Commercial SCR Catalyst Research Articles

Evaluation of V 2O 5–WO 3–TiO 2 and alternative SCR catalysts in the abatement of VOCs

Two different commercial SCR catalysts belonging to the V 2O 5–WO 3–TiO 2 system, and different alternative catalysts based on Mn, Fe, Cr, Al and Ti oxides have been tested in the conversion of VOCs in excess oxygen in a temperature range typical of the SCR process (500–700 K). Propane, propene, isopropanol, acetone, 2-chloropropane and 1,2-dichlorobenzene have been fed with excess oxygen and helium. The industrial catalysts are poorly active in the conversion of propane, giving mainly rise to propene by oxy-dehydrogenation. The conversion of propene is higher with CO as the predominant product. In any case, the oxidation activity depends on the vanadium content of the catalyst. Isopropanol is mainly converted into acetone and propene, while acetone is burnt predominantly to CO. Mn- and Fe- containing systems are definitely more active in the conversion of hydrocarbons and oxygenates, giving rise almost exclusively to CO 2. 2-Chloropropane is selectively dehydrochlorinated to propene and HCl starting from 350 K, propene being later burnt to CO on the industrial V 2O 5–WO 3–TiO 2 catalysts, whose combustion activity is, apparently, not affected by chlorine. On the contrary, chlorine strongly affects the behavior of Mn-based catalysts, that are active in the dehydrochlorination of 2-chloropropane, but are simultaneously deactivated with respect to their combustion catalytic activity. The conversion of 1,2-dichlorobenzene gives rise to important amounts of heavy products in our experimental conditions with relatively high reactant concentration.

Characterization and Reactivity of V2O5–MoO3/TiO2 De-NOx SCR Catalysts

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 Brønsted sites. The addition of Mo and V causes the formation of Brønsted 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.

Kinetics of the Selective Reduction of NO with NH3 over a V2O5(WO3)/TiO2 Commercial SCR Catalyst

In order to clarify the mechanism of the selective catalytic reduction of nitric oxide with ammonia over a V2O5(WO3)/TiO2 commercial SCR catalyst, measurements were made on the reaction rate, rNO, as a function of partial pressure of nitric oxide, PNO, partial pressure of ammonia, PNH3, and partial pressure of oxygen, PO2, from 513 to 553 K under steady-state conditions. The adsorption of NO and NH3 on the catalyst was also observed by infrared spectroscopy (DRIFT). The apparent reaction orders with respect to NO were observed to be less than unity, 0.6–0.8. The reaction rate was nearly independent on PNH3 at lower temperatures. As temperature increased, rNO became slightly increased with increasing PNH3 at lower partial pressures of ammonia and tended to be saturated with further increases of PNH3. The dependence of rNO on PO2 was similar to that of PNH3: rNO increased with increasing PO2 at lower partial pressures of oxygen and was saturated with further increase of PO2. The spectroscopic study showed that NO does not adsorb significantly on the oxidized nor on the NH3 preadsorbed surface of catalysts above at least 473 K. The SCR reaction was considered to proceed as follows. NH3 adsorbed on the Brønsted acid sites as ammonium ions. Ammonium ions were activated with the terminal oxygen groups, V5+=O, prior to the reaction with gaseous NO. Subsequent reaction with NO produced N2, H2O, and the hydroxyl groups bonded to the reduced vanadium, V4+−OH, which would be reoxidized by oxygen to the V5+=O species. The Brønsted acid sites where NH3 adsorbed were then recreated. The results obtained in this study suggested that the Brønsted acid sites and/or the V5+=O species were equilibrated with the other species on the surface, implying that the number of each site changed with the experimental conditions such as PO2. The relative amount of the V5+=O species would vary from ∼0.1 to ∼0.4 with increasing PO2.