Defects metal oxide catalysts

Before getting over-excited about the importance of the electronic stmcture of the metal oxide catalyst it is worth noting the other mechanisms that may affect the kinetics of MgH2 such as the metal oxide catalysts acting as a milling aid, their high defect density, size and surface area effect, crystal structure and availability of sites for OH groups. It is therefore a complex system to which there is a temptation to oversimplify. 

Defect sites present in the oxide are not covered by metal particles, as in a conventional metal/oxide catalyst. The oxide/metal system is an attractive model to investigate the role of the oxide in a catalytic process, and also can be used to study reaction mechanisms... 

Active catalyst sites can consist of a wide variety of species. Major examples are coordination complexes of transition metals, proton acceptors or donors in a solution, and defects at the surface of a metallic, oxidic, or sulphidic catalyst. Chemisorption is one of the most important techniques in catalyst characterization (Overbury et al., 1975 Bartley et al, 1988 Scholten et at, 1985 Van Delft et al, 1985 Weast, 1973 and Bastein et al., 1987), and, as a consequence, it plays an essential role in catalyst design, production and process development. 

Oxidative catalysis over metal oxides yields mainly HC1 and C02. Catalysts such as V203 and Cr203 have been used with some success.49 50 In recent years, nanoscale MgO and CaO prepared by a modified aerogel/hypercritical drying procedure (abbreviated as AP-CaO) and AP-MgO, were found to be superior to conventionally prepared (henceforth denoted as CP) CP-CaO, CP-MgO, and commercial CaO/MgO catalysts for the dehydrochlorination of several toxic chlorinated substances.51 52 The interaction of 1-chlorobutane with nanocrystalline MgO at 200 to 350°C results in both stoichiometric and catalytic dehydrochlorination of 1-chlorobutane to isomers of butene and simultaneous topochemical conversion of MgO to MgCl2.53-55 The crystallite sizes in these nanoscale materials are of the order of nanometers ( 4 nm). These oxides are efficient due to the presence of high concentration of low coordinated sites, structural defects on their surface, and high-specific-surface area. 

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. 

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... 

Over most reported Au catalysts, CO oxidation takes place at the junction perimeter between Au NPs and the metal oxide supports. Carbon monoxide is adsorbed on the edges, corners and steps of Au NPs. Molecular oxygen is adsorbed on the support surfaces and may be activated at the oxygen defect sites at the perimeter interfaces, where the two adsorbates react to form C02 in the gas phase. At the perimeter interfaces Au is assumed to exist as Au3 +, which might be stabilized through bonding with OH-. 

More than 90% of the natural metallic elements of the periodic table form perovskites the wide range of cations, the possibility of partial substitution of A or B cation sites, and the remarkable capacity to accommodate a multitude of different kinds of defects result in a wealth of properties of these solids leading to applications ranging from superconductors (33) to oxidation catalysts (34). 

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