Langmuir 1990,6, 74-81
74
Probing the Electrocatalytic Properties of Bimetallic Interfaces by Chemisorption of Redox-Active Species? George J. Cali, John E. Harris, Michael E. Bothwell, and Manuel P. Soriaga*” Department of Chemistry, Texas A&M University, College Station, Texas 77843 Received June 26, 1989 The electrocatalytic properties of (i) a smooth polycrystalline Au90Pt,o alloy interface and (ii) Ag-onPt electrodeposited overlayers have been characterized via the changes in the redox reactivities of iodine and 2,5-dihydroxythiophenol (DHT) when these materials are chemisorbed onto the respective bimetallic surfaces. This approach is possible since the redox properties of I and DHT in the chemisorbed state are strongly dependent upon the metallic composition of the electrode surface. Experimental measurements were based upon thin-layer voltammetry and coulometry. The results accumulated thus far suggest that the electrocatalytic properties of the subject bimetallic interfaces are largely dependent upon the relative strengths of interaction between the reactant and the individual components of the mixedmetal surface.
Introduction The introduction of a foreign metal into a metallic catalyst is a well-established procedure for modifying surface electronic and geometric structures in order to manipulate catalytic selectivity.’ Although the study of bimetallic catalysts a t gas-solid interfaces has been pursued extensively,’!’ parallel investigations at the electrodesolution interface are not as abundant. Attempts have been made to determine the surface composition and reactivity of platinum group elements alloyed with the coinage metals by cyclic ~ o l t a ” e t r y . ~ For - ~ example, in studies with VIIIA-IB bimetallic alloys, the amount of chemisorbed hydrogen has been correlated with the surface composition of the group VIIIA component on the surface since hydrogen chemisorption occurs on these metals but not on group IB electrode^.^-^ It is now well-accepted that the catalytic properties of bimetallic systems are governed primarily by the composition and structure at the mixed-metal interface.’ As shown in Figure 1, at least three types of surfaces can arise when two different metals are mixed together, depending upon the native electronic and geometric characteristics of the individual components. Type I represents a true bimetallic alloy in which the metals M, and M, are homogeneously distributed within the bulk and on the surface. Type I1 is for a bimetallic catalyst in which the surface is enriched with one of the metals. Type I11 corresponds to a catalyst, such as the Cu-Ru system,6 in which the two metal components are completely immiscible. An interface similar to that of type I11 can be formed by electrochemical deposition of one metal onto another even if both metals form bulk alloys * Author t o whom correspondence should be addressed. Presented a t the symposium entitled “Photoelectrochemical and Electrochemical Surface Science: Microstructural Probes of Electrode Processes”, sponsored jointly by t h e Divisions of Analytical Chemistry a n d Colloid and Surface Chemistry, 197th National meeting of t h e American Chemical Society, Dallas, April 9-14, 1989. Presidential Young Investigator. (1) Sinfelt, J. H. Bimetallic Catalysts; Wiley: New York, 1983. (2) Bond, G. C. Catalysis B y Metals; Academic Press: New York, 1962. (3) Woods, R. Electrochim. Acta 1971, 16, 655. (4) Breiter, M. W. J . Phys. Chem. 1965, 69, 901. (5) Rand, D. A. J.; Woods, R. J . Electroanal. Chem. 1972, 36, 57. (6) Goodman, D. W.; Stuve, E. M. In Electrochemical Surface Science; Soriaga, M. P., Ed.; American Chemical Society: Washington, DC. 1988.
*
provided the electroplated interface is not subjected to high temperatures. The interfacial properties of type I1 catalysts generally follow those of the surface-segregated metal, especially if the M,-enriched film is fairly thick. Types I and 111, on the other hand, present far more interesting possibilities since the properties of such interfaces are expected to be hybrids of those of the individual metals. Our research interests lie in electrode surfaces of these two types. Over the past few years, we have been studying the influence of electrode material on the chemisorption-induced changes in the reversible redox and related properties of selected organic and inorganic reagent^.^ Our studies have established the fact that the electrochemical properties of species in the chemisorbed state are profoundly dependent upon the composition of the electrode surface. For example, on Ir, Pt, and Au, iodide in aqueous solutions undergoes spontaneous oxidative chemisorption to form a close-packed monolayer of zerovalent iodine. Desorption of iodine from these surfaces can be effected by reduction a t sufficiently negative potentials in which the chemisorbed iodine is converted to aqueous iodide ions; the redox potential EoI(ads) for the I(ads) I-(aq) reaction was found to decrease in the order Ir > P t > A u . ~In another set of studies, it was found that the reversible redox properties of 2,5-dihydroxythiopheno1 (DHT) were dependent upon the electrode material.8 As an example, the redox peak width of DHT chemisorbed at full coverage was much larger on P t than on Au; this result was interpreted in terms of substratemediated DHT-DHT interactions occurring at the Pt electrodes8 The fact that the reversible redox properties of adsorbed I and DHT are strong functions of the composition of the electrode surface makes it attractive to employ these and similar reagents as electrochemical probes of the electrocatalytic properties of bimetallic interfaces. In the present paper, we present results which lend validation to this particular approach. We describe here data obtained by using I and DHT as redox-active probes in surface electrochemical studies of the following bimetal-
-
(7) Soriaga, M. P. In Electrochemical Surface Science; Soriaga, M. P., Ed.; American Chemical Society: Washington, DC, 1988. (8) Mebrahtu, T.; Berry, G. M.; Bravo, B. G.; Michelhaugh, S. L.; Soriaga, M. P. Langmuir, in press.
0 1990 American Chemical Society
Electrocatalytic Properties of Bimetallic Interfaces
Langmuir, Vol. 6, No. 1, 1990 75
000 I
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111
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Figure 1. Schematic illustration of three types of bimetallic electrocatalysts. lic interfaces: (i) A@tl0 alloy and (ii) Ag overlayers elecis a true alloyg and repretrodeposited on Pt. Au,Pt,, sents a type I bimetallic interface, whereas the unannealed Ag-on-Pt electrodeposited overlayers provide type 111 bimetallic interfaces.
Experimental Section The electrode materials were purchased from The Wilkinson Co. (Thousand Oaks, CA). The mnnometal electrodes were of 99.99% purity by weight, whereas the gold-platinum alloy consisted of 90.0% Au and 10.0% Pt by atomic composition (Au,Pt,,). Experimental measurements were based upon thinlayer electrochemistry. The fabrication of the thin-layer cells has been described elsewhere.1° The preparation of atomically smooth Au,Pt,, electrode surfaces followed the procedure outlined previously for the pure metals.'' After metallographic polishing, the alloy electrode was treated in hot, concentrated HNO, and then thermally annealed hy heating to dull redness in a flame. In between experimental trials, the electrodes were cleaned in 1 M H,SO, by sequential oxidation at potentials just helow that for the oxygen evolution reaction (1.2 V for Pt; 1.35 V for the Au-based electrodes) and reduction at potentials just helow that for the hydrogen evolution reaction (43.2 V for Pt; 4.35 V for both Au and A%Pt,,); all electrode potentials were referenced against a Ag/AgCI (1 M C11 electrode. The surface area for the Pt electrode (1.18 cm') was determined by underpotential hydrogen chemisorption," while those for the Aucontaining electrodes (1.10 cmz for Au, 1.12 emz for A%Pt,,) were based upon the iodine chemisorption method." Surface roughness facton helow 1.1can he maintained if potentials above the oxygen evolution reaction are not applied." Experiments done at pH 0 employed 1 M H,SO, as supporting electrolyte, whereas those performed at pH 7 and 10 utilized appropriately phosphatehuffered 1M NaCIO, electrolyte. All solutions were prepared with pyrolytically triply distilled water.13 Preparation of Ag overlayers on Pt was done hy electroehemical deposition from AgCIO, in 1M H,SO,. Monolayer or lower coverages were obtained by underpotential deposition; deposition from single thin-layer cell aliquots of dilute AgCIO, was used to prepare submonolayer Ag coverages." Multilayer Ag electrodeposits were formed by using multiple thin-layer cell aliquots of AgCIO,, the concentration C, of which was such that a single aliquot would lead to exactly one layer of electrodeposit:" C, = Ar,/V
(1)
where A is the surface area of the Pt electrode, r, the packing density of the underpotentially deposited Ag, and V the volume of the thin-layer cell. Formation of n Ag overlayers thus required electrolysis of n aliquots of this AgCIO, solution. Pretreatment of the electrode surfaces with a full monolayer of iodine or 2.5-dihydroxythiophenol (DHT) consisted simply of rinsing the clean electrodes with a M aqueous solution (9) Rinorv Alloy and Phase Diwroms: Massalaki. T. B.. Ed.: American Society for Mktals: Metals Park, OH,1986. (IO) Hubbard, A. T. Crit. Reo. Anal. Chem. 19?3,3,201. (11) White. J. H.: Soriaea. " . M. P.: Hubbard. A. T. J . Electraonal. Chim. 1984, 177.89. (12) Rudriguez. J. F.; Mebrahtu. T.; Soriaga. M.P. J. Electroonol. rhem 77 .. 7, 7RR ... .. 1 .9 .8.77 ., . . . .. . (13) Conway. B. E.; Angerstein-Kalowska. H.;Sharp,W. B. A,; Criddle, E. E. A n d . Chem. 1913.45, 1331. (14) Harris, J. E.; Soriaga, M.P. Electroehim. Acta.
In press.
10
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Figure 2. Thin-layer current-potential curves in the hydrogen chemisorption/evolution region in pH 10 buffered 1 M NaCIO, for (i) clean, smooth polycrystalline Pt, (ii) Pt after deposition of 0.5 layer of Ag, (iii) P t after deposition of 1.0 layer of Ag. and (iv) Pt after deposition of 20 layers of Ag. Volume of the thin-layer cell, V = 3.76 rL. Area of electrode. A = 1.14 cm2. Sweep rate, r = 2 mV/s. Temperature, T = 298 K. of the adsorbate. The surface coverage of iodine was determined by the iodate reduction method in which TI,the amount of chemisorbed iodine, is given by the electrolytic charge for the reduction of aqueous IO,., formed from prior anodic oxidation of [(ads) to aqueous I,:
ri = (Q - Qb)rrd,ioS-/5FA
(2)
where Qh is the background charge obtained in the absence of surface iodine. A t full coverages on Pt and Au, DHT is bound exclusively through the sulfur end, and the pendant diphenol displays quinone/diphenol redox activity similar to that in the unadsorbed state. On the basis of this reactivity, rDmcan he extracted from the following equation: (3)
In this equation, (Q - Qh)elis the electrolytic charge for the quinone/diphenol redox reaction of the chemisorbed DHT measured in the absence of unadsorhed species?
Results and Discussion I. Ag Overlayers on Pt. A. Iodine Chemisorption. Figure 2 shows thin-layer current-potential curves in the hydrogen evolution region for (i) a clean Pt electrode, (ii) a Pt electrode electrodeposited with one-half monolayer of Ag, (iii) a Pt electrode deposited with one layer of Ag, and (iv) a Pt electrode deposited with 20 layers (hulk) of Ag. As can he seen from this figure, hydrogen chemisorption on the Pt electrode is completely suppressed by the presence of even 0.5 layer of Ag, although reversible hydrogen evolution still takes place at the latter interface. T h e absence of distinct hydrogen chemisorption peaks for the Pt surface half-covered with Ag is surprising since hydrogen is small enough to adsorb on the Ag-free Pt sites. In this sense, the usefulness of hydrogen chemisorption as a diagnostic tool conceming the presence or absence of group IB metal on group VIIIA surfaces is limited since it does not provide quantitative information concerning t h e group IB adatom coverage. However, the data in Figure 2 indicate that, under the present conditions, the submonolayer Ag electrodeposit is uniformly distributed on the Pt surface; that is, islands or aggregates of Ag d o not exist at the interface. T h e results shown in Figure 2 may he compared with those shown in Figures 3-5, which correspond to data on the reductive desorption of Chemisorbed iodine ((I(ads) I-(aq)) at Pt electrode surfaces pretreated with vari-
-
76 Langmuir, Vol. 6, No. I, 1990
Cali et al.
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ous coverages of Ag. Figure 3 shows OI-.VS-Eplots, where 6, is the fractional surface coverage of iodine defined by I'I/rI,max at pH 0, 7 , and 10 for (i) a Ag-free Pt surface, (ii) a Pt surface covered with half a monolayer of Ag, and (iii) a Pt surface deposited with bulk or 20 layers of Ag. Changes on the I reductive desorption reaction brought about by the presence of Ag overlayers are evident in this figure. For example, whereas the cathodic stripping reaction on Pt is pH-dependent due to concomitant hydrogen chemisorption at sites vacated by the iodine ions,7 no such pH-dependent behavior is shown by the surface
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Figure 5. 6 vs E plots for smooth polycrystalline Pt containing 1, 2, anb20layers of electrodeposited Ag. The solid lines connect the experimental points and do not represent any theoretical curve. Other experimental conditions were as in Figure 2.
containing bulk Ag. In addition, two segments in the 0,vs-E plots can be observed for the Pt surface treated with one-half monolayer of Ag. One segment, from 0, = 1.0 to 0.5, is pH-dependent; the other, a t 6, < 0.5, is pH-invariant. The implications of these results are discussed below. Figure 4 compares 0,-vs-E plots at selected pH for Pt surfaces on which (i) Ag is electrodeposited before I chemisorption and (ii) Ag is deposited after I chemisorption. It can be concluded from the data shown that, within the experimental error of &8%, there is no difference in the I reductive desorption reaction when I is present before or after Ag electrodeposition. This is in agreement with earlier studies of well-defined Pt(111)single-crystal electrodes, which showed that Ag deposition onto an Icoated Pt surface always resulted in I being the outermost layer.15 Figure 5 shows the influence of Ag overlayer coverage on the reductive desorption of chemisorbed iodine. The top portion compares 01-vs-E plots at pH 0,7, and 10 for a Pt surface electrodeposited with 1 and 20 layers of Ag; the bottom part compares 61-vs-E plots at the same selected pH for Pt plated with 2 and 20 layers of Ag. The results presented in Figures 3-5 clearly suggest the following: (i) The interfacial properties of a Pt surface deposited with one-half monolayer of Ag are intermediate between those for pure Pt and bulk Ag. (ii) Two monolayers of Ag behave identically with bulk material. (iii) The bimetallic interface consisting of one Ag layer on a Pt substrate possesses unique electrocatalytic properties distinct from those of either of the pure metals. As was mentioned above, the 61-vs-E plots for a Pt surface containing one-half monolayer of Ag (Figure 3) were characterized by two segments, one pH-dependent and the other pH-invariant. Earlier studies of Ag deposition onto I-coated Pt(ll1) surfaces showed that, a t quiescent or equilibrium potentials, the crystallographic structure of the interface consisting of I on Pt covered with one(15) Stickney, J. L.; Rosasco, s. D.; Soriaga, M. P.;Song,D.; Hubbard, A. T.Surf. Sci.
Electrocatalytic Properties of Bimetallic Interfaces
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Figure 6. Thin-layer current-potential curves in the hydrw gen chemisorption/evolution region in pH 10 buffered 1 M NaClO for (i) smooth polycrystalline Pt containing 0.5 layer of Ag, hi) Pt containing 0.5 layer of Ag and full coverage of I, and (iii) Pt containing 0.5 layer of Ag and full coverage of I after reductive desorption of I from the Pt surface sites. Other experimental conditions were as in Figure 2.
Langmuir, Vol. 6, No. I , 1990 77 surface I is an important factor in the negative potential induced surface reconstruction of the Ag submonolayer; in this context, it is relevant to mention that the chemisorption strength of I is much greater on Ag than on Pt.14 (ii) The potential-induced surface reconstruction creates distinct, bulk-like Ag and Pt domains a t the interface. (iii) Hydrogen chemisorption does not provide a potent enough driving force to cause surface reconstruction of the Ag submonolayer a t the same negative potentials. B. 25-Dihydroxythiophenol Chemisorption. Chemisorption of 2.5-dihydroxythiophenol (DHT) at fuil coverages (IIDHT = 0.54 f 0.03 nmol/cm*) on both Pt and Au surfaces results in the identical formation of closepacked layers in which the DHT molecules are attached solely through the sulfur moiety. In this orientation, the diphenolic group is pendant and thus exhibits reversible quinone/diphenol redox at potentials similar to those for unadsorbed DHT.8 Recent studies with aliphatic and aromatic mercapto compound^'^^'^ have demonstrated that the S H group is spontaneously oxidized upon chemisorption: R-SH(aq)
-
R-S(ads)
1 + ZH2(g)
(4)
Hence, the reversible quinone/diphenol reaction for DHT Chemisorbed at full coverages can be represented as
(51
Figure 7. Schematic illustration of the reconstruction of the Ag overlayer during the reductive desorption of iodine at the 0.5 layer of Ag on Pt interface. half monolayer of Ag was uniform. If this structure was kept uniform a t all potentials, the Ops-E plots would not have shown segmented pH-dependent behavior; in fact, the opposite behavior was observed. Surface reconstruction of the Ag overlayer at negative potentials, in which islands or domains of Ag and Pt sites are formed on the surface, is therefore indi~ated.'~Additional information concerning this surface reconstruction can be gleaned from Figure 6, which shows thin-layer cyclic voltammetric curves in the hydrogen chemisorption region for (i) a Pt surface containing 0.5 layer of Ag, (ii) a Pt surface coated with 0.5 layer of Ag and iodine, and (iii) the same I-Ag-Pt interface as in ii but after cathodic removal of chemisorbed iodine at -1.0 V. The peaks just before hydrogen evolution for the I-Ag-Pt interface represent cathodic stripping of the I attached to the Pt surface sites; this reaction is reversible, provided that the desorbed iodide ions are not rinsed away from the thinlayer cell. The voltammogram which results if desorbed I-,aq, ions are removed from the thin-layer cell is characterized by the emergence of a reversible peak a t about 0.6 V. Comparison of this voltammogram with that for clean Pt (Figure 2) suggests that the reversible peak at -0.6 V is due to hydrogen chemisorption on the now Ifree Pt surface sites, as illustrated in Figure 7. This observation should be contrasted with the result previously noted in Figure 2 that, on an I-free Pt surface containing only 0.5 layer of Ag, no distinct hydrogen chemisorption peaks are observed. On the basis of these data, the following conclusions can be made: (i) The presence of
There is one significant difference in the redox activity of the pendant diphenol at Pt and Au surfaces: the redox peak width is much larger a t Pt than at Au. This difference has been attributed to considerableDHT-DHT interactions on Pt which occur through the metal and not through space. The diphenol group itself is strongly reactive toward Pt but is inert toward Au; this difference in adsorbahility is the driving force for the substrate-mediated DHT-DHT interactions on Pt.8 Thin-layer current-potential curves for DHT chemisorbed on Pt at pH 0 and 7 are displayed in Figure 8. The top portion of this figure gives voltammetric curves in t h e potential region where reversible quinone/ diphenol redox occurs a t pH 7 for DHT (i) initially chemisorbed at pH 0 but run a t pH 7 and (ii) directly chemisorbed at pH 7. It can he seen here that the peak for the pH 7 chemisorbed DHT is significantly smaller than for pH 0 adsorbed DHT. One possible explanation for this difference in peak size is that the DHT undergoes self-desulfurization upon chemisorption a t the higher pH. In view of the S H oxidative chemisorption reaction (eq 41, the self-desulfurization reaction may be written as mR,-SH(aq)
-
(m - n)R,-%ads)
+ &(ads) +
nR,H(aq) (6) where R, is C,H,(OH),. Support for this postulate of DHT self-desulfurization upon chemisorption at pH 7 is provided by the anodic oxidation voltammetric curves in the bottom portion of Figure 8. These curves were obtained in 1 M H,S04 for (i) DHT Chemisorbed a t pH (16) Hubbsrd, A. T.Chem. Re". 1988.88.633, (17) Friend, C. M.;Roberts, J. F.Ace. Chem. Res. 1988.21.394.
78 Langmuir, Vol. 6, No. I , 1990
Cali et al.
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Figure 8. (Top) Thin-layer current-potential curves showing (i) reversible quinone/diphenol redox at pH 7 for DHT chemisorbed on smooth Pt at pH 0 and 7. (Bottom) Anodic oxidation in 1 M H SO, of DHT chemisorbed at pH 0 and 7 before and after hy&odesulfurization (HDS) at -0.25 V in 1 M H,SO,. Other experimental conditions were as in Figure 2. 0, (ii) DHT chemisorbed a t pH 0 after hydrodesulfurization (HDS) at -0.25 V in 1 M H,SO,, (iii) DHT chemisorbed a t pH 7, and (iv) DHT chemisorbed a t pH 7 after HDS. It is easy to see in this figure that the peak potential "Ep and the electrolytic charge QOx for the anodic oxidation of pH 7 chemisorbed DHT are intermediate between those for pH 0 adsorbed DHT before and after HDS. As expected, HDS of the residual intact pH 7 adsorbed DHT yields an anodic oxidation voltammogram identical with that €or hydrodesulfurized pH 0 chemisorbed DHT. Figure 9 presents data which summarize the influence of electrodeposited Ag on the self-desulfurization of pH 7 chemisorbed DHT. The top portion of the figure shows complete cyclic voltammograms depicting the quinone/ diphenol redox of intact DHT; the bottom portion presents an expanded view of the oxidation (diphenol-toquinone) part of the reaction. Close examination of the data reveals the following: (i) As little as a quarter monolayer of electrodeposited Ag considerably minimizes the self-desulfurization reaction. (ii) One-half monolayer of deposited Ag suppresses virtually all of the self-desulfurization reaction. (iii) The quinone/diphenol redox peaks are sharper for DHT chemisorbed on the Ag overlayer than on the pure Pt surface; this signifies that the Pt substrate-mediated DHT-DHT interactions are rendered inactive by the presence of Ag adatoms. (iv) A direct correlation exists between the extent of substratemediated DHT-DHT interactions and the degree of DHT self-desulfurization. That the self-desulfurization reaction occurs only a t pH 7 and not a t pH 0 is probably because oxidative chemisorption of the SH group occurs more readily in basic media. 11. AuQOPtl,Interface. A. Iodine Chemisorption. The electrocatalytic selectivity of true bimetallic alloys such as AuQ,Ptlo is not a simple matter to rationalize. A naive expectation would be that the interfacial properties of such alloys are some weighted combinations of the properties of the homogeneously interact-
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ing individual components. Examples which provide useful insights into this question are given in Figures 10 and 11. The upper portion of Figure 10 shows thin-layer voltammetric curves in 1 M H,SO, for (i) pure Pt, (ii) pure Au, and (iii) Au,Pt,,. As is well-known, the voltammetric curves for pure Au and Pt differ significantly from one another. For example, the hydrogen evolution reaction takes place more readily on Pt than on Au. In addi-
Langmuir, Vol. 6, No. 1, 1990 79
Electrocatalytic Properties of Bimetallic Interfaces
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*
tion, dissociative chemisorption of molecular hydrogen occurs on Pt but not on Au, as manifested by the appearance of the two underpotential deposition peaks just prior to the hydrogen evolution reaction. Formation of surface oxides is also retarded on Au relative to Pt. The cyclic voltammogram for the Aug0Pt,, electrode is characterized by the following features: (i) The onset of the hydrogen evolution reaction occurs a t potentials intermediate between those a t pure Au and Pt; the enhancement of the hydrogen evolution reaction relative to Au is brought about by the presence of surface Pt atoms. (ii) Although the hydrogen evolution reaction is promoted by a comparatively small fraction of surface Pt atoms, hydrogen underpotential deposition peaks similar to those for pure Pt could not be detected even on an expanded scale. (iii) The formation of surface oxides on the alloy surface starts at a potential only slightly less positive than that on a pure Au surface. This means that the Pt surface atoms are not oxidized independently of the Au surface atoms. (iv) Reduction of the oxidized AugoPtlo surface yields two peaks: the most prominent occurs at 0.92 V, identical with that a t which the pure Au oxide is reduced. A smaller peak can be seen at 0.50 V, similar to that a t which the oxide formed a t pure Pt is reduced. The relative sizes of these two oxide reduction peaks reflect the 9O:lO Au:Pt ratio in the bulk and on the surface.la The bottom part of Figure 10 shows voltammetric curves for the AugoPtlo electrode in 1 M H,S04 when the solution contains gaseous 0, (solid curve). In this figure, it can be seen that reduction of O,(aq) occurs at a potential very close to that for reduction of Pt surface oxide. In view of this result, it can be asserted that (i) O,(aq) reduction a t this alloy interface occurs principally a t the Pt sites and (ii) for the O,(aq) reduction reaction, the Pt atoms behave independently of the surrounding Au sites. The dotted curve in Figure 10 was obtained when O,(aq) was removed from the solution. The relative sizes of the Pt and Au oxide surfaces are the same as those obtained prior to the O,(aq) reduction experiments, a result which signifies that, even if O,(aq) reduction took place at the Pt sites, no surface segregation of Pt, at least of the irreversible type, occurred during the reaction. Figure 11 displays cyclic voltammograms in 1 M H,S04 of (i) pure Pt, (ii) pure Au, and (iii) AugOPtloafter treatment with a full monolayer of iodine. The voltammetric curves for the I-coated pure metals are quite different. For example, anodic oxidation of chemisorbed (18) Cali, G. J.; Soriaga, M. P. J. Electroanal. Chem. In press.
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Vvvs AgCl
Figure 12. (Top) Thin-layer cyclic voltammetric curves in the hydrogen evolution region for iodine-coated Au, Pt, and Au,Pt,, in 1 M NaClO buffered at pH 7. (Bottom)Superimposed plots of the cathodic current and rIin the hydrogen region for iodine-coated Aug,Ptlq at pH 7. The solid lines serve only
to connect the rI data points and do not represent any theoretical fit. Experimental conditions were as in Figure 10.
iodine to aqueous iodate occurs at about 1.1V on Pt but takes place at 1.2 V on Au. The reduction of aqueous iodate to aqueous iodine is also different at the two monometal surfaces. On Pt, the IO,-(aq) to 12(aq) reaction (0.8 V) occurs before reduction of the surface oxide (0.5 V); in contrast, the same reduction on Au (0.5 V) takes place only after the surface gold oxide has been reduced (0.9 V). The influence of Pt on the cyclic voltammograms of the I-coated AugoPtlo surface is manifested by the following features: (i) The anodic oxidation of the surface iodine starts sooner on the alloy, 1.0 V, than on the pure Au surface, 1.1 V, although the peak potential .Ep itself, 1.2 V, is unchanged. (ii) The IO
