Review on manganese dioxide for catalytic oxidation of airborne formaldehyde

Indoor formaldehyde (HCHO) pollution is becoming an important issue with the increase of space confinement. Manganese dioxide (MnO2) has attracted great attention due to its high catalytic activity, thermal stability, facile synthesis with low-cost materials and availability in various crystal morphologies. This review covers recent progress on MnO2-based materials for catalytic oxidation of HCHO, with a particular emphasis on the enhancement of the catalytic activity at low temperature. According to different modification strategies, MnO2 catalysts are divided into three categories, namely, single MnO2 (generally showing tunneled or layered structures), doped or composite MnO2, and supported MnO2 catalysts. Specifically, modification of single MnO2, especially layered MnO2, is deeply discussed in terms of morphology control, and defect engineering, aiming at regulating surface active species; doping or composite MnO2 could alter the properties of the catalysts and facilitate the entire reaction process (i.e., adsorption, reaction and desorption), favoring the rate-limiting step from the thermodynamic and kinetic points of view; supporting MnO2 on carriers is necessary for practical application. Moreover, reaction mechanism of HCHO oxidation by single or composite MnO2 is also reviewed. Finally, perspective on the challenges and opportunities for exploring advanced MnO2-based catalysts is presented.