Formation of V 2 O 5 -based mixed oxides in flames
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INTRODUCTION Vanadium-based mixed oxides are well-known catalysts for the selective oxidation of hydrocarbons, particularly the oxidation of o-xylene to phthalic anhydride,1 and for the selective reduction of nitrogen oxide with NH 3 . 2 There is general agreement that high activity and high selectivity are achieved when the vanadia are present in the form of highly dispersed, amorphous species. This occurs when the amount of vanadium present corresponds to that necessary to form a bidimensional layer (called a "monolayer") of vanadium oxide on the surface of the support.3"5 With larger amounts of vanadium, the presence of crystalline V 2 O 5 can be detected (crystalline V 2 O 5 is thought to be inactive). The dependence of the activity and selectivity of such catalysts on the composition and phase of the support remains a controversial matter. Titanium and aluminum oxides are the most studied components for use as the vanadium oxide support. Vanadium-based catalysts are usually obtained by wet impregnation of the support with ammonium metavanadate solution,5"11 by mechanical mixing and grinding of the support with V2O5 powders, 1213 or by vapor phase reaction of VOC13 with hydroxylated supports.4-6'14 However, these "traditional" preparation methods require a very high number of steps, i.e., pretreatment of the a)
Present address: Cabot Corp., Cab-O-Sil Division, P.O. Box 188, Tuscola, Illinois 61953-0188. b )Address correspondence to this author. 2404
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J. Mater. Res., Vol. 8, No. 9, Sep 1993
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support, chemical reaction, filtration, purification, and drying of the final product. We recently published two papers15'16 showing that mixed oxide powders can be produced in a single process and in a highly controlled environment by using a flame process. This process allows the formation of powders with precise size, morphology, and crystalline structure. In this paper, we present our initial results of the study of the formation of V 2 O 5 -TiO 2 and V2O5-AI2O3 mixed oxide powders in a counterflow diffusion flame burner. We shall show that the operating conditions can be adjusted to produce powders whose vanadium structure matches that observed in active catalysts. II. EXPERIMENTAL SECTION Flame reactor: A counterflow diffusion flame burner was used to study the formation of the mixed oxide powders (see Fig. 1). Since the advantages and the geometry of this burner have been described repeatedly in detail elsewhere,15'17'18 only a synoptic description is given here. This burner consists of two vertically opposed tubes of rectangular cross section separated by a distance of 15 mm. The fuel (H2, 99.995%, diluted with Ar, 99.998%) flows upward from the lower tube, and the oxidizer (O 2 , 99.6%, diluted with Ar, 99.998%) flows downward from the upper tube. A flame is generated in the region where the two opposed gas streams impinge. This flame is very flat and uniform in the horizontal plane, and thus, accurate measurements of temperature and scattering intensity can be
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