Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

222 EXPERIMENT A L METHODS F 0 R HE TE R 0 GENE 0 US RE A C TI 0 N S(e) Carriers andpromotors's2The main functions of a carrier or support are usually to lend mechanicalstrength, increase stability to sintering and provide a larger active surface areathan would otherwise be available. There is evidence that, in many instances,compound or complex formation takes place between the catalyst and the support,with a consequent effect on the catalytic properties. The most commonly usedsupport materials are silica, alumina, silica-alumina, titania, silicon carbide,diatomaceous earths, magnesia, zinc oxide, iron oxide and activated carbon.A promotor is generally defined as a substance added to a catalyst, in smallconcentration, which gives a marked improvement in performance, but whichdoes not possess appreciable activity by itself. Promotors may modify the electronicproperties of the catalyst, retard sintering or increase the number of lattice defects.In most cases satisfactory promotors have been found by trial and error, withouttheir function being fully understood.2.2.2 DifusionAny catalytic reaction can be thought of as occurring by the following processes:(1) Diffusion of the reactants to the catalyst surface.(2) Adsorption of the reactants on the surface.(3) Reaction on the surface.(4) Desorption of the products from the surface.(5) Diffusion of the products away from the surface.It is generally assumed, for reasons of mathematical simplification, that one ofthese processes is slower than the rest and that this is the rate determining step.Because the reaction mechanism is rarely known in sufficient detail, steps 2,3 and4 are usually considered together. Where the rates of adsorption/reaction/desorptionare slower than the rate of diffusion, the true 'chemical' kinetics will be observed.If, however, the reverse is true, 'diffusion' kinetics will be observed andthe results will not be chargcteristic of the chemical reactivity of the system.The influence of diffusional limitations in gas phase reactions has been extensivelytreated by WheelerI8 3, 184 and from a chemical engineering viewpoint by Hougenand Watson'85i 186. More recently a monograph by Satterfield and Sherwood"'has appeared. The problem of diffusion can be separated into two parts, the firstis diffusion or mass transfer to the external surface of thecatalyst and second,for those catalysts which are porous, diffusion within the catalyst pores. Whendiffusion is the rate limiting process, reaction rate, selectivity and activation energyare affected.(a) Mass trartflerFor very fast reactions, only the external surface of the catalyst particle is