Study Of V₂O₅-WO₃/TiO₂and Its F-doped Catalyst For Preparation, Characteristic Reaction And Application In NH₃-SCR DeNO_x

The selective catalytic reduction (SCR) of NOx with NH3in the presence of excess oxygen has been proven to be an efficient process for the NOx removal. V2O5+WO3/TiO2has been applied as an industrial catalyst for many years as the core of the SCR technology. Based on the advances of non-medium temperature NH3-SCR process and the relative catalysts, this study has described the promotional effect of WO3on O2-over V2O5/TiO2catalyst for selective catalytic reduction of NO with NH3and V2O5-WO3/TiO2was modified with fluorine. The reactive properties of the F-doped V2O5-WO3/TiO2catalyst in low-temperature SCR process have been systematically investigated. Based on these studies, molecular simulation and pilot experiment for the F-doped catalyt have been studied. The results would be benefit for the design of new low-temperature SCR catalyst and contribute to a better understanding of the low-temperature SCR processes.Firstly, V2O5-WO3/TiO2catalyst was prepared by a sol-gel method for titania preparation, followed by impregnation for W and then V. The experimental results showed that temperature window for SCR reaction was greatly widened and NO conversion was improved at non-medium temperature by WO3over the V2O5-WO3/TiO2catalyst. The trend was obvious as WO3loadings to6%. Characterized by XRD, XPS, PL and EPR spectra, the effects of WO3on the structural and surface properties of the V2O5-WO3/TiO2catalyst were studied. It showed that WO3could interact with oxygen vacancies of the TiO2surface that could inhibit the formation of crystallinity of rutile TiO2to improve the catalyst stability at high-temperature. Furthermore, WO3could store electrons and transfer electrons over V2O5-WO3/TiO2catalyst to enhance the interaction of V2O5with TiO2. It was facilitated that electrons could be transferred from TiO2to V2O5to improve the formation of the reduced V2O5. The reduced V2O5over the catalyst was the active sites for the formation of superoxide ions and WO3could increase the amount of superoxide ions that improve the NO convertion of V2O5-WO3/riO2catalyst.Then, the aim of the V2Os-WO3/TiO2catalyst design was to improve the activity of a catalyst with low V2O5and WO3loadings. Based on improving the interaction of active species with supporter to increase the amount of superoxide ions, it was described that F-doped catalyst formed by partly substituting the lattice oxygen of the catalyst with fluorine. The F-doped V2Os-WO3/TiO2catalyst was prepared by a sol-gel method for titania preparation, followed by impregnation for W and then V. The experimental results showed that NO conversion of the catalyst was improved by F doping below250℃. Characterized by XRD, XPS, PL and EPR spectra, online processing and offline characterization, and instantaneous reaction, the effects of F doping on the structural and surface properties of the F-doped V2O5-WO3/TiO2catalyst have been studied. It showed that F doping could improve the formation of oxygen vacancies with electron, F doping improved the interaction of WO3with TiO2by oxygen vacancies to facilitate the formation of the reduced WO3, reduced WO3could interact with O2to form superoxide ions. Furthermore, NH3acted as electron donor in the SCR reaction. F doping could improve the NO oxidation. And the catalyst with [F]/[Ti]=1.35×10-2showed the highest NO removal efficiency in SCR reaction at low temperatures. When the reacting mixture contained500ppm NO+500ppm NH3+5vol%O2balanced with N2at a gas hour space velocity of15,000h-1, The NO removal of this F-doped catalyst with1%V2O5and3%WO3was96.4%at240℃.Furthermore, based on the study of structural and surface properties, the cluster of the catalyst was created. The structural properties of the F-doped V2O5-WO3/TiO2catalyst were further studied on the basis of bond length, mulliken charge and bond energy of the molecular simulations results using density functional theory. It showed that the structure of F-doped TiO2supporter that F atom instead bridging oxygen of TiO2was stable and oxygen vacancies that formed at bridging oxygen were stable. The oxygen vacancies by F doping exhibited high affinity to oxygen to improve the interaction with W species. It facilitated the formation of reduced W species over WTiF and VWTiF catalyst by F doping. Furthermore, V species could also improve the formation of reduced W species and F doping could enhance the formation of reduced V species. Therefore, these aspects resulted in the increase of the superoxide ions over WTiF and VWTiF catalystFinally, based on the above results, development of monolith honeycomb catalyst of V2O5-WO3/TiO2catalyst and its pilot experiment were studied. The preparation process of the monolith honeycomb catalyst involved vacuum pugging, staleness, kneading, extrusion molding, drying and calcining technology were optimalized. The F-doped catalyst preparation of75×75×800mm was succeed. The axial maximum crack strength of the F-doped monolith honeycomb catalyst was1.75MPa and its radial maximum crack strength was0.6Mpa. The NO removal evaluation results of F-doped catalyst that was prepared by commercial grade materials by the test of fixed bed reactor had the same trend with the one that was prepared by a sol-gel method for titania preparation, followed by impregnation for W and then V. The F-doped monolith honeycomb catalyst that was prepared by commercial titanium tungsten powder had a higher effect of NO removal. The NOx removal results of70kW De-NOx reactor of coal-fired simulated flue gas and100kW generator reactor showed that F-doped V2O5-WO3/TiO2monolith honeycomb catalyst had an ideal NOx conversion and its NOx conversion remained stable in60hours.