2702
J . Phys. Chem. 1988, 92, 2102-2106
Redox-Actlvated Adsorptlon/Desorption Processes: IodineiIodide at Polycrystalline Iridium in Aqueous Solvents Jose F. Rodriguez, Michael E. Bothwell, John E. Harris, and Manuel P. Soriaga* Department of Chemistry, Texas A&M University, College Station, Texas 77843 (Received: August 18, 1987; In Final Form: November 13, 1987)
The interaction of aqueous iodide with polycrystalline iridium has been studied as a function of solution pH and electrode potential; experimental measurements were based on thin-layer electrochemical techniques. The principal findings of the study are as follows: (1) Iodide is spontaneously and oxidatively chemisorbed on Ir as zerovalent iodine. (2) The open-circuit limiting coverage of iodine is dependent upon the pH at which chemisorption is carried out; the limiting coverage is lower at higher pH, postulated to occur due to coadsorption of hydroxo species, but the changes in coverage resulting from changes in pH are reversible. (3) Complete reductive desorption of iodine from the Ir surface is attained by application of ample amounts of hydrogen and/or sufficiently negative potentials. (4) The cathodic stripping of iodine is a pH-dependent process since it is coupled with reductive chemisorption of hydrogen. (5) In molar acid, desorption of iodine occurs after hydrogen gas is evolved, whereas in basic solutions, removal of iodine occurs before evolution of hydrogen gas. ( 6 ) The redox potential for the couple is about 720 mV negative of the redox potential for the 12(aql/I-(aq)couple; this large negative shift indicates that zerovalent iodine is more stable than iodide in the chemisorbed state by about 145 kJ/mol.
Introduction Extensive studies' of the interaction of polycrystalline and single-crystal platinum surfaces with aqueous iodide and gaseous hydrogen iodide have demonstrated that iodide is oxidatively chemisorbed on Pt as zerovalent iodine, the surface structures of which are identical with those formed from the dissociative chemisorption of molecular iodine.2 We recently examined the redox activity of this chemisorbed zerovalent iodine species at smooth polycrystalline Pt in protic3 and aprotic4 solvents. In aqueous media, it was found that reduction of the adsorbed iodine to iodide results in desorption of the ionic species accompanied by the reductive chemisorption of hydrogen atoms. Consequently, the overall cathodic stripping process is a two-electron, singleproton r e a ~ t i o nthis ; ~ coverage-dependent process is reversible in the sense that oxidative removal of chemisorbed hydrogen immediately leads to oxidative readsorption of iodide as iodine atoms. This pH-dependent behavior also occurs at single-crystal Pt electrodes.5 In anhydrous acetonitrile, an irreversible one-electron I(ab+I-(wl) desorption peak was noted at a potential 0.91 V more negative than a reference 12~sol~-to-I~~s,,~~ reduction peak;3 the nonreversibility of the stripping process is due to irreversible adsorption of (undetermined) acetonitrile-derived species following iodide desorption. The large adsorption-induced negative shift in the iodine/iodide redox potential indicates dramatic stabilization of iodine relative to iodide in the chemisorbed state. The interaction of aqueous iodide with polycrystalline gold has also been studied;6 the initial postulate6bpcthat iodide is oxidatively chemisorbed on Au as zerovalent iodine was recently verified by thin-layer electrochemical experiments that allowed accurate measurements of the absolute surface packing density and redox activity of the chemisorbed Investigations of the reductive desorption of iodine at polycrystalline Au showed that, in contrast to the results at Pt, the stripping reaction is a simple one-electron, pH-independent process; presumably, the pH in(1) (a) Lane, R. F.; Hubbard, A. T.J . Phys. Chem. 1975, 79, 808. (b) Soriaga, M. P.; Hubbard, A. T. J . A m . Chem. SOC.1982, 104, 2742. (c) Garwood, G. A.; Hubbard, A. T. Surf. Sci. 1980, 92, 617. (d) Stickney, J. L.; Rosasco, S. D.;Salaita, G. N.; Hubbard, A. T.Langmuir 1985, I , 66. (2) Felter, T. E.; Hubbard, A. T. Electroanal. Chem. 1979, 100, 473. (3) Mebrahtu, T.;Rodriguez, J. F.; Bravo, B. G.; Soriaga, M. P. J . Electroanal. Chem. 1987, 219, 327. (4) Bravo, B. G.; Mebrahtu, T.; Rodriguez, J. F.; Soriaga, M . P. J . Electroanal. Chem. 1987, 221, 281. (5) Lu, F.; Salaita, G. N.; Baltruschat, H.; Hubbard, A. T.Electroanal. Chem. 1987, 222, 327. (6) (a) Rodriguez, J. F.; Soriaga, M. P. J . Electrochem. Soc., in press. (b) Gao, P.; Weaver, M. J. J . Phys. Chem. 1986, 90, 4057. (c) Tadayyoni, M. A.; Gao, P.; Weaver, M. J. J . Electroanal. Chem. 1986, 198, 125. (7) Rodriguez, J. F.; Mebrahtu, T.; Soriaga, M. P.J . Electroanal. Chem.. in press.
0022-3654/88/2092-2702$01.50/0
dependence is due to the known fact that dissociative chemisorption of hydrogen does not occur on Au as readily as it does on Pt. The reversible I ~ a ~ J I - ~ sredox o l ) reaction at Au is coverage-dependent. The potential at which the iodine coverage is half maximum is about 900 mV negative of the redox potential for the TZ(aq)/I-!aq) couple; this indicates that the relative stabilization of surface iodine is approximately the same on A d a as it is on Pt.4 In order to help achieve a general picture of the surface electrochemical properties of the noble transition metals as a group of electrocatalyst materials, the above studies of the redox-activated adsorption-desorption reactions of chemisorbed iodine/iodide have been extended to iridium. In this study, the absolute surface coverage and redox activity of iodine chemisorbed at polycrystalline Ir electrodes have been examined in aqueous solutions as functions of electrode potential and solution pH.
Experimental Section Experimental measurements were based upon thin-layer electrochemical methods identical with those employed in studies with polycrystalline Pt2 and Au3 electrodes. The fabrication of the thin-layer electrochemical cell and the surface preparation of the Ir electrode were also as described for Pt thin-layer electrodes8 although, because Ir is a very hard material, metallographic polishing had to be done exclusively with diamond powder and paste. It is well-known that Ir electrodes in acidic media have a propensity to form hydrous oxide layers that ( I ) increase in thickness upon potentiodynamic cycling from the oxygen- to the hydrogen-evolution regions and (2) are not easily reduced by potentiostatic control in the hydrogen r e g i ~ nthe ; ~ nature of this hydrous oxide layer has been a subject of recent interest.1° Since surface oxide prevents iodine chemisorption, electrochemical cleaning of the Ir surface prior to the iodine chemisorption experiments was performed in such a manner that formation of the hydrous oxide film was eliminated; the procedure employed was based on a recently proposed model for anodic hydrous oxide growth at Ir electrodes.1° According to this model, initial application of positive potentials in the oxygen-evolution region results in the formation of a compact inner oxide on top of which a monolayer of hydrated surface oxide layer exists. When the potential is made negative in the hydrogen-evolution region, only (8) (a) Hubbard, A. T. C R C Crit. Reu. Anal. Chem. 1973, 3. 201. (b) White, J. H.; Soriaga, M. P.; Hubbard, A. T. J . Electroanal. Chem. 1984, 177, 89. (9) (a) Rand, D. A. J.; Woods, R. J . Electroanal. Chem. 1974, 55, 375. (b) Breiter, M. W. J . Electroanal. Chem. 1983, 157. 327. (c) Motoo, S.; Furuya, N. J . Electroanal. Chem. 1984, 167, 309. ( I O ) Pickup, P. G.; Birss, V . I. J . Electroanal. Chem. 1987, 220, 83.
0 1988 American Chemical Society
The Journal of Physical Chemistry, Vol. 92, No. 9, 1988 2703
Iodine/Iodide at Polycrystalline Iridium Clean and I-coated Iridium
has been shown to exist at gold7 and platinum'-5 electrodes. ( 2 ) Iodate Reduction. The amount of chemisorbed iodine is also directly proportional to the electrolytic charge for reduction of aqueous 103- (obtained in method 1) to aqueous I,; from this reaction (cf. peak 4 in Figure l), the iodine coverage is given by
-- I 10
I I
where Q is the total reductive charge and Qb is the background charge obtained in the absence of chemisorbed iodine. The electrolytic charges were obtained by potential-step coulometry from 0.9 to 0.7 V. This method is limited to thin-layer electrodes. It will be emphasized that in eq 2 the use an n value of 5 is independent of the chemisorption valency of iodine; hence, if iodine is indeed zerovalent in the chemisorbed state, then the following equality must hold:
a
.-1
Clean .a"
.OS
0.0
0.5
1 .o
1.5
(3)
E N vs. AgCl
Figure 1. Thin-layer cyclic current-potential curves for a clean (solid curve) and an iodine-pretreated (dashed curve) polycrystalline iridium electrode in 1 M H,S04. The peak numbers are as discussed in the text. The volume of the thin-layer cell V = 3.74 pL; the surface area of the electrode A = 1.40 cm2; voltammetric sweep rate r = 2 mV/s; temperature T = 298 K.
the inner oxide is reduced to the metal. If the potential is made positive again, the inner oxide-hydrated surface layer formation repeats, and, since the hydrated oxide layers are not reducible, the quantity of oxide left on the surface following each oxidation-reduction cycle builds up. The model also states that application of potentials within the double-layer region slowly dehydrates the hydrated surface oxide layer, which then becomes reducible to Ir metal at more negative potentials. On the basis of this model, electrochemical cleaning of the Ir electrodes in 1 M H2S04consisted of the following steps: (1) The electrode was oxidized at 1.2 V [Ag/AgCl (1.0 M C1-) reference electrode] for 60 s. (2) The potential was stepped to 0.2 V and held there for 60 s, following which the thin-layer cell was rinsed several times. (3) Steps 1 and 2 were repeated at least three times. (4) After the last cleaning cycle, the electrode potential was held a t -0.2 V for 60 s. ( 5 ) Prior to the chemisorption experiments, the electrode was equilibrated at 0.2 V. Surface cleanliness was verified by thin-layer cyclic voltammetry in 1 M H2S04. Iodine chemisorption-desorption experiments were carried out in 1 M H2SO4 (taken as pH 0), in 1 M NaCIO,, buffered at pH 7 with NaOH/NaH2P04," and in 1 M NaC104 buffered at pH 10 with NaHC03/Na2C03;" all aqueous solutions were prepared in pyrolytically triply distilled water.12 Determination of the Surface Coverage of Iodine. Pretreatment of the Ir surface with a monolayer of iodine consisted simply of rinsing the clean electrode with 1 mM NaI at the same pH that the reductive desorption experiments were to be carried out. The surface coverage of iodine was determined by two methods7 ( I ) Anodic Oxidation of Chemisorbed Iodine. This method is based on the fact that the total quantity of chemisorbed iodine is directly proportional to the charge for anodic oxidation of adsorbed iodine to aqueous IO,- (cf. peaks 3a and 3b in Figure 1):
Determination of the Surface Area of Polycrystalline Ir. In a recent paper, we suggested the use of iodine chemisorption as a method to determine the active surface area of polycrystalline Au electrodes7 for which the hydrogen chemisorption method is not applicable. Although hydrogen chemisorption occurs at clean Ir, its use for the measurement of the active surface area of polycrystalline Ir electrodes is limited by the facts that (1) not all of the underpotential hydrogen deposition peaks are well separated from the hydrogen-evolution reaction9*loand (2) the use of the hydrogen chemisorption method requires knowledge of the surface crystallographic orientationi3that cannot be known a priori for a polycrystalline electrode. In the iodine chemisorption method, the surface area A of the electrode is obtained from the following e q ~ a t i o n : ~ (4) where (Q and nl are, respectively, the electrolytic charge and number of electrons for the specific electrode reaction assois the ciated with chemisorbed iodine (cf. eq 1 and 2), and rI,Micd calculated iodine packing density. Calculation of rIis based on the premise that the chemisorption of iodine is space-limited; that is, the chemisorbed iodine layer consists of close-packed but unassociated atoms. If one assumes hexagonal close-packing of chemisorbed iodine, which has been shown to exist even on a nonhexagonal Pt( 100) substrate,1c%2 the average area occupied by the spherical iodine atom, 'JI,calcd is given by qcalcd
= rvd,22(31/2)= 0.160 nm2
(5)
where rvdwis the van der Waals radius14of the iodine atom, 0.21 5 nm. rI,calcd is related to 'JI,calcd by the following equation:
(1)
where N A is Avogadro's constant. rI,Mlcd is therefore equal to 1.04 nmol cm-2. This method of calculating rrhas been tested with well-defined Pt single crysta1slband polycrystalline Au electrode^.^ For example, the I'l/I'I,calcd ratios for Pt( 1 11) and Pt( 100) were found to be 1.03 and 1.04, respectively. It must be understood, of course, that the use of eq 4-6 to determine the active surface area of the Ir surface ensures that the maximum iodine coverage is 1.04 nmol cm-*.
where rl is the absolute surface coverage (mol cm-2) of chemisorbed iodine, Q is the total charge for oxidation of both surface iodine and Ir, Qb is the background charge for oxidation of the Ir surface in the absence of chemisorbed iodine, F is the Faraday constant, and A is the electrode surface area; a procedure for determining the active surface area of Ir is discussed below. The electrolytic charges were measured by potential-step coulometry from 0.2 to 1.2 V. The use of an n value of 5 in eq 1 implies that iodine is zerovalent in the chemisorbed state; zerovalent iodine
Results and Discussion Fkure 1 shows cyclic thin-layer current-potential curves, initially scanned in the negative direction starting from 0.2 V, for a clean (solid curve) and an iodine-pretreated (dashed curve) Ir surface in 1 M H,SO,. These clean-iridium voltammetric curves are similar to those previously r e p ~ r t e d . ~ .Peaks '~ l a and l b correspond to underpotential hydrogen deposition on presumably the low-index planes of the Ir surface; the second hydrogen chemisorption peak is not well resolved from the hydrogen-evo-
(1 1) Weast, R. C. Handbook of Chemistry; CRC Press: Boca Raton, FL, 1986. (12) Conway, B. E.; Angerstein-Kozlowska, H.; Sharp, W. B. A,; Criddle, E. E.Anal. Chem. 1974, 45, 133 1.
(13) Hubbard, A. T.; Ishikawa, R. M.; Katekaru, J. J. Electroanal. Chem. 1978, 86, 271. (14) Pauling, L . The Nature o f t h e Chemical Bond Cornell University
FI = (Q - Q b ) o x , ~ / 5 F A
Press: Ithaca, NY, 1960.
2704
The Journal of Physical Chemistry, Vol. 92, No. 9, 1988
Rodriguez et al.
I-coated Iridium 1 M Sulfuric acid
,
Iodine on Iridium I
Oi-
-10
1
.20
t
1
1 30' 02
' 0 0
'
'
0 2
0 4
"
06
'
08
' 1 0
"
1 2
"
-0 2
1 4
0 1
E'V vs. AgCl
lution reaction. Peak 2 , a very broad peak which commences at 0.4 V, is due to formation of Ir surface oxide. There is no well-defined surface oxide reduction peak; however, the cathodic Faradaic current below 0.4 V is considerably higher than that prior to the anodic oxidation scan, an indication that oxide reduction occurs over a wide range of potential down to the hydrogen-evolution region. It can be seen in Figure 1 that chemisorbed iodine does not show reversible iodine/iodide redox in the same potential region (ca. 0.4 V) where the solution species react. It can also be seen from this figure that pretreatment of the Ir surface with iodine completely suppresses the hydrogen chemisorption peaks, as expected based on previous results with Pt and Au. The early start of the surface oxidation is also inhibited by chemisorbed iodine. As will be discussed below, peaks 3a and 3b are both associated with the oxidative desorption of chemisorbed iodine to aqueous iodate; this is the reaction referred to in eq 1. Peak 4 corresponds to reduction of IO,-(,,) to 12(aq) and is the reaction referred to in eq 2. At potentials just after iodate reduction, the surface is still covered with an oxide layer that prevents the readsorption of iodine from Iz(aq). Hence, an Iz(aq) 21-(aq)reduction peak appears at about 0.4 V (peak 5 ) ; the charge under this peak is five times smaller than the charge under peak 4. Figure 2 shows thin-layer current-potential curves, with the potential initially scanned in the positive direction starting from 0.2 V, for the iodine-pretreated Ir in 1 M HzS04;the dashed curve is identical with that shown in Figure 1, whereas the solid curve was obtained by reversal of the potential immediately after completion of peak 3a. The data in this figure demonstrate that peak 3a is part of the overall oxidative desorption of chemisorbed iodine to iodate: First, peak 3a cannot be attributed to solution species since no I-(aq) IZ(aq)peak occurs near 0.4 V on the first scan. Iz(4 and Iz(ad I-(aq) reductions Second, corresponding IO
