POSITRON EMISSION IMAGING IN CATALYSIS

In situ imaging technology developed for nuclear medicine is now being applied to study the kinetics of heterogeneous catalytic reactions un der actual process conditions. Minute quantities of molecules (ca. 10- (15) mel), radio-labeled with positron-emitting isotopes such as C-11; N-13; or O-15, are injected as pulses into the feed streams of chemica l reactors. Subsequent coincident detection of pairs of gamma photons produced via positron-electron annihilation allows the concentration o f reactants, intermediates and products to be mapped as a function of both time and position within the reactor bed. As well as providing qu alitative information regarding the mechanism of the reaction under in vestigation the data obtained can be compared with mathematical models based on the reaction kinetics in order to refine parameters such as activation energies and pre-exponential factors for elementary reactio n steps. Since the technique is capable of imaging transient phenomena , information is provided that is not accessible to steady-state techn iques.