Study On Mn-based Catalysts For Low-temperature Selective Catalytic Reduction Of No_x

Selective catalytic reduction (SCR) of NOx(x=1,2) by ammonia is one of the majortechnologies for reducing nitrogen oxides emitted from stationary sources such as powerstations, industrial heaters, and cogeneration and has been successfully commercialized.Vanadium-based catalysts such as V2O5/TiO2(anatase) mixed with WO3or MoO3are typicalcommercial catalysts for this process; however, they are only active within a narrowtemperature window of300~400oC, and are susceptible to deactivation by dust deposition orSO2poisoning. Thus, there has been a very strong incentive to develop highly efficientdenitration catalysts for low-temperature SCR processes in which these catalysts would beplaced downstream of the desulfurizer and electrostatic precipitator in the power generationsystem. In this dissertation, three kinds of Mn-based catalysts were investigated for thelow-temperature SCR and systematically characterized and the structure-activity relationshipwere discussed and the catalytic kinetics were also studied.Novel Mn-Zr mixed-oxide catalysts have been prepared by citric acid method for thelow-temperature SCR of NOxwith ammonia in the presence of excess oxygen compared tothe co-precipitation method and solid state reaction method. It was found that anMn(0.5)-ZrOx-450catalyst prepared by citric acid method showed the highest activity, givingalmost100%NOxconversion at100oC with a gas hourly space velocity of30,000h-1. Thecatalyst showed a certain level of sulfur tolerance and water resistance. The effect of H2Ocould be quickly eliminated after its removal, whereas deactivation by SO2proved to beirreversible. The formation of (NH4)2Mn2(SO4)3on the catalyst surface results in deactivationand the catalyst can be recovered by water washing but not thermal regeneration because(NH4)2Mn2(SO4)3is hard to decomposition but can dissolved in water. The characterizationresults suggested that an Mn-Zr solid solution was formed in the Mn(0.5)-ZrOx-450(CA)catalyst, with highly dispersed MnOx. An appropriate NH3adsorption ability was beneficialfor the low-temperature SCR. The kinetic study results show that the reaction order for NO,NH3and O2respectively is0.6,0and0.5.Mn-TiO2catalysts have been prepared by the solvothermal method for the low-temperature SCR of NOxwith ammonia in the presence of excess oxygen with comparison to wetimpregnation method and the effect of Mn resource was also investigated. It was found thatthe Mn-TiO2catalyst prepared by solvothermal method using Mn(CHCOO)2as Mn precursorshowed the highest activity, giving almost100%NOxconversion at120oC with a gas hourlyspace velocity of30,000h1. The catalyst showed a certain level of sulfur tolerance and waterresistance. The effect of H2O could be quickly eliminated after its removal, whereasdeactivation by SO2proved to be irreversible. The characterization results showed that a highdispersity of Mn on nano TiO2particles and good reducibility and high surface area isbenefited for the low temperature SCR. The kinetic study results showed that the reactionorder for NO, NH3and O2respectively is1,0and0.5and the active energy is24.2kJ·mol-1which is lower than similar catalysts in the literatures.Fe-Co mixed oxides were prepared by citric acid method and investigated for lowtemperature SCR. The Fe4Co1Ox(mole ratio of Fe/Co for4) gave the best activity. Nearly100%NOxconversion was achieved between200~300oC with a gas hourly space velocity of30,000h-1. The activity at low temperature (<200oC) can be improved by added Mn intoFe4Co1Oxand high low temperature activity can be obtained on Mn(0.3)-Fe4Co1Oxcatalystwith mole ratio of Mn/(Fe+CO) for0.3as100%NOxconversion can be obtain at80oC,which is also higher than Fe-Mn catalyst previous reported. The catalysts were examined byvarious characterization techniques and it was found that mixed crystal phases of Fe2O3andCoFe2O4spinel existed in Fe4Co1Oxwhile only CoFe2O4spinel crystal existed inMn(0.3)-Fe4Co1Ox. Highly dispersed Mn and Fe on the catalyst surface and good reducibilitycan be accounted for the high activity of Mn(0.3)-Fe4Co1Oxcatalyst.