Heats of adsorption of oxygen, ethylene, and butadiene on Al 2O 3-supported silver

Abstract

Heats of adsorption of oxygen on small, clean dispersed Ag crystallites have been measured directly. On the largest (27 nm) Al 2O 3-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 Al 2O 3 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 C 2H 4 adsorption on clean Ag because adsorption on the support was significantly reduced. On an oxygen-covered Ag surface, Qad values for C 2H 4 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 Al 2O 3 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 C 2H 4, it increased markedly to 65 kcal mol −1 at 443 K. The nearly identical values at 443 K for C 2H 4 and C 4H 6 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 CC bond.