Moritz F. Kuehnel

Synthesis of Fluorinated Building Blocks by Transition‐Metal‐Mediated Hydrodefluorination Reactions

The activation and functionalization of carbon-fluorine bonds can be considered as a major challenge in organometallic chemistry. The growing demand for means to introduce fluorine into new materials or into biologically active molecules has inspired the development of diverse synthetic strategies. Hydrodefluorination is regarded as a promising approach to access partially fluorinated building blocks from readily available perfluorinated bulk chemicals. We provide an overview of transition-metal-based complexes and catalysts that were developed to mediate hydrodefluorination reactions. Special emphasis will be placed on discussing the underlying mechanistic patterns and their impact on scope and selectivity. In addition, future requirements for further developing this field will be highlighted.

Selective Photocatalytic CO2 Reduction in Water through Anchoring of a Molecular Ni Catalyst on CdS Nanocrystals

Photocatalytic conversion of CO2 into carbonaceous feedstock chemicals is a promising strategy to mitigate greenhouse gas emissions and simultaneously store solar energy in chemical form. Photocatalysts for this transformation are typically based on precious metals and operate in nonaqueous solvents to suppress competing H2 generation. In this work, we demonstrate selective visible-light-driven CO2 reduction in water using a synthetic photocatalyst system that is entirely free of precious metals. We present a series of self-assembled nickel terpyridine complexes as electrocatalysts for the reduction of CO2 to CO in organic media. Immobilization on CdS quantum dots allows these catalysts to be active in purely aqueous solution and photocatalytically reduce CO2 with >90% selectivity under UV-filtered simulated solar light irradiation (AM 1.5G, 100 mW cm-2, λ > 400 nm, pH 6.7, 25 °C). Correlation between catalyst immobilization efficiency and product selectivity shows that anchoring the molecular catalyst on the semiconductor surface is key in controlling the selectivity for CO2 reduction over H2 evolution in aqueous solution.

Synthesis of the Smallest Axially Chiral Molecule by Asymmetric Carbon–Fluorine Bond Activation

Surprisingly, thecarbon backbones of 3 and 5adeviate by 4.4(3)8 and 4.6(2)8from the linearity that would be expected based on their sp-hybridized central carbon atoms (Figure 2). Furthermore, thetwo terminal F-C-H planes in5a have a torsion angle of91.9(2)8. The previously reported structures derived frommicrowave spectroscopy do not show the same distortions(Supporting Information, Table S2), because erroneously, therefinement was based on a constrained linear geometry.

Photocatalytic Formic Acid Conversion on CdS Nanocrystals with Controllable Selectivity for H2 or CO

Formic acid is considered a promising energy carrier and hydrogen storage material for a carbon-neutral economy. We present an inexpensive system for the selective room-temperature photocatalytic conversion of formic acid into either hydrogen or carbon monoxide. Under visible-light irradiation (λ>420 nm, 1 sun), suspensions of ligand-capped cadmium sulfide nanocrystals in formic acid/sodium formate release up to 116±14 mmol H2 gcat−1 h−1 with >99 % selectivity when combined with a cobalt co-catalyst; the quantum yield at λ=460 nm was 21.2±2.7 %. In the absence of capping ligands, suspensions of the same photocatalyst in aqueous sodium formate generate up to 102±13 mmol CO gcat−1 h−1 with >95 % selectivity and 19.7±2.7 % quantum yield. H2 and CO production was sustained for more than one week with turnover numbers greater than 6×105 and 3×106, respectively.

Metal-encapsulated organolead halide perovskite photocathode for solar-driven hydrogen evolution in water

Lead-halide perovskites have triggered the latest breakthrough in photovoltaic technology. Despite the great promise shown by these materials, their instability towards water even in the presence of low amounts of moisture makes them, a priori, unsuitable for their direct use as light harvesters in aqueous solution for the production of hydrogen through water splitting. Here, we present a simple method that enables their use in photoelectrocatalytic hydrogen evolution while immersed in an aqueous solution. Fields metal, a fusible InBiSn alloy, is used to efficiently protect the perovskite from water while simultaneously allowing the photogenerated electrons to reach a Pt hydrogen evolution catalyst. A record photocurrent density of −9.8 mA cm−2 at 0 V versus RHE with an onset potential as positive as 0.95±0.03 V versus RHE is obtained. The photoelectrodes show remarkable stability retaining more than 80% of their initial photocurrent for ∼1 h under continuous illumination.

Tuning Product Selectivity for Aqueous CO2 Reduction with a Mn(bipyridine)-pyrene Catalyst Immobilized on a Carbon Nanotube Electrode

The development of high-performance electrocatalytic systems for the controlled reduction of CO2 to value-added chemicals is a key goal in emerging renewable energy technologies. The lack of selective and scalable catalysts in aqueous solution currently hampers the implementation of such a process. Here, the assembly of a [MnBr(2,2′-bipyridine)(CO)3] complex anchored to a carbon nanotube electrode via a pyrene unit is reported. Immobilization of the molecular catalyst allows electrocatalytic reduction of CO2 under fully aqueous conditions with a catalytic onset overpotential of η = 360 mV, and controlled potential electrolysis generated more than 1000 turnovers at η = 550 mV. The product selectivity can be tuned by alteration of the catalyst loading on the nanotube surface. CO was observed as the main product at high catalyst loadings, whereas formate was the dominant CO2 reduction product at low catalyst loadings. Using UV–vis and surface-sensitive IR spectroelectrochemical techniques, two different intermediates were identified as responsible for the change in selectivity of the heterogenized Mn catalyst. The formation of a dimeric Mn0 species at higher surface loading was shown to preferentially lead to CO formation, whereas at lower surface loading the electrochemical generation of a monomeric Mn-hydride is suggested to greatly enhance the production of formate. These results emphasize the advantages of integrating molecular catalysts onto electrode surfaces for enhancing catalytic activity while allowing excellent control and a deeper understanding of the catalytic mechanisms.

Solar Hydrogen Generation from Lignocellulose

Abstract Photocatalytic reforming of lignocellulosic biomass is an emerging approach to produce renewable H2. This process combines photo‐oxidation of aqueous biomass with photocatalytic hydrogen evolution at ambient temperature and pressure. Biomass conversion is less energy demanding than water splitting and generates high‐purity H2 without O2 production. Direct photoreforming of raw, unprocessed biomass has the potential to provide affordable and clean energy from locally sourced materials and waste.

Aerobic Conditions Enhance the Photocatalytic Stability of CdS/CdO x Quantum Dots

Abstract Photocatalytic H2 production through water splitting represents an attractive route to generate a renewable fuel. These systems are typically limited to anaerobic conditions due to the inhibiting effects of O2. Here, we report that sacrificial H2 evolution with CdS quantum dots does not necessarily suffer from O2 inhibition and can even be stabilised under aerobic conditions. The introduction of O2 prevents a key inactivation pathway of CdS (over‐accumulation of metallic Cd and particle agglomeration) and thereby affords particles with higher stability. These findings represent a possibility to exploit the O2 reduction reaction to inhibit deactivation, rather than catalysis, offering a strategy to stabilise photocatalysts that suffer from similar degradation reactions.

Research data supporting "Solar-driven reforming of lignocellulose to H2 with a CdS/CdOx photocatalyst"

Raw data supporting Nature Energy publication: Solar-driven reforming of lignocellulose to H2 with a CdS/CdOx photocatalyst

Raw Data supporting article: ZnSe qantum dots modified with a Ni(cylam) catalyst for efficient visible-light driven CO2 reduction in water

Raw Data supporting article: ZnSe qantum dots modified with a Ni(cylam) catalyst for efficient visible-light driven CO2 reduction in water