Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

Chapter 3Experimental Methods for the Study ofHeterogeneous ReactionsD. SHOOTER1. General introductionA wide variety of methods have been used to study the kinetics of heterogeneousreactions. The emphasis in this chapter is on those methods which have beenparticularly developed for the study of such reactions, and as a consequence, thestudy of kinetics by conventional measurements, e.g. chemical analysis, pressurechanges etc., is only briefly mentioned.The subject matter really covers several fields of work and it is not possible toinclude more than a fraction of the available references. Where good reviews of thesubject exist, references to individual publications have been deliberately limited.More direct references have been given where suitable reviews have not been found.2. The solid-gas interface2.1 ADSORPTIONThere are two usually distinguishable types of adsorption at the gas-solid interface,these are physicaladsorptionandchemisorption. Physical adsorption, or Van derWaals adsorption, occurs when theadsorbed molecule is held to the surface by weakVan der Waals or dispersion forces of the type responsible for cohesion in liquids.Chemisorption is a true chemical reaction involving electron transfer between thesolid and the gas. The difference between the two types of adsorption is sometimesblurred but anumber of criteria can beused to distinguish betweenthem'- '.Physicaladsorption is non-specific and generally takes place at low temperatures through forcesofattraction similar to thosecausing liquefaction. Multilayer adsorptioncan takeplace and the heat of adsorption is less than 5 kcal.mole -I. Chemisorption ismuch more specific for solid-gas pairs and can occur at temperatures well abovethe boiling point of the gas. A single adsorbed layer is formed, with a heat ofchemisorption which can be as high as 150 kcal.mole-'. The data of physicaladsorption are beyond the scope of this article, although the process is widely usedfor the measurement of the surface area and pore structure of solids's4r5. Furtherinformation can be obtained from the many reference sources on this ~ubject'*~*~.