J . Phys. Chem. 1990, 94,6013-6021
6013
Metal Crystallinity Effects in Electrocatalysis As Probed by Real-Time FTIR Spectroscopy: Electrooxidation of Formic Acid, Methanol, and Ethanol on Ordered Low-Index Platinum Surfaces Si-Chung Chang, Lam-Wing H. Leung, and Michael J. Weaver* Department of Chemistry, Purdue University, West Lafayette, Indiana 47907 (Received: December 27, 1989; In Final Form: February 27, 1990)
The electrooxidation kinetics of 0.05-0.25 M formic acid, methanol, and ethanol in 0.1 M HCIO4on ordered Pt( 11I), Pt( IOO), and Pt( 110) surfaces were examined by means of FTIR spectra obtained during slow (2-10 mV s-I) thin-layer potential sweeps. This procedure enables the potential-dependent CO coverage, Oca, formed by dissociative reactant chemisorption to be followed in real time in relation to the appearance of C 0 2and partial electrooxidation products as assayed quantitatively from their characteristic infrared bands. Terminally bonded (Le., on-top) CO is the major form of this adsorbate detected under most conditions from the characteristic C-0 stretching frequencies at ca. 2030-2060 cm-l, although bridging CO was also observed in some cases. For formic acid electrooxidation, high CO coverages (eco 2.0.7) are formed on Pt( 100) and Pt( 1 10) that inhibit severely the reaction at low overpotentialsduring the positive-going sweep. The more facile kinetics observed on Pt( 11 1) under these conditions are consistent with the observed low coverage of CO and other chemisorbed poisons. Only low or moderate CO coverages (eco 5 0.3) were obtained on all three Pt faces from methanol or ethanol chemisorption. For the latter, direct spectroscopicevidence for chemisorbed poisons in addition to CO was obtained from their electrooxidation to C02. On Pt( 11 I), ethanol undergoes primarily four-electron oxidation to acetic acid. On Pt( 100) and Pt( 1 IO), in contrast, acetic acid formation is inhibited almost entirely at potentials below where electrooxidation of chemisorbed poisons to C 0 2 occurs, two-electron ethanol oxidation to acetaldehyde dominating under these conditions.
Understanding the manner in which the crystalline state of a metal electrode influences its electrocatalytic properties constitutes a problem of central fundamental importance in electrode kinetics, as part of the overall issue of surface structure-reactivity relationships in heterogeneous catalysis. A useful and well-known tactic for this purpose entails comparing the kinetics of a given reaction on different oriented monocrystalline faces of the same metal. The electrooxidations of formic acid’ and methanol2 on low-index and stepped platinum faces in acidic aqueous media have received particular attention in this regard. While such electrode kinetic, primarily voltammetric, studies have shed substantial light on the overall reaction mechanisms, deducing the nature of the adsorbed reaction intermediates or poisons involved is necessarily rather indirect. At least in principle, surface infrared spectroscopy can provide more direct molecular information on these issues. Aside from the substantial number of such studies devoted to polycrystalline surfaces, potential-modulation infrared spectroscopy has recently been applied to the characterization of low-index platinum electrodes in the presence of formic acid3 or methanol electrooxidationS4 These latter studies have detected terminal (“linearly”) bound CO formed by dissociative reactant chemisorption, most likely acting as a surface poison, along with other spectroscopic features that have been attributed to further adsorbed poisons or ( I ) (a) Adzic, R. R.; OGrady, W. E.; Srinivasan, S. Surf Sci. 1980, 94, L191. (b) Calvilier, J.; Parsons, R.; Durand, R.; Lamy, C.; Leger, J. M. J . Elecrroanal. Chem. 1981, 124, 321. (c) Motoo, S.;Furuya, N. J . Electroanal. Chem. 1985, 184, 303. (d) Clavilier, J.; Sun, S.G. J . Electroanal. Chem. 1986, 199, 471. (e) Adzic, R. R.; Tripkovic, A. V.; Vesovic, V. B. J . Electroanal. Chem. 1986,204,329. (f) Motoo, S.;Furuya, N. Ber. Bunsen-Ces. fhys. Chem. 1987, 91, 457. (g) Markovic, N. M.; Tripkovic, A. V.; Marinkovic, N . S.;Adzic, R. R. ACS Symp. Ser. 1988, No.378, 497. (2) (a) Clavilier, J.; Lamy, C.; Leger, J. M. J . Elecrroanal. Chem. 1981, 125, 249. (b) Adzic, R. R.; Tripkovic, A. V.; OGrady, W. E. Narure 1982, 296, 137. (c) Sun, S.G.; Clavilier, J. J . Elecrroanal. Chem. 1987, 236, 95. (d) Juanto, S.;Beden, B.; Hahn, F.; Leger, J. M.; Lamy, C. J . Elecrroanal. Chem. 1987,237, 119. ( e ) Beden, B.; Juanto, s.; Leger, J. M.; Lamy, C. J . Elecrroanal. Chem. 1987, 238, 323. (f) Bowmann, J. M.; Bittman, J. S.; Harding, L. B. J . Chem. fhys. 1986,85,91 I . (g) Bittins-Cattaneo, B.; Sento, E.; Vielstich, W.; Linke, U. Electrochim. Acta 1980, 33, 1499. (3) Sun, S.G.; Clavilier, J.; Bewick, A. J . Elecrroanal. Chem. 1988, 240, 147. (4) (a) Juanto, S.;Beden, B.; Hahn, F.;Leger, J.-M.; Lamy, C. J . Electroanal. Chem. 1987,237. 119. (b) Beden, B.; Juanto, S.;Leger, J. M.; Lamy, C. J . Elecrroanal. Chem. 1987, 238, 323.
0022-3654/90/2094-6013$02.50/0
possibly reaction intermediate^.^^^ Whereas such potential-modulated “EMIRS” tacticsScan offer excellent sensitivity, they are restricted inherently to the detection of adsorbed species exhibiting rapid reversible potential-dependent alterations in their spectra. While particularly useful for probing adsorbates at potentials below the onset of oxidation, this can provide a significant limitation for examining the irreversible potential-induced compositional changes anticipated for multistep organic electrooxidations. Partly to circumvent this difficulty, we have recently been utilizing an alternative potential-difference infrared (PDIR) procedure that employs a signal-potential sweep or step during the Fourier transform data acquisition. This tactic, which we have dubbed “single-potential alteration infrared spectroscopy” (SPAIRS),7s8 can be utilized to yield quantitative information on irreversible potential-induced compositional changes for adsorbates and in the thin-layer s o l ~ t i o n .Although ~ typically less sensitive than repeated potential difference or potential-modulation procedures, a particular virtue of SPAIRS is that it can be coupled directly with simultaneous voltammetric sweep measurements, thereby aiding the molecular interpretation of the In addition to studies involving polycrystalline we have recently begun to employ this technique for the examination of organic electrooxidation mechanisms on monocrystalline surfaces, initially Pt(ll1) and Rh( 111).Io Related infrared studies in our laboratory have involved the adsorption and electrooxidation of CO on low-index platinum” and rhodium surfaces.12 Described (5) EMIRS = electrochemically modulated infrared spectroscopy. This acronym is usually applied to relatively rapid ( > I Hz) potential modulation tactics that employ a grating, rather than a Fourier transform, spectrometer.“ ( 6 ) For a review, see: Bewick, A,; Pons, S. In Advances in Infrared and Raman Spectroscopy; Clark, R. J. H., Hester, R. E., Eds.; Wiley Heyden: New York, 1985; Vol. 12, Chapter I . (7) (a) Corrigan, D. S.;Leung, L.-W. H.; Weaver, M. J. Anal. Chem. 1987, 59, 2252. (b) Corrigan, D. S.;Weaver, M. J. J . Elecrroanal. Chem. 1988, 241, 143. (c) Leung, L.-W. H.; Weaver, M. J. J . Elecrroanul. Chem. 1988,240, 341. (d) Leung, L.-W. H.; Weaver, M. J. Lungmuir 1990,6,323. (E) For a brief overview, see: Weaver, M. J.; Corrigan, D. S.; Gao, P.; Gosztola, D.; Leung, L.-W. H. J . Electron Spectrosc. Relat. fhenom. 1987, 45, 291. (9) Leung, L.-W. H.; Weaver, M. J. J . Phys. Chem. 1988, 92, 4019. (10) (a) Leung, L.-W. H.; Chang, S.-C.; Weaver, M. J. J . Electroanal. Chem. 1989,266,317. (b) Leung, L.-W. H.; Weaver, M. J. J . fhys. Chem. 1989, 93, 7218.
0 1990 American Chemical Society
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"he Journal of Physical Chemistry, Vol. 94, No. 15, 1990 I
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Chang et al.
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E / V vs SCE Figure 1. Cyclic voltammograms obtained at 0.05 V s-' initially positive-going sweeps from -0.25 V for the electrooxidation of 50 mM formic acid in 0.1 M HCIO, on ordered Pt(lll), Pt(100), and R(llO), as indicated. The dashed-dotted trace for Pt( 110) refers instead to 5 mM
formic acid. The geometric electrode areas are 0.75,0.8, and 0.75 cm2, respectively, for these surfaces. here is a comparative examination by means of coupled SPAIRS-voltammetric measurements of three electrooxidation reactions on ordered Pt( 1 1 I ) , Pt( IOO), and Pt(1 IO) surfaces in acidic aqueous media. Besides the two reactants, formic acid and methanol, noted above, ethanol is chosen for examination here. These reactions have previously been characterized by SPAIRS on Pt( 1 1 1) and Rh( 1 1 l).Io Ethanol provides an interesting example of a simple unifunctional reactant which yields several solution species-acetaldehyde, acetic acid, and C0,-upon electrooxidation under these conditions. Taken together, the present results show how the presence of adsorbed C O and other dissociatively chemisorbed species plays inherently synergistic roles in controlling the voltammetric response of these electrocatalytic systems.
Experimental Section Most experimental details regarding the infrared instrumentation and procedures are given in refs 7 and loa. A Bruker-IBM IR 98-414 Fourier transform instrument was employed, with a globar light source and either an InSb or a MCT narrow-band detector (Infrared Associates). The spectral resolution was 4 cm-]. The electrochemical infrared measurements were performed by pushing the electrode up to a CaF2 or ZnSe optical window, so to form a thin (ca. 5 pm) solution layer. The latter window was required in order to extend the observable frequency range below 1000 cm-I. The Pt( 1 1 I), Pt( loo), and Pt( 110) crystals (ca. 9-mm diameter, 4 mm thick) were obtained from the Material Preparation Facility of Cornell University; they are oriented correctly within l o , as ( I I ) (a) Leung, L.-W. H.; Wieckowski, A.; Weaver, M. J. 1.Phys. Chem. 1988,92,6985. (b) Chang, S.-C.; Leung, L.-W. H.; Weaver, M. J. J . Phys. Chem. 1989,93,5341. (c) Chang, S.-C.; Weaver, M. J. J . Chem. Phys. 1990, 92, 4582. (d) Chang, S.-C.; Weaver, M. J. Surf. Sci. 1990, 230, 222. (e) Chang, S.-C.; Weaver, M . J. J . Phys. Chem. 1990, 94, 5095. (12) (a) Leung, L.-W. H.; Chang, S.-C.; Weaver, M . J. J . Chem. Phys. 1989, 90, 7426. (b) Chang, S.-C.; Weaver, M. J. J . Elecrroanal. Chem., in press.
I
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Figure 2. Effect of altering the positive potential limit upon the cyclic voltammetric oxidation of 50 mM formic acid in 0.1 M HCIO, on Pt(100). Lower trace was obtained as in Figure 1; upper traces were obtained similarly, but for increasingly positive potential limits, as in-
dicated. verified by X-ray diffraction. The preparation of the ordered surfaces followed an annealing procedure similar to that of Wieckowski et al.,13 as detailed in refs 10a and lla,b. The various organic reactants used here were procured (as "high purity" grades) from Aldrich or Fluka. Perchloric acid (double distilled) was from G. F. Smith. Water was purified by means of a "Milli-Q" system (Millipore). All potentials are quoted versus the saturated calomel electrode (SCE), and all measurements were performed at 23 f 1 O C .
Results and Discussion Formic Acid Electrooxidation. Figure 1 shows typical cyclic voltammograms obtained for the electrooxidation of 50 mM formic acid on Pt(l1 l), Pt(100), and Pt(ll0) in 0.1 M HC104, sweeping initially positive from -0.25 V at 50 mV s-I. The positive potential limits in each case were chosen so to avoid the surface disordering that is engendered by irreversible anodic oxide formation. The crystal-face-dependent morphology of these voltammograms, which are comparable to those reported previously,' certifies the strong sensitivity of the electrochemical kinetics to the surface crystallography. On Pt( 11 1) and Pt( loo), the voltammograms were essentially reproducible upon repetitive potential cycling, although some (10-15%) decreases in current were observed during the first few cycles on Pt( 110) (cf. ref 1b). As an illustration of the catalytic consequences of surface disordering, Figure 2 shows the effects of altering the positive potential limit for formic acid electrooxidation on Pt(100). While the bottom trace refers to conditions identical with those in Figure 1, the upper traces were obtained by employing progressively more positive potential limits, as indicated. For potential excursions beyond ca. 0.7 V, an additional anodic peak at ca. 0.45 V appears during the negative-going potential sweep. This feature may be attributed to electrooxidation on "stepped-facet" sites.'g A comparably large effect upon the voltammetry for this reaction on Pt( 11 1) has also been seen upon surface disordering.Ioa As for related studies on polycrystalline Pt,' Pt( 11 1) and Rh(1 1 1),Io it is of interest to ascertain whether both the extent of (13) (a) Zurawski, D.; Rice, L.; Hourani, M.; Wieckowski, A. J . ElecChem. 1987, 230, 221. (b) Hourani, M.; Wieckowski, A. J . ElecChem. 1987, 227, 259.
rroanal. iroanal.
TABLE I: Fractional Coverages of Hydrogen, e,, rad Carbon
Monoxide, ,e F o d by Msrrociative Cbemisorption of Some Organic Molecules on Ordered Platinum Low-Index Surfaces at -0.25 V in 0.1 M H W 4
Pt(l1I)
concn, reactant carbon monoxide formic acid methanol ethanol acetaldehyde
-
M I 0-3 0.05 0.25 0.05 0.25 0.05 0.25 0.05 0.25
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0.4 0.4 0.35 0.35 0.25
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8',
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0.6 0.15 0.15 0.2 0.35 0.2 0.3 0.25 -0.1 0.1 0.25 0.15 0.25 0.15 0.15
=
The Journal of Physical Chemistry. Vol. 94, No. 15. 1990 6015
Metal Crystallinity Effects in Electrocatalysis
Pt(ll0) 8~0'
0.85 1.o 0.8 0.7 0.85 0.9 0.15 0.25 0.35 0.35 0.15 0.2 0.25 0.2 0.55 0.2 0.5 0.3 0.1 0.25 0.15 0.1 0.05 50.05
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'Coverage of hydrogen (Le., ratio of adsorbate atoms to Pt surface atoms), obtained from faradaic charge for reversible formation/removal in cyclic voltammogram from ca. -0.25 to 0.2 V vs SCE combined with Pt surface packing densities6 [Values of 0, in 0.1 M HC104above are as follows: Pt(l1 I), 0.7; Pt( IOO), 0.9; Pt(1 IO), 1.0.1 bFor P t ( l 1 I ) , 1.5 X 1015atoms cm-2; Pt(lOO), 1.28 X 1015atoms cm-2; Pt(l IO), 0.92 X 10" atoms c ~ - ~ . I ' 'Coverage of terminally bound CO, from integrated absorbance of infrared vco band at ca. 2020-2060 cm-I. (For CO adsorbate formed from solution CO, coverages determined additionally from faradaic charge required for voltammetric electrooxidationof irreversibly bound CO.IIb) See text for further details. dissociative chemisorption to form C O and the potentials at which this adsorbate is electrooxidatively removed can be related to the overall electrooxidation kinetics of the solution-phase reactant on different low-index platinum surfaces. For this purpose, Table 1 contains values of the fractional CO coverage, Oc0, present at the initial voltammetric potential, -0.25 V, for a pair of formic acid concentrations, 0.05 and 0.25 M, at all three platinum lowindex faces. These concentrations were chosen in part so to provide sufficient reactant within the thin-layer reservoir to avoid major problems from reactant depletion during the voltammogram (vide infra); the use of higher concentrations can yield complications from leakage of product species from the thin layer. As usual, Oco refers to the surface concentration of adsorbed CO ratioed to the surface density of platinum atoms.'Ib These OCO values were obtained from the integrated absorbances, Ai, of the terminal C 4 stretching band, uco, seen at 2030-2060 cm-' in the surface infrared spectrum at -0.25 V, relative to the absorbances for known CO coverages formed on the same surface by solution CO dosage" (cf. refs 7 and 10). This procedure takes advantage of the roughly linear Aidco relationships found typically under these conditions." We estimate the OCO values to be reliable to within 0.05-0.1. Examples of surface infrared spectra utilized for this purpose are shown in Figure 3. Figure 3A shows a typical set of potential-difference infrared (PDIR) spectra obtained for 50 mM formic acid in 0.1 M HCIO4 on Pt(100). As is conventional, repetitive potential steps were made every ca.20 s between a "base" value (-0.25 V) and the series of "sample" potentials indicated during spectral data acquisition in order to minimize instrument drift. The positive- and negative-going features refer to bands present at the base potential (-0.25 V) and the sample potentials, respectively. In addition to the major bipolar uco feature at 2030-2060 cm-', a weak unipolar band is seen in Figure 3A at 1866 cm-I. The latter indicates the presence of some twofold bridging CO at -0.25 V, which is essentially absent at potentials E 2 0 V. This finding is in harmony with the potential-dependent appearance of bridging YCO bands for the adsorption of solution CO on Pt( 100).llc Although sensitive spectral measurements can be made in this manner, one limitation alluded to above is that this modulation approach faces difficulties a t potentials beyond the onset of irreversible CO electroo~idation.'~Figure 3B shows a typical sequence of single-potential alteration infrared (SPAIR) spectra, obtained on Pt( 1 IO) for 50 mM formic acid in 0.1 M HC104. Each spectrum was acquired by using 30 interferometer scans
Figure 3. (A) Potential-difference infrared (PDIR) spectra in the 1700-2150-~m-~region for 50 mM formic acid in 0.1 M HCIO, on Pt( 100). Spectra were obtained by acquiring 1024 interferometerscans at 'base" and 'sample" potentials (former was -0.25 V; latter are as
indicated), the potential being altered after every 32 scans. (B) Singlepotential alteration infrared (SPAIR) spectra in the 2000-21 50-cm-l region for 50 mM formic acid in 0.1 M HC104on Pt(l10). Spectral sequence was obtained during 2 mV s-I positive-going potential sweep from -0.25 V. Each spectrum was acquired by using 30 interferometer scans; the potentials indicated beside each spectrum are the average values during the data acquisition. A corresponding set of interferograms obtained subsequently, upon completion of CO electrooxidation, was subtracted from each spectrum so to remove solvent and other spectral interferences. (consuming ca. 20 s), obtained during a 2 mV s-l positive-going potential sweep from -0.25 V. The potentials indicated beside each spectrum are the average values during the data acquisition. Subtracted from each set of interferometer s a n s is a corresponding set obtained subsequently at more positive potentials (>0.5 V) upon completion of CO electrooxidation, in order to cancel solvent and other spectral interferences. Unlike the spectra in Figure 3A, extracted by using repetitive potential-difference conditions, absolute unipolar (rather than bipolar) u a bands are obtained under these SPAIRS conditions (Figure 3B), which aid the quantitative measurement of Ai and hence Oco. [The frequency region shown in Figure 3B is narrower than that in Figure 3A since only terminal bands were detected on Pt(1 lo).] In addition to yielding approximate 0, values at potentials prior to C O electrooxidation, it is apparent from Figure 3B that such potential-sweep SPAIRS data can yield valuable information on the Oco-E dependence at more positive potentials during the voltammetric oxidation of solution reactants such as formic acid. The spectra can also provide a quantitative measure of specific product formation, in this case CO, from the asymmetric W-0 stretch at 2343 cm-I, since the solution products (or intermediates) remain essentially trapped within the thin layer on the voltammetric time scale.' The infrared band intensities together with corresponding effective absorptivities, ten, enable the potentialdependent (and hence time-dependent) quantity, Q,of species formed (or consumed) to be obtained from9-10 While teff will depend somewhat on the optical thin-layer geom-
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Chang et al.
The Journal of Physical Chemistry, Vol. 94, No. 15, 1990
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Figure 4. Plots of quantity, Q,of C 0 2 (solid traces) and adsorbed CO (dashed traces) present as a function of electrode potential during the voltammetric oxidation of formic acid on ordered Pt(100) at 2 mV s-I (circles) and IO mV s-I (triangles). Open and filled symbols refer to formic acid concentrationsof 50 and 250 mM, respectively. Data were extracted from the SPAIR spectra as outlined in the text. The Q ( C 0 )
values are magnified IO-fold for clarity. e t r ~the , ~ present arrangement yields teff = 3.5 X lo4 M-' cm-, for the C 0 2 2343-cm-I band; more generally teff = 2.5tb, where tb is the corresponding "bulk solution" absorptivity obtained by using a conventional infrared transmittance e ~ p e r i m e n t . ~ Plots of Q(C0,) versus electrode potential obtained during the thin-layer voltammetric oxidation of formic acid in 0.1 M HCIO, on Pt( 100) are shown as solid traces in Figure 4. The circles and triangles refer to voltammetric sweep rates of 2 and 10 mV s-l, positive-going from -0.25 V; the open and filled symbols refer to formic acid concentrations of 0.05 and 0.25 M, respectively. Since CO, is the exclusive electrooxidation product, the slopes of these Q(C02)-E [and hence Q(C02)-time] plots are proportional to the faradaic current for formic acid oxidation. (The thin-layer voltammograms exhibit very similar morphologies to those observed under more conventional conditions, as in Figure 1 .) Also included in Figure 4 are plots of the C O surface concentration, I'(CO), versus E (dashed traces) obtained simultaneously from the SPAIR spectra, as noted above. The I'(C0) values (mol cm-2) can be related to the corresponding fractional coverages, 8c0, by I'(C0) = ~ C d R / r . (2) where L is Avogadro's number and N R is the number of surface Pt atoms per cm2. [For P t ( l l l ) , Pt(100), and Pt(llO), N R = 1.5 X I O l 5 , 1.28 X and 0.92 X I O l 5 atoms For clarity, I'(C0)-E plots obtained only at 2 mV s-I are shown in Figure 4. However, the corresponding traces obtained at 10 mV s-l are essentially identical, except that the potential region where CO electrooxidation occurs [i.e., where I'(C0) diminishes rapidly] is shifted positive by ca. 0.05 V. Several significant features can be discerned by a careful inspection of Figure 4. Most prominently, although some C 0 2 production occurs prior to the onset of adsorbed CO electrooxidation at 0.25-0.3 V, the reaction rates [Le., the Q(C02)-E slopes (14) (a) Norton, P. R.;Goodale, J. W.; Creber, D. K. Surf. Sci. 1982,119, 4 1 I . (b) Norton, P. R.; Davies, J. A.; Creber, D. K.; Sitter, C. W.; Jackman, T. E. Surf. Sci. 1981, 108, 205. (c) Jackman, T. E.; Davies, J. A.: Jackson, D. P.; Unertl. W. N.; Norton, P. R. Surf Sci. 1982, 120, 389.
/ V vs SCE Figure 5. As for Figure 4, but on ordered Pt(l10). The Q ( C 0 ) values are magnified 20-fold for clarity.
at a given sweep rate] are essentially independent of the reactant concentration; Le., the reaction order is zero. Once adsorbed C O electrooxidation commences, however, the rates become substantially dependent upon the reactant concentration. This indicates that the reaction in the presence of adsorbed C O is limited by the availability of surface sites, so that the coverage of the adsorbed intermediate, and hence the overall reaction rate, is only mildly dependent on the solution reactant concentration. Release of surface sites held by adsorbed C O enables the reactive intermediate to form more extensively, leading to the observed rate increases above ca. 0.4 V. Also worthy of note is the dependence of Q(C0,) on the voltammetric sweep rate at a given reactant concentration at potentials prior to C O electrooxidation. If the C 0 2 arises from an adsorbed species that is largely unable to be re-formed from the solution reactant during the voltammetric sweep (i.e., unable to "turn over"), then Q(C0,) at a given potential should be virtually independent of sweep rate. Otherwise, Q(C0,) should increase markedly with decreasing sweep rate (Le., with increasing reaction time); in the limit where the turnover is extremely facile, then Q(C0,) at a given potential should be inversely proportional to the sweep rate (i.e., proportional to the reaction time). Figure 4 shows that an intermediate situation is observed at potentials prior to adsorbed CO electrooxidation, indicating that some limited "turnovern with solution reactant is achieved. The corresponding Q(C0,)-sweep rate dependence at higher overpotentials is complicated somewhat by the effects of reactant depletion in the thin-layer solution. These are most pronounced at low formic acid concentrations and slow sweep rates (e.g., open circles, Figure 4, for 0.05 M, 2 mV s-l), where the Q(C0,)-E plots ultimately approach a plateau. Nevertheless, in the absence of these complications the reaction rates for E > 0.4 V become less dependent upon the sweep rate, indicating that sufficient catalytic surface sites are available to allow extensive turnover to take place. While such reactant depletion effects precluded quantitative SPAIRS analysis during the return (negative-going) potential sweep, no significant re-formation of adsorbed CO was detected until E S 0.1 V, where hydrogen adsorption commences on Pt(100) in 0.1 M HC104. This observation provides a ready explanation of the large currents for formic acid electrooxidation discerned during the return potential sweep (Figure I). Efficient electrocatalytic oxidation of formic acid on Pt( 100) can therefore proceed in the absence of adsorbed CO. Corresponding Q(C0,)-E and I'(CO)-E plots for formic acid electrooxidation on Pt( 1 10) are displayed in Figure 5. In contrast
rhe Journal of Physical Chemistry, Vol. 94, No. 15. 1990 6017
Metal Crystallinity Effects in Electrocatalysis to Pt( 100) (Figure 4), the rates of formic acid electrooxidation on Pt( 1 IO) at potentials prior to adsorbed CO electrooxidation are significantly dependent on the solution reactant concentration. Although rather sluggish, the reaction rates are therefore not limited entirely by adsorbate site availability under these conditions, even given that high C O coverages are present (& = 0.7-0.9, Table I). The I'(CO)-E profiles on Pt(1 lo), in contrast to Pt( IOO), are significantly dependent on the formic acid concentration in that both the C O coverage and the potentials at which electrooxidation occurs are greater at the higher reactant concentration (Figure 5). Especially the latter observation suggests that the adsorbed CO might be re-formed significantly from formic acid during the voltammetric sweep, thereby acting partly as a reaction intermediate rather than a poison under these conditions (cf. ref 7b). However, the likely inhibiting role of adsorbed C O on Pt(1 lo), at least at lower overpotentials, is exposed by the observation that complete CO readsorption occurs during the return voltammetric sweep by 0.3 V. This finding, which contrasts with the behavior on Pt( 100) (vide supra), accounts for the virtually complete inhibition of formic acid electrooxidation for E 5 0.3 V noted from the voltammetry under these conditions (Figure 1). Interestingly, voltammograms for markedly lower formic acid concentrations yield larger currents during the initial positive-going sweep. An example is shown for 5 mM formic acid as the dashed-dotted trace in Figure I ; the initial currents under these conditions are seen to be larger than for 50 mM reactant. This unusual effect was traced to the marked dependence of Oco upon the reactant concentration; for 50 mM formic acid, Bco = 0.7, whereas for 5 mM reactant, Oco = 0.25 (vide infra). In contrast to Pt( 100) and Pt( 1 lo), formic acid electrooxidation on Pt( 1 1 1 ) commences at low overpotentials during the forward voltammetric sweep, and the voltammogram exhibits markedly less hysteresis between the forward and reverse scans (Figure 1). This observation can be rationalized readily from the low C O coverages (eco = 0.15) apparent from the SPAIR spectra for formic acid on Pt(l11) (Table I). Indeed, the onset of C 0 2 production on Pt( 1 1 1) occurs at significantly lower overpotentials, ca. 0 V, than those where the adsorbed C O commences electrooxidation, ca. 0.2-0.3 V, as deduced from the SPAIR spectra. Methanol Electrooxidation. Figure 6 displays representative cyclic voltammograms at 50 mV s-I for the electrooxidation of 50 mM methanol in 0.1 M HCIO, on P t ( l l l ) , Pt(100), and Pt( 110) (ca. Figure I ) . Again, these results are similar to voltammograms reported earlier.* Noticeable decreases in the currents (ca. 10-20%) were observed upon repetitive cycling on Pt(100) and Pt(1 lo), but not on Pt(l1 I ) . While significant differences in the electrocatalytic behavior are seen for methanol oxidation on the three low-index faces (Figure 6 ) , these are somewhat less marked than for formic acid oxidation. Inspection of the relevant C O coverage data (Table I) shows that whereas formic acid engenders near-saturation OCo values on Pt(100) and Pt(1 lo), methanol yields only small or moderate C O coverages on all three Pt faces. As for formic acid, the adsorbed C O formed from methanol is bound predominantly in the terminal configuration. Nevertheless, a weak band at 1810-1 830 cm-', characteristic of twofold bridging CO, was also detected for methanol on Pt( 11 1). Measurements of the coverage due to adsorbed hydrogen, 8H, at negative potentials together with Oco can yield information on the presence of additional adsorbates since OH should be depressed by such competitive adsorption (cf. refs 7 and lob). The OH values listed in Table I were obtained from the faradaic charge required for oxidation of the adsorbed hydrogen, appearing as reversible waves from -0.25 to ca. 0.2 V (see Figure 1 of ref 1 lb). (As for Om the OH values are normalized to the density of surface platinum atoms, N R . ) While the (6, + 8co) sum for methanol on Pt(l11) is com0.7) or saturated parable to that for adsorbed hydrogen (OH C O adsorption (eco = 0.6) alone, the (e, Oca) values for methanol on both Pt(100) and P t ( l l 0 ) faces, ca. 0.5-0.6, are significantly below those expected on this basis. [On Pt(100), 0.9, Oc0 for saturated hydrogen and C O adsorption alone, OH
-+
-
~
I
...(110) ..... .*. .....,,......,....*-
I
I
I
I
I
I
,%e
*
0 f ,.......*. ..'
I
0
0.4
I 0.8
E / V vs SCE Figure 6. As for Figure 1, but for the electroooxidation of 50 m M
methanol.
-
i= 0.85, and for Pt( 1 lo), 1.0, Oco i= 1.0; see Table I.] For partial dosages of solution C O onto these surfaces, however, (OH )0, was found to remain essentially constant (within ca. f0.05) over the entire coverage range. The smaller (6, 8co) values obtained for methanol, then, are indicative of the presence of additional adsorbed species, either chemisorbed poisons or reaction intermediates. Consistent with this notion, the methanol electrooxidation rates on Pt(100) and P t ( l l 0 ) were found to be virtually independent of reactant concentration in the range 0.05-0.25 M, even at potentials beyond where CO electrooxidation is essentially complete (E R 0.45 V) as deduced from the SPAIR spectra. Ethanol Electrooxidation. Typical cyclic voltammograms at 50 mV s-] for the electrooxidation of 50 mM ethanol in 0.1 M HClO, on Pt(l1 l), Pt(100), and P t ( l l 0 ) are shown in Figure 7. The stability of the voltammograms upon repetitive cycling was similar to that for methanol. As for the reactants already considered, similarly shaped voltammograms were obtained at least over the concentration range 0.05-0.25 M and for the slower thin-layer voltammetric sweeps suitable for SPAIRS. Again, there is a substantial dependence of the voltammetric features upon the surface crystallographic orientation. In this case, Pt( 110) provides the most potent electrocatalyst, both in terms of the onset of electrooxidation and the peak currents achieved. As already noted, unlike formic acid and methanol electrooxidation, ethanol undergoes predominantly partial oxidation to form acetaldehyde and acetic acid according to
+
+
CzHsOH - 2e- - 2H+
-
and CIHSOH - 4e- - 4H+
+ H20
CH3CH0
(34
-
(3b)
CH3COOH
The potential-dependent contributions of these processes to the overall voltammetric response can be deduced quantitatively from the SPAIR spectra.IQ8Typical sequences of relative transmittance SPAIR spectra in the 900-2500-~m-~region obtained for electrooxidation of 0.25 M ethanol in 0.1 M HCIO4 on P t ( l l l ) , Pt( lOO), and Pt( 110) are displayed in Figure 8, A-C, respectively. The voltammetric sweep rate was 2 mV s-I, from -0.25 V to 0.8,
6018
Chang et al.
The Journal of Physical Chemistry, Vol. 94, No. IS, 1990 I
I
I
I
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I
. . . .......
. . : ... . . . ..
.I).
+; ..
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I
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I
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'....I
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............
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0
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E / V vs SCE Figure 7. As for Figure 1. but for the electrooxidation of 50 mM ethanol.
A
IC
--p#
0.27V
0.4
0.6
0.8
E / V YS S C E Figure 9. Plots of quantity of acetic acid (solid trace), acetaldehyde (dotted trace), CO, (dashed trace), and adsorbed CO (dashed-dotted trace) present as a function of potential during the voltammetric oxidation at 2 mV sT1 of 0.25 M ethanol in 0.1 M HCIO, on ordered Pt(l1 I), obtained from SPAIR spectra as in Figure 8A as outlined in the text. The Q ( C 0 , ) and r ( C 0 ) values are magnified 5- and IO-fold, respectively, for clarity.
respectively. Each spectrum involved acquiring 40 interferometer scans (consuming ca. 25 s) during the voltammetric sweep at the average potentials indicated and ratioing to a corresponding "reference" spectrum obtained at the initial potential just prior to starting the potential sweep. As outlined previously for reaction of ethanol on Pt( 1 1 1),Ioa several negative-going features signal the formation of specific electrooxidation products. The bands a t 1280, 1370-1 390, and ca. 17 15 cm-l in Figure 8 are assigned to the C-O stretch, CH3 bend, and C = O stretch for aqueous acetic acid.'$ A complication
is that the 1715-cm-I feature can also be due to the C 4 stretch for aqueous acetaldehyde. Fortunately, however, a band at 933 cm-' (Figure 8) provides a unique monitor of acetaldehyde formation; this feature is tentatively assigned to an 0-C-0 asymmetric stretch of hydrated acetaldehyde.Ioa Besides the negative-going band at 11 IO cm-I, due primarily to the potential-induced migration of perchlorate anions,I6 the 2343-cm-' band signals (as before) the formation of C 0 2 . The appearance of adsorbed C O on the Pt( 100) and Pt( 1 IO) faces can also be discerned at the least positive potentials from the bipolar feature at ca. 2050-2060 cm-]. (The bipolar nature of this band arises from using a reference spectrum in Figure 8 at -0.25 V, Le., prior to C O electrooxidation.) These SPAIR spectra were utilized as before to generate plots of the quantities of acetaldehyde, acetic acid, and C02, Q(Ald), Q(Ac), and Q(C02),respectively, formed as a function of electrode potential. The 933-cm-l acetaldehyde band (eb = 2.2 X IO3 M-I cm-*) and the 1280-cm-I acetic acid feature (eb = 5.8 X lo3 M-I were used for this purpose.'" The resulting Q-E plots, along with the corresponding r(CO)-E data obtained as noted above, are shown for ethanol electrooxidation on Pt( 11 l), Pt( loo), and Pt(l10) in Figures 9-1 1, respectively. The Q(Ald), Q(Ac), and Q(C0,) values are plotted in each case as triangles/dotted traces, squares/solid traces, and filled circles/dashed traces, respectively; the I'(C0) values are displayed as open circles/dashed-dotted traces. Careful intercomparison of these plots provides several points of interest. While the initial appearance of acetaldehyde occurs at potentials, ca. 0.3-0.4 V, that do not vary greatly with the crystallographic orientation, the initial formation of acetic acid occurs at markedly (ca. 0.2-0.3 V) higher overpotentials on Pt(100) and Pt( 1 10) than on Pt( 11 1). This greater surface structural sensitivity exhibited by the four-electron process is not surprising: acetic acid formation requires oxygen transfer to occur, probably from adsorbed water or hydroxyl radicals (eq 3b), whereas the production of acetaldehyde from ethanol involves merely a coupled
(15) Corrigan, D. S.;Krauskopf, E. K.; Rice, L. M.; Wieckowski, A,; Weaver, M. J. J . Phys. Chem. 1988, 92, 1596.
(16) Corrigan, D. S.;Weaver, M. J. J . Elecrroanal. Chem. 1988, 239, 55.
1116
12ao
0.55---+-+ pfE0.02
o8 ' 00 , 6 8 + 3
iiio
0.40 0.26
~
2dS3
I
2500
L 90
2500
v
900
2500
900
/ cm-'
Figure 8. Single-potential alteration infrared (SPAIR) spectra obtained during the voltammetric oxidation of 0.25 M ethanol on ordered Pt( I 1 I ) , pt(lOO), and Pt(l10) (A, B, and C, respectively) in 0.1 M HCIO4., Potential sweep rate was 2 mV s-l, positive-going from -0.25 V. Each spectrum, displayed as relative reflectance (ARIR), was acquired from 40 interferometer scans at the average potentials indicated, ratioed to a corresponding spectrum obtained at the initial potential immediately prior to initiating the potential sweep.
0.65, and 0.7 V and return for Pt(l1 I), Pt(100), and Pt(l IO),
Metal Crystallinity Effects in Electrocatalysis
E / V vs SCE Figure 10. As for Figure 9, but on Pt(lO0). The Q(C0,) and r(C0) values are magnified 5 - and IO-fold, respectively, for clarity.
The Journal of Physical Chemistry, Vol. 94, No. 15, 1990 6019
.. I
I
0
I
I
0$4
I
I
0.8
E / V vs SCE Figure 12. As for Figure 1, but for the electrooxidation of 50 mM acetaldehyde.
E / V vs SCE Figure 11. As for Figure 9, but on Pt(l IO). The Q(C0,) and l'(C0) values are magnified IO-fold for clarity. two-electron/proton transfer (eq 3a). A related observation is that the potentials where adsorbed CO removal is consummated during ethanol electrooxidation are significantly (ca. 0.2 V) more positive on Pt( 100) and Pt( 110) (Figures 10 and 1 1) than on Pt( 11 1) (Figure 9); these potentials correspond closely to the onset of acetic acid formation. This correlation may arise from an inhibiting action of the adsorbed CO toward four-electron ethanol electrooxidation, sites occupied by the former needing to be released before the latter process can proceed, as for formic acid electrooxidation. While plausible, however, the relatively low C O coverages obtained under these conditions (eco 5 0.2) argue against this explanation. Nevertheless, electrooxidation of adsorbed C O also requires oxygen
transfer from a coadsorbed species, most likely hydroxy1.l' The rough correspondence between the potentials at which adsorbed C O electrooxidation to C02and ethanol electrooxidation to acetic acid occur may well therefore reflect the need for a common adsorbed coreactant. While the production of COz from ethanol electrooxidation is obviously due in part to the electrooxidative removal of adsorbed CO formed at the initial potential (-0.25 V), significantly greater amounts of C 0 2 are formed on all three faces than can be accounted for on this basis; i.e., total Q(C0,) > initial I'(C0) (Figures 9-1 1). That this additional COz also arises from oxidation of adsorbed species is evident from the observation that the total Q(C0,) is essentially independent of the voltammetric sweep rate and the ethanol concentration, at least in the range 0.05-0.25 M. Further evidence that other adsorbed species are present, at least initially at -0.25 V, is provided by the relatively low (0, + Bco) values, 50.4-0.5, found in the presence of ethanol (Table I). The likely significance, as well as nature, of this additional adsorbate(s) is unclear; it is particularly prevalent on Pt(100) and Pt(ll0) (Figures 10 and 11). Noteworthy, however, is the especially large total Q(C0,) observed on Pt( loo), 1.3 X mol cm-* (Figure 10); assuming that one carbon is bound per Pt yields an effective overall reactant coverage of ca.0.6. That this adsorbate does not undergo extensive electrooxidation until relatively positive potentials can rationalize the observed inhibition of acetaldehyde formation on Pt(100) (Figure 10); note that the sudden onset of acetaldehyde production at ca. 0.4 V coincides closely with the initial formation of C 0 2 and hence the creation of vacant surface sites. An important consequence of the different crystal-face dependencies of the acetaldehyde and acetic acid formation kinetics is that the proportion of these two species produced varies substantially with the crystallographic orientation. For Pt( 11 l ) , comparable amounts of acetaldehyde ind acetic acid are formed during the positive-going portion of the potential cycle. Furthermore, the former species is entirely oxidized further to acetic acid during the return potential sweep (Figure 9). For Pt(100) (17) For example: Gilman, S. J . Phys. Chem. 1%3,67, 78; 1964,68, 70.
6020 The Journal of Physical Chemistry, Vol. 94, No. 15, 1990 TABLE 11: Terminal C - O Stretching Frequencies, v b for CO Formed on Pt(ll0) at -0.25 V in 0.1 M HCIO, from Organic Reactants As Comtmred witb Adsomtion from Solution CO U&
reactant formic acid
concn. M
5 X IO-) 5X 0.25 methanol 0.05 ethanol 0.05 acetaldehyde 0.05
",,S 0.25 0.7 0.9 0.35 0.25 0.1
cm-'
co
co
2022 2055 2068 2033 2022 -2010
2066 2061 2068 2061 2066 2065
reactantb dosingc strippingd 2040 2058 2068 2044 2048 2040
"Coverage of terminal CO formed from reactant indicated, as determined from integrated absorbance of infrared uc0 band (cf. Table I). See text for details. bFrequency of terminal uc0 band formed from reactant at coverage indicated at -0.25 V. CFrequencyof terminal uc0 band formed by dosing with solution CO to coverage indicated at -0.25 V. Taken from Fig. 4 of ref 1 Id. Frequency of terminal um band for CO adlayer at coverage indicated at -0.25 V, formed by saturation adsorption from solution CO followed by partial electrooxidative stripping. Taken from Fig. 4 of ref I Id. and Pt( 1 IO), on the other hand, comparable amounts of acetaldehyde and acetic acid are formed by the end of the potential cycle (Figures IO and 11). In an attempt to shed further light on the ability of acetaldehyde to undergo further electrooxidation under these conditions, corresponding voltammetric and SPAIRS experiments were conducted using acetaldehyde as the initial reactant. Figure 12 shows typical cyclic voltammograms (50 mV s-l) obtained for 50 mM acetaldehyde in 0.1 M HCIO4 on Pt( 11 l), Pt( loo), and Pt( 110). The marked inhibition of acetaldehyde electrooxidation on Pt( 1 11) seen during the positive-going sweep has been considered in detail previously and attributed on the basis of SPAIRS data to inhibition from a sluggishly oxidized chemisorbed poison.10a Corresponding SPAIRS results obtained for acetaldehyde on Pt( 100) also implicate the role of C O and other chemisorbed species in delaying the onset of electrooxidation until ca. 0.45 V (Figure 12), in that C 0 2 production from these species begins sharply at this point. For acetaldehyde electrooxidation on Pt(1 IO), no significant C 0 2 production is detected until ca. 1 .O V, beyond where surface disordering occurs. This finding, coupled Oco) values, 50.2, observed for acetaldehyde with the low (0, on Pt( 1 IO) implicates the role of chemisorbed poisons in severely inhibiting acetaldehyde electrooxidation under these conditions (Figure 12). This sluggish nature of acetaldehyde electrooxidation on Pt(100) and Pt(l IO), even during the return voltammetric sweep (Figure 12), can account in part for the relative importance of this species as a reaction product, rather than an intermediate, from ethanol electrooxidation on these surfaces. CO Adlayer Structure. Some information on the structure of the CO adlayers formed by reactant dissociative adsorption can be gained by examining the uc0 terminal frequencies, uko, and bandshapes in relation to those for corresponding coverages formed from solution CO. This is prompted by our recent observation that substantial (up to ca.60 cm-I) upshifts in uLo with increasing OCo (20.05) can be obtained on Pt low-index faces by dosing from dilute CO solutions, whereas the high uko values corresponding to saturation CO coverages decrease by only a small extent (610 cm-I) when Oc0 is diminished over the same range by electrooxidative stripping.J1n By means of infrared measurements for mixed 1%O/13C0isotopic layers, the large U ~ ~ - O C dependencies O obtained under the former "direct dosing" conditions were identified as being due primarily to variations in the dynamic dipoledipole coupling. The retention of such coupling for the latter "stripping" conditions provides strong evidence for the formation of extensive CO domains ("islands") having high local coverages during electrooxidation.llE The ~ ~ variations ~ seen - under 0 direct dosing conditions, on the other hand, are indicative of the presence of relatively dispersed C O structures. Table 11 compares typical uko values obtained for chemisorption of the four organic species considered here on Pt( 1 10) with the
+
Chang et al.
corresponding frequencies obtained at the same coverage for adlayers formed from solution CO by direct dosing and partial electrooxidative stripping, as indicated. Although the uko values for CO formed by reactant chemisorption vary significantly with Oco, these variations are smaller than obtained for adlayers formed by C O dosing. This finding suggests that some C O island formation takes place under reaction conditions, perhaps engendered by coadsorption of other molecular fragments. Comparable results are obtained for the other two Pt low-index faces. The Pt( 110) surface is of particular interest since there is evidence for surface reconstruction on this face that can be lifted by CO adsorption.'Ic This issue will be considered in detail elsewhere. Overall Mechanistic Implications. Although the electrooxidation reactions considered here not surprisingly exhibit distinctly different features, sufficient commonalities are evident so to enable some overall mechanistic deductions to be made regarding the nature of the observed surface crystallinity effects. Most importantly, broad-based connections can be made between the extent of initial dissociative chemisorption to form CO, along with the kinetics of its electrooxidative removal, with the electrocatalytic oxidation of solution reactant. The relative inability of Pt( 1 11) to engender formation of adsorbed C O is clearly responsible for the onset of electrooxidation of formic acid, methanol, and ethanol at especially low overpotentials on this surface. Marked reaction inhibition by adsorbed C O and other chemisorbed "poisons" is evident on both Pt( 100) and Pt( 110). Formic acid electrooxidation on these latter surfaces provides the clearest indication of this phenomenon, in that CO constitutes the predominant adsorbate. Even though the onset of solution electrooxidation can precede the initial removal of adsorbed CO, the limited availability of surface sites for this purpose is apparent in the voltammetric SPAIRS data, especially on Pt(100) (Figure 4). The involvement of a "weakly" adsorbed species acting as the reaction intermediate, by utilizing such surface sites in the face of a strongly adsorbed yet electroactive "surface poison", forms the basis of the oft-considered "dual-pathway" mechanism, considered especially for formic acid electrooxidation.18 A recent EMIRS study of formic acid on Pt( 1 1 1) and Pt( 100) reported broad bands around 1750 cm-I, obtained by modulating to potentials where adsorbed C O electrooxidation o c c ~ r r e d .These ~ features were attributed to a reactive intermediate, possibly 'COOH. Although the present less sensitive SPAIRS technique precluded the observation of such species, at least on the voltammetric time scale, clear evidence was also obtained for the presence of other adsorbed inhibitors. This evidence is most direct for ethanol and acetaldehyde electrooxidation, since the C 0 2 detected by SPAIRS arises exclusively from initially adsorbed species. A relevant finding, applicable to ethanol on all three faces (Figures 9-1 1) and especially to acetaldehyde on Pt( 11 l),loais that the electrooxidative removal of the other adsorbates to form C 0 2 occurs in part at higher overpotentials than for CO itself. [This point can be deduced readily by subtracting the I'(C0)-E trace from the corresponding Q(C02)-E plot, thereby eliminating the source of C 0 2 due to adsorbed CO.loa] Such species can therefore exert substantial inhibiting effects upon the reaction even after the adsorbed CO is largely electrooxidized. This latter finding for the present reactants on ordered low-index platinum surfaces differs from corresponding results obtained on polycrystalline platinum in that the latter exhibit markedly higher C O coverages.7d The onset of facile solution electrooxidation on the polycrystalline surface tends to correspond closely with the removal of this C0.7d The ability of polycrystalline platinum to generate adsorbed CO from alcohols, aldehydes, and amides usually occurs for molecules featuring at least one hydrogen bound to the a carbon atom.'lCvdAlthough such dissociative chemisorption occurs to a smaller extent on ordered low-index Pt surfaces, the observation of significant CO coverages (OCO k 0.1) for a range demonstrates that a qualitatively ~ of such ~ species on Pt( 11 similar adsorption process is operative. The present reactants yield (18) Capon, A.; Parsons, R. J . Electround. Chem. 1973,44, 1; 1973,45, 205.
J. Phys. Chem. 1990, 94, 6021-6028 uniformly higher CO coverages on Rh( 11 1) compared to Pt( 1 1 I), a trend consistent with the greater chemisorbing ability of the former metal.l% Such strong C O chemisorption provides Rh(l11) with markedly inferior electrocatalytic properties compared with Pt( 1 1 I ) , due to the relative inability of the chemisorbed C O to undergo electrooxidation.iOb Further studies along these lines, including the examination of the electrocatalytic properties of other rhodium low-index faces
6021
in comparison with platinum by means of real-time infrared spectroscopy, are planned.
Acknowledgment. This work is supported by the National Science l?"tion. Registry No. HCIO,, 7601-90-3; Pt,7440-06-4; CO, 630-08-0; C02, 124-38-9; formic acid, 64-1 8-6; methanol, 67-56-1; ethanol, 64-17-5; acetic acid, 64-19-7; acetaldehyde, 75-07-0.
Reverse Micelles in Supercritical Fluids. 3. Amino Acid Solubilization in Ethane and Propane Richard M. Lemert, Rob A. Fuller, and Keith P. Johnston* Department of Chemical Engineering, The University of Texas at Austin, Austin, Texas 7871 2 (Received: January 9, 1990)
The solubilization of the amino acids tryptophan and proline in bis(2-ethylhexyl) sodium sulfosuccinate (AOT) reverse micelles and microemulsions is described for the solvents supercritical fluid (SCF) ethane and liquid propane. Two different types of condensed phases, a solid crystalline amino acid phase and an aqueous phase, are investigated at 37 "C from 73 to 325 bar. The concentration of surfactant in both the aqueous and dense fluid phases is reported for the first time, giving insight into the natural curvature of the micelles. In regions where pressure has little effect on the water-to-surfactant ratio, Wo, solubilization in the micelles is relatively constant. At lower pressures, there is a pronounced effect of pressure on the partitioning of surfactant and water, and thus on solubilization. The partitioning of an amino acid between the water pool inside the micelles and the micellar interfacial region is described by a surface monolayer model. This partitioning is influenced by pressure in regions where Woand thus the curvature and rigidity of the interface are variable. The model explains the experimental result that the micelles are highly selective for proline versus tryptophan. Based on the measured concentrations of AOT and tryptophan in both the fluid and aqueous phases, a process is presented for separating hydrophilic substances from water.
Introduction Dense fluid solvents, such as supercritical C 0 2 ( T , = 31.1 OC, P, = 73.8 bar), ethane ( T , = 32.3 OC, P, = 48.8 bar), and liquid propane ( T , = 96.7 OC, P, = 49.5 bar), offer certain advantages for separation and reaction pr0cesses.l For example, the solvent strength may be adjusted over a continuum with pressure and temperature, and minimal solvent residue is left in the products after depressurization. Supercritical fluids (SCFs) such as COz and ethane have a low dipolarity/polarizability and are appropriate solvents for lipophilic substances.2 In order to solubilize significant amounts of nonvolatile polar substances, it is usually necessary to add several mole percent of a cosolvent such as ethan01.~ However, such a mixed solvent does not dissolve hydrophilic substances such as proteins and amino acids. In nonpolar solvents, certain surfactants such as bis(2-ethylhexyl) sodium sulfosuccinate (AOT) aggregate to form reverse micelles in which the hydrophilic head groups form a core and the lipophilic tails project into the oil-continuous phase. Water dissolves into the cores of these micelles to give transparent, thermodynamically stable water-in-oil microemulsions. There is not a clear distinction between the terms reverse micelle and microemulsion, and they are often used interchangeably. A variety of techniques have been used to determine the nature of the water in reverse micelle^.^ Other studies have focused on the solubi(1) Johnston, K. P., Penninger, J. M. L., Eds. Supercritical Fluid Science and Technology; ACS Symposium Series 406; American Chemical Society: Washington, DC, 1989. Brennecke, J. F.; Eckert, C. A. AIChE J . 1989,35, 1409. Lee, L. L.; Debenedetti, P. G.;Cochran, H. D. Supercritical Fluid Technology; CRC Press: Boca Raton, FL, in press. (2) Stahl, E.; Quirin, K.-W.; Gerard, D. Dense Gases for Extraction and Refining, Springer-Verlag: Berlin 1987. (3) Johnston, K. P.; Peck, D. G.; Kim, S . Ind. Eng. Chem. Res. 1989,28, 1 1 15. Wong, J. M.; Johnston, K. P. Biotechnol. Prog. 1986, 2, 29. Chang, C. J.; Randolph, A. D. AIChE J . 1989, 35, 1876. Schaeffer, S. T.; Zalkow, L. H.;Teja, A. S.Ind. Eng. Chem. Res. 1989.28, 1017. Larson, K. A.; King, M. L. Biotechnol. Prog. 1986, 2, 73.
0022-3654/90/2094-6021$02.50/0 I
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lization of peptides, amino acids, and salts,5 and their location within the micellar structure.6 Reverse micelles may be used to extract hydrophilic substances selectively from aqueous solution.' Reverse micelles are considered to be models of biological membranes and have been used to gain insights into protein solubilization, structure, and activity within these membranes.* Supercritical fluid solvents could be useful for investigating the general nature of reverse micelles and microemulsions, since the solvent strength may be adjusted over a continuum by varying pressure. This could be used to manipulate micellar properties, without changing chemical properties such as pH or salinity. Recently, reverse micelles have been studied in dense fluids such as S C F COz, ethane, and liquid propane. The formation of small aggregates of spin-labeled cholesterol in C 0 2 was observed with EPR ~pectroscopy.~The phase behavior for a variety of types of surfactants has been explored in COZ.'O Most studies of reverse (4) Wong, M.; Thomas, J. K.;Graetzel, M. J . Am. Chem. Soc. 1976, 98, 2391. DAprano, A,; Liuio, A,; Liveri, V. T.; Aliotta, F.; Vasi, C.; Migliardo, P. J . Phys. Chem. 1988.92.4436. Jain, T. K.; Varshney, M.;Maitra, A. J . Phys. Chem. 1989, 93, 7409. Eicke, H. F.; Kvita, P. In Reverse Micelles; Luisi, P. L., Straub, B. E., Eds.; Plenum: New York, 1984. ( 5 ) Dossena, A.; Rizzo, V.; Marchelli, R.; Casnati, G.; Luisi, P. L. Biochim. Biophys. Acta 1976, 446, 493. Fendler, J. H.; Nome, F.; Nagyvary, J. J . Mol. Euol. 1975,6.215. Fletcher, P. D. I. J . Chem. Soc.,Faraday Trans. 1986,82, 265 1. Leodidis, E. B.; Hatton, T. A. In Structure and Reactiuity in Reuersed Micelles; Pileni, M. P., Ed.; Elsevier: Amsterdam, 1988. Leodidis, E. B.; Hatton, T. A. Lungmuir 1989, 5, 741. ( 6 ) Fendler, J. H.; Nome, F.; Nagyvary, J. J . Mol. Euol. 1975, 6 , 215. Fletcher, P. D. I. J . Chem. Soc., Faraday Trans. 1986,82, 2651. Luisi, P.
L.; Giomini, M.; Pileni, M. P.; and Robinson, B. H. Biochim. Biophys. Acta 1988,947,209. Rodgers, M. A. J.; Lee, P. C. J . Phys. Chem. 1984,88, 3480. (7) Armstrong, D. W.; Li, W. Anal. Chem. 1988,60,86. Goklen, K.E.; Hatton, T. A. Sep. Sci. Technol. 1987, 22, 831. Thien, M. P.; Hatton, T. A.; Wang, D. I. C. Biotechnol. Biwng. 1988, 32, 604. (8). Martinek, K.; Levashov, A. V.; Klyachko, N.; Khmelnitski, Yu. L.; Berezin, I. V. Eur. J . Biochem. 1986,155,453. Luisi, R. L. Angew. Chem. 1985, 24, 439. (9) Prausnitz, J. M.; Randolph, T. W.; Clark, D. S.; Blanch, H. W. Science 1988, 238, 387.
0 1990 American Chemical Societv
