Layered birnessite-type MnO2 with surface pits for enhanced catalytic formaldehyde oxidation activity

Catalytic reaction active sites tend to be structural defects, such as edges, corners and ribs which have a low coordination number. Here, a kind of layered MnO2 (birnessite) with irregular surface pits was successfully synthesized and investigated for the catalytic oxidation of formaldehyde. The surface pits were generated by a series of chemical surface modifications. Tetrabutylammonium hydroxide (TBAOH) played a key role in generating surface pits, which entered the interlayer from the edge of acid-treated MnO2 and weakened the van der Waals force between interlayers. The steric effects of TBAOH lead to the generation of irregular surface pits. The presence of surface pits not only provided larger specific surface areas but also led to more unsaturated manganese and oxygen atoms, which promoted formaldehyde adsorption and oxygen activation. As a result, birnessite-type MnO2 with surface pits exhibited a lower apparent activation energy than the pristine one (37.6 versus 55.4 kJ mol−1) for 200 ppm of HCHO under a GHSV of 120 000 mL g−1 h−1 and ∼45% relative humidity. These findings open up a new way to generate surface pits on layered 2D materials, holding promise for designing highly efficient catalysts by creating active centers.