Toward an Understanding of Selective Alkyne Hydrogenation on Ceria: On the Impact of O Vacancies on H₂ Interaction with CeO₂(111)

Abstract

Ceria (CeO₂) has recently been found to be a promising catalyst in the selective hydrogenation of alkynes to alkenes. This reaction occurs primarily on highly dispersed metal catalysts, but rarely on oxide surfaces. The origin of the outstanding activity and selectivity observed on CeO₂ remains unclear. In this work, we show that one key aspect of the hydrogenation reaction-the interaction of hydrogen with the oxide-depends strongly on the presence of O vacancies within CeO₂. Through infrared reflection absorption spectroscopy on well-ordered CeO₂(111) thin films and density functional theory (DFT) calculations, we show that the preferred heterolytic dissociation of molecular hydrogen on CeO₂(111) requires H₂ pressures in the mbar regime. Hydrogen depth profiling with nuclear reaction analysis indicates that H species stay on the surface of stoichiometric CeO₂(111) films, whereas H incorporates as a volatile species into the volume of partially reduced CeO_2-x(111) thin films (x ∼ 1.8-1.9). Complementary DFT calculations demonstrate that oxygen vacancies facilitate H incorporation below the surface and that they are the key to the stabilization of hydridic H species in the volume of reduced ceria.