Immobilized Cobalt Bis(benzenedithiolate) Complexes: Exceptionally

Sep 21, 2017 - Quantitative comparisons, based on this approach, are hampered by the fact that consistency in the RGO coverage and topography is gener...
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Immobilized Cobalt Bis(benzenedithiolate) Complexes: Exceptionally Active Heterogeneous Electrocatalysts for Dihydrogen Production from Mildly Acidic Aqueous Solutions Shawn C. Eady, Molly M. MacInnes, and Nicolai Lehnert* Department of Chemistry, University of Michigan, 930 North University Avenue, Ann Arbor, Michigan 48109, United States S Supporting Information *

ABSTRACT: A series of cobalt bis(benzenedithiolate) complexes with varying benzenedithiolate (general abbreviation: bdt2−) ring substitutions (S2C6X42−) were prepared and adsorbed on inexpensive electrodes composed of (a) reduced graphene oxide (RGO) electrodeposited on fluorine-doped tin oxide (FTO) and (b) highly ordered pyrolytic graphite (HOPG). The catalystadsorbed electrodes are characterized by X-ray photoelectron spectroscopy. Catalyst loading across the ligand series improved notably with increasing halide substitution [from 2.7 × 10−11 mol cm−2 for TBA[Co(S2C6H4)2] (1) to 6.22 × 10−10 mol cm−2 for TBA[Co(S2C6Cl4)2] (3)] and increasing ring size of the benzenedithiolate ligand [up to 3.10 × 10−9 mol cm−2 for TBA[Co(S2C10H6)2] (6)]. Electrocatalytic analysis of the complexes immobilized on HOPG elicits a reductive current response indicative of dihydrogen generation in the presence of mildly acidic aqueous solutions (pH 2−4) of trifluoroacetic acid, with overpotentials of around 0.5 V versus SHE (measured vs platinum). Rate constant (kobs) estimates resulting from cyclic voltammetry analysis range from 24 to 230 s−1 with the maximum kobs for TBA[Co(S2C6H2Cl2)2] (2) at an overpotential of 0.59 V versus platinum. Controlled-potential electrolysis studies performed in 0.5 M H2SO4 at −0.5 V versus SHE show impressive initial rate constants of over 500 s−1 under bulk electrolysis conditions; however, steady catalyst deactivation over an 8 h period is observed, with turnover numbers reaching 9.1 × 106. Electrolysis studies reveal that halide substitution is a central factor in improving the turnover stability, whereas the ring size is less of a factor in optimizing the long-term stability of the heterogeneous catalyst manifolds. Catalyst deactivation is likely caused by catalyst desorption from the electrode surfaces.

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catalysts immobilized on the electrode surfaces.7−10 The latter approach has the advantage that the molecular catalysts can be improved in a systematic manner and derivatized to meet given application needs, as demonstrated in this work. However, a substantial challenge in this regard is to design suitable interfaces that allow for the easy, yet stable and affordable, attachment of the catalysts to the electrode surfaces. In this paper, we further demonstrate that reduced graphene oxide (RGO) films can serve as inexpensive, yet extremely versatile, interfaces to adsorb a variety of cobalt bis(benzenedithiolate), [CoIII(bdt)2]2−, dihydrogen production catalysts to semiconductor electrode surfaces, yielding robust and highly active platforms for electrocatalytic dihydrogen generation.

INTRODUCTION Devices capable of generating dihydrogen from a renewable energy source are crucial for meeting the global dihydrogen consumption demand in a sustainable way and for efforts to replace carbon-based fuel technologies with more sustainable alternatives.1−4 Large-scale dihydrogen production from protons and electrons, whether directly coupled to water oxidation photoanodes or electolyzers powered by renewable energy sources, must inevitably be facilitated by heterogeneous catalysts to allow for application of the catalysts in large-scale flow reactors.5−7 To this end, electrocatalysts for dihydrogen production must be designed that meet the demands of these systems and, at the same time, are inexpensive and easy to assemble and that can be prepared on a large scale without difficulty. These electrocatalysts could be built either from catalytically active electrode materials or from molecular © 2017 American Chemical Society

Received: June 25, 2017 Published: September 21, 2017 11654

DOI: 10.1021/acs.inorgchem.7b01589 Inorg. Chem. 2017, 56, 11654−11667

Article

Inorganic Chemistry In addition to the interface, fast, efficient, and inexpensive electrocatalysts are necessary to obtain platinum-free manifolds for dihydrogen production. Previous research in the renewable energy field has “unearthed” a number of inexpensive iron, nickel, and cobalt electrocatalysts for dihydrogen production.2,4,11−55 Solid-state and molecular catalysts alike have displayed impressive proton reduction activity, rivaling platinum in overpotential (η), turnover frequency (TOF), and turnover number (TON).46,47,56−62 In some cases, these systems have nearly matched or even exceeded platinum with regard to stability in poisoning conditions or for dual catalysis applications, a milestone that should be held in high regard, considering the decades of catalyst development spent toward platinum substitution.1,46,47,63 Still, the design of versatile and inexpensive heterogeneous catalyst manifolds remains a major challenge for the development of cost-efficient dihydrogen production systems.48,49,64 Cobalt(III) bis(benzenedithiolate) and corresponding cobalt(III) bis(dithiolene) complexes, discovered nearly 5 decades ago, have historically been researched for various applications based on their strong absorption features and unique electronic structures.26,27,50,65−76 Recently, these complexes have been found to be active for dihydrogen production in acidic organic solutions by McNamara et al., with a maximum TOF of 3400 h−1 and a TON of 9000 in 12 h in solution.26,27 Later, we reported adsorption of a molecular cobalt bis(benzenedithiolate) species onto graphitic supports to provide a heterogeneous proton reduction system with high activity from mildly acidic aqueous solutions.77 In parallel studies, these catalyst systems have also been investigated as two-dimensional polymer materials on graphitic supports, as well as one-dimensional light-permeable materials on silicon surfaces for photoelectrocatalysis.67,68 Curiously, these heterogeneous manifolds, whether prepared directly by the adsorption of molecular catalysts onto a support or designed from a polymer material transferred to a surface, show substantially higher TOFs than the corresponding catalysts in homogeneous phase. As demonstrated in this work, cobalt bis(benzenedithiolate) based heterogeneous catalyst manifolds approach even the catalytic activities of nickel bis(diphosphine) systems (albeit under different conditions), which have become the gold standard for homogeneous and heterogeneous dihydrogen production systems in a number of applications.19,46,47,49,78 Moreover, cobalt bis(benzenedithiolate) complexes provide versatility for incorporation into devices because both the graphitic supports (e.g., graphene and RGO) used to adsorb these complexes and their polymeric forms can be immobilized on a variety of electrode materials. While the polymeric systems potentially offer more variety in the choice of electrode materials, one challenge with such materials is functionalization and thereby the systematic improvement of their catalytic performance. For this reason, investigation of the molecular catalysts, which are easily derivatized, is of great interest to determine the effects of varying the functionality on the overall performance of the immobilized cobalt bis(benzenedithiolate) systems, which is the key point of this study. Herein, we report a series of cobalt bis(benzenedithiolene) derivatives (Chart 1) adsorbed onto graphitic surfaces and investigations into their dihydrogen production activity in acidic aqueous solutions, including the quantification of catalyst loadings and activity data. Because of the ease of preparation and potential future applications of these systems, initial studies

Chart 1. Cobalt Bis(benzenedithiolate) Derivatives

were conducted on RGO electrodeposited on fluorine-doped tin oxide (FTO), which provides a design strategy of how these catalysts could be immobilized on a variety of supports, especially semiconductor electrodes, using RGO as the universal interface. Quantitative comparisons, based on this approach, are hampered by the fact that consistency in the RGO coverage and topography is generally low and there is a lack of stability of the FTO electrodes at low pH. To complement these tests, the catalysts were also adsorbed on highly ordered pyrolytic graphite (HOPG) electrodes. HOPG electrodes provide a more consistent surface area and topography, therefore allowing for a quantitative comparison between different catalyst derivatives in the surface-immobilized state. The resulting series of heterogeneous electrocatalysts provides detailed insight into the effect of benzenedithiolate ligand substitution on the surface adsorption and coverage, electrocatalytic activity, and lifetime of the catalysts, allowing us to optimize the catalytic properties of these complexes.

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EXPERIMENTAL SECTION

General Methods. Chemicals were of the highest purity grade commercially available and were used without further purification (unless mentioned). Methanol (anhydrous, ACS grade) was purchased from Fisher, distilled over calcium hydride, and then degassed via extended dinitrogen purges prior to use. Acetonitrile (ACS grade), sodium methoxide, trifluoroacetic acid (TFA), sulfuric acid, potassium ferricyanide, and potassium ferrocyanide were purchased from Fisher. 1,2-Benzenedithiol, toluene-3,4-dithiol, and 3,6-dichloro-1,2-benzenedithiol were purchased from Sigma and used without further purification. Graphite powder was purchased from MTI Corp. All procedures were performed under a dinitrogen atmosphere unless otherwise specified. Preparation of Dithiol Ligand Derivatives. Tetrahalidedithiol ligands were prepared using a slightly modified method from that reported by Gray and co-workers.65 Naphthalenedithiol was prepared by first following procedures reported by Hart et al. to give odibromonaphthalene, followed by alkylthiol formation and alkyl cleavage as outlined by Montanucci and co-workers.79,80 3,4,5,6-Tetrachlorobenzenedithiol. A mixture of hexachlorobenzene (1 g, 3.5 mmol), sodium hydrogen sulfide (0.75 g, 13 mmol), sulfur (0.08 g, 2.5 mmol), and iron powder (0.18 g, 3.2 mmol) was heated in 50 mL of N,N-dimethylformamide at 140 °C for 8 h. After cooling, 100 mL of distilled water was added, and the mixture was allowed to briefly stir at room temperature, during which time a black precipitate formed. The precipitate was filtered off, washed with water, and then dried in vacuo. The dried solid was added to a suspension of zinc oxide in a 1:1 methanol/1 M aqueous sodium hydroxide solution, which was refluxed for 1 h. After cooling, the mixture was filtered, and the yellow filtrate was acidified with a 1 M HCl solution to precipitate the dithiol product. The solid was dried and recrystallized from benzene to afford 3,4,5,6-tetrachlorobenzenedithiol 0.44 g (45% yield) 11655

DOI: 10.1021/acs.inorgchem.7b01589 Inorg. Chem. 2017, 56, 11654−11667

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Inorganic Chemistry as a pale-yellow powder. 1H NMR (400 MHz, CD2Cl2): δH 4.79 (s, 2H). 3,4,5,6-Tetrafluorobenzenedithiol. A mixture of 1,2-dibromo3,4,5,6-tetrafluorobenzene (1 g, 3.2 mmol), sodium hydrogen sulfide (0.75 g, 13 mmol), sulfur (0.08 g, 2.5 mmol), and iron powder (0.18 g, 3.2 mmol) was heated in 50 mL of N,N-dimethylformamide at 120 °C for 8 h. After cooling, 100 mL of distilled water was added, and the mixture was allowed to briefly stir at room temperature, during which time a black precipitate formed. The precipitate was filtered off, washed with water, and then dried in vacuo. The dried solid was added to a suspension of zinc oxide in a 1:1 methanol/1 M aqueous sodium hydroxide solution, which was refluxed for 1 h. After cooling, the mixture was filtered, and the yellow filtrate was acidified with a 1 M HCl solution to precipitate the dithiol product. The solid was dried and recrystallized from benzene to afford 0.12 g (17% yield) of 3,4,5,6tetrafluorobenzenedithiol as a pale-yellow powder. 1H NMR (400 MHz, CD2Cl2): δH 4.03 (s, 2H). 6,7-Dibromo-1,4-dihydronaphthalene-1,4-epoxide. To a stirred solution of 1,2,4,5 tetrabromobenzene (8 g, 20 mmol) and furan (10 mL) in dry toluene (200 mL) at −23 °C under argon was added dropwise over 3 h n-BuLi (22 mmol in 200 mL of hexane). After the mixture slowly warmed to room temperature, methanol (1 mL) was added, the mixture was washed with water and dried, and the solvent was removed (rotavap). The resulting yellow oily solid was triturated with hexane to give an off-white solid. This crude product was chromatographed on silica gel using dichloromethane/hexane (1:1) as the eluent to give 0.7 g (74% yield) of 6,7-dibromo-1,4dihydronaphthalene-1,4-epoxide as a white powder. 1H NMR (400 MHz, CD2Cl2): δH 5.62 (s, 2H), 6.95 (s, 2H), 7.44 (s, 2H). 1,2-Dibromonaphthalene. A suspension of zinc powder (2 g) in 60 mM dry tetrahydrofuran (THF) under a dinitrogen atmosphere was cooled to 0 °C. Titanium tetrachloride (2 mL) was added dropwise, and the mixture was heated to reflux for 30 min. The reaction was then again cooled to 0 °C, and a solution of 6,7-dibromo-1,4dihydronaphthalene-1,4-epoxide (1.0 g, 3.3 mmol) in 20 mL of THF was added dropwise. The mixture was refluxed overnight, cooled, and poured into 100 mL of cold 10% HCl. The mixture was extracted with dichloromethane, washed with water, dried with sodium sulfate, and reduced in vacuo to an off-white powder. The crude product was chromatographed on silica gel using dichloromethane/hexane (1:1) as the eluent to afford 0.7 g (74% yield) of 1,2-dibromonaphthalene as a white powder. 1H NMR (400 MHz, CD2Cl2): δH 8.15 (s, 2H), 7.75 (m, 2H,), 7.5 (m, 2H). 1,2-Naphthalenedithiol. A solution of 1,2-dibromonaphthalene (1 g, 3.5 mmol) and sodium propanethiolate (1.7 g, 17.5 mmol) in 50 mL of N,N-dimethylacetamide was stirred under a dinitrogen atmosphere at 100 °C for 12 h. Sodium metal (0.6 g, 26 mmol) was cut into small pieces and added to the solution, which was allowed to continue stirring at 100 °C for 12 h. The resulting mixture was poured into 100 mL of a 0.1 M HCl solution and subsequently extracted three times with ether. The combined organic layer was washed with water, dried with sodium sulfate, and reduced in vacuo to give 0.6 g (89% yield) of 1,2-naphthalenedithiol as a yellow solid. 1H NMR (400 MHz, CD2Cl2): δH 7.9 (s, 2H), 7.42 (m, 2H), 6.64 (m, 2H), 3.95 (s, 2H). Preparation of Cobalt Bis(benzenedithiolate) Derivatives. 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DOI: 10.1021/acs.inorgchem.7b01589 Inorg. Chem. 2017, 56, 11654−11667