Reaction-relevant gold structures in the low temperature water-gas shift reaction on Au-CeO₂

Combined in situ X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) studies have been conducted to follow gold structural changes of low-content (< l%Au) gold-ceria catalysts in water-gas shift (WGS) reaction tests at 100 and 200°C; and after heating the used catalysts in oxygen gas at 150°C. Gold in the fresh (400°C-calcined) material was atomically dispersed in cerium oxide. Under WGS reaction conditions, reduction of the oxidized gold species was observed, accompanied by gradual gold aggregation. The Au-Au coordination number is zero for the fresh material, but increases with the reaction temperature, to 6.5 ± .4 (after use at 100°C) and to 8.7 ±1.5 (after 200°C) in a gas mixture containing 5% CO- 3% H₂O in helium. The second important parameter is the reaction gas composition which determines the extent of Au-O reduction. The lower the reduction potential of the reaction gas mixture, the more oxidized the gold is in the used catalyst, and the higher its activity. The maximum activity of Au-CeO₂ was that of the fully dispersed Au-O-Ce fresh material. Loss of surface oxygen took place during reaction, as measured by H₂-TPR of the used samples, and it was commensurate with the activity loss. Attempts to reoxidize and redisperse the gold by heating in oxygen gas at 150°C were not effective. However, we report here that complete recovery of the surface oxygen amount and redispersion of gold in ceria was possible after a 400°C- oxygen treatment of both the 100°C- and 200°C- used catalyst samples, with concomitant recovery of the initial catalyst activity. These tests were conducted by consecutive H ₂-TPR/steady-state catalyst activity measurements in the same microreactor.