Authors: Schilling, C; Hess, C
ACS Catal.. vol: 9. page: 2155-5435.
Date: FEB. 2019.
Supported-metal (Au, Pt) ceria-based catalysts are considered as promising candidates for the water-gas shift (WGS) reaction at low temperatures. Two main mechanisms have been proposed in the literature, the redox and associative mechanisms. A key step in both mechanisms has been considered to be the cleavage of O-H bonds. In this mechanistic study, the role of surface and bulk oxygen species involved in the WGS reaction over ceria-supported gold catalysts (Au/CeO2) was elucidated directly using operando Raman spectroscopy combined with isotope labeling and supported by DFT+U calculations. Exposure of Au/CeO2 to pure (H2O)-O-18 results in a complete replacement of surface O-16 ions by O-18 ions as rationalized by dissociative adsorption of (H2O)-O-18 in the presence of a surface oxygen vacancy and a subsequent backward reaction restoring lattice oxygen as O-18 and releasing (H2O)-O-16. This reaction pathway is accessible even in the absence of CO. Exposure to reaction conditions leads to (i) a complete disappearance of the Ce-O surface modes due to hydroxyl formation, (ii) a Raman F-2g redshift due to reduction of the ceria subsurface, leading to a change in stoichiometry from CeO1.947-x (in argon) to CeO1.873-x (in CO/(H2O)-O-16), and (iii) large amounts of O-18 in the subsurface of the ceria support due to oxygen transfer from the surface to the ceria subsurface, highlighting the oxygen dynamics of the ceria support. While the results of this study are fully consistent with a redox mechanism involving a reaction pathway for replenishment of surface oxygen ions O2- from terminal hydroxyl groups (O-H) accessible also in the absence of CO in the gas phase, other reaction mechanisms cannot be ruled out..