Methanol decomposition and oxidation on Ir(111)

The adsorption, decompn., and oxidn. of methanol (CH3OH) has been studied on Ir(111) using temp.-programmed desorption and high-energy resoln. fast XPS. Mol. methanol desorption from a methanol-satd. surface at low temp. shows three desorption peaks, around 150 K (alpha ), around 170 K (beta 1), and around 220 K (beta 2), resp. 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 sep. component in the C 1s core level spectra. A part of the initially adsorbed methanol decomps. into COad and Had around (or even below) 175 K. Intermediate CHxO species of CH3OH decompn. were not obsd. The formation of a small amt. of CHxad indicates that (Hx)C-O(H) bond scission occurs as well. Temp.-programmed desorption expts. confirm that CHxad species form, as evidenced by a high-temp. (500 K) H2 formation peak due to decompn. of CHad. The presence of Oad causes a downward shift in the C 1s and O 1s BEs of molecularly adsorbed methanol, but the desorption barrier for mol. methanol desorption is not significantly influenced by the presence of Oad. A stable reaction intermediate, most probably methoxy (CH3Oad), was obsd. in the presence of Oad, between 160 and 220 K. It is an intermediate in the formation of both formate (HCO2ad) and COad, which occurs around 220 K. Formate decomps. around 350 K into CO2 (g) and Had (which reacts with the remaining oxygen to H2O), whereas the COad reacts with Oad around 400 K. [on SciFinder (R)]