Bulk metal Molybdate surface composition

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

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). 

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. 

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. 

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). 

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