The reactions of soot with gaseous and adsorbed NO2were tested over the K/MgAlO catalyst.After the reaction intermediates were identified by combination of in situ IR characterization and first-principles calculation,the different mechanisms were elucidated.It was found that the reactivity of adsorbed NO2is lower than that of the gas form.The adsorbed NO2reacts with soot in the form of nitrates,leading to the observation of two IR bands at 2,234and 2,110 cm-1,which are ascribed to the vibration frequencies of cyanates on K sites and cyanides on the MgAlO support,respectively.On the contrary,the isocyanates were confirmed as intermediates in the reaction of soot with gaseous NO2.Because the adsorbed NO2species(i.e.,nitrates)are restricted by the electrostatic field of K+,the cyanates are produced and readily cracked into cyanides,which transfer to the MgAlO support.The gaseous NO2favours the production of isocyanates due to their higher stability.The weaker reactivity of adsorbed NO2at lower temperatures can be attributed to the restriction of the electrostatic field of K+.
A series of MnMgA10 samples with different amounts of Ce doping were facilely prepared using coprecipitation method and their catalytic soot combustion activity was evaluated by temperature programmed oxidation reaction （TPO）. The methods of X-ray diffraction （XRD）, Brumauer-Emmett-Teller （BET）, H2-TPR, NO-TPO and in situ 1R were used to characterize the physio- chemical properties of these samples. Dopant Ce improved the soot combustion performance of MnMgA10 catalyst due to the en- hanced redox ability. Introduction of NOx led to the further increase of catalytic soot oxidation activity on these samples. Over Ce-containing samples, the catalytic activity was slightly decreased as the amount of dopant Ce increased in 02. Diftbrently, in NO＋O2, a certain amount of dopant Ce was much more favorable and excess amount of Ce resulted in a sharp drop of the catalytic soot combustion activity. Both NO： and nitrates were found to have great contributions to the effects of NOx on the soot combustion activity of Ce-doped catalysts. More NO2 was generated as dopant Ce increased. When appropriate amount of Ce was introduced, the as-formed NO2 was stored as bridging bidentate nitrate on Mn-Ce site, which was confirmed to have higher reactivity with soot than nitrite or monodentate nitrate on Mn and/or Ce sites. Overall, Mno.sMg2.sCeo.lAlo.90 was considered as the most potential catalyst for soot combustion.
