Stability of formate species on b-Ga2O3: a combined DFT and IR

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

Lyon, Francia

Conferencia; 13th International Conference on the Applications of Density Functional Theory in Chemistry and Physics; 2009

Gallia (gallium oxide) has 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-Ga2O3revealed the formation of several formate species. The b -Ga2O3 (1 0 0) surface was modelled bymeans of periodic DFT calculations with the VASP program (PBE functional, 400 eV cutoff).The stability of said species and their vibrational mode assignments are discussed together withthe formate interconversion barriers. A possible mechanism is proposed based on theexperimental and theoretical results: first CO inserts into surface (monocoordinate) hydroxylgroups leading to monocoordinate formate; this species might evolve to the thermodynamicallymost stable dicoordinate formate, or might transfer hydrogen to the surface oxidizing to CO2creating an oxygen vacancy and a hydride group. The barrier for the first step, CO insertion, iscalculated to be significantly higher than that of the monocoordinate formate conversion steps.Monocoordinate formates are thus short-lived intermediates playing a key role in the COoxidation reaction, while bidentate formates are mainly spectators