758
Langmuir 1997, 13, 758-764
Adsorption of Ethyne on Cu(110): Experimental and Theoretical Study Julian R. Lomas,† Christopher J. Baddeley,‡ Mintcho S. Tikhov, and Richard M. Lambert* Department of Chemistry, Cambridge University, Lensfield Road, Cambridge CB2 1EW, England
Gianfranco Pacchioni Dipartimento di Chimica Inorganica, Metallorganica e Analitica, Universita` di Milano, via Venezian, 20133 Milano, Italy Received September 5, 1996. In Final Form: November 4, 1996X High-resolution electron energy loss spectroscopy and angle-resolved ultraviolet photoelectron spectroscopy data indicate that ethyne adopts a low symmetry (most likely C1) adsorption geometry on Cu(110). Detailed ab initio Hartree-Fock cluster calculations identify a minimum on the potential energy surface for ethyne in a C1 adsorption geometry. This structure also provides the best agreement between the experimental and calculated vibrational frequencies of the geometries investigated. In addition, the calculations show that the internal structure of the ethyne molecule is relatively insensitive to the adsorption site and that the adsorbed molecule is essentially sp2 hybridized.
1. Introduction The adsorption and reactions of ethyne on metal surfaces have been extensively studied in the past and are still the subject of numerous experimental and theoretical investigations.1-33 A few low index metal single crystal planes are active for the cyclization reaction of ethyne to * Corresponding author.
[email protected]. † Current address: EPSRC, Polaris House, North Star Avenue, Swindon SN2 1ET. ‡ Current address: Leverhulme Centre, Department of Chemistry, University of Liverpool, P.O. Box 147, Liverpool L69 3BX, U.K. X Abstract published in Advance ACS Abstracts, January 15, 1997. (1) Lomas, J. R.; Baddeley, C. J.; Tikhov, M. S.; Lambert, R. M. Langmuir 1995, 11, 3048. (2) Weinelt, M.; Huber, W.; Zebisch, P.; Steiru¨ck, H.-P.; Ulbricht, P.; Birkenheuer, U.; Boettger, J. C.; Ro¨sch, N. J. Chem. Phys. 1995, 102, 9709. (3) Tysoe, W. T.; Nyberg, G. L.; Lambert, R. M. J. Chem. Soc., Chem. Commun. 1983, N11, 623. (4) Sesselman, W.; Woratschek, B.; Ertl, G.; Kuppers, J.; Haberland, H. Surf. Sci. 1983, 130, 245. (5) Avery, N. R. J. Am. Chem. Soc. 1985, 107, 6711. (6) Tysoe, W. T.; Nyberg, G. L.; Lambert, R. M. Surf. Sci. 1983, 135, 128. (7) Hoffman, H.; Zaera, F.; Ormerod, R. M.; Lambert, R. M.; Wang, L. P.; Tysoe, W. T. Surf. Sci. 1990, 232, 259. (8) Baddeley, C. J.; Lee, A. F.; Lambert, R. M.; Giessel, T.; Fernandez, V. T.; Schaff, O.; Bao, S.; Theobald, A.; Hirschmugl, C.; Lindsey, R.; Bradshaw, A. M. In preparation. (9) Patterson, C. H.; Lambert, R. M. J. Phys. Chem. 1988, 92, 1266. (10) Patterson, C. H.; Lambert, R. M. J. Am. Chem. Soc. 1988, 110, 6871. (11) Ormerod, R. M.; Lambert, R. M.; Hoffman, H.; Zaera, F.; Yao, J. M.; Saldin, D. K.; Wang, L. P.; Bennett, D. W.; Tysoe, W. T. Surf. Sci. 1993, 295, 277. (12) Pacchioni, G.; Lambert, R. M. Surf. Sci. 1994, 304, 208. (13) Rucker, T. G.; Logan, M. A.; Gentle, T. M.; Muetterties, E. L.; Somorjai, G. A. J. Phys. Chem. 1986, 90, 2703. (14) Baddeley, C. J.; Jefferson, D. A.; Lambert, R. M.; Ormerod, R. M.; Rayment, T.; Schmid, G.; Walker, A. P. Mater. Res. Soc. Symp. 1992, 272, 85. (15) Abdelrehim, I. M.; Thornberg, N. A.; Sloan, Caldwell, T. E.; Land, D. P. J. Am. Chem. Soc. 1995, 117, 9509. (16) Bao, S.; Schindler, K. M.; Hoffman, P.; Fritzsche, V.; Bradshaw, A. M.; Woodruff, D. P. Surf. Sci. 1993, 291, 295. (17) Bandy, B. J.; Chesters, M. A.; Pemble, M. E.; McDougall, G. S.; Sheppard, N. Surf. Sci. 1984, 139, 87. (18) Hermann, K.; Witko, M. Surf. Sci. 1995, 337, 205. (19) Clotet, A.; Pacchioni, G. Surf. Sci. 1996, 346, 91.
S0743-7463(96)00865-7 CCC: $14.00
form benzene, most notably Pd(111)3,4,6-15 and Cu(110).1,5 In a previous paper we presented a study of this reaction over Cu(110)1 in which a rather detailed insight into the reaction mechanism and kinetics was gained and a number of important differences are found with the reaction over Pd(111). We have shown that ethyne cyclization to benzene over Cu(110) is an efficient reaction that proceeds at low temperatures with close to 100% efficiency. On the clean surface, C2H2 adsorbs into islands, there is no threshold coverage for the onset of reaction, and benzene evolution into the gas phase occurs in a single TPR peak due to a surface reaction rate limited process. In each of these four respects the behavior is very different from that found on Pd(111). The reaction mechanism has been established as
2C2H2(a) f C4H4(a)
(1)
C4H4(a) + C2H2(a) f C6H6(a)
(2)
C6H6(a) f C6H6(g)
(3)
with step 1 rate limiting overall; again in marked contrast to the reaction over Pd(111) where step 3 is rate limiting (20) Lee, L.-Q.; Shi, D.-H.; Zhao, Y.-J.; Cao, P.-L. J. Phys. Cond. Matter 1995, 7, 6449. (21) Marinova, T. S.; Stefanov, P. K. Surf. Sci. 1987, 191, 66. (22) Demuth, I. E. IBM J. Res. Dev. 1978, 22, 265. (23) Stuve, E. M.; Madix, R. J.; Sexton, B. A. Surf. Sci. 1982, 123, 491. (24) Bao, S.; Hoffman, P.; Schindler, K. M.; Fritzsche, V.; Bradshaw, A. M.; Woodruff, D. P.; Casado, C.; Asensio, M. C. Surf. Sci. 1994, 307309, 722. (25) Ibach, H.; Lehwald, S. J. Vac. Sci. Technol. 1981, 18, 625. (26) Chesters, M. A.; Canning, N. D. S. Vacuum 1981, 31, 695. (27) Chesters, M. A.; Canning, N. D. S. Surf. Sci. 1981, 111, 441. (28) Ibach, H.; Lehwald, S. J. Vac. Sci. Technol. 1978, 15, 407. (29) Chesters, M. A.; McCash, E. M. J. Electron Spectrosc. Relat. Phenom. 1987, 44, 99. (30) Gates, J. A.; Kesmodel, L. L. J. Chem. Phys. 1982, 76, 4281. (31) Timbrell, P. Y.; Gellman, A. J.; Lambert, R. M.; Willis, R. F. Surf. Sci. 1988, 206, 339. (32) Sellers, H. J. Chem. Phys. 1990, 94, 8329. (33) Tong, S. Y.; Li, C. H.; Mills, D. L. Phys. Rev. Lett. 1980, 44, 407; Phys. Rev. B 1981, 24, 806; Phys. Rev. B 1980, 21, 3057.
© 1997 American Chemical Society
Adsorption of Ethyne on Cu(110)
overall3,9 and step 2 is rate limiting for the overall surface reaction.15 Here we present a high-resolution electron energy loss spectroscopy (HREELS), angle-resolved UV photoelectron spectroscopy (ARUPS), and ab initio Hartree-Fock cluster model study of the adsorption of ethyne on Cu(110). HREELS data previously reported in a brief communication by Avery5 led to the tentative conclusion, from the similarity of the spectra on Cu(110)5 and Cu(100),21 that the ethyne molecule is likely to occupy “a common adsorption site, most probably a two-fold bridge site”. We have obtained HREEL spectra for the Cu(110)/ethyne adsorption system in which the Cu(110) sample is mounted with the [11h 0] contained within the scattering plane. This has enabled us to present a new and different interpretation of the spectra based on the an analysis of the normal modes predicted for ethyne in various adsorption geometries under the HREELS selection rules. These conclusions are in accord with our ARUPS data and are verified and extended by our cluster model calulations of ethyne geometries and vibrational frequencies on (110)-oriented Cu clusters.
Langmuir, Vol. 13, No. 4, 1997 759
2.1. Experimental Details. ARUPS data were collected using a modified VG ADES 400 (angle dispersive electron spectrometer) ultrahigh vacuum (UHV) system (chamber 1), which has been described in detail previously.34 The unpolarized photon source was a double differentially pumped inert gas (He) discharge lamp, and the analyzer was a rotatable concentric hemispherical analyzer. All the ARUP spectra were collected using He I (21.2 eV) radiation and are the result of five averaged scans collected over a 7 min period. The HREELS data were collected using a VSW ARIES UHV chamber (chamber 2) incorporating a VSW HREELS system, also described in detail previously.35 Specular HREEL spectra were collected at an incident angle of ∼45° with a primary beam energy of 13 eV; all the HREEL spectra are the result of 99 averaged scans collected over a 8.5 h period. In both the ARUPS and HREELS experiments the crystal was mounted with the [11 h 0] direction contained within the scattering plane. Adsorption temperatures of 160 and 140 K were used in chamber 1 and chamber 2, respectively. The sample was cleaned using the sputter/anneal procedure described in ref 1 until no impurities were detectable by either XPS or X-ray excited AES. Gas exposures were carried out by back-filling the chamber, and coverages were calibrated by comparison of the C(1s) XPS integrated intensity with that from a saturation coverage of ethyne, known to correspond to 0.5 ML1 (i.e., 1 monolayer of C atoms). 2.2. Computational Details. We have determined ab initio Hartree-Fock self-consistent field (SCF) wavefunctions for clusters with and without a single adsorbed ethyne molecule. The metal atoms of the substrate have been described using effective core potentials (ECPs). Two types of ECP have been used, an 11-electron ECP where the 1s2 to 3p6 electrons are treated as core36 and a 1-electron ECP where only the 4s electron is treated as valence.37 This latter ECP has only been used for the Cu atoms at the cluster periphery while the Cu atoms in the nearest neighbor positions to the ethyne molecule have been
treated with the 11-electron ECP. In a previous work19 test calculations have shown that the use of 1-electron ECPs for the Cu atoms close to the adsorbate results in too short adsorbate-cluster distances and an overestimation of the interaction energy due to insufficient representation of the core-core repulsions. However the use of 1-electron ECPs for the Cu atoms on the periphery gives almost identical results to those obtained with the 11electron ECPs. In the present work the effects of the position of the 1-electron ECPs on the adsorption properties have been carefully checked in order to avoid artifacts due to the reduced repulsion of the adsorbate with these Cu atoms. The [2s1p1d]36 contracted Gaussian-type orbital basis sets have been used for the Cu 11-electron ECPs, and for the Cu 1-electron ECP a [2s1p]37 basis set has been used. The ethyne molecule has been described using the reduced MIDI-2 [3s2p]38 basis set for C and a double-zeta (DZ) [2s]38 basis for H. The basis sets used give rise to the well-known basis set superposition errors (BSSEs) which for Cu-ethyne cluster systems have been calculated previously19 using the standard Boys-Bernardi counterpoise method.39 For ethyne on Cu clusters the BSSEs have been found, in SCF, to be ∼0.10 eV for the Cu clusters and ∼0.10 eV for the ethyne molecule giving a total BSSE for the Cu-C2H2 adsorption energies of ∼0.20 eV: i.e. the interaction energy is overestimated by this value. However, given the other difficulties found in interpreting the energetics of these systems18,19 (see below), the binding energies quoted in this study will not be corrected for the BSSE. In some cases it has been necessary to use open shell clusters, and in these cases only the restricted HartreeFock (RHF) formalism has been used. Several, low-spin (i.e., singlet for closed shell and doublet for open shell clusters), electronic configurations have been considered to ensure that the lowest energy bound state is found for each cluster. In all cases the adsorbate geometries have been fully optimized, within the constraints of the point group symmetry, using analytical gradients. During the optimizations the Cu cluster atoms are fixed at their original geometry while no constraints (other than those of symmetry) are placed on the adsorbate geometry. The resulting stationary points on the potential energy surface (PES) have been characterized by means of a full vibrational analysis determined using numerical derivatives evaluated by finite difference of analytical first derivatives (harmonic frequencies only). A general problem is found in interpreting the energetics found for Cu-C2H2 clusters in that they are unbound with respect to the dissociation limit but with local minima on the energy surface. This phenomenon has been investigated in detail by Clotet and Pacchioni19 for the Cu(111) surface and is known to involve a crossing or avoided crossing of electronic states. A similar situation is found here for the Cu(110) surface. This presents a major difficulty in interpreting the adsorbate binding energies obtained for these systems. Introduction of correlation has been shown to be essential in order to achieve more reliable estimates of adsorption energies.18,19,40,41 However, in order to achieve the accuracy required to distinguish between adsorption
(34) Badyal, J. P. S.; Gellman, A. J.; Judd, R. W.; Lambert, R. M. Catal. Lett. 1988, 1, 41. (35) Horton, J. H.; Moggridge, G. D.; Ormerod, R. M.; Kolobov, A. V.; Lambert, R. M. Thin Solid Films 1994, 237, 134. (36) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (37) Bagus, P. S.; Bauschlicher, C. W.; Nelin, C. J.; Laskowski, B. C.; Seel, M. J. Chem. Phys. 1984, 81, 3594.
(38) Tatewaki, H.; Huzinaga, S. J. Comput. Chem. 1980, 1, 205. (39) Boys, S. F.; Bernardi, F. Mol. Phys. 1970, 19, 553. (40) Pacchioni, G., Bagus, P. S., Parmigiani, F., Eds. Cluster Models for Surface and Bulk Phenomena, NATO ASI series B; Plenum Press: New York, 1992; Vol. 283. (41) Illas, F.; Rubio, J.; Ricart, J. M. J. Mol. Struct. (THEOCHEM) 1993, 287, 167.
2. Experimental and Computational Details
760
Langmuir, Vol. 13, No. 4, 1997
Lomas et al.
Figure 1. Schematic representation of the eight C2v structures for ethyne on Cu(110) investigated using cluster models.
sites on the basis of energetics, larger clusters and a multiconfiguration treatment are required which are not accessible given the number of valence electrons and the computing power available in this study. A further method to avoid the cluster size dependency of the binding energies has been proposed for simple atomic adsorbates,42 known as the “bond preparation” approach where the reference cluster is that of an excited state which better represents the situation of the infinite surface. However, the choice of bond prepared cluster is not always straightforward and implies an a priori knowledge of the bonding mechanism, and therefore this method has not been employed in this study. Given the difficulties in interpreting the energetics for these systems highlighted above and the problem of the cluster size dependency of interaction energies (see below), no conclusions in this study will be based on the grounds of energetics alone. Several clusters were used in this study and Figures 1 and 2 show the adsorption geometries investigated. The Cu-Cu distances in the cluster have been taken from the bulk value of 2.54 Å. The effect of cluster size has also been assessed in selected cases, and the results for one of the geometries investigated (X2b) are presented in Table 1. Clearly there are no significant changes in the adsorbate geometry with cluster size; however, there are large oscillation in the interaction energy (Eb). Finally, selected calculations have been performed at MP2 level, in order to assess to effects of correlation on the adsorption geometry and binding energy of ethyne on (110) oriented Cu clusters. Selected results of MP2 geometry optimizations for ethyne on (110) oriented Cu clusters are presented in Table 2. It is well-known that calculations of hydrocarbons at the Moller-Plesset second-order perturbation theory (MP2) level overestimate single bond distances by, on average, 0.02 Å19,43 and that multiple bond distances are even less well reproduced, giving an average overestimate
Figure 2. (a) Ball and stick representations of the eight optimized C2v structures for ethyne on (110) oriented Cu clusters. These structures are detailed in Table 6. (b) Two views of the low symmetry (C1) ethyne structure on the Cu12(6,6) cluster. The shortest Cu-C distances are 2.03 Å for the C atom nearest the surface and 2.30 Å for the Cu atom furthest above the surface. The CdC axis is tilted 18° away from the surface plane. The top figure is a plan view of the surface and the lower view is a view down the [11h 0] direction in the surface. The structure is detailed in Figure 5.
(42) Panas, I.; Schule, J.; Siegbahn, P. E. M.; Wahlgren, U. Chem. Phys. Lett. 1988, 149, 265.
of 0.06 Å.43 This is clearly demonstrated by comparison of the MP2 optimized geometry for free ethyne with the
Adsorption of Ethyne on Cu(110)
Langmuir, Vol. 13, No. 4, 1997 761
Table 1. Optimized Geometries and Binding Energies for Ethyne in the X2b Site on (110) Oriented Clusters of Various Sizesa cluster
Eb
r(CC)
r(CH)
R(CCH) (deg)
r(C-Cu)
Cu8(2,6) Cu10(4,6) Cu18(12,6) Cu22(12,10)
+2.22 +2.38 +0.78 +1.09
1.36 1.36 1.37 1.36
1.10 1.10 1.10 1.10
121 120 119 118
2.03 2.04 2.05 2.05
a All distances are given in Å and the binding energies are given in eV. The sign convention is that a positive value indicates that the adsorbed molecule is unbound with respect to the separate fragments.
Table 2. Selected Results of MP2 Optimizations of Ethyne on (110) Oriented Cu Clustersa
site
cluster
no. of 11-electron ECPs
Eb
D∞h C2H2 computed in SCF D∞h C2H2 computed in MP2 4 +1.06 X2b Cu8(2,6) (+2.22) 2b +0.40 X2b Cu10(4,6) (+2.36) 4 +1.26 Y1b Cu8(2,6) (+2.55) 4 +1.81 Y2a Cu8(6,2) (+3.07)
r(CC) r(CH) 1.20 1.24 1.39 (1.36) 1.36 (1.35) 1.40 (1.38) 1.41 (1.38)
1.06 1.08 1.12 (1.10) 1.11 (1.10) 1.12 (1.10) 1.13 (1.11)
R(CCH) (deg) r(Cu-C) 180 180 122 (121) 121 (119) 125 (123) 116 (115)
2.03 (2.03) 1.97 (2.04) 2.19 (2.19) 2.09 (2.12)
a All distances are given in Å. E is the binding energy, in eV, b with respect to the bare Cu10(4,6) cluster computed at MP2 level and free ethyne (D∞h) computed at the MP2 level. In each case the corresponding SCF values are given in parentheses. In addition both the calculated geometries of free ethyne at SCF and MP2 level are given. b This structure was calculated using a larger doublezeta plus polarization (DZP) quality basis sets for Cu ([2s1p2d]36) and C ([3s2p1d]37).
SCF ethyne structure where the C-H single bond distance is found to be 0.02 Å longer in MP2 and the C-C triple bond distance is longer by 0.04 Å. Given this artifact of MP2 calculations, it is clear that there are no significant differences in the internal geometry of ethyne adsorbed on (110) oriented Cu clusters when correlation is introduced at MP2 level providing further confidence in the SCF internal geometries for the adsorbates obtained in this study. However, in some cases a shorter Cu-C carbon distance is found which indicates that the SCF calculation overestimates this distance due to the absence of correlation effects. In general SCF calculations of inorganic complexes tend to overestimate metal to hydrocarbon bond distances by, on average, 0.08 Å.43 There is also a large difference in the binding energies obtained in SCF and MP2 which is partially accounted for by the larger BSSE for MP2 calculations. The MP2 BSSE for ethyne on Cu clusters is somewhat larger than the value of ∼0.20 eV obtained in SCF and has previously been determined as being of the order of 1.20 eV.19 All the calculations were performed on IBM Risc 6000 workstations by means of the HONDO 8.5 program package.44 3. Experimental Results and Discussion Figure 3 shows HREEL spectra collected at specular reflection for 0.5 ML and 0.09 ( 0.03 ML of C2H2 and 0.5 ML of C2D2 on Cu(110). The 0.5 ML C2H2 spectrum is in good agreement with that published previously by Avery.5 Bands are clearly identified at 2880, 1140 (with a broad, high-frequency tail), 960, and 620 cm-1. Other than (43) Hehre, W. J.; Radom, L.; Schleyer, P. V. R.; Pople, J. A. In Ab Initio Molecular Orbital Theory; Wiley: New York, 1986. (44) Dupuis, M.; Johnston, F.; Marquez, A. HONDO 8.5 for CHEMStation; IBM Co.: Kingston, 1988.
Figure 3. HREEL spectra of 0.5 and ∼0.09 ML C2H2 and 0.5 ML C2D2 adsorbed at 140 K on Cu(110), collected at specular reflection with an incidence angle of ∼45° and a primary beam energy of 13.0 eV. All the spectra presented are the result of 99 averaged scans collected over a period of 8.5 h. Table 3. Comparison of the HREELS Frequencies for 0.5 ML C2H2 Adsorbed on Cu(110) Obtained in this Work with Those of Avery5 a ν(CH)
ν(CC) δ(CCH)as δ(CCH)s γ(CH)s
frequencies in 2880 b present data (∼2150)c (1260) frequencies from 2900 1305 Avery5 (2190) (1280)
1140 (950) 1140 (930)
960 (680) 940 (680)
620 (- - -) 640 (510)
a All frequencies are given in cm-1 and have been found to be reproducible to (10 cm-1. The frequencies obtained from 0.5 ML of C2D2 are given in parentheses. b Not resolved in this work. c Overlaps with the band at ∼2090 cm-1 due to a small amount of coadsorbed CO.
overall intensity changes, there are no significant differences between the spectra collected at the two coverages. This provides strong support for the previous assertion1 that ethyne adsorbs into islands on Cu(110). Table 3 compares these results with those of Avery.5 The vibrational mode assignments are made by comparison with previous HREELS studies of adsorbed ethyne and by comparison with the calculated vibrational frequencies of ethyne on Cu cluster models presented in section 4. Due to instrumental limitations, it is not possible to resolve the ν(CC) mode which Avery5 observed at 1305 cm-1; however, this mode is clearly visible at 1260 cm-1 in the C2D2 spectrum. The C2D2 frequencies are compared with Avery’s results in Table 3. It is interesting to note that the intensity of the ν(CC) band in our spectrum is significantly weaker than it is in Avery’s spectrum; in Avery’s spectrum it is one of the most intense bands. This suggests that this mode has a significant impact scattering contribution, given the only difference between the two experiments is the orientation of the [11h 0] direction in the Cu(110) surface. Table 4 shows the irreducible representations to which each of the ethyne vibrations belong in various point group symmetries. Due to the selection rules imposed on impact scattering in the specular direction in the presence of a mirror plane or a C2 axis,45 only those modes which are totally symmetric are allowed in either dipole or imact scattering in specular reflection. Symmetry analysis of the normal modes of ethyne adsorbed on Cu(110) in various geometries reveals that ethyne on Cu(110) must be located in a low symmetry adsorption site. This conclusion holds regardless of the (45) Woodruff, D. P.; Delchar, T. A. Modern Techniques of Surface Science (CUP); 1986.
762
Langmuir, Vol. 13, No. 4, 1997
Lomas et al.
Table 4. Showing the Irreducible Representations to Which Each of the Vibrations of Adsorbed Ethyne Correspond in Various Symmetries on Cu(110)a symmetry CdC axis ıı [11 h 0]
CdC axis ıı [001]
site
C2v
C2
Csxz
Csyz
Csxz
Csyz
ν(CH)as ν(CH)s ν(CC) δ(CCH)as δ(CCH)s γ(CH)as γ(CH)s
b1 a1 a1 b1 a1 a2 b2
b a a b a a b
a′ a′ a′ a′ a′ a′′ a′′
a′′ a′ a′ a′′ a′ a′′ a′
a′′ a′ a′ a′′ a′ a′′ a′
a′ a′ a′ a′ a′ a′′ a′′
a The z-axis is taken as parallel to the surface normal and the x-axis is taken as being parallel to the [11 h 0] direction.
mechanism of excitation of the vibrations in the specular HREEL spectrum due to the selection rules imposed on impact scattering in the specular direction with the [11h 0] direction contained within the scattering plane. Clearly if there are no symmetry elements in the adsorbate/ substrate point group, then all the modes become allowed for both dipole and impact scattering in the specular and off-specular directions. The above analysis assumes that only a single type of adsorption site was occupied at the coverages used in the HREELS experiments; this seems most likely given the evidence for adsorption of ethyne into islands presented in ref 1. The selection rules summarized above suggest that the none of the point group symmetries considered in this table are consistent with the experimental data and that the molecule/surface complex must have no symmetry elements at all. However, the γ(CH)s band is thought to be due to a negative ion resonance for ethyne adsorbed on Pd(111)31,32 and therefore will obey different selection rules which allow this band to occur in specular HREELS. As a result, two more symmetries become “allowed”: Csxz (i.e., with x-z plane as a symmetry plane) with the CdC axis parallel to the [11h 0] (x) direction and Csyz with the CdC axis parallel to the [001] (y) direction. In an attempt to determine the mechanism of excitation of the γ(CH)s mode for specular scattering, a spectrum was recorded at 6 eV primary beam energy (5 eV is the practical limit of the instrument). There was no discernible change in the relative intensity of the γ(CH)s band compared with the other bands in the spectrum. This suggests that the band is not due to a negative ion resonance although this conclusion is tentative as the resonant behavior may occur at much lower energies. Additionally, off-specular HREELS indicates that the intensity of this band is not entirely due to dipole scattering. Therefore the two Cs symmetries referred to above cannot be ruled out. However, there appears to be at least some dipole scattering contribution to this band, and therefore the C1 symmetry would appear most likely. There appears to be at least one other known case of C2 hydrocarbon adsorption at a low symmetery site on an fcc(110) surface. In a recent paper, Gutdeutsch et al.46 found that ethene on Ni(110) occupies a low symmetry site halfway between the atop and bridge site on top of the [11h 0], close-packed rows of the surface. This results in Csxz symmetry with the CdC axis parallel to the [11h 0] direction, i.e., one of the two Cs symmetries that cannot conclusively be ruled out for the ethyne Cu(110) system. Finally, it is possible that the γ(CH)as mode is present in either or both of the specular or off-specular HREEL (46) Gutdeutsch, U.; Birkenheuer, U.; Bertel, E.; Cramer, J.; Boettger, J. C.; Ro¨sch, N. Surf. Sci. 1996, 345, 331.
spectra as it is found to be almost degenerate with the δ(CCH)s mode in the vibrational spectra calculated for section 4. However, this γ mode can only be active if C1 symmetry holds, and therefore the presence or absence of this band does not significantly affect the arguments above. He I ARUP spectra have been collected with unpolarized UV light from a saturation coverage (0.5 ML1) of ethyne on Cu(110) at many different combinations of angles of incidence (0°-70°) and emission (0°-70°) and Figure 4 shows representative ARUP spectra collected at three typical experimental geometries. Assignments of the resonances are made by comparison with gas phase47 spectra and previous work2,48 and are given in Table 5 as well as being indicated in the figures. If a high-symmetry (say C2v) site geometry were adopted, one would expect to find significant angular intensity variations in the UP spectra due to the changes in magnitude of the electric field perpendicular to the surface as the angle of incidence is varied and due to the changes in Cartesian components of the momentum of the detected photoelectrons as the angle of emission is varied. The ethyne-derived features in the ARUP spectra collected from 0.5 ML of ethyne on Cu(110) show limited angular variation in the intensity providing a further indication of a low-symmetry adsorption site. 4. Cluster Model Results and Discussion Table 6 gives the fully optimized geometries for ethyne adsorbed in each of the C2v sites on (110) oriented Cu clusters. Ball and stick representations of the eight optimized structures are presented in Figure 2a (see section 2.2). For each cluster we give the number of Cu atoms in each layer of the cluster and the number of Cu atoms treated with 11-electron ECPs. Each of these optimized geometries has been characterized by means of a full vibrational analysis (harmonic frequencies only) and those structures which have been found to have modes with a significant negative frequency and therefore are transition states are indicated by italic type in the table. Full vibrational analyses of each adsorption geometry are presented in Table 7. It is clear that the internal geometry of the adsorbed ethyne molecule is relatively insensitive to the choice of binding site, with the CdC distances, r(CC), all lying in the range 1.36-1.40 Å, the C-H distances, r(CH) all in the range 1.10-1.12 Å, and the CCH angles, R(CCH) all in the range 116°-122°: these compare with 1.34-1.39 Å and 115-120° for previous calculations on (111) oriented Cu clusters.18,19 From these data one is immediately drawn to the conclusion that the carbon atoms in the ethyne molecule are rehybridized from sp character to very close to sp2 character, signified by the CCH bond angle of around 120°: the CCH bond angles and CdC bond lengths in gas phase ethylene and benzene (both sp2 hybridized) are 121° and 1.34 Å and 120° and 1.40 Å, respectively.49 This is particularly interesting because the dominant reaction pathway for ethyne on Cu(110) is formation of benzene.1,5 Given the conclusion from the experimental data that the ethyne is adsorbed in a low symmetry (either Cs or C1) site on Cu(110), a number of other calculations have been performed in low symmetry adsorption sites consistent with the experimental HREELS data. The sites investigated were as follows: a C2 site with the center of the CdC bond above a short bridge site; two Csxz sites with (47) Bieri, G.; Åsbrink, L. J. Electron Spectrosc. Relat. Phenom. 1980, 20, 149. (48) Demuth, J. E. Surf. Sci. 1979, 84, 315. (49) CRC Handbook of Chemistry and Physics, 73rd ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 1992.
Adsorption of Ethyne on Cu(110)
Langmuir, Vol. 13, No. 4, 1997 763
Figure 4. Three representative ARUP spectra for ethyne adsorbed on Cu(110) at 160 K: (a) at an incidence angle (Ψ) of 40° and normal emission (Φ); (b) Ψ ) 40°, Φ ) 30°; (c) normal incidence and Φ ) 35°. In each case the lower trace (solid line) is the result of subtracting the clean surface spectrum (dashed line) from the saturation benzene spectrum (bold line). The legend line at the bottom of each figure is intended to indicate the expected positions of the resonances due to these orbitals. Table 5. Comparison of the UPS Binding Energies of Ethyne Molecular Orbitals at Saturation Coverage on Cu(110), Ni(110), and Pd(111)a C2H2 orbital 1πg 1πud 3σg 2σu 2σg
binding energy on Cu(110)/eV, this workb 5.2-7.3 9.1 11.2
binding energy on Ni(110)2/eVc
binding energy on Pd(111)47/eVb
4.5 6.0-7.5 9.0 11.2 16.8
4.6-6.2 9.0 11.0
a All binding energies are given with respect to the Fermi energy. Binding energies are given from normal emission spectra. c Binding energies are given at the Γ-point (k ) 0). d The 1π orbitals ıı u are degenerate (pz and py) in gas phase ethyne; however, this degeneracy is lifted on adsorption. b
the CdC axis parallel to the [11 h 0] direction, one with the CdC axis above the rows and another with the CdC axis above the troughs; a Csyz site with the CdC axis perpendicular to the [11 h 0] direction derived from the Y1b site (Figures 1 and 2a) by tilting the molecule into the trough; and the C1 site described below. Starting from these low symmetry geometries, we found that the molecule moves into one of the local C2v minima listed in Table 6 and Figure 2a; thus no local minima were found on the PES for ethyne adsorbed in any of the C2 or Cs geometries investigated, including that known for ethene on Ni(110).46 As the structure of an fcc(110) surface can be visualized as consisting of (111) microfacets, a C1 structure based on the known bridging adsorption site for ethyne on Cu(111)16 was tried. For this purpose we used a Cu12(6,6) cluster shown in Figure 2b; the two peripheral second layer atoms are present to simulate the second layer of a (111)-oriented cluster. Optimization of this structure reveals that there is a minimum on the PES corresponding to the molecule adsorbed in a C1 geometry and the final structure is shown in Figure 2b. This adsorption site bears a marked similarity with the known adsorption site for ethyne on Cu(111).16 The internal structure of the ethyne molecule is detailed in Figure 5. The 3° torsion of the C-H bonds
Table 6. Optimized Geometries and Binding Energies for Ethyne in the Eight C2v Adsorption Sites on Various (110) Oriented Cu Clustersa
site gas gas X1a X1b X2a X2b Y1a Y1b Y2a Y2b
cluster
no. of 11-electron ECPs
Eb
exptl48 calcd (SCF) Cu8(6,2) Cu11(8,3) Cu7(3,4) Cu10(4,6) Cu7(3,4,1) Cu10(4,6) Cu8(6,2) Cu11(8,3)
4 7 3 2 3 2 2 5
+4.10 +2.91 +3.28 +2.38 +3.82 +3.07 +3.05 +0.19
r(CC) r(CH)
R(CCH) (deg) r(Cu-C)
1.20 1.20
1.06 1.06
180 180
1.39 1.40 1.40 1.36 1.38 1.38 1.37 1.36
1.12 1.10 1.10 1.10 1.10 1.10 1.11 1.10
117 118 117 120 122 121 116 116
2.44 2.30 2.07 2.04 2.07 2.19 2.11 2.16
a All distances are given in Å and E is the binding energy, in b eV, with respect to the bare cluster and free ethyne (D∞h). The sign convention is that a positive value indicates that the adsorbed molecule is unbound with respect to the separate fragments. In addition both the experimental and calculated (SCF) geometries of free ethyne are given. The rows in italic correspond to the structures found to be transition states. The numbers in parentheses refer to the number of Cu atoms in the first and second layers (and in one case third layer) of the cluster used.
out of the HCCH plane implies that within the accuracy of these calculations the C-H and CdC bonds in the adsorbed ethyne structure are essentially coplanar. It is interesting to note that the internal structure of the molecule is essentially the same as that found for the C2v adsorption sites again, supporting the assertion that the adsorbate structure is almost independent of the adsorption site and is essentially rehybridized to sp2. The interaction energy of the adsorbed ethyne in this case is +0.69 eV, which is more stable than all the C2v structures with the exception of Y2b. Therefore this structure is clearly a candidate for the “real” structure. Note that the asymmetry of the cluster could lead to an asymmetric electrostatic potential outside the surface. This in turn could lead to spurious minima on the potential energy
764
Langmuir, Vol. 13, No. 4, 1997
Lomas et al.
Table 7. Calculated Shifts in Vibrational Frequency from the Free Ethyne Values for Ethyne Adsorbed on (110) Oriented Cu Clusters in the Optimized Geometries Presented in Table 6a site exptl values (this work) exptl values5 X1a X1b X2a X2b Y1a Y1b Y2a Y2b C1
ν(CH)s ν(CH)as ν(CC) δ(CCH)as δ(CCH)s -494b -474b -660 -853 -692 -630 -475 -632 -503 -685 -519 -698 -510 -680 -591 -782 -495 -550 -481c -498d
-669 -775 -852 -909 -673 -770 -752 -681 -647 -700
+410 +410 +504 +498 +503 +475 +393 +474 +552 +549 +466
+230 +210 +219 +216 +239 +260 +189 +200 +325 +343 +200
a Frequencies are given in cm-1. The rows in italic correspond to the structures found to be transition states. b Not resolved in HREELS. c Stretching of the CH bond pointing away from the surface. d Stretching of the CH bond pointing into the surface.
size of the basis sets used and the absence of correlation effects. Additionally, the calculation of only harmonic frequencies tends to result in the vibrational frequencies found being higher than the “real” anharmonic frequencies. Table 7 compares the experimental and calculated shifts in vibrational frequencies on adsorption. This provides a more reliable quantitative comparison between experiment and theory than the often used method of using a scaling factor to bring the SCF frequencies into line with the experimental values.12,19 No values are given for the γ(CH) modes of adsorbed ethyne as these have no direct analogue in the free ethyne molecule because they do not involve any changes in the bond distance or bond angles within the adsorbed molecule. Clearly the best agreement between the experimental and calculated data is found for the C1 structure. These findings strongly reinforce the conclusion from the previous section that this C1 structure is a credible candidate for the “real” structure which is known to occupy a low-symmetry site from HREELS (see Chapter 3). 5. Conclusions
Figure 5. Detail of the internal structure of the ethyne molecule in the C1 adsorption geometry found in the cluster calculations.
surface for electrostatically bound molecules. However the C2H2 is chemically and not electrostatically bonded. More importantly, as already mentioned in section 2.2, we do not rely on the energetics for the assignment of adsorption sites given the limitations of the cluster approach. On the contrary the vibrational data, both experimental and calculated, of the adsorbed species provide a much more useful tool for this tentative assignment. Vibrational analysis of this structure reveals that there is an imaginary vibrational frequency with significant amplitude on the ethyne molecule at -92 cm-1. However, this is attributed to the numerical “noise” in the geometrical optimization. In fact, for all the geometry optimizations performed in this work, the optimization is considered converged when the energy gradients are smaller than a given threshold, T (T ) 0.0005 au), and so the gradients for the final structure are not numerically zero. Therefore this geometry remains a credible candidate for the “real” structure which is from HREELS known to have low symmetry (see section 3). For hydrocarbons it is well-known that SCF gives frequencies which are consistently higher than the “true” experimental values; i.e., the use of single determinant wavefunctions (SCF) yields too steep a potential in the vicinity of the equilibrium structure on the PES. This is a well-known characteristic of SCF19,50 calculations of vibrational frequencies and is not surprising given the
We have presented HREELS data for ethyne on Cu(110) in an experimental geometry ([11h 0] direction contained within the scattering plane) which has allowed us to place a new and different interpretation on these data based on an analysis of the normal modes predicted for ethyne in various adsorption symmetries given the HREELS selection rules. Our experimental data (HREELS and ARUPS) strongly indicate that ethyne adsorbs into a low symmetry adsorption site on Cu(110), probably with no symmetry elements at all (C1). Ab initio Hartree-Fock cluster calculations have identified a low-symmety adsorption geometry which is a minimum on the PES and has C1 symmetry. The internal structure of the ethyne molecule in this site is very similar to those found for the C2v adsorption sites, and this relative structure insensitivity of the geometry provides confidence in the conclusion that the ethyne molecule is essentially sp2 hybridized when adsorbed on Cu(110). Finally the calculated vibrational frequencies for ethyne adsorbed in the C1 site are clearly in the best agreement with the experimental frequencies providing further evidence that this structure is indeed a strong candidate for the “real” adsorption geometry. Acknowledgment. The experimental work was supported under Grant No GR/K45562 awarded by the U.K. Engineering and Physical Sciences Research Council and the computation work was supported by the Italian Ministry of Research. J.R.L. thanks the support of the EPSRC under studentship number 93305671 and St John’s College, Cambridge, for assistance in funding his visit to Milan. We are grateful to Johnson Matthey plc for the loan of precious metals. LA960865J (50) Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. In Ab Initio Molecular Orbital Theory; Wiley: New York, 1986.
