Characterization and catalytic function of Re0 and Re4+ in Re/Al2O3 and PtRe/Al2O3 catalysts
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Electron spin resonance (ESR) spectroscopy and CO adsorption measurements have shown that Re 0 and Re 4+ species coexist on the surface of reduced 0.2 wt% Re Al 2 O 3 and 0.3-0.2 wt% PtRe Al 2 O 3 catalysts. The catalysts were prepared by incipient wetness impregnation with metallic salt precursors, calcined in air at 500 C, and dried in He at 500 C prior to reduction. The Re 4+ species, which is stabilized by the Al 2 O 3 and resists reduction up to 500 C, yields an ESR signal which exhibits hyperfine splitting and accounts for less than 20% of the total Re in Re Al 2 O 3 and less than 10% of the Re in PtRe Al 2 O 3 . This species does not chemisorb CO but interacts weakly with O 2 . The Re 0 does not give rise to a resonance signal, but adsorbs CO strongly. The double bond shift (DBS) of 1-pentene, hydrogenation of 1-pentene, and the skeletal isomerization (SI) of 3,3-dimethyl-l-butene were tested using mild reaction conditions. The results indicate that Re 4+ slightly enhances the Brnsted acidity in Re Al 2 O 3 , although this effect is negligible in comparison with the acidity induced by Cl - ions. The Re 0 sites are very active for olefin hydrogenation; however, after presulfiding the Re Al 2 O 3 , this hydrogenation activity is drastically lowered, but the DBS activity is still high which indicates that surface ReS species are active for the DBS at the Conditions employed. 1987.
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