High-Pressure NO-Induced Mixed Phase on Rh(111): Chemically

Jan 30, 2015 - Kazuhiko Mase,. ‡. Bongjin Simon Mun,. § and Hiroshi Kondoh*. ,†. †. Department of Chemistry, Keio University, 3-14-1 Hiyoshi, K...
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High-Pressure NO-Induced Mixed Phase on Rh(111): Chemically Driven Replacement 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, Kanagawa 223-8522, Japan Institute of Materials Structure Science, High Energy Accelerator Research Organization, and The Graduate University for Advanced Studies, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan § Department of Physics and Photon Science, and Ertl Center for Electrochemistry and Catalysis, Gwangju Institute of Science and Technology, Gwangju 500-712, Republic of Korea ‡

S Supporting Information *

ABSTRACT: The interaction between nitric oxide (NO) and Rh(111) surface has been investigated by a combination of near-ambient-pressure X-ray photoelectron spectroscopy, low energy electron diffraction, and density functional theory calculations. Under lowtemperature and ultrahigh vacuum conditions, our experimental and computational results are consistent with the previous reports for NO adsorption phases on Rh(111). While at room temperature and upon exposure to gaseous NO of 100 mTorr, NO molecules partially dissociate followed by chemical removal of atomic nitrogen by NO from the surface, and the remaining atomic oxygen and NO form a NO/O mixed phase. Interestingly, this mixed phase is stable even after NO evacuation and shows a well-ordered (2 × 2) periodicity. These observations provide a new insight into the NO/Rh(111) system under nearambient-pressure condition.

1. INTRODUCTION Rhodium (Rh) is known as a good catalyst for the reduction of harmful nitroxides. The adsorption of nitric oxide (NO) on Rh surfaces has been studied extensively from both experimental and theoretical points of view.1−25 Recently, high-pressureinduced formation of various high-density overlayers of simple molecules such as CO, NO, and O2, accompanied by surface restructuring and distinct catalytic activities on Pt-group metal surfaces, has been found using high-pressure surface science techniques.18−30 Although Rh is a highly active catalyst for NO conversion, the number of studies on the interaction of NO on Rh(111) surfaces under high-pressure conditions is limited.18−21 Rider et al. found that a densely packed adsorption structure with (3 × 3) periodicity was formed under exposure to several tens mTorr NO using high-pressure scanning tunneling microscopy (HP-STM).18 A structure model consisting of 7 NO molecules (hollow:top = 6:1) per unit cell was proposed for the (3 × 3) structure and supported by a density functional theory (DFT) calculation.19 Although these studies provided information on the periodicity and the arrangement of the NO molecules, the possibility of dissociation of NO molecules was not taken into account.18,19 On the other hand, Wallace et al. reported from polarization-modulated infrared reflection absorption spectroscopy (PM-IRAS) measurements that a single vibrational peak was observed for the NO/Rh(111) system under the presence of 1 Torr NO, which was assigned to NO molecules located at top sites.21 They suggested that the hollow sites are blocked by © 2015 American Chemical Society

dissociated species under 1 Torr NO. Chemical identification of the surface species including the dissociated species is required to understand the high-pressure-induced phase. Near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) is an element-specific surface-sensitive technique via core-level shifts (CLSs) operating under realistic conditions.20,24,25,28 It provides chemical information on both substrate and adsorbate under various pressure and temperature conditions. A combination with DFT-based calculations enables one to characterize the electronic and geometric structures for the observed adsorption system. In this work, we studied NO adsorption behavior on a Rh(111) surface over wide pressure ranges from ultrahigh vacuum (UHV) to 1 Torr by the use of NAP-XPS and low energy electron diffraction (LEED). Experimental results are paralleled by computational simulations; that is, the results are interpreted on the basis of optimized geometries and CLSs for NO overlayers reported by the previous studies.3−9 We found a formation of a mixed overlayer of NO and atomic oxygen (O) under a high-pressure condition. This mixed overlayer is formed via dissociation of NO and selective removal of atomic nitrogen (N). This finding provides an understanding of the puzzled NO-induced structure reported previously and may Received: July 27, 2014 Revised: January 14, 2015 Published: January 30, 2015 3033

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BEs of core electrons and the resultant CLSs were estimated using the Slater−Janak transition approach.24

2. EXPERIMENTAL SECTION A Rh(111) surface (SPL, 8 mmϕ × 1 mm, 99.995% quality) was cleaned by a standard procedure: repeated cycles of the Ar+ sputtering at 1 kV at room temperature (RT) for 20 min, an O2 treatment at 1000 K for 5 min, followed by a brief annealing with an electron beam heating to ∼1300 K. XPS measurements were performed at an undulator soft X-ray beamline 13A/B at the Photon-Factory of High Energy Accelerator Research Organization (KEK-PF) in Tsukuba, Japan.34 The base pressures of the analysis chamber and the separated preparation chamber were of the order of 10−10 Torr. Pure NO gas (Takachiho chemical industrial, 99.9%) was introduced to the analysis chamber up to 1 Torr via a variable metal leak valve. We checked contaminations in the NO gas using a mass spectrometer at a NO pressure of 10 mTorr. As a result, the signals of O2 (m/e = 32), N2O (44), and NO2 (46) were under the detection limit. The sample temperature could be varied to 155 K using liquid N2 cooling in the analysis chamber. More detailed information on the experimental apparatus is described elsewhere.35 The incident photon energies were varied so that the emitted photoelectron has almost the same kinetic energy (i.e., O 1s, 650 eV; N 1s, 500 eV; and Rh 3d5/2, 400 eV). With the same kinetic energy of photoelectron, the property of each peak carries the information from similar physical origin beneath the surface. The binding energy (BE) scale was calibrated with respect to the Fermi edge. Because the photoemission intensity is attenuated by ambient-pressure gas, the O 1s and N 1s intensities were normalized by the integrated intensity of Rh 3d5/2 photoelectrons from the Rh substrate. X-ray irradiation effects on adsorbed layers were checked, and the absence of such effects was confirmed. The XP spectra were deconvoluted using a convolution function of Doniach−Šunjić and Gaussian line shapes, and Shirley-type background subtraction. As a result of analytical process, the Rh 3d5/2 bulk component is fitted at 307.3 eV with a Lorentzian width of 0.31 eV and an asymmetric factor α of 0.20.

4. RESULTS AND DISCUSSION 4.1. NO Adsorption under UHV Conditions. First, we checked the LEED pattern for clean and NO-adsorbed Rh(111) surfaces with different coverage as shown in Figure 1. All of the patterns were obtained at 155 K with a beam

Figure 1. Observed LEED patterns from clean and NO-exposed Rh(111) surfaces and corresponding model structures; clean surface (a and b), and NO exposure of 3 L (c and d) and 60 L (e and f). The substrate was cooled to 155 K. Here, the electron energy E of 73 eV was applied. Dashed tetragons show unit cells of the NO-induced superstructure. The Rh, N, and O atoms in the models are color-coded in gray, blue, and red, respectively. High-symmetry sites for adsorption are indicated in (b) as fcc-hollow (F), hcp-hollow (H), bridge (B), and top (T).

3. COMPUTATIONAL METHODS We employed DFT-based calculations, the Vienna ab initio simulation package (VASP). A projector augmented wave (PAW) method was used with a plane wave cutoff energy of 400 eV for electron−ion interactions. The generalized gradient approximation (GGA)−Perdew−Burke−Ernzerhof (PBE) was used for the exchange-correlation functional. The Rh metal slab was modeled by seven-layer thickness. Brillouin zone integration of (9 × 9 × 1) Monkhorst−Pack k-point mesh was applied for the (2 × 2) supercell. We used coequal k-point meshes for the other supercells to be the same density. For the bare Rh metal, the calculated equilibrium lattice constant was a0 = 3.82 Å, which was slightly larger than an experimental value of 3.80 Å. Adsorbates were placed on both sides of surfaces to reduce a long-range dipole−dipole interaction. The first to third Rh layers of both side surfaces and adsorbed molecules then are fully relaxed, whereas the middle Rh layer is fixed to the bulk lattice constant. Although there is a small margin for each adsorption structure, the adsorbed NO molecules basically have the perpendicular configuration to the surface parallel after the relaxation process (see Supporting Information Table S1).

energy of 73 eV. Before NO exposure, a sharp (1 × 1) pattern was observed as shown in Figure 1a. The model structure is illustrated in the right panel as Figure 1b. High-symmetry sites are also indicated in Figure 1b. Figure 1c shows a LEED pattern taken after 3 Langmuir (L, 1 L = 1 × 10−6 Torr s) exposure at 155 K. Because NO dissociation into N and O does not take place at 155 K,3 the ordered structures arise from the adsorbed NO molecules. Additional diffraction spots denoted as c(4 × 2) are seen with the substrate spots. According to the previous reports on this structure, two NO molecules are packed at the hcp- and fcc-hollow sites in a unit cell, resulting in a coverage of 0.50 monolayers (ML) with respect to the number of surface Rh atoms as shown in Figure 1d.3,4,7,8,17 Further NO exposure causes a phase transformation to p(2 × 2), which is completed at 60 L as shown in Figure 1e, and the pattern no longer changes even under continuous dosing at 10−7 Torr. NO molecules occupy the top sites above 0.5 ML.3−8 It is noted that the present LEED observation cannot exclude coexistence 3034

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Figure 2. XP spectra from O 1s, N 1s, and Rh 3d5/2 levels from a Rh(111) surface taken under different surface conditions (i.e., before and after NO gas exposures). The photon energies are indicated in each figure. The bottom spectra were taken from a clean Rh(111) surface (a−c). NO adsortion phases with a c(4 × 2) structure (d−f) and a p(2 × 2) structure (g−i) were prepared by NO exposures of 5 and 60 L, respectively. The substrate temperature was kept at 155 K. O 1s and Rh 3d5/2 spectra were deconvoluted into distinct components (solid black lines). The dark gray plots show the experimental result, and the bold black line is a sum of the fitting curves, while the black dashed line indicates a background.

of p(2 × 2)-2NO and p(2 × 2)-3NO structures. However, most of the p(2 × 2) overlayers contain three NO molecules in one unit cell as indicated by XPS results described later. We estimated the adsorption energies for NO, N, and O on each site using DFT calculation. Details of the calculation method are described in the Supporting Information. At 0.25 ML, the energetically most favorable site is the hcp-hollow (H) site, and the adsorption energy for each site decreases in the following order: H > fcc-hollow (F) > bridge (B) > top (T). We compared the average adsorption energies of the NO structures at 0.5 and 0.75 ML. The average adsorption energy per molecule results in −2.41, −2.33, and −2.07 eV for the c(4 × 2)-2NO, p(2 × 2)-2NO, and p(2 × 2)-3NO, respectively. These adsorption energies are consistent with previous DFT calculation results.4,17,19 Figure 2 shows XP spectra for O 1s, N 1s, and Rh 3d5/2 levels taken after exposures of 0, 5, and 60 L. The bottom three spectra shown in Figure 2a−c correspond to a clean Rh(111) surface. The O 1s and N 1s spectra in Figure 2a and b show no peak, while in the Rh 3d5/2 spectrum of Figure 2c a fitting analysis reveals two distinct components at 306.8 and 307.3 eV. The signal at lower BE side is attributed to the first-layer Rh atoms (denoted by Rh0), whereas the higher one is assigned as the bulk Rh atoms including a contribution from the second layer.31−33 The middle three spectra of Figure 2d−f were measured after 5 L exposure at 155 K. At this moment, a single peak is visible in the O 1s level of Figure 2d, which is assigned to NO molecules at the hcp- and fcc-hollow sites.14 The N 1s level of Figure 2e also indicates the adsorption of NO molecule at BE of 400.3 eV.14,20,24 The Rh 3d5/2 level of Figure 2f exhibits a spectral change; the surface component shifts to the higher BE side, and it splits into two components at 307.1 and 307.5 eV, indicating that two Rh atoms with different chemical states are induced by the NO adsorption on the surface. We attribute the lower and higher BE components to the Rh atoms bonded by one and two NO molecule(s), respectively, at the hollow site(s). Because one NO molecule is shared by three Rh atoms, the effective coordination number of the Rh atom is 1/3 (Rh1/3) or 2/3 (Rh2/3). The intensity ratio of Rh2/3/Rh1/3 is estimated to be 1.4. If the complete c(4 × 2)-2NO phase covers

the surface, the peak intensity ratio is simply assumed as unity. However, the actual ratio is higher than unity, which suggests that extra NO molecules are accommodated into the c(4 × 2)2NO phase, and/or a photoelectron diffraction effect contributes to the deviation from the nominal ratio for the c(4 × 2)-2NO phase. The upper three spectra of Figure 2g−i were recorded after 60 L NO exposure at 155 K, where the surface reached the saturation coverage under the UHV condition. The O 1s XP spectrum of Figure 2g shows two distinct peaks, one of which newly appears at the higher BE side from the hollow-NO peak at 530.8 eV. This new peak at 532.8 eV is assigned to NO molecules located at top sites.14 The coverage of top-NO is estimated to be 0.20 ML. Because the hollow-NO peak remains unchanged in intensity, the coverage of hollow-NO is 0.50 ML. Therefore, the total NO coverage reaches 0.70 ML. The ideal coverage for the p(2 × 2)-3NO phase is 0.75 ML (see Figure 1f). The observed slightly lower coverage may be explained by the presence of domain boundaries of the p(2 × 2)-3NO structure, which can decrease the coverage of particularly topNO according to the structure model of the domain boundaries.4 The N 1s level of Figure 2h slightly shifts to the lower BE side (∼ −0.2 eV). However, the spectral shape shows no clear coverage dependence, indicating that the BE is insensitive to difference in adsorption site. A previous study for NO/Pd(100) reported that the top-NO led a negative shift in N 1s level.24 The Rh 3d5/2 level shifts to the higher BE side as shown in Figure 2i. The Rh1/3 component completely disappears, whereas the Rh2/3 one increases in intensity, and a new component appears at 307.8 eV. This new component is attributed to Rh atoms bonding to the top-NO (Rh1). The experimental intensity ratio of Rh1/Rh2/3 is 0.27, which is a little smaller than the ideal value of 0.33 and associated with the domain boundaries. To confirm the above assignments, we calculated CLSs of Rh 3d5/2 level for the c(4 × 2)-2NO and p(2 × 2)-3NO structures. Table 1 shows a comparison of the calculated and experimental CLSs with respect to the BE for the bulk Rh atoms. The calculated CLSs of Rh0 (−0.46 eV), Rh1/3 (−0.27 eV), Rh2/3 (+0.07 and +0.10 eV), and Rh1 (+0.57 eV) are in good agreement with the experimental CLSs of −0.5, −0.2, +0.2, and 3035

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from the surface after dissociation. Because the associative desorption of N2 from atomic N occurs above 500 K on Rh(111),1,2 the recombination channel should not be dominant. Note that a moderate-pressure (1 × 10−7 Torr) NO dose at room temperature also induces NO dissociation as mentioned above but results in atomic N remaining on the surface after evacuation (see Supporting Information Figure S2). This suggests that the removal of atomic N needs the presence of high-pressure NO. A possible channel for the removal of atomic N is the formation of N2O by the reaction with gaseous NO.9 Such atomic N removal has been also observed in previous NAP-XPS studies on Pd(100)24 and Pt(111).25 The Rh 3d5/2 level is decomposed into three components (denoted as bulk, Rh2/3, and Rh1). It should be noted that the Rh 3d5/2 spectrum is very similar to that for the p(2 × 2)-3NO phase shown in Figure 2i. Another remarkable point is that the XP spectra hardly changed even after NO evacuation (Pbackground ≈ 10−8 Torr). The chemical states of both the substrate and the adsobates remain unchanged when going from the presence of 100 mTorr NO to the high-vacuum condition. Once the coadsorption layer of NO and O is formed under the high-pressure condition, it is quite stable. We have confirmed that the NO/O mixed overlayer is formed after 1 Torr NO exposure. Figure 4a shows a difference spectrum of the O 1s level between the 100 mTorr ambience at room temperature and the 60 L exposure at 155 K. It is obvious that the decrement of the NO and increment of the atomic O are compatible; therefore, the hollow-NO is replaced by the atomic O. On the other hand, the top-NO peak shows no difference, indicating that the topNO keeps the same adsorption states. If the NO domains are separated from the O domains, the top-NO intensity should change in the same manner as that for the hollow-NO. Furthermore, if the O atoms are aggregated into twodimensional islands, the Rh 3d5/2 level of Figure 3c and f should give a lower-BE component than the bulk one (see Supporting Information Figure S1).33 The absence of such a lower-BE component and the lack of intensity change of the top-NO peak provide the evidence of incorporation of the O atoms into the NO domains. Figure 4b shows a LEED pattern observed for the NO/O mixed phase after NO evacuation. A sharp p(2 × 2) pattern is clearly observed, which indicates that the NO/O mixed phase has a well-ordered structure. Considering the fact that all of the XP spectra (O 1s, N 1s, and Rh 3d5/2 levels) taken after evacuation are the same as the corresponding spectra before evacuation (Figure 3), it is most

Table 1. Calculated and Experimental CLSs of Rh 3d5/2 Level for Clean and NO-Adsorbed Rh(111) Surfacesa coverage (ML) 0 0.50 0.75

species

calcd CLS (eV)

exp. CLSb (eV)

exp. BEb (eV)

Rhbulk Rh0 Rh1/3 Rh2/3 Rh2/3 Rh1

0.0 −0.46 −0.27 +0.10 +0.07 +0.57

0.0 −0.5 −0.2 +0.2 +0.2 +0.5

307.3 306.8 307.1 307.5 307.5 307.8

a For the NO/Rh(111) surfaces, the c(4 × 2)-2NO (0.50 ML) and p(2 × 2)-3NO (0.75 ML) structures were assumed for the calculations. All of the CLSs are indicated with respect to the bulk Rh atom. bThe experimental CLSs and BEs are deduced from XP spectra shown in Figure 2.

+0.5 eV, respectively. Therefore, the present XPS results for the c(4 × 2)-2NO and p(2 × 2)-3NO phases support the previously proposed structure models formed under UHV conditions.3−10 4.2. NO Adsorption under a NAP Condition. Next, we performed XPS measurements under a NAP condition at room temperature. The NAP experiment was carried out at room temperature, while the UHV one was done at 155 K. It is noted that the sticking coefficient and adsorption process can be changed depending on the difference in temperature. Figure 3 shows XP spectra taken under 100 mTorr of NO (bottom) and after NO gas evacuation (upper). An additional component appears at 529.5 eV in the O 1s level of Figure 3a. The new peak is assigned to a chemisorbed O species located at the hollow site.11,33 This assignment is supported by the XP spectra taken from the O-covered Rh(111) surface (see Supporting Information Figure S1). It is noted that the NO dissociation is not a high-pressure-induced process but a thermally activated process. While adsorbed NO molecules do not dissociate at 155 K, dissociated species were observed on the surface at room temperature after NO gas exposure at 10−7 Torr (Supporting Information Figure S2). Therefore, the atomic O could be generated by NO dissociation at the initial stage of highpressure NO dose at room temperature. Besides, we can recognize that NO molecules are adsorbed on the hollow and the top sites. As a result, the atomic O and molecular NO are coadsorbed on the surface under 100 mTorr of NO. Here, the experimental surface coverage is estimated to be 0.71 ML. On the other hand, no signal from atomic N is detected in the N 1s level of Figure 3b, which means that the atomic N is removed

Figure 3. XP spectra from O 1s, N 1s, and Rh 3d5/2 core levels from a Rh(111) surface taken under 100 mTorr of NO at room temperature (a−c) and after NO gas evacuation and cooling to 155 K (d−f). The photon energies are indicated in each figure. O 1s and Rh 3d5/2 spectra were deconvoluted into several components. 3036

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signal. The lack of the hollow-NO signal may be explained in terms of weakening of the dipole moment of the hollow-NO due to electronic effects arising from coadsorption of the topNO as was seen for NO/Pt(111).16 From all of the experimental results and calculated adsorption energies (Supporting Information Table S2), we propose adsorption models for the NO/O mixed phase with the p(2 × 2) periodicity as shown in Figure 4c and d. In model 1, within the unit cell two NO molecules occupy the hcphollow site and the top site, and an O atom occupies the fcchollow site as shown in Figure 4c. In model 2, an NO molecule is sitting on the top site, and two O atoms are adsorbed on both the hcp- and the fcc-hollow sites as shown in Figure 4d. Corresponding CLSs for the two NO/O mixed phases are shown in Table 2. The surface Rh atoms bonding to both the Table 2. Calculated and Experimental CLSs of Rh 3d5/2 Level of NO/O/Rh(111) Systemsa

Figure 4. (a) The difference spectrum (red ○) of O 1s level is obtained by subtraction of the spectrum taken after 60 L exposure (●; Figure 2g) from the spectrum taken under 100 mTorr dosing condition (gray ●; Figure 3a). An ex situ LEED pattern from a Rh(111) surface at 155 K taken after NO gas evacuation (b). The electron energy E of 80 eV was applied. (c,d) Structure models for adsorption phases on Rh(111) formed by the high-pressure NO treatment (see text).

model

species

calcd CLS (eV)

exp. CLSb (eV)

exp. BEb (eV)

1: p(2 × 2)-2NO/O

Rh2/3‑NO/O Rh1 Rh2/3‑2O Rh1

+0.17 +0.60 +0.35 +0.62

+0.2 +0.5 +0.2 +0.5

307.5 307.8 307.5 307.8

2: p(2 × 2)-NO/2O a

All of the CLSs are indicated with respect to the bulk Rh atom. bThe experimental CLSs and BEs are deduced from XP spectra shown in Figure 3.

likely that the NO/O mixed phase has the same ordered structure under the presence of 100 mTorr NO. In fact, the previous high-pressure STM measurements confirmed that a p(2 × 2) structure is formed under a similar NO pressure condition.18 It is noted that the actual surface coverages of the hollow-NO and the hollow-O are estimated to be 0.38 and 0.12 ML, respectively, The molecular adsorption and dissociation of NO are competitive processes on the Rh(111) surface at room temperature. The dissociation needs unoccupied sites for dissociated species (atomic N and O) in addition to an adsorption site for NO. The presence of preadsorbed NO molecules suppresses the dissociation due to a lack of vacant sites for the dissociated species. Consequently, the coverage of dissociated species (hollow-O) does not reach the saturation (0.25 ML), and the hollow sites are occupied by both NO and atomic O. The ratio between NO and O at the hollow sites may be kinetically influenced by the NO pressure and the substrate temperature. Another possibility is the coexistence of the NO/ O mixed phase and the p(3 × 3)-7NO (0.78 ML) phase, which was reported under high-pressure NO conditions by the HPSTM study.18 In the p(3 × 3)-7NO phase, the hollow/top intensity ratio should be 6 in O 1s level. However, we could not recognize such an enhancement in hollow-NO intensity. Besides, although the total NO coverage should increase in the case of coexistence of the p(3 × 3)-7NO phase, the experimental coverage hardly changed between in situ and ex situ conditions. The discrepancy may be explained in that the densely packed p(3 × 3)-7NO phase is not a dominant phase under the pressures used here, or it consists of both NO and O species within the unit cell. Anyway, the p(3 × 3)-7NO phase seems to give a minor effect under the present conditions, even if it coexists. The replacement of hollow-NO with dissociated species was suggested in the previous PM-IRAS study,21 which is well consistent with our results that the replacement by O atom takes place under the high-pressure conditions. However, the PM-IRAS study reported complete absence of the hollow-NO

hollow-NO and the hollow-O and bonding to two hollow-Os are denoted by Rh2/3‑NO/O and Rh2/3‑2O, respectively. The calculated CLSs based on model 1 are in better agreement with the observed ones. Consequently, it is most plausible that spontaneous formation of the p(2 × 2)-2NO/O mixed phase is induced by the high-pressure NO treatment. A part of the fcchollow sites are replaced by atomic O with respect to the p(2 × 2)-3NO phase. This replacement is attained by NO dissociation and subsequent removal of atomic N, the latter of which proceeds via reaction with NO. In this sense, this replacement is not a physical replacement induced by collision and displacement, but a chemically driven replacement.

5. CONCLUSION We presented both in situ and ex situ XPS observations for the NO/Rh(111) system under a wide NO pressure range. We first confirm that high-pressure dose of NO on Rh(111) induces the formation of a stable NO/O mixed phase via NO dissociation and removal of atomic N. The LEED observation and the DFTbased CLS calculation suggest that the NO/O mixed phase has a p(2 × 2) structure where two NO molecules sit on the hcphollow site and the top site and one O atom on the fcc-hollow site. The combination of the NAP-XPS technique and the DFT-based CLS calculation is a powerful approach to characterize high-pressure-induced adsorption phases.

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ASSOCIATED CONTENT

S Supporting Information *

Calculated results of geometric parameters and adsorption energies of NO/Rh(111) and NO/O/Rh(111) systems (Table S1 and S2). XP spectra from an O-covered surface (Figure S1). XPS observations at room temperature under different NO pressures together with a LEED pattern after 10−7 Torr NO 3037

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The Journal of Physical Chemistry C

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exposure (Figure S2). 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 thank the Photon Factory staff for their technical support. This study was supported by a Grant-in-Aid for scientific research (nos. 20245004 and 26248008) and the MEXTSupported 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. 2012G093 and 2012S2006). R.T. acknowledges support from the JSPS Research Fellowship for Young Scientists.

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REFERENCES

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DOI: 10.1021/jp507542h J. Phys. Chem. C 2015, 119, 3033−3039

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DOI: 10.1021/jp507542h J. Phys. Chem. C 2015, 119, 3033−3039