Chemical and physical characteristics of solid catalysts

In heterogeneous catalysis, the reaction takes place at the interface between the catalyst and the less dense phase. Adsorption is defined as the preferential concentration of gas molecules at a fresh solid surface, caused by the existence of a field force that attracts molecules of the contacting fluid. Two major types of adsorption have been recognized, namely, physical adsorption and chemisorption [5, 6]. 

Physical adsorption, which is similar to the condensation of vapor molecules onto a liquid surface of the same composition, 

Langmuir used fractional surface coverage by the adsorbate gas, 9a, as a measure of the amount of gas adsorbed and 

Two other well-known isotherms that do not involve an assumption regarding energetic equivalence of the active surface 

This isotherm may be derived from kinetic considerations for intermediate surface coverages (0.2 0 0.8), but it does not lend itself to multicomponent adsorption and also fails to predict the limiting conditions of — 0 when Pa 0 and 9 — when 00. Even though it was used for correlating the kinetics of ammonia synthesis, the Temkin isotherm has not found much use in the kinetic analysis of solid-catalyzed gas-phase reactions. 

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