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

Ceria-supported transition metal oxide (such as CoO_x) catalysts are promising, more cost-effective candidates to replace platinum group metal catalysts in the NO reduction process. A series of CoO_x (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 CoO_x/CeO₂ catalysts. It was observed that there are structural changes with varied Co loadings, such as (1) sub-monolayer: <2.3 Co/nm², (2) monolayer: 2.3-2.7 Co/nm², and (3) over-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. The results provided several structure-property relationships and a possible reaction mechanism for NO reduction by CO reaction over CoO_x/CeO₂ catalysts.