In Situ XPS Studies of Perovskite Oxide Surfaces ... - ACS Publications

known about how the surface characteristics respond to oxida- tion and reduction. ...... Mars van Krevelen (MVK)61 mechanism, are commonly used to exp...
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J. Phys. Chem. B 2005, 109, 2445-2454

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In Situ XPS Studies of Perovskite Oxide Surfaces under Electrochemical Polarization† Greg Vovk,§ Xiaohua Chen,‡ and Charles A. Mims* Departments of Chemistry and Chemical Engineering and Applied Chemistry, UniVersity of Toronto, Toronto, Ontario M5S 3E5, Canada ReceiVed: March 26, 2004; In Final Form: August 11, 2004

An in situ XPS study of oxidation-reduction processes on three perovskite oxide electrode surfaces was carried out by incorporating the materials in an electrochemical cell mounted on a heated sample stage in an ultrahigh vacuum (UHV) chamber. Electrodes made of powdered LaCr1-xNixO3-δ (x ) 0.4, 1) showed changes in the XPS features of all elements upon redox cycling between formal Ni3+ and Ni2+ oxidation stoichiometries, indicating the delocalized nature of the electronic states involved and strong mixing of O 2p to Ni 3d levels to form band states. The surface also showed changes in adsorption capacity for CO2 upon reduction as a result of increased nucleophilicity of surface oxygen. Another perovskite oxide, La0.5Sr0.5CoO3-δ, laser deposited as highly oriented thin films on (100) oriented yttria-stabilized zirconia (YSZ), also showed evidence of both local and nonlocal effects in the XPS features upon redox cycling. In contrast to LaCr1-xNixO3-δ, redox cycling mainly affected the XPS features of cobalt with little effect on oxygen. This signifies reduced participation of O 2p states in the conduction band of this material. Small changes in surface cation stoichiometry in this film were observed and attributed to mobility of the A-site Sr dopant under polarization.

Introduction Surface redox chemistry is a central feature in oxidation catalysis and electrocatalysis.1,2 In these processes, various active surface oxygen-containing species are formed by the reaction of oxygen with the surface, and these species react in turn with adsorbed or gas-phase reactants such as hydrocarbons or carbon monoxide. Furthermore, under reaction conditions, the average oxidation state of a catalyst, for both its bulk and surface, is determined under reaction conditions by a dynamic balance between these simultaneous reduction and oxidation processes. Increased understanding of the redox chemistry of the surfaces of these materials is central to improvement of their performance. An important class of oxidation catalysts and electrocatalysts contains the rare earth perovskite oxides, ABO3-δ (A ) lanthanide and aliovalent substitutions, B ) 3d transition metal, 3-δ indicates variable oxygen stoichiometry).3 These materials find broad application as deep oxidation catalysts for environmental applications,4-20 as oxygen permeation membrane materials for air separation and membrane oxidation reactors,21-30 and as electrocatalytic materials, including electrode and interconnect materials in solid oxide fuel cells (SOFCs).31-44 The wide range of possible substitutions at both the A and B sites in the perovskite lattice allows the tailoring of electronic, thermodynamic, and catalytic properties for specific applications.3,41,45-47 Both the surface and bulk properties are affected by these compositional changes. Many of the uses for these materials rely on the mixed (oxide ion and electron) conductivity associated with various perovskite oxide compositions. Little is known about the surface structure of complex oxides,48-52 including the surface oxygen forms, nor is much †

Part of the special issue “Michel Boudart Festschrift”. * Corresponding author: [email protected], (416) 978 4575. Current address: FMC Lithium Division, Hwy. 161, Box 795, Bessemer City, NC, 28016. ‡ Current address: JDS Uniphase, 80 Rose Orchard Way, San Jose, CA 95134. §

known about how the surface characteristics respond to oxidation and reduction. Furthermore, many of the surface species and characteristics critical to their operation are not preserved in the vacuum environment required for traditional surface techniques. In this paper, we apply an electrochemical technique to vary the oxidation state of mixed conducting perovskite oxide materials in vacuo for study by X-ray photoelectron spectroscopy (XPS). The perovskite material with mixed electronicionic conductivity (ABO3) is incorporated in an electrochemical cell (ABO3 | YSZ | Pd:PdO), where YSZ is yttria-stabilized zirconia, an oxide ion conducting solid electrolyte, and a mixture of palladium:palladium oxide (Pd:PdO) serves as a reference electrode with fixed oxygen potential. At temperatures where the kinetics (ion mobilities, etc.) are sufficiently high, the oxygen activity (expressed as an equivalent oxygen partial pressure) at the “working” ABO3 electrode can be varied over a wide range by varying the voltage on the cell.53 The equilibrium working electrode oxygen potential, p′O2, expressed as the equivalent oxygen partial pressure, is given (from a rearrangement of the Nernst equation) by

p′O2 ) p′′O2 exp

(4EF RT )

(1)

where p′′O2 is the oxygen potential at the reference electrode, fixed by the Pd:PdO redox couple. F is the Faraday constant, E is the applied cell potential, R is the gas constant, and T is the absolute temperature. The number 4 in the expression is the number of electrons required for the reduction of the oxygen molecule to oxide ions in the electrolyte. The maximum experimentally available oxygen potential under anodic potentials (positive voltage) is limited by oxygen recombination and desorption from the perovskite electrode. The minimum oxygen potential under cathodic bias is limited by the reductive decomposition of the perovskite or loss of electronic conductivity in the perovskite oxide electrode. An applied voltage forces oxide ions through the electrolyte (changing the oxidation

10.1021/jp0486494 CCC: $30.25 © 2005 American Chemical Society Published on Web 12/15/2004

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potential of the working electrode) until equilibration to the applied potential is reached. This technique has been used occasionally to alter the surface characteristics or to modify the surface reactivity in electrochemical reactors.53-56 We studied two classes of perovskite oxides that exhibit good electronic conductivity when fully oxidized. The first included two members of the B-site substituted series, LaCr1-xNixO3 (x ) 0.4, 1), occasionally referred to herein as LCNO and LNO, respectively. Substitution of Ni and other reducible metal ions on the B sites in LaCrO3 produces large changes in electronic57-60 and catalytic10,20 properties while preserving the innate robustness of the chromite perovskite lattice. These materials have been previously studied for their catalysis of methane oxidation.20 The kinetic rate laws derived in this study were consistent with a Mars-van Krevelen redox cycle61 and the redox site was plausibly linked to Ni dimer ensembles in the oxide. The proposed redox cycle site involved the following transformation,

Ni3+-O-Ni3+ T Ni2+-0-Ni2+ (+1/2O2)

(2)

where 0 is an oxygen vacancy at the surface. The reducible Ni cation substituted into the stable LaCrO3 thus provides the redox activity required for methane oxidation. Both materials show high electronic conductivity when fully oxidized, with LaNiO3-δ exhibiting metallic conductivity. The conductivities of both materials decrease when reduced. The second material we study here is La0.5Sr0.5CoO3-x (LSCO), a single member of the wellstudied series of A-site substituted lanthanum cobaltite, LaCoO3.44,62-64 Substitution of Sr for lanthanum on the A site also induces changes, both in the electronic properties and ionic conductivity 37,65-67 of the material. Of the ABO3 materials, the cobaltites have the highest rates of oxygen activation and are under development as improved SOFC cathodes in combination with advanced electrolytes.41 The La1-xSrxCoO3-δ class of materials are metallic or semimetallic conductors 41,68,69 whose conductivities also decrease upon reduction. In addition to this general interest in cobaltite perovskites, we had access to high quality, highly oriented thin films of this material and had previously investigated oxygen activation and permeation through these thin film materials.71 Experimental Section LaCr1-xNixO3 Cell Preparation. As previously described,10 the LaCr1-xNixO3 (x ) 0.4, 1) powders were prepared by the Pechini gel precipitation method72 and were shown to be single phase by XRD. Two methods were used to incorporate these materials into electrochemical cells.73 In one method, a paste (1-2 drops of poly(ethylene glycol) + 100 mg of perovskite) was screen printed through a 200 µm screen onto a fully sintered (1250 °C) YSZ (8 wt % Y2O3) disk, onto which an electroformed screen of platinum had been laid as a current collector. This partial cell was sintered in air at 1050 °C for 2 h. The reference electrode was applied as a slurry of Pd powder with small amounts (10%) of YSZ, and the entire cell assembly was sintered in air at 750 °C for 1 h. Another method involved careful layering in sequence into a pill press die (Carver) with intervening hand pressing of the following: (1) a mixed PdPdO powder, (2) YSZ powder, (3) the current collector, and (4) sufficient perovskite powder to cover the electrolyte. After pressing at 50 MPa, with or without mild sintering, the cells were robust enough and had sufficient conductivity for these experiments. Cells with electrodes of both compositions, LaCr0.6Ni0.4O3-δ and LaNiO3-δ, were constructed using the screen-printed electrode method. The layered powder technique was used to construct cells of LaCr0.6Ni0.4O3-δ only.

La0.5Sr0.5CoO3-x Cell Preparation. The LSCO thin filmYSZ single crystal composite was supplied by Allan Jacobson (University of Houston) and has been previously described.71 Briefly, a purchased (Superconductive Components, Inc.) La0.5Sr0.5CoO3-x target was used to deposit a 300 nm thick film by laser ablation onto the polished side of a 1 mm thick YSZ single crystal with (100) orientation (ESCETE Single-Crystal Technology). During deposition, the crystal was held at high temperature (800-860 °C) and in an oxygen pressure of 250300 mTorr (1 Torr ) 133.3Pa). The film thickness was measured by SIMS depth profiling. The film was highly oriented with respect to the underlying YSZ substrate but characterized by columnar growth of domains approximately 100 nm in width. The effective film resistance measured in air was 100-500 Ω. The reference electrode was applied to the other side of the YSZ crystal as a paste consisting of Pd, PdO, and YSZ-based ceramic glue. A 0.1 mm Pt wire was embedded in the paste before the assembly was cured for 4 h at 200 °C in air. Cell Voltammetry and XPS Analysis. The cells were assembled onto a custom-modified sample holder (Surface/ Interface, Inc.), approximately 6 × 6 × 3 cm in dimension.73 The entire sample holder could be moved to and from the vacuum chamber via an airlock onto a custom-designed receiver station. This station was incorporated into a rotatable mechanism mounted on a three-axis translation manipulator (Vacuum Generators model MX600). Seven “banana plug” connectors on the sample holder mated to electrical feeds on the receiving station to supply heating power, cell polarization (one lead to each electrode), and thermocouple connections. The receiving station could be cooled by liquid nitrogen while the cell was mounted onto an electrically heated platform on the sample holder (either a silicon wafer or a BN encased heater), thus allowing the sample temperature to be controlled in the range 150-1000 K. The cells were held in place on the heated platform by a screw-mounted gold plate with a hole of approximately 0.8 cm2 in the center to expose the electrode. This plate also served as the electrical lead to the working electrode by making contact with the electrode, in the case of the LSCO thin film or the electrode current collector in the case of the powder electrodes. After initial analysis of the electrode surface composition, the cell was heated to an operating temperature sufficient to achieve cell currents of several microamperes upon initial polarization (usually near 400 °C). The heating and voltammetry measurements were accomplished with an automated PC-based LabVIEW system. Preliminary to cell voltammetry, the adventitious carbon initially present on the surface was burned off under anodic bias. As long as oxygen remained in the reference electrode, a transient current was observed upon changing the cell voltage. This current decayed with time to a small value, showing that there were no electronic short circuits in the cell and that the cell current could be attributed to cell reactions. When a transient current reached a low level (less than 5% of the initial value), the voltage was tuned to achieve zero current, and this voltage was recorded as the open circuit voltage of the cell. The total cell discharge for each voltage was measured by integrating the cell current. This value was related to the perovskite electrode stoichiometry by comparing the number of electrons transferred to the known (from mass and composition) number of formula units in the electrode. For example, one electron transferred per LaNiO3 formula unit is required to reduce the formal oxidation state of nickel from 3+ to 2+. Simultaneously, the oxygen stoichiometry (with a formal charge of 2-) would change from LaNiO3 to LaNiO2.5, a change of

In Situ XPS Studies of Perovskite Oxide Surfaces the nonstoichiometry parameter, δ in LaNiO3-δ, from 0 to 0.5. Both descriptions of the degree of reduction are used in this paper. The reference electrode contains a finite amount of oxygen, and when this inventory was exhausted, the transient currents consisted only of sharp capacitance spikes followed by negligible current flow. At this stage, the sample could have been removed and recharged by calcination of the cell in air. This was rarely done, since such thermal treatments usually caused spallation of the reference electrode. XPS spectra were obtained by a Leybold-Heraeus electron energy analyzer (EA-11), and dual (Mg, Al) anode X-ray source (LH) mounted at 90° to the analyzer and housed in a stainless steel UHV chamber with a base pressure of 1.6% 10-10 Torr. The perovskite electrode lead was held at ground and cell polarization achieved by applying bias to the reference Pd:PdO electrode. In this manner, XPS spectra could be obtained while the cell was under bias while avoiding wholesale shifts in the electron energy distribution. An excitation energy of 1486.6 eV (Al KR) was used throughout. A constant pass energy of 100 eV was used as a compromise between the need for acceptable energy resolution and the need to acquire spectra quickly in view of the limited oxygen inventory in the reference electrode. The binding energy scale calibration was periodically checked by analyzing the gold plate on the sample holder. Spectra were taken both at operating temperatures and at room temperature (having been cooled under bias) during the various sample treatments. The latter method was used to minimize the escape of oxygen from the cell during analysis. In addition to survey scans, the regions used for analysis of the LaCr1-x.NixO3-δ materials were La 3d, Cr 2p, Ni 3p (La 3d3/2 interferes with Ni 2p), O 1s, C 1s and (for the Na contaminated materials) Na 2s, and the valence band region. For the LSCO electrodes, the relevant regions were La 3d, Sr 3d, Co 2p, O 1s, and C 1s. Quantification of the XPS signals used basic sensitivity factors for our 90° analysis geometry.74 Both perovskite oxides have substantial electronic conductivity when fully oxidized, and absolute binding energies can be referenced to the Au features from the sample holder. However, both materials show substantial decreases in electronic conductivity when reduced,59,60,69 and some charging was observed in these cases. We used the La 3d features in our materials as a reference for relative binding energy measurements. Lanthanum is not thought to participate in redox chemistry in these materials, remaining in the fully ionized 3+ state throughout. In support of this method, our previous 10 studies of the oxidized LaCr1-x.NixO3-δ series of materials showed that the La 3d features did not show any variation in binding energy with respect to adventitious carbon, unlike the B cation and oxygen features. In the present study, the La 3d binding energies for the two LaCr1-x.NixO3-δ (x ) 0.4, 1) materials, when fully oxidized, were identical when referred to the Au features on the holder. Finally, CO2 adsorption on the LaCr1-x.NixO3-δ materials was accomplished in the sample introduction chamber. An exposure to pressures around 1 Torr (130 Pa) for several minutes was used to reach saturation. CO2 adsorption was not studied on LSCO because of interference by carbonate species associated with surface strontium. Results and Discussion Reversible cell behavior was obtained on all three materials. Because of the limited oxygen inventory in the reference electrode, we did not have the opportunity to obtain data for a highly resolved series of oxygen activities. Nevertheless, a general picture of the effects of oxidation and reduction on the surface electronic structure is shown by the XPS results.

J. Phys. Chem. B, Vol. 109, No. 6, 2005 2447

Figure 1. Cell current versus time following a change in cell bias from 0 to -0.7 V. Screen-printed LaCr0.6Ni0.4O3-δ electrode at 450 °C.

LaCr1-xNixO3-δ Results. Cell BehaVior and Redox ReVersibility. All cells showed the ability to undergo reversible oxidation-reduction. Figure 1 shows the current transient observed upon changing the cell bias from 0 to -0.7 V for a screen-printed cell with a LaCr0.6Ni0.4O3-δ electrode at 450 °C. The decrease in current with time results from a combination of relaxation in the polarization of the cell and changes in electrode resistances. The remaining current could be effectively nulled by adjusting the cell bias. Remaining cell leakage currents were very small as long as the cell was under cathodic polarization and no oxygen was being desorbed from the perovskite. For the experiment shown in Figure 1, the final open circuit voltage (0.61 V) corresponded to an oxygen activity of 10-23, or equivalent to a water vapor atmosphere with approximately 0.1% hydrogen at 450 °C. Under anodic polarization, oxygen evolution from the perovskite continued to provide a pathway for leakage current and, if continued, would pump all the available oxygen from the reference electrode. At higher temperatures, oxygen evolved from the reference electrode itself and contributed to the depletion of the oxygen reservoir. For this reason, the lifetimes of the cell were limited, the number of analysis cycles was limited to approximately 10, and a maximum of 3 redox cycles was achieved for any of the cells. The total charge passed in the transient shown in Figure 1 was approximately equal to one electron per nickel atom in the electrode (calculated from mass of electrode and stoichiometry) and therefore sufficient to reduce nickel from a formal oxidation state of 3+ to 2+, that is, from LaCr0.6Ni0.4O3 to LaCr0.6Ni0.4O2.8. This process was reversible and preserved the perovskite structure as long as the degree of reduction stayed within this limit. Previous temperature programmed reduction in H2O/ H2 atmospheres10 also demonstrated reversible redox cycling of the perovskite oxide within these stoichiometric limits. In wet hydrogen, reduction beyond this limit occurs for x > 0.5 and results in destruction of the perovskite structure. Under electrochemical polarization in this study, it was possible to over reduce both LaCr0.6Ni0.4O3-δ and LaNiO3-δ and destroy the perovskite lattice. Surface Stoichiometry and Changes upon Reduction. No significant changes, within the experimental precision, were observed in the surface cation composition upon redox cycling. The relative uncertainties in the Ni 3p intensities were 4%, and, for other components, 2%. Absolute stoichiometry determination by XPS on powdered samples is not straightforward and angleresolved XPS is of little use in determining possible surface

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Figure 4. Cr 2p XPS spectra of oxidized (c) and reduced (d) LaCr0.6Ni0.4O3 electrodes. The conditions are the same as in Figures 2 and 3. The features were fit with a single peak at the energies indicated by the dashed lines.

Figure 2. La 3d XPS spectral region for LaNiO3 ((a) and (b)) and LaCr0.6Ni0.4O3 ((c) and (d)) screen-printed electrodes. The spectra were obtained on oxidized electrodes ((a) and (c)) and on electrodes reduced to Ni2+ equivalent ((b) and (d). The La features are split into two finalstate components, appearing for La 3d5.2 at binding energies 833 (LaI) and 837 eV (LaII), are shown at the right. The Ni 2p3/2 region overlaps with the La 3d3/2 features at the left.

lifetime. Only the screen-printed cells provided useful data for LaCr0.6Ni0.4O3-δ. These cells had initial (surface (XPS)) cation stoichiometry of La0.97Cr0.62Ni0.38, indistinguishable from the bulk,10 and did not change upon reduction. The layered powder cells suffered from sodium contamination in the YSZ powder. Sodium migration to the electrode was observed under cathodic polarization of these cells. The initial cation stoichiometry of La1.18Cr0.63Ni0.37 changed to Na1.65La1.15Cr0.62Ni0.38 upon reduction. This sodium migration was completely reversible upon subsequent anodic polarization. Oxygen stoichiometries were also measured. When oxidized, both materials showed oxygen contents somewhat (10%) higher than those predicted by the cation stiochiometry. Upon reduction, the O 1s intensity decreased, but not in quantitative agreement with the change implied by the total cell discharge. For example, reduction by one electron per nickel atom (LaNi(3+)O3 to LaNi(2+)O2.5) would entail a decrease in oxygen content by 1/6. In one such case, we observed a reduction of 9%. In other cases, there was little apparent change in oxygen stoichiometry upon reduction. One possible explanation is the reaction of background water vapor in the vacuum chamber with oxygen vacancies:

H2O (g) + O2- (s) + 0 f 2 OH- (s)

Figure 3. Ni 3p XPS spectra for oxidized ((a) and (c)) and reduced ((b) and (d)) forms of LaNiO3 and LaCr0.6Ni0.4O3 screen-printed electrodes. Conditions are as in Figure 2. The Ni 3p region consists of the unresolved 2p3/2 and 2p1/2 components shown below. The Cr 3s feature is evident on the LaCr0.6Ni0.4O3 electrode. The peaks were fit with a principal peak at the energies indicated by the dashed lines and indicate a shift in binding energy upon reduction.

enrichment.75 Nevertheless, changes in the relative abundances can be measured with some confidence if the surface topography remains fixed. The LaNiO3-δ electrode had an initial (after cleaning) cation composition of La0.92Ni, which appeared to change to La0.96Ni upon reduction. This change was smaller than the experimental uncertainties, but appeared, in multiple determinations, to recover partially upon reoxidation. Longer term changes were not measured because of the limited cell

(3)

where 0 represents an oxygen vacancy. This process neutralizes any change in the oxygen stoichiometry due to reduction. Evidence supporting this premise was observed in the O 1s XPS spectra, shown in the next section. XPS Changes in Chemical State. The changes in the XPS features show consistent evidence of the delocalized nature of the reduction of the lattice. These can be seen in the raw data, shown in Figures 2-5. Briefly, all features of the (Ni,Cr)O3-δ3sublattice show a generalized shift with respect to the La 3d features upon reduction without any noticeable changes in shape or the appearance of any new features. One exception is the O 1s region, where a broadening was observed, consistent with the presence of hydroxide in the reduced material. In addition to shifts in binding energies, a change in the La 3d region was also seen as shown in Figure 2. The La 3d5/2 is split into two final-state components at binding energies 833.5 (LaI) and 837.2 eV (LaII), shown at the right side of Figure 2. The La 3d3/2 feature is also split and overlaps with the Ni 2p3/2 region on the left. XP spectra (a) and (c) were taken on oxidized LaNiO3 and

In Situ XPS Studies of Perovskite Oxide Surfaces

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Figure 5. O 1s XPS spectra of LaNiO3 and LaCr0.6Ni0.4O3 screenprinted electrodes. Spectra (a) and (c) are from oxidized samples. Spectra (b) and (d) were taken after reduction. Conditions as in Figures 2-4. The spectra were fit with peaks at the energies shown by the dashed lines.

LaCr0.6Ni0.4O3 samples, respectively. Spectra (b) and (d) were taken on these same samples after electrochemical reduction. The splitting of the La peaks into two features, LaI and LaII, labeled I and II respectively in Figure 2, is common in lanthanum compounds.76-80 The two features are generally agreed to result from hybridization/charge transfer between valence levels and La 4f levels in the presence of the 3d core hole. Increased overlap of occupied ligand orbitals with La 4f during XPS produces an increase in the relative intensity of the LaI feature.79 LaCr0.6Ni0.4O3 (spectrum (d)) and LaNiO3 (spectrum (b)) taken after Ni3+-Ni2+ reduction both show significant increases in the LaI/LaII ratio. The change in LaI/ LaII is more pronounced in LaNiO3 than in LaCr0.6Ni0.4O3, consistent with the larger charge addition per formula unit. Further reduction of LaNiO3 under more severe conditions causes further, irreversible, changes to the LaI/LaII ratio as the perovskite lattice is destroyed. Figure 3 shows the Ni 3p XPS spectra for the oxidized ((a) and (c)) and reduced ((b) and (d)) LaNiO3 and LaCr0.6Ni0.4O3 electrodes. The two LaCr0.6Ni0.4O3 XPS spectra, (c) and (d), contain an additional Cr 3s XPS feature at 75 eV. There are no noticeable changes in peak shapes, nor are any new features observed upon reduction. Both sets of spectra show a simple shift of the Ni 3p peak to lower binding energy upon electrochemical reduction. The binding energies in the reduced states (67.5 and 67.35 for LNO and LCNO respectively) are consistent with reported values of 67.381 and 67.582 for NiO. Both the Cr 2p3/2 and Cr 2p1/2 features in LaCr0.6Ni0.4O3 appear to shift without changing shape and without new features appearing (see Figure 4). This behavior is also echoed in Figure 3, where the Cr 3s peak also shifts toward lower binding energy in step with the Ni 3p. The value for the principal 2p3/2 peak in the reduced LCNO material (576.1) is consistent with values reported for Cr2O3 of 576.283 and 576.1.82 Figure 5 shows that reduction of both materials causes changes in the O 1s region. Oxygen XPS on oxides is complex,3,84 and the peak shapes here are similar to those usually seen on complex oxides. Substantial intensity is contained in a symmetric peak “oxide” peak near 529 eV along with additional intensity on the higher binding energy side, which occasionally forms a separate peak. The high binding energy oxygen has

Figure 6. Changes in XPS features in LaNiO3 and LaCr0.6Ni0.4O3 screen-printed electrodes (solid squares). The arrow indicates the change upon reduction. These values are compared to those for an oxidized series of LaCr1-xNixO3 materials taken from reference [10] (diamonds). The values for two reference materials, La2O3 and La2NiO4, are shown to the left and right, respectively, of the main series. Panel (a) shows changes in the LaII/LaI ratio. Panel (b) shows changes in the XPS binding energies indicated. See text for discussion.

been assigned as “active oxygen” in other studies.4,85-87 However, intensity in the binding energy region 531-532 eV on oxides can also arise from carbonate (adsorbed carbon dioxide)84,88,89 or hydroxide (adsorbed water).84,89 The intensity in the high binding energy region appears to decrease upon reduction in the case of LaNiO3 and increase in LaCr0.6Ni0.4O3. This variation in behavior could be due to the presence of varying amounts of hydroxide via the reaction in eq 3. A clear decrease in the average binding energy is observed for LaNiO3, while the “oxide” portion of the LaCr0.6Ni0.4O3 spectrum appears to shift to lower binding energy as well. The XPS spectra were processed by Shirley background subtraction74 and fit to constant-width Gaussian components corresponding to LaI, LaII, Cr 2p, Ni 3p, O 1s (I, low binding energy), and usually one higher binding energy O 1s feature (O 1s II, etc.). One measure of the binding energy shifts is given by the changes in these fits to the spectra. When no significant changes in peak shape was involved (i.e., for most of the data), the binding energy shifts were also measured by recording the relative shift of the two spectra that produced a minimum difference signal. These two methods gave results that agreed to better than 0.2 eV in all cases. Figure 6 summarizes these results. The LaII/LaI ratios are shown in panel (a) and the binding energies results for Cr 2p, Ni 3p, and O 1s (“oxide”) relative to LaII are shown in panel (b). For comparison, previously reported data are included in the figure. The diamonds are data from a previous study of the oxidized forms of a complete series of LaCr1-xNixO3-δ compounds.10 The triangles in the wings of the panels are data for two model compounds. The results of the current study are indicated by solid squares. The arrows indicate the change upon reduction. As discussed,10 the trend in the full series of oxidized materials shows that as more nickel (formally in the 3+ charge state) is substituted for chromium, the O, Cr,

2450 J. Phys. Chem. B, Vol. 109, No. 6, 2005 and Ni features shift to higher binding energies and the probability of hybridization/exchange with La during XPS is reduced. The data from this study show that electrochemical reduction of these materials restores the XPS features to those found when nickel is in the 2+ state (La2NiO4) or absent (La2O3, LaCrO3). As discussed more thoroughly below, the mixing of Ni 3d levels with O 2p levels in the valence band produces itinerant states responsible for the electronic conductivity seen in these materials. Changes induced either by changes in Ni substitution levels, or by reduction of the lattice, are reflected in smooth, simultaneous shifts in the charge density associated with all components of the BO3 sublattice and thus smooth and simultaneous shifts of all of their XPS features. CO2 Adsorption BehaVior. A preliminary investigation of the effect of redox state on adsorption was performed by cooling the materials to room temperature and exposing them to 130 Pa of CO2 in the sample introduction chamber for one minute. Such an exposure of the oxidized LaNiO3 surface resulted in no measurable carbonate formation (less than 1% surface coverage). When reduced to Ni2+ stoichiometry, however, CO2 exposure gave rise to a new feature in the C 1s spectra at 289.2 eV that we attribute to surface carbonate formation.84,88 These amounts were roughly 10% of the surface composition. This dramatic change upon reduction can be attributed to an increase in nucleophilicity as charge density on oxygen increases. La0.5Sr0.5CoO3-δ Results. The thin film electrodes of La0.5Sr0.5CoO3-δ also showed reversible redox behavior. Similarities and differences to LaCr1-xNixO3-δ were seen in the response of the various XPS features to oxidation/reduction cycles. Because of interference from the carbonate signal associated with strontium, the carbon dioxide adsorption experiments on this material were not informative. Cell BehaVior and Redox ReVersibility. The current transients in the thin film cells were similar to those in the powdered cells of LaCr1-xNixO3. Somewhat lower temperatures were used for the thin film cells (350-370 °C for most of the work), and smaller transient currents were induced (several microamperes maximum current). The time required for the cell to reach equilibrium was similar to the previous LaCr1-xNixO3 cells, however, because the reaction involved a much smaller amount of electrode material in the thin film. One of the cells was taken through three oxidation-reduction cycles of approximately 0.6 electrons per formula unit (equivalent to an oxygen stoichiometry change, δ, of 0.30) and back again. The XPS changes seen upon reduction were generally reversible upon reoxidation. The second cell was stepped through a progressive sequence of partial reductions, δ ) 0.0, 0.07, 0.17, and terminating at 0.38, or 0.76 electrons per unit cell. The PdO reservoir was exhausted by this time and this cell was not reoxidized. The XPS features and their changes upon reduction were similar in both cells. The LSCO perovskite structure can also be irreversibly destroyed by over-reduction. Surface Stoichiometry and Changes upon Reduction. The surface cationic composition (by XPS) was La0.50Sr1.05Co, substantially enriched in strontium by comparison with the bulk, and was similar for takeoff angles of 15° (Sr/Co ) 1.15) and 45° (Sr/Co ) 1.05). Although this indicates a slight surface enrichment, the finite surface roughness in these films71 makes the interpretation of these data uncertain. The apparent strontium enrichment at the surface may have been caused by the exposure of the material to the atmosphere. The carbon and oxygen signals showed evidence for carbonate (C 1s intensity at 289.5 eV and O 1s intensity near 531 eV) on the as-received surface, and this decreased somewhat upon heating and during the reduction

Vovk et al.

Figure 7. La 3d5/2 XPS region of a La0.5Sr0.5CoO3-δ thin film electrode when fully oxidized (a) and after a reduction (b) equivalent to a nonstoichiometry (δ) of 0.38. The two peaks are labeled La I (right) and La II (left), as in Figure 2. A slight decrease in the LaII/LaI ratio is evident upon reduction.

cycles. The oxygen stoichiometry was consistent with the cation stoichiometry, given the uncertainties introduced by the strontium enrichment and the presence of carbonate. Both reversible and irreversible changes occurred upon cell cycling. Upon reduction, the surface strontium enrichment (Sr/(La + Co)) increased irreversibly by another 5% while the La/Co ratio did not change. The XPS oxygen stoichiometry, complicated by the possible roles of hydroxide and carbonate, varied both reversibly and irreversibly during the redox cycles. The irreversible component was associated with a net loss of O 1s intensity at high binding energy (see below) as well as loss of C 1s intensity associated with carbonate. At the maximum degree of reduction, equivalent to a stoichiometry change δ ) 0.38, a total decrease in oxygen stoichiometry (O/Co) of 7% was observed. This corresponds to a stoichiometry change with respect to cobalt (δ in CoO3-δ) of 0.43, in reasonable agreement with XPS results. Less than half of this change in XPS was regained upon reoxidation; however, again this was a possible result of carbonate and hydroxide. XPS Changes in Chemical State. The behavior of the XPS features in La0.50Sr1.05CoO3-δ showed similarities (in La and Co) and differences (Co, O, Sr) when compared to the LaCr1-xNixO3-δ. Both local and nonlocal effects were observed. The oxygen results were complicated by the features associated with strontium enrichment. The binding energies measured on the oxidized film agree well with those measured by van der Heide on the same material.90 As shown in Figure 7, the finalstate doublet LaI, LaII also dominates the La 3d features in this material. Similar to the LaCr1-xNixO3 results, a reversible decrease in the LaII/LaI ratio was observed upon reduction. The change in ratio progressed with the degree of reduction, but the changes were smaller than those seen in LNO and LCNO. The La II/LaI ratio decreased from 1.04 to 0.97 in LSCO with 0.76 electrons added per formula unit. By comparison, the LaII/ LaI ratio decreased from 1.03 to 0.86 in LNO with a somewhat higher degree of reduction (approximately 1 electron per formula unit). Even LCNO, with approximately 0.5 electrons added per unit cell, showed a decrease in ratio from 0.97 to 0.88, proportionally larger than the LSCO result. The cobalt 2p3/2 region is shown in Figure 8 for the end members of the reduction

In Situ XPS Studies of Perovskite Oxide Surfaces

Figure 8. Cobalt 2p region of a La0.5Sr0.5CoO3-δ thin film electrode before (a) and after (b) reduction equivalent to a nonstoichiometry of 0.38. The principal features were fit to peaks at the energies indicated by the dashed line. Additional shake-up features, indicated by /, appear near 782 and 801 eV at this high degree of reduction but not at lower degrees of reduction.

series. The primary 2p3/2 binding energy for the oxidized material (average cobalt oxidation state ) 3.5 is 780.7 eV, compared to 780.4 for this film by van der Heide90). Values for Co2O3 of 780.091 and for Co3O4 of 780.792 are in agreement with our results, although there is substantial scatter in the literature.84 As Figure 8 shows, the binding energy decreases to 780.2 and a separate peak appears at higher binding energies (around 787 eV). Such features have been observed previously in cobalt compounds and are generally assigned to a “shakeup” feature associated with Co2+,90,93 although satellite peaks in this region have been reported for both Co2O3 and Co3O4.94 It is interesting that this feature only appears at the highest degree of reduction studied here, the only condition where the calculated cobalt formal oxidation state drops below 3+. In addition to the appearance of this separate peak, the binding energy of the principal feature shifts to lower binding energies throughout the reduction series without any apparent change in peak width or shape (see binding energy discussion below). This gradual shift is consistent with the delocalized changes seen for the Ni and Cr features in the LaCr1-xNixO3 materials. The O 1s spectrum consists of a prominent “oxide” feature at 528.5 eV in the reduced material, similar to the “oxide component in LNO and LCNO, and a shoulder at higher binding energy (531.2 eV). Part of the intensity in the shoulder is due to the surface species such as carbonate and possibly hydroxide. Figure 9 shows that a modest decrease in the relative intensities of the shoulder with respect to the main peak is observed upon reduction of the film. This decrease is a major factor in the oxygen stoichiometry decrease described above and was not recovered during redox cycling in the first cell. Aside from the decrease in the high binding energy shoulder, there were no changes in the shape of the principal peak. The Sr 3d XPS on this material has a complex peak shape composed of two distinct 5/2, 3/2 doublets as is shown in the high-resolution spectrum shown in Figure 10. The energy separation between the two is 2.0 eV, such that the 3d3/2 component of one set overlaps the 3d5/2 component of the other. Such peak shapes are common in Sr containing complex oxides.90,93,95 The assignment is not well established and could arise either from a shake-up feature or two distinct chemical environments. Angle-resolved measurements in van der Heide90 suggest that the higher binding energy component is surface enriched. In view of the excess Sr at the surface of this material, we cannot rule out two distinct environments. Nevertheless, as the material is reduced, the

J. Phys. Chem. B, Vol. 109, No. 6, 2005 2451

Figure 9. Oxygen 1s region of La0.5Sr0.5CoO3-x thin film electrode before (a) and after (b) reduction equivalent to a nonstoichiometry of 0.38. The spectra were fit with peaks at the energies indicated by the dashed lines. The shoulder at higher binding energies decreases upon prolonged heating.

Figure 10. Sr 3d region of an oxidized La0.5Sr0.5CoO3-x thin film electrode. The fit was generated from two 5/2,3/2 doublets indicated at energies indicated at the bottom of the figure.

complex peak shape does not change despite the fact that a small binding energy shift occurs (see below). The effects of electrode reduction on the measured binding energies of the key components are summarized in Figure 11. For the oxidized material, the binding energies (BEs) difference between the Co 2p3/2 main feature and the La II peak was -56.4 eV. For the O 1s (I-oxide) feature the difference was -308.6 eV, and for the Sr 3d3/2 low BE feature the difference was -705.5 eV. The changes from these initial values upon reduction are shown in Figure 11. The binding energy of Co 2p decreases smoothly as reduction proceeds while O 1s “oxide” and Sr 3d increase only slightly. The binding energy shifts calculated by both spectrum shifting and peak fitting methods agreed to within 0.1 eV, and the average is shown in Figure 11. The smooth change in cobalt binding energy implies delocalized band filling as in the nickelates. The lack of significant response in the O 1s upon reduction implies that the states affected during redox are associated more with cobalt than with oxygen, in contrast to the more equal involvement seen in the LaCr1-xNixO3-δ materials. It is tempting to assign the increase in the Sr binding energy to differential charging of a nonconducting strontium phase, but the absence of any change in the complex peak shape shows that all of the Sr, not just the excess, was affected.

2452 J. Phys. Chem. B, Vol. 109, No. 6, 2005

Figure 11. Binding energy changes for various XPS features of La0.5Sr0.5CoO3-x thin film as a function of the degree of reduction, expressed as equivalent oxygen nonstoichiometry. Binding energy changes are expressed relative to the La 3d5/2(II) feature. Triangles ) Co 2p3/2 main feature, circles ) O 1s “oxide” feature, diamonds ) Sr 3d3/2 low BE feature.

Discussion Nonlocalized changes in electronic structure are indicated in both the La0.50Sr1.05CoO3-δ and LaCr1-xNixO3-δ perovskite oxides upon redox cycling. Primarily, the involvement of delocalized electronic states is shown by the smooth shifts of B cation (Ni, Cr, and Co) binding energies to lower values during reduction without noticeable changes in peak shape. This occurs without the appearance of new features, as would be expected upon localized reduction of the B cations. One possible indication of localized reduction in La0.50Sr1.05CoO3-δ is the appearance of the Co 2p region at high degrees of reduction, although this is not definitive. Further evidence of a delocalized electronic structure in LaCr1-xNixO3-δ arises from the simultaneous shift of oxygen and B cation features in XPS. This implies that both oxygen and B cation-derived states participate in the electronic structural changes. By contrast, the principal O 1s binding energy in La0.5Sr0.5CoO3-δ was almost unaffected by the reduction of the material, indicating little involvement of oxygen levels in the affected electronic states in LSCO. That the two classes of materials differ in the participation of oxygen levels is also indicated in the La 3d XPS features. Substantial changes were seen in the LaII/LaI ratios upon reduction of the LaCr1-xNixO3-δ materials, indicative of increased O 2p-La 4f mixing/charge exchange during the XPS event. The La 3d features in LSCO changed in a similar direction, but to a smaller extent. Thus, both the O 1s binding energy changes and the LaII/LaI ratio changes are consistent with less involvement of oxygen levels in the affected band states of LSCO than in LNO and LCNO. These findings can be interpreted in the light of what is known about the band structure in these materials. It is generally agreed that metallic electronic conductivity in perovskite oxides requires formation of partially delocalized states based on covalent mixing of O 2p and transition metal 3d states.41,68-70,96-107 In the absence of sufficient overlap in these levels, the materials exhibit semiconducting (small polaron hopping conductivity) properties. Based on simple ligand field concepts, Goodenough and co-workers96-98 were able to show that O 2p-Ni 3d covalent mixing was sufficient in LaNiO3 to produce metallic conductivity, while such mixing in LaCoO3 is inadequate to guarantee metallic conductivity. Strontium substitution, which changes the formal charge on the CoO3 sublattice, and/or high

Vovk et al. temperatures, can produce metallic conductivity in cobaltite materials.69,70,99 This trend to decreasing metal-oxygen overlap as one proceeds to the left in the periodic chart continues in iron-containing perovskites. The cation-anion mixing in these materials is sufficiently weak that they are generally found to exhibit hopping conductivities that are thought to arise from localized states on iron centers.41,101-107 This basic picture of the origin of electronic conductivity in these materials is also in qualitative agreement with our results, which show weaker involvement of O 2p states in the affected valence band states of La0.5Sr0.5CoO3-δ than in LaCr1-xNixO3-δ. Although the gross features of the electronic structures of these materials are consistent with this basic concept, there are many important issues that require very detailed theoretical work. Large changes in conductivity are caused by slight distortions from cubic symmetry in LaNiO3100 and by subtle details of spin ordering in LSCO.68,99 Current calculational capabilities cannot adequately treat such important complexities as the presence of oxygen vacancies and the influence of random cation substitutions. It is hoped that results such as ours under controlled oxidation stoichiometry will aid in the development and validation of more detailed electronic structure calculations in the future. The delocalized electronic structure has general implications for oxidation catalysis. Redox cycles are integral to the function of oxidation catalysts. Simple mechanistic schemes, such as the Mars van Krevelen (MVK)61 mechanism, are commonly used to explain catalytic oxidation kinetics. Similar to simple enzyme kinetics, this mechanism involves two irreversible processes: (1) the initiation of the oxidation chemistry by an oxidized catalytic site, O;

RH + O f _ + products

(4)

followed by (2) the reoxidation of the reduced site, _, by molecular oxygen.

/2O2 + _ f O

1

(5)

In such simple models, the average oxidation level of the catalytic sites depends on a dynamic balance between site oxidation and site reduction. Depending on the relative facility of the two processes, the surface sites at steady state can be oxidized to a greater or lesser degree. Simple mechanistic models such as MVK usually assume that the rate coefficients for oxidation and reduction vary only with temperature, as would be expected if the catalytic sites operate independently. Nonlocal effects on the surface properties such as those shown in this study can influence the individual site reactivities and produce rate coefficients that depend on the oxidation state of the surface. Thus, the dynamic balance during an oxidation reaction can affect the catalyst on two levels: the balance at the local site of the reaction and the balance affecting the oxidation state of the bulk of the catalysts. The latter, in turn, influences the reactivity of the individual surface sites. The general state of the surface, therefore, is a “hidden variable” in the kinetic rate expressions and is subsumed into the gas-phase terms of the kinetic rate laws. Nonlocal effects of the total oxygen inventory in silver are thought to play an important role in the epoxidation selectivity of surface oxygen toward ethylene,108-111 but such effects are not commonly discussed in relation to oxide catalysis. In addition to the general nonlocal effect on reactivity mentioned above, a delocalized electronic structure implies that mechanistic steps occurring at localized sites can induce delocalized

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