MODIFICATION OF CHEMISORPTION PROPERTIES BY ELECTRONEGATIVE ADATOMS: Hz AND CO ON CHLORIDED, SULFIDED, AND PHOSPHIDED

The effect of preadsorbed electronegative atoms Cl, S and P on the adsorption-desorption behavior of CO and Hz on Ni(lOO) has been studied using thermal desorption, LEED and AES. It is found that the presence of the electronegative atoms causes a reduction of the adsorption rate, the adsorption bond strength and the capacity of the Ni(lOO) surface for CO and Hz adsorption. The poisoning effect becomes stronger with increasing electronegativity of the preadsorbed atoms. In the case of CO adsorption the most tightly bound Pz-state is suppressed most significantly. The variation of the initial sticking coefficient of CO as a function of the adlayer precoverage shows that at low Cl and S coverages the effective number of the influenced Ni surface atoms is more than four. The observed reduction of CO and Hz adsorption and the difference in the poisoning effect of Cl, S and P is generally interpreted in terms of the changes in the surface electron density in the presence of electronegative atoms.