Langmuir 1990,6, 1234-1237
1234
Oxidation-State Changes of Molecules Irreversibly Adsorbed on Electrode Surfaces As Monitored by in Situ Fourier Transform Infrared Reflection Absorption Spectroscopy Takeshi Sasaki, In Tae Bae, and Daniel A. Scherson* Case Center for Electrochemical Sciences and The Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106
Beatriz G.Bravo and Manuel P. Soriaga Department of Chemistry, Texas A&M University, College Station, Texas 77843 Received November 2, 1989. In Final Form: January 22, 1990 In situ Fourier transform infrared reflection absorption spectroscopy has been used to monitor the changes in the vibrational properties of 2,5-dihydroxythiophenol (DHT) irreversibly adsorbed on polycrystalline Au as a function of the applied potential. The results obtained have indicated that the normalized difference spectra of DHTin its oxidized state exhibit features consistent with the formation of the quinone derivative of the compound, benzoquinonethiol (BQT), in agreement with the suggestions put forward earlier in the literature on the basis of other measurements. Strong similarities were also found between the normalized difference spectra of adsorbed and solution-phaseBQT. The relatively large intensity of the signals observed for the adsorbed species has provided strong evidence that both DHT and BQT are adsorbed on Au with the molecular plane forming a small rather than large angle with respect to the normal to the surface.
Introduction Species irreversibly adsorbed on electrode surfaces have been the subject of much study during recent years, particularly in connection with electrocatalytic phenomena.'?* Part of the effort has been aimed at achieving a detailed understanding of the nature of adsorbatesubstrate interactions a t such interfaces and the way in which these influence the overall properties of the adsorbate. Irreversibly adsorbed species which in addition can undergo well-defined oxidation and/or reduction processes represent a very interesting subclass of such systems. I n particular, correlations between t h e spectroscopic properties and the surface coverage can be made in a rather straightforward fashion by using the charge involved in the oxidation and/or reduction reactions as a measure of the number of species present on the electrode surface. Perhaps the most widely investigated system of this type is the adsorption of CO on polycrystalline and single-crystal Pt surfaces by means of Fourier transform infrared reflection absorption spectroscopy (FTIRRAS), in which the amount of CO adsorbed on the surface has been determined fairly accurately by integrating the most prominent infrared band associated with solutionphase COZ generated by the oxidation of C0.3-6 The present work will illustrate the use of FTIRRAS as an in situ probe of the vibrational properties of 2,sdihydroxythiophenol (DHT) adsorbed on polycrystalline Au surfaces as a function of potential. This species
appears particularly suited for this type of investigation as judged by a number of considerations: (i) Adsorption of D H T occurs spontaneously on Au (and Pt)' upon exposure of such surfaces to aqueous solutions of the material.8 (ii) Both DHT and its oxidized form (denoted hereafter as BQT (benzoquinone thiol)) display negligibly small rates of desorption over periods of hours in a wide potential range. This enables measurements to be conducted in solutions which do not contain DHT (or BQT) for times long enough for t h e spectroscopic experiments to be completed. (iii) Unlike CO, D H T exhibits a reversible redox process, making it possible to perform numerous experiments under practically identical conditions.8 (iv) The cyclic voltammetry of DHT adsorbed on Au (and Pt) is characterized by a prominent twoelectron redox peak in a region in which Au (and to a lesser extent Pt) behaves as an ideally polarizable electrode with essentially constant ~ a p a c i t y .This ~ allows the charge associated with the redox process to be determined and thus the number of electroactive species on the electrode surface without interference from other pseudocapacitive processes. (v) Many of the electrochemicalaJOand ex situs electron-based spectroscopic properties of DHT adsorbed on polycrystalline Pt and Au and single-crystal Pt(111)have been thoroughly examined. This study was motivated to a large extent by the need of drawing still-lacking correlations between vibrational
* Author to whom correspondence should be addressed.
(1) Parsons, R.; VanderNoot, T. J. Electroanal. Chem. 1988, 257, 9. (2) Murray, R. InElectroanulytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1984. (3) Kunimatsu, K. J . Electroanal. Chem. 1986,126, 149. (4) Furuya, N.; Motoo, S.; Kunimatsu, K. J . Electroanal. Chem. 1988, 239, 347. ( 5 ) Corrigan,D. S.;Weaver, M. J. J. Electroanal. Chem. 1988,241,143. (6)Leung, L. W.; Wieckowski, A,; Weaver, M. J. J. Phys. Chem. 1988, 92. 6985.
(7) Soriaga, M. P.; Hubbard, A. T. J. Am. Chem. SOC.1982,104,2735.
(8)Bravo, B. G.; Mebrahtu, T.; Soriaga, M. P.; Zapien, D. C.; Hubbard, A. T.; Stickney, J. L. Langmuir 1987,3, 595. (9) 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, 110,4885. (10) Mebrahtu, T.;Berry, G. M.; Bravo, B. G.; Michelhaugh,S. L.; Soriaga, M. Langmuir 1988, 4, 1147.
0743-7463/90/2406-1234$02.50/00 1990 American Chemical Society
Langmuir, Vol. 6, No. 7, 1990 1235
Oxidation-State Changes of Adsorbed Molecules properties of molecules adsorbed on electrode surfaces obtained by in situ and ex situ techniques.
Experimental Section DHT was synthesized by the method of Alcalayll and later extracted and recrystallized in ether. Both the synthesis and the extraction were performed under a nitrogen atmosphere to minimize decomposition. The electrochemical cell used in the FTIRRAS experiments was similar to that reported recently by Bae et al.,l* except that the flat CaFz window was replaced by a 60' CaF2 dove prism. In this fashion, gains of a factor of 2-3 could be achieved in terms of sensitivity over the flat window arrangement for p-polarized radiation.13J4 Solutions were made with Ultrex grade HC104 (Baker) and ultrapure water and were thoroughly degassed with highpurity N2. All potentials were measured with respect to a saturated calomel electrode (SCE). All voltammetry measurements were performed in the FTIRRAS cell using 0.1 M HC104 as the base electrolyte. After cell assembly, the electrode was cycled in this solution until features believed to be characteristic of the clean metals could be observed. The 0.1 M HC104electrolyte was then removed from the cell with a syringe and replaced by a 10 mM DHT in 0.1 M HC104solution. After 10 min, the DHT-containing electrolyte was removed and the cell filled with the base solution to wash excess DHT left as droplets or otherwise adsorbed on the cell components. The solution was subsequently removed and replaced again by 0.1 M HC104 solution. The latter step was repeated 3 times in order to decrease to negligible levels possible spectral contributions due to solution-phase DHT. After completion of this procedure, the amount of adsorbed DHT was determined by cyclic voltammetry. A series of experiments were conducted to verify that the adsorption of DHT was irreversible within the time scale of the spectroscopic experiments. For these measurements, the electrode was polarized at 0.1 V (reduced state) after recording a voltammetry curve and then pushed against the window. After 30 min it was pulled away from the window, and a new voltammogram was recorded. The electrolyte was then removed, the cell filled with 0.1 M HC104 solution, and a cyclic voltammogram once again obtained. The electrode was then polarized at 0.6 V (oxidized state) and the same procedure repeated. No detectable decrease in the amount of adsorbed DHT could be obtained during the whole procedure as judged by the fact that all voltammetric curves were found to be essentially identical. The spectroscopic measurements were conducted with the electrode polarized at a fixed potential for a time long enough (ca. 4 min) to acquire 500 scans. The spectrum was then stored and the potential scanned with the electrode always pushed against the window to a new value before acquisition of a new spectrum of the same number of scans. The spectra presented in this work represent the normalized difference between averaged spectra obtained at a given potential and the averaged spectrum at +0.1 V denoted herein as the reference. In addition to gold, spectroscopic data were also acquired for platinum for which the results were found to be somewhat similar.
Results and Discussion Cyclic Voltammetry. Figure 1 shows a voltammetric curve obtained for DHT adsorbed irreversibly on Au in t h e spectroelectrochemical cell with t h e electrode assembly pushed against the optical window. Much of the distorsion in the curve including the difference in the peak potentials for the oxidation and reduction reactions may be attributed to the large resistance due to the thin layer (11) Alcalay, W. Helu. Chim. Acta 1947, 30, 578.
(12) Bae, I. T.; Xing, X.; Yeager, E. B.; Scherson, D. A. Anal. Chem. 1989,61, 1164.
(13) Greenler, R. J. Chem. Phys. 1966,44, 310. (14) Bethune, D. S.; Luntz, A. C.; S a , J. K.; Roe, D.K. Surf.Sci. 1988,
197, 44.
-0.40
0.00
0.40
0.80
V v s . SCE
Figure 1. Cyclic voltammetry of DHT adsorbed irreversibly on a Au electrode. This curve was obtained in the spectroelectrochemical cell with the electrode pushed against the optical window (see text for details regarding the adsorption procedure). Scan rate, 1mV.s-1; electrode area, 1.7 cm2. The dashed line represents the capacitive contribution of the interface, which was used as a base line for the integration of the peaks.
-AR/R
T0.002
1800
1600
1400
1200
Wavenumber, cm-'
Figure 2. Series of normalized difference FTIRRAS spectra of DHT adsorbed on polycrystallinegold in the region between 1100 and 1800 cm-1. The spectum obtained at +0.1 V was used as a reference. These curves were recorded in the sequence +0.2 (A), +0.4 (B),+0.5 (C), +0.6 (D),+0.4 (E) and +0.1 V (F). The latter represents the normalized difference spectra of the original reference and the spectra obtained at the same potential after the potential excursion to +0.6 V.
of electrolyte trapped between the working electrode and the cell window. Integration of either of the redox peaks using the rather featureless capacitive current as a base line (see dashed line) yielded values for t h e charge corresponding t o ca. 7.3 X 10-lo mol (or 4.4 X 1014 molecules)/cm2, which compare well with those obtained earlier by Soriaga and co-workers.sJ0 In Situ FTIRRAS Spectra. A series of normalized difference spectra in the region between 1100and 2000 cm-l obtained with Au electrodes using as reference the spectra obtained at +0.1 V are shown in Figure 2. As indicated, no features could be detected at a potential of +0.2 V (curve A). This is consistent with the cyclic voltammetry in Figure 1,which indicates that this potential is negative to the onset of the redox peak. A number of positive- and negativegoing peaks could be clearly identified, however, at potentials more positive than +0.4 V (curves B-D). The rather strong character of the signals observed suggests that the various normal modes involved have associated dynamic dipole moments with substantial components along a direction perpendicular to the electrode surface
Sasaki et al.
1236 Langmuir, Vol. 6, No. 7, 1990
PO Po HE
HO
+
-AR/R
I
,
,
'570
i
1547
1277
r\'
1631.
2H'
*
ze
A &
1319
Figure 4. Schematic diagram of the oxidation and reduction of adsorbed DHT. 1454
!
c. 1800
1211
+
+
1600
1400
'
1 .. . + A 1200
Wavenumber, cm-'
Figure 3. Normalized difference FTIRRAS spectra of a solution of 50 mM DHT in 0.1 M HC104 electrolyzed for 150 min at a potential of 0.5 V with the spectra obtained at +0.1 V as a reference.
and thus are capable of interacting with the electric field vector for light polarized parallel to the plane of incidence. This provides evidence that the adsorbate both in the reduced and oxidized forms is coordinated to the surface with the molecular plane perpendicular rather than parallel to the substrate. Spectra collected for potentials in decreasing sequence (curves E and F) yielded similar results, indicating that the process is indeed reversible. It may be noted that the increase in the background a t 1106 cm-' is due to the perchlorate,'5 which is forced into the thin-layer cell to counterbalance the increase in the proton concentration brought about by the oxidation reaction. Solution-Phase Spectra. Considerable insight into the nature of these features could be obtained from FTIR reflection absorption experiments involving a thicker layer of a DHT solution interposed between the window and the electrode surface. Figure 3 shows the normalized (reference potential = +0.1 V) FTIRRAS spectra obtained with unpolarized light for a solution of 50 mM DHT in 0.1 M HCIOr that had been electrolyzed for 150 min at a potential of 0.5 V. This spectrum displays a number of prominent features: (i) a negative-going band a t 1211 cm-', which is characteristic of C-0 stretchingzs arising from the hydroquinone form of the compound; (ii) two positive-going bands a t 1277 and 1319 cm-1 associated with C-C stretching modes;" (iii) two negative-going bands a t 1501 and 1454 cm-' which may be assigned to C=C skeletal deformations of the benzene ring;l6-lS and (iv) a negativegoing band at 1693 cm-' in the solution-phase spectra, which may be attributed to water OH bending.lg The first three observations are consistent with a hydroquinone-quinone-type redox process, as has been inferred from other types of measurements. These features are very similar to those observed for the surface-bound forms of these species, indicating that the chemical attachment modifies only slightly their vibrational properties. A schematic diagram of the reaction involving surface-bound species is shown in Figure 4. Support for a S-bridge-type attachment involving a loss of the mercaptan hydrogen may be found in the known affinity of Au for thio-type functionalities and in the absence of the S-H stretch a t 2500 cm-' and S-H bend a t 1020 cm-' in (15) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds; Wiley: New York, 1986;p 130. (16) Pouchert,C . J. The Aldrich Library of Infrared Spectra, 3rd ed.; Aldrich Co.: Milwaukee, 1981. (17) Davies, M.; Prichard, F. E. Trans. Faraday SOC.1963,59, 1248. (18)Prichard, F.E.Spectrochim. Acta 1964,20, 1283. (19)Herzberg, G.Molecular Spectra and Molecular Structure. II. Infrared and Raman Spectra of Polyatomic Molecules; van Nostrand Reinhold Co.: New York, 1945;p 384.
Table I. Spectral Features of a Normalized Difference Spectra of DHT Irreversibly Adsorbed on a Polycrystalline Au Electrode frequency, cm-' solution Pt(ll1) gold phase" (HREELS)g assignment Negative-Goina Peaks 1207,1180 1211 1171 C-0 stretching in-plane 0-H deformation 1342 1342,1381 1463 C = C skeletal stretching 1445,1497 1454,1501 C=C stretching 1574 1595 1593 Positive-GoingPeaks 1286,1306 1277,1319 1531,1561 1547,1570 1647 1631
the spectra of reference.
c IT
at +0.1
C-C stretching C=C stretching C=O stretching ' vs SCE was used as a
-AR/R 1106
'i P I
1
7
I
T 0.002
2500
2000
1500
Wavenumber, cm-'
Figure 5. Normalized difference FTIRRAS spectra of DHT adsorbed on polycrystallinegold recorded at +0.8 V. The prominent band at 2343 cm-1 is due to CO2 generated by the partial decomposition of BQT. See caption to Figure 2 for other conditions. The arrows point to the spectral features listed in Table I.
the high-resolution electron energy spectra (HREELS) of DHT adsorbed on Pt(ll1) surfaces (vide infra). The energies of the spectral features obtained for the adsorbed (both in situ and ex situ) and solution-phasespecies including the proposed peak assignments are given in Table I. Although the similarities between the spectra obtained in solution and on the surface in situ are indeed marked, there are some differences that deserve further attention. These include the much decreased relative intensity of the peak a t about 1211 cm-1 for the surface-bound species compared with those a t 1319 and 1277 cm-l and the slight shifts in energies for these and other bands. In fact, a careful inspection of Figure 3 affords some evidence for a splitting of the 1211-cm-1 peak into two bands. This effect may be related to the inequivalency of the two C-0 bonds, which may become particularly pronounced upon adsorption of DHT to the surface. Spectroscopic Evidence for Electrochemically Induced Degradation of Adsorbed BQT. Normalized difference spectra were also recorded with the Au electrode polarized at +0.8 V. As indicated in Figure 5, the spectra displayed a prominent band a t 2343 cm-', which may be
Langmuir 1990,6, 1237-1245 attributed to C02,19 and a much increased Clod- band a t 1106 cm-1. Also to be noted is the very much diminished 1561-cm-l band ascribed to C=C stretch compared with the normalized difference spectra a t +0.6 V. These findings afford evidence that a t +0.8 V the adsorbed BQT undergoes partial oxidation to yield carbon dioxide and other products which still remain to be identified. Comparison of in S i t u a n d ex S i t u Results. Hubbard and co-worker$ have recently reported Auger electron spectra (AES),high-resolution electron energy loss spectra (HREELS), and cyclic voltammetry data for DHT adsorbed on Pt(ll1) from aqueous solutions. I t was concluded on the basis of those results that about 50% of the adsorbed DHT undergoes cleavage generating hydroquinone (which presumably remains in solution phase) and atomic S on the Pt(ll1) surface. In addition to losses a t energies higher than 1000 and smaller than 3000 cm-l, these authors reported bands a t 1574,1463,and 1171cm-l which, although slightly shifted, compare well with those observed in this work. The in-plane 0-H deformation modes a t ca. 1350 cm-l (ref 20) and the two different C-0 stretches could not be resolved by HREELS and hence were not listed in that work. Although HREELS offers a much higher sensitivity than conventional reflection absorption infrared spectroscopy, (20) (a) Silverstein, R. M.; Bassler, G . C.; Morril1,T. C. Spectrometric Identrfrcation of Organic Compounds; Wiley: New York, 1981; p 114. (b)Bellamy, L J. The Infrared Spectra of Complex Molecules; Chapman and Hall: London, 1975; p 122. (c) Yates, P.; Ardao, M. I.; Fiester, L. F. J . Am. Chem. SOC.1956, 78, 650.
1237
there are a t least two issues that may limit the use of this technique in the study of electrochemical interfaces: its intrinsic ex situ character and, perhaps most important, its much poorer resolution, a factor that will preclude the detection of subtle modifications in the position and relative intensities of various spectral features induced by changes in the applied potential. Summary The results obtained in this study have demonstrated the following: (i) In situ FTIRRAS in its present stage of development has enough sensitivity to examine the spectral properties of D H T (and a number of other species irreversibly adsorbed on Au (and Pt) electrodes) as a function of potential. (ii) The overall spectral characteristics of DHT and its quinone derivative (BQT) in the adsorbed state resemble closely those of their solution-phase counterparts. (iii) The relatively large intensity of the signals observed affords strong evidence t h a t D H T and BQT are adsorbed on both Pt and Au with the molecular plane at a small rather than large angle with respect to the normal to the surface. Acknowledgment. This work was supported by the Gas Research Institute. Registry No. DHT, 2889-61-4; BQT, 91751-34-7;Au, 744057-5.
Adsorption of Alkyl-Substituted Phenols onto Montmorillonite: Investigation of Adsorbed Intermediates via Visible Absorption Spectroscopy and Product Analysis Debra D. Sackett and Marye Anne Fox* Department of Chemistry, University of Texas at Austin, Austin, Texas 78712 Received November 9, 1989. In Final Form: February 16, 1990 The mechanism of oxidative adsorption of substituted phenols on H+- and Cu2+-exchangedmontmorillonite has been investigated via visible absorption spectroscopy. A characteristic red absorption maximum at approximately 520 nm appears to derive from a phenoxonium ion. The presence of water increased phenol reactivity by increasing the pH of the clay and thus facilitating phenol oxidation. The products extracted from the clays were identified, and a mechanism was proposed for their formation. Molecular oxygen also increased reactivity, most significantly on H+-exchanged clays. The oxidation potential and steric bulk of the phenols are clearly correlated to their reactivity at the clay surface. Introduction
A wide variety of organic molecules are known to react a t clay surfaces via initial oxidation of the substrate by the clay.' Of these, adsorbed aromatic molecules have been the most widely studied. The products resulting from oxidation of adsorbed substituted benzene molecules tend to be dimers.l.2 In some instances, however, oligomers and polymers are produced.3* The factors which control oligomerization remain unknown, however, and we have 0743-7463/90/2406-1237$02.50/0
sought to determine in this study the relative importance of structural variables in governing observable chemical reactivity profiles on cation-exchangedclays. Such studies would be important contributions to mechanistic surface(1) (a) Voudrias, E. A.; Reinhard, M. In Geochemical Procesees at Mineral Surfaces;Davis, J. A., Hayes, K. F., E&.; ACS Symposium Seriea 323; American Chemical Society: Washington, DC, 1986, pp 462-488. (b) Theng, B. K. G . In International Clay Conference; van Olphen, H., Veniale, F., Eds.;Elsevier: Amsterdam, 1982. (c) Laszlo, P.Acc. Chem. Res. 1986, 19, 121.
0 1990 American Chemical Society
