The Study Of Selective Catalytic Reduction Of NO_x Over CuO_x-based Catalysts

Nitrogen oxides (NOx) discharged from industrial processes and human being’s daily lives are one of the major sources of air pollutants, which can induce photochemical smog, acid rain, ozone depletion and greenhouse effects. Selective catalytic reduction (SCR) of NOx with NH3 in the presence of oxygen is one of the most effective methods to control NOx emission. And the key point for SCR technique is to develop novel and effective catalyst. V2O5-based catalyst, which is toxic and only active within a narrow and high temperature at 300-400℃, is widely used as an industrial catalyst now. The developing of environmental-friendly catalysts is necessary. However, the catalytic performance of the catalysts was still not very high at low temperature, and the reaction mechanism was not clear. Also the deactivation of the catalysts by SO2 at low temperature was still a big problem. All these disadvantages limited the industrial application of new catalysts.Novel catalysts, which used CuOx as the main active component and FeOx and CeOx as the auxiliary component, were prepared to investigate the SCR reaction mechanism in this paper. Many new discoveries were obtained and listed as follows:(1) The revelation of SCR reaction mechanism at low temperature over Fe-Cu-Ox/CNTs-TiO2 catalyst.Sol-gel method was used to prepare Fe-Cu-Ox/CNTs-TiO2 catalyst which showed very high catalytic performance. The conversion of NO reached 99% at 175-250℃ in the presence of oxygen on this catalyst. NH3 was adsorbed on the surface of the catalyst during reaction process. NO2 was formed from NO oxidation and can absorb on the surface of the catalyst. The reaction between adsorbed NH3 and adsorbed NOx was the main reason for the improvement of NH3-SCR at low temperature.(2) The innovation of SCR reaction mechanism at high temperature over Fe-Cu-Ox/CNTs-TiO2 catalyst.The catalyst was pretreated by SO2 in the absence of NH3 at different temperatures. The S species adsorbed on the catalyst surface affected the behaviors of adsorption and oxidation of NH3 and NO, thus influenced the NO reduction route. Over catalysts pretreated at low temperatures, the de-NOx reaction is mainly through the adsorbed NH3 and gaseous NOx. While over catalysts pretreated at high temperatures, the reaction between adsorbed NO and gaseous NH3 becomes the main reaction route at high temperature. The reduction of Cu2+ to Cu+ by SO2 is a possible reason for catalyst deactivation at low temperatures, but it also suppressed NH3 and SO2 oxidation, which improved the NO removal efficiency at high temperature.(3) The revelation of SO2 deactivation mechanism over M-Cu-Ox/CNTs-TiO2 catalyst.M-Cu-Ox/CNTs-Ti02(M=Fe, Mn, Ce, Zn, Bi, Sn) catalysts were prepared by sol-gel method and used to investigate the effect of SO2 on the NH3-SCR performance at different temperatures. The SCR activity of the catalysts was deactivated by SO2 at 150-300℃. The deposition of ammonia salts blocked the active sites on the surface of the catalyst, which reduce the adsorption of NH3 and NOX.Also the oxidation of NO was weakened. However, the catalytic performance was improved when the deactivation temperature was 350℃. The sulfation of the catalyst at high temperature restrained the oxidation of NH3 and strengthened the adsorption of NO.(4) The innovation of new catalyst which can restrain the SO2 deactivation at low temperature.CuO-CeO2 catalyst was prepared by chemical deposition method to further investigate the deactivation of SO2 to metal oxides catalysts in NH3-SCR. The catalytic performance of the catalyst was affected by oxygen concentration in SCR system. The NO conversion was improved by increasing oxygen concentration through the fast reaction between NO2 and adsorbed NH3 at low temperature. However, the NH3 oxidation was significantly increased at high temperature, which inhibited the catalytic ability of the catalyst. The deactivation of the catalyst by SO2 was depended on oxygen concentration at 240℃. The deposition of ammonia salts on the surface of the catalyst reduced the NO conversion obviously at 1 vol% O2. The reaction between ammonia salts and NOX (NO and NO2) and gaseous NH3 could recover the catalytic performance of the catalyst at 5 vol% O2. The deposited ammonia salts decomposed when the oxygen concentration increased to 10 vol%, which recovered the activity of the catalyst and restrain the deactivation of SO2.