Langmuir 1991, 7,2544-2547
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Surface Structure and Reactivity of Aromatic Molecules: Can We Track Down Trends in the Periodic Table? Falko P. Netzer Institut fiir Physikalische Chemie, Universitat Innsbruck, A-6020 Innsbruck, Austria Received April 10,1991. I n Final Form: July 19, 1991 The adsorption characteristics of benzene on transition-metal surfaces as a prototype molecule for aromaticsystemsare investi ated to search for trends across the periodic table. Data on adsorbatestructure and orientation,intramolec$r structure, and molecular stability are collectedand discussed comparatively. The strength of benzene-metal surface interactions increases in going from the right to the left in the periodic table and thus follows the general trend of chemical reactivity. However, the second period seems to behave exceptionally, in that remarkable molecular stability is encountered on these surfaces. The details of the surface chemical bond of aromatic molecules are still poorly understood, and the need for more rigorous theoretical treatment is emphasized.
Introduction Aromatic moleculesare excellent model systems to study molecule-surface interactions, and a wealth of information on the surface properties of these molecules has accumulated in recent years. Structure-sensitive techniques such as angle-resolved UV photoemission (ARUPS), highresolution electron energy loss spectroscopy (HREELS), and near-edge X-ray absorption fine structure spectroscopy (NEXAFS) have been applied successfully to study the adsorption characteristics of aromatic molecules on metal single crystal surfaces, and it is timely to look for general aspects in the surface reactivity of these molecules. The purpose of this article is to look for trends in the periodic table, and as a representative example I will consider the benzene adsorption systems, because most experimental evidence has been accumulated for this molecule. Important issues in the discussion will be adsorbate structure and orientation, intramolecular geometry, surface bonding, and molecular stability at surfaces.
Ordered Adsorbate Structures The compilation in Table I shows the existence of ordered adsorbate structures as detected by low energy electron diffraction (LEED). In Table I only those structures are considered, which have been observed at or near saturation adsorption conditions. In several cases other LEED structures have been observed at lower surface ~
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(1)Steinrack, H.-P.; Huber, W.; Pache, T.; Menzel, D. Surf. Sci. 1989, 218, 293. (2) Bertolini, J. C.; Dalmai-Imelik, G.; Rousseau, J. Surf. Sci. 1977,67, 478. (3) Steinmtiller, D.;Ramsey, M. R.; Netzer, F. P. Santaniello, A.; Schedel, T.; Lloyd, D. R. Surf. Sci. 1991,251/252, 979. (4) Netzer, F. P.; Mack, J. U. J. Chem.Phys. 1983, 79, 1017. (5) Nyberg, G. L.; Richardeon, N. V. Surf, Sci. 1979,85,335. (6) Netzer, F. P.; Rangelov, C.; Rosina, G.; Saalfeld, H. B.; Neumann, M.; Lloyd, D. R. Phys. Reu. E: Condens. Matter 1988,37, 10399. (7) Mate, C. M.; Somorjai, G. A. Surf. Sci. 1985, 160, 542. (8) Lin, R. F.; Koestner, R. J.; Van Hove, M. A.; Somorjai, G. A. Surf. Sci. 1983, 134, 161. (9) Neumann, M.; Mack, J. U.; Bertel, E.; Netzer, F. P. Surf. Sci. 1985, 155, 629. (10) Netzer, F. P.; Rosina, G.; Bertel, E.; Saalfeld, H. Surf. Sci. 1987, 184, L397. (11)Jakob, P.; Menzel, D. Surf. Sci. 1988, 201, 503. (12)Richardson, N. V.; Palmer, N. R. Surf. Sci. 1982, 114, L1. (13) Mack, J. U.; Bertel, E.; Netzer, F. P. Surf. Sci. 1985, 159, 265. (14) Netzer,F. P.; Graen, H. H.; Kuhlenbeck, H.; Neumann, M. Chem. Phys. Lett. 1987, 133,49. (15) Graen,H. H.;Neuber,M.;Neumann,M.;Illing, G.;Freund,H.-J.; Netzer, F. P. Surf. Sci. 1989, 223, 33. (16) Graen, H. H.; Neuber, M.; Neumann, M. private communication, 1990. (17) Dudde, R.; Frank, K.-H.; Koch, E. E. Surf. Sci. 1990,225, 267.
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Table I. Ordered Benzene Surface Structures as Observed by LEED Near Saturation Adsorption Conditions substrate Ni(ll1) (100) (110) Pd( 111) (100) (110) Rh(ll1) Ru(001) Pt(ll1) (100) Ir(111)
Os(001) Re(001) Ad11 1)
structure (v'7Xv'7)R19.1° 44x4) ~(4x2) disordered 44x4) ~(4x2) (243X3)rect (Zv'3x4)rect (v'7Xv'7)R19.1° disordered c(4X4) (3x3) (47Xv'7)R19.1° disordered (3x3)
unit ceU area, A* 37.6 49.6 35.1 60.5 42.8 37.3 49.8 44.5 61.4 57.7 45.5 65
refs 1 2 3 4 5 6 7 8-10,33 11 7 12 13 14,15 16 17
coverages, but not all benzene adsorption systems have been characterized carefully as a function of coverage; in order to avoid confusion, lower coverage LEED structures have therefore been omitted in this compilation. There seem to be a number of common LEED structures on different metal surfaces, e.g. the (d7Xd7)R19.1° structures on the hexagonal faces of Ru, Os, and Ni, or the 44x2) structures on the (110) faces of Ni and Pd, or the more open 44x4) structures on the (100) faces of Ni, Pd and Pt. With these LEED data the areas allowed to the benzene molecules in the different unit cells can be calculated, and these are included in Table I, column 3. Comparison can then be made with the van der Waals area of the molecule to get an idea of the packing density on various surfaces. The van der Waals area of the benzene hexagon is 35.2 A2, and the area of the smallest rectangle which can incorporate benzene within its van der Waals dimensions is -50 A2. Two groups of structures can be distinguished in Table I structures with unit cell areas A > 50 A2,and structures with A < 45 A2. In the latter group the adsorbate packing density can become rather high. Consider, e.g., the ( v ' 7 X d 7 ) structure of benzene on Ni(ll1) (A = 37.6 A2), or the 44x2) structure on Ni(llO), where A = 35.1 Az;the latter is exactly the van der Waals area of the benzene hexagon. One may wonder whether a surface orientation with the ring plane parallel to the surface can be adopted in these structures-note that this is the favorable adsorption geometry for intact benzene-or whether the molecules have to be inclined with respect to the surface to release intermolecular strain. Recent investigations by ARUPS and NEXAFS have however determined an essentially flat adsorbate geometry in the benzene ~ ( 4 x 2 ) 0 1991 American Chemical Society
Structure and Reactivity of Aromatic Molecules
Langmuir, Vol. 7, No. 11, 1991 2545
Table 11. Thermal Desorption from Saturated Benzene Adlayere. CsHs desorpt onset of Hz substrate max, K desorption, K refs Ni Pd Rh Ru
Mo Pt Ir
os
Re cu 43
-300-390 430-530 400 360 360
410 420
