Carbon monoxide desorption from platinum chemically modified by sulfur

Abstract

Carbon monoxide (CO) desorption energetics from clean polycrystalline platinum [Pt(Poly)] and polycrystalline platinum with sulfur (S) pre-coverage [S/Pt(Poly)] have been studied using temperature programmed desorption (TPD) over a wide range of CO and S coverages. The results for low CO coverages strongly indicate that sulfur modifies the binding strength of platinum for CO in addition to blocking favorable CO adsorption sites. From clean, well-annealed (1200 K) Pt(Poly), CO desorbs in two broad peaks that have been attributed to CO from three adsorption states: defect sites, strongly bound to (111) sites, and weakly bound to (111) sites. With increasing S coverage, CO desorption energies for CO continuously decrease even for dilute CO coverages (less than 1.5% of saturation). The desorption energy for dilute CO coverages from clean Pt(Poly) is 143 kJ/mol. On a S/Pt(Poly) surface with a decreased adsorption capacity of 60% of the saturation CO coverage, the CO desorption energy remains the same. The CO desorption energy decreases to 130 kJ/mol on a S/Pt(Poly) surface with a larger S coverage which has 43% of the saturation CO adsorption capacity. From a S/Pt(Poly) surface with 28% of the saturation CO adsorption capacity, the CO desorption energy is decreased further to 115 kJ/mol. Thus, sulfur not only acts as a site blocker, but also has long-range effects that decrease the binding strength of Pt(Poly) for CO.