Preparation And Catalytic Performance Of Pd/Ce_xCu_1-xO_2-δ^-Y₂O₃ Monolithic Catalysts For VOCs Combustion

Among technologies for elimination of volatile organic compounds(VOCs) emissions, catalytic combustion has been paid much attention because of low energy consumption, high efficiency and simple equipment.Moreover,there is no associated pollution such as nitrogen oxides(NO_x),as it operates at low temperature.A catalyst with high activity is the key to catalytic combustion.Monolithic catalysts with cordierite honeycomb ceramics developed in last decades are novel catalysts.For the catalysts,the cordierite honeycomb usually used as the substrate and a washcoat of Al₂O₃ is coated on its surface as the second support,and then active species such as Pt,Pd is deposited.As Al₂O₃ has poor thermal stability,and it can easily react with the promoters to form spinel phases(AB₂O₄) at high temperature,which is detrimental to the catalytic performance.Instead of Al₂O₃,the Pd monolithic catalysts prepared with the Y₂O₃ washcoat exhibit excellent activity for catalytic combustion of toluene,but its activity for catalytic combustion of ethyl acetate is poor.At present,non-noble metals such as Cu and Mn show good activities for catalytic combustion of oxygenated compounds and noble metals show good activities for catalytic combustion of aromatics.Since the industrial exhaust gases usually contain various kinds of VOCs,a dual-functional catalyst was developed with the Ce_xCu_1-xO_2-δ solid solution washcoat supported Pd,which are effective for both oxygen-free and oxygenated VOCs. The catalysts were characterized by XRD,Raman spectroscopy,H₂-TPR,BET and CO chemisorption.The detailed contents of this thesis are as follows:Using Y₂O₃ as a binder,the Ce_xCu_1-xO_2-δ solid solution was adhered to the substrate of cordierite honeycomb,and the Pd/Ce_xCu_1-xO_2-δ-Y₂O₃ catalysts were prepared by supporting Pd on the washcoat.Model reactions for the catalytic combustion of toluene and ethyl acetate were carried out to evaluate the activity and thermal stability of the catalysts.The results show that the Pd/Ce_xCu_1-xO_2-δ-Y₂O₃ catalysts possess dual-function properties,and exhibit good activity and thermal performance for catalytic combustion of both toluene and ethyl acetate.The catalytic activities of the Pd/Ce_xCu_1-xO_2-δ-Y₂O₃ catalysts are closely related to the component of Ce_xCu_1-xO_2-δ,and the Ce_0.9Cu_0.1O_1.9-Y₂O₃ catalyst exhibits the best activity.Further study of the Pd/Ce_0.9Cu_0.1O_1.9-Y₂O₃(Pd/CCY) catalyst found that the Ce_0.9Cu_0.1O_1.9 or CuO plays a main role in catalytic combustion of ethyl acetate,while Pd-PdO entities are the active site for oxidation of toluene,and the addition of Pd does not inhibit the activity for ethyl acetate combustion.The active sites of the catalyst calcined at low temperature are well dispersed,leading to a high activity.However,when calcined at high temperature,the Pd/CCY exhibits a degraded activity for ethyl acetate oxidation, which is probably related to the growth in the particle size of Ce_0.9Cu_0.1O_1.9 or CuO.For the catalytic combustion of toluene,the reactivity first declines with increasing calcination temperature up to 900℃,then recovers to some extent when the catalyst was calcined at 1000℃.This probably due to the fact that increased metallic Pd may compensate deactivation of the catalyst to some extent,since metallic Pd is possibly more active than PdO,through providing more adsorption sites for toluene.In summary,the formation of the Ce_0.9Cu_0.1O_1.9 solid solution before coating is the key to high activity and thermal stability.Moreover,Pd-PdO species are the active phase for oxidation of toluene,while Ce_0.9Cu_0.1O_1.9 or CuO is responsible for oxidation of ethyl acetate,and the formation of the Pd probably impels the enhancement of activity for the catalyst calcined at 1000℃.The Pd/CCY catalysts are dual-functional for both oxygen-containing and oxygen-free VOCs compounds,which may have applicable potentials.