Interaction of methanol with lanthanum oxide surfaces and LaOx/Cu(111) interfaces. Part I. Adsorption and thermal decomposition

Abstract

The adsorption of methanol on ordered-epitaxial layers of lanthanum oxide, possessing a La₂O₃(001)-like structure and grown on a Cu(111) substrate, has been studied using X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), molecular beam scattering (MBS), temperature-programmed desorption (TPD) and Fourier-transform reflection/absorption infrared spectroscopy (FT-RAIRS). Methanol adsorbs dissociatively at 300 K, with a high sticking probability, to yield surface methoxy species. The methoxy species on the (001)-type terraces of the oxide films are stable to temperatures in excess of 500 K, but then decompose to yield almost exclusively H₂ and CO, with only trace quantities of methanol and formaldehyde being simultaneously desorbed. IR spectra also indicate the formation of some formate species on well-oxidised surfaces, and at high coverages the adsorbed methoxy is seen to partially convert upon mild heating to a new species which has characteristics of methyl formate. When regions of the Cu(111) surface are exposed, a greater proportion of the methoxy is converted to formaldehyde.