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

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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.