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Molecular and Dissociative Adsorption of Oxygen on Solid Oxide Fuel Cell Cathode Materials of La SrCoO, La SrxCo FeO, and La SrMnO Studied by Temperature Programmed Desorption 1-x

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Keisuke Higuchi, Hiromi Sugiyama, and Jun Kubota J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b02653 • Publication Date (Web): 14 Jun 2017 Downloaded from http://pubs.acs.org on June 17, 2017

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Molecular and Dissociative Adsorption of Oxygen on Solid Oxide Fuel Cell Cathode Materials of La1-xSrxCoO3, La1-xSrxCo1-yFeyO3, and La1-xSrxMnO3 Studied by Temperature Programmed Desorption

Keisuke Higuchi, Hiromi Sugiyama, and Jun Kubota*

Department of Chemical Engineering, Fukuoka University, 8-19-1, Nanakuma, Jonan-ku, Fukuoka 814-0180, Japan

ABSTRACT: Molecular and dissociative adsorption of oxygen on the surfaces of solid oxide fuel cell (SOFC) cathodes of La0.8Sr0.2CoO3, La0.6Sr0.4CoO3, La0.6Sr0.4Co0.2Fe0.8O3, and La0.8Sr0.2MnO3 were investigated using temperature programmed desorption measurements for molecularly- and atomically-adsorbed oxygen. The desorption of molecularly-adsorbed oxygen occurred at 140–150 K, and the adsorption sites for molecularly-adsorbed oxygen were mostly oxygen vacancy sites that were formed by combinative desorption of surface oxygen atoms at

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550–580 K. In the case of La1-xSrxCoO3, the recovery of oxygen vacancy sites required oxygen adsorption above 523 K, which indicates that the dissociative adsorption of oxygen at the oxygen vacancy sites was an activation process. Oxygen adsorption is the first step in the cathode reactions of fuel cells, and the oxygen adsorption properties of each cathode material is discussed with respect to the activity of the cathode.

INTRODUCTION Solid oxide fuel cells (SOFCs) are promising devices for electric power generation from various fuels, and are expected to be utilized in SOFC-combined power plants and house/building cogeneration systems.1 There have been many challenges in the large scale commercialization of SOFCs, such as the development of oxide electrolytes, cathode materials for lower temperature operation, anode materials for use with a variety of fuels, and materials that offer durability against heat stress.1-6

Conventional SOFCs typically operate at high

temperatures of 1100~1300 K, whereas intermediate temperature SOFCs that operate around 900 K are expected to allow for a greater thermal expansion mismatch of materials and thus a wider selection of materials for sealing, electrical connection, and manifolding.1,2 The cathode is an essential part of an SOFC, and La1-xSrxMnO3 has been widely used in conventional and commercial SOFCs due to its high activity for the oxygen reduction reaction (ORR), thermal expansion matching with electrolyte materials, and lower reactivity toward solid-state reactions with electrolytes at high temperature.1-6 However, other types of cathode materials such as La1-xSrxCoO3, La1-xSrxCo1-yFeyO3, and La1-xSrxNiO4 have been proposed that are more active for the ORR at lower temperature.1-6 Such perovskite materials have both appropriate electronic and


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ionic conductivities, and they are expected to be applied as cathode materials especially for lower temperature next-generation SOFCs. The development of active cathode materials is most important to achieve more efficient SOFCs.

The ORR includes oxygen adsorption and

dissociation on cathode surfaces, which should thus be investigated to understand the nature of cathodes; however, there have been few fundamental studies regarding oxygen adsorption on cathode materials, especially with respect to molecular (non-dissociative) adsorption. In this work, some of typical cathode materials of La0.8Sr0.2CoO3, La0.6Sr0.4CoO3, La0.6Sr0.4Co0.2Fe0.8O3, and La0.8Sr0.2MnO3 were selected among variety of cathode materials for the study on oxygen adsorption. Figure 1 shows a schematic diagram of the ORR at a cathode surface.7-9 Gaseous O2 is first adsorbed molecularly on the surface without dissociation. It is possible for O2 molecules to become adsorbed at oxygen vacancy sites, steps/kinks, and flat surface sites. There have been a few electronic structures for molecularly-adsorbed oxygen proposed on the basis of previous spectroscopic studies; O2, O2-, and O22-.10.11 The adsorption of molecular oxygen on cathode surfaces is the initial step in the ORR. The adsorbed O2 molecules dissociate to two O atoms, which may occur at oxygen vacancy or step/kink sites, and the O atoms are incorporated into the bulk as O2– ions. O2- ions in the bulk or on the surface of the cathode diffuse to the oxide electrolyte, and the cathode can dissociate the next oxygen molecule at oxygen vacancy sites. During this process, a total of 4 electrons are transferred to one O2 molecule to form two O2– ions. The adsorption of O2 molecules is a basic first step in cathode reactions, and aspects of the adsorption process such as the adsorption energy and the amount of oxygen adsorbed, have a fundamental influence on the performance of a cathode material for the ORR.


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The molecular adsorption of O2 has been reported for materials such as Pt,12,13 Ag,14 TiO2,15-17 Cr2O3,18,19 and V2O5.11 On Pt(111) metal surfaces,12,13 molecularly-adsorbed oxygen is known to desorb at ca. 170 K from temperature programmed desorption (TPD) measurements.

On

polycrystalline silver surfaces, molecularly-adsorbed oxygen has been observed based on O-O stretching vibrations by infrared absorption for the desorption peaks at 180 and 265 K.14 On particulate TiO2 surfaces, the molecular adsorption of O2 is known to be complicated, in that there are two adsorption states. One is weakly adsorbed O2 on TiO2, which is adsorbed in equilibrium with the gas phase pressure, even at 120 K.15 The infrared absorption spectra of this adsorbed O2 species have indicated that it is electrically neutral to maintain the O-O double bond.15

The other state of molecular O2 adsorption has the form of O2- (superoxo) or

O22- (peroxo) species, for which desorption peaks occur around 400 K.16-17 In both cases, the molecular adsorption of O2 is considered to take place at oxygen vacancy sites on oxide surfaces, where metal cations are not fully covered by oxide ions. The TPD method has been utilized to estimate the desorption energy of atomic oxygen (oxide ions) from cathode materials in several previous works.20-28 Surface oxygen atoms are desorbed from La1-xSrxCo1-yFeyO3 at 523 K with heating.25-26 The heat of adsorption was estimated to be 101–121 kJ mol-1 and the amount of desorption was about 5% of a compact monolayer of oxide ions. The desorption peak at 523 K has been assigned to the combinative desorption of oxygen atoms at the surface and/or subsurface. In the case of La1-xSrxMnO3, a similar desorption peak has been reported.23

Another temperature programmed method has been performed for

La1-xSrxCoO3 cathode materials using an isotope for temperature programmed isotope exchange (TPIE).28 The surface oxide ions are exchanged by isotopic O2 at 439 K, which is lower than the temperature for TPD measurements of atomic oxygen.

Desorption and isotope exchange


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reactions of surface oxide ions can be characterized by TP methods. The isotope exchange reaction seems to take place at lower temperatures than that for desorption because the exchange reaction is an adsorption-assisted desorption processes with a heat balance.29 In the case of oxide/nitride cathode materials for polymer electrolyte fuel cells (PEFCs), the desorption of molecularly-adsorbed oxygen, which occurs below 170 K, was examined by TPD, and it has been proposed to contribute to the ORR activity.30 It has been reported that the amount and strength of molecular oxygen adsorption provides an indication of the activity of the cathode material.

The molecularly-adsorbed oxygen species can be practically observed at low

temperatures below 200 K. Therefore, it seems that this species cannot contribute to the ORR at high temperatures such as those for the operation of SOFCs. However, even if the absolute coverage of molecularly-adsorbed oxygen is negligibly small at high temperatures, there is no doubt that the ORR, which involves the dissociative adsorption of oxygen with electron transfer, occurs from a precursor of molecularly-adsorbed oxygen. In this work, we examined the molecular adsorption of O2 on La0.8Sr0.2CoO3, La0.6Sr0.4CoO3, La0.6Sr0.4Co0.2Fe0.8O3, and La0.8Sr0.2MnO3 cathode materials, from which molecularly adsorbed oxygen desorbs at 130–140 K. The nature of molecular O2 adsorption on cathode materials of SOFCs is discussed with respect to oxygen vacancy sites.

EXPERIMENTAL La0.8Sr0.2CoO3 and La0.6Sr0.4CoO3 samples were provided from Noritake Co. Ltd., and La0.6Sr0.4Co0.2Fe0.8O3 and La0.8Sr0.2MnO3 samples were obtained from Aldrich.

Scanning

electron microscopy (SEM; Jeol, JSM-6060) and X-ray diffraction (XRD; Shimadzu, XRD6100) were applied after pretreatment at 673 K in 10 kPa of O2 to analyze the morphological and


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crystalline properties of the samples. The surface areas of the samples were evaluated from Brunauer-Emmett-Teller (BET) adsorption isotherms measured with an adsorption analyzer (Shimadzu, TriStar-3000). The apparatus for TPD measurements is schematically illustrated in Figure 2. The vacuum system consisted of two parts; a quadrupole mass spectrometer (QMS; Pfeiffer Vacuum, Prisma Plus QMG220) with a turbo molecular pump and a mechanical rotary pump, and a sample cell with a gas handling system that can be evacuated by an oil diffusion pump with another mechanical rotary pump. The sample cell could be heated with an electrical heater and cooled with liquid nitrogen, and the temperature was controlled using a programmable temperature controller (Omron, E5AN-H). 100 mg of sample was placed into a quartz tube sample cell. The cell was then evacuated with heating at 673 K. 10 kPa of O2 was introduced into the cell and the sample was calcined at 673 K for 1 h. The cell was cooled to a specific temperature and then evacuated to high vacuum using an oil diffusion pump. The sample was cooled in vacuum to 90 K using liquid N2, then 10 kPa of O2 was introduced into the cell. After 3 min of O2 exposure, the cell was evacuated to high vacuum again. The cell was connected to the QMS system by slowly opening a variable-leak valve. The sample was heated at 10 K min-1 and desorbed gases were monitored with the QMS. Several ions, such as H2O (m/z=18), N2 and/or CO (m/z=28), O2 (m/z=32), were monitored simultaneously during the heating stage. Oxygen desorption, as monitored at m/z=32, was recorded with respect to the temperature. The QMS signal was calibrated against an absolute amount of O2 and the desorption rate was expressed in units of moles per second (mol s-1).

The details of TPD apparatus are described in supporting

information (SI).


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RESULTS AND DISCUSSION SEM and XRD analysis Figure 3 shows SEM images of the La0.8Sr0.2CoO3, La0.6Sr0.4CoO3, La0.6Sr0.4Co0.2Fe0.8O3, and La0.8Sr0.2MnO3 samples that were pretreated at 673 K in 10 kPa of O2, as with the samples for TPD measurements.

All samples consist of particle aggregations with diameters of a few

hundred nanometers.

For La0.6Sr0.4Co0.2Fe0.8O3 and La0.8Sr0.2MnO3, large particles with

diameters of approximately 5 µm were identified in the SEM images. The BET surface areas for La0.8Sr0.2CoO3, La0.6Sr0.4CoO3, La0.6Sr0.4Co0.2Fe0.8O3, and La0.8Sr0.2MnO3 were estimated to be 5.5, 7.8, 3.9, and 3.9 m2 g-1, respectively. The BET surface areas for La0.6Sr0.4Co0.2Fe0.8O3 and La0.8Sr0.2MnO3 were slightly smaller than for the other samples because they contained large particles. The morphologies of the samples were typical of cathode materials and these samples were applied to the following TPD studies. Figure 4 shows XRD patterns for La0.8Sr0.2CoO3, La0.6Sr0.4CoO3, La0.6Sr0.4Co0.2Fe0.8O3, and La0.8Sr0.2MnO3. Each sample consists of a single phase that is assignable to the perovskite structure. The XRD results are typical for La0.8Sr0.2CoO3, La0.6Sr0.4CoO3, La0.6Sr0.4Co0.2Fe0.8O3, and La0.8Sr0.2MnO3.

Oxygen TPD from La1-xSrxCoO3 Figure 5 shows TPD profiles for O2 from La0.8Sr0.2CoO3 for different evacuation temperatures after the pretreatment.

Two types of O2 species were desorbed from the surface; a low-

temperature desorption peak exists at 140 K with a broad shoulder at 220 K and a hightemperature desorption peak appears at 570 K followed by continuous desorption of O2 at higher


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temperatures. As reported previously for La1-xSrxCo1-yFeyO3, the oxygen desorption at 570 K can be assigned to the formation of oxygen vacancy sites on the surfaces by combinative desorption of surface oxygen atoms (ions). The continuous desorption of O2 at higher than 570 K indicates that oxygen was released from the La0.8Sr0.2CoO3 bulk. The absolute amount of desorbed oxygen represented by the 570 K peak was estimated to be about 0.08 molecules-O2 nm-2 for the samples evacuated at 373 and 473 K, which indicates that 0.2 oxygen vacancies per nm-2 were present at the surface and subsurface when La0.8Sr0.2CoO3 was evacuated at 673 K.

The

assignment to combinative desorption of oxygen atoms at the surface and subsurface seems to be reasonable because the amount of desorption at 570 K is sufficiently smaller than the number of surface oxide ions. The peak-like appearance of this desorption suggests that it originates from a limited area of the surface. The assignment of the peak at 570 K is also consistent with previous reports.25,27 The low-temperature desorption peak at 140 K with a broad shoulder at 220 K originates from the molecular adsorption of O2. It should be noted that the amount of molecular O2 adsorption was significantly high for the sample evacuated at 673 K. The absence of a high-temperature peak at 570 K for the sample evacuated at 673 K indicates that this surface has a considerable number of oxygen vacancy sites. Therefore, the increase in intensity of the 140 K peak with increasing evacuation temperature suggests that molecular adsorption of O2 on La0.8Sr0.2CoO3 occurred mostly at oxygen vacancy sites. For the sample evacuated at 373 K, oxygen vacancy sites were not formed; therefore, these sites were occupied with oxygen, as indicated by the TPD peak at 570 K, and molecular adsorption of O2 could not occur. In particular, the broad shoulder at 220 K was significant for the sample evacuated at 673 K, and this species strongly corresponds to desorption from oxygen vacancy sites. The amount of adsorption associated with


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the 140 K peak for the sample evacuated at 673 K was estimated to be 0.05 molecules-O2 nm-2. The amount of molecularly-adsorbed oxygen is thus four times smaller than the number of oxygen vacancies at 0.2 per nm-2. As discussed later with respect to the results for different samples, the amount of molecularly-adsorbed oxygen tends to increase with the number of oxygen vacancies, although the ratio of the amount of molecularly-adsorbed oxygen and the number of oxygen vacancies is not unity. Based on the simple assumption that the activation energy for desorption in kilojoules per mole (kJ mol-1) is approximately equal to the peak temperature (K) multiplied by 0.25,31 the desorption temperatures of 140 and 570 K lead to activation energies for desorption of approximately 35 and 140 kJ mol-1, respectively. The activation energy obtained for the combinative desorption of oxygen atoms is consistent with the activation energy for the isotope exchange reaction at the surface reported previously.32 Figure 6 shows TPD profiles for O2 from La0.6Sr0.4CoO3. The amount of oxygen desorption represented by the 570 K peak, which is due to combinative desorption of oxygen atoms with the formation of oxygen vacancy sites at the surfaces, was significantly higher than that for La0.8Sr0.2CoO3. The amount of desorption associated with the 570 K peak was 0.08 moleculesO2 nm-2 for La0.8Sr0.2CoO3, but 0.4 molecules-O2 nm-2 for La0.6Sr0.4CoO3. The density of oxygen vacancies was determined to be 0.8 vacancies for an area of 1 nm2. This amount is in a similar range to that reported previously, in which about 5% of a compact monolayer of oxide ions was proposed as the amount of vacancies (0.5 molecules-O2 nm-2) present on the surface.25 This indicates that La0.6Sr0.4CoO3 can have five times the number of oxygen vacancies at the surface than La0.8Sr0.2CoO3. A low-temperature peak was also observed at 150 K; the difference in the temperature of this peak for La0.8Sr0.2CoO3 and La0.6Sr0.4CoO3 was insignificant. The area of the low-temperature desorption peak at 150 K with a broad shoulder at 220 K for La0.6Sr0.4CoO3


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indicated the amount of adsorption was 0.04 molecules-O2 nm-2, which was almost the same as that for La0.8Sr0.2CoO3, although there was a large difference in the density of oxygen vacancy sites. Therefore, it is proposed that not all oxygen vacancy sites can adsorb molecular oxygen. It is supposed that the adsorption capability of molecular oxygen is influenced by the detailed structure surrounding the oxygen vacancy sites, i.e., some oxygen vacancy sites are positioned at the subsurface but not at the outmost layer. Summarizing the results for the two La1-xSrxCoO3 compositions, the amount of oxygen desorption from surface oxygen atoms with the formation of oxygen vacancies is strongly dependent on the Sr content. Although the molecular adsorption of oxygen, which has a TPD peak at 140 K, is considered to occur at oxygen vacancy sites, the amount of adsorbed oxygen was similar for both samples.

Dissociative adsorption of oxygen at oxygen vacancies The dissociative adsorption of oxygen at oxygen vacancy sites is the next step in the ORR mechanism. Figure 5 shows that molecularly-adsorbed oxygen at oxygen vacancy sites was desorbed at 140–220 K from the sample evacuated at 673 K, and the peak at 570 K was not subsequently recovered. This indicates that molecularly-adsorbed oxygen at oxygen vacancy sites cannot dissociate to form oxygen atoms and occupy the vacancies at these low temperatures. Thus, the La0.8Sr0.2CoO3 sample evacuated at 673 K was exposed to oxygen at higher temperatures of 473, 493, 523, and 573 K, cooled to 90 K, evacuated at 90 K, and then TPD measurements were performed. The results are shown in Figure 7. To recover the 570 K peak, which is due to the combinative desorption of surface oxygen atoms with the formation of vacancies, oxygen exposure above 523 K was required. The dissociative adsorption of oxygen at


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oxygen vacancy sites is considered to require two oxygen vacancies. Therefore, it is suggested that the oxygen vacancies can migrate on the surface at temperatures above 523 K and possibly encounter neighboring vacancies. The dissociative adsorption of oxygen molecules at oxygen vacancy sites was determined to be a process activated at 523 K. The required temperature for recovery at 523 K, which is close to the desorption temperature, indicates that the activation energy for the dissociative adsorption at vacancy sites is almost the same as that for desorption (about 140 kJ mol-1) by the combinative desorption of oxygen atoms. This indicates that the kinetics for the dissociative adsorption and combinative desorption of oxygen atoms is controlled by the migration of vacancy sites at the surface. The precursor for dissociative adsorption of oxygen at vacancy sites is considered to be molecularly-adsorbed oxygen at an oxygen vacancy site, and the nature of the molecularly-adsorbed oxygen is also another key for dissociative adsorption. A schematic illustration of the oxygen adsorption discussed in this paper is presented in Figure 8.

Oxygen TPD from La0.6Sr0.4Co0.8Fe0.2O3 La0.6Sr0.4Co0.8Fe0.2O3 is another cathode candidate for lower temperature SOFCs. TPD profiles for oxygen from La0.6Sr0.4Co0.8Fe0.2O3 were examined and the results are shown in Figure 9. The peak at 140–220 K for molecularly-adsorbed oxygen is negligibly small. The shape of the peak at 580 K is similar to that for La1-xSrxCoO3 and the amount of desorption was estimated to be 0.2 molecules-O2 nm-2. Although oxygen vacancy sites were formed on the surface, molecular adsorption of oxygen was not observed for La0.6Sr0.4Co0.8Fe0.2O3. It is supposed that there are


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several types of oxygen vacancy sites on the surface, such as oxygen vacancies at the outmost surface, those at the subsurface, and those with steric hindrance for oxygen adsorption. The dissociative adsorption of oxygen at oxygen vacancies was examined for La0.6Sr0.4Co0.8Fe0.2O3, as with La0.8Sr0.2CoO3 in the previous subsection, and the results are shown in Figure 10. The intensity of the peak at 550 K increased above 473 K, which indicates that the oxygen vacancies started to recover by the adsorption of oxygen above 473 K. This is a slightly lower temperature than that for La1-xSrxCoO3. Therefore, it can be concluded that La0.6Sr0.4Co0.8Fe0.2O3 is more active for oxygen dissociative adsorption at vacancy sites than La1-xSrxCoO3.

Oxygen TPD from La0.8Sr0.2MnO3 As explained in the introduction, La0.8Sr0.2MnO3 has been widely applied to conventional SOFCs. The oxygen adsorption properties were next examined. Figure 11 shows TPD profiles for oxygen from La0.8Sr0.2MnO3.

The characteristic feature of oxygen adsorption on

La0.8Sr0.2MnO3 is that there is no desorption peak at 570 K. For the Co-based materials, an oxygen desorption peak is observed at 570 K, which is assigned to the combinative desorption of oxygen atoms at the surface that leads to the formation of oxygen vacancies. A desorption peak at 540 K has been reported previously for La0.85Sr0.15MnO3.27 The reason why the peak at about 540 K was not observed in this work is that the amount of desorption was much smaller than that for La1-xSrxCoO3 and La0.6Sr0.4Co0.8Fe0.2O3. While there is no desorption peak at 570 K, the low temperature peak at 140 K due to molecular adsorption was observed for La0.8Sr0.2MnO3. The amount of desorption increased with evacuation temperature, which indicates that molecular adsorption occurs at the oxygen


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vacancy sites. It should be noted that the amount of desorption of molecularly-adsorbed oxygen at 140 K was clearly smaller than that for La1-xSrxCoO3. The density of molecularly-adsorbed oxygen was estimated to be 0.001 molecules-O2 nm-2 (for the sample evacuated at 673 K), which is approximately fifty times smaller than that for La1-xSrxCoO3. Therefore, it is considered that the number of oxygen vacancy sites at the surface of La0.8Sr0.2MnO3 is smaller than that for La1-xSrxCoO3 and La0.6Sr0.4Co0.8Fe0.2O3. From the amount of molecular oxygen desorption, the number of oxygen vacancy sites at the surface can be considered to be approximately fifty times smaller than that for La1-xSrxCoO3. However, as discussed in the previous sections, the ratio of the amount of molecular adsorption and oxygen vacancies is strongly dependent on the type of material, so that this estimation of fifty times is very rough. Summarizing the results for La0.8Sr0.2MnO3, it is concluded that the amount of combinative desorption with oxygen vacancy formation is negligibly small compared with the other materials. Thus, the amount of desorption from the molecular adsorption of O2 is also small, although it was clearly observed at 170 K.

General Discussion As explained in the introduction, La1-xSrxCoO3 and La0.6Sr0.4Co0.8Fe0.2O3 have relatively high activity at low temperature, and are thus expected to be utilized in intermediate temperature SOFCs. The characteristic feature of these materials for oxygen TPD was determined to be the combinative desorption of surface oxygen atoms by the TPD peak at 570 K prior to the discharge of oxygen from the oxide bulk. This relates to the higher mobility of oxygen vacancies and oxygen atoms at the surface at about 523–570 K, which is probably linked with the high ORR activity.


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On the other hand, the nature of molecularly-adsorbed oxygen, which desorbs mostly at 140– 150 K, is quite complicated. Molecularly-adsorbed oxygen is considered to be adsorbed at oxygen vacancy sites. For the same type of sample, the amount of molecularly-adsorbed oxygen increases with evacuation temperature, which leads to an increase in the number of oxygen vacancies. However, molecularly-adsorbed oxygen was observed for La1-xSrxCoO3 but not for La0.6Sr0.4Co0.8Fe0.2O3. The ratio of the amount of molecularly-adsorbed oxygen and the number of oxygen vacancies was dependent on the type of sample. In the case of La0.8Sr0.2MnO3, the peak associated with combinative desorption of surface oxygen atoms was not observed, probably because it was negligibly small, and the amount of molecularly-adsorbed oxygen was about 50 times smaller than that for La1-xSrxCoO3.

The absence of a TPD peak due to

combinative desorption of surface oxygen for La0.8Sr0.2MnO3 indicates that there are much less oxygen vacancies at the surface than for the other materials. As reported in the literatures on theoretical calculations, the molecular adsorption states of oxygen have the adsorption energy over 1 eV (96 kJ mol-1).8,9,33 The energy for desorption (at 140 K) of molecularly adsorbed O2 of 35 kJ mol-1 seems to be obviously smaller than that reported by theoretical calculations. This indicates that the charge transfer from the adsorption site to oxygen molecules is not significant in the case of molecular adsorption in the present study. Therefore, it is speculated that the molecular adsorption state in the present case is not assignable to O2- (superoxo) or O22- (peroxo). However, the desorption energy of molecularly adsorbed O2 of 35 kJ mol-1 estimated in this study is 5 times higher than the heat of condensation of O2 of ca. 7 kJ mol-1. This clearly indicates that the observed molecularly adsorbed O2 is not in the physisorption state.

Moreover, the molecular adsorption O2 in the present work is


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remarkably stronger than that observed in the adsorption equilibrium at low temperature as reported spectroscopically.15 Although it is difficult to systematically explain all the experimental results at present, they make a strong contribution towards understanding the activity of these SOFC cathode materials. Low-temperature adsorption, such as with molecularly-adsorbed oxygen, frequently tends to be ignored in discussions on the high-temperature chemical reactions of SOFCs.

However,

molecularly-adsorbed oxygen, which is dominant only at low temperatures, is a key precursor for oxygen dissociation, even at high temperatures. The nature of the reaction precursor is the most important factor for the reaction. In the case of La0.6Sr0.4Co0.2Fe0.8O3, molecularly-adsorbed oxygen was not observed in the TPD profiles. It should be noted that the experimental apparatus employed in this work could analyze the oxygen TPD above 90 K. If the molecularly-adsorbed oxygen signal had been weaker for La0.6Sr0.4Co0.8Fe0.2O3, then it could not have been observed in the TPD measurements. The adsorption states identified in this work are not all of the adsorption states for oxygen and further studies are required to achieve clarification.

CONCLUSIONS Oxygen

adsorption

on

La0.8Sr0.2CoO3,

La0.6Sr0.4CoO3,

La0.6Sr0.4Co0.2Fe0.8O3,

and

La0.8Sr0.2MnO3 was investigated using TPD. Two types of oxygen desorption were determined; desorption from molecularly-adsorbed oxygen (140–150 K) and combinative desorption of surface oxygen atoms with the formation of oxygen vacancies (550–580 K). Some molecularlyadsorbed oxygen was considered to be adsorbed at oxygen vacancies that were produced by evacuation above 673 K. To recover the oxygen vacancies, exposure to gaseous oxygen above 523 K was required for La0.8Sr0.2CoO3, and this temperature was close to the desorption


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temperature for surface oxygen atoms. In the case of La0.6Sr0.4Co0.2Fe0.8O3, recovery of oxygen vacancies was observed above 473 K. In the case of La0.8Sr0.2MnO3, combinative desorption with formation of oxygen vacancy did not show a peak, whereas molecularly-adsorbed oxygen was observed at 140 K. The chemistry of molecularly-adsorbed oxygen at oxygen vacancies is considered to play an important role in the ORR process.

ASSOCIATED CONTENT Supporting Information This information is available free of charge via the Internet at http://pubs.acs.org

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (J.K.) Author Contributions The authors, K.H., H.S., and J.K., contributed to this work equally.

ACKNOWLEDGMENTS This work was mainly supported by one of the projects in “Phase Interface Science for Highly Efficient Energy Utilization” in the CREST program of the Japan Science and Technology Agency (JST). This work partly contributes one of the projects in “Creation of Innovative Core


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Technology for Manufacture and Use of Energy Carriers from Renewable Energy” in the CREST program of JST.

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Figure 1. Schematic diagram of ORR on SOFC cathode. VO denotes an oxygen-vacancy site at the surface. Molecular oxygen can also be adsorbed at VO.


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Figure 2. Schematic illustration of experimental apparatus used for TPD measurements. RP: mechanical rotary pump, FT: fore-line trap, TMP: turbo molecular pump, QMS: quadrupole mass spectrometer, CCG: cold cathode gauge, VLV: variable leak valve, V1-V3: metal diaphragm valves, PG: semiconductor pressure gauge, TC: thermocouple, LN2: liquid nitrogen, DP: oil diffusion pump.


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(a)

(b)

(c)

(d)

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Figure 3. SEM images of (a) La0.8Sr0.2CoO3, (b) La0.6Sr0.4CoO3, (c) La0.6Sr0.4Co0.2Fe0.8O3, and (d) La0.8Sr0.2MnO3 after pretreatment at 673 K in 10 kPa of O2.


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Figure 4. XRD patterns for (a) La0.8Sr0.2CoO3, (b) La0.6Sr0.4CoO3, (c) La0.6Sr0.4Co0.2Fe0.8O3, and (d) La0.8Sr0.2MnO3. The samples were pretreated at 673 K in 10 kPa of O2.


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Figure 5. TPD profiles for O2 from La0.8Sr0.2CoO3 evacuated at stated temperatures. O2 was introduced into the cell at 90 K. The heating rate was 10 K min-1.


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Figure 6. TPD profiles for O2 from La0.6Sr0.4CoO3 evacuated at stated temperatures. O2 was introduced into the cell at 90 K. The heating rate was 10 K min-1.


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Figure 7. TPD profiles for O2 from La0.8Sr0.2CoO3 evacuated at 673 K. 10 kPa of O2 was introduced into the cell at the stated temperatures and the sample was cooled to 90 K, followed by the evacuation of O2 prior to TPD measurements. The heating rate was 10 K min-1.


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Figure 8. Schematic illustration of oxygen adsorption mechanism proposed in this work.


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Figure 9. TPD profiles for O2 from La0.6Sr0.4Co0.2Fe0.8O3 evacuated at stated temperatures. O2 was introduced into the cell at 90 K. The heating rate was 10 K min-1.


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Figure 10. TPD profiles for O2 from La0.6Sr0.4Co0.2Fe0.8O3 evacuated at 673 K. 10 kPa of O2 was introduced into the cell at the stated temperatures and the sample was cooled to 90 K, followed by evacuation of O2 prior to TPD measurements. The heating rate was 10 K min-1.


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Figure 11. TPD profiles for O2 from La0.8Sr0.2MnO3 evacuated at stated temperatures. O2 was introduced into the cell at 90 K. The heating rate was 10 K min-1.


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Molecular and Dissociative Adsorption of Oxygen ... - ACS Publications

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