Promotional catalytic oxidation of airborne Formaldehyde over mineral-supported MnO2 at ambient temperature

Achieving low-level formaldehyde (HCHO) catalytic oxidation at ambient temperature under a high gas-hourly space velocity (GHSV) remains challenging. In this study, MnO2-based catalysts were prepared via a facile in-situ redox reaction using palygorskite (PG), montmorillonite (MT) and diatomite (DT) as support materials. The results indicated that these minerals substantially enhanced the performance of HCHO oxidation compared with that of commercial Al2O3 at ambient temperature. The as-prepared MnO2/PG exhibited the superior catalytic performance, where 100% removal efficiency of 1 ppm HCHO in 1500 min remained under a GHSV of 150,000 h(-1). The performance order of the as-prepared catalysts at 1500 min was MnO2/PG (100%) > MnO2/MT (87%) > MnO2/DT (74%) > MnO2/Al2O3 (59%). More importantly, the MnO2/PG catalyst exhibited the highest CO2 selectivity (72.1%) and excellent long-term stability after five cycles. The kinetic results for HCHO oxidation followed the second-order kinetics. The well-distributed MnO2, abundant hydroxyl species, and Mn3+ and Mn4+ couples were responsible for the excellent catalytic performance of MnO2/PG. In addition, the incorporation of MnO2 with PG increased the electron density of the MnO2/PG surface, which was also related to the excellent performance for HCHO oxidation. A possible reaction mechanism for HCHO oxidation was proposed based on XPS, in-situ DRIFTS and electrochemical analyses. This study paves a road for the development of MnO2/mineral hybrid catalysts for indoor air purification at ambient temperature.