Interaction of Sulfur Dioxide and Near-Ambient Pressures of Water

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Interaction of Sulfur Dioxide and Near-Ambient Pressures of Water Vapor with Cuprous Oxide Surfaces Markus Soldemo, Joakim Halldin Stenlid, Zahra Besharat, Niclas Johansson, Anneli Önsten, Jan Knudsen, Joachim Schnadt, Mats Göthelid, Tore Brinck, and Jonas Weissenrieder J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06486 • Publication Date (Web): 05 Oct 2017 Downloaded from http://pubs.acs.org on October 8, 2017

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The Journal of Physical Chemistry

Interaction of Sulfur Dioxide and Near-Ambient Pressures of Water Vapor with Cuprous Oxide Surfaces Markus Soldemo1, Joakim Halldin Stenlid2, Zahra Besharat1, Niclas Johansson3, Anneli Önsten1, Jan Knudsen3,4, Joachim Schnadt3,4, Mats Göthelid1, Tore Brinck2, Jonas Weissenrieder1,* 1

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KTH Royal Institute of Technology, Material Physics, SE-164 40 Kista, Sweden

KTH Royal Institute of Technology, Applied Physical Chemistry, School of Chemical Science and Engineering, SE-100 44 Stockholm, Sweden

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Lund University, Department of Physics, Division of Synchrotron Radiation Research, Box 118, SE-221 00 Lund, Sweden 4

MAX IV Laboratory, Lund University, Box 118, SE-221 00 Lund, Sweden

* Corresponding author: [email protected]

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Abstract

The interaction of water vapor and sulfur dioxide (SO2) with single crystal cuprous oxide (Cu2O) surfaces of (100) and (111) termination was studied by photoelectron spectroscopy (PES) and density functional theory (DFT). Exposure to near-ambient pressures of water vapor, at 5×10-3 %RH and 293 K, hydroxylates both Cu2O surfaces with OH coverage up to 0.38 copper monolayers (ML) for (100) and 0.25 ML for (111). O 1s surface core level shifts indicate that the hydroxylation lifts the (3,0;1,1) reconstruction of the clean (100) surface. On both clean Cu2O terminations, SO2 adsorb to unsaturated surface oxygen atoms to form SO3-species with coverage, after a saturating SO2 dose, corresponding to 0.20 ML on the Cu2O(100) surface and 0.09 ML for the Cu2O(111) surface. Our combined DFT and PES results suggests that the SO2 to SO3 transformation is largely facilitated by unsaturated copper atoms at the Cu2O(111) surface. SO3 terminated surfaces exposed to low doses of water vapor (≤ 100 L) in ultra-high vacuum show no adsorption or reaction. However, during exposure to near-ambient pressures of water vapor, the SO3 species dissociate and sulfur replace Cu2O lattice oxygen in a reaction that forms Cu2S. The hydroxylation of the Cu2O surfaces is believed to play a central role in the reaction.

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Introduction Copper is among the most important metals in the today’s society. In metallic form, it has excellent

electrical and thermal conduction properties and is especially suitable for applications in electrical generation and wiring, as well as in heat exchangers. Since copper is both malleable and corrosion resistant, it is also frequently used as plumbing and roofing material.1 In such applications the most prevalent initial corrosion product is cuprous oxide (Cu2O); a thin film of Cu2O is rapidly formed on copper upon exposure to an ambient environment.1-3 Besides being an abundant corrosion product, Cu2O has properties making it suitable for applications within e.g. catalysts for CO oxidation,4-6 photocatalytic water splitting,7 and propene epoxidation.8-11 Sulfur dioxide (SO2) is a molecule that, depending on the context, may have negative or positive connotations; on the one hand it is corrosive to several construction materials,12 while on the other hand it is a common feedstock for production of other chemicals, mainly sulfuric acid, and used as a preservative for food.12 In atmospheric corrosion, sulfur dioxide is one of the most important agents for accelerating the corrosion rate of metals and other materials.13 The emission of SO2 from human activities mainly stems from combustion of fossil fuels containing sulfur impurities. The global SO2 emissions significantly increased during the industrial revolution and peaked in the 1970s.14 However, the trend of declining emissions on the global scale was reversed in the early 21st century as emissions from growing economies such as China14 and India15 are increasing. Furthermore, reactions involving SO2 have been reported in literature, e.g. reduction of SO2 by hydrocarbon16 and the Claus reaction (where SO2 + H2S → 3/2S + 2H2O is a reaction step)17. Catalysts for converting exhaust gas from combustion of petroleum based fuels, in e.g. the automotive industry, are commonly based on noble metals, such as platinum.18 As noble metals are expensive, development of catalysts based on other, cheaper, materials is desirable. However, sulfur poisoning has 3



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historically been an issue for the alternative catalysts since the sulfur content in the fuels form e.g. SO2 upon combustion. Recent development of refining processes of oil has permitted fuels with significantly lower levels of sulfur impurities. This may pave the path for replacing noble metal based catalysts with catalysts based on cheaper materials where copper is one of the more promising candidates and Cu2O has shown high activity to the CO oxidation reaction, see e.g. ref 6. Corrosion of copper is in general unwanted and has been subject to a large number of studies over the years. Early works investigating the impact of sulfur dioxide and humidified air on the corrosion were conducted by Vernon already in the 1930s.19 In the presence of humidified air the Cu2O surfaces may become hydroxylated.20 Formation of bulk Cu(OH)2 is, however, unfavorable compared to Cu2O and H2O, but the compound can be kinetically stabilized.21 Combining several oxidizing agents may lead to total corrosion rates higher than the summed corrosion rates of separate exposures, e.g. Aastrup et al.22 showed how a combined exposure to humidified air and sulfur dioxide resulted in a higher corrosion rate than equivalent individual exposures. Sulfur dioxide adsorption has been studied on copper, oxidized copper, and bulk copper oxides. On Cu(100), it has been reported that SO2 molecules dissociates23 while on Cu(111), at 300 K, no adsorption on SO2 occurs24. With an oxygen adsorbate layer on Cu(100), Cu(100)-c(2×2)O, the dissociation rate of SO2 is lower23 while forming thin films of Cu2O and CuO on Cu(111) increases the adsorption rate of SO2 and SO3 species are formed24. On bulk oxide Cu2O(111), Önsten et al. showed that SO2 adsorbs as SO3species at room temperature with a saturated surface coverage reached already at a dose of 1 L sulfur dioxide.25 Extensive doses of SO2 on Cu2O have been reported to result in Cu2S formation.24 Also, on polycrystalline copper in UHV, sulfide species have been reported after SO2 dosing.26 The dissociation rate of SO2 into adsorbed S or sulfide on the Cu2O surface increases in the presence of oxygen.24,

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Further, in aqueous solutions containing Na2S the lattice oxygen atoms in Cu2O can be replaced by sulfur atoms through an anion exchange reaction, see e.g. refs 27-31. Sulfur dioxide interaction with other metal 4


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oxide surfaces has also been studied extensively, e.g. the oxides of iron,32 aluminum,33-34 titanium,34-37 chromium,38 magnesium,34, 38 zinc,39-40 zirconium,34 and cerium.33-34, 41 Studies of sulfur dioxide interaction with clean metal surfaces includes iron 42, copper 23-24, 26, nickel 43, tin 44, zinc 39-40, and cesium45. Cu2O has a cubic crystal structure (space group 3) with a lattice parameter of 4.27 Å. This study uses the two well-defined low-index surfaces (100) and (111) of single crystalline Cu2O as model for oxidized copper. The prepared surfaces exhibit a (3,0;1,1)-reconstruction (matrix notation) for the (100) surface46 and a (√3×√3)R30°-reconstruction for the (111) surface. The Cu2O(111) (√3×√3)R30°reconstruction is suggested to be formed from 1/3 ML oxygen vacancies at the surface

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, mixed with

minority patches of (1×1) structure and full oxygen coverage. The exact atomic structure and composition of the (111) surface are, however, yet to be determined and the presence or absence of unsaturated copper atoms (CuCUS) on the surface is still under debate 25, 48. This leaves us with two plausible models: model A and model B. The model A has ⅓ ML of coordinatively unsaturated oxygen atoms (OCUS) vacancies

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while model B has both ⅓ ML OCUS and 1 ML coordinatively unsaturated copper atoms (CuCUS) vacancies. Adsorption of probe molecules, e.g. water and SO2, may determine the most likely surface structure. In Table 1, the coverage of a copper monolayer is provided for the different surfaces together with the relevant surface oxygen coverage. The objective of this study is to investigate how the surfaces are impacted at the early stage of being exposed to the two corrosion agents sulfur dioxide and water. This is done by studying how sulfur dioxide binds to the (100)- and (111)-surfaces of Cu2O and subsequently how near-ambient pressures of water at room temperature affects the surfaces. Photoelectron spectroscopy (PES) enables the study of species present at the surface before, during, and after exposure to the different oxidizing agents. The density functional theory (DFT) study sheds light on what species are likely to be present at the surface together with how and where the species interact with the surfaces. Water adsorption at near-ambient pressures results in hydroxylation of both the Cu2O(100) and Cu2O(111) surfaces, with the (100) surface exhibiting 5



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a higher OH coverage. Adsorption of sulfur dioxide on the clean surfaces results in SO3 formation on both the Cu2O surfaces. DFT simulations suggest that the conversion of SO2 to SO3 indeed is exothermically favorable both on the Cu2O(100) surface and also on the Cu2O(111) surfaces with CuCUS present. When exposing the SO3 surface species to water under low pressures (UHV conditions), no reaction is detected experimentally. However, when increasing the water pressure to near-ambient conditions the adsorbed SO3 species dissociate and react with the Cu2O substrate to form Cu2S. The results may be interpreted such as the hydroxylation of the Cu2O surfaces is required for SO3 dissociation and the following sulfur and oxygen ion exchange reaction to take place. We further note that the relative humidity (%RH) dependency of the anion exchange process closely resembles the behavior of Cu2O upon exposure to mixed H2S and H2O gases,49 which may suggest a common rate-limiting reaction step.

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Experimental methods The near-ambient pressure (and UHV) PES measurements were carried out at the NAP-PES endstation

at the beamline I511 on the MAX II ring of the National Swedish Synchrotron Facility MAX IV Laboratory in Lund, Sweden.50 The analysis chamber is equipped with a SPECS Phoibos 150 NAP analyzer and a high-pressure cell for near-ambient pressure studies. In direct connection to the analysis chamber is a preparation chamber equipped with facilities for argon ion sputtering, sample annealing, high precision leak valves, and a LEED-apparatus. Two different surfaces of cuprous oxide, (100) and (111), were used in the study. The single crystal samples were acquired from a commercial retailer (Surface Preparation Laboratories, the Netherlands). The cleaning procedure consists of multiple cycles of argon ion sputtering (1 kV) and annealing in oxygen gas at 550 - 600 °C. The surface structures obtained were the (3,0;1,1)-structure for Cu2O(100)46 and predominately (√3×√3)R30° (with a minor (1x1) contribution) for Cu2O(111)47. The energy scale of the

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photoelectron spectra was calibrated to the Au 4f7/2 peak (binding energy of 84.00 eV) of a gold foil in direct electric connection with the sample. The SO2 gas was acquired from a commercial retailer (AGA Linde, > 99.9%). Deionized water was used for the water dosing in the high-pressure cell and in UHV after it had been further purified through several freeze-pump-thaw cycles.

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Computational methods All DFT calculations were carried out with the VASP simulation package51-55 using the PBE56

exchange-correlation functional with Grimme’s D3 dispersion corrections57 and Becke-Johnson damping58. Hubbard + U on-site corrections59 with an U-j value of 3.6 eV, as recommended in ref. 60, were added for the Cu 3d-states. Spin-polarization was allowed throughout. The valence electrons (Cu: 4s13d10, O: 2s22p4, S: 3s23p4) were represented by a plane-wave basis set with a 400 eV cut-off, whereas the core states were treated by standard PBE PAW potentials. Γ-centered 4×4×1 k-point meshes and the tetrahedron method with Blöchl corrections61 were used to sample the Brillouin zone for the slab calculations. Only the Γ-point was, however, considered for the molecular SO2 calculations (employing a 15×15×15 Å3 cell). The forces were allowed to relax to 0.03 eV/Å during the structural optimizations and the transition state structures were identified using the Nudge-elastic band method62-63. O1s core level shifts were calculated according to the final state approximation64-65 as described in ref. 46. The SO2 adsorption energies (Ead) are reported as electronic energies (i.e. omitting zero-point energies and thermal effects), and determined as the energy difference between the Cu2O-SO2 adduct (ECu2O-SO2) and the separated surface (ECu2O) and adsorbate (ESO2) by: = + −

(1)

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Periodic slab models comprising six Cu2O layers were used to represent the Cu2O(111) and (100) surfaces. The cubic unit cell lattice parameter was set to 4.316 Å based on previous references for the same computational settings.46 Vacuum distances of at least 20 Å were used throughout to avoid interactions between the slabs. SO2 was asymmetrically adsorbed on one side of the slabs: to the Cuterminated side of (100), whereas for (111) both sides are identical. In order to best match the experimental adsorbate coverage, surface super-cells of the c(2×2) [resulting in 0.25 ML coverage] and (√3×√3)R30° [0.09 ML] sizes were chosen for the (100) and (111) surfaces, respectively. The surface termination for (100) was taken as the recently identified low-energy structures of reference 46 with surface Oad atoms. For the (111) surface, the ideal surfaces as well as all three combinations of ⅓ML oxygen vacancies (coordinatively unsaturated, OCUS) and 1 ML copper vacancies (coordinatively unsaturated, CuCUS) were considered.

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Experimental results

High pressure water exposures As reference to the water exposure experiments with SO2 molecules pre-adsorbed on the surfaces, the clean (100) and (111) surfaces were exposed to ambient pressure of water vapor at room temperature. The O 1s photoelectron spectra for both surfaces are shown in Figure 1, where contributions from surface oxygen atoms and OH-groups have been deconvoluted. The UHV spectra from the freshly prepared Cu2O surfaces are shown as comparison. For spectra collected in the near-ambient pressure cell (HP cell), the analyzer was optimized for high transmission (wide analyzer slits and 100 eV pass energy) to minimize the risk for beam damage. The sample was also scanned during acquisition. The relative humidity (%RH) in the present experiments were 5×10-3 %RH. The OH-coverages were estimated following the same method as used by Stenlid et al.,20 where the attenuation of photoelectrons due to their limited inelastic mean-free path in Cu2O and OH-layer was taken into account. The inelastic mean-free path of Cu2O was 8


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calculated using the Tanuma-Penn-Powell algorithm from the NIST Electron Inelastic-Mean-Free-Path Database.66 The OH-coverage were estimated to 0.38 ML (4.2 OH-groups/nm2) and 0.25 ML (3.2 OHgroups/nm2) for Cu2O(100) and Cu2O(111), respectively. In comparison to the clean surface spectra, it is noticed that the component on the low-binding energy side, corresponding to surface oxygen atoms, have vanished completely for Cu2O(100) while it partially remains on Cu2O(111). This indicates that the reconstruction of the clean surface of Cu2O(100) is lifted during the hydroxylation at near-ambient water pressures, in agreement with what has been suggested for cold H2O adsorption in UHV.20 Here the OHpeaks are shifted 1.27 eV and 1.13 eV towards higher binding energies from the bulk oxide peak for Cu2O(100) and Cu2O(111), respectively. This is in reasonable agreement with previous studies by Deng et al.67 where the observed shift is 0.7 eV - 0.9 eV, depending on the relative humidity. Stenlid et al. reported OH-shifts and H2O-shifts on Cu2O(100) of 1.1-1.2 eV and 2.3-2.4 eV under UHV-conditions, respectively.20 Further, Önsten et al. reported the OH-shift and H2O-shift on Cu2O(111) of 1.1-1.2 eV and 2.5 eV under UHV conditions in the temperature range 150 K - 180 K, respectively.25 Adsorption of SO2 in UHV The clean surfaces of Cu2O(100) and Cu2O(111) were exposed to 25 L sulfur dioxide in UHV at room temperature. The PES S 2p- and O 1s-regions were measured with photon energies of 490 eV and 750 eV, respectively (see Figure 2). The S 2p spin-orbit splits are 1.19 eV for Cu2O(100) and 1.23 eV for Cu2O(111). Comparing the shape of the spectra from the two surfaces, it is apparent that the full width half maximum (FWHM) of the components is different for the two surfaces. Curve fitting analysis extracts S 2p3/2 FWHM of 0.70 eV and 0.95 eV for Cu2O(100) and Cu2O(111), respectively. As the Cu2O(111) surface is known to consist of both (√3×√3)R30° and (1×1) terminations47, it is likely that the difference in binding energy of these two terminations contribute to the observed wider FWHM for Cu2O(111) than for Cu2O(100), The binding energy of the S 2p3/2-component on Cu2O(100) was 166.61 eV and 166.35 eV on Cu2O(111). The binding energy of SO2 molecules adsorbed on Cu2O(111) as 9



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SO3-species has been reported at 166.3 eV.25 Similar binding energies have been reported for SO3-species binding to other metal oxides, e.g. 166.7 eV for TiO2(110),37 166.3 eV for Cr2O3(0001),38 167.5 eV for Al2O3 and CeO2,33 and 166.2 eV for Fe3O4(100).32 From the S 2p results, it is determined that SO3-species are formed exclusively on both surfaces. This agrees well with results from DFT calculations (vide infra). In line with this interpretation the highbinding energy component in the O 1s-region that appears upon SO2 exposure, see Figure 2, is associated with the SO3-species. The experimentally determined binding energy of the O 1s SO3 component is 531.23 eV and 530.97 eV for Cu2O(100) and Cu2O(111), respectively. For Cu2O(111), the high-binding energy component is shifted 0.89 eV from the bulk oxide peak, which is marginally larger than the reported literature data of 0.8 eV.25 For the Cu2O(100) surface, the shift of the high-binding energy component relative the bulk oxide component is 0.99 eV. The component on the low-binding energy side of the bulk oxide component in the O 1s-region is associated with Ocus for both Cu2O(100) and Cu2O(111) surfaces.25, 46 It is observed in the spectra that the surface component decreases as the high-binding energy side-component increases, indicative of that the Ocus takes part in the binding of the SO2 molecules. Modeling the SO2 adsorption with a single bond to a Ocus, and assuming that all three oxygen atoms in the formed SO3-unit are contributing equally to the O 1s spectra, the decrease of the low-binding energy component of the O 1s shall equal a third of the growth of the high-binding energy O 1s SO3 component. For the Cu2O(111) surface, the low-binding energy component decreases with 67 % upon SO2 exposure, the decrease in the integrated area equals precisely one third of the component associated with the SO3 species, in agreement with the above suggested adsorbate model. However, for the Cu2O(100) surface, the decrease in intensity of the under-coordinated surface oxygen component exceeds a third of the high-binding energy peak (SO3 species). The closest distance between two surface oxygen atoms on the (3,0;1,1) reconstructed Cu2O(100) surface is quite large, 3.7 Å,46 rendering a bridge bonded configuration unlikely. However, a tentative explanation to the 10


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discrepancy involves the fact that the (3,0;1,1) reconstruction comprise more oxygen than those protruding the most out of the surface. Previous studies have reported that the reconstruction may be lifted by adsorption of atomic hydrogen or water.20, 68 The present results indicate that reconstruction of the (100) surface is also lifted by the adsorption of SO2 and that the O 1s binding energy of oxygen atoms in the reconstruction that do not bind directly to the SO3-species will shift under the bulk oxide component as the reconstruction is lifted. The integrated signal of the different components in the O 1s-region was used to estimate the coverage of SO3-species on the surfaces. The contribution to the O 1s signal from oxygen in the bulk was estimated by calculating the decreasing signal from atomic planes at increasing depth from the surface taking into account the attenuation due to the limited inelastic mean free path of the photoelectrons. The inelastic mean-free path for 220 eV photoelectrons in Cu2O was calculated to 5.37 Å using the Tanuma-PennPowell algorithm in the NIST Electron Inelastic-Mean-Free-Path Database.66 The SO3-species were modeled as a homogenous layer of tunable thickness to fit the relative signal difference between the peaks. For simplicity, it was further assumed that all three oxygen atoms in the SO3-like species contributed equally to the signal of the SO3-component. The calculated coverage of SO3-species after a saturation dose at 293 K corresponds to 0.20 copper monolayers on the Cu2O(100) surface (or 2.2 SO3-species/nm2) and to 0.09 copper monolayers on the Cu2O(111) surface (or 1.1 SO3-species/nm2). This indicates that the SO3 coverage on the Cu2O(100) surface is approximately twice that on the Cu2O(111) surface and suggests that the Cu2O(100) surface is more prone to interact with SO2. Water dosing of SO2 pre-covered surface The 25 L SO2 pre-dosed Cu2O(100) surface was exposed to water vapor both under UHV and at nearambient pressure conditions. In UHV conditions, the surface was dosed with 1 L, 10 L, and 100 L of water at 293 K. Comparing to the spectra for the SO2 pre-dosed surfaces, water dosing in UHV does not lead to any detectable changes in the O 1s- and S 2p-regions, see Figure 3. This suggests that either the water 11



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vapor pressure or the total dose is too low to result in any significant reaction. Interestingly, on Fe3O4(100) low doses of water has been reported to result in significant dissociation of adsorbed SO2-molecules.32 An important difference between Cu2O(100) and Fe3O4(100) is the capacity of OH-group formation at low doses of water in UHV. In contrast to Fe3O4(100), Cu2O(100) lacks the ability of significant hydroxyl formation at room temperature in UHV and low doses of water. At near-ambient water vapor pressure conditions, it was observed on both the Cu2O surfaces that the pre-adsorbed SO3-species converted into CuxS-like species. Figure 4 shows a color map of the S 2p-region during the course of a H2O exposure of the Cu2O(100) surface. The water pressure is stepwise increased in the experiment (each increment is indicated by a red solid line in the figure). The trend graph in the right panel shows the relative surface coverage of SO3-like species and CuxS-like species throughout the experiment. The water pressures (and relative humidities) throughout the experiment are supplied in Table 2. It is noticed that the SO3 to CuxS reaction rate increases significantly at a water vapor pressure of 3.86×10-4 mbar (or 1.54×10-3 %RH). The SO3 to CuxS reaction was completed at a total H2O dose of approximately 2×108 L. The total water dose of the experiment was ~3×108 L. In a separate experiment where the H2O pressure was immediately regulated to 8.78×10-3 mbar the SO3 to CuxS reaction was completed already at a total dose of around 2×106 L (not shown here). Similar observations have been reported regarding the sulfidation of Cu2O by H2S, where the anion exchange rate was found to be significantly increased at elevated humidity 49. In a similar experiment, the Cu2O(111) surface was exposed to a constant water pressure of 8.78×10-3 mbar at room temperature (3.51×10-3 %RH) and a total dose of ~2×107 L. Analogous to the Cu2O(100) surface, it was found that the amount of SO3-like species converted into CuxS-species increased with the dose of water. Beam damage checks were performed for both surfaces by shifting the sample position during acquisition, with shift distances exceeding the beam spot size. No photo-induced reactions were observed at the photon flux used in the current experiment. Further, post-exposure analysis in UHV 12


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comparing non-photon beam exposed surfaces with surfaces exposed to the X-ray beam showed no detectable difference. Post high-pressure water dosing analysis in UHV Both samples were studied in UHV after the near-ambient pressure exposure of water. The S 2p-regions for both surfaces are shown in Figure 5. It is evident that most of the remaining sulfur on the surfaces have been converted from SO3-species into CuxS. By curve fitting, the binding energy for S 2p3/2 is determined to 161.90 eV and 161.85 eV for Cu2O(100) and Cu2O(111), respectively. This rules out atomic sulfur at 164.2 eV,69 but is within the reported region of both CuS70-75 and Cu2S70-73, 76-77. Since large doses of SO2 on Cu2O reportedly lead to Cu2S formation24 and the exchange reaction replacing oxygen in Cu2O with sulfur to form an isostructural Cu2S is beneficial,27-28 Cu2S is deemed as the most likely product. From coverage estimations by integrating the area for Cu2S- and SO3-species, it is found that approximately 77 % and 82 % of the remaining sulfur have dissociated for Cu2O(100) and Cu2O(111), respectively. Of the total sulfur signal, it is found that 81 % and 61 % of the initial sulfur coverage remains on the surface in any form after the near-ambient pressure water dosage for Cu2O(100) and Cu2O(111), respectively. In Figure 6, the PES O 1s-region at different stages of the study are presented for the two surfaces. As previously noticed, the contribution of the Ocus for the clean surfaces decrease upon sulfur dioxide adsorption and a high-binding energy shoulder for the SO3-like species arises. After exposing the surfaces with SO3-species present to an ambient-pressure of water, the amount of SO3-species decreases significantly. The components attributed to surface oxygen atoms do not reappear upon Cu2S-formation, suggesting either sulfur atoms have taken their lattice positions or the surfaces have reconstructed. Beside formation of Cu2S-species, OH-groups and trace amount of adsorbed water molecules can be observed after the exposure. Both surfaces show approximately 0.22 copper ML of OH-groups on the surface after the exposure, correlating to 2.4 OH-groups/nm2 and 2.2 OH-groups/nm2 for Cu2O(100) and Cu2O(111), respectively. In addition, the Cu2O(111) surface also shows 0.035 copper ML of water molecules adsorbed 13



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to the surface, corresponding to 0.4 water molecules/nm2. The O 1s binding energy shifts for the OHgroups and the water molecules are the same as for the ambient-pressure water dosing on the clean Cu2O surfaces. After the extended exposures of H2O, low levels of carbonaceous species was detected that may contribute with a minor fraction of the O 1s high binding energy component (less than 10% of the intensity). It is further noticed that after near-ambient pressure water dosing on the SO2 pre-dosed surfaces, the bulk oxide O 1s component shifts towards higher binding energies. The binding energy shift after adsorption of SO2 is 0.14 eV and 0.00 eV for Cu2O(100) and Cu2O(111), respectively. After nearambient pressure water exposure, the binding energy shift relative the clean surfaces is 0.07 eV and 0.09 eV for the Cu2O(100) and Cu2O(111) surfaces, respectively. The binding energy shifts observed are likely due to band bending induced by the adsorption.

5

Computational investigation DFT calculations were used to further investigate the adsorption behavior of SO2 on the (100) and (111)

surface facets of Cu2O. Different adsorption sites were considered for each surface starting from a number of plausible initial structures, including e.g. adsorption via O- and S-down, and mixtures of these adsorption modes. In addition, intact molecular SO2 adsorption was compared to adsorption followed by surface oxygen (Osurf) abstraction leading to an adsorbed SO3 species via SO2(g) + Osurf (SO3)ad. The results are summarized in Table 3. Figures showing the favored SO2 and SO3 adsorption structures are shown in Figure 7 for the c(2×2) Cu2O(100) surface, and in Figure 8 for the four different terminations considered for the Cu2O(111) surface (vide infra). Additional details are included in Table S1 in the supporting information. For the (100) surface facet, it was found that SO2 adsorption onto the reconstructed c(2×2) surface is accompanied by reduction of the surface leading to adsorbed SO3. At a potential first state, molecular SO2 binds parallel to the surface towards the copper atoms via both its oxygen atoms (top site: 2.0 Å) and the 14


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sulfur atom (bridge site: 2.3 Å and 2.5 Å), see Figure 7. This adsorption mode is exothermic by -1.31 eV and involves the reincorporation of the surface Oad in its lattice position. From this state, formation of SO3 via abstraction of surface O by SO2 is further exothermic by -1.04 eV. SO3 binds to the surface via the oxygen atoms towards surface copper atoms at an average distance of 1.6 Å. An alternative, and arguable more plausible, adsorption mechanism is via direct formation of SO3 from the combination of gaseous SO2 and surface residing Oad atoms, which in total is exothermic by -2.35 eV. Regardless of the mechanism, SO3 is the most favorable surface state under the considered conditions. Upon SO3 formation the surface adopts a mixed (1×1)/c(2×2) structure that lacks the lattice oxygen surface atoms since these are now part of the SO3 species. Although, the SO3 formation was here modeled on the basis of the c(2×2) surface reconstruction, it is anticipated that SO2 interaction with the low-energy (3,0;1,1) reconstruction will result in the same final state because Oad atoms that can react with SO2 to form SO3 are present also on the (3,0;1,1) structure. Also for the (3,0;1,1) surface, the SO3 formation is expected to be accompanied by the formation of a (1×1)/c(2×2) surface termination similar to the case of water and methanol adsorption.20, 78 This is largely in line with the interpretation of the experimental PES data, which suggest that SO3 is formed on the surface upon exposure to SO2, and that the vacuum (3,0;1,1) reconstruction is lifted in favor for a (1×1)-like structure. Our calculated O1s shift of ~0.5 eV towards higher binding energy for the SO3 compared to the bulk atoms is in reasonable agreement with the experimental shift of 0.99 eV. The study of the (111) surface is more involving than the (100) study. This is due to the lack of consensus in the surface science community regarding the atomic structure and composition of this surface; although experimental and computational data have been presented in favor for the various surfaces terminations (vide infra), conclusive support is yet to be presented. The ideal (1×1) surface is terminated by a layer of coordinatively unsaturated (CuCUS) and saturated (CuCS) copper atoms that are sandwiched between an outer layer of unsaturated oxygen atoms (OCUS), and an inner layer of saturated oxygen atoms (OCS), see Figure 9. According to periodic plane-wave DFT calculations using standard 15



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GGA functionals, the thermodynamically most stable surface termination under all conceivable oxygen pressures is the (1×1) structure with 1 ML of CuCUS vacancies [(1×1)-Cuvac in Figure 8b].79-80 More elaborate DFT calculations with hybrid functionals suggest, however, that the ideal (1×1) surface (Figure 8a) becomes more stable than the CuCUS vacant structure under oxygen lean conditions similar to those in the current study.81 Although DFT favors a (1×1) structure, structures with a (√3×√3)R30° symmetry pattern (Figure 8c-d) are the most commonly observed experimentally, see e.g. reference 47. The (√3×√3)R30° pattern is attributed to the absence of ⅓ ML of OCUS at the surface. Also for the (√3×√3)R30° reconstructed surface, there is an on-going debate on whether CuCUS atoms are present or not. In the following, the surface termination notation from reference 47 will be adopted; accordingly, the (√3×√3)R30° surface model with ⅓ ML of OCUS vacancies will be referred to as the model A surface (Figure 8c), whereas the (√3×√3)R30° surface model with both ⅓ ML OCUS and 1 ML CuCUS vacancies will be called the model B surface (Figure 8d). The assignment of the expected surface terminations may be facilitated by the comparison of computational and experimental results for the adsorption behavior of different probe molecules, e.g. SO2 or H2O. Computationally, this has proven a difficult task due to the close concurrence between results from different surface terminations. For methanol adsorption on Cu2O(111) we have e.g. recently found that the DFT results based on the model B surface best reproduce experimental data.78 Nevertheless, the model A structure could not be excluded. In the following we argue that the results of the SO2 adsorption study strongly suggests that the model A surface, with CuCUS atoms present, is the active surface structure upon interaction with SO2. The possibility of mixed A and B phases could provide an explanation for the deviant results of SO2 and methanol. In order to comprehensively study SO2 on Cu2O(111), we have investigated four different terminations of the Cu2O(111) surface: the ideal (1×1) structure, the (1×1) structure with 1 ML of CuCUS vacancies [(1×1)-Cuvac], the (√3×√3)R30° model A structure, and the (√3×√3)R30° model B structure in Figure 8. It was found that the coordinatively unsaturated atoms (CuCUS and OCUS) have a large effect on the SO2 16


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adsorption; the first indication of this is that the most favorable adsorption mode for the molecular SO2 is with sulfur directed towards the OCUS for all but the model B surface. The S-OCUS distance is 1.9 Å, except for the model B surface where adsorption to the OCUS position yields a distance of 2.3 Å. In addition, if CuCUS is present on the surface it binds one of the SO2 oxygen atoms (1.9 Å), while the other oxygen atom is free. In the absence of CuCUS, both O atoms are free upon adsorption to the OCUS site. This is not a beneficial adsorption structure and on the oxygen and copper vacant (√3×√3)R30° model B surface, the most favorable adsorption mode is instead with SO2 parallel to the surface and positioned on top of the three CuCS atoms that are exposed under the oxygen vacancy at a distance of 2.2 Å (S-CuCS) and 2.0 Å (OCuCS). The SO2 adsorption is exothermic on all surfaces but most favorable on the ideal (1×1) surface (Ead = 1.33 eV). This is followed by the model A surface (Ead = 1.24 eV), and the model B surface (Ead = 1.08 eV). Adsorption onto the copper vacant (1×1) surface is by far the weakest (Ead = 0.56 eV), which likely is a reflection of the low surface energy of this surface termination as well as the lack of copper binding sites on the surface for the SO2 oxygen atoms. We further note that for the Cu2O(111) surfaces where Cu-OSO2 bonds are formed, the adsorption of SO2 leads to rather large local rearrangements with copper atoms (especially CuCUS) protruding the surface significantly. In addition to the above, all surfaces have sites that are close in adsorption energy to the most favorable site: in particular the oxygen and copper vacancies are identified as possible adsorption sites (see Table S1 in supporting information). Regardless of the adsorption mode, the adsorption of one SO2-molecule per (√3×√3)R30° surface unit cell (all surfaces where modeled by this unit cell size) is consistent with the experimentally determined 0.09 ML (≈1/12 ML i.e. one per cell) coverage. Interestingly, this exactly matches the coverage of OCUS vacancies on the (√3×√3)R30° surfaces. The critical role of the CuCUS atom is further illustrated by the DFT result in that the formation of SO3 can only be achieved on the surfaces with CuCUS present (Table 3), i.e. the ideal (1×1) and the model A surfaces. On e.g. the latter surface, the positional flexibility of the CuCUS atoms aids in the conversion from 17



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the SO2 molecule to the adsorbed SO3 species; as the OCUS underneath SO2 forms a covalent O-S bond, the coordination of a SO2 oxygen atom to the CuCUS facilitates the reaction. The barrier is merely 0.10 eV on this surface and the reaction is exothermic (by -1.54 eV compared to the surface and SO2(g), and by -0.30 eV compared to the SO2 adsorption mode). Upon SO3 formation, SO3 binds O-down towards surface copper atoms at an average distance of 1.6 Å. The reaction is exothermic also on the ideal (1×1) surface, however only by -0.12 eV compared to SO2 adsorption. This is in stark contrast to the (1×1) surface with 1 ML CuCUS vacancies, where the same reaction step is endothermic by +0.42 eV, and to the model B surface, where the reaction is endothermic by +1.19 eV. From the experimental data, formation of SO3 is expected over molecular SO2 adsorption. Since SO3 formation is only conceivable on the surfaces with CuCUS atoms, our DFT results suggest that the Cu2O(111) surface is terminated by a (√3×√3)R30° structure with ⅓ ML of OCUS vacancies and no copper vacancies, i.e. the model A surface. Calculated O1s shifts compared to the bulk O for the three oxygen atoms of SO3 adsorbed on the proposed surface termination are 0.8-0.9 eV. This is in close agreement with the experimental shift of +0.89 eV. In contrast, the O1s shift of SO2 on the same surface is -0.15 eV, which does not agree with experimental data. Hence the calculated O1s shift corroborates the conclusion that SO3 is formed upon interaction between SO2 and the Cu2O(111) surface. In order to gain further understanding on the SO2 induced Cu2O-Cu2S conversion observed under humid conditions, co-adsorption of H2O and SO2 onto the (100) and model A (111) surfaces was considered. This shows that H2O adsorption is more exothermic with SO3 present on the surfaces (SO3 is formed upon SO2 adsorption via abstraction of surface an O atom, vide supra) due to strong H-bond interactions between H2O and SO3. H2O dissociation is, however, hampered by the presence of SO3 on the surface (see Table S2 in supporting information). Similar to the case of the related conversion to Cu2S by H2S, a S2- surface species is a likely precursor to the reaction, and in both the case of exposure to SO2 (in the present work) as well as to H2S,

49, 82

humid and wet conditions have been found to speed up the conversion reaction. 18


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Interactions with H2O, and potentially OHad and Had, offer possibilities for catalytic reduction of SO3 to S2at the Cu2O surface. The interactions between co-adsorbed H2O and SO2 as described above is an initial indication of this. Mapping of the full mechanism for SO2 induced conversion of Cu2O to Cu2S is, however, beyond the scope of the present study.

6

Discussion

Ambient-pressure water dosing on the clean surfaces Dosing water on the clean Cu2O surfaces showed a significant difference between UHV conditions and near-ambient pressure conditions. In contrast to the near-ambient pressure conditions, no OH-groups were formed on the surfaces when dosing water at UHV conditions. In this study, the lowest near-ambient pressure for water dosing was 1.1×10-3 mbar (5×10-3 %RH), this is multiple orders of magnitude higher water vapor pressure compared to the study by Deng et al. where OH-groups were formed on polycrystalline Cu2O-films already at 5×10-7 %RH.67 The lack of OH-groups under UHV conditions may suggest that either the adsorption rate of water molecules is too low, the lifetime of water molecules on the surface is too low, and/or that the dissociation rate is too low. At near-ambient water vapor pressures conditions (or lower temperatures) OH-groups are observed on both surfaces. The polar nature of the (100) surface may explain the stabilization of a higher OH coverage (0.38 ML OH) compared to the nonpolar (111) surface (0.25 ML OH). Sulfur dioxide adsorption High-resolution PES measurements of the S 2p-region identified that the SO2-molecules adsorb as SO3like species on both the Cu2O surfaces under UHV-conditions. The binding energy of S 2p3/2 was determined to approximately 166.1 eV and 166.4 eV for Cu2O(100) and Cu2O(111), respectively. The binding energies are close to literature data reported for the binding energy of SO3-species on Cu2O(111) but is outside the expected binding energy range of SO4-species,25 leading to the conclusion that the 19



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observed sulfur species on the copper oxide surfaces are SO3-like. This is consistent with the results of the DFT calculations on the (100) surface as well as on the (111) surfaces including under-coordinate copper atoms (CuCUS) but not the copper vacant structures. The PES results of the O 1s-region show that the component for the under-coordinated surface oxygen atoms, on the low-binding energy side of the bulk oxygen peak, decreases when the SO3-component grows. This indicates that the surface oxygen atoms are active in binding the SO2-molecules to the surface. The suggested reaction path for the SO2 adsorption is Cu2O + SO2,gas → Cu2SO3,ads. DFT suggests a barrier of 0.1 eV for the conversion. In the literature, it has been reported that on Cu2O/Cu(111), at large doses of SO2, Cu2S starts being formed beside the SO3species.24 However, formation of Cu2S is not observed in the limited SO2 dose range of the present study. The DFT calculations of SO2 adsorption on the A and B models of the Cu2O(111) (√3×√3)R30° surface find that the model A is in better agreement with the experimental observations. However, the calculations show that the model B cannot be completely ruled out and that there might be a mixture of the two on the surface. DFT calculations of other molecules, e.g. methanol, suggest the model B over the model A.78 The coexistence of the two models may explain the wider FWHM observed in the experimental PES S 2pspectra of sulfur dioxide adsorbed on the Cu2O(111) surface than on the Cu2O(100) surface. The possible coexistence of the two different models of the Cu2O(111) (√3×√3)R30° termination may not be the only source to the experimentally observed wider FWHM as domains of Cu2O(111) (1×1) are also present. Water dosing on surfaces with SO3-species present Water dosing at UHV conditions of the Cu2O(100) surface with SO3-species present does not render any observable change in the PES S 2p-region. This indicates that either the water pressure is too low to initialize any reactions involving the SO3-species or the water dose is too low to obtain a detectable amount of reaction product. At near-ambient pressures, however, drastic changes in the S 2p-region were observed, where the majority of the sulfur remaining on the surfaces after exposure were in the form of Cu2S. A central difference between water dosing in UHV and at near-ambient pressures is the formation 20


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of OH-groups under near-ambient pressure conditions on the Cu2O surfaces, which is revealed by the PES O 1s-region. The observation that a lower total H2O dose is required for completing the SO3 to CuxS reaction at higher H2O pressure underline that importance of OH stabilization on the surface for the reaction rate. A recent PES/DFT-study of water interaction with Cu2O(100) shows that the hydrogen atoms from the dissociated water molecules associate and desorb from the surface as hydrogen gas when the OH-coverage exceeds 0.25 ML.20 In this study, the OH-coverage on Cu2O(100) is estimated to 0.22 ML when having SO3-species present on the surface, a coverage slightly below the conditions for H2 desorption determined in the theoretical study. Thus, adsorbed hydrogen atoms may play a role in the reaction of the SO3 adsorbate. This suggests both OH-groups and hydrogen atoms on the surface may be necessary for the reaction converting sulfur from SO3-species into Cu2S, potentially through the over-all reactions Cu2O + SO2,gas Cu2SO3,ads and Cu2SO3,ads + 6H Cu2S + 3H2Ogas. As the hydrogen atoms in the reaction come from dissociated water molecules, the remaining part of the dissociated water molecule, OHads, has also to desorb in order to prevent saturation and keep the reaction running. This could suggestively follow via 2OHads H2Ogas + Oads and 2Oads O2,gas. The importance for the sulfide formation of having OH-groups present on the surface has been observed for Fe3O4(100),32 where OH-groups was formed by pre-dosing the surface with water in UHV before the dosing of sulfur dioxide. The study found that FexSy-species as well as SO3- and SO4-species were formed on the hydroxylated Fe3O4(100) surface, while no FexSy formation was observed on the clean surface. No intermediate reaction steps for the transition of sulfur atoms between SO3-like species to Cu2S could be detected during dosing in the near-ambient pressure photoelectron spectroscopy analysis. Possible explanations to this are that the reaction is too quick and/or that the coverage of the intermediates is below the limit of detection in the present analysis. Consequently, the reaction path cannot be experimentally determined here but similar systems have been described in literature where exchange reactions are 21



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central. Baxter et al. suggested the reaction path Cu2O + SO2(g) Cu2SO3 Cu2S + 1½O2 when exposing the surface to high doses of SO2 or co-exposing the Cu2O/Cu(111) surface for SO2 and O2.24 This is similar to the reaction path observed in this study when exposing the SO3-species on the surface with water. However, the mechanism when going from Cu2SO3 to Cu2S is not understood. Several studies have reported anionic exchange reaction where lattice oxygen atoms in Cu2O are replaced by sulfur atoms when being exposed to Na2S in solvents.27-28,

30-31

Also exposing ZnO to H2S results in oxygen atoms being

replaced by sulfur atoms through an ion exchange reaction.83 Though, for ion exchange reactions to occur, the ions must have the same oxidation number. In the current study, the sulfur in the SO3-species has oxidation number +IV while the surface oxygen atoms have the oxidation number -II. This suggests intermediate steps have to take place in order to reduce the sulfur atoms before proceeding with the ion exchange reaction. Without determining the exact reaction path, the OH-groups, hydrogen atoms, water molecules, and SO3-species present on the surface are likely to contribute to the intermediate steps. Formation of H2S, where the sulfur atom has the same oxidation number as the surface lattice oxygen, could be a step in the potential reaction path. The similar dependency on the humidity upon sulfidation of Cu2O by SO2 (reported here) and H2S49 is indicative of a common mechanistic pathway. It is further observed that the total amount of sulfur atoms on the surface, in any form, decreases during the water dosing. This suggests that the intermediates containing sulfur are not necessarily always bonded to the surface. It should be noted that minor amounts of carbon contaminations were observed on the SO2 pre-dosed surfaces after the extended near-ambient pressure water dosing. The binding energies of the carbon contamination, measured in PES C 1s-region, were in the regions of formate and carbonate. The carbon contamination origins from the walls of the high-pressure cell, the contamination on the surface accumulates slowly with the water dose. Despite being a minor contaminant, the role of the contamination on the dissociation of the SO3-like species cannot be ruled out from the experimental data. It has been 22


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shown that metalxSOy-species can react with organic molecules forming H2S. Through the thermochemical sulfate reduction, Worden and Smalley showed that H2S can be formed from e.g. CaSO4 and methane through CaSO4 + CH4 CaCO3 + H2S + H2O.16 This opens for the possibility that other organic molecules may react similarly with the SO3-species on the Cu2O surfaces.

7

Conclusions The adsorption of SO2 on two low-index Cu2O surfaces has been studied by DFT and photoemission

spectroscopy at room temperature under ultra-high vacuum conditions. It was found, for both (100) and (111) surfaces, that SO2-molecules bind to coordinatively unsaturated surface oxygen atoms and form SO3-like surface species. This is in agreement with DFT results for both the (100) and (111) surfaces, where the latter must contain under-coordinated copper atoms, CuCUS, in order for SO3 formation to occur. On the Cu2O(100) surface, the formation of SO3 tentatively results in a relaxation of the (3,0;1,1) surface reconstruction. After a saturation dose of SO2, the resulting SO3 coverage corresponds to 0.20 ML on the Cu2O(100) surface and 0.09 ML for the Cu2O(111) surface. On the Cu2O(100) surface, the full width half maximum of the peaks in the S 2p-region is smaller than for the Cu2O(111) surface. The wider FWHM for the Cu2O(111) surface may partly be explained by the coexistence of two surface structures of the (√3×√3)R30° termination, in this study called model A and model B, and partly by the coexistence of (√3×√3)R30° domains and (1×1) domains on the surface. Upon exposure to near-ambient water vapor at 5×10-3 %RH and 293 K, hydroxylation is observed to occur for both Cu2O surfaces with OH coverage up to 0.38 copper monolayers (ML) for the (100)-surface and 0.25 ML for the (111)-surface. Surface core level shifts indicate that the hydroxylation completely lifts the reconstruction of the clean Cu2O(100) surface while the (111) surface remains partially reconstructed. Exposing the SO3 terminated surfaces to low doses of water vapor (≤ 100 L) in ultra-high vacuum results in no reactions. However, during exposure to near-ambient pressures of water vapor, the 23



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SO3 surface species dissociate and sulfur atoms replace Cu2O lattice oxygen atoms in a reaction that forms Cu2S. Other studies have reported that both OH-groups and atomic hydrogen are stabilized at the higher water pressures. In this study, OH-groups were detected, which suggests that the OH-groups and/or the hydrogen atoms from the dissociated water molecules are involved in the observed reaction. Supporting information •

DFT adsorption details for: o

SO2 adsorption

o

H2O and SO2 co-adsorption.

Acknowledgements We gratefully acknowledge the support from the MAX IV Laboratory staff during beamtimes. This work was funded by the Swedish Research Council (VR), the Knut och Alice Wallenbergs stiftelse, the Swedish nuclear fuel and waste management company (SKB), and the Excellence award (to JHS) of the School of Chemical Science and Engineering at KTH.

References

1. 2. 3. 4.

Leygraf, C.; Graedel, T. E., Atmospheric Corrosion, 1st ed.; Wiley-Interscience: New York, 2000. Platzman, I.; Brener, R.; Haick, H.; Tannenbaum, R., Oxidation of Polycrystalline Copper Thin Films at Ambient Conditions. J. Phys. Chem. C 2008, 112, 1101-1108. Kleber, C.; Weissenrieder, J.; Schreiner, M.; Leygraf, C., Comparison of the Early Stages of Corrosion of Copper and Iron Investigated by in Situ TM-AFM. Appl. Surf. Sci. 2002, 193, 245-253. Royer, S.; Duprez, D., Catalytic Oxidation of Carbon Monoxide over Transition Metal Oxides. ChemCatChem 2011, 3, 24-65. 24


Page 25 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.

20. 21. 22.


Huang, T. J.; Tsai, D. H., CO Oxidation Behavior of Copper and Copper Oxides. Catal. Lett. 2003, 87, 173-178. White, B.; Yin, M.; Hall, A.; Le, D.; Stolbov, S.; Rahman, T.; Turro, N.; O'Brien, S., Complete CO Oxidation over Cu2O Nanoparticles Supported on Silica Gel. Nano Lett 2006, 6, 2095-2098. Paracchino, A.; Laporte, V.; Sivula, K.; Grätzel, M.; Thimsen, E., Highly Active Oxide Photocathode for Photoelectrochemical Water Reduction. Nat Mater 2011, 10, 456-461. Hua, Q.; Cao, T.; Gu, X.-K.; Lu, J.; Jiang, Z.; Pan, X.; Luo, L.; Li, W.-X.; Huang, W., Crystal-Plane-Controlled Selectivity of Cu2O Catalysts in Propylene Oxidation with Molecular Oxygen. Angew. Chem. Int. Ed. 2014, 53, 4856-4861. Schulz, K. H.; Cox, D. F., Propene Oxidation over Cu2O Single-Crystal Surfaces: A Surface Science Study of Propene Activation at 1 Atm and 300 K. J. Catal. 1993, 143, 464480. Reitz, J. B.; Solomon, E. I., Propylene Oxidation on Copper Oxide Surfaces: Electronic and Geometric Contributions to Reactivity and Selectivity. J. Am. Chem. Soc 1998, 120, 11467-11478. Schulz, K. H.; Cox, D. F., Propene Adsorption on Cu2O Single-Crystal Surfaces. Surf. Sci. 1992, 262, 318-334. Kreysa, G.; Schütze, M., Corrosion Handbook, 2 ed.; Wiley, 2008. Corrosion Mechanisms in Theory and Practice, Second ed.; Marcel Dekker, Inc, 2002. Smith, S. J.; van Aardenne, J.; Klimont, Z.; Andres, R. J.; Volke, A.; Delgado Arias, S., Anthropogenic Sulfur Dioxide Emissions: 1850–2005. Atmos. Chem. Phys. 2011, 11, 11011116. Lu, Z.; Streets, D. G.; de Foy, B.; Krotkov, N. A., Ozone Monitoring Instrument Observations of Interannual Increases in SO2 Emissions from Indian Coal-Fired Power Plants During 2005–2012. Environ. Sci. Technol. 2013, 47, 13993-14000. Worden, R. H.; Smalley, P. C., H2S-Producing Reactions in Deep Carbonate Gas Reservoirs: Khuff Formation, Abu Dhabi. Chem. Geol. 1996, 133, 157-171. Elsner, M. P.; Menge, M.; Müller, C.; Agar, D. W., The Claus Process: Teaching an Old Dog New Tricks. Catal. Today 2003, 79–80, 487-494. Chorkendorff, I.; Niemantsverdriet, J. W., Environmental Catalysis. In Concepts of Modern Catalysis and Kinetics, Wiley-VCH Verlag GmbH & Co. KGaA: 2005; pp 377-400. Vernon, W. H. J., A Laboratory Study of the Atmospheric Corrosion of Metals. Part I.-the Corrosion of Copper in Certain Synthetic Atmospheres, with Particular Reference to the Influence of Sulphur Dioxide in Air of Various Relative Humidities. Trans. Faraday Soc. 1931, 27, 255-277. Stenlid, J. H.; Soldemo, M.; Johansson, A. J.; Leygraf, C.; Gothelid, M.; Weissenrieder, J.; Brinck, T., Reactivity at the Cu2O(100):Cu-H2O Interface: A Combined DFT and PES Study. Phys. Chem. Chem. Phys 2016, 18, 30570-30584. Zhang, J.; Richardson, H. W., Copper Compounds. In Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH Verlag GmbH & Co. KGaA: 2000. Aastrup, T.; Wadsak, M.; Leygraf, C.; Schreiner, M., In Situ Studies of the Initial Atmospheric Corrosion of Copper Influence of Humidity, Sulfur Dioxide, Ozone, and Nitrogen Dioxide. J. Electrochem. Soc. 2000, 147, 2543-2551. 25



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23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38.

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Lu, H.; Janin, E.; Dávila, M. E.; Pradier, C. M.; Göthelid, M., 2nd Swedish Vacuum Meeting Adsorption of SO2 on Cu(100) and Cu(100)-C(2 × 2)-O Surfaces Studied with Photoelectron Spectroscopy. Vacuum 1998, 49, 171-174. Baxter, J. P.; Grunze, M.; Kong, C. W., Interaction of SO2 with Copper and Copper Oxide Surfaces. J Vac Sci Technol A 1988, 6, 1123-1127. Önsten, A.; Weissenrieder, J.; Stoltz, D.; Yu, S.; Göthelid, M.; Karlsson, U. O., Role of Defects in Surface Chemistry on Cu2O(111). J. Phys. Chem. C 2013, 19357-19364. Galtayries, A.; Grimblot, J.; Bonnelle, J. P., Interaction of SO2 with Different Polycrystalline Cu, Cu2O and CuO Surfaces. Surf. Interface Anal 1996, 24, 345-354. Kuo, C.-H.; Chu, Y.-T.; Song, Y.-F.; Huang, M. H., Cu2O Nanocrystal-Templated Growth of Cu2S Nanocages with Encapsulated Au Nanoparticles and in-Situ Transmission X-Ray Microscopy Study. Adv. Funct. Mater. 2011, 21, 792-797. Wu, H.-L.; Sato, R.; Yamaguchi, A.; Kimura, M.; Haruta, M.; Kurata, H.; Teranishi, T., Formation of Pseudomorphic Nanocages from Cu2O Nanocrystals through Anion Exchange Reactions. Science 2016, 351, 1306-1310. Kristiansen, P. T.; Massel, F.; Werme, L.; Lilja, C.; Duda, L.-C., Sulfidation of SinglePhase Oxide on Copper and as Powder Studied Using Soft X-Ray Spectroscopy. J. Electrochem. Soc. 2015, 162, C785-C791. Hollmark, H. M.; Keech, P. G.; Vegelius, J. R.; Werme, L.; Duda, L. C., X-Ray Absorption Spectroscopy of Electrochemically Oxidized Cu Exposed to Na2S. Corros Sci. 2012, 54, 85-89. Smith, J. M.; Wren, J. C.; Odziemkowski, M.; Shoesmith, D. W., The Electrochemical Response of Preoxidized Copper in Aqueous Sulfide Solutions. J. Electrochem. Soc. 2007, 154, C431-C438. Stoltz, D.; Onsten, A.; Karlsson, U. O.; Gothelid, M., High Resolution Spectroscopic and Microscopic Signatures of Ordered Growth of Ferrous Sulfate in SO2 Assisted Corrosion of Fe3O4(100). Appl Phys Lett 2007, 91. Smirnov, M. Y.; Kalinkin, A. V.; Pashis, A. V.; Sorokin, A. M.; Noskov, A. S.; Kharas, K. C.; Bukhtiyarov, V. I., Interaction of Al2O3 and CeO2 Surfaces with SO2 and SO2 + O2 Studied by X-Ray Photoelectron Spectroscopy. J. Phys. Chem. B 2005, 109, 11712-11719. Ziolek, M.; Kujawa, J.; Saur, O.; Aboulayt, A.; Lavalley, J. C., Influence of Sulfur Dioxide Adsorption on the Surface Properties of Metal Oxides. J. Mol. Catal. A: Chem. 1996, 112, 125-132. Smith, K. E.; Mackay, J. L.; Henrich, V. E., Interaction of SO2 with Nearly Perfect and Defect TiO2(110) Surfaces. Phys. Rev. B 1987, 35, 5822-5829. Smith, K. E.; Henrich, V. E., Interaction of SO2 and CO with the Ti2O3(10-1 2) Surface. Phys. Rev. B 1985, 32, 5384-5390. Román, E.; de Segovia, J. L.; Martín-Gago, J. A.; Comtet, G.; Hellner, L., 5th European Vacuum Conference Study of the Interaction of SO2 with TiO2 (110) Surface. Vacuum 1997, 48, 597-600. Rodriguez, J. A.; Jirsak, T.; Pérez, M.; Chaturvedi, S.; Kuhn, M.; González, L.; Maiti, A., Studies on the Behavior of Mixed-Metal Oxides and Desulfurization: Reaction of H2S and SO2 with Cr2O3(0001), MgO(100), and CrxMg1-XO(100). J. Am. Chem. Soc 2000, 122, 12362-12370. 26


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56.


Önsten, A.; Stoltz, D.; Palmgren, P.; Yu, S.; Claesson, T.; Göthelid, M.; Karlsson, U. O., So2 Interaction with Zn(0001) and ZnO(0001) and the Influence of Water. Surf. Sci. 2013, 608, 31-43. Chaturvedi, S.; Rodriguez, J. A.; Jirsak, T.; Hrbek, J., Surface Chemistry of SO2 on Zn and ZnO: Photoemission and Molecular Orbital Studies. J. Phys. Chem. B 1998, 102, 70337043. Rodriguez, J. A.; Jirsak, T.; Freitag, A.; Hanson, J. C.; Larese, J. Z.; Chaturvedi, S., Interaction of SO2 with CeO2 and Cu/CeO2 Catalysts: Photoemission, XANES and TPD Studies. Catal. Lett. 1999, 62, 113-119. Furuyama, M.; Kishi, K.; Ikeda, S., The Adsorption of SO2 on Iron Surfaces Studied by XRay Photoelectron Spectroscopy. J. Electron. Spectrosc. Relat. Phenom. 1978, 13, 59-67. Galtayries, A.; Cousi, C.; Zanna, S.; Marcus, P., SO2 Adsorption at Room Temperature on Ni(111) Surface Studied by XPS. Surf. Interface Anal. 2004, 36, 997-1000. Rodriguez, J. A.; Jirsak, T.; Chaturvedi, S.; Hrbek, J., Surface Chemistry of SO2 on Sn and Sn/Pt(111) Alloys: Effects of Metal−Metal Bonding on Reactivity toward Sulfur. J. Am. Chem. Soc. 1998, 120, 11149-11157. Rodriguez, J. A.; Jirsak, T.; Hrbek, J., Reaction of SO2 with Cesium and Cesium-Promoted ZnO and MoO2. J. Phys. Chem. B 1999, 103, 1966-1976. Soldemo, M.; Stenlid, J. H.; Besharat, Z.; Yazdi, M. G.; Onsten, A.; Leygraf, C.; Gothelid, M.; Brinck, T.; Weissenrieder, J., The Surface Structure of Cu2O(100). J Phys Chem C 2016, 120, 4373-4381. Önsten, A.; Göthelid, M.; Karlsson, U. O., Atomic Structure of Cu2O(111). Surf. Sci. 2009, 603, 257-264. Li, C.; Wang, F.; Li, S. F.; Sun, Q.; Jia, Y., Stability and Electronic Properties of the OTerminated Cu2O(111) Surfaces: First-Principles Investigation. Phys. Lett. A 2010, 374, 2994-2998. Sharma, S. P., Reaction of Copper and Copper Oxide with H 2S J. Electrochem. Soc. 1980, 127, 21-26. Schnadt, J., et al., The New Ambient-Pressure X-Ray Photoelectron Spectroscopy Instrument at Max-Lab. J. Synchrotron Radiat. 2012, 19, 701-704. Kresse, G.; Hafner, J., Ab Initio Molecular Dynamics for Liquid Metals. Phys. Rev. B 1993, 47, 558-561. Kresse, G.; Hafner, J., Ab Initio Molecular Dynamics for Open-Shell Transition Metals. Phys. Rev. B 1993, 48, 13115-13118. Kresse, G.; Hafner, J., Ab Initio Molecular-Dynamics Simulation of the Liquid-Metal– Amorphous-Semiconductor Transition in Germanium. Phys. Rev. B 1994, 49, 1425114269. Kresse, G.; Furthmüller, J., Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169-11186. Kresse, G.; Furthmüller, J., Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set. Comput. Mater. Sci. 1996, 6, 15-50. Perdew, J. P.; Burke, K.; Ernzerhof, M., Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865-3868. 27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 75.

Page 28 of 42

Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H., A Consistent and Accurate Ab Initio Parametrization of Density Functional Dispersion Correction (DFT-D) for the 94 Elements H-Pu. J. Chem. Phys. 2010, 132, 154104. Grimme, S.; Ehrlich, S.; Goerigk, L., Effect of the Damping Function in Dispersion Corrected Density Functional Theory. J. Comput. Chem. 2011, 32, 1456-1465. Dudarev, S. L.; Botton, G. A.; Savrasov, S. Y.; Humphreys, C. J.; Sutton, A. P., ElectronEnergy-Loss Spectra and the Structural Stability of Nickel Oxide: An LSDA+U Study. Phys. Rev. B 1998, 57, 1505-1509. Yu, K.; Carter, E. A., Communication: Comparing Ab Initio Methods of Obtaining Effective U Parameters for Closed-Shell Materials. J. Chem. Phys. 2014, 140, 121105. Blöchl, P. E.; Jepsen, O.; Andersen, O. K., Improved Tetrahedron Method for BrillouinZone Integrations. Phys. Rev. B 1994, 49, 16223-16233. Henkelman, G.; Uberuaga, B. P.; Jónsson, H., A Climbing Image Nudged Elastic Band Method for Finding Saddle Points and Minimum Energy Paths. J. Chem. Phys. 2000, 113, 9901-9904. Mills, G.; Jónsson, H.; Schenter, G. K., Reversible Work Transition State Theory: Application to Dissociative Adsorption of Hydrogen. Surf. Sci. 1995, 324, 305-337. Nilsson, V.; Van den Bossche, M.; Hellman, A.; Grönbeck, H., Trends in Adsorbate Induced Core Level Shifts. Surf. Sci. 2015, 640, 59-64. Köhler, L.; Kresse, G., Density Functional Study of CO on Rh(111). Phys. Rev. B 2004, 70, 165405. Powell, C. J.; Jablonski, A. Nist Electron Inelastic-Mean-Free-Path Database, Version 1.2, Srd 71, National Institute of Standards and Technology: Gaithersburg, MD, 2010. Deng, X.; Herranz, T.; Weis, C.; Bluhm, H.; Salmeron, M., Adsorption of Water on Cu2O and Al2O3 Thin Films. J. Phys. Chem.C 2008, 112, 9668-9672. Schulz, K. H.; Cox, D. F., Surface Hydride Formation on a Metal-Oxide Surface - the Interaction of Atomic-Hydrogen with Cu2O(100). Surf. Sci. 1992, 278, 9-18. Lindberg, B. J.; Hamrin, K.; Johansson, G.; Gelius, U.; Fahlman, A.; Nordling, C.; Siegbahn, K., Molecular Spectroscopy by Means of Esca Ii. Sulfur Compounds. Correlation of Electron Binding Energy with Structure. Phys. Scr. 1970, 1, 286. Gebhardt, J. E.; McCarron, J. J.; Richardson, P. E.; Buckley, A. N., The Effect of Cathodic Treatment on the Anodic Polarization of Copper Sulfides. Hydrometallurgy 1986, 17, 2738. Nakai, I.; Sugitani, Y.; Nagashima, K.; Niwa, Y., X-Ray Photoelectron Spectroscopic Study of Copper Minerals. J. Inorg. Nucl. Chem. 1978, 40, 789-791. Brion, D., Etude Par Spectroscopie De Photoelectrons De La Degradation Superficielle De FeS2, CuFeS2, ZnS Et Pbs a L'air Et Dans L'eau. Appl. Surf. Sci. 1980, 5, 133-152. Perry, D. L.; Taylor, J. A., X-Ray Photoelectron and Auger Spectroscopic Studies of Cu2S and CuS. J. Mater. Sci. Lett. 1986, 5, 384-386. Kutty, T. R. N., A Controlled Copper-Coating Method for the Preparation of ZnS: Mn Dc Electroluminescent Powder Phosphors. Mater. Res. Bull. 1991, 26, 399-406. Scheer, R.; Lewerenz, H. J., Photoemission-Study of Evaporated CuInS2 Thin-Films .2. Electronic Surface-Structure. J Vac Sci Technol A 1994, 12, 56-60. 28


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76. 77. 78. 79. 80. 81. 82. 83.


Yu, X.-R.; Liu, F.; Wang, Z.-Y.; Chen, Y., Auger Parameters for Sulfur-Containing Compounds Using a Mixed Aluminum-Silver Excitation Source. J. Electron. Spectrosc. Relat. Phenom. 1990, 50, 159-166. Strohmeier, B. R.; Levden, D. E.; Field, R. S.; Hercules, D. M., Surface Spectroscopic Characterization of CuAl2O3 Catalysts. J. Catal. 1985, 94, 514-530. Besharat, Z.; Stenlid, J. H.; Soldemo, M.; Marks, K.; Önsten, A.; Johnson, M.; Öström, H.; Weissenrieder, J.; Brinck, T.; Göthelid, M., Dehydrogenation of Methanol on Cu2O(100) and (111). J. Chem. Phys. 2017, 146, 244702. Bendavid, L. I.; Carter, E. A., First-Principles Predictions of the Structure, Stability, and Photocatalytic Potential of Cu2O Surfaces. J. Phys. Chem. B 2013, 117, 15750-15760. Soon, A.; Todorova, M.; Delley, B.; Stampfl, C., Thermodynamic Stability and Structure of Copper Oxide Surfaces: A First-Principles Investigation. Phys. Rev. B 2007, 75, 125420. Nilius, N.; Fedderwitz, H.; Gro; Noguera, C.; Goniakowski, J., Incorrect DFT-GGA Predictions of the Stability of Non-Stoichiometric/Polar Dielectric Surfaces: The Case of Cu2O(111). Phys. Chem. Chem. Phys. 2016, 18, 6729-6733. Stenlid, J. H.; Johansson, A. J.; Leygraf, C.; Brinck, T., Computational Analysis of the Early Stage of Cuprous Oxide Sulphidation: A Top-Down Process. Corros. Eng. Sci. Techn. 2017, 52, 50-53. Dloczik, L.; Könenkamp, R., Nanostructure Transfer in Semiconductors by Ion Exchange. Nano Lett 2003, 3, 651-653.

29



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(100)

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(111)

(1×1) (3,0;1,1) (1×1) (√3×√3)R30° Copper coverage [atoms/nm2]

11.0

Oxygen coverage [atoms/nm2] 5.5

11.0

12.7

12.7

5.5

3.3

2.1

Table 1: Coverages for one monolayer of copper and oxygen for the different surface structures. For the oxygen coverage only coordinatively unsaturated O atoms (OCUS) are counted.

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Time [s]

0

803

975

1267

1825

2161

2845

3180

H2O pressure [mbar]

2.88E-04

2.92E-04

3.32E-04

3.86E-04

6.15E-04

1.13E-03

4.53E-03

8.78E-03

%RH

1.15E-03

1.17E-03

1.33E-03

1.54E-03

2.46E-03

4.52E-03

1.81E-02

3.51E-02

Table 2: Water vapor pressures and relative humidities (%RH) during near-ambient pressure exposure.

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Table 3: Adsorption sites and energies (∆Ead) in eV for SO2 (and SO3) on the studied Cu2O(100) and (111) surface models. Adsorption details, including binding distances and energies, for additional adsorption modes are provided in the supporting information.

Surface (100) c(2×2) c(2×2) (111) (1×1) ideal (1×1) ideal (1×1) 1 ML (1×1) Cu 1 ML f) Cu Model A Model A f) Model B g) Model B g) a)

Site

Species

∆Ead

Cua) Cub)

SO2 SO3

1.31 2.35

O c) d) O O e) d) O O c) d) O h) O d) O

SO2 SO3 SO2 SO3 SO2 SO3 SO2 SO3

1.33 1.45 0.56 0.12 1.24 1.54 1.08 -0.11

SO2 adsorbs parallel to surface with S binding to a Cu bridge site and the two O atoms binding Cu top sites; b) O-down: one O binding to a Cu bridge site and the two other O atoms binding Cu top sites; c) S binds OCUS, one O binds CuCUS, the other O is free; d) at the CuCS atoms under the O vacancy created by g) OCUS abstraction; e) binds only via S-down to OCUS, O atoms free; f) (√3×√3)R30° with ⅓ ML O ; h) (√3×√3)R30° with ⅓ ML O and 1 ML Cu; parallel to surface at O with S and O atoms binding to one CuCS top site each.

32


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Figure 1: PES O 1s-region at different stages of the experiment for Cu2O(100) (a) and Cu2O(111) (b). The spectra show (from top to bottom): the clean surfaces as prepared in UHV, after transfer into the ambient pressure cell, and during exposure to water vapor at 5 x 10-3 %RH inside the ambient pressure cell.

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Figure 2: PES results before and after exposure to 25 L SO2. (a) and (b) show the S 2p-region for Cu2O(100) and Cu2O(111), respectively. (c) and (d) show the O 1s-region before and after exposure to 25 L sulfur dioxide region for Cu2O(100) and Cu2O(111), respectively. For all spectra, the intensities are normalized to the background signal and a linear background has been subtracted.

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Figure 3: PES O 1s- and S 2p-regions for Cu2O(100) predosed with SO2 at different stages of the UHV water dosing study. All spectra have been normalized to the background intensity.

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Figure 4: (Left) Colormap of the PES S 2p-region during the near-ambient pressure water vapor exposure experiment for the SO2 predosed Cu2O(100) surface. The red horizontal lines in the colormap indicate changes in water vapor pressure, the pressures are presented in Table 2. (Right) Relative coverage of the SO3-species (black dots) and CuxS-species (red dots) on the surface. Note that the total sulfur coverage decreases during the dosing of water vapor.

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Figure 5: PES S 2p-region after exposure to near-ambient water vapor pressures, the spectra are collected in UHV. The spectra are normalized to the background intensity for both surfaces.

37



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Figure 6: PES O 1s-region for (bottom to top) the clean surfaces, after exposure to 25 L SO2, and after near-ambient pressure water vapor exposure. All spectra are collected in UHV and are normalized to the background intensity. The left panel shows the results for the Cu2O(100) surface and the right panel shows the results for the Cu2O(111) surface.

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Figure 7: Surface structure of the Cu2O(100) surface with the c(2×2) unit cell marked out in green (top). The bottom figures show the structures for the SO2 adsorption (left) and SO3 formation (right). Note that the Oad atom present at the clean surface is incorporated in the lattice in b), and abstracted in the SO3species in c). Coloring: Cu (light brown), surface Cu (dark brown), O (red), Oad (purple), and S (yellow). The SO2 and SO3 adsorbates are distinguished from the surface by a reduced atomic radius.

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Figure 8: Surface structures of Cu2O(111) including the favored SO2 and SO3 adsorption structures for the considered surface terminations. The (1×1) and (√3×√3)R30° unit cells are marked in green. The model A structure in c) includes ⅓ ML OCUS vacancies, while the model B structure in d) includes both ⅓ ML OCUS and 1 ML CuCUS vacancies. Note that the pristine crystalline surface structure is shown in a) for the ideal (1×1) surface, whereas the relaxed surface with CuCUS-CuCS dimers is energetically favorable (not shown in the figure). The other surfaces are shown at their lowest energy structure. Note also that (√3×√3)R30° unit cells was used in the (1×1) simulations. Coloring: Cu (light brown), CuCUS (dark brown), O (red), OCUS (purple), and S (yellow). The SO2 and SO3 adsorbates are distinguished from the surface by a reduced atomic radius.

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Figure 9. Shows the surface structure of the ideal (1×1) (111) surface facet of Cu2O. The top figure displays the surface viewed along the [111] direction.

41



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Table of content graphics

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Interaction of Sulfur Dioxide and Near-Ambient Pressures of Water

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