Acetylene hydrogenation catalyzed by bare and Ni doped CeO(110): the role of frustrated Lewis pairs.

Ceria (CeO) has recently been found to catalyze the selective hydrogenation of alkynes, which has stimulated much discussion on the catalytic mechanism on various facets of reducible oxides. In this work, H dissociation and acetylene hydrogenation on bare and Ni doped CeO(110) surfaces are investigated using density functional theory (DFT). Similar to that on the CeO(111) surface, our results suggest that catalysis is facilitated by frustrated Lewis pairs (FLPs) formed by oxygen vacancies (Os) on the oxide surfaces. On bare CeO(110) with a single O (CeO(110)-O), two surface Ce cations with one non-adjacent O anion are shown to form (Ce-Ce)/O quasi-FLPs, while for the Ni doped CeO(110) surface with one (Ni-CeO(110)-O) or two (Ni-CeO(110)-2O) Os, one Ce and a non-adjacent O counterions are found to form a mono-Ce/O FLP. DFT calculations indicate that Ce/O FLPs facilitate the H dissociation a heterolytic mechanism, while the resulting surface O-H and Ce-H species catalyze the subsequent acetylene hydrogenation. With CeO(110)-O and Ni-CeO(110)-2O, our DFT calculations suggest that the first hydrogenation step is the rate-determining step with a barrier of 0.43 and 0.40 eV, respectively. For Ni-CeO(110)-O, the reaction is shown to be controlled by the H dissociation with a barrier of 0.41 eV. These barriers are significantly lower than that (about 0.7 eV) on CeO(111), explaining the experimentally observed higher catalytic efficiency of the (110) facet of ceria. The change of the rate-determining step is attributed to the different electronic properties of Ce in the Ce/O FLPs - the Ce f states closer to the Fermi level not only facilitate the heterolytic dissociation of H but also lead to a higher barrier of acetylene hydrogenation.