Surface Oxygen Vacancy and Hydride Species on Ceria Are Detrimental to Acetylene Semihydrogenation Reaction

Reactivity of OH and hydride species in oxide-catalyzed hydrogenation reactions has attracted great interest. Herein, we report a combined in situ spectroscopic characterization and density functional theory (DFT) calculation study of ceria-catalyzed acetylene semihydrogenation reaction. The ceria surface is fully hydroxylated during the adopted reaction condition. C2H2 adsorbs molecularly on the stoichiometric CeO2 surface and hydrogenates with OH groups selectively to produce C2H4. Semihydrogenation of C2H2 to C2H4 with either OH groups or hydride species on ceria surfaces with surface oxygen vacancies proceeds more facilely than on a stoichiometric CeO2 surface, but C2H4 adsorbs more strongly and further hydrogenates to C2H6 more facilely; moreover, dissociative adsorption of C2H2 to C2H species occurs, which facilely hydrogenates with the hydride species eventually to form C2H6 and react with each other to produce oligomers, decreasing the catalytic selectivity and stability, respectively. These results demonstrate that the ceria catalyst with a stoichiometric surface is extremely selective in catalyzing C2H2 semihydrogenation reaction to C2H4, whereas surface oxygen vacancies or hydride species on ceria are harmful to the catalytic performance.