Structural Distortion of Molybdenum-Doped Manganese Oxide Octahedral Molecular Sieves for Enhanced Catalytic Performance

Abstract

Due to the excellent catalytic performance of manganese oxide (K-OMS-2) in a wide range of applications, incorporation of various dopants has been commonly applied for K-OMS-2 to acquire additional functionality or activities. However, the understanding of its substitution mechanism with respect to the catalytic performance of doped K-OMS-2 materials remains unclear. Here we present the structural distortion (from tetragonal to monoclinic cell) and morphological evolution in K-OMS-2 materials by doping hexavalent molybdenum. With a Mo-to-Mn ratio of 1:20 (R-1:20) in the preparation, the resultant monoclinic K-OMS-2 shows a small equidimensional particle size (∼15 nm), a high surface area of 213 m(2) g(-1), and greatly improved catalytic activity toward CO oxidation with lower onset temperatures (40 °C) than that of pristine K-OMS-2 (above 130 °C). HR-TEM analyses reveal direct evidence of structural distortion on the cross-section of 2 × 2 tunnels with the absence of 4-fold rotation symmetry expected for a tetragonal cell, which are indexed using a monoclinic cell. Our results suggest that substitution of Mo(6+) for Mn(3+) (rather than Mn(4+)) coupled with the vacancy generation results in a distorted structure and unique morphology. The weakened Mn-O bonds and Mn vacancies associated with the structural distortion may be mainly responsible for the enhanced catalytic activity of monoclinic K-OMS-2 instead of dopant species.

Preparation of nano-structured cryptomelane materials for catalytic oxidation reactions

“…The peak detected at 544 cm -1 is assigned to the vibration that is due to the displacement of the oxygen anion (relative to manganese ion) along the direction of the octahedral chains, whereas the vibration frequency at 607 cm -1 is assigned to Mn-O stretching mode in tetrahedral sites environment, as confirmed by FT-IR. The signals at 142, 250 and 360 cm -1 are due to the Mn-O-Mn bending vibration in the MnO 2 octahedral lattice [20].…”

Enzyme‐free Cu2O@MnO2/GCE for Hydrogen Peroxide Sensing

“…The Cu 2 O@MnO 2 composite materials was synthesized using the single-step reflux method which was adjusted from the earlier report [28]. First, a 0.32 M KMnO 4 solution was prepared and then CuSO 4 · 5H 2 O was added into 100 mL of the 0.32 M KMnO 4 solution (stoichiometric ratio of Cu : Mn as 1 : 5) -Solution A.…”

The Role of Cation Vacancies in Electrode Materials for Enhanced Electrochemical Energy Storage: Synthesis, Advanced Characterization, and Fundamentals

“…There are several ways for incorporating cation vacancies and tailoring its content in transition metal oxides/carbides through synthetic approaches, such as replacing the constituent cations or anions with aliovalent substitutes, [ 94–96,104,105 ] equilibration at solutions with different pH values, [ 89,90,99,106,107 ] selective removal of constituent cations, [ 101,102,108 ] thermal annealing in defects inducing atmospheres, [ 109,110 ] and plasma etching. [ 111 ] The characterization of cation vacancies, however, is usually challenging.…”

“…[ 90,95,107 ] Other powerful techniques for detecting cation vacancies in nanomaterials, especially the materials without long‐range order, are extended X‐ray absorption fine structure (EXAFS) [ 77,78,97,105,111 ] and X‐ray pair distribution function (PDF) analysis, [ 89,94,96,106 ] both of which are sensitive to the local structure variations that enable accurate determination of cation vacancies. Additionally, multiple analytical techniques, such as X‐ray photoelectron spectroscopy (XPS), [ 91,105,113 ] nuclear magnetic resonance (NMR), [ 94–96 ] Raman/Fourier transform infrared (FTIR) spectroscopy, [ 104 ] electron paramagnetic resonance (EPR), [ 110 ] photoluminescence (PL), [ 110 ] and positron annihilation spectrometry (PAS), [ 114 ] have also been reported to serve as complementary tools for cation vacancies characterization. Some of these techniques are commonly used together to elucidate the presence and content of cation vacancies, which will give a better understanding of the physical nature of these cation‐deficient nanomaterials.…”