Density vanadia catalysts

The turnover frequencies (TOP), defined as the number of methanol molecules converted to formaldehyde per surface vanadia site per second, are presented for the different supported vanadia catalysts, at monolayer coverages, in Table 2. There is a dramatic variation in the TOFs with the specific oxide support and the variation spans approximately three orders of magnitude at monolayer coverages (the same surface density of surface vanadia species). The TOFs were also relatively independent of surface vanadia coverage... 

The direct way to improve the volumetric activity of monolithic catalysts is to increase its cell density, by which the volumetric surface is tremendously enlarged [15,34], Figure 9.8 shows that an increase in cell density from 300 to 400 cpsi is more effective on the DeNO performance than an increase of the vanadia concentration from 1.9 to 3%, which is, moreover, accompanied by the above mentioned problems. 

Although platinum catalysts once were used in the manufactme of sulfuric acid, the only catalysts presently in use employ supported vanadia. ° For om problem we shall use a catalyst studied by Eklimd, whose work was echoed extensively by Donovan" in his description of the kinetics of SO, oxidation. The catalyst studied by Eklimd was a Reymersholm V2O5 catalyst deposited on a pumice carrier. The cylindrical pellets had a diameter of 8 mm and a length of 8 mm, with a bulk density of 33.8 Ib/ft. Between 818 and 1029°F, the rate law for SO, oxidation over this particular catalyst was... 

Common cell densities used for heavy-duty diesel applications with vanadia SCR catalysts are 300 cpsi [19, 36] and 400 cpsi [6, 18, 35, 37, 40]. For these cell densities, the wall thicknesses for cordierite substrates range typically from 4 mil (100 pm) to 8 mil (200 pm) [18, 19, 37, 41]. In Fig. 3.12, with wall thickness is here considered the total wall thickness resulting from the substrate including the catalyst washcoat. The washcoat thickness for a coated vanadia SCR catalyst depends on the washcoat loading and could range from 20 to 100 pm [37]. For a washcoated SCR catalyst, the wall thickness could of course be reduced either by decreasing the inert substrate wall thickness or reduce the washcoat thickness as long as this does not effect the catalyst performance. 

In ethane ODH, the activity of VjOj catalysts increases with the loading. The rates per V atom reach a maximum on domains of intermediate size. There exists a balance between the aetivity of surface VO species and their accessibility to reaetants. The seleetivity to ethylene reaches its maximum at intermediate values of loading eorresponding to the formation of the monolayer structure. The ethylene formation rate per V atom depends on the vanadia surface density and reaches its maximum value at intermediate surfaee densities [36-37]. Surface oxygen, 0-H groups and especially oxygen vaeaneies are considered as the most prevalent reactive intermediates during ODH on active VO domains [38-39]. Klose et al. observed that the adsorbed O species [40] mainly contribute to formation. 

The reactivity of vanadium oxide in ethane ODH is highly dependent on the type of support [18, 37, 42, 47-51]. The performance of the catalysts depends on the characteristics of the supports and the nature of the active species formed on these supports. At a given surface density, the vanadia-support interactions determine the type of VO structure formed. Banares and co-workers [52-53] found that in ethane ODH, the turnover frequency values vary with the specific oxide support for both the isolated and polymeric surface VO species. Thus, the support has a significant influence on the reaction parameters. The support cation directly affects the reactivity of the bridging V-O-support bond. 

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