Bridging V-O-Support Bonds

THE BRIDGING V 0-SUPP0RT BOND CONTROLS THE SPECIFIC CATALYTIC ACTIVITY (TOF) 

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Figure 2 The bridging V-O-Support bond controls the specific catalytic activity.

These structural changes are accompanied by significant reactivity modifications of the surface vanadia species. The addition of the surface potassium oxide species decreases the reducibility of the surface vanadia species in temperature programmed reduction (TPR) studies and the TOF for methanol oxidation.23,50 The most likely reason for this behavior is that the surface potassium oxide species is intimately coordinated to the bridging V-O-Support bond and retards its participation in these redox processes. Thus, all oxidation reactions, involving one surface vanadia site as well as dual surface vanadia-acidic sites, will be retarded by the surface potassium oxide additive. The basic properties of the surface potassium oxide additive may also affect the product selectivity by... 

Essentially the same methanol oxidation TOFs were obtained on the different oxide supports. The Degussa P-25 titania support (90% anatase 10% rutile) was also examined, as shown in Figure 6, because it possesses very low levels of surface impurities and represents a good reference sample. The invariance of the methanol oxidation TOF with the specific phase of the titania support reveals that the oxidation reaction is controlled by a local phenomenon, the bridging V-O-Support bond, rather than long range effects, the structure of the 2 support. Thus, the phase of the oxide support does not appear to influence the molecular structure or reactivity of the surface vanadia species. 

The reactivity of vanadium oxide in ethane ODH is highly dependent on the type of support [18, 37, 42, 47-51]. The performance of the catalysts depends on the characteristics of the supports and the nature of the active species formed on these supports. At a given surface density, the vanadia-support interactions determine the type of VO structure formed. Banares and co-workers [52-53] found that in ethane ODH, the turnover frequency values vary with the specific oxide support for both the isolated and polymeric surface VO species. Thus, the support has a significant influence on the reaction parameters. The support cation directly affects the reactivity of the bridging V-O-support bond. 

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