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Timestamp: 2019-04-24 14:31:40+00:00

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The nature of the oxygen species active in ethylene epoxidation is a long-standing question. While the structure of the oxygen species that participates in total oxidation (nucleophilic oxygen) is known the atomic structure of the selective species (electrophilic oxygen) is still debated. Here, we use both in situ and UHV X-ray Photoelectron Spectroscopy (XPS) to study the interaction of oxygen with a silver surface. We show experimental evidence that the unreconstructed adsorbed atomic oxygen (Oads) often argued to be active in epoxidation has a binding energy (BE) ≤ 528 eV, showing a core-level shift to lower BE with respect to the O-reconstructions, as previously predicted by DFT. Thus, contrary to the frequent assignment, adsorbed atomic oxygen cannot account for the electrophilic oxygen species with an O 1s BE of 530–531 eV, thought to be the active species in ethylene epoxidation. Moreover, we show that Oads is present at very low O-coverages during in situ XPS measurements and that it can be obtained at slightly higher coverages in UHV at low temperature. DFT calculations support that only low coverages of Oads are stable. The highly reactive species is titrated by background gases even at low temperature in UHV conditions. Our findings suggest that at least two different species could participate in the partial oxidation of ethylene on silver.
On Ag(111), Carlisle et al.36 reported that unreconstructed atomic oxygen could be present only at oxygen coverages below 0.05 ML, as seen by STM. In line with this observation, an investigation15 with fast in situ XPS measurements performed with a Ag(111) single crystal in O2 at 10−4 mbar showed that the O 1s spectra had a peak with BE < 528 eV that could only be seen for the first minutes of dosing, shifting with time to slightly higher BE. The low BE component was only observed for an O-coverage of ∼0.05 ML. Although the shift in BE is small for this surface, it was interpreted as a transition from atomic oxygen on the unreconstructed silver surface to the formation of islands of O-reconstructions as coverage increased, since the reconstruction is thermodynamically favoured.15 In contrast, the computed BE difference of unreconstructed adsorbed atomic oxygen with respect to the O-reconstructions on the Ag(110) surface is larger.15 However, in the same study, for the Ag(110) surface no such low BE peak was observed when exposed to 10−4 mbar of O2. The sticking coefficient for the dissociative adsorption of O2 on Ag(110) has been reported to be two to three orders of magnitude higher than on Ag(111).28,37–42 Thus, it can be expected that the O-coverage would increase faster on Ag(110) than on Ag(111), implying a reconstructed surface would form faster on Ag(110) under O2.
Herein, we perform in situ experiments on a Ag(110) surface by exposing the single crystal to O2 at 10−5 mbar and 10−6 mbar at 423 K and taking fast O 1s spectra (30 s per spectra). We show that a low BE species is only present at low coverages (θO < 0.04 ML) which for Ag(110), can only be obtained at low O2 pressure (10−6 mbar). We assign this species with BE ≤ 528 eV to unreconstructed atomic O, as predicted by DFT and in line with previous interpretations.15 We confirm the assignment to unreconstructed atomic oxygen by XPS measurements of O/Ag(110) at 120 K in UHV where the O-reconstruction, although thermodynamically favoured,15 is kinetically hindered, and higher coverages (∼0.1 ML) of atomic adsorbed O on the unreconstructed silver can be obtained.
The in situ XPS measurements were performed at the ISISS beamline in the BESSY II synchrotron radiation facility of the Helmholtz-Zentrum Berlin. Details about the system can be found elsewhere.44,45 The Ag(110) single crystal was polished and oriented to an accuracy <1°. The crystal was cleaned by repeating cycles of O2 treatment at 10−3 mbar at 423 K for 20 min, followed by Ar sputtering at 1.5 kV for 20 min and annealing at 673 K in vacuum (5 × 10−8 mbar) for 5 min. Large amounts of C, Cl, S and Si segregated to the surface after the initial O2 exposure, but after several cleaning cycles, only Ag and O were observed for exposures shorter than 1–2 h. The crystal was placed in a sapphire sample holder and held by a tungsten wire. Heating was done from the backside with an IR laser on a stainless-steel plate in contact with the crystal. The sample temperature was measured with a K-type thermocouple and controlled by adjusting the laser power using a PID feedback loop. Photon energies were chosen so that photoelectrons with the same kinetic energy of 150 eV could be measured for the different elements, giving an equivalent to an inelastic mean free path of 0.5 nm.
The low temperature measurements were performed in UHV at the BACH beamline in Elettra Sincrotrone Trieste.46,47 For these measurements the Ag(110) single crystal was cleaned by cycles of Ar sputtering at 1.5 kV for 20 min, annealing in O2 at 10−6 mbar at 453 K and subsequent annealing in UHV at 673 K. The cycles were repeated until no C was observed on the surface. A photon energy of 670 eV was used for acquisition of all the core-lines. For the low temperature measurements, the sample was cooled by flowing liquid N2 through the manipulator. Sample heating was done by a tungsten filament placed behind the sample holder. The sample temperature was measured with a N-type thermocouple. Oxygen dosing was done by backfilling the UHV chamber with O2 at pressures in the range 10−7 to 10−6 mbar.
In all cases, the binding energy scale for each spectra was calibrated by the Fermi edge measured with the same photon energy.
Calculations were performed with the Quantum ESPRESSO package48 using the Perdew, Burke, and Ernzerhof (PBE) exchange and correlation potential49 and the exchange-hole dipole moment (XDM)50,51 dispersion correction. Projector augmented wave (PAW) potentials were taken from the PS library,52 and we employed a kinetic energy (charge density) cutoff of 70 Ry (700 Ry). Surfaces were modeled as five-layer slabs separated by 15 Å of vacuum with the bottom two layers were held fixed at their computed bulk values. A k-point mesh equivalent to at least (12 × 12) for the (1 × 1) surface-unit-cell was used with Marzari–Vanderbilt cold smearing53 and a smearing parameter of 0.01 Ry. Core level shifts were computed with the ΔSCF (Self Consistent Field) method to capture both initial and final state effects.54 Climbing image nudged elastic band (NEB) calculations were performed using 8–10 images with a single climbing image, a k-point mesh equivalent to (8 × 8) for the (1 × 1) surface-unit-cell, and a kinetic energy (charge density) cutoff of 30 Ry (300 Ry), unless otherwise noted. This reduced convergence criteria results in adsorption energy changes of only 0.1 eV.
Fig. 1 O 1s time evolution on Ag(110) during exposure to O2 at 10−5 mbar and 10−6 mbar at 423 K.
Fig. 2 (a) O 1s BE as a function of time for Ag(110) in 10−6 mbar O2 at 423 K, (b) predicted maximum coverage of unreconstructed atomic oxygen as a function of temperature, assuming the added-row Ag–O chains in the p(N × 1) contribute no configurational entropy. The dashed line shows the predicted coverage at the temperature of the experiment.
For simplicity, the configurational entropy of an O atom in the surface reconstruction is taken to be zero.
Because we are concerned with low-coverage phases, we take the adsorption energies of oxygen in their low-coverage limits, the p(4 × 1) reconstruction and 1/16 ML oxygen in the fourfold hollow sites of the Ag(110) surface (see ESI for more details, Table 3S and Fig. 1S†). With these adsorption energies, the maximum coverage of adsorbed oxygen on the unreconstructed surface can be found by setting Δγ(T) = 0.
The predicted maximum coverage of oxygen on the unreconstructed surface is shown in Fig. 2b. This approach predicts a maximum coverage of atomic oxygen on unreconstructed Ag(110) of ∼0.02 to 0.03 ML at 423 K, in good agreement with the experimental results.
With the preceding analysis in mind it is worth returning to the experiments performed at 10−5 mbar on the Ag(110). The absence of the low BE feature in these experiments can now be understood as due to the faster increase in coverage at this pressure (see Fig. 3S†). The first measured spectra after exposure of O2 at 10−5 mbar corresponds to an estimated coverage of ∼0.08 ML, a coverage at which both our experimental and theoretical (see Fig. 2) results show unreconstructed atomic O is no longer stable.
By this, it may be argued that for the pressure used in ref. 15 (10−4 mbar O2) the absence of a low BE on the Ag(110) surface is probably due to a O-coverage higher than 0.04 ML already at very short times.
It is well known that on clean silver surfaces steps act as a source of mobile Ag atoms even at room temperature56,57 and the detachment of these Ag atoms is a thermally activated process.56 Ostwald ripening of two dimensional islands on Ag(110) has been observed at temperatures above 160 K.56,58 The growth rate of O-reconstructions on Ag(110) at 190 K was found to be lower than the supply rate of oxygen atoms, remaining constant while the O-coverage increased.56 The equilibrium Ag adatom density might not be sustained at low temperatures if the Ag atom detachment from the steps is kinetically limited, even if thermodynamically favoured. Consequently, at lower temperatures unreconstructed atomic O can be expected to be present at the silver surface. Thus, we turn to low temperature experiments with the aim of obtaining unreconstructed atomic O, since this should be a stable species for longer time at low temperature and in UHV conditions.
Fig. 3 shows the O 1s and Ag 3d spectra of a Ag(110) surface exposed to different amounts of O2 at different temperatures (indicated in the figure). The spectra acquired for the clean surface is also shown for comparison. The resulting binding energies are summarized in Table 4S.† The clean surface shows a single component with a BE of 368.25 eV for the Ag 3d5/2 core-line. When 600 L O2 is dosed at 453 K, the O 1s spectra shows a main peak at 528.3 eV. This BE is in agreement with the measured15,28 and calculated15 BE for the added row reconstructions on Ag(110) and in line with the measured BE from our in situ experiments (528.2 eV, see Fig. 1). Other small contributions are observed at 529.2 eV and 530 eV. The BEs of these species have previously been assigned to oxide-like layers and electrophilic oxygen, respectively.6,15 The corresponding Ag 3d spectra shows a main component with a BE of 368.25 eV and a smaller component at a BE of 367.85 eV, which is due to the Agδ+ formation due to the presence of oxygen atoms on the surface.6,23 Although most metals show a core-level shift (CLS) to higher BE for higher oxidation states, it is well known that for silver there is a CLS to lower BE.62 The O 1s spectra for the Ag(110) exposed to 60 L and 120 L at 120 K shows two components. One at 529.7 eV was assigned to molecularly adsorbed oxygen based on our DFT calculations showing O2 adsorbed in a fourfold hollow (FFH) site with the interatomic axis parallel to the  or [1−10] direction (adsorption geometries in Fig. 2S†) has a computed O 1s BE of 529.7 eV, consistent with literature values.28 A second oxygen species at 527.9–528 eV is consistent with the computed O 1s binding energy of unreconstructed atomic oxygen15 and the in situ measurements of this species (see Fig. 1). Additionally, the corresponding Ag 3d spectra also show a small component at lower BE, 367.9 eV, due to Agδ+ sites.
Fig. 3 O 1s and Ag 3d5/2 spectra of clean and oxygen covered Ag(110) surface. Temperature and oxygen dosing are indicated in the figure.
The question then remains if the oxygen species identified in this work is possibly also active in epoxidation. To answer this we turned to testing the mechanism of the reaction of oxygen on the unreconstructed Ag(110) with ethylene. Following the convergence tests (see Table 5S†), we employed a (4 × 4) cell with a (3 × 3) k-point mesh, a kinetic energy (charge density) cutoff of 40 Ry (400 Ry), and XDM dispersion corrections. The results are summarized in Fig. 4 (see also Table 6S†). Inspection of Fig. 4 reveals ethylene can react with a low coverage (θ = 1/16 ML) of oxygen adsorbed on the unreconstructed Ag(110) through an oxometallacycle (OMC) mechanism, as has been observed for other surfaces31,66,69,70 and a high coverage (θ = 1/4 ML) of oxygen on the unreconstructed Ag(110).31 The first step in the reaction is ethylene adsorption, which is 0.31 eV exothermic when dispersion corrections are included. Following adsorption, the (partial) oxidation of ethylene proceeds by OMC formation, as the C–H bond is too strong to make hydrogen abstraction from ethylene feasible.31 We find that OMC formation is weakly activated, with Ea = 0.09 eV.
Fig. 4 Reaction of ethylene with Oads on Ag(110) in a (4 × 4) cell. The black lines indicate the formation of the OMC (step 2). The blue (red) lines show the activation energy associated with EO (AcH) formation through the transition state labeled state 3 (5).
Both EO and AcH can be formed through decomposition of the surface OMC. Assuming the EO produced in this reaction does not further decompose—isotope labeling studies on silver sponge suggest 10% of the EO is burned71—and noting that AcH rapidly combusts72,73 allows the branching ratio associated with EO and AcH formation to be viewed as a computational measure of the maximum selectivity afforded by an OMC mechanism.10,31,66,69,74–76 From this measure one would predict the low-coverage phase of Oads investigated in this work is not selective to EO, with the barrier to AcH formation (0.76 eV) 0.14 eV lower than the barrier to EO formation (0.90 eV).
It is interesting to note, however, that at a higher coverage the OMC is thought to decompose more selectively towards EO.31,77 To test this we recomputed the branching ratio using a (2 × 2) cell and found no evidence increasing the OMC coverage will change the preference to AcH (see Table 7S and 8S†). We further verified the absence of dispersion corrections and a different pseudopotential library do not appreciably alter the computed branching ratio (see Table 7S†).
Our results then suggest the oxygen adsorbed on unreconstructed Ag(110) identified in this work will not be selective towards EO. However, the small difference in AcH/EO activation energy implies Oads may produce EO as a minority product with AcH, though perhaps less so than oxygen on unreconstructed Ag(111) or Ag(100).31,66,69 This behavior is in contrast to reconstructed atomic oxygen on the Ag(110) which—with an Ea to AcH more than 0.4 eV lower than that to EO10—is selective towards AcH, and hence, CO2. Furthermore, in the presence of promoters the AcH/EO branching ratio associated with the reaction of Oads on Ag(110) may shift towards EO. Such behavior has already been found in calculations of the OMC mechanism in the presence of halogens74 and Cs76,78 on Ag(111).
The combination of the experimental evidence show that unreconstructed O and electrophilic oxygen are different species and DFT indicates that O on the unreconstructed surface may participate in the partial oxidation of ethylene. Thus, two species may be active in epoxidation. First, the covalently bond type of oxygen16 (of debated structure) with a BE of 530–531 eV (electrophilic oxygen).5,6,13,29,79 Second, the O adsorbed on the unreconstructed surface with a BE ≤ 528 eV as shown herein. The ultimate test for the later will be the experimental epoxidation of ethylene with a low coverage of O adsorbed on an unreconstructed surface. While DFT calculations have shown that the reaction of ethylene with Oads can produce EO through an oxometallacycle (OMC) intermediate,17,33 the experimental evidence on this mechanism has relied on the production of EO from an OMC formed after EO adsorption on a silver single crystal.77 Here we have shown that Oads can be prepared and identified in UHV, opening the opportunity to test the complete route of reaction C2H4 + Oads → OMC → EO predicted by DFT, although the high reactivity of Oads towards clean-off reactions will make such studies challenging.
We have determined experimentally that on unreconstructed Ag(110) adsorbed atomic O has a BE ≤ 528 eV, as earlier predicted by DFT, which is lower than the p(N × 1) reconstruction and thus, cannot give rise to the O 1s feature with BE of 530–531 eV (electrophilic oxygen) believed to be responsible for epoxidation.
Atomic O adsorbed on the unreconstructed silver surface is present during in situ experiments upon O2 exposure at low O-coverages (<0.04 ML) at 423 K. At higher coverages, the thermodynamically favored O-reconstructions are formed. These findings are supported by DFT. At low temperatures, ca. 120 K, unreconstructed O can be obtained in UHV and the atomic O coverage reaches 0.1 ML, due to kinetic limitations to form the O-reconstructions, which is a thermally activated process. This species is highly reactive towards clean-off reactions even at 120 K and reacts rapidly with background gases. These findings suggest that only very low coverage of unreconstructed atomic O is likely to be present at the silver surface under ethylene epoxidation conditions. Although present at low coverage, the computed barriers to EO and AcH indicate that on unpromoted silver EO might be produced as a minority product through reaction with oxygen adsorbed on unreconstructed Ag(110). Our findings suggest that at least two different species, a covalently bond oxygen—electrophilic oxygen—(with a BE of 530–531 eV) and unreconstructed atomic oxygen (with a BE ≤ 528 eV) might participate in the partial oxidation of ethylene. This points to a new way of thinking about one of the most well-studied reactions in chemistry. The fact that not one but multiple oxygen species can participate in epoxidation. This has important implications for the understanding of the mechanism of ethylene epoxidation on silver and of the role of the different oxygen species.
We thank the Helmholtz-Zentrum Berlin for providing support of the in situ electron spectroscopy activities of the FHI at ISISS beamline in BESSY II and the Max-Planck Gesellschaft for generous founding. We gratefully acknowledge Höchstleistungsrechenzentrum Stuttgart (HLRS) for generous access to the supercomputer HazelHen through the SEES2 project. This project has received funding from the EU-H2020 research and innovation programme under grant agreement No. 654360 having benefitted from the access provided by CNR-IOM in ELETTRA Trieste, Italy, within the framework of the NFFA-Europe Transnational Access Activity. T. E. J. acknowledges the Alexander-von-Humboldt foundation for financial support. Open Access funding provided by the Max Planck Society.
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