Anal. Chem. 1991, 63, 1603-1606
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Potential-Difference Surface Infrared Spectroscopy under Forced Hydrodynamic Flow Conditions: Control and Elimination of Adsorbate Solution-Phase Interferences Joseph D. Roth and Michael J. Weaver*
Department of Chemistry, Purdue University, West Lafayette, Indiana 47907
The utffltatlon of a thln-layer 8pectrod.ctrachomkaI cell feauhg f0fc.d h y d ” k rduHon flow for the separatbn of potentlaklltference Infrared (PDIR) spectral components ~f”lcompodtknrrlchangesatthedectrode Interface and hthr bulc 8oMb11b W a t d f o r t h e ol culck and cyanate Ion at polycrystalline rllver. I n a conventlonal (stagnant) tMn4ayer conliguratlon, Mpdar PDIR band8 are obtdnrd,twacHngthe 0oupkdChrmg.rlnth. Interfacld and rdutlon-phaseadsorbate compodtbn that adso from potentlakkp”i a w b n equlllbda. I n the presence of rulllclent hydrodynamlc flow, however, only the unlpolar band component for the adsorbed specks remalns dnce the solutlon compodtlon rmnaln, Invarlant. Separate extractlon of the solutlon-phare lnfrarad component of the Mpolar bands can be obtalned by appropriate cwbtractlon of c o r r ~ r p . c t r a o b t a i m dIn the p”nceandab8ence of dutlon flow. Such tacUc8 enable a quantltatlve assessment of the coveragdopendent Infrared band characterktks, shethe -band absorbance, AXlol), In the PDIR spectra provides a rellable monltor of the potentlal-in&sad change In adsorbate surface concentratbn. The rdatlonshlps between the coveragedepandmt Integrated absorbances In the adsorbed and solutlon state are dhcumed forthe asy”bk N-N-NandC-Ndrotchingbandsdcukk and cyanate, mpectlvdy. The g m ~ aut#y l of such f l o w 4 tactks for dkslqukhhg between surface and solutlon-phase component8 of PDIR spectra lo polnted out.
INTRODUCTION Applications of reflection-absorption infrared spectroscopy, especially utilizing FTIR spectrometers, to the examination of adsorptive and reactive electrochemical systems are becoming increasingly diverse as well as widespread (see ref l for a recent broad-based review). The use of various potential-difference infrared (PDIR) techniques along with thinlayer spectmelectrochemical arrangements has proved a potent means of eliminating (or at least minimizing) the influence of bulk-phase solvent and other spectral interferences, enabling interfacial species to be detected. In addition to potential modulation, PDIR techniques involving only a single potential alteration (SPAIRS)during the spectral data acquisition have been utilized to advantage, the latter being appropriate for the examination of irreversible electrochemicalsystems (2). These methods rely for their surface sensitivity either on the adsorbate vibrational frequency shifts brought about by changes in the electrode charge and attendant inner-layer fields or on the potential-induced alterations in the adsorbate surface concentration. There are several circumstances, however, where the removal of vibrational bands associated with solution-phase, rather than adsorbed, speciesby using these PDIR tactics will be incomplete as a result of potential-induced changes in the thin-layer solution composition. Firstly, bands associated with
supporting-electrolyte ions will usually appear, arising from migration of these charged species between the thin-layer cavity and the solution reservoir so as to balance the alterations in the electronic double-layer charge (3,4). Secondly, substantial potential-induced changes in the overall thin-layer composition can also be brought about by the occurrence of redox reactions. These may involve adsorbed reactants, such as the electrooxidation of adsorbed CO on transition metals to form solution C02 (2) or solution-phase reactants such as organic molecules (5). Thirdly, potential-induced shifts in adsorption-desorption equilibria can yield substantial changes in the solution composition, even though the overall quantity of the adsorbing species in the thin-layer cavity remains constant (6). At least the last two situations can be, and have been, utilized to gain quantitative information on adsorbate coverage (2,5,6)or electrochemical reaction pathways (5). The former application, appropriate in general for interconversions between adsorbed and solution species, exploits the quantitative adherance to Beer’s law exhibited by the latter species to provide adsorbate coverage information (vide infra) (6). Nevertheless, there are numerous circumstances where such solution-phase spectral contributions constitute serious unwanted interferences, hindering or even preventing the acquisition of surface infrared spectra. In particular, the differences in vibrational frequencies for a given species between the solution and adsorbed state are often small. Potentialdifference epectra acquired by utilizing the corresponding potential-induced shifts in adsorption-desorption equilibria can therefore yield bipolar bands, having components arising from corresponding adsorbed and solution species that are overlapping and hence exhibit mutual interference. Described herein is a straighffonvard means by which such solution-phase interferences can be identified and removed. The method utilizes a flow cell, with continuous replenishment of the thin-layer electrolyte so that the components of the infrared absorbance arising from solution-phase species remain independent of potential and therefore do not contribute to the PDIR spedrum. The coupled use of PDIR measurements with and without solution flow as a means of separating infrared components arising from interfacial and bulk solution species, and assaying the former, is also outlined. While flow cells for infrared spectrochemical purposes have been described recently (7-1 O), such applications have, surprisingly, not been discussed previously. Specific illustrative applications of these procedures are made to the adsorption of azide (Nf) and cyanate (OCN-) at silver electrodes. Both adsorbate systems have been the subject of earlier detailed PDlR studies utilizing a stagnant thin-layer configuration (3, 6, 11, 12). EXPERIMENTAL SECTION Most aspects of the experimental arrangement, including a detailed description of the infrared spectroelectrochemicalflow cell, are given in ref 10. Briefly, the flow cell was constructed chiefly from Kel-F and featured a micrometer adjustment of the gap between the metal electrode disk (ca. 0.9-cm diameter) and the CaF2optical window. The latter featured a central small hole
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into which was epoxyed a 0.7-mm-0.d. Pt tube. Solution could be flowed through the thin layer in concentric fashion via this tube, under pressure from a syringe pump (Harvard Model 975 compact infusion pump). The outlet was provided in the cell wall. The counter electrode consisted of Pt wire encircling the working electrode; the Ag/AgC1(3 M KCl) reference electrode (Bioaualytical Systems) was located in a separate compartment adjacent to the flow-cell inlet. The solution flow rate through the spectral thin layer was adjusted directly from the syringe pump; it was typically about 1mL min-'. The thin-layer thickness was about 5-15 pm (IO);such parameters correspond to average thin-layer residence times t, d 0.1 s (IO). The spectrometer was an IBM (Bruker) IR 98-4A Fourier transform instrument with a helical Globar light source and a liquid nitrogen cooled InSb detector (Infrared Associates). The spectral resolution utilized here was f 4 cm-'. The incident light was ppolarized by means of a KRS-5 wiregrid polarizer (Harrick) and was incident on the CaFzwindow at about 60° to the surface normal. The polycrystalline silver electrode was polished with successively fmer grades of alumina, with the finest grade (0.05 pm) immediately prior to insertion into the flow cell. The most reproducible infrared spectra were obtained after a few surface "oxidation-reduction" cyclea, involving stepping the potential from 4.50and 0.45 V (for ca.5 s) and return (6. ref 11). Sodium azide and sodium perchlorate (reagent-gradesalta) were recrystallized twice from water, sodium c y w t e was purified as in ref 13. Water was purified by means of a Milli-Q system (Millipore Inc.). All potentials are quoted versus the Ag/AgCl (3 M KCl) reference electrode, and all measurements were made at room temperature, 23 f 1 "C.
RESULTS AND DISCUSSION The adsorption of azide on silver provides an interesting example of reversible potential-dependent adsorption. Analysis of differential capacitance electrode potential data in mixed NaC104/NaN3 electrolytes at fixed ionic strength containing, say, 10 mM azide shows that the azide surface concentration, rNS, increases reversibly from very low values, 51 X 10-lomol cm-2, at large negative potentials (beyond ca. -1.0 V) to near-saturated values, 21.5 X lo4 mol cm-2, at potentials, ca. 0.25 V, close to the onset of silver oxidation in these electrolytes (6, 14). Figure 1A shows part of a sequence of absorbance PDIR spectra in the N-N-N asymmetric stretch region from 1950 to 2200 cm-' obtained in the absence of solution flow (i.e., for a stagnant thin layer) for silver in 0.1 M NaC104 + 0.01 M NaN3 at the various sample potentials as indicated. The reference potential was selected to be sufficiently negative, -1.0 V, so as to minimize the extent of N3- adsorption. The data in Figure 1A were obtained by acquiring sets of interferometer scans (100 each set) while sweeping the potential once in the positive direction from -1.0 V at 1mV s-'. While the reversible nature of the potential-dependent azide adsorption also enables potential-modulation tactics to be obtained (6), this SPAIRS procedure was employed here for simplicity. (Sufficient signal-to-noise ratio was achieved in both cases; the extent of azide mass transfer between the thin-layer cavity was small or negligible on the measurable time scale.) As expected, the data in Figure 1A show a close resemblance to that in Figure 1 of ref 6. (Note that the latter, being displayed in "relative transmittance" units, are inverted with respect to the present absorbance spectra.) The prominent negative-going feature in Figure 1A at 2048 cm-', increasing in intensity toward more positive potentials, is due to the loss of azide in the thin-layer solution arising from azide adsorption (6). This deduction is confirmed from the potential independence of the N-N-N asymmetric stretch frequency, u, = 2048 cm-', and its identity with solution-phase azide spectra. The weaker positive-going band at higher frequencies, ca. 2075-2085 cm-l, observed at the most positive potentials in
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Figure 1. Potentialdifference infrared absorbance spectra obtained for the adsorption of azide on polycrystalline silver in aqueous 0.1 M NacrO, + 0.01 M Nab. Referpot&hl is -1.0 V WKSUS Ag/AgCi (3M KCI); sample potentials are as indicated. Spectra extracted from sets of 100 interferometer scans acquired during 1 mV s-' positlvegoing potential excursion from -1.0 V. (A) Spectra obtained in the absence of solution flow (Le., in stagnant thin layer). (B) Spectra obtained with 1 mL min-I solution flow. (C) "Doubiedtfference" spectra, obtained by subtracting spectra at each sample potential in B from those in A.
Figure 1A arises from the adsorbed azide itself. This 2535-cm-' frequency upshift compared with solution azide is consistent with end-on N coordination to the silver surface (6). The apparent lack of the adsorbed azide infrared feature except at the most positive sample potentials is indicative of NS- binding chiefly in a "flat" orientation, whereupon the uy feature will be absent on the basis of the usual surface infrared selection rule (15). The appearance of the surface u, band, associated on this basis with "vertical" (or tilted) azide, only at potentials where the azide coverages are high suggesta that this configuration forms when the surface is near-saturated with the flat azide form (6, 12). It is possible, however, that this adsorbed azide feature is obscured at more negative potentials by the dominant presence of the corresponding solution-phase infrared band. To investigate this possibility, spectral sequences were obtained in the presence of sufficient solution flow through the spectral cavity so as to maintain constancy of the thin-layer solution composition. Figure 1B displays such a typical sequence of PDIR spectra obtained under identical conditions as in Figure 1A but in the presence of 1 mL min-' solution flow. The negative-going v, feature in Figure 1A is entirely absent in Figure lB, as expected since the azide concentration in the thin-layer solution will remain constant throughout the potential excursion and therefore will cancel entirely when the band absorbances between the reference and sample potentials are subtracted in the usual PDIR procedure. Significantly, removal of this bulk-solution feature allows unobstructed observation of the positive-going u, band. As seen in Figure lB, the latter feature is substantially more pronounced, and extends to more negative potentials, than
ANALYTICAL CHEMISTRY, VOL. 63, NO. 15, AUGUST 1, 1991
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Flguro 2. Plot of integrated absorbance of asymmetric N-N-N ~ ~ b a n d f a s o l u t k n g h a s e a n d ~ e z l d e ( s q u a r drcles, es, respecthrdy), obtained from PDIR spectra such 88 In Figwe 1, against electrode patentlal of "sample spectrum".
might be anticipated from Figure 14that is, the solution azide band providea a marked interference in PDIR spectra obtained by using the usual stagnant thin-layer arrangement. While the qualitative conclusions drawn in ref 6 from spectra obtained in the latter c o n f i a t i o n are unaffected, the present observations show that the presence of end-on coordinated azide is somewhat more prevalent on silver than previously apparent. An additional virtue of the flow-cell tactics described here is that subtraction of appropriate pairs of PDIR spectra (referring to a given sample and reference potential) obtained in the absence and presence of solution flow (as in Figure 1A and B, respectively) enable spectra consisting of only the solution-phase Y, band component to be extracted. Such a sequence of such "double-difference" infrared spectra is displayedtin Figure 1C. When unfettered from the surface u, component in this manner, the solution band intensities provide a reliable quantitative monitor of the adsorbate coverage. Admittedly, the alteration in the solution-phase adsorbate concentration resulting from the potential-induced changes in surface coverage may affect slightly the extent of azide adsorption, so that the surface u- component might not cancel out entirely in Figure 1C. This complication, however, is negligible in the present case, since the alterations in the solution azide concentration are only 10% or less, which yield even smaller (Sfew %) changes in the adsorbate concentration. Consequently, then, spectra such as those in Figure 1B and C provide an essentially complete deconvolution of the infrared bands depicting the surface and solution-phase partners involved in the potential-dependent adsorption-desorption equilibria. These components appear alongside, and partly obscure each other, in the conventional PDIR spectra obtained in a conventional stagnant thin-layer cell, as in Figure 1A. A plot of the integrated absorbance, Ai, of the solution-phase and adsorbed u, azide bands (square and circles, respectively), obtained from data such as in Figure 1, versus the electrode potential is shown in Figure 2. As originally enunciated in ref 6, the surface band component is virtually absent at lower azide coverages, corresponding to the lower half of the solution Ai values, Ai(sol),in Figure 2. As noted above, Ai(sol)provides a reliable monitor of the adsorbate surface concentration, r (or coverage), since Ai(sol) r as a consequence of the adherence of the thin-layer solution species to Beer's law (6). (Absolute I? values can also be extracted on this basis but Q:
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Figm 3. Asin Figme 1 butforadsaptkn of cyanateon pdyaystalne 0.01 M NaOCN.
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require accurate knowledge of the band absorptivity within the thin-layer optical geometry ( 5 , s ) ) . At higher coverages, the increases in the surface and solution Ai values roughly mirror each other, indicating that vertical (or tilted) azide coordination provides an important, and possibly predominant, component of this additional adsorbed azide. Also worthy of note here is the substantial (up to 35-cm-9 upshifts in the u, frequency of the adsorbed azide observed as the azide coverage increases (Figure 1B). A portion of these frequency changes can be ascribed to the concomitant changes in electrode potential that are involved, although the major component of this effect probably arises from coverage-induced increases in adsorbate dipole-dipole coupling (cf. ref 16). The presence of these coverage-induced frequency shifts was apparently obscured entirely in the earlier stagnant thin-layer data (6), in that the surface u, bands at lower coverages overlap extensively with the more intense solution-phase u, feature. Cyanate Adsorption. Similarly to azide, the adsorption of cyanate (OCN-) on silver is markedly potential dependent in mixed aqueous perchlorate electrolytes, increasing by altering the potential from large negative values (beyond ca. -1.0 V) to values close to silver oxidation, ca. 0.2 V (6). The C-N stretching mode, U C N , located at about 2150-2200 cm-', provides a convenient infrared spectral monitor of cyanate adsorption (6). To a greater extent than u, for azide, however, uCN for adsorbed cyanate on silver overlaps with the corresponding band for the solution-phase species. A typical sequence of absorbance PDIR spectra obtained on silver in 0.1 M NaC104 + 0.01 M NaOCN in the absence and presence of solution flow (1 mL min-l) is depicted in Figure 3A and B, respectively. As in the azide case (Figure 1A and B), these data were obtained during a 1mV s-l positivegoing potential sweep, the prior reference spectrum b e i taken at -1.0 V. The positive- and negative-going features in Figure 3A can be attributed to the appearance of adsorbed cyanate (N-coordinated) and the loss of solution-phase cyanate,respectively, arising from alterations in the potential f" -1.0 V to the more positive "sample" values indicated. Es-
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Flgwo 4. Plot of integrated absorbance of C-N stretching band for solutionphase and adsorbed cyanate (squares and circles, respectively), obtained from PDIR spectra such as in Figure 3, against electrode potential of "sample spectrum".
sentially similar spectra in a stagnant thin-layer confiiation are reported in Figure 4 of ref 6. In this earlier study, a rough deconvolution of the overlapping surface and solution-phase UCN components was undertaken by using an approximate procedure that utilizes cyanate surface concentrations extracted from capacitance-potential data (6). This deconvolution is made more reliably here by utilizing the flow cell. In addition to the potential-dependent um bands for adsorbed cyanate seen in Figure 3B,the corresponding solution-phase uCN bands are shown in Figure 3C. The latter were obtained, as before, from the difference between correspondingPDIR spectra in Figure 3A and B. A resulting plot of the integrated absorbance of the solution-phase and absorbed UCN bands (squares and circles, respectively) vemus electrode potential, obtained from such data (cf. Figure 2), is given in Figure 4. This plot for cyanate adsorption exhibits a rough similarity with the corresponding azide data (Figure 2) in that the surface u a band only appears at larger Ai values for solution cyanate, Corresponding to large (25 X 10-lomol cm-2 (6))cyanate surface concentrations. These results differ noticeably from the earlier estimated absorbance data in ref 6 for which the coverage-dependent Ai valuea for the surface and solution-phaseu a bands, Ai(surf) and Ai(ml),were deduced to be closely similar,so that Ai(surf) a r. The present, more reliable, analysis shows instead that the dependence of Ai(surf) upon I' is clearly nonlinear for cyanate adsorption (Figure 4). This nonproportionalitymay arise at least in part from a coverage-induced alteration in the adsorbate orientation, as inferred above for azide.
CONCLUDING REMARKS The present results demonstrate the effectiveness of thinlayer spectroelectrochemicalcells utilizing forced hydrodynamic flow for achieving a separation between components of potential-difference infrared spectra arising from compositional changes at the electrode-solution interface and in the bulk electrolyte. As such, these tactics provide a useful means of identifying reliably the often sought-after interfacial components of the infrared spectra in the face of closely coupled solution-phase interferences. This issue deserves more attention than it has received typically in the literature since it is not uncommon to misidentify the latter contribution as the former. For systems featuring potential-dependent adsorption4wrption equilibria, such as the examples presented here, the separate extraction of the coupled infrared band components arising from the reequilibrating species in the solution as well as interfacial environments also provides a means of assaying the adsorbate coverage. The use of the present flow-cell tactics can also aid the interpretation of PDIR spectral components originating from migration of supporting electrolyte ions to and from the thin-layer cavity in response to alterations in the double-layer charge (3,4). In a stagnant thin layer, the appearance of such PDIR bands signals only the net gain (or loss) of a given ionic species from the spectral cavity and provides no information as to the disposition of the ion between the double-layer and solution-phase regions. In the presence of appropriate solution flow, however, such spectral features will arise only from compositional changes within the double layer since the ionic concentrations in thin-layer solution will necessarily be held constant. LITERATURE CITED (1) Christensen, P. A.; Hamnett, A. I n f2"s/ve Chmh/KhstlCs; Compton, R. G., Hamnett, A., Eds.; Elsevler: ,"A 1989 Vd. 29,chapter 1. (2) Corrlgan, D. S.; Leung, L.-W. H.; Weaver, M. J. Am/. chem.1987, 59, 2252-2256. (3) Conigen, D. S.; Weaver, M. J. J . Ebcbuanal. Chem. 1988, 239, 55-86. (4) Iwaslta, T.; Nart, F. C. J . €lecime&. Chem. 1990, 295, 215-224. (5) Leung, L.-W. H.; Weaver, M. J. J . phys. Chem. 1988, 92, 4019-4022. (6) Corrigan, D. S.; Weaver, M. J. J. Fhys. chsm.1986, 00, 5300-5306. (7) Nlchols, R. J.; BewICk, A. € k W O C h h . Acte 1988, 33, 1691-1694. (8) Bockris, J. OM.; Yang, 6. J . Ehscbuand. Chem. 1968, 252. 209-2 14. (9) Zhang, J.; Lu, J.; Cha, C.; Feng, 2. J . €&cbuam/.Chem. 1989, 265. 329-334. IO) Roth, J. D.; Weaver, M. J. J . Ebcbuaml. Chm., In press. 11) Conigan, D. S.; G o , P.; Leung, L.-W. H.; Weaver, M. J. Langnnk 1986, 2, 744-752. 12) Conigen, D. S.; Brandt, E. S.; Weaver, M. J. J . €bctfmm/.Chem. 1987, 235, 321-341. 13) Fernellus. W. C. I"/c S y n M k ; McQraw-HIII: New York, 1948; Vd. 11. p 88. (14) Hlrpp, J. T.; Larkln, D.; Weaver, M. J. suf.Sd. 1989, 725,429-451. (15) Pear-, H. A.; ShePperd, N. Suf. &/. 1976, 50, 205-217. (18) Chang. S . 4 . ; Weaver, M. J. J . Ct".phys. 1990. 92, 4582-4594.
RECEIVED for review February 15,1991.Accepted May 16, 1991. This work is supported by the National Science Foundation.
