Langmuir 1988, 4, 268-276
268
and both the parallel and perpendicular polarizability of CO are needed.42 The final term in eq B1 is a barrier to chemisorption described more fully in the text. The CO molecule itself is treated as a rigid rotor of length 1.13 A. The internal vibratiohs of CO are so high in frequency (~1800 cm-l) compared to the surface Debye frequency of Ni ( ~ 1 5 cm-’) 0 that there is little coupling between them. Table I lists all the potential parameters used in this study. One notable feature of this potential surface is the relatively small variation in energy for different orienta(42) Harris, J.; Feibelman, P. J. Surf. Sci. 1982, 115, L133.
tions or lateral positions for a fixed height of the molecule above the surface. This and the fact that there are only two lateral and two orientational degrees of freedom means that the integral of eq 1 can be calculated for a fixed surface by direct Monte Carlo sampling; that is, Metropolis sampling is unnecessary. In calculating the potentials of mean force in Figures 5-7, we have evaluated the integrand for each z a t 1000 randomly chosen points in the fourdimensional space of CO configurations. The results appear to be converged to within a few percent, though we have made no quantitative error estimates. These calculations are quite fast, and there is little difficulty in calculating the free energy at all values of z.
Adsorbate-Induced Reconstruction of p(2X2)X Adlayers on Ni( 100)f Jay Benziger,* Gregory Schoofs,f and Andrea Myers Department of Chemical Engineering, Princeton University, Princeton, New Jersey 08544 Received July 17, 1987. In Final Form: August 24, 1987 The interactions between CO and Ni(100)-p(2X2)X,X = C, N, 0, S, C1, have been examined experimentally with LEED, AES, temperature-programmed desorption, and reflection absorption infrared spectroscopy and modeled with a semiempirical tight-binding model. Adsorption at 170 K was reduced on all the surfaces with adlayers relative to clean Ni(100). The adsorption enthalpies, as estimated from TPD desorption energies, were in the order clean < 0 < C1< C < S < N. Infrared spectroscopy found that at 170 K both bridge and on-top CO species were found on the surfaces with adlayers of C, N, and C1; only bridge-bonded CO was found on surfaces with 0 and S adlayers. Above 300 K only on-top bonded species were seen on surfaces with 0, S, and C1 adlayers. No CO remained on the p(2X2)N surface above 300 K, and both bridgeand on-top-bonded CO persisted above 300 K on the carbided surface. The calculations indicated that CO adsorption in on-top sites is greatly inhibited by the presence of a p(2X2)X adlayer, with sulfur and chlorine being the most detrimental to CO adsorption. The effect of the adatoms on CO adsorption is dominated by the CO(5u) adatom p overlap, such that CO should be preferentially adsorbed on bridge sites on the surfaces with p(2X2)X adlayers. The experimental results have been explained by considering the reconstruction of the p(2X2)X adlayer into islands of C(2x2)X and clean surface. It is shown that this reconstruction makes energetically more favorable on-top binding sites available for CO, and the thermodynamic driving force for this reconstructionis favorable for all the adatoms except nitrogen. This model is able to account for the experimentally observed CO binding-site transformationsfrom bridge sites to on-top sites, and it also accounts for why nitrogen had the most deleterious effect on CO adsorption.
Introduction The effects of adatoms on the adsorption of carbon monoxide on metal surfaces have received a tremendous amount of attention. On metal surfaces the adsorption of carbon monoxide has generally been likened to the bonding of CO in metal carbonyls as suggested by B1yholder.l In this model CO adsorption occurs with the CO(5a) acting as a donor into the metal d-orbitals, while the C0(2n*) acts as an acceptor for back-donation from the metal d-orbitals. The resulting interaction is bonding with respect to the C-M bond and antibonding with respect to the C-0 bond. One of the implications of this theory is that adatoms that are more electronegativethan CO should deplete the metal surface of electrons, thereby weakening the M-C bond and strengthening the C-0 bond. Baerends and Ros have
* Author to whom inquiries should
be addressed. 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. Present address: Department of Chemical Engineering, Stanford University, Stanford, CA 94305.
*
attempted to quantify how the C-0 bond stretching frequency should vary with the population of the CO(5a) level: and Doyen and Ertl have calculated the dependence of the CO adsorption energy on the occupancy of the CO(2a*)0rbita1.~ We are attempting a systematic experimental test of the concepts outlined above using temperature-programmed desorption (TPD) and reflection absorption infrared spectroscopy (RAIS) of carbon monoxide on a series of well-defined Ni(lOO)-p(2x2)X surfaces with carbon, nitrogen, oxygen, sulfur, and chlorine adlayers. Presumably the behavior of CO on these different surfaces relative to the clean Ni(100) surface should reveal the chemical and steric influences of the various adatoms. Several other studies had been performed which examined the influence of adatom surface coverage on Ni(100) for various adatoms to try to identify whether the bonding effects were short-range (such as site blocking) or longer range elec(1) Blyholder, G. J. Phys. Chem. 1964,68,2772.
(2)Baerenlis, E. J.; Ros, P. J. Quantum Chem., Quantum Chem. Symp. 1978,12,169. (3) Doyen, G.; Ertl, G. Surf. Sci. 1974,43, 197.
0743-7463/88/2404-0268$01.50/0 0 1988 American Chemical Society
Langmuir, Vol. 4,No.2, 1988 269
Reconstruction of ~ ( 2 x 2Adlayers ) on Ni(100) tronic e f f e ~ t s . ~ These -~ studies have suggested that the effects of adatoms are mostly short range (-4 A), as either site blocking or electrostatic effects. Kiskinova and Goodmans compared C1, S, and P overlayers and suggested that the changes in CO adsorption could generally be interpreted in terms of the changes in the surface electron density in the presence of electronegative adatoms. Shustorovich has examined the effect theoretically using an analytical bond order conservation approach and predicted that the CO heat of chemisorption decreases as the adatom heat of chemisorption increases.1° In the course of our studies we found no correlations between electronegativity and adsorption energy or CO stretching frequency, nor did we find any obvious correlations with adatom heats of adsorption. The experimental results also showed that the CO binding site was temperature dependent and dependent on the adatom. To help us try to understand the effects of the adatoms we modeled the interaction of CO with a Ni(100)-p(2X2)X adlayer with a tight-binding approximation. The results of these calculations are presented in this paper. The calculations are in agreement with the results predicted by Shustorovich but did not agree with the experiments. Reexamination of the experimental results suggested that the energetics of the CO binding-site transformations were related to the energetics of restructuring the p(2X2)X adlayers. In this paper we present arguments for the adsorbate-induced restructuring of adlayers and show how the experimental findings can be accounted for by this phenomena.
clean x h2
I-
2 W
LT LT
3 0
W
2 0
cc Io w a m
m m
a E
Experimental Section The experiments were carried out in an ultrahigh vacuum system that has been described previously." The chamber was equipped for low-energy electron diffraction (LEED), Auger electron spectroscopy(AES),temperatureprogrammed desorption (TPD),and reflection infrared spectroscopy. The operation of the reflection spectrometer has been described in detail elsewhere.12 The spectrometer has been carefully calibrated with a polystyrene standard, atmospheric water vapor, and carbon dioxide. Peak positions are believed to be accurate to *6 cm-l; any uncertainty is more likely systematicthan random, and hence all frequencies would be shifted in the same direction. Carbon, nitrogen, oxygen, sulfur, and chlorine all form ~ ( 2 x 2 ) adlayers on Ni(100) at 0.25-monolayer coverage (1monolayer = 1.6 X 1Ol6 cmT2).Oxygen, sulfur, and chlorine also form c(2X2) adlayers at 0.5-monolayer coverage. The carbon adlayer was deposited by repeated adsorption of propene at 180 K and subsequent heating to 600 K until little dihydrogen evolution was ) observed during the heating cycle and a clear ~ ( 2 x 2 LEED pattern was observed. The p(2X2)CLEED pattern represented saturation coverage of carbon. Heating to above 600 K caused the LEED pattern to fade and the C(KLL) Auger transition to decrease in intensity. Nitrogen was adsorbed on the Ni(100) surface by repeated exposure to hydrazine (N2H4)at 180K and heating to 600 K until little dihydrogen evolution was observed and a sharp ~ ( 2 x 2 ) LEED pattern was observed. The p(2x2)N LEED pattern represented saturation coverage, and this surface structure was stable (4)Johnson, S.;Madix, R.J. Surf. Scl. 1981,108, 77. (5)Madix, R. J.; Thornberg, M.; Lee, S.-B.Surf. Sci. 1983,133,L447. (6)Kiskinova, M.; Goodman, D. W. Surf. Sci. 1981,108, 64. (7)Goodman, D.W.; Kiskinova, M. Surf. Sci. 1981,105, L265. (8)Gland, J. L.;Madix, R.J.; McCabe, R. W.; DeMaggio, C. Surf, Sci. 1984,143, 46. (9) Hardegree, E. L.; Ho, P.; White, J. M. Surf. Sci. 1986,165, 488. (10)Shustorovich, E.Surf. Sci. Rep. 1986,6,1. (11)Schoofs, G. R.;Benziger, J. B. J. Phys. Chem., submitted for uublication. (12)Benziger, J. B.; Preston, R. E.; Schoofs, G. R. J. Applied Optics 1987,26,343.
200 400 600 TEMPERATURE ( K )
Figure 1. Temperature-programmed desorption of CO from ) Ni(100) surfaces with ~ ( 2 x 2adlayers.
to 750 K. Dinitrogen desorption was observed at temperatures above 800 K. Oxygen was adsorbed on the Ni(100) surface by low exposures ) to dioxygen at 180 K and annealing to 800 K. A ~ ( 2 x 2LEED pattern was observed over a narrow range of oxygen exposures; LEED pattern was observed. at higher oxygen exposures a ~(2x2) The AES O(KLL)/Ni(LMM)peak ratios indicated these formed at 0.25- and 0.50-monolayer coverages of oxygen. Sulfur was adsorbed on Ni(100) by exposures to H2Sat 180 K and annealing to 700 K. A ~ ( 2 x 2LEED ) pattern that was pattern at higher observed to initidy develop gave way to a ~(2x2) coverages of sulfur. The AES S(LMM)/Ni(LMM)peak height ratios indicated that these occurred at 0.25- and 0.50-monolayer coverages. Chlorine was adsorbed on Ni(100) by exposures to C1, at 180 K and subsequent annealing. After annealing to 800 K a 42x2) LEED pattern was observed. Heating to 1000 K resulted in ) desorption of NiC12and C1 at 880 K and a resultant ~ ( 2 x 2LEED pattern. Further heating to 1200 K resulted in the remaining chlorine desorbing at 1080K and leaving the surface clean. The AES Cl(LMM)/Ni(LMM) peak height ratios were consistentwith the surface coverages of chlorine being 0.25 and 0.5 monolayers for the p(2X2)Cl and c(2X2)Cl surface, respectively. The LEED/AES results for the five adatoms are summarized in Table I. All the adlayers were stable to temperatures of 600 K in the absence of CO adsorption.
Results Figure 1 shows the TPD data of carbon monoxide desorption from clean, carbided, nitrided, oxided, sulfided,
270 Langmuir, Vol. 4 , No. 2, 1988
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Table I. LEED/AES Correlation for Adatoms on Ni( 100)" AES peak adatom carbon nitrogen oxygen sulfur chlorine
LEED pattern P(2X2) ~(2x2) ~(2x2) c(2X2) P(2X2) 42x2) ~(2x2) 42x21
coverage, monolayers 0.25 0.25 0.25 0.50 0.25 0.50 0.25 0.50
height ratio, adatom/Ni 0.78 0.38 0.18 0.34 4.2 8.4 5.0 9.7
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a One monolayer corresponds to one adatom per substrate nickel atom, 1.6 X l O l 6 cm-2. The following Auger transitions were used in determining the AES peak height ratio: carbon, KLL a t 272 eV; nitrogen, KLL at 379 eV; oxygen, KLL a t 503 eV; sulfur, LMM at 152 eV; chlorine, LMM at 181 eV; nickel, LMM a t 848 eV. The carbon KLL transition refers to carbon exhibiting a metal carbide fine structure.
i
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Figure 3. Infrared spectrum of CO on Ni(100)-p(2x2)C (a, top) after adsorption a t 170 K and (b, bottom) after adsorption a t 170 K and heating to 290 K. I BOO
I 1950
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Figure 2. Infrared spectrum of CO on clean Ni(100) after adsorption a t 170 K and annealing to the temperatures shown.
and chlorided Ni(100) surfaces, respectively. All promoted surfaces had the adatoms distributed in a ~ ( 2 x 2pattern ) prior to carbon monoxide adsorption a t 170 K. After the CO desorption experiments the ~ ( 2 x 2LEED ) patterns were recovered. No distinct LEED patterns were observed for the surfaces with CO adsorbed on the p(2X2)X surfaces at 170 K. TPD data for CO on carbided, nitrided, sulfided, and chlorided Ni(100) surfaces presented here are similar t o the results obtained previously by Kiskinova and However, they found that most of the CO adsorbed on Ni(100)-p(2X2)N and 15% of the CO adsorbed on Ni(100)-p(2X2)Sdesorbed at 150 K, a temperature below which we could cool our crystal. Figures 2-7 present the infrared spectra for carbon monoxide adsorbed on clean Ni(100) and Ni(100)-p(2X2)X (X = C, N, 0, S, Cl) recorded after adsorption at 180 K and after heating to 300 f 25 K; this temperature was chosen by attempting to record RAIS data in the valley of the TPD spectra. Features above 2000 cm-' have been attributed to CO bound on top of a nickel atom, whereas features below 2000 cm-' have been associated with bridge-bonded C0.13914 The infrared spectra for CO adsorbed on clean Ni(100) (shown in Figure 2) have been previously presented.15 At 180 K a (542Xd2)R45 LEED pattern was observed at a CO coverage of 8co = 0.6. The IR spectra indicated a relative site population of two-thirds bridged (peak at 1965 cm-') and one-third on-top (peak at 2035 cm-l), and a (13) Eischens, R. P.; Francis, S. A.; Pliskin, W. A. J.Phys. Chem. 1956,
surface structure has been proposed to account for this.15 Heating to 280 K resulted in little desorption, and the same (5d2Xd2)R45 LEED pattern observed at 180 K persisted, but the infrared spectrum indicated a reversal of relative population in the bridged and on-top sites. Further heating to 325 K resulted in desorption of CO, reducing the CO coverage to ec0 = 0.50. The LEED pattern was a c(2X2), and the IR spectrum indicated that all the CO molecules then occupy on-top sites (peak a t 2038 cm-I). On a p(2X2)C surface the infrared spectra of CO adsorbed a t 170 K showed CO primarily adsorbed in on-top sites with the main feature a t 2081 cm-' (Figure 3). This is shifted up in frequency by approximately 40 cm-' from the frequency observed for on-top adsorption on clean Ni(100). There was also a weak band a t 1937 cm-' due to bridge-bonded CO. Heating the carbided surface to 290 K resulted in desorption of approximately half the CO. The infrared spectra indicated that CO from on-top sites desorbed, while the band corresponding to bridge-bonded CO remained constant. Adsorption of CO on the p(2X2)N surface (Figure 4) was reduced more than adsorption on any of the other Ni surfaces studied. The coverage at 165 K was estimated from TPD to be approximately 15% saturation coverage on a clean surface. The infrared spectra of CO adsorbed at 165 K showed three features. A feature a t 1934 cm-' may be ascribed to bridge-bonded CO, while there appeared to be two bands corresponding to on-top-bonded CO at 2046 and 2067 cm-'. These bands almost vanish entirely after heating the sample to 290 K. The RAIS results for CO adsorbed on a p(2x2)O (Figure 5) and a ~ ( 2 x 2 )surface s (Figure 6) appear quite similar. A t CO adsorption temperature circa 170 K the infrared spectra for both surfaces indicated a single CO band corresponding to bridge-bonded CO a t 1957 cm-' on the oxide surface and 1946 cm-' on the sullide surface. Heating these surfaces to circa 325 K resulted in desorption of
60, 194.
(14) Hoffman, F. M. Surf. Sci. Rep. 1983, 3, 107.
(15) Benziger, J. B.; Schoofs, G.R. Surf. Sci. 1986, 171, L401.
Langmuir, Vol. 4, No. 2, 1988 271
Reconstruction of ~ ( 2 x 2Adlayers ) on Ni(100)
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Figure 6. Infrared spectrum of CO on Ni(lW)-p(2X2)S(a, top) after adsorption at 170 K and (b, bottom) after adsorption at 170 K and heating to 325 K.
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Figure 4. Infrared spectrum of CO on Ni(lW)-p(2x2)N(a, top) after adsorption at 165 K and (b, bottom) after adsorption at 165 K and heating to 290 K.
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Figure 5. Infrared spectrum of CO on Ni(lW)-p(2x2)0(a, top) after adsorption at 175 K and (b, bottom) after adsorption at 175 K and heating to 330 K.
Figure 7. Infrared spectrum of CO on Ni(lW)-p(2x2)Cl(a, top) after adsorption at 170 K and (b, bottom) after adsorption at 170 K and heating to 325 K.
approximately half the CO adsorbed a t 170 K and a shift of the remaining CO into on-top sites with peak positions a t 2017 cm-' on the oxide surface and 2021 cm-l on the sulfide surface. The infrared spectra of CO adsorbed a t 170 K on the p(2X2)Cl surface (Figure 7) shows three features. Two bridge-bonded CO features appear at 1945 and 1955 cm-l, and an on-top-bonded CO feature is evident a t 2026 cm-'. After heating to 325 K there is little desorption as evi-
denced by TPD, and the infrared spectrum shows a very intense feature a t 2028 cm-'.
I Model Calculations The experimental results presented above were examined to see if any correlations existed between adatom electronegativity and adsorption energy as estimated from TPD and CO stretching frequencies for both bridgebonded CO and on-top-bonded CO a t 180 and circa 300
212 Langmuir, Vol. 4, No. 2, 1988
Benziger et al.
U"lt Cell
k=(O,O)
k=(n/a.n/a)
Figure 8. Unit cells used for model calculations. The on-top and bridge CO bmdw sited are denoted as C, and CB mpectively.
Table 11. Bond Distances, SLatcr Exponent, and Atomic Orbital Energies (in eV) for Adatoms adatom carbon nitrogen oxygen 8dfW
chlorrne nickel
atom-surfaee distance. A 0 90 0 90 0.90 1.30 1.24
CO(on-top)
1.17
Cotbridge)
1.50
valence
Slater
energy
level
exponent
level, eV
2P 2P 2P 3P
1.625 1.950 2.275 1 R17
3P
2.033
3d(0.568)' 3dt0.629Y 4% 4P 5d2pJ 2r'(2p.,) 5d2PJ 2r.rip.J
5 75 2.00
2.1W 2.00 1.625
l.fi25 1325 1825
-11.8 -15.4 -17.2 -11.9 -13.8
-9.02 -7.38 -3.50 -14.9 -1 0 -14.9 -7.0
"These are the coefficients and associated Slater exponents for the double I potential used for the d-orbitals.
K. The data were always scattered, and no discernible correlations were observed. We also looked for correlations of the CO adsorption energy and stretching frequencies with the heats of formation of bulk nickel compounds for the various adatoms, and no discernible correlation could be found. T o see what parameters are important in CO bonding in the presence of adatoms, a semiempirical method for looking at CO adsorption on metal slabs was employed. CO adsorption on a two-layer Ni(100) slab with and without p(2X2)X adlayers waa examined by a tightbinding approximation. The tight-binding approximation represented the wave functions of the surface aa a set of Bloch functions of the form
In this Bloch function the sum extends over a surface of N atoms at positions rJ with individual atomic wave functions g(r). Unit cells of eight nickel atoms with (100) symmetry two layers deep were used aa shown in Figure 8. The positions of the nickel atoms were chosen to be identical with the positions of a bulk nickel crystal with nearest-neighbor spacings of 2.5 A. The adatoms with ~ ( 2 x 2symmetry ) were assumed to sit in the 4-fold hollow on the top layer at distances above the surface shown in Table 11. These distances were chosen baaed on experimental values for oxygen and sulfur adlayers,'"IT with the distances for the other adatoms scaled to those distances according to their covalent radii. CO molecules were assumed to bond either in on-top or bridge sites as shown in Figure 8 at distances above the surface consistent with experimental values,'* as shown in Table 11. The eight (16) Vsn Have, M. A.;Tong, S.Y.J. Voe.Sei. Teehnol. 1976,1Z, 230. (11)Stohr, J.; Jaeger, R; Kendelewicz, T. Phys. Re". Lett. 1982,49, 142. (18) Andersson, S.;Pendry, J. B. Phys. Re". Lett. 1979.43, 363.
Figure 9. Symmetry of unit cell at the four corners of the Brillouin zone used for the tight-binding calculations.
nickel atom cluster, with one adatom and one CO molecule, modeled a surface with swface coverages of 0.25 monolayer of both the adatom and CO. Only valence orbitals were used for each of the atomic species. For nickel the 3d, 49, and 4p levels were all necessary as they are closely spaced. For the adatoms either the 2p or 3p levels were used, and the 50 and 2s* levels were used for CO. The individual orbital wave functions were approximated aa Slater wave functions for the nickel atoms and the adatoms, with energy levels taken from the literature and summarized in Table II. The CO molecular orbitals were approximated as carbon 2p orbitals with energies corresponding to a compromise between extended Huckel calculation^'^ and self-consistent-field calculations.20 The imprtant feature about c h w i n g the energy levels for the CO molecule was to make sure that the 50 level waa well below the calculated nickel Fermi level and that the 2s* level was above the calculated nickel Fermi level, aa is consistent with established experimental findings. The CO(5a) is primarily carbon 2p, in character and the CO (2r*)is largely carbon 2p, and 2pr in character, so the approximation of the molecular orbitals by atomic Slater orbitals should not significantly distort the results. One then proceeds to solve the determinant for the energy levels of the system detlH,, - ~~S,syl = 0 where the overlap integral is given by
and the resonance integral is approximated as Hjj = (K/2)(Ei+ Ej)SjJ The overlap integral is only evaluated over nearest neighbors and is assumed to vanish for non-nearest neighbors. Each nickel atom has eight nearest-neighbor nickel atoms, one adatom, and one CO molecule. Adatom-adatom and CO-CO interactions are neglected, but a d a t o m 4 0 interactions are considered. The energy levels E, are given in Table 11, and K is a constant, similar to the (19) Anderaon, A. B. Surf. Sci. 1977,62,119. (20) Jorgeasen, W.L.; Salem, L. The Organic Chemist'J Book of Orbitals; Academic: New York, 1973.
Langmuir, Vol. 4, No. 2, 1988 273
Reconstruction of ~ ( 2 x 2Adlayers ) on Ni(100) a
so-
CO ON Ni(lOO)-p(ZxZJS, TOP SITE
CO ON NI(100), TOP SITE
C
2
4
-
2K'
-
10
so-
CO ON N1(100), BRIDGE SITE
CO ON Ni(lOO)-p(2x2)S, BRIDGE SITE
Figure 10. Density of states for CO adsorbed on Ni(100): (a) C( in on-top site on Ni(100), (b) CO in on-top site on Ni(100)-p(2x2)S, (c) CO in bridge site on Ni(100), (d) CO in bridge site on Ni(l( I)-p(2X2)S. constant in extended Huckel theory, with a value of 1.75. These calculations have been repeated at the center of the Brillouin zone and a t the zone edges. The symmetry of the orbitals at these four points for the top layer of nickel atoms is shown in Figure 9. The total energy of the system is then found by filling up the energy levels with 4(80 + 2 n ) electrons, where n is the number of electrons from the adatom. The binding energy of CO is taken as the total energy of the system with the adatoms and CO minus the energy of the slab with the adatoms minus the energy of four isolated CO molecules (twice the energy of the 5a level). These calculations were done for CO bonded in both on-top and bridge positions. The results for the binding energies of CO are summarized in Table III. These results indicate that the CO is more strongly bound in on-top sites than in bridge sites on clean Ni(100). Adatoms greatly destabilize CO bonding to on-top sites and have a much smaller effect for CO bonding in bridge sites, resulting in an inverstion of the relative stability of CO bonding sites on p(2X2)X surfaces. The energy band pictures for CO adsorption in bridge and on-top sites with and without a sulfur adlayer are shown in Figure 10. These results show that when CO is bonded to an on-top site there is a strong repulsive coupling between the CO(5n) and the S(3pJ level that greatly reduces the CO bonding to the surface. When the CO is in a bridge site away from the adatom this coupling is greatly reduced, so the CO bonding is not
+
Table 111. CO Binding Energies on Ni(lOO)-p(2X2)X Surfaces binding energy, kJ/mol adlayer on-top bridge none carbon
-59
nitrogen
-8 -8 +20
oxygen sulfur
chlorine
-6
+24
-35 -29 -30 -28 -12 -6
significantly affected. Analogous effects are seen for all the adlayers, and the controlling factor in determining the magnitude of destabilization is the overlap between the CO(5a) and the adlayer pz orbitals. These calculations are not meant to be highly quantitative, but the trends they represent should closely follow experiment. We have examined the effecta of valying bond distances, energy levels, the value of K in the resonance integral, and the number of points in the Brillouin zone. These variations change the quantitative values but do not alter the qualitative features of the CO bonding discussed above. The results suggest that in the presence of a p(2X2)X adlayer CO bonding to on-top sites is strongly inhibited for the five adatoms examined here. The calculations also suggest that sulfur and chlorine adlayers should have the most detrimental effects for CO adsorption, contrary to the experimental TPD results that in-
274 Langmuir, Vol. 4, No. 2, 1988
Benziger et al. Table IV. Energetics for Adlaser Restructuriue AH(p(2x2)x'- C(ZX2)X). adlayer carbon nitrogen oxygen sulfur chlorine
kJ/mol of X 45a
80 40 50 50
"The value for carbon represents heat of solution into the b u k from a p(2x2)C adlayer.
-
Fmre 11. Restructuring of a ~ ( 2 x 2 )structure s to a ~(2x2)s structure and the creation of on-top binding sites for CO.
dicated chlorine was the least detrimental and nitrogen exerted the greatest destabilization effect.
Discussion The experimental results presented here are in good agreement with results presented by previous investigators who have examined the effects of adatoms on CO adsorption on Ni(100). The work presented here is unique in attempting to compare similar adlayer structures and identify how different adatoms affect CO adsorption. However, the results are perplexing in that stronger electron-withdrawing species (C1 and 0 )and larger adatoms (Cl and S) have much less of a detrimental effect for CO adsorption than an adlayer of nitrogen. Furthermore, the experimentalresults show that CO adsorbs on on-top sites on Ni(lOO)-p(2X2)X (X = 0,S, C1) surfaces after heating to 300 K with little perturbation of the CO bond relative to adsorption on a clean Ni(100) surface; calculations suggest such bonding should be inhibited. These rather bizarre results can be accounted for by considering the possibility of restructuring of the adlayer due to CO adsorption. We will show what the energetic criteria for restructuring are and show that the experimental results are consistent with the known thermodynamics of adlayer structures. To understand the thermodynamic driving force for restructuring of the adlayer, consider a ~ ( 2 x 2 ) adlayer s as shown in Figure 11. The sulfur atoms are drawn t o scale assuming a radius of 1.3 A, intermediate to the covalent and ionic radii and the radius for CO is 1.7 kZ1 The sulfur atom and the CO are in close proximity when the CO is in an on-top site, and from the model calculations discussed above the CO is nonbonding. With the CO sitting in the bridge s i t s shown in Figure 11the C&ulfur interaction is reduced, but the CO binding energy is also reduced relative to the bonding on an on-top site. Now if the adatom were to move from its original ~ ( 2 x 2 site ) to a 42x2) position as shown in Figure 11, there would be two on-top binding sites made available for CO adsorption. These two on-top sites no longer have adatoms blocking the adsorption, and hence the CO adsorption energy is comparable to the clean surface adsorption energy. The movement of the adatom from its ~ ( 2 x 2site ) to a ~ ( 2 x 2site ) will clearly require some energy, and the restructuring of the adlayer will only occur provided the energy gain from increased CO binding is greater than the energy expenditure for restructuring. (21)This is bassd on the dosepsckeddensity of CO of 1.1 x 10" em* ohserved for CO on Ni(100)and a variety of other surfaces (Tracy, J. C. J. Chem. Phya. 1912,56,2736).
The enthalpies of the p(2X2)X c(2X2)X transition me available or may be estimated from existing data in the literature. For the carbon system Isett and Blakely measured the heat of segregation for carbon dissolved in Ni to the surface of Ni(100) to form a p(2X2)C adlayer to be -45 kJ/mo1?2 Fan et al?3 formed a c(2X2)N adlayer on Ni(100) by nitrogen ion bombardment followed by annealing to 500 K. They observed a c(2X2)N p(ZX2)N transition with LEED and AES at 550 K, and nitrogen desorption from the p(2X2)N adlayer occurred at 880 K. The desorption energies can be approximated as E = 0.25Tp where E is the activation energy for desorption and Tpis the desorption peak temperature in degrees Kelvin." The difference in the enthalpies of adsorption may be approximated as the difference in the activation energies for desorption, AH(p(2X2)N c(2X2)N) = 80 kJ/mol. The enthalpy for the p(2XZ)O c(2X2)O transition has been measured by Grabke and Viefhaus under equilibrium conditions to be 40 kJ/mol." The enthalpy change for sulfur can be inferred from the data of McCarty and Wise,%who measured adsorption isostere8 on supported nickel catalysts. They observed a increase in the isosteric heat of adsorption of sulfur of approximately 50 kJ/mol when the sulfur coverage increased from a coverage of 4.7 X lOI4 cm-2 to a coverage of 8 X l O I 4 cm-z; this coverage change corresponds to the coverage changes between p(2x2)s and ~ ( 2 x 2 )adlayers s on Ni(100). The enthalpy change for chlorine can be estimated from the TPD data for the chlorine adlayers presented here. The enthalpy change can be approximated from the difference in desorption energies of chlorine from the p(ZX2)Cl and c(2X2)Cl surfaces to be 50 kJ/mol. The enthalpy changes associated with the p(2X2)X c(2X2)X transition are summarized in Table IV. The adsorption enthalpy of CO on Ni(100) has been measured by several investigators by a variety of methods. There is reasonable agreement on a value of 120 kJ/mol for CO coverages up to 0.5 monolayer.M~2'A t higher CO coverages the binding energy decreases due to CO-CO interactions. For CO coverages up to 0.5 monolayer the CO occupies on-top binding sites.'"18 A t higher CO coverages or in the presence of adlayers CO occupies bridge sites.8J5J8 The multitude of desorption peaks for CO at high coverages or in the presence of adlayers and the possible restructuring of the adlayer make the determination of the binding energy of CO in bridge sites ambiguous. The magnitude of the binding energy reduction for bridgebonded CO may be approximated by scaling the experimental CO binding energy on the clean Ni(100) surface to the ratio of the calculated CO binding energies
-
--
-
Isett, L. C.; Blalreb, J. M.Sur/. Sei. 19'76,58,397. (23~Fan.Y.-N.:Tu.L.-X.:Sun.Y.-Z.:Li.R.-S.:Kuo.K.H.Surf.Sei. . . . . . , . . (22)
1980,94,L203. (24) Rhodin, T.N.; Adams, D. L. In Treatise on Solid State Chemistry; Hannay, N. B., Ed.;Plenum: New York, 1976;Vol. 6A, p 343. (25) Grabke, H.J.; Viefiaus, H. Surf. Sci. 1981, 112, L779. (26) McCarty, J. G.; Wise, H.J . Chem. Phys. 1980, 72,6332. (27) Benzigar, J. B.; Madir, R. J. Sur/,Sei. 1979, 79, 394.
Langmuir, Vol. 4, No. 2, 1988 275
Reconstruction of ~ ( 2 x 2Adlayers ) on Ni(100) for on-top bonding and bridge bondhig. This assumes that the relative changes in the calculated binding energies scale, even though the absolute numbers may be in error. This gives an approximate range for CO binding energies in bridge sites with a p(2X2)X adlayer of 20-60 kJ/mol. The estimated value for the p(2x2)N surface may be compared with the desorption energy for CO desorption from Ni(100)-p(2X2)N,where the predominant desorption peak occurred a t 150 K.6 The experimental desorption energy from this surface is approximately 40 kJ/mol, in reasonable agreement with the estimated value. The restructuring of the adlayer due to CO adsorption may be considered thermodynamically favorable provided the free energy decreases. When adsorbed species are considered the entropy changes associated with restructuring will be relatively small and may be neglected to a first approximation. In that case the thermodynamic criteria for reconstruction is m(C0bridge
-
< -m(p(2x2)x
co~n-bp)
-
c(2x2)x)
This simply states that the decrease in enthalpy from moving a CO from a bridge to an on-top site must offset the enthalpy increase from causing the adatoms to be in closer proximity. The enthalpy decrease from CO moving from a bridge to an on-top site is approximately 60-80 kJ/mol. On the basis of the data for reconstruction all the surfaces except the p(2X2)N surface should reconstruct to accommodate CO in the energetically more favorable on-top site. The thermodynamic criteria are more restrictive if the restructuring of the adlayer is to occur during the adsorption of CO. Adsorption of gas-phase CO is accompanied by an entropy decrease of approximately 200 J/(mol.K).2s This substantial entropy change changes the thermodynamic criteria to m(CObridge
-
coon-top)
< -m(p(2x2)x
-
c(2X2)X) - 200T kJ/mol
The substantial entropy loss during adsorption suggests that the coupling of CO adsorption and the adlayer reconstruction are probably restrictive. The more likely route is CO adsorption, followed by a mutual restructuring of the adlayer and the CO adsorption site. The TPD data shown in Figure 1 show that all the p(2X2)X surfaces other than the p(2X2)N surface show a significant amount of desorption at approximatey 420 K, where CO desorption from the clean surface occurs. In addition the oxygen, sulfur, and chlorine surfaces all show a temperature-induced change in binding site from a bridge site to an on-top site (see Figures 5-7). During CO adsorption at 170 K the adatoms may not be sufficiently mobile to restructure, in which case CO would be forced to occupy the bridge sites. As the temperature increased to above 300 K the 0, S, and C1 adatoms became sufficiently mobile to restructure into 42x2) structures and allow the CO to occupy on-top sites. If one assumes that mobility, or surface diffusivity, is related to the adatom binding energy, then the temperature of this restructuring should be adatom dependent. Chlorine is less strongly bound to Ni(100) than oxygen and sulfur (as evidenced by a lower desorption temperature), so the restructuring of the chlorine adlayer may be expected to occur a t lower temperatures than the restructuring of the oxygen and sulfur adlayers. This was indeed the case and can be seen from the infrared spectrum shown in Figure 7, where much of the CO occupies on-top sites a t 170 K on the chlorided surface. In addition carbon is known to easily diffuse in (28) Benziger, J. B. Appl. Surf. Sci. 1980, 6, 105.
Ni, as evidenced by the diffusion of carbon into the nickel crystal at temperatures as low as 600 K, even in the absence of any other adsorbates. The relatively high mobility of carbon is also suggested by the infrared spectra shown in Figure 3, where the CO also sits predominantly in on-top sites after adsorption a t 170 K. The restructuring of the adlayer explains why the nitrogen adlayer had the most drastic effect on CO adsorption. Energetically the p(2X2)N adlayer will not reconstruct to accommodate CO into on-top binding sites, whereas the other adlayers all have favorable energetics for reconstruction. The reconstruction also suggests an explanation for the frequency shifts of the infrared absorption bands for CO. The restructuring of the adlayer could result in island formation with patches of c(2X2)X and patches of clean surface. If the CO was adsorbed in islands of clean Ni(100) the CO stretching frequency would be very similar to the frequency observed on the clean surface. As seen in Figures 5-7 the CO stretching frequencies for CO in on-top sites for the oxided, sulfided, and chlorided surfaces are all very close to that for the clean surface, all being shifted slightly downward in frequency. The downward shift in frequency may be due to a polarizable dielectric surrounding the islands of CO, similar to the effect observed by Wang and YatesZ9for Xe coadsorbed with dinitrogen. The frequency shift is in the opposite direction suggested by the electron-withdrawing nature of the adatoms. It may be pointed out that Gland et ala8also noted a slight downward shift in the CO stretching frequency due to the presence of sulfur adatoms on Ni(100). Trenary et al. have examined the effect of sulfur adlayers on CO adsorption on Ni(111).30 They did not see a significant frequency shift for bridge-bonded CO, but they did observe a weakly bound CO under a CO atmosphere with a stretching frequency shifted up close to the stretching frequency of gas-phase CO. Gland et al. saw a similar weakly perturbed CO on Ni(100) with sulfur adlayers when CO was adsorbed at 90 K, but the feature disappeared by 120 K. This high-frequency CO was attributed to a short-range CO sulfur interaction, though it is not possible to definitively identify the binding site for this species. In contrast to the oxided, sulfided, and chlorided surfaces, the CO stretching frequency for on-top CO on both the carbided and nitrided surfaces showed the frequency shifted upward significantly. As neither carbon nor nitrogen forms 4 2 x 2 ) islands, these surfaces probably accommodated CO by a local disordering phenomena. For the p(2X2)C surface, carbon atoms would diffuse into the crystal, leaving a pair of sites available for CO adsorption, but the CO would not be adsorbed in islands that appeared like a clean surface. On the p(2X2)N surface, defects and a small amount of restructuring of the surface would accommodate CO into on-top sites, but again there would not be large patches of clean surface. In these cases the electron-withdrawing effects of the adatoms predominate and result in reduced electron donation into the T* orbitals of the CO molecule and hence an upward shift in the stretching frequency. The model proposed here can quantitatively predict when surface rearrangements can occur. It accounts very well for the experimental infrared results of CO adsorbed on Ni(lOO)-p(2X2)Xsurfaces. The model also accounts qualitatively for the CO TPD results from the various surfaces. A quantitative model to account for the TPD results requires additional information concerning the (29) Wang, H.P.;Yates, J. T.,Jr. J. Phys. Chem. 1984, 88, 852. (30) Denary, M.; Uram,K. J.; Yates, J. T., Jr. Surf. Sci. 1985, 157,512.
276 Langmuir, Vol. 4, No. 2, 1988 2.0,
, , , ,
I
,, , ,
Benziger et al.
,,,,
,
I
, , , ,
,
2
5 : '
>
I
I
i N
Cl
c
is that the electronegative adatoms make the metal atoms a t the surface more electron deficient and hence more electronegative. When CO adsorbs the electron density from the CO(5a) and CO(la), orbitals will be shifted toward the surface making the C-0 bond more strongly polarized. Increasing the polarization of the C-0 bond would increase the dynamic dipole and hence the infrared absorbance.
Conclusions 1
S
o
l
r
4
C
9 0.5
1
a. 0
( , , , , , , , , , , , , , I
I
, , , ,
0. 5 1.0 1. 5 ELECTRONEGATIVITY OIFFERENCE.
1
2.0 X-Ni
Figure 12. Correlation of CO absorbance per molecule on Ni(lOO)-p(2x2)Xsurfaces with adatom electronegativity.
energetics of adatom mobility as the infrared data show that the restructuring occurs a t the same time that desorption is occurring, which gives rise to the complex TPD results shown in Figure 1. This model also explains why there were no correlations between the TPD results of infrared stretching frequencies and adatom electronegativity or heat of adsorption. We did find one interesting correlation between the experimental results and adatom electronegativity. In Figure 12 the infrared absorbance per CO molecule on the various surfaces, as measured by the integrated infrared absorbance divided by the area under the TPD curve, is shown relative to the electronegativity of the adatoms. The absorbance per molecule is related to the dynamic dipole of the CO, and the data suggest that the dynamic dipole increases as the electronegativity of the adatoms increases. A possible explanation of this effect
The results from the experiments and calculations indicate the following. Adatoms may rearrange to accommodate more energetically favorable adsorption of an adsorbate. In particular, 0, s,and C1 adlayers appear to undergo a ~ ( 2 x 2 ) to 42x2) rearrangement to accommodate CO into on-top adsorption sites on Ni(100). Carbon appears to diffuse into the bulk from a p(2X2)C adlayer to allow CO to occupy an on-top binding site. Semiempirical tight-binding calculations suggest that atoms of C, N, 0, S, and C1 block CO adsorption a t nearest-neighbor on-top sites on Ni(100). The calculations suggest that the overlap between the adatom porbitals and the CO(5a) and C0(27r*) orbitals are the dominant effect on CO adsorption. On Ni(lOO)-p(2X2)X (X = C, N, 0, S, C1) surfaces nitrogen adlayem cause the greatest reduction in CO binding energy due to the poor energetics for restructuring the nitrogen adatoms into a 42x2) arrangement. This inhibits CO from occupying energetically more favorable on-top binding sites. Adatoms appear to polarize the surface resulting in increased polarization of the adsorbed CO molecule.
Acknowledgment. We thank the Air Force Office of Scientific Research (Grant 86-0050) for financial support of this work. A. K. Myers thanks the National Science Foundation for a graduate fellowship. Registry No. CO, 630-08-0;Ni, 7440-02-0;C, 7440-44-0;N2, 7727-37-9;02,7782-44-7;S, 7704-34-9;C1, 7782-50-5.
