Catalysis mixed oxides

Transition-metal mixed oxides active in combustion catalysis have been prepared by two main procedures i) classical coprecipitation / calcination procedures starting from metal nitrates and/ or alkoxides ii) preparation based on the supercritical drying of gels prepared from organic complexes (alkoxides, acetylacetonates or acetates), producing aerogels . Details on the second preparation can be found in Ref. 13. 

For the sake of simplicity, a 0eO2—Zr02 (70/30) mixed oxide will be now used as material. This mixed oxide has been previously shown to be able to proceed to three-way catalysis, the general concept for N2 formation over a metal cation being the same NO decomposition and oxygen species scavenging, in stoichiometric conditions, by CO as reductant [10,11],... 

Fernandez-Garcia, M., Fuerte, A., Hemandez-Alonso, M.D., Soria, J., and Martinez-Arias, A. (2007) Platinization of sunlight active Ti-W mixed oxide photocatalysts. Journal of Catalysis,... 

Centi, G. Perathoner, S. Vazzana, F., Catalysis using guest single and mixed oxides in host zeolite matrices, in Catalysis by Unique Metal Ion Structures in Solid Matrices, Centi, G. Bell, A.T. Wichterlova, B. (eds.), Springer-Verlag. Dordrecht (The Netherlands), 2001,165-186. 

Heterogeneous catalysis is clearly a complex phenomenon to understand at the molecular level. Any catalytic transformation occurs through a sequence of elementary steps, any one of which may be rate controlling under different conditions of gas phase composition, pressure, or temperature. Furthermore, these elementary processes occur catalytically on surfaces that are usually poorly understood, particularly for mixed oxide catalysts. Even on metallic catalysts the reaction environment may produce surface compounds such as carbides, oxides, or sulfides which greatly modify... 

Zhang, RD Alamdari, H Kaliaguine, S. Crystal structure, redox properties and catalytic performance of Ga-based mixed oxides for NO reduction by CsHg, Catalysis Communications, 2008, Volume 9, 111-116. 

Zeolite catalysts may also be regarded as mixed oxides, but the crystallographic structures differ from the solids discussed above in that active sites for catalysis occur within the open lattice framework. In consequence, rate data are not directly comparable with similar observations for other heterogeneous reactions since the preexponential factors are calculated and reported on a different basis. For completeness, however, it is appropriate to mention here that instances of compensation behavior on zeolite catalysts are known. Taylor and Walker (282) described such an effect for the decomposition reactions of formic acid and of methyl forma te on cation-exchanged 13X molecular sieves, and comparable trends may be found in data reported for reactions of propene on similar catalysts (283). 

Ethane ODH catalysis by mixed oxides of nickel and iron has been evaluated by both the gas sensor and an FID gas chromatograph. Fig. 8.11(a) shows the C2H4... 

Hoang, et a1. (2003), Redox Behaviour of La-Cr Compounds Formed in Cr0x/La203 Mixed Oxides and Cr0x/La203/Zr02 Catalysts , Applied Catalysis A General, 239, 95-110. 

Once the multi-step reaction sequence is properly chosen, the bifunctional catalytic system has to be defined and prepared. The most widely diffused heterogeneous bifunctional catalysts are obtained by associating redox sites with acid-base sites. However, in some cases, a unique site may catalyse both redox and acid successive reaction steps. It is worth noting that the number of examples of bifunctional catalysis carried out on microporous or mesoporous molecular sieves is not so large in the open and patent literature. Indeed, whenever it is possible and mainly in industrial patents, amorphous porous inorganic oxides (e.g. j -AEOi, SiC>2 gels or mixed oxides) are preferred to zeolite or zeotype materials because of their better commercial availability, their lower cost (especially with respect to ordered mesoporous materials) and their better accessibility to bulky reactant fine chemicals (especially when zeolitic materials are used). Nevertheless, in some cases, as it will be shown, the use of ordered and well-structured molecular sieves leads to unique performances. 

Baca M, Pigamo A, Dubois JL, Millet JMM. Propane oxidation on MoVTeNbO mixed oxide catalysts Study of the phase composition of active and selective catalysts. Topics in Catalysis. 2003 23(l-4) 39 46. 

Tu X, Furuta N, Sumida Y, Takahashi M, Niiduma H. A new approach to the preparation of MoVNbTe mixed oxide catalysts for the oxidation of propane to acrylic acid. Catalysis Today. 2006 117(l-3) 259-264. 

Beato P, Blume A, Girgsdies F, Jentoft RE, Schlogl R, Timpe O, Trunschke A, Weinberg G, Basher Q, Hamid FA, Hamid SBA, Omar E, Mohd Salim L. Analysis of structural transformations during the synthesis of a MoVTeNb mixed oxide catalyst. Applied Catalysis, A General. 2006 307(1) 137-147. 

Various kinds of oxide materials, including single oxides, mixed oxides, molybdates, heteropoly-ions, clays, and zeolites, are used in catalysis they can be amorphous or crystalline, acid or basic. Furthermore the oxides can be the actual catalysts or they can act as supports on which the active catalysts have been deposited. Silica and alumina are commonly used to support both metals and other metal oxide species. Amorphous silica/alumina is a solid acid catalyst, it is also used as a support for metals, when bifunctional (acid and metal) catalysis is required, e.g., in the cracking of hydrocarbons. Other acid catalysts are those obtained by the deposition of a soluble acid on an inert support, such as phosphoric acid on silica (SPA, used in the alkylation of benzene to cumene. Section 5.2.3). They show similar properties to those of the soluble parent acids, while allowing easier handling and fixed bed operation in commercial units. 

Thus, in ammonia synthesis, mixed oxide base catalysts allowed new progress towards operating conditions (lower pressure) approaching optimal thermodynamic conditions. Catalytic systems of the same type, with high weight productivity, achieved a decrease of up to 35 per cent in the size of the reactor for the synthesis of acrylonitrile by ammoxidation. Also worth mentioning is the vast development enjoyed as catalysis by artificial zeolites (molecular sieves). Their use as a precious metal support, or as a substitute for conventional silico-aluminaies. led to catalytic systems with much higher activity and selectivity in aromatic hydrocarbon conversion processes (xylene isomerization, toluene dismutation), in benzene alkylation, and even in the oxychlorination of ethane to vinyl chloride. 

Surface Vanadate, Molybdate and Tungstate Species The pure and mixed oxides and the salts of vanadium, molybdenum and tungsten in their higher oxidation states are used widely as heterogeneous catalysts, for selective oxidation as well as for acid catalysis. Similarly, supported chromia and rhenium oxides find wide application in different catalytic processes. 

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