Vacancies metal oxide catalysts

For a large group of metal oxide catalysts, it has been proved that a redox mechanism occurs, as originally proposed by Mars and van Krevelen [204], The oxidation of the hydrocarbons, methanol, etc. is effected by oxygen supplied by the catalyst, very often by oxygen contained in the crystal lattice a vacancy results which is reoxidized by oxygen from the gas phase, viz. 

Some of the oxides of vanadium and molybdenum catalyze the selective oxidation of hydrocarbons to produce valuable chemical intermediates. In a reaction path proposed by Mars and van Krevelen (see Section 10.5), the hydrocarbon first reduces the surface of the metal oxide catalyst by reaction with lattice oxygen atoms. The resulting surface vacancies are subsequently re-oxidized by gaseous O2. The elementary steps of this process are shown below. Electrons are added to the sequence to illustrate the redox nature of this reaction. 

To finish with another trend for NO removal consisting in NO direct decomposition, we would like to depict the infrared study of NO adsorption and decomposition over basic lanthanum oxide La203 [78], In this case, the basic oxygens are proposed to lead to N02 and N03 spectator species, whereas the active sites for effective NO decomposition are described as anion vacancies, which are often present in transition metal oxides. This last work makes the transition with the study of DeNO, catalysts from the point of view of their ability to transfer electrons, i.e. their redox properties. 

In the case of oxide catalysts or alkali metal-doped oxide catalysts, basic surface sites can be generated by decarboxylation of a surface metal carbonate exchange of hydroxyl hydrogen ions by electropositive cations thermal dehydroxylation of the catalyst surface condensation of alkali metal particles on the surface and reaction of an alkali metal with an anion vacancy (AV) to give centers (e.g., Na + AV — Na + e ). 

The results that CS planes (which eliminate anion vacancies in supersaturation by shearing and collapsing the lattice) are detrimental to catalysis are also consistent with me fact mat if the catalyst structure continues to collapse to form CS planes, after a period of time me catalyst is no longer an efficient oxidation catalyst. An efficient catalyst is essential for prolonged catalytic activity. This has led to me discovery of a novel glide shear mechanism (Gai et al 1995, Gai 1997). The role of mis mechanism in mixed-metal practical (commercial) catalysts will be examined when we discuss butane oxidation technology. 

The migration of oxide ion is mainly accelerated by lattice vacancies. Knowledge of how to introduce such vacancies into the catalyst by the substitution of part of a component metal cation for another one having a different valence is widely realized for the preparation of excellent industrial oxidation catalysts. 

More complex Bi-Mo oxide catalysts are claimed in patents, such as Bi-Mo-M +-M +-M+-X-Y-0, where M+ is an alkali metal, X=Sb, W, and Y=P, B, and so on. In multicomponent catalysts, the metal molybdates M +MoOd and M2 +(Mo04)3 serve as supports for the active phase, a -Bi2M03Oi2. Bulk migration of ions through lattice vacancies plays an important role in enhancing catalytic... 

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