Activity of manganese oxides supported on halloysite towards the thermal catalytic oxidation of formaldehyde: Constraint from the manganese precursor

Abstract Mn-based catalysts have been widely studied for the oxidation of volatile organic compounds (VOCs) and appear to be the most active catalysts among the transition metal oxides. In this study, manganese oxides are supported on halloysite, a nanoscale and porous clay mineral, by an impregnation method using manganese nitrate, manganese acetate, and potassium permanganate as precursors. The nitrate results in scattered 35–249 nm agglomerates of pyrolusite (β-MnO2) on the external surface of the halloysite nanotubes; the acetate produces hausmannite (Mn3O4) particles with sizes of 15–40 nm dispersed on the external surface, while the permanganate creates nanoparticles and agglomerates of amorphous K-containing MnO2 on the external surface as well as in the lumen. These differences in structure and distribution are related to the oxidizability of the precursor, surface charge and the cylindrical lumen of halloysite, and the interaction between manganese ions (Mn2+ or MnO4−) and the surface groups of halloysite (Si O Si or Al OH). The obtained samples were tested for the thermal catalytic oxidation of formaldehyde. The halloysite-supported manganese oxide with the permanganate precursor exhibits the best activity and achieves 90% of CO2 generation at 149 °C. The activity of formaldehyde oxidation is positively correlated to the reducibility, which depends on the phase, oxidation state, and dispersion of manganese oxides. This study illuminates the constraint of the manganese precursor on halloysite-supported manganese oxides for the thermal catalytic oxidation of formaldehyde and will be beneficial for the development of transition metal oxide-based catalysts in the abatement of VOCs.