The activity of exposed crystal facets directly determines its physicochemical properties. Thus, acquiring a high percentage of reactive facets by crystal facet engineering is highly desirable for improving the catalytic reactivity. Herein, single-crystalline α-MnO 2 nanowires with major exposed highindex {310} facets were synthesized via a facile hydrothermal route with the assistance of a capping agent of oxalate ions. Comparing with two other low-index facets ({100} and {110}), the resulting α-MnO 2 nanowires with exposed {310} facets exhibited much better activity and stability for carcinogenic formaldehyde (HCHO) oxidation, making 100% of 100 ppm of HCHO mineralize into CO 2 at 60 °C, even better than some Ag supported catalysts. The density functional theory (DFT) calculations were used to investigate the difference in the catalytic activity of α-MnO 2 with exposed {100}, {110}, and {310} facets. The experimental characterization and theoretical calculations all confirm that the {310} facets with high surface energy can not only facilitate adsorption/activation of O 2 and H 2 O but also be beneficial to the generation of oxygen vacancies, which result in significantly enhanced activity for HCHO oxidation. This is a valuable report on engineering surface facets in the preparation of α-MnO 2 as highly efficient oxidation catalysts. This study deepens the understanding of facetdependent activity of α-MnO 2 and points out a strategy to improve their catalytic activity by crystal facet engineering.

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Formaldehyde (HCHO) causes increasing concerns, because of its ubiquitous presence in the indoor environment and its irritating and carcinogenic nature, with regard to humans. The fast abatement of HCHO is of significant practical interest at room temperature. In this paper, we fabricate a three-dimensional manganese dioxide framework (3D-MnO2), which has interconnected network structures, low mass density (∼7.3 mg cm–3), and high absorption capacity for organic liquids. In particular, the 3D-MnO2 showed excellent activity and stability for HCHO oxidation at room temperature, achieving 45% of 100 ppm of HCHO mineralized into CO2 under high gas hourly space velocity (GHSV = 180 L gcat –1 h–1). The excellent performance of 3D-MnO2 catalysts in decomposing HCHO can be ascribed to their quick reversibility and high water content for replenishing the consumed surface hydroxyl groups during HCHO decomposition, and fully exposed active reaction sites. It is valuable to know that inexpensive metal oxides such as MnO2 can transform ppm-level HCHO into harmless CO2 in a timeframe as brief as a subsecond at room temperature.

The relationship between K+ and Mn vacancies and the significant effect of the K+ content on the structure, morphology and catalytic activity of birnessite-type MnO2 for HCHO oxidation was systematically studied.

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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.