Characterization of supported rhenium oxide catalysts: effect of loading, support and additives
The nature of the surface rhenium oxide species present in supported rhenium oxide catalysts was determined as a function of the oxide support, rhenium oxide loading and secondary metal oxide additives. The catalysts were prepared using the incipient wetness impregnation method with dilute aqueous perrhenic acid as the precursor and characterized using Raman, FTIR and X-ray photoelectron spectroscopy (XPS) and temperature programmed reduction (TPR) experiments. The Raman and XPS studies reveal that the surface rhenium oxide species are well dispersed on the Al2O3 and TiO2 supports. Under dehydrated conditions, Raman and IR studies reveal that the surface rhenium oxide species is independent of oxide support (Al2O3 and TiO2) and it appears that 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 surface rhenium oxide species depends strongly on the specific oxide support: the surface rhenium oxide species on TiO2 is more reducible than the surface rhenium oxide species on Al2O3 . The difference in reducibility is related to the bridging Re–O–support bond strength. The secondary metal oxide additives, sodium and vanadium, can be categorized as interacting and non-interacting, respectively. The non-interacting additive (vanadium oxide) coordinates directly to the oxide supports without significantly affecting the structure and reducibility of the surface rhenium oxide species. The interacting additive (sodium oxide), however, directly coordinates with the surface rhenium oxide species and changes its structure and hydrogen reducibility.