Characterization of supported rhenium oxide catalysts: effect of loading, support and additives

The nature of the surface rhenium oxide species present in supported rheniu m oxide catalysts was determined as a function of the oxide support, rheniu m oxide loading and secondary metal oxide additives. The catalysts were pre pared using the incipient wetness impregnation method with dilute aqueous p errhenic acid as the precursor and characterized using Raman, FTIR and X-ra y photoelectron spectroscopy (XPS) and temperature programmed reduction (TP R) experiments. The Raman and XPS studies reveal that the surface rhenium o xide species are well dispersed on the Al2O3 and TiO2 supports. Under dehyd rated conditions, Raman and IR studies reveal that the surface rhenium oxid e species is independent of oxide support (Al2O3 and TiO2) and it appears t hat the same isolated rhenium oxide species is formed for all loadings. The structural details of this dehydrated isolated rhenium oxide species are, however, not clear. TPR measurements suggest that the reducibility of the s urface rhenium oxide species depends strongly on the specific oxide support : the surface rhenium oxide species on TiO2 is more reducible than the surf ace rhenium oxide species on Al2O3. The difference in reducibility is relat ed to the bridging Re-O-support bond strength. The secondary metal oxide ad ditives, sodium and vanadium, can be categorized as interacting and non-int eracting, respectively. The non-interacting additive (vanadium oxide) coord inates directly to the oxide supports without significantly affecting the s tructure and reducibility of the surface rhenium oxide species. The interac ting additive (sodium oxide), however, directly coordinates with the surfac e rhenium oxide species and changes its structure and hydrogen reducibility .