Probing the Fate of Mn Complexes in Nafion: A Combined

Article pubs.acs.org/JPCC

Probing the Fate of Mn Complexes in Nafion: A Combined Multifrequency EPR and XAS Study Alexander Schnegg,*,† Joscha Nehrkorn,† Archana Singh,‡,§ Irati Alonso Calafell,† Shannon A. Bonke,‡ Rosalie K. Hocking,∥ Klaus Lips,† and Leone Spiccia‡ †

Berlin Joint EPR Laboratory (BeJEL), Institute Nanospectroscopy, Helmholtz-Zentrum Berlin für Materialien und Energie, Kekuléstrasse 5, 12489 Berlin, Germany ‡ School of Chemistry and Australian Centre of Excellence for Electromaterials Science, Monash University, Melbourne, Victoria 3800, Australia § INSPIRE Faculty, Council of Science and Industrial Research Center-AMPRI, Hoshangabad Road, Bhopal, Madhya Pradesh 462026, India ∥ Discipline of Chemistry, School of Pharmacy and Molecular Sciences, James Cook University, Townsville, Queensland 4811, Australia S Supporting Information *

ABSTRACT: Multifrequency electron paramagnetic resonance (EPR, 9.4 and 263 GHz) and X-ray absorption spectroscopy (XAS) were employed to study structural and electrochemical changes of selected Mn complexes in contact with dried Nafion films upon electro-oxidation and after longterm illumination. It was found that when in contact with Nafion the Mn-Me3TACN complexes are reduced into Mn(II) complexes with an octahedral geometry (Me3TACN = 1,4,7-trimethyl-1,4,7triazacyclononane). The reduction process involves an intermediate product in which Mn has been reduced from the initial +III or +IV state of the precursor Mn complex but is still coordinated to the TACN ligand. Electro-oxidation yields a MnOx mineral with a birnessite structure, in which the Mn(III) or Mn(IV) ions exhibit very strong magnetic coupling. Long-term illumination of the oxide-containing Nafion film while it is exposed to an aqueous electrolyte partially decomposes the mineral and forms a Mn(II) species with octahedral coordination.

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INTRODUCTION The human need for sustainable and eco-friendly energy systems has stimulated intense research in technologies that can provide a pathway to hydrogen fuel.1−4 Photoelectrochemical (PEC) water splitting offers a method to produce hydrogen and oxygen in a carbon free process.5 A limiting factor of PEC efficiency is the overvoltage required for the water oxidation reaction. This situation can be improved by using catalysts capable of achieving oxygen evolution at potentials closer to the thermodynamic value.6 Advances in PEC water splitting have included the use of catalysts based on molecular metal complexes and oxides.7−36 However, there is a growing trend in the literature that suggests that under particular experimental conditions some of the molecular complexes used as water oxidation catalysts are transformed in situ into the corresponding metal oxide which is the actual catalyst.12,19,35−38 In this context, we recently investigated a range of molecular Mn complexes as precursors for the formation of MnOx nanoparticles in Nafion polymer.35 Among the different Mn complexes, [MnIV(Me3TACN)(OCH3)3]PF6 (MnIV) and [(Me3TACN)2MnIII2(μ-O)(μ-CH3COO)2](ClO4)2 (MnIIIMnIII) (abbreviations in brackets will be used hereafter to refer to these two Mn complexes; Me3TACN = 1,4,7-trimethyl1,4,7-triazacyclononane) were found to yield MnOx nano© XXXX American Chemical Society

particles in Nafion whose turnover frequencies (TOFs) for water oxidation were the highest, being approximately 10 times higher than those derived from [Mn(OH2)6](ClO4)2, referred to as Mn2+ hereafter (41−44 Mn1− hr−1 compared to 0.2−2 Mn −1 hr−1).35 On the basis of knowledge gained from different analytical methods, including XAS, transmission electron microscopy (TEM), and EPR, it was concluded that origin of the catalytic activity is the metal oxide nanoparticles formed with a birnessite-like structure with its catalytic activity being correlated with particle size. Moreover, simulations of the electron diffraction patterns, obtained from high-resolution TEM, revealed that layer misregistration and a high number of lattice vacancies are features of the birnessite material formed within Nafion from the Mn(IV) precursor.39 X-band EPR spectroscopy revealed that upon doping into Nafion the Mn precursors change their oxidation state to Mn(II) independent of their initial molecular structure and oxidation state. Electro-oxidation of doped films results in strongly reduced EPR signal intensities with changed spectral Received: October 25, 2015 Revised: December 18, 2015

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DOI: 10.1021/acs.jpcc.5b10451 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C shape.35 Further long-term illumination of the electro-oxidized films in the presence of an electrolyte partly regenerated ( 1/2), the interpretation of X-band EPR spectra can become very difficult when ZFS and Zeeman splittings are comparable. As a result, so-called “forbidden” transitions are detected, which originate from the fact that mS quantum numbers are not well-defined. These additional transitions lead to pronounced line broadening, which deteriorates the spectral resolution. In particular in the case of Mn(II), the EPR spectra can be significantly simplified by going to higher resonance frequencies/fields, where Zeeman splittings exceed ZFS.46,47,53 For the case of a Mn(II) ion with small ZFS, this reduction is shown in Supporting Information Figure 1. Reduction in complexity, however, is a precondition for the extraction of structural information via the electron spin coupling parameters. These parameters are obtained via spectral simulations based on a spin Hamiltonian (SH). For the high-spin (S > 1/2) Mn states the SH is54

Despite its high sensitivity and spectral resolution, particularly to systems with unpaired spin, even high-field EPR may fail to detect strongly coupled paramagnetic ions. However, a comparison of EPR and XAS data can help to assign Mn oxidation and binding states.19 In the present work, we exploit the complementary power of multifrequency EPR (at 9.4 and 263 GHz) and XAS to extract information that can assist us to resolve electrochemical and structural changes of Mn species embedded in the Nafion matrix. These experiments will help in the determination of the binding environment of Mn(II) formed in contact with Nafion and after the application of potential bias and light. To study the Nafion Mn precursor reactions, the reaction of MnIIIMnIII, MnIV, and Mn2+ in acetonitrile solution with dried Nafion films is investigated by means of multifrequency EPR. In a second step, the fate of the Mn(II) embedded in Nafion films is followed by 263 EPR and XAS. On the basis of these experiments, conclusions are drawn about the incorporation of Mn complexes into Nafion films and the Mn products formed on electro-oxidation and their photoreduction on illumination.

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MATERIALS AND METHODS Materials. All chemicals and solvents were purchased from Sigma-Aldrich and used as received. Nafion was purchased from Sigma-Aldrich in the form of acidic polymer dispersion, 117 Nafion, 5% solution in alcohol. Millipore water was used throughout to prepare the electrolyte solutions. Synthesis. The ligand N,N′,N″-trimethyl-1,4,7-triazacyclononane (Me3TACN) and complexes [MnIV(Me3TACN)(OCH3)3]PF6 (MnIV) and [(Me3TACN)2MnIII2(μ-O)(μCH3COO)2](ClO4)2 (MnIIIMnIII) were prepared and characterized according to the literature procedures.56,57 Electrochemistry. Electrochemical experiments were conducted at 22 °C on a Biologic Science Instrument, VSP 300, using FTO as a working electrode. A conventional threeelectrode setup was used in which the Nafion-coated electrodes, doped with the Mn compounds, served as the working electrode, an Ag/AgCl electrode (3 M NaCl, 0.200 V potential vs NHE) served as reference electrode, and Pt wire or mesh served as counter electrode. Electrochemical measurements were carried out in 0.10 M Na2SO4 (pH 6.5) aqueous solution prepared in distilled water. The potentials are quoted with respect to the Ag/AgCl reference electrode. Nafion Film Preparation. For both the EPR and XAS experiments, the Nafion films were produced by drop-casting on FTO glass using 5% water−ethanol solution of Nafion, drying at room temperature and then heating at 120 °C for 20 min. X-ray Absorption Spectroscopy. Mn K-edge XAS spectra were recorded at either the Australian National Beamline Facility (ANBF; beamline 20B at the Photon Factory, Tsukuba, Japan) or the Australian Synchrotron (12ID). The beam energy was 2.5 GeV, and the maximum beam current was 400 mA. Harmonic rejection was achieved by detuning the channel-cut Si[111] monochromator by 50%. Glassy-carbon electrodes were placed 45° to both the incoming beam and the Fe−Li fluorescence detector. The energy scale was calibrated internally using a Mn foil (calibration energy 6,539.0 eV, corresponding to the first inflection point of the foil). At least two sequential measurements were taken at room temperature. The Average program58 was used to average raw data files, and PySpline was used to subtract the background.59 XAS data was measured directly from a glassy-carbon electrode at room

⎛ 2 1 ⎞ ̂ Ĥ = g isoμB S ·̂ B0 + A isoS ̂ ·I55Mn + D⎜Sẑ − S(S + 1)⎟ ⎝ ⎠ 3 2 1 ̂2 + E (Sx − Sŷ ) (1) 2

Here the first term denotes the electron Zeeman interaction, which couples the electron spin Ŝ to B0 via giso. The second term represents the hyperfine interaction (HFI), which quantifies coupling between electron spin and 55Mn nuclear spin (I = 5/2) by the isotropic HFI-value Aiso. The last two terms are the ZFS terms, parametrized by axial and transverse ZFS parameters, D and E, respectively. giso, Aiso, D, and E contain information on the electronic and local structure of the paramagnetic states. In the case of high-spin Mn(II) (S = 5/2), most information may be gained from Aiso and D. Aiso for Mn(II) typically ranges from 160 to 300 MHz and was found to be a measure of the manganese ligand field and the ionicity.40,55 ZFS values are mainly influenced by geometry variations. D values of 1 GHz or smaller are typically assigned to six-coordinate Mn(II), whereas five-coordinate systems frequently exhibit D values up to many GHz.41,42 Much larger ZFS values are found for Mn(IV) S = 3/243 and for low (S = 1)44 and high (S = 2)45 integer spin Mn(III) states. B


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Figure 1. Left: X-band cw EPR spectra (T = 30 K, νmw = 9.4 GHz) of 4 mM MnIIIMnIII in acetonitrile with different amounts of dried Nafion (from top to bottom: 10, 20, 30, and 40 μL, respectively) added to the liquid solution. Arrows indicate resonance peaks due to “forbidden transition”. Right: Experimental 263 GHz cw EPR (green) spectra of MnIIIMnIII in frozen acetonitrile solution with varying amounts of Nafion added: 5 μL (first row, T = 190 K, νmw = 263.09 GHz), 15 μL (second row, T = 190 K, νmw = 263.09 GHz), 20 μL (third row, T = 170 K, νmw = 263.09 GHz), and 80 μL (fourth row, T = 180 K, νmw = 263.25 GHz). X-band intensities are plotted in absolute units given by the spectrometer, whereas 263 GHz spectra are normalized to 1. Blue traces represent simulations made by assuming two Mn(II) populations, species A and B, with Aiso, giso, D, and E values given in the text. Spectral simulations depicted in the fourth row were obtained assuming only contributions of species A.

containers of the 263 GHz spectrometer and rapidly frozen with liquid nitrogen. Doped Nafion Films. State 1. Dried Nafion films (1 cm2 in area, preparation as described above) on FTO glass were doped by dipping them for 10 min into acetonitrile solution containing the Mn precursor ([Mn]total = 8 mM). After rinsing and drying at room temperature, half of the Mn-doped film was peeled from the FTO and studied by EPR (hereafter termed “doped in Nafion”). State 2. The film remaining on the FTO electrode was exposed for 10 min to a potential of 1.1 V (vs Ag/AgCl) in a 0.10 M Na2SO4 aqueous solution contained in an electrochemical cell. Nafion films containing the electro-oxidized Mn products were peeled off the electrode and studied by EPR. State 3. In a third step, a 0.10 M Na2SO4 solution was added to the EPR sample tube containing the electro-oxidized Mn products embedded in Nafion at room temperature. Subsequently, the sample was irradiated with light from a 500 W metal tungsten lamp for 16 hours. After light exposure, EPR spectra were again recorded at X-band and 263 GHz.

temperature at different stages of electrochemical cycling. The electrode was prepared by drop-casting Nafion onto a glassycarbon electrode as has been described previously, followed by dipping the electrode for 10 min into an acetonitrile solution containing the Mn precursor ([Mn]total = 4 mM). A single electrode was cycled through the sequence described above, and the data was taken on each state, after which it was returned to electrolyte (0.1 M Na2SO4) for conversion to the next state. EPR Spectroscopy. EPR was performed on a Bruker Elexsys 780 (263 GHz/12 T) EPR spectrometer equipped with a nonresonant sample insert, a He-cooled sample cryostat, and a Bruker ESP 300 X-band EPR spectrometer modified for quantitative EPR measurements. Spectral simulations of cw Xband and 263 GHz Mn(II) spectra were carried out with EasySpin, a Matlab toolbox.60 Because electrochemical cycling cannot be performed inside the 263 GHz EPR probe, samples containing Mn in the following states were prepared outside of the 263 GHz sample probe according to the protocols below. Mn Solutions. Defined amounts of dried Nafion film were added to three different acetonitrile solutions (30 μL), containing MnIV [Mn]total = 8 mM, MnIIIMnIII [Mn]total = 4 mM, or Mn2+ [Mn]total = 8 mM, respectively. After adding the Nafion, the samples were transferred to the Teflon sample

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RESULTS AND DISCUSSION Mn Solutions. To study the initial reaction of Mn precursors with Nafion, the effect of adding different amounts C


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Figure 2. cw 263 GHz EPR spectra of MnIIIMnIII (left) and MnIV (right) doped into Nafion films. First and second row: Mn complexes doped into Nafion (state 1) measured at 280 and 5 K (MnIIIMnIII) and 150 and 5 K (MnIV), respectively. Third row: doped Nafion films after applying a potential bias (state 2). Fourth row: doped Nafion films after illumination with white light for 16 h (state 3). EPR spectra depicted in the third and fourth rows were measured at 280 K (MnIIIMnIII) and 150 K (MnIV), respectively. Green traces, experimental data; blue traces, spectral simulations with Aiso, giso, D, and E values given in the text. All spectra were measured at a mw frequency of 263.09 GHz, except the MnIV spectrum in the first row, which was recorded at 262.93 GHz.

of dried Nafion film to MnIIIMnIII, MnIV, and Mn2+ acetonitrile solutions was investigated. Figure 1 depicts frozen-solution Xband (9.4 GHz, left column) and 263 GHz (right column) cw EPR spectra of 4 mM MnIIIMnIII acetonitrile solutions with different amounts of Nafion added to 30 μL of the liquid solutions. X-band and 263 GHz spectra depicted in the first, second, and third rows of Figure 1 were taken from samples that were immediately frozen in liquid nitrogen after addition of Nafion. The spectra depicted in the fourth row of Figure 1 were taken after waiting times of 16 h after the addition of 40 μL (Xband) and 80 μL (263 GHz) of Nafion, respectively. Recently, we found that adding increased amounts of Nafion to MnIIIMnIII acetonitrile solutions, which show no EPR signal in the absence of Nafion (Supporting Information of ref 35), finally leads to a complete conversion of all the Mn into Mn(II).35 However, in addition to the growing Mn(II) EPR signal intensity upon increasing the amount of Nafion, a pronounced change of the EPR line shape can be observed. This change is most pronounced between the X-band spectra recorded after the addition of 30 and 40 μL of Nafion. The Xband spectrum in the third row (30 μL) exhibits at least 12 resonant peaks (arrows in Figure 1) spanning from 280 to 400 mT, whereas the spectrum in the fourth row (40 μL with extended waiting times) is dominated by 6 Mn(II) resonance lines and a broad background signal. Because of the abovementioned restrictions, a clear-cut assignment of the observed

spectral changes was not possible on the basis of X-band EPR data alone. Applying 263 GHz/12 T cw EPR, we found that the addition of small amounts of Nafion leads to a six-line Mn(II) spectrum with additional splitting on the outer high and low field lines (Figure 1 right; first, second, and third row). The observed spectral shape can be rationalized assuming two Mn(II) populations, with different Aiso but similar giso values. Mn(II) high-field EPR spectra exhibit six well-separated EPR lines, which originate from mS = −1/2 to +1/2 transitions (red circle in Supporting Information Figure 1B) split by Aiso. In contrast to X-band EPR on Mn(II), which is very sensitive to ZFS, high-field EPR spectra are mainly dominated by Aiso and giso. A superposition of two Mn(II) species with different HFI and g values leads to 12 individual lines. However, if the difference in giso is small compared to the inhomogeneous line width, then only the outer lines will be split, whereas the inner lines overlap. This situation may be best seen in the second and third row of Figure 1. On the basis of spectral simulations assuming two different Mn(II) species the following can be postulated. At low Nafion concentrations (5 μL, upper spectrum on the right side of Figure 1), the 263 GHz EPR spectrum is dominated by a Mn(II) species with Aiso = 246(1) MHz and giso = 2.0007(1) and a minor contribution from a second species Aiso = 268(1) MHz and giso = 2.0007(1). A change of 20 MHz in Aiso is quite significant and for Mn(II); we will therefore refer to Mn(II) with Aiso ranging from 265 to 269 D


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Figure 3. (a) Mn K-edge XANES and (b) k3 weighted EXAFS spectra of standard Mn(II) reference material (magenta) in comparison to those of the products of doping Nafion films (state 1) with Mn2+ (light blue) and MnIIIMnIII (green), respectively. (c) k3 weighted EXAFS spectra of synthetic sodium birnessite (black), in comparison to MnIIIMnIII measured in different states of photochemical cycling. state 2 = initial load +1.1 V potential (red), state 3 = state 2 + 2 h of light excitation (green), state 4 = state 3 + 1 h 1.1 V potential (purple), and state 5 = state 4 + 5 h of light excitation.

MHz as Mn species A, whereas Mn(II) with Aiso between 244 and 248 MHz will be labeled as Mn species B. Upon increasing the Nafion content, the contribution of species A to the 263 GHz spectra increases relative to that of species B. Finally, the spectrum is completely dominated by species A (lower spectrum on the right side of Figure 1). For the simulation of the X-band EPR spectra, we employed Aiso and giso values obtained for species A and B from the 263 GHz EPR spectra. Because Mn(II) X-band EPR spectra are much more affected by ZFS, D = 700 (200) MHz and E = 250 (100) MHz and D = 500 (100) MHz and E = 100 (50) MHz and strains in the same order of magnitude as the ZFS values were used for the simulation of species A and B. respectively. In accordance with the 263 GHz spectra, X-band spectra in the third row were simulated assuming a superposition of species A and B with equal weights, whereas simulations in the fourth row of Figure 1 are obtained assuming solely contributions from species A. Xband simulations reproduce the main features of the experimental data very well and confirm that all spectral features can be assigned to isolated Mn(II). Similar shifts of Aiso (55Mn) from 265 MHz to values between 240 and 245 MHz were previously assigned to changes from octahedral binding in a hydration sphere to strongly distorted tetrahedral binding.40,46 In the present case, the smaller Aiso found for species B may be due to the presence of Mn(II) intermediates coordinated to nitrogen atoms from the TACN ligands in a strongly distorted geometric environment. Upon longer waiting times or the addition of larger amounts of Nafion, these nitrogen donors are successively replaced by oxygen donors from the Nafion network, which finally leads to full conversion into species A. Analogous experiments with MnIV acetonitrile solutions yielded similar spectra, which could again be modeled by a superposition of species A and B (Supporting Information Figure 2). In contrast, however, the Mn2+ acetonitrile solutions did not show any contribution of species B after adding Nafion (Supporting Information Figure 3). This clearly shows that the change in Aiso is related to the

detachment of the TACN ligands and not to interactions with solvent molecules. Doped Nafion Films. Figure 2 displays the cw 263 GHz EPR spectra after doping MnIIIMnIII (left) and MnIV (right) into Nafion films. From top to bottom are depicted the spectra of Mn complexes doped into Nafion (state 1) measured at kT ≫ hν (first row, where k denotes the Boltzmann constant, T the sample temperature, ℏ denotes Planck’s constant, and ν denotes the EPR excitation frequency) and kT ≪ hν (second row), after applying a potential bias (state 2, third row), and finally illumination with white light for 16 h (state 3, fourth row). Alongside the experimental spectra (green continuous lines) are plotted the spectral simulations (dashed blue lines).60 Figure 3 shows complementary XAS (Figure 3a) and EXAFS (Figure 3b) spectra of Mn(II) reference material in comparison to that of the products of doping Nafion films (state 1) with MnIIIMnIII, and Mn2+, respectively. In Figure 3c, the EXAFS spectra of synthetic sodium birnessite and MnIIIMnIII doped in Nafion films at the various stages of the transformation are shown. Comparing high-field EPR and XAS, the following conclusions may be drawn. State 1. The 263 GHz cw EPR spectra of both MnIIIMnIII and MnIV doped in Nafion show well-defined Mn(II) six-line spectra with spin coupling parameters matching those of species A found in MnIIIMnIII and MnIV acetonitrile solutions after the addition of Nafion. This assignment is in line with our previous findings35 and is further supported by XANES and EXAFS on Nafion films doped with Mn2+ and MnIIIMnIII (Figure 3 a,b, respectively). Clearly, independent of the precursor chemical structure and oxidation state, doping in the Nafion films results in Mn(II). Changing the temperature from room temperature to 20 K leaves the spectral shape of the cw 263 GHz spectrum unchanged (data not shown). However, upon lowering the temperature down to 5 K (second row of Figure 2), kT ≪ hν, an additional broad feature appears in the 263 GHz spectra of both precursor molecules. This broad component is due to E


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The Journal of Physical Chemistry C transitions between the mS = −5/2 to mS = −3/2 spin levels. At higher temperatures or lower resonance frequencies, this component contributes only as a broad, barely visible background to the EPR spectrum. Because of the chosen resonance frequency of 263 GHz and corresponding resonance fields around 9.4 T, at 5 K splitting between the field dependent energy levels exceed kT, which leaves mainly the energetically lowest lying mS = −5/2 state populated47 (black circle in Supporting Information Figure 1B). On the contrary, energetically higher lying states, e.g. mS = ± 1/2, are depleted. As a result, the low-temperature spectra are dominated by transitions from the energetically lowest lying states, which are much more affected by D and E than transitions between the mS = ± 1/2 levels. To corroborate our assignment and determine the spin coupling parameters, spectral simulations were performed assuming one single Mn(II) species. The simulated spectra are depicted in the first and second rows of Figure 1, together with the experimental spectra. From simultaneous simulations of the high- and low-temperature spectra of species A in MnIIIMnIII and MnIV, the following spin coupling parameters were obtained: giso = 2.0007(1), Aiso = 268(1) MHz, D = 700(200) MHz, and E = 250(100) MHz for MnIIIMnIII and giso = 2.0008(1), Aiso = 269(1) MHz, D = 860(200) MHz, and E = 215(100) MHz for MnIV. In both cases, distribution functions for D and E with fwhm in the same order as the main values had to be included in the simulations. Within experimental errors, the same set of spin coupling parameters are obtained for both precursor molecules. The good match between simulated and experimental spectra in low- and high-temperature spectra strongly support the formation of a single Mn(II) species after incorporation of the MnIIIMnIII and MnIV precursors into Nafion. The observed ZFS and Aiso values match those typically assigned to octahedral Mn(II) complexes.46 Previously, nearly identical values were determined by very high-frequency/high-field EPR for Mn(II) symmetrically ligated by six oxygen molecules.40 In the present case, these oxygen atoms could be either from residual water molecules in the Nafion pores or from sulfonate groups in the Nafion matrix. State 2. EXAFS spectra depicted in Figure 3 c clearly indicate that Mn is incorporated in a mineral with birnessite structure after applying a potential bias. This finding is in accordance with previous X-band EPR and EXAFS results.35 However, these studies also revealed a residual X-band EPR signal that could not be assigned so far.35 The corresponding 263 GHz EPR spectra are shown in the third row of Figure 2. In accordance with the findings from Xband EPR, the signal intensity of the 263 GHz EPR spectra drastically decreases by a factor of 10−30 (depending on the experimental conditions) after applying an external potential. Again, the spectral changes from state 1 to state 2 are very similar for both precursor molecules. In both cases, the residual EPR spectra consist of 10 narrow lines centered around g ≈ 2, which resemble a superposition of two Mn(II) species, with different Aiso, but identical giso values. Surprisingly, the 263 GHz EPR spectra of the electro-oxidized films could be simulated with nearly identical spin coupling parameters (species A + species B) as the two component spectrum observed in MnIIIMnIII and MnIV solutions after the addition of Nafion (right column of Figure 1). In addition to Mn(II) species A detected in state 1, species B was assigned with giso = 2.0008 and Aiso = 247 MHz (MnIIIMnIII) and giso = 2.0008(1) and Aiso = 248(1) MHz (MnIV). Because the residual signals in state 2

were much weaker than in state 1, D and E could not be measured directly from low-temperature measurements. Therefore, for the simulation of species A and B in state 2, ZFS values in the same range as those experimentally determined for species A in state 1 were used. This could be rationalized by the assumption that some of the Mn(II), which is not changing its oxidation state upon applying a potential bias, is still bound to N atoms from the TACN ligands. The fact that species B is mainly visible in doped Nafion films after applying a potential (state 2), whereas species A dominates in state 1 and 3, may indicate, that species B is not involved in the electro-oxidation. Our simulations show EPR signals for the electro-oxidized products originating from Mn(II) states in two different binding environments, which could not be assigned in our previous study.12 The residual EPR signal found in electro-oxidized Nafion films doped with MnIIIMnIII or MnIV is a peculiarity of these compounds. Complementary quantitative X-band EPR measurements on Mn2+-doped Nafion films revealed that electro-oxidation of the doped films decreases the EPR signal below 0.1% (Supporting Information Figure 5), which is close to the detection limit of this method and far below the EPR signal intensity found for electro-oxidized Nafion films obtained from MnIIIMnIII or MnIV precursors. To check for other Mn states (e.g., Mn(III) or Mn(IV)) with unknown spin coupling parameters, the magnetic field was swept between 0 and 11 T (Supporting Information Figure 2). However, no additional EPR signatures were observed. This means that for the Nafion films doped with MnIIIMnIII or MnIV despite a minor contribution from a Mn(II) species Mn is very effectively incorporated in the birnessite mineral which was assigned as the dominating contribution by means of EXAFS35 (Figure 3 c for MnIIIMnIII). State 3. Light exposure of MnIIIMnIII- or MnIV-doped Nafion films after applying a 1.1 V potential leads to a significant increase of Mn(II) (fourth row in Figure 1) but only for very long exposure times (>12 h) and in the presence of an electrolyte. The EPR spectrum recorded for MnIIIMnIII-doped films (left) in state 3 slightly deviates from that obtained from MnIV-doped (right) films. The EPR spectra of both precursors after irradiation are dominated by species A, whereas for MnIIIMnIII doped films an additional contribution of species B could be assigned. Spectral simulations of experimental 263 GHz spectra revealed spin coupling parameters for MnIIIMnIII of giso = 2.0008(1) and Aiso = 267(1) MHz (species A) as well as giso = 2.0008(1) and Aiso = 253(1) MHz (species B), whereas for MnIV, only contributions from species A were detected, with giso = 2.0009(1) and Aiso = 269(1) MHz. Quantitative X-band EPR revealed35 that under these conditions most of the Mn is still in an oxidation state different from Mn(II). Evidence on the formation mechanism of the residual Mn(II) in the photoreduced products may be drawn from a comparison of EPR and EXAFS data. Figure 3 c shows that the electro-oxidized (state 2) and photoreduced products (state 3) exhibit identical EXAFS spectra, which were recently assigned to birnessite. This indicates that the majority of the Mn-doped in Nafion films is incorporated into birnessite after electro-oxidation and remains in this form even after photoreduction. However, long-term illumination leads to a change in the birnessite EXAFS spectrum, and at the same time, isolated Mn(II) species in an octahedral environment are generated, as can be seen from 263 GHz EPR. The lightassisted formation of Mn(II) from birnessite is in accordance F



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with previous studies, which showed that birnesite61 and other Mn oxides62 release Mn(II) upon light exposure.

AUTHOR INFORMATION

Corresponding Author

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*E-mail: [email protected].

CONCLUSIONS In the following, we summarize the information on the incorporation of Mn complexes in Nafion films and the fate of doping-induced Mn(II) upon electro-oxidation and photoreduction, drawn from multifrequency EPR and XAS. In accordance with previous findings, Nafion effectively reduces MnIIIMnIII and MnIV complexes to Mn(II).35 In contact with Nafion, the nitrogen atoms from Me3TACN ligands are substituted by oxygen atoms from either residual water molecules in the Nafion pores or its sulfonate groups. The Mn(II) species are in a well-defined single octahedral geometry. The change in the binding environment of the Mn-TACN complexes to this state proceeds via a distorted geometry in which the TACN ligands remain coordinated. This is accompanied by characteristic changes in Aiso. Electro-oxidation of the Mn-containing Nafion films in the absence of light converts Mn(II) ions to a mineral with birnessite structure as has been reported recently19 and is confirmed here by EXAFS measurements. This conversion results in a strongly reduced Mn(II) EPR signal intensity. In films doped with MnIIIMnIII and MnIV complexes, the residual Mn(II) signal is a superposition of species in octahedral and a second, more distorted geometry. Because the latter seems less effective at changing its oxidation state, it is assumed to play a minor role in the photocatalytic activity of doped Nafion films. Nevertheless, the formation of Mn(II) species B from MnIIIMnIII and MnIV in contact with Nafion may influence the incorporation of Mn into the polymer matrix and subsequent conversion into the MnOx mineral via electrooxidation. Neither X-band nor 263 GHz EPR showed any evidence for isolated Mn(III) or Mn(IV) states in doped electro-oxidized Nafion films. Long-term illumination of the doped Nafion films after electro-oxidation recovers part of the signal attributable to octahedrally coordinated Mn(II) species. This state appears only after long-term illumination and in the presence of an aqueous electrolyte solution. The formation of octahedral Mn(II) species is further accompanied by change in the EXAFS signal. These facts may be explained by a lightassisted formation of Mn(II) species from birnessite. Our findings show the potential of combining multifrequency EPR and XAS to track the fate of catalytically active materials, in particular Mn, and to draw conclusions about the structure of the intermediate states of the catalytic compound.

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Monash University for scholarship support (A. Singh) as well as the Australian Research Council (through the ARC Centre of Excellence for Electromaterials Science, L.S.) and DFG (through priority program SPP 1601, A. Schnegg and J.N.) for financial support. L.S. is very grateful to the Alexander von Humboldt for a Senior Research Award and to the Helmholtz Association for the award of a Helmholtz International Fellowship. In addition, we gratefully acknowledge technical assistance by J. Rappich and S. Greil (HZB) and the use of facilities at the Australian National Beamline Facility (ANBF; beamline 20B at the Photon Factory, Tsukuba, Japan) and the Australian Synchrotron (12ID).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b10451. Calculated X-band and 263 GHz EPR Mn(II) spectra, 263 GHz EPR spectra of MnIV acetonitrile solution in the absence and presence of Nafion, 263 GHz EPR spectra of Mn2+ acetonitrile solution in the absence and presence of Nafion, 263 GHz EPR spectra of the electro-oxidized product of MnIV-doped Nafion films recorded with different magnetic field widths, and double-integrated Xband EPR signal of Nafion films doped with Mn2+ before (solid black) and after electro-oxidation. (PDF) G


Article


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Probing the Fate of Mn Complexes in Nafion: A Combined

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