The Chemistry of Acetates on the Rh(111) Surface

Abstract

The formation of acetates by the dissociation of acetic acid on the clean Rh(111) surface and by reactions of acetic acid, acetaldehyde, and ethanol with preadsorbed oxygen atoms was examined using HREELS and TPD. Acetate intermediates were identified by the characteristic vibrations of the OCO moiety symmetrically bonded to the surface via both oxygen atoms. Acetates formed from acetic acid on the clean Rh(111) surface decomposed via two pathways. CO scission by 278 K released adsorbed carbon monoxide and atomic oxygen; decarboxylation at 372 K liberated carbon dioxide and hydrogen. Acetates were formed after the adsorption of both ethanol and acetaldehyde on the Rh(111)-(2 × 2)O surface, as well as after the adsorption of acetic acid. For the alcohol and the aldehyde, the adsorbed intermediate involved in the reaction with atomic oxygen was likely η 2(C,O)-acetaldehyde. For acetic acid on the Rh(111)-(2 × 2)O surface, acetates were formed via direct transfer of the acid hydrogens to the surface oxygen atoms to form surface hydroxyls. When oxygen atoms were present on the surface, the only pathway for acetate decomposition was decarboxylation at 430K. Thus coadsorbed oxygen atoms provide significant stabilization of acetates on Rh(111), as previously observed for formates on the same surface. In the presence of adsorbed oxygen, a portion of the methyl groups of surface acetates was oxidized to CO at temperatures below those expected for reactions of atomic carbon and oxygen species. This observation suggests that oxygen intervenes in the acetate decomposition pathway prior to the complete fragmentation of these intermediates.