Langmuir 1994,10,3663-3667
3663
Reflection Infrared Study of the Xe-CO Interaction on Pt(335): A New Method for Site Assignment of Chemisorbed Species Jiazhan Xu, Peter N. Henriksen,? and John T. Yates, Jr.* Surface Science Center, Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 Received April 4,1994. I n Final Form: August 5, 1994@ TheinteractionofCO andXeonPt(335){Pt[4(lll)x(lOO)l} hasbeenstudiedbytemperature-programmed desorption (TPD) and infrared reflection-absorption spectroscopy (IRAS). TPD results show desorption peaks at -400 K and -500 K from the pure CO layer. The high-temperature peak is related to CO adsorbed on step sites, while the low-temperature peak is related mainly to CO adsorbed on terrace sites. IRAS studies,performed on CO layers after annealingto 290 K, show four subsequentlyappearingvibrational modes: -2075 and -1880 cm-l for low Oco, and -2092 and -1900 cm-' for high Oco. Experiments involving Xe coadsorption and isotopic CO site exchange have been conducted, and the four vibrational modes are assigned to terminal and bridged CO adsorbed on step sites for low OCO and on terrace sites for high Oco, respectively. The stepwise sequence for CO adsorption on Pt(335) is thus: terminal-bound CO followed by bridge-bound CO on step sites at low Oco, then terminal-bound CO followed by bridgebound CO on terrace sites at high Oco. Altogether, this study demonstrates the usefulness of Xe coadsorption in identifylng adsorption sites as well as the adsorption sequence.
1. Introduction Identification of reaction sites for adsorption and catalysis on stepped single crystals has been a long-term goal of surface science.' It is generally accepted that adsorption is preferential at step sites on metal surfaces because of the coordinately unsaturated atoms a t these sites. However, contrary behavior was found in a recent study of NO adsorbed on Pd(112) where NO adsorption favored terrace sites over step^.^,^ Therefore, to identify adsorption sites, and to characterize these sites for catalytic reactions, it is necessary to use experimental techniques that identify and follow the sequential steps of the adsorption process. Xenon is helpful when probing adsorption sites since it interacts with both the substrate and a d ~ o r b a t e . ~For -~ example, when Xe is coadsorbed with CO, there is a reduction in intensity of the CO absorption band as well as a red shift in the CO vibrational f r e q ~ e n c y . ~These -~ effects are mainly due to the physical interaction between Xe and its neighboring CO molecule. Since Xe adsorbs preferentially on uncovered step sites,'~~ it is possible to probe CO on step sites by measuring the red shift and reduction in intensity of the CO vibrations. This paper reports the use of this technique to probe the interaction between chemisorbed CO and Xe as a function ofcoverage at both step and terrace sites on F't(33.5). t Permanent address: Department of Physics, The University of Akron, Akron, OH 44325. Abstract published in Advance ACS Abstructs, September 15, 1994. (1)Wandelt, K. In Physics and Chemistry of Solid SurfacesVZZC Vanselow, R., Howe, R., Eds.; Springer Series in Surface Sciences Springer: Berlin, 1990; Vol. 22,p 289. (2) Gao, Q.; Ramsier, R. D.; Waltenburg, H. N.; Yates, J. T., Jr. J. Am. Chem. SOC.1994,116,3901. (3)Ramsier, R. D.;Gao, Q.; Waltenburg, H. N.; Yates, J. T., Jr. Submitted for publication in Surf. Sci. (4) Xu,Z.; Sherman, M. G.; Yates, J. T., Jr.; Antoniewicz, P. R. Surf. Sci. 1993,276,249. (5) Hoffmann, F. M.; Lang, N. D.; Nmskov, J.K.Surf.Sci.Lett. 1990, 226,L48. (6)Ehlers, D.H.; Esser, A. P.; Spitzer, A.; Luth, H. Surf. Sci. 1987, 191,466. (7)Siddiqui, H.R.;Chen, P. J.; Guo, X.;Yates, J. T., Jr. J. Chem. Phys. 1990,92,7690. (8)Rettner, C. T.;Bethune, D. S.; Schweizer, E. K. J. Chem. Phys. 1990,92,1442.
The chemisorption of CO on F't single crystals is one of the most thoroughly investigated systems in surface science. A general feature of this system is that CO adsorbs first on step sites followed by adsorption on terrace sites at high c ~ v e r a g e . ~ -These l~ sites are identified through the vibrational bands associated with terminal CO adsorption. The vibrational frequency assigned to CO terminally adsorbed on step sites is -2075 cm-', and that of terminal CO adsorption on terrace sites is -2100 ~ m - l . ~ - lVibrational l modes associated with bridged CO are not well defined, and, prior to this report, no unambiguous assignment of bridged CO on terrace sites has been made.g In this report temperature-programmed desorption (TPD) and infrared absorption-reflection spectroscopy (IRAS) were used, in conjunction with isotopic CO site exchange and Xe coadsorption, to study the adsorption of CO on Pt(335). As a result, four vibrational modes are assigned to terminal and bridged CO adsorption on both step and terrace sites. A complete stepwise sequence is thus qualitatively revealed where terminal CO adsorption is followed by bridged CO adsorption on step sites a t low &o, and then, at high Oca, terminal CO adsorption is followed by bridged CO on terrace sites.
2. Experimental Section Experimentswere performed in an ultrahighvacuum chamber with a base pressure < 1 x 10-10 mbar as described previ0us1y.l~ Briefly, the system contains facilities for Auger electron spec-
@
(9)Luo, J.s.;Tobin, R. G.; Lambert, D. K.; Fisher, G. B.; DiMaggio, C. L. Surf.Sci. 1992,274,53, and references therein. (10)butt-Robey, J. E.;Doren, D. J.; Chabal, Y. J.; Christ", S. B.
J. Chem. Phvs. 1990.93.9113. (11)Hayclen, B. E.: Kretzschmar, K.; Bradshaw, A. M.; Greenler, R. G. Surf.Sci. 1985,149,394. (12)McClellan, M. R.;Gland, J. L.; McFeeley, F. R.Surf.Sci. 1981, 112,63. (13)Henderson, M. A.; Szab6, A.; Yates, J. T., Jr. J. Chem. Phys. 1989,91,7245. (14)Henderson, M.A.; Szab6, A.; Yates, J. T., Jr. J. Chem. Phys. 1989,91,7255. (15)Henderson, M. A.;Szab6, A.; Yates, J. T., Jr. Chem Phys. Lett. 19w),168,51. (16)Banholzer, W. F.;Parise, R. E.; Masel, R. I. Surf.Sci. 1985,155, RF;R VVV.
(17)Trenary, M.; Uram, K. J.; Yates, J. T., Jr. Surf.Sci. 1985,157, 512.
0743-7463/94/2410-3663$04.50/00 1994 American Chemical Society
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3664 Langmuir, Vol. 10, No. 10, 1994
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Figure 1. Temperature-programmed desorption of CO on Pt(335). Heating rate was 1.3Ws. CO was dosed at 90 K. The coverages were determined with respect t o the saturation coverage of 0.63 CO/Pt.$ troscopy (AES),X-ray photoelectron spectroscopy (XPS),low energy electron diffraction (LEED),temperature-programmed desorption (TPD), infrared reflection-absorption spectroscopy (IRAS),and work function (A41 measurements. IRASmeasurementswere performed using a modified Fourier transform infrared spectrometer (Mattson Cygnus 100) which contained a polarizerwith the polarization axis orientedparallel to the normal t o the crystal surface.l8 A liquid-nitrogen cooled MCT detector was used to collect the spectra. Each spectrum was averaged over 900 scans (or 200 scans)at 4 cm-l resolution. The peak frequency could be measured to hO.1 cm-l, as shown by the smoothness of our data during increasing Xe adsorption. TPD measurements were performed using a UTI lOOC quadrupole mass spectrometer with multichannel computer control. The heating rate for annealing and TPD measurement was 1.3 Ws. Coverages were measured relative to saturation coverage using TPD. The CO (99.99%)and Xe (99.995%)gases were obtained from Matheson in glass flasks. The R(335) single crystal was a 9 mm diameter and 1 mm thick disk, oriented to be within 0.5" of the (335) direction. Prolonged cycles ofAr+ sputtering,oxidation,and annealingwere performed to clean the surface. A typical cycle includes (1) sputtering, 1000eVAr+,1.6pNcm2,1800sat room temperature, (2)oxidation,1x lO-5mbar 02,900s at 1000K, and (3)annealing, 1273K, 900 s. The surface ordering and the step structure were confirmed by the LEED pattern giving the expected beam splitting for this plane. The surface cleanliness was checked by AES and the maximum impurity levels of 0.004 atomic fraction for Ca and 0.01 atomicfraction for C were measured in the depth of Auger ~amp1ing.l~
3. Results 3.1. Temperature ProgrammedDesorptionMeasurements of CO on Pt(335). The desorption of CO adsorbed on Pt(335)was studied by TPD with a heating rate of 1.3 Ws and the results are shown in Figure 1.The coverage measurements are based on a saturation coverage of 0.63 COI'P~.~There are two desorption features with one peaked at -400 K and another at -500 K. At low coverage (Figure la-c), only the high-temperture desorption peak exists, and the peak temperature shifts downward as coverage increases. Upon increasing the coverage (Figure Id-f), an additional low-temperature (18)Xu, Z.; Yates, J. T.,Jr. J. Vac. Sei. Technol. 1990,A8, 3666. (19)Davis, L.E.; MacDonald, N. C.; Palmberg, P. W.; Weber, G. E. Handbook ofAuger Electron Spectroscopy, 2nd ed.;Perkin-Elmer: Eden Prairie, MN, 1976.
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Figure 2. I U S studies of CO stepwise adsorption on Pt(335) monitoring adsorption site occupancy. The spectra were averagedover 900 scans at 4 cm-l resolution. The spectra were taken at 90 K after annealing the adsorbate-crystal surface to 290 K. desorption peak appears. These results are consistent with previous TPD reports in both temperature dependence and relative peak area.9J1-15 The high-temperature desorptionpeak is related to chemisorbed CO on step sites while the low-temperature desorption peak is related to CO on terrace sites, as will be discussed in section 4. 3.2. IRAS Study of CO Adsorption on Pt(335). IRAS studies of the stepwise sequence of CO adsorption on Pt(335) are shown in Figure 2. The CO coverage, measured separately from the IRAS studies, was determined using TPD. For IRAS measurements, the sample was dosed at 90 K, heated to 290 K for annealing, and then cooled back to 90 K to obtain the spectra which were averaged over 900 scans at 4 cm-' resolution. Throughout the coverage range there are four clearly resolved bands corresponding to different species subsequently appearing on the surface. At low coverage (Figure 2a-b), the vibrational band at -2075 cm-I is due to terminal CO adsorbed on step sites. As the coverage increases (Figure 2c-d), a vibrational band at -1880 cm-' appears which is due to bridged CO adsorbed on step sites. At higher coverage (Figure 2e-0, terminal CO starts to populate the terrace sites as indicated by the vibrational band at -2092 cm-l. Finally, bridged CO on terrace sites produces a band at -1900 cm-l (Figure 2g-h). These assignments are consistent with other studies on stepped Pt surface^^-^^ and will be discussed in section 4.1. For both the terminal and bridged CO species it is noted that as the coverage increases the absorption bands shift from lower to higher frequencies for each species. This is due to intermolecular vibrational coupling effects at higher coverage on both step and terrace sites.20 Figure 3 shows the effect of site exchange between isotopic CO species on Pt(335). The spectra were averaged over 900 scans at 4 cm-l resolution at 90 K. The insertion (20) HoEmann, F. M. Surf. Sei. Rep. 1983,3,107.
Xe-CO Znteraction on Pt(335) I
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Langmuir, Vol. 10, No. 10, 1994 3665 I
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Wavenumber (cm-I) Figure 3. Isotope mixing experiment of CO on Pt(335) monitoring adsorption site exchange. The spectra were averaged over 900 scans at 4 cm-l resolution at 90 K. is a TPD spectrum of CO at saturation coverage. Figure 3a shows the IRAS spectrum of CO(29) on Pt(335) after saturating the surface with CO(29) and annealing to 445 K to desorb the phase responsible for the low-temperature desorption peak. There are vibrational modes at -2034 and -1838 cm-l associated with the high-temperature desorption feature. These two vibrational modes correspond to terminal and bridged CO(29) on the step sites. Addition of CO(2S) onto the C0(29)/Pt(335) surface gives rise to another vibrational mode which is due t o terminal CO(28) on terrace sites. Subsequent heating of the crystal to 290 K causes the CO isotopes to exchange sites, producing a total of six vibrational modes. In the terminal CO range (2000-2100 cm-l), two higher frequency modes are related to CO(28) and CO(29) on terrace sites, while two modes a t lower frequency are related to CO(28) and CO(29) on step sites. CO(28) has a higher vibrational frequency than CO(29) due to its smaller reduced mass. The modes in the range 1800-1900 cm-' are due to bridged CO on the step sites (see section 4.1 for details on this vibrational mode assignment). 3.3. Effect of Xe on the CO Adsorption. The effect of physisorbed Xe on chemisorbed CO on Pt(335)has been studied and the results are shown in Figure 4. The spectra were averaged over 200 scans a t 4 cm-' resolution a t 90 K. The CO coverage, measured separately using TPD, is 0.24 monolayer (ML). Figure 4a shows the CO spectrum after annealing to 290 K. The vibrational mode a t -2090 cm-l is due to terminal CO on terrace sites and the vibrational mode a t -2070 cm-l is due to terminal CO on step sites. The 1890-cm-l mode is due to bridged CO on the step sites. Xe is then added in a controlled manner at 90 K. As the Xe coverage increases, both the CO vibrational frequency and intensity of all the three vibrational modes decrease. This result is in general agreement with other s t ~ d i e s . ~ - ~ Figure 5 shows the effect of Xe coadsorption on the vibrational frequency of terminal CO a t both step and terrace sites. These data are obtained from experiments similar to those shown in Figure 4. The dotted lines are drawn to indicate the deviation of the v(C0) frequency shift from linearity for CO adsorbed on terrace sites. The inserts show the effect oflow Xe coverage on the vibrational frequencies of CO at step and terrace sites. From these inserted graphs it is seen that the frequency of the step-
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Wavenumber (cm-'1 Figure 4. IRAS study of the effect of Xe on CO molecule on Pt(335). The spectra were averaged over 200 scans at 4 cm-l at 90 K. 2100 2095
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Figure 5. CO vibrational frequency vs Xe exposure onPt(335). The data were obtained at 90 K. The dotted line is drawn as a basis for seeing the deviation from linearity.
adsorbed CO is shifted linearly as a function of Xe exposure, while the frequency of the terrace-adsorbed CO is initially almost unaffected and then gradually shifts to a linear dependence on Xe exposure for exposures greater than 0.7 x 10%m2. These results are in agreement with the fact that Xe prefers to adsorb on step sites-even if covered with CO, thus influencing the step-bound CO at the onset of Xe adsorption, and having little effect on terrace-bound CO before moderate amounts of Xe are adsorbed. Therefore, the interaction between Xe and terrace-bound CO is delayed while Xe first adsorbs on step sites. 4. Discussion 4.1. Assignment of CO Vibrational Modes. In this section, we discuss the rationale for the assignments of
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Xu et al.
3666 Langmuir, Vol. 10, No. 10, 1994 the CO vibrational modes given in section 3. As shown in Figure 2, there are a total of four subsequently appearing C-0 vibrational bands. The assignment of CO vibrational modes is related to the higher binding energy of CO on the step sites, an effect related to lower metal atom selfcoordination on these sites. Thus a t low coverages, the vibrational modes observed are due to terminal and bridged CO on the step sites; these species are associated with the high-temperature desorption peak in the TPD data (Figure 3). As CO coverage increases, two later appearing vibrational modes are related to terminal and bridged CO on the terrace sites. The ability to discern bridged CO species on terrace sites and step sites is complicated by possible CO-CO interactional effects and should be considered tentative a t this point. Additional evidence for the assignment comes from the study of the effect of physisorbed Xe on CO adsorption on Pt(335) (Figure 5) and the isotope site exchange experiments (Figure 3). Many studies reveal that Xe prefers to adsorb on the step defect sites.7,8,21The binding energy ofXe on uncovered step sites is about 3.0 kcaYmol higher than Xe on the terrace For coadsorption of CO and Xe on a a metal surface, the Xe-CO interaction is shown to be attractive (-1 kcal/mol on Ni(lll)).4 Therefore, it is expected that the preferential adsorption site of Xe is on the step sites. Since Xe can interact with the neighboring CO molecule and cause the CO vibrational frequency and intensity to decrease, the vibrational mode (-2070 cm-l) influenced by Xe a t low coverage is assigned to CO on step sites. Another vibrational mode (-2090 cm-'), which is only significantly influenced by Xe a t higher coverages, is assigned to CO on the terrace sites. Site assignments of bridged CO using Xe coadsorption is not feasible since the modes are too broad and the intensity is too low. However, on another basis, we can assign the mode at -1875 cm-' to bridged CO on step sites since it appears with terminal CO on step sites as the CO coverage is systematically increased. Other evidence for the assignment of bridged CO on step sites is found in the isotope exchange experiment (Figure 31, where bridged CO(28) on step sites is not observed when CO(28) is adsorbed (at 90 K) after the adsorption of a CO(29) layer which previously saturated the step sites. Bridged CO(28) on step sites appears only after isotope site exchange. Another mode a t -1910 cm-I is assigned to bridged CO on terrace sites since its appearance is accompanied with the formation of terminal CO on terrace sites. 4.2. Stepwise Adsorption of CO on Pt(335). The stepwise adsorption sequence of CO on Pt(335) is shown clearly in Figure 2. Terminal CO on step sites appears first, followed by the appearance of bridged CO on step sites as CO coverage increases. Thus CO molecules prefer to adsorb on the step sites compared to the terrace sites. These two CO species on step sites are related to the hightemperature desorption peak (-500 K) in the TPD spectra. As the coverage increases, the terrace sites begin to populate. Again, terminal CO on terrace sites appears followed by bridged CO on terrace sites. This order of CO species formation also occurs on Pt(ll1) surfaces with terminal CO being followed by bridged CO. This qualitative picture of the CO stepwise adsorption sequence is in general agreement with the model proposed by Luo et al.9 One exception is that bridged CO on the step sites at saturation CO coverage is not predicted by their model. The binding strength of CO on Pt single crystals is dependent on both the CO 50 dative bond and the back donation into the 2x* orbital from the metal, in addition to the effect of intermolecular interactions. Preferential (21)Wandelt, K. Surf. Sei.1991,251/252,387, and references therein.
adsorption of CO on step sites indicates a stronger interaction between CO and the metal atoms a t step sites for both terminal and bridged CO. Terminal CO is more stable than bridged CO and the enthalpy difference (AH = Hb - Ht)decreases as CO coverage increase^.^^-^^ This effect may be due to direct repulsive interactions between CO molecules or indirect substrate mediated interactions. Therefore terminal CO appears first followed by bridged
co.
4.3. Interaction of Xe with CO on Pt(335). An examination of Figures 4 and 5 shows two features for the interaction of chemisorbed CO and physisorbed Xe on Pt(335): 1. The vibrational frequency of CO decreases as Xe exposure increases. The red shifl of step CO occurs a t low Xe exposure, while that of terrace CO shows a n exposure delay. This is due to the preferential adsorption of Xe on the step sites. 2. The vibrational intensity of CO decreases as the Xe exposure increases. By use of the spectral peak heights to indicate the intensity change, Xe causes a total of -62% attenuation in intensity of step CO species while only a -35% decrease occurs for terrace CO. The interaction of Xe with chemisorbed CO has been demonstrated to cause a red shift in the CO vibrational frequency and a decrease in the CO vibrational int e r m i t ~ . ~The - ~ red shift is mainly correlated with a decrease in the work function of the metal due to the adsorption of Xe and is a result of the positive outward electrostatic field associated with the polarized Xe which influences neighboring CO molecules. The reduction in CO infrared absorption band intensity occurs because coadsorbed Xe is polarized by the dynamic dipole field of the CO molecules. The net or effective dynamic dipole moment of the very weakly bound CO-Xe complex is reduced, thereby reducing the intensity of the CO absorption band. The larger decrease in intensity of step CO by Xe coadsorption may be due either to closer Xe-CO distance for step CO compared to terrace CO or to C-0 orientation changes caused by neighbor Xe. This effect is not due to the displacement of CO from step sites to terrace sites as shown by experiments with CO on the step sites only. 5. Conclusions Several results have been obtained by studying CO adsorption on bare and Xe covered Pt(335): 1. Temperature programmed desorption experiments show two CO desorption features on Pt(335). The hightemperature desorption peak is related to CO chemisorbed on the step sites and the low-temperature desorption peak is related to CO on the terrace sites. 2. Four vibrational bands appear subsequently in the I U S spectra. They are assigned to terminal and bridged CO on step and terrace sites, respectively. 3. The physical interaction of Xe with CO causes a red shift in CO vibrational frequency and a reduction in vibrational intensity. The interaction of step CO and Xe occurs a t low Xe coverage while a delay of the CO-Xe interaction is shown for terrace CO. The total attenuation in spectral peak intensity of step CO (-62%) is larger than that ofterrace CO (-35%) for the completeinteraction with Xe. (22)Mieher, W.D.;Whitman, L. J.;Ho, W. J.Chem. Phys. 1989,91, 3228,and references therein. (23)Hopster, H.; Ibach, H. Surf. Sei. 1978,77,109. (24)Olsen, C.W.; Masel, R. I. Surf. Sci. 1988,201,444.
Xe-CO Interaction on Pt(335) 4. The combination of IRAS and Xe adsorption experiments provides a new method for assigning adsorption sites on single crystal surfaces.
Acknowledgment. We thank Professor Roger Tobin of Michigan State University and Dr. David Lambert of
Langmuir, Vol. 10, NO. 10,1994 3667 the GM Research Laboratories for loaning US the Pt(335)crystal. J.X. acknowledges support of an A. W. Mellon Predoctoral Fellowship from the University of Pittsburgh. "his work is supported by the Department of Energy (DOE), Ofice of Basic Energy Sciences.
