J. Phys. Chem. 1987, 91, 6192-6197
6192
Angular Distribution of Desorption of COP Produced on Potassium-Covered Palladium (1 11) Surfaces Tatsuo Matsushima Research Institute for Catalysis, Hokkaido University, Sapporo 060, Japan (Received: August 26, 1986)
The desorption of C 0 2 produced on potassium-covered Pd( 11 1) surfaces was studied with angle-resolved thermal desorption and isotope tracer techniques. The desorption peaks of C 0 2 were observed around 220, 370,490, and 800 K. The angular distributions of the first two desorptions were sharp along the surface normal and became broad with increasing coverage of potassium. The others showed a simple cosine distribution. The latter two peaks are due to the desorption from chemisorbed C 0 2 and the decomposition of carbonate. This assignment was confirmed by isotope experiments. The change in the former distributions is discussed with several factors.
1. Introduction The angular distribution of the flux of desorbing molecules gives a microscopic insight into the dynamics of surface processes.] This determination is the first step in the analysis of the translational energies. The angular distribution (or velocity distribution) depends on the amount of surface species such as sulfur2x3and ~ x y g e n . ~It is also affected by the local structure surrounding the reaction sites.5$6 Further, the presence of surface sulfur changes the internal energy distribution.'^^ These facts suggest that chemical modifications of catalyst surfaces can be characterized through the analysis of the energy distribution. The aim of this work is along this line. The C O oxidation over Pd( 1 1 1) surfaces is a suitable model reaction for this type of study, since the product C 0 2 is desorbed immediately after the formatioa6 In this paper, we will report the angular distribution of C 0 2 produced on potassium-covered Pd( 11 1) surfaces. Several new factors may contribute to the angular distribution, since alkalis on palladium are expected to increase the heat of adsorption of CO9-I3 and also the surface density of o ~ y g e n . ~Heating ~ . ~ ~ of adlayers consisting of CO and oxygen on the K-covered surfaces showed the C 0 2 desorption in four peaks. These desorptions were analyzed in the angle-resolved form and were also studied with tracer techniques. The peaks were assigned as follows: to the desorption, immediately after the formation on different reaction sites, after the trapping in the chemisorption sites, and to the decomposition of carbonate.
2. Experimental Section The experimental apparatus and procedures were reported p r e v i o ~ s l y . ~ .Briefly, ' ~ ~ ~ ~ the apparatus consisted of a reaction ( 1 ) Comsa, G.; David, R. Surf. Sci. Rep. 1985, 5, 145. (2) Bradley, T. L.; Dabiri, A. E.; Stickney, R. E. Surf. Sci. 1972, 29, 590. Bradley, T. L.; Stickney, R. E. Surf. Sci. 1973, 38, 313. (3) Comsa, G.; David, R.; Schumacher, B. J. Surf. Sci. 1979,85,45;Surf. Sci. 1980, 95, L210. Comsa, G.; David, R. Surf. Sci. 1982, 117, 77. (4) Cosser, R. C.; Bare, S. R.; Francis, S. M.; King, D. A. Vacuum 1981, 31, 503. (5) Segner, J.; Campbell, C. T.; Doyen, G.; Ertl, G. Surf. Sci. 1984, 138, 505. (6) Matsushima, T.; Asada, H. Chem. Phys. Lett. 1985,120,412; J . Chem. Phys. 1986, 85, 1658. (7) Thorman, R. P.; Anderson, D.; Bernasek, S. L. Phys. Rev. Lett. 1980, 44, 743. Thorman, R. P.; Bernasek, S. L. J . Chem. Phys. 1981, 74, 6498. (8) Kubiak, G. D.; Sitz, G. 0.;Zare, R. N. J . Vac. Sci. Techno/.. A 1985, 3, 1649. (9) Crowell, J. E.; Garfunkel, E. L.; Somorjai, G. A. Surf. Sci. 1982, 121, 303. Garfunkel, E. L.; Crowell, J. E.; Somorjai, G. A. J . Phys. Chem. 1982. 86, 310. (10) Crowell, J. E.; Somorjai, G. A. Appl. Surf. Sci. 1984, 19, 73. Crowell, J. E.; Tysoe, W. T.; Somorjai, G. A. J . Phys. Chem. 1985, 89, 1598. (11) dePaola, R. A.; Hrbek, J.; Hoffmann, F. M. J . Chem. Phys. 1985, 82, 2484. (12) Lee, J.; Arias, J.; Hanrahan, C. P.; Martin, R. M.; Metiu, H. J . Chem. Phys. 1985.82, 485. (13) Whiteman, L. J.; Ho, W. J . Chem. Phys. 1985, 83, 4808. (14) Pirug, G.; Bonzel, H.; Broden, G. Surf. Sci. 1982, 122, 1. ( 1 5 ) Garfunkel, E. L.; Somorjai, G. Surf. Sci. 1982, 115, 441. (16) Matsushima, T. Surf. Sci. 1983, 127, 403. (17) Matsushima, T. J . Card 1985, 96, 420
0022-3654/87/2091-6192$01.50/0
chamber (with LEED-AES), an analyzer chamber, and a collimator placed between them. The sample was a disk-shaped slice (10-mm diameter X 0.8-mm thickness; purity 99.995% from Metal Crystal Ltd., Cambridge, U.K.). It was mounted on a rotatable axis of the manipulator, in such a way as to face LEED-AES optics, an Ar' gun, a Saes Getter potassium source, and the analyzer chamber at a desired desorption angle. The sample crystal was prepared in the same way as beforeS6 It was bombarded by Ar' at room temperature up to 800 K and then heated in oxygen, until it was clean as judged by AES. Before each adsorption experiment, the sample was flashed up to 1100 K. A trace amount of potassium remained on the surface even after this treatment, as judged by thermal desorption spectra of oxygen from the molecular adsorption states. The surface was subject to contamination by vanadium and titanium during the deposition of potassium. Therefore, it was frequently bombarded by Ar', heated briefly in 02,and annealed up to 1100 K. Potassium was deposited on both faces of the crystal below 200 K to keep the deposition rate constant. During the deposition, the Saes Getter was biased up to +9 V against the sample to monitor the current due to Ar' ion. This surface was exposed to oxygen (I8O2)and/or to carbon monoxide (CI6O). Then the surface was heated to proceed the reaction. The desorption of oxygen, carbon monoxide, and the product C 0 2 was analyzed both in the angle-integrated form with a mass spectrometer in the reaction chamber and also in the angle-resolved form with another mass spectrometer in the analyzer chamber. The usage of 1 8 0 2 improved the signal-to-noise ratio of the mass spectrometer in the analyzer chamber. Oxygen- 18 is frequently designated simply as 0 in the following description.
3. Results 3.1. Coverage of Potassium. The coverage of potassium was estimated from the ion current due to the depositing K+ and the deposition intervals. It is difficult to determine directly the coverage with AES, since the K 252-eV Auger signal is superimposed on the Pd 243-eV signal (see Auger spectra inserted in Figure 1). The method used in the previous work" was applied to the determination of the interval required to produce one monolayer at a fixed evaporation rate. Thermal desorption (TD) of potassium was followed by monitoring the amount of K remaining on the surface after sequential heating. In T D spectra of potassium deposited on transition metals, an extremely sharp peak is usually observed from the desorption of a multilayer above the first layer.9s11J5~18 The desorption from the first layer yields a broad peak in the temperature range of 500-1200 K. Therefore, the completion of the first layer can be examined from a sharp decrease in the amount of K during sequential heating procedures. Figure 1 shows the intensity ratio of the K Auger signal at 252 eV (which is partially contributed from the Pd signal at 243 eV) to the main Pd signal at 330 eV as a function of the temperature up to which the sample was heated. Potassium was first deposited (18) Doering, D. L ; Semancik, S. Surf. Sci. 1983, 129, 177
0 1987 American Chemical Society
The Journal of Physical Chemistry, Vol. 91, No. 24, 1987 6193
Angular Distribution of COz on K-Covered Pd( 1 11)
,
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K/Pd(lII)
330
/I\I / I \A I Tp
/IO2 K
Figure 1. Intensity ratio of the K Auger signal at 252 eV to the Pd signal at 330 eV as a function of annealing temperature, Tp. The surface covered by K in various amounts was heated to Tp,and the amount of K was measured after cooling below 200 K. Such procedures were repeated to Tp = 1100 K. The evaporation interval and the initial potassium coverage: 0, teVap= 300 s (6'K/6'K,max = 1.9); e, 240 s (1.3); 0, 180 s (1.1); and A, 120 s (0.7). Inserted Auger spectra (modulation = 240 s. Arrows show the position of Tp. voltage 5 eV) were at
around 100 K for an interval tWap at a fixed evaporation rate. The sample was heated slowly (about 2 K/s) to the desired temperature T p and cooled again below 200 K. The amount of remaining potassium was measured with AES. The intensity ratio decreased > 180 s. Above this temperature sharply around 325 K, when tWap the ratio decreased fairly linearly with the increasing Tp and followed the same line independent of tevap. Above 800 K it appeared constant, since the small K signal was obscured by the Pd signal. When tevap< 160 s, the K signal showed a smooth decrease with the increasing T,. The sharp decrease around 325 K is due to the desorption from the multilayer. The completion of the first layer was determined from the cross point of the vertical dashed-dotted line at 325 K and the curve of the Auger signal intensity above 325 K. This point indicates tevap= 165 s as the interval for the completion of the first monolayer. This was further examined through observation of LEED patterns. Distinguished LEED patterns were observed after the surface was annealed above 320 K when t,, > 180 K. The LEED observations were always conducted below 200K. When T p = 320 K, a ( ~ ' 3 x 4 3 ) structure with a weak spot intensity was observed. With increasing Tp,it changed to a ringlike structure and eventually was accompanied by a ( d 3 X d 3 ) R 3 0 ° structure. The LEED spots coalesced, producing purely the ( d 3 X d 3 ) R 3 0 ° structure at Tp= 395 K. The spot intensity of this structure, which was followed by a spot photometer, decreased with the increasing Tpabove this temperature. At this stage ( Tp = 395 K) the surface potassium coverage was concluded to be OK = I/,. By using this coverage as a reference, the maximum coverage in the first layer was estimated to be OK,max = 0.36. This conclusion agrees well with the work on Pt( 11 1) by Garfunkel and S ~ m o r j a i . ' ~The surface atom density on Pt( 111) is quite close to that on Pd( 11 1). In the following the coverage of potassium is represented as the evaporation interval relative to the interval required for OK,max = 0.36, as oK/eK,max. The evaporation rate of potassium varied from day to day. Therefore the evaporation interval for was frequently determined. With increasing Tp, the K overlayer changed LEED patterns as ( 4 3 x 4 3 ) (43Xd3)R30° ~ ( 2 x 2 ) .The intensity of the last pattern was low. This structure was reported on Ru(0001)" but not on P t ( l 1 l).15 3.2. Oxygen Adsorption on Potassium-Covered Surfaces. The adsorption and desorption of oxygen on clean Pd( 11 1) surfaces at low temperatures were previously reported in detail.lg Oxygen
-
(19) Matsushima, T. SurJ Sci. 1985, 157, 297
-
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1.0 1.5 2.0 2.5 '5 6
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Figure 2. Desorption spectra of 0, from potassium-covered surfaces. Potassium was deposited below 200 K. The surface was exposed to 0, at 100 K. The heating rate was changed at 300 K.
adsorbs molecularly below 130 K. No dissociation occurs at the adsorption temperature. The dissociation proceeds during subsequent heating procedures. This conclusion has recently been confirmed by EELS experiments.*O For small 0, exposures, only a single desorption peak appears above 600 K. With the increasing exposure, two additional peaks, cy1 at 200 K and a2at 160 K, are observed. The former is due to the combination of adatoms and the others are due to the desorption from the molecular adsorption states. The admolecules do not react with carbon m0n0xide.l~ The addition of potassium drastically changed the desorption spectra (Figure 2). When the surrface was covered by potassium at OK N 0.050K,max, the desorption of a , - 0 2peaked at 185 K and showed a peak almost twice as high as that on the clean surface. With the increasing potassium coverage, the amount of O2 desorbed from the molecular state decreased rapidly and disappeared beyond the completion of the first monolayer. On the other hand, surface potassium greatly increased the desorption from adatoms above 500 K. When the surface was initially clean, one-fourth monolayer of adatoms is attainable at saturation.2' Potassium increased the amount of saturation almost 4 times. This oxygen does not necessarily mean surface adatoms. Of course, part of it may be adsorbed directly onto the surface and also may be found in an oxide form of potassium. We were not successful in determining the individual amount. The stoichiometry of the oxide is not clear, since several forms are possible in the case of potassium the desorption started around 550 oxide. Above 0K = 0.508K,max, K and showed an additional sharp peak at 660 K. This peak disappeared predominantly when the surface was exposed to CO at room temperature before heating. No oxygen desorption was observed after large exposures of CO. All oxygen, which could be desorbed above 550 K, is reactive toward CO. The sharp peak at 660 K may be due to the decomposition of the potassium oxide and/or the desorption from a dense oxygen adlayer in which the repulsive interaction between oxygen adatoms is pronounced. The desorption above 1000 K, which is due to the combination of absorbed oxygen,21.22 also increased with the increasing potassium coverage. This oxygen is produced during heating procedures above 500 K. The amount depended on the history of the sample. The desorption was hardly detected in the experiments after Arf -
~ _ _ _ _ _ _ _
~~~~
~~
~
(20) Imbihl, R.; Demuth, J. E . Surf Sci. 1986, 173, 395. (21) Conrad, H.; Ertl, G.;Kuppers, J.; Latta, E. E . Surf. Sci. 1977, 65, 235, 245. (22) Matsushima, T., unpublished results.
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The Journal of Physical Chemistry, Vol, 91, No. 24, 1987
Matsushima 1
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Pd(IIl)-K, 2.IL '*02/280K +3.0LC160/100K t ,
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Figure 3. Desorption spectra of C 0 2 produced on potassium-covered surfaces: (a) angle-integrated form; (b) angle-resolved form at the
normal direction. bombardments. No effect on the dynamics of C O oxidation is expected. 3.3. Carbon Monoxide Adsorption on Potassium-Covered Surfaces. The desorption of CO peaked around 500 K on the clean surfaces. An additional peak appeared around 670 K with the increasing coverage of potassium. This is due to the desorption from potassium-modified sites. The new peak around 670 K was maximized around OK = 0.60K,,,,. The activation energy for desorption was increased from 31 kcal/mol for clean surfaces to 41 kcal/mol on the potassium-modified sites. No CO desorptions were observed above OK = OK,max. These features are quite similar to CO desorption on potassium-covered transition-metal surface~.~-~~ A general overview of the CO, formation on the potassiumcovered surfaces will be summarized in the next section. The other following sections will deal with the angular distribution of the C 0 2 desorption and isotope tracer experiments. 3.4. General Features of CO,Spectra. Desorption spectra of CO, produced depended strongly on the coverage of potassium. Typical spectra were reproduced in Figure 3. The surface covered by potassium was exposed to 2.1 langmuirs (1 langmuir = 1 X lo4 Torr s) of lsO2 a t 1.1 X Torr and at 280 K. This temperature is high enough to remove oxygen admolecules. The surface was almost saturated with oxygen adatoms at this stage. It was further exposed to carbon monoxide in large amounts (3.0 langmuirs of CI6O) at 100 K. The surface was heated at a rate of 18 K/s. The desorption of C 1 6 0 1 8 produced 0 was monitored in both the angle-integrated and angle-resolved forms. The latter was recorded at 0 = Oo, where 0 (the desorption angle) is the angle between the surface normal and the collimator axis. No change was found in C 0 2 spectra with more exposure of CO. The carbon dioxide produced during the above thermal desorption procedures contained mostly C ' 6 0 1 8 0below 600 K. Angle-integrated spectra in Figure 3a consists of three peaks, at 220-230 K (P4-CO2),at 380-390 K (&CO,), and at 490-500 K (y1-C02). The details of CO, involved in the first two peaks were discussed in previous papers.6 p4-CO2 is formed in a mixed adlayer of C O and oxygen with (2x1) periodicity. & C 0 2 is produced between separate domains of C O and oxygen with a ( d 3 X d 3 ) R 3 0 ° structure. The peak height increased first and decreased above OK = O.4OK,,,,. It means that the mixed adlayer described above is extended first. This is reasonable because this adlayer with (2x1) periodicity has a half monolayer of oxygen and the addition of potassium increases oxygen first toward this
1
2
3 4 T/102 K
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,
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Figure 4. Angle-resolved C 0 2 spectra observed at various desorption angles at small potassium coverage.
level. The peak of &CO2 decreased and disappeared around OK = 0.50K,max. The third peak was newly observed. This increased with the increasing potassium coverage and was maximized around OK = 0.50K,max.NO C 0 2 was observed above OK = 0.70K,max. It was probably caused by the decreasing amount of C O adsorbed. The amount of oxygen deposited on the surface increases sharply with the increasing OK. The dense oxygen layer may prevent C O from adsorbing. The angular distributions of the desorption of p4-C02 and & C 0 2 were very sharp on the surface free of potassium.6 These became broad with the increasing potassium coverage. The ratio of the peak height of PcCOz in the angle-integrated form to that in the angle-resolved form in the normal direction was increased by 3 times with the increasing OK in the range of OK/OKsnax = 0-0.5. A similar ratio of P3-CO2 also increased. On the other hand, the ratio of y I - C 0 2was large as compared with that of &- or p4-C02. Thus, the angular distribution of each CO, depends upon potassium in different modes. 3.5. Angular Distribution. Over the surfaces with OK N O.05OK,,,,, the peak height ratio of p4-C02 to P3-C02 in the angle-integrated form was about 2 (Figure 3a), whereas a similar ratio of the peak height in the normal direction in an angle-resolved form attained about 5 (Figure 3b). These values are in good agreement with the results on clean surfaces.6 No effect of potassium was observed on the angular distribution at small coverages. The desorption of P4-C02 is distributed more sharply than that of & C 0 2 along with the surface normal. Typical angle-resolved spectra of C 1 6 0 1 8 0observed at various desorption angles for different coverages of potassium were reproduced in Figures 4 and 5. No significant change was noticed in the spectra at OK = O.18OK,,,, (Figure 4). The peak height of p4- and &-C02, however, decreased more slowly with the increasing desorption angle, as compared with those on the clean surfaces. This effect became clear at OK = 0.5BK,max (Figure 5). The peak height of p4-C02decreased with an increase in the desorption angle more slowly than that shown in Figure 4. The peak height of -yl-C02decreased much more slowly. The angular distributions of desorption of the above CO, are separately summarized in Figures 6-8. Several power series of cos B are also drawn for comparison. The desorption of &CO2 shows the power of d = 30 on the clean surface. The power d
The Journal of Physical Chemistry, Vol. 91, No. 24, 1987 6195
Angular Distribution of C 0 2 on K-Covered Pd( 111) I
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2.IL I802/280K+3.0L C160/100K
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T/102 K Figure 5. Angle-resolved C02 spectra observed at various desorption angles at moderate potassium coverage.
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o.o'-;o
A 2b \
40\60 8 (degree)
>o
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Figure 6. Angular distribution of ,B,-CO2 at various potassium coveragas. Experimental conditions are given in Figure 3. The solid line (d = 30) is from ref 6 .
decreased rapidly from 30 to 3 with the increasing potassium coverage. A similar effect was observed for the desorption of &CO2 (Figure 7). The desorption shows d = 10-15 on the clean surfaces.6 It shows nearly a simple cosine form at OK = 0.180K,On the other hand, the desorption of r l - C 0 2(Figure 8) always obeys a cosine distribution. It suggests that this C 0 2 is completely thermalized before the desorption, for example, the desorption from the chemisorption state. In order to confirm this point, two kinds of experiments were conducted. The first was to analyze isotope distribution in rl-COzand the second to compare with the desorption after exposure of C 0 2 itself. 3.6. Isotope Experiments. The oxygen exchange is expected to occur between COz and oxygen combined with potassium, if rl-C02is due to the desorption from chemisorption sites on the potassium oxide. The exchange was examined during thermal
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0.6C
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io
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40 60 eo 100' 8 (degree) Figure 8. Angular distribution of y,-CO,. Experimental conditions are given in Figure 3. O 0 L 0
desorption followed by CI6O exposure around room temperature, in such a way that the signal of yI-C02was not obscured by large amounts of &- and p4-CO2formation. In the following experiments, the surface covered by potassium at 270 K and further to 3.0 was exposed to 2.8 langmuirs of 1802 langmuirs of C l 6 0 at the same temperature. The exposure pressure of CO was 5 X lo4 Torr. Most of surface oxygen were removed as C 0 2 during the CO exposure. The following thermal desorption produced a small amount of C02, which peaked around 330 and 470 K and further around 800 K. Typical results are displayed in Figure 9, where the angle-resolved spectra recorded simultaneously are also reproduced. The first peak is assigned to p3-C02,which is produced from the remaining adsorbed oxygen and CO. The second is due to the desorption of y1-C02and the third yz-C02. r-COz was observed even above OK = 0.70Kqmax in these experiments. The signals of CI6O2and CI8Ozwere also followed during thermal desorption procedures. p3-C02involved only C'60180.
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The Journal of Physical Chemistry, Vol. 91, No. 24, 1987
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Figure 10. Angle-resolved desorption spectra of C 0 2 trapped on potassium-covered surfaces. The surface precovered with K was exposed to (a) 1.4 langmuirs of I8O2at 280 K and 1.5 langmuirs Cl60 at 100 K, (b) 0.3 langmuir of CI6O2at 100 K, (c) 1.4 langmuirs of 1802 at 280 K and 1.5 langmuirs of CI6O2at 100 K, (d) 2.8 langmuirs of 180at 2 280 K and 1.5 langmuirs of CI6O2at 100 K, and (e) 1.4 langmuirs of 1802 at 280 K and 1.5 langmuirs of Cr602at 280 K. The C 0 2 signal except for (a) indicates only the signal intensity due to CI6O2.
cI6o2
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T/102 K Figure 9. (a) Isotopic composition of 740,. (b) Angle-resolved C 0 2
spectra observed at various desorption angles. On the other hand, significant amounts of C1602and C1802were found in 7,- and y2-C02. The degree of isotope exchange was small in yl-C02,Le., Ci602,C1802
