Bulk metal Molybdate

Briand, L., Hirt, A. and Wachs, I. (2001). Quantitative Determination of the Number of Surface Active Sites and the Turnover Frequencies for Methanol Oxidation over Metal Oxide Catalysts Application to Bulk Metal Molybdates and Pure Metal Oxide Catalysts, J. Catal., 202, pp. 268-278. 

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. 

Briand, L.E., Hirt, A.M., and Wachs, I.E. Quantitative determination of the number of surface active sites and the turnover frequencies for methanol oxidation over metal oxide catalysts Application to bulk metal molybdates and pure metal oxide catalysts. J. Catal. 2001, 202, 268-278. 

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],... 

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 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... 

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... 

Surface analyses were investigated mainly by using XPS (Fig. 7). It was clearly indicated that many composite oxides found by XRD are located un-homogeneously in the catalyst particle. Molybdenum and bismuth are undoubtedly concentrated in the surface layer of the catalyst particle and divalent and trivalent metal cations are found in the bulk of the catalyst. As a result, it is clear that bismuth molybdates, especially its a-phase, is located on the surface of each particle, and metal molybdates of divalent and trivalent cations are situated in the bulk of the catalyst. 

More complex Bi-Mo oxide catalysts are claimed in patents, such as Bi-Mo-M +-M +-M+-X-Y-0, where M+ is an alkali metal, X=Sb, W, and Y=P, B, and so on. In multicomponent catalysts, the metal molybdates M +MoOd and M2 +(Mo04)3 serve as supports for the active phase, a -Bi2M03Oi2. Bulk migration of ions through lattice vacancies plays an important role in enhancing catalytic... 

Bulk Mixed Oxide Catalysts. - Raman spectroscopy of bulk transition metal oxides encompasses a vast and well-established area of knowledge. Hie fundamental vibrational modes for many of the transitional metal oxide complexes have already been assigned and tabulated for systems in the solid and solution phases. Perhaps the most well-known and established of the metal oxides are the tungsten and molybdenum oxides because of their excellent Raman signals and applications in hydrotreating and oxidation catalysis. Examples of these two very important metal-oxide systems are presented below for bulk bismuth molybdate catalysts, in this section, and surface (two-dimensional) tungstate species in a later section. 

The bulk and surface compositions of tungstates and molybdates can be modified through ionic exchange process, such as those used to obtain transition metal molybdate from sodium molybdate [5]. In this topic, as a specific example of preparation and uses of modified tungstates and molybdates, will be presented the preparation of Eu(III) compounds for optical purposes. 

Bulk mixed metal oxide catalytic materials consist of multiple metal oxide components. Such mixed metal oxide catalysts find wide application as selective oxidation catalysts for the synthesis of chemical intermediates. For example, bulk iron-molybdate catalysts are employed in the selective oxidation of CH3OH to H2CO [122], bulk bismuth-molybdates are the catalysts of choice for selective oxidation of CH2=CHCH3 to acrolein (CH2=CHCHO) and its further oxidation to acrylic acid (CH2=CHCOOH) [123], selective ammoxidation of CH2=CHCH3 to acrylonitrile (CH2=CHCN) [123], and selective oxidation of linear CH3CH2CH2CH3 to cyclic maleic anhydride consisting of a flve-membered ring (four carbons and one O atom) [124]. The characterization of the surface... 

Analytical electron microscopy permits structural and chemical analyses of catalyst areas nearly 1000 times smaller than those studied by conventional bulk analysis techniques. Quantitative x-ray analyses of bismuth molybdates are shown from lOnm diameter regions to better than 5% relative accuracy for the elements 61 and Mo. Digital x-ray images show qualitative 2-dimensional distributions of elements with a lateral spatial resolution of lOnm in supported Pd catalysts and ZSM-5 zeolites. Fine structure in CuLj 2 edges from electron energy loss spectroscopy indicate d>ether the copper is in the form of Cu metal or Cu oxide. These techniques should prove to be of great utility for the analysis of active phases, promoters, and poisons. 

One possible conclusion is that under reducing conditions, metal cation movement occurs. Another possible conclusion is that despite the similar surface layer composition of bismuth and molybdenum for the three phases of bismuth molybdate, the three bismuth molybdate phases possess different catalytic activities, catalytic selectivities, adsorption properties, surface oxomolybdenum species, and reducibilities because the surface properties of the active bismuth molybdates are dependent upon the foundation upon which they exist, i.e., upon the bulk structure and its chemical and electronic properties. 

During the electrolytic preparation of composite cathodes from solutions of Ni or Co salts with molybdate or tungstate, the current efficiency for deposition of the two metals is far from 1(X)%, so cathodic Hj evolution, with codeposition (sorption) of the H intermediate, is unavoidable. Hence it is virtually certain that these composite cathode materials are formed as hydride materials. It was suggested in Ref. (75) that this may be one of the reasons for their excellent electrocatalytic behavior in the HER, in contrast to that of bulk, thermally prepared alloys of the same metals, Ni and Mo. In this respect, hydrided metals may behave like Pt cathodes where the HER proceeds with good electrocatalysis on a full monolayer of UPD H and, under appreciable applied current densities, on a Pt surface region containing apparently some significant quantity of three-dimensionally sorbed H (136). 

This may well have effects on the spedation of transition metal precursors. It is sometimes hard to evidence because one needs a sensitive spectroscopic probe able to distinguish a surface spedes, possibly present in small amounts, against the backgroxmd of unmodified bulk spedes. For the molybdate/ y alumina system, Mo NMR has shown that the alumina surface could promote the basic hydrolysis of heptamolybdate to monomolybdate in its vidnity... 

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