HEATS OF ADSORPTION OF OXYGEN, ETHYLENE, AND BUTADIENE ON AL2O3-SUPPORTED SILVER

Heats of adsorption of oxygen on small, clean dispersed Ag crystallites have been measured directly. On the largest (27 nm) Al2O3-supported particles, the integral Qad values of 34-41 kcal mol-1 at 443 K were very consistent with those reported for Ag single crystals, and Qad increased to 50 kcal mol-1 on the smallest (3.7 nm) particles. The heats of adsorption measured at 300 K were slightly lower but showed the same trend with crystallite size. The heat of adsorption of ethylene on pure Al2O3 was 10.1 kcal mol-1 at 300 K, which, when combined with the sizable adsorption on the support, precluded any measurement on Ag; however, at 443 K a value of 10 kcal mol-1 could be estimated for C2H4 adsorption on clean Ag because adsorption on the support was significantly reduced. On an oxygen-covered Ag surface, Qad values for C2H4 were 5.2 and 64 kcal mol-1 at 300 and 443 K, respectively. Similar measurements for butadiene gave heats of adsorption of 10.9 and 13.1 kcal mol-1 on pure Al2O3 at 300 and 443 K, respectively, and on clean Ag at 443 K, the Qad value was 9 kcal mol-1 On oxygen-covered Ag, Qaad for butadiene was 21 kcal mol-1 at 300 K but, as with C2H4, it increased markedly to 65 kcal mol-1 at 443 K. The nearly identical values at 443 K for C2H4 and C4H6 on both clean and O-covered Ag imply a similar mode of bonding on each surface. The high values on the latter surface indicate a strong interaction with surface oxygen atoms, presumably involving only a single CC bond. © 1991.