Acidity of a microporous amorphous alumina measured by intermittent temperature-programmed desorption of ammonia

Abstract

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.