Cu(II) Aliphatic Diamine Complexes for Both Heterogeneous and

Nov 12, 2015 - Simply mixing a Cu(II) salt and 1,2-ethylenediamine (en) affords precursors for both heterogeneous or homogeneous water oxidation catal...
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Cu(II) aliphatic diamine complexes for both heterogeneous and homogeneous water oxidation catalysis in basic and neutral solutions Cui Lu, Jialei Du, Xiao-Jun Su, Ming-Tian Zhang, Xiaoxiang Xu, Thomas J. Meyer, and Zuofeng Chen ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b02173 • Publication Date (Web): 12 Nov 2015 Downloaded from http://pubs.acs.org on November 24, 2015

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Cu(II) aliphatic diamine complexes for both heterogeneous and homogeneous water oxidation catalysis in basic and neutral solutions Cui Lu,† Jialei Du,† Xiao-Jun Su,‡ Ming-Tian Zhang,‡ Xiaoxiang Xu,† Thomas J. Meyer,§ and Zuofeng Chen*,† †

Shanghai Key Lab of Chemical Assessment and Sustainability, Department of Chemistry, Tongji University, Shanghai 200092, China

‡ §

Center of Basic Molecular Science (CBMS), Department of Chemistry, Tsinghua University, Beijing 100084, China

Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, United States

Supporting Information ABSTRACT: Simply mixing a Cu(II) salt and 1,2ethylenediamine (en) affords precursors for both heterogeneous or homogeneous water oxidation catalysis depending on pH. In phosphate buffer at pH 12, the Cu(II) en complex formed in solution is decomposed to give a phosphate-incorporated CuO/Cu(OH)2 film on oxide electrodes that catalyzes water oxidation. A current density of 1 mA/cm2 was obtained at an overpotential of 540 mV, a significant enhancement compared to other Cu-based surface catalysts. The results of electrolysis studies suggest that the solution en complex decomposes by en oxidation to glyoxal following Cu(II) oxidation to Cu(III). At pH 8, the catalysis shifts from heterogeneous to homogeneous with a single-site mechanism for Cu(II)/en complexes in solution. A further decrease in pH to 7 leads to electrode passivation by formation of a Cu(II) phosphate film during electrolyses. As the pH is decreased, en, with pKb ~6.7, becomes less coordinating and precipitation of the Cu(II) film inhibits water oxidation. The Cu(II)based reactivity toward water oxidation is shared by Cu(II) complexation to the analogous 1,3-propylenediamine (pn) ligand over a wide pH range. KEYWORDS: water oxidation, copper, ligand dissociation, heterogeneous catalysis, homogeneous catalysis Introduction. In photoelectrochemical water splitting, water oxidation (2H2O → O2 + 4H+ + 4e–) plays a crucial role in providing the reductive equivalents and protons for proton reduction to hydrogen.1-4 The multi-electron, multi-proton character of this reaction however typically leads to slow kinetics and high overpotentials. In addition, successful catalysis requires robust and efficient water oxidation in aqueous solutions under relatively benign conditions. Significant progress has been made recently on Cu-based water oxidation catalysis with simple Cu(II) complexes5-15 or salts16,17 as homogeneous single-site or dual-site catalysts in solution, or as precursors to catalytically active metal oxide films. However, the reactivity of these Cu catalysts is usually limited to basic solutions and typically requires high overpotentials especially for homogeneous catalysts. A complication with these and other first row catalysts is oxidation or dissociation of a ligand to form electroactive nanoparticles, which may act as more efficient catalysts for water oxidation than the original complexes. While there are an increasing number of reports on interfacial catalysis by decomposition of Cu(II) and other transition metal

complexes,10,14,18-21 the complex decomposition chemistry is usually not well elucidated due to the complexity of the ligands used and their irreversible redox chemistries. Reaction conditions - solution pH, electrolyte, applied potential - can add an additional layer of complexity. Elucidation of complex decomposition requires relative simplicity with access to procedures for characterizing oxidation products under diverse experimental conditions. We report results here on a Cu(II) 1,2-ethylenediamine (en) complex self-assembled from the metal ion and ligand with the complex functioning both as a precursor for heterogeneous water oxidation catalysis in basic solutions and as a homogeneous catalyst in weakly basic to neutral solutions. Experimental studies provide insight into relationships among catalyst structure/composition, reactivity, and stability. Based on the observation of Cu(III)-induced interfacial catalysis and glyoxal as the ligand oxidation product, oxidative decomposition of the ligand is proposed to occur in basic solutions following oxidation of the complex and oxidative deamination of the ligand. Ligand decomposition is the precursor to a shift from homogeneous catalysis at relatively low pH to 1

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heterogeneous catalysis at elevated pH accompanied by ligand oxidation and complex decomposition.

both self-assembled complexes and pre-synthesized complexes were the same.

Pre-catalyst and catalyst structures. The structures of Cu(II)/en complexes at different Cu(II):en ratios are shown in Figure 1A-C. In aqueous solution, these complexes are octahedrally coordinated with en coordinated as a bidentate ligand and the remaining coordination sites occupied by coordinated water.22 As shown in UV-visible spectra at pH 12 in Figure 1D, light absorption by Cu(II) d-d transitions in the visible is enhanced with λmax shifting to higher energy as the en concentration is increased, consistent with stepwise en coordination. Beyond a Cu(II):en 1:3 ratio there are barely discernible changes in the spectra consistent with formation of the tris complex, [CuII(en)3]2+ as the dominant form in solution.23 The successive stability constants, defined in eqs 1 and 2, for [CuII(en)(OH2)4]2+, [CuII(en)2(OH2)2]2+ (shown as the trans isomer) and [CuII(en)3]2+ are shown in Table 1. Given the magnitude of these stability constants, the mono and bis complexes are the only forms of Cu(II) at Cu(II):en ratios of 1:1 and 1:2, respectively; At a Cu(II):en 1:3 ratio, the tris complex is the dominant form (> 90%) with a small amount of the bis complex. Comparable spectral changes were observed at pH > 8, Figure S1. The coordination chemistry is more complex at pH 7 given pKb (20 °C in water) ~6.7 for en.24 Under these conditions, there is a continuous increase in absorptivity and in the d-d λmax with increasing en concentration, Figure 1E. The coordination chemistry is complicated by the acid-base properties of the en ligand and formation of mixed aqua-en complexes.

Heterogeneous electrocatalysis. Figure 2A shows cyclic voltammograms (CVs) obtained at an ITO (Sn(IV)-doped In2O3) electrode in a 0.2 M phosphate buffer solution (PBS, pH 12). In the absence of both Cu(II) and en, a small background current is observable with an onset at ~0.92 V vs. NHE (normal hydrogen electrode). Upon addition of 2 mM en, the CV profile is nearly unchanged showing that the free ligand is stable toward oxidation at the ITO electrode, which is attributable to the hydrophilicity of the ITO surface.25 Upon further addition of 1 mM Cu(II) to the en solution, a dramatic current enhancement is observed above background. Note that adding only Cu(II) to basic PBS leads to Cu(OH)2 or Cu3(PO4)2 precipitates. Under the solution condition of added both en and Cu(II), the onset potential for water oxidation appears at ~0.90 V, an overpotential of ~380 mV at this pH, followed by two catalytic waves. At slow scan rates, these waves sharpen and deviate from a diffusive waveform, Figure S2. On the reverse scan, a current “cross-over” appears along with a small re-reduction wave at Ep,c ~0.78 V (Ep,c is the reductive peak potential). Continued CV scans show increased catalytic currents which were stabilized after 25 continuous scans, Figure S3. These features are consistent with water oxidation by interfacial catalysis and formation of a catalytically active surface-bound intermediate. Similar CV behavior was observed at an FTO (Fdoped SnO2) electrode, Figure S4. Controlled potential electrolysis was conducted at an ITO electrode at 1.15 V under the same solution conditions, Figure 2B. In contrast to the background electrolyses, with added Cu(II) and en a rising current density appears reaching a steady state value of ~2.7 mA/cm2 after ~1 h which was sustained for at least 6 h. Measurement of oxygen evolved by a fluorescence based oxygen sensor (Ocean Optics), gave 38.6 µmol of O2 over an electrolysis period of 6 h with a Faradaic efficiency of 92% for O2 production. The non-unit Faradaic efficiency is due to oxidation of en ligands, as discussed below. Under the experimental conditions, a theoretic Faradaic efficiency of 90.4% is calculated by excluding the charge consumed by oxidation of all en, assuming an electrochemical stoichiometry of 4 for oxidizing each en ligand. During long term electrolysis, there was visible evidence for formation of a precipitated orange film on the ITO electrode. At its rest state, the surface-bound solid is stable and was not dissolved in 0.2 M PBS (pH 12). Upon oxidation, it appears to be highly active toward water oxidation in Cu(II) and en-free PBS solution at pH 12, Figure 2C. Controlled potential electrolysis under these conditions at 1.15 V leads to a steady state current density of ~2.5 mA/cm2 sustained for least 6 h, Figure 2C inset. Effervescence from the orange coating on the electrode is vigorous under these conditions. The catalytic current density, j, obtained at the film prepared during electrolysis with Cu(II)/en complex was measured as a function of the overpotential, η, Figure 2D, with η the difference between the applied potential minus the cell resistance and the standard potential for water oxidation at pH 12. From the Tafel plot of log j vs. η, the slope is ~62 mV/decade for current densities ranging from 3.2 mA/cm2 to 0.5 mA/cm2 with a current density of 1 mA/cm2 at η = ~540 mV. The Tafel slope of the film formed in the basic en solution is close to the 59 mV/decade values obtained for phosphate-incorporated Co and Ni films26,27 and the reactivity

MAn-1 + A ⇄ MAn

(1)

kn = [MAn]/[MAn-1][A]

(2)

Table 1. Successive stability constants (logkn) for Cu(II) to form the mono, bis and tris complexes in H2O at 30 °C.23 [CuII(en)(OH2)4]2+

[CuII(en)2(OH2)2]2+

[CuII(en)3]2+

10.55

9.05

1.0

Figure 1. Illustrating the structures of the mono, bis, and tris en complexes of Cu(II), [CuII(en)(OH2)4]2+ (A), trans[CuII(en)2(OH2)2]2+ (B), [CuII(en)3]2+ (C), and UV-visible spectra at pH 12 (D) and pH 7 (E) with different ratios of Cu(II) and en. The catalyst was independently synthesized as described in the literature22 and its absorption spectrum is the same as for the selfassembled complex in solution at pH 7-12 when Cu(II) and en were simply mixed in a designed ratio in water. The reactivities of

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in terms of observed current density at 1 mA/cm2 is greater than other reported Cu-based interfacial catalysts.10,16,17

Figure 2. (A) CVs at an ITO electrode, with addition of 2 mM en, and with 1 mM Cu(II) & 2 mM en in 0.2 M PBS at pH 12 at 100 mV/s. (B) As in (A), electrolysis at 1.15 V. Inset: headspace O2 measurement during electrolysis. (C) CVs at an ITO electrode and an as-prepared film at pH 12 at 100 mV/s. Inset: electrolysis at 1.15 V. (D) Tafel plot, η = (Vapplied – iR) – E(pH 12) of an asprepared film at pH 12, corrected for the iR drop of the solution. The morphology of the electrode coating formed during electrolysis was examined by scanning electron microscopy (SEM), Figure 3A. The electrodeposited film consists of particles with diameters in the tens of nanometers range and completely covers the ITO substrate. Film thickness gradually increases over the course of the electrodeposition procedure as seen by the change in ITO color. Under steady state electrolysis conditions, the film was 400 nm thick, Figure 3B. The X-ray diffraction (XRD) pattern of a film of the electrodeposited catalyst includes broad amorphous features and no peaks indicative of crystalline phases other than the peaks associated with the ITO layer, Figure 3C. In the absence of detectable crystallites, the composition of the electrode coating was analyzed by energy-dispersive X-ray (EDX) and the X-ray photoelectron (XPS) spectroscopies. The EDX results in Figure 3D show that the amorphous solid was a mixture containing Cu, O and P with negligible N and Na. In Figure 3E-H, all of the XPS peaks are assignable based on the elements detected by EDX. The presence of Cu(OH)2/CuO is confirmed by the binding energies of Cu 2p3/2 at 934.1 eV and 2p1/2 at 953.9 eV with obvious shake-up satellite peaks.28 The P 2p peak at 132.8 eV is consistent with phosphate. Together, the EDX and XPS results indicate that electrolysis with 1 mM Cu(II) and 2 mM en in PBS at pH 12 results in electrodeposition of an amorphous CuO or Cu(OH)2 particle film which incorporates a substantial amount of phosphate anion at a stoichiometric ratio of ~3:1 Cu:P.

Figure 3. (A) Top-down, (B) cross-sectional SEM images, (C) XRD, (D) EDX, and (E-H) XPS of the as-prepared film with 1 mM Cu(II) & 2 mM en. For comparison, in (D)-(H), EDX and XPS for the film prepared with 1 mM Cu(II) & 5 mM en are also present. The simplicity of the ligand system in this study enables the detection of ligand oxidation product providing a probe for the decomposition chemistry leading to the nanoparticle CuO film. As mentioned above, two catalytic waves were observed in CVs at a 1:2 Cu(II):en ratio. Under these conditions the diaqua complex, [CuII(en)2(H2O)2]2+ is the dominant form in solution with further, sequential oxidation to Cu(III) and Cu(IV) (or Cu(III)-O•) the origin of the oxidative waves in the voltammograms. There is experimental evidence for decomposition following 1e– oxidation based on switching potential measurements. As shown in Figure S5, the profile of CV and electrolysis with potential limited within the first anodic peak (≤ 1.15 V) is consistent with interfacial catalysis. After electrolysis at 1.15 V for 30 min in D2O solutions, analysis of the solution by 1H NMR revealed glyoxal as the product of ligand oxidation decomposition, Figure S6. Other potential small organic products such as formaldehyde, acetaldehyde, glycol, methanol and ethanol were not detected by NMR. Based on the product analysis, a ligand decomposition mechanism induced by Cu(III) is proposed in Scheme 1. Earlier work on oxidation of coordinated diamines in bis(2,2'-bipyridine) complexes of ruthenium, [RuII(bpy)2(en)]2+, showed that the diamine oxidative dehydrogenation reactions are initiated by oxidation of ruthenium(II) to ruthenium(III), and that the reactions probably occur in a stepwise manner via monoimine intermediates, eqs 3-6.29

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In our study, electrochemical oxidation to Cu(III) is presumably followed by intra-complex electron transfer from a deprotonated –NH2 group of an en ligand to Cu(III) to give the singly oxidized nitrogen radical –(H)N• coordinated to Cu(II). At the potential used, further oxidation to Cu(III) followed by ligand oxidation would lead to the imine,–CH=NH, on one arm of a en ligand. As shown in Scheme 1, further oxidation of the monoimine would lead to the coordinated di-imine, HN=CH-CH=NH, which is labile in aqueous solution toward hydrolysis to generate glyoxal and ammonia, eq 7.30,31 With loss of the en ligand, uncoordinated Cu(II) is precipitated as Cu(OH)2/CuO with the incorporation of some PO43–, presumably as Cu3(PO4)2, on the electrode surface which is electroactive toward water oxidation. At potentials where Cu(IV) is accessible, oxidative deamination may involve prior two-electron oxidation of metal followed by oxidation of the en ligand. In both schemes, strong proton acceptor bases, in this case OH– or PO43– (pKa ~12.5 for HPO42–), play an important role as the proton-coupled electron transfer (PCET) bases in the oxidative decomposition of the ligands.32 HN=CH-CH=NH + 2H2O → H(O)C-C(O)H + 2NH3

(7)30,31

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species in solution, a change in CV response occurs with the onset potential increased positively. By further increasing the en concentration, and decreasing the fraction of mixed en-aqua complexes in solution, water oxidation via Scheme 1 is further inhibited with catalytic currents negligibly small. This observation is consistent with the important role played by aqua ligands for many single-site polypyridyl Ru33,34 and Ir35,36 water oxidation catalysts. For the Ru(II) polypyridyl catalysts, stepwise oxidation from RuII-OH2 to RuV=O initiates water oxidation. Similarly, for the Cu(II)-en complexes oxidative activation to oxo stabilized Cu(III) or Cu(IV) intermediates by proton-coupled electron transfer (PCET) in eq 8, prior to decomposition, presumably plays an important role as illustrated in Scheme 1.

Although catalysis during the forward CV scan was inhibited with increasing en concentrations, the current level during the reverse scan increased, reaching a maximum level at Cu(II):en 1:5, Figure 4B. Because the reverse current appears to arise largely following formation and oxidation of the surface-bound nanoparticle film, the voltammetric response points to maximized surface precipitation/film formation at a Cu(II):en ratio of 1:5. The forward catalytic wave at a 1:2 Cu(II):en ratio includes a large component from molecular catalysis, see below, which does not contribute to the interfacial reaction during the reverse scan. At a Cu(II):en ratio of 1:5, the current density during electrolysis at 1.15 V increases dramatically, by a factor of 34, from 0.07 mA/cm2 to a steady state current level of 2.4 mA/cm2 within 1 h, Figure 4C. By contrast, the catalytic current is enhanced by ~2.5 at a 1:2 Cu(II):en ratio. EDX and XPS spectra in Figure 3 reveal no significant compositional differences between films prepared from 1:5 and 1:2 Cu(II):en ratios. However, as shown by the SEM images in Figure 4D, the film made from 1:5 Cu(II):en is more compact and fragile with clear cracking in the film structure. At ratios of Cu(II):en lower than 1:5, the catalytic current is further inhibited during the forward scan resulting in decreasing interfacial catalysis during the reverse scan. These results point to a significant impact on complex decomposition and film buildup by the ligand composition in the reaction solution.

Scheme 1. Proposed mechanism for Cu(III)-induced, ligand oxidation decomposition of trans-[CuII(en)2(H2O)2]2+ during water oxidation at pH 12 with the trans aqua ligands omitted for clarity. The Cu(II):en ratio has a significant effect on catalytic activity in solution before the film is deposited. For clarity, forward and reverse CV curves are shown separately in Figure 4A and Figure 4B with a full CV scan cycle in Figure S7. During the forward CV scan, related observations are made at Cu(II):en ratios of both 1:1 and 1:2. At a ratio of 1:3, with [CuII(en)3]2+ as the dominant

Figure 4. Forward (A) and reverse (B) CV scans at an ITO electrode with increasing en in a solution 1 mM in Cu(II) at pH 12 (0.2 M PBS) at 100 mV/s. (C) Electrolysis without and with 1 4

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mM Cu(II) and 5 mM en at pH 12 at 1.15 V. (D) SEM images of the film from 1:5 Cu(II):en. Homogeneous electrocatalysis. Unlike heterogeneous water oxidation at pH 12, water oxidation by 1:2 Cu(II):en is homogeneous at pH ≤ 10 with no evidence for heterogeneous catalysis. Heterogeneous catalysis is typically characterized by cross-over effects in the CV profile with the appearance of apparent re-reduction features in the reverse CV scan and rising current densities during electrolysis due to the buildup of surfacebound catalyst. As shown in Figure 5, there is no evidence for this behavior for water oxidation at pH 8. Following long-term electrolyses, there is also no evidence for precipitation on the electrode surface by SEM and XPS analysis. As noted in Scheme 1, the shift from heterogeneous to homogeneous catalysis as the pH is decreased is consistent with the pH/buffer base effects for PCET oxidation of the en ligand following oxidation to the higher oxidation states of Cu. Under homogeneous catalytic conditions from pH 7 to 10, the catalytic onset potential shifts positively by ~60 mV/pH, which tracks the pH dependence of the expected high-oxidation-state Cu couples involved in water oxidation, Figure S8. At pH 8, the catalytic current increases linearly with catalyst concentration from 0.1-2.5 mM, Figure 5A inset. The linear dependence is further indicative of homogeneous catalysis presumably by a single-site mechanism in the rate determining step(s).37 In the proposed scheme at pH 8, nucleophilic attack of water occurs on high-oxidation-state CuIII=O or CuIV=O intermediates to form the O---O bond.6 In this case, oxidative activation to an oxo intermediate and O---O bond formation occur successfully in competition with oxidative deamination of the en ligand. A more detailed mechanistic study on homogeneous water oxidation catalysis is currently under investigation. Homogeneous water oxidation under electrolysis condition in Figure 5B was confirmed by O2 measurement with a Faraday efficiency of 75%. This level of Faraday efficiency indicates that en oxidation may still occur slowly as a side reaction, although not dominant, during catalytic water oxidation at this pH. CVs were recorded as a function of scan rate at pH 8. When normalized by the square root of scan rate to account for diffusion, j/υ1/2 increases with decreasing scan rate, Figure S9A, consistent with a rate-limiting step prior to electron transfer to the electrode. At slow scan rates (5 mV/s), there is evidence for two anodic waves, consistent with sequential oxidation to Cu(III) and Cu(IV), Figure S9B. In return scans, reductive waves were observed at Ep,c = ‒0.2 V and ‒0.45 V attributable to reduction to Cu(I) and Cu(0), respectively, Figure S9C. A linear relationship exists between peak current density (jd) at ‒0.45 V and the square root of scan rate from 5-1000 mV/s, Figure S9D, consistent with a diffusion-controlled process and the Randles-Sevcik equation in eq 9.38

electrolysis by using eq 10 and assuming DCu+ as the diffusion coefficient for the oxidized catalyst with ncat = 4 for water oxidation to oxygen. Use of eq 10 is an approximation based on an EC model with water oxidation a 4e‒ process. The value of kcat ~0.4 s–1 is comparable to values reported earlier for other molecular water oxidation catalysts.8,9,11 jcat = (ncatF)[Cu](kcatDCu)1/2

(10)38

Figure 5. (A) CVs at an ITO electrode (red), with addition of 2 mM en (green), and with 1 mM Cu(II) & 2 mM en (blue) in 0.2 M PBS at pH 8 at 100 mV/s. Inset: plot of catalytic current in CVs at 1.55 V vs. catalyst concentration at 1:2 Cu(II):en. (B) As in (A), electrolysis at 1.55 V. The current-potential and current-time profiles for the CV and electrolysis traces at pH 7 in Figure 6A,B are consistent with homogeneous catalysis. However, the catalytic current observed in CVs deviates from a linear variation with catalyst concentration beyond ~1 mM, Figure 6A inset. After electrolysis, an azury film was observed on the ITO surface. In contrast to the orange film formed at pH 12, this film is inactive toward water oxidation, Figure 6B inset. It passivates the ITO electrode resulting in the decrease in catalytic current for CVs at high catalyst concentrations and small catalytic currents during electrolyses. The SEM image in Figure 6C shows that the passivating film on the ITO electrode consists of flower bud-like crystalline particles having diameters ~15-20 µm, Figure 6C. The XRD pattern of the film in Figure 6D exhibits a new diffraction peak at 2θ = ~7.6° above the ITO background. In a control experiment, a film of Cu3(PO4)2 was prepared by mixing Cu(II) and PBS at pH 5. The resulting precipitate cast on ITO gives a diffraction peak at ~7.6° with an additional small peak at ~12.6° consistent with a Cu(II) hydroxide species.39 The EDX in Figure 6E identifies Cu, P, and O as the principal elemental components of the passivated film with a Cu:P ratio of 3:2.1. These results are consistent with formation of a passivating film of Cu3(PO4)2 at pH 7. Under these conditions, near pKb ~6.7 for en,24 formation and precipitation of solid Cu3(PO4)2 becomes competitive further complicating interfacial effects in water oxidation catalysis by Cu(II).

jd = 0.4463nd3/2F[Cu](FυDCu/RT)1/2 (9) In eq 9, nd (= 1) is the number of electrons transferred at ‒0.45 V, F is the Faraday constant, [Cu] is the catalyst concentration in mol/cm3, υ is the scan rate in V/s, DCu is the catalyst diffusion coefficient in cm2/s, R is the gas constant, and T is the temperature in K. From Figure S9D and eq 9, the diffusion coefficient for once-reduced catalyst, DCu+, is 2.5 × 10‒6 cm2/s. A rate constant for catalytic water oxidation of kcat ~0.4 s–1 was estimated from the steady state catalytic current density jcat during 5

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the de-coordinated Cu(II) forming a catalytically active layer of CuO/Cu(OH)2 on the electrode surface which has been characterized by a variety of techniques. In the basic medium, the oxide film overlayer is active toward sustained water oxidation catalysis with O2 produced in a high Faradaic yield and at an overpotential superior to other Cu-based surface catalysts. Initial experiments with the di-amine ligand 1,3-propylenediamine point to a generality for this type of Cu(II) reactivity.

ASSOCIATED CONTENT Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Figure 6. (A) CVs at an ITO electrode (red), with addition of 2 mM en (green), and with 1 mM Cu(II) & 2 mM en (blue) in 0.2 M PBS at pH 7 at 100 mV/s. Inset: plot of catalytic peak current in CVs at 1.60 V vs. catalyst concentration at 1:2 Cu(II):en. (B) As in (A), electrolysis at 1.60 V. Inset: CVs of an ITO electrode and the as-formed passivated film at pH 7. (C) SEM, (D) XRD, (E) EDX of the passivated film. Extension to a en analog. In an initial series of experiments, the generality of the di-amine ligand coordination chemistry at Cu(II) for water oxidation was investigated by using 1,3propylenediamine (pn) as the chelating ligand. In 0.2 M PBS at pH 12 containing 1 mM Cu(II) and 2 mM pn, the available evidence in both CV (cross-over feature) and electrolysis (rising current densities) clearly indicates heterogeneous catalysis of water oxidation by the Cu(II)/pn complex, Figure S10A,B. Electrolysis of the solution at 1.15 V resulted in the buildup of a film or precipitate which was highly active and robust toward water oxidation catalysis, Figure S10C,D. At pH 7, the electrochemical behavior is also consistent with the shift from heterogeneous to homogeneous catalysis of water oxidation, Figure S11. These results presage a general water oxidation reactivity for this class of Cu(II) amine complexes. Conclusions. The results described here are important in extending the known chemistry of Cu(II) water oxidation catalysis to a new family of coordination complexes with water oxidation catalysis observed over a wide pH range. The Cu(II)/en catalysts or catalyst precursors are formed spontaneously above pKb ~6.7 for the en ligand by simply adding the ligand in controlled amounts to aqueous solutions of Cu(II). A variety of catalytic behaviors has been documented under these conditions in phosphate buffer solutions as a function of pH. Homogenous water oxidation catalysis occurs in solutions from pH 7 to 10 for mixed Cu(II) en-aqua complexes such as [CuII(en)2(OH2)2]2+. At pH 8, oxidation of the Cu(II) complex to Cu(III) and Cu(IV) is followed by rate limiting O---O bond formation which occurs with kcat ~0.4 s–1. At pH 7, near pKb ~6.7 for en, long term electrolysis in the phosphate buffer solution is complicated by formation of a passivating layer of Cu3(PO4)2 which inhibits electrocatalysis. At higher pH, oxidation of the complex is followed by oxidative deamination of the diamine ligand to give glyoxal with

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS We thank The National Natural Science Foundation of China (21405114, 21573160), The Recruitment Program of Global Youth Experts by China, and Science & Technology Commission of Shanghai Municipality (14DZ2261100) for support.

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