The effect of surface chemical functional groups on the adsorption and desorption of a polar molecule, acetone, from a model carbonaceous surface, graphite

The role of surface chemical heterogeneity in adsorption on a model carbonaceous surface (highly oriented pyrolitic graphite, HOPG) was investigated by temperature programmed desorption of acetone, a representative polar volatile organic compound. It was observed that oxygen-containing functional groups exist on air cleaved HOPG. The presence of surface functional groups reduces the binding energy, while increasing the uptake kinetics at low coverage. At high coverage, i.e., beyond a monolayer, the amount of acetone adsorbed is independent of surface heat treatment and depends only on total exposure. Thermal treatment (>1000 K) appears essential for the removal of functional groups from a carbonaceous surface. Removal of these groups allows acetone molecules to adsorb directly on the HOPG surface. The binding energy of the monolayer adsorption was determined by a combination of desorption line shape simulation and variation of desorption heating rates. The monolayer is characterized by a zero coverage binding energy of 55.5±3 kJ/mol and a pre-exponential factor of 1020±1.5 s−1. Coverage dependent, attractive intermolecular interactions at about 5% of the binding energy were determined in the monolayer.