Research On Purifying NO_X On Catalysts Surpported By Rayon-based Activated Carbon Fibers At Low Temperature

Active carbon fibers （ACF）,which has large specific surface areas and vast micro-pores that most of them distribute on the ACF surface, can be used as ideal SCR catalyst carrier at low temperature （< 120℃）.At present, the air pollution caused by nitrogen oxides is drawing increasing concerns, and these major pollutants cause a range of issues such as acid rain, photochemical smog and so on, which destruct the earth’s ecological environment and influence humans’health. So the effective removal method of nitrogen oxide pollution is an important research topic. Using the activated Carbon Fiber to purify the NO_x has become a very heated issue in realm of catalysis. In recent years, the automobile as one of modern means of transport has become more and more popular, and the air pollution caused by the auto exhaust are becoming increasingly serious. Nitrogen oxides as the major pollutants of the vehicle exhaust has seriously affected the air quality of some environmentally sensitive areas such as schools, commercial centers.At present, there are many methods to purify NO_x from air, and the most widely applied denitrogenatien technology in industry is selective catalytic reduction （SCR） that used NH₃ as reducing agent. However, it is inappropriate on many occasions because using NH₃ as reducing agent in reaction requires stringent conditions such as temperature and so on. Nowadays, most researches focus on purification under high and low temperature conditions, which cannot be applied under normal temperature condition. So, it is still a difficult issue that how to conveniently and efficiently achieve NO_x purification around Schools, institutions, hospitals, senior business districts and other environmentally sensitive areas where large-scale purifying and heating devices can not be built.As to the high atmospheric environmental quality standards of environmental sensitive areas, it has become an urgent need of NO_x purification technology that can be efficient, convenient and especially applied under normal temperature around some environmental sensitive areas.In this paper, we used a relative low price of viscose-based ACF, loading urea, cerium oxide, manganese oxide, ceria mixed with urea and manganese oxide mixed with urea respectively, which contain 18 kinds of catalyst of total 5 categories for low-temperature experimental study. Characterization Experiment of catalyst shows that as crop load increases, the catalyst specific surface area decrease continuously. It also can be found through scanning by electron microscopy that ACF surface is smooth, with the increase of crop load; the nanoparticles on the surface gradually increase. By X-ray diffraction experiments, it can be found that most metal oxide is not existed as crystal form.The results of orthogonal experiments show that the first important factor affected Catalytic performance is space velocity （R = 11.753）, the next percentage volume of oxygen （R = 10.283）, then are temperature （R = 3.227） and urea load （R = 1.99）. Through orthogonal experiment design, the optimum technological recipe is obtained, namely, the volume of oxygen concentration of 10%, space velocity 10000h-1.The SCR experimental results show that the transformation of NO_x is based on catalytic reduction reaction. As to the series experiment of urea/ACF, urea load has significant impact on the NO_x purification performance. Although loading amount 20% urea at 20℃, the highest conversion rate of NO_x is obtained, its performance is greatly affected by temperature change. Accessing the NO_x conversion rate bases on the efficiency and stability of the 6 kinds of catalysts, the optimal conditions are load 60% urea. However, if only for constant temperature 20℃, load 20% is the best option.A series research on CeO₂/ACF demonstrated that load 20% is optimal conditions attaining the highest catalyst activity. But comprehensively considering the changes in catalytic activity with temperature, load 10% CeO₂ is adopted to make urea-CeO₂/ACF.Series experiment studies of urea-CeO₂/ACF catalyst have shown that ACF loaded by a certain degree of urea and cerium oxide can effectively improve the stability of catalytic properties under different temperatures, but not the more the better. The 4:1 urea-CeO₂/ACF catalyst can achieve high NO_x conversion ratio and the temperature change has little effect on its catalytic ability.MnO_x/ACF catalyst experiments stated that the experimental conditions of ACF load MnO_x alone for the low-temperature purifying NO_x is not feasible, it need further improvement.A group researches on urea-MnO_x/ACF show that when payload urea on MnO_x/ACF can greatly improve its NO_x purifying ability at low temperature. At 20℃NO_x conversion rate of the three catalyzes are above 70%; 5:2urea-MnO_x/ACF and 6:2 urea-MnO_x/ACF approximate 80%. 5:2 urea-MnO_x/ACF is superior to other uniformity catalysts.After generally accessing the five kinds catalysts it is clearly that the catalyst loading urea achieves significantly higher NO_x conversion rate than that of no urea catalyst loaded. In terms of kinds of urea-x/ACF catalysts, 4:1 urea-CeO₂/ACF has a significantly higher NO_x conversion ratio than the load 60% urea/ACF and 5:2 urea-MnO_x/ACF, while NO_x conversion ratio of 5:2 urea-MnO_x/ACF is also higher than that of the 60% urea / ACF when the temperature is lower than 60℃, so it could be applied to NO_x purification at normal temperature. But generally compared 4:1 urea-CeO₂/ACF with 5:2 urea-MnO_x/ACF, it can be found that 4:1 urea-CeO₂ has more steady and higher catalytic activity than the 5:2 urea-MnO_x/ACF. Between the two catalysts loading urea and no loading urea the reaction mechanism is distinct and the process of chemical reaction is also different.