Multimetallic CO2 Reduction Catalysts as Artificial Cofactors

Carbon dioxide (CO2) levels on Earth have reached an all-time high, and mitigating man-made climate change is a defining challenge of our era. However the global economy and our society are critically dependent on fossil fuels, which meet 80% of the worlds energy demands and account for the production of 95% of all chemical commodities we rely on in our everyday lives. One example, ethylene, is a two-carbon molecule that is currently produced from fossil fuels in a highly energy-consuming and polluting petrochemical process. Ethylene is called a platform chemical as it used to synthesise a wide range of other chemicals, and is a crucial monomer in many common plastics.
One emerging technology is to enable the direct conversion of CO2 emissions into carbon-based chemicals, thereby much reducing the environmental damage caused by their production. Nature uses this 'waste' gas as its primary one-carbon building-block for biomass, and the chemical industry is beginning to realise the potential of CO2 as a cheap, renewable feedstock for producing of vital chemicals such as ethylene.
Conversion of CO2 is challenging as the molecule is very stable and unreactive, and a vast energy input is required to make it react. Catalysts are needed to lower this energy requirement and interest is growing in new transition metal catalysts for CO2 activation. The scalable industrial application of this technology is currently held back by poor catalyst efficiency and low selectivity for a particular carbon-containing product. Although catalysts have been developed to generate one-carbon products from CO2, there are very few examples in which multi-carbon (C2+) products are formed. This is a key barrier to be overcome: C2+ compounds like ethylene represent the best trade-off between high economic value and a reduction in global warming potential, if they could be produced from CO2 using renewable electrical energy. This is an opportunity with massive potential impact for decarbonising innovation at scale in the chemicals industry, since more ethylene is produced each year than any other organic compound, and its annual production releases around 200 million tons of CO2. Of the existing catalysts, none are sufficiently active or selective for C2+ products. This is partly due to a lack of fundamental understanding about the requirements for C-C bond forming; this theoretical underpinning is needed to make rational steps to design improved catalysts.

To address these challenges, this research takes inspiration from enzyme catalysts which are able to reduce CO2 to C2+ hydrocarbons with good activity and selectivity - but cannot be scaled. The model enzymes are the carbon monoxide dehydrogenases for CO2 reduction to CO in Nature, and nitrogenases for reduction of nitrogen to ammonia. These enzymes harbour multiple transition metals in their active sites, positioned where coupling of two CO2 units can occur. In a collaboration initiated by recent discoveries in the researchers' laboratories, hybrid catalysts that combine the benefits of synthetic catalysts and enzymes will be developed. Their use in synthetic CO2 conversion will be tested, taking this principle of confined catalyst sites to promote C-C bond formations between multiple metal sites.
Producing chemicals from CO2 requires an energy input, and energy must come from a decarbonised source to reduce emissions. Our catalysts will use electrons to drive the reaction, since renewable electrical energy is becoming increasingly available at low cost. Studying the reaction mechanisms 'on-the-fly' will inform the design of more efficient catalysts, with the ultimate aim of realising a catalytic method for converting CO2 into any carbon-containing molecule.
Borrowing a trick or two from enzymes, this research will move the chemical industry a step closer to becoming part of a true, waste-free, circular economy, as well as helping to make the goal of generating negative CO2 emissions a reality.