Low temperature water-gas shift: Examining the efficiency of Au as a promoter for ceria-based catalysts prepared by CVD of a Au precursor

Increasing the Au loading had a significant positive impact on the catalytic activity. The partial reduction of ceria is necessary for generating bridging OH groups on the surface of ceria, which serve as the active sites. The surface shell reduction process in H-2 was monitored by TPR, XANES, and in situ DRIFTS spectroscopy. Either the oxygen deficiencies are first formed, followed by dissociative adsorption of H2O to generate the Type II bridging OH groups or, they may be formed directly by spillover of dissociated H-2 from the metal to the ceria surface. For each pair of bridging OH groups formed, two cerium atoms in the surface shell change from the Ce4+ to Ce3+ oxidation state. Addition of Au facilitates the surface reduction process and thereby decreases the reduction temperature from 450 degrees C for the unpromoted catalyst to 100 degrees C for the 5% Au/ceria catalyst. A systematic decrease in the required temperature for ceria surface shell reduction was observed by increasing the Au promoter loading as follows: 0.1, 0.25, 0.5, 1, 2.5, 5%. Gold and platinum promoted catalysts were compared in a suitable reaction temperature range after first ensuring that metal-oxide interactions were overcome (ca. 5% metal loading). Approximately 20 times the amount of 5% Au/ceria catalyst was required to achieve a lightoff curve similar to that of a 5% Pt/ceria in the temperature range 200-300 degrees C, and 5% Pt/ceria also exhibited higher steady-state activity (about double that of An) at 175 degrees C. This result suggests that, in addition to the role the metal plays in facilitating the formation of the active site bridging OH groups, it also influences the intrinsic rate of the WGS reaction. Transient formate decomposition experiments carried out at 140 degrees C indicated that the rate of formate decomposition was approximately 20 times higher for 2.5% Pt/ceria than that of 2.5% Au/ceria, suggesting that the metal (in addition to the promoting effect of H2O previously reported) plays a role in facilitating the decomposition of surface formate intermediates, the proposed rate limiting step of the reaction mechanism. (c) 2005 Published by Elsevier B.V.