Surface Chemistry of Methanol on Clean and Atomic Oxygen Pre-Covered Au(111)

Abstract

Desorption kinetics of methanol on Au(111) and the oxidation of methanol on atomic oxygen pre-covered Au(111) have been investigated employing temperature programmed desorption (TPD) and molecular beam reactive scattering (MBRS). On clean Au(111), methanol is weakly adsorbed and desorbs molecularly. Three adsorption states, which are assigned to methanol molecules desorbing from the monolayer (β phase), amorphous multilayers (α2 phase), and crystallized multilayers (α1 phase), are observed. For the β peak, detailed numerical analysis, based on mathematical inversion of the Polanyi-Wigner equation, leads to an optimized pre-exponential factor (log ν = 11.3 ± 1.1 s-1) and coverage-dependent desorption energies, which are further used to accurately simulate TPD spectra for submonolayer initial methanol coverages. For methanol coverages of 0.2 and 1.0 ML, desorption energies of 40.6 ± 1.6 kJ/mol and 34.8 ± 1.4 kJ/mol are extracted. In the presence of atomic oxygen, TPD of adsorbed methanol exhibits desorption of only CH3OH, H2O, CO, and CO2. No other partial oxidation products or derivatives such as hydrogen, formaldehyde, formic acid, or methyl formate are detected during TPD or MBRS measurements. On the basis of our experimental results and previous similar studies on other coinage metals, we suggest that abstraction of hydrogen is the initial step in the surface decomposition of methanol and that the adsorbed/surface bound methoxy group [CH3O(a)] is the primary intermediate in the oxidation of methanol.