Electric field effects on CO and NO adsorption at the Pt(111) surface

The electric field dependence of CO and NO adsorption onto a Pt(lll) cluste r model surface is studied by quantum-chemical density functional calculati ons. It is found that CO starts favoring the hollow site over the atop site for negative potentials, whereas NO adsorbs hollow sites, at ail potential s. These differences can be understood by referring to the Blyholder 5 sigm a donation - 2n* back donation model, to the influence of the field in shif ting the adsorbate levels with respect to the metal levels, and to the lowe r energy of the NO 5 sigma and 2 pi* adsorbate levels with respect to CO. I t is also found that the interfacial Stark tuning rate (i.e. the change in intramolecular stretching frequency with the field) is higher for NO and hi gher in the high-coordination site, which can be understood in terms of the back donation influence. These findings are in good agreement with experim ental results. (C) 1999 Elsevier Science S.A. All rights reserved.