Direct Formation of 2D-MnOx under conditions of Water Oxidation

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Direct Formation of 2D-MnOx under conditions of Water Oxidation Catalysis Rosalie K. Hocking, Rosalind Gummow, Hannah J King, Mayada Sabri, Peter Kappen, Christian Dwyer, and Shery LY Chang ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00095 • Publication Date (Web): 05 Apr 2018 Downloaded from http://pubs.acs.org on April 12, 2018

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ACS Applied Nano Materials

Direct Formation of 2D-MnOx under Conditions of Water Oxidation Catalysis

Rosalie K. Hocking*1,2, Rosalind J. Gummow2, Hannah J. King1,2, Mayada Sabri1,2,3, Peter Kappen4, Christian Dwyer5, Shery L. Y. Chang*6

*Authors to whom correspondence should be addressed [email protected] ; [email protected]

1

Department of Chemistry and Biotechnology, Faculty of Science, Engineering and Technology, Swinburne University of Technology, Hawthorn, Melbourne, Victoria, 3122, Australia 2

College of Science, Technology, and Engineering, James Cook University, Townsville, Queensland, 4811 3

University of Baghdad- College of Education for Pure Science –Ibn Al-Haitham, 10071, Iraq.

4

5

Australian Synchrotron, 800 Blackburn Road Clayton, Victoria, Australia, 3168

Department of Physics, Arizona State University, Tempe, AZ 85287, USA.

6

LeRoy Eyring Centre for Solid State Science, Arizona State University, Tempe, 85287, USA.

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Abstract We describe the synthesis and characterization of a novel 2D-MnOx material using a combination of HR-TEM, XAS, XRD, and reactivity measurements. The ease with which the 2D material can be made and the conditions under which it can be made implies that water oxidation catalysts previously described as “birnessite-like” (3D) may be better thought of as 2D materials with very limited layer stacking. The distinction between the materials as being “birnessite-like” and “2D” is important because it impacts on our understanding of the function of these materials in the environment and as catalysts. The 2D-MnOx material is noted to be a substantially stronger chemical oxidant than previously noted for other birnessite-like manganese oxides. The material is shown to both “directly” and “catalytically” oxidize water in the presence of Ce4+, and to directly oxidize H2O2 in the absence of any other oxidant.

TOC

Key words: Catalysis, birnessite, metal oxide, nanoparticles, redox, water-oxidation, X-ray absorption Spectroscopy, Transmission Electron Microscopy


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Introduction Efficient water oxidation catalysts are considered one of the most important components in the development of carbon-neutral energy technologies.1 A popular strategy for designing these catalysts is the so called “bio-inspired” approach to catalyst design, where materials are developed to mimic the catalytic functions of metallo-enzymes found in natural systems.2-3 While many such “protein mimics” have now been developed4-8, simple minerals are amongst the most catalytically functional. An excellent example is the mineral phase birnessite, which has recently been developed as a water oxidation catalyst.9-16 Structurally the mineral-phase birnessite bears striking similarities to photosystem II (PSII)17-20, the enzyme that oxidizes water in nature, Figure 1, and the relationship has fascinated many.9-10, 12, 21

c

a) The active site of PSII (ref 17-20) b

a

c) δ-MnO2 (birnessite (ref 22-23)

b) Fragment of Birnessite (ref 9-16)

Figure 1. (a) The active site of PSII, (b) a single layer fragment of birnessite, (c) the layer structure of birnessite, and (d) turbostratic (i.e. disordered layer stacking) birnessite. 18, 22

One of the fundamental challenges in understanding how catalysts work is to characterize the active phase of a material by analyzing the sample while in its “functional” or “active” state.23 This is particularly challenging for electro-catalysts, such as thin film oxides, which can largely only be characterized using X-ray Absorption Spectroscopy (XAS), owing to the diffraction amorphous nature of these materials, low sample yield, and other constraints inherent in the study of actual electrode surfaces.24-25



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One of the problems with relying on XAS alone in characterizing catalysis is that it can miss key parts of the structure, such as layer stacking or size effects. XAS is excellent for characterizing atomic-level structural disorder but is unable to probe larger-scale structural disorder. Therefore, XAS should be used in complement to other analytical techniques to determine the importance of structure to reactivity. To enable us to characterize the disorder typical of electrode materials in more detail we developed a simple synthetic techniques to synthesize bulk quantities of materials that mimicked the disordered structures found on electrode surfaces by XAS.24 Manganese oxide (MnOx) materials were synthesized by direct precipitation from a sodium hypochlorite (OCl-, pH 11) and Mn2+ solution.4, 26 This synthetic method was developed from the observation that molecular water oxidation catalysts (molecular PSII mimics) were converted to birnessite-like phases when exposed to these test conditions for water oxidation (0.7 M ClO-, pH 10). 12, 23, 27 However, the birnessite-like phase formed under these conditions appeared far more reactive than other similar phases which prompted us to characterize them further. Herein, we find that this material - while simple to make - is in fact better described as 2DMnOx rather than “birnessite-like” as the material does not have the coherent/ordered layer stacking that is characteristic of birnessite structures, (Figure 1). The relationship between 2D-MnOx and birnessite is analogous to the relationship between graphene and graphite. This material is shown to be a substantially stronger chemical oxidant when compared to K+ birnessite and other birnessite-like materials described in the literature. The material directly oxidizes (i.e. 2MnOx(coll.) + 4H+(aq) → O2(g) + 2Mn2+(aq) + 2H2O(l)) as well as catalytically oxidize water in the presence of Ce4+. This implies that 2D-MnOx is a stronger chemical oxidant than other manganese oxides isolated previously. The observation of direct oxidation (alongside catalytic oxidation) is important in the context of a self-healing mechanism.8, 28 This observation enables us to uncouple the water oxidation and the deposition steps in a way that is simple and helpful in catalyst design. Since the direct oxidation reaction consumes protons, by thermodynamic considerations, a chemical reaction that is both direct (i.e. sacrificial) and catalytic at low (pH=1-2) may be only catalytic at higher pH values where water oxidation is typically tested electrochemically.


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The ease with which the 2D material can be made (i.e. by direct precipitation) and the conditions under which the material precipitates (i.e. under oxidative and catalytic conditions) implies that 2D-MnOx may be an important component in a number of water oxidation catalysts. The ubiquity of manganese oxides in natural systems also implies that this 2D-material and materials closely related to 2D-MnOx may in fact be important mediators for much of the oxidative chemistry in natural systems, more generally ascribed to birnessitelike phases.29-30

Results and Discussion Identification of the 2D phase- as distinct from birnessite The highly reactive MnOx sample was characterized by multiple techniques, including X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HR-TEM). Figure 2 shows the comparative XAS of the 2D-MnOx material and high temperature “stacked” K+ birnessite materials. The K+ birnessite material was chosen for comparison because symmetry of the layer structure is very close to hexagonal making the two phases closely related in their structure.31 The Extended X-ray Absorption Fine Structure (EXAFS) and the Fourier transform (of the EXAFS) of the two materials are nearly identical (Figure 2c and d), which indicates, as expected that the two materials are very closely related structurally. Explicitly, the EXAFS gives evidence for a very similar intra-layer structure for both samples. While the EXAFS indicates that the two materials have a very similar intra-layer structure, the slight broadening of the X-ray Absorption Near Edge Structure (XANES) by the 2D material indicates that there are differences in their electronic structures. The XANES of the 2D-MnOx material is broader than that of K+ birnessite, as indicated by the arrows in Figure 2a. The edge shift and XANES broadening is consistent with there being a larger distribution of final states in the MnOx material. These final states may originate from differences in the distribution of oxidation states within the sample, which could be caused by inter-layer faults such as Mn absences32 and the associated Mn2+ ions sitting above those absences. They could also indicate a secondary or mixed phase, however no evidence was found for this in the XANES or EXAFS or in any of the other techniques used to analyze the 2D material. A 5 ACS Paragon Plus Environment


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comparison of the XAS data with a set of standards is given in Figure S1. The 2D-MnOx and K+ birnessite pre-edge regions both contain 2 peaks that are attributed mostly to MnIV and are sensitive to ligand field (t2g and eg orbitals and associated states). However, the 2D-MnOx peaks have reduced intensity similar to what has been noted before in other catalytic materials studied ex situ, (Figure S2). The presence of Mn2+ in the inter-layer will reduce the intensity of pre-edge features caused predominantly by Mn4+.32 The pre-edge and XANES regions of a previously published active manganese oxide catalyst (prepared by doping molecular compounds into Nafion33) (Figure S2) and that of SP6-MnOx reported by Saratovsky34 were compared. (Figure S3). The shape and intensity of the edge feature and the intensity reduction in the pre-edge peaks of the highly active catalyst are consistent with the material we describe herein.34 This suggests that this “2D-MnOx” material may be a common component in both biogenic and catalyst systems.


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a.

1s→t2g

1s→eg

Mn-O Mn-Mn

Minor differences in Fourier Transform Mn-Mn Mn-O

2D-MnOx

Mn-Mn-Mn

K+ Birnessite

Figure 2. Comparison of Mn K-edge XAS spectra. Blue: K+ birnessite and orange: 2D-MnOx. The data is represented as: (a) XANES, (b) pre-edge, (c) k-weighted EXAFS, and (d) Fourier transform of the EXAFS labelled with the bonding interactions responsible for the dominant peaks.



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XRD While XAS is sensitive to the intra-layer structure, the large inter-layer spacing of MnOx (7 Å or more) is beyond the range of sensitivity of EXAFS analysis. For this reason, powder XRD was used to investigate the long-range stacking order in these MnOx materials. The synthesis of 2D-MnOx (and other electrode material mimics using this synthesis) yields a bulk quantity of sample which allowed the sample to be analyzed by XRD without further processing after synthetic isolation. It is difficult to synthesize the required quantity of the thin layered electro-catalyst for XRD and to analyze data on materials with such limited crystalline order. Figure 3 shows the experimental powder X-ray diffraction patterns for the 2D-MnOx material (Figure 3: orange trace), K+ birnessite (Figure 3: blue trace), and a biogenic manganese oxide (L. discophora SP6)35 (Figure 3: purple trace). The K+ birnessite pattern shows sharp and welldefined peaks, with dominant (00l) peaks.31 This is typical of a birnessite phase with ordered layer stacking (Figure 1c) and is consistent with previous reports31. The XRD peaks for the 2DMnOx and the biogenic sample, however, are much weaker and broader compared to K+ birnessite, indicating that the crystallinity of these materials is perturbed from the highly ordered standard. The source of these perturbations can include the finite size of the crystals, the limited stacking coherence in the inter-layers due to the finite number of layers, or the disordered layer stacking (turbostratic disorder, Figure 1d).32, 35-38 To examine the cause of this perturbed crystallinity in the 2D-MnOx material, a series of simulations were performed to determine the effects of limiting the crystallite dimensions (a x b x c), where a and b are the crystallographic dimensions in the MnOx intra-layer planes, and c is the inter-layer crystallographic dimension (i.e. the layer stacking axis). Limiting the crystallite dimensions to 300 Å in a, b and c results in peak broadening (comparison of Figure 3: simulations g and f). This effect is consistent across the simulated series with decreasing intra-layer dimensionality causing a systematic increase in peak broadening. Limiting the number of MnOx layers reduces the relative intensities of the (00l) peaks that are associated with c-directional stacking (comparison of Figure 3: simulations g and f). This trend is systematic across the simulated series (Figure 3: simulations f, d and c).


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The experimental 2D-MnOx XRD pattern is best fit with a finite intra-layer dimension (~30 nanometers) and a reduced inter-layer dimension (< 3 layers). In support of this conclusion; the X-ray diffraction patterns of the 2D-MnOx material and the biogenic MnOx sample (L. discophora SP6)35 (Figure 3a vs. b) are very similar. The 2D material is thus likely a good mimic of a biogenic oxide which has been noted previously (by XRD) to be composed of 2-3 layers35. The conclusions of this XRD analysis cannot alone confirm the 2D nature of the synthesized material as the analysis could permit disordered or buckled MnOx layers to be interpreted as limited layers. TEM is thus required to confirm that the material is in fact comprised of a few layers rather than exhibiting turbostratic stacking.

a

a

b

b

b a

10

20

30

40 50 2θ (degrees)

60

70

Figure 3. Powder X-ray diffraction data. The (a) 2D material is compared to the experimental data of (h) K+ birnessite, (b) a biogenic MnOx from Jurgenson35, and a series of simulations that examine changes in lattice dimensions (simulations (c), (d), (e), (f) and (g)). ±Detailed simulations of K+ birnessite with a 2 layer hexagonal unit cell are given in the supporting information.39



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TEM High Resolution – TEM was used to further verify the morphology and structural differences between K+ birnessite and the 2D-MnOx material. The bright-field (BF-) TEM image of K+ birnessite (Figure 4a) shows aggregates of well-defined spherical-like particles of sizes around 40nm. The K+ birnessite particles are highly crystalline, as seen in the HR-TEM image given in Figure 4b. The 2D-MnOx material, however, has a very different morphology from K+ birnessite (Figure 4c), with aggregates that show a characteristic curved sheet morphology. This curved sheet morphology is consistent with what other have noted for 2D metal oxides. 40-45

An enlarged area from the edge of the aggregates confirms that the material is composed

of nano-sized sheets (Figure 4d). The aberration-corrected high-resolution TEM image of an isolated sheet (Figure 4e) reveals well-resolved atomic columns.

The atomic structure shows the atomic hexagonal

arrangement expected in the two-dimensional MnOx material (as viewed along the [001] direction). Well-defined atomic structures would not be observed along the c-axis if the material exhibited turbostratic stacking. Therefore, the TEM images of the MnOx material are consistent with a material composed of only a few layers, i.e. are 2D. The HR-TEM image also shows variations of the (110) spacing. A reduction in the lattice spacing of 0.2 Å occurs in every other (110) lattice plane and resulted in a (1/2 1/2 0) superlattice. A plausible (and likely) origin of this variation is the presence of Mn vacancies (Figure 3f). Mn vacancies can distort (by shortening) the positions of the oxygen atoms surrounding the Mn vacancy - as suggested by DFT calculations performed by Kwon46-47. This causes the observed variations of the lattice spacing. The presence of the (110) spacing variations concurs with the (110) peak broadening observed in the XRD, and supports the conclusion that Mn vacancies and the compensating Mn2+ interlayer ions are the likely origin of the XANES K-edge broadening observed for 2D-MnOx and for other manganese oxide electrocatalysts12 The patterns observed in Figure 4e are not consistent with Jahn Teller distortions / oxygen vacancies which would cause a clear pattern of short and long distances not differences in atomic column focus.48 We note that stabilization of Mn(III) states have been implicated in a number of water oxidation catalysts.15, 49-51 However we do not find any evidence for Jahn-Teller distorted Mn(III) here.


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b.

20nm

5nm

d.

10nm

V

Figure 4. TEM data. (a) BF-TEM and (b) HR-TEM Images of K+ birnessite. (c) BF-TEM and (d) HR-TEM images of 2D-MnOx. (e) Atomically resolved image of 2D-MnOx viewed along [001] orientation. (f) Atomic model of MnOx; where pink = manganese atom, and red = oxygen atom. The image illustrates that the original red oxygen atoms distort (due to the Mn vacancy) from their shown position to the dashed position.

Overall, the combined results from the XAS, XRD and TEM demonstrate that the MnOx material is distinct from 3D K+ birnessite by having the morphology and structure of a material 11 ACS Paragon Plus Environment


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with 2D nano-sheets. The structural differences have important consequences to both reactivity and our understanding of catalytic mechanism as for water oxidation the faces of the sheets may play an important role in ways not previously recognized.

Catalytic Activity and Oxidative Efficiency of 2D vs. 3D MnOx The challenge in studying the room temperature material in catalytic reactions is preserving its 2D nature.52 There are two established test systems for “true” water oxidation catalysts: (i) a test implementing a chemical oxidant such as CeIV or [Ru(bpy)3]3+, and (ii) electro-catalytic studies where the material is immobilized on an electrode. During the course of testing these materials, we observed that the 2D-MnOx material decomposed to Hollandite if it was subjected to the usual conditions of electrode preparation from powder samples.

53

Experimental conditions were therefore carefully considered to avoid changes to the 2D nature of this material.

9, 26, 54

Zhang and Sun et al.11 have recently demonstrated that a

materials prepared in situ on electrodes with a near identical XRD spectrum gives efficiencies similar to nickel and cobalt electro-catalysts at near neutral pH. One of the least ambiguous way to test a manganese oxide for catalysis is to use a sacrificial oxidant of which CeIV is considered the best26, 54-55 as it has a high enough potential for water oxidation , but not too high, and it does not transfer oxygen. The results of CeIV water oxidation tests for 2D-MnOx and K+ birnessite are presented in Figure 5a. The 2D material produces ~20x times more oxygen than K+ birnessite. Given the similar short range order for both materials and that there is less than a 4x difference in surface area, the substantial increase likely arises - at least in part - from the “2D-nature” of the material. Interestingly, the 2D-MnOx material behaves differently to other manganese oxides26, 54-55 in the Ce4+ test, in that the material dissolves over time. To quantify this effect further, the samples subject to the cerium test were freeze dried at 3 time points and analyzed by XAS. The Mn XANES of 2D MnOx clearly show that the evolution of O2 is accompanied by the breakdown of the MnOx phase to Mn2+, Figure 6. The stoichiometry of this observation is such that under conditions tested MnOx acts both as a catalytic and a sacrificial oxidant of water. The observation is important to the understanding of catalytic mechanism and the functionality of metal oxide materials ex situ and on an electrode surface. The test allows the function of the electro-catalyst to be uncoupled into two steps; the first being direct oxidation (i.e. 2MnOx

(colloidal)+

4H+(aq) ⇌ O2(g) + 2Mn2+(aq) + 12

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2H2O(l)); the second being electrodeposition (i.e. Mn2+(aq) + 2H2O(l) ⇌2 MnOx(colloidal) + 2e- +

4H+(aq)). By thermodynamic considerations alone a material that is a sacrificial oxidant at low pH, will be an efficient electro-catalyst at higher pHs where the sacrificial reaction is less spontaneous.

a

Ce4+ test for

800 700

mmols of O2 produced / mol Mn

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


600

Better Catalyst, an Stronger O

500 400 300 200 100 0

0

10

20

30 Time (min)

40

50

60

Figure 5. (a) A comparison of the oxygen evolution curve for the 2D-MnOx material and K+ birnessite, (b) A comparison of the oxygen evolved for the reaction of the 2D-MnOx material with H2O2, (c) BET surface area measurements



1.2

a. 2D-MnOx

Absorption Intensity (Normalized)

1.0 0.8 0.6

0hr

0.4

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Absorption Intensity (Normalized)

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1.0

6548 6568 Energy (eV)

6588

b. K+Birnessite

0.8 0.6

0hr

0.4

1hr

0.2

48 hours

0.0

6528

6548 6568 Energy (eV)

6588

Figure 6. Mn K –edge XANES taken on cerium treated manganese oxides. a) 2DMnOx. The formation of Mn2+ can been seen in the XANES indicated by the red arrow. B) K+ birnessite (no decomposition is noted).

In natural systems, birnessite-like phases are important mediators of oxidative reactions which occur via the reductive decomposition of the oxides into a Mn2+ state.29-30 This oxidative chemistry is distinct from catalytic chemistry where the catalyst is unchanged. The reaction with hydrogen peroxide (H2O2) was used as a further test to study oxidative efficiency. H2O2 was chosen in this test for simplicity as most chemists are familiar with the well-known catalytic disproportionation of H2O2 by MnO2. However, when H2O2 is added to a manganese oxide the material can act as both an oxidant - in which this material is reduced to Mn2+- and a catalyst for the disproportionation of H2O2 to O2 and H2O - in which the material 14 ACS Paragon Plus Environment

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is unchanged. The less susceptible a material is to oxidative decomposition, the better it is as a catalyst for the disproportionation of H2O2. More disordered manganese oxides are better mediators of sacrificial (redox) chemistry.46-47 The efficiency of 2D-MnOx as an oxidant is clear from its reaction with H2O2, where it readily dissolves in preference to acting as a catalyst. This is quantified by the moles of O2 gas evolved in Figure 5b. A video showing the striking difference in reactivity of 2D vs 3D MnOx with H2O2 is provided with the Supporting Information. Herein, a number of manganese oxide water oxidation catalysts exhibit structural and reactivity properties associated with 2-dimensionality.

The effects on reactivity are

particularly pertinent when examining the sacrificial versus catalytic nature of a 2D material and how this relates to our understanding of the function of electro-catalysts. Our study of a 2D material shows that the 2D nature decreases the thermodynamic stability of a material and allows the material to perform as an oxidant as well as a catalyst. This conclusion is in support of “self-healing” catalysts56 where a number of “catalyst” materials are broken down in the water oxidation reaction but are reformed (from solution) at potentials where water oxidation occurs.28, 57 The high reactivity of “birnessite-like” MnOx materials in nature may also be better explained by the presence of a 2D phase. XRD patterns of biogenic materials that have previously been ascribed to a turbostratic structure could equally be attributed to a 2D material. In summary, a manganese oxide material formed under conditions of water oxidation has very limited c-direction stacking and is better described as a 2D –MnOx instead of a “birnessite-like” phase. The 2D material formed this way has vacancies that change its electronic structure in ways that are likely key for reactivity. Vacancies can have distinct behavior in 2D materials58-59 and have important consequences for reactivity. Specifically, the presence of Mn vacancies in this material likely play a significant role in enhancing the catalytic properties of natural birnessite-like phases and catalytic MnOx phases.



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Methods Synthesis of 2D-MnOx: The 2D-MnOx material was made by direct precipitation from a solution of Mn(II) (manganese(II) acetate tetrahydrate, Mn(OCOCH3)2(OH2)4, 200mM stock solution) and hypochlorite (4% ClO- solution) that had been combined in a 1:1 volume ratio of Mn(II) stock to hypochlorite. The samples were then separated by centrifugation with water washing (exchanged 10x with water following each separation cycle).The extracted material was then washed with acetone (2x). Once the sample had dried, it was used without further physical manipulation in the analytical analyses described below (unless otherwise stated). Synthesis of K+ Birnessite: Potassium (K+) birnessite was made according to a published method. KMnO4 was heated to 800°C in a muffle furnace for 12 hours. The K+ birnessite product was separated by filtration and air dried.34, 60 Powder X-ray Diffraction: Samples were ground into a fine powder prior to XRD analysis. A sample was prepared by loading the fine powder into a standard XRD tray and pressing the solid to form a flat surface flush with the rim of the tray. Each sample was scanned between 10-70° at 0.02° increments at a rate of 5 seconds per increment. XRD data was background corrected and normalised using WINPLOTR61 - part of the FULLPROF62 suite. XRD data was analysed further with Microsoft Excel 2010. A SIEMENS D5000 X-Ray Diffractometer with a Cu anode X-ray tube (λ=0.154 nm) was used for powder X-ray diffraction analysis. X-ray diffraction simulations of “c-disordered” and 2D-MnOx samples were carried out with the Fortran program DIFFaX63. The DIFFaX software models a crystalline solid as a set of stacked layers and derives the XRD pattern by integrating the diffraction intensities layer by layer. The atomic positions in the layer structure of MnOx were taken from the literature using a hexagonal unit cell of Galliot31. Identical layers were stacked using (001) as the stacking vector, as a first approximation. The dimensions of the layers in the a and b directions were adjusted to simulate nano-sized layers and the number of stacked layers was varied from 43 to 1. Inductively Coupled Plasma–Atomic Emission Spectroscopy: The manganese content of the MnOx materials was measured using Inductively Coupled Plasma – Atomic Emission Spectroscopy (ICP-AES). The solid MnOx samples were dissolved in nitric acid and H2O2 prior 16 ACS Paragon Plus Environment

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to ICP-AES analysis (based on the method described in reference 64). In this method, a known amount (0.1 g) of material was heated (90 ± 5°C) in 70% nitric acid (HNO3, 20 mL) for 2 hours. The addition of hydrogen peroxide (H2O2, 30%, 5 mL) ensured total dissolution of the MnOx sample. The digestion solution was cooled and made up to 100 mL using distilled water. The ICP-AES analyses were carried out using a Varian Liberty Series II instrument.

X-ray Absorption Spectroscopy: Mn K-edge XAS spectra were recorded at the wiggler XAS beam-line 12 ID at the Australian Synchrotron (3.0 GeV storage ring; 200 mA beam current in top-up mode). The incident beam energy was controlled using a Si(111) double-crystal monochromator. Higher harmonic content was minimised using mirrors, including a short harmonic rejection mirror in the endstation upstream near the sample position. Spectra on the oxide powders were collected in transmission mode using ionisation chambers (Oken; ~1.5 L/min He flow; supply voltage U = 250V). Samples were prepared by grinding the materials to fine powders using an agate mortar, mixing with boron nitride and loading into a 1mm thin aluminium sample holder sealed with thin self-adhesive Kapton tape.65 XAS data were analysed using a combination of PySpline66 and Microsoft Excel67 for background subtractions, and Artemis68-69 for EXAFS fitting. Hydrogen peroxide oxidation experiments: A calibrated suspension of MnOx (5.057x10-5 M) was added to hydrogen peroxide (H2O2, 30%, 2 mL) in acetate buffer (CH3COONa, 0.1 M, pH=5.5, 5 mL) or phosphate buffer (Na2HPO4, 0.1M, pH=7.0, 5 mL). The gas evolved was measured in triplicate using a gas burette. Transmission Electron Microscopy: High Resolution Transmission Electron Microscopy was performed using an aberration corrected TEM, Titan (FEI Company) at 300kV at LeRoy Eyring Center for Solid State Science, Arizona State University. The imaging condition was tuned to so-called negative Cs imaging condition. In this case, spherical aberration Cs = -15 µm and the defocus C1 ~ 20 nm. The images were acquired using the single electron detection camera K2 (Gatan, Inc) under "counted" mode. The dose rate was kept at 10 electrons/pixel/second to minimize the potential electron beam induced vacancies. HR-TEM image simulations were performed using in-house program70 where the specimen exit wave functions were calculated using the multislice formula. The image intensities were 17 ACS Paragon Plus Environment


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then simulated according to the experimental imaging conditions. The atomic coordinates of pure hexagonal birnessite (with water and inter-layer cation omitted) was obtained based on Kwon’s DFT calculations of vacancies in hexagonal birnessite46-47. The atomic coordinates of hexagonal birnessite with Mn vacancies placed in the centre of a 2x2 supercell was obtained based on the relaxed structure calculated using DFT46-47. Tests for Water oxidation: The Frey, Wiechen and Kurtz9, 54, 71 gas chromatography (GC) analytical method was used to determine the water oxidation efficiency of the manganese oxides. Each reaction solution consisted of the MnOx (ca. 10 mg for 2D-MnOx and 5 mg for K+ birnessite) and cerium(IV) oxidant ((NH4)2Ce(NO3)6, 675 mg) mixed with distilled water (5 mL) in a Labco Exetainer© vial (12 mL, flat bottomed, non-evacuated, double wadded cap). At the time steps of 10 minutes, 20 minutes and 60 minutes; 0.5 mL (injection volume) of the sample vial headspace was hand drawn for analysis. Molecular oxygen and nitrogen was measured using a Shimadzu GC-2014 equipped with a Restek Molesieve 5 Å 80/100 column (10 ft × 2.1 mm) and thermal conductivity detector (TCD). The column temperature was 70°C, the injector temperature was 150°C and the TCD was 380°C. Ultra-high purity He was used as a carrier gas at a flow rate of 30 mL / min. Preparation of freeze dried samples of Ce4+/MnOx for analysis by XAS: XAS was used to determine the fate of the MnOx samples as they participated in the Ce4+ test for water oxidation described above. The reaction was studied at different time points (0 hours (hr), 1 hr and 48 hr) by quenching the suspension at the required time (in a -20°C freezer) and freeze drying in preparation for XAS analysis. A new suspension was prepared for each time point. Each suspension was prepared in a 50 mL Falcon tube and contained the MnOx sample (ca. 10 mg for 2D-MnOx and 5 mg for K+ birnessite), cerium(IV) oxidant ((NH4)2Ce(NO3)6, 200 mg) and water (5 mL). After the suspension had completely frozen (ca. 24 hours in the freezer), it was freeze dried on one of the 8 valve manifolds of a ‘CRYODOS -50’ freeze dryer until all the solvent had evaporated (ca. 24 hours). The samples were then diluted 1/3 with boron nitride and analysed by XAS in fluorescence mode.

Formulation of 2D-MnOx: The chemical formula of 2D-MnOx was characterised using a combination of Ion Chromatography, AES, iodiometric titration and weight changes upon 18 ACS Paragon Plus Environment

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heating to different temperatures.

Upon heating to 500oC 2D-MnOx decomposes to

Hollandite with sodium in the inter-layer, and upon heating to 800°C decomposes to Mn2O3. The materials kept below 200°C were noted to reversibly absorb and desorb water. Materials freshly heated to 100 °C were found to have a composition of Mn2+0.16Na0.12(H2O)0.9 [Mn4+0.95,(MnV)0.05] unheated materials had the composition Mn2+0.16Na0.12 (H2O)3.0 [Mn4+0.95,(MnV)0.05], where (MnV) is a Mn vacancy. Materials heated were noted to be hydroscopic, and for this reason ion chromatography was used in addition to AES to accurately determine the Na:Mn ratio. Supporting Information Available: XANES and pre-edge of 2D-MnOx and K+ birnessite are compared to MnO (Mn2+), Mn2O3 (Mn3+) and MnO2 (Mn4+); XANES pre-edge and EXAFS data are compared to Spiccia’s catalyst, XAS data of the material taken under formation conditions; Comparison of the XAS data with that of Biogenic MnOx formed by L. disophora SP6). EXAFS fits for both K+ Birnessite and 2D-MnOx; a blow up of the powder diffraction data. In addition a video showing the dissolution of 2D-MnOx by peroxide is given as a *.mp4 file); Electrochemical data of the material drop cast on ITO and the calculation of electrochemically active surface area is provided. Acknowledgements. Dr C. Glover and B. Johannesson are thanked for their assistance with the collection of the XAS data. Dr S. P. Best is thanked for useful discussions around the topic of reactivities of 2D materials. Part of this research was undertaken on the X-ray Absorption Spectroscopy beamline at the Australian Synchrotron -a part of the Australian Nuclear Science and Technology Organisation (ANSTO). Dr. Hocking acknowledges Swinburne University for a Vice Chancellor’s Women in STEM Fellowship and James Cook University for supporting the early stages of this research.



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Manganese Pyrophosphate with Tunable Mn Valency. J. Am. Chem. Soc. 2014, 136, 42014211. 51. Takshima T.; Hashimoto K.; Nakamura R., Inhibition of Charge Disproportionation of MnOx Electrocatalysts for Efficient Water Oxidation Under Neutral Conditions. J. Am. Chem. Soc. 2012, 134, 18153-18156. 52. During the course of these experiments we noted that if we heated 2D-MnOx above 300 °C it turned into hollandite, this limited our electrochemical analysis to drop cast materials with very mild heating (

Direct Formation of 2D-MnOx under conditions of Water Oxidation

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