J. Phys. Chem. 1987, 91, 2230-2232
2230
that the shift of the O H frequency from 3610 cm-’ in H-RHOS773 to 3640 cm-’ in H - R H O S 7 7 3 and the weak acidity accompanying the presence of the 3640-cm-’ band in H-RHO-SS773 is related to the occurrence of the N F A in the six-ring. Acknowledgment. We thank Dr. A. J. Vega for communicating the N M R results, Dr. Vega and Dr. J. D. Jorgensen for discus-
sions, the North Atlantic Treaty Organization for Research Grant No. 149/84 and the Computer Center of the University of Illinois at Chicago, where part of the work was done, for computer time. R.X.F. and W.H.B. thank duPont de Nemours & Co. for a grant. The work at the Intense Pulsed Neutron Source was supported by the U S . Department of Energy, BES-Materials Science, under contract W-31-109-ENG-38.
Comparlson of Benzene Adsorption on Ni( 111) and Ni( 100) A. K. Myers, G. R. Schoofs, and J. B. Benziger* Department of Chemical Engineering, Princeton University, Princeton, New Jersey 08544 (Received: January 12, 1987)
The adsorption of benzene on the Ni( 100) and the Ni( 111) crystal faces was compared in order to investigate the effect of crystallographic orientation on the interaction of benzene with nickel. Temperature programmed reaction (TPR) was used to characterize adsorption bond strengths and determine product distributions. Benzene was found to adsorb 44 kJ/mol less strongly on the Ni( 111) plane than on the Ni( 100) surface. Di-hydrogen evolution formed after decompositionof benzene was similar for both surfaces. Benzene chemisorption was modeled by using extended Hilckel theory (EHT), a semiempirical molecular orbital method. The calculations predict bonding of benzene over a threefold hollow site on Ni( 11 1). Multicenter bonding of the benzene carbon atoms with the nickel atoms is indicated by the calculations. The binding strength of benzene is controlled by the degree of overlap of the carbon x orbitals with the nickel atom orbitals. Benzene binds more strongly to the Ni( 100) surface because the carbon x orbitals can overlap with four nickel atoms on the fourfold hollow site, whereas on Ni(ll1) the carbon atoms are closely associated with only three nickel atoms on the threefold hollow site.
It is known that the crystallographic orientation of the surface of metal catalysts may be an important factor in determining catalytic activity. Thus, it is important to understand how sensitive the chemisorption and reaction of a particular molecule is to the arrangement of surface metal atoms. In this study, we have investigated the influence of surface structure on the chemisorption of benzene on nickel. Adsorption on the (100) and (1 11) crystal faces of nickel was compared by using both experiment, temperature programmed reaction (TPR), and a molecular orbital model, extended Huckel theory (EHT). Experiments were performed on clean Ni( 100) and Ni( 11 1) crystals in a stainless steel ultrahigh vacuum chamber described previously.1,2 The crystallographic orientations were verified by low-energy electron diffraction (LEED). Benzene (99%) was obtained from Mallinckrodt and was used as received. Prior to each experiment the cleanliness of the crystal was verified by Auger electron spectroscopy (AES). Adsorption was carried out a t 250 K or below; exposure was sufficient to ensure saturation coverage. TPR was used to determine product distributions. After adsorption, the crystal temperature was linearly ramped to 600 K, and the desorbing gases were monitored by a quadrupole mass spectrometer located 6 cm directly in front of the crystal face. Five mass-to-charge ratios ( m / q ) were recorded simultaneously by a microcomputer. Adsorbed carbon remaining on the surface after an experiment was measured by AES. Figure 1 compares the TPR spectra for benzene and di-hydrogen desorption from Ni(100) and Ni(ll1) following saturation coverage at 200 K. On the (100) face, benzene desorbed in a peak at 475 K with a long tail trailing to lower temperatures. Competing with molecular desorption is the thermal decomposition of benzene, which produces H2desorption between 350 and 500 K with the primary H2 peak at 500 K. After heating to 600 K, AES indicates a surface contaminated with carbidic carbon at a coverage of 0.20 monolayers. The benzene TPR spectrum following adsorption at 250 K on the (1 11) nickel crystal exhibited a sharp peak at 300 K with a broad shoulder trailing down to 450 K. Di-hydrogen is evolved in a broad peak centered at 450 K. *Author to whom inquiries should be addressed.
0022-3654/87/2091-2230%01.50/0
After the temperature was raised to 600 K carbon coverage was 25% of a monolayer. Excluding benzene fragments formed in the mass spectrometer, no carbon-containing reaction products were detected from either surface. On both crystal planes of nickel, benzene decomposition, as evidenced by the di-hydrogen evolution and formation of adsorbed carbon, competed with molecular desorption. H2 was produced from benzene decomposition at roughly the same temperature for reaction of benzene on both the Ni( 11 1) and the Ni( 100) face. However, the TPD data show that, while molecular benzene desorbed at about 475 K on Ni(100), desorption of benzene on Ni( 1 11) occurred a t much lower temperatures (the main peak a t 300 K). Thus we may estimate a difference in molecular benzene binding energy’ on the two nickel crystal faces of about 44 kJ/mol. Extended Huckel calculations with a correction for repulsion have been performed previously by us for benzene on a 17-atom nickel cluster with (100) symmetry! Those results indicated that benzene bonds flat on the metal surface, most probably over a fourfold hollow site (see Figure 2a). In this study we have extended the E H T calculations to benzene on Ni( 11 1 ) in order to explain the difference in adsorption strength on the (1 1 1) and (100) surfaces. In most applications of EHT, the total energy of the system is given as the sum of the energies of the occupied molecular orbitals, Eh. Total energies calculated simply as the sum of EHT orbital energies omit internuclear repulsion and a u n t electron-electron repulsion twice and are only valid if the internuclear repulsion and the electron-electron repulsion contributions cancel one another. In general, and particularly for small internuclear distances, this is not the case. In our calculations, the total energy of the molecule adsorbed on the cluster is given as E, + E,, where E, is a repulsive term to compensate for the neglect of the difference in energy between the internuclear and interelectronic repulsion terms.5 A more detailed discussion of the (1) Schoofs, G.R. Ph.D. Thesis, Princeton University, 1986. (2) Benziger, J. B.; Preston, R. E. J . Phys. Chem. 1985, 89, 5002. (3) Redhead, P. A. Vacuum 1962, 12, 203. (4) Myers, A. K.; Benziger, J. B. Langmuir, in press. ( 5 ) Anderson, A. B. J. Chem. Phys. 1975,62,1187. 0 1987 American Chemical Societv
The Journal of Physical Chemistry, Vol. 91, No. 9, 1987
Letters
1
0 Benzene on Ni(ll1)
TABLE I: Adsorption Energies and Equilibrium Geometries of Benzene on Ni( 11 1) and Ni( 100)
10.8 amp
RNi-ringr
surface
site
Ni( 1 11)
threefold hollow CdUJ C3u(ud) bridge C,(X--X) C,,(X--Y) on-top c'6
c 6
b
Ni(100)
kJ/mol
2.5 2.2
84 68
2.4 2.4
83 60
2.5 2.5
40 65 75
fourfold hollow
2.2
129 119
experimental
Benzene on Ni(lO0)
300
400
500
Ebind,
A
experimental
1
2231
6C
Temperature ( O K ) Figure 1. T P R spectra of benzene: (a) on N i ( l l 1 ) ; (b) on Ni(100).
a
' L J W W W W
b
nnn
N i ( l l 1 ) (243 x 243) R30" Benzene Figure 4. Adsorption of benzene on the threefold hollow site to form a (2v'3X2v'3)R30° overlayer on Ni(ll1). The large circles indicate the van der Waals extension of benzene.
Ni(l11)
N(100)
Figure 2. Benzene adsorption sites: (a) fourfold hollow site on Ni(100); (b) on-top, bridge, and threefold sites on N i ( l l 1 ) .
2.49A F i p e 3. Cluster model of 19 nickel atoms used to represent the Ni( 111) surface. Unfilled circles correspond to atoms in the first layer, dots represent atoms in the second layer. For the threefold hollow sites, benzene was centered over atom 1; for the bridge site benzene bridged atoms 2 and 3; for adsorption on the on-top site the cluster was flipped over and benzene was centered over atom 1.
theory and parameterization is given in ref 4. The (1 1 1) surface was modeled as a 19 nickel atom cluster as shown in Figure 3 and the parameters were the same as those used in the calculations for Ni(100). Twelve nickel atoms are in the top layer with the
symmetry of the (1 11) face; seven more metal atoms lie below. A cluster of at least this size is necessary for modeling adsorption of a large molecule such as benzene to ensure that all adsorbate atoms have underlying metal atoms. The binding site for benzene on Ni( 11 1) has not yet been unequivocally established by experiment. Lehwald et al. conducted HREELS studies of benzene on Ni( 111) at temperatures between 140 and 320 K and concluded that benzene adsorbs on a threefold hollow or top site.6 Another extended Huckel-type study of benzene on Ni( 111)' predicted the threefold hollow site to be the most stable although absolute values of binding energies were greatly overestimated. The Slater orbital exponents and atomic orbital energy levels employed in our study differ from those used in ref 7. Our parameters were previously chosen in agreement with a number of literature sources, and the calculated binding energy for benzene over the Ni( 100) fourfold site closely matches the experimental values4 Consequently, we believe that the magnitudes of our calculated binding energies are more realistic. In order to determine the most stable site, we modeled adsorption on each of the six sites shown in Figure 2b (two each of threefold hollow, bridge, and top sites, differing by in-the-ring-plane rotations of 30'). The adsorbed benzene structure was assumed to be unperturbed from its stable gas-phase structure. Benzene was also assumed to adsorb with the ring parallel to the nickel atom plane, as suggested by experiment6V8-l0and analogy with benzene adsorption on Ni( 100).1$4Adsorption energies and equilibrium geometries are presented in Table I. (6) Lehwald, S . ; Ibach, H.; Demuth, J. E. Surf. Sci. 1978, 78, 577. (7) Anderson, A. B.; McDevitt, M. R.; Urbach, F. L. Surf. Sci. 1984,146,
80. (8) Bertolini, J. C.; Rousseau, J. Surf. Sci. 1979, 89, 467. (9) Demuth, J. E.; Eastman, D. E. Phys. Reu. 1976, 13, 1523. (10) Bertolini, J. C.; Dalmai-Imelik, G.; Rousseau, J. Surf. Sci. 1977, 67, 478.
2232 The Journal of Physical Chemistry, Vol. 91, No. 9, 1987 Our calculations determine the most stable site for benzene on N i ( l l 1 ) to be with the center of the ring above the C3,(uv) threefold hollow site, with adsorption over the C,(X X) bridge site as a very close second. Figure 4 illustrates benzene adsorption over the threefold hollow site; the arrangement of the molecules is consistent with the ( 2 d 3 X 2 d 3 ) R 3 0 ° benzene superstructure observed by LEED on Ni( 11l).Io In another experimental study of benzene adsorption on various crystal faces of the bonding geometry on Ni( 11 1) was predicted by using a model which emphasizes the importance of a possible multicenter bond between carbon, hydrogen, and the metal atoms. The authors speculated that the on-top site should be the most stable for the Ni( 1 11) surface, since here the hydrogen atoms would be able to lie directly over six nickel atoms and probably bend toward them, thus providing the possibility of a three-center carbonhydrogen-nickel bond. However, our calculations suggest that, if anything, the hydrogen-surface interaction is repulsive. In addition to a small repulsive energy, E,, between the hydrogen and nickel atoms we find the overlap population between the hydrogen atoms and the underlying metal atoms to be small and negative, suggesting an antibonding interaction. Thus, if anything, one would expect the hydrogen atoms to tilt slightly away from the surface to minimize this interaction, not toward it. We find adsorption on the on-top site to be weakest since all six carbon atoms are situated between nickel atoms rather than over them, resulting in decreased overlap between the carbon and nickel and decreased bonding. Our conclusions are similar to those reached both in the previously mentioned study of Anderson et al.' and in a theoretical study of benzene adsorption on Rh(l1 l).13 The on-top site was found to be the most weakly bound for benzene on Ni( 1 1 1)7and unstable for benzene chemisorption on Rh( 111) (benzene was repelled by the metal c l ~ s t e r ) . 'In ~ both of these studies, the most stable adsorption geometry was with benzene over a threefold hollow site with the hydrogen atoms tilted slightly away from the surface in order to optimize overlap between the carbon pz orbitals and the nickel orbitals. The C3,(uv)threefold hollow site places three carbon atoms directly over three nickel atoms, facilitating a strong bonding interaction between the carbon T orbitals and the nickel orbitals. Thus the controlling factor in determining adsorption strength on a particular site seems to be the degree of nickel orbitalarbon pr orbital overlap made possible by the geometric configuration of the underlying metal atoms. The equilibrium benzene-surface distances shown in Table I are larger by several tenths of an Angstrom than the distances calculated in aforementioned works. This difference may arise from end effects in the other studies due to the smaller number of metal atoms used (3-7 rhodium atoms in ref 13 and 7-17 nickel atoms in ref 7 vs. our 19-atom cluster). A small cluster would allow the benzene molecule to come closer to the surface than a larger cluster, since outerlying atoms which add extra repulsion are not present. Great care must be taken in cluster calculations to assure that the cluster is large enough so that edge effects are minimal. Our calculated difference in binding energy between the C30(uv) threefold hollow site on Ni( 11 1) and the fourfold hollow site on Ni(100) is 45 kJ/mol, in good agreement with the experimental finding of 44 kJ/mol. The repulsion term E, between benzene and the nickel cluster is found to be slightly greater for Ni( 11l ) ,
-
(1 1) Muetterties, E. L.; Friend, C. M. J. Am. Chem. SOC.1981, 103, 773. (12) Muetterties, E. L. In Rencfiuiiy ofMefal-MefolBonds, Chisholm, M. H., Ed.; American Chemical Society: Washington, DC, 1981; ACS Symp. Ser. No. 155, p 273. (13) Garfunkel, E. L.; Minot, C.; Gavezzotti, A,; Simonetta, M. Surf Sci. 1986, 167, 177.
Letters since nickel atoms are more tightly packed on the (1 11) face than on the (100) face; however, the effect of this contribution to the overall difference in binding energy on the two nickel surfaces is slight. The smaller binding energy of benzene on Ni( 111) relative to Ni(100) is mainly due to the substantial decrease in the magnitude of the E H electronic energy. This decrease may be traced to a smaller overlap of the benzene R orbitals with the nickel orbitals on the Ni( 111) threefold site relative to the Ni( 100) fourfold hollow. On the threefold hollow site of the (1 1 1) plane, the six carbon atoms are closely associated with three nickel atoms. On the fourfold site of the (100) surface, however, benzene R orbitals lie above four nickel atoms and may thus bind more strongly. Note that, if a larger factor in adsorption of benzene was the formation of a carbon-hydrogen-metal bond as suggested in ref 12, we would expect binding strength to be greater on the N i ( l l 1 ) surface where a 1:l correspondence of H-Ni atom positions is possible (the on-top C6 site) than on the Ni(100) plane, where the symmetry of the benzene molecule and the surface are different. The TPR spectrum for molecular benzene desorption from Ni( 11 1) exhibited a broad shoulder from about 325 to 400 K trailing from the sharp 300 K peak. The presence of this shoulder indicates that some benzene molecules are bound more strongly to the Ni( 11 1) surface than the benzene desorbing in the main 300 K peak. Multiple molecular desorption peaks are also found for toluene and xylene desorption from Ni( One possibility is that some benzene molecules lose one or more hydrogen atoms upon adsorption or heating; this partially dehydrogenated species would be more strongly bound to the surface than the associatively adsorbed benzene. Extended Huckel theory is not suited to comparing intact structures with those which have partially dissociated as the absolute energies calculated by this method are not very accurate. In contrast to the very significant difference in molecular benzene desorption from N i ( l l 1 ) and Ni( loo), the evolution of di-hydrogen from benzene decomposition on each surface was remarkably similar. The principal desorption peak for H2 from benzene decomposition on both surfaces was close to 500 K. Di-hydrogen desorption from adsorbed hydrogen occurs between 350 and 400 K on both surfaces,14indicating that the di-hydrogen desorption at 500 K resulted from decomposition of a surface intermediate. Similar di-hydrogen desorption results have also been obtained for the ring hydrogens in toluene and xylenes on Ni( 100) and Ni( 11l).4*"315 These results suggest that similar hydrocarbon intermediates are formed from all these benzenederived molecules on both nickel planes and that they are probably bound to the surface through u-type bonds which are not sensitive to crystallographic orientation. In conclusion, the arrangement of the surface metal atoms has a marked effect on the chemisorption strength of benzene on nickel due to metal-carbon overlap in multicenter bonds. Benzene binding strength is found to be greater on the Ni( 100) face, where the benzene molecule can arrange itself to closely associate its carbon R orbitals with more nickel atoms than on the (1 11) plane. Acknowledgment. Support from the Air Force Office of Scientific Research (Grant 82-302) is acknowledged with thanks. A. K. Myers is appreciative of a fellowship from the National Science Foundation. (14) Christmann, K.; Schober, 0.; Ertl, G.; Neumann, M. J . Chem. Phys. 1914, 60, 4528. (15) Klarup, D. G.; Muetterties, E. L.; Stacy, A. M. Lnngmuir 1985, I ,
164.
