Molecular-Level Understanding of CeO₂ as a Catalyst for Partial Alkyne Hydrogenation

Abstract

The unique catalytic properties of ceria for the partial hydrogenation of alkynes are examined for acetylene hydrogenation. Catalytic tests over polycrystalline CeO₂ at different temperatures and H₂/C₂H₂ ratios reveal ethylene selectivities in the range of 75–85% at high degrees of acetylene conversion and hint at the crucial role of hydrogen dissociation on the overall process. Density-functional theory is applied to CeO₂(111) in order to investigate reaction intermediates and to calculate the enthalpy and energy barrier for each elementary step, taking into account different adsorption geometries and the presence of potential isomers of the intermediates. At a high hydrogen coverage, β-C₂H₂ radicals adsorbed on-top of surface oxygen atoms are the initial reactive species forming C₂H₃ species effectively barrierless. The high alkene selectivity is owed to the lower activation barrier for subsequent hydrogenation leading to gas-phase C₂H₄ compared to that for the formation of β-C₂H₄ radical species. Moreover, hydrogenation of C₂H₅ species, if formed, must overcome significantly large barriers. Oligomers are the most important byproduct of the reaction and they result from the recombination of chemisorbed C₂H_<i>x</i> species. These findings rationalize for the first time the applicability of CeO₂ as a catalyst for olefin production and potentially broaden its use for the hydrogenation of polyunsaturated and polyfunctionalized substrates containing triple bonds