(14d) Effects of Surface Species and Dispersion of CeO2 Supported Transition Metal Oxide Catalysts for NO Reduction By CO Reaction

Nitrogen oxides (NO_x) are a main contributor to global air pollution ^[1]. During the past few decades, much attention has been paid to eliminating NO_x pollution and technologies such as NH₃-SCR and NO_x storage and reduction have been developed for NO_x reduction ^[2]. As of now, most commercial catalytic converters use platinum group metals (PGMs) and zeolites for NO_x abatement ^[3]. Although these catalysts are highly active in NO_x reduction reactions, NO_x decomposition over PGMs and zeolites has some practical issues such as high cost and high light-off temperature ^[4]. However, transition metal oxide catalysts were found to also be active in NO_x reduction reactions. By far, transition metal oxide catalysts have been investigated for many reactions including NO_x decomposition reaction by CO, which is a primary reaction in three-way catalysis (TWC).

In this study, series of CeO₂ supported CoO_x, FeO_x, or NiO_x catalysts were prepared by incipient wetness impregnation method for NO reduction by CO. Catalytic activity and catalyst physicochemical property were investigated by FTIR, BET, and Raman spectroscopy. No diffraction peaks due to crystalline CoO or Co₃O₄ were observed in XRD patterns under to 10 wt%, while nanocrystaline Co₃O₄ peaks at ~610cm^-1 was observed in the Raman spectra with 5 wt% sample, indicating that 5-6 wt% CoO_x/CeO₂ contains monolayer coverage. Similarly, 1 wt% NiO_x/CeO₂ and 10 wt% FeO_x/CeO₂ also showed monolayer coverage. It was observed that both surface species and dispersion are closely related to the catalytic activity in NO reduction by CO. For example, for CoO_x/CeO₂ catalysts, monolayer and above monolayer coverage catalysts showed 80~90% CO and NO conversion at 300~400^oC with high N₂ selectivity (>90%), whereas sub-monolayer coverage catalysts had significantly low activity.