Abstract
What is catalysis? How does reaction with solid occur? Catalysts are substances or materials that increase the reaction rates while emerged unaltered after reaction. Solid material can participate in reaction either as catalysts or as reactants. Reactions occurring on solid surface start with reactants or catalyst from the fluid phase collide and associate with the active centers (or sites) on the surface, ie, adsorption. Adsorption can be described as Langmuir isotherm if uniform surface activity (or ideal surface) is assumed. More commonly, surfaces are not uniform and thus interactions can vary. Nonideal surfaces can be idealized by either distribution or multiple distinct levels of interaction energy. UniLan is a model based on uniform distributions of active sites with adsorption energy. The cooperative adsorption model idealizes the sequential adsorption on the active centers to multiple levels of interactions, likening multiple layers. The adsorption isotherms exhibit different dependence on the bulk adsorbate concentration due to the variation of surface interactions: Types I through V. Langmuir-Hinshelwood-Hougen-Watson kinetics is derived based on the Langmuir adsorption isotherm. Both UniLan and cooperative adsorption isotherms can also be employed to derive at reaction rate expressions. The rate expression can also appear to be of power-law form if Freundlich or Temkin approximation were applied to UniLan isotherms. Rate expressions for noncatalytic reactions involving solids can be derived in the same manner. The participation of the surface active centers in actual reactions can affect the site balance, either active center renewal or depletion occurs. For catalytic surfaces, the activity can decrease with increase duration of service. The surface activity decline can be treated the same way as the concentration of a reacting component in the mixture.