Influence of Sulfate Ratio on the Catalytic Performance of Cobalt Oxide in Low-Temperature Ammonia Oxidation

Abstract Cobalt oxide catalysts with different sulfate loadings (1, 3, and 5 wt %) were synthesized via precipitation and subsequent sulfation, followed by calcination at 500°C. The structural, textural, and acidic properties of the catalysts were systematically investigated using X-ray diffraction (XRD), Fourier Transform Infrared (FTIR) spectroscopy, N2 adsorption–desorption, and pyridine-adsorbed FTIR spectroscopy. Results revealed that moderate sulfation (3 wt %) significantly enhanced surface area (118.70 m2/g), mesoporosity, and surface acidity, while excessive sulfate loading (5 wt %) led to pore blockage and reduced accessibility of active sites. Catalytic tests demonstrated that the Co-3S catalyst achieved complete NH3 conversion (100%) with excellent N2 selectivity at a remarkably low temperature of 200°C, outperforming both pure Co3O4 and samples with lower or higher sulfate ratios. The superior activity was attributed to the synergistic effect of balanced Brønsted–Lewis acidity and optimized textural properties. Furthermore, Co-3S exhibited outstanding stability and reusability over multiple reaction cycles. These findings highlight that a moderate sulfate ratio is crucial for tailoring the surface and catalytic properties of cobalt oxide, offering a cost-effective and efficient alternative for selective ammonia oxidation.