Catalytic reactions and reactors: a surface science approach

Abstract

An integrated analysis of surface chemical reactions and reactors is central to the rational design of a catalytic reactor. Developments have, however, proceeded along two different paths; and much of the interaction that has occurred has been through the analysis of the reaction-diffusion problem within a catalyst pellet, and of multistationarity and instability. It is only during the last 10-15 years that such studies have tended to include a major emphasis on surface reactions with all their nonidealities (such as heterogeneity and lateral interaction between adsorbed molecules), and their role in inducing multiplicity and instability. Basic to a consideration of reactions on nonideal surfaces is the formulation of the adsorption integral equation and its solution under various conditions, including the use of a nonideal local isotherm (such as the Fowler-Guggenheim or the quasichemical) as the local isotherm in the integral equation. In the present monographic review, various aspects of adsorption pertinent to the subject are considered, including the derivation and solution of the adsorption integral equation. Methods of modelling catalytic reactions are also discussed, with a brief overview of the latest statistical methods used. The concept of the statistical rate on a nonuniform (heterogeneous) surface is then introduced, and procedures for formulating expressions for these rates are presented both for simple and complex reactions involving single as well as multicomponent adsorption. This is followed by a brief discussion of the characterization of surface heterogeneity in which the use of information theory and fractal analysis is highlighted. The role of the catalyst surface in inducing multiple steady states is explained, and various situations under which this can happen are stressed. Multiplicity can also occur as a result of feedback of mass or energy within a pellet or reactor. Although the literature in this area is extensive, the use of some of the newer mathematical tools such as the catastrophe and singularity theories in dealing with this problem is discussed, though very selectively. And, finally, the effects of incorporating surface nonidealities on the performance of a fixed-bed catalytic reactor are explored, and it is shown how these can be quite dramatic under certain conditions. It has so far only been possible to incorporate some very rudimentary effects of nonideality in the analysis, such as the use of the Fowler-Guggenheim or the quasichemical isotherm. No attempt has yet been made to include surface heterogeneity also through the adsorption integral equation. With the increasing use of parallel computing, it should be possible to explore the surface space and its role in directing a reaction in greater detail (through Monte Carlo simulation), and possibly, within the foreseeable future, one can also hope to formally incorporate these effects in reactor design for simple reactions.