Oxides MO

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.)...

Catalytic oxidation of n-butane at 490° over a cerium chloride, Co-Mo oxide catalyst produces maleic anyhydride ... 

Bimetallic Co-Mo oxide specimens were prepared via co-impregnation of calculated amounts of cobalt nitrate and ammonia heptamolybdate on y-alumina to achieve a total metal loading of 20wt% with an equimolar Co Mo ratio. Nitridation of catalysts was carried in a fixed bed... 

In contrast to MoSx/NaY, the Fourier transform for MoSj/NaY clearly showed Mo-O bondings as well as Mo-S and Mo-Mo bondings as summarized in Table 1. It is evident that Mo oxide species in calcined MoOj/NaY are only partially sulfided. XPS results corroborated the EXAFS results. The incomplete sulfidation of the Mo species in MoS /NaY may explain in part the relatively low HDS and HYD activities of the cat ysts in Figs.l and 2. 

Pt-Mo/y-AljO catalyst (1) lowers the activity but increases the selectivity of the Pt catalyst for the NO reduction by H2, and (2) increases the activity for the NO reduction by CO and the activity and selectivity for the NO reduction by CO + H2> Based on the TPR and IR data, we attribute these results to a strong interaction between Pt and Mo oxides on the y-A170, support. Such interaction facilitates the removal of the surface... 

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. 

The high activity of V-W-Ti-0 and of V-Mo-Ti-O is due to a synergistic effect between V and W (Mo) oxide species and is related to the superior redox properties of the ternary catalysts. 

Baeyer-ViUiger oxidation, 786-7 fatty add hydroperoxides, 690 olefin oxidation, 792-3 Tin complexes, Mo-oxide, 428 Tiron, Fe(III) enhancement, 658, 659 Tissue... 

A third problem is related to the slow baek-desorption of the produets of reaetion, when they form on metal-oxide nanopartieles within a host ordered porous siliea matrix. For example, in toluene oxidation to benzaldehyde over Fe-Mo-oxide nanopartieles stabilized within a siliealite matrix, the slow rate of reoxidation of the redueed Fe-Mo-oxide, due to the low nanopartiele size, inereases the presenee of redueed molybdenum sites, whieh, interaeting with the earbonyl group of benzaldehyde, slow down the desorption and enhanee the rate of the eonseeutive oxidation. " ... 

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. 

The two bands at around 1700 cm may be reasonably attributed to Vc=o two different adsorbed species, probably acrolein and acrylic acid. In this compound, in fact, the vc=o band is found at a frequency about 20 cm higher than in acrolein (9). In these adsorbed compounds [for example, on V-Mo oxides (9)], the vc=c band is expected at a nearly the same value as in 7C-bonded propylene (around 1625 cm ), whereas other IR active bands are covered by the stronger bands due to physisorbed propane. A more clear identification of the above species, therefore, is not possible. The shoulder at about 1425 cm" may be attributed to VsCOO in adsorbed acrylate, but the VasCOO band expected at around 1550 cm is absent. A more reasonable interpretation is the formation of alkene oligomers. In fact, propene adsorbed on HNaY gives rise to the formation of a main band at about 1460 cm (9), apart from vch 5ch bands that, in our case, are covered by the band of physisorbed propane. However, all adsorbed species are removed by evacuation, indicating their weak interaction with the surface. 

An isotherm of C3H6 conversion on y is shown in figure 3.16, where the products are monitored at 450 °C. The figure shows various stages of activity and the activity begins to decrease after 1 hr. A drop in the selectivity to acrolein is also observed above 450 °C. Sample C, for example, shows the presence of y, and a structure with dimensions 8.4 A x 10.8 A. Sample D has more metallic Bi and reduced Mo-oxides. 

Erom HRTEM studies, it is proposed that the majority of the bismuth molybdate phases can be derived from the fluorite structure, in which both the cation and anion vacancies can be accommodated within the fluorite framework (Buttrey et al 1987). Several industrial processes containing multicomponent bismuth molybdates may suffer loss of Mo oxides by volatilization under operating conditions, resulting in the loss of catalytic activity. Monitoring the catalyst microstructure using EM is therefore crucial to ensuring the continuity of these catalytic processes. 

---