A static SIMS/TPD study of the kinetics of methoxy formation and decomposition on O/Pt(111)

Abstract

The kinetics and mechanism of the decomposition of methanol (CH 3OD) on oxygen-covered Pt(111) were studied using static secondary ion mass spectrometry (SIMS) and temperature programmed desorption (TPD). The initial sticking coefficient and the saturation first layer coverage of CH 3OD are unity and 0.36 ML, respectively. The maximum amounts decomposed in TPD on O/Pt(111) and clean Pt(111) are 0.19 and 0.047 ML, respectively. At low methanol coverages (< 0.05 ML) on O/Pt(111) the only reaction products were CO 2, H 2O and D 2O, whereas at saturation CO, H 2O, D 2O and H 2 were observed. The decomposed amount did not saturate at or before the onset of molecular methanol desorption, but increeased monotonically until saturation of the first layer. No oxygen exchange was observed between CH 3OD and preadsorbed 18O. A decomposition mechanism involving methoxy and hydroxyl type species is proposed. Methanol coverages as low as 0.002 ML could be detected with SIMS. The CH 3 + ion was the most intense ion in the positive SIMS spectrum of both methanol and methoxy. Larger ion clusters such as (CH 3OD) n + ( n = 2, 3) developed successively at specific multilayer coverages. At low coverages on O/Pt(111), methoxy formation occurs above 125 K and its decomposition becomes detectable above 150 K during temperature programming. In the isothermal mode, the SIMS CH 3 + ion was used to follow the kinetics. Over the temperature range 120–145 K, the second order Arrhenius rate parameters for methoxy formation are E = 5.5±0.7 kcal/mol and A = 1.5×10 −7±0.6 cm 2/s·molecule for an initial methanol coverage of 0.05 ML. Methoxy decomposition was studied in the temperature range 150–165 K and at an initial coverage of 0.015 ML. The first order kinetic parameters, E = 11.4±0.5 kcal/mol and A = 5.3×10 13±1 s −1 were derived. Advantages and limitations of using SIMS as a tool for kinetic studies are discussed.