Effect of Solvent Washing and Drying Strategies on the Optimization of Cobalt Oxide Catalysts for CO Oxidation

Abstract In this study, nanostructured cobalt oxides (CoO x ) were synthesized hydrothermally from Co(NO 3 ) 2 ·6H 2 O and urea at 110 °C for 12 h, and the effects of washing (water or ethanol) and drying (oven drying or freeze-drying) pretreatments on their physicochemical properties and catalytic performance toward CO oxidation were systematically investigated. The catalysts were characterized using XRD, FTIR, SEM/TEM, BET, TPR, Raman, XPS, and EPR analysis, and their catalytic activities were evaluated in a custom-designed micro-reactor. The results show that pretreatment plays a decisive role in governing CO oxidation efficiency. Among all samples, the ethanol-washed and freeze-dried catalyst (CoO x E–F) displayed the highest activity, achieving T 50 and T 100 temperatures of 62 °C and 125 °C, respectively. The superior performance of CoO x E–F is attributed to the efficient removal of residual ions by ethanol washing and the structural preservation afforded by freeze-drying, which together produce highly dispersed nanostructures with large surface area, uniform mesoporosity, weakened Co–O bond strength, abundant Co 3+ species, and a high concentration of oxygen vacancies. These results demonstrate that combining ethanol washing with freeze-drying is an effective pretreatment strategy for optimizing defect formation and enhancing the intrinsic catalytic properties of Co 3 O 4 -based catalysts for low-temperature CO oxidation. Graphical Abstract