Formation of a Stable Surface Oxametallacycle that Produces Ethylene Oxide

Temperature programmed desorption, high-resolution electron energy loss spectroscopy (HREELS), and density functional theory (DFT) were used to investigate the adsorption and reaction of ethylene oxide (EO) on the Ag(111) surface. When EO is dosed onto Ag(111) at 140 K it adsorbs molecularly, desorbing without reaction at approximately 200 K. On the other hand, when EO is dosed at 250 K, the ring-opening of EO is activated, and a stable surface intermediate is formed. This intermediate reacts at 300 K to re-form EO plus a few other products. HREELS and DFT studies suggest that this stable intermediate is a surface oxametallacycle. Moreover, the activation energies observed for the reaction of the oxametallacycle to form EO are in an excellent agreement with the values reported for the steady-state ethylene epoxidation process. This work represents the first demonstration of surface oxametallacycle ring-closure to form EO. Comparison of the spectroscopic results obtained from silver single crystals and supported catalysts strongly suggests that oxametallacycles are important intermediates in silver-catalyzed ethylene epoxidation.