Felix Gärtner

Efficient Dehydrogenation of Formic Acid Using an Iron Catalyst

Iron-catalyzed hydrogen generation raises prospects for a cheap hydrogen storage medium. Hydrogen is one of the essential reactants in the chemical industry, though its generation from renewable sources and storage in a safe and reversible manner remain challenging. Formic acid (HCO2H or FA) is a promising source and storage material in this respect. Here, we present a highly active iron catalyst system for the liberation of H2 from FA. Applying 0.005 mole percent of Fe(BF4)2·6H2O and tris[(2-diphenylphosphino)ethyl]phosphine [P(CH2CH2PPh2)3, PP3] to a solution of FA in environmentally benign propylene carbonate, with no further additives or base, affords turnover frequencies up to 9425 per hour and a turnover number of more than 92,000 at 80°C. We used in situ nuclear magnetic resonance spectroscopy, kinetic studies, and density functional theory calculations to explain possible reaction mechanisms.

Photocatalytic Hydrogen Generation from Water with Iron Carbonyl Phosphine Complexes: Improved Water Reduction Catalysts and Mechanistic Insights

An extended study of a novel visible-light-driven water reduction system containing an iridium photosensitizer, an in situ iron(0) phosphine water reduction catalyst (WRC), and triethylamine as sacrificial reductant is described. The influences of solvent composition, ligand, ligand-to-metal ratio, and pH were studied. The use of monodentate phosphine ligands led to improved activity of the WRC. By applying a WRC generated in situ from Fe(3) (CO)(12) and tris[3,5-bis(trifluoromethyl)phenyl]phosphine (P[C(6)H(3)(CF(3))(2)](3), Fe(3)(CO)(12)/PR(3)=1:1.5), a catalyst turnover number of more than 1500 was obtained, which constitutes the highest activity reported for any Fe WRC. The maximum incident photon to hydrogen efficiency obtained was 13.4% (440 nm). It is demonstrated that the evolved H(2) flow (0.23 mmol H(2) h(-1) mg(-1) Fe(3)(CO)(12)) is sufficient to be used in polymer electrolyte membrane fuel cells, which generate electricity directly from water with visible light. Mechanistic studies by NMR spectroscopy, in situ IR spectroscopy, and DFT calculations allow for an improved understanding of the mechanism. With respect to the Fe WRC, the complex [HNEt(3)](+)[HFe(3)(CO)(11)](-) was identified as the key intermediate during the catalytic cycle, which led to light-driven hydrogen generation from water.

Water Oxidation with Molecularly Defined Iridium Complexes: Insights into Homogeneous versus Heterogeneous Catalysis

Molecularly defined Ir complexes and different samples of supported IrO(2) nanoparticles have been tested and compared in the catalytic water oxidation with cerium ammonium nitrate (CAN) as the oxidant. By comparing the activity of nano-scaled supported IrO(2) particles to the one of organometallic complexes it is shown that the overall activity of the homogeneous Ir precursors is defined by both the formation of the homogeneous active species and its conversion to Ir(IV)-oxo nanoparticles. In the first phase of the reaction the activity is dominated by the homogeneous active species. With increasing reaction time, the influence of nano-sized Ir-oxo particles becomes more evident. Notably, the different conversion rates of the homogeneous precursor into the active species as well as the conversion into Ir-oxo nanoparticles and the different particle sizes have a significant influence on the overall activity. In addition to the homogeneous systems, IrO(2)@MCM-41 has also been synthesized, which contains stabilized nanoparticles of between 1 and 3 nm in size. This latter system shows a similar activity to IrCl(3)⋅xH(2)O and complexes 4 and 5. Mechanistic insights were obtained by in situ X-ray absorption spectroscopy and scanning transmission electron microscopy.

Hydrogen Evolution from Water/Alcohol Mixtures: Effective In Situ Generation of an Active Au/TiO2 catalyst

Gold standard: Au/TiO(2) catalysts, easily prepared in situ from different Au precursors and TiO(2), generate hydrogen from water/alcohol mixtures. Different alcohols, and even glucose, can serve as sacrificial reductants. The best system produces hydrogen on a liter scale, and is stable for more than two days. Deuteration studies show that proton reduction is likely the rate-limiting step in this reaction.

Insights into the Mechanism of Photocatalytic Water Reduction by DFT‐Supported In Situ EPR/Raman Spectroscopy

Considering the foreseeable shortage of fossil resources and global warming, the development of sustainable-energy technologies is of vital interest. An attractive option for the production of more benign energy vectors is the generation of hydrogen by photocatalytic water reduction. This concept facilitates the transformation of sunlight as the ultimate energy source into transportable energy carriers such as hydrogen. Hence, significant efforts are currently being undertaken to increase the activity and stability of suitable water-splitting catalysts. 3] The overall process can be divided into the two half reactions: water oxidation and water reduction. Studying these half reactions in detail, in particular the formation, operation, and decomposition of the catalyst, provides essential information for the development of new more efficient and environmentally benign catalysts. Recently, the Beller group disclosed an efficient water-reduction catalyst system consisting of [Ir(ppy)2(bpy)]PF6 (ppy = 2-phenylpyridine, bpy = 2,2’-bipyridine) as photosenzitizer (IrPS), [Fe3(CO)12] as water-reduction catalyst (WRC), and triethylamine (TEA) as sacrificial reductant (SR; Scheme 1). It is supposed that the catalytic cycle starts by photoexcitation of IrPS and charge separation, and subsequent reduction of its excited state by TEA (SR, cycle I). From the reduced state IrPS an electron is transferred to the WRC, which subsequently reduces aqueous protons to H2 (cycle II). To date, the only intermediate that has been experimentally identified by in situ IR spectroscopy in the water-reduction cascade (Scheme 1) is the anion [HFe3(CO)11] , which is considered to be the catalytically active species. However, the preceding steps leading to its formation as well as pathways responsible for the observed deactivation with time are still not known. Thus, more comprehensive in situ studies using additional methods are highly desired. It is probable that the one-electron-transfer processes in the catalytic cycles I and II (Scheme 1) lead to paramagnetic radical intermediates. Such species are accessible by EPR spectroscopy, while the diamagnetic [HFe3(CO)11] anion is EPR-silent but can be observed by vibrational in situ spectroscopic methods. To gain a more detailed insight into catalytic cycles I and II and to identify possible deactivation processes, we have monitored the reaction simultaneously by in situ EPR/Raman spectroscopy. To the best of our knowledge, photocatalytic water-splitting reactions have never been studied by these coupled techniques. The interpretation of our experimental data is supported by DFT calculations and additional in situ IR studies. First, catalytic cycle I was investigated. As expected, the IrPS complex (low-spin d, diamagnetic) showed no EPR signal in a solution containing THF/TEA/H2O (8:2:1) in the absence of [Fe3(CO)12] without light irradiation. However, if this solution is irradiated at 300 K, an intense isotropic signal at g = 1.9840 is observed (Figure 1). This signal corresponds to the reduced form of the iridium photosensitizer (IrPS ), which is formed by reductive quenching of the excited state (IrPS*) by TEA. A similar signal was formed neither in pure THF nor in THF/H2O, suggesting that 1) TEA is needed as a reducing agent and 2) excitation by light is essential to initiate the electron transfer. However, it must also be mentioned that the signal rapidly declines with time, probably because of ligand dissociation from IrPS (for additional information see Figure SI1 in the Supporting Information). In a reaction mixture containing all the necessary components of the waterreduction system (THF, H2O, TEA, IrPS, and Fe-WRC), no Scheme 1. General principle of H2 formation through the photocatalytic water-reduction cascade.