ABSTRACT

A pronounced influence of geometry on the pathway of the catalytic reaction is visible in the case of the oxidation of 2-xylene on V205 studied by Gasior and Machej [39]. Figure 4. 24 shows the selectivity for phthalic anhydride and the selectivity for products of total oxidation as a function of the textural factor of V205 crystallites, which is expressed as the ratio of intensities of (001) to (110) reflections. Comparison with the arrangement of V06 octahedra on these crystal faces shown in Fig. 8.13 indicates that high selectivity for phthalic anhydride is observed in the case of samples composed of crystallites exposing mainly the (001) faces with the V=O groups sticking out of the surface. However, when the crystallites expose

20 '---_.L..-_..L-_..L--_.....L...-_--L-_......L.....- -J 0.1 0.2 03 0.4 0.5 0.6 Ip101

"0 '7.01:

FIGURE 4.24 Selectivities in oxidation of 2-xylene on V205 as a function of the textural factor. (After [39].)

mainly the (110) faces, at which the shear planes may be nucleated and whole perpendicular layers of oxygen may be extracted, total oxidation becomes the predominant reaction pathway. A general conclusion may thus be forlnulated that in compounds of those transition elements, in which the phenomenon of the displacement stabilization results in the strong anisotropy of properties, differences in surface and catalytic properties of different crystal faces may be encountered.