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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].
Oxide Based Supercapacitors I-Manganese Oxides
Published in Ling Bing Kong, Nanomaterials for Supercapacitors, 2017
Ling Bing Kong, Wenxiu Que, Lang Liu, Freddy Yin Chiang Boey, Zhichuan J. Xu, Kun Zhou, Sean Li, Tianshu Zhang, Chuanhu Wang
Figure 4.11 shows XRD patterns of the MnO2 powders with different [14]. According to Fig. 4.11(a), the birnessite phase of MnO2 had a monoclinic type crystal structure, with space group C2/m and unit cell parameters of a = 5.174 Å, b = 2.850 Å, c = 7.336 Å and β = 103.18° [50]. The broadened diffraction peaks indicated that the sample exhibited a relatively poor crystallinity. For (Fig. 4.11c) powders, the XRD patterns of Fig. 4.11(b) and (c) demonstrated the pure phases of cryptomelane and pyrolusite, respectively, corresponding to a tetragonal crystal structure [51]. High crystallinity of the two samples was confirmed by the sharp diffraction peaks. The tetragonal cryptomelane MnO2 possessed a 2 × 2 tunnel structure, with space group I4/m and cell parameters a = 9.815 Å and c = 2.847 Å [12]. The MnO2 pyrolusite possessed cell parameters of a = 4.38 Å and c = 2.85 Å, with space group P42/mnm [52].
Mn vein-type deposits in the provinces of Santiago del Estero and Cordoba, Argentina
Published in Adam Piestrzyński, Mineral Deposits at the Beginning of the 21st Century, 2001
M.K. Brodtkorb, P. R. Leal, M. J. Correa, V.A. Ramos
The ore minerals, studied by x-ray and microprobe techniques, are hollandite, cryptomelane, and minor coronadite in fine grained masses and botryoidal textures with alternating bands of these species, and tabular ramsdellite in some districts. The gangue minerals are opal, along with chalcedony, barite, fluorite, calcite and some rhodochrosite. Opal is ubiquous, in different colours such as white, brown, olive-green, light and dark blue, pink and black.
Low thermal oxidation of gaseous toluene over Cu/Ce single-doped and co-doped OMS-2 on different synthetic routes
Published in Chemical Engineering Communications, 2022
Kitirote Wantala, Fe Corazon L. See Go, Vince Carlo C. Garcia, Prae Chirawatkul, Narong Chanlek, Pinit Kidkhunthod, Ralf Ruffel M. Abarca, Mark Daniel G. de Luna
Cryptomelane has been synthesized using various precursors such as manganese sulfate, potassium permanganate, manganese acetate, and manganese (II) nitrate (Pahalagedara et al. 2017; Sultana et al. 2018). Simple routes for cryptomelane fabrication, such as the hydrothermal method, have been developed where manganese oxides are autoclaved and recrystallized at relatively low temperatures. For example, K-OMS-2 produced via the hydrothermal route achieved 80% toluene removal at mild temperatures (Millanar et al. 2018). Sol-gel routes were also explored to produce a birnessite precursor then later converted into cryptomelane by metal impregnation (Yu et al. 2011; T. Jakubek et al. 2019). Solid-state reactions, though slightly crude, are also effective ways of producing cryptomelane (H. Sun et al. 2011). Newer cryptomelane synthesis routes using low temperatures of 25 °C have also been developed (Tian et al. 2011; Birkner and Navrotsky 2017).
Optimization of the preparation conditions of cordierite honeycomb monoliths washcoated with cryptomelane-type manganese oxide for VOC oxidation
Published in Environmental Technology, 2021
Diogo F. M. Santos, Olívia S. G. P. Soares, José L. Figueiredo, Manuel Fernando R. Pereira
Catalytic oxidation at low temperatures is one of the most efficient and economically viable VOC removal technologies [2,5]. This process has high removal efficiencies at low temperatures (typically lower than 400°C), no additional fuels are needed and, usually, no harmful by-products are formed [2,6,7]. However, the presence of poisons, such as chlorinated and sulphur-containing compounds in the feed gas, can decrease the efficiency of the process significantly [8]. The volatilization and thermal degradation of the metal catalyst and the formation of coke deposits can also result in the deactivation of the catalyst [9]. As such, for an efficient reaction, the catalysts used need to be very efficient (able to treat large volumes of gas with low concentration of VOCs, typical of environmental applications, at low temperatures), produce only CO2 and H2O, and present high stability under the operating conditions. Noble metal and transition metal oxide catalysts have been developed and successfully applied in VOCs oxidation reactions. Noble metals are the most used catalysts due to their high catalytic activity. Platinum and palladium are the most common [8,10–16], but other noble metals such as ruthenium [17], gold [18] and silver [19] can also be used. However, these catalysts present very high cost and are sensitivity to poisons, which limits their application. Several transition metal oxides have been studied as an alternative, and good activities have been reported [20–23]. Cryptomelane-type manganese oxides (KMn8O16) are receiving considerable attention due to their exceptional catalytic properties [24–26]. For example, Wu et al. [27] achieved 100% conversion of o-xylene (500 ppm) into CO2 at 190°C, with a space velocity of 8000 h−1, over a cryptomelane prepared via wet chemical methods from KMnO4/benzyl alcohol. Cryptomelane is an octahedral molecular sieve (OMS-2) composed of 2 × 2 edge and corner shared MnO6 chains forming a tunnel structure, with potassium ions and small amounts of water present inside the tunnels, providing charge balance and stabilizing the structure [28]. The activity of cryptomelane has been attributed to the mixed valence state of manganese (Mn (IV) and minor amounts of Mn (III) [29]) and to the mobility of the lattice oxygen on the cryptomelane surface [21,28].