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Electrochemistry of Porous Oxides and Related Materials
Published in Antonio Doménech-Carbó, Electrochemistry of Porous Materials, 2021
Among the first studied synthetic porous oxides were the so-called manganese octahedral molecular sieves (OMS) with the structures of todorokite (OMS-1) and hollandite (OMS-2) [6]. The potassium form of the mineral hollandite (cryptomelane), KMn8O16, includes one-dimensional tunnels among rigid MnO2 framework composed of edge-shared and corner-shared MnO6 octahedra with a tunnel size of 4.6 × 4.6 Å, while the composition of OMS-1 is Mg0.98–1.35MnII1.89–1.94MnIV4.38–4.54O12·(4.47–4.55)H2O and its structure defines cavities of size 6.9 Å. The electrochemistry of such materials is complicated by the fact that both Mn(IV) and Mn(II) centers coexist. The mixed valence of manganese makes this material a good semiconductor and oxidation catalyst, and the possibility of ion intercalation modulates their structural and catalytic properties [7].
Manganese Particles in Freshwaters
Published in Jacques Buffle, Herman P. van Leeuwen, Environmental Particles, 2018
Richard R. De Vitre, William Davison
Distinct mineral-forms of manganese (Table 2) have mainly been identified as components of iron and manganese-rich nodules or crusts.58 Manganese is mainly present as amorphous oxyhydroxides, but todorokite (y-MnOOH), bimessite (δ- MnO2), psilomelane (MnBaMn8O18.2H2O), and rhodochrosite (MnCO3) have all been identified.13,66 The unique freshwater occurrence of pisilomethane has been attributed to there being insufficient sulphate to form barite, which allows the coprecipitation of barium with manganese.58 Generally the concentration of barium in manganese nodules is found to be closely correlated with the concentration of manganese.58 Minor transition metals, including Cu, Co, Ni and Zn, are also associated with the manganese phase.
The Effect of Heat Treatment in Different Atmospheres on Tungsten-doped MnO2 for Ozone Decomposition
Published in Ozone: Science & Engineering, 2021
Yajie Yang, Dan Shen, Pengyi Zhang
The valence state of Mn and W in the original W-MnO2 was 3.45 and 5.85, respectively, which implied oxygen vacancies existed according to the principle of electroneutrality. This is confirmed by the high ratio of Oads/Ototal (0.35), because usually oxygen and/or water molecules are adsorbed at the oxygen vacancy sites to form surface-adsorbed oxygen species such as O2−, O− and OH. After heat-treated in air, helium, or hydrogen atmosphere, the valence states of Mn and W both decreased with the increase of the heating temperature, except for the Air-200 sample with weak heating effect. This result is somewhat different from the finding in our previous study, in which after the cerium-modified todorokite-type manganese dioxides (Ce-MnO2) were heated in air, the valence states of Mn element slightly increased with the increase of heating temperature (Liu and Zhang 2017b). This difference could be explained by the change of crystal structure in this case. It can be seen in Table 2, the Mn valence state significantly decreased to close to 3 (3.13 after 400 °C in air, 3.08 after 300 °C in helium and 3.14 after 200 °C in hydrogen), and at higher temperature, it even decreased to below 3. The Mn valence state was as low as 3 or below 3 implied that other manganese oxide structure rather than MnO2 formed during heat treatment. In addition to the identifiable crystal fringe spacings in HRTEM images of the samples such as Air-400 and He-300, there were still many amorphous areas, at least part of them can be assigned to amorphous Mn2O3 because its clear XRD peaks could be observed in the sample of Air-500 and He-500.
Basin-scale seawater lead isotopic character and its geological evolution indicated by Fe-Mn deposits in the SCS
Published in Marine Georesources & Geotechnology, 2020
Xiaoxia Tu, Huaiyang Zhou, Chonghui Wang, Qunhui Yang, Benduo Zhu
XRD data (Figure 2) indicate that the dominant manganese mineral of the samples is vernadite (δ-MnO2) with a broad peak at ∼2.45 Å, and some nodules contain todorokite with a peak at ∼10 Å. All the samples have a large number of detrital minerals, which are dominated by quartz (peaks at 4.3 Å, 3.3 Å and 1.8 Å) and feldspar (peak at 3.2 Å). The mineral components of the samples show that the crusts and nodules of the SCS are typical of hydrogenetic origin.