&Metal–Organic Frameworks

Surface and Structural Investigation of a MnOx Birnessite-TypeWater Oxidation Catalyst Formed under Photocatalytic Conditions

Benjamin J. Deibert,[a] Jingming Zhang,[a] Paul F. Smith,[a] Karena W. Chapman,[b]

Sylvie Rangan,[c] Debasis Banerjee,[a] Kui Tan,[d] Hao Wang,[a] Nicholas Pasquale,[a]

Feng Chen,[e] Ki-Bum Lee,[a] G. Charles Dismukes,[a, f] Yves J. Chabal,[d] and Jing Li*[a]

Abstract: Catalytically active MnOx species have been re-ported to form in situ from various Mn-complexes during

electrocatalytic and solution-based water oxidation when

employing cerium(IV) ammonium ammonium nitrate (CAN)oxidant as a sacrificial reagent. The full structural characteri-zation of these oxides may be complicated by the presenceof support material and lack of a pure bulk phase. For the

first time, we show that highly active MnOx catalysts formwithout supports in situ under photocatalytic conditions.

Our most active 4MnOx catalyst (~0.84 mmol O2 mol Mn¢1 s¢1)

forms from a Mn4O4 bearing a metal–organic framework.4MnOx is characterized by pair distribution function analysis

(PDF), Raman spectroscopy, and HR-TEM as a disordered, lay-ered Mn-oxide with high surface area (216 m2g¢1) and small

regions of crystallinity and layer flexibility. In contrast, theSMnOx formed from Mn2 + salt gives an amorphous speciesof lower surface area (80 m2g¢1) and lower activity(~0.15 mmol O2 mol Mn¢1 s¢1). We compare these catalysts tocrystalline hexagonal birnessite, which activates under the

same conditions. Full deconvolution of the XPS Mn2p3/2 corelevels detects enriched Mn3 + and Mn2 + content on the sur-

faces, which indicates possible disproportionation/compro-

portionation surface equilibria.

Introduction

Solar energy is one of the most abundant renewable energyresources available, and is therefore considered to be a key el-

ement for energy sustainability. Hydrogen produced from solar

driven water splitting poses a particularly attractive alternativeto fossil fuels due to the availability of water and solar resour-ces. The overall water splitting reaction is comprised of twohalf-reactions: water oxidation (O2 evolution reaction, OER) and

proton reduction (H2 evolution reaction, HER), in which theformer half generates protons and electrons for the latter half.Thermodynamic and kinetic limitations make the oxidationhalf-reaction the bottleneck of the overall process, which re-

quires two water molecules and a four-electron oxidation;a minimum potential of 1.23 V vs. NHE at pH 0 is needed todrive this reaction [Eq. (1)] .[1]

2 H2O!O2 þ 4 Hþ þ 4e¢ ð1Þ

Pt, Ru, and Ir based materials have proven effective as water

oxidation catalysts (WOCs) ; however, their low abundance andsubsequent high cost make them ill-suited for widespread ap-plication.[2] With this in mind, catalysts based on common tran-

sition metals, such as Mn, Fe, Co, Ni, and Cu, are attractive al-ternatives. Of these, Manganese is of particular interest ; for the

past few decades immense effort has been spent resolving thestructure and OER mechanism of the naturally occurring Pho-tosystem II (PSII) oxygen evolving complex (OEC) “hetero-

cubic” CaMn4O5.[3] Various Mn-oxides and multinuclear Mn-complexes have been studied, with special attention paid to

materials containing OEC-like structural motifs.[4] Despite theseefforts, few Mn-complexes have proven themselves as stable

WOCs;[1d] however, several polymorphs of Mn-oxide are knownto be active. Our previous work tested a number of crystalline

[a] B. J. Deibert,+ J. Zhang,+ P. F. Smith, D. Banerjee, H. Wang, N. Pasquale,Prof. K.-B. Lee, Prof. G. C. Dismukes, Prof. J. LiDepartment of Chemistry and Chemical BiologyRutgers, The State University of New Jersey, Piscataway, NJ 08854 (USA)E-mail : jingli@rutgers.edu

[b] Dr. K. W. ChapmanX-ray Science Division, Advanced Photon SourceArgonne National Laboratory, Argonne, IL 60439 (USA)

[c] Dr. S. RanganDepartment of Physics and AstronomyRutgers, The State University of New Jersey, Piscataway, NJ 08854 (USA)

[d] K. Tan, Prof. Y. J. ChabalDepartment of Material Science & EngineeringUniversity of Texas at Dallas, Richardson, TX 75080 (USA)

[e] Prof. F. ChenDepartment of Chemistry, Biochemistry, and PhysicsRider University, Lawrenceville, NJ 08648 (USA)

[f] Prof. G. C. DismukesThe Waksman Institute, Rutgers, The State University of New JerseyPiscataway, NJ 08854 (USA)

[++] These authors contributed equally to this work.

Supporting information for this article is available on the WWW underhttp ://dx.doi.org/10.1002/chem.201501930. It contains materials, syntheticprocedures, WLED illumination profile, additional Clark electrode data, XPSsurvey scans, XPS splitting parameters, b-MnO2 Mn 2p3/2 spectra, synchro-tron powder diffraction pattern, PDF bond length comparisons, TGA data,and color versions of the manuscript figures.

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Full PaperDOI: 10.1002/chem.201501930

Mn-oxide phases under photochemical conditions, which mayreflect possible performance in a photochemical cell.[4h] Our ini-

tial results have shown Mn2O3, Mn3O4, and l-MnO2 (spinel) asactive when driven by photogenerated [Ru(bpy)3]3 + (bpy =

2,2’-bipyridine). Activity from these materials trends with flexi-ble Mn3 + features, such as long Mn–Mn distances and weakMn¢O bonds.

Some birnessite MnO2 polymorphs have also been reportedas active, but frequently under alternate conditions.[4a–f, 5] Bir-

nessite is a hydrated lamellar phyllomanganate comprised ofnegatively charged sheets of edge-sharing MnO6 octahedraand ~7 æ periodicity from layer to layer (Figure 1d).[6] The neg-ative charge arises from either the presence of Mn3 + atoms

within the octahedral layer (Mn3þlayer, triclinic- or monoclinic-

type) when formed under alkaline conditions, or Mn4þlayer vacan-

cies (hexagonal-type), which is typically formed under acidic

conditions. Triclinic-type is generally charge balanced by inter-layer cations (Na+ , K+ , Mg2+ , Ca2 + , Sr2 + , etc.) and hexagonal-

type by Mn3 + triple corner-sharing octahedra above or belowthe layer vacancy (Mn3þ

interlayer).[7] Trends in catalytic activity have

been correlated to degree of disorder (higher disorder usuallyresulting in higher activity), structure flexibility, and interlayer

species, with Ca2 + birnessites showing some of the highest

levels of activity by Kurz et al.[4d,f, 8] It has been suggested bySuib et al. that activity in birnessites may correspond to inter-

layer/surface Mn3+ bound above and below vacancies.[4e] Nota-bly, the specific structural polytype is rarely discussed in con-

text of performance, likely owing to birnessites’ intrinsic poorcrystallinity, structural disorder, and small particle sizes, thereby

complicating unambiguous characterization.[4d] The fact thatthese materials readily undergo structural changes in response

to solution pH further complicates structural analysis. Ceri-um(IV) ammonium ammonium nitrate (CAN) solution is com-

monly employed as sacrificial chemical oxidant to evaluate bir-nessite OER catalysts, but has an intrinsically low pH (~1).[4c,d,f]

As documented, birnessite is well known to transition from thetriclinic to hexagonal phase under ~pH 5, which is virtually un-detectable to most analytical methods.[7d–f]

Recent literature has focused upon the transformation ofmolecular Mn species into amorphous MnOx during CAN oxi-dant and electrochemically driven water oxidation. The activespecies has been loosely characterized as birnessite-like based

on bonding features present in extended X-ray absorption finestructure (EXAFS) studies. However, structural characterization

is limited by the necessary use of support materials, such as

clay in the case of CAN reactions, and Nafion in electrocatalyticsystems.[9] Recently, Driess, Dau, and co-workers studied the

formation of an amorphous MnOx catalyst through partial cor-rosion of a support-free crystalline MnO precursor.[10] The re-

sulting MnOx was found to be active under both CAN and pho-tochemical conditions; however, the former was required to

oxidize the MnO to MnOx and generate an active phase. Be-

cause the precursor is only partially oxidized, a heterogeneousmaterial very high in Mn2 + content was obtained (average oxi-

dation state of Mn reported as 2.5). It is worth noting that re-cently the solution structure and role of CAN as a one-electron

outer sphere oxidant has been called into question, promptingus to limit the current study to photocatalytic conditions.[11] .

Metal–organic frameworks (MOFs) are an emerging class of

materials comprised of crystalline coordination networks be-tween metal ions and organic linkers, and have been demon-

strated as important candidates for multiple applications.[12] Byutilizing an MnII

4O4 based MOF precursor approach, we show

for the first time that MnOx catalysts form under photochemi-cal water oxidation conditions and without the use of supportmaterial. Highly active discreet MnOx particles (4MnOx) were

isolated that can be best described as resembling hexagonal-type birnessite. We compare our results to amorphous precipi-tate formed from Mn2+ salt (SMnOx) under the same condi-tions, as well as crystalline hexagonal K+ birnessite (HexBir).Bulk characterization of the catalysts was carried out usingpowder X-ray diffraction (PXRD) and X-ray photoelectron spec-

troscopy (XPS) was used to investigate the active surfaces. Ad-ditional detailed structural analysis was performed on the mostactive catalyst, 4MnOx, using high-energy synchrotron X-ray dif-

fraction with pair distribution function (PDF) analysis, Ramanspectroscopy, and high resolution transmission electron mi-

croscopy (HR-TEM).

Results and Discussion

Photochemical water oxidation

For facile characterization of active Mn oxides generated in

situ during water oxidation, we developed a precursor methodemploying photocatalytic conditions to convert an Mn2+ cube

Figure 1. a) Microscope image of as-synthesized Mn4-MOF (inset shows bulksynthesis product, scale = 1.0 mm); b) zoom out HR-TEM image of catalyst4MnOx ; c) structure-plot of Mn4-MOF (polyhedrons represent cube-shapedSBU); d) generic conceptual drawing of layered MnO2 (triclinic-type) with dis-ordered interlayer.

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bearing MOF, Mn4(m3-OMe)4(nic)4 (Mn4-MOF, nic = nicotinate), toan active MnOx material.[13] Amber crystals of as-synthesized

Mn4-MOF were obtained using the reported synthesis method(Figures 1a and c) and phase purity was confirmed by PXRD

(Figure 2a). The as-made crystals were ground and added toa visible-light driven water oxidation assay containing

[Ru(bpy)3]2+ photosensitizer and Na2S2O8 sacrificial oxidant inpH 7 NaHCO3 buffer. O2 evolution was monitored usinga Clark-type electrode and four white LEDs as the light source

(see Figure S1 in the Supporting Information for luminescenceprofile). Upon illumination, the system gradually changed colorfrom orange to brown and subsequently began generating

a significant amount of O2 after an initial lag period of ~1 min

(Figure S2a in the Supporting Information). The catalyst was re-covered by centrifugation, which had changed from amber

crystals into black powder termed 4MnOx (Figure 1b and d).When the experiment was repeated with 4MnOx, the same rate

of oxygen evolution was measured, except without a significantlag period (Figure S2a, Supporting Information). We ascribe the

difference in catalytic onset time to the conversion of Mn4-MOF to 4MnOx. Based on these initial results, a scale-up proce-

dure was devised to convert Mn4-MOF into the catalyst parti-cles for further study (see the Experimental Section for details).

Initial PXRD analysis revealed that the Mn4-MOF is oxidized toa manganese oxide, characterized by the presence of low in-

tensity broad peaks, indicative of semi-crystallinity and smallparticle size (Figure 2a). The peaks can be roughly indexed to

those of birnessite-type MnO2, and look reminiscent of turbo-

stratic-type patterns.[7f] The peak positions at ~2q= 12.5 and258 correlate well to the 00l reflections (001 and 002 usinga single layer model), while the main peak at 378 can be in-dexed to 100 layer reflections. The BET surface area of the cata-lyst was measured to be 216 m2g¢1, which is among the high-est reported values for these types of materials and likely con-

tributes to its high performance.

When re-evaluated in the photoassay using 125 ppm con-centration (Figure 2b, solid black line), an approximate initial

O2 evolution rate of ~0.84 mmol O2 mol Mn¢1 s¢1 was calculatedbased on the slope of the linear region from 30–60 (see the

Supporting Information for Mn content approximation). Wealso confirmed the photocatalytic nature of O2 production by

performing intermittent 30 s exposures to the light source. In

the absence of light, the O2 curve leveled off, which was recov-ered by turning the lights back on again (Figure 2b, dashed

line). In the absence of [Ru(bpy)3]2 +/S2O82¢, little to no oxygen

was generated (Figure 2b, dotted line). Further experiments

were performed in the absence of catalyst. The system was ni-trogen purged and the Clark electrode showed minimal O2

production (Figure 2b, light grey line). Our previous work has

shown that in the absence of catalyst in unpurged [Ru(bpy)3]2 +

/S2O82¢ systems, an O2 consumption trace results that is as-

cribed to oxidation of [Ru(bpy)3]3 + .[14] However, the presenceof an active catalyst helps to inhibit the decomposition of the

photosensitizer, as previously described by Hill and co-work-ers.[15] To confirm these findings, reactions for 4MnOx were per-

formed in both purged and unpurged solutions (Figure S2c,

Supporting Information). Similar results were obtained for bothmeasurements, confirming that oxygen consumption by

[Ru(bpy)3]3+ oxidation is negligible in the presence of an activespecies.

The long term stability and catalytic nature of 4MnOx wasconfirmed by performing six sequential one hour long reac-

tions monitored by gas chromatography (GC) using 150 ppmof catalyst, which was recovered and reused for each trial (seethe Experimental Section for details). It is known that thissystem is not suitable for determining long-term catalytic per-formance due to sacrificial reagent degradation; however, we

employed this method to confirm that the catalyst does notdeactivate under these conditions. Virtually the same amount

of O2 was produced in each trial (1st = 0.0060 mmol, 6th =

0.0058 mmol), with the total amount of oxygen (~0.072 mmol)exceeding the amount of oxygen present in 4MnOx

(~0.029 mmol). The approximate TON over these six trials is2448 mmol O2 mol Mn¢1. Inductively coupled plasma with opti-

cal emission spectrometry (ICP-OES) conducted on the super-natant solution after the first and last reactions show negligi-

Figure 2. a) Simulated and measured PXRD patterns for Mn4-MOF, 4MnOx,SMnOx, and HexBir. b) Dissolved oxygen concentration measured by a Clark-type electrode at 20 8C using 125 ppm catalyst concentration in bicarbonatebuffer (2 mL, pH 7) with [Ru(bpy)3Cl2]·6 H2O (0.5 mm) and Na2S2O8 (10 mm). Il-lumination begins at time t�25 s.

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ble Mn leeching (<0.01 and <0.02 ppm, respectively). The de-viation between the O2 yield predicted by the initial rate of the

Clark data and the yield measured by GC can be attributed todegradation/consumption of the sacrificial reagents during the

hour long reactions.We also studied the precursor role of the Mn4-MOF as com-

pared to Mn2 + salt ; under the same conditions used to gener-ate 4MnOx (see the Experimental Section), the salt also resultedin an active Mn-oxide (SMnOx). In contrast to 4MnOx, the PXRD

pattern shows only a single broad peak at ~2q = 378 (Fig-ure 2a) and lacks the low-angle peaks associated with orderedstacking in birnessites. Additionally, SMnOx results in a lowerlevel of catalytic activity when evaluated in the photoassay; an

approximate initial O2 evolution rate of about 0.15 mmolO2 mol Mn¢1 s¢1 (Figure 2b, dark grey line) was calculated as-

suming MnO2 stoichiometry (see the Supporting Information).

The BET surface area for SMnOx was measured to be 80 m2g¢1.When comparing catalytic performance relative to surface area,4MnOx still results in a higher level of activity (0.038 mmol O2/m2

vs. 0.021 mmol O2/m2). Hence, the crystalline MOF serves as

a valuable precursor for the synthesis of high-surface area, par-tially ordered Mn oxides with high catalytic activity for photo-

catalytic OER.

Models for performance of 4MnOx

We compared oxygen evolution to a well-characterized model

hexagonal-type K+ birnessite (HexBir). There is significant diffi-

cultly in structurally characterizing birnessites unambiguouslydue to vast structural diversity, small particle sizes, and semi-

crystallinity.[7a] The HexBir used for this study is a rare case ofan extremely well-studied and characterized polymorph by

synchrotron microdiffraction and XANES analysis.[16] Lansonet al. report the structural formula of this material to be

K+0.231Mn3 +

0.077(Mn4 +0.885 & 0.115)O2·0.60 H2O, in which & repre-

sents lattice vacancies. This phase was obtained using the re-ported synthesis method and confirmed by PXRD (Figure 2a);

the BET surface area was measured to be 5 m2g¢1.[17] Whenevaluated as an OER catalyst in the nitrogen purged photoas-say using the same mass loading as 4MnOx and SMnOx

(125 ppm), very little oxygen was produced (Figure 2b, ^).However, the O2 level above the catalyst-free run prompted usto increase catalyst loading to probe if the low activity was

due to HexBir’s very low surface area, which could limit theavailability of reactive sites. When using 1100 ppm of catalyst,O2 evolution was observed after a short lag time (Figure S2c,

Supporting Information), with an initial rate corresponding to~0.08 mmol O2 mol Mn¢1 s¢1. While this rate is much lower than

either 4MnOx or SMnOx on a per Mn basis, normalizing to sur-face area results in a much higher perceived level of activity

(0.15 mmol O2/m2), which suggests the importance of high sur-

face area for layered Mn oxides in the [Ru(bpy)3]2 +/S2O82¢

driven systems.

To probe the fate of the bulk structure during the reactions,PXRD measurements were made after 1 hour of reaction time

(HexBir AR, Figure 2a). Using the two-layer model proposed byLanson et al. , the peak positions of HexBir at ~2q = 12.5 and

258 correlate to 002 and 004 reflections arising from layerstacking along the crystallographic c-axis, representing distan-

ces of ~7.12 and 3.56 æ, respectively (Figure 2a).[16] For HexBirAR, both of these peaks shift to lower angles relative to the

higher angle peaks (2D in plane layer reflections), which sug-gests that the layer-to-layer spacing increases. Additionally, the

peak intensities relative to the higher angle peaks are reduced,which suggests diminished stacking order and may correspond

to the exchange of K+ for Na+ and H+ as charge balancing

species, as indicated by XPS measurements (Figure S3, Sup-porting Information). We note the interlayer spacing (~5 æ) isnot large enough to allow entry of [Ru(bpy)3]2 + oxidant mole-cules (~10.2 æ), thereby limiting any activation to the surface

and leaving the bulk phase unaffected.

XPS surface measurements

XPS spectra of the catalysts were measured after the OER and

the Mn 2p3/2 core levels were analyzed using a strict set of pro-tocols to estimate the oxidation state of Mn at the active sur-

face regions. Following the precedent set by Nesbitt and Ban-

erjee, and based on the multiplet structure calculation byGupta and Sen, the Mn 2p3/2 core levels were deconvoluted

into five multiplets for each oxidation state of Mn according tothe method described in the Experimental Section.[18] A table

of exact parameters used can be found in the Supporting In-formation (Table S1). It needs to be noted that the values ob-

tained using this method are best described as close esti-

mates; however, using the same strict parameters for eachsample allows for relative comparisons and observation of

trends. Rarely are Mn 2p3/2 spectra for birnessite water oxida-tion catalysts analyzed in detail, and none that we are aware

of have attempted to deconvolute the region into their multi-plet and satellite peaks.[10b, 19]

The overall fit for Mn4 + is characterized by a peak maximum

near 641.9 eV with a shoulder peak on the high energy side atabout 642.5 eV, and is readily apparent from our deconvoluted

b-MnO2 reference spectra (Figure S4, Supporting Information).This general shape dominated all spectra except SMnOx, which

also has a pronounced shoulder on the low energy side (641–642 eV). The main contribution to the Mn 2p3/2 spectrum of4MnOx (Figure 3a) is Mn4 + , whereas Mn3 + and Mn2+ are inlower quantities, resulting in an average surface oxidation

state of ~3.5. As Mn2 + has ~45 % larger atomic radii thanMn4+ , it is unlikely that it resides within the octahedral sheets,but rather exists as a charge balancing species on the surface

or in the interlayer. Mn3 + has been found to exist within thelayer as well as occupying vacant Mn4+ sites, which may con-

tribute to layer disorder and possibly promotes layer flexibility(atomic radii : Mn2+ = 0.972, Mn3 + = 0.785, Mn4 + = 0.670 æ).[18a]

The presence of Na+ (~2 %) was also detected by our survey

scan, and is likely acting as a charge-balancing species. In con-trast, the Mn 2p3/2 spectrum for SMnOx shows a lower surface

oxidation state (~3.3) and the main contributor to spectrum isMn3+ , which can be viewed as an increase in the low energy

shoulder at ~641.5 eV. The lack of any ordered stacking appar-ent from the low-angle region of the PXRD pattern for this

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sample suggests a mainly amorphous oxide material, whichmay be comprised of 2D sheets. The lack of any Na+ signal

from our survey scan may suggest the importance of an inter-layer region for the presence of charge-balancing group 1 cat-

ions.Mn 2p3/2 spectra for HexBir and HexBir AR show minimal

change between the two samples. In both cases, features fromMn4 + dominate the spectra and show an estimated surface ox-idation state of ~3.5. There appears to be a very slight increase

in Mn3 + relative to Mn4 + in the HexBir AR sample, which canbe observed as a slight decrease in the shape of the high

energy shoulder at ~643 eV and an increase in the low energyshoulder between ~641 eV. The observed increase in surface

Mn3 + , although subtle, may suggest a pathway for activation

by photoreduction and subsequent ejection of an Mn4þlayer atom

to an Mn3þinterlayer occupying the newly formed vacancy as very

recently described by PeÇa et al. ,[20] or surface disproportiona-tion/comproportionation equilibria. Survey scans of both sam-

ples suggest an exchange of K+ during the reaction for Na+

and H+ to maintain charge neutrality. This is not surprising

considering reaction conditions (0.1 m NaHCO3) and the ten-dency of these materials to undergo cation exchange.[7a]

To compare the spectra directly, the Mn 2p3/2 regions werenormalized and overlaid (Figure 3b). From this figure, the sur-

face Mn oxidation state similarity between 4MnOx and HexBir isapparent. The main difference lies in the shapes of the spectra;4MnOx has a better defined high energy shoulder (~642.5–643 eV) and a slightly narrower profile ; however, the differen-ces are minimal and better expressed with deconvolution of

the original spectra. We were surprised to find that any surfaceMn2+ persists through the oxidizing conditions and might

question the validity of this assumption if not for the compari-son to the b-MnO2 reference, which shows minimal Mn2 + con-

tribution (~3 %). We note that catalytic activity does notappear to scale with total surface Mn3 + content, as suggested

by the performance of SMnOx.

4MnOx structural characterization

Bulk structural analysis of the most active catalyst, 4MnOx, was

performed to better understand the nature of the material.

High energy PXRD with PDF analysis has emerged as a power-ful total scattering technique for collecting detailed local and

long-range structural information for poorly crystalline ornanoscale materials.[21] The PDF, G(r), reports all atom–atom

distances in a sample as a weighted histogram, with peaks ob-served at r-values corresponding to well-defined bond lengths

and atomic distances within the material. This method, as

noted in previous work, provides similar information as EXAFSexcept covers the full range of atom–atom distances and is

free of Debye–Waller effects (peak broadening at longer dis-tances), resulting in more reliable distances past the first coor-

dination shell and higher resolution. PDF has proven valuablein elucidating minor structural features in cobalt oxide

WOCs.[22]

Although broad, the most intense peaks in the high-energydiffraction data for 4MnOx correspond to the principal diffrac-

tion features for birnessite-type MnO2 (Figure S5, SupportingInformarion). The corresponding PDF data (Figure 4, solid) wasfit within PDFgui by a real space structural model correspond-ing to a refined monoclinic birnessite structure (R ~27 %; C2m ;

a = 4.97, b = 2.84, c = 7.29 æ; b= 98.78 ; Figure 4, dashed).[23]

There was no evidence of other semicrystalline manganese

oxide phases, even as a minor component. Atom–atom corre-lations within individual layers of edge-sharing MnO6 octahe-dra were well-defined and dominated the PDF data; however,

correlations between atoms in different layers or interstitialspecies were not clear, which suggests a large distribution/dis-

order in the interlayer structure. Bond lengths were deter-mined by fitting Gaussian functions to peaks in the PDF within

Fityk (nonlinear curve fitting and data analysis software).[24]

The average Mn¢O bond length for 4MnOx was determinedto be 1.912 æ, but a small shoulder peak is observed at

~2.15 æ, possibly arising from Jahn–Teller elongated axialMnIII¢O bonds present in MnIIIO6 octahedra.[7d] The most in-

tense peak at 2.868 æ corresponds to di-m-oxido bridging be-tween Mn atoms and is common for birnessites. The other

Figure 3. Mn 2p3/2 XPS spectra. a) Deconvoluted spectra for 4MnOx (top left),SMnOx (top right), HexBir (bottom left), and HexBir AR (bottom right). b) In-tensity and binding energy normalized Mn 2p3/2 spectral data overlay (exper-imental).

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bond lengths illustrated in Figure 4 occurring at 3.455, 4.461,

4.943, 5.682, and 6.046 æ are commensurate with common in-tralayer distances. Notably, 4MnOx shows the most significant

intensity misfit at ~3.5 æ, which can be assigned to triplecorner sharing Mn3þ

interlayer (Figure 5) located above or below

layer vacancies. The model used to fit the data was refinedfrom a monoclinic-type birnessite structure, characterized by

rows of Mn3þlayer, minimal lattice vacancies, and thus minimal

Mn3þinterlayer contribution, which helps to explain the misfit at this

distance. The caption image in Figure 4 shows the shows

atomic distances expanded to 20 æ, and matches well with the

refined model. Both the average Mn¢O bond length and the

Mn–Mn distance are similar to those reported by Lanson et al.for the HexBir sample used in this study (1.914 and 2.84 æ, re-

spectively).[16] They also show that the averageMn3þ

interlayer–Mn4þlayer interatomic distance is ~3.540 æ, which corre-

lates with the increased peak intensity in our PDF data at thisvalue.

Our results can be compared to those of Dau, Kurz, and co-workers, who studied the bond lengths of a number of syn-

thetic birnessite catalysts using EXAFS.[4d,f] Compared to theirwork, the current system shows a slightly longer average Mn¢O bond length (1.912 vs. 1.89 æ) and a slightly shorter Mn–Mndistance (2.868 vs. 2.88 æ). They attribute bonding features oc-curring at about 3.3 æ to the presence of cubane-like motifs

arising from triple-corning sharing interlayer cations, such asSr2 + and Ca2 + , which act as charge balancing species for

Mn3þlayer. This particular feature was discussed in terms of possi-

ble catalytic reaction sites, owing to structural similarity to the

naturally occurring OEC.[4f] In contrast, 4MnOx has a very minorcharge balancing contribution from Na+ (~0.07 per MnO2),

and instead Mn3þinterlayer acts to balance Mn4þ

layer vacancies. As also

suggested by Suib and co-workers, we speculate that this fea-ture may be contributing to the observed catalytic activity

through coordinatively unsaturated oxygen atoms that canassist in proton-coupled electron transfer (PCET) through inter-

mediate m-OH units.[4e] Additionally, HexBir structural studies byLanson et al. show that Mn3þ

interlayer coordinate as many as 0.24

of the 0.60 structural water molecules, which suggests that

these sites may have an additional role to play in bindingwater molecules during catalysis (see Figure 5b).

We also compared our findings with those of Sparks and co-workers, who performed PDF analysis on a variety of birnes-

sites (Figure S6, Supporting Information).[25] 4MnOx shows a simi-lar ratio of Mn–O to Mn–Mn peak intensities and widths (re-

flects vacancy concentration) to ‘hexagonal birnessite’ (also no-

tated as HexBir in Figure S6, Supporting Information, not to beconfused with the HexBir used in our study), which indicates

a high fraction of lattice vacancies. In contrast, 4MnOx hasa longer Mn¢O bond length

than any of their studied birnes-sites. For example, their hexago-

nal birnessite was reported to

have an average Mn¢O bondlength of only 1.893 æ, which isshorter than 4MnOx and the re-ported distance for HexBir by

Lanson et al.[16] Instead, our aver-age Mn¢O bond distance is

more akin to their triclinic (TrBir,1.904 æ) or manganite (g-MnOOH, 1.917 æ) distances. Thisis possibly due to differences insample preparation methods, as

birnessites are known for theirstructural diversity arising from

even minor differences in syn-thetic conditions.

Additional structural studies were carried out using Raman

spectroscopy. Raman has been noted as an excellent techniqueto study the structure of amorphous materials due to its sensi-

tivity to short range order.[26] Spectra were collected for theparent MOF structure, 4MnOx, and a birnessite formed under al-

Figure 4. Comparison of the experimental G(r) obtained for 4MnOx (solid)with the calculated G(r) for the refined birnessite structure model (dashed);residue shown below (dotted). Illustrations correlate distances obtained bythe PDF study with distances found within a single birnessite layer. Insetshows PDF data expanded to 20 æ and the dashed line shows the residualpattern.

Figure 5. Idealized conceptual drawing of 4MnOx along the crystallographic c-axis (a) and b-axis (b). Interlayer/sur-face Mn3 + species over vacant sites are represented as the triply bonded Mn atoms in (a) and as the Mn octahe-dra out of the layer plane in (b).

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kaline conditions the Mn3+ content of which is mostly limitedto the layer that our group has characterized previously (d-

MnO2, Figure 6).[4 h, 27] Both 4MnOx and d-MnO2 show features

common to birnessite-typeMnO2.[26a] The three main fea-

tures are the weaker peak at~475–510 cm¢1 and two stron-

ger peaks at ~550–570 and 620–

650 cm¢1. The band at 650 cm¢1

is the most notable; it is as-

signed to the symmetric stretch-ing mode n2(Mn¢O) perpendicu-

lar to the chains of MnO6 octa-hedra. The peak at ~575 cm¢1 isassigned to the n3(Mn¢O)

stretching in the basal plane andis similar for both 4MnOx and d-

MnO2. Compared to the spec-trum of d-MnO2, the n2(Mn¢O)mode in 4MnOx shifts to a higherwave number and intensity relative to the peak at 575 cm¢ 1,

which can be viewed as reflecting lattice vacancy concentra-tion and layer–layer interactions. Chen et al. showed that asNa-birnessite interlayer sodium atoms were replaced by water,

the n2 mode shifted to a higher wavenumber and becamemore intense relative to the n3 mode, which is consistent with

our results.[26b] These results are also consistent with those ofPereira–Ramos et al. , who demonstrate similar findings when

comparing Li–Bir (similar to d-MnO2, ordered interlayer) to SG-

Bir (HexBir-like).[26a] We assign the weak double peak at thelower wavenumbers (480, 508 cm¢1) to interlayer ordering and

guest identity/binding modes; as interlayer species changes,the distinctive features of this region change in response. Per-

eira–Ramos et al. show very distinctive separated sharp peakswhen Li+ is the guest, as opposed to our double peak in the

case of potassium. As the interlayer species is replaced by dis-ordered Mn3+ or Na+ , the double peak is lost and instead

a largely featureless single peak is obtained.Finally, the morphology and lattice disorder of 4MnOx was

studied using HR-TEM. The zoom-out images in Figure 1bdepict pebble/flake-like morphology ranging from ~30–

100 nm in size. Each flake is comprised of multiple layers/sheets (Figure 7a). The lattice images (Figure 7b and c) depicta highly disordered and flexible layer structure; a SAED (select-

ed area electron diffraction) pattern was unobtainable due tothe level of disorder. The inset image in Figure 7b shows MnO6

octahdra lattice fringes, which were measured to be ~2.8 æand is commensurate with our PDF data. There appears to be

regular disruption of the fringes, resulting in the disorderedappearance of the layer and is a likely cause for the deviation

from the typical crisscross pattern expected for crystalline bir-

nessite.[23] These features may be a combined result of randomlocations of both Mn3þ

layer atoms and Mn4þlayer vacancies. Lattice

fringe curvature is observed in Figure 7c, as there appears tobe entire semi-ordered regions that curve/flex throughout the

edge region of the flake. We speculate that these may corre-spond to flexible regions dominated by Mn3þ

layer, in which the

columns form to minimize lattice strain caused by Jahn–Teller

distortion.

4MnOx formation

Oxidation of Mn2 + in solution has been previously studied byHem and Lind, whereby various manganese oxides were pre-cipitated by bubbling air through dilute (0.01 m) solutions of

MnXy (X = Cl¢ , NO3¢ , SO4

2¢, or ClO4¢).[28] They found that de-

pending upon the pH, temperature, and counter-ion, that

metastable forms of Mn3 + species could be expected. In turn,Mn3+ disproportionates into MnO2 and Mn2 + , but pH and

ligand environment can control the extent of this

disproportionation. For instance, oxidation of Mn2 + is knownto proceed fully to MnO2 at alkaline pH and in the presence of

weak field ligands (e.g. , NO3¢ , Cl¢). On the other hand, strong

field ligands have been observed to stabilize Mn3+ in other

systems, and both pyrophosphate and sulfate anions havebeen observed to favor the formation of MnOOH.[29] We sus-

Figure 6. Raman spectra of Mn4-MOF (bottom), 4MnOx (top, black), and d-MnO2 (top, grey).

Figure 7. HR-TEM images of 4MnOx. a) 50 nm scale showing layered flake-like morphology; b) 5 nm scale showingdisordered basal plane, the inset is a magnification of the indicated region showing ~2.8 æ lattice fringes;c) 10 nm scale showing lattice fringe curvature.

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pect this complex equilibrium accounts for the variety of Mnoxidation states observed by XPS (Table 1), and the high

degree of disorder found across both of the in-situ generatedmaterials.

Conclusions

Catalytically active MnOx species were found to form under

commonly used photocatalytic water oxidation conditions for

the first time. Using an Mn4-MOF precursor results in the mostactive material studied in this work, 4MnOx. Detailed structural

analysis suggests that 4MnOx is similar to hexagonal birnessite-type MnO2, and can be characterized by the presence of

Mn3þlayer, Mn3þ

interlayer, and Mn4þlayer vacancies, giving rise to layer dis-

order and possible flexibility. There are multiple reports of OER

activity on MnOx materials prepared using a variety of methods

and comparing the activity of these various materials to oneanother is difficult due to variations in reaction conditions and

methods. With that said, 4MnOx is on the high end of the per-formance range typically reported for MnOx catalysts evaluated

in the [Ru(bpy)3]2+/S2O82¢/visible-light-driven systems,[8] which

may be partially attributed to its very high surface area

(216 m2g¢1). In contrast, the SMnOx catalyst formed from Mn2 +

salt shows a lower average surface oxidation state, lower sur-face area (80 m2g¢1), apparent lack of ordered stacking, and

a lower level of activity, even when surface area normalized.Both materials show significantly higher levels of activity thancrystalline hexagonal K+ birnessite when employed in thesame photoassay.

The surface Mn oxidation states of the MnOx catalysts werestudied using XPS and compared to those of hexagonal-type

birnessite before and after reaction. All species showed en-riched Mn3 + and Mn2 + content near the surface that be possi-bly attributed to surface comproportionation/disproportion-

ation equilibria of MnOOH species. PDF analysis results inhighly resolved 4MnOx bond distances and indicates the pres-

ence of Mn3þinterlayer occupying layer vacancies, highlighting this

technique as an alternative to EXAFS for studying semi-crystal-

line materials. The description of 4MnOx as hexagonal-birnes-

site-like was further confirmed through the use of Raman spec-troscopy, and disordered flexible layers were illustrated by HR-

TEM. This work supports our previous finding that proposescatalysis is promoted by structural flexibility and Mn3 + content.

Finally our study suggests that many Mn2 + species have po-tential form similar active MnOx materials under photocatalytic

water-splitting conditions, and that rigorous post catalyticanalysis is necessary to confirm the active species.

Experimental Section

Light-driven conversion to MnOx

Conversion from Mn4-MOF to birnessite-type 4MnOx was first notedwhen Mn4-MOF was evaluated for photochemical WOC activity.Following this initial experiment, 4MnOx was synthesized directlyby decomposition of Mn4-MOF. Mn4-MOF (10 mg) was added toa vial (20 mL) containing pH 7 bicarbonate buffer solution (10 mL,0.1 m, adjusted with ~4 m HNO3) with [Ru(bpy)3Cl2]·6 H2O (0.5 mm,photosensitizer) and Na2S2O8 (20 mm, electron scavenger). The vialwas then illuminated with four white LEDs (see Figure S1 in theSupporting Information for illumination profile) under constantmagnetic stirring at room temperature. Within one minute of illu-mination, a solution color change from orange to brown was ob-served indicating light-induced conversion. After one hour of reac-tion time, the reaction suspension was centrifuged, washed withdeionized water to remove any surface adsorbed impurities, andleft to air dry under ambient lab conditions. Conversion from Mn2+

salt to birnessite-type SMnOx was performed in an identical manneras 4MnOx, except that stoichiometric quantities of the constituentswithin the MOF were used in its place, with nicotinic acid (6.2 mg)substituting for the linker and Mn(ClO4)2·6 H2O (18.2 mg) acting asthe Mn source. These reactions were later scaled up directly toobtain enough material to carry out additional measurements.

Photochemical water oxidation

Clark-electrode : Photochemical water oxidation reactions wereconducted in a headspace-free 2 mL quartz reaction chamber sur-rounded by a circulating water jacket maintained at 20 8C. Fora typical reaction, a catalyst suspension (1 mL, 250 ppm, or2200 ppm in the case of the HexBir high-loading experiment) inpH 7 NaHCO3 buffer (0.1 m, adjusted with HNO3,) was injected intothe photoassay chamber containing the same buffer (1 mL) with[Ru(bpy)3Cl2]·6 H2O (1.0 mm, 0.5 mm concentration after dilutionfrom catalyst suspension) and Na2S2O8 (20 mm, 10 mm total con-centration after dilution). After establishing a stable baseline fortwo minutes in both N2 purged and nonpurged systems, the reac-tions were illuminated by four white LEDs as described above anddissolved O2 concentrations were measured with a Clark-type elec-trode (Hansatek), which was calibrated with both sodium dithio-nate and N2 purged solutions daily.

Gas chromatography : The long-term stability and catalytic natureof 4MnOx was confirmed by GC. Data was recorded using an Agi-lent 6890N GC, equipped with a 30 m, 0.53 mm ID, 50 mm Rt-Msieve 5 A capillary column and thermal conductivity detector(TCD). 4MnOx (1.5 mg) was added into a crimp-cap septum-sealedglass vial (23 mL) containing pH 7 bicarbonate buffer (10 mL, 0.1 m,adjusted with HNO3) with [Ru(bpy)3Cl2]·6 H2O (0.5 mm) and Na2S2O8

(20 mm). While remaining covered with aluminum foil to inhibitthe photoreaction, the solution was purged with He for 5 min fol-lowed by illumination for 1 h under magnetic stirring at room tem-perature (monitored by a laser thermometer at ~24 8C); manualheadspace aliquots (10 mL) were injected into the GC. After eachreaction the catalyst was recovered by centrifugation, washed withDI water three times and transferred to a fresh aluminum foil cov-ered reaction vial using NaHCO3 buffer (10 mL) to prevent loss ofcatalyst. [Ru(bpy)3Cl2]·6 H2O and Na2S2O8 were added to the vial inthe same concentration as for the first trial and sonicated to aid

Table 1. Estimated relative abundances of Mn oxidation states on cata-lyst surfaces as measured by XPS.

Sample Mn4 + [%] Mn3 + [%] Mn2 + [%] FWHM [eV]

b-MnO2 73 24 3 1.164MnOx 63 27 9 1.20SMnOx 42 45 13 1.25HexBir 57 35 8 1.19HexBir AR 54 37 9 1.25

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dissolution. The vial was then purged and illuminated for an addi-tional hour. This process was repeated for six cycles.

Powder X-ray diffraction

PXRD patterns were recorded on a Rigaku Ultima IV X-ray diffrac-tometer, using CuKa radiation (l= 1.5406 æ). A graphite monochro-mator was used and the generator power settings were set to40 kV and 44 mA. Data were collected be-tween 2q of 3–508 witha step size of 0.028 and a scan speed of 1.5 deg/min.

N2 sorption measurements

BET surface area measurements were made by N2 gas adsorptionat 77 K on a volumetric gas sorption analyzer (Autosorb-1 MP,Quantachrome Instruments). Samples (~100 mg) were prepared byoutgassing under vacuum at 120 8C overnight.

Inductively-coupled plasma optical emission spectroscopy

Catalyst leeching was determined by ICP-OES using a SPECTROARCOS EOP ICP: 4MnOx (150 ppm) was added to a crimp-cap vial(23 mL) containing [Ru(bpy)3Cl2]·6 H2O (0.5 mm) and Na2S2O8

(20 mm) in pH 7 bicarbonate buffer (10 mL). After 2 h of illumina-tion the catalyst was removed from the solution by centrifugation.The resulting supernatant was collected and diluted to with DIwater (20 mL). An aliquot of the solution was then acidified withnitric acid (2 %) and injected into the instrument.

X-ray photoelectron spectroscopy

A Thermo K-Alpha spectrometer was used with charge compensa-tion to measure the surface elemental composition of the catalysts.Solid samples were fixed on carbon tape and a beam of 150 mm di-ameter was focused on an area covered by the powdered sample.

Peak deconvolution : In order to deconvolute the Mn 2p3/2 spectra,we followed the precedent set by Nesbitt and Banerjee, which arebased on free ion calculations reported by Gupta and Sen.[18] Eachoxidation state of Mn results in a set of five multiplets (15 total) ;relative peak intensities and differences in binding energies (D eV)from peak to peak were set as fixed values (the highest energymultiplet of Mn4 + was allowed variable intensity) based on thevalues used in their study and those calculated by Gupta and Sen.The lowest energy Mn4 + multiplet was used as a reference pointto set the binding energy of the lowest energy Mn3 + and Mn2 +

multiplets at fixed values. The other four higher energy multipletsof each group were set at a specific D eV from the lowest energypeaks, but were allowed a 0.2 eV variance. These same D eV valueswere used for each sample measured. The full widths at half max(FWHM) were constrained to a specific value for each sample (30 %Lorentzian/70 % Gaussian), ranging from 1.16 to 1.23 eV. The initialpeak position for Mn4 + was determined from our referencesample, b-MnO2, and the O1s spectra peak maxima was used tocalibrate the Mn4 + position for the other samples. Survey scanswere also taken for each sample and suggest overall MnO2 stoichi-ometry in each case—the excess oxygen in HexBir AR likely arisesfrom incomplete washing and the presence of CO3

2¢, as suggestedby the abundance of carbon present.

Synchrotron powder X-ray diffraction and pair distributionfunction analysis

High-energy X-ray scattering data suitable for PDF analysis was col-lected on beamline 11-ID-B at the Advanced Photon Source (APS)

at Argonne National Laboratory. Dried samples were containedwithin polyimide capillaries of ~1 mm diameter. Monochromatic X-rays (l= 0.2128 æ, ~58 keV) were combined with a large Perkin–Elmer amorphous silicon-based area detector to collect data tohigh value of momentum transfer (Q). The two-dimensional scat-tering images were reduced to one dimensional intensity data asa function of scattering angle within fit2D. The PDFs were extract-ed within PDFgetX2 using data up to Qmax = 23 æ¢1 and were cor-rected for background scattering, Compton scattering, and obliqueincidence.

Structural refinement : The PDF data was fit within PDFgui bya real-space structural model corresponding to a monoclinic bir-nessite structure (R ~27 %; C2 m; a = 4.97, b = 2.84, c = 7.29 æ; b=98.78).[23] The lattice parameters, atomic positions (as constrainedby the C2m symmetry) and atomic displacement parameters forthe model MnO2 layers were structurally refined using a largevalue of the atomic displacement parameter normal to the layer di-rection (U33) ; U33 was constrained to be a factor of 20 times largerthan U11 = U22. Maximum particle size was constrained to 50 nm.The original sub-cell used for refinement has the unit cell parame-ters, a = 5.175, b = 2.850, c = 7.337 æ, b= 103.188.

Raman spectroscopy

Raman measurements were collected using a Thermo-Fisher Nico-let Almega XR Dispersive Raman spectrometer equipped witha 50x objective microscope. A 532 nm solid-state laser was usedfor excitation; the output power was reduced to 1 % (0.117 mW)and the acquisition time varied from 5 to 10 min to avoid sampledecomposition or phase-transition induced by laser heating. Thespectra were obtained from 50 to 2000 cm¢1. For Mn4-MOF, thebands at 1595 and 1040 cm¢1 are due to the n(C=C) and b(C¢H) vi-brations of the ring modes, while the 1397 cm¢1 band is ascribedto the ns(COO¢) units coordinated to metal ions in MOF structure.Ring deformation modes d6a and d6b are observed at 848 and614 cm¢1, mixed with the d(COO) vibration.[30]

High-resolution field emission TEM

Particle morphology and lattice images were taken using a JEOL2010F field emission TEM operating at 200 kV. A suspension of4MnOx in water was sonicated and allowed to stand for 24 + hours.Aliquots were drawn from the top of the vial, diluted with water,and a single droplet was placed onto a carbon-coated copper gridand dried under vacuum, overnight.

Thermogravimetric analysis

The water content of 4MnOx and SMnOx was determined using ther-mogravimetric analysis (TGA, see Supporting Information). Mea-surements were performed using a TA Q5000 ThermogravimetricAnalyzer with a temperature ramp of 1 8C min¢1 from room tem-perature to 600 8C under nitrogen gas flow.

Acknowledgements

The RU and UTD team would like to acknowledge the supportby the U.S. Department of Energy, Office of Basic Energy Scien-

ces, Materials Sciences and Engineering Division through grantDE-FG02–08ER46491 for the synthesis, catalytic reactions and

data analysis, and materials characterizations including PXRD,IR, TGA, Raman, XPS, and sorption experiments. This work was

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also partially supported by the National Science Foundation,Chemistry of Life Processes grant no. 434878 and CHE-1213772

(G.C.D.), and NSF Graduate Research Fellowship under GrantNo. DGE0903675 (P.F.S.). The use of the Advanced Photon

Source (APS) was supported by the U.S. Department of Energy,Office of Science, Office of Basic Energy Sciences, under Con-

tract No. DE-AC02–06CH11357.

Keywords: birnessite structure · manganese oxide · metal–

organic frameworks · water oxidation catalyst · water splitting

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Received: May 16, 2015Published online on August 11, 2015

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