An Ambient Pressure X-ray Photoelectron Spectroscopy and Sc

Article pubs.acs.org/JPCC

Reaction of CO with Preadsorbed Oxygen on Low-Index Copper Surfaces: An Ambient Pressure X‑ray Photoelectron Spectroscopy and Scanning Tunneling Microscopy Study Baran Eren,† Leonid Lichtenstein,† Cheng Hao Wu,†,§ Hendrik Bluhm,‡ Gabor A. Somorjai,‡,§ and Miquel Salmeron*,†,∥ †

Material Sciences Division and ‡Chemical Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, United States § Department of Chemistry, University of California, Berkeley, California 94720-1460, United States ∥ Department of Materials Science and Engineering, University of California, Berkeley, California 94720-1760, United States S Supporting Information *

ABSTRACT: The reaction of CO with chemisorbed oxygen on three low-index faces of copper was studied using ambient pressure X-ray photoelectron spectroscopy (XPS) and high-pressure scanning tunneling microscopy. At room temperature, the chemisorbed oxide can be removed by reaction with gas-phase CO in the 0.01−0.20 Torr pressure range. The reaction rates were determined by measuring the XPS peak intensities of O and CO as a function of time, pressure, and temperature. On Cu(111) the rate was found to be one order of magnitude faster than that on Cu(100) and two orders of magnitude faster than that on Cu(110). The apparent activation energies for CO oxidation were measured as 0.24 eV for O/Cu(111), 0.29 eV for O/ Cu(100), and 0.51 eV for O/Cu(110) in the temperature range between 298 and 473 K. These energies are correlated to the oxygen binding energies on each surface.

1. INTRODUCTION Cu-based catalysts are commonly used in industry for various CO conversion reactions, such as oxidation,1−3 methanol synthesis,4 and water−gas shift.5 CO oxidation is especially important because it is considered a prototype reaction for heterogeneous catalysis.6,7 Therefore, single-crystal Cu surfaces, Cu films, and Cu powders have been investigated in the past decades.1,2,8,9 Ambient pressure techniques such as highpressure scanning tunneling microscopy (HPSTM)10−13 and ambient pressure X-ray photoelectron spectroscopy (APXPS)14,15 have enabled the monitoring of the surface structure with atomic resolution and chemical sensitivity under reaction conditions.16,17 To date, only the CO oxidation reaction on Cu(111) has been reported using these techniques.18,19 Here we use these two techniques to study the CO oxidation reaction on the three lowest-index surfaces of Cu, reconstructed with preadsorbed oxygen. The Cu−O structures on each surface were prepared by dosing 10−1000 Langmuirs of O2 at temperatures between 473 and 600 K. The O-covered Cu surfaces were exposed to CO in the pressure range of 0.01 to 0.2 Torr to simulate oxygen-lean conditions under which no buildup of oxide phases occurs. We found that at room temperature (RT) and under 0.02 Torr of CO, the CO-oxidation reaction rates on Cu(100) and © XXXX American Chemical Society

Cu(110) are one and two orders of magnitude times smaller than on that on Cu(111), repsectively. Reaction kinetics analysis was performed in the temperature range between 298 and 473 K by measuring the oxygen coverage as a function of time during CO exposure. In this way we obtained apparent activation energies of 0.24 eV for O/Cu(111), 0.29 eV for O/ Cu(100), and 0.51 eV for O/Cu(110).

2. METHODS HPSTM and APXPS measurements were performed separately in two different chambers under identical conditions of pressure and temperature. Auger electron spectroscopy (AES) and low-energy electron diffraction (LEED) were used to monitor the chemical composition and structure before and after the reactions. 2.1. Sample Preparation. Clean Cu surfaces were prepared by several cycles of Ar sputtering (1 keV, 15 min) and annealing (793−823 K, 15 min). The clean samples were exposed to 100 Langmuirs of O2 at 473 K for Cu(111),20 1000 Special Issue: Steven J. Sibener Festschrift Received: December 23, 2014 Revised: February 20, 2015

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DOI: 10.1021/jp512831f J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. STM images of the Cu(110)−(2 × 1)-O surface at RT (a) in UHV and (b) under 0.01 Torr CO pressure ∼1 h after CO introduction. Scale bars are 4 nm. Inset: schematic diagram of the reconstructed surface, with white, blue, and red circles representing first layer of Cu atoms, bulk Cu, and oxygen atoms, respectively. (c) Temporal variation of the chemisorbed oxygen coverage measured from the XPS peak areas at RT under 0.02 Torr CO. (d) XPS O 1s spectral region during reaction at 0.2 Torr revealing the presence of chemisorbed O at 529.9 eV, molecularly adsorbed CO at 531.0 eV, and gas-phase CO at 538 eV. (e) Evolution of the adsorbed oxygen and CO coverage obtained from the area of Gaussian− Lorentzian fits to the XPS peaks under 0.2 Torr CO at RT. (f) Temporal variation of the BE and fwhm of the chemisorbed oxygen peak RT at 0.2 Torr CO pressure. The chemisorbed oxygen coverage decreases with time, while the adsorbed molecular CO coverage increases. Coverages are calibrated with respect to the initial XPS intensity which is reproducible within ±10% error between different experiments.

Langmuirs at 520 K for Cu(100),21 and 10 Langmuirs at 600 K for Cu(110),22 which produced ordered O-saturated structures on each crystal. No carbon or other contaminants were detected on the sample surfaces prior to reaction experiments. CO gas was leaked into the chamber, with an initial base pressure in the low 10−10 Torr range, after passing through a carbonyl trap at 513 K. The pressure was measured with a MKS 722A Baratron capacitance pressure gauge. The temperature was measured with chromel−alumel thermocouples attached to the back of the samples. 2.2. APXPS Measurements. APXPS experiments were performed at the BL 11.0.2 endstation of the Advanced Light Source, the Berkeley Lab Synchrotron Facility. Photon energies of 275 eV for Cu 3p, 490 eV for C 1s, and 735 eV for O 1s were used to produce photoelectrons with kinetic energies around 200 eV in all cases. The peak positions were referred to the Fermi level, measured in the same spectrum at the corresponding photon energy. Peak areas and widths were measured from Gaussian−Lorentzian fits of the spectra. Spectra were collected in ultrahight vacuum (UHV) and under CO pressures of 0.02 and 0.2 Torr. Prior to system bake-out, a nitrogen plasma was ignited in the chamber to deplete hydrocarbon and other contaminants emanating from the chamber walls. This greatly reduced adventitious carbon buildup on the sample surface during subsequent high-pressure experiments. 2.3. HPSTM Measurements. HPSTM measurements were performed at RT with a home-built STM instrument housed in a high-pressure cell.12 Gold-coated Pt/Ir tips were used to ensure chemical stability under CO environments. The STM instrument was operated in the constant current mode, with I = 0.8 nA, V = −1.2 V for Cu(111) and Cu(100), and I = 0.8 nA,

V = 1.4 V for Cu(110) (bias voltage applied to the sample). Images were obtained in UHV, and under 0.01 and 0.2 Torr of CO.

3. RESULTS AND DISCUSSION 3.1. Cu(110)−(2 × 1)-O. On Cu(110), oxygen forms a (2 × 1) structure, sometimes referred to as the “added-row structure”, consisting of Cu−O chains along the [001] direction with a coverage of 0.5 monolayers (ML). It was proposed from STM studies at cryogenic temperatures that CO adsorbs on the Cu atoms in the added rows by slightly displacing the Cu atoms from their initial positions.23 Also using STM, it was proposed that around 400 K the Cu−O chains become mobile and that defects form that serve as active sites for the reaction.24,25 STM images of the Cu(110)−(2 × 1)-O surface were obtained in UHV and under 0.01 Torr of CO. Under 0.01 Torr of CO and RT, removal of O is slow, with 5% of the O reacted away in 30 min. Under 0.02 Torr of CO, the coverage decreased from 0.50 to 0.38 after 1 h, as can be observed from the evolution of the APXPS peak intensity in Figure 1c. The LEED patterns, obtained after pumping away the CO, remained unchanged (Figure S1a and b in the Supporting Information), indicating that the oxygen-covered regions of the surface retain the (2 × 1) structure. When the CO pressure was increased to 0.2 Torr, the O-removal rate not only increased but accelerated with time because of the increased availability of clean Cu sites,26 resulting in the complete removal of O after 15 min, as shown in Figure 1e and Figure S1c in the Supporting Information. The evolution of the XPS O 1s region during the reaction is shown in Figure 1d. The spectra reveal the presence of two Ocontaining species: chemisorbed O at 529.9 eV and molecularly adsorbed CO, which produces the peak at 531.0 eV. The main B


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Figure 2. STM images of the Cu(100)−(2√2 × √2 R45°)-O surface at RT (a) in UHV and (b) under 0.01 Torr of CO after 0.5 h following CO introduction. The second image is presented in derivative mode to enhance contrast. Scale bars: 5 nm. Inset in (a) is a schematic of the reconstructed surface. (c) Temporal variation of the chemisorbed O coverage at RT under 0.02 Torr. (e) Same under 0.2 Torr CO, including the adsorbed CO coverage. (f) Temporal variation of the subsurface oxygen peak intensity under 0.2 Torr CO pressure at RT. Panels e and f are obtained after fitting the O 1s spectra. In panels c and e, the O coverage is given after the contribution from the subsurface oxygen was subtracted.

Figure 3. (a) Schematic of the surface oxide layer on Cu(111). STM images of the surface at RT in UHV (panels b and c, repsectively) and after reaction under 0.01 Torr CO (d). Scale bars are 4 nm. The ordered features in panels b and c correspond to the 73 R5.8° × 21 R10.9° surface oxide structure. In panel d, no ordered structure can be resolved on the terraces and the step edges appear fuzzy, characteristic of the clean metallic Cu. Image b is shown as a derivative mode to enhance contrast. (e) Temporal evolution of the XPS O 1s spectrum at 0.02 Torr CO pressure.

and satellite peaks in the O 1s and C 1s region of the spectra are discussed in the Supporting Information. The binding energy and full width at half-maximum (fwhm) of the chemisorbed O peak remained unchanged during the reaction (Figure 1f), indicating that oxygen maintains the same chemical environment during the reaction. In the STM images the disappearance of the (2 × 1)-O structures is revealed by the lack of features and by the appearance of frizzled step edges separating the clean terraces, due to fast diffusion of Cu atoms, as found by previous authors.27 Figure 1e shows the increase in the amount of adsorbed CO as free Cu sites are created as a result of the reaction. From the peak intensity, the total CO coverage after

complete removal of the chemisorbed O was estimated to be 0.22 ML under 0.2 Torr CO pressure at RT. 3.2. Cu(100)−(2√2 × √2 R45°)-O. Oxygen produces a 2√2 × √2 R45° (“missing-row”) structure on Cu(100) with 0.5 ML coverage, where every fourth Cu row is squeezed out while oxygen occupies pseudohollow sites similar to the hollow sites on the bare surface but is 3-fold coordinated because of the missing Cu rows (Figure 2a inset).28 Missing rows can appear both in the [001] and [010] directions because these are equivalent on the (100) surface. Figure 2a shows an STM image acquired in UHV of the initial surface structure as explained in ref 29. Under 0.01 Torr of CO the two domains of missing-row reconstruction are visible for up to 1 h, as shown in Figure 2b, indicating that the CO + O reaction is slow at this C


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Figure 4. Semilog plots of the oxygen coverage ratio at 0.02 Torr CO pressure for (a) Cu(110) and (b) Cu(100), used to obtain the Arrhenius plots in panels c and d. The slopes provide the apparent reaction activation energies. In panel d, the black and red data points are extracted from fits to the initial and final linear regimes in panel b.

previous surfaces, such that after ∼15 min in 0.01 Torr of CO the in situ STM images revealed no features due to the surface oxide layer. The XPS O-peak vanished completely in ∼260 s in 0.02 Torr CO at RT (Figure 3e). 3.4. Reaction Kinetics. In the reactions studied here the concentration of chemisorbed O, set at the start, decreases as a result of the reaction with CO:

pressure at RT. Under 0.02 Torr of CO, the O coverage measured in situ by its XPS peak area decreases more rapidly, as shown in Figure 2c, so that after 3100 s almost half of the chemisorbed O was removed. The LEED pattern obtained in UHV afterward showed that the 2√2 × √2 R45° periodicity persists after 3100 s (Figure S2a in the Supporting Information), although with decreased intensity of the spots from the O-structure. At 0.2 Torr of CO at RT the initial removal of chemisorbed oxygen proceeds 4 to 5 times faster than at 0.02 Torr, as shown in Figure 2e. The evolution of the XPS peaks in the O 1s region is shown in Figure 2d, with the peaks of chemisorbed O and CO appearing at approximately the same binding energies as in Cu(110). An additional feature, in the form of a shoulder at 0.4 eV lower binding energy from the main peak, is also visible. This can be attributed to subsurface oxygen, as proposed in previous work.30 This subsurface oxygen is likely the result of the high oxygen exposure used to prepare the sample. As can be seen in Figure 2e, the removal rate of chemisorbed oxygen decreases after approximately 10 min. We attribute this to replenishment of surface O by diffusion of species from the subsurface region. Figure 2f shows the increase of the intensity of the subsurface oxygen peak with time, which is due to the decreased attenuation from the surface oxygen layer as it is being reacted away (the photoelectron inelastic mean free path is 0.88 nm for 530 eV electrons31). After around 1500 s at 0.2 Torr CO the STM images were found to be similar to those from the clean surface, and the intensity of the superlattice spots in the LEED pattern had decreased substantially (Figure S2b in the Supporting Information). The saturation CO coverage in 0.2 Torr at RT was about 4 times smaller than on Cu(110) (Figure 2e), which agrees with the expected values calculated using the reported CO adsorption energies on Cu(100) and Cu(110) (0.53 eV versus 0.57 eV).32 3.3. Cu(111)−( 73 R5.8° × 21 R10.9°)-O. Upon O chemisorption on Cu(111), a complex surface oxide structure is formed (Figure 3a), which has been interpreted as consisting of a buckled hexagonal Cu−O layer with additional oxygen atoms at the center of the hexagons forming a 73 R5.8° × 21 R10.9° periodicity.33 It was previously shown that reduction of this structure by CO is a two-step process, involving first the removal of the central oxygen atom, followed by the removal of the ring oxygen.18,19 Figure 3b,c shows STM images in UHV of the Cu(111) surface after formation of the surface O-layer. Removal of O by reaction with CO occurred much faster than that on the

CO(g) → CO(ads) followed by CO(ads) + O(ads) → CO2(ads) → CO2 (g) (1)

To determine the kinetics parameters of the reaction and the apparent activation energies (Eapp), we measured surface coverages using the oxygen O 1s XPS intensities and followed their evolution as a function of time. We analyze the results using the mean-field approximation: dno = −k rnOnCO dt

(2)

where kr is reaction rate, nx concentration of x species (O and CO) on the surface corresponding to a coverage θx = nx/N, and N the number of surface Cu sites per unit area. We assume also that the chemisorbed molecular CO is always in steady-state equilibrium with the gas phase CO (i.e., the CO desorption rate is much faster than the CO−O reaction rate): θCO = (1 − θO)

1 = (1 − θO)A 1 + (kdN )/(ICOσCO)

(3)

where kd is CO desorption rate, ICO incident CO flux (ICO = PCO /(2πmkTgas)1/2), and σCO CO sticking coefficient assumed to be constant. Under this assumption, A is a constant for a given temperature and pressure that can be directly measured using the 0.2 Torr data presented in Figures 1e and 2e. The measurements of θCO and θo from the XPS peak areas during the reaction on Cu(100) and for the later stages of the reaction (after 300 s) on Cu(110) validated the assumption. On Cu(111), however, this could not be verified because of the low value of θCO. The rate eq 2 can be integrated to give θO/(1 − θO) = Be−krNAt

(4)

The integration constant B = 1 is determined from the measured initial O coverage θO = 0.5. Combining eqs 3 and 4 D


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The Journal of Physical Chemistry C and using the fact that θCO is very small at 0.02 Torr and T > RT, we obtain ln(θO/(1 − θO))/t = −(k r /kd)ICOσCO

(5)

Finally, kr/kd depends on Eapp through the Arrhenius’ equation: k r /kd = k 0 exp( −(Eact − E b)/kT )

(6)

where Eapp = (Eact − Eb) is the apparent activation energy, equal to the difference between the reaction activation energy and the CO binding energy. With this mean-field approximation and using the slopes of Figure 1c and 4a, we obtained a linear fit for Cu(110) with Eapp = 0.51 eV (Figure 4c). For Cu(100), we used the initial phase of the reaction where kr is faster than the oxygen replenishment from the subsurface and the second phase where kr and oxygen diffusion reached steady state. The nonlinear transient regime is not analyzed. From Arrhenius plots of these two linear regimes (Figures 2c and 4b), we obtain Eapp values of 0.29 eV for the initial stage and 0.37 eV for the final stage where the reaction is probably bulk diffusion limited. By adding published values of the CO−Cu binding energies32 to Eapp we obtain true activation energies of 1.08 eV for Cu(110) and 0.82 eV for Cu(100). Experiments on Cu(111) were performed at a pressure of 5 × 10−3 Torr of CO and lower temperature range (310−355 K) to slow the oxygen removal to a measurable rate. From the kinetic and Arrhenius plots, shown in Figure S4 in the Supporting Information, we obtain Eapp = 0.24 eV, corresponding to a true activation energy of 0.74 eV. 3. 5. Comparison between Different Crystal Faces. As we have seen, at 0.02 Torr at RT the initial CO + O reaction rates on Cu(100) and Cu(110) are one and two orders of magnitude slower than on Cu(111), respectively. Such a substantial difference can be related to differences in oxygen binding energies, which were calculated as 2.0−2.1, 1.8, and 1.5−1.6 eV for Cu(110), Cu(100), and Cu(111), respectively.34,35 The reaction conditions are less favorable at higher oxygen binding energies, a manifestation of the Sabatier effect. From our results and from literature data, the energy diagram in Figure 5 can be constructed. The energy level at the transient state is not very different on different faces, which leads to the conclusion that the difference in activation energies between each crystal face originates mostly from differences in oxygen binding energies on different surfaces. Again, this is a consequence of the Sabatier effect, i.e., poisoning of the catalyst surface with the more strongly adsorbed species. The binding energy of O is therefore a good descriptor of the catalytic activity for CO oxidation on the different surface orientations of Cu, similar to the case of gold catalysts studied by Nørskov and co-workers.36

Figure 5. Energy diagram for the CO oxidation reaction on different Cu faces. The CO binding energies (0.5, 0.53, and 0.57 on Cu(111), Cu(100), Cu(110), respectively) are from ref 32. The O binding energies (1.6, 1.8, and 2.1 eV on Cu(111), Cu(100), and Cu(110), respectively) are from refs 34 and 35. The energy levels in the transition state (TS) are not much different on the 3 surfaces, indicating that the activation energies are highly correlated with the oxygen binding energies on each surface. The overall change in free energy of 5.47 eV is the sum of half of the O2 dissociation energy and the CO2 formation energy in the gas phase.

measuring the CO and O coverage with APXPS as a function of time and temperature, we obtained apparent activation energies of 0.24 eV for Cu(111), 0.29 eV for Cu(100), and 0.51 eV for Cu(110). The difference in reactivity on the three crystal surfaces is correlated to the oxygen binding energies on the surfaces, indicating that the O chemisorption energy is a good descriptor of the CO oxidation reaction on Cu surfaces.

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ASSOCIATED CONTENT

S Supporting Information *

LEED images, sample C 1s XPS spectra, and reaction rate plots on Cu(111). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +1 510-486-6704. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Office of Basic Energy Sciences (BES), Division of Materials Sciences and Engineering, of the U.S. Department of Energy (DOE) under Contract DE-AC02-05CH11231, through the Chemical and Mechanical Properties of Surfaces, Interfaces and Nanostructures program. B.E. acknowledges the Early Postdoc Mobility fellowship from the Swiss National Research Funds (SNF). L.L. acknowledges support by the Alexander von Humboldt Foundation. C.H.W. acknowledges the ALS Doctoral Fellowship in Residence.

4. CONCLUSION We used in situ imaging and spectroscopy (HPSTM and APXPS) techniques to study the CO oxidation reaction on lowindex Cu surfaces with a preadsorbed oxygen layer. This is a model system for the catalytic oxidation of CO in O-lean conditions. We have shown that the structure of the surfaces with the ordered O overlayer remains unaltered during the reaction as long as there is O on the surface. At room temperature under 0.02 Torr of CO, the CO + O reaction rate on Cu(111) is one order of magnitude higher than on Cu(100) and 2 orders of magnitude higher than on Cu(110). By

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REFERENCES

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