Steady-State Isotopic Transient Kinetic Analysis (SSITKA)

The principles and practical use of steady-state isotopic transient kinetic analysis (SSITKA) for studying heterogeneous catalytic reactions are presented. SSITKA combines the advantages of steady-state and transient techniques. A basic requirement for SSITKA is that steady-state conditions are maintained during the isotope switch, and for 12C/13C, 14N/15N, and 16O/18O, the isotope effect is small and these isotopes are all used. The use of H2/D2 is not straightforward since a H2/D2 switch may induce kinetic isotopic effects. Hydrogen spillover to the support and exchange with surface hydroxyl groups may also take place during a H2/D2 switch. The SSITKA principle is outlined based on a simple reaction with adsorbed intermediates. The presentation of the technique is then extended to include complex multi-pool SSITKA and to reactivity distributions. A standard experimental set-up is described and several examples of reactions studied with SSITKA are included. The technique makes it possible to identify the abundance of intermediates and their kinetic parameters. It is not possible to determine the composition of the adsorbed intermediates, but recent developments combining SSITKA with spectroscopic techniques are promising developments. Examples of the combination of SSITKA and spectroscopic techniques are included.