Tailored Mesostructured Copper/Ceria Catalysts with Enhanced Performance for Preferential Oxidation of CO at Low Temperature

Abstract

Hydrogen as the most efficient and cleanest energy source for fuel cell power is produced mainly by reformation of hydrocarbons, followed by the water gas shift reaction. The CO (0.5–2%) present in the hydrogen stream must be selectively removed because CO is highly poisonous to the electrocatalyst in proton-exchange membrane fuel cells (PEMFCs). Preferential oxidation (PROX) of CO in excess H2 is therefore a key reaction for the practical use of H2 in PEMFCs. [1,2] Among the catalysts reported to be active for PROX, copper/ceria-based catalysts have been considered as promising candidates because of their low cost and high selectivity compared to catalysts based on gold or platinum. However, they usually only show noticeable activities above 100 8C, while the operating temperature of PEMFCs is around 80 8C. Furthermore, the catalytic properties depend strongly on the preparation method and the CuO/CeO2 interfacial area. Despite numerous studies about PROX catalysts, little is known concerning the influence of pore size and pore structure on the catalytic performance. Transition-metal oxides exhibiting mesoporous structures, for example, Co3O4 and CuO/Fe2O3, are active for CO oxidation at low temperature and show higher activity than the corresponding bulk materials. The high activity of mesoporous metal oxides was correlated to their ordered mesostructure and high surface area. Hard templating is a method known to enable the synthesis of materials that possess a highly defined pore architecture and a very high surface area, thus leading to unique physicochemical properties. However, studies of surface redox reactivity and the confinement of reactions near to the surface owing to the dimension of the pores have been limited to a few compositions of catalysts for CO oxidation. Herein, we report the catalytic performance in CO-PROX of Cu/CeO2 and CuM/CeO2 catalysts prepared by the nanocasting method. In this study, mesoporous catalysts with various compositions were synthesized using an improved hard templating method that we have recently developed. The pore size, specific surface area, and pore structure were tailored by changing the type of mesoporous silica used as solid template (e.g., KIT-6 aged at different temperatures, SBA-15, and MCM-48 nanospheres). The resulting metal oxide materials possess a high surface area (up to 200 mg ) and a pore size ranging from 3 nm to 12 nm. The catalytic performance of these materials is among the best reported thus far for copper/ceria-based catalysts with respect to the CO conversion and CO2 selectivity at low temperature. The effect of their mesostructure and composition on the reducibility and catalytic properties are also substantiated. Mesoporous silica templates with different pore structures (KIT-6, SBA-15, andMCM-48) were synthesized according to the literature. The pore size of the KIT-6 was varied by changing the aging temperature (40, 100, and 130 8C). The nanocast catalysts were prepared by one-step-impregnation hard templating (see Experimental Section). The samples prepared from KIT-6 were labeled as Cu(x)CeM(y)-K-T, with T representing the aging temperature of KIT-6, x (x= 10–30) and y (y= 20) are nominal molar percentages of Cu or M to Ce (M=Co or Fe), respectively. The samples using SBA-15 and MCM-48 hard templates were denoted as Cu(x)Ce-SBA and Cu(x)Ce-MCM, respectively. Representative TEM images of the nanocast materials replicated from KIT-6 and SBA-15 templates confirm the long-range periodic order of the mesopores (Figure 1A and B, and Figure S1 in the Supporting Information). The TEM image of Cu/CeO2 replicated from MCM-48 spheres clearly show the mesoporous spherical particle morphology. Mesoporosity was further confirmed by N2 adsorption–desorption measurements (Figure S2). All of the samples casted from SBA-15, as well as from KIT-6 aged at 100 and 130 8C, showed type IV isotherms with a capillary condensation step above p/po= 0.4, which are rather typical for mesoporous metal oxide nanocasts. 4d] Narrow pore size distributions were observed for all the samples except for Cu/CeO2 produced from KIT-6-40. Poresize analysis, obtained from the adsorption branch by NLDFT methods (see characterization section in the Supporting Information), indicated mesopores of approximately 5 nm [*] H. Yen, Prof. F. Kleitz Department of Chemistry and Centre de Recherche sur les Mat riaux Avanc s (CERMA), Universit Laval Quebec, G1V 0A6 (Canada) E-mail: freddy.kleitz@chm.ulaval.ca