Probing the reaction intermediates for the water-gas shift over inverse CeOx/Au(111) catalysts

The water-gas shift (WGS) is an important reaction for the production of molecular H-2 from CO and H2O. An inverse CeOx/Au(1 1 1) catalyst exhibits a very good WGS activity, better than that of copper surfaces or Cu nanoparticles dispersed on a ZnO(0 0 0 (1) over bar) substrate which model current WGS industrial catalysts. In this work we report on intermediates likely to arise during the CO + H2O reaction over CeOx/Au(1 1 1) using soft X-ray photoemission (sXPS) and near-edge X-ray absorption fine structure (NEXAFS). Several potential intermediates including formates (HCOO), carbonates (CO3) and carboxylates (HOCO) are considered. Adsorption of HCOOH and CO2 is used to create both HCOO and CO3 on the CeOx/Au(1 1 1) surface, respectively. HCOO appears to have greater stability with desorption temperatures up to 600 K while CO3 only survives on the surface up to 300 K. On the CeOx/Au(1 1 1) catalysts, the presence of Ce3+ leads to the dissociation of H2O to give OH groups. We demonstrate experimentally that the OH species are stable on the surface up to 600 K and interact with CO to yield weakly bound intermediates. When there is an abundance of Ce4+, the OH concentration is diminished and the likely intermediates are carbonates. As the surface defects are increased and the Ce3+/Ce4+ ratio grows, the OH concentration also grows and both carbonate and formate species are observed on the surface after dosing CO to H2O/CeOx/Au(1 1 1). The addition of ceria nanoparticles to Au(1 1 1) is essential to generate an active WGS catalyst and to increase the production and stability of key reaction intermediates (OH, HCOO and CO3). (C) 2010 Elsevier Inc. All rights reserved.