(2022) Separation and Purification Technology_Broccoli-like CeO₂ with Hierarchical/Porous Structures, and promoted oxygen vacancy as an enhanced catalyst for catalytic diesel soot elimination

Tsai Y.-C., Kwon E., Park Y.-K., Nhat Huy N., Lisak G., Hsu P.-S., Hu C., Lin K.-Y.A.

(Elsevier B.V.) Separation and Purification Technology ISSN: 13835866 Vol.281 Issue. Article No.119867 DOI: 10.1016/j.seppur.2021.119867

While CeO2 is a promising catalyst for soot elimination, it is essential to develop CeO₂ with higher contact areas, and reactivity for effective soot oxidation as catalytic soot oxidation is dominantly controlled by structures, and surficial properties of catalysts. In this study, a Ce-Metal organic frameworks (MOFs) consisting of Ce and trimesic acid (TA) is employed as the precursor as CeTA exhibits a unique broccoli-like hierachitecture which is transformed into CeO₂ with a hierarchical structure consisting of nanofibers of CeO₂ bundled together, forming a broccoli-like CeO₂ nanostructure. More importantly, these CeO₂ nanofibers in this broccoli-like CeO₂ (BCL-CeO2) possesses porous structures, and also more oxygen vacancies, enabling BCL-CeO₂ to become a promising catalyst for soot oxidation. Thus, BCL-CeO₂ shows a much higher catalytic activity than commercial CeO₂ nanoparticle (com-CeO₂) for soot oxidation with a significantly lower ignition temperature (Tig). More importantly, while soot oxidation by com-CeO₂ leads to production of CO together with CO₂, BCL-CeO₂ can completely convert soot to CO₂. The tight contact mode also enables BCL-CeO₂ to exhibit a very low Tig of 295 °C, whereas the existence of NO and H₂O also enhances the soot oxidation by BCL-CeO₂ to reduce the Tig. The mechanism of NO-assisted soot oxidation is also examined, and validated by DRIFTS to identify the presence and transformation of nitrogen-containing intermediates. BCL-CeO₂ is also recyclable over many consecutive cycles and maintained its high catalytic activity for soot oxidation. These results demonstrate that BCL-CeO₂ is a promising and easily-prepared hierarchitectured Ce-based catalyst for soot oxidation. © 2021 Elsevier B.V.