Langmuir 1990,6, 1273-1281
1273
Multinitrogen Heteroaromatics Studied at Pt( 111) Surfaces by EELS, Auger Spectroscopy, and Electrochemistry: Pyrazine, Pyrimidine, Pyridazine, 1,3,5-Triazine, and Their Carboxylic Acid Derivatives Scott A. Chaffins, John Y. Gui, Chiu-Hsun Lin, Frank Lu, Ghaleb N. Salaita, Donald A. Stern, and Arthur T. Hubbard* Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221 -01 72 Received September 29, 1989 Reported here are surface electrochemical studies of several multinitrogen heteroaromatics and their carboxylic acid derivatives adsorbed at well-defined Pt(111)electrode surfaces from aqueous solutions. The adsorbates studied include pyrazine (PZ), pyrimidine (PM), pyridazine (PD), 2-pyrazinecarboxylic acid (PZCA),2,3-pyrazinedicarboxylicacid (23PZDCA),4-pyridazinecarboxylicacid (4PDCA),and 1,3,5triazine (TZ). Packing densities (moles adsorbed per unit area) of each compound adsorbed from solution at controlled pH and electrode potential are measured by means of Auger spectroscopy. All of the above compounds except TZ adsorb strongly to Pt(ll1). Surface vibrational spectra of these adsorbed layers are obtained by electron energy-loss spectroscopy(EELS)and are comparedwith either vapor- or condensedphase infrared (IR) spectra of the parent compounds. The similarity of EELS and IR spectra indicates that the adsorbates retain their molecular structure unchanged as a result of adsorption. Adsorbed layers of these materials are stable in contact with aqueous fluoride solutions and are stable also under vacuum. The adsorbed layer is electrochemically unreactive and tends to passivate the Pt surface. Carboxylic acid Substituents of these compounds interact with the Pt surface to an extent which depends upon the electrode potential. Long-range order is absent from the adsorbed layer, based upon LEED observations, although the Pt(ll1) surface remains ordered. The adsorbed molecules are attached to the surface primarily through the least hindered ring nitrogen atom and are arranged in tilted-vertical orientation with the average ringto-surface angles ranging from 73" to 86".
Introduction Adsorption of aromatic compounds from solution onto annealed Pt(ll1) surfaces has been shown to produce oriented adsorbed layers.' In particular, unhindered pyridyls and bipyridylslnlbadopt tilted vertical orientations, with surface attachment primarily through the nitrogen atom, while compounds hindered by bulky groups at both positions ortho to nitrogen display horizontal orientation. Metacarboxylic acid substituents of such compounds tend to be pendant from the surface a t negative electrode potentials but coordinatively bound to the Pt surface at positive potentials. The molecular orientation of the adsorbate profoundly influences surface vibrational spectra, electrochemical oxidation/reduction processes, surface chemical reactions, and heterogeneous catalysis. Accordingly, surface spectroscopy is very helpful in ~
~~
(a) 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. SOC.1989,111,877. (b) Chaffins, S. A.; Gui, J. Y.; Kahn, B. E.; Lin, C.-H.; Lu, F.; Salaita, G. N.; Stern, D. A.; Zapien, D. C.; Hubbard, A. T. Langmuir, in press. (c) Lu, F.; Salaita, G. N.; LagurenDavidson, L.; Stern, D. A.; Wellner, E.; Frank, D. G.; Batina, N.; Zapien, D. C.; Walton, N.; Hubbard, A. T. Langmuir 1988,4,637. (d) Stem, 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, 120,4885. (e) 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. Electroanat. Chem. 1988,245,253. (0 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. (g) 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. (h) Gui, J. Y.; Kahn, B. E.; Lin, C.-H.; Lu, F.; Salaita, G. N.; Stern, D. A.; Zapien, D. C.; Hubbard,A. T. J. Electroaml. Chem.1988,252,169. (i) Frank, D. G.; Batina, N.; Golden, T.; Lu, F.; Hubbard, A. T. Science 1990,247, (1)
137. 182.
understanding the voltammetric behavior of adsorbed molecules, while voltammetry in turn yields valuable clues as to the mode of attachment of adsorbed species. Adsorbed pyridines strongly passivate the Pt(111) surface toward electrochemical reactions. In the present work, we explore the adsorption behavior at Pttlll) of aromatic compounds containing two or more ring nitrogens, with emphasis upon the strength of adsorption, and the nature of bonding to the surface. In these experiments, a Pt(ll1) surface was immersed at controlled electrode potential into solutions of KF/HF electrolyte (10 mM, pH 3) containing a subject compound; the layer formed by adsorption was characterized by electron energy-loss spectroscopy (EELS), Auger spectroscopy, and cyclic voltammetry. Adsorbed layers found to be stable in contact with liquids are found to be similarly stable under vacuum. EELS spectra of the adsorbed layers closely resemble the vapor-phase infrared spectra, except as adsorption alters the molecular structure of the adsorbate. Pyrazine, pyrimidine, pyridazine, and their carboxylic acid derivatives form nearly vertical oriented layers at Pt(lll),but 1,3,5-triazinedoes not chemisorb at Pt(ll1). The adsorbed layers are relatively inert to electrochemical oxidation, as expected for nitrogen heteroaromaticsadsorbed in nearly vertical orientations. None of the adsorbed layers shows any LEED pattern, indicating that they adsorb from aqueous solutions onto Pt(ll1) without long-range order.
Experimental Section Reported here are experiments in which an electrode surface containing an adsorbed layer is investigated by means of specially constructed instrumentation: surface structure is ex-
amined by means of low-energy electron diffraction (LEED),
0 1990 American Chemical Society
1274 Langmuir, Vol. 6, No. 7, 1990 surface elemental composition and molecular packing density are determined by Auger spectroscopy, adsorbed layer vibrational bands are observed by electron energy-loss spectroscopy (EELS), and electrochemical reactivity of the surface is explored by voltammetry and coulometry. The Pt(ll1) surfaces employed for this work are oriented and polished such that all six faces are crystallographically equivalent. All faces are cleaned simultaneously by Ar+ ion bombardment at 700 eV under 5 X 10-6 Torr Ar pressure and are annealed a t about lo00 K under ultrahigh vacuum. Cleaning and annealing of the Pt surface are continued until Auger spectroscopy and LEED show that the surface is free from detectable impurities and disorder. This well-defined Pt(ll1) electrode is then isolated in an argon-filled antechamber, followed by 3-min immersion into an aqueous electrolyte (pH 3 or pH 7) containing the subject compound a t a specific electrode potential. After removal from the adsorbate solution, the electrode is rinsed 3 times with a 2 mM HF (pH 3) or 0.1 mM KOH (pH 10) solution at the same potential to wash off excess solute. The Pt electrode with adsorbed layer is then evacuated in the antechamber followed by transfer into the main UHV chamber, where surface characterization (EELS, Auger, LEED) takes place. Electrode potentials are measured and controlled by means of three-electrode electrochemical circuitry based upon operational amplifiers. The electrochemical cell is constructed of Pyrex glass. Solutions and gases are transferred through glass and Teflon-jacketed tubing. The jacket is 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 KCl) and Pt auxiliary electrode is introduced into the antechamber by means of a bellows assembly and gate valve; there are no sliding seals or other sources of contamination in the apparatus. All potentials are referred to standard Ag/AgCl (1 M KCl); however, to minimize contamination of the adsorbate solution with chloride, dilute KCl solution (10 mM) was employed in the reference electrode, which was calibrated against a standard Ag/ AgCl reference half-cell. Electrolytes employed for adsorption and electrochemical measurements contain 10 mM KF (adjusted with HF to pH 3) to provide adequate conductivity and buffer capacity. The acidic rinsing solution is 2 mM HF (pH 3), and the basic rinsing solution is 0.1 mM KOH (pH 10). Water used in the experiments is pyrolytically distilled in pure 0 2 through a hot (800 "C)Pt gauze catalyst and distilled again. All glassware is cleaned by heating a t 425 OC overnight before use.2 All adsorbates studied in the present work were obtained from Aldrich Chemical Co. (Milwaukee, WI 53201) and were used as received: pyrazine (PZ), pyrimidine (PM), 1,3,5triazine (TX), 2-pyrazinecarboxylicacid (PZCA), 2,3-pyrazinedicarboxylic acid (23PZDCA), and 4-pyridazinecarboxylic acid (4PDCA). Pyridazine (PD) was distilled under vacuum in order t o remove traces of red-brown residue.
COOH
PZCA
23 PZDCA
4 PDCA
Electron energy-loss spectra (EELS) were obtained by means of 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 a t a resolution of about 10 meV (80 cm-') in these experiments. (2) Larkin, N. A,; Gibbs, E. L. In Vitro 1973,8, 480.
Chaffins et al. Vapor-phase infrared spectra (PZ,PM, PD, and 2PZCA) were obtained from Sadtler.3 The FTIR spectrum of 23PZDCA in Nujol was obtained from Aldrich,4 from which the Nujol spectrum was digitally subtracted. The IR spectra of 4PDCA and the potassium salts of the above acids were obtained by using a Perkin-Elmer Model 1420 spectrometer. Auger electron spectra were collected with a cylindrical mirror analyzer equipped with an integral electron gun (Model 9812707, Varian Associates, Inc., Palo Alto, CA 94303, or Model 10155G, Perkin-Elmer, Eden Prairie, MN 55344). Lock-in amplifiers (Model 128A and 5101, Princeton Applied Research, Princeton, NJ 08540) were used to acquire the first derivatives of the spectrum. The equipment was interfaced to a computer (Hewlett-Packard Model 3497A interface and Model 9920 computer, Hewlett-Packard, 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/cm2) or r (moles of adsorbed molecules/cm2) were obtained as follows: Auger signals, ZX,due to each element X were measured and normalized by the Auger signal at 161 eV due to the clean Pt surface ZptO. Elemental packing density was obtained from (ZX/ ZptO) by means of the equation
where Bx was calibrated by means of hydroquinone (Bc, Bo) and L-DOPA ( B N ) ;Bc = 0.314 cm2/nmol; Bo = 0.574 cmz/ nmol; BN = 0.633 cm2/nmol; 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); fx is the attenuation factor for Auger electrons of element X by light atoms such as C, N, or 0; fx = 0.70 for X = C, N, or 0, based upon the observed attenuation of Pt Auger electons (235 eV) by a (3 X 3) layer of horizontally oriented hydroquinone; and Mi is the number of non-hydrogen atoms located on the average path from the emitting atom to the detector. Self-scattering attenuation of the Auger signal &) depends upon the directions of detection of the Auger signal;" on the basis of previous data,' such errors are less than 15% for the analyzer used in the present work. Molecular packing density (r)is determined by two independent measurements: (i) Molecular packing density is related to the elemental packing density r x of each element X by
r = rx/n where n is the total number of atoms of element X in the molecule. X = carbon in the present article because carbon is most abundant in the subject compounds, has a favorable Auger electron yield, and thus affords the best precision. (ii) Molecular packing density is also related to the attenuation of the substrate Pt Auger signal due to the adsorbed layer
where Zpt/ZptO is the ratio of Pt Auger signals at 161 eV measured before (Zpto) and after (Zpt) application of the adsorbed layer. Ji represents the number of non-hydrogen atoms per molecule located at the ith level of the adsorbed layer, and N is the outermost layer. K X (X = C, 0, N) is equal to 0.165 cm2/nmol as reported earlier.' Method ii serves primarily as a quantitative check of method i because packing density measurements based upon Zpt/ZptO are statistically about fourfold less precise than measurements based upon ZC/Zpt.l
Results and Discussion Pyrazine (PZ).The m o l e c u l a r p a c k i n g d e n s i t y calculated from Auger data (Table I) for PZ adsorbed from (3) Sadtler Standard Infrared Vapor Spectra; Sadtler Research Laboratories: Philadelphia, PA,.1986. (4) Pouchert, C. J. The Aldrrch Library of FTIR Spectra; Aldrich Chemical Co.: Milwaukee, WI, 1985.
Langmuir, Vol. 6, No. 7,1990 1275
Multinitrogen Heteroaromatics at Pt(l11) Surfaces
compd PZ PZ PZ PZ PZ PZ PZ PZ PZ PZ
Table I. Auger Data for Heterocyclic Aromatics Adsorbed at Pt(ll1). packing densities, nmol/cm2 molecular r based on -log C electrode potential, V pH Ipt/Ipto Ic/Ipto Io/Ipto I N / I P t o I ~ / l p t O rc ro r N r K * IclIpP Ipt/IptO 3.00 -0.3 3.0 0.54 0.50 0.41 2.04 0.75 0.51 0.53 0.0 3.0 0.56 2.22 1.01 3.00 0.54 0.54 0.56 0.51 0.47 3.0 0.53 0.00 0.59 0.0 2.41 0.88 0.60 0.55 7 0.55 0.00 0.50 0.42 -0.10 2.06 0.42 0.51 0.52 1.00 0.51 0.46 7 0.53 -0.10 0.52 2.09 0.46 0.56 7 0.50 2.00 0.41 2.00 0.49 0.41 -0.10 0.50 0.59 7 0.52 1.97 0.43 3.00 0.48 0.38 -0.10 0.49 0.57 7 0.56 4.00 0.51 0.39 -0.10 0.52 2.06 0.39 0.52 7 0.57 1.90 0.29 5.00 0.46 0.29 -0.10 0.47 0.49 7 0.54 1.46 0.29 6.00 0.36 0.29 -0.10 0.37 0.54
PM PM PM PM PM PM PM PM PM PM
3.00 3.00 0.00 0.00 1.00 2.00 3.00 4.00 5.00 6.00
-0.3
0.58 0.55 0.57 0.51 0.47 0.50 0.51 0.50 0.53 0.58
0.48 0.52 0.58 0.54 0.53 0.54 0.52 0.50 0.49 0.37
0.39 0.48 0.44 0.44 0.39 0.42 0.45 0.40 0.42 0.21
1.86 2.05 2.29
-0.10 -0.10 -0.10
3.0 3.0 3.0 7 7 7 7 7 7 7
PD PD PD PD PD PD PD PD PD PD
3.00 3.00 3.00 0.00 1.00 2.00 3.00 4.00 5.00 6.00
-0.3 -0.2 0.0 0.00 0.00 0.00 0.00 0.00 0.00 0.00
3.0 3.0 3.0 7 7 7 7 7 7 7
0.78 0.60 0.56 0.52 0.53 0.53 0.54 0.57 0.52 0.62
0.43 0.46 0.50 0.49 0.45 0.41 0.39 0.38 0.37 0.23
PZCA PZCA PZCA PZCA PZCA PZCA PZCA PZCA PZCA PZCA
3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00
-0.3 -0.2
3.0 3.0 3.0 10.0 3.0 10.0 3.0 3.0 10.0 3.0
0.58 0.67 0.51 0.45 0.56 0.39 0.50 0.44 0.47 0.48
0.41 0.47 0.51 0.51 0.60 0.56 0.52 0.52 0.46 0.51
23PZDCA 23PZDCA 23PZDCA 23PZDCA 23PZDCA 23PZDCA
3.00 3.00 3.00 3.00 3.00 3.00
-0.2 -0.2 0.0 0.0 0.5 0.5
3.0 10.0 3.0 3.0 10.0
0.63 0.57 0.49 0.49 0.53 0.63
4PDCA 4PDCA 4PDCA 4PDCA 4PDCA 4PDCA
3.00 3.00 3.00 3.00 3.00 3.00
-0.2 -0.2 0.0 0.0 0.5 0.5
3.0 10.0 3.0 10.0 3.0 10.0
0.56 0.40 0.49 0.51 0.49 0.59
0.0 0.0
-0.10 -0.10 -0.10 -0.10
-0.1 -0.1 0.0 0.0
0.3 0.5 0.5 0.6
10.0
2.07 2.13 2.04 1.96 1.91 1.44
0.80 0.99 0.91 0.47 0.43 0.46 0.49 0.44 0.46 0.23
0.47 0.51 0.51 0.53 0.52 0.53 0.51 0.49 0.48 0.37
0.48 0.52 0.50 0.58 0.61 0.60 0.60 0.59 0.56 0.48
0.38 0.37 0.36 0.28 0.31 0.28 0.26 0.24 0.27 0.18
1.59 1.71 1.87 1.82 1.69 1.55 1.46 1.43 1.37 0.86
0.85 0.84 0.80 0.34 0.38 0.34 0.32 0.29 0.33 0.22
0.40 0.43 0.47 0.45 0.42 0.39 0.36 0.36 0.34 0.22
0.23 0.46 0.51 0.54 0.56 0.55 0.54 0.50 0.57 0.43
0.24 0.39 0.33 0.54 0.38 0.63 0.27 0.41 0.38 0.39
0.33 0.48 0.52 0.54 0.54 0.50 0.39 0.52 0.55 0.51
1.78 2.03 2.22 2.21 2.62 2.45 2.28 2.25 2.00 2.24
0.46 0.73 0.63 1.02 0.71 1.19 0.50 0.77 0.71 0.74
0.61 0.89 0.97 1.00 0.40 1.01 0.94 0.67 0.73 0.96 1.01 0.10 0.94
0.36 0.41 0.44 0.44 0.52 0.49 0.46 0.45 0.40 0.45
0.32 0.24 0.38 0.43 0.34 0.49 0.39 0.44 0.42 0.40
0.32 0.37 0.44 0.48 0.49 0.32
0.29 0.39 0.41 0.41 0.48 0.28
0.34 0.44 0.44 0.46 0.51 0.58
1.40 1.60 1.94 2.13 2.17 1.40
0.62 0.84 0.87 0.88 1.03 0.60
0.64 0.81 0.23 0.82 0.85 0.33 0.94 1.08 0.19
0.23 0.27 0.32 0.35 0.36 0.23
0.21 0.25 0.30 0.30 0.27 0.21
0.51 0.52 0.64 0.66 0.54 0.52
0.38 0.42 0.48 0.48 0.40 0.42
2.12
1.22
2.03 0.31
0.70 0.99 0.59
0.42 2.05 0.72 0.95 0.41 0.33 0.43 0.65 2.10 0.79 0.97 0.22 0.42 0.48 0.45 2.58 0.90 1.02 0.52 0.39 0.51 0.84 2.66 0.90 1.15 0.28 0.53 0.38 0.37 2.18 0.75 0.84 0.44 0.39 0.38 2.10 0.79 1.15 0.13 0.42 0.51 0.31 a Experimental Conditions: beam current, 100 nA, 2000 eV, at normal incidence; modulation, 5 V pp; reference electrode, Ag/AgCl (1 M KC1); adsorption from 10 mM KF adjusted to the pH indicated; rinsing with HF (pH 3); 0.1 mM KF (pH 7) or 0.1 mM KOH (pH 10). * indicates potassium signal adjusted for contributions due to immersion.
1mM aqueous solution at 0.00 V onto Pt(ll1) surface is J? = 0.56 nmol/cm2, which is similar to that obtained from 1M solution. This indicates that PZ reaches the limiting packing density from a 1 mM solution. If PZ had been adsorbed in the horizontal or vertical orientation, the packing density would have been lower, 0.438, or higher, 0.748 nmol/cm2, on the basis of molecular models.5 ( 5 ) (a) Pauling, L. C. The Nature of the Chemical Bond, 3rd ed.; Corne11 University Press: Ithaca, NY, 1960. (b) Structure Reports, Pearson, W. B., Ed.; 1957; Vol. 21,p 610. (c) Structure Reports; Pearson, W. B., Ed.; 1MO;Vol. 24, p 689. (d) Almenningen, A,; Bjornsen, G.;Ottersen, T.; Seip, R.; Strand, T. Acta Chem. Scand. 1977,31A, 63.
By analogy with preceding studies of pyridine adsorption at Pt(lll),lBwe interpret this result as indicating that PZ adsorbs through one of its nitrogen atoms, in a tilted vertical orientation with an angle of 79" between the ring and the Pt surface: 16.61/I' = a(b cos 4 + 3.4 sin 4) (4) where u is the area of the "molecular footprint" (Az/ molecule) projected onto the surface, a is the molecular width (6.53 A), b is the molecular height (6.36 A), and 3.4 A is the molecular thicknemsa A model illustrating the PZ structure is shown in Figure 2A. u=
Chaffins et al.
1276 Langmuir, Val. 6, No. 7, 1990
Table 11. Formulas for Obtaining Packing Density from Auger Spectra. compd formulas PZ
PM
PD
PZCA
r = rc/5 r N = ( ~ N / I P ~ ' ) / [ B N+(f/2)1 ~/~ ro = (zo/zpt')/[B0(3/4 + f/4)1 (zPt/iPtO) = (1 - 3~172 rc = (Ic/~~t')/[Bc(1/12+ llf/12)1 r = rc/6 r N = (IN/ZP~')/[BN(~/~ + f/2)1 r o = (zo/Zpto)/[Bo(3/8 + 5f/8)1
ENERGY LOSS Ism-I)
IskHz
23PZDCA
(IptlIpt') = (1- 6Kr)' rc = (Ic/~pt0)/[Bc(3/10+ 7f/10)1 r = rc/5 r N = (IN/IPto)/(BNf) To = (Zo/Ipto)/[Bo(3/4 + f/4)1 (IpJlptO) = (1- 3 K r ) ( l - 6Kr)
4PDCA
a Constants: Bc = 0.314 cm2/nmol; BN = 0.633 cm2/nmol; Bo = 0.574 cmZ/nmol; BK = 3.03 cm2/nmol; K = 0.165 cm2/nmol; f = 0.70.
0
IO00
zoo0
3000
4
0
ENERGY LOSS ( c m - I )
Table 111. Assignments. of EELS Bands for Adsorbed Layers of Parent Two-Nitrogen Heteroaromatics
PZ at pH 3/0.00 V 3071 1409 1127
PM at pH 3/0.00 V 3080 1545 1397 1186 1078
PD at pH 3/0.00 V 3080 1561 1363 1159 907
776 442
798 693 450
448
description CH stretch CC, CN stretch CC, CN stretch CH bend, ring breathing ring breathing CH bend CH bend, ring bend ring bend ring bend, Pt-N stretch
Experimental conditions: adsorption from 1 mM subject compound in 10 mM KF/HF (pH 3) solution, followed by rinsing with either HF (pH 3, 2 mM) or KOH (pH 10, 0.1 mM) solution as indicated in the table. Electrode potential applied during adsorption and rinsing processes is the same; other conditions as in Figure 1. Assignments are band frequencies in cm-l. a
ENERGY LOSS (cm-1)
Figure 1. Vibrational spectra of dinitrogen heteroaromatics. (A) Upper curve: EELS spectrum of adsorbed pyrazine at Pt(ll1). Lower curve: mid-IR spectrum of pyrazine vapor.3 (B)Upper curve: EELS spectnun of adsorbed pyrimidine at Pt(ll1). Lower curve: mid-IR spectrum of pyrimidine vapor.3 (C)Upper curve: EELS spectrum of adsorbed pyridizine at Pt(ll1). Lower curve: mid-IR spectrum of pyridazine vapor.3 Experimental conditions: adsorption from 1mM solution of adsorbate in 10 mM KF/HF (pH 3) aqueous electrolyte, followed by rinsing with 2 mM H F (pH 3); electrode potential, 0.00 V; EELS incidence and detection angle, 62O from surface normal; beam energy, 4 eV; beam current, about 200 PA; EELS resolution, about 10 meV (80cm-l).
The EELS spectrum of PZ is shown in Figure 1A (upper curve). The spectrum is essentially the envelope of the IR spectrum of vapor-phase PZ (lower curve). The peak at 442 cm-' is a t least partially due to the Pt-N stretch. Detailed assignments of the EELS spectrum based upon accepted IR assignments'3J are given in Table III. Although (6) Pearce, C. K.; Grosse, D. W.; Hessel, W. J. Chem. Eng. Data 1970, 15,567. (7) Bus,J.; Liefltens, T. J.; Schwaiger, W. Recuell 1973, 92,123.
the nitrogen atom furthest from the surface might be at least partially protonated at pH 3, the proton would leave the surface (as HF) during evacuation. Adsorbed PZ is electrochemically inert, as illustrated by the cyclic voltammogram shown in Figure 3A. The PZ layer passivates the Pt(ll1) surface to about the same degree as is observed for pyridine.'" Pyrimidine (PM). The molecular packing density of PM at Pt(ll1) is 0.51 nmol/cm2, Tables I and 11. When compared with the theoretical molecular packing densities of 0.383 nmol/cm2 for the horizontal orientation or 0.735 nmol/cm2 for the vertical orientation,6 the experimental packing density points to the structure shown in Figure 2B, in which PM is oriented nearly vertically with a planeto-surface angle of 77", eq 4 ( a = 6.65 A, b = 7.07 A). The EELS spectrum of PM adsorbed from 1 mM aqueous solution (pH 3) at 0.00 V, Figure 1B (upper curve), is very similar to the vapor-phase IR spectrum (lower curve). Assignments of the EELS vibrational bands of PM appear in Table 111.
Langmuir, Vol. 6, No. 7, 1990 1277
Multinitrogen Heteroaromatics at Pt(11 1 ) Surfaces A. PYRAZINE
6. PYRIMIDINE
p~ 3
pH 3
C. PYRIDAZINE p~ 3
O.OV
E. 23PZDCA
pH3
F. 4PDCA -0.2v
pH3
ONn
0 0.5v
Figure 2. Structural models of adsorbed dinitrogen heteroaromatics. Values of 0 based upon Auger data for adsorption from 1mM adsorbate (pH 3) solution at 0.00 V: (A) PZ (e = 79O), (B) PM (e = 77O), (C) P D (8 = 73O), (D) PZCA (8 = 86O), (E) 23PZDCA (0 = 80°), (F)4PDCA (0 = 86'). Table IV. Assignments. of EELS Bands for Adsorbed Layers of PZCA PH PH PH PH PH PH PH PH 314.20 V 314.10 V 3/0.00 V 310.30 V 310.50 V 10/4.10 V lO/O.00 V 10/0.50 V 3567 3570 3408 3444 3454 3450 3420 3085 3075 3084 3081 3083 3070 3051 3061 1732 1756 1746 1742 1750 1642 1633 1627 1375 1377 1396 1385 1404 1399 1380 1390 1215 1190 1171 1180 1050 1061 1182 1029 1029 844 844 849 759 759 715 669 659 669 536 470 485 ' 439 430 478 a
description 0-H stretch 0-H-N stretch CH stretch C=O stretch OCO, CC stretch CC, CN stretch; CH bend CC, CN, OCO stretch; CH bend CH, OH bend; C-O stretch; ring breathing CH bend; ring breathing CH ring bend CH, ring, OCO bend CH, ring, OCO bend CH, ring bend ring bend ring bend; Pt-N stretch
Band frequencies in cm-1. Experimental conditions in Table 111.
Table V. Assignments. of EELS Bands for Adsorbed Layers of 23PZDCA PH 3/4.20 V PH 3/0.00 V pH 3/0.50 V pH 10/4.20 V pH lO/O.00 V pH 10/0.50 V description 3606 0-H stretch 3383 3400 3400 3417 3466 3426 0-He-N stretch 3067 3100 3088 3054 3100 3087 CH stretch 1722 1769 1774 C 4 stretch 1750 1620 1635 1666 OCO, CC stretch 1374 1403 1402 CC, CN stretch; CH bend 1359 1400 1393 CC, CN, OCO stretch; CH bend 1112 1167 1095 1120 1178 CH, OH bend; C-0 stretch ring breathing 728 800 870 CH, ring, OCO bend 850 CH, ring bend 634 702 663 CH, ring bend 494 ring bend; Pt-N stretch 476 0 Band frequencies in cm-l. Experimental conditions in Table 111.
Adsorbed PM is electrochemically inert, similar to PZ and pyridine, Figure 3B. Pyridazine (PD).The packing density of PD at Pt(111) adsorbed from aqueous solutions (10 mM KF, and 0.1 mM KOH, pH 7) at 0.00 V i s g r a p h e d versus PD concentration in Figure 4. An interesting feature of t h i s isotherm, compared with those of PA, PM, and pyridine,'a
is that the molecular packing density increases steadily
with concentration above 0.1 mM, unlike the others, which exhibit constant packing densities above 0.1 mM. These differences in adsorption behavior suggest that bonding b y u donation predominates i n the attachment of PD to platinum. At low packing densities, double-u attachment to the Pt surface (two nitrogen atoms) accounts for the
1278 Langmuir, Vol. 6, No. 7, 1990
Chaffins et al.
Table VI. AssiPnments. of EELS Bands for Adsorbed Lasers of IPDCA pH 3/0.00 V pH 3/0.50 V pH 10/-0.20 V pH lO/O.00 V pH 10/0.50 V description 3579 3589 0-H stretch 3091 3074 3087 3110 3083 CH stretch 1700 1739 C = O stretch 1609 1653 1645 OCO, CC stretch 1571 CC, CN stretch 1395 1336 1373 CC, CN stretch; CH bend 1358 1385 1370 CC, CN, OCO stretch; CH bend 1170 1204 1166 CH, OH bend; ring breathing 1142 CH bend; ring breathing 1001 998 CH bend; C-0 stretch 832 825 814 CH, ring, OCO bend 686 729 786 719 740 CH, ring bend 504 512 516 ring bend; Pt-N stretch a Band frequencies in cm-1. Experimental conditions in Table 111. I
pH 3/-0.20 V 3561 3072 1728
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Figure 3. Cyclic voltammagrams of adsorbed dinitrogen heteroaromatics at Pt(ll1): (A) PZ, (B)PM, (C) PD, (D)PZCA, (E) 23PZDCA, (F)4PDCA. Experimental conditions: electrolyte, 10 mM K F adjusted to p H 3 with H F scan rate, 5 mV/s; reference electrode, Ag/ AgCl (1 M KCI).
Langmuir, Vol. 6, No. 7, 1990 1279
Multinitrogen Heteroaromatics at P t ( l 1 l ) Surfaces I 0.6
u
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results, giving way gradually to nearly vertical single-u bonding at the highest concentrations. In either range, T bonding contributions are evidently limited for PD, Figure 2C. Theoretical packing densities based upon molecular models6 are 0.383 nmol/cm2 for horizontal orientation, 0.664 nmol/cm2 for N-N edgewise vertical orientation, and 0.735 nmol/cm2 for perpendicular vertical orientation with attachment through only one nitrogen. Based upon eq 4 and the Auger data in Table 11,the ringto-surface angle is 73O at 0.00 V (1 mM PD, pH 3 KF/
HF electrolyte) ( a = 6.65 A, b = 7.08 A) if the endwise, N-$, orientation is assumed to predominate. The EELS spectrum of PD (Figure lC, upper curve) closely resembles its vapor-phase IR spectrum (lower curve). Assignments are given in Table 111. Cyclic voltammetry demonstrates inertness of the PD layer similar to that of pyridine,la PZ, and PM. 2-PyrazinecarboxylicAcid (PZCA). Carboxylic acid derivatives of nitrogen heterocycles have the interesting property that the carboxylate moieties undergo acidbase reactions with the solution phase, as well as surface bonding with platinum, to a degree which depends upon electrode potential.' Orientation changes probably accompany this potential dependence. EELS spectra of PZCA adsorbed from solution at various electrode potentials are shown in Figure 5, along with the IR spectrum of PZCA vapor. As the potential is increased from -0.2 to 0.00 V, the peak at 3450 cm-l due to 0-H stretching grows while that at 3570 cm-l shrinks. The peak at 3450 cm-1 then begins to shrink as the potential is increased above 0.00 V. These findings appear to be related to intramolecular hydrogen bonding between the carboxylic acid moiety and the nitrogen atom farthest from the surface in the adsorbed state (Nl) illustrated in Figure 2D. An equilibrium may exist between the hydrogenbonded conformation (3450 cm-1) and the free acid form (3570 cm-l) similar to that in PZCA vapor (bottom curve, Figure 5). Evidently, the inductive effect of increasingly positive electrode potential shifts the hydrogen-bonding equilibrium in favor of the bound form at 3450 cm-l. As the potential increases beyond 0.00 V, the carboxylate r--"l"
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Figure 5. Vibrational spectra of PZCA. (A-E) EELS spectra of 2PZCA adsorbed a t Pt(ll1) from 1 mM solution (10 mM KF/HF, pH 3), followed by rinsing with 2 mM HF (pH 3) at -0.20 V (A), -0.10 V (B), 0.00 V (C), 0.30 V (D), and 0.50 V (E). The lowest curve is the mid-IR spectrum of PZCA vapor.3 (F-H) EELS spectra of PZCA adsorbed at Pt(ll1) from 1 mM solutlons (10 mM KF/HF, pH 3), followed by rinsing with 0.1 mM KOH (pH 10) a t -0.10 V (F), 0.00 V (G),and 0.50 V (H). The lowest curve is the mid-IR spectrum of the potassium salt of PZCA. Other experimental conditions as in Figure 1.
1280 Langmuir, Vol. 6,No. 7, 1990
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Chaffins et al.
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Figure 6. Vibrational spectra of 23PZDCA. (A-C) EELS spectra
of 23PZDCA adsorbed a t Pt(ll1) from 1mM solutions (10 mM KF/HF, pH 3), followed by rinsing with 2 mM HF (pH 3) at -0.20 V (A), 0.00 V (B), and 0.50 V (C). The lowest curve is the midIR spectrum of solid 23PZDCA.4 (D-F) EELS spectra of 23PZDCA adsorbed a t Pt(ll1) from 1mM solution (10 mM KF/ HF, pH 3) followed by rinsing with 0.1 mM KOH (pH 10) at -0.20 V (D), 0.00 V (E), and 0.50 V (F). The lowest curve is the midIR spectrum of potassium salt of 23PZDCA. Other experimental conditions as in Figure 1.
Figure 7. Vibrational spectra of 4PDCA. (A-C) EELS spectra of 4PDCA adsorbed at Pt(ll1) from 1mM solutions (10 mM KF/ HF, pH 3), followed by rinsing with 2 mM HF (pH 3), and rinsing with 2 mM HF (pH 3) at -0.20 V (A), 0.00 V B), and 0.50 V (C). The lowest curve is mid-IR spectrum of solid 4PDCA. (D-F) EELS spectra of 4PDCA adsorbed at Pt(ll1) from 1mM solution (10 mm KF/HF, pH 3), followed by rinsing with 0.1 mM KOH (pH 10) at -0.20 V (D), 0.00 V (E), and 0.50 V (F). The lowest curve is mid-IR spectrum of potassium salt of 4PDCA. Other experimental conditions as in Figure 1.
group enters into bonding with the Pt surface, leading to disappearance of t h e O-H stretching bands, as is characteristic of such compounds,le,b,g Figure 2D, and
decreasing amplitudes of bands corresponding to CH and ring bending modes (659-715 cm-l). The fact that PZCA binds to the surface predominantly through the nitrogen
Multinitrogen Heteroaromatics at P t ( l l 1 ) Surfaces
atom in position 4 (N4) is probably due to steric hindrance at N1; inductive effects may also contribute. Orientational changes are also probably as shown schematically in Figure 2. When the adsorbed layer of PZCA is rinsed with KOH (pH lo), the carboxylic acid is neutralized, and K + ions a r e r e t a i n e d (0.40-0.67 n m o l / c m 2 ) in approximately the stoichiometric amount. The packing density of PZCA at 0.00 V is relatively high (0.52 nmol/cm2). The theoretical packing density is 0.599 nmol/cm2 based upon a molecular model of the N-at,tached vertical orientation, with the carboxylate moiety coplanar with the ring, or 0.282 nmol/cm2 for the horizontal orientation. Based upon eq 4 (with a = 8.15 A and b = 7.78 A) and the observed packing density, the ring-tosurface angle is 86O. That is, PZCA is oriented nearly vertically at 0.00 V. The decline in packing density at very positive potentials is thought to be due to coordination of the carboxylate group to the surface, which increases the size of the molecular footprint, and also to interference by surface oxides. The decline at very negative potentials is probably due to interference from adsorbed hydrogen. Also, the fact that the packing density is high, approaching that for vertical orientation, indicates coplanarity of the carboxylate with the ring in the adsorbed state. Cyclic voltammetry demonstrates the inertness of adsorbed carboxylic acid derivatives of nitrogen heterocycles PZ, 23PZCA, and 4PDCA, Figure 2E-G. 2,3-PyrazinedicarboxylicAcid (23PZDCA). EELS spectra of 23PZDCA adsorbed a t various electrode potentials are shown in Figure 6. The presence of a prominent 0-H vibration at 3417 cm-l a t -0.12 V is evidence for a pendant carboxylic acid group which is involved in hydrogen bonding; the shoulder near 3606 cm-l is assignable to free carboxylic acid 0-H stretching. Analogous behavior is displayed by PZCA, Figure 5. Evidently, the hydrogen-bonded forms of the carboxylic acid moiety predominate in adsorbed 23PZDCA, Figure 2E. Predominance of the hydrogen-bonded form suggests that intramolecular hydrogen bonding between the carboxylic acid in the 3-position and the nitrogen atom in the 4-position is favorable, although intra- and intermolecular hydrogen bonding undoubtedly both occur to at least some extent. Adsorption of 23PDCA at positive potentials leads to decreased intensity of the 0-H stretching bands, an indication that coordination of carboxylate groups to the Pt surface is more favorable at positive than at negative potentials, as expected, Figure 6. The carboxylate closest to the surface in the adsorbed state is evidently coordinated to Pt a t all potentials, as reported previously for picolinic acid.l* These trends are illustrated in Figure 2E. Adsorbed 23PDCA behaves as a relatively weak acid, being incompletely neutralized at pH 10 (Table I1 and Figure 6). Potassium ion retention amounts to 0.33 nmol/cm2, corresponding to only one K+ ion per molecule containing two carboxylate groups. The molecular packing density of 23PZDCA at 0.00 V is relatively small, 0.32 nmol/cm2 (Table I). This suggests a tilted, vertical orientation in which the ring-to-surface angle is 80° 16.61/r = a ( b cos $J + 5.07 sin 4) where a = 8.15 A, b = 7.78 A, and the effective width of the molecule is equal to 5.07 A, the width of the carboxylate group. Theoretical packing densities based upon molecular models are 0.236 nmol/cm2 for the horizontal orientation and 0.402 nmol/cm2 for a vertical orientation in which at least one carboxylate moiety is rotated outof-plane. a=
Langmuir, Vol. 6, No. 7, 1990 1281
4-Pyridazinecarboxylic Acid (4PDCA). T h e molecular packing density of 4PDCA at 0.00 V is relatively high, 0.52 nmol/cm2. Three modes of surface attachment are possible:
The theoretical packing density based upon molecular models of the +orientation is 0.638 nmol/cm2, while that for the N2-7’ orientation is 0.591 nmol/cm2 or for the N1q1 orientation is 0.741 nmol/cm2. That is, on the basis of packing density alone, one cannot distinguish between vertical-edgewise (q2) and tilted-endwise (ql) orientations. EELS spectra of 4PDCA adsorbed at negative potentials, Figure 7, contain a distinct 0-H stretching peak of moderate height and high frequency, 3561 cm-l. This peak decreases markedly at positive potentials of adsorption. Rinsing the surface layer with KOH results in retention of K+ ions which is most pronounced at negative potentials, Table 11, and changes in the EELS spectrum appropriate to an adsorbed carboxylate salt, Figure 7. The observed potential dependence of the 0-H stretching band indicates the presence of at least some N2-+ or N1N2-q2material, by analogy with results previously reported for nicotinic acid and isonicotinic acidel* The high frequency of 0-H stretching is as expected since these compounds are structurally incapable of intramolecular hydrogen bonding. The prominent carboxylate anion EELS peaks (814,1370, and 1645 cm-l) at all potentials are an indication that N17‘ is the predominant adsorbed state. The fact that the 0-H stretching band is only moderately strong is an indication that intermolecular hydrogen bonding occurs to at least some extent in the 4PDCA layer. The 4PDCA adsorbed layer is electrochemically inert, as shown by the cyclic voltammogram in Figure 3F. 1,3,5-Triazine (TZ). Auger spectra of the Pt(ll1) surface after immersion into an aqueous TZ solution (1 mM TZ, 10 mM KF/HF, pH 3, 0.00 V) displayed only peaks due to platinum. Evidently, TZ is not chemisorbed a t P t ( l l 1 ) under the conditions of these experiments. The possibility that TZ could be chemisorbed from solution but desorbed under vacuum is ruled out by the results of the following experiment: immersion of Pt(ll1) into a solution containing 10 mM TZ and 0.1 mM hydroquinone (HQ) (in 10 mM KF/HF aqueous electrolyte, pH 3, -0.1 V) resulted in only a 20% decrease in packing density of HQ, Table 11,relative to that observed when TZ was omitted. That is, TZ is adsorbed only weakly or not at all, as a 100-fold excess of TZ had little influence upon the adsorption of HQ from solution. Apparently, the presence of three nitrogen atoms in the six-membered aromatic ring largely deactivates the ring toward a bonding and K bonding with a Pt surface.
Acknowledgment. This work is supported by the National Institutes of Health. Instrumentation was provided by the National Science Foundation, the Air Force Office of Scientific Research, and the University of Cincinnati. The technical assistance of Arthur Case, Frank Douglas, Douglas Hurd, and Richard Shaw is gratefully acknowledged Registry No. Pt, 7440-06-4;pyrazine, 290-37-9; pyrimidine, 289-95-2;pyridazine, 289-80-5; 2-ppazinecarboxylicacid, 98-975; 2,3-pyrazinedicarboxylicacid, 89-01-0; 4-pyrazinecarboxylic acid, 50681-25-9.
