Article révisé par les pairs
Résumé : Hydrogen adsorption is studied on two Rh/Al2O3 catalyst: 1.04% Rh(∼100% dispersion) and 4.8% Rh (<10% dispersion). Hydrogen adsorption is decomposed into a fast process and a slow activated process, of almost equal importance. A systematic study of the influence of the reduction temperature, outgassing temperature and temperature at which hydrogen is put in presence of the catalyst is conducted. Adsorption isotherms in a large range of pressure (10-3-50 torr) and temperature (193K-500K) are measured allowing to calculate the isosteric heat of adsorption Q as a function of coverage. For both catalysts, very high initial values of Q are found(∼50 kcal/mole), much higher than on model unsupported catalysts, that seems characteristic of the adsorbed H during the slow process. It is proposed that Cl atoms bound to the metal and introduced during the preparation of the catalyst, are responsible of the slow adsorption. © 1989.