Reaction Mechanisms in Environmental Catalysis: The Scarlet Pimpernel of Catalysis

09:00 AM - 11:00 AM
Professor Robbie Burch, Queen's University, Belfast
NTRC Seminar
National Transportation Research Center, Room L-115
Email: Bill Partridge

While the empirical approach to the discovery of new catalysts and catalytic processes is well established, and still frequently used in an industrial context, there is potential merit in a more fundamental approach which is designed to develop an understanding of the reaction mechanism and the role of different reaction intermediates. In this presentation we will demonstrate the value of trying to understand the mechanisms of gas phase reactions. The investigation of the mechanisms of gas phase reactions has been studied in great detail for decades, principally because these reactions are easily investigated using transient, and kinetic isotope techniques which only require relatively simple and inexpensive equipment. Moreover, in the gas phase the response time of the equipment can be on the millisecond time scale, whereas for liquid phase reactions the timescale is of the order of tens of seconds. This allows much faster reactions to be investigated in the gas phase. However, it is not always recognized that in “conventional” mechanistic studies there can be serious problems due to retention of species on the surface of the support. It is often the case that these species will have characteristics, such as their infrared spectra, that are indistinguishable from true reaction intermediates adsorbed at active sites on a metal. Therefore it becomes impossible to monitor changes in the concentrations of the true reaction intermediates because of the large and dominant concentration of similar species on the support. However, it is generally the case that the active metal will very rapidly come to equilibrium with reactants, intermediates and products whereas adsorption on the support will be slower, perhaps by orders of magnitude. Therefore, by monitoring changes in the surface species after a very short exposure to the reactants it should be possible to differentiate between true reaction intermediates and spectator species that may appear identical in any spectroscopic measurement. In this presentation we will describe our Short Time On Stream (STOS) transient kinetic isotope technique and show how it can be used to investigate reaction mechanisms for a variety of reactions. Specifically, we will describe the use of the technique to investigate the reduction of nitrogen oxides (NOx) under lean-burn automotive emission control conditions and in which we identify an isocyanate intermediate that may be utilized to enhance the overall NOx reduction. In the case of the Toyota Lean NOx Trap (LNT) process we show that an isocyanate species is probably responsible for producing a second pulse of nitrogen when the rich mixture is switched to lean conditions, a surprising but technically important result which is now being developed for commercialization by Toyota. Finally, we will demonstrate the power of using isotopic labeling techniques to elucidate the mechanism of propene oxidation and carbon monoxide oxidation on diesel oxidation catalysts.


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