Kinetics and Dynamics of N2 Formation in a Steady-State N2O+ CO

cross-correlation time-of-flight techniques. The reaction proceeded steadily above 400 K, and the kinetics was switched at a critical CO/N2O pressure ...
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Langmuir 2005, 21, 9529-9536

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Kinetics and Dynamics of N2 Formation in a Steady-State N2O + CO Reaction on Pd(110) Yunsheng Ma,† Song Han,‡ and Tatsuo Matsushima*,† Catalysis Research Center, Hokkaido University, Sapporo 001-0021 Japan, and Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060-0810, Japan Received March 16, 2005. In Final Form: July 22, 2005 The N2O decomposition kinetics and the product (N2 and CO2) desorption dynamics were studied in the course of a catalyzed N2O+CO reaction on Pd(110) by angle-resolved mass spectroscopy combined with cross-correlation time-of-flight techniques. The reaction proceeded steadily above 400 K, and the kinetics was switched at a critical CO/N2O pressure ratio. The ratio was about 0.03 at 450 K and reached ∼0.08 at higher temperatures. Below it, the reaction was first order in CO, and negative orders above it. Throughout the surveyed conditions, the N2 desorption sharply collimated along about 45° off the normal toward the [001] direction. Desorbing N2 showed translational temperatures in the range of 2000-5000 K. It is proposed that the decomposition proceeds in N2O(a) oriented along the [001] direction. On the other hand, the CO2 desorption sharply collimated along the surface normal, showing a translational temperature of about 1600 K.

I. Introduction The nitrous oxide (N2O) decomposition on rhodium and palladium surfaces has received much attention because N2O is concomitantly produced in the catalytic NO reduction by CO, H2 and hydrocarbon in automobile gas converters. N2O is harmful and has a remarkable greenhouse effect. This species has recently been assigned to the key intermediate in controlling the selectivity to N2 because its decomposition highly shares the surfacenitrogen removal pathways in the catalyzed NO reduction.1-4 However, knowledge of the removal pathway of adsorbed N2O is still limited under both steady-state and transient conditions.5 This paper delivers detailed studies of N2O reduction kinetics in the presence of gaseous CO on Pd(110) and its product desorption dynamics. The reaction proceeded steadily above 400 K, and the kinetics was switched over at a critical N2O/CO pressure ratio. Throughout the surveyed conditions, the N2 desorption merely collimated along about 45° off the normal toward the [001] direction. Desorbing N2 showed translational temperatures in the range of 2000-5000 K, whereas the CO2 desorption collimated along the surface normal with lower translational temperatures. This N2O species is very reactive on open surfaces such as Cu(110), stepped Ni(557), Ni(110), Ni(100), Rh(110), Ir(110) and Pd(110).6-12 The decomposition proceeds at * To whom correspondence should be addressed. Fax: (+81-11706-9120). E-mail: [email protected]. † Catalysis Research Center. ‡ Graduate School of Environmental Earth Science. (1) Ohno, Y.; Kimura, K.; Bi, M.; Matsushima, T. J. Chem. Phys. 1999, 110, 8221. (2) Kobal, I.; Kimura, K.; Ohno, Y.; Matsushima, T. Surf. Sci. 2000, 445, 472. (3) Rzeznicka, I. I.; Ma, Y.-S.; Cao, G.; Matsushima. T. J. Phys. Chem. B 2004, 108, 14232. (4) Ma Y.-S.; Rzeznicka I.; Matsushima T. Chem. Phys. Lett. 2004, 388, 201. (5) Matsushima, T. Surf. Sci. Rep. 2003, 52, 1. (6) Spitzer, A.; Lu¨th, H. Phys. Rev. B 1984, 30, 3098. (7) Kodama, C.; Orita, H.; Nozoye H. Appl. Surf. Sci. 1997, 121/122, 579. (8) Sau, R.; Hudson, J. B. J. Vac. Sci. Technol. 1981, 18, 607. (9) Hoffman, D. A.; Hudson, J. B. Surf. Sci. 1987, 180, 77. (10) Li, Y.; Bowker, M. Surf. Sci. 1996, 348, 67.

around 100 K or even below it. To keep its decomposition continuous, however, reducing reagent such as CO or H2 is necessary to remove deposited surface oxygen. The steady-state decomposition would be controlled by the adsorption of either N2O or reducing reagent. This condition, i.e., the fast surface processes of N2O(a) f N2(g) + O(a) and O(a) + CO(a) f CO2(g) compared with the supply of N2O(a), O(a) or CO(a), is very similar to that in the CO oxidation on noble metals, i.e., the steady-state CO oxidation is controlled by either CO adsorption or O2 dissociation and not by the CO2 formation step.13-16 Thus, ordinary kinetic measurements at the steady state are not informative for the removal processes of N2O(a) as reported for the CO2 formation step in the CO oxidation.17 On the other hand, the angle-resolved product desorption measurements are useful whenever any step becomes ratedetermining because the angular distributions do not involve the reaction rate and are always related to the desorption process.5 In the steady-state N2O reduction, peculiar N2 desorption is expected in its angular distribution as observed in angle-resolved (AR) temperatureprogrammed desorption (TPD) work.11 In general, a chemical reaction should be characterized from both chemical kinetics and reaction dynamics. The former concerns the reaction rate and the latter deals with the energy partition during an event. The observed quantities in both approaches must be understood consistently. However, no good model reaction has been found on metal surfaces although such an approach is common in gas-phase reactions. This N2O reduction will be a good candidate because of the simplicity and applicability of state-selective measurements toward the product N2.18 Velocity measurements of desorbing products are the first (11) Horino, H.; Liu, S.; Hiratsuka, A.; Ohno, Y.; Matsushima, T. Chem. Phys. Lett. 2001, 341, 419. (12) Haq S.; Hodgson A. Surf. Sci. 2000, 463, 1. (13) Matsushima, T.; Musset, C. J.; White, J. M. J. Catal. 1976, 41, 397. (14) Matsushima, T.; White J. M. Surf. Sci. 1977, 67, 122. (15) Matsushima, T.; Hashimoto, M.; Toyoshima I. J. Catal. 1979, 58, 303. (16) Ehsasi, M.; Matloch M.; Frank, D.; Block J. H.; Christmann, K.; Rys F. S.; Hirschwald W. J. Chem. Phys. 1989, 91, 4949. (17) Engel, T.; Ertl, G. Adv. Catal. 1979, 28, 1. (18) Hodgson, A. Prog. Surf. Sci. 2000, 63, 1.

10.1021/la0507030 CCC: $30.25 © 2005 American Chemical Society Published on Web 09/13/2005

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step in dynamic studies of surface reactions.19 Since the pioneering work by Auerbach et al., the modulated molecular beam (MMB) has been used in this field.20-22 Later, Matsushima et al. combined AR-TPD with timeof-flight (TOF) techniques, and Hodgson combined TPD with resonance-enhanced multiphoton absorption ionization (REMPI).5,23 However, the relation between the energy transfer to product and reaction kinetics has been reported in only a few cases.5,24 This is because neither of the above methods can establish steady-state conditions. Furthermore, it has been difficult to analyze the decomposition dynamics of N2O on well-defined single-crystal surfaces under steady-state conditions because of the small product yield and the large fragmentation of N2O in an ultrahigh vacuum (UHV) chamber with ion pumps as well as in the ionizer of a mass spectrometer.25 In the present work, angle-resolved product desorption was successfully analyzed under both steady-state and transient conditions for the N2O + CO reaction. II. Experimental Section Experiments were performed in a UHV system composed of a reaction chamber, a chopper house and an analyzer.3,5 The reaction chamber was evacuated by a turbo-molecular pump with a pumping rate of 1.5 m3s-1 and equipped with low-energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS) optics, an ion gun, and a mass spectrometer (MS) for angleintegrated (AI) measurements. The chopper house, which had a large pumping rate of about 7 m3s-1 for high angle resolution,26 had a narrow slit facing the reaction chamber and a crosscorrelation random chopper blade.27 It was connected to the analyzer through a narrow tube, in which another MS was set for AR-product desorption and time-of-flight (TOF) analyses. The distance from the ionizer to the chopper blade was 377 mm, and the time resolution was selected at 20 µs.28 A Pd(110) surface was cleaned by repeated cycles of Ar+ ion bombardments (2 keV, 1 µA), oxygen treatments (850 K in 5 × 10-8 Torr of O2) and annealing to 1000 K until a sharp (1 × 1) LEED structure was observed. The cleanliness of the surface was verified by the absence of CO and CO2 desorption in TPD after oxygen exposure. Before each experiment, the surface was annealed at 1000 K for 5 min. 15N O (the purity as nitrous oxide; 99.99%, and the isotope 2 purity; 99%) was introduced without further purification through a gas doser with a small orifice (diameter; 0.1 mm) about 2 cm from a sample crystal while 13CO (the isotope purity; 99%) was backfilled. The use of this orifice was effective to reduce N2 formation on the reaction chamber wall. Nevertheless, the product signals of 15N2 and 13CO2 were mostly monitored in the AR form. Hereafter, 15N and 13C are simply denoted as N and C in the text. The AR signal was obtained by the MS in the analyzer as the difference between the signal at the desired angle and that when the crystal was away from the line-of-sight position. The desorption angle (polar angle; θ) was scanned in the plane along the [001] direction.5,24 In this report, all of the partial pressures were corrected by the sensitivities of a BA gauge. The N2O pressure in front of the single crystal was corrected by a factor of 2.5. This calibration against the BA reading was performed by comparing the sticking probability of CO supplied from the (19) Dabiri, A. E.; Lee, T. J.; Stickney, R. E. Surf. Sci. 1971, 26, 522. (20) Auerbach, D., Becker, C. A.; Cowin, J. P.; Wharton, L. Appl. Phys. 1977, 14, 141. (21) D′Evelyn, M. P.; Madix, R. J. Surf. Sci. Rep. 1983, 3, 413. (22) Arumainayagam, C. R.; Madix, R. J. Prof. Surf. Sci. 1991, 38, 1. (23) Wright, S.; Skelly, J. F.; Hogdson, A. Faraday Discuss. 2000, 117, 133. (24) Matsushima, T.; Rzeznicka, I. I.; Ma, Y.-S. Chem. Rec. 2005, 5, 81. (25) Masson, D. P.; Lanzendorf, E. J.; Kummel, A. C. J. Chem. Phys. 1995, 102, 9096. (26) Kobayashi, M.; Tuzi, Y. J. Vac. Sci. Technol. 1979, 16, 685. (27) Comsa, G.; David, R. Surf. Sci. Rep. 1985, 5, 145. (28) Comsa, G.; David, R.; Schumacher, B. J. Rev. Sci. Instrum. 1981, 52, 789.

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Figure 1. Surface temperature dependence of the AR-15N2 signal at the collimation angle (43°) in the steady-state 15N2O + CO reaction at different PCO values and a fixed PN2O of 3.3 × 10-6 Torr. The starting temperature shifted with increasing PCO. doser with that of back-filled CO. The same factor was obtained by comparing the AR-N2 yield in the NO + CO reaction when NO was introduced by backfilling with that by the doser. Under this construction, the flux of incident N2O toward the unit surface area decreases proportional to the cosine of the desorption angle when the angle is shifted from the normal direction even if the incident reactant density in the effused beam is homogeneous around the crystal sample. This effect was corrected as previously described in detail.3 The fragmentation of N2O into N2 in both mass spectrometers was separately estimated by introducing N2O. It was about 25% of the N2O signal in the MS in the analyzer. On the other hand, the relative AI-signals at the mass/charge ratios m/e ) 30 (15N2), 31 (15N16O), and 46 (15N216O) were 0.250.30, 0.32-0.34, and 1, respectively. Only the m/e ) 30 signal was about doubly enhanced as compared with the standard mass spectra.29 Hence, the fragmentation to N2 took place both in the ionizer and on the reaction chamber wall.

III. Results A. Kinetic Transition and Angular Distribution. The AI signal for the product N2 involved large experimental uncertainty because its intensity was always less than 10% of the total signal due to the high background. On the other hand, the AR signal was measurable on account of the sharply collimated desorption. The AR N2 signal at the collimation angle of 43° is shown versus the surface temperature (TS) with different CO pressures at fixed PN2O of 3.3 × 10-6 Torr (Figure 1). As described in detail below (Figure 2), N2 desorption collimated sharply at 45 ( 2° off the surface normal in the plane along the [001] direction, whereas the maximum flux of CO2 was located at θ ) 0°. The N2 signal became noticeable above 400 K, increased quickly to the maximum and then decreased slowly at higher temperatures. With decreasing CO pressure, the starting temperature shifted to lower values and coalesced to around 420 K, suggesting the initiation by the removal of CO(a). The optimum temperature also shifted in a similar way. For example, the formation rate reached the maximum at 430 K at PCO ) 0.2 × 10-7 Torr, while it shifted to 530 K at PCO) 5 × 10-7 Torr. There was a critical CO pressure where the kinetics changed sharply (Figure 2). Below the critical value, the N2 yield increased linearly with increasing PCO (first order in CO). It dropped sharply at the critical point or was suppressed above it (negative orders in CO). The CO2 formation also showed an identical kinetic transition with CO pressure. Hereafter, the side below the critical PCO (29) Ed. S. Oka In Bunseki-Kagakubinran (Analytical Chemistry Tables; Maruzen: Tokyo, 1961; p 1365 (Japanese).

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Figure 2. PCO dependence of (a) the AR-15N2 signal at θ ) 43° and (b) the AR-13CO2 signal at θ ) 0° in the steady-state 15N2O + 13CO reaction at a fixed PN2O of 3.3 × 10-6 Torr and different TS values. A kinetic transition at TS ) 450 and 470 K is drawn by the vertical lines. (c), (d) Angular distribution of 15N2 and 13CO2 desorption in the active region and the inhibited region at TS ) 470 K.

value is named the “active region” and that above it is named the “inhibited region”. The critical CO pressure shifted to higher values at higher TS or higher N2O pressures. The CO/N2O pressure ratio at the critical point was about 0.03 at TS ) 450 K and increased to 0.08 at TS ) 520 K. Furthermore, no hysteresis was found when PCO was increased and then decreased. The angular distribution of desorbing N2 collimated far from the surface normal in both the active and inhibited regions (Figure 2c,d). The signal in the active region was approximated as a form of {cos24(θ + 46) + cos24(θ - 46)}. The angle-dependence remained fairly invariant even in the inhibited region. The distribution became broader at higher temperatures (for example, a cos20(θ - 46) form at 750 K). The CO2 desorption sharply collimated along the surface normal in a similar way in both regions.30,31 B. Transient Behavior. The amount of surface species at the steady-state condition can be estimated through analysis of the transient signal decays. After the reaction reached a steady-state level, the CO supply was quickly stopped while the N2O pressure was kept constant and the AR-signals of N2 at θ ) 43° and CO2 at θ ) 0° were monitored as a function of time. Both CO2 and N2 signals decreased quickly when they were followed from the active region. At TS ) 470 K and PN2O ) 3.3 × 10-6 Torr, the CO2 (30) Kimura K., Ohno Y.; Matsushima, T. Surf. Sci. 1999, 429, L455. (31) Moula Md. G.; Wako, S.; Cao, G.; Kimura, K.; Ohno, Y.; Kobal, I.; Matsushima, T. Phys. Chem. Chem. Phys. 1999, 1, 3677.

signal disappeared completely in about 4 s, whereas the CO pressure decreased slightly faster than that of CO2 (Figure 3a). Interestingly, the N2 signal was still significant even until 15 s. These delays indicate that the N2O dissociation still proceeds without CO supply. This measurable capacity means that some active sites available for N2O dissociation are present in the active region. Considering that the reaction of CO(a) with O(a) is very fast in the temperature range studied,17 the CO2 formation must be suppressed as fast as the CO pressure decay when no active sites are available in the active region. This consideration leads the idea that the decay curves deliver the amounts of CO(a) and active sites. Similar behavior was observed at 450 K where the CO2 signal decay was slowed (Figure 3b). The signal decay from the inhibited region was very different (Figure 4). At TS ) 470 K, the AR CO2 signal first decreased, then increased to the maximum at about 6 s, and finally dropped to the background level. The CO adsorption was negligible because its pressure decay was completed within 3 s. This CO2 signal must come from the reaction of CO adsorbed on the surface before cutting off the CO supply. Hence, the area under the CO2 curve conserved the CO coverage during the steady-state reaction because the reaction rate of CO(a) + O(a) was much faster than the CO desorption.13 On the other hand, the AR N2 signal increased similarly to the maximum at 6 s and then decreased slowly to the background at about

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Figure 3. Transient 15N2 and 13CO2 formation curves after stopping the 13CO supply from the steady-state 15N2O reduction in the active region at PN2O ) 3.3 × 10-6 Torr and PCO ) 0.1 × 10-6 Torr at TS ) (a) 470 K and (b) 450 K. The crosshatched areas show the amount of N2 formation without CO(a), indicating the amount of active sites.

Figure 4. Transient 15N2 and 13CO2 formation curves after stopping the 13CO supply from the steady-state 15N2O reduction in the inhibited region at PN2O ) 3.3 × 10-6 Torr and PCO) 0.5 × 10-6 Torr at TS ) (a) 470 K, (b) 450 K, and (c) 430 K. The crosshatched areas show the amount of N2 formation without CO(a).

18 s. At lower TS values, the peak position shifts to longer periods. The induction period defined from the CO valve closure to the N2 peak position was 6 s at 470 K and 24

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Figure 5. (a) Intensity of the fractional order spot at (-1, -2/3) in a c(2 × 3)-1D LEED structure. (c-d) Typical LEED structures at different N2O/CO ratios in the steady-state N2O + CO reaction (PN2O ) 3.3 × 10-8 Torr, TS ) 470 K). The vertical line indicates the kinetic transition point.

s at 430 K. It should be noted that the peak position of the N2 signal always appeared at the point for the disappearance of the CO2 signal. C. LEED Studies. Before introducing N2O gas, the surface showed a clear (1 × 1) LEED pattern. After the surface was exposed to a constant N2O flow, a streaky c(2 × 4) structure was observed at TS values below 350 K. Increasing the TS value to 470 K resulted in the appearance of (2 × 3)-1D superstructure spots. Only oxygen desorption was detected in the subsequent TPD procedures, indicating that N2O could dissociate on the surface even below 350 K to release N2 and only oxygen was left.11,12 Similar c(2 × 4) and (2 × 3)-1D patterns were reported previously for O2 adsorption on Pd(110) with the oxygen coverage of a 0.5 monolayer (ML) and a 0.2-0.3 ML.32-36 In separate experiments, a clear c(2 × 4) pattern was also observed when the surface was saturated by oxygen at TS ) 470 K, and the oxygen coverage of this structure was assumed to be 0.5 ML. If we compared the amount of oxygen desorption in the subsequent heating after the surface was saturated by N2O at TS ) 470 K, then the oxygen coverage of this N2O-saturated surface with the (2 × 3)1D structure would be estimated as 0.23 ML, which agrees well with previous results based on the structures after oxygen adsorption.32,36 On Rh(110) and Ru(001), saturated oxygen coverage after N2O adsorption was also found to be about half of that after O2 adsorption.10,37 Typical LEED patterns under steady-state N2O reduction conditions at 3.3 × 10-8 Torr and different CO pressures are shown in Figure 5b-d. The intensity of the fractional spot in (2 × 3)-1D became weaker with increasing CO pressure (Figure 5a). When the CO/N2O ratio was around the kinetic (32) He, J. W.; Memmert, U.; Norton P. R. J. Chem. Phys. 1989, 90, 5082 (33) Jo, M.; Kuwahara, M.; Onchi, M.; Nishijima, M. Chem. Phys. Lett. 1986, 131, 106. (34) Brena, B.; Comelli, G.; Ursella, L.; Paolucci G. Surf. Sci. 1997, 375, 150 (35) Bennett, R. A.; Poulston, S.; Jones, Z.; Bowker, M. Surf. Sci. 1998, 401, 72. (36) Goschnick, J.; Wolf, M.; Grunze, M.; Unetrl, W. N.; Block, J. H.; Loboda-Cackovic, J. Surf. Sci. 1986, 178, 831. (37) Shi, S. K.; White, J. M. J. Chem. Phys. 1980, 73, 5889.

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Figure 6. (a) Velocity distributions of desorbing product 15N2 at θ ) 45° (PN2O ) 3.3 × 10-6 Torr, PCO ) 0.5 × 10-6 Torr and TS ) 520 K). The average kinetic energy is indicated in 〈 〉 in temperature units. Typical deconvolutions are drawn by broken curves. The solid line indicates their summation. (b) Average kinetic energy and flux of the fast component versus the desorption angle.

Figure 7. (a) Velocity distributions of desorbing product 13CO2 at θ ) 0° (PN2O ) 3.3 × 10-6 Torr, PCO ) 0.1 × 10-6 Torr and TS ) 470 K). The average kinetic energy is indicated in 〈 〉 in temperature units. Typical deconvolutions are drawn by broken curves. The solid line indicates their summation. (b) Average kinetic energy and flux of the fast component versus the desorption angle.

transition or above it, the surface was converted into a (1 × 1) structure, suggesting that the surface was free from oxygen. D. Desorption Parameters. Angular and velocity distributions of the products were examined in both active and inhibited regions. It is surprising to find no differences in the angular distributions of CO2 and N2 desorption between the two regions (Figure 2). The N2 distribution was commonly approximated in a two-directional form as {cos24(θ + 45) + cos24(θ - 45)}. The collimation angle was θ ) 45 ( 2°. The velocity distribution of desorbing N2 was broad, involving high velocity components (Figure 6a). The translational temperature calculated from the average kinetic energy (〈E〉) as T〈E〉 ) 〈E〉/2k, where k is the Boltzmann constant, is shown in 〈 〉 in the figure. The value was maximized to 3250 K around the collimation angle. It decreased slowly with an increasing shift from the collimation angle (Figure 6b). The distribution did not involve the component expected by the Maxwell distribution at the surface temperature (thermalized component). This is consistent with the absence of the cosine component in the angular distribution but is in high contrast to the results in the NO + CO reaction where the cosine component was significant.3,4 The velocity distribution of N2 was wide even at the collimation angle compared with a Maxwellian distribution. The speed ratio (SR) defined as (〈v2〉/〈v〉 2 -1)1/2/(32/9π -1)1/2 was about 1.1, where v is the velocity of the molecule, 〈v〉 is the mean velocity, and 〈v2〉 is the mean square velocity. The SR value is unity for a Maxwellian distribution and becomes smaller for the distributions of molecules with hyper-thermal energy.27 Thus, the distribution was further deconvoluted into two components by assuming the modified Maxwellian distribution, f(v) ) v3 exp {-(v-v0)2/R2}, where v0 is the stream velocity and R is the width parameter. As described previously, a common width was assumed in this deconvolution.3 The resultant deconvolutions are shown by broken curves in Figure 6a. At θ ) 45°, the faster component shows a translational temperature of 5210 K

and the slower component, one of 1770 K. This result is quite similar to that for the NO + CO reaction on Pd(110).3,4 These fast components may be due to different vibrational states. A velocity analysis with higher time resolutions as well as state-resolved forms is required for future investigations.18 The total flux calculated from the velocity distribution also shows the maximum at θ ) ca. 45°, which is consistent with the above angular distribution without velocity analysis. The apparent translational temperature of CO2 reached 1260 K at the normal direction (Figure 7a). It decreased with increasing the shift from the surface normal and reached the surface temperature around θ ) 45° (Figure 7b). Two components can be resolved in the velocity curve of desorbing CO2: one is the slow component following the Maxwell distribution at the surface temperature, and the other is the fast component with the translational temperature of 1670 K, which is close to the values reported in NO + CO and CO + O2 reactions.3,30,31 The flux of the fast component follows the cos13θ distribution, which supports the above deconvolution of angular distribution. The kinetic energy of CO2 increased monotonically with increasing TS, with a slope of unity. On the other hand, that of N2 was scattered at higher surface temperatures due to small signals. The enhancement of the kinetic energy at high temperatures was not confirmed. IV. Discussion A. Kinetic Transition and Adsorbed CO. A clear kinetic transition was observed in the steady-state N2O + CO reaction (Figure 2). This is reminiscent of the kinetic behavior reported in the CO + O2 reaction on noble metals.13-17 The similarity is reasonable since the surfacespecies removal processes, N2O(a) f N2(g) + O(a) and CO(a) + O(a) f CO2(g), are very fast (not rate-determining), and the overall reaction is controlled by either the CO adsorption or N2O adsorption. The amounts of adsorbed N2 and N2O may be very small because of the

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finally prevented N2O from dissociation. Therefore, this peak area after the maximum can be used to estimate the total available sites for N2O dissociation on the clean surface. In fact, this value was invariant at TS ) 420-500 K (see Figure 4). On the other hand, the N2 signal area until the peak position increased with decreasing TS value although it was erratic below 430 K. This N2 formation proceeded on the vacant sites left by the removal of CO(a). Thus, the active sites at the steady-state condition in the inhibited region can be estimated from the difference between both signal areas before (Sb,N2) and after (Sa,N2) the peak position. The amount of active sites in the inhibited region was estimated by Figure 8. PCO dependence of the CO and oxygen coverage in the steady-state N2O + CO reaction at TS ) 470 K and PN2O ) 3.3 × 10-6 Torr. The amount of the active sites (ΘV; 9) was estimated in different ways in the active and inhibited regions (see text). The CO coverage (ΘCO;O) was determined by transient CO2 decays. The oxygen coverage (ΘO) was referred to ref 12.

limited heat of adsorption.1,11 The products CO2 and N2 are immediately desorbed. Thus, the surface species are mostly CO(a) and/or O(a). Below about 470 K, the supply of O(a) by N2O decomposition is balanced with CO2 formation as well as with the removal of CO(a) because O(a) can be removed as only CO2 and the reaction of CO(a) with O(a) is much faster than its desorption.13 For steadystate N2O dissociation at TS ) 470 K, the amounts of adsorbed CO and vacant active sites were estimated in the following way. The amount of adsorbed CO was first obtained as the peak area under CO2 transient curves (Figures 3a and 4a) and then normalized to the saturated CO coverage (ΘCO) at room temperature, which is assumed to be 0.7 monolayer (ML).38 The estimated CO coverage vs CO pressure is shown in Figure 8. The CO coverage obtained in this way was fairly overestimated below the kinetic transition by CO adsorption during transient procedures, i.e., after the CO valve closure. However, it is clear that there is a sharp increase in the CO coverage around the kinetic transition, consistent with the kinetic switching. The CO coverage above the transition point was estimated to be about 0.25 ML. B. Active N2 Formation Site and Kinetic Transition. There are two kinds of active sites that contribute to the transient N2 formation; one is the vacant site present under steady-state conditions and the other is that activated by either O(a) or CO(a) removal as CO2 during the transient procedure. In the active region (Figure 3a), N2O still dissociated after the CO supply was terminated, indicating the presence of the vacant site. Therefore, the amount of the active sites in the active region can be calculated by the following equation

active sites Θv ) SN2 /SN2, max

(1)

where Θv is the relative amount of vacant active sites under the steady-state condition, SN2 is the peak area of the N2 transient curve after the disappearance of the CO2 signal and SN2, max is the peak area under transient N2 formation from N2O dissociation on the clean surface at the same surface temperature (TS ) 470 K). The latter quantity was estimated from the N2 peak area after the maximum of the transient signal from the inhibited region (Figure 4a). At this maximum N2 signal point, CO(a) had mostly been removed as CO2 and the surface became clean because of the fast reaction of O(a) with CO(a). The deposited oxygen began to accumulate after this peak and (38) Jones I. Z.; Bennett R. A.; Bowker M. Surf. Sci. 1999, 439, 235.

Θv ) (Sa,N2 - Sb,N2) /SN2, max

(2)

This estimated value of active sites is also shown in Figure 8. It increased almost linearly with increasing CO pressure in the active region and dropped at the kinetic transition. It decreased with a further increase in the CO pressure, consistent with the kinetic behavior. This suppression is induced by increasing CO(a). It should be noted that the actual AR-signal count rate of CO2 is very different from that of N2 at both steadystate and transition conditions because of the differences in collimations, distribution sharpness, and mass sensitivities. Actually, the AR-signal of CO2 at the surface normal was about one-fourth of that of N2 at the collimation angle. Thus, the CO2 signal in Figures 3 and 4 was multiplied by a factor of 3.5 so that the peak area is drawn to be comparable with that of N2. In the active region, the surface is covered by oxygen as confirmed by the LEED results. The observed condition of CO(a) , O(a) indicates that the reaction rate is determined by CO adsorption. The intensity of fractional spots due to the oxygen-induced super-lattices decreased with increasing CO pressure, indicating a decrease in the oxygen coverage (Figure 5). Concomitantly, the number of active sites increased with an enhancement of the reaction rate. Around the critical point, the fractional spots disappeared and the surface shows the (1 × 1) structure, indicating that the surface was free from oxygen. At this point, the N2O dissociation was maximized, yielding the maximum N2 and CO2 yield. The kinetic transition point shifted to a smaller pressure ratio of CO/N2O (less than 0.2) compared with the CO/O2 ratio around unity in the CO oxidation. This was due to differences in the sticking probability between N2O and O2, i.e., the sticking probability of N2O dissociation above 400 K linearly decreased from 0.2 with an increase in the oxygen coverage,12 whereas that for O2 was close to 0.4.38 The oxygen coverage was estimated from this sticking probability data as drawn in Figure 8, where the oxygen coverage at lower PCO pressures was assumed to be half of the value in the paper cited since the N2O maximum was half a monolayer.11,12 Above the critical point, the O(a) supply is overcome by the removal with CO and un-reacted CO(a) can accumulate to the equilibrium level with CO pressure and retard the N2O dissociation. Thus, the amount is controlled by the CO partial pressure, and N2O adsorption/dissociation then becomes rate-limiting. C. Inhibition by O(a) and CO(a). We consider the relation between the number of active sites and the amount of adsorbed oxygen or adsorbed CO. In the active region, the number of active sites is mostly controlled by O(a) since CO(a) , O(a). It increases almost linearly with increasing PCO, as seen in the LEED intensity due to the oxygen superstructure. This is also consistent with the linear decrease of the sticking probability of N2O against

N2 Desorption in N2O + CO on Pd(110)

the oxygen coverage.12 The surface oxygen level in the active region is controlled by the removal of O(a) by CO. The O(a) supply, i.e., N2O dissociation, is merely controlled by the oxygen coverage since the N2O pressure is fixed. The resulting coverage decreases quickly with increasing PCO. The fitting line for the active site was fairly described as (1- xΘO)n, where x was 4 and n was unity. This is not unreasonable for the surface covered by a (1 × 3)-O lattice. In the inhibited region, on the other hand, the surface is covered by CO, i.e., CO(a) . O(a). The N2O dissociation is prevented by CO(a), where the reaction rate decreases quickly with a small decrease of vacant area. The number of the active site is close to the calculated line as (1 xΘCO)n with either x ) 2.5 and n ) 1 or x ) 1 and n ) 4. The latter was predicted on the kinetic simulation by Zhdanov et al.39 It reveals that CO(a) effectively blocks the site for N2O dissociation. There are two possibilities for the CO poisoning effect on N2O dissociation. One is a site-blocking effect; CO adsorbs randomly on short-bridge sites of Pd rows when the CO coverage is below 0.3 ML or TS > 450 K.40,41 On the other hand, N2O adsorbs on-top sites with an upright or parallel form.42 Therefore, bridge-adsorbed CO(a) will certainly prohibit N2O adsorption on the top sites. Another possibility may come from the change of electronic structure of the substrate. The CO adsorption on Pd(110) results in an increase of work function by ∼1.2 eV, which suggests charge donation from the substrate into CO antibonding orbital.41 It may lower the possibility of N2O adsorption/dissociation on the substrate. A similar repulsive lateral interaction between CO(a) and N2O(a) has also been reported on Ru[0001].43 D. Desorption Dynamics and Surface Species. The mechanism of the inclined N2 desorption on Pd(110) was first examined in the reaction of 14NO(a) with preadsorbed 15 N(a) by Ikai et al.44 These researchers observed the inclined desorption of 14N15N and the normally directed 15N desorption, although they proposed the desorption2 mediated mechanism without N2O formation for this characteristic desorption. Later, Ohno et al. observed similar inclined desorption of N2 in both N2O and NO decomposition on the same surface by using TPD-TOF techniques.1 They argued that the inclined N2 desorption was due to the decomposition of N2O oriented along the [001] direction and, furthermore, proposed that nascent N2 molecules received strong Pauli repulsion from the surface in addition to the repulsive force along the ruptured bond axis, parallel to the surface during N2O dissociation. However, by using infrared reflection-absorption spectroscopy (IRAS) techniques, Hodgson and co-workers found that N2O adsorbed upright on Pd(110) with the terminal nitrogen atom interacting with the surface.12 Even at 85 K, N2O can dissociate to produce oxygen on the surface, which will cause surface reconstruction with the (1 × 2) LEED pattern. It was proposed that the decomposition of N2O on the (1 × 2) surface led to the off-normal N2 desorption. Our recent study confirmed that even on the surface with a (1 × 1) structure, the inclined desorption was observed in steady-state NO+CO reactions.2-4 Furthermore, density functional theory (DFT) calculations in a generalized gradient approximation level predict two (39) Zhdanov, V. P.; Ma Y.; Matsushima, T. Surf. Sci. 2005, 583, 36. (40) Raval, R.; Haq, S.; Harrison M. A.; Blyholder G.; King, D. A. Chem. Phys. Lett. 1990, 167, 391 (41) He, J. W.; Norton P. R. J. Chem. Phys. 1988, 89, 1170. (42) Kokalj, A.; Kobal, I.; Matsushima, T. J. Phys. Chem. B 2003, 107, 2741. (43) Huang, H. H.; Seet, C. Z.; Zou Z.; Xu, G. Q. Surf. Sci. 1996, 356, 181. (44) Ikai, M.; Tanaka, K. I. J. Phys. Chem. B 1999, 103, 8277.

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stable adsorption forms on Pd(110), i.e., one lying along the [001] direction and the other standing with the terminal nitrogen-metal bond.42 Such adsorption structures were also confirmed by NEXAFS measurements and STM work.45,46 On the basis of similar inclined desorption of N2 in steady-state N2O + CO and NO + CO reactions, it is concluded that a common intermediate N2O(a) was involved in both reactions.47 In the steady-state N2O reduction, it is interesting to find the two different types of product desorption: one is due to the associative process of CO(a) + O(a). This desorption process cannot provide information on the reaction mechanism for the nitrogen removal. The product CO2 receives the repulsive force from the surface at the formation, and the collimation carries the information about the orientation and symmetry of reaction sites.5 The other is due to the dissociative desorption from N2O(a), providing the orientation of the intermediate molecules immediately before product emission. The N2 desorption in the steady-state N2O + CO reaction always shows similar angular and velocity distributions before ((2 × 3)-1D structure) and after the kinetic transition ((1 × 1) structure). The desorption dynamics do not change when the oxygen-covered surface is converted into the CO-covered condition. The above insensitive desorption dynamics suggests that N2O dissociation always takes place on oxygen-uncovered (1 × 1) patches. In fact, the N2O dissociation on Pd(110) and Rh(110) surfaces was reported to be severely suppressed by adsorbed oxygen except for the very small oxygen coverage.11,48,49 Thus, it is proposed that this (1 × 1) area is surrounded by O(a) in the active region, whereas it is surrounded by CO(a) in the inhibited region. N2O dissociation proceeds on this (1 × 1) area. Similar angular and velocity distributions of N2 desorption on O(a)- or CO(a)-covered surface also suggest that the interaction between O-N2O or CO-N2O has no significant effect on the N2 desorption dynamics. Indeed, its dissociation can proceed only below a small coverage of CO(a) or O(a). The removal of O(a) keeps the (1 × 1) structure from the oxygenstabilized (1 × 2) form above 360 K.50 Atomic oxygen in the (2 × 3)-1D structure, which was observed in the active region, was proposed to have adsorbed on the 3-fold hollow sites into a zigzag chain ((111) facets) along Pd paired rows based on the (1 × 3) reconstruction.34,35 Therefore, N2O decomposition cannot proceed on such oxygen-covered surfaces. This insensitive dynamics was also observed for CO2 desorption. The normally directed CO2 desorption as well as its kinetic energy does not change even when the reaction condition is switched from the active region to the inhibited region. In the CO + O2 reaction on Pd(110), it is reported that the velocity of desorbing CO2 changed suddenly at the critical point.31 In the pressure range of 10-3 Torr to 10-6 Torr, the kinetic energy was high in the active region, whereas it decreased sharply at the critical point or above it. On the other hand, below 10-6 Torr, the kinetic energy slightly (about 10%) increased at the critical (45) Horino, H.; Rzeznicka, I.; Matsushima, T.; Takahashi, K.; Nakamura, E. In. UVSOR Activity Report 2002; Institute for Molecular Science, 2003; p 209. (46) Watanabe, K.; Kokalj, A.; Inokuchi, Y.; Rzeznicka, I.; Ohshimo, K.; Nishi, N.; Matsushima, T. Chem. Phys. Lett. 2005, 406, 474. (47) Ma, Y.; Matsushima, T. J. Phys. Chem. B 2005, 109, 689. (48) Liu, S.; Horino, H.; Kokalj, A.; Rzeznicka, I.; Imamura, K.; Ma, Y.-S.; Kobal, I.; Ohno, Y.; Hiratsuka, A.; Matsushima, T.; J. Phys. Chem. B 2004, 108, 3828. (49) Ma, Y.-S.; Rzeznicka, I. I.; Matsushima, T. Appl. Surf. Sci. 2005, 244, 558. (50) Rzeznicka, I. I.; Matsushima, T. Chem. Phys. Lett. 2003, 377, 279.

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point with increasing CO pressure.30 The critical point in the present work is in the latter condition, consistent with the previous results. E. Collimated Fragment Desorption. The above N2 emission from N2O on Pd(110) indicates the orientation of adsorbed N2O. Such collimated fragment desorption has frequently been reported in electron-stimulated desorption ion angular distribution (ESDIAD), in which the flux of desorbing ions or neutral species is analyzed in angle-resolved form when the surface is exposed to electron or photon beams.51,52 In this method, the desorbing species frequently have translational energies in the 1001000 kJ mol-1 range. The collimation angle at which the flux of desorbing species maximizes appears in the ruptured bond direction. Fragments resulting from ruptured bonds receive a strong repulsive force close to the stable bonding configuration because electronic transitions induced by electron impacts, from the bonding state to the antibonding state, are completed much more quickly than nuclear motions. The resultant fragment desorption collimates sharply along the ruptured bond axis. In thermal reactions, however, the translational energy of repulsively desorbed products barely reaches 100 kJ mol-1 (about 6000 K). This is far below that observed in ESDIAD, and, therefore, it causes the structural information from the spatial distribution to become unclear. The relaxation time of vibrationally excited molecules is in the order of 10-12 s on metal surfaces.53 The molecule vibrates several hundred times before relaxing, and nascent products are likely to maintain high energy immediately after forming. However, the thermal reaction product being desorbed will be trapped by attractive dispersion forces exerted from the surface if it is not repulsively desorbed. In fact, collimated desorption from (51) Madey, T. E.; Ramaker, D. E.; Stockbauer, R. Annu. Rev. Phys. Chem. 1984, 35, 215. (52) Stulen, R. H. Prog. Surf. Sci. 1989, 32, 1. (53) Heiweil, E. J.; Casassa, M. P.; Cavanagh, R. R.; Stephensen, J. T. Annu. Rev. Phys. Chem. 1989, 40, 143.

Ma et al.

metal surfaces has been limited to some associative desorption such as CO(a) + O(a) f CO2(g) and N(a) + N(a) f N2(g).5 The desorbing molecules with excess energy appear only in some exothermic processes of which the reverse (dissociative) processes require significant activation energy. Therefore, the above N2 emission from N2O is exceptional because it is directly emitted in a thermal dissociation event (without associative process). A similar collimated fragment desorption was once reported in a hydrazine decomposition on Ir(111) in 1980.54 The product N2 peaked around 290 K and 440-500 K in the thermal desorption procedures after N2H4 exposures at 260 K. The N2 desorption at 290 K collimated along the surface normal in a (cosθ)∼100 form. However, the reliability of the ARsignal is not clear since the performance of the angleresolution in an apparatus with a single slit is very poor in nontime-resolved measurements.26 Repeating this decomposition in the AR way would be worthwhile. V. Summary The kinetics and dynamics in the steady-state N2O + CO reaction were studied on Pd(110) by angle-resolved measurements and cross-correlation TOF techniques. A clear kinetic transition was observed, where the ratelimited step was changed from CO adsorption (surfaceoxygen removal) to N2O adsorption/dissociation. Under the conditions studied, N2 desorption always collimated around 45° off surface normal in the plane along the [001] direction, showing high translational temperatures in the range of 2000-5000 K. The inclined N2 desorption was proposed due to the decomposition of N2O(a) oriented along the [001] direction. On the other hand, the CO2 desorption collimated along the surface normal with a translational temperature of about 1600 K. LA0507030 (54) Sawin, H. H.; Merrill, R. P. J. Chem. Phys. 1980, 73, 996.