Preparation of carbon-coated CeO₂–Co₃O₄ and optimization study on degradation efficiency of methylene blue

Synthetic dye, a major contributor to global water pollution, poses substantial and irreversible threats to human health and ecological systems. Recently, water treatment technologies based on peroxymonosulfate (PMS) have emerged as a promising solution. The decomposition of PMS generates reactive oxygen species (ROS) with high redox potential, but the slow reaction rate limits its practical application in water remediation. To increase the PMS decomposition efficiency, a comprehensive experiment is designed to synthesize carbon-coated CeO₂–Co₃O₄ composite catalysts.

In this comprehensive experiment, a hollow CeO₂–Co₃O₄ precursor was initially prepared via the solvothermal method. Subsequently, a resorcinol–formaldehyde (RF) resin layer was applied to the precursor surfaces using the sol–gel process, followed by carbonization to form a core–shell structured material. Concurrently, by introducing silicon dioxide (SiO₂) interlayers of different thickness values as templates, two types of carbon-coated Co₃O₄–CeO₂ with a “rattle” structure were fabricated. The structural characteristics of the materials were analyzed using X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and N₂ adsorption–desorption techniques. The performance of the catalysts was evaluated using methylene blue (MB) as a model organic pollutant, and scavengers were employed to identify the predominant ROS involved in the reaction.

The successful preparation of the designed materials was confirmed by TEM. Compared with the uncoated product (Co₃O₄–CeO₂), the RF and SiO₂ layers effectively inhibited grain growth during carbonization. The rattle-type composites exhibited significantly higher BET surface areas, particularly the one made with 0.75 mL tetraethyl orthosilicate or TEOS (Co₃O₄–CeO₂@h-C-1, 222.1 m²/g). In contrast, the BET surface area of the core–shell structured sample (Co₃O₄–CeO₂@C, 24.8 m²/g) was even lower than that of Co₃O₄–CeO₂ (55.8 m²/g) due to the sealing effect of dense carbon shell. The Co₃O₄–CeO₂@h-C-1 demonstrated the best catalytic performance with ¹O₂ and ${\text{SO}}_4^{·-}$ as the main ROS in MB degradation. This superiority is due to its highest BET surface area and unique rattle-type structure. Moreover, the small grain size of the metal oxides creates more defects and active sites on their surface, which is beneficial for PMS activation.

This comprehensive experiment simulates the entire scientific research process, encompassing the preparation, characterization, performance testing, and data analysis of carbon-coated Co₃O₄–CeO₂ composites with different structures. This experimental project trains students to understand the structure–activity relationship of catalysts and the comprehensive application of knowledge from chemistry and material disciplines, enhancing students' experimental design capabilities and fostering scientific thinking and innovative awareness.