Bio-inspired sulfide nanocatalysts: From proof of concept to 'real' catalysis

Sustainable energy and climate change are areas of global societal concern, which is a recognised strategic priority area of the RCUK through their Energy Programme, managed by EPSRC. Catalysis, moreover, is the lynchpin of a large number of industrial processes, which are instrumental in maintaining global wealth and health, as well as playing a key role in developing processes that are both environmentally and economically sustainable.

Despite the high thermodynamic stability of CO2, biological systems are capable of both activating the molecule and converting it into a range of organic molecules, all of which under moderate conditions. It is clear that, if we were able to emulate Nature and successfully convert CO2 into fuel or useful chemical intermediates without the need for extreme reaction conditions, the benefits would be enormous: One of the major gases responsible for climate change would become an important feedstock for the fuel, chemical and pharmaceutical industries!

Iron-nickel sulfide membranes formed in the warm, alkaline springs on the Archaean ocean floor are increasingly considered to be the early catalysts for a series of chemical reactions leading to the emergence of life. The anaerobic production of acetate, formaldehyde, amino acids and the nucleic acid bases - the organic precursor molecules of life - are thought to have been catalyzed by small cubane (Fe,Ni)S clusters (for example Fe5NiS8), which are structurally similar to the surfaces of present day sulfide minerals such as greigite (Fe3S4) and mackinawite (FeS).

Contemporary confirmation of the importance of sulfide clusters as catalysts is provided by a number of proteins essential to modern anaerobic life forms, such as ferredoxins, hydrogenases, carbon monoxide dehydrogenase (CODH) or acetylcoenzyme A synthetase (ACS), all of which retain cubane (Fe,Ni)S clusters with a greigite-like local structure, either as electron transfer sites or as active sites to metabolise volatiles such as H2, CO and CO2.

In Phase 1 of the project, we have used a comprehensive combination of computational, synthetic and electrochemical expertise to mimic Nature and produce Fe-S and Ni-doped Fe-S nanoparticles to catalyse the conversion of CO2. Careful and sensitive testing of the computationally designed materials, prepared through novel synthesis routes, has shown unequivocably that the nanoparticles have the power to adsorb CO2 and reduce it to formic acid - a useful chemical intermediate. A particularly promising aspect is that the catalytic conversion of CO2 takes place at room pressure and temperature and at the sort of low voltages that could be obtained from solar energy, thus making it a sustainable process. Following this success, in Phase 2 of the project we aim to optimise the catalysts to improve yield and adapt for further product formation e.g. methanol, acetate and, eventually, dimethyl ether (DME) - all proven pre-cursors to fuels and fine chemicals - and to develop materials and processes that are robust enough to perform under 'real' conditions.

Work in this area, in collaboration with a number of industrial partners, requires the dove-tailed interplay of experiment and computation to design, synthesise, characterise and catalytically test the potential transition metal-sulfide nano-catalysts, followed by scale-up of the nanoparticle production and evalulation in an industrial environment. The aim at the end of Phase 2 is to have created a commercially viable catalytic system for CO2 reduction, that performs in an industrially relevant environment.

The research in this project will clearly have impact on:

* Society, by developing effective catalysts for the utilisation of CO2, instead of rapidly dwindling fossil fuel resources, for the production of fuels and chemicals under mild and sustainable reaction conditions, thus assisting in stabilising energy supplies and maintaining our quality of life, as well as alleviating pressure on the environment;

* The Economy, through the design of new (photo)catalytic materials - for commercialisation by catalyst manufacturers - and devices for alternative and sustainable routes to important products. Catalysis is at the heart of the chemical industry - an immensely successful and important part of the overall UK economy, generating in excess of £50 billion per annum.

* Knowledge, both academic and commercial, as the new atomic-level insight into the structure and performance of the active site(s) and the nano-catalysts as a whole - obtained from the computational models, in situ synchrotron experiments and electrochemical screening - will deliver significant advances in catalyst design and optimisation and more widely in materials (nano-)science, and will thus enhance the knowledge base of chemical, energy and other relevant industries;

* People, through the technical expertise developed by the researchers during the project, the training received by them in societal and ethical issues, the fruitful links with industry initiated through the project, and the transferable skills developed in engagement with the media, the general public, policy makers and legislators.

In addition to the obvious benefits to academic researchers in the field (see Academic Beneficiaries section), the research will benefit in particular (i) ) the UK and global commercial sector, but also (ii) the general public, (iii) the public sector, and, more speculatively (iv) voluntary workers and charities.

(i) Commercial sector
Many industrially crucial processes, e.g. the water-gas shift reaction, still take place under extreme conditions of pressure and temperature, thus making them environmentally unsustainable in the long term. Furthermore, existing catalysts often depend on the use of expensive noble metals (Pt, Pd, Au) and transition metal compounds (e.g. ceria), which are often only available from limited sources and countries, or toxic elements, such as chromium in iron-oxide catalysts, whose use is increasingly limited by EU legislation. The development of novel catalysts, which operate under milder conditions and can utilise a sustainable alternative to fossil fuels, will clearly benefit both the catalyst manufacturing industry and the companies employing these catalysts in their production processes, e.g. pharmaceuticals and chemicals manufacturers and CO2-to-fuel producers.

(ii) The General Public
Everyone, whether living in highly industrialised countries or in the developing world, is affected by climate change and rising costs of energy and commodities. A robust and sustainable route for the recycling of CO2 into synthetic fuels and chemicals will ensure stable and predictable energy prices, especially as synthetic fuels, unlike other energy alternatives, do not require extensive changes to existing infrastructure

(iii) Government/Public Sector
With ever more stringent legislation put in place to guarantee a cascade of international agreements to reduce CO2 and other greenhouse gases to acceptable levels, viable routes to CO2 utilisation are clearly of prime importance to policy makers and legislators.

(iv) Third Sector
More speculative beneficiaries of this research are charities and voluntary organisations, who are called on for help in weather-induced disasters due to climate change, or disruption and hardship caused by rising energy costs.