ACIDITY OF A MICROPOROUS AMORPHOUS ALUMINA MEASURED BY INTERMITTENT TEMPERATURE-PROGRAMMED DESORPTION OF AMMONIA

Mathematical models used to achieve the deconvolution of temperature-programmed desorption (TPD) spectra from heterogeneous surfaces generally rely on assumptions about the pre-exponential factor or on the variation of the activation energy of desorption with coverage. In contrast, intermittent temperature-programmed desorption (ITPD) is a method that allows the apparent desorption activation energies E(d) to be determined without making such restrictive assumptions. ITPD has been applied to ammonia, used as a basic probe molecule desorbed from an amorphous, microporous alumina sample. Five different chemisorbed states have been evidenced. These five states of adsorbed ammonia are characterized by apparent E(d) values ranging from 109 to 234 kJ/mol. These results are compared with those of differential microcalorimetry found in the literature and the consequences of the discovery of discrete ammonia adsorbed states are discussed.