Construction of Supported MnOx/MgAl Hydrotalcite Catalysts and Their Highly Efficient Catalytic Performance for Low-Temperature Formaldehyde Removal

A series of supported MnOx/MgAl-layered double hydroxide (LDH) catalysts were prepared by hydrothermal co-precipitation to investigate their catalytic performances for low-temperature formaldehyde oxidation reactions. Activity tests show that the 10Mn/Mg3Al1-LDH catalyst exhibits higher efficiency for low-temperature formaldehyde oxidation with a high CO2 yield. It also shows remarkable long-term operational stability as well as good adaptability to different velocities and humidities. Various characterizations were carried out to establish the possible structure–activity correlations. The results show that there were a large number of hydroxyl groups in the 10Mn/MgAl-LDH catalysts, and the hydroxyl groups were positively correlated with Mg2+ content. The outstanding catalytic performance of 10Mn/Mg3Al1-LDH can be attributed to abundant surface hydroxyl groups, surface adsorbed oxygen and higher Mn4+/Mn3+ ratios. Through in situ Fourier-transform infrared spectroscopy (in situ FTIR), it was revealed that formaldehyde was gradually converted into CO2 and water with dioxymethylene (DOM), formate and carbonate as the major intermediates under the action of both active oxygen and active hydroxyl groups. The active oxygen and active hydroxyl groups consumed in the process are continuously replenished by the effective reaction between the oxygen molecules in the air and the active site of the catalyst. The low-temperature asynchronous conversion of formaldehyde results in the accumulation of some intermediates on the catalyst surface covering the active center, which induces catalyst deactivation.