Study On Low-temperature Selective Catalytic Reduction Of NO With NH₃ Over Modified Activated Carbon

The selective catalytic reduction (SCR) is a widly-used commercial process to control the exhaust of NO_x from stationary sources. And recently, low-temperature SCR became to attracts interests for its potential application. Activated carbons (AC), due to their high surface area, bulk porosity and low price, are often used as good adsorbent and support for catalysts in many applications. Therefore, activated carbon can be also employed as the catalyst support for the low-temperature SCR of NO. In this investigation, activated carbon based catalyst have been repared and used for low-temperature SCR of NO with NH₃.Firstly, the SCR performance of commercial AC was studied. The results showed that the NO conversion droped quickly from 80% to 11% as the temperature increase from 30℃to 200℃. It could be concluded that the reaction temperature window of AC was too narrow, thus the AC was not qualified to be an efficient low-temperature SCR catalyst.Secondly, in order to broden the reaction temperature window of AC in SCR, the MnO_x/AC was prepared by pore volume impregnation method. Before impregnation, the AC was washed by concentrated nitric acid to eliminate the influence of transition metal, which was despited on the surface of original AC, on the performance of MnO_x/AC in SCR reaction. The purified and Mn-loaded AC was then characterized. The special surface area and micropore volume increased slightly and the surface of AC became cleaner and there was almost no other element except C, H,O and N after purification; After MnO_x loading, The special surface area and micropore volume both decreased by some extent.Then, the low-temperature SCR performance of MnO_x/AC was appraised in a continuous flow fixed catalytic reactor. The effects of operating condtions such as calcinations temperature, the content of MnO_x loading, NH₃ and O₂ concentration, GHSV and reaction temperature were also studied. The optimal operating parameter was found as : Mn loading content =5 wt.%, calcination temperature = 350℃, O₂ concentration > 5%, [NH₃]/[NO]=0.95-1. Finally, nitrogen doping of AC and MnO_x/AC were performed by annealing in ammonia and to improve the stability and activity of AC-based catalysts in low-temperature SCR of NO with NH₃. It was found that the existence of MnO_x could enhance the amount of surface oxygen complexes and facilitate nitrogen incorporation in carbon, especially in the form of pyridinic nitrogen. Pyridinic nitrogen species seem to play an important role in SCR of NO with NH₃, since these species may be responsible for NO adsorption and oxidation to NO₂ in the presence of O₂. The activities of both AC and MnO_x/AC in low-temperature SCR were enhanced and the increament of MnO_x/AC was more notable than AC, which indicated that the cooperative effect of MnO_x and the surface nitrogen species would give a positive support on the improvement of catalytic activity.