Transition Metal Doped Ceria Oxide Catalysts On Catalytic Combustion Of Toluene

Catalytic combustion is regarded as one of the most effective and promising ways todeal with VOCs because of its efficient economic, high removal efficiency, convenientoperation and non-secondary pollution, such as NO_xand SO_x. Catalyst, one of the keyfactors in catalytic combustion, gives a decisive affect on removal efficiency and energyconsumption. Ceria is rare earth oxide and fluorite structure. Because ceria possessunique capability of oxygen storage and releasing by oxidation reduction cycle betweenCe⁴⁺and Ce³⁺, it has been widely used in the catalytic reaction. But a single ceria easilyappear sintering, which may reduce its catalytic activity.It has been found that when part of the cerium elements was replaced by otherscation with appropriate radius in the ceria lattice, the activation of ceria lattice oxygencan be promoted and the oxygen storage capacity can be increased, so its catalyticactivity was improved, which is because radius diversity between different cations leadto distortion and surface defects into ceria lattice. The doping elements mainly involvealkaline element, rare earth and transition elements. The catalysts of transition elementsdoping ceria show good activity for VOCs combustion.A series of catalysts were prepared with five different transition metals doping ceriaacting as active components and γ-Al₂O₃acting as the carrier through an impregnationmethod. In the series of catalyst MxCe_1-xO₂/γ-Al₂O₃,（M=Mn, Cu, Fe, Co and Ni）. Thecatalytic activity and optimal doping value of each transition element were studied bycombustion of target pollutant toluene, the catalysts microstructure was characterized byXRD, BET, SEM and ICP-MS, and so on.Results indicated that the activity and stability of complex oxide catalysts of dopingceria with five difference transition metals were usually better than the supported puretransition metal oxides or ceria in γ-Al₂O₃.When different transition metals doping ceria, the catalytic activity has appeared quite different. Series catalysts of Mn_xCe_1-xO₂/γ-Al₂O₃showed the best activity for the catalytic oxidation of toluene, T₁₀of which werebetween80℃and120℃and T₉₀between165℃and220℃. Secondly, Cu doping ceriaformed a series catalysts of CuxCe_1-xO₂/γ-Al₂O₃, which showed better activity thanothers catalysts.When toluene conversion rate is percent ten, the reaction temperatureis between160℃and170℃; temperature of complete catalytic combustion is between280℃and300℃.Among the catalysts of same cation doped ceria, their catalytic activity greatlychanged with the change of doping amount. There is an optimum doping level betweeneach transition element and Ceria, the optimum ratio of five transition element follow:Cu_0.2Ce_0.8O₂/γ-Al₂O₃, Fe_0.2Ce_0.8O₂/γ-Al₂O₃, Mn_0.8Ce_0.2O₂/γ-Al₂O₃, Co_0.3Ce_0.7O₂/γ-Al₂O₃,Ni_0.2Ce_0.8O₂/γ-Al₂O₃. For these five catalysts, the test of100hours stability show thatthe catalysts of Cu_0.2Ce_0.8O₂/γ-Al₂O₃and Mn_0.8Ce_0.2O₂/γ-Al₂O₃have the best stabilitythan the others, which as follows Fe_0.2Ce_0.8O₂/γ-Al₂O₃> Co_0.3Ce_0.7O₂/γ-Al₂O₃>Ni_0.2Ce_0.8O₂/γ-Al₂O₃.For the best optimal catalyst Mn_0.8Ce_0.2O₂/γ-Al₂O₃, further research has shown thatthe content of active component and calcination temperature has some impact oncatalytic activity. When the content of active component up to20%and calcining at550℃, the catalyst of Mn_0.8Ce_0.2O₂/γ-Al₂O₃exhibited the best extraordin-arily highcatalytic activity for toluene combustion.The reaction space velocity and import concentration of toluene have certainlyimpact on the catalysts of Mn_0.8Ce_0.2O₂/γ-Al₂O₃.The catalytic activity remainedunchanged when the space velocity was between3600h^-1and7200h^-1, but with furtherincre-aseing to between10800h^-1and21600h^-1, the conversion rate rapidly decreased,the conversion rate decreased from percent of78to67at the reaction temperature of220℃. Its conversion rate did not obviously changed.When inlet concentration oftoluene was between500mg/m³and1000mg/m³, but when increasing to3000mg/m³,the toluene conversion rate declined in percent of9at the reaction temperature of220℃.