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Supported vanadia catalysts, Reducibility, Vanadia phases; Ethane oxidation; Selective oxidation, Structure-activity relations
Abstract:
In supported vanadate catalysts the V atoms are commonly assumed to be mostly in oxidation state +5 in the oxidized state. However, H/V ratios of less than 2 are often observed in TPR experiments, and significant fractions of V(IV) species are observed using the XPS, UV-VIS or ESR techniques.
The aim of this contribution is to clarify this discrepancy. From the TPR, XPS, BET, XRD, TEM, DRIFTS and pyridine TPD data as well as from a structure-activity analysis in ethane oxidation, it can be concluded that in isolated and polymerized vanadates only 1/3 of V (but 80 % of V in crystalline V2O5 phases) can reach the oxidation state +5. The other V centers remain at oxidation state +4 even after oxidizing or various redox treatments. This implies the existence of two different kinds of V species within the vanadate structures. However, M-O-V(V) bonds as active sites and V=O bonds should be present to reach consistence with spectroscopic data and to explain the support effects. The smallest unit which meets these requirements is [V3O8]3- (3 support bonds) for the isolated vanadates. For polyvanadates a structure with the sequence (-V(IV)-O-V(V)-O-V(IV)-O-)n is postulated.
