ADSORPTION ISOTHERMS AND CATALYST SELECTIVITY FOR A UNIFORM SURFACE: THE ROLE OF MULTIPLE LANDING AREAS

In general a gas phase species can be present simultaneously in more than one adsorbed form on a uniform solid surface. The different adsorbed forms tie up different combinations of surface sites. Each such combination is termed a landing area. The different adsorbed forms present on the surface of a catalyst may form different products. (In the more general case, each different adsorbed form yields a different distribution of a set of products.) Hence the relative amount of each adsorbed form and landing area present on the surface determines the selectivity of the solid catalyst. The variation of the landing area distribution with temperature and partial pressure of the gas phase species is examined in the first part of this work. In general, increasing the pressure or decreasing the temperature leads to the relative decrease of molecules adsorbed on landing areas consisting of larger number of sites and the relative increase of molecules adsorbed on smaller landing areas. The kinetics of metal catalyzed skeletal isomerization and hydrogenoly-sis of hydrocarbons on supported metal catalysts is examined in the framework of the landing area in the second part of this work. A simple model for the variation of reaction order with pressure agrees with experimental data from other laboratories.