The activity and selectivity of oxygen atoms adsorbed on a Ag/?-Al2O3 catalyst in ethene epoxidation

Abstract

The bonding of the oxygen species held on a Ag/α-Al2O3 catalyst has been studied by temperature programmed desorption and their reactivity in ethene epoxidation by temperature programmed reduction using ethene as the reductant. The Ag/α-Al2O3 catalyst was produced by the thermal decomposition of a Ag oxalate/α-Al2O3 precursor. Oxygen desorbs from this Ag/α-Al2O3 catalyst in two states, one (peak maximum temperature 520 K) having a desorption activation energy of 140 kJ mol−1 – oxygen desorbing from Ag(111), and one (peak maximum temperature 573 K) having a desorption activation energy of 155 kJ mol−1 – oxygen desorbing from a highly stepped or defected Ag surface. Temperature programmed reduction of the two oxygen states existing on the surface of the Ag/α-Al2O3 catalyst using ethene as the reductant produced two peaks at 373 and 473 K in which ethene epoxide and CO2 evolved coincidently. The peak at 373 K derives from the reduction of oxygen atoms adsorbed on Ag(111). The higher temperature peak (473 K) corresponds to the reduction of oxygen atoms adsorbed on highly stepped or defected Ag surface. The selectivity to ethene epoxide for the 373 K peak is ~ 57%, while that of the 473 K peak is 34%. The coincident evolution of ethene epoxide and CO2 shows that the selective and unselective reaction pathways have a common surface intermediate – probably an oxametallacycle. The higher selectivity of the oxametallacycle formed by the bonding of ethene to the weaker Ag-O bond is considered to result from its having a lower activation energy to cyclisation than that produced by ethene bonding to the higher Ag-O bond.