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Halide Electrocatalysts for Efficient Chlorine Evolution Reaction

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Halide Electrocatalysts for Efficient Chlorine Evolution Reaction Chemical Science View Article Online EDGE ARTICLE View Journal | View Issue Macrocycle-assisted synthesis of non- stoichiometric silver(I) halide electrocatalysts for Cite this: Chem. Sci.,2017,8, 5662 efficient chlorine evolution reaction† Qiong-You Zhang, ‡ Xin He ‡ and Liang Zhao * The electrocatalytic oxidation of chloride to chlorine is a fundamental and important electrochemical reaction in industry. Herein we report the synthesis of non-stoichiometric silver halide nanoparticles through a novel macrocycle-assisted bulk-to-cluster-to-nano transformation. The acquired positively charged nanoparticles expedite chloride transportation by electrostatic attraction and facilitate the formation of silver polychloride catalytic species on the surface, thus functioning as efficient and selective electrocatalysts for the chlorine evolution reaction (CER) at a very low overpotential and within Received 7th February 2017 a wide concentration range of chloride. The formation of uncommon non-stoichiometric nanoparticles Accepted 9th June 2017 prevents the formation of a AgCl precipitate and exposes more coordination unsaturated silver atoms to DOI: 10.1039/c7sc00575j Creative Commons Attribution-NonCommercial 3.0 Unported Licence. catalyze CER, finally causing a large enhancement of the atomic catalytic efficiency of silver. This study rsc.li/chemical-science showcases a promising approach to achieve efficient catalysts from a bottom-up design. Introduction based anodes tend to be deactivated by surface poisoning8 and are likely to degrade under harsh conditions (low pH, high À À The chlorine evolution reaction (CER, 2Cl / Cl2 +2e ) is one current density, etc.). Therefore, homogeneous catalysts, such of the most important electrochemical reactions in industry as polypyridine Ru-aqua complexes,9 Ru-centered coordination (e.g. the chlor-alkali process).1 The great signicance of CER, as polymer derivatives,10 and ferrocene in micellar media11 are This article is licensed under a reected by the extensive applications of chlorine in polymers, desirable in order to avoid instability. Although silver(I) has long disinfectants, and drugs etc.,2 has stimulated a tremendous been recognized as an excellent oxidant and applied in many number of studies focused on developing efficient catalysts and oxidative coupling reactions,12 it has been seldom employed as 3,4 Open Access Article. Published on 09 June 2017. Downloaded 9/24/2021 9:56:52 PM. an in-depth understanding of related catalytic mechanisms. an electrocatalyst for CER due to the easy formation of AgCl ¼ Â À10 Moreover, CER has also been deemed as a potential alternative precipitate (Ksp 1.77 10 at 25 C) and the high potential + À o ¼ to the oxidation of water to oxygen (2H2O / 4H +O2 +4e )in required for accessing the Ag(II/I) couple (E 1.98 V). Recently, a water-splitting cell because the two-electron CER would Chen and co-workers manifested that the silver(I) polychloride À 2À consume less energy than the four-electron oxygen evolution species [AgCl2] and [AgCl3] provided access to the higher reaction (OER). From a thermodynamic viewpoint, OER (equi- formal oxidation states of silver by delocalizing the oxidative librium potential Eo ¼ 1.23 V vs. NHE) is preferred over CER charge over the chloride anions and thus catalyzed CER at a low (Eo ¼ 1.36 V).5 Therefore, in addition to the pursuit of efficient overpotential.13 However, a high concentration of chloride is catalysts for making CER proceed at low overpotentials, much required as a prerequisite to avoid AgCl precipitation and to nÀ1 ¼ – effort has also been devoted to achieving high selectivity of the form the catalytic species [AgCln] (n 2 4). Herein, we anodic reaction toward CER over OER.6 disclose a facile synthesis of non-stoichiometric silver halide (mÀn)+ ¼ Among the existing classes of electrocatalysts for CER, RuO2, nanoparticles [AgmXn] (m > n,X Cl, Br, I) through a novel together with iridium and titanium oxides, constitute the most macrocycle-assisted bulk-to-cluster-to-nano transformation À 7 (m n)+ extensively applicable catalyst family. However, such RuO2- (Scheme 1). The [AgmXn] nanoparticles can be steadily dispersed in NaCl solution without the formation of AgCl (mÀn)+ The Key Laboratory of Bioorganic Phosphorus Chemistry & Chemical Biology, precipitate. The positively charged nature of [AgmXn] Department of Chemistry, Tsinghua University, Beijing 100084, China. E-mail: expedites chloride transportation by electrostatic attraction and [email protected] facilitates the formation of a silver polychloride catalytic species † Electronic supplementary information (ESI) available: Supporting gures, on the surface of the nanoparticles, thus functioning as a highly high-resolution ESI-MS, TEM images, CV plots and NMR, FT-IR, UV-vis, XPS efficient and selective electrocatalyst for CER at a very low and AES spectra. Renement details and crystal data for 1–3 are also available. – overpotential (10 mV) and within a wide concentration range of CCDC 1527294 1527296. For ESI and crystallographic data in CIF or other – electronic format see DOI: 10.1039/c7sc00575j chloride (0.05 1 M). The non-stoichiometric elemental ratio ‡ These authors contributed equally. between the silver and halogen atoms in the newly synthesized 5662 | Chem. Sci.,2017,8, 5662–5668 This journal is © The Royal Society of Chemistry 2017 View Article Online Edge Article Chemical Science Scheme 1 Macrocycle-assisted bulk-to-cluster-to-nano trans- formation for the fabrication of non-stoichiometric silver halide nanoparticles as electrocatalysts for CER. nanoparticles makes the coordination unsaturated silver atoms easily exposed to catalyze the chloride oxidation, consequently Fig. 1 Crystal structures and silver halide cluster core structures of (a) fl resulting in a large enhancement of the atomic catalytic effi- 1, (b) 2 and (c) 3. Some uncoordinated tri ates are omitted for clarity. Color coding: Ag, purple; C, black; H, gray; N, blue; O, red; F, cyan; S, ciency of silver. yellow; Cl, green; Br, brown; I, dark yellow. Results and discussion ˚ 2.725(1)–2.740(1) A. The expansion of the Ag3 triangle from 1 to Creative Commons Attribution-NonCommercial 3.0 Unported Licence. To date the synthesis of stoichiometric AgX nanoparticles has 3 results in the inclusion of one more silver atom capping the – been extensively reported via direct precipitation,14 reverse silver triangle in 3 through a linkage of Ag I coordination and – ˚ 19 micellar/microemulsion,15 hydrothermal16 methods, etc. It is threefold argentophilic interactions (2.867(4) 2.909(4) A). The ˚ a formidable challenge to achieve non-stoichiometric silver iodide anion nally bonds to four silver atoms by 0.92 A above – – halide complexes because the stoichiometric compounds AgX the Ag1 Ag2 Ag2A plane. are thermodynamically stable. We envisioned that a polydentate The formation of the silver halide clusters [Ag3–4X] inside Py macrocyclic ligand would facilitate the formation of a poly- [7] in solution was also con rmed by electrospray ionization mass spectrometry (ESI-MS) and elemental analysis (see Fig. S1– nuclear silver halide cluster [AgnX] (n > 1) inside it. The subse- This article is licensed under a S3 in the ESI for details†). The ESI-MS spectrum of 1 displayed quent aggregation of [AgnX] would engender the formation of two isotopically well-resolved peaks at m/z ¼ 1251.00 and non-stoichiometric silver halide complexes, as shown in + 551.03, corresponding to [(Ag3Cl)(Py[7])(CF3SO3)] and [(Ag3- Scheme 1. With reference to our previous synthetic methods for 2+ 17 Cl)(Py[7])] , respectively. In addition, the other two observed Open Access Article. Published on 09 June 2017. Downloaded 9/24/2021 9:56:52 PM. silver-sulde clusters, azacalix[7]pyridine (Py[7]) was applied ¼ as an outer template to induce the formation of silver halide peaks at m/z 1506.86 and 678.95 can be ascribed to the above two charged species plus a silver triate group, respectively clusters. Diffusion of diethyl ether into the AgX-AgCF3SO3-Py[7] ¼ (Fig. S1†). Similarly, several [Ag3Br]-related isotopically well- (X Cl, Br and I) mixture produced yellow crystals of three silver + resolved peaks consistent with [(Ag3Br)(Py[7])(CF3SO3)] (m/z ¼ halide cluster complexes. X-ray crystallographic analysis 2+ 1295.45), [(Ag3Br)(Py[7])] (m/z ¼ 573.19), and [(Ag3Br)(Py revealed their formulae as [Ag4Cl(CF3SO3)3(Py[7])(CH3OH)] (1), + [7])(CF3SO3) + (AgCF3SO3)] (m/z ¼ 1552.39) were observed in [Ag5Br(CF3SO3)2(H2O)4(Py[7])](CF3SO3)2$H2O(2) and [Ag4I(H2- † O) (Py[7])](CF SO ) (3). As shown in Fig. 1, 1–3 all comprise the ESI-MS spectrum of complex 2 (Fig. S2 ). These ESI-MS 2 3 3 3 a central halide that is encompassed by three or four silver experiments con rm the dominant presence of the Py[7]-stabi- atoms. Each halogen-bonded silver atom is coordinated by one lized [Ag3Cl] and [Ag3Br] clusters in the solutions of 1 and 2 and or two pyridyl nitrogen atoms, plus oxygen atoms of triate the possible involvement of a silver tri ate molecule, respec- – groups and solvent molecules, and is further supported by tively. The ESI-MS spectrum of the AgI AgCF3SO3-Py[7] reaction ¼ – p – – ˚ mixture revealed two peaks at m/z 1342.94 and 595.99, cor- silver aromatic interactions (Ag C distances: 2.515 2.705 A). + 2+ The restricted coordination of Py[7] gives rise to a systematic responding to the [(Ag3I)(Py[7])(CF3SO3)] and [(Ag3I)(Py[7])] † variation of the silver–halide bonding from 1 to 3. In complex 1, species, respectively (Fig. S3 ). This result is not consistent with the Ag–Cl bond lengths are in the range of 2.473(3)–2.560(3) A,˚ the [Ag4I] cluster, as shown in the crystal structure of 3, ˚ although the composition of complex 3 has been further evi- approximately 0.2 A shorter than in reported pyramidal [Ag3Cl] clusters.18 Thus, the chlorine atom is 0.34 A˚ above the Ag1–Ag2– denced by elemental analysis.
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