Uranium-mediated electrocatalytic dihydrogen production from water

Journal name:
Nature
Volume:
530,
Pages:
317–321
Date published:
DOI:
doi:10.1038/nature16530
Received
Accepted
Published online

Depleted uranium is a mildly radioactive waste product that is stockpiled worldwide. The chemical reactivity of uranium complexes is well documented, including the stoichiometric activation of small molecules of biological and industrial interest such as H2O, CO2, CO, or N2 (refs 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11), but catalytic transformations with actinides remain underexplored in comparison to transition-metal catalysis12, 13, 14. For reduction of water to H2, complexes of low-valent uranium show the highest potential, but are known to react violently and uncontrollably forming stable bridging oxo or uranyl species15. As a result, only a few oxidations of uranium with water have been reported so far; all stoichiometric2, 3, 16, 17. Catalytic H2 production, however, requires the reductive recovery of the catalyst via a challenging cleavage of the uranium-bound oxygen-containing ligand. Here we report the electrocatalytic water reduction observed with a trisaryloxide U(iii) complex [((Ad,MeArO)3mes)U] (refs 18 and 19)—the first homogeneous uranium catalyst for H2 production from H2O. The catalytic cycle involves rare terminal U(iv)–OH and U(v)=O complexes, which have been isolated, characterized, and proven to be integral parts of the catalytic mechanism. The recognition of uranium compounds as potentially useful catalysts suggests new applications for such light actinides. The development of uranium-based catalysts provides new perspectives on nuclear waste management strategies, by suggesting that mildly radioactive depleted uranium—an abundant waste product of the nuclear power industry—could be a valuable resource.

At a glance

Figures

  1. Electrochemical characterization of catalyst 1.
    Figure 1: Electrochemical characterization of catalyst 1.

    a, Cyclic voltammogram of the electrochemical H2O reduction in THF with 0.1 M TBAPF6 (20 μl H2O in 5 ml THF, 0.22 M) on a glassy carbon electrode, without catalyst (black) and with 0.4 mol% catalyst 1 (blue). The onset potential for the water reduction is reduced by 0.5 V, and the reductive current density at the vertex potential increases from −0.027 mA cm−2 to −0.382 mA cm−2 after addition of the catalyst (5 mg, 0.4 mol%). For comparison, the H2O reduction on a platinum electrode under similar conditions is shown (red). E is the potential, measured in volts. b, Plot of the charge Q passed during a 300-s electrolysis per run at different potentials E for uncatalysed H2O electrolysis (black), in the presence of 0.4 mol% catalyst 1 (blue), and 0.4 mol% UI3 (green), in THF with 0.1 M TBAPF6. c, Close-up of the Nyquist plot in d for H2O electrolysis at −3.25 V (versus Fc+/Fc) with and without catalyst 1, focusing on the catalysed reaction, in which Z′ corresponds to the real part and Z″ to the imaginary part of the impedance Z. Nyquist plots were simulated with the instrument’s software to extract resistances and capacities (see Supplementary Information). d, Nyquist plot of the uncatalysed H2O electrolysis at −3.25 V (versus Fc+/Fc). The charge transfer resistance is three orders of magnitude greater in the uncatalysed reaction than it is in the catalysed reaction, demonstrating the catalytic effect of compound 1.

  2. Independent synthesis and characterization of the uranium(IV) hydroxo complex [((Ad,MeArO)3mes)U–OH] (2–OH).
    Figure 2: Independent synthesis and characterization of the uranium(IV) hydroxo complex [((Ad,MeArO)3mes)U–OH] (2–OH).

    a, Synthesis of 2–OH with concomitant H2 evolution. b, Molecular structure of the crystallographically characterized complex 2–OH in crystals of C67H84O5U · 3(C4H8O), with thermal ellipsoids at 50% probability. All hydrogen atoms except for the hydroxo H were omitted for clarity. c, Infrared vibrational spectra of 2–OH (black) and its isotopomer 2–OD (blue), showing the expected isotopic shift for the O–H stretching vibration ν. The inset is a close-up of the 2–OH spectrum, showing the two OH stretching frequencies at ν = 3,659 cm−1 and ν = 3,630 cm−1.

  3. Postulated mechanism for the reduction of H2O by the U(iii) complex 1, based on EPR results.
    Figure 3: Postulated mechanism for the reduction of H2O by the U(iii) complex 1, based on EPR results.

    The addition of H2O to 1 probably yields a U(iii) aquo species, which forms a fleeting U(v) hydroxo–hydrido intermediate, [((Ad,MeArO)3mes)U(OH)(H)], by intramolecular insertion; this hydroxo–hydrido species then decays to a U(v) oxo species by elimination of H2 (reaction (1)). Subsequently, the U(iv) hydroxo complex 2–OH is formed in a comproportionation reaction between the U(v) oxo and the U(iii) aquo species (reaction (2)). In the net reaction, two U(iii) aquo complexes form two molecules of 2–OH and one equivalent H2.

  4. X-band EPR spectrum of a frozen 10 mM toluene solution of 1 with a sub-stoichiometric amount of H2O.
    Figure 4: X-band EPR spectrum of a frozen 10 mM toluene solution of 1 with a sub-stoichiometric amount of H2O.

    The EPR data show a convoluted spectrum of two species: the U(iii) starting material and a well-defined rhombic species, tentatively assigned to the fleeting U(v) hydroxo–hydrido species. Experimental conditions are as follows: temperature T = 7.5 K, frequency ν = 8.96286 GHz, power P = 1 mW, modulation width of 1.0 mT. The experimental spectrum (black) and simulation (red) under these conditions are shown. The best fit for the experimental spectrum is a convolution of the signal of 1 in toluene (simulated, green; g values at g1 = 1.56, g2 = 1.48, g3 = 1.20, with line widths of W1 = 21.4 mT, W2 = 30.5 mT, W3 = 14.4 mT; relative weight of 1.0) and the signal of an additional, rhombic transient U(v) species (simulated, blue; g values at g1 = 2.73, g2 = 1.83, g3 = 1.35, with line widths of W1 = 18.9 mT, W2 = 25.5 mT, W3 = 26.5 mT; relative weight of 0.70). The spectra are offset for ease of viewing.

  5. Postulated electrocatalytic cycle for H2 generation from H2O in the presence of the homogeneous U(iii) catalyst [((Ad,MeArO)3mes)U] (1).
    Figure 5: Postulated electrocatalytic cycle for H2 generation from H2O in the presence of the homogeneous U(iii) catalyst [((Ad,MeArO)3mes)U] (1).

    Step 1 (top to bottom-right), H2 evolution and formation of [((Ad,MeArO)3mes)U(OH)(THF)] (2–OH) through oxidation of 1 with H2O. Step 2 (bottom-right to bottom-left), electrochemical reduction of 2–OH, forming the transient anion 2–OH. Step 3 (bottom-left to top), elimination of OH from 2–OH to regenerate catalyst 1.

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Author information

Affiliations

  1. Department of Chemistry and Pharmacy, Inorganic Chemistry, Friedrich-Alexander University Erlangen-Nürnberg (FAU), Egerlandstrasse 1, D-91058 Erlangen, Germany

    • Dominik P. Halter,
    • Frank W. Heinemann,
    • Julien Bachmann &
    • Karsten Meyer

Contributions

D.P.H., J.B., and K.M. planned the research and prepared the manuscript. D.P.H. performed the experiments. F.W.H conducted the XRD analyses and refined structures. K.M. supervised the project in all aspects.

Competing financial interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to:

Atomic coordinates and structure factors for the reported crystal structures have been deposited in the Cambridge Crystallographic Data Centre under the accession code CCDC-1413741 (for 2–OH from THF/n-pentane), CCDC-1401838 (for 2–OH from THF), and CCDC-1437872 (for [((Ad,tBuArO)3tacn)U(OH)]).

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  1. Supplementary Information (4.2 MB)

    This file contains Supplementary Text and Data, Supplementary Figures 1-34, Supplementary Tables 1-5 and Supplementary References – see contents for details.

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