Facile and green synthetic strategy of birnessite-type MnO2 with high efficiency for airborne benzene removal at low temperatures

Volatile organic compounds (VOCs) are notorious for global air pollution. It is key to find a material with high and stable activity to catalytically oxidize VOCs at low temperatures. In this paper, benzene, as a typical VOC contaminant, could be completely oxidized on a highly efficient and moisture-resistant birnessite MnO2 at temperatures significantly lower than the reported values in literature. The best catalyst (H-MnO2 (30-0.2-6)) could be tailored via a facile and green process by engineering a proper combination of treatment temperature (30 degrees C), HNO3 aqueous concentration (0.2 M) and treatment period (6 h). The thus-obtained MnO2 exhibited a stable removal efficiency of 94% for 318 ppm of benzene under a high space velocity of 120 L.g(-1) h(-1) and 1.5 vol.% H2O at 250 degrees C. XRD, SEM, (HR)TEM, EDS mapping, XPS, H2-TPR, O-2-TPD and pyridine adsorption IR were combined with the deliberately designed C6H6-TPD, surface oxidation reaction of benzene and CO2-TPD to disclose the tremendous role of acid treatment in benzene oxidation. The increased acid sites and acidity of the acid-treated surface promoted the adsorption and activation of gaseous benzene, and the lattice oxygen and surface adsorbed oxygen became more facile and reactive due to the generated active oxygen vacancies via acid treatment, these two favorable factors together with the easy desorption of reaction products resulting in the excellent performance of the acid-treated sample. Finally, the acid treatment method can be extended to most of other crystal structures of manganese dioxides except alpha-MnO2.