N atoms were selectively doped at substitutional or interstitial sites in the MnO2 lattice using N2 plasma. This research provides a site-selective N-doping method and a deep insight into the different effects of doping sites.

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Formaldehyde (HCHO) is a priority pollutant in the indoor environment, which is irritative and carcinogenic to humans. The non-noble metal oxides have a wide application prospect in the decomposition of HCHO. Defects in metal oxides have been widely accepted as active sites in heterogeneous catalysis. Compared with the extensive study of oxygen defects, the effect of cation defects has not been clearly addressed. Herein, Mn defect-rich Mn 3 O 4 was synthesized by pyrolysis of Ce-doped MnCO 3 . It is found for the first time that the content of Mn defects in Mn 3 O 4 can be adjusted by introducing Ce. The introduction of Ce resulted in the higher contents of Mn defects, which significantly enhances the HCHO decomposition. Moreover, Mn defect can effectively narrow the half-metallic gap of Mn 3 O 4 , regulate the electronic structure and coordination environment of surrounding oxygen, and further improve the activity and mobility of neighboring oxygen atoms. Importantly, Mn defects are not only beneficial to the generation of neighboring oxygen vacancy but also conducive to enhancing the activation ability of oxygen vacancy for O 2 . The advantages resulting from Mn defects significantly enhance the HCHO decomposition. This research proposes a strategy to adjust cation defects and deepens the comprehension of the function of cation defects.

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Crystal facet engineering, the selective exposure of reactive crystal facets, has emerged as an important technique for the design of efficient catalysts. However, in the facet-controlled synthesis of nanocrystals by either traditional top-down or bottom-up routes, the selection of capping agents is crucial and challenging. Herein, a phase transition strategy that does not require the assistance of capping/etching agents was developed to achieve the selective exposure of {103}, {101}, and {112} facets in Mn 3 O 4 . Facet-dependent activity in the photothermal decomposition of the carcinogen formaldehyde was investigated. The resulting Mn 3 O 4 with exposed {103} facets showed the best photothermal catalytic activity, achieving the complete mineralization of formaldehyde at ambient temperature. The synergistic mechanism of the photothermal catalysis of Mn 3 O 4 was discovered to be a thermal-assisted photocatalytic process rather than solar-light-driven thermal catalysis. In addition, the photothermal synergistic decomposition process of HCHO was also revealed. The photothermal decomposition of HCHO undergoes the processes of HCHO → DOM (dioxymethylene) → formates → carbonates → CO 2 . The findings proposed in this work not only broaden the study of crystal facet engineering but also provide suggestions for the purification of volatile organic compounds (VOCs) in indoor air.

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Developing high-efficiency formaldehyde (HCHO) catalytic oxidation catalysts is of significance and practicability for indoor air decontamination. It is generally considered that anionic doping is a traditional strategy to regulate the reaction activity of catalyst materials. However, one of the concerns worth exploring is the recognition of doping sites and their impact on catalytic performance. Herein, nitrogen atoms are successfully introduced into the MnO2 structure by high-temperature calcination with urea as the nitrogen source for the first time. By regulating the usage of urea, nitrogen atoms are selectively and successfully doped into the interstitial sites and substitutional sites. Furthermore, the effect of nitrogen-doped MnO2 on the catalytic oxidation of HCHO is studied in detail. Results indicate that nitrogen doping can promote the formation of oxygen vacancies, strengthen the activation of adsorbed O2, and enhance the adsorption of HCHO, which is conducive to catalytic decomposition of HCHO. Importantly, nitrogen doping at various doping sites has a considerable effect on catalytic decomposition activity. Although both interstitial and substitutional nitrogen doping can boost the decomposition performance of HCHO, the interstitial sites are the most suitable doping sites in the α-MnO2 lattice. The chemical environment around the interstitial sites is favorable for the creation of oxygen vacancies, as well as the adsorption/activation of HCHO and O2. Interstitial nitrogen doping can dramatically increase the HCHO catalytic activity of α-MnO2, and the complete catalytic decomposition temperature is reduced from 130 to 90 °C at the condition of GHSV = 90 L/gcat·h. This study provides a facile and effective method for site-selective nitrogen-doping and an in-depth understanding of its effect on catalytic activity.