The adsorption and coadsorption of sulfur and carbon monoxide on rhenium single crystal surfaces

Abstract

The structure of sulfur overlayers on Re(0001) and Re(10 10) , and their effect upon CO chemisorption have been studied using Auger electron spectroscopy, thermal desorption spectroscopy, and low energy electron diffraction. In addition, calculations based upon a kinematic analysis of electron scattering have yielded possible two-dimensional (2D) real space structures based upon the observed LEED patterns for sulfur adsorbed on Re(0001). Four sulfur overlayer structures were seen on Re(0001): starting with a p(2×2) at 0.25 monolayers ( θ s = 0.25), a “ (3 3 ×3 3 ) R30° ” with missing spots at θ s = 0.35, a 3 1 1 3 at θ s = 0.4, and a (2 3 ×2 3 ) R30° at θ s = 0.5, with saturation occurring at approximately θ s ≅ 0.6. Two sulfur structures are observed on Re(10 10) , a p(2×2) at θ s = 0.5, and a p( l ×2) at θ s = 1.0. Desorption energies for sulfur on both surfaces were determined to range from 104 kcal/mol at low sulfur coverage to 85 kcal/mol near saturation. Comparisons with calculations based upon a kinematic analysis of the high sulfur coverage diffraction patterns on Re(0001) suggest that close proximity between adsorbed sulfur atoms may be responsible for lowering the adsorption energy. On Re(10 10) , an adsorption site for sulfur is proposed which involves first and second layer atoms. CO chemisorption was inhibited linearly by increasing the concentration of pre-adsorbed sulfur. A slight decrease in the CO desorption peak temperature at low sulfur coverage on Re(0001) was also observed. Comparison with other systems such as Ru(0001) enables us to conclude that the decrease in peak temperature is due to repulsive interactions between CO and S. On the other hand, pre-adsorbed sulfur on Re(10 10) blocks CO desorption peaks at low temperature, suggesting that pre-adsorbed sulfur blocks CO-CO repulsion.