| The NH3-selective catalytic reduction (NH3-SCR), combined with the combustion optimization, is one of the most promising technical routes to meet the future emission regulation. It is generally accepted that the most significant problem of the NH3-SCR to reduce NOx is that the NOx removal efficiency decreases dramatically at low exhaust temperatures (< 250℃),that the active reaction temperature window is too narrow (300-400℃), and that the N2 selectivity deteriorates at the temperature above 400℃. Therefore, in this paper, two solutions are proposed:①The novel self-propagating high-temperature synthesis (SHS) method is applied to synthesize a series of nanocomposites catalysts TiMO2-δ. We make progress in improving the low temperature activity and broadening the active temperature window.②We use the non-thermal plasma facilitated NH3-SCR hybrid system to improve the NOx removal efficiency of the commercial vanadium-based catalyst V2O5-MoO3/TiO2 at low temperatures. The main results and conclusions in this work are summarized as follows:(1) By comparing the performance of the TiVO2-δcatalyst prepared by the SHS method and that of the V2O5/TiO2 catalyst prepared by the conventional impregnation method, we find that TiVO2-δhas high activity at low temperatures with more than 90% NOx reduced in the range of 170-390℃and excellent N2 selectivity (> 95%) above 350℃. The catalysts with the active component of Mn, V, Ce, and Fe have high activity. According to the principle of complementary advantages, we prepare the binary compound metal oxide catalysts Ti0.9M0.05N0.05O2-δby the doping method, and find that the Mn doped catalysts of Ti0.9Mn0.05V0.05O2-δ and Ti0.9Mn0.05Fe0.05O2-δare highly active at low temperatures (< 150℃). The N2 selectivity at high temperature is significantly improved by doping 5% Ce into Ti0.9V0.1O2-δcatalyst. The Ti0.9Ce0.05Fe0.05O2-δcatalyst substituted by Ce is more active than Ti0.9Ce0.1O2-δand Ti0.9Fe0.1O2-δ.(2) By comparing the performance of catalysts with tetrabutyl titanate and titanium isopropoxide as Ti precursor, we find that the NOx removal efficiency of the former is better than that of the later, while the selectivity is reverse. By comparing the performance of TixMn0.5-0.5xV0.5-0.5xO2-δand TixCe0.5-0.5xV0.5-0.5xO2-δ(x = 0.85, 0.9 or 0.95) series catalysts, we also find that the NOx removal efficiency of Ti0.9Mn0.05V0.05O2-δ and Ti0.9Ce0.05V0.05O2-δ are obviously higher than that of others.(3) We investigate the effect of the ignition temperature in the combustion synthesis on the catalysts performance. The NOx removal efficiency and the N2 selectivity decrease with the ignition temperature increasing, and the N2O selectivity increases as the ignition temperature increases. The higher ignition temperature improves the crystallized degrees, increases the particle size and average pores diameter, decreases the specific surface area, the BJH desorption pore volume, the surface fractal dimension, and the Br?nsted acid sites, and as a result, decreases the apparent SCR activity. The concentration of surface OA (the chemisorbed oxygen) and the ratio of OA / (OA + OL) (OL: the lattice oxygen) decrease dramatically with the ignition temperature increasing, which may be one of the main reasons for the decrease of the N2 selectivity. The surface key active nitrate species and the amount of NH3 desorption decrease continually as the ignition temperature increases, which is the key factor for the deterioration of the catalytic performance.(4) The TiCeVO2-δcatalyst prepared by the SHS method has robust H2O and SO2 tolerance. The results suggest that the poisoning effect of H2O is reversible, while the poisoning effect of SO2 is partially reversible. The competition adsorption between NH3 and NO, the formation of ammonium nitrate and sulfate salts over the catalyst are the main reasons for the catalyst poisoning, and the later can be heat-treated to regenerate.(5) Through the investigation of the surface adsorption, desorption, and SCR reaction over the catalysts using in situ DRIFTS, we find that the reductant NH3 can absorb quickly and strongly on the SHS catalyst surface. The adsorbed species of NH3 are mainly the coordinated NH3 bound to Lewis acid sites and the ionic NH4+ bound to the Br?nsted acid sites. The adsorbed species of NO adsorption are monodentate nitrate, bidentate nitrate, and bridging nitrate. The presence of O2 can greatly promote the formation of these nitrate species. The adsorption capability of NH3 is stronger than that of NO; the ionic NH4+, the monodentate nitrate, and the bridging nitrate are highly active, which can quickly participate in the SCR reaction, and the reaction rate increases with the increasing reaction temperature. However, the coordinated NH3 and the bidentate nitrate are relatively inactive and can not participate in the SCR reaction. Moreover, they occupy the active sites over the catalyst surface, which is not beneficial to the NH3-SCR reaction. The adsoption and active of NH3 is the initial step for the SCR reaction; the oxidation of NO to the weak adsorption NO2, and the formation of the monodentate nitrate and bridging nitrate are the rate-controlling steps of the SCR reaction. Within a relatively low temperature range (T < 250℃), the NH3-SCR process over Ti0.9Ce0.05V0.05O2-δ mainly follows the L-H (Langmuir-Hinshelwood) mechanism, while in a relatively high temperature range (T > 250℃), it mainly follows the E-R (Eley-Rideal) mechanism.(6) Non-thermal plasma facilitated NH3-SCR hybrid system is used to reduce NOx, we find that the energy density has a significant effect on the NOx removal efficiency at low reaction temperatures (< 250℃), and the effect almost disappears at higher temperatures in the range of 300-450℃. Meanwhile, the effects of the O2 content, the C3H6 concentration, and the SO2 concentration as well as the temperature of the DBD reactor are investigated. The NOx removal efficiency of the hybrid system increases dramatically with the increasing O2 content and decreases when the O2 content exceeds 10%. Therefore, there is an optimal O2 content at a certain energy density. The oxidation efficiency of NO to NO2 is dramatically promoted by the addition of C3H6, thus improving the reaction of SCR at low temperatures. It is discovered that the intermediate products produced by C3H6 and O radicals can further facilitate the NOx reduction. The NOx removal efficiency decreases with the increasing SO2 concentration at the temperatures below 200℃, and increases slightly by SO2 at higher temperatures above 250℃. The deterioration effect of SO2 at low temperatures may come from the instable sulfite ion, which can react with the chemisorbed oxygen to form sulfate and lead to the decrease of the active site of the catalyst, and thus the activity of the system. The promoted effect of SO2 at high temperatures mat attributed to the formation of SO42- on the catalyst surface, which can increase the catalyst surface acidity. The oxidation process for nonthermal plasma is selectivity, and the rate for the NO oxidation to NO2 reaction is much larger compared to that for the SO2 oxidation to SO3.(7) The formation of byproducts like HCHO and CH3CHO in the non-thermal plasma facilitated NH3-SCR hybrid system is also investigated. The concentration of HCHO and CH3CHO increases as the energy density as well as the C3H6 addition increases, and decreases with the reaction temperatures increasing. HCHO and CH3CHO are almost completely removed when the feed gas is processed in the catalytic reactor, i.e., the concentration of HCHO and CH3CHO at the outlet of the plasma catalytic reactor is only a few ppm, indicating that HCHO and CH3CHO can make a difference to the NOx removal in the NH3-SCR reactions.(8) For the reaction mechanism of the synthetic effect of the non-thermal plasma facilitated NH3-SCR hybrid system, the addition of C3H6 changes the oxidation reaction mechanism significantly. C3H6 is easily decomposed by being combined with O radical to produce CH3 and C2H5 radicals, which can rapidly combine with O2 to form the peroxyl radicals CH3O2 and C2H5O2, and the hydro-peroxyl radical HO2. Theses radicals are strong oxidizing radicals capable of oxidizing NO to NO2 efficiently, thus dramatically improving the NOx removal efficiency of NH3-SCR at low temperatures below 250℃. Additionally, a great number of active radicals such as OH, HO2, and RO2 produced during the plasma discharge process, and the partial oxidation products of C3H6 like aldehydes, nitrogen-containing organic compounds, and other excited state CxHyOz radicals and intermediated species, may all contribute to further facilitate the efficiency of the activity of the NH3-SCR system.
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