Classifications

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  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
  • B01J37/02—Impregnation, coating or precipitation
  • B01J37/03—Precipitation; Co-precipitation
• • H—ELECTRICITY
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  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
  • H01M4/90—Selection of catalytic material
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  • B01J23/38—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals
  • B01J23/40—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals of the platinum group metals
  • B01J23/42—Platinum
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  • B01J35/00—Catalysts, in general, characterised by their form or physical properties
  • B01J35/60—Catalysts, in general, characterised by their form or physical properties characterised by their surface properties or porosity
  • B01J35/61—Surface area
  • B01J35/615—100-500 m2/g
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  • B01J37/02—Impregnation, coating or precipitation
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  • B01J37/08—Heat treatment
  • B01J37/082—Decomposition and pyrolysis
  • B01J37/088—Decomposition of a metal salt
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  • B01J37/16—Reducing
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• • H—ELECTRICITY
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  • H01M4/00—Electrodes
  • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
  • H01M4/88—Processes of manufacture
• • H—ELECTRICITY
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  • H01M4/90—Selection of catalytic material
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• • H—ELECTRICITY
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  • H01M4/90—Selection of catalytic material
  • H01M4/92—Metals of platinum group
  • H01M4/925—Metals of platinum group supported on carriers, e.g. powder carriers
  • H01M4/926—Metals of platinum group supported on carriers, e.g. powder carriers on carbon or graphite
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  • B01J21/00—Catalysts comprising the elements, oxides, or hydroxides of magnesium, boron, aluminium, carbon, silicon, titanium, zirconium, or hafnium
  • B01J21/18—Carbon
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  • B01J23/24—Chromium, molybdenum or tungsten
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  • B01J23/24—Chromium, molybdenum or tungsten
  • B01J23/30—Tungsten
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  • B01J23/38—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals
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  • B01J23/38—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals
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  • B01J23/56—Platinum group metals
  • B01J23/64—Platinum group metals with arsenic, antimony, bismuth, vanadium, niobium, tantalum, polonium, chromium, molybdenum, tungsten, manganese, technetium or rhenium
  • B01J23/648—Vanadium, niobium or tantalum or polonium
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  • B01J23/56—Platinum group metals
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  • B01J23/56—Platinum group metals
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• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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  • Y02E60/30—Hydrogen technology
  • Y02E60/50—Fuel cells

Definitions

• the invention relates to a process for producing a catalyst, where the catalyst comprises a catalytically active material and a modified carbon-comprising support.
• the invention further relates to a catalyst comprising a modified carbon-comprising support and a catalytically active material.
• Catalysts comprising a catalytically active material and a carbon-comprising support are used, for example, as heterogeneous catalysts for electrochemical reactions.
• catalytically active material for electrochemical reactions use is usually made of metals of the platinum group or alloys of the metals of the platinum group.
• Alloying components used are generally transition metals, for example nickel, cobalt, vanadium, iron, titanium, copper, ruthenium, palladium, etc., in each case individually or in combination with one or more further metals.
• Such catalysts are used, in particular, in fuel cells.
• the catalysts can be used both on the anode side and on the cathode side. Particularly on the cathode side, it is necessary to use active cathode catalysts which are also corrosion-stable. Alloy catalysts are generally used as active cathode catalysts.
• the catalysts are usually supported.
• the support used has to be electrically conductive.
• Carbon for example in the form of conductive carbon blacks, is generally used as support.
• Carbon supports used usually have a high specific surface area which allows fine dispersion of the particles of the catalytically active material, which are usually present as nanopar- ticles.
• the BET surface area is generally above 100 m 2 /g.
• these carbon supports for example Vulcan XC72 having a BET surface area of about 250 m 2 /g or Ket- jen Black EC-300J having a BET surface area of about 850 m 2 /g, have the disadvantage that they corrode very rapidly.
• the corrosion of carbon-comprising supports can be compared, for example, by subjecting them to potentials above 1 V in the presence of water, for example in a humid stream of nitrogen or in an aqueous electrolyte solution, if appropriate at elevated temperature.
• the carbon is converted into carbon dioxide and the carbon dioxide formed can be measured.
• the higher the temperature and the higher the potential the more rapidly does the carbon-comprising support cor- rode.
• Vulcan XC72 at potentials of 1 .1 V, about 60% of the carbon is corroded away by oxidation to carbon dioxide after 15 hours.
• the carbon is provided with a metal carbide layer.
• Metals used for producing the metal carbide layer are, for example, titanium, tungsten or molybdenum.
• the catalytically active material is subsequently deposited on the metal carbide layer.
• a metal salt solution is firstly applied to the surface of the carbon-comprising support and this solution is then reduced to the metal.
• the support is subsequently heated to convert the metal into metal carbide.
• Heating to form the metal carbide layer is carried out at a temperature in the range from 850 to 1 100°C.
• the carbide layer produced as described in WO-A 2006/002228 is not sufficiently stable to bring about a satisfactory improvement in the corrosion stability.
• the corrosion of the carbon-comprising support leads to detachment of the particles of the catalytically active material and thus to a decrease in performance.
• the catalyst particles can also sinter, which significantly reduces the catalytically active surface area.
• a catalyst whose catalyst particles interact with the surface area in such a way that the particles change only little on the support, i.e. barely sinter and do not become detached from the support, should be provided.
• the object is achieved by a process for producing a catalyst, where the catalyst comprises a catalytically active material and a carbon-comprising support, which comprises the following steps: (a) impregnation of the carbon-comprising support with a metal salt solution, heating of the carbon-comprising support impregnated with the metal salt solution to a temperature of at least 1200°C to form a metal carbide layer, application of the catalytically active material to the carbon-comprising support provided with the metal carbide layer.
• a stable metal carbide layer is formed. Due to the metal carbide layer on the support, the carbon is bound on the surface and no longer undergoes any reaction with the oxygen surrounding the support. The corrosion of the carbon-comprising support can in this way be reduced or even be avoided completely.
• a further advantage is that the catalytically active surface of the catalyst is not changed significantly by formation of the metal carbide layer and a constantly high catalytic activity and long-term stability are thus achieved. In addition, the loss of catalytically active material can be prevented by the metal carbide layer, so that the catalytic activity of the catalyst is not reduced by lost catalytically active material.
• the improved adhesion of the catalytically active material can be examined, for example, by means of transmission electron microscopy.
• Journal of Power Sources, 2008, 185, pages 734-739 it is possible to produce an image of an elec- trocatalyst at the same place before and after electrochemical treatment and observe the changes in the catalyst caused thereby.
• Suitable carbon-comprising supports for the catalyst of the invention are preferably carbon blacks.
• the carbon black can be produced by any process known to those skilled in the art. Carbon blacks which are usually used are, for example, furnace black, flame black, acetylene black or any other carbon black known to those skilled in the art.
• the use of graphitized carbon, in particular carbon having a low surface area, is particularly preferred.
• low surface area means a BET surface area of not more than 250 m 2 /g, more preferably not more than 100 m 2 /g.
• Suitable carbons which can be used as support are, for example, SKW Carbon having a BET surface area of 72 m 2 /g, DenkaBlack having a BET surface area of 53 m 2 /g or XMB206 or AT325 from Evonik Degussa GmbH, having a BET surface area of about 30 m 2 /g.
• a metal carbide layer is applied to the appropriate carbon support.
• the catalytically active material used comprises, for example, a metal of the platinum group, a transition metal, an alloy of these metals or an alloy comprising at least one metal of the platinum group.
• the catalytically active material is preferably selected from among platinum and palladium and alloys of these metals and alloys comprising at least one of these metals.
• the catalytically active material is very particularly preferably platinum or a platinum-comprising alloy. Suitable alloying metals are, for example, nickel, cobalt, iron, vanadium, titanium, ruthenium and copper, in particular nickel and cobalt.
• Suitable alloys comprising at least one metal of the platinum group are, for example, selected from the group consisting of PtNi, PtFe, PtV, PtCr, PtTi, PtCu, PtPd, PtRu, PdNi, PdFe, PdCr, PdTi, PdCu and PdRu. Particular preference is given to a platinum-nickel alloy or a platinum-cobalt alloy.
• the proportion of metal of the platinum group in the alloy is preferably in the range from 25 to 85 atom% and more preferably in the range from 40 to 80 atom%, even more preferably in the range from 50 to 80 atom% and in particular in the range from 60 to 80 atom%.
• alloys comprising more than two different metals, for example ternary alloy systems. It is also possible for further components to be comprised, usually in a proportion of less than 1 % by weight, for example metal oxides.
• the carbon-comprising support is impregnated with a metal salt solution in a first step.
• a metal salt solution it is possible, for example, to disperse the carbon- comprising support in the metal salt solution and subsequently concentrate the dispersion.
• the metal salt solution penetrates into the pores of the carbon-comprising support.
• a metal salt layer is also formed on the outer surface of the carbon-comprising support.
• the surface is preferably converted into a metal carbide.
• the metal salt solution for impregnating the carbon-comprising support is preferably added in a subs- toichiometric amount.
• substoichiometric means that less than 90% by weight of metal based on the sum of metal and carbon is used.
• the proportion of metal is usually from 5 to 75% by weight, preferably from 20 to 50% by weight, in each case based on the sum of metal and carbon.
• the metal of the metal salt solution is tungsten, molybdenum, titanium, vanadium or zirconium, preferably tungsten or molybdenum.
• the metal carbide layer formed on the carbon-comprising support is a tungsten carbide layer or molybdenum carbide layer.
• the layer can also comprise mixed carbides of two or more metals. It is also possible for the metal carbide layer to be doped with a second metal.
• the carbon-comprising support impregnated with the metal salt solution is, in a second step, heated to a temperature of at least 1200°C in an inert atmosphere.
• Inert atmosphere means that the atmosphere does not comprise any materials which can react with the carbon of the support or the metal salt.
• a suitable atmosphere is, for example, a noble gas atmosphere or a nitrogen atmosphere.
• the inert atmosphere is preferably a nitrogen atmosphere.
• the temperature to which the carbon-comprising support impregnated with the metal salt solution is heated is at least 1200°C, preferably at least 1300°C and in particular at least 1500°C.
• the carbon-comprising support impregnated with the metal salt solution is maintained for at least 30 minutes, preferably at least one hour, in particular at least 2 hours, at the temperature to which the carbon-comprising support impregnated with the metal salt solution has been heated.
• the heat treatment being carried out at a temperature of 1500°C for a period of 2 hours.
• the carbon-comprising support provided with the metal carbide layer is cooled and the catalytically active material is applied.
• Application of the catalytically active material can be effected by any method known to those skilled in the art.
• the application of the catalytically active material can, for example, be carried out by deposition in solution.
• the metal can be bound covalently, ionically or by complexation.
• the metal it is also possible for the metal to be deposited reductively, as precursor or by means of alkali to precipitate the corresponding hydroxide.
• the catalytically active material When the catalytically active material is applied by precipitation, it is possible to carry out, for example, a reductive precipitation, for example of platinum from platinum ni- trate, in ethanol or by means of NaBH 4 .
• a reductive precipitation for example of platinum from platinum ni- trate
• decomposition and reduction in an H 2 /N 2 gas mixture for example of platinum acetylacetonate mixed with the carbon-comprising support provided with the metal carbide layer, is also possible.
• Preference is given to carrying out a reductive precipitation by means of ethanol.
• palladium or an alloy comprising a metal of the platinum group is used instead of platinum as catalytically active material, the catalytically active material is applied analogously.
• a catalyst produced by the process of the invention comprises a carbon-comprising support and a catalytically active material, with the carbon-comprising support having a metal carbide layer and the catalytically active material having been applied to the carbon-comprising support provided with the metal carbide layer.
• the corrosion of the carbon support and thus the detachment and loss of catalytically active material can be significantly reduced by the metal carbide layer.
• the specific surface area and thus also the BET surface area of the carbon-comprising support provided with the metal carbide layer is dependent on the carbon-comprising support originally used. Preference is given to the carbon-comprising support having a BET surface area of not more than 250 m 2 /g. Particular preference is given to the carbon-comprising support having a BET surface area of not more than 100 m 2 /g.
• the catalytically active material being a metal of the platinum group or an alloy comprising at least one metal of the platinum group.
• Suitable metals of the platinum group are, in particular, platinum and palladium. It is also possible for platinum and palladium as a mixture to form the catalytically active material.
• the catalytically active material is an alloy comprising the at least one metal of the platinum group
• this alloy is preferably selected from the group consisting of PtNi, PtFe, PtV, PtCr, PtTi, PtCu, PtPd, PtRu, PdNi, PdFe, PdCr, PdTi, PdCu and PdRu.
• the metal of the metal carbide layer of the catalyst is preferably selected from the group consisting of tungsten, titanium, molybdenum, zirconium, niobium, vanadium and mixtures thereof.
• the metal of the metal carbide layer is particularly preferably tungsten.
• the catalyst of the invention is particularly suitable for use as electrocatalyst in a fuel cell.
• the catalyst is particularly suitable as cathode catalyst. Examples
• Carbon corrosion is critical since a large amount of carbon can corrode away even in a short time at potential peaks of up to 1.5 V in operation of a fuel cell.
• the surface-modified carbon support produced in this way will hereinafter be referred to as WC/Denka.
• 7.0 g of the support produced in this way were dispersed in 500 ml of H 2 0 and homogenized by means of an Ultra-Turrax at 8000 rpm for 15 minutes. 5.13 g of platinum nitrate were dissolved in 100 ml of H 2 0 and slowly added to the support dispersion. 200 ml of H 2 0 and 800 ml of ethanol were subsequently added to the mixture and the mixture was refluxed for 6 h. After cooling overnight, the suspension was filtered, the solid was washed free of nitrate with 2 I of hot water and dried under reduced pressure. The platinum loading was 29.8% and the average crystallite size in the XRD was 3.4 nm.
• the preparation was carried out in a manner analogous to the method described in comparative example 1 with the exception of the carbon black support. Carbon black C2 was used instead of carbon black C1 .
• the platinum loading was 27.4% and the average crystallite size in the XRD was 3.1 nm.
• the modification of the surface was carried out in a manner analogous to the method described in example 2, but the carbidization step was carried out at a temperature of 850°C for 6 h (analogous to WO 2006/002228) with an intermediate temperature stage at 400°C for 1 h.
• the tungsten loading was 7%.
• the calculated value was 20%, i.e. the tungsten could not be deposited quantitatively. No tungsten carbide phase was observed in the XRD, only H 2 W0 4 * H 2 0.
• the platinum catalyst produced in this way (analogous to example 2) had a platinum loading of 28.9% and an average crystallite size of 3.4 nm.
• the preparation was carried out in a manner analogous to the method described in WO 2006/002228.
• 8 g of Vulcan XC72 were suspended in 1000 g of H 2 0 and homogenized by means of an Ultra-Turrax at 8000 rpm for 30 minutes.
• 3.2 g of ammonium tungstate were dissolved in 200 ml of H 2 0 and slowly added to the suspen- sion.
• a further 750 ml of H 2 0 were added to the mixture and the mixture was refluxed for 4 h.
• 30.4 g of NaBH 4 were subsequently dissolved in 100 ml of water and added dropwise over a period of one hour with vigorous evolution of gas and the mixture was refluxed for a further 20 minutes.
• the reaction mixture was filtered and the solid was washed with 2 I of H 2 0.
• the still moist filter cake was heated in a tube furnace, firstly at 100°C for 1 h and subsequently at 900°C for 1 h.
• a platinum catalyst was produced on the support produced in this way.
• the platinum loading was 28.2% and the average crystallite size in the XRD was 2.0 nm. Only traces of tungsten could be detected (0.05%).
• Comparative example 4 The preparation was carried out in a manner analogous to the method described in comparative example 1 with the exception of the carbon black support.
• a carbon black XC72 was used instead of the carbon black C1 .
• the platinum loading was 27.7% and the average crystallite size in the XRD was 1.9 nm.
• the preparation was carried out in a manner analogous to the method described in comparative example 1 with the exception of the carbon black support. DenkaBlack carbon black was used instead of the carbon black C1 .
• the platinum loading was 27.7% and the average crystallite size in the XRD was 3.7 nm.
• Carbon black C1 is XMB206 from Evonik Degussa GmbH, carbon black C2 is AT325 from Evonik Degussa GmbH and WC/Denka is a surface-modified carbon support produced as described in example 1 . It can be seen that the corrosion rate of the sample C1 and WC/Denka do not differ significantly. Observed differences between catalysts comprising the respective supports thus arise only from the interaction between catalyst particles and support.
• the decrease in performance of electrocatalysts can also be estimated by means of accelerated aging tests.
• the catalytic activity in respect of the reduction of oxygen (cathode reaction) can be determined before and after potential cycles.
• 150 potential cycles between 0.5 and 1 .3 V were carried out at a rate of 50 mV/s in the oxygen-saturated electrolyte.
• Table 2 WC/Denka is tungsten carbide on DenkaBlack carbon black, WC/C1 is tungsten carbide on carbon black C1 and WC/C2 is tungsten carbide on carbon black C2.
• Figure 2 shows the catalyst of comparative example 1 after exposure to an electrochemical process
• Figure 3 shows a catalyst as per example 1 before exposure to an electrochemical process
• Figure 4 shows a catalyst as per example 1 after exposure to an electrochemical process.
• the uncoated support is denoted by reference numeral 1
• the support coated with carbide is denoted by reference numeral 3
• the platinum particles are denoted by reference numeral 2.
• TEMs Transmission electron micrographs (TEMs) by means of which the same catalyst region was examined before and after exposure to an electrochemical process were taken for the catalysts of example 1 and comparative example 1.

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