Langmuir 1988,4, 599-606
599
Adsorption and Oxidation of Benzoic Acid, Benzoate, and Cyanate at Gold and Platinum Electrodes As Probed by Potential-Difference Infrared Spectroscopy Dennis S. Corrigan and Michael J. Weaver* Department of Chemistry, Purdue University, West Lafayette, Indiana 47907 Received September 28, 1987. In Final Form: November 24, 1987 Potential-difference infrared spectroscopy (PDIRS) utilizing a Fourier transform spectrometer has been employed to examine the modes of electrooxidation as well as adsorption of benzoic acid, benzoate, and cyanate at gold and platinum electrodes. In addition to conventional PDIRS involving the repeated modulation of the potential between the base and sample values, spectra were also obtained during a single potential step or sweep. The latter, “single-potential alteration infrared” (SPAIR), technique enables irreversible potential-dependent processes to be monitored. Reversible potential-dependent adsorption of benzoic acid was observed at gold from the characteristic bipolar bands arising from the potential-induced changes in the surface composition and, correspondingly, in the thin-layer solution. The extent of adsorption, as deduced from the intensities of the latter features, reaches a maximum at the onset of anodic oxide formation. The observation of a single symmetric carboxylate stretching mode at 1380-1390 cm-’ for adsorbed benzoic acid is consistent with adsorption via both oxygens in a CZusymmetry. The corresponding PDIR spectra for benzoate indicate a similar surface coordination, although the data are complicated by interferences from surface and solution-phase vibrations and potential-induced pH changes in the thin layer. Although the irreversible adsorption of those species on platinum thwarts their characterization by conventional PDIRS, corresponding SPAIR spectra indicate that the irreversibly adsorbed, but not the solution, species undergoes irreversible eledrooxidationto COzat potentials where surface oxide formation commences. Examination of C6H613COOHelectrooxidation showed that COz is formed at essentially equal rates from all seven carbon atoms. Potential-sweep SPAIR spectra obtained for cyanate at gold and platinum electrodes indicate that electrooxidation to COP commences on both surfaces at potentials prior to surface oxide formation, although the consumption of solution cyanate to form adsorbed cyanate and isocyanic acid competes with C02 production even at far positive potentials. Although conventional electrochemical methods provide a myriad of kinetic and mechanistic information for heterogeneous redox processes, it is desirable to supplement this with the molecular structural insight that can now be provided by in situ surface spectroscopic techniques. This is especially true when examining multistep reactions involving adsorbed intermediates since the spectral data can yield valuable information on the identity as well as reactivity of the interfacial species. Of the techniques available, infrared reflection-absorption spectroscopy (IRRAS) is well suited for this task.’ We have been employing one variant of IRRAS, potenas configured tial-difference infrared spectroscopy (PDIRS) for an FTIR spectrometer: for the investigation of anionic and molecular adsorption at silver,a’Oagold: and platinum6 electrodes. More recently, we have examined the rates of adsorption and electrooxidation at platinum of adsorbed carbon monoxide formed from small oxygen-containing organic molecules in relation to the overall reaction kinetics in order to ascertain the possible mechanistic role of this adsorbate in these proce~ses.~J (1) For a recent review, see: Foley, J. K.; Koneniewski, C.; Dashbach, J. L.; Pons, S. In Electroanalytical Chemistry-A Series of Aduances; Bard, A. J. Ed.; Marcel Dekker: New York, 1986, Vol. 14, p 309. (2) This approach has also been referred to as “SNIFTIRS” (subtractively normalized Fourier transform infrared spectroscopy).’ (3) (a) Corrigan, D. 5.;Weaver, M. J. J. Phys. Chem. 1986,90,5300. (b) Corrigan, D. S.; Brandt, E. S.; Weaver, M. J. J.Electroanal. Chem. 1987,236, 327. (4) (a) Corrigan, D. S.; Gao, P.; Leung, L.-W. H.; Weaver, M. J. Langgmuir 1986,2,744. (b) Corrigan, D. S.; Foley, J. K.; Gao, P.; Pons, S.; Weaver, M. J. Langmuir 1985, I , 616. (5) Corrigan, D. S.; Krauskopf, E. K.; Rice, E.; Wieckowski, A.; Weaver, M. J. J. Phys. Chem., in press. (6) (a) Corrigan, D. S.; h u n g , L.-W. H.; Weaver, M. J. Anal. Chem. 1987,59,2252. (b) Comgan, D. S.;Weaver, M. J. J.Electroanal. Chem. 1988,241,143. (c) Leung, L.-W. H.; Weaver, M. J. J.Electroanal. Chem. 1988, 240, 341. (7) Weaver, M. J.; Comgan, D. S.;Gao, P.; Gosztola, D.; Leung, L.-W. H., J. Electron. Spectrosc. Relat. Phenom. 1987, 45,291.
The PDIR technique involves the subtraction of spectra obtained at pairs of electrode potentials in order to cancel the bulk solvent interference^.'.^ The versatility of Fourier transform instrumentation allows for several different methods of data acquisition. The simplest way is to acquire two seta of spectral scans sequentially: one at a base (reference) potential followed by another set at a sample potential. We have referred to this approach as singlepotential alteration infrared spectroscopy (SPAIRS)! This method is not always applicable since a large number of interferometer scans often need to be acquired at the two potentials in order to achieve a satisfactory signal-to-noise ratio, so that the effects of instrumental drift can be deleterious over the long time periods required to record such spectra. In order to minimize these effects, it is usual to obtain PDIR spectra by altering the potential periodically between the base and sample values until the desired number of co-added interferometer scans has been accumulated. [A related potential-modulationtechnique is also required when obtaining PDIR spectra by using dispersive infrared spectrometers (so-called electrochemically modulated infrared spectroscopy,l EMIRS) since each modulation needs to occur at an essentially constant wavelength during the spectral scan.] Such potential-modulation approaches are clearly applicable to the examination of potential-dependent systems such as reversible redox reactions or adsorption-desorption processes. However, they are usually inapplicable to the study of irreversible potential-dependent processes such as are often encountered with multistep electrode processes since the reactant cannot be regenerated continually by potential step reversal. On the other hand, SPAIRS is especially applicable to such irreversible systems since the single-potential alteration can be arranged so as to initiate the electrode process in the desired direction. For example, SPAIRS was utilized recently to examine the irreversible electrooxidation of formic acid and methanol at plati-
0743-7463/88/2404-0599$01.50/00 1988 American Chemical Society
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num.6aib A potential sweep variant of this approach was found to be especially useful, whereby seta of interferometer scans are taken during the acquisition of a linear sweep volta"ogram.6byC In the present paper, we illustrate the application of both conventional PDIRS and SPAIRS to the characterization of benzoic acid, benzoate, and cyanate adsorbed a t platinum and gold electrodes. In addition to potential-dependent adsorption, each of these species is seen to undergo irreversible electrooxidation under suitable conditions. The delineation of the extent as well as mechanism of electrooxidation is seen to be aided considerably by the surface infrared measurements for such systems where concomitant metal surface oxidation hinders the interpretation of conventional electrochemical (i.e., current-potential) data. The adsorption and oxidation of cyanate on platinum electrodes have recently been examined with polarization-modulation infrared spectroscopy (PM-IRRAS).8 These latter data together with corresponding SPAIRS results reported here provide a direct comparison of these two techniques as applied to irreversible electrochemical reactions. Experimental Section Most experimentaldetails of the surface infrared measurements are given in ref 3a. The infrared spectrometer, a Brucker-IBM IR 98-4A Fourier transform instrument, was equipped with a helical globar light source and either a liquid-nitmgen-mled MCT or InSb narrow-band detector (Infrared kssociatea). Spectra were acquired with 8-cm-' resolution. The infrared radiation was p-polarized by means of a KRS-5 wire-grid polarizer (Harrick) and was incident on the CaFz windowlair interface at an angle of ca. 65' to the normal. The polycrystalline platinum and gold electrodeswere 9.5m"meter disks mounted on glaas plungers. The electrode surfaces were pretreated immediatelyprior to use by mechanical polishing and dipping in hot chromic acid, followed by rinsing and potential cycling at 50 mV s-' between -0.25 and 1.4 V vs SCE in 0.1 M HC104. The acquisition of the interferometer scans was synchronizedwith the potential alterations by means of trigger pulses from the spectrometerto the potentiostat (PAR 1731179) as outlined in ref 9. A PAR 175 potential programmer was employed when the electrode potential was swept during the spectral data acquisition. Aqueous-phase integrated molar absorptivities for infrared modes of interest here were measured with a transmission IR cell having CaFzwindows and a Teflon spacer. The path length was 0.026 mm as determined from the interference fringes in the mid-infrared region. The solute concentrations were comparable to those employed in the surface infrared measurements. Benzoic acid (Mallinckrodt),sodium benzoate (Eastman), and sodium perchlorate (G. F. Smith) were reagent grade and recrystallized at least once from water. Carboxylate 13C benzoic acid was obtained from Aldrich Co. Perchloricacid (G.F. Smith) was "double distilled" and used as supplied. The sodium cyanate required further purification as described in ref 10. Water was purified by means of a Milli-Q system (Millipore, Inc.). All solutions used for electrochemical measurements were purged previously with nitrogen. Electrode potentials are quoted versus the saturated calomel electrode (SCE),and all measurements were made at room temperature, 23 k 1 OC. Results and Discussion Benzoic Acid and Benzoate at Gold Electrodes. Benzoic acid is an interesting adsorbate to examine with surface vibrational spectroscopies in view of the multiple possible modes of surface coordination. While the fre(8) Kitamwa, F.: Takahashi, M.; Ito, M. Chem. Phys. Lett. 1987, 136,
I
[A
0.2v
B
0.2v
1545
0.4
0.6
I
2500
I
v(cm-')
I
= 6.5 x 10-3
1000
Figure 1. Potential-difference infrared (PDIR) spectra in the
1000-2500-cm-' region at gold for (A) 0.01 M CBH5COOH+ 0.1 M HCIOl and (B) 0.01 M CBH5COONa+ 0.1 M NaC104. The base potential is -0.2 and -0.4 V va SCE for A and B, respectively; sample potentials are as indicated. Spectra are an average of 512 interferometer scans at each potential. quency downshifts in the ring-breathing modes observed in the surface-enhanced Raman (SER) spectra at gold are indicative of direct binding of the benzene ring to the metal surface,ll the importance of the carboxylate moiety in the adsorption process is not yet clear. Figure 1A contains illustrative PDIR spectra for the adsorption of benzoic acid at gold in the 1000-2500-~m-~ region. The aqueous solution contained 0.01 M C6H5COOH + 0.1 M HC104,and the potential steps were from a base value of -0.2 V to the series of sample potentials indicated. As is conventional,' the spectra are reported as relative changes in the reflected infrared intensities, ARIR. It is seen in Figure 1A that for sample potentials between 0 and 1.2 V four positive-going bands at 1700, 1455,1320, and 1280 cm-' and two negative-going bands at ca. 1390 and 1110 cm-' are observed. On the basis of its frequency and bandwidth, the last feature (1110 cm-') is identified with the major infrared-active C1-0 stretch, vc1-0, for perchlorate anions. This band arises from the migration of perchlorate anions into the thin-layer cavity upon stepping the potential from the base to more positive sample values in order to maintain electroneutrality.'* The positive-going bands are identical in form with that observed for solution benzoic acid, with the 1700-, 1455-, 1320-, and 1280-cm-' bands attributed to the C 4 stretch (vc4), two C-C stretches (v&, and the C-0 stretch (vc-o), re~pective1y.l~The increasing intensity of these bands up to a maximum around 1.0 V (Figure 1A) is indicative of the loss of solution benzoic acid resulting from increasing specific adsorption as the potential is altered from the base to the more positive sample value^.^^^ This behavior is similar to that seen for acetic acid at platinum and gold electrode^.^ The sharp and relatively intense
62.
(9) Corrigan, D. S.; Milner, D. F.; Weaver, M. J . Reu. Sci. Instrum. 1985.56.1965. (10) Fernelius, W. C. Inorganic Synthesis; McGraw Hill: New York, 1946; Vol. 11, p 88.
(11) Gao, P.; Weaver, M. J . J.Phys. Chem. 1985,89, 5040. (12) Corrigan, D. S.; Weaver, M. J. J. ElectroanaL Chem. 1988, 239,
55. (13) Gonzalez-Sanchez, F. Spectrochim. Acta 1958, 12, 17.
Langmuir, Vol. 4, No. 3, 1988 601
Adsorption at Gold and Platinum Electrodes r
1' 7
7
0
8
04
08
12
E/ V vs SCE
Figure 3. Anodic-cathodic cyclic voltammograms obtained at 0.1 V s-l at gold in 0.1 M HCIOI (dashed trace) and after addition of 0.01 M C6H5COOH(solid trace). Anodic current is plotted upwards.
Figure 2. Plot of the estimated changes in benzoic acid surface concentration from -0.2 V, Ar (mol cm-2),as a function of potential, E, for a gold electrode in 0.01 M C6H&OOH + 0.1 M HCIOc extracted from the PDIR spectra in Figure 1(see text for details).
negative-going feature at 1390 cm-' can as a consequence be identified with the corresponding adsorbed species. The peak frequency of this negative-going band increases slightly with increasing positive potential (dv/dE = 15 em-' V-1). Since an essentially fixed amount of benzoic acid is trapped in the thin-layer cavity on the time scale of the PDIR m e a s ~ r e m e n t s , the ~ ~ ~intensity ~J~ of the positivegoing bands can be used as a measure of the extent of potential-induced adsorption between the base and sample values. (The use of a large excess of perchlorate supporting electrolyte essentially eliminates any migration of other charge solutes, such as benzoate considered below, to and from the thin-layer cavity when the potential is altered.) As detailed elsewhere,b5this procedure involves plotting the difference spectra in absorbance units and integrating the area under one or more of these bands. From a knowledge of the molar absorptivity, Ai, of the solution infrared bands in question, the integrated intensities can be converted into the corresponding change in surface concentration, AI' (mol cm-2),between the base and sample potentials. The vc4 band, with an Ai value of 1.88 X lo4 M-l cm-2 (base 10 logarithms scale), was employed for this purpose. A complication in this analysis, however, involves the strong spatial and angular variation of the p-polarized light intensity in the thin-layer cavity, so that the effective Ai value can differ somewhat from that evaluated by using the transmittance cell? Although the procedure employed here has yielded r values for azide adsorption on silver within ca. 20-3070 of those obtained from capacitancepotential (Cd-E) measurements,38 more recent results showed infrared estimates of I' for acetic acid on platinum and gold to be as much as 3 times larger than those determined directly via radiotracer methods.6 Nevertheless, the shape of the latter I'-E plots obtained by these two methods are in good agreement, indicating that the PDIR measurements provide a reliable means of evaluating relative potential-dependent surface concentrations, AI',
between the base and sample potentials. The resulting plot of AI' versus electrode potential for benzoic acid on gold extracted from the PDIR spectra in Figure 1A is shown in Figure 2. Note that AI' is plotted here rather than I' because there is considerable adsorption of benzoic acid even at the base potential of -0.2 V as evidenced by both Cd-E14 and SERS" measurements. The surface coverage increases steadily as the potential becomes more positive, peaking at ca. 1.0 V and decreasing sharply at more positive potentials. Interestingly, this decrease in surface concentration coincides with the onset of surface oxide formation as determined from the cyclic voltammogram shown in Figure 3 (dashed trace) and is similar to that observed for the acetic acid adsorption at gold! In view of the systematic discrepancies between the infrared and radiotracer I' values noted above for acetic acid, we anticipate that the y-axis scale in Figure 2 may well also be ca. 3-fold too large. In addition to providing information concerning the extent of surface coverage, analysis of the peak frequencies and relative intensities of the negative- versus the positive-going vibrational bands in the PDIR spectra (Figure 1A) can yield an understanding of the adsorbate structure. The frequency of the single negative-going vibrational band is very similar to that observed for adsorbed acetic acid at gold5 and is consistent with a symmetric carboxylate stretching mode, v,(C02-). As was the case for acetic acid adsorption, the appearance of the v,(C02-) band together with the absence of the v,(CO,) partner expected at higher frequencies is suggestive of a carboxylate binding geometry having a 12% (or C,)symmetry with both oxygens oriented toward the metal surface.15 This is because the latter mode will then be inactive on the basis of the "dipole surface selection rule", whereby only vibrations having a dipolar component normal to the surface can interact with the p-polarized light.16 Although it is possible that the v, band is masked by a positive-going feature, this is unlikely since no PDIR bands are observed in the frequency region 1500-1600 cm-', where the v, mode is anticipated. On the basis of infrared frequencies of bulk-phase carboxylate c~mplexes,~' the similarity of the v,(C02) frequency with that for the free benzoate anion, 1390 cm-l,18 is consistent with a "bridging" surface coordination, whereby the benzoic acid is deprotonated and the two (14) Katoh, K.; Schmid, G. M. Bull. Chem. SOC.Jpn. 1971,44,2007. (15) For a similar case observed at a metal%= interface, gee: Sexton, B. A. Surf. Sci. 1979,88, 319. (16) (a) Pearce, H. A.; Sheppard, N. Surf. Sci. 1976, 59, 205. (b) Moskovits, M.; Huse, J. E. Surf. Sci. 1978, 78, 397. (17) Deason, G. B.; Phillips, R. J. Coord. Chem. Reu. 1980, 33, 227.
(18) Green, J. H. S.; Kynaston, W.; Lindsey, A. S. Spectrochim. Acta
1961, 17,486.
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oxygens are bound to different metal atoms. This is therefore regarded as the most likely adsorbate binding geometry. Although the PDIR spectra for acetic acid adsorption on gold are similar to those for benzoic acid on gold, the adsorbate structure in this latter case was suggested to involve hydrogen bonding between the carbonyl oxygen and inner-layer water molecules, or self-association to form interfacial dimers or higher oligomer^.^ Evidence favoring this conclusion included the lack of detectable adsorption of acetate anions from neutral media under otherwise comparable conditions. Although a similar adsorbate structure may also apply to benzoic acid on gold, the evidence for significant benzoate adsorption (vide infra) suggests that direct surface coordination via oxygen bridging is more likely in this case. The absence of negative-going bands associated with the u c x modes in Figure 1A also deserves comment. It is possible that these features are buried underneath the corresponding poeitive-going (i.e., solution-phase) PDIR bands observed at 1320 and 1455 cm-l. However, the intensities of these bands relative to the vW and uc-o features are essentially identical with those in the corresponding absolute solution (i.e., transmittance) spectrum, so that the u c x band intensities for the adsorbate are presumably markedly smaller than those for solution benzoic acid. Such intensity decreases upon adsorption can result from either physical or chemical factors.% Thus, a diminution of the absorptivities of the ring modes can be expected on the basis of the infrared surface selection rule when the aromatic ring adsorbs in a predominantly flat orientation such that there is little or no dipole component for these modes perpendicular to the surface. This possibility, however, is not entirely consistent with the observation of the intense u,(COz-) for the adsorbate since the dipole selection rule would then also disfavor this mode. Alternatively, adsorption even in a more vertical orientation could yield a smaller absorptivity relative to the solution species because of the alterations in the adsorbate structure that can accompany surface coordination. The presence of a vertical or tilted adsorbate orientation is also consistent with the SERS data Spectral measurements were also made at higher pH values such that benzoate was the predominant solution species (pK, of benzoic acid = 4.2).19 Typical PDIR spectra obtained in 0.01 M C6HSCOONa+ 0.1 M NaC104 at gold with -0.4 V as the base potential are shown in Figure 1B. At potentials more positive than 0.4 V, three positive-going bands at 1595, 1545, and 1390 cm-' are clearly observed and are identified as a C-C stretch and the uaa(CO2-)and the v,(COp) modes of benzoate.ls As for benzoic acid, +his spectral form is consistent with a decrease in the benzoate concentration in the thin-layer solution as the potential is altered from the base to more positive sample values. Besides the 1110-cm-' perchlorate feature, a single weak negative-going band at 1280 cm-l is observed. A t first sight, the PDIR spectral features in Figure 1B appear to arise from increasing benzoate adsorption as the potential is made progressively more positive, similarly to benzoic acid in Figure 1A. However, these spectra undoubtedly arise in part from potential-induced pH changes in the thin-layer cavity. Thus the cyclic voltammograms in the unbuffered benzoate-containing electrolyte (dashed curve, Figure 3) indicate that anodic oxide formation occurs for sample potentials beyond ca. 0.8 V. This process
''
(19) Serjeant, E. P.; Dempsey, B. Ionization Constants of Organic Acids in Aqueous Solution; IUPAC Chemical Data Series No. 23, Pergamon Press: Oxford, 1979.
0-
I, -04
I
0
0.4
0.0
12
E/Vvs.SCE
Figure 4. Anodic-cathodic cyclic voltammograms obtained at 0.1 V s-' at gold in 0.1 M NaCIO, (dashed trace) and after the addition of 0.01 M C6H5COONa(solid trace). Anodic current is
plotted upwards.
releases protons which will reversibly convert an equivalent quantity of benzoate anions into benzoic acid as the potential is altered from the base to sample values. This phenomenon can account almost entirely for the form of the PDIR spectra obtained at sample potentials of 1.0 and 1.2 V. Thus the negative-going 1280-cm-' feature is consistent with the uc-o band for solution benzoic acid; although the V C band ~ at 1700 cm-' is expected on this basis, ita absence is probably due to the presence of a broad interfering water band in this frequency region. While the relative intensities of the positive-going uaa(C02-)and us(COz-) benzoate bands at the most positive sample potentials are the same as found in the solution transmittance spectrum, for potentials below 0.8 V the 1390-cm-I feature becomes significantly less intense than the 1545-cm-' band (Figure 1B). This behavior suggests the presence of a negative-going component a t 1390 cm-l, arising from adsorbed benzoate, which partly cancels the positive-going band at the same frequency. The inferred coincidence in the infrared spectra for benzoic acid and benzoate supporb the structure involving oxygen surface coordination noted above. Such benzoate adsorption from neutral solutions on gold is also indicated from SER spectra." In this respect, the present PDIR spectra for benzoate on gold differ from those for acetate in that the latter can be accounted for entirely in terms of potential-dependent changes in the thin-layer cavity. As noted above, while such PDIR spectra obtained by repeated potential modulation will reveal reversible potential-dependent alterations in the surface and/or thinlayer solution composition, the detection of irreversible changes requires the application of single-potential alteration (SPAIR) techniques. However, SPAIR spectra obtained by employing positive-going potential steps for both benzoic acid and benzoate at gold were found to be essentially identical with those observed with potential modulation. In particular, no significant electrooxidation to Cog occurred over the accessible potential range, up to 1.4 V on gold, as indicated by the absence of the antisymmetric O=C=O stretch at 2343 cm-' (vide infra). Benzoic Acid and Benzoate at Platinum. In contrast to gold, adsorption of aromatic molecules on transition metals is often characterized by irreversible surface binding.20 Some radiotracer and other measurements
Langmuir, Vol. 4, No. 3, 1988 603
Adsorption at Gold and Platinum Electrodes 0.12
-
0
'5 2 0.08
0
0
0
0 0
c 0.04
n
a
i
-
0
1
I
1
0
0.4
E/V
I
0.8 VS.
I
I.2
SCE
Figure 5. Plot of the integrated absorbance of the C 0 2 band for benzoic acid electrooxidation at platinum (circles) as a function
of potential extracted from single-potential alteration infrared (SPAIR)spectra during a positive-goingpotential sweep at 2 mV 8' from a base value of -0.2 V. Electrolyte was 0.01 M C$ISCOOH + 0.1 M HCIOI. Each spectrum was acquired by the coaddition of 5 interferometer scans. The solid trace is the corresponding anodic-cathodic cyclic voltammogram obtained simultaneously, and the dashed trace is for 0.1 M HC104alone. Anodic current is plotted upwards.
indicate that benzoic acid is indeed strongly and irreversibly adsorbed on platinum.21 Conventional potential-modulated PDIR spectra obtained for benzoic acid at platinum under the same conditions as in Figure 1A did not display any spectral features in the 1000-2500-cm-' region. This finding is consistent with irreversible adsorption in that no significant adsorption/desorption presumably occurs on the time scale (ca. 30 s) between each potential alteration. Somewhat different results were obtained, however, under SPAIRS conditions. These spectra were obtained by acquiring a given number, typically 5-30, of interferometer scans (consuming ca. 3-20 s) first at the base potential, -0.2 V, and then after a series of potential steps or during a potential sweep, the spectra at each of the sample potentials being ratioed against the initial spectrum. A single negative-going band at 2343 cm-' was observed for potentials beyond 0.5 V. From its frequency and bandshape, this feature is identified as the antisymmetric O=C=O stretch of C02, indicating that benzoic acid electrooxidation to form COPoccurs at platinum under these conditions. Since the C02 is a bulk-phase species formed in the spectral thin layer, the band absorbance can provide a quantitative measure of the extent of benzoic acid electrooxidation! Plotted in Figure 5 is the integrated absorbance of the C02band (circles) against the electrode potential, extracted from SPAIR spectra obtained for 0.01 M C6H&OOH + 0.1 M HCIOl at platinum during a positive-going potential sweep at 2 mV s-' from -0.2 V. The solid trace is the corresponding anodic-cathodic cyclic voltammogram obtained simultaneously with the SPAIR spectra, and the dashed trace is the voltammogram for 0.1 M HCIOl alone. Given that these results indicate clearly that benzoic acid electrooxidation occurs, one might expect the corresponding appearance of positive-going features associated with the consumption of adsorbed and/or solution benzoic (20) For example: Damaskin, B. B.; Kazarinov, V. E. In Comprehensive Treatise of Electrochemistry; Bockris, J. O'M.,Conway, B. E., Yeager, E., Eds.; Plenum: New York, 1980; Vol. I, Chapter 8. (21) (a) Horanyi, G.; Nagy, F. J. Electroanul. Chem. 1971,32,275. (b) Shchukin, I. V.; Kazarinov, V. E.; Vasil'ev, Yu. B.; Babkin, V. A. Ser. Electrochem. 1985, 21, 674.
acid in the thin-layer cavity. The observed lack of such factors is indeed surprising if the C02 is produced by electrooxidation of solution benzoic acid. The COz integrated absorbance values, Ai plotted in Figure 5 can be converted into a quantity of COz formed by a calibration procedure involving the oxidation of irreversibly adsorbed carbon monoxide;6bby this means we deduce that about 3.5 X mol cm-2C02are produced under the conditions of Figure 5. This quantity of C02 corresponds to the consumption of only 5 X lo-" mol cm-2 on benzoic acid if all seven carbons undergo oxidation (vide infra), which is expected to yield only very weak positive-going bands even if solution benzoic acid were involved. In any case, strong evidence that the electrooxidation involves only irreversibly adsorbed, rather than solution, benzoic acid was obtained from SPAIR spectra for the oxidation of irreversibly adsorbed benzoic acid in the absence of solution species. This involved prior adsorption of benzoic acid at 0.1 V in a separate solution followed by transferral of the electrode to the infrared cell containing 0.1 M HCIOI. The resulting potential-dependent intensity of the C02 band was found to be identical with that obtained in the presence of solution benzoic acid (Figure 2). These SPAIRS experiments were also repeated with carboxylate 13C-labeledbenzoic acid in order to ascertain the ability of the carboxylate versus the aromatic carbons to undergo electrooxidation to C02. At all potentials, the ratio of the quantity of 12C02with respect to I3CO2determined from the relative infrared band absorbances (after correction for the slight isotope effect upon A t 2 )was close to 6:l over the entire potential range, indicating that all seven carbons undergo electrooxidation with approximately the same propensity. Corresponding PDIR and SPAIR spectra obtained at platinum for 0.01 M sodium benzoate in 0.1 M NaC104 yielded results similar to those observed at gold (vide supra), with positive-going bands at 1595, 1545, and 1390 cm-' attributed to loss of solution benzoate by formation of benzoic acid via release of protons associated with anodic oxide formation. One difference with the results for gold, however, is that the relative intensities of these bands are essentially the same as in bulk-phase (transmittance) spectrum, indicating the absence of interference from infrared features due to adsorbed benzoate. Similarly to benzoic acid, SPAIR spectra indicated that benzoate oxidation proceeded at potentials above about 0.4 V, as evidenced by the appearance of the 2343-cm-' C 0 2 band (Figure 6). Cyanate at Gold and Platinum Electrodes. As noted above, the electrooxidation of cyanate has recently been examined at platinum by using PM-IRRAS.8 Since this approach enables absolute spectra to be obtained at a given potential, irreversible electrode processes may be monitored by obtaining a series of spectra for an appropriate sequence of potentials. With this procedure, cyanate was found to electrooxidize to COz,with the onset at potentials in the vicinity of platinum surface oxide formation.8 Parts A and B of Figure 7 show typical SPAIR spectra obtained during positive-going sweeps at 2 mV s-l for 0.01 M NaOCN + 0.1 M NaC104 at gold and platinum electrodes, respectively. The corresponding anodic-cathodic cyclic voltammograms recorded simultaneously with the acquisition of these spectra at gold and platinum are shown (solid curves) in Figures 8 and 9, respectively. Each of the SPAIR spectra was obtained by co-adding five interferometer scans during the potential sweep and ratioing these (22) P i n c h , S.; Laulicht, I. Infrared Spectra of Labelled Compounds; Academic: New York, 1971; Chapter 9.
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604 Langmuir, Vol. 4,No. 3, 1988
0.2
I
1,
, -02
,
, 02
,
,
\! ,
06 E / V vs SCE
,
I O
Figure 8. Cyclic voltammograms at 2 mV s-l obtained at gold in 0.1 M NaCIOl (dashed trace) and after addition of 0.01 M NaOCN (solid trace). The latter trace was obtained simultaneously with the spectral data of Figure 7. The anodic current is plotted upwards.
2500
v(cm-l)
1000
Figure 6. Potential-step SPAIR spectra in the 100&2500-~m-~ region at platinum for 0.01 M C6H5COONa+ 0.1 M NaC104, obtained by acquiring 64 interferometer scans successively at the base potential,-0.4 V, and then at the sample potentials indicated.
0
A 2168
0334v
T
-
l
-0 2
V
06
02
E/
IO
V vs SCE
Figure 9. Conditions the same as for Figure 8, but for platinum electrode.
I 013
0 772
I
1
2343
2500
v/cm-l
1850 2500
v/cm-l
1850
Figure 7. SPAIR spectra obtained simultaneously with slow linear sweep voltammograms at 2 mV s-l for 0.01 M NaOCN + 0.1 M NaC104 at (A) gold and (B) platinum. A sequence of single-beam spectra was acquired, each generated by the coaddition of 5 interferometer scans. Each of these was ratioed to the correspondingspectrum at the initial potential obtained prior to the onset of the potential sweep, -0.4 V. The various potentials indicated are the average values during the acquisition of each spectrum.
to the corresponding spectrum at the initial potential, 4 . 4 V. The potentials given alongside each spectrum denote the average value during each spectral acquisition period (ca. 3 s). In contrast to benzoic acid and benzoate, similar
spectral features are obtained for both these surfaces. Three prominent bands are observed: a positive-going feature at 2168 cm-' and two negative-going bands at 2260 and 2343 cm-'. On the basis of the frequencies and bandshapes, the first and third bands can easily be assigned to the asymmetric stretching modes for solution cyanate% and C02, respectively. The appearance of the former feature in PDIR spectra for cyanate-containing electrolytes on silver and gold has been noted previously and ascribed to the loss of solution cyanate species by specific adsorption as the potential is made more posiIn support of this explanation, a corresponding negative-going band has been noted at 2170-2200 and 21W2220 cm-' at silver*& and gold,&J2respectively, the frequencies increasing with potential. Similar results for cyanate on gold have also been obtained by using an emersion infrared t e c h n i q ~ e . These ~ ~ frequency upshifts relative to the solution band are consistent with "end-onn surface coordination via the cyanate nitrogen.3a On the basis of the SPAIR spectra in Figure 7, the appearance of the positive-going 2168 cm-' band can be ascribed to potential-induced depletion of solution cyanate from irreversible electrooxidation to C02, as well as from increasing adsorption. The latter process is dominant for sample potentials below ca. 0.6 and 0.3 V for gold and platinum, respectively, as gleaned from the absence of the negative-going C02band in the SPAIR spectra under these conditions (Figure 7). (Cyanate electrooxidation was also (23) Hofmann, 0.; Doblhofer, K.; Gerischer, H. J. Electroanal. Chem. 1984, 161, 337.
Adsorption at Gold and Platinum Electrodes
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absent in our earlier s t ~ d i e s hsince ~ ~ Jless ~ positive sample potentials were employed.) The appearance of a weak broad feature around 2220-2230 cn-' in the SPAIR spectra on gold (Figure 7A) is most likely associated with cyanate adsorption, although no corresponding feature can be discerned clearly on platinum (Figure 7B). The relative importance of cyanate adsorption and electrooxidation at more positive potentials may be extracted from the spectra in Figure 7 by comparing the integrated infrared absorbancesof the solution cyanate and C02 bands. These absorbances can be converted into corresponding quantities of cyanate, Q(0CN-), and C02, Q(C02), consumed and produced, respectively, by the potential alteration from a knowledge of the integrated molar absorbances, Ai. The required relative values for cyanate and C02,Ai(OCN-) and Ai(COZ),were determined from the aqueous solution transmittance spectra for OCNand the C-H bending mode S(C-H) for formic acid at 1400 cm-' combined with the relative intensities of the S(C-H) and C 0 2 bands obtained in SPAIR spectra for the electrooxidation of formic acid on platinum. (This latter reaction was chosen since COz is likely to be formed quantitatively from formic acid ~ x i d a t i o n . ~This ~ ) procedure yielded Ai(OCN-)/Ai(CO2) = 1.50. Applying this analysis to the spectra in Figure 7 showed that while Q(C0,) = Q(0CN-) on platinum, i.e., the production of C 0 2 consumes most of the cyanate, other processes contribute significantly on gold in the intermediate potential region of ca. 0.6-0.8 V, where Q(C0,) 0.5Q(OCN-). Two additional modes of solution cyanate consumption can readily account for the latter finding. Firstly, as noted above, cyanate adsorption will undoubtedly occur to an increasing extent at more positive potentials. Although it will presumably be nullified in part by electrooxidation, cyanate adsorption should be particularly prevalent on gold since the onset of surface oxide formation occurs at more positive potentials than on platinum. Secondly, similar to the benzoate case above, the formation of surface oxide will release hydrogen ions, which can consume OCN- by protonation. Since HNCO is a relatively weak acid (pK, = 3.725), this proton release will form essentially an equivalent amount of isocyanic acid. This mechanism is consistent with the appearance of a negative-going 2260cm-' band observed at both gold and platinum (Figure 7); similar frequencies have been reported for the v, mode of liquid-phase HNC0.26 It is interesting to compare these findings with the PMIRRAS data obtained at platinum under similar conditioms At the most negative potentials a single band at 2170 cm-' was observed, arising from incomplete cancellation of the solution cyanate band. This is because the greater effective absorptivity of surface species with prelative to s-polarized light in the IRRAS experiment also applies in part to solution species within the entire thinlayer cavity since the thickness of this region is comparable to the infrared wavelengths employed.n As a consequence, the electrochemical PM-IRRAS technique will provide only a partial removal of solution interferences from the desired surface infrared spectra. Altering the potential to progressively more positive potentials yielded changes in the PM-IRRAS spectra in ref 8 that, broadly speaking, match the present SPAIRS
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(24) Further details of this procedure will be given elsewhere: Leung, L.-W. H.; Weaver, M. J. J. Phys. Chem., submitted. (25) Amell, A. R. J. Am. Chem. SOC.1956, 78,6234. (26) Rasko, J.; Solymosi, F. J. Catal. 1981, 71, 219. (27) For example: Seki, H.; Kunimatsu, K.; Golden, W. G. Appl. Spectrosc. 1985, 39,437.
results. Thus bands a t 2342 and 2260 cm-' appear in the former spectra8 at comparable potentials to those seen in Figure 7B. While these features, as in the present work, were ascribed to the formation of C 0 2 and HNCO, respectively, the formation of the latter was attributed instead in ref 8 to reaction between adsorbed NCO and atomic hydrogen. This proposed mechanism is very unlikely since the coverage of adsorbed hydrogen is vanishingly small in the potential region, >0.6 V, where the HNCO is formed as signaled by the appearance of the 2260-cm-' band. The appearance of the 2260-cm-l feature on gold as well as platinum (Figure 7) is also inconsistent with the explanation given in ref 8. The mechanism for NCO- electrooxidation on platinum suggested in ref 8 involves reaction between adsorbed NCO- and adsorbed oxygen ("oxide") to form C02and N2. While not inconsistent with the present results on gold as well as platinum, another possibility involves NCO- electrooxidation via reaction with adsorbed water. Reaction with adsorbed oxygen (or hydroxyl) would only be expected to occur at potentials beyond the onset of surface "oxide" formation. Especially in the presence of cyanate, the lack of pronounced oxide reduction peaks in the return (cathodic) segment of the cyclic voltammograms on both gold (Figure 8) and platinum (Figure 9) indicates that such oxide formation is not extensive under these conditions. On the other hand, the observed rough correspondence between the onset of NCO- electrooxidation as seen from the appearance of the 2343-cm-l C02band and that of the 2260-cm-l HNCO feature on both gold and platinum (Figure 7) is suggestive of an involvement of surface oxide in the reaction mechanism.
Conclusion In some respects, the comparison of the P M - I R W data with the present SPAIRS results demonstrates the advantages of the latter approach for gaining electrode mechanistic information. In particular, the occurrence of potential-induced changes in the surface and/or solution thin-layer composition can be monitored quantitatively from the corresponding SPAIRS band intensities. A distinction between reversible and irreversible potentialdependent process can also readily be discerned from corresponding potential-modulation PDIR and SPAIR spectra, since irreversible transformations will only be observable with the latter technique. In contrast to these potential-difference techniques, PM-IRRAS will yield bands associated with both surface and solution species even if they are not influenced by the electrode potential. Nevertheless, the occurrence of irreversible as well as reversible potential-dependent phenomena can also be assessed by suitable sequences of PM-IRRAS measurements. In any case, a major virtue of such infrared techniques is the ability to sense specific molecular transformations under conditions where such information cannot be obtained readily by conventional electrochemical means. This point is particularly germane to the electrooxidation processes considered here for which concomitant surface metal oxidation occurs since it is difficult to distinguish between these two contributions to the voltammetric response. Thus inspection of Figures 8 and 9 shows that the current-potential curves obtained in the presence (solid curves) and absence (dashed curves) of cyanate contain no obvious characteristics that enable the rates, or even the occurrence, of cyanate oxidation to be extracted. This is especially true given that the presence of cyanate will inevitably alter the current-potential component associated with surface oxidation, vitiating the use of the blank electrolyte data for this purpose. Similar comments also
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Langmuir 1988,4,606-610
apply to the electrooxidation of benzoic acid at platinum (Figure 5). The combination of such voltammetric data together with SPAIR spectra obtained simultaneously, however, provides a powerful means by which the extent as well as the nature of specific molecular transformations associated with the passage of Faradaic current can be unraveled. Another example of this procedure applied to the electrooxidation of a number of small organic molecules has been described recently!b*c Its application to an increasing
range of multistep electrode reactions is under investigation in our laboratory.
Acknowledgment. Helpful additional experiments were performed by Lam Leung. This work is supported by the National Science Foundation. D.S.C. acknowledges a Purdue Chemistry Department fellowship from the Conoco Corp. Registry No. C6HsCOOH, 65-85-0; CBHsCOO-, 766-76-7; OCN-, 661-20-1; HOCN, 75-13-8; Au, 7440-57-5; Pt, 7440-06-4.
Effect of Surfactant Neutralization on Hexadecane/Water/l -Pentanol/Oleic Acid/Ethanolamine Microemulsions-A SANS Study E. Caponetti,+W. L. Griffith,*z J. S. Johnson, Jr.,i R. Triolo,? and A. L. Compere$ Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831,and Istituto Chimica Fisica, University of Palermo, Palermo, Italy Received July 22, 1987. I n Final Form: December 2, 1987 Microemulsions are an attractive method of modifying diesel fuel and ita emissions through the incorporation of renewable materials and water. Practical fuel systems are often partially neutralized with ashless bases, such as ammonia or ethanolamine. The effect of variations in surfactant concentrations and neutralization was analyzed by small-angle neutron scattering (SANS). The system hexadecanel water/l-pentanol/oleic acidjethanolamine was varied in oleic acid concentration and degree of neutralization. Changes in isotopic labeling of water and hexadecane were used to provide contrast permitting discrimination of scattering by particle core and shell. Least-squares fits were obtained by using models incorporating penetrable shells within oblate ellipsoid particles. To a first approximation, aqueous core size is a function of the concentration of neutralized surfactant, giving similar particle sizes for half the concentration of fully neutralized surfactant and a full concentration of half-neutralized surfactant.
Introduction Incorporation of fuel additives in microemulsion dispersions provides wider options of modifying compositions without degrading engine performance by inhomogeneity than one has when the additives have to be hydrocarbon soluble. For example, water dispersed in diesel fuel by fatty acid surfactants has been shown to affect substantially NO,, SO,, particulates, hydrocarbon and CO emissions. However, reports are conflicting with respect to the extent, and even the direction, of effects with microemulsions comprised of the same components. To improve reproducibility, we have been carrying out studies of the effect of composition on properties and stability. One aspect is investigation of the size and structure of the dispersions by small-angle neutron scattering (SANS). We hope to correlate our results with tests of engine performance, carried out by others. Although a number of components have been tried or suggested, the most commonly investigated microemulsion fuels contain water, a (generally unsaturated) fatty acid, an alcohol, and an ashless base, all dispersed in diesel fuel. The fatty acid, in contrast to conventional practice in research and in other applications, is usually partially, rather than fully, neutralized. Simple phase studies showed that degrees of neutralization of 0.3 and higher *Author to whom correspondence should be sent. Istituto Chimica Fisica, University of Palermo, Palermo, Italy. *Oak Ridge National Laboratory, Oak Ridge, T N 37831. 0743-7463/88/2404-0606$01.50/0
were sufficient for ready formation of stable systems. We report here SANS results on the effects of partial neutralization on the structure and size of the disperse phase. To facilitate data analysis, we used hexadecane as the oil component, rather than the complex mixture comprising diesel fuel. We have found stability ranges of microemulsions prepared with diesel fuel and with hexadecane to be similar. The experimental matrix included a reference composition, full concentration of oleic acid, fully neutralized by ethanolamine (FCFN); a full concentration, half-neutralized (FCHN); a half-concentration, fully neutralized (HCFN); and a half-concentration, half-neutralized (HCHN). A composition with a mole ratio of ethanolamine/oleic acid of about 1.5 is included, to confirm that the base is strong enough for complete neutralization of the acid. For FCFN and FCHN, scattering patterns were measured for three different contrasts by variations in the labeling of components: D20 only, hexadecane-d, only, and both D20 and hexadecane-dw The differences in scattering properties of H and D results in very different patterns for the same molar composition, and the ability of a model to fit all patterns increases confidence in its validity.
Materials and Methods The compositionsof the experimental matrix are summarized in Table I. Previously,scattering patterns from fully neutralized potassium oleate (KOL)systems were measured and analyzed by a group including some of the present authors.' To facilitate comparison, the benchmark composition (B) here approximates 0 1988 American Chemical Society
