Heterogeneous Catalysis: Fundamentals

Abstract

Heterogeneously catalyzed reactions are composed of purely chemical and purely physical reaction steps. For the catalytic process to take place, the starting materials must be transported to the catalyst. Thus, apart from the actual chemical reaction, diffusion, adsorption, and desorption processes are of importance for the progress of the overall reaction.We will now consider the simplest case of a catalytic gas reaction on a porous catalyst. The following reaction steps can be expected ( Fig. 5-1) [T20, T26]: 1) Diffusion of the starting materials through the boundary layer to the catalyst surface. 2) Diffusion of the starting materials into the pores (pore diffusion).3) Adsorption of the reactants on the inner surface of the pores. 4) Chemical reaction on the catalyst surface. 5) Desorption of the products from the catalyst surface. 6) Diffusion of the products out of the pores. 7) Diffusion of the products away from the catalyst through the boundary layer and into the gas phase.In heterogeneous catalysis chemisorption of the reactants and products on the catalyst surface is of central importance, so that the actual chemical reaction (step 4) can not be considered independently from steps 3 and 5. Therefore, these steps must be included in the microkinetics of the reaction. In cases where the other transport processes discussed above play a role, the term macrokinetics is used.The measured reaction rate, known as the effective reaction rate, is determined by the most strongly inhibited and therefore slowest step of the reaction sequence. This rate-determining step also determines the reaction order.The effective reaction rate r eff is influenced by many parameters, including the nature of the phase boundary, the bulk density of the catalyst, the pore structure, and the transport rate in the diffusion boundary layer. If the physical reaction steps are rate determining, then the catalyst capacity is not fully exploited.If one wishes to determine the mechanism and to describe it exactly in terms of rate equations, then one must ensure that only steps 3-5 are rate determining.