Study On The Design Of Heteropoly Acids Catalyst For NH₃-SCR Reaction

According to China Environmental Status Bulletin 2014,China’s total nitrogen oxide emissions reached 20.78 million tons,the stationary sources emissions accounted for 67.6%.Selective catalytic reduction(NH3-SCR)technology is currently the most widely used technology to remove nitrogen oxides in flue gas emissions from stationary sources.So far,the commercial NH3-SCR catalyst is V2O5-WO3(MoO3)/TiO2,which has excellent NOx removal efficiency and resistance to SO2 poisoning in the medium temperature range(350℃-450℃).However,this material was limited to high and narrow operating temperature as well as toxicity of vanadium oxide.Consequently,the development of an ideal catalyst in the low temperature range is highly needed.Based on heteropoly acids,this paper designed catalysts such as heteropoly acids-oxides,heteropoly acids-zeolite to explore a feasible strategy for heteropoly acids to be used for DeNOx.HPW(phosphotungstic acids)modified Mn-based NH3-SCR catalysts with excellent NO conversion and N2 selectivity be designed.N2 yield was hardly more than 75%over MnOx/TiO2 for NH3-SCR reaction,whereas the NH3-SCR performance has been significantly improved by using 50wt.%HPW-MnOx/TiO2.100%NO conversion and more than 95%N2 yield was obtained in wide operating temperature window(150-400℃),suggesting that the addition of HPW could effectively improve the NO reduction conversion.After that,the catalysts were further characterized by XRD,H2-TPR,NH3-TPD,XPS and in situ DRIFT.DRIFT analysis implied that the introduction of HPW significantly enhanced the capacity of N14+species adsorbed on Br1nsted acid sites accompanied with inhibiting the formation and consumption of nitrite species.It proved that the non-selective catalytic reduction reaction over HPW-MnOx/TiO2 catalysts are restrained.HPW could accelerate the formation and consumption of NH4+species adsorbed on Br(?)nsted acid sites with deactivation of nitrate species.Therefore,NH3(ad)could be hardly oxidized to NH species and then reacted with nitrate species(L-H mechanism)and gaseous NO(E-R mechanism).More importantly,the oxidation of NH3 was also suppressed,which plays a dominate role to form N2O above 300℃.Besides,the deactivation of potassium poisoning on the SCR activity significantly weakened for modified samples compared to parent catalyst.At the same time,long-term test at 300℃ for 80 h found that 30wt.%HPW-MnOx/TiO2 and MnOx/TiO2 had stable NO conversion of about 100%and 92%,respectively,indicating that the catalyst has excellent thermal stability.After that,we designed heteropoly acid salt/SBA-15 catalysts.NO conversion of the Cu3/2PW/SBA-15 sample has a significant improvement in SCR performance,achieving about 90%NO conversion at 350℃.A better catalytic activity is obtained in the presence of less metal species(8‰～1%).After the introduction of oxide,we found that the 10wt.%CuO-10wt.%HPW/SBA-15 sample achieved more than 85%NO conversion over a wide temperature range of 250-400℃.Compared with Cu3/2PW/SBA-15,there is a significant increase in SCR activity,and the SBA-15 molecular sieve support is also superior to the anatase TiO2 support.