Molecular orbital study of CO chemisorption on a Pt(111) surface in the presence of potassium

Abstract

A molecular orbital study was mode, using an atom superposition and electron delocalization (ASED) technique, to examine the chemisorptive properties of CO on a Pt(111) surface in the presence of coadsorbed potassium atoms. To understand the effect of preadsorbed K on CO chemisorption, we have studied the adsorption of K on a cluster model of a Pt(111) surface. K. Is predicted to be preferentially adsorbed at a three-fold position and the calculated binding energy is in reasonable agreement with other estimates. K adsorption is found to lead to a significant charge transfer which in turn increases the electron density of all surface Pt atoms. The presence and increasing coverage of K on the Pt(111) surface has been simulated in our present theoretical study by decreasing the valence state ionization potential of Pt atoms in our cluster models. With the decreasing values (i.e. with increasing K coverage), the CO molecule is predicted to change its site of occupation from a one-fold position tot he two-fold or three-fold positions. The CO stretching force constant constant is predicted to decrease for all the sites with decreasing VSIP values of Pt. We find the decrease in CO force constant on going from the one-fold to two-fold site is smaller than the decrease on going from the one-fold to three-fold site. Our theoretical results corroborate with the recent experimental findings of Garfunkel et al. for this system.