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Langmuir 2000, 16, 2689-2696

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Velocity of Desorbing CO2 in Steady-State CO Oxidation on Palladium(110): Pressure and Temperature Effects Sugio Wako,† Md. Golam Moula,†,§ Gengyu Cao,‡ Kazushi Kimura,† Ivan Kobal,‡,| Yuichi Ohno,†,‡ and Tatsuo Matsushima*,†,‡ Catalysis Research Center and Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060-0811, Japan Received July 27, 1999. In Final Form: November 22, 1999 The velocity distribution of desorbing CO2 was studied in steady-state CO oxidation on Pd(110) in a wide range of both surface temperature and reactant pressure by means of cross-correlation time-of-flight techniques. The velocity distribution curve always involved two components (fast and slow). The slow component showed a Maxwellian velocity distribution at the surface temperature. The energy of the fast component increased with increasing ratio of oxygen to CO pressure from 2200 to 4000 K, suggesting compressed oxygen lattices enhancing the velocity. This component was abruptly suppressed at the critical CO pressure, where the rate-determining step switched from CO adsorption to oxygen dissociation. The energy of the fast component increased with increasing surface temperature, suggesting a change in the energy partition itself. Above about 3 × 10-4 Torr of O2, the slow component did not become predominant even far above the critical pressure of CO.

* Corresponding author. Fax: +81-11-706-3695. E-mail: [email protected]. † Graduate School of Environmental Earth Science. ‡ Catalysis Research Center. § Also at Open School, Bangladesh Open University, Gazipur1704, Bangladesh. | Also at J. Stefan Institute, Ljubljana, Slovenia.

in the product concomitant with kinetic transitions, whereas, above 3 × 10-4 Torr, such an abrupt change was highly suppressed. The dynamic (energy transfer) and kinetic properties of chemical processes on catalysts are controlled by surface structures including adsorbates. Both properties are likely to vary concomitantly with experimental conditions. Actually, such simultaneous occurrence was observed in steady-state CO oxidation on Pt(557), Pt(113), and Pt(133) in the 10-5 Torr range of the reactant pressures.12-16 The excess translational energy of desorbing CO2 was reduced around the condition in which the ratedetermining step switched from CO adsorption to oxygen dissociation. Generally, the oxygen occupancy of adsorption sites, which provide CO2 formation places,6 should decrease in places further from the dissociation site because of its limited surface mobility;17 that is, CO2 formation is likely to occur on or near oxygen dissociation sites at lower coverages. This tendency in the site shift was clearly seen on Pt(113) below 1 × 10-5 Torr.13-15 In this paper, we examined this tendency on Pd(110) over a wide pressure range of CO and oxygen. The dependence of product velocity on the surface temperature was also studied to get information on the energy accommodation in the reactive desorption. Both product desorption dynamics and reaction kinetics are sensitive to reactant coverages and surface temperature. Modulated molecular beam scattering (MBS) causes the oxygen and CO coverages to change throughout the conditions critical for chemical kinetics.18,19 Both reactant coverages and surface temperature must be scanned in

(1) Wei, J. Adv. Catal. 1975, 24, 57 and the references therein. (2) Kummer, J. T. J. Phys. Chem. 1986, 90, 4747. (3) Delabie, L.; Honore´, M.; Lenaerts, S.; Huyberechts, G.; Roggen, J.; Maes, G. Sens. Actuators 1997, B44, 446. (4) Langmuir, I. Trans. Faraday Soc. 1921, 17, 621. (5) Engel, T.; Ertl, G. Adv. Catal. 1979, 28, 1. (6) Matsushima, T. Heterog. Chem. Rev. 1995, 2, 51. (7) Becker, C. A.; Cowin, J. P.; Werton, L.; Auerbach, D. J. J. Chem. Phys. 1977, 67, 3394. (8) Barker, J. A.; Auerbach, D. J. Surf. Sci. Rep. 1985, 4, 1. (9) Brawn, L. S.; Bernasek, S. L. J. Chem. Phys. 1985, 82, 2110. (10) Coulston, G. W.; Haller, G. L. J. Chem. Phys. 1991, 95, 6932. (11) Watanabe, K.; Ohnuma, H.; Uetsuka, H.; Kunimori, K. Surf. Sci. 1996, 368, 366.

(12) Cao, G.; Seimiya, Y.; Matsushima, T. J. Mol. Catal. 1999, 141, 63. (13) Cao, G.; Seimiya, Y.; Ohno, Y.; Matsushima, T. Chem. Phys. Lett. 1998, 294, 419. (14) Cao, G.; Moula, M. G.; Ohno, Y.; Matsushima, T. Surf. Sci. 1999, 427/428, 272. (15) Cao, G.; Moula, M. G.; Ohno, Y.; Matsushima, T. J. Phys. Chem. 1999, B103, 3235. (16) Seimiya, Y.; Cao, G.; Ohno, Y.; Yamanaka, T.; Matsushima, T.; Jacobi, K. Surf. Sci. 1998, 415, L988. (17) Von Oertzen, A.; Rotermund, H. H.; Nettesheim, S. Surf. Sci. 1994, 311, 322. (18) Engel, T.; Ertl, G. J. Chem. Phys. 1978, 69, 1267.

1. Introduction CO oxidation on platinum metals is one of the most important catalytic combustion processes, particularly in the catalytic treatments of exhausting gases and in gas sensors.1-3 This reaction has also provided model examinations of new concepts and advanced techniques for surface reactions and kept a leading position in both mechanistic and dynamic studies.4-6 A hyperthermal translational energy was found in the product CO2.7,8 Combined with internal energy measurements, the determination of this energy will provide the energy partition in reactive desorption events.9-11 However, the experimental results reported so far are difficult to be compared because there are large pressure gaps in experiments between translational and internal energy. Most velocity work has been performed below 10-5 Torr, whereas reactant pressures above 1 × 10-2 Torr have been used for infrared emission experiments to determine the internal energy. For the first time, this paper reports velocity measurements extended up to the pressure above which scattering by gaseous molecules becomes significant, that is, to 3 × 10-2 Torr. Below approximately 1 × 10-4 Torr, a sharp translational energy change was found

10.1021/la991010k CCC: $19.00 © 2000 American Chemical Society Published on Web 01/17/2000

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angle-resolved thermal desorption spectroscopy combined with time-of-flight techniques (TDS-TOF).6 In the present work, therefore, the velocity of product CO2 was determined by cross-correlation TOF over a Pd(110) sample subjected to a constant flow of reactant gases and at fixed surface temperatures. 2. Experimental Section The apparatus consisted of three chambers for controlling reaction conditions, chopping the flow of the desorbing product, and detecting CO2 molecules.12 The first had low-energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS) optics, an Ar+ gun, a quadrupole mass spectrometer (QMS), and a gas-handling system for back-filling CO and oxygen gases. A chopper disk in the second chamber had slots of equal width (1 mm × 6 mm) ordered in a pseudorandom sequence (with a double sequence of 255 elements each).20 The analyzer chamber had another QMS operating in a pulse-counting mode. The arrival times at the ionizer of the QMS were registered on a multichannel scaler synchronized with the chopper rotation. Velocity distributions were obtained after the raw TOF spectra had been deconvoluted using a standard cross-correlation deconvolution technique.20 The distance between the chopper blade and the ionizer of the QMS was 377 mm. The time resolution of 20 µs was obtained at a chopper speed of 98.032 Hz. The ion drift-time of CO2+ in the mass spectrometer of the analyzer was determined in separate experiments. A palladium (110) crystal (supplied by MaTeck, Germany) was mounted on the top of a manipulator and rotated to change the desorption angle (θ; polar angle) in a plane in the [001] direction. This is because the angular distribution in a plane in the [110] direction was broad.21,22 The crystal was cleaned by repeated cycles of Ar+ bombardment and heating in O2 and was finally annealed in vacuo up to 1100 K.21 The LEED pattern at this stage showed a sharp (1 × 1) structure. The surface temperature (TS) was monitored with a chromel-alumel thermocouple welded on the side of the crystal. The partial pressures of CO (PCO) and O2 (PO2) below 10-4 Torr were monitored with the mass spectrometer in the reaction chamber. With higher pressures, an ionization gage in another attached chamber differentially pumped was used after calibration. Desorption of product CO2 was monitored in angle-resolved form with the mass spectrometer in the analyzer chamber and also in angle-integrated form with the other mass spectrometer in the reaction chamber.

3. Results 3.1. Kinetics. The steady-state CO2 formation rate was monitored in the normal direction in angle-resolved form with the mass spectrometer in the analyzer chamber. It was determined as the difference in the CO2 signal between the normal direction and that when the crystal was moved away from the line-of-sight position. The rate was negligible below TS ) 370 K and increased rapidly to a maximum with increasing temperature before decreasing at higher values, as shown in Figure 1a. Both the starting temperature of CO2 formation and the maximum level shifted to higher values with increasing PCO. The PCO dependence of the rate was characterized by sharp transitions at critical PCO values.23 Below the value, the CO2 desorption almost linearly increased with increasing PCO, whereas, above the critical pressure, it decreased with a further increase in CO pressure. Hereafter, the former is named the “active region” and (19) Campbell, C. T.; Ertl, G.; Kuipers, H.; Segner, J. J. Chem. Phys. 1980, 73, 5862. (20) Comsa, G.; David, R.; Schumacher, B. J. Rev. Sci. Instrum. 1981, 52, 789. (21) Matsushima, T. J. Chem. Phys. 1989, 91, 5722. (22) Matsushima, T.; Shobatake, K.; Ohno, Y.; Tabayashi, K. J. Chem. Phys. 1992, 97, 2783. (23) Moula, M. G.; Wako, S.; Cao, G.; Kimura, K.; Kobal, I.; Ohno, Y.; Matsushima, T. PCCP 1999, 1, 3677.

Figure 1. (a) Variation of the CO2 signal in the surface normal direction with surface temperatures at the fixed O2 pressure of 3.0 × 10-5 Torr and different CO pressures: PCO/10-5 Torr ) 12 (O), 3.0 (9), and 1.0 (2). The CO/O2 pressure ratio is inserted. (b) Temperature dependence of the translational energy of the fast component (O) and the average energy (b) of desorbing CO2 at θ ) 0° and PCO/PO2 ) 4. The vertical bars indicate typical experimental errors. (c) Velocity distributions of desorbing CO2 at different surface temperatures at the ratio PCO/PO2 ) 4. The translational energy of the fast component is shown in the temperature unit. Typical deconvolutions are shown by dotted lines. The solid lines indicate the sum of both components. The ordinate was normalized to the maximum at 573 K.

the latter the “inhibited region.”15 This sharp kinetic transition is caused by the switching of the ratedetermining step from CO adsorption to oxygen dissociation.24-27 The surface is mostly covered by oxygen in the (24) Matsushima, T.; Musset, C. J.; White, J. M. J. Catal. 1976, 41, 397. (25) Matsushima, T.; Hashimoto, M.; Toyoshima, I. J. Catal. 1979, 58, 303. (26) Matsushima, T. Bull. Chem. Soc. Jpn. 1978, 51, 1956. (27) Ehsasi, M.; Matloch, M.; Frank, D.; Block, J. H.; Christmann, K.; Rys, F. S.; Hirschwald, W. J. Chem. Phys. 1989, 91, 4949.

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Figure 3. Velocity distribution curves of desorbing CO2 at different desorption angles. The surface temperature was 673 K. PCO was 1.2 × 10-4 Torr. Typical deconvolutions are shown by dotted lines. The solid lines indicate the sum of both components. The ordinate was normalized to the maximum in the normal direction.

Figure 2. (a) Variation of the steady-state CO2 formation rate at θ ) 0° with oxygen pressures. (b) Translational energy of the fast component (O) and its average value (b) under the above conditions. The vertical bars indicate typical experimental errors. (c) Velocity distributions of desorbing CO2 at selected PO2 values under the above conditions. The solid lines indicate the sum of both dotted lines, showing typical deconvolutions. The ordinate was normalized to the maximum value at 2 × 10-6 Torr.

active region, whereas it is highly covered by CO in the inhibited region. The PO2 dependence of the rate was also characterized by a sharp transition at a critical value (Figure 2a). Below the value, the CO2 desorption linearly increased with PO2. It remained constant above the value in a wide pressure range. The latter corresponds to the active region. This condition is suitable to examine the oxygen coverage effect on desorption dynamics. 3.2. Surface Temperature and Velocity. Velocity distributions at different temperatures are summarized in Figure 1c. The oxygen pressure was fixed at 3 × 10-5 Torr, and that of CO, at 1.2 × 10-4 Torr. The reaction at TS ) 473 K was in the inhibited region. The ordinate was normalized to the maximum intensity at 573 K. The distribution curve always consisted of two components, a

fast one and a slow one. The slow component was described by a Maxwellian distribution at the surface temperature. The remaining value after subtraction of the slow component from the experimental signal was fitted to a modified Maxwellian form. The fitted curve (the broken lines in Figure 1c) was drawn by plotting the flux multiplied by the velocity v, that is, v4 exp{-(v - v0)2/a2} versus v. v0 is the stream velocity, and a is the width parameter. The energy of the fast component was determined in this way and inserted in the temperature unit as T〈E〉 ) 〈E〉/2k. 〈E〉 is the mean translational energy,28 and k is the Boltzmann constant. The velocity distribution at 473 K involved a relatively high contribution from the slow component. On the other hand, the curves above 573 K, where CO did not retard the reaction, mostly consisted of the fast component. The maximum position of the velocity curve significantly shifted to higher values with increasing temperature, yielding an enhanced velocity at high temperatures. In fact, the translational energy of the fast component increased with increasing TS, yielding the slope of T〈E〉/TS Z 2 shown in Figure 1b. The average translational energy was also increased in a similar way as shown in the figure. It should be noticed that the slow component was in a significant amount in the CO2 formation even at high temperatures. The slow component was highly involved in desorbing CO2 at large desorption angles. The velocity distribution curves at different angles and TS ) 673 K are shown in Figure 3. The curves at angles > 30° were clearly bimodal. The signal of the fast component decreased quickly with increasing desorption angle, yielding an angular distribution in a cos10 θ form, whereas that of the slow one was almost constant, indicating a cosine distribution. The translational energy of the fast component decreased with increasing angle, consistent with the angle dependence in repulsive desorption.28,29 3.3. Velocity and Oxygen Pressure. The velocity distribution of desorbing CO2 suddenly changed at the (28) Comsa, G.; David, R. Surf. Sci. Rep. 1985, 5, 145. (29) Comsa, G.; David, R.; Schumacher, B. J. Surf. Sci. 1979, 85, 45.

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Figure 4. Velocity distribution curves of CO2 at different CO pressures. The O2 pressure was fixed at (a) 5 × 10-6 Torr and (b) 3 × 10-4 Torr. The slow and fast components are shown by dotted lines, and the sum is shown by solid lines. The ordinate was normalized to the maximum at 2 in part a and at 7 in part b.

critical pressure. The distribution curves at θ ) 0° in both regions are summarized in Figure 2c. The CO pressure was fixed at 3 × 10-6 Torr, and TS was 460 K. The curve at 1 × 10-6 Torr of O2 in the inhibited region was bimodal, involving a high contribution of the slow component. The signal peak of the fast component largely shifted from that in a Maxwellian form at the surface temperature. Above 2 × 10-6 Torr of O2, the slow component intensity remained fairly invariant, whereas the fast component was enhanced. With increasing O2 pressure, the maximum intensity position shifted to higher velocities, yielding high translational energies up to 4000 ( 200 K. 3.4. Kinetic Transition. The above variation in the velocity curve around the critical O2 pressure became clearer when the distributions at θ ) 0° were compared at different CO pressures, as shown in Figure 4a. The distribution curves were again bimodal especially when the CO pressure exceeded the critical value of 7 × 10-6 Torr. The ordinate was normalized to the maximum value at 6 × 10-6 Torr of PCO in 2. The fast component was suddenly suppressed in the inhibited region (3-5). It should be noticed that the curves in the active region (1, 2) significantly involved the slow component. It is interesting to examine the CO pressure dependence of the flux of each component when the reaction passes the transition region. The flux in the surface-normal direction was calculated by considering the velocity and plotted versus the CO pressure on the logarithmic scale in Figure 5a. Both components increased in the active

region almost linearly with increasing PCO and changed their slopes to negative values at the critical pressure. At the critical CO pressure at a fixed PO2 of 5 × 10-6 Torr, only the fast component was suddenly suppressed to the level of the slow one. The slow component became major in the inhibited region because of its broader angular distribution. 3.5. Component Change. The above sudden and large suppression of the fast component was observed at critical PCO values below approximately 5 × 10-5 Torr. Above this level, the fast component significantly remained in the inhibited region although the kinetics still showed sharp transitions. The distribution curves at the fixed O2 pressure of 3 × 10-4 Torr and 460 K are shown in Figure 4b, where the critical CO pressure was 8 × 10-5 Torr. Curves at pressures below this critical level (6, 7) were observed in the active region and showed a single peak. The ordinate was normalized to the maximum at 7 × 10-5 Torr of CO (7). The curves in the inhibited region showed a sign of bimodality (curves 8-10). The slow component was clear but not major. The translational energy of the fast component remained constant throughout the kinetic transition. The flux of each component under these conditions was plotted versus PCO again on the logarithmic scale in Figure 5a. The absolute signal intensity cannot be compared with the data at 5 × 10-5 Torr of PO2 because the mass spectrometer sensitivity was adjusted for high pressure conditions. However, it is clear that only the fast

CO2 Velocity on Pd(110)

Figure 5. (a) Flux variation of the fast and slow components with CO pressures at fixed O2 pressures of (O, b) 5 × 10-6 Torr and (0, 9) 3 × 10-4 Torr. (b) PCO dependence of the fraction of the fast component into the normal direction (data points and solid lines) and that in the total CO2 formation (dotted lines). Fixed O2 pressures are shown by downward arrows with data symbols. The vertical broken lines show kinetic transition positions.

component was slightly and suddenly suppressed at the critical point. The above kinetic and dynamic transitions were examined in a wide pressure range. The results are summarized in Figure 5b. The fraction of the fast component in the normal direction was plotted as a function of PCO for various fixed oxygen pressures (the downward arrows). The kinetic transition pressure is shown by vertical broken lines. This pressure shifted to higher values with increasing fixed O2 pressure. The ratio of the critical CO pressure to the fixed PO2 value exceeded unity below approximately 1 × 10-5 Torr, and above this pressure, it decreased quickly with increasing total pressure. 3.6. Scattering by Gaseous Reactants. It is interesting to examine the pressure above which product molecules are noticeably scattered by gaseous reactants. This mean-free-path problem becomes serious in angleresolved IR-emission experiments, because measurements are performed at pressures as high as 1 × 10-2 Torr.9-11 The real pressure around the sample surface, however, is hardly determined in an apparatus with a high pumping rate. We examined the pressure of the reactants to cause significant scattering of desorbing CO2 in our apparatus. The velocity distribution curves above the total pressure of 0.9 × 10-2 Torr are summarized in Figure 6. The CO pressure was kept at the fraction 0.2-0.3 of PO2, and TS was 540 K. The actual distribution at 0.9 × 10-2 Torr showed a significant slow component with a translational temperature of approximately 380 ( 40 K (Figure 6a). This component remained above this pressure (Figure 6b); however, the fast component was completely suppressed at 4 × 10-2 Torr (Figure 6c). 4. Discussion 4.1. Kinetics and Mechanism. The observed kinetics agrees well with the literature.5,24-27,30-32 The reported mechanism explains all the observed kinetic results; that

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Figure 6. Velocity distribution curves at pressures high enough to scatter CO2 by gaseous reactants. The total pressure Ptot is shown.

is, the reaction rate is limited by CO adsorption below the critical CO pressure, whereas, above it, the rate is controlled by dissociative adsorption of oxygen. It is wellknown that, in the CO pressure region of 10-7 to 10-5 Torr, surface species are sharply replaced from O(a) > CO(a) to O(a) , CO(a) at a critical PCO value with increasing CO pressure.27,33 This is because the CO adsorption rate balances with the reactive removal of adsorbed CO at the critical pressure, and furthermore, the removal reaction proceeds much faster than either CO or oxygen adsorption. At the oxygen pressure of 10-3 Torr or higher, however, the replacement of the surface species may become unclear, since experimental conditions are close to those for chemical oscillations of the catalytic CO oxidation.34-37 During this oscillation, Ertl and co-workers showed the growth and disappearance of peculiar patches of O(a)and CO(a)-covered surfaces in the size range from millimeter to micrometer.38,39 The amounts of surface species may not be switched quickly at the critical CO pressure under these conditions. The reaction rate is controlled by oxygen dissociation at the temperatures showing a steep increase below 500 K in Figure 1a, and after the maximum, it is governed by several processes, such as the adsorption and desorption of both CO and oxygen as well as the CO2 formation reaction.5 Above 550 K, the steady-state CO coverage is very small.33 However, the surface residence time of CO is long enough to visit several adsorption sites, because its surface residence time until its removal as CO2 is (30) Matsushima, T.; Almy, D. B.; White, J. M. Surf. Sci. 1977, 67, 89. (31) Matsushima, T.; White, J. M. Surf. Sci. 1977, 67, 122. (32) Golchet, A.; White, J. M. J. Catal. 1978, 53, 266. (33) Bowker, M.; Jones, I. Z.; Bennett, R. A.; Esch, F.; Baraldi, A.; Lizzit, S.; Comelli, G. Catal. Lett. 1998, 51, 187. (34) Ehsasi, M.; Seidel, C.; Ruppender, H.; Drachsel, W.; Block, J. H.; Christmann, K. Surf. Sci. 1989, 210, L198. (35) Ehsasi, M.; Berdau, M.; Rebitzki, T.; Charle′, K.-P.; Christmann, K.; Block, J. H. J. Chem. Phys. 1993, 98, 9177. (36) Ertl, G.; Norton, P.; Ru¨stig, J. Phys. Rev. Lett. 1982, 49, 171. (37) Imbihl, R. Prog. Surf. Sci. 1993, 44, 185. (38) Rotermund, H. H.; Engel, W.; Kordesch, M. E.; Ertl, G. Nature 1990, 343, 355. (39) Ertl, G. Surf. Sci. 1993, 287/288, 1.

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around several hundred microseconds, and the lifetime on each adsorption site is of the order of magnitude of nanoseconds.18,19,40 Above 700 K, oxygen can be desorbed without reaction.41,42 This consideration leads to the idea that the surface is kept clean above 600 K when PCO is higher than approximately twice PO2 because the removal rate of oxygen is faster than its supply to the surface.43 In fact, the CO2 formation was kept fairly constant at 550-700 K when the CO/O2 ratio was 4. The surface can provide a suitable stage for the temperature dependence of the velocity without being disturbed by coverage variation. However, the temperature dependence of the velocity would involve the coverage effect when the CO/ O2 pressure ratio was unity or less. 4.2. Velocity and Surface Temperature. The velocity distribution clearly showed the two components. The translational energy of the fast component increased with increasing TS, yielding a slope of about 2. This slope would be underestimated when the oxygen coverage effect is involved. The oxygen coverage must decrease significantly, yielding reduced velocity, as expected from the O2 pressure effect. Nevertheless, the observed energy was enhanced more than an increment of the surface temperature. This increase may be attributed to a change in the energy partition itself. In fact, recent infrared chemiluminescence analysis by Kunimori et al. shows that the internal energy of the product CO2 on Pd(111), Pd(110), and Pd(335) is 2-3 times higher than the surface temperature and increases further with increasing surface temperature above 650 K.44,45 Thus, the total energy which CO2 holds increases with the surface temperature. This is not unreasonable because only about half of the potential energy (measured from the vacuum level) of the activated complex is transferred into several modes in CO2. Furthermore, the energy accommodation with the surface becomes inefficient at higher surface temperatures. The potential energy of the activated complex of CO2 formation on Pd(111) is about 30 kcal/mol above the vacuum level at small coverages although it shifts toward higher values at higher coverages.5 Only about 20% of the transferable energy is converted into the translation. The internal modes of CO2 receive more energy, about 25% of the potential energy. The energy is largely (about 55%) transferred into surface modes. This transfer may be reduced at higher temperatures, as predicted from smaller energy accommodation coefficients at elevated temperatures in scattering experiments of CO2 over platinum.46 With decreasing TS, the average translational energy decreased more rapidly than the translational energy of the fast component, indicating a relatively enhanced slow component at lower temperatures. The fraction of the slow component in the normal direction decreased from 0.3 at 573 K to 0.1 at 873 K. It should be noticed that the slow component was always noticeable even when the surface was free from either CO or oxygen. The formation of this component is not due to interaction with adsorbates. The present surface appears to have intrinsic sites for the formation of the slow component. This site does not play

direct roles in producing the fast component because of its rapid decrease in the inhibited region. 4.3. Oxygen Pressure and Velocity. The translational temperature of the fast component smoothly decreased from 4000 to 2400 K when the O2 pressure decreased from 2 × 10-4 to 1 × 10-6 Torr, as shown in Figure 2b. The average translational energy decreased also smoothly in the above region and dropped suddenly at around 1 × 10-6 Torr because of the suppression of the fast component. The surface lacked oxygen in the inhibited region below the critical oxygen pressure. The oxygen coverage must increase toward saturation in the constant CO2 formation range in Figure 2a.31 The lattice due to adsorbed oxygen may change from (1 × 2) to c(2 × 4), c(2 × 6), and (1 × 3) because of temperatures higher than 500 K.42,47 The enhanced velocity of CO2 molecules at higher pressures is explained by the increasing density of oxygen in surface lattices.21,22,48-51 The increasing reactant density may cause sharper angular distributions and higher velocities because of enhanced repulsive forces between adsorbates. The activated complex of the reaction will be expanded from a smaller volume in its chemical-adsorption state to a larger volume of CO2 (being formed) because of no chemical bonding to the surface (Pauli’s repulsion). The repulsive force exerted toward this state will be enhanced in lattices with higher density. A similar enhancement in velocity was caused by adsorbed CO.52 On Pd(110), Kimura et al. found a sudden increase of the translational temperature, approximately 150 K, at the critical CO pressure of (2-3) × 10-7 Torr and at a fixed oxygen pressure of 2 × 10-7 Torr, far below the values in the present work, and a rapid decrease with a further increase in the CO pressure. The velocity distribution curve became slightly broad below the critical CO pressure, enhancing slower velocity components. These components were suppressed above the critical CO pressures. It is well-known that CO(a) can compress the surface oxygen lattices to a higher density, yielding an enhanced repulsive force between adsorbed reactants.21,22,48-51 However, at high coverages, the repulsion of CO(a) becomes strong enough to prevent oxygen from dissociating. 4.4. Surface Structure. The nascent product CO2 is repulsed along the surface normal direction, yielding the structural information of reaction sites.6 The CO2 formation on Pd(110) was concluded to proceed in surface troughs, because the product desorption showed an isotropic spatial distribution correlated to the symmetry of the trough structure.21,22 The Pd(110)-c(2 × 4)-O lattice was clearly seen by LEED in the active region of CO(a) < O(a) for the steady-state CO oxidation around 500 K.47 This lattice was once concluded to be due to an oxygen superlattice on the (1 × 1) form; however, it was later determined to be reconstructed into a missing-row structure.53-58 This missing-row structure, which consists of three-atom-wide (111) terraces declining alternatively in the [001] direction, is metastable without oxygen at

(40) Reutt-Robey, J. E.; Doren, D. J.; Chabal, Y. T.; Christman, S. B. Phys. Rev. Lett. 1988, 61, 2778. (41) Matsushima, T. Surf. Sci. 1989, 217, 155. (42) He, J. W.; Memmert, U.; Griffiths, K.; Norton, P. R. J. Chem. Phys. 1989, 90, 5082. (43) Goschnick, J.; Wolf, M.; Grunze, M.; Unertl, W. N.; Block, J. H.; Loboda-Cackovic, J. Surf. Sci. 1986, 178, 831. (44) Watanabe, K.; Ohnuma, H.; Kimpara, H.; Uetsuka, H.; Kunimori, K. Surf. Sci. 1998, 402, 100. (45) Uetsuka, H.; Watanabe, K.; Ohnuma, H.; Kunimori, K. Chem. Lett. 1996, 227. (46) Mantell, D. A.; Ryali, S. B.; Haller, G. L.; Fenn, J. B. J. Chem. Phys. 1983, 78, 4250.

(47) Ertl, G.; Rau, P. Surf. Sci. 1969, 15, 443. (48) Conrad, H.; Ertl, G.; Ku¨ppers, J. Surf. Sci. 1978, 76, 323. (49) Matsushima, T.; Asada, H. J. Chem. Phys. 1986, 85, 1658. (50) Stuve, E. M.; Madix, R. J.; Brundle, C. R. Surf. Sci. 1984, 146, 146. (51) Ohno, Y.; Matsushima, T.; Shobatake, K.; Nozoye, H. Surf. Sci. 1992, 273, 291. (52) Kimura, K.; Ohno, Y.; Matsushima, T. Surf. Sci. 1999, 429, L455. (53) Yagi, K.; Higashiyama, K.; Fukutani, H. Surf. Sci. 1993, 295, 230. (54) Tanaka, H.; Yoshinobu, J.; Kawai, M. Surf. Sci. 1995, 327, L505. (55) Yasui, Y.; Sawada, M.; Takagi, N.; Aruga, T.; Nishijima, M.Surf. Sci. 1996, 365, 422.

CO2 Velocity on Pd(110)

room temperature and is lifted into the (1 × 1) form above 370 K.57,58 On such missing-row surfaces, the CO2 desorption would be split into a two-directional way in a plane in the [001] direction, as observed on both reconstructed Pt(110) and Ir(110).59-61 However, no splitting was found in the present steady-state measurements. The desorption was sharply collimated along the normal direction in the active region. There are two possibilities. On one hand, the oxygenstabilizing missing rows on declining terraces may be less reactive toward CO and the oxygen in the bottom of the valley may yield the product.60 On the other hand, the CO2 formation may mostly proceed on (1 × 1) facets.21 A highly roughened surface after repeated reconstructionlifting cycles is not expected in this case, because the CO2 desorption showed a sharp distribution collimated along the normal direction. 4.5. Composition Change. The slow component in the active region was already noticeable in the surface normal direction and became more significant at higher desorption angles. This is due to differences in the angular distributions between the slow and fast components. The fast component had a distribution approximated as cos10 θ in a plane in the [001] direction, and the other component showed a simple cosine form. It should be noted that the signal due to the fast component is much more enhanced than that of the slow one in the normal direction. The fraction of the fast component in the normal direction showed a sharp change at the critical PCO value, especially below 3 × 10-5 Torr of oxygen. On the other hand, above 3 × 10-4 Torr of oxygen, the fraction decreased only slightly. This component change became clearer when the fraction was considered for the total CO2 formation. This fraction of the total was estimated from the signal in the normal direction and the angular distribution of each component. It should be noted that the relation between the observed signal and the total flux depends on the sharpness of the angular distribution.15 The total flux of desorbing molecules Q is given in the following expression

Q ) I0(n + 1)-1/2(m + 1)-1/2 where I0 is the flux in the surface normal direction. This relation was derived by integrating over a half sphere the angular distribution described as cosm θ in a plane in the [001] direction and as cosn θ in a plane perpendicular to it. The Q value of the slow component was estimated by assuming n ) m ) 1, and that of the fast one was estimated with n ) 10 and m ) 2.21,22 The resultant fraction of the fast component is shown by dotted curves in Figure 5b. The fraction suddenly decreased from about 0.9 to 0.1 at 3 × 10-7 Torr of oxygen. A similar change was found below 3 × 10-5 Torr of oxygen. The slow component became major in the inhibited region. The preference of CO2 formation mostly changed from the fast component to the slow one. We may expect a switching of the CO2 formation place in both reaction regions. However, the fraction decreased only slightly in the pressure range of 10-4 Torr or above. The fast component (56) Niehus, H.; Achete, C. Surf. Sci. 1996, 369, 9. (57) Dhanak, V. R.; Comelli, G.; Paolucci, G.; Prince, K. C.; Rosei, R. Surf. Sci. 1992, 260, L24. (58) Brena, K. B.; Comelli, G.; Ursella, L.; Paolucci, G. Surf. Sci. 1997, 375, 150. (59) Matsushima, T. J. Chem. Phys. 1990, 93, 1464. (60) Ohno, Y.; Matsushima, T.; Uetsuka, H. J. Chem. Phys. 1994, 101, 5319. (61) Matsushima, T.; Ohno, Y.; Nagai, K. J. Chem. Phys. 1991, 94, 704.

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highly remained in the inhibited region (about 70%). The reaction place for the fast component, which is probably due to patches covered highly by oxygen, is still present in the inhibited region. The stability of such domains of oxygen was already conformed by Ertl and co-workers under the conditions for chemical oscillations as described in Section 4.1.37-39 In fact, those are close to the present high pressures. The remaining fast component in the inhibited region was expected from IR emission work performed around 1 × 10-2 Torr of the total pressure, in which vibrationally excited CO2 molecules were found. It should be emphasized that, under such high pressures, the translational energy of CO2 is much higher than that determined in the pressure range 10-8 to 10-5 Torr and the reaction places are different from those in low-pressure experiments. 4.6. Translational Energy. CO2 desorption is definitely divided into two channels on this surface; that is, the product is desorbed with an excess energy, or the energy is completely dissipated. The fast component was major in the active region and mostly suppressed at the critical CO pressure. Complete dissipation occurs for only about 10-15% of the product in the active region below 2 × 10-5 Torr. On the other hand, the slow component was not abruptly suppressed at the critical pressure and became major in the inhibited region. It was still noticeable above 600 K in both regions, where the steady-state coverage of CO was very small in the active region or both coverages were very small in the inhibited region. This comparison suggests that the slow component is produced on or near the dissociation sites of oxygen. Oxygen admolecules are highly mobile at the reaction conditions and are likely to dissociate on intrinsic dissociation sites, step edges, or surface structural defects where admolecules and the resultant adatoms adsorb strongly.63-66 Oxygen adatoms are populated around their dissociation sites at low coverages and then distributed also to further places at higher coverages. Thus, CO2 formation at high oxygen coverages may take place far from the dissociation sites. This CO2 might have a high translational energy. This model requires a mechanism to explain the quick flow of the excess energy to the oxygen dissociation sites or nearby. The formation of structural defects is likely on the present surface because the surface may be repeatedly reconstructed and lifted in the course of reactions, yielding many defects in both the active and inhibited regions.62 Segner, Campbel, Kuippers, and Ertl (SCKE) proposed that CO2 had no excess energy when it was formed on structural defects.67 Their speculation came from the observation that the angular distribution of desorbing CO2 became broad at lower coverages on Pt(111) with structural defects. However, such irregular defects oriented in different directions may cause a broad distribution even if desorbing CO2 keeps an excess energy. In fact, CO2 desorption was split into two directional ways, closely along the (111) and (001) facet normal directions on stepped Pt(113) ) [(S)2(111) × (001)]. The observed angular distribution was rather broad and CO2 kept high (62) Bennett, R. A.; Poulston, S.; Jones, I. Z.; Bowker, M. Surf. Sci. 1998, 401, 72. (63) Wintterlin, J.; Schuster, R.; Ertl, G. Phys. Rev. Lett. 1996, 77, 123. (64) Barth, J. V.; Zambelli, T.; Wintterlin, J.; Ertl, G. Chem. Phys. Lett. 1997, 270, 152. (65) Rar, A.; Matsushima, T. Surf. Sci. 1994, 318, 89. (66) Heyd, D. V.; Scharff, R. J.; Yates, J. T., Jr. J. Chem. Phys. 1999, 110, 6939. (67) Segner, J.; Campbell, C. T.; Doyen, G.; Ertl, G. Surf. Sci. 1984, 138, 505.

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excess translational energies.13,15,68,69 Actually, this stepped surface mostly consists of structural defects. On palladium surfaces, however, this consideration does not necessarily exclude SCKE’s model that protruding surface atoms can receive the energy being produced from CO2 more easily than flat planes. This is because a palladium atom having a mass closer to that of CO2 than platinum can more easily receive an excess energy during the impact event. Poehlmann, Schmitt, Hoinkes, and Wilsch (PSHW) proposed another model showing that the slow component was produced on sites induced by high oxygen exposures.70 This slow component might be due to the formation on or near surface impurities because it disappeared after Ar+ bombardments. PSHW’s model might be considered when new reaction sites can be created in the inhibited region. 4.7. Scattering. The velocity distribution curves at high pressures (Figure 6) indicated an additional component appearing rapidly around 1 × 10-2 Torr. The translational energy was around 380 ( 50 K, clearly below the surface temperature. This was enhanced at higher pressures of either CO or oxygen and much less than the maximum intensity of the fast component below 0.9 × 10-2 Torr. (68) Yamanaka, T.; Moise, C.; Matsushima, T. J. Chem. Phys. 1997, 107, 8138. (69) Stefanov, P. K.; Ohno, Y.; Yamanaka, T.; Seimiya, Y.; Kimura, K.; Matsushima, T. Surf. Sci. 1998, 416, 305. (70) Poehlmann, E.; Schmitt, M.; Hoinkes, H.; Wilsch, H. Surf. Rev. Lett. 1995, 2, 741.

Wako et al.

This component may involve CO2 scattered by gaseous molecules. However, it is difficult to separate the fraction of scattered CO2 molecules from the slow component formed on the surface. The mean free path of CO2 molecules was estimated to be around 5 mm at the CO pressure 2 × 10-2 Torr.71 The distance from the sample surface to the first slit measured 25 mm in the present apparatus. At 4 × 10-3 Torr, about 60% of desorbed CO2 is estimated to be scattered by gaseous reactants before the first slit. The observable signal due to CO2 without scattering should be maximized around this pressure and mostly attenuated around 2 × 10-2 Torr.71 This was actually observed. Thus, the observed signal above 1 × 10-3 Torr must be somewhat underestimated by this scattering. Acknowledgment. The authors thank Ms. Atsuko Hiratsuka for her drawings. M. G. Moula is indebted to the Ministry of Education, Science, Sports and Culture of Japan for a scholarship in 1998-2002. Ivan Kobal acknowledges the support he received through the foreignresearcher (COE) invitation program in 1998-1999 of the above Ministry. This work was partly supported by a COE special-equipment program in 1996 of the said Ministry. LA991010K (71) Melville, H.; Gowenlock, B. G. Experimetnal Methods in Gas Reactions; Macmillan & Co. Ltd.: London, 1964; p 6.