Engineering Crystal Facet of α‑MnO₂ Nanowire for Highly Efficient Catalytic Oxidation of Carcinogenic\nAirborne Formaldehyde

Abstract

The activity of exposed crystal facets\ndirectly determines its\nphysicochemical properties. Thus, acquiring a high percentage of reactive\nfacets by crystal facet engineering is highly desirable for improving\nthe catalytic reactivity. Herein, single-crystalline α-MnO₂ nanowires with major exposed high-index {310} facets were\nsynthesized via a facile hydrothermal route with the assistance of\na capping agent of oxalate ions. Comparing with two other low-index\nfacets ({100} and {110}), the resulting α-MnO₂ nanowires\nwith exposed {310} facets exhibited much better activity and stability\nfor carcinogenic formaldehyde (HCHO) oxidation, making 100% of 100\nppm of HCHO mineralize into CO₂ at 60 °C, even better\nthan some Ag supported catalysts. The density functional theory (DFT)\ncalculations were used to investigate the difference in the catalytic\nactivity of α-MnO₂ with exposed {100}, {110}, and\n{310} facets. The experimental characterization and theoretical calculations\nall confirm that the {310} facets with high surface energy can not\nonly facilitate adsorption/activation of O₂ and H₂O but also be beneficial to the generation of oxygen vacancies, which\nresult in significantly enhanced activity for HCHO oxidation. This\nis a valuable report on engineering surface facets in the preparation\nof α-MnO₂ as highly efficient oxidation catalysts.\nThis study deepens the understanding of facet-dependent activity of\nα-MnO₂ and points out a strategy to improve their\ncatalytic activity by crystal facet engineering.