Mechanism Of N₂ Selectivity And H₂O/SO₂ Resistance Of CrCeMoO_x Catalytic For NH₃-SCR Of NO_x At Medium-low Temperature

Nitrogen oxides(NOx)emitted from industrial furnaces are major precursors of acid rain,photochemical smog and ozone.With the promotion of ultra-low emission transformation of power plant boilers and industrial boilers,the weight of NOx emissions from ceramic/glass industrial kilns has become increasingly prominent.It will become the priority consideration of NOx emission control in the "14th Five-Year Plan".Ammonia selective catalytic reduction(NH3-SCR)technology is the most widely used high-efficiency deNOx method.But the flue gas conditions of ceramic and glass industrial furnaces are very complex.The key technical bottleneck is to make the catalyst have high activity and N2 selectivity at medium and low temperature(180～260℃),while have excellent performance on SO2 and H2O resistance.Therefore,it is the key to achieve industrial furnaces ultra-low NOx emission by developing new and high efficiency SCR catalyst that can adapt to the complex flue conditions.It has important scientific significance to realize the application of NH3-SCR technology in the industrial application,especially ceramic/glass furnace.In this paper,Cr-Ce composite oxide was used as the main active component for NH3-SCR catalyst due to high activity at medium and low temperature.Firstly,Mo was selected as the doping modification element,and the preparation conditions were further optimized by citric acid method.The best activity and selectivity of Mo(0.3)-CrCeOx was obtained.The conversion of NOx,selectivity and sulfur resistance of the Mo(0.3)-CrCeOx were tested compared with CrCeOx.The XRD,XPS,NH3-TPD and H2-TPR were used to characteristic the physicochemical properties and structure-activity relationship on catalyst.In situ DRIFTS was used to obtain the mechanism of Mo doping on the significant improvement of SO2 resistance and N2 selectivity,and the reaction mechanism was proposed.Furthermore,CTAB-assisted was used to improve the coexist of SO2 and H2O resistance on Mo(0.3)-CrCeOx at medium and low temperature(260℃).The XRD,BET,SEM,XPS,NH3-TPD and H2-TPR were used to characteristic the effect mechanism of CTAB-assisted.Meanwhile,study the cause of deactivation on CA method in the coexistence of water and sulfur.In situ DRIFTS was used to study the mechanism of SO2 and H2O resistance by CTAB-assisted,and the reaction mechanism was proposed.The main research conclusions of this paper are as follows:(1)The preparation conditions of Mo doped CrCeOx composite metal oxide catalyst was optimized via citric acid(CA)method.The Cr/Ce ratio was 7:3,Mo/(Cr+Ce)was 0.3,the roasting temperature was 500℃,the roasting time is 3 h,the amount of citric acid added was 1.3 times the total metal molar amount.It has higher than 92%NOx conversion and 80%N2 selectivity at 175～275℃.At the same time,the NOx conversion was 73%at 150℃,which can widen the low temperature activity temperature window to the greatest extent.(2)Mo(0.3)-CrCeOx was systematically characterized.It was found that amorphous MoO3 was highly dispersed on the catalyst surface,and the CrCeOx crystallinity was not affected.Mo and Ce3+formed Ce4+-O-Mo-O-Ce4+by ionic bond and exposed more strong acid sites.By covalent bonding,Cr3+has structural deviation and plays a regulating role in alkalinity.The formation of active NO2 species makes it difficult for gaseous NO to interact with NH3 site.Inhibit the E-R reaction.Inhibition of Cr6+contributes to the formation of reactive intermediates and enhances selectivity.The formation of inactive dinitro compounds forms competitive adsorption with SO2.In-situ DRIFTS showed that the oxidative dehydrogenation of coordination NH3 and NH4+to-NH2.Oxidation of NO to NO2-,further strong reduction to an important active intermediate cis-N2O22-for SCR reaction.The resulting inactive-NH instantly reacts with NH3 to-NH2.The chain cycle reaction is formed,following L-H mechanism.(3)Mo doping provides a sacrificial site for SO2 binding to Mo-O-Ce lattice oxygen to form SO3.Then it forms low-coordination Ce2(SO4)3 with neighboring Ce3+by Mo-O-S-O-Ce3+.The H2-TPR curve was demonstrated the lower reduction temperature of sulfate formation.It indicated that the adsorption of SO2 is inhibited on the surface.At the same time,the alkalinity regulation of Cr site makes NO stronger than SO2 competitive adsorption on the surface.(4),Mo(0.3)-CrCeOx was constantly destroyed due to the tension and disordered expansion of H2O via CA method,causing the inactivation of NH3-SCR by blocking the electron transport reaction chain.Then,ammonium sulfate[(NH4)2SO4/NH4HSO4]was formed on the surface,and the active component site was vulcanized.CTAB-assisted is conducive to form a certain structural strength,and the outer layer is coated by clusters of components,protecting the body phase from being destroyed.The SCR mechanism was proposed.The introduction of SO2 stimulated the formation of new acid sites on the surface,which increase the reaction capacity.And at the same time,it effectively reduced the decomposition temperature of sulfate.Therefore,it can further improve sulfur resistance.(5)Research on the mechanism of CTAB-assisted illustrated that the Lewis acid on the surface was dominant,which promoted the activity of N-H and could reduce NO3-directly.NH4+ belonging to Br(?)nsted acid site can also participate in the reaction,although its activity of N-H is not as active as the Lewis acid site,the rate of activation of NH4+ is faster than the formation.The reaction follows the L-H mechanism.An auxiliary verification of acid activation energy was obtained from the NH3-TPD results.