Temperature Dependence Of Selective Catalytic Reduction Of NO_X And Mechanisms Of Its Resistance To SO₂ Induced Catalyst Deactivation

The development of the industry, the progress of the society, and the improvement of people’s living standard inevitably cause the emission of various pollutants into the environment. Among them, nitrogen oxides (NOx) emitted from power plants, engines, waste incinerators and automobiles are one series of the most serious pollutants, which have caused not only the formation of acid rain but also photochemical smog. In fact, NOx have given rise to a variety of increasingly harmful impacts on plant growth and even human health. Strict reduction of NOx emission has become one of the hot research topics all over the world. Selective catalytic reduction (SCR) of NOx by NH3 is undoubtedly one of the most effective methods to decrease the NOx levels in gaseous emissions. The key point for SCR technique is the development of high-performance and cheap catalysts. As a widely used catalyst, the detrimental of vanadium-based catalyst is its high working temperature with a narrow temperature window, high activity for SO2 oxidation, short service life and high toxicity of Vanadia, which stimulated the continuing efforts to develop new effective catalysts which could be used at low reaction temperature. Recently, novel catalysts used Cu, Fe and Ce oxides as active components have attracted much interest because of their remarkable catalytic activity at low temperature. Meanwhile, Zeolites were recently well studied as potential catalyst supports. Up to now, however, Due to relatively complicated circumstance of low temperature SCR compared to the normal high-temperature SCR, the mechanisms for the low-temperature active SCR reaction is still not fully understood and deserve further study. Furthermore, the catalyst deactivation caused by SO2 at low temperature is still a big problem which restricted its application.With these as background, in this study we prepared a series of ZSM-5 zeolite supported metal oxides (CuOx and FeOx) catalysts, and the SCR of NO by NH3 was investigated systematically, mainly we focused on the temperature dependent of reaction mechanism and its SO2 induced catalyst deactivation. At the same time, we also clarified the mechanism of the resistance to SO2 induced deactivation over CuO-CeO2 catalyst at low temperature in NH3-SCR. The main achievements we obtained were listed as follows:(1) Clarification of SCR mechanism and reaction route over Fe-ZSM-5.Ion exchange method was used to prepare a series of Fe-ZSM-5 with different Fe loading. The Fe3+ mainly exchanged the strong acid H+ on zeolite, and existed as isolated ions at low Fe3+ concentration. All the strong acid H+ were exchanged by Fe3+ at a Fe3+ concentration of 2.67 wt%. NO2 were mainly adsorbed on Fe3+ ions, while NH3 was adsorbed on weak acid sites. According to the calculated apparent activation energy, we revealed that at temperature below 250℃, the NOx reduction followed the L-H mechanism through the reaction between adsorbed NO2 and adsorbed NH3. While at reaction above 250℃, the main reaction followed the E-R mechanism through the reaction between the adsorbed NO2 and gaseous NH3.(2) Clarification of temperature dependent SCR reaction mechanism over highly dispersed Fe2O3 loaded on ZSM-5 catalyst.For FexOy clusters heavy loaded on zeolite (FeH-ZSM-5), Fe3+ mainly existed as highly dispersed FexOy clusters, with further increasing of Fe3+ concentration. By changing the preparation process, part of strong acid H+ could be remained on FeH-ZSM-5, which changed its performance for NH3 adsorption. The synergy between Fe2O3 clusters and ZSM-5 also modified the NOx adsorptivity of FeH-ZSM-5, thus the SCR reaction routes changed accordingly. Based on the calculated apparent activation energy, at temperature below 200℃, the NOx reduction followed L-H mechanism through the reaction between adsorbed NO2 and non-activated but adsorbed NH3. At temperature between 200℃ and 325℃, fast SCR reaction between NO, NO2 and non-activated NH3 dominated the NOx removal, and the reaction followed E-R mechanism. The NH3 activation at temperature above 300℃ promoted the reaction between NO and activated NH3, which compensated thermodynamic limitation induced suppression of fast SCR. And the suppression of NH3 to NO over oxidation promised a high NO reduction efficiency of 91% at 400 ℃.(3) Clarification of SO2 deactivation mechanism over FeH-ZSM-5 and CuH-ZSM-5 catalyst.The performance of SO2 induced catalyst deactivation was significantly different over the heavy loaded FeH-ZSM-5 and CuH-ZSM-5 catalyst. Over the FeH-ZSM-5, SO2 induced the crystallization of FexOy clusters on the surface of ZSM-5. This catalyst structure change process is not reversible, as a result, the activity of FeH-ZSM-5 catalyst can not be recovered by post heat treatment. The inhibition of NO oxidation and NO2 adsorption was found to be the main reason for the suppression of de-NOx activity. The structure of CuH-ZSM-5 was not destroyed after the SO2 deactivation, on the contrary, the NO and NH3 adsorption were promoted compared to the fresh catalyst. Though the deposition of sulfate and sulfite blocking the active sites on the catalyst surface which suppressed its NH3-SCR performance at temperature below 275℃. At temperature above 275℃, however, the existence of sufficient NO2 promoted the decomposition of sulfate and thus improved its NOx removal efficiency. Because the catalyst structure was not destroyed during SO2 induced deactivation, the deactivation caused by SO2 could be recovered by post heat treatment.(4) Clarification of SO2 deactivation mechanism over CuO-CeO2 catalyst at low temperature.CuO-CeO2 catalyst was prepared by chemical deposition method, and its SO2 induced deactivation of selective catalytic reduction of NO over was studied at low temperature. In the case of reaction under low O2 concentration of 1.0 vol%, SO2 severely deactivated the catalyst at 240℃ with a surface S atomic concentration as low as 1.34%. However, the deactivated catalyst could be reactivated during online NO reduction under 5.0 vol% O2 without decreasing the surface S concentration of the catalyst, which could be attributed to the involvement of NO2 in the reactions. NO2 could promote the NO removal through three reaction routes:fast SCR reaction, reaction between NO2 and NH3, and reaction between NO2 and NH4+. Especially under conditions of 10.0% O2, the reaction between NO2 and NH3/NH4+ induced the formation of extra NHx<3 species which promoted the decomposition of surface-deposited sulfate to SO2 with the assistance of Ce2O3, further suppressed the accumulation of sulfate on the catalyst surface, and finally suppressed the SO2-induced catalyst deactivation at 240℃.