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The adsorption sites of CO on Ni(111) as determined by infrared reflection-absorption spectroscopy

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302 J.C. Camputano, R.G. Greenler /Adsorption sites of CO on Nifll 1)

with electron energy-loss spectroscopy (EELS) (5,8]. The higher resolution of the infrared technique reveals some information not seen with EELS.

The chemisorption of CO on transition metal surfaces is thought to be similar to

the bonding in metal carbonyls. Blyholder [9] has described it in terms of the Htickel molecular orbital theory, considering the bonding to be due to donation of electrons in the 5a level of CO to the metal, and back-donation of metal electrons to the antibonding 2n” level of CO. This simple view has been confirmed by more accurate theories, the HFSCF calculation of Ellis et al. [IO], the HF calculation of Hermann and Bagus (II], the Anderson hamiltonian model of Doyen and Ertl [ 121, etc. The conclusion that one can derive from the theory [16] is that the C-O frequency is very sensitive to the population of the 2n* level, which in turn is very sensitive to the position of the molecule with respect to the metal surface [lo]. But there is another effect that can change the C-O stretching frequency independently of any electronic changes in the molecule: the dipole-dipole coupling between adsorbed molecules. Crossley and King [14] conclude that the 37 cm-’ frequency shift with coverage of CO on a polycrystalline Pt ribbon recrystallized to (111) is due entirely to dipole-dipole interactions, although Mahan and Lucas [17] calcu- late that the frequency shift due to dipole coupling should be at most 10 cm-‘.

In this paper we will try to attribute the frequency shift observed for CO on Ni(l11) to its major causes: changes in adsorption site, back-donation, or dipole- dipole coupling.

2. Experimental details

The experiments were performed in a UHV chamber with a base pressure of 6 X lo-” Torr. The chamber is equipped with windows through which the infrared radiation from a Beckman IR-9 spectrophotometer can be focused on the sample at 8” from the surface plane, with an angular spread of +7”. This permits the simulta- neous use of a 4-grid retarding field analyzer for LEED-AES experiments. A self-

compensating Kelvin probe can be positioned next to the surface of the sample for work function measurements. The chamber is equipped with a quadrupole mass spectrometer in line~f-sight to the sample for TDS experiments. The apparatus will be described in detail in another publication f18].

The 99.995% pure Ni sample (from Materials Research Corporation) was cut in the form of a slab, 0.95 cm X 4 cm X 2 mm, exposing the (111) face with an error of less than 0.5”. The sample was cleaned as described in ref. [ 151. CO gas, 99.9 ~01% purity, was admitted to the UHV chamber, at a constant pressure of 2 X IO-* Torr, the coverage B being determ~ed by changes in work function according to the experimentally-determined relationship ]lS]

0 = 0.50 A@, (1)

where A@ is the work function change in volts. This expression is valid between 6 = 0 and @ = 0.57. Within experimental error, this ielationship is temperature-indepen- dent.

J.C. Campuzano, R.G. Greenler /Adsorption sites of CO on Ni(I 11) 303

Id50 cm-l lgbocm-1 n’

WAVENUMBER (CM-‘)

Fig. 1. Direct copy of original recorded spectra. Spectra of the background and of with adsorbed CO are superimposed to illustrate relative size of noise.

the sample

For the isotope experiments, 99.9 ~01% pure CO gas (92% 180, 8% 160) was mixed in different ratios with 12C160 in the gas manifold, and subsequently admit- ted to the UHV chamber. The ’ 2C160/1 2C1 *O ratio was checked by mass-analyzed TDS after adsorption, because the 12C180 exchanges readily with 12C160 adsorbed on the walls of the gas manifold at the relatively high pressures used there.

The C-O stretching infrared absorption bands occur in the region between 1700 and 2155 cm-‘. Since the Beckman IR-9 spectrophotometer has a grating order break in this spectral range, the region was scanned in two steps: from 1700 to 1980, and from 1980 to 2155 cm-‘. To obtain the infrared bands, spectra with a clean sample, and with a gas-covered sample are obtained. Fig. 1 shows two such recorded spectrometer traces superimposed. The prominent bands common to both spectra result from uncompensated atmospheric water vapor. The CO absorption is represented by the difference between the curves in the ?25 cm-’ region around 1850 cm-‘. Wide spectrometer slits were used in obtaining these spectra, giving a spectral slit width of about 8 cm-‘. (This resolution can be verified by looking at the width of the water bands in fig. 1.) At present, our data handling system is rather crude: we drew a central line through each trace independently, digitized the resulting curves and fed the results to a computer which subtracts the two spectra and plots the resulting absorption bands.

3. Results

A reflection-absorption infrared band of CO on Ni(ll1) for six different cover- ages is shown in fig. 2. The fist five spectra, up to a coverage of fI = 0.50 were run

304 J.C. Campuzano, R.G. Greenler /Adsorption sites of CO on Ni(1 I I)

. . ...‘. . . . . .

tl=Q.O6 . ..*.....*.... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..,. . . . . . . ‘... . . . .

g $ &O.lO .. ‘..“‘.“.:“‘..‘.‘. ,.............,... : . .

. . . . . . . .

FE1 a= g

8=0.20 ‘. .,................,.~,..~..~,...~.. ....“.“““. “““.... . . . . . . . . .’

Z . . . .,.

a$ I

494.33 . . . . . . . . . . . . . . . . . . . .,,...“.. . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . x . ..*............

(J.o.50 . . . . . . . . . . . ........’ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .:.‘. ..,.

. . . . . . . . . . . . . 8457 . . . . . . . ...’

. . . . . . . . . . . . . . . . ‘.,. . . . ., . . . . . . . .

1975 1950 1925 1900 1875 1850 B25 1800

WAVENUMBER (CM-‘)

Fig. 2. Infrared bands for different coverages of CO on Ni(ll1). Spectra of alI but the highest

coverage were measured with the sample at room temperature. The spectrum for 0 = 0.57 was measured with the sample at -3O’C.

at room temperature. The last one (0 = 0.57) was run at -30°C. At coverage less than 0.5, and temperatures >O”C, no other bands could be found in the region between 1700 and 2155 cm-‘. For a coverage of 0.05 of a monolayer, the band appears at 1817 cm-’ (all peak positions are determined to +2.5 cm-‘), and as the coverage is increased, the band shifts to higher frequency, until at B = 0.50, when the c(4 X 2) structure is observed in the LEED pattern, the band appears at 1910 cm-‘. If the coverage is increased to 0 = 0.57 (where LEED shows a (p/2 X fi/2)R19.1” pattern, as analyzed by Conrad et al. [4]) the band shifts to 1915

cm-’ , and another band appears at 2045 cm-’ (fig. 3). The bands are quite broad, with a full width at half maximum (FWHM) of about

50 cm-’ at 0 = 0.50. Note that the actual width of the bands is large; 50 cm-’ is six times the spectrometer resolution limit for the recording conditions of these spec-

tra. The integrated intensities of the subtracted spectra of fig. 2 appear not to vary

linearly with coverage. In the data from which these curves are derived, the wide, shallow bands lie on a background that is not flat. We have trouble with baseline drift in our spectrometer system at these high scale expansions, and a slight shift in the background spectrum can cause a relatively large error in the band area. Modern spectrometers with more sophisticated data-handling facility can do considerably better than our instrument.

A plot of frequency versus coverage (fig. 4) shows a large shift of 98 cm-‘. The shift is not uniform, but shows significant structure, suggesting that there is more than one factor causing the shift. Among the possible causes are: (1) changes in back-donation in the adsorbate binding [9]; (2) changes in adsorption site [19]; (3)

J.C. Campuzano, R.G. Greenler /Adsorption sites of CO on Ni(l11) 305

u-l m a$

.I :’ .::.....

e-057 ..* . . :.: . . . . . . . .‘... *.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .._. * . . . . . . . . . . . . . . .

2090 2040 2000 1960 1920 1880 1840 It300

WAVENUMBER (CM-‘)

Fig. 3. Infrared spectrum showing two bands from adsorbed CO on Ni(ll1) for a coverage of

0 = 0.57, adsorbed at a sample temperature at -3O’C.

dipole-dipole interactions [14,17,20]. There are other effects that could possibly affect the C-O stretching frequency, but Lucas [21] has argued that they are small compared to the effects named above.

1920 -

I800 I , I , I , I , I , I , I, I , I , I , I , I I I _

0 0.1 0.2 0.3 0.4 0.5 0.6

COVERAGE

Fig. 4. Experimental measure of infrared band position vs. coverage for CO/Ni( 111). Solid shows calculated shift due to dipole--dipole interaction between adsorbed CO molecules.

liiC

306 J.C. Campuzano. R.G. Greenler /Adsorption sites of CO on Ni(lI1)

4. Discussion

Hammaker, Francis, and Eischens [20] and Crossley and King [14] have suggested that the shift due to dipole interactions could be determined with the use of isotopes as follows: the dipole shift can be simulated at constant coverage (to eliminate frequency shifts due to changes in either adsorption site or back-donat-

ion) by varying the ratio of two isotopes of the same molecule. When the molecules of the two isotopes are adsorbed, their vibrations couple through the dipole inter- action, giving rise to two bands. Hammaker et al. have shown that as the ratio of the two isotopes is varied (keeping the coverage at some fixed 0) one of the bands

will shift by almost the same amount as the shift in the band of one isotope, as its coverage is varied from 0 to 0. Crossley and King [14] did this experiment for 12C160 and 13C160 on Pt(l1 l), and concluded that the observed shift (37 cm-‘) is entirely accounted for by dipole interactions. We carried out a similar experiment for CO on Ni(ll1) using the isotopes r2C160 and 12C180. If 12C1*0 is adsorbed,

it should give rise to a band whose wavenumber is related to that of “Cl 6O as

where m is the reduced mass of the molecule. If at maximum coverage the band due to 12C160 appears at 1915 cm-‘, the band due to 12C180 should appear at 1869 cm-‘. The bands for different ratios of the two isotopes are shown in fig. 5. All bands in the figure are for maximum coverage, 1’3 = 0.57. The band for 92% pure ’ 2C1 ‘0 occurs at 1869 cm-‘, as predicted by eq. (2). Unfortunately, Nature is not always cooperative; the bands for CO adsorbed on Ni(ll1) are quite wide, with a FWHM of 50 cm-‘, compared with 15 cm-’ for CO on Pt( 111). This causes the two bands to overlap, so that they cannot be followed as function of isotope ratio. Lacking a separate experimental determination of the dipole-dipole shift, we will use the theory of Mahan and Lucas [17] to calculate the shift, so it can be sepa- rated from the total experimentally-observed shift shown in fig. 4.

The expression given by Mahan and Lucas for the dipole shift is

(n/o,)2 = 1 + BaJ:,/(l + &Y,Ce), (3)

where we is the wavenumber of the unshifted C-O vibration; a is the wavenumber of the shifted C-O vibration; ol, is the vibrational polarizability of the CO mole-

cule; 01, the electronic polarizability of the CO molecule; and Xc, is a dipole sum,

&(1/R:), where Ri is the distance from an adsorbed molecule to the ith molecule in a filled layer. Mahan and Lucas take account of the images of the adsorbed dipoles, and therefore include both the direct interaction, t(O), and the image terms, V(O), in the total dipole sum

& = t(0) + V(0) (4)

J.C. Campuzano, R.G. Greenler /Adsorption sites of CO on Niflll) 307

50% &o . . . . . . . . . . . . . . . . . . . . . . . . . . .

60% 160 . . . . . . . . . . . , . . . . . . . . . . . .

9039~160 ...’ ..,.: . . . . . . . . . . . . . . .

,oo%160 .:..“’ . . . .

..,.....,..... . L

1975 1950 1925 1900 1875 1850 1825 1800

WAVENUMBER (CM-‘)

Fig. 5. Infrared bands for various mixtures of 12C160 and 12C180 adsorbed on Ni(ll1). AU spectra are at a coverage of 0 = 0.57.

The direct dipole sums have been reported in the literature [ 171, with the result

t(0) = cr1312, (9

where n is the surface density of adsorbed molecules; c = 9.0336 for a square lat-

tice; c = 8.8904 for a planar triangular lattice. CO on Ni(ll1) at half a monolayer coverage forms neither a square, not a triangular lattice [4], but something in between. Eq. (5) is not sensitive to the structure of the lattice (since the c-value

changes only slightly) but only to the density of molecules. At f3 = 0.50, the CO overlayer has a density of 9.35 X 10r4 molecules/cm2, so eq. (5) yields

t(0) = 0.257 A-3. (6)

The dipole sums are not calculated for a coverage greater than 0.50, because at those coverages, there is more than one infrared band, indicating that the molecules are adsorbed on two types of sites. This will be discussed in more detail later. The image dipole term in (3) is given by

V(0) = -2rrn g G exp(-2raG) + 1/(4ri), (7)

where the G’s are the reciprocal lattice vectors of the CO overlayer, and r. is the distance of the point dipole from the surface. The first term includes all of the image dipoles, more easily evaluated in reciprocal space than in real space. The sec- ond term subtracts the dipole’s own image, since its interaction is already included

308 J.C. Cmpuzano, R.G. Greenler /Adsorption sires of CO on Nifl II)

in the experimentally-measured frequency of the isolated, adsorbed molecule. As- mming Pi = 1 8, we obtain from eq. (7)

~(~)=0.~~07 8-3. (8)

Although the dipole’s image term depends strongly on the choice of re, the com- plete sum, V(O), is rather insensitive to its value because the dipole’s image term is subtracted from the sum.

To complete the evahration of eq. (3) we need the electronic and vibrationaf ~oiar~zabiI~t~es of the adsorbed CO molecule. For gas-phase CO, the electronic polarizabifity along the C-O axis is [22]

For an ionic osciflator, the vibrational poIarizability is

where f is the oscillator strength and e is the electronic charge. We will consider a

case of high polarizability, where the adsorbed molecule has an oscillator strength of 1. For an experimentally measured frequency of 1817 cm-’ for an isolated mole- cule (considering the molecules at 0 = 0.05 isolated), eq. (10) yields

Then eq. (3) yields

sx,- 1817[(1 + S.O2@/(1 tO.958)]? (12)

For B = 0.50, 52 = 1836 cm-‘, eq. (12) predicts a dipote-dipole induced shift of 19 cm-r. The theoretical dipole shift, together with the experimental shift is shown

in fig. 4. The slope of the calculated dipole shift matches the slopes of the two plateaus of the experimental curve,

The striking feature displayed by this curve is the step-like behavior of the CO stretching frequency vs. coverage, which leads us to the following conclusions about the adsorpion of CO on Ni(ll1):

(1) At few coverages, f) G 0.20, CO is adsorbed on three-fold coordinated sites. The band frequency agrees with the spectra of metal carbonyls, where three-fofd coordinated CO generally has stretching frequencies below 1800 cm-r [23]. in an EELS study of CO on Ni(lOO), which does not have three-fold sites, Andersson [6] observes the lowest CQ stretching frequency to be 1930 cm-‘. The same situation occurs for CO on Pd [24], which is chemically similar to Ni, where the low fre- quency bands are found on the (Ill) face, but not on (100).

(2) As 8 goes from 0.20 to 0.25, all molecules shift from three-fold to two-fold coordinated sites, and stay in two-fold sites until 8 = 0.50. As the coverage changes over the narrow range 0.20 to 2.25, the band shifts from 1850 to 1895 cm-‘, strongly suggesting a change in structure. At 6 = 0.50, when LEED shows the ~(4 X 2) pattern, only one infrared band is observed at 1910 cm-‘. Assuming that

J.C. Campuzano, R.G. Greenler / Adsorption sites of CO on Ni(I 11) 309

the single infrared band implies that all molecules are in equivalent sites, we must conclude that they are in two-fold sites. There is no way that a c(4 X 2) structure can be arranged with all molecules on either three-fold or on single sites. When the dipole shift has been subtracted from the observed frequency (fig. 4) the band stays constant in frequency between 0 = 0.25 and 0 = 0.50. We conclude that in this coverage range, all CO molecules are adsorbed on two-fold sites, and the small fre- quency shift that is observed results from dipole-dipole interactions. Another ob- servation supports the conclusion that above 6’ = 0.25 all molecules are in one type of site: at 8 = 0.33, the infrared band narrows to a FWHM of 32 cm-‘, compared to a FWHM of 55 cm-’ for the band at 0 = 0.20. The band widens again at higher cov- erages. What we have described as one band shifting over almost 100 cm-’ would probably be described as two bands correspond~g to CO on three-foId and two- fold sites if the band widths were less, allowing them to be separated.

(3) Above B = 0.50, another band appears at 2045 cm-*. As indicated by infra- red data of Ni(C0)4 [23], the 2045 band is assigned to linearly-bonded CO, that is, CO molecules adsorbed on top of Ni atoms. Andersson and Pendry’s work [7] also supports this assignment; They have done a full dynamics LEED analysis of CO on NiflOO), showing l~earl~bo~ded CO at the coverage where Andersson [6] ob- serves the CO vibrational frequency to be 2064 cm-‘. In the present study, when the band at 2045 cm-’ appears, the band at 1910 cm-’ is still present without much change in size, only shifted further to 1915 cm-‘. At this coverage, LEED shows a sharp (fi/2 Xfi/2)R19.1° pattern, as described by Conrad et al. [4]. Such an adsorbate structure can be arranged only one way on a Ni( 111) surface if

only two types of sites are to be occupied, and if so, one fourth of the molecules are adsorbed on top sites. This model fits our infrared data well, and should enable us to calculate the relative absorption coefficient of CO molecules in the two sites.

Fig. 3 shows the two bands which exist at the same coverage. Averaging the inte- grated intensities of two spectra and assuming that there are three times the number of molecules in two-fold sites as in top sites, we conclude that the molecules in the top sites have an integrated absorption coefficient 1.1 + 0.5 times as large as those in two-fold sites, or alternatively, the dynamic dipole moment, d&/dr lr=e, of top- bonded CO is 1.05 + 0.2 times larger than bridge-bonded CO.

The rather large uncertainty on the ratios of integrated absorption coefficients requires some comment. We have mentioned our difficulty with spectrometer drift, that causes a relatively large un~e~ainty in the integrated intensities of the bands. The limits on the quoted values represent realistic extremes in the uncertainty of our measurement at 8 = 0.57 and indicate that the dynamic dipole moment of CO in the two sites differ by no more than 20%. This is in clear disagreement with the EELS result of Erley, Wagner and Ibach [8], from which they infer that the inte- grated absorption coefficient for badge-bonded CO is 9 times greater than for lin- early-bonded CO, with the dynamic dipoles differing by a factor of 3.

(4) In fig. 4, there is a plateau at about 1846 cm-‘, where the coverage varies between 0 = 0.10 and 0.20, and the CO molecules are on three-fold sites. Then, as

310 J.C. Canzpuzano, R.G. Greenler /Adsorption sites of CO on Ni(l1 i}

the molecules shift to two-fold sites above @ = 0.20, the frequency jumps abruptly to above 1895 cm-‘.

The frequency also shifts abruptly below 0 = 0.10, even though we expect the molecules to be in three-fold sites. This effect could be due to significant changes in back-donation at very low coverages, possibly due to surface states that are removed by a small coverage of CO. This effect has been studied in detail by Plummer and Gadtuk [25] and Holmes and King [26] on the (100) and (110) sur-

faces of tungsten respectively. They fmd that the surface state present in W can be removed completely by ad~rption of a monolayer of hydrogen. Surface states were predicted theoretically by Forstmann and Pendry f27J on 3-d transition metals. If the effect is due to a surface state, the number of electrons in it can be

estimated from the formula given by Baerends and Ros [16], which relates the C-O stretching frequency to the population of the 27r” and 5a levels:

o~__~ = -962.2 F(5a) - 707.2 P(Zn*) + 4039.1.

In free CO, P(5o) = 2, and P(2n*) = 0. Upon adsorption, F’(Su) changes only by 5 to lo%, while P(2n*) changes from 0 to 0.64 [lo]. So, for the smaller changes in fre- quency of adsorbed CO with coverage, P(5a) can be considered constant. Then

AG+_~ = 707.2 AF(2z*).

If the frequency shift going from 0 = 0.1 to 3 = 0.06 is due to the increasing ratio of surface state electrons to adsorbed CO molecules, the shift would be caused by a change of 0.04 2n” electrons per CO molecule. If the surface electrons are equally shared among the 2n* states of the adsorbed CO molecules at 0 = 0.06, the shift would result from only 0.0024 surface state electrons per Ni surface atom. One might suspect that the frequency shift at very low coverages is due to some pecu- liarity of this particular Ni sample, such as a certain amount of disorder in the (11 I) face. This appears not to be the case. The isosteric heat of adsorption for CO on Ni(1 11) as measured by Christmann et al. [3], shows a rapidly decreasing value at low coverages, which correlates with the infrared frequency shift. On Pd(l1 l), when the isosteric heat of adsorption remains constant at low coverages [28], so does the infrared frequency [24].

Recently, Bertolini, Dalmai-Imelik, and Rousseau [5] have published an electron energy-loss study of the vibrations of CO adsorbed on Ni(ll1). Since the experi- ment was carried out at room temperature, they observe only one band, which ini- tially appears at 225 meV (1815 cm-*), and shifts with increasing coverage to 237 meV (1911 cm-“). These results and the results of the present experiment are in good agreement, However, the EELS experiment of Bertolini, et al. achieves a reso-

lution of only 175 cm-‘, not adequate to follow in detail the CO stretching fre-

quency as a function of coverage. As a consequence, they were not able to make any definite statements as to the causes of the vibrational frequency shift, or the structure of the CO overlayer, as a function of coverage. Erley, Wagner, and Ibach [8] report another EELS investigation of the CO/Ni(l 11) system with a resolution

J.C. Cumpuzano, R.C. Greenler f Adsorption sites of CO on Nil1 111 311

of about 60 cm-‘. They find a band at 1810 cm-’ that shifts to 1910 cm-‘, with

increasing coverage, and another band at 2050 cm -*. Furthermore, they can detect

the carbon-metal vibration at 400 cm- l, The total frequency shift of the CO

stretching band is the same as we have observed, but the structure of the frequency versus coverage curve is quite different. Some of the difference results from the accuracy with which the band center is measured, but the main difference is due to the different experimental conditions under which the curves were obtained. Erley et al. adsorb CO at a low temperature, 140 K, and find that the band due to lin- early-bonded CO already appears at coverages lower than that associated with the c(4 X 2) structure (0 = 0.50). At this low temperature the CO molecules impinging on linear sites apparently remain there, not having enough mobility to shift to the lower-energy two-fold or three-fold sites. Therefore, at 140 K, there is adsorption on mixed sites at almost all coverages, and the frequency versus coverage plot given by Erley et al. does not show a sharp transition from three-fold to two-fold coordi- nation for the CO molecules.

Our interpretation of the data indicates that CO is most strongly bound to the three-fold sites on Ni(l11) and most weakly bound to the top sites. The transition from the c(4 X 2) structure at B = 0.50 (with all molecules on two-fold sites) to the (fi/2 X fl/2)R19.1° structure at 0 = 0.57 (with one quarter of the molecules on top sites) is easily understood. At 8 = 0.50 the activation energy for adsorption [15] has risen as a result of a repulsive interaction between neighboring CO mole- cules. However a net reduction in the energy of the system results from adsorbing more molecules, even though it requires shifting some already adsorbed molecules

from bridge sites to less-favorable linear sites. No such simple explanation appears to explain why the molecules, adsorbed on three-fold sites at low coverages, have all shifted to two-fold sites before the (6 X fi)R30” structure is observed at r9 = 0.33. Unlike the c(4 X 2) structure, the (4 X fi)R30” structure could be located

on three-fold sites, as well as two-fold sites. This feature remains to be explained.

Acknowledgements

We are glad to acknowledge the helpful discussions we had with David A. King in the early stages of this work and with Ryszard DuS in the later stages.

The research was supported by a grant, DMR76-82937, from the National Science Foundation.

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