Isotopic study of ceria-catalyzed soot oxidation in the presence of NOx

The ceria-catalyzed soot oxidation mechanism has been studied by a pulse technique with labeled O2 in the absence and presence of NO, using ceria–soot mixtures prepared in the loose contact mode. In the absence of soot, the ceria-catalyzed oxidation of NO to NO2 takes place with ceria oxygen and not with gas-phase O2. However, the oxygen exchange process between gas-phase O2 and ceria oxygen (to yield back O2, but with oxygen atoms coming from ceria) prevailed with regard to the ceria-catalyzed oxidation of NO to NO2. Gas-phase O2 did not react directly with soot when pulsed to a soot–ceria loose contact mixture. Instead, ceria oxygen is transferred to soot (this step does not require gas-phase molecular oxygen to be present), and gas-phase O2 fills up the vacancies created on the oxide in a further step. The transfer of oxygen between ceria and soot occurred directly in the absence of NO. However, in the presence of NO, NO2 is expected to be additionally generated by ceria oxygen oxidation, which also reacts with soot. The main reaction products of the ceria-catalyzed soot oxidation reaction with NO/O2 were CO2 and NO. Additionally, evidence of the reduction of NOx to N2 was found.

The ceria-catalyzed oxidation of soot by O2/NO takes place with ceria oxygen (directly and via NO oxidation to NO2), and gas-phase O2 fills up the oxygen vacancies created on the catalyst.Figure optionsDownload high-quality image (106 K)Download as PowerPoint slideHighlights
► Ceria oxygen, and not gas-phase molecular oxygen, oxidizes NO to NO2.
► Ceria oxygen exchanges with gas-phase O2 and prevails with regard to NO oxidation.
► Ceria oxygen oxidizes soot directly (active oxygen) or mediated by NO2.
► Gas-phase O2 does not react with soot, but fills up oxygen vacancies on ceria.