PROBING THE MODIFIER PRECURSOR STATE - ADSORPTION OF CO ON SN PT(111)SURFACE ALLOYS/

The presence of a second, inert metal component in an alloy surface ca n have a dramatic effect on the adsorption kinetics compared to that p redicted from simple site-blocking. This is due to the important role that is often played by the modifier precursor state [Xu and Koel, J. Chem. Phys., in press]. In order to more clearly reveal details concer ning this state, the adsorption kinetics and thermodynamics of CO on P t(111) and two ordered Sn/Pt(111) surface alloys have been studied. Th e thermodynamics of CO adsorption on Pt(111) is only slightly influenc ed by the presence of Sn. In contrast to the adsorption thermodynamics , the nature of the adsorption kinetics depends strongly on the presen ce of Sn. This difference can be explained by the fact that the thermo dynamics of strong chemisorption is mainly determined by chemical bond ing forces, while the adsorption kinetics can be determined by physiso rption forces because of their influence on precursor states.