Te oxide

For Se and Te, oxidation of the adatom takes place at potentials higher than that of CO oxidation. The adatom is always in its reduced state, and no bifunctional catalysis through the transfer of oxygen from the adatom to the CO molecule can take place. 

Zhiznevskii et al. chose another group of catalysts for the production of methacrolein, based on the solid solution systems Mo—Te oxides [110,360] and Sb—Te oxides [361] which were investigated in a pulse reactor at 340°C. Selectivities of 70—90% are reported. The Mo—Te catalyst (1 4) has a unique dependence of the activity upon the degree of reduction of the catalyst surface. Initial reduction to 6% increases activity, but further reduction lowered this again. The selectivity changes in a similar manner. X-Ray investigations led the authors to the conclusion that some Te—Mo—P-phase, which is formed with reduction but decomposed if the reduction is taken too far, is responsible. Combinations of Sb2Os and TeOz can likewise be selective catalysts. The relatively low activity at 400°C is much improved by the addition of molybdenum (60 mol.%). 

Dibenzotellurophene-5,5-dichloride and Chloramine-T combine to form Te-tosyl-iminodibenzolellurophene, which is very easily hydrolyzed to dibenzotellurophene Te-oxide and reacts with 2,2 -dilithiobiphenyl to give bis[2,2 -biphenyldiyl] tellurium2. 

Even though the two-stage process recommends itself for the flexibility it offers for the optimization of each oxidation step, the single-pass option is attractive as it requires lower investment for fewer reactors and other equipment. The catalysts claimed in the patents are generally multicomponent metal oxides based on Mo with Te as the promoter. However the yields are lower (50-60%) and deactivation faster owing to the volatility of Te oxide. 

Figure 4 Selectivity changes of acrylic acid (circle), propene (triangle), and further oxidation products (lozenge) as a function of contact time in the oxidation of propane over Mo-V-Te oxide (open symbol) and Mo-V-Te-Nb oxide (closed symbol) catalysts...

Figure 4.51. Crystal structure schematic of Co02-based TE oxides. Shown are (a) NaxCo02, (t>) Ca3Co409, and (c) Bi2Sr2Co203 . Reproduced with permission from Koumoto, K. Terasaki, L ...

The results concerning the influence of the rotation speed on Hg0.79Cdo.2]Te oxidation are presented in table I. 

The aim of the present study was to prepare mixed Mo-V-Nb-Te oxides using a surfactant route, and to check in which extend modification of the composition and of the atmosphere in which these oxides are activated may lead to a phase corresponding to active catalysts for ammoxidation of propane. 

Bhatnagar D, Ehrlich KC, Cleveland TE. Oxidation and reduction reactions in biosynthesis of secondary metabolites. Bhatnagar D, Liilehoj EB, Arora OK, eds. Handbook of Applied Mycology, Vot. 5 Mycotoxins in Ecological Systems. New York Marcel Dekker, 1992 255-285. 

Allyl is fromed from propylene by a-hydrogen abstraction by the Te " " oxide moiety (59). 

Mixed composite particles have been prepared by simultaneous vaporization of two or more metals. In this case beam splitters are used to focus the laser on the different targets. Figure 11 displays SEM micrographs obtained for the mixed Si/Al oxide. The particles appear to have a uniform size and shape. Energy dispersed analysis (shown in Figure 12) indicates that particles, within an area of 1 ivcfi, contain both Si and A1 with a composition that is identical throughout the sample. Other examples of composite particles prepared by this method include Si/B oxide, Mg/Fe oxide and Cd/Te oxide. 

In this chapter, we have tried to give an overview on the emerging field of thermoelectric (TE) oxides. Dealing with this class of materials, the first important conclusion lies in the physics difference between the p- and n-type TE oxides. As a large part of that research has been devoted to the p-type NaxCo02 layer cobaltates and derived phases, one major output lies in the important role played by the electronic correlations. Usually the physics of conventional TEs is based on models of degenerate semiconductors and thus strongly correlated materials have opened new perspectives for the search of TE materials. 

Finally, the possibility to create artificial superlattices to control the thickness of the conducting layer in order to take advantage of the S increase with the structure dimensionality decrease is also a promising route to enhance the figure of merit of oxides. This should help the solid state scientist in designing bulk TE oxides to use in TEG of the future. 

---