Methanol Decomposition and Oxidation on Ir(111)

Abstract

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 (α), around 170 K (β1), and around 220 K (β2), respectively. The α peak is assigned to methanol adsorbed on top of the first, chemisorbed layer, whereas β1 and β2 are both assigned to methanol directly coordinated to the metal surface atoms (chemisorbed). The CH3OHad responsible for the β2 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 Had around (or even below) 175 K. Intermediate CH x O species of CH3OH decomposition were not observed. The formation of a small amount of CH x ad indicates that (H x )C−O(H) bond scission occurs as well. Temperature-programmed desorption experiments confirm that CH x ad species form, as evidenced by a high-temperature (500 K) H2 formation peak due to decomposition 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 molecular methanol desorption is not significantly influenced by the presence of Oad. A stable reaction intermediate, most probably methoxy (CH3Oad), was observed 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 decomposes 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.

Ethanol Adsorption, Decomposition and Oxidation on Ir(111): A High Resolution XPS Study

“…[14,15] The Ir single crystal was cleaned using Ar + sputtering (3 kV) and annealing cycles A C H T U N G T R E N N U N G (%1400 K) followed by oxygen treatment. The oxygen was removed either by flashing to 1400 K or by heating in the presence of H 2 .…”

“…A similar shift was observed for methanol adsorption on O ad -covered IrA C H T U N G T R E N N U N G (111). [15] The oxygen adatoms on the surface are negatively charged which influences the adsorbed ethanol both in the initial state and the final state of the photoemission process. Both destabilization of the initial state and stabilization of the final state could result in a shift to lower BE values.…”

“…A similar effect is observed during methanol adsorption on an O ad -covered surface. [15] In the latter case desorption of methanol results in the reappearance of the O ad signal.…”

Detection of adsorbate overlayer structural transitions using sum-frequency generation spectroscopy

“…However, TPD is limited to monitoring gas-phase products and does not detect reaction steps involving only adsorbed molecules. In order to access the kinetics of these steps other temperature-programmed techniques, such as X-ray photoelectron (XPS) [2][3][4][5][6][7][8][9] and absorption (XAS) [10] spectroscopy, infrared reflection absorption spectroscopy (IRAS) [11], and vibrational sum-frequency generation (SFG) spectroscopy have been applied [12,13]. The SFG technique provides a high inherent surface sensitivity that can be used to probe adsorbate coverages to below 0.001 ML [14].…”

Oxidation of C213H5OH by NO and O2 on the surface of stepped Pt(332): Relationship to selective catalytic reduction of NO with hydrocarbons

“…7,8 The reduction of NO in the presence of CO, NH 3 , and H 2 on the surfaces of various single crystals has been extensively investigated in this way. [18][19][20][21][22][23] Recently, we have studied the reactions of NO with hydrocarbons including C 2 H 4 and C 6 H 6 on the surface of the Pt͑332͒ under UHV condition, 24,25 elucidating the reaction mechanisms and the dependence of the reaction efficiency on the nature of the hydrocarbon. [18][19][20][21][22][23] Recently, we have studied the reactions of NO with hydrocarbons including C 2 H 4 and C 6 H 6 on the surface of the Pt͑332͒ under UHV condition, 24,25 elucidating the reaction mechanisms and the dependence of the reaction efficiency on the nature of the hydrocarbon.…”