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

Mathematical models used to achieve the deconvolution of temperature-p rogrammed desorption (TPD) spectra from heterogeneous surfaces general ly rely on assumptions about the pre-exponential factor or on the vari ation of the activation energy of desorption with coverage. In contras t, intermittent temperature-programmed desorption (ITPD) is a method t hat allows the apparent desorption activation energies E(d) to be dete rmined without making such restrictive assumptions. ITPD has been appl ied to ammonia, used as a basic probe molecule desorbed from an amorph ous, microporous alumina sample. Five different chemisorbed states hav e been evidenced. These five states of adsorbed ammonia are characteri zed by apparent E(d) values ranging from 109 to 234 kJ/mol. These resu lts are compared with those of differential microcalorimetry found in the literature and the consequences of the discovery of discrete ammon ia adsorbed states are discussed.