Chemistry · Ch 10 — Surface Chemistry
Adsorption Isotherms and Isobars
Adsorption Isotherms and Isobars
The amount of gas held on a solid adsorbent is jointly a function of both temperature and pressure (or concentration, for adsorption from solution), so this relationship can be examined in two complementary ways. Holding the temperature fixed and varying pressure gives an adsorption ISOTHERM — the amount of adsorption plotted against pressure at constant temperature. Holding the pressure fixed and varying temperature instead gives an adsorption ISOBAR — the amount of adsorption plotted against temperature at constant pressure.
The adsorption isobars of physisorption and chemisorption look strikingly different from one another when placed side by side, and this difference is itself diagnostic of which type of adsorption is occurring in a given system. If x is the mass of adsorbate held on a mass m of adsorbent, the quantity x/m is plotted, on the vertical axis, against temperature on the horizontal axis (at constant pressure).
For physical adsorption, x/m decreases steadily and monotonically as temperature rises — there is no rise at any point, only a continuous fall — because the weak van der Waals forces holding the physisorbed layer together are progressively overcome as the adsorbate molecules gain more thermal kinetic energy. …
What this figure shows. Both graphs plot the extent of adsorption, x/m (the mass x of adsorbate held on mass m of adsorbent), on the vertical axis against temperature on the horizontal axis, at constant pressure. Graph (a), for physical adsorption, is a curve that falls steadily as temperature rises — x/m is highest at low temperature and decreases monotonically, because the weak van der Waals forces holding the adsorbate are easily overcome by increasing thermal (kinetic) energy. Graph (b), for chemical adsorption, instead rises from a low value, reaches a maximum at some intermediate temperature, and then falls — the initial rise reflects the activation energy needed to form the adsorbent–adsorbate chemical bond (so a little heating actually helps chemisorption begin), while the eventual fall at high temperature is due to desorption, as the adsorbate's kinetic energy becomes larg …