Are multiple oxygen species selective in ethylene epoxidation on silver?

Abstract

We show atomic oxygen on an unreconstructed Ag(110) surface has a O 1s binding energy ≤ 528 eV and its stable at low coverages. Our findings point to the idea of multiple selective oxygen species in ethylene epoxidation on Ag.

Kinetics of Ethylene Epoxidation on a Promoted Ag/α‐Al2O3 Catalyst—The Effects of Product and Chloride Co‐Feeds on Rates and Selectivity

“…Schlögl and co‐workers demonstrated that the yield of EO, monitored by proton‐transfer reaction mass spectrometry, correlates with the amount of electrophilic oxygen (O1s/Ag3d 5/2 ) measured in situ using XPS on a polycrystalline Ag foil at 420 K. A concurrent increase in the atomic percentage of Cl (from 1.7 to 7.9 %, O decreased from 16.7 to 14.9 % with balance Ag, determined by XPS with a probing depth of ≈0.6 nm), in EO selectivity (from 9.5 % to 55 %), and in the ratio of electrophilic (O1s binding energy 530–531 eV) to nucleophilic (528–529 eV) oxygen species (from 1.24 to 2.07) was observed as the number of ethylchloride (C 2 H 5 Cl) pulses was varied from zero to three during ethylene epoxidation on a commercial Ag powder (≈120 000 h −1 space velocity, 0.3 mbar total pressure, 503 K) . Subsequent studies under UHV conditions demonstrated that a third species, with O1s binding energy <528 eV, was formed on Ag(110) at <0.04 monolayer oxygen coverage which rapidly (<180 s) recombined to form nucleophilic oxygen with increasing exposure time . DFT calculations predicted this species to be unreconstructed oxygen adatoms which could form EO and acetaldehyde via an OMME‐type intermediate, though with low selectivity to EO .…”

“…Subsequent studies under UHV conditions demonstrated that a third species, with O1s binding energy <528 eV, was formed on Ag(110) at <0.04 monolayer oxygen coverage which rapidly (<180 s) recombined to form nucleophilic oxygen with increasing exposure time . DFT calculations predicted this species to be unreconstructed oxygen adatoms which could form EO and acetaldehyde via an OMME‐type intermediate, though with low selectivity to EO . Bukhtiyarov et al .…”

“…[8] Subsequents tudies under UHV conditions demonstrated that at hirds pecies, with O1s binding energy < 528 eV,w as formed on Ag(110) at < 0.04 monolayer oxygen coveragew hich rapidly( < 180 s) recombined to form nucleophilic oxygen with increasing exposure time. [33] DFTcalculationsp redicted this speciest ob eu nreconstructed oxygen adatomsw hich could form EO and acetaldehyde via an OMME-type intermediate, though with low selectivity to EO. [33] Bukhtiyarov et al [32] showed on the basis of experiments with isotopically labeled oxygen over as ilver foil that an ucleophilic oxygen specieswith O1s binding energyo f5 28.4 eV participated solely in total oxidation, while an electrophilic oxygen species with an O1s binding energy of 530.5 eV performede thylene epoxidation.…”

“…[33,34] These unreconstructedOatoms are highly reactivea nd becomeu nstable at oxygen coverages greater than 0.02 monolayers (ML), and thus couldb eu ndetectableb yi ns itu techniques under ethylene epoxidationc onditions. [33] We build on these precepts in the literaturer egarding different oxygen species on the surface of Ag catalysts to postulate ar ole of surface chlorine coverageo nt he relative densities of the types of adsorbed oxygen speciesa nd consequently on the rate and selectivity of ethylene epoxidation.…”

“…Both species were required to form ethylene oxide however, as ethylene did not adsorb to a silver foil prepared without O nucl . In situ XPS studies and DFT calculations on Ag(110) showed that two different oxygen species can be active for EO formation: 1) the aforementioned electrophilic oxygen species (O elec , BE(O1s) ≈530.5 eV), as well as 2) an atomic oxygen species on an unreconstructed silver surface (BE(O1s) ≈527.9 eV) . These unreconstructed O atoms are highly reactive and become unstable at oxygen coverages greater than 0.02 monolayers (ML), and thus could be undetectable by in situ techniques under ethylene epoxidation conditions .…”