Abstract: Background, aim, and scope Formaldehyde (HCHO), a potential carcinogen, usually exists at high concentration in indoor air, which poses a serious threat on human health. Therefore, it is essential to explore efficient methods to remove formaldehyde. The catalytic technologies for formaldehyde removal at room temperature can fully degrade formaldehyde with low energy consumption. It is the most promising method to remove indoor formaldehyde. The valence state of Mn element ranges from +2 to +7, and MnO2 possesses four different crystal phases consisting of unique [MnO6] octahedral structure, which allows MnO2-based materials to have excellent low-temperature redox ability. Thus, MnO2-based materials are the most promising catalysts to degrade HCHO of low concentration at room temperature. It is of great practical significance to review the application of MnO2-based materials in the catalytical degradation of formaldehyde at room temperature. In this paper, the structure-activity relationship of doped MnO2, MnO2 composites and supported MnO2 at room temperature were discussed in terms of formaldehyde degradation. And the potential application of MnO2-based materials in the degradation of formaldehyde at room temperature was also discussed. Materials and methods A total of 63 articles were retrieved by WOS (Web of Science) retrieval TS = (HCHO* AND MnO2) from all databases before 2019-04-03, and more relevant literatures were found through the references, similar literatures and cited literatures of these articles. And the factors affecting the catalytic activity of MnO2-based materials for HCHO degradation at room temperature and the corresponding catalytic mechanism were summarized based on these articles. The potential research areas for the catalytic removal of formaldehyde at ambient temperature were proposed. Results Among the formaldehyde catalytic materials, the precious metal-based catalysts can convert formaldehyde of high concentration into CO2 and H2O at room temperature, while the MnO2-based materials of the transition metal oxide-based catalysts are abundant and have showed excellent activity for HCHO of low concentration at room temperature. The actual concentration of HCHO in indoor air is relatively low. Discussion At present, more attentions are focused on noble metal materials to remove HCHO at room temperature. However, the precious metal-based catalyst is very expensive, sintering easily and not convenient for industrial application, while MnO2-based materials possess special structures and strong redox ability. It is most likely to replace precious metals to degrade formaldehyde of low concentration at room temperature. However, the relationship between the activity and structure of MnO2-based materials in formaldehyde catalysis at room temperature, and its practical application have been rarely reviewed. Conclusions Supported-MnO2 is expected to completely degrade formaldehyde of low concentration and does have great application prospect in the field of indoor air purification. Recommendations and perspectives (1) MnO2-based materials combined with AC, PET, ACF and other materials may be more effective and suitable to remove formaldehyde at low concentration in indoor air; (2) The room temperature catalytic activity of MnO2-based materials for formaldehyde at low concentration in some complex environments with high humidity and other VOCs pollutants still needs to be further investigated.
Keywords: MnO2-based catalysts; formaldehyde degradation; room-temperature catalysis; reaction mechanism; influence factor