Mechanism of diethyl ether formation on Ag(110) and its dependence on coadsorbed oxygen species

The reactions of adlayers consisting of ethyl ligands, ethoxide groups, and oxygen atoms on Ag(110) were investigated using temperature-programmed des orption and high-resolution electron energy-loss spectroscopy. Iodoethane a dsorbs dissociatively at 150 K to produce surface-bound ethyl ligands. Thes e species react to form butane, which desorbs at 221 K, diethyl ether (236 K), ethylene (245 K), water (258 K), acetaldehyde (268 K), and ethanol (268 K). To determine the mechanism of formation of diethyl ether (the most abu ndant product), several surface mixtures of ethyl and ethoxide species were prepared by adsorption of different coverages of oxygen, iodoethane, and e thanol. While ethyl groups and ethoxides did couple to form diethyl ether i n some cases, this reaction occurred only in the presence of additional coa dsorbed oxygen atoms on the surface. The kinetics of diethyl ether formatio n are identical for the two reaction channels investigated: ethyl-ethyl-oxy gen coupling and ethyl-ethoxide coupling. Therefore, formation of diethyl e ther from two ethyl ligands and an oxygen atom appears to involve the seque ntial reaction of these species to first form an ethoxide. followed by coup ling of the ethoxide with an ethyl group in the presence of additional oxyg en atoms. This pathway was confirmed by experiments in which C-13-labeled e thoxides and unlabeled ethyl groups were deposited on the surface in the pr esence of oxygen; the product ethers contained either zero or one labeled l igands. The requirement of coadsorbed oxygen atoms for ether synthesis by a lkyl-alkoxide coupling is reminiscent of the need for subsurface oxygen in the silver-catalyzed formation of ethylene oxide (a cyclic ether) from ethy lene.