Mechanistic assessment of NO oxidative activation on tungsten-promoted ceria catalysts and its consequence for low-temperature NH3-SCR

Ceria has absorbed extensive interests in functioning as catalysts for low-temperature (LT) selective catalytic reduction (SCR) of NOx due to its abundant surface oxygen species and salient redox property. The mechanistic interpretations of how such redox property contributes to its LT-SCR activity, however, still remain debated. Here, we use a model tungsten-promoted ceria catalyst (WO3/CeO2), known as preeminent in LT-SCR reactions, and combine steady-state kinetic, structural characteristic and in situ spectroscopic experiments with theoretical treatments to reveal the mechanistic connections between NO oxidative activation and LT-SCR turnovers. We show that, compared with NO oxidation to NO2, NO oxidative activation to nitrite intermediates is both kinetically and thermodynamically more favorable; surface nitrate species are not detected in the present case, indicating their more difficult formation than nitrites. All these results thus suggest the prevalence of NO activation to nitrites over NO to nitrates or oxidation to NO2 and subsequent occurrence of fast SCR in the WO3/CeO2-catalyzed LT-SCR reaction. These findings progress the understanding of LT-SCR reaction mechanism on CeO2-based catalysts and convey a detailed perspective of different reaction intermediates and their consequences for LT-SCR turnovers, which may contribute to the rational design of catalysts towards further improved LT-SCR activity.