Metal-oxide catalysis selected applications

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. 

Lin MM. Complex metal-oxide catalysts for selective oxidation of propane and derivatives. I. Catalysts preparation and application in propane selective oxidation to acrylic acid. Applied Catalysis, A General. 2003 250(2) 305-318. 

Another important application of selective extra absorption is the use of chemical shifts in the various X-ray emission channels. This opportunity yields the possibility of measuring the valence-selective X-ray absorption spectra, both XANES and EXAFS. This technique has been applied successfully to Fe(III) as a test case (Glatzel et al., 2002). At least in principle, many applications of this methodology are possible. A metal will have a different X-ray emission spectrum than an oxide. This difference can be used to measure the X-ray absorption spectra of the metal and the oxide in a mixed metal oxide. Such mixtures are encountered frequently in heterogeneous catalysis. 

A particularly important application of nanoscale catalysis is the oxidation of atmospheric pollutants such as carbon monoxide to less harmful products [47-49]. One challenge has been to find a suitable catalytic material with a sufficient active lifetime to induce the oxidation of CO. Recent findings by Harata and cowoikers danonstrated that nanosized gold particles deposited onto selected metal oxides exhibited high activities for the low-tanperature oxidation of carbon monoxide [9-12]. These studies also established... 

Abstract Immobilized metallic and bimetallic complexes and clusters on oxide or zeolite supports made from well-defined molecular organometaUic precursors have drawn wide attention because of their novel size-dependent properties and their potential applications for catalysis. It is speculated that nearly molecular supported catalysts may combine the high activity and selectivity of homogenous catalysts with the ease of separation and robustness of operation of heterogeneous catalysts. This chapter is a review of the synthesis and physical characterization of metaUic and bimetallic complexes and clusters supported on metal oxides and zeohtes prepared from organometaUic precursors of well-defined molecularity and stoichiometry. 

Strength (FLS) empirical approach are discussed in Section 3 as methods for determining the molecular structures of metal-oxide species from their Raman spectra. The state-of-the-art in Raman instrumentation as well as new instrumental developments are discussed in Section 4. Sampling techniques typically employed in Raman spectroscopy experiments, ambient as well as in situ, are reviewed in Section S. The application of Raman spectroscopy to problems in heterogeneous catalysis (bulk mixed-oxide catalysts, supported metal-oxide catalysts, zeolites, and chemisorption studies) is discussed in depth in Section 6 by selecting a few recent examples from the literature. The future potential of Raman spectroscopy in heterogeneous catalysis is discussed in the fmal section. 

In the last years a great interest was paid to the catalytic properties of iron-containing zeolites that show interesting activities in different industrial reactions. The Fe-BEA zeolite is reported to be a good catalyst in the vapour phase alkylation processes [1], the Fe-TON zeolite shows very high activity and selectivity in the olefin isomerization [2, 3]. Finally, new applications of zeolitic catalysts in the partial oxidation catalysis, such as the Solatia Inc. processes for benzene hydroxylation to phenol using Fe-MFI, open a novel route for the use of zeolites in oxidation processes [4, 5]. On the other hand, the catalytic properties of the metal-modified MOR type zeolite in the isomerization process are well known. 

---