Metal oxides multi component interaction

It is important to select stoichiometric co-reductants or co-oxidants for the reversible cycle of a catalyst. A metallic co-reductant is ultimately converted to the corresponding metal salt in a higher oxidation state, which may work as a Lewis acid. Taking these interactions into account, the requisite catalytic system can be attained through multi-component interactions. Stereoselectivity should also be controlled, from synthetic points of view. The stereoselective and/or stereospecific transformations depend on the intermediary structure. The potential interaction and structural control permit efficient and selective methods in synthetic radical reactions. This chapter describes the construction of the catalytic system for one-electron reduction reactions represented by the pinacol coupling reaction. 

The multi-component systems developed quite recently have allowed the efficient metal-catalyzed stereoselective reactions with synthetic potential [75-77]. Multi-components including a catalyst, a co-reductant, and additives cooperate with each other to construct the catalytic systems for efficient reduction. It is essential that the active catalyst is effectively regenerated by redox interaction with the co-reductant. The selection of the co-reductant is important. The oxidized form of the co-reductant should not interfere with, but assist the reduction reaction or at least, be tolerant under the conditions. Additives, which are considered to contribute to the redox cycle directly, possibly facilitate the electron transfer and liberate the catalyst from the reaction adduct. Co-reductants like Al, Zn, and Mg are used in the catalytic reactions, but from the viewpoint of green chemistry, an electron source should be environmentally harmonious, such as H2. 

Physical adsorption is one of the important ways for the precise characterization of the surface structure of the catalyst. In the chemisorption, the interactive force between adsorption molecule and the solid surface is of the chemical affinity, which makes the chemical bond form between the adsorbed molecule and the solid surface. In general, they form covalent bond or coordinated bond containing enough parts of ion-bond on the metal surface, and obviously ionic bond on the surface of semiconductor oxide as well as some compounds, so chemisorption has significant selectivity. By the use of the selectivity of chemisorption, the surface area of metal components and the munber of active sites in the multi-component catalyst and supported catalyst can be measured. Thus a lot of useful information can be achieved. 

A variety of metal catalysts are commonly used with their oxides as precursors. The oxide may undergo chemical reaction with support, and reactions may take place between the components in multi-component metal catalysts during preparation process. Thus, TPR peaks of each oxide will be different from its pure oxides. In other words, the interaction between metal components and support or between metal components can be studied by TPR method for metal catalysts. The sensitivity is so high that it can detect the reduction reaction with consumption of only 10- mol H2. 

Background. The formalism presented in the previous section for predicting the stability of oxide surfaces in equilibrium with a multi-component gas phase is readily extended to systems that contain catalytic metal particles supported on oxide surfaces. Identifying stable particle-support constructions is indispensable for predicting the catalytic activity of the particle-support interface. This section will outline studies on reducible oxides (Ti02 and Ce02) that display unique particle-support interactions where the oxide support plays an active role in the catalytic mechanism. These examples demonstrate the ability of ab initio thermodynamics to determine the stability of metal clusters on oxide supports under realistic catalytic conditions. Such calculations can be used in concert with DFT reactivity studies... 

The interaction of nitric oxide (NO) with metal ions in zeolites has been one of the major subjects in catalysis and environmental science and the first topic was concerned with NO adsorbed on zeolites. NO is an odd-electron molecule with one unpaired electron and can be used here as a paramagnetic probe to characterize the catalytic activity. In the first topic focus was on a mono NO-Na" complex formed in a Na -LTA type zeolite. The experimental ESR spectrum was characterized by a large -tensor anisotropy. By means of multi-frequency ESR spectroscopies the g tensor components could be well resolved. The N and Na hyperfine tensor components were accurately evaluated by ENDOR spectroscopy. Based on these experimentally obtained ESR parameters the electronic and geometrical structures of the NO-Na complex were discussed. In addition to the mono NO-Na complex the triplet state (NO)2 bi-radical is formed in the zeolite and dominates the ESR spectrum at higher NO concentration. The structure of the bi-radicai was discussed based on the ESR parameters derived from the X- and Q-band spectra. Furthermore the dynamical ESR studies on nitrogen dioxides (NO2) on various zeolites were briefly presented. 

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