Journal of the Chemical Society, Faraday Transactions 1: Physical Chemistry in Condensed Phases p. 767 - 784 (1986)
Update date:2022-08-11
Topics:
Jonson, Bo
Rebenstorf, Bernd
Larsson Ragnar
Andersson, S. Lars T.
Lundin, Sten T.
Vanadium oxide catalysts based on Al2O3-C (γ-Al2O3) as support contain at least four different forms of vanadium.The distribution between these depends on the vanadium loading.The activity (per gram of vanadium) of these catalysts for toluene oxidation increases with increassing loading.The selectivity for benzene formation decreases from 2percent for the support to 0percent for 0.5 wtpercent V, while the selectivity for benzaldehyde formation first appears at this concentration and rises to 29percent for 10 wtpercent V.It is suggested that benzene is formed at Lewis acid sites on this support, whereas benzaldehyde is formed on vanadium sites.At low loadings (0.1 and 0.2 wtpercent V) single vanadium species with tetrahedral coordination are formed.The oxidised forms have u.v. bands at 35500 and 42000 cm-1 and the reduced forms have i.r. bands of adsorbed CO at 2200 cm-1 (room tmperature) and at 2190 and 2158 cm-1 (133 deg K).At medium loadings, vanadium surface clusters with varying degrees of agglomeration are formed in addition to the other species.These are suggested to be single chain species with vanadium in tetrahedreal coordination, double chain species with vanadium in square pyramidal coordination and aggregates of octahedral vanadium formed by the coupling of double chains.The oxidised forms have u.v. bands at 34500 and 43000 cm-1 and the reduced forms have i.r. bands of adsorbed CO at 2178 cm-1 (room temperature) and at 2178 and 2158 cm-1 (133 deg K).The agglomerates are more active than the isolated species and show some selectivity for benzaldehyde.Both the activity and the selectivity appear to increase with the degree of agglomeration.At high vanadium loadings (10 wtpercent V), surface crystallites of vanadium oxide are formed in addition to the other species.The oxidised forms have u.v. bands at 30000 and 43000 cm-1 and the reduced forms have i.r. bands of adsorbed CO at 2181 cm-1 (room temperature) and at 2181 and 2145 cm-1 (133 deg K).These crystallites are more active and selective than other species with less agglomeration.It is suggested that the increased activity for the larger species is due to possibility of a transition from corner to edge sharing octahedra at the release of oxygen, which increases the activity of the double-bonded oxygen.
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(1986)
