Monolayers, vanadium oxide, supported

To overcome the problems encountered in the homogeneous Wacker oxidation of higher alkenes several attempts have been undertaken to develop a gas-phase version of the process. The first heterogeneous catalysts were prepared by the deposition of palladium chloride and copper chloride on support materials, such as zeolite Y [2,3] or active carbon [4]. However, these catalysts all suffered from rapid deactivation. Other authors applied other redox components such as vanadium pentoxide [5,6] or p-benzoquinone [7]. The best results have been achieved with catalysts based on palladium salts deposited on a monolayer of vanadium oxide spread out over a high surface area support material, such as y-alumina [8]. Van der Heide showed that with catalysts consisting of H2PdCU deposited on a monolayer vanadium oxide supported on y-alumina, ethene as well as 1-butene and styrene... 

The surface structure and reactivity of vanadium oxide monolayer catalysts supported on tin oxide were investigated by various physico-chemical characterization techniques. In this study a series of tin oxide supported vanadium oxide catalysts with various vanadia loadings ranging from 0.5 to 6. wt.% have been prepared and were characterized by means of X-ray diffraction, oxygen chemisorption at -78°C, solid state and nuclear magnetic resonance... 

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. 

M. C., Moulijn, J.A., Medema, J., de Beer, V.H.J. and Gellings, P.J. (1980) Vanadium oxide monolayer catalysts. 3. A Raman spectroscopic and temperature-programmed reduction study of monolayer and crystal-type vanadia on various supports. J. Phys. Chem., 84, 2783. 

In the preceding sections the use of catalysts in which vanadium oxides are supported on a more or less inert carrier has been mentioned quite often. Because of the importance of this type of catalyst they are discussed more extensively in this section. Often a distinction is made between the normal supported catalysts and so called monolayer catalysts. In the latter the vanadium oxide is supposed to be dispersed in a monomolecular layer on the support, which may be covered completely or only partly. The normal supported catalysts are usually made by impregnation, either wet or dry, of the porous carrier with an aqueous solution, often of NH4V03, sometimes with oxalate added.12 14,75,95,139,140... 

The spectroscopic study of the nature of vanadium oxide [VOx] supported on a high surface area Ti02 (anatase) indicated the formation of three different VOx structures [48] a) isolated V4+ ions, part of which was coordinatively unsaturated, strongly bonded to the surface hydroxy groups of the support b) bidimensional clusters of VOx with mainly V5+ after calcinations, reducible under mild conditions to V4+ and also to V3+ to some extent (these species weakly interact with the support surface) c) V205 appeared when cove-rage was about the monolayer and was presented as bulk multiplayer structures. The authors observed the existence of at least two different isolated surface V4+ species, which caused splitting of the low-field hfs lines in parallel orientation. 

FIGURE 11 TPR-Raman spectra of silica-supported vanadium oxide near monolayer coverage (0.8 V atoms/nm2) (source, M.A. Banares). 

V—0-Support (930cm ) and V—O—V (625 cm ) bonds. Similar distributions of monomeric and polymeric surface VO4 species are found on other oxide supports with the exception of Si02 [30]. For the supported V20s/Si02 catalyst system, only isolated surface VO4 species are present below the maximum dispersion limit (<3 V atoms/nm ). For all supported vanadium oxide catalysts, crystalline V2O5 NPs are also present above the monolayer surface coverage or maximum dispersion limit [31]. 

Figure 11.3 Number of exposed redox catalytic active sites for supported V2O5/AI2O3 catalysts as a function of vanadium oxide loading. Monolayer coverage corresponds to the 20% V2O5/AI2O3 (-7.9V atoms/nm ) catalyst sample.

The preparation and characterization of titania-supported vanadia catalysts have been reviewed by Bond and Flamerz Tahir and provide a guide to the literature on preparation, structure and catalytic properties of vanadium oxide monolayer catalysts [5]. Preparative methods such as grafting, heating mechanical mixtures, or coprecipitation are also discussed. 

Partial Oxidation of Ethane over Monolayers of Vanadium Oxide. Effect of the Support and Surface Coverage. 

The nature of supported oxides and of the support plays a critical role in the partial oxidation of hydrocarbons since the support is not only providing a high surface area, but also dispersing the oxide. The interaction between the metal oxide overlayer and the imderlying support similarly determines the performance of the catalyst, which may also be affected by the exposed sites of the support. To fully understand these effects, a series of supported vanadium oxide catalysts at monolayer and submonolayer coverage have been prepared. The monolayer coverage was determined hy Raman spectroscopy and X -ray photoelectron spectroscopy. The activity of the supported vanadium oxide catalysts is determined by the specific support and surface vanadia coverage. 

Figure 3. Selectivity conversion plots for the monolayers of vanadium oxide on the different supports. Reaction conditions Total flow 30 mL/min. Reaction temperature 740-883 K. W = 20 mg. O2/C2H6 = 2 molar and He/02 = 4 molar.

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