Langmuir 1990,6, 957-970 work, since basically every type of technical electrode can now be used. We achieved also a good sensitivity: although the real surface area of the electrode was only 0.3 cm2, it was possible to detect 14 different m/z values for the adsorbate (corresponding to 15 isomers of both benzene and cyclohexane), which means that the sensitivity is at least in the range of a few percent of a monolayer. We studied the adsorption of benzene, because it is the parent compound of aromatic molecules, and found that (1) adsorbed benzene is partially desorbed in the
957
hydrogen region, (2) all remaining adsorbate can be desorbed after hydrogenation a t potentials where the hydrogen evolution takes Place, and (3) the degree of H / D exchange suggests that no C-H bond is broken upon adsorption.
Acknowledgment. Thanks are due to Prof. Heitbaum for stimulating discussions. Financial support by the Deutsche Forschungsgemeinschaft is gratefully acknowledged. Registry No. CeHs, 71-43-2;HO,1333-74-0;Pt, 7440-06-4.
Adsorption of Bipyridyls and Structurally Related Compounds at Pt( 11 1) Electrodes: Studies by Vibrational Spectroscopy (EELS),Auger Spectroscopy, and Electrochemistry Scott A. Chaffins, John Y. Gui, Bruce E. Kahn,? Chiu-Hsun Lin, Frank Lu, Ghaleb N. Salaita,l Donald A. Stern, Donald C. Zapien, and Arthur T. Hubbard* Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221 -01 72
C. Michael Elliott Department of Chemistry, Colorado State University, Ft. Collins, Colorado 80523 Received June 1,1989. I n Final Form: November 7, 1989 The adsorption behavior of bipyridyls and structurally related compounds from solution at well-defined Pt(ll1) surfaces is examined in this study to explore the influence of adsorbate molecular structure on surface bonding, molecular orientation, vibrational spectroscopy,acid-base reactivity, and electrochemical behavior of adsorbed aromatic molecules. Molecules were selected to represent various degrees of steric hindrance at the ring nitrogens: 2,2’-bipyridyl(22BPY), 2,4-bipyridyl (24BPY), 3,3’-bipyridyl (33BPY), 4,4’-bipyridyl (44BPY), 2-phenylpyridine (2PPY), 3-phenylpyridine (3PPY), 4-phenylpyridine (4PPY), 2,6-diphenylpyridine (26DPPY), 2-methylpyridine (PMPY), and 2,6-dimethylpyridine (26DMPY). A series of carboxylic acids was also studied in order to explore the interactions between acidic moieties and the Pt(ll1) surface: 2,2’-biphenyldicarboxylicacid (22BPDC), 4,4’-biphenyldicarboxylic acid (44BPDC), 2,2’-bipyridyl-4,4’-dicarboxylic acid (44DC), 2,2’-bipyridyl-4-methyl-4’-carboxylic acid (4M4C), 2,2’-bipyridyl-5,5’-dicarboxylic acid (55DC), and 2,2’-bipyridyL4,4’,5,5’-tetracarboxylic acid (4455TC). Packing densities (moles adsorbed per unit area) were measured by means of Auger spectroscopy. Linear potential scan voltammetry was used to determine the reactivity of the adsorbed layers toward electrochemical oxidation; electrochemicalreactivity of these compounds provides important clues to their mode of bonding to the surface. Surface vibrational spectra were obtained by electron energy-loss spectroscopy (EELS) and are assigned by comparison with the IR spectra of the pure compounds. The Pt(ll1) surfaces used in this study were characterized by LEED. Most of the subject compounds are adsorbed with the ring plane nearly perpendicular to the platinum surface; the exceptions are those prevented from doing so by steric constraints such as bulky substituents adjacent to the aromatic nitrogen atoms. All of the bipyridyl carboxylic acids studied adsorb strongly and have pendant carboxylic acid moieties that give vibrational spectra that are noticeably dependant on the electrode potential and that react readily with KOH. Adsorption at relatively positive potentials (+0.4 V vs Ag/AgCl) shows increased interaction of the carboxylic acid moieties with the metal surface compared with relatively negative potentials (-0.1V vs AgJAgCl), as evidenced by diminution of the intensities of bands due to 0-H and C=O stretching, as well as shifts in the frequency and intensity of aromatic CC modes. The biphenyl carboxylic acids 22BPDC and 44BPDC allow the carboxylic acid to interact with the metal surface, even at relatively negative (-0.1 V vs Ag/AgCl) electrode potentials. This is borne out by low-intensity 0-H and C=O stretching vibrations. Introduction Adsorption of aromatic compounds from solution onto annealed Pt surfaces has been shown to produce orit Present address: Eastman Kodak Company, Ultra Technologies, Route 88 South, PO Box 267, Newark, New York 14513. Present address:Union Carbide Corporation, PO BOX 8361, South Charleston, West Virginia 25303.
*
0743-7463/90/2406-0957$02.50/0
ented adsorbed 1ayers.l In particular, pyridine adsorbed at Pt(111) from aqueous solutions (10-6-1 M) or liquid pyridine forms an oriented layerlf having an average ringto-surface angle of 71°, based upon average molecular Packing densities obtained from Auger spectroscopic data. The pyridine adsorbed layer has a long-range ordered structure as judged by LEED, Pt(lll)(3.3224x4.738, 77.1°)R340-PYR, in which the ordered molecular layer 0 1990 American Chemical Society
958 Langmuir, Vol. 6, No. 5, 1990
adopts a specific rotational alignment with respect to the Pt(l11) surface but nonperiodic (incommensurate) with Pt(ll1). That is, adsorbed pyridine correlates with the surface structure of Pt(lll),although only to the extent of rotational alignment without occupying specific surface sites. Bonding of pyridine to Pt(ll1) is predominantly through the ring nitrogen atom. Adsorbed pyridine carboxylic acids exhibit covalent interaction between the carboxylate moiety and the Pt surface, particularly when adsorbed at positive electrode potentials, as evidenced by profound variations in the EELS spectrum of the carboxylic acid OH stretching band as a function of the electrode potential. Adsorbed 3-pyridylhydroquinone is reversibly electroactive, forming a pendant quinone/ hydroquinone redox couple. Adsorbed pyridines are otherwise remarkably inert and exert a passivating influence upon the host Pt(ll1) surface. Adsorbate molecular orientation has fundamental as well as practical implications, as each orientational state displays unique surface vibrational spectra, electrochemical oxidation/reduction processes, surface chemical reactions, and heterogeneous catalysis.’ In the present work, layers formed at Pt(ll1)by adsorption of bipyridyls and closely related compounds from solution were characterized by electron spectroscopic techniques under ultrahigh vacuum (UHV) and by cyclic voltammetry at atmospheric pressure in aqueous electrolyte. These experiments represent a new departure in surface electrochemistry in which well-characterized adsorbed species are formed at electrode surfaces and their electrochemical reactivity examined by use of voltammetric techniques. Surfaces are cleaned by Ar+ ion bombardment, annealed in UHV, immersed a t controlled electrode potential into buffered electrolyte containing the adsorbate, and then characterized by means of Auger spectroscopy, EELS, LEED, and related techniques. Auger spectra characterize the cleanliness of the surface, the elemental composition of the surface layer, the average molecular packing density (mol/cm2), and ultimately the average molecular orientation of the adsorbate in most cases. EELS spectra reveal the modes of bonding of the adsorbate to the surface, the molecular constitution of the adsorbate, and the nature of adsorbate-adsorbate intermolecular interacti0ns.l LEED probes the long-range order of the surface and adsorbed layer. Adsorption of pyridine at Pt(ll1) in UHV has been studied by use of EELS and related techniques (reviewed in ref If). Agreement between the gas-solid and liquidsolid adsorption experiments is excellent. However, the (1)(a) Lu, F.; Salaita,G. N.; Laguren-Davidson, L.; Stern, D. A.; Wellner, E.; Frank, D. G.; Batina, N.; Zapien, D. C.; Walton, N.; Hubbard, A. T. Langmuir 1988,4, 637. (b) Stern, D. A,; Wellner, E.; Salaita, G. N.; Laguren-Davidson, L.; Lu, F.; Batina, N.; Frank, D. G.; Zapien, D. C.; Walton, N.; Hubbard, A. T. J. Am. Chem. SOC.1988, 220,4885. ( c ) Salaita, G. N.; Laguren-Davidson, L.; Lu, F.; Walton, N.; Wellner, E.; Stern, D. A,; Batina, N.; Frank, D. G.;Lin, C. H.; Benton, C. S.; Hubbard, A. T. J.Electroanal. Chem. 1988,245,253. (d) Batina, N.; Frank, D. G.; Gui, J. Y.; Kahn, B. E.; Lin, C. H.; Lu, F.; McCargar, J. W.; Salaita, G. N.; Stern, D. A.; Zapien, D. C.; Hubbard, A. T. Electrochim. Acta 1989,34,1031. (e) Stern, D. A.; Salaita, G. N.; Lu, F.; McCargar, J. W.; Batina, N.; Frank, D. G.; Laguren-Davidson, L.; Lin, C. H.; Walton, N.; Gui, J. Y.; Hubbard, A. T. Langmuir 1988, 4 , 711. (f) Stern, D. A,; Laguren-Davidson, L.; Frank, D. G.; Gui, J. Y.; Lin, C. H.; Lu, F.; Salaita, G. N.; Walton, N.; Zapien, D. C.; Hubbard, A. T. J. Am. Chem. Sac. 1989, 121, 877. (9) Cui, J. Y.; Kahn, B. E.; Lin, C. H.; Lu, F.; Salaita, G. N.; Stern, D. A.; Hubbard, A. T. Langmuir, in press. (h) Batina, N.; Gui, J. Y.; Kahn, B. E.; Lin, C. H.; Lu, F.; McCargar, J. W.; Salaita, G. N.; Stern, D. A,; Hubbard, A. T.; Mark, H. B., Jr.; Zimmer, H. Langmuir 1989,5, 588. (i) Gui, J. Y.; Kahn, B. E.; Lin, C. H.; Lu, F.; Salaita, G. N.; Stern, D. A.; Zapien, D. C.; Hubbard, A. T. J.Electroanal. Chem. 1988,252,169. 0‘) Gui, J. Y.; Kahn, B. E.; Lin, C. H.; Lu, F.; Salaita, G. N.; Stern, D. A.; Zapien, D. C.; Hubbard, A. T. Electroanalysis 1989,1, 213. (k) Kahn, B. E.; Chaffins, S. A.; Gui, J. Y.; Lu, F.; Stern, D. A.; Hubbard, A. T. Chemical Physics, in press.
Chaffins et al.
present work appears to be the first study of bipyridine and related compounds by EELS. It also appears to be the first time that the adsorption of such compounds from solution at electrodes of any metal has been studied by means of Auger spectroscopy, EELS, LEED, and related methods. Bipyridyls were chosen for study in order to explore the relationship between the location of the aromatic nitrogen and the behavior of the adsorbate. In particular, the ring nitrogens of 2,2’-bipyridyl are situated such that their access to a smooth metal surface should be hindered by steric interference from the adjoining C and H atoms at the 3- and 3’-positions of the rings. Analogous steric hindrance to surface coordination of the ring nitrogen should exist in pyridine derivatives substituted at the 2- and 6-positions, as for 2,6-dimethylpyridineor 2,6-diphenylpyridine. In contrast, compounds unhindered at the positions ortho to nitrogen should interact with metal surfaces without steric hindrance, in a manner similar to pyridine.lf This work also explores the structural relationships controlling the potential-dependent interaction of carboxylate substituents with the Pt electrode surface. Carboxylate substituents of 2,2’-bipyridyl tend to be pendant from the surface at negative electrode potentials and coordinated to the surface a t positive potentials. In contrast, biphenyl carboxylate moieties are bound to the surface even at negative potentials. Pendant carboxylic acid groups react readily with KOH. Adsorbate structure, orientation, and mode of attachment exert a profound influence on the reactivity of the Pt(ll1) surface and the adsorbed layer toward electrochemicaloxidation. Accordingly, surface spectroscopy is helpful in understanding the voltammetric behavior of adsorbed molecules, and in turn voltammetry yields valuable clues to the mode of attachment of adsorbed species. Adsorbed layers of bipyridyl and related pyridine analogues are stable in contact with solutions and are found to be similarly stable under vacuum. Accordingly, EELS spectra of adsorbed layers closely resemble the vaporphase infrared spectra, except as adsorption alters the molecular structure of the adsorbate. Each of the subject adsorbates forms an oriented layer at Pt(ll1). However, none of the adsorbed layers studied in the present work exhibited long-range order as judged by LEED. Unhindered pyridyl rings adopt tilted vertical orientations with surface attachment primarily through the nitrogen atom, while pyridyl rings hindered by bulky groups at both positions ortho to nitrogen display horizontal orientation. Experimental Section Reported here are experiments in which an electrode surface containing an adsorbed layer was investigated with specially built instrumentation? surface structure was examined by using low-energy electron diffraction (LEED), surface elemental composition and molecular packing density were determined by using Auger spectroscopy, adsorbed layer vibrational bands were observed by electron energy-loss spectroscopy (EELS),and electrochemical reactivity of the surface was explored by using voltammetry and coulometry. The Pt(ll1) surfaces used for this (2) (a) Hubbard, A. T. J. Vac. Sci. Technol. 1980, 17, 49. (b) Hubbard, A. T.; Stickney, J. L.; Soriaga, M. P.; Chia, V. K. F.; Rosaeco, s. D.; Schardt, B. C.; Solomun, T.; Song, D.; White, J. H.; Wieckowski, A. J. Electroanal. Chem. 1984, 168, 43. (c) Hubbard, A. T. In Comprehensive Chemical Kinetics; Bamford, C. H., Tipper, D. F. H., Compton, R. G., Eds., Elsevier: Amsterdam, 1988; Vol. 28, Chapter 1. (d) Hubbard, A. T. Chem. Reu. 1988,88,633.
Langmuir, Vol. 6, No. 5, 1990 959
Adsorption of Bipyridyls and Related Compounds work were oriented3 and polished4 such that all six faces are crystallographically equivalent. All faces were cleaned simultaneously by bombardment with Ar+ ions a t 700 eV and were annealed a t about lo00 K under ultrahigh vacuum. Cleaning and annealing of the Pt surface were continued until Auger spectroscopy and LEED showed that the surface was free from detectable impurities and disorder. The Pt surfaces were isolated in an argon-filled antechamber for immersion into buffered aqueous electrolytes or hexane solutions, which contain the subject adsorbates. Electrode potentials were measured and controlled with threeelectrode electrochemical circuitry based on operational amplifiers. The electrochemical cell was made of Pyrex glass. Solutions and gases were transferred through Teflon-jacketed tubing. The jacket was purged with argon to minimize diffusion of air into the tubes conveying the solutions and inert gases. The electrochemical cell containing the reference electrode (Ag/ AgCl prepared with 10 mM KC1) and Pt auxiliary electrode was introduced into the antechamber by using a bellows assembly and gate valve; there are no sliding seals or other sources of contamination in the apparatus. All potentials are referred to Ag/AgC1(1 M KC1). Aqueous solutions used for adsorption and voltammetric or coulometric measurements contained 10 mM KF pH-adjusted with HF as indicated to provide adequate conductivity and buffer capacity. Water used in the experiments was pyrolytically distilled in pure 0 2 through a Pt gauze catalyst at 800 OC and distilled again. The following adsorbates studied in the present work were obtained from Aldrich Chemical Co. (Milwaukee, WI 53201) and were used as received: 2-methylppidine (BMPY), 2,6-dimethylpyridine (26DMPY), 2,2’-biphenyldicarboxylic acid (22BPDC), 4,4’-biphenyldicarboxylic acid (44BPDC), 2,2‘bipyridyl(22BPY), 2,4-bipyridyl(24BPY),3,3’-bipyridyl(33BPY), 4,4‘-bipyridyl (44BPY), 2-phenylpyridine (ZPPY), 3-phenylpyridine (3PPY), 4-phenylpyridine (4PPY), and 2,6-diphenylpyacid ridine (26DPPY). 2,2’-Bipyridyl-4-methyl-4’-carboxylic (4M4C),S82,2’-bipyridyl-4,4’-dicarboxylic acid (44DC)pband 2,2‘bipyridyL5,5’-dicarboxylic acid (55DC)Sbwere synthesized from the corresponding dimethyl compounds according to published acid (4455TC) procedures. 2,2’-Bipyridyl-4,4’,5,5’-tetracarboxylic was prepared from 4,4’,5,5’-tetramethyL2,2’-bipyridine& and isolated in a manner similar to 44DC and 55DC (‘H NMR (D20, Na2C03): 6 (NaTMS) 8.84 (s, 1 H), 8.04 (s, 1 H)). The proton NMR spectrum of each of the bipyridyl carboxylic acids is consistent with their structure and indicates greater than 98% purity in every case. Electron energy-loss spectra (EELS) were obtained with an LK Technologies EELS spectrometer (Bloomington, IN 47405). Beam current at the sample was approximately 200 PA; beam energy was 4 eV. The spectrometer was operated at a resolution of about 10 meV (80 cm-1) in these experiments. Vapor-phase infrared spectra (LMPY, 26DMPY, PPPY, 4PPY, 22BPY, 24BPY, 33BPY, and 44BPY) were obtained from ref 6a. FTIR spectra of solid 26DPPY or neat 3PPY were obtained from ref 6b. The infrared spectrum of 22BPDC, 44BPDC, 4M4C, 44DC, 55DC, and 4455TC in Nujol on ZnS was obtained by using a Perkin-Elmer Model 1420 spectrometer. Auger electron spectra were collected by use of a cylindrical mirror analyzer equipped with an integral electron gun (Model 981-2707, Varian Associates, Inc., Palo Alto, CA 94303; or Model 10-155, Perkin-Elmer, Eden Prairie, MN 55344). A lock-in amplifier (Model 128, Princeton Applied Research, Princeton, NJ 08540) was used to acquire the first derivative of the spectrum; the modulation amplitude was 5 V peak-to-peak at 1000 Hz. The equipment was interfaced to a computer (Hewlett-Pack(3) Wood, E. A. Crystal Orientation Manual, Columbia University Press: New York, 1963. (4)Samuels, L. E.Metallographic Polishing- by - . - Mechanical Methods; Pittman: London, 1967. ( 5 ) (a) Telser, J.; Kruikshank, K. A,; Schanze, K. S.; Netzel, T. L. J. Am. Chem. SOC.,in press. (b) Spritachnik, G.; Spritschnik, H. W.; Kirch, P. P.; Whitten, D. G. J.Am. Chem. SOC.1977,99,4947. (c) Elliott, C. M.; Freitag, F. A.; Blaney, D. D. J. Am. Chem. SOC. 1985,107, 4647. (6)(a) Sadtler Standard Spectra, ZR Vapor Phase; Sadtler Research Laboratories: Philadelphia, PA, 1987. (b) Pouched, C. J. The Aldrich Library of FTIR Spectra; Aldrich Chemical Co.: Milwaukee, WI, 1985.
QCH,
H3C @CH,
2MPY
2 2 BPY
2PPY
4M4C
26DMPY
33 BPY
4 4 BPY
2 4 BPY
4PPY
26DPPY
3PPY
44DC
55DC
CO-H
4 4 5 5 TC
22BPDC
44BPDC
ard Model 3497A interface and Model 9920 computer, HewlettPackard, Palo Alto, CA 94304) so that the data could be collected and stored on disk for later manipulation. The incident beam current was only 0.1 PA at 2000 eV to minimize the effect of beam damage and was controlled to within 1 % to limit scatter in the resulting data. Packing densities, rx (moles of adsorbed X atoms/cmZ) or r (moles of adsorbed molecules/cm2), were obtained as follows:ld Auger signals, Ix,due to each element X were measured and normalized by the Auger signal at 161 eV due to the clean Pt surface IptO. Packing density was obtained from (Ix/Zpt0)with equations of the following form:
rx =
IX/IPtO N
ill
Bx was calibrated by using hydroquinone (Bc, Bo)la or L-DOPA ( B ~ ) l eBc , = 0.314 cm2/nmol, Bo = 0.574 cm2/nmol, BN = 0.747 cm2/nmol, and Li is the fraction of element X located in level i (i = 1 is adjacent to the Pt surface and N is the outermost layer). f x is the attenuation factor for Auger electrons of element X by light atoms such as C, N, or 0, f x = 0.70 for X = C, N, or 0, based on the observation of Pt Auger electrons (235 eV) by a (3x3) layer of horizontally oriented hydroquinone,la and Mi is the number of non-hydrogen atoms located on the average path from the emitting atom to the detector.
Results and Discussion Surface bonding of bipyridines at P t ( l l 1 ) is easier t o understand i n light of t h e EELS and Auger spectra of t h e simpler, analogous methyl- a n d phenylpyridines. Accordingly, t h e behavior of those latter two classes will be described prior t o t h e bipyridines. Methylpyridines. Molecular orientation has been d e t e r m i n e d a l r e a d y for various p y r i d i n e derivatives adsorbed at P t ( l l 1 ) from aqueous solution.lf T h e pyrid i n e ring adsorbs at the m e t a l surface via t h e ring nitrogen in a nearly vertical orientation, except when prevented from doing so by steric hindrance. I n particular, o r t h o substituents that d o n o t coordinate t o t h e metal surface may introduce steric interactions that prohibit vertical or near-vertical orientation, leading to other orientations. Examples are t h e ortho-methylated pyridine derivatives 2-methylpyridine (2MPY) and 2,6-dimethylpyridine (26DMPY) discussed below.
960 Langmuir, Vol. 6, No. 5, 1990 compd 2MPY 26DMPY 4PPY BPPY BPPY 26DPPY 44BPY 44BPY 44BPY 44BPY 44BPY 44BPY 33BPY 22BPY 22BPY 22BPY 22BPY 22BPY 22BPY 24BPY 4M4C 4M4C 44DC 44DC 55DC 55DC 4455DC 4455TC 22BPDC 22BPDC 44BPDC 44BPDC
Chaffins et al.
Table I. Auger - Data for Molecules Adsorbed at Pt Electrodes -log C solvent electrode potential rinse pH I R / I ~ ~ OIK/IRO Ic/IptO I N / I ~ ~ Io/lpto O
3.0 3.0 3.0 3.0 3.0 2.4 6.0 5.0 4.0 3.0 2.0 1.5 3.0 6.0 5.0 4.0 3.0 2.0 1.5 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0
water water water water water hexane water water water water water water water water water water water water water water water water water water water water water water water water water water
0.0 0.0
-0.15 -0.15 -0.15 OCP -0.2 -0.2 -0.2
-0.2 -0.2 -0.2
-0.2 -0.2 -0.2 -0.2 -0.2 -0.2 -0.2 -0.2
-0.2 -0.2 -0.2 -0.2 -0.2 -0.2 -0.2
-0.2 -0.2 -0.2
3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 10 3 10 3 10 3 10 3 10
-0.2
3
-0.2
10
0.581 0.649 0.410 0.453 0.570 0.565 0.694 0.598 0.455 0.391 0.346 0.331 0.405 0.641 0.530 0.508 0.554 0.541 0.616 0.310 0.583 0.638 0.541 0.528 0.582 0.583 0.674 0.664 0.638 0.565 0.583 0.497
1. 2-Methylpyridine (2-Picoline) (2MPY). BMPY is an adsorbate which readily forms the nearly vertical N-bonded adsorbed state characteristic of pyridine in spite of the presence of an inert ortho substituent. The packing density of SMPY and inertness toward electrochemical oxidation parallel that of pyridine. In particular, the molecular packing density obtained from the Auger data (Table I) for 2MPY adsorbed at Pt(ll1) is 0.37 nmol/cm2. This observed packing density is greater than the calculated value based on molecular models of horizontally oriented SMPY (0.29 nmol/ cm2)' but is smaller than that calculated for vertical BMPY (0.51 nmol/cm2). Pyridine adsorbs at Pt(ll1) from the gas phase at room temperatures as well as from aqueous solutions1f through its nitrogen atom in a nearly vertical orientation with an average angle of 4 = 71' between the aromatic ring and the surface. Similarly, an average angle of 4 = 76' is obtained from the observed molecular packing density of BMPY by use of eqs 2 and 3: molecular area, u (A') = a ( b cos 4 + c sin 4)
(2)
packing density, r (nmol/cm') = 16.61/u
(3)
where a = 7.76 A,b = 7.41 A, and c = 4.17 A. Evidently, the steric repulsion resulting from one ortho methyl group still permits formation of a tilted vertical orientation of adsorbed BMPY, Figure 1A. Cyclic voltammetry of adsorbed BMPY is shown in Figure 2A. The potential scan was started at 0.00 V, the same potential at which the electrode was immersed. The adsorbed layer was characterized in UHV for approximately 1 h prior to voltammetry. This voltammogram is similar to that reported previously for pyridine.lf The (7) (a) Pauling, L. C. The Nature of the Chemical Bond, 3rd ed.; Cornel1 University Press: Ithaca, NY, 1960. (b) Bak, B.; Hansen-Hygaard, L.; Rastrup-Andersen, J. J. Mol. Spectrosc. 1958,2, 361. (8)Johnson, A. L.; Muetterties, E. L.; Stohr, J.; Sette, F. J. Phys. Chem. 1985,89,4071.
0.580 0.502 0.646 1.138 0.342 0.673
0.591 0.523 1.089 1.108 0.744 0.788 0.374 0.458 0.762 0.911 1.056 1.034 1.001 0.439 0.478 0.584 0.672 0.628 0.702 1.078 0.503 0.481 0.650 0.678 0.653 0.608 0.457 0.419 0.710 0.638 0.750 0.621
0.128 0.146 0.122 0.130 0.093 0.063 0.100 0.153 0.250 0.293 0.326 0.354 0.336 0.141 0.159 0.153 0.175 0.181 0.222 0.306 0.154 0.188 0.182 0.213 0.201 0.256 0.192 0.236
G
rc
rN
2.21 1.66 4.98 4.36 2.51 2.51 1.79 2.19 3.64 4.35 5.04 4.94 3.96 1.59 1.73
0.244 0.196 0.334 0.249 0.178 0.084 0.180 0.275 0.449 0.527 0.585 0.635 0.641 0.269 0.303 0.292 0.334 0.346 0.424 0.689 0.294 0.360 0.349 0.408 0.383 0.490 0.367 0.450
2.11
0.256 0.277 0.277 0.356 0.270 0.360 0.274 0.402 0.420 0.493 0.486 0.483
A
C
rK
2.43 2.27 2.54 4.93 1.86 0.191 1.77 2.44 0.166 2.54 2.38 0.213 2.22 1.81
0.376 1.65 2.26 0.113 2.03 2.39 0.222 1.98
ro
r
0.447 0.482 0.481 0.619 0.469 0.628 0.477 0.699 0.732 0.858 0.847 0.841
0.369 0.238 0.452 0.400 0.228 0.148 0.179 0.219 0.364 0.435 0.504 0.494 0.396 0.159 0.173 0.211 0.243 0.227 0.254 0.493 0.155 0.148 0.203 0.212 0.198 0.185 0.129 0.118 0.162 0.145 0.171 0.141
B
E
D
F
I
H
J
4
&/-$7 M
L
u
0
N
P
Figure 1. Structural models of pyridines at Pt(ll1): (A) BMPY, (B) 26DMPY, (C) 4PPY, (D) BPPY, (E) SPPY, (F) 26DPPY, (G)44BPY, (H)33BPY, (I) 22BPY, (J) 24BPY, (K)4M4C, (L) 44DC, (M) 55DC, (N) 4455TC, (0)22BPDC, and (P)44BPDC.
adsorbed BMPY layer passivates the electrode surface toward both adsorption of OH (oxidation) and of hydrogen, as evidenced by noticeably less current a t both extremes of potential (Figure 2A, solid curve) relative to that obtained for a clean surface (Figure 2A, dotted curve).
Langmuir, Vol. 6, No. 5, 1990 961
Adsorption of Bipyridyls and Related Compounds
C:
-04
00
POTENTIAL,
04
08
I2
D:
H:
E:
I :
F-
J. 1
VOLT vs. Ag/AgCI .04
00
POTENTIAL,
0 4
VOLT
08
YS
12
Ag/AgCI
Figure 2. Cyclic voltammetry of adsorbed pyridines at Pt(ll1). (A) Solid curve: immersion into 1 mM BMPY at 0.0 V, pH 3, followed by rinsing with 0.1 mM HF, first scan. Dotted curve: third scan. (B) Solid curve: immersion into 1 mM 26DMPY a t 0.0 V, pH 3, followed by rinsing with 0.1 mM HF, first scan. Dotted curve: third scan. (C) Solid curve: immersion into 1 mM 4PPY at -0.1 V, pH 3, followed by rinsing with 0.1 mM HF, first scan. Dotted curve: second scan. (D) Solid curve: immersion into 1 mM 3PPY at -0.1 V, pH 3, followed by rinsing with 0.1 mM HF, first scan. Dotted curve: second scan. (E) Solid curve: immersion into 1 mM 2PPY at -0.1 V, pH 3, followed by rinsing with 0.1 mM HF, first scan. Dotted curve: second scan. (F) Solid curve: immersion into 10 mM 26DPPY in hexane, open circuit, followed by rinsing with hexane, first scan. Dotted curve: second scan. (G) Solid curve: immersion into 1 mM 44BPY a t -0.2 V, pH 3, followed by rinsing with 0.1 mM HF, first scan. Dotted curve: second scan. (H) Solid curve: immersion into 1mM 33BPY at -0.2 V, pH 3, followed by rinsing with 0.1 mM HF, first scan. Dotted curve: fourth scan. (I) Solid curve: immersion into 1 mM 22BPY at -0.2 V, pH 3, followed by rinsing with 0.1 mM HF, first scan. Dotted curve: third scan. (J) Solid curve: immersion into 1 mM 24BPY at -0.2 V, pH 3, followed by rinsing with 0.1 mM HF, first scan. Dotted curve: third scan. (K) Solid curve: immersion into 1 mM 44DC at -0.3 V, pH 10, followed by rinsing with 10 mM KF (pH 3) a t -0.3 V, first scan. Dotted curve: second scan. (L) Solid curve: immersion into 1 mM 55DC at -0.2 V, pH 10, followed by rinsing with 10 mM KF (pH 3) a t -0.2 V, first scan. Dotted curve: fourth scan. (M) Solid curve: immersion into 1 mM 44BPDC a t -0.2 V, pH 10, followed by rinsing with 10 mM KF (pH 3) a t -0.2 V, first scan. Dotted curve: fourth scan. Experimental conditions: scan rates, 5 mV/s; electrolyte, 10 mM KF adjusted to pH 3 with HF; temperature, 23 f 1 “C.
The EELS spectrum of adsorbed BMPY (Figure 3A, upper curve) shows vibrational frequencies similar to those in the vapor-phase IR spectrum of 2MPY (Figure 3A, lower curve). Proposed assignments of the EELS vibrational bands based on literature IR assignmentseaare given in Tables 11-VI. As was observed in the EELS spectrum of pyridine,lf the intensities of the in-plane C-H bending vibrations (1013,1162 cm-l) are larger than the out-of-planeC-H bending vibration (768 cm-l). Although it would be tempting to invoke an explanation based upon dipole orientation, such an explanation is not consistent with data for other adsorbates. The presence of a prominent band at 221 cm-’ in the EELS spectrum of BMPY suggests that weakening of the Pt-N bond occurs due to the o-methyl substituent, lowering the frequency of the Pt-N stretch (from about 416 cm-l for pyridine). As reported in a series of recent articles, EELS is primarily an impact scattering technique which does not show strong dipole orientation dependence. Molecular orientation can affect EELS through surface chemical bonding, however.lk (9) (a) Green, J. H. S.; Kynaston, W.; Paisley, H. M. Spectrochim. Acta 1963, 19, 549. (b) Vymetal, J.; Hejda, Z. Collect. Czech. Chem. Commun. 1978,43 (ll),3024. (c) Colucci, S.; Garrone,E.; Morterra, C. J. Phys. Chem. 1981, 124, 201. (d) Pearce, C. K.; Grosse, D. W.; Hessel, W. J. Chem. Eng. Data 1970, 15 (41,567. (e) Strukl, J. S.; Walter, J. L. Spectrochim. Acta 1971, 27A, 209. (d) Pearce, C. K.; Grose, D. W.; Hessel, W. J. Chem. Eng. Data 1970, 15 (4), 567. (e) Strukl, J. S.; Walter, J. L. Spectrochim. Acta 1971, 27A, 209. (0Muniz-Miranda, M.; Angeloni, L.; Castellucci, E. Spectrochim. Acta 1983,39A (2), 97.
2. 2,6-Dimethylpyridine (Lutidine) (26DMPY). With both ortho positions occupied by methyl groups, 26DMPY is fully hindered at the ring nitrogen. As a result, the molecular packing density obtained from the Auger data (Table I) for 26DMPY adsorbed at Pt(ll1) is only 0.24 nmol/cm2. This packing density is very close to that calculated from molecular models’ of horizontally oriented 26DMPY (0.26 nmol/cm2) but much smaller than that observed for pyridine and calculated for vertically oriented 26DMPY (0.45 nmol/cm2). Evidently, presence of two o-methyl groups causes enough steric repulsion to change the bonding mode from predominantly u to predominantly ll interaction, leading to a nearly horizontal orientation, Figure 1B. Therefore, 26DMPY affords the interesting opportunity to study the surface electrochemical and spectroscopic behavior of a horizontally oriented adsorbed pyridine ring. Voltammetric behavior of adsorbed 26DMPY, Figure 2B, is very different from that of adsorbed BMPY (Figure 2A) or of pyridine.lf A distinct oxidation feature is observed near 0.9 V, in contrast t o the profound inertness of adsorbed pyridine and 2MPY. Also, the adsorbed 26DMPY layer does not passivate the electrode surface as did BMPY and pyridine. This voltammetric reactivity of adsorbed 26DMPY is further evidence of its horizontal mode of adsorption. The average number of electrons transferred in oxidation of adsorbed 26DMPY can be obtained from eq 4:
nox= (Qox - Q J / F A r
(4)
Chaffins et al.
962 Langmuir, Vol. 6, No. 5, 1990
Table 11. Assignments (cm-1) of EELS Bands. BMPY 26DMPY descrhtion 3183 C-H stretch 3033 C-H stretch 3062 1795 N-H stretch 1603 CC stretch 1546 1415 CC, CN stretch 1433 1348 CC, CN stretch 1162 1151 C-H bend (in plane) 1027 1013 (C-H, CN) bend (in plane) C-H bend (out of plane) 933 781 C-H bend (out of plane) 768 619 ring bend 532 468 X-sens 221 X-sens X-sens 148
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Table 111. Assignments (cm-I) 4PPY 3PPY 2PPY 26DPPY 3082 3069 3071 3059 2980 1599 1620 1601 1437 1443 1441 1449 1178 1217 1245 1175
0
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F i g u r e 3. Vibrational spectra of methylpyridines. (A) Upper curve: EELS spectrum of BMPY at P t ( l l 1 ) adsorbed from 1 mM BMPY in 10 mM K F (adjusted to p H 3 with HF) a t 0.00 V (vs Ag/AgCl), followed by rinsing with HF (pH 3). Lower curve: vapor-phase IR spectrum of 2MPY. (B) Upper curve: EELS spectrum of 26DMPY at Pt(ll1) adsorbed from 1 mM 26DMPY in 10 mM K F (adjusted to pH 3 with HF) at 0.00 V (vs Ag/AgCl), followed by rinsing with H F (pH 3). Lower curve: vapor-phase IR spectrum of 26DMPY. Experimental conditions: EELS incidence and detection angle, 62' from surface normal; beam energy, 4 eV; beam current, about 200 PA; EELS resolution, 10-meV (80 cm-l) fwhm; IR resolution, 4 cm-l.
where Qox and Qb are the coulometric charges for the oxidation and background (first and second scans), respectively, F is Faraday's constant, A is the electrode area, and r is the molecular packing density obtained by Auger spectroscopy. A value of 22 is obtained for nor. This value shows that the oxidation does not proceed to completion, as complete oxidation would have resulted in no, = 42 (eq 5). Instead, the oxidation process is likely to start with the methyl groups (eq 6), followed by incomplete oxidation of the pyridine ring. C,H,N
+
+ 17H,O
-
-
+ NO; + 43H' + 42eC,H,N(CO,H), + 12H' + 12e7C0,
(5)
C,H,N 4H,O (6) The dependence of adsorbate oxidizability on orientation has been shown before, and as in the present case, oxidation of horizontally oriented adsorbates proceeds very differently from vertical orientation.le The EELS spectrum of 26DMPY (Figure 3B, upper curve) exhibits frequencies similar to the vapor-phase IR spectrum (Figure 3B, lower curve). Proposed assignments of the EELS vibrational bands based on literature IR assignmentsgbare given in Tables 11-VI. EELS features at 1603 and 1795cm-1 are observed for 26DMPY but not for 2MPY or pyridine. As was noted previously in the EELS spectra of BMPY and pyridine, the intensities of the in-plane C-H bending vibrations (1027,1151 cm-l) are larger than that of the out-of-plane C-H bend-
1060 964 778 429
1097 900 750
996 711 412
of EELS Bands.
descrbtion C-H stretch C-H stretch asym ring stretch s y m ring stretch C-H bend (in plane), ring-ring stretch C-H bend (in plane) C-H bend ring breath, C-H bend C-H bend
Adsorbed at -0.1 V, pH 3; rinse pH 3.
Table IV. Assignments (cm-I) of EELS Bands. 44BPY 33BPY 22BPY 24BPY description 3062 3062 3063 3079 C-H stretch 1575 1580 1595 asym ring stretch 1436 1421 1442 1476 sym ring stretch 1270 1253 1253 C-H bend (in plane), ring-ring stretch 1095 1159 1172 1132 C-H bend (in plane) 1075 C-H bend (in plane) 982 958 1038 C-H bend (out of plane) 792 795 818 869 C-H bend (out of plane) 773 C-H bend (out of plane) 653 ring bend 428 475 ring bend a
Adsorbed at 0.0 V, pH 3; rinse pH 3.
ing vibration (781 cm-l). Evidently, impact scattering cross section rather than molecular orientation is the dominant factor in EELS amplitudes of these compounds. Phenylpyridine. The adsorption behavior of all three phenylpyridine isomers was investigated at a Pt(ll1) surface for comparison with the analogous methylpyridines (above) and bipyridyls (below). The ortho isomers behave differently from the meta and para isomers. None of the phenylpyridine adsorbed layers yields fractional-index LEED beams; evidently, the phenylpyridines form an adsorbed layer which is lacking in long-range order as judged by LEED. 1. 4-Phenylpyridine (4PPY). The phenyl substituents of 4PPY evidently do not significantly influence the adsorption of the pyridine moiety. The molecular packing density calculated from the Auger data (Table I) for 4PPY adsorbed at Pt(ll1) is 0.45 nmol/cm2. This packing density is greater than that calculated based on molecular models' of horizontally oriented 4PPY (0.22 nmol/ cmz) but less than that calculated for vertical 4PPY (0.73 nmol/cmz). By use of e s 2 and 3 (where a = 6.72 A, b = 11.37 A, and c = 3.4 ), an average tilt angle of 4 = 79O is obtained for 4PPY. This structure for 4PPY obtained from Auger data, Figure lC, is similar to that determined for pyridine itself, indicating that the pyri-
w
Langmuir, Vol. 6, No. 5, 1990 963
Adsorption of Bipyridyls and Related Compounds Table V. Assignments (cm-')of EELS Bands. 4M4C
44DC
55DC
4455TC
3575
3573
3572
3065
3080
3075
3594 3500 3368 3245 3092
1758
1730
1747
1741
1545
1544
1549
1395
1358
1324
1600 1460 1384
1192
1173
1137
977
1034
1014
22BPDC
44BPDC
3090 1823
3054
1646
1657
1421 1261
1425 1257
1206 1111
0
829
791
817
651
641
656
418
430
968 914 819 715 640 546 400
934 790 633 474
description 0-H stretch 0-H stretch 0-H stretch 0-H stretch C-H stretch C=O stretch C=O stretch C-0 stretch asym ring stretch sym ring stretch sym ring stretch asym ring stretch sym ring stretch (C-H, 0-H) bend (in plane) (C-O, ring-ring) stretch C-H bend, ring breath OCO bend (out of plane) C-H bend (out of plane) ring bend C-H bend (out of plane) ring bend ring bend
Adsorbed at -0.2 V, pH 10;rinse pH 3. Table VI. Assignments (cm-')of EELS Bands. 4M4C
55DC
4455TC
3062
3069
3066
3584 3355 3062
1607 1386
1612 1341
1566 1444 1245
1609 1394 1204
1110
1147 988
832 730 e
44DC
22BPDC
980 821
746 595
44BPDC
3075 1785 1645 1421 1234
3047
962
978
560
763 530
description 0-H stretch 0-H stretch 0-H stretch C = O stretch asym ring stretch, CCO stretch sym ring stretch C-H bend (in plane), ring breath C-H bend (in Dlane),. ring- breath ring breath OCO bend, C-H bend OCO bend, C-H bend ring bend
1659 1230
Adsorbed at -0.2 V, pH 10; rinse pH 3, rinse pH 10.
dine ring binds to the surface in a tilted orientation having a pendant phenyl ring. Cyclic voltammetry of adsorbed 4PPY is shown in Figure 2C. This voltammogram is similar to that observed for pyridine and 2MPY. The adsorbed 4PPY layer is unreactive toward oxidation and passivates the electrode surface in a manner similar to that of other vertically oriented pyridine derivatives. The EELS spectrum of 4PPY (Figure 4A, upper curve) is similar to the envelope of the vapor-phase IR spectrum (Figure 4A, lower curve). Proposed assignments of the EELS vibrational bands are given in Tables 11-VI. The intensity of the out-of-plane C-H bending vibration (738 cm-l) is substantial, due to the contribution from the pendant phenyl group, being relatively unaffected by the surface. 2. 3-Phenylpyridine (3PPY). The metal phenyl substituent of 3PPY is essentially noninteractive with the Pt(ll1) surface: the molecular packing density obtained from the Auger data (Table I) for 3PPY adsorbed at Pt(ll1) is 0.40 nmol/cm2. This packing density is nearly the same as that observed for 4PPY and close to that calculated from molecular models7 of vertically oriented 3PPY (0.47 nmol/cm2), Figure 1D. Cyclic voltammetry of adsorbed 3PPY (Figure 2D) shows the lack of reactivity characteristic of pyridine derivatives having tilted vertical orientations. The EELS spectrum of 3PPY (Figure 4B, upper curve) is similar to the vapor-phase IR spectrum (Figure 4B, lower curve). Proposed assignments of the EELS vibra-
tional bands are given in Tables 11-VI. The intensity of the out-of-plane C-H bending vibration (778 cm-l) is substantial, as for 4PPY, due to the contribution from the pendant phenyl group. 3. 2-Phenylpyridine (2PPY). In contrast to the behavior of the m- and p-phenylpyridines, the molecular packing density obtained from Auger data (Table I) for 2PPY adsorbed at Pt(ll1)is relatively low, 0.23 nmol/ cm2. There are two possible conformations consistent with the observed packing density: horizontally oriented 2PPY would lead to a packing density of 0.22 nmol/ cm2.7 The other possible conformer, having a theoretical packing density of 0.22 nmol/cm2, consists of a nearly vertically oriented pyridine ring and a phenyl ring parallel to the metal surface, Figure 1E. Cyclic voltammetry of adsorbed BPPY (Figure 2E) displays significantly greater susceptibility to electrochemical oxidation than 4PPY and 3PPY, for which the phenyl ring is further from the electrode surface. The observed nor value of 21 is close to the value expected for complete oxidation of the phenyl portion of 2PPY, no, = 27 (eq 7): C,,H,N
-
+ 12H20
C,H,NC02H
+ 5C0, + 28H' + 28e-
(7)
Evidently, the phenyl ring of adsorbed BPPY is in covalent contact with the Pt(ll1) surface, allowing oxidation to dccur, unlike the inertness of 3PPY and 4PPY. The EELS spectrum of BPPY (Figure 4C, upper curve) resembles its vapor-phase IR spectrum (Figure 4C, lower
964 Langmuir, Vol. 6, No. 5, 1990
Chaffins et al.
A m
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Figure 4. Vibrational spectra of phenylpyridines. (A) Upper curve: EELS spectrum of 4PPY at Pt(ll1) adsorbed from 1 mM 4PPY in 10 mM KF (adjusted to pH 3 with HF) at -0.1 V (vs Ag/AgCl), followed by rinsing with HF (pH 3). Lower curve: vaporphase IR spectrum of IPPY. (B) Upper curve: EELS spectrum of 3PPY at Pt(ll1) adsorbed from 1 mM 3PPY in 10 mM KF (adjustedto pH 3 with HF) at -0.1 V (vs Ag/AgCl), followed by rinsing with HF (pH 3). Lower curve: IR spectrum of neat 3PPY. (C) Upper curve: EELS spectrum of 2PPY at Pt(ll1) adsorbed from 1 mM BPPY in 10 mM KF (adjustedto pH 3 with HF) at -0.1 V (vs Ag/AgCl), followed by rinsing with HF (pH 3). Lower curve: vapor-phase IR spectrum of 2PPY. (D) Upper curve: EELS spectrum of 26DPPY at Pt(ll1) adsorbed from 1 mM 26DPPY in hexane, followed by rinsing with hexane. Lower curve: IR spectrum of solid 26DPPY (Nujol mull). Experimental conditions as in Figure 3.
"V
6
5
4
3
-LOG
2
1
0
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Figure 5. Adsorption isotherms of 44BPY and 22BPY. curve). Proposed assignments of the EELS vibrational bands are given in Tables 11-VI. Out-of-plane C-H bending (750 cm-') is weak for BPPY compared with 3PPY and 4PPY, as expected due to the relatively close proximity of the phenyl ring of BPPY to the Pt(ll1) surface and the resulting H-Pt interactions. 4. 2,6-Diphenylpyridine (26DPPY). 26DPPY illustrates the behavior of adsorbates in which the ring nitrogen of pyridine is hindered by aromatic substituents. The molecular packing density based on the Auger data (Table I) for 26DPPY adsorbed a t Pt(ll1) is 0.15 nmol/cm2. This relatively low packing density compares with that calculated from molecular models' of horizontally oriented 26DPPY (0.13 nmol/cm2), Figure 1F. This horizontal orientation is as expected if steric hindrance by
the two ortho phenyl substituents prevents vertical orientations, analogous to 26DMPY (see 2,6-Dimethylpyridine, above). Cyclic voltammetry of 26DPPY (adsorbed from hexane and transferred to aqueous electrolyte) is shown in Figure 2F. This voltammogram displays the reactivity of horizontally oriented adsorbed pyridine derivatives and is similar to that for 26DMPY. This horizontally oriented 26DPPY layer does not passivate the electrode surface, and considerable electroactivity is observed at relatively positive potentials. The noxvalue of 38 obtained for 26DPPY is close to twice that obtained for 2PPY (nor = 22) and is consistent with the proposed horizontal orientation of 26DPPY which evidently leads to extensive oxidation of the two phenyl rings. The pathway of complete oxidation of the phenyl rings is shown in eq 8: C,,H,,N
+ 24H,O
-
C,H,N
+ 12C0, + 56H' + 56e-
(8)
The EELS spectrum of 26DPPY (Figure 4D, upper curve) resembles its vapor-phase IR spectrum (Figure lD, lower curve). Proposed assignments of the EELS vibrational bands are given in Tables 11-VI. Once again, outof-plane C-H bending is relatively weak, probably due to interaction of the two phenyl rings with the Pt(ll1) surface. Bipyridyls. Having gathered vital background information from studies of the adsorption and spectroscopy of methyl- and phenylpyridines, let us now proceed to examine the behavior of bipyridines and derivatives. In view of the importance of steric hindrance at the ring
Adsorption of Bipyridyls and Related Compounds
A
C
1
Langmuir, Vol. 6, No. 5, 1990 965
IZShh
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4
8
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Figure 6. Vibrational spectra of bipyridyls. (A) Upper curve: EELS spectrum of 44BPY a t Pt(ll1) adsorbed from 1 mM 44BPY in 10 mM KF (adjusted to pH 3 with HF) a t -0.2 V (vs Ag/AgCl), followed by rinsing with HF (pH 3). Lower curve: vapor-phase IR spectrum of 44BPY. (B) Upper curve: EELS spectrum of 33BPY at Pt(ll1) adsorbed from 1 mM 33BPY in 10 mM KF (adjusted to pH 3 with HF) a t -0.2 V (vs Ag/AgCl), followed by rinsing with HF (pH 3). Lower curve: vapor-phase IR spectrum of 33BPY. (C) Upper curve: EELS spectrum of 22BPY at Pt(ll1) adsorbed from 1 mM 22BPY in 10 mM KF (adjusted to pH 3 with HF) at -0.2 V (vs Ag/AgCl), followed by rinsing with HF (pH 3). Lower curve: vapor-phase IR spectrum of 22BPY. (D) Upper curve: EELS spectrum of 24BPY a t Pt(ll1) adsorbed from 1 mM 24BPY in hexane, followed by rinsing with hexane. Lower curve: vaporphase IR spectrum of 24BPY. Experimental conditions as in Figure 3.
nitrogens, we have studied four isomers, including a para isomer (44BPY), ortho (22BPY), meta (33BPY), and an ortho-para combination (24BPY). 1. 4,4'-Bipyridyl (44BPY). The two ring nitrogens of 44BPY are both relatively unhindered. Accordingly, 44BPY is expected to exhibit efficient surface packing and inertness in its adsorbed state analogous to 4PPY or pyridine. The adsorption isotherm of 44BPY at Pt(ll1) at -0.20 V is shown in Figure 5. A striking characteristic of this isotherm is that the packing density of 44BPY at Pt(ll1) increases with concentration up to 10 mM, in contrast to pyridine, which shows essentially constant packing density (0.45 nmol/cm2) over a wide concentration range. The comparatively gradual concentration dependence of 4BPY density may be due to a gradual transition in average adsorbate molecular conformation, such as twisting of the two aromatic rings toward a coplanar arrangement, leading to an orientation which is more nearly vertical. Structural studies of a crystalline 44BPY derivative have reported similar substantial variations in planarity of the 44BPY moiety.'O The molecular packing density at a 10 mM concentration of 44BPY is 0.50 nmol/ cm2, which is between the packing density predicted for the horizontal orientation (0.23 nmol/cm2) and the vertical orientation (0.73 nmo1/cm2).' Based on eqs 2 and3 (a = 6.72 A, b = 10.64 A, and c = 3.4 A), the tilt angle for 44BPY at Pt(ll1) is estimated to be 81' (Figure 1G). (IO) Munavalli, S.;Poziomek, E.J.; Day, C.S.J. Mol. S t m c t . 1987, 160, 311.
The cyclic voltammogram of adsorbed 44BPY in Figure 2G exhibits inertness of the adsorbed layer to oxidation and passivation of the Pt surface typical of vertically oriented pyridine derivatives. The EELS spectrum of 44BPY (upper curve, Figure 6A) closely resembles the vapor-phase IR spectrum (lower curve, Figure 6A). Proposed assignments of the EELS vibrational bands based on literature IR assignmentsgCsgdare given in Table 11-VI. The intensities of the out-of-plane C-H bending vibrations (653 and 792 cm-l) are substantial, as expected, due to the presence of a pendant pyridine group that is relatively unaffected by the surface. 2. 3,3'-Bipyridyl (33BPY). The ring nitrogens of 33BPY are relatively unhindered but are situated to one side of the major axis of the molecule. The molecular packing density obtained from the Auger data (Table I) for 33BPY adsorbed at Pt(ll1) is 0.40 nmol/cm2. This packing density is much greater than that calculated for a molecular model' of horizontally oriented 33BPY (0.22 nmol/cm2) but slightly less than that calculated for vertically oriented 33BPY (0.464 nmol/cm2). These Auger data and eqs 2 and 3 (where a = 10.54 A, b = 7.13 A, and c = 3.4 A) suggest an orientation in which the adsorbed pyridine ring forms an angle 80' with respect to the surface (Figure 1H) and in which the other pyridine ring is pendant and perpendicular to the metal surface. A structure in which both pyridine rings are attached to the surface through the ring nitrogens is unlikely due to the severe molecular distortion of the interring bond that would exist
966 Langmuir, Vol. 6, No. 5, 1990
Chaffins et al.
ENERGY_ LOSS ( c m - 1 )
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Figure 7. Vibrational spectra of bipyridylcarboxylic acids. (A) Upper curve: EELS spectrum of 4M4C at Pt(ll1) adsorbed from 1 mM 4M4C in 10 mM K F (adjusted to pH 10 with KOH) at -0.2 V (vs Ag/AgCl), followed by rinsing with H F (pH 3). Lower curve: IR spectrum of solid 4M4C (Nujol mull). (B)Upper curve: EELS spectrum of 4M4C a t Pt(ll1) adsorbed from 1 mM 4M4C in 10 mM K F (adjusted to pH 10 with KOH) a t -0.2 V (vs Ag/AgCl), followed by rinsing with H F (pH 3) and rinsing with KOH (pH 10). Lower curve: IR spectrum of solid K[4M4C] (Nujol mull). (C)Upper curve: EELS spectrum of 44DC at Pt(ll1) adsorbed from 1 mM 44DC in 10 mM K F (adjusted to pH 10 with KOH) at -0.2 V (vs Ag/AgCl), followed by rinsing with H F (pH 3). Lower curve: IR spectrum of solid 44DC (Nujol mull). (D) Upper curve: EELS spectrum of 44DC a t Pt(ll1) adsorbed from 1 mM 44DC in 10 mM K F (adjusted to pH 10 with KOH) at -0.2 V (vs Ag/AgCl), followed by rinsing with H F (pH 3) and rinsing with KOH (pH 10). Lower curve: IR spectrum of solid K2[44DC] (Nujol mull). (E) Upper curve: EELS spectrum of 55DC a t Pt(ll1) adsorbed from 1 mM 55DC in 10 mM K F (adjusted to pH 10 with KOH) at -0.2 V (vs Ag/AgCl), followed by rinsing with H F (pH 3). Lower curve: IR spectrum of solid 55DC (Nujol mull). (F) Upper curve: EELS spectrum of 55DC a t Pt(ll1) adsorbed from 1 mM 55DC in 10 mM K F (adjusted to pH 10 with KOH) at -0.2 V (vs Ag/AgCl), followed by rinsing with H F (pH 3) and rinsing with KOH (pH 10). Lower curve: IR spectrum of solid K2[55DC] (Nujol mull). (G) Upper curve: EELS spectrum of 4455TC a t Pt(ll1) adsorbed from 1 mM 4455TC in 10 mM K F (adjusted to pH 10 with KOH) at -0.2 V (vs Ag/AgCl), followed by rinsing with H F (pH 3). Lower solid curve: IR spectrum of solid 4455TC (Nujol mull). (H) Upper curve: EELS spectrum of 4455TC at Pt(ll1) adsorbed from 1 mM 4455TC in 10 mM K F (adjusted to pH 3 with HF) at -0.2 V (vs Ag/AgCl), followed by rinsing with H F (pH 3) and rinsing with KOH (pH 10). Lower curve: IR spectrum of solid K4[4455TC] (Nujol mull). Experimental conditions as in Figure 3. in s u c h a structure. The proposed 33BPY orientation is s i m i l a r t o that p r o p o s e d for 3 - p y r i d y l h y d r o q u i n o n e (3PHQ),for which the adsorbed pyridine ring is present
i n a tilted vertical orientation, while the hydroquinone moiety is pendant and reversibly electroactive ( l b ) . The cyclic voltammogram of adsorbed 33BPY, Figure
Langmuir, Vol. 6, No. 5, 1990 967
Adsorption of Bipyridyls and Related Compounds
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(cm-I)
8
IO00
2000 3000 ENERGY LOSS ( c m - 1 )
4000
D
ENERGY L O S S
(cm-I1
Figure 8. Vibrational spectra of biphenylcarboxylic acids. (A) Upper curve: EELS spectrum of 22BPDC at Pt(ll1) adsorbed from 1 mM 22BPDC in 10 mM KF (adjusted to pH 10 with KOH) at -0.2 V (vs Ag/AgCl), followed by rinsing with HF (pH 3). Lower curve: IR spectrum of solid 22BPDC (Nujol mull). (B) Upper curve: EELS spectrum of 22BPDC at Pt(ll1) adsorbed from 1 mM 22BPDC in 10 mM KF (adjusted to pH 10 with KOH) at -0.2 V (vs Ag/AgCl), followed by rinsing with HF (pH 3), and rinsing with KOH (pH 10). (C) Upper curve: EELS spectrum of 44BPDC at Pt(ll1) adsorbed from 1mM 44BPDC in 10 mM KF (adjusted to pH 10 with KOH) at -0.2 V (vs Ag/AgCl), followed by rinsing with HF (pH 3). Lower curve: IR spectrum of solid 44BPDC (Nujol mull). (D) Upper curve: EELS spectrum of 44BPDC at Pt(ll1) adsorbed from 1 mM 44BPDC in 10 mM KF (adjusted to pH 10 with KOH) at -0.2 V (vs Ag/AgCl), followed by rinsing with HF (pH 3) and rinsing with KOH (pH 10). Experimental conditions
as in Figure 3.
2H, reflects the inertness characteristic of vertically oriented pyridine derivatives. The EELS spectrum of 33BPY (upper curve, Figure 6B) closely resembles the vapor-phase IR spectrum of 33BPY (lower curve, Figure 6B). Proposed assignments of the EELS vibrational bands based on literature IR assignmentsgdare given in Tables 11-VI. 3. 2,2'-Bipyridyl (22BPY). The structure of 22BPY is such that their approach to a smooth surface is sterically hindered. Figure 5 shows the adsorption isotherm of 22BPY at Pt(ll1). This isotherm shows very different behavior from that of 44BPY at Pt(ll1) (Figure 5 ) . The 22BPY packing density exhibits a less pronounced concentration dependence (50% increase from lo4 to lo-' M) than does 44BPY (200% increase). This difference in isotherm behavior may be due to the steric barrier to coplanarity observed for 44BPY and the likelihood that adsorbed 22BPY has a fixed noncoplanar conformation, Figure 1G and 11. Futhermore, the observed limiting packing density of 22BPY (Figure 5) is relatively small, 0.24 nmol/cm2, compared with that of 44BPY, 0.50 nmol/ cm2. There are two possible types of conformation of adsorbed 22BPY: a horizontal orientation having coplanar rings (theoretical packing density, 0.2 n m ~ l / c m ~ ) ~ J ' or a twisted conformation such as shown in Figure 11. Rotation of the rings out of coplanarity would minimize steric repulsion and would facilitate coordination of both (11) Chisholm, M. H.; Huffman, J. C.; Rothwell, I. P.; Bradley, P.G.; Kress, N.; Woodruff, W. H. J.Am. Chem. SOC.1981,102,4945.
pyridine nitrogens to the metal surface, Figure 11. The inertness toward oxidation and the observed electrode surface passivation of 22BPY (Figure 21) support the twisted orientation, Figure 11. Horizontally oriented 22BPY would have been expected to be electrochemically oxidizable, as was observed for 26DMPY and 26DPPY. Furthermore, EELS spectra of 22BPY carboxylic acid derivatives also suggest that 22BPY is not oriented horizontally (see below). The EELS spectrum of 22BPY (upper curve, Figure 6C) closely resembles the vapor-phase IR spectrum of 22BPY (lower curve, Figure 6C), as expected for the structure shown in Figure 11, because one of the pyridine rings is vertically oriented. Proposed assignments of the EELS vibrational bands based on literature IR assignment^^^-^ are given in Tables 11-VI. 4. 2,4-Bipyridyl (24BPY). The compound 24BPY, which contains nitrogen atoms in two distinguishable positions, has the potential to coordinate to the metal surface in two different ways. The molecular packing density observed for 24BPY (0.49 nmol/cm2,Table I) is much closer to that observed for 44BPY (0.43 nmol/cm2, Table 1)than for 22BPY (0.21 nmol/cm2),suggesting that adsorp tion of 24BPY occurs primarily through the 4-pyridyl ring, Figure l J , with the other ring being pendant. This orientational assignment is reinforced by the unreactivity of 24BPY toward voltammetric oxidation (Figure 251, resembling 44BPY (Figure 2G). The EELS spectrum of 24BPY (upper curve, Figure
968 Langmuir, Vol. 6, No. 5, 1990 Table VII. Formulas for Obtaining Packing Density from Auger SDectra.
26DMPY 4PPY 3PPY 2PPY 26DPPY 44BPY 33BPY 22BPY 24BPY 4M4C
44DC
55DC
4455TC
22BPDC 44BPDC
a Constants: Bc = 0.314 cm2/nmol; BN = 0.747 cm2/nmol; Bo = 0.574 cm2/nmol; BK = 3.03 cm2/nmol; f = 0.70.
6D) closely resembles the vapor-phase IR spectrum of 24BPY (lower curve, Figure 6D). Proposed assignments of the EELS vibrational bands based on literature IR assignmentsgdare given in Tables 11-VI. Carboxylic Acids. Carboxylic acids are convenient probes of the molecular structure of adsorbed layers including acid-base effects, surface bonding of substituents, and intermolecular interactions.lfsh For example, carboxylate moieties in adsorbed molecular layers have been found to be either reactive or inert, as a function of the applied potential and the structure of the adsorbate. Since the locations of the carboxylate moieties and of the nitrogen atoms are both expected to be important, the present study includes a series of isomeric bipyridyl and biphenyl carboxylic acids. 1. Bipyridylcarboxylic Acids. a. 2,2'-Bipyridyl4-methyl-4'-carboxylic Acid (4M4C). The ring nitrogen of 4M4C are hindered while the carboxylate groups
Chaffins et al. are moderately accessible to the surface. The molecular packing density of 4M4C based on Auger data (Table I) is 0.16 nmol/cm2. As for 22BPY, this packing density is consistent with either a horizontal orientation or a twisted vertical orientation. However, the 3575-cm-1 band in the EELS spectrum of 4M4C, Figure 7A, is characteristic of a pendant carboxylic acid 0-H stretching mode,lf.h Figure lK, and rules out the horizontal orientation as the principal conformation. A further indication of the presence of pendant carboxylic acid groups in the 4M4C layer is provided by experiments in which the adsorbed layer is rinsed with M KOH (pH lo), followed by Auger spectroscopy. The resulting packing density of K+ ions, 0.19 nmol/cm2 (Table I), is close to the molecular packing density of 4M4C (0.15 nmol/cm2),indicating that each molecule retains one K+ ion as expected for a pendant monocarboxylic acid. The 0-H/C-H stretching peak height ratio in the EELS spectrum of 4M4C, Figure 7A, when compared to the results for nicotinic acid,lf also supports that conclusion. The EELS spectrum obtained for the 4M4C layer after rinsing with KOH corresponds to the IR spectrum of the solid potassium salt, Figure 7B, as expected if the pendant carboxylic acid moiety reacts as an acid. b. 2,2'-Bipyridyl-4,4'-dicarboxylic Acid (44DC). 44DC resembles 4M4C but contains two carboxylate substituents. The molecular packing density of 44DC is 0.20 nmol/cm2, consistent with the twisted vertical conformation shown in Figure 1L. Cyclic voltammetry of 44DC, Figure 2K, demonstrates the inertness of the adsorbed layer, which resembles nicotinic acid,lf suggesting vertically oriented rings. The EELS spectrum of 44DC, Figure 7C, contains a carboxylic acid 0-H stretching band (3573 cm-l) and C=O stretching band (1730 cm-l), also consistent with the proposed structure, Figure 1L. Rinsing the 44DC layer with KOH led to retention of K+ ions (0.17 nmol/cm2), compared with an expected value of 0.4 nmol/cm2 if both carboxylates were involved in K+ retention. Evidently, neutralization of the surface layer is not quantitative at pH 10, due to covalent interaction of the carboxylate groups with the Pt surface, as was observed for various pyridinedicarboxylic acids.lf However, the EELS spectrum of the 44DC layer following KOH rinsing is qualitatively that expected for the adsorbed potassium salt, as can be seen by comparing the EELS and IR spectra, Figure 7D. c. 2,2'-Bipyridyl-5,5'-dicarboxylic Acid (55DC). The ring nitrogens of 55DC are hindered as for all of the 2,2'bipyridines, but the carboxylate groups are situated such that access to the surface in the adsorbed state is facilitated. The molecular packing density of 55DC is 0.20 nmol/cm2,which is consistent with the conformation shown in Figure lM, in which both rings are bonded to the surface, one vertically and the other horizontally. Cyclic voltammetry, Figure 2L, shows that adsorbed 55DC is difficult to oxidize, although more reactive than 22BPY, Figure 21. Apparently, the carboxylate groups of 55DC act to stabilize the oxidizable horizontal orientation of one of the two pyridyl rings. In keeping with this hypothesis, the related isomer, 44DC, Figure lL, in which the carboxylate group attached to the vertically oriented pyridine ring cannot readily interact with the surface is relatively unreactive. The analogous 4,4'-biphenyldicarboxylic acid (44BPDC) is horizontally oriented and accordingly is relatively easy to oxidize, Figure 2M. The EELS spectrum of 55DC, Figure 7E, contains a carboxylic acid 0-H stretching band (3572 cm-l) and a C=O stretching band (1747 cm-l) consistent with the proposed structure. Rinsing the 55DC layer with KOH led to retention
Adsorption of Bipyridyls and Related Compounds of K+ ions (0.21 nmol/cm2), as expected. The K+ packing density would have been 0.4 nmol/cm2 if both carboxylates had been involved in K+ retention. As was observed for 44DC, neutralization of the surface layer is incomplete at pH 10, due in part to stepwise dissociation of the adsorbate and also to covalent interactions of one of the carboxylate groups with the Pt surface, Figure 1M. The EELS spectrum of the 55DC layer following KOH rinsing exhibits the various characteristics expected for the adsorbed potassium salt, Figure 7F. d. 2,2’-Bipyridyl-4,4’,5,5’-tetracarboxylicAcid (4455TC). The molecular packing density of 4455TC is 0.12 nmol/cm2, which is consistent with the conformation shown in Figure 1N. While 4455TC contains two types of carboxylic acid groups, the EELS spectrum of 4455TC adsorbed at -0.2 V, Figure 7G, contains a C=O stretching band (1741 cm-l) and four reproducible carboxylic acid O-H stretching vibrations (3245,3368,3500, and 3594 cm-l), indicative of four different carboxylic acid environments in the adsorbed layer. Evidently, the two pyridyl rings of 4455TC are dissimilarly bonded to the surface. Rinsing the 4455TC layer with KOH led to retention of K+ ions (0.38 nmol/cm2) in almost the amount expected (0.5 nmol/cm2) if each of the four carboxylates retained a K+ ion. Evidently, neutralization of adsorbed 4455TC is nearly complete in pH 10. The EELS spectrum of the 4455TC layer following KOH rinsing is as expected based upon the IR spectrum of the potassium salt, Figure 7H. Adsorption of 4455TC at a positive potential (0.2 V) results in profound attenuation of the O-H stretching vibrations and the C=O stretch (1741 cm-l), along with substantial changes in the C-H bending and in the C-0 and CC stretching regions (1000-1500 cm-1). Evidently, potential-dependent coordination of the carboxylate groups to the Pt surface is occurring, Figure lN, as reported previously for nicotinic acid and related carboxypyridines.lf 2. Biphenyldicarboxylic Acids. Comparison of data for biphenylcarboxylic acids with those for bipyridylcarboxylic acids reveals some of the unique properties of heteroaromatic nitrogen atoms. a. 2,2’-BiphenyldicarboxylicAcid (22BPDC). The molecular packing density of 22BPDC, 0.16 nmol/cm2, indicates the all-horizontal orientation shown in Figure 10, in contrast to the vertical or twisted structures exhibited by the bipyridyls, Figure 1A-N. The EELS spectrum of 22BPDC, Figure 8A, displays only small carboxylic acid O-H stretching bands. That these bands are weak is probably due to the strong interaction of the carboxylic acid moieties with the metal surface (monodentate at negative potentials, but bidentate at positive), as reported previously for benzoic acid,lf induced at least in part by the close proximity of carboxyl groups to the Pt surface in a horizontally oriented, adsorbed state. A stretching band (1646 cm-l) characteristic of carboxylate carbon-oxygen stretching is also observed, consistent with the proposed structure. Rinsing the 22BPDC layer with KOH led to relatively little retention of K+ ions (0.11 nmol/cm2), as expected for a layer in which the carboxylate moieties are strongly coordinated to the surface. The EELS spectrum of the 22BPDC layer following KOH rinsing is as expected for a layer in which the carboxylate moieties are coordinated to the surfacelf or are present as potassium salts, Figure 8B. b. 4,4’-BiphenyldicarboxylicAcid (44BPDC). Adsorbed 44BPDC is likewise oriented horizontally, as evidenced by its molecular packing density of 0.17 nmol/ cm2, Figure 1P. The cyclic voltammogram of 44BPDC, Figure 2M, shows a relatively large oxidation wave near
Langmuir, Vol. 6, No. 5, 1990 969 1.1V as expected for the proposed structure. The EELS spectrum of 44BPDC, Figure 8C, contains a negligible carboxylic acid O-H stretch and exhibits a carbon-oxygen stretching band (1657 cm-l) characteristic of carboxylate, consistent with the proposed structure. Rinsing the 44BPDC layer with KOH led to retention of K+ ions (0.22 nmol/cm2) amounting to neutralization of more than half of the carboxylate groups. Apparently, an appreciable proportion of the anionic charge of the carboxylate moieties is retained in the adsorbed state, thus leading to retention of K+ ions. The EELS spectrum of the 44BPDC layer following KOH rinsing is similar to that from acidic solution, Figure 8D, as expected if the adsorbed species is present in anionic form in both cases. Conclusions Adsorption behavior of pyridine and related nitrogen heteroaromatic compounds is sensitive to steric hindrance at the positions ortho to the ring nitrogens. Compounds in which all ortho positions are unsubstituted achieve the highest possible packing densities at Pt(ll1) allowed by their molecular sizes; included in this category are 44BPY, 44BPY, 4PPY, 3PPY, and 24BPY. BMPY, hindered at only one position ortho to its ring nitrogen, packs almost as densely as the unhindered compounds and adopts a similar tilted-vertical orientation. These relatively unhindered, vertically oriented adsorbates are virtually unreactive toward electrochemical oxidation. In contrast, the fully methyl-hindered pyridine derivative 26DMPY adsorbs in its bulky “horizontal” orientation in which the pyridine ring is approximately parallel to the Pt(ll1) surface. Adsorbed 26DMPY undergoes electrochemicaloxidation relatively readily involving both methyl groups and the pyridine ring. Similarly, 26DPPY adsorbs in a horizontal orientation with the pyridine ring and both phenyl rings parallel to the surface. Adsorbed 26DPPY readily undergoes electrochemicaloxidation. The 2,2’-bipyridyls belong to a third category of pyridine derivatives in which one pyridine ring is vertically oriented and N-attached to Pt while the other aromatic ring is parallel to the Pt surface, 22BPY and 2PPY; the carboxylic and derivatives of 2,2’-bipyridine also belong to this category: 4M4C, 44DC, 55DC, and 4455TC. The carboxylic acid derivatives of 2,2’-bipyridyl also adopted “twisted” surface conformations in which one pyridine ring is N-bonded to the Pt surface in a vertical orientation, while the other ring is horizontally oriented: 4M4C, 44DC, 55DC, and 4455TC. O-H stretching bands are seen in the EELS spectra of these compounds, indicating the presence of pendant carboxylic acid moieties. The adsorbed layer retains K+ ions when rinsed with KOH at pH 10. Having no ring nitrogens, the biphenylcarboxylic acids, 22BPDC and 44BPDC, adopt horizontal orientations, analogous to that of benzoic acid.“ O-H stretching and bending bands are not seen in the EELS spectra of these adsorbates, indicating strong interaction of the carboxylate moieties with the Pt surface for these compounds. Acknowledgment. This work was supported by the National Institutes of Health (A.T.H.) and the US. Department of Energy Office of Basic Energy Sciences (DEFG02-87ER13666) (C.M.E.). Instrumentation was provided by the National Science Foundation, the Air Force Office of Scientific Research, and the University of Cincinnati (A.T.H.). The technical assistance of Arthur Case, Frank Douglas, Douglas Hurd, and Richard Shaw is gratefully acknowledged.
970
Langmuir 1990, 6, 970-973
Registry No. 22BPY, 366-18-7; 24BPY, 581-47-5; 33BPY, 581-46-4; 44BPY, 553-26-4; BPPY, 1008-89-5; 3PPY, 1008-88-4; 4PPY, 939-23-1;26DPPY, 3558-69-8;BMPY, 109-06-8;26DMPY,
108-48-5; 22BPDC, 482-05-3; 44BPDC, 787-70-2; 44DC, 681338-3; 4M4C, 103946-54-9; 55DC, 1802-30-8; 4455TC, 12522788-5; P t ( l l l ) , 7440-06-4.
Pyridine Adsorption on Polycrystalline Platinum Studied by the Radioactive-Labeling Method E. K. Krauskopf, L. M. Rice-Jackson, and A. Wieckowski* University of Illinois, Urbana-Champaign, 1209 W. California St., Urbana, Illinois 61801 Received October 9, 1989. In Final Form: December 5, 1989 The adsorption of pyridine on a polycrystalline platinum electrode was measured by using a radioactive-labeling technique. The potential dependence and concentration dependence of adsorption were measured in 0.1 M HC104, and two exchange experiments were performed. The potential dependence revealed behavior typical for a strongly adsorbed organic species; a broad plateau of maximum adsorption occurs in the double-layer region with decreased adsorption toward potential extremes. No exchange of surface species with added unlabeled solution pyridine occurred at 0.2 V (vs Ag/AgCl, 1 M Cl-1, but slow exchange was observed at -0.2 V over about 2 h; slow hydrogenation is the most likely cause of the loss of adsorbate at this potential. Two differentconcentration dependence profiles were observed depending on the treatment of the platinum electrode surface; a clean surface gave consistently higher packing densities as a function of pyridine concentration than one that had been pretreated at 1 0 4 M. The observed packing densities correspond to horizontally oriented pyridine, formed at low concentrations, and vertically oriented pyridine, formed at high concentrations.
Introduction The adsorption of aromatic compounds on metal surfaces is interesting to study because of the multiple orientations the molecules can adopt with respect to the substrate. Pyridine, in particular, is of interest because it has two dominant sites for the formation of a surface bond: the nitrogen atom in the ring or the entire ring system itself. For example, for gas-phase dosed pyridine at low coverages on Ag(lll), the aromatic *-cloud interacts strongly with the metal surface, and the pyridine molecule is adsorbed with the ring plane parallel to the surfaceel A t higher exposures, pyridine becomes a-bonded through the nitrogen lone pair and stands up from the surface at an inclined angle.’ The molecular orientation of a wide range of adsorbed aromatic compounds has been studied by Hubbard and c o - w ~ r k e r s . ~They - ~ used a thin-layer electrochemical (TLE) cell to measure the loss of electroactive analyte from a small volume of solution due to adsorption by a platinum electrode. This method is limited to electroactive species, so pyridine was not studied. For hydro(1)Demuth, J. E.;Christmann, K.; Sanda, P. N. Chem. Phys. Lett. 1980,76,201. (2)Soriaga, M.P.; Hubbard, A. T. J. Am. Chem. SOC.1982,104,2735. (3)Soriaga,M.P.; Wilson,P. H.; Hubbard, A. T. J.Electroanal. Chem. 1982,142,317. (4)Soriaga, M.P.; Stickney, J. L.; Hubbard, A. T. J. Mol. Catal. 1983,21,211. ( 5 ) Soriaga, M. P.; Hubbard, A. T. J . Electroanal. Chem. 1984,167, 79. (6) Soriaga, M.P.; Stickney, J. L.; Hubbard, A. T. J . Electroanal. Chem. 1983,144,207. (7)Soriaga, M.P.; Hubbard, A. T. J. Phys. Chem. 1984,88,1089. (8)Soriaga, M. P.; Hubbard, A. T. J. Electroanal. Chem. 1984,167, 79. (9) White, J. H.; Soriaga, M. P.; Hubbard, A. T. J.Electroanal. Chem. 1984,177,89.
0743-763190/2406-0970$02.50/0
quinone, two plateaus were observed in the packing density versus solution concentration profile, corresponding to a flat orientation (low packing density) at low concentrations and a vertical orientation (high packing density) at high concentration^.^ The shape of the transition between the two orientations depended on whether the electrode was clean or “predosed”; the presence of flat molecules already on the surface inhibited the transition to a more tightly packed vertical state. Kuwana and co-workers developed a related technique, the long optical path length thin-layer cell method, which can measure the adsorption of nonelectroactive compounds as long as there is measurable solution absorbance.1° With this method, Kuwana’s group studied both hydroquinone-related compounds and pyridine-related c o m p ~ u n d s . l l -The ~ ~ pyridine work was focused on the potential dependence of adsorption, and their study to 1.5 X was limited to a concentration range of M, over which only a slight increase in coverage was reported.13 The vertical orientation of adsorption was assumed, and the predosing effect was not addressed. The objective of our experiments was to see if the predosing effect identified by Hubbard would also be observed for pyridine. In contrast to Hubbard‘s and Kuwana’s work, our radiotracer technique is not based on measuring the loss of adsorbate from a small volume but rather is based on measuring the surface signal directly. In this paper, we discuss our results of the adsorption of [2,614C]pyridineon polycrystalline platinum, which includes characterizing the potential dependence of adsorption, (10)Zak, J.; Porter, M. D.; Kuwana, T. Anal. Chem. 1983,55, 2219. (11)Gui, Y.-P.; Porter, M. D.; Kuwana, T. Anal. Chem. 1985,57, 1474. (12)Gui, Y.-P.; Kuwana, T. Langmuir 1986,2,471. (13)Gui, Y.-P.; Kuwana, T. J. Electroanal. Chem. 1987,222,321.
0 1990 American Chemical Society
