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An infrared study of CO adsorption on Pt(111)
444
Surface Science 201 (1988) 444-460 North-Holland, Amsterdam
AN INFRARED STUDY OF CO ADSORPTION ON P t ( l l l ) C.W. OLSEN and R.I. MASEL * Chemical Engineering Department, University of Illinois 1209 W. California Street, Urbana, IL 61801, USA Received 9 September 1987; accepted for pubfication 1 March 1988
Infrared spec~oscopy of isotopic mixtures is used to examine island formation during carbon monoxide (CO) adsorption on Pt(111) at 300 K. It was found that the linear CO band shifts from 2084.0~cm -1 at low coverage to 2094.7 cm -1 at saturation. This shift can be decomposed into a 14.4 cm -1 downward shift due to changes in the binding of the CO with coverage, and a 25.1 cm-1 upward shift due to dipole interactions within the adlayer. Analysis of the measured band shifts with coverage and isotopic dilution shows that no islands are formed upon CO adsorption on P t ( l l l ) at low coverage. The interpretation of the data is less clear at high coverage. However, all of the obsm'vations can be explained without invoking island formation.
1, Introduction The adsorption of CO on Pt(111) is one of the most heavily studied systems in the surface science literature. Nonetheless, there are still several unresolved questions about the CO adsorption process. One outstanding question is whether islands form when CO adsorbs on Pt(111) at 300 K. Infrared (IR) data has been inte,'preted as indicatiag that when the CO adsorbs on Pt(111), the CO molecules cluster into highly ordered islands even at low coverage [4-6]. The coverage dependency of the sticking coefficient has also been interpreted as suggesting island formation [2], although the presence of a mobile precursor state could account for the observed behavior as well [4]. Yet, temperature programmed resorption (TPD) and isosteric heat data [1-5] indicate that the binding energy of the CO decreases with increasing coverage. The TPD data have been interpreted as indicating that the interactions between adjacent CO's are repulsive. Island formation would be difficult to understand if the interactions between the adjacent CO's were repulsive. Further, at room temperature, low energy electron diffraction (LEED) shows diffuse spots characteristic of a high degree of disorder in the adsorbed layer [1]. At low temperatures, a series of distinct LEED patterns are seen [2]. It is difficult to explain the observed LEED patterns if highly ordered islands form. * To whom correspondence should be addressed.
0039-6028/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
C.W. Olsen, R.L Masel / IR study of CO adsorption on Pt( l l l )
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Thus, there is an apparent contradiction between the IR results which suggest island formation, and the TPD, isosteric heat, and LEED results which do not show behavior consistent with island formation. Much of the evidence for island formation comes from a pioneering paper [4] by Crossley and King where IR was used to examine CO adso~tion on Pt(100) and P t ( l l l ) . Previous work by Hammaker, Francis and Eischens [7] showed that if island formation occurs there should be an increase in the coupling between the vibrating dipoles on adjacent molecules in the adlayer. The increase in the dipole coupling produces a shift in the position of the CO IR band which can be detected experimentally. Of course, the position of the CO absorption band is also affected by changes in the bonding between the adsorbed molecule and the surface. These changes are called chemical effects and can be significant in some systems [8-12]. However, Crossley and King pointed out that if a 12CO layer were diluted with 13CO, at fixed total coverage, the binding of the ~2CO would be independent of the isotopic ratio. Nonetheless, the strength of the dipole interactions change, since the vibrational coupling between ~2CO and ~3CO is weak. As a result, an isotopic dilution technique can be used to isolate the dipole shifts from the chemical effects. Consequently, Crossley and King proposed that one should be able to tell whether islands form when CO adsorbs on metal surfaces using the isotopic dilution technique. Crossley and King [4] used the tecl~nique to examine CO adsorption on Pt(100) and P t ( l l l ) . On Pt(100), they fcand that at saturation coverages the linear 12CO band shifted by 35 cm -1 with isotopic dilution. In contrast, the linear 12CO band only shifted by 17 cm -1 in going from low coverages to saturation. At the time Crossley and King did these measurements, it was thought that if there were a chemical shift, the shift with coverage should have been larger than the shift due to dipole effects alone [20]. Yet, Crossley and King were observing a smaller shift with coverage than with isotopic dilution. Consequently, Crossley and King suggested that the chemical shifts were negligible, and that the whole shift observed with coverage was caused by dipole coupling alone. Crossley and King then noted that if there were no chemical shifts the singleton frequency (the frequency of a 12CO molecule isolated from other riCO's) would be independent of coverage. As a result, one could calculate the dipole shift as a function of coverage directly from their data. The calculation indicated that L,hedipole interaction between adjacent CO's did not go to zero in the limit of zero coverage. Notice, however, that it should have gone to zero if the CO molecules adsorb randomly on the surface. The shift would not go to zero in the zero coverage limit only if CO islands form. As a result, Cross~ey and King concluded that islands form when CO adsorbs on Pt(100). Crossley and King also applied the isotopic dilution technique to CO adsorption on a (111) oriented platinam foil. The foil data were more
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C. IV. Olsen, R,L Masel / IR study of CO adsorption on P t ( l l l )
complicated than those on Pt(100). There were two bands; a large band with characteristics simil&r tO the main band on Pt(100) and a smaller shoulder, whose intensity varies with adsorption conditions [141. The main band showed the same characteristics as those described above. The shift with coverage was less than the shift with isotopic dilution. Crossley and King also noted that the shift with isotopic dilution was greater than the shift with coverage measured by Horn and Pritchard [5] for CO adsorption on a P t ( l l l ) single crystal. As a result Crossley and King argued that islands also form on P t ( l l l ) . Crossley and King went on to present a method for estimating the island dimensions from vibrational spectra. Over the years, there have been several papers which applied Crossley and King's technique to other systems. It has been found there are some subtleties in the isotopic dilution experiment which were not apparent when Crossley and King did their work [19]. For example, it is now known that there are often important chemical effects when CO adsorbs on transition metals [8-12]. The chemical effects can cause either upward or downward shifts in the singleton frequency with coverage. The shifts with coverage earl be smaller than the shifts with isotopic dilution even in the presence of significant chemical effects. As a result, one has to be careful before concluding that the singleton frequency for CC,. dsorbed on P t ( l l l ) is not varying with coverage. In fact, it is quite likely th,.t the singleton frequency of CO adsorbed on P t ( l l l ) is coverage dependent. Note that the binding energy of CO on P t ( l l l ) varies significantly with coverage [1-3]. Low energy electron diffraction (LEED) shows that the structure of the adsorbed layer changes from V~× ¢~'R30 ° geometry at low coverage to a c(4 × 2) geometry at saturation [1]. Singleton frequencies are dependent on geometry and bond energy. Thus, there is reason to suspect that the IR spectrum of CO adsorbed on Pt(111) should show chemical shifts with coverage. If there are chemical shifts, then one has to reconsider the interpretation of Crossley and King's data. With this in mind, CO adsorption on Pt(111) was reexamined with IR. The procedure was similar to Crossley and King's, except that the isotopic dilution experiment was done at many coverages so it would be possible to see if the singleton frequency were coverage dependent. The data were then used to see whether islands form when CO adsorbs on Pt(111). Modifications to the techniques used to estimate island dimensions from IR data are also suggested.
2. ~xperimental The experiments reported here were conducted using the apparatus and procedures described previously [8,13]. The apparatus consisted of an ultra-high vacuum chamber with an operating base pressure of 2 × 10 -1° Torr, inter-
C W. Oisen, R.I. Masei / IR study of CO adsorption on Pt(lll)
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faced to a Nicolet Fourier transform infrared spectrometer. The spectrometer allowed infrared spectra of submonolayer quantities of adsorbates to be obtained. Resolutions of much less than a wavenumber were possible. However, most of the experiments reported here were run at a resolution (FWHM) of 4 c m -I. The Pt(lll) crystalused here was cut from a Metron singlecrystalrod. The rod was aligned to within 0.5° of the (111) plane using Lane back reflection.A sample was then cut from the rod, polished with diamond paste and cleaned in I atm of flowing oxygen at 900 ° C for 24 h. The sample was then washed in hydrofluoric acid, nitricacid and acetone and mounted in the vacuum system. The sample was then cleaned using a cycle of argon ion bombardment followed by annealing and heating in I0 -6 Torr oxygen at 1000°C. Isotopic dilution experiments were done by dosing with a mixture of 12CO and 13CO. The 13CO was obtained from Stohler Isotope Chemicals at 99,% isotopic purity. There was some difficultywith exchange between 12CO and 13C O on the chamber walls during dosing. As a result,it was never possible to get pw~,re13CO onto the crystal.However, isotopicratios of up to 4/1 ~3CO to 12C O could be obtained. The isotopic ratios reported below are those actually measured with ~ Analvac quadrupole mass spectrometer.
3. Results
Fig. 1 shows the development of the CO infrared absorption band with coverage at room temperature. There are two main CO bands centered at 1850 and 2090 cm-2, respectively. They are at the positions expected for linear and two-fold bridge bound CO. There is also some evidence for the weak third band around 1820 cm -1. It is at the position expected for triply coordinated CO. All three bands are similar to those reported previously [14]. The frequency and coverage dependence (2084.0-2094.7 cm-1) of the linear band are in good agreement with previous work [4-6, 14-16]. The bridging mode is quite broad and weak (0.1%) at high coverage which is consistent with the few RAIRS studies reporting a bridging mode for CO on Pt(111) [6,14,18]. The triply coordinated band is weaker still; extensive noise reduction was required to resolve the band fully and that was not the purpose of our work. Hayden and Bradshaw report a small shoulder to higher energies on the linear CO band after adsorption at 300 K. The shoulder was thought to be associated with defec.'s. "i'nere is no evidence for the shoulder in "'-m~-u~t~J=~-u,:- !~.~:~ 1. A shoulder could be observed after adsorption of CO on Pt(lll) at 90 K. On a freshly sputtered sample, the intensity of the shculder was nearly the same as that of the main feature at very low coverages. However, in all case~:, the shoulder was much less intense than the one reported by Hayden and Bradshaw and it disappeared upon annealing. All of the data here were taken
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C. IV. Olsen, R.L Masei / IR study of CO adsorption on Pt(l 11)
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Fig. 1. The development of the CO infrared absorption band as a function of coverage on P t ( l l l ) at 300 K.
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Fig. 2. A comparison of the linear CO infrared absorption bands seen after dosing 6 L of a variety of 12CO/13CO mixtures onto a 300 K P t ( l l l ) sample.
C W. Olsen, R.I. Masel / IR study of CO advorption on Pt(lll)
449
.2L
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Fig. 3. A comparison of the linear CO infrared absorption bands seen after dosing 0.2 L of a variety of 12CO/13CO mixtures onto a 300 K P t ( l l l ) sample.
under conditions where the shoulder was absent. Hence, it appears that our sample had an unusually low defect density. The effects of isotopic dilution were Lnvestigated following the method of Crossley and King [4]. Fig. 2 shows a series of isotopic dilution spectra taken at a 6 L exposure. The 12CO band shows a shift from 2077 to 2092 cm -1 in going from 25~ to 100% 12CO while the 13CO band shifts from 2036 so 2027 cm-1. Notice also that the peak intensities do not reflect the isotopic composition (intensity stealing) and the bands broaden upon dilution. Both results are characteristic of the presence of significant dipole coupling within the CO layer. Thus, it appears that there are substantial dipole interactions on the surface at 6 L exposure. Fig. 3 shows the effects of isotopic dilu';,..~ at low coverage. In contrast to the results in fig. 2, the i2CO peak in fig. 3 only sh;~fts about I cm -1 in going from 30% to 100% 12CO. A 1 cm -1 shift is actually smaller than the scatter observed in the data and hence, ma~,' not be due to dipole coupling interactions. Thus, it does not appear that significant dipole coupling is occurring at these low coverages even though significant dipde interactions were seen at high coverages. Conseq,.~entl2~, it appears that the strength of the dipole interactions varies significantly with coverage. One can quantify the effects by using data such as that shown in figs. 2 and 3 to estimate the ~ g ! e t o n f~,~ac~cy as a function of coverage. The most eoaaaon way to estimate singleton frequencies is to empirically extrapolate the position of the linear 12CO band to infinite dilution. However, such a procedure does not work well unless very dilute ~2CO mixtures can be
C W. Olsen, IL1. Masel / IR study of CO adsorption on Pt(l l l)
450
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obtained. Experimentally, dilute 1 2 C O mixtures are difficult to obtain at low coverages due to exchange in the doser. Consequently, an alternative procedure was used to analyze the data here. The procedure comes about because at low to moderate coverages the frequency of the 12CO absorption band, ~,, should vary with the fraction of 12CO ha the layer, D, according to: v2= Vs2 + KD,
(1)
where Ps is the singleton frequency and K is a constant which depends on the coverage and the geometry of the adlayer, provided the vibrational coupling between the 1 2 C O and 13CO is negligible. Hence, one can obtain a fairly accurate value of the singleton frequency, Ps, if one uses a least-squares procedure to fit eq. (1) to isotopic dilution data. The data here has been analyzed in this manner, and the results are presented in fig. 4. The points in the figure are the measured positions of the linear 12CO band as a function of isotopic dilution while the lines are the best fit to the dz, a using eq. (1). Notice that quite good agreement between the data and eq. (1) has been obtained. Thus, it appears that eq. (1) gives a good representation of the experimental results. According to eq. (1), the Lntercepts of the ~nes in fig. 4 correspond to the singleton frequency at each coverage. Notice that the singleton frequencies vary from 2082 to 2069.6 cm -1 with increasing coverage. Thus, it seems that the singleton frequency is indeed coverage dependent. Notice also that the lines in the figure cross. "?he crossing of the lines is further evidence that the singleton frequencies are coverage dependent.
C.W. Olsen, R.L Masel / IR study of CO adsorption on Pt(l 11)
451
The figure also shows that the strength of the dipole shifts varies substantially with coverage. Notice that with a total exposure of 20 L, the linear 12CO band shifts from 2069.6 to 2094.7 c m - ~ in going from infinite dilution to 100% 12CO. At a constant total exposure, there are no chemical shifts, so the whole shift in the linear 12C0 band must be caused by a change in the dipole interactions between adjacent ]2CO's. As a result, we conclude that at saturation, (20 L) there is a 25.1 cm -1 dipole shift in the frequency of the linear 12CO band. Notice, however, that the size of the shift decreases with decreashlg coverage until at 0.2 L the shift is only about 2 cm-1. Again, a 2 c m - ] shift is small enough to be attributed to experimental error. Thus, it is apparent that while there are strong dipole interactions at moderate to high coverages, the dipole interactions do aot appear to be important at low coverages. It should tdso be mentioned that virtually the same conclusions are reached if the analysis above is done on the 13CO band rather than ]2CO band. The 13CO band exhibits a - 14.2 cm -1 chemical shift with coverage. This compares to the - 1 4 . 4 cm -1 chemical shift with 12CO. The lSCO band also exhibits a 26.6 c m - ] dipole shift at high coverage. This compares to the 25.1 cm-~ dipole shift seen with lZcO. For a completely decoupled system the shifts in the 12CO and 13CO bands should be equal, and these numbers are close enough to say that equality was nearly achieved. The results of this analysis are finally summarized in fig. 5 which shows the contributions of dipole interactions and chemical effects to the observed shift as a function of coverage. The figure clearly shows that the rather small shift observed with coverage is actually composed of two larger opposing shifts. Thus, one does have to consider both shifts in the analysis of the data.
4. Discussion Looking back at the data reveals that all of the results presented here are in good agreement with those of previous investigators. The peak positie~ls and their dependence on the isotopic ratio reported here are virtually the same as those reported by Crossley and King [4] and Hayden and Bradshaw [14]. The saturation coverage singleton frequency found here is about 3 cm -1 higher than that reported by Crossley and King. Crossley and King, however, did their work on a {lll}-oriented ribbon, not a (111) crystal. There was an extra ~ 1 v~a~, ," ~- t a,1L..~ c .C'_.'1 t u . spectt-arn, and so a deconvolution procedure was needed to calculate the position of the main CO band. Hence, it is not surprising there is a small difference between Crcssley and King's results and those reported here. Qualitatively, our results are very similar to Crossley and King's which suggests that our results are characteristic of Pt(111). Of course, we do observe that the singleton frequency is coverage depen-
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C W. OIsen, R.I. Masel / IR study of CO adsorption on Pt(ill)
dent. Crossley and King did their isotopic dilution experiments at constant (saturation) coverage. They then assumed that the singleton frequency did not change with coverage. As noted previously, since Crossley and King did their original work, it has become apparent that singleton frequencies are often coverage dependent. Coverage dependent singleton frequencies have been observed during CO adsorption on Pt(410) [8], C u ( l l l ) [11], Cu(ll0) [12,17], Cu(100) [9], and Pd(100) [10], for example. The binding of CO is known to change with coverage on Pt(111). Thus, it is not surprising that the singleton frequency is coverage dependent. Of course, the data in fig. 4 show that the singleton frequency decreases by about 15 cm -1 with increasing coverage. A decreasing singleton frequency indicates a weakening of the C - O bond relative to the gas phase. The observation of a singleton frequency which decreases with increasing coveraged is somewhat unexpected. Many years ago, Blyholder [20] developed a model for the changes in the singleton frequency that occur with increasing coverage. This model asserts that changes in the singleton frequency will arise from a reduction in metal back-donation into the CO 2~r* antibonding orbital due to increased competition for the metal d-electrons with coverage. Note, however, that Blyholder's model predicts that the singleton frequency increases with increasing coverage. It cannot account for downward shifts such as those observed here. Of course, downward shifts in the singleton frequency have previously been observed during CO adsorption on Cu(111) [11], and polycrystalline silver and gold [21]. Pritchard and co-workers [21,22], explained the downward shifts by suggesting that CO forms a bond which is almost entirely o in character with copper, silver and gold. Pritchard et al. then argued that with increasing coverage there is an accumulation of surface charge. The accumulation of surface charge lowers the heat of adsorption of the CO. The charge accumulation can be relieved through metal back-donation into the CO 2~r* orbital. The result is a decrease in the singleton frequency with coverage. Bagus et al. [28] calculated the shift expected for such a model, and showed that one can get an increase or a decrease in the singleton frequency due to such effects. It is not clear whether Prichard's model wov.ld apply to CO on P t ( l l l ) . However, the heat of adsorption of CO on Pt(111) decreases with ;r,creasing ccverage [5] in a manner consistent with Prichard's model. Thus, Prichard's model can •.Av--- ,**,..,,.a~,,,,o In the o ~ . ~ . . . . . . ,,,1. . . . . j . . . . . . . . . . . . . . This is not the only possibility, however. As noted previousiy, LEED shows that the geometry of the CO adlayer on Pt(ll 1) changes with coverage. At low coverages the LEED pattern is disordered. However, at 0 = 1 / 3 a diffuse × v~'R30 ° pattern is observed. Further increases in cove,'~ge cause the LEED pattern to be continuously comr~essed until at 0 = 0.5 0.e. saturation) a c(4 x 2) pattern is seen. Such geometric changes might a]s~ b~ ~e~p~n~ihle for the coverage dependency of the singleton frequency,
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Admittedly, at present, it is not clear theoretically to what extent geometric changes affec~ ~,he singleton frequency. Calculations for CO on Pd single crystals imply that the C - O stretching frequency is relatively insensitive to site geometry [23]. In contrast, calculations for CO on Ni clusters suggests a change in the site geometry can produce a significant change in the degree of d-back-bonding [24]. Changes in the degree of d-back-bonding would produce a significant change in the singletun frequency. As a result, it is not known to what extent geometric effects will cause changes in the singleton frequency. At the same time, there is an apparent correlation between the changes in singleton frequency observed here and the geometric changes in the CO layer seen in LEED. For example, fig. 5 shows that the singleton frequency shifts only about 4 em-1 at low coverages. This corresponds to the range where a disordered LEED pattern is seen, and not much can be said about ~.lle site geometry. Near 0 --- 1 / 3 up to saturation, which is where compression of the overlayer is seen, a significant chemical shift of 11 cm -1 is observed. Compression of the ovedaye: would be expected to weaken the C - O bond for reasons similar to those -~:. . . . . . . ~ ~k . . . . D~,.~,I , h ~ , ,h.:. i,*,~r~,-*i,,,~ h,~tween adjacent molecules are repulsive, so forcing them closer together could induce significant changes in the binding of CO. These binding energy changes should be reflected in the C - 0 stretching frequency. At present, it is unclear to what extent geometric and electronic effects are contributing to the obser~red changes in the singleton frequency. However, since both effects could produce changes in the singleton frequency, it is not surprising that changes in the singleton frequency are observed.
C.W. Olsen, R.L Masei / IR study of CO adsorption on Pt(l l l)
454
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Fig. 6. Comparison of the dipole shifts measured here to the shifts one would estimate using the procedure of Crossley and King [4]. (,'-) Measured shifts, (o) shifts calculated by the procedure o? Crossley and King. The dashed line is a least-squares fit to the measured shifts. The solid line is a least squares fit to the shifts calculated by the procedure of Crossley and King.
Given that the singleton frequenc/is changing, one does have to reconsider whether islands are forming during CO adsorption on Pt(111). As noted previously, Crossley and King argued that islands form during CO adsorption on Pt(100) and Pt(111) because they found that the shift in the CO IR band due to the dipole interactions between adjacent CO's did not go to zero in the limit of zero coverage. Note, however, that Crossley and King did not actually measure the dipole shift as a function of coverage. Instead they calculated a dipole shift assuming that the singleton frequency was constant. The results here show that the singleton frequency actually varies significantly with coverage. Hence, one needs to re-evaluate Crossley and King's conclusions. Fig. 6 shows a plot of the actual dipole shift measured here as a function of coverage. Dipole shifts calculated by the procedure of Crossley and King are included for comparison. Notice that the actual dipole shift varies roughly linearly with coverage. The best f:,t line though the data goes approximately to zero in the limit of zero coverage. Thus, it does not appear that there are significant shifts due to dipole ~nteractions between adjacent CO's in the limit of zero coverage. Hence, Crossley and King's arguments for island formation do not seem to apply. One should consider w h a t the absence of significant dipole shifts tells us about the adsorption of CO. Many years ago, Hammaker, Francis and
C W. Olsen, R.L Masel / IR study of CO adsorption on Pt(lll)
455
Eischens [7] showed that the dipole shift, p 2 ~$2, can be calculated approximately from , _
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where v is the frequency of the riCO band, in the absence of 13CO, vs is the singleton frequency, 81~/8r is the dynamic dipole moment, Mr is the reduced mass of the oscillator, and R u is the distance between molecules i and j+ Note that the sum in eq. (2) should go over all of the atoms in the lattice. Fig. 7 shows how the dipole shift, calculated from eq. (2) should vary with cluster size for a series of isolated clusters on the surface. The experimental data are included for comparison. Notice that if islands form, one would expect to observe a significant dipole shift, even in the limit of zero coverage. Thus, the fact that no significant shift is observed in the zero coverage limit, implies that there are no islands on the surface in the limit of zero coverage. coverage (CO molecules/Pt atom) 0.10
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Fig. 7. A comparison o f the experimental dipole shift to the dipole shift t h a t one would calculate if there were a series o f noninteract hag clusters o f C O ,. u. .u. .~. u. .,.~. on "'-~,,~o. ......¢_^~ . . . . . . .,rh~ . . . . . .M,;~ ~nes . ~r~o the results of calculations which assumed that the C O layer had a V~ × V~'R30 o structure. T h e d a s h e d lines are the results of calculations which assumed that the C O layer had a c ( 4 x 2) structure. ~ateractions between C O molecules in adjacent clusters were ignored. The triangles a r e the experimental data. T h e actual calculations to generate the figure were done using the m e t h o d of H a n u n a k e r et al. [7]. However, we have also d o n e calculations using the methods of M a h a n a n d L u c a s [25] Persson a n d R y b e r g [26] and Scheffler [27] and virtually identical results were obtained.
456
C W. Oisen, R.I. Masel / IR study of CO adsorption on Pt(lll)
The situation is less clear at finite coverages. A dipole shift is observed at all finite coverages. Thus, one cannot tale out the presence of islands without further analysis. In order to do the analysis, one needs a model. Crossley and King proposed a model which assumed that CO formed clusters of known size and structure on the surface and that there were no interactions between adjacent clusters even at high coverages. Fig. 7 shows a plot of the dipole shifts which one would predict from Crossley and King's model as a function of island size. The experimental shifts as a function of coverage are included for comparison. The figure shows that one could explain the dipole shifts in fig. 6 by assuming that CO was clustering on the surface, and that the average cluster size increases from 2 to > 100 molecules with increasing coverage. The same conclusions are obtained if one uses the methods of Mahan and Lucas [25], Persson and Ryberg [26] or Scheffler [27] to calculate the dipole shift. Mahan and Lucas, Persson and Ryberg, and $cheffler each propose different ways of calculating the equivalent of the lattice sum in eq. (2). However, one still has to fit an effective dynamic dipole moment to experimental data before one can calculate an accurate dipole shift. In our calculations the dynamic dipole moment was adjusted so each of the methods gave the correct dipole shift at full coverage. The dipole shift was then calculated as a function of the island size. However, the results were virtually identical ~o those in fig. 7 no matter which of the methods was used. Hence the conclusion that, if there are isolated islands, the island size must vary from 2 to 100 molecules with coverage is independent ~,~ which of the methods is used to calculate the dipole shift. One should, however, consider whether it is appropriate to assume there are no interactions between adjacent clusters on the surface. This is probably a reasonable assumption when all of the CO clusters are well separated from each other. Notice, however, that fig. 7 shows that one needs to have 12 atom clusters to get the shift that is observed at a coverage of 25% of saturation. However, one can show that it is impossible to pack enough 12 molecule clusters on Pt(111) to get 25% of saturation and still keep the clusters far enough apart to neglect any interactions between them. Hence, Crossley and King's model is probably not a good representation of the data reported here. An alternate model is to assume that (1) CO, or ch~sters of CO0 randomly fill a series of fixed adsorption sites on the surface, and that (2) the strength of the interaction between adjacent clusters is given by the equations in refs. [7,25-27]. Fig. 8 shows how the dipole shift should vary with cluster size if this model is correct. The figure actually shows results calculated using the method of Hammaker et al. [7]. However, again virtually identical results were obtained when the dipole shifts were calculated using the methods of Mahan and Lucas [25], Persson and Kyberg [26], or Scheffler [27]. Notice that the dipole shift is predicted to be coverage dependent even in the absence of
C W. Olsen, R.L Masel / IR study of CO adsorption on Pt(lll)
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percent saturation Fig. 8. A comparison of the experimental dipole shift to the dipole shift that one would calculate if there were a series of CO clusters which randomly fill the known CO adsorption sites on the P t ( l l l ) surface and the dipole interactions between adjacent clusters were considered in the calculations. The solid lines are the results of calculations which assumed that the CO layer randomly filled a ~f3 x V ~ ' R 3 0 ° structure while the dashed i~-.es are the results of calculations which assumed that the CO layer randomly filled a c(4 × 2) structure. The curved line is the result of a calculation which assumed that the surface was completely covered by a mixture of xfJ ×¢~'R30 ° and c ( 4 x 2 ) domains with no bare spots. The actual calculations to generate the figure were done using the method of Hammaker et al. [7]. However, we have also clone calculations using the methods of Mahan and Lucas [25] and Persson and Ryberg [26] and Scheffler [27] and virtually identical results were obtained.
islands because as the coverage increases the interactions between adjacent clusters grow. The figure shows calculations for the random adsorption of single CO molecules, and for random adsorption of CO dimers, trimers and quadrimers on Pt(111). Independent cflculations are shown for a CO overlayer with a V~ x V'3R30 ° structure with random bare patches, a CO overlayer with a c(4 × 2) structure with random bare patches, and a structure with random domains of ¢~" x ~ R 3 0 ° and c(4 × 2) structure. One can see from the figure that sig, lficant dipole interactions can be expected at moderate coverages even for a random adsorption of individual molecules (ringlets) on the v/3 x~/3-R30 ° sites of the surface. The calculated dipole interaction increases if even small ordered clusters are formed. This effect, however, is small unless the clusters become quite large. The calculations also indicate that from moderate to high coverages there should be significant dipole interactions in the adsorbed layer even in the absence of organized clustering. Hence, the presence of a dipole shift at high coverages does not necessarily imply that islands form.
458
C.W. Oisen, R.L Masel / IR study of CO adsorption on Pt(l l 1)
A comparison of the data to the calculations in fig. 8 shows that there is little clustering of the CO on P t ( l l l ) . The low coverage data in fig. 8 all fall between the lines corresponding to cluster sizes of I to 4 molecules. Within the accuracy of the data it is not clear whether any clustering is actually occurring. There could be some small clusters of perhaps 2 or 3 molecules on the surface. However, in contrast to what has been suggested previously, there is no evidence for large scale clustering of CO on the Pt(111) surface at low coverage. The situation is less clear at moderate to high coverages. There is a significant dipole shift at moderate to high coverages. While this shift could be explained if islands form on the surface, there is no reason to insist that islands form. Notice that even at high coverage the actual shift observed here is of the magnitude that one would expect if the CO adsorbed randomly on the surface. Admittedly at the highest coverages used here, island formation would not make a large change in the measured CO peak position. Hence, one cannot rule out the presence of islands at the highest coverages used here. However, there is no reason to suspect that islands form. Afterall, as noted in the introduction, the existing TPD, LEED and isosteric heat data are not consistent with island formation. In fact the best picture of our results is that there are a series of sites for CO adsorption on the P t ( l l l ) surface, and that the CO's adsorb randomly on these sites. Notice that at low coverage, the data here are best explained if it is assumed that the CO's adsorb randomly on the surface. If the CO adsorbed randomly, the LEED pattern would be diffuse, as is observed. Of course, the LEED pattern begins to show V~ x v~R30 ° spots when the coverage reaches 40%-50% of saturation. Notice, however, that the data in fig. 8 fall fight along the line that one expects for random adsorption on a V~ x f 3 R 3 0 ° lattice. There is no cddence for organized clustering of CO. At coverages between 60% and 80% of saturation things get more complicated. L E E D shows evidence for a compression structure between a V~ x v~-R.~0 ° structure and a c(4 x 2) structure, while the data here all fall between the lines for random adsorption on the V~ x v~R30 ° and c(4 x 2) lattices. The shifts are slightly larger than one would expect if the surface were completely covered by domains of V~ × v ~ R 3 0 ° and c(4 x 2) CO. However, if one assumes that the are some bare spots on the surface and that some extra CO's are compressed onto the c.(4 x 2) sites to compensate, one can explain all of the observed shifts. LEED [1] shows the presence of c(4 × 2) spots even before the ~./3"3× . ~ R 3 0 ° structure is filled (i.e. below 67% of saturation). Hence, it seems that domains of f 3 x v~R30 ° and c(4 × 2) can coexist even when there are bare patches on the surface. Of course, the bare patches fill up at coverages above 80% of saturation. The data fall on the line expected for a c(4 x 2) structure. A c(4 × 2) structure is seen in LEED. There is no evidence for islands. However, as noted above,
C W. Olsen, R.I. Masel / IR study of CO adsorption on Pt(lll)
459
islands cannot be ruled out at coverages approaching saturation, since island fociiiation would not make a large difference in the position of the linear IR band. Still, there is no reason to suppose that island form; all of the data can be explained if it is assumed that there is a random adsorption of CO molecules onto the c(4 × 2) sites on the Pt(111) surface.
5. Conclusions The adsorption of CO on Pt(111) has been re-examined with infrared spectroscopy. Both the linear and bridge bound modes were observed. The linear band shifted from 2084.0 to 2094.7 cm-1 with increasing coverage while the bridge band remained around 1850 cm -~. Isotopic dilution experiments done for various coverages showed that the shift m the linear CO band is actually composed of a 14.4 cm-1 downward shift due to chemical effects and a 25.1 cm- ~ upward shift due to dipole interactions. There is no evidence for CO islands at low coverage. The situation is less clear at high coverage. However, all of the results are completely consistent with a random filling of adsorption sites with single CO molecules up to high coverage and no island formation.
Acknowledgements This work was supported by the National Science Foundation under Grant CPE 83-51648 and by the General Motors Corporation, Chevron Research Corporation and Shell USA. Sample preparation was done using the facilities of University of Illinois Center of Microanalysis of Materials, which is supported, as a national facility, under National Science Foundation Grant D M R 86-12860.
References [1] [2] [3] [4] fql
[6] [7] [8] [9] [10] [11]
G. Ertl, M. Neumann and K. Streit, Surface Sci. 64 (1977) 393. H. Steininger, S. Lehwald and H. Ibach, Surface Sci. 123 (1982) 264. R.W. McCabe and L.D. Schmidt, Surface Sci. 66 (1977) 101. A. Crossley and D.A. King, Surface Sci. 68 (1977) 528; 95 (1980) 131. I[/ T-.1"~rf~ and w Pfitchard, J. Ph,~ ~ P a . d ~ gs i1077"~ CA-Ifi4. H J . Krebs and H. Luth, Appl. Phys. 14 (1977) 337. R.M. Hammaker, S.A. Francis and R.P. Eischens, Spectrochim. Acta 21 (1965) 1295. W.F. Banholzer, R.E. Parise and R.I. Masel, Surface Sci. 155 (1985) 653. R. Ryberg, Surface Sci. 114 (1982) 627. A. Ortega, F.M. Hoffmann and A.M. Bradshaw, Surface Sci. 119 (1982) 79. P. Hollins and J. Pritchard, Surface Sci. 89 (1979) 486.
460 [12] [13] [14] [15] [16] [17] [18]
C. IV. Olsen, R~L Masel / IR study of CO adsorption on Pt(l l l )
D.P. Woodruff, B.I~. Hayden, K. Prince and A.M. Bradshaw, Surface Sci. 123 (1982) 397. W.F. Banholzer and R.I. Masel, Surface Sci. 132 (1984) 339. B.E. Hayden and A.M. Bradshaw, Surface Sci. 125 (1983) 787. A.M. Bradshaw and F.W. Hoffmann, J. Catalysis 44 (1976) 328. R. Shigeishi and D.A. King, Surface Sci. 58 (1976) 379. K. Horn, M. Hussain and L Pritchard, Surface Sci. 63 (1977) 244. M.D. Baker and M.A. Chester, in: Vibrations at Surfaces, Eds. R. Candano, J.M. Gilles and A.A. Lucas (Plenum, New York, 1982) p. 289. [19] P. Hollins and J. Pritchard, Progr. Surface Sci. 19 (1985) 275. [20] G. Biyholder, J. Phys. Chem. 68 (1964) 2772. [21] M.A. Chester~ J. Pfitchard and M.L. Sims, in: Adsorption-Desorption Phenomena, Ed. P. Ricca (Academic Press, London, 1972) p. 277. [22] P. Hoilins and J. Pritchard, ACS Symp. Ser. 137 (1980) 51. [23] G. Doyden and G. Ertl, Surface" Sci. 43 (1974) 197. [24] G. Blyholder, J. Phys. Chem. 79 (1975) 756. [25] G.D. Mahan and A.A. Lucas, J. Chem. Phys. 68 (1978) 1334. [26] B.N.J. Persson and R. Ryberg, Phys. Rev. B 23 (1981) 6954; B.N.J. Persson and A. Liebsch, Surface Sci. 110 (1981) 356. [27] M. Scheffler, Surface Sci. 81 (1979) 562. [28] P.S. Bagus and W. Muller, Chem. Phys. Letters 115 (1985) 540.
Surface Science 201 (1988) 444-460 North-Holland, Amsterdam
AN INFRARED STUDY OF CO ADSORPTION ON P t ( l l l ) C.W. OLSEN and R.I. MASEL * Chemical Engineering Department, University of Illinois 1209 W. California Street, Urbana, IL 61801, USA Received 9 September 1987; accepted for pubfication 1 March 1988
Infrared spec~oscopy of isotopic mixtures is used to examine island formation during carbon monoxide (CO) adsorption on Pt(111) at 300 K. It was found that the linear CO band shifts from 2084.0~cm -1 at low coverage to 2094.7 cm -1 at saturation. This shift can be decomposed into a 14.4 cm -1 downward shift due to changes in the binding of the CO with coverage, and a 25.1 cm-1 upward shift due to dipole interactions within the adlayer. Analysis of the measured band shifts with coverage and isotopic dilution shows that no islands are formed upon CO adsorption on P t ( l l l ) at low coverage. The interpretation of the data is less clear at high coverage. However, all of the obsm'vations can be explained without invoking island formation.
1, Introduction The adsorption of CO on Pt(111) is one of the most heavily studied systems in the surface science literature. Nonetheless, there are still several unresolved questions about the CO adsorption process. One outstanding question is whether islands form when CO adsorbs on Pt(111) at 300 K. Infrared (IR) data has been inte,'preted as indicatiag that when the CO adsorbs on Pt(111), the CO molecules cluster into highly ordered islands even at low coverage [4-6]. The coverage dependency of the sticking coefficient has also been interpreted as suggesting island formation [2], although the presence of a mobile precursor state could account for the observed behavior as well [4]. Yet, temperature programmed resorption (TPD) and isosteric heat data [1-5] indicate that the binding energy of the CO decreases with increasing coverage. The TPD data have been interpreted as indicating that the interactions between adjacent CO's are repulsive. Island formation would be difficult to understand if the interactions between the adjacent CO's were repulsive. Further, at room temperature, low energy electron diffraction (LEED) shows diffuse spots characteristic of a high degree of disorder in the adsorbed layer [1]. At low temperatures, a series of distinct LEED patterns are seen [2]. It is difficult to explain the observed LEED patterns if highly ordered islands form. * To whom correspondence should be addressed.
0039-6028/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
C.W. Olsen, R.L Masel / IR study of CO adsorption on Pt( l l l )
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Thus, there is an apparent contradiction between the IR results which suggest island formation, and the TPD, isosteric heat, and LEED results which do not show behavior consistent with island formation. Much of the evidence for island formation comes from a pioneering paper [4] by Crossley and King where IR was used to examine CO adso~tion on Pt(100) and P t ( l l l ) . Previous work by Hammaker, Francis and Eischens [7] showed that if island formation occurs there should be an increase in the coupling between the vibrating dipoles on adjacent molecules in the adlayer. The increase in the dipole coupling produces a shift in the position of the CO IR band which can be detected experimentally. Of course, the position of the CO absorption band is also affected by changes in the bonding between the adsorbed molecule and the surface. These changes are called chemical effects and can be significant in some systems [8-12]. However, Crossley and King pointed out that if a 12CO layer were diluted with 13CO, at fixed total coverage, the binding of the ~2CO would be independent of the isotopic ratio. Nonetheless, the strength of the dipole interactions change, since the vibrational coupling between ~2CO and ~3CO is weak. As a result, an isotopic dilution technique can be used to isolate the dipole shifts from the chemical effects. Consequently, Crossley and King proposed that one should be able to tell whether islands form when CO adsorbs on metal surfaces using the isotopic dilution technique. Crossley and King [4] used the tecl~nique to examine CO adsorption on Pt(100) and P t ( l l l ) . On Pt(100), they fcand that at saturation coverages the linear 12CO band shifted by 35 cm -1 with isotopic dilution. In contrast, the linear 12CO band only shifted by 17 cm -1 in going from low coverages to saturation. At the time Crossley and King did these measurements, it was thought that if there were a chemical shift, the shift with coverage should have been larger than the shift due to dipole effects alone [20]. Yet, Crossley and King were observing a smaller shift with coverage than with isotopic dilution. Consequently, Crossley and King suggested that the chemical shifts were negligible, and that the whole shift observed with coverage was caused by dipole coupling alone. Crossley and King then noted that if there were no chemical shifts the singleton frequency (the frequency of a 12CO molecule isolated from other riCO's) would be independent of coverage. As a result, one could calculate the dipole shift as a function of coverage directly from their data. The calculation indicated that L,hedipole interaction between adjacent CO's did not go to zero in the limit of zero coverage. Notice, however, that it should have gone to zero if the CO molecules adsorb randomly on the surface. The shift would not go to zero in the zero coverage limit only if CO islands form. As a result, Cross~ey and King concluded that islands form when CO adsorbs on Pt(100). Crossley and King also applied the isotopic dilution technique to CO adsorption on a (111) oriented platinam foil. The foil data were more
446
C. IV. Olsen, R,L Masel / IR study of CO adsorption on P t ( l l l )
complicated than those on Pt(100). There were two bands; a large band with characteristics simil&r tO the main band on Pt(100) and a smaller shoulder, whose intensity varies with adsorption conditions [141. The main band showed the same characteristics as those described above. The shift with coverage was less than the shift with isotopic dilution. Crossley and King also noted that the shift with isotopic dilution was greater than the shift with coverage measured by Horn and Pritchard [5] for CO adsorption on a P t ( l l l ) single crystal. As a result Crossley and King argued that islands also form on P t ( l l l ) . Crossley and King went on to present a method for estimating the island dimensions from vibrational spectra. Over the years, there have been several papers which applied Crossley and King's technique to other systems. It has been found there are some subtleties in the isotopic dilution experiment which were not apparent when Crossley and King did their work [19]. For example, it is now known that there are often important chemical effects when CO adsorbs on transition metals [8-12]. The chemical effects can cause either upward or downward shifts in the singleton frequency with coverage. The shifts with coverage earl be smaller than the shifts with isotopic dilution even in the presence of significant chemical effects. As a result, one has to be careful before concluding that the singleton frequency for CC,. dsorbed on P t ( l l l ) is not varying with coverage. In fact, it is quite likely th,.t the singleton frequency of CO adsorbed on P t ( l l l ) is coverage dependent. Note that the binding energy of CO on P t ( l l l ) varies significantly with coverage [1-3]. Low energy electron diffraction (LEED) shows that the structure of the adsorbed layer changes from V~× ¢~'R30 ° geometry at low coverage to a c(4 × 2) geometry at saturation [1]. Singleton frequencies are dependent on geometry and bond energy. Thus, there is reason to suspect that the IR spectrum of CO adsorbed on Pt(111) should show chemical shifts with coverage. If there are chemical shifts, then one has to reconsider the interpretation of Crossley and King's data. With this in mind, CO adsorption on Pt(111) was reexamined with IR. The procedure was similar to Crossley and King's, except that the isotopic dilution experiment was done at many coverages so it would be possible to see if the singleton frequency were coverage dependent. The data were then used to see whether islands form when CO adsorbs on Pt(111). Modifications to the techniques used to estimate island dimensions from IR data are also suggested.
2. ~xperimental The experiments reported here were conducted using the apparatus and procedures described previously [8,13]. The apparatus consisted of an ultra-high vacuum chamber with an operating base pressure of 2 × 10 -1° Torr, inter-
C W. Oisen, R.I. Masei / IR study of CO adsorption on Pt(lll)
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faced to a Nicolet Fourier transform infrared spectrometer. The spectrometer allowed infrared spectra of submonolayer quantities of adsorbates to be obtained. Resolutions of much less than a wavenumber were possible. However, most of the experiments reported here were run at a resolution (FWHM) of 4 c m -I. The Pt(lll) crystalused here was cut from a Metron singlecrystalrod. The rod was aligned to within 0.5° of the (111) plane using Lane back reflection.A sample was then cut from the rod, polished with diamond paste and cleaned in I atm of flowing oxygen at 900 ° C for 24 h. The sample was then washed in hydrofluoric acid, nitricacid and acetone and mounted in the vacuum system. The sample was then cleaned using a cycle of argon ion bombardment followed by annealing and heating in I0 -6 Torr oxygen at 1000°C. Isotopic dilution experiments were done by dosing with a mixture of 12CO and 13CO. The 13CO was obtained from Stohler Isotope Chemicals at 99,% isotopic purity. There was some difficultywith exchange between 12CO and 13C O on the chamber walls during dosing. As a result,it was never possible to get pw~,re13CO onto the crystal.However, isotopicratios of up to 4/1 ~3CO to 12C O could be obtained. The isotopic ratios reported below are those actually measured with ~ Analvac quadrupole mass spectrometer.
3. Results
Fig. 1 shows the development of the CO infrared absorption band with coverage at room temperature. There are two main CO bands centered at 1850 and 2090 cm-2, respectively. They are at the positions expected for linear and two-fold bridge bound CO. There is also some evidence for the weak third band around 1820 cm -1. It is at the position expected for triply coordinated CO. All three bands are similar to those reported previously [14]. The frequency and coverage dependence (2084.0-2094.7 cm-1) of the linear band are in good agreement with previous work [4-6, 14-16]. The bridging mode is quite broad and weak (0.1%) at high coverage which is consistent with the few RAIRS studies reporting a bridging mode for CO on Pt(111) [6,14,18]. The triply coordinated band is weaker still; extensive noise reduction was required to resolve the band fully and that was not the purpose of our work. Hayden and Bradshaw report a small shoulder to higher energies on the linear CO band after adsorption at 300 K. The shoulder was thought to be associated with defec.'s. "i'nere is no evidence for the shoulder in "'-m~-u~t~J=~-u,:- !~.~:~ 1. A shoulder could be observed after adsorption of CO on Pt(lll) at 90 K. On a freshly sputtered sample, the intensity of the shculder was nearly the same as that of the main feature at very low coverages. However, in all case~:, the shoulder was much less intense than the one reported by Hayden and Bradshaw and it disappeared upon annealing. All of the data here were taken
448
C. IV. Olsen, R.L Masei / IR study of CO adsorption on Pt(l 11)
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C W. Olsen, R.I. Masel / IR study of CO advorption on Pt(lll)
449
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under conditions where the shoulder was absent. Hence, it appears that our sample had an unusually low defect density. The effects of isotopic dilution were Lnvestigated following the method of Crossley and King [4]. Fig. 2 shows a series of isotopic dilution spectra taken at a 6 L exposure. The 12CO band shows a shift from 2077 to 2092 cm -1 in going from 25~ to 100% 12CO while the 13CO band shifts from 2036 so 2027 cm-1. Notice also that the peak intensities do not reflect the isotopic composition (intensity stealing) and the bands broaden upon dilution. Both results are characteristic of the presence of significant dipole coupling within the CO layer. Thus, it appears that there are substantial dipole interactions on the surface at 6 L exposure. Fig. 3 shows the effects of isotopic dilu';,..~ at low coverage. In contrast to the results in fig. 2, the i2CO peak in fig. 3 only sh;~fts about I cm -1 in going from 30% to 100% 12CO. A 1 cm -1 shift is actually smaller than the scatter observed in the data and hence, ma~,' not be due to dipole coupling interactions. Thus, it does not appear that significant dipole coupling is occurring at these low coverages even though significant dipde interactions were seen at high coverages. Conseq,.~entl2~, it appears that the strength of the dipole interactions varies significantly with coverage. One can quantify the effects by using data such as that shown in figs. 2 and 3 to estimate the ~ g ! e t o n f~,~ac~cy as a function of coverage. The most eoaaaon way to estimate singleton frequencies is to empirically extrapolate the position of the linear 12CO band to infinite dilution. However, such a procedure does not work well unless very dilute ~2CO mixtures can be
C W. Olsen, IL1. Masel / IR study of CO adsorption on Pt(l l l)
450
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obtained. Experimentally, dilute 1 2 C O mixtures are difficult to obtain at low coverages due to exchange in the doser. Consequently, an alternative procedure was used to analyze the data here. The procedure comes about because at low to moderate coverages the frequency of the 12CO absorption band, ~,, should vary with the fraction of 12CO ha the layer, D, according to: v2= Vs2 + KD,
(1)
where Ps is the singleton frequency and K is a constant which depends on the coverage and the geometry of the adlayer, provided the vibrational coupling between the 1 2 C O and 13CO is negligible. Hence, one can obtain a fairly accurate value of the singleton frequency, Ps, if one uses a least-squares procedure to fit eq. (1) to isotopic dilution data. The data here has been analyzed in this manner, and the results are presented in fig. 4. The points in the figure are the measured positions of the linear 12CO band as a function of isotopic dilution while the lines are the best fit to the dz, a using eq. (1). Notice that quite good agreement between the data and eq. (1) has been obtained. Thus, it appears that eq. (1) gives a good representation of the experimental results. According to eq. (1), the Lntercepts of the ~nes in fig. 4 correspond to the singleton frequency at each coverage. Notice that the singleton frequencies vary from 2082 to 2069.6 cm -1 with increasing coverage. Thus, it seems that the singleton frequency is indeed coverage dependent. Notice also that the lines in the figure cross. "?he crossing of the lines is further evidence that the singleton frequencies are coverage dependent.
C.W. Olsen, R.L Masel / IR study of CO adsorption on Pt(l 11)
451
The figure also shows that the strength of the dipole shifts varies substantially with coverage. Notice that with a total exposure of 20 L, the linear 12CO band shifts from 2069.6 to 2094.7 c m - ~ in going from infinite dilution to 100% 12CO. At a constant total exposure, there are no chemical shifts, so the whole shift in the linear 12C0 band must be caused by a change in the dipole interactions between adjacent ]2CO's. As a result, we conclude that at saturation, (20 L) there is a 25.1 cm -1 dipole shift in the frequency of the linear 12CO band. Notice, however, that the size of the shift decreases with decreashlg coverage until at 0.2 L the shift is only about 2 cm-1. Again, a 2 c m - ] shift is small enough to be attributed to experimental error. Thus, it is apparent that while there are strong dipole interactions at moderate to high coverages, the dipole interactions do aot appear to be important at low coverages. It should tdso be mentioned that virtually the same conclusions are reached if the analysis above is done on the 13CO band rather than ]2CO band. The 13CO band exhibits a - 14.2 cm -1 chemical shift with coverage. This compares to the - 1 4 . 4 cm -1 chemical shift with 12CO. The lSCO band also exhibits a 26.6 c m - ] dipole shift at high coverage. This compares to the 25.1 cm-~ dipole shift seen with lZcO. For a completely decoupled system the shifts in the 12CO and 13CO bands should be equal, and these numbers are close enough to say that equality was nearly achieved. The results of this analysis are finally summarized in fig. 5 which shows the contributions of dipole interactions and chemical effects to the observed shift as a function of coverage. The figure clearly shows that the rather small shift observed with coverage is actually composed of two larger opposing shifts. Thus, one does have to consider both shifts in the analysis of the data.
4. Discussion Looking back at the data reveals that all of the results presented here are in good agreement with those of previous investigators. The peak positie~ls and their dependence on the isotopic ratio reported here are virtually the same as those reported by Crossley and King [4] and Hayden and Bradshaw [14]. The saturation coverage singleton frequency found here is about 3 cm -1 higher than that reported by Crossley and King. Crossley and King, however, did their work on a {lll}-oriented ribbon, not a (111) crystal. There was an extra ~ 1 v~a~, ," ~- t a,1L..~ c .C'_.'1 t u . spectt-arn, and so a deconvolution procedure was needed to calculate the position of the main CO band. Hence, it is not surprising there is a small difference between Crcssley and King's results and those reported here. Qualitatively, our results are very similar to Crossley and King's which suggests that our results are characteristic of Pt(111). Of course, we do observe that the singleton frequency is coverage depen-
452
C W. OIsen, R.I. Masel / IR study of CO adsorption on Pt(ill)
dent. Crossley and King did their isotopic dilution experiments at constant (saturation) coverage. They then assumed that the singleton frequency did not change with coverage. As noted previously, since Crossley and King did their original work, it has become apparent that singleton frequencies are often coverage dependent. Coverage dependent singleton frequencies have been observed during CO adsorption on Pt(410) [8], C u ( l l l ) [11], Cu(ll0) [12,17], Cu(100) [9], and Pd(100) [10], for example. The binding of CO is known to change with coverage on Pt(111). Thus, it is not surprising that the singleton frequency is coverage dependent. Of course, the data in fig. 4 show that the singleton frequency decreases by about 15 cm -1 with increasing coverage. A decreasing singleton frequency indicates a weakening of the C - O bond relative to the gas phase. The observation of a singleton frequency which decreases with increasing coveraged is somewhat unexpected. Many years ago, Blyholder [20] developed a model for the changes in the singleton frequency that occur with increasing coverage. This model asserts that changes in the singleton frequency will arise from a reduction in metal back-donation into the CO 2~r* antibonding orbital due to increased competition for the metal d-electrons with coverage. Note, however, that Blyholder's model predicts that the singleton frequency increases with increasing coverage. It cannot account for downward shifts such as those observed here. Of course, downward shifts in the singleton frequency have previously been observed during CO adsorption on Cu(111) [11], and polycrystalline silver and gold [21]. Pritchard and co-workers [21,22], explained the downward shifts by suggesting that CO forms a bond which is almost entirely o in character with copper, silver and gold. Pritchard et al. then argued that with increasing coverage there is an accumulation of surface charge. The accumulation of surface charge lowers the heat of adsorption of the CO. The charge accumulation can be relieved through metal back-donation into the CO 2~r* orbital. The result is a decrease in the singleton frequency with coverage. Bagus et al. [28] calculated the shift expected for such a model, and showed that one can get an increase or a decrease in the singleton frequency due to such effects. It is not clear whether Prichard's model wov.ld apply to CO on P t ( l l l ) . However, the heat of adsorption of CO on Pt(111) decreases with ;r,creasing ccverage [5] in a manner consistent with Prichard's model. Thus, Prichard's model can •.Av--- ,**,..,,.a~,,,,o In the o ~ . ~ . . . . . . ,,,1. . . . . j . . . . . . . . . . . . . . This is not the only possibility, however. As noted previousiy, LEED shows that the geometry of the CO adlayer on Pt(ll 1) changes with coverage. At low coverages the LEED pattern is disordered. However, at 0 = 1 / 3 a diffuse × v~'R30 ° pattern is observed. Further increases in cove,'~ge cause the LEED pattern to be continuously comr~essed until at 0 = 0.5 0.e. saturation) a c(4 x 2) pattern is seen. Such geometric changes might a]s~ b~ ~e~p~n~ihle for the coverage dependency of the singleton frequency,
C HI. Olsen, R.L Masel / IR study of CO adsorption on Pt(l[l) ,
i
t
i _
i
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,
i
t
,
i
,
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,
i
i
i
i
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453 i
A o b s e r v e d s h i l l f r o m 2 0 8 4 clx* t o c h e m i c a l c o n L r i b u L i o n Lo Lhe s h i l l o dipole conLribution to the shill
20 ¸
i~-10
-20
-30
0.01
,
,
,
,O, 10 i . . . .
O.PO . . . .
0.30 ' . . . .
0 . 4 '0
. . . .
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coverage Fig. 5. The eon:ributions o f dipole coupling a n d -~hemical interactions to the total shift of the linear C O b a n d with increasing coverage. T h e p o i n t s are the data. T h e lines are the best fits to the data.
Admittedly, at present, it is not clear theoretically to what extent geometric changes affec~ ~,he singleton frequency. Calculations for CO on Pd single crystals imply that the C - O stretching frequency is relatively insensitive to site geometry [23]. In contrast, calculations for CO on Ni clusters suggests a change in the site geometry can produce a significant change in the degree of d-back-bonding [24]. Changes in the degree of d-back-bonding would produce a significant change in the singletun frequency. As a result, it is not known to what extent geometric effects will cause changes in the singleton frequency. At the same time, there is an apparent correlation between the changes in singleton frequency observed here and the geometric changes in the CO layer seen in LEED. For example, fig. 5 shows that the singleton frequency shifts only about 4 em-1 at low coverages. This corresponds to the range where a disordered LEED pattern is seen, and not much can be said about ~.lle site geometry. Near 0 --- 1 / 3 up to saturation, which is where compression of the overlayer is seen, a significant chemical shift of 11 cm -1 is observed. Compression of the ovedaye: would be expected to weaken the C - O bond for reasons similar to those -~:. . . . . . . ~ ~k . . . . D~,.~,I , h ~ , ,h.:. i,*,~r~,-*i,,,~ h,~tween adjacent molecules are repulsive, so forcing them closer together could induce significant changes in the binding of CO. These binding energy changes should be reflected in the C - 0 stretching frequency. At present, it is unclear to what extent geometric and electronic effects are contributing to the obser~red changes in the singleton frequency. However, since both effects could produce changes in the singleton frequency, it is not surprising that changes in the singleton frequency are observed.
C.W. Olsen, R.L Masei / IR study of CO adsorption on Pt(l l l)
454
c o v e r a g e (CO m o l e c u l e s / P t 0.00
120000
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,
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,
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,
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80000
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percent saturation
Fig. 6. Comparison of the dipole shifts measured here to the shifts one would estimate using the procedure of Crossley and King [4]. (,'-) Measured shifts, (o) shifts calculated by the procedure o? Crossley and King. The dashed line is a least-squares fit to the measured shifts. The solid line is a least squares fit to the shifts calculated by the procedure of Crossley and King.
Given that the singleton frequenc/is changing, one does have to reconsider whether islands are forming during CO adsorption on Pt(111). As noted previously, Crossley and King argued that islands form during CO adsorption on Pt(100) and Pt(111) because they found that the shift in the CO IR band due to the dipole interactions between adjacent CO's did not go to zero in the limit of zero coverage. Note, however, that Crossley and King did not actually measure the dipole shift as a function of coverage. Instead they calculated a dipole shift assuming that the singleton frequency was constant. The results here show that the singleton frequency actually varies significantly with coverage. Hence, one needs to re-evaluate Crossley and King's conclusions. Fig. 6 shows a plot of the actual dipole shift measured here as a function of coverage. Dipole shifts calculated by the procedure of Crossley and King are included for comparison. Notice that the actual dipole shift varies roughly linearly with coverage. The best f:,t line though the data goes approximately to zero in the limit of zero coverage. Thus, it does not appear that there are significant shifts due to dipole ~nteractions between adjacent CO's in the limit of zero coverage. Hence, Crossley and King's arguments for island formation do not seem to apply. One should consider w h a t the absence of significant dipole shifts tells us about the adsorption of CO. Many years ago, Hammaker, Francis and
C W. Olsen, R.L Masel / IR study of CO adsorption on Pt(lll)
455
Eischens [7] showed that the dipole shift, p 2 ~$2, can be calculated approximately from , _
=
(2)
'
where v is the frequency of the riCO band, in the absence of 13CO, vs is the singleton frequency, 81~/8r is the dynamic dipole moment, Mr is the reduced mass of the oscillator, and R u is the distance between molecules i and j+ Note that the sum in eq. (2) should go over all of the atoms in the lattice. Fig. 7 shows how the dipole shift, calculated from eq. (2) should vary with cluster size for a series of isolated clusters on the surface. The experimental data are included for comparison. Notice that if islands form, one would expect to observe a significant dipole shift, even in the limit of zero coverage. Thus, the fact that no significant shift is observed in the zero coverage limit, implies that there are no islands on the surface in the limit of zero coverage. coverage (CO molecules/Pt atom) 0.10
0.00
I
120000
t
I
,I
0.20 I
I
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I
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e x p e r i m e n t a l shift
maximum
for c(4x2)
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Fig. 7. A comparison o f the experimental dipole shift to the dipole shift t h a t one would calculate if there were a series o f noninteract hag clusters o f C O ,. u. .u. .~. u. .,.~. on "'-~,,~o. ......¢_^~ . . . . . . .,rh~ . . . . . .M,;~ ~nes . ~r~o the results of calculations which assumed that the C O layer had a V~ × V~'R30 o structure. T h e d a s h e d lines are the results of calculations which assumed that the C O layer had a c ( 4 x 2) structure. ~ateractions between C O molecules in adjacent clusters were ignored. The triangles a r e the experimental data. T h e actual calculations to generate the figure were done using the m e t h o d of H a n u n a k e r et al. [7]. However, we have also d o n e calculations using the methods of M a h a n a n d L u c a s [25] Persson a n d R y b e r g [26] and Scheffler [27] and virtually identical results were obtained.
456
C W. Oisen, R.I. Masel / IR study of CO adsorption on Pt(lll)
The situation is less clear at finite coverages. A dipole shift is observed at all finite coverages. Thus, one cannot tale out the presence of islands without further analysis. In order to do the analysis, one needs a model. Crossley and King proposed a model which assumed that CO formed clusters of known size and structure on the surface and that there were no interactions between adjacent clusters even at high coverages. Fig. 7 shows a plot of the dipole shifts which one would predict from Crossley and King's model as a function of island size. The experimental shifts as a function of coverage are included for comparison. The figure shows that one could explain the dipole shifts in fig. 6 by assuming that CO was clustering on the surface, and that the average cluster size increases from 2 to > 100 molecules with increasing coverage. The same conclusions are obtained if one uses the methods of Mahan and Lucas [25], Persson and Ryberg [26] or Scheffler [27] to calculate the dipole shift. Mahan and Lucas, Persson and Ryberg, and $cheffler each propose different ways of calculating the equivalent of the lattice sum in eq. (2). However, one still has to fit an effective dynamic dipole moment to experimental data before one can calculate an accurate dipole shift. In our calculations the dynamic dipole moment was adjusted so each of the methods gave the correct dipole shift at full coverage. The dipole shift was then calculated as a function of the island size. However, the results were virtually identical ~o those in fig. 7 no matter which of the methods was used. Hence the conclusion that, if there are isolated islands, the island size must vary from 2 to 100 molecules with coverage is independent ~,~ which of the methods is used to calculate the dipole shift. One should, however, consider whether it is appropriate to assume there are no interactions between adjacent clusters on the surface. This is probably a reasonable assumption when all of the CO clusters are well separated from each other. Notice, however, that fig. 7 shows that one needs to have 12 atom clusters to get the shift that is observed at a coverage of 25% of saturation. However, one can show that it is impossible to pack enough 12 molecule clusters on Pt(111) to get 25% of saturation and still keep the clusters far enough apart to neglect any interactions between them. Hence, Crossley and King's model is probably not a good representation of the data reported here. An alternate model is to assume that (1) CO, or ch~sters of CO0 randomly fill a series of fixed adsorption sites on the surface, and that (2) the strength of the interaction between adjacent clusters is given by the equations in refs. [7,25-27]. Fig. 8 shows how the dipole shift should vary with cluster size if this model is correct. The figure actually shows results calculated using the method of Hammaker et al. [7]. However, again virtually identical results were obtained when the dipole shifts were calculated using the methods of Mahan and Lucas [25], Persson and Kyberg [26], or Scheffler [27]. Notice that the dipole shift is predicted to be coverage dependent even in the absence of
C W. Olsen, R.L Masel / IR study of CO adsorption on Pt(lll)
0.00 120000
,
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m
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,
coverage
(CO m o l e c u l e s / P t
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,
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,
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-I-" ~ .
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,,'
40000
20000
0
i
0
i
i
i
20
+
i
i
40
i
i
i
i
i
60
I
80
+
'
i
i
100
percent saturation Fig. 8. A comparison of the experimental dipole shift to the dipole shift that one would calculate if there were a series of CO clusters which randomly fill the known CO adsorption sites on the P t ( l l l ) surface and the dipole interactions between adjacent clusters were considered in the calculations. The solid lines are the results of calculations which assumed that the CO layer randomly filled a ~f3 x V ~ ' R 3 0 ° structure while the dashed i~-.es are the results of calculations which assumed that the CO layer randomly filled a c(4 × 2) structure. The curved line is the result of a calculation which assumed that the surface was completely covered by a mixture of xfJ ×¢~'R30 ° and c ( 4 x 2 ) domains with no bare spots. The actual calculations to generate the figure were done using the method of Hammaker et al. [7]. However, we have also clone calculations using the methods of Mahan and Lucas [25] and Persson and Ryberg [26] and Scheffler [27] and virtually identical results were obtained.
islands because as the coverage increases the interactions between adjacent clusters grow. The figure shows calculations for the random adsorption of single CO molecules, and for random adsorption of CO dimers, trimers and quadrimers on Pt(111). Independent cflculations are shown for a CO overlayer with a V~ x V'3R30 ° structure with random bare patches, a CO overlayer with a c(4 × 2) structure with random bare patches, and a structure with random domains of ¢~" x ~ R 3 0 ° and c(4 × 2) structure. One can see from the figure that sig, lficant dipole interactions can be expected at moderate coverages even for a random adsorption of individual molecules (ringlets) on the v/3 x~/3-R30 ° sites of the surface. The calculated dipole interaction increases if even small ordered clusters are formed. This effect, however, is small unless the clusters become quite large. The calculations also indicate that from moderate to high coverages there should be significant dipole interactions in the adsorbed layer even in the absence of organized clustering. Hence, the presence of a dipole shift at high coverages does not necessarily imply that islands form.
458
C.W. Oisen, R.L Masel / IR study of CO adsorption on Pt(l l 1)
A comparison of the data to the calculations in fig. 8 shows that there is little clustering of the CO on P t ( l l l ) . The low coverage data in fig. 8 all fall between the lines corresponding to cluster sizes of I to 4 molecules. Within the accuracy of the data it is not clear whether any clustering is actually occurring. There could be some small clusters of perhaps 2 or 3 molecules on the surface. However, in contrast to what has been suggested previously, there is no evidence for large scale clustering of CO on the Pt(111) surface at low coverage. The situation is less clear at moderate to high coverages. There is a significant dipole shift at moderate to high coverages. While this shift could be explained if islands form on the surface, there is no reason to insist that islands form. Notice that even at high coverage the actual shift observed here is of the magnitude that one would expect if the CO adsorbed randomly on the surface. Admittedly at the highest coverages used here, island formation would not make a large change in the measured CO peak position. Hence, one cannot rule out the presence of islands at the highest coverages used here. However, there is no reason to suspect that islands form. Afterall, as noted in the introduction, the existing TPD, LEED and isosteric heat data are not consistent with island formation. In fact the best picture of our results is that there are a series of sites for CO adsorption on the P t ( l l l ) surface, and that the CO's adsorb randomly on these sites. Notice that at low coverage, the data here are best explained if it is assumed that the CO's adsorb randomly on the surface. If the CO adsorbed randomly, the LEED pattern would be diffuse, as is observed. Of course, the LEED pattern begins to show V~ x v~R30 ° spots when the coverage reaches 40%-50% of saturation. Notice, however, that the data in fig. 8 fall fight along the line that one expects for random adsorption on a V~ x f 3 R 3 0 ° lattice. There is no cddence for organized clustering of CO. At coverages between 60% and 80% of saturation things get more complicated. L E E D shows evidence for a compression structure between a V~ x v~-R.~0 ° structure and a c(4 x 2) structure, while the data here all fall between the lines for random adsorption on the V~ x v~R30 ° and c(4 x 2) lattices. The shifts are slightly larger than one would expect if the surface were completely covered by domains of V~ × v ~ R 3 0 ° and c(4 x 2) CO. However, if one assumes that the are some bare spots on the surface and that some extra CO's are compressed onto the c.(4 x 2) sites to compensate, one can explain all of the observed shifts. LEED [1] shows the presence of c(4 × 2) spots even before the ~./3"3× . ~ R 3 0 ° structure is filled (i.e. below 67% of saturation). Hence, it seems that domains of f 3 x v~R30 ° and c(4 × 2) can coexist even when there are bare patches on the surface. Of course, the bare patches fill up at coverages above 80% of saturation. The data fall on the line expected for a c(4 x 2) structure. A c(4 × 2) structure is seen in LEED. There is no evidence for islands. However, as noted above,
C W. Olsen, R.I. Masel / IR study of CO adsorption on Pt(lll)
459
islands cannot be ruled out at coverages approaching saturation, since island fociiiation would not make a large difference in the position of the linear IR band. Still, there is no reason to suppose that island form; all of the data can be explained if it is assumed that there is a random adsorption of CO molecules onto the c(4 × 2) sites on the Pt(111) surface.
5. Conclusions The adsorption of CO on Pt(111) has been re-examined with infrared spectroscopy. Both the linear and bridge bound modes were observed. The linear band shifted from 2084.0 to 2094.7 cm-1 with increasing coverage while the bridge band remained around 1850 cm -~. Isotopic dilution experiments done for various coverages showed that the shift m the linear CO band is actually composed of a 14.4 cm-1 downward shift due to chemical effects and a 25.1 cm- ~ upward shift due to dipole interactions. There is no evidence for CO islands at low coverage. The situation is less clear at high coverage. However, all of the results are completely consistent with a random filling of adsorption sites with single CO molecules up to high coverage and no island formation.
Acknowledgements This work was supported by the National Science Foundation under Grant CPE 83-51648 and by the General Motors Corporation, Chevron Research Corporation and Shell USA. Sample preparation was done using the facilities of University of Illinois Center of Microanalysis of Materials, which is supported, as a national facility, under National Science Foundation Grant D M R 86-12860.
References [1] [2] [3] [4] fql
[6] [7] [8] [9] [10] [11]
G. Ertl, M. Neumann and K. Streit, Surface Sci. 64 (1977) 393. H. Steininger, S. Lehwald and H. Ibach, Surface Sci. 123 (1982) 264. R.W. McCabe and L.D. Schmidt, Surface Sci. 66 (1977) 101. A. Crossley and D.A. King, Surface Sci. 68 (1977) 528; 95 (1980) 131. I[/ T-.1"~rf~ and w Pfitchard, J. Ph,~ ~ P a . d ~ gs i1077"~ CA-Ifi4. H J . Krebs and H. Luth, Appl. Phys. 14 (1977) 337. R.M. Hammaker, S.A. Francis and R.P. Eischens, Spectrochim. Acta 21 (1965) 1295. W.F. Banholzer, R.E. Parise and R.I. Masel, Surface Sci. 155 (1985) 653. R. Ryberg, Surface Sci. 114 (1982) 627. A. Ortega, F.M. Hoffmann and A.M. Bradshaw, Surface Sci. 119 (1982) 79. P. Hollins and J. Pritchard, Surface Sci. 89 (1979) 486.
460 [12] [13] [14] [15] [16] [17] [18]
C. IV. Olsen, R~L Masel / IR study of CO adsorption on Pt(l l l )
D.P. Woodruff, B.I~. Hayden, K. Prince and A.M. Bradshaw, Surface Sci. 123 (1982) 397. W.F. Banholzer and R.I. Masel, Surface Sci. 132 (1984) 339. B.E. Hayden and A.M. Bradshaw, Surface Sci. 125 (1983) 787. A.M. Bradshaw and F.W. Hoffmann, J. Catalysis 44 (1976) 328. R. Shigeishi and D.A. King, Surface Sci. 58 (1976) 379. K. Horn, M. Hussain and L Pritchard, Surface Sci. 63 (1977) 244. M.D. Baker and M.A. Chester, in: Vibrations at Surfaces, Eds. R. Candano, J.M. Gilles and A.A. Lucas (Plenum, New York, 1982) p. 289. [19] P. Hollins and J. Pritchard, Progr. Surface Sci. 19 (1985) 275. [20] G. Biyholder, J. Phys. Chem. 68 (1964) 2772. [21] M.A. Chester~ J. Pfitchard and M.L. Sims, in: Adsorption-Desorption Phenomena, Ed. P. Ricca (Academic Press, London, 1972) p. 277. [22] P. Hoilins and J. Pritchard, ACS Symp. Ser. 137 (1980) 51. [23] G. Doyden and G. Ertl, Surface" Sci. 43 (1974) 197. [24] G. Blyholder, J. Phys. Chem. 79 (1975) 756. [25] G.D. Mahan and A.A. Lucas, J. Chem. Phys. 68 (1978) 1334. [26] B.N.J. Persson and R. Ryberg, Phys. Rev. B 23 (1981) 6954; B.N.J. Persson and A. Liebsch, Surface Sci. 110 (1981) 356. [27] M. Scheffler, Surface Sci. 81 (1979) 562. [28] P.S. Bagus and W. Muller, Chem. Phys. Letters 115 (1985) 540.