Selective photocatalytic conversion of CO2 to olefins: a feasibility study

The quest for sustainable resources to meet demands of a constantly rising global population is one of the main challenges for mankind this century. Worldwide concern over the impact of CO2 emissions on climate change means there is an urgent need to reduce our dependency on oil as a source of chemicals. Oil accounts for the vast majority of chemical feedstocks, however to be truly viable alternative feedstocks must be sustainable, that is "have the ability to meet 21st century energy needs without compromising those of future generations." The recent UK Fourth Carbon Budget set the ambitious target of a 50 % cut in CO2 emissions by 2025 compared with 1990 levels. CO2 utilisation as a chemical feedstock is a particularly attractive strategy to ameliorate carbon emissions while offering sustainable, safe and useful carbon capture. Current CO2 utilisation for chemical synthesis (principally urea) accounts for only 2 % of emitted CO2, but forecasts predict such approaches could mitigate 300-700 Mt (megatons) CO2 per year, far larger than the combined potential for CO2 abatement by nuclear, wind and cellulosic biofuel technologies (~50 Mt CO2 per year). Indeed the recent CS3 White Paper "A Sustainable Global Society" highlights photocatalytic CO2 conversion to chemicals as an area where comprehensive fundamental materials chemistry research is essential.

Olefins and their polymers are the single largest chemical commodity in the world, with global ethene and propene production capacity in 2010 estimated to be 123 and 77 Mt/year respectively. Commercial ethene and propene manufacture from oil involves steam or catalytic cracking of naphtha, gasoil and condensates to hydrocarbon mixtures followed by distillation. Steam cracking is the most energy-consuming process in chemistry, accounting for 8% of the sector's primary energy use and annual CO2 emissions of 180-200 Mt! Photocatalytic CO2 reduction (PCR) offers a potentially economical and environmentally-benign CO2 utilisation process, facilitating long-term carbon entrainment within e.g. plastics and polymers, and the creation of new chemical supply chains free of current dependencies on oil, coal and natural gas.

This feasibility study will develop novel photocatalysts critical to achieving the selective photoreduction of CO2 to ethene (i.e. 2CO2 + 2H2O -> C2H4 + 3O2), thereby underpinning resubmission of the TranSChem Programme Grant application that seeks to integrate such nanostructured inorganic photocatalysts with the exceptional light-harvesting properties of biological pigments, inside novel solar photoreactors for maximum process intensification of CO2 PCR to olefins.

This project will provide a pathway to alternative chemical resources, while helping to mitigate emissions and climate change associated with current fossil fuel use. Success will contribute to achieving the UK's greenhouse gas emission targets, and therefore, impact positively on human health and the environment, and in turn on the robustness of the UK economy. Direct solar conversion of CO2 to chemicals would also stimulate a new manufacturing base. There is a very strong international dimension to the proposed research, since CO2 abatement and renewable energy are areas of intense global research featuring highly on the political agendas of our economic competitors. This project represents a critical step to establishing the scientific and technical principles underpinning photocatalyst design and the associated knowledge base, establishing the UK at the forefront of CO2 solar chemical technologies. Outputs will guide the formulation of new photocatalysts (and photoreactors) selective for olefins production from CO2, and provide design principles pertinent to other important photochemical reactions such as water splitting, with ancillary benefits to basic nanotechnology, energy and environmental research.

This proposal aligns with the EPSRC Delivery Plan 2011-1015 highlighting Catalysis and Energy efficiency as areas of primary importance to the Manufacturing the future and Energy challenge themes, wherein sustainable high-value manufacturing and low-carbon energy are areas of strategic importance. Our project also addresses the EPSRC priorities identified in the solar energy portfolio '...to develop novel light harvesting technologies such as artificial photosynthesis that offer the potential for innovative low cost alternatives', conventional generation '...more efficient and reduce its environmental impact', and catalysis '...development of new catalytic processes'. Solar chemicals production from CO2 also aligns closely with the EPSRC CO2Chem Grand Challenge Network, and policy reports from stakeholders such as the RSC whose 2012 Solar Fuels and Artificial Photosynthesis report highlights photocatalytic CO2 conversion as an area where "The UK needs to develop strategies for investing in the next generation of solar energy researchers.." notably "..PhD students, postdoctoral researchers and university faculty members who are progressing towards leadership positions". This feasibility study also complements existing EPSRC-funded projects on solar routes to H2/fuels (EP/F056230/1, EP/F047851/1, EP/H046305/1), photovoltaics (EP/F056702/1), and our own recently funded Challenging Engineering award for 'Solar fuels via engineering innovation' (EP/K021796/1), but focuses on an entirely different goal, namely that of solar chemicals, and methodology, namely hierarchical photocatalyst design (rather than photoreactor modelling/optimisiation as in the latter).

We anticipate that our research will benefit academic and industrial researchers both within the UK and globally across catalysis, materials science, condensed matter physics, sustainable chemistry and reaction engineering, and the outcomes offer new knowledge specifically in solar energy conversion and carbon dioxide utilisation. Specifically, this feasibility study will directly underpin future funding applications by the TranSChem consortium, comprising 14 UK and international academics from across the biophysical and social sciences, backed by commercial project partners from the petrochemical, catalyst manufacturing and energy sectors, which seeks to develop solar chemical technologies based upon the scientific breakthroughs targeted in the present grant application. The PDRA will also receive extensive interdisciplinary training in the physical and engineering sciences.