Anal. (?hem. 1986, 58,2961-2964 Koryta, J ; Vanqsek, P.; Biezina, M. J . Nectraanal. Chem. 1977, 75, 211. Samec, 2.; MareEek, V.; Weber, J. J . Elechoanal. Chem. 1979, 700, 841. Kihara, S.:Yoshida, 2 . Talanta 1984, 3 7 , 789. MareEek, V. Nectrochemical Detectors ; Plenum Press; New York, 1984; p 141. Samec, 2.; Homolka, D.; MareEek, V. J . Electroanal. Chem. 1982, 735,265. Samec, 2.: MareEek, V.; Weber, J.; Homolka, D. J . Electroanal. Chem. 1979, 9 9 , 385. Koczorowski, 2.; Geblewicz, G. J . Electroanal. Chem. 1982, 139, 177. Hundhammer, 6 . ;Solomon, T. J . Nectroanal. Chem. 1983, 157, 19. Solomon, T.: Alemu, H.; Hundhammer, B. J . Electroanal. Chem. 1984, 169, 303. Koryta, J. Anal. Chlm. Acta 1984, 159, 1. Vangsek, P.; Buck, R. P., J . Electroanal. Chem. 1984, 163, 1. Koryta, J.: Vanqsek, P.; Biezina, M. J . Elechoanal. Chem. 1978, 6 7 , 263. Klhara, S.;Yoshida, 2.;Fujinaga, T. BunsekiKagaku, 1982, 3 1 , E297. Ueno, K.; Saitoh, M.: Tamaoki, K. Bunseki Kagaku, 1968, 17, 1548. Kihara, S.; Suzuki, M.; Maeda. K.; Ogura, K.; Matsui, M. J . Electroanal. Chem., in press. Senda, M.; Kakutani, T.; Osakai, T. Denkl Kagaku oyobi Kogyo Butsurl Kaaaku 1981. 49. 322. (19) Fujkaga, T.; Kihara, S.; Yoshida, 2. BunsekiKagaku 1982, 3 1 , E301.
2961
(20) Grunwald, E.; Baughman, G.; Kohnstam, G. J . Am. Chem. SOC.1960, 82, 5801. (21) Paker, J. Chem. Rev. 1989, 6 9 , 1. (22) Popovych, 0.CRCCrit. Rev. Anal. Chem. 1970, 1 , 7 3 . (23) Solomon, T.: Aiemu, H. J . Electroanal. Chem. 1984, 769, 311. (24) Bockris, J. O'M.; Reddy, A . K. N. Modern Electrochemistry; Plenum Press; New York, 1970. (25) Samec. 2.: MareEek, V.; Homolka. D. J . Electroanal. Chem. 1985, 187, 31. (26) Osakai, T.: Kakutani, T.; Senda, M. Bull. Chem. SOC.Jpn. 1985, 58, 2626. (27) Girault, H. H. J.; Schiffrin, D. J. J . Electroanal. Chem. 1984, 770, 127. (28) Maeda. K.; Kihara, S.:Suzuki, M.; Ogura, K.: Matsui, M., to be submitted for publication in J . Electroanal. Chem. (29) Marcus, Y. Pure Appl. Chem. 1983, 5 5 , 977. (30) Sillen, L. G.: Martell, A. E. Stability Constants of Metal-Ion Complexes : The Chemical Society: London, 197 1.
RECEIVED for review June 6, 1986.
Accepted July 30, 1986. This work was partly supported by a Grant in Aid for Scientific Research (59470025) from the Ministry of Education, Science and Culture of Japan and partly by a grant from the Nissan Science Foundation.
Determination of Electrochemical Heterogeneous Electron-Transfer Reaction Rates from Steady-State Measurements at Ultramicroelectrodes A n d r e a Russell,' K a r i Repka,' T i m o t h y Dibble,' J a m a l Ghoroghchian,' J e r r y J. Smith,' M a r t i n Fleischmann,* Charles H.Pitt,3a n d S t a n l e y Pons*'
Department of Chemistry and Department of Metallurgical Engineering, University of Utah, Salt Lake City, Utah 84112, and Department of Chemistry, The University, Southampton, Hampshire SO9 5NH, England
I n this work, we demonstrate the use of very thin ring ultramlcroelectrodes In measurlng the heterogeneous rate constants of several fast one-ektron-transfer reactions under steadydate dlffudon condmons. Under these condltlons, the theoretlcal and Instrumental efforts are greatly slmpilfled compared to those requlred for relaxation technlques.
The determination of the heterogeneous rate constants of electrode reactions has hitherto been carried out by the use of steady-state or transient techniques using conventional electrodes (characteristic dimensions 1 mm). The upper limit of the rate constants which can be measured by using steady-state methods is -0.05 cm s-l, a limit that is set by the maximum rate of convection diffusion to appropriately shaped electrodes such as rotating disks or tubular sections. Transient (relaxation) methods, which involve the perturbation of systems initially a t equilibrium or in a steady state and the measurement of the resulting response, are able to reach rate constants as high as 10 cm s-l in view of the high rates of non-steady-state mass transfer to the electrode surface. The accuracy of all of these transient techniques is however limited a t short times by the low impedance of the double layer which appears in parallel across the equivalent circuit of the electrode processes; furthermore, all of these techniques require relatively sophisticated and costly instrumentation. The mini-
-
-
'University of U t a h , Department of Chemistry.
T h e University, Southampton. University of U t a h , Department of Metallurgical Engineering.
mum configuration will almost always include a potentiostat with a fast rise time and high voltage compliance, a signal generator, and high speed transient recording devices. Recent advances in the construction of ultramicroelectrodes (1-18) have opened up an alternative route to the determination of the kinetics of fast electrode processes. A high rate of steady-state diffusion is rapidly established in the spherical or quasi-spherical diffusion fields surrounding the microstructures (the mass transfer coefficient is of order k , = D / a cm s-l, where a (cm) is the characteristic dimension of the microstructure and D (cm2s-l) is the diffusion coefficient). The rate parameters of fast electrode reactions therefore become readily accessible by means of steady-state or quasi-steady-state measurements (e.g., by the use of slow linear sweep voltammetry). An important feature of these measurements is that they are not limited to the region close to equilibrium (the entire polarization curve can be determined) and that the accuracy is not limited by the double layer capacitance. Moreover, only relatively simple instrumentation is required, viz., a linear sweep generator, a current follower, and an appropriate recorder. A variety of ultramicroelectrode structures have been proposed and developed, including single hemispherical droplets and collections of droplets (8),dispersions of spherical particles (9),disks ( l o ) ,and thin rings (17, 18). In this paper we report measurements of the rates of a range of redox processes using the latter structure which has a number of advantages: the characteristic dimension (the thickness of the ring) can be made very small so that high rates of mass transfer are easily achieved; a t the same time current levels are determined by the radius of the ring and are therefore
0003-2700/86/0358-2961$01.50/00 1986 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986
relatively high; difficulties with regard t o the nonuniform accessibility of the surface of planar disk electrodes are markedly reduced; lastly, such rings are relatively simple to construct.
EXPERIMENTAL SECTION Tetracyanoethylene (TCNE) and 7,7,8,8-tetracyanoquinodimethane (TCNQ) were obtained from Aldrich Chemical Co. and used as received. Naphthalene, anthracene, 9,lGdiphenylanthracene, and ferrocene were also obtained from Aldrich and recrystallized from acetonitrile. Pyrene was obtained from Eastman Kodak and recrystallized in a similar manner. The supporting electrolytes tetrabutylammonium tetrafluoroborate (TBAF) and lithium perchlorate were obtained from ChemBiochem Research, Inc., and G. Frederick Smith Chemical Co., respectively, and dried for 24 h. under vacuum at 65 "C. The solvent acetonitrile, HPLC grade, was obtained from J. T. Baker Chemical Co. The solvent was dried over Woelm super grade VI neutral alumina. The base electrolyte solution was either 0.1 M LiCIOl or 0.1 M TBAF in acetonitrile; the potential window was established by cyclic voltammetry using a three-electrode cell (vide infra). The solutions were 0.5 to 5.13 mM in the analyte. The solution was degassed by purging with dry helium for at least 5 min prior to the voltammetric measurement; the lifetime of the charged ion radical conjugate species was found to correspond, in the worst case, to a value that would result in a concentration which was within 4 % of the predicted value of a totally stable species. All experiments were run in a 18 X 18 X 18 in.3 Faraday cage with d / 8 in. thick aluminum walls to eliminate stray spurious capacitative coupling t o the detection circuitry. A Hi Tek Instruments waveform generator was used to apply potential ramps to the cell, the scan rates being in the range 1-100 mV s-l. The scan rate independent current was monitored by a current follower (Keithley 619 Electrometer/Multimeter) and the resulting signal was output to a recorder. For measurements of the diffusion coefficients of the electroactive species, when required, dual compartment cells (working solution cell volume 1mL) of high electrode area to solution volume ratio were used. These had gas purge ports and openings for electrodes. A three-electrode system consisting of a platinum-wire working electrode, a Ag/Ag+ reference electrode, and a frit-isolated secondary platinum electrode was used for preliminary tests of each solution and for partial electrolysis of the solutions in order to electrogenerate the conjugate redox species. For the systems investigated herein, we either used the same values for the diffusion coefficients that previous workers used for determining their values of the heterogeneous rate constant (see Table I) or determined them from chronoamperometric experiments at electrodes of known area on solutions of carefully measured concentrations at 25.0 "C. We did not measure significant differences between the diffusion coefficients for the oxidized and reduced forms in any case. The coefficients for the charged form of the redox couple were determined from solutions where the neutral form had been partially electrolyzed. The concentration was checked by the amount of charge used in the electrolysis. The solutions were kept under an atmosphere of dry helium during measurements to stabilize the ion radicals. For measurements with the microelectrodes only two electrodes were required: the working microelectrode, and the same auxiliary electrode (Ag/Ag+ reference). The reference/auxiliary electrode was mounted in a compartment that terminated with a fine tipped Luggin capillary which was positioned so as to be in close proximity to the microelectrode. These measurements were thus performed in the working compartment of the same cell as quickly as possible after the conjugate redox species was generated. The gold ring microelectrodes were constructed as follows: A 2-mm quartz rod was drawn out over a flame to make a fine fiber attached to the thicker rod. The quartz fibers were then painted with Engelhard liquid bright gold paint and baked in a pyrolysis oven for 15 min at 500 "C to reduce the organometallic gold complex to gold metal. After the fibers had cooled, contact was made by soldering a wirewrap wire to the painted part of the rod. The rods were then sealed in either pipet tubes or capillary tubing with epoxy resin. After the epoxy resin had dried, the electrode was either cut off or sanded with sandpaper to expose the mic-
-
Table I. Values for Heterogeneous Rate Constantsn k . cm
compound
SK'
expt (expt no.)
pyrene
6.6 f 0.1 (4)
ferrocene naphthalene anthracene
0.09 f 0.005 (3)
TCNE
0.15 f 0.01 (6)
0.88 f 0.02 (7) 3.33 f 0.05 (4)
lit. 7.0 (Hg, TEAP, DMF (19) 0.60 (11) (Hg, TEAP, DMF(19)) 0.09 (Pt, TBAP, ACN(2O)) 1.0 (Hg, TBAI, DMF(21)) 3.46 f 0.55 (Au, TEAP, ACN(22)) >4.0 (I) (Hg, NBuJ, DMF(23)) 0.0091 (11) (Hg, NBu,I, DM"3)) 4 (I) (Hg, TBAI, DMF(21)) 9.1 X (11) (Hg, TBAF, DMF(21)) 0.16 f 0.02 (Pt, LiClO,, AC N (24)) 0.39 f 0.05 (Pt, LiC104, ACN(24)) (3.8 f 0.1) X 10-5 (11) (Pt, TBAF, ACN(25)) (1.4 0.1) x 10-3 (c, TBAF, ACN(25)) 1.1 f 0.1) x (C. TEAP, AC N ( 2 5 ) ) (1.8 f 0.1) x 10-3 (Pt, TEAP, ACN(25)) 0.26 f 0.03 (Pt, LiC104, ACN(24)) 0.20 f 0.03 (Au, LiClO,, ACN(24))
*
TCNQ
0.23 f 0.01 (6)
9,lO-diphenyl- 5.7 f 0.1 (3) anthracene "The electrodes used for the various systems were electrode A for pyrene, anthracene, and diphenylanthracene, electrode B fer naphthalene, TCNE, and TCNQ, and electrode C for ferrocene. roring. Electron micrographs were taken of the final electrode assembly to determine the diameter and thickness of the ring. The thicknesses of the several gold electrodes used in this work were 0.09, 0.2, and 0.7 pm for electrodes A-C, and the corresponding ring diameters were 10, 41, and 101 pm, respectively.
RESULTS AND DISCUSSION The behavior of the microring electrode has been discussed in detail elsewhere (17). In the development of the theory, we assume that the flux over the surface (per unit area) of the thin ring electrode is uniform and that the ratio of the thickness of the ring t o the diameter of the ring is less than 0.01. We have shown that, under these conditions, the mass transport coefficient for diffusion to the ring for a species i is given by
k , = 0.2347Di/Ar
(1)
where Ar is the half thickness of the thin ring and Di is the diffusion coefficient of the i species (for further definitions see the Glossary). For a simple single-electron-transfer reaction, to a species i the limiting current at the ring was also shown to be i,im,ring = 4a2nFDir'ci" /Ci,"
(2)
where Ci,= rDiCi/SiAr,a dimensionless concentration parameter in the steady state. ( S is the strength of the continuous surface source (mol cm-2 s-l).) This parameter has a value of 13.37. The ratio of the limiting current of the ring t o that of a disk of the same radius was therefore found to be 73.75%, which explains the increased efficiency to reduction in ohmic loss and increased rates of mass transport: the ring contains only a fraction of the active surface area of a disk of similar dimensions. In addition, diffusion to the ring was shown to rapidly approach a true steady state just as does a spherical geometry; this cannot be claimed with nonspherical
ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986
2963
geometries. As a result, analysis of the data for kinetic experiments is considerably simplified. For a simple redox reaction
(3)
O + e + R
we obtained the following form for the polarization plots:
I:;-[
[
i = ~ T ~ F ~ ' A ~ D exp ~ , C~ ~ C R ~
[
1 - exp( ~ ) l ) / [ ~ ~ C O m C+ R 13.37 m Ark,
X
+ Cornexp Introducing the limiting current and the standard heterogeneous rate constant
k, = ks(com)l-"(cRrn)'
(5)
we obtain a general expression for the kinetically controlled case
[E( A-) F] clim,anodic
i/ilim,cathodic
=
-CY7.lF
ex.[
Ilim,cathodic
Flgure 1. Polarization curve (points) for the anthracenelanthracene anion system and the calculated polarization plot (line) corresponding to a standard heterogeneous rate constant of 3.33 cm s-I. The solution was a partially oxidized solution of anthracene (initially 1.32 mM) prepared in acetonitrile (0.1 M tetra-n-butylammonium tetrafluoroborate or 0.1 M lithium perchlorate).
and a similar equation for the anodic branch. The value of ks is most easily obtained from the slope of the polarization curve a t 11 = 0. Taking the derivative of eq 6 and rearranging, we obtain
(
k s = (slope)/ -(slope)
i( )"( !lim,anodic clim,cathodic
] +- ( F
Llim,anodic
kmRT
1+
ilim,anodic ilim,cathodic
))
the data obtained with the thin ring electrodes will also be less affected by the assumptions of uniform accessibility than is the case, for example, for microdisk electrodes or, indeed, for electrodes of conventional size and design. With suitable techniques, it is perfectly feasible to fabricate very thin rings and to measure the kinetics of fast electrode reactions. Further reductions in the ring thickness should allow the determination of the forms of the complete polarization curves from such processes; such measurements will provide information important to the development of heterogeneous electron-transfer theory as it should prove possible to derive the form of the variation of (Y with the standard overpotential.
(7)
ACKNOWLEDGMENT This paper is dedicated to Cheves Walling on the occasion of his 70th birthday.
where
i
GLOSSARY (8)
We point out here that the effect of increasing k , is analogous to increasing the rotation speed of a rotating disk electrode; both high rotation rates for a rotating disk or decreasing Ar of a stationary ring ultramicroelectrode will have the effect of shifting the polarization curve to irreversible behavior. When the curves are in this irreversible region, the slopes a t the origin are an accurate indicator of the rate constant. Figure 1 represents a typical experimental result, the polarization curve for anthracene in acetonitrile a t a gold ring electrode and the theoretical plot. The results obtained for several commonly studied electrochemical systems are shown in Table I. It can be seen that there is good agreement between the data obtained with ring ultramicroelectrodes and those derived from measurements with relaxation techniques. The rate constants obtained with the ring electrodes however are generally more precise than those derived from transient methods as the low impedance of the double layer does not affect the steady-state measurements. It should be noted that
a Ci
C" Ci,-
D E F
i
im kB 0 r' Ar
R R Si
T
ff
'I
characteristic dimension (cm) concentration of a given species (mol ~ m - ~ ) bulk concentration of a given species (mol ~ m - ~ ) dimensionless concentration parameter in the steady state diffusion coefficient of a given species (cm2 s-l) electrode potential (V) Faraday's constant (C mol-') current (A) mass transfer coefficient (cm s-l) the heterogeneous rate constant (mol cm-2 s-l) the standard heterogeneous rate constant (cm s-l) oxidized species of a redox couple the radius of the ring (cm) the half thickness of the ring (cm) reduced species of a redox couple the gas constant (J mol-') continuous surface source strength (mol cm-2 s-l) temperature ("C) transfer coefficient overpotential (V)
Subscripts
Anal. Chem. 1986, 58, 2964-2968
2964
0 R i
denoting the oxidized form denoting the reduced form pertaining to species 0 and R C cathodic lim pertaining to the diffusion controlled current Registry . No. TCNE, 670-54-2; TCNQ, 1518-16-7;Au, 744057-5; pyrene, 129-00-0;ferrocene, 102-54-55naphthalene, 91-20-3; anthracene, 120-12-7; 9,10-diphenylantracene, 1499-10-1; anthracene radical anion, 34509-92-7.
LITERATURE CITED (1) Wightman, R. M. Anal. Chem. 1981, 5 3 , 1125A. (2) Bond, A. M.; Fleischmann, M.; Robinson, J. Extended Abstract, 165th Meeting of the Electrochemical Society, May, 1984; p 523. (3) Bond, A. M.; Fleischmann. M.; Khoo, S. B.; Pons, S.; Robinson, J. Indian J . Techno/., in press. (4) Blndra, P.; Brown, A. P,; Fleischmann, M.; Pletcher, D. J . flectroanal. Chem . Interfacial Nectrochem . 1975, 58, 3 1. (5) Bindra, P.; Brown, A. P.; Fleischmann, M.; Pletcher, D. J . flectroanal. Chem. Interfacial Nectrochem. 1975, 58, 39. (6) Flelschmann, M.; Lasserre, F.; Robinson, J.; Swan, D. J . flectroanal. Chem. Interfacial Electrochem. 1984, 177,97 (7) Fleischmann, M.; Lasserre, F.; Robinson, J. J . Nectroanai. Chem. interfacial Electrochem. 1985, 177,115. (8) Scharifker, B.; Hills, G. J. J . flectroanal. Chem. Interfacial Electrochem. 1981, 130, 81. (9) Fleischmann, M.; Ghoroghchian, J.; Pons, S. J . Phys. Chem. 1985, 8 9 , 5530. (10) Lui, H. Y. S. M.Ph. Thesis, University of Southampton, 1975.
(11) Rolison, D. R.; Nowak, R. J.; Pons, S.; Fleischmann, M.; Ghoroghchian, J., unpublished work. (12) Fleischmann, M.; Ghoroghchian, J.; Rolison, D.; Pons, S. J . Phys. Chem., in press. (13) Aoki, K.; Osteryoung, J. J . flectroanal. Chem. Interfacial flectrochem. 1981, 122, 19. (14) Heinze, J. J . flectroanal. Chem. InterfacialNectrochem. 1981, 124, 73. (15) Bond, A. M.; Fleischmann, M.; Robinson, J J flectroanal Chem. Interfacial flectrochem. 1984, 180, 257. (16) Bond, A. M.; Fleischmann, M.; Robinson, J. J . flectroanal. Chem Interfacial Electrochem. 1984, 168, 299. (17) Fleischmann, M.; Bandyopadhyay. S.; Pons, S. J . Phys. Chem. 1985, 89, 5537. (18) Ghoroghchian, J.; Sarfarazi, F.; Dibble, T.; Cassidy, J.; Smith, J. J.; Russell, A.; Fleischrnann. M.; Pons, S. Anal. Chem. 1986, 5 8 , 1757-1 761. (19) Koizumi. N.; Aoyagui, S.; J . flectroanal. Chem. Interfacial Nectrochem. 1984, 139, 69. (20) Kadish, K.: Ding, J.; Malinski, T. Anal. Chem. 1984, 5 6 , 1741. (21) Hale, J. M. I n Reactions of Molecules at Electrodes; Hush, N. S., Ed.; Wley-lnterscience: London, 1971. (22) Howell, J.; Wightman, M. Anal. Chem. 1984, 5 6 , 524. (23) Koryta. J. Dvorak, J.; Bohackova, V. Hectrochemistry; Methuen: London, 1970. (24) Sharp, M. J . Electroanai. Chem. Interfacial Electrochem. 1978, 88, 193. (25) Khoo, S. Ph.D. Thesis, University of Alberta, 1983.
RECEIVED for review March 14, 1986. Accepted August 15, 1985. The authors thank the Office of Naval Research for support of this work.
Analysis of Products from Reactions of Chemisorbed Monolayers at Smooth Platinum Electrodes: Electrochemical Hydrodesulfurization of Thiophenol Derivatives Kenneth L. Vieira,' Donald C. Zapien, Manuel P. Soriaga,2 and Arthur T. Hubbard* D e p a r t m e n t of Chemistry, University of California, S a n t a Barbara, California 93106 Karen P. Low and Stanley E. Anderson W e s t m o n t College, S a n t a Barbara, California 93108 The product mixtures from electrochemical hydrodesulfurization of selected thiophenolic compounds chemisorbed through the -SH moiety at smooth Pt electrodes in molar acid have been analyzed quantitatively by using thinlayer electrochemical methods in conjunctlon with caplllary gas chromatography and llquld chromatography. The following compounds were studied pentafluorothiophenol (PFT), mercaptohydroqulnone (MHO), and 2-mercaptobenzoic acid (MBA). A comparathrely hlgh area, large-volume preparative thin-layer electrode (TLE) was constructed to facilitate sample analysls. The results obtalned from TLE, GC, and HPLC analysis were in good agreement. The extent of hydrodesulfurlzatlon (defined here as simple cleavage of the C-S bond without impairment of the aromatic functionality) depended on the nature of the pendant aromatic ring, decreasing in the order PFT (100%) >> MHQ ( 5 0 % ) >> MBA (15 % ). Only one desulfurization product was observed for MHO and MBA; the absence of other products was probably because ring hydrogenation (io form alkyl-type groups) competed wlth simple desulfurization, and detachment of the alkyl moieties from the -SH anchor occurred with greater difficulty than that of the aromatic group.
Present address: Clorox Technical Center, The Clorox Co., Pleasanton, CA 94566. Present address: Department of Chemistry, Texas A&M University, College Station, TX 77843. 0003-2700/86/0358-2964$01.50/0
Extensive studies on the chemisorption and orientation of a variety of aromatic compounds a t smooth polycrystalline Pt electrodes by means of thin-layer electrochemical techniques (1-4) have demonstrated that thiophenol derivatives are bound to the surface through the SH moiety ( 5 ) . This mode of attachment is strong as illustrated by the fact (2)that the adsorbed thiophenols are not displaced by iodide, a well-known potent reagent for Pt surfaces (6). Thin-layer voltammetric and coulometric experiments have also shown that in aqueous electrolyte these chemisorbed thiophenols undergo irreversible electrochemical oxidation and reduction to various degrees depending upon the nature of the group directly linked to the SH moiety (7-11). For example, in 1 M perchloric or sulfuric acid, pentafluorothiophenol was found to be completely resistant toward oxidative desorption even at 1.3 V (Ag/AgCl (1M KC1) reference) but showed a welldefined two-electron cathodic peak a t a potential slightly positive of that for hydrogen evolution (7,8);stoichiometric considerations indicated that this peak was due to quantitative hydrodesulfurization (cleavage of the C-S bond without impairment of the aromatic framework) to yield unadsorbed pentafluorobenzene and chemisorbed hydrogen sulfide. Insertion of a -CH2- group between the aromatic ring and the thiol anchor (as in benzyl mercaptan) altered the electrochemical behavior of the adsorbed mercaptan: the chemisorbed species was no longer resistant to oxidation, but the ease of desulfurization also decreased. In the absence of the 0 1986 American Chemical Society
