Bulk Metal Vanadate

The significant difference between the TOP and selectivity of bulk metal molybdates and vanadates compared with pure metal oxides was a key factor in uncovering the true surface composition of those bulk catalysts. Table 11.3 and Table 11.4 show the number of surface active sites, redox TOP, and selectivity toward methanol selective oxidation products of bulk metal vanadates and the corresponding metal oxide, respectively. Similar results were obtained for bulk metal molybdates. Bulk metal vanadates possess a high selectivity to formaldehyde with some selectivity to dimethoxy methane (nickel vanadate), dimethyl ether (niobium, chromium, and aluminum vanadates), methyl formate (magnesium, chromium, and copper vanadates), and CO2 (niobium and silver vanadates). 

The observation that bulk metal vanadates posses a high selectivity toward formaldehyde strongly suggests that the surface of bulk metal vanadates is composed of vanadium oxide sites with redox properties that cover the counter-cation sites (Mg, Ni, Mn, Cr, Co, Zn, Al, Nb, Fe, Cu, and Ag) thus, inhibit methanol total oxidation. The TOF values of the bulk metal vanadates are similar indicating that there is no significant influence of the specific nature of the metal oxide counter-cation on the catalytic behavior. Moreover, there are significant differences between the TOF values of bulk metal vanadates and pure metal oxides. 

TOF and Selectivity of Bulk Metal Vanadates Toward Methanol Oxidation at Low Methanol Conversion... 

The TOF values of bulk metal molybdates were extrapolated to 300 C in order to compare their values with the corresponding bulk metal vanadate catalysts. Figure 11.11 shows that, in general, bulk metal vanadates possess one order of magnitude higher TOF values ( 2 to 14 sec ) than their corresponding bulk metal molybdates ( 0.1 sec ) for methanol selective oxidation. The TOF values of pure... 

Figure 11.10 Comparison of the TOFs towards methanol selective oxidation products of monolayer supported vanadium (A symbols) and bulk metal vanadate catalysts ( ) at 300°C (From Briand, L.E., Jehng, J.-M., ComagUa, L.M., Hirt, A.M., and Wachs, l.E. Catal. Today 2003, 78, 257-268. With permission.)...

Tian, H., Wachs, I. and Briand, L. (2005). Comparison of UV and Visible Raman Spectroscopy of Bulk Metal Molybdate and Metal Vanadate Catalysts, J. Phys. Chem. B., 109, pp. 23491-23499. 

Surface Composition of Bulk Metal Molybdate and Vanadate Catalysts... 

The number and nature of the active surface sites and the catalytic activity of bulk metal molybdates and vanadates were also investigated through methanol chemisorption [51-53]. These materials proved to be equally or more active and stable than the industrial catalyst Mo03/Fe2(Mo04)3 in formaldehyde production [54-56],... 

V2O5 (9.8 sec ) and M0O3 (0.6 sec ) crystals are also presented to demonstrate that the difference in the activity of bulk metal molybdates and vanadates is based on the nature of the surface species (in this case, VO versus MoO c). The results... 

In certain instances of poisoning, especially in the case of base metal catalysts, the deactivation can be simply explained by the formation of new bulk solid phases between the base metal and the poison. Examples are the formation of lead vanadates (14) in vanadia catalysts, or of sulfates in copper-chromite and other base metal catalysts (81). These catalyti-cally inactive phases are identifiable by X-ray diffraction. Often, the conditions under which deactivation occurs coincide with the conditions of stability of these inert phases. Thus, a base metal catalyst, deactivated as a sulfate, can be reactivated by bringing it to conditions where the sulfate becomes thermodynamically unstable (45). In noble metal catalysts the interaction is assumed to be, in general, confined to the surface, although bulk interactions have also been postulated. 

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