Redox-Mediated Reconstruction of Copper during Carbon Monoxide Oxidation

Redox-Mediated Reconstruction of Copper during Carbon MonoxideOxidationFang 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 rapidlydeactivates under reaction conditions. In an effort to obtain a full picture of the dynamicmorphological and chemical changes occurring on the surface of catalysts under COoxidation conditions, a complementary set of in situ ambient pressure (AP) techniquesthat include scanning tunneling microscopy, infrared reflection absorption spectroscopy(IRRAS), and X-ray photoelectron spectroscopy were conducted. Herein, we report in situAP CO oxidation experiments over Cu(111) model catalysts at room temperature.Depending on the CO:O2 ratio, Cu presents different oxidation states, leading to thecoexistence of several phases. During CO oxidation, a redox cycle is observed on thesubstrate’s surface, in which Cu atoms are oxidized and pulled from terraces and step edgesand then are reduced and rejoin nearby step edges. IRRAS results confirm the presence ofunder-coordinated Cu atoms during the reaction. By using control experiments to isolateindividual phases, it is shown that the rate for CO oxidation decreases systematically asmetallic copper is fully oxidized.

1. INTRODUCTION

CO oxidation has long been a prototypical reaction forfundamental research as well as in critical practical applicationssuch as the elimination of automotive exhaust pollution,1,2 andthe water gas shift3−5 and preferential oxidation (PROX)6−8

reactions used during the purification of hydrogen. Tradition-ally, heterogeneous catalytic materials, such as noble metals (Pt,Pd, Rh, and Ru)9 and oxide-supported catalysts10 are widelyused. In an effort to reduce the high expense of the noble metalcatalysts, transition-metal oxides11−13 (cobalt oxides and copperoxides) and unsupported/supported transition metals14,15 (Co,Cr, Cu, Ni, Zn) have been studied as alternatives. Extensivecatalytic and kinetic studies have been conducted on theoxidation of CO over Cu-based catalysts.16−20 It was reportedthat, for Cu thin films, the activity decreases as the degree ofoxidation increases.20 Cu-based powder catalysts have a varietyof facets and oxidation states,21 and their complexity has provento be a challenge for atomic scale studies. On the basis ofresearch over powder catalysts, it has been suggested that Cu2Ois more active than metallic copper,1 which contradicts the Cufilm results mentioned above. For copper oxides, it has beensuggested that Cu+ sites are the most active for CO oxidation.These could be found at surface defects in grain boundaries onpowder oxide samples and have been studied theoretically usingCu2O surfaces.22,23 The oxygen from subsurface layers22 or thegas phase23 replenishes the oxygen vacancies created by CO2

formation during CO oxidation on the oxide. An even moreextensive body of research exists on the study of CO oxidationover Pt-group catalysts24 (and references therein), where theoxidation state of the most active phase for these metal catalystshas similarly been widely debated.The facile oxidation of copper limits the possibility of

maintaining a single phase of these catalysts over the course ofthe CO oxidation reaction: either ex situ measurements aremade postmortem after high-pressure experiments20 or in situmeasurements are made under vacuum conditions.14 However,the topographies and catalytic activities of heterogeneousmetal-based catalysts have been reported to behave differentlyunder high vacuum as compared to high pressure.25−30

Morphological studies of the substrate and adsorbates havebeen conducted on Cu(110) using scanning tunnelingmicroscopy (STM) during CO oxidation.14,31,32 STM imagesof 10−5 Torr of CO exposed to preoxidized Cu(110) showedthat defects on the oxygen p(2 × 1) overlayer structure servedas active positions and, once defects were created, the reductiontook place rapidly along oxygen rows. Oxygen defects werefilled by gas-phase oxygen, and reduced Cu atoms aggregated atstep edges.14 A steady-state adsorbate study was conducted

Received: May 22, 2014Revised: June 27, 2014Published: July 4, 2014

Article

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

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using STM on oxygen pretreated Pt(111), and as 10−8 Torr ofCO was introduced, CO adsorption regions formed, and thepreadsorbed oxygen regions compressed and disappeared asCO oxidation occurred at the boundaries between the tworegions.33 We have previously investigated the reduction of awell-ordered Cu2O film supported on Cu(111) in the presenceof a CO environment using ambient pressure (AP)-STM andwere able to track the substrate reduction with nanoscaleresolution in situ.29 The most common and thermally stablefacet of Cu is Cu(111), which is used as a model catalyticsystem in surface science studies. For CO oxidation onCu(111), the Langmuir−Hinshelwood mechanism has beenpredicted theoretically.34 Here, we report an in situ study ofCO oxidation over Cu(111) at ambient pressures by STM,infrared reflection absorption spectroscopy (IRRAS), and X-rayphotoelectron spectroscopy (XPS), to elucidate the activity forCO oxidation on metallic copper and Cu2O films and reconcileprevious conflicting reports in the literature.

2. EXPERIMENTAL METHODS

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

chamber ultra-high-vacuum (UHV) pressures. Images wererecorded using an etched tungsten tip with a constant currentimaging mode. High-purity CO (GTS-WELCO, 99.999%) andO2 (GTS-WELCO, 99.9999%) gases were further purified inliquid nitrogen traps prior to being dosed into the high-pressurecell. In situ AP-IRRAS experiments were carried out in anelevated pressure reactor combined with a UHV surfaceanalysis chamber.35 Both the STM and the IRRAS chambersare housed in the Chemistry Department at BrookhavenNational Lab. In situ AP-XPS experiments were performed in aSPECS XPS with a PHOIBOS 150 AP analyzer36 at beamlineI511-1,37 at the MAX II storage ring at MAX-Lab in LundUniversity, Sweden.

3. RESULTS AND DISCUSSION

3.1. In Situ Reaction of Carbon Monoxide and Oxygenon Copper(111). In situ AP-IRRAS and AP-XPS were used toprobe the surface of Cu(111) under CO oxidation reactionconditions at 300 K (Figure 1). CO is used as a probe moleculein IR experiments due to its high sensitivity to the oxidationstate of adsorption sites, in particular, on well-defined singlecrystals such as Cu(111).38,39 Table 1 summarizes the IRassignments 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 Figure1A). Upon the addition of O2, two main peaks for COadsorbed on the surface appear at 2103 and 2115 cm−1. The

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 showsexposure 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 afunction 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 afterevacuation of the AP cell.

Table 1. CO Stretch IR Frequency on Copper Sites

metal phase coordination number wavenumber (cm−1) ref.

Cu(111) 9 2071 39−41

Cu(100) 8 2085 40,42

Cu(110) 7 2091 40,42

Cu(211) 6 2100 40

Cu/SiO2/Mo(110) nanoparticle 2106 43

oxide phaseCu2O(111) well-ordered 2098 39

Cu2O disordered 2115 40,41 present workCuO disordered 2148 present work

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peak at 2115 cm−1 is assigned to CO on disordered Cu2O, asreported previously.44,45 The peak at 2103 cm−1 can beassigned to adsorbed CO on the under-coordinated Cu0, basedon the reported assignments for the stretching frequency of COadsorbed on under-coordinated Cu atoms of high-index singlecrystals [2100 cm−1 for Cu(211) and 2102 cm−1 forCu(311)42] and on small Cu nanoparticles, 2106 cm−1.43 COadsorption on a well-ordered film of Cu2O(111) presents afeature at 2098 cm−1, but this feature blue shifts on disorderedCu2O structures above 2110 cm−1.41 The Cu2O(111) well-ordered phase requires annealing to elevated temperatures tobe formed and is not present in the current experiments carriedout at 300 K. Further evidence pointing to the assignment ofthe 2103 cm−1 feature to under-coordinated Cu0 sites is that itdisappears in Figure 1 under longer exposure to O2. Withlonger exposures to oxygen, the Cu is further oxidized, resultingin an increase in the intensity of the peak at 2115 cm−1. Thepeak at 2148 cm−1 is assigned to CO adsorbed on CuO regions,where the CO stretching frequency is similar to the resultsreported on fully oxidized Ru(001)46 and Pd(100).24 Theformation of CuO films has not been reported under UHVconditions, but it can be stabilized under elevated pressure andtemperature conditions.47 The growth of the peak associatedwith the presence of CuO slows down after 8.0 min of thereaction, corresponding to a decrease in the reaction rate, whichis shown as an inset in Figure 1A. Early experiments overoxidized Cu also showed a decrease in activity of CuO ascompared to Cu2O for the CO oxidation reaction.48 Notingthat the IRRAS peak intensities relate to the absorption crosssection of the particular vibrational mode as well as theorientation and coverage of the species, the higher intensity ofthe peak at 2148 cm−1 as compared to the peak at 2115 cm−1

does not guarantee the presence of more CO adsorbed ontoCuO 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 onlyobserved peak under 22 mTorr of CO. Upon the addition ofO2, several new peaks are observed. The features at 538.3 and539.3 eV correspond to gas-phase O2.

50 Under CO oxidationconditions on Cu(111) at room temperature, the oxidation ofthe surface was observed in the O 1s region, and peaks wereobserved for CuO (at 528.9 eV),47 Cu2O (at 529.9 eV),47 andsurface oxygen (at 530.9 eV),50 which is in agreement with theformation of CuO and Cu2O deduced from the IRRAS data.According to the XPS peak intensities, Cu2O is the majorityoxide species that forms. The broad feature at ∼4 eV above theCu2O signal matches data collected upon adsorption of CO onCu2O(111) at low temperature (data not shown). Uponevacuation of the cell, the sample was moved to UHVconditions and the three peaks (at 530.9, 529.9, and 528.9 eV)were still observed, showing that all three oxide phases arestable in UHV. By comparing the relative peak intensity in situand under UHV conditions, it is observed that CuO partiallydecomposes under vacuum.The morphological surface changes experienced by the

Cu(111) model catalyst were captured via AP-STM in situimaging, under similar conditions to the AP-IRRAS and AP-XPS results from Figure 1 and are shown in Figure 2. All imagesare recorded with the scanning direction from the bottom tothe top of the image. Figure 2A shows a clean Cu(111) surfacewith one step edge prior to gas exposure. At the bottom ofFigure 2B, a valve was opened to fill the high-pressure cell withthe 2:1 CO and O2 gas mixture and horizontal lines are

observed in the image as a result of the vibrations from openingthe gas valve. Upon exposure of the Cu surface to CO and O2,structural changes were observed at the step edge, as seen in thetop of Figure 2B and more evidently in Figure 2C. The changesobserved in Figure 2B,C correspond to the initial oxidation ofthe Cu surface, similar to the previously reported changesobserved by STM during the exposure of Cu(111) to pureoxygen under intermediate pressures.51−53 Furthermore, thedisordered oxide that forms on Cu(111) at room temperatureis active in oxidation reactions, as shown by STM via theoxidation of methanol.54 The oxidation occurs at the stepedges, forming a step oxide,51 and the step edges begin tofacet52 as the oxide forms along the Cu(111) close-packed⟨110⟩ direction (green lines in Figure 2C). Less than 1 minafter Figure 2C was recorded, the Cu(111) surface was coveredby a disordered structure, as seen in Figure 2D,E. Themorphology of the disordered oxide structure after Figure 2Dappeared to reach an equilibrium configuration, where onlysmall changes at a local level were observed. It is well-knownthat the room-temperature oxidation of Cu(111) leads to adisordered oxide structure,52,53,55 which becomes well-orderedat higher temperatures.30,51 Figure 2 shows the appearance of adisordered structure under CO oxidation conditions at roomtemperature, which is assigned as oxidized Cu, and this oxidestructure 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 thereaction progresses and the copper is further oxidized as thereaction progresses. Even at highly reducing conditions, with alarger CO:O2 ratio of ∼8:1, rapid formation of Cu+ and Cu2+ isobserved, as shown in Figure 3. For all of the experimentspresented in Figures 1−5, the reaction temperature was 300 K.

At higher temperatures, the fast oxidation of Cu dominates theprocess and translates into larger copper oxide domains on thesurface of the catalyst. In the CO oxidation experiments on thinCu films reported in the literature,20 a very high reducingCO:O2 ratio of 97:3 was used to calculate the apparentactivation energy for CO oxidation on metallic Cu attemperatures ∼ 550 K. The amorphous thin Cu films likely

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 close-packed ⟨110⟩ direction. (F) Surface morphology after evacuating thegases for ∼1 min. Scale bar = 10 nm. Scanning conditions: I = 0.36 nA,V = 1.40 V.



consist of several facets and domains of Cu atoms. Polycrystal-line Cu films and open facets are more reactive and undergooxidation more rapidly than Cu(111) single crystals. Weconducted AP-IRRAS experiments at temperatures andpressures where the adsorption of CO on metallic Cu(111)can be spectroscopically detected. At a CO:O2 ratio of 97:3 and300 K, rapid formation of Cu2O regions was still detected.Figure 3B shows that, even under highly reducing conditionswith a CO:O2 ratio of 99:1 and room temperature, theintroduction of O2 induces a reconstruction of the surface bysuccessive oxidation−reduction cycles. A background spectrum

was taken after exposing the Cu(111) sample to 99 mTorr ofCO, which did not change over time. After the addition of 1mTorr of O2, a positive feature appeared at 2068 cm−1,indicating the disappearance of CO adsorbed onto Cu0 due tothe adsorption of oxygen on the surface, and a new peakdevelops over time at 2100 cm−1, indicating the formation ofunder-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 areobserved for CO oxidation over Cu(111) at a 3:1 ratio (30mTorr of CO:10 mTorr of O2) at 300 K. Figure 4A shows twostep edges on clean Cu(111) highlighted by white lines prior tothe exposure of CO and O2. After 3.0 min of reaction, the stepedges have retreated, as seen in Figure 4B, and faceted alongthe close-packed ⟨110⟩ direction, as shown with green lines inFigure 4C. The roughened appearance of the surface in thevicinity of the original step edge indicates the formation of adisordered oxide, in agreement with previous room-temper-ature STM Cu(111) oxidation experiments51 and the results inFigure 2. However, in Figure 4D,E, after ∼10 min of thereaction, the growth of the roughened oxide regions has slowedand the appearance of small islands was observed. Similar islandgrowth was observed due to O2 exposure (6 × 10−5 Torr) on aCu(100) surface at 373 K, and the islands were assigned asCu2O, which facilitate oxygen penetrating to the subsurface.56

The height difference between the smooth areas on adjacentterraces and the small islands seen in Figure 4D is 0.21 ± 0.1nm, corresponding to the height of a single metallic Cu layer.Similar Cu islands were observed during the in situ COreduction of Cu2O/Cu(111) due to the release of Cu atomsduring the consumption of the copper oxide by CO.29 Theformation of Cu islands (indicated by white circles) in Figure4D and their subsequent disappearance in Figure 4E show acycle between oxidized and reduced domains. It is important tomention that, during CO oxidation over Pt and Pd singlecrystals, nonlinear chemical oscillation has long been reportedin open systems, where a constant flow is applied andthermodynamic equilibrium is not reached.57,58 In Figure4G,H, the terraces continue to grow, without the appearance ofislands, and the step edges are faceted along the Cu ⟨110⟩direction (green lines). STM images show that pure CO at this

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: totalpressure 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 under-coordinated Cu sites, respectively, and increase in intensity as a function of time after exposure to CO + O2. Spectra in (B) were obtained aftersubtracting the CO gas-phase absorption.

Figure 4. In situ AP-STM images during CO oxidation (CO:O2 = 30mTorr:10 mTorr (3:1)) on Cu(111) at 300 K. Green lines indicatethe 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 theoriginal step edge position. White circles in (D) and (E) highlightnewly 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.



pressure does not facilitate the faceting of Cu step edges alongthe ⟨110⟩ direction at 300 K (see the Supporting Information),whereas O2 alone oxidizes the Cu and forms stablestructures.52,53,55 Previous AP-STM experiments show therelease of highly mobile Cu atoms to nearby step edges duringthe reduction of a Cu2O film under a CO environment.29 Thus,we propose that Cu2O forms locally on Cu terraces in the earlystages of the reaction,48,56 and the movement of step edges isdue to the diffusion of the highly mobile Cu atoms released inthe reduction step of the redox cycle during CO oxidation.Upon evacuating the gases after the reaction in Figure 4I, thesurface immediately appears pitted (at the top of Figure 4I),which is similar to the copper oxide structure observed inFigure 2F. Since CO has a smaller adsorption coefficient thanO2, any oxygen remaining in the system readily oxidizes the Cusurface. The pitted structure is stable under UHV conditions at300 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

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), allof the step edges appear etched, as was observed under the 3:1ratio in Figure 4B. After another 4 min, in Figure 5C and itsinset, a highly resolved image shows the Cu terraces with lowmobility point defects (depressions) and ring structures of thecopper oxide precursor, most likely hexagonal and five- andseven-membered (“5−7” structure) Cu2O rings,29,59 made upof surface oxygen,51 with a height of 0.06 ± 0.01 nm. Greenlines show the faceting of step edges along the ⟨110⟩ direction.As the reaction progresses from Figure 5C to Figure 5G, thecopper oxide ring structure gradually disappears and islands

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 growin the ⟨110⟩ direction (Figure 5G) as the reaction progresses.From Figure 5F to 5G, the terrace structure remains the sameas the step edge changes, similar to what is observed in Figure4. Upon the evacuation of the cell in Figure 5H, a pitted oxidesurface forms, which then decomposes within 4 min ofevacuation (Figure 5I), unlike the stable copper oxide formedin Figure 4I, showing that the oxide domains formed under the4:1 ratio are not as stable under vacuum conditions as thoseformed 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 clearlyestablish the difficulty of preparing homogeneous phases of agiven oxide domain to establish the most active phases in asystem. This may be the source of the controversy in theliterature with respect to the relative activity among Cu0, Cu+,and Cu2+.1 Because of the easy oxidation of copper, the mostdifficult phase to isolate for determining its catalytic activity ismetallic copper. It has been predicted theoretically that therate-limiting step in the oxidation of CO on Cu(111) is not thedissociation 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 tothe ones reported on the oxidation of CO on Pt(111) by Ertl etal.33 CO desorbs from Cu(111) at 160 K, a lower temperaturethan on Pt(111).39 When the sample is pressurized with 1.0mTorr, adsorption of CO on clean metallic Cu(111) can bedetected in equilibrium with the gas phase up to 250 K. Asurface saturated by chemisorbed oxygen was prepared byexposing Cu(111) to 150 Langmuirs of O2 at 300 K, where theinitial formation of under-coordinated Cu sites is observed bythe adsorption of a small amount of CO with a peak at 2089cm−1 (Figure 6). This oxygen-saturated surface was thenexposed to 1.0 mTorr of CO at 200 K, and the adsorption of

Figure 5. In situ AP-STM images during CO oxidation (4:1, 32 mTorrof 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 highlyresolved 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.

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



CO on metallic Cu sites was followed as a function of timewhile the adsorbed CO reacted with preadsorbed oxygen toform CO2 (Figure 6). By carrying out similar experiments tothe one shown in Figure 6 at various temperatures below 200K, where the initial rate of CO oxidation was measured, wewere able to determine an apparent activation energy for theoxidation of CO on an oxygen-saturated copper surface, whichis equal to Ea(O−Cu) = 0.10 ± 0.03 eV. We also prepared well-defined Cu2O/Cu(111) films and measured their initial rate ofreduction at temperatures ∼ 300 K and CO pressures of 1mTorr. After an initial induction period required for theformation of vacancies, the apparent activation energy for COoxidation on Cu2O was measured to be Ea(Cu2O) = 0.22 ±0.01 eV. A previous result on the reduction of oxygen-saturatedCu(111) by CO at ∼300 K, which lacked spectroscopiccharacterization, but based on our current studies, may haveinvolved a mixture of Cu0 and Cu+ sites, reported an apparentactivation energy Ea = 0.15 ± 0.01 eV, in agreement with anaverage value of the two activation energies discussed above.61

We are not able to prepare homogeneous phases of Cu2+onCu(111) to test their catalytic activity, but the AP-IRRAS datadiscussed above show that the reaction rate is significantlylowered 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, thesurface of copper catalysts under CO oxidation conditions isheterogeneous and, in general, consists of all possible Cuoxidation states. In the stoichiometric case of the 2:1 (CO:O2)ratio, the Cu(111) surface was oxidized to a relatively staticstructure, on which CO oxidation may partially proceedthrough a Mars−van Krevelen mechanism,62 where CO reactswith lattice oxygen and then O2 fills in O vacancies instantly toprotect the surface from reduction. Under more reducingconditions, the surface structure is very dynamic and undergoesreconstructions related to the oxidation/reduction of coppermetal/oxide domains. The increase of surface mobility is due toreleased Cu atoms from the redox cycle, to which CO can bindat room temperature and aid Cu diffusion.29 The released Cuatoms form faceted step edges to minimize surface energy.Metallic copper is the most efficient phase for the oxidation ofCO via a Langmuir−Hinshelwood pathway. Although thereported overall CO oxidation activity of noble metal-basedcatalysts is better than that of copper-based catalysts, theapparent activation energy for the process on metallic copper isthe lowest reported in the literature, compared with values of0.5 eV on Pt(111)33 and Ru(001).46 The previously reportedhigher value for Cu films of 0.4 eV20 was most likely due to thepresence of less efficient Cu+ and Cu2+ regions. The trend inthe efficiency for the oxidation of CO is Cu0 > Cu+ > Cu2+,which agrees with the trend observed on Pt-group metals.24

4. CONCLUSIONSBy using a complementary set of in situ microscopic andspectroscopic techniques, a clear picture of the dynamic natureof the surface of copper catalysts during the oxidation of COhas been obtained. Copper oxide domains form even underheavily reducing conditions at 300 K, and the surfacereconstructs constantly through a redox cycle between CuO,Cu2O, and Cu. The Cu atoms released during the redox cyclediffuse to the step edges, leading to a flat surface with highlymobile step edges that are faceted along the ⟨110⟩ direction.Metallic copper is the most active phase, but it cannot bestabilized under reaction conditions. Cu+ is also very active,

more active than Pt-group metals at 300 K, and the formationof Cu2+ deactivates the catalysts. Strategies to stabilizestructures with Cu+ cations, such as the formation of mixedoxides,63 could lead to very efficient and stable oxidationcatalysts.

■ ASSOCIATED CONTENT*S Supporting InformationScanning tunneling microscopy images show no mass transferof Cu below 100 mTorr of CO at room temperature. Thismaterial is available free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*Phone: 631-344-4378. E-mail: [email protected] authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe thank the U.S. Department of Energy for financial supportunder contract No. DE-AC02-98CH10886. The Swedishresearch council (VR) is acknowledged for their financialsupport and the MAX-lab staff for its support duringbeamtimes.

■ REFERENCES(1) Royer, S.; Duprez, D. Catalytic Oxidation of Carbon Monoxideover Transition Metal Oxides. ChemCatChem 2011, 3, 24−65.(2) Prasad, R.; Singh, P. A Review on CO Oxidation over CopperChromite 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 CuNanoparticles 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 Studiesof the Water-Gas Shift Reaction by Ambient-Pressure X-ray Photo-electron Spectroscopy. Angew. Chem., Int. Ed. 2013, 52, 5101−5105.(6) Liu, Y.; Fu, Q.; Flytzani-Stephanopoulos, M. PreferentialOxidation of CO in H2 over CuO-CeO2 Catalysts. Catal. Today2004, 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 IrradiationMethod 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 fromHydrogen-Rich Fuel-Cell Feedstreams by Selective Catalytic-Oxida-tion. J. Catal. 1993, 142, 254−262.(10) Marino, F.; Descorme, C.; Duprez, D. Noble Metal Catalysts forthe Preferential Oxidation of Carbon Monoxide in the Presence ofHydrogen (PROX). Appl. Catal., B 2004, 54, 59−66.(11) Saalfrank, J. W.; Maier, W. F. Directed Evolution of Noble-Metal-Free Catalysts for the Oxidation of CO at Room Temperature.Angew. Chem., Int. Ed. 2004, 43, 2028−2031.



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(12) Huang, T. J.; Tsai, D. H. CO Oxidation Behavior of Copper andCopper Oxides. Catal. Lett. 2003, 87, 173−178.(13) Xie, X. W.; Li, Y.; Liu, Z. Q.; Haruta, M.; Shen, W. J. Low-Temperature Oxidation of CO Catalysed by Co3O4 Nanorods. Nature2009, 458, 746−749.(14) Crew, W. W.; Madix, R. J. A Scanning Tunneling MicroscopyStudy of the Oxidation of CO on Cu(110) at 400 K: Site Specificityand Reaction Kinetics. Surf. Sci. 1996, 349, 275−293.(15) Marino, F.; Descorme, C.; Duprez, D. Supported Base MetalCatalysts for the Preferential Oxidation of Carbon Monoxide in thePresence of Excess Hydrogen (PROX). Appl. Catal., B 2005, 58, 175−183.(16) Ono, Y.; Shibata, M.; Inui, T. Non-Linear Change in OxidationState of Cu During Co Oxidation on Supported Copper CatalystsMeasured by the Forced-Oscillating Reaction Method. J. Mol. Catal. A:Chem. 2000, 153, 53−62.(17) Liu, X. Y.; Wang, A. Q.; Zhang, T.; Su, D. S.; Mou, C. Y. Au−CuAlloy Nanoparticles Supported on Silica Gel as Catalyst for COOxidation: Effects of Au/Cu Ratios. Catal. Today 2011, 160, 103−108.(18) Mrabet, D.; Abassi, A.; Cherizol, R.; Do, T. O. One-PotSolvothermal Synthesis of Mixed Cu-Ce-Ox Nanocatalysts and TheirCatalytic Activity for Low Temperature CO Oxidation. Appl. Catal., A2012, 447, 60−66.(19) Domagala, M. E.; Campbell, C. T. The Mechanism of COOxidation over Cu(110): Effect of CO Gas Energy. Catal. Lett. 1991,9, 65−70.(20) Jernigan, G. G.; Somorjai, G. A. Carbon-Monoxide Oxidationover Three Different Oxidation-States of Copper: Metallic Copper,Copper (I) Oxide, and Copper (Ii) Oxide - A Surface Science andKinetic-Study. J. Catal. 1994, 147, 567−577.(21) Simson, G.; Prasetyo, E.; Reiner, S.; Hinrichsen, O. ContinuousPrecipitation of Cu/ZnO/Al2O3 Catalysts for Methanol Synthesis inMicrostructured Reactors with Alternative Precipitating Agents. Appl.Catal., A 2013, 450, 1−12.(22) Sadykov, V. A.; Tikhov, S. F.; Bulgakov, N. N.; Gerasev, A. P.Catalytic Oxidation of CO on CuOx Revisited: Impact of the SurfaceState on the Apparent Kinetic Parameters. Catal. Today 2009, 144,324−333.(23) Le, D.; Stolbov, S.; Rahman, T. S. Reactivity of the Cu2O(100)Surface: Insights from First Principles Calculations. Surf. Sci. 2009,603, 1637−1645.(24) Gao, F.; Goodman, D. W. Reaction Kinetics and PolarizationModulation Infrared Reflection Absorption Spectroscopy Investiga-tions of CO Oxidation over Planar Pt-Group Model Catalysts.Langmuir 2010, 26, 16540−16551.(25) Nguyen, L.; Cheng, F.; Zhang, S. R.; Tao, F. Visualization ofSurfaces of Pt and Ni Model Catalysts in Reactive Environments UsingAmbient Pressure High Temperature Scanning Tunneling Microscopyand Understanding the Restructurings of Surfaces of Model MetalCatalysts under Reaction Conditions at near Ambient Pressure. J. Phys.Chem. C 2013, 117, 971−977.(26) Zhu, Z. W.; Butcher, D. R.; Mao, B. H.; Liu, Z.; Salmeron, M.;Somorjai, G. A. In Situ Scanning Tunneling Microscopy and X-rayPhotoelectron Spectroscopy Studies of Ethylene-Induced StructuralChanges on the Pt(100)-Hex Surface. J. Phys. Chem. C 2013, 117,2799−2804.(27) Zafeiratos, S.; Piccinin, S.; Teschner, D. Alloys in Catalysis:Phase Separation and Surface Segregation Phenomena in Response tothe Reactive Environment. Catal. Sci. Technol. 2012, 2, 1787−1801.(28) Tao, F.; Zhang, S. R.; Nguyen, L.; Zhang, X. Q. Action ofBimetallic Nanocatalysts under Reaction Conditions and DuringCatalysis: Evolution of Chemistry from High Vacuum Conditions toReaction Conditions. Chem. Soc. Rev. 2012, 41, 7980−7993.(29) Baber, A. E.; Xu, F.; Dvorak, F.; Mudiyanselage, K.; Soldemo,M.; Weissenrieder, J.; Senanayake, S. D.; Sadowski, J. T.; Rodriguez, J.A.; Matolin, V.; et al. In Situ Imaging of Cu2O under ReducingConditions: Formation of Metallic Fronts by Mass Transfer. J. Am.Chem. Soc. 2013, 135, 16781−16784.

(30) Yang, F.; Choi, Y. M.; Liu, P.; Hrbek, J.; Rodriguez, J. A.Autocatalytic Reduction of a Cu2O/Cu(111) Surface by CO: STM,XPS, and DFT Studies. J. Phys. Chem. C 2010, 114, 17042−17050.(31) Crew, W. W.; Madix, R. J. CO Adsorption and Oxidation onOxygen Precovered Cu(110) at 150 K: Reactivity of Two Types ofAdsorbed Atomic Oxygen Determined by Scanning TunnelingMicroscopy. Surf. Sci. 1996, 356, 1−18.(32) Hartmann, N.; Madix, R. J. Dynamical Rearrangements of the (2× 1)O Adlayer During CO Oxidation on Cu(110). Surf. Sci. 2002,516, 230−236.(33) Wintterlin, J.; Volkening, S.; Janssens, T. V. W.; Zambelli, T.;Ertl, G. Atomic and Macroscopic Reaction Rates of a Surface-Catalyzed Reaction. Science 1997, 278, 1931−1934.(34) Wei, Z. Z.; Li, D. C.; Pang, X. Y.; Lv, C. Q.; Wang, G. C. TheMechanism of Low-Temperature CO Oxidation on IB Group Metalsand Metal Oxides. ChemCatChem 2012, 4, 100−111.(35) Hrbek, J.; Hoffmann, F. M.; Park, J. B.; Liu, P.; Stacchiola, D.;Hoo, Y. S.; Ma, S.; Nambu, A.; Rodriguez, J. A.; White, M. G.Adsorbate-Driven Morphological Changes of a Gold Surface at LowTemperatures. J. Am. Chem. Soc. 2008, 130, 17272−17273.(36) Schnadt, J.; Knudsen, J.; Andersen, J. N.; Siegbahn, H.; Pietzsch,A.; Hennies, F.; Johansson, N.; Martensson, N.; Ohrwall, G.; Bahr, S.;et al. The New Ambient-Pressure X-ray Photoelectron SpectroscopyInstrument at MAX-Lab. J. Synchrotron Radiat. 2012, 19, 701−704.(37) Denecke, R.; Vaterlein, P.; Bassler, M.; Wassdahl, N.; Butorin,S.; Nilsson, A.; Rubensson, J. E.; Nordgren, J.; Martensson, N.;Nyholm, R. Beamline I511 at MAX II, Capabilities and Performance. J.Electron Spectrosc. Relat. Phenom. 1999, 101, 971−977.(38) Borguet, E.; Dai, H. L. Probing Surface Short Range Order andInter-Adsorbate Interactions through Ir Vibrational Spectroscopy: COon Cu(100). J. Phys. Chem. B 2005, 109, 8509−8512.(39) Mudiyanselage, K.; An, W.; Yang, F.; Liu, P.; Stacchiola, D. J.Selective Molecular Adsorption in Sub-Nanometer Cages of a Cu2OSurface Oxide. Phys. Chem. Chem. Phys. 2013, 15, 10726−10731.(40) Wadayama, T.; Kubo, K.; Yamashita, T.; Tanabe, T.; Hatta, A.Infrared Reflection Absorption Study of Carbon Monoxide Adsorbedon Submonolayer Fe-Covered Cu(100), (110), and (111) BimetallicSurfaces. J. Phys. Chem. B 2003, 107, 3768−3773.(41) Hollins, P.; Pritchard, J. Interactions of CO Molecules Adsorbedon Cu(111). Surf. Sci. 1979, 89, 486−495.(42) Pritchard, J.; Catterick, T.; Gupta, R. K. Infrared Spectroscopyof Chemisorbed Carbon-Monoxide on Copper. Surf. Sci. 1975, 53, 1−20.(43) Xu, X. P.; Vesecky, S. M.; He, J. W.; Goodman, D. W. SurfaceSpectroscopic Studies of a Model Silica-Supported Copper Catalyst:Adsorption and Reactions of CO, H2O, and NO. J. Vac. Sci. Technol., A1993, 11, 1930−1935.(44) Hollins, P.; Pritchard, J. Interactions of CO Molecules Adsorbedon Oxidized Cu(111) and Cu(110). Surf. Sci. 1983, 134, 91−108.(45) Mudiyanselage, K.; Kim, H. Y.; Senanayake, S. D.; Baber, A. E.;Liu, P.; Stacchiola, D. Probing Adsorption Sites for CO on Ceria. Phys.Chem. Chem. Phys. 2013, 15, 15856−15862.(46) Peden, C. H. F.; Goodman, D. W.; Weisel, M. D.; Hoffmann, F.M. In situ FT-IRAS Study of the CO Oxidation Reaction overRu(001): I. Evidence for an Eley-Rideal Mechanism at High-Pressures.Surf. Sci. 1991, 253, 44−58.(47) Jiang, P.; Prendergast, D.; Borondics, F.; Porsgaard, S.;Giovanetti, L.; Pach, E.; Newberg, J.; Bluhm, H.; Besenbacher, F.;Salmeron, M. Experimental and Theoretical Investigation of theElectronic Structure of Cu2O and CuO Thin Films on Cu(110) UsingX-ray Photoelectron and Absorption Spectroscopy. J. Chem. Phys.2013, 138, 024704.(48) Garner, W. E.; Stone, F. S.; Tiley, P. F. The Reaction betweenCarbon Monoxide and Oxygen on Cuprous Oxide at RoomTemperature. Proc. R. Soc. London, Ser. A 1952, 211, 472−489.(49) Qadir, K.; Kim, S. M.; Seo, H.; Mun, B. S.; Akgul, F. A.; Liu, Z.;Park, J. Y. Deactivation of Ru Catalysts under Catalytic CO Oxidationby Formation of Bulk Ru Oxide Probed with Ambient Pressure XPS. J.Phys. Chem. C 2013, 117, 13108−13113.



(50) Bluhm, H.; Havecker, M.; Knop-Gericke, A.; Kleimenov, E.;Schlogl, R.; Teschner, D.; Bukhtiyarov, V. I.; Ogletree, D. F.;Salmeron, M. Methanol Oxidation on a Copper Catalyst InvestigatedUsing in Situ X-ray Photoelectron Spectroscopy. J. Phys. Chem. B2004, 108, 14340−14347.(51) Matsumoto, T.; Bennett, R. A.; Stone, P.; Yamada, T.; Domen,K.; Bowker, M. Scanning Tunneling Microscopy Studies of OxygenAdsorption on Cu(111). Surf. Sci. 2001, 471, 225−245.(52) Wiame, F.; Maurice, V.; Marcus, P. Initial Stages of Oxidation ofCu(111). Surf. Sci. 2007, 601, 1193−1204.(53) Lawton, T. J.; Pushkarev, V.; Broitman, E.; Reinicker, A.; Sykes,E. C. H.; Gellman, A. J. Initial Oxidation of Cu(hkl) Surfaces Vicinal toCu(111): A High-Throughput Study of Structure Sensitivity. J. Phys.Chem. C 2012, 116, 16054−16062.(54) Lawton, T.; Kyriakou, G.; Baber, A.; Sykes, E. An Atomic ScaleView of Methanol Reactivity at the Cu(111)/CuOx Interface.ChemCatChem 2013, 5, 2684−2690.(55) Niehus, H. Surface Reconstruction of Cu(111) Upon Oxygen-Adsorption. Surf. Sci. 1983, 130, 41−49.(56) Lahtonen, K.; Hirsimaki, M.; Lampimaki, M.; Valden, M.Oxygen Adsorption-Induced Nanostructures and Island Formation onCu{100}: Bridging the Gap between the Formation of SurfaceConfined Oxygen Chemisorption Layer and Oxide Formation. J.Chem. Phys. 2008, 129, 124703.(57) Imbihl, R.; Ertl, G. Oscillatory Kinetics in HeterogeneousCatalysis. Chem. Rev. 1995, 95, 697−733.(58) Krischer, K.; Eiswirth, M.; Ertl, G. Oscillatory CO Oxidation onPt(110): Modeling of Temporal Self-Organization. J. Chem. Phys.1992, 96, 9161−9172.(59) Yang, F.; Choi, Y.; Liu, P.; Stacchiola, D.; Hrbek, J.; Rodriguez,J. A. Identification of 5−7 Defects in a Copper Oxide Surface. J. Am.Chem. Soc. 2011, 133, 11474−11477.(60) Kandoi, S.; Gokhale, A. A.; Grabow, L. C.; Dumesic, J. A.;Mavrikakis, M. Why Au and Cu Are More Selective Than Pt forPreferential Oxidation of CO at Low Temperature. Catal. Lett. 2004,93, 93−100.(61) Vankooten, W. E. J.; Vannispen, J. P. C.; Gijzeman, O. L. J.;Geus, J. W. Reduction of Cu(111)/O at 298-K with CO. Surf. Sci.1994, 315, 219−226.(62) Sun, B. Z.; Chen, W. K.; Xu, Y. J. Reaction Mechanism of COOxidation on Cu2O(111): A Density Functional Study. J. Chem. Phys.2010, 133, 154502.(63) Baber, A. E.; Yang, X. F.; Kim, H. Y.; Mudiyanselage, K.;Soldemo, M.; Weissenrieder, J.; Senanayake, S. D.; Al-Mahboob, A.;Sadowski, J. T.; Evans, J.; et al. Stabilization of Catalytically Active Cu+

Surface Sites on Titanium−Copper Mixed-Oxide Films. Angew. Chem.,Int. Ed. 2014, 53, 5336−5340.



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Redox-Mediated Reconstruction of Copper during Carbon Monoxide Oxidation