The Structure-activity Relationship Of Mn And Cr Based Spinel-type Catalysts And Its NH_3- SCR Reaction

Nitrogen oxides(NOx)is one of the main atmospheric pollutants,it will cause acid rain,haze and other environmental pollution events.Efficient control of NOx emissions is of great significance for improving the atmospheric environment quality in China.With the completion of ultra-low emission technology transformation of coal-fired power plants,the main battlefield of NOx emission control of industrial flue gases has been transferred to non-electric industries such as steel,cement,petrochemical,g lass,etc.Selective catalytic reduction(SCR)of NOx with NH3 is the most extensive,mature and eficient technology for flue gas de-NOx in coal-fired power plants.The core of SCR technology lies in the choice of catalyst.It is not suitable to use V2O5-WO3/TiO2 catalyst in non-electric industries for NOx emission control directly,because flue gas produced by cement rotary kilns,glass melting kilns,and metallurgical sintering machines,etc.has a high demand for de-NOx under low and medium temperature conditions.Moreover,the resistance of the SCR catalysts to water vapor,sulfur and alkali metals need to be improved as well.Therefore,the development of low and medium temperature SCR catalysts with excellent comprehensive performance is of great significance for NOx control in typical non-electric industries flue gas.With its high mechanical strength,stable chemical properties and easy regulation of the active components,spinel-type oxides(AB2O4)are ideal materials for low temperature SCR catalysts.Mn and Cr are effective active components for low temperature SCR catalysts.In this paper,we have made efforts to constructed Mn-Cr spinel-type oxide catalysts and investigated the best preparation method for MnCr2O4.It was found that MnCr2O4 prepared by the sol-gel method exhibited the best denitrification activity and N2 selectivity with a high purity spinel phase.The T90 temperature window is 169-341? under a GHSV of 60,000 h-1,and N2 selectivity is near 100%below 250?.In order to further enhance the low-temperature activity and N2 selectivity of MnCr2O4,Zr4+ cations were introduced onto the Mn2+sites of MnCr2O4 prepared by sol-gel method.The feasibility of Zr4+substitution and the influence of the amount of Zr4+on the structure-performance relationship and reaction mechanism of MnCr2O4 were investigated.Characterizations preliminary demonstrated that Zr4+cations can successfully occupy the A2+ sites of MnCr2O4 spinel and form ZrxMn1-xCr2O4 solid solution,in which Zr is mainly present in the form of Zr3+and Zr4+.The SCR activity of MnCr2O4 can be significantly enhanced by appropriate amount of zirconium doping,and Zr0.05 Mn0.95Cr2O4 exhibited the best SCR performance.Its T90 temperature window is 202-333? a GHSV of 112000 h-1,and its N2 selectivity and water vapor resistance were also significantly improved compared to MnCr2O4.The acidity and redox properties of Zr0.05Mn0.95Cr2O4 are significantly stronger than those of MnCr2O4,and the surface lattice oxygen is activated by Zr doping and participates in the SCR reaction,resulting in a significant increase in catalytic activity.In addition,in situ diffuse reflectance infrared Fourier transform spectroscopy(in situ DRIFTS)combined with density functional theory(DFT)to further investigate the active sites and reaction mechanisms.Cr was found to be the main active site of the SCR reaction,and the energy barriers for beneficial NH2 and NO2 species formation were significantly lower on Zr0.05Mn0.95Cr2O4 than on MnCr2O4.The reaction mechanism of MnCr2O4 was temperature-dependent,and the reaction followed the L-H mechanism below 250?and the E-R mechanism above 250?.However,The reaction mechanism of Zr0.05Mn0.95Cr2O4 is temperature-independent at 100-400?,and the reaction is mainly carried out at 100-400?,following the L-H mechanism.Subsequently,the doping effect of bivalent Fe,Co,Ni,and Cu ions at the Mn2+site of MnCr2O4 was continued to be investigated,and it was found that the SCR efficiency of MnCr2O4 increased after doping with Fe,Co,Ni or Cu,and Co0.1Mn0.9Cr2O4 catalyst exhibited the best catalytic efficiency.By changing the relative content of Mn to Co,it was found that Mn0.1Co0.9Cr2O4 exhibited significantly better SCR catalytic activity than MnCr2O4 and CoCr2O4 and outperformed most of the reported metal oxide catalysts.Its T90 temperature range is 166-393? and showed good resistance to 5 vol.%water vapor and 200 ppm SO2,and the retention of the spinel-type structure is beneficial for its high catalytic activity.Compared to CoCr2O4,the specific surface area of Mn0.1Co0.9Cr2O4 increased by 36.5%,Co3+and Cr5+increased by 15.8%and 4%,respectively,and the chemisorbed oxygen concentrations also increased significantly.The amounts of Lewis acid sites increased by 2.6 times,and the reducibility has become stronger.Thus,Mn0.1Co0.9Cr2O4 shows much better SCR performance.The SCR reaction of Mn0.1 Co0.9Cr2O4 and Mn0.1 Co0.9Cr2O4 followed the L-H mechanism throughout the test temperature range,but the amount of adsorbed NH3 and mononitrates and other active substances on Mn0.1Co0.9Cr2O4 was significantly higher than that of CoCr2O4,and the reaction rates between adsorbed NH3 and nitrates was also faster.The effect of alkali metals on the SCR performance of Mn0.1Co0.9Cr2O4 catalysts was next investigated using Na+as a representative.It was found that Na+causes a severe efficiency loss in Mn0.1Co0.9Cr2O4 catalysts,with T90 narrowing down to 219-300Cr204 when 1 wt.%Na+was added.The sulfuric acid modification of Mn0.1Co0.9Cr2O4 catalyst showed that 0.1 M sulfuric acid not only had a small enhancement effect on the activity of fresh catalyst,but also significantly enhanced the resistance to Na+ poisoning of the acid-modified catalyst with a T90 window of 199-400Cr204.The reason is that the binding of sodium salt to the catalyst was inhibited after the acid treatment and the production of Na2O was reduced,while the increased amount of Br(?)nsted acid on the surface of the catalyst after the acid treatment provided more sites for Na+binding,reducing the poisoning of the active sites of the catalyst,thus enough active sites were preserved.