Different mechanisms between reactions of soot with gaseous and adsorbed NO2

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+.