Ethylene Epoxidation at the Phase Transition of Copper Oxides

Article pubs.acs.org/JACS

Ethylene Epoxidation at the Phase Transition of Copper Oxides Mark T. Greiner,* Travis E. Jones, Alexander Klyushin, Axel Knop-Gericke, and Robert Schlögl Fritz-Haber-Institut, Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany S Supporting Information *

ABSTRACT: Catalytic materials tend to be metastable. When a material becomes metastable close to a thermodynamic phase transition it can exhibit unique catalytic behavior. Using in situ photoemission spectroscopy and online product analysis, we have found that close to the Cu2O−CuO phase transition there is a boost in activity for a kinetically driven reaction, ethylene epoxidation, giving rise to a 20-fold selectivity enhancement relative to the selectivity observed far from the phase transition. By tuning conditions toward low oxygen chemical potential, this metastable state and the resulting enhanced selectivity can be sustained. Using density functional theory, we find that metastable O precursors to the CuO phase can account for the selectivity enhancements near the phase transition.

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INTRODUCTION Determining the chemical structure of active sites is a key step in understanding how a heterogeneous catalyst works. It is often found that active sites are high-energy, metastable configurationssuch as step edges,1−3 stacking faults,4 vacancy defects,5−7 lattice-strained sites,8 and surfaces of amorphous materials.9−11 In some cases, catalytically active, metastable ensembles can only form in situ by the combined effect of many kinetically and thermodynamically driven processes.12,13 Unfortunately, the scale and complexity of such dynamic processes preclude the identities of metastable sites from being predicted ab inito using modern theoretical methods. Contemporary in situ microscopy and spectroscopy techniques are used to learn more about such phenomena, identify metastable active sites, and understand how they are formed and how they behave catalytically. One of the most informative clues about the metastable nature of active catalysts comes from observations of catalysts under conditions close to a thermodynamic phase transition. By “phase transition”, we are referring to the scenario in which a set of thermodynamic variables (i.e., temperature or pressure) results in two phases of a material having identical free energies. When a catalytic material is held in such conditions, small local perturbations in temperature, chemical potential, and concentration can stimulate the surface to flip from one phase to the other with minimal energetic consequence. In a sense, the catalyst becomes thermodynamically frustrated. This bistability gives rise to such catalytic phenomena as spatiotemporal pattern formation,14 oscillatory reactivity,15 and activity enhancements.16−18 The spatiotemporal pattern formation and reaction oscillations associated with bistable phases were intensively researched in the 1980s and 1990s, where the focus was mainly on reactions relevant to the automotive industry, such as CO oxidation and NO reduction.15,18−21 In these cases, the two coexisting phases were surfaces terminated with different adsorbates (e.g., CO- and O-covered surfaces in the case of CO oxidation). The bistability of the two terminations ensures © 2017 American Chemical Society

that both reactants are present on the surface in high concentrations and in close proximity to one another. It remains unclear how influential phase transitions are to general catalysis, e.g., in cases where oscillations and pattern formation are not observed. Under reaction conditionsdue to interactions between catalyst and gas phasegenerally many surface phases coexist, and dynamic transitions between them occur continuously. If such phase transitions are coupled with a step in a catalytic pathway, they can have a large influence on catalytic performance. Indeed, the development of new in situ characterization methods has provided many examples of phase coexistence under reaction conditions;22−28 far fewer studies have so far demonstrated a correlation between phase coexistence or phase transitions and catalytic performance.23,26,29,30 Based on the diversity of surface phases with similar free energies that can be present under reaction conditions and the dynamic nature of catalyst surfaces,27 it is quite possible that phase transitions play a much larger and more general role in catalysis than is currently recognized by a large part of the research community. It is important to point out, however, that the notion of “phase” here must be defined to include not only bulk phases but also surface phases, such as surface reconstructions and surface terminations. In the present work, we report on enhanced catalytic activity for a kinetically controlled reactionethylene epoxidation near the phase transition from Cu2O to CuO. In contrast to the rate enhancements near the phase transition in CO oxidation, where the rate enhancement is a result of a maximum in coadsorption of reactants, we propose that the epoxide selectivity enhancement near the phase transition on copper oxides is due to the presence of metastable adsorbed oxygen species on Cu2O that are precursors for CuO formation and form close to the Cu2O−CuO phase transition. Density functional theory (DFT) calculations indicate that such species offer an alternative reaction pathway to ethylene epoxide that is kinetically favored over the total oxidation pathway. These Received: May 15, 2017 Published: July 28, 2017 11825

DOI: 10.1021/jacs.7b05004 J. Am. Chem. Soc. 2017, 139, 11825−11832

Article

Journal of the American Chemical Society

Figure 1. (a) In situ Cu L3-edge spectra measured while heating Cu2O in a 1:1 mixture of O2:C2H4 at 0.3 mbar total pressure. (b) Ethylene epoxide selectivity measured using gas chromatography during the temperature ramp, with the corresponding proportions of Cu2O and CuO on the surface, as determined from the spectra in (a).

findings suggest that metastable surface species formed near oxide phase transitions could be utilized to enhance catalytic properties.

CuO phase transition. The maximum in EO selectivity (ca. 21%) is obtained at a temperature of 180 °C. At higher temperatures, the EO selectivity rapidly drops by more than a factor of 10 (to ca. 1.5%). Note selectivity was calculated as [EO]/([EO] + [AcH] + 2[CO2]). EO, CO2, and AcH were the only ethylene oxidation products detected. The same experiment was performed using CuO as the staring material (shown in Figures S1 and S3). In this case, there was no change to the copper oxidation state during the experiment and no high-selectivity EO production. The only EO production was of low-selectivity (1.5%) at higher temperatures (>240 °C), similar to what was observed after Cu2O was completely oxidized to CuO in the previous results. These findings are consistent with those reported by Jayamurthy et al.32 If conditions are tuned such that Cu2O does not transform into CuO at high temperature (i.e., by using lower O2 partial pressures) then the high selectivity epoxidation reaction is also observed during cooling (see Figure S5). This finding confirms that the high selectivity epoxidation is not simply due to the burning off of contaminant surface carbon, because such species are completely absent at high temperature. While Figure 1b shows that the high-selectivity epoxidation reaction is clearly correlated with the Cu2O−CuO phase transition, one could argue that this correlation is only coincidental and not causally related to the Cu2O−CuO phase transition. One could argue, for instance, that Cu2O is the active phase for epoxidation, and if one could keep the catalyst in a state of Cu2O at higher temperatures, the epoxide activity would continue to increase. However, as Figure S5 shows, in an oxygen poor gas feed of 1:10 O2:C2H4 the complete transition of Cu2O to CuO can be avoided, such that the catalyst remains Cu2O at high temperatures. Under these conditions, where the Cu2O−CuO phase transition is no longer imminentbecause oxygen chemical potential decreases with increasing temperaturethe catalyst does not exhibit high selectivity epoxidation, but rather a low selectivity of ca. 1% (similar to CuO). Thus, high selectivity on Cu2O is only observed under conditions where the Cu2O−CuO transition is imminent. It should be noted that the GC used for these experiments also resolves AcH. Interestingly, detectible amounts of AcH were produced during the high temperature reaction (with a selectivity for aldehyde of only ca. 1%). During epoxidation on silverthe de facto standard ethylene epoxidation catalyst

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RESULTS In the ethylene oxidation reaction, several products are possiblenamely ethylene epoxide (EO), acetaldehyde (AcH), and CO2, with CO2 being the most thermodynamically stable product. EO and AcH, being partial oxidation products, are thermodynamically less stable than CO2 but can be produced preferentially when the path toward partial oxidation products and subsequent desorption is faster than the total oxidation pathway. It has previously been shown that metallic copper generally oxidizes to CuO when exposed to epoxidation conditions and exhibits a selectivity to epoxide of only ca. 1%;31 however, it was also found that, during the initial stages of oxidation, a transient selectivity of ca. 28% occurs.32 This selectivity enhancement was attributed to the presence of Cu2O, forming as an intermediate structure during Cu oxidation, but is not stable under epoxidation conditions. To begin our investigation, we examined the behavior of Cu2O under epoxidation conditions using near-ambient pressure (i.e., millibar range) photoemission spectroscopy. Figure 1 shows a series of in situ Cu L3 NEXAFS spectra from Cu2O powder measured during a temperature ramp (from 50 to 450 °C, 4 °C/min) in a 1:1 gas mixture of O2:C2H4 at 0.3 mbar total pressure. One can clearly recognize the transformation from Cu2O to CuO by the change in position of the main absorption line seen in Figure 1a. At low temperatures, the main line is positioned at 933.56 eV (indicating Cu+). When the temperature reaches 120 °C, the first signs of Cu2+ become apparent, with the formation of a peak at 931.26 eV (indicating Cu2+), and by 350 °C the transformation of the surface from Cu2O to CuO is complete. In tandem with the in situ NEXAFS spectra, the reaction products were monitored using gas chromatography (GC), proton-transfer-reaction mass spectrometry (PTR-MS), and electron ionization mass spectrometry (EI-MS). The selectivity to EOas measured with GCis shown in Figure 1b as well as the corresponding catalyst surface compositionas determined by fitting the NEXAFS spectra of Figure 1a using reference spectra. From this figure, one can see that the selectivity to EO is greatly enhanced at the onset of the Cu2O− 11826


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Figure 2. (a) Partial oxidation products and (b) selectivity data, measured with PTRMS, during temperature ramps of Cu2O (blue lines) and CuO (red lines) in 0.3 mbar of 1:1 O2:C2H4.

AcH is never detected because it is rapidly oxidized to CO2 before it can desorb from the surface. This observations indicates a distinctly different behavior compared to conventional epoxidation catalysts. In order to investigate the high-selectivity epoxidation process further, we repeated the temperature-programmed reaction experiments with and without gas-phase oxygen in the reaction feed, using both Cu2O and CuO as starting materials. The PTRMS results are presented in Figure 2, representing the production of mass 44 (note that PTRMS was chosen due to its extremely high sensitivity to C2H4O and relative insensitivity to CO2 due to their very different proton affinities).33 From Figure 2a, it is clear that the boost in EO selectivity near the phase transition is due to increased EO production rather than decreased CO2 production. Second, one sees from Figure 2b that when gas-phase oxygen is removed from the reaction feed, Cu2O does not produce any detectible EO. This finding suggests that a form of adsorbed oxygen is required for the high selectivity EO reaction to occur. Last, comparison of Figure 2c,d shows that CuO produces EO only at high temperatures (>240 °C), with low selectivity (1−3%), and produces EO even in the absence of O2. This observation, in addition to the NEXAFS spectra corresponding to the experiment in Figure 2d (see Supporting Information)showing that CuO is reduced during heating in ethylenesuggests that EO is produced on CuO via a Mars van Krevelen mechanism. To understand how adsorbed oxygen species existing under conditions close to the Cu2O−CuO phase transition might influence ethylene epoxidation activity, we performed a variety of computational experiments. For these tests, we examined the Cu2O(110) surface because it has been predicted from ab initio atomistic thermodynamics to be the thermodynamically most stable Cu2O surface under the conditions used in our experiments.34 A rendering of the Cu2O(110) surface is shown in Figure 3a. The surface shares a structural similarity with the CuO(110)

Figure 3. Images of the (a) Cu2O(110), (b) Cu2O(110) with interstitial oxygen, and (c) CuO(110) surfaces.

surface, consisting of rows of Cu ions and a zigzag arrangement of O ions along the rows of Cu. It is clear from Figure 3c that the CuO(110) surface can be constructed by filling all the empty zigzag sites (the tetrahedral holes) of the Cu2O(110) surface, along with a concomitant distortion to the lattice. To model how the Cu2O(110) surface might look prior to the onset of oxidation to CuO, we have placed an additional O ion into a tetrahedral site of the Cu2O(110) surface. Such interstitial oxygen ions could potentially form at defect sites such as O vacancies (as elaborated further in the Discussion). The geometry shown in Figure 3b is the site with the lowest energy O-interstitial configuration on the Cu2O(110) surface. This surface represents Cu2O with an excess of oxygen, and recognizing the structural similarity of this interstitial O ion with the stoichiometric CuO(110) surface, one could expect it to represent a precursor to the nucleation of CuO. It should be noted that the first elementary step in Cu2O−CuO transformation and the fitst elementary step in ethylene epoxidation are the samenamely, O2 activation. In the geometry of Figure 3b, only one interstitial O ion is present (instead of two) because the geometry was formed by relaxing O2 close to an Ovacancy defect on the Cu2O(110) surface. The surfaces shown in Figure 3a,bi.e., Cu2O(110) and Cu2O(110) with an interstitial oxygen defectwere used for subsequent minimum-energy path (MEP) calculations to determine barriers in the ethylene epoxidation mechanism. 11827


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Figure 4. Calculated reaction pathway for epoxidation on (a) a pristine Cu2O(110) surface and (b) a Cu2O(110) surface containing an O-interstitial ion. The zero energy represents desorbed EO and the respective surface.

Figure 5. (a) Cu L3 spectra, (b) epoxide production, and (c) epoxide selectivity (measured with GC) of Cu2O during a 10-h isothermal reaction in 1:7 O2:C2H4 at 170 °C and 0.3 mbar.

Turning to the Cu2O surface containing the O interstitial (Figure 4b), we see that adsorbed EO can be formed by overcoming a barrier of only 0.8 eV. Again, no oxametallacycle intermediate is involved in the pathway. Interestingly, due the instability of the interstitial O ion, the EO adsorption energy is only ca. 0.3 eV, indicating that once formed, EO quickly desorbs. When considering the isomerization of EO to AcH, we find that the reaction passes through an ethylenedioxy (EDO) intermediate, with a barrier of ca. 1.5 eV, before forming AcH, reminiscent of what has been computed for partially oxidized silver surfaces.37 Furthermore, no direct path from ethylene to AcH was found (i.e., the MEP was always EO → EDO → AcH). In summary, these calculations suggest that an O-rich Cu2O surface would have both a higher activity and higher branching ratio to EO through a common intermediate than the stoichiometric surface would. While it is interesting that epoxidation activity can be enhanced near a phase transition, the question arises whether a catalyst can be sustained in this metastable state. To answer this question, we performed isothermal reactivity and in situ NEXAFS measurements using various O2:C2H4 ratios. We found that by tuning the oxygen-to-ethylene ratio in the gas feed it was possible to hold the catalyst in the state of Cu2O indefinitely. Figure 5 shows Cu L3 spectra and GC data from Cu2O in a 1:7 mixture of O2:C2H4 (0.3 mbar) held at 170 °C (i.e., near the high-selectivity reaction maximum) for 10 h.

The results from these calculations are summarized in Figure 4. On both surfaces, we calculated the pathway from adsorbed ethylene to EO and to AcH. For the stoichiometric surface, the O ion used for the reaction was taken from a normal lattice site, while for the non-stoichiometric surface, the interstitial O ion was used in the reaction. Considering first the stoichiometric surface, the energy diagram shows that EO is formed from ethylene directly (i.e., without passing through the oxametallacycle intermediate typically found on metal surfaces).35 This result is consistent with previously published calculations that showed a direct pathway to EO on Cu2O(100).36 The barrier to direct epoxidation on the stoichiometric surface was calculated to be ca. 1.7 eV. Due to the strong oxygen−copper bond, breaking the bond and forming EO is endothermic relative to adsorbed ethylene on the stoichiometric surface. Once formed, EO can either desorb from the surface, revert back to ethylene and lattice oxygen, or isomerize to AcH. The desorption energy is nearly the same as the barrier of reverting back to ethylene (ca. 0.8 eV). To form AcH it was found that the molecule must revert back to ethylene, followed by formation of an oxametallacycle, then formation of AcH. This process has a barrier significantly smaller (by 0.2 eV) than the barrier to formation of EO from ethylene and lattice O from Cu2O. In summary, the stoichiometric surface should preferentially form AcH (or CO2), as opposed to EO, because the barrier to AcH formation is smaller than the barrier to EO formation. 11828


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and theoretical studies.40−44 Several native defects are present in significant concentrations in Cu2O. The most dominant defect under most conditions is the neutral Cu vacancy; however, O vacancies are also relatively stablesecond only to Cu vacanciesand increase in concentration with temperature. At elevated temperatures O vacancies can even become the dominant defect.40 The interstitial structure used here was obtained computationally by relaxing the geometry of O2 in the vicinity of an O vacancy, indicating a barrier-less pathway to formation. A significant number of O vacancies will be present on the Cu2O surface at elevated temperatures. The proposed interstitial oxygen structure used in this study is both a relatively stable structure, with an adsorption energy of −0.5 eV (relative to gas phase O2 and the pristine Cu2O surface), and structurally analogous to the CuO surface. One would expect that such species would be in relatively high concentrations under conditions near to the Cu2O → CuO phase transition and prior to the nucleation of CuO. In this structure, the over-coordinated Cu ion would be formally Cu2+, and one would expect a steady-state population of Cu2+ species to be present, whose concentration depends on the balance of their formation and disappearance rates. There are numerous possible routes to the formation of such species. As mentioned, O2 splitting at O vacancies, or at under-coordinated Cu sites at step edges or kinks are potential routes. Recent atomically resolved in situ TEM studies have shown that Cu2O grows via terrace growth along step edges, suggesting that O2 activation may occur at Cu2O step edges.45 An O2 activation step would be needed for both ethylene epoxidation and the Cu2O−CuO transformation. If activated O species are not removed by ethylene, they will eventually result in the nucleation of CuO. The present data do not enable us to quantitatively assess the formation and disappearance rate of such species to determine whether the Cu2+ concentration measured at steady state (0.3 atomic percent) is a reasonable concentration. The NEXAFS spectra at steady state would suggest that 3 out of every 1000 Cu surface sites are Cu2+. Interestingly, the proposed activated oxygen species does not behave the same as a terminal oxygen on the CuO surface, even though they have structural similarities. As shown in Figure S7, ethylene removes the lattice O ions from CuO and gives rise primarily to the formation of CO2. One can understand this by considering that, on a fully oxidized CuO surface, the terminating oxygen species are in a minimum energy configuration, with bond and lattice distortions at a minimum. In contrast, a single CuO-like site in an otherwise Cu2O-like surface is only metastable. The interface between such a site and the surrounding lattice results in a strained bonding environment. This highly strained site gives rise to the exothermic epoxidation step (predicted on the MEP calculations) and accounts for the ease at which epoxide desorbs from these sites. In stoichiometric CuO, the O ion that gives rise to epoxide is taken from the surface lattice, as is evident from the reduction of CuO during epoxidation (see Figure S7). In the recent work by Jayamurthy et al., where an epoxidation selectivity of 28% was observed from supported copper catalysts in a flow reactor, they postulated that the Cu+ species gives rise to the high-selectivity epoxidation. The high selectivity that was observed was only transient, and over the course of tens of minutes, it decreased to ca. 1%. They found that, after the reaction, the Cu catalyst had oxidized to CuO, and that by reducing the oxidized catalyst back to Cu they

Under these conditions, the epoxide production and selectivity reach a steady state, with a selectivity of 10% (note, the selectivity was generally found to decrease with O2 partial pressure). The NEXAFS spectra measured during the reaction show that the surface remains primarily Cu2O; however, a close examination of the pre-edge reveals a small concentration of Cu2+ (ca. 0.3 atomic percent) as is evident by the small shoulder at 931.26 eV in Figure 5d. Whether this Cu2+ represents over-coordinated Cu ions in Cu2O or a small amount of CuO on the surface is uncertain. It should also be noted that, under steady-state conditions, a small amount of carbonaceous species forms on the surface and contain CC, CO, and CO bonds according to the XPS spectra (see Figure S6). These species are believed to be adsorbed AcH species that become strongly bound to the surface, as predicted from the DFT calculations from Figure 4. Note that the formation of carbon does not increase with time, but reaches a steady state.

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DISCUSSION The results shown in this work demonstrate several important findings. First, Cu2O can exhibit low-temperature, highselectivity ethylene epoxidation under conditions in which it is metastable. Complete oxidation to the thermodynamically more stable CuO deactivates this process, and CuO does not exhibit low-temperature, high-selectivity epoxidation activity. The high-selectivity reaction can be sustained on Cu2O if the reaction is carried out under conditions in which the catalyst stays as Cu2O but is close enough to the phase transition to enable the formation of the hypothesized adsorbed metastable species. These conditions are ones where the oxygen chemical potential in the gas phase is high enough to form the metastable species of an O-rich Cu2O surface, while the ethylene chemical potential is high enough to prevent complete oxidation of Cu2O to CuO. It is possible, based on the observed presence of carbonaceous species at steady state, that strongly bound CxHyOz species slow the Cu2O oxidation by blocking sites that would otherwise be susceptible to oxidation. The inference of an adsorbed oxygen species was based on the tests with and without O2 in the gas phase. Gas-phase O2 is needed for the low-temperature, high-selectivity reaction on Cu2O, and in the absence of gas-phase O2, Cu2O is inactive to epoxidation at the corresponding temperatures (ca. 150−220 °C). Thus, we conclude that an adsorbed oxygen species is needed in the reaction. In contrast, for the high-temperature reaction on CuO, no gas-phase oxygen is needed to form epoxide because the source of oxygen is the CuO lattice. This means that, without O2 in the gas phase to reoxidize the CuO, it would eventually deactivate via reduction. At high temperatures in the absence of O2, Cu2O produces CO2 in a similar manner, where the oxygen source for forming CO2 is the lattice oxygen of the Cu2O surface. Regarding the choice of adsorbed oxygen species to model, we considered several possibilities. While some oxides can form charged molecular oxygen species like superoxo and peroxo, evidence based on EPR suggests that these intermediate structures are not stable on Cu2O.38 Furthermore, stoichiometric Cu2O surfaces have been shown to be relatively incapable of O2 activation, with activation barriers on the stoichiometric Cu2O(111) surface in the range of 1.3 eV.39 For O2 activation to occur, it is believed that defects are required, such as step edges or O vacancies. The thermodynamic stability of defects in Cu2O has been the subject of many experimental 11829


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monochromator to provide photons in the energy range of 80−2000 eV, with a flux of in the range of 6 × 1010 photons/s/100 mA and a spot size at the sample of 100 μm × 80 μm. The end station consists of a near-ambient pressure X-ray photoemission spectrometer (NAPXPS) produced by SPECS GmBH. The spectrometer uses a Phoibos 150 analyzer with three differentially pumped stages separating it from the sample chamber. The pressure in the sample chamber was held constant using mass flow controllers to flow reactive gases into the chamber and a PID-controlled throttle valve to pump the gases out and maintain constant pressure in the chamber. Samples were heated from the back side using a focused infrared laser that uniformly illuminates, over an area of 10 mm × 10 mm, a stainless steel backplate in contact with the sample. Temperature was monitored using type K thermocouples clamped to the surface of the sample and regulated using a PID controller connected to the laser power source. XPS spectra were measured with the spectrometer in fixed-analyzer transmission (FAT) mode. Photon energies were always tuned to keep the photoelectron kinetic energies at 150 eV, making the technique very surface sensitive (λ ≈ 6 Å in Cu2O). NEXAFS spectra were either measured in total electron yield (TEY) mode or Auger electron yield (AEY) mode. TEY mode has better signal intensity but cannot be used for the O K-edge when gas-phase oxygen is present due to the large gas-phase absorption signal. In TEY mode, the drain current is measured by detecting the secondary electrons that hit the nozzle in front of the sample surface (i.e., the nozzle that leads to the differentially pumped stages and hemispherical analyzer). In AEY mode, the analyzer is set to collect electrons of a specified kinetic energy that corresponds to an Auger emission line (in the case of oxygen, we used a kinetic energy of 493.0 eV). In both TEY and AEY, the signals are recorded while the monochromator is scanned across the desired absorption edge. The excitation energy scale was calibrated for the Cu L-edge using the absorption edge of metallic Cu (932.67 eV at the adsorption edge inflection point), and in the case of the O Kedge it was calibrated using the π* transition of gas-phase O2 (absorption peak at 530.8 eV). Copper oxide powders were purchased from Sigma-Aldrich and were of 99.999% metals purity. Powders were pressed into 8 mm diameter pellets using a pressing tool. After loading onto vacuum, the samples underwent a standard pretreatment procedure. They were first heated in 10−2 mbar of O2 to 400 °C to remove adsorbed carbon, while monitoring with C 1s XPS peak. The surface composition was then checked using two survey spectra: one using a photon energy of 1486.7 eV for a broad overview, and one using a photon energy of 355 eV that is very sensitive to low atomic mass impurities such as S, Cl, and Si. In general the samples were clean after initial oxidation, but if samples contained impurities they would be exchanged for new samples. Note that sputter cleaning is not an effective technique for cleaning powders that are to be tested in catalysis because the surface within the pores of the powder that are not exposed to the ion beam will remain contaminated. The Cu oxidation state of the starting material was confirmed using Cu L-edge NEXAFS, O K-edge NEXAFS, Cu 2p XPS, and O 1s XPS. During initial oxidation, it can happen that CuO begins to form on the Cu2O surface. This can be removed either by heating to a high temperature in vacuum (>550 °C) or by heating in a mild reducing agent like ethylene (>350 °C). Gas analysis methods included gas chromatography (GC), quadrupole mass spectrometry (QMS), and proton-transfer-reaction mass spectrometry (PTR-MS). The three methods were used in parallel for every experiment. All gas analytical methods were attached, using highvacuum connections, to the exhaust line of the differentially pumped sample chamber. The GC was a Varian CP-4900 MicroGC with four columns: two silica molecular sieve columns, one PPQ column, and one alumina column. With this equipment we could detect all the partial and total oxidation products. The PTR-MS was from Ionic Analytik and was used to measure C2H4O (both EO and AcH) with very high sensitivity and without detection of CO2. This ability is afforded by the vastly different proton affinities of C2H4O and CO2. The QMS (Prisma from Pfeiffer GmbH) was used to detect CO2 with a higher sensitivity than the GC. All detection equipment was

could reproducibly form the initial structure that generated the higher selectivity epoxidation. It was concluded that the oxidation state of Cu plays an important role in the epoxidation reaction, but without in situ spectroscopy it was uncertain which oxidation state was correlated to the epoxide activity. It was proposed that a Cu+ plays an important role in the epoxidation, and they even postulated that a synergy between oxidation states may play a role. In the present work, through the use of in situ spectroscopic methods correlated with online product analysis, we can confirm that such a synergy between oxidation states exists between Cu+ and Cu2+. The synergy occurs at the onset of the phase transition between Cu2O and CuO. We can confirm that Cu2O does not intrinsically exhibit this enhanced epoxide selectivity (i.e., when far from the phase transition). This observation is clear from an experiment where epoxidation was performed at high temperature (>300 °C) with an oxygen-lean reaction feed of O2:C2H4 = 1:10 (see Figure S5). Under these conditions, the epoxide selectivity peaks at ca. 150−220 °C, then decreases at higher temperature, while the surface remains Cu2O. The selectivity at high temperature is similar to the hightemperature reaction on CuO (ca. 1%). The explanation for this observation is that, as temperature increases, O2 chemical potential decreases. At elevated temperature and low O2 partial pressure, Cu2O becomes thermodynamically preferred over CuO and thus is not close to a phase transition. 46 Consequently, the proposed metastable oxygen species would rapidly recombine and desorb from the surface. In contrast, near the phase transition, Cu2O has the highest oxygen chemical potential it can have without forming CuO. This finding is an indication that the high-selectivity reaction occurs only when Cu2O is operated in conditions near to the phase transition and does not intrinsically exhibit high epoxide selectivity. We propose that the presence of metastable oxygen surface species is necessary.

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CONCLUSION We have found that operating a Cu2O catalyst in ethylene oxidation, under conditions at the onset of the Cu2O−CuO phase transition, gives rise to a large increase in ethylene epoxide activity. Due to this increase in epoxide production, the epoxide selectivity increases by a factor of 20 (from ca. 1% away from the phase transition to over 20% at the onset of the phase transition). When fully oxidized to CuO, or when present as Cu2O, but far away from the phase transition, catalysts exhibit a ca. 1% epoxide selectivity. In the absence of gas-phase O2, the epoxidation reaction on Cu2O is absent, leading us to conclude that the reaction involves an adsorbed oxygen species that forms on the Cu2O surface at the onset of oxidation to CuO. By modeling this surface as a Cu2O(110) surface with interstitial O ions (a precursor to the CuO structure), DFTbased minimum energy pathway calculations reveal that such an oxygen species would give rise to increased epoxidation activity due to its high energy. These findings provide new insights into how catalyst metastability near phase transitions yields valuable catalytic behavior, and how a kinetically controlled reaction can be enhanced near an oxide phase transition.

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EXPERIMENTAL SECTION

All experiments were carried out at the Innovative Station for In-Situ Spectroscopy (ISISS) beamline at the Helmholz-Zentrum Berlin für Materialien und Energie (HZB) synchrotron light source (BESSY II). The beamline uses a bending magnet and a plane-grating 11830



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calibrated using a five-point calibration curve made by dosing of test gases containing the analytes of interest in known amounts. The temperature ramps (example shown in Figure 1) were performed by first cooling the sample to 50 °C, filling the chamber with the desired composition of O2 and C2H4 up to 0.3 mbar total pressure, then initiating a programmed temperature ramp from 50 to 450 °C at a rate of 4 °C/min. Photoemission spectra (NEXAFS or XPS) and gas product analysis were measured during the ramps. For temperature ramps on both Cu2O and CuO, gas compositions of 1:1, 1:2, 1:5, 1:7, and 1:10 (O2:C2H4) were tested (see Supporting Information). Steady-state tests were performed only on Cu2O, using a temperature of 170 °C, a total pressure of 0.3 mbar, and gas compositions of 1:1, 1:2, 1:4, and 1:7 (O2:C2H4). The duration of the tests was either 10 h or until the activity was expended. Calculations. DFT calculations were performed with the Quantum ESPRESSO package47 using ultrasoft pseudopotentials from the PS Library48 with a kinetic energy (charge density) cutoff of 30 Ry (300 Ry) and the PBE exchange and correlation potential.49 We performed calculations with and without the dispersion corrections.50 Only the dispersion corrected results are reported. A k-point mesh equivalent to (2×4) for the (110) surface unit cell was used. We employed four layers of Cu2O for all slabs, which were separated by 20 Å of vacuum, and the bottom two layers were held fixed during ionic relaxation. Cell parameters for the 2×2 slab were 12.02 and 8.50 Å. MEPs were computed with the climbing image nudged elastic band algorithm. The paths were considered to be converged when the force on the climbing image was less than 0.05 eV/Å and the energy change dropped below 10−3 eV.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b05004. Figure S1, GC data of Cu2O and CuO in 1:1 O2:C2H4; Figure S2, NEXAFS spectra of Cu2O during temperature ramp in 1:1 O2:C2H4; Figure S3, NEXAFS spectra of CuO during temperature ramp in 1:1 O2:C2H4; Figure S4, NEXAFS spectra and PTRMS data of Cu2O during several heating cycles in 1:1 O2:C2H4; Figure S5, GC data and NEXAFS spectra of heating and cooling ramps of Cu2O in dilute-O2 epoxidation feed; Figure S6, C 1s spectra of Cu2O during epoxidation reaction; Figure S7, NEXAFS spectra measured during heating of CuO in C2H4 (PDF)

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Article

AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Mark T. Greiner: 0000-0002-4363-7189 Travis E. Jones: 0000-0001-8921-7641 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank the Max-Planck Gesellschaft and the Alexander von Humboldt Foundation for their generous funding. We would also like to thank the Max-Planck Gesellschaft and the Helmholtz-Zentrum Berlin for use of their infrastructure. We thank Höchstleistungsrechenzentrum Stuttgart (HLRS) for access to the supercomputer HazelHen. 11831


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