On the mechanism of alkyne hydrogenation catalyzed by Ga-doped ceria

The hydrogenation of acetylene by Ga-doped ceria is investigated using density functional theory (DFT). Different from a previously proposed mechanism that contains an unattainable barrier, our models are based on two Ga-doped ceria surfaces with oxygen vacancies, representing respectively low and high Ga dopant concentrations. The doping of Ga is shown to promote the formation of surface oxygen vacancies, which create a template containing frustrated Lewis pairs to facilitate heterolytic H2 dissociative chemisorption. This mechanism is similar in spirit to our recently proposed mechanism for bare and Ni-doped CeO2 catalysts (J. Am. Chem. Soc., 2018, 140, 12964–12973) and calls for the formation of GaH hydride species along with OH ones on the ceria surface. The former has recently been experimentally observed, thus providing strong support for this mechanism. The rate-limiting step in doped CeO2(1 1 1) with low Ga concentrations has a barrier that is 0.18 eV for the Ga/O FLP and 0.05 eV for the Ce/O FLP higher than that of undoped CeO2(1 1 1), while that for high Ga concentrations is 0.21 eV lower, consistent with the experimental observed concentration dependence of catalytic activity. However, different from the Ni dopant, which was found to be a promoter, the Ga dopant is not only a promoter but also an active participant in the catalysis.