Langmuir 1986,2, 471-476
in detail elsewhere.18 When infrared radiation is reflected from a metal surface, only the component polarized parallel to the plane of incidence (p polarized) has any amplitude a t the surface after reflection. Light polarized perpendicular to the plane of incidence (s polarized) undergoes a phase shift of close to 180' for all angles of incidence resulting in a standing wave that has little amplitude near the surface. Infrared radiation will interact with an oscillating dipole of a species when both the electric field of the radiation and the oscillator have spatial components in the same direction. Thus, only molecules that have a component of the dipole derivative (the change in the dipole moment with respect to the normal coordinate) oriented in a direction perpendicular to the surface can interact with the p-polarized radiation. The s-polarized radiation is blind to species adsorbed near the surface. For a molecule adsorbed flat on the surface, absorption of infrared radiation is forbidden by the surface selection rule. However, if a dipole moment is induced in the species perpendicular to the surface, for example, by external fields or bonding effeds, a vibrational transition can be observed by using infrared radiation. The appearance of symmetry-forbidden bands in the spectra of molecules adsorbed on metal surfaces has been observed. The interpretation of such bands includes mechanisms involving chemical bonding of the molecule to the surface and interaction of the molecule with electric fields near the metal surface. The chemical mechanism suggests that bonding to the surface decreases the symmetry of the molecule causing disallowed modes to become active.lg In addition, distortion of the molecule by donation of electrons from the metal to orbitals on the molecule has also been suggested. More quantitative explanations based on electric fields present near the metal surface have been discussed. Sass et aL20have shown that electric field gradients arising from interaction of radiation with the metal surface are strong enough to couple with quadrupole moments in the molecule, giving rise to activation of infrared-forbidden modes. In electrochemical systems, it has
471
been shown quantitatively that large electric fields which exist across the electrical double layer are strong enough to interact with electrons of highly polarizable molecules. This interaction results in a dipole moment which can oscillate normal to the metal surface? Applying this calculationgto pyrene predids a ARIR on the order of 10-4 for an electric field strength of lo6 V/cm. Therefore, we believe that the mechanism for appearance of bands in the SNIFTIRS difference spectrum of Figure 3 is through interaction of polarizable electrons in the molecule with the large static electric field across the double layer. This interaction can induce a dipole moment normal to the surface which can oscillate at the vibrational frequency of the ABring mode. The band appears at potentials very close in energy to those observed for ring stretching modes in the Raman spectrum (which are infrared-forbidden). Thus, for adsorbed pyrene a dipole moment can be induced normal to the surface by coupling the highly polarizable electrons in the aromatic ring of the molecule to the electric field across the double layer. The aromatic C-H stretching modes would not be expected to be enhanced by the electric field because of the small polarizability of the C-H bond.
Conclusion This report demonstrates that the electric field in the double layer is sufficiently strong to induce infrared activity in modes which are forbidden by normal infrared selection rules. Field induced absorption can be used to study the electric field in the double layer. Acknowledgment. We thank the Office of Naval Research for support of this work. We thank John Foley for many helpful discussions of the work, and we also acknowledge the assistance of and discussion with Dr. Michael Hunnicutt and Professor Joel M. Harris regarding pyrene. These workers have investigated similar effects of pyrene adsorbed a t dielectric surfaces.22 Registry No. Pt, 7440-06-4; pyrene, 129-00-0.
Long Optical Path Length Thin-Layer Spectroelectrochemistry: Hydrogenation of Adsorbed Aromatics by Coadsorbed Hydrogen Atoms at Platinum Yu-Peng Gui* and Theodore Kuwana* Department of Chemistry, The Ohio State University, Columbus, Ohio 43210 Received January 14,1986. In Final Form: April 18, 1986
The long optical path length thin-layer cell (LOPTLC), which allows the high-sensitivityoptical monitoring of solution species, is employed for quantitating the extent of and the electrode potential for the hydrogenation of adsorbed aromatics at Pt. The compounds studied are 1,4-hydroquinone,1,2,4-benzenetriol, p-aminophenol, 1,2-dihydroxynaphthalene,and p-nitrophenol. The experimental results indicate that (1) hydrogenation occurs at electrode potentials corresponding to adsorbed hydrogen, (2) some displacement of the adsorbed aromatics is observed with the adsorption of hydrogen, (3) hydrogenation rate depends on the electrode potential and the structure of the aromatic, (4) complete hydrogenation of the aromatic ring occurs to produce cyclohexane including reduction of the nitro group to amine in the case of p nitrophenol, and ( 5 ) hydrogenation is a surface-controlledreaction between the adsorbed hydrogen and the adsorbed aromatic.
Introduction Several catalytic hydrogenation have demonstrated that aromatic compounds can be hydrogenated to
* Present address: Center for Bioanalytical Research, University of Kansas, Lawrence, KS 66046.
their corresponding ring-saturated products under H2 pressure with Pt as a catalyst. For example, phenol3 has
(1)Freifelder, M. Practical Catalytic Hydrogenation; Wiley: New York, 1971; Chapter XXIV.
0743-7463/86/2402-0471$01.50/00 1986 American Chemical Society
472 Langmuir, Vol. 2, No. 4 , 1986
been found to adsorb on Pt with subsequent hydrogenation to cyclohexamol. The hydrogenation mechanism is believed to be a surface-controlled reaction. In recent years, electrochemical methods4-' have been applied to study hydrogenations in liquids where conditions could be more readily controlled in contrast to gas-solid phase reactions. Hubbard and co-workers&1°have reported on the orientation and catalytic oxidation of adsorbed aromatics at Pt in a thin-layer electrochemical cell. Pons and Bewick'l have examined the orientation of difluorobenzene at Pt by FTIR. The application of the LOPTLC to the quantitation of adsorption and orientation of 1,4-hydroquinone and 1,2,4-benzenetriol at Pt was reported in our laboratory.12 This cell, with its long optical path length and large ratio of electrode area to solution volume, provides optical quantitation at nanomole levels of solution species. Such quantitation, when correlated to electrochemical information, gives evidence to support conclusions regarding processes involving the electrode surface. During the above study it was observed that the optical absorption decreased contrary to expectations of an increase at potentials corresponding to adsorbed hydrogen. A decrease indicated a loss of the aromatics from the solution rather than an increase due to displacement of the adsorbed aromatic from the Pt surface by the adsorption of hydrogen. This observation was deemed significant from the standpoint of being able to quantitate and determine the potential dependence of hydrogenation, particularly by adsorbed hydrogen. Reports on the electrochemical hydrogenation of aromatic compounds at Pt are limited. Misra and Sharma6 and Sasaki and co-workers7have investigated the hydrogenation of some aromatics at Pt in aqueous acidic solutions. Although these investigators implicated adsorbed hydrogen as being responsible for the hydrogenation, direct proof of adsorption and of the identity of the nature of adsorbed hydrogen was not provided. Hubbard et al.1° suggested that partial hydrogenation could occur when the applied potential was less than 4.10 V. What is addressed in this paper is the extent and potential dependence of hydrogenation at Pt with the LOPTLC. Experimental Section Chemicals and Solutions. 1,4-Hydroquinoneand p-aminophenol were obtained from Matheson. 1,2,4-Benzenetriol and 1,4-dihydroxynaphthalenewere obtained from Aldrich. p Nitrophenol was obtained from Eastman and potassium iodide was obtained from Mallinckrodt. The supporting electrolyte was 1 M H2S04(Mallinckrodt). The aromatic concentrations were lo4 to M in 1M H2S04.This concentration range was chosen because aromatic molecules are known to adsorb a t Pt in a flat orientation within this range.*J2 All solutions were freshly prepared as needed with NANOpure water (Sybron Barnstead). The (2) Bag, A.; Egupov, T.; Volokitin, D. Prom-st. Org. Khim. 1936,141, 2.
(3) Smith, H. A.; Stump, B. L. J. Am. Chem. SOC. 1961, 83, 2739. (4) Miller, L. L.; Christensen, L. J. Org. Chem. 1978, 43, 2095. (5) Bagnell, L.; Jeffery, E. Aust. J . Chem. 1980, 33, 2565. (6) Misra, R. A.; Sharma, B. L. Electrochim. Acta 1978, 24, 727. (7) Sasaki, K.; Kunai, A.; Harada, J.; Nakabori, S. Electrochim. Acta 1983, 3, 671. (8) Soriaga, M. P.; Wilson, P. H.; Hubbard, A. T. J. Electroanal. Chem. 1983, 142, 317. (9) Soriaga, M. P.; Stickney, J. L.; Hubbard, A. T. J. Mol. Catal. 1983, 21,211. (10) Chia, V. K.; Soriaga, M. P.; Hubbard, A. T. J. Electroanul. Chem. 19R4. 97. - _7fi7. _. 7
- - - I
(11)Pons, S.; Bewick, A. Langmuir 1985, 1, 141. (12) Gui, Y.; Porter, M. D.; Kuwana, T. Anal. Chem. 1985,57, 1475. (13)McNicol, B. D.;Miles, R.; Short, R. T. Electrochim. Acta 1983, 28, 1285. (14) Kinoshita, K.; Lundquist, J.; Stonehart, P. J. Catal. 1973,31,325.
Gui and Kuwana
9'
Side view
1
E--
A
C
Cell body
Top view
1 Figure 1. (A) First ledge for placement of Pt. (B) Second ledge for placement of Kalrez gasket. (C) Slots for placement of quartz windows. (D) Auxiliary electrode channel. (E) Solution flow and reference electrode channel. (F) LOPTL cell cavity. 1
purity of the NANOpure water was checked by examining the area decrease with time of the H-adsorption peaks at an active Pt ele~trode;'~ its purity was similar to water prepared by the Millipore system. N2 used to deoxygenate the solutions was treated with BTS catalyst (Chemical Dynamics Corporation) and then successively paased through two wash bottles which contained 1 M KOH and 1 M H,S04 solutions, respectively. Cell Fabrication a n d Dimensions. The basic LOPTL cell (LOPTLC) configuration is shown in Figure 1. The cell body was machined from a Teflon rod. The distance between the two window slots is the optical path length. There were two ledges along the optical axis of the cell; the first one was for the placement of the Pt working electrode and the second one was for a Kalrez (Du Pont) gasket, which seala the back of the Pt. The height of the first ledge determined the thickness of the cell cavity. Two optical windows were cut from a piece of S1-UV quartz plate (ESCO Products, Inc.), polished successively with sandpaper for precise fit into the slots of the cell body and sealed vacuum-tight with silicon cement (RTV, General Electric Co.) on the outside edges of the windows. A silicon gasket is placed on top of this assembly and clamped by means of stainless steel plates for a vacuum seal. The electric contact to the Pt was made through a spring between the working electrode and the steel plate. Two outlets, made from CTFE female luer (Hamilton Co.), were threaded into the holes of the cell body. One port served as a channel for the Pt wire auxiliary electrode; the other was used as a combination solution flow port and reference electrode channel. The Ag/AgCl (saturated KCl) reference electrode was connected to this port via 30 cm of Teflon tubing and a five-way Hamilton valve. All potentials reported herein are referred to this reference electrode. The working electrode was a sheet of 0.5-mm-thick Pt foil (Alfa, 99.99%), the preparation of which has been described previously.12 The LOPTLC volume ( u ) and optical pathlength ( I ) were determined from the cyclic voltammograms and absorption spectra of HQ and Triol in 1M H&04 electrolyte solution. The calculated values for the cell were u = 2.3 pL and E = 0.85 cm. From a Pt geometric surface area of 0.55 cm2, the solution thickness was calculated to be ca. 42 pm. Instrumentation. The spedroelectrochemical system included the LOPTL cell and a five-way Hamilton valve with two solution inlet ports, a vacuum port, and a reference electrode port. The five-way Hamilton valve was used to "open" the cell to one of the following lines: vacuum, pure electrolyte solution, sample solution, and reference electrode. The cell and associated parts were fabricated from inert materials such as Teflon, Kel-F, and glass. A conventional in-house-built three-electrode potentiostat was used for electrochemical experiments. Spectral measurements were made with a DMS 90 UV-vis spectrophotometer (Varian). All spectroelectrochemical experiments were performed a t room temperature. Data were recorded on a Houston 2000 X-Y recorder (Houston Instrument Co.). GC/MS experiments were carried out using a Finnigan GC/MS unit with a Carbopack (0.1% SP 1000) column. Experimental Procedures. Three types of LOPTLC experiments were conducted in this study. The first type of experiment consisted of simultaneously monitoring current (i) and
Langmuir, Vol. 2, No. 4, 1986 473
Hydrogenation of Adsorbed Aromatics absorbance (A)(monitored at the A- of aromatic) as a function of applied potential (E). The potential was scanned from the rest potential region (0.55-0.13 V) to the various limits in the hydrogen adsorption region (0.13 to -0.13 V), then scanned back. The second type of experiment consisted of measuring the optical absorbance (A) as a function of wavelength (A) at a fixed rest potential before and after a potential cycle performed in the type 1 experiment. Finally, in the third type of experiment, the absorbance (A) was recorded as a function of time (t) under a constant potential for 80-120 s at the .,A The potential was stepped from a rest potential to successively more cathodic values. The correlation of results from the above experiments gave information regarding (1)aromatic adsorption and desorption as a function of electxode potential, (2) discrimination between simple adsorption and hydrogenation of aromatics in the H adsorption region, (3) possible identification of the hydrogenation products, (4)measurement of the hydrogenation rate as a function of potential, and ( 5 ) direct proof of the hydrogenation being a surface-controlled reaction. The experimental cell preparation procedure is as follows: (1) Dissolved O2is removed from the pure electrolyte and aromatic solution by vacuum, then the LOPTLC cell is evacuated and filled with either the pure electrolyte or aromatic solution by positive N2 pressure; (2) a potential is applied in the rest potential region to the Pt after the cell is filled; (3) electrochemical and spectroelectrochemical data of i-E, A-E, and A-X are obtained; and (4) steps 1-3 are repeated if necessary in the case of aromatic solution fillings until the spectral results show that the surface is saturated with the aromatic. Additional experimental details are given in the text as necessary. Between experiments the Pt electrode was cleaned electrochemically in 1 M HzS04 by repetitively scanning at 20 mV/s between the potential limits of 1.30 and -0.14 V until a steady-state voltammetric response was obtained. Bulk electrolysis-GC/MS experiments were conducted to identify the hydrogenation products of HQ. A fritted H-cell containing a 4.7-cm2 Pt foil working electrode, a Ag/AgCl reference, and a Pt wire auxiliary electrode was used. The Pt working electrode, pretreated in the same way as in the LOPTLC experiments, was placed into the bulk cell containing deoxygenated lo4 M HQ solution in both the working and auxiliary chambers. A potential of -0.13 V was then applied while purging the solution with Np During the electrolysis,small portions of solution were removed for optical measurements to monitor the process. Electrolysiswas stopped when 25% of the HQ had been converted. The electrolyzed solutions were then neutralized with sodium bicarbonate, extracted with ethyl ether, dried over sodium sulfate, and evaporated under N2to ca. 1mL samples. GC/MS spectra of these samples were compared to those of standard solutions of HQ containing either 1,4-cyclohexanedione or l,4-cyclohexanediol at lo4 M.
Results and Discussion Evidence for Hydrogenation. The types of spectroelectrochemical experiments conducted with the aromatics in t h e LOPTLC are illustrated in Figure 2A with p-aminophenol as an example and compared to a background of 1 M H2S04(Figure 2B). Diagrams labeled a, b, and c show traces of cyclic voltammograms (&E), optical absorbances measured as a function of electrode potentials (A-E), and optical spectra (A-A), respectively. T h e characteristic hydrogen absorption waves with peak potentials of 0.05 a n d -0.07 V (trace a, Figure 2B) are assigned to t h e strongly (H,) and weakly (H,) adsorbed respectively. T h e peak heights of these hydrogen at Pt,14J6 waves (trace a, Figure 2A) are greatly suppressed in t h e presence of p-aminophenol, which was preadsorbed onto Pt in t h e double-layer region of potential (e.g., 0.3 V). It should be noted that for the aromatic adsorbed on Pt there is no obvious distinction between H,a n d H,, as there is (15) Woods, R. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1976; Vol. 9, p 37.
A
E
/I
.............................
...........................
I
I
............................
I
I
11
Figure 2. i-E, A-E, and A-X responses: (A) 0.657 X lo4 M p-aminophenol in 1 M HzS04;(B) 1M HzSO4. (a) and (b) are the i-E and A-E responses, respectively. First cycle; (-) second cycle, (- - -) third cycle. All scans began at 0.30 V and are then scanned cathodically to the negative value of potential shown. Finally scan is back to 0.30 V. Scan rate = 2 mV/s. Monitoring wavelength X = 271 nm. All i-E curves were corrected for the background curves, which were obtained with iodine-pretreated Pt in H&301. (c) A-X responses. Before first ≤ (-) after fiit cycle; (-- -) after second cycle; (--) aftei third cycle. All spectra were taken at E = 0.30 V. Arrows indicate a position change of recorder pen for clarity. (-a)
(e-)
for clean R,because the adsorbed aromatic molecules may change the Pt surface properties. For convenience in later discussions these notations of H, and H , will still be used; however, they only indicate potential regions corresponding to t h e strongly and weakly adsorbed hydrogen potentials on clean Pt. All potential scans began at 0.3 V and were scanned in a decreasing direction to various potential limits and then cycled back to 0.3 V. For the A-E traces, the wavelength maximum of 271 nm was chosen for monitoring the solution concentration of p-aminophenol (see spectra, traces in Figure 2A,c). Since absorbance at 271 n m does not change for 1 M HzS04(Figure 2B,b), the Pt surface changes are not reflected in t h e optical absorbance (compare traces a a n d b, Figure 2B); that is, t h e absorbance remains independent of potential for H2S04. I n t h e case of p-aminophenol, the absorbance, A, does not change if the potential is scanned only in t h e H, region (see dotted trace of f i i t scan, Figure 2A,b) to a potential limit of -0.01 V. If the potential is scanned more negative t h a n -0.01 V into t h e H, region (see solid line trace of second scan, Figure 2A,b), t h e absorbance initially increased slightly.
474 Langmuir, Vol. 2, No. 4, 1986
Gui and Kuwana
Table I. Experimental N H / N Aand Theoretical N H / N A Ratios with Proposed Hydrogenation Products
b ....... ,........ ....... ,
e
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I
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F,
I
a2
0.3
I
oa
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-0 .I
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Figure 3. A-E responses for 0.684 >! lo4 M hydroquinone in 1M HaOI. (-1 First cycle; (-) second cycle. Scan rate = 2 mV/s. X = 289 nm.
Calculated the proposed hydrogenation products. Calculated from the i-E and A-E experimental data. Average deviations (zt2.5) were taken from three experimental trials except for 1,2-dihydroxynaphthalene, for which only one trial was performed. (I
-D.;-------,-------: e l
20s
However, on returning the potential back to 0.3 V after scanning to a limit of -0.12 V, the absorbance had decreased from its initial starting value of the second scan. The absorbance decreased markedly if the potential was scanned to the limit of -0.15 V, as seen for the third scan (dashed line, Figure 2A,b). These results are consistent with the changes in the spectra of solution p-aminophenol taken prior to and after the first CV scan (dotted and solid lines spectra coincide in Figure 2A,C) and after the second and third scans. The absorbances a t the wavelength maxima of 271 and 217 nm decreased only in the latter two cases. This absorbance decrease indicates the loss of solution p-aminophenol, which is confirmed by the charge decrease for the reversible oxidation and reduction (about 0.4 V) of unadsorbed p-aminophenol in the solution phase. Assuming that the decrease of solution p-aminophenol was due to hydrogenation via adsorbed hydrogen and that the hydrogenated products adsorb very weakly if at all on Pt a t resting potentials compared to p-aminophenol, quantitation should be possible by calculating the moles (NH) of adsorbed hydrogen consumed to the moles (Npm) of p-aminophenol lost from the solution. These two quantities are easily computed from the i-E and A-E results with the relationships NPAP= (Af Ai)u/el NH = (8,- Q.&/F where Ai and Af are the initial and final values of the optical absorbance from a CV scan and Q, and Q, are the cathodic and anodic charges for hydrogen adsorption and desorption a t Pt. The terms u, e, and 1 are the solution volume, molar absorptivity, and cell length, respectively. The ratio NH/Npm for p-aminophenol is 5.8. For complete hydrogenation of the aromatic ring, the theoretical ratio is 6. The compounds of 1,4-hydroquinone,1,2,4benzenetriol, and 1,4-dihydroxynaphthalene exhibited similar spectroelectrochemical behavior to p-aminophenol. The experimental and theoretical ratios of the moles of adsorbed hydrogen consumed to the loss of moles of solution species are summarized in Table I. The experimental uncertainty
1
I
I
I
12
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I
I
I
I
13
6
I
I
14
I
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,s
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1
is quite large because of the difficulty in obtaining precise background-subtracted values of Q, and 8,. However, evidence for complete ring hydrogenation came from the analysis by GC-MS of 1,4-cyclohexanediol which was isolated from the bulk reduction of 1,Chydroquinone a t a potential of -0.13 V a t a large Pt electrode. Thus, it is being tentatively proposed that complete hydrogenation occurs with all of the compounds examined. Potential Dependence of Displacement and Hydrogenation. As previously discussed in the case of paminophenol, the optical absorbance is observed to increase slightly as the potential is scanned into the H region. This increase is being interpreted as the displacement of some surface-adsorbed aromatics by the coadsorption of hydrogen. The extent of this displacement and onset of hydrogenation are potential-dependent and vary with the structure of the aromatic. For example, displacement begins at ca. 0.13 V and hydrogenation at ca. -0.06 V with hydroquinone as seen by the A-E trace taken during a CV scan in Figure 3. What is interesting in this trace as well as in the third scan of p-aminophenol (see Figure 2A,b) is the continued decrease of the absorbance on scan reversal into the H,potential region. This decrease is due to the readsorption of the aromatic from the solution as the amount of adsorbed H atoms decreases, Le., oxidative desorption of adsorbed hydrogen as the potentia1 moves toward more positive values. The hydrogenation rate is observed to be potential-dependent. This rate can be qualitatively assessed by
Langmuir, Vol. 2, No. 4, 1986 475
Hydrogenation of Adsorbed Aromatics 0:s
0:4
Oh
0:2
b. A - E
0:l
d.0
-0:l
=.A-C
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Figure 5. Dependence of hydrogenation rate on potential for ( 0 )hydroquinone, (A)p-aminophenol, and (v)1,2,4benzenetriol, calculated from A-t experimental results.
monitoring the time-dependent loss of the aromatics from solution. In Figure 4, A-t traces a t various applied potentials are shown for the compounds of benzenetriol, p-aminophenol, and hydroquinone. When hydrogenation occurs, the optical absorbance decreases linearly with time. The zero-order rate suggests that the reaction is surfacecontrolled. When the data of Figure 4 are replotted as the rate of loss vs. potential in Figure 5, it appears that the order of hydrogenation is hydroquinone > p-aminophenol > benzenetriol. Also, the rate vs. E plots in Figure 5 indicate that finite hydrogenation begins as the potential moves into the H, region. Preliminary experiments with p-nitrophenol indicate that hydrogenation occurs at much more positive potentials and at much faster rates than with the other compounds studied. The ease of reduction can be rationalized by the electron-withdrawingeffect of the nitro group. The enhanced rate was evident from the fact that p-nitrophenol (6.0 X M) was completely removed from the solution during a single CV scan between the potential limits of 0.3 and -0.13 V. It had previously been reported that hydrogenation of p-nitrophenol involved mainly the nitro group.7 However, the spectral results suggest that both the nitro and the aromatic ring are hydrogenated. If so, the product would be aminocyclohexanediol,the equivalent of a 12-electron reduction. We are currently uncertain whether the nitro group is hydrogenated and reduced prior to, simultaneously with, or after the aromatic ring. Further quantitation and mechanistic studies are needed to verify our preliminary observations. Inhibition of Adsorption and Hydrogenation by Iodine. Iodine adsorbs strongly onto the Pt surface, thus blocking it for adsorption by other ~pecies.'~J' Thus, pretreating the Pt with iodide (adsorption of an iodide ion results in an adsorbed iodine atom on PtI8)would have a marked effect on the hydrogenation of aromatics, if this reaction occurs by means of adsorbed hydrogen. This blockage of hydrogenation by adsorbed iodine was illustrated by adsorbing iodide on activated Pt and then determining whether 1,2,4-benzenetriol was hydrogenated. The solution containing 8 X lo4 M KI in 1M H2S04was introduced into the LOPTL cell after the Pt electrode was activated by CV cycling as previously described. Next, the KI solution was removed and the cell was washed 3 times (16) Hubbard, A. T.; Ishikawa, R. M.; Katekaru, J. J. E l e c t r o a i i Chem. 1978,86, 271. (17)Soriaga, M.P.;White, J. H.; Song, D.; Hubbard, A. T. J. Electroanal. Chem. 1984, 171, 359. (18)Stickney, J. L.; Rosasco, S. D.;Salaita, G. N.; Hubbard, A. T. Langmuir 1985, 1 , 66.
I
i A lnml
Figure 6. Spectroelectrochemicaldata for iodine blocking ex-
periment. (a) and (b) are i-E and A-E curves, respectively. (-) First cycle; (--) second cycle. Scan rate = 2 mV/s. X = 287 nm. The i-E curves were corrected for the background curves, which were obtained with iodine-pretreated Pt in 1 M H2S04. (c) represents absorbance spectra (A-X) taken at E = 0.30 V before the first cycle and the second cycle. The two spectra were identical.
with 1 M HzS04before being filled with 8.1 X M benzenetriol in 1 M HzS04. It should be noted that the adsorbance values from A-X spectra for the first and second fillings with the benzenetriol solution (thoroughly washed between fillings) are identical. This indicates that benzenetriol does not adsorb onto the iodine-pretreated Pt, consistent with previous resu1ts.l6J7 The results of the spectroelectrochemical experiments are shown in Figure 6. The first CV scan is from a potential of +0.3 V to the anodic limit of +0.5 V. On scan reversal, the potential is cycled to a cathodic limit of -0.13 V and back to +0.3 V; the second CV is scanned from +0.3 V anodically to +0.5 V and finally is reversed to the initial +0.3 V. The reversible cyclic voltammogram with a peak anodic potential of ca. +0.40 V is due to the oxidation of the benzenetriol in the solution phase to 2-hydroxy-1,4-benzoquinone and then back to the benzenetriol on the reverse cathodic scan. It is clearly shown from Figure 6a that the areas under the oxidation or reduction peak at ca. 0.40 V are identical before and after the first potential scan (up to -0.13 V). This simply indicates that there is no loss of benzenetriol in the solution phase in the H adsorption potential region. This result is confirmed by the identical spectra (Figure 6c) of the benzenetriol, with an absorbance maximum at 287 nm, taken prior to and after the first CV scan. The result for monitoring the optical absorbance at 287 nm during the CV scan is shown in Figure 6b. The absorbance decreased during the oxidation of the benzenetriol to the quinone form and then returned to the original value upon scan reversal since the reduction results in restoring the benzenetriol without any loss of concentration. What is interesting, however, is that the optical absorbance does not change as the potential is scanned even to a limit of -0.15 V. This means that within this potential region iodine always adsorbs much more strongly on Pt than does benzenetriol. From the above experimental results, it can be concluded that adsorbed iodine has effectively inhibited the adsorption of both benzenetriol and hydrogen on Pt, thereby preventing the hydrogenation reaction from occurring. This conclusion agrees very well with previous results reported by Hubbard.lg On the other hand, it is interesting that adsorbed iodine does not inhibit the electron-transfer reaction for the oxidation of benzenetriol (19)Soriaga, M. P.; Hubbard, A. T. J.Electroanal. Chem. 1983,159, 101.
476 Langmuir, Vol. 2, No. 4, 1986
to the quinone form and then back to benzenetriol.
Summary and Conclusions The results indicate that the rates of displacement and hydrogenation of adsorbed aromatics are dependent on the molecular structure and the applied potential at Pt. Although displacement can begin in the H, region of potential, significant rates of hydrogenation occur only in the H, region of potential except in the case of p-nitrophenol. It is proposed that in the H, potential region, the H atoms coadsorb on the Pt surface with the aromatics and not only displace but also react and hydrogenate the aromatics. The hydrogenation of the aromatic ring appears to be quantitative, although the fairly large experimental errors in the ratios of N A / N Hdo not preclude some finite loss of aromatics through other routes, i.e., hydrogenolysis and ammonolysis (in the case of p-aminophenol). Pons and Bewick'l have reported the observation that the concentration of p-ditluorobenzene increased in the solution phase when the applied potential was varied from 0.203 to 0.003 V. Their studies were concerned with the orientation of adsorbed p-difluorobenzene on Pt with FTIR. The increase that they reported could have been due to adsorbed p-dfluorobenzene being displaced by coadsorbed H at the potential of 0.003 V. The hydrogenation is a surface-controlled reaction as evidenced by the linear decrease in the quantity of aromatics in the solution during set potential, timedependent experiments, as shown by the data in Figure 4. Also, the complete inhibition of hydrogenation of adsorbed iodine on Pt lends further credence to the importance and involvement of the surface. In accordance with previous results, the aromatic molecules are believed to adsorb in the "flat" configuration on the Pt surface. These flat adsorbed aromatic molecules then react with the adjacent H atoms which adsorb on Pt sites not covered by the aromatic molecules. The hydrogenation undoubtedly proceeds through several steps before complete ring reduction is achieved. The hydrogenation product, a cy-
Gui and Kuwana
clohexane, is then displaced from the Pt surface by an aromatic from the solution phase and the catalytic hydrogenation continues. A similar mechanism has been proposed previously in nonelectrochemically initiated hydrogenation studies with Pt,3,20i21 where the rate of the hydrogenation was found to be first order in H2 pressure, zero order in concentration of the aromatic, and proportional to the mass of the Pt catalyst. However, a detailed understanding of the individual steps in the hydrogenation mechanism, particularly for something like p-nitrophenol, will require additional studies. It should be mentioned that the nature of H, and H, has received considerable attention r e ~ e n t l y .Despite ~~~~~ some discrepancies in experimental results, it appears that H, and H, can be assigned to hydrogen atoms that are adsorbed on different Pt planes. For example, Hubbard et a1.22have assigned H,and H, to hydrogen atoms adsorbed on Pt(100) and Pt(ll1) planes, respectively. It would be interesting to study the hydrogenation of aromatics in the LOPTLC with single-crystal Pt electrodes.
Acknowledgment. This work was supported by a grant from the National Science Foundation and by funds from the Distinguished Scholars Award at The Ohio State University. We are grateful for their support. The contributions of B. Kazee, T. Hu, G. Hance, and M. Koppang to this work are greatly appreciated. Registry No. Pt,7440-06-4;H (atomic),12385-13-6; iodide, 20461-54-5;1,4-hydroquinone,123-31-9;1,2,4-benzenetriol, 53373-3;p-aminophenol,123-30-8;1,2-dihydroxynaphthalene,57400-5; p-nitrophenol,100-02-7;1,4-cyclohexanediol, 556-48-9;4aminocyclohexanol, 6850-65-3;1,2,4-trihydroxycyclohexane, 79817-82-6;1,4-dihydroxydecahydronaphthalene,19054-33-2; iodine (atomic),14362-44-8. (20) Pritzkow, W.Chem. Ber. 1954,87, 1668. (21) Smith, H.A.; Thompson, R. G. Advances in Catalysis; Academic Press: New York, 1957; Vol. 9, p 727. (22) Hubbard, A. T.;Ishikawa, R. M.; Katekaru, J. J. Electroanal. Chem. 1978,86, 271. (23) Will, F.J.Electrochem. SOC.1965, 112, 451.
