Structure of Adsorbed Sulfur and Carbon on Molybdenum and Rhenium Single Crystal Surfaces, and Their Influence on Carbon Monoxide and Hydrocarbon Chemisorption

摘要

An ultra-high vacuum (10 sup -10 Torr) study was performed on the chemisorption and structures of S and C adsorbates on Mo(100), Re(0001), and Re(1010) single crystal surfaces. Both S and C adsorb strongly on Mo(100), Re(0001), and Re(1010), with adsorption energies >70 kcal/mol for coverages less than saturation. S was proposed to adsorb in the highest symmetry sites on all surfaces except for theta/sub s/ > 0.75 on Mo(100), where studies suggest two different adsorption sites. C adsorbs in a ''carbidic'' or active phase on Mo(100), where it is also proposed to adsorb in the highest symmetry sites. However, C adsorbs in a ''graphitic'' or inactive phase on Re(0001) and Re(1010). CO chemisorption on the S and C overlayers was found to be blocked (except for C on Mo(100)), with S blocking adsorption more efficiently than C. Changes in adsorption energy were determined to be caused by local crowding of CO molecules by S or C, rather than a long-range electronic interaction. Unsaturated hydrocarbons decomposed completely on Mo(100), Re(0001), and Re(1010). Similar to the results for CO chemisorption, strong adsorption of unsaturated hydrocarbons (leading to decomposition) was blocked by pre-adsorbed S, allowing only physisorption to occur (adsorption energies < 11 kcal/mol). The effect of pre-adsorbed ''graphitic'' C on Re(0001) and Re(1010) on unsaturated hydrocarbon chemisorption was the same; strong adsorption (leading to decomposition) was blocked allowing only physisorption. However, Mo(100) with pre-adsorbed ''carbidic'' carbon blocks only decomposition while allowing strong reversible molecular chemisorption (12 to 23 kcal/mol). Differences in inhibition efficiency of S and C are proposed to be caused by differences in bond distances of the adsorbates to the surface. Greater distance from the metal surface causes more interaction with neighboring metal atoms. These differences also suggest explanations for catalytic hydrodesulfurization of thiophene. (ERA citation 13:001192)