The dependence of H2O adsorption and reaction on the structure of the carbon substrate

Abstract

We have examined the interaction of H2O vapor with three structurally different carbon surfaces. The edge and basal surface of crystalline graphite as well as a ‘‘paracrystalline’’ glassy carbon were characterized with LEED, UPS, and AES. Structural characterization by LEED showed the typical ‘‘ring’’ pattern for the basal surface; the edge surface showed evidence of structural order distinct from the basal plane and the glassy carbon showed only diffuse scattering. UPS from the glassy carbon and edge graphite surfaces showed increases in emission within 2 eV of EF relative to the basal plane. This is associated with the presence of less saturated surface valence structure. Exposure to H2O vapor at 30 °C produced saturation oxygen coverage after 10−2 Torr s on the glassy carbon and after 10−1 Torr s on the edge surface. Adsorption on the basal surface saturated at lower coverages and the residual adsorption activity is attributed to surface irregularities. UPS of the glassy carbon and edge graphite surfaces provided evidence for the dissociative adsorption of water. The adsorbed oxygen species produced are strongly held and desorbed as CO between 650–800 °C. Our results show that the edge surface of graphite and less ordered carbon surfaces are active for H2O dissociation.