Study On The Catalytic Performance Of Ceria-based Catalysts For NO Elimination

With the rapid development of automobile industry,motor vehicles brought convenience to people,but also caused serious atmosphere pollution.The treatment of motor vehicle exhausts has become imperative.NO reduction by CO is an important model reaction in the purification of motor vehicle exhausts,which attracts much attention of the researchers due to the simultaneous catalytic elimination of CO and NO pollutants.Ceria-based catalysts were widely applied in this model reaction because of their excellent redox behavior and high storage/release oxygen capacity.In order to further improve the catalytic performance and broaden the research scope of ceria-based catalysts,we carried out a series of systematic investigation through modulating the kinds and amounts of dopants,as well as the preparation methods of ceria-based catalysts,as follows:(I)A series of ceria-based solid solutions(Ce0.67M0.33O2(M=Zr4+,Ti4+,and Sn4+))were prepared by inverse co-precipitation method.The synthesized samples were studied in detail by means of XRD,Raman,TEM,UV-Vis DRS,N2-physisorption,H2-TPR,OSC,XPS,and in situ FT-IR technologies.Furthermore,NO reduction by CO was chosen as a model reaction to evaluate the catalytic performance of these catalysts.This work focuses on investigating the effect of different dopants(Zr4+,Ti4+,and Sn4+)on the structure,texture,reduction,and adsorption behavior of CeO2,and further exploring the relationship between the physicochemical properties and catalytic performance of these ceria-based catalysts.The obtained results indicate that the incorporation of Zr4+,Ti4+,and Sn4+ into the lattice of CeO2 leads to smaller crystallite size and enhanced reduction behavior.Moreover,the catalytic performance test shows that the activity and selectivity of these ceria-based solid solutions are higher than those of pure CeO2,and the Sn4+-doped sample(Ce0.67Sn0.33O2)is the best one.The reasons may be that:(1)the decrease of crystallite size results in the enlargement of BET specific surface area and the increase of surface Ce3+content,the former is conducive to the sufficient contact between catalyst and reactant molecules,the latter contributes to the adsorption of CO species;(2)the enhanced reduction behavior is beneficial to generating more surface oxygen vacancies during the reaction process,which can weaken the N-O bond to promote the dissociation of NO,effectively.(?)In the Part ?,we found that Ce0.67Sn0.33O2 solid solution,which obtained by incorporating Sn4+ into the lattice of CeO2,exhibited the optimal catalytic performance for NO+CO reaction.As a result,we carried out a more detailed study on the Sn4+-doped samples in the present part.Firstly,a series of CexSn1-xO2 mixed oxides with different Ce:Sn mole ratios and single oxides(CeO2 and SnO2)were prepared by inverse co-precipitation method and calcined at 450 and 750?;Secondly,the obtained samples were characterized in detail by means of XRD,UV-Raman,N2-physisorption,H2-TPR,XPS,and in situ DRIFTS technologies;Finally,the catalytic performance of these samples was evaluated through NO reduction by CO model reaction.The emphasis of the present work is that:investigating the influence of different Ce:Sn mole ratios on the physicochemical properties and catalytic performance of these CexSn1-xO2 catalysts.The characterization results indicate that the incorporation of Sn4+ into the lattice of CeO2 can result in the decrease of crystallite size,the increase of lattice strain and the improvement of reduction behavior,which are beneficial to the enhancement of catalytic performance.Furthermore,the catalyst with the optimal mole ratio(Ce:Sn=2:1 mole ratio)exhibits the best catalytic performance for NO reduction by CO,because that more catalytic domains(CD,-Ce3+-?-Sn2+-species,? represents the surface synergetic oxygen vacancy)can be generated in the reaction process due to the enhancement of reduction behavior and the formation of uniform solid solution without crystalline SnO2 blocking the active sites.Ce3+ is beneficial to the adsorption of CO species,while the surface synergetic oxygen vacancy can promote the dissociation of NO species,the synergistic interaction of them plays a significant role in NO+CO reaction.(?)Based on the investigation of the Part I,a series of ceria-based solid solutions(Ce0.67Zr0.33O2,Ce0.67Ti0.33O2,and Ce0.67Sn0.33O2)were still synthesized by inverse co-precipitation method,and then used as supports to prepare CuO/Ce0.67M0.33O2(M=Zr4+,Ti4+,and Sn4+)catalysts through wetness impregnation method in the present part.The obtained samples were investigated in detail by means of XRD,Raman,N2-physisorption,H2-TPR,XRF,XPS and in situ FT-IR techniques.Furthermore,the catalytic reduction of NO by CO as a model reaction was chosen to evaluate the catalytic performance of these ceria-based catalysts.The present work focuses on exploring the relationship among the intrinsic properties of dopants(Zr4+,Ti4+,and Sn4+),the physicochemical properties and catalytic performance of these ceria-based catalysts.The obtained results suggest that:(1)the reduction of CuO/Ce0.67Zr0.33O2 is easier than CuO/Ce0.67Ti0.33O2 and CuO/Ce0.67Sn0.33O2 catalysts,which may be attributed to the difference in the electronegativity of dopants;(2)the reduced state Cu+species is present in CuO/Ce0.67Zr0.33O2 catalyst at ambient temperature due to the shifting of redox equilibrium(Cu2++Ce3+(?)Cu++Ce4+)to right;(3)the adsorbed NO species on the surface of CuO/Ce0.67Zr0.33O2 catalyst are more liable to be desorbed/transformed/decomposed than those on the surface of CuO/Ce0.67Ti0.33O2 and CuO/Ce0.67Sn0.33O2 samples,which may be because that the d-electrons of the low-valence copper species donate to the anti-bonding orbital of NO to weaken the N-O bond.Moreover,the results of catalytic performance show that Cu+/Cuo species play a key role in NO reduction by CO,and the activity is closely related to the electronegativity of dopants,the reduction and adsorption behavior of these catalysts.(IV)This work mainly focuses on investigating the influence of preparation methods on the physicochemical properties and catalytic performance of CuO-CeO2 catalysts.Firstly,five different preparation methods have been used to synthesize CuO-CeO2 catalysts:mechanical mixing method(MMM),impregnation method(IM),grinding method(GM),hydrothermal treatment method(HTM)and co-precipitation method(CPM);Secondly,all of these samples were characterized by a series of techniques such as N2-physisorption,XRD,Raman,H2-TPR,ICP-AES,XPS,and in situ FT-IR;Finally,NO reduction by CO was chosen as a model reaction to compare the catalytic performance of these CuO-CeO2 catalysts obtained by different preparation methods.The obtained results show that the catalytic performance of these CuO-CeO2 catalysts can be ranked by CuCe-IM>CuCe-CPM>CuCe-GM>CuCe-HTM>CuCe-MMM,which is in agreement with the orders of the surface oxygen vacancy concentration,reducibility,and surface Cu+ content,suggesting that the synergistic effect between surface Cu+species and surface oxygen vacancy of these CuO-CeO2 catalysts plays an important role in NO+CO reaction,because surface Cu+ species is conducive to the adsorption of CO,surface oxygen vacancy can promote the dissociation of NO.