Redox-Mediated Reconstruction of Copper during Carbon Monoxide

Article pubs.acs.org/JPCC

Redox-Mediated Reconstruction of Copper during Carbon Monoxide Oxidation Fang Xu,†,‡ Kumudu Mudiyanselage,†,§ Ashleigh E. Baber,† Markus Soldemo,∥ Jonas Weissenrieder,∥ Michael G. White,†,‡ and Darío J. Stacchiola*,† †

Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973, United States Department of Chemistry, Stony Brook University, Stony Brook, New York 11794, United States § Department of Science, BMCC-CUNY, New York, New York 10007, United States ∥ KTH Royal Institute of Technology, Material Physics, 164 40 Stockholm, Sweden ‡

S Supporting Information *

ABSTRACT: Copper has excellent initial activity for the oxidation of CO, yet it rapidly deactivates under reaction conditions. In an effort to obtain a full picture of the dynamic morphological and chemical changes occurring on the surface of catalysts under CO oxidation conditions, a complementary set of in situ ambient pressure (AP) techniques that include scanning tunneling microscopy, infrared reflection absorption spectroscopy (IRRAS), and X-ray photoelectron spectroscopy were conducted. Herein, we report in situ AP CO oxidation experiments over Cu(111) model catalysts at room temperature. Depending on the CO:O2 ratio, Cu presents different oxidation states, leading to the coexistence of several phases. During CO oxidation, a redox cycle is observed on the substrate’s surface, in which Cu atoms are oxidized and pulled from terraces and step edges and then are reduced and rejoin nearby step edges. IRRAS results confirm the presence of under-coordinated Cu atoms during the reaction. By using control experiments to isolate individual phases, it is shown that the rate for CO oxidation decreases systematically as metallic copper is fully oxidized.

1. INTRODUCTION CO oxidation has long been a prototypical reaction for fundamental research as well as in critical practical applications such as the elimination of automotive exhaust pollution,1,2 and the water gas shift3−5 and preferential oxidation (PROX)6−8 reactions used during the purification of hydrogen. Traditionally, heterogeneous catalytic materials, such as noble metals (Pt, Pd, Rh, and Ru)9 and oxide-supported catalysts10 are widely used. In an effort to reduce the high expense of the noble metal catalysts, transition-metal oxides11−13 (cobalt oxides and copper oxides) and unsupported/supported transition metals14,15 (Co, Cr, Cu, Ni, Zn) have been studied as alternatives. Extensive catalytic and kinetic studies have been conducted on the oxidation of CO over Cu-based catalysts.16−20 It was reported that, for Cu thin films, the activity decreases as the degree of oxidation increases.20 Cu-based powder catalysts have a variety of facets and oxidation states,21 and their complexity has proven to be a challenge for atomic scale studies. On the basis of research over powder catalysts, it has been suggested that Cu2O is more active than metallic copper,1 which contradicts the Cu film results mentioned above. For copper oxides, it has been suggested that Cu+ sites are the most active for CO oxidation. These could be found at surface defects in grain boundaries on powder oxide samples and have been studied theoretically using Cu2O surfaces.22,23 The oxygen from subsurface layers22 or the gas phase23 replenishes the oxygen vacancies created by CO2 © 2014 American Chemical Society

formation during CO oxidation on the oxide. An even more extensive body of research exists on the study of CO oxidation over Pt-group catalysts24 (and references therein), where the oxidation state of the most active phase for these metal catalysts has similarly been widely debated. The facile oxidation of copper limits the possibility of maintaining a single phase of these catalysts over the course of the CO oxidation reaction: either ex situ measurements are made postmortem after high-pressure experiments20 or in situ measurements are made under vacuum conditions.14 However, the topographies and catalytic activities of heterogeneous metal-based catalysts have been reported to behave differently under high vacuum as compared to high pressure.25−30 Morphological studies of the substrate and adsorbates have been conducted on Cu(110) using scanning tunneling microscopy (STM) during CO oxidation.14,31,32 STM images of 10−5 Torr of CO exposed to preoxidized Cu(110) showed that defects on the oxygen p(2 × 1) overlayer structure served as active positions and, once defects were created, the reduction took place rapidly along oxygen rows. Oxygen defects were filled by gas-phase oxygen, and reduced Cu atoms aggregated at step edges.14 A steady-state adsorbate study was conducted Received: May 22, 2014 Revised: June 27, 2014 Published: July 4, 2014 15902

dx.doi.org/10.1021/jp5050496 | J. Phys. Chem. C 2014, 118, 15902−15909

The Journal of Physical Chemistry C

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Figure 1. In situ spectroscopic data for CO oxidation (CO:O2 = 2:1) over Cu(111) at 300 K. (A) AP-IRRAS results: The top spectrum shows exposure to pure CO, and the following correspond to CO + O2 reaction times of 3.0, 4.3, 6.0, 8.0, 10.8, 14.5, and 19.0 min. Inset: total pressure as a function of reaction time. (B) AP-XPS O 1s spectra: Spectra from bottom to top obtained after exposure of Cu(111) to CO, CO + O2, and after evacuation of the AP cell.

Table 1. CO Stretch IR Frequency on Copper Sites metal phase Cu(111) Cu(100) Cu(110) Cu(211) Cu/SiO2/Mo(110) oxide phase Cu2O(111) Cu2O CuO

wavenumber (cm−1)

coordination number

ref.

9 8 7 6 nanoparticle

2071 2085 2091 2100 2106

39−41

well-ordered disordered disordered

2098 2115 2148

39

using STM on oxygen pretreated Pt(111), and as 10−8 Torr of CO was introduced, CO adsorption regions formed, and the preadsorbed oxygen regions compressed and disappeared as CO oxidation occurred at the boundaries between the two regions.33 We have previously investigated the reduction of a well-ordered Cu2O film supported on Cu(111) in the presence of a CO environment using ambient pressure (AP)-STM and were able to track the substrate reduction with nanoscale resolution in situ.29 The most common and thermally stable facet of Cu is Cu(111), which is used as a model catalytic system in surface science studies. For CO oxidation on Cu(111), the Langmuir−Hinshelwood mechanism has been predicted theoretically.34 Here, we report an in situ study of CO oxidation over Cu(111) at ambient pressures by STM, infrared reflection absorption spectroscopy (IRRAS), and X-ray photoelectron spectroscopy (XPS), to elucidate the activity for CO oxidation on metallic copper and Cu2O films and reconcile previous conflicting reports in the literature.

40,42 40,42 40 43

40,41

present work present work

chamber ultra-high-vacuum (UHV) pressures. Images were recorded using an etched tungsten tip with a constant current imaging mode. High-purity CO (GTS-WELCO, 99.999%) and O2 (GTS-WELCO, 99.9999%) gases were further purified in liquid nitrogen traps prior to being dosed into the high-pressure cell. In situ AP-IRRAS experiments were carried out in an elevated pressure reactor combined with a UHV surface analysis chamber.35 Both the STM and the IRRAS chambers are housed in the Chemistry Department at Brookhaven National Lab. In situ AP-XPS experiments were performed in a SPECS XPS with a PHOIBOS 150 AP analyzer36 at beamline I511-1,37 at the MAX II storage ring at MAX-Lab in Lund University, Sweden.

3. RESULTS AND DISCUSSION 3.1. In Situ Reaction of Carbon Monoxide and Oxygen on Copper(111). In situ AP-IRRAS and AP-XPS were used to probe the surface of Cu(111) under CO oxidation reaction conditions at 300 K (Figure 1). CO is used as a probe molecule in IR experiments due to its high sensitivity to the oxidation state of adsorption sites, in particular, on well-defined single crystals such as Cu(111).38,39 Table 1 summarizes the IR assignments for CO adsorption on various Cu environments. At 300 K and 30 mTorr of CO, a gas-phase CO peak is present in the IRRAS data (black spectrum at the top of Figure 1A). Upon the addition of O2, two main peaks for CO adsorbed on the surface appear at 2103 and 2115 cm−1. The

2. EXPERIMENTAL METHODS A SPECS Aarhus 150 HT STM chamber with a high-pressure cell was used for imaging experiments, with a base pressure of 5 × 10−10 Torr. A Cu(111) single crystal (Princeton Scientific Corp.) was prepared by consecutive Ar+ sputtering (5 μA, 2 keV, 20 min) and annealing (800 K, 10 min) cleaning cycles. The high-pressure cell is housed inside of a vacuum chamber and was sealed during the experiments to protect the main 15903



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peak at 2115 cm−1 is assigned to CO on disordered Cu2O, as reported previously.44,45 The peak at 2103 cm−1 can be assigned to adsorbed CO on the under-coordinated Cu0, based on the reported assignments for the stretching frequency of CO adsorbed on under-coordinated Cu atoms of high-index single crystals [2100 cm−1 for Cu(211) and 2102 cm−1 for Cu(311)42] and on small Cu nanoparticles, 2106 cm−1.43 CO adsorption on a well-ordered film of Cu2O(111) presents a feature at 2098 cm−1, but this feature blue shifts on disordered Cu2O structures above 2110 cm−1.41 The Cu2O(111) wellordered phase requires annealing to elevated temperatures to be formed and is not present in the current experiments carried out at 300 K. Further evidence pointing to the assignment of the 2103 cm−1 feature to under-coordinated Cu0 sites is that it disappears in Figure 1 under longer exposure to O2. With longer exposures to oxygen, the Cu is further oxidized, resulting in an increase in the intensity of the peak at 2115 cm−1. The peak at 2148 cm−1 is assigned to CO adsorbed on CuO regions, where the CO stretching frequency is similar to the results reported on fully oxidized Ru(001)46 and Pd(100).24 The formation of CuO films has not been reported under UHV conditions, but it can be stabilized under elevated pressure and temperature conditions.47 The growth of the peak associated with the presence of CuO slows down after 8.0 min of the reaction, corresponding to a decrease in the reaction rate, which is shown as an inset in Figure 1A. Early experiments over oxidized Cu also showed a decrease in activity of CuO as compared to Cu2O for the CO oxidation reaction.48 Noting that the IRRAS peak intensities relate to the absorption cross section of the particular vibrational mode as well as the orientation and coverage of the species, the higher intensity of the peak at 2148 cm−1 as compared to the peak at 2115 cm−1 does not guarantee the presence of more CO adsorbed onto CuO than Cu2O. In the corresponding XPS data in Figure 1B, the O 1s peak at 537.5 eV is assigned to gas-phase CO49 and is the only observed peak under 22 mTorr of CO. Upon the addition of O2, several new peaks are observed. The features at 538.3 and 539.3 eV correspond to gas-phase O2.50 Under CO oxidation conditions on Cu(111) at room temperature, the oxidation of the surface was observed in the O 1s region, and peaks were observed for CuO (at 528.9 eV),47 Cu2O (at 529.9 eV),47 and surface oxygen (at 530.9 eV),50 which is in agreement with the formation of CuO and Cu2O deduced from the IRRAS data. According to the XPS peak intensities, Cu2O is the majority oxide species that forms. The broad feature at ∼4 eV above the Cu2O signal matches data collected upon adsorption of CO on Cu2O(111) at low temperature (data not shown). Upon evacuation of the cell, the sample was moved to UHV conditions and the three peaks (at 530.9, 529.9, and 528.9 eV) were still observed, showing that all three oxide phases are stable in UHV. By comparing the relative peak intensity in situ and under UHV conditions, it is observed that CuO partially decomposes under vacuum. The morphological surface changes experienced by the Cu(111) model catalyst were captured via AP-STM in situ imaging, under similar conditions to the AP-IRRAS and APXPS results from Figure 1 and are shown in Figure 2. All images are recorded with the scanning direction from the bottom to the top of the image. Figure 2A shows a clean Cu(111) surface with one step edge prior to gas exposure. At the bottom of Figure 2B, a valve was opened to fill the high-pressure cell with the 2:1 CO and O2 gas mixture and horizontal lines are

observed in the image as a result of the vibrations from opening the gas valve. Upon exposure of the Cu surface to CO and O2, structural changes were observed at the step edge, as seen in the top of Figure 2B and more evidently in Figure 2C. The changes observed in Figure 2B,C correspond to the initial oxidation of the Cu surface, similar to the previously reported changes observed by STM during the exposure of Cu(111) to pure oxygen under intermediate pressures.51−53 Furthermore, the disordered oxide that forms on Cu(111) at room temperature is active in oxidation reactions, as shown by STM via the oxidation of methanol.54 The oxidation occurs at the step edges, forming a step oxide,51 and the step edges begin to facet52 as the oxide forms along the Cu(111) close-packed ⟨110⟩ direction (green lines in Figure 2C). Less than 1 min after Figure 2C was recorded, the Cu(111) surface was covered by a disordered structure, as seen in Figure 2D,E. The morphology of the disordered oxide structure after Figure 2D appeared to reach an equilibrium configuration, where only small changes at a local level were observed. It is well-known that the room-temperature oxidation of Cu(111) leads to a disordered oxide structure,52,53,55 which becomes well-ordered at higher temperatures.30,51 Figure 2 shows the appearance of a disordered structure under CO oxidation conditions at room temperature, which is assigned as oxidized Cu, and this oxide structure was stable at 300 K during the reaction (Figure 2D,E) and after evacuating the gas reaction mixture (Figure 2F). The in situ AP-IRRAS data shown in Figure 1 indicate that the reaction rate, or total pressure drop, decreases as the reaction progresses and the copper is further oxidized as the reaction progresses. Even at highly reducing conditions, with a larger CO:O2 ratio of ∼8:1, rapid formation of Cu+ and Cu2+ is observed, as shown in Figure 3. For all of the experiments presented in Figures 1−5, the reaction temperature was 300 K.

Figure 2. In situ AP-STM images of Cu(111) during CO oxidation (CO:O2 = 30 mTorr:15 mTorr (2:1)). Reaction time from (A) to (E) is 0, 0.7, 1.5, 2.2, and 20.4 min. Green lines in (C) indicate the closepacked ⟨110⟩ direction. (F) Surface morphology after evacuating the gases for ∼1 min. Scale bar = 10 nm. Scanning conditions: I = 0.36 nA, V = 1.40 V.

At higher temperatures, the fast oxidation of Cu dominates the process and translates into larger copper oxide domains on the surface of the catalyst. In the CO oxidation experiments on thin Cu films reported in the literature,20 a very high reducing CO:O2 ratio of 97:3 was used to calculate the apparent activation energy for CO oxidation on metallic Cu at temperatures ∼ 550 K. The amorphous thin Cu films likely 15904



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Figure 3. In situ IRRAS for CO oxidation over Cu(111) at 300 K. (A) CO:O2 = 8:1; the top spectrum shows exposure to pure CO. Inset: total pressure as a function of the reaction time. (B) CO:O2 = 99:1; peaks at 2068 and 2100 cm−1 represent CO adsorbed on Cu(111) and undercoordinated Cu sites, respectively, and increase in intensity as a function of time after exposure to CO + O2. Spectra in (B) were obtained after subtracting the CO gas-phase absorption.

was taken after exposing the Cu(111) sample to 99 mTorr of CO, which did not change over time. After the addition of 1 mTorr of O2, a positive feature appeared at 2068 cm−1, indicating the disappearance of CO adsorbed onto Cu0 due to the adsorption of oxygen on the surface, and a new peak develops over time at 2100 cm−1, indicating the formation of under-coordinated Cu sites on the surface. In situ AP-STM data sets for CO:O2 ratios greater than 2:1 are shown in Figure 4. Changes in the surface morphology are observed for CO oxidation over Cu(111) at a 3:1 ratio (30 mTorr of CO:10 mTorr of O2) at 300 K. Figure 4A shows two step edges on clean Cu(111) highlighted by white lines prior to the exposure of CO and O2. After 3.0 min of reaction, the step edges have retreated, as seen in Figure 4B, and faceted along the close-packed ⟨110⟩ direction, as shown with green lines in Figure 4C. The roughened appearance of the surface in the vicinity of the original step edge indicates the formation of a disordered oxide, in agreement with previous room-temperature STM Cu(111) oxidation experiments51 and the results in Figure 2. However, in Figure 4D,E, after ∼10 min of the reaction, the growth of the roughened oxide regions has slowed and the appearance of small islands was observed. Similar island growth was observed due to O2 exposure (6 × 10−5 Torr) on a Cu(100) surface at 373 K, and the islands were assigned as Cu2O, which facilitate oxygen penetrating to the subsurface.56 The height difference between the smooth areas on adjacent terraces and the small islands seen in Figure 4D is 0.21 ± 0.1 nm, corresponding to the height of a single metallic Cu layer. Similar Cu islands were observed during the in situ CO reduction of Cu2O/Cu(111) due to the release of Cu atoms during the consumption of the copper oxide by CO.29 The formation of Cu islands (indicated by white circles) in Figure 4D and their subsequent disappearance in Figure 4E show a cycle between oxidized and reduced domains. It is important to mention that, during CO oxidation over Pt and Pd single crystals, nonlinear chemical oscillation has long been reported in open systems, where a constant flow is applied and thermodynamic equilibrium is not reached.57,58 In Figure 4G,H, the terraces continue to grow, without the appearance of islands, and the step edges are faceted along the Cu ⟨110⟩ direction (green lines). STM images show that pure CO at this

Figure 4. In situ AP-STM images during CO oxidation (CO:O2 = 30 mTorr:10 mTorr (3:1)) on Cu(111) at 300 K. Green lines indicate the close-packed ⟨110⟩ direction. Reaction time from (A) to (H) is 0, 3.0, 6.0, 9.0, 12.1, 24.5, 26.6, and 50.1 min. (B) White lines show the original step edge position. White circles in (D) and (E) highlight newly formed Cu islands. The gases were evacuated in the middle of (I). Scale bar = 10 nm. Scanning conditions: I = 0.41 nA, V = 1.44 V.

consist of several facets and domains of Cu atoms. Polycrystalline Cu films and open facets are more reactive and undergo oxidation more rapidly than Cu(111) single crystals. We conducted AP-IRRAS experiments at temperatures and pressures where the adsorption of CO on metallic Cu(111) can be spectroscopically detected. At a CO:O2 ratio of 97:3 and 300 K, rapid formation of Cu2O regions was still detected. Figure 3B shows that, even under highly reducing conditions with a CO:O2 ratio of 99:1 and room temperature, the introduction of O2 induces a reconstruction of the surface by successive oxidation−reduction cycles. A background spectrum 15905



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appear with random (Figure 5C) and hexagonal (Figure 5F) shapes and the height of a single Cu layer, 0.21 ± 0.01 nm, similar to the islands in Figure 4D. Step edges continue to grow in the ⟨110⟩ direction (Figure 5G) as the reaction progresses. From Figure 5F to 5G, the terrace structure remains the same as the step edge changes, similar to what is observed in Figure 4. Upon the evacuation of the cell in Figure 5H, a pitted oxide surface forms, which then decomposes within 4 min of evacuation (Figure 5I), unlike the stable copper oxide formed in Figure 4I, showing that the oxide domains formed under the 4:1 ratio are not as stable under vacuum conditions as those formed under more oxidizing conditions (2:1 and 3:1 ratios). The combined spectroscopic and microscopic in situ studies of the CO oxidation on a copper surface at 300 K clearly establish the difficulty of preparing homogeneous phases of a given oxide domain to establish the most active phases in a system. This may be the source of the controversy in the literature with respect to the relative activity among Cu0, Cu+, and Cu2+.1 Because of the easy oxidation of copper, the most difficult phase to isolate for determining its catalytic activity is metallic copper. It has been predicted theoretically that the rate-limiting step in the oxidation of CO on Cu(111) is not the dissociation of molecular oxygen, like in Pt-group metals, but, instead, the last step of the reaction, e.g., COad + Oad → CO2.60 To study this last step, we carried out similar experiments to the ones reported on the oxidation of CO on Pt(111) by Ertl et al.33 CO desorbs from Cu(111) at 160 K, a lower temperature than on Pt(111).39 When the sample is pressurized with 1.0 mTorr, adsorption of CO on clean metallic Cu(111) can be detected in equilibrium with the gas phase up to 250 K. A surface saturated by chemisorbed oxygen was prepared by exposing Cu(111) to 150 Langmuirs of O2 at 300 K, where the initial formation of under-coordinated Cu sites is observed by the adsorption of a small amount of CO with a peak at 2089 cm−1 (Figure 6). This oxygen-saturated surface was then exposed to 1.0 mTorr of CO at 200 K, and the adsorption of

pressure does not facilitate the faceting of Cu step edges along the ⟨110⟩ direction at 300 K (see the Supporting Information), whereas O 2 alone oxidizes the Cu and forms stable structures.52,53,55 Previous AP-STM experiments show the release of highly mobile Cu atoms to nearby step edges during the reduction of a Cu2O film under a CO environment.29 Thus, we propose that Cu2O forms locally on Cu terraces in the early stages of the reaction,48,56 and the movement of step edges is due to the diffusion of the highly mobile Cu atoms released in the reduction step of the redox cycle during CO oxidation. Upon evacuating the gases after the reaction in Figure 4I, the surface immediately appears pitted (at the top of Figure 4I), which is similar to the copper oxide structure observed in Figure 2F. Since CO has a smaller adsorption coefficient than O2, any oxygen remaining in the system readily oxidizes the Cu surface. The pitted structure is stable under UHV conditions at 300 K. Figure 5 shows AP-STM results from CO oxidation over Cu(111) upon increasing the CO:O2 ratio to 4:1 (32 mTorr of

Figure 5. In situ AP-STM images during CO oxidation (4:1, 32 mTorr of CO:8 mTorr of O2) on Cu(111). Green lines indicate the Cu(111) close-packed ⟨110⟩ directions. Reaction time from (A) to (G) is 0, 8.2, 12.3, 14.4, 18.6, 27.0, and 77.2 min. The inset of (C) shows the highly resolved oxide structure from the black square (16 × 16 nm2). (H) After evacuation. (I) 4.2 min after evacuation. Scale bar = 10 nm. Scanning conditions: I = 0.41 nA, V = 1.40 V.

CO:8 mTorr of O2). Figure 5A shows an area of the Cu(111) surface with five terraces before the exposure to CO and O2. After 8.2 min of exposure to the reactant gases (Figure 5B), all of the step edges appear etched, as was observed under the 3:1 ratio in Figure 4B. After another 4 min, in Figure 5C and its inset, a highly resolved image shows the Cu terraces with low mobility point defects (depressions) and ring structures of the copper oxide precursor, most likely hexagonal and five- and seven-membered (“5−7” structure) Cu2O rings,29,59 made up of surface oxygen,51 with a height of 0.06 ± 0.01 nm. Green lines show the faceting of step edges along the ⟨110⟩ direction. As the reaction progresses from Figure 5C to Figure 5G, the copper oxide ring structure gradually disappears and islands

Figure 6. In situ AP-IRRAS during the reduction of an oxygensaturated Cu(111) surface at 200 K under 1.0 mTorr of CO. The peak at 2089 cm−1 corresponds to a small amount of CO adsorbed on under-coordinated Cu sites, while the increase in intensity of the peak at 2069 cm−1 over time corresponds to adsorption of CO on Cu(111) sites upon the removal of chemisorbed oxygen by formation of CO2. 15906



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more active than Pt-group metals at 300 K, and the formation of Cu2+ deactivates the catalysts. Strategies to stabilize structures with Cu+ cations, such as the formation of mixed oxides,63 could lead to very efficient and stable oxidation catalysts.

CO on metallic Cu sites was followed as a function of time while the adsorbed CO reacted with preadsorbed oxygen to form CO2 (Figure 6). By carrying out similar experiments to the one shown in Figure 6 at various temperatures below 200 K, where the initial rate of CO oxidation was measured, we were able to determine an apparent activation energy for the oxidation of CO on an oxygen-saturated copper surface, which is equal to Ea(O−Cu) = 0.10 ± 0.03 eV. We also prepared welldefined Cu2O/Cu(111) films and measured their initial rate of reduction at temperatures ∼ 300 K and CO pressures of 1 mTorr. After an initial induction period required for the formation of vacancies, the apparent activation energy for CO oxidation on Cu2O was measured to be Ea(Cu2O) = 0.22 ± 0.01 eV. A previous result on the reduction of oxygen-saturated Cu(111) by CO at ∼300 K, which lacked spectroscopic characterization, but based on our current studies, may have involved a mixture of Cu0 and Cu+ sites, reported an apparent activation energy Ea = 0.15 ± 0.01 eV, in agreement with an average value of the two activation energies discussed above.61 We are not able to prepare homogeneous phases of Cu2+on Cu(111) to test their catalytic activity, but the AP-IRRAS data discussed above show that the reaction rate is significantly lowered with the formation of large Cu2+ domains. 3.2. Active Phase of Copper during Carbon Monoxide Oxidation. According to the in situ data discussed above, the surface of copper catalysts under CO oxidation conditions is heterogeneous and, in general, consists of all possible Cu oxidation states. In the stoichiometric case of the 2:1 (CO:O2) ratio, the Cu(111) surface was oxidized to a relatively static structure, on which CO oxidation may partially proceed through a Mars−van Krevelen mechanism,62 where CO reacts with lattice oxygen and then O2 fills in O vacancies instantly to protect the surface from reduction. Under more reducing conditions, the surface structure is very dynamic and undergoes reconstructions related to the oxidation/reduction of copper metal/oxide domains. The increase of surface mobility is due to released Cu atoms from the redox cycle, to which CO can bind at room temperature and aid Cu diffusion.29 The released Cu atoms form faceted step edges to minimize surface energy. Metallic copper is the most efficient phase for the oxidation of CO via a Langmuir−Hinshelwood pathway. Although the reported overall CO oxidation activity of noble metal-based catalysts is better than that of copper-based catalysts, the apparent activation energy for the process on metallic copper is the lowest reported in the literature, compared with values of 0.5 eV on Pt(111)33 and Ru(001).46 The previously reported higher value for Cu films of 0.4 eV20 was most likely due to the presence of less efficient Cu+ and Cu2+ regions. The trend in the efficiency for the oxidation of CO is Cu0 > Cu+ > Cu2+, which agrees with the trend observed on Pt-group metals.24

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ASSOCIATED CONTENT

* Supporting Information S

Scanning tunneling microscopy images show no mass transfer of Cu below 100 mTorr of CO at room temperature. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 631-344-4378. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the U.S. Department of Energy for financial support under contract No. DE-AC02-98CH10886. The Swedish research council (VR) is acknowledged for their financial support and the MAX-lab staff for its support during beamtimes.

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REFERENCES

(1) Royer, S.; Duprez, D. Catalytic Oxidation of Carbon Monoxide over Transition Metal Oxides. ChemCatChem 2011, 3, 24−65. (2) Prasad, R.; Singh, P. A Review on CO Oxidation over Copper Chromite Catalyst. Catal. Rev.: Sci. Eng. 2012, 54, 224−279. (3) Rodriguez, J. A.; Graciani, J.; Evans, J.; Park, J. B.; Yang, F.; Stacchiola, D.; Senanayake, S. D.; Ma, S. G.; Perez, M.; Liu, P.; et al. Water-Gas Shift Reaction on a Highly Active Inverse CeOx/Cu(111) Catalyst: Unique Role of Ceria Nanoparticles. Angew. Chem., Int. Ed. 2009, 48, 8047−8050. (4) Rodriguez, J. A.; Liu, P.; Wang, X.; Wen, W.; Hanson, J.; Hrbek, J.; Perez, M.; Evans, J. Water-Gas Shift Activity of Cu Surfaces and Cu Nanoparticles Supported on Metal Oxides. Catal. Today 2009, 143, 45−50. (5) Mudiyanselage, K.; Senanayake, S. D.; Feria, L.; Kundu, S.; Baber, A. E.; Graciani, J.; Vidal, A. B.; Agnoli, S.; Evans, J.; Chang, R.; et al. Importance of the Metal-Oxide Interface in Catalysis: In Situ Studies of the Water-Gas Shift Reaction by Ambient-Pressure X-ray Photoelectron Spectroscopy. Angew. Chem., Int. Ed. 2013, 52, 5101−5105. (6) Liu, Y.; Fu, Q.; Flytzani-Stephanopoulos, M. Preferential Oxidation of CO in H2 over CuO-CeO2 Catalysts. Catal. Today 2004, 93−95, 241−246. (7) Kugai, J.; Moriya, T.; Seino, S.; Nakagawa, T.; Ohkubo, Y.; Nitani, H.; Mizukoshi, Y.; Yamamoto, T. A. Effect of Support for Pt− Cu Bimetallic Catalysts Synthesized by Electron Beam Irradiation Method on Preferential CO Oxidation. Appl. Catal., B 2012, 126, 306−314. (8) Han, J.; Kim, H. J.; Yoon, S.; Lee, H. Shape Effect of Ceria in Cu/ Ceria Catalysts for Preferential CO Oxidation. J. Mol. Catal. A: Chem. 2011, 335, 82−88. (9) Oh, S. H.; Sinkevitch, R. M. Carbon-Monoxide Removal from Hydrogen-Rich Fuel-Cell Feedstreams by Selective Catalytic-Oxidation. J. Catal. 1993, 142, 254−262. (10) Marino, F.; Descorme, C.; Duprez, D. Noble Metal Catalysts for the Preferential Oxidation of Carbon Monoxide in the Presence of Hydrogen (PROX). Appl. Catal., B 2004, 54, 59−66. (11) Saalfrank, J. W.; Maier, W. F. Directed Evolution of NobleMetal-Free Catalysts for the Oxidation of CO at Room Temperature. Angew. Chem., Int. Ed. 2004, 43, 2028−2031.

4. CONCLUSIONS By using a complementary set of in situ microscopic and spectroscopic techniques, a clear picture of the dynamic nature of the surface of copper catalysts during the oxidation of CO has been obtained. Copper oxide domains form even under heavily reducing conditions at 300 K, and the surface reconstructs constantly through a redox cycle between CuO, Cu2O, and Cu. The Cu atoms released during the redox cycle diffuse to the step edges, leading to a flat surface with highly mobile step edges that are faceted along the ⟨110⟩ direction. Metallic copper is the most active phase, but it cannot be stabilized under reaction conditions. Cu+ is also very active, 15907



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Redox-Mediated Reconstruction of Copper during Carbon Monoxide

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