Influence of phosphorus on the catalytic properties of V2O5/TiO2 catalysts for toluene oxidation

Abstract

Monolayer-type vanadium catalysts, 2 wt % V on TiO₂(Degussa P25), have been prepared with phosphorus additions of a P/V atom ratio of 0.25–5.0. The catalytic performance in the oxidation of toluene was investigated and correlated with features shown by X.r.d., ESCA and F.t.i.r. studies of adsorbed CO. Spectroscopic and activity results suggest that phosphorus species are highly dispersed on the monolayer vanadium structure at low P/V ratios. Above P/V = 1.25 agglomeration of P–V–O phases occurs, and crystalline vanadium phosphates are detected at a P/V ratio of 5.0. The anatase / rutile ratio changes very little with increasing P/V ratio. Surface areas are constant, except for a decrease at a P/V ratio of 5.0, probably due to P–V–O phases. ESCA data suggest the presence of V⁴⁺ in calcined catalysts at a P/V ratio higher than 1.0, which is supported by the i.r. studies of adsorbed CO. Furthermore, the i.r. studies suggest the formation of V³⁺ in Co-reduced samples without phosphorus additions, whereas V⁴⁺ seems to be stabilized with increasing P/V ratio. Phosphorus shows a strong negative influence on the activity in toluene oxidation. The rate correlates linearly with the surface V/(V + P) atom ratio determined by ESCA. Apparent activation energies do not change, indicating that the negative effect of phosphorus on the activity is dominated by a steric effect. Selectivities also change with phosphorus addition. The selectivity for side-chain oxidation products decreases, and those for carbon oxides and oxidative coupling products increase with increasing P/V ratio. A model for surface P–V–O species is presented and the change in selectivities are suggested to be caused by an increased life time of the initial intermediate, probably the benzylic radical.