Single metal oxide catalysts

In situ dynamic ETEM studies in controlled environments of oxide catalysts permit direct observations of redox pathways under catalytic reaction conditions and provide a better fundamental understanding of the nucleation, growth and the nature of defects at the catalyst surface and their role in catalysis (Gai 1981-1982 92). The following paragraphs describe the methods of observation and quantitative analyses of the surface and microstmctural changes of the catalyst, and correlation of microstmctural data with measurements of catalytic reactivity. We examine examples of pure shear and crystallographic (CS) shear defects that occur under catalytic conditions. 

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Meta/ Oxides. The metal oxides aie defined as oxides of the metals occurring in Groups 3—12 (IIIB to IIB) of the Periodic Table. These oxides, characterized by high electron mobiUty and the positive oxidation state of the metal, ate generally less active as catalysts than are the supported nobel metals, but the oxides are somewhat more resistant to poisoning. The most active single-metal oxide catalysts for complete oxidation of a variety of oxidation reactions are usually found to be the oxides of the first-tow transition metals, V, Cr, Mn, Fe, Co, Ni, and Cu. 

When single metallic oxide catalysts such as magnesium oxide supported on wood charcoal are used at a temperature of 420° to 430° C., a mixture of butanol, ethyl acetate, and aldehyde is obtained from ethanol. When manganese carbonate or zinc oxide supported on wood charcoal is used at 450°, ethanol decomposes into only butanol and aldehyde.65... 

C. J. Heyes, J. G. Irwin, H, A. Johnson, and R.L. Moss, "The Catalytic Oxidation of Organic Air Pollutants. Part 1. Single Metal Oxide Catalysts", J. Chem. Tech. Biotechnol.. 1982,22, 1025-1033. 

The role of defects in heterogeneous catalysis, including the role of CS planes which form near the catalyst operating temperature in single metal oxides such... 

Isolation and identification of surface-bonded acetone enolate on Ni(l 11) surfaces show that metal enolate complexes are key intermediates in carbon-carbon bond-forming reactions in both organometaUic chemistry and heterogeneous catalysis. Based on studies on powdered samples of defined surface structure and composition, most of the results were reported for acetone condensation over transition-metal oxide catalysts, as surface intermediate in industrially important processes. With the exception of a preoxidized silver surface, all other metal single-crystal surfaces have suggested that the main adsorption occurs via oxygen lone-pair electrons or di-a bonding of both the carbonyl C and O atoms. 

Promotion of catalyst nanoparticles, electrochemical promotion (NEMCA) of porous and of single-crystal catalyst films, and metal nanoparticle-support interactions are three, at a first glance, independent phenomena that can all dramatically affect catalytic activity and selectivity on metal and metal oxide catalyst surfaces. 

The light-off curves for acetone combustion over the catalysts based on the single Mn and Co oxides are presented in Fig. 1, and the corresponding values of the temperature T50 at which the acetone conversion reaches 50% are included in Table 1. It is clear from these results that the behaviour of the alumina-supported single metal oxides contrasts strongly with that of the silica-supported ones. The performance of Mn/Si-823 is very similar to the one of Co/Si-823 in accordance with the tendency of the respective unsupported Mn and Co oxides for this reaction... 

Mixed metal oxides are used quite often in industrial partial oxidation reactions, examples being Bi Oj-MoOj for the oxidation of propene to acrolein and V O -MoOj for oxidation of benzene to maleic anhydride. Some mixed oxides also are quite active deep oxidation catalysts, a good example being MnOj-CuO. The difficulties in understanding mixed oxides are of course more formidable than they are for single metal oxides. It is a well-established empirical fact that mixed oxides behave quite differently than as individual oxides in most catalytic reactions. This situation is further complicated by the often dramatic effect of promoters, such as alkali metal oxides that are added to the catalyst intentionally. 

One of the most recent design approaches for the identification of effective partial oxidation catalysts has considered the activation of reactant and product molecules with catalytically active metal oxide materials (442,443). It is clear that the most effective selective oxidation catalysts are based on metal oxide systems, and it is by this precedent that the activation of methane, oxygen, and methanol has been examined on single metal oxides. The aim of the approach is to search for reactivity factors for catalyst components that when combined in two-component metal oxide catalysts will lead to catalyst formulations with a high degree of synergy for selective methane oxidation. The approach involves the identification of components that... 

Watanabe, N. and Ueda, W. (2006). Comparative Study on the Catalytic Performance of Single-Phase Mo-V-O-Based Metal Oxide Catalysts in Propane Ammoxidation to Acrylonitrile, Ind. Eng. Chem. Res., 45, pp. 607-614. 

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