Application of 11C and 15O positron emitters in exhaust catalysis research

The need for environmental conservation has fostered the research, development and application of environmentally acceptable processes. In this respect automotive exhaust catalysis has grown in importance ever since in the USA in 1970 Congress passed a series of amendments to the Clean Air Act. In Europe the implementation of automotive exhaust catalysis started in the 1980s and still is going on. At present, the technological answer to emission standards is the ‘controlled three-way catalyst’, named after its ability to reduce hydrocarbons, CO and NO emission levels simultaneously. The main ingredients of most commercial automotive exhaust catalysts are platinum, palladium, rhodium and ceria (Ce02)15. In a strong simplification Pt and/or Pd are mainly present for the oxidation of CO and hydrocarbons6, while Rh serves for the reduction of NOX7~° and the low temperature oxidation of CO1’. The most important function of the CeO2 is the low temperature catalysis of the water gas shift reactio&2. The activity of the catalyst is fatally affected by the presence of small amounts of lead oxide or phosphorous oxide in the exhaust gases13. The kinetic behaviour of automotive exhaust catalysts is complex as many interrelated reactions take place between the various components present in exhaust gases and the different active materials present on the catalyst. On top of that, the dynamic response of automotive exhaust catalysts has to be fast and should be able to cope with a variety of conditions as the composition of exhaust gases emitted by the engine of a driving car vary rapidly. Three-way exhaust catalysis is strongly diffusion limited complicating a study of the kinetics under actual process conditions (in-situ) even more. This complexity makes it difficult to obtain information on in-situ exhaust catalysis. Information about the state of a catalyst surface under actual reaction conditions can be derived using Fourier transform infra-re&4 and Rama&3 spectroscopic techniques. However, quantitative evaluation of the recorded spectra is virtually impossible in systems where many distinct components are present at the catalyst surface. Labelling techniques16 utilising 3H and ‘4C may provide information about reaction mechanisms and rate limiting steps in reaction kinetics, but with these radionuclides no in-situ information from the catalyst surface can be obtained.