Importance of water content in birnessite-type MnO2 catalysts for HCHO oxidation: Mechanistic details and DFT analysis.

Water is featured in an indispensable role during the process of catalytic oxidation of HCHO. In this work, a rich water-containing birnessite-type MnO2 was synthesized, and its water content was adjusted through calcination. Phase structure and texture properties of the prepared birnessite were characterized. It was revealed that three types of water (namely absorbed water, molecular water, and structural hydroxyl) existed in birnessite. With the loss of water content, the interlayer distance of samples had decreased which changed the structure of birnessite to cryptomelane. This converted the morphology from an initial layered shape to a rod-like shape. Besides, the underlying mechanism for this effect on HCHO catalytic oxidation was elucidated. Results indicated that hydroxyl groups could slowly and sequentially oxidize HCHO to DOM, formate, and carbonate species. The hydroxyl groups also promoted the formation of oxygen vacancy which could activate O2 to O- 2 and O-. The hydroxyl groups which were consumed had originally been supplied by the reaction between O- 2, O-, and H2O (absorbed and interlayer water in birnessite) which was then replenished from air stream. Clearly, water is favorable to the catalytic reaction. It is the main reason why birnessite can continuously decompose HCHO.