Monolayer, molybdate

Russell and Stokes (9) and Sonnemans and Mars (11) have presented strong evidence for the formation of a molybdate monolayer. It appears from their experiments that each molybdate group covers 20-25 A2 of the alumina surface.The surface of the alumina support, which has been used in this study, is high enough for a complete spreading of 15 wt% M0O3, so no bulk molybdenum oxide is expected to be present. This has been confirmed by X-ray diffraction measurements. 

Hexacyanoferrates were immobilized on Au covered with SAM of 3,3 -thiodipropionic acid [86]. It has been found from voltammetric studies that the surface coverage of hexacyanoferrate is close to one monolayer and such an electrode exhibits very good surface redox behavior. Cheng et al. [87] have described the formation of an extremely thin multilayer film of polybasic lanthanide heteropolytungstate-molybdate complex and cationic polymer of quaternary poly(4-vinylpyridine), partially complexed with osmium bis(2,2 -bipyridine) on a gold electrode precoated with a cysteamine SAM. Consequently, adsorption of inorganic species might also be related to the properties of SAMs. This problem will be discussed in detail in a separate section later. 

In such a case, one must conclude that a real two-dimensional aluminum molybdate is formed. As long as the quantity of aluminum molybdate corresponds to less than a monolayer, no three-dimensional aluminum molybdate forms. 

Chromates and molybdates protect the steel by passivation and form a chromium oxide-iron oxide in the case of chromate. While the oxide film of chromium may be several monolayers, molybdenum oxide film is of the order of a monolayer.73... 

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). 

When the cooperating phases in catalytic oxidation have been found to be clearly separated in no epitaxial position, another traditional explanation was put forward, namely that an element of one phase migrated to the other phase for making a contamination layer of molecular thickness, or monolayer. The idea has been based on the observation that M0O3 spreads spontaneously on Y-AI2O3 and, to a certain extent, on bismuth molybdates during calcination in air. But a review of literature shows that M0O3 has much lower ability to... 

Examples of synergistic effects are now very numerous in catalysis. We shall restrict ourselves to metallic oxide-type catalysts for selective (amm)oxidation and oxidative dehydrogenation of hydrocarbons, and to supported metals, in the case of the three-way catalysts for abatement of automotive pollutants. A complementary example can be found with Ziegler-Natta polymerization of ethylene on transition metal chlorides [1]. To our opinion, an actual synergistic effect can be claimed only when the following conditions are filled (i), when the catalytic system is, thermodynamically speaking, biphasic (or multiphasic), (ii), when the catalytic properties are drastically enhanced for a particular composition, while they are (comparatively) poor for each single component. Therefore, neither promotors in solid solution in the main phase nor solid solutions themselves are directly concerned. Multicomponent catalysts, as the well known multimetallic molybdates used in ammoxidation of propene to acrylonitrile [2, 3], and supported oxide-type catalysts [4-10], provide the most numerous cases to be considered. Supported monolayer catalysts now widely used in selective oxidation can be considered as the limit of a two-phase system. 

The selective oxidation of methanol to give formaldehyde is in practice performed in two different processes, one using metallic silver, the other using iron molybdate as catalyst. Vanadium oxide has been shown to be a good selective catalyst in a variety of oxidation processes (refs. 1-2) and we have previously shown that it is also selective for methanol oxidation (refs. 3-5) when the V Og is applied as a very thin layer (monolayer) on different supports the support can have a significant influence on the activity and selectivity of these monolayer catalysts, as was shown by Roozeboom (ref. 6). In a previous paper (ref. 5), it was shown that both the type of support (A Og or TiC ) and the crystal structure of the TiO have an influence on the selectivity of the catalyst for the production of formaldehyde in general, production of the formaldehyde increases with a decrease in the reducibility of the vanadia. 

The XPS analysis (Figure 2) of the average sample from the deeper reactor layer, with no considerable bulk properties changes, indicated fine restructuring of the active phase. The monolayer structure of molybdenum Mo(VI) phase is partially changed to multilayer structure [9,10], while the reduced Mo(IV)/Mo(V) phase remained stable [11]. An indication of nucleation of aluminum molybdate on the surface reveals that a part of molybdenum phase... 

Molybdates, as adsorbent additives, 175 Monolayer adsorption, 41 Monolayer capacity, determination of, 58 Montmorillonite, see Organo clays Multilayer adsorption, 41 effect on isotherm, 59 Multiple development, 30-34 Multistage separations, 352-353... 

Figure 11.2 IR spectra of methanol adsorbed over Fe203, M0O3, and monolayer snppor-ted molybdennm over iron oxide and bnlk iron molybdate at 110°C (From Burcham, L.J., Briand, L.E., and Wachs, LE. Langmuir 2001, 17, 6164—6174 and 6175-6184. With permission.)...

The authors determined the adsorption and the surface reaction kinetic parameters of methanol reaction over monolayer supported vanadium, molybdenum, chromium, rhenium catalysts, and bulk iron molybdate. 

Figure 11.9 Comparison of the TOFs toward methanol selective oxidation products of monolayer supported molybdenum (square symbols) and bulk metal molybdate (triangles) catalysts at 380°C (From Briand, L.E., Hirt, A.M., and Wachs, l.E. J. Catal. 2001, 202, 268-278. With permission.)...

In this work, a monolayer film of PPy nanopartides dispersed into a nonconducting matrix was applied on iron. This monolayer film was then coated by a nonconductive top coat and a delamination experiment was started. The PPy was doped with polyphospho-molybdate (PMo) anions that decompose upon release into mainly molybdate and some phosphate, both inhibiting corrosion on iron. The nonconducting matrix polymer successfully prevented particles from forming macroscopic percolation networks [243] within the... 

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