Surface acidity of catalytic solids studied by base desorption : experimental and modelling approaches

A thermogravimetric analyzer (TGA) was used to collect thermodesorption curves of 2-phenylethylamine (PEA) from acidic surfaces with the aim of determining the amount and distribution of the acid sites of the samples. Oxides widely used as active phase supports as well as catalytic phases were selected for this study: alumina, silica, silica-alumina, silica-zirconia, and silica-titania. The thermodesorption curves were collected at different heating rates (5 ≤ β (°C/min) ≤ 30) in inert atmosphere. The activation energies of PEA desorption from the acid sites were calculated from the dependence upon the heating rate β of the displacements of the observed desorption peaks (Tmax) as determined from the derivative of the TGA profiles (DTGA). For a more accurate kinetic study of the desorption phenomenon, a kinetic model based on parallel reactions of desorption, each one running with its own kinetic parameter in a temperature-dependent manner in accordance with Arrhenius's law, was applied to the experimental desorption data at the different heating rates. The quantitative acid site energy distribution was optimized for each sample, and kinetic parameters for each type of acid site were determined. The conclusions drawn from PEA thermodesorption were compared with the results obtained from the differential heat curves of ammonia adsorption measured at 80 °C in a volumetric-calorimetric line.