Mechanism of diethyl ether formation on Ag(110) and its dependence on coadsorbed oxygen species
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The reactions of adlayers consisting of ethyl ligands, ethoxide groups, and oxygen atoms on Ag(110) were investigated using temperature-programmed desorption and high-resolution electron energy-loss spectroscopy. Iodoethane adsorbs dissociatively at 150 K to produce surface-bound ethyl ligands. These 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 abundant product), several surface mixtures of ethyl and ethoxide species were prepared by adsorption of different coverages of oxygen, iodoethane, and ethanol. While ethyl groups and ethoxides did couple to form diethyl ether in some cases, this reaction occurred only in the presence of additional coadsorbed oxygen atoms on the surface. The kinetics of diethyl ether formation are identical for the two reaction channels investigated: ethyl-ethyl-oxygen coupling and ethyl-ethoxide coupling. Therefore, formation of diethyl ether from two ethyl ligands and an oxygen atom appears to involve the sequential reaction of these species to first form an ethoxide, followed by coupling of the ethoxide with an ethyl group in the presence of additional oxygen atoms. This pathway was confirmed by experiments in which 13C-labeled ethoxides and unlabeled ethyl groups were deposited on the surface in the presence of oxygen; the product ethers contained either zero or one labeled ligands. The requirement of coadsorbed oxygen atoms for ether synthesis by alkyl-alkoxide coupling is reminiscent of the need for subsurface oxygen in the silver-catalyzed formation of ethylene oxide (a cyclic ether) from ethylene. 1999 American Chemical Society.
