Mechanisms of methanol decomposition on Pd {111}

Abstract

The decomposition of methanol (CH3OD) on a Pd{111} single crystal surface has been investigated by combined static secondary ion mass spectrometry (SSIMS), x-ray photoelectron spectroscopy (XPS), and temperature programmed desorption (TPD). The SSIM spectra contain a number of ions, including CH+x(x=0–3), PdCH+3, PdO+ (or PdOD+ ), PdOCH+3 and PdnCO+(n=1–3). These ions indicate the presence of methyl, oxygen (hydroxyl), methoxy, and carbon monoxide in the adsorbed layer. The absolute and relative ion intensities have been found to depend on the methanol gas pressure and surface temperature. The latter has been varied in a linear time program (TPSSIMS) in order to monitor changes in the composition of the adsorbed layer. The reaction route leading to CO and hydrogen involves methoxy formation and decomposition. Thus O–H and C–H bond breaking occur in successive steps. Steady methoxy surface concentrations, maintained by adsorption and thermal desorption (after recombination with hydrogen), build up with a delay time after accumulation of adsorbed CO. The detection of methyl and hydroxyl/oxygen suggests that O–C bond activation takes place as well. Adsorbed methyl is thermally and chemically stable up to temperatures of 500 K. TPD reveals the formation of H2O, HDO, and D2O as a result of the recombination reaction between OHad/Oad and hydrogen/deuterium atoms. The origin of the CH+3 ions detected in (TP)SSIMS is twofold. They are formed in some amounts by fragmentation of a methoxy species; large amounts, however, result from the direct sputter desorption of a methyl adsorbate. At temperatures above 300 K the surface concentration of methoxy is small and, consequently, the contribution from fragmentation can be disregarded. C 1s XP spectra have been taken at temperatures between 90 and 453 K. The data give corroborating evidence for the presence of surface species containing carbon atoms in different chemical environments. A clear distinction between adsorbed CO and CHx has become possible.