The adsorption of H 2 and CO on strained Ni overlayers on W(110) and W(100)

Abstract

The structural and chemisorptive properties of thin films (0.01–20 monolayers, ML) of Ni evaporated onto W(110) and W(100) single crystal surfaces have been examined by low energy electron diffraction (LEED), Auger electron spectroscopy (AES), and temperature programmed desorption (TPD). LEED results indicate that at coverages up to 1 ML the Ni films grow pseudomorphically with respect to the W(110) and W(100) substrates. This growth pattern leads to Ni monolayer densities on W(110) and W(100) which are 21% and 38% less than the corresponding monolayer densities for Ni(111) and Ni(100), respectively. AES data indicate layer-by-layer growth of Ni on both surfaces at 100 K. TPD of Ni from W(110) and W(100) yield two discrete features which are related to Ni desorption from the monolayer and multilayer structures. For both surfaces heats of sublimation of an annealed Ni multilayer determined from the TPD data are consistent with three-dimensional island formation upon annealing Ni coverages in excess of 1 ML. TPD results following the chemisorption of CO onto monolayer Ni films on both W(110) and W(100) indicate a significant weakening of the CO binding strength, corresponding to a decrease of approximately 75 K in the CO desorption temperature maxima, compared with CO on bulk Ni. TPD of H 2 from similarly prepared Ni films indicate a weakening of the adsorbed hydrogen, H(ads), binding strength on Ni/W(100), and a stabilization of the H(ads) binding strength on Ni/W(100).