Active Sites for Heteroatom Removal

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The basis of catalytic activity for heteroatom (sulfur or nitrogen) removal is explored. Model surfaces are prepared by sulfiding single-crystal surfaces of tungsten or molybdenum. Corrugated (211) and stepped (532) surfaces are employed to create sulfided surfaces similar to the edge sites of molybdenum disulfide, claimed by some to be the active sites in commercial hydrodesulfurization catalysts. The structures of the sulfided surfaces are characterized with low-energy electron diffraction, Auger electron spectroscopy, and scanning tunnelling microscopy. The latter provides a direct means to image the complex restructuring that occurs on these surfaces during sulfiding. Adsorption and reaction of organosulfur and organonitrogen compounds on these sulfur- modified surfaces are followed with temperature-programmed reaction, reflection/absorption infrared spectroscopy, and photoelectron spectroscopy. Substituted thiophenes and pyridines are used to determine the effects of altered chemical reactivity and steric hindrances on interaction with the surface. Experimental findings are complemented with semiempircial molecular-orbital calculations of the organic compounds with modified metal clusters; both extended Huckel and INDO methods are used. Removal of sulfur and nitrogen from fuels is the preferred method for reduction of both photochemical smog and acid rain. As low-sulfur and low-nitrogen reserves of coal and petroleum are exhausted this problem becomes more acute. This work is designed to explain how catalysts known to remove heteroatoms work and to provide a basis for development of improved systems for nitrogen and/or sulfur removal from fuels or fuel precursors.