Enrico Barsch

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.

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.

Base‐Free Non‐Noble‐Metal‐Catalyzed Hydrogen Generation from Formic Acid: Scope and Mechanistic Insights

The iron-catalyzed dehydrogenation of formic acid has been studied both experimentally and mechanistically. The most active catalysts were generated in situ from cationic Fe(II) /Fe(III) precursors and tris[2-(diphenylphosphino)ethyl]phosphine (1, PP3 ). In contrast to most known noble-metal catalysts used for this transformation, no additional base was necessary. The activity of the iron catalyst depended highly on the solvent used, the presence of halide ions, the water content, and the ligand-to-metal ratio. The optimal catalytic performance was achieved by using [FeH(PP3 )]BF4 /PP3 in propylene carbonate in the presence of traces of water. With the exception of fluoride, the presence of halide ions in solution inhibited the catalytic activity. IR, Raman, UV/Vis, and EXAFS/XANES analyses gave detailed insights into the mechanism of hydrogen generation from formic acid at low temperature, supported by DFT calculations. In situ transmission FTIR measurements revealed the formation of an active iron formate species by the band observed at 1543 cm(-1) , which could be correlated with the evolution of gas. This active species was deactivated in the presence of chloride ions due to the formation of a chloro species (UV/Vis, Raman, IR, and XAS). In addition, XAS measurements demonstrated the importance of the solvent for the coordination of the PP3 ligand.

Iron-catalyzed photoreduction of carbon dioxide to synthesis gas

Photocatalytic processes to convert CO2 to useful products including CO and HCOOH are of particular interest as a means to harvest the power of the sun for sustainable energy applications. Herein, we report the photocatalytic reduction of CO2 using iron catalysts and visible light generating varying ratios of synthesis gas. In most cases, either a 1 : 1 CO : H2 ratio was observed or the selectivity was skewed slightly towards CO with combined TONs reaching nearly 100. Operando FTIR studies revealed the favourable activity of FeBr2 as an iron precursor to generate the catalytically active species Fe(CO)3bpy. The Fe(CO)3bpy complex was then synthesized and successfully applied as a catalyst at various loadings providing almost 300 total TONs for syngas formation. This represents the highest activity reported thus far for an iron-based system in photocatalytic CO2 reduction.

Mechanistic Study of Photocatalytic Hydrogen Generation with Simple Iron Carbonyls as Water Reduction Catalysts

This study provides new insights into light‐driven hydrogen generation using an iridium photosensitizer (IrPS) and simple iron carbonyls as water reduction catalysts (WRCs). Stopped‐flow rapid‐scan FTIR and operando continuous‐flow FTIR spectroscopy as well as time‐dependent density functional theory (TD‐DFT) has been applied to study the reaction. The conversion of the WRC precursor [Fe3(CO)12] into the radicals [Fe3(CO)11].− and [Fe2(CO)8].− as well as [Fe(CO)5] in the absence of light in a solvent mixture of tetrahydrofuran, triethylamine, and water has been studied quantitatively. During light‐induced hydrogen production in the presence of the IrPS, the trimeric [HFe3(CO)11]− and the monomeric [HFe(CO)4]− anion could be identified as major WRC species. The equilibrium between both species can be shifted completely towards [HFe(CO)4]− by increasing the water content of the solvent mixture. Application of other iron(0) carbonyl compounds as WRC precursors also results in the exclusive formation of [HFe(CO)4]−. Kinetic experiments show that the stability of the system is primarily influenced by the applied amount of WRC precursor, whereas the reaction rate is mainly determined by the concentration of the IrPS. At least two loss channels could be identified: light‐induced CO dissociation from the WRC and decomposition of the IrPS at high IrPS/WRC ratios, accompanied by a ligand transfer from the iridium towards the iron center of the WRC. To reveal the nature of the catalytically active complex, binding energies and charge‐transfer probabilities of all coordination geometries of various IrPS⋅⋅⋅WRC complexes have been calculated. These computations indicate an increased probability of charge transfer for dimeric and trimeric iron carbonyl species.

Peak group analysis for the extraction of pure component spectra

Structure elucidation for the reactive or catalytic species of a chemical reaction system can significantly be supported by spectroscopic measurements. If the spectroscopic data contains isolated signals or groups of partially separated peaks, then the identification of correlations between these peaks can help to determine the pure components by their functional groups. A computational method is presented which constructs from a certain frequency window, which contains a single peak or a peak group,an associated pure component spectrum on the full frequency range.This global spectrum reproduces the spectrum in the local frequency window or, at least, reproduces the contribution from the dominant component in the local window. The method is called the peak group analysis (PGA).The methodological background of the PGA are a multivariate curve resolution method and the solution of a minimization problem with weighted soft constraints.The method is tested for two experimental FT-IR data sets from investigations into equilibria of hydroformylation catalysts based on rhodium and iridium.An implementation of the PGA is presented as a part of the FACPACK software.

Diferrate [Fe2(CO)6(μ‐CO){μ‐P(aryl)2}]− as Self‐Assembling Iron/Phosphor‐Based Catalyst for the Hydrogen Evolution Reaction in Photocatalytic Proton Reduction—Spectroscopic Insights

This work is focused on the identification and investigation of the catalytically relevant key iron species in a photocatalytic proton reduction system described by Beller and co-workers. The system is driven by visible light and consists of the low-cost [Fe3 (CO)12 ] as catalyst precursor, electron-poor phosphines P(R)3 as co-catalysts, and a standard iridium-based photosensitizer dissolved in a mixture of THF, water, and the sacrificial reagent triethylamine. The catalytic reaction system was investigated by operando continuous-flow FTIR spectroscopy coupled with H2 gas volumetry, as well as by X-ray absorption spectroscopy, NMR spectroscopy, DFT calculations, and cyclic voltammetry. Several iron carbonyl species were identified, all of which emerge throughout the catalytic process. Depending on the applied P(R)3 , the iron carbonyl species were finally converted into [Fe2 (CO)6 (μ-CO){μ-P(R)2 }]- . This involves a P-C cleavage reaction. The requirements of P(R)3 and the necessary reaction conditions are specified. [Fe2 (CO)6 (μ-CO){μ-P(R)2 }]- represents a self-assembling, sulfur-free [FeFe]-hydrogenase active-site mimic and shows good catalytic activity if the substituent R is electron poor. Deactivation mechanisms have also been investigated, for example, the decomposition of the photosensitizer or processes observed in the case of excessive amounts of P(R)3 . [Fe2 (CO)6 (μ-CO){μ-P(R)2 }]- has potential for future applications.