Potassium-modulated δ-MnO2 as robust catalysts for formaldehyde oxidation at room temperature

Abstract

Engineering MnO2 with rich surface active oxygen species is critical to effectively eliminate formaldehyde (HCHO) under mild conditions. Herein, we introduced a facile redox method to fabricate a series of δ-MnO2 samples by varying the concentration of K+, which efficiently modulated the layer size, morphology, crystallinity, redox properties, and thus the surface active oxygen species of the obtained δ-MnO2. The medium potassium concentration led to the optimized MnO bond strength, the abundant surface active oxygen species, and the complete conversion of ca. 22 ppm HCHO at 30 °C under a weight hourly space velocity (WHSV) of 200,000 mL/(gcat h). Surface adsorbed oxygen species (e.g., O2− and O−) and surface hydroxyl groups, were suggested to oxidize HCHO into intermediates (i.e., DOM, formate, and carbonate species). Water was critical for further transforming the intermediates into CO2. A Langmuir-Hinshelwood (LH) mechanism was proposed involving in the whole oxidation process.