Promotional effects of cerium doping and NOx on the catalytic soot combustion over MnMgAIO hydrotalcite-based mixed oxides

A series of MnMgAlO 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 IR were used to characterize the physiochemical properties of these samples. Dopant Ce improved the soot combustion performance of MnMgAlO catalyst due to the enhanced redox ability. Introduction of NO_x 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 O₂. Differently, 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 NO₂ and nitrates were found to have great contributions to the effects of NO_x 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, Mn_0.5Mg_2.5Ce_0.1Al_0.9O was considered as the most potential catalyst for soot combustion.