Performance and Mechanism of Cobalt-Aluminum Spinel Catalyzed Oxidation of Nitric Oxide
Cobalt-aluminum spinel metal oxides derived from hydrotalcite were synthesized via hydrothermal, coprecipitation, and sol-gel methods, and their catalytic performance for NO oxidation was systematically evaluated. Characterization by X-ray photoelectron spectroscopy (XPS), O2 temperature-programmed desorption (O2-TPD), H2 temperature-programmed reduction (H2-TPR), and Raman spectroscopy revealed that the synthesis method significantly influences the surface Co2+/Co3+ ratio, which in turn modulates the formation of surface oxygen vacancies. The hydrothermally synthesized catalyst (CoAlO-H) exhibited the highest density of surface oxygen vacancies, leading to enhanced adsorption and activation of gaseous oxygen and superior NO oxidation activity compared to coprecipitation (CoAlO-C) and sol-gel (CoAlO-S) counterparts. Mechanistic studies using NO-TPD, NO+O2-TPD, and in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) identified nitrates as key intermediates. Notably, CoAlO-C and CoAlO-S followed the Langmuir-Hinshelwood (L-H) mechanism, whereas CoAlO-H operated via both L-H and Mars-van Krevelen (MvK) mechanisms. The exceptional performance of CoAlO-H is attributed to its abundant surface oxygen vacancies, high surface oxygen mobility, and low decomposition temperature of reaction intermediates. These findings provide a rational basis for designing efficient non-precious metal catalysts for NO oxidation in diesel exhaust aftertreatment.