Energy and Hydrogen

Figure 1. a) Structure of [NiFe] hydrogenase from D. gigas (PDB 1FRV). The two subunits are indicated in blue (small subunit) and green (large subunit). The structure of the active site is shown enlarged at the bottom (see text). The arrow indicates the 6th coordination site at Ni which is found to be unoccupied. b) Structure of [FeFe] hydrogenase from D. desulfuricans (PDB 1HFE). The H-cluster (hydrogen-activating cluster) and the two additional [4Fe-4S] clusters are all located in the large subunit. The molecular structure of the H-cluster with the cubane [4Fe-4S]H and the dinuclear [2Fe]H subclusters is shown enlarged at the bottom. The arrow indicates the free coordination site at the distal iron Fed. The figure is reproduced from Lubitz , W.; Reijerse, E.J.; van Gastel, M. Chem. Rev. 2007, 107, 4331-4365.

The hydrogen economy represents a way of fulfilling our need for energy by using molecular hydrogen as an energy carrier and using reactions in which polluting products like greenhouse gases are avoided. The prospects of such an economy require the development of clean and efficient ways of producing and storing molecular hydrogen, or, in an extended sense, of molecules by which energy is stored in their chemical bonds.

Presently, the most common methods for the production of molecular hydrogen on an industrial basis concern firstly, steam reforming, a process in which steam is allowed to react with fossil fuels at high temperatures. The energy required (i.e., the enthalpy change ΔH), for, e.g., methane steam reforming
CH4 + H2O → CO + 3 H2
amounts to +49 kcal/mol.1 Secondly, a well-established and widely applied method for hydrogen production, introduced during the early days of electrochemistry in 1800, and which has recently become commercially available, concerns electrolysis of water,
2 H2O → 2 H2 + O2
which requires +116 kcal/mol.1 For comparison, the bond dissociation energy of H2 amounts to +104 kcal/mol.1 These numbers indicate that the production of molecular hydrogen, the key ingredient of the hydrogen economy, is by no means a trivial task, and that the presently used industrial processes to produce molecular hydrogen will in the long term likely not be sustainable.
Fortunately, Nature, has found a way to produce molecular hydrogen very efficiently, by using proteins as catalysts for this reaction. The proteins that evolve hydrogen are called hydrogenases and they catalyze both the formation and the decomposition of molecular hydrogen
H2 → 2 H+ + 2e−          (1)

he family of hydrogenases is presently subdivided into three classes, depending on the metal content of the active site.[2] The classes comprise [NiFe], [FeFe] and [Fe] hydrogenases. The turnover numbers for hydrogen production of the [FeFe] hydrogenases amount to 9000 molecules per second.[3] A disadvantage is their oxygen sensitivity, especially for the [FeFe] hydrogenases. The active sites of the [NiFe] and [FeFe] hydrogenases display an unusual arrangement, which includes inorganic CO and CN ligands (see figure 1). In the case of the [FeFe] hydrogenase, the enzyme contains a [4Fe4S] cluster coupled to a [2Fe2S] cluster, which are both clusters are only embedded in the protein by the cysteine thiolate ligands.

In general, the active sites of both classes of hydrogenases contain one free coordination position, which is most likely the catalytically relevant coordination position.[2] For the [NiFe] hydrogenases, which display a rich redox structure in which the nickel atom switches back and forth between the 3+ and 2+ redox state and the iron is 2+, low spin, a hydride ligand has been detected using HYSCORE spectroscopy,[4] which is direct evidence that the catalytic activity indeed occurs at the free coordination position of nickel. Presently, research emphasizes on improving the issue of oxygen sensitivity. In this respect the hydrogenases from extremophile organisms are promising candidates, since these enzymes are much more robust, oxygen insensitive and even function at elevated temperatures.

In a broader sense, besides the H-H bond, Nature often stores energy in chemical bonds of reduced molecules. Well known examples are, e.g., nicotinamide adenine dinucleotide (NADH) or Nicotinamide adenine dinucleotide phosphate (NADPH). As with molecular hydrogen, the energy stored in the respective C-H bonds can be released at any convenient time in an oxidizing environment. Moreover, by the storage of energy in chemical bonds as opposed to charge separation, Nature has greatly simplified the issue of energy storage. Besides NADH, another common molecule is adenosine triphosphate (ATP), in which energy (app. −7 kcal/mol) is stored in the P-O bond. These molecules are common “fuels” employed by Nature to store energy. Additionally, in plant photosynthesis, light energy is converted and stored as sugars (see section e).

The active sites of hydrogenases have been a focal point for inorganic chemists with the aim to prepare biomimetic inorganic molecules that possess catalytic activity. One of the first molecules, which did show catalytic activity towards the splitting of hydrogen in aqueous solution, has been reported by Ogo et al.[5] This molecule features a nickel-ruthenium dinuclear metal center, which is shown to be bridged by a hydride by neutron diffraction studies. The hydride could additionally be shown to be the product of the heterolytic cleavage of molecular hydrogen. Other systems which show catalytic activity in organic solvents involve homonuclear metal centers featuring ruthenium or iridium.

Given these promising initial successes, the challenges for the future still remain many in number. On the one hand, it is of crucial importance to examine the oxygen sensitivity of hydrogenases. On the other hand, the synthesis of inorganic complexes has recently resulted in catalytic activity towards hydrogen production, and should be extended to optimize activity and stability. Thirdly, research efforts are also going into the direction of storing energy in the form of other reduced molecules, i.e., sugars and biomass in a broader sense. Eventually, either of these paths will have to be upscaled to mass production. With fossil fuels being available for only a few more decades, it is clear that the process of energy conversion and storage from alternate sources is a pressing problem and it presently still unknown which path will lead to definite successes first.

References

  1. Handbook of Chemistry and Physics, 81st edition, CRC Press New York (2000).
  2. Chemical Reviews 2007, 107, issue 10 "Hydrogen"
  3. Cammack, R., Nature 1999, 397, 214-215.
  4. Brecht, M.; van Gastel, M.; Buhrke, T.; Friedrich, B.; Lubitz, W. J. Am. Chem. Soc. 2003, 125, 13075-13083.
  5. Ogo, S.; Kabe, R.; Uehara, K.; Kure, B.; Nishimura, T.; Menon, S. C.; Harada, R.; Fukuzumi, S.;  Higuchi, Y.; Ohhara, T.; Tamara, T.; Kuroki, R. Science 2007, 316, 585-587.
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