The effect of potassium on the chemisorption of carbon monoxide on the Rh(111) crystal face

Abstract

The adsorption of CO on the clean and potassium-predosed Rh(111) crystal surface has been studied using TPD and HREELS. When 10% of a monolayer of potassium is preadsorbed, the thermal desorption spectrum of CO broadens up to 100 K higher temperature. In the presence of 10–36% of a monolayer of coadsorbed potassium, two new high temperature desorption maxima develop due to dissociation of CO. The CO stretching vibrations decrease continuously with either increasing potassium coverage or decreasing CO coverage. Substantial population and adsorption sequence shifts in site occupancy from the atop to the bridge site occur as a function of increasing potassium coverage. The results indicate both strengthening of the M-C bond and weakening of the C-O bond in the presence of potassium, due presumably to increased electron occupancy of the 2 π ∗-orbital of CO. Broadening and asymmetry of the vibrational peaks suggest the proximity of the CO molecules to the potassium adatoms influence the chemisorption behavior, although nonlocal interactions are also indicated.