Catalytic soot oxidation using hierarchical cobalt oxide microspheres with various nanostructures: Insights into relationships of morphology, property and reactivity

While diesel engines are extensively employed, soot from diesel engines lead to serious air pollution, which should be eliminated through catalytic soot oxidation processes. As Co₃O₄ represents a practical catalyst for oxidizing soot, it is critical to further investigate relationships of morphology-property-reactivity of Co₃O₄ catalysts for optimizing design of Co₃O₄ for soot oxidation. Thus, the present study, as the first attempt, aims to manipulate morphologies of Co₃O₄ microspheres to exhibit different surficial hierarchical nanostructures for comparing their corresponding catalytic activities. Specifically, four hierarchical Co₃O₄ microspheres (HCOMs) are fabricated, including Co₃O₄ with flower morphology (COF), Co₃O₄ with urchin morphology (COU), Co₃O₄ with sheet morphology (COS), and Co₃O₄ with pyramid morphology (COP), for comparisons in catalytic soot oxidation. The results indicate that these HCOMs showed much higher catalytic activities than the commercial spherical Co₃O₄ nanoparticles and most of reported cobalt-based catalysts as these HCOMs possessed high surface areas and porous nanostructures. In particular, among these HCOMs, COP stood out as the most advantageous HCOM for soot oxidation as COP contained more reactive surface and more superior redox properties than other HCOMs, thereby enabling COP to accelerate soot oxidation. These HCOMs could be also recyclable for soot oxidation over multiple cycles without loss of catalytic activities. These results and findings demonstrate that HCOMs are certainly promising cobaltic catalysts for oxidizing soot, and, through this present study, the relationship between morphologies, properties and catalytic activities can be correlated to provide important insights to design and apply hierarchically nanostructured Co₃O₄ catalysts for soot oxidation.