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Operando Observation of NO Reduction by CO on Ir(111) Surface Using NAP-XPS and Mass Spectrometry: Dominant Reaction Pathway to N2 Formation under Near Realistic Conditions Kohei Ueda,† Masaaki Yoshida,† Kazuhisa Isegawa,† Naoki Shirahata,† 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, SOKENDAI (The Graduate University for Advanced Studies), 1-1 Oho, Tsukuba 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 nitric oxide (NO) reduction by carbon monoxide (CO) on Ir(111) surfaces under near ambient pressure conditions was studied by a combination of near-ambient-pressure X-ray photoelectron spectroscopy (NAPXPS) and mass spectrometry (MS), particularly paying attention to the dominant reaction pathway to formation of molecular nitrogen (N2). Under a relatively low CO pressure condition (50 mTorr NO + 10 mTorr CO), two reaction pathways to form N2 are clearly observed at different ignition temperatures (280 and 400 °C) and attributed to a reaction of NO adsorbed at atop site (NOatop) with atomic nitrogen (Nad) and associative desorption of Nad, respectively. Since the adsorption of NOatop is inhibited by CO adsorbed at atop site (COatop), the ignition of the NOatop + Nad reaction strongly depends on the coverage of COatop; the ignition temperature shifts to higher temperature as increasing CO pressure. In contrast, for the Nad + Nad reaction the ignition temperature keeps almost constant (∼400 °C). The online MS results indicate that the latter reaction is the dominant pathway to N2 formation and the former one less contributes to N2 formation with accompanying a small amount of nitrous oxide (N2O). No evidence for contribution of the isocyanate (NCO) species as an intermediate was observed in the operando NAP-XP spectra.

1. INTRODUCTION The reduction of harmful nitric oxide (NO) is one of the most important reactions in the three-way catalyst for automobile exhaust gas. Catalytic reduction of NO under coexistence of excess oxygen has been paid much attention particularly for lean burn engines. Wang et al. reported that iridium (Ir) supported on a zeolite (ZSM-5) shows a high NO conversion reduced by carbon monoxide (CO) in the presence of excess oxygen compared to the most used platinum group metals (PGMs) such as platinum (Pt), palladium (Pd), and rhodium (Rh).1 Shimokawabe and Umeda investigated NO conversion by CO and nitrogen (N2) selectivity over nitrous oxide (N2O) of Ir and Rh catalysts supported on various oxides and found that Ir/WO3 exhibits the highest NO conversion and N2 selectivity.2 In addition, NO reduction by CO on Ir-based catalysts were examined from the viewpoints of additive effect,3−7 support effect 6−12 and additional gas phase effect.3,7−11,13−15 In order to elucidate the unique catalytic properties of Ir, the reduction of NO by CO has been studied on several Ir single crystal surfaces.16−22 Since single crystals provide well-defined uniform surfaces, the use of a single crystal as a model catalyst © 2017 American Chemical Society

enables to obtain information on adsorption sites of reactants and chemical states of catalysts at the atomic level. Fujitani and co-workers found from temperature programed desorption (TPD) measurements for CO + NO coadsorbed Ir(111) surfaces that two reaction pathways contribute to desorption of N2; a disproportionation reaction between NO and atomic N (NO + N → N2 + O) and the recombination of atomic N (N + N → N2) where the atomic N is generated from dissociation of NO and the remaining atomic O is removed by reaction with CO (O + CO → CO2).17 They also reported reaction kinetics of NO reduction by CO on Ir single-crystal surfaces under near ambient pressure (NAP) conditions together with polarization modulation infrared reflection adsorption spectroscopy (PMIRAS) results.18 They addressed that the rate-limiting step of this reaction is the NO dissociation step and isocyanate (NCO) species contributes to the reaction process as an essential reaction intermediate (N + CO → NCO, NCO + NO → N2 + CO2). Although three distinct reaction pathways have been Received: November 17, 2016 Revised: December 28, 2016 Published: January 10, 2017 1763

DOI: 10.1021/acs.jpcc.6b11583 J. Phys. Chem. C 2017, 121, 1763−1769

Article

The Journal of Physical Chemistry C

intensity of Ir 4f7/2 so as to cancel out the attenuation effect from ambient pressure gases. The XP spectra were fitted by the convolution function of Doniach-Šunjić and Gaussian functions after subtraction of a Shirley-type background.

proposed for NO reduction by CO on the Ir surfaces, it is unclear which pathway mainly contributes to the N2 formation under realistic conditions. In this decade, a number of new surface science techniques have emerged which allow us to overcome the pressure-gap problem and understand the catalytic reactions under near realistic conditions. NAP X-ray photoelectron spectroscopy (NAP-XPS) is one of such techniques and provides a powerful approach for elemental, chemical, and surface sensitive analysis of both catalysts and adsorbates under NAP conditions. The NAP-XPS has the capability to detect NO dissociation (i.e., adsorbed atomic nitrogen and oxygen) and NCO formation directly as well as adsorption of NO and CO. In our previous report,16 NO reduction by CO on Ir(111) surfaces was investigated by the NAP-XPS technique. Because of an upper limit of heating, however, only the initial stage of NO + CO reaction was studied. In this work, the NO reduction by CO on Ir(111) surface is investigated over a wider range of temperature under various CO pressures by the combination of NAP-XPS and mass spectrometry (MS). We observed that N2 is produced by two reactions: (i) N2 formation from NO adsorbed at atop site (NOatop) and atomic nitrogen (Nad) and (ii) associative desorption of Nad. The ignition temperature of the former reaction shifts higher as increasing CO pressure, while that of the latter reaction appears almost constant (∼400 °C) irrespective of CO pressure. The latter reaction is the dominant pathway to form N2 and contribution from the NCO intermediate is excluded under the present NAP reaction conditions.

3. RESULTS AND DISCUSSION 3.1. NO Reduction Monitoring with Online MS. Figure. 1 shows partial pressures of NO (m/e = 30), 13CO (m/e = 29),

2. EXPERIMENTAL SECTION NAP-XPS experiments were performed at the soft X-ray undulator beamline 13B of the Photon Factory (PF) at the High Energy Accelerator Research Organization (KEK). The details of the NAP-XPS apparatus and BL-13B of the PF are described elsewhere.23,24 The base pressure of the analysis and preparation chamber were 2 × 10−9 Torr and 2 × 10−10 Torr, respectively. The Ir(111) single-crystal (MaTeck, 10 mm o.d. × 1 mm, 99.999% purity) surface was cleaned by repeated cycles of Ar+ sputtering (PAr = 5.0 × 10−7 Torr, 3 keV) at room temperature, oxygen treatment (PO2 = 5.0 × 10−7 Torr, 780 °C) and annealing (∼880 °C). The periodicity and cleanness of the surfaces were confirmed by low energy electron diffraction (LEED) and XPS, respectively. NO (Takachiho chemical industrial, 99.9%) and 13CO (99.99%, isotopic enrichment ≥99%) were introduced to the analysis chamber via variable metal leak valves. Isotopic CO (i.e., 13CO) was used to distinguish N2 from CO (m/e = 28) and N2O from carbon dioxide (CO2) (m/e = 44) in the MS measurements. The rate of increasing temperature was almost constant in the MS experiments (∼10 °C/min), whereas in XPS measurements with MS monitoring, increasing temperature was stopped when XPS was measured. The incident photon energies used for XPS measurements were 160, 400, 500, and 650 eV for Ir 4f7/2, C 1s, N 1s, and O 1s levels, respectively, such that the photoelectron kinetic energies are almost the same (Ek ≈ 100 eV) which enables to obtain information with the same surface sensitivity for each core level. Beam-induced damages were checked by contiguous scans of XP spectra and no detectable radiation damage is found during the XPS measurements. The binding energy (BE) was calibrated by the Fermi edge of the Ir substrate. All XP spectra were normalized by integrated

Figure 1. Online MS intensities of 13CO (green), NO (dark yellow), N2(blue), 13CO2 (orange), and N2O (brown) from Ir(111) under 50 mTorr NO and 13CO with pressures of (a) 10, (b) 30, and (c) 250 mTorr. The surface temperature was linearly increased with time at a constant rate (∼10 °C/min). The vertical bands indicate ignition temperatures of the N2 formation via N + NO (orange) and via N + N (pink) (see text for details).

N2 (m/e = 28), 13CO2 (m/e = 45), and N2O (m/e = 44) taken under exposure of the Ir(111) surface to NO and CO gases at different 13CO pressures (10, 30, and 250 mTorr) with a fixed NO pressure (50 mTorr). In the case of 13CO pressure of 10 mTorr (Figure 1a), N2 and CO2 starts to appear at 280 °C, which is accompanied by formation of a small amount of N2O. The formation of N2O as a byproduct was also observed for Ir black at temperatures below 350 °C.25 As the temperature is increased further, the formation rate of N2 and CO2 starts to increase more at around 400 °C, whereas that of N2O keeps unchanged. Since the heating rate was constant, this two-step increase in formation rate as a function of temperature indicates the contribution of two different reaction pathways. The presence and the absence of accompanying N2O also support that the observed two activations are associated with different pathways; the activation at 280 °C includes the formation of N2O as a byproduct, while another one at 400 °C does not accompany the N2O formation. In the case of 13CO pressures of 30 and 250 mTorr, however, the activation appears only once at around 400 °C. Furthermore, N2O is produced under 30 mTorr 13CO at almost the same temperature as the activation (400 °C), whereas under 250 mTorr 13CO no clear evidence for N2O formation is observed at least in the 1764


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Figure 2. XP spectra of O 1s, N 1s, C 1s, and Ir 4f7/2 levels measured at various temperatures under 50 mTorr NO and 10 mTorr 13CO. The temperature was increased stepwise. Surface Ir atoms give different binding energies in Ir 4f7/2 level depending on the coordination number. The Ir 4f7/2 spectra are deconvoluted into several components corresponding to Ir atoms with different coordination numbers shown at the bottom. The bottom inset is illustrated by VESTA.26 The gas-phase species give peaks at higher binding energies outside the indicated ranges.

temperature range studied here (below 470 °C). Thus, one of the reaction pathways accompanies N2O formation and its activation temperature strongly depends on CO pressure and seems to shift to higher temperature with increasing CO pressure. The other pathway does not accompany N2O formation, and its activation temperature is almost independent of CO pressure and appears at ∼400 °C. Since the amount of produced N2O is at most 1/10th of the N2, the Ir(111) exhibits a high N2 selectivity for NO reduction by CO, which is in good agreement with previous reports.1,2,17,18 3.2. NAP-XPS and MS Results Taken under a Reaction Condition at 10 mTorr of CO. Figure 2 shows O 1s, N 1s, C 1s, and Ir 4f7/2 XP spectra taken under 50 mTorr NO + 10 mTorr 13CO with increasing temperature from room temperature to 518 °C. At room temperature, the Ir(111) surface is almost covered by CO adsorbing at atop sites (COatop), which is indicated by observation of prominent peaks at 532.3 eV in O 1s and 286.3 eV in C 1s.16 A small peak is observed at 400.3 eV in N 1s at room temperature, which is assigned to adsorbed NO molecule (NOad). This NO molecule is adsorbed at hollow sites, because the O 1s spectrum exhibits a small feature at 530.5 eV which is ascribed to NO adsorbed at hollow sites (NOhollow).17 In Ir 4f7/2 level, the spectrum can be deconvoluted into four components, of which core-level-shifts (CLSs) with respect to the bulk component (60.85 eV) are −0.50, −0.15, 0 and +0.25 eV. According to the previous work,16 these components can be assigned to adsorbate-free surface Ir atom (Ir0), surface Ir atom coordinated to one adsorbate located at 3fold hollow site (Ir1/3), bulk Ir atom and surface Ir atom bonded to COatop (Ir1), respectively. The Ir1 component is

overlapped by another component from surface Ir atom coordinated to two adsorbates located at 3-fold hollow sites (Ir2/3).16 These Ir atoms and the adsorption sites schematically illustrated at the bottom of Figure 2. In addition to these Ir atoms, the surface Ir atom coordinated to NOatop could give a peak at 61.70 eV (CLS of 0.85 eV).16 Because under the present condition the component of NOatop is absent and instead the Ir1/3 component is observed at room temperature, the NOad peak in N 1s should be attributed to NOhollow. This is supported by the observation of the 530.5 eV feature in O 1s and consistent with the previous IRAS results.17 As the temperature increases, the COatop peak decreases and disappears at 518 °C as shown in Figure 2. At 518 °C a new peak appears at 530.0 eV in O 1s, which is attributed to atomic oxygen adsorbed at hollow sites (Oad). In N 1s spectra, a peak appears at 397.5 eV at 256 °C. This is associated with Nad at hollow sites, which is resulted from dissociation of NO at hollow sites.16,17 The atomic oxygen generated from the NO dissociation is removed from the surface by reaction with COatop. However, at 518 °C the generation rate of Oad surpasses the consumption rate of Oad by CO due to increasing dissociation of NO and decreasing adsorption of CO with temperature. As a result, the Oad remains on the surface with a coverage of 0.05 monolayer (ML, defined as the adsorbate density with respect to the first layer of Ir(111)), while the COatop disappears at 518 °C. The Nad peak grows from 256 °C due to an enhanced dissociation of NO with temperature. It takes a maximum at 440 °C and diminishes at 518 °C, which indicates that the consumption rate of Nad is increased faster than the dissociation rate of the NO at the elevated 1765


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effects are not significant (see Figure S1). At room temperature, no reaction takes place and the surface is mostly covered by COatop. The COatop coverage is estimated from ratio of the surface Ir components (Ir0, Ir1/3, and Ir1) to be 0.56 ML. Here the contribution of Ir2/3 is assumed to be zero, since Ir2/3 could appear exclusively at high coverages (>0.33 ML) of N, O and NO. This situation keeps almost unchanged up to 256 °C, though the COatop coverage is slightly decreased to 0.44 ML. However, at around 300 °C, the Ir(111) surface is activated and the first reaction pathway sets in. At this moment, the COatop coverage decreases to 0.30 ML, while the Nad coverage increases to 0.14 ML. The heating of the surface at 300 °C promotes CO desorption and NO dissociation resulting in a decrease in COatop coverage and an increases in Nad coverage. Although Oad species should be generated by the NO dissociation, it is immediately removed from the surface by the reaction with CO to form CO2. This is indicated by generation of gaseous CO2 and absence of the Oad species at 312 °C as shown in parts a and b of Figure 3. The previous IRAS work reported that the NOatop species can be observed exclusively at coadsorbed COatop coverages below 0.27 ML at room temperature.17 Interestingly, the COatop coverage at the ignition of the first reaction pathway (0.30 ML) is almost the same as this critical coverage. Actually a small amount of molecular NO is observed above room temperature in N 1s level (see Figure 2) and this could be attributed to NOatop as mentioned above. Thus, it is most likely that the decrease of COatop coverage allows NO molecules to be adsorbed at atop sites and react with the Nad to form N2 via N2O. As the temperature increases further, the dissociation probability of NO becomes higher and the Nad coverage increases to 0.2 ML at 391 °C. Whereas the adsorption probability of the NOatop should be reduced with increasing temperature. As a result the rate of Nad + NOatop reaction exhibits no significant increase. Nevertheless above 440 °C the N2 formation rate increases again, which indicates that the second reaction pathway starts to contribute to the N2 formation. At 518 °C, the Nad coverage decreases to 0.11 ML. The enhanced N2 formation rate and the reduction in Nad coverage with increasing temperature are explained by thermal activation of the Nad + Nad reaction pathway. Since the Nad species sufficiently exist before the thermal activation, this reaction pathway is regarded as reaction-limited. It should be noted that no NCO-associated peak is observed at any temperatures used here. The NCO-relating reaction does not seem to contribute to the NO reduction under the present conditions. 3.3. NAP-XPS and MS Results Taken under a Reaction Condition at 250 mTorr of CO. Figure 4 shows O 1s, N 1s, C 1s, and Ir 4f7/2 XP spectra taken under 50 mTorr NO + 250 mTorr 13CO with increasing temperature from room temperature to 497 °C. At room temperature, the COatop and the NOhollow peaks were observed in O 1s level unlike the case of 10 mTorr 13CO where only the COatop is observed. This is because NO gas was first dosed, which is followed by CO dose in the case of 250 mTorr CO. At 327 °C the NOhollow dissociates into Nad and Oad, latter of which is removed from the surface by COatop. As the temperature increases, the COatop gradually desorbs from the surface but still remains even at 497 °C, while the Nad increases until 418 °C. The Ir 4f7/2 curves are deconvoluted into Ir0, Ir1/3, bulk and Ir1 (+Ir2/3) components. The Ir1 decreases in accordance with the COatop, while the Ir1/3 increases following the Nad. Note that the Ir0 keeps observable

temperatures. Changes in O 1s, N 1s and C 1s spectra as a function of temperature are in agreement with those in Ir 4f7/2 spectra; the Ir 1 component is decreased and the Ir 1/3 component is increased with increasing temperature, which means that the COatop desorbs and instead the population of Nad is increased. The Ir0 (adsorbate-free surface Ir) component remains almost unchanged from room temperature to even above 500 °C, although the reaction readily proceeds under near ambient pressure condition. It should be noted that a small amount of NOad is observable up to 440 °C in N 1s spectra. At room temperature, the NOad is associated with NOhollow as evidenced by the 530.5 eV feature in O 1s level. However, since the NOhollow dissociates into atomic N and O above room temperature,17 the NOad species observed in N 1s from 256 to 440 °C s should be attributed to NOatop. This is also consistent with the IRAS results.17 Figure 3 shows MS intensities of 13CO, NO, N2, and 13CO2 under the reaction condition at 10 mTorr of 13CO plotted as a

Figure 3. (a) MS intensities of 13CO, NO, N2, and 13CO2 under the reaction condition at 10 mTorr of 13CO are plotted as a function of temperature together with (b) coverages of adsorbates (COatop, NOad, Nad, and Oad) and (c) fractions of the surface Ir components (Ir0, Ir1/3 and Ir1 (+Ir2/3)) estimated from the peak areas of XP spectra shown in Figure 2.

function of temperature (Figure 3a) together with coverages of adsorbates (COatop, NOad, Nad, and Oad) (Figure 3b) and fractions of the surface Ir components (Ir0, Ir1/3, and Ir1(+Ir2/3)) (Figure 3c) estimated from the peak areas of XP spectra shown in Figure 2. Photoelectron diffraction (PD) effects on the Ir components are checked by photon-energy dependence of peak intensities and it is confirmed that the PD 1766


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Figure 4. XP spectra of O 1s, N 1s, C 1s, and Ir 4f7/2 levels measured at various temperatures under 50 mTorr NO and 250 mTorr 13CO. The temperature was increased stepwise in the same way as Figure 2.

in the same manner as the case of 10 mTorr CO. The Oad peak is not observed at any temperature due to rapid consumption by CO. In Figure 5 are shown pressures of 13CO, NO, N2, and 13CO2 under the reaction condition at 250 mTorr of 13CO plotted as a function of temperature (Figure 5a) together with the coverages of adsorbates (COatop, NOad, Nad and Oad) (Figure

5b) and the fractions of the surface Ir components (Ir0, Ir1/3 and Ir1 (+Ir2/3)) (Figure 5c) estimated from the peak areas of XP spectra shown in Figure 4. The coverage of COatop at room temperature is estimated to be 0.50 ML. At 327 °C, no reaction takes place, though the Nad + NOatop reaction readily occurs at the same temperature under 10 mTorr CO. The higher pressure of CO (250 mTorr) induces the higher equilibrium coverage of COatop (0.37 ML) even at the same temperature, which prevents NO molecules from being adsorbed at atop sites and hence suppresses the Nad + NOatop reaction pathway. Above 400 °C, the N2 formation is observed. Since no N2O is formed at these temperatures (not shown in Figure 5 but confirmed in Figure 1c), this N2 formation is attributed to the Nad + Nad reaction pathway. Note that the Ir1/3 fraction is almost three times larger than the Nad coverage. This means that each Nad atom occupies three Ir atoms in the hollow configuration. If it is assumed that no Ir2/3 exists on the Ir(111) surface, the nearest-neighbor distance between the Nad atoms corresponds to the lattice constant of a (√3 × √3)R30° superstructure which is fairly long (0.47 nm). This suggests a strong repulsive interaction between the Nad atoms. Thermal activation to overcome a large potential barrier due to the strong repulsive interaction may be required to attain the recombination. No NCO-associating peak is observed in N 1s level again even under 250 mTorr 13CO as confirmed in Figure 4, although the Nad species is clearly formed and exposure of the Nad species to the higher-pressure CO could induce a formation of NCO. Thus, under 250 mTorr 13CO, only the Nad + Nad and Oad + COatop reactions contribute to the NO reduction by CO. 3.4. Reaction Mechanism for NO Reduction by CO on Ir(111). Two reaction pathways to form N2 are observed for NO reduction by CO on Ir(111) under 10 mTorr CO as shown in Figure 6. The Nad + NOatop reaction starts at 280 °C and the Nad + Nad reaction at ∼400 °C. It should be noted that the fraction of adsorbate-free surface Ir atoms (Ir0) stays fairly large (approximately 30% of the first Ir layer) during the whole reaction as shown in Figure 3c. The adsorbate-free surface Ir atoms could be formed within the (m√3 × m√3)R30° − nCOatop lattices ((m, n) = (1,1), (2,7), (3,19)) on Ir(111)16 and might play a role to accommodate NO molecules at atop sites and supply them to the Nad + NOatop reaction. The Nad + NOatop reaction should once yield N2O and dissociates into N2 and Oad. N2 desorption via decomposition of N2O has been observed for various platinum-group metal surfaces.27 In fact a small amount of N2O is accompanied by the N2 formation in

Figure 5. (a) MS intensities of 13CO, NO, N2 and 13CO2 under the reaction condition at 250 mTorr of 13CO are plotted as a function of temperature together with (b) coverages of adsorbates (COatop, NOad, Nad and Oad) and (c) fractions of the surface Ir components (Ir0, Ir1/3, and Ir1 (+Ir2/3)) estimated from the peak areas of XP spectra shown in Figure 4. 1767


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observed two pathways are attributed to the Nad + NOatop and the Nad + Nad reactions. The ignition temperature of the Nad + NOatop reaction is significantly influenced by CO pressure, while that of the Nad + Nad reaction stays almost constant (∼400 °C). The former reaction is a NOatop-adsorption-limited process and the latter one is a reaction-limited process. The Nad + Nad reaction is the dominant pathway to form N2. The pathway including the NCO species as an intermediate could not be observed under the present reaction conditions. The absence of lower-temperature activity below 280 °C can be ascribed to CO poisoning. The high N2 selectivity of Ir catalysts is related to nearly perfect decomposition of N2O formed from the Nad + NOatop reaction. Theoretical studies on the Nad + NOatop reaction pathway would uncover the physical origin of the almost complete decomposition of the N2O intermediate on the Ir surfaces.

Figure 6. N2 MS intensity from the Ir(111) surface under 50 mTorr NO and 10 mTorr 13CO as a function of temperature (same as that in Figure 3a). It exhibits two distinct pathways. Reaction models for the two pathways are illustrated in the figure.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b11583. Photoelectron diffraction effects on the Ir 4f7/2 peaks checked by photon-energy dependence of peak intensities (Figure S1) (PDF)

the present study. This is a sharp contrast to the case of Rh, where a significant amount of N2O is produced.2 This difference in product could be associated with the different elementary steps; on the Ir(111) surface the NOatop species exclusively contributes to reaction with the Nad species, while on Rh(111) the NOhollow species is reactive to the Nad.28 Since the Nad species are located at hollow sites on the both surfaces,29−31 the difference in adsorption site of reactive NO could be contributing to the difference in product. Theoretical studies on the dynamics of the elementary steps are desirable to understand the origin of the difference in the product. The ignition temperature of 280 °C for the Nad + NOatop reaction under 10 mTorr CO is much higher than those for the Nad + NOatop reaction on CO-precovered Ir(111) (150 °C)17,28 and for the Nad + NOhollow reaction on Rh(111) (150−200 °C).28,32 This is probably due to CO poisoning under 10 mTorr CO for Ir(111) where the surface is dominated by high-coverage COatop as shown in Figure 3(b). CO molecules are more strongly adsorbed on Ir(111) than NO molecules,16,31 which is an opposite trend to the case of Rh(111).33 The absence of activity at a low temperature (150 °C) for NO + CO reaction on Ir particles supported by alumina2 under ambient pressure conditions could be explained by the CO poisoning. Since the N2 MS intensity for the vertical axis of Figure 6 corresponds to formation rate of N2, the N2 formation rate via the Nad + Nad reaction is significantly higher than that via the Nad + NOatop reaction. Another reaction pathway via the NCO intermediate is not observed under the present reaction conditions as mentioned above. Therefore, the associative desorption of Nad species is the dominant reaction pathway for NO reduction to N2 by CO on the Ir(111) model catalyst under the NAP conditions studied here. The contribution of the Nad + NOatop reaction is suppressed by the coadsorbed CO and almost disappears at the higher CO pressures.

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AUTHOR INFORMATION

Corresponding Author

*(H.K.) Telephone: +81-45-566-1701. Fax: +81-45-566-1697. E-mail: [email protected]. ORCID

Masaaki Yoshida: 0000-0002-3656-4444 Hiroshi Kondoh: 0000-0003-3877-5891 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Photon Factory staffs for their technical supports. This study was supported by the Grants-in-Aid for scientific research (Nos. 20245004 and 26248008) and the MEXTsupported program for the Strategic Research Foundation at Private Universities, 2009−2013. We would like to thank N. Saida of the workshop of Keio University for fabricating sample holders and other relating metal parts. K.U. acknowledges the Keio University Doctorate Student Grant-in-aid Program. The experiments were performed under the approval of the Photon Factory Program Advisory Committee (PF PAC No. 2015S2008).

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REFERENCES

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4. CONCLUSION We conducted operando observations of NO reduction by CO (NO + CO → N2 and CO2) on Ir(111) surfaces under near ambient pressure conditions by NAP-XPS and MS at various temperatures (room temperatureca. 500 °C) and pressures (PNO = 50 mTorr and PCO = 10, 30, and 250 mTorr). The online MS results clearly indicate that two distinct reaction pathways contribute to the NO reduction. On the basis of the NAP-XPS results taken under the reaction conditions, the 1768


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