CO2SF · Solar Fuel Chemistry: Design and Development of Novel Earth-abundant Metal complexes for the Photocatalytic Reduction of Carbon Dioxide

Solar Fuel Chemistry: Design and Development of Novel Earth-abundant Metal complexes for the Photocatalytic Reduction of Carbon Dioxide

Project Summary and Results The primary goal of the project was for the development of a synthetic molecular photocatalyst, in an aqueous solution, that can absorb photons under solar light illumination and carry out the proton-coupled multi-electron transfer steps required to convert dissolved gaseous CO2 to CO, one of the primary gases important in syngas that is a key step in the development of a liquid ‘solar fuel’. The other gas is H2, and it was here where the project was most successful, though it did deviate significantly from the initial proposal. The need to develop of photocatalytic systems that make use of Earth abundant metals, and to move away from precious metals due to both cost and availability. It is also crucial to develop cheap and efficient light absorbing materials that make better use of the entire solar spectrum. Whereas many of the most well known light absorbing materials, such as titanium dioxide, use most of the UV portion of the spectrum, synthetic alternatives to these materials would represent a significant advance in the field. Crucial to the assembly of a photosynthetic device on a useable scale, is the optimization of the interaction between the fuel forming catalyst and the light absorber. To this end, the project has addressed two of these challenges, by making use of a water soluble nickel catalyst and a H2 producing enzyme in combination with two metal-free light absorbing materials. Investigations into the solar fuel producing ability of both the synthetic Ni catalyst and the enzyme in aqueous solution has been successfully carried out in combination with both a carbon nitride and carbon quantum dots. Carbon nitride is a non-toxic, and cost effective polymeric carbon based material made by the condensation of melamine. It has been shown to be capable of light absorption in the visible region of the solar spectrum, but had not been used as a photosensitizer for anything other than precious metals. This project made use of this material, specifically to act in suspension in aqueous solutions, to absorb light, and provide excited electrons to drive catalytic proton reduction by a water-soluble nickel catalyst (Caputo, C.A., Gross, M., Lau, V.W., Cavazza, C., Lotsch, B.V., Reisner, E. “Photocatalytic Hydrogen Production using Polymeric Carbon Nitridie with a Hydrogenase and a Bioinspired Synthetic Ni Catalyst” Angew. Chem. Int. Ed. 2014, 126, 11722-11726). The project was a collaboration between researchers at Ludwig-Maximilians-Universität München, the Max Planck Institute for Solid State Research and Cambridge University. The carbon nitride material proved to be very stable, remaining active for a significantly longer amount of time that other known photosensitiser systems that photobleach or degrade over extended irradiation times. In addition, the use of hydrogenase enzyme as the proton reduction catalyst was also very novel, as there have been few examples of the use of hydrogenase in photocatalytic schemes for solar fuel synthesis. Examination the interactions between the material and the catalysts, have allowed for further optimization of the system. The collaborative project was carried out between researchers at Universität Rhür and at Cambridge University (Caputo, C.A., Wong, L., Beranek, R., Reisner, E. “Carbon Nitride-TiO2 Hybrid Modified with Hydrogenase for Visible Light Driven Hydrogen Production” Chem. Sci. 2015, 6, 5690-5694). Another material, carbon quantum dots, were prepared and used as photosensitizers in combination with a molecular Ni catalyst for solar light driven H2 production. (Martindale, B.C.M., Hutton, G.A.M., Caputo, C.A., Reisner, E. “Solar Hydrogen Production Using Carbon Quantum Dots and a Molecular Nickel Catalyst” J. Am. Chem. Soc. 2015, 137, 6018-6025). A straightforward hydrothermal synthesis of the carbon quantum dots was carried out using a very cheap starting material, citric acid. They were then shown to be active as light absorbers in hybrid photocatalytic experiments using a molecular Ni-based catalyst in aqueous solution. Measurable H2 production occurred even under visible light irradiation. The work is particularly significant, since it uses such a common, widely available, non-toxic and cheap starting material as the precursor to the photosensitizer and eliminates the need for toxic metal based materials. This result will be transformative and has already received significant attention, as it was highlighted in the “JACS Spotlights” section, which draws attention to significant results, typically the top 4 articles in each issue (C. Brownlee “Carbon Quantum Dots Have Their Day in the Sun” JACS Spotlight, 2015, 137, 5853-5854). It has also make it to the top list of most read articles in the Journal of the American Chemical Society for the month of August 2015 and is currently the 15th most downloaded articles in JACS for the whole of 2015. Conclusions The use of novel metal-free, cheap and abundant materials, including carbon nitride and carbon quantum dots, in hybrid photocatalytic schemes has been shown to be effective when used in combination with non-noble metal based molecular catalysts and enzymes. Significant research into the action and mechanism of hydrogenase enzymes has been carried out, to aid and inform the design and synthesis of functional synthetic molecular mimics. However, very little had been done to probe the interactions between enzymes and materials prior to this work. This work thus contributes greatly to this area. We have shown that the interactions between hydrogenase enzymes specifically and photosensitization materials can be further optimized by taking advantage of its known affinity for titania. Dissemination of Results This work has fulfilled a many of the objectives outlined in the initial proposal, including Photochemical Testing and Catalyst Evaluation and Synthesis of Modified Catalysts, Surface Functionalization and Catalytic Evaluation with the modified goal of producing H2 rather than CO. Four high impact scientific publications have resulted from this work and several spin-off projects continue to make use of the initial results reported, with those results expected to be published within the next calendar year. The results of this work have also been presented at several international scientific conferences including the 1st International Solar Fuels Conference in Uppsala, Sweden (April 2015), the 250th American Chemical Society National Conference & Exhibition (August 2015) and will be presented at the Boston Regional Inorganic Chemistry Regional Meeting (October 2015) and the Pacifichem Conference, Honolulu, Hawaii (December 2015). Contributions to European Excellence Significant contributions to European knowledge in the general area of solar fuels have been made through the funding of this project. The record of publications, dissemination of results to both the scientific and general public (through outreach activities such as Science Open Day at Cambridge University and through the development of a YouTube video explaining “Solar Fuels”) have contributed to the advancement of the science in the development of novel strategies for the assembly of hybrid photocatalytic systems and toward the optimisation of the interactions between catalyst and light absorbing material. In addition, through the collaborations initiated in Germany, this knowledge transfer will continue to develop by providing links between the UK, Germany and the USA. I hope to maintain close contact with all the researchers I was able to work with throughout the course of this project. I am keen to use my network to set up a student exchange (USA to/from Europe) once my independent career at The University of New Hampshire is established. Wider Societal Implications This work advances the use of hybrid photocatalytic schemes by integrating highly active electrocatalysts with advanced light absorbing materials such as carbon nitride-TiO2, which is shown to be compatible with hydrogenases in aqueous solution. The wider societal impact of the results of this project will gradually become apparent as the field continues to progress. The initial studies reported here may indeed pave the way for the development of a truly useful system for the generation of solar fuel technology. Undoubtedly, these studies have highlighted the importance of surface interaction in photocatalytic systems and will hopefully be a consideration for the development of future systems of this kind. If indeed a system can be developed that leads to adoption of solar fuel technologies within out global society, an immediate drastic and lasting impact. We can limit the damage to the natural environment by decreasing the global dependence of fossil fuels, especially for our transportation demands. Progress towards this ambitious goal, no matter how small, will contribute to the massive global movement to utilize clean energy technology.

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