Effects of contact model and NOx on soot oxidation activity over Pt/MnOx-CeO2 and the reaction mechanisms

- NO and NO2 has no promotion for the soot oxidation in the tight contact.
- The promotion of NOx in loose contact is mainly related to active oxygen species.
- NO shows better promotion than NO2 due to formation of more surface nitrates.
- Active oxygen species would play a key role in NOx-assisted soot oxidation.

This study aims to compare the promoting roles of NO and NO2 additions on soot oxidation over Pt/MnOx-CeO2 under tight and loose contacts and further reveal the mechanisms for the promoted reactions by DRIFTS (diffuse reflectance infrared fourier transform spectroscopy) and GCMS (gas chromatograph mass spectrometer). The soot oxidation tests show that the promotion of NOx cannot be exhibited under tight contact and NO addition displays a greater promoting role than NO2 addition under loose contact. The mechanism investigations indicate that more surface nitrates form on the catalyst in NO and their decomposition effectively enhances the formation and desorption of more active oxygen species, which would be a key role in the accelerated soot oxidation reactions. However, the promoting role of NO can not be found in tight contact, it may be due to the inhibition of nitrate formation by soot coverage on the surface of the catalyst. This work also confirms that active oxygen would be main contributor for soot oxidation despite the presence of NO2.

The improved activity of Pt/MnOx-CeO2 in NOx + O2 would be mainly attributed to the formation of active oxygen species (O∗) during surface nitrate decomposition. Furthermore, the NO in reaction gas shows greater promoting roles than NO2 due to the formation and decomposition of more surface nitrates.153