Some surfaces, especially catalysts, are sufficiently reactive to form chemical bonds with certain gases. In contrast to physisorption, chemical adsorption (chemisorption) involves the formation of strong bonds between adsorbate molecules and specific surface locations known as active sites.
Chemisorption is thus used primarily to evaluate quantitatively the number of surface active sites which are likely to promote (catalyze) chemical reactions. Both static, adsorption isotherms and dynamic pulse titrations yield monolayer uptake, metal area, nanocluster (crystallite) size and active metal area of heterogeneous catalysts. Hydrogen and carbon monoxide are the two most commonly employed gases. Oxygen or other gases are sometimes suitable. In an analogous manner, the amount of acidic or basic sites is determined from the adsorption of a basic or acidic gas, such as ammonia or carbon dioxide, respectively. Isothermal results can be used to calculate heats of adsorption.
Related temperature programmed (i.e. non-isothermal) methods such as TPR, TPO and TPD are used to determine relative ease of reduction (of oxides), oxidation (of low valency species, particularly carbons) and desorption (of ammonia from acid sites for example). Activation energy for a given chemical process can be gleaned by employing different heating rates.
