过渡金属掺杂对镧锡烧绿石催化碳烟燃烧性能的影响

Catalytic Combustion of Soot Particulates over Rare Earth Pyrochlore Oxides Doped with Transition Metals

催化燃烧是去除机动车排放碳烟颗粒物的有效方法之一.利用CTAB辅助法制备了不同过渡金属掺杂的镧锡烧绿石型La_2Sn_(1.8)TM_(0.2)O_7(TM为Sn、Mn、Fe、Co和Cu)复合氧化物催化剂.采用XRD、氮气吸脱附、SEM、FT-IR、H2-TPR和荧光光谱(PL)等手段表征了催化剂的理化性质,采用程序升温氧化(TPO)技术评价了其催化碳烟燃烧的活性.研究发现,催化剂经900℃焙烧后呈球形形貌,具有相对较大的比表面积(~20 m2·g^(-1)).低价过渡金属离子的掺杂使氧空位的整体浓度增加,有利于催化剂活化吸附氧分子,改变材料的氧移动和氧化还原性能.富氧气氛下,少量过渡金属掺杂提高了烧绿石催化碳烟燃烧的活性和选择性,这与氧空位浓度的增加以及氧化还原能力的提高有关,其中Co-LSO具有较好的催化性能,起燃温度(T10)为379℃,CO_2选择性接近100%;NOx气氛存在可以进一步提升催化剂氧化去除碳烟的活性.利用等温反应和碳烟厌氧滴定实验进行了活性氧浓度的定量和反应动力学分析,并计算出富氧气氛下催化反应的转化频率(TOF),其中Co-LSO样品的TOF值最大为3.20×10^(-3)s^(-1),基于TOF的活性顺序与TPO法得到的起燃性能的结果基本一致.

Catalytic combustion is an efficient way to remove soot particulates from automobile exhausts.A series of rare earth pyrochlore oxides La_2Sn_（1.8）TM_（0.2）O_7（TM = Sn,Mn,Fe,Co,Cu） were prepared with CTAB-assisted sol-gel method.The products were characterized by XRD,N_2 Adsorption-Desorption,SEM,FT-IR,H2-TPR and PL techniques.Their catalytic activity for soot oxidation was investigated by TPO under tight conditions in both O_2 and NO ＋ O_2 atmospheres.After calcination at 900℃,the as-synthesized oxides with pure pyrochlore phase displayed nanospheres with diameter of 30-60 nm and relatively large surface areas.The partial substitution of Sn with transition metals largely influenced the reduction behavior of pyrochlores especially on the low temperature range in H2-TPR profiles,which was due to the interactions between Sn and transition metals.The improved oxygen mobility might be derived from the structure defects induced by transition metals doping,which could be helpful in oxidation reactions.In comparison with uncatalyzed reactions,the La_2Sn_2O_7 catalyst exhibited modest catalytic activity for soot combustion,while transition metals incorporation further enhanced the activity and selectivity.The improved activity of transition metals doped samples was likely to be associated with the improved reducibility and increased surface oxygen vacancies on the pyrochlore oxides.The presence of NO in the gas phase significantly enhanced the soot oxidation activity,which was due to the promotion effect of NO_2.Especially,the densities of active oxygen sites and turnover frequency（TOF） values of the catalysts,quantified by isothermal anaerobic titration with soot as a probe molecule,were used to explain the different soot combustion behaviors.Among the pyrochlore oxides,the Co-doped pyrochlore sample displayed the highest ignition activity and the largest intrinsic activity with TOF of 3.20 × 10^（-3）s^-1.