J. Jehng, I. Wachs
Oct 25, 1990
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2
Influential Citations
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Journal
Catalysis Today
Abstract
The molecular structure-reactivity relationships for supported niobium oxide catalysts were achieved by combining Raman spectroscopy structural studies with chemical probes that measured the acidity and reactivity of the surface niobium oxide sites. The Raman spectra of niobium oxide compounds are related to the specific niobium oxide molecular structures. The molecular structures of the surface niobium oxide phases present in supported niobium oxide catalysts under ambient conditions, where adsorbed moisture is present, are controlled by the surface pH of the system. Basic surfaces result in the formation of highly distorted NbO6 groups and acidic surfaces result in the formation of slightly distorted NbO6, NbO7, and NbO8 groups. Under in situ conditions the adsorbed moisture desorbs upon heating and the surface niobium oxide overlayer on oxide supports become dehydrated. The dehydration process further distorts the highly distorted NbO6 octahedra due to removal of the coordinated water, but does not perturb the slightly distorted NbO6 octahedra. The highly distorted NbO6 octahedra possess NbO bonds and are associated with Lewis acid sites. The slightly distorted NbO6 octahedra as well as NbO7 and NbO8 groups only possess NbO bonds and are associated with Bronsted acid sites. The Lewis acid surface sites are present on all the supported niobium oxide systems, but the Bronsted acid surface sites are limited to the Nb2O5/Al2O3 and Nb2O5/SiO2 systems. The surface niobium oxide Lewis acid sites are significantly more active when coordinated to the Al2O3 and SiO2 surfaces than to the TiO2, ZrO2, and MgO surfaces (surface oxide-support interaction). Furthermore, these surface niobium oxide sites on SiO2 behaves as redox sites and the surface niobium oxide on Al2O3 are acid sites during partial oxidation reactions.