Red mud (RM), a metal oxide-rich solid waste, was subjected to three different acid treatments to evaluate its catalytic performance in toluene oxidation. The acetic acid-modified red mud (HAC-RM) demonstrated excellent catalytic activity, achieving complete toluene conversion at 450 °C. XRD, XRF, N2-BET and SEM results show acetic acid treatment can effectively remove pore-blocking inert components such as Na2O and CaO, thus increased the Fe2O3 content, and significantly enhanced both the specific surface area and pore size of the catalyst. Furthermore, this modification enhanced reducibility and generated additional oxygen vacancies, verified by H2-TPR and O2-TPD, thereby improving the overall catalytic performance. In contrast, oxalic acid treatment under ultraviolet irradiation led to the formation of calcium carbonate via reaction with Ca2+ ions in RM, which resulted in reduced catalytic activity. To further enhance performance, MnO2 was loaded onto the modified HAC-RM via an impregnation method to develop a low-cost and highly active catalyst. Among the prepared samples, 20%MnO2/HAC-RM exhibited the highest catalytic efficiency, achieving 100% toluene conversion at 300 °C. XPS, H2-TPR, and O2-TPD results indicate the synergistic interaction between Fe2O3 and MnO2 facilitated electron transfer and enhanced surface oxygen mobility. Additionally, the catalytic oxidation mechanism of 20% MnO2/HAC-RM was elucidated. A detailed reaction pathway for toluene degradation is proposed by in situ DRIFT, as follows: toluene → benzyl alcohol → benzaldehyde/benzoyl peroxide → benzoate → CO2 and H2O. These findings are expected to contribute to the development of efficient, sustainable, and cost-effective catalysts for volatile organic compound (VOC) abatement.
Wenjun et al. (2026) studied this question.