Kinetics and Mechanism of Heterogeneous Water Oxidation by α

Research Article pubs.acs.org/acscatalysis

Kinetics and Mechanism of Heterogeneous Water Oxidation by α‑Mn2O3 Sintered on an FTO Electrode Zaki N. Zahran,*,†,‡ Eman A. Mohamed,† and Yoshinori Naruta*,†,§ †

Institute of Science and Technology Research, Chubu University, Kasugai 487-8501, Japan Faculty of Science, Tanta University, Tanta, Egypt § JST ACT-C, Kawaguchi, Saitama 332-0012, Japan ‡

S Supporting Information *

ABSTRACT: Kinetic studies of heterogeneous water oxidation by an α-Mn2O3/FTO electrocatalyst in nonaqueous (CH3CN/0.1 M n-Bu4NPF6 and DMF/0.1 M n-Bu4NPF6) and aqueous 0.1 M KPi (pH 7.0) solutions showed that the rate of water oxidation is first order in catalyst concentration and in H2O concentration. The square wave and cyclic voltammetry measurements reveal the stepwise proton-coupled electron transfer (PCET) oxidations of the active MnII−OH2 site to MnIII−OH and then to MnIVO and finally an electron transfer oxidation of MnIVO to MnVO species. The MnVO species undergoes a rate-limiting O atom transfer to H2O to give a MnIII−OOH2 species that, in turn, undergoes further oxidations to release O2.

KEYWORDS: manganese(III) oxide, water oxidation catalyst, electrolysis, intermediate, proton-coupled electron transfer mechanism

S

terminology are required for heterogeneous WOCs in studying their mechanism of catalysis. Because of their involvement in the active site of the photosystem II water oxidation enzyme,34−38 a variety of manganese oxides have been intensively studied as heterogeneous WOCs.39−46 The systematic investigation of these oxides revealed that α-Mn2O3 exhibits the highest activity for water oxidation as either a bulk or heterogeneous WOC.47,48 We recently optimized the preparation of a highly active and robust nanocrystalline α-Mn2O3 thin film on an FTO electrode (α-Mn2O3/FTO) that showed, in a neutral aqueous 0.1 M KPi solution, a high electrocatalytic water oxidation activity with a turnover frequency of 2.1 s−1, TOF (based on the surface active Mn), and a Faradaic efficiency of 96.7% for oxygen evolution at an overpotential η of 470 mV.49 Here we report, for the first time, the kinetic and mechanistic studies of heterogeneous water oxidation with α-Mn2O3/FTO by using electrochemical (square wave and cyclic voltammetry), XPS (X-ray photoelectron spectroscopy), and UV−vis spectroelectrochemical methods in nonaqueous and aqueous 0.1 M KPi buffer solutions. The measurements revealed the stepwise protoncoupled electron transfer (PCET) oxidations of the active MnII−OH2 site on the surface to MnIII−OH and then to MnIVO and finally an electron transfer oxidation of MnIVO to MnVO species. Comparing the nonaqueous and aqueous

olar energy storage in the form of hydrogen and/or carbon resource fuels requires an electron source. For sustainable and massive-scale fuel production, water, via its oxidation, is the one and only attractive source.1−6 However, water oxidation is a challenging process because of the high kinetic barriers and the high thermodynamic stability of water.7−9 Plenty of precious and non-precious metal-based water oxidation catalysts (WOCs) have been reported to reduce the kinetic barrier and yield efficient water oxidation.10−32 These catalysts are generally classified into two major groups: homogeneous and heterogeneous WOCs. To design more efficient and robust WOCs, understanding the mechanistic models of the sequence of elementary steps and the active species based on atomic-scale experiments is essential for both WOC classes. Despite the detailed atomic-scale mechanistic investigation of water oxidation with homogeneous WOCs, much work with heterogeneous WOCs focused on the route of their preparation (e.g., electrodeposition, thermal oxidation, and sol−gel), morphology (e.g., smooth, rough, and cracked), and annealing temperature, with scarce focus on the mechanistic models and hypotheses of the atomic-scale sequence of elementary steps. Indeed, most heterogeneous water oxidation mechanistic and rate-determining step studies rely on Tafel slopes derived from semilogarithmic potential−current plots.33 Physical terminology such as adsorption and hydroxide discharge is used, although hydroxide discharge could lead to formation of either a HO• or a HO− bound to the catalyst surface that leads to a 1 unit increase in its formal charge. Atomic-scale mechanistic investigation and a transition from the physical to chemical © 2016 American Chemical Society

Received: February 11, 2016 Revised: May 14, 2016 Published: June 2, 2016 4470

DOI: 10.1021/acscatal.6b00413 ACS Catal. 2016, 6, 4470−4476

Research Article

ACS Catalysis measurements revealed that MnVO is the active species in the water oxidation process that undergoes a rate-limiting O atom transfer to H2O to give MnIII−OOH2 that, in turn, undergoes further oxidations to release O2. The brown transparent α-Mn2O3/FTO WOC was prepared, as we previously reported, by sintering an electrodeposited MnO2/FTO precursor at 550 °C for 1 h in air. The identity of the α-Mn2O3/FTO was confirmed via X-ray diffraction (XRD), UV−vis, and X-ray absorption near-edge structure (XANES) spectroscopic techniques.49 The redox properties of α-Mn 2 O 3 /FTO have been investigated by square wave and cyclic voltammetry in nonaqueous (CH3CN and DMF) and aqueous (neutral 0.1 M KPi) solutions. Figure 1 shows the square wave voltammo-

grams at a scan rate of 0.3 mV/s in the positive direction of αMn2O3/FTO (0.52 μmol/cm2 Mn content) in CH3CN/0.1 M n-Bu4NPF6 (blue line), DMF/0.1 M n-Bu4NPF6 (black line), and neutral 0.1 M KPi (red line) solutions (n-Bu4NPF6 = tetran-butylammonium hexafluorophosphate). Three successive oxidations are observed in the nonaqueous CH3CN/0.1 M nBu4NPF6 and DMF/0.1 M n-Bu4NPF6 solutions assigned to MnII/III, MnIII/IV, and MnIV/V oxidations (eqs 1a, 1b−3a, and 3b) (1a)

Ep1(DMF) = 0.57 V

(1b)

Mn II−OH 2 ⇌ Mn III−OH + H+ + e− Ep1(H 2O) (1c)

= 0.62 V

Mn III−S ⇌ Mn IV −S + e− Ep2(CH3CN) = 1.54 V

(2a)

Ep2(DMF) = 1.35 V

(2b)

Mn III−OH ⇌ Mn IV O + H+ + e− Ep2(H 2O) = 1.17 V (2c) IV

V

−

Mn −S ⇌ Mn −S + e Ep3(CH3CN) = 1.71 V

(3b)

Mn IV −OH ⇌ Mn V O + H+ + e−

(3c)

where S stands for the solvent. The peak potentials of the MnII/III and MnIII/IV oxidations observed in CH3CN/0.1 M nBu4NPF6 at Ep1(CH3CN) = 1.16 V and Ep2(CH3CN) = 1.54 V are shifted to more negative potentials in DMF/0.1 M nBu4NPF6 [Ep1(DMF) = 0.57 V; Ep2(DMF) = 1.35 V], consistent with the strong coordination properties of DMF compared to those of CH3CN. However, the MnIV/V peak potentials occur at quite similar values (eqs 3a and 3b) in both solvents. In the aqueous 0.1 M KPi solution, the MnII/III and MnIII/IV oxidations are observed at Ep1(H2O) = 0.62 V and Ep2(H2O) = 1.17 V, respectively (eqs 1c and 2c). The MnIV/V oxidation observed in the nonaqueous solvents is replaced with a strong catalytic current for water oxidation, indicating clearly the involvement of the electro-generated MnV species in the water oxidation process. Studies of the pH dependences of Ep values for α-Mn2O3/ FTO over the pH range of 5.0−9.0 reveal that the MnII/III and MnIII/IV couples shift reductively by approximately 59 mV/pH decade (Figure S1), suggesting stepwise proton-coupled electron transfer (PCET) oxidations of the active site MnII− OH2 to MnIII−OH and MnIII−OH to MnIVO (eqs 1c and 2c). On the other hand, the onset potential of the catalytic water oxidation peak potential does not change with pH, suggesting an electron transfer oxidation of MnIVO to MnVO (eq 3c). These results indicate the involvement of MnVO species in the catalytic water oxidation without any contribution of the MnIVO species. The UV−vis spectroelectrochemical measurements of a transparent α-Mn2O3/FTO electrode (0.15 μmol/cm2 Mn content) at applied potential Eapp = 1.55 V for 15 min in a CH3CN/0.1 M n-Bu4NPF6 solution showed more absorption of the light with a small peak at 454 nm compared with αMn2O3, indicating the formation of MnIV species. Setting the potential at 2.0 V for 15 min showed more absorption of the light, disappearance of the peak at 454 nm, and appearance of a new peak at 420 nm attributed to MnV species (Figure S2). To gain further insight into the Mn oxidation states at the surface, XP spectra of the Mn 2p and O 1s regions were observed for both the as-prepared α-Mn2O3 and that electrolyzed at different applied potentials (selected on the basis of the square wave voltammograms in Figure 1) in neutral 0.1 M KPi and nonaqueous DMF/0.1 M n-Bu4NPF6 solutions. On the basis of the previous reports,48,50−62 the Mn oxidation states were assigned by the position of the Mn 2p3/2 peak, where MnII shows peak values in the range of 640.10−641.12 eV, MnIII in the range of 641.40−641.90 eV, and MnIV in the range of 641.85−643.0 eV. Moreover, the O 1s peaks attributed to Mn−O−Mn, Mn−OH, and H−O−H were observed in the ranges of 529.3−530.3, 530.5−531.5, and 531.8−532.8 eV, respectively. Figure 2 depicts the deconvoluted XP spectra of Mn 2p3/2 and O 1s of both the as-prepared α-Mn2O3 film deposited on the FTO electrode and that electrolyzed at different applied potentials in DMF/0.1 M n-Bu 4 NPF 6 solutions. The deconvoluted XP spectra of Mn 2p3/2 and O 1s of both the as-prepared α-Mn2O3 film deposited on the FTO electrode and that electrolyzed at different applied potentials in a neutral 0.1 M KPi solution are shown in Figure S3. Table S1 summarizes the XPS peak values of Mn 2p3/2 and O 1s. As previously reported,48 the as-prepared α-Mn2O3 revealed a

Figure 1. Square wave voltammograms at a scan rate of 0.3 mV/s of αMn2O3/FTO (0.52 μmol/cm2 Mn content) in CH3CN/0.1 M nBu4NPF6 (blue), DMF/0.1 M n-Bu4NPF6 (black), and neutral 0.1 M KPi (red) solutions.

Mn II−S ⇌ Mn III−S + e− Ep1(CH3CN) = 1.16 V

Ep3(DMF) = 1.80 V

(3a) 4471

DOI: 10.1021/acscatal.6b00413 ACS Catal. 2016, 6, 4470−4476

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ACS Catalysis

intensity of the MnII peak in both DMF/0.1 M n-Bu4NPF6 (Figure 2) and KPi (Figure S3) solutions, indicating the reduction of the surface MnIII to MnII. The O 1s spectra revealed the presence of Mn−O−Mn binding in both solutions and a significant difference in the Mn−OH binding region indicating, as the square wave voltammetry revealed, the binding of DMF to the surface Mn in the nonaqueous solution. Applying potential beyond the MnII/III oxidation peak in both DMF (Eapp = 0.80 V) and KPi (Eapp = 0.75 V) solutions resulted in XP spectra consistent with the oxidation of the surface MnII to MnIII. Similarly, at potentials beyond MnIII/IV, oxidation generated XP spectra consistent with the formation of MnIV at the suface in both DMF and KPi solutions. At a 2.1 V applied potential in a DMF/0.1 M n-Bu4NPF6 solution, the deconvoluted XP spectra revealed the presence of a new species at 643.57 eV that is higher than that reported for MnIV species and lower than that reported for MnVI species (MnVI at 643.8 eV).62 This new species is most likely MnV species. This species was not detected at a high potential (Eapp = 1.40 V) in an aqueous KPi solution by XP spectroscopy. However, the MnIV species, as previously reported,48 was detected. These results indicate the involvement of the MnV in the water oxidation to oxygen process. The MnVO species has been detected and/or proposed as the active species for water oxidation in several reported Mncontaining WOCs studies. We previously generated MnVO species in cofacial Mn porphyrin dimers WOCs that catalyzed water oxidation in a CH3CN/5% H2O solution.63,64 It is proposed as the active species for water oxidation with a [(OH2)(terpy)MnIII(μ-O)2MnIV(terpy)(OH2)]3+ WOC.65 Collins and co-workers proposed it as an intermediate in natural water oxidation.66 Furthermore, Åkermark and co-workers generated a MnVO species in nitrophenylcorrole Mn complex that undergoes nucleophilic hydroxide attack to form O2.67 It is known that α-Mn2O3 has a complex orthorhombic crystal structure (space group Pcab) that crystallizes in a distorted bixbyite structure with MnIII atoms octahedrally coordinated by oxygen, and the Mn atom at the surface is coordinately unsaturated and freely binds H2O.68−71 On the basis of the square wave voltammetric and XPS behavior, we hypothesize that the mechanism of electrocatalytic water oxidation with α-Mn2O3 proceeds as shown in Scheme 1. Two successive PCET oxidations of the surface MnII−OH2 to MnIII−OH and then to MnIVO [species 1−3 (Scheme 1)] and then an electron transfer oxidation of MnIVO to MnV O (species 3 to 4) that undergoes a rate-limiting O atom transfer to H2O to give MnIII−OOH2 [species 4−6, based on kinetic isotope effect results (see below)] that, in turn, undergoes further oxidations to release O2. If the O atom transfer from MnVO to H2O is the ratelimiting step, as we hypothesize in Scheme 1, the rate law and the current expression for heterogeneous catalysis will be as shown in eqs 4 and 5, respectively.71−73

Figure 2. Deconvoluted XP spectra of Mn 2p3/2 and O 1s of the αMn2O3 film deposited on an FTO electrode for both the as-prepared one and that electrolyzed at different applied potentials in a DMF/0.1 M n-Bu4NPF6 solution.

main deconvoluted Mn 2p3/2 peak at 641.42 eV with a small shoulder at 640.35 eV (Figure 2, Figure S3, and Table S1), indicating the presence of MnIII as the main component with a low MnII content. The O 1s XP deconvoluted spectra revealed the presence of Mn−O−Mn binding at 529.62 eV, typical for Mn oxides, and a small peak at 531.57 eV for Mn−OH binding. At a 0.2 V applied potential where, according to the square wave voltammograms, the surface MnIII will be reduced to MnII, the deconvoluted XP spectra revealed the increase in the

rate = kcat[Mn V O] = k O − O[Mn V O][H 2O]

(4)

icat = nFAkcat[α‐Mn2O3] = nFAk O − O[H 2O][α‐Mn2O3] 4472

(5) DOI: 10.1021/acscatal.6b00413 ACS Catal. 2016, 6, 4470−4476

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ACS Catalysis

also observed in a DMF/0.1 M n-Bu4NPF6 solution; however, saturation of the catalytic current occurs at 2.0 M H2O (Figure S4). The catalytic current, icat, also varies linearly with the concentration of α-Mn2O3 sintered on an FTO electrode as obtained from the study of its dependence on α-Mn2O3 concentration in a neutral 0.1 M KPi solution (Figure S5). These observations of the dependence of icat on H2O concentration and α-Mn2O3 concentration are consistent with our hypothesized electrocatalytic water oxidation mechanism with α-Mn2O3/FTO involving a rate-limiting O atom transfer from MnVO to H2O (Scheme 1). The H−D kinetic isotope effect (KIE) is measured by comparing the CVs of α-Mn2O3/FTO in CH3CN/3.3 M H2O and CH3CN/3.3 M D2O solutions (Figure S6a). kcat(H2O)/ kcat(D2O) = icat(H2O)/icat(D2O) ≤ 1.2 KIE. This low KIE value is consistent with a mechanism involving a single H2O molecule and a direct O atom transfer to H2O in the O−O bond-forming step to give a coordinated hydrogen peroxide MnIII−OOH2, not MnIII−OOH, intermediate, in nonaqueous solutions containing some H2O (eq 6). The H2O/D2O KIE measured in H2O and D2O each containing 0.1 M NaClO4 (Figure S6b) shows also a lower value of 1.4 indicating behavior similar to that of the reactions in nonaqueous solutions containing some H 2 O. Indeed, a Ru hydrogen peroxide, Ru III −OOH 2 , intermediate was also proposed in the mechanism of water oxidation catalysis with a (Ru(Mebimpy)(bpy)OH2)2+ molecular catalyst in nonaqueous solutions [Mebimpy = 2,6-bis(1methylbenzimidazol-2-yl)pyridine; bpy =2,2′-bipyridine], and the initial density functional theory gas-phase calculations show that it is favored over the RuIII−OOH isomer by ∼8 kcal/ mol.73

Scheme 1. Proposed Mechanism of Water Oxidation at the Coordinately Unsaturated Surface of α-Mn2O3 Sintered on an FTO Electrode

where n = 4 is the electrochemical stoichiometry, F is the Faraday constant, A is the electrode surface area (1 cm2), and [α-Mn2O3] is the concentration of the catalyst. We studied the dependence of icat on H2O concentration in nonaqueous CH3CN and DMF solvents with H2O as a limiting reagent by cyclic voltammetry. Figure 3a shows CVs at a scan rate of 10

O atom transfer

Mn V O···OH 2 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ [Mn III(OOH 2)]+

(6)

The catalytic peak current normalized to the scan rates, icat/ν, increases with a decreasing scan rate in nonaqueous and aqueous solutions (Figures S7a and S8a). This behavior consists of a rate-limiting O atom transfer from MnVO to H2O followed by electron transfer to the electrode and release of the O2 like the mechanism shown in Scheme 1.75 In the absence of substrate, the peak current of the heterogeneous catalysis is linearly changed with scan rate ν according to eq 7.72−74 To eliminate the dependence on catalyst concentration and electrode area A, eq 5 is divided by eq 7 to give eq 8 for heterogeneous catalysis:72−74 ip =

Figure 3. (a) CVs at a scan rate of 10 mV/s of α-Mn2O3/FTO (0.52 μmol Mn content) in a CH3CN/0.1 M n-Bu4NPF6 solution upon addition of increasing amounts of H2O as indicated. (b) Plot of icat at 1.78 V vs [H2O].

n2F 2A[α‐Mn2O3]ν 4RT

(7)

icat 4RTkcat 4RTko − o[H 2O] = = ip nFν nFν

(8)

−1

mV/s of α-Mn2O3/FTO (0.52 μmol Mn content) in a CH3CN/0.1 M n-Bu4NPF6 solution with added H2O. In the absence of H2O and as observed by square wave voltammetry (Figure 1), three oxidation peaks appear at Ep values of 1.21, 1.56, and 1.73 V for the MnII/III, MnIII/IV, and MnIV/V couples, respectively. Addition of H2O to a concentration of 6.5 M resulted in a shift in the oxidation potential peak of the MnIII/IV couple to a more negative potential. Moreover, the catalytic peak current for H2O oxidation, icat, is enhanced and shifts to a more negative potential. The catalytic current at 1.78 V, icat, varies linearly with H2O concentration (Figure 3b) with a concentration of added H2O of up to 5.0 M. Similar behavior is

Plotting icat/ip versus ν from the scan rate-dependent measurements, we estimated kcat and kO−O values to be 60.0 s−1 and 9.1 M−1 s−1, respectively, at 1.78 V in CH3CN/6.5 M H2O (Figure S7b). In DMF/3.3 M H2O (Figure S8b), kcat and kO−O values are 40.9 s−1 and 12.4 M−1 s−1, respectively. In a 0.1 M KPi solution, as expected, the kcat increased to 121.8 s−1. Figure 4 shows the square wave voltammograms at a scan rate of 0.3 mV/s in the negative direction after setting the αMn2O3/FTO (0.52 μmol/cm2 Mn content) electrode at a certain potential for 5 s in a 0.1 M KPi (pH 7.0) solution (positive direction scanning shown for comparison). Setting the α-Mn2O3/FTO electrode at 1.78 V where the water oxidation 4473

DOI: 10.1021/acscatal.6b00413 ACS Catal. 2016, 6, 4470−4476

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ACS Catalysis

Figure 5. Schematic representation of α-Mn2O3 octahedra of the bulk and of the surface exposed to the electrolyte during the application of anodic potentials. The model summarizes the mechanism proposed in Scheme 1. The anodic polarization oxidizes the labile MnIII−OH at the surface to MnVO species that rapidly transfer the O atom to the H2O molecules and release O2 upon further electron transfer to the FTO electrode. Figure 4. Square wave voltammograms at a scan rate of 0.3 mV/s of αMn2O3/FTO in a 0.1 M KPi (pH 7.0) solution in the positive and negative directions after setting the electrode for 5 s at a certain potential as indicated.

have both bridged and nonbridged oxo sites. The Mn−O bond lengths of the bridged oxo are nearly equivalent and shorter in length. The nonbridged oxo sites, on the other hand, are relatively longer and consequently more labile.79 Our data show that the anodic polarization oxidizes the labile MnIII−OH in the polyhedral coordination located at the surface to form an active MnVO species through a PCET process. Once formed, the MnVO species rapidly transfer the O atom to the H2O molecules and release O2 upon further electron transfer to the FTO electrode. All data on the kinetic, XPS, and electrochemical measurements shown above rule out the O−O coupling mechanism between adjacent two MnIV atoms for O2 evolution.48 In conclusion, we investigated the mechanism and kinetics of heterogeneous water oxidation with the most active Mn oxide water oxidation catalyst sintered on an FTO electrode, αMn2O3/FTO, by electrochemical measurements in nonaqueous and aqueous solutions. The results show that α-Mn2O3 undergoes stepwise proton-coupled electron transfer (PCET) oxidations of the active MnII−OH2 site to MnIII−OH and MnIII−OH to MnIVO and finally an electron transfer oxidation of MnIVO to MnVO species. MnVO is the active species in the water oxidation process that undergoes O atom transfer to a H2O molecule in the rate-limiting step before the electron transfer to the electrode to release O2.

rate is very high for 5 s and scanning in the negative direction shows reduction peaks for the MnIVO/MnIII−OH and MnIII−OH/MnII−OH2 couples (black line, negative direction). Decreasing the setting potentials at the α-Mn2O3/FTO electrode, where the rate of water oxidation decreases, results in the appearance of a broad peak in the region of the MnIV/III; its current also increases with decreases in the setting potentials of the electrode. We tentatively assign this broad peak to the MnIV−OO−/MnIII−OOH (species 7, 6 in Scheme 1) reduction couple proposed in Scheme 1 in addition to the MnIVO/ MnIII−OH reduction couple (species 3, 2). The broad peak has a shoulder that appears at a more positive potential, presumably because of the MnIVO/MnIII−OOH2 reduction couple (species 4, 5). The Tafel plot corrected for the iR drop of α-Mn2O3/FTO in a neutral 0.1 M KPi solution (Figure S9) shows a linear correlation with a Tafel slope of 120 mV decade−1 indicative of a water discharge step (eq 9) controlling the oxygen evolution reaction.76−78 M + H 2O → M−OH + H+ + e−

(9)

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where M stands for a Mn site at the electrode surface. Combining the results of the Tafel plot with the mechanism proposed in Scheme 1 indicates that the Tafel explanation of the mechanism is an outline that needs details. Equation 9 is an outline of the mechanism, and its details are explained in the conversion from species 3 to species 6 in Scheme 1. The behavior observed by the electrochemical, XPS, and kinetic measurements of α-Mn2O3 could be summarized by a simple model shown in Figure 5. In α-Mn2O3, the MnIII atom is octahedrally coordinated by oxygen, i.e., MnIIIO6. There are five symmetrically inequivalent MnIIIO6 sites in the unit cell. Three MnIIIO6 sites have different Mn−O bond lengths (e.g., 1.88, 1.91, 1.97, 2.00, 2.22, and 2.27 Å). The other two sites have very similar Mn−O bond lengths (e.g., 1.96, 1.99, and 2.05 Å). Although the three MnIIIO6 sites with different Mn−O bond lengths have orthorhombic symmetry, they are considered as distorted tetragonal with two long (trans) and four shorter (cis) bonds; i.e., they have a trigonal antiprismatic symmetry (D3d). These MnIIIO6 octahedral sites are connected by corner-sharing bridges to other MnIIIO6 sites. The MnIIIO6 units at the surface

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.6b00413. Experimental details for α-Mn2O3/FTO preparation and characterization and additional figures (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by JST ACT-C, KAKENHI Grant 23245035, and a Chubu University Research Grant. 4474

DOI: 10.1021/acscatal.6b00413 ACS Catal. 2016, 6, 4470−4476

Research Article

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