A theoretical study of dioxymethylene, proposed as intermediate in the oxidation of formaldehyde to formate over copper

The density functional theory-based hybrid method B3LYP was used to study t he interaction of the dioxymethylene species with the copper(lll) surface. This species has been proposed experimentally as one possible intermediate in the oxidation of methanol catalysed by metal surfaces. The H2CO2 species is very unstable, and this makes the experimental study too difficult. As far as we know, there are no direct theoretical or experimental studies of H2CO2 adsorption on metal surfaces in the literature. The experimental know ledge is limited to the IR frequencies obtained for adsorption on metal oxi des. In this study, two different clusters and two different adsorption sites ha ve been studied. A two-layer Cu-7 cluster was used to model the H2CO2 and [ H2CO2](2-) adsorption on a small copper island, and a large three-layer Cu- 30 cluster was used to model the H2CO2 adsorption on a copper (111) surface . These clusters were used to extract information concerning the energetics , geometry and IR frequencies for the dioxymethylene adsorption. When compa red with a similar species, formate, dioxymethylene is stabilized more effi ciently on the cross-bridge site than on the aligned-bridge site, which is the preferred orientation for formate. However, the oxygen-to-surface dista nces are similar, and the same is observed for the bonding type, which is m ainly ionic. A bridge-bonded conformation is predicted for adsorption on th e two sites considered. The comparison of the adsorption energy of the diox ymethylene species and the adsorption energy of atomic oxygen and free form aldehyde yields an interesting result: H2CO2(ads) is energetically more sta ble than adsorbed (C)((ads)) and H2CO(g). The IR frequencies are in good ag reement with experimental data obtained for dioxymethylene adsorption on se veral oxide surfaces. (C) 2000 Published by Elsevier Science B.V. All right s reserved.