Determination of the heat of adsorption of ammonia on zeolites from temperature-programmed desorption experiments

A general equation, obtained from the classical TPD theory and statistical thermodynamics, has been designed for determining the enthalpy of adsorptio n, DeltaH, from a series of temperature-programmed desorption (TPD) runs ca rried out with various sample masses, heating rates, or inert gas flow rate s. This equation applies to TPD performed either under vacuum or under an i nert gas stream, provided that readsorption occurs freely during the experi ments. It has been applied to ammonia TPD (under vacuum) from HY zeolite sa mples of different masses. The pressure of ammonia over the sample was in t he range 1 x 10(-4)-15 Pa. The value of DeltaH obtained is -136 kJ/mol. Res ults showed that the molecule of ammonia in the gas phase completely loses its translational motion when it is adsorbed. Finally, a simple experimenta l method has been proposed to rapidly investigate whether readsorption occu rs during the TPD runs. It has been found that, under our experimental cond itions, free readsorption effectively takes place at temperatures under 750 K but that desorption is irreversible above this temperature.