Experiments and processes in the transient regime for heterogeneous catalysis

Experiments in the transient regime are now widely used in mechanistic studies of heterogeneous catalysis. They permit the quantitative determination of the surface composition during catalysis and also give important information on the sequence of steps that underlie the observed global reaction. In particular, by computer-based modeling and simulation, the forward and backward rate constants of the steps can be estimated so that the behavior of the reacting system can be simulated by numerical methods over a wide range of variables. Results from transient experiments lead to reliable models for reactor design, and they are essential for reactors operating in the transient regime, a situation becoming increasingly common. In this review, I consider the general principles of common experiments in the transient regime: step response, pulse response, frequency response, and temperature-programmed response. The use of feeds of stable isotopes is widespread in transient experiments, greatly increasing the power of these methods. All these methods can be applied at atmospheric pressure and higher, and also under surface science conditions. Following a discussion of general principles, many examples have been chosen from the literature for more detailed case studies. Most of the reactions chosen have a practical application since the design of large-scale reactors requires quantitative information that is best obtained by transient experiments. Among the examples treated are those involving methane or carbon monoxide as reactants, and attention is also devoted to the reduction of the concentration of pollutants in exhaust gases. The response of the laboratory reactor is manifested by the concentration of the various components in the gas leaving the reactor as a function of time. Mass spectrometry is well suited for these measurements, but it is extremely useful to also measure the response of the surface phase composition of the catalyst. This measurement is usually made by infrared spectroscopy (including diffuse reflection from powders), but interesting results have also been obtained by Raman spectroscopy. X-ray spectroscopy, Mossbauer spectroscopy, and scanning tunneling microscopy. These are all applicable at atmospheric pressure; under vacuum conditions, there are many other techniques available. In the search for better catalysts or for a better understanding of structure-performance relations, the determination of the rate parameters of the elementary steps of a reaction is of great utility. © 2000 Academic Press Inc.