The Reduction of Carbon Dioxide by Enzymes Adsorbed on Electrodes: from Mechanistic Studies to Bioinspired Catalysts

Carbon dioxide (CO2) is produced by the combustion of carbon-containing molecules, such as fossil fuels, to power industry, for transportation, and in our homes. Burning fossil fuels is not only causing the level of CO2 in the atmosphere to increase (CO2 is a greenhouse gas and a major contributor to global warming) it is also depleting valuable resources that are required for the manufacture of plastics, chemicals, fertilizers - and countless other requirements of modern society. Unfortunately, CO2 is a very stable and unreactive molecule, and the only large scale process known that can remove CO2 from the atmosphere and use it to regenerate a carbon-containing fuel is biological photosynthesis (growing plants and trees). Therefore, an industrial process that could use the energy from sunlight, or a green-electricity source, to take CO2 out of the atmosphere and turn it into a useful fuel or chemical would revolutionise modern society, by supplying both our energy and material demands. Of course, no such process currently exists. The aim of this proposal is to explore a new approach for the development of such a process. Some bacteria use enzymes called formate dehydrogenases (FDHs) to catalyse the oxidation (= burning) of formate to CO2, and extract energy from this reaction in order to survive. Chemically, formate is one of the simplest hydrocarbons - it is already used as a chemical building block (feedstock) in industry, and formate 'fuel cells' are being developed. Turning a formate dehydrogenases 'in reverse' would turn CO2 into formate, a useful product. In fact, a small number of specialised bacteria use special 'tungsten-containing' FDHs to catalyse this reverse reaction - and live off the tiny amount of energy that they extract. In a pilot study we showed that the tungsten FDH from a bacterium called Syntrophobacter fumaroxidans can act as an extremely efficient electrically-driven catalyst for the reduction of CO2 to formate. In this project we aim to find out 'how the tungsten formate dehydrogenase does it'. We will start by looking for and characterising tungsten FDHs from different organisms, to find those that are the best for our experiments. Then we will apply sophisticated biochemical, electrochemical and physical techniques to aim to find out how they work - and why they work so well. Finally, we will compare our biological catalysts with available synthetic catalysts, aiming to find out how to improve the synthetic catalysts, and to develop 'demonstration devices' that show how efficient CO2 reduction catalysts can be powered by solar radiation and used in fuel cells.

Carbon dioxide (CO2) is a thermodynamically and kinetically stable molecule. It is easily formed by the oxidation of organic molecules, during combustion or respiration, but difficult to chemically activate or reduce. The production of reduced carbon compounds from CO2 is an attractive proposition, because carbon-neutral energy sources could be used to generate fuel resources and sequester atmospheric CO2. However, available methods for CO2 reduction are slow, energetically wasteful, and produce mixtures of products. In a preliminary study we demonstrated that a tungsten-containing formate dehydrogenase (W-FDH) enzyme can be adsorbed to an electrode surface, to catalyse the efficient electrochemical reduction of CO2 to formate: catalysis is fast, thermodynamically reversible, and specific. Formate is an important feedstock, a stable intermediate in the conversion of CO2 to methanol and methane, and a viable energy source in its own right. This proposal aims to define the mechanism of the electrocatalytic reduction of CO2 to formate by W- FDH enzymes, using an interdisciplinary approach that combines state of the art electrochemical studies with an array of biochemical and mechanistic techniques. We aim also to 'narrow the gap' between the highly active enzymes and the most promising synthetic catalysts: we aim to provide proof-of-principle devices for exploiting the interconversion of CO2 and formate, and to compare the enzymes and synthetic catalysts directly. Therefore, we aim to establish an experimental and theoretical foundation for the development of robust synthetic catalysts for future application in carbon capture, energy storage, and regenerative fuel cell devices.

The inexpensive capture and conversion of carbon dioxide into a valuable and sustainable energy carrier such as formic acid is of major and immediate economic interest. The UK White Paper on Energy 2007 underlined the fact that energy is essential for our lives and our economy. The reduction of carbon dioxide emissions, and ensuring a secure supply of clean and affordable energy were identified as major objectives. Thus, it is imperative that we react fast to develop renewable energy technologies. The target of this proposal is to lay a foundation for a new direction of industrially relevant research in the renewable production of carbon-based fuels. In this project we aim to understand how to activate and reduce CO2, by studying enzymes as model systems for the development of synthetic catalysts. At this stage, these are basic research aims, with academic beneficiaries, and commercialisation of a product is not an aim of the current proposal (as enzymes are very precious materials and not cost-competitive with current energy generation). But, after successful completion of this project, we will seek a close industrial partnership to develop a catalyst and devices, to apply the principles learnt from this BBSRC-funded project and to replace our enzymes with small molecule equivalents. Our ultimate aim is the production of a low-cost product capable of reducing carbon dioxide, ideally using sunlight. Our proposed research project combines a high degree of novelty and impact, with a high probability of achieving our stated goals, with immediate impact on UK academic science, and longer-term impact on UK industry. This project will establish a new academic partnership between Judy Hirst, Medical Research Council, and Erwin Reisner, University of Cambridge, forming a nucleus around which future networks and collaborations will be built. Within this project we will provide top-quality cross-disciplinary training for two BBSRC PDRAs (plus at least four University of Cambridge undergraduate students, who will undertake projects related to this proposal), to provide expertise in the development of alternative energy technologies, an area of critical scientific, technological and economic importance for the future.