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Title: On the Role of Subsurface Oxygen and Ethylenedioxy in Ethylene Epoxidation on Silver.

Abstract

The research described in this product was performed in part in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. The thermochemical stability of various three-component phases containing oxygen, ethylene, and Ag(111) was determined as a function of oxygen and ethylene chemical potential using periodic, self-consistent density functional theory calculations. Ethylenedioxy is stable over a wide range of conditions, although its formation may be kinetically hindered in some cases. Ethylene and ethylene-containing oxametallacycles are also found to be stable over a reasonably large range of chemical potentials, particularly if ethylenedioxy formation is neglected. Furthermore, subsurface oxygen (Osb) is seen to be present in the three-component systems at a variety of conditions; minimum energy path calculations performed at a coverage of 1/2 ML Osb suggest that this species may actually increase the reaction barrier for ring closure leading to ethylene oxide elimination from Ag(111).

Authors:
;
Publication Date:
Research Org.:
Pacific Northwest National Laboratory (PNNL), Richland, WA (US), Environmental Molecular Sciences Laboratory (EMSL)
Sponsoring Org.:
USDOE
OSTI Identifier:
989062
DOE Contract Number:
AC05-76RL01830
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Physical Chemistry C, 111(22):7992-7999; Journal Volume: 111; Journal Issue: 22
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; CLOSURES; ETHYLENE; FUNCTIONALS; OXIDES; OXYGEN; SILVER; STABILITY; Environmental Molecular Sciences Laboratory

Citation Formats

Greeley, Jeffrey P., and Mavrikakis, Manos. On the Role of Subsurface Oxygen and Ethylenedioxy in Ethylene Epoxidation on Silver.. United States: N. p., 2007. Web. doi:10.1021/jp070490i.
Greeley, Jeffrey P., & Mavrikakis, Manos. On the Role of Subsurface Oxygen and Ethylenedioxy in Ethylene Epoxidation on Silver.. United States. doi:10.1021/jp070490i.
Greeley, Jeffrey P., and Mavrikakis, Manos. Sat . "On the Role of Subsurface Oxygen and Ethylenedioxy in Ethylene Epoxidation on Silver.". United States. doi:10.1021/jp070490i.
@article{osti_989062,
title = {On the Role of Subsurface Oxygen and Ethylenedioxy in Ethylene Epoxidation on Silver.},
author = {Greeley, Jeffrey P. and Mavrikakis, Manos},
abstractNote = {The research described in this product was performed in part in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. The thermochemical stability of various three-component phases containing oxygen, ethylene, and Ag(111) was determined as a function of oxygen and ethylene chemical potential using periodic, self-consistent density functional theory calculations. Ethylenedioxy is stable over a wide range of conditions, although its formation may be kinetically hindered in some cases. Ethylene and ethylene-containing oxametallacycles are also found to be stable over a reasonably large range of chemical potentials, particularly if ethylenedioxy formation is neglected. Furthermore, subsurface oxygen (Osb) is seen to be present in the three-component systems at a variety of conditions; minimum energy path calculations performed at a coverage of 1/2 ML Osb suggest that this species may actually increase the reaction barrier for ring closure leading to ethylene oxide elimination from Ag(111).},
doi = {10.1021/jp070490i},
journal = {Journal of Physical Chemistry C, 111(22):7992-7999},
number = 22,
volume = 111,
place = {United States},
year = {Sat May 12 00:00:00 EDT 2007},
month = {Sat May 12 00:00:00 EDT 2007}
}
  • The authors have performed electronic structure calculations on the chemisorption of atomic oxygen on Ag(110) and on the subsequent reaction of this chemisorbed oxygen with ethylene. These calculations show that the presence of subsurface oxygen (i) reduces the bond energy between silver and adsorbed oxygen and (ii) converts the repulsive interaction between adsorbed oxygen and (gas-phase) ethylene into an attractive one, thus making possible the epoxidation reaction. The presence of subsurface oxygen diminishes an important four-electron destabilizing interaction (Pauli repulsion) between the occupied ethylene {pi} orbital and a surface oxygen lone-pair orbital by shifting the band of ethylene {pi}-oxygen lone-pairmore » antibonding orbitals largely above the Fermi level. As for total combustion of ethylene, we do not find any C-H bond activation for several different geometries in which ethylene approaches the adsorbed oxygen atom.« less
  • The silver-catalyzed, oxidative coupling of methane to C{sub 2} hydrocarbons (OCM) is shown to be an extremely structure-sensitive reaction. Reaction-induced changes in the silver morphology lead to changes in the nature and extent of formation of various bulk and surface-terminating crystal structures. This, in turn, impacts the adsorption properties and diffusivity of oxygen in silver which is necessary to the formation of subsurface oxygen. A strongly bound, Lewis basic, oxygen species which is intercalated in the silver crystal structure is formed as a which a intercalated in the silver crystal structure is formed as a result of these diffusion processes.more » This species is referred to as O{sub {gamma}} and acts as a catalytically active crystal structure is formed as a result of these diffusion process. This species is referred to as O{gamma} and acts a catalytically active site for the direct dehydrogenation of a variety of organic reactants. It is found that the activation energy for methane coupling over silver of 138 kJ/mol is nearly energy for methane coupling over silver of 138 kJ/mol is nearly identical to the value of 140 kJ/mol for oxygen diffusion in silver measured under similar conditions. This correlation between the diffusion kinetics of bulk-dissolved oxygen and the reaction kinetics of the oxidative coupling of methane to C{sub 2} hydrocarbons suggests that the reaction is limited by the formation of P{gamma} via surface segregation of bulk dissolved oxygen. Catalysis over fresh silver catalysts indicates an initially preferential oxidation of CH{sub 4} to complete oxidation projects. This is result of the reaction of methane with surface bound atomic oxygen which forms preferentially on high-index terminating crystalline planes. Reaction-induced facetting of the silver results in a restructuring of the catalysts from one which initially catalyzes the complete oxidation of methane to CO{sub x} and water to a catalyst which preferentially catalyzes the formation of coupling products. This represents an extremely dynamic situation in which a solid-state restructuring of the catalyst results in the formation of a Lewis-basic, silver-oxygen species which preferentially catalyzes the dehydrogenation of organic molecules.« less
  • An {alpha}-alumina support and a Cs-promoted Ag/{alpha}-Al{sub 2}O{sub 3} ethylene-epoxidation catalyst prepared on this same support have been characterized using secondary electron microscopy (SEM), electron dispersive X-ray spectroscopy (EDS), ion scattering spectroscopy (ISS), Auger electron spectroscopy (AES), and X-ray photoelectron spectroscopy (XPS). The SEM micrographs indicate that the {alpha}-alumina is crystalline with a well-defined plane exposed and that the Ag covers the support surface uniformly as a thin film. The EDS data also support the assertion that both surfaces are fairly uniform with regard to composition. ISS data exhibit features due to many contaminants such as C, N, Ca, andmore » Ni on the {alpha}-alumina, as well as Na and Si binder materials which are also observed with AES and XPS. The XPS chemical-state data for the O, Al, Na, C, and Ag are complex, indicating that multiple chemical states of each element are present. Apparently, Ag is present predominantly as AgO, but Ag{sub 2}O and Ag metal are also present. 20 refs., 8 figs., 1 tab.« less
  • The aging effects of an {alpha}-alumina-supported Ag, ethylene epoxidation catalyst have been examined as a function of reaction time in a Berty backmix reactor. The catalyst was prepared by impregnation of commercial {alpha}-alumina pellets (1.3 m{sup 2}/g) in a Ag{sub 2}O/lactic acid solution and contained 13.87 wt% Ag and 420 ppm Cs by weight. The fresh and used catalysts have been characterized using scanning electron microscopy (SEM), ion scattering spectroscopy, Auger electron spectroscopy, and X-ray photoelectron spectroscopy. With aging, the fresh catalyst becomes more active (percentage of conversion to ethylene oxide increases), and then the activity drops with increasing reactionmore » time. Large changes are observed in the spectra obtained from these surfaces which are due primarily to morphological changes caused by sintering as observed in the SEM micrographs. The surface Cl content is increased at the support surface by adsorption of ethyl chloride, added in small amounts to the feed gas as a moderator, from the gas phase. The surface also becomes enriched in Na due to migration of the Na binder material to the surface where it associates with the Cl to from NaCl. The SEM micrographs indicate that both the Ag and alumina crystallites grow with age and that the large Ag crystallites do not interact with the aged alumina surface suggesting that the formation of NaCl plays a role in the sintering process. 12 refs., 5 figs.« less
  • A nonpromoted Ag/{alpha}-Al{sub 2}O{sub 3} catalyst and a Ag/{alpha}-Al{sub 2}O{sub 3} catalyst promoted with 420 ppm Cs used for ethylene epoxidation were studied using scanning electron microscopy (SEM), ion scattering spectroscopy (ISS), Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and secondary ion mass spectrometry (SIMS). SEM, ISS, XPS, and AES data indicate that the Cs-promoted catalyst consists of a thin film of Ag covering most of the support surface. Both a thin film and small Ag clusters exist on the nonpromoted catalyst, but the alumina is only partially covered. ISS and SIMS depth profiling data taken from the promotedmore » catalyst indicate that most of the Cs lies beneath the surface although a small amount is present at the surface. The surface characterization data suggest that the Cs coats the support material during the preparation and acts as a binder between the Ag and the support resulting in a larger Ag coverage of the {alpha}-alumina. Total oxidation of ethylene and ethylene oxide occur primarily on the alumina surface. The enhancement in selectivity toward ethylene epoxidation may result from the fact that Cs addition results in the thin Ag film covering the alumina. 16 refs., 9 figs.« less