J . Phys. Chem. 1989, 93, 2610-2614
2610
Reductive Elimination of Surface-Coordinated Iodine at Platinum Electrodes: The Influence of Codeposited Silver John E. Harris, Michael E. Bothwell, Jose F. Rodriguez, Manuel P. Soriaga,* Department of Chemistry, Texas A & M University, College Station, Texas 77843
and John L. Stickney Department of Chemistry, University of Georgia, Athens, Georgia 30602 (Received: August 3, 1988)
Previous studies have shown that deposition of silver onto an iodine-pretreated platinum electrode results in place exchange between the Ag and I atoms. It has also been demonstrated that I surface-coordinatedon Pt is zerovalent and that its reduction to iodide leads to its desorption from the surface; this reductive elimination process is pH-dependent due to accompanying formation of surface hydrides. In the present investigation, the influence of codeposition of moderately electropositive Ag adatoms on the reductive desorption of I initially surface-coordinated at smooth polycrystalline Pt electrodes in aqueous solutions was examined. The study, based upon thin-layer electrochemical methods, focused on the potential and pH dependencies of the surface coverage of iodine TI when the amount of electrodeposited Ag was increased from half a monolayer to 20 layers (bulk). The main findings of this study are as follows. (i) The reductive elimination of iodine in the presence of bulk Ag is pH-independent, as expected since Ag does not dissociatively chemisorb hydrogen. (ii) Complete desorption of I was difficult to achieve in the presence of bulk Ag; at -1.2 V, only 70% of the I was removed from the surface. (iii) The reductive in the presence of bulk Ag was determined to be -1.1 V (AgCI reference). This means that desorption potential EoI(ads) the ratio of the surface-complex formation constants Kf,I/Kf,l-is approximately 3 X lo3*and that chemisorption of iodide onto bulk Ag is an oxidative addition process. (iv) Codeposition of half-monolayer Ag yielded results which were intermediate between those for pure Pt and bulk Ag. (v) rIvs E data measured after deposition of 2 monolayers of Ag were identical with those for bulk Ag, signifying that 2 monolayers of Ag behave just like bulk material. (vi) Electrodeposition of 1 monolayer of Ag exhibited unique rIvs E plots which indicated that the bimetallic interface consisting of one Ag monolayer on a Pt substrate possesses surface coordination and electrochemical properties distinct from those of either of the pure metals.
\
Introduction We have been pursuing studies of the interaction of surfaceactive and reversibly electroactive moieties with noble-metal electrocatalyst surfaces.’” Our interest in these systems stems from the fact that chemisorption-induced changes in the redox properties of the subject adsorbates yield important information concerning the coordination, electrochemical, and catalytic properties of the electrode surface. For example, the alteration in the standard potential of a redox center is a measure of the surface coordination strength of the oxidized form relative to the reduced state; this behavior is expected to depend upon the electrode material. In our studies, we have made extensive use of the iodide ion to probe the interfacial properties of the noble-metal electrodes Ir3, Pt’, and AuZsince it is strongly surface-active and its reversible redox activity in the solution phase is well documented. On these three metals, aqueous iodide undergoes spontaneous oxidative addition or chemisorption to form a close-packed monolayer of zerovalent iodine, the saturation coverage of which is limited by the van der Waals radius of the iodine Elimination of iodine from these surfaces can be achieved by its reduction to the anion either by application of sufficiently negative potentials or by exposure to ample amounts of hydrogen gas. On Pt and Ir, the reductive elimination of iodine is coupled with reductive chemisorption of hydrogen; consequently, the overall reaction is a two-electron, pH-dependent process. From a plot of E,/*, the potential at which the iodine coverage is decreased to half its maximum value, against pH, it was determined that the redox potential for the I(ads) I-(ads) reaction is 4 . 3 6 V (AgCI reference) at Pt and -0.32 V at Ir. On Au, where formation of surface hydrides is not favorable, the iodine reductive desorption process is a pH-independent, one-electron reaction for which is -0.50 V. When compared to the potential for the Z2(aq) I-(aq) redox couple, 0.40 V, these values indicate that surface coordination of zerovalent iodine is overwhelmingly favored over iodide.’-3
-
-
*To whom correspondence should be addressed
0022-3654/89/2093-2610$01.50/0
The present study reexamines the iodine cathodic stripping reaction at smooth polycrystalline Pt as it is affected by electrodeposition of silver adatoms. This study has been motivated from the fact that when silver is electrodeposited onto an I-pretreated Pt surface, place exchange between the I atoms and the incoming silver occurs such that iodine is always the outermost layer of the bimetallic surface.’ It thus becomes an important extension to the above-cited studies’-3 to investigate how the reductive desorption of iodine from Pt is perturbed as the amount of codeposited silver is progressively increased from submonolayer to multilayer (bulk) coverages. Such a study is expected to provide valuable insights as to when the properties of the electrode interface changes from that of pure Pt to that of bulk Ag as the electrodeposition process progresses.
Experimental Section Smooth polycrystalline platinum electrodes were utilized in this study. Experimental measurements were based upon thin-layer electrochemical methods. The fabrication of the thin-layer cells* and the preparation of smooth electrode surfacesgwere as described in the literature. Removal of electrode-surface contaminants between experimental trials involved sequential electrochemical oxidation at 1.2 V [AgCI (1 M C1-) reference] and reduction at ( 1 ) Bravo, B. G.; Rodriguez, J. F.; Mebrahtu, T.; Soriaga, M. P. J . Phys. Chem. 1987, 91, 5660. ( 2 ) Rodriguez, J . F.; Soriaga, M. P. J . Electrochem. SOC.1988, 135, 617. (3) Rodriguez, J. F.; Bothwell, M. E.; Harris, J. E.; Soriaga, M. P. J. Phys. Chem. 1988. 92. 2703. ~1
~
~~
(4) Bravo, B. G.; Mebrahtu, T.; Soriaga, M. P.; Zapien, D. C.; Hubbard, A. T.; Stickney, J. L. Langmuir 1987, 3, 595.
(5) Mebrahtu, T.; Berry, G. M.; Bravo, B. G.; Michelhaugh, S. L.; Soriaga, M . P.Langmuir 1988, 4, 1147. (6) Bothwell, M. E.; Rodriguez, J. F.; Soriaga, M. P. J . Elecrroanal. Chem., in press. ( 7 ) Hubbard, A. T.; Stickney, J. L.; Rosasco, S. D.; Soriaga, M. P.; Song, D. J . Electroanal. Chem. 1983, 150, 165. (8) Hubbard, A. T. Crir. Reu. Anal. Chem. 1973, 3, 201. (9) White, J. H.; Soriaga, M. P.; Hubbard, A. T. J . Electroanal. Chem. 1984, 177, 89.
0 1989 American Chemical Society
Surface-Coordinated Iodine at Pt Electrodes
The Journal of Physical Chemistry. Vol. 93, No. 6, 1989 2611
-0.2 V in 1 M H2SO4. Surface cleanliness was verified by cyclic voltammetry? Experiments were carried out in 1 M H2S04 (taken as pH 0), I M NaC104 buffered at pH 7 with NaOH/NaH2P04,10 and in 1 M NaCIO4 buffered at pH 10 with Na2C03/NaHC0310; all aqueous solutions were prepared with pyrolytically triply distilled water." Surface coordination of iodine onto the Pt surface was carried out by exposing the clean electrode to a buffered 1 mM NaI for 180 s. Unreacted iodide was removed by rinsing the thin-layer cell with supporting electrolyte. The absolute surface coverage of iodine rI was determined by thin-layer coulometry in 1 M H 2 S 0 4by oxidation of the surface at 1.2 (during which all the chemisorbed iodine is oxidized to aqueous iodate) followed by coulometric reduction at 0.7 V (during which the aqueous iodate is reduced to aqueous iodine). rr is given by the following equation'-3
r~= (Q - Qb)red,103-/5FA
"
'
~
"
'
I
"
'
I
"
'
I
"
'
I
'
'
'
"
"
8-
4 -
4 .-.
0-
-4
-
-8
-
(1)
where (Q - Qb)rcd,I03- is the background-corrected I03-(aq) to 12(aq) reductive charge, F is the Faraday, and A is the surface area (1.08 cm2) measured by underpotential hydrogen deposition9 or iodine chemisorption.12 It is important to mention that this technique of determining rris applicable even if other species are present on the Pt surface. Hence, this same iodate-reduction method was employed when rIwas determined in the presence of electrodeposited silver. Silver electrodeposition onto the I-pretreated polycrystalline Pt electrode was done by using 2 mM AgC104 in 1 M HzSO4. Under these conditions, Ag deposition consists of two underpotential deposition (UPD) peaks at 0 . 6 5 and 0.50 V (peaks 1 and 2, respectively, in Figure 1 ) and a bulk deposition peak at 0.30 V (peak 3 in Figure I)'. The total background-corrected charge (Q - Qb)upD under the two UPD peaks was measured to be 0.203 (6) mC cm-2, which corresponds to a Ag monolayer packing density rA8,UpD of 2.10 (6) nmol cm-2. Deposition of half a monolayer of Ag can be accomplished either by scanning the potential just beyond the first UPD peak or by a single filling of the thin-layer cell at 0.45 V with a solution of Ag'; the concentration CH is given by
V C =~ o . 5 r A g , U P ~= 1.05 x 1 0 - 9 ~
12
(2)
where Vis the cell volume (3.76 KL). Deposition of additional layers of silver was based upon the charge (Q - Qb)UPD. For example, when two layers of silver were desired, the potential was scanned into the bulk peak until the total deposition charge was twice that of (Q - Qb)UPD;deposition of 20 layers of Ag was achieved by accumulation of a total reductive charge equal to 20 (Q - Qb)upD. Following the Ag deposition onto the I-coated Pt surface in 1 M H2S04, the electrode was transferred to another H cell containing only supporting electrolyte buffered at the appropriate pH. In the latter solution, the thin-layer cell was rinsed and the electrode held for 60 s at a preselected potential below 0.1 V; at these potentials, no anodic stripping of the electrodeposited Ag is possible. The cell was again rinsed following the potentiostatic control in order to remove any iodide that may have been cathodically desorbed at this potential. The amount of iodine remaining on the surface was then quantitated by using the iodate reduction method (eq 1) described above.
Results Figure 1 shows thin-layer cyclic current-potential curves for a smooth polycrystalline platinum electrode pretreated with a full monolayer of iodine in a solution of 2 mM AgC104 in 1 M H2S04; in this experiment, the potential was initially scanned in the (10) Weast, R. C. Handbook of Chemistry; CRC Press: Boca Raton, FL, 1986. (1 1) Conway, B. E.; Angerstein-Kozlowska, H.; Sharp, W. B. A.; Criddle, E . E. Anal. Chem. 1973, 45, 1331. (12) Rodriguez, J. F.; Mebrahtu, T.; Soriaga, M. P. J. Electroanat. Chem. 1987, 233, 283.
-12
~
~
0.2
"
0.4
0.6
"
0.8
~
1.0
1.2
"
1.4
E N vs. AgCl Figure 1. Thin-layer cyclic current-potential curves for electrodeposition of Ag from a solution containing 2 mM AgC104 in 1 M H2S04onto a smooth polycrystalline platinum electrode pretreated with a full coverage of iodine. The potential scan was initially in the negative direction starting from 0.85 V. A change of scale in the ordinate should be noted for the anodic scan above 0.85 V. The peak numbers are as discussed in the text. Volume of thin-layer cell, V = 3.76 p L ; electrode surface area, A = 1.08 cm2; sweep rate = 2 mV/s; T = 298 K.
negative direction starting from 0.85 V. The first two cathodic peaks (peak 1 at 0.65 V and peak 2 at 0.50 V) are independent of the Ag+ concentration and are due to the underpotential deposition (UPD) of Ag onto the I-coated Pt.' Peak number 3 at 0.30 V varies with the Ag+ concentration and is attributed to multilayer (bulk) deposition of silver onto the surface which now contains I over a Ag-plated Pt surface.' Reversal of the potential scan in the positive direction shows the corresponding anodic stripping peaks for the bulk and underpotential deposited silver. At more positive potentials, a large anodic oxidation peak can be seen at 1.1 V due to oxidation of the surface-coordinated iodine (to aqueous iodate) and the formation of surface oxides. The sharp cathodic peak at 0.8 V (peak 4) arises from reduction of aqueous iodate to aqueous iodine and is the basis for quantitation of the coverage rIof chemisorbed iodine (eq 1); it will be mentioned that, at 0.7 V, the Pt surface is still covered with an oxide film which prevents reattachment of 1 or electrodeposition of Ag. The data in Figure 1 demonstrate that (i) electrodeposition of Ag onto an I-pretreated Pt surface does not lead to desorption of the halogen, and (ii) oxidation of the surface-coordinated I to iodate does not occur until all the silver is anodically removed from the surface. As already mentioned above, the total UPD charge under peaks 1 and 2 was measured to be 0.203 (6) mC cmb2 which corresponds to a Ag monolayer coverage rAg,$pD of 2.10 (6) nmol cm-2. rptfor a smooth polycrystalline9 surface is 2.35 nmol cm-2; = 0.89 (3), a value consistent with the fact that hence, rAg/rpt the metallic radius of Ag is 5 % larger than that of Pt.I3J4 The charge after only the first UPD peak, 0.102 (3) mC corresponds to 0.5 monolayer of silver, rAg/rpt = 0.44 (3). The influences of UPD and bulk electrodeposited Ag on the thin-layer current-potential curves for an I-coated Pt electrode are illustrated in Figure 2 for experiments carried out at pH 10. The voltammograms for clean and I-pretreated Pt in the absence of Ag have been discussed previously;' The redox peaks observed for the I-coated surface are due to the reversible reductive elimination/oxidative addition of iodide accompanied by reductive chemisorption/oxidative desorption of hydrogen; the overall process (13) Pauling, L. The Nature of the Chemical Bond; Cornell University Press: Ithaca, NY, 1960. (14) Dean, J. Lange's Handbook of Chemistry; McGraw-Hill: New York, 1985.
~
2612
Harris et al.
The Journal of Physical Chemistry, Vol. 93, No. 6, 1989 11
IiPt
L i
1
09b
mOI
Eb
u -
2 .-0 e Y
4 .-.
07:
.-
05-
-
03-
01
1 -10' -1
.o
-
"
-0.8
"
pHONoAg
-b-
pHlONoAg
--+--
pHOl LAg
---t.
pH71LAg
pH7NoAg
t- pH 101 L A g
E N vs. AgCl
Bulk Ag
"
-
'
I
-0.6
"
"
-0.4
'
"
'
Figure 4. The fractional coverage of iodine BI plotted as a function of potential at pH 0, 7, and 10 for an I-pretreated Pt before and after electrodeposition of 1 layer (L) of Ag. The solid lines interconnect the data points and do not represent any theoretical fit. Experimental conditions were as in Figure 3 .
-0.2
E N vs. AgCl Figure 2. Thin-layer cyclic voltammetric curves in the potential region
near the hydrogen evolution reaction for (i) clean Pt, (ii) I-pretreated Pt, (iii) I-coated Pt electrodeposited with 1 monolayer (UPD) of Ag, and (iv) I-coated Pt electrodeposited with 20 layers (bulk) of Ag in 1 M NaC104 buffered at pH 10. Other experimental conditions were as in Figure 1.
1 m
i
09
m
11
1
1t
f 1 0" t
I 091
m
e$
s2
L
-
i
t
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pH7BulkAg pH l O N o A g
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.-0
I
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-1 4
pH705layerAg
--t
'
-0 1- 1 4
pH 10 0 5 Layer Ag l
-1 0
-06
-0 2
02
06
E N vs. AgCl
Figure 3. The fractional coverage of iodine Or (=FI/rr,max) plotted as a function of potential at pH 0 (1 M H2S04),7 (phosphate-buffered1 M NaC104),and 10 (carbonate-buffered1 M NaC104) for an I-pretreated Pt before and after electrodeposition of half monolayer of Ag. The solid lines interconnect the data points and do not represent any theoretical fit. Experimental conditions were as in Figure 1.
is therefore 2-electron and pH-dependent. The main feature to be noted in Figure 2 is that the peaks associated with the redox-activated adsorption/desorption reactions of iodide are no longer observed when the electrodeposited Ag is present on the surface. The influence of codeposited Ag on the reductive elimination of iodine chemisorbed on smooth platinum can be seen more clearly from coverage vs potential plots at selected pH values. Such plots, expressed as BI vs E , where O1 is the fractional coverage and rI,mx was measured for each of defined by I'I/rr,max O1 vs E curve and averaged 1.1 (1) nmol cm-2, are given in Figures 3 (halfmonolayer Ag), 4 (monolayer Ag), and 5 (bulk Ag). In Figure 3, two portions can be seen in the 4vs E plots: One, in the region from Or = 1 to Or 0.5, obeys the same pH dependence and steep slope of the plots when no Ag is codeposited; the other, in the region 0, C 0.5, is pH-independent and characterized by a less steep slope. When compared to the bulk Ag data given in Figure 5, where the O1 vs E plots are independent of solution pH and less steep, the results in Figure 3 suggest that the surfacecoordinated iodine is bonded to two different surface sites; those on Pt sites give rise to the pH-dependent behavior, and those on
-
'
'
-1 0
-06
-0 2
02
06
E N vs. AgCl Figure 5. The fractional coverage of iodine OI plotted as a function of potential at pH 0, 7, and 10 for an I-pretreated Pt before and after electrodeposition of bulk (20 layers) Ag. The solid lines interconnect the
data points and do not represent any theoretical fit. Experimental conditions were as in Figure 3. Ag sites account for the pH-independent character. It is most interesting to note in Figure 4 that the O1 vs E plots obtained when exactly 1 monolayer of Ag has been deposited are not simple combinations of those for pure Pt and bulk Ag. This suggests that the coordination, electrochemical, and catalytic properties of the bimetallic interface consisting of one Ag monolayer on a Pt substrate are distinct from those of either of the pure metals. Figure 6 compares O1 vs E plots obtained after electrodeposition of 2 and 20 layers of Ag. It is obvious from this figure that codeposition of only two layers of Ag already produces bulklike behavior. Figures 7 and 8 compare the OI vs E plots for submonolayer, monolayer, and bulk codeposited Ag; similarities or differences for these three cases are depicted more clearly in these figures. Discussion An earlier study of silver deposition onto an I-pretreated Pt( 1 11) single crystal showed that three exceedingly sharp underpotential deposition (UPD) peaks precedes the bulk deposition of Ag;' the third UPD peak is very close and barely resolved from the bulk peak. In the present study using polycrystalline surfaces, only two UPD peaks have been observed; since the UPD peaks on polycrystalline Pt are broad, the third UPD peak is probably unresolved from the bulk deposition peak. Support for this conjecture lies in the subsequent positive scan where a shoulder appears in the anodic stripping peak for bulk Ag. The amount of silver, expressed as rAg/rpt(lll), deposited after the first de-
Surface-Coordinated Iodine at Pt Electrodes
1.1
The Journal of Physical Chemistry, Vol. 93, No. 6,1989 2613
L
1.1
0.9 -
P
0.7
-
0.5 pH 0 Bulk Ag pH 0 2 Layers Ag pH 7 Bulk Ag pH 7 2 Layers Ag pH 10 Bulk Ag pH 10 2 Layers Ag
-2-
-
-E-
.--+--A -
4 + +
0.1
b -0.1
-1 .o
-1.5
-0.5
0.0
0.5
E N vs. AgCl Figure 6. The fractional coverage of iodine OI plotted as a function of potential at pH 0, 7, and 10 for an I-pretreated Pt after electrodeposition of 2 and 20 monolayers of Ag. The solid lines interconnect the data points and do not represent any theoretical fit. Experimental conditions were as in Figure 3 .
m -
09 -
07-
-
05-
-t-
03-
--t
011
-0.1
.1 4
;
,
,
-1 0
,
,
,
,
-0.6
,
,
,
,
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,
pH 0 0.5 Layer Ag
pH 7 0.5 layer Ag pH 10 0.5 Layer Ag
pH OEulkAg pH 7EulkAg pH 10BulkAg ,
,
,
0.2
,
,
,
,I 0.6
E N vs. AgCl
-0.1
-1.5
-1 .o
pHO2OLAg pHOlLAg pH720LAg pH71LAg pH1020LAg pH101 LAg
-0.5
0.0
0.5
E N vs. AgCl Figure 8. The fractional coverage of iodine O1 plotted as a function of potential at pH 0, 7, and 10 for an I-pretreated Pt after electrodeposition of 1 and 20 monolayers (bulk) of Ag. The solid lines interconnect the data points and do not represent any theoretical fit. Experimental conditions were as in Figure 3.
plexes which might be postulated in view of the fact that AgC1, complexes have been observed in surface-enhanced Raman experiments. On polycrystalline surfaces, a similar structure probably exists in the absence of applied potential, since the most stable coadsorbed layer on both single-crystal and polycrystalline surfaces is formed when the Ag and I atoms are present in equimolar amounts, a condition which only exists just after the first UPD peak. The existence of pH-independent and pH-dependent portions in the O1 vs E plots for half-monolayer Ag (Figures 3 and 7), however, suggests surface reconstruction at sufficiently negative potentials. The implication of the data in Figures 3 and 7 is that the iodine is being reductively desorbed from two different surface sites: those being eliminated from Pt sites give rise to the pH-dependent behavior, whereas those desorbed from Ag sites account for the pH-independent character: I on Ag pH-independent reductive desomtion
Figure 7. The fractional coverage of iodine O1 plotted as a function of potential at pH 0, 7, and 10 for an I-pretreated Pt after electrodeposition of half monolayer and 20 monolayers (bulk) of Ag. The solid lines
1on Pt: pH-dependent reductive desorption
interconnect the data points and do not represent any theoretical fit. Experimental conditions were as in Figure 3.
position peak, based upon coulometric and Auger electron spectroscopic (AES) measurements, is equal to 0.46 (2); the amount remains constant at 0.46 (2). In comparison, of iodine, I'I/I'R(III), the values obtained in the present investigation are rAg/rPt = 0.44 (3), and rl/rR= 0.46 (5). It is interesting and important to note that the surface coverages of Ag and I after the first UPD peak are, within experimental error, identical. After the second deincreases to 0.83 (2); this corresponds position peak, I'Ag/I'R(lll) to the deposition of 1 monolayer of Ag. In the present study, a value of 0.88 (3) was measured after the second UPD peak. Good agreement between the polycrystalline and single-crystal studies, insofar as the first two UPD peaks is concerned, is thus evident. Based upon low-energy electron diffraction (LEED) and Auger electron spectroscopic (AES) data, it has been postulated15 that the entire surface layer formed after the first deposition peak on I-coated Pt( 11 1) is uniformly that of the (1 11) plane of silver iodide in the so-called zinc blende structure. In this model, only Ag-I bonding is possible. The absence of countercations on the I-Ag-Pt interface precludes the existence of AgI; surface comA.
(15) Stickney, J. L.; Rosasco, S.D.; Soriaga, M. P.; Song, D.; Hubbard, T.Surf. Sci. 1983, 130, 326.
-
It is critical to realize that the pH-dependent portion of the 0.5; at lower Or,the 8, vs E plots become pH-independent. This trend is most significant since it can be shown from surface coverage calculationsI2 that, if the half-monolayer Ag is aggregated as islands, then half of the chemisorbed iodine would lie on top of the Ag domain (I'lA/I'lm = 0.5) while the other half remains on the Pt substrate (I'I,R/rlsnax = 0.5). The driving force for reconstruction from a uniform zinc blende Ag-I surface structure to patches of Ag-I and Pt-I domains could arise from the instability of the Pt sites toward reductive elimination of I and the formation of surface hydrides at rather negative potentials. An increase in the electron density of the electrode may also enhance island formation (and bulklike behavior) of submonolayer Ag in view of the fact the electrodeposition process can be considered in terms of the following processes: vs E plots terminates when O1
nM+ + ne-
-
nM(ads)
-
M,
(3)
2614
J . Phys. Chem. 1989, 93, 2614-2620
Potential excursions in the cathodic direction would shift the equilibrium to the right, favoring the formation of the M, aggregate whose interfacial properties should not be very different from those of the bulk material. The Hz(g) evolution reaction on Pt is a well-known example of the reaction depicted in eq 3. The earlier cited studies of Ag deposition onto an I-coated have shown that codeposition of a full Pt( 1 1 1) single monolayer of Ag results in total place exchange between the Ag and I adatoms. Since the iodine is now completely the outermost layer, formation of direct I-Pt bonds is prohibited. In such a case, one might expect that the resulting 81 vs E plots would be very similar to those for bulk Ag. This is not the case, however, as can be seen from the data in Figure 8. The unique action of monolayer Ag is most evident at pH 0 where removal of the first 20% of the surface-coordinated iodine occurs even sooner than at pure Pt. These results can only mean that the electrochemical properties of the bimetallic interface consisting of one Ag monolayer atop a Pt substrate are distinct from those of either of the pure metals. The nonbulk behavior of monolayer Ag is not unexpected since, as evidenced by the existence of two UPD peaks, the adsorbate-substrate (Ag-Pt) interactions are stronger than the adsorbate-adsorbate (Ag-I and Ag-Ag) interactions. Consequently, the Ag-I binding strength for monolayer Ag is weaker than for bulk deposited material where the innermost Ag-Pt interactions are no longer felt by the outermost Ag-I layer. Closer inspection of the Orvs E data for monolayer Ag in Figure 4 reveals a trace of pH dependence in the reductive desorption of the first 20% of the surface iodine. This result suggests that a small but nonnegligible interaction occurs between the outer I atoms with the inner Pt substrate despite the fact that a gap of one Ag monolayer exists between them. Such through-the-silver interaction is anticipated to vanish when a second layer of Ag atoms is electrodeposited. This expectation is indeed borne out by the data in Figure 6 which compare 8, vs E plots for 2 and 20 layers of Ag. The evidence is indisputable that two layers of codeposited Ag behave just like bulk Ag. From Figure 6, one can
readily extract EoI(,&),the redox potential for the following surface rea~tionl-~ I(ads)
+ e-
-
I-(ads)
(4)
from the potential at which OI = 0.5; this EoI(ads,) value for bulk Ag is -1.1 V. In comparison, Eo1(,d,) for Ir3, Pt , and AuZhave been measured to be -0.32, -0.36, and -0.50 V, respectively. The where the latter term is the redox difference [EoI(ads)potential for the I,(aq)/I-(aq) couple, 0.40 V, is a measure of the relative chemisorption strengths of zerovalent iodine and iodide.'" A(AGo) E AGo1(,k) - AG0I-(,ds) nFIEoI(ads)- EoI(aq)l
- AGod ( 5 )
In eq 5 , AGoI(,dS! and AGoI-(ads)are the respective free energies of adsorption of iodine and iodide, and 2AGod is the energy in2I(aq) dissociation, = 150 kJ/mol.I6 volved in the Iz(aq) Application of data from Figure 6 to eq 5 yields A(AGo) = -200 kJ/mol; that is, the surface-coordination strength of zerovalent iodine is greater than that of iodide by about 220 W/mol. A(AGo) in turn can be used to calculate the ratio of the surface-complex formation constants for zerovalent iodine and iodide, Kf,I/Kf,I:
-
Kf,I/Kf,I- = exp[-A(AGO)/RT]
(6)
The ratio Kf,I/Kf,I-is approximately 3 X lo3*which signifies an overwhelming preference for surface coordination of zerovalent iodine over iodide. Acknowledgment. Acknowledgment is made to the Robert A. Welch Foundation and the Regents of Texas A&M University through the AUF-sponsored Materials Science and Engineering Programs for support of this research. Registry No. I*, 7553-56-2; Ag, 7440-22-4; Pt, 7440-06-4. (16) Cotton, F. A.; Wilkinson, G.Advanced Inorganic Chemistry; Wiley: New York, 1980.
Prediction of Velocity Cross-Correlation Coefficients of Binary Liquid Mixtures Thomas M. Bender7 and R. Pecora* Department of Chemistry, Stanford University, Stanford, California 94305 (Received: February 8, 1988; In Final Form: September 16. 1988)
Velocity cross-correlation coefficients (VCC's) may be used as measures of the coupling between the velocities of different molecules in binary liquid mixtures. The hydrodynamic expression for the VCC's proposed by Friedman and Mills is used to derive expressions for the VCC's in terms of molecular diameters and the Kirkwood-Buff parameters. The main approximation is the use of the Ornstein-Zernike form for the liquid radial distribution functions supplemented by a small separation cutoff. An ideal reference system for the VCC's is proposed in terms of hypothetical ideal Kirkwood-Buff parameters. Experimental VCC's, experimental Kirkwood-Buff parameters, theoretical VCC's using our expression, and reference VCC's are computed from literature data for eight binary liquid systems ranging from nearly thermodynamically ideal to very nonideal. Good agreement is shown between the experimental and predicted VCC's over the entire composition range of these mixtures.
Introduction There are many common binary liquid mixtures for which abundant information is available regarding the system thermodynamic properties and diffusion coefficients. In some cases, it has been possible for specific links to be established between these various static and dynamic properties. In other cases, however, our understanding of the quantitative and qualitative relation between these variables is lacking. Some success has been had in reconciling various of these properties of certain binary liquid 'Present address: Rohm and Haas, Spring House, PA 19477.
0022-3654/89/2093-2614$01.50/0
mixtures by the use of kinetic aggregation models (e.g., clathrate-type models for the tert-butyl alcohol/water system or the 2-butoxyethanol/water system's2). The question must be raised, however, as to whether multistep aggregation, system-specific models are desirable ways for describing binary liquid behavior. In this paper we present a new model relating thermodynamic to diffusion properties of binary liquid mixtures. Through the (1) Ito, N.; Saito, K.; Kato, T.; Fujiyama, T. Bull. Chem. Sac. Jpn. 1981, 54, 991. (2) Bender, T. M.; Pecora, R. J. Phys. Chem. 1986, 90, 1700.
0 1989 American Chemical Society
