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A Comparative Kinetic Study of the CO-O2 Reaction Over Pt-Rh (111), (100), (410) and (210) Single Crystal Surfaces.
A. Crucq (Editor), Catalysis andAutomotive Pollution Control II 0 1991 Elsevier Science Publishers B.V., Amsterdam
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A COMPARATIVE KINETIC STUDY OF THE C O - 0 2 REACTION OVER Pt-Rh (lll), (loo), (410) AND (210) SINGLE CRYSTAL SURFACES. J. Siera, R. van Silfhout, F. Rutten and B.E. Nieuwenhuys Gorlaeus labaratories, Leiden University, P.O. BOX 9502, The Netherlands ABSTRACT The CO-02 reaction has been studied as a function of temperature and partial pressure of CO and 0 2 over the (1 1l), (loo), (210) and (410) surfaces of a Pt025-Rho.75single crystal. The effects of alloying and surface structure are discussed. It is found that the reaction rates on the alloy surfaces show a positive order in oxygen at high temperature (%OK), whereas under these conditions the reported order in oxygen is negative for Rh(ll1). Furthermore, considerable differences are found in the steady-state rate of C02 formation for the four surfaces. On Pt-Rh(210)the CO oxidation starts around 500K, while over Pt-Rh(ll1) the rate has already reached its maximum value at this temperature. Comparison of the four surfaces shows that CO oxidation over terraces can proceed at lower temperature than over step sites. Hence, CO oxidation over stepped surfaces can proceed in two different stages. The first one occurs at relatively low temperature on terrace sites, and the second one on step sites at higher temperature. The possible origin of the controversy in the literature concerning surface structure sensitivity or insensitivity in the CO oxidation over metals is briefly discussed.
INTRODUCTION
Supported Pt-Rh catalysts are used as active components in the automotive three-way catalyst for controlling pollution from combustion products such as nitric oxide (NO), carbon monoxide (CO) and hydrocarbonsl. The reaction mechanism and kinetics of the CO-02 reaction over noble metals have been extensive y studied in the past [2-111. The formation of C 0 2 is usualTABLE 1 ly thought to proceed through a Langmuir-Hinshelwood mechanism between adsorbed CO and adsorbed oxygen atoms, as confirmed by molecular beam experiments [ 101.The reaction steps are summarised in table I . In this study we present results or the CO-02 reaction over Pt-Rh(ll1 Pt-Rh( loo), Pt-Rh(410) and Pt-Rh(210) under low pressure conditions. It will 9
396
be shown that there exists large differences in the behaviour of the four surfaces studied. At low temperature the inhibition of the steady state rate of CO2formation by CO is very high on the (210) surface. This effect is less pronounced for the two flat surfaces (1 11) and (loo), while the (410) surface shows an intermediate behaviour. The C 0 2 production found for the four surfaces differs also at high temperatures where CO inhibition does not take place. Under these conditions the (111) surface shows only low activity, probably caused by a low sticking probability of oxygen. EXPERIMENTAL
All experiments were carried out in a standard UHV system with a base pressure better than 2x10-10 mbar. It was equipped for quadrupole mass spectrometry (QMS), Auger electron spectroscopy (AES) and low energy electron diffraction (LEED). The single crystal was oriented by means of the Laue back reflection method to within 0.50 of the desired direction. The samples were cut by spark erosion and mechanically polished down to 0.25 pm grain size using standard techniques. The samples were cleaned by cycles of Ar-ion bombardment, oxidising treatments and flashing to high temperature (1400K) in vacuum. The crystals were heated by passing current through tantalum leads spotwelded to the edges of the crystals. The temperature of the crystals was measured by means of a Pt/Pto. 10-Rho.90 thermocouple spotwelded to the edge of the crystal. The system is pumped by an ion pump, a turbo-molecular pump and a titanium sublimation pump. During reactions the valve separating the ion pump from the main chamber was closed and the system was only pumped by the turbo-molecular pump. In this way, reaction rates were measured by using the main chamber of the UHV system as a continuous flow reactor. Gases were dosed to the main chamber through variable valves. It was assured that the background pressure contributed to less than 1% of the total reactant pressure. RESULTS
A
-
Surface composition
In the case of an alloy sample knowledge of the surface composition is required to understand the obtained kinetic results. The surface composition of Pt-Rh alloys has been studied extensively in the past few years [8,12-171. In general, a large surface Pt enrichment is found. In table 2 the platinum surface concentrations for the four surfaces are given as observed by AES after annealing the samples at 1300K in vacuum. The AES measurements on (210), (100) and (111) were done by directly measuring the signal intensities of the Rh222, Rh256, Rh302, Pt64 and Pt168 eV Auger transitions. In the evaluation of
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the data the Gallon model was applied [18]. Calibration with pure Pt and Rh samples is needed to determine quantitatively the surface composition by means of this model. Furthermore, the assumption is made that the second layer has the same Pt concentration as the bulk. Tsong et a1 1141 found that the second layer is enriched in Rh atoms. However, the deviation from the bulk concentration is small and our assum tion will not lead to large errors. In the case of Pt-Rh(21O) it was Table 2. also assumed that the backscattering Composition of various factors and escape depths of the surfaces of a Pt0.25-Rh0.75 generated Auger electrons are the same as for the (100) surface. The AES alloy single crystal (at 9%) following annealing at 1300K. determination of the surface composition of Pt-Rh(410) has been done by Surface Pt concentration(%) oxygen titration, a method described by van Delft et a1 [ 121. Information conPt-Rh( 11 1) cerning the distribution of Pt and Rh 32f5 Pt-Rh( 100) atoms along the steps and terraces is, 40 f 5 P t - R h ( 2 10) unfortunately, not available.The open 55 k 5 Pt-Rh(4 10) (210) surface shows the largest Pt 4 0 k 10 enrichment, implying that on stepped I surfaces the Pt segregation may be irger at steps than on the terraces. Van Delft et a1 [17] that oxygen induces Rh surface segregation in the temperature range from 600 to 1000K. All experiments described in the rest of this paper were done after a standard annealing procedure in order to produce a well known surface concentration. However, it can not be excluded that the surface concentration changes during the course of the reaction, especially under oxidizing conditions at temperatures higher than 600K.
B - Temperature dependence of the reaction rate under steady state conditions. The formation of C02 was followed by monitoring the intensity of the amu=44 mass signal. It was assured that the reaction rate reached its steadystate value. In figures I to 4 the results are shown for the CO-02 reaction carried out over Pt-Rh(l1 l), Pt-Rh( loo), Pt-Rh(410) and Pt-Rh(210), respectively. Under stoichiometric reaction conditions the total reactant pressure was 3x 10-7 Torr. Partial pressures were also varied from oxidizing to reducing conditions in order to study the order of the reaction in CO or 0 2 at different temperatures. The first observation to notice is that there exists striking differences in the rate of COT formation as a function of increasing temperature for the four single crystal surfaces. For all the surfaces studied the reaction rate increases with increasing temperature until a maximum rate is
398
0
m
I-
Y
-t -
-0
0
.O b
0
m 0
0
x
m
I-
-
t Y
-0 0
0
.O PI
0 I n 0
0
0
399
observed at a temperature Tm. This value of Tm is one parameter which strongly depends on the surface structure. For Pt-Rh(ll1) the value of Tm is only 490K, the lowest Tm measured in this study. Pt-Rh(210) and Pt-Rh(410) have Tm 's of 615K and 650K, respectively. The Pt-Rh(100) shows a different behaviour, for this surface two local maxima are observed in the rate of C02 formation. The first maximum lies around 500K and the second one around 580K. A similar behaviour has also been observed for pure Rh(100) [7]. The positions of the local maxima in the case of pure Rh(100) are almost similar to those on Pt-Rh( 100). The relative contribution of the low temperature maximum is larger for Pt-Rh(100) than for Rh(100). In addition to the variation of the value of Tm for the four surfaces, also the increase in the rate of C 0 2 formation with increasing temperature is different in the low temperature range. These differences are reflected by the values of the activation energy of the process. At low temperature (<480K) the reaction rate increases with an apparent activation energy Ea of 65 k 5 kJ/mol for the Pt-Rh( 111) surface. The apparent activation energy was calculated from the rates measured in the temperature traject of 360-49OK. When a slightly smaller temperature traject is choosen, i.e. 360 to 465K a somewhat higher Ea is found, namely 85 f 5 kJ/mol. The latter value is close to the value of 84 k 8 kJ/mol reported for the CO-02 reaction over Rh(ll1) [7].
C
-
Dependence on partial pressures.
Pt-Rh(ll1) At distinct temperatures the partial pressure of one reactant is varied over a wide range while the other is kept constant. In this way, the effect of excess CO or oxygen is easily studied, i.e. information is obtained concerning the order of the reaction in the two reactants. In figures 5a and 5b the results for Pt-Rh(ll1) are shown. In these figures the logarithm of the C02 yield at a certain temperature subtracted by the logarithm of the C 0 2 yield at stoichiometric conditions at the same temperature is plotted versus the logarithm of the partial pressure of the reactant which is in excess present in the reactor. The reaction order in CO depends strongly on the reaction temperature. The reaction order in CO at 415K is -0.20, whereas the order switches to +0.17 at 528K. At 503K the order in CO pressure is only slightly positive, i.e. +0.04, The order in oxygen is positive for the three temperatures studied ranging from +0.85 to +1.00. It is interesting to compare these results with those reported for Rh(ll1) [7]. At low temperature (415K) the order in oxygen is positive and negative in CO, suggesting that CO saturates the Rh(ll1) surface below 500K. However, there is a striking difference between Pt-Rh(ll1) and Rh(ll1) at 528K. At this temperature the order in 0 2 is positive, as described above, however the order is negative in 0 2 in the case of Rh(ll1) [7]. Thus, it seems that at this temperature the oxygen coverage on Pt-
400
Rh( 111) must be considerably smaller than on Rh( 111). The negative order in 0 2 reported for Rh(ll1) was ascribed to oxidation of the Rh surface. Apparently, the oxidation of the alloy surface is lower under the influence of Pt atoms distributed in the surface.
Fig 5a Pt-Rh(ll1) 0.5
I
Fiaure 5
1t ,
-0.5-
41 5K I
-16
I
I
I
I
I
-1 5 In(P(C0)
Fig 5b Pt-Rh(l11)
I
I
I
I
,:J
-14 4
I
The logarithm of the C02 formation in excess of CO minus the logarithm of the C O 2 formation under stoichiometric conditons is plotted versus the logarithm of the CO (5a) pressure ( T o r r ) and 0 2 ( S b ) pressure (Torr) f o r PtRh(1 I I ) . The CO pressure is varied from 1x10-7 to I X I 0 - 6 Torr, the 0 2 pressure is varied from 2x10-7 to 1x10-6 Torr.
Pt-Rh(100) The experiments were carried out in the same way as described above for Pt-Rh(ll1). The results for Pt-Rh(100) are shown in f i g . 6. The CO inhibition on Pt-Rh(100) is similar to the CO inhibition on PtRh(ll1). At low temperature (415K)the order is -0.12 in CO, changing to
40 1
0.14 at 578K. At 503K the order in CO is +0.10. The order in oxygen is +0.33, +0.413 and +0.16 for 415K, 503K and 578K, respectively.Hence, both for the Pt-Rh( 111) and (100) surfaces no negative order in 0 2 is observed. The effect of oxygen is less pronounced as compared with Pt-Rh( 111). Even at the position of the second local maximum a slightly positive order in 0 2 is observed. Fig 6a Pt-R h(100) -
I
578K
.... ....
-16
-2
.-...
-
-15 In(P(C0))
--w.
The logarithm of the C02 formation in excess of CO minus the logarithm of the C O 2 formation under stoichiometric conditons is plotted versus the logarithm of the CO (6a) pressure (Torr) and 0 2 (6b)pressure (Torr) for Pt-Rh(l00). The CO pressure is varied from 1 x 1 0 7 to 1x10-6 Torr, the 0 2 pressure is varied from 2x10-7 to 1x10-6 Torr.
-14
Fig 6b Pt-Rh(lO0)
528K
lrl(P(02))
-
+
The results for the Pt-Rh(210) surface are shown infig.7. Pt-Rh(210) CO inhibition at low temperature could not be measured because of the low steady-state rate of C02 formation. At 533K and 553K the order in CO was found to be positive, 0.11 and 0.15, respectively. The order in oxygen is positive, 0.43 and 0.45, respectively, just as was the case with Pt-Rh(100).
402 Fig.7a Pt-Rh(210) 0.5 1
I
FiPure 7
-16
-15 In(P(C0))
-14 -----C
Fig. 7 b Pt-Rh(210) 1.5
-
/
The logarithm of the C 0 2 formation in excess of CO minus the logarithm of the C 0 2 formation under stoichiometric conditons is plotted versus the logarithm of the CO (7a) pressure (Torr) and 0 2 (7b)pressure (Torr) for Pt-Rh(210). The CO pressure is varied from 1 x 1 0 - 7 to 1x10-6 Torr, the 0 2 pressure is varied from 2x10-7 to 1x10-6 Torr.
1 -
0.5
0
,-/
-15.5
__.I-
__-.I.-
533K
_.I-
-15 -14.5 ln(P(02)) A
-14
The results for the (410) surface have been reported elsewhere17. In this study the surface concentrations of CO and 0 2 were also measured by AES. It was found that at low temperatures (T<580K) the surface is saturated with CO. Above this temperature the oxygen concentration increases and the CO concentration decreases. Around 600K the product of CO and 0 coverages is at maximum.
Pt-Rh(410)
403 DISCUSSION
The CO-02 reaction has been studied under varying conditions over four Pt-Rh alloy single crystal surfaces, i.e. ( l l l ) , (loo), (210) and (410). The rate of C02 formation strongly depends on the surface studied. The surface composition of the surfaces was also determined to gain insight into the effect of alloying. All the Pt-Rh surfaces show a Pt-enrichment relative to the bulk concentration. It is known that the CO oxidation on metals like Pd and Pt proceeds at the boundary of CO islands and 0 islands [3]. One possible beneficial effect of alloy formation might be that one of the reactant molecules has a strong preference for adsorption on one of the alloy components while the other molecule is preferentially adsorbed on the other. For example, 0 is bound with a higher heat of adsorption on Rh than on Pt. Hence, it is likely that oxygen will be adsorbed on the Rh sites of a Pt-Rh alloy surface. This effect might result in an easier mixing of CO and 0 on the alloy surface and hence, in a faster reaction at lower temperature. No indication of such an effect was found in the present study, as will be discussed later on. Large differences were found in the steady-state rates of C02 formation for the four surfaces studied. These differences are found in the CO inhibition regime at low temperature, as well as in the regime at higher temperature where the orders in both CO and 02pressures are positive. The different behaviour of the four Pt-Rh alloy surfaces is well reflected in the values of Tm where the reaction rate has reached its maximum value: 490K for Pt-Rh( 1 1l), 615K for (210), 650K for (410) and 500K-580K for (100). At the temperature T m the concentrations of 0 and CO have reached their optimum concentrations for the reaction. A comparison between Pt-Rh(lOO), Pt-Rh(21O) and Pt-Rh(410) is very interesting because both (210) and (410) consist of (100) terraces, with similar structure of the step sites. The (210) surface has only 2 atoms wide terraces, whereas (410) consists of 4 atom wide terraces. In this way, by comparing the three surfaces not only information is obtained concerning the effect of the introduction of a surface defect, or step site, but also on the effect of the step concentration. At low temperature, around 500K, the reaction is faster on (100) than over (410) and it is slowest over (210). Hence, increasing the concentration of steps results in a reduced CO oxidation at low temperature. It has been reported that step sites can have an activation energy of desorption of CO enhanced by 29 kJ/mol relative to terrace sites [19]. McCabe and Schmidt [28] studied the CO desorption from Pt(l1 l), Pt( loo), Pt(1 lo), Pt(211) and Pt(210). It was found that activation energies for desorption of the most tightly bound states varies from 145kJ/mol on Pt(210) and (211) to 105kJ/mol on Pt(ll0) and Pt(ll1). In the CO inhibition regime it is therefore likely that CO
404
oxidation on terraces proceeds easier over terraces than on step sites. It is not very likely that the differences are caused by differences in noble metal atom concentration at the surface. The step atoms on (210) and (410) have the same coordination, it is therefore expected that the concentration of Pt and Rh along the steps is the same, and therefore the difference between the two surfaces must be due to the difference in concentration of steps. The Pt-Rh(l00) shows two local maxima in the C02 steady-state rate of formation. This was also observed for Rh(l00). The reason for the occurence of two maxima is not yet clear. It is planned to follow the concentration of the adsorbates CO and 0 during the reaction. Hopefully, these experiments will shed some light upon this matter. A comparison of the behaviour of Rh(100) with that of Pt-Rh(100) shows that the relative contributions of the first and second oxidation stage is different. The contribution of the high temperature maximum for Rh(100) is larger. The value of Tm in the case of Pt-Rh(ll1) is 490K, the lowest for the four surfaces studied. This can be understood in terms of a combination of a low adsorption energy of CO and a low sticking probability of oxygen. The low adsorption energy of CO will lead to desorption of CO at relative low temperature, thereby creating room for oxygen to become adsorbed. However, the oxygen concentration remains as a result of the low sticking probabilty of oxygen on the flat (1 11) surface. CO oxidation has mostly been studied on the densely packed (111) surfaces, and many similarities have been found for Pt( 111) and Rh( 111). A major difference between Pt( 111) and Rh(ll1) is that oxygen desorbs from Pt(ll1) in TDS below 900K [32,33], while it remains on Rh until 1300K [34]. Another major difference is the value of the sticking probability of oxygen, which is very low in the case of Pt(ll1) and Pt(l00) [32]. Based on these observations a positive or zero dependence in oxygen pressure is expected for Pt(l1 l ) , while for Rh(ll1) a negative one-half order kinetics in 0 2 is observed. The Pt-Rh( 111) surface shows a positive order in oxygen pressure at high temperatures (>500K). This observation may suggest that oxidation of CO under these conditions proceeds largely on Pt-sites. An alternative explanation may be that Rh sites are more difficult to oxidise in the presence of Pt atoms. Above the rate maximum the C02 production decreases giving rise a negative apparent activation energy. The value of this apparent activation energy is -9 kJ/mol for Pt-Rh(ll1). In the case of Rh(ll1) a value of -28 kJ/mol was reported. This difference illustrates the fact that the C02 production over the Pt-Rh alloy decreases less steeply with increasing temperature than on the Rh( 111) surface. This may be caused by an enhanced oxidation of Rh( 111) compared with Pt-Rh(ll1). The order in CO is negative at low temperature, switching to positive at higher temperature. At low temperature dense CO islands prevent oxygen adsorption, thereby inhibiting the reaction. At a temperature where CO
405
desorption starts sites are created on which oxygen can adsorb and subsequently react with adsorbed CO to form C02, which is released into the gas phase. The CO inhibition on the rough Pt-Rh(210) lasts up to relatively high temperature, i.e. 500K,at this temperature the CO oxidation rate over PtRh(ll1) has already reached its maximum. CO adsorbs in several binding states on Pt(210) [20], i.e. strongly bound and more weakly held states. Steadystate C02 production starts probably after considerable desorption of the tightly bound CO. This was also observed in the case of the CO oxidation over Ir surfaces [21]. The CO oxidation is usually considered as a typical example of a reaction that is insensitive to the surface structure [22]. For example, Oh et a1 reported that the specific reaction rate is the same for Rh(lll), Rh(100)and for high surface area supported Rh under high pressure conditions [4]. However, Fisher et a1 171 reported that at a total pressure of 2x10-7 TOITthe reaction proceeds much faster on Rh( 100) than on Rh( 111) at a temperature of 600K. FEM observations by Gorodetskii et a1 [30] also point to a large effect of the surface structure on the CO oxidation over Rh under low pressure conditions. Recently, Yates Jr. showed by ESDIAD that CO adsorbed on terraces of Pt(211) reacts at a lower temperature than CO adsorbed on step sites [19]. Rate oscillations in the C02 formation on Pt(100) are caused by a reconstruction of the surface from a (1x1) type of surface to a (5x20) type of surface. When the surface switches between the two different types of surfaces oscillations arise [23,31]. This model relies completely on the fact that the CO0 2 reaction on Pt is structure sensitive under the given experimental conditions. Structure sensitivity of the reaction is also reported for Ru [24,25] and Ir [21] surfaces at low pressures. However, Boudart [22] reported that the rate is independent of the particle size of Pd at high pressure. Moreover, the specific reaction rate was similar for Pd( 11l), (100) and (1 10) under low pressure conditions [3,26]. More recently, Kruse et a1 [27] found some evidence for a different behaviour of small Pd particles for the CO oxidation. On Ir(ll1) and (110) it was found that substantial desorption of the most tightly bound state of CO is required in order to achieve maximum activity in the steady state reaction of CO and oxygen to form C02 [21]. Furthermore, it was found that the maximum rate of C02 production occurs at lower temperature for Ir(ll1) than for Ir(ll0) [21]. Therefore, under various conditions structure sensitivity or insensitivity has been reported for several metals. At low temperature the CO-02 reaction is hampered by a large amount of adsorbed CO. Therefore, at low temperature the rate determining step is usually considered to be desorption of CO into the gas phase. After the desorption of a part of the CO, oxygen adsorption can take place. The last step, the reaction between CO and oxygen occurs relatively fast. Any structure sensitivity at low temperature must therefore be related to a difference in
406
adsorption energy of CO for different type of surface structures. For Pt it is reported that step sites show a desorption energy which is enhanced by 29 kJ/mol relative to the (111) flat surfaces [19]. However, the strong repulsion between adsorbed CO molecules could erase the structural heterogeneity, as has been suggested by Boudart in the case of Pd [22]. CO-CO repulsion on Pt(ll1) can only account for a 12-16 kJ/mol [28] decrease in adsorption energy of CO, if it assumed that the CO coverage does not exceed 0.5. Given the mild experimental conditions this assumption is justified. Thus an influence of step sites could still be expected, and has been observed in the present study. In conclusion, the CO oxidation by 0 2 is a surface sensitive reaction. However, under certain conditions the reaction could appear as being structure insensitive. Boudart suggested that lateral interactions between adsorbates erase the effect of different types of adsorption states, leading eventualy to structure insensitivity. A reactive mixture of CO and 0 2 could also modify the catalyst particle shape in such a way that one type of surface structure dominates. Another explanation could be that only the activity of one type of site is measured under experimental conditions. Recent results of Yates Jr [19]. et a1 are consistent with this model. They found that the CO + 0 2 reaction takes place at lower temperature on the terraces than on the steps of the Pt(211) surface using temperature programmed reaction conditions. However, at higher temperature the high diffusion rate of 0 adatoms over the surface obscures the differences of step and terrace sites.
References 1. 2. 3. 4.
5. 6. 7. 8. 9. 10. 11. 12. 13.
14.
15.
16. 17. 18. 19.
K.C. Taylor, Automotive Catalytic Converters (Springer, Berlin, 1984) I. Langmuir, Trans. Faraday SOC.17 (1922), 672 T. Engel and G. Ertl, Advances in Catalysis, 28 (1979), 1-78 S.H. Oh, G.B. Fisher, J.E. Carpenter and D.W. Goodman, J. of Cat. 100 (1986), 360-376 C.T. Campbell and J.M.J. White, J. Catal. 54 (1978), 289 W.M. Daniel and J.M. White, Int. J. of Chem. Kin., Vol 17 (1985), 413-417 S.B. Schwartz, L.D. Schmidt and G.B. Fisher, J. Phys. Chem. 90 (1986), 6194-6200 R.M. Wolf, J. Siera, F.C.M.J.M. van Delft and B.E. Nieuwenhuys, Faraday Discussions of the Chemical Society 87 (1989), 275 A.G. van de Bosch-Driebergen, M.N.H. Kieboom, A. van Dreumel, F.C.M.J.M. van Delft, R.M. Wolf and B.E. Nieuwenhuys, Cat. Let. 2 (1989), 73 T. Engel and G.Ertl, J. Chem. Phys., 69 (1978), 1267 B.E. Nieuwenhuys, Surf. Sci. 126 (1983), 307 F.C.M.J.M. van Delft and B.E. Nieuwenhuys, Surf. Sci. 162 (1985), 538 F.C.M.J.M. van Delft. A.D. van Langeveld and B.E. Nieuwenhuys Surf. Sci.189/190 (1987L 1129 T.T. Tsong, Surf. Sci. Rep. 8 (1988) 127 R.M. Wolf, M.J. Dees and B.E. Nieuwenhuys, J. Phys. (Pans) (1986), c7-419-/11 F.L. Williams and G.C. Nelson. ADD].Surf. Sci., 3 (1979). 409 F.C.M.J.M. van Delft, thesis Leidki, (1988) T.E. Gallon, Surf. Sci. 17 (1969), 486 J.T. Yates Jr., Private Communication v
407 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30.
31. 32.
33. 34.
M. Ehsasi, J.H. Block, K. Christman and W. Hirschwald, J. Vac. Sci. Technol. A5(4), Jul/Aug (1987),821 W.H. Weinberg, W.F. Egelhoff Jr., V.P. Ivanov, V.L. Tataurov and G.K. Boreskov, R o c 7th Intern. Vac. Congr. and 3rd Intern. Conf. Solid Surfaces (Vienna 1977), 1151 M. Boudart and F. Rumpf, React. Kinet. Catal. Lett. Vol. 35,Nos 1-2, (1987)95-105 S.B. Schwartz and L.D. Schmidt, Surf. Sci. 206 (1988), 169-186 T.E. Madey, H.A. Engelhardt and D. Menzel, Surf. Sci. 48 (1975),304 P.D. Reed, C.M. Comrie and R.M. Lambert, Surf. Sci. 64 (1977),603 J. Koch, Thesis, Technische Universitat, Hannover (1972) V. Matolin, E. Gillet and N. Kruse, Surf. Sci. 186 (1987),L541 G. Ertl, M. Neumann and K.M. Streit, Surf. Sci. 64(1977),393 R.W. McCabe and Schmidt, Surf. Sci. 66 (1977),101-124 V.V. Gorodetskii, B.E.Nieuwenhuys, W.M.H. Sachtler and G.K. Boreskov, Appl. of Surf. Sci. 7 (1981),355-371 R.Imbihl, M.P. Cox and G. Ertl, J. Chem. Phys. 84 (1986),3519 J.L. Gland, B.A. Sexton and G.B. Fisher, Surf. Sci. 95 (1980),587 L. Gland, Surf. Sci. 93 (1980),487 T. Matsushima, J. Cat. 85 (1984),98-104
395
A COMPARATIVE KINETIC STUDY OF THE C O - 0 2 REACTION OVER Pt-Rh (lll), (loo), (410) AND (210) SINGLE CRYSTAL SURFACES. J. Siera, R. van Silfhout, F. Rutten and B.E. Nieuwenhuys Gorlaeus labaratories, Leiden University, P.O. BOX 9502, The Netherlands ABSTRACT The CO-02 reaction has been studied as a function of temperature and partial pressure of CO and 0 2 over the (1 1l), (loo), (210) and (410) surfaces of a Pt025-Rho.75single crystal. The effects of alloying and surface structure are discussed. It is found that the reaction rates on the alloy surfaces show a positive order in oxygen at high temperature (%OK), whereas under these conditions the reported order in oxygen is negative for Rh(ll1). Furthermore, considerable differences are found in the steady-state rate of C02 formation for the four surfaces. On Pt-Rh(210)the CO oxidation starts around 500K, while over Pt-Rh(ll1) the rate has already reached its maximum value at this temperature. Comparison of the four surfaces shows that CO oxidation over terraces can proceed at lower temperature than over step sites. Hence, CO oxidation over stepped surfaces can proceed in two different stages. The first one occurs at relatively low temperature on terrace sites, and the second one on step sites at higher temperature. The possible origin of the controversy in the literature concerning surface structure sensitivity or insensitivity in the CO oxidation over metals is briefly discussed.
INTRODUCTION
Supported Pt-Rh catalysts are used as active components in the automotive three-way catalyst for controlling pollution from combustion products such as nitric oxide (NO), carbon monoxide (CO) and hydrocarbonsl. The reaction mechanism and kinetics of the CO-02 reaction over noble metals have been extensive y studied in the past [2-111. The formation of C 0 2 is usualTABLE 1 ly thought to proceed through a Langmuir-Hinshelwood mechanism between adsorbed CO and adsorbed oxygen atoms, as confirmed by molecular beam experiments [ 101.The reaction steps are summarised in table I . In this study we present results or the CO-02 reaction over Pt-Rh(ll1 Pt-Rh( loo), Pt-Rh(410) and Pt-Rh(210) under low pressure conditions. It will 9
396
be shown that there exists large differences in the behaviour of the four surfaces studied. At low temperature the inhibition of the steady state rate of CO2formation by CO is very high on the (210) surface. This effect is less pronounced for the two flat surfaces (1 11) and (loo), while the (410) surface shows an intermediate behaviour. The C 0 2 production found for the four surfaces differs also at high temperatures where CO inhibition does not take place. Under these conditions the (111) surface shows only low activity, probably caused by a low sticking probability of oxygen. EXPERIMENTAL
All experiments were carried out in a standard UHV system with a base pressure better than 2x10-10 mbar. It was equipped for quadrupole mass spectrometry (QMS), Auger electron spectroscopy (AES) and low energy electron diffraction (LEED). The single crystal was oriented by means of the Laue back reflection method to within 0.50 of the desired direction. The samples were cut by spark erosion and mechanically polished down to 0.25 pm grain size using standard techniques. The samples were cleaned by cycles of Ar-ion bombardment, oxidising treatments and flashing to high temperature (1400K) in vacuum. The crystals were heated by passing current through tantalum leads spotwelded to the edges of the crystals. The temperature of the crystals was measured by means of a Pt/Pto. 10-Rho.90 thermocouple spotwelded to the edge of the crystal. The system is pumped by an ion pump, a turbo-molecular pump and a titanium sublimation pump. During reactions the valve separating the ion pump from the main chamber was closed and the system was only pumped by the turbo-molecular pump. In this way, reaction rates were measured by using the main chamber of the UHV system as a continuous flow reactor. Gases were dosed to the main chamber through variable valves. It was assured that the background pressure contributed to less than 1% of the total reactant pressure. RESULTS
A
-
Surface composition
In the case of an alloy sample knowledge of the surface composition is required to understand the obtained kinetic results. The surface composition of Pt-Rh alloys has been studied extensively in the past few years [8,12-171. In general, a large surface Pt enrichment is found. In table 2 the platinum surface concentrations for the four surfaces are given as observed by AES after annealing the samples at 1300K in vacuum. The AES measurements on (210), (100) and (111) were done by directly measuring the signal intensities of the Rh222, Rh256, Rh302, Pt64 and Pt168 eV Auger transitions. In the evaluation of
397
the data the Gallon model was applied [18]. Calibration with pure Pt and Rh samples is needed to determine quantitatively the surface composition by means of this model. Furthermore, the assumption is made that the second layer has the same Pt concentration as the bulk. Tsong et a1 1141 found that the second layer is enriched in Rh atoms. However, the deviation from the bulk concentration is small and our assum tion will not lead to large errors. In the case of Pt-Rh(21O) it was Table 2. also assumed that the backscattering Composition of various factors and escape depths of the surfaces of a Pt0.25-Rh0.75 generated Auger electrons are the same as for the (100) surface. The AES alloy single crystal (at 9%) following annealing at 1300K. determination of the surface composition of Pt-Rh(410) has been done by Surface Pt concentration(%) oxygen titration, a method described by van Delft et a1 [ 121. Information conPt-Rh( 11 1) cerning the distribution of Pt and Rh 32f5 Pt-Rh( 100) atoms along the steps and terraces is, 40 f 5 P t - R h ( 2 10) unfortunately, not available.The open 55 k 5 Pt-Rh(4 10) (210) surface shows the largest Pt 4 0 k 10 enrichment, implying that on stepped I surfaces the Pt segregation may be irger at steps than on the terraces. Van Delft et a1 [17] that oxygen induces Rh surface segregation in the temperature range from 600 to 1000K. All experiments described in the rest of this paper were done after a standard annealing procedure in order to produce a well known surface concentration. However, it can not be excluded that the surface concentration changes during the course of the reaction, especially under oxidizing conditions at temperatures higher than 600K.
B - Temperature dependence of the reaction rate under steady state conditions. The formation of C02 was followed by monitoring the intensity of the amu=44 mass signal. It was assured that the reaction rate reached its steadystate value. In figures I to 4 the results are shown for the CO-02 reaction carried out over Pt-Rh(l1 l), Pt-Rh( loo), Pt-Rh(410) and Pt-Rh(210), respectively. Under stoichiometric reaction conditions the total reactant pressure was 3x 10-7 Torr. Partial pressures were also varied from oxidizing to reducing conditions in order to study the order of the reaction in CO or 0 2 at different temperatures. The first observation to notice is that there exists striking differences in the rate of COT formation as a function of increasing temperature for the four single crystal surfaces. For all the surfaces studied the reaction rate increases with increasing temperature until a maximum rate is
398
0
m
I-
Y
-t -
-0
0
.O b
0
m 0
0
x
m
I-
-
t Y
-0 0
0
.O PI
0 I n 0
0
0
399
observed at a temperature Tm. This value of Tm is one parameter which strongly depends on the surface structure. For Pt-Rh(ll1) the value of Tm is only 490K, the lowest Tm measured in this study. Pt-Rh(210) and Pt-Rh(410) have Tm 's of 615K and 650K, respectively. The Pt-Rh(100) shows a different behaviour, for this surface two local maxima are observed in the rate of C02 formation. The first maximum lies around 500K and the second one around 580K. A similar behaviour has also been observed for pure Rh(100) [7]. The positions of the local maxima in the case of pure Rh(100) are almost similar to those on Pt-Rh( 100). The relative contribution of the low temperature maximum is larger for Pt-Rh(100) than for Rh(100). In addition to the variation of the value of Tm for the four surfaces, also the increase in the rate of C 0 2 formation with increasing temperature is different in the low temperature range. These differences are reflected by the values of the activation energy of the process. At low temperature (<480K) the reaction rate increases with an apparent activation energy Ea of 65 k 5 kJ/mol for the Pt-Rh( 111) surface. The apparent activation energy was calculated from the rates measured in the temperature traject of 360-49OK. When a slightly smaller temperature traject is choosen, i.e. 360 to 465K a somewhat higher Ea is found, namely 85 f 5 kJ/mol. The latter value is close to the value of 84 k 8 kJ/mol reported for the CO-02 reaction over Rh(ll1) [7].
C
-
Dependence on partial pressures.
Pt-Rh(ll1) At distinct temperatures the partial pressure of one reactant is varied over a wide range while the other is kept constant. In this way, the effect of excess CO or oxygen is easily studied, i.e. information is obtained concerning the order of the reaction in the two reactants. In figures 5a and 5b the results for Pt-Rh(ll1) are shown. In these figures the logarithm of the C02 yield at a certain temperature subtracted by the logarithm of the C 0 2 yield at stoichiometric conditions at the same temperature is plotted versus the logarithm of the partial pressure of the reactant which is in excess present in the reactor. The reaction order in CO depends strongly on the reaction temperature. The reaction order in CO at 415K is -0.20, whereas the order switches to +0.17 at 528K. At 503K the order in CO pressure is only slightly positive, i.e. +0.04, The order in oxygen is positive for the three temperatures studied ranging from +0.85 to +1.00. It is interesting to compare these results with those reported for Rh(ll1) [7]. At low temperature (415K) the order in oxygen is positive and negative in CO, suggesting that CO saturates the Rh(ll1) surface below 500K. However, there is a striking difference between Pt-Rh(ll1) and Rh(ll1) at 528K. At this temperature the order in 0 2 is positive, as described above, however the order is negative in 0 2 in the case of Rh(ll1) [7]. Thus, it seems that at this temperature the oxygen coverage on Pt-
400
Rh( 111) must be considerably smaller than on Rh( 111). The negative order in 0 2 reported for Rh(ll1) was ascribed to oxidation of the Rh surface. Apparently, the oxidation of the alloy surface is lower under the influence of Pt atoms distributed in the surface.
Fig 5a Pt-Rh(ll1) 0.5
I
Fiaure 5
1t ,
-0.5-
41 5K I
-16
I
I
I
I
I
-1 5 In(P(C0)
Fig 5b Pt-Rh(l11)
I
I
I
I
,:J
-14 4
I
The logarithm of the C02 formation in excess of CO minus the logarithm of the C O 2 formation under stoichiometric conditons is plotted versus the logarithm of the CO (5a) pressure ( T o r r ) and 0 2 ( S b ) pressure (Torr) f o r PtRh(1 I I ) . The CO pressure is varied from 1x10-7 to I X I 0 - 6 Torr, the 0 2 pressure is varied from 2x10-7 to 1x10-6 Torr.
Pt-Rh(100) The experiments were carried out in the same way as described above for Pt-Rh(ll1). The results for Pt-Rh(100) are shown in f i g . 6. The CO inhibition on Pt-Rh(100) is similar to the CO inhibition on PtRh(ll1). At low temperature (415K)the order is -0.12 in CO, changing to
40 1
0.14 at 578K. At 503K the order in CO is +0.10. The order in oxygen is +0.33, +0.413 and +0.16 for 415K, 503K and 578K, respectively.Hence, both for the Pt-Rh( 111) and (100) surfaces no negative order in 0 2 is observed. The effect of oxygen is less pronounced as compared with Pt-Rh( 111). Even at the position of the second local maximum a slightly positive order in 0 2 is observed. Fig 6a Pt-R h(100) -
I
578K
.... ....
-16
-2
.-...
-
-15 In(P(C0))
--w.
The logarithm of the C02 formation in excess of CO minus the logarithm of the C O 2 formation under stoichiometric conditons is plotted versus the logarithm of the CO (6a) pressure (Torr) and 0 2 (6b)pressure (Torr) for Pt-Rh(l00). The CO pressure is varied from 1 x 1 0 7 to 1x10-6 Torr, the 0 2 pressure is varied from 2x10-7 to 1x10-6 Torr.
-14
Fig 6b Pt-Rh(lO0)
528K
lrl(P(02))
-
+
The results for the Pt-Rh(210) surface are shown infig.7. Pt-Rh(210) CO inhibition at low temperature could not be measured because of the low steady-state rate of C02 formation. At 533K and 553K the order in CO was found to be positive, 0.11 and 0.15, respectively. The order in oxygen is positive, 0.43 and 0.45, respectively, just as was the case with Pt-Rh(100).
402 Fig.7a Pt-Rh(210) 0.5 1
I
FiPure 7
-16
-15 In(P(C0))
-14 -----C
Fig. 7 b Pt-Rh(210) 1.5
-
/
The logarithm of the C 0 2 formation in excess of CO minus the logarithm of the C 0 2 formation under stoichiometric conditons is plotted versus the logarithm of the CO (7a) pressure (Torr) and 0 2 (7b)pressure (Torr) for Pt-Rh(210). The CO pressure is varied from 1 x 1 0 - 7 to 1x10-6 Torr, the 0 2 pressure is varied from 2x10-7 to 1x10-6 Torr.
1 -
0.5
0
,-/
-15.5
__.I-
__-.I.-
533K
_.I-
-15 -14.5 ln(P(02)) A
-14
The results for the (410) surface have been reported elsewhere17. In this study the surface concentrations of CO and 0 2 were also measured by AES. It was found that at low temperatures (T<580K) the surface is saturated with CO. Above this temperature the oxygen concentration increases and the CO concentration decreases. Around 600K the product of CO and 0 coverages is at maximum.
Pt-Rh(410)
403 DISCUSSION
The CO-02 reaction has been studied under varying conditions over four Pt-Rh alloy single crystal surfaces, i.e. ( l l l ) , (loo), (210) and (410). The rate of C02 formation strongly depends on the surface studied. The surface composition of the surfaces was also determined to gain insight into the effect of alloying. All the Pt-Rh surfaces show a Pt-enrichment relative to the bulk concentration. It is known that the CO oxidation on metals like Pd and Pt proceeds at the boundary of CO islands and 0 islands [3]. One possible beneficial effect of alloy formation might be that one of the reactant molecules has a strong preference for adsorption on one of the alloy components while the other molecule is preferentially adsorbed on the other. For example, 0 is bound with a higher heat of adsorption on Rh than on Pt. Hence, it is likely that oxygen will be adsorbed on the Rh sites of a Pt-Rh alloy surface. This effect might result in an easier mixing of CO and 0 on the alloy surface and hence, in a faster reaction at lower temperature. No indication of such an effect was found in the present study, as will be discussed later on. Large differences were found in the steady-state rates of C02 formation for the four surfaces studied. These differences are found in the CO inhibition regime at low temperature, as well as in the regime at higher temperature where the orders in both CO and 02pressures are positive. The different behaviour of the four Pt-Rh alloy surfaces is well reflected in the values of Tm where the reaction rate has reached its maximum value: 490K for Pt-Rh( 1 1l), 615K for (210), 650K for (410) and 500K-580K for (100). At the temperature T m the concentrations of 0 and CO have reached their optimum concentrations for the reaction. A comparison between Pt-Rh(lOO), Pt-Rh(21O) and Pt-Rh(410) is very interesting because both (210) and (410) consist of (100) terraces, with similar structure of the step sites. The (210) surface has only 2 atoms wide terraces, whereas (410) consists of 4 atom wide terraces. In this way, by comparing the three surfaces not only information is obtained concerning the effect of the introduction of a surface defect, or step site, but also on the effect of the step concentration. At low temperature, around 500K, the reaction is faster on (100) than over (410) and it is slowest over (210). Hence, increasing the concentration of steps results in a reduced CO oxidation at low temperature. It has been reported that step sites can have an activation energy of desorption of CO enhanced by 29 kJ/mol relative to terrace sites [19]. McCabe and Schmidt [28] studied the CO desorption from Pt(l1 l), Pt( loo), Pt(1 lo), Pt(211) and Pt(210). It was found that activation energies for desorption of the most tightly bound states varies from 145kJ/mol on Pt(210) and (211) to 105kJ/mol on Pt(ll0) and Pt(ll1). In the CO inhibition regime it is therefore likely that CO
404
oxidation on terraces proceeds easier over terraces than on step sites. It is not very likely that the differences are caused by differences in noble metal atom concentration at the surface. The step atoms on (210) and (410) have the same coordination, it is therefore expected that the concentration of Pt and Rh along the steps is the same, and therefore the difference between the two surfaces must be due to the difference in concentration of steps. The Pt-Rh(l00) shows two local maxima in the C02 steady-state rate of formation. This was also observed for Rh(l00). The reason for the occurence of two maxima is not yet clear. It is planned to follow the concentration of the adsorbates CO and 0 during the reaction. Hopefully, these experiments will shed some light upon this matter. A comparison of the behaviour of Rh(100) with that of Pt-Rh(100) shows that the relative contributions of the first and second oxidation stage is different. The contribution of the high temperature maximum for Rh(100) is larger. The value of Tm in the case of Pt-Rh(ll1) is 490K, the lowest for the four surfaces studied. This can be understood in terms of a combination of a low adsorption energy of CO and a low sticking probability of oxygen. The low adsorption energy of CO will lead to desorption of CO at relative low temperature, thereby creating room for oxygen to become adsorbed. However, the oxygen concentration remains as a result of the low sticking probabilty of oxygen on the flat (1 11) surface. CO oxidation has mostly been studied on the densely packed (111) surfaces, and many similarities have been found for Pt( 111) and Rh( 111). A major difference between Pt( 111) and Rh(ll1) is that oxygen desorbs from Pt(ll1) in TDS below 900K [32,33], while it remains on Rh until 1300K [34]. Another major difference is the value of the sticking probability of oxygen, which is very low in the case of Pt(ll1) and Pt(l00) [32]. Based on these observations a positive or zero dependence in oxygen pressure is expected for Pt(l1 l ) , while for Rh(ll1) a negative one-half order kinetics in 0 2 is observed. The Pt-Rh( 111) surface shows a positive order in oxygen pressure at high temperatures (>500K). This observation may suggest that oxidation of CO under these conditions proceeds largely on Pt-sites. An alternative explanation may be that Rh sites are more difficult to oxidise in the presence of Pt atoms. Above the rate maximum the C02 production decreases giving rise a negative apparent activation energy. The value of this apparent activation energy is -9 kJ/mol for Pt-Rh(ll1). In the case of Rh(ll1) a value of -28 kJ/mol was reported. This difference illustrates the fact that the C02 production over the Pt-Rh alloy decreases less steeply with increasing temperature than on the Rh( 111) surface. This may be caused by an enhanced oxidation of Rh( 111) compared with Pt-Rh(ll1). The order in CO is negative at low temperature, switching to positive at higher temperature. At low temperature dense CO islands prevent oxygen adsorption, thereby inhibiting the reaction. At a temperature where CO
405
desorption starts sites are created on which oxygen can adsorb and subsequently react with adsorbed CO to form C02, which is released into the gas phase. The CO inhibition on the rough Pt-Rh(210) lasts up to relatively high temperature, i.e. 500K,at this temperature the CO oxidation rate over PtRh(ll1) has already reached its maximum. CO adsorbs in several binding states on Pt(210) [20], i.e. strongly bound and more weakly held states. Steadystate C02 production starts probably after considerable desorption of the tightly bound CO. This was also observed in the case of the CO oxidation over Ir surfaces [21]. The CO oxidation is usually considered as a typical example of a reaction that is insensitive to the surface structure [22]. For example, Oh et a1 reported that the specific reaction rate is the same for Rh(lll), Rh(100)and for high surface area supported Rh under high pressure conditions [4]. However, Fisher et a1 171 reported that at a total pressure of 2x10-7 TOITthe reaction proceeds much faster on Rh( 100) than on Rh( 111) at a temperature of 600K. FEM observations by Gorodetskii et a1 [30] also point to a large effect of the surface structure on the CO oxidation over Rh under low pressure conditions. Recently, Yates Jr. showed by ESDIAD that CO adsorbed on terraces of Pt(211) reacts at a lower temperature than CO adsorbed on step sites [19]. Rate oscillations in the C02 formation on Pt(100) are caused by a reconstruction of the surface from a (1x1) type of surface to a (5x20) type of surface. When the surface switches between the two different types of surfaces oscillations arise [23,31]. This model relies completely on the fact that the CO0 2 reaction on Pt is structure sensitive under the given experimental conditions. Structure sensitivity of the reaction is also reported for Ru [24,25] and Ir [21] surfaces at low pressures. However, Boudart [22] reported that the rate is independent of the particle size of Pd at high pressure. Moreover, the specific reaction rate was similar for Pd( 11l), (100) and (1 10) under low pressure conditions [3,26]. More recently, Kruse et a1 [27] found some evidence for a different behaviour of small Pd particles for the CO oxidation. On Ir(ll1) and (110) it was found that substantial desorption of the most tightly bound state of CO is required in order to achieve maximum activity in the steady state reaction of CO and oxygen to form C02 [21]. Furthermore, it was found that the maximum rate of C02 production occurs at lower temperature for Ir(ll1) than for Ir(ll0) [21]. Therefore, under various conditions structure sensitivity or insensitivity has been reported for several metals. At low temperature the CO-02 reaction is hampered by a large amount of adsorbed CO. Therefore, at low temperature the rate determining step is usually considered to be desorption of CO into the gas phase. After the desorption of a part of the CO, oxygen adsorption can take place. The last step, the reaction between CO and oxygen occurs relatively fast. Any structure sensitivity at low temperature must therefore be related to a difference in
406
adsorption energy of CO for different type of surface structures. For Pt it is reported that step sites show a desorption energy which is enhanced by 29 kJ/mol relative to the (111) flat surfaces [19]. However, the strong repulsion between adsorbed CO molecules could erase the structural heterogeneity, as has been suggested by Boudart in the case of Pd [22]. CO-CO repulsion on Pt(ll1) can only account for a 12-16 kJ/mol [28] decrease in adsorption energy of CO, if it assumed that the CO coverage does not exceed 0.5. Given the mild experimental conditions this assumption is justified. Thus an influence of step sites could still be expected, and has been observed in the present study. In conclusion, the CO oxidation by 0 2 is a surface sensitive reaction. However, under certain conditions the reaction could appear as being structure insensitive. Boudart suggested that lateral interactions between adsorbates erase the effect of different types of adsorption states, leading eventualy to structure insensitivity. A reactive mixture of CO and 0 2 could also modify the catalyst particle shape in such a way that one type of surface structure dominates. Another explanation could be that only the activity of one type of site is measured under experimental conditions. Recent results of Yates Jr [19]. et a1 are consistent with this model. They found that the CO + 0 2 reaction takes place at lower temperature on the terraces than on the steps of the Pt(211) surface using temperature programmed reaction conditions. However, at higher temperature the high diffusion rate of 0 adatoms over the surface obscures the differences of step and terrace sites.
References 1. 2. 3. 4.
5. 6. 7. 8. 9. 10. 11. 12. 13.
14.
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16. 17. 18. 19.
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M. Ehsasi, J.H. Block, K. Christman and W. Hirschwald, J. Vac. Sci. Technol. A5(4), Jul/Aug (1987),821 W.H. Weinberg, W.F. Egelhoff Jr., V.P. Ivanov, V.L. Tataurov and G.K. Boreskov, R o c 7th Intern. Vac. Congr. and 3rd Intern. Conf. Solid Surfaces (Vienna 1977), 1151 M. Boudart and F. Rumpf, React. Kinet. Catal. Lett. Vol. 35,Nos 1-2, (1987)95-105 S.B. Schwartz and L.D. Schmidt, Surf. Sci. 206 (1988), 169-186 T.E. Madey, H.A. Engelhardt and D. Menzel, Surf. Sci. 48 (1975),304 P.D. Reed, C.M. Comrie and R.M. Lambert, Surf. Sci. 64 (1977),603 J. Koch, Thesis, Technische Universitat, Hannover (1972) V. Matolin, E. Gillet and N. Kruse, Surf. Sci. 186 (1987),L541 G. Ertl, M. Neumann and K.M. Streit, Surf. Sci. 64(1977),393 R.W. McCabe and Schmidt, Surf. Sci. 66 (1977),101-124 V.V. Gorodetskii, B.E.Nieuwenhuys, W.M.H. Sachtler and G.K. Boreskov, Appl. of Surf. Sci. 7 (1981),355-371 R.Imbihl, M.P. Cox and G. Ertl, J. Chem. Phys. 84 (1986),3519 J.L. Gland, B.A. Sexton and G.B. Fisher, Surf. Sci. 95 (1980),587 L. Gland, Surf. Sci. 93 (1980),487 T. Matsushima, J. Cat. 85 (1984),98-104