Langmuir 1988, 4 , 289-293
289
CO/O Interaction on Cr(110): Spectroscopic Manifestation of Poisoning+ Neal D.Shinn Sandia National Laboratories, Division 1134, Albuquerque, New Mexico 87185 Received July 17, 1987. In Final Form: August 28, 1987 Studies of carbon monoxide and oxygen coadsorption on Cr(ll0)below 150 K using high-resolution electron energy loss and ultraviolet photoemission spectroscopies have found direct evidence of coadsorbate interactions that alter the CO dissociation pathway on Cr(ll0).These interactions include the blocking of a surface-stabilized CO dissociation intermediate (denoted by a1-C0) by preadsorbed oxygen and an oxygen-induced binding state conversion from the q - C O state to a terminally bonded state (denoted by a2-CO) at low temperatures. Quantitative analyses of these data imply that both electronic and ensemble (steric) effects are operative in these interaction phenomena.
Introduction Understanding both the dynamics of adsorption/desorption and the energetics of adsorbate interactions a t solid surfaces is a necessary prerequisite to the elucidation of the mechanisms of surface chemical processes. Although adsorbate-adsorbate interactions frequently are manifested in the kinetics of catalytic reactions (e.g., via the promotion or poisoning of a specific reaction step by a coadsorbate'), it is less common to observe, spectroscopically, changes in the adsorbate-surface bonding that can be correlated to the measured kinetic Several vibrational studies have identified coveragedependent shifts in the CO stretching frequency that are manifestations of interactions among CO molecules adsorbed in equivalent surface sites.- These are, of course, to be distinguished from vibrational shifts due to adlayer inhomogeneity. Also, dramatic changes in the vibrational spectrum of CO adsorbed in the presence of alkali adatoms have been reported for several metals,lD-l6with a comparable number of controversial explanations for these observations. The opposite is true for CO adsorption with electronegative adatoms; only subtle increases in the CO stretching frequency have been o b s e r ~ e d ,which ~ * ~ are attributed to a decrease in the m e t a l 4 0 27r* back-donation. This paper reports vibrational and photoemission data for mixed CO/O adlayers on Cr(ll0)which provide unambiguous evidence for coadsorbate electronic interactions that block the formation of a stable CO dissociation intermediate (al-CO)and, consequently, inhibit the dissociation of carbon monoxide. Although steric effects and direct competitition for surface adsorption sites can account for reduced sticking probabilities and subsequent variations in reaction kinetics, the stoichiometry of the CO/O interaction effects on Cr(ll0)and the low oxygen coverages involved imply that electronic interactions are responsible, a t least in part, for these observations. Results As in the previous Cr(llO)/CO&ndCr(llO)/Ochemisorption high-resolution electron energy loss spectroscopy (EELS), Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), electron-stimulated desorption (ESD), and ultraviolet photoemission spectroscopy (UPS)using synchrotron radiation were used in this CO/O investigation. Prior to a discussion of the mixed adlayer data, the Cr(llO)/CO and Cr(llO)/O Presented at the symposium entitled 'Molecular Processes at Solid Surfaces: Spectroscopy of Intermediates and Adsorbate Interactions", 193rd National Meeting of the American Chemical Society, Denver, CO, April 6-8, 1987.
0743-7463/88/2404-0289$01.50/0
chemisorption results are summarized. For CO coverages less than 0.25monolayer, where 1 monolayer is defined to be the surface atomic density of the ideal Cr(ll0)plane (1.705 X 1015 c m 3 , only one molecular binding state (al-CO) is observed. The atypical nature of this binding state is most evident in the EELS data summarized in Figure 1. Prior to CO doses, a small peak is observed at 595 cm-' (figure la), which is due to the residual 2-3% of a monolayer of surface oxygen."Jg Each vibrational frequency assignment reported in this paper corresponds to the frequency a t which the energy loss intensity is maximized in the raw data (as shown). The convolution of the natural vibrational line width (unknown) with the instrumental resolution (full width a t half-maximum = 65-90 cm-') of the electron energy loss spectrometer operating at a primary energy of 2.3 eV and the superposition of vibrational loss peaks on a sloping background a t low frequencies lead to vibrational assignments with an associated uncertainity of f15 cm-'. This resolution is sufficient to distinguish the two classes of chemisorbed carbon monoxide in this study of poisoning effects. For CO exposures of less than -0.8 langmuir (1langmuir = lo4 Torr s), carbon monoxide stretching frequencies are found in the range 1150-1330 cm-' (Figure (1) Goodman, D. W. Annu. Reu. Phys. Chem. 1986,37,425. (2)Gland, J. L.; Madix, R. J.; McCabe, R. W.; DeMaggio, C. Surf. Sci. 1984,143, 46. (3)Trenary, M.; Uram, K. J.; Yates, J. T. Jr. Surf. Sci. 1985,157,512. (4)Moskovits, M.; Hulse, J. E. Surf. Sci. 1978,78,397. (5)Hollins, P.; pritchard, J. Surf. Sci. 1979,89,486. (6)Crossley, A.;King, D. A. Surf. Sci. 1980,95,131. (7)Peramp, B. N. J.; Ryberg, R. Solid State Commun. 1980,36,1421. ( 8 ) Pfnur, H.; Menzel, D.; Hoffmann, F. M.; Ortega, A,; Bradshaw, A. M. Surf. Sci. 1980,93,431. (9)Persson, B. N.J.; Ryberg, R. Phys. Rev. B: Condens. Matter 1981, 24,6954. (10)Crowell, J. E.;Garfunkel, E. L.; Somorjai, G. A. Surf. Sci. 1982, 121,303. (11)Wallden, L.Surf. Sci. 1983,134,L513. (12)dePaola, R, A,; Hrbek, J.; Hoffmann, F. M. J. Chem. Phys. 1985, 82,2484. (13)Seip, U.; Bassignana, I. C.; Kuppers, J.; Ertl, G. Surf. Sci. 1985, 160,400. (14)Lackey, D.; Surman, M.; Jacobs, S.; Grider, D.; King, D. A. Surf. Sci. 1985,1521153,513. (15)Crowell, 3. E.;Somorjai, G. A. Appl. Surf. Sci. 1985,154,L261. (16)Dubois, L. H.; Zegarski, B. R.; Luftman, H. S. J . Chem. Phys., in press. (17)Shinn, N. D.;Madey, T. E. Phys. Reu. Lett. 1984,53,2481;J. Chem. Phys. 1985,83,5928. (18)Shm, N. D.; Madey, T. E. Phys. Reu. E: Condens. Matter 1986, 33, 1464. Shinn, N.D. Phys. Rev. B Condens. Matter submitted for publication. (19)Shinn, N. D.;Madey, T. E. Surf. Sci. 1986,173,379;1986,176, 635.
0 1988 American Chemical Society
Shinn
290 Langmuir, Vol. 4 , No. 2, 1988 Cr( 1 1O)ICO __
595495 \I
-1150
EELS 1
' r-1330
120K
- 1865
ENERGY LOSS
(cm-'I
Figure 1. High-resolutionelectron energy loss spectra of molecular CO adsorbed on Cr(ll0) at 120 K. Spectra shown are for (a) the sputter-annealedsurface and CO exposures of (b)0.3, (c) 0.7,and (d) 0.9 langmuir. I
lb), indicative of (1)substantial C-O bond weakening and (2) unusual Cr-CO bonding. Numerous experiments were performed to investigate the coverage dependence of these two vibrational features; no systematic correlation was observed in their behavior, suggesting that these are not two normal modes of the same species." At present, these peaks are believed to be due to inequivalent adsorbed CO molecules on the Cr(ll0) surface due to subtle variations in the surface morphology (defect concentrations) and residual contamination levels. Infrared absorption experiments are planned to examine these vibrational peaks in greater detail with much higher resolution. The reduction in vco to -1200 cm-' is inconsistent with the conventional Blyholder mode120 of CO 5a electron donation to the metal and synergistic electron backdonation from the metal d-states to the antibonding CO 27r* orbitals, analogous to the bonding in transition-metal carbonyl complexes.21 Further insights into the details of the CO-metal bonding are provided by recent theoretica122-uand e ~ p e r i m e n t awork, l ~ ~ but these studies all involve cases where the CO valence orbitals are much less perturbed- and the C-0 stretching frequency is lower than the gas-phase value of 2143 cm-' by only -200 cm-'-than in the case of al-CO on Cr(ll0). At 0.25 monolayer coverage, a 4 4 x 2 ) al-CO ordered overlayer is observed by LEED," a t which point the al-CO binding state is saturated. Subsequent addition of carbon monoxide results in the disordering of the overlayer and the onset of three new vibrational features in the EELS data (Figure lc,d): two new C-O stretching frequencies at 1865 and 1975 cm-l and a Cr-CO stretching mode a t 495 cm-l. These vibrational frequencies are indicative of carbonyl-bonded CO molecules on Cr(llO), analogous to CO chemisorption states on all other transition and noble metals studied to date.26 (20) Blyholder, G. J. Phys. Chem. 1964, 68, 2772. 1985, 107, 578. (21) Sung, SA.; Hoffman, R. J . Am. Chem. SOC. (22) Bagus, P. S.; Nelin, C. J.; Bauschlicher, C. W., Jr. Phys. Reu. E: Condens. Matter 1983,28, 5423. (23) Messmer, R. P. Surf. Sci. 1985, 158, 40. (24) Kao, C. M.; Messmer, R. P. Phys. Reo. B: Condens. Matter 1985, 31, 4835. (25) Houston, J. E.; Peden, C. H. F.;Feibelman, P. J.; Hamann, D . R. Surf. Sci. submitted for publication. (26) Yates, J. T., Jr.; Madey, T. E.; Campuzano, J. C. In The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis; King, D. A,, Woodruff, D. P., Eds.; Elsevier: Amsterdam, 1986.
I
I
I
I
1 6 1 4 1 2 10 6
I
I
I
6
4
2
BINDING ENERGY
I 0
I
(eV)
Figure 2. Angle-integrated ultraviolet photoemission spectra of CO chemisorbed on Cr(ll0) at 90 K. Inset shows experimental geometry for these data. CO exposures are (a) clean; (b) 0.4; (c) 0.8; (d) 1.0; (e) 1.6;(f)2.0; and (8) 3.0 langmuirs. Binding energies for the CO 4a photoemission peaks in the q - C O and cu&O molecular binding states are shown by dashed and solid vertical
lines, respectively.
Annealing studies'l have shown that molecules in the al-CO state dissociate at a lower surface temperature those in the terminally bonded a2-COstate, further supporting the designation of the al-CO state as a surface-stabilized dissociation intermediate. Angle-integrated,coverage-dependentUPS data,18summarized in Figure 2, confirm the sequential population of two molecular binding states at low temperature. The Cr 3d valence band (extending from the Fermi level to a binding energy of -3 eV) and a chromium satellite peak at -7-eV binding energy (corresponding to a final state with two d holes on the same atomn) are seen in the clean spectrum of figure 2a. Carbon monoxide features appear a t 7-8 and 11.6 eV (Figure 2b). The broad 7-8-eV peak is a composite of the CO la and CO 5a peaks, while the weak 11.6-eV peak is due to the nominally nonbonding CO 4a molecular orbital.% Beginning with the -0.8-langmuir CO exposure, Figure 2c, and becoming more evident after a 1.0-langmuirexposure, Figure 2d, two significant changes are seen: (1) the CO 4a peak center shifts to a lower binding energy (10.8 eV) and (2) the 4a peak intensity increases above that of the composite (la + 5a) band. These spectral changes have been analyzed in greater detail elsewhere.18 In addition to confifming the existence of the sequentially populated a-CO and a2-COmolecular binding states, these observations demonstrate that the chromium-CO bonding is significantly different for the two states. The first observation is particularly significant because the CO 4a orbital is spatially localized on the oxygen end of the molecule28and usually is not involved in the m e t a l 4 0 bonding.21 The peak intensity increase implies that either (27) Chandesris, D.; Lecante, J.; Petroff, Y. Phys. Reu. B: Condens. Matter 1983, 27, 2630. (28) Cotton, F.A.; Wilkinson, G. Aduanced Inorganic Chemistry, 3rd ed.; Interscience: New York, 1972.
COIO Interactions on Cr(110)
Langmuir, Vol. 4, No.2,1988 291
Real Space Models: Cr(1lO)iCO
(a)
,:
AES
(a)
O X Y G E N EXPOSURE I L )
I
(b)
a,CO
P ( 4 X 2 ) O MODEL
... ..... ..... .....
+ a,CO
J .
.
0
.
.
6 .
o . . .
0
. . 0 . .
T [ti01
0 . . . 0
. . 0 . .
o t l x l ) .D14X2)
L o o 1-, Figure 3. Models for the (a) ~ ( 4 x 2q-CO ) overlayer at 0.25 monolayer and the (h) mixed al-CO and arCO disordered overlayer for Eco > 0.25.
the q-CO and arc0 molecular orientations on the surface or their 40 differential photoionization cross sections are different;" significant differential photoionization cross section changes would not he unexpected in the case of a CO valence orbital rehybridization in the al-CO binding State. Thus molecular CO chemisorption on Cr(ll0) is characterized by the sequential population of a dissociation intermediate, q-CO, to form a ~ ( 4 x 2overlayer ) at a coverage of 0.25 monolayer, followed by the onset of terminally bonded a&O to result in a disordered mixed adlayer. Models for these overlayers are shown in parts a and b of Figure 3, respectively. It should be noted that both the local adsorption site and molecular orientation shown for al-CO (Le., inclined in the C , surface hollow sites) are speculative and as yet not mnfiied by experiment. Other authors as well have proposed that CO dissociation intermediates with weakened C-0 bonds on clean metal surfaces necessitate CO frontier orbital rehybridization and an associated inclined molecular axis.ws2 Conclusive experiments and ah initio theory investigating these models in detail are lacking. The results of oxygen chemisorption s t u d i e ~are ' ~ summarized in Figure 4. Oxygen dissociatively adsorbs at temperatures above 120 K until diffusion of the atomic oxygen into the lattice becomes rate limiting and surface oxygen blocks adsorption sites; this is reflected in the temperature-dependent saturation exposures of Figure 4a. Only one vibrational feature, a t 605 cm-l, is found for dissociative adsorption (Figure 4b), corresponding t o the Cr-0 stretching mode. An ordered overlayer is formed at a coverage of monolayer with the proposed structure of Figure 4c,which results in the LEED pattern of Figure 4d. The diffraction pattern is observed throughout the exposure range -1.0 1.6 langmuirs, which has been inte~preted'~ as being indicative of overlayer island growth.
-
(29) Davenport, J. W. Ph.D. Dissertation, University of Pennsylvania. (30) Benndorf, C.; Kruger, B.; Thieme, F. Surf. Sei. 1985,163, L675. (31) Mwn, D. W.; Bernasek, S. L.; Dwyer, D. J.; Gland, J. L. J. Am. Chem. Sac. 1985, 107, 4363. Moon, D. W.; Cameron, S.;Zaera, F.; Eberhardt,W.; Can,R.; B e m e k , S. L.; Gland, J. L.; Dwyer, D. J.to be
publihed. (32) k a ,F.;Kollin, E.;Gland, J.L. Chem. F'hys. Lett. 1985,121,464.
Figure 4. Summary of oxygen chemisorption data: (a) 0(KLL):Cr(LMM)Auger peak-to-peak ratios as a function of exp u r e at two surface temperatures; (b) representative vibrational spectrum of adsorbed atomic oxygen; (e) model for the p(4X2)O overlayer at monolayer; and (d) schematic LEED pattern for p(4X2)O overlayer.
crdio)/d'ico
EELS
I46561io 1975
I
ENERGV LOSS (cm-' )
Figure 5. Vibrational spectra illuntrating the blocking of a,-CO by the preadsorption of atomic oxygen. Gas exposures are (a) 2.5 langmuirs of O2at 300 K, followed by CO exposures at 120 K of (b) 0.3, (e) 0.45, and (d) 0.75 langmuir.
At surface temperabelow 150 K, a minority molecular O,(ads) state is seen by EELS (yo, = 1020 cm-') as well as by electron-stimulated de~orption.'~ Because the a,-CO state is a stable dissociation intermediate, it is an excellent candidate for detecting surface electronic changes (long or short range) induced by electronegative adatoms that inhibit dissociation or "poison" the surface.' Atomic oxygen was the first coadsorbate to be studied to determine what effect, if any, the residual 2-370 of a monolayer Of surface oxygen has on the CO adsorption. A series of vibrational spectra were recorded with increasing CO expoaures for various precoverages of atomic oxygen. Figure 5 illustrates one such series using 'W2;identical experiments were performed using I6O2with the same physical results (different Cr-0 and 0-0frequencies, of course). The red-shifted Cr-W stretch at 570 m-'and the oxygen-ass~ciated'~ weak shoulder at 285 cm-' are shown in figure 5 a The broadening of the tail of the
292 Langmuir, Vol. 4, No. 2, 1988
Shinn
Cd 1 1O)/CO/O, EELS
,I
tl
1
120K
I
~
1
O2EXPOSURElL1
,
.
L - 1 0
~ 1 0 0
1000
,
2000
(a),
1
I
3000
ENERGY LOSS (cm-l)
Figure 6. Vibrational data highlighting the conversion of al-CO to a&O by the addition of oxygen at 120 K (a) 0.25 monolayer of alCO, (b) after the addition of 0.5 langmuir of oxygen. 10.8
elastic peak with increasing adsorbate coverage (CO, oxygen, or both) obscures the weak 285-cm-' feature in subsequent spectra of Figure 5. (The same has been seen in the Cr(llO)/O data a10ne.l~)At this oxygen coverage, the terminally bonded a&O state (YCO = 1975 cm-' and VCSO = 465 cm-') appears after very low CO doses (Figure 5b) and masks the presence of any al-CO as 8 ~ increases 0 (Figure 5e-d). This result could be due to the preadsorbed oxygen simply blocking the adsorption sites (e.g., surface hollow sites) or a more subtle electronic interaction. Evidence for the latter is seen in Figure 6, where the al-CO intermediate state is adsorbed first (Figure 6b) and then subsequently converted to the terminally bonded ap-CO state by the addition of low oxygen exposures (Figure 6 4 . An estimated CO-0 stoichiometry is derived by noting that only 0.5 langmuir of O2(much less than the monolayer p(4X2)O exposure needed to form the ~verlayer'~) is sufficient to effect the q - C O az-CO conversion for the 1/4 monolayer of CO. This experiment, which assumes that the vibrational cross section for the q-CO molecules is independent of the nature of the mixed surface adlayer, suggests that each oxygen atom strongly perturbs a minimum of two nearby CO molecules; on the basis of reasonable estimates for the sizes of the adsorbed CO and oxygen, this stoichiometry is inconsistent with a site-blocking or steric explanation alone. The dependence upon vibrational cross sections for quantitative interaction measurement^^^ is avoided by using photoemission spectroscopy to demonstrate the oxygen-induced a-CO cuz-CObinding state conversion a t low temperatures. For the UPS experiment, the spectroscopic signature of the conversion is the decrease in the CO 4a binding energy from 11.6 to 10.8 eV and an associated increase in this peak intensity. Note that in this conversion experiment these spectroscopic differences are due not to the addition of more CO molecules to the surface, as was the case in Figure 2, but to the alteration of the molecular binding states (and perhaps sites) while conserving Oca. Figure 7 shows the UPS data, starting with 0.25 monolayer of q C 0 a t 90 K (Figure 7a). As oxygen is added, the oxygen 2p peak a t -6.0 eV increases as expected and, more importantly, the two anticipated CO 40 peak changes are indeed observed (Figure 7b-f). To verify these UPS observations, additional CO was added
-
-
(33) Shinn, N. D.,Madey, T.E.J. Vac. Technol. A 1985, A3, 1673.
0.0
1 15
I 10
I 5
I O=E,
E l e c t r o n Binding Energy
(ev)
Figure 7. Angle-integrated UPS data for the oxygen-induced CO binding state conversion at 90 K. Experimental geometry is given in the inset of Figure 2. Exposures for the spectra shown are (a) 0.7 langmuir of CO followed by oxygen exposures of (b) 0.1, (c) 0.2, (d) 0.3, (e) 0.4, and (f) 0.5 langmuir. Finally, (g) 3.0 langmuirs of CO is added to saturate the surface with molecular CO after the oxygen exposures. a t 90 K after the oxygen to saturate the surface (Figure 7g); this increases17J8the population of the terminally bonded a2-C0. The agreement in CO 4a binding energies and relative CO 4a to CO (la + 5a) intensity ratios for parts f and g of Figure 7 confirms that the molecular binding state conversion is occurring even a t 90 K. Finally, it is important to note that the interactions between adsorbed CO and oxygen on Cr(ll0) are not limited to this low-temperture CO binding state conversion. For example, in these coadsorption studies and other annealing studies,17 a 1530-cm-' vibrational loss has been observed (see Figure 5b,c). This could be due to another perturbed CO binding state on oxygen-dosed surfaces or alternately to a surface carbonate. A definitive assignment of this vibrational peak cannot be made at this time. Also, it is possible that the presence of preadsorbed CO might alter the sticking probability or saturation coverage of oxygen on Cr(ll0). The data shown in Figures 6 and 7 indicate that the oxygen atoms quite readily displace the molecular al-CO so that low coverages of al-CO ( 1 9
