• • Hydrothermal synthesis yields CoAlO-H with the highest surface oxygen vacancy density, directly enhancing NO oxidation activity compared to coprecipitation and sol-gel methods.
• • CoAlO-H operates via dual L-H and MvK mechanisms, whereas CoAlO-C and CoAlO-S follow only L-H, indicating a mechanistic shift driven by oxygen vacancy concentration.
• • Nitrate and nitrite species are critical intermediates; CoAlO-H exhibits lower decomposition temperatures for these intermediates, facilitating faster NO-to-NO2 conversion.
• • The Co2+/Co3+ ratio is a key descriptor: higher ratios correlate with more oxygen vacancies, providing a design principle for optimizing cobalt-based spinel catalysts.