Ceria-based mixed-metal oxide structure, including method of making and use

Abstract

A homogeneous ceria-based mixed-metal oxide, useful as a catalyst support, a co-catalyst and / or a getter, is described. The mixed-metal oxide has a relatively large surface area per weight, typically exceeding 150 m<2> / g, a structure of nanocrystallites having diameters of less than 4 nm, and including pores larger than the nanocrystallites and having diameters in the range of 4 to about 9 nm. The ratio of the pore volumes, VP, to skeletal structure volumes, VS, is typically less than about 2.5, and the surface area per unit volume of the oxide material is greater than 320 m<2> / cm<3>, such that the structural morphology supports both a relatively low internal mass transfer resistance and large effective surface area for reaction activity of interest. The mixed metal oxide is made by co-precipitating a dilute metal salt solution containing the respective metals, which may include Zr, Hf, and / or other metal constituents in addition to Ce, replacing water in the co-precipitate with a water-miscible low surface-tension solvent, and relatively quickly drying and calcining the co-precipitate at moderate temperatures. A highly dispersive catalyst metal, such as Pt, may be loaded on the mixed metal oxide support from a catalyst-containing solution following a selected acid surface treatment of the oxide support. The mixed metal oxide, as catalyst support, co-catalyst or getter, is applied in various reactions, and particularly water gas shift and / or preferential oxidation reactions as associated with fuel processing systems, as for fuel cells and the like.

Description

[0001] This invention relates to mixed metal oxides, and more particularly to ceria-based mixed-metal oxide structures, for use as catalyst supports, as co-catalysts, as getters, and the like. The invention relates further to methods of preparing such ceria-based mixed-metal oxide structures, and further still to metal loading of such structures. The invention relates still further to the application of such mixed-metal oxide structures as catalyst supports, co-catalysts, and / or getters in, for instance, fuel processing systems.[0002] Various metal oxides have found use in chemically reactive systems as catalysts, supports for catalysts, gettering agents and the like. As used herein, a gettering agent, or getter, is a substance that sorbs or chemically binds with a deleterious or unwanted impurity, such as sulfur. In those usages, their chemical characteristics and morphologies may be important, as well as their ease and economy of manufacture. One area of usage that is of particula...

AI Technical Summary

Benefits of technology

[0018] The inventive process for making the Ce-based mixed metal oxide material having the constituents, properties and morphology of the invention avoids the need for using surfactants and lengthy aging steps, and includes the steps of 1)dissolving salts of the cerium and at least one other constituent in water to form a dilute metal salt solution; 2) adding urea, either as a solid or aqueous solution; 3) heating the solution of metal salt and urea to near boiling (which may include boiling) to coprecipitate homogeneously a mixed-oxide of the cerium and the one or more other constituent(s) as a gelatinous coprecipitate; 4)optionally maturing the gelatinous coprecipitate in accordance with a thermal schedule; 5) replacing water in the solution with a water miscible, low surface-tension solvent, such as dried 2-propanol; 6) drying the coprecipitate and solvent to remove substantially all of the solvent; and 7) calcining the dried coprecipitate at an effective temperature, typically moderate, for an interval sufficient to remove adsorbed species and strengthen the structure against premature aging. In the dilute metal salt solution, the metal concentration is less than 0.16 mol / L, is preferably less than about 0.02 mol / L, and is most preferably less than about 0.016 mol / L, and the urea concentration is relatively high, being greater than 0.5 mol / L and preferably at least about 2 mol / L. The maturing of the coprecipitate is accomplished in less than 72 hours, and preferably less than about 24 hours, for example in the range of 3 to 8 hours. The calcining of the dried coprecipitate occurs for 1-6 hours, and preferably 2-4 hours, at a heating rate of about 2.degree. C. / min with a final calcining temperature in the range of 250.degree.-600.degree. C., and preferably in the range of 350.degree.-500.degree. C.

[0022] It is desirable to efficiently maximize the effective surface area of a catalyst support, particularly for use in water gas shift (WGS) reactions and / or PROX oxidation reactions to process hydrocarbon feedstocks into hydrogen-rich fuels for fuel cells, in order to make the resulting reaction as efficient as possible. Consequently, the proper combination of relatively high surface area per unit skeletal density coupled with relatively, though not excessively, large pores that minimize internal mass transfer resistance without creating excessive pore volume, results in a highly effective catalyst that increases catalyst efficiency by maximizing the amount of effective surface area within a given reactor volume. By increasing the efficiency of a catalyst in such a reaction, it is possible then to either increase the reaction flow for a given reactor volume or to decrease the reactor volume for a given reaction flow, or a combination of the two. The use of such fuel processing systems in mobile applications places considerable emphasis on reducing size / volume, as will be understood. The improved catalyst support / catalyst / getter of the invention contribute to this objective.

[0026] Referring to the process by which supports having the aforementioned structure and characteristics are formed, a novel method of synthesis by homogeneous coprecipitation is used. While homogeneous coprecipitation methods are known, including the use of urea as in the present invention, the steps and parameters of the process of the invention are specific and unique, and yield the improved ceria-based mixed-metal oxide support in a novel and efficient manner. The synthesis method used in the invention has the advantage of relatively short aging, or maturing, time, the avoidance of expensive reagents like alcoxides, and the avoidance of super-critical solvent removal.

[0030] Following drying, the oxide, or the aforedescribed formed and dried mixed metal oxide may be calcined at 250.degree. C.-600.degree. C., and preferably about 350.degree. C. -500.degree. C., for an interval sufficient to remove adsorbed species and strengthen the structure against premature aging. Lower temperatures typically mean more physisorbed and chemisorbed solvent and / or carbonates, while higher temperatures and longer times mean the reverse. In an exemplary process, the calcining required about 4 hours with a heating rate of about 2.degree. C. / minute. The calcining process typically begins at a temperature of about 70.degree. C., and the calcining temperature selected is a balance of increased surface area at the lower end of the time / temperature range vs. assured removal of contaminants at the upper end.

[0075] It will be noted from Table 3 that the catalyst is relatively more active at the higher temperatures over the practical range of operating temperatures between about 200.degree. C. and 320.degree. C. , as would be anticipated. Moreover, for a feed of 1.5% CO, a relatively greater proportion (%) of the CO is removed at lower temperatures than for the 4.9% feed. Importantly also, a significant degree of catalyst stability is indicated by the fact that its activity at 230.degree. C. at 165 hours is at least 90% of its activity at that temperature at 60 hours.

Problems solved by technology

However, if that value becomes too small because of excessive pore size and / or volume, the effective number of sites per crystallite aggregate necessarily decreases and the amount of effective surface area per unit reactor volume also decreases.