Jinge Li

Room-Temperature Oxidation of Formaldehyde by Layered Manganese Oxide: Effect of Water

Layered manganese oxide, i.e., birnessite was prepared via the reaction of potassium permanganate with ammonium oxalate. The water content in the birnessite was adjusted by drying/calcining the samples at various temperatures (30 °C, 100 °C, 200 °C, 300 °C, and 500 °C). Thermogravimetry-mass spectroscopy showed three types of water released from birnessite, which can be ascribed to physically adsorbed H2O, interlayer H2O and hydroxyl, respectively. The activity of birnessite for formaldehyde oxidation was positively associated with its water content, i.e., the higher the water content, the better activity it has. In-situ DRIFTS and step scanning XRD analysis indicate that adsorbed formaldehyde, which is promoted by bonded water via hydrogen bonding, is transformed into formate and carbonate with the consumption of hydroxyl and bonded water. Both bonded water and water in air can compensate the consumed hydroxyl groups to sustain the mineralization of formaldehyde at room temperature. In addition, water in air stimulates the desorption of carbonate via water competitive adsorption, and accordingly the birnessite recovers its activity. This investigation elucidated the role of water in oxidizing formaldehyde by layered manganese oxides at room temperature, which may be helpful for the development of more efficient materials.

The effects of Mn loading on the structure and ozone decomposition activity of MnOx supported on activated carbon

Abstract Manganese oxide catalysts supported on activated carbon (AC, MnOx/AC) for ozone decomposition were prepared by in situ reduction of the permanganate. The morphology, oxidation state, and crystal phase of the supported manganese oxide were characterized by scanning electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, electron spin resonance, Raman spectroscopy, and temperature-programmed reduction. The supported MnOx layer was distributed on the surface of AC with a morphology that changed from a porous lichen-like structure to stacked nanospheres, and the thickness of the MnOx layer increased from 180 nm to 710 nm when the Mn loading was increased from 0.44% to 11%. The crystal phase changed from poorly crystalline β-MnOOH to δ-MnO2 with the oxidation state of Mn increasing from +2.9–+3.1 to +3.7–+3.8. The activity for the decomposition of low concentration ozone at room temperature was related to the morphology and loading of the supported MnOx. The 1.1%MnOx/AC showed the best performance, which was due to its porous lichen-like structure and relatively high Mn loading, while 11%MnOx/AC with the thickest MnOx layer had the lowest activity owning to its compact morphology.

Facile Synthesis of Activated Carbon-Supported Porous Manganese Oxide via in situ Reduction of Permanganate for Ozone Decomposition

Lichen-like porous manganese oxides (MnOx) with birnessite-type structure, which were assembled with curled nanosheets (less than 10 nm in thickness), were deposited in situ on the surface of activated carbon (AC) via facile reduction of permanganate by AC, and were tested for the decomposition of ozone, a common air pollutant. Despite the low Mn loading (typically 0.04%–0.14%), the as-synthesized MnOx/AC catalysts exhibited high catalytic activity and stability for ozone decomposition at room temperature and high relative humidity, which can be attributed to the enrichment of the Mn element on the catalyst surface and a well-organized porous surface morphology of MnOx.