Monolithic catalysts loaded with ZIF-derived Co3O4 on copper foam for the catalytic oxidation of toluene: the impact of synthetic methods

Abstract

A series of copper foam-based monolithic catalysts were prepared by the hydrothermal method, electrophoretic deposition method, and impregnation method using ZIF-67 as a sacrificial template agent. Among them, the Co₃O₄@CF-H catalyst prepared by the hydrothermal method exhibited excellent toluene catalytic oxidation activity with a T₉₀ (temperature at 90% conversion of toluene) of 218 °C when the weight hour space velocity (WHSV) was 15 000 mL g⁻¹ h⁻¹. The favorable catalytic performance for toluene was maintained even at higher space velocity (WHSV = 60 000 mL g⁻¹ h⁻¹ and T₉₀ = 234 °C) and in an oxygen-poor environment (T₉₀ = 232 °C at 3 vol% O₂ content). Particularly, the Co₃O₄@CF-H catalyst exhibited excellent resistance to CO₂ interference and stability. The excellent catalytic performance of the Co₃O₄@CF-H monolithic catalysts, analyzed in conjunction with a variety of characterization results, was attributed to their strong cobalt–copper interaction, excellent low-temperature reduction and oxygen mobility, as well as the abundance of reactive oxygen and Co³⁺ species. In situ DRIFTS results revealed that the degradation path of toluene on the Co₃O₄@CF-H catalyst may be as follows: toluene → benzyl alcohol → benzaldehyde → benzoic acid → maleic anhydride → acetone → CO₂ and H₂O. DFT calculations illustrated that the loading of Co₃O₄ and the presence of Cu⁰ in the copper foam skeleton were both beneficial for improving the catalytic activity. Specifically, it favored the adsorption of oxygen and toluene, and facilitated the dissociation and activation of O₂. Overall, this work provides a monolithic catalyst with potential applications for the treatment of VOC exhaust gases.