Selective Hydrogenation of Propyne to Propene Promoted by Synergistic Effect of Surface Oxygen Vacancies and Hydride Species on Ceria

The high activity and selectivity of ceria in selective hydrogenation of alkynes have attracted much attention. However, the high operating temperature and the high H2/alkyne ratios required hamper the practical application of ceria catalysts, and the complex H2-ceria interaction as well as the ambiguous role of oxygen vacancies (Ov) prevent the further reactivity optimization of ceria-based catalysts. To elucidate the role of Ov sites and hydride (Ce–H) species that can easily generate on ceria in the selective hydrogenation of propyne reaction, we constructed two model surfaces: CeO2(111) and CeO2–x(111)–H with H– ions preoccupied in almost all of the Ov sites. From the catalytic performance measurements, both surfaces exhibit high selectivity for propene, while the CeO2–x(111)–H surface shows a propene production three times higher than CeO2(111). Using in situ ambient pressure X-ray photoelectron spectroscopy, we studied the formation and evolution of Ov, Ce–H, and carbon-containing species on the two surfaces during the hydrogenation reaction and correlated with their catalytic performance. Assisted by density functional theory calculations, we found that surface-exposed Ov sites are required for the formation of Ce–H species and the dissociative adsorption of propyne. Meanwhile, Ce–H species possess high hydrogenation activity and can help weaken the adsorption of CH3CCH2* to form gas-phase propene. The propene production and selectivity are optimal only in the coexistence of Ov sites and Ce–H species with sufficient concentration. Our study has thus demonstrated the crucial synergetic roles of Ov sites and hydride species on ceria for the selective hydrogenation reaction.