(128b) Metal Promoted Binary Oxides of Ceria and Zirconia for Low Temperature Water-Gas Shift

Mixed oxides (e.g., solid solutions) of ceria and zirconia, promoted with noble metal, like Pt or Au, offer a synergism on the reducibility over metal/ceria or metal/zirconia catalysts alone. For example, by temperature programmed reduction, we observed a shift of more than 50 degrees C to lower temperature in the peak assigned to reduction of the oxide surface shell. This is an important step for activating the Type II bridging OH groups on the surface. A series of mixed oxides of ceria and zirconia were prepared and characterized by a number of techniques, including TPR, XRD, EXAFS, and infrared spectroscopy, and tested for the low temperature water-gas shift reaction, an important reaction for the purification of hydrogen for fuel cell applications. The feed utilized contained CO, H₂O, H₂ and CO₂, in order to simulate the low temperature stage of a fuel processor. Increasing the Zr content, activity was found to pass through a maximum, and then decrease, at a Ce:Zr ratio of 1:1. Results are explained in terms of the number of active sites on the catalyst surface, and the rate of decomposition of surface formate intermediates, the proposed rate limiting step of the mechanism. Inhibition of the WGS rate to carbon dioxide was also examined.