REACTIVITY OF NI ON OXYGEN-COVERED W(110) SURFACES

The adsorption at 90 K of hydrogen, carbon monoxide, and benzene on Ni -n/O-0.5/W(110) (with n = 1-10) has been studied by thermal desorption , x-ray photoemission spectroscopy, ultraviolet photoemission spectros copy, low-energy electron diffraction, and work function measurements. The surfaces are relatively inert to hydrogen adsorption, with estima ted quasisaturation uptake being H/W = 0.18 for n = 1 and increasing t o 0.36 for n = 10. In contrast, CO and C6H6 adsorb readily on Ni-1/O-0 .5/W(110), with saturation coverages of CO/W = 0.85 +/- 0.09 and C6H6/ W = 0.18 +/- 0.03, respectively. These values are comparable to covera ges obtainable on clean W(110), namely, CO/W = 0.70 and C6H6/W = 0.18. Coadsorption of Ni and O on W(110) and subsequent annealing to 500-10 00 K leads to segregation of Ni and 0, with the formation of Ni crysta llites largely [111] oriented along the surface normal. The heights of these Ni ''towers'' can be adjusted by varying the amounts of coadsor bed oxygen and Ni. The Ni crystallites resist sintering, even when ann ealed at 1000 K for 3600s. Adsorption of H-2 and CO on them at 90 K wa s also investigated, and found to be less than expected from the estim ated overall Ni surface area. (C) 1995 American Vaccum Society.