J. Pkys. Ckem. 1988, 92, 973-978 conventional concepts of physical organic chemistry can hardly be justified. Conclusion The present work shows that an extensive set of X-ray diffraction intensities, accurately measured a t very low temperature and properly corrected for truncation losses, allows a reliable deconvolution of the electron distribution from its thermal motion when interpreted by a rigid-pseudoatoms (multipoles) model. The positional parameters we have determined for L-alanine give its “state-of-the-art” molecular geometry in all details at a very high degree of confidence. Bond distances and angles involving nonhydrogen atoms are more accurate and precise than those obtained in a previous room temperature neutron study? which suffers from uncertainties in the interpretation of the libration and/or vibration motion. Modelling the H atoms as polarized along their covalent bonds, hence including monopole and dipole terms in their scattering factors, has resulted in C-H and N-H bond lengths in strict agreement with those derived from the neutron diffraction work, although the esd’s for the X-ray values are 3-4 times larger than those of the neutron study. Significant features of the hydrogen-bonding network are found in the experimental deformation maps, both of charge density and electrostatic potential. All features of these maps closely match the electron population analysis of the multipole model, but caution against overin-
973
terpretation of the results is in order: the extent to which least-squares parameters such as our multipole coefficients, even when giving excellent fit of a chosen model to the experimental data, bear full physical meaning is open to speculation. Our results, based on a set of data of particularly high quality, confirm that a special warning is needed when comparing atomic (or even group) charges derived from electron density analysis of X-ray data with those obtained by ab initio LCAO-SCF-MO calculations: the different grounds, as well as the inherent limitations of both methods, can lead, at the best, to merely qualitative agreement.
Acknowledgment. This investigation was supported in part by Public Health Service Research Grant No. GM-16966 from the National Institutes of Health. R.D. acknowledges .NATO Senior and Fulbright fellowships. R.B. expresses his gratitude to R. F. Stewart for help and advice during his stay at the CarnegieMellon University. We thank G . Morosi for valuable help in ab initio calculations. Registry No. L-Alanine, 56-41-7. Supplementary Material Available: Tables of anisotropic thermal parameters and electron population parameters both for models D and E (3 pages); list of observed and calculated structure factors for L-alanine (17 pages). Ordering information is given on any current masthead page.
Organic Monolayers on Transition-Metal Surfaces. The Catalytically Important Sites Gabor A. Somorjai,* M. A. Van Hove, and B. E. Bent Materials and Chemical Sciences Division, Lawrence Berkeley Laboratory, and Department of Chemistry, University of California, Berkeley, California 94720 (Received: June 16, 1987)
The structure and bonding of organic monolayers have been explored by combined low-energy electron diffraction and high-resolution electron energy loss spectroscopy studies. Unusual bonding arrangements for adsorbed ethylene and benzene have been found. There is a temperature-induced sequential fragmentation of molecules that is a unique feature of the surface chemical bond. All the surface species appear very similar to multinuclear organometallic clusters revealing that the surface chemical bond is similar to that found in these clusters. The strongly bound organic surface species are not reaction intermediates during ethylene hydrogenation at 300 K but are spectators that play only secondary roles in this catalytic reaction. The coadsorption of electron acceptor molecules (CO or NO) facilitates the ordering of benzene and other organic molecules which are electron donors to the transition-metal substrates. This ordering indicates the importance of adsorbate-adsorbate interactions in forming surface structures.
Introduction The development of organic surface chemistry was the aim of Professor Simonetta and of our group, and we collaborated closely over the years. In this paper, we shall describe the state of understanding of the structure of monolayers of small organic molecules on metal surfaces, where much of our effort has been concentrated. The unique features of the surface chemical bonds of these molecules are (1) the temperature-dependent variation of their bonding and sequential fragmentation, which leads to the formation of organic moieties that are stable only in a well-defined temperature range on a given metal surface; (2) their dependence on coverage and the predominance of ordered structures of coadsorbed molecules reflecting the importance of adsorbateadsorbate interactions. The bonding of these organic molecules shows very close similarity to multinuclear organometallic clusters that contain a minimum of three. metal atoms. Thus the surface chemical bonds of organic molecules can be well approximated by the bonding of stable organometallic clusters. It has been one of the conclusions of the collaborative research between our Milano and Berkeley groups that the organometallielike species that appear to form on surfaces are not usually, intermediates in reactions catalyzed by metal surfaces, but are 0022-3654/88/2092-0973$01.50/0
spectators that are strongly bound to the metal surface and play secondary roles in influencing reaction rates. Our detailed studies of ethylene hydrogenation revealed that the turnover rate to produce ethane is over a million times faster than the rate of renewal of the stable ethylidyne species that are adsorbed and formed in an ordered layer on platinum and rhodium surfaces under reaction conditions at 300 K. While the reaction mechanism varies greatly with temperature and experimental conditions, our results imply that, at least at 300 K and on the P t ( l l 1 ) and R h ( l l 1 ) crystal faces, the hydrogenating ethylene molecule is weakly adsorbed either on top of or between the stable ethylidyne surface species. We shall describe these issues in more detail in the next several sections. Experimental Techniques To Investigate the Structure and Reactivity of Organic Monolayers Low-energy electron diffraction’ (LEED) and high-resolution electron energy loss spectroscopy* (HREELS) were the primary (1) Van Hove, M. A.; Tong, S.Y. Surface Crystallography by Low EnergV Electron Diffraction: Theory, Computation and Structural Results; Springer: Heidelberg, 1979.
0 1988 American Chemical Society
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The Journal of Physical Chemistry, Vol. 92, No. 4, 1988 Surface and Cluster Bonding of Acetylide
Known Cluster Coordination
Proposed Surface Geometry
Figure 1. Ethylidyne adsorbed on metal surfaces (left) and bonding in metal clusters (right).
techniques used to study the structure and bonding of the organic monolayers. LEED has developed to the point where a complete set of diffraction beam intensities can be obtained in about 5 min over a 100-150-eV electron energy range by using computer-interfaced video camera instr~mentation.~The recent development4 of digital LEED permits the use of nanoampere instead of microampere currents of the incident electron beam, so as to minimize or completely eliminate beam damage of the adsorbed species during the experiments. The theory of LEED has developeds rapidly over the past 10 years to permit the structure analysis of ordered layers of organic molecules of ever increasing size, including benzene and naphthalene. Recent theoretical advances permit the structure analysis of disordered adsorbed monolayers as long as the substrate is ordered.6 HREELS determines the vibrational spectrum of adsorbed layers, ordered or disordered, with a typical energy resolution of 50 cm-'. This technique permits us to independently obtain structural information2 on the same adsorbate system that was studied with LEED. In addition, the vibrational spectra can be readily compared with spectra obtained from the organometallic molecules or multinuclear clusters. A wealth of infrared spectroscopy data exists in the literature on the vibrational spectra of these molecules, which permits us to compare their structure with those of the surface adsorbed species.' Both LEED and HREELS studies have to be carried out at the low ambient pressures of ultra-high-vacuum (10-9-10-6 Torr). In order to investigate the behavior of organic monolayers when subjected to high-pressure gases or under catalytic reaction conditions at high pressures, a high-pressure/low-pressurereaction chamber is utilized.*.9 The single-crystal sample (usually 1 cm2 in size) can be isolated in a tube that is placed in the middle of the utra-high-vacuum system containing the LEED or HREELS equipment. The tube can then be pressurized after closing and the catalytic reaction, such as ethylene hydrogenation, can be carried out at atmospheric pressures. The reaction rate and the product distribution can be monitored by a gas chromatograph. When the reaction is completed the reaction tube is evacuated and opened and the crystal surface is subjected to further studies by LEED, HREELS, or other surface techniques (Auger electron spectroscopy, for example). After surface analysis the crystal may be repeatedly exposed to high ambient pressures if d e ~ i r e d . ~ The Structures Produced following Ethylene Adsorption on Metal Surfaces When ethylene (C2H4) adsorbs on platinum or rhodium (1 1 1) crystal faces at 200 K or below, an ordered overlayer forms.lO+ll (2) Ibach, H.; Mills, D. L. Electron Energy Loss Spectroscopy and Surface Vibrations; Academic: New York, 1982. (3) Ogletree, D. F.; Katz, J. E.; Somorjai, G. A. Reu. Sci. Insrrum. 1986, 57( 12). 301 2-301 8. (4) Ogletree, D. F.; Blackman, G. S.;Katz, J. E.; Somorjai, G. A,, to be published. (5) Van Hove, M. A,; Lin, R. F.; Somorjai, G. A. Phys. Rev. Len. 1983, $1
77R
. I _ .
( 6 ) Saldin, D. K.; Pendry, J. B.; Van Hove, M. A,; Somorjai, G. A. Phys. Rev. B 1985, 831, 1216. (7) Albert, M. R.; Yates, Jr., J. T. The Surface Scientist's Guide to Organometafic Chemistry; American Chemical Society: Washington, DC, 1987. (8) Blakely, D. W.; Kozak, E. I.; Sexton, B. A.; Somorjai, G. A. J . Vac. Sci. Technol. 1976, 13, 1091. Cabrera, A. L.; Spencer, N . D.; Kozak, E. I.; Davies, P. W.; Somorjai, G. A. Rev. Sci. Insrrum. 1982, 53, 1888. (9) Somorjai, G. A. Chemistry in Two Dimensions; Cornell University Press: Ithaca, NY, 1981.
Surface and Cluster Bonding of Methylidyne Known Cluster Coordination
Proposed Surface Geometry on R h ( l l 1 )
Ht
co'c~co
\1/ co
H
C I'
dr Figure 2. Cluster bonding geometries known for the C2Hand CH ligands in organometallic chemistry along with proposed C2H and CH bonding geometries on R h ( l l 1 ) and Rh(100).
HREELS studies indicate that the adsorbed molecules lie with their C-C bonds parallel to the surface and are elongated by 0.05-0.2 A with respect to their bond lengths in the gas phase (1.34 A).Io.J1 This implies a decrease in bond order from 2.0 to about 1.5 or less due to charge donation from the ?r bond to the metal and/or back donation of charge from the metal into the T * orbital of ethylene. In all the cases of adsorption of organic molecules on metal surfaces that we are familiar with, a decrease in the metal work function is observed, which suggests net electron donation from the adsorbate to the metal. Upon heating above 250 K the adsorbed molecule rearranges to form the ethylidyne species C2H3. This species is shown in Figure 1. The molecule occupies a 3-fold site on the (1 11) faces of Pt and Rh,12 but it also exists on the Rh(100) face where it must have a different site symmetry." Its C-C bond is perpendicular to the metal surface and is elongated to 1.45 A or more, indicating a bond order of about 1.5 or less. Ethylidyne has been (IO) Steininger, H.; Ibaeh, H.; Lehwald, S. Surf. Sci. 1982, 117, 685. Bent, B. E.; Mate, C. M.; Somorjai, G. A,, to be published. (a) Stbhr, J.; Sette, F.; Johnson, A. L. Phys. Reu. Lett. 1984, 53, 1684. (11) Bent, 3.E. Ph.D. Thesis, University of California, Berkeley, 1986. (12) Koestner, R. J.; Van Hove, M. A.; Somorjai, G . A. Surf.Sci. 1982, 121, 321; J . Phys. Chem. 1983.87, 203.
Organic Monolayers on Transition-Metal Surfaces observed on many metal surfaces around room temperature and appears to be a prevalent surface species formed upon adsorbing various olefins. Propylene, 1-butene, and 1-pentene also yield "alkylidyne" species, in which the C-C bond closest to the metal is perpendicular to the metal surface.' 1-13 When the ethylidyne molecule is heated to above 350 K, it rearranges to produce C2H and C H species; the latter requires C-C bond rupture in addition to C-H bond scission. These species are shown in Figure 2a,b. Because of extensive disorder in the surface monolayer that is associated with the formation of these molecules, LEED crystallography studies have not been carried out for them as yet. However, HREELS vibrational spectroscopy studies in several laboratories all identify the formation of these As the molecules on a number of single-crystal metal temperature is increased further, hydrogen evolution and changes in the HREELS spectrum suggest the formation of polymeric Cfi species, followed by complete dehydrogenation of the monolayer to yield a graphite overlayer. In Figures 1 and 2 we also show examples of the multinuclear organometallic cluster analogues of these surface species that have been investigated and identified. For every organic molecule or molecular fragment we have identified on surfaces, there are multinuclear organometallic clusters of similar structure and bonding. Similar C2H3, C2H, C H species have also been found upon adsorption and heating of ethylene on nickel, palladium, and ruthenium crystal From metal to metal there are small variations in the temperatures at which these structural rearrangements occur; the stabilities and structures of these fragments are also somewhat sensitive to the atomic structure of the metal substrate."
The Probable Reaction Paths for the Surface Rearrangements of Adsorbed Ethylene Recent theoretical c a l c ~ l a t i o n ssuggest ~~ smaller activation energies for the conversion of adsorbed molecular ethylene to ethylidyne if the molecule is hydrogenated first to C2H5and then dehydrogenated to C2H3 rather than losing a hydrogen atom directly. These two possible reaction pathways are shown in Figure 3a. In the hydrogenation/dehydrogenationmechanism, the source of the adsorbed hydrogen needed for the initiating hydrogenation step is either the ambient of the ultra-high-vacuum chamber that always contains residual amounts of H2or the lower temperature dehydrogenation of an ethylene at a surface imperfection. Each successive conversion of ethylene to ethylidyne then produces an additional surface hydrogen atom. Figure 3b shows the calculated14 activation energies for the formation of C2H species from the ethylidyne. The experimental resultslO*llcorrelate well with these computational findings. Structures Produced following Benzene Adsorption on Metal Surfaces
The ordering of adsorbed benzene monolayers requires the presence of coadsorbed carbon monoxide or other electron acceptor molecules (NO, for example).15 In their presence ordering of the benzene occurs readily above 250 K. LEED surface crystallography has been performed on several of these adsorbed benzene layers'619 and the results of their structure analyses are (13) Avery, N. R.; Sheppard, N. Proc. R. Soc. London A 1986,405, 1. (a) Demuth, J. E.; Ibach, H. Surf. Sci. 1978, 78, 1238. Baro, A. M.; Ibach, H. J. Chem. Phys. 1981, 74, 4194. Kesmodel, L. L.; Waddill, G. D.; Gates, J. A. Surf. Sci. 1984, 138, 464. Stroscio, J. A,; Bare, S. R.;Ho, W. Surf. Sci. 1984, 148, 499. Hills, M. M.; Parmeter, J. E.; Mullins, C. B.; Weinberg, W. H. J. Am. Chem. SOC.1986, 108, 3554; ref 22. (14) Kang, D. B.; Anderson, A. B. Surf.Sci. 1985, 155, 639. (15) Mate, C. M.; Somorjai, G. A. Surf. Sci. 1985, 160(2), 542. (16) Van Hove, M. A.; Lin, R. F.; Somorjai, G. A. J. Am. Chem. SOC.
1986, 108, 2532. (17) Van Hove, M. A,; Lin, R. F.; Ogletree, D. F.; Blackman, G. S.; Mate, C. M.; Somorjai, G. A. J. Vac. Sci. Technol. 1987, AS, 692. (18) Ogletree, D. F.; Van Hove, M. A,; Somorjai, G. A. Surf. Sci. 1987, 183. 1. (19) Ohtani, H.; Van Hove, M. A,; Somorjai, G. A., to be published.
The Journal of Physical Chemistry, Vol. 92, No. 4, 1988 975 Proposed Mechanisms For Ethylidyne Formation
Hydrogenation / Dehydrogenation
PY kcallmol
Proposed Surface Reaction Mechanism for Fragmentation of Ethylidyne (CCH3) to Vinylidene (CCH2) and Acetylide (CCH)
H. C H.H
6
/ 1 \
-
H C-C.H
H
-
&-C-H , H, H,
Figure 3. (a, top) Schematic representation of the surface intermediates and energetics for the previously postulated dehydrogenation/hydrogenation mechanism and the newly proposed hydrogenation/dehydrogenation mechanism for ethylidyne formation on transition-metal surfaces. (b, bottom) A proposed mechanism for conversion of ethylidyne (CCH,) to vinylidene (CCH,) and acetylide (CCH). Energetics shown were calculated by molecular orbital methods for a Pt( 111) slab.I4
shown in Figure 4a-d. Benzene lies with its ring parallel to the metal surface. The center of the ring is either above a 3-fold hollow or a bridge site. The benzene adsorbed on the palladium( 111) surface appears to best maintain its gas-phase-like structure, showing very little distortion or elongation of its C-C bonds. On the other metal surfaces the C-C bonds are elongated to various degrees dependent on the location of the molecule (2-fold or 3-fold site) and on the metal surface. Similar distortions have been observed in multinuclear clusters.21 The distortions on the metal surface are greater than in the complexes, presumably reflecting interaction with a larger number of metal atoms. The surface-adsorbed benzene molecule does not show buckling: the carbon ring appears to be flat when the molecule adsorbs on the metal surface. The decomposition of benzene upon heating yields CH and C2H species similar to those found upon heating ethylidyne.22 A (20) Bent, B. E.; Mate, C. M.; Crowell, J. E.; Koel, B. E.; Somorjai, G. A. J. Phys. Chem. 1987, 91, 493. (21) Gomez-Sal, M. P.; Johnson, B. F. G.; Lewis, T.; Raithby, P. R.; Wright, A. H. J. Chem. SOC.,Chem. Commun. 1985, 1682. (22) Koel, B. E.; Crowell, J. E.; Bent, B. E.; Mate, C. M.; Somorjai, G. A. J. Phys. Chem. 1986, 90, 2949.
976
The Journal of Physical Chemistry, Vol. 92, No. 4, 1988
Somorjai et al. b
a
1 50
/
-r
'
/\-
I
V
I
'
- pprn
1
1
'-)
d
C
145
I
Pt(ll1) - ( 2 ~ 5 X 4 ) r e c-2C& l
I
+ 4CO
Pd (1 11) - (3 X 3) - CH ,, + 2CO Figure 4. Four coadsorption structures of benzene with CO on (a, b) Rh(ll1); (c) Pt(ll1); and (d) Pd(ll1). The benzene to CO ratios are 1:l in (a) and 1:2 in (b), (c), and (d). Top and bottom panels show side and top views, respectively. Van der Waals radii are used for the adsorbates, touching spheres (open circles) for the substrate. Small dots between substrate atoms indicate second-layer metal atoms. The hydrogen positions are guessed.
The dashed lines indicate one two-dimensional unit cell of the periodically repeating structure.
The Mechanism of Ethylene Hydrogenation on Platinum and Rhodium Crystal Surfaces at 300 K Ethylene readily hydrogenates to ethane in the presence of excess hydrogen on most transition-metal surfaces at 300 K.23 This facile reaction is known to be structure insensitive: it occurs with similar rates (rate/surface atom) and activation energies on
wires, on powders, and on dispersed metal particles on oxide supports. When carried out on the (1 11) crystal faces of rhodium and platinum in our low-pressure/high-pressure apparatus, the high reaction rates could be monitored by gas chromatography and they compare favorably with those for high surface area catalysts."p After evacuation of the reaction chamber, inspection of the catalyst surfaces revealed the presence of a monolayer of ordered ethylidyne molecules by both LEED and HREELS. In order to monitor the role of the adsorbed ethylidyne during reaction, CzH4was labeled with I4C which is a @-emitterand then adsorbed on the Pt surfaces to produce an ethylidyne monolayer before the hydrogenation reaction commence^.'^ In this way the
(23) Horiuti, J.; Miyahara, K. "Hydrogenation of Ethylene on Metallic Catalysts"; Nail. Stand. Ref Data Ser. 1969, NSRDS-NBS-I 3.
(24) Zaera, F.; Somorjai, G. A. J . Am. Chem. SOC.1984, 106, 2288. Godbey, D.;Zaera, F.; Yeates, R.; Somorjai, G. A. Surf. Sci. 1986, 167, 150.
comparison of the fragmentation pathways of benzene and ethylene on the Rh( 111) crystal face is shown in Figure 5 . It appears that fragmentation of benzene occurs through the formation of an acetylene intermediate which then undergoes C-H and/or C-C bond breaking to produce the C2H and CH species.22
The Journal of Physical Chemistry, Vol. 92, No. 4, 1988 977
Organic Monolayers on Transition-Metal Surfaces
Rh(ll1)
f
J
L
Figure 7. Proposed mechanisms for ethylene hydrogenation on Pt( 111) and Rh( 111) at 300 K at high pressures. Ordering of Benzene on Pd( 1 1 1) by Addition
of CO (T=300K) ( 1 x 1 ) clean P d ( l l 1 )
1
100
.
1
.
1
.
1
.
1
.
300 500 Temperature (K)
1
.
1
.
700
1
Figure 5. Hydrogen thermal desorption from benzene (top curve) and ethylene (bottom curve) adsorbed on Rh( 11l), with schematic indication of the fragmentation pathways.
r T = 310 K qo,ol=latm.
B.
a pt
\1
+co
O h
....
PI
Rh
C.
z2 H,
t H2
c 2 Hs
+
I
X C H , +H2
+
C, H, +C2 H,
I
ECCH,
J +co
+% +
\1 +co
ZCCH2D t HD
(3X3)-Benzene+CO
Figure 6. Comparison of the rates of catalytic ethylene hydrogenation, ethylidyne rehydrogenation, and ethylidyne H,D exchange over Pt( 111) and Rh(ll1). residence time of the C2H3 could be monitored by detecting the @-emissionafter hydrogen treatments. H,D exchange in the surface ethylidyne species when deuterium was used as a reactant Figure was monitored by HREEL vibrational 6 compares the rate of ethylene hydrogenation on Pt and Rh( 1 11) crystal surfaces with the rate of removal of the C2H3 and with the rate of deuteriation of the methyl group in the adsorbed ethylidyne. The rate of the hydrogenation reaction is about a million times greater than the rate of ethylidyne removal or even the deuteriation of its methyl group. Thus adsorbed C2H3 is a spectator rather than a reactant on the metal surface. This conclusion is also consistent with recent in situ infrared spectroscopy studies of ethylene hydrogenation over a supported Pd (25) Davis, S. M.;Zaera, F.; Gordon, B. E.;Somorjai, G. A. J. Cutul. 1985, 92, 240. (26) Koel, B. E.;Bent, B. E.;Somorjai, G. A. Surf. Sci. 1984,146,211.
Ffgure 8. LEED patterns that occur as a function of exposure of Pd( 111) to CO or benzene or both. The bottom pattern corresponds to the structure drawn in Figure 4d.
catalyst in which the steady-state ethylidyne coverage was varied by using different H2/C2H4ratios without substantially altering the catalytic hydrogenation rate.27 Since the close-packing of the C2H3 species on Pt( 1 11) and Rh( 111) does not leave space for ethylene to directly contact the metal surface, either these species must diffuse apart slightly under reaction conditions to allow ethylene to reach the surface, or the (27)
Beebe, Jr., T.P.;Yates, Jr., J. T.J. Am. Chem. Soc. 1986,108,663.
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The Journal of Physical Chemistry, Vol. 92, No. 4, 1988
Coadsorption of Adsorbates on the R h ( l l 1 ) : Adsorbates
Coadsorbed LEED Pattern ~(4x2)
~(4x2) ~(4x2)
~(4x2) (3x3) NO
k
Na
+
co
Disorder
C2H2
Disorder
Na -k zC2H3
Disorder
Na 4- CgH6
Segregates
Proposed Model : Donor-Acceptor Coadsorbates hdused Ordering Donor -
Donor-Donor
Acceptor [
D
Acceptor-Acceptor I
D-D
A Surlace
Figure 9. (Top) Coadsorption structures found with LEED on Rh( 11 I), and (bottom)charge-transfer model giving rise to attractive and repulsive adsorbate-adsorbate forces.
surface hydrogen atoms which are present despite the high coverage of e t h ~ l i d y n e must * ~ somehow be transferred to ethylene weakly adsorbed on top of the ethylidyne monolayer. Examples of these two types of mechanisms are shown in Figure 7. Both mechanisms presume that the catalytic reaction is indeed structure insensitive and occurs over the entire surface. These models, while imperfect due to the lack of direct identification of the short-lived intermediate, do explain all of the experimental results." It should be noted that other reaction mechanisms may also exist as the experimental conditions are altered. At higher temperature (above about 400 K) there is a finite rate of rehydrogenation of the ethylidyne and other organic fragments from the metal surface. This exposes the bare metal which can also perform the hydrogenation reaction. Nevertheless, at 300 K ethylidyne is a spectator rather than a reactant for ethylene hydrogenation.
Somorjai et al.
The Role of Molecular Coadsorption on Transition-Metal Surfaces It was found that the ordering of benzene is greatly aided by coadsorbing with carbon m0n0xide.l~ The influence of CO coadsorption on the ordered surface structure of benzene on the Pd(ll1) crystal faces is shown in Figure 8. While benzene forms a disordered overlayer the presence of CO converts this to an ordered phase. In fact NO has a similar ordering effect. Detailed coadsorption studies2s indicate that when an electron donor (like benzene or another organic molecule) is coadsorbed with an electron acceptor (like CO or NO) ordering in the adsorbed monolayer is facilitated. On the other hand, when two donors or two acceptors are coadsorbed a disordered layer forms. Figure 9 shows the various systems that exhibit ordering or form disordered monolayers as a result of their attractive or repulsive adsorbate-adsorbate interactions. Thus, in addition to the adsorbate-substrate interaction the adsorbate-adsorbate interaction also plays a significant role in determining the properties of the adsorbed monlayer. Conclusions The structure and bonding of organic monolayers can be explored by combined LEED and HREELS studies. Unusual bonding arrangements for adsorbed ethylene and benzene have been found. There is a temperature-induced sequential fragmentation of molecules that is a unique feature of the surface chemical bond. All the surface species appear very similar to known species in multinuclear organometallic clusters, revealing that the surface chemical bond is similar to that found in these clusters. This analogy with clusters enhances the importance of Professor Simonetta's recent structural fluctuation model for catalytic reactions.29 The model states that catalytic activity is related to the ability of the catalyst particles to rapidly fluctuate between different geometrical configurations of its constituent atoms. Experimental investigation of this model is most desirable. Acknowledgment. This work was supported in part by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division, US. Department of Energy, under Contract No. DE-AC03-76SF00098; by the Italian Ministero della Publica Istruzioni; and by an NSF-Italy Cooperative Science Grant. Supercomputer time was also provided by the Office of Energy Research of the Department of Energy. Registry No. CO, 630-08-0; NO, 10102-43-9; ethylene, 74-85-1; benzene, 7 1-43-2; ethane, 74-84-0; ethylidyne, 4218-50-2; platinum, 7440-06-4; rhodium, 7440- 16-6. ~~
~
~
(28) Kao, C. T.; Blackman, G. S.; Mate, C. M.; Bent, B. E.; Somorjai, G. A,, to be published. (29) Simonetta, M. Nouu. J . Chim. 1986, 10, 533.
