J . Phys. Chem. 1993, 97, 10518-10520
10518
Adsorbate-Catalyzed Layer-by-Layer Metal Dissolution in Halide-Free Solutions: Pd( 111)(43X43)R3Oo-I Jane A. Schimpf, John R. McBride, and Manuel P. Soriaga'J Department of Chemistry, Texas A & M University, College Station, Texas 77843 Received: June 1 1 , 1993; In Final Form: September 2, 1993"
The anodic dissolution of Pd in inert (halide-free) solution (0.05 M H2S04), catalyzed by a single adsorbed layer of iodine, was studied with a well-ordered, single-crystal electrode surface, Pd( 11 1)(d3Xd3)R3Oo-I. Experimental measurements were based upon a combination of electrochemistry (voltammetry and coulometry) and electron spectroscopy (low-energy electron diffraction and Auger electron spectroscopy). The significant results were as follows: (i) Pd dissolution occurred only when iodine was present on the surface, (ii) the amount of adsorbed iodine was not affected by the dissolution process, and (iii) the interfacial structure of the iodine adlattice remained highly ordered even after prolonged corrosion. These observations strongly suggest a n adsorbate-catalyzed layer-by-layer anodic dissolution process.
Introduction
In an earlier study with polycrystalline electrodes, we found that the anodic dissolution of Pd in inert (halide-free) 1 M HzSO4solution was promoted by a single chemisorbed layer of iodine atoms.Iv2 Coulometric and inductively coupled plasma spectroscopy indicated a Pdo-to-Pd2+ oxidation process. X-ray photoelectron spectroscopy showed that the adsorbed iodine was not removed from the surface even after prolonged (several hours) Pd dissolution. In the same H2SO4 solution, but in the absence of the iodine adlayer, no Pd dissolution was observed;2only surface oxide formation was noted. This type of anodic stripping does not appear to follow the conventional view that catalyzed anodic dissolution occurs primarily in the presence of appreciable quantities of corrosive electrolyte, such as C1-, in ~ o l u t i o n . ~ - ' ~ We have now extended the study to examine the I(,d,,-catalyzed dissolution at a well-ordered I-treated Pd( 1 11) single-crystal electrode, Pd( 111)(d3Xd3)R30°-I. The primary objective was to ascertain whether or not the corrosion reaction destroyed the adlattice structure of the adsorbed iodine. Preservation of the ordered adlayer structure, as would be demonstrated by the identity of the low-energy electron diffraction (LEED) patterns before and after dissolution, provides evidence for a layer-bylayer process. The results are described in the present paper.
Halide-free 0.05M HzS04
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E/V vs. AgCl(1 mM CI')
Figure 1. Current-potential curves in the surface oxidation region in 0.05 M H2S04 for Pd( 11 1) (solid curve) and Pd( 11 1)(d3Xd3)R30°-I (dashed curve) single-crystalelectrode surfaces. Potential sweep rate r = 10.0 mV s-I, temperature T = 298 K, and geometric area of electrode = 1.44 cm2.
Experimental Section
Anodic Oxidation of Pd(l1 1)(d3xd3)R3O0-I
Experiments were performed in an ultrahigh-vacuum electrochemistry (UHV-EC) apparatusll that integrated (i) a gate valve-isolable chamber back-filled with high-purity argon for electrochemical measurements, (ii) LEED optics (Perkin-Elmer, Eden Prairie, MN), and (iii) a cylindrical mirror analyzer (PerkinElmer, Eden Prairie, MN) for Auger electron spectroscopy ( A S ) . A 99.999% pure, commercially oriented and polished Pd( 11 1) single crystal (Aremco, Ossining, NY) was initially cleaned by multiple cycles of thermal oxidation and Ar+ ion bombardment followed by high-temperature annealing to restore atomic smoothness;l2 the geometric area of the electrode was mesured to be 1.44 cm2. Electrolytic solutions were made from Milli-Q Plus water and contained 0.05 M H2SO4 prepared from fuming sulfuric acid stock solution (Aldrich, Milwaukee, WI) to eliminate C1contamination. Iodine chemisorption was carried out by immersion of the UHV-prepared Pd( 1 11) electrode in 0.05 M H2SO4 containing 0.5 mM NaI for 180 s at potentials within the double-layer region, nominally 0.1 V [Ag/AgCl (1 mM C1-) Presidential Young Investigator. .Abstract published in Aduance ACS Abstracts, October 1, 1993. +
0022-3654/93/2097-10518$04.00/0
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Figure 2. Plots of anodic oxidation charge Qox as a function of reaction time at applied (dissolution) potentials of 0.8, 0.9, and 1.0 V for a Pd(1 11)(d3Xd3)R30°-I interface. Data for an iodine-free Pd( 111) electrode are also given for reference. Experimental conditions were as in Figure 1.
0 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97, No. 41, 1993 10519
Letters
AES of Pd(lll)(./3x./3)R3O0-I After Etch
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KE/eV Figure 3. Auger electron spectra of the Pd( 111) and Pd( 111)(d3Xd3)R30°-I single-crystal electrode surfaces before and after anodic dissolution at the potentials indicated. Incident beam energy = 2 keV; beam current = 1 PA.
Pd(l11)-(.\/3~.\/3)R30"-I
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Figure 4. Low-energy electron diffraction patterns for a Pd( 111)(d3Xd3)R30°-I adlattice before and after anodic dissolution of approximately 40 Pd monolayers (90 s at 1.0 V), 20 monolayers (90 s at 0.9 V), and 10 monolayers (90 s at 0.8 V). Beam energy = 52.4 eV; beam current = 2.8 PA.
reference electrode]. Anodic oxidation of the Pd( 111) and Pd(1 11)(d3xd3)R30°-I electrodes was done at selected potentials below 1.15 V. The reference electrode was separated from the working electrode compartment by a fine glass frit; the absence of C1- contamination at a clean Pd( 11 1) surface was used as evidence that there was no leakage of C1- anions into the sample solution.
Results and Discussion Slow-sweepcurrent-potential curves in the surface oxide region for Pd( 111) and Pd( 111)(2/3xd3)R300-1 single-crystal electrode surfaces in 0.05 M &SO4 are shown in Figure 1. The anodic oxidation current for the Pd(ll1) surface is minimal, indicative of the formation of a passivating surface oxide film. In contrast, there is an enormous oxidation current for the surface.
10520 The Journal of Physical Chemistry, Vol. 97, No. 41, 1993 At the other noble metals, such as Pt or Au, the zerovalent iodine adatoms can be oxidatively desorbed to aqueous 103- ions at potentials slightly above 1.0 V. However, such a five-electron process cannot account for the large anodic oxidation charge observed for the I-coated Pd; in addition, the anodic current does not decay to zero, as would be expected for the surface-limited I(a.j&o-103-(aq) reaction, when the potentiodynamic scan is arrested and maintained at potentials below the anodic peak potential (1.05 V). Figure 2 shows plots of anodic charge Qoxaccumulated over 90 s at preselected dissolution potentials for the clean and I-pretreated Pd(ll1) electrodes. It can be seen that Qoxfor Pd(1 1 1) reaches a constant value after only 20 s at 1.O V, whereas those for Pd(l1 1)(d3Xd3)R30°-I show a linear increase with time; the slope, which is a measure of the corrosion rate, increases as the dissolution potential is increased (but not beyond 1.05 V). These results are consistent with those for polycrystalline Pd.IV2 For a Pdo-to-Pd2+corrosion reaction,'v2 the Q,values accumulated over 90 s at 1.0 V (27 mC), 0.9 V (12 mC), and 0.8 V (6 mC) correspond to dissolution of approximately 40, 20, and 10 monolayers. XPS measurements obtained subsequent to the corrosion experiments with polycrystalline Pd showed the presence of chemisorbed iodine at unaltered coverages.l.2 The same postcorrosion constant-iodine-coverage result was also observed in the present study, as can be seen in Figure 3. This figure shows AESdata thePd( 111)(d3Xd3)R30°-I surface beforeand after 90-s etches at dissolution potentials of 0.8 and 1.OV. Quantitative analysis of the AES data indicates that there were no changes in the chemisorbed iodine coverages after the controlled dissolution. Of specific interest in this study are dissolution-induced alterations, if any, in the LEED pattern of the Pd( 1 1l ) ( d 3 X d 3 ) R30°-I interface. The results are shown in Figure4. It is obvious from the clarity and sharpness of the LEED patterns that the postcorrosion I adlattice remains as well-ordered as the freshly prepared adlayer. No perceptible changes in the LEED patterns
Letters were observed when longer corrosion times ( 5 min) were employed; surface disorder occurred, however, when potential above 1.05 V were applied. The above results, especially the striking LEED data, provide evidence that the Z(,d,)-catalyzed anodic dissolution of Pd(l1 l), at least under the conditions of the present study, occurs layer by layer. A more drastic dissolution reaction yields pits that would dramatically disorder the surface; disrupted interfacial layers are manifested by highly diffuse LEED patterns.''-13 The present experiments do not provide information on the specific mechanism by which the corrosion of Pd takes place. Dissolution may occur via I-Pd place exchange; it could also proceed directly at steps or defects, provided those sites are in close proximity to an adsorbed iodine atom. Zn situ scanning tunneling microscopy may be able to shed light on this subject.
Acknowledgement is made to the National Science Foundation (Presidential Young Investigator program, DMR-8958440), the Robert A. Welch Foundation, and the Center for Mineral and Energy Resources at Texas A & M University for support of this research. References and Notes (1) McBride, J. R.; Soriaga, M. P. J . Electrounul. Chem. 1989,303,255. ( 2 ) Schimpf, J. A. M.S. Thesis, Texas A&M University, 1991. (3) Pourbaix, M. Lectures on Electrochemicol Corrosion; Plenum Press: New York, 1973. (4) Sato, N. Corros. Sci. 1987, 27, 421. ( 5 ) Asawa, M. Corrosion 1987, 43, 198. (6) Genesca, J.; Durand, R. Electrochim. Actu 1987, 32, 541. (7) Ryu, W. S.;Kang, Y. H.; Lee,J.-Y. J . Nucl. Muter. 1988,52, 194. ( 8 ) Wilhemsen, W.; Grande, A. P. Electrochim. Actu 1987, 32, 1469. (9) DeChialvo, M. R. G.; De Mele, M. F. L.; Salvarezza, R. C.; Arvia, A. J. Corros. Sci. 1988, 28, 121. (IO) Sato, N . Corrosion 1989, 45, 354. (1 1) Soriaga, M. P. Prog. Surf.Sci. 1992, 39, 525. (12) Rodriguez, J. F.; Mebrahtu, T.; Soriaga, M. P. J . Elecrrounul.Chem. 1989. 264. 29 1 . (13) Rodriguez, J. F.; Bothwell, M. E.; Cali, G . J.; Soriaga, M. P. J . Am. Chem. SOC.1990, 112, 7392.
