0 Copyright 1993 American Chemical Society
The ACS Joumal of
Surfaces and Colloids DECEMBER 1993 VOLUME 9, NUMBER 12
Letters Adsorbate-Catalyzed Dissolution in Inert Electrolyte: Layer-by-Layer Corrosion of Pd(lOO)-c(2X2)-1 Jane A. Schimpf, Juan B. Abreu, and Manuel P. Soriaga’st Department of Chemistry, Texas A&M University, College Station, Texas 77843 Received June 25, 199P The anodic dissolution of Pd in halide-free sulfuric acid, catalyzed by a single adsorbed layer of iodine, ) adlattice. Experimental was studied with a Pd(100) single crystal that contained an ordered ~ ( 2 x 2iodine measurementswere based upon ultrahigh vacuum electrochemistry(UHV-EC),an approachthat combined voltammetry, coulometry, low-energyelectron diffraction,and Auger electron spectroscopy. It was found that Pd dissolution occurred only when the surface was pretreated with iodine. More significantly, the surfacecoverage and structure of the iodine adlattice were unaffected by the corrosionreaction, an indication that the anodic dissolution process occurs essentially one interfacial metal-layer at a time.
Introduction In halide-free sulfuric acid solution, the anodic dissolution of polycrystalline Pd does not occur unless a single chemisorbed layer of iodine is present on the surface;’ even after prolonged corrosion, the iodine adatom layer remains attached to the surface. This type of anodic stripping, clearly catalyzed by the iodine adlayer, is unexpected since the conventional view holds that anodic dissolution transpires only in the presence of an abundant supply of corrosive reactants in sol~tion.”~It has been postulated that the reaction occurs one interfacial metal+ Presidential Young Investigator. Abstractpublishedin Advance ACSAbstracts, October 15,1993. (1) McBride, J. R.; Soriaga, M. P. J. Electroanul. Chem. 1989, 303, 255.
(2) Pourbaix, M. Lectures on Electrochemical Corrosion; Plenum Press: New York, 1973. (3) Sato, N. Corros. Sci. 1987,27, 421. (4) Aaawa, M. Corrosion 1987,43, 198. (5) Geneeca, J.; Durand, R. Electrochim. Acta 1987, 32, 541. (6) Ryu, W. S.; Kang,Y. H.; Lee, J.-Y. J. Nucl. Mater. 1988,52,194. (7) Wilhemeen, W.; Grade, A. P. Electrochim. Acta 1987,32, 1469. (8)DeChialvo, M. R. G.; De Mele, M. F. L.; Salvarezza, R. C.; Arvia, A. J. Corros. Sci. 1988, 28, 121. (9) Sato, N. Corrosion 1989, 45, 354.
layer at a time, as depicted in the following chemical equations:
Pd,b,,-Pd,-Pd3-Pd,-Pdl-I(ah) Pd,b,)-Pd4-Pd3-Pd2-I,ah)+ P d F + 2e- (1) +
Pd(b,,,-Pd4-Pd3-Pd,-I(ah)
-
Pd,,,,-Pd4-Pd3-I,,, Pd(bulk)-Pd4-Pd3-1(ah)
-
Pd(b,)-Pd4-I(ah)
+ P d p + 2e-
(2)
+ Pd? + 2e-... (3)
where the numerical subscripts label the first four interfacial layers of Pd atoms. Such layer-by-layer corrosion mechanism is indicated from recent work, based upon tandem ultrahigh vacuum electrochemistry (UHV-EC), which showed that the low-energy electron diffraction (LEED) pattern of a highly ordered interfacial layer, Pd(111)(d3Xd3)R30°-I, was the same before and after anodic dissolution.lO On fundamental grounds, it is important to ascertain if, for a different crystallographic (10)McBride, J. R.; Schimpf, J. A.; Soriaga, M. P. J. Electroanul. Chem. 1993,350, 317.
0743-7463/93/2409-3331$04.00100 1993 American Chemical Society
Letters
3332 Langmuir, Vol. 9, No. 12,1993 Anodic Oxidation of Pd(lO0) and Pd(1OO)-l
0.0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
E N vs. AgCl(1 mM CI -)
Figure 1. Current-potential curves in the surface-oxidation region in 0.05 M H2S04 for Pd(100) (solid curve) and Pd(100)c(2X2)-I (dashed curve) single-crystal electrode surfaces: potential sweep rate, r = 10 mV s-l; temperature, T = 298 K; geometric area of electrode = 1.4 cm2. AES Of Pd(l00)-~(2~2)-I
I
After 180s Corrosion
0
100
200
3 0 0 4 0 0
500
Kinetic Energy/eV Figure 2. Auger electron spectra (AES) of the Pd(100) and Pd(lll)-c(2x2)-1 single-crystal electrode surfaces before and after anodic dissolution for 180 s at 0.8 V incident beam energy = 2 keV; beam current = 1 pA.
plane, the same layer-by-layer adsorbate-catalyzed dissolution is observed. The present article describes a study that is addressed to this question; reported here are LEED and Auger electron spectroscopy (AES) data of an I-pretreated Pd(1 ) single-crystalelectrode surface, Pd(100)c(2x2)-1, be ore and after anodic dissolution.
T
Experimental Section UHV-EC experiments" were performed in all-stainless steel apparatus that integrated (i) an ante-chamber that, by means of a gate valve, could be isolated and back-filled with high-purity argon for electrochemical measurements, (ii) LEED optics, (iii) a cylindrical mirror analyzer for Auger electron spectroscopy (AES), and (iv) a quadrupole mass analyzer for temperature(11)Soriaga, M. P.h o g . Surf. Sei. 1992, 39,525.
Figure 3. Low-energyelectron diffraction (LEED) patterns for a Pd(lOO)-c(ZxZ)-Iadlattice before and after anodic dissolution of about 20 Pd monolayers: beam energy = 62.4 eV; beam current = 2.2 pA. programmed desorption.12 A Pd(100) single crystal, 99.999 % pure and commerciallyoriented, was metallographicallypolished and cleaned over several weeks by multiple cycles of high temperature (750 K) to promote surface segregation of trace impurities and Ar+-ionbombardment (2 pA in 106 Torr Ar) to remove sputter away those impurities.13 Thermal annealing of the clean crystal at 750 K restored atomic smoothness. The geometric area of the electrode was measured to be 1.6 cm2. Electrolyticsolutions,made from Milli-Q Plus water, contained 0.05 M H2S04 prepared from fuming sulfuric acid stock solution in order to preclude C1- contamination. Iodine chemisorption was carried out by immersion of the UHV-prepared Pd(ll1) electrode in a 0.05 M H2S04 + 1mM NaI solution at potentials within the double-layer region (nominally 0.1 V). Anodic oxidation of the Pd(100) and Pd(lOO)-c(2X2)-1 electrodes was ~
~~~
(12) Rodriguez, J. F.;Mebrahtu, T.; Soriaga, M. P. J. EZectroanuL Chem. 1989,264, 291. (13) McBride,J.R. Ph.D. Dissertation,Texas AkMUniversity, College Station, TX,1992.
Langmuir, Vol.9,No.12, 1993 3333
Letters done at selected potentials below 1.1 V. A Ag/AgC1(1 mM C1-) reference electrode was employed. A fine-glass frit separated the reference/auxiliaryand sample compartments;experiments with a cleanPd(100)surface showed no surfaceC1-contamination, an indication that leakage of halide into the sample solution did not occur.
Results and Discussion The current-potential curves in the surface-oxidation region for a clean (dotted curve) and an iodine-coated (solid curve) Pd(100) single-crystal electrode in 0.05 M HzS04 are shown in Figure 1. The voltammogram for the iodinefree Pd(100) surface is characterized by the surface-limited formation of a passive surface-oxide film, similar to that on polycrystalline1 and Pd(lll).lo Pretreatment of the Pd(100) electrode with a monolayer of zerovalent iodine aroms, however, results in a large increase in the anodic oxidation current. In addition, as was noted previously with polycrystalline Pd and monocrystalline Pd(ll1) electrodes, the current does not decrease, and the electrolyte solution attains a yellow coloration due to formation of solvated Pd2+ species when the applied potential is maintained at a fixed value above 0.6 V but below the anodic peak potential (1.0 V in Figure 1). XPS measurements obtained on polycrystalline Pd subsequent to the corrosion experiments revealed the presence of chemisorbed iodine at unaltered c0verages.l The same postcorrosion constant-iodine-coverageresult was observed with the Pd(lOO)-c(2X2)-1 surface, as evidenced by the AES spectra (Figure 2) obtained before and after a 180-sdissolution at 0.8 V; quantitative analysis of the AES data indicates no alterations in the I(&) coverage after the controlled corrosion reaction. Corrosion-induced changes, if any, in the LEED pattern of the I-modified surface are of primary interest in this study. The top photograph in Figure 3 shows the LEED pattern for the freshly prepared Pd(100)42X2)-1. The same surface was then subjected to a potential of 0.8 V for 180 s. The accumulated electrolytic charge (13 mC) was equivalent to dissolution of about 20 monolayers. The
LEED pattern obtained after the 20-monolayer etch is shown in the bottom photograph of Figure 2. It is clear that the postcorrosion LEED pattern is identical to that prior to the controlled dissolution. It will be mentioned that identical LEED patterns were observed when longer corrosion times were employed. The results presented in this paper provide compelling evidence that the I(,&)-catalyzed Pd dissolution in Pd(100)-~(2X2)-I,underthe conditions described here, occurs essentially layer-by-layer. A disordered surface brought about by multilayer, random-depth corrosion would have rendered the LEED pattern totally diffuse. The layer-by-layer Pd corrosion in Pd(lOO)-c(2X2)-1 can be expressed in terms of the following chemical equations:
-
Pd(100),-Pd( 100),-Pd( 100),-Pd( 100),-Pd( loo),C( 2X2)-I Pd(100),-Pd( 100),-Pd( 100),-Pd( loo),c(2X2)-1+ PdI2++ 2e- (4) Pd(100),-Pd( 100),-Pd( loo),-Pd( 100),-~( 2X2)-I Pd(100),-Pd(100),-Pd(100),-c(2X2)-1+ P d p + 2e(5) +
Pd(lOO),-Pd( 100),-Pd( 100),-~(2x2)-1Pd(100),-Pd(100),-c(2x2)-1+ Pd?
+ 2e-
(6)
Layer-by-layer dissolution was likewise indicated at the more closely-packedPd(ll1) electrode surface. Evidently, the ordered I(,&)-catalyzedcorrosion of Pd is independent of the surface crystallographic orientation. Acknowledgment. Acknowledgment is made to the National Science Foundation (Presidential Young Investigator program, DMR-8958440), the Robert A. Welch Foundation, and the Center for Energy and Mineral Resources at Texas A&M University for support of this research.
