Performance And Kinetics Of SCR Catalyst Surface Acidification

Nowadays, environmental pollution is more and more serious, for example, industry and transportation emission of nitrogen oxides （NOx） pollution into the atmosphere is increasing year by year. NOx in the air and atmosphere is not only bad for people, animals but also plants and building land. Currently, using SCR technology to the removal of NOx in exhaust gas from fuel combustion has been gradually matured and put into commercial operation. The commercial SCR catalysts are mainly V-TiO₂ catalyst. In this article, we introduced acid into the V-catena catalysts to improve the catalysts’ surface acid sites and change the load form of V-species on the surface of catalysts. In addition, we added additive Ce to improve the redox ability of the catalysts in the catalytic process, and thus to broaden the catalysts’ reactivity temperature window and enhance the catalytic activity.V-species is unilaminar when V is not too much, and it turns to polymeric vanadate species with the increase of V on the surface of the nano-TiO₂ supported catalysts.Appropriate polymeric vanadate species is very beneficial to the activity of denitration. In this paper,we obtained the best catalytic activity when the mass fraction of vanadium pentoxide was 12%.After the acidification treatment to the carrier of the catalyst loading 12% V with different amount of acid, we found the catalyst had the best activity when the amount of acid was 0.1% relative to the carrier. After acidification,the catalyst’s surface acid sites increased significantly. We compared carrier acidification, loading acid and active substances together and catalyst surface acidification, and then found that carrier acidification was the most helpful, the catalyst’s denitrification activity was the best for there were the most surface acid sites. It illustrated that acidification not only enhanced the number of acid sites on the catalyst’s surface,but also made the acid occupy a portion of load position of V,which brought out more polymeric vanadate species on the catalyst’s surface, leading the catalytic performance improved.The addition of Ce enhanced the catalytic performance, for the lattice oxygen of CeO₂ accelerated the catalytic oxidation-reduction reaction rate. It was discovered that there were B acid sites and L acid sites on the catalyst’s surface by using in situ infrared spectroscopy,and the presence of the two kinds of acidic sites improved the reaction activity. By XRD tests, we found V was amorphous but not crystals on the catalyst’s surface which was very beneficial for the catalytic activity. From the SEM observation, we knew that the more uniform active substance distributed on the catalyst surface and the smaller the particles,the better the catalytic activity.The catalyst with V:Ce=3:1 and carrier acidification was conducted various dynamics tests, it was discovered the reaction followed the ER mechanism. We also found O₂ concentration had a great influence on the activity of the catalyst when it was less than 3% but no obvious impact when it was larger than 3%. When NO concentration on the catalyst surface was 1.1×10-3（volume ratio）, it was more readily to disperse into the catalyst pore surface, which was proved to be conducive to the reaction. NH3 concentration had no effect on the catalytic activity when the proportion of ammonia and nitrogen was greater than or equal to 1, in addition, NH3 adsorption was zero-order reaction. It was also noticed that the catalyst activity decreased as airspeed increased. The kinetics analysis of SCR standard reaction was conducted to establish reaction rate equation, and through experiments the equation of SCR standard reaction was simplified to the first-order reaction. We calculated the reaction rate constant k of the catalyst with V:Ce=3:1 and acidified carrier at each temperature according to denitrification activity data of different contact time, and then ciphered out the activation energy of V:Ce=3:1 and acidified catalyst was 12938 J/mol by Arrhenius equation through the relationship between temperature and the reaction rate constant accordingly.