Mechanism of Ammonia Formation on Rh(111) Studied by Dispersive

Jan 15, 2010 - Kenta Amemiya , Yuka Kousa , Shuichi Nakamoto , Taiga Harada , Shogo Kozai , Masaaki Yoshida , Hitoshi Abe , Ryohei Sumii , Masako ...
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Mechanism of Ammonia Formation on Rh(111) Studied by Dispersive Near-Edge X-ray Absorption Fine Structure Spectroscopy Masanari Nagasaka,† Hiroshi Kondoh,*,‡ Kenta Amemiya,§ Ikuyo Nakai,| Toru Shimada,⊥ Reona Yokota,# and Toshiaki Ohta∇ Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan, Department of Chemistry, Keio UniVersity, Yokohama 223-8522, Japan, Institute of Materials Structure Science, High Energy Accelerator Research Organization, Tsukuba 305-0801, Japan, Department of Chemistry, Graduate School of Science, Kyoto UniVersity, Kyoto 606-8502, Japan, Fachbereich Physik, Freie UniVersita¨t Berlin, 14195 Berlin, Germany, Sekisui Chemical Co., Ltd., Tsukuba 300-4292, Japan, and SR Center, Ritsumeikan UniVersity, 1-1-1 Noji-Higashi, Kusatsu, Shiga 525-8577, Japan ReceiVed: July 19, 2009; ReVised Manuscript ReceiVed: NoVember 17, 2009

The reaction mechanism of ammonia formation on a Rh(111) surface was investigated by means of dispersive near-edge X-ray absorption fine structure (dispersive-NEXAFS) spectroscopy. Nitrogen-covered Rh(111) surfaces were exposed to gaseous hydrogen (1.0-5.0 × 10-7 Torr) at constant surface temperatures (330-390 K) to form ammonia which desorbs immediately from the surface. Continuous data acquisition of nitrogen K-edge NEXAFS spectra enables us to monitor coverage changes of surface species during the progress of the reaction. The obtained NEXAFS spectra were well reproduced by summation of the standard spectra for N and NH. We found that the NH species, which is considered as a stable intermediate, is not only hydrogenized to NH3, but also decomposed into N and H under H2-lacking conditions. The reaction-order analyses indicate that the NH decomposition occurs at periphery of 2-dimensional islands of NH while the NH3 formation takes place most likely over the entire NH islands. We propose a two-step reaction mechanism: (1) formation of NH islands and (2) hydrogenation of the NH species to NH3. The former process reaches equilibrium with decomposition after a short period. k1

I. Introduction

Nad + Had 98 NHad

Rhodium is one of key catalysts that convert automotive exhaust gas to harmless species, particularly effective for NO reduction. By the reaction of NO and CO on rhodium surfaces, adsorbed NO species is dissociated into atomic N and O1,2 and the atomic N desorbs as N2 gas.3 In addition to the N2 formation,4,5 a substantial amount of NH3 is also produced under realistic conditions, since NO is reduced by hydrogen that is one of components of the exhaust gas and also formed from hydrocarbons on metal particles of the catalysts. Although the NH3 formation on Rh surfaces has been so far studied both from experimental6,7 and theoretical8 points of view, the reaction mechanism has not been fully understood yet. In the previous studies on the ammonia formation, the reaction system was simplified such that Rh(111) surfaces were once partially covered by atomic nitrogen and then exposed to gaseous hydrogen.6,7 Therefore, the effects of NO adsorption and dissociation on the ammonia formation were excluded, and the resultant reaction system was considered as a model of successive hydrogen association reaction.8 This reaction has been believed to proceed step by step from nitrogen atom to ammonia gas:6,8 * To whom correspondence should be addressed. Phone/Fax: +81-45566-1701. E-mail: [email protected]. † Institute for Molecular Science. ‡ Keio University. § Institute of Materials Structure Science. | Kyoto University. ⊥ Freie Universita ¨ t Berlin. # Sekisui Chemical Co., Ltd. ∇ Ritsumeikan University.

k2

NHad + Had 98 NH2,ad k3

NH2,ad + Had 98 NH3v

(1)

(2)

(3)

Here the intermediates NHx (x ) 1, 2) play an important role in the reaction and have been studied by temperature programmed desorption (TPD)9,10 and static secondary ion mass spectrometry (SSIMS).6,7 The TPD studies concluded that NH is more stable than NH2, while the SSIMS results indicated that NH2 is the predominant intermediate. According to the SSIMS studies, step 3, the NH2 hydrogenation, is rate limiting and most of the formed NH3 molecules are immediately desorbed from the surface above 330 K.6,7 The energy diagram for the NHx hydrogenation on Rh(111), obtained from density functional theory (DFT) calculations,8 indicates that NH species is the most stable intermediate as proposed from the TPD measurements.9,10 High resolution electron energy loss spectroscopy (HREELS) studies support the existence of NH during the NO + H2 reaction on a Rh(100) surface.11 On the other hand, NH2 species is dominant during the NO + H2 reaction on the Pt(100)-(1 × 1) surface.12 Further spectroscopic monitoring of the surface species under the reaction conditions is strongly desirable to understand the reaction mechanism. The NO + H2 reaction on a Rh(111) surface has been attracting much attention because of its strange behavior under specific reaction conditions: oscillation of the formation rate of

10.1021/jp906833v  2010 American Chemical Society Published on Web 01/15/2010

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N2 gas.13-15 The surface distribution of the adsorbed species exhibits spiral waves with a mesoscopic scale.16,17 The main factor of these characteristic phenomena is assumed to be NH islands formed by the attractive interactions between NH species.17-19 For revealing the mechanism of the NO + H2 reaction, it is necessary to study the mechanism of the ammonia formation, which is one of the elementary steps in the reaction. Near edge X-ray absorption fine structure (NEXAFS) is a very promising method to obtain the quantitative information of surface adsorbed species.20 However, it takes usually more than several minutes to measure a spectrum from submonolayer adsorbates, which restricts the investigation of surface reaction. Recently, we have developed a dispersive-NEXAFS spectroscopy,21 which enables us to obtain a spectrum within approximately 10 s. Thus, the in situ observations of various surface reactions have been conducted to obtain the real-time coverage changes of reaction species.22-25 In this work, the real-time coverage changes of surface species were measured during the ammonia formation on Rh(111) by the dispersive-NEXAFS spectroscopy. The decomposition of NH to form N was also observed when the supply of the hydrogen gas was stopped. On the basis of the kinetics analyses of the ammonia formation and the NH decomposition, we proposed a mechanism for the ammonia formation on the Rh(111) surface. II. Experimental Section The experiments were performed at the soft X-ray beamline, BL-7A,26 of the Photon Factory, Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK-PF), using an ultrahigh vacuum chamber (base pressure, below 2.0 × 10-10 Torr). The chamber at the beamline end is equipped with a position sensitive electron energy analyzer (GAMMADATA-SCIENTA, SES-2002) for X-ray photoelectron spectroscopy (XPS) and dispersiveNEXAFS spectroscopy, a microchannel plate for partialelectron-yield NEXAFS, a low energy electron diffraction (LEED) optics, and a quadrupole mass spectrometer. The dispersive NEXAFS is a technique recently developed to measure NEXAFS spectra with a certain energy range simultaneously by the combination of the dispersed X-rays and the position sensitive electron energy analyzer.21 The Hettrick mount monochromator is used as a polychromator by fully opening the exit slit.27 Dispersed X-rays from the polychromator irradiate the sample surface (typically 10 eV at N K-edge). Each position of the irradiated surface emits Auger electrons, whose number is proportional to the X-ray absorption coefficient. Integrating the Auger electron counts from each sample position28 yields a NEXAFS spectrum in one shot. It takes typically 10 s to acquire a single spectrum from submonolayer adsorbates with a high signal-to-noise ratio. A Rh(111) single crystal surface was cleaned by the cycles of Ar+ sputtering and annealing at 1250 K. The cleanliness and ordering of the surface were checked by XPS and LEED. The nitrogen atom covered Rh(111) surfaces were prepared by exposing to 1.0 L (1 L ) 1.0 × 10-6 Torr s) NO at 150 K, annealing at 400 K for NO dissociation, and exposing to 2.0 L H2 gas at 400 K for removal of oxygen adsorbates. Nitrogen adsorbates also react with hydrogen gas, but the reactivity of the nitrogen atom is lower than that of the oxygen atom.6 Thus, a certain amount of the nitrogen atoms remains after the exposure of the hydrogen gas. The coverage of initial nitrogen atoms was monitored by XPS. After preparing the nitrogen atom covered surface, the formation of ammonia gas was performed

Figure 1. (a) N K-edge NEXAFS spectra of an N-covered Rh(111) surface taken by the dispersive mode as a function of time, under exposure to 1.0 × 10-7 Torr hydrogen gas at 350 K. It took 14 s to obtain one spectrum, and some selected spectra are shown. H2 exposure started at 0 min. (b) Selected N K-edge NEXAFS spectra recorded during the reaction. Spectral shape clearly changes with time due to contribution from a reaction intermediate.

by exposing to hydrogen gas (1.0-5.0 × 10-7 Torr) at several surface temperatures (330-390 K). During the exposure of hydrogen gas, the dispersive-NEXAFS spectra were recorded with a time interval of 14 s. Although the partial pressure measurements of the formed NH3 gas by using the mass spectrometer are desirable, under the experimental conditions used here, the reaction proceeds too slowly to detect the formed NH3 gas with our mass spectrometer. Therefore, the amount of the formed NH3 gas was estimated as the difference between the initial N coverage and the coverage of all N-containing surface species, since the formed NH3 desorbs immediately from the surface.29,30 III. Results and Discussion A. Typical Example of Ammonia Formation. Figure 1a shows a series of N K-edge NEXAFS spectra taken with the dispersive mode during the ammonia formation. We adopted an X-ray incident angle of 55° with respect to the surface parallel in order to estimate coverages of the surface species without the X-ray polarization effect.20 The NH3 formation reaction proceeded by exposing the N atom covered surface to 1.0 × 10-7 Torr H2 gas at 350 K. It is obviously recognized that a

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Figure 3. Coverage changes of N and NH and the amount of the formed NH3 gas as a function of time during NH3 formation on a N-covered Rh(111) surface. The amount of NH3 gas is estimated by subtraction of N and NH coverages from the initial N coverage. The reaction proceeded by exposing to 1.0 × 10-7 Torr H2 gas at 350 K. The exposure started at 0 min.

Figure 2. (a) Standard N K-edge NEXAFS spectra for N and NH adsorbed on Rh(111). (b) An example of the least-squares fitting to estimate the N and NH coverages. Dispersive-NEXAFS spectrum was fitted by the superposition of the standard NEXAFS spectra.

first peak due to N (398.6 eV) reduces in intensity in the progress of the reaction, and a broad structure observed from 401 to 404 eV, which is associated with NH as mentioned below, appears at first and then decreases. These spectral features are clearly recognized from the selected spectra shown in Figure 1b. As the N coverage decreases, the contribution from atomic N becomes small as seen for the main peak at 398.6 eV. However, the relative intensity at the postedge region from 401 to 404 eV is increased. This increase is associated with contribution from an intermediate species, probably NH, whose standard spectrum is shown in Figure 2a. The main peak is gradually red-shifted during the reaction, which can be explained by increased contribution of the NH species to this peak because the peak position of the NH species is slightly lower than that of the N atom as shown in Figure 2a. To obtain coverage changes of N and NH during the reaction, each spectrum was fitted by a superposition of the standard NEXAFS spectra of N and NH adsorbed on Rh(111). The standard spectrum for N was taken from the initial surface, while that for NH was obtained from the decomposition of H2NCHO;9 an NH + CO mixed surface was formed by exposing to 5 L H2NCHO at 200 K, followed by annealing at 300 K. Because NH is the only N-containing species on this surface, the standard NH spectrum was successfully obtained, although CO molecules are coadsorbed. We confirmed that the NH species was not decomposed by the beam-induced damage under our experimental conditions. Since a pure NH2 adlayer has not been so far prepared on Rh(111) to our knowledge, we could not obtain the standard NH2 spectrum. Figure 2a shows the standard dispersive-NEXAFS spectra for N and NH adsorbed on Rh(111). All the spectra were divided by the spectrum taken from the clean Rh(111) surface in order to remove the effects from inhomogeneous sensitivity of the MCP detector and photonintensity distribution as a function of energy. The N and NH coverages were 0.13 and 0.09 ML, respectively. An example of the fitting result is shown in Figure 2b, where the observed spectrum was well reproduced by superposition of the standard spectra after the least-squares fitting. We confirmed that several NEXAFS spectra taken at different elapsed times were well

fitted by summation of the two standard spectra. The good agreement between the experimental and fitted curves suggests that the contribution from the NH2 species (if it exists) is negligibly small, although we cannot completely exclude the contribution from the NH2 species. Hereafter we assume that the N and the NH species are N-containing ones which are observable with the NEXAFS technique. The same fitting procedure was applied to all the NEXAFS spectra taken during the reaction to obtain the N and NH coverages as a function of time. All the spectra were well reproduced by this fitting procedure. The amount of produced NH3 was estimated by subtraction of the N and NH coverages from the initial N coverage because the NH3 desorbs immediately as mentioned above. Figure 3 shows the coverage changes of N and NH during the ammonia formation obtained by the fitting procedure. The amount of the formed NH3 gas is also shown as a function of time. The coverage of N decreases quickly, and NH is formed instead in the first 5 min. The NH coverage reached a maximum of ca. 0.04 ML. Then, both of the N and NH coverages decrease slowly, accompanied by the NH3 formation. We observed that a small amount of N remained on the surface even after a long exposure of hydrogen gas. B. NH Decomposition on Rh(111). In the previous section, the formation of the NH species during ammonia formation was confirmed by the fact that all the NEXAFS spectra were well reproduced by summation of the N and NH standard spectra. Here we investigated the reaction behavior when the exposure of hydrogen gas was stopped during the reaction. Because adsorbed hydrogen atoms desorbs from the surface at this temperature range,31 the lack of surface hydrogen after ceasing the H2 dose would cause a significant change in the reaction kinetics. The N-covered surface was first exposed to 5.0 × 10-7 Torr hydrogen gas at 370 K for 5 min. Just after ceasing to supply hydrogen gas, we found that the intermediate NH species is decomposed into N (and H) as shown in Figure 4a. We performed a reaction order analysis to understand the NH decomposition on Rh(111). The reaction equation and the rate of NH decomposition are expressed as k4

NHad 98 Nad + Had

(4)

d[NH] ) -k4[NH]m dt

(5)

Because there would be some transient processes during evacuation H2 gas, we analyze the NH coverage changes without

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Figure 4. (a) Coverages of N and NH during the NH decomposition at 370 K as a function of time. Hydrogen gas was supplied at a pressure of 1.0 × 10-7 Torr and stopped after 5 min. The results of the fitting procedures are also shown by a solid curve. (b) Double logarithmic plots of the decomposition rates versus the NH coverages. The slope m is found to be approximately 1/2. The double logarithmic plots of the decomposition rates versus the NH coverages at (c) 360 K and (d) 350 K.

considering a few data points recorded just after stopping of the dose. As shown in Figure 4b, the NH decomposition rate exhibits reaction order of 0.5 ( 0.5 with respect to the NH coverage. Although it has a large error due to insufficient statistics, we observed almost the same reaction order around 0.5 at other temperatures above 350 K as shown in Figure 4c,d. Therefore, it is likely that the NH decomposition obeys the halforder kinetics (m ) 1/2). The half-order dependence of NH decomposition rate suggests that the NH species form islands by attractive interactions possibly via hydrogen bonding, and the decomposition occurs exclusively at the periphery of the NH islands. Below 350 K, however, the decomposition does not exhibit the half-order kinetics, and there remains unreactive NH species. It might be due to reaction blocking by N atoms formed by the decomposition which decorates the peripheries of the NH islands because the diffusion of N atoms becomes more difficult at lower surface temperatures. The remaining N atoms prevent the peripheral NH species from decomposition possibly due to the absence of vacant sites available for the decomposition. From the above analysis, we deduced the theoretical kinetics of the NH decomposition as

1 2 [NH] ) [NH]1/2 0 - k4t 2

(

)

(6)

By using the above equation, the rate constant k4 of the NH decomposition has been obtained. Figure 4a shows the results of the fitting procedure. The rate constant k4 at 370 K was determined to be (4.9 ( 0.3) × 10-4 ML1/2 s-1. From the results of fitting procedures between 350 and 390 K, the activation energy and the pre-exponential factor of the rate constant k4 were estimated to be 0.91 ( 0.04 eV and (1 ( 1) × 109 ML1/2 s-1, respectively. C. Temperature and Pressure Dependence of Ammonia Formation. As shown in Figure 3, the amount of N, NH, and NH3 at 350 K were obtained as a function of time. At the initial stage, N atoms convert to NH very rapidly. After 5 min, the coverages of both N and NH decrease gradually to form NH3 gas. As described in the preceding section, we also observed the NH decomposition. Therefore, it is considered that the NH formation and NH decomposition reach equilibrium after 5 min,

Figure 5. Coverage changes of N and NH and the amount of the formed NH3 gas as a function of time during exposure to 1.0 × 10-7 Torr hydrogen gas at a surface temperature of (a) 350, (b) 370, and (c) 390 K, respectively.

and the total amount of N and NH gradually decreases by the formation of NH3 gas. In order to reveal the reaction kinetics, the ammonia formation experiment was carried out at several temperatures, as shown in Figure 5. We confirmed that the maximum coverage of NH decreases as the surface temperature increases. Note that a certain amount of N remained on the surface at 390 K after the long time exposure of hydrogen gas. It means the NH decomposition is more favored than the NH formation at higher surface temperatures. Therefore, the activation energy of NH decomposition is higher than that of the NH formation. The

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reduced maximum coverage of NH at higher temperatures would be caused by the enhancement of the NH decomposition.

d[N] ) -k1[N][H] + k4[NH]1/2 dt

The pressure dependence of ammonia formation was also investigated. Figure 6a shows the coverage changes of the reaction species monitored by the dispersive-NEXAFS spectroscopy. The maximum coverage of NH was almost the same irrespective of pressure. After passing the maximum coverage of NH, however, the N coverage decreased faster as the pressure of the hydrogen gas increased. Hydrogen atoms are directly involved both in the formation of NH and NH3, and these reaction rates should increase as the pressure of hydrogen gas becomes higher. On the other hand, NH decomposition is a selfreaction of NH species, which would be much less influenced by hydrogen pressure. The relative contribution of the NH decomposition is reduced at the higher pressure, as was seen in the faster consumption of N. The boxes in Figure 6a show the regions where the decrease of N coverage becomes relatively faster than that of NH as the increase of hydrogen pressures.

d[NH] ) k1[N][H] - k4[NH]1/2 - k2[NH]n[H] dt d[NH3] ) k2[NH]n[H] dt

As described in the previous section, the NH decomposition obeys the half-order kinetics, indicating that the decomposition occurs at the periphery of the NH islands. The N atoms diffuse very fast and do not form any islands, as is confirmed by the fact that N is not visible in the image of scanning tunneling microscopy (STM).32 Thus, the reaction order of the NH formation should be one with respect to both N and H coverages. However, no information is available for the reaction order of the NH3 formation from NH. The investigation of the pressure dependence of hydrogen gas is effective to reveal the order of the NH3 formation. Here we simulated the reaction kinetics at several hydrogen pressures by integrating the sets of reaction rate equations, which are expressed as

(7) (8) (9)

The hydrogen coverage is expected to be constant because it reaches equilibrium by the H adsorption and the H desorption. Note that the hydrogen atom adsorbs at the fcc hollow site,33 while N and NH occupy the hcp hollow sites.8,34 Thus, the adsorption of hydrogen atom is not blocked by preadsorbed N and NH species. Since the formation of NH3 from NH2 can be assumed to proceed fast as mentioned above, this path was not considered in the simulation. The rate constant k4 of the NH decomposition was set to 8.8 × 10-5 ML1/2 s-1, which is the value obtained from the decomposition experiment at 350 K. Because the main purpose of the simulation is the investigation of the pressure dependence of hydrogen gas, the rate constants k1 and k2 were adjusted in such a way that the experimental kinetics is well reproduced at the pressure of 1.0 × 10-7 Torr.35 The pressure dependence of the ammonia formation was calculated in the cases of the reaction order of n ) 1 and 1/2. When the hydrogen pressure increases by P times, the hydrogen coverage is expected to become P times larger. The simulations were also performed at pressures of 3.0 × 10-7 and 5.0 × 10-7 Torr using the same rate constants as used at 1.0 × 10-7 Torr.35 Figure 6b,c shows the pressure dependence of the simulated coverage changes during the ammonia formation under the condition of n ) 1 and 1/2, respectively. When the reaction order n is 1, the simulated consumption rate of N is faster at the higher pressure, and the change of N coverage almost coincides with that of NH after passing the

Figure 6. Coverage changes of N and NH and the amount of the formed NH3 gas as a function of time at 350 K during exposure to different pressures of hydrogen gas obtained from (a) the experiment, (b) the simulation at n ) 1, and (c) the simulation at n ) 1/2, respectively. From the bottom of the figures, the pressure of hydrogen gas is increased as 1.0 × 10-7, 3.0 × 10-7, and 5.0 × 10-7 Torr. The regions indicated by the boxes are used for the discussion of the reaction order of NH3 formation (see text).

Ammonia Formation on Rh(111)

Figure 7. Schematics of the reaction model proposed for the ammonia formation on a Rh(111) surface. The solid arrows indicate the diffusion of the adsorbed species. The dashed one means the adsorption or desorption of the molecules.

maximum of NH coverage. These regions were shown by the boxes in Figure 6b. The reaction order n ) 1 means that the formation of NH3 from NH proceeds over the entire NH island. The simulated pressure dependence is in good agreement with that obtained by the experiment (Figure 6a), where the decrease of N coverage becomes faster than that of the NH coverage. In the case of n ) 1/2, on the other hand, there remains N atoms at the final stage of the reaction although the NH coverage becomes almost zero. The boxes in Figure 6c show the regions where the NH species disappears with N atoms remaining on the surface. This is inconsistent with the experimental results (Figure 6a). The NH3 formation is assumed to occur only at the periphery of the NH island in the case of n ) 1/2. The NH consumption proceeds under competition between the NH3 formation and the NH decomposition at the edge of the NH islands. The disappearance of NH species during the course of the reaction was always found in the simulation irrespective of kinetic parameters.36 It is unreasonable that the intermediate NH islands disappear even though we assume the NH3 formation occurs exclusively at the periphery of the NH islands in the case of n ) 1/2. On the basis of these simulated results, we propose that n ) 1 is more plausible than n ) 1/2. It means that the formation of NH3 from NH proceeds over the entire NH islands. D. Mechanism of Ammonia Formation on Rh(111). From the above results, the mechanism of the ammonia formation on Rh(111) is proposed as follows: Figure 7 shows a scheme of the reaction model. In the initial condition, the adsorbed nitrogen atoms diffuse solely, and do not form islands. When the surface is exposed to hydrogen gas at 350 K, the surface density of the hydrogen atom stays constant because it reaches equilibrium by rapid adsorption and desorption of hydrogen due to the substrate temperature being higher than the desorption temperature of hydrogen.31 The adsorbed hydrogen atoms are also wellknown to diffuse very fast on the surface.37,38 NH species are created from N and H atoms on the surface, and condensed into two-dimensional islands by attractive lateral interactions. The attractive interactions would be caused by the hydrogen bonding of NH as seen on a Ni(110) surface.39 At the periphery of the NH islands, the decomposition of NH to form N and H and the attachment of diffusing NH simultaneously occur and probably reach an equilibrium. The hydrogen atoms can diffuse inside the NH islands. The hydrogenation of NH to ammonia takes place over the NH islands. Therefore, the size of the NH islands decreases with keeping the equilibrium with N and H.

J. Phys. Chem. C, Vol. 114, No. 5, 2010 2169 We believe that the hydrogenation from NH to NH2 and that from NH2 to NH3 proceed sequentially and the second hydrogenation from NH2 to NH3 occurs so quickly that the lifetime of NH2 is too short to be observed with our fairly slow technique (14 s/spectrum). To evidence this hypothesis it needs molecular dynamics calculations. In this study, we regarded the reaction intermediate as NH. However, there exists controversial discussion about the reaction intermediate. The DFT and TPD studies suggested NH is the stable species.8,9 On the other hand, NH2 was considered as the reaction intermediate in the SSIMS studies.6,7 Because the NEXAFS spectra taken during the reaction were reproduced by the summation of the standard spectra of N and NH, we here consider NH as the stable reaction intermediate. Once the NH species is hydrogenized into NH2, it is assumed to be quickly further hydrogenized into NH3. Since we could not obtain the standard spectrum of NH2 in the present study, however, we cannot completely exclude the existence of the NH2 species as mentioned above. The contribution of the NH2 species as an intermediate is a future subject. The NH islands would have a hydrogen-bonding network, which may prevent the NH species from decomposing inside the islands. To confirm the validity of the model, it is desirable to observe the structure of the surface species during the ammonia formation. Because the reaction-order analyses with kinetics simulations provided a view that the ammonia formation is significantly influenced by the formation of the NH islands, in situ STM observation of microscopic behavior of the surface species will be useful to understand how the intermediate NH species form islands and where they react with hydrogen to form NH3. IV. Conclusions The ammonia formation reaction was studied by exposing the N-covered surface to hydrogen gas at a constant surface temperature. The coverage changes during the course of the reaction were monitored with the dispersive NEXAFS. The obtained data were analyzed assuming that NH is an only observable intermediate based on the fact that the NEXAFS spectra were well reproduced by summation of the N and NH standard spectra. The data analyses indicated the following results: First, the coverage of N decreases rapidly and NH is formed instead. Subsequently, the coverages of both N and NH decrease slowly, leading to the formation of NH3 gas. The reverse reaction path was also found, where NH is decomposed into N and H. From the obtained kinetics, a reaction model is proposed as follows. First, randomly distributed N and H atoms react to form NH, and the NH species form islands with their attractive interactions. Because the islanding NH species are decomposed at the periphery, the formation and decomposition of NH are in equilibrium. Comparison between the experimental and simulated kinetics suggests that NH3 species are formed over the entire NH islands. As a result, the size of the NH islands decreases with maintaining the equilibrium between N and NH species during the reaction. Acknowledgment. This study was supported by the Grantsin-Aid for scientific research (no. 17205002) and the 21st century COE Program from the Ministry of Education, Culture, Sports, Science, and Technology. The present work has been performed under the approval of the Photon Factory Program Advisory Committee (PF PAC nos. 2001S2-003, 2004G-320, and 2006G-355).

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Supporting Information Available: Simulated coverage changes during ammonia formation on Rh(111) at different kinetic parameters for rate constants k1 and k2, assuming reaction order of n ) 1 (Figure S1) and n ) 1/2 (Figure S2) with respect to hydrogen pressure.This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Borg, H. J.; Reijerse, J. F. C.-J. M.; Van Santen, R. A.; Niemantsverdriet, J. W. J. Chem. Phys. 1994, 101, 10052. (2) Root, T. W.; Fisher, G. B.; Schmidt, L. D. J. Chem. Phys. 1986, 85, 4679. (3) Zaera, F.; Gopinath, C. S. J. Chem. Phys. 2002, 116, 1128. (4) Bugyi, L.; Solymosi, F. Surf. Sci. 1991, 258, 55. (5) Belton, D. N.; Dimaggio, C. L.; Ng, K. Y. S. J. Catal. 1993, 144, 273. (6) Van Hardeveld, R. M.; Van Santen, R. A.; Niemantsverdriet, J. W. J. Phys. Chem. B 1997, 101, 998. (7) Van Hardeveld, R. M.; Van Santen, R. A.; Niemantsverdriet, J. W. J. Vac. Sci. Technol., A 1997, 15, 1558. (8) Liu, Z.; Hu, P.; Lee, M. J. Chem. Phys. 2003, 119, 6282. (9) Wagner, M. L.; Schmidt, L. D. Surf. Sci. 1991, 257, 113. (10) Wagner, M. L.; Schmidt, L. D. J. Phys. Chem. 1995, 99, 805. (11) Tanaka, K.; Yamada, T.; Nieuwenhuys, B. E. Surf. Sci. 1991, 242, 503. (12) Smirnov, M. Y.; Zemlyanov, D. J. Phys. Chem. B 2000, 104, 4661. (13) Cobden, P. D.; Janssen, N. M. H.; Van Breugel, Y.; Nieuwenhuys, B. E. Surf. Sci. 1996, 366, 432. (14) Makeev, A. G.; Slinko, M. M.; Janssen, N. M. H.; Cobden, P. D.; Nieuwenhuys, B. E. J. Chem. Phys. 1996, 105, 7210. (15) Cobden, P. D.; Nieuwenhuys, B. E.; Esch, F.; Baraldi, A.; Comelli, G.; Lizzit, S.; Kiskinova, M. Surf. Sci. 1998, 416, 264. (16) Schaak, A.; Imbihl, R. J. Chem. Phys. 2002, 116, 9021. (17) Makeev, A. G.; Janssen, N. M. H.; Cobden, P. D.; Slinko, M. M.; Nieuwenhuys, B. E. J. Chem. Phys. 1997, 107, 965. (18) Cobden, P. D.; Nieuwenhuys, B. E.; Esch, F.; Baraldi, A.; Comelli, G.; Lizzit, S.; Kiskinova, M. J. Vac. Sci. Technol., A 1998, 16, 1014. (19) Cholach, A. R.; Van Tol, M. F. H.; Nieuwenhuys, B. E. Surf. Sci. 1994, 320, 281. (20) Sto¨hr, J. NEXAFS Spectroscopy; Springer: New York, 1992.

Nagasaka et al. (21) Amemiya, K.; Kondoh, H.; Nambu, A.; Iwasaki, M.; Nakai, I.; Yokoyama, T.; Ohta, T. Jpn. J. Appl. Phys. 2001, 40, L718. (22) Nagasaka, M.; Kondoh, H.; Amemiya, K.; Nambu, A.; Nakai, I.; Shimada, T.; Ohta, T. J. Chem. Phys. 2003, 119, 9233. (23) Nakai, I.; Kondoh, H.; Amemiya, K.; Nagasaka, M.; Nambu, A.; Shimada, T.; Ohta, T. J. Chem. Phys. 2004, 121, 5035. (24) Nakai, I.; Kondoh, H.; Shimada, T.; Nagasaka, M.; Yokota, R.; Amemiya, K.; Orita, H.; Ohta, T. J. Phys. Chem. B 2006, 110, 25578. (25) Nakai, I.; Kondoh, H.; Shimada, T.; Nagasaka, M.; Yokota, R.; Katayama, T.; Amemiya, K.; Orita, H.; Ohta, T. J. Phys. Chem. C 2009, 113, 13257. (26) Amemiya, K.; Kondoh, H.; Yokoyama, T.; Ohta, T. J. Electron Spectrosc. Relat. Phenom. 2002, 124, 151. (27) Hettrick, M. C. Nucl. Instrum. Methods Phys. Res., Sect. A 1988, 266, 404. (28) Mårtensson, N.; Baltzer, P.; Bru¨hwiler, P. A.; Forsell, J.-O.; Nilsson, A.; Stenborg, A.; Wannberg, B. J. Electron Spectrosc. Relat. Phenom. 1994, 70, 117. (29) Frechard, F.; Van Santen, R. A.; Siokou, A.; Niemantsverdriet, J. W.; Hafner, J. J. Chem. Phys. 1999, 111, 8124. (30) Van Hardeveld, R. M.; Van Santen, R. A.; Niemantsverdriet, J. W. Surf. Sci. 1996, 369, 23. (31) Yates, J. T., Jr.; Thiel, P. A.; Weinberg, W. H. Surf. Sci. 1979, 84, 427. (32) Xu, H.; Ng, K. Y. S. Appl. Phys. Lett. 1996, 68, 496. (33) Fukuoka, M.; Okada, M.; Matsumoto, M.; Ogura, S.; Fukutani, K. Phys. ReV. B 2007, 75, 235434. (34) Mavrikakis, M.; Rempel, J.; Greeley, J.; Hansen, L. B.; Nørskov, J. K. J. Chem. Phys. 2002, 117, 6737. (35) When the pressure of hydrogen gas is 1.0 × 10-7 Torr, the hydrogen coverage is set to be 0.1 ML. To reproduce the experimental results at 1.0 × 10-7 Torr, the rate constants k1 and k2 were chosen as k1 ) 7.0 × 10-3 ML-1 s-1 and k2 ) 7.0 × 10-3 ML-1 s-1 for n ) 1 and 1.3 × 10-3 ML-1/2 s-1 for n ) 1/2, respectively. (36) Supporting Information. (37) Seebauer, E. G.; Kong, A. C. F.; Schmidt, L. D. J. Chem. Phys. 1988, 88, 6597. (38) Mann, S. S.; Seto, T.; Barnes, C. J.; King, D. A. Surf. Sci. 1992, 261, 155. (39) Ruan, L.; Stensgaard, I.; Lægsgaard, E.; Besenbacher, F. Surf. Sci. 1994, 314, L873.

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