Stability of formate species on beta-Ga2O3

M. CALATAYUD, S.E. COLLINS, M.A. BALTANÁS, A.L. BONIVARDI

PHYSICAL CHEMISTRY CHEMICAL PHYSICS

Royal Society of Chemistry

Año: 2009 vol. 11 p. 1397 - 1405

1463-9076

Gallia (gallium oxide) has been proved to enhance the performance of metal catalysts in a varietyof catalytic reactions involving methanol, CO and H2. The presence of formate species as keyintermediates in some of these reactions has been reported, although their role is still a matter ofdebate. In this work, a combined theoretical and experimental approach has been carried out inorder to characterize the formation of such formate species over the gallium oxide surface.Infrared spectroscopy experiments of CO adsorption over H2 (or D2) pretreated b-Ga2O3 revealedthe formation of several formate species. The b-Ga2O3 (100) surface was modelled by means ofperiodic DFT calculations. The stability of said species and their vibrational mode assignmentsare discussed together with the formate interconversion barriers. A possible mechanism isproposed based on the experimental and theoretical results: first CO inserts into surface(monocoordinate) hydroxyl groups leading to monocoordinate formate; this species might evolveto the thermodynamically most stable dicoordinate formate, or might transfer hydrogen to thesurface oxidizing to CO2 creating an oxygen vacancy and a hydride group. The barrier for thefirst step, CO insertion, is calculated to be significantly higher than that of the monocoordinateformate conversion steps. Monocoordinate formates are thus short-lived intermediates playing akey role in the CO oxidation reaction, while bidentate formates are mainly spectators.