Active Surface Oxygen for Catalytic CO Oxidation on Pd(100

Letter pubs.acs.org/JPCL

Active Surface Oxygen for Catalytic CO Oxidation on Pd(100) Proceeding under Near Ambient Pressure Conditions Ryo Toyoshima,† Masaaki Yoshida,† Yuji Monya,† Kazuma Suzuki,† Bongjin Simon Mun,§,∥ Kenta Amemiya,‡ Kazuhiko Mase,‡ and Hiroshi Kondoh*,† †

Department of Chemistry, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522, Japan Institute of Materials Structure Science, High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan § Department of Applied Physics, Hanyang University, ERICA 426-791, Republic of Korea ∥ Ertl Center for Electrochemistry and Catalysis, Gwangju Institute of Science and Technology, Gwangju 500-712, Republic of Korea ‡

S Supporting Information *

ABSTRACT: Catalytic CO oxidation reaction on a Pd(100) single-crystal surface under several hundred mTorr pressure conditions has been studied by ambient pressure X-ray photoelectron spectroscopy and mass spectroscopy. In-situ observation of the reaction reveals that two reaction pathways switch over alternatively depending on the surface temperature. At lower temperatures, the Pd(100) surface is covered by CO molecules and the CO2 formation rate is low, indicating CO poisoning. At higher temperatures above 190 °C, an O−Pd−O trilayer surface oxide phase is formed on the surface and the CO2 formation rate drastically increases. It is likely that the enhanced rate of CO2 formation is associated with an active oxygen species that is located at the surface of the trilayer oxide. SECTION: Surfaces, Interfaces, Porous Materials, and Catalysis

C

reactivity.16 Theoretical simulations have been employed to shed light on the reaction mechanisms.22−29 For instance, they suggest the Eley−Rideal (ER) mechanism on oxide surfaces24 and another reaction mechanism where CO is adsorbed on metallic patches and reacts with oxygen atoms on the edges of oxide islands.25,26 The clear understanding of the reaction mechanism at the atomic level under realistic conditions remains unclear and requires much further investigation. In the case of Pd(100), the interactions of oxygen with the substrate surface have been studied under various pressures and temperatures conditions,31−37 which reveal that the metal surface is partly or completely oxidized under mTorr O2 pressures and at elevated temperatures to form a (√5 × √5)R27° surface oxide and a PdO bulk oxide. For the (√5 × √5)R27° surface oxide, a detailed atomic-scale structure model has been proposed based on surface science techniques.31−34 Recent in situ CO oxidation observation under high pressure conditions using surface X-ray diffraction (SXRD) demonstrated that the catalytic activity drastically increased when the surface was oxidized to the (√5 × √5)R27° surface oxide and/or the PdO bulk oxide.12,13 The ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) technique provides information on the chemical state of the oxide as well as the metallic Pd. It also enables us to distinguish oxygen species with different chemical environments in the oxide. In this sense, the

atalytic oxidation of carbon monoxide (CO) on Pt-group metal surfaces has been widely studied over the past decades. In these studies, single crystals or metal particles supported on oxides are used for the fundamental understanding of the prototypical catalytic surface reaction or industrial technologies (i.e., automotive exhaust converter, methane combustion system).1−29 As well known, under ultrahigh vacuum (UHV) conditions,1−7 the reaction generally proceeds via the Langmuir−Hinshelwood (LH) mechanism where a chemisorbed CO and a chemisorbed O encounter and react on the metal surface. In the LH model, the reacting surface remains as metallic state, and no oxide species are formed during the reaction. Recently, with the development of high-pressure experiment techniques, several studies under realistic pressure conditions have been conducted and suggested another possible reaction mechanism involving oxide species on the metal substrate,8−16 e.g., the Mars−van Krevelen (M-vK) mechanism,30 which is different from the conventional LH mechanism. In the M-vK mechanism, the catalytically active phase occurs when the surface forms the oxide species during reaction. Between these two models, i.e., LH versus M-vK, the relationship between chemical state and catalytic activity has been under debate for many years. Some of in situ surface observations have proposed that oxide phases are active species for CO oxidation reaction.8−15 Oxygen species chemisorbed on the metallic surfaces also have been proposed as highly active species from in situ reaction monitoring under ambient-pressure conditions.17−20 Recent CO titration experiments revealed that the chemisorbed O and the surface oxide exhibited almost the same © 2012 American Chemical Society

Received: September 13, 2012 Accepted: October 16, 2012 Published: October 19, 2012 3182

dx.doi.org/10.1021/jz301404n | J. Phys. Chem. Lett. 2012, 3, 3182−3187

The Journal of Physical Chemistry Letters

Letter

AP-XPS is a technique complementary to the SXRD and gives more detailed chemical information. In this work, we studied the CO oxidation reaction on a Pd(100) single crystal surface under 100 mTorr pressures of CO and O2 gases at 30−370 °C by means of in situ observation based on the AP-XPS and differential-pumping mass spectroscopy (MS). The present study provides a direct spectroscopic observation of a transition from a CO-poisoning state to a completely surface-oxide-covered state concomitantly with a drastic jump of CO2 formation rate. Furthermore, our in situ observation gives evidence for site-specific CO oxidation of the surface oxide. Such chemical identification of the reactive oxygen species as well as the active oxide phase has been achieved by the operando analysis for the CO oxidation on Pd(100). All experiments were performed with an AP-XPS system at soft X-ray beamline 13A at the Photon-Factory of High Energy Accelerator Research Organization (KEK-PF) in Tsukuba, Japan.38 A clean ordered surface of Pd(100) single-crystal (MaTech, 10 mmφ × 1 mm, 99.999% quality) was achieved by using the well-established procedure and checked by XPS in a high-pressure chamber. The CO and O2 gases were introduced into the high-pressure chamber up to several hundred millitorr by a variable leak valve and a pyrolytic boron nitride (PBN) heater was applied for the sample heating. The photon energy of 435 eV was used for Pd3d5/2 and C1s, 650 eV for O1s region. Details about the experimental methods are described in the Supporting Information and elsewhere.15 We first report the CO adsorption behavior on Pd(100). Previously, the CO adsorption on Pd single crystal surfaces has been investigated under wide ranges of pressure and temperature.39−45 It is known that CO molecules are adsorbed at bridge sites on the (100) surface, and they form several adsorption structures depending on the substrate temperature. Under UHV conditions, a p(2√2 × √2)R45° (θ = 0.50) structure is formed at room temperature, and a further exposure at lower temperatures gives rise to a p(3√2 × √2)R45° (240 K, θ = 0.67) and a p(4√2 × √2)R45° (160 K, θ = 0.75) structure.40 Andersen et al. reported core-level binding energies of Pd3d5/2 and C1s for each CO adsorption structure using a high-resolution XPS measurement under UHV conditions.41 They revealed that the CO adsorption structure affected the electronic structure of surface Pd atoms effectively. Figure 1 shows the XPS observation during the CO exposure up to 1 × 10−2 Torr at room temperature in Pd3d5/2 (a) and C1s (b) regions. Before CO gas introduction (UHV), the Pd3d5/2 XP spectrum exhibits surface (light green colored) and bulk (blue colored) components. A small hump at 284 eV of the C1s XP spectrum is due to residual carbon. At 1 × 10−7 Torr CO pressure, the bulk metal component and an additional higher binding energy component at 335.5 eV (CO(I); green colored) are observed in Pd3d5/2, and a single peak is observed at 285.9 eV in C1s region. These results indicate the formation of the p(2√2 × √2)R45° structure, being consistent with the previous study,41 as shown in Figure 1c. The CO molecules are bound to bridge sites, and all the surface Pd atoms are coordinated by one CO molecule. At a higher pressure of 1 × 10−2 Torr, a new component was observed at 335.9 eV (CO(II); orange colored) in Pd3d5/2 region. The binding energy is in good agreement with that of the two CO-coordinated Pd reported by measurements at lower temperatures under UHV.41 We also confirmed the assignments for the CO(I) and CO(II) components using core-

Figure 1. XP spectra of a Pd(100) surface taken under different CO pressures up to 1 × 10−2 Torr at room temperature for Pd3d5/2 (a) and C1s (b). The Pd3d5/2 spectrum for a Pd surface under UHV is deconvoluted into two components; surface component (334.4 eV; light green colored) and bulk component (334.9 eV; blue colored). Under exposure of CO at 1 × 10−7 Torr, the surface component shifts to a higher binding energy of 335.5 eV (CO(I); green colored). The C1s spectrum exhibits a single peak, which is attributed to CO chemisorbed on bridge sites. This CO adsorption phase is identified as a p(2√2 × √2)R45° structure as shown in (c). CO gas exposure at 1 × 10−2 Torr causes the appearance of another component at 335.9 eV (CO(II); orange colored). This phase is attributed to a p(3√2 × √2)R45° structure as shown in (d). The green and orange balls show one-CO and two-CO coordinated Pd atoms, respectively. The dark red balls correspond to CO molecules adsorbed at bridge sites. Rectangles indicate the unit cells of the CO adsorption structures.

level shift (CLS) calculations based on the density functional theory (DFT) (see Supporting Information). The intensity ratio of CO(II)/CO(I) is about 0.5 at 1 × 10−2 Torr. This ratio corresponds well to the p(3√2 × √2)R45° structure as illustrated in Figure 1d, where orange and green colored Pd atoms are coordinated by two and one CO molecule(s), respectively. Only a slight shift of approximately 0.1 eV toward higher binding energy side is observed in the corresponding C1s spectrum, which suggests that all the CO molecules still occupy the bridge sites. Thus, these Pd3d5/2 and C1s XP spectra lead us to consider that the CO molecules form the p(3√2 × √2)R45° structure at 1 × 10−2 Torr even at room temperature, though it can be formed at a low temperature under UHV conditions. This CO adsorption phase appears reversibly depending on the CO gas pressure. Up to now, it has been suggested that high-pressure CO exposure causes the CO dissociation on supported Pd nanoparticles,46 and stepped and even close-packed Pt singlecrystal surfaces.47,48 In our experimental conditions, however, we confirm that the CO dissociation does not occur and no carbide component appears. The experiments on the single3183



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carbon. Its integrated intensity is about 1/4 of the total C1s intensity. Even if all of the residual carbon segregates at the surface, a major part of the surface is dominated by adsorbed CO. However, the presence of residual carbon affects the Pd3d5/2 spectra, because the Pd atoms binding to the residual carbon give a Pd3d5/2 component at around 335.4 eV. In fact, the Pd component at 335.5 eV (green colored) shown in Figure 2b appears larger than that of the CO-only experiment (see Figure 1a). At point B (175 °C), the CO2 MS intensity is still low, but the coverage of adsorbed CO is decreased, which corresponds to the p(2√2 × √2)R45°-like structure judging from the Pd3d5/2 regions (Figure 1c). This indicates that both the CO adsorption layers hinder the oxygen adsorption and hence suppress the catalytic oxidation reaction. The O1s XP spectrum exhibited no component induced by chemisorbed O (not shown). Note that the CO2 intensity is almost constant between 30 and 175 °C. This suggests that the activation energy for the CO oxidation is close to zero under the CO-poisoning condition. In our previous fast-XPS study on CO oxidation on Pd(111) under UHV conditions we found two different reaction pathways depending on the density of adsorbed oxygen: (1) a high-density oxygen domain obeys the first-order reaction kinetics with an activation energy of 0.29 ± 0.03 eV and (2) a low-density oxygen domain exhibits CO oxidation exclusively at the domain boundary with an activation energy of 0.04 ± 0.02 eV.4 Such an extremely small activation energy for the latter path could be explained in terms of a precursor-mediated reaction mechanism where a weakly bound species at the domain boundary is involved in the reaction as a precursor.4 Under the CO-poisoning condition as indicated by the present XPS results, the CO oxidation reaction proceeds probably at the domain boundaries. Therefore it is not unreasonable to assume a very small activation energy for the present COpoisoning state. Another possibility for the absence of the temperature dependence is that the MS intensity of CO2 observed between 30 and 175 °C is not due to CO2 formation from the CO-poisoning Pd surface but mainly associated with CO2 generation on the chamber wall and/or a cross-talk signal when O2 and CO gases are introduced. Although the origin for the absence of the temperature dependence is not clear, it is certain that the CO2 formation rate is low under the COpoisoning conditions. Just above the critical temperature (190 °C), at point C, three new components appear in the Pd3d5/2 XP spectrum, while the CO-associated components completely disappear both from the Pd3d5/2 and C1s XP spectra. Since these binding energies are in good agreement with those of the (√5 × √5)R27° surface oxide on a metallic surface,33−35 each component is assigned as follows: (1) the 334.6 eV peak (gray colored): Pd atom in the interface layer situating between the surface-oxide layer and the metallic bulk Pd, (2) the 335.4 eV peak (pink colored): 2-fold Pd atom of the (√5 × √5)R27° surface oxide which is coordinated by two oxygen atoms, and (3) the 336.2 eV peak (red colored): 4-fold Pd of the surface oxide. These assignments are consistent with our calculated CLSs as shown in Table S1. No evidence for the presence of the chemisorbed oxygen species on the metallic Pd or the surface oxide is found from the Pd3d5/2 region. We confirm that the chemisorbed O species gives peaks experimentally at 335.5 and 529.6 eV in the Pd3d5/2 and O1s regions, respectively (see Figure S1). The binding energy of Pd bound to the chemisorbed oxygen is close to that of the 2-fold

crystal Pt surfaces were carried out above 40 Torr of CO pressure,47,48 The absence of surface defects and the difference in CO pressure in the present case may explain the nondissociation behavior. Next we present in situ monitoring data of MS and AP-XPS taken for CO oxidation reaction under a near ambient-pressure condition in Figure 2. The CO and O2 gases were introduced

Figure 2. (a) Temperature dependence of MS intensity of CO2 under exposure to 2 × 10−1 Torr of O2 and 2 × 10−2 Torr of CO. The CO2 background contribution from the residual gas was subtracted. Each label (A, B, C, and D) indicates temperature where XP spectra were measured. (b,c) XP spectra recorded at Pd3d5/2 region and C1s region, respectively, which indicate a drastic change in surface chemical state at around 190 °C. XP spectra denoted by “E” were taken at 370 °C under PO2 = 2 × 10−1 Torr, after stopping the CO gas dose.

with pressures of 2 × 10−2 and 2 × 10−1 Torr, respectively. Figure 2a shows MS intensity changes of CO2 (m/e = 44) as a function of surface temperature. As increasing the temperature, the CO oxidation reaction is drastically increased above 190 °C. However, a further heating causes a decay of the reactivity. Labels A, B, C, and D in Figure 2a indicate temperatures where XPS measurements are done and correspond to each XP spectrum denoted by the labels in the Pd3d5/2 and C1s regions (Figure 2b,c). At room temperature (point A in Figure 2a), the CO2 MS intensity is low. The corresponding XP spectra show that a large amount of CO molecules are adsorbed on the metallic Pd surface. From the peak deconvolution, it is likely that the CO molecules form the p(3√2 × √2)R45° structure (θ = 0.67) shown in Figure 1d. This phase can be regarded as “COpoisoning”. No carbonyl species is detected in the C1s region. It is noted that a weak and broad feature can be seen at around 283−285 eV in the C1s region. This is ascribed to residual 3184



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surface oxide, and the bulk oxide formation is hindered even at the higher temperatures above 230 °C. Under the highly reactive condition (point C and D), we measured O1s XP spectra as shown in Figure 3a. Two O1s

Pd. However, the peak intensity ratio of 2-fold Pd/4-fold Pd in Figure 2b is almost unity, i.e., the (√5 × √5)R27° surface oxide includes the 2-fold Pd and the 4-fold Pd with an atomic ratio of 1:1 as shown in Figure S3. That is, the surface oxide does not coexist with the chemisorbed oxygen. In addition, the absence of bare surface (metallic) Pd at 334.4 eV supports this explanation. In the case of CO oxidation on Pt(111), Li et al. have suggested that metallic and oxide domains coexist under reaction conditions and that CO molecules adsorbed on the metallic domain react with oxygen atoms at the edge of oxide domain.25 However, since our XPS results do not show the presence of the metallic surface Pd nor CO adsorbed on the metallic Pd surface, the surface oxide prevails over the surface under the active reaction condition. The C1s XP spectrum exhibits no peak. We confirm that the gas introduction sequence (CO first followed by addition of O2 and vice versa) is not crucial to the formation of the reactive surface state (see Figures S4 and S5). It is noted that when only the CO is introduced to the surface above the critical temperature, CO molecules can readily adsorb on the metallic Pd surface as observed in Pd3d5/2 XPS (see Figure S5). This suggests that the formation of the reactive surface structure is not induced by spontaneous CO desorption and subsequent oxidation but by destruction and removal of the poisoning CO layer due to faster O2 adsorption compared to CO adsorption. Since the CO adsorption proceeds via the nonactivated precursor-mediated adsorption mechanism, the adsorption rate decreases with temperature, while in the case of O2 adsorption, it is promoted at elevated temperatures due to a thermal activation of dissociation. The adsorption rate of CO is much faster at lower temperatures but is surpassed by the O2 adsorption rate at higher temperatures. The critical temperature (190 °C), where the surface situation is completely reversed, would be associated with the switchover of adsorption rate. The CO2 formation rate is gradually decreased with temperature above the critical temperature. At 370 °C, point D, the Pd3d5/2 XP spectrum clearly shows that the surface oxide peaks are reduced in intensity. Instead, a new component appears at 335.9 eV, which is assigned as a “partly reduced” surface Pd species.35 This is also supported by the CLS calculations as shown in Table S1 (see Supporting Information). A part of Pd atoms in the surface oxide is reduced to less coordinated Pd by the removal of oxygen via the reaction with CO. The drop in reaction rate could be originated from the decrease in the amount of the surface oxide. At high temperatures, since the catalytic reaction itself proceeds more quickly, the bottleneck of the reaction is recovery of the surface oxide. If the reaction path obeys the M-vK mechanism, the CO adsorbs on the oxide and reacts with oxygen included in the oxide. Therefore, the generation of reduced species decreases the amount of reaction active site and CO2 formation rate. When we stopped CO exposure with maintaining O2 exposure at 370 °C, the PdO bulk oxide was clearly observed as seen at E, where the 4-fold Pd species in the bulk PdO newly appears at 336.5 eV (brown colored).35 This result indicates that CO is certainly provided to the X-ray irradiation spot under the reaction conditions. Therefore the spot is not under the CO-depleted condition during the course of reaction (A− D). The PdO bulk oxide is formed above 230 °C under the O2only dose (2 × 10−1 Torr) condition (not shown). Under the reaction conditions, however, the Pd surface is covered by the

Figure 3. O1s XP spectra taken under CO oxidation reaction conditions: at point C, where the reaction rate is the maximum (bottom), and at point D, where the reaction rate decreases a little (top). (b) Side view of a model structure for the (√5 × √5)R27° surface oxide on Pd(100). Large and small balls show Pd and O atoms, respectively. All balls are color-coded, based on the colors of XPS components deconvoluted in Figure 2a and panel a. Note that a broad feature at a higher binding energy (black solid line) is attributed to Pd 3p3/2 photoelectrons from the substrate.

peaks are observed at 528.8 eV (yellow colored) and 529.6 eV (deep blue colored) with almost equal intensities at point C and the both peaks are associated with the (√5 × √5)R27° surface oxide.33,35 The (√5 × √5)R27° surface oxide has an O−Pd−O trilayer structure. The yellow colored component is attributed to the upper-side O species, while the deep blue colored component arises from the oxygen species located below the surface Pd layer33−35 as illustrated in Figure 3b, where each color of balls corresponds to each color of peaks deconvoluted in Figures 2b and 3a. These assignments are also supported by the CLS calculations (Table S1). At point D, where the reactivity became lower than that at point C, the intensity of the lower-binding-energy component decreases, while the higher-binding-energy component remains almost unchanged, which indicates that the upper-side O species is reactive and exclusively consumed. This behavior is interpreted as selective reduction of the surface oxide via the M-vK like mechanism. It is difficult to distinguish the lower-side O species and chemisorbed oxygen from O 1s XPS. Then, even if there is a trace of chemisorbed oxygen, it is not the active phase. 3185



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(4) Nakai, I.; Kondoh, H.; Shimada, T.; Resta, A.; Andersen, J. N.; Ohta, T. Mechanism of CO Oxidation Reaction on O-Covered Pd(111) Surfaces Studied with Fast X-ray Photoelectron Spectroscopy: Change of Reaction Path Accompanying Phase Transition of O Domains. J. Chem. Phys. 2006, 124, 224712−224719. (5) Nagasaka, M.; Kondoh, H.; Nakai, I.; Ohta, T. CO Oxidation Reaction on Pt(111) Studied by the Dynamic Monte Carlo Method Including Lateral Interactions of Adsorbates. J. Chem. Phys. 2007, 126, 044704−044710. (6) Nagarajan, S.; Thirunavukkarasu, K.; Gopinath, C. S. A Revisit to Carbon Monoxide Oxidation on Pd(111) Surfaces. J. Phys. Chem. C 2009, 113, 7385−7397. (7) Zheng, G.; Altman, E. I. The Reactivity of Surface Oxygen Phases on Pd(100) Toward Reduction by CO. J. Phys. Chem. B 2002, 106, 1048−1057. (8) Hendriksen, B. L. M.; Frenken, J. W. M. CO Oxidation on Pt(110): Scanning Tunneling Microscopy Inside a High-Pressure Flow Reactor. Phys. Rev. Lett. 2002, 89, 046101−046104. (9) Hendriksen, B. L. M.; Bobaru, S. C.; Frenken, J. W. M. Oscillatory CO Oxidation on Pd(100) Studied with In Situ Scanning Tunneling Microscopy. Surf. Sci. 2004, 552, 229−242. (10) Ackerman, M. D.; Pedersen, T. M.; Hendriksen, B. L. M.; Robach, O.; Bobaru, S. C.; Popa, I.; Quiros, C.; Kim, H.; Hammer, B.; Ferrer, S.; et al. Structure and Reactivity of Surface Oxides on Pt(110) during Catalytic CO Oxidation. Phys. Rev. Lett. 2005, 95, 255505− 255508. (11) Gustafson, J.; Westerström, R.; Balmes, O.; Resta, A.; van Rijn, R.; Torrelles, X.; Herbschleb, C. T.; Frenken, J. W. M.; Lundgren, E. Catalytic Activity of the Rh Surface Oxide: CO Oxidation over Rh(111) under Realistic Conditions. J. Phys. Chem. C 2010, 114, 4580−4583. (12) van Rijn, R.; Balmes, O.; Resta, A.; Wermeille, D.; Westerström, R.; Gustafson, J.; Felici, R.; Lundgren, E.; Frenken, J. W. M. Surface Structure and Reactivity of Pd(100) during CO Oxidation near Ambient Pressures. Phys. Chem. Chem. Phys. 2011, 13, 13167−13171. (13) Hendriksen, B. L. M.; Ackermann, M. D.; van Rijn, R.; Stoltz, D.; Popa, I.; Balmes, O.; Resta, A.; Wermeille, D.; Felici, R.; Ferrer, S.; Frenken, J. W. M. The Role of Steps in Surface Catalysis and Reaction Oscillations. Nat. Chem. 2010, 2, 730−734. (14) Blume, R.; Hävecker, M.; Zafeiratos, S.; Teschner, D.; Kleimenov, E.; Knop-Gericke, A.; Schlögl, R.; Barinov, A.; Dudin, P.; Kiskinova, M. Catalytically Active States of Ru(0001) Catalyst in CO Oxidation Reaction. J. Catal. 2006, 239, 354−361. (15) Toyoshima, R; Yoshida, M.; Monya, Y.; Kousa, Y.; Suzuki, K.; Abe, H.; Mun, B. S.; Amemiya, K.; Mase, K.; Kondoh, H. In Situ Ambient Pressure XPS Study of CO Oxidation Reaction on Pd(111) Surfaces. J. Phys. Chem. C 2012, 116, 18691−18697. (16) Butcher, D. R.; Grass, M. E.; Zeng, Z.; Aksoy, F.; Bluhm, H.; Li, W.-X.; Mun, B. S.; Somorjai, G. A.; Liu, Z. In Situ Oxidation Study of Pt(110) and Its Interaction with CO. J. Am. Chem. Soc. 2011, 133, 20319−20325. (17) Chen, M. S.; Cai, Y.; Gath, K. K.; Axnanda, S.; Goodman, D. W. Highly Active Surfaces for CO Oxidation on Rh, Pd, and Pt. Surf. Sci. 2007, 601, 5326−5331. (18) Gao, F.; McClure, S. M.; Cai, Y.; Gath, K. K.; Wang, Y.; Chen, M. S.; Guo, Q. M.; Goodman, D. W. CO Oxidation Trends on PtGroup Metals from Ultrahigh Vacuum to Near Atmospheric Pressures: A Combined In Situ PM-IRAS and Reaction Kinetics Study. Surf. Sci. 2009, 603, 65−70. (19) Gao, F.; Wang., Y.; Cai, Y.; Goodman, D. Y. CO Oxidation on Pt-Group Metals from Ultrahigh Vacuum to Near Atmospheric Pressures. 2. Palladium and Platinum. J. Phys. Chem. C 2009, 113, 174−181. (20) Chen, M.; Wang, X. V.; Zhang, L.; Tang, Z.; Wan, H. Active Surfaces for CO Oxidation on Palladium in the Hyperactive State. Langmuir 2010, 26, 18113−18118. (21) Chung, J. Y.; Aksoy, F.; Grass, M. E.; Kondoh, H.; Ross, P., Jr; Liu, Z.; Mun, B. S. In-Situ Study of the Catalytic Oxidation of CO on a

The switch-over behavior from an inactive to an active condition has been observed under various pressures of O2 and CO in previous studies.12,13,18 They reported that after the switch-over, some oxide species were formed on the surface. From their results, it seems that this switch-over occurs for wide pressure ranges of CO and O2. Since the oxidation usually proceeds via surface-oxide formation, the jump of activity may be associated with the formation of the surface oxide irrespective of the CO and O2 pressures. In summary, we have observed both the Pd(100) surface and the gas-phase product under the CO oxidation reaction conditions focusing on the relation between the catalytic activity of CO oxidation and surface chemical state using MS and AP-XPS. The CO2 production rate has a great relevance to surface chemical states. At lower temperatures below 175 °C, the Pd surface keeps metallic and is covered by a large amount of CO molecules, which results in a low reaction rate (i.e., CO poisoning). With raising the temperature above 175 °C, the CO molecules are replaced by oxygen, and above the critical temperature (190 °C), an O−Pd−O trilayer surface oxide is formed on the Pd(100) surface and the CO oxidation rate is drastically enhanced. We found that the upper-side O species of the surface oxide plays a key role to the high activity.

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ASSOCIATED CONTENT

S Supporting Information *

Details of the experimental methods, XPS observation of the chemisorbed oxygen on Pd(100) surface, MS observation of CO oxidation reaction, assignments of XPS peaks using DFT calculations, and sequence dependence of O2 and CO gas introduction are given. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +81-45-566-1701. FAX: +81-45-566-1697. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank Prof. Wei-Xue Li for his help with conducting the CLS calculations. This study was supported by a Grants-in-Aid for scientific research (No. 20245004) and the MEXT-Supported Program for the Strategic Research Foundation at Private Universities, 2009-2013. The experiments have been performed under the approval of the Photon Factory Program Advisory Committee (PF PAC Nos. 2010G151 and 2012G093).

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REFERENCES

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