Mo oxidation states

Fig. XVin-6. Curve-fitted Mo XPS 3d spectra of a 5 wt% Mo/Ti02 catalyst (a) in the oxidic +6 valence state (b) after reduction at 304°C. Doublets A, B, and C refer to Mo oxidation states +6, +5, and +4, respectively [37]. (Reprinted with permission from American Chemical Society copyright 1974.)...

Figure 5. An oxidation state diagram for Mo, Cr, Fe and Mn. For Mo and Cr, N = III for Fe and Mn, N = II. Potentials are given at standard states in acid solution relative to the hydrogen electrode. On such a diagram, die slope between any two points equals the redox potential. In conh ast to most other metals, multiple Mo oxidation states are accessible over a small range of potentials. Note also that Mo is oxidized to Mo(VI) at relatively low potential (similar to Fe(III). Figure modified after Frausto da Silva and Williams (2001).

O.lA shorter. Third, the linear clusters exhibit no reaction chemistry with small molecules at Mo, but such chemistry is well-established for the single cubane MoFe3S clusters (71). It is clear that new strategies to produce synthetic Mo-Fe-S clusters with proper stoichiometry. Mo oxidation state, and reactivity are necessary two such approaches are described below. 

Mo oxidation state. These Mo catalytic systems show similar ethylene-ethane product ratios in the reduction of acetylene (see Table VI). The ESR result appears to correspond well to the recent electrochemical result (20, 21). 

In addition to the membrane-anchored enzymes that support anaerobic respiration, bacteria express a number of functionally, and structurally, distinct nitrate reductases related by the presence of an Mo[MGD]2 containing active site. PFV has demonstrated tunnel-diode behaviour from two of these the periplasmic Rhodobacter sphaeroides NapAB and the assimilatory Synechococcus elongatus NarB. In both cases preferential binding of nitrate to the Mo , over Mo , oxidation state provides an explanation for the catalytic voltammetry and this may prove to be a conserved feature of the catalytic cycle in these enzymes. 

Commercial heterogeneous HDS catalysts for refinery use consist, almost without exception, of nickel- and/or cobalt-promoted molybdenum oxide located on a high surface area (approx. 300 m g ) alumina or silica-alumina support. Cobalt and nickel promoters increase the catalytic activity, particularly towards thiophenes whether Co or Ni is used as a promoter depends on the specific function for which the catalyst should be optimal. The catalyst material is shaped into porous pellets, a few millimeters in size, and these pellets are loaded into the reactor, forming a catalyst bed of 30-200 m volume. During start-up of a freshly loaded reactor, the catalyst bed, which is in the oxidic form, is sulfided, typically by treatment with an oil feed which has been spiked with a reactive sulfur compound that readily generates H2S in situ. The oxidic precursor phases (non-stoichiometric CoMo or NiMo surface oxides) are thereby converted into sulfidic phases termed Co-Mo-S and Ni-Mo-S. The conversion from the oxidic phase to the sulfidic is accompanied by a reduction in Mo oxidation state from +6 to +4. 

Figure 1. Effective Mo oxidation state vs. oxygen coverage.

Figure 19.12. Sketch of the correlation between Mo oxidation state and oxidation of propene. At Mo(-l-VI) sites the partial oxidation to acrolein dominates, whereas at Mo(H-V) sites the total oxidation occurs.

Costentin, G., Lavalley, J.C., and Studer, R Mo oxidation state of Cd, Fe, and Ag catalysts under propane mild oxidation reaction conditions. J. Catal 2001, 200, 360-369. 

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