Methanol decomposition and oxidation on Ir(111)

The adsorption, decomposition, and oxidation of methanol (CH3OH) has been studied on Ir(111) using temperature-programmed desorption and high-energy resolution fast XPS. Molecular methanol desorption from a methanol-saturated surface at low temperature shows three desorption peaks, around 150 K (alpha), around 170 K (beta(1)), and around 220 K (beta(2)), respectively. The alpha peak is assigned to methanol adsorbed on top of the first, chemisorbed layer, whereas beta(1) and beta(2) are both assigned to methanol directly coordinated to the metal surface atoms (chemisorbed). The CH3OHad responsible for the beta(2) desorption peak appears as a separate component in the C 1s core level spectra. A part of the initially adsorbed methanol decomposes into COad and H-ad around (or even below) 175 K. Intermediate CHxO species of CH3OH decomposition were not observed. The formation of a small amount of CHxad indicates that (H-x)C-O(H) bond scission occurs as well. Temperature-programmed desorption experiments confirm that CHxad species form, as evidenced by a high-temperature (500 K) H-2 formation peak due to decomposition of CHad. The presence of O-ad causes a downward shift in the C 1s and O 1s BEs of molecularly adsorbed methanol, but the desorption barrier for molecular methanol desorption is not significantly influenced by the presence of O-ad. A stable reaction intermediate, most probably methoxy (CH3Oad), was observed in the presence of O-ad, between 160 and 220 K. It is an intermediate in the formation of both formate (HCO2ad) and COad, which occurs around 220 K. Formate decomposes around 350 K into CO2 (g) and H-ad (which reacts with the remaining oxygen to H2O), whereas the COad reacts with O-ad around 400 K.