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EPR, Raman and hydrogen production

Der Fokus unserer Gruppe liegt auf dem Verständnis allgemeiner Prozesse in der Katalyse durch spektroskopische Untersuchung der elektronischen Struktur von Reaktions-Zwischenprodukten. Unsere Hauptforschungsmethoden sind die Resonanz-Raman-Spektroskopie, die Elektronen-Paramagnetische Resonanz-Spektroskopie (EPR) sowie die Quantenchemie. Die Kenntnis der elektronischen Struktur von katalytischen Zwischenprodukten ist von entscheidender Bedeutung, denn es sind die Elektronen, die in nahezu allen katalytischen Prozessen, die fast ausnahmslos auf der Redox-Chemie basieren, eine Hauptrolle spielen. Wann immer Elektronen in Bewegung sind, ist die Kenntnis, wo sich die redoxaktiven Elektronen befinden, z.B. in Form von Donor- und Akzeptororbitalen, sowohl für das Verständnis als auch für die schrittweise Verbesserung der Katalysatoren entscheidend. Darüber hinaus kann die durch die Kombination dieser Methoden gewonnene Erkenntnis auch die Identifizierung von Übergangszuständen und Energiebarrieren ermöglichen, die möglicherweise überwunden werden müssen, um die Katalyse weiter zu verbessern.

Maurice van Gastel

Dr. Maurice van Gastel

seit 2018
Gruppenleiter am Max-Planck-Institut für Kohlenforschung (KOFO)
2011-2017
Gruppenleiter am MPI für Chemische Energieconversion (CEC)
2009-2011
Hochschullehrer an der Universität Bonn
2005-2008
Gruppenleiter am MPI für Bioanorganische Chemie; heute: MPI CEC
2002-2005
Postdoc am MPI für Bioanorganische Chemie; heute MPI CEC
2000
Promotion an der Universität von Leiden, Niederlande
1995
Master (Diplom) an der Universität von Leiden, Niederlande
seit 2015
Ombudsmann für gute wissenschaftliche Praxis
 

Forschungsthemen

Hydrogen und Nitrogen

Hydrogen und Nitrogen

Hydrogen

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. We have investigated a catalytically active nickel-bis-dithiolate complex by theory and spectroscopy and conclusively identified the energetically most favourable energetic pathway.

Figure 1. Overview of reorganization steps with thermodynamic parameters (green box) and schematic overviews including ligand field of the intermediates and transition states (blue boxes).[1]

In addition, we have performed similar investigation for the naturally occurring enzymes, [NiFe] hydrogenases. A recurring theme is the binding of the substrate, electron donation into empty metal d orbitals, and backdonation into antibonding orbitals of the ligand, thus doubly weakening the chemical bond in the substrate.

Figure 2. (left) Schematic overview of the orbitals that play a role in hydrogen oxidation in [NiFe] hydrogenases; (right) orbital mixing scheme after McGrady and Guilera leading to weakening of the H-H bond.[1]

Nitrogen

Nitrogen is one of the most inert molecules on the planet. It features a triple bond. Presently, nitrogen is turned into ammonia in the Haber-Bosch process, which requires high temperature and pressure. In nature, nitrogenase enzymes, featuring an iron-molybdenum (FeMoCo) active site are able to catalytically turnover N2 into ammonia in an impressive 8-electron reduction process, in which additionally an H2 molecule is produced. We investigated a simplified molecular complex that features some of the structural elements of the FeMoCo, bound NO+, which is iso-electronic to N2, and derived on the electronic level how the weakening of the NN triple bond occurs.

Figure 3. 3d orbital manifolds of the irons atoms as well as the 5 valence orbitals of the NO+ ligand for pictorially representing the resonance structure of NO+ with one s and two p orbitals. The orbital structure reflects s donation of NO+ into multiple iron 3d orbitals as well as a spectacular quadruple π backdonation into the π* orbitals of the substrate analogue. All 6 phenomena weaken the triple bond.[2]

 

References

[1] Das, R.; Neese, F.; van Gastel, M. Phys. Chem. Chem. Phys. 2016, 18, 24681-24692.

[2] Kalläne, S.I.; Hahn, A.; Weyhermüller, T.; Bill, E.; Neese, F.; DeBeer, S.; van Gastel, M. Inorg. Chem. 2019, 58, 5111-5125.

Energy und Hydrogen
Energy und Hydrogen

Energy und Hydrogen

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.
Calculation of EPR observables
Calculation of EPR observables

Calculation of EPR observables

In the past few years, great steps forward have been achieved in the field of quantum chemistry with respect to the calculation of EPR parameters, including g values, hyperfine coupling constants and zero field splitting parameters. In the latter case, however, the comparison with calculated zero field splitting parameters by state-of-the-art quantum chemical methods still sometimes gives rise to mediocre agreement. Though the D parameter is usually satisfactorily reproduced when DFT-based methodology is employed, the calculated E parameter does not show a correlation with its experimental counterpart. And even worse, when calculations are performed within a spin-unrestricted formalism, the correlation between the calculated and experimental D parameter is lost completely. In a recent publication, our group has shown that spin polarization is critical for the calculation of zero-field-splitting parameters. Spin polarization effects are difficult to calculate for larger molecules, since it requires the correlation of many electrons – two for each sigma bond present in the molecule - by configuration interaction. In this project, we use a basis of spin-adapted correlated wave functions as a replacement for the widely used Slater determinant and investigate whether these basis functions allow a more accurate calculation of EPR parameters.

 

Magnetic Raman Spectroscopy

Magnetic Raman Spectroscopy

Raman and resonance Raman spectroscopy are well established methods to investigate the vibrational frequencies of molecular systems. In particular, in the area of catalysis, one of the focal points of the MPI für Kohlenforschung, this spectroscopy is a valuable tool for acquiring experimental information about the electronic structure of catalysts and thereby of the catalytic mechanism. This can be achieved by either measuring the vibrational fingerprint of the final products, the educts, the metalorganic catalyst, or even of reaction intermediates, provided their lifetime is sufficiently large so that they can be trapped with freeze quench methods.A second spectroscopy, equally relevant in particular for the elucidation of catalytic mechanisms that involve one-electron redox steps, is electron paramagnetic resonance (EPR) spectroscopy. When one-electron redox chemistry occurs, paramagnetic reaction intermediates will occur.The energy window accessible by conventional Raman spectroscopy roughly spans a range from 100 cm-1 to several 1000 cm-1, although present-day monochromators allow detection of bands down to 5 cm-1 relative to the Rayleigh line. On the other side, EPR spectroscopy, including the latest generation of high-field spectrometers, is typically used in the range from 0.1 cm-1 to 9 cm-1. Yet, oftentimes, the magnetic interactions in the system are larger than those accessible with even a state-of-the-art high-frequency EPR spectrometer. Although such systems are intrinsically magnetic, they are generally qualified as EPR silent systems and little experimental information is typically be retrieved. A prime example is high-spin FeII, 3d6, with a total spin S =2. In this integer spin system, the interaction between the unpaired electrons, including second-order spin-orbit contributions, known together as zero-field splitting, is notoriously difficult to measure directly by EPR spectroscopy. A second example of a field of research where EPR cannot be used to its fullest extent is the field of molecular magnetism, where magnetic interactions on the order of 20 cm-1 to 150 cm-1 occur. This energy window has to the best of our knowledge been dark for any spectroscopy, and has become generally known as the “Terahertz gap”.

The extractable information from the THz gap is manifold. First and foremost THz spectroscopy is relevant for catalysis: besides being able to for the first time directly measure electronic structure parameters in this frequency regime and analyzing these parameters with the aim to determine the reaction mechanisms together with state-of-the-art quantum chemical methods, for which the Neese group has established itself as one of the world-leading groups, the energy window of the THz gap is also the window where electron-phonon coupling occurs. Thus, the ability to directly measure frequencies that correspond to electron-phonon couplings allows direct information about the onset of energy transfer processes themselves. Thirdly, by building a magnetic Resonance-Raman spectrometer for operation in the THz regime, one could even envision actively steering energy transfer in situ, since the magnetic field allows for a coupling or decoupling of the electron-phonon interactions. Lastly, recent developments in the field of FTIR spectroscopy resulted in the construction of a narrowband (<0.0007 cm-1) THz source which can go down to 3 cm-1 in energy. As such, the development of a magnetic Resonance-Raman spectrometer combined with a THz source is timely and allows a full flexibility of experiments in terms of Raman-detected magnetic resonance, as well as employing the strength of the underlying Raman and EPR methods, e.g., in terms of studying crystalline materials under polarized-light conditions.

These experiments have never been systematically attempted before. As such, even the theory of magnetic Resonance-Raman spectroscopy is in its infancy. As part of this project, the theory of magnetic Resonance-Raman spectroscopy will be established and a magnetic Raman spectrometer will be built.

References

[1] Das, R.; Neese, F.; van Gastel, M. Phys. Chem. Chem. Phys. 2016, 18, 24681-24692.

[2] Kalläne, S.I.; Hahn, A.; Weyhermüller, T.; Bill, E.; Neese, F.; DeBeer, S.; van Gastel, M. Inorg. Chem. 2019, 58, 5111-5125.

 

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