Dirk Hollmann

Water Reduction with Visible Light: Synergy between Optical Transitions and Electron Transfer in Au-TiO2 Catalysts Visualized by In situ EPR Spectroscopy†

Golden electrons: Visible light excites conduction electron transfer from gold particles to support vacancies where they are taken up by protons to produce hydrogen. This transfer process was visualized by in situ EPR spectroscopy.

Ruthenium‐catalyzed Selective Monoamination of Vicinal Diols

The monoamination of vicinal diols in the presence of in situ generated ruthenium catalysts has been investigated. Among the various phosphines tested in combination with [Ru(3)(CO)(12)], N-phenyl-2-(dicyclohexyl-phosphanyl)pyrrole showed the best performance. After optimization of the reaction conditions this system was applied to different secondary amines and anilines as well as a number of vicinal diols. With the exception of ethylene glycol, monoamination of the vicinal diols occurred selectively and the corresponding amino alcohols were obtained in good yields, producing water as the only side product.

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.

Boosting Visible‐Light‐Driven Photocatalytic Hydrogen Evolution with an Integrated Nickel Phosphide–Carbon Nitride System

Solar light harvesting by photocatalytic H2 evolution from water could solve the problem of greenhouse gas emission from fossil fuels with alternative clean energy. However, the development of more efficient and robust catalytic systems remains a great challenge for the technological use on a large scale. Here we report the synthesis of a sol-gel prepared mesoporous graphitic carbon nitride (sg-CN) combined with nickel phosphide (Ni2 P) which acts as a superior co-catalyst for efficient photocatalytic H2 evolution by visible light. This integrated system shows a much higher catalytic activity than the physical mixture of Ni2 P and sg-CN or metallic nickel on sg-CN under similar conditions. Time-resolved photoluminescence and electron paramagnetic resonance (EPR) spectroscopic studies revealed that the enhanced carrier transfer at the Ni2 P-sg-CN heterojunction is the prime source for improved activity.

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.

Advances in Asymmetric Borrowing Hydrogen Catalysis

Tremendous efforts have been directed towards the conversion of stoichiometric reactions into catalytic processes leading to a minimized E factor. In this regard, asymmetric catalysis has enabled the selective synthesis of chiral products to circumvent the separation of racemic mixtures. Therefore, it has become one of the most important topics in organic chemistry. To avoid multi-step reactions and expensive separation processes, new concepts have been developed. Over the years, immense expenditure to utilize sequential one-pot procedures without purification has been conducted. A powerful strategy is the borrowing hydrogen (BH) methodology, which combines transfer hydrogenation (avoidance of direct usage of hydrogen) with an intermediate reaction, such as condensation or aalkylation, without necessary separation processes. Utilizing this highly efficient technique, carbon–carbon or carbon–nitrogen bonds can be formed (Scheme 1).

Complementing Graphenes: 1D Interplanar Charge Transport in Polymeric Graphitic Carbon Nitrides.

Charge transport in polymeric graphitic carbon nitrides is shown to proceed via diffusive hopping of electron and hole polarons with reasonably high mobilities >10(-5) cm(2) V(-1) s(-1). The power-law behavior of the ultrafast luminescence decay exhibits that the predominant transport direction is perpendicular to the graphitic polymer sheets, thus complementing 2D materials like graphene.

Photoassisted TiO Activation in a Decamethyltitanocene Dihydroxido Complex: Insights into the Elemental Steps of Water Splitting†

One of the major challenges for mankind concerns power supply. Renewable forms of energy are being investigated intensively as an alternative to the fossil resources commonly used nowadays. As all types of such renewable energies (except geothermal and tidal power) originate in the solar radiation, direct utilization of this ubiquitous sustainable energy source appears to be most reasonable. Apart from photovoltaic electricity production, for example, by Gr tzel cells, the conversion of sunlight into chemical energy is a very promising research topic. After the discovery of the Honda–Fujishima effect, which describes the photoassisted generation of dihydrogen and dioxygen from water using a TiO2/Pt electrode array, [3]

Hydrogen Generation by Water Reduction with [Cp*2Ti(OTf)]: Identifying Elemental Mechanistic Steps by Combined In Situ FTIR and In Situ EPR Spectroscopy Supported by DFT Calculations

A detailed mechanism of hydrogen production by reduction of water with decamethyltitanocene triflate [Cp*2 Ti(III) (OTf)] has been derived for the first time, based on a comprehensive in situ spectroscopic study including EPR and ATR-FTIR spectroscopy supported by DFT calculations. It is demonstrated that two H2 O molecules coordinate to [Cp*2 Ti(III) (OTf)] subsequently forming [Cp2 *Ti(III) (H2 O)(OTf)] and [Cp*Ti(III) (H2 O)2 (OTf)]. Triflate stabilizes the water ligands by hydrogen bonding. Liberation of hydrogen proceeds only from the diaqua complex [Cp*Ti(III) (H2 O)2 (OTf)] and involves, most probably, abstraction and recombination of two H atoms from two molecules of [Cp*Ti(III) (H2 O)2 (OTf)] in close vicinity, which is driven by the formation of a strong covalent TiOH bond in the resulting final product [Cp*2 Ti(IV) (OTf)(OH)].

Active Sites for Light Driven Proton Reduction in Y2Ti2O7 and CsTaWO6 Pyrochlore Catalysts Detected by In Situ EPR

Abstract In situ EPR spectroscopy proved to be a versatile tool to identify active sites for photocatalytic hydrogen generation in modified Y2Ti2O7 and CsTaWO6 catalysts of pyrochlore structure, in which the metal cations are located in two different positions A and B. It was found that the B-sites exclusively occupied by titanium (Y2Ti2O7) and tantalum/tungsten (CsTaWO6) act as electron traps on the surface. From these sites, electron transfer to the co-catalysts proceeds. Thus, the B-sites are responsible for photocatalytic water reduction.Graphical Abstract