Mesoporous MnO2 hollow spheres for enhanced catalytic oxidation of formaldehyde

In this work, hollow MnO2 spheres were synthesized via a sacrificial templating method using SiO2 nanospheres as hard templates. The MnO2 coating on SiO2 was achieved by a newly devised low-temperature, controlled precipitation by redox (CPR) method which is based on the controlled redox reaction between KMnO4 and Mn(NO3)2 at 30 °C. After calcination of the SiO2@MnO2 core shell spheres at 300 °C, hollow spheres were obtained following template removal by NaOH etching. The hollow spheres were characterized by XRD, Raman spectroscopy, TEM, HRTEM, N2 adsorption, XPS and H2-TPR. Two different MnO2 crystal phases (γ- and δ-MnO2) could be obtained by making simple changes in the precursor addition protocols during synthesis. All the samples had high BET surface areas between 104 m2 g−1 and 236 m2 g−1, hierarchical pore size distribution with high mesoporosity and the presence of oxygen vacancies for high mobility of oxygen species on the catalyst surface. The catalysts were used for the complete catalytic oxidation of formaldehyde in dry air (100 ppmv; GHSV: 30,000 h−1). Long-term stability tests showed that γ-MnO2 deactivated gradually with time potentially due to structural collapse of the hollow spheres. The interlayer spacing in the δ-MnO2 sample, however, was responsible for increasing the expected conversion due to the regeneration of oxygen/hydroxyl species from adsorbed water formed from the oxidation of formaldehyde on the catalyst surface.