New reduction mechanism of CO dimer by hydrogenation to C2H4 on a Cu(100) surface: theoretical insight into the kinetics of the elementary steps

Abstract

A systematic DFT study that examines the role of the kinetics of the elementary reaction steps during the course of the reduction of a CO dimer, OCCO*, to C₂H₄ on Cu(100) is presented for the first time in the present study, and a new mechanism is introduced. Kinetic analysis of the elementary reaction steps has suggested that the further reduction of CO is the key selectivity-determining step for the formation of C₂H₄ and CH₄ on Cu(100) and Cu(111), respectively. The main reaction pathway on Cu(111) proceeds through the reduction of CO to a CHO* intermediate, which may eventually result in CH_x species by the breaking of a C–O bond and production of CH₄. On Cu(100), OCCO* is first formed by CO dimerization, which is the first step and a more favorable pathway than the further hydrogenation of CO. This explains why only C₂ species and not C₁ species are observed experimentally on Cu(100). For the formation of C₂H₄ on Cu(100), the results suggest that the hydrogenation of OCCO* to the OCCHO* intermediate is the most likely reaction path, followed by the formation of intermediate OHCCHO* through further hydrogenation of the OCCHO* intermediate. The formation of OCCO* may be the rate-determining step in the reduction mechanism of the CO dimer. Kinetic analysis of the elementary steps gives a different mechanistic explanation for the selectivity of C₂H₄ production, which is in contrast to a previous suggested thermodynamic theoretical study on the reduction mechanisms of a CO dimer to C₂H₄. This present reduction pathway is consistent with the latest experimental results and explains the experimental uncertainty regarding the reaction intermediates. At present, it appears that the mechanism proposed in this study is most agreeable with the present experimental results.