Methanol decomposition on the alfa-Ga2O3 (100) surface: A DFT approach

M. M. BRANDA AND N. J. CASTELLANI

Bariloche

Conferencia; 13th International Conference on Solid Films and Surfaces; 2006

The use of proton exchange membrane fuel cells (PEMFC) in vehicular applications has massive potential since they offer environmental and operational benefits better than those obtained from conventional technologies. Thus, steam reforming of methanol (SRM), i.e.: CH3OH + H2O = 3 H2 + CO2, is a promising process to generate hydrogen. Ga2O3 is a hopeful catalytic material for both the synthesis and reforming of methanol. At the present, the study of surface physical and chemical properties of such technological materials by means of model systems has become an important tool to understand its behavior in the involved reaction mechanisms. In this work, Density Functional Theory (DFT) quantum-mechanical calculations were carried out for methanol adsorbed on Ga2O3 clusters by using a Gaussian basis set and the B3LYP method. The electronic properties of the adsorbed molecules have been monitored by computing adsorption energies, optimized geometry parameters, overlap populations, NBO atomic charges and vibrational frequencies. As the methanol-gallia interaction could show different scenarios according to the local surface chemical composition, adsorption sites consisting in tetrahedral and octahedral gallium ions with a variable number of oxygen vacancies have been considered. The calculations show that methanol can react in three different ways with the gallia surface giving rise to a non dissociative adsorption, a dissociative adsorption and a oxidative decomposition. The adsorption energy (Eads) values obtained when the methanol molecule is non-dissociatively adsorbed goes from ~ 1.0 eV to ~ 1.5 eV (with the BSSE correction) [J. Phys. Chem. B, 110 (2006) 11847]. The surfaces without oxygen vacancies are more reactive than the others and produce the methanol decomposition. The methanol molecule oxidizes on these surfaces, forming H2CO, on tetrahedral sites (T0), and CO2 and H2O, on octahedral sites (O0). In both situations, surface hydroxyls are produced, too. All these molecules remain linked to the surface oxide. The Eads obtained for these sites are large: ~6 eV and ~12 eV, respectively. Hence, the high basicity of these surfaces brings about  the loose of hydrogen atoms by steps; while formate is the final species on T0 site, this molecule undergoes  the dehydrogenation to CO2 on O0 site. These results indicate that the (100) gallia surface without vacancies is very reactive, leading to the total decomposition of the methanol molecule.