Method for synthesizing carbon-supported platinum group metal or metal alloy nanoparticles

Abstract

The present invention relates to a method for the synthesis of carbon-supported platinum group metal or metal alloy nanoparticles, comprising the steps of adsorbing on a carbon support a complex of a platinum group metal with a urea complexing agent selected from the group consisting of urea, a urea derivative, a mixture of urea and at least one urea derivative, and a mixture of at least two urea derivatives, and reducing the complex adsorbed on the carbon support to metal nanoparticles to form the carbon-supported metal nanoparticle product. The present invention also provides the use of carbon-supported platinum group metal or metal alloy nanoparticles obtained by the method of the present invention as a catalyst. The present invention further relates to a method for adsorbing a precursor of a platinum group metal on the surface of a carbon support, and to the use of a complex of a platinum group metal with a urea complexing agent for adsorbing a precursor of a platinum group metal on a carbon support.

Description

[Technical Field]

[0001] The present invention relates to a method for the synthesis of carbon-supported platinum group metal or metal alloy nanoparticles. The present invention also provides the use of carbon-supported platinum group metal or metal alloy nanoparticles obtainable by the method of the present invention as catalysts. [Background technology]

[0002] The literature describes many methods for the synthesis of carbon-supported platinum group metals. Among them, the most commonly applied methods include the impregnation reduction method, the Boenneman method, the water-in-oil microemulsion method, and the polyol method (Non-Patent Document 1). Depending on the method used, the resulting catalysts are characterized by different morphologies, nanoparticle size and distribution, and catalytic properties. The method of the present invention can be classified as a variant of the impregnation reduction method. However, it has many advantages over the methods known in the prior art, such as simplicity, versatility, better utilization of the precious metal due to a larger active surface area, and low cost.

[0003] The method of the present invention utilizes urea or urea derivative complexes of platinum group metals as precursors for the synthesis of carbon-supported platinum group metal nanoparticles. To date, the only use of urea in nanoparticle synthesis has been to adjust the pH of reagents during the reduction of a non-urea precursor of Pt (HPtCl) (Non-Patent Document 2). In the background art, it should be emphasized that the only use of urea in the synthesis of precious metal nanoparticles is to control the pH of the reducing agent. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] C. Coutanceau, S. Baranton and TW Napporn (2012). Platinum Fuel Cell Nanoparticle Syntheses: Effect on Morphology, Structure and Electrocatalytic Behavior, The Delivery of Nanoparticles, Dr. Abbass A. Hashim (Ed.), ISBN: 978-953-51-0615-9, InTech [Non-patent document 2] Baizeng Fang, Nitin K. Chaudhari, Min-Sik Kim, Jung Ho Kim, and Jong-Sung Yu* Homogeneous Deposition of Platinum Nanoparticles on Carbon Black for Proton Exchange Membrane Fuel Cell J. Am. Chem. Soc. 2009, 131, 15330-15338. Yu et al. US 8,993,198 B2 Summary of the Invention

[0005] The present invention relates to a method for the synthesis of carbon-supported platinum group metal or metal alloy nanoparticles. As used herein, platinum group metals (PGMs) consist of six elements: platinum (Pt), palladium (Pd), rhodium (Rh), iridium (Ir), ruthenium (Ru), and osmium (Os), while metal alloys include alloys of any two or more components of the PGMs, including alloys with other platinum group metals, alloys with metals other than the PGMs, and alloys with non-metallic elements. Because carbon-supported platinum group metal or metal alloy nanoparticles can be used as catalysts, the method of the present invention is a novel method for catalyst synthesis.

[0006] The synthesis method according to the present invention comprises the following steps: (b) adsorbing a platinum group metal complex onto a carbon support using a urea complexing agent; (c) reducing the metal from the complex adsorbed on the carbon support obtained in step (b) to metal nanoparticles to form carbon-supported metal nanoparticle products.

[0007] As used herein, the terms "metal-urea complexing agent complex" or "metal-urea complexing agent complex" are defined as coordination complexes containing a platinum group metal as the central ion and at least one molecule of urea or a urea derivative as the ligand(s). This definition also includes mixed-ligand complexes in which, in addition to the urea or urea derivative ligand, other ligands are present, such as ammonia, chloride ions, cyanide ions, or nitrite ions. According to the present invention, the urea complexing agent is selected from the group consisting of urea and a urea derivative, a mixture of urea and at least one urea derivative, and a mixture of at least two urea derivatives. As used herein, a urea derivative is to be understood as a compound containing a -HN-CO- functional group, preferably a -HN-CO-NH- functional group. In particular, such urea derivatives are selected from the group comprising methylurea, N,N-dimethylurea, N,N'-dimethylurea, ethylurea, trimethylurea, N,N-diethylurea, N,N'-diethylurea, N,N'-bis(hydroxymethyl)urea, and analogs thereof such as bis(hydroxymethyl)urea, urea condensates including biuret, biurets, triuret, biuret derivatives containing -(HN-CO-)N- functionality, other carbamides, urea-containing oligomers or polymers such as urea-formaldehyde, and other ureas and carbamates. Preferably, the complex of a platinum group metal with a urea complexing agent is selected from the group consisting of urea, methylurea, N,N-dimethylurea, N,N'-dimethylurea, ethylurea, trimethylurea, N,N-diethylurea, N,N'-diethylurea, N,N'-bis(hydroxymethyl)urea, and bis(hydroxymethyl)urea. More preferably, the complex of a platinum group metal with a urea complexing agent is selected from the group consisting of urea, methylurea, and N,N'-dimethylurea. It is important that the method of the present invention uses a compound containing a -HN-CO- functional group, preferably a -HN-CO-NH- functional group, as the complexing agent. Results obtained by the present inventors suggest that the urea complexing agent binds to the platinum group metal via a nitrogen atom and to the surface of the carbon support via an oxygen atom. This results in very good coverage of the platinum group metal on the carbon support.

[0008] It should also be understood that the platinum group metal complex adsorbed on the carbon support may comprise a complex of a single platinum group metal, preferably Pt, Pd, or Ir, or a complex of two or more platinum group metals. In the latter case, metal alloy nanoparticles are formed as a result of reduction of the adsorbed complex. Two or more platinum group metal complexes can be adsorbed simultaneously on the carbon support or sequentially (i.e., one after the other). The simultaneous adsorption process is carried out when two or more complexes of different platinum group metals are present in a solution in which the carbon support is immersed. In this case, at least two platinum group metal complexes are simultaneously adsorbed on the surface of the carbon support. The process in which different platinum group metal complexes are sequentially adsorbed on the surface of the carbon support involves multiple adsorption steps, in which adsorption of a first platinum group metal complex is followed by adsorption of a complex of a second, different platinum metal complex. This latter process can be carried out by immersing the carbon support in a first solution of the platinum group complex, and after the desired level of complex adsorption is achieved, the carbon support is transferred to another solution containing a complex of a second, different platinum group metal, where the complex undergoes adsorption. It should be noted that the platinum group complex adsorption process can be repeated multiple times.

[0009] Preferably, steps (b) and (c) of the method of the present invention are preceded by step (a), in which a platinum group metal precursor, preferably K2PtCl4, K2PdCl4, and IrCl4, in solution, is reacted with a urea complexing agent selected from the group comprising urea derivatives and mixtures of urea and at least one urea derivative, or mixtures of at least two urea derivatives, to form a metal-urea complexing agent complex, wherein the urea derivative is defined above.

[0010] In one embodiment of the method of the present invention, steps (a) and (b) are carried out simultaneously (i.e., they form one step). In this embodiment, a platinum group metal precursor and urea or a urea derivative are mixed with a carbon support to form a suspension, and the suspension is heated, thereby forming a metal-urea or a urea derivative complex and adsorbing it onto the carbon support. The complex adsorbed on the carbon support then undergoes a reduction step (c), resulting in the formation of metal nanoparticles.

[0011] In another embodiment, steps (a) and (b) are carried out separately (i.e., a multi-step process is carried out), and the complex obtained in step (a) is subsequently mixed with a carbon support to allow adsorption of the complex onto the carbon support. The complex adsorbed on the carbon support is then subjected to a reduction step (c), resulting in the formation of metal nanoparticles.

[0012] According to any of the above embodiments, step (a) of forming a metal-urea or urea derivative complex and step (b) of adsorbing said metal-urea or urea derivative complex onto a carbon support are preferably carried out in an aqueous solution.

[0013] Preferably, in step (b), in addition to the complex of a platinum group metal and a urea complexing agent, other platinum group metal precursors (salts, hydrates, or complexes) can be adsorbed onto the surface of the carbon support. Alternatively, in step (b), in addition to the complex of a platinum group metal and a urea complexing agent, precursors (salts, hydrates, or complexes) of metals other than platinum group metals can be adsorbed onto the surface of the carbon support. Subsequently, a combination of at least two different metal precursors undergoes a reduction process. As a result, alloy nanoparticles are obtained. The metal other than the platinum group metal used to obtain the alloy nanoparticles supported on the carbon support includes a metal selected from the group consisting of vanadium, chromium, manganese, iron, cobalt, nickel, copper, gold, silver, and tin. Preferably, the metal other than the platinum group metal used to obtain the alloy nanoparticles supported on the carbon support is nickel, silver, or gold. As already mentioned above, the adsorption process of the platinum group metal complex with urea complexing agent and the precursor of the metal other than the platinum group metal can be carried out simultaneously (i.e., the platinum group metal complex and the precursor of the metal other than the platinum group metal can be present in one solution) or sequentially (i.e., the carbon support is transferred from one complex or precursor solution to another to effect adsorption of the respective complex and precursor).

[0014] Alternatively, in step (b), in addition to the complex of a platinum group metal with a urea complexing agent, other precursors (salts, hydrates, or complexes) of the same platinum group metal can be adsorbed onto the surface of the carbon support. Subsequently, the combination of at least two different metal precursors in the form of different complexes of the same platinum group metal undergoes a reduction process. As a result, nanoparticles of a single platinum group metal are obtained. As discussed above, the adsorption of the complex of a platinum group metal with a urea complexing agent and the adsorption of other precursors of the same platinum group metal can be carried out simultaneously or sequentially.

[0015] The term "metal precursor" as used herein should be understood as any metal salt, hydrate, or complex that can be reduced to the metallic form that is deposited on the carbon substrate. The term also includes mixtures of metal precursors that undergo reduction to form alloys. The term "metal precursor" also encompasses any metal salt, hydrate, or complex that can be used to form a complex with a urea complexing agent.According to the present invention, the platinum group metal precursors used in the method of the present invention are preferably selected from the group consisting of HPtCl, HClN, Pt, PtCl, PtBr, K[PtCl], Na[PtCl], Li[PtCl], HPt(OH), Pt(NO), [Pt(NH)]Cl, [Pt(NH)](HCO), [Pt(NH)](OAc), (NH)Pt(NO), (NH)PtBr, KPtCl, PtSO, Pt(HSO) )2, Pt(ClO4)2, K2PtI6, K2[Pt(CN)4], cis-[Pt(NH3)2Cl2], H2PdCl6, H6Cl2N2Pd, PdCl2, PdBr2, K2[PdCl4], Na2[PdCl4], Li2[Pd Cl4], H2Pd(OH)6, Pd(NO3)2, [Pd(NH3)4]Cl2, [Pd(NH3)4](HCO3)2, [Pd(NH3)4](OAc)2, (NH4)2PdBr6, (NH3)2PdCl6, PdSO4, Pd(H SO4)2, Pd(ClO4)2, Pd(OAc)2, RuCl2, ((CH3)2SO)4, RuCl3, [Ru(NH3)5(N2)]Cl2, Ru(NO3)3, RuBr3, RuF3, Ru(ClO4)3, K2RuCl6, Os I, OsI2, OsBr3, OsCl4, OsF5, OsF6, OsOF5, OsF7, IrBr4, IrF6, IrCl3, IrF4, IrF5, Ir(ClO4)3, K3[IrCl6], K2[IrCl6], Na3[IrCl6] , Na2[IrCl6], Li3[IrCl6], Li2[IrCl6], [Ir(NH3)4Cl2]Cl, RhF3, RhF4, RhCl3, [Rh(NH3)5Cl]Cl2, RhCl[P(CH5)3]3, K[Rh(CO)2Cl2], Na[Rh(CO)2Cl2]Li[Rh(CO)2Cl2], Rh2(SO4)3, Rh(HSO4)3, and Rh(ClO4)3, their hydrates and mixtures of these salts and / or hydrates. The most preferred PGM precursors are selected from the group comprising K2PtCl4, K2PdCl4, and IrCl4. Alternatively, the PGM precursor can be selected from the group of water-insoluble salts such as PtCl2, provided that it is capable of forming a metal complex with urea or a urea derivative.

[0016] The preferred precursor for metals other than platinum group metals to be used with platinum group metals to obtain alloy nanoparticles supported on a carbon support is VSO(H2O). X , VCl3, VBr3, VF3, VI3, V2O3, V2(SO4)3, VO(acac)2, CrBr3, CrCl3, [Cr(H2O)6](NO3)3·3H2O, CrI3, [C r3O(O2CCH3)6(OH2)3]Cl(H2O)6, Cr(ClO4)3, Cr2(SO4)3·x(H2O), Mn(O2CCH3)3, Mn(CH3CO2)2·(H2O) n , MnBr2, MnCl2, Mn(NO3)2·(H2O) n , Fe(O2CCH3)2, FeCl2, FeI2, FeF2, [Fe(H2O)6] 2+ {[Fe(C6H5O7)(H2O)] -}2·2H2O, Fe(NO3)2·6H2O, FeSO4·xH2O, Fe(BF4)2, Fe(C3H5O3))(H2O) n , Fe2(SO4)3(H2O) n , Fe(NO3)3·xH2O, FeF3(H2O) X , FeCl3, Co(CH3CO2)2·4H2O, CoBr2, Co(ClO3)2, CoCl2, CoF2, Co(HCO2)2, CoI2, Co(NO3)2·xH2O, CoSO4(H2O) X , Co(NO3)3, [Co(NH3)5Br]Br2, [Co(NH3)5(NO2)]Cl2, [Co(NH3)6]C13, Ni(NO3)2, Ni(NO2)2, NiSO4(H2O)6, NiCl2, NiCl2·6H2O, NiBr2(H2O) X , Nil2, NiF2, Ni(CH3CO2)2·xH2O, CuCl2, CuSO4(H2O) X ,Cu(NO3)2(H2O) X , Cu(ClO3)2, CuBr2, Cu(OSO2CF3)2, Cu(H2O) X (BF4)2, Cu2(OAc)4(H2O)2, AuCl3, AuBr3, HAuCl4·(H2O) X, AgNO3, AgC2H3O2, AgNO2, AgF, Ag2SO4, SnCl2, SnCl4, SnF2, SnSO4, SnBr4, hydrates thereof, and mixtures of these salts and / or hydrates.

[0017] In a preferred embodiment, the method of the present invention also includes a step between steps (b) and (c) in which adsorption of the complex is carried out and the carbon support with the adsorbed complex is isolated from the solution and then washed. The isolation step is preferably carried out by filtration. The washing step is carried out using water. In some embodiments, the carbon support with the adsorbed complex can be air-dried or dried under an inert gas atmosphere.

[0018] Also preferably, step (c) of the method of the present invention, i.e., reducing the metal-urea or urea derivative complex on the carbon support, is carried out using gaseous hydrogen. In a specific embodiment, the method of the present invention also includes a step of carrying out adsorption of the complex and isolating the carbon support with the adsorbed complex from the solution. When gaseous hydrogen is used, the reduction step is carried out at a temperature of 50 to 200 °C by placing the carbon support with the adsorbed complex in a flow of a gas mixture consisting of an inert gas, preferably argon or nitrogen, and hydrogen. The hydrogen content in the gas mixture is preferably in the range of 1 to 10%, more preferably 3 to 7%, and most preferably 4 to 6%. The reduction step is carried out at a flow rate of the gas mixture in the range of 5 to 30 ml / min, preferably for 1 to 6 hours, more preferably 2 to 5 hours, and most preferably 3 to 4 hours. The hydrogen content and flow rate in the gaseous reduction stream should be adjusted to provide a significant stoichiometric excess of hydrogen (approximately 10 times) relative to the metal-urea complexing agent complex undergoing reduction. Preferably, before the reduction process, the carbon support with the adsorbed complex is subjected to a flow of inert gas, such as argon or nitrogen, to remove residual oxygen. This step of removing residual oxygen is carried out at ambient temperature or higher, but not exceeding 200°C, typically in the range of 20-150°C. Preferably, the removal of residual oxygen is carried out for about 1-3 hours. Alternatively, after the reduction step, a heat treatment (e.g., in the temperature range of 50-350°C) can be carried out for 1-6 hours under an inert gas atmosphere to remove impurities or unreacted urea complexing agent adsorbed on the carbon support.

[0019] Alternatively, the reduction step in the method of the present invention is carried out by thermal decomposition of the adsorbed complex of the metal and urea complexing agent under an inert atmosphere. The decomposition process is carried out at a temperature ranging from 190 to 550°C for 1 to 6 hours. Preferably, the reduction by decomposition is carried out in an inert gas selected from the group consisting of argon and nitrogen.

[0020] In yet another embodiment, the metal complex reduction step (step (c)) is carried out in solution using a reducing agent, preferably L-ascorbic acid, sodium borohydride, formaldehyde, or citric acid. Most preferably, when the metal complex reduction step is carried out in solution, L-ascorbic acid, sodium borohydride, or citric acid is used as the reducing agent. Other reducing agents may also be considered, such as glucose, hydrazine, hydrazine sulfate, hydrazine nitrate, sodium hypophosphite, lithium borohydride, aluminum borohydride, lithium tetraethylborohydride, methanol, ethanol, formic acid, ethylene glycol, 1,2-hexadecanediol, hydroxylamine, and dimethylborazane DMAB.

[0021] Regardless of the reduction method, in preferred embodiments of the process of the present invention, the molar ratio of urea complexing agent to metal used in step (a) is in the range of 1 to 20:1, preferably 1 to 10:1, more preferably 1 to 6:1, and most preferably 1 to 4:1. More preferably, the molar ratio of urea complexing agent to metal used in step (a) is selected from the group consisting of the following ratios: 20:1, 19:1, 18:1, 17:1, 16:1, 15:1, 14:1, 13:1, 12:1, 11:1, 10:1, 9:1, 8:1, 7:1, 6:1, 5:1, 4:1, 3:1, 2:1, 1.5:1, and 1:1. Most preferably, the molar ratio of urea complexing agent to metal used in step (a) is selected from the group consisting of the following ratios: 4:1, 3:1, 2:1, 1.5:1, and 1:1, especially 4:1, 2:1, and 1:1.

[0022] In the method of the present invention, the amounts of metal-urea complexing agent complex and carbon support used in step (b) are adjusted to obtain a product containing 0.001 to 60 wt. %, preferably 5 to 40 wt. %, more preferably 10 to 40 wt. %, and most preferably 20 to 40 wt. % metal, calculated based on the total weight of the product (i.e., metal nanoparticles and carbon support). It should be noted that the upper limit of 60% metal content in the final product obtained by the method of the present invention is arbitrary. It is selected with consideration for the use of the final product as a catalyst. However, it is possible to obtain carbon-supported metal nanoparticle products with higher metal contents. The metal content is limited only by the surface area of ​​the carbon support onto which the metal-urea complexing agent complex is adsorbed.

[0023] The concentrations of the metal precursor solution and urea complexing agent solution used in step (a) of the method of the present invention range from 1 mM to 5 M, preferably from 1 mM to 4 M, more preferably from 1 mM to 3 M, and most preferably from 1 mM to 2 M. By varying the concentrations of the starting solutions of metal precursor and urea complexing agent and their amounts, the ratio of metal to urea complexing agent in the complex formed in step (a) can be easily adjusted.

[0024] In the method of the present invention, a solution containing a platinum group metal precursor and a urea complexing agent is heated under reflux at 40 to 100°C, preferably 50 to 90°C, and more preferably 60 to 80°C, in step (a) to form a complex. This step (step (a)) is carried out for 10 minutes to 10 hours, preferably 20 minutes to 5 hours, more preferably 30 minutes to 4 hours, even more preferably 30 minutes to 3 hours, and most preferably 30 minutes to 2 hours. Alternatively, in step (a) of the method of the present invention, a solution containing a platinum group metal precursor and urea or a urea derivative is heated at 40 to 100°C, preferably 50 to 90°C, and more preferably 60 to 80°C, until all liquid has evaporated.

[0025] In another embodiment, an organic solvent, preferably isopropanol or ethanol, is added to the aqueous solution used in step (a) in a volume ratio of 0.05 to 30:1, calculated based on the volume of water in the solution. The ratio is adjusted depending on the effect to be achieved. Addition of a small amount of organic solvent (a volume ratio of 0.05 to 0.5:1 relative to water) results in the formation of various complexes of the metal and the urea complexing agent. If a significant amount of organic solvent is used (a volume ratio of 10 to 30:1 relative to water), the formed complex of the metal and the urea complexing agent precipitates. This precipitate can be washed and subsequently dissolved in water for further use in the method of the present invention (i.e., step (b) in which the complex of the platinum group metal and the urea complexing agent is adsorbed onto the carbon support).

[0026] In one embodiment, as described above, the method of the present invention comprises a one-step process for complex formation and complex adsorption, in which all reagents are mixed together at the start of the reaction and the metal complex is formed in situ in the presence of the carbon support. In this embodiment, a pre-weighed amount of carbon support is dispersed in water. Preferably, the carbon support is dispersed using an ultrasonic bath for approximately 1 to 30 minutes, more preferably 1 to 5 minutes. As used herein, carbon support refers to any support consisting essentially of carbon. Suitable carbon supports include all commercially available carbon supports classified into classes such as carbon black, sooth, carbon nanotubes, carbon nanofibers, ordered mesoporous carbon, or carbon xerogels. A preferred carbon support for use in the method of the present invention is carbon black. In particular, commercially available carbon supports sold under trade names such as Vulcan® XC-72R (oil furnace carbon black) and Ketjenblack® EC-300J (acetylene black) can be used. Generally, the choice of carbon support does not significantly affect the properties of the method of the present invention or the resulting products. Therefore, the method of the present invention has a universal characteristic regarding the carbon support used. The carbon-to-water ratio ranges from 1 to 50 mg, preferably 5 to 20 mg, and most preferably 5 to 10 mg of carbon per ml of water. Next, a solution of metal precursor and urea or a urea derivative is added to the carbon suspension. The term "urea derivative" refers to any urea derivative capable of forming a complex with PGM, and "urea derivative" should be understood as a compound containing a -HN-CO- functional group, preferably a -HN-CO-NH- functional group. In particular, such urea derivatives are selected from the group comprising methylurea, N,N-dimethylurea, N,N'-dimethylurea, ethylurea, trimethylurea, N,N-diethylurea, N,N'-diethylurea, N,N'-bis(hydroxymethyl)urea, and analogs thereof such as bis(hydroxymethyl)urea, urea condensates including biuret, biurets, triuret, biuret derivatives containing -(HN-CO-)N- functionality, other carbamides, urea-containing oligomers or polymers such as urea-formaldehyde, and other ureas and carbamates.Preferably, the complex of a platinum group metal with a urea complexing agent is selected from the group consisting of urea, methylurea, N,N-dimethylurea, N,N'-dimethylurea, ethylurea, trimethylurea, N,N-diethylurea, N,N'-diethylurea, N,N'-bis(hydroxymethyl)urea, and bis(hydroxymethyl)urea. More preferably, the complex of a platinum group metal with a urea complexing agent is selected from the group consisting of urea, methylurea, and N,N'-dimethylurea. It is important that the method of the present invention uses a compound containing a -HN-CO- functional group, preferably a -HN-CO-NH- functional group, as the complexing agent. The present invention also contemplates the use of a mixture of urea and / or urea derivatives in the synthetic method of the present invention. In addition to urea and its derivatives, mixtures thereof are collectively referred to below as urea complexing agents. Also, whenever urea or a urea derivative is referred to, it should be understood to include a mixture of urea and at least one urea derivative or a mixture of different urea derivatives.

[0027] The concentrations of both the precursor and urea complexing agent solutions are typically in the range of 1 mM to 5 M. The amounts of solution used in the method of the present invention are adjusted to obtain the desired metal mass content in the final product, i.e., carbon-supported platinum group metal or metal alloy nanoparticles (preferably corresponding to 0.001 to 60% of the total mass in the final sample of carbon-supported platinum group metal or metal alloy nanoparticles), and the desired urea:metal molar ratio in the suspension (preferably 1 to 10:1). The suspension is then heated under reflux, preferably at 40 to 100°C, for 1 to 10 hours, most preferably 2 to 6 hours. Subsequently, the suspension (containing the metal-urea complexing agent complex adsorbed on the carbon) is filtered, washed thoroughly with water, and then either 1) reduced with a solution of a reducing agent, preferably an aqueous solution (i.e., a wet reduction process), 2) dried under an inert gas atmosphere (preferably under an argon or nitrogen atmosphere) or in vacuum, and then reduced under a hydrogen atmosphere, preferably under an atmosphere of an inert gas, preferably a mixture of hydrogen with argon or nitrogen (i.e., a dry reduction process), or 3) reduced by thermal decomposition of the adsorbed metal-urea complex under an inert atmosphere at temperatures in the range of 190-550°C, preferably 250-350°C. The reducing agent used in the solution in the wet reduction step of the method of the present invention is preferably selected from the group consisting of glucose, hydrazine, hydrazine sulfate, hydrazine nitrate, sodium hypophosphite, sodium borohydride, lithium borohydride, aluminum borohydride, lithium tetraethylborohydride, methanol, ethanol, formaldehyde, formic acid, ethylene glycol, 1,2-hexadecanediol, hydroxylamine, and dimethylborazane (DMAB). L-ascorbic acid and citric acid are particularly preferred reducing agents for use in the method of the present invention.

[0028] In an alternative embodiment, as described above, the synthesis of the present invention involves a step of complex formation followed by another step of complex adsorption on a carbon support (a multi-step process), after which the metal complex adsorbed on the surface of the carbon support undergoes a reduction process. In this embodiment, in a first step, a metal-urea complex is synthesized using one of the following procedures: (i) A solution of urea complexing agent is added to a solution of metal precursor, followed by heating under reflux at 40-100°C for 10 minutes to 10 hours, preferably 10 minutes to 2 hours. The concentration of the solution is typically in the range of 1 mM to 5 M, and the amounts of reagents are adjusted to obtain a molar ratio of urea complexing agent to metal of 1-20:1, most preferably 1-4:1. (ii) A urea complexing agent solution is added to the metal precursor solution, which is then heated at 40-100°C until the liquid evaporates. The amounts of reagents are adjusted to obtain a molar ratio of urea complexing agent to metal of 1-20:1, most preferably 1-4:1. The resulting precipitate is further heated at 40-100°C or left under a fume hood to evaporate excess liquid for 1-60 minutes, and then dissolved in water to produce a solution of the metal-urea complex. The evaporation-dissolution procedure can be repeated several times, resulting in various metal-urea complexing agent complexes. For the evaporation process, a container with a flat bottom, such as a Petri dish, is preferred to ensure more uniform and reproducible crystallization of the metal-urea complex. (iii) A water-insoluble metal precursor (such as PtCl) is mixed with the urea solution, stirred, and heated under reflux at 40-100°C for 1-10 hours. The amounts of reagents are adjusted to obtain a molar ratio of urea complexing agent to metal of 1-20:1, most preferably 1-6:1.

[0029] Under certain conditions set out in variants i), ii), and iii), especially at elevated temperatures (e.g., above 100°C), or when using high urea complexing agent:metal ratios (e.g., greater than 10:1 urea complexing agent:metal), and / or during prolonged heating or evaporation processes (e.g., more than 5 hours), water-insoluble metal-urea complexing agent complexes can be obtained. Such compounds cannot be used in further steps of the synthesis of carbon-supported nanoparticles. If the synthesis yields two phases (solid and liquid) (both containing the metal-urea complexing agent complex), the liquid phase is used in further steps. The formation of insoluble complexes depends on the metal and urea complexing agent used in the process. Nevertheless, the specific reaction conditions leading to the formation of insoluble complexes vary for different reagents.

[0030] Importantly, the ongoing hydrolysis process leads to different metal-urea complexing agent complexes depending on the age of the metal precursor solutions used and their storage conditions (such as temperature).

[0031] The above-described method for the synthesis of complexes (all variants (i), (ii), and (iii)) can be modified by the addition of an organic solvent. In particular, after the metal-urea complexing agent complex is obtained in solution, a small amount (volume ratio of 0.05 to 0.5:1 with respect to water) of organic solvent is added, resulting in a solution containing a different complex of the metal-urea complexing agent compared to the aqueous solution without the addition of organic solvent. In a preferred embodiment, the organic solvent is selected from the group comprising isopropanol or ethanol.

[0032] If a larger amount of alcohol is added, the metal-urea complexing agent complex is precipitated. In particular, after a solution of the metal-urea complex is obtained, adding a large amount of organic solvent (10-30:1 volume ratio to water) results in the precipitation of the metal-urea complexing agent complex. The precipitate is then washed several times with organic solvent and then dissolved in water to produce a solution of the metal-urea complexing agent. The organic solvent used for the precipitation of the complex is preferably selected from the group including isopropanol or ethanol.

[0033] It should be noted that when certain solvents (such as alcohols) are used in the above modifications of the complex synthesis, the metal-urea complexing agent complex can be reduced by the solvent, resulting in the formation of precipitates of metal nanoparticles, which is not a desirable process in the synthesis of highly dispersed carbon-supported nanoparticles.

[0034] In the second step of the multi-step synthesis method of the present invention, a pre-weighed amount of carbon support is dispersed in water. The carbon-to-water ratio ranges from 1 to 50, preferably 5 to 20, and most preferably 5 to 10 mg of carbon per ml of water. Preferably, the carbon support is dispersed using an ultrasonic bath for approximately 1 to 30 minutes, more preferably 1 to 5 minutes. Next, a controlled amount of solution or solutions containing a metal-urea complexing agent complex (preferably corresponding to 0.001 to 60% of the platinum group metal by total mass in the final sample) is added. The sample is then allowed to stand for 1 minute to 10 hours or heated under reflux at 40 to 100°C for 1 to 4 hours until the metal-urea complex adsorbs onto the carbon surface (usually indicated by a color change in the solution). Most preferably, the sample is not stirred to limit the amount of oxygen present in the sample. Without being bound by theory, oxygen has a detrimental effect on the adsorption process due to the formation of hydrophobic domains on the carbon surface. The resulting suspension (containing the precursor of the metal-urea complexing agent adsorbed on the carbon) is then filtered, washed thoroughly with water, and then reduced.

[0035] The reduction step is carried out as described above for the one-step synthesis method of the present invention. In particular, the suspension (containing the metal-urea complexing agent complex adsorbed on the carbon) is filtered, thoroughly washed with water, and then reduced with a solution of an aqueous reducing agent, preferably an aqueous solution (i.e., a wet reduction step), or dried under an inert gas atmosphere (preferably under an argon or nitrogen atmosphere) and then reduced under a hydrogen atmosphere, preferably an inert gas, preferably a hydrogen mixture with argon or nitrogen atmosphere (i.e., a dry reduction step), or reduced by pyrolysis under an inert atmosphere. The reducing agent in the solution used in the wet reduction step of the present method is preferably selected from the group consisting of glucose, hydrazine, hydrazine sulfate, hydrazine nitrate, sodium hypophosphite, sodium borohydride, lithium borohydride, aluminum borohydride, lithium tetraethylborohydride, methanol, ethanol, formaldehyde, formic acid, ethylene glycol, 1,2-hexadecanediol, hydroxylamine, and dimethylborazane DMAB. L-ascorbic acid and citric acid are particularly preferred reducing agents for use in the methods of the present invention.

[0036] The most preferred metal reduction method for any of the above-presented embodiments is a dry reduction method in which the metal in the metal-urea complexing agent is reduced under a hydrogen atmosphere.

[0037] In both the single-step and multi-step synthesis methods according to the present invention, aqueous solutions are preferably used. However, in alternative embodiments, any other solvent (e.g., a water-isopropanol or water-acetone mixture) can be used, provided that the urea or urea derivative and the metal precursor are soluble in such a solvent mixture. In multi-step processes, where the synthesis is carried out until the liquid evaporates (the method described in item (ii) above), the use of solvents with a lower surface tension than water or their mixtures with water results in a more uniform and reproducible crystallization process of the metal-urea complexing agent complex. The choice of solvent has a significant impact on the type of metal-urea complexing agent complex obtained and, consequently, on the properties of the carbon-supported nanoparticles obtained using these complexes.

[0038] Caution should be exercised when readily flammable solvents (such as acetone) are used in the synthesis and when high temperatures are applied in any of the steps described.

[0039] In a preferred embodiment, the metal reduction step in the method of the present invention is carried out by exposing the carbon substrate, onto which the metal-urea complexing agent complex is adsorbed, to a hydrogen gas atmosphere in a dry reduction process in which a filtered carbon sample containing the adsorbed metal-urea complexing agent (obtained by either a single-step or multi-step synthesis) is first placed in a stream of inert gas (preferably argon) at a temperature in the range of 20-150°C and held until residual oxygen is removed, e.g., for about 1-3 hours.

[0040] Next, the sample is reduced in a H2 / inert gas flow (gaseous reduction flow), preferably H2 / Ar or H2 / N. The reduction process preferably takes 1 to 6 hours. Preferably, the gaseous reduction flow contains 1 to 10% hydrogen. In a preferred embodiment, the reduction process is carried out at a high temperature, such as 50 to 200°C. In a final step, the gas atmosphere is switched back to an inert gas (preferably argon or nitrogen), and the sample is allowed to cool to ambient temperature. Optionally, after the reduction step, the sample is heat-treated in a flow of inert gas (preferably argon or nitrogen), typically at 50 to 350°C for 1 to 6 hours. This step allows for the removal of impurities or unreacted urea / urea derivatives adsorbed on the carbon support.

[0041] It should be emphasized that urea decomposes at temperatures above 190°C (Phys. Chem. Chem. Phys., 2019, 21, 16785). Nitrogen from urea or urea decomposition products can be bound to the carbon support surface, thereby modifying the support's surface properties. Scientific literature describes the beneficial effect of nitrogen in the carbon support on PGM / carbon catalysts. In particular, heating the carbon support in ammonia flow incorporates nitrogen atoms into the carbon structure, resulting in improved catalytic activity and durability of Pt and Pt-alloys (Enhancement of Pt and Pt-alloy fuel cell catalyst activity and durability via nitrogen-modified carbon supports, “Energy Environ. Sci., 2010, 3, 1437-1446|1439). Urea, when decomposed into ammonia and carbon dioxide, can be a similar nitrogen source, especially at sufficiently high temperatures, resulting in similar structural modifications. The application of carbon supports modified with nitrogen atoms or urea derivatives is a possible means for obtaining carbon-supported metal nanoparticles with modified catalytic properties. It should be emphasized here that the presence of nitrogen after adsorption of the metal-urea complexing agent complex was observed by the inventors using X-ray photoelectron spectroscopy. In addition, if the adsorption process is carried out from a molar excess of urea compared to the metal precursor, excess uncomplexed urea can be adsorbed onto the carbon support during the adsorption step, leading to competition between urea and the metal-urea complexing agent complex. Therefore, a large stoichiometric excess of urea compared to the metal precursor (i.e., 20:1) can result in low carbon coverage with metal nanoparticles and high nitrogen doping of the carbon support. This provides a simple method for adjusting the nitrogen-to-carbon ratio of the support and the metal-to-carbon ratio of the final product.

[0042] At any stage of the reduction process, the flow rate of the inert gas or H2 / inert gas mixture is in the range of 5-30 ml / min.

[0043] The method of the present invention is highly versatile, as it allows the synthesis of catalysts with a wide range of metal:carbon mass ratios. Therefore, the catalysts obtained by the method of the present invention can be used in a variety of fields. Furthermore, the materials obtained by the method of the present invention (i.e., carbon-supported metal nanoparticles) exhibit high activity. The nanoparticles obtained on carbon supports by the method of the present invention have very small average sizes (diameters ranging from 0.8 to 1.6 nm) and a uniform distribution of nanoparticle sizes, resulting in a highly developed surface area. Due to the highly developed surface area, the materials obtained by the method of the present invention exhibit high activity per mass of precious metal used. In addition, nitrogen from the decomposition of urea or urea derivatives can be introduced into the final catalyst, which can be beneficial to catalytic activity. Therefore, the method of the present invention allows the use of small amounts of precious metal precursor salts to synthesize catalysts with the desired activity.

[0044] Due to the fact that complexes of platinum group metals and urea complexing agents provide very good coverage when adsorbed onto the surface of a carbon support, the present application also provides a method for adsorbing a platinum group metal precursor onto the surface of a carbon support, wherein the carbon support is immersed in a solution of a complex of a platinum group metal and urea complexing agent, the urea complexing agent being selected from the group consisting of urea, urea derivatives, mixtures of urea and at least one urea derivative, and mixtures of at least two urea derivatives. This method is particularly useful for further nanoparticle synthesis by reduction of the platinum group metal precursor on the carbon support. Terms such as "precursor," "carbon support," and "urea complexing agent" have the same meanings as those described above in connection with the method for synthesizing carbon-supported platinum group metal or platinum group metal alloy nanoparticles according to the present invention.

[0045] In a preferred embodiment, the formation of the complex of the platinum group metal with the urea complexing agent is carried out in the presence of a carbon support, and the complex of the platinum group metal with the urea complexing agent is adsorbed directly onto the carbon support after formation.

[0046] The present invention also relates to the use of complexes of platinum group metals with urea complexing agents for the adsorption of platinum group metal precursors on carbon supports. [Brief explanation of the drawings]

[0047] The subject matter of the present invention is illustrated in the following drawings.

[0048] [Figure 1] Figure 1 shows cyclic voltammograms recorded in 0.5 M H2SO4 for samples with different nominal Pt mass contents. Voltammograms were recorded at a scan rate of 5 mV s-1. [Figure 2a] TEM images and EDX maps for samples with different nominal Pd mass contents, 1% Pd and 99% C, are shown. [Figure 2b] TEM images and EDX maps for samples with different nominal Pd mass contents, 5% Pd and 95% C, are shown. [Figure 3] Cyclic voltammograms recorded in 0.5 M H2SO4 for a sample containing nominal 5% Rh and 95% C are shown. For comparison, a cyclic voltammogram of a sample containing pure carbon has been added to the graph. Voltammograms were recorded at a scan rate of 5 mV s-1. [Figure 4] TEM images and EDX maps are shown for a sample containing nominal 5% Rh and 95% C. [Figure 5] Cyclic voltammograms recorded in 0.5 M H2SO4 for samples containing nominal 5% Pt and 95% C obtained on different carbon black supports and at different reduction temperatures are shown: (a) Ketjenblack EC300J as the carbon support, and (b) Vulcan® XC-72 as the carbon support. For comparison, cyclic voltammograms for samples containing 100% carbon (Ketjenblack EC300J or Vulcan® XC-72) have been added to the graph. Voltammograms were recorded at a scan rate of 5 mV s-1. [Figure 6a]C support obtained by reduction at 100° C.: TEM images and EDX maps for a sample containing nominal 5% Pt and 95% C supported on Ketjenblack EC300J. [Figure 6b] TEM images and EDX maps are shown for a sample containing nominal 5% Pt and 95% C supported on a C support: Vulcan® XC-72 obtained by reduction at 100° C. [Figure 7] Figure 6b shows the histogram of Pt nanoparticles calculated based on the image shown at a scale of 5 nm. [Figure 8] Cyclic voltammograms recorded in 0.5 M H2SO4 for samples containing nominal 5% Pd and 95% C obtained using different amounts of carbon are shown. Voltammograms were recorded at a scan rate of 5 mV s-1. [Figure 9] Cyclic voltammograms recorded in 0.5 M H2SO4 for a sample containing nominal 5% Pd and 95% C obtained using urea and urea derivatives: Pd1 - Pd complex with urea; Pd2 - Pd complex with N,N'-dimethylurea; Pd3 - Pd complex with N,N'-dimethylurea; Pd4 - Pd complex with methylurea; Pd5 - Pd complex with trimethylurea; and Pd6 - Pd complex with N,N'-diethylurea. Voltammograms were recorded at a scan rate of 5 mV s-1. [Figure 10] Cyclic voltammograms recorded in 0.5 M H2SO4 for samples containing nominal 20% Ir and 80% C obtained with urea, N,N-dimethylurea, and N,N'-dimethylurea are shown. Voltammograms were recorded at a scan rate of 5 mV s-1. [Figure 11a] Shown are a TEM image and EDX map for a sample containing nominal 5% Ir and 95% C. The sample was obtained using IrCl4 as the precursor salt. [Figure 11b]Shown are TEM images and EDX maps for a sample containing nominal 5% Ir and 95% C. The sample was obtained using IrCl3 as the precursor salt. [Figure 12] Cyclic voltammograms recorded in 0.5 M H2SO4 for samples containing nominal 20% Ir and 80% C obtained from fresh (48 h) or aged (6 months) solutions of IrCl4 are shown. Voltammograms were recorded at a scan rate of 5 mV s-1. [Figure 13] Cyclic voltammograms recorded in 0.5 M H2SO4 for samples containing nominal 5% Ir and 95% C obtained with urea complexes synthesized at different temperatures: a) narrow potential window, and b) wide potential window. Voltammograms were recorded at a scan rate of 5 mV s-1. [Figure 14] Cyclic voltammograms recorded in 0.5 M H2SO4 for a sample containing nominal 45% Pt and 55% C obtained by one-step synthesis using different temperatures are shown. Voltammograms were recorded at a scan rate of 5 mV s-1. [Figure 15] Cyclic voltammograms recorded in 0.5 M H2SO4 for samples containing nominal 45% Pt and 55% C obtained by one-step synthesis using different synthesis times are shown. Voltammograms were recorded at a scan rate of 5 mV s-1. [Figure 16] Cyclic voltammograms recorded in 0.5 M H2SO4 for samples containing nominal 45% Pt and 55% C obtained with different Pt:urea ratios are shown. Voltammograms were recorded at a scan rate of 5 mV s-1. [Figure 17] Cyclic voltammograms recorded in 0.5 M H2SO4 for samples containing nominal 45% Pt and 55% C obtained with different Pt:urea ratios are shown. Voltammograms were recorded at a scan rate of 5 mV s-1. [Figure 18]Cyclic voltammograms recorded in 0.5 M H2SO4 for samples containing nominal 20% Pt and 80% C obtained with different initial concentrations of Pt and urea are shown. Voltammograms were recorded at a scan rate of 5 mV s-1. [Figure 19] Figure 1 shows cyclic voltammograms recorded in 0.5 M H2SO4 for samples containing nominal 45% Pt and 55% C obtained with and without heating during the complex adsorption step, with urea:Pt ratios equal to (a) 20:1 and (b) 30:1. Voltammograms were recorded at a scan rate of 5 mV s-1. [Figure 20] Cyclic voltammograms recorded in 0.5 M H2SO4 for samples containing nominal 40% Pt and 60% C obtained using the one-step and two-step synthetic routes are shown. Voltammograms were recorded at a scan rate of 5 mV s-1. [Figure 21] Cyclic voltammograms recorded in 0.5 M H2SO4 for a sample containing nominal 5% Pd and 95% C obtained using different reducing agents are shown. Voltammograms were recorded at a scan rate of 5 mV s-1. [Figure 22] Cyclic voltammograms recorded in 0.5 M H2SO4 for samples containing nominal 5% Pd and 95% C obtained using various reduction temperatures during reduction with H2 / Ar. Voltammograms were recorded at a scan rate of 5 mV s-1. [Figure 23] TEM images are shown for samples containing nominal 5% Pd and 95% C obtained using various reduction temperatures during reduction with H2 / Ar. [Figure 24] Figure 23 shows histograms of the diameters of Pd nanoparticles obtained using different reduction temperatures during reduction with H2 / Ar: (a) 50 °C, (b) 100 °C, and (c) 150 °C. The histograms were calculated based on the images shown in Figure 23. [Figure 25]Cyclic voltammograms recorded in 0.5 M H2SO4 for samples containing nominal 5% Ir and 95% C obtained using different reduction temperatures during reduction with H2 / Ar: a) narrow potential window, and b) wide potential window. Voltammograms were recorded at a scan rate of 5 mV s-1. [Figure 26] Cyclic voltammograms recorded in 0.5 M H2SO4 for samples containing nominal 45% Pt and 55% C obtained by synthesis with a urea:Pt ratio equal to 2:1, with or without post-synthesis heat treatment at 350 °C. Voltammograms were recorded at a scan rate of 5 mV s-1. [Figure 27a] TEM images and EDX maps are shown for a sample containing nominal 45% Pt and 55% C without post-synthesis heat treatment. [Figure 27b] TEM images and EDX maps are shown for a sample containing nominal 45% Pt and 55% C with post-synthesis heat treatment. [Figure 28] Figure 27 shows histograms of Pt nanoparticle diameters in a sample containing nominal 45% Pt and 55% C (a) without and (b) with post-synthesis heat treatment at 350 °C. The histograms were calculated based on the images shown in Figure 27. [Figure 29] Cyclic voltammograms recorded in 0.5 M H2SO4 for samples containing nominal 45% Pt and 55% C obtained by synthesis with urea:Pt ratios equal to (a) 2:1 or (b) 4:1, with or without post-synthesis heat treatment at 300 °C. Voltammograms were recorded at a scan rate of 5 mV s-1. [Figure 30] Cyclic voltammograms recorded in 0.5 M H2SO4 for samples containing nominal 5% Rh and 95% C obtained using metal precursor salts or metal-urea complexes in the adsorption step are shown. For comparison, the cyclic voltammogram of a sample containing 100% carbon has been added to the graph. Voltammograms were recorded at a scan rate of 5 mV s-1. [Figure 31]Cyclic voltammograms recorded in 0.5 M H2SO4 for a sample containing nominal 20% metal and 80% C with different Pt:Ir nanoalloys: 100% Ir; 75% Ir, 25% Pt; 50% Ir, 50% Pt; 25% Ir, 75% Pt; and 100% Pt. Voltammograms were recorded at a scan rate of 5 mV s-1. [Figure 32] TEM images of (a) a 20% Pt 80% C (Vulcan®) catalyst from BASF and (b) a 20% Pt 80% C (Vulcan®) sample obtained by the method of the present invention. [Figure 33] Figure 32 shows a histogram of the diameter of Pt nanoparticles in a sample of (a) a 20% Pt 80% C (Vulcan®) catalyst from BASF and (b) a 20% Pt 80% C (Vulcan®) sample obtained by the method of the present invention. The histogram was calculated based on the TEM image shown in Figure 32 at a scale of 10 nm. [Figure 34] TEM images of (a) a 40% Pt 60% C (Vulcan®) catalyst from E-TEK and (b) a 40% Pt 60% C (Vulcan®) sample obtained by the method of the present invention. [Figure 35] Figure 34 shows a histogram of the diameter of Pt nanoparticles in samples of (a) 40% Pt 60% C (Vulcan®) catalyst from E-TEK and (b) a 40% Pt 60% C (Vulcan®) sample obtained by the method of the present invention. The histogram was calculated based on the TEM images shown in Figure 34 at a scale of 10 nm. [Figure 36](a) Mass-normalized cyclic voltammograms recorded in 0.5 M H2SO4 for a sample containing nominal 40% Pt and 60% C (Ketjenblack®) obtained by the method of the present invention and a commercially available catalyst, 40% Pt 60% C (Vulcan®) catalyst from E-TEK, and (b) TEM images corresponding to the samples shown in the voltammograms. The voltammograms were recorded at a scan rate of 5 mV s-1. The charge values ​​plotted on the voltammogram plots are the total charge values ​​corresponding to the desorption process of a hydrogen monolayer from the platinum catalyst surface. [Figure 37] Cyclic voltammograms recorded in 0.5 M H2SO4 for a sample containing nominal 20% Pt and 80% C obtained with different reducing agents are shown. Voltammograms were recorded at a scan rate of 5 mV s-1. [Figure 38] Ion currents determined by mass spectrometry recorded during the thermal decomposition of a urea complex of iridium(III) deposited on carbon black (20% Ir and 80% C) at a heating rate of 0.6 °C / min are shown. The ion current at m / z = 17 can be assigned to ammonia, m / z = 18 to water, and m / z = 44 to carbon dioxide. [Figure 39] TEM images and EDX maps for a sample containing 20% ​​Ir and 80% C after adsorption of Ir-urea complex, a-c) before and d) after thermal decomposition of the Ir-urea complex in an inert atmosphere. DETAILED DESCRIPTION OF THE INVENTION [Example]

[0049] Example 1. Synthesis of carbon-supported Pt nanoparticles—different metal-to-carbon mass ratios 0.01 mol l in 10 ml -1 2 mol l of K2PtCl4 and 0.125 ml -1Urea (urea:Pt ratio equal to 2.5:1) was mixed and heated at 90 °C in a Petri dish until all the liquid had evaporated. The precipitate was left to stand for 5 min under a fume hood without heating and then dissolved in 10 ml of water, thereby obtaining 0.01 mol l of platinum-urea complex. -1 The synthesis was repeated twice, resulting in a total of 30 ml of 0.01 mol l solution. -1 The resulting solution was allowed to stand for 5 hours, and then a calculated amount of solution was added to approximately 50 mg of carbon black sample to achieve the desired metal:carbon ratio in the dry mass of the sample (1.28, 6.09, and 16.23 ml for nominal Pt contents of 5, 20, and 40%, respectively). The resulting suspension of carbon in the platinum-urea complex solution was then heated at 90 °C under reflux for 2 hours. The suspension was then filtered and thoroughly washed with water. The sample was then placed in a tube furnace and dried at 150 °C under argon for 4 hours, subsequently reduced in a hydrogen-argon mixture (10% H, 90% Ar) at 150 °C for 4 hours, and then allowed to cool to ambient temperature under argon. The flow rate of the gas flow (argon or hydrogen-argon mixture) was 15 sccm. The presence of nitrogen after precursor adsorption and before reduction was confirmed using X-ray photoelectron spectroscopy. Additionally, nitrogen was present after the reduction step as measured using the same method, and can be removed in large excess from the final sample by washing with a water-ethanol solution.

[0050] The resulting material, carbon-Pt nanoparticles, was examined by cyclic voltammetry. A few milligrams of the synthesized material were mixed with a 5% Nafion® solution (DuPont) in an amount such that Nafion® constituted approximately 32% of the dry mass of the mixture. An equal volume of isopropanol was then added. For example, 80 μl of 5% Nafion® (density = 0.8 g cm) was added to 6.8 mg of material. -3) and 80 μl of isopropanol were used. 1 μl of the resulting suspension was then dropped onto a gold disk electrode and allowed to stand and dry under an argon atmosphere. The resulting electrode, containing the synthesized material (mixed with Nafion) deposited on the gold disk electrode, was then used as the working electrode in a three-electrode setup for cyclic voltammetry measurements. The gold disk electrode was also used as the counter electrode, and a mercury-sulfur electrode was used as the reference electrode. 0.5 M H2SO4 was used as the supporting electrolyte in all measurements. Prior to measurements, the supporting solution was degassed using argon bubbling through the solution. An example of a recorded cyclic voltammogram is shown in Figure 1. All potentials were given versus a reversible hydrogen electrode.

[0051] The described method allows for a wide range of Pt:carbon mass ratios to be obtained, which allows for the preparation of catalysts that can potentially be applied in a variety of fields.

[0052] Example 2. Synthesis of carbon-supported Pd nanoparticles—different metal-to-carbon mass ratios 0.01 mol l in 10 ml -1 1 mol l of K2PdCl4 and 0.25 ml -1 A solution of urea (urea:Pd ratio equal to 2.5:1) was mixed and heated at 80 °C in a Petri dish until all liquid evaporated. The precipitate was dissolved in 10 ml of water, and the evaporation / dissolution procedure was repeated twice. The calculated amount of the resulting 0.01 mol l was used to achieve the desired metal:carbon ratio in the dry mass of the sample. -1 The Pd-urea complex solution was added to approximately 50 mg of carbon black sample (2.26 ml and 0.43 ml for 5% Pd 95% C and 1% Pd 99% C, respectively). The resulting suspension of carbon in the palladium-urea complex solution was allowed to stand for 30 minutes, then filtered and thoroughly washed with water. The sample was then placed in a tube furnace and dried at 100°C under argon for 4 hours, subsequently reduced in a hydrogen-argon mixture (10% H, 90% Ar) at 100°C for 4 hours, and then allowed to stand and cool to ambient temperature under argon. The gas flow (argon or hydrogen-argon mixture) was 15 sccm.

[0053] Figure 2(a) and (b) show the TEM image and EDX map of the resulting material, a carbon-supported Pd catalyst.

[0054] Example 3. Synthesis of carbon-supported Rh nanoparticles 0.01 mol l in 10 ml -1 1 mol l of RhCl3 and 0.25 ml -1 Urea (urea:Rh ratio equal to 2.5:1) was mixed and heated at 90 °C in a Petri dish until all the liquid evaporated. The resulting precipitate was dissolved in 10 ml of water, and the evaporation-dissolution procedure was repeated twice. Approximately 2.43 ml of the resulting 0.01 mol l -1 The rhodium urea complex solution was added to approximately 50 mg of carbon black to obtain the desired rhodium:carbon mass ratio (5% Rh, 95% C). The resulting suspension was allowed to settle for approximately 5 hours, after which it was filtered and thoroughly washed with water. The sample was then placed in a tube furnace and dried at 150°C under an argon atmosphere for 4 hours, subsequently reduced in a hydrogen-argon mixture (10% H, 90% Ar) at 150°C for 4 hours, and then allowed to settle and cool to ambient temperature under an argon atmosphere. The gas flow rate (argon or hydrogen-argon mixture) was 15 sccm.

[0055] The obtained samples were analyzed by cyclic voltammetry to evaluate their activity in the hydrogen adsorption-desorption process. For comparison, the cyclic voltammogram of a sample containing 100% carbon was measured. The samples were prepared and measurements were recorded in the same manner as described in Example 1. An example of the recorded cyclic voltammogram is shown in Figure 3. The obtained Rh / C catalyst exhibits clear activity in the hydrogen adsorption-desorption process (current peaks observed between approximately -0.3 and -0.65 V).

[0056] FIG. 4 shows the TEM image and EDX map of the resulting material, a carbon-supported Rh catalyst.

[0057] Example 4. Synthesis of carbon-supported Pt nanoparticles on different types of carbon supports 0.01 mol l in 10 ml -1 2 mol l of K2PtCl4 and 0.125 ml -1 Urea (urea:Pt ratio equal to 2.5:1) was mixed and heated at 90 °C in a Petri dish until all the liquid had evaporated. The precipitate was left to stand for 5 min under a fume hood without heating and then dissolved in 10 ml of water, thereby obtaining 0.01 mol l of platinum-urea complex. -1 A solution was obtained. The solution was allowed to stand for 5 hours, and then a calculated amount of solution was added to approximately 50 mg of pre-weighed carbon sample to achieve the desired metal:carbon ratio (5% metal, 95% carbon) in the dry mass of the sample (1.26 ml and 1.28 ml for Ketjenblack® EC300J and Vulcan® XC-72, respectively). The resulting suspension was allowed to stand for 1 hour, filtered, and thoroughly washed with water. The sample was then placed in a tube furnace and dried at 100°C or 150°C under argon atmosphere for 4 hours, subsequently reduced in a hydrogen-argon mixture (10% H2, 90% Ar) at 100°C or 150°C for 4 hours, allowed to stand, and cooled to ambient temperature under argon atmosphere. The flow rate of the gas flow (argon or hydrogen-argon mixture) was 15 sccm.

[0058] Materials reduced with different C supports and at different temperatures were examined by cyclic voltammetry as described in Example 1. Examples of recorded cyclic voltammograms are shown in Figure 5. Regardless of the type of carbon support, it can be observed that higher applied reduction temperatures result in a decrease in activity in the hydrogen adsorption-desorption region compared to lower reduction temperatures. This is caused by a higher probability of sintering of the nanoparticles at higher reduction temperatures, which should result in a smaller surface area and a decrease in activity.

[0059] Selected TEM images and EDX maps of the obtained samples are shown in Figure 6. A histogram of nanoparticle diameters is also shown in Figure 7 for a sample containing nominal 5% Pt and 95% Vulcan® XC 72 as the carbon support. This histogram was obtained based on the 5 nm scale image shown in Figure 6(c), i.e., the image of a sample containing nominal 5% Pt and 95% C-Vulcan® XC 72 obtained at a reduction temperature of 100 °C. Based on this histogram, it can be clearly seen that a narrow distribution of very small nanoparticles (most of the nanoparticles are in the diameter range of 0.8–1.6 nm) is obtained in the case of the 5% Pt / Vulcan® catalyst.

[0060] Example 5. Synthesis of carbon-supported Pd nanoparticles using different amounts of carbon support 15ml of 0.05mol l -1 1 mol l of K2PdCl4 and 1.88 ml of -1 The urea solution was mixed and heated in a Petri dish at 80 °C until all liquid had evaporated. The precipitate was dissolved in 10 ml of water and the evaporation was repeated. The precipitate was then dissolved in 15 ml of water. The resulting 0.05 mol l was calculated to achieve the desired metal:carbon ratio (5% Pd, 95% C) in the dry mass of the sample. -1 The Pd-urea complex solution was added to a pre-weighed amount of carbon (0.45, 2.7, and 9 ml for approximately 50, 300, and 1000 mg of carbon black, respectively; the carbon sample was first dispersed in water for 5 minutes using an ultrasonic bath (10 mg of carbon per ml of water)). The resulting suspension of carbon in the palladium-urea complex solution was allowed to stand for 30 minutes, then filtered and thoroughly washed with water. The sample was then placed in a tube furnace and dried at 100 °C under an argon atmosphere for 4 hours, subsequently reduced in a hydrogen-argon mixture (10% H, 90% Ar) at 100 °C for 4 hours, and then allowed to stand and cool to ambient temperature under an argon atmosphere. The flow rate of the gas flow (argon or hydrogen-argon mixture) was 15 sccm.

[0061] The resulting samples were examined using cyclic voltammetry as described in Example 1. Examples of recorded voltammograms are shown in Figure 8. Only minor differences in the cyclic voltammograms could be observed between samples prepared with different amounts of carbon support. This indicates that the synthesis method according to the present invention can be easily used with various amounts of reagents, resulting in the scalability of the synthesis method.

[0062] Example 6. Synthesis of carbon-supported Pd nanoparticles using different urea derivatives 0.01 mol l in 10 ml -1 1 mol l of K2PdCl4 and 0.25 ml -1 Solutions of urea or urea derivatives (methylurea, N,N-dimethylurea, N,N'-dimethylurea, trimethylurea, N,N'-diethylurea) were mixed and heated at 80 °C in a Petri dish until all liquid evaporated. The precipitate was dissolved in 10 ml of water and the evaporation / dissolution procedure was repeated. To achieve the desired metal:carbon ratio (5% Pd, 95% C) in the dry mass of the sample, 2.26 ml of the resulting 0.01 mol l -1 Each of the Pd complex solutions was added to a pre-weighed sample of approximately 50 mg of carbon black. The resulting suspension was allowed to settle for 30 minutes, then filtered and thoroughly washed with water. The sample was then placed in a tube furnace and dried at 100°C under an argon atmosphere for 4 hours, subsequently reduced in a hydrogen-argon mixture (10% H, 90% Ar) at 100°C for 4 hours, and then allowed to settle and cool to ambient temperature under an argon atmosphere. The gas flow rate (argon or hydrogen-argon mixture) was 15 sccm.

[0063] The obtained samples were examined using cyclic voltammetry as described in Example 1. Examples of recorded voltammograms are shown in Figure 9. The activity of the materials observed in the hydrogen adsorption-desorption process (current peaks between about -0.6 and -0.7 V) was of similar magnitude for all prepared samples, but the samples prepared using urea, methylurea, and trimethylurea showed the best performance (i.e., the highest charge associated with the adsorption-desorption process).

[0064] Example 7. Synthesis of carbon-supported Ir nanoparticles using different urea derivatives The following solutions were prepared: a) 10 ml of 0.01 mol l -1 IrCl4, 0.25 ml of 1 mol l -1 In addition to urea (urea:Ir ratio equal to 2.5:1), b) 10 ml of 0.01 mol l -1 IrCl4, 0.25 ml of 1 mol l -1 In addition to N,N-dimethylurea (N,N-dimethylurea:Ir ratio equal to 2.5:1), and c) 10 ml of 0.01 mol l -1 IrCl4, 0.25 ml of 1 mol l -1 N,N'-dimethylurea was added (N,N'-dimethylurea:Ir ratio equal to 2.5:1).

[0065] The solutions were heated under reflux at 80°C for 1.5 hours. Next, 3.17 ml of each solution was added to a pre-weighed sample of approximately 25 mg of carbon black to achieve the desired iridium:carbon ratio (20% Ir, 80% C) in the dry mass of the sample. The resulting suspension was allowed to settle for approximately 15 hours, after which it was filtered and thoroughly washed with water. The samples were then placed in a tube furnace and dried at 150°C under an argon atmosphere for 4 hours, subsequently reduced in a hydrogen-argon mixture (10% H, 90% Ar) at 150°C for 4 hours, and then allowed to settle and cool to ambient temperature under an argon atmosphere. The gas flow rate (argon or hydrogen-argon mixture) was 15 sccm.

[0066] The obtained samples were examined using cyclic voltammetry as described in Example 1. Examples of recorded voltammograms are shown in Figure 10. In the case of carbon-supported Ir nanoparticles, significantly higher activity (approximately -0.3 to 0.65 V) in the hydrogen adsorption-desorption process was observed for samples in which urea derivatives (N,N-dimethylurea and N,N'-dimethylurea) were used in the synthesis instead of urea.

[0067] Example 8. Synthesis of carbon-supported Ir nanoparticles using different precursor salts Carbon-supported Ir nanoparticles were obtained as described above using IrCl3 and IrCl4 as precursor salts for the synthesis of urea complexes. Selected TEM images and EDX maps of the obtained samples are shown in Figure 11. No differences were observed in the obtained materials despite the use of different salts as starting reagents in the synthesis.

[0068] Example 9. Synthesis of carbon-supported Ir nanoparticles - Effect of precursor salt hydrolysis 0.01 mol l in 10 ml -1 IrCl4 (solution 48 hours or 6 months after preparation) and 0.25 ml of 1 mol l -1 The Ir complexes were mixed with N,N-dimethylurea (N,N-dimethylurea:Ir ratio equal to 2.5:1) and heated at 80°C under reflux for 5 hours. Next, 3.17 ml of each of the resulting Ir complex solutions was added to a pre-weighed sample of approximately 25 mg of carbon black to achieve the desired metal:carbon ratio (20% Ir, 80% C) in the dry mass of the sample. The resulting suspension was allowed to stand for approximately 15 hours, after which it was filtered and thoroughly washed with water. The sample was then placed in a tube furnace and dried at 150°C under argon for 4 hours, subsequently reduced in a hydrogen-argon mixture (10% H, 90% Ar) at 150°C for 4 hours, and then allowed to cool to ambient temperature under argon. The gas flow rate (argon or hydrogen-argon mixture) was 15 sccm.

[0069] The obtained samples were examined using cyclic voltammetry as described in Example 1. An example of a recorded voltammogram is shown in Figure 12. In the described example, the beneficial role of the hydrolysis of IrCl4 can be seen. Presumably, as a result of the hydrolysis process, mixed chloride and hydroxy complexes of iridium are produced, which influence the type of Ir complex formed with the urea derivative and thus the activity of the final sample obtained.

[0070] Example 10. Synthesis of carbon-supported Ir nanoparticles - Effect of complex synthesis temperature 0.01 mol l in 10 ml -1 1 mol l of IrCl4 and 0.25 ml -1 N,N'-dimethylurea (N,N'-dimethylurea:Ir ratio equal to 2.5:1) was prepared. The solutions were heated under reflux at 35°C, 55°C, or 80°C for 5, 3, and 1.5 hours, respectively. Next, 1.28 ml of each solution was added to a pre-weighed sample of approximately 50 mg of carbon black to achieve the desired iridium:carbon ratio (5% Ir, 95% C) in the dry mass of the sample. The resulting suspension was allowed to stand for approximately 15 hours, after which it was filtered and thoroughly washed with water. The sample was then placed in a tube furnace and dried at 100°C under argon for 4 hours, subsequently reduced in a hydrogen-argon mixture (10% H, 90% Ar) at 100°C for 4 hours, and then allowed to cool to ambient temperature under argon. The gas flow rate (argon or hydrogen-argon mixture) was 15 sccm.

[0071] The obtained samples were examined using cyclic voltammetry as described in Example 1. An example of a recorded voltammogram is shown in Figure 13. The lack of a clear distinction between hydrogen adsorption and desorption regions expected for the cyclic voltammogram of Ir in H2SO4 makes it difficult to assess the influence of synthesis temperature and time on catalytic activity. The presence of a current peak suggests the presence of Ir nanoparticles. Therefore, it can be concluded that Ir nanoparticles on carbon supports can be obtained by synthesizing Ir complexes over a wide temperature range.

[0072] Example 11. One-step synthesis of carbon-supported Pt nanoparticles - Effect of complex synthesis temperature Two samples were prepared, where 15.1 ml of 0.01 mol l -1 2 mol l of K2PtCl4 and 0.302 ml -1 Urea (urea:Pt ratio equal to 4:1) was added to approximately 40 mg of carbon black, giving a nominal content of 45% Pt and 55% C by dry mass, and heated at 90°C or 100°C under reflux for 220 minutes. The resulting suspension was then filtered and thoroughly washed with water. The sample was then placed in a tube furnace and dried at 150°C for 4 hours under argon, subsequently reduced in a hydrogen-argon mixture (10% H, 90% Ar) at 150°C for 4 hours, and then allowed to cool to ambient temperature under argon. The flow rate of the gas stream (argon or hydrogen-argon mixture) was 15 sccm.

[0073] The obtained samples were examined using cyclic voltammetry as described in Example 1. An example of a recorded voltammogram is shown in Figure 14. It can be observed that for samples obtained using higher temperatures during the adsorption process, a significantly higher activity of the hydrogen adsorption-desorption process (from about 0.3 to -0.65 V) is observed.

[0074] Example 12. One-step synthesis of carbon-supported Pt nanoparticles - effect of synthesis time Two samples were prepared, where 18.9 ml of 0.01 mol l -1 2 mol l of K2PtCl4 and 0.199 ml -1Urea (urea:Pt ratio equal to 2.1:1) was added to approximately 50 mg of carbon black, giving a nominal content of 45% Pt and 55% C by dry mass, and heated at 100°C under reflux for 120 or 220 minutes. The resulting suspension was then filtered and thoroughly washed with water. The sample was then placed in a tube furnace and dried at 150°C for 4 hours under argon, subsequently reduced in a hydrogen-argon mixture (10% H, 90% Ar) at 150°C for 4 hours, and then allowed to cool to ambient temperature under argon. The flow rate of the gas flow (argon or hydrogen-argon mixture) was 15 sccm.

[0075] The resulting samples were examined using cyclic voltammetry as described in Example 1. Examples of recorded voltammograms are shown in Figure 15. No difference in activity was observed between samples prepared by synthesis with shorter (120 min) and longer (220 min) heating times during adsorption.

[0076] Example 13. One-step synthesis of carbon-supported Pt nanoparticles—effect of Pt:urea ratio Two samples were prepared, where 18.9 ml of 0.01 mol l -1 2 mol l of K2PtCl4 and 0.199 ml -1 urea (urea:Pt ratio equal to 2.1:1) or 0.378 ml of 2 mol l -1 Urea (Pt:urea ratio equal to 4:1) was added to approximately 50 mg of carbon black, giving a nominal content of 45% Pt and 55% C by dry mass, and heated at 100°C under reflux for 220 minutes. The resulting suspension was then filtered and thoroughly washed with water. The sample was then placed in a tube furnace and dried at 150°C for 4 hours under an argon atmosphere, followed by reduction in a hydrogen-argon mixture (10% H, 90% Ar) at 150°C for 4 hours, and then allowed to cool to ambient temperature under an argon atmosphere. The flow rate of the gas stream (argon or hydrogen-argon mixture) was 15 sccm.

[0077] The obtained samples were examined using cyclic voltammetry as described in Example 1. Examples of recorded voltammograms are shown in Figure 16. The difference between the samples obtained using both ratios of urea:Pt is negligible, indicating that even very small ratios such as 2:1 can be used to obtain highly active catalysts.

[0078] Example 14. Synthesis of carbon-supported Pt nanoparticles—effect of Pt:urea ratio Several solutions were prepared. a) 0.6 ml of 2 mol l -1 Urea, 20 ml of 0.01 mol l -1 K2PtCl4 plus (urea:Pt ratio equal to 6:1) b) 1 ml of 2 mol l -1 Urea, 20 ml of 0.01 mol l -1 K2PtCl4 plus (urea:Pt ratio equal to 10:1) c) 2 ml of 2 mol l -1 Urea, 20 ml of 0.01 mol l -1 K2PtCl4 plus (urea:Pt ratio equal to 20:1) d) 3 ml of 2 mol l -1 Urea, 20 ml of 0.01 mol l -1 KPtCl4 (urea:Pt ratio equal to 30:1)

[0079] Each solution was heated at 80°C under reflux for 1 hour. Next, a calculated amount of solution (10.05, 10.26, and 10.71 ml of solution prepared in steps a), b), and c), respectively, was added to a pre-weighed approximately 25 mg sample of carbon black to achieve the desired metal:carbon ratio (45% Pt, 55% C) in the dry mass of the sample. The resulting suspension of carbon in platinum-urea complex solution was then heated at 100°C under reflux for 2 hours. The suspension was then filtered and thoroughly washed with water. The sample was then placed in a tube furnace and dried at 100°C under argon for 4 hours, subsequently reduced in a hydrogen-argon mixture (10% H, 90% Ar) at 100°C for 4 hours, and then allowed to cool to ambient temperature under argon. The gas flow rate (argon or hydrogen-argon mixture) was 15 sccm.

[0080] The resulting samples were examined using cyclic voltammetry as described in Example 1. Examples of recorded voltammograms are shown in Figure 17. Performance for all samples was similar, but was best at the lowest and highest urea:Pt stoichiometry (6:1 and 30:1). This suggests that more than one effect is likely influenced by the urea:Pt stoichiometry, including: 1) the effect of the urea:Pt ratio on the type of complex formed; 2) inaccessibility of free sites to the metal due to the adsorption of urea on the carbon that could occupy these free sites.

[0081] Example 15. Synthesis of carbon-supported Pt nanoparticles - varying initial concentrations of metal precursors and urea at constant Pt:urea ratio 0.1 mol l -1 K2PtCl4, 2 mol l -1 Urea and water solutions were mixed in various amounts as follows: a) 5 ml of 0.1 mol l -1 2 mol l of K2PtCl4, 0.625 ml -1 urea, and 4.375 ml of water b) 2 ml of 0.1 mol l -1 2 mol l of K2PtCl4, 0.25 ml-1 urea, and 7.75 ml of water c) 0.5 ml of 0.1 mol l -1 2 mol l of K2PtCl4, 0.063 ml -1 urea, and 9.437 ml of water

[0082] Each mixture was heated in a Petri dish at 90 °C until all the liquid had evaporated. The precipitate was allowed to stand for 5 min under a fume hood without heating, and then 0.025 mol l of platinum-urea complex was added. -1 The platinum-urea complexes were dissolved in water in amounts sufficient to obtain solutions (20, 8, and 2 ml of HO for a), b), and c), respectively. The solutions were allowed to stand for 4 hours, and then 1.95 ml of each of the resulting Pt-urea complex solutions was added to approximately 40 mg of carbon black samples to achieve the desired metal:carbon ratio (20% Pt, 80% C) in the dry mass of the sample. The resulting suspension of carbon in the platinum-urea complex solution was then heated at 90°C under reflux for 2 hours. The suspension was then filtered and thoroughly washed with water. The samples (containing the platinum-urea complexes adsorbed on the carbon) were then placed in a tube furnace and dried at 150°C under argon for 4 hours, subsequently reduced in a hydrogen-argon mixture (10% H, 90% Ar) at 150°C for 4 hours, and allowed to cool to ambient temperature under argon. The gas flow (argon or hydrogen-argon mixture) was 15 sccm.

[0083] The resulting samples were examined using cyclic voltammetry as described in Example 1. An example of a recorded voltammogram is shown in FIG.

[0084] Similar activity is observed for carbon-supported Pt nanoparticles obtained from precursor solutions of different concentrations (the highest concentration used being close to the maximum obtainable for the K2PtCl4 precursor salt).

[0085] Example 16. Synthesis of carbon-supported Pt nanoparticles - Effect of temperature on the adsorption of urea Pt complexes on carbon supports Two solutions were prepared. a) 2 ml of 2 mol l -1 Urea, 20 ml of 0.01 mol l -1 KPtCl4 (urea:Pt ratio equal to 20:1) a) 3 ml of 2 mol l -1 Urea, 20 ml of 0.01 mol l -1 KPtCl4 (urea:Pt ratio equal to 30:1)

[0086] The solution was then heated at 80°C under reflux for 1 hour. Next, calculated amounts of solution (10.71 ml and 11.21 ml of solution obtained in steps a) and b), respectively, were added to two pre-weighed samples of carbon black (approximately 25 mg) to achieve the desired metal:carbon ratio (45% Pt, 55% C) in the dry mass of the sample. One of the resulting carbon suspensions was then heated at 100°C under reflux for 2 hours. Two other samples were prepared in a similar manner using the solution prepared in step b) and approximately 25 mg of carbon black.

[0087] The suspension was then filtered and thoroughly washed with water. The sample was then placed in a tube furnace and dried at 100°C under argon for 4 hours, then reduced in a hydrogen-argon mixture (10% H, 90% Ar) at 100°C for 4 hours, and then allowed to cool to ambient temperature under argon. The gas flow (argon or hydrogen-argon mixture) was 15 sccm.

[0088] The resulting samples were examined using cyclic voltammetry as described in Example 1. An example of a recorded voltammogram is shown in Figure 19. The beneficial effect of heating the sample during adsorption on the catalytic activity of the final sample can be observed.

[0089] Example 17. Synthesis of carbon-supported Pt nanoparticles - Comparison of one-step and multi-step synthesis routes One-step synthesis of platinum-urea complexes adsorbed on carbon. 18.9 ml of 0.01 mol l -12 mol of K2PtCl4 and 0.236 ml -1 Urea (urea:Pt ratio equal to 2.5:1) was added to 50 mg of carbon black and heated at 100° C. under reflux for 3 hours, giving a sample with a nominal content of 40% Pt and 60% C by dry mass.

[0090] Multistep synthesis of platinum-urea complexes adsorbed on carbon 0.02 mol l in 10 ml -1 2 mol l of K2PtCl4 and 0.25 ml -1 Urea (urea:Pt ratio equal to 2.5:1) was mixed and then heated at 90°C until all liquid evaporated. The resulting precipitate was allowed to stand for 5 minutes and then dissolved in 20 ml of water. The resulting solution was allowed to stand for 4 hours, after which 15.89 ml was added to approximately 50 mg of carbon black to give a sample with a nominal content of 40% Pt and 60% C by dry mass.

[0091] The suspensions obtained by both routes were filtered and thoroughly washed with water. The samples were then placed in a tube furnace and dried at 150 °C under argon for 4 h, then reduced in a hydrogen-argon mixture (10% H, 90% Ar) at 150 °C for 4 h, and then allowed to cool to ambient temperature under argon. The gas flow (argon or hydrogen-argon mixture) was 15 sccm.

[0092] The obtained samples were examined using cyclic voltammetry as described in Example 1. Examples of recorded voltammograms are shown in Figure 20. Carbon-supported Pt nanoparticles obtained by different routes show similar activity. A more convenient method can be used for the synthesis.

[0093] Example 18. Synthesis of carbon-supported Pd nanoparticles using different reducing agents 0.01 mol l in 10 ml -1 1 mol l of K2PdCl4 and 0.25 ml -1The urea solution was mixed and heated in a Petri dish at 80°C until all liquid evaporated. The precipitate was dissolved in 10 ml of water, and the evaporation / dissolution procedure was repeated twice. 1.35 ml of the resulting solution was added to three approximately 30 mg samples of carbon black to achieve the desired metal:carbon ratio (5% Pd, 95% C) in the dry mass of the sample. The resulting suspension of carbon in the palladium-urea complex solution was allowed to stand for 30 minutes, then filtered and washed thoroughly with water. The samples were then reduced by one of the following methods: The samples were placed in a tube furnace, dried at 100 °C under argon atmosphere for 4 h, then reduced in a hydrogen-argon mixture (10% H, 90% Ar) at 100 °C for 4 h, and allowed to cool to ambient temperature under argon atmosphere. The gas flow (argon or hydrogen-argon mixture) was 15 sccm. - 1.3 ml of a 0.4% NaBH4 solution in 0.02 M NaOH was added and the sample was left to stand for 1 hour at ambient temperature (NaBH4:Pd ratio approximately equal to 10:1). - 1.35 ml of 0.1 M citric acid was added and the sample was heated at 60°C under reflux for 1 hour (citric acid:Pd ratio equal to approximately 10:1). - 1.35 ml of 0.1 M L-ascorbic acid was added and the sample was heated at 60°C under reflux for 0.1 h (L-ascorbic acid:Pd ratio equal to approximately 10:1).

[0094] The resulting samples were examined using cyclic voltammetry as described in Example 1. An example of a recorded voltammogram is shown in Figure 21. The reduction process carried out using L-ascorbic acid results in a material with the best catalytic properties. Reduction with H2 or citric acid is not as beneficial as reduction with L-ascorbic acid, and reduction with NaBH4 results in the worst catalytic performance.

[0095] Example 19. Synthesis of carbon-supported Pd nanoparticles using hydrogen as the reducing agent and different reduction temperatures 0.01 mol l in 10 ml -1 1 mol l of K2PdCl4 and 0.25 ml-1 The urea solution was mixed and heated in a Petri dish at 80°C until all liquid evaporated. The precipitate was dissolved in 10 ml of water, and the evaporation / dissolution procedure was repeated twice. 2.26 ml of the resulting solution was added to a pre-weighed amount of carbon (approximately 50 mg of carbon black) to achieve the desired metal:carbon ratio (5% Pd, 95% C) in the dry mass of the sample. The resulting suspension of carbon in the palladium-urea complex solution was allowed to stand for 30 minutes, after which it was filtered and thoroughly washed with water. The sample was then placed in a tube furnace and dried at 100°C under argon for 4 hours. It was then reduced in a hydrogen-argon mixture (10% H, 90% Ar) at 50°C, 100°C, or 150°C for 4 hours, allowed to stand, and cooled to ambient temperature under argon. The gas flow rate (argon or hydrogen-argon mixture) was 15 sccm.

[0096] The resulting samples were examined using cyclic voltammetry as described in Example 1. Examples of recorded voltammograms are shown in Figure 22. Selected TEM images of the resulting samples and the corresponding nanoparticle diameter histograms are shown in Figures 23 and 24, respectively.

[0097] It can be observed that as the reduction temperature increases, the distribution of nanoparticle diameters shifts towards higher values. This likely suggests enhanced sintering of the nanoparticles at higher temperatures. However, this is not reflected in the CVs obtained, as the most active sample was the one reduced at 100 °C. This indicates that sintering is not the only process that may be affected by the reduction temperature.

[0098] Example 20. Synthesis of carbon-supported Ir nanoparticles using hydrogen as a reducing agent and different reduction temperatures 0.01 mol l in 10 ml -1 1 mol l of IrCl4 and 0.25 ml -1N,N'-dimethylurea (N,N'-dimethylurea:Ir ratio equal to 2.5:1) was mixed with the solution. The solution was heated at 80°C under reflux for 1.5 hours. Next, 1.28 ml of the solution was added to a sample of approximately 50 mg of carbon black to achieve the desired iridium:carbon ratio (5% Ir, 95% C) in the dry mass of the sample. The resulting suspension was allowed to stand for approximately 15 hours, after which it was filtered and thoroughly washed with water. The sample was then placed in a tube furnace and dried at 100°C under argon for 4 hours, subsequently reduced in a hydrogen-argon mixture (10% H, 90% Ar) at 100°C or 150°C for 4 hours, and then allowed to stand and cool to ambient temperature under argon. The gas flow rate (argon or hydrogen-argon mixture) was 15 sccm.

[0099] The resulting samples were examined using cyclic voltammetry as described in Example 1. Examples of recorded voltammograms are shown in Figure 25. In the case of carbon-supported Ir nanoparticles, higher reduction temperatures resulted in higher activity of the material (higher currents associated with the hydrogen evolution process between about -0.6 and -0.7 V).

[0100] Example 21. Effect of post-treatment temperature on carbon-supported Pt nanoparticles obtained by one-step synthesis Several samples were prepared. a) 15.1 ml of 0.01 mol l -1 2 mol l of K2PtCl4 and 0.159 ml -1 Urea (urea:Pt ratio equal to 2.1:1) was added to 40 mg of carbon black. b) 15.1 ml of 0.01 mol l -1 2 mol l of K2PtCl4 and 0.159 ml -1 Urea (urea:Pt ratio equal to 2.1:1) was added to 40 mg of carbon black. c) 15.1 ml of 0.01 mol l -1 2 mol l of K2PtCl4 and 0.302 ml -1 Urea (urea:Pt ratio equal to 4:1) was added to 40 mg of carbon black.

[0101] The nominal mass content of Pt in each sample was 45% by dry mass. Each sample was heated under reflux at 100°C for 220 minutes. The resulting suspension was then filtered and thoroughly washed with water. The samples were then placed in a tube furnace and dried at 150°C for 4 hours under an argon atmosphere. They were then reduced in a hydrogen-argon mixture (10% H, 90% Ar) at 150°C for 4 hours and allowed to cool to ambient temperature under an argon atmosphere. The gas flow rate (argon or hydrogen-argon mixture) was 15 sccm. Each of the resulting samples was then divided into two portions, and one portion was heated at 300°C or 350°C for 4 hours under an argon atmosphere.

[0102] Both samples before and after temperature treatment were examined using cyclic voltammetry as described in Example 1. An example of a voltammogram recorded for a sample containing nominal 45% Pt and 55% C (Ketjenblack) obtained by synthesis using a urea:Pt ratio equal to 2.1:1 is shown in Figure 26 (thermal treatment at 350°C). Selected TEM images and corresponding nanoparticle diameter histograms for these samples before and after temperature treatment are shown in Figures 27 and 28, respectively. A voltammogram recorded for a sample containing nominal 45% Pt and 55% C obtained by synthesis using a urea:Pt ratio equal to 2.1:1 or 4:1 is shown in Figure 29 (thermal treatment at 300°C).

[0103] The 45% Pt and 55% C sample obtained in this example showed excellent temperature stability, which was confirmed by no change in activity (indicated by similar values ​​of peak current in the hydrogen adsorption-desorption region of the CV) and no significant change in the size distribution of the nanoparticles, i.e., no significant sintering.

[0104] Example 22. Comparison of adsorption of metal precursors with adsorption of metal-urea complexes 0.01 mol l in 10 ml -1 1 mol l of RhCl3 and 0.25 ml -1Urea (urea:Rh ratio equal to 2.5:1) was mixed and heated in a Petri dish at 90 °C until all liquid evaporated. The resulting precipitate was dissolved in 10 ml of water, and the evaporation-dissolution procedure was repeated twice. Next, 2.33 ml of the resulting rhodium-urea complex solution was added to 50 mg of carbon black to obtain the desired rhodium:carbon mass ratio (5% Rh, 95% C). The resulting suspension was allowed to stand for approximately 5 hours, after which it was filtered and thoroughly washed with water. The sample was then placed in a tube furnace and dried at 150 °C under an argon atmosphere for 4 hours. Subsequently, it was reduced in a hydrogen-argon mixture (10% H, 90% Ar) at 150 °C for 4 hours, allowed to stand, and cooled to ambient temperature under an argon atmosphere. The flow rate of the gas stream (argon or hydrogen-argon mixture) was 15 sccm. The same procedure was applied to a second sample, where, instead of the Rh-urea complex, 0.01 mol l -1 A RhCl3 solution was used.

[0105] The resulting samples were examined using cyclic voltammetry as described in Example 1. For comparison, the cyclic voltammogram of a sample containing 100% carbon was measured. An example of a recorded voltammogram is shown in Figure 30. The sample obtained using the Rh-urea complex exhibits significantly higher activity (higher current in the hydrogen adsorption-desorption region between approximately -0.3 and -0.65 V) than the sample obtained using RhCl3 as the adsorbed compound. The cyclic voltammogram of the sample obtained using RhCl3 as the adsorbed compound is not significantly different from that of the sample containing 100% carbon, likely due to the lower amount of RhCl3 adsorbed onto the carbon during the adsorption process. Based on the comparison of the cyclic voltammograms shown, it can be concluded that the resulting Rh-urea complex adsorbs more readily onto the carbon surface than the precursor salt, RhCl3.

[0106] Example 23. Synthesis of carbon-supported Pt / Ir alloy nanoparticles Two solutions were prepared. a) 10 ml of 0.01 mol l -10.21 ml of 1 mol l IrCl4 -1 added to urea (urea:Ir ratio equal to 2.1:1) b) 10 ml of 0.01 mol l -1 0.21 ml of 1 mol l of K2PtCl4 -1 added to urea (urea:Pt ratio equal to 2.1:1)

[0107] Solution a) was heated at 80°C until all the liquid evaporated. Next, 10 ml of water was added, and the resulting solution was heated under reflux at 80°C for 5 hours. Solution b) was heated at 80°C until all the liquid evaporated. Next, 10 ml of water was added, and the evaporation-dissolution procedure was repeated twice. The resulting solution of Pt-urea and Ir-urea complexes was allowed to stand for approximately 5 hours, after which it was added to a pre-weighed amount of carbon (approximately 25 mg) calculated to achieve the desired platinum:iridium and metal:carbon ratios in the dry mass of the sample. The resulting suspension was allowed to stand for approximately 15 hours, after which it was filtered and thoroughly washed with water. The sample was then placed in a tube furnace and dried at 150°C under an argon atmosphere for 4 hours, subsequently reduced in a hydrogen-argon mixture (10% H, 90% Ar) at 150°C for 4 hours, and then allowed to cool to ambient temperature under an argon atmosphere. The gas flow (argon or hydrogen-argon mixture) was 15 sccm.

[0108] The resulting samples were examined using cyclic voltammetry as described in Example 1. Figure 31 shows the voltammograms recorded for a sample containing nominal 20% metal and 80% C and the following Pt:Ir ratios: 100% Ir; 75% Ir, 25% Pt; 50% Ir, 50% Pt; 25% Ir, 75% Pt; 100% Pt.

[0109] All the obtained samples show good activity in the hydrogen adsorption-desorption process. Based on the charge associated with hydrogen adsorption and desorption (between approximately -0.3 and -0.65 V), it can be concluded that the activity does not vary linearly with the Pt (or Ir) mass content, and therefore an effect of alloying on the catalytic properties is observed.

[0110] Example 24. Comparison of carbon-supported metal nanoparticles of the present invention with commercial catalysts - nanoparticle size and distribution The carbon-supported metal nanoparticles of the present invention were compared with commercially available catalysts. Figure 32 shows TEM micrographs of a 20% Pt 80% C (Vulcan®) catalyst from BASF and a 20% Pt 80% C (Vulcan®) sample obtained using the method of the present invention. Figure 33 shows a histogram of the corresponding Pt nanoparticle diameters for both samples, calculated based on 10 nm-scale TEM images. Figure 34 shows a TEM micrograph of a 40% Pt 60% C (Vulcan®) catalyst from E-TEK and a 40% Pt 60% C (Vulcan®) sample obtained by the method of the present invention. Figure 35 shows a histogram of the corresponding Pt nanoparticle diameters for both samples, calculated based on 10 nm-scale TEM images.

[0111] Comparison of samples with the same Pt:C ratio shows that the samples prepared by the method of the present invention have a more uniform distribution of nanoparticles and a smaller average size of the nanoparticles compared to the commercial catalysts tested, which leads to a higher active surface area.

[0112] Example 25. Comparison of carbon-supported metal nanoparticles of the present invention with commercial catalysts - active surface area The carbon-supported metal nanoparticles of the present invention were compared with a commercial catalyst with the same Pt:C ratio (40% Pt, 60% C) using cyclic voltammetry as described in Example 1. An example of a recorded voltammogram is shown in Figure 36. The charge values ​​listed in the figure are the total charge values ​​corresponding to the process of desorption of a hydrogen monolayer from the platinum catalyst surface. Based on the charge values, the specific active surface area of ​​the catalyst was estimated to be 54 m for the commercial catalyst (E-TEK) and the catalyst obtained by the method of the present invention, respectively. 2 g -1 pt and 86m 2 g -1 ptThe calculation was performed using the method described by Coutanceau et al. (C. Coutanceau, S. Baranton and TW Napporn (2012). Platinum Fuel Cell Nanoparticle Syntheses: Effect on Morphology, Structure and Electrocatalytic Behavior, The Delivery of Nanoparticles, Dr. Abbass A. Hashim (Ed.), ISBN: 978-953-51-0615-9, InTech). That is, the current associated with hydrogen desorption (in the potential region between about 0.05 and 0.4 V vs. RHE in the anodic scan), corrected for the current associated with double layer charging, was integrated and divided by the scan rate used in the experiment according to the following equation:

number

[0113] A comparison of the calculated values ​​shows that the samples prepared by the method of the present invention have a significantly higher specific active surface area than the commercial catalysts tested, which is likely a result of a more uniform distribution of the nanoparticles and a smaller average size of the nanoparticles. The highly developed active surface area of ​​the catalysts obtained by the method of the present invention is a very important feature in terms of the cost of catalyst synthesis due to the fact that a smaller mass of precious metal precursor can be used to obtain a catalyst with the desired activity.

[0114] Example 26. Synthesis of carbon-supported Pt nanoparticles using different reducing agents 0.01 mol l in 10 ml -1 2 mol l of K2PtCl4 and 0.125 ml -1 Urea (urea:Pt ratio equal to 2.5:1) was mixed and heated at 90 °C in a Petri dish until all the liquid had evaporated. The precipitate was left to stand for 5 min under a fume hood without heating and then dissolved in 10 ml of water, thereby obtaining 0.01 mol l of platinum-urea complex. -1 The synthesis was repeated, resulting in a total of 20 ml of 0.01 mol l solution. -1 A solution was obtained. The solution was allowed to stand for 5 hours, and then 6.09 ml of the resulting solution was added to three approximately 50 mg samples of carbon black to achieve the desired metal:carbon ratio (nominal content of 20% Pt) in the dry mass of the sample. The resulting suspension of carbon in platinum-urea complex solution was then heated at 90° C. under reflux for 2 hours. The suspension was then filtered and washed thoroughly with water. The samples were then reduced in one of the following ways: The samples were placed in a tube furnace, dried at 150 °C under argon atmosphere for 4 h, and then reduced in a hydrogen-argon mixture (10% H, 90% Ar) at 150 °C for 4 h, and then allowed to cool to ambient temperature under argon atmosphere. The gas flow (argon or hydrogen-argon mixture) was 15 sccm. - 6.09 ml of 0.1 M citric acid was added and the sample was heated at 60°C under reflux for 1 hour (citric acid:Pt ratio equal to approximately 10:1). - 6.09 ml of 0.1 M L-ascorbic acid was added and the sample was heated at 60°C under reflux for 1 hour (L-ascorbic acid:Pt ratio equal to approximately 10:1).

[0115] The resulting samples were examined using cyclic voltammetry as described in Example 1. An example of a recorded voltammogram is shown in Figure 37. Reduction with H2 is more beneficial than reduction with citric acid or L-ascorbic acid.

[0116] Example 27. Synthesis of carbon-supported Ir nanoparticles using thermal decomposition of adsorbed Ir-urea complexes 0.01 mol l in 10 ml -1 Fresh IrCl4 solution and 0.25 ml of 1 mol l -1 Urea (urea:Ir ratio equal to 2.5:1) was mixed and heated at 90°C in a Petri dish until all liquid evaporated. The resulting precipitate was dissolved in 10 ml of water. 4.28 ml of the resulting iridium-urea complex solution was then added to 32.6 mg of carbon black to obtain the desired iridium:carbon mass ratio (21% Ir, 79% C).

[0117] The resulting suspension was allowed to stand for approximately 15 hours, after which it was filtered and thoroughly washed with deionized water. The sample was then placed in a tube furnace and dried under vacuum at 70 °C for 4 hours, followed by heating under argon at 350 °C for 2 hours at a heating rate of 0.6 °C / min. The argon flow rate was 30 sccm. During the decomposition stage, mass spectrometry was used to monitor the decomposition products. As can be seen in Figure 38, the adsorbed Ir-urea complex decomposes to ammonia and carbon dioxide above 270 °C. The same procedure was used to remove excess urea from the carbon substrate.

[0118] Example 28. Synthesis of carbon-supported Cu / Pd alloy nanoparticles on carbon supports A pre-weighed amount (approximately 100 mg) of carbon black was transferred to a small beaker containing 20 ml of a 2 mM aqueous solution of Cu(NO3)2, allowed to stand for 30 min, then filtered and washed thoroughly with water. -1 1 mol l of K2PdCl4 and 0.25 ml -1The urea solution was mixed and heated in a Petri dish at 80 °C until all liquid evaporated. The precipitate was dissolved in 10 ml of water, and the evaporation / dissolution procedure was repeated twice. To achieve the desired metal:carbon ratio (2.5% Cu, 2.5% Pd, 90% C) in the dry mass of the sample, 2.26 ml of the resulting solution was added to a beaker containing the copper precursor deposited on carbon. The resulting suspension of Cu / carbon in the palladium-urea complex solution was allowed to settle for 30 minutes, after which it was filtered and thoroughly washed with water. The sample was then placed in a tube furnace and dried at 100 °C under argon for 4 hours. It was then reduced in a hydrogen-argon mixture (10% H, 90% Ar) at 50 °C, 100 °C, or 150 °C for 4 hours, and then allowed to settle and cool to ambient temperature under argon. The gas flow rate (argon or hydrogen-argon mixture) was 15 sccm.

Claims

1. 1. A method for the synthesis of carbon-supported platinum group metal or platinum group metal alloy nanoparticles, said method comprising: (b) adsorbing a complex of a platinum group metal and a urea complexing agent onto a carbon support, the complex of a platinum group metal and a urea complexing agent is a coordination complex comprising a platinum group metal as a central ion and at least one molecule of urea or a urea derivative as a ligand; the urea complexing agent is selected from the group comprising urea, a urea derivative, a mixture of urea and at least one urea derivative, and a mixture of at least two urea derivatives; the urea derivative is selected from the group consisting of methylurea, N,N-dimethylurea, N,N'-dimethylurea, ethylurea, trimethylurea, N,N-diethylurea, N,N'-diethylurea, N,N'-bis(hydroxymethyl)urea, bis(hydroxymethyl)urea; (c) reducing the complex of platinum group metal and urea complexing agent adsorbed on the carbon support in step (b) to metal nanoparticles to form carbon-supported metal nanoparticle products; A method comprising:

2. 2. The method of claim 1, wherein the adsorption of the complex of platinum group metal and urea complexing agent onto the carbon support in step (b) is accompanied by the adsorption of another precursor of the platinum group metal.

3. 3. The method of claim 1, wherein the adsorption of the complex of a platinum group metal and a urea complexing agent onto the carbon support in step (b) is accompanied by the adsorption of a precursor of a metal other than the platinum group metal.

4. A method according to any one of claims 1 to 3, wherein the complex of platinum group metal with urea complexing agent comprises a mixed complex in which other ligands are present in addition to urea or a urea derivative.

5. The method according to any one of claims 1 to 4, wherein the adsorption of the metal-urea complexing agent complex onto the carbon support in step (b) is carried out in an aqueous solution.

6. 6. The method of any one of claims 1 to 5, wherein step (a) is carried out simultaneously with or prior to step (b), by reacting in solution the platinum group metal precursor with a urea complexing agent selected from the group comprising urea, a urea derivative, a mixture of urea and at least one urea derivative, and a mixture of at least two urea derivatives to form a complex.

7. 7. The method of claim 6, wherein steps (a) and (b) are carried out simultaneously by mixing a platinum group metal precursor and a urea complexing agent with a carbon support to form a suspension, and subsequently heating said suspension, whereby a complex of platinum group metal and urea complexing agent is formed and adsorbed onto said carbon support, and the adsorbed complex of platinum group metal and urea complexing agent undergoes reduction step (c), resulting in the formation of metal nanoparticles.

8. 7. The method of claim 6, wherein steps (a) and (b) are carried out separately, and the complex of platinum group metal and urea complexing agent obtained in step (a) is subsequently mixed with the carbon support to allow adsorption of the complex of platinum group metal and urea complexing agent on the carbon support, and the complex undergoes reduction in step (c) to result in the formation of metal nanoparticles.

9. 9. The method of any one of claims 6 to 8, wherein step (a) of forming the metal-urea complexing agent complex is carried out in an aqueous solution.

10. The method of any one of claims 1 to 9, wherein the reduction in step (c) is carried out using gaseous hydrogen.

11. 11. The method of claim 10, wherein the reduction in step (c) is carried out at a temperature of 50 to 200°C by placing the carbon support having the adsorbed platinum group metal and urea complexing agent complex in a stream of a gas mixture of hydrogen and an inert gas.

12. The method of claim 11, wherein said gas mixture of hydrogen and an inert gas comprises 1 to 10% hydrogen.

13. 13. The method according to any one of claims 10 to 12, wherein the reduction is carried out in a flow of a gas mixture of hydrogen and an inert gas for 1 to 6 hours.

14. The method according to any one of claims 11 to 13, wherein the inert gas is argon or nitrogen.

15. 10. The method of any one of claims 1 to 9, wherein the reduction in step (c) is carried out by thermal decomposition of the adsorbed metal-urea complexing agent under an inert atmosphere.

16. 16. The method of claim 15, wherein the reduction in step (c) is carried out at a temperature of 190 to 600°C by placing the carbon support having the adsorbed platinum group metal and urea complexing agent complex in a stream of inert gas.

17. 17. The method of claim 16, wherein the inert gas is argon or nitrogen.

18. The method according to any one of claims 1 to 9, wherein the reduction in step (c) is carried out in solution by using a reducing agent.

19. 19. The method of claim 18, wherein the reducing agent is L-ascorbic acid or citric acid.

20. 10. The process of any one of claims 6 to 9, wherein the molar ratio of urea complexing agent to metal used in step (a) is in the range of 1 to 20:

1.

21. 21. The method of any one of claims 1 to 20, wherein the amounts of platinum group metal complex and urea complexing agent and carbon support in step (b) are adjusted to obtain a product containing 0.001 to 60 wt. % metal, calculated on the total weight of the product.

22. 21. The method according to any one of claims 6 to 9 and 20, wherein the concentrations of the metal precursor in the metal precursor solution and the urea complexing agent in the urea complexing agent solution used in step (a) are each in the range of 1 mM to 5 M.

23. 23. The method of any one of claims 6 to 9, 20 and 22, wherein in step (a), the solution containing the platinum group metal precursor and urea or a urea derivative is heated at 40 to 100°C under reflux.

24. 24. The method of claim 23, wherein in step (a), the solution comprising the platinum group metal precursor and the urea complexing agent is heated for 10 minutes to 10 hours.

25. 23. The method of any one of claims 6 to 9, 20 and 22, wherein in step (a), the solution comprising the platinum group metal precursor and the urea complexing agent is heated at 40 to 100°C until all liquid has evaporated.

26. 26. The method of any one of claims 6 to 9, 20 and 22 to 25, wherein in step (a), the organic solvent is added to the solution in a volume ratio of 0.05 to 30:1 calculated based on the volume of water in the solution.

27. The metal precursor is K 2 PtCl 4 , K. 2 PdCl 4 , and IrCl 4 The method of any one of claims 6 to 9, 20 and 22 to 26, selected from the group comprising:

28. 28. The method according to any one of claims 1 to 27, wherein the urea derivative is a compound containing a -HN-CO- or -HN-CO-NH- functional group.

29. 29. Use of carbon-supported platinum group metal or metal alloy nanoparticles obtainable by the process according to claims 1 to 28 as a catalyst.

30. 1. A method for adsorption of a platinum group metal precursor on the surface of a carbon support, wherein the carbon support is immersed in a solution of a complex of a platinum group metal with a urea complexing agent, the urea complexing agent being selected from the group consisting of urea, a urea derivative, a mixture of urea and at least one urea derivative, and a mixture of at least two urea derivatives, the complex of the platinum group metal with the urea complexing agent being a coordination complex comprising a platinum group metal as a central ion and at least one molecule of urea or a urea derivative as a ligand, the urea derivative being selected from the group consisting of methylurea, N,N-dimethylurea, N,N'-dimethylurea, ethylurea, trimethylurea, N,N-diethylurea, N,N'-diethylurea, N,N'-bis(hydroxymethyl)urea, bis(hydroxymethyl)urea.

31. 31. The method of claim 30, wherein the complex of platinum group metal and urea complexing agent is formed by reacting, in solution, a platinum group metal precursor with a urea complexing agent selected from the group consisting of urea, urea derivatives, mixtures of urea and at least one urea derivative, and mixtures of at least two urea derivatives.

32. 32. The method of claim 31 , wherein the formation of the complex of the platinum group metal and the urea complexing agent is carried out in the presence of the carbon support, and the complex of the platinum group metal and the urea complexing agent is adsorbed onto the carbon support after its formation.

33. 1. Use of a complex of a platinum group metal and a urea complexing agent for the adsorption of a platinum group metal precursor on a carbon support, wherein the complex of a platinum group metal and a urea complexing agent is a coordination complex comprising a platinum group metal as a central ion and at least one molecule of urea or a urea derivative as a ligand, and the urea derivative is selected from the group consisting of methylurea, N,N-dimethylurea, N,N'-dimethylurea, ethylurea, trimethylurea, N,N-diethylurea, N,N'-diethylurea, N,N'-bis(hydroxymethyl)urea, and bis(hydroxymethyl)urea.

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