Additional Bistability in a CO Oxidation Reaction on Pt(111

Article pubs.acs.org/JPCC

Second/Additional Bistability in a CO Oxidation Reaction on Pt(111): An Extension and Compilation Rafal J. Wrobel,*,†,‡ Stefan Becker,† and Helmut Weiss† †

Otto-von-Guericke Universität, Chemisches Institut, Universitätsplatz 2, 39106 Magdeburg, Germany West Pomeranian University of Technology Szczecin, Institute of Chemical and Environment Engineering, Pulaskiego 10, 70-322 Szczecin, Poland

‡

ABSTRACT: The CO oxidation reaction on Pt(111) was investigated in a pressure range of reactants 10−7−10−5 mbar and a temperature range 364−643 K. In the studied dependence of reaction rate vs temperature, two loops of hysteresis were found in reaction rate measurements instead of one, as expected. The explanation for the observed phenomena leads to two regions of bistability in the kinetic phase diagram caused by two phase transitions. These two detected phase transitions were confirmed independently by variation of two external control parameters, temperature and CO partial pressure, respectively. It means that the two kinetic phase diagrams were crossed in two parameter spaces, i.e., by variation of temperature with constant partial pressure of reactants and by variation of the CO partial pressure in isothermal conditions. The presented experimental data and comparison with literature data suggest that both phase transitions were observed previously but interpreted erroneously as one phenomenon. A unifying explanation of all observed phenomena is proposed.

■

INTRODUCTION Due to the technical importance in automotive exhaust catalysis, the CO oxidation on noble metals of the platinum group (e.g., platinum, palladium, and rhodium) is one of the most thoroughly studied chemical reactions. This catalytic reaction, which is, for example, taking place in a commercial three-way catalyst (TWC), is considerably complex due to the variety of factors affecting the reaction. Therefore, scientists following the fundamental advice of Langmuir,1 despite its limits, apply ideas of reductionism and investigate the model catalysts, i.e., highly simplified systems, for example, monocrystals. These systems allow elucidation of unity phenomena and thus give insight into the nature of complex systems. One of such interesting phenomena is, depending on the crystal plane and on external control parameters, the nonlinear behavior of the CO oxidation reaction, for example, spatiotemporal kinetic oscillations and hysteresis phenomena leading to a bistability region.2−9 In contrast to the crystallographic open Pt planes, kinetic oscillations are not observed at the CO + O/Pt(111) system in UHV conditions, which makes the Pt(111) plane a suitable surface for studying exclusively the phenomenon of bistability in detail. On the macroscopic scale, two stable states of the CO + O/ Pt(111) system are observed: a CO dominated low reactive state and a high reactive state, where oxygen is dominating the surface. It is established that a fully CO covered surface (at room temperature 0.5 ML) is inactive for an oxidation reaction because oxygen cannot adsorb dissociatively on adjacent adsorption places. On Pt(111), a c(4 × 2) structure is reported © 2012 American Chemical Society

for this case. On the other hand, the maximal oxygen coverage is 0.25 ML realized in a p(2 × 2) loose structure which allows CO coadsorption and thus reaction.11−14 Not only are the differences of adsorbate structure responsible for bistability phenomenon but also it is well established that the adsorbed species (CO and oxygen) influence the sticking coefficients of the reactants and thus influence significantly the surface reaction.10 As a result of the nonlinear behavior, the phase transition between these two stable regions leads to a bistable region, which was observed for given parameters.5,7,8 The other structures of the adsorbates (mainly of CO) founded by STM and LEED investigations15−19 have been so far not directly connected with the CO oxidation reaction. The bistability phenomenon was also found for Pd(111) and Pd(210) planes.6,9 The experimental results are supported by theoretical calculations, for example, using Monte Carlo simulations.20,21 The regions of mono- and bistability are commonly presented in two-dimensional space of CO pressure and inverse temperature as a kinetic phase diagram as proposed by Schlögl.22,23 Judging from the vast amount of both theoretical and experimental efforts devoted to CO oxidation bistability on platinum surfaces, one could conclude that everything was explained in this system. The Pt(111) surface is used in our research as a reference sample in investigation of the influence of cerium oxide nanoislands on the CO oxidation reaction.7 To Received: March 8, 2012 Revised: September 11, 2012 Published: September 19, 2012 22287

dx.doi.org/10.1021/jp302270n | J. Phys. Chem. C 2012, 116, 22287−22292

The Journal of Physical Chemistry C

Article

is the molar volume of a perfect gas at 298 K [dm3·mol−1]; and SPt(111) is the surface area of a Pt(111) crystal available for the surface reaction [m2].

our surprise during detailed kinetic measurements of CO oxidation on the Pt(111) system, the phenomena which can not be explained with the classical interpretation mentioned above have been observed. The goal of this work is presentation of experimental data of reaction rate measurements exhibiting two phase transitions of adsorbate structures during the CO oxidation reaction on Pt(111) instead of one phase transition observed previously by other researchers. It will be shown that the classical phase transition from the oxygen covered Pt(111) surface to the totally CO poisoned Pt(111) surface may occur through an intermediate structure. Moreover, the bistability for this intermediate structure will be proven. On the basis of the here presented results compared with the literature data, it will be shown that the phenomena were partially observed before but interpreted erroneously as one phenomenon. Finally, an interpretation, which unifies the here presented results with the results of others, will be given.

■

RESULTS AND DISCUSSION Catalytic Activity of Pt(111) vs Temperature. The reaction rate was measured in the temperature range from 643 to 364 K over clean Pt(111) for a constant partial pressure of reactants. Figure 1 presents the results for three different O2/

■

EXPERIMENTAL SECTION The experimental studies were performed in a surface analysis UHV system (XPS, LEED, STM, AES) (SPECS) operating at a base pressure below 10−10 mbar range. The preparation chamber of the UHV system allows cleaning (Ar+ bombardment and annealing up to 1400 K) and control of the surface composition (AES, LEED; SPECS, ErLEED 150). The analysis chamber is equipped with monochromatic and nonmonochromatic X-ray photoelectron spectroscopy (XPS; SPECS, Phoibos-150) and scanning tunnelling microscopy (STM; Omicron NanoTechnology, VT-AFM/STM). The Pt(111) monocrystal (disk of 10 mm diameter and 2 mm thickness, orientated 0.2° within the bulk (111) plane) was mounted on Mo sample holders equipped with a chromel− alumel thermocouple welded to the crystal surface. The atomically clean Pt(111) surface was prepared as described in detail in ref 24. After this procedure, the quality of the surface was confirmed by a sharp hexagonal LEED pattern and by STM. The absence of impurities was confirmed with XPS, AES, and CO titration methods.24 The XP spectra presented in this work were recorded with a 150 mm hemispherical energy analyzer (Phoibos-150, SPECS) calibrated with a clean gold foil (Au 4f7/2 signal). A monochromatic Al/Ag twin anode X-ray source (XR-50M, SPECS) with the Kα-radiation of Al (1486.6 eV) has been used. The CO oxidation reaction investigations were done in the described high vacuum system which was differentially pumped and used as a constant flow reactor. Due to the low pressure range (below 10−5 mbar), there are virtually no collisions between the gas molecules, and they were considered as perfect gas with ideal mixing. The reaction on the clean Pt(111) surface was monitored by CO, CO2, and O2 signals measured by a quadrupole mass spectrometer (QMS; MKS Instruments, VacCheck). In the steady flow conditions, eq 1 holds

Figure 1. CO2 production rate vs temperature for different CO/O2 ratios. (a) Set of three curves denoted with a roman number for three different CO partial pressures. (b) Curve (iii) in magnification. Different colors of the points denote the different repetitions of the measurements. Visible loops of hysteresis are denoted with arrows. The different surface phases occurring at a given temperature range are denoted.

CO ratios denoted by (i), (ii), and (iii). Three regions of the reaction can be distinguished in each curve. Those are: the region of negligible reaction rate (region I), followed by the region up to the curve maximum (region II), and the region beyond the curve maximum (region III). Region I. At lower temperatures, there is no noticeable significant surface reaction. In this temperature range, the surface is saturated with CO (θCO = 0.5, c(4 × 2)) which leads, due to the Langmuir−Hinshelwood mechanism (LH), to a poisoning of the surface by CO.25 Region II. In the second region, an increase of the reaction rate with a rise of the temperature can be observed. In this temperature range, a significant desorption of CO from the Pt(111) surface occurs (see the TDS curve presented in ref 26). This means that at these conditions the number of adjacent adsorption places necessary for dissociative adsorption of the molecular oxygen increases significantly which leads to an increase of the reaction rate on the Pt(111) surface.

•

rCO2 =

V ·pout ·NA ·XCO2 24.8·SPt(111)

(1) •

where rCO2 is reaction rate; V is volume pumping speed of a turbo molecular pump [dm3·s−1]; pout is pressure on the reactor outlet normalized to normal pressure; NA is Avogadro's number; XCO2 is molar fraction of CO2 in the gas phase; 24.8 22288



Article

Additionally, the reaction rate increases also due to kinetic reasons. Higher CO pressure hinders CO desorption (curve (i) and (ii)), and a pronounced shift of the reaction onset toward higher temperature can be noticed. Region III. The maximum of the curves determines the beginning of region III. The reaction rate grows exponentially with temperature in region II until, depending on the partial pressure ratio of the reactants, the reaction is limited by the adsorption rate of one of the reactants. Due to the decrease of sticking coefficients with temperature, the curve decays.27 The CO oxidation reaction on platinum is known to be bistable; i.e., at the same values of thermodynamic parameters, two different reaction rates are possible. Therefore, the state of the system is history dependent, and by variation of one thermodynamic parameter the phenomenon of hysteresis in reaction rate dependencies may appear.5−7,28 Such a phenomenon can be expected also in curves presented in Figure 1a. Indeed, in the case of curve (iii) (Figure 1b, Figure 2), there are surprisingly two regions of bistability clearly visible instead of one expected.5,7,8

Furthermore, an experiment with a zero sample excludes apparatus artifacts. The construction of the sample holder excludes the possibility of temperature gradient as well. Also, a reaction on the other side of the crystal can be excluded because this side was fully blocked with the Mo-sample holder. Therefore, the origin of the two hysteresis loops has to be a phenomenon related with the Pt(111) surface. Catalytic Activity of Pt(111) vs CO Pressure. The bistability phenomenon manifests itself also in dependencies of the reaction rate vs CO partial pressure in isothermal conditions presented in Figure 3. All hysteresis loops were

Figure 3. Typical hysteresis loop. (a) The whole loop. The transition points were denoted with τA and τB. The surface phases corresponding to a given stage of the loop were denoted. (b) Magnification of the low CO pressure feature characteristic of the intermediate phase transition. The loop with better resolution of transition points was obtained by 2.5 times higher flux of reactants. The transition points for the intermediate phase were denoted with τ′A and τ′B.

Figure 2. Magnified loops of hysteresis present in curve (iii) of Figure 1. (a) Low-temperature hysteresis characteristic for the transition between structure c(4 × 2) and (√3·√3)-R30. (b) High-temperature loop characteristic for the transitions between oxygen p(2 × 2) and CO(√3·√3)-R30.

obtained by variation of CO partial pressure in about 15 min per cycle. In the hysteresis presented in Figure 3a, a set of transition points denoted by τA and τB are visible. Moreover, it may be noticed that in hysteresis loops (Figure 3a,b) additional features, indicating the occurrence of an additional phase transition (τ′A and τ′B), are visible. By an increase of the

Curve (iii) in Figure 1b was produced by lowering temperature from 643 to 364 K, followed by raising the temperature again up to 643 K. This cycle was repeated three times during 15 h continued experiment, and all the data have undisputable reproducibility. The different colors of the presented data points correspond to different measurement cycles. The structures of CO and oxygen given in Figure 1b and Figure 2 are attributed based on the commonly accepted results presented in the literature.11,17,30

•

•

reactant flux (VO2 = 1.0 × 10−9 mol·s−1 and VO2 = 2.5 × 10−9 mol·s−1, respectively), a better resolution of this phase transition point (Figure 3b) was obtained. A closer inspection of ref 7 reveals the same features in hysteresis (like shown in Figure 3), but in this work this phenomenon was not discussed. This indicates that there was 22289



Article

excluded that it is realized by a different surface structure or by a disordered phase. Whatever the true nature of the intermediate structure is, the presented experimental data strongly support the idea of extension of the kinetic phase diagram by an additional surface structure. A possible explanation for the encountered τ′A−τ′B bistability cannot be that commonly accepted for the τA−τB transition, i.e., only by blocking the adsorption places for dissociative oxygen adsorption. If that were the case, than after poisoning the surface by CO the reaction rate could not rise with the CO pressure like is observed in hysteresis (Figure 3a,b). A similar behavior was observed also in previous studies5,7,8,28 but was not discussed in them. However, it is well-known that the molecules present on the surface influence the adsorption sticking coefficient of the gases.10 Therefore, the bistability can be explained by different values of the sticking coefficients of reactants for the Pt(111) surface precovered with oxygen or loosely precovered with CO. Additionally, it should be underlined that the width of the bistable region is dependent on the hysteresis sampling rate.5 Therefore, a quantitative comparison of the width of the bistable region obtained at different experiment and by different authors is problematic. Alternative Explanation. The surface of the precleaned Pt(111) monocrystal surface may be recontaminated with carbon by segregation from the bulk24 which may result in hysteresis in the high-temperature region (Figure 2b). However, in situ XPS measurements immediately performed after kinetic studies show no traces of carbon which excludes this scenario. Another phenomenon which might be responsible for abovementioned results is the presence of defects like steps of the platinum terraces. These defects and atoms adjacent to such defects have different catalytic activity toward the CO oxidation reaction. Therefore, one should expect an overlapping effect of phenomena (τA−τB transitions) occurring on plain platinum and defected platinumhence two bistable regions. The role of defects should be similar like the presence of adatoms, on the Pt(111) surface, promoting the CO oxidation reaction, e.g., CeOx/Pt(111).7 We do not observe an additional phase transition resulting from the presence of CeOx. However, we do observe the shift of the phase diagram. The shift of the phase diagram indicates the change of catalytic activity of platinum atoms adjacent to the CeOx island, i.e., the same effect like is expected from defects. This reasoning does not fully rule out this alternative scenario. There are other possible mechanisms which, potentially, could cause multiple hysteresis. The work of Blakely et al. shows that surfaces can reconstruct upon interaction with oxygen or carbon.31 The reversible doubling of the step height on a stepped Pt(111) surface may occur. As the amount of surface oxygen varies during the hysteresis loop, this may result in more than one bistable region. The work of Wang et al. suggests that on platinum steps the one-dimensional PtO2 structure forms after oxygen exposure even at low pressure.32 It has been found that this oxide reacts more easily with CO than oxygen chemisorbed on the platinum terraces. Therefore, observed phenomena could be linked to reversible structural changes or to an oxidation−reduction mechanism. On the other hand, Nakai et al. have found that oxygen over Pd(111) changes its surface structure depending on CO partial pressure.33 Furthermore, they claim that the observed three oxygen structures have different reactivity. As the Pt(111)

also an additional phase transition measured, which cannot be explained by the classical interpretation of the transition points. The classical interpretation of the transition points in the bistable region assumes that the transition occurs only between the oxygen covered active metal surface and the nonactive one poisoned by CO.5,29 Oxygen forms an island on the Pt(111) surface realized in a p(2 × 2) structure.11,30 The CO structure which leads to a total inhibition of the CO oxidation reaction is commonly assumed to be a dense c(4 × 2) superstructure. However, in the case of CO, it is generally accepted that it forms another structure: A (√3·√3)-R30 at θCO = 0−0.3 and c(4 × 2) which coexists with the saturated structure at θCO = 0.3−0.5 p(2 × 2).17 At elevated temperatures, experimental problems occur in determination of the CO structures. Moreover, the loose CO structure is claimed to be a disordered phase.17 Therefore, the CO transitory phase may be expected in switching from the high reactivity side (oxygen covered) to the low reactivity side (dense CO coverage). Kinetic Phase Diagrams. By determining the transition points τA,τB and τ′A,τ′B, respectively, in the corresponding hysteresis loops as a function of temperature at which the hysteresis was measured, the kinetic phase diagrams can be built. The measurement procedure for these points is analogous like described in ref 7, and the results are presented in in Figure 4.

Figure 4. Kinetic phase diagram. The surface phase structures of adsorbed reactants are denoted. The oxygen p(2 × 2) and CO c(4 × 2) phases are characteristic for high and low reactivity, while the phase CO (√3·√3)-R30 corresponds to an intermediate activity of transitory phase. The horizontal lines denoted with (i), (ii), and (iii) correspond to the thermodynamic conditions for the curves from Figure 1a.

The main τA and τB transitions, found in the hysteresis loop (Figure 3), obscure effectively the more subtle transitions τ′A and τ′B. Therefore, not all points τ′A and τ′B in the phase diagram were able to be determined. The area above and below phase diagrams corresponds to the low and high reactivity states, respectively. The transitory phase between the kinetic phase diagrams (Figure 4) can be attributed to the intermediate CO structure on the Pt(111) surface, i.e., the (√3·√3)-R30 phase.17 It has to be underlined that we have no possibility to confirm the presence of the CO (√3·√3)-R30 phase, as to be responsible for the τ′A−τ′B intermediate phase transition. The applied experimental conditions exclude the possibility of LEED measurements with the used experimental setup. Therefore, it cannot be 22290



Article

Final Interpretation. At low CO pressures the oxygen p(2 × 2) is present on the Pt(111) surface (Figure 3). By rising of the CO pressure, the reaction rate increases linearly until the τ′A is reached. At this point, the oxygen is reacted off from the Pt(111) surface, and CO appears like observed by Berdau et al.5 The CO forms a disordered phase or/and islands of CO (√3·√3)-R30 structure.17 The CO coverage is loose and allows the dissociative oxygen adsorption. The transition from the oxygen covered surface to the loose CO covered affects the sticking coefficients of reactants and thus affects the reaction rate. By a further increase of the CO pressure, the reaction rate increases until the τA is reached like observed by Suchorski et al.7 This point may correspond to the Pt(111) surface with CO (√3·√3)-R30 phase (θCO = 0.33) and nucleation of the CO c(4 × 2) phase. A further rise of the CO pressure results in the formation of a dense CO c(4 × 2) phase (θCO = 0.5), and thus the reaction rate decreases down to total poisoning. The phases (√3·√3)-R30 and c(4 × 2) may coexist, but a fraction of the CO c(4 × 2) phase increases asymptotically with the CO pressure, which results in an asymptotic decay of the CO2 production.17 The observed two hysteresis loops in the temperature dependence can be explained analogously (Figure 1).

surface is very similar to the Pd(111) one, there might be more than two surface structures of oxygen and CO, resulting in two bistability regions. The mechanism presented in this work with a CO transitory phase, although it explains all observed experimental phenomena in the whole temperature range, does not rule out other mechanisms. Comparison with the Literature Data. Table 1 presents sets of thermodynamic parameters for transition point τA at Table 1. Thermodynamic Parameters for Transition Points at Temperature ∼413 K for the Pt(111) System pO2 [mbar] τA, Berdau et al.5 τA, Losovyj et al.8 τA, Suchorski et al.7 τA, this work τ′A, this work τA, Berdau et al. for Pt(210)6

1.3 2.0 1.3 7.6 7.6 5.3

× × × × × ×

10−5 10−5 10−5 10−6 10−6 10−5

pCO [mbar] 4.7 1.1 3.5 2.0 7.0 1.5

× × × × × ×

10−7 10−6 10−6 10−6 10−7 10−7

CO/O2 ratio 0.04 0.06 0.27 0.26 0.09 0.28

∼413 K in different works. A strong discrepancy between works, e.g., ref 5 and ref 7, occurs as the CO/O2 ratio differs by a factor of 6.75. This discrepancy strongly indicates that the transition points τA defined in ref 5 and ref 7, respectively, are of a different origin. The values for phase transition τA given in ref 5 and ref 8 are in good agreement with the here defined transition point τ′A. The CO/O2 ratio for τA defined in ref 7 is in good agreement with the results presented herein. This supports the idea that the classical work of Berdau et al. on Pt(111)5 concerns the here named τ′A transition at very low CO partial pressures, and probably due to the applied pressure conditions (see Table 1) the phase transition at higher CO/O2 ratios was not observed. On the other hand, in the work of Suchorski et al.7 it was erroneously assumed that the observed phenomenon is of the same nature as shown by others,5,8 while the small feature τ′A was not discussed. Therefore, those works show probably two different bistable regions. The work of Berdau et al. seems to show the phase transition from the oxygen covered to loosely CO covered (here named transitory phase) Pt(111) surface. This loosely CO covered surface does not prevent the dissociative adsorption of oxygen which explains also that the reaction rate is still rising with increasing CO partial pressure even after reaching the transition point (see hysteresis presented in ref 5). On the basis of the LH mechanism including a total poisoned surface by CO (ΘCO = 0.5), it should not be the case, and it is indeed not observed after reaching the true poisoning point τA (see Figure 3a). For the Pt(210) a hysteresis was shown which, based on shape, exhibits an apparently real poisoning effect (no further increase of the reaction rate after reaching τA is observed).6 Furthermore, τA occurs in a pressure ratio similar to the reactants like given in ref 7 (see Table 1). The kinetic phase diagram, presented in ref 7, shows also an apparently real poisoning effect, which means that the transition point τA marks the transition from loose CO covered Pt(111) to the CO dense covered Pt(111) surface (ΘCO = 0.5). The here presented experimental data and their interpretation seem to unify all above-mentioned works.5−8 If the presented interpretation is true, then ref 7 shows unwittingly probably the first ever kinetic phase diagram concerning the real poisoning effect on Pt(111).

■

CONCLUSIONS The kinetic investigation of the CO oxidation reaction on a well-defined Pt(111) surface for a variety of thermodynamic external control parameters (temperature and partial pressure of reactants) was performed. The reaction rate dependency vs temperature for oxygen atmosphere revealed the first ever two regions of bistability instead of one region, like expected from the classical CO−oxygen reaction bistability interpretation. The reaction rate vs CO pressure at isothermal conditions exhibits also more than one surface phase transition, which is in contrast with the classical bistability observed on Pt(111). The intermediate CO surface structure (√3·√3)-R30 or disordered loose structure was proposed to be responsible for those phenomena. The observed bistability for the transitory phase was attributed to different reactant adsorption sticking coefficients for the CO and oxygen precovered Pt(111) surface. Additionally, an alternative origin of the observed phenomena was discussed. The kinetic phase diagram of the CO oxidation reaction on Pt(111) was updated and modified into two separate bistable regions.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail address: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS S.B. acknowledges the financial support by Fonds der Chemischen Industrie e.V. (Scholarship no. 182221). RJW acknowledges the support by EU Contract HPRN-CT-200200191.

■

REFERENCES

(1) Langmuir, I. Trans. Faraday Soc. 1922, 17, 615. (2) Eiswirth, M.; Ertl, G. Surf. Sci. 1986, 177, 90. (3) Ladas, S.; Imbihl, R.; Ertl, G. Surf. Sci. 1988, 198, 42. (4) Kasai, H.; Yamamoto, T.; Okiji, A. Surf. Sci. 1989, 220, L709− L713.

22291



Article

(5) Berdau, M.; Yelenin, G. G.; Karpowicz, A.; Ehsasi, M.; Christmann, K.; Block, J. H. J. Chem. Phys. 1999, 110, 11551. (6) Berdau, M.; Karpowicz, A.; Yelenin, G. G.; Christmann, K.; Block, J. H. J. Chem. Phys. 1997, 106, 4291. (7) Suchorski, Y.; Wrobel, R.; Becker, S.; Weiss, H. J. Phys. Chem. C 2008, 112, 20012. (8) Losovyj, Ya.B; Ketsman, I. V.; Kostrobij, P. P.; Suchorski, Yu. Vacuum 2001, 63, 277. (9) Karpitschka, S.; Wehner, S.; Küppers, J. J. Chem. Phys. 2009, 130, 054706. (10) Goschnick, J.; Grunze, M.; Loboda-Cackovic, J.; Block, J. H. Surf. Sci. 1987, 189/190, 137. (11) Wintterlin, J.; Völkening, S.; Janssens, T. V. W.; Zambelli, T.; Ertl, G. Science 1997, 278, 1931. (12) Gland, J. L.; Sexton, B. A.; Fisher, G. B. Surf. Sci. 1980, 95, 587. (13) Campbell, C. T.; Ertl, G.; Kuipers, H.; Segner, J. Surf. Sci. 1981, 107, 220. (14) Steininger, H.; Lehwald, S.; Ibach, H. Surf. Sci. 1982, 123, 1. (15) Olsen, C. W.; Masel, R. I. Surf. Sci. 1988, 201, 444. (16) Pedersen, M.Ø.; Bocquet, M.-L.; Sautet, P.; Lægsgaard, E.; Stensgaard, I.; Besenbacher, F. Chem. Phys. Lett. 1999, 299, 403. (17) Fichthorn, K. A.; Gulari, E.; Ziff, R. M. Surf. Sci. 1991, 243, 273. (18) Longwitz, S. R.; Schnadt, J.; Vestergaard, E. K.; Vang, R. T.; Stensgaard, I.; Brune, H.; Besenbacher, F. J. Phys. Chem. B 2004, 108 (38), 14497. (19) Vestergaard, E. K.; Thostrup, P.; An, T.; Lægsgaard, E.; Stensgaard, I.; Hammer, B.; Besenbacher, F. Phys. Rev. Lett. 2002, 88, 259601−1. (20) Völkening, S.; Winterlin, J. J. Chem. Phys. 2001, 114, 6382. (21) Petrova, N. V.; Jakovkin, I. N. Surf. Sci. 2005, 578, 162. (22) Schlögl, R. Z. Phys. 1971, 248, 446. (23) Schlögl, R. Ber. Bunsenges. Phys. Chem. 1980, 84, 351. (24) Wrobel, R.; Becker, S. Vacuum 2010, 84, 1258. (25) Campbell, C. T.; Ertl, G.; Kuipers, H.; Segner, J. J. Chem. Phys. 1980, 73, 5862. (26) Steininger, H.; Lehwald, S.; Ibach, H. Surf. Sci. 1982, 123, 264. (27) Becker S.; Wrobel R.; Weiss H. The role of subsurface oxygen in the catalytic CO oxidation on Pd(111). To be published. (28) Suchorski, Y.; Wrobel, R.; Becker, S.; Strzelczyk, B.; Drachsel, W.; Weiss, H. Surf. Sci. 2007, 601, 4843. (29) Imbihl, R. New J. Phys. 2003, 5, 62.1. (30) Gland, J. L.; Sexton, B. A.; Fisher, G. B. Surf. Sci. 1980, 95, 587. (31) Blakely, D. W.; Somorjai, G. A. Surf. Sci. 1977, 65, 419. (32) Wang, J. G.; Li, W. X.; Borg, M.; Gustafson, J.; Mikkelsen, A.; Pedersen, T. M.; Lundgren, E.; Weissenrieder, J.; Klikovits, J.; Schmid, M.; et al. Phys. Rev. Lett. 2005, 95, 256102. (33) Nakai, I.; Kondoh, H.; Shimada, T.; Resta, A.; Andersen, J. N.; Ohta, T. J. Chem. Phys. 2006, 124, 224712.

22292


Additional Bistability in a CO Oxidation Reaction on Pt(111

Recommend Documents