Our research integrates molecular catalysis with nanomaterials to generate solar fuels. We utilise the excellent light absorption properties of quantum dots to generate charge-separated states that can drive redox processes. Immobilised catalysts allow us to control the selectivity of these redox reactions to produce chemical fuels such as H2 or CO from water and waste. See below for examples of our recent work.
Solar-driven H2 Generation from Waste
The conversion of waste materials into fuels is an attractive approach to simultaneously produce fuels and mitigate pollution. We are developing photocatalysts to drive this process with solar energy.
Lignocellulosic biomass is the most abundant raw material on earth, but its utilisation for fuel production is limited to energy-intensive processes due to low solubility and reactivity at ambient conditions. This work is the first example of direct photochemical conversion of unprocessed waste biomass into clean hydrogen fuel. We adapted concepts of solar fuels research for biomass valorisation by replacing commonly used sacrificial electron donors with waste. Through protection of CdS quantum dots with a CdOx layer to overcome the well-documented photocorrosion of CdS, we developed an efficient and robust photocatalyst. This visible-light responsive system produces H2 fuel from various types of unprocessed lignocellulose such as wood, grass and waste paper at ambient conditions using sunlight as the sole energy input.
We have recently extended this approach to the photochemical generation of hydrogen fuel from plastics. Plastic waste has become a growing environmental challenge of global dimensions that urgently requires new mitigation technologies. Using our CdS/CdOx photocatalyst, we can generate H2 fuel from common plastics such as PET, PLA and PU upon solar irradiation. During this process, the plastics are degraded to useful organic compounds, e.g. photochemical treatment of PET generates terephthalic acid, one of the precursors needed to produce new PET. Remarkably, we can use real-world plastic waste cut from an old dirty water bottle without lowering the performance of our catalyst. We aim for this low-energy process to become a complement to conventional recycling, targeting mixed and contaminated waste that is currently challenging to recycle.
“Solar-driven reforming of lignocellulose to H2 with a CdS/CdOx photocatalyst”, Nat. Energy, 2017, 2, 17021 (link to article)
“Solar hydrogen generation from lignocellulose”, Angew. Chem. Int. Ed., 2018, 57, 3290–3296 (link to article).
“Plastic waste as a feedstock for solar-driven H2 generation”, Energy Environ. Sci., 2018, 11, 2853–2857 (link to article)
Selective reduction of aqueous CO2 using immobilised molecular catalysts
Direct solar-driven conversion of H2O and CO2 into feedstock chemicals is a promising strategy to mitigate greenhouse gas emissions and simultaneously store solar energy in chemical form. We have developed entirely precious metal-free photocatalysts for the selective reduction of CO2 to CO in water, based on a hybrid design that combines the photo-physical properties of semiconductor nanocrystals with the selectivity of a well-defined molecular base-metal catalyst. We developed a series of nickel terpyridine complexes with promising electrocatalytic activity for selective CO2 reduction at low overpotential in organic media. Designing suitable surface anchors allowed us to attach these catalysts to CdS quantum dots to drive CO2 reduction with visible light. Immobilisation enables the Ni catalysts to operate in pH neutral aqueous solution without compromising their selectivity. Under visible light, this precious-metal free CdS-[Ni] hybrid catalyst gave CO with >90% selectivity.
A second-generation photocatalyst based on ZnSe nanocrystals and an immobilised Ni(cyclam) catalyst shows a much higher activity and stability. Selectivity tuning was achieved by targeted inhibition of hydrogen evolution from the ZnSe particles through surface modification. Recently, we have translated these suspension systems to a photoelectrode, by immobilising a Co terpyridine catalyst on a mesoporous Si-TiO2 photocathode for photoelectrochemical CO2 reduction.
“Solar-driven reduction of aqueous CO2 with a cobalt bis(terpyridine)-based photocathode”, Nat. Catal., 2019, 2, 354–365 (link to article).
“ZnSe quantum dots modified with a Ni(cyclam) catalyst for efficient visible-light driven CO2 reduction in water”, Chem. Sci. 2018, 9, 2501–2509 (link to article).
“Tuning Product Selectivity for Aqueous CO2 Reduction with a Mn(bipyridine)-pyrene Catalyst Immobilized on a Carbon Nanotube Electrode”, J. Amer. Chem. Soc., 2017, 139, 14425–14435 (link to article)
“Selective photocatalytic CO2 reduction in water through anchoring of a molecular Ni catalyst on CdS nanocrystals”, J. Amer. Chem. Soc., 2017, 139, 7217–7223 (link to article)
Metal-encapsulated Organolead Halide Perovskite Photocathode for Solar-driven Hydrogen Evolution in Water
Organolead halide perovskites have emerged as the fastest-developing technology in photovoltaics. Their inherent instability towards water, however, has hampered their use in solar water splitting. This work demonstrates how cross-disciplinary thinking is key to modern materials research. Classic inorganic chemistry allowed us to stabilise a perovskite photocathode in water by using a eutectic alloy as a conductive encapsulation. We developed a metal protection layer processable below the perovskite decomposition temperature. Electrodes thus protected exhibit a much enhanced life time when immersed in an aqueous solution without compromising the photovoltaic performance. In the presence of Pt as hydrogen evolution catalyst, benchmark photocurrents up to 10 mA cm–2 were achieved at 0 V vs. RHE with near unity Faradaic efficiency.
M. Crespo-Quesada, L. M. Pazos-Outón, J. Warnan, M. F. Kuehnel, R. H. Friend and E. Reisner, Nat. Commun. 2016, 7, 12555 (link)
Photocatalytic Formic Acid Conversion on CdS Nanocrystals with Controllable Selectivity for H2 or CO
Formic acid is a promising hydrogen storage material that can be derived from renewable sources. Although release of the stored hydrogen is exothermic, a precious metal catalyst and/or elevated temperatures are required. In this work, we showed that CdS quantum dots can convert formic acid at ambient conditions under solar light, and with switchable selectivity. Depending on the conditions and particle surface groups, irradiated CdS nanocrystals produce H2 and CO2 through dehydrogenation, or CO and H2O through dehydration of formic acid. This bifunctional reactivity was achieved by careful matching of engineered particle surfaces with optimised reaction media. Activity and stability of this photocatalyst are unprecedented when compared to any analogous system. Photochemical CO generation from formic acid had not been reported before.
M. F. Kuehnel, D. W. Wakerley, K. L. Orchard and E. Reisner, Angew. Chem. Int. Ed., 2015, 54, 9627 (link)
Synthesis of the Smallest Axially Chiral Molecule via Asymmetric Carbon-Fluorine Bond Activation
This landmark paper describes the first example of an asymmetric C-F bond cleavage and brings together two important synthetic challenges: Asymmetric synthesis and selective deconstruction of C-F bonds. We used transition-metal mediated C-F bond activation to synthesise the smallest isolated chiral molecule, 1,3-difluoroallene. Collaboration with leading experts allowed us to characterise this chiral gas by an unprecedented combination of X-ray crystallography of a frozen gas, theoretical calculations at the coupled-cluster level, supramolecular enantiopurity determination and absolute configuration determination by gas-phase vibrational circular dichroism spectroscopy.
M. F. Kuehnel, T. Schlöder, S. Riedel, B. Nieto-Ortega, F. J. Ramírez, J. T. López Navarrete, J. Casado and D. Lentz, Angew. Chem. Int. Ed. 2012, 51, 2218-2220 (link)
Titanium-Catalyzed C-F Activation of Fluoroalkenes
Fluorine forms the strongest known single bond to carbon. The construction and deconstruction of such inert bonds is of great interest to medicinal and environmental chemistry. Cleavage under mild conditions had previously only been achieved under drastic conditions or using precious metals. This work was a breakthrough in this field, because it demonstrates that a simple titanium complex is a highly active catalyst for the selective conversion of perfluoroalkenes to hydrofluoroalkenes at ambient temperature. Our catalyst system is not only environmentally benign and inexpensive, but also an order of magnitude more active than all previously known catalysts, allowing the use of C-F activation for synthetic applications.
M. F. Kühnel and D. Lentz, Angew. Chem. Int. Ed. 2010, 49, 2933-2936 (link)