脉冲介质阻挡放电下MnO_x-CeO_2催化氧化碳烟的关键表面物种研究

Evolution of key surface species on MnO_x-CeO_2 in pulse dielectric barrier discharge towards soot oxidation

采用柠檬酸络合法制备了Mn/(Mn+Ce)原子比为0.4的MnO_x-CeO_2复合氧化物,使其分别在有和无脉冲介质阻挡放电条件下,多次催化氧化碳烟.采用XRD、BET、Raman和XPS对新制备未反应过的和各次反应后的MnO_x-CeO_2进行表征.结果发现,在初次反应中,脉冲介质阻挡放电对反应具有强化作用,使MnO_x-CeO_2具备更好的活性.在放电和无放电2种条件下,催化剂第2次反应的活性均较第1次有所降低,且放电条件下降幅更大,而在第3次反应时催化剂活性在两种条件下均保持稳定状态.以上现象原因为,初次反应,两种条件下高温使MnO_x-CeO_2中Mn-Ce-O固溶体发生相分离,部分Mn析出生成了Mn_3O_4;未引入放电时催化剂表面发生不可逆还原的Mn^(4+)量更少,且拥有更多Ce^(3+)并产生了本征氧空位及更多的表面氧;放电的引入,加剧了初次反应时MnO_x-CeO_2表面Mn^(4+)向Mn^(3+)和Mn^(2+)的转化,使其具备更高的活性,但也抑制了Mn^(4+)的恢复,降低了催化剂的活性.在随后的反应中,有无放电催化剂均保持了稳定.脉冲介质阻挡放电对MnO_x-CeO_2反应的关键因素Mn^(4+)、本征氧空位和表面氧产生了重要影响,这种影响在初次反应时并不可逆,进而减弱了放电对反应的强化作用,该现象不可忽视,值得进一步研究.

MnOx-CeO2catalyst（ Mn/（ Mn＋Ce） molar ratio = 0.4） was prepared by citric acid complex method for repeated catalytic combustion of model soot with and without pulse dielectric barrier discharge（ PDBD）. Fresh and reacted MnOx-CeO2 were characterized by XRD,BET,Raman and XPS.MnOx-CeO2 with PDBD showed higher catalytic activity in the first combustion of model soot. The activity decreased without discharge and to a larger extent with discharge in the second round and then kept steady in the third round under the both conditions. It is found that the phase separation and irreversible reduction of Mn^4＋of MnOx-CeO2 after first round was responsible for the decreased catalytic activity. PDBD facilitated the irreversible transformation of Mn^4＋to Mn^3＋and Mn^2＋and inhibited its recovery with less intrinsic oxygen vacancies and surface oxygen,leading to reduced activity.Although the catalyst remained stable after the first round,the irreversible impact on MnOx-CeO2 of PDBD deserves further study.