Molybdate active sites

The activity shown by unsupported Mo sulfide or Co molybdate catalysts W Is not Inconsistent with the nature of the active sites postulated. The essential pair members and Interactions could all exist on unsupported catalysts. Either Co or Mo alone can cause desulfurization. The support serves mainly to Increase the amount of exposed Co and Mo In some desirable configuration. 

Bismuth iron molybdate, 27 207-209 X-ray diffraction, TlilW Bismuth molybdate, 27 184-187, 189, 191-194, 196, 199--204, 30 124-125 active site, 27 210-213 alumina supported, 27 203, 204 ammoxidation, 30 159 P phase, 27 201 catalyst... 

Finally, Arrhenius treatments of the catalytic data were examined for the HTAD synthesized substitutional series, Bi(2-2x) 2x 030i2, and the binary bismuth molybdate series where Bi/Mo ratios were varied fi-om pure Mo oxide to pure Bi oxide. The noteworthy aspect of the oxidation results is that in the most reactive regime of x = 0-5% atom fi-action Fe, before separate phase Fe3Mo30j2 begins to dominate the catalyst composition in the iron series, the apparent activation energies were all in the range of 19-20 kcal/mol. Furthermore, the activation energies for the pure Bi-Mo series were between 19-20 kcal/mol while the activities were considerable different. Thus, the chief difference in the reactivities in both series is in the preexponential factor, i.e. the number of active sites. 

Molybdenum comprises usually 50% or a little more of the total metallic elements. Most of molybdenum atoms form (Mo04)2 anion and make metal molybdates with other metallic elements. Sometimes a little more than the stoichiometric amount of molybdenum to form metal molybdate is included, forming free molybdenum trioxide. Since small amounts of molybdenum are sublimed continuously from the catalyst system under the working conditions, free molybdenum trioxide is important in supplying the molybdenum element to the active catalyst system, especially in the industrial catalyst system. In contrast, bismuth occupies a smaller proportion, forming bismuth molybdates for the active site of the reaction, and too much bismuth decreases catalytic activity somewhat. The roles of alkali metal and two other additives are very complicated. Unfortunately, few reports refer to these elements, except patents. In this article, discussion is directed only at the fundamental structure of the multicomponent bismuth molybdate catalyst system with multiphase in the following paragraphs. 

Many studies have been devoted to the clarification of the selective oxidation mechanism and the nature of the active sites. In the following, the known bismuth molybdate phases and their significance will first be briefly reviewed, followed by a discussion of the mechanism and kinetics. 

Investigating a bismuth molybdate catalyst with Bi Mo = 1 oxidizing pentenes to isoprene, Watanabe and Echigoya [344] found that isomers of pentenes were less reactive than those of the corresponding butenes in flow experiments, but the reverse was true in pulse experiments. Heat of adsorption measurements make it clear that the active sites are not uni-... 

The occurrence of an almost constant, albeit rather low, activity level, which is reached after a number of pulses, signifies that a certain quasiequilibrium concentration of active sites is maintained by transport of bulk oxygen anions to the surface. Such a mobility of oxygen is particularly observed for bismuth molybdates and some related catalysts (see below). Typical examples of catalysts which completely loose their activity at a low degree of reduction are the antimonates this is primarily caused by the absence of anion mobility. 

Even though the studies discussed have brought about considerable advancement in the level of our understanding of propylene oxidation and selective oxidation catalysts, not all of the details concerning active sites have been reconciled. However, it appears that one reasonable schematic representation of the active sites for propylene oxidation on molybdate catalysts may be given by... 

Moffat, Johnson, and Clark86)found the propylene disproportionation reaction on tungsten oxide-silica catalyst to be limited by interphase diffusional effects in spite of calculations which predict that no diffusional limitation should occur. They postulate that widely separated and very active sites could have their inherent activity limited by localized film diffusional effects which are functions of Reynolds and Schmidt numbers. Activity of cobalt molybdate-alumina was not limited by interphase or prediffusional effects. 

Scheme 5.5. Schematic of the active site for acrolein formation from propene over bismuth molybdate...

ALLISON AND GODDARD Active Sites on Molybdate Surfaces... 

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