Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

220 EXPERIMENTAL METHODS FOR HETEROGENEOUS REACTIONS2.2 CATALYSISThe literature of catalysis is so vast that it is impossible to do fill justice in onepart of a chapter, even to the experimental methods that are in use. Most catalyticreactions are quite complex and a complete reaction mechanism can rarely bedetermined from a measurement of kinetics alone. It is safe to say that no catalyticgas-solid reaction has yet been fully characterised. Because of the many difficulties,work on catalytic reactions tends to veer towards two opposite poles. One of thesecan be described as the “clean surface approach”, which uses the simplest reactionsunder high vacuum conditions and attempts to dehe the catalyst surface completely.The other approach is to study much more complex reactions in order toobtain overall kinetic information which describes the process. Where the mechanismis not understood, it is common to employ expressions similar to those usedfor homogeneous reactions, or even purely empirical equations.2.2.1 Catalyst preparation’77* 17’The practice of catalysis involves an extensive knowledge of the procedures andexperimental, techniques used in the preparation of solid catalysts. Chemical compositionis not a sufficient guide. In many cases, the physical characteristics such assurface area, pore volume, pore size, particle size and crystal structure modifyor control the catalytic properties of the material.Catalysts for the “clean surface” approach are usually prepared by methodssimilar to those used for obtaining suitable surfaces for adsorption measurements(Section 2.1.1). In general, evaporated fihs have been most widely usedbecause of their relatively large surface area compared to other forms of catalyst.The more conventional catalysts are usually prepared by one of the techniquesoutlined below. For preparation of any particular catalyst it is usually necessary torefer to the original publication. Even this approach is not a guarantee of successbecause there is rarely sufficient detail to define the preparation exactly.(a) Ignited catalystsOne of the simplest methods of preparation is by decomposition of a thermallyunstable compound. The nitrate or chloride is often preferred, sulphates tend todecompose at higher temperatures. Where the presence of residual traces of anionis to be avoided, the metal salts of organic acids are particularly useful. Formates,oxalates, acetates etc., decompose at low temperatures and often reduce the metalat the same time. For the preparation of catalysts from anions, the ammoniumsalt is frequently used. Metallic salts of complex acids can be used as a source ofmetal oxide mixtures. Decomposition of the appropriate chromate, tungstate,molybdate or vanadate will produce the mixed oxide.