Effects of Vacancy-Defected, Dopant and the Adsorption of Water upon Mn2O3 and Mn3O4 (001) Surfaces: A First-Principles Study (original) (raw)
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Rutile (β-)MnO2 Surfaces and Vacancy Formation for High Electrochemical and Catalytic Performance
Journal of the American Chemical Society, 2014
MnO 2 is a technologically important material for energy storage and catalysis. Recent investigations have demonstrated the success of nanostructuring for improving the performance of rutile MnO 2 in Li-ion batteries and supercapacitors and as a catalyst. Motivated by this we have investigated the stability and electronic structure of rutile (β-)MnO 2 surfaces using density functional theory. A Wulff construction from relaxed surface energies indicates a rod-like equilibrium morphology that is elongated along the c-axis, and is consistent with the large number of nanowire-type structures that are obtainable experimentally. The (110) surface dominates the crystallite surface area. Moreover, higher index surfaces than considered in previous work, for instance the (211) and (311) surfaces, are also expressed to cap the rod-like morphology. Broken coordinations at the surface result in enhanced magnetic moments at Mn sites that may play a role in catalytic activity. The calculated formation energies of oxygen vacancy defects and Mn reduction at key surfaces indicate facile formation at surfaces expressed in the equilibrium morphology. The formation energies are considerably lower than for comparable structures such as rutile TiO 2 and are likely to be important to the high catalytic activity of rutile MnO 2 .
Spin-Coated vs. Electrodeposited Mn Oxide Films as Water Oxidation Catalysts
Materials, 2016
Manganese oxides (MnO x), being active, inexpensive and low-toxicity materials, are considered promising water oxidation catalysts (WOCs). This work reports the preparation and the physico-chemical and electrochemical characterization of spin-coated (SC) films of commercial Mn 2 O 3 , Mn 3 O 4 and MnO 2 powders. Spin coating consists of few preparation steps and employs green chemicals (i.e., ethanol, acetic acid, polyethylene oxide and water). To the best of our knowledge, this is the first time SC has been used for the preparation of stable powder-based WOCs electrodes. For comparison, MnO x films were also prepared by means of electrodeposition (ED) and tested under the same conditions, at neutral pH. Particular interest was given to α-Mn 2 O 3-based films, since Mn (III) species play a crucial role in the electrocatalytic oxidation of water. To this end, MnO 2-based SC and ED films were calcined at 500˝C, in order to obtain the desired α-Mn 2 O 3 crystalline phase. Electrochemical impedance spectroscopy (EIS) measurements were performed to study both electrode charge transport properties and electrode-electrolyte charge transfer kinetics. Long-term stability tests and oxygen/hydrogen evolution measurements were also made on the highest-performing samples and their faradaic efficiencies were quantified, with results higher than 95% for the Mn 2 O 3 SC film, finally showing that the SC technique proposed here is a simple and reliable method to study the electrocatalytic behavior of pre-synthesized WOCs powders.
Catalysis Today, 2005
The characterization of a series of manganese oxides prepared by different methods were performed by X-ray diffraction (XRD) and X-ray absorption near-edge structure (XANES) analysis. Low crystallized phases were found by means of XRD. The XANES results indicated that the mean oxidation state value of all the samples was between 3+ and 4+. XANES analysis of spectra and extended X-ray absorption fine structure (EXAFS) data revealed that Mn was present in three different octahedral environments; one of them corresponded to the environment of Mn in the pyrolusite phase while the other two can be associated to Mn in ramsdellite-like environment and Mn in a octahedral site with Mn-vacancies in the second shell of coordination. The different oxides were analyzed in the reaction of total oxidation of ethanol and we found that the catalytic activity was enhanced when the couple Mn 3+ -Mn 4+ was present in the structure of the oxides. #
Electronic structural insights into efficient MnO x catalysts
J. Mater. Chem. A, 2014
Soft X-ray absorption and resonant inelastic X-ray scattering at the Mn L-edge are established as tools for gaining electronic structural insights into water oxidation catalysis. The MnO x catalyst with the lowest d-d transitions, strongest charge transfer and a higher proportion of Mn 3+ over Mn 2+/4+ produces itinerant electrons that contribute to a higher catalytic activity.
The effect of crystal structure on the surface properties of a series of manganese dioxides
Journal of Colloid and Interface Science, 1966
The zero-points-of-charge for a series of manganese dioxides have been determined by both electrophoretic and coagulation-sedimentation techniques. The zpc was found to increase from pit 1.5 for the amorphous ~ form of MnO2 up to pit 7.3 for /~-MnO~, the most well-defined crystalline form. This variation is interpreted in terms of, and is shown to be in agreement with, the electrostatic field strength concept modified to describe more complex solids. The change in cation-anion spacing, as derived from the volume per unit cell per cation in the crystal lattice, corresponds to a change in the polarization of the water dipoles and their subsequent dissociation to establish the electrochemical double layer at the oxide-water interface.
Advanced Functional Materials, 2018
Holey 2D nanosheets of low-valent Mn 2 O 3 can be synthesized by thermally induced phase transition of exfoliated layered MnO 2 nanosheets. The heat treatment of layered MnO 2 nanosheets at elevated temperatures leads not only to transitions to low-valent manganese oxides but also to the creation of surface hole in the 2D nanosheet crystallites. Despite distinct phase transitions, highly anisotropic 2D morphology of the precursor MnO 2 material remains intact upon the heat treatment whereas the diameter of surface hole becomes larger with increasing heating temperature. The obtained holey 2D Mn 2 O 3 nanosheets show promising electrocatalyst performances for oxygen evolution reaction, which are much superior to that of nonporous Mn 2 O 3 crystal. Among the present materials, the holey Mn 2 O 3 nanosheet calcined at 500 °C displays the best electrocatalyst functionality with markedly decreased overpotential, indicating the importance of heating condition in optimizing the electrocatalytic activity. Of prime importance is that this material shows much better catalytic activity for Li-O 2 batteries than does nonporous Mn 2 O 3 , underscoring the critical role of porous 2D morphology in this functionality. This study clearly demonstrates the unique advantage of holey 2D nanosheet morphology in exploring economically feasible transition metal oxide-based electrocatalysts and electrodes for Li-O 2 batteries.