Biomimetic Hydrogen Evolution

Other Unique Engineering Ideas

The electrochemical hydrogen evolution process whereby protons and electrons are combined into molecular hydrogen is catalyzed most effectively by the Pt group metals. The search for alternatives to the Pt group metals as catalysts for hydrogen evolution is currently being intensified, as molecular hydrogen, H2, is being considered as an energy carrier.                                                      

1. Description

2. Why

3. How

4. Future Trends

5. Related Links

Description

In the foreseeable future, fossil fuel reservoirs will be depleted and alternative energy carriers will be needed. Unlike the hydrocarbon fuels used today hydrogen produces only water during oxidation, for instance in a fuel cell. In order for hydrogen to be a real alternative to hydrocarbons it must be produced in a sustainable fashion. This requires an efficient catalyst for hydrogen evolution, and alternatives to the expensive and scarce platinum catalyst are needed.

  • One possible energy carrier, which is being considered is H2.
  • One way to produce H2 is the electrochemical hydrogen evolution, which is catalyzed most effectively by the Pt group metals.
  • Many organisms metabolize hydrogen gas by means of an enzyme called hydrogenase.
  • The nature of the catalytic sites in the hydrogenases has long been a subject of conjecture and debate.

In their Perspective, Adams and Stiefel discuss results reported in the same issue by Peters et al. in which x-ray crystallography was used to provide the first structural glimpse of the iron-only hydrogenase from the hydrogen-producing, anaerobic bacterium Clostridium pasteurianum.

Why 

However, the need for these catalysts will increase and alternatives to the scarce and expensive Pt group catalysts will be needed. We have used insight from hydrogen-producing enzymes to find a new hydrogen evolution catalyst, MoS2, which is both cheap and effective.The fields of molecular biology and inorganic chemistry overlap in the study of metalloenzymes in the form of enzyme mimics among other. Hydrogenases and nitrogenases are effective catalysts for the hydrogen evolution process, even though the catalytically active site of these enzymes contains the much less noble metals Fe, Ni, and Mo, and the nitrogenase active site contains S.A necessary criterion for high catalytic activity is that the binding free energy of atomic hydrogen to the catalyst is close to zero. This criterion enables us to search for new catalysts, and inspired by the nitrogenase active site, it is found that MoS2 nanoparticles supported on graphite are a promising catalyst. They catalyze electrochemical hydrogen evolution at a moderate overpotential of 0.1-0.2 V.

How

Hydrogenases were first discovered in the 1930s, and they have since attracted interest from many researchers including inorganic chemists who have synthesized a variety of hydrogenase mimics. Understanding the catalytic mechanism of hydrogenase might help scientists design clean biological energy sources, such as algae, that produce hydrogen.

  • The potentials at which the systems mediate H2 evolution are significantly more positive than those of other molecular systems that catalyze H2  evolution.
  • It typically require potentials in the range of 21 V to 22 V in organic solvents.
  • Catalysis occurs in the presence of acids with modest (8.7–12.7) to low (0.1) pKa values in acetonitrile.

Under analogous conditions but in the absence of added catalyst, analogous proton reduction was found to occur at a Pt electrode at ca. 20.58 V with CF3COOH and ca. 20.26 V with HCl, with equilibrium potentials at 20.12 V and 0.0 V,respectively.According to this principle, a necessary (but not sufficient) criterion for a good hydrogen evolution catalyst is that the free energy for hydrogen binding is close to zero. For nitrogenase and hydrogenase, we find that the free energy for hydrogen binding is close to zero so that this criterion holds.The interesting thing here is that we can explicitly show this natural requirement to hold for both inorganic and biological systems using the quantum chemical calculations. This means that we can use the same calculations to search for other systems, which could be candidates as catalysts for hydrogen evolution. 

  • The structure of the heterodimeric Fe-only hydrogenase from Desulfovibrio desulfuricans - the first for this class of enzymes.
  • With the exception of a ferredoxin-like domain, the structure represents a novel protein fold.
  • The so-called H cluster of the enzyme is composed of a typical [4Fe-4S] cubane bridged to a binuclear active site Fe center.
  • It contains putative CO and CN ligands and one bridging 1, 3-propanedithiol molecule.

The conformation of the subunits can be explained by the evolutionary changes that have transformed monomeric cytoplasmic enzymes into dimeric periplasmic enzymes. Plausible electron- and proton-transfer pathways and a putative channel for the access of hydrogen to the active site have been identified.

Future Trends 

The advantage of indirect modeling or enzyme mimicry is high-resolution crystal structures and well-defined spectral data from which comparisons can be made to low-resolution crystal structures and poorly defined spectral data obtained from the native enzymes.For transition metal surfaces, it has recently been shown that the free energy for hydrogen binding is a key parameter determining activity for hydrogen evolution in an electrochemical cell.

Keywords

Biomimetic hydrogen, MoS2 Nanoparticles, hydrogen evolution, cobalt, difluoroboryl-diglyoximate

Related Articles

Related Links