Enhancement of adsorption on graphite (HOPG) by modification of surface chemical functionality and morphology

The effects of chemical functional groups and surface morphology on the adsorption/desorption behavior of a model non-polar organic adsorbate (propane) on model carbonaceous surfaces [air-cleaved highly oriented pyrolytic graphite (HOPG) and plasma-oxidized HOPG], were investigated using temperature-programmed desorption (TPD). Oxygen- and hydrogen-containing functional group,; exist on both air-cleaved HOPG and plastma-oxidized HOPG. The presence of these groups almost completely suppresses propane adsorption at 90 K. However, these group, can be removed from both air-cleaved and plasma-oxidized HOPG by thermal treatment (>500 K), leading to more than an order of magnitude increase in adsorption capacity. It is essential for both air-cleaved HOPG and plasma-oxidized HOPG to be outgassed at over 1273 K for all the adsorption sites to be chemically accessible for propane molecules. The effect of morphological heterogeneity is evident for plasma-oxidized HOPG as this substrate provides greater surface area available for adsorption, as V ell as higher energy binding sites. (C) 2002 Elsevier Science Ltd. All rights reserved.