Chlorine promoters in selective ethylene epoxidation over Ag(111): a comparison with Ag(110)
Chlorine promoters are added in the silver-catalyzed oxidation of ethylene (C/sub 2/H/sub 4/ + 1/2 O/sub 2/ ..-->.. C/sub 2/H/sub 4/O) in order to improve the selectivity for ethylene epoxide production. The author has recently modeled the role of chlorine promoters in this reaction by depositing chlorine adatoms onto the Ag(110) surface in ultrahigh vacuum (UHV) and monitoring the resultant effects on the medium-pressure (approx. 1 atm) kinetics of the epoxidation reaction. This was accomplished with an apparatus that allowed rapid (< 20 s) transfer of the model catalyst between the UHV chamber for surface analysis (XPS, AES, LEED, AES) and a high-pressure microreactor for kinetics. That work led to a detailed picture of the role of chlorine promoters in this reaction. This present study is an extension of that work to include the Ag(111) surface, which behaves much differently than Ag(110) in dissociate oxygen adsorption, but nevertheless displays very similar catalytic behavior for this reaction. The results indicate that the effects of chlorine addition are very similar on Ag(111) and (110). They can be explained with the same reaction mechanism involving adsorbed oxygen and ethylene. The steady-state atomic oxygen coverage (under catalytic reaction conditions) goes through a large maximum with chlorine coverage on Ag(111), not seen with Ag(110). This is due to the very low reactivity of atomic oxygen on Ag(111) in the presence of chlorine. Evidence is presented that suggests that the standard method of measuring the Ag surface area of supported catalysts via oxygen chemisorption is very unreliable.
- Research Organization:
- Los Alamos National Lab., NM (USA)
- OSTI ID:
- 5182306
- Journal Information:
- J. Catal.; (United States), Vol. 99:1
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
02 PETROLEUM
CHLORINE
CATALYTIC EFFECTS
EPOXIDES
SYNTHESIS
ETHYLENE
OXIDATION
SILVER
ADSORPTION
AUGER ELECTRON SPECTROSCOPY
CHEMICAL REACTION KINETICS
CRYSTAL FACES
ELECTRON DIFFRACTION
EMISSION SPECTROSCOPY
OXYGEN
PHOTOELECTRON SPECTROSCOPY
PROMOTERS
SURFACE AREA
X-RAY SPECTROSCOPY
ALKENES
CHEMICAL REACTIONS
COHERENT SCATTERING
DIFFRACTION
ELECTRON SPECTROSCOPY
ELEMENTS
HALOGENS
HYDROCARBONS
KINETICS
METALS
NONMETALS
ORGANIC COMPOUNDS
ORGANIC OXYGEN COMPOUNDS
REACTION KINETICS
SCATTERING
SORPTION
SPECTROSCOPY
SURFACE PROPERTIES
TRANSITION ELEMENTS
400201* - Chemical & Physicochemical Properties
020500 - Petroleum- Products & By-Products