Research On SCR Catalysts With Low-temperature H₂O Resistance

Nitrogen oxides（NO_x）,which is one of the major air pollutants,is a crucial source of haze.Therefore,the emission control of NO_x is becoming increasingly stringent.Currently,selective catalytic reduction of NO_x with NH₃（NH₃-SCR）is the dominated NO_x treatment technology and the core of the technology is the catalyst.With the accelerated implementation of the"coal to gas"project,the consumption of natural gas will increase dramatically.And natural gas is mainly consumed in urban gas and industrial fuel.Considering the high content of water vapor in the flue gas of gas-fired boilers and its adverse effects on catalytic performance,the research on SCR catalysts with low-temperature H₂O resistance will be widely concerned.Based on Mn-Zr and Mn-Eu catalysts,modified Mn-Zr-Cr and Mn-Eu-Fe catalysts with excellent low-temperature H₂O resistance were obtained by the addition of transition metallic oxides.The catalysts were characterized by XRD,BET,SEM,XPS,H₂-TPR,TPD and in situ DRIFTS.And the causes of enhanced SCR activity and H₂O resistance were analyzed.In addition,the effects of carriers on the SCR activity and H₂O resistance of supported catalysts were investigated.（1）In this thesis,Mn-Zr-Cr catalysts were prepared.And the effects of Zr/Mn and Cr/Mn molar ratios,the addition of other transition metallic oxides on the SCR activity and H₂O resistance of the Mn-Zr catalyst were investigated.It was found that the Mn-Zr-Cr=5:2:2.5 catalyst had the best H₂O resistance,excellent stability and long-lifetime H₂O resistance.For the Mn-Zr-Cr=5:2:2.5 catalyst,improved redox properties and higher quantities of Mn⁴⁺and Lewis acid sites made its H₂O resistance improved.（2）In this thesis,Mn-Eu-Fe catalysts were also prepared,and the effects of Fe/Mn molar ratios,calcination temperatures and reaction space velocities（GHSVs）on the SCR activity and H₂O resistance were investigated.It was found that the Mn-Eu-Fe（1）-500 catalyst possessed the same excellent stability and long-lifetime H₂O resistance with the Mn-Zr-Cr=5:2:2.5 catalyst.NO conversion levels were still maintained at roughly 90%even 50 h after reactions occurred in the presence of 15%H₂O at 230°C and a space velocity of 75000 h^-1.For the Mn-Eu-Fe（1）-500 catalyst,the combination of the three metallic oxides made the specific surface area larger,the concentrations of O_αhigher,the redox properties improved and the amounts of Br?nsted acid sites increased.Therefore,there were more adsorption amounts of NO/NH₃ in the presence of H₂O,so that the low-temperature activity and H₂O tolerance levels of the Mn-Eu-Fe（1）-500 catalyst were improved.（3）Supported Mn-Zr-Cr and Mn-Eu-Fe catalysts with different carriers were prepared,and the Mn-Eu-Fe/γ-Al₂O₃ catalyst exihibited the best H₂O resistance among the tested samples.The NO removal efficiency value of the Mn-Eu-Fe/γ-Al₂O₃catalyst could maintain at roughly 88%even in the presence of 15%H₂O at 200°C.Thus,it is a practical application potential SCR catalyst with low-temperature H₂O resistance in NO_x treatment of gas-fired boilers exhaust gas.