A molecular molybdenum-oxo catalyst for generating hydrogen from water

Journal name:
Nature
Volume:
464,
Pages:
1329–1333
Date published:
DOI:
doi:10.1038/nature08969
Received
Accepted

A growing awareness of issues related to anthropogenic climate change and an increase in global energy demand have made the search for viable carbon-neutral sources of renewable energy one of the most important challenges in science today1. The chemical community is therefore seeking efficient and inexpensive catalysts that can produce large quantities of hydrogen gas from water1, 2, 3, 4, 5, 6, 7. Here we identify a molybdenum-oxo complex that can catalytically generate gaseous hydrogen either from water at neutral pH or from sea water. This work shows that high-valency metal-oxo species can be used to create reduction catalysts that are robust and functional in water, a concept that has broad implications for the design of ‘green’ and sustainable chemistry cycles.

At a glance

Figures

  1. Reaction of [lsqb](PY5Me2)Mo(CF3SO3)[rsqb]1+ with water to form [lsqb](PY5Me2)MoO[rsqb]2+ and release H2.
    Figure 1: Reaction of [(PY5Me2)Mo(CF3SO3)]1+ with water to form [(PY5Me2)MoO]2+ and release H2.

    The generation of H2 was confirmed by mass spectrometry, and the oxo ligand was shown to originate from water through observation of the expected isotopic shift for νMo = O in the infrared spectrum, using H218O. The structures depicted are the results of single-crystal X-ray analyses of compounds 5 and 6, with green, yellow, light blue, red, blue and grey spheres representing Mo, S, F, O, N and C atoms, respectively; H atoms are omitted for clarity. Selected interatomic distances and angles for compounds 5 and 6 are as follows. Mo–O: 2.117(9)Å, 1.685(9)Å; mean Mo–Nequatorial: 2.14(2)Å, 2.154(3)Å; Mo–Naxial: 2.097(9)Å, 2.297(8)Å; mean O–Mo–Nequatorial: 93(3)°, 98(1)°; O–Mo–Naxial: 176.2(4)°, 179(1)°; mean Nequatorial–Mo–Nequatorial: 90(10)°, 89(8)°.

  2. Cyclic voltammograms of compounds 2 and 7.
    Figure 2: Cyclic voltammograms of compounds 2 and 7.

    a, A 5mM acetonitrile solution of 2. b, A 2mM acetonitrile solution of compound 7. c, A 4.2µM aqueous solution of compound 7. Measurements in a and b were performed using 0.1M (Bu4N)PF6 as the electrolyte with a scan rate of 100mVs-1, whereas the measurement in c was performed in 1M KCl with a scan rate of 100mVs-1. In b and c, red lines indicate the initial scans and black lines indicate subsequent scans.

  3. Electrochemical data for a 7.7[thinsp][micro]M solution of [lsqb](PY5Me2)MoO[rsqb](PF6)2 (7) in a 0.6[thinsp]M phosphate buffer at pH[thinsp]7.
    Figure 3: Electrochemical data for a 7.7µM solution of [(PY5Me2)MoO](PF6)2 (7) in a 0.6M phosphate buffer at pH7.

    a, Cyclic voltammograms of the buffer with (red line) and without (blue line) compound 4 at a scan rate of 50mVs-1. b, Charge build-up versus time at various overpotentials. c, Turnover frequency versus overpotential. The background solvent activity has been subtracted from the plots in b and c. Overpotential = |applied potential minus E(pH7)|. Turnover frequency calculations assume (see Supplementary Fig. 6) that every electron is used for the generation of hydrogen, and provide only a lower bound, given that not all catalyst molecules are in proximity to the electrode surface at a given time.

  4. Extended electrolysis.
    Figure 4: Extended electrolysis.

    Electrolysis data for a 2μM solution of [(PY5Me2)MoO](PF6)2 (compound 7) in a 3M pH7 phosphate buffer, showing charge build-up and turnover number versus time (red circles), and data for the buffer solution alone showing charge build-up versus time (blue circles) with the cell operating at a potential of -1.40V versus SHE.

  5. Speculative electrocatalytic cycle for the reduction of water to release hydrogen and hydroxide anions.
    Figure 5: Speculative electrocatalytic cycle for the reduction of water to release hydrogen and hydroxide anions.

    Although formal metal oxidation states are given for electron counting purposes, it should be recognized that there is probably significant delocalization of the added electrons onto the PY5Me2 ligand. An alternative cycle, in which each reduction step is immediately followed by or even coupled to a proton transfer, is depicted in Supplementary Fig. 7.

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Author information

Affiliations

  1. Department of Chemistry, University of California, Berkeley, California 94720, USA

    • Hemamala I. Karunadasa,
    • Christopher J. Chang &
    • Jeffrey R. Long
  2. Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

    • Hemamala I. Karunadasa,
    • Christopher J. Chang &
    • Jeffrey R. Long
  3. Howard Hughes Medical Institute, University of California, Berkeley, California 94720, USA

    • Christopher J. Chang

Contributions

H.I.K., C.J.C. and J.R.L. planned the research, and H.I.K. performed the experiments. H.I.K., C.J.C. and J.R.L. prepared the manuscript.

Competing financial interests

The authors declare no competing financial interests.

Corresponding authors

Correspondence to:

X-ray coordinates from the crystal structure determinations have been deposited with the Cambridge Crystallographic Data Centre with reference codes 720362 (compound 1), 720363 (compound 2), 753993 (compound 5), 753992 (compound 6) and 720364 (compound 7).

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Supplementary information

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  1. Supplementary Information (742K)

    This file contains Supplementary Methods, a Supplementary Discussion, Supplementary Tables S1-S2, Supplementary Figures S1-S9 with legends and References.

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