Electron spin resonance (ESR) spectroscopy and CO adsorption measurements have shown that Re0 and Re4+ species coexist on the surface of reduced 0.2 wt% Re Al2O3 and 0.3-0.2 wt% PtRe Al2O3 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 Re4+ species, which is stabilized by the Al2O3 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 Al2O3 and less than 10% of the Re in PtRe Al2O3. This species does not chemisorb CO but interacts weakly with O2. The Re0 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 Re4+ slightly enhances the Brønsted acidity in Re Al2O3, although this effect is negligible in comparison with the acidity induced by Cl- ions. The Re0 sites are very active for olefin hydrogenation; however, after presulfiding the Re Al2O3, 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.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry