Vanadia surface coverage

The molecular structures of the hydrated surface metal oxides on oxide supports have been determined in recent years with various spectroscopic characterization methods (Raman [34,37,40 3], IR [43], UV-Vis [44,45], solid stateNMR [32,33], and EXAFS/XANES [46-51]). These studies found that the surface metal oxide species possess the same molecular strucmres that are present in aqueous solution at the same net pH values. The effects of vanadia surface coverage and the different oxide supports on the hydrated surface vanadia molecular structures are shown in Table 1.2. As the value of the pH at F ZC of the oxide support decreases, the hydrated surface vanadia species become more polymerized and clustered. Similarly, as the surface vanadia coverage increases, which decreases the net pH at PZC, the hydrated surface vanadia species also become more polymerized and clustered. Consequently, only the value of the net pH at PZC of a given hydrated supported metal oxide system is needed to predict the hydrated molecular structure(s) of the surface metal oxide species. 

Therefore, it is inferred that near the maximum surface coverage, under reducing conditions, the surface vanadium oxide species compensate for the oxygen removal by sharing oxide ions. Any effect that promotes interactions among surface vanadia species must lead to the formation of crystalline V205. 

Surface Characterization. In the present case we started with XPS which, due to the low energy of the ejected electrons, yields results on the top 5-15 atomic layers. Since the energy of the electrons is characteristic of the element found, and even of its valence state, a semi-quantitative picture of the surface coverage of elements can be deduced. If accurate results of an overall elemental analysis are available and a set of models is applied for example, one of the models states that vanadia is only found mixed with titania, then the analytical results can be quantified and... 

Number of methanol molecules adsorbed per surface vanadia species at monolayer surface coverages (adsorption temperature of 100 C)... 

C. Zhao and l.E. Wachs, Selective oxidation of propylene over model supported V2O5 catalysts Influence of surface vanadia coverage and oxide support, J. Catal, 257, 181-189 (2008). 

The TOFs for methanol oxidation to formaldehyde (95-99% selectivity), butane oxidation to maleic anhydride and C0/C02 (30% maleic anhydride selectivity) and S02 oxidation to SO3 are independent of surface vanadia coverage. This observation suggests that these oxidation reactions do not depend on the surface concentration of bridging V-O-V bonds since the reaction TOFs do not correlate with the surface density of bridging V-O-V bonds. Furthermore, the constant TOFs with surface vanadia coverage suggest that only one surface vanadia site is required for the activation of these molecules during the oxidation reactions. 

Information about the number of surface sites required for an oxidation reaction, or activation of the reactant molecule, can be obtained by examination of the variation of the TOF with surface vanadia coverage. In general, reactions requiring only one surface site will exhibit a TOF that is independent of the surface vanadia coverage (surface density of sites) and reactions requiring multiple surface sites will exhibit a TOF that increases with the surface vanadia coverage (surface density of sites). From such an analysis, the number of surface vanadia sites required for various oxidation reactions is presented in Table 3. 

Promoters. - Many supported vanadia catalysts also possess secondary metal oxides additives that act as promoters (enhance the reaction rate or improve product selectivity). Some of the typical additives that are found in supported metal oxide catalysts are oxides of W, Nb, Si, P, etc. These secondary metal oxide additives are generally not redox sites and usually possess Lewis and Bronsted acidity.50 Similar to the surface vanadia species, these promoters preferentially anchor to the oxide substrate, below monolayer coverage, to form two-dimensional surface metal oxide species. This is schematically shown in Figure 4. 

Poisons. - Unlike secondary surface metal oxide additives that indirectly interact with the surface vanadia sites via lateral interactions, poisons are surface metal oxide additives that directly interact with the surface vanadia sites and decrease the TOF. For example, the addition of surface potassium oxide to supported vanadia catalysts results in both a structural change and a reactivity change of the surface metal oxide species.50 This interaction, at submonolayer coverages, reflects the attractive interaction between these two surface metal oxide species. The presence of the surface potassium oxide poison alters the V-O bond lengths and the ratio of polymeric and isolated surface vanadia species (favoring isolated surface vanadia species). The interaction of the surface potassium oxide poison with the surface vanadia species is schematically shown in Figure 5. 

Although UV-vis DR spectra of vanadia on other oxide supports (such as Ti02, Ce02, and Nb205) cannot be readily interpreted because of the overlap of their strong absorptions with those of vanadia, equivalent shifts of the Raman bands as a function of vanadia coverage suggest that the surface VO4 species also polymerize on these supports. 

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