Ultrasonic-assisted green synthesis of δ-MnO2 promoting formaldehyde degradation: regulation of oxygen vacancies

It remains challenging to prepare δ -MnO 2 catalyst with high catalytic performance by facile and low-energy green synthesis method. In this work, an ultrasonic-assisted method using plant-derived green reducing agents (quercetin, apigenin, and glucose) was employed to prepare δ -MnO 2 catalysts at room temperature. Due to the type and quantity of hydroxyl groups (phenolic or glycosyl), and the ultrasonic treatment time, the crystallinity and surface lattice defects, specific surface area, and surface Mn 3 +  concentration are modulated, thereby effectively regulating the oxygen vacancies of the δ -MnO 2 samples. The results showed that the δ -MnO 2 -Q30 catalyst prepared with quercetin containing the highest number of phenolic hydroxyl groups exhibited outstanding catalytic activity and stability. Specifically, under static test conditions at room temperature, the removal efficiency of HCHO reached 96% within 30 min for approximately 1 ppm HCHO, and remained at 92% after five cycles. This indicates that the increase in the number of phenolic hydroxyl groups, along with appropriate reaction time, can improve the oxygen vacancies in the δ -MnO 2 structure, promote the activation of surface-adsorbed oxygen species, accelerate the generation of ROS, and thus facilitating the oxidative degradation of HCHO. Furthermore, EPR and in situ DRIFTS were employed to systematically detect ROS and reaction intermediates, elucidating that oxygen vacancy-induced ROS are the key factor for the conversion and oxidation of intermediates. This work provides new insights for further developing green synthetic δ -MnO 2 particles and enhancing their catalytic performance for formaldehyde removal at room temperature. Graphical