SURFACE CATALYSIS STUDIED BY INSITU POSITRON EMISSION

A COMPLETE understanding of heterogeneous catalytic processes requires quantitative in situ information on the concentrations of reactants present on the catalyst surface, but few techniques are available to supply this information. Here we show that positron-emitter labelling and scanning, using the isotopes C-11, N-13 and O-15, can be used to study the reactions taking place during the catalytic conversion of automotive exhaust. By introducing small pulses of labelled molecules into a reactant stream passing through the catalyst, in situ quantitative information on the concentrations and residence times of reactants in the reactor is obtained. The labels are detected using a positron camera, an imaging device adapted from nuclear medicine, and the recorded data are presented as 'reaction images', showing quantitatively the distribution of the label in the catalyst bed as a function of position and time. These data can then be used to quantify reaction kinetics by serving as the input to mathematical simulations based on elementary reaction steps.