Temperature-Dependence of the Electro-oxidation of the Irreversibly

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Temperature-Dependence of the Electro-oxidation of the Irreversibly Chemisorbed As on Pt(111) Sonia Blais and Gregory Jerkiewicz* De´ partement de chimie, Universite´ de Sherbrooke, 2500 boul. Universite´ , Sherbrooke, Que´ bec, J1K 2R1 Canada

Enrique Herrero and Juan M. Feliu* Departamento de Quimica Fisica, Universidad de Alicante, Apartado 99, E-03080 Alicante, Spain Received May 18, 1999. In Final Form: February 26, 2001 An overlayer of chemisorbed arsenic, Aschem, on Pt(111) having a surface coverage of θAs is prepared, and the impact of T variation on the surface-oxidation behavior in 0.5 M aqueous H2SO4 is examined. Temperature variation in the 273-313 K range does not affect the Aschem surface coverage, but the cyclicvoltammetry (CV) profiles undergo qualitative changes. The anodic and cathodic peaks shift toward morepositive potentials, and a charge-density redistribution between two anodic and cathodic features is observed, the latter effect being more pronounced in the anodic scans. At T ) 313 K, the CV transients are symmetrical with respect to the potential axis. In the case of Aschem overlayers having θAs well below 0.33, the CV profiles representative of the Aschem oxidation reveal only one peak. An analysis of the possible reaction pathways indicates that the surface oxidation proceeds in one step involving transfer of three electrons and addition of three OH groups, thus in formation of As(OH)3chem. An analysis of the As3+ and OH- radii indicates that the As(OH)3chem layer is densely packed. The existence of two anodic and two cathodic peaks in CV transients is explained in terms of formation two energetically slightly different structures of As(OH)3chem, their origin being two different ways of coordinating As(OH)3chem to Pt(111). The standard enthalpy of the surface process is determined from the slope of the E/T versus 1/T plots, and the standard enthalpy of formation of As(OH)3chem, ∆H°f(As(OH)3chem), is found to be -678 kJ mol-1. The value of ∆H°f(As(OH)3chem) is characteristic of As being in the 3+ oxidation state and being attached to three OH groups. The conclusion that As(OH)3chem is the species formed agrees with the CV, IR spectroscopy, and ex-situ scanning tunneling microscopy data which point to an oxygenated As3+ surface compound formed in the course of Aschem electro-oxidation.

Introduction Elements of groups V and VI of the periodic table of the elements such as As, Bi, Sb, S, Se, and Te reveal the unique ability to form spontaneously and irreversibly a chemisorbed overlayer on single-crystal or polycrystalline Pt substrates.1-8 The overlayer development is typically accomplished within seconds by immersing Pt in an aqueous solution of a salt or an oxide of the respective element undergoing chemisorption. Theoretically, the formation of the chemisorbed overlayer on Pt can proceed via three distinct pathways: (a) oxidative adsorption, for instance the Schem monolayer formation on Pt(111) from aqueous solution of Na2S,7,8

Pt(hkl) + Saq2- + 2H2O f Pt(hkl)-Schem + 2OH- + H2 (1) (1) Adzic, R. R. In Advances in Electrochemistry and Electrochemical Engineering; Gerisher, H., Ed.; Wiley-Interscience: New York, 1984; Vol. 13. (2) Watanabe, M.; Furuuchi, Y.; Motoo, S. J. Electroanal. Chem. 1985, 191, 367. (3) Clavilier, J.; Feliu, J. M.; Aldaz, A. J. Electroanal. Chem. 1988, 243, 419. (4) Feliu, J. M.; Ferna´ndez-Vega, A.; Aldaz, A.; Clavilier, J. J. Electroanal. Chem. 1988, 256, 149. (5) Feliu, J. M.; Go´mez, R.; Llorca, M. J.; Aldaz, A. Surf. Sci. 1993, 289, 152. (6) Feliu, J. M.; Go´mez, R.; Llorca, M. J.; Aldaz, A. Surf. Sci. 1993, 297, 209. (7) Sung, Y.-E.; Chrzanowski, W.; Zolfaghari, A.; Jerkiewicz, G.; Wieckowski, A. J. Am. Chem. Soc. 1997, 119, 194.

(b) adsorption without change of the oxidation state of the chemisorbing species, for instance As(OH)3chem submonolayer formation on Pt(111) and Pt(100) from an acidic aqueous solution of As2O3,4

Pt(hkl) + AsO2- + 2H2O f Pt(hkl) - As(OH)3chem + OH- (2) (c) adsorption with disproportionation of the chemisorbing species, for instance Sn chemisorption from an acidic aqueous solution of SnSO4 leading to a reduced state of Sn upon chemisorption.9

Pt(111) + 2Snaq2+ f Pt(111) - Snchem + Snaq4+ (3) None of these processes is stimulated by an external charge-transfer, albeit an interfacial redox process takes place. It should be added that, for the purpose of general presentation, eqs 1-3 depict only the irreversible formation of the chemisorbed adlayers but do not imply that the atomic adsorbate-to-substrate stoichiometry is unity (see refs 3-8 and 10 for the respective surface chemical compositions). The adlayers of irreversibly formed inorganic species are effective catalytic surface modifiers that (8) Sung, Y.-E.; Chrzanowski, W.; Wieckowski, A.; Zolfaghari, A.; Blais, S.; Jerkiewicz, G. Electrochim. Acta 1998, 44, 1019. (9) Rodes, A.; Feliu, J. M.; Aldaz, A.; Clavilier, J. J. Electroanal. Chem. 1988, 256, 455. (10) Orts, J. M.; Rodes, A.; Feliu, J. M. J. Electroanal. Chem. 1997, 434, 121.

10.1021/la990598s CCC: $20.00 © 2001 American Chemical Society Published on Web 04/17/2001

Electro-oxidation of Chemisorbed As onPt(111)

can either play the role of a surface promoter or a surface poison of a given electrochemical process of interest.1,11,12 For instance, the irreversibly chemisorbed Aschem, Bichem, and Schem overlayers suppress the underpotential deposition of hydrogen (UPD H) and poison the hydrogen evolution reaction (HER) on Pt(hkl).13,14 On the other hand, the Bichem layer promotes the formic acid oxidation on single-crystal and polycrystalline Pt electrodes.2,15 In recent years, there has been an increased interest in such chemically and electronically modified Pt electrodes owing to (i) their electrocatalytic properties13,16 and (ii) the feasibility of providing molecular-level images of their surface structures by application of scanning tunneling microscopy (STM) and the possibility of relating them to the catalytic activity.10 An interesting feature of the above-mentioned adsorbates is that all of them can chemisorb spontaneously on Pt under open-circuit conditions, an aspect that is of importance to (i) electrocatalysis, where coadsorbed inorganic or organic species can significantly reduce the real surface area available to the intermediate and can modify the mechanism of the process, as in the case of the HER; (ii) corrosion science and technology, where H embrittlement, caused by parasitic H entry into the metallic host, can be diminished by a preadsorbed siteblocking species (SBS); (iii) design of fuel-cell catalysts that might become poisoned already during the manufacturing or handling; and (iv) metal-hydride science and technology where the desired H interfacial transfer can be effectively enhanced by a surface promoter. The impact of the Schem overlayer is not only limited to the suppression of the UPD H and anion adsorption, as in the case of Pt(111) electrodes,7,8 because it can have far-reaching effects. For instance, Schem on Pd or Fe electrodes can enhance the absorption of H and the mechanism of the HER,17 or it can enhance the anodic metal dissolution, as is observed in the case of Ni(100) electrodes.18 Other group VI adsorbates such as Sechem and Techem on Au(hkl) electrodes can be applied as suitable substrates in the growth of structured semiconductor layers; the process, developed in recent years, is referred to as electrochemical atomic layer epitaxy (ECALE).19 The irreversible chemisorption under opencircuit conditions is not limited to one adsorbate. For instance, mixed Bichem-Aschem adlayers on Pt(111) can be prepared and the coverage of the chemisorbed species can be controlled.20 The Aschem and Bichem adlayers on Pt(111) reveal a reversible surface redox behavior upon potential cycling in the 0.05-0.85 V range; the chemisorbed layers remain on the Pt(111) substrate even upon their oxidation. The pH dependence of the Aschem oxidation on Pt(111) indicates that this surface reaction involves addition/stripping of O-containing species, most likely OH groups.21 Yet, there exists some conflicting interpretations with regard to the number of electrons transferred per Aschem adatom in the process; thus, the stoichiometry of the oxidized Pt(111)(11) Conway, B. E.; Jerkiewicz, G. J. Electroanal. Chem. 1993, 357, 47. (12) Jerkiewicz, G.; Borodzinski, J. J.; Chrzanowski, W.; Conway, B. E. J. Electrochem. Soc. 1995, 142, 3755. (13) Clavilier, J.; Feliu, J. M.; Ferna´ndez-Vega, A.; Aldaz, A. J. Electroanal. Chem. 1990, 294, 193. (14) Go´mez, R.; Ferna´ndez, A.; Feliu, J. M.; Aldaz, A. J. Phys. Chem. 1993, 97, 4769. (15) Clavilier, J.; Ferna´ndez-Vega, A.; Feliu, J. M.; Aldaz, A. J. Electroanal. Chem. 1989, 258, 89. Leiva, E.; Iwasita, T.; Herrero, E.; Feliu, J. M. Langmuir 1997, 13, 6287. (16) Herrero, E.; Rodes, A.; Pe´rez, J. M.; Feliu, J. M.; Aldaz, A. J. Electroanal. Chem. 1995, 393, 87.

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Aschem species is uncertain.4,20 To examine the stoichiometry of the surface redox process, additional insight into it was sought by performing in-situ Fourier transform infrared (FTIR) spectroscopy measurements and ex-situ STM characterization.10 The results indicate that (i) there are no coadsorbed anionic species that could be contributed to the redox process, and thus to the experimentally measured charged density, and (ii) when the Aschem surface coverage, θAs, is equal to 0.33, the adlayer has the Pt(111)(x3 × x3)R30°-As structure.10 The existing cyclicvoltammetry (CV), STM, and FTIR data are rather conclusive with regard to the relation between the Aschem coverage, the charge transferred in the course of the surface redox process, and the structure formed by the Aschem adlayer. In this contribution, we provide new data on the temperature-dependence of the surface redox process as investigated using CV for a well-defined Aschem overlayer (namely, for θAs ) 0.33). This first ever T-dependent study followed by detailed thermodynamic treatment sheds additional light on the nature of the species formed in the course of Aschem electro-oxidation through determination of the standard enthalpy of the process, ∆H°. The thermodynamic data combined with the existing CV, STM, and FTIR results provide a more detailed picture of the physicochemical nature of the surface redox process and an insight into its mechanism. Experimental Section Pt(111) Electrode Preparation. A bead-shaped Pt(111) single crystal was prepared and oriented according to the procedure developed by Clavilier.22 It was polished with diamond paste (0.25 µm, Struers) to a mirrorlike finish followed by etching and thermal annealing at ∼1100 °C. Its quality was verified by recording a CV profile in 0.5 M aqueous H2SO4 at potentials between 0.06 and 0.90 V, RHE; the CV scan agreed with the CV transient that is representative of a well-ordered Pt(111).23,24 The electrode diameter, d, was measured with a Vernier microscope and was found to be 0.209 ( 0.002 cm, and its surface area, A, was 0.0342 ( 0.0006 cm2. Prior to each measurement, the crystal was annealed in a hydrogen flame (at ∼1100 °C) and cooled in a mixture of hydrogen and argon followed by attachment of a droplet of ultrahigh purity water to protect the surface from contamination.22,23 Then, a CV profile was recorded at each temperature in order to verify the surface orientation and the solution cleanliness. As Overlayer Preparation. The As overlayer was prepared by immersion of the Pt(111) single crystal for 5 s in 0.5 M aqueous H2SO4 solution saturated with As2O3 (the As2O3 solubility is ∼37 g dm-3, thus ∼0.19 mol dm-3) followed by thorough rinsing with Millipure water and transfer to an electrochemical cell.3,10 Since the procedure always resulted in formation of an As overlayer initially having θAs > 0.33,3,10 the removal of the Aschem excess was accomplished by repetitive cycling of the Pt(111) electrode in a 0.5 M aqueous H2SO4 solution in the 0.05-0.90 V, RHE, range at the scan rate of 50 mV s-1 until the Aschem coverage of (17) Qian, S. Y.; Conway, B. E.; Jerkiewicz, G. Phys. Chem. Chem. Phys. 1999, 1, 2805. (18) Suzuki, T.; Yamada, T.; Itaya, K. J. Phys. Chem. 1996, 100, 8954. Ando, S.; Suzuki, T.; Itaya, K. J. Electroanal. Chem. 1996, 412, 139. (19) Gregory, B. W.; Stickney, J. L. J. Electroanal. Chem. 1991, 300, 543. (20) Dollard, L.; Evans, R. W.; Attard, G. A. J. Electroanal. Chem. 1993, 345, 205. (21) Feliu, J. M.; Ferna´ndez-Vega, A.; Orts, J. M.; Aldaz, A. J. Chim. Phys. 1991, 88, 1493. (22) Clavilier, J.; Armand, D.; Sun, S. G.; Petit, M. J. Electroanal. Chem. 1986, 205, 266. Clavilier, J. In Interfacial Electrochemistry; Wieckowski, A., Ed.; Marcel Dekker: New York, 1999; Chapter 14. (23) Clavilier, J.; El Achi, K.; Petit, M.; Rodes, A.; Zamakhchari, M. A. J. Electroanal. Chem. 1990, 295, 333. (24) Rodes, A.; Zamakhchari, M. A.; El Achi, K.; Clavilier, J. J. Electroanal. Chem. 1991, 305, 115.

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0.33 was reached. It should be added that the Aschem overlayer having θAs ) 0.33 gives an explicit CV profile that cannot be confused with CV transient characteristics of Aschem overlayers having different coverages. Solution and Electrochemical Cell. The 0.5 M aqueous H2SO4 solution was prepared from BDH Aristar grade H2SO4 and Nanopure (Brandstead) or Milli-Q Plus (Millipure) water (18 MΩ cm). The experiments were conducted in a Pyrex, twocompartment electrochemical cell. The cell design enabled its immersion in a water bath for T-dependent measurements.25 The glassware was precleaned according to a well-established procedure.26 During the experiments, ultrahigh-purity H2 gas, presaturated with water vapor, was bubbled through the reference electrode (RE) compartment in which a Pt/Pt-black electrode was immersed. It served as a reversible hydrogen electrode, RHE. Ultrahigh-purity Ar gas, presaturated with water vapor, was passed through the working electrode (WE) compartment; prior to each experiment, Ar was passed through the electrolyte and, during the experiment, above the solution level. The counter electrode (CE) was a Pt wire (99.998% purity, Aesar). Temperature-Dependent Measurements. The electrochemical cell was immersed in a water bath (Haake W13), and the temperature was controlled to within (0.5 K by means of a thermostat (Haake D1). The water level in the bath was maintained above that of electrolyte in the cell to ensure uniformity of temperature distribution. The temperature in the water bath and the electrochemical cell was controlled by means of thermometers ((0.5 K) and a K-type thermocouple (80 TK Fluke), and was found to agree to within (0.5 K. The experiments were conducted in the 273-318 K range with an interval of 5 K.25 Electrochemical Measurements. The experiments consisted of CV measurements in the potential range corresponding to the UPD H, anion adsorption, and the Aschem surface layer oxidation in 0.5 M aqueous H2SO4, thus mainly in the 0.05-0.90 V, RHE, potential range. The CV experiments were conducted at various scan rates varying between 10 and 50 mV s-1. The electrochemical instrumentation included (a) an EG&G model 263A or HQ Instruments model 101 potentiostat; (b) an EG&G 175 analog programmer; (c) a Philips model PM8133 X-Y recorder; (d) an IBM-compatible 80486 PC computer; and (e) EG&G M270 electrochemistry software. All potentials were measured with respect to a Pt/Pt-black RHE immersed in the same electrolyte. The data interpretation was done by examination of the CV transients corresponding to the UPD H, anion adsorption, and Aschem oxidation as well as reduction of the Aschemoxidized moiety. Since the potential of the CV features was determined versus the RHE,25 it became necessary to convert it to the standard hydrogen electrode (SHE) scale for subsequent data treatment (the standard conditions are aH+ ) 1 and fH2 ) 1 at any T27). This was achieved by application of eq 4.

ESHE,T ) ERHE(0.5M H2SO4) - 0.021 V

(4)

This formula was derived on the basis of the Nernst and Davis equations,28 while bearing in mind that the second dissociation constant for H2SO4 is low (K2 ) 1.2 × 10-2 M). In general, the Davis relation allows determination of the activity coefficients of the proton and the corresponding anion(s)-constituting electrolyte as a function of their ionic strengths. This conversion was sufficient when dealing with full-cell reactions and thus when plotting E/T versus 1/T in order to derive the standard enthalpy of the process (see below). However, the SHE scale varies with T and this dependence affects the potential values of the halfcell reactions. If the potentials of half-cells are to be compared, then a common scale has to be used: the SHE scale at T ) 298 K. The conversion from the SHE to the SHE at T ) 298 K is (25) Zolfaghari, A.; Jerkiewicz, G. J. Electroanal. Chem. 1997, 422, 1. (26) Angerstein-Kozlowska, H. In Comprehensive Treatise of Electrochemistry; Yeager, E., Bockris, J. O’M., Conway, B. E., Sarangapani, S., Eds.; Plenum Press: New York, 1984; Vol. 9, Chapter 1. (27) Winn, J. S. Physical Chemistry; Harper Collins: New York, 1995; pp 334-335. (28) Lurie, J. Handbook of Analytical Chemistry; Mir Publishers: Moscow, 1975.

achieved through application of eq 5,29 bearing in mind that the ∂E/∂T factor equals 8.4 × 10-4 V K-1.30

∂E ESHE,T)298K ) ESHE,T*298K + (T - 298) ∂T

(5)

For the sake of comparison, all CV profiles have been plotted versus SHE at T ) 298 K. However, the thermodynamic treatment has been performed on data with the potential expressed versus SHE at any T.

Results and Discussion Temperature Dependence of the Electro-Oxidation of the Irreversibly Chemisorbed As on Pt(111). The impact of T variation on the oxidation/reduction behavior of the irreversibly chemisorbed As layer on the Pt(111) substrate was verified by recording CV transients in the 0.05-0.75 V, RHE, potential range at 273 e T e 328 K with an interval of ∆T ) 5 K (Figure 1; we show CV transients of the surface redox process for three selected temperatures which are representative of the whole set of experiments). The data reveal findings that can be summarized as follows. (a) The anodic and cathodic parts of the CV profiles are slightly asymmetric with respect to the potential axis, and the asymmetry is an intrinsic property of the system; the CV profiles point to the existence of two overlapping peaks in both the anodic and the cathodic transients. (b) At T ) 273 K, the anodic CV part reveals two overlapping peaks of different heights; the first one (PAN1) being a sharp peak and the second one (PAN2) being a small one which overlaps PAN1 at more-positive potentials. The cathodic part of the CV transient shows two overlapping peaks (PCATH1 and PCATH2) having almost the same peak currents. (c) An increase of T from 273 to 313 K leads to charge redistribution, both on the anodic and cathodic sides. On the anodic side, the peak current of PAN1 drastically decreases and the peak broadens; PAN2 increases slightly as well. On the cathodic side, the change is less dramatic; however, PCATH1 increases slightly whereas PCATH2 decreases. (d) At T g 313 K, the anodic and cathodic parts of the transients are almost perfectly symmetrical with respect to the E axis; thus, T enhances the reversibility of the process. To assess if the charge density corresponding to the Pt(111)-Aschem surface oxidation and the surface reduction of the corresponding oxidized species was affected by T variation, we integrated the CV transients and determined the charge density, q/µC cm-2, related to the surface redox process. Conversion of the anodic and cathodic charge densities to the surface coverage by Aschem was accomplished using eq 6, bearing in mind that the surface redox process involved 3 e (see ref 10).

θAs )

q(µC cm-2)/3 240.8 µC cm-2

(6)

The results (Figure 2) reveal that (i) the Aschem surface coverage was in the 0.31 e θAs e 0.35 range, and thus that it was practically constant and θAs ) 0.33 was within the experimental uncertainty of the Aschem coverage determination, namely ∆θAs ) (0.02, and (ii) the surface coverage by Aschem or the oxidized Aschem species was unaffected by T variation. These findings point to the (29) Bratsch, S. G. J. Phys. Chem. Ref. Data 1989, 18, 1. (30) Conway, B. E.; Angerstein-Kozlowska, H.; Sharp, W. B. A. J. Chem. Soc., Faraday Trans. 1 1978, 74, 1373.

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Figure 1. CV profiles showing the surface redox behavior of the irreversibly chemisorbed As, Aschem, on Pt(111) in 0.5 M aqueous H2SO4 solution at 273, 293, and 313 K; A ) 0.0342 cm2 and s ) 50 mV s-1.

Figure 2. Relation between the Aschem surface coverage, θAs, and temperature, T, determined by integration of anodic (b) and cathodic (O) cyclic-voltammetry transients.

chemical stability of these overlayers on Pt(111) under the above pH, potential, and temperature conditions. The stability of the layers of Aschem and the oxidized Aschem species is of importance to electrocatalysis and to the examination of the thermodynamics of this surface redox process, described below. It might be beneficial to elaborate on the tiny feature that appears at ∼0.35-0.45 V on the cathodic side of some CV transients in Figure 1.10 At T ) 273 and 293 K this feature seems to be a little peak overlapping PCATH1, while at T ) 313 K it resembles a tail of PCATH1. The charge density associated with this feature (∼6-8 µC cm-2; see ref 10) corresponds to θAs ) 0.01 (see Figure 2) and thus to less than the experimental uncertainty of the Aschem coverage determination in our measurements (∆θAs ) (0.02). This feature might be associated with a tiny excess of Aschem, namely θAs ) 0.01, because the two specific CV transients shown in Figure 1 (for 273 and 293 K) refer to Aschem overlayers whose coverage is slightly greater than 0.33, namely 0.34. Even if it corresponds to a small excess of Aschem, it does not affect the main CV features (peaks PAN1, PAN2, PCATH1, and PCATH2) and their behavior upon temperature variation. Because this bump in the CV profiles disappears only when θAs ) 0.31,31 it would be

Figure 3. Relation between the potential of the anodic peaks, PAN1 (b) and PAN2 (O), and temperature, T.

premature to state that its pronouncement upon T decrease refers to an excess of Aschem and that is not an intrinsic feature of the system (no other T-dependent data for this system exist, and any other statement would be premature). We examined the shift of the anodic and cathodic peaks, representative of the surface redox process, brought about by T variation (Figures 3 and 4). The EPAN1 versus T and EPAN2 versus T relations presented in Figure 3 show that T augmentation leads to displacement of PAN1 and PAN2 toward more-positive potentials and that the slope of these relations, ∂E/∂T, equals 0.355 × 10-3 and 0.536 × 10-3 V K-1, respectively. The shift of PCATH1 and PCATH2 is also toward more-positive potentials, but the respective slopes, ∂E/∂T, have significantly higher values, namely 0.543 × 10-3 V K-1 for PCATH1 and 0.726 × 10-3 V K-1 for PCATH2. At present, it is difficult to assess the meaning of this slope, especially since the CV transients reveal charge redistribution upon T variation. However, the current results are of significance because they contribute to a better understanding of the system under investigation. Nature of the Surface Redox Process. Elsewhere,10 it was demonstrated on the basis of ex-situ STM measurements that the Aschem adlayer (θAs ) 0.33) on Pt(111) had the (x3 × x3)R30° structure and that its electro-

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and two cathodic peaks implies the existence of two energetically or structurally different species.

Second Pathway: Pt(111)-Aschem + 3H2O T Pt(111)-As(OH)3chem + 3H+ + 3e (9)

Figure 4. Relation between the potential of the cathodic peaks, PCATH1 (b) and PCATH2 (O), and temperature, T.

oxidation involved three electrons. The CV4,13 as well as in-situ FTIR and ex-situ STM results10 led to important conclusions regarding the initial and the final states of the respective oxidized and reduced overlayers but did not permit a detailed examination of the process’s mechanism. The new T-dependent experimental research and thermodynamic analysis presented in this paper lead to a new insight into this surface redox process and thereupon represents a novel and important contribution. It is essential to recognize that the CV profiles shown in Figure 1 always comprise two overlapping peaks and that their shape at 273 K is slightly asymmetrical with respect to the potential axis while at 313 K it is almost ideally symmetrical. The existence of the two oxidation/ reduction peaks and their temperature dependence inspired us to consider two different reaction pathways and thus two distinct interpretations. In agreement with the previous results,4,10 we make the initial hypothesis that the anodic scan refers to As(OH)3chem formation and the cathodic one to its subsequent reduction to Aschem. The validity of this assumption will be explained in a subsequent section on the thermodynamics of the surface redox process. First Interpretation. The surface redox process occurs in two steps and involves the transfer of two electrons in the first stage and one electron in the second (eqs 7 and 8). This proposal is based on the observation that the charge density under the first peak (PAN1 and PCATH1) is greater than that under the second (PAN2 and PCATH2) as well as on the similarity of the Pt(111)-Aschem system to the Pt(111)-Bichem one.3,4,20

First Pathway: Pt(111)-Aschem + 2H2O T Pt(111)-As(OH)2chem + 2H+ + 2e (7) Pt(111)-As(OH)2chem + H2O T Pt(111)-As(OH)3chem + H+ + e (8) This reaction pathway infers that the CV profiles could be deconvoluted into two peaks, the first one comprising 2/3 of q and the second one 1/3 of q. However, we were not able to resolve the CV transients into two components which would meet the above requirement. Thus, the existing experimental evidence does not support the first pathway and another explanation has to be sought. Second Interpretation. The surface redox process occurs in one step that involves transfer of three electrons and discharge of three water molecules (eq 9). However, if the process occurs in one step, then the existence of two anodic

The notion that the Aschem electro-oxidation can occur in one step is supported by the results presented in Figures 5 and 6. The data shown in Figure 5 reveal that as the Aschem surface coverage decreases from 0.33 to 0.23, the CV profiles undergo essential qualitative changes, namely from two anodic and two cathodic peaks to one anodic and one cathodic peak. Moreover, in the case of the Aschem overlayer having θAschem significantly less than 0.33, T variation does not result in the appearance of two CV peaks. Therefore, we may conclude that the two CV peaks shown in Figure 1 are characteristics of the electrooxidation behavior of the Aschem overlayer whose coverage is equal to 0.33. In Figure 6, we demonstrate a series of CV transients that exhibit further changes brought about by diminution of the Aschem surface coverage from 0.12 to 0.02. These CV transients show that the electro-oxidation of the Aschem overlayer proceeds in one step (one CV feature), with addition of three OH groups and direct oxidation of As0 to As3+ without any intermediate acting in the process. Because the charge density associated with the CV transients (Figures 1, 5, and 6) corresponds to the transfer of 3 e per Aschem adatom, the results support the reaction mechanism depicted in eq 9. This reaction pathway is compatible with the existence of two peaks in the CV transients (Figures 1 and 5) on the assumption that the Aschem oxidation leads to two coexisting, yet energetically and possibly structurally distinct, overlayers of As(OH)3chem on Pt(111), an assumption that yet has to be proven. Our ex-situ STM data10 on the As(OH)3chem characterization at T ) 298 K demonstrate the existence of the (x3 × x3)R30° structure that contains a certain degree of heterogeneity and a number of (x3 × x3) islands despite the surface being well ordered and defect free within the same domain (the domain analyzed by STM). These ex-situ STM results indicate that, indeed, the Aschem surface electro-oxidation could lead to formation of two chemically identical, yet energetically and structurally distinct, species. On the other hand, the two peaks for the Aschem oxidation process only appear at coverage values above 0.25. One possible explanation for the appearance of the two peaks is the transformation of an initial structure found at a Aschem coverage of 0.25 to the (x3 × x3)R30° structure, with the Aschem coverage increasing beyond 0.25. Each peak then could be assigned to a given, coexisting surface structure. To check that possibility, ex-situ STM measurements were performed to examine the structure of Aschem on Pt(111) at θAschem ) 0.25. No atomically resolved images were obtained for such a coverage, suggesting that the As overlayer was not ordered. Therefore, we can conclude that the two peaks appearing for the Aschem oxidation process are characteristic of the well ordered (x3 × x3)R30° structure. One possible answer to the existence of the two CV peaks can come from an analysis of the atomic separation between the Pt surface atoms in Pt(111) and the size of each As(OH)3chem adspecies comprising the chemisorbed overlayer. A hard-sphere model proposed in Figure 7 (we emphasize that it is a proposed model that has not been proven by

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Figure 5. CV profiles showing the surface redox behavior of the irreversibly chemisorbed As, Aschem, on Pt(111) in 0.5 M aqueous H2SO4 at three different Aschem coverages: 0.33, 0.30, and 0.23; T ) 298 K; A ) 0.0342 cm2, and s ) 50 mV s-1.

Figure 6. Set of CV profiles showing the surface redox behavior of the irreversibly chemisorbed As, Aschem, on Pt(111) in 0.5 M aqueous H2SO4 at T ) 298 K. The arrows indicate changes brought about by a gradual decrease of the Aschem surface coverage from 0.12 to 0.03; A ) 0.0342 cm2, and s ) 50 mV s-1.

in-situ STM) and an analysis of the ionic and atomic radii of As3+, OH-, and Pt32,33 summarized in Table 1 support the idea that the electro-oxidation of Aschem could lead to a densely packed (compact) As(OH)3chem overlayer on Pt(111). If we assume that As(OH)3chem has an sp3 hybridization (expected for an element of group VB) and that the nonbonding electron pair in one sp3 orbital is involved in the bond formation with the Pt(111) substrate, then the three OH groups bound to the Aschem adatom form a (31) Fernandez-Vega, A.; Feliu, J. M.; Aldaz, A.; Clavilier, J. J. Electroanal. Chem. 1991, 305, 229. (32) Pauling, L. The Nature of the Chemical Bond; Cornell University Press: Ithaca, NY, 1989. (33) Conway, B. E. Ionic Hydration in Chemistry and Biophysics; Elsevier: Amsterdam, The Netherlands, 1981.

Figure 7. Hard-sphere model of the As(OH)3chem on Pt(111) with Aschem in the threefold hollow site (A) and the on-top site (B). Both structures reveal that that As(OH)3chem layers are densely packed regardless of the Aschem adsorption site (proposal supported by ex-situ STM measurements). Table 1. Atomic and Ionic Radii species

Pt

As3+

OH-

radius

0.139

0.058

0.140

distorted tetrahedral structure. We are unable to predict if the OH-Aschem-OH angle would be greater or smaller than the 109° angle characteristic of an ideal sp3 structure. This reasoning is consistent with the most fundamental concepts of inorganic chemistry and surfaces science.34,35 At present, we do not possess any experimental data on the 3D structure of As(OH)3chem on Pt(111) (apart from our ex-situ STM results) nor are we aware of any modeling of the structure of this surface compound. In light of the above-proposed concept that is visualized in Figure 7 (34) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry; John Wiley & Sons: New York, 1966. (35) Somorjai, G. A. Introduction to Surface Chemistry and Catalysis; Wiley-Interscience: New York, 1994.

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(while bearing in mind the sp3 hybridization of Aschem and excluding the coplanarity of OH groups and Aschem adatoms), it is apparent that the As(OH)3chem overlayer is so densely packed that the OH groups constituting As(OH)3chem are in close contact with each other and thus they are within the limit of van der Waals interactions (these could lead to lateral repulsions). Besides, we recognize that (i) Aschem might occupy the threefold hollow site (octahedral or tetrahedral, Figure 7A) or/and the ontop site (Figure 7B),13 and (ii) the surface sites occupied by the OH groups are directly related to the adsorption site of the Aschem adatom within As(OH)3chem. Thus, if a well-ordered As(OH)3chem overlayer is formed on Pt(111), then it might possess two distinct structures originating from either different chemisorption sites occupied by Aschem (Figure 7) and/or different structural ordering of the OH groups around Aschem in order to minimize their lateral repulsions. The existence of As(OH)3chem in two distinct surface structures is conceivable, and it might lead to two CV features. However, if the two structures have a similar energy of formation and if the activation energy of transition from one structure to the other is low, then one might have two coexisting As(OH)3chem structures that undergo fast mutual transition. Consequently, if it were the case, then one would be unable to visualize them by in-situ STM. To examine this possibility and to explain why we were not able to obtain an in-situ STM image, we perform a theoretical analysis of the temperature-dependent CV transients and determine the energetics of the process and the standard enthalpy of formation of As(OH)3chem. Thermodynamics of the Surface Redox Process. The standard enthalpy of the surface redox process taking place in the cell, ∆H°, may be readily determined on the basis of eq 10,36 provided that (a) the surface species does not undergo desorption upon oxidation or reduction; (b) its surface coverage is unaffected by T variation; (c) the overall chemical process is well-defined; and (d) one can examine the T-dependence of the peak potential corresponding to the surface oxidation or reduction.

∆H°reduction ∂(E/T) )nF ∂(1/T)

∆H°oxidation ∂(E/T) )+ nF ∂(1/T)

(10)

where n is the number of electrons transferred. The overall surface chemical process taking place in the cell may be represented by summation of the following single-electrode processes:

Working Electrode Pt(111)-Aschem + 3H2O f Pt(111)-As(OH)3chem + 3H+ + 3e (11) Reference Electrode +

3

3H + 3e f /2H2

(12)

Overall Chemical Process ∆H° oxidation

Pt(111)-Aschem + 3H2O 98 Pt(111)-As(OH)3chem + 3/2H2 (13) Summation of the respective thermodynamic Born-

Haber cycles36 leads to the following equation describing the standard enthalpy of the electro-oxidation of Aschem:

∆H°oxidation ) ∆H°f(Pt(111)-As(OH)3chem) + 3

/2∆H°f(H2) - 3∆H°f(H2O) - ∆H°f(Pt(111)-Aschem) (14)

where ∆H°f(Pt(111)-As(OH)3chem) is the standard enthalpy of formation of Pt(111)-As(OH)3chem, ∆H°f(H2) is the standard enthalpy of formation of H2(g) and it equals 0.0 kJ mol-1, ∆H°f(H2O) is the standard enthalpy of formation of H2O(l) and it equals -258.8 kJ mol-1,37 and ∆H°f(Pt(111)-Aschem) is the standard enthalpy of formation of Pt(111)-Aschem. Two enthalpies of formation that appear in eq 14 are unknown, namely ∆H°f(Pt(111)-As(OH)3chem) and ∆H°f(Pt(111)-Aschem). The alteration of the Pt(111)-Aschem surface bond energy, δEPt(111)-Aschem, brought about by the new chemical environment (the surface compound formation) may be expressed by the following equation:

δEPt(111)-Aschem ) EPt(111)-Aschem(elemental) EPt(111)-Aschem(compound) (15) where EPt(111)-Aschem(elemental) is the Pt(111)-Aschem bond energyfortheelementalAschem overlayerandEPt(111)-Aschem(compound) is the Pt(111)-Aschem(compound) bond energy of the chemisorbed compound. At present, we are unaware of any thermodynamic data for the system nor of any semiempirical approach that would allow us to examine δEPt(111)-Aschem upon formation of As(OH)3chem. The value of δEPt(111)-Aschem could be determined using the Flores et al. model38 if the values of the hardness, η, of Aschem and As(OH)3chem were known (η is a measure of the ability of a surface compound to share electrons with the substrate). Since the values of η for Aschem and As(OH)3chem are unknown, we are forced to make the assumption that the electro-oxidation of Aschem does not modify the Pt(111)Aschem surface bond energy. This hypothesis might not be far from the reality because As is a strongly and irreversibly chemisorbed surface modifier (in a subsequent section we will elaborate on the validity of this assumption). Thus, on the basis of our assumption, we may write the following formula:

∆H°f(Pt(111)-As(OH)3chem) ) ∆H°f(Pt(111)-Aschem) + ∆H°f(As(OH)3chem) (16) Substitution of eq 16 into eq 14 leads to the following formula which relates the standard enthalpy of the process to the enthalpy of formation of As(OH)3chem:

∆H°oxidation ) ∆H°f(As(OH)3chem) + 3/2∆H°f(H2) 3∆H°f(H2O) (17a) or

-∆H°reduction ) ∆H°f(As(OH)3chem) + 3/2∆H°f(H2) 3∆H°f(H2O) (17b) because ∆H°oxidation ) - ∆H°reduction. Thus, on the basis of (36) Jerkiewicz, G.; Zolfaghari, A. In Solid-Liquid Electrochemical Interfaces; Jerkiewicz, G., Soriaga, M. P., Uosaki, K., Wieckowski, A., Eds.; ACS Symposium Series 656; American Chemical Society: Washington, DC, 1997; Chapter 4. (37) Lide, D. R. CRC Handbook of Chemistry and Physics, 74th ed.; CRC Press: Boca Raton, FL, 1993. (38) Flores, F.; Gabbay, I.; March, N. H. Chem. Phys. 1980, 63, 391.

Electro-oxidation of Chemisorbed As onPt(111)

Figure 8. E/T versus 1/T relations for the anodic peaks, (b) PAN1 and (O) PAN2.

Langmuir, Vol. 17, No. 10, 2001 3037

there are no thermodynamic data for As(OH)3 or As(OH)3chem. However, the standard enthalpy of formation of Bi(OH)3 is known and its value is -711.3 kJ mol-1.37 On the other hand it is well-established that the standard enthalpies of formation of various oxides, sulfides, and halides of As, Sb, and Bi are very close to each other.39 Thus, the proximity of the value of ∆H°f(As(OH)3chem) to the value of ∆H°f(Bi(OH)3) (they are within 33 kJ mol-1 from each other) supports the validity of the theoretical approach applied in the data analysis. Moreover, the magnitude of ∆H°f(As(OH)3chem) implies that As(OH)3 is the surface species formed in the course of the Aschem surface oxidation. It is essential to examine if As2O3 could be the species formed in the course of the process. First, if As2O3 were the product of Aschem oxidation, then it would desorb in the form of AsO2-. Second, if As2O3 were the species formed during the electro-oxidation (assuming that it could stay on the Pt(111) substrate), then the standard enthalpy of formation of As2O3 determined using eq 10, the corresponding Born-Haber cycles, and the E/T versus 1/T data (see eq 18 for the overall process; here n ) 6) would be ∼ -500 kJ mol-1.39

2Pt(111)-Aschem + 3H2O f Pt(111) - As2O3chem + 3H2 (18)

Figure 9. E/T versus 1/T relations for the cathodic peaks, (b) PCATH1 and (O) PCATH2.

the T-dependent cyclic-voltammetry study one may assess the standard enthalpy of the overall surface chemical process and afterward determine ∆H°f(As(OH)3chem). In the course of research, we plotted E/T versus 1/T for the anodic peaks (PAN1 and PAN2, Figure 8) and the cathodic peaks (PCATH1 and PCATH2, Figure 9), and then we fitted them into linear relations whose correlation coefficients were over 0.99. The slope of the E/T versus 1/T relations had the following values: 0.659 for PAN1, 0.634 for PAN2, 0.602 for PCATH1, and 0.574 V for PCATH2, respectively. Application of eq 10 while bearing in mind that the number of electrons exchanged in the process is n ) 310 allowed us to determine the standard enthalpy of the surface oxidation and reduction processes represented by PAN1, PAN2, PCATH1, and PCATH2 in the CV profiles (Table 2). Compilation of the data presented in Table 2, eq 17, and the standard enthalpies of formation of H2O(l) and H2(g) permits determination of the enthalpy of formation of the chemisorbed As(OH)3. The values of ∆H°f(As(OH)3chem) derived on the basis of the anodic and cathodic CV transients and the above-presented thermodynamic approach are summarized in Table 2. An analysis of the data shown in Table 2 reveals that the values of ∆H°f(As(OH)3chem) determined on the grounds of T-dependent cyclic-voltammetry research are close to each other, namely within 27 kJ mol-1, or 4% of the average value of ∆H°f(As(OH)3chem), which is -678 kJ mol-1. It is important to assess the validity of the thermodynamic approach and the assumption that the oxidation of Aschem leads to the formation of As(OH)3chem through examination of the magnitude of ∆H°f(As(OH)3chem). Incidentally, it should be stated that

Clearly, As2O3 cannot be the product of the process because its enthalpy of formation would be expected to be close to -300 kJ mol-1;39 thus, there exists a difference of 200 kJ mol-1 than cannot be accounted for. In the course of the discussion, we suggested that Aschem can exist in two energetically, and possibly structurally, different states. The above values of ∆H°f(As(OH)3chem) are close to each other, say within 8-10 kJ mol-1 (Table 2). Their proximity indicates that the two distinct states of Aschem and As(OH)3chem, proposed on the basis of Tdependent CV studies followed by a detailed thermodynamic analysis, are in a similar energetic state. It is worthwhile mentioning that the enthalpies of formation of the cubic and monoclinic As2O3 are within 2.1 kJ mol-1 of each other and that their As-O bond energies are practically the same.39 These data show that As can form structurally different, yet energetically alike, compounds. The above-presented experimental results, thermodynamic data, and reasoning suggest that we might form two coexisting As(OH)3chem structures on Pt(111) that have very similar values of ∆H°f. Interestingly, the abovepresented CV data followed by a thermodynamic analysis point to an unsurpassed sensitivity of cyclic-voltammetry as a surface-science technique, both in determination of the surface coverage by the adsorbed species and in analysis of the energetics of the process. In combination with other techniques such as FTIR and STM it provides an additional input into the process and sheds light on the nature of the species formed. During the data treatment, we assumed that the energy of the Pt(111)-Aschem surface bond does not change upon attachment of OH groups to Aschem. To sustain impartiality, we wish to add that there might be some modification of this bond energy upon the OH group addition, originating from local electronic effects. These local effects are expected to be related to the adsorption site of As, and they would be expected to be more pronounced for As occupying the threefold hollow site than the on-top one (Figure 7). (39) Bailar, J. C., Emele´us, H. J., Nyholm, R., Trotman-Dickenson, A. F., Eds. Comprehensive Inorganic Chemistry; Pergamon Press: Oxford, 1973; Chapter 21.

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Table 2. Standard Enthalpy of the Surface Oxidation and Reduction Processes and Standard Enthalpy of Formation of As(OH)3chem CV feature

AN1 (oxidation)

AN2 (oxidation)

CATH 1 (reduction)

CATH 2 (reduction)

∆H°/kJ mol-1 ∆H°f(As(OH)3)/kJ mol-1

+191 ( 2 -664 ( 5

+183 ( 2 -674 ( 5

-174 ( 2 -683 ( 5

-166 ( 2 -691 ( 5

However, any Pt(111)-Aschem bond energy modification might be small, especially if the orbitals involved in the As-OH bond formation (As having the sp3 hybridization) are different from those involved in As chemisorption. At present, this assumption cannot be verified due to the lack of thermodynamic data and lack of knowledge of the hardness, η, of Aschem and As(OH)3chem (making any semiempirical calculations such as those proposed by Flores et al.38 impossible), and plainly more work on the system is needed using modern surface-science techniques, which would lead to assessment of the Pt-Aschem bond length, bond energy, and electronic structure prior to and after electro-oxidation. Nonetheless, this T-dependent research represents a new and important contribution to the study of irreversibly chemisorbed adlayers that can act as surface modifiers in electrocatalysis. The abovepresented discussion is also an extension of electrochemical thermodynamics to systems other than underpotential deposition.36,40

2. The CV peaks, which correspond to the Aschem oxidation and the respective reduction of As(OH)3chem, displace toward more-positive potential upon T increase. The corresponding peak potential versus T relations are linear. 3. The new T-dependent study supports the notion that the Aschem electro-oxidation (θAs ) 0.33) proceeds in one step with transfer of three electrons and addition of three OH groups. 4. The existence of two anodic and two cathodic peaks might be assigned to two chemically identical, yet energetically and structurally different, species (different surface coordination). 5. Theoretical analysis of the data leads to determination of the standard enthalpy of formation of As(OH)3chem, ∆H°f(As(OH)3chem), which is -678 kJ mol-1. The magnitude of this value indicates that As(OH)3chem is the surface species formed in the course of Aschem oxidation.

Conclusions 1. Temperature variation in the 273-313 K range affects neither the surface-chemical nature nor the surface coverage of the Aschem overlayer (θAs ) 0.33) on Pt(111) in aqueous H2SO4 solution; thus, the overlayer is chemically stable. (40) Zolfaghari, A.; Jerkiewicz, G. J. Electroanal. Chem. 1999, 467, 177.

Acknowledgment. An acknowledgment is made to the NSERC of Canada, le FCAR du Que´bec, Ministerio de Ciencia y Tecnologia, and DGI (Grant No. BQU 20000240) for support of this research project. G.J. gratefully acknowledges a Visiting Professorship from Generalitat Valenciana, Spain. LA990598S