Revealing the boosting role of NO for soot combustion over CeO2(111): A first-principles microkinetic modeling

Abstract

Co-feeding NO can show an interesting boosting role in the soot combustion reaction, but the molecular-level mechanism remains unsettled and thus hinders the rational optimization of such an important catalytic system. Herein, a first-principles study integrating with microkinetic model is performed to quantitatively explore the catalytic mechanism of soot combustion on CeO2(111) in the presence of NO, aiming to unravel the origin of promotional effect of NO. The catalytic activities of different N-containing species evolved from NO oxidation, i.e. nitrite/nitrate trapped at O vacancy sites and the surface-bound nitrate, which are the generally supposed species for boosting soot combustion, are explicitly examined. The microkinetic result indicates that the soot combustion activity in the presence of NO at the typical temperature of 600 K is nearly two orders of magnitude higher than that in the absence of NO. Despite the improved reactivities relative to the pristine CeO2(111), we find that these oxidative species themselves are unexpectedly not the most crucial in facilitating soot combustion at the typical experimental condition; the in-situ generated O vacancies from the NO oxidation side-reaction largely enhance the adsorption of more reactive peroxide O22−, which indirectly contribute more to the overall soot combustion efficiency. Moreover, the effects of temperature and NO partial pressure on modulating the soot combustion are discussed with the optimal value provided. These insights deepen understanding of the NO effect on soot combustion and could facilitate the optimization of after-treat system in diesel exhaust emission.