OXYGEN-ADSORPTION ON SUPPORTED SILVER CATALYSTS INVESTIGATED BY MICROGRAVIMETRIC AND TRANSIENT TECHNIQUES

Oxygen adsorption on α-Al2O3-supported Ag catalysts was investigated in the temperature range 40 to 400°C. The adsorption kinetics, studied with a vacuum ultramicrobalance and analyzed with the Elovich equation, indicate that three kinetically distinguishable processes take place, with activation energies of ∼ 1, 10.5, and 24 kcal/mol. Temperature-programmed desorption (TPD) experiments confirm the existence of three adsorbed oxygen species. A weakly adsorbed species which is found to desorb below 150°C, is related to the 10.5 kcal/mol adsorption process and is assigned to molecularly adsorbed oxygen. The adsorption peak at 285°C is related to the almost nonactivated process and is ascribed to atomic oxygen, which is probably multicoordinated on the surface. At high adsorption temperatures (300 and 400°C) a third peak, desorbing above 400°C. is observed in the TPD profiles and is related to the highly activated process of 24 kcal/mol which is revealed by the gravimetric experiments. This species is suggested to be subsurface oxygen based on the results of TPD experiment conducted after isotope exchange of preadsorbed 16O2, and 18O2. Temperature-programmed reaction (TPR) experiments showed that all three species are active toward ethylene combustion. © 1993 by Academic Press, Inc.