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Predicting the effectiveness of metal catalysts
17 November 2009
CNRS (Délégation Paris Michel-Ange)
Catalysis is a process that is widely used in industry to synthesize molecules or materials. However, determining catalytic mechanisms is a major challenge for modern chemistry. Researchers at the Laboratoire de Chimie de Lyon (CNRS / Ecole normale supérieure de Lyon) have used numerical simulation methods to show how the selectivity of reaction mechanisms at the surface of a metal catalyst can be understood far more simply. This discovery marks an important step forward in the identification of competitive catalytic mechanisms and therefore in the design of innovative, cheaper and cleaner processes. The results are published in the 9 November 2009 issue of the journal Angewandte Chemie International Edition.
In industry, catalysis is used for the synthesis of plastics, drugs, cosmetics and fuels. Catalysts not only accelerate chemical reactions, but can also be used to drive them along pathways through possible reaction networks, thus getting more efficiently to the desired product while avoiding undesirable by-products. In this way, controlling the reaction mechanisms of a catalyst, according to its nature and structure, enables the reaction to be directed towards the desired compound. In order to identify these mechanisms, intensive computations are needed to determine the energy barriers involved, which are associated with the identification of transition states, intermediate points on the synthetic pathway of a product. This method is therefore currently limited to simple reaction schemes. In order to synthesize more complex molecules of use to society, such as drugs, the number of possible reaction networks rapidly becomes very great, and an exhaustive exploration of the associated mechanisms is almost impossible. The selectivity of the reaction (ie the pathway that will be followed) is therefore extremely difficult to understand and to control.
The researchers showed that the activation energy for one step in a series of catalytic reactions can be predicted by simply calculating the bond energy between the reactants and the surface of the catalyst, which is a much simpler quantity to evaluate than the energies of transition states. The reaction they studied was the interaction between hydrogen and an organic compound on a platinum surface. This molecule contains four possible sites for the reaction, which leads to a complex network of 32 elementary steps that need to be considered in the mechanism. How does the catalyst drive the reaction through this maze? The researchers showed that, for each set of steps related to a reactive site, there is a correlation between the activation cost of conversion and the bond energy of the reactants at the surface just before the reaction occurs. Besides its simplicity, the correlation found provides especially reliable predictions that can be extended to a wide family of organic molecules.