In Situ Photoemission Observation of Catalytic CO Oxidation Reaction

Article pubs.acs.org/JPCC

In Situ Photoemission Observation of Catalytic CO Oxidation Reaction on Pd(110) under Near-Ambient Pressure Conditions: Evidence for the Langmuir−Hinshelwood Mechanism Ryo Toyoshima,† Masaaki Yoshida,† Yuji Monya,† Kazuma Suzuki,† Kenta Amemiya,‡ Kazuhiko Mase,‡ Bongjin Simon Mun,§,∥ 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 Physics and Photon Science, Gwangju Institute of Science and Technology, Gwangju 500-712, 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: CO oxidation reaction on a Pd(110) single crystal surface at various temperatures under near-ambientpressure conditions has been investigated using in situ X-ray photoemission spectroscopy and mass spectroscopy. At lower temperature conditions, the CO2 formation rate is low, where the surface is covered by CO molecules (i.e., CO poisoning). Above a critical temperature 165 °C the Pd(110) surface converts to a catalytically active surface and is dominated by chemisorbed oxygen species. Further at the higher temperatures up to 320 °C, the CO2 formation rate is gradually decreased to about 80% of the maximum rate. At this moment, the amount of chemisorbed O was also decreased, which suggests that the CO oxidation reaction proceeds via the conventional Langmuir−Hinshelwood mechanism even under nearambient pressure conditions. spectroscopy (NAP-XPS).15−20 Consequently, some of these studies, including ours, have found a close relationship between the existence of the oxide species and the catalytic activity, and hence suggest a reaction mechanism named as the Mars−van Krevelen (MvK) mechanism, which involves oxide species formed on the metal substrate controls the reaction.7,8,13,14,16−18 In contrast, Chung et al. reported that a Pt(110) surface was not oxidized during the catalytic active conditions and that the reaction proceeds via the conventional LH mechanism.15 Goodman and co-workers suggest that the most reactive surface is covered by chemisorbed O (not an oxide phase) with undetectable amount of CO.9−12 Butcher et al. found that the chemisorbed oxygen and surface oxide exhibited the almost same reactivity for the CO titration process.19 There is a long-standing issue, which is the actual reaction mechanism, LH or MvK, under (near) realistic pressure conditions. The interaction between the oxygen and palladium (Pd) single crystal surface has been extensively investigated over wide pressure and temperature ranges.21−34 At room temper-

1. INTRODUCTION The oxidation of carbon monoxide (CO) to carbon dioxide (CO2) on metal surfaces is considered as a prototypical heterogeneous catalytic reaction and has been extensively studied over the past several decades to obtain the fundamental understanding of the reaction mechanism.1−20 The CO oxidation itself is also very important for industrial technologies, i.e., automobile exhaust gas convertor, gas purification, and so on. A large number of mechanistic studies using single crystal surfaces have been performed by means of various surface science techniques under ultrahigh vacuum (UHV) conditions.1−6 In the past time, it is concluded that the oxidation reaction proceeds via the Langmuir−Hinshelwood (LH) mechanism, in which both the adsorbed CO and oxygen species diffuse and react on the metallic surfaces under UHV conditions. Contribution of subsurface oxygen species3 and gas phase CO molecules4−6 has also been proposed as a key factor. In recent years, new high-pressure surface science techniques have been developed, and in situ observations of CO oxidation reaction under (sub-)Torr pressure ranges have been reported; for instance, high-pressure scanning tunneling microscopy (HPSTM),7,8 polarization-modulation infrared reflection absorption spectroscopy (PM-IRAS), 9−12 surface X-ray diffraction (SXRD),13,14 and near-ambient-pressure X-ray photoelectron © 2013 American Chemical Society

Received: June 1, 2013 Revised: August 11, 2013 Published: September 30, 2013 20617

dx.doi.org/10.1021/jp4054132 | J. Phys. Chem. C 2013, 117, 20617−20624

The Journal of Physical Chemistry C

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in the preparation chamber by repeated cycles of Ar+ sputtering at 1 kV at RT for 20 min, flashing with electron bombardment to 900 °C, and O2 treatment at 700 °C for 5 min. The absence of contaminated species was checked by XPS. The surface ordering was checked by low energy electron diffraction (LEED). The O2 and CO gases were introduced to the highpressure cell though variable leak valves. XP spectra were measured using X-rays introduced with an incidence angle of 15° from the surface parallel and emitted photoelectrons were collected with a differentially pumped electron analyzer via an aperture. The quadrupole mass spectrometer (HIDEN, HAL201) was mounted at the first stage of the differentialpumping system of the high pressure chamber to monitor the partial pressures of the reactant and product gases. More detailed information of our AP-XPS apparatus is described elsewhere.17,18 The incident photon energies used here were hυ = 435 eV for Pd 3d5/2 and C 1s, and 650 eV for O 1s, respectively. The average escape depth (inelastic mean free path; IMFP) of photoelectrons (Ekinetic ≈ 100 eV) is approximately 4.8 Å for the Pd 3d5/2 region, which makes the measurement highly surface sensitive. The binding energies (BEs) of photoelectrons were calibrated with respect to the Pd Fermi-level. All the XP spectra were fitted by using a convolution of Doniach−Šunjić and Gaussian line shapes after Shirley-type background subtraction. As a result, the peak position of Pd 3d5/2 for the bulk component was found at 335.0 eV, which is consistent with the previous photoemission studies on Pd(110) singlecrystal surfaces.34 It is noted that photoemission intensities taken under different pressure conditions cannot be directly compared because the presence of the near-ambient pressure gas attenuates the photoelectron intensity.

ature (RT) molecular O2 coming from gas phase adsorbs atomically on the surface (chemisorbed oxygen), and it forms several periodic adsorption structures, which sometimes involve a substrate reconstruction, depending on the oxygen coverage. It is well-known that the O2 exposure under higher temperature conditions leads to the formation of stable ultrathin oxide film (surface oxide) on the Pd(111),21−23 Pd(100),24−26 and stepped Pd surfaces.27,28 Then the surface is deeply oxidized to form the PdO bulk oxide with a large mass transport. Some in situ observations demonstrated the phase transition from a metallic surface with the chemisorbed oxygen to a surface oxide and then to the completely oxidized one under Torr pressure conditions.22,23,26 A recent photoemission study finds that O2 exposure to the Pd(110) surface does not form a surface oxide phase even at higher temperature and pressure conditions, but induces the dense chemisorbed oxygen phases, which have the (7 × √3) and (9 × √3) surface periodicity, with the bulk oxide phase coexisting on the surface, that is named as complex structure.33,34 The CO adsorption on low-index Pd surfaces has been also investigated using surface sensitive techniques.35−41 In the case of Pd(111), CO molecules occupy the 3-fold hollow, 2-fold bridge and on-top sites irrespective to the CO background pressure from 10−6 to 0.1 mbar (1 mbar = 0.75 Torr).35,36 On the contrary, the CO/Pd(100) system exhibited a clear pressure dependence; the CO adsorption periodicity changes from the (2√2 × √2)R45° to (4√2 × √2)R45° unit cell and then to a further dense structure in the range of 10−7 to 0.5 Torr even at RT.37,38 The Pd(110) surface also interacts with CO molecules and forms several adsorption structures.39−41 At the coverage (θ) of 0.75, the (4 × 2)-6CO structure is stably formed at RT accompanying a substrate surface reconstruction identified as the (1 × 2) missing-row structure. CO molecules adsorb on the (111)-facets of the reconstructed surface. At a higher coverage (θ = 1), the substrate periodicity lifts to the original (1 × 1), and an adsorption superstructure with (2 × 1)p2 mg-2CO is formed. CO molecules occupy the 2-fold bridge sites of topmost one-dimensional Pd chains with a tilted configuration from the surface normal. To our knowledge, the CO/Pd(110) system has not been explored under high pressure conditions yet. In this work, first we checked the pressure dependence of CO and O adsorption on Pd(110). Then we observed the temperature dependence of CO2 formation rate and the chemical states of both the metal substrate and the reactant species during the catalytic reaction under near-ambient pressure conditions by a combination of NAP-XPS and differential pumping mass spectroscopy (MS). We found that a catalytically active surface was dominated by the chemisorbed oxygen under a reaction condition, which could be interpreted in terms of the conventional LH mechanism. This is a sharp contrast to the previous results for the Pd(111)17 and Pd(100)18 surfaces, which obey the MvK mechanism.

3. RESULTS AND DISCUSSION 3.1. Clean Pd(110) Surface. Before the gas exposure, we measured XP spectra for a clean Pd(110) surface just after the cleaning procedure under UHV condition (RT, the base pressure of 4 × 10−10 Torr). Figure 1a−c shows the XP spectra taken for the Pd 3d5/2, C 1s, and O 1s regions, respectively. The photon energy was chosen to keep almost constant kinetic energy for detected photoelectrons. The Pd 3d5/2 exhibited two distinct components assigned to the bulk centered at 335.0 eV and the surface, which is associated with the topmost and second Pd layers. The corelevel shift (CLS) with respect to the bulk component, which is estimated to be −0.5 eV, is consistent with the previous XPS measurements.34 Atomic carbon (∼284 eV) and adsorbed CO (∼286 eV) are known as the typical contamination species on metal surfaces. In this case, no peak was observed in C 1s region. A broad feature due to the Pd 3p3/2 level was observed near O 1s level. A surface component of Pd 3p3/2 cannot be distinguished from the bulk one because of an insufficient resolution. However, no O 1s feature was detected, which generally appears at 528−530 eV. It is noted that the Pd 3p3/2 photoemission peak significantly overlaps with the O 1s one. Figure 1d shows a LEED pattern obtained from a clean Pd(110) surface. From the sharp (1 × 1) spots, we confirm that the clean Pd(110) surface is well ordered. Figure 1e shows an illustration of the clean (110) surface. The first-layer and the second-layer Pd atoms are displayed in different gray colors. The (1 × 1) unit cell is also drawn in the illustration. 3.2. Oxygen Adsorption on Pd(110) Surface. The oxidation process of Pd(111)22,23 and Pd(100)26 surfaces under

2. EXPERIMENTAL SECTION All experiments were carried out at an undulator soft X-ray beamline 13A at the Photon-Factory of High Energy Accelerator Research Organization (KEK-PF) in Tsukuba, Japan.42,43 AP-XPS measurements were performed using a homemade vacuum system consisting of a high-pressure cell with a differential pumping system and a preparation chamber, of which base pressures were in 10−10 Torr range. A Pd(110) surface (SPL, 10 mmϕ × 1 mm, 99.999% quality) was cleaned 20618



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Figure 1. XP spectra of a clean Pd(110) surface taken at RT for Pd 3d5/2 (a), C 1s (b), and O 1s (c) regions. The photon energy and peak assignments are shown in the figures. The Pd 3d5/2 XP spectrum was deconvoluted into two components; surface (334.5 eV; light blue line) and bulk (335.0 eV; black line). No feature was detected for C 1s and O 1s levels. A broad feature centered at 532 eV is due to the Pd 3p3/2 photoemission. (d) A LEED pattern (E = 86 eV) taken after cleaning process. (e) Illustration for the (1 × 1) fcc-(110) surface.

Figure 2. XP spectra of Pd 3d5/2 (a) and O 1s (b) regions from a Pd(110) surface taken at various O2 pressures and substrate temperatures. At 1 × 10−7 Torr and at RT, the XP spectra are deconvoluted into several components; surface (334.5 eV; light blue line), bulk (335.0 eV; black line), and two O-induce components (335.4 and 335.6 eV; red lines). The middle spectrum indicates the formation of dense chemisorbed O phases (335.4 and 335.6 eV; green lines). The pink line is attributed to the second-layer Pd atoms (335.2 eV). The bulk oxide (336.3 eV; orange line) appeared under heating conditions. Illustrations of the c(2 × 4) (c) and the (7 × √3) (d) structures of atomic oxygen adsorbed on Pd(110) and the PdO bulk oxide (e). Pd atoms are indicated in gray colors, whereas O atom is colored in dark red. The black open square (box) indicates the 2D (3D) unit cell.

near-ambient-pressure conditions has been studied previously using XPS. In the case of Pd(110), however, in situ XPS measurements under elevated pressures have not been reported yet, though detailed structure analyses for oxygen-adsorbed Pd(110) surfaces have been recently conducted under UHV conditions by means of LEED, XPS, STM, XRD, and density functional theory (DFT) calculations.34 Therefore, we performed in situ XPS measurements for Pd(110) under O2 gas exposure at 10−7 Torr and 2 × 10−1 Torr. Figure 2 shows XP spectra of Pd 3d5/2 (a) and O 1s (b) regions for a Pd(110) surface taken under different temperatures and pressures. At PO2 = 10−7 Torr and at RT, the Pd 3d5/2 spectral line shape changed drastically from the cleansurface one shown in Figure 1a; the center of spectrum shifted to the higher BE side. The spectrum was deconvoluted into three components in addition to the bulk one, which are attributed to surface Pd species with different chemical states. The O 1s XP spectrum exhibits a peak centered at 529.2 eV, with a broad feature contributed from the Pd 3p3/2. The 529.2 eV peak is well fitted by a single component with asymmetry. This spectral feature is consistent with the formation of the c(2 × 4) structure of oxygen with the missing-row reconstruction.34 According to the previous studies,30−32,34 the c(2 × 4) structure, illustrated in Figure 2c, is a well identified adsorption structure that is formed by a lower-pressure dose of O2 gas. It contains three types of surface Pd atoms; (1) oxygen-free Pd (second layer Pd), (2) Pd binding to one oxygen atom, and (3)

Pd binding to two oxygen atoms. Because the electron transfer from Pd to O atom, the higher coordination number around a Pd atom gives a higher BE.34 On the basis of the illustrated periodicity, the three surface components in Pd 3d5/2 should give the same contribution; however, actually obtained peak intensities are not equal; that is, (1):(2):(3) = 1:1:2, which means that a part of Pd atoms have a higher coordination number than the complete c(2 × 4) structure, and/or a modulation due to the photoelectron diffraction. At 10−5 Torr exposure, all of the surface components shifted to the higher BE side. The O-induced components are enhanced in intensity (335.4 and 335.6 eV; green lines). The spectrum shows a formation of dense chemisorbed O phase. The O 1s spectrum was fitted by three components, and the Pd 3p3/2 peak becomes broad. On the basis of the previous studies, these features can be attributed to the (7 × √3) and (9 × √3) high density chemisorbed oxygen phases.32−34 The pink line is attributed to the Pd atoms lying at the second layer. An atomicscale illustration of the (7 × √3) structure is shown in Figure 2d. The missing-row reconstruction lifts to the (1 × 1) surface, the atomic row splits into strings consisting of seven Pd atoms, 20619



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Figure 3. XP spectra of Pd 3d5/2 (a), C 1s (b), and O 1s (c) regions from a Pd(110) surface taken at various CO pressures at RT. At 1 × 10−7 Torr, the XP spectra are deconvoluted into three components; bulk (335.0 eV; black line) and two CO-induce components; CO(I) (335.6 eV; purple line) and CO(II) (336.0 eV; blue line). Adsorbed CO gives a peak around 286 eV in C 1s. A photoemission peak from atomic C is detected at around 284 eV above the 10−3 Torr. A single peak observed at 531.8 eV is attributed to the adsorbed CO (c). Illustrations of the (4 × 2)-6CO structure (d) and the (2 × 1)p2 mg-2CO structure (e) formed on the Pd(110) surface. Pd atoms are indicated in gray colors, whereas C and O atoms are colored in brown and dark red, respectively. The CO molecule of the (2 × 1)p2 mg structure has a tilted configuration. The black open square indicates the unit cell.

components in O 1s is also changed, which is resulted from the contribution of the bulk oxide. It is a common trend that the bulk oxide gives the higher BE peak than chemisorbed oxygen on Pd.17,18,22,23,26 The structure model of the bulk oxide is shown in Figure 2e. 3.3. CO Adsorption on Pd(110) Surface. Next, we present results for the CO/Pd(110) system under from UHV to near-ambient pressure conditions. Figure 3a,b shows the photoelectron spectra obtained at different CO pressures up to 0.1 Torr for Pd 3d5/2 and C 1s, respectively. First, at 10−7 Torr, three distinct components were observed in Pd 3d5/2 and they are denoted as bulk; 335.0 eV, CO(I); 335.6 eV and CO(II); 336.0 eV. We could not fit the CO-induced Pd component by a single curve. Adsorbed CO gives a single peak at 286.0 and 531.8 eV in C 1s and O 1s, respectively. The CLS of CO(I) (+ 0.6 eV) with respect to the bulk component is close to the reported CLS for the (4 × 2)-6CO structure (θ = 0.75) illustrated in Figure 3d, where all the CO molecules adsorb at bridge sites of the (1 × 2) missing-row reconstructed surface.40 On the other hand, the CLS of CO(II) (+ 1.0 eV) suggests formation of the (2 × 1)p2 mg-2CO structure (θ = 1) shown in Figure 3e. In the (2 × 1)p2 mg-2CO structure, the CO molecules adsorb at the bridge sites of atomic rows of the (1 × 1) surface with a tilted configuration.39,40

and each string rotates a little from the atomic-row direction of the substrate. The oxygen atoms adsorb at the both sides of the Pd string with a zigzag chain configuration.32−34 This oxygen species does not form a O−Pd−O trilayer structure, and the BEs of the O-binding Pd atoms are close to that of the c(2 × 4) chemisorbed oxygen. Therefore, we conclude that the oxygen species observed at 10−5 Torr is not associated with a surface oxide but with a chemisorbed oxygen. Then the dose pressure was increased to the 2 × 10−1 Torr at RT. At this moment the line shapes of Pd 3d5/2 and O 1s hardly changed except for the appearance of gas-phase O2 peaks at around 538 eV (not shown). The (7 × √3) and (9 × √3) chemisorbed oxygen phases are sustained under near-ambient pressure conditions at RT. Next, the Pd sample was heated to 300 °C under the O2 exposure at 2 × 10−1 Torr. The line shape of Pd 3d5/2 slightly changed; a small peak appeared at 336.3 eV, which is attributed to the PdO bulk oxide,33,34 while the chemisorbed oxygen components keep observable but their relative contribution becomes smaller (Figure 2a, top) because a part of the chemisorbed oxygen atoms are transformed into the bulk oxide. The chemisorbed oxygen and the PdO bulk oxide coexist on the surface (complex structure).33,34 A Pd atom included in the bulk oxide are coordinated by four oxygen atoms, resulting in the highest BE. The intensity ratio of the three oxygen 20620



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Figure 4. (a) Temperature dependence of MS intensity of CO2 under exposure of PO2 = 2 × 10−1 Torr and PCO = 2 × 10−2 Torr. Above the critical temperature, the CO2 intensity increased drastically; however, further heating causes a gradual decay of the CO2 signal (dashed line). Each label (A, B, C, and D) indicates temperature where XP spectra were measured. XP spectra obtained for Pd 3d5/2 (b), C 1s (c), and O 1s (d). Gas phase CO2 peak is observed at around 536 eV.

CO(I) component is predominantly observed, while the CO(II) component is missing (see Supporting Information). The absence of the CO(II) component indicates that the CO molecules first form the (4 × 2)-6CO structure at the low exposure. Further exposure induces the lifting of the reconstruction to accommodate more CO molecules. The transformation from the (4 × 2)-6CO to (2 × 1)p2 mg-2CO structure causes a gradual higher-BE shift for the surface Pd component in Pd 3d5/2 upon CO exposure. That is consistent with a previous high-resolution XPS study.41 3.4. CO Oxidation Reaction on Pd(110) Surface. Figure 4 presents in situ observations of mass signal intensities (a) and NAP-XP spectra (b−d) for catalytic CO oxidation on Pd(110) surface under a constant reactant gas dose (PO2 = 2 × 10−1 Torr, PCO = 2 × 10−2 Torr). Figure 4a shows changes in MS signal intensity for O2 (m/e = 32), CO (28), and CO2 (44) as a function of surface temperature. The CO and CO2 intensities are plotted on a magnified scale by a factor of 10. The CO2 background contribution from the residual gas was subtracted. Since partial pressures are measured at the first differential pumping stage, the measured pressures are not the actual partial pressures in the reaction cell but can be quantitatively correlated to them. At lower temperatures (165 °C), the CO2 formation rate is approximately 6-times increased. The COinduced components completely disappeared both from the Pd 3d5/2 and C1s XP spectra (labeled C). Instead, three O-binding components newly appear in Pd 3d5/2, which are attributed to the chemisorbed O phases as discussed in section 3.2. A small contribution from the PdO bulk oxide is also detected. The C 1s spectrum shows no peak, meaning that the CO poisoning disappears. Thus, the chemisorbed O and the bulk oxide phases dominate the whole surface. The O 1s exhibited distinct three peaks indicating a formation of the complex structure. A new peak at around 536 eV is associated with gaseous CO2. Hence, we confirm spectroscopically that the catalytic reaction proceeds under this condition. Since the chemisorbed O stably exists on the (110) surface, an adsorbed CO reacts with chemisorbed O on the surface. Therefore, the oxidation process is assumed to proceed via the conventional LH mechanism.1,2 It is noted that the Pd(111)17 and Pd(100)18 surfaces are oxidized to the (√6 × √6) and (√5 × √5)R27° surface oxides leading to a high catalytic reactivity. This discrepancy is discussed later. The CO2 formation rate is gradually decreased with increasing temperature above the critical temperature. At point D, the amount of chemisorbed O is obviously reduced in Pd 3d5/2 (Figure 4bD). It clearly shows a close relationship between the catalytic reactivity and amount of chemisorbed O. The intensity of the PdO bulk oxide slightly increased in going

from C to D. We have confirmed that the PdO bulk oxide peak grows up depending on the substrate temperature under O2only dosing condition. The growth of a peak at 529.5 eV, attributed to the PdO component, indicates the phase transition from chemisorbed O to bulk oxide. No CO species is observed above the critical temperature as seen in C and D of C1s level. Although the CO-induced peak is invisible with XPS, a major part of the arrival CO molecules readily react with the chemisorbed oxygen as evidenced by the MS monitoring. This suggests that the adsorbed CO molecule immediately reacts and desorbs as CO2, and hence, the residence time of CO on the surface is very short under the highly active conditions. With increasing temperature the CO sticking probability should be decreased, which is another possible factor for the decay of reaction rate with temperature. 3.5. Comparison with Pd(111) and Pd(100) Surfaces. The CO oxidation on Pt-group metal surfaces under (near) realistic pressure conditions has been studied by in situ techniques7−20 (e.g., HP-STM, PM-IRAS, SXRD, NAP-XPS, etc.). Some of the studies suggested that the metal surfaces are not oxidized under reaction conditions, while the others proposed that formation of surface oxide promotes the reaction rate. Previously, we performed in situ XPS observations of catalytic CO oxidation reaction over Pd(111) 17 and Pd(100)18 surfaces under constant gas pressures (PO2 = 2 × 10−1 Torr, PCO = 2 × 10−2 Torr). The results are summarized as follows; (1) CO molecules dominate the surface at lower temperatures, which blocks the O2 dissociation, resulting in quite a low reaction rate. (2) Pd surface is oxidized to the surface oxide and CO oxidation proceeds quickly above the critical temperature. (3) Upper-side oxygen of the surface oxide has a highly catalytic reactivity (i.e., MvK mechanism). It is a general trend that the CO adsorption is dominant on low-index Pt-group metal surfaces at lower temperatures and that the O adsorption is blocked. The oxidation reaction slowly proceeds via the LH mechanism in this situation. It is noted that the coverages of CO and O are in equilibrium as a function of temperature. The critical temperature where the reaction rate is drastically enhanced varies depending on surface orientations; (111) >200 °C, (100) 190 °C, and (110) 165 °C. The removal of CO adlayer is an important factor for the activation of the catalysts. The more open surface may facilitate the CO removal via facile access of O2 molecules to the Pd atoms. A surface oxide, which has an O−metal−O trilayer structure, is formed on the (111)21−23 and (100)24−26 surfaces, with a two-dimensional periodicity of (√6 × √6) and (√5 × √5)R27°, respectively. In these unit cells half of O atoms are sitting on the Pd layer and the other half are situated underneath the Pd layer resulting in a sandwich structure with a high oxygen density (0.8−1 ML). Such sandwich structures are formed on the relatively smooth (111) and (100) surfaces. However, the Pd(110) surface exhibited a different behavior, which does not form any surface oxide probably due to a significant lattice mismatch between the smooth O−metal−O layer and the ridge-and-trough structure of the (110) surface. Another possible reason for the absence of surface oxide is that high density oxygen atoms (approximately 0.9 ML)33,34 can be accommodated on the (1 × 1)-(110) surface without significant restructuring. The distance between adjacent Pd rows of the pristine (110) surface would be long enough to accept the inter-row repulsion arising from high-density adsorption of 20622



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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. 2012G093 and 2012S2-006).

oxygen. At the most reactive condition for each surface, the surface structures consist of oxygen atoms with similar densities; 0.8 ML for √6 oxide/Pd(111), 1.0 ML for √5 oxide/Pd(100), and 0.86−0.89 ML for chemisorbed-O/ Pd(110). The adsorption energy per O atom is reduced with increasing O density on the surface, which would decrease the activation energy and accelerate the reaction rate. In this sense, the three surfaces have similarly highly active oxygen atoms. However, the adsorption of oxygen for the (110) surface is different from those for the other two surfaces from the structural point of view. Since all the oxygen atoms are adsorbed on the Pd rows of (110) surface with the high density, they are all available for the CO oxidation via the LH mechanism. While in the case of (111) and (100) surfaces, only the half of oxygen atoms of the surface oxide can react with CO via the MvK mechanism. Both of the LH and MvK mechanisms can be responsible for the CO oxidation reaction depending on surface orientation as well as substrate temperature. Recently, Blomberg et al. found from AP-XPS measurements for Pd(100) surfaces that the both of the chemisorbed O (PO2/ PCO = 0.25:0.25 Torr) and surface oxide (PO2/CO = 0.4:0.1 Torr) exhibit a high reactivity for the CO oxidation after overcoming the CO poisoning.20 The active species is controlled by the partial pressures of reactant gases as well as surface temperature. The present results indicate that the surface orientation also gives influences on the active species and the activation temperature.

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4. CONCLUSIONS We have investigated the catalytic CO oxidation reaction on a Pd(110) surface under sub-Torr steady-state conditions using a combination of near-ambient pressure XPS system and differentially pumping mass spectroscopy. Below the critical temperature (

In Situ Photoemission Observation of Catalytic CO Oxidation Reaction

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