Mechanism Study On Synergistically Catalytic Removal NO_x And Toluene In Coal-Fired Flue Gas Based On SAPO-34

Besides the conventional pollutants such as NO_x and SO₂,coal-fired flue gas also contains organic pollutants seriously endangering environmental safety and human health.In this paper,the mechanism of synergistic removal of NO_xand toluene were systematically investigated.The design and modification methods of bifunctional catalysts were established.The main research content is as follow:Firstly,the Cu and Ce were directly introduced into SAPO-34 framework as redox centers.The results showed that the formation of Cu²⁺on cation sites of zeolite was promoted by appropriate introduction of Ce and optimizing surface acidity of catalysts as well as SCR activity.In contrast,the active Cu²⁺sites were reduced with excess Ce,which was negative for NO_x reduction.The 0.05Cu0.02Ce-SAPO-34 exhibited more than 80%conversion of NO_x during 168～500℃.In addition,in situ diffuse reflectance infrared spectroscopy（in situ DRIFTS）revealed that the SCR over Cu Ce-SAPO-34 followed E-R mechanism with the Cu²⁺as both Lewis acid sites and redox sites.The core reaction process is dehydrogenation of adsorbed NH₃,which generated NH^2*and reacted with NO to form N₂ and H₂O.Then,the MnO_x was supported on SAPO-34 to study its performance and mechanism of toluene oxidation.The results showed that using oxalic acid as precipitant could not only form MnO_x with high charge imbalance on SAPO-34 promoting formation of oxygen active sites,but also form micro-mesoporous hierarchical structure enhancing the exposure of active sites and diffusion as well as mass-transfer of reactants.It could realize more than90%conversion of toluene at only 237℃with～100%CO₂ selectivity.The long-term stability was maintained at 260℃even with 5%H₂O.Moreover,it was found that the oxidation of toluene followed Mv K mechanism according to in situ DRIFTS.The active lattice oxygen was key site to gradually oxidize toluene into benzyl alcohol,benzaldehyde,benzoic acid,and finally become CO₂.Furthermore,the reaction characteristics and mechanism of Mn/SAPO-34 synergistic catalytic reduction NO_x and oxidation toluene were researched.When toluene was not completely oxidized,the NO_x removal efficiency was reduced by 27%～58%compared with that of SCR alone in process of synergistic removal.As toluene was totally oxidized,the NO_x conversion was still less than 80%due to the side reaction caused by over-oxidation of NH₃.In addition,the inhibition mechanism for multi-reactants was analyzed.It is found that toluene inhibited the oxidative dehydrogenation of NH₃to form NH₂^*blocking E-R reaction path.Meanwhile,toluene inhibited the conversion of NO to NO₂ cutting off the fast SCR reaction.On this basis,Cu²⁺were introduced into cation site of MnO_x/SAPO-34 framework,the reaction characteristics and mechanism of synergistic removal of NO_x and toluene were explored.The SAPO-34 framework prevents toluene from entering its interior and reacting with Cu²⁺avoiding deactivation to SCR active sites.Meanwhile the redox properties of catalyst were adjusted by Mn loading to obtain the suitable temperature range for synergistic removal.The 20Mn/Cu-SAPO-34 achieved more than 80%conversion of NOx and toluene simultaneously at 300～400℃.In addition,the temperature-programmed experiments revealed that the SCR reaction followed E-R and L-H mechanisms,while the catalytic oxidation of toluene followed Mv K mechanism.No mutual inhibition between them was detected.Finally,the bifunctional catalyst was optimized by replacing MnO_x with Ce and Cr oxides to research its anti-poisoning characteristics to SO₂ and H₂O.There is a redox reaction between Ce⁴⁺and Cr³⁺that can generate Ce³⁺and Cr⁶⁺,thereby enhancing redox ability of catalysts and anti-sulfation ability of active sites.The 1Ce-1Cr/Cu-SAPO-34 had the best activity,which could reach more than 80%conversion of NO_x and toluene simultaneously with co-existence of SO₂ and H₂O at 330～400℃.Meanwhile,in long-term experiment at 400℃for 1000 minutes,the SCR activity only decreased by 2%and finally maintained at～80%.The toluene conversion decreased by～20%and finally maintained at～77%.