Template-Free Synthesis of Mesoporous β-MnO₂ Nanoparticles: Structure, Formation Mechanism, and Catalytic Properties

Abstract

Mesoporous β-MnO2 nanoparticles were synthesized by a template-free low-temperature crystallization of Mn4+ precursors (low-crystallinity layer-type Mn4+ oxide, c-distorted H+-birnessite) produced by the reaction of MnO4 – and Mn2+. The Mn starting materials, pH of the reaction solution, and calcination temperatures significantly affect the crystal structure, surface area, porous structure, and morphology of the manganese oxides formed. The pH conditions during the precipitation of Mn4+ precursors are important for controlling the morphology and porous structure of β-MnO2. Nonrigid aggregates of platelike particles with slitlike pores (β-MnO 2 -1 and -2) were obtained from the combinations of NaMnO4/MnSO4 and NaMnO4/Mn­(NO3)2, respectively. On the other hand, spherelike particles with ink-bottle shaped pores (β-MnO 2 -3) were formed in NaMnO4/Mn­(OAc)2 with pH adjustment (pH 0.8). The specific surface areas for β-MnO 2 -1, -2, and -3 were much higher than those for nonporous β-MnO2 nanorods synthesized using a typical hydrothermal method (β-MnO 2 -HT). On the other hand, c-distorted H+-birnessite precursors with a high interlayer metal cation (Na+ and K+) content led to the formation of α-MnO2 with a 2 × 2 tunnel structure. These mesoporous β-MnO2 materials acted as effective heterogeneous catalysts for the aerobic oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) as a bioplastic monomer and for the transformation of aromatic alcohols to the corresponding aldehydes, where the catalytic activities of β-MnO 2 -1, -2, and -3 were approximately 1 order of magnitude higher than that of β-MnO 2 -HT. β-MnO 2 -3 exhibited higher catalytic activity (especially for larger molecules) than the other β-MnO2 materials, and this is likely attributed to the nanometer-sized spaces.

Development of α‐MnO2 Nanowire with Ni‐ and (Ni, Co)‐Cation Doping as an Efficient Bifunctional Oxygen Evolution and Oxygen Reduction Reaction Catalyst

“…[40] Figure S3 represents the XPS spectra recorded for Ni-doped α-MnO 2 nanowire catalyst prepared in this work. The Mn 2p spectrum (Figure S3a) demonstrates two major peaks at 642.6 and 654.3 eV are belongs to Mn 2p 3/2 and Mn 2p 1/2 respectively, [47,48] which are the characteristic peaks of α-MnO 2 . [45] The observed spin energy separation value (ΔE) of 11.8 eV indicates the predominant oxidation state is Mn 4 + present in MnO 2 nanowires.…”

Aerobic oxidative CC bond cleavage of aromatic alkenes by a high valency iron-containing perovskite catalyst

“…Therefore, we have focused on the liquid-phase catalysis of hexagonal perovskites with unique face-sharing octahedral units based on high valency metal species. During the course of our investigation on crystalline first-row metal oxide catalysts, [12][13][14][15][16][17] we have successfully synthesized various hexagonal perovskite nanoparticle catalysts for the liquid phase selective oxidation of various organic substrates with O 2 as the sole oxidant. [15][16][17] In particular, high valency iron-containing BaFeO 3−δ was found to act as an efficient heterogeneous catalyst for the aerobic oxidation of alkanes to the corresponding alcohols and ketones, in sharp contrast to Fe 3+ /Fe 2+ oxides.…”

High surface area MnO2nanomaterials synthesized by selective cation dissolution for efficient water oxidation

“…39,42,43 With increasing the heating temperature of the AT-CMO-60 sample, shoulder vibrational bands started appearing and become clear till 400 C. The shoulder bands developed at 325, 550, and 650 cm À1 represent the characteristic Mn-O vibrational modes in the MnO 6 octahedral co-ordination of b-MnO 2 . [44][45][46] At 400 C a pure phase of b-MnO 2 formed. On further increasing the heating temperature to 450 C, the vibrational spectrum matches with the Mn 2 O 3 phase.…”