Are multiple oxygen species selective in ethylene epoxidation on silver?

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.


Introduction
Although extensively studied, 1-4 the structure of the O/Ag system during ethylene epoxidation is still not completely understood. Under reaction conditions, two broad types of oxygen species are present at the O/Ag surface during epoxidation. [5][6][7] These species can be distinguished by X-ray Photoelectron Spectroscopy (XPS). One type (nucleophilic oxygen) has an O 1s binding energy (BE) of 528-528.5 eV and the other type (electrophilic oxygen) has a BE of 530-531 eV. [5][6][7] The former, an electron rich oxygen, has been proposed to activate C-H bond breaking 8,9 and has been shown to participate in the complete oxidation of ethylene. 10 The latter, an electron decient oxygen, has been proposed to open the C]C double bond of ethylene, forming the COC ring through O insertion, 9,11,12 and has been shown to participate in epoxidation. 13 Yet, while the atomic structure of nucleophilic oxygen is known, 10,14,15 the atomic structure of the active species for ethylene epoxidation is debated. 4,6,[15][16][17] This electrophilic species is thought to be weakly bound oxygen and, following early assignments, 18,19 has been extensively interpreted as being unreconstructed adsorbed or dissolved atomic O. 6,7,[20][21][22][23][24][25][26][27] However, even in early publications such assignments appeared inconsistent. Campbell et al., 28 using TPR (Temperature Programmed Reaction) and XPS, reported on an unreactive subsurface form of oxygen with BE of $528.5 eV and proposed that it would be similar in nature to atomic O. Similarly, in two later publications Bukhtiyarov et al. 29,30 observed that XPS spectra would only show a main component at 528.4 eV, although the presence of dissolved O could be detected by TPD (Temperature Programmed Desorption). Consistent with the assignment of electrophilic oxygen to unreconstructed atomic oxygen, an oxometallacycle (OMC) mechanism has been developed wherein ethylene reacts with adsorbed O on the unreconstructed surface to make ethylene oxide and acetaldehyde. 17,[31][32][33] More recent publications from Rocca et al. 34,35 report on O species with BE < 528 eV for a Ag(210) surface exposed to O 2 at low temperatures. The authors attribute the low BE O 1s component to two O species: adsorbed O atoms in 4-fold hollows and O-Ag rows at the steps. Moreover, it was recently demonstrated by means of Density Functional Theory (DFT) calculations combined with experimentally measured O 1s BE and NEXAFS (Near Edge X-ray Absorption Fine Structure), that the spectroscopic features of unreconstructed atomic oxygen do not agree with those measured for the electrophilic species with a BE of 530-531 eV. 15 Furthermore, it was shown by DFT that when Ag/O bonding is ionic, as is the case for atomic O on Ag, more weakly bound atomic oxygen will have a lower O 1s BE and thus, it cannot account for the electrophilic species. 16 Despite this, adsorbed atomic O is still assigned in the literature to unreconstructed atomic oxygen based on the BE of 530-531 eV. 26 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 investigation 15 with fast in situ XPS measurements performed with a Ag(111) single crystal in O 2 at 10 À4 mbar showed that the O 1s spectra had a peak with BE < 528 eV that could only be seen for the rst minutes of dosing, shiing with time to slightly higher BE. The low BE component was only observed for an Ocoverage of $0.05 ML. Although the shi 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 À4 mbar of O 2 . The sticking coefficient for the dissociative adsorption of O 2 on Ag(110) has been reported to be two to three orders of magnitude higher than on Ag(111). 28,[37][38][39][40][41][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 O 2 .
It is well known that when a Ag(110) surface is exposed to O 2 at conditions such that dissociative adsorption occurs, a series of p(N Â 1) added row reconstructions can easily be produced. 28,42 Moreover, due to the relatively high sticking coefficient of oxygen on the Ag(110) surface, these reconstructions can be made under low O 2 dosing in UHV compatible system. 28,42 Such conditions are necessary when studying highly reactive species, which can be undetectable if clean-off reactions with background gases, for instance CO, become relevant. These types of unwanted reactions are typical for adsorbed atomic O, as shown by low temperature STM measurements. 43 Herein, we perform in situ experiments on a Ag(110) surface by exposing the single crystal to O 2 at 10 À5 mbar and 10 À6 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 (q O < 0.04 ML) which for Ag(110), can only be obtained at low O 2 pressure (10 À6 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 conrm 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.

Experimental details
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 O 2 treatment at 10 À3 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 Â 10 À8 mbar) for 5 min. Large amounts of C, Cl, S and Si segregated to the surface aer the initial O 2 exposure, but aer 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 O 2 at 10 À6 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 owing liquid N 2 through the manipulator. Sample heating was done by a tungsten lament placed behind the sample holder. The sample temperature was measured with a N-type thermocouple. Oxygen dosing was done by backlling the UHV chamber with O 2 at pressures in the range 10 À7 to 10 À6 mbar.
In all cases, the binding energy scale for each spectra was calibrated by the Fermi edge measured with the same photon energy.
More details about the tting procedure and other information are given in the ESI. †

Computational details
Calculations were performed with the Quantum ESPRESSO package 48 using the Perdew, Burke, and Ernzerhof (PBE) exchange and correlation potential 49 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 ve-layer slabs separated by 15Å of vacuum with the bottom two layers were held xed at their computed bulk values. A k-point mesh equivalent to at least (12 Â 12) for the (1 Â 1) surface-unit-cell was used with Marzari-Vanderbilt cold smearing 53 and a smearing parameter of 0.01 Ry. Core level shis were computed with the DSCF (Self Consistent Field) method to capture both initial and nal 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 Â 8) for the (1 Â 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.

Results and discussion
In Situ measurements We begin our investigation studying the interaction of O 2 with a Ag(110) surface by performing fast in situ XPS measurements at low pressures, in order to characterize the species present at low coverages. When exposed to 10 À5 mbar O 2 at 423 K, the O 1s spectra shows only a main peak at 528.2 eV and does not change signicantly with time (see Fig. 1a). This BE indicates that an Oreconstruction is formed 15,28 immediately aer exposure. The situation changes when the experiment is done at 10 À6 mbar, which is close to the predicted minimum pressure needed to stabilize the p(N Â 1) reconstructions at 423 K. 15 Thus, we might expect unreconstructed atomic oxygen to be stable, at least transiently. We nd that at 10 À6 mbar O 2 at 423 K, an oxygen species with a BE of 527.9 eV is present aer 30 seconds of dosing (Fig. 1b), with an estimated coverage of $0.02 ML. This BE is in line with that expected for unreconstructed atomic O from DFT calculations. 15,16 The O 1s BE shis rapidly to higher BEs within the rst three minutes (see Fig. 2a) reaching a BE of $528.15 eV and a coverage of $0.04 ML, indicating a surface reconstruction has precipitated at this coverage. Aer this point, coverage increases at a slower rate from $0.04 ML to $0.09 ML in 8 minutes and the BE of the O 1s spectra is approximately constant, reaching a value of 528.2 eV, which corresponds to that of the p(N Â 1) reconstructions. 15,28 This behavior is similar to what was previously observed on Ag(111) at 10 À4 mbar 15 and is attributed to the transition from O adsorbed on the unreconstructed silver to the formation of reconstructed p(N Â 1) islands. These ndings are supported by ab initio atomistic thermodynamics. 15,55 Previous work has shown that the adsorption energy of oxygen is higher in a p(N Â 1) reconstruction than on the unreconstructed surface. 15 Inspection of Tables 1S and 2S in ESI † conrm this nding even for a low coverage of oxygen on the unreconstructed surface. Thus-ignoring changes in the vibrational free energy, which are expected to be small 55unreconstructed atomic oxygen will only be favored over the oxygen induced p(N Â 1) reconstructions when the congurational entropy difference between the two phases is high. The difference in surface free energy for equal coverages of reconstructed and unreconstructed can then be given by: where DE ads is the difference in adsorption energy between oxygen on the unreconstructed Ag(110) and a p(N Â 1) reconstruction: The difference in congurational entropy in eqn (1) is: where the congurational entropy for an adatom at coverage q on the unreconstructed surface is given by: For simplicity, the congurational 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 Â 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 Dg(T) ¼ 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 À5 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 rst measured spectra aer exposure of O 2 at 10 À5 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 À4 mbar O 2 ) 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 temperature 56,57 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.

Low temperature measurements
We continue by analysing the interaction of O 2 with a Ag(110) surface by dosing O 2 at 453 K and then at 120 K. Although at 120 K the main adsorption form of oxygen is molecular, dissociation is expected due to low activation energy, which we computed to be only 0. 4 Fig. 2S †). In fact, dissociation has been observed to occur to some extent already at this temperature by STM, 36,59 though the formation of the added row reconstructions was found to start at 170-200 K. 43,59,60 Taking into consideration the results obtained for the in situ measurements, and that the sticking coefficient for O 2 is higher at lower temperatures, 38-40,42 the O 2 exposure was done using pressures in the range 10 À7 to 10 À6 mbar, in order to obtain low O-coverages. Moreover, low O 2 exposures are preferred to minimize the formation of OH and CO 3 (ref. 28 and 61) that might occur due to the fast reaction of unreconstructed atomic O with background gases CO and H 2 O. 43 Fig . 3 shows the O 1s and Ag 3d spectra of a Ag(110) surface exposed to different amounts of O 2 at different temperatures (indicated in the gure). 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 3d 5/2 coreline. When 600 L O 2 is dosed at 453 K, the O 1s spectra shows a main peak at 528.3 eV. This BE is in agreement with the measured 15,28 and calculated 15 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 Ag d+ formation due to the presence of oxygen atoms on the surface. 6,23 Although most metals show a core-level shi (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 O 2 adsorbed in a fourfold hollow (FFH) site with the interatomic axis parallel to the [001] or [1À10] 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 oxygen 15 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 Ag d+ sites.
We observe that the amount of the low BE species in the O 1s core-level spectra decreases with time. For instance, aer 35 minutes at 120 K with a maximum base pressure of 10 À9 mbar, the amount decreases by at least half (see Fig. 4S †). This is consistent with the previously reported high reactivity of unreconstructed atomic oxygen. 59,60 Two kinetically different atomic O species have been previously described for CO oxidation to CO 2 . 40,63,64 For O pre-covered Ag(110) exposed to CO, the formation of CO 2 was observed to be slow at high oxygen coverages and to accelerate at lower coverages, 40 due to the more reactive unreconstructed adsorbed O atoms present at the low coverage limit. Furthermore, unreconstructed adsorbed O has been reported to react with CO at temperatures as low as 70 K. 43 It was observed by low temperature STM measurements, that the amount of adsorbed O atoms on the unreconstructed silver would decrease due to clean-off reactions with the background gas of the UHV chamber even at a base pressures lower than 1 Â 10 À10 mbar at 110 K. 43 As mentioned before, DFT calculations predict unreconstructed atomic oxygen on silver to have a BE # 528 eV (ref. 15 and 16) in contrast to the p(N Â 1) reconstructions on Ag(110) for which it predicts a BE of 528.5 eV. 15 This gives a CLS of $0.5 eV to lower BE for O on the unreconstructed Ag(110) surface. Thus, the BE predicted by DFT, the measured BE of 527.9-528 eV and the high reactivity observed for this species, are all characteristics consistent with atomic O adsorbed on unreconstructed Ag(110). This is consistent with the low BE component seen in the in situ experiments (see Fig. 1). The higher coverage attainable at low temperatures is attributed to the reconstructions' formation being kinetically hindered at 120 K, 59,60 due to the lower mobility of Ag atoms at low temperature. 56

Adsorbed atomic O in ethylene epoxidation
The generally accepted idea that reconstructed atomic oxygen, assigned to nucleophilic oxygen, participates in the complete oxidation of ethylene to CO 2 and that unreconstructed adsorbed atomic oxygen, oen assigned to electrophilic oxygen, can participate in ethylene oxide (EO) production, motivated a series of theoretical investigations on the reaction mechanism for ethylene epoxidation. 17,31-33 On unpromoted silver, experimental evidence 10,14,27 shows that the reaction of ethylene with O-reconstructions favors total combustion giving CO 2 as a product, which is supported by theory. 10,32 For the reconstructions, DFT calculations show that the reaction of ethylene and atomic O has a low barrier towards acetaldehyde (AcH) formation, 10 which rapidly burns. 10,14,31 Conversely, for unreconstructed adsorbed atomic oxygen on silver, DFT calculations have been reported to give similar barriers for the conversion of ethylene to EO and AcH, 31,33,65 predicting a $50% EO selectivity for unpromoted silver catalysts. 31 This agrees with many experimentally observed selectivities, 1,2,66 that were believed to take place on unpromoted silver catalysts thus, unreconstructed atomic O was thought to be the active species in ethylene epoxidation and was assigned to electrophilic oxygen. 19 However, we demonstrate that unreconstructed atomic oxygen is present at very low O-coverages and can be only detected transiently during in situ measurements. At the temperature and pressure relevant for ethylene epoxidation only 2 oxygen species have been predicted to be stable by ab initio atomistic thermodynamic studies: the surface reconstruction (nucleophilic oxygen) and unreconstructed atomic O. 55,67 The O-reconstruction is always thermodynamically favored under ethylene epoxidation conditions 15 making unlikely that unreconstructed O would be present at the surface under equilibrium conditions. However, low coverages of such species might be present 55,67 if the formation of the reconstruction was limited by kinetics, as suggested by microkinetic modeling. 68 Now consider that on silver under O 2 at 423-520 K nucleophilic oxygen is the rst O species detected while electrophilic oxygen is obtained only aer longer times of exposures. 6,18 Moreover, electrophilic oxygen is more rapidly obtained by increasing the O 2 pressure and substrate temperature 6,29 or by exposing the silver catalyst to the reaction mixture (C 2 H 4 + O 2 ). 13,27 Thus, adsorbed atomic oxygen on the unreconstructed silver surface which is (as shown herein) a highly reactive species detected only at low coverages its unlikely to survive under such conditions used to obtain high concentration of electrophilic oxygen for UHV studies. 13,27,29 Furthermore, the maximum obtainable coverage of unreconstructed atomic O of ca. 0.02 ML (see Fig. 1 and 2) is much lower than to the Oreconstruction coverage of 0.5-0.7 ML observed in UHV 14,28,39 and under reaction conditions. 6,15 These combined characteristics of high reactivity and very low coverage have prevented the species from being accurately characterized in the past by spectroscopic techniques both in UHV and during in situ measurements. Considering that there is small spectroscopic difference between these species it is reasonable to think that a very low coverage of unreconstructed O atoms would remain "undetected" even by in situ techniques. Thus, although predicted to exist, unreconstructed atomic O has been erroneously assigned to the so called electrophilic oxygen with a BE of 530-531 eV observed on silver catalysts in UHV 13,27,29 and under reaction conditions 5,7,20,21 and shown to participate in epoxidation. 13 The question then remains if the oxygen species identied 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 31 The rst 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 nd that OMC formation is weakly activated, with E a ¼ 0.09 eV.
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 burned 71 -and noting that AcH rapidly combusts 72,73 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][75][76] From this measure one would predict the low-coverage phase of O ads 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 Â 2) cell and found no evidence increasing the OMC coverage will change the preference to AcH (see Table 7S and 8S †). We further veried 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) identied in this work will not be selective towards EO. However, the small difference in AcH/EO activation energy implies O ads 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 E a to AcH more than 0.4 eV lower than that to EO 10 -is selective towards AcH, and hence, CO 2 . Furthermore, in the presence of promoters the AcH/EO branching ratio associated with the reaction of O ads on Ag(110) may shi towards EO. Such behavior has already been found in calculations of the OMC mechanism in the presence of halogens 74 and Cs 76,78 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 oxygen 16 (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 O ads 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 aer EO adsorption on a silver single crystal. 77 Here we have shown that O ads can be prepared and identied in UHV, opening the opportunity to test the complete route of reaction C 2 H 4 + O ads / OMC / EO predicted by DFT, although the high reactivity of O ads towards clean-off reactions will make such studies challenging.

Conclusions
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 Â 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 O 2 exposure at low Ocoverages (<0.04 ML) at 423 K. At higher coverages, the thermodynamically favored O-reconstructions are formed. These ndings 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 ndings 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 ndings 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.

Conflicts of interest
There are no conicts to declare.