Static SIMS/TPD (Secondary Ion Mass Spectrometry/Temperature Programed Desorption) Study of the Kinetics of Methoxy Formation and Decomposition on o/Pt(111)

摘要

The kinetics and mechanism of the decomposition of methanol (CH30D) 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 CH30d are unity and 0.36 ML, respectively. The maximum amounts decomposed in TPD on 0/Pt(111) and clean Pt(111) are 0.19 and 0.047 ML, respectively. At low methanol coverages (<0.05 ML) on 0/Ptu(111) the only reaction products were C02, H20 and D20, whereas at saturation Co, H20, D20 and H2 were observed. The decomposed amount did not saturate at or before the onset of molecular methanol desorption, but increased monotonically until saturation of the first layer. No oxygen exchange was observed between CH30D and preadsorbed oxygen 18 0. A decomposition mechanism involving methoxy and a hydroxyl type species is proposed. Methanol coverages as low as 0.002 ML could be detected with SIMS. The Ch3+ ion was the most intense ion in the positive SIMS spectrum of both methanol and methoxy. Larger ion clusters such as (CH30d)+n (n=2,3) developed successively at specific multilayer coverages. At low coverages on 0/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 CH3+ ion was used to follow the kinetics. Advantages and limitations of using SIMS as a tool for kinetic studies are discussed.