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Gerbrand Ceder - One of the best experts on this subject based on the ideXlab platform.
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kinetic origins of the metastable zone width in the manganese oxide Pourbaix Diagram
Journal of Materials Chemistry, 2021Co-Authors: Borrong Chen, Wenhao Sun, Daniil A Kitchaev, Kevin H Stone, Ryan C Davis, Gerbrand Ceder, Laura T Schelhas, Michael F ToneyAbstract:Pourbaix Diagrams show solid–aqueous phase stability as a function of pH and redox potential and can be a valuable tool to guide the hydrothermal synthesis of transition metal oxides. However, the Pourbaix Diagram is based on thermodynamics, and nucleation kinetics are not readily apparent in this framework. Here, we conduct a combined experimental and theoretical study to measure the onset of MnO2 precipitation from an MnO4−(aq) solution at various pH conditions, which we then compare against Pourbaix Diagram phase boundaries. Using a combination of in situ X-ray absorption spectroscopy and X-ray wide-angle scattering, we directly observe the transformation kinetics from the tetrahedral MnO4−(aq) ion to octahedrally coordinated Mn pre-nuclei, as well as its initiation into crystalline δ′-MnO2 at various pH. The kinetics of octahedral-Mn precursor availability is observed to govern induction times of crystalline MnO2 nucleation, which results in a metastable zone width in the Mn–H2O Pourbaix Diagram. These results suggest that synthesis conditions often need to be prepared far beyond the phase boundaries in a Pourbaix Diagram to initiate crystallization within a reasonable timescale.
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non equilibrium crystallization pathways of manganese oxides in aqueous solution
Nature Communications, 2019Co-Authors: Wenhao Sun, Daniil A Kitchaev, Gerbrand Ceder, Denis KramerAbstract:Aqueous precipitation of transition metal oxides often proceeds through non-equilibrium phases, whose appearance cannot be anticipated from traditional phase Diagrams. Without a precise understanding of which metastable phases form, or their lifetimes, targeted synthesis of specific metal oxides can become a trial-and-error process. Here, we construct a theoretical framework to reveal the nanoscale and metastable energy landscapes of Pourbaix (E-pH) Diagrams, providing quantitative insights into the size-dependent thermodynamics of metastable oxide nucleation and growth in water. By combining this framework with classical nucleation theory, we interrogate how solution conditions influence the multistage oxidation pathways of manganese oxides. We calculate that even within the same stability region of a Pourbaix Diagram, subtle variations in pH and redox potential can redirect a non-equilibrium crystallization pathway through different metastable intermediates. Our theoretical framework offers a predictive platform to navigate through the thermodynamic and kinetic energy landscape towards the rational synthesis of target materials.
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Non-Equilibrium Crystallization Pathways of Manganese Oxides in Aqueous Solution
2018Co-Authors: Wenhao Sun, Daniil A Kitchaev, Denis Kramer, Gerbrand CederAbstract:Aqueous precipitation of transition metal oxides often proceeds through non-equilibrium phases, whose appearance cannot be anticipated from traditional phase Diagrams. Without a precise understanding of which metastable phases form, or their lifetimes, targeted synthesis of specific metal oxides can become a trial-and-error process. Here, we derive a new thermodynamic potential for the free-energy of a metal oxide in water, which reveals a hidden metastable energy landscape above the equilibrium Pourbaix Diagram. By combining this ‘Pourbaix potential’ with classical nucleation theory, we interrogate how solution conditions can influence the multistage oxidation pathways of manganese oxides. We calculate that even within the same phase stability region of a Pourbaix Diagram, subtle variations in pH and redox potential can redirect a crystallization pathway through different metastable phases. Our theoretical framework offers a predictive platform to navigate through the thermodynamic and kinetic energy landscape towards the rational synthesis of target metal oxide phases.
J M R Genin - One of the best experts on this subject based on the ideXlab platform.
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on the stoichiometry and Pourbaix Diagram of fe ii fe iii hydroxy sulphate or sulphate containing green rust 2 an electrochemical and mossbauer spectroscopy study
Corrosion Science, 1996Co-Authors: J M R Genin, A Olowe, Ph Refait, L SimonAbstract:A new preparation of sulphate containing green rust 2, GR2(SO4−−), is shown to reach its chemical formula at stoichiometry. It consists of mixing FeCl2 · 4H2O and NaOH to precipitate Fe(OH)2 before adding immediately Na2SO4. Eh and pH vs time curves are recorded during oxidation: the formula is established to be [Fe(II)4 Fe(III)2 (OH)12]++ · [SO4 · 2H2O]−−. The chemical potential is determined to be μ0[GR2(SO4−−)] = − 1 014 500 ± 2500 cal mol− and the Pourbaix Diagram of iron in sulphated aqueous media is drawn. Mossbauer spectra display two quadrupole doublets in the 2:1 intensity ratio attributed to ferrous and ferric states, respectively. The unique ferrous environment which is detected indicates that all Fe++ ions are in the first neighbourhood of one oxygen atom which belongs to a SO4−− anion or to a water molecule.
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the preparation and thermodynamic properties of fe ii fe iii hydroxide carbonate green rust 1 Pourbaix Diagram of iron in carbonate containing aqueous media
Corrosion Science, 1995Co-Authors: S H Drissi, Ph Refait, Mustapha Abdelmoula, J M R GeninAbstract:Abstract Carbonate-containing green rust 1, GR1(CO32−), is prepared by oxidation of Fe(OH)2 in aqueous solution. Ferrous hydroxide is precipitated from NaOH and FeSO4·7H2O solutions and carbonate ions are added as a Na2CO3 solution. For sufficiently large concentrations of sodium carbonate, SO42− ions do not play any role during the oxidation process and, at the end of the first stage of reaction, Fe(OH)2 oxidizes into GR1(CO32−). In the second stage of reaction, GR1(CO32−) oxidizes into α-FeOOH goethite except when the transformation of ferrous hydroxide is partial, which leads to the formation of magnetite. From the X-ray diffraction analysis of GR1(CO32−), lattice parameters of its hexagonal cell are found to be a = 3.160 ± 0.005 A and c = 22.45 ± 0.05 A . From the Mossbauer analysis of the stoichiometric GR1(CO32−), which leads to a Fe2+:Fe3+ ratio of 2:1, the chemical formula is established to be: [Fe4(II)Fe2(III)(OH)12][CO3·2H2O]. The 78 K Mossbauer spectrum of the compound can be fitted with three quadrupole doublets, two Fe2+ doublets d1 and D2 corresponding to isomer shifts (IS) of 1.27 and 1.28 mm s−1 and quadrupole splittings (QS) of 2.93 and 2.67 mm s−1, respectively, and one Fe3+ doublet D3 with an IS of 0.47 mm s−1 and QS of 0.43 mm s−1. These three doublets were already used to fit the Mossbauer spectrum of chloride-containing GR1(Cl−) [see J.M.R. Genin et al., Mat. Sci. Forum8, 477 (1986) and J.M.R. Genin et al., Hyp. Int. 29, 1355 (1986)]and therefore are characteristic of GR1 compounds. From the recording of electrode potential E and the pH of the suspension versus time during the oxidation, the standard free enthalpy of formation of stoichiometric GR1(CO32−) is estimated to be ΔG °f = − 966.250 cal mol−1. Knowing the chemical formula and ΔG °f of GR1(CO32−) the Pourbaix Diagram of iron in carbonate-containing aqueous solutions is drawn.
L Simon - One of the best experts on this subject based on the ideXlab platform.
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on the stoichiometry and Pourbaix Diagram of fe ii fe iii hydroxy sulphate or sulphate containing green rust 2 an electrochemical and mossbauer spectroscopy study
Corrosion Science, 1996Co-Authors: J M R Genin, A Olowe, Ph Refait, L SimonAbstract:A new preparation of sulphate containing green rust 2, GR2(SO4−−), is shown to reach its chemical formula at stoichiometry. It consists of mixing FeCl2 · 4H2O and NaOH to precipitate Fe(OH)2 before adding immediately Na2SO4. Eh and pH vs time curves are recorded during oxidation: the formula is established to be [Fe(II)4 Fe(III)2 (OH)12]++ · [SO4 · 2H2O]−−. The chemical potential is determined to be μ0[GR2(SO4−−)] = − 1 014 500 ± 2500 cal mol− and the Pourbaix Diagram of iron in sulphated aqueous media is drawn. Mossbauer spectra display two quadrupole doublets in the 2:1 intensity ratio attributed to ferrous and ferric states, respectively. The unique ferrous environment which is detected indicates that all Fe++ ions are in the first neighbourhood of one oxygen atom which belongs to a SO4−− anion or to a water molecule.
Ph Refait - One of the best experts on this subject based on the ideXlab platform.
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on the stoichiometry and Pourbaix Diagram of fe ii fe iii hydroxy sulphate or sulphate containing green rust 2 an electrochemical and mossbauer spectroscopy study
Corrosion Science, 1996Co-Authors: J M R Genin, A Olowe, Ph Refait, L SimonAbstract:A new preparation of sulphate containing green rust 2, GR2(SO4−−), is shown to reach its chemical formula at stoichiometry. It consists of mixing FeCl2 · 4H2O and NaOH to precipitate Fe(OH)2 before adding immediately Na2SO4. Eh and pH vs time curves are recorded during oxidation: the formula is established to be [Fe(II)4 Fe(III)2 (OH)12]++ · [SO4 · 2H2O]−−. The chemical potential is determined to be μ0[GR2(SO4−−)] = − 1 014 500 ± 2500 cal mol− and the Pourbaix Diagram of iron in sulphated aqueous media is drawn. Mossbauer spectra display two quadrupole doublets in the 2:1 intensity ratio attributed to ferrous and ferric states, respectively. The unique ferrous environment which is detected indicates that all Fe++ ions are in the first neighbourhood of one oxygen atom which belongs to a SO4−− anion or to a water molecule.
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the preparation and thermodynamic properties of fe ii fe iii hydroxide carbonate green rust 1 Pourbaix Diagram of iron in carbonate containing aqueous media
Corrosion Science, 1995Co-Authors: S H Drissi, Ph Refait, Mustapha Abdelmoula, J M R GeninAbstract:Abstract Carbonate-containing green rust 1, GR1(CO32−), is prepared by oxidation of Fe(OH)2 in aqueous solution. Ferrous hydroxide is precipitated from NaOH and FeSO4·7H2O solutions and carbonate ions are added as a Na2CO3 solution. For sufficiently large concentrations of sodium carbonate, SO42− ions do not play any role during the oxidation process and, at the end of the first stage of reaction, Fe(OH)2 oxidizes into GR1(CO32−). In the second stage of reaction, GR1(CO32−) oxidizes into α-FeOOH goethite except when the transformation of ferrous hydroxide is partial, which leads to the formation of magnetite. From the X-ray diffraction analysis of GR1(CO32−), lattice parameters of its hexagonal cell are found to be a = 3.160 ± 0.005 A and c = 22.45 ± 0.05 A . From the Mossbauer analysis of the stoichiometric GR1(CO32−), which leads to a Fe2+:Fe3+ ratio of 2:1, the chemical formula is established to be: [Fe4(II)Fe2(III)(OH)12][CO3·2H2O]. The 78 K Mossbauer spectrum of the compound can be fitted with three quadrupole doublets, two Fe2+ doublets d1 and D2 corresponding to isomer shifts (IS) of 1.27 and 1.28 mm s−1 and quadrupole splittings (QS) of 2.93 and 2.67 mm s−1, respectively, and one Fe3+ doublet D3 with an IS of 0.47 mm s−1 and QS of 0.43 mm s−1. These three doublets were already used to fit the Mossbauer spectrum of chloride-containing GR1(Cl−) [see J.M.R. Genin et al., Mat. Sci. Forum8, 477 (1986) and J.M.R. Genin et al., Hyp. Int. 29, 1355 (1986)]and therefore are characteristic of GR1 compounds. From the recording of electrode potential E and the pH of the suspension versus time during the oxidation, the standard free enthalpy of formation of stoichiometric GR1(CO32−) is estimated to be ΔG °f = − 966.250 cal mol−1. Knowing the chemical formula and ΔG °f of GR1(CO32−) the Pourbaix Diagram of iron in carbonate-containing aqueous solutions is drawn.
Herbert Over - One of the best experts on this subject based on the ideXlab platform.
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extraordinary stability of iro2 110 ultrathin films supported on tio2 110 under cathodic polarization
Journal of Physical Chemistry Letters, 2020Co-Authors: Tim Weber, Vedran Vonk, Marcel J S Abb, Jonas Evertsson, Martina Sandroni, Jakub Drnec, A Stierle, Edvin Lundgren, Herbert OverAbstract:Down to a cathodic potentials of -1.20 V versus the reversible hydrogen electrode, the structure of IrO2(110) electrodes supported by TiO2(110) is found to be stable by in situ synchrotron-based X-ray diffraction. Such high cathodic potentials should lead to reduction to metallic Ir (Pourbaix Diagram). From the IrO2 lattice parameters, determined during cathodic polarization in a H2SO4 electrolyte solution (pH 0.4), it is estimated that the unit cell volume increases by 1% due likely to proton incorporation, which is supported by the lack of significant swelling of the IrO2(110) film derived from X-ray reflectivity experiments. Ex situ X-ray photoelectron spectroscopy suggests that protons are incorporated into the IrO2(110) lattice below -1.0 V, although Ir remains exclusively in the IV+ oxidation state down to -1.20 V. Obviously, further hydrogenation of the lattice oxygen of IrO2(110) toward water is suppressed for kinetic reasons and hints at a rate-determining chemical step that cannot be controlled by the electrode potential.
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chlorine evolution reaction on ruo2 110 ab initio atomistic thermodynamics study Pourbaix Diagrams
Electrochimica Acta, 2014Co-Authors: Kai S Exner, Josef Anton, Timo Jacob, Herbert OverAbstract:Abstract Constrained ab initio thermodynamics in the form of a Pourbaix Diagram can greatly assist kinetic modeling of a particular electrochemical reaction such as the chlorine evolution reaction (CER) over RuO 2 (110). Pourbaix Diagrams reveal stable surface structures, as a function of pH and the potential. The present DFT study indicates that the Pourbaix Diagram in the CER potential region above 1.36 V and pH values around zero is dominated by a stable surface structure in which all coordinatively undercoordinated Ru sites (Ru cus ) are capped by on-top oxygen (O ot ). This oxygen saturated RuO 2 (110) surface is considered to serve as the catalytically active phase in the CER, quite in contrast to the heterogeneously catalyzed HCl oxidation (Deacon process), for which the active RuO 2 (110) surface is mainly covered by on-top chlorine. The active sites in the CER are suggested to be Ru cus O ot surface complexes, while in the Deacon process both undercoordinated surface Ru and oxygen sites must be available for the activation of HCl molecules.