| Selective catalytic reduction(SCR) technology is widely used in domestic and overseas to denitration from flue gas, which has the advantages of stable and efficient. Catalysts are the crucial factor for the SCR technology, which determinate the efficiency of the system directly. However, the commonest commercial catalysts is V2O5-WO3(Mo O3)/Ti O2 that the operation the temperature window is range of 300~400℃.In order to obtain high denitration efficiency, SCR reactor is put upstream of the desulfurizer and particulate removal device. This made the catalysts be scoured, blocked and poisoned by high concentration of dust and SO2. Additionally, due to the toxicity of V2O5, waste catalysts should be treated correctly to reduce the harm to the environment. So developing environmentally friendly low-temperature SCR catalysts is one of the important ways to solve the above problems. Since low temperature SCR catalyst deactivation in SO2 and H2 O easily and cannot adjust to the actual working condition. To enhance SO2 and H2 O resistance of the catalysts is significant in the practical application of low temperature SCR technology. Base on the structural properties and SO2/H2 O deactivation mechanism of Mn Ox/MWCNTs catalysts, improvement and optimization were implemented on the samples to obtain catalysts with excellent performance on low-temperature activity and resistance to SO2 and H2 O.Firstly, the effect of preparation method on SCR activity of the catalysts with zirconium addition was investigated. The results show that the addition of zirconium could improve the SCR activity of Mn Ox/MWCNTs. The catalysts that were prepared by a method of Zr Mn substep impregnation have the best SCR activity. The results of X-ray powder diffraction(XRD), H2 temperature-programmed reduction(H2-TPR) and X-ray photoelectron spectroscopy(XPS) reveal that the method of substep impregnation gives rise to fewer mixing of Mn and Zr on the support. There are a larger proportion of Mn4+ existing on the surface of the catalysts. These are the main reasons for the catalyst having better low-temperature SCR activity.Secondly, Zr Mn substep impregnation can also be regarded as a process of zirconium modified MWCNTs and then Mn was supported. Catalysts prepared by Zr modified MWCNTs, were studied in terms of SCR activity and structural properties. The catalysts with 30% Zr loading were found to have the highest activity. The results of XRD, Raman spectroscopy, transmission electron microscopy(TEM) and N2 adsorption-desorption show that the modification of zirconium could enhance the dispersion of Mn Ox on the support as well as the interaction between metal oxides and MWCNTs. Additionally, zirconium could also increase the specific surface area, total pore volume and the average pore size of the catalysts. Moreover, the results of XPS, H2-TPR and temperature-programmed desorption of NH3(NH3-TPD) show that zirconium could increase the atomic concentration of chemisorbed oxygen on the catalyst surface and promot the conversion of Mn3+ to Mn4+. Therefore the active sites on the surface of the catalysts and the redox ability of the catalysts were improved. Additionally, the amount and strength of acid on catalyst surface increased.Finally, based on the theory of ionic polarization, cerium and yttrium were screened to resist the catalysts deactivation caused by SO2 and H2 O. The structural properties of the catalysts were characterized by XRD, H2-TPR, XPS and infrared spectroscopy(FTIR) before and after reaction. The results show that Ce could improve the activity of catalyst further and broaden the temperature window, while Y has an inhibitory effect on SCR activity. Ce or Y exist in the catalysts with the form of high dispersion or amorphous and prefer to reacting with SO2 in containing sulfur atmosphere. Therefore, the sulfation of Mn near to the Ce or Y was suppressed and then SO2 resistance of the catalysts improved. Ce could improve H2 O resistance of the catalysts to some extent, while the effect of Y was not obvious. |
