Studies On The Structure And Catalytic Properties Of Nano Mn-Ti-O Composite Oxide Catalysts For Selective Catalytic Reduction Of NO By NH₃

It is well known that the selective catalytic reduction (SCR) of NO by NH3 as reductant is one of the most effective methods for elimination of NOx in flue gases from stationary sources, and the low-temperature SCR has attracted much attention recently because of longer life of catalysts used and lower investment for the control of NOx emission. In this dissertation, the structure and catalytical properties of supported MnOx/TiO2 catalysts and MnOx-TiO2 composite oxide catalysts for selective catalytic reduction of NO by NH3 have been systematically studied in depth, as well as the promotional effect of CeOx in CeOx-MnOx-TiO2 composite oxide catalysts on SCR activity and the SO2 resistance of MnOx-TiO2 and CeOx-MnOx-TiO2 catalysts have been studied by using micro-reactor tests combined with BET surface area measurement, X-ray diffraction (XRD), Laser Raman spectra (LRS), X-ray photoelectron spectroscopy (XPS), H2 temperature-programmed reduction (H2-TPR), TG-DSC-MS, infrared spectroscopy of chemisorbed NH3 (NH3-IR), and NH3 temperature-programmed desorption (NH3-TPD). The experimental results have shown that: 1. The manganese oxide loadings on the two TiO2 supports exert great influences on the SCR activity. For the rutile supported manganese oxide catalysts, increasing manganese oxide loading leads to the increase of reducibility of dispersed manganese oxide species and the rate constant k, which reaches a maximum around 9.6×10-6 mol (gMn s)-1 at 0.5 mmol Mn/100 m2 TiO2. When the manganese oxide loading is beyond this value, the existence of amorphous MnOx multiple layers will certainly reduce the ratio of manganese oxide species exposed on the surface and the reducibility of dispersed manganese oxide species, resulting in the rapid decrease of rate constant k.2. The LRS and XPS results have revealed that for the anatase supported manganese oxide catalysts manganese oxide species exist in Mn+4 as a major species with Mn+3 species and partially undecomposed Mn-nitrate as the minor species. Under the SCR reaction conditions, Mn+3 species on anatase are oxidized to Mn+4 species, inserting in the surface of anatase and promoting the anatase-to-rutile transformation in the surface layers of anatase support. Since these Mn4+ cations are actually dispersed on the support with a rutile shell-anatase core structure and its concentration is very near to that of MnOx/TiO2 (R) catalyst, the relation between the rate constant k and the MnOx loading on anatase support is similar to that on rutile support, and that the rate constant k values for anatase and rutile supported manganese oxide catalysts are very close at the same MnOx loading.3. The MnOx-TiO2 catalysts were prepared by sol-gel method and the structure of MnOx-TiO2 catalysts has been studied deeply by using various characterization techniques. The results indicate that the Mn ions incorporate into the TiO2 lattice decreasing the c axis of TiO2 lattice and promoting the surface anatase-to-rutile transformation. The nano-particles of MnOx-TiO2 solid solutions exist as the rutile shell-distorted anatase core structures. 4. The low-temperature SCR activities of MnOx-TiO2 nano-particles catalysts are much higher than the supported MnOx/TiO2 catalyts prepared by conventional impregnation method. The relation between the low-temperature SCR activity and the composition of MnOx-TiO2 nano-particles catalyst is correlated not only to its higher specific surface area but also to the surface structure of solid solution. When the Mn/Ti molar ratio is very low, the Mn4+ ions exist mainly as the isolated MnOx species and interact with Ti4+ ions through Mn-O-Ti bonds. With the increase of Mn/Ti molar ratio, more Ti4+ ions in the surface layers of MnOx-TiO2 solid solutions are replaced by Mn4+ ions, resulting in the increase of the ratio of Mn4+-OMn4+ to Mn4+-O-Ti4+ bonding as well as the relative amount of the polymerized to isolated MnOx species, leading to the increases of reducibility and the NO turnover frequencies (NO TOFs) of the catalysts. With the increase of Mn/Ti molar ratio from 0.20 to 0.30, the increased Mn species in the surface layers of the particles mainly consist of two dimensional polymerized Mn+4 species with very similar reducibility and NO TOFs. When the Mn/Ti molar ratio is higher than 0.40, the excess manganese oxides segregated in a form of Mn5O8 (MnO2·2Mn2O3) and the NO TOFs decrease drastically due to the lower reducibility of the Mn-O-Mn bonding on the MnOx multiple layers and the higher concentration of less reactive Mn+3 species.5. The addition of cerium oxide component remarkably increases the low-temperature SCR activity of Mn-Ti-O catalyst. With increasing cerium content, NO conversion increases greatly, reaching a maximum around Ce/Mn molar ratio= 0.08. Further increasing cerium content results in the decrease of NO conversion over the catalyst. The results indicate that the addition of proper Ce species in Mn-Ti-O catalyst hardly changes the surface acidity, but increase the surface concentration of active Mn species, relative content of Mn+4 species in surface active Mn species and the reducibility of Mn species, which increases the low-temperature SCR activity of Ce-Mn-Ti-O catalysts. when the Ce/Mn molar ratio is beyond 0.08, the surface layers Mn/Ti molar ratio and SCR activity decrease, probably because of the appearance of multiple layers aggregated Ce-O-Mn species composed by Ce and Mn species on the surface of Ce-Mn-Ti-O catalysts.6. The SO2 treatment could poison and deactivate the MnOx-TiO2 and CeOx-MnOx-TiO2 composite oxide catalysts for selective catalytic reduction of NO by NH3, and the SCR activities of MnOx-TiO2 and CeOx-MnOx-TiO2 composite oxide catalysts decrease with increasing the time of SO2 treatment. The experimental results indicate that the Mn species in MnOx-TiO2 and CeOx-MnOx-TiO2 catalysts were sulphated after SO2 treatment, which decreases the reducibility of Mn species in MnOx-TiO2 and CeOx-MnOx-TiO2 catalysts and leads the weak Lewis acid sites on the surface of catalysts to change to strong Br(?)nsted acid sites, thus poisoning and deactivating the MnOx-TiO2 and CeOx-MnOx-TiO2 catalysts.