Characterization of Catalytic Surfaces by Isotopic-Transient Kinetics during Steady-State Reaction

Steady-state isotopic-transient kinetic analysis (SSITKA) is a very useful technique for the kinetic study of heterogeneous catalytic reactions. Initially developed by Happe1,l Bennett,2 and Biloen? SSITKA has been used in a number of studies to determine in situ kinetic information about the reaction mechanism and the catalyst-surface reaction intermediates. The technique is based upon the detection of isotopic labels in the reactor effluent species versus time following a switch (step change) in the isotopic labeling of one of the reactant species in the reactor feed. In addition to maintaining isothermal and isobaric reaction conditions, the reactant and product concentrations and flow rates remain undisturbed during the step change. Thus, in the absence of isotopic mass effects, steady-state reaction conditions are maintained under isotopic-transient operation. The reaction intermediates present on the catalyst surface do not change, and unlike for other transient technique^,^-^ analysis of the steadystate kinetic behavior of the catalyst surface is possible. An isotopic-step input is typically used so that the transient behavior of the less-active catalytic sites is included in the kinetic determinations, information which may be lost when using an isotopic-pulse technique. Steady-state kinetic information which has been obtained from SSITKA includes concentrations of different types of adsorbed reaction intermediates, coverages, surface lifetimes, site heterogeneity, activity distributions, and identification of possible mechanisms. This overview of SSITKA includes discussions of the technique of steady-state isotopic-transient labeling for kinetic study, the mathematical formalisms used in transient analysis, the kinetic parameters