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-programmed desorption (TPD) experiments suggest that hydrogen interacts strongly with the transition-metal component of the aluminides. TPD spectra obtained for the hydrogen product resulting from water decomposition on TiAl, FeAl, and NiAl are similar to the hydrogen TPD spectra obtained from corresponding clean aluminide surfaces that were exposed to molecular hydrogen. Oxygen resulting from water decomposition preferentially resides on the aluminum component as determined by X-ray photoelectron spectroscopy (XPS). Ultraviolet photoelectron spectroscopy (UPS) shows that a near-surface boron concentration of 20 at% decreases the density of states at the Fermi level on FeAl. Boron is found to dramatically reduce the surface reactivity of FeAl toward hydrogen, consistent with the change in electronic structure. A similar decrease in reactivity for molecular hydrogen is found on TiAl when boron is present. The reactivity of NiAl for hydrogen is found to be unaffected when boron is present, under our experimental conditions.