Mechanism of the hydrogenation of CO2 to methanol on a Cu(100) surface: dipped adcluster model study

The mechanism of the hydrogenation of CO2 to methanal on a Cu(100) surface was studied using the dipped adcluster model (DAM) combined with ab initio Hartree-Fock (HF) and second-order Moller-Plesset (MP2) calculations. The L angmuir-Hinshelwood (LH) mode, which corresponds to the reaction between co adsorbed species on the surface, was adopted. Our calculations show that hy drogen and formate are adsorbed at short-bridge sites. The coadsorption of hydrogen and CO2, in which CO2 is chemisorbed in the bent anionic state, is described well by the DAM. Five successive hydrogenations are involved in the hydrogenation of adsorbed CO2 to methanol. the intermediates are format e, dioxomethylene, formaldehyde and methoxy. The geometries of these interm ediates and the transition states, as well as the energy diagrams in the re action process, are presented. The rate-limiting step is the hydrogenation of adsorbed formate. Subsequent steps occur relatively readily, and lead to the formation of methanal. Clearly, any factor that could enhance the hydr ogenation of formate on copper should lead to enhanced activity in methanal synthesis. (C) 1999 Elsevier Science B.V. All rights reserved.