Studies to the Functioning of Automotive Exhaust Catalysts Using In-Situ Positron Emission Tomography

Studying the reactions in an automotive exhaust catalyst is complex as the exhaust gases contain many components of which the composition constantly varies and the catalyst contains different active components. To elucidate reaction kinetics in exhaust catalysis, the application of a technique stemming from nuclear medicine, i.e. Positron Emission Tomography, has been developed.

Short lived positron emitting nuclides ¹¹C (t_1/2 = 20 min), ¹³N (t_1/2 = 10 min) and ¹⁵O (t_1/2 = 2 min) have been used to synthesise actual reactant molecules such as ¹¹CO, ¹¹CO₂, ¹¹CH₄, ¹³N₂, ¹⁵OO, C¹⁵O and C¹⁵OO. Pulses of picomole amounts of these labelled reactants have been added to synthetic exhaust gases, which were led over commercial and model automotive catalysts in a small plug flow reactor. The labelled compounds were detected and identified in both reactant and product stream using a gas chromatograph equipped with a proportional counter. On top of that, the labelled reactants in the catalyst bed could be monitored right through the reactor wall using a Positron Emission Tomograph, with a resolution of 1 cm in place and 1 s in time. Hereby, a large amount of information on the residence time of the reactants is obtained directly from the catalyst surface under actual reaction conditions.

By means of these experiments the rate limiting step in the carbon monoxide oxidation on platinum and rhodium under cold start conditions of a car were found to be the availability of free noble metal surface for the dissociative oxygen adsorption. The different behaviour of rhodium and platinum in the CO oxidation in the presence of NO was found to be the higher tendency for NO dissociation of rhodium. A strong interaction was found between gas phase CO₂ and CeO₂ at the catalyst surface, whereby the oxygen atoms contained in lattice CeO₂ exchange rapidly with the oxygen atoms of the adsorbed CO₂. A mathematical model for the reaction kinetics was constructed and by simulating the experiments with this model the kinetic parameters were quantified.

The experiments demonstrate the utility of short-lived positron emitter nuclides such as ¹¹C, ¹³N and ¹⁵O, for non-invasive and in-situ catalyst research. The importance of using in-situ catalyst surface information in understanding the working of automotive exhaust catalysts under actual reaction conditions is emphasised.