Preparation of Porous Birnessite Nanospheres Derived from Pyrolusite–Pyrite Acid Leach Solution and Their Evaluation as a Catalyst for Formaldehyde Decomposition

Manganese oxide is a promising catalyst used for HCHO oxidation. However, its unsatisfactory catalytic performance hinders its practical applications at ambient temperature. This study reported porous birnessite nanospheres with high thermal catalytic and photocatalytic properties for HCHO decomposition, which were synthesized via a seed-mediated growth method. The spherical birnessite seeds with a particle size of ∼100 nm were first obtained from pyrolusite–pyrite acid leaching solution and then grew to porous spheres with particles size of 600–800 nm through a hydrothermal process at a temperature of 200 °C for 20 h. With the particles growing by regulating the synthesis temperature and time, the peak intensity of the Mn–O–Mn band and hydroxyl groups in birnessite gradually increased, and the average oxidation state of Mn decreased. These changes improved the catalytic ability of birnessite, further confirmed by catalytic tests. The birnessite nanospheres having optimum catalytic performance demonstrated 59.5 and 64.8% HCHO conversion for thermal catalysis and photocatalysis, respectively. Evidently, this work provides a short process for the direct synthesis of birnessite from an untreated pyrolusite–pyrite acid leaching solution, and its thermal catalytic and photocatalytic oxidation performance in decomposition of formaldehyde had an obvious improvement compared with that of previous reports.