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The University of Southampton
Chemistry

Research project: Evans: Heterogeneous Catalysis and Surface Organometallic Chemistry

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Our in situ heterogeneous catalyst studies have concentrated on the automotive exhaust catalyst, which typically involves rhodium, palladium and platinum to remove NO, CO and hydrocarbons from the exhaust stream.

Molecular assembly
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It is known that these catalysts do not operate optimally at the stoichiometric fuel ratio, so finding out a way to optimise this is of some importance. This mixture of oxidising and reducing gases has a dramatic effect on the composition of rhodium catalysts, which changes with the partial pressure of the gases and the operating temperature. (http://www3.interscience.wiley.com/cgi-bin/abstract/96516344/ABSTRACT)

But it is the catalyst composition that governs activity and selectivity. So unravelling this synergistic relationship between gas streams and catalyst is the key to further developments of these catalysts. We are members of the new EC Network of Excellence IDECAT which has a holistic approach to catalyst design from the design of specific sites, through the nanoscale architecture of catalyst supports right through to the reactor design.

In situ studies of automotive exhaust catalysts
The interrelationships between the platinum group metals and the oxide support is the key to optimising the performance of these three-way catalysts over the varied reaction conditions from a cold start to cruising at high speed. We have found that palladium protects rhodium form oxidation by NO, (http://www3.interscience.wiley.com/cgi-bin/abstract/109561381/ABSTRACT) But there are many interactions – involving platinum and the active oxide supports – that we have not yet probed and are fascinating prospects.

Synthesis of new site-specific catalysts
The complexity of the materials formed by conventional synthesis of heterogeneous catalysts makes them difficult to study and also they are not optimised. So we aim to utilise what we are learning from these conventional catalysts to design task-specific catalysts.

Surface organometallic species
On a molecular level, creating a reactive centre which is anchored to a benign support has many advantages. This eases the separation of catalyst from the products, and can also much reduce the reactor volume as compared to homogeneous catalysts under high dilution. By using the support as a ligand, a positive synergy can be achieved which provides unique catalyst sites. The principles of organometallic chemistry can be applied to the synthesis of transition metal species bound to the surface of oxide supports. For example, rhodium carbonyl centres can be readily synthesised on oxides such as alumina and titania. These allow some fundamental reactions to be investigated, such as the formation of metal clusters and the reaction with nitric oxide, one of the car exhaust gases. The diagram shows the monitoring of the reversible reaction of Rh(CO)2 on alumina with NO using mass spectrometry and X-ray absorption spectroscopy. These experimental results are being, linked to theoretical approaches which can aid in establishing likely structures, and estimate the a kinetic and thermodynamic parameters of fundamental reaction steps.
(http://pubs.acs.org/cgi-bin/abstract.cgi/jpcbfk/2002/106/i16/abs/jp013749k.html)

Molecular Assembly
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Spatially confined nanocatalysts
The relatively new oxidic materials, the mesoporous silicas have ordered arrays of pores of 25 - 40Å which are well matched to accommodate organometallic and cluster species and yet still have space to allow a catalyst substrate to enter the pore and react at the catalyst centre. These pores will generate an electric field at the catalyst which in term may affect its reactivity. So we aim to compare the catalytic activity of species in these pores with that on external surfaces. The spatial confinement should also enhance selectivity..(http://pubs.acs.org/cgi-bin/abstract.cgi/jpcbfk/2005/109/i07/abs/jp0404394.html)

Catalytic nanodots
Some catalytic reactions require ensembles of centres rather than specific molecular sites. Some heterogeneous metal catalysts consist of a dispersion of metallic nanodots. And some key reactions, for example the cleavage of the NO molecule on rhodium nanoparticles, are not evident on mononuclear sites. So engineering nanodots to optimise these key reaction steps is an important target.

Related research groups

Functional Inorganic, Materials and Supramolecular Chemistry
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