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J. Phys. Chem. 1985,89, 1910-1914
Adsorption on Catalyst Surfaces Studied by Enhanced Raman Scatterlng A. Wokaun,*+ A. Baiker,* S. K. Miller,* and W. Fluhrt Physical Chemistry Laboratory and Department of Industrial and Engineering Chemistry, Swiss Federal Institute of Technology, ETH Zentrum, CH-8092 Zurich, Switzerland (Received: November 5, 1984)
Surface enhanced Raman scattering is used to investigate the adsorption and bonding of m-toluidine on a metallic copper surface. The enhanced Raman spectra of toluidine on copper exhibit significant shifts from the bulk spectra. These shifts are interpreted by comparison with the spectra of model compounds and by normal coordinate analysis. The results indicate that for room-temperature adsorption, toluidine binds to the surface via the nitrogen lone pair.
1. Introduction Surface characterization techniques are of crucial importance for the investigation of reaction mechanisms in heterogeneous catalysis. Powerful methods are nowadays available to determine the geometry and elemental composition of catalyst s ~ r f a c e s . l - ~ Of particular interest to the chemist are methods capable of identifying molecular species adsorbed onto the surface; high sensitivity is required as catalytically important species are often present in small concentrations. Photoelectron spectroscopy and absorption/reflection spectroscopy can satisfy these requirements in favorable cases. Vibrational spectroscopy4 is particularly attractive because any molecule has a vibrational spectrum, the frequencies are highly molecule specific, and the vibrational bands are usually narrow such that a reasonable number of species on the surface can be detected simultaneously. For this reason, the use of infrared spectroscopy in catalysis has early been investigated; a survey on potential and problems of this technique has been given by Bell and Hair.s Electron energy loss spectroscopy6 is emerging as a powerful tool to probe adsorbate vibrations, although resolution is still limited at present. Recently surface enhanced Raman has been proposed as a new technique for characterizing adsorbate layers on catalysts. This method combines the specificity of vibrational s p e c t r m p y with high sensitivity; coverages of 1% of a monolayer have been detected. Signals from the first adsorbed monolayer experience the strongest enhancement; thus the technique is inherently sensitive to those molecules which are actively participating in the catalytic process. The second and subsequent physisorbed molecular layers, if present, give rise to weaker yet detectable Raman signals.I0 Raman frequency shifts and additional metal-adsorbate vibrational modes contain information on the bonding to the surface;'** chemisorbed and physisorbed molecules can be distinguished from their Raman frequencies. * The present study uses enhanced Raman scattering to investigate catalyst surfaces in a reaction environment, i.e., metal catalysts prepared under laboratory conditions and exposed to reactant gases at operating (=atmospheric) pressures. Rather than selecting model compounds according to the strength of their SERS signals, we demonstrate that the method contributes to the understanding of a catalytic system of current interest. The interaction of amines with metallic copper surfaces is being studied. In a previous publicationI2 the preparation of copper substrates suitable for the in situ measurement of surface enhanced Raman spectra has been described. A roughening procedure was indicated and characterized which results in strong Raman enhancement for adsorbed monolayers. Copper catalysts have been found to be most suitable for the dehydroamination of alcohols.13 The process has been thoroughly studied and characterized by Baiker and co-workers.IeI6 The interaction of the reactant and product amines with the surface plays a dominant role in the dehydroamination m e ~ h a n i s r n . ' ~It J ~is in search of direct spectroscopic evidence for surface species which were postulated in this mechanismI4 that the present investigation is undertaken. Physical Chemistry Laboratory. Department of Industrial and Engineering Chemistry.
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As a first stop the adsorption of an amine on copper from the saturated vapor is characterized; m-toluidine CH3C6H4NH2is chosen as an example. Raman scattering experiments performed on this system are described in section 2. Surface enhanced Raman spectra are presented and discussed in section 3, together with the spectra of model compounds. Results are interpreted with normal coordinate analysis, reported in section 4. In section 5 the present conclusions are summarized, and an outlook to further work is given.
2. Experiments Copper foil, cut into strips 5 X 20 mm in size, was etched as described in ref 12, thoroughly rinsed with deionized water, and dried in a nitrogen stream. The surface of the etched strip was wetted with a drop of m-toluidine and the excess liquid shaken off. The sample was placed into a small cylindrical glass reactor, 1 cm in diameter and 5 cm in height, which was briefly flushed with nitrogen and then sealed. After thermal equilibration at room temperature, small quantities of m-toluidine condensed at the wall of the reactor, showing that the copper surface was in adsorption/desorption equilibrium with a saturated toluidine vapor. Raman scattering was excited at a wavelength of 662 nm (15 100 cm-') by using the output of a DCM dye laser. Ten milliwatts of power was focused through the walls of the reactor onto a 1 X 10 mm area on the sample, a t an incidence angle of 70". Raman scattered light was collected at 90" to the incident (1) Delannay, F., Ed. "Characterization of Heterogeneous Catalysts"; Marcel Dekker: New York, 1984. (2) Thomas, J. M., Lambert, R. M., Eds. "Characterisation of Catalysts"; Wiley: Chichester, England, 1980. (3) Baiker, A. Chimia 1981,35,408,440,485; In?. Chem. Eng. 1985,25, 16, 30, 38. (4) Willis, R. F.,Ed. "Vibrational Spectroscopy of Adsorbates"; Springer-Verlag: West Berlin, 1980. ( 5 ) Bell, A. T., Hair, M.L., Eds. "Vibrational Spectroscopiesfor Adsorbed Species"; American Chemical Society: Washington, DC, 1980; ACS Symp. Series, No. 137. (6) Ibach, H.; Mills, D. L. "Electron Energy Loss Spectroscopy and Surface Vibrations"; Academic Press: New York, 1982. (7) (a) Van Duyne, R. P. In "Chemical and Biochemical Applications of Lasers"; Moore, C. B., Ed.; Academic Press: New York, 1979; Vol. 4, p 101. (b) Chang, R.K., Furtak, T. E., Eds. 'Surface Enhanced Raman Scattering"; Plenum Press: New York, 1982. (8) Otto, A. In 'Light Scattering in Solids"; Cardona, M., Ed.; Springer-Verlag: West Berlin, 1983; Vol. 4, p 289. (9) Wokaun, A. In 'Solid State Physics"; Ehrenreich, H., Seitz, F., Turnbull, D., Eds.;Academic Press: New York, 1984; Vol. 38, p 223. (10) Rowe, J. E.; Shank, C. V.; Zwemer, D. A,; Murray, C. A. Phys. Reo. Lett. 1980, 44, 1770. (1 1) Wood, T. H.; Zwemer, D. A.; Shank, C. V.; Rowe, J. E. Chem. Phys. Lett. 1981, 82, 5. (12) Miller, S. K.; Baiker, A.; Meier, M.; Wokaun, A. J . Chem. SOC., Faraday Trans. I 1984, 80, 1305. (13) Baiker, A.; Richarz, W. Ind. Eng. Chem. Prod. Res. Dev. 1977, 16, 261; Synth. Commun. 1978, 8, 27. (14) Baiker, A.; Caprez, W.; Holstein, W. L. Ind. Eng. Chem. Prod. Res. Deu. 1983, 22, 217. (15) Baiker, A. Ind. Eng. Chem. Prod. Res. Deu. 1981, 20, 615. (16) Baiker, A.; Monti, D.; Song Fan, Y . J . Catal. 1984, 88, 81.
0 1985 American Chemical Society
The Journal of Physical Chemistry, Vol. 89, No. 10, 1985 1911
Adsorption on Catalyst Surfaces
a
UI .-c
1
a
fm 1618
1
,-
UI E
u
1
E c
C
d
200
500
800
Raman s h i f t
1100
1400
1700
Icrn-’]
Figure 1. Raman spectra of the rn-toluidine-copper system. The surface enhanced Raman spectrum of rn-toluidine adsorbed on a roughened copper surface is shown in trace (b). Raman spectra of m-toluidine neat liquid (c) and of the model compounds toluidinium chloride (a) and lithium toluidide (d) are used for comparison. The breaks in the spectra at 650 and 1100 cm-’ arise from the different amounts of background subtracted for the low, medium, and high wavenumber regions. Experimental details are given in the text.
beam and dispersed in a SPEX double monochromator run at 8-cm-l resolution. Raman signals were detected by photon counting and accumulated under computer control. The Raman shift range‘ of interest was divided into three overlapping regions, Le., 190-790, 650-1250, and 1100-1700 cm-I. These regions were alternately scanned in a time of 30 to 60 min, and the spectra stored for off-line signal averaging and smoothing. Typically five to ten such runs had to be coadded to obtain acceptable signal-to-noise ratios. Raman spectra of neat m-toluidine liquid, excited at 514 nm, were recorded for comparison. To interpret the shifts in Raman frequencies observed for the surface species, to be discussed below, the spectra of two model compounds have also been measured. m-Toluidinium chloride was precipitated by slowly adding an excess of 2 M hydrochloric acid to a stirred volume of m-toluidine; after removal of the liquid the precipitate was dried under vacuum. From a saturated solution of the hydrochloride in water Raman spectra were readily obtained with good sensitivity. Lithium m-toluidide (CH3C6H4NH-Li) was synthesized from m-toluidine and buty1lithium.l’ The lithium toluidide, which precipitated from the reaction mixture as a white powder, was isolated under N2 atmosphere, dried, filled into a 1-mm cuvette, and sealed. Raman spectra were excited at 488 nm; the same geometry as (17) Lithium toluidide was synthesized by Mr. G. Grassi following the procedure given in: Houben-Weyl “Methoden der organischen Chemie”, 4th ed.; Thieme: Stuttgart, 1970; Vol. 13(1), p 99.
for the copper substrates was used; Le., the laser was incident through the front face of the quartz cuvette at an angle of 70°, and scattered light was collected perpendicular to the excitation. Raman spectra of lithium toluidide were recorded at 8-cm-’ resolution, while a 2-cm-’ resolution setting was used for the liquid samples (neat m-toluidine and hydrochloride solution). To support the assignments described below, deuteration studies are being performed. N,N-dideuterated m-toluidine is synthesized by exchanging the amine proteins in a large excess of D2S04, raising the pH by an excess of NaOD, and isolating the product by ether extraction. IR and Raman spectra of the N D / N H stretching region showed that the product was 80-90% deuterated. Raman spectra of the deuterated molecule on copper were obtained as above.
3. Surface Enhanced Raman Spectra Raman spectra of neat m-toluidine liquid, of toluidine on Cu, and of the two model compounds are shown in Figure 1. The spectrum of the liquid (Figure IC) agrees with published reference spectra. Marked differences between the liquid spectrum and the enhanced Raman spectrum of toluidine on Cu are immediately apparent upon inspection of Figure 1b. These differences concern Raman frequencies, relative peak intensities, and the presence of a large unstructured backgrond in the surface enhanced Raman spectrum, comparable in intensity to the molecular Raman bands. This background is quite generally observed in surface enhanced Raman spectra’ and has been interpreted a s luminescence due
1912 The Journal of Physical Chemistry, Vol. 89, No. 10, 1985
Wokaun et al.
to electron-hole pair recombination. As the background does not l.05h H carry molecule-specific information, it may be reduced by subtracting a constant number of counts from each spectrum. This procedure emphasizes the relevant Raman peaks and facilitates the use of peak finder routines. Such a background subtraction has been applied to the spectra shown in Figure 1. The partial spectra for the wavenumber regions 190-650, 650-1 100, and 1100-1700 cm-l have been offset by different amounts, which gives 2.0A rise to the breaks visible at 650 and 1100 cm-'. Relative band intensities in surface enhanced Raman spectra are quite generally different from those of the free m o l e c ~ l e . ~ ~ ~ Basically this difference arises from the fact that the local field Figure 2. Molecular geometry used in the normal coordinate analysis of component perpendicular to a metallic surface is much stronger the amino group fragment R-NH2 and of the R-NH,-.Cu surface comthan the parallel component. Considering this inequality and the plex. modification of the Raman selection rules by the presence of a symmetry-reducing surface,lS relative SERS intensities for several TABLE I: Aniline Vibrations Involving the Amino Group" small molecules adsorbed on silverI9 have been quantitatively Phe-NH,b Phe-NH2b interpreted. For such an interpretation the adsorption geometry exptl calcd exDtl calcd must be well-known. As we feel that the sensitivity in the surface 3485 3505 1279 1285 spectra (Figure l b ) is at present not sufficient to warrant a 3400 3388 1054 963 quantitative interpretation of peak intensities, we shall concentrate 1619 1608 570 559 on the Raman frequencies which may be accurately determined from the spectra. "Frequencies in cm-]. bPhe- = C,H,-. Rather than giving a complete table of Raman frequencies, quency skeletal motions are upshifted to = 1640 cm-l. Next to which is available from us, we comment on the most prominent a weak signal at 1295 cm-', a new band at 1248 cm-' is present differences between the surface spectrum (Figure lb) and the which resembles the 1254-cm-' vibration of the surface species. spectrum of the liquid (Figure IC). The highest frequency band The remaining bands of toluidinium correlate quite closely to those in the skeletal motion region, at 1630 cm-', is upshifted by _N 12 of the neutral parent molecule. cm-' on copper compared to the free molecule. Aromatic ring One might have expected that the copper-adsorbed toluidine stretching modes appear at 1517 and = 1580 cm-' on copper and would exhibit similarities with the lithium compound. The exare apparently downshifted from the free molecule values (1 570 perimentally observed similarity between the surface spectrum and 1605 cm-I). The peak at 1296 cm-' shows a smaller but and that of toluidinium prompted us to investigate a different significant upshift. A new intensive vibration appears on the bonding model, i.e., an undissociated amino group interacting via surface at 1254 cm-I, which has no counterpart in the Raman or the nitrogen lone pair with a metallic surface, which acts as an IR spectra of unbound toluidine. Another new band is observed electron acceptor. This is the subject of the normal coordinate at 801 cm-I, overlapping with the 774-cm-' band which is also analysis presented in the next section. found in the spectrum of the liquid. In the low wavenumber region, the well-structured spectrum of the free molecule merges into two 4. Normal Coordinate Analysis broad bands for the adsorbed species, centered around 520 and A strategy was chosen to interpret the adsorption-induced 220 cm-l. frequency shifts by normal coordinate analysis using a minimum The observation of frequency shifts and metaladsorbate modes number of adjustable parameters. Consideration of the entire has first been demonstrated in the pioneering SERS work on molecule is not very satisfying in this respect. Toluidine with its pyridine,'^^ and is a most valuable source of information on the 17 atoms has 45 normal vibrations. Inspection of a normal cobonding to the surface. Here we demonstrate that this information ordinate analysis published for the simpler problem of tolueneZo enables us to decide between two models of bonding to the surface. shows that 67 force constants were introduced to reproduce the Our interpretation of the surface spectrum rests on comparison spectrum of this molecule. The elementary stretching and bending with the spectra of model compounds and on a partial normal coordinates are coupled by numerous off-diagonal elements, and coordinate analysis. As mentioned above, the toluidinium ion the calculated frequencies are sensitive to subtle changes of the CH3C6H4NH3+and the lithium "salt" CH3C6H4NH-Li were parameter values. In attempting to fit the frequencies of the chosen as models. Compared to the free molecule, toluidinium aromatic ring one is facing the following problem: (i) several has a partial positive charge and the lithium compound a partial stretching and bending force constants will experience at least negative charge on the nitrogen. In the toluidinium ion, an adsmall changes upon adsorption; (ii) the calculated frequencies will ditional covalent bond between nitrogen and the proton is formed. be quite sensitive to these changes; (iii) if agreement between In the lithium compound, a hydrogen is substituted by the metal theory and experiment is achieved by introducing a large number such that the product resembles the surface species R-NH* of adjustable parameters, the uniqueness and the physical sigformulated in the dehydroamination mechanism,14 where the nificance of the results must be questioned. asterisk symbolizes a bond to the copper surface. We have therefore decided to restrict ourselves to the amino The spectra of lithium toluidide are shown in Figure Id. The group fragment, and to study the effect of introducing a bond "square" appearance of several bands, which are intrinsically between the nitrogen lone pair and the surface. Calculations have narrow, is an artifact due to the 8-cm-' spectrometer band-pass been performed for R-NHz, R-NH,-.Cu, and R-NH3+, where used. The Raman bands appear superimposed to a broad the aromatic ring R was treated as a single mass as suggested in fluorescence background rising toward longer wavelengths. There ref 21. Aniline Phe-NH, was used as a model for the free is no apparent similarity between the spectra of the lithium molecule. Force constants for the four-atom fragment are adjusted compound and those of toluidine on copper; the frequency shifts to the experimental frequencies of Phe-NHz and Phe-ND,. The relative to the free molecule are quite different. experimental geometry from ref 21 shown in Figure 2 was used. The spectrum of toluidinium, on the other hand, exhibits striking The obtained fit for the six frequencies of the four-atom fragment similarities to that of the adsorbed toluidine. The highest fre-
1
(18) Moskovits, M. J . Chem. Phys. 1982, 77, 4408. (19) Moskovits, M.; DiLella, D. P. In "Surface Enhanced Raman
Scattering"; Chang, R. K., Furtak, T. E., Eds.; Plenum Press: New York, 1982; p 243.
(20) Lan, C. L.; Snyder, R. G . Spectrochim. Acta, Part A 1971, 27A, 2073. Snyder, R. W.; Painter, P. C. Spectrochim. Acta 1980, 36. 331. Snyder, R.W.; Painter, P. C. Polymer 1981, 22, 1629. (21) Evans, J. C. Spectrochim. Acta 1960, 16,428. Hamburg, E.; Grecu, R.; Fernea. M. Rev. Roum. Chim. 1972, 17, 1845.
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The Journal of Physical Chemistry, Vol. 89, No. 10, 1985 1913
TABLE 11: Vibrations Involving the Amino Group and Their Shifts Due to Surface Bonding and Protonation" exptlb calcdc Tol-NH, Tol-NH,-Cu Tol-NH?' Phe-NH1 Phe-NHrCu Phe-NHq' assignmente 3432 3505 (3506) 3501d N-H stretch N-H stretch 3388 (3391) 3342 3366 1608 1633 1620d H-N-H bend 1618 1630 =1620 438 1436 R-N stretch, R-N-H bend, H-N-H bend 1292 1296 285 1296 R-N-H bend 1248 1254 Cu-N-H bend, R-N-H bend 248 1228d R-N-H bend 1077 963 R-N stretch, H-N-H bend 1029 1005 Cu-N-H bend 80 1 787 Cu-N stretch, R-N stretch 520 & 90 R-N stretch, Cu-N stretch, R-N-H bend 599 541 534 559 624 R-N stretch, R-N-H bend Cu-N-R bend, Cu-N stretch 88 "Frequencies in cm-'. bTol- = m-(CH3)C6H4-. Phe- = C6H5-. dDoubly degenerate mode. Major contributions. is given in Table I. Force constants within the R-NH2 entity were not adjusted subsequently when discussing the effects of introducing an additional bond to Cu or to the proton in the ion. For the surface-bound species, we used the geometry shown in Figure 2. The Phe-NH, fragment was left unchanged; a bond to the surface was introduced perpendicular to the Phe-N bond. A very large mass was attributed to the surface "atom". The Cu-N stretching and Cu-N-H and Cu-N-Phe bending force constants were treated as adjustable parameters. The coppernitrogen bond length was varied between 1.5 and 2.5 A, which represent extreme values found in transition-metal amino complexes. The quality of the fit was found to be nearly independent of the bond length chosen. For the final fit the bond length was fixed at a value of 2.0 A, which is reasonable in view of the spatial extent of the aromatic *-electron system. For the anilinium ion Phe-NH3+ a tetrahedral geometry around the nitrogen was assumed. The N-H stretching and Phe-N-H bending force constants were given the same values as in Phe-NH2. These parameters were not adjusted; rather, the frequencies of Phe-NH3+ were computed in a forward calculation. By this procedure we prove that the experimentally observed trends are reproduced by theory without an adjustable parameter. Experimental and calculated Raman frequencies are juxtaposed in Table 11. The pertinent bands have been labeled in Figure 1. The overall agreement is seen to be quite satisfactory. Let us first compare surface-bound toluidine with the neat liquid. The upshift of the H-N-H and R-N-H bending motions (observed at 1630 and 1296 cm-I, respectively, on copper) is well reproduced by theory. The prominent surface mode at 1254 cm-', which is absent in the Raman and IR spectra of the liquid, appears in the calculation as a bending motion of the molecule relative to the surface. Substantial involvement of the surface bond is also predicted in lower frequency vibrations at 1005, 181, 599, and 88 cm-'. Hints of bands that might correspond to these motions are seen in the spectra at 1029 and 801 cm-I, although these assignments are tentative due to limited signal-to-noise ratio. Calculations for the anilinium ion, as mentioned, have been performed by using aniline force constants without adjustment. The H-N-H bending motion is predicted to upshift by 12 cm-I. Experimentally the group of peaks centered around 1600 cm-' in liquid toluidine is also seen to upshift and broaden in the toluidinium ion; it is centered around 1620 cm-' with the highest frequency component at about 1640 cm-I. The second feature of the toluidinium spectrum which resembles toluidine on Cu is the peak at 1248 cm-'. Again the forward calculation for anilinium readily predicts a peak in this frequency region, Le., the mode at 1228 cm-I. Unfortunately, we were not able to observe the N-H stretching region for toluidine on Cu due to experimental limitations. The choice of an excitation wavelength in the red spectral region (1 5 100 cm-') was dictated by the requirements for surface enh a n c e m e n t ~ . ~ - With ~ , ~ ~their large Raman shift of -3400 cm-', (22) See ref 12, footnote on p 1306.
the N-H stretching motions were then emitting beyond the long wavelength cutoff of our GaAs photomultiplier tube. As no coupling between the nitrogen-pper bond and the N-H stretches was introduced in the calculation, the calculated N-H stretching frequencies appear essentially unshifted and should not be considered significant. Finally, calculations have also been performed for the deuterated species Phe-ND2 both free and on copper, using the same force constants as for the protonated form. Generally the surface-induced shifts are predicted to be smaller for the deuterated species. Experimental spectra for a 90% deuterated sample, both as the neat liquid and adsorbed on copper, have been recorded. However, the presence of -220% of the monodeuterated species R-NHD, together with the richness of the spectrum in low-intensity peaks (cf. Figure l ) , resulted in very crowded Raman spectra both for the liquid and the adsorbed species, such that a quantitative interpretation is not feasible at present. 5. Conclusions Adsorption of m-toluidine on a copper surface has been studied by enhanced Raman spectroscopy. The results have allowed us to decide between two models for the bonding between the molecule and the surface. Covalent bonding by substitution of a hydrogen, to form a R-NH-Cu chemisorbed species, can be ruled out from the spectra. Upon room-temperature adsorption toluidine appears to be bonding to the surface via the nitrogen lone pair (R-NH2..Cu); i.e., the molecule donates electron density to the metal. This was establish i by modeling of the surface complex in a normal coordinate analysis of the amino group fragment and by comparison with the Raman spectrum of the toluidinium ion R-NH3+ in solution. At present no statement about the interaction of the aromatic 7-electron system with the surface can be made. Of the ring vibrations resolved in the surface spectrum (Figure lb), many appear unshifted, as, e.g., the ring breathing mode a t 995 cm-l. On the other hand, profound changes are occurring in the 1600-cm-' region. Interpretation of the 15 17- and 1578-cm-' bands seen on the surface would require a normal coordinate analysis of the aromatic moiety, where a large number of force constants will experience slight changes due to the surface. The mentioned bands are not seen in the absence of toluidine, so they are not due to "graphite" type surface impurities frequently no evidence for seen in SERS ~ o r k At . ~room ~ ~temperature ~ corrosion of the copper foil by the adsorbed m-toluidine has been found. The fact that the surface spectrum in Figure 1b is surface enhanced, and not due to a thick layer of a copperamine complex, is supported by two observations. First, the Raman signals show the same dependence on the surface roughness as previously observed in surface enhanced resonance Raman scattering from adsorbed dyes.12 Second, the Raman signal shows a pronounced wavelength dependence which is characteristic for surface enhancement by copper; i.e., no signal is observed for excitation in (23) Otto, A. Surf.Sci. 1978, 75, L392.
J . Phys. Chem. 1985,89, 1914-1921
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the blue or green spectral region. The present results point out the direction of further work. Infrared transmission measurements of toluidine adsorbed on suitable supported copper catalysts will be performed to study changes in the N-H stretching region upon adsorption. As a next step the adsorption of m-toluidine at elevated temperatures up to 200 O C will be studied. It will be interesting to see whether the transition to the covalently bound form R-NH-Cu can be observed in the surface enhanced Raman spectra. As judged from the spectrum of the lithium model compound shown in Figure Id, profound changes in the Raman frequencies can be expected. The present study demonstrates the potential of enhanced Raman scattering as a diagnostic tool to study adsorption in environments and at pressures corresponding to catalytic operating conditions. This knowledge will be used to perform and interpret in situ measurements during heterogeneously catalyzed reactions,
such as the dehydrogenation reaction of alcohols.
Acknowledgment. We are grateful to Professors R. Ernst, W. Richarz, and U. Wild for their continued interest and generous support of this work. We thank Prof. U. Wild for the use of spectroscopic and computer equipment. We are indepted to Mrs. G. Grassi, A. Kallir, J. Keller, W. Jaeggi, and M. Meier for help with experimental and computational problems. Financial support by the Swiss National Science Foundation and the Branco Weiss Foundation is gratefully acknowledged. We are indebted to one of the reviewers for suggesting SERS experiments in an electrochemical environment, which could provide additional information on the bonding of toluidine to copper surfaces. By varying the applied electrode potential, the degree of bonding to the surface via the nitrogen lone pair could be influenced. Registry No. m-Toluidine, 108-44-1; copper, 7440-50-8.
Effect of Surface Atom Vibrations on the DMractlon of 'H and 'H Beams from the Basal Plane of Graphlte S. Iannotta; G. Stoles,* and U. Valbusat Centrefor Molecular Beams and Laser Chemistry, University of Waterloo, Waterloo, Ontario, Canada N2L 3Gl (Received: September 20, 1984)
The diffractive scattering of 'H and 2H beams from the basal plane of graphite has been studied as a function of the crystal temperature in the range 65-350 K. The data at low temperatures have been interpreted by using the hard corrugated wall (HCW) model which gives a peak-to-peak corrugation trn= 0.090 8, which is about a factor of 2 lower than the value reported in the literature for He-(0001)C. The agreement between the experimental probabilities and the predictions of the HCW model as a function of the incident angle is discussed. Furthermore, the temperature dependence of the experimental intensities of the (0,O)and (T,O) peaks have been studied at several incident angles in order to test the applicability of a Debye-Waller factor theory. Although the high-temperature range of the data seems to be well reproduced by a DWF theory when a Beeby correction is included, a comparison with recent accurate calculation of the surface mean square displacements shows that the Armand correction is needed to compare the theoretical results with the experiment. The present data are also compared with similar data obtained for He by Boato, G. et al. Surf. Sci. 1982, 114, 485.
1. Introduction
The basal plane of graphite is a surface of great interest because of its characteristic structure and properties. It is, in fact, a surface of good uniformity, crystallographically simple, and chemically inert. Furthermore, graphite is interesting also from a theoretical point of view because it is one of the best examples known for a solid with very large anisotropy in most of its properties. From the experimental point of view, the interest in the (0001) surface of graphite and in its adsorption properties has been rising since uniform samples with large surface area became available. Since then several different experimental techniques have been applied to the study of physisorption on graphite. Systems such as graphitized carbon blacks and, more recently, different types of pyrolitic graphite (papiex, graphoil, etc.) have been shown to have a great degree of uniformity. Therefore these systems are uniquely suitable for studying the problem of physisorption and the phenomena of formation of bidimensional phases on a crystal substrate. Similar experiments on graphite single crystals have been carried out only using LEED (low energy electron diffraction), THEED (transmission high energy electron diffraction),Ia and X-rays.Ib More recently atomic beam diffraction has been shown to be very effective in the studies of physisorption on 'Present address: Centro C.N.R. Per I'Impianto Ionico e la Fisica degli Stati Aggregati 38050 Povo di Trento, Italy. *Present address: Dipartimento di Fisica, Universiti degli studi di Genova, Viale Dodecanneso 33, 16100 Genova, Italy.
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graphite single crystals.lc In the present paper the interest is focused onto the study of the substrate: the (0001) surface of graphite itself. The first atomic diffraction experiment from a graphite crystal was carried out by the Genova groups2 From the intensity patterns interpreted in terms of the Eikonal approximation to the H C W model a peak-to-peak corrugation lPp = 0.21 A has been determined which, in units of the lattice parameter, is about 8.5%. This value, although 10 times higher than the one observed for the Ag( 111) ~ u r f a c e ,is~ ,signifidantly ~ smaller than in the case of the alkali halides. No other intensity measurements with an He beam are available in the literature up to now. A great deal of work has been devoted instead to the study of bound-state resonances in this system. It turns out that He-(0001)C is an ideal system for studying resonances and band-structure effects. In fact the number of diffraction peaks is small, and the only sizeable Fourier component of the potential is the (10). The Genova g r o ~ p ~ , ~ , ~ (1) (a) *Ordering in Two Dimensions"; Sinha, S . K., Ed.;(North-Holland: Amsterdam, 1980. Fain, Jr., S.C. Springer Ser. Chem. Phys. 1982,20, 203. (b) Heiney, P. A,; Birgeneau, R. J.; Brown, G. S.; Horn,P. M.; Moncton, D. E.; Stephens, P. W. Phys. Rev. Lett. 1982,48, 104. (c) Ellis, T.H.; Iannotta, S.; Scoles, G.; Valbusa, U. Phys. Rev. E 1981, 24, 2307. Ellis, T. H. Ph.D. Thesis, University of Waterloo, 1984. Bracco, G.; Cantini, P.; Glachant, A.; Tatarek, R. Surf.Sci. 1983, 725, L81. (2) Boato, G.; Canthi, P.; Tatarek, R. Phys. Reu. Lett. 1978, 40, 887. (3) Boato, G.; Cantini, P.;Tatarek, R. Proc. 7th Int. Vac. Congr. Vienna, 1977.
0 1985 American Chemical Society
