The adsorption of hydrogen on tungsten (112); co-adsorption of hydrogen with oxygen

Abstract

The adsorption of hydrogen and the co-adsorption of hydrogen with oxygen on W (112) have been studied by means of flash desorption mass-spectrometry, work function and LEED measurements. After saturating a clean surface with hydrogen, three desorption peaks can be identified at peak temperatures of approximately 600, 500 and 400°K. The kinetics of hydrogen adsorption are considered through measurements of the amounts of adsorbed hydrogen in each of the three binding states after different exposures. A plot of change in work function versus total coverage has changes in curvature which can be correlated with the occurrences of particular desorption peaks. The maximum work function increase is 0.72 eV. After exposing a clean surface to oxygen, the adsorbed oxygen cannot be reduced by hydrogen at the experimental temperatures and pressures. Adsorption of hydrogen upon a surface containing preadsorbed oxygen raises the work function above the increase produced by oxygen alone. The increase produced by saturation hydrogen exposure falls linearly with oxygen precoverage from the clean surface value of 0.72 eV to zero at θ oxygen = 1. The saturation amount of hydrogen adsorbed also falls linearly with oxygen precoverage to zero at θ oxygen = 1. On a surface containing both oxygen and hydrogen a rearrangement of the adsorbed species is produced by the electron beam at room temperature, or in the absence of the beam by heating to about 400°K. The heterogeneity of single-crystal surfaces is discussed in terms of the formation of ordered adsorbate structures and also of site heterogeneity, with particular reference to the hydrogen/tungsten system and to our present results on tungsten (112).