3884
Ind. Eng. Chem. Res. 1996, 35, 3884-3892
Steady-State and Transient Reactivity Study of TiO2-Supported V2O5-WO3 De-NOx Catalysts: Relevance of the Vanadium-Tungsten Interaction on the Catalytic Activity Luca Lietti,† Pio Forzatti,†,* and Fiorenzo Bregani‡ Dipartimento di Chimica Industriale ed Ingegneria Chimica “G. Natta”, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italy, and Centro Ricerche Ambiente e Materiali, ENEL-DSR, Via Monfalcone 15, 20132 Milano, Italy
The reactivity of ternary V2O5-WO3/TiO2 de-NOx catalysts (V2O5 ) 0-1.47% w/w, WO3 ) 0-9% w/w) in the selective catalytic reduction (SCR) reaction is investigated under steady-state and transient conditions. The results indicate that over the investigated catalysts the SCR reaction occurs via a redox mechanism and that the rate-determining step of the reaction is the catalyst reoxidation process. The reactivity of the V2O5-WO3/TiO2 catalysts increases on increasing either the V2O5 or the WO3 loading; the reactivity of V and/or W in the ternary catalysts is higher than that of the corresponding binary samples. A synergism between the TiO2-supported V and W surface oxide species in the SCR reaction is suggested, that is exploited in the enhancement of the catalyst redox properties of the samples. Accordingly, tungsta increases the rate of the SCR reaction of V2O5/TiO2 catalysts by favoring the catalyst reoxidation by gasphase oxygen. Introduction Catalytic systems constituted by V2O5 and WO3 dispersed over a TiO2-anatase support are widely used in the selective catalytic reduction (SCR) of NOx with NH3 in SO2-containing power plant stack gases (Bosch and Janssen, 1988; Forzatti and Lietti, 1996). V2O5 is very active in the reduction of NOx by NH3 (Went et al., 1992; Svachula et al., 1993a; Schneider et al., 1994; Marshneva et al., 1995) but also in the undesired oxidation reactions of SO2 (Svachula et al., 1993b) and of NH3 (Ozkan et al., 1993). WO3 presents a much lower activity in both denitrification and SO2 oxidation reactions and is used to preserve the structural and morphological characteristics of TiO2-anatase upon addition of vanadia (Alemany et al., 1995). For these reasons, in typical commercial catalyst formulations, the V2O5 content is low (
