Electronegativity metal oxide catalysts

Fig. 4.34 Correlation between the selectivity of allylic oxidation of propene over metal oxide catalysts and the electronegativity of the metal ion. ...

Table IV presents the results of the determination of polyethylene radioactivity after the decomposition of the active bonds in one-component catalysts by methanol, labeled in different positions. In the case of TiCU (169) and the catalyst Cr -CjHsU/SiCU (8, 140) in the initial state the insertion of tritium of the alcohol hydroxyl group into the polymer corresponds to the expected polarization of the metal-carbon bond determined by the difference in electronegativity of these elements. The decomposition of active bonds in this case seems to follow the scheme (25) (see Section V). But in the case of the chromium oxide catalyst and the catalyst obtained by hydrogen reduction of the supported chromium ir-allyl complexes (ir-allyl ligands being removed from the active center) (140) C14 of the...

In this chapter, we have discussed the application of metal oxides as catalysts. Metal oxides display a wide range of properties, from metallic to semiconductor to insulator. Because of the compositional variability and more localized electronic structures than metals, the presence of defects (such as comers, kinks, steps, and coordinatively unsaturated sites) play a very important role in oxide surface chemistry and hence in catalysis. As described, the catalytic reactions also depend on the surface crystallographic structure. The catalytic properties of the oxide surfaces can be explained in terms of Lewis acidity and basicity. The electronegative oxygen atoms accumulate electrons and act as Lewis bases while the metal cations act as Lewis acids. The important applications of metal oxides as catalysts are in processes such as selective oxidation, hydrogenation, oxidative dehydrogenation, and dehydrochlorination and destructive adsorption of chlorocarbons. 

Solid metal sulphates and phosphates also exhibit acid—base properties their acid strength is lower than that of silica—alumina but they are stronger acids than some oxide catalysts [5]. Correlation of activity with electronegativity of cations has been obtained for several reactions [9, 50,51],... 

Fig. 6.7 Relationship between the ammonia conversion on Ru/M catalysts (667K) and electronegativity of the metal oxide (M)...

In the case of platinum catalysts the addition of some metal oxides has also resulted in enhanced catalytic activity, which has been attributed to the modification of platinum crystallite size, and especially to the modification of the oxidation state of platinum. It has been reported that promoters with large electronegativities, such as molybdenum, vanadium, tungsten and niobium, enhance the catalytic activity compared with the unpromoted Pt/Al203, since more electronegative promoters present a higher resistance to oxidation, and platinum remains less oxidised than those with electropositive promoters, such as alkaline and alkaline-earth metals, which are even less reactive than the unpromoted Pt/Al203 catalyst. ... 

In conclusion, XPS is among the most frequently used techniques in characterizing catalysts. It readily provides the composition of the surface region and also reveals information on both the oxidation state of metals and the electronegativity of any ligands. XPS can also provide insight into the dispersion of particles over supports, vrhich is particularly useful if the more common techniques employed for this purpose, such as electron microscopy or hydrogen chemisorption, can not discriminate between support and active phase. 

P, N, O, S, or C based, which favor covalent bonding and stabilize low oxidation states) due to the metals higher electronegativity and lower oxidation states [24], In recent years, late transition metal catalysts [25-29] have attracted attention not only for the polymerization of a-olefins, but more importantly for the copolymerization of hydrocarbon monomers with readily available polar monomers such as acrylates, vinyl ethers, and vinyl acetate [27 and references therein]. 

This treatment of coordination polymerization with Ziegler type catalysts is independent of whether the growing polymer end is attached to a Group I—III metal or to a transition metal. Both types can be obtained depending upon the alkylating ability of the alkyl metal with respect to the transition metal component. In both cases the ionic character of the M-R bond can vary appreciably with the cation electronegativity, i. e., with the metal, its oxidation state and ligands. The... 

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