Solid-State NMR of Oxidation Catalysts

Detailed studies were conducted by infrared, TPD, XPS but also by more sophisticated techniques such as CP-MAS solid state NMR or EXAFS, on the various steps by which molybdenum can be deposited on alumina supports starting from [Mo(CO)e]. Indeed, thin films of molybdenum or of its oxides have wide application as gas sensors or solar cell catalysts. 

Solid-state NMR spectra of V20s/Sn02 catalysts and pure V2O5 alone are shown in Figure 2. Two types of distinct signals, with varying intensities depending on the vanadia content on tin oxide support, are the main features of these spectra. Unsupported and crystalline pure V2 Os exhibits a line with an axial anisotropy of the chemical shift tensor ( = -310 ppm and 6 = - 1270 ppm) with small peaks due to... 

The technique of solid-state NMR used to characterize supported vanadium oxide catalysts has been recently identified as a powerful tool (22, 23). NMR is well suited for the structural analysis of disordered systems, such as the two-dimensional surface vanadium-oxygen complexes to be present on the surfaces, since only the local environment of the nucleus under study is probed by this method. The nucleus is very amenable to solid-state NMR investigations, because of its natural abundance (99.76%) and favourable relaxation characteristics. A good amount of work has already been reported on this technique (19, 20, 22, 23). Similarly, the development of MAS technique has made H NMR an another powerful tool for characterizing Br 6nsted acidity of zeolites and related catalysts. In addition to the structural information provided by this method direct proportionality of the signal intensity to the number of contributing nuclei makes it a very useful technique for quantitative studies. 

Monolayer coverage of vanadium oxide on tin oxide support was determined by a simple method of low temperature oxygen chemisorption and was supported by solid-state NMR and ESR techniques. These results clearly indicate the completion of a monolayer formation at about 3.2 wt.% V2O5 on tin oxide support (30 m g" surface area). The oxygen uptake capacity of the catalysts directly correlates with their catalytic activity for the partial oxidation of methanol confirming that the sites responsible for oxygen chemisorption and oxidation activity are one and the same. The monolayer catalysts are the best partial oxidation catalysts. 

This chapter focuses on the application of solid-state NMR techniques for the characterization of oxidation catalysts. Initially, a brief introduction to these techniques is provided (Section 5.2), within which methods suitable for the study of both bulk structure (Section 5.2.1) and surface characteristics (Section 5.2.2), are described. Examples of the application of these techniques are then provided in Section 5.3, for bulk oxides, and Section 5.4, for surface properties. Finally, Section 5.5 provides an outlook as to future directions in this area. 

Metal oxides are widely used as catalyst supports. For instance, a-Al203 is employed as a support for catalysts in the partial oxidation of ethylene to ethylene oxide, because a non-reactive material is essential for such applications [141]. However, aluminas are also important catalysts in their own right. Transition aluminas are known to catalyze the isomerization of alkenes, the dehydration of alcohols, H/D exchange reactions and C—H bond activation [142]. Consequently, the development of an understanding of both their bulk and their surface structure has been a key goal in catalysis, with solid-state NMR being widely employed to this end. 

In addition to aluminas, other oxides such as Si02, Sn02 and Ga203 are also employed as catalyst supports. As these materials also contain NMR-active nuclei, such as Sn or Ga, it is also possible to apply solid-state NMR to their characterization. In general the study of such oxides is not as well developed as that of Al NMR. However, recent technical and theoretical advances have led to an increasing interest in these materials. 

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