Catalysis by metal oxides

One feature of oxides is drat, like all substances, they contain point defects which are most usually found on the cation lattice as interstitial ions, vacancies or ions with a higher charge than dre bulk of the cations, refened to as positive holes because their effect of oxygen partial pressure on dre electrical conductivity is dre opposite of that on free electron conductivity. The interstitial ions are usually considered to have a lower valency than the normal lattice ions, e.g. Zn+ interstitial ions in the zinc oxide ZnO structure. 

An effect which is frequently encountered in oxide catalysts is that of promoters on the activity. An example of this is the small addition of lidrium oxide, Li20 which promotes, or increases, the catalytic activity of dre alkaline earth oxide BaO. Although little is known about the exact role of lithium on the surface structure of BaO, it would seem plausible that this effect is due to the introduction of more oxygen vacancies on the surface. This effect is well known in the chemistry of solid oxides. For example, the addition of lithium oxide to nickel oxide, in which a solid solution is formed, causes an increase in the concentration of dre major point defect which is the Ni + ion. Since the valency of dre cation in dre alkaline earth oxides can only take the value two the incorporation of lithium oxide in solid solution can only lead to oxygen vacaircy formation. Schematic equations for the two processes are 

The reaction shown above for the steam reforming of methatie led to die formation of a mixture of CO and H2, die so-called synthesis gas. The mixture was given this name since it can be used for the preparation of a large number of organic species with the use of an appropriate catalyst. The simplest example of this is the coupling reaction in which medrane is converted to ethane. The process occurs by the dissociative adsorption of methane on the catalyst, followed by the coupling of two methyl radicals to form ethane, which is then desorbed into the gas phase. 

A closer analysis of die equilibrium products of the 1 1 mixture of methane and steam shows the presence of hydrocarbons as minor constituents. Experimental results for die coupling reaction show that the yield of hydrocarbons is dependent on the redox properties of the oxide catalyst, and the oxygen potential of the gas phase, as well as die temperamre and total pressure. In any substantial oxygen mole fraction in the gas, the predominant reaction is the formation of CO and the coupling reaction is a minor one. 

The reaction of CH4 with hydrogen, at the odrer end of die oxidation scale, produces mainly acetylene, C2H2, edrylene C2H4 and ethane, C2H6. These reactions are favoured by operating at high temperatures. In fact the production of acetylene is most efficient if die gas mixmre is passed drrough an arc struck between carbon electrodes, which probably produces a reaction temperature in excess of 2500 K. 

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Catalysis by Metal Oxides and Zeolites. Metal oxides are common catalyst supports and catalysts. Some metal oxides alone are industrial catalysts an example is the y-Al202 used for ethanol dehydration to give ethylene. But these simple oxides are the exception mixed metal oxides are more... 

Mark A. Barteau is Robert L. Pigford Professor and Chair of the Department of Chemical Engineering at the University of Delaware. He received his B.S. degree from Washington University in 1976 and his M.S. (1977) and Ph.D. (1981) from Stanford University. His research area is chemical engineering with specialized interests in application of surface techniques to reactions on nonmetals, hydrocarbon and oxygenate chemistry on metals and metal oxides, scanning probe microscopies, and catalysis by metal oxides. 

Busca, G. (1996) The use of vibrational spectroscopies in studies of heterogeneous catalysis by metal oxides an introduction. Catal. Today, 27, 323-352. 

Guerrrero-Perez, M.O., and Banares, M.A., Topics Catal. (2004) Special issue on the Symposium on Catalysis by Metal Oxides, Philadelphia, August 2004. 

Cluster Compounds Inorganometallic Compounds Containing Transition Metal Main Group Elements Heterogeneous Catalysis by Metals Oxidation Catalysis by Transition Metal Complexes Polyoxometalates Tungsten Organometallic Chemistry. Tungsten Proteins. 

The Use of Infrared Spectroscopic Methods in the Field of Heterogeneous Catalysis by Metal Oxides... 

The initiation step in the classic mechanism for the pyrolysis of ethane involves the formation of methyl radicals, but of more relevance to catalysis by metal oxides is the role of O ions in generating these radicals. Bohme and Fehsenfeld [Ref. 11] have shown that the reaction... 

Reductive processes utilizing carboxyhc acid derivatives also provide routes to ketenes. These include the conversion of biomass to useful oils with catalysis by metal oxides or zeolites, which imphcate the possible role of ketene formation in the conversion of esters, as su ested by the necessity of a-hydrogens in the ester substrates (Eqn (4.16)). An outline of the process of dehydration of the acid to form the ketene on the metal surface is shown in Figure 4.7. 

Busca, G. (2009). Use of infiared spectroscopy methods in the field of heterogeneous catalysis by metal oxides, in S. Jackson and J. Hargreaves (eds.). Metal Oxide Catalysis, Vol. 1, WUey-VCH, New York, pp. 95-175. 

G. Busca, in Metal Oxide Catalysis, ed. by S.D. Jackson, J.S.J. Hargreaves. The Use of Infrared Spectroscopy Methods in the Field of Heterogeneous Catalysis by Metal Oxide, vol 1 (Wiley-VCH, Weinheim, 2009), p. 95... 

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