Effects of Molecular and Electronic Structures in CoOx/CeO2 Catalysts on NO Reduction by CO

Ceria-supported transition metal oxide (such as CoOx) catalysts are promising, more cost-effective candidates to replace platinum group metal catalysts in the NO reduction process. A series of CoOx (0.2–31.3 Co/nm2) catalysts supported on CeO2 were prepared by the incipient wetness impregnation method and were tested for NO reduction by CO reaction in this work. Various characterization techniques, including Brunauer–Emmett–Teller, Raman spectroscopy, powder X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) were used to investigate the molecular and electronic structures of CoOx/CeO2 catalysts. It was observed that there are structural changes with varied Co loadings, such as (1) sub-monolayer: 2.7 Co/nm2. The highest molar rate was observed at the 2.7 Co/nm2 sample. In the case of over-monolayer samples, such as 7.1 Co/nm2, the oxidation state of Co affected the catalytic activity. Using in situ XAS, an oxidation state change from Co3+ to Co2+ between 200 and 300 °C was identified. Catalyst deactivation was also affected by the change of Co oxidation states from the fresh sample (Co3+) to the used sample (Co3+/Co2+). N2O formation and decomposition were affected by the reaction temperature in a two-step procedure, where NO converts into N2: (1) NO → N2O and (2) N2O → N2. N2 selectivity monotonically increased with an increasing reaction temperature between 200 and 400 °C. Furthermore, the results provided several structure–property relationships and a possible reaction mechanism for NO reduction by CO reaction over CoOx/CeO2 catalysts.