Study On Combined Catalysts For The Selective Catalytic Reduction Of NOX With NH₃

With the development of China’s industry and the increasing number of personal cars,a large quantity of coal and oil have been burned to use.The exhaust gas has seriously polluted the Earth’s atmosphere.A following series of air pollution problems such as acid rain,haze,and global warming have had a tremendous impact on the lives of people and the development of human society.Considering that air pollution is threatening our health and living environment.Nitrogen oxides(NOx)mainly including NO and NO2 and a small amount of N2O is a major air pollutant.China’s emissions of nitrogen oxides becomes more and more strict.Selective catalytic reduction of NOx with NH3(NH3-SCR)is a mature,reliable,high efficiency denitrification method.It also has good selectivity and high performance-price ratio.Although the VW(Mo)/TiO2 catalyst has been maturely applied in the power plant,the vanadium catalyst still has some problems such as high operating temperature,a narrow operating temperature window,a decrease in N2 selectivity at high temperature,and SO2 to SO3 oxidation.Therefore,the searching of new NH3-SCR catalysts with better effect is still the direction that we should strive for.In the third chapter,several kinds of single metal oxides with different oxidation properties were prepared.It was found that the oxidation of the metal oxide catalyst had an important effect on the NH3-SCR by comparing the activities of the single metal oxides which have different oxidation properties.The metal oxide catalyst with better oxidizing property had better catalytic activity at low temperature.However,it would produce much NO2 and N2O and caused poor N2 selectivity.The MnOx prepared by precipitation method was mainly Mn5O8,and the MnOx synthesized by hydrothermal synthesis was mainly MnO2.The activity of MnOx prepared by hydrothermal synthesis was better than the MnOx prepared by precipitation method,but the N2 selectivity was still poor.The SO42-/MnOx catalyst was prepared by sulfuric acid treatment of precipitated MnOx.It greatly improve the selectivity and activity of the reaction by sulfuric acid treatment.It was found that the introduced SO42-played a role as a passivation to the MnOx catalyst by a series of characterization.It not only reduced the oxidation of MnOx activity but also increased the amount of surface oxygen of the MnOx which could improve the catalytic activity of MnOx.In the fourth chapter,a series of acidic metal oxides were prepared.By evaluating the NH3-SCR activity of these acidic metal oxide catalysts,we found that the acidity of the metal oxides played an important role in the reaction.Strong acidic metal oxides tended to have better high temperature activity,and their N2 selectivity was better.The super acid catalyst SO42-/Fe2O3 was prepared from acid treatment to Fe2O3.After the sulfuric acid treatment,the activity and selectivity of SO42-/Fe2O3 catalyst had greatly improved.A series of characterization had been carried out to analyze the SO42-/Fe2O3 catalyst.It was found that the addition of SO42-improved the surface activity of the catalyst as well as the oxygen content.It increased the activity of the reaction too.In the fifth chapter,we choosed a series of Mn-based composite metal oxides with advantages at low temperature and Ce-based metal oxides with high activity at high temperature.By evaluating the activity of these two series of composite metal oxides,we found that the Mn-based composite metal oxide had a good NO conversion rate at a low temperature.However,manny by-products appared during the reaction.It reduced the N2 selectivity of the reaction.Ce-based catalysts had good high temperature activity and N2 selectivity.The SO42-/MnFeOx catalyst was prepared by acidifying the MnFeOx composite metal oxide.The activity of SO42-/MnFeOx was decreased at low temperature.However,the activity and selectivity had been greatly improved at higher temperature.In the sixth chapter,several single metal oxides,composite metal oxides and modified single metal oxides were choosed.The combined catalysts prepared by three different combinations were tested for NH3-SCR.It was found that when the catalyst is combined,the high temperature activator with better acidity is often placed in the first half and the catalyst with the better oxidative stability is placed in the second half and the combined catalyst has the best catalytic activity with selectivity.