Supported vanadium oxide, catalyst for

The reactivity of the supported vanadium oxide catalysts for other oxidation reactions also show similar trends as the oxide support is varied from titania to silica [13]. The activity and selectivity for partial oxidation products of vanadium oxide supported on titania being higher than vanadium oxide supported on silica. The oxidation activity of the supported vanadium oxide catalysts is related to the ability to donate oxygen to form the required oxidation products. The... 

In experiments run over a number of cycles, the activity was observed to increase after the first cycle, unlike the y-A Os counterpart which deactivated. Using BN, no Pt sintering occurred and this was ascribed to the high thermal conductivity of BN, ensuring that no local hot-spots were formed. On the basis of XPS, the locus of Pt particle attachment was proposed to be surface boron oxide impurities. Taylor and Pollard have compared the activities of silica (194 m g ) and boron nitride (7 m g ) supported vanadium oxide catalysts for propane oxidation. The use of boron nitride was reported to significantly... 

T. Shikada and K. Fujimoto, "Effect of Added Alkali Salts on the Activities of Supported Vanadium Oxide Catalysts for Nitric Oxide Reduction", Chem. Lett., 1983,77-80. 

Liu, J., Zhao, Z., Xu, C., etal. (2010). Ce02-supported Vanadium Oxide Catalysts for Soot Oxidation The Roles of Molecular Structure and Nanometer Effect, J. Rare Earths, 28, pp. 198-204. 

Luciani S., Ballarini N., Cavani R, et al (2009). Anatase-supported vanadium oxide catalysts for o-Xylene oxidation Rrom consolidated certainties to unexpected effects, Catal. Today, 142, pp. 132-137. 

Cortez, G., Fierro, J. and Banares, M. (2003). Role of Potassium on the Structure and Activity of Alumina-Supported Vanadium Oxide Catalysts for Propane Oxidative Dehydrogenation, Catal. Today, 78, pp. 219-228. 

The vanadium oxide species is formed on the surface of the oxide support during the preparation of supported vanadium oxide catalysts. This is evident by the consumption of surface hydroxyls (OH) [5] and the structural transformation of the supported metal oxide phase that takes place during hydration-dehydration studies and chemisorption of reactant gas molecules [6]. Recently, a number of studies have shown that the structure of the surface vanadium oxide species depends on the specific conditions that they are observed under. For example, under ambient conditions the surface of the oxide supports possesses a thin layer of moisture which provides an aqueous environment of a certain pH at point of zero charge (pH at pzc) for the surface vanadium oxide species and controls the structure of the vanadium oxide phase [7]. Under reaction conditions (300-500 C), moisture desorbs from the surface of the oxide support and the vanadium oxide species is forced to directly interact with the oxide support which results in a different structure [8]. These structural... 

Table II. The TON and selectivity to formaldehyde for the methanol oxidation reaction on various 1% supported vanadium oxide catalysts...

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. 

Analysis of structure-activity relationships shows that various species characterized by different reactivities exist on the surface of vanadium oxide-based catalysts.339 The redox cycle between V5+ and V4+ is generally accepted to play a key role in the reaction mechanism, although opposite relationships between activity and selectivity, and reducibility were established. More recent studies with zirconia-supported vanadium oxide catalysts showed that vanadium is present in the form of isolated vanadyl species or oligomeric vanadates depending on the loading.345,346 The maximum catalytic activity was observed for catalysts with vanadia content of 3-5 mol% for which highly dispersed polyvanadate species are dominant. 

Figure 9.15 Comparison of selectivity to propene obtained by cyclic feed (redox mode) (open symbols) and co-feed (filled symbols), for supported vanadium oxide catalysts [76m]. V205-Al203 (0, ), V205-Ti02 (O, ), V205-Si02 ( , ), V205-Ti02/Al203 (A, A).

The structure-activity relationships characteristic of supported vanadium oxide catalysts have been evaluated for S02 oxidation to S03 by Wachs and coworkers (Dunn et al., 1998, Dunn et al., 1999). It was demonstrated for individual supported vanadium oxide catalysts that the oxygen of the bridging V-0-Msupport bond was involved in the ratedetermining step of S02 oxidation (Dunn et al., 1998). The specific reaction rate (TOF) was found to be the same for both isolated and polymeric surface vanadia species, and depended only on the oxide support changes in the support led to dramatic changes in the TOF (Dunn et al., 1998). 

Recent studies of supported vanadium oxide catalysts have revealed that the vanadium oxide component is present as a two-dimensional metal oxide overlayer on oxide supports (1). These surface vanadium oxide species are more selective than bulk, crystalline V2O5 for the partial oxidation of hydrocarbons (2). The molecular structures of the surface vanadium oxide species, however, have not been resolved (1,3,4). A characterization technique that has provided important information and insight into the molecular structures of surface metal oxide species is Raman spectroscopy (2,5). The molecular structures of metal oxides can be determined from Raman spectroscopy through the use of group theory, polarization data, and comparison of the... 

In the present investigation, the interaction ot vanadium oxide with dif-ferent alumina phases (7, 6-6, and a) is examined with Raman spectroscopy. Comparison ot the Raman spectra ot the supported vanadium oxide catalysts with those obtained -from vanadium oxide reference compounds allows for the structural assignment of these supported species. The present Raman data demonstrate that the molecular structures of the surface vanadium oxide phases are significantly influenced by the presence of surface impurities on the alumina supports and this overshadows the influence, if any, of the alumina substrate phase. 

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]. 

The values of n have been determined for a number of oxidation reactions over supported vanadium oxide catalysts and are given in Table 11.1. 

Although this chapter focuses on oxidation reactions involving redox supported vanadium oxide catalysts, similar trends with surface coverage and specific oxide support also apply for other redox supported transition metal oxide catalysts, such as supported M0O3 [51], CrOs [52] and Re207 [53], The redox supported vanadium oxide catalytic system was chosen for this review because of the extensive studies that these catalysts have received in recent years as well as their widespread industrial appHcations. 

Infrared spectrum of pyridine adsorbed on undoped alumina-supported vanadium oxide catalyst, after evacuation at 150 °C, shows an absorption band at 1450 cm 1, characteristic for pyridine retained on Lewis acid sites, which has been related to V-free alumina [4,10, 11]. The intensities of the bands at 1450 cm l (related to Lewis acid sites) and 1545 cm I (related to Bronsted acid sites) have been used to determine the numer of Lewis and Bronsted acid sites on the surface of catalysts. The results are outlined in Table 1. 

Figure 5 shows the variation of the catalytic activity for the oxidation of propane at 500°C with the reducibility of V-based catalysts, determined as the inverse of the temperature of maximum H2-consumption (l/TJ. A parallelism between the reducibiiity of catalyst and the catalytic activity for propane conversion similar to those observed on supported vanadium oxide catalysts [1, 10] can be proposed. 

For the preparation of aliphatic nitriles, mainly acetonitrile, 2-4 carbon olefins have been reacted over a supported vanadium oxide catalyst in a... 

Oxidation of toluene with large excess of air over an alumina- (Alfrax-) supported vanadium oxide catalyst has been claimed to be an effective method for benzoic acid production. Thus, the use of air toluene weight ratios of 39-49 1, temperatures on the order of 410-430 C, and contact times of 0.25-0.75 sec are claimed to result in yields of benzoic acid of about 34 per cent with corresponding maleic acid yields of 7-11 per cent based on toluene consumed in the process. 

Supported vanadium oxide catalysts have some activity for the olefin metathesis reaction but are not very selective (Banks 1969). In combination with Rc207 a very active catalyst system is obtained with alumina as the support (Nakamura, R. 1977b Xu, X. 1985b). When V2O5/AI2O3 is preheated at about 575°C and treated with Me4Sn it forms an effective catalyst system (V/Al = 3/97 Sn/V = 0.05-0.08) for propene metathesis at 25°C (Ahn, H-G. 1993). 

Gao X, Wachs IE (2000) Investigation of surface structures of supported vanadium oxide catalysts by UV-vis-NIR diffuse reflectance spectroscopy. J Phys Chem B 104 1261-1268 Gas kov A, Rumyantseva M (2009) Metal oxide nanocomposites Synthesis and characterization in relation with gas sensing phenomena. In Baraton MI (ed) Sensors for Environment, Health and Security. Springer Science + Business Media B.V., Dordrecht, the Netherlands, pp 3-29... 

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