Alkali Metal Doped MnO_x Catalysts for Formaldehyde Oxidation

The catalytic oxidation of formaldehyde to CO₂ and H₂O under low temperature is of great significance and insistent demand for indoor air purification. In this work, through alkali metal doping, we significantly improved the formaldehyde oxidation activity of Mn-based catalysts. At a temperature as low as 97 °C, 300 ppm of formaldehyde can be completely eliminated over 5%Cs/MnO_x. The results showed that the presence of alkali metals markedly increased the redox ability of MnO_x catalyst and the proportion of reactive oxygen species. The adsorption and reaction path of formaldehyde on the surface of the catalysts were studied by in-situ infrared spectroscopy. It was found that the adsorption form of formaldehyde on the surface of alkali metals doped MnO_x catalyst was different from that of MnO_x, except for monodentate formate detected over MnO_x, more easily decomposed bridged adsorbed formate was another prominent adsorbed species over 5%Na/MnO_x and 5%Cs/MnO_x catalysts. The difference in reaction paths may be the key to the higher activities of alkali doped MnO_x catalysts. This finding may provide some new ideas for the design of low temperature formaldehyde oxidation catalysts.

The intermediate species during formaldehyde oxidation on MnO_x and Na⁺/Cs⁺-doped MnO_x are different, which may be the key reason why the latter exhibit higher activities than MnO_x.