In Situ Ambient Pressure XPS Study of CO Oxidation Reaction on Pd

Article pubs.acs.org/JPCC

In Situ Ambient Pressure XPS Study of CO Oxidation Reaction on Pd(111) Surfaces Ryo Toyoshima,† Masaaki Yoshida,† Yuji Monya,† Yuka Kousa,† Kazuma Suzuki,† Hitoshi Abe,‡ Bongjin Simon Mun,§,∥ Kazuhiko Mase,‡ Kenta Amemiya,‡ and Hiroshi Kondoh*,† †

Department of Chemistry, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 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, Korea ‡

S Supporting Information *

ABSTRACT: The CO oxidation reaction on the Pd(111) model catalyst at various temperatures (200−400 °C) under hundreds mTorr pressure conditions has been monitored by in situ ambient pressure X-ray photoelectron spectroscopy and mass spectroscopy. In situ observation of the reaction revealed that the Pd(111) surface is covered by CO molecules at a lower temperature (200 °C), while at higher temperatures (300−400 °C) several oxide phases are formed on the surface. We found that the reactivity is enhanced in the presence of a surface oxide and significantly suppressed by formation of a cluster oxide and the PdO bulk oxide. CO titration experiments suggest that less coordinated oxygen atoms are more reactive for CO oxidation.

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INTRODUCTION The catalytic CO oxidation reaction on Pt-group metal surfaces has been widely studied over the past several decades not only for the importance for industrial technologies such as automotive exhaust converter or methane combustion system for gas-powered turbines, but also for the fundamental understanding of the prototypical catalytic surface reaction.1−28 Many model studies in surface chemistry have been conducted on single crystal surfaces with various surface science techniques under ultrahigh vacuum (UHV) conditions.1−7 So far it is generally believed that most of the surface reactions proceed via the Langmuir−Hinshelwood (LH) mechanism on the metallic surfaces under UHV conditions. However, under realistic conditions used for the practical catalysts, the reaction mechanism has been found to be different from those under the UHV conditions.18−28 Using high pressure scanning tunneling microscopy (STM) and mass spectroscopy (MS), Hendriksen et al. studied the CO oxidation on Pt(110)18 and Pd(100)19 and showed the improved surface reactivity with the formation of surface oxide, suggesting that the surface reaction obeys the Mars−van Krevelen mechanism. On the other hand, Goodman and co-workers reported from reaction kinetics measurements and polarization modulationinfrared reflection absorption spectroscopy (PM-IRAS) that the CO-free (metallic) surface, which is transiently formed during phase transition from a CO-poisoned metallic surface to a Pd oxide surface, has the highest catalytic activity.20−23 More recently, Gustafson et al. found from surface X-ray diffraction © 2012 American Chemical Society

(SXRD) measurements for the CO oxidation on Rh(111) that a surface oxide is formed during the reaction condition and appears as a more active phase compared with the metallic phase.25 In the case of Pd, the interactions of oxygen with Pd surfaces have been studied over wide ranges of pressure and temperature, and it was revealed that several Pd oxide phases are formed depending on the pressure and temperature.29−37 Under UHV conditions, the exposure of oxygen to Pd(111) surface results in formation of the well-known p(2 × 2) overlayer with a coverage of 0.25 mono layer (ML). Further oxygen exposure leads to formation of several oxide phases; Pd5O4 (two-dimensional) surface oxide,29−32 subsurface oxide,31 supersaturated O adyer,31,32 and PdO bulk oxide. It is intriguing to study the catalytic activity of these oxide surfaces as well as the metallic Pd surfaces under realistic conditions. It should be noted that CO molecularly adsorbs on the Pd(111) surfaces either at the 3-fold hollow, bridge, or on-top sites and causes no surface restructuring irrespective of CO pressure.38,39 In this work, we investigated the CO oxidation reaction on Pd(111) surfaces under near realistic conditions by the ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) and the differential-pumping MS technique. We observed the evolution of oxide phases during the reaction conditions under Received: February 19, 2012 Revised: August 13, 2012 Published: August 24, 2012 18691

dx.doi.org/10.1021/jp301636u | J. Phys. Chem. C 2012, 116, 18691−18697

The Journal of Physical Chemistry C

Article

confirmed the formation of surface and bulk oxides on the Pd surface under elevated pressures and temperatures. Figure 1 shows XP spectra of O1s/Pd3p3/2 (a) and Pd3d5/2 (b) regions for a Pd(111) surface taken at various temperatures

the near ambient pressures of the reactant gases, which suggests the different reaction mechanism to the various oxide phases.

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EXPERIMENTAL SECTION The experiments were carried out at soft X-ray beamlines 7A and 13A at the Photon-Factory of High Energy Accelerator Research Organization (KEK-PF) in Tsukuba, Japan. AP-XPS measurements were performed using a homemade vacuum system consisting of a high-pressure chamber with a differential pumping system and a preparation chamber, the base pressures of which are 5 × 10−10 and 2 × 10−10 Torr, respectively. The high-pressure chamber is equipped with an electron energy analyzer modified for high pressure experiments (OMICRON, EA125HP) via the differential-pumping system as shown in Figure S1. A Pd(111) single-crystal (MaTech, 10 mmφ × 1 mm, 99.999% quality) surface was cleaned in the preparation chamber by repeated cycles of Ar+ sputtering and annealing with the electron bombardment up to 800 °C. The cleanliness was checked by XP spectra. The sample in the high pressure chamber was annealed by a pyrolytic boron nitride (PBN) heater from the back side, and the temperature was monitored by an alumel−chromel thermocouple attached to the sample holder. The temperature difference between the sample and the holder is estimated to be 30 °C at maximum under the present experimental conditions. The O2 and CO gases were introduced up to several hundred mTorr by a variable leak valve. We confirmed there are no significant gas impurities by the mass spectroscopy. Particularly we checked the absence of Fe- or Ni-carbonyl species from CO and did not detect any Feor Ni-including species with XPS under 0.5 Torr CO. XP spectra were measured using X-rays introduced with an incidence angle of 15° from the surface parallel via a silicon nitride window with a thickness of 100 nm and the photon energy used here was 650 eV unless otherwise stated. At this photon energy, the average escape depth (inelastic mean free path) of photoelectrons is approximately 8 Å for the Pd3d5/2 region. The binding energies of photoelectrons were calibrated with respect to the Pd Fermi edge. All spectra were deconvoluted by the software XPSPEAK41, using asymmetric Lorentzian−Gaussian sum-type line-shapes, preceded by a subtraction of the Shirley-type background. Gas-phase peaks from O2, CO, and CO2 under ambient-pressure conditions were observed in O1s/Pd3p3/2 region, and we confirmed that they do not overlap with the peaks from the sample surface. As for the XPS intensity, it is noted that the intensities taken under different conditions cannot be directly compared, because the presence of the ambient-pressure gas affects the photoelectron intensity. Therefore, the intensity was normalized first by the baseline level and subsequently by the attenuation factor due to the presence of gas. The quadrupole mass spectrometer (HIDEN, HAL201) was mounted on the way of the differential-pumping system of the high pressure chamber to monitor the partial pressures of the reactant and product gases. The partial pressures of CO, O2, and CO2 gases are calibrated using a pressure gauge directly mounted to the high-pressure chamber.

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Figure 1. XP spectra of O1s/Pd3p3/2 (a) and Pd3d5/2 (b) regions for a Pd(111) surface taken at various temperatures under the presence of O2 gas at 200 mTorr. The XP spectra are deconvoluted into several components, as indicated by different colors.

under the presence of O2 gas at 200 mTorr. Note that Pd3p3/2 XPS peaks significantly overlap with O1s XPS peaks. The photon energies used here were 650 eV for O1s/Pd3p3/2 and 435 eV for Pd3d5/2. At 200 °C, the O1s XP spectrum exhibits a broad peak centered at 530 eV. Based on the previous reports,30,32 this broad peak can be decomposed into three components, O(I), O(II), and chemisorbed O on a surface oxide. The intensity ratio of O(I) and O(II) was almost 1:1. These peaks were observed in the previous AP-XPS study and assigned as two oxygen species of surface oxide Pd5O4,30 which is illustrated in Figure 2a. According to the structure model of

Figure 2. Structure models of the Pd5O4 surface oxide (a) and the PdO bulk oxide with the (101) surface orientation (b).

Pd5O4 proposed from STM and density functional theory (DFT) calculations, the O(I) and O(II) species are attributed to 3- and 4-fold coordinated oxygen atoms, respectively.29 By increasing the surface temperature to 300 °C, the chemisorbed O desorbs from the surface, which is consistent with the previous study reporting that this O species adsorbs on the oxide and desorbs above 200 °C.32 After desorption of the chemisorbed O species, the O(I) and O(II) peaks are more clearly observed with the intensity ratio of 1:1 (300 °C). At this moment, the Pd3d5/2 XP spectrum exhibits three peaks at 334.9, 335.4, and 336.2 eV, which are attributed to the metallic bulk Pd, 2-fold coordinated Pd and 4-fold coordinated Pd atoms, respectively.30,32 The increase in coordination number

RESULTS AND DISCUSSION

Oxidation of Pd(111). The oxidation process of Pd(111) surfaces under near ambient pressure conditions has been studied previously using AP-XPS.30,32 Several high pressure induced O-containing species have been reported. We also 18692



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of oxygen causes the higher binding energy shift of Pd. The surface oxide Pd5O4 includes two types of Pd atoms with 2- and 4-fold coordination, the atomic ratio of which is 4:1. However, the observed ratio appears almost 1:1, which is interpreted as contribution of a subsurface oxide based on ref 30. The subsurface oxide containing the 4-fold coordinated Pd atoms is formed underneath the surface oxide and regarded as a precursor to the PdO bulk oxide. Increasing temperature to 400 °C gives rise to the formation of the PdO bulk oxide, as evidenced by appearance of Pd3d5/2 XPS peak at 336.6 eV,30,32 which is accompanied by growth of the 4-fold coordinated O peak (O(II)) in the O1s XP spectrum. Further increase of temperature up to 500 °C causes significant growth of the PdO bulk oxide, instead the metallic Pd bulk component becomes small. Even at this moment, the 3-fold coordinated O peak is still observed, which is consistent with formation of the PdO bulk oxide with the (101) surface orientation as shown in Figure 2b. The PdO(101) surface consists of the 3-fold coordinated O atoms. Kan et al. reported from low-energy electron diffraction (LEED) observations for Pd(111) surfaces oxidized with molecular beam under UHV conditions,34−36 that PdO(101) preferentially grows on the surface at the final stage of oxidation. Furthermore, the 2-fold coordinated Pd peak is also discernible in the Pd3d5/2 XP spectrum at 500 °C, which may indicate that the bulk oxide is partially covered by the surface oxide including the lowcoordination Pd. Recently, X-ray induced oxidation of metal surfaces has been observed under ambient-pressure O2 conditions for Au40 and Pt(111) surfaces.41 We checked Xray irradiation dependence of the oxide formation (not shown) and confirmed that such an X-ray induced oxidation does not occur on the Pd(111) surface under our experimental conditions. CO Oxidation at 200 °C. First we measured AP-XP spectra for a Pd(111) surface at 200 °C with monitoring partial pressures of O2, CO, and CO2. Figure 3 shows (a) partial pressures of O2, CO and CO2, (b) O1s/Pd3p3/2, and (c) Pd3d5/2 XP spectra under exposure of O2 and CO at 200 °C. When we introduced 200 mTorr O2 and 20 mTorr CO in the beginning (region α), almost no CO2 was produced. At this moment, the O1s (b) and the Pd3d5/2 (c) XP spectra indicate that CO molecules are adsorbed at hollow, bridge, and on-top sites of the metallic Pd surface, which is consistent with the previous studies.38,39 C1s XP spectra also confirmed that CO is chemisorbed on the surface. However, neither chemisorbed oxygen nor oxide species can be seen for region α. These results indicate that the CO molecules are covering the metallic Pd surface, resulting in CO poisoning where the O2 adsorption is strongly hindered by the adsorbed CO overlayer under this condition. This corresponds well to the CO poisoning commonly observed for the Pt-group metals.20 As increasing O2 pressure, however, the CO2 production gradually increases (region β) and a new peak (O(0)) appears at 529.8 eV in the O1s/Pd3p3/2 XP spectrum. This binding energy is higher than that for the chemisorbed O at 529.237 or 529.1 eV (see Supporting Information). Because this binding energy is in good agreement with that of the subsurface species observed in a previous study,37 the new peak is tentatively attributed to a subsurface O species. This assignment is supported by behavior of this peak under CO exposure: We stopped the dose of O2 and the Pd surface was exposed to CO atmosphere (region γ). In region γ, this peak was still observed without loss of intensity, which indicates that this species is not

Figure 3. CO oxidation reaction at 200 °C. (a) Partial pressures of O2, CO, and CO2 monitored by mass spectroscopy, (b) O1s/Pd3p3/2, and (c) Pd3d5/2 AP-XP spectra of a Pd(111) surface under exposure to O2 and CO gases, up to 500 and 20 mTorr, respectively.

reactive to CO. The lack of reactivity suggests that this species is not located at the surface but situated underneath the surface Pd layer. This behavior is the same as that of the subsurface species reported in the previous study.37 The subsurface species formed underneath a metallic Pd(111) surface could be stable against reaction with surface CO. Also, Nagarajan et al. reported the existence of subsurface O species under CO oxidation reaction conditions and proposed that it enhances the reactivity of surface CO,11 which is consistent with the present observation (region β). The defect sites might play a role as channels for oxygen diffusion into the subsurfaces.11 CO Oxidation at 300 °C. Figure 4 shows in situ monitoring of the CO oxidation at 300 °C. Each XP spectrum denoted by α, β, and γ corresponds to each region α, β, and γ in the MS monitoring (a). CO molecules do not adsorb on the Pd surface at this temperature under UHV conditions42 and even under ambient pressure conditions used here (PCO = 20 mTorr). The absence of surface CO and the higher temperature induce a drastic change in surface state from that at 200 °C. When the O2 pressure was increasing after introduction of CO at 20 mTorr, the partial pressure of CO substantially dropped while the CO2 partial pressure significantly increased, indicating most of the CO gas was consumed by the oxidation reaction (region α). The corresponding XP spectrum in the O1s/Pd3p3/2 region exhibits an asymmetric oxygen-associated peak at 530 eV. This peak can be deconvoluted into two components O(I) and O(II) at 529.1 and 529.8 eV, respectively, which are in a good agreement with those for the surface oxide Pd5O4.29−32 Also in the Pd3d5/2 region, in addition to the bulk component, two oxygen-induced Pd peaks were observed at 335.4 and 336.2 eV with an intensity ratio of 4:1. This intensity ratio is consistent with that for the surface 18693



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oxide Pd5O4.29−32 From these observations, it is most likely that the surface oxide Pd5O4 is formed on the highly reactive surface seen in region α. We call this surface as “active state”. Next, we increased the O2 partial pressure up to 600 mTorr (region β). This further increase of O2 pressure causes a decrease in the CO2 formation rate, indicating that the surface turns to “less active state” under the O2 excess condition. The O1s XP spectrum taken under this condition (region β) revealed that a new component appears at 530.5 eV, as denoted by O(III). The O(III) species would contribute also to the peak at 335.5 eV in the Pd3d5/2 XP spectrum, because its intensity is grown with respect to the Pd bulk one. Thus, the difference between “active state” and “less active state” is correlated to the presence of O(III). As the third step, we stopped the dose of O2 and allowed the surface to be reduced by CO (region γ). Interestingly, the O(I) component completely disappeared from the O1s XP spectrum. On the other hand, the O(II) and O(III) components stayed almost unchanged under the CO reduction condition. These observations indicate that O(II) and O(III) are not reactive but O(I) is a reactive species to the CO oxidation. CO Oxidation at 400 °C. In situ observation of the CO oxidation was performed at a higher temperature of 400 °C, as shown in Figure 5. In the beginning (region α) under exposure of CO (20 mTorr) and O2 (0−200 mTorr), the CO 2 production was rapidly enhanced, which is again attributed to “active state”. In fact, the O1s XP spectrum taken at region α exhibits both the O(I) and O(II) components similar to the case at 300 °C. However, as time passed, the CO2 formation rate drastically decreased even under keeping the reaction condition constant (region β), which suggests that spontaneous formation of O(III) takes place and suppresses the CO oxidation. Actually, the O1s XP spectrum supports the

formation of O(III) in region β. Note that in the region β the O2 pressure was 200 mTorr, which is lower than those at 200 and 300 °C. Another remarkable point in this spectrum is a significant increase of the O(II) peak, which is also observed when the bulk oxide is formed (see Figure 1: 400 °C). Under the CO reduction condition (region γ), the O(I) component disappeared, while the O(II) peak remained observable, which again supports that O(I) is reactive to the CO oxidation. Note that the O(III) peak is rather increased in spite of reduction condition. This might be explained as formation of the O(III) species by the reduction of the surface oxide via removal of the O(I) species. Finally, binding energies of each component of Pd3d5/2 and O1s are summarized in Table 1. Reactivity of the Oxygen Species. From the in situ monitoring of CO oxidation reaction over Pd(111) under various conditions, we could find very reactive conditions where most of the CO gas is consumed and converted to CO2, that is, region α at 300 and 400 °C. The O1s XP spectra taken under these conditions exhibit both the O(I) and O(II) peaks with almost equal intensities in common (Figure 4b-α and Figure 5b-α), which can be interpreted as the presence of the surface oxide Pd5O4 on the surface. It should be noted here that a chemisorbed oxygen species on metallic Pd surfaces has been considered as a highly reactive species for CO oxidation.8,12,20 We prepared a chemisorbed-oxygen-covered surface by exposing a clean Pd(111) surface to O2 gas (200 mTorr) at room temperature and measured an O1s XP spectrum, as shown in Figure S4 (see Supporting Information). The O1s XP spectrum gives a single peak at 529.1 eV, which is the same binding energy of the O(I) peak and difficult to be distinguished from it. Therefore the contribution of the chemisorbed oxygen species to the “active state” cannot be 18694



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The growth of the O(II) peak is associated with formation of the subsurface oxide and the PdO bulk oxide as mentioned above. The subsurface oxide is regarded as a precursor to the bulk oxide and transformed into the bulk oxide under O2 atmosphere.30 Even the mixture gas of O2 and CO gives rise to generation of the subsurface oxide and the bulk oxide at 400 °C as shown in Figure 5b,c. When these oxides are formed, the reactivity drastically decreases (Figure 5a). Therefore, the PdO bulk oxide and its precursor are less active to CO oxidation. The appearance of the O(III) species does not promote the reactivity but rather causes a decrease in reactivity, as seen in region β at 300 and 400 °C. To identify the O(III) species is not straightforward, because this species is not observed during the oxidation of Pd(111) but formed exclusively by the CO dose onto the surface oxide (see Figures 1a and 5b). The O(III) peak appears at 530.5 eV and slightly shifts to 530.8 eV at 400 °C, which is close to that of carbonate species (CO32−) on a metal surface.43 The carbonate species could be formed under CO oxidation reaction on Pt(110).44 In the present case, however, formation of carbonate species can be completely excluded, because the C1s XP spectra taken at the same time showed no peak (not shown). It should be noted here that the O(III) peak is accompanied by the 2-fold coordinated Pd peak, as seen in Figure 5b,c. Therefore, a possible candidate for O(III) is an oxygen species of an oxygen-deficient Pd oxide. Because the binding energy of the O(III) species is the highest (530.5−530.8 eV) among the O species observed, it might be poorly screened in the final state of O1s photoemission. This suggests that the oxygen-deficient Pd oxide is in the form of small clusters isolated on the surface. Such cluster oxides are certainly formed under CO oxidation reaction on Pd(100).19,26,27 The O(III) species would be highly coordinated by Pd atoms in the oxygen-deficient cluster and strongly bound to the cluster, which leads to low availability for CO oxidation. Furthermore, the cluster oxide could cover the surface oxide. Thus, it is likely that the decrease of the reactivity concomitantly with appearing of O(III) is due to formation of the oxygen-deficient cluster oxide. The CO oxidation reaction on Pt-group nanoparticles also has been extensively studied by means of various approaches.8,10,33,45 Recently, Alayon et al. suggested that partially oxidized Pt particles exhibit higher activity for CO oxidation.10,45 However, obviously there are differences between single crystals and supported nanoparticles; for example, oxygen storage at the particle−substrate interface8 and CO dissociation on the particle surfaces.33 It is intriguing to observe the supported nanoparticles under the reaction conditions with AP-XPS, particularly focusing on the relation between the chemical state at the surface/interface and the catalytic activity. Reaction Mechanism Involving the Surface Oxide. So far the CO oxidation on Pd surfaces under ambient pressure conditions has been studied by CO2 formation rate measurements,20 PM-IRAS,21 high-pressure STM,19 SXRD,26,27 and DFT calculations.16 On the Pd(100) surfaces, a (√5 × √5)R27° surface oxide and a bulk-like PdO are formed under reaction conditions at ambient pressures of reactants.16,19,26,27 Goodman and co-workers reported that a highly active phase is transiently formed when a CO-poisoned metal surface is transformed into a Pd oxide.20,21 During the transformation from the CO-covered metallic Pd to the bulk oxide, a coexisting surface of the metallic Pd and the surface oxide might be once formed, which may give rise to a jump in reactivity. The surface

Table 1. Binding Energies of Each Species in the Pd3d5/2 and O1s Regions are Summarizeda region Pd3d5/2

O1s

binding energy (eV) 334.9 335.5

336.2 336.6 335.4 335.6 336.1 529.8 529.1 529.8 530.5 531.2 530.7

species Pd bulk 2-fold Pd oxygen-deficient Pd oxide 4-fold Pd PdO bulk oxide bridge CO hollow CO on-top CO O(0); subsurface O O(I); 3-fold O O(II); 4-fold O PdO bulk oxide O(III); oxygen-deficient Pd oxide chemisorbed CO chemisorbed O on surface oxide

200 °C

300 °C

400 °C

○ − −

○ ○ ○

○ ○ ○

− − ○ ○ ○ ○ − − − −

○ − − − − − ○ ○ − ○

○ ○ − − − − ○ ○ ○ ○

○

− −

− −

b

a

The presence (○) and the absence (−) of each species under the reaction conditions at different temperatures are also indicated in the right three columns. bChemisorbed O on the surface oxide is observed exclusively under the Pd oxidation by O2 exposure.

excluded, although it is suggested from the 1:1 atomic ratio of the O(I) and O(II) species that a major part of the surface may be covered by the Pd5O4 surface oxide. So far, it has been pointed out that under the highly active conditions, a lack of CO supply to the surface (CO depletion) is observed for Pd(110) and other Pt-group-metal surfaces.20 There is a possibility that the formation of the surface oxide might be induced by the CO depletion. However, the surface oxide keeps observable even under relatively CO rich conditions (see region β of Figures 4 and 5) though the additional chemical components (the O(III) species and the bulk oxide) suppress the activity. Therefore, the formation of the surface oxide is not due to CO depletion but an intrinsic phenomenon under the reaction conditions used here. The Pd5O4 surface oxide phase contains the two oxygen species with different coordination numbers as shown in Figure 2a. CO titration experiments of Figures 4 and 5 revealed that the 3-fold coordinated O (O(I)) in the Pd5O4 phase is reactive, while the 4-fold coordinated O (O(II)) exhibits no reactivity. The low coordination may enhance the availability for the CO oxidation. In case of the PdO bulk oxide, however, it does not show high activity although the surface could be at least partially covered by the 3-fold O atoms as illustrated in Figure 2b. This difference in reactivity of 3-fold O atoms might be correlated to differences in electronic structure and chemical environment between the surface oxide and the bulk oxide. For example, since the surface oxide is sitting on the metallic Pd surface, the transition state of the CO oxidation involving the 3fold O may be able to be screened and stabilized by the free electrons of the metallic Pd. On the other hand, the bulk oxide lacks such free electrons leading to poor screening and hence may increase the activation energy of the CO oxidation. Theoretical calculations are desirable to understand the microscopic environmental effects on the CO oxidation by the 3-fold O atoms. 18695



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oxide as well as the chemisorbed oxygen provides lowcoordination oxygen atoms. The presence of such an oxygen species may be important to facilitate the CO oxidation. Li and Hammer proposed from DFT calculations for CO oxidation on Pt(111) a unique reaction model where CO molecules adsorbed on the metallic surface react with oxygen atoms involved in a 2D cluster oxide.15 This 2D cluster oxide is regarded as a monolayer island of α-PtO2, that is a kind of surface oxide. In the present case, we observe no CO molecules adsorbed on the surface, when we found the high reactivity in the presence of the surface oxide, which indicates that the surface residence time of CO is fairly short. Therefore, the CO molecules adsorbed on the surface may diffuse quickly and react with the oxygen atoms followed by immediate desorption as CO2. A recent STM/XPS study on oxidation of Pt(110) revealed that at high pressure (0.5 Torr O2), islands of α-PtO2like surface oxide form along with chemisorbed oxygen on the surface and the chemisorbed oxygen and the surface oxide exhibit comparable consumption rates for CO titration.46 Recently, Hirivi et al. proposed reaction pathways of CO oxidation on PdO surfaces based on the DFT calculations.14 They found that a Pd-assisted Eley−Rideal-type pathway exhibits the lowest activation energy, where the trajectory of arriving CO molecules is guided by the Pd atom and led to the adjacent 3-fold oxygen atom effectively.14 Measurements of the activation energy for the surface oxide under ambient pressure conditions will examine the possibility of this reaction pathway with a low activation energy.

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 This study was supported by the 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 No. 2010G151).

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CONCLUSIONS We studied CO oxidation reaction on Pd(111) surfaces under near ambient pressure conditions (up to 600 mTorr) at three different temperatures by using the AP-XPS technique combined with the differentially pumping mass spectroscopy. At a relatively low temperature (200 °C), the metallic Pd surface is still remaining but poisoned by CO molecules resulting in a low reaction rate. At 300 °C the reaction rate is significantly increased. At this moment a surface oxide, probably in the form of Pd5O4, exists on the surface. Coexistence of a chemisorbed oxygen species is not excluded. This surface oxide contains two different oxygen species with different coordination numbers (three and four). We found that only the three-coordinated oxygen atoms are reactive. During the progress of the reaction, formation of a cluster oxide takes place, which is not so reactive but rather hinders the reaction on the surface oxide. At a high temperature (400 °C), the PdO bulk oxide is formed, which is less reactive compared to the surface oxide, although the topmost surface layer of the bulk oxide should include the 3-fold oxygen species. The chemical environment of the 3-fold oxygen species is crucial to the reaction.

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ASSOCIATED CONTENT

S Supporting Information *

The AP-XPS apparatus we used in this study and Fe2p and Ni2p XPS data to confirm the absence of metal carbonyl species in the CO gas, an O1s XP spectrum of gas-phase species taken under a reaction condition and an O1s XP spectrum of a chemisorbed oxygen species are given. This material is available free of charge via the Internet at http://pubs.acs.org. 18696



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In Situ Ambient Pressure XPS Study of CO Oxidation Reaction on Pd

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