Engineering Crystal Facet of alpha-MnO2 Nanowire for Highly Efficient Catalytic Oxidation of Carcinogenic Airborne Formaldehyde

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 alpha-MnO2 nanowires with major exposed high-index {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 alpha-MnO2 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 CO2 at 60 degrees 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 alpha-MnO2 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 H2O 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 alpha-MnO2 as highly efficient oxidation catalysts. This study deepens the understanding of facet dependent activity of alpha-MnO2 and points out a strategy to improve their catalytic activity by crystal facet engineering.