Substrate-dependent reactivity of water on metal carbide surfaces

The interaction of water with two transition metal carbides, titanium carbide (TIC) acid vanadium carbide (VC), has been investigated. The adsorption, reaction, and desorption of water on the (100) face of single-crystal samples of these materials have been studied as a function of substrate temperature over the range 100-600 K. The adsorption state of water on these surfaces has been probed with high resolution electron energy loss spectroscopy (HREELS). The reactivity of water has been directly measured with HREELS and X-ray photoelectron spectroscopy (XPS). The desorption of molecular water and the products of surface reactions has been followed with temperature programmed desorption. Collectively, these measurements indicate that water adsorbs both molecularly and dissociatively on TiC and VC however, a greater degree of reactivity at cryogenic temperatures is observed on TiC. Dissociation of water produces surface bound hydrogen and hydroxyl groups on both surfaces and a fully dissociated surface oxide on TIG. Furthermore, a greater participation of the surface carbon atoms is observed at the TIC surface through the evolution of CO, species at elevated temperatures. The differences in surface bonding and desorption profiles are discussed in terms of differences in electronic structure of the two metal carbides. Some possible implications of these studies for the use of TiC and VC as tribological materials are also discussed.