Robert B. Grant

A single crystal study of the silver-catalysed selective oxidation and total oxidation of ethylene

Abstract Ethylene oxidation has been investigated on a well-characterised Ag(111) single crystal surface at pressures of up to 50 Torr. In the absence of promoters and moderators, chemisorbed atomic oxygen reacts with adsorbed ethylene to yield both ethylene oxide and (CO 2 + H 2 O). Chemisorbed dioxygen, though present, appears to play no direct role in either of these reactions; the presence of subsurface oxygen is necessary for selective oxidation but not for total oxidation. Batch reactor studies yield rate parameters for both partial and total oxidation which are consistent with the values reported for conventional supported catalysts; selectivity decreases with increasing temperature, pressure, and ethylene coverage. Acetaldehyde, acetic acid, and oxalic acid are identified as reaction intermediates in the pathway to CO 2 formation. Results for the oxidation of C 2 D 4 confirm these observations, and the observed kinetic isotope effect indicates that H-transfer rather than CC cleavage is rate-determining in the combustion of both ethylene and ethylene oxide. Possible reaction pathways and mechanisms are examined.

Basic studies of the oxygen surface chemistry of silver: Chemisorbed atomic and molecular species on pure Ag(111)

Abstract The interaction of oxygen with Ag(111) has been studied over the pressure range 10 −2 −1.0 Torr. Thermal desorption measurements using isotopically labelled molecules unambiguously establish the presence of a stable chemisorbed dioxygen species which co-exists with adsorbed atomic oxygen. Dissolved oxygen undergoes exchange with the latter species but not with the former. The maximum dioxygen population is found to be markedly sensitive to gas dosing pressure; a model is proposed which accounts for these observations and for related observations on alkali-doped Ag. XP and UP spectral features can be correlated with the two types of oxygen species; angle-resolved XP and Auger spectra indicate that O 2 (a) resides on the metal surface whereas O(a) is located within the surface. The XP spectra also suggest that in the case of O 2 (a) the molecular axis may lie perpendicular to the surface.

Ethylene oxide isomerisation on single-crystal Ag(111) in atomically clean and Cs-moderated conditions

Abstract The isomerisation of ethylene oxide to acetaldehyde has been investigated over a single crystal Ag(111) surface between 300 and 500 K and at pressures of up to 2 Torr. It is found that the nature of the rate-determining step changes at ~410 K; at lower temperatures the rate of isomerisation of the adsorbed reactant is rate limiting, at higher temperatures the rate of adsorption becomes rate controlling. There is qualitative and quantitative agreement between the high-pressure reaction rate data and the measured adsorption/desorption properties of the reactant and product molecules. The rate parameters and reaction order are in satisfactory agreement with data obtained for conventional supported Ag catalysts indicating that it is indeed a metal-catalysed reaction which determines the isomerisation behaviour of such materials. The promoter and moderator actions of dissolved oxygen and preadsorbed Cs are characterised and a model for the observed behaviour is presented.

Chlorine-oxygen interactions and the role of chlorine in ethylene oxidation over Ag(111)

Abstract Coadsorbed chlorine enhances the surface → bulk transport of chemisorbed oxygen on Ag(111). Preadsorbed chlorine progressively blocks the chemisorption of atomic oxygen (O(a)); O(a) uptake is completely suppressed at θ Cl = 0.25. Reactor studies of ethylene oxidation in the presence of adsorbed chlorine show that Cl reduces overall activity and increases selectivity towards ethylene oxide formation. All catalytic activity is quenched for chlorine fractional coverages ≥0.25. This strongly suggests that O(a) is the crucial surface species responsible for both selective and total oxidation of ethylene. Temperature-programmed reaction data indicate that under certain conditions Cl exerts an apparent antipromoter effect. The relevant conditions are remote from those which prevail in a typical flow reactor.

Secondary chemistry in the selective oxidation of ethylene: Effect of Cl and Cs promoters on the adsorption, isomerisation, and combustion of ethylene oxide on Ag(111)

Abstract The adsorption/desorption, isomerisation, and combustion of ethylene oxide on single crystal Ag(111) have been studied under conditions which are pertinent to ethylene epoxidation. The effects of chlorine on the uptake and isomerisation of ethylene oxide are examined, as is the effect of Cs on the burning reaction. These data complement our earlier results and suggest that the observed promoter effects are largely electronic in origin. CO 2 strongly inhibits the combustion reaction, apparently by converting chemisorbed oxygen to a carbonate species.

Alkali metal, chlorine and other promoters in the silver-catalysed selective oxidation of ethylene

Ethylene oxidation over well characterised Ag(111) surfaces has been investigated by temperature-programmed reaction measurements and by differential batch reactor studies at pressures up to 50 Torr. The influence of chlorine predosing on catalytic activity indicates that a chemisorbed atomic oxygen species is responsible for both partial oxidation and complete oxidation to CO2+ H2O. This tends to be confirmed by experiments using N2O as the oxidant, both with the single crystal speciemen and with a practical Ag–αAl2O3 catalyst in a flow microreactor. Dissolved oxygen, like adsorbed chlorine, is found to be a selectivity promoter. Adsorbed Cs also increases the rate of ethylene oxide production but can also positively influence the overall activity of the system. The results suggest that chlorine and dissolved oxygen promoters principally affect the primary chemistry, whereas the main effect of Cs is on the secondary chemistry (further oxidation of ethylene oxide). This view tends to be confirmed by temperature programmed reaction measurements and by direct experiments on the influence of CI and Cs on the isomerisation and combustion of ethylene oxide itself. It is suggested that these effects are primarily electronic in origin and a mechanism based on this view is presented. In the presence of Cs, both CO2 and NOx can act as selectivity promoters for the formation of ethylene oxide.

The silver-catalysed decomposition of N2O and the catalytic oxidation of ethylene by N2O over Ag(111) and Agα-Al2O3

Abstract Single-crystal measurements show that N 2 O decomposition is significantly activated on atomically clean Ag(111). The presence of preadsorbed atomic oxygen accelerates the process, as do small amounts of dissolved oxygen; larger amounts of dissolved oxygen have an opposite effect. Isotope distribution data show that the reaction tends to occur preferentially in the vicinity of preexisting surface oxygen atoms. Atomic oxygen deposited by N 2 O decomposition is shown to be active for ethylene epoxidation in three different types of experiment, in each of which the effect of gaseous oxygen was negligible. These results provide strong support for the “atomic oxygen” mechanism of ethylene epoxidation.

Pressure dependence of ethylene oxidation kinetics and the effects of added CO2 and Cs: A study on Ag(111) and Ag/α-Al2O3 catalysts

Abstract The kinetics of ethylene oxidation on a well-characterised Ag(111) single crystal surface have been investigated over a range of pressure, temperature and gas composition. Similar measurements on an α-A 2 O 3 supported Ag catalyst are compared with the single crystal data and discussed in terms of a specific model for the reaction mechanism. Cs is an important selectivity promoter and CO 2 an effective poison in the industrial epoxidation of ethylene. Single crystal data on the effect of these two additives are also presented. In particular, it is found that the simultaneous presence of both species can lead to markedly enhanced selectivity towards ethylene oxide formation.

Mechanism of the silver-catalysed heterogeneous epoxidation of ethylene

Chemisorbed atomic oxygen is the crucial surface species which selectively oxidises ethylene to ethylene oxide; adsorbed dioxygen plays no direct role in this reaction.

Pressure Broadening of H2O Absorption Lines in the 1.3 μm Region Measured by Continuous Wave-Cavity Ring-Down Spectroscopy: Application in the Trace Detection of Water Vapor in N2, SiH4, CF4, and PH3

A continuous wave cavity ring-down (cw-CRD) spectrometer has been developed for the measurement of trace levels of water vapor by absorption spectroscopy at wavelengths in the vicinity of 1358 nm and 1392 nm. The speed of data acquisition and selectivity make cavity ring-down spectroscopy potentially more useful than current techniques for measurement of trace water in process gases and vacuum environments used for semiconductor manufacture where water vapor contamination has a detrimental effect on the final product. The pressure broadening coefficients (γ) for bath gases N2, air, and Ar and semiconductor process gases SiH4, PH3, and CF4 were determined for a range of absorption lines in the 2v1 and v1 + v3 bands of H2O. For the transitions investigated, the concentration of water vapor in the sample gas varied from 1.7 × 1012 to 2.9 × 1015 molecule cm−3 in N2 at a total pressure of <10 mbar and was mixed with the bath gas of increasing pressure up to ∼200 mbar. The values of γ quantify the reduction in peak absorption cross-sections with bath gas pressure and, thus, their effects on the detection limit of water vapor. For a CRD spectrometer with a ring-down time of τ = 12.0 μs measured with a precision of 0.6%, detection limits for the measurement of water vapor in 1 atm N2 and of CF4 were estimated to be 18 and 14 ppbv, respectively. Competing absorption by SiH4 and PH3 in the 1.3 μm wavelength region results in respective detection limits for water vapor of 98 and 319 ppbv (relative to 1 atm) in 0.2 atm SiH4 and 0.37 atm PH3.