The interaction of hydrogen with Ni-rich (111)Cu-Ni surfaces

Abstract

The hydrogen chemisorption on Ni-rich (111)CuNi alloy surfaces with different surface compositions has been studied by means of thermal desorption spectroscopy (TDS), low energy electron diffraction (LEED), and work function measurements during absorption and desorption. With respect to the interaction with hydrogen these alloy surfaces may, in a first approximation, be described as consisting of active Ni ensembles diluted by inert Cu. Desorption spectra observed after adsorption at 120 and 250 K show nearly the same overall features as those known from pure (111)Ni with two desorption states (β 1, and β 2) and second order desorption kinetics indicating dissociative adsorption. Increasing Cu surface content leads to a rapid decrease of the adsorbed amount of hydrogen, indicating a strong ensemble effect, and to a higher relative intensity of the low temperature desorption maximum (β 1) as compared to the high temperature one (β 2). This result shows that the hydrogen adsorption on the Ni ensembles is influenced by the surrounding Cu atoms. This may be due to the fact that Cu prevents lateral ordering of the adsorbed hydrogen, at least on smaller Ni islands. These findings are in keeping with those of our LEED investigations and work function measurements. As with pure Ni the work function increases during the development of the β 2-state and decreases if β 1 is filled. The initial work function increase rapidly becomes smaller for higher Cu surface content and even vanishes for higher concentrations. The (2 × 2) LEED adsorption structure associated to the β 2-state can only be observed at small Cu content. Studies of the adsorption kinetics indicate that Cu atoms may form precursor sites for the adsorption on Ni sites.