Anal. Chem. 1990, 62, 1197-1200
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TECHNICAL NOTES Monltoring of Electrochemical Reactions by Nuclear Magnetic Resonance Spectrometry Daryl
W.Mincey,* M a r c J. Popovich, and P a t r i c k J. Faustino
Department of Chemistry, Youngstown State University, Youngstown, Ohio 44555 Marilyn M. Hurst a n d Joseph A. Caruso
Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221 Previously, Richards and Evans have shown nuclear magnetic resonance spectrometry, NMR, to be a potentially useful aid for monitoring electrochemical reactions ( 1 , 2 ) . The usefulness of NMR as a detection means lies in its ability to distinguish quite similar species. In their studies the electrode generating the species to be monitored was not placed in the actual detection region of the NMR. However, it would have been desirable to observe events as they occur a t or near the electode surface besides observations of the bulk solution. For many years it has been realized that considerably more information could be retrieved by performing simultaneous electrochemical and spectrometric experiments as opposed to considering the experiments on an individual basis. Generation of reactive intermediates could be studied before decomposing or being altered by other constituents of the cell, such as glass surfaces, other solutes, or solvents. In 1964, the application of the first in situ spectrophotometric electrochemical cell was reported (3). This cell incorporated an optically transparent electrode for the generation of the species of interest, whether they were intermediates or products. This electrode was transparent to light because of a thin film of tin oxide n-type semiconductor material on glass (Nesa Glass). Light could be transmitted directly through this generating surface without time delay in observing events occurring at the surface. Since the first radiation transparent electrolysis cell, other spectrometric techniques monitoring in situ events were developed. Several review articles detailing the progress of such techniques are in the literature ( 4 , 5 ) . The cell developed by Richards and Evans was not radiation transparent due to its asymmetric and metallic nature. Any surface, if asymmetric, could produce inhomogeneity. Furthermore, metallic electrodes could possibly cause loss of resolution and sensitivity by altering and diminishing the radio frequency (flfield due to the presence of conduction electrons. NMR studies of metals have shown that the rf field can penetrate a conducting surface without appreciable attenuation or phase change provided at least one dimension of that surface is small compared to a property of the conducting surface termed the skin depth. The distance, s, of a specific conductor through which the amplitude of the wave is reduced by a factor e-' (0.3679) is defined as the skin depth. To permit studies of metals by NMR, the samples consist of fine powders that are suspended in insulating media or thin foils. A full analysis of this effect has been given by Bloembergen (6). The skin depth for any particular conductor can be calculated from s = (2p/wp)'/2
(1)
where p is the material resistivity, w is the angular frequency of the rf field, and p is the permeability, which is 1.26 x lo4 henry/m. The material resistivity is a property of the specific conductor and, likewise, of the skin depth. The angular 0003-2700/90/0362-1197$02.50/0
Table I. Skin Depths metal
p, 0 M
skin depth, A, at 60 MHz
Ag
1.59 X lo-* 1.72 X 10" 2.44 X
8 182 8 520 10 136 21 125 22 941
cu AU
Pt Nb
1.06 x 10-7 1.25 x 10-7
frequency in the equation, for this discussion, will refer to the rf field of the NMR spectrometer. A listing of the skin depth of various metals is given in Table I. These thicknesses, which are considerably larger than typical thin film depositions, suggest a possible procedure for the in situ generation of electroactive species within the detection volume of a NMR spectrometer. EXPERIMENTAL SECTION Electrochemical Cell. The cell for this preliminary study is shown in Figure 1. It consisted of an 18 cm long 5 mm 0.d. NMR tube (507-PP, Wilmed Glass Co., Buena, NJ) opened at both ends. The lower 15 cm of the interior wall was coated with an antimony-doped tin oxide (Sb-Sn02) n-type semiconductor film. This film comprised the working electrode of the electrochemical cell. Its length was chosen to be slightly longer than the 10-cm detection length of the NMR. A liquid seal at the bottom of the NMR tube was fabricated from a graphite rod. This graphite plus was divided into three sections of different diameter. A length of 3 cm was tooled to have a diameter just slightly less than the internal diameter of the NMR tube. The second 5 cm was machined to have the same diameter as the external diameter of the NMR tube. A final section of approximately 20 cm in length possessed a very narrow diameter. The utility of this section will be discussed later. The graphite plug was held in place by an 8-cm band of Teflon heat-shrink tubing. Electrical contact was achieved between the tin oxide conductive surface and the graphite device by cementing the two with silver epoxy (Eccobond Solder 58C, Emerson and Cumming, Inc.). Eccobond Solder 58C is a one-component conductive epoxide-based adhesive. When cured, the adhesive has a conductivity comparable to metallic conductors. It requires curing in an oven at a temperture of 150 "C or greater for 120 min. It is comprised of a fine silver powder that gives the epoxy its low electrical resistance. It is also observed to be chemically resistant to organic solvent such as DMF and DMSO. The NMR electrochemical tube was placed into the spinner assembly of the spectrometer. Visual inspection of the tube while spinning showed no apparent wobble. The weight of the entire plug was such that only slightly more air was required to effect the same spin rate as that necessary for a conventional tube. The NMR spectrum of a sample contained in such a tube revealed no abnormal effects. The reference electrode consisted of a 0.25-mm silver wire (Alfa Chemical) suspended in a 1 M KCl saturated AgCl solution contained in a 1.8-mm capillary tube. The potential of this 0 I990 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 62, NO.
11, JUNE 1, 1990
Figure 1. Electrochemical cell inside Varian A-60 probe assembly: (a) l.8-mtn capiRary tub; (b)Plexiglas cap;(c)copper wke leads; (d)5-mm NMR tube; (e) NMR spinner twblne; (f) NMR spinner assembly; (8)glass NMR probe tube; (h) reference and auxiliary electrode assembly; (i) Sb-Sn02 film; (j)detector coil length; (k) graphite plug; (I) narrow graphite rod; (m) mercury pool; (n) graphite cup.
electrode versus a NHE was +0.23 V. Contact was maintained with the sample solution through a porous porcelain plug (Fisher Scientific). This plug was attached to the bottom of the capillary tube by a 5-mm section of Teflon heat-shrink tubing. The auxiliary electrode was 0.25-mm silver wire spiraled around the capillary tube approximately 15 times and held in place by additional heabshrink tubing at the top and bottom. This exposed portion of wire was as close to the bottom of the tube as possible and was 15 mm in length. The capillary tube was held stationary and concentric so that it did not interfere with the normal spinning of the 5-mm NMR tube. The method of providing electrical contact to a spinning electrode was a mercury pool device. A 1-mL pool of mercury was contained in a graphite cup positioned in the glass tube normally found in the NMR probe assembly. It was around this tube that the receiver coil of the NMR was wrapped. The third section of the graphite plug rotated freely in the mercury while maintaining electrical mntinuity. A shielded copper wire was silver epoxied into the graphite cup to provide external contact to the working electrode connection of the potentiostat. Deposition Technique. Sb-Sn02 films were applied to the modified NMR tube interior by using the apparatus sketched in Figure 2. It was an adaptation of a device employed by Kane, Schweizer, and Kern to apply from 1000 to 3000 A films of SbSNOBon soda lime, borosilicate, and quartz glass plates (7). It consisted of a source of tin and liquid dibutyl tin diacetate heated to approximately 100 "C. At this temperature dibutyl tin diacetate had a sufficient vapor-phase concentration to permit chemical vapor deposition, CVD. It had been observed that an antimony dopant produced an n-type semiconductor pcmessing greater conductivity than pure SnOz (8). In order to introduce antimony into the gas phase, antimony pentachloride was contained in another vessel. SbC15was volatile at room temperature and was not heated. Since Sb was to be a dopant in the semiconductor material, ita vapor-phase concentration was appreciably less than that of tin. Dry nitrogen served as a carrier gas to transport the volatile sources of tin and antimony up a heated (150 "C) glass column where they were mixed with moist oxygen. The nitrogen was dried to prevent reaction of SbCl, and H,O to form a white antimony
Figure 2. Sb-SnO, chemical vapor deposition apparatus: (a) 13mm i.d. furnace; (b) 5-mm NMR tube (opened at both ends); (c) N, Inlets; (d) teflon connection; (e) moist O2 inlet; (1) heated mlxlng column: (9) dibutyltln diacetate; (h) antimony pentachlorlde dopant.
oxide precipitate, which clogged the tubing. The nitrogen was passed through a Drierite gas-drying column for the removal of water. Wet oxygen, on the other hand, provided for better film characteristics; the explanation of which was not fully understood. Varying amounts of water were added by passing part of the oxygen required through a 500-mL fritted disk gas washing bottle (Fisher Scientific) containing distilled water. The mixing of reactants had to be close to the place of desired deposition or a poorly conductive film would result. After mixing in the heated column, the reactant vapors were passed through a normal NMR tube, which was open at both ends. This tube was heated from 450 to 520 "C. Heat was provided by a 100 mm long by 13 mm i.d. micro preheater furnace (Arthur Thomas Co.). The temperature of the NMR tube was monitored by a Fisher indicating pyrometer with a A270 accuracy. A 22-A variable transformer was employed to regulate the temperature. Oxidation of the Sn and Sb sources occurred on the interior glass surface of the NMR tube located inside the furnace. Kane et al. listed 14 variables that could affect the resistance of the resulting Sb-Sn02 film (7).The thickness of the resulting films was not measured. Ten to twenty minutes wm required to produce a sufficiently conductive f i i , which was the important quantity in this study. The resistance was measured with a four-point probe. The greater the incorporation of antimony the bluer was the tint of the f i i . Depositions with a faint blue color exhibited the most conductivity. The graphite plug described earlier was then inserted. Current Source. Potentials were applied with a Princeton Applied Research Model 173 potentiostat with a 179 digital coulometer. Cyclic voltammetry was performed with a PAR 175 universal programmer to produce sweep potentials. Potentials were monitored with a Fluke Model 8000A digital multimeter. Cyclic voltammograms were plotted on a Houston Instruments Omnigraphic 2000 X-Y recorder. Reagents. Certified ACS reagent grade solvents and electrolytes were used except as otherwise specified. RESULTS AND DISCUSSION Characterization of N M R and Electrochemical Prop erties of the Cell. A 5-mm uncoated NMR tube containing a 10% CHC13 sample in CCll was inserted into the spectrometer and the resulting spectrum was recorded as set forth
ANALYTICAL CHEMISTRY, VOL. 62, NO. 11, JUNE 1, 1990
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Flgwe 5. Cyclic voltammogram of 1 mM potassium ferricyanide in 0.5
M KCI on a Sb-SnO, film: scan rate, 10 mV s-'; initial scan cathodic.
C
B
A
Figure 3. Demonstration of radio frequency filed transparency:
(a)
uncoated 5 mm 0.d. NMR tube containing 10% CHCI, in CCI,; (b) Sb-SnO, coated (interior wall) 5 mm 0.d. NMR tube; (c) B after remaximization of shim controls.
I
Y
Figure 4. Demonstration of resolution: Sb-SnO,
coated tube, 5 %
CH,CH*O in CCI,
in Figure 3. Another portion of this solution was placed in a 5-mm Sb-Sn02 coated tube and a second spectrum recorded. The settings that could alter sensitivity and resolution were not changed. A final spectrum was taken of the chloroform resonance after remaxization of the rf field strength and shim controls. The difference in sensitivity between A and C was 10%. To deted any possible loss in resolution, a 5% acetaldehyde sample was monitored in a coated tube. The spectrum recorded appears in Figure 4. The resulting width at half peak height was measured to be 0.4 Hz. With this particular instrument, the resolution of the same resonance in an uncoated tube also was 0.4 Hz. The ability of the Sb-Sn02 film to generate electoactive species was investigated. A cyclic voltammogram (CV) of a 0.5 M KCl background was performed. The effective cathodic limit was about -1.0 V and an anodic limit was approximately 1.5 V.
The CF of a 5 mM potassium ferricyanide solution on the Sb-Sn02 is presented in Figure 5. Ferricyanide ion was reduced with a peak potential of 35 mV to ferrocyanide on the tin oxide film (upper curve). It could subsequently be oxidized back to ferricyanide with a peak potential of 180 mV (lower curve). It was assumed that stationary reference and auxiliary electrodes placed in a cylindrical spinning working electrode would cause convectional mixing of the solution. Such mixing should provide shorter electrolysis times. Detection of Electrolysis Products. NMR was utilized to monitor the reduction of p-quinone (I) to hydroquinone (11). This was a two-electron process occurring at 450 mV versus Ag/AgCl (9). 0
OH
0 I
I1
OH
This reaction required the presence of a proton donor such as HC1 or phenol. It was reported that the radical and dianion intermediates would be present. It was not expected that the NMR spectra of compounds I and I1 would be very different. Both spectra should be singlets for the four phenyl protons with only a slight chemical shift difference. The resulting, in situ, reduction as monitored by NMR spectrometry is displayed in Figure 6. Spectrum A was the resonance attributed with I before the onset of electrolysis. Spectrum B revealed the upfield appearance of the resonance accounted to 11. A resonance with the same chemical shift was present when a prepared sample of I1 was recorded. This resonance was also observable when the product of the partial reduction of I is placed in an NMR tube. Spectra C through K depict the increasing strength of the resonance due to I1 and the decreasing peak produced by the presence of I. The resonances of I and I1 are about equal by the time of spectrum F. This required approximately 4 min. By spectrum L, only the resonance due to I1 could be identified. This cell was able to generate electrochemical products directly in the detection volume of the NMR tube while simultaneously monitored by NMR spectrometry, with no observable interference with the NMR spectrometer. This design would be adequate for the maintenance of a specific potential and the identification of products. This cell also could be useful for the generation of an electroactive reagent for further
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 11, JUNE 1, 1990
I 3
J
K
L
Figwe 6, Monitoring of an electrochemical reaction by NMR spectrometry: (A) 0.1 M p-quinone in D,O with 0.1 M KCI as suppcdng ektrolyte and 0.3 M DCI 8s proton donor, before Initlation of ektrolysk; (B) decrease in resonance assodated with pquinone and increase in that associated wkh hydroquinone, control potential -500 mV, after approxlmately 30 s; (C) after 80 s; (D) after 140 s. Monitoring of an electrochemical reactlon by NMR spectrometry: (E) after 200 s; (F) after 250 s; (G) after 308 s; (H) after 360 s. Monitoring of an electrochemical reaction by NMR spectrometry: (I) after 410 s; (J) after 480 s; (K) after 550 s; (L) resonance associated with 0.1 M hydroquinone only, after 680 s.
reactions. The advantages of such a cell over a flow type electrode would include total degassing capabilities, small sample sizes, cycling abilities, and signal averaging of a constant solution in equilibrium. If methods could be developed to increase the sensitivity of the NMR technique or increase of rate of electrolysis and isolate products generated from the anode, the NMR also may be able to provide mechanistic information concerning intermediates and reaction pathways. The resistive nature of the Sb-Sn02 film was believed to slow the rate of product generation. Metallic or carbon-based films, if applied in the same manner as Sb-Sn02, may provide faster electrolysis. Any method that would produce a greater surface area to volume ratio also would decrease generation time. It has been observed that when an rf field penetrates a multiple number of conducting surfaces, the loss in amplitude was almost the same as if it had penetrated only one surface. This would be true provided a distance of at least one skin depth separates each conducting surface. In this way, the surface area to volume would greatly be increased, if a symmetrical arrangement of films could be produced. Larger NMR tube cells of different design are presently under study and possess significantly faster conversion abilities. Fourier transform NMR will be employed in future studies. The utilization of electron-nuclear double resonance (ENDOR) techniques may be useful in monitoring paramagnetic species (IO). In conventional spectrometry, only diamagnetic species would be detectable. Compounds possessing unpaired electrons produce proton resonances too broad for detection. Simultaneous irradiation of the electron resonance and proton resonance would remove such broadening with as much as a
600-fold increase in sensitivity. This is a result of the electron Overhauser effect associated with the cross-relaxation between electron and nuclei of interest. Such a double resonance technique could expand the utility of the cell described above. It is hoped that having demonstrated the penetration ability of the rf field through thin metallic film, further research on techniques producing sufficient concentration of electroactive species required for mechanism will occur.
ACKNOWLEDGMENT The authors thank Marion Harding, Edgar Younginger, Erv Kuhlmann, and William R. Heineman for their aid and advice during this endeavor. Registry No. I, 106-51-4; Sb, 7440-36-0; Sn02, 18282-10-5; dibutyltin &acetate, 1067-33-0;antimony pentachloride, 7647-18-9. LITERATURE CITED (1) Richards, J. A.; Evans, D. H. Anal. Chem. 1975, 47, 964-966. (2) Richards, J. A. Ph.D. Thesis, Unlversity of Wisconsin, 1975. (3) Kuwana, T.; Darlington, R. K.; Leedy, D. W. Anal. Chem. 1964, 36, 2023-2025. (4) Kuwana, T.; Heineman, W. R. Acc. Chem. Res. 1976, 9 , 241-248. (5) Heineman, W. R. Anal. Chem. 1978, 50, 390A-402A. (6) Bloembergen,N. J . Appl. Phys. 1952, 23, 1383-1389. (7) Kane, J.; Schweizer, H. P.; Kern. W. J . Electrochem. SOC. 1976, 123, 270-277. (8) Kane, J.; Schweizer, H. P.; Kern, W. J . Electrochem. SOC. 1975, 722. 1144-1 149. (9) Tomilou, A. P.; Mairanovskii, S. G.; Fioshin, M. Ya.; Smirnov, V. A. The Electfmhemlstry of Ofganic Compounds, 1st ed.: John Wiley and Sons: New York, 1972; Chapter 4. (10) Carrington, A.; Mdachlan, A. D. Introduction to Magnetic Resonance, 1st ed.;Harper and Row: New York, 1967.
RECEIVED for review December 18,1989. Accepted February 23, 1990.
