Study Of F-doped Vanadia/Titania Catalyst And Its Use In Low-temperature NH₃-SCR DeNO_x

To develope a low-temperature SCR catalyst for NOx removal in coal-fired power plant, the advances of low-temperature NH3-SCR process and the relative catalysts were critically reviewed. It showed that the catalyst design was almost focused on the metal part of the oxide catalyst and changing carriers. The aim of this study was to approach the problem from a different angle, by targeting the non-metal part of the catalyst. This study has described a novel F-doped vanadia/titania catalyst formed by partly substituting the lattice oxygen of the catalyst with fluorine. The preparation of the F-doped vanadia/titania and its reactive properties in low-temperature SCR process have been systematically investigated. The results would benefit for the design of new low-temperature SCR catalyst and contribute to a better understanding of the low-temperature SCR processes over a vanadia/titania catalyst.Firstly, characterized by N2 physisorption, XRD, XPS, ICP, Raman, ESR, PL,FT-IR and UV-vis DRS spectra, the effects of fluorine contents, fluorine precursors and preparation methods on the structural and surface properties of this new catalyst have been studied, combinding with the SCR activity of the catalyst at low temperatures. It showed that fluorine doping enhanced the crystallinity of anatase TiO₂, facilitated the formation of V⁴⁺ and Ti³⁺ ions mainly by charge compensation, promoted the distribution of vanadium on the catalyst surface, and increased the amounts of surface superoxide ions. Within the scope of this study, It showed that the sol-gel method was more suitable than hydrothermal method for preparing fluorine-contained catalyst. The catalytic activity of NO removal was promoted by F-adding, whatever the fluorine precursor was. The activity of catalyst using (NH₄)₂TiF₆ as precursor was higher than those using NH4F and HF as precursors. And the catalyst with [F]/[Ti]= 1.35×10^-2 showed the highest NO removal efficiency in SCR reaction at low temperatures. When the reacting mixture contained 500 ppm NO+600 ppm NH₃+5 vol% O₂ balanced with N₂ at a gas hour space velocity of 38,893 h^-1, the kinetic constants of this catalyst with 0.9 wt%V₂O₅ loading were both about 3.2 times than those of the comparative catalyst at 483 K and 513 K. The NO removal of this F-doped catalyst was up to 55% and 78% at 483 K and 513 K, respectively. There was a synergistic inhibitory effect of H₂O and SO₂ on the low-temperature NH3-SCR over F-doped catalyst.Then, based on the information of above characterization, the molecular cluster models were built and the affinity to H₂O over titania surface was calculated by density fuctional theory method. The result stated that the affinity to H₂O over titania surface was promoted over vanadia/titania surface after F doping. The effect of charge compensation was also confirmed by density fuctional caculation.Furthermore, compared with a commerciallized SCR catalyst, the adsorption of reactants over F-doped vanadia/titania suface was characterized by EPR and Raman spectra to state the relative reaction process. A two-step process was proposed in the adsorption of reactants over vanadia/titania catalyst, that was, the vanadium sites were firstly reduced by NO or NH3 as electron donor, and then the superoxide ions were formed over vanadium. This two-step process was enhanced by F doping. The adsorption amount of NO over the catalyst was promoted by F doping. And the affinity to NO of different F content catalysts varies in agreement with the activity of catalysts.Finally, based on the above results, the inhibition mechanism of SO₂ and H₂O on the SCR process was thoroughly elucidated by the two-step process. During NO+O₂ adsorption, addition H₂O mainly blocked the second step of the two-step process through competing for oxygen vacancies with O₂. However, During NH₃+O₂ adsorption, the reduction of V sites was almost stopped by addition H₂O. When SO₂ was present, there were both promotion and inhibition effects to the two-step process, which should have made a balance. This effect was caused by the inhibition of H₂O and SO₂ on the two-step process. When H₂O and SO₂ co-existed, a synergistic inhibitory effect on the formation of V⁴⁺ and superoxide ions was observed. In conclusion, the results, obtained in this study, have revealed that the Langmuir-Hinshelwood mechanism was included in the low-temperature NH3-SCR process over F-doped vanadia/titania catalyst. The results also confirmed the importance of the two-step process in the mechanisms of low-temperature NH3-SCR over F-doped vanadia/titania catalyst.