Hydrogen sulfide adsorption on alumina, modified alumina, and molybdenum trioxide/alumina

Abstract

A systematic investigation of H/sub 2/S adsorption on Al/sub 2/O/sub 3/, modified Al/sub 2/O/sub 3/, and MoO/sub 3//Al/sub 2/O/sub 3/ catalysts was carried out by using temperature-programmed desorption techniques, coupled with IR and XPS. SiO/sub 2/-Al/sub 2/O/sub 3/ and AlPO/sub 4/ were also studied for comparison. It was found that Al/sub 2/O/sub 3/ showed two distinctly different chemisorption modes of H/sub 2/S, associative (..cap alpha..) and dissociative (..beta..) H/sub 2/S, whereas SiO/sub 2/-Al/sub 2/O/sub 3/ and AlPO/sub 4/ provided predominantly ..cap alpha..-H/sub 2/S. The heats of adsorption were 13 and 34 kcal mol/sup -1/ for ..cap alpha..- and ..beta..-H/sub 2/S, respectively, and the corresponding site densities were 4.8 and 5.5 x 10/sup 13/ cm/sup -2/. The adsorption sites are concluded to be cus (coordinately unsaturated) Al/sub tet//sup 3 +/ cations for ..cap alpha..-H/sub 2/S and cus Al/sub tet//sup 3 +/-strong base pair sites for ..beta..-H/sub 2/S. The effects of calcination and evacuation temperatures on the H/sub 2/S adsorption behavior of Al/sub 2/O/sub 3/ are also interpreted in terms of the above site models and a surface reconstruction of ..gamma..-Al/sub 2/O/sub 3/. On the basis of IR studies of CO/sub 2/-H/sub 2/S coadsorption, cus Al/sub tet//sup 3 +/-strongly basic oxide anionmore » pair sites are proposed to be responsible for the reaction. When the loading level of MoO/sub 3/ was low (< 3 wt%), molybdenum anions were found to react with the surface hydroxyl groups on Al/sub 2/O/sub 3/ as bidentate ligands similar to SO/sub 4//sup 2 -/ anions, forming a monomolecular layer of molydena over Al/sub 2/O/sub 3/. The H/sub 2/S adsorption on sulfided MoO/sub 3//Al/sub 2/O/sub 3/ catalysts was also examined.« less