First-principles analysis of the hydrogenation of carbon monoxide over palladium

Abstract

The reaction paths for the hydrogenation of CO to methanol over Pdx (x = 1–4 and 19) cluster models were examined using first-principle density functional quantum chemical calculations. The predicted adsorption energies for the most favorable binding modes for CO, H2, HCO, H3CO, CH3OH, C, O and H on a Pd19 model Pd(111) clusters were -147, -62, -340, -51, -195, -33, -610, -349 and -251 kJ/mol, respectively. The most favorable modes for CO, CH3O, H, C and O on Pd(111) were all found to be the 3-fold fcc site. The most favorable modes for the formyl and formaldehyde surface intermediates at low coverage were the 3-fold (ζ2µ3), and the di-σ sites, respectively. At higher surface coverages, however, the atop ζ1 (C) and the π modes for the formyl and formaldehyde intermediates were more likely. The computed adsorption energies were subsequently used to compute overall reaction energies for the hydrogenation of CO to methanol. The initial hydrogenation of CO to the ζ1 (C) HCO intermediate was found to be +52 kJ/mol endothermic and has been speculated as a possible rate-limiting step. The remaining surface hydrogenation steps become increasingly more exothermic as more hydrogen was added. The elementary steps of formyl to formaldehyde, formaldehyde to methoxide and methoxide to methanol were computed to be -9, -26 and -33 kJ/mol, respectively. The overall energy for CO dissociation was found to be highly unlikely at +260 kJ/mol and a clear indication that methanation and chain growth chemistry is not very likely over Pd. The most favorable reaction coordinate for the hydrogenation of CO to the ζ1 (C) formyl intermediate was that which proceeds over a single Pd site where there is a migratory insertion of the CO into a Pd–H bond. The barrier for this path was computed to be +78 kJ/mol on the Pd19 cluster. There was a very weak dependence on cluster size. This is a likely indication that this reaction is structure insensitive. A second path which involved the coupling of H and CO over a bridge site was found to be +130 kJ/mol which is less likely, but may also occur under different conditions.