Layered manganese oxides for formaldehyde-oxidation at room temperature: the effect of interlayer cations

Abstract

A series of K-, Mg-, Ca-, and Fe-containing birnessites were prepared by a facile comproportionation reaction of Mn²⁺ and MnO₄⁻ in the presence of different metal cations. The as-synthesized birnessite samples were characterized by FESEM, XRD, TG, and XPS. The catalytic activity toward decomposition of HCHO was evaluated under ambient temperature. Fe-birnessite showed the highest HCHO oxidation activity due to its highest content of surface hydroxyl groups. However, the CO₂ generation by Fe-birnessite was relatively low due to the accumulation of formate species without further oxidation. The influence of different interlayer cations on the activity of birnessite was studied by H₂-TPR, O₂-TPD, and HCHO in situ DRIFTS. The result indicates that K⁺ leads to a considerable enhancement of surface oxygen activity which then facilitates the regeneration of surface hydroxyls by activating H₂O, therefore K-birnessite showed the highest CO₂ generation performance during HCHO removal at ambient temperature. HCHO of 0.5 mg m⁻³ and the gas hourly space velocity (GHSV) of 1 200 000 h⁻¹ (the corresponding contact time is 0.003 s) were selected to check the stability of the samples. K-Bir still showed stable activity with the removal efficiency reaching 40% under these critical test conditions. Considering the effect of introducing different metal cations, this work provides new insight into designing high performance catalysts for indoor air purification.