Fundamental Sulphur-Chemistry of Molybdenum Carbide Surfaces: Towards Catalytic Exploitation of Transition Metal Carbides

Many of the most important chemical reactions underlying the modern world are made possible only by the use of catalysts. These are substances that either speed up a chemical reaction or improve its selectivity towards the desired product - preferably both - whilst not themselves being used up in the process. Classic examples include ammonia synthesis from nitrogen and hydrogen, using an iron catalyst, and the manufacture of synthetic petrol or diesel from carbon monoxide and hydrogen, using a cobalt catalyst. Both of these processes are crucial to future economic development (ammonia for fertiliser production, and synthetic fuels for carbon-neutral transportation) and it is indeed fortunate that the metals involved as catalysts are in plentiful supply.

More often, the catalysts in use today are based on expensive and rare precious metals, such as platinum, palladium or rhodium (all used, for example, in the catalytic converters that remove harmful gases from car exhausts prior to emission) and the search for cheaper or better alternatives is correspondingly urgent. Another perennial issue, besides the cost and scarcity of certain catalysts, is one of gradual deactivation by the build-up of contaminants at the surface of the catalyst. Although the catalyst itself is not used up, the microscopic active sites where the chemical reactions actually occur can become blocked by unreactive atoms, and removing these to reverse the 'poisoning' of the catalyst can often involve considerable effort or expense. Again, the search for unconventional catalysts that are less prone to poisoning is extremely pressing.

In the present project, we will study the fundamental surface chemistry of a particularly unconventional catalyst, molybdenum carbide, which is extremely promising as a cheaper alternative to platinum and similar precious metals in many types of catalysis. One of the most interesting aspects of this chemistry relates to the behaviour of sulphur, which is a notorious catalyst poison that is deposited upon the decomposition of sulphur-containing molecules. Molybdenum carbide is known to be particularly resistant to sulphur poisoning, and indeed can be used to remove sulphur from the mixture of molecules produced by oil refineries (processing either traditional fossil fuels or green carbon-neutral biofuels) by enhancing the reaction of sulphur compounds with hydrogen in a process known as hydrodesulphurisation. Not only can this be of importance in reducing automotive and power-plant sulphur emissions (responsible for acid rain) but it can also massively improve the suitability of refinery products for use as feedstocks in the production of commodity chemicals, where the presence of sulphur compounds would poison many of the common catalysts. At present, this important function of hydrodesulphurisation is carried out with a catalyst containing cobalt and molybdenum sulphide, but molybdenum carbide could represent a leap forward in industrial practice from both the economic and the environmental perspectives.

In order to understand the interaction of sulphur compounds with molybdenum carbide, we will carry out infra-red spectroscopic measurements, capable of identifying the various products of decomposition that end up bound to the surface, and supersonic molecular beam measurements that allow us to determine reaction rates as a function of surface conditions and the state of incoming molecules. We will work under ultra-high vacuum conditions, and with extremely well-characterised samples, so as to obtain the most detailed results possible with state-of-the-art techniques. The information we can gather in this way will be of use to other scientists, in both academia and industry, who seek to optimise working catalysts based on this material.

In addition to academic beneficiaries, working particularly in the field of heterogeneous catalysis through surface science studies, we believe that our proposed work will significantly impact upon a range of individuals and institutions throughout the wider world. Many of these impacts derive from the possible significance of our fundamental research in guiding more applied catalytic studies that will in turn eventually lead to industrial deployment. The development of transition metal carbide catalytic technologies has undoubted potential to enhance wealth creation and environmental sustainability through opportunities for hydrodesulphurisation of refinery products (both petroleum-based and biomass-derived) destined for use as fuels or as feedstock for commodity chemical production. Additional applications in a range of catalytic processes, including hydrogen production, ammonia synthesis and aromatisation, will also benefit from our findings. We furthermore identify a variety of stakeholders in our research, including industrial partners, taxpayers and policy-makers, and propose specific strategies to ensure that an appropriate dialogue is maintained throughout the grant period and beyond.

Our engagement with industry already has a solid foundation through our links with collaborative partners (recently Johnson Matthey, BP and Shell) and our direct communication with the research staff of these companies provides an excellent basis for efficient knowledge transfer. In a number of cases, individuals trained within the Surface Science group have progressed to work within our partner companies in research-based positions, which again provides fruitful avenues through which to pursue collaboration. We nevertheless seek also to diversify our industrial contacts, and plan to make use of initiatives available at Departmental and University level to achieve this (the Corporate Associates Scheme within the Department of Chemistry; the Research Horizons magazine and seminar series within the University of Cambridge).

Engagement with the public is primarily addressed at present through the group's website, which maintains staff profiles, list of publications, explanation of techniques and summarised highlights of recent research. Our aim is to communicate enthusiasm for the fundamental blue-sky aspects of our work, whilst simultaneously outlining practical applications that may eventually follow. Our work is also publicised on the Department of Chemistry website, and within the Department's broadly distributed Chem@Cam newsletter. We intend to ensure greater coverage on the University of Cambridge website in future through contact with the Office of External Affairs and Communications, which hosts a staff member dedicated to promoting the public profile of the University's scientific research. We believe this will also prove to be a valuable resource in helping us achieve coverage in the national and international press (newspapers, popular scientific periodicals, websites, etc).

In respect of engagement with policy-makers, we aim to develop contact with the Liberal Democrat MP for Cambridge, a working scientist who completed his PhD within our Department only six years ago. We anticipate that he will be keen to represent an informed view of science within Westminster, and we will invite him to visit our laboratories at the earliest opportunity, so that he can be brought up to speed on the benefits to society of surface science and heterogeneous catalysis. At the same time, the Department's Chemistry Advisory Board provides another opportunity to present our work to influential figures within the establishment, as does the Cambridge College system. We furthermore expect that our strong links with key individuals at the Smith School of Enterprise and the Environment, notably the former Chief Scientific Advisor to H.M. Government Professor Sir David King, will yield additional routes to inform policy-makers in general terms.