Design and optimization of cobalt-based and cobalt-promoted catalysts for enhanced low-temperature toluene combustion

Abstract

Volatile organic compounds (VOCs), predominantly emitted from anthropogenic sources such as industrial processes, transportation, and indoor activities, are recognized as major contributors to environmental degradation and public health risks, primarily through their role as precursors to ozone and secondary organic aerosols. In response, various technologies have been developed for VOC removal, including biological degradation, adsorption, plasma catalysis, photocatalytic oxidation, and catalytic oxidation. Among these, low-temperature catalytic combustion stands out as a promising approach due to its lower operating temperature and minimal secondary pollution. While noble metal catalysts exhibit excellent low-temperature activity and complete mineralization, their widespread application is constrained by high cost, susceptibility to sintering, and poisoning by sulfur or chlorine. Consequently, significant research efforts are directed toward developing non-noble metal alternatives.
As a typical spinel-type transition metal oxide, Co₃O₄ has garnered considerable attention for VOC combustion, owing to its favorable redox properties and high oxygen mobility. However, a well-known limitation of Co₃O₄ is its poor tolerance to water vapor, which is commonly present in practical waste streams. To address this challenge, the initial part of our study focused on elucidating the influence of water vapor on the toluene degradation mechanism. Through in-situ DRIFTS analysis, we discovered that water vapor alters the reaction pathway, promoting the formation of maleate and quinone species. These intermediates are resistant to further decomposition even at elevated temperatures, leading to their accumulation on the catalyst surface and a consequent decline in activity. This insight also provides a direct explanation for the superior performance observed in subsequent studies of cobalt-promoted catalysts: systems that circumvent the formation of maleate intermediates demonstrate best catalytic behavior. The performance of spinel Co₃O₄ can be further enhanced through morphology and facet control. In our second investigation, the Co₃O₄-R catalyst, dominated by facets {220} (53.9 %), achieved the highest normalized reaction rate of 1.26 μmol·s⁻¹·gcat⁻¹. In contrast, the Co₃O₄-T sample, with the highest proportion of {111} planes (46.9 %), exhibited the lowest activity (0.03 μmol·s⁻¹·gcat⁻¹), strongly indicating that the {220} facet is more active than the {111} facet for toluene oxidation. The final phase of our research, centered on cobalt-promoted catalysts, highlighted the significant potential of strategic cobalt incorporation. With only 1.5 wt% Co loading, the CoCeO₂-400 catalyst achieved a T₉₀ of 241 °C—representing a 50 % improvement over pristine CeO₂.
Collectively, this thesis presents a systematic investigation into cobalt-based catalysts for toluene oxidation, investigating the intrinsic properties of pure Co₃O₄, controlling its catalytic performance through crystal facet engineering, and incorporating cobalt as a promoter into CeO₂ to create highly active composite systems.

Date of Award	18 Jul 2026
Original language	English
Awarding Institution	University of Nottingham
Supervisor	Tao Wu (Supervisor) & Hongfeng Yin (Supervisor)