15096
J. Phys. Chem. B 2004, 108, 15096-15102
In Situ XAS Investigation of Transformation of Co Monolayer on Carbon-Supported Platinum Clusters Underpotential Control B. J. Hwang,*,† Y. W. Tsai,† Loka S. Sarma,† C. H. Chen,† J.-F. Lee,‡ and H. H. Strehblow§ Nanoelectrochemistry Laboratory, Department of Chemical Engineering, National Taiwan UniVersity of Science and Technology, Taipei, 106, Taiwan, R.O.C. ReceiVed: June 22, 2004; In Final Form: July 15, 2004
A complete electrochemical oxidation-reduction cycle for a Co monolayer on carbon-supported Pt clusters in alkaline solution has been monitored in situ by X-ray absorption spectroscopy (XAS). The near range order and oxidation state of the Co monolayer as a function of potential has been evaluated by in situ XAS and it was found that the oxidation state of the Co monolayer remains +2 even at a potential of -0.8 V vs Ag/AgCl and becomes +3 at +0.5 V vs Ag/AgCl. No desorption of Co was observed during the oxidationreduction cycle. Two main peaks in the Fourier transformed Extended X-ray Absorption Fine Structure (EXAFS) spectra were found. The first peak is attributed to the nearest oxygen. The second peak is ascribed to the nearest Co neighbors upon reduction and to the nearest Co and Pt neighbors upon oxidation. It is interesting that the deposited cobalt appears to form a structure similar to that of Co(OH)2 at the reduced conditions but the incorporation of cobalt oxide into the platinum oxide/hydroxide surface layer was found at the oxidized conditions. The transformation between the reduced Co and the incorporated cobalt oxide is reversible during the reduction-oxidation cycle.
1. Introduction The rate and mechanism of several electrochemical reactions are well-known to be dependent on the surface-specific properties of the electrode encompassing the electrocatalyst material. The catalysis of electrochemical reactions occurring at the electrode/electrolyte interface may be divided into two main branches: the first one is the heterogeneous catalysis by electroactive species in which the reaction mainly happens at the electrode surface and the second one is the homogeneous catalysis of electrode reactions by free species in the bulk of the solution. The formation of two-dimensional sub- and monolayer surface coverage by the adsorption of heavy metal atoms on noble and transition metal electrodes at potentials positive to the Nernst potential of the corresponding M/Mz+ electrode is a convenient method for modifying electrode surfaces with respect to their electrocatalytic activity.1-4 This phenomenon, referred to as underpotential deposition (UPD), has attracted considerable interest recently from both the fundamental and practical points of view. Interest in such processes stems from the recognition that sub- and monolayers may have profound influence on electrode substrates. This kind of electrode substrates have been widely used for electrocatalytic reactions involved in the field of electrosynthesis, sensor, and fuel cell technologies. A greater understanding of the UPD process can be obtained from the literature studies.5 During the last two decades, some fundamental investigations were undertaken on the platinum group, modified by heavymetal adatoms deposited at underpotential.6 Due to the simple atomic nature of the adsorbed species these modified electrodes * Corresponding author. E-mail:
[email protected]. † National Taiwan University of Science and Technology. ‡ Present address: National Synchrotron Radiation Research Center, Hsinchu 300, Taiwan. § Present address: Institute of Physical Chemistry and Electrochemistry, Heirich-Heine University, Duesseldorf, Germany.
provide useful model systems for studying the influence of surface structure on heterogeneous catalysis by foreign metal adatoms. The foreign metal adatoms may enhance the electrocatalytic activity of metal substrates in different ways, i.e., by (i) formation of a bifunctional catalyst, providing different active sites for adsorption of molecules or radicals participating in the electrode reaction, (ii) acting as heterogeneous redox mediators; (iii) modifying the electronic properties of the surface, and (iv) preventing the surface of the electrode from being poisoned by strongly adsorbed intermediates. The enhancement of the electrocatalytic activity of metal electrodes by an UPD sub- and monolayer coverage can (i) permit electron-transfer processes to occur at a much lower overpotential, (ii) improve the selectivity of the electrocatalyst surface, and (iii) improve the stability of electrocatalysts. The decrease of the overvoltage of electrochemical reactions and increase of the current density may prove to be of practical significance, particularly in electrochemical energy conversion. Examples of such systems include electroreduction of oxygen, electrooxidation of hydrogen, and some organic fuels. In recent years a number of in situ and ex situ structuresensitive techniques have been applied to characterize electrochemical systems at the atomic level. The conventional and sensitive ultrahigh vacuum (UHV) techniques, which rely on the emission or scattering of electrons, have been used ex situ by coupling the UHV chambers to the electrochemical cell with appropriate transfer systems. While these techniques can identify species and give information about both energetics and structure, they are clearly subject to a number of restrictions: surfaces may restructure during transfer and measurements on adsorbates are restricted to strongly chemisorbed species. To overcome these limitations, it is desirable to examine the electrodes in situ, that is, in an environment that closely duplicates one of a real device. This optimal strategy, however, elicits a new set of challenges derived from the need of finding suitable probes with
10.1021/jp047273r CCC: $27.50 © 2004 American Chemical Society Published on Web 08/31/2004
Co Monolayer on Pt/C Clusters: In Situ XAS Investigation
J. Phys. Chem. B, Vol. 108, No. 39, 2004 15097
which to view inside the cell. The recent advent of high-intensity tunable sources of X-rays, now available at synchrotron facilities worldwide, made X-ray absorption spectroscopy (XAS) a powerful tool for examining electrodes in situ and have opened new prospects for studying materials to an extent far beyond the simple imaging or crystal structure determination. The penetration power and elemental specificity of X-rays is the key to the development of new experimental strategies, which are now beginning to unveil various aspects of operating electrodes. The XAS method yields information about the near range order of the adsorbed atoms as well as their chemical state7 under reaction conditions. The full exploitation of these capacities will undoubtedly lead to a better understanding of electrocatalysts by allowing us to monitor the interior while the device is at work. Meanwhile, surface reactions involving dispersed metal clusters are of great significance in various fields such as catalysis, fuel cells, and energy storage.8,9 These reactions include adsorption and desorption of atoms and ions as well as chemical processes in which these adsorbates participate. A detailed knowledge of the reactions at metal cluster surfaces is therefore of special interest. During the last two decades new methods have been developed, particularly scanning tunneling microscopy and surface X-ray scattering, which have significantly improved our knowledge about the structure of solid surfaces and the processes occurring at the solid/ambient interfaces. Most of the published works, however, deal with the structure of bare and adsorbate-covered single crystal surfaces.10 Contrary to that, there exist only a few reports about the structure of adsorbates deposited on the surface of metal clusters.11-14 A characteristic feature of metal-cluster materials is their large specific surface area of up to several 100 m2 per gram. Due to the large ratio of surface area to mass, metal clusters covered with a monolayer or even submonolayer of an adsorbate open a unique possibility for the in situ study of adsorbed species with use of X-ray absorption spectroscopy (XAS). In contrast, the presence of a liquid phase next to the surface under study prevents the use of most of the techniques employed in UHV for in situ investigations. The oxidation of a metal surface is one of the most fundamental electrochemical phenomena. Whereas the processes of oxide formation and reduction have been intensively studied with electrochemical, spectroelectrochemical, and ex situ UHV methods,15 there exist only a few reports about the structure of oxide layers formed during the very first stages of the bulk metal oxidation16 and to the best of our knowledge there are no reports in the literature on the structure and the chemical state of the Co monolayer on metal clusters supported on carbon during their electrochemical oxidation and reduction. The objectives of the current study are to investigate in situ the transformation of a Co monolayer, which was formed by an UPD process on highly dispersed carbon supported Pt clusters, using X-ray absorption spectroscopy. The change of chemical state and near-range order of a Co monolayer during oxidation and reduction will also be discussed.
The electrode preparation procedure was similar to that published by McBreen.12 Prior to cobalt deposition the electrode was cleaned by cycling in 0.5 M H2SO4 in the potential range between the hydrogen-evolution and the oxygen-evolution region. The cycling was performed until a current-potential curve similar to that for massive polycrystalline platinum electrodes was obtained. The deposition of cobalt was then performed in a solution containing 0.1 M HNO3 and 30 mM Co(NO3)2 by holding the electrode for several hours at a potential of -0.55 V vs Ag/AgCl/KCl (saturated in water), i.e., slightly above the Co/Co2+ equilibrium potential for a 30 mM Co2+ solution. After the deposition, the electrode was removed from the acidic solution and was immediately immersed into a Co2+ free 0.1 M NaOH solution and the electrode was operated at a potential of -0.6 V vs Ag/AgCl/KCl (saturated in water) and then transferred into the cell containing 0.1 M NaOH for XAS measurements.18 X-ray Absorption Spectroscopy (XAS) Measurements. The XAS measurements were performed at the Wiggler-C beam lines in the vicinity of the Co absorption edge at the National Synchrotron Radiation Research Center (NSRRC) of Taiwan. The intensity of the incoming X-ray beam as well as the transmitted intensity was measured with use of the gas-filled ionization chambers. A third ionization chamber was used for a simultaneous recording of cobalt foil spectra. The X-ray beam from the storage ring was monochromatized by a double-crystal Si(111) monochromator. After the measurement at a particular electrode potential, the value was scanned to another one with a scan rate of 2 mV/s and the next spectrum was recorded after the equilibrium states were obtained. EXAFS Data Analysis. Standard procedures were followed to analyze EXAFS data. First, the raw absorption spectrum in the preedge region was fit to a straight line and the background above the edge was fit with a cubic spline. The EXAFS function “χ” was obtained by subtracting the postedge background from the overall absorption and then normalizing with respect to the edge jump step. The normalized χ(E) was transformed from energy space to k-space, where “k” is the photoelectron wave vector. The χ(k) data could be used to describe the oscillation of the backscattering wave through the local environment, ca. 10 Å. The k3‚χ(k) for the Co absorber was calculated to compensate the damping of EXAFS oscillations in the high k-region. Subsequently, k3-weighted χ(k) data in the k-space ranging from 1.33 to 14.83 Å-1 for the Co K-edge was Fourier transformed (FT) to r-space to separate the EXAFS contribution from different coordination shells. A nonlinear least-squares algorithm was applied for the curve fitting of EXAFS in r-space between 1.0 and 3.2 Å for the Co K-edge. The standards for Co-Pt, Co-Co, and Co-O interactions were calculated theoretically from a crystallographic model of PtCoOx. All the computer programs were implemented in the UWXAFS package.19 The structure parameters for a specific absorber, such as coordination numbers (N), bond distance (R), and the DebyeWaller Factor (σ2), were extracted by curve-fitting analysis based on FEFF601.20
2. Experimental Section
3. Results and Discussion
Electrochemical Measurements. Carbon-supported platinum cluster material was obtained from Merck, Darmstadt, Germany. The mean diameter of platinum clusters supported on carbon has been determined previously by XAS and was found to be 2 nm17 and it is consistent with the data given by the supplier. The electrodes were fabricated with use of carbon-supported platinum cluster material with a platinum loading of 10 wt %.
Electrochemical Results. Figure 1 shows typical cyclic voltammograms (CV) recorded for a Co monolayer/Pt/C electrode (solid line) and a bare Pt/C electrode (dotted line) in 0.1 M NaOH solution. In the case of the bare Pt/C electrode, peaks designated as C1 and A1 are due to hydrogen adsorption and desorption, respectively. The two characteristic peaks A2 and C2 appeared at around -0.2 and -0.3 V, respectively, and
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Figure 1. Cyclic voltammograms for a Pt/C electrode in 0.1 M NaOH: electrode without adsorbate layer (Dotted line) and after deposition of a cobalt monolayer (solid line) at -0.55 V vs Ag/AgCl/ KCl (saturated in water) in a solution of 0.1 M HNO3 containing 30 mM Co(NO3)2. Scan rate: 1 mV/s.
indicate the oxide formation and reduction of Pt clusters for the bare Pt/C electrode in the alkaline solution. As can be seen in Figure 1, the most characteristic feature in the CV for the Pt/C electrode after Co UPD is the absence of the hydrogen adsorption and desorption peaks, which are present in the curve of the bare Pt/C electrode. This strongly suggests that, after electrode transfer to the alkaline solution, Pt clusters were still completely covered with a Co monolayer or at least that all of the hydrogen adsorption sites were blocked by a Co monolayer during a cathodic scan. It is noteworthy that two new anodic peaks A3 and A4 at potentials -0.3 and 0.1 V, respectively, and a new cathodic peak C3 at -0.6 V associated with a cobalt monolayer appear in the voltammetric profile. The peak A3 is ascribed to the oxidation of the Co monolayer and the peak A4 is assigned to be the oxide formation of the Pt or Pt-Co alloys. The shoulder before the A3 peak may be assigned to the adsorption of OH-. The cathodic peak C3 is ascribed to the reduction of Pt or PtCo alloy oxides formed during the anodic scan. We have calculated the coulometric charge corresponding to the redox process [Co(OH)2 + H2O T Co(OH)3 + H+ + e-] related to the A3 peak at -0.3 V, and a value of 230 µC cm-2 was obtained. In the literature,16a for a monolayer of Co on Pt, a coulometric charge of 250 µC cm-2 associated with the redox process [Co(OH)3 + H+ + e- T Co(OH)2 + H2O] at -0.355 V was claimed. This value is very close to the theoretical charge corresponding to a process that exchanges one electron per surface Pt atom at a Pt(111) surface (243 µC cm-2). As these values are close to our calculated value we assume the cobalt coverage on Pt clusters is nearly monolayered assuming that each cobalt exchanges one electron. The literature dealing with the Co UPD process is rather scarce and to the best of our knowledge, there is no thorough study concerning the behavior of the Co UPD layer on platinum clusters. Abruna et al.16a have studied the chemical state of a Co monolayer irreversibly adsorbed onto a Pt(111) surface by means of an electrochemical, in situ surface EXAFS and crystal truncation rod (CTR) studies. They found that the Co monolayer in an alkaline solution of 0.1 M NaOH shows two voltammetric peaks and they ascribed one happening at a potential of -0.355 V to the CoIII(OH)3/CoII(OH)2 redox process and the other occurring at -1.04 V that was assigned to the Co0 generation process. We too have observed the oxidation of CoII(OH)2 at lower potentials of ca. -0.3 V. However, during the forward scan at the high potentials of ca. 0.1 V there is a possibility for the formation of Pt-Co alloy oxides. This formation is more probable due to the fact that we used 2-nm Pt nanoparticles
Figure 2. Near-edge absorption spectra of Co monolayer on Pt/C in 0.1 M NaOH for various potentials. The spectrum recorded at +0.5 V was measured at the beginning of the oxidation/reduction cycle.
supported on carbon for Co UPD experiments. We will confirm this later from the in situ XAS studies. The electrochemical behavior of the Co monolayer/Pt/C electrode is completely different from that of the bare electrode. It further implies that the surface of the Pt clusters was completely covered with the Co monolayer. The peak at +0.5 V shows the oxygen evolution. It was found that the oxygen evolution is depressed. There are two reasons for this observation. One is that most of the active sites are blocked by the Co monolayer on the surface of the Pt clusters and the other is the formation of mixed oxides of Co and Pt. Since only one reduction peak (C3) was found in the cathodic scan, it is suggested that the oxides of Pt-Co alloy clusters are formed in the anodic scan. This will be further confirmed from the in situ investigation of XAS. X-ray Absorption Results. The X-ray absorption near-edge spectra of a Co monolayer on Pt/C clusters in 0.1 M NaOH for various potential series are shown in Figure 2. The position and the shape of the near-edge absorption curve are indicative for the electronic structure of the absorbing atom and sensitive to the site environment, i.e., coordination number and metal oxidation state.21 The preedge peak arising from 1s to 3d absorption transition is observed in the case of tetrahedral cobalt environments but is forbidden for an octahedral environment.22 These features are absent in the case of preedges present in Figure 2, indicating that Co atoms in a cobalt monolayer on carbon-supported Pt clusters and in reference Co(OH)2 are in an octahedral environment. In the case of LiCoO2, 1s f 3d transition is theoretically forbidden because of its octahedral symmetry. But this transition is observable with moderate intensity due to electric quadrupole couplings. The relative Co 3d f 4p orbital mixing arises in the noncentrosymmetric environment around Co ions of distorted CoO6 octahedra.23 This is the reason for the appearance of preedge peak in the XANES spectra. The XANES curves of the cobalt monolayer at the oxidized state (+0.5 V) and again at +0.5 V after cycling through negative potentials were superimposable, revealing the structural and hence electrochemical reversibility of the redox processes.
Co Monolayer on Pt/C Clusters: In Situ XAS Investigation
J. Phys. Chem. B, Vol. 108, No. 39, 2004 15099
Figure 4. The edge shift of different species against its oxidation state formed during the Co monolayer transformation. Figure 3. First derivatives of the near-edge spectra in Figure 2. The left dashed line corresponds to the edge position of the Co adsorbate in the reduced state and the right one marks the edge position for the +3 oxidized state.
The changes in near-edge spectra can be better visualized by taking the 1st derivatives of the absorption spectra, as shown in Figure 3. As can be seen from Figure 3 (left dashed line) the edge positions of the cobalt monolayer at negative potentials of -0.6 and -0.8 V are similar and comparable with the reference Co(OH)2, indicating that the oxidation state and the near-range order around the cobalt ions in the monolayer is similar to that of Co(OH)2 at the negative potentials of -0.6 and -0.8 V. However, the edge position shifts toward the higher energy side (right dashed line) when the potential was switched to -0.1 V and then to +0.5 V, indicating the increase in oxidation state of oxidized monolayer species. However, the oxidation state and the near-range order of the oxidized form of cobalt are different from those of the LiCoO2. Figure 4 shows the edge shift of the different species against their oxidation states. The edge shifts were measured by taking energies at the inflection points of their corresponding XANES spectra. At +0.5 V a shift of the absorption edge by ca. 2 eV relative to that at -0.8 V is observed with a simultaneous change of its shape corresponding to the formation of Co(III) species. The oxidation state does not change with the subsequently applied potential of -0.1 V. At -0.6 V a shift of the edge position toward lower energy is detectable. At potentials of -0.6 and -0.8 V, the shape and position of the edge matches exactly with those of reference Co(OH)2. It is interesting that the Co monolayer is unable to be reduced to cobalt metal even at the potential of -0.8 V. This situation is different from that in the case of Cu monolayer in which the Cu monolayer on Pt clusters can be completely reduced to copper metal at a cathodic potential.18 The near-edge spectra indicate a transition from Co(III) to Co(II) during the cathodic part of the cycle as well as an oxidation of the Co(II) to Co(III) during the reverse potential scan. Additionally, the reduction of the bulk oxide layer to Co(II) occurs at a significantly more negative potential
Figure 5. EXAFS spectra of a Pt/C electrode with a Co monolayer in 0.1 M NaOH at +0.5 V, i.e., at the beginning of the oxidation cycle, at -0.8 V, the most negative potential of the cycle, and at +0.5 V, after the subsequent reduction of the Co monolayer on Pt/C electrode.
compared to the metal monolayer deposited on the surface of platinum clusters. We have performed the EXAFS experiments on Co monolayer deposited on Pt clusters dispersed on carbon in 0.1 M NaOH during an electrochemical oxidation/reduction cycle. It was observed that there was no loss of the adsorbates even after several oxidation/reduction cycles within the potential range of -0.8 to +0.5 V (vs Ag/AgCl) corresponding to a repeated oxidation of the Co monolayer and its subsequent reduction. Figure 5 shows EXAFS spectra for
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Figure 6. Two-shell fits (circle line) with the back transformed experimental data (solid line) of Figure 7 of a Pt/C electrode with a Co monolayer in 0.1 M NaOH at +0.5 (a), -0.1 (b), -0.6 (c), and -0.8 V (d).
the cobalt adsorbate in the oxidized state (+0.5 V) at the beginning of the potential cycle. Then the potential was swept to and kept at -0.1, -0.6, -0.8, and again +0.5 V, respectively. For comparison, spectra recorded for Co(OH)2 and LiCoO2 are also shown in Figure 5. These spectra suggest that the cobalt monolayer deposited on the surface of platinum clusters can be reversibly oxidized and reduced. The two-shell fit (dashed line) matches closely with the backtransformed experimental data of Figure 7 (solid line) for the whole series of potentials, as shown in panels a, b, c, and d of Figure 6 at +0.5, -0.1, -0.6, and -0.8 V, respectively. This fine structure is closely related to the near-range order in the vicinity of the scattering atom. Similar to the near-edge spectra, we observe characteristic changes in the oscillatory structure of the spectra during the cathodic and the anodic scan. The most obvious changes occur between -0.1 and -0.6 V indicating significant changes in the near-range structure around the cobalt atoms.
The Fourier transform of EXAFS yields the nearest neighbors distribution function correlated to the near-range order structure around the scatterer. The potential dependent EXAFS data corresponding to the spectra of Figure 5 in the k-range of 1.33 to 14.83 Å-1 were Fourier transformed into the r-space and are shown in Figure 7. There are two peaks observed in the spectra both at the anodic (-0.1 and 0.5 V) and at the cathodic parts (-0.6 and -0.8 V). The first peak corresponds to the backscattering of oxygen atoms ligating Co and the second peak may be responsible for the backscattering of Co and Pt atoms beyond the O coordination shell. It was found that the interatomic distance for both shells at cathodic potentials is obviously longer than that at anodic potentials. Reversing the potential scan at a potential above the oxide formation leads to a reversible change of the absorption spectra. The FT -spectra at 0.5 V in the reverse scan closely match those measured at the beginning of the CV cycle indicating, again, a full reversibility of the adsorbate structure and oxidation state. According to the EXAFS data and the near-edge spectra, the
Co Monolayer on Pt/C Clusters: In Situ XAS Investigation
J. Phys. Chem. B, Vol. 108, No. 39, 2004 15101 TABLE 1: Near-Range Order Structure Parameters According to the Evaluation of the EXAFS Data at Different Potentialsa
samples
E/V vs (Ag/AgCl satd KCl) atoms
UPD Co/Pt/C OCP Co/Pt/C -0.1 Co/Pt/C -0.6Co/Pt/C -0.8Co/Pt/C +0.5+ Co/Pt/C +0.5+ Figure 7. EXAFS data of Figure 5 between 1.33 and 14.83 Å-1 are Fourier transformed into the r-space shown here.
cobalt monolayer is reduced at below -0.6 V and oxidized at -0.1 V. For a mean Pt particle size of 2 nm as used in our study, the surface of a particle has been suggested to be composed of 5% (100) faces and 50% (111) faces, with the reminder being edge and corner atoms.24 Depending on the applied potential, the oxidation of the Co atoms will start at the most active sites of the Pt clusters. The FT-EXAFS spectra shown in Figure 7 have been used to derive the structural parameters based on a three-shell model involving the Co-O, Co-Co, and Co-Pt shells characterizing the near-range structure in the vicinity of Co atoms which are summarized in Table 1. Figure 6 suggest a good agreement between the experimental results and theoretical fitting. The number of nearest oxygen, Co, and Pt neighbors is 6.0, 4.9, and 1.3, respectively, at the potential of -0.6 V. Meanwhile the distance of the Co-O and Co-Co bonds is 2.10 and 3.20 Å, respectively. The near-range structure of Co atoms at -0.8 V is similar to that at -0.6 V. The coordination numbers of O and Co around the absorbed Co atoms in the Co monolayer at the negative potentials are similar to those in Co(OH)2. This suggests that the Co monolayer is in the form of Co(OH)2 at the negative potentials. Note that the contribution of Pt neighbors is negligible at the negative potential. Similar results have also been observed in the case of the Cu monolayer in our previous work.18 It implies that the Co(OH)2 is weakly adsorbed onto the surface of Pt clusters and forms clusters of Co(OH)2 at negative potentials. The Co-O, Co-Co, and Co-Pt distances at anodic potentials of -0.6 and -0.8 V are similar and around 2.10, 3.20, and 2.88 Å, respectively, and the values obtained at -0.1 V are also similar. The coordination numbers of O, Pt, and Co neighbors in the vicinity of cobalt at the potential of -0.1 V are 3.8, 4.8, and 5.4, respectively. These values are similar to those obtained at the 0.5 V potential. The high number of Pt nearest neighbors suggests that at the anodic potentials the cobalt hydroxide is oxidized and incorporated into the Pt clusters simultaneously. These results further confirm the CV behavior of the Co monolayer on carbon-supported Pt cluster electrodes. The two peaks corresponding to the reduction of Co oxides and Pt oxides were not observed in the CV. However, only one reduction peak has been observed in the cathodic scan of CV and it is believed to arise from the reduction of the oxides
Co(OH)2
reference
LiCoO2
reference
N
R/Å
σ2/Å2
∆E0/eV
GF
O Co Pt O Co Pt O Co Pt O Co Pt O Co Pt O Co Pt
3.7 5.2 5.4 3.8 5.4 4.8 6.0 4.9 1.3 6.0 4.0 1.3 3.4 4.7 7.9 4.3 4.8 6.9
1.90 2.86 2.88 1.91 2.85 2.88 2.10 3.20 2.88 2.10 3.20 2.88 1.89 2.84 2.88 1.91 2.84 2.88
0.0034 0.0060 0.0106 0.0024 0.0060 0.0011 0.0085 0.0067 0.0170 0.0080 0.0064 0.0181 0.0026 0.0066 0.0190 0.0037 0.0062 0.0184
-8.00 -7.73 4.07 -5.27 -8.51 5.35 -3.59 -3.29 14.40 -3.77 -3.22 13.66 -7.30 -8.82 3.44 -5.07 -9.34 2.36
3.071
O Co O Co
6.0 6.0 5.7 5.6
2.11 3.19 1.92 2.82
0.0058 0.0062 0.0021 0.0022
-4.53 -5.11 11.06 7.96
3.172 5.410 5.236 3.907 4.615
5.045 5.897
a OCP is the open circuit potential, N is the coordination number, R is the neighbor shell distance, σ2 is the structural disorder term, ∆E0 is the threshold energy, and GF is the goodness of fitting; + and - mean positive and negative scans, respectively.
SCHEME 1: Cobalt Monolayer Transformation on Carbon-Supported Pt Clusters under Potential Control
of the Pt-Co alloy. Finally, understanding the results from CV and in situ XAS analysis allowed us to build a model to account for the transformation of Co monolayer on Pt clusters dispersed on carbon underpotential control that is shown in Scheme 1. Discussion on the Structure of Cobalt Monolayer on Pt Clusters. We have attempted to deduce the probable structure of the cobalt monolayer on Pt clusters at higher potentials based on the CV and in situ XAS results. We have assigned the oxidation peak in the CV at -0.3 V to the oxidation of the Co monolayer. At more negative potentials the EXAFS spectral parameters indicated that the cobalt monolayer gives parameters similar to the close-packed cubic octahedral structure with nearly six oxygen atoms at 2.10 Å. At this stage we did not observed any contribution from Pt in the second coordination shell. However, at potentials close to OCP and above, the Pt contribution increases. As we have observed another oxidation peak in the CV at 0.1 V, we assumed that this peak could be due to the mixed oxides of Pt and Co. As found from the EXAFS results, at higher potentials (+0.5 V) first neighbor coordination around Co of about 4.3 O atoms at 1.9 Å and second neighbor coordination of about 4.8 Co atoms at 2.88 Å and 6.9 Pt atoms at 2.88 Å indicat that the cubic close-packed cobalt oxide octahedra is incorporated into the face centered
15102 J. Phys. Chem. B, Vol. 108, No. 39, 2004
Figure 8. Structure of the cobalt monolayer at high potentials.
cubic platinum(II) oxide. A corresponding probable structure with an octahedral environment for the mixed Pt and Co oxides formed is shown in Figure 8. 4. Conclusion The near- range order and the variation of the oxidation state of a cobalt monolayer in alkaline solution adsorbed on the surface of platinum clusters dispersed on carbon have been evaluated by using the transmission mode in situ X-ray absorption spectroscopy under reactions conditions. Analysis of cyclic voltammograms indicates that a cobalt monolayer is formed during the application of potential in the UPD region and it is weakly adsorbed on the surface of the Pt cluster, which resulted in changes in the properties of Pt clusters. The Co(OH)2 is oxidized and incorporated into the structure of Pt clusters at higher potentials. The oxide formation of Pt-Co alloy was observed. XAS results were consistent with the presence of an oxidized cobalt layer with Co, O, and Pt nearest neighbors. The cobalt monolayer was found to be very stable during several oxidation/reduction cycles. Acknowledgment. We thank the National Science Council (NSC-92-2214-E-011-007) and the National Taiwan University of Science and Technology, Taipei for financial support and National Synchrotron Radiation Research Center, Hsinchu, Taiwan for providing us with the beam time. References and Notes (1) Schmickler, W. Interfacial Electrochemistry; Oxford University Press: Oxford, 1996; Chapter 4. (2) Levia, E. Electrochim. Acta 1996, 41, 2185.
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