Substrate-dependent reactivity of water on metal carbide surfaces

The interaction of water with two transition metal carbides, titanium carbi de (TIC) acid vanadium carbide (VC), has been investigated. The adsorption, reaction, and desorption of water on the (100) face of single-crystal samp les of these materials have been studied as a function of substrate tempera ture over the range 100-600 K. The adsorption state of water on these surfa ces has been probed with high resolution electron energy loss spectroscopy (HREELS). The reactivity of water has been directly measured with HREELS an d X-ray photoelectron spectroscopy (XPS). The desorption of molecular water and the products of surface reactions has been followed with temperature p rogrammed desorption. Collectively, these measurements indicate that water adsorbs both molecularly and dissociatively on TiC and VC however, a greate r degree of reactivity at cryogenic temperatures is observed on TiC. Dissoc iation of water produces surface bound hydrogen and hydroxyl groups on both surfaces and a fully dissociated surface oxide on TIG. Furthermore, a grea ter participation of the surface carbon atoms is observed at the TIC surfac e through the evolution of CO, species at elevated temperatures. The differ ences in surface bonding and desorption profiles are discussed in terms of differences in electronic structure of the two metal carbides. Some possibl e implications of these studies for the use of TiC and VC as tribological m aterials are also discussed.