Decomposition and Oxidation of Methanol on Ir(111): A First-Principles Study

The adsorption, decomposition, and oxidation of methanol on Ir(111) were studied based on periodic density functional calculations. Each elementary step in the methanol decomposition reaction on clean Ir(111) via O-H, C-H, and C-O bond scissions was considered. The formation mechanisms of CO, CO2, H2O, and CHx (x = 1-3) were elucidated. The results show that the desorption and decomposition of methanol are competitive on a clean surface, and the presence of 0 or OH has a larger effect on some specific reaction steps. The surface-assisted decomposition of methanol mainly follows two competitive dehydrogenation pathways initialed with O-H and C-H bond scissions, respectively, i.e, CH3OH -> CH3O -> HCHO -> CHO -> CO and CH3OH -> CH2OH -> CHOH -> CHO -> CO. The predosed O enhances the dehydrogenation of CH3OH into CH3O, while the surface is slightly more active toward the C-H bond breaking of CH3O than O and OH. HCHO would like to dehydrogenate into CHO assisted by the surface or OH, followed by OH-assisted dehydrogenation into CO. CO combines with O to yield CO2. However, if the surface 0 coverage is higher, CO2 could be formed via the oxidation pathway of HCHO, i.e., HCHO ->(+O) H2CO2 ->(or+OH) HCO2 -> CO2 The comparison between theoretical results and experimental observation was made.