Adsorption energies and prefactor determination for CH3OH adsorption on graphite.

In this paper, we have studied adsorption and thermal desorption of methanol CH₃OH on graphite surface, with the specific aim to derive from experimental data quantitative parameters that govern the desorption, namely, adsorption energy E_ads and prefactor of the Polanyi-Wigner law. In low coverage regime, these two values are interconnected and usually the experiments can be reproduced with any couple (E_ads), which makes intercomparison between studies difficult since the results depend on the extraction method. Here, we use a method for determining independently the average adsorption energy and a prefactor value that works over a large range of incident methanol coverage, from a limited set of desorption curves performed at different heating rates. In the low coverage regime the procedure is based on a first order kinetic law, and considers an adsorption energy distribution which is not expected to vary with the applied heating rate. In the case of CH₃OH multilayers, E_ads is determined as 430 meV with a prefactor of 5 × 10¹⁴ s^-1. For CH₃OH submonolayers on graphite, adsorption energy of 470 ± 30 meV and a prefactor of (8 ± 3) × 10¹⁶ s^-1 have been found. These last values, which do not change between 0.09 ML and 1 ML initial coverage, suggest that the methanol molecules form island-like structure on the graphite even at low coverage. [ABSTRACT FROM AUTHOR]