A theory of the catalytic surface

Abstract

The problem of the mechanism of a catalytic action occurring at the surface of a solid catalyst can be studied in several ways. Kinetic studies of such reactions supply important information with regard to the manner by which the process is achieved. On the other hand, attention may be focussed, not on the reactions which occur at the catalyst surface, but on the properties of the surface itself, and, from the results of such studies, generalizations may be made with respect to a variety of reactions which occur at such a surface. It is this latter method of attack on the problem of contact catalysis which has been adopted in the last few years in a comprehensive series of investigations on the properties of solid catalytic surfaces conducted in Princeton by the author and his co-workers. Taken in conjunction with the results of other investigators in the field of reaction kinetics, these studies have led to a concept of the catalytic surface which is, perhaps, more comprehensive than earlier efforts and which leads to interesting general conclusions with reference to matter in the solid state. The theory of Bodenstein and Fink, based on kinetic studies, assumed that reaction velocity was determined by the rate of diffusion of reactant gases through a layer of gas adsorbed on the catalyst surface. More recent studies, especially those of Langmuir, have led to the conclusion that multi-molecular layers of gas do not occur on plane surfaces of an adsorbent. Langmuir’s view is that the surface of a catalyst is to be regarded as an area containing a definite number of elementary spaces in which some of the spaces are vacant, while others are covered with adsorbed atoms or molecules. The activity of the surface is conditioned by the nature of the arrangement, and the spacing of the atoms, in the surface layer. The velocity of reaction on such surfaces depends on the fraction of such surface which is covered by adsorbed substances. Langmuir developed two cases in detail, one in which the surface was relatively bare, the other in which the surface was nearly completely covered with adsorbed gases.