining lightness and mechanical strength requirements with a certain resistance to flames in the event of fire. However, these tanks are exposed to the risk of rupture, with consequent uncontrolled release of hydrogen, and of explosion, in particular during the step of compression.

[0012] To reduce these risks, use is often made of technology for hydrogen storage in liquid form. This technology is widely used by virtue of its high gravimetric and volumetric density and rapid kinetics of hydrogen storage and release. However, this technology too is not devoid of risks in use, mainly ascribable to rupture of the tank and uncontrolled release of the hydrogen contained in it, freezing, and increase in temperature beyond safe thresholds. Then there is the risk of catastrophic pressurization One of the principal drawbacks is the high energy costs owing to the need to perform a sudden and intense cooling of the fluid.

[0013] Furthermore, even though liquefied hydrogen offers the advantage that it can be kept at pressures not much higher than atmospheric pressure, this technology presents the risk of release of certain amounts of hydrogen into the atmosphere, especially if the storage containers are not adequately thermally insulated.

[0014] The most mature storage techniques therefore use cryogenic temperatures of around -253°C, or extreme pressures.

[0015] An alternative physics-based storage method is cryo-compressed hydrogen storage. This technology combines the advantages and disadvantages of the two previous technologies.

[0016] In “cryo-compressed” hydrogen, the gas is stored at less extreme cryogenic temperatures, varying between -120 and -190 °C, in containers under pressure, typically at 25-30 MPa.

[0017] However, the combination of low handling temperatures and high pressures required to implement this technology requires infrastructure that is technologically advanced and particularly expensive.

[0018] A second type of hydrogen storage technology is based on the use of specific “materials” I substances capable of storing hydrogen, typically by chemisorption or physisorption. In adsorption, hydrogen is stored in a material, typically MOFs and hydrides, by means of the reversible formation of a chemical bond, or by complexing. These technologies have the advantage of avoiding the use of high storage pressures and permitting a greater flexibility of handling.

[0019] A first “materials”-based or solid medium-based technology involves the use of MOFs (Metal Organic Frameworks), high-porosity crystalline materials constituted by metallic clusters coordinated by organic bonds. These materials can adsorb great amounts of hydrogen. For example, at 100 bar and 77 K, densities of 7.2 MJ / L can be obtained.

[0020] Other types of “materials” used are based on COF (Covalent Organic Framework) or on PAF (Porous Aromatic Framework), for which ratios of mass of hydrogen stored to mass of material are observed of higher than 17% (T=-196 °C; P=80 bar). In these cases the hydrogen is stored by superficial interaction or by incorporation into the solid material, which for example can be based on nanoporous carbon for which a ratio of mass of hydrogen stored to mass of material has been obtained of higher than 10%, at T=-196°C and P=60 bar. The stored hydrogen is then released through a chemical reaction, under specific experimental conditions.

[0021] As an alternative to the abovementioned materials, solid chemical vectors are used, for example metal hydrides (MgH2, LiH, AIH3), complex hydrides (NaBF , NaAIF , Mg2FeHe), or chemical hydrides (BH3NH3). However, hydrides have the drawback of exhibiting pyrophoric characteristics, and a certain sensitivity to oxygen and humidity. Furthermore, these systems, although having a high thermal stability, have a lower gravimetric density which limits their use in industrial applications. Owing to the complex hydride, these technologies require high temperatures and modest pressures of H2 to promote dehydrogenation, and often they have poor reversibility and slow reaction kinetics.

[0022] Further materials-based technologies for storing H2 make use of liquid organic molecules known as LOHCs (Liquid Organic Hydrogen Carriers), transporters capable of giving rise to hydrogen storage / release cycles through catalytic hydrogenation / dehydrogenation reactions.

[0023] The use of LOHC technologies has the advantage of offering good compatibility with pre-existing infrastructure and allowing hydrogen to be handled in liquid form, so reducing the risks of explosion and fire. However, even LOHC technology presents drawbacks, such as the high cost of implementation, mainly linked to the use of catalytic systems based on precious metals and energy efficiency problems. In any case, use of these technologies is associated with a higher risk of fire and toxicity for the human body, dictated by their chemical, typically aromatic, nature.

[0024] In an attempt to overcome these drawbacks, experiments have been conducted using some specific LOHC substrates such as alcohols and formates / bicarbonates in an aqueous solution. This technology enables storage and release in safety, with yields comparable to mature technologies and currently already in use. One of the limitations of this technology is the low energy density of formate and bicarbonate solutions, which limits the amount of hydrogen that can be stored per unit of volume. To increase the energy density, high-performing catalysts and higher-concentrated solutions are required, but this entails higher viscosity, lower stability and greater corrosiveness.

[0025] In an attempt to overcome these problems, new catalysts are being developed which will be combined with technologies using LOHC materials, new solvents and new hydrogen separation and purification systems.

[0026] As to catalysts, currently the main technical limitations encountered are due mainly to complexity and production costs. In fact, the current methods use complex morphologies of the mediums, which moreover are obtained under strictly controlled conditions, followed by pyrolysis and / or calcination at high temperature and by impregnation and consequent chemical reduction through the use of expensive and hazardous reagents at high temperatures. Furthermore, the catalysts used at present are often unstable and made using precious metals that are extremely expensive.

[0027] Therefore, although suitable technologies are available for storing H2, the need is still felt for a process that meets the requirements dictated by the various energy, economic and safety profiles that are the prerequisite for the proliferation of hydrogen technologies.

[0028] Currently therefore the need is felt to identify innovative systems and technologies for hydrogen storage that offer a low risk profile together with contained running costs.

[0029] Therefore, a general aim of the present invention is to provide a catalyst for bicarbonate / formate catalytic conversion in an aqueous solution and its application in a reversible H2 storage and release cycle that combines a high safety profile with high conversion yields.

[0030] An additional object of the invention is to provide a process for making a selected catalyst for bicarbonate / formate catalytic conversion in a reversible cycle of hydrogen storage and release that does not require multiple preparation steps and minimizes the use of chemical reagents.

[0031] SUMMARY OF THE INVENTION

[0032] In their research activities in the sector of materials-based chemical systems for storing H2, the inventors have observed that by photodepositing nanoparticles of Pd / Ag in specific quantitative ratios on a photoactive semiconductor medium based on selected crystalline forms of TiO2 and / or WO3, a category of catalysts is obtained which promote a selective and higher catalytic activity in NaHCOa hydrogenation phases and HCOONa dehydrogenation in the liquid phase, compared to conventional binary alloys supported on semiconductors.

[0033] The inventors then identified Pd and Ag bimetallic alloy-based heterogeneous catalysts with selected ratios by weight as the active catalytic phase photodeposited on specific crystalline forms of TiC>2 and / or WO3.

[0034] These systems are used in a formate-bicarbonate conversion cycle in the liquid phase within a reversible hydrogen storage and release system.

[0035] The use of the heterogeneous catalyst described here in hydrogen storage makes it possible to meet the necessary energy, economic and safety requirements for the proliferation of current and future hydrogen technologies.

[0036] According to an aspect, a heterogeneous catalyst for hydrogen storage is supplied by means of an aqueous solution of formate / bicarbonate comprising a Pd / Ag bimetallic alloy as the active catalytic phase, and a TiC - and / or WOs-based photoactive semiconductor medium, wherein the Pd / Ag bimetallic alloy is photodeposited on the TiO2- and / or WOs-based photoactive semiconductor medium.

[0037] According to a preferred embodiment, the present invention relates to a heterogeneous catalyst for hydrogen storage by means of an aqueous solution of formate / bicarbonate comprising:

[0038] - a Pd / Ag bimetallic alloy as the active catalytic phase, - a TiO2- and / or WOa-based photoactive semiconductor medium, wherein

[0039] - the metals Pd and Ag of the alloy are in a weight ratio respectively of 70-95 Pd I 5-30 Ag p / p and

[0040] - the TiO2-based photoactive semiconductor medium comprises from 70% to 95% by weight of anatase and from 5% to 30% of rutile.

[0041] Preferably the nanoparticles of the active metals Pd and Ag of the alloy are deposited on the photoactive semiconductor medium by means of photodeposition. Therefore a further object of the invention is to provide a heterogeneous catalyst for hydrogen storage by means of an aqueous solution of formate / bicarbonate comprising a Pd / Ag bimetallic alloy as the active catalytic phase, and a photoactive semiconductor medium as described herein, obtained by means of a method of photodeposition of the bimetallic nanoparticles of Pd / Ag on the surface of the photoactive semiconductor medium.

[0042] In the catalyst described herein the nanoparticles of the TiO2- and / or WOa-based medium can have a wide variability of sizes, for example from 5 nm to 100 pm, preferably they have a size of from 5 to 50 nm or from 30 to 50 pm, for example measured by means of the DLS zeta sizer analysis method, suitable for nano- / microparticles.

[0043] In one embodiment of the catalyst described herein, the Pd / Ag bimetallic alloy is present in an amount of from 2 to 20% by weight with respect to the weight of the TiO2- and / or WOa-based photoactive semiconductor medium.

[0044] The specific application of the catalyst described herein is in the sector of chemical systems for hydrogen storage, by means of an aqueous solution of formate / bicarbonate.

[0045] Advantageously, the catalyst described herein can be used in a hydrogen storage process which uses reversible chemical reactions of dehydrogenation and hydrogenation, which involve formates and bicarbonates in the liquid phase, preferably in an aqueous solution. Typically, the dehydrogenation step comprises the release of hydrogen from the formate-based liquid organic molecule (LOHC), and in the process acts as a vector. The hydrogenation step comprises loading the bicarbonate with hydrogen.

[0046] The use of the catalyst according to the invention in a dehydrogenation / hydrogenation system with formates / bicarbonates in an aqueous solution offers the following advantages:

[0047] - reduction of costs by virtue of the simplicity of the synthesis procedure, the partial presence of silver in the active phase, since silver has a market price two orders of magnitude lower than Pd;

[0048] - recovery of the active phases, which is beneficial for circularity of raw materials;

[0049] - use of photodeposition, which makes it possible to prepare the catalyst in situ, under ambient-like conditions and without the need to use reducing chemical reagents and / or hazardous conditions;

[0050] - preparation of materials according to protocols with high reproducibility;

[0051] - possibility to activate the photodeposition process, not only with UV radiation, for example for TiC>2, but also with visible radiation, for example when using WO3.

[0052] According to another aspect, the invention relates to a process for producing a heterogeneous catalyst for storing H2 by means of aqueous solutions of formate / bicarbonate, which comprises a Pd / Ag bimetallic alloy as a metallic active phase and a TiO2- and / or WOa-based photoactive semiconductor medium, this process comprising a step of reductive photodeposition, preferably simultaneous, of nanoparticles of a Pd / Ag bimetallic alloy on a TiC - and / or WOa-based photoactive semiconductor medium wherein the TiOa medium comprises both rutile and anatase crystalline forms, preferably from 70% to 95% anatase, from 5% to 30% rutile, more preferably from 75 to 85% anatase and from 15 to 25% rutile. According to some embodiments of the process described herein, the metallic Pd and Ag nanoparticles are photodeposited simultaneously on a TiOa-based P25 semiconductor medium containing 80% anatase and 20% rutile.

[0053] Preferably, in the process described herein, the metals Pd and Ag deposited on the photoactive TiOa-based medium are in a weight ratio respectively of 70-95 : 5-30. According to preferred embodiments, the load of active Pd / Ag bimetallic alloy for the preparation of the catalyst is comprised from 2 to 20% by weight with respect to the TiOa- and / or WOa-based photoactive semiconductor medium.

[0054] Preferably, in the process described herein, the salts PdCh (oxidation no. +2) and AgNOa (oxidation no. +1 ) are used, dissolved in an aqueous solution, as precursors of the Ag and Pd metals. Preferably, these salts are deposited simultaneously on the semiconductor medium by means of irradiation (photodeposition) with light emitted by a suitable irradiation source, for example a lamp with emission in the UV / Vis spectrum.

[0055] According to some embodiments, the production process entails the addition and photodeposition on the semiconductor medium of the precursors PdCh, AgNOa, in the presence of a sacrificial agent, for example, methanol and / or ethanol, more preferably without adding further chemical reagents.

[0056] According to certain aspects, a process is provided for the production of the catalyst described herein, which comprises the following steps: a) Preparation of an aqueous solution containing the precursors PdCh and AgNOa of the bimetallic alloy; b) Addition and dispersion of a TiOa- and / or WOa-based photoactive semiconductor medium, preferably in powder form, in an aqueous solution, c) Addition of an organic sacrificial substance, preferably methanol and / or ethanol, to promote the photodeposition process, d) Preferably degassing of the suspension with bubbling of inert gas to reduce the risks of oxygen interference with the photodeposition process, e) irradiation, preferably by means of immersion in the solution of a fluorescent mercury vapor lamp with emission in the UV / Vis spectrum with peaks at wavelengths preferably of 305, 313, 366, 405, 408, and 436 nm, preferably up to a maximum of 2 hours in order to achieve the photodeposition of the metals.

[0057] Preferably in the step of photodeposition of the bimetallic alloy, the Pd / Ag bimetallic alloy on the TiC - and / or WOa-based photoactive semiconductor medium, the metallic active phase precursors are first dissolved in oxidized ionic form in solution in order to facilitate the photodeposition on the TiOa- and / or WOa-based photoactive semiconductor medium.

[0058] Preferably, in step a) of the process, the amounts of PdCh and AgNOa salts that represent the Pd / Ag bimetallic alloy precursors are calculated on the basis of the volume of solution in order to obtain a quantitative ratio expressed by weight of

[0059] - from 70 to 95%, preferably 75-85% of Pd;

[0060] - from 5 to 30%, preferably from 15 -25% of Ag; preferably a Pd:Ag quantitative ratio equal to 80:20 by weight. An additional object of the present invention is a process for hydrogen storage in a formate / bicarbonate cycle with a heterogeneous catalyst, based on a Pd / Ag bimetallic alloy and / or a Pd- and / or Ag-based catalyst as an active phase photodeposited on a TiO2- and / or WOa-based photoactive semiconductor medium according to any embodiment described herein.

[0061] Use of the catalyst according to an embodiment described herein, in a reactive cycle of hydrogenation / dehydrogenation of an aqueous solution of formates / bicarbonates, makes it possible to store hydrogen and release it in safety and to store amounts of hydrogen comparable to amounts stored with mature technologies currently already in use.

[0062] Furthermore, the process for hydrogen storage with the catalytic system based on Pd / Ag photodeposited on a medium as described herein makes it possible to increase the reaction kinetics and the yield of the process.

[0063] In the technical field of the invention, formate / bicarbonate in an aqueous solution are organic liquid transporters of H2 that can perform hydrogen storage / release cycles, by means of catalytic hydrogenation / dehydrogenation reactions.

[0064] The formates used in the hydrogenation process are alkaline metal salts or earth alkaline metal salts of formic acid which can constitute a non-corrosive source of hydrogen.

[0065] In decaying, formates release hydrogen and generate bicarbonates, which is why in the present discussion reference is made to the formates / bicarbonates-based cycle or system.

[0066] Typically, the bicarbonates generated in the H2 storage process can be recovered catalytically with alkaline formates under suitable pressure conditions.

[0067] The formates / bicarbonate cycle as described herein can therefore be considered a vehicle for safely storing hydrogen and energy. Advantageously the entire cycle / process is substantially zero-emission, reversible, and environmentally sustainable.

[0068] Another advantage of using the catalyst described herein is the fact that it is not consumed during the cycle of hydrogenation and dehydrogenation.

[0069] Some aspects of the present invention are described in detail, with reference to the accompanying figures. BRIEF DESCRIPTION OF THE DRAWINGS

[0070] Some features and advantages of the present invention will be evident from the accompanying drawings, which are of embodiments wherein:

[0071] Figure 1 is a schematic diagram of the steps of preparation of a Pd / Ag bimetallic alloy-based catalyst photodeposited on a TiO2- and / or WOa-based photoactive semiconductor medium, according to Example 1 ;

[0072] Figure 2 is a diagram of the dehydrogenation reactor and process according to Example 2 which entails the use of a Pd / Ag bimetallic alloy-based catalyst photodeposited on a TiO2- and / or WOa-based photoactive semiconductor medium; Figure 3 is a graph of the trend of concentration of MHCOa salt (bicarbonate of M wherein M is Na+, K+or NH4+) over time, using a Pd catalyst on three different mediums, according to Example 2. The graph shows the low yield obtained with an alternative Pd / ZnO-based semiconductor medium in the prior art, the use of which offers lower yields than the medium / catalyst according to the invention;

[0073] Figure 4 is a graph showing the percentage formate conversion as the percentage of Ag in the bimetallic catalyst varies, in the active phase. The curve shows how formate conversion dips when the amount of Ag present in the bimetallic alloy exceeds the amount of 25% by weight. The conversion curve reaches zero if a non- metallic catalyst is used, wherein Ag is the only active phase.

[0074] Figure 5 contains two graphic representations showing the variation over time of the concentration of metal (Pd, Ag) of MHCO3 bicarbonate produced using the Pd / Ag-based catalyst (co-catalyst) on a TiC -based medium according to an embodiment of the invention, and a catalyst containing only Pd on a TiC -based medium. The values defined by the curve for PdAg / TiC according to an embodiment of the invention are considerably higher than the values defined by the curve for Pd / TiO2.

[0075] DETAILED DESCRIPTION OF THE INVENTION

[0076] According to a first aspect of the invention, a heterogeneous catalyst is provided as defined in the accompanying claim 1 .

[0077] In particular, the heterogeneous catalyst of the invention is used in hydrogen storage by means of aqueous solutions of formate / bicarbonate, and it is obtained by photodeposition of a bimetallic alloy, preferably the Pd / Ag bimetallic alloy or Pd / Ag-based catalyst, on a TiO2- and / or WOs-based photoactive semiconductor medium.

[0078] Advantageously, the palladium- and silver-based bimetallic alloy used in the process for storing H2 described herein can be obtained at production costs that are appreciably lower than the production costs of catalysts in the prior art made using alloys of noble metals.

[0079] Furthermore, the Pd / Ag alloy of the heterogeneous bimetallic catalyst according to the invention is active for the formates / bicarbonates reactive cycle and respectively for the dehydrogenation and hydrogenation reactions, with reaction kinetics and yields that are compatible with the technological applications.

[0080] Advantageously the two metals (Me) Pd and Ag of the catalyst described herein are present in selected amounts, as defined in claim 1 .

[0081] According to certain embodiments of the invention, in the heterogeneous catalysts the Pd / Ag bimetallic alloy is present in an amount of from 2 to 20% by weight with respect to the weight of the TiO2- and / or WOa-based photoactive semiconductor medium.

[0082] According to some embodiments, in the heterogeneous catalysts described herein the photoactive semiconductor medium contains or consists of TiC preferably in the mineral forms of anatase and rutile, preferably in a ratio of 80 / 20.

[0083] Preferably, the anatase and rutile are in the form of nanoparticles with a size of from 5 to 50 nm or from 30 to 50 pm. These selected sizes increase the efficiency of the Pd / Ag catalyst according to any of the embodiments described herein.

[0084] Advantageously, in the catalyst described herein, the two selected metals, Pd and Ag, and mixtures thereof, are coupled to a photoactive semiconductor medium, selected from TiC , WO3, and mixtures thereof, by means of a process of simultaneous photodeposition, so providing a heterogeneous catalyst that increases the efficiency of the hydrogen storage process using aqueous solutions of formate / bicarbonate.

[0085] According to another aspect, the invention relates to a process for producing a heterogeneous catalyst as defined in the accompanying claim 5.

[0086] Advantageously, the catalyst described herein is obtained by photodeposition, preferably simultaneous, of a bimetallic alloy, of Pd / Ag, on a medium based on TiC>2, WO3 and mixtures thereof. In this description, the terms “catalyst” and “catalytic phase” are to be understood as synonyms.

[0087] In the photodeposition technique used in the production process of the catalyst according to the invention, preferably the precursors of the metallic active phase are PdCh and AgNOa and they are initially dissolved in oxidized ionic form in a solution. This preliminary step promotes their consequent photodeposition on the TiOa- and / or WOa-based photoactivatable semiconductor medium without resorting to the use of further chemical reagents except for the sacrificial agent (methanol or ethanol).

[0088] Therefore, according to preferred embodiments, the Pd of the bimetallic alloy of the catalyst is obtained starting from PdCh (oxidation no. +2) and the Ag starting from AgNOa (oxidation no. +1 ).

[0089] In the heterogeneous catalysts described herein the semiconductor mediums enable the energetic promotion of electrons from the valence band to the conduction band, if they are irradiated with an electromagnetic radiation that is sufficiently energetic to overcome the energy difference between the bands, known as the band gap.

[0090] The band gap represents the energy difference between the valence band and the conduction band. In this manner, an electronic gap is generated in the valence band, which predisposes the semiconductor to become the site of an oxidation / reduction reaction. In particular, the oxidation reactions occur on the surface of the valence band, while the reduction reactions occur at the conduction band.

[0091] In this manner, the metal salts proximate to the conduction band of the medium, i.e. the Pd2+and Ag+ions, are reduced, acquiring the electrons excited by the UV photons, which have migrated from the valence band of TiC>2, then bonding to the semiconductor medium, becoming Pd and Ag with a zero oxidation state.

[0092] Given the reversibility of the photon excitation process, it is preferable to prevent the electrons from reoccupying the gaps left in the valence band. To this end, preferably an organic sacrificial substance is used, for example ethanol (EtOH) and / or methanol (MetOH), which cedes electrons when it reacts, so becoming oxidized and so occupying the gaps.

[0093] Experimental tests conducted by the inventors show that the optimal load of the bimetallic alloy that represents the active phase of the catalyst preferably falls in the range of 2-20% by weight with respect to the semiconductor medium.

[0094] According to preferred embodiments, the photoactivatable semiconductor medium is TiO2-based, preferably for example in its P25 form which creates a homojunction between the two crystalline forms of titanium dioxide: anatase and rutile in a ratio of 80 / 20; preferably the particles of these crystalline forms of TiC have a size of from 5 nm to 50 nm or from 30 to 50 pm.

[0095] According to some embodiments, the "one-pot" process for preparing the photocatalyst can comprise the following steps:

[0096] - Preparation of an aqueous solution of PdCh and AgNOa salts, containing the precursors of the two metallic active phases, preferably PdCh and AgNOa. In this preliminary step, the amounts of the two precursors can be calculated on the basis of the volume of solution in order to obtain a maximum quantitative ratio between Pd and Ag that falls in the range 85:15 and preferably in the range 80:20 described above;

[0097] - Addition and dispersion of the catalyst medium in powder form in an aqueous solution in order to obtain a suitable ratio between the weight of the catalyst medium and the weight of the precursors of the active phase, so as to obtain a bimetallic alloy load preferably from 2 to 20 % by weight.

[0098] - Addition of an organic sacrificial compound, for example methanol and / or ethanol, in order to facilitate the photodeposition process, in a dose of 2-10 % by volume with respect to the solution.

[0099] - Preferably degassing of the suspension with bubbling using inert gas to guard against the interference of oxygen with the photodeposition process,

[0100] - simultaneous photodeposition by means of immersion in the solution of a fluorescent mercury vapor lamp with emission in the UV / Vis spectrum, preferably with peaks at wavelengths preferably of 305, 313, 366, 405, 408, and 436 nm, preferably up to a maximum of 2 hours for complete photodeposition of the metals. The catalyst thus obtained is ready for use in a hydrogenation / dehydrogenation system in a formates / bicarbonates cycle, according to any of the embodiments described herein.

[0101] According to another aspect, a process is provided for storing H2 by means of a formates / bicarbonates cycle, in the presence of a catalyst as defined in claims 9- 10.

[0102] Preferably, in the storage process described herein, the formate is formic acid salified with an alkaline or alkaline earth metal.

[0103] According to some embodiments, the hydrogen storage process comprises: a) a step of dehydrogenation, preferably at a pressure higher than or equal to atmospheric pressure and more preferably from 1 to 4 bar, and at a temperature preferably higher than or equal to 70°C, more preferably from 75 to 90°C, and b) a step of hydrogenation with a pressure higher than or equal to 10 bar and preferably between 50 and 150 bar, at a temperature higher than or equal to 25°C and preferably between 50 and 80°C.

[0104] The cyclic formate-bicarbonate interconversion for H2 storage in which the catalyst according to the invention is used can be shown schematically as follows:

[0105] M = K+e / oNa+e / oNH4Preferably, in the MHCO3 bicarbonate M is an alkaline or alkaline earth metal; preferably M is sodium or potassium, more preferably potassium.

[0106] One of the advantages of using the catalyst according to the invention in this reaction is working under mild operating conditions, in terms of pressure and temperature, with respect to other, traditional methods of hydrogen storage.

[0107] For example, storage using LOHC requires average temperatures of 200-350°C for the dehydrogenation. The storage of compressed gas requires a pressure of 350- 850 bar to reach an adequate energy density. Storage in the liquid phase requires cryogenic temperatures of the order of 20 K.

[0108] It is therefore evident that the conditions of the processes in the prior art are more severe and imply higher costs, reduced energy efficiency, complex heat management, and operating risks. Furthermore, the process and the catalyst described herein have other advantages, such as hydrogen storage with formates with a high degree of safety, together with high chemical and heat stability, ease of transport and distribution, in addition to the possibility of integration with existing infrastructure.

[0109] Furthermore, another advantage of the process described herein is represented by the reaction AH for the formates / bicarbonates reactive cycle which is approximately half that of some classes of LOHC, such as for example formates. A low value of AH implies a lower energy expenditure, with consequent reduction of the overall operating costs of storage.

[0110] From the foregoing it is evident that the process described herein enables hydrogen storage with an ample margin of safety, and furthermore it follows the twin logic of circularity and sustainability.

[0111] The following example is provided mainly to illustrate some embodiments of the present invention.

[0112] According to some alternative embodiments, following the catalyst preparation technique described above, it is possible to proceed with calcination of the catalyst. This is a high-temperature heat treatment, for example from 300 °C to 600 °C, and is performed in order to eliminate volatile substances trapped in the grains of solid and better anchor the active phase onto the medium, so reinforcing the catalyst. EXAMPLE 1

[0113] PREPARATION OF A SELECTED Pd / Aq CATALYST BY MEANS OF PHOTODEPOSITION REACTION

[0114] The synthesis of a catalyst made of Pd / Ag bimetallic alloy according to an embodiment described herein was conducted in a glass batch reactor suitable for photodeposition reaction, provided with three openings: two to allow inertization of the internal environment by way of a continuous flow of nitrogen, and the third for inserting a fluorescent mercury vapor lamp, correctly jacketed to control the temperature and prevent overheating.

[0115] Then a catalyst in Pd / Ag bimetallic alloy was made, with a weight ratio between the two metals of 80:20 (% p / p) , by means of photodeposition on a TiC -based medium, containing the crystalline forms of anatase 80% and rutile 20% p / p.

[0116] PdCh and AgNOa were used as precursors in the liquid phase in order to obtain the reduction and photodeposition of nanoparticles of the two metals on the selected TiO2-based medium described above.

[0117] The calculations of the weights to be used must be performed on chloride and nitrate, and the masses to be used for the synthesis can be obtained from the following equations:

[0118] The production process comprised the following steps of preparation: supplying the precursors of the active phases (metals) and the medium (PdCh, AgNOa, TiC ). The amounts of the reaction products were weighed for the experimental test;

[0119] The precursors of the metals were then added to an aqueous solution;

[0120] Then between 2 and 10 mL of organic sacrificial product, for example methanol and / or ethanol, was added for each 100 mL of water to promote the photodeposition process;

[0121] The solution thus prepared is kept under continuous agitation to ensure the uniformity of the liquid phase and degassing of the suspension was performed with bubbling using N2 to guard against the interference of oxygen with the photodeposition process.

[0122] After the necessary time for inertization, the UV lamp / Vis was inserted in the reactor and therefore in the solution; the reaction was conducted for a period of 2 hours, which was deemed suitable to obtain the photodeposition of the metals.

[0123] The process was conducted at ambient temperature and pressure.

[0124] The catalyst thus obtained can be separated by filtration or separation and is ready for use in a hydrogenation / dehydrogenation catalytic cycle.

[0125] DEHYDROGENATION TEST

[0126] Tests were performed of dehydrogenation with catalysts of binary metal alloys containing different amounts by weight of Pd and Ag with a Pd > Ag load, observing unexpected increases in yield by combining selected ratios by weight of Pd / Ag alloys photodeposited on a TiO2-based medium with specific ratios of crystalline forms.

[0127] The values given in the graphs of the process yields are commented below:

[0128] An aqueous solution of HCO2M (formate of M = K+and / or Na+and / or NH4+) was prepared according to the initial concentration required by the specific reaction conducted, for example variable from 0.5M up to 15, preferably in the range from 2 to 10 M. This mixture was inserted in the reactor for dehydrogenation under continuous agitation; the reactive environment was inertized in an atmosphere of N2;

[0129] The reactor, shown schematically in Figure 3, is fitted with a heating jacket which makes it possible to control the process temperature;

[0130] After inertization, the catalyst was inserted in the solution, according to the selected amounts;

[0131] The system was closed and the flow of nitrogen stopped; to monitor the progress of the conversion over time, six samples of solution were taken using a syringe, at t= 0.15, 30, 60, 120, 180, 240 and 300 min; the contents of each sample were sent for HPLC analysis.

[0132] In detail, some of the tests conducted observe the following process specifications: The reactive system used is shown schematically in Figure 2 and is conveniently closed; in such conditions, the maximum obtainable conversion is that of thermodynamic equilibrium, which is reached in the different tests upon reaching the plateau, as shown in Figures 3 and 5. The closed system configuration, although it limits the conversion and the kinetics as the maximum conversion of equilibrium is reached, was used experimentally to increase the reproducibility of the data obtained and to allow an adequate comparison between the various experimental conditions adopted.

[0133] The first tests, in Figure 3, were conducted in order to identify the best medium. They were conducted in the absence of a co-catalyst (Ag), at an initial concentration of HCOOM salt chosen to be 0.5 mol L’1, at T = 75°C, pH = 8 and with a catalyst load in solution equal to 3000 ppm, at ambient pressure. As can be seen, the mediums selected, TiC -based (mix of anatase 80% and rutile 20% p / p) and WO3- based, combine high activity with a superior yield of bicarbonate (and also of H2).

[0134] The disclosures in Italian Patent Application No. 102024000017521 from which this application claims priority are incorporated herein by reference.

[0135] Where the technical features mentioned in any claim are followed by reference numerals and / or signs, those reference numerals and / or signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly, such reference numerals and / or signs do not have any limiting effect on the interpretation of each element identified by way of example by such reference numerals and / or signs.

Claims

CLAIMS1 . A heterogeneous catalyst for hydrogen storage by means of an aqueous solution of formate / bicarbonate comprising a Pd / Ag bimetallic alloy as the active catalytic phase, and a TiO2- and / or WOa-based photoactive semiconductor medium, characterized in that:- the metals Pd and Ag of the alloy are in a weight ratio respectively of 70-95 1 5-30 % p / p and- the TiOa-based photoactive semiconductor medium comprises from 70% to 95% by weight of anatase and from 5% to 30% of rutile.

2. The catalyst according to claim 1 , wherein the metals Pd I Ag of the alloy are in a weight ratio respectively of 75-85 Pd : 15-25 Ag % p / p, preferably 80 I 20 % p / p.

3. The catalyst according to claim 1 or 2, wherein the TiOa-based photoactive semiconductor medium comprises from 75 to 85% by weight of anatase and from 15% to 25% of rutile.

4. The catalyst according to any one of claims 1 -3, wherein the Pd / Ag bimetallic alloy is present in an amount from 2 to 20% by weight with respect to the weight of the TiO2- and / or WOa-based photoactive semiconductor medium.

5. A process for producing a Pd / Ag heterogeneous catalyst according to any one of claims 1 -4, which comprises the photodeposition of nano- / microparticles of Pd and Ag on a TiC - and / or WOa-based photoactive semiconductor medium, wherein the metals Pd and Ag are deposited on the TiOa-based photoactive medium in a weight ratio respectively of 70-95 : 5-30 % p / p, preferably 75-85 : 15- 25 % p / p, and wherein said TiOa-based photoactive semiconductor medium comprises from 70% to 95%, preferably from 75 to 85% by weight of anatase and from 5% to 30%, preferably from 15 to 25% by weight of rutile.

6. The process according to claim 5, wherein the metals Pd and Ag are supplied in the form of the precursors PdCh and AgNOa.

7. The process according to claim 5 or 6, wherein the TiOa-based photoactive semiconductor medium contains anatase and rutile in the form of nano- or microparticles, respectively with a size of from 5 to 50 nm, or from 30 to 50 pm.

8. The process according to any one of claims 5-7, wherein the step ofphotodeposition of the nanoparticles of Pd / Ag-based bimetallic alloy on a TiO2- and / or WOa-based photoactive semiconductor medium is simultaneous and comprises a step of immersion in an aqueous phase of PdCh and AgNOa and exposure to an electromagnetic radiation preferably emitted by a fluorescent mercury vapor lamp, preferably with emission in the UV / Vis spectrum with peaks at wavelengths preferably of 305, 313, 366, 405, 408, and 436 nm, preferably lasting up to 2 hours.

9. The process according to any one of claims 5-8, comprising one or more of the following steps: a) Preparation of an aqueous solution containing the precursors PdCh and AgNOa of the Pd / Ag bimetallic alloy; b) Addition and dispersion of a Ti02-and / or WOa-based photoactive semiconductor medium, preferably in powder form in an aqueous phase, c) Addition of an organic sacrificial substance, preferably methanol and / or ethanol, to promote the photodeposition process by forming a suspension, d) Optionally degassing of the suspension with bubbling of inert gas to reduce the risks of oxygen interference with the photodeposition process, e) Immersion in the solution of a fluorescent mercury vapor lamp with emission in the UV / Vis spectrum with peaks at wavelengths preferably of 305, 313, 366, 405, 408, and 436 nm, preferably up to 2 hours in order to achieve simultaneous photodeposition of the metals.

10. The process according to any one of claims 5-9, wherein the metals Pd and Ag that form the bimetallic alloy are added in an amount from 2 to 20% by weight with respect to the weight of the TiO2- and / or WOa-based photoactive semiconductor medium.1 1. A process for storing and / or releasing hydrogen, which comprises reactions of hydrogenation of a bicarbonate of an alkaline metal, and of dehydrogenation of a formate in the liquid phase, in the presence of a catalyst according to any one of claims 1 -4.

12. The process according to claim 1 1 , comprising: a) a step of catalytic dehydrogenation of a solution of a formate at a pressure higher than or equal to atmospheric pressure and more preferably from 1 to 4 bar, and at a temperature preferably higher than or equal to 50°C, more preferably from 75 to90°C, and b) a step of catalytic hydrogenation of a bicarbonate of an alkaline metal at a pressure higher than or equal to 10 bar and preferably between 50 and 150 bar, at a temperature higher than or equal to 25°C and preferably between 50 and 80°C.

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