Mechanism and efficiency of methylene blue degradation by microwave-induced birnessites with different Mn average oxidation states

Tao, Juan; Song, Kenan; Lv, Guocheng; Mei, Lefu; Li, Zhaohui; Liao, Libing; Wang, Xiaoyu; Xiao, Xing; Xu, Ben

doi:10.1166/mex.2014.1194

Cited by 5 publications

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“…Except P0.50, the spectra of the products have similar shape, suggesting these solids have the same phase (birnessite). However, the edge position of the spectra is blue-shifted with increasing P/Mn ratio, which can be attributed to increased Mn(III) concentrations, decreasing particle size at the nanometer level, ,, and/or the lattice contraction due to light absorption moving to higher photon energy driven by faster vibrating of atoms at shorter atomic distance. , In addition, the absorbance in the range of 500–800 nm decreases with increasing P/Mn ratio to 0.3 due to decreasing Mn(IV) concentration, because Mn(IV) has lower spin magnetic moment than Mn(III), resulting in more microwave-absorbing capability. , The direct and indirect band gaps of the P series samples are probed optically via Tauc–Mott plot (Figure S14). With increasing P/Mn ratio, the allowed direct band gap increases from 1.48 to 1.82 eV, while the allowed indirect band gap increases from 0 to 0.27 eV (Figure B).…”

Section: Resultsmentioning

confidence: 99%

“…24,55 In addition, the absorbance in the range of 500−800 nm decreases with increasing P/Mn ratio to 0.3 due to decreasing Mn(IV) concentration, because Mn(IV) has lower spin magnetic moment than Mn(III), resulting in more microwave-absorbing capability. 56,57 The direct and indirect band gaps of the P series samples are probed optically via Tauc−Mott plot (Figure S14). With increasing P/Mn ratio, the allowed direct band gap increases from 1.48 to 1.82 eV, while the allowed indirect band gap increases from 0 to 0.27 eV (Figure 9B).…”

Section: ■ Materials and Methodsmentioning

confidence: 99%

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Synthesis of Birnessite in the Presence of Phosphate, Silicate, or Sulfate

Wang

Liao

et al. 2016

Inorg. Chem.

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Layered manganese (Mn) oxides, such as birnessite, are versatile materials in industrial applications and common minerals mediating elemental cycling in natural environment. Many of birnessite properties are controlled by Mn(III) concentration and particle sizes. Thus, it is important to synthesize birnessite nanoparticles with controlled Mn(III) concentrations and sizes so that one can tune its properties for many applications. Birnessite was synthesized in the presence of oxyanions (phosphate, silicate, or sulfate) during reductive precipitation of KMnO by HCl and characterized using multiple synchrotron X-ray techniques, electron microscopy, and diffuse reflectance UV-vis spectroscopy. Results indicate that all three anions decrease MnO sheet sizes, attributed to oxyanion adsorption on edges of the sheets, inhibiting their lateral growth. As a result of decreased sizes, sheets undergo significant structural contraction. Meanwhile, Mn(III) concentration significantly increases with increasing oxyanion/Mn ratio. The increased Mn(III) concentration, along with the decreased size, enlarges both direct and indirect band gaps of birnessite. Phosphate imposes the strongest influence, followed by silicate and then by sulfate, consistent with their decreasing adsorption affinity. Reacting with 1 M KOH solution effectively removed the adsorbed oxyanions while leading to increased sheet sizes, probably due to oriented attachment-driven particle growth mechanisms. The results have important implications for developing highly performed birnessite materials, for example, small size Mn(III)-rich birnessite for photochemical and catalytic applications, as well as for understanding chemical compositional variations of naturally occurring birnessite.

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