SURFACE SCIENCE LETTERS INTERACTION OF Hz AND CO WITH CARBIDED AND NITRIDED Ni(lOO)

In our previous paper [I] we reported that the presence of P, S and Cl adatoms on Ni(lO0) surface causes a reduction of CO and Hz adsorption rate and adsorption binding energy. In the case of Cl and S the observed poisoning effect is stronger than the predicted one by a simple adsorption site blocking model and it increases in the sequence P, S, Cl. Since Cl, S and P have similar atomic radii we ascribed the observed difference in their poisoning effect to the increasing electronegativity from P to Cl. It was demonstrated that the observed adsorption-desorption behavior of CO and Hz correlates with the vacations in the kinetics of the methanation reaction on Ni(lOO) in the presence of S and P [2]. This work is a continuation of our systematic study on the effect of foreign atoms on CO and Ha adsorption on Ni(lOO). We have chosen C and N because they have the same electronegativity as S and Cl (2.5 and 3.0 respectively) but smaller atomic radii. Both elements form the same ordered overlayer structure at saturation, so as discussed in ref. [1], we expect the difference in their poison~g behavior to be due mainly to the differences in their electronegativities. All experiments were performed in a stainless steel UHV chamber, equipped with a double pass Auger spectrometer, quadrupole mass spectrometer and LEED optics. The preparation of a clean Ni(lOO) sample and experimental procedures have been described in detail in ref. [3]. The carbon coverage was built up by adsorption of ethylene at 100 K and subsequent heating to 600 K which causes a cracking of the adsorbed ethylene and evolution of hydrogen. This procedure is the same described by Ko and Madix [4] and