On the morphological properties of tungsta-titania de-NO x ing catalysts
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M. Bellotto CISE, P.O. Box 12081, 20134 Milano, Italy
P. Forzatti Dipartimento di Chimica Industriale e Ingegneria Chimica del Politecnico, Piazza Leonardo da Vinci 32, 1-20133 Milano, Italy
F. Bregani ENEL-CRTN, Via Monfalcone 15, 20100 Milano, Italy (Received 3 February 1992; accepted 2 April 1993)
Tungsta-titania samples with different W loadings up to 15 wt. % and calcined at different temperatures have been prepared and characterized by surface area measurements, mercury porosimetry, x-ray diffraction, microstructural analysis, and laser Raman spectroscopy. It has been found that W inhibits the initial sintering of TiO2 (anatase) and the anatase —• rutile transformation. The morphological and structural properties of the samples (surface area, porosity, and phase composition) have been related to the microscopic properties of the materials such as crystallite dimensions and defects concentration. A model for the sintering of TiO 2 is discussed. This model is based on the diffusion of surface hydroxyls, formed upon adsorption of water on surface oxygen vacancies. A role for W is proposed in terms of stabilization of both material defects and surface hydroxyls.
I. INTRODUCTION The Selective Catalytic Reduction (SCR) of nitrogen oxides by ammonia is widely used for the control of NOj emissions from thermal power plants due to its efficiency, selectivity, and economics.1'2 Commercial catalysts consist of W and V oxides supported on high surface area TiO 2 anatase. The vanadium content controls the activity of the system in the reduction of NO* and also catalyzes the oxidation of SO2 to SO 3 . Under typical de-NO^ing conditions, the formation of NH4HSO4 may take place by reaction of SO 3 , water vapor, and slip ammonia, causing pressure drop and corrosion problems. Accordingly, the vanadium content is generally low, and it is reduced below 1 wt. % in high sulfur applications. Tungsten oxide provides thermal stability to the system, presents a much lower activity in both denitrification and SO 2 oxidation reactions, and is typically used in larger amounts («40 wt. %). Silicates are also added to the catalysts as mechanical promoters. It has been demonstrated that the performances of SCR catalysts strongly depend on the morphology of the catalysts as well as on the composition and structure of the materials.3 In fact, an optimal balance is required between micropores (to sustain the reaction) and macropores (to speed intraparticle gas diffusion). J. Mater. Res., Vol. 8, No. 8, Aug 1993
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Micropores provide for high surface area and, consequently, high potential catalytic activity; parallel macropores guarantee a good intraparticle diffusion of the reagents (NH3 and NO*), increasing the accessible catalyst volume. In this respect it is worth noting that the oxidation of SO 2 is not affected by the catalyst morphology since this reaction is controlled by chemical kinetics. In the current literature many investigations have been published on the characteristics
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