Probing the Release and Uptake of Water in Î±-MnO2Â·xH2O

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Probing the Release and Uptake of Water in #-MnO•xHO Zhenzhen Yang, Denise C. Ford, Yang Ren, Joong Sun Park, Soojeong Kim, Hacksung Kim, Timothy T. Fister, Maria K. Y. Chan, and Michael M. Thackeray Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b03721 • Publication Date (Web): 27 Dec 2016 Downloaded from http://pubs.acs.org on January 2, 2017

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Chemistry of Materials

Probing the Release and Uptake of Water in α-MnO2•xH2O Zhenzhen Yang,a# Denise C. Ford,b,e# Joong Sun Park,a Yang Ren,c Soojeong Kim,a Hacksung Kim,a,f Timothy T. Fister,a Maria K. Y. Chan,*d Michael M. Thackeray*a a

Chemical Sciences and Engineering Division, b Materials Science Division, c X-ray Science Division, Advanced Photon Source, d Center for Nanoscale Materials, Argonne National Laboratory, Argonne IL 60439, USA e Mechanical Engineering and Materials Science, Duke University, Durham NC 27708, USA f Center for Catalysis and Surface Science, Northwestern University, IL 60208, USA #

These authors contributed equally

*Corresponding authors: Maria K. Y. Chan ([email protected]) and Michael M. Thackeray ([email protected])

Abstract α-MnO2 is of interest as a cathode material for 3 V lithium batteries and as an electrode/electrocatalyst for higher energy, hybrid Li-ion/Li-O2 systems. It has a structure with large tunnels that contain stabilizing cations such as Ba2+, K+, NH4+, and H3O+ (or water, H2O). When stabilized by H3O+/H2O, the protons can be ion-exchanged with lithium to produce a Li2O-stabilized α-MnO2 structure. It has been speculated that the electrocatalytic process in LiO2 cells may be linked to the removal of lithium and oxygen from the host α-MnO2 structure during charge, and their reintroduction during discharge. In this investigation, hydrated α-MnO2 was used, as a first step, to study the release and uptake of oxygen in α-MnO2. Temperatureresolved in-situ synchrotron X-ray diffraction (XRD) revealed a non-linear volume change profile, which with the aide of X-ray absorption near-edge spectroscopy (XANES), redox titration, and density functional theory (DFT) calculations, is interpreted as the release of water 1 ACS Paragon Plus Environment


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from the α-MnO2 tunnels. The two stages correspond to H2O release from intercalated H2O species at lower temperatures and H3O+ species at higher temperature. Thermogravimetric analysis confirmed the release of oxygen from α-MnO2 in several stages during heating – including surface water, occluded water, and structural oxygen – and in-situ UV resonance Raman spectroscopy corroborated the uptake and release of tunnel water by revealing small shifts in frequencies during the heating and cooling of α-MnO2. Finally, DFT calculations revealed the likelihood of disordered water species in binding sites in α-MnO2 tunnels and a facile diffusion process. 1. Introduction Natural and synthetic polymorphs of MnO2 that can accommodate guest ions such as H+ and Li+ are particularly well known for their application as cathodes in primary and secondary (rechargeable) battery systems.1, 2 For example, γ-MnO2 (Fig. 1a), a structural intergrowth of βMnO2 (rutile-type, Fig. 1b) and ramsdellite-MnO2 (Fig. 1c), is used ubiquitously in primary 1.5 V Zn/MnO2 Leclanché cells and alkaline cells that operate by a proton insertion mechanism. λMnO2, a defect spinel (Fig. 1d), serves as a lithium insertion cathode in 4 V lithium-ion batteries,3 while α-MnO2 with its cation-stabilized hollandite-type structure and large onedimensional ‘2×2’ tunnels, ~5Å in width4 (Fig. 1e), is of interest as (i) an insertion electrode for 3 V lithium batteries, particularly when stabilized by lithia and ammonia, (ii) a supercapacitor,4 and (iii) a catalyst for hybrid Li-ion/Li-O2 systems.5-7 Manganese oxide electrode structures are also of interest for sodium8 and magnesium9-12 batteries.

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Fig. 1 Schematic structures of (a) γ-MnO2; (b) β-MnO2; (c) ramsdellite-MnO2; (d) λ-MnO2; and (e) cation-stabilized α-MnO2. Alpha-MnO2 minerals found in nature are always stabilized by cations that reside at the center of the (2×2) tunnels, such as Ba2+ (hollandite), K+ (cryptomelane), Pb2+ (coronadite) and Na+ (manjiroite). In synthetically-prepared α-MnO2, which is hydrated, the oxygen anion of the water/hydronium species, like a stabilizing metal cation, is located at the center of the tunnels of the structure. When heated to ~400 °C, this oxygen is removed from the structure, presumably as H2O, thereby destabilizing the empty or proton-lined tunnels, and making them energetically favorable for immediate reincorporation of water into the α-MnO2 framework if exposed to a moist ambient environment, akin to dehydrated zeolite structures that are known to behave as effective desiccants. In fact, it has been observed that on exposing a dehydrated α-MnO2 sample to moist air, the sample becomes extremely hot, which we conclude is evidence of an exothermic reaction upon reabsorption of water (similar to activated zeolites) from the atmosphere.13 Despite the many technological challenges thwarting the development of Li-O2 batteries, Bruce et al. have demonstrated that, within the family of manganese oxides, α-MnO2 has shown



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the greatest promise as an electrocatalyst to promote the reaction between lithium and oxygen – at least during the early cycles of the cell.5 The key to this superior performance appears to be related to: 1) the presence of stabilizing H3O+ cations (or H2O species) at the center of the large 2×2 tunnel that can be ion-exchanged with lithium to generate a Li2O-stabilized structure;6, 14 2) the possibility that lithium and oxygen might be electrochemically extracted from, and reintroduced into, the Li2O-stabilized α-MnO2 framework in a reversible manner during charge and discharge; and 3) the concomitant insertion of lithium into α-MnO2 that occurs close to the potential of the lithium-oxygen reaction (~3 V),6, 15, 16 which increases the mixed valence Mn4+/3+ character and likely the electronic conductivity of the α-MnO2 framework and imparts hybrid Liion/Li-O2 character to the overall cell reaction.7 Electrochemical reactions that occur within a Li2O-stabilized α-MnO2 electrode structure in Li-O2 cells are highly complex and poorly understood; little is known about the structural changes that occur during cell operation, both physical and electronic, or the thermodynamics of the reactions between lithium and oxygen in the presence of α-MnO2. As a first step to understand these reactions, a base-line study of the removal of oxygen from, and reinsertion into, a hydrated α-MnO2 host structure has been undertaken. Unlike the Li2O component in lithiastabilized α-MnO2, the release of H2O (and hence oxygen), by heat treatment and its reaccommodation by subsequent exposure to moisture on cooling to room temperature can be readily monitored. This study has relevance to high-capacity, lithium-rich metal oxide electrodes for lithium-ion batteries, in which a partially reversible oxygen oxidation reaction accompanied the concomitant displacement of 5-9% of the O atoms from a close-packed oxygen array at high potentials during the early cycles, has recently been reported.17


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Several characterization techniques, notably temperature-resolved in-situ synchrotron powder X-ray diffraction (XRD), thermogravimetric analysis (TGA), redox titration, X-ray absorption near-edge spectroscopy (XANES), and in-situ UV resonance Raman spectroscopy were employed to monitor changes to the composition, chemical state, and structure on heating hydrated α-MnO2 to 450 °C and cooling to room temperature in air. First-principles density functional theory (DFT) calculations were used to predict the thermodynamics and kinetics of H2O/H3O+ intercalation into α-MnO2 tunnels and the charge state of the tunnel species. 2. Methods 2.1 MnO2 synthesis α-MnO2 was synthesized from a Mn2O3 precursor that was prepared by heating electrolytic manganese dioxide (Energizer) in air at 700 °C for 24h. Refluxing the Mn2O3 precursor in 4.5M H2SO4 for 16 hours at 60 °C yielded a dark brown product, which was then filtered and rinsed with deionized H2O. The product was dried in an oven at 275 °C for 16 hours and stored in a desiccator. 2.2 Redox titrations The average oxidation state of manganese in a hydrated α-MnO2 sample and in two standard manganese oxide compounds, Mn2O3 (Aldrich) and β-MnO2 (Energizer), was determined by a standard redox titration process. Samples of 50mg each were reacted with a known amount of ammonium ferrous sulfate ((NH4)2FeSO4•6H2O) in 4N H2SO4 solution to reduce the manganese to Mn2+. The original manganese oxidation state was then determined by back titrating the unreacted Fe2+ with a 0.1N potassium permanganate (KMnO4) solution, as described more fully elsewhere.18



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2.3 X-ray diffraction and structure refinement High resolution, temperature-resolved XRD patterns of hydrated α-MnO2 and a standard β-MnO2 sample were collected continuously in air and under Ar at beam line 11ID-C of the Advanced Photon Source (APS) at Argonne National Laboratory. The wavelength and size of the high-energy X-ray beam were 0.11165 Å and 0.6 mm × 0.6 mm, respectively. The samples were heated from room temperature to 450 °C at a rate of 1°C/min with a TMS94 controller (Linkham Scientific Instruments) and then cooled to room temperature; XRD patterns were collected, insitu, every 1°C during both heating and cooling with an integration time of one minute per data point. A second sample was heated in similar fashion to 600 °C. The temperature was monitored with a platinum resistor sensor to a stated accuracy of ±0.1°C. A Perkin-Elmer large area detector covered the 0-40o 2θ range of the complete scan. Two-dimensional diffraction patterns were calibrated and converted to a conventional intensity vs. 2θ format using the program Fit2D.19 All refinements were performed using the General Structure Analysis System (GSAS) package with the EXPGUI interface.20 Rietveld refinement of the XRD data was carried out to calibrate the background, zero point and the X-ray source profile for accurate cell parameter determination. 2.4 First principles modeling Density functional theory (DFT) calculations of pure and hydrated α-MnO2 structures were performed using the plane wave code VASP.21,

22

Projector augmented wave (PAW)

potentials22, 23 were used. Due to the reduced computational cost and negligible effects on lattice parameters (~0.1%) and water intercalation energies (-0.39 eV vs -0.36 eV/H2O for the lowenergy configuration tested), as compared to the regular oxygen PAW potential, the soft version of the PAW potential was used for oxygen. To treat the exchange–correlation, the generalized 6 ACS Paragon Plus Environment

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gradient approximation (GGA) of Perdew–Burke–Ernzerhof (PBE)24 was employed with the Hubbard-U correction in the implementation of Dudarev et al.25 The value of U was chosen to be 3.5 eV in order to reproduce experimental formation enthalpies of 5 compounds in the Li–Mn–O system.15,16 Van der Waals corrections in the Grimme approach26 (D2) were also included. A kinetic energy cutoff of 353.6 eV was used for the plane wave basis set. Bulk calculations were performed in supercells containing 16 and 32 formula units, with 3×3×5 and 3×3×3 k-point grids respectively. The volume of the cell was optimized, while maintaining tetragonal symmetry, until the total energy converged to 10-4 eV. The nudged elastic band method (NEB)27 was used to determine the diffusion barrier for H2O and H3O between two adjacent sites in the α-MnO2 tunnels, with the intermediate images converged to 0.05 eV/Å. Thermal properties were calculated with Phonopy28 using the atomic forces calculated with VASP. Bader charge analysis was performed29, 30 to obtain the charges of the tunnel species and the manganese ions. Figures of atomistic structural models were produced using the VESTA visualization code.31 The sampling of H2O and H3O+ insertion in α-MnO2 was performed as follows. One H2O or H3O+ was inserted per 16-formula-unit cell, with a=b=9.82Å, c=5.85Å, and α=β=γ=90°. The position of the oxygen atom, determined previously by neutron diffraction to be along the axis at the center of the 2×2 tunnels,14 was varied varying along the tunnel axis. The hydrogen atoms were placed bonded to the oxygen atom while maintaining the bond lengths and angles of freestanding H2O/H3O+ molecules. These structures were then fully relaxed with PBE+U+D2. One hundred orientations each of H2O and H3O+ insertion were tested. Fixed-volume ab initio molecular dynamics simulations (AIMD) were performed for the lowest energy structures, at 300, 500, and 700K, for 3 ps, with a time step of 0.3 fs, to further explore the configurational space.



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2.5 X-ray Absorption Near-Edge Spectroscopy (XANES) XANES data were collected in transmission mode for hydrated/de-hydrated α-MnO2 and two standard Mn2O3 and β-MnO2 samples at beam line 10-BM at the APS. A Mn reference (foil) was measured simultaneously for energy calibration. Data analysis was performed using IFEFFIT and Athena software packages.32 2.6 Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC) Thermogravimetric analyses (TGA) and differential scanning calorimetry (DSC), respectively, were performed to monitor 1) the water loss on heating a hydrated α-MnO2 sample, and 2) the MnO2 to Mn2O3 phase transition that occurs by oxygen loss. The TGA/DSC analyses were conducted with a NETZSCH instrument (TG 449 F3 Jupiter); the sample was heated from room temperature to 600 °C at a rate of 1 °C/min under compressed air using a flow rate of 60 ml/min. 2.7 UV Raman spectroscopy For the Raman experiments, α-MnO2 samples were mounted in a specially designed cell, shown schematically in Fig. 2, heated from room temperature to 435 °C with a ramp rate of 1 °C/min in flowing O2 (60 ml/min), held at 435 °C for 4 h, and then cooled to room temperature at 1 °C/min. Two different flow methods were used, 1) ‘open continuous flow’, and 2) ‘closed circulation flow’; in both methods, O2 gas flows through the α-MnO2 sample during heating and cooling. In the ‘open continuous flow’ method, O2 gas continuously carries the H2O released from the heated α-MnO2 into the hood exhaust, whereas in the ‘closed circulation flow’ method, O2 gas and H2O circulate in a closed loop configuration using a gas circulation pump. The UV Raman spectra of the samples were recorded in-situ at room temperature in flowing O2 gas before and after heat treatment. 8 ACS Paragon Plus Environment

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The 325 nm line from a Helium-Cadmium laser (Kimmon Koha) was used for excitation. The collimated laser light was focused on the sample, with the laser power delivered to the sample kept at 2.5 mW. A home-made 90° off-axis ellipsoidal reflector with the backscattering geometry was used to collect the scattered light, as described in detail elsewhere.33 A triplegrating spectrometer (Princeton Instruments, Trivista 555) equipped with a liquid N2-cooled UVenhanced CCD detector was used. The triple-grating spectrometer efficiently filters out Rayleigh light and the stray light. Cyclohexane, chloroform, and trichloroethylene were used as a frequency standard to calibrate the Raman shifts, the accuracy of which is estimated to be ±1cm-1.

Fig. 2 UV Raman system (for the ‘continuous flow’ method), consisting of a 325 nm UV laser, an in-situ Raman cell, an ellipsoidal reflector, and a triple-grating spectrometer equipped with a UV-enhanced CCD.

3. Results and Discussion 3.1 Mn oxidation state and composition – experiment and theory Redox titration was used to determine the Mn oxidation state and to estimate the composition of the hydrated α-MnO2 sample; the Mn oxidation state of two commerciallyavailable standard materials, Mn2O3 and β-MnO2, were also determined for comparison (Table 1). The manganese oxidation state in the Mn2O3 and β-MnO2 standards was found to be 3.02+ and



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4.02+, respectively, consistent with the values for stoichiometric compounds. In contrast, the manganese oxidation state in the hydrated α-MnO2 sample was 3.83+, consistent with a hydronium-stabilized structure with composition 0.17H3O•MnO2. This composition correlates well with the nominal formula of other well-known cation-stabilized α-MnO2 minerals such as KxMn8O16 (cryptomelane) and BaxMn8O16 (hollandite) in which x is ∼1 and the Mn oxidation state ~3.875+ and ~3.750+, respectively.1, 34, 35 Figure 3 shows the Mn-K edge XANES absorption spectra of the Mn2O3, β-MnO2, as well as hydrated and dehydrated α-MnO2 samples. The spectra of hydrated and dehydrated αMnO2 are remarkably similar, with one data set essentially overlapping the other. This finding strongly suggests that dehydration has no significant effect on the oxidation state of Mn. The Mn K edge (6545-6560eV) of the hydrated and ‘dehydrated’ α-MnO2 samples is positioned between those of Mn2O3 and β-MnO2, but lies significantly closer to the Mn K edge of β-MnO2, consistent with the redox titration data. Least squares fitting of the K edge shows an effective Mn oxidation state of 3.73+ in the hydrated α-MnO2 sample. The difference between the XANES data of the two standard samples is also clearly evident in the first derivative of the spectra, in which the amplitude of the Mn3+ (Mn2O3) signal decreases at 6555-6560eV, whereas the Mn4+ (β-MnO2)18 signal increases over the same range, as shown in Fig. 3 (inset). In this respect, the first derivative spectra of the two α-MnO2 samples are similar to β-MnO2, confirming that the Mn valence is close to 4+. The data are consistent with the hypothesis that, on heating hydrated α-MnO2, water is removed from the stabilizing hydronium (H3O+) ions to leave the residual protons (H+) within a now unstable α-MnO2 framework, thereby maintaining the slightly reduced oxidation state of the manganese in the structure; however, the co-existence of charged


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H3O+ and neutral H2O species within the structure cannot be discounted. These data are consistent with related reports on α-MnO2.18, 36

Fig. 3 Mn-K edge XANES spectra of hydrated and dehydrated α-MnO2 samples and Mn2O3 (Mn3+) and β-MnO2 (Mn4+) standards. Inset: first derivative of the XANES spectra.

An analysis of the expected oxidation states of Mn in xH2O•MnO2 and xH3O•MnO2 based on DFT calculations, together with the experimental data, is summarized in Table 1. Mn oxidation states were calibrated using both magnetic moments and the Bader charges of Mn in Mn2O3, β-MnO2, MnOOH, Mn3O4, and MnO structures. A least-squares fit, involving all of the phases, was used to determine the linear relationships between the magnetic moment and the oxidation state, and also between the Bader charge and the oxidation state. The equations for these least square fits are shown in Table 1. Using the calibration, the oxidation state of Mn in 0.125H2O•MnO2 and 0.125H3O•MnO2 structures, i.e., those in which the interstitial sites in the tunnels of α-MnO2 were half occupied by oxygen, were deduced. The results show that the experimentally observed oxidation states from XANES fitting is also consistent, in terms of electronic structure, with insertion of 0.1-0.2 H2O/H3O+ per MnO2. In this paper, we have



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therefore used the composition of the material inferred by the redox titration data, i.e., 0.17H3O•MnO2, as the reference point for the discussions. Table 1 Mn oxidation states, magnetic moments, and Bader charges of various manganese compounds from DFT calculations, and Mn oxidation states from redox titration(a) and XANES fitting (b).

Phase

Formal Mn charge (QFMn)

Magnetic Moment µMn

Bader charge QMn

β-MnO2 standard (experimental value) α-MnO2 (anhydrous)

4 (4.02a) 4

2.96

1.83

3.19

1.77

Mn2O3 standard (experimental value) MnOOH

3 (3.02a) 3

3.87

1.66

3.98

1.65

Mn3O4

3

3.85

1.64

Mn3O4

2

4.52

1.41

MnO

2

4.61

1.42

Least square fit from above standards

Hydrated α-MnO2 (experimental value)

(3.83a) (3.73b)

µMn = -0.745QFMn+6.089 R2 = 0.977 -

QMn = 0.193QFMn+1.048 R2=0.966 -

0.125H2O•MnO2

3.87 (from µMn) 3.91(from QMn)

3.21

1.80

0.125H3O•MnO2

3.73 (from µMn) 3.59(from QMn)

3.31

1.74

3.2 In-situ XRD studies of hydrated α-MnO2 during heating and cooling Hydrated α-MnO2 crystals have tetragonal, I4/m, symmetry.6 Rietveld refinement of room temperature synchrotron XRD data yielded lattice constants of a = 9.7765 Å and c = 2.8616 Å, in good agreement with previous reports and DFT calculations.3,6 The XRD data and contour maps of the variation in 2θ of two strong reflections, (200) and (411), centered at approximately 1.30° and 3.50° 2θ, respectively, on heating a 0.17H3O•MnO2 sample from room 12 ACS Paragon Plus Environment

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temperature to 450 °C, and on cooling back to room temperature at 1°C/min in air, are shown in Figs. 4(a-b). Both reflections shift slightly to lower angles with increasing temperature and back again on cooling, consistent with the expected thermal expansion of the structure. Close inspection of the (200) reflection (Fig. 4b) reveals an unusual thermal behavior, as the contour lines bend several times during the heating process, but less so during cooling. Despite this behavior, the tetragonal α-MnO2 framework appears to remain intact when heated below 450 °C. A second experiment, in which heating was continued to 600 °C, indicated the onset of a structural change at about 500 °C, consistent with the transformation of α-MnO2 to Mn2O3, the XRD data of which are provided later in the paper.

Fig. 4 (a): Synchrotron XRD patterns (λ=0.11165 Å) of hydrated α-MnO2, collected in-situ, when heated to 450 °C and cooled to room temperature (ramp rate = 1 °C/min). (b): Contour maps showing the movement of the (200) and (411) reflection positions (2θ) during heating and cooling.

Figure 5 provides plots of the lattice parameter and unit cell volume changes vs. temperature for an initially hydrated α-MnO2 sample when heated at 1 °C/min from room temperature to 450 °C, a) in air and b) under argon and, thereafter, cooled to room temperature.



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Because the c lattice parameter does not vary significantly on dehydration, the increase in this lattice parameter value is predominantly a result of thermal expansion. In contrast, the a(=b) parameter, and consequently the unit cell volume, shows abnormal behavior for both air and argon environments. When heated in air (Fig. 5a), a remains essentially constant between room temperature and ~120 °C, as it does again between ~275 and ~375 °C, implying that a loss of water counters the thermal expansion of the unit cell; the same trend appears in the unit cell volume vs. temperature plot. When heated under argon (Fig. 5b), similar behavior is observed but, in this case between room temperature and ~160 °C, there is a significant decrease in a, yet a small increase in volume. On cooling from 450 °C to room temperature, whether in air or under argon, the lattice parameters and cell volume decrease linearly, without the plateaus observed on heating, to values slightly above their initial values. We attribute this behavior either to a structural change that limits or prevents the return of water molecules into the framework, or to a process in which the water from the surrounding atmosphere is slowly re-absorbed during the cooling process. This anomaly is discussed in further detail later in the paper.

Fig. 5 Plots of lattice parameters and volume vs. temperature of hydrated α-MnO2, obtained insitu with synchrotron XRD data, when heated at 1°C/min in (a) air and (b) argon.


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3.3 Thermal expansion of α-MnO2 In order to differentiate the effects of thermal expansion from those of H2O/H3O+ intercalation on lattice parameters, we performed first principles DFT calculations of α-MnO2 at various volumes. The imposed increase in the c lattice parameter was smaller in magnitude than the a and b parameters, in accordance with the XRD data taken at temperatures between 150°C and 250 °C in Fig. 5a. Phonon calculations were then performed to determine the volumes that minimize the Helmholtz free energy at temperatures

Probing the Release and Uptake of Water in Î±-MnO2Â·xH2O

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