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Surface Reactivity at “Chiral” Platinum Surfaces Ahmad Ahmadi and Gary Attard* Department of Chemistry, Cardiff University, P.O. Box 912, Cardiff CF1 3TB, U.K.
Juan Feliu and Antonio Rodes Department de Quimica Fisica, Universitat d’Alacant, Apartat 99, E-03080 Alacant, Spain Received August 24, 1998. In Final Form: December 4, 1998 The electro-oxidation of D- and L-glucose has been investigated using the chiral electrode Pt{643}R and its enantiomorph Pt{643}S. Both electrodes are demonstrated to be enantioselective. We ascribe this behavior to the inherent (left or right) “handedness” of kink sites present at the surface. In contrast, no difference in D- and L-glucose oxidation could be detected using stepped Pt{211} and Pt{332} electrodes. Stepped surfaces are achiral, since they lack the prerequisite necessary for the observation of chirality, namely kink sites. For Pt{531}, a surface which contains only kink sites, a diastereomeric product excess of ∼80% is estimated for D- and L-glucose oxidation. This compares with a value of ∼60% for Pt{643}. Hence, the chiral discrimination appears to scale with the surface density of kink sites. These findings constitute the first experimental proof that chiral discrimination is an intrinsic property of kinked single-crystal surfaces.
Introduction An area of intense research interest remains the identification of effective asymmetric catalysts for both the synthesis of pure enantiomers and their separation and detection.1 This is hardly surprising, considering the enormous range of chemicals whose utility depends critically upon the extent of enantioselectivity which they display. A fruitful strategy adopted in performing enantiomeric separations or catalysis has often been to adsorb a chiral modifier onto the surface of an achiral substrate.2-5 Although significant enantiomeric excesses have been achieved by this procedure,6 the precise mechanism of enantiodifferentiation is not well understood. Nonetheless, a full understanding of the origins of enantioselectivity at catalyst surfaces would herald a major breakthrough in many applied areas of science, including asymmetric synthesis, electroanalysis, and heterogeneous catalysis. In a recent paper by McFadden and co-workers, the conditions thought necessary to observe enantioselective adsorption on single-crystal surfaces were described.7 In ref 7 it was proposed that so long as the step lengths comprising the kink site of a single-crystal surface were of unequal magnitude, reflection through a plane normal to such a surface produces a new surface which itself cannot be superimposed upon the originalsthat is, such kink sites should be chiral. An example of such a kinked chiral substrate would be the {643} surface of a facecentered cubic (fcc) crystal. We will show later that this interpretation of surface chirality is actually an approximation and that the lengths of steps, the confluence of which forms the kink site, may be equal and still display a chiral effect. McFadden et al. were also the first to test * To whom correspondence should be addressed. (1) Hutchings, G. J.; Wells, R.; Feast, S.; Siddiqui, M. R. H.; Willock, D.; King, F.; Rochester, C. H.; Bethell, D.; Bulman Page, P. C. Catal. Letts. 1997, 46, 249. (2) Izumi, Y. Adv. Catal. 1983, 32, 215. (3) Blaser, H.-U. Tetrahedron Asymm. 1991, 2, 843. (4) Webb, G.; Wells, P. B. Catal. Today 1992, 12, 319. (5) Baiker, A. J. Mol. Catal. 1997, 115, 473. (6) Meheux, P. A.; Ibbotson, A.; Wells, P. B. J. Catal. 1991, 128, 387. (7) McFadden, C. F.; Cremer, P. S.; Gellman, A. J. Langmuir 1996, 12, 2483.
their hypothesis experimentally.7 Using as a model system the adsorption and oxidation of the chiral alcohol 2-butanol on Ag{643} and its enantiomorph Ag{6 h4 h3 h }, they examined the difference in desorption enthalpies of the alcoholic stereoisomers using temperature-programmed desorption (TPD). Unfortunately, no measurable difference in desorption enthalpies could be detected to within (0.1 kcal mol-1.7 In addition, no difference was observed in the decomposition kinetics of the enantiomeric alkoxides formed on the preoxidized Ag{643} and Ag{6 h4 h3 h } surfaces. However, a recent theoretical paper8 detailing calculations of the adsorption properties of a number of chiral hydrocarbons on chiral platinum lent further support to the expectation of enantiospecificity in asymmetric kinked surfaces of the McFadden type described above. In the present study, we repeat the search for an enantiomeric response first attempted in ref 7, but replacing the less reactive silver substrate by platinum and using a relatively “large” probe molecule containing five chiral centers (glucose). Both of these modifications were predicted to enhance the likelihood of a successful outcome to the work. Moreover, the surface sensitivity of the electrochemical method, especially toward the initial stages of adsorption (in that molecular adsorption at the various substrate sites may be monitored continuously from the outset), makes it particularly sensitive to the subtle differences in adsorption kinetics expected when examining enantiomeric effects. Electrochemical investigation of glucose oxidation using polycrystalline electrodes9-14 has now been expanded to include surfaces of well-defined crystalline structure.15-18 The structure-sensitive nature of glucose (8) Sholl, D. S. Langmuir 1998, 14, 862. (9) Rao, M. L. B.; Drake, R. F. J. Electrochem. Soc. 1969, 116, 334. (10) Yao, S. J.; Appleby, A. J.; Wolfson, S. K., Jr. Z. Phys. Chem. N. F. 1972, 82, 225. (11) Skou, E. Electrochim. Acta 1977, 22, 313. (12) Ernst, S.; Heitbaum, J.; Hamman, C. H. J. Electroanal. Chem. 1979, 100, 173. (13) Vassilyev, Y. B.; Kharzova, G. A.; Nikolaeva, N. N. J. Electroanal. Chem. 1985, 196 (105), 127. (14) Essis Yei, L. H.; Beden, B.; Lamy, C. J. Electroanal. Chem. 1988, 246, 349. (15) Kokkiniois, G.; Leger, J. M.; Lamy, C. J. Electroanal. Chem. 1988, 242, 221.
10.1021/la9810915 CCC: $18.00 © 1999 American Chemical Society Published on Web 03/05/1999
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oxidation was confirmed in refs 15-18, particularly in relation to the dependence of oxidation rate on terrace width. In addition a “dual-path” mechanism for glucose electro-oxidation was proposed whereby, in parallel with the electrochemical reaction, a surface poison is produced, identified as chemisorbed carbon monoxide, which if not removed at more positive overpotentials eventually quenches all electrocatalytic activity.17,18 Besides these purely structural aspects of glucose oxidation, it should be remembered that electrochemical oxidation of organic molecules in general has been intensively studied because the electrical current generated in a “fuel cell” type configuration may be related to the amount of electroactive species present. Hence, a fuel-cell-based sensor incorporating chiral discrimination would have profound implications for electroanalysis and electrosynthesis. Experimental Section The platinum bead electrodes used in these experiments were prepared using the method developed by Clavilier et al.19 Briefly, the end of a 0.5 mm diameter high-purity platinum wire is melted and cooled carefully, without vibration, to generate a singlecrystal bead. The bead is then fixed in position at the center of a goniometer cradle placed approximately 2 m away from a helium-neon laser supported on an optical bench. Diffraction of the incident laser light by the (111) facet edges of the bead crystal allows rapid orientation of the crystal with respect to the grinding/ polishing wheel. After grinding/polishing to a mirror finish using finer and finer grades of diamond paste, an approximated hemispherical form is produced, the flat surface of which is orientated parallel to the required crystal plane. The crystal diameter was approximately 3 mm, and the clean, well-ordered surface was obtained by flame-annealing and subsequent cooling in a H2/H2O atmosphere. Clean transfer of the platinum crystal from the cooling vessel to the electrochemical cell was achieved using a droplet of ultrapure water attached to the electrode to protect the active surface from the ambient and in order to form a meniscus contact with the electrolyte solution. Two identical electrochemical cells for cyclic voltammetry measurements were used in tandem, containing aqueous acidic solutions of D- and L-glucose, the cell design having been described previously.20 Sulfuric acid solutions were prepared from ARISTAR grade concentrated acid (BDH) using ultrapure water from a Millipore Milli-Q system. The D- and l-glucose were purchased from Aldrich and Fluka (99.9% optical purity), respectively. To allow for the effect of mutarotation,21 glucose solutions were prepared at least 12 h prior to any electrochemical measurements to enable time for equilibration between the R- and β-glucose moieties present in aqueous solution. All electrolyte solutions were degassed for 30 min before each experiment using purified argon. Electrochemical potentials are quoted with reference to a saturated palladium-hydrogen electrode in direct contact with the electrolyte.
Results Note on the Conditions Necessary To Observe Chirality in Single-Crystal Surfaces. In the original paper by McFadden et al.,7 a necessary and sufficient condition for observing chirality in single-crystal substrates was to note whether the magnitudes of the step lengths comprising the kink sites were different. If they (16) Popovic, K.; Tripkovic, A.; Markovic, N.; Adzic, R. R. J. Electroanal. Chem. 1990, 295, 79. (17) Llorca, M. J.; Feliu, J. M.; Aldaz, A.; Clavilier, J.; Rodes, A. J. Electroanal. Chem. 1991, 316, 175. (18) Rodes, A.; Llorca, M. J.; Feliu, J. M.; Clavilier, J. An. Quim. Int. Ed. 1996, 92, 118. (19) Clavilier, J.; Armand, D.; Sun, S. G.; Petit, M. J. Electroanal. Chem. 1986, 205, 267. (20) Evans, R. W.; Attard, G. A. J. Electroanal. Chem. 1993, 345, 337. (21) Morrison, R. T.; Boyd R. N. Organic Chemistry, 3rd ed.; Allyn and Bacon: Boston, 1973.
Figure 1. (a) Hard sphere model of Pt{643}S and Pt{643}R surfaces. The {111} (gray), {110} (white), and {100} (black) sites comprising the kink are indicated. The kink edges are also highlighted. (b) Cahn-Ingold-Prelog analogy used to define the absolute stereochemistry of the kink site as either S or R. (c) Relative dimensions of Pt{643}R and Pt{643}S surfaces in comparison with the size of D-glucose.
were, the surface would be chiral. This statement, however, is only partially correct, since it ignores the basic symmetry of individual step and terrace sites ({111}, {110}, and {100}) constituting the kink (see later for Pt{531}). In fact, a detailed analysis of the surface crystallography associated with fcc systems reveals that all single-crystal kinked surfaces are chiral, irrespective of the magnitudes of step lengths comprising the kink (save for meso forms such as “stepped” surfaces of the n(110) × (100) or n(100) × (110) type).22 Hence, a modification to the nomenclature originally introduced in ref 7 and used to define the absolute stereographic configuration of chiral single crystals is proposed in the present study. It is based on an analogy with the Cahn-Ingold-Prelog sequence rules found in introductory textbooks on organic chemistry,21 whereby the groups associated with the stereogenic center are given a particular order of priority. The kink site is viewed from above (i.e. from the vacuum or electrolyte phase). The sequence of sites forming the kink is noted (see Figure 1). If the sequence {111} f {100} f {110} is found to run clockwise, the surface is denoted “R” (from the Latin “rectus”). If however, the sequence {111} f {100} f {110} runs anticlockwise, the surface should be denoted “S” (from the Latin “sinister”). Using this convention, the original assignments made in ref 7 of the kinked silver surface Ag{643} as being Ag{643}S and of the Ag{6 h4 h3 h }surface as being Ag{643}R are preserved. (22) Clavilier, J.; Attard, G. A.; Feliu, J.; Rodes, A. In preparation.
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Figure 2. Pt{211} voltammogram in (a) 0.05 M sulfuric acid + 5 × 10-3 M D-glucose or (b) 0.05 M sulfuric acid + 5 × 10-3 M L-glucose. Pt{332} voltammogram in (c) 0.05 M sulfuric acid + 5 × 10-3 M D-glucose or (d) 0.05 M sulfuric acid + 5 × 10-3 M L-glucose. Sweep rate ) 50 mV/s.
Figure 3. Pt{643}S voltammogram in (a) 0.05 M sulfuric acid + 5 × 10-3 M D-glucose or (b) 0.05 M sulfuric acid + 5 × 10-3 M L-glucose. Pt{643}R voltammogram in (c) 0.05 M sulfuric acid + 5 × 10-3 M D-glucose or (d) 0.05 M sulfuric acid + 5 × 10-3 M L-glucose. Sweep rate ) 50 mV/s. In contrast to Figure 2, a clear enantiomeric difference in oxidation rate at 0.3 V is observed using the kinked electrode surface.
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Figure 4. Electro-oxidation of D- and L-glucose using a highly kinked Pt{531}R electrode. All electrolyte concentrations as in Figure 3. Sweep rate ) 50 mV/s.
Glucose Electro-Oxidation on Platinum. Since the major thrust of this paper is to demonstrate an enantiomeric response in a chiral platinum electrode rather than a detailed exposition of glucose oxidation, only the salient features pertinent to the present discussion in relation to cyclic voltammetry will be outlined. The details of the voltammetric peaks are not paramount; rather, their overall shape should be regarded as a fingerprint of the adsorption processes taking place at a particular singlecrystal orientation. So long as a surface is achiral, whether D- or L-glucose is being adsorbed, the voltammetric curves should be the same. However, when there is a difference in the voltammetry (all other parameters constant), it represents a difference in the kinetics of the reaction and hence an enantiomeric response. Figure 1 also shows a hard sphere model of the Pt{643} surface. The three different types of adsorption sites present at the surface have been highlighted: the {110} monatomic step forming part of the kink, the {100} sites forming the rest of kink, and the {111} terrace sites. Figure 2a is the voltammogram obtained from an acidic solution of D-glucose using a Pt{211} electrode. Pt{211} has the same {111} terrace width as Pt{643}, but the terraces are separated by {100} × {111} steps rather than {310} × {111} steps; that is, Pt{211} is not chiral because it does not possess kink sites. The voltammetry of Pt{211} in sulfuric acid has already been published in ref 23, and the corresponding voltammetry of Pt{643} will be appearing in ref 24. The first positive-going potential sweep corresponding to the initial adsorption and decomposition of D-glucose is very similar to that obtained in ref 18. The peaks at 0.22 and 0.3 V may be ascribed to adsorption of D-glucose at {100} × {111} steps and {111} terraces, respectively.17,18 The peak at 0.5 V arises from D-glucose oxidation at sites influenced by the poisoning intermediate. The {111} terrace sites (peak at 0.3 V) appeared to block more rapidly than step sites. This finding is consistent with an earlier study in which the rate of D-glucose adsorption and oxidation was found to increase with increasing terrace width.17,18 Figure 2b shows the (23) Furuya, N.; Koide, S. Surf. Sci. 1989, 220, 18. (24) Attard, G. A.; Ahmadi, A.; Feliu, J.; Rodes, A.; Herrero, E.; Blais, S. J. Phys. Chem. B, in press.
electro-oxidation of L-glucose under the same condition as before using the second electrochemical cell. All the features reported in Figure 2a are reproduced with very minor variations in intensity. This is also true for electrooxidation of D- and L-glucose on Pt{332} (Figure 2c and d). Pt{332} may alternatively be written as 6{111} × {111}. Hence, it consists of six-atom-wide {111} terraces separated by {111} × {111} steps. The lack of asymmetric kink sites makes Pt{332} an achiral surface. Therefore, as expected, negligible differences in the voltammetry of the surface toward D- and L-glucose oxidation are observed. Because of the wider terrace width exhibited by Pt{332}, the magnitudes of the electro-oxidation currents of 0.3 V (associated with glucose adsorption at {111} terraces) are larger than those for Pt{211}. In addition, the peak corresponding to adsorption at {100} × {111} sites (0.22 V) is completely absent, since “100” sites are not present on Pt{332}, although the initial adsorption of glucose at {111} × {111} steps is observed at 0.07 V. Figure 3 shows the voltammetry derived from electro-oxidation of glucose on Pt{643}S and Pt{643}R. In contrast to the achiral surfaces, the reactivity of Pt{643}S toward glucose manifests itself as a clear enantiomeric response dependent on the precise stereochemical configuration of the glucose molecule. In fact, the oxidation peak at 0.3 V ({111} terraces) appears already to be blocked on the first sweep for L-glucose oxidation on Pt{643}S. Hence, we conclude that the Pt{643}S electrode possesses a greater reactivity toward L-glucose as compared with D-glucose; that is, the Pt{643}S surface is chiral. Figure 3 also shows the corresponding glucose electro-oxidation voltammetry obtained using the enantiomorph of Pt{643}S, Pt{643}R. In contrast to the case for Pt{643}S, it is now the D-glucose stereoisomer which gives rise to the greatest interaction with the {111} terrace sites, as signified by the absence of the peak at 0.3 V. The L-glucose adsorption peak at 0.3 V is clearly visible using Pt{643}R. In fact, D-glucose electro-oxidation on Pt{643}S is identical to L-glucose electro-oxidation on Pt{643}R. Similarly, L-glucose electrooxidation on Pt{643}S is indistinguishable from D-glucose electro-oxidation on Pt{643}R. This finding further sup-
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ports the view that a true enantiomeric interaction is taking place at the surface of the kinked platinum electrode. Figure 4 shows the electro-oxidation of D-and L-glucose using a Pt{531}R electrode. This particular surface is unusual in that it contains only kink sites. Therefore, it does not fall into the category of chiral surface proposed in ref 7 and should not be chiral. However, it is clear from Figure 4 that very pronounced chiral discrimination is occurring, as signified by the difference in rates of electrooxidation (current densities) at 0.31 V. Using the values of current densities at 0.31 V, a diastereomeric excess of ∼80% is evaluated for glucose electro-oxidation on Pt{531}.25 This compares with a value of ∼60% in the case of Pt{643}. We conclude from these calculations that the chiral discrimination scales with the surface density of kink sites. This finding has profound implications for heterogeneous catalysis. Supported metal catalysts contain a large number of defect sites by virtue of their small particle size. For polycrystalline materials, of course, equal numbers of R and S type kinks are present at the surface. However, if a chiral promoter is adsorbed onto a polycrystalline substrate, it should be expected that kink sites of a particular stereochemistry will be favored compared (25) Mason, S. F. Molecular Optical Activity and the Chiral Discriminations; Cambridge University Press: Cambridge, U.K., 1982.
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to the enantiomorphic sites. Therefore, enantioselective heterogeneous catalysis may be associated with preferential blocking of kinks of one stereochemistry, leaving the enantiomorphic site free for catalysis. Future studies using kinked single-crystal surfaces should resolve this speculation. Conclusion Using D- and L-glucose electro-oxidation as a probe reaction, it has been demonstrated that single-crystal surfaces orientated to give kink sites are chiral. It represents the first experimental evidence that chiral discrimination is an intrinsic property of kink sites formed on single-crystal surfaces. The realization of an enantiomeric electrochemical response is expected to have significant consequences for electroanalysis and, at a fundamental level, enable researchers to examine chirality at surfaces in a new and controlled manner. Acknowledgment. The work has been carried out under the auspices of the Accion Integrada HB 1996-0187 sponsored by the British Council and the Ministerio de Educacion y Cultura. G.A. and A.A. would also like to acknowledge the financial support of the EPSRC (Grant No. GR/K58982). J.F. and A.R. would like to acknowledge the support of DGES (Grant No. PB96-0409). LA9810915
