Controlled ensemble size and elucidation of structure-sensitivity: a new approach in nitride catalysis

Most industrial chemical processes use catalysts to enhance the rate at which a reaction proceeds or to favour a specific product. Catalysts thus lead to significant savings in the energy used for large scale processes and a reduction in the amount of waste generated. Amongst the catalysts applied, heterogeneous catalysts are the most practical due to the ease of their separation from product streams.
This project will combine synthetic and application based advances to metal nitrides which are an emerging class of heterogeneous catalyst with interesting and distinctive properties. The approach taken is to exert control over both the size and composition of active centres dispersed within a host matrix. Controlled ensemble size has proven to be a powerful approach with other categories of catalysts. We propose to develop novel materials with enhanced activity for ammonia synthesis, a vitally important reaction on a global scale for which some metal nitrides have already shown activity of potential interest for industrial application. We also plan to target high activities in two other important processes - ammonia decomposition, which is a potential clean hydrogen source for renewable energy applications, and ammoxidation, important in chemicals synthesis. The latter is a particularly challenging target which requires catalyst design that combines different functionalities within active centres.
Our programme combines the development of catalyst synthesis routes, which will be used to control the size and elemental composition of the catalytic sites with extensive catalyst testing, involving reactions performed under industrially relevant conditions.

The work will design new catalysts and develop new catalytic processes. Heterogeneous catalysis is of key economic importance for the chemicals industry, which contributes £50 billion per annum to the UK economy. Furthermore, the identification of more active and/or more selective catalysts leads to significant environmental benefits by reducing energy requirements of processes, utilising starting materials more efficiently and reducing the amount of waste generated. We will concentrate on three reactions of significant economic and environmental impact:
(i) Ammonia synthesis uses the Haber-Bosch Process, directly combining N2 and H2 with iron-based catalysts. This 90 year-old high pressure, high temperature process accounts for 1% of global energy demand including purification of N2 and H2. However, it can be directly credited with sustaining the world's population through access to synthetic fertilisers and provides an important chemical reagent. Liquid ammonia is also a high energy density fuel that can readily be transported at moderate pressure, and hence ammonia synthesis is a potential means to store renewable energy in a versatile form. It is readily apparent that any improvement in the ammonia synthesis process could yield massive returns. Testing will be carried out in conjunction with Johnson Matthey Catalysts, an internationally-leading leading British-based manufacturer of industrial ammonia synthesis catalysts.
(ii) Ammonia decomposition is of current interest in relation to the development of the hydrogen economy. The liberated H2 is free from the presence of COx and can therefore be applied directly in PEM fuel cells, which are susceptible to poisoning by trace CO present in hydrogen generated by reforming. Some power stations employ selective catalytic reduction using NH3 to remove NOx from waste gas generated in fossil fuel combustion - NH3 decomposition catalysts could also be used to guard against NH3 slippage to the atmosphere.
(iii) Ethane ammoxidation yields acetonitrile, which is a solvent for many large-scale applications. Using an alkane as the hydrocarbon reactant is economically and environmentally desirable as the commoner alkene reactants are produced by an energy-intensive high temperature steam cracking route. In 2008/9, a worldwide shortage of acetonitrile resulted from the global reduction in capacity for acrylonitrile from which it is a by-product. Alternative production routes are needed to maintain stability of supply.
The project will develop generic methodology which will extend beyond the three targets outlined above to further catalytic processes and other applications. The postdoctoral researchers will need to develop new skills in their own fields (materials and catalysis) but will also benefit strongly from the broad combination of expertise required in this interdisciplinary project. The interactions with Johnson-Matthey will provide a strong industrial perspective to all those involved in the project, and will provide an opportunity to gain a high level of familiarity with industrial catalysis through direct involvement in their testing programmes.