Previous Patent: Imidazole carbenes
Next Patent: Ceria-based mixed-metal oxide structure, including method of making and use
Next Patent: Ceria-based mixed-metal oxide structure, including method of making and use
[0001] The present development is a high efficiency catalyst for use in the water-gas-shift reaction suitable for production of hydrogen. The catalyst includes a Group VIII or Group IB metal and a transition metal promoter on a ceria-based support. The transition metal promoter is selected from the group consisting of rhenium, niobium, silver, manganese, vanadium, molybdenum, titanium, tungsten and a combination thereof. The support may further include gadolinium, samarium, zirconium, lithium, cesium, lanthanum, praseodymium, manganese, titanium, tungsten or a combination thereof.
[0002] Large volumes of hydrogen gas are needed for a number of important chemical reactions and since the early 1940's the water-gas-shift (WGS) reaction has represented an important step in the industrial production of hydrogen. For example, the industrial scale water-gas-shift reaction is used to increase the production of hydrogen for refinery hydro-processes and for use in the production of bulk chemicals such as ammonia, methanol, and alternative hydrocarbon fuels.
[0003] The hydrogen gas is produced from the reaction of hydrocarbons with water or oxygen and from the reaction of carbon or carbon monoxide with water. The hydrocarbons are typically reacted with water and/or oxygen in the presence of supported nickel catalysts and at high temperatures to produce a combination of carbon oxides and hydrogen gas, commonly referred to as synthesis gas or syngas (see equations 1-3):
[0004] Alternatively, the syngas can be produced through the gasification of coal (equation 4):
[0005] In the subsequent water-gas-shift reaction (equation 5),
[0006] the composition of the so-called water gas can be adjusted to the desired ratio of hydrogen and carbon monoxide. (For a more detailed review of synthesis gas generation and application, see for example E. Supp, Rohstoff Kohle,
[0007] As is known in the art, the water-gas-shift reaction (equation 5) is believed to proceed either through an associative mechanism or through a regenerative mechanism. According to the associative mechanism, the active metal of the catalyst reacts with water causing the water molecule to dissociate on the metal surface into a hydroxyl group and a hydrogen atom. The hydroxyl group can then react with adsorbed carbon monoxide to generate a formate ligand. The formate ligand can decompose to release carbon dioxide leaving a hydrogen atom associated with the metal. The hydrogen from the formate can then combine with the hydrogen from the water to produce hydrogen gas (H
[0008] Typically, the catalysts used in the industrial scale water-gas-shift reaction include either an iron-chromium (Fe—Cr) metal combination or a copper-zinc (Cu—Zn) metal combination. The Fe—Cr oxide catalyst works extremely well in a two stage CO conversion system for ammonia synthesis and in industrial high temperature shift (HTS) converters. In the two stage ammonia synthesis Fe—Cr oxide catalyzed reaction, the catalyst is heated to temperatures ranging from about 320° C. to about 400° C. and the CO level is reduced from about 10% to about 3500±500 ppm. However, in single stage converters the Fe—Cr oxide catalysts are not as effective and the CO level is only reduced to about 1%. The industrial HTS converters—which have reactor inlet temperatures of from about 300° C. to about 380° C.— exclusively use the Fe-based catalysts because of their excellent thermal and physical stability, poison resistance and good selectivity. These attributes are especially beneficial when low steam to CO ratios are used and the formation of hydrocarbons is favored. (See K. Kochloefl, ‘Water Gas Shift and COS Removal’ in “Handbook of Heterogeneous Catalysis”, G. Ertl, H. Knözinger, and J. Weitkamp (Ed.), VCH, Ludwigshafen, 4, Chapter 3.3, pp. 1831-1843 (1997), incorporated herein by reference, for a more extensive discussion of HTS catalysts.) Typically, the commercial catalysts are supplied in the form of pellets containing 8-12% Cr
[0009] The copper-based catalysts function well in systems where the CO
[0010] Although Fe—Cr and Cu—Zn catalysts are efficient when used in a commercial syngas generation facility, they are not readily adaptable for use in stationary fuel cell power units or mobile fuel cells which generate hydrogen from natural gas or liquid fuel. For example, the catalysts used in the fuel cell reformer must have a high level of activity under high space velocity operation conditions because relatively large volumes of hydrocarbons are passed over the catalyst bed in a relatively short period of time. Moreover, the catalyst bed volume must be extremely small as compared to a commercial syngas generation facility. A typical syngas generation facility uses reformer catalyst beds having average volumes ranging from about 2 m
[0011] The present development is a catalyst for use in the water-gas-shift reaction. The catalyst includes a Group VIII or Group IB metal, a transition metal promoter selected from the group consisting of rhenium, niobium, silver, manganese, vanadium, molybdenum, titanium, tungsten and a combination thereof, and a ceria-based support. The support may further include gadolinium, samarium, zirconium, lithium, cesium, lanthanum, praseodymium, manganese, titanium, tungsten or a combination thereof.
[0012] In one embodiment, the catalyst includes platinum metal and a rhenium promoter on a ceria support. More particularly, the catalyst comprises platinum at a concentration of up to about 20 wt %, rhenium at a concentration of up to about 20 wt %, and ceria at a concentration of greater than about 10 wt %. Optionally, the catalyst formulation may further include zirconia in the range of from about 0 wt % to about 90 wt %.
[0013] The present development also includes a process for preparing a platinum and rhenium promoted catalyst having a ceria support for use in the water-gas-shift reaction. In a preferred embodiment, the process involves providing “clean” precursors as starting materials in the catalyst preparation.
[0014]
[0015]
[0016]
[0017]
[0018]
[0019] The catalyst of the present invention is intended for use as a water-gas-shift (WGS) catalyst in a reaction suitable for conversion of hydrogen for chemical processing. The catalyst composition comprises a Group VIII metal or a Group IB metal or a combination thereof, and a transition metal promoter supported on a ceria-based material. The Group VIII metal or Group IB metal or the combination thereof is preferably present at a concentration of up to about 20 wt %. The transition metal promoter is selected from the group consisting of rhenium, niobium, silver, manganese, vanadium, molybdenum, titanium, tungsten and a combination thereof, and is preferably present in the catalyst at a concentration of up to about 20 wt %. The cerium oxide support is present in the catalyst at a concentration of greater than about 10 wt %. Optionally, the support may include an additive, such as gadolinium, samarium, zirconium, lithium, cesium, lanthanum, praseodymium, manganese, titanium, tungsten or a combination thereof, which may be added to the support at a concentration of from about 0 wt % to about 90 wt %.
[0020] As used herein, the terms “Group VIII” and “Group IB” refer to the Periodic Table of the Elements period labelling used by the Chemical Abstract Services. Alternative terminology, known in the art, includes the old IUPAC labels “Group VIIIA” and “Group IB”, respectively, and the new IUPAC format numbers “Groups 8, 9, 10” and Group 11, respectively. Further, throughout the specification a short-hand notation is used when referring to the support. Specifically, the short-hand notation can be generalized as M1
[0021] The term “weight percent (wt %)” as used herein refers to the relative weight each of the above specified components contributes to the combined total weight of those components. As is known in the art, catalysts may be loaded onto a variety of substrates depending on the intended application. The present catalyst may similarly be delivered on a variety of substrates, such as monoliths, foams, spheres, or other forms as are known in the art. When delivered in these forms and for the purposes of illustration herein, unless otherwise noted, any weight added by the substrate is not included in the wt % calculations.
[0022] The present invention can be illustrated and explained through a series of examples presented herein, which are not to be taken as limiting the present invention in any regard. Examples 1 and 2 describe general catalyst preparation procedures for preparing a water-gas-shift catalyst made according to the present invention. For the purpose of the illustration, the catalyst of Example 1 or Example 1A includes 3 wt % platinum on a cerium oxide support, with the platinum precursor being chloroplatinic acid. For the purpose of the illustration, the catalyst of example 2 includes 3 wt % platinum and 1 wt % rhenium on a cerium zirconium oxide support, with the platinum precursor being chloroplatinic acid and the rhenium precursor being ammonium perrhenate. Examples 3-91 follow either the general preparation procedure described in Example 1 or Example 1A or the general preparation procedure described in Example 2, with the particular general procedure and any variations noted for the specific example(s).
[0023] A 100 g sample of a water-gas-shift catalyst having about 3 wt % platinum on a cerium oxide (CeO
[0024] A 100 g sample of a water-gas-shift catalyst having about 3 wt % platinum on a cerium oxide (CeO
[0025] Samples of water-gas-shift catalysts are prepared according to the general procedure of Example 1 or Example 1A except the cerium oxide support (CeO
[0026] The Primary Transition Metal
[0027] Catalysts designed for use in fuel cell reformer beds must have a high level of activity under high space velocity operation conditions because relatively large volumes of hydrocarbons are passed over the catalyst bed in a relatively short period of time. Moreover, the stationary and mobile fuel cell catalyst bed volume is extremely small (generally being from about 0.01 m
[0028] In the present development, platinum functions well as a primary transition metal for the catalyst because of its efficiency in carbon monoxide elimination and in hydrocarbon oxidation. However, other metals or combinations of metals, and particularly the Group VIII and Group IB transition metals, such as iron, cobalt, nickel, copper, ruthenium, rhodium, palladium, silver, osmium, iridium gold, and cadmium and rhenium may be substituted for or may be added to the platinum as appropriate to alter the equilibrium product mix.
[0029] Samples of water-gas-shift catalysts are prepared according to the general procedure of either Example 1 or Example 1A but the chloroplatinic acid is replaced by a series of different metal precursors, as indicated in Table 1, so as to deliver the specified transition metal on the support surface.
TABLE I Transition metal and Transition metal Example concentration precursor 1 platinum 3 wt % H platinum 3 wt % H 3 iron 3 wt % Fe(NO 4 cobalt 5 wt % Co(NO 5 nickel 3 wt % Ni(NO 6 copper 3 wt % Cu(NO 7 ruthenium 3 wt % Ru(NO)(NO 8 rhodium 2 wt % Rh(NO 9 palladium 2 wt % (NH 10 palladium 2 wt % (NH 11 silver 5 wt % AgNO 12 osmium 3 wt % OsO 13 iridium 2 wt % H 14 platinum 1 wt % H 15 platinum 1 wt % (NH 16 platinum 3 wt % (NH 17 gold 5 wt % [H 18 rhenium 4 wt % NH 19 cadmium 2 wt % Cd(NO
[0030] The primary transition metal—as a single metal or as a combination of metals—may be present in the catalyst composition at a concentration of up to about 20 wt %, including the weight of the primary transition metal. The concentration selected is dependent on the anticipated reaction conditions and the desired product mixture, and may be optimized using known experimental procedures, such as performance versus concentration studies, as are known in the art.
[0031] The Transition Metal Promoter
[0032] It is known in the art that promoters may be added to a catalyst formulation to improve selected properties of the catalyst or to modify the catalyst activity and/or selectivity. Because fuel cell reformer beds must have a high level of activity under high space velocity operation, judicial selection of the promoter can produce a highly efficient catalyst at a relatively low cost. In the present invention, the primary transition metal and the transition metal promoters—individually or in combination—may be select as desired and as appropriate to alter the equilibrium product mix. Preferably, the transition metal promoter is selected from the group consisting of lithium, potassium, rubidium, cesium, titanium, vanadium, niobium, molybdenum, tungsten, manganese, rhenium, ruthenium, rhodium, iridium, silver, the Group VIII metals, the Group IB metals and a combination thereof.
[0033] When platinum is selected as the primary transition metal, rhenium is a particularly effective promoter for the conversion of carbon monoxide. However, other transition metal promoters may be substituted for or may be added to the rhenium as warranted by the reaction conditions. Further, when a primary transition metal other than platinum is selected, the optimum promoter may be rhenium, or rhenium used in combination with another transition metal promoter, or one or more of the other transition metal promoters as appropriate for the specific application.
[0034] The transition metal promoter is present in the water-gas-shift catalyst of the present invention at a concentration of up to about 20 wt %, including the weight of the promoter. The concentration used is dependent on the transition metal promoter selected, the primary transition metal used, the concentration of the primary transition metal, and upon the anticipated reaction conditions.
[0035] Platinum impregnated water-gas-shift catalysts are prepared according to the general procedure of Example 1. A promoter is then added to the platinum impregnated catalyst following the procedure generally outlined in Example 2 except that the ammonium perrhenate is replaced by the designated promoter precursor, as indicated in Table II, to deliver the desired promoter to the catalyst surface.
TABLE II Primary transition Example metal, concentration Promoter, concentration Promoter precursor 20 platinum 1 wt % lithium 10 wt % Li 21 platinum 1 wt % potassium 20 wt % K 22 platinum 3 wt % rubidium 2 wt % Ru 23 platinum 5 wt % cesium 1 wt % Cs 24 platinum 3 wt % titanium 2 wt % Ti[OCH(CH 25 platinum 3 wt % vanadium 2 wt % VO(SO 26 platinum 3 wt % niobium 2 wt % NbCl 27 platinum 3 wt % molybdenum 2 wt % MoCl 28 platinum 3 wt % tungsten 2 wt % WCl 29 platinum 3 wt % manganese 2 wt % MnNO 30 platinum 1 wt % rhenium 3 wt % NH 31 platinum 3 wt % rhenium 1 wt % NH 32 platinum 1 wt % ruthenium 0.3 wt % Ru(NO)(NO 33 platinum 1.6 wt % rhodium 0.4 wt % Rh(NO 34 platinum 3 wt % iridium 2 wt % H 35 platinum 3 wt % silver 2 wt % AgNO
[0036] Because the promoter is used in combination with the primary transition metal, the concentration of the promoter may be evaluated in terms of its weight percent contribution to the catalyst or in relative terms as compared to the primary transition metal. For example, for a water-gas-shift catalyst including a primary transition metal of 3 wt % platinum and a promoter of 1 wt % rhenium, the efficiency of the catalyst for carbon monoxide conversion over the temperature range of from about 200° C. to about 400° C. may be affected by the catalyst having a total metal concentration of about 4 wt % and/or by the catalyst including 1 wt % rhenium in the composition and/or by the catalyst having a platinum metal to rhenium metal ratio of about 3:1.
[0037] A series of water-gas-shift catalysts are prepared according to the general procedure of Example 2 except that a zirconium oxide (ZrO
TABLE III Example Pt concentration Re concentration [Pt]/[Re] 36 0 wt % 4 wt % 0 37 1 wt % 3 wt % 0.3 38 2 wt % 2 wt % 1 39 3 wt % 1 wt % 3 40 4 wt % 0 wt % — 41 3 wt % 0 wt % —
[0038] As shown in
[0039] Samples of water-gas-shift catalysts are prepared according to the general procedure of Example 2 except that chloroplatinic acid is replaced by platinum tetra-amine hydroxide and, as shown in Table IV, the amount of platinum tetra-amine hydroxide and the amount of ammonium perrhenate added to the composition are varied while maintaining a platinum to rhenium ratio of about 3:1.
TABLE IV Example Pt concentration Re concentration [Pt]:[Re] 42 0.5 wt % 0.167 wt % 3:1 43 1.0 wt % 0.33 wt % 3:1 44 2.0 wt % 0.67 wt % 3:1 45 3 wt % 1 wt % 3:1 46 6 wt % 2 wt % 3:1 47 9 wt % 3 wt % 3:1 48 12 wt % 4 wt % 3:1 49 15 wt % 5 wt % 3:1 50 21 wt % 7 wt % 3:1
[0040]
[0041] Further as shown in
[0042] A series of water-gas-shift catalysts are prepared according to the general procedure of Example 2 except that the chloroplatinic acid is replaced by platinum tetra-amine hydroxide, and the amount of platinum tetra-amine hydroxide and the ammonium perrhenate are varied as necessary to deliver the platinum metal and rhenium concentrations as shown in Table V. Examples 51 and 52 are prepared according to the general procedure of Example 1 or Example 1A with platinum tetra-amine hydroxide replacing the chloroplatinic acid and the cerium zirconium oxide replacing the CeO
TABLE V Example Pt concentration Re concentration [Pt]/[Re] 51 3 wt % 0 wt % — 52 3.5 wt % 0 wt % — 53 3 wt % 3 wt % 1 54 3 wt % 2 wt % 1.5 55 3 wt % 1.5 wt % 2 56 3 wt % 1 wt % 3 57 3.2 wt % 0.8 wt % 4 58 6 wt % 1 wt % 6 59 3 wt % 0.43 wt % 7 60 3.5 wt % 0.5 wt % 7 61 9 wt % 1 wt % 9
[0043]
[0044] The Support
[0045] The water-gas-shift catalyst support of the present invention comprises a ceria-based material that is present at a concentration of greater than about 10 wt %. Cerium oxide is generally recognized as an efficient support for water-gas-shift catalysts because ceria can essentially function as a promoter. For example, in general, precious metals such as platinum, rhodium and palladium are not good water gas shift catalysts because they are not easily oxidized by water. However, it has been shown that when these metals are ceria supported, they are active shift catalysts. The activity is believed to result from the thermodynamically favorable oxidation of Ce
[0046] The water-gas-shift reaction, and particularly the CO conversion, can also be affected by the inclusion of additives to the cerium oxide. To enhance the CeO
[0047] Samples of water-gas-shift catalysts are made according to the general procedure of Example 2 except that the cerium oxide support is substituted with the support material noted in Table VI for the particular example.
TABLE VI Example Support 62 Ce 63 Ce 64 Ce 65 cerium metal 66 CeO 67 20% ZrO 68 50% ZrO 69 80% ZrO
[0048] Mixed cerium zirconium oxide is a preferred support for the platinum/rhenium containing catalyst. The cerium to zirconium ratio can be varied as necessary to optimize the catalyst performance. In the present development using a platinum primary metal and a rhenium promoter, it has been found that a cerium zirconium oxide support which is rich in cerium, i.e. in which the weight percent added to the support by the cerium is greater than the weight percent added to the support by the zirconium, demonstrates a surprisingly improved level of CO conversion without concommitant significant methane formation. For example, for the catalyst comprising about 3 wt % platinum and about 1 wt % rhenium, a preferred support is Ce
[0049] Samples of water-gas-shift catalysts are prepared according to the general procedure of Example 1 or Example 1A except that the cerium oxide support is substituted with the support material noted in Table VII for the particular example.
[0050] Samples of water-gas-shift catalysts are prepared according to the general procedure of Example 2 except that the cerium zirconium oxide support is substituted with the support material noted in Table VII for the particular example.
TABLE VII Example Pt concentration Re concentration Support 70 3 wt % 0 wt % CeO 71 3 wt % 0 wt % Ce 72 3 wt % 0 wt % Ce 73 3 wt % 0 wt % Ce 74 3 wt % 0 wt % Ce 74 3 wt % 0 wt % ZrO 76 6 wt % 0 wt % Ce 77 3 wt % 1 wt % CeO 78 3 wt % 1 wt % Ce 79 3 wt % 1 wt % Ce 80 3 wt % 1 wt % Ce 81 3 wt % 1 wt % Ce 82 3 wt % 1 wt % Ce 83 3 wt % 1 wt % Ce 84 3 wt % 1 wt % ZrO 85 6 wt % 1 wt % Ce
[0051] Precursor Ligands and Catalyst Preparation
[0052] The preparation method can affect the performance of the water-gas-shift catalyst. For example, as is known in the art, the primary transition metal(s) and the transition metal promoter are generally provided in the form of a metal-based precursor for impregnation on a support material. The metal-based precursor generally includes one or more substituents or ligands which separate from the metal when the metal is impregnated on the support material. Although the ligands of the precursor are not believed to be active materials of the finished catalyst, they may affect how the support receives the transition metal and/or the promoter. Further, as is known in the art, certain ligands or substituents may negatively affect the support surface and may effectively “poison” the catalyst.
[0053] In the present development, the primary transition metal and the promoter are preferably based on clean precursors, wherein the term “clean” refers to a precursor which does not include one or more potentially catalytically poisonous substituents or to a precursor from which the potentially catalytically poisonous substituents can be removed with relative ease during the catalyst preparation process. As is known in the art, a potentially poisonous substituent is any element which can adsorb to the support surface in such a manner so as to prevent one or more sites on the support surface from participating in the desired catalytic reaction. For water-gas-shift catalysts, some commonly recognized poisons are sulfur, chlorine, sodium, bromine, iodine or combinations thereof. Depending on the particular support material selected, other substituents may be included in the list of potential poisons based on their reactivity.
[0054] In the present development, some representative “clean” precursors would include complexes having ligands selected from the group consisting of ammonia, primary amines, secondary amines, tertiary amines, quaternary amines, nitrates, nitrites, hydroxyl groups, carbonyls, carbonates, aqua ions, oxides, oxylates, and combinations thereof. For example, for the platinum containing catalysts, the platinum may be delivered to the support in the form of a platinum tetra-amine hydroxide solution, a platinum tetra-amine nitrate, a platinum di-amine nitrate, platinum oxalate, platinum nitrate or other similar platinum-based complexes. When the platinum is delivered to the support in the form of the platinum tetra-amine hydroxide solution the resultant water-gas-shift catalyst has a slightly greater carbon monoxide conversion profile than when other precursor materials are used. Similarly, the rhenium may be provided as a clean precursor in the form of ammonium perrhenate or as one of the known rhenium oxide complexes, such as ReO
[0055] Alternatively, the primary transition metal precursor and the promoter precursor may include substituents which may potentially be poisonous to the catalyst, but which can be removed with relative ease during the catalyst production process to a sufficient extent so as to make the catalyst “clean.” For example, as indicated in Example 1 or Example 1A and several related examples herein, chloroplatinic acid may be used as a platinum source with the chlorine being removed by air calcination. Depending on the concentration of chlorine present in the catalyst following calcination, the catalyst may be washed by various methods known in the art such as water washing, washing with basic solution, steam calcination, reducing the catalyst with hydrogen and/or other reducing agents followed by washing.
[0056] As is known in the art, catalysts are frequently calcined to drive off volatile matter or to effect changes in the catalyst. The calcination time and temperature can affect the catalyst performance, and it is recommended that the calcination conditions be optimized for the particular desired catalyst composition and intended use. In the present invention, the catalyst is calcined after the primary transition metal is added to the support. When the primary transition metal is platinum which is delivered to the catalyst in the form of chloroplatinic acid, and the support comprises ceria, the catalyst is calcined in a furnace set at from about 440° C. to about 500° C. for from less than about 1 hour to greater than about 16 hours with a heating rate of about 10° C. per minute in air. If a transition metal promoter is added to the primary transition metal catalyst, the catalyst is calcined after the addition of the promoter in a furnace set at from about 440° C. to about 500° C. for from less than about 1 hour to greater than about 3 hours with a heating rate of about 10° C. per minute in air.
[0057] Samples of water-gas-shift catalysts are prepared according to the general procedure of Example 1 or Example 1A and are calcined at about 500° C. for either about 1 hour or for about 15 hours as noted in Table VIII. Examples 88-89 vary from Example 1 or Example 1A by having a zirconium oxide support substituted for the cerium oxide support. Examples 90-91 vary from Example 1 or Example 1A by having a cerium zirconium oxide support substituted for the cerium oxide support.
TABLE VIII Calcination Re Time, Residual Example Pt conc. conc. Support Temp Chlorine 86 3 wt % 0 wt % CeO 500° C., 1 2.0% hour 87 3 wt % 0 wt % CeO 500° C., 15 0.6% hours 88 3 wt % 0 wt % ZrO 500° C., 1 0.8% hour 89 3 wt % 0 wt % ZrO 500° C., 15 0.2% hours 90 3 wt % 0 wt % Ce 500° C., 1 1.6% hour 91 3 wt % 0 wt % Ce 500° C., 15 0.6% hours
[0058] As shown in
[0059] The catalyst may be delivered on substrates other than monoliths, foams, spheres, or similar substrates. For example, the present catalyst may be delivered in the form of extrudates, tabs, pellets, multi-passage substrates or similarly prepared materials. When delivered in these forms, the catalytic activity is dependent on the relative amounts of the active components on the substrate surface because it is essentially only the surface components which can participate in the water-gas-shift reaction. As is known in the art, when delivered in these alternative forms, the concentration of the components is more accurately referred to in terms of the surface concentration or in grams of specific metal per liter of catalyst.
[0060] There are numerous ways in which metals can be combined with supports to produce catalysts. In the examples presented herein, the metals have been combined with the support using known impregnation techniques. However, other methods may be used, such as co-precipitation, sol-gel, vapor deposition, chemical vapor deposition, deposition precipitation, sequential precipitation, mechanical mixing, decomposition and other methods which are known in the art. Any means for combining metals with a support to produce a catalyst which has the composition described herein is believed to fall within the scope of this invention.
[0061] On-Stream Performance
[0062] Like all catalysts, the water-gas-shift catalyst itself is not permanently altered in the water-gas-shift reaction. However, over time the catalyst efficiency can be diminished by contamination of the active sites, for example, by deposition of carbon or other contaminants in the material feed, thus requiring the catalyst bed to be cleaned or regenerated. Because fuel cells, and particularly mobile fuel cells, are being considered for use in consumer vehicles, proper routine maintenance may be difficult to ensure. Thus, a desirable water-gas-shift catalyst should be able to remain on stream for an extended period between catalyst regeneration.
[0063] The primary transition metal, promoter and support affect the on-stream performance, and may be combined to optimize the on-stream performance as desired. In the present development, the platinum on a cerium zirconium oxide support performs adequately for extended periods on-stream and following regeneration. However, the addition of rhenium significantly improves the on-stream performance before, and particularly following, the regeneration cycles.
[0064] It is understood that variations may be made which would fall within the scope of this development. For example, precursor materials other than those expressly listed may be employed to deliver the desired primary transition metal(s) and/or the promoter(s), or the processing conditions may be varied without exceeding the scope of this development. Further, the active catalyst may be delivered in a form that includes essentially inert components. In the latter case, the inert components should be disregarded in any calculations when determining the relative weight percentages of the active components.