Cobalt-Bisglyoximato Diphenyl Complex as a Precatalyst for

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Cobalt-Bisglyoximato Diphenyl Complex as a Precatalyst for Electrocatalytic H2 Evolution Elodie Anxolabehere-Mallart, Cyrille Costentin, Maxime Fournier, and Marc Robert J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp500813r • Publication Date (Web): 02 Jun 2014 Downloaded from http://pubs.acs.org on June 2, 2014

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The Journal of Physical Chemistry

Cobalt-Bisglyoximato Diphenyl Complex as a Precatalyst for Electrocatalytic H2 Evolution Elodie Anxolabéhère-Mallart,* Cyrille Costentin,* Maxime Fournier, Marc Robert* Univ Paris Diderot, Sorbonne Paris Cité, Laboratoire d'Electrochimie Moléculaire, UMR 7591 CNRS, 15 rue JeanAntoine de Baïf, F-75205 Paris Cedex 13, France. Supporting Information Placeholder

ABSTRACT: Electrochemical investigation of the title CoII compound in acetonitrile with a strong acid (HClO4) showed no sign of proton catalysis at the CoII/CoI wave, but instead revealed the formation of Co based nanoparticles at the surface of the carbon electrode. Catalytic proton reduction of the resulting nanometer sized cobalt particles at pH 7 was found to occur efficiently. Partial coverage of the carbon substrate by the particles leads to an apparent exchange current density as high as those obtained at a pure cobalt electrode or cobalt films.

Keywords: Co complex, Co particles, proton reduction, hydrogen, precatalyst Molecular based materials are expected to play a central role in the development of a renewable source of energy.1 This is fuelled from the fact that nature uses molecular metal complexes at the active catalytic sites of most enzymes participating in the main energetic transduction pathways, i.e. from photosynthesis to respiration.2 The principle lesson we learn from these natural catalysts is the presence of earth abundant metal ions and a set of organic chelates provided by the amino acids. Therefore, it comes as no surprise that there is an intense activity in the development of low molecular weight metal complexes that can replicate these reactivity patterns.3-5 Synthetic molecular catalysts are less protected than the active site of a protein matrix and are not self healing, at variance with natural systems like, e.g. photosystem II, being thus prone to degradation and deactivation processes. A trial and error strategy may lead to the design of more robust catalysts, while in some cases, the initial metal complex actually behaves as a pre-catalyst, which undergoes severe chemical modifications, as e.g. ligand loss, finally heading to the real catalytic molecules or particles.6-11 Many molecular cobalt complexes have recently been investigated as potential catalysts both for the hydrogen evolving reaction (HER)12-17 and oxygen evolving reaction (OER).4, 18-21 Among these, cobalt bis glyoximato complexes as, e.g., the dimethyl substituted compound, have been particularly studied as catalysts for the HER in aprotic solvents22-27 and concomitantly different mechanistic pathways have been proposed. Conversely, in a recently reported electrochemical investigation of a fluoroboryl-capped tris(glyoximato) diphenyl cobalt clathrochelate complex in acetonitrile in presence of acid, we have demonstrated that the catalytic activity toward hydrogen evolution results from a deligation of the cobalt center followed by an electrodeposition of cobalt-containing nanoparticles on the electrode surface at a modest cathodic potential.6 The deposited particles act as remarkably active catalysts for H2 production in water at pH 7. We have also shown that replacement of the

phenyl substituent by a methyl group or the fluoro Bsubstituent by a phenyl group lead to the same behaviour.28 Very recently, it was revealed that two bis pyridine methyloxime Co complexes are electrochemically converted to cobalt-based particles at the electrode surface, in acetonitrile containing large concentration of trifluoroacetic acid.29 Co bis-glyoximes complexes remain however commonly viewed as efficient molecular catalysts for H2 evolution in aprotic solvents. As mentioned above, difluoroboryl bis glyoximato dimethyl complex has been proposed as an active catalyst in N,N-dimethylformamide with Et3NHCl as an acid source,26 in acetonitrile with CF3COOH, HCl,27 paratoluenesulfonic acid (TsOH),22 as well as an adsorbed molecular species on graphite electrode, following electrolysis in acetonitrile in the presence of a large excess of TsOH.23 Difluoroboryl bis glyoximato diphenyl CoII complex (Scheme 1, [1]) has been presented as a molecular catalyst in acetonitrile with HCl, HBF4 27 as well as with TsOH.22 With this complex, it was proposed that electrochemical generation of the CoI species (CoII/CoI wave close to -0.3 V vs. SCE) triggers catalytic reduction of protons. We have discovered that in acetonitrile with a strong acid (HClO4), complex [1] does not catalyze H2 formation along a molecular pathway at the CoII/CoI electrochemical wave but rather decomposes after ligand reduction, shaping the formation of nanoparticles that act as the real catalyst, thus showing that within this promising class of compounds, electron assisted deactivation of the starting complex should be considered as an important pathway towards H2 formation.

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Scheme 1. Investigated CoII-bisglyoximato diphenyl complex. In solution, L = acetonitrile. Cyclic voltammetry of Co(dpgBF2)2 [1] (Fig. 1a) gives rise in acetonitrile to a one electron reversible cathodic wave leading to the generation of the corresponding CoI species E0(CoII/CoI) = -0.26 V vs. SCE) in agreement with previous report, and to an anodic irreversible wave leading to the generation of an unstable CoIII species (Epeak = 0.78 V vs. SCE at 0.1 V/s). The more cathodic, irreversible fourelectron wave peaking at -1.39 V vs. SCE, corresponds to the reduction of the diphenylglyoxime ligand, as previously checked with a solution of the ligand alone.6 Comparison with the electrochemical signature of the dimethyl complex Co(dmgBF2)2 in the same conditions indicates that the CoII/CoI cathodic wave is shifted ca. 300 mV towards positive values. This shift results from the electron-withdrawing nature of the phenyl ring on the glyoxime ligands in Co(dpgBF2)2 by comparison with the methyl groups, making complex [1] an attractive candidate as a catalyst for H2 evolution, as already mentioned in the literature. a 3

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Figure 1. Cyclic voltammetry of bis-cobaloxime complex Co(dpgBF2)2 [1] (0.1 mM in acetonitrile + 0.1 M NaClO4, v = 0.1 V/s) at a glassy carbon electrode (2 mm diameter): (a) alone, (b) in the presence of HClO4 : 0 mM (black), 0.5 mM (blue), 1 mM (cyan), 2 mM (green), 3 mM (yellow), 5 mM (orange), 7 mM (red), 9 mM (wine), 11 mM (purple) and 15 mM (magenta). Upon addition of perchloric acid, the CoII/CoI wave of [1] (noted 1, Fig. 1b) becomes irreversible and a second wave noted 2 appears at a more negative potential (ca. -0.7 V vs. SCE). At the first cathodic wave 1, it is proposed that protonation of the CoI(dpgBF2)2 produced by the electrochemical reduction of the starting CoII complex yields the corresponding hydride CoIIIH(dpgBF2)2. The CoII/CoI wave is not affected by the addition of a weaker acid (trifluoroacetic acid, Fig. S1) and no second wave appears in that case, indicating that wave 2 involves the reduction of the hydride formed at the level of the first wave. Thus, no catalysis occurs at the CoII/CoI wave, showing that the CoIIIH hydride is not basic enough to react with proton, in line with the electron-withdrawing nature of the phenyl ring on the glyoxime ligands. Upon varying the acid strength (HBF4 (pKa 0.1), H2SO4 (pKa 7.2), no catalysis was observed at the CoII/CoI wave.30 Confirmation that no catalysis occurs on larger timescale on wave 1 was given by the absence of H2 formation upon electrolysis of [1] (0.1 mM) in the presence of 6 mM HClO4, at a potential negative to the CV peak (E = -0.55 V vs. SCE). A closer look at wave 2 (reduction of the cobalt hydride formed at wave 1) shows that upon addition of perchloric

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acid, its heights increases and then levels off at high acid concentration (Fig. 1b), the consumption of the acid within the diffusion-reaction layer being then negligible. In these conditions, the 4e− stoichiometry of wave 2 points to the reductive hydrogenation of the four C-N bonds of the two diphenylglyoxime ligands, requiring the transfer of two electrons and two protons each.31 This process is similar in intensity to the reduction of the diphenylglyoxime ligands in the complex in the absence of acid (cathodic wave peaking at -1.39 V vs. SCE, see Fig. 1a). Diphenylglyoxime hydrogenation in the presence of HClO4 takes place at a much more positive potential, through an intramolecular catalysis of the hydrogenation by the CoIIH species generated from the CoIIIH hydride reduction. Thus, in the case of [1], proton-coupled reduction of the ligand, starting from the CoIIH species and acting as a catalyst deactivation pathway overcomes catalytic formation of H2. Electrolysis of [1] (0.12 mM) in the presence of 9 mM HClO4 at -0.70 V vs. SCE (wave 2 peak) does not produce hydrogen. The current rapidly drops to essentially zero, while cyclic voltammetry shows that the initial complex has disappeared (see Fig. S2), thus confirming that no catalysis occurs on wave 2 on longer times, while the ligand is reduced. We then investigated the behaviour of [1] at more negative potentials. As seen on Fig. 2, wave 2 is followed by a small shoulder (noted "dep") and by a broad wave (noted 3) whose height increases with addition of acid. This later wave can be attributed to the catalytic reduction of proton helped by the presence of free cobalt cation as asserted by comparison with a voltammogram from a cobalt salt Co(NO3)2 solution in the presence of the same acid (Fig. 2). The data previously reported on fluoroboryl-capped tris(glyoximato) diphenyl cobalt complex strongly suggest that wave "dep" is due to an electrodeposition process leading to the formation of an electrocatalytic species, active towards proton reduction. The electrodeposition is made possible by deligation of the cobalt cation following ligand reduction. This means that in addition to proton coupled catalytic reduction of the glyoximate ligands (corresponding to a 4e− stoichiometry), the reduction of two N-O bonds or the reduction of the four N-O bonds may also take place. At these negative potentials, one may not exclude that some hydrogen evolution could also occur in parallel from a molecular catalyst (reduced form of a cobalt hydride), at least in the time scale of the cyclic voltammetry. Note that such a molecular mechanism may also account for H2 evolution in photochemical experiments where a photosensitizer and a sacrificial electron donor are used instead of an electrode. Polarizing the electrode at -0.9 V vs. SCE for 30 s enhances surface modification by an adsorbed, catalytic species as indicated by the growing of a new wave "dep*" (Fig. 2) observed upon scanning after polarization. Note that the initial voltammogram could be restored upon careful polishing of the electrode surface. i (µA)

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Figure 2. Cyclic voltammetry of [1] (0.1 mM) in acetonitrile (0.1 M NaClO4, v = 0.1 V/s) at a glassy carbon electrode (2 mm diameter) in the presence of 9 mM HClO4: (black) initial scan on freshly polished electrode; (red) after microelectrolysis at -0.9 V vs. SCE during 30 s ; 0.1 mM Co(NO3)2 in the presence of 5 mM HClO4 (green). Confirmation that deligation of the complex leads to an electrodeposition on the electrode surface was inferred from control SEM images and EDX analysis of the carbon surface, before and after electrolysis at -0.9 V vs. SCE, showing nanoparticles (average size 100-200 nm) deposited on the surface (Fig. 3). Energy-dispersive X-ray spectroscopy (EDX) analysis revealed that the particles contain mainly cobalt and oxygen (see Fig. S4 a). The nanoparticles deposited from complex [1] are very similar to those deposited from fluoroboryl-capped tris(glyoximato) diphenyl cobalt complex.6 We have also found that a solution of bisglyoximato dimethyl Co complex (Co(dmgBF2)2, 0.1 mM) leads, in the presence of 5 mM HClO4, to the formation of Co containing nanoparticles (ca. 200 nm diameter, Fig. S5) upon electrolysis at -0.94 V vs. SCE, thus illustrating that this robust molecular catalyst may also act as a pre-catalyst towards an active nano-structured material, depending on the applied potential and experimental conditions (acid content). In this connection, it has also recently been discovered that electrolysis of bis-glyoximato dimethyl Co complex leads, in phosphate buffered aqueous solutions (pH = 7), to the formation of dense cobalt nanoparticles on semi-conducting electrode surface phosphate buffered aqueous solutions (pH = 7).32

to a linear relationship over two orders of magnitude with a slope of 200 mV/decade (-dE/dlog(j)) and to an apparent exchange current density j0 = 10-5.7 A/cm2 (geometric surface). This value compares favorably to the one recently reported for an electrodeposited Co-based film at pH 7, 105.5 A/cm2 (the surface coverage is 10-6 mol of cobalt per cm2 for the electrodeposited film, vs. 10-5.5 mol per cm2 as determined by ICP for the metallic particles (see SI)).32 Note also that j0 is close to the value obtained for pure, metallic cobalt (Fig. 4). Finally, preparative scale electrolysis experiments with the activated GC film as electrode, poised at -1 V vs. SCE, led to the results summarized in Fig. 5a and 5b. The catalyst appears as stable over long electrolysis times and the faradaic yield for H2 production is 95 %. a 2.4

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Figure 4. (a) Cyclic voltammetry in water at pH 7 (0.1 M Phosphate buffer, v = 1 mV/s, stirred solution) of a carbon disk (black line), a cobalt disk (blue line) and the activated carbon disk from electrodeposition of Co nanoparticles (microelectrolysis at -0.9 V vs. SCE during 600 s in a 0.15 mM solution of Co(dpgBF2)2 [1] + 13.5 mM HClO4 in acetonitrile) (red line). (b) Tafel plot analysis: cobalt disk (blue line), modified carbon disk (red line). a 10

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Figure 3. SEM micrograph of nanoparticles deposited on glassy carbon surface (GC foil) after 16 h electrolysis at -0.9 V vs. SCE from an acetonitrile solution containing 2 mM Co(dpgBF2)2 + 100 mM HClO4. The catalytic activity toward H2 production in water of these nanoparticles deposited on a carbon electrode (either a GC film or a GC disk electrode) was investigated after surface preparation at a controlled potential of -0.9 V vs. SCE in acetonitrile. After electrolysis, the so-called activated electrode was rinsed and transferred under argon to water containing a phosphate buffer at pH 7 and used as working electrode for H2 evolution. At pH 7, overpotential (η) for proton reduction (defined as the difference between the apparent potential of the H+/H2 couple at pH 7, -0.42 V vs. NHE, and the potential necessary to obtain a given current density) is ca. 600 mV at 2.4 mA/cm2 (Fig. 4a), a performance similar to those obtained with nanoparticles electrodeposited from tris(glyoximato) diphenyl Co clathrochelate complex. A Tafel analysis (Fig. 4b) of the data recorded at low scan rate to avoid interference of mass transport leads

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Figure 5. Controlled potential electrolysis (-1 V vs. SCE) in water at pH 7 (0.5 M phosphate buffer), at a 1.2 cm2 doubleface GC foil working electrode. (a) Charge vs. time: nonactivated electrode (black line); electrode activated by deposition of cobalt containing nanoparticles (electrolysis at -0.9 V vs. SCE during 16 h in a 2 mM solution of Co(dpgBF2)2 and 60 mM HClO4 in acetonitrile). (b) H2 produced (µmol) at the activated electrode vs. time. We have found that the cobalt glyoximato complex Co(dpgBF2)2 is not catalytically active towards protons at the CoII/CoI wave at a graphite electrode in acetonitrile, in the presence of both weak and strong acids. With HClO4, the cobalt complex is electrochemically altered at potentials

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negative to -0.9 V vs. SCE, and one may not exclude some concurrent hydrogen evolution from a molecular active form of the complex occurs in the time scale of the voltammetric experiments. The reasons why the pre-catalyst is transformed stand from a series of electrochemically proton-coupled electron transfer reductions of the ligand (C=N double bonds reduction) and, then, at more negative potential (-0.9 V vs. SCE) the complex is definitively altered, presumably through the reductive cleavage of the bonds capping the coordination sphere (N-O bonds) ending into metallic nanoparticles electrodeposition, thus providing a general alteration scheme for metal complex catalysts comprising ligands prone to similar reduction processes. The resulting cobalt materials are efficient catalysts for the HER. This new example of a molecular complex being a precatalyst joins a growing list of related examples. These results may help in the understanding of previous electrocatalytic performance of the cobalt glyoximato molecular complexes in water 33-34 and it may also help in the design of precatalysts for getting new, active catalytic materials towards proton reduction and other electrocatalytic transformation of small molecules, like e.g., H2O, O2, N2 and CO2. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (C. C.); [email protected] (E. A. M.); [email protected] (M. R.). Univ Paris Diderot, Sorbonne Paris Cité, Laboratoire d'Electrochimie Moléculaire, UMR 7591 CNRS, 15 rue Jean-Antoine de Baïf, F75205 Paris Cedex 13, France. Fax: +33 1 57 27 87 88; Tel: + 33 (0) 1 57 27 - 87 90 (M. R.), - 87 84 (E. A. M.), - 87 96 (C. C.). ACKNOWLEDGMENT Financial support from the Agence Nationale de la Recherche Scientifique (ANR 2010 BLAN 0808) is gratefully acknowledged. Dr. Sophie Nowak (Univ. Paris Diderot) and Prof. Rémi Losno (Univ. Paris Diderot) are gratefully thanked for SEM/EDX, and ICP analysis respectively. ASSOCIATED CONTENT Supporting Information Available Experimental details (chemicals, synthesis, physical methods), Cyclic voltammograms of CoII/CoI wave (complex [1]) with CF3COOH and during electrolysis experiment, EDX and SEM analysis of activated surfaces. This information is available free of charge via the Internet at htpp://pubs.acs.org. REFERENCES 1. Eisenberg, R.; Nocera, D. G., Preface: Overview of the Forum on Solar and Renewable Energy. Inorg. Chem. 2005, 44, 6799-6801. 2. Magnuson, A.; Anderlund, M.; Johansson, O.; Lindblad, P.; Lomoth, R.; Polivka, T.; Ott, S.; Stensjö, K.; Styring, S.; Sundström, V. et al., Biomimetic and Microbial Approaches to Solar Fuel Generation. Acc. Chem. Res. 2009, 42, 1899-1909. 3. Du, P.; Eisenberg, R., Catalysts Made of EarthAbundant Elements (Co, Ni, Fe) for Water Splitting: Recent

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Page 6 of 6

TOC (graphic)

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

E = -0.9 V vs. SCE

(ACN + HClO4 )

2H+

2e–

Co

E = -1 V vs. SCE water, pH = 7

Co nanoparticles on electrode surface

H2

6


Cobalt-Bisglyoximato Diphenyl Complex as a Precatalyst for

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