Effect of varying titania surface coverage on the chemisorptive behavior of nickel

Temperature-programmed desorption (TPD) of CO and H₂ from nickel surfaces containing varying amounts of titania showed that the effects of titania adspecies are predominantly short-ranged. Titania surface species block CO adsorption at strongly bound sites (believed to be sites atop individual Ni atoms) corresponding to heats of adsorption in the range 134-139 kJ · mol^-1; in addition, titania enhances adsorption into more weakly bound sites (believed to be bridging and hollow sites between Ni atoms) with heats of adsorption from 91 to 107 kJ · mol^-1. For hydrogen adsorption, overall adsorption strength was increased with increasing concentration of surface titania. In addition, a new, activated adsorption state was created and is believed to be associated with hydrogen at sites on titania. The variations of the initial sticking coefficients of CO and H₂ as a function of titania precoverages show that at low titania coverage each Tio_x moiety influences approximately 4-10 Ni surface atoms. The long-range electronic influence of titania at a concentration of 0.1 monolayer was estimated to be responsible for only a 6-kJ · mol^-1 decrease in the CO heat of adsorption. Qualitatively, similar results were found for an alumina-containing Ni surface, although alumina appears to be more poorly dispersed than titania on Ni. This suggests that the effects of metal oxide species on metal surfaces may be generalized to include irreducible support materials.