Monitoring of Surface Processes

McCarroU and Thomson have described the use of a solid-state windowless device for the direct monitoring of surfaces under UHV conditions. The detection system had excellent vacuum properties, a high detection efficiency for [14-C] 3 particles, and good background characteristics. However, since the detector was light sensitive, the apparatus had to be operated in total darkness. 

Tritium, another widely used isotope in catalytic investigations, emits a 

Self-absorption problems were encountered by Paal and Thomson in a study of the retention of [3-H] by a Pt black catalyst. After various pretreatments residual radioactivity was detected by scintillation counting of a suspension of catalyst in a liquid scintillator. The results provided evidence that considerable amounts of tritium were retained by the catalyst. However, the absolute amount present could not be obtained because some of the tritium may have been in the pores and interstices of the Pt, resulting in severe self-absorption effects. 

Mechanistic Studies. - The application of radioactive tracer techniques to mechanistic studies in heterogeneous catalysis is, in principle, the same as their application to homogeneous reaction systems. A labelled compound, for example, a possible intermediate, is added to the reaction mixture. Gas-chromatographic separation of the reaction products and immediate radioactive assay of the separate components yields information about the chemical identity of the radioactive products and gives a quantitative measure of the degree of incorporation of the radioactive label. 

The choice of detector is determined by the radionuclide to be assayed and the required temperature of operation. A paper by Cacace compared Geiger-Muller counters, internal flow proportional counters, scintillation counters, and ionization chambers. Perhaps the simplest to use, in conjunction with a chromatographic column, is the internal flow proportional counter, which has considerable advantages for the detection of [14-C] and [3-H]. ° Schmidt-Bleek and Rowland have described a counter, constructed of brass and Teflon with a stainless-steel anode, which is very robust, inert 

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