Ultra-light 3D MnO₂-agar network with high and longevous performance for catalytic formaldehyde oxidation

Under the background of indoor air formaldehyde decontamination, a freestanding ultra-light assembly was fabricated via ice-templating approach starting from MnO₂ nanoparticles and environmentally benign agar powder. The 3D composite of agar and MnO₂ (AM-3D) was comparatively studied with powdered counterparts (including pure MnO₂ and mixture of agar and MnO₂) and the 3D-structured agar for formaldehyde oxidation, and their physicochemical properties were examined with XRD, ATR, SEM, XPS, isothermal N₂ adsorption, ESR, Raman, CO-TPR and O₂-TPD. For the single test of formaldehyde oxidation, the AM-3D catalyst exhibited 62.0%-67.0% removal percentage for ~400 mg/m³ formaldehyde, which did not demonstrate significant advantage over the control samples. However, thanks to the porous 3D agar scaffold with large spatial volume that could promote a rapid gas-phase formaldehyde concentration reduction, and the strong interaction between the dispersed MnO₂ particles and agar substrate that could afford a large amount of reactive oxygen species to further oxidize the adsorbed formaldehyde, the AM-3D composite was a much better HCHO-to-CO₂ converter and possessed much more advantageous stability for repeated cycles of formaldehyde oxidation: even after ten cycles, there was still 41.7% of formaldehyde removed. Furthermore, the viable sunlight irradiation could easily restore the activity of the used AM-3D catalyst back to the level approaching that of the fresh one. Finally, reaction pathways were put forward via the infrared spectroscopic and ion chromatographic investigations on the surface intermediates of the spent materials.