Nickel phosphonate MOF as efficient water splitting photocatalyst
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Energy, Advanced Porous Materials Unit (APMU), Avda. Ramón de la Sagra 3, E-28935 Móstoles, Madrid, Spain Departamento de Química Inorgánica I. Fac. CC. Químicas, Universidad Complutense de Madrid, Avda. Complutense s/n, 28040 Madrid, Spain 3 Departamento de Química and Instituto de Tecnología Química (CSIC-UPV), Universitat Politècnica de València, C/Camino de Vera, s/n, 46022 Valencia, Spain 4 Institut Charles Gerhardt Montpellier, UMR 5253 CNRS UM ENSCM, Université Montpellier, Place E. Bataillon, 34095 Montpellier Cedex 05, France 2
© Tsinghua University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020 Received: 14 February 2020 / Revised: 14 July 2020 / Accepted: 14 August 2020
ABSTRACT A novel microporous two-dimensional (2D) Ni-based phosphonate metal-organic framework (MOF; denoted as IEF-13) has been successfully synthesized by a simple and green hydrothermal method and fully characterized using a combination of experimental and computational techniques. Structure resolution by single-crystal X-ray diffraction reveals that IEF-13 crystallizes in the triclinic space group Pī having bi-octahedra nickel nodes and a photo/electroactive tritopic phosphonate ligand. Remarkably, this material exhibits coordinatively unsaturated nickel(II) sites, free –PO3H2 and –PO3H acidic groups, a CO2 accessible microporosity, and an exceptional thermal and chemical stability. Further, its in-deep optoelectronic characterization evidences a photoresponse suitable for photocatalysis. In this sense, the photocatalytic activity for challenging H2 generation and overall water splitting in absence of any co-catalyst using UV–Vis irradiation and simulated sunlight has been evaluated, constituting the first report for a phosphonateMOF photocatalyst. IEF-13 is able to produce up to 2,200 mol of H2 per gram using methanol as sacrificial agent, exhibiting stability, maintaining its crystal structure and allowing its recycling. Even more, 170 mol of H2 per gram were produced using IEF-13 as photocatalyst in the absence of any co-catalyst for the overall water splitting, being this reaction limited by the O2 reduction. The present work opens new avenues for further optimization of the photocatalytic activity in this type of multifunctional materials.
KEYWORDS metal-organic framework, phosphonates, photocatalysis, water splitting
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Introduction
Metal-organic frameworks (MOFs) or porous coordination polymers (PCPs), combining a huge versatility and regular porosity [1–3], are promising multifunctional materials in different economically-key fields (separation, sensing, energy, biomedicine and catalysis, among others) [4–6]. While most of the reported structures are based on carboxylate and/or N-donor organic spacers; phosphonate-based MOFs (P-MOFs) are comparatively much less explored (the ratio of articles referring to P-MOFs vs. carboxylate-MOFs ≈ 1:12; according to Web of Science). This is mainly due to the non-trivial synthesis of phosphonates (e.g. limited commercial availability, poor solubility) and the diff
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