Redox vanadia catalysts

Promoters. - Many supported vanadia catalysts also possess secondary metal oxides additives that act as promoters (enhance the reaction rate or improve product selectivity). Some of the typical additives that are found in supported metal oxide catalysts are oxides of W, Nb, Si, P, etc. These secondary metal oxide additives are generally not redox sites and usually possess Lewis and Bronsted acidity.50 Similar to the surface vanadia species, these promoters preferentially anchor to the oxide substrate, below monolayer coverage, to form two-dimensional surface metal oxide species. This is schematically shown in Figure 4. 

In addition to the structure in the dehydrated state, the structure of supported vanadia catalysts under redox reaction conditions is directly related to the catalytic performance. Vanadia catalysts are usually reduced to some extent during a redox reaction, and the reduced vanadia species have been proposed as the active sites [4, 19-24]. Therefore, information on the valence state and molecular structure of the reduced vanadia catalysts is of great interest. A number of techniques have been applied to investigate the reduction of supported vanadia catalysts, such as temperature programmed reduction (TPR) [25-27], X-ray photoelectron spectroscopy (XPS) [21], electron spin resonance (ESR) [22], UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS) [18, 28-32], X-ray absorption fine structure spectroscopy (XAFS) [11] and Raman spectroscopy [5, 26, 33-41]. Most of these techniques give information only on the oxidation state of vanadium species. Although Raman spectroscopy is a powerful tool for characterization of the molecular structure of supported vanadia [4, 29, 42], it has been very difficult to detect reduced supported... 

The physicochemical properties of potassium-, bismuth-, phosphorous- and molybdenum-doped (MeA7 atomic ratios of 0 to 1) V2O5/Y-AI2O3 catalysts and their catalytic behavior in the oxidative dehydrogenation of propane have been compared. The incorporation of metal oxides modifies the catalytic behavior of alumina-supported vanadia catalysts by changing both their redox and their acid-base properties. In this way, the addition of potassium leads to the best increase in the selectivity to propylene. This performance can be related to the modification of the acid character of the surface of the catalysts. The possible role of both redox and acid-base properties of catalysts on the selectivity to propylene during the oxidation of propane is also discussed. 

It has been suggested that the Incorporation of alkali metals on Ti02-vanadia catalysts decreases both the V=0 stretching frequencies and their polarizing power, while the incorporation of acid anions produces an opposite trend [20]. In addition, the presence of alkali ions decreases the heat of the propylene adsorption [17,18, 21]. Thus the different catalytic behavior of doped alumina supported vanadia catalysts, could be explained on the bases of the influence of the acid-base character of catalysts on the adsorption/desorption of propane and propene. In any case, the redox properties must be also considered. In this way, it will be interesting to study if, realy, a lower reducibility of the active sites could favor a lower rate of the consecutive reactions, as it has been observed in the case of K-doped catalysts. 

Scheme 7.H Proposed redox mechanism over Mg loaded vanadia catalyst [133]...

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. 

Redox properties of the catalysts can be determined, on the contrary, by submitting them to TPR (Thermal Programmed Reduction) with hydrogen (see for example Fig.l). Reduced catalysts can be reoxidized with pulses of oxygen so determining also the catalyst dispersion. These determinations have been made for vanadium based catalysts. It is very interesting to observe, for example, that clusters of vanadia of different sizes, corresponding to different dispersion indexes, can show very different redox properties, as it can be seen in Table 4 and Fig. 1. 

Two tetrahedral complexes and one octahedral complex of vanadia species were found when vanadium and titanium were deposited simultaneously. Tetrahedral complexes are less active in redox reactions. All complexes differ from those found on the V2O5 on Si02 and the V2O5 on Ti02 catalysts. 

The large-scale production of sulfuric acid, overall one of the most important products in the chemical industry, exploits the redox activity of vanadium(V) oxide. Vanadia-, molybdena-, and tin oxide-based catalysts are used further for a host of selective hydrocarbon oxidation processes including dehydrogenation, oxidative coupling, and oxygenation (3,41. Much recent activity in this area has been aimed at the development of more active and more selective catalysts by dispersing oxides in the form of monolayers on bulk oxide supports. This approach has proven extremely successful for tailoring catalyst properties to... 

Hydrides on the palladium are continually removed as water by oxygen present no redox component is necessary. However, other oxidizing components were included in heterogeneous catalysts for the uncommercialized oxidation (Pd-phosphomolybdate) and oxycyanation (olefin/HCN/02 over Pd-vanadia) of ethylene to acetaldehyde (ethanal)/acetic (ethanoic) acid and acrylonitrile (propenonitrile) respectively. 

In the 1990s and the first part of the 2000s, noble metal and mixed oxide-based catalysts were mostly investigated, with interest first focused on alternative materials, especially metal oxides and protonic and metal zeohtes. More recently, efforts have been focused on formulations with enhanced redox properties and oxygen storage capacity, such as combinations of ceria, zirconia, titania and vanadia. Noble metals (Pt, Pd) show higher specific activity when they are prepared in a highly... 

Notably, catalysts with redox properties, such as molybdenum-, chromium-, and vanadia-based catalysts, show high activity in various oxidative dehydrogenation reactions of hydrocarbons [45 8]. Factors influencing the reaction also include acid-base bifunctionality, which plays an important role in CO2-mediated dehydrogenation reactions [49]. Both basic sites and Lewis-acid vacant sites are important for hydrocarbons activation [50]. In fact, an enhanced basicity results in an improved performance because of the rapid desorption of the electron-rich alkenes, whereas Lewis acid sites enhance the dehydrogenation process [51]. In addition, in the presence of CO2 as feed, surface basicity favors the adsorption and reactivity of the acid CO2 molecules [52] (see also previous chapters). 

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