(337f) Transient Kinetics in Transition: A Case of Heterogeneous Catalysis

The evolution of different approaches to transient kinetics since the 1960s until today is presented focusing on heterogeneous ‘gas-solid’ catalysis. Relaxation methods implemented into heterogeneous catalytic studies were originally sparked by Manfred Eigen’s work on measuring fast chemical reactions. In 1960-1980s, a battery of transient kinetic methods was developed, i.e. relaxation studies in CSTR (Bennett), tracer studies in PFR (Kobayashi H and Kobayashi M.), dynamic adsorption measurements (Tamaru), SSITKA-experiment for determining the number of active catalytic sites (Happel, Bilouen). Most of these methods were supplemented by corresponding computer calculations. However, all these approaches did not provide ‘model-free’ non-steady-state kinetic information obtained under conditions of small chemical non-uniformity within the reactive zone. Also, the rigorous mathematical theory for the non-steady-state experiments was not build up.

Since the end of 1980s, the development of Temporal Analysis of Products (TAP), pulse-response technique, created a new situation in catalytic transient kinetics [1-2]. Instead of advective transport, the well-defined Knudsen diffusion was chosen as a ‘measuring stick’ for revealing the intrinsic chemical information. The contemporary TAP strategy, especially the Thin-Zone TAP (TZTR)-approach, is intended to fulfill three principles: (1) Uniformity of the active zone regarding the composition; (2) Insignificant change of catalyst composition during the single pulse; (3) Controlled change of catalyst composition during the series of pulses.

During first two decades of TAP studies, most of achievements have been obtained in two areas: (1) development of the TAP reactor configuration with illustration of the technique on oxidative catalytic reactions; (2) development of the new methodological and theoretical framework of interrogative kinetics (‘chemical calculus’). In present TAP studies, the mainstream is focused on the systematic application of the pulse-response methods to improving/designing catalysts and catalytic systems using detailed kinetic information. New perspectives and goals have arisen: (a) development of the standard protocol for the precise non-steady-state catalyst characterization based on the ‘catalyst state scale’ (e.g. reduction-oxidation degree); (b) decoding the mechanism of the complex catalytic procedure based on the vast non-steady-state information (rates, concentrations, uptakes/releases) and special interrogative procedure [3].

The Renaissance of transient kinetics in heterogeneous catalysis may, potentially, change a contemporary “structure-activity” paradigm for understanding the catalytic activity. It can be replaced by the new paradigm “composition-kinetics” where from this paradigm, kinetically significant structures that emerge during catalyst functioning and can be recognized on kinetic peculiarities.

References 1. Gleaves, J. T., Ebner, J. R. and Kuechler, T. C. (1988) Temporal analysis of products (TAP), A unique catalyst evaluation system with submillisecond time resolution. Catal. Rev. -Sci. Eng. 30, 49-116.; 2. Gleaves, J. T., Yablonskii, G. S., Phanawadee, P., and Schuurman, Y. (1997) TAP-2: An interrogative kinetics approach. Appl. Catal. A 160, 55-88; 3. G. S. Yablonsky, D. Constales, S. Shekhtman, J. Gleaves, “Y-Procedure: How to Extract the Chemical Transformation Rate from Reaction-Diffusion Data with no Assumption on the Kinetic Model”, Chem. Eng. Sci, 62 (2007) 6754-6767.