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TiO2-supported V2O5–MoO3 catalysts, having V and Mo loadings representative of commercial SCR catalysts, are considered in this study. These catalysts are constituted by TiO2 anatase supporting the V and Mo active components. MoO3 acts as a “structural” promoter preventing the TiO2 matrix from sintering upon vanadia addition. The Mo and V oxide are present on the catalyst surface in the form of molybdenylic and vanadylic species, respectively, and the presence of polymeric MoxOy species cannot be excluded. The features of the V and Mo surface oxide species closely resemble those observed over the binary V2O5/TiO2 and MoO3/TiO2 catalysts, thus pointing out the vibrational independence of the V and Mo surface vanadyl and molybdenyl oxide species. However, in spite of their structural and vibrational independence, the presence of electronic interactions between the TiO2-supported V and Mo oxides is also apparent. These interactions may operate via the TiO2 support or may involve mixed V–Mo surface oxide species who were, however, not observed. The catalyst surface is characterized by strong acidity, probed by NH3-TPD and FT-IR. Ammonia is coordinatively held over Lewis acid sites (associated with Ti, V, and Mo surface cation species) and is protonated as NH4+ ions over Mo–OH or V–OH Bronsted sites. The addition of Mo and V causes the formation of Bronsted sites and of stronger Lewis acid sites, if compared to TiO2. The V2O5–MoO3/TiO2 catalysts are very active in the reduction of NO by NH3, and exhibit a higher reactivity with respect to the corresponding binary V2O5/TiO2 and MoO3/TiO2 samples. Calculations show that the reactivity of V and/or Mo in the ternary catalysts is higher than that measured over V2O5/TiO2 and MoO3/TiO2 having the same V and Mo loading: hence it is suggested that a synergism operates in the SCR reaction between the V and Mo surface oxide species. Accordingly in these catalysts molybdenum also acts as a “chemical” promoter for the SCR reaction. On the basis of the characterization data, it is suggested that the observed synergism in the SCR reaction is related to the existence of the V–Mo electronic interactions. This picture closely resembles that obtained in the case of the analogous V2O5–WO3/TiO2 system and indicates that the effects of the addition of WO3 and MoO3 to V2O5/TiO2 are similar, both oxides acting as “chemical” promoters besides playing a “structural” function as well. However the V2O5–MoO3/TiO2 samples show higher formation of N2O and lower NO conversions at high temperatures: these differences are possibly associated with the different electronic characteristics of Mo compared to W and to their higher reactivity in the ammonia oxidation reactions. It is found that water addition in the feed improves the catalyst performance in that it preserves high NO conversions and high N2 selectivities at high temperatures: this is due to its strong inhibiting effect on the ammonia oxidation reactions occurring simultaneously with the SCR reactions. |
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