SURFACE-CHEMISTRY OF WATER AND HYDROGEN ON POLYCRYSTALLINE TIAL, FEAL, AND NIAL AND THE EFFECT OF BORON

The chemistry of water and hydrogen on polycrystalline TiAl, FeAl, and NiAl has been investigated with and without boron. Temperature-progra mmed desorption (TPD) experiments suggest that hydrogen interacts stro ngly with the transition-metal component of the aluminides. TPD spectr a obtained for the hydrogen product resulting from water decomposition on TiAl, FeAl, and NiAl are similar to the hydrogen TPD spectra obtai ned from corresponding clean aluminide surfaces that were exposed to m olecular hydrogen. Oxygen resulting from water decomposition preferent ially resides on the aluminum component as determined by X-ray photoel ectron spectroscopy (XPS). Ultraviolet photoelectron spectroscopy (UPS ) shows that a near-surface boron concentration of 20 at% decreases th e density of states at the Fermi level on FeAl. Boron is found to dram atically reduce the surface reactivity of FeAl toward hydrogen, consis tent with the change in electronic structure. A similar decrease in re activity for molecular hydrogen is found on TiAl when boron is present . The reactivity of NiAl for hydrogen is found to be unaffected when b oron is present, under our experimental conditions.