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

Abstract

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.

Progress of Catalytic Oxidation of Formaldehyde over Manganese Oxides

“…[97,98] Among them, hydrothermal and redox methods are the most widely investigated ways to prepare the manganese oxides. Generally, KMnO 4 , MnSO 4 , Mn (CH 3 COO) 2 and Mn(NO 3 ) 2 are used as the raw materials. Different synthesis methods and raw materials are used to tailor the manganese oxides' crystal form, morphology and microstructure, reducibility, and surface properties.…”

“…First, formaldehyde reacts reversibly with active oxygen species adsorbed on the surface of the catalyst (1,2) to form the intermediate adsorbed species, followed by the decomposition of the intermediate species to form the adsorbed formate species (3). Then the adsorbed formate species were further decomposed to form CO 2 and H (a) (4). At the same time, H 2 O was formed by the combination of H (a) and O (a) (5).…”

“…At the same time, H 2 O was formed by the combination of H (a) and O (a) (5). Where subscript (g) and (a) represent gas and adsorbed, respectively: (3,4) were the rate-determined steps for the oxidation of formaldehyde over manganese oxide.…”

“…The presence of variable chemical valences and defects will benefit the mobility of surface oxygen and improvement of oxygen storage capacity of manganese oxides. [3,4] Therefore, manganese oxides have been widely used in field of battery anode material, [5][6][7][8] super capacitor, [9][10][11][12][13] pollutant degradation, [14][15][16][17][18][19] heterogeneous catalysis, [4,[20][21] and so on.…”

Eu2+ and Eu3+ Doubly Doped ZnWO4 Nanoplates with Superior Photocatalytic Performance for Dye Degradation

“…Previous explorations have shown the profound influence of facets on the photocatalysis. For example, Yuan et al demonstrated the crystal facet-correlated photocatalytic activity of α-Fe 2 O 3 for water splitting [35]; Wu et al reported that {010} faceted BiOBr nanocrystals displayed a better photo-oxidative capability than {001} faceted nanocrystals for water oxidation and formic acid degradation [36]; Qi et al found that {101} faceted TiO 2 showed superior catalytic activity to {001} and {010} faceted TiO 2 for anthracene degradation [37]; Rong et al reported that {310} faceted α-MnO 2 nanowires exhibited much better activity than {100} and {110} facets for formaldehyde oxidation [38]. It is clear that little attention is paid on the facet-dependent photocatalytic activity of ZnWO 4 , although such a phenomenon is well studied in a number of functional materials [31,32,33,34,35,36,37,38].…”