CO ADSORPTION SITE OCCUPATION ON PT(335) - A QUANTITATIVE INVESTIGATION USING TPD AND EELS

In a study of CO on Pt(335) at various coverages using TPD and EELS we observe bridge-bonded CO. Bridge-bonded CO on Pt(335) is inconsistent with existing models of CO adsorption on highly stepped Pt surfaces. Previous studies of CO on Pt(335) and on the similar Pt(112) surface identified two atop CO species: edge atop and terrace atop. We observe two additional species: edge bridge and terrace bridge. Edge atop and edge bridge CO both adsorb before CO adsorbs at terrace sites. Activation energies are estimated for CO desorption from edge and terrace sites. A model that explains our data, based on a quantitative analysis of the TPD and EELS spectra, is presented. Possible overlayer structures are proposed at selected coverages. At saturation, all edge sites are occupied with atop CO, half of the terrace sites are occupied, and on the terrace the atop-to-bridge ratio is 2:1. At low coverage the overlayer reaches equilibrium within a few minutes at 100 K. At high coverage equilibrium is not reached in tens of minutes at 100 K, but annealing 10 s at 280 K does allow the overlayer to reach equilibrium.