J. Phys. Chem. 1984, 88, 469-411 surfaces. CO was exposed to a Ni surface precovered with atomic oxygen (formed at low exposures of 0,) at 80 K. In Figure 1, we have shown He I1 spectra of a Ni/atomic oxygen surface exposed to CO. We could not observe features of the C 0 2 spectrum at low exposures of CO. When we allowed an atomic oxygen precovered Ni surface to stand in flowing C O at precovered for long periods (100-300 s), the spectrum of C 0 2 could be seen distinctly. While C O and atomic oxygen do not desorb around 120 K, C 0 2 desorbs completely around this temperature. We have therefore shown in Figure 7 the difference spectrum of the surface covered by 0 + CO at 80 K and the same annealed at 120 K. The difference spectrum clearly shows the spectrum of C02. The 0 + C O reaction could also be monitored in XPS by following the photoemission signals in the C( 1s) and O( 1s) regions. On a (0 CO)/Ni surface the C(1s) region shows a peak at -290 eV corresponding to the C 0 2 species; the peak vanishes on warming, as expected (Figure 8). Similar changes are also
+
469
seen in the O(1s) region as shown in Figure 8. The 0 C O reaction was not observed when the sequence of exposures was carried out in the reverse order. It appears as though the preadsorption of C O blocks the adsorption of oxygen on metal surfaces. Oxygen adsorption, on the other hand, is known not to hinder C O adsorption.22 Oxidation of CO has been suggested to occur by the surface diffusion of CO to the preadsorbed 0 sites.22 It is also possible for CO, to be formed by the EleyRideal mechanism between C O in the gas phase and adsorbed oxygen.
+
Acknowledgment. We thank the Department of Science and Technology, Government of India, for support of this research. Registry No. Oxygen, 7782-44-7; carbon dioxide, 124-38-9; nickel, 7440-02-0; copper, 7440-50-8; palladium, 7440-05-3; silver, 7440-22-4; gold, 7440-57-5; carbon monoxide, 630-08-0. (22) Gland, J. L.; Kollin, E. B. J. Chem. Phys. 1983,78, 963
Structure of CO Adlayers on P t ( l l 1 ) As Inferred from the Infrared Spectrum Mark W. Severson, Wade J. Tornquist, and John Overend* Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455 (Received: May 25, 1983)
New experimental measurements of the infrared spectrum of CO adsorbed on Pt( 11 1) have been made at 85 and 300 K. The study of adsorbates formed from mixtures of l2C0and 13C0has shed new light on the dynamical coupling of CO molecules adsorbed contiguously. To interpret our results we have used a very general model for this coupling which does not depend on whether the origin is in dipole-dipole interactions or some other electronic effect which involves electrons from the platinum substrate.
Introduction The vibrational spectrum of the stretching fundamental of C O adsorbed on P t ( l l 1 ) has been extensively studied by EELS' and by IRRAS2-7and one might reasonably question the need for yet another paper on this subject. Indeed, we had originally thought the experimental data to be essentially complete and we were engaged in the examination of models for the explanation of coverage-dependent wavenumber shifts in the light of our findings of potential-induced wavenumber shifts in the IRRAS spectrum of C O adsorbed on a platinum electrode.* A number of small questions aroused our concern about the exact wavenumbers of the C O absorption peaks under various conditions and, since we had the experimental facilities readily available, we decided to remeasure the IRRAS spectrum of CO on Pt(l11). The results that we obtained were surprising to us and, as we shall report in this paper, have led us to a number of postulates about the structure of a C O adsorbate on Pt( 11 1) and to new insight into the origin of the wavenumber shifts. Experimental Method The basic design of the IRRAS experiment has been reported previo~sly.~For the measurement of the spectra described in (1) Froitzheim, H.; Hopster, H.; Ibach, I.; Lehwald, S.Appl. Phys. 1977, 13, 147. (2) Shigeishi, R. A.; King, D. A. Surf. Sci.1976,58, 379. (3) Krebs, H. J.; Liith, H. Appl. Phys. 1977, 14, 337.
(4) Golden, W. G.; Dunn, D. S.; Pavlik, C. E.; Overend, J. J. Chem. Phys. 1979,70, 4426. ( 5 ) Horn, K.; Pritchard, J. J . Phys. (Orsay, Fr.) 1977,38, C4, 164. (6) Crossley, A,; King, D. A. Surf. Sci.1977,68, 528. (7) Crossley, A,; King, D. A. Surf. Sci.1980,95, 131. (8) Russell, J. W.; Overend, J.; Scanlon, K.; Severson, M.; Bewick, A. J . Phys. Chem. 1982, 86, 3066. Russell, J. W.; Severson, M.; Scanlon, K.; Overend, J.; Bewick, A. Ibid. 1983,87, 293.
0022-3654/84/2088-0469$01.50/0
this paper we used a 0.5-m Ebert-Fastie monochromator in addition to the circularly variable dielectric wedge filter and in this way we were able to achieve a resolution of 4 cm-' (0.5 meV) with reasonable signal-to-noise ratio when we used an InSb detector. However, since we found that the C O bands were quite broad, we found that a resolution of 8 cm-I was sufficient for the study in hand and all spectra reported in this paper are taken at that resolution. The vacuum system was operated at a base pressure of 5 X torr. Gas composition in the vacuum system was monitored with a quadrupole mass spectrometer which was also used to measure TPD spectra of the adsorbate to determine fractional coverage and to determine the isotopic composition of adsorbed isotopic mixtures. The platinum substrate was cut from a single crystal as a disk, 7 mm in diameter and 1 mm thick. The substrate was spot-welded to two tantalum flaps which were attached to the copper posts of an ultrahigh-vacuum (UHV), high-current, feed-through flange. This arrangement allowed the substrate to be resistively heated. A chromel/alumel thermocouple, spot-welded to the back of the substrate, was referenced to a room-temperature junction outside the vacuum system. The thermocouple was calibrated at high temperature against an optical pyrometer and the high-temperature calibration was extrapolated to room temperature by fitting to a polynomial. Temperatures below room temperature were read directly from the NBS thermocouple calibration table. The initial cleaning of the substrate prior to each set of measurements was as follows: the crystal was heated for 10 min in 5 X torr of O2at 1173 K. The O2was then pumped out and the crystal was heated briefly to 1400 K under UHV and allowed to cool under UHV to the temperature of the experiment.I0 Within each series (9) Golden, W. G.; Dunn, D. S.; Overend, J. J . Catal. 1981, 71, 395.
0 1984 American Chemical Society
470 The Journal of Physical Chemistry, Vol. 88, No. 3, 1984
Severson et al.
2 0 8 6
h
2 0 9 9
2150
2100
c m-
'
Figure 1. Increasing coverage sequence for 12C160 on Pt( 11 1) at 300 K. Coverages: (a) 0.03, (b) 0.04, (c) 0.05, (d) 0.08, (e) 0.15, (f) 0.18, (g) 0.46.
of experiments CO adsorbates were removed by heating to > 1000 K under UHV. For the measurements at 85 K, the copper feed-throughs supporting the sample were immersed in liquid nitrogen. Since the spectra have been measured only over the limited wavenumber region 1985-2125 cm-', we were able to obtain curves with quite flat or only slightly sloped base lines and we had no need to subtract the base-line spectrum of the cleaned substrate.
Experimental Results Adsorbates of I2CO were established at 300 K by dosing for successively longer times. After each spectrum was recorded, the adsorbate was removed by heating the substrate and the TPD spectrum recorded to determine the estimated fractional coverage. Typical spectra obtained are illustrated in Figure 1. There is, as previously observed,2a monotonic shift to higher wavenumbers as coverage is increased. Because CO was a primary constituent of the background gas, the lowest coverage that we were able to obtain resulted in spectrum a, Figure 1; other workers have reported a band at ca. 2065 cm-' at the lowest coverages on both Pt( 111)3,5and polycrystalline Pt foil^.^,^^^ The substrate was next cooled to 85 K and the series of experiments at increasingly higher doses was repeated. At this temperature the spectra, shown in Figure 2, appear very different. At intermediate coverage there is an obvious doublet structure which is not apparent in the higher temperature spectra (cf. Figure 1). Moreover, and we believe quite importantly, the absorption feature at saturation coverage appears significantly sharper and at a slightly different frequency when the spectrum is measured (10) Full details of the cleaning and purity of this crystal have been given by: Dum, D. S.; Severson, M. W.; Hylden, J. L.; Overend, J. J. C u d . 1982, 78, 225.
2150
2100 -1
cm
Figure 2. Increasing coverage sequence for 1zC160 on Pt( 1 11) at 85 K. Coverages: (a) 0.09, (b) 0.14, (c) 0.165, (d) 0.173, (e) 0.24, (fj 0.245, (g) 0.34, (h) 0.38. See Figure 4f for the spectrum at 85 K, coverage.
at 85 K than when it is measured at room temperature. But, to us, the most remarkable feature of the 85 K spectra is that neither component of the doublet appeared to exhibit the coverage-dependent wavenumber shift which we have come to accept6 as a characteristic of this absorption feature of CO on Pt(ll1) surfaces. Of course, the relative intensities of the two components of the doublet change with coverage and, if the resolution is degraded such that the doublet is unresolved (ca. 20 cm-I), the absorption maximum of the single feature shows a coverage-dependent wavenumber shift. It should be noted that the spectra shown in Figure 1, which were measured at room temperature, do show
Structure of C O Adlayers on Pt( 11 1)
The Journal of Physical Chemistry, Vol. 88, No. 3, 1984 471
a
a
2150
2100
cm-' Figure 3. 12C160, 8 = ca. 0.25, adsorbed at 83 K. Spectra recorded at (a)83, (b) 151, (e) 156,(d) 165,and ( e ) 183 K.
the coverage-dependent wavenumber shifts, even at a resolution of 4 cm-'. This observation prompted us to examine the temperature dependence of the spectrum of the C O adlayer on Pt(ll1). The spectra shown in Figure 3 were obtained by establishing a CO adsorbate at 83 K with a coverage corresponding roughly to that of the spectrum shown in Figure 2d at a coverage of 0 = 0.17. As the temperature of the substrate and adsorbate was raised in a stepwise manner to 15 1 K, there was virtually no change in the spectrum. Indeed, the spectrum that we measured at 151 K, illustrated in Figure 3b, is almost identical with that at 83 K shown in Figure 3a. Through the range 151-183 K there are dramatic changes in the spectrum and at 183 K the spectrum as it appears in Figure 3e consists of just a single band, corresponding to the observations that we made at room temperature. Our discovery of the effect of temperature on the spectrum of the CO adsorbate on Pt( 11 1) gives rise to some questions about the previous observations of the spectra of isotopic mixtures6,' and we decided to repeat some of the earlier measurements at controlled temperatures. The spectra of a number of mixtures of 12C160 and l3CI6Omeasured at 85 K are illustrated in Figure 4; all these spectra were measured at saturation coverage established by dosing with 10 langmuirs of the appropriate isotopic mixture. A corresponding series of spectra measured at 300 K are shown in Figure 5. We also measured spectra of a 50:50 mixture of l2Cl60and l3CI6Oat increasing coverage. The spectra measured at 85 K are shown in Figure 6 and the spectra measured at 300 K are shown in Figure 7. The spectra shown in Figures 1-7 are only a small fraction of the ones that we have measured; we have found the results to be quite reproducible and to present a consistent picture, the interpretation of which we shall discuss in the following sections of this paper.
12
-\
2106
2 1 0 0
2 0 5 0
- 1
cm
Figure 4. Mixtures of l2CI6Oand '3Ci60on Pt(ll1) at 84 K,10-langmuir doses. Fraction of 12C160:(a) 0.1,(b) 0.16,(e) 0.3,(d) 0.35,( e ) 0.72, (f) 1.0.
Our qualitative interpretation of the spectra shown in Figure 3 is that the structure of the adsorbate at low temperature differs from the structure at high temperature. The fact that there are two distinct spectral features at low temperature and only one at high temperature leads us to believe that the adsorbate exists in two phases at low temperature and in only one at high temperature. The rather sudden change of the spectrum at 160 K suggests that there is a phase change or critical behavior close to this temperature. It is quite well established that CO adsorbed on Pt(ll1) exhibits two distinct LEED patterns; at lower coverages the LEED pattern is 3'i2 X 31i2 R30° but at higher coverages, a second c 4 X 2 pattern emerges." The c 4 X 2 pattern corresponding to a coverage 0 = is fully formed only at temperatures of about 270 K and below. It disappears completely around 350 K. At low temperatures and under conditions of very
-
(11) Ertl, G.; Neumann, M.; Streit, K. M. Surf. Sci. 1977, 64, 393.
472
The Journal of Physical Chemistry, Vol. 88, No. 3, 1984
L E
/L
V
f
2086
6 1
2090
I
2100
I
2050
cm
- 1
Figure 5. Mixtures of 12C'60and 13C'60on Pt(ll1) at 300 K, 10langmuir doses. Fraction of 1zC'60:(a) 0.15, (b) 0.19, (c) 0.38, (d) 0.52, (e) 0.7, (f) 0.87.
high dosage, additional CO adsorption has been reported to occur giving coverages up to about 0 = 0.68." However, we do not believe that such an adsorbate structure will be formed under the conditions of our experiment. We observed increases in coverage for doses up to 10 langmuirs and a very slow increase in coverage for higher doses. In what follows we shall assume that our 10langmuir, 85 K doses correspond to the c 4 X 2,0 = 0.5 structure. There is a generally held view, as originally suggested by Froitzheim, Hopster, Ibach, and Lehwald,' based on an earlier and more general suggestion by Ertl, Neumann, and Streit," that the 3Il2 X 3Ii2 R30° LEED pattern is associated with the adsorbate structure illustrated in Figure 8a and the c 4 X 2 pattern with the more compressed structure illustrated in Figure 8b. Since we have no reason to question these proposed structures, we shall base the interpretation of our spectroscopic observations upon them. The high-wavenumber feature at 2100 cm-l in the spectra
I
I
2100
2050
- 1
cm
Figure 6. Increasing coverage sequence for mixture of l2CI6Oand 13C'60 on Pt(ll1) at 85 K. Fraction of l2CI6Ois 0.5. Doses: (a) 1, (b), 2, (c) 3.5, (d) 4, and (e) 5 langmuirs.
of Figure 2 increases in intensity with increasing coverage and, if we are to assign the two features to the CO-stretching vibrations of two phases of the adsorbate, it seems reasonable to associate the high-wavenumber band with the compressed structure shown in Figure 8b and to associate the low-wavenumber band with a more open structure of which the adsorbate structure illustrated in Figure 8a is the dominant component. If we examine the spectroscopic behavior of the isotopic mixtures at 85 and 300 K (cf. Figures 4 and 5), there is a marked difference. In the spectrum of a 10:90 mixture of 12C*60:J3C160 shown in Figure 4a, the feature at 2074 cm-' is clearly more intense than that at 2049 cm-I whereas in the spectra shown in Figure 5, a and b, which are of mixtures of composition 1535 and 1931 the reverse distribution of intensities is found, Theory In order to understand the implications of our experimental results, let us consider, in simple terms, our theoretical expectations in this experiment. If we make the approximation that the COstretching mode may be separated from the other modes of the adsorbatesubstrate system, the potential energy function for that mode may be written as
i#j
where N is the number of adsorbed CO molecules in the system and ci, is the interaction force constant coupling the local CO-
-
The Journal of Physical Chemistry, Vol. 88. No. 3, 1984 473
Structure of C O Adlayers on Pt(lI1)
a
We then make the approximation, originally suggested by Decius, Malan, and Thompson,12that the only mode contributing with significant intensity to the infrared spectrum is that where all CO molecules vibrate with the same phase, Le., the s = 0 mode, for which the normal coordinate Q = N-'l2Ciqi. Thus, for every qi in eq 1, qi = N-'/2Q and, if all C O molecules have the same Xo, eq 1 may be written as
2 0 7 6
L
N
+ 2 C c1j)Q2
2 V = (Ao
(4)
j=2
The summation over j in eq 2 now indicates a sum over all the inteiaction force constants important for a particular ( i = 1 ) adsorbed C O molecule; it is written as a sum over all the other CO molecules in the adsorbate but clearly is limited to those within the range of the significant interaction force constants. If one believes one has a realistic model for the estimation of the interaction force constants, the sum in eq 2 may be evaluated as is done routinely for models based on dipole-dipole interaction where cIjvaries as r1y3and the sum over interaction force constants is replaced by a single parameter multiplying into a sum over a function of interaction distances. But, even if one does not have a clear idea of the origin of the interaction force constants, one may treat the sum of the cy as a parameter although one must recognize that it most probably has a dependence on adsorbate structure, Le., on coverage and on the structural details of the adsorbate. The vibrational frequency of the CO-stretching mode, v, follows immediately from eq 4 as
2 0 7 7
47r2c2v2= 'A I
I
2100
2050 - 7
cm
Figure 7. Increasing coverage sequence for mixture of 1*C160 and '3C160 on P t ( l l 1 ) at 300 K. Fraction of I2Cl6Ois 0.5. Doses: (a) 1, (b) 2, (c) 3, (d) 5 , and (e) 10 langmuirs. f i r ' \ - , - -
+ 2 C c l j = A' + k
(5)
The spectroscopic behavior of a mixture of two coadsorbed isotopic species such as I2CI6Oand 13Ci60which have different Ao values may be similarly inferred. In that case the sum in eq 5 is broken up into 12-12, 13-13, and 12-13 interactions; if we assume that there are M I2CO molecules and N - M 13C0 molecules and if we further assume statistical behavior, the number of 12-12 interactions is proportional to @, the number of 13-13 interactions to ( N - W2,and the number of 12-13 interactions to 2M(N - M). If we then express the effective interaction force constant, 2Cclj, in eq 4, by the symbol k , we find that the value of X calculated for the l2CI6Omolecules is X = Xo ( M / N ) k (6)
+
and A' for the I3Cl6Omolecules by" A' = Xd ( ( N - M ) / N ) k '
+
(7)
These two normal modes, one consisting of the I2CI6Omolecules and one consisting of the I3Cl6Omolecules, are connected by an effective interaction force constant, (2M(N- M)/Nz)k";the values of X corresponding to the true energy levels are given by the eigenvalues of the matrix
a
b
Figure 8. Structure models for CO on Pt(l11): (a) 3II2 X 31/2R30° structure with all molecules in linear sites. A possible structure of the border region between two arrays out of registry with one another is shown. (b) c 4 X 2 structure, with half the molecules in bridging and half in terminal sites.
stretching normal coordinates, qi and 4,. At this stage it is unnecessary to specify the form of the interaction force constant or its physical origin; indeed, there is no problem if several physical effects make significant contributions to cij and there is no need at this time to identify them or separate them. The normal coordinates of the entire adlayer have the form
Q, = N-'/2Cexp(27risj-ri)qi i
i
{[M(N-M)]"Zl.V}k"
{ [ M ( N - M) ] "',"}k"
A'
]
(8)
and the stabilized wave functions as linear combinations of the wave functions for the l2Cl60and 13C'60arrays by the eigenvectors of the above matrix. If we represent the higher-wavenumber eigenvector as (a,b) and the lower-wavenumber eigenvector as (b,-a), it follows that the intensities are given by I ( + ) = (urn + bml2
(9)
I(-) = Ibm - amI2
(10)
(2)
and, since this is a unitary transformation, its inverse is given by q i = N-1/2Cexp(-27risj.ri)Qj
.=[
(3)
where sj is a reciprocal lattice vector and ri is the position vector of the ith adsorbed molecule.
(12) Decius, J. C.; Malan, 0.G.; Thompson, H. W. Proc. R. SOC.London, Ser. A 1963, 275, 295. ( 1 3) The value of the effective interaction force constant depends inversely on the kinetic energy metric of the local oscillator; Le., for a diatomic species such as CO, the effective force constant k'= k ~ ~ ~ and ( w ~ 3 k(p12/~13)'!~ where l / p 1 2= 1/12 1/16, etc. This scaling is implicit In eq 1 where q, IS defined as a local normal coordinate. (14) Mahan, G. D.; Lucas, A. A. J . Chem. Phys. 1978, 68, 1344.
+
,"=
474
The Journal of Physical Chemistry, Vol. 88, No. 3, 1984
2080
/ , .
,,x ’
4
.I
:
Severson et al.
2080
2060 cm-’
2040
I
I
0.2
I
I
I
0.4
I
0.6
I
I
0.8
1 I
Figure 9. Calculated dependence of peak wavenumber (cm-I) on coverage Nl2~160for CO on Cu(100) using the following models: (-) Persson and Ryberg (dipole-dipolecoupling),ref 15; (-.-) Mahan and this work, effective interLucas (dipole-dipole coupling), ref 14; action force constant varies linearly with coverage; (---) this work, effective interaction force constant varies quadratically with coverage (see (-*e)
text).
I
1
I
I
I
I
I
I
12 16
c o
Figure 10. Calculated dependence of peak wavenumbers (cm-I) for mixtures of l2CI6Oand 12C’80on Cu(100) using the following models: (-) Persson and Ryberg (dipole-dipole coupling), ref 15; (---) this work; (I) average and standard deviation for 10 arrays of 100 randomly distributed molecules, using a nearest-neighbor coupling model similar to Moskovits and Hulse. ref 20.
where m and m’are the expected (uncoupled) transition moments mdyn/A and k = 1.23850 - 0.58302 mdyn/A; Le., we took the attributable to the l 2 C 0 and T O species, respectively. If we cij coefficients of eq 4 to have a linear dependence on 0. In Figure then assume that m = M1i2moand m‘= ( N - h4)1/2(p12/~13)1/2m~ 9 we also show the curve developed from eq 1, 13, 25, and 26 of where mo is a transition moment of a single l 2 C 0 molecule, it ref 16; it is clear that our model may be brought into exact follows that the fraction of the total intensity, I , in the highagreement with that of Persson and Ryberg by appropriate sewavenumber band is given by lection of the coverage dependence of the interaction force con+ 2ab(M(N - M) X stant, but it is equally clear that the arbitrary assumption of a Z ( + ) / Z = (a2M + b2(Nsimple coverage-dependent value of k will give a very similar (b12/b13))1’21/(M + ( N - 1K)(blZ/b13)) (11) prediction of coverage-dependent wavenumber shifts. In principle, it should be possible by very careful measurement of the frequency and that in the low-wavenumber band by of the CO-stretching mode as a function of coverage to choose the model which most closely matches experimentI6 but at this [ ( - ) / I = (b2M + a2(N- M)(b12/b13) - 2ab(M(N - M) x time it does not appear to be experimentally feasible. (b12/b13))1~21/wf+ ( N - 1K)(bIZ/Kl3)1 (12) The same value of the interaction force constant, k , determined to fit the coverage-dependent frequency shift should also give An important assumption in the foregoing argument is that mo calculated frequencies and relative intensities of the bands observed may be taken to be an effective constant, i.e., that an average value in the spectra of isotopic mixtures at saturation coverage. Figure of the transition moment may be assumed for the adsorbed 10 shows the dependence of frequency on isotopic composition molecules. of I2Cl6Oand 12C180mixtures on Cu(100). The dashed curve One can then introduce the idea of fractional coverage by in Figure 10 was calculated from eq 5 by using the same pascaling N and M by multiplying by a total coverage factor, 0. In rameters as in the first calculation above, Le., X, = 16.8613 that case, k , k’, and k” may be replaced by Bk, Ok’, and 0k” in mdyn/A and k = 0.71720 mdyn/A. Also shown in Figure 10 are the above but one must realize that such a simplistic replacement several points calculated for arrays of randomly distributed is dependent on the fractional coverage being completely statistical, molecules using a nearest-neighbor coupling model similar to that Le., that island formation or other effects which could give rise proposed by Moskovits and Hulse?’ The gross shape of the curves to coverage-dependent cij coefficients are unimportant. in Figures 9 and 10 may be compared with experiment; if both We may compare this model with others which have been do not fit the experimental data with the same value of k, a simple advanced; for example, Persson and RybergI5 have developed assumption of a coverage dependence of the principal force conmodels for the coupling based on dipole-dipole interactions, taking stant of a single CO molecule is all that is needed to disconnect into account the screening effects of the dielectric constant of the the two curves. adlayer. Since the latter is coverage dependent, these models imply It is unclear to us exactly how to adapt the model calculations a particular coverage dependence of the ci, coefficients. Some to the case where the adsorbate structure changes significantly idea of the effectiveness of the simple treatment of eq 5 may be as coverage is increased. If particular adsorbate structures gained from Figure 9, in which we have plotted the coveragedominate over particular ranges of coverage, it should be relatively dependent shift of the CO-stretching frequency calculated in two straightforward to model the vibrational spectrum of the adlayer ways. We took force constants appropriate for CO adsorbed on Cu(100). We show two lines calculated from eq 5; for the first we assumed k to be proportional to 0 and we took Xo = 16.8613 (16) All these models would need to be modified to take into account a mdyn/A and k = 0.71720 mdyn/A where 0 was allowed to vary coverage dependence of the adsorbate structure. from 0 to 1. For the second line we assumed Xo = 16.9035 (15) Persson, B. N.J.; Ryberg, R. Phys. Reo. B 1981, 24, 6954.
(17) (18) (19) (20)
Blyholder, G. J . Phys. Chern. 1964, 68, 2772. Overend, J.; Scherer, J. Spectrochim. Acza 1960, 16, 773. Overend, J. J . Electron Spectrosc. Relai. Phenom. 1983, 30, 1 . Moskovits, M.; Hulse, J. E. Surf. Sci. 1978, 78, 397.
Structure of C O Adlayers on Pt( 111)
The Journal of Physical Chemistry, Vol. 88, No. 3, 1984 475 1.o
I 2100-
0.8 2080cm-'
1
-
2060-l
0.2
0.4
0.6
0.8
1.o
Nl2
Figure 11. Calculated and observed wavenumbers (cm-I) for mixtures of 12C160 and I3Cl6Oon Pt( 111): (-) calculated for 85 K, eq 8, ho = 17.2664, k = 0.6738; (---) calculated for 300 K, eq 8, Xo = 17.193, k = 0.5375; (a)observed, 85 K, this work; (+) observed, 300 K, this work ( 0 ) observed, 300 K, ref 7; (A) observed, 300 K, ref 6.
in each coverage region. But it may well be impossible to express the principal and interaction force constants as a simple function of coverage.
Discussion With this understanding of the general theoretical model we return to the quantitative interpretation of the observed spectra of C O on Pt( 11 1). The observed wavenumbers of the absorption maxima in isotopic mixtures of '*Cl60 and 13C160are plotted as a function of composition in Figure 1 1. It is quite clear that the data measured at 300 K fall on one line and those measured at 85 K on another, at least for the high-frequency component. The low-frequency component does not show this correlation as clearly, perhaps because the low-frequency band is quite broad and weak, making assignment of its wavenumber rather subjective. The relative intensity of the high-frequency component is plotted as a function of l2CI6O fraction in Figure 12, along with curves calculated by using eq 11 and the same parameters as in Figure 11. Here the relative intensity is lower at 300 K for a given l2CI6O fraction than it is at 85 K. In Figures 11 and 12 we show data taken from Crossley and King697as well as our new measurements. The solid and dashed lines represent our best efforts to fit the experimental data with the model represented by eq 8. Since it is most probable that the adsorbate structure is different at the two temperatures, we have allowed the principal and interaction force constants to differ although, since the spectra were all taken at saturation coverage, we have used a single value of each force constant for each curve. If we assume that at saturation coverage there are compressed structures as shown in Figure 8b and that a t 85 K these compressed structures are more preponderant, it appears that the essential differences between the curves at the two temperatures in Figure 11 result from differences in the average structure of the adsorbate. There is also a real possibility that the linearly bound CO molecules in the compressed structure are coupled with the bridge-bound molecules.' It is clear by reference to Figure 8, a and b, that the linear-bridge physical separation is less than the linear-linear separation and one might reasonably expect the interaction force constant coupling linear to bridge-bound C O molecules to be fairly large and to give rise to significant coupling even though the separation of the unperturbed states is close to 200 cm-'. An effect of this interaction, if the force constant is assumed to be positive, is to transfer intensity from the bridge
I
I 0.2
I
I 0.4
I
I 0.6
I
I
0.8
1.o
N,, Figure 12. Calculated and observed relative intensity for tke highwavenumber band for isotope mixtures of I2CI60and I3CI6Oon 't(ll1): (-) calculated for 85 K as in Figure 11; (---) calculated for 300 K as in Figure 11; (a) observed, 85 K, this work; (+) observed, 30C K, this work; (A) observed, 300 K, ref 6.
mode to the linear mode and this effect may well account wholly or partially for the fact that the intensity of the bridge riode is always found to be low compared with the linear mode a id also that the intensity of the bridge mode appears to decrc:ase as conditions are made more favorable for the buildup of com iressed structures. One possible way to model the coupling between linear and bridge-bonded molecules is to take XI0 = 17.2264 mdyn, A and Abo = 13.826 mdyn/A corresponding to 2065 and 1850 c n-' for the linear and bridge bands, respectively. If the shift in wavenumber of the linear band from 2065 to 2097 cm-' as 8 is ini:reased is due to coupling with other linearly bound molfrom 0 to ecules, this gives a linear-linear coupling force constant of kIl = 1.61401mdyn/A where 8, is the fractional coverage of linear molecules. As the coverage is increased to 6 = 1/2, the wavenumber of this band increases to 2105 cm-' whereas, if the structure shown in Figure 8b is correct, the fractional coverage of linear molecules decreases to 8, = 1/4. Assuming that this increase in wavenumber between one-third and one-half cc verage is due to coupling between linear and bridged molecules, we obtained a linear-bridged interaction force constant, klb = 3.9843(818b)1/2mdyn/A, since the fractional coverage of bridged molecules at one-half coverage is 8 b = (cf. Figure 8b). We have used this model to calculate the expected wavenum )ers of mixtures of I2CI6Oand I3Cl6O;the results that we obtair ed are essentially identical with those calculated by assuming n o linear-bridge coupling if we use a coverage-dependent, linear -linear interaction force constant. The assumption of coupling bc:tween the linear and bridge-bonded molecules, however, gives an in tensity ratio for the two bands of 3:1, respectively. Our observaticins are generally that this ratio is about 4:l but Froitzheim et a1 have reported a value of 3: 1. On comparison of Figures 4 and 5 we are led to the contlusion that the auerage coupling between adsorbed molecules a 85 K is larger than that at 300 K since the disparity betwem the intensities of the high- and low-wavenumber peaks for comrarable isotopic mixtures is higher at 85 K than at 300 K. This same conclusion is suggested by the fact that we had to use a iigher coupling force constant to fit the 85 K wavenumbers than t le 300 K wavenumbers (cf. Figure 11). However, even casual inspection of the spectra illustrs ted in Figure 6 reveals that the two features at ca. 2100 and 2050 cm-'
476
T h e Journal of Physical Chemistry, Vol. 88, No. 3, 1984
are of roughly comparable intensity which, on the basis of any coupling theory, means that they must be assigned to some adsorbed CO species which are relatively weakly coupled. Also, since we intuitively believe that any interaction force constant coupling adjacent CO molecules should be positive in sign, i.e., that the energy is higher when both interacting C O bonds are simultaneously stretched or compressed than when they are displaced in opposite senses, the fact that 2100 and 2050 cm-' are highwavenumber modes suggests that, if they are not strongly coupled, the principal CO-stretching force constant of the responsible species is relatively high. An experimental study of the potential-induced wavenumber shift of the CO-stretching mode of CO adsorbed on a platinum electrodes has previously led us to the conclusion that a transfer of electrons from the metal substrate to the a* orbital of the adsorbed C O is responsible for the lowering of the principal CO-stretching force constant.I7 We also believe that competition between adjacent CO molecules for these electrons results in a significant, possibly major, contribution to the interaction force constant. A logical conclusion appears to be that a general withdrawal of these electrons from the a* orbital of the CO results in an increase in the principal CO-stretching force constant and a concomitant decrease in the interaction force constant. Empirically we have observeds that the net effect of withdrawing electrons is to increase the wavenumber of the CO-stretching mode, i.e., that the shift in the CO-stretching wavenumber is dominated by changes in the principal rather than in the interaction force constant. Further, the compressed structure of the adsorbate illustrated in Figure 8b has half of the C O molecules in bridge sites bound to two Pt atoms. The well-established spectral feature at ca. 1850 cm-I has been assigned as the CO-stretching mode of these bridge-bound CO species; this wavenumber is considerably lower than that associated with the linear species which is bound to only a single platinum atom. The lower wavenumber of the bridgebound C O is partly due to the effect of the different geometry as described by Overend and Scherer; Is we calculate a lowering of 32 cm-I if the C O bond is assumed to have the same principal force constant in both linear and bridged configurations. The remainder of the lowering must be due to a lowering of the effective principal force constant of C O when it is bound in the bridged configuration and we attribute this to an enhanced transfer of electrons from the metal into the a* orbital of the CO. The implication of this is that the linear-bound CO molecules adjacent to the bridge-bound ones will not have as high an occupation of the a* orbital as the linear-bound C O molecules in the structure shown in Figure 8a and, hence, we expect the linear CO molecules in the structure of Figure 8b to have a higher principal force constant and to be less strongly coupled to one another. In order to reconcile our spectroscopic observations with these intuitive insights we start out by assigning the two bands at 2100 and 2050 cm-' in the low-coverage spectra of Figure 6 to linearly bound CO molecules in islands with compressed structure. The observation that these two modes are only weakly coupled is quite consistent with our expectations. This leads us to assign the higher-wavenumber component of the doublet in the spectra of Figure 2 to the same adsorbate system; the lower-wavenumber component at ca. 2090 cm-' in Figure 2 and at 2082/2035 cm-' in Figure 6 we assign to open structures in which the sites shown in Figure 8a are partially filled. The coupling between the C O molecules in these latter sites is significantly stronger as is evidenced by the relative intensities of the 2082- and 2035-cm-' bands in Figure 6 and the fact that, in the 50:50 mixtures of l2CI6Oand 13C160,the high-wavenumber component is red shifted only 5 cm-' from the same coverage of pure 12C160whereas the low-wavenumber component is red shifted by 20 cm-I. There is one feature of the spectra in Figure 6 which we have some difficulty explaining; that is the apparent shift of the 2100-cm-' band from 2105 to 2099 cm-' and that of the 2050-cm-' band from 2056 to 2051 cm-1 as coverage is increased. This red shift implies a decrease in the principal force constant or a decrease in the interaction force constant with increasing coverage. The
Severson et al. first of these effects is contrary to our expectation that, as coverage is increased, there is increasing competition for the metal electrons which, by transfer to the a* orbital, decrease the CO-bond principal force constant. The second is contrary to our general sense that higher coverage favors strong interaction (cf. eq 4). We have considered a number of qualitative explanations including coupling between linear and bridge-bonded C O molecules but at present we favor the following: it seems likely that the CO molecules at the periphery of an island with compressed structure are linearly bound rather than bridged since, if bridge-bound molecules are formed only at high coverage, we should expect them to relax if they were at the edge of an island structure. If this supposition is correct, these peripheral CO molecules are peculiar in that they have only one bridge-bound nearest neighbor and may therefore be expected to have greater population of the a* orbitals, a lower principal force constant, and a higher interaction force constant coupling them to neighboring linearly bound C O molecules, both in the islands with compressed structure and in the sea of linearly bound C O molecules with the structure shown in Figure 8a. The lowering of the principal force constant should result in a red shift of the CO-stretching wavenumbers of the peripheral molecules, moving them, we believe, under the band envelopes at 2080 and 2035 cm-'. The interaction with the CO molecules in the interior of the island will shift the interior wavenumber to the blue and the peripheral one further to the red. In small islands, this interaction will be stronger than in large ones since in small islands there is a possibility that individual interior molecules will be coupled to more than one peripheral molecule and we may expect a larger effective interaction force constant. Consequently, if the islands are small as we expect at low coverage, we should see a displacement of the mode of the interior molecules to even higher wavenumber. As coverage is increased and island size develops, this interaction is reduced and the mode of the interior C O molecules relaxes to lower wavenumber. Thus, we are led to the conclusion that, although the interaction force constant associated with the compressed structures decreases with increasing coverage, the effective interaction force constant associated with the 3lIz X 3112R30° structure increases as coverage increases and, ultimately, both modes lie in the single spectral feature at 2100 cm-' (cf. Figure 2). This understanding implies that the significant difference between the 300 K spectra shown in Figure 1 and the 85 K spectra shown in Figure 2 arises because at 300 K the islands of compressed structure are not formed at the intermediate coverage where we should expect them to show as a separate spectral feature. It further implies that the change in the spectrum with temperature, illustrated in Figure 3, results from a breakup of compressed structures forming the more open 31/2X 3112 R30° structures. The fact that the process does not appear to be reversible (cf. Figure 4 of ref 19) suggests that it is kinetically controlled, possibly with an activation energy. At the higher temperatures favoring the 3112 X 3112 R30° structure, the island breakup is relatively fast but the reconstruction of the islands at low temperatures is slow compared with the time of our experiments (ca. 15 min). In this study we had available sufficiently high spectral resolution that we are able to assert with confidence that the widths of the absorption features are virtually unaffected by an instrument transfer function. It appears reasonable to conclude that all the bands are stochastically broadened and that the band width may be taken as a measure of inhomogeneity in the adlayer. We note first that all spectra taken at 300 K are significantly broader than those taken at 85 K. Further, in the 85 K spectra the features assigned to the more open 3llz X 3lI2 R30° structure are generally broader than those assigned to the c 4 X 2 compressed structure. These observations are consistent with our experimental results that the effective coupling between l2Cl60and 13C160is enhanced at 85 K since, of course, the extent of the coupling depends just as much on the separation of the uncoupled wavenumbers as on the size of the interaction force constant. Although we have already commented, we again stress that our interpretation does not depend on the molecular origin of the interaction force constant and consequently our observations do
J . Phys. Chem. 1984, 88, 477-479 not give any insight into this chemically important question. In order to distinguish experimentally the chemical (electron transfer) contributions from those resulting from dynamic dipole coupling it will be necessary to conduct further experiments. We hope eventually to be able to repeat the isotopic mixture measurements on an electrode surface and to determine the dependence of the coupling on electrode potential.
477
Acknowledgment. We are grateful to Dr. W. G. Golden for assistance with the experiment and for helpful discussions. M.W.S. acknowledges financial support from the Amoco Foundation. The work was supported by a grant from the National Science Foundation (NSF-DMR-8016509). Registry No. Carbon monoxide, 630-08-0; platinum, 7440-06-4.
Kinetics and Mechanism of Oxygen Atom Transfer from Iodosylbenzene to Certain Cobalt( I I I ) Nitrosyl Complexes Tian-Lang Chen, P. N. Venkatasubramanian, and Fred Basolo* Department of Chemistry, Northwestern University, Evanston, Illinois 60201 (Received: April 11, 1983; In Final Form: October 11, 1983)
It was observed that certain cobalt nitrosyl complexes react with icdosylbenzene to yield the corresponding cobalt nitro complexes. The rates of reaction are first order in concentrationsof both the complexes and iodosylbenzene. Reactions do not take place in the absence of added base. The rates of reaction depend on the nature and the concentration of base, but at high base concentrations the rates reach limiting values. A mechanism is proposed which involves a rapid preequilibrium followed by a rate-determining, second-order process.
Introduction A decade ago, Clarkson and Basolo' reported on the kinetics and mechanism of the reaction of several cobalt(II1) nitrosyl complexes with dioxygen to form the corresponding cobalt(II1) nitro complexes (eq 1). Six years later Tovrog, Diamond, and o.; .;Lo, .* .$Q, N
N
TABLE I: Wavelengths a t Which the Reactions of the Co(L)(B)(NO) with C,H,IO Were Monitored
-
A, nm B
dtc
benacen
Im 1-MeIm 2-MeIm 1-BzIm
363 363 363 363 368 375
325 325 325 325 335 315
PY 4-CNpy B
M a r e 2 reported the important observation of a nonradical catalytic specific oxidation of organic substrates by molecular oxygen based on reaction 1, along with the transfer of an oxygen atom from the coordinated nitro ligand to the organic substrate (eq 2). Since
the oxidation state of cobalt does not change and the oxidation and reduction (eq 1 and 2) take place exclusively on the nitro ligand, this precludes the usual metal-assisted nonspecific radical reaction process. Groves, Nemo, and Myers3 and Chang and Kuo4 independently reported the oxidation of hydrocarbon with iodosylbenzene, using an iron porphyrin as a catalyst. The reactions appear to involve free radicals, and they do produce a mixture of products. It was decided to determine if iodosylbenzene will oxidize Co"'-NO to C0"'-NO2, because then the nitro compound can in turn oxidize certain organic substrates2and a catalytic cycle could be set up. We report here the fact that such a reaction (eq 3) between CO(L)(B)(NO) + C ~ H J O CO(L)(B)(NO,) + C,H,I (3) cobalt(II1) nitrosyl complexes and iodosylbenzene does take place, and we also report the result of a kinetic and mechanism study of the reaction.
-
(1) Clarkson, S. G.; Basolo, F. Inorg. Chem. 1973, 12, 1528-34. (2) Tovrog, B. S.;Diamond, S. E.; Mares, F. J . Am. Chem. SOC.1979, 101, 270-2. Tovrog, B. S.;Diamond, S. E.; Mares, F.; Szalkiewicz, A. Ibid. 1981, 103, 3522-6 and references therein. (3) Groves, J. T.; Nemo, T. E.; Myers, R. S. J . Am. Chem. SOC.1979, 101, 1032-5. (4) Chang, C. K.; Kuo, M. S. J . Am. Chem. SOC.1979, 101, 3413-5.
0022-3654/84/2088-0477$01.50/0
Experimental Section Compounds and Solvents. All experimental operation were carried out under an atmosphere of N2. Toluene was distilled under nitrogen from sodium benzophenone ketyl, and methanol was refluxed over activated alumina for 5 h before being distilled under N2. The unidentate axial bases, imidazoles and pyridines (Im and pys), were purchased from Aldrich and used as received or purified if necessary. The cobalt nitrosyl complexes
Co(benacen)(NO)
were prepared and purified by the methods reported in the literat~re.~.'Elemental analyses and spectra were in good agreement with the two cobalt nitrosyl complexes. The corresponding nitro products, Co(dtc),(B)(NO,) and Co(benacen) (B)(N02),resulting from the reaction of the nitrosyl complexes with C,H,IO (eq 3) (5) Abbreviations: L, equatorial ligand or ligands occupying 4-coordination sites on cobalt; B, unidentate axial base; dtc, N,N-dimethyldithiocarbamate ion; benacen, N,N'-ethylenebis(benzoy1acetiminate) ion; Im, imidazole; 1MeIm, 1-methylimidazole; I-BzIm, I-benzylimidazole; 2-MeIm, 2-methylimidazole; py, pyridine; 4-CNpy, 4-cyanopyridine. (6) Enemark, J. H.; Feltham, R. D. J . Chem. SOC.,Dalton, Trans. 1972, 718-23. (7) Tamaki, M.; Masuda, I.; Shimra, K. Bull. Chem. SOC.Jpn. 1969, 42, 2858-62.
0 1984 American Chemical Society
