Dynamic Investigation of the Role of the Surface Sulfates in NO_x Reduction and SO₂ Oxidation over V₂O₅−WO₃/TiO₂ Catalysts

Abstract

Transient experiments performed over synthesized and commercial V2O5−WO3/TiO2 catalysts during catalyst conditioning and during step changes of the operating variables (SO2 inlet concentration and temperature) show that conditioning of the catalyst is required to attain significant and reproducible steady-state data in both the reduction of NO x and the oxidation of SO2 . The response time of conditioning for NO x reduction is of a few hours and that for SO2 oxidation is of several hours. Fourier transform infrared spectroscopy temperature programmed decomposition, and thermogravimetric measurements showed that catalyst conditioning is associated with a slow process of buildup of sulfates: the different characteristic conditioning times observed in the reduction of NO x and in the oxidation of SO2 suggest that the buildup of sulfates occurs first at the vanadyl sites and later on at the exposed titania surface. Formation of sulfates at or near the vanadyl sites increases the reactivity in the de-NO x reaction, possibly due to the increase in the Brønsted and Lewis acidity of the catalyst, whereas the titania surface acts as SO3 acceptor and affects the outlet SO3 concentration during catalyst conditioning for the SO2 oxidation reaction. The response time to step changes in SO2 concentration and temperature is of a few hours in the case of SO2 oxidation and much shorter in the case of NO x reduction. The different time responses associated with conditioning and with step changes in the settings of the operating variables have been rationalized in terms of the different extent of perturbation of the sulfate coverage experienced by the catalyst.

Study of the V2O5-WO3/TiO2 Catalyst Synthesized from Waste Catalyst on Selective Catalytic Reduction of NOx by NH3

“…The FT-IR spectra were taken with KBr pressed disks exposed to the air and the spectrum of the TiO 2 support has been subtracted. Compared with the fresh catalyst, the waste catalyst showed a broad and strong band at 1122 cm −1 , which was assigned to SO 4 2− [10,35], and might result from the deposition of sulfates during operation. After OA leaching, the peaks of sulfates on the spectrum were clearly weakened, indicating that the deposited sulfates can be removed, favoring the improvement of catalytic activity of the resynthesized catalysts.…”

A commercial V2O5-WO3/TiO2 catalyst used at an NH3-SCR deNOx process in an oil-fired power plant: cause of an increase in deNOxing and NH3 oxidation performances at low temperatures

“…Thus, it is believed that a promoting effect similar to that proposed in the literature occurred during the deNO x reaction with our Com-F catalyst containing 0.59% S. In case that the sulfur is SO 2 with concentrations of ppm levels in flue gases, its role in NH 3 -SCR reaction with non-sulfated TiO 2 -supported V 2 O 5 -WO 3 catalysts is shown to be somewhat complicated. In the presence of gas-phase SO 2 in a feed gas stream for the reaction, the formation of sulfates at or near vanadyl sites present in such catalysts occurs until they are saturated and then does on the surface of the TiO 2 , which has been not yet covered by V 2 O 5 and WO 3 [6,22]. All these sulfates could significantly improve deNO x performances of the catalysts, which was probably associated with an increase in the Lewis acidity of the vanadyls [23] as well as with a creation of new acidic sites by the sulfates [6].…”

“…In the presence of gas-phase SO 2 in a feed gas stream for the reaction, the formation of sulfates at or near vanadyl sites present in such catalysts occurs until they are saturated and then does on the surface of the TiO 2 , which has been not yet covered by V 2 O 5 and WO 3 [6,22]. All these sulfates could significantly improve deNO x performances of the catalysts, which was probably associated with an increase in the Lewis acidity of the vanadyls [23] as well as with a creation of new acidic sites by the sulfates [6]. However, such a positive effect of the sulfation of vanadyl sites on deNO x reaction could be observed only at surface V 2 O 5 coverages up to half a monolayer that requires approximately [22,24].…”

“…(1); however, for an on-site operation of commercial NH 3 -SCR processes, actual NH 3 amounts injected into flue gas streams to which they are applied are somewhat less than the theoretical value to avoid downstream problems, representatively, severe corrosion and pressure drop by ammonium salts that can be produced from a reaction between SO 3 and residual NH 3 , so-called NH 3 slip, when exceeding approximately 5 ppm, and that are deposited primarily onto an air preheater [5,6].…”

SnO2 modified Ce–Ti–Ox catalyst for the selective catalytic reduction of NOx with NH3

“…The commercial catalysts for this process are V 2 O 5 -WO 3 /TiO 2 and/or V 2 O 5 -MoO 3 /TiO 2 [1]. However, some inevitable disadvantages exist in the corresponding catalyst system, such as too low catalytic activity at low temperature, toxicity of the active V 2 O 5 and rapid decrease of the SCR activity at high temperatures [2][3][4]. Therefore, great efforts have been made by more groups to develop new catalysts.…”