Effect Of Nitrogen Doping And Heat Treatment On Reduction Of NO With Activated Carbons And Mechanisms Thereof

NO_x is the major pollutant in the atmosphere,and it can also result in some secondary pollution,so it has been regarded as the main object in the treatment of air pollution.The pollution of NO_x from industrial sources is serious,and these sources usually locate in some cities and industrial areas,so the hazardness and potential hazardness is severe,therefore,it's very meaningful to harness the high concentration NO_x from industry.At present,many technologies to deal with the high concentration NO_x have both advantages and disadvantages,so the development of the efficient and economical technology is an interesting subject in the field.Compared with the gaseous reducing agents,activated carbons can also show good performance in the reduction of NO_x. Moreover,they are cheap and free of secondary pollution besides abundant sources, which make them a well applicable perspective in the field of DeNOx.Because the reduction of NO by carbon is a heterogeneous reaction,the effect of surface chemistries on the reaction is very important,but there some few related studies,so it is necessary to do more basic research.Some activated carbons were modified by nitrogen doping and heat treatment,and the regularity for change of the surface physicochemical characters was studied with kinds of measurement technologies such as elemental analyzer,XRD,XPS,TPD-MS, TGA and so on.At the same time,the reduction of NO with the modified carbons was investigated in a fixed-bed reactor when oxygen was both present and absent. Combined with the above analysis of the surface chemistries,the mechanisms for the reduction of NO with the modified carbons were studied in details.Results show that:1.The effect of both nitrogen doping and heat treatment on the surface chemistries is significant.When the carbons had been annealed in ammonia for 1～4h,the surface oxygen content decreased greatly,while the surface nitrogen content increased by 1～4 times.The main change of the surface chemistries was the surface oxygen content by the heat treatment,and it deceased by 45%when the carbon had been heated at 800℃for 4h.2.The active site for nitrogen doping by annealing in ammonia is C-OH,and most incorporated nitrogen atoms locate at the edges of graphene structures,especially in the form of N-6 and N-5.The existence of Fe₂O₃ particles can facilitate the nitrogen doping,and C=O is the active site for the nitrogen doping.3.The reduction of NO with the carbon becomes noticeable at about 500℃in the presence of oxygen,but the reaction begins at about 300℃with the nitrogen-doped carbon,and the NO_x conversion is as high as 18%at 500℃.It may be attributed to the formation of NO₂ with the help of surface nitrogen species as catalytic centers. Compared with the pure carbon,both the reactivity and selectivity of the modified carbons increase greatly,and the apparent activation energy of the reduction of NO by the modified carbons decreases by 60～80%.4.The carbon that is incorporated both nitrogen and Fe₂O₃ shows excellent activity for the reduction of NO in the presence of oxygen,and the NO_x conversion is higher than 40%at 500℃,which is due to the cooperative effect of Fe₂O₃ and the surface nitrogen species.Moreover the selectivity of modified carbon increases by 40～60%, and the apparent activation energy decreases by 30～60%.5.When oxygen is absent,the heat treatment has a negative effect on the activity of the carbon,and the deeper heat treatment results in the lower activity,which is directly relevant to the decrease of the amount of C-OH which plays an important role in the reduction of NO with carbon.But the effect can be neglected in the presence of oxygen.