High surface area α-aluminas II. Adsorption of oxygen, ethylene, carbon dioxide and carbon monoxide on silver dispersed on HSA α-alumina

Abstract

High surface area (HSA) α-Al 2O 3 (ca. 100 m 2 g −1) prepared from unground (U) and ground (G) diaspore was utilized to obtain two very good silver dispersions of 0.43 and 0.56 for 0.83% AgAl 2/O 3(U) and 0.71% AgAl 2/O 3(G) , respectively, as measured by oxygen adsorption and hydrogen titration at 443 K. The corresponding integral heat of adsorption ( Q ad ) for oxygen at 300 and 443 K was 43 ± 1kcal mol −1 , which is similar to values for η-alumina-supported silver. The adsorption of other reactants and products in ethylene oxidation, i.e., C 2H 4, CO 2, and CO was also examined. On the reduced silver catalysts, no irreversible adsorption of ethylene or carbon monoxide occurred, and average Q ad values for reversible adsorption were 5.1 and 12.8 kcal mol −1, respectively. However, after oxygen adsorption on these silver catalysts, significant irreversible adsorption along with an enhancement of reversible adsorption occurred for ethylene and carbon monoxide. The integral heat of adsorption for ethylene on the oxygen-covered silver catalysts was around 11 kcal mol −1 for the reversible uptake, while values were around 37 kcal mol −1 at 443 K and 14 kcal mol −1 at 300 K for irreversible ethylene adsorption on the oxygen-covered silver surfaces. Integral heats of carbon dioxide adsorption at 300 K were 8 kcal mol −1 for reversible adsorption, while irreversible adsorption on the reduced and oxygen-covered samples gave Q ad values of 15 and 13 kcal mol −1, respectively. The integral heats of carbon monoxide adsorption at 300 K were near 13 kcal mol −1 for reversible adsorption and 34 kcal mol −1 for irreversible adsorption on the oxygen-covered silver surfaces. The CO (a)/O (a) and CO 2(a)/O (a) ratios after oxygen exposure imply the formation of carbonate species.