Laser synthesis of vanadium-titanium oxide catalysts
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G. De Michele ENEL-CRTN, Via A. Pisano 120, Pisa, Italy (Received 8 January 1992; accepted 17 June 1992)
A laser-induced process has been studied and developed for the synthesis either of pure TiO2 powders for use as a catalyst support or of mixed Ti^VjOa oxides to be used directly as catalysts in the selective catalytic reduction of nitrogen oxides with ammonia. The radiation source is a 600 W cw CO2 laser and the precursors are titanium and vanadium alkoxides whose vapors are carried under the laser beam by controlled flows of an inert gas and of a reaction sensitizer. The study of the process has been mainly devoted to the optimization of the production rate and of the reaction efficiency. The powder characteristics, related to the process parameters, have been investigated through different techniques. The results show that the particles are monodispersed, unaggregated, monocrystalline with anatase structure and their diameters can be as small as 50 A. Vanadium contents up to X = 0.25 have been obtained in Ti^V^C^ compounds, without V 2 O 5 segregation. Finally, a comparison between the behavior of surface VOX active sites has been carried out in catalysts prepared by chemical impregnation of TiO2 and in the mixed Ti—V oxide powders.
I. INTRODUCTION Vanadium oxides supported on anatase titanium dioxide constitute a well-known heterogeneous catalyst for several industrial processes; in particular, it has been proved to be the most efficient catalyst for the selective catalytic reduction (SCR) of nitrogen oxides with ammonia, today extensively employed for the abatement of NOX emissions from combustion sources. In this system it is well established that the TiO2 support plays an active role on the reactivity of the surface vanadium oxide sites. The nature of the involved interaction, however, has not yet been completely understood, and different interpretations are suggested. As a matter of fact, the experimental results often disagree because the preparation method affects the strength of the specific interaction of V with TiO2 , thus leading to different VOX surface species.1 The conventional preparation method consists of the deposition of vanadium oxides on the surface of TiO2 powders up to achieving one monolayer coverage, generally through chemical impregnation and subsequent calcination. The characteristics of the obtained catalysts, however, strongly depend on the quality of TiO2 support and particularly on its surface area, hydration degree, crystallinity, and porosity. In order to understand better the nature of the different vanadium oxide species, other nonconventional preparation methods have been investigated, such as coprecipitation of Ti and V precursors2'3 or solid state reaction between V2O5 and TiO 2 . 4 The re2846 http://journals.cambridge.org
J. Mater. Res., Vol. 7, No. 10, Oct 1992
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sults of these works suggest that formation of solid solutions of the two oxides can be obtained only in the rutile crystalline phase, whereas surface VOX layers are grown on anatase T
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