Effects of Molecular and Electronic Structures in CoOx/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/nm 2 ) catalysts supported on CeO 2 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 /CeO 2 catalysts. It was observed that there are structural changes with varied Co loadings, such as (1) sub-monolayer: <2.3 Co/nm 2, (2) monolayer: 2.3–2.7 Co/nm 2, and (3) over-monolayer: >2.7 Co/nm 2 . The highest molar rate was observed at the 2.7 Co/nm 2 sample. In the case of over-monolayer samples, such as 7.1 Co/nm 2, the oxidation state of Co affected the catalytic activity. Using in situ XAS, an oxidation state change from Co 3+ to Co 2+ between 200 and 300 °C was identified. Catalyst deactivation was also affected by the change of Co oxidation states from the fresh sample (Co 3+ ) to the used sample (Co 3+ /Co 2+ ). N 2 O formation and decomposition were affected by the reaction temperature in a two-step procedure, where NO converts into N 2: (1) NO → N 2 O and (2) N 2 O → N 2 . N 2 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 /CeO 2 catalysts.