ARTICLE pubs.acs.org/JPCC
Stability of Ceria Supports in Pt CeOx/C Catalysts Ding Rong Ou,*,†,‡ Toshiyuki Mori,‡ Keisuke Fugane,‡,§ Hirotaka Togasaki,‡,|| Fei Ye,^ and John Drennan# †
)
Laboratory of Fuel Cells, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China ‡ Innovation Center of Nanomaterials Science for Environment and Energy, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan § Graduate School of Chemical Sciences and Engineering, Hokkaido University, Kita 13, Nishi 8, Kita-ku, Sapporo, Hokkaido 060-8628, Japan Department of Chemistry, Graduate School of Science, Hokkaido University, Kita 13, Nishi 8, Kita-ku, Sapporo, Hokkaido 060-8628, Japan ^ Key Laboratory of Materials Modification, School of Materials Science and Engineering, Dalian University of Technology, 2 Linggong Road, Dalian, Liaoning 116024, China # Centre for Microscopy and Microanalysis, The University of Queensland, St. Lucia, Qld 4072, Australia ABSTRACT: Pt CeOx/C (1.5 e x e 2) catalysts are promising electrode materials in polymer electrolyte membrane fuel cells (PEMFCs), especially in direct methanol fuel cells. In this study, the chemical and microstructural features of the catalysts before and after electrocatalytic reactions were compared, and then the stability of ceria supports in the catalysts was studied. It was found that, during the impregnation of Pt on ceria, Pt particles will invade into the ceria supports and the ceria will be partly reduced. Subsequently, in an acidic condition that is required in a fuel cell using an acid-based polymer electrolyte, the ceria supports were instable and easily dissolved into the sulfuric acid solution. As a result, only a small amount of ceria remains around/between the Pt particles. The experimental result suggested that the dissolution of ceria could be due to the partial reduction of the ceria supports in the as-prepared catalysts. Although only a small amount of ceria could remain in the catalysts, it can effectively prevent the growth and sintering of Pt particles and contribute to the CO tolerance and higher catalytic activities of the Pt CeOx/C catalysts.
1. INTRODUCTION The fuel cell is a developing technology that can convert chemical energy into electrical energy with high energy conversion efficiency and minimal pollutant emission.1 To reduce the price of this technology, the improvement in catalyst performance is of great importance. For polymer electrolyte membrane fuel cells (PEMFCs), the most suitable catalysts are Pt nanoparticles loaded on conductive carbon so far. However, these catalysts are still with some problems that need to be overcome. (1) On cathode, due to the inhibition of O2 reduction caused by OH adsorption on Pt surface, the slow kinetics of the oxygen reduction reaction (ORR) at the surface of Pt dramatically raise the overpotential loss and lower the performance of PEMFCs.2 (2) For anode catalysts, the poison of carbon monoxide, which may come from the impurity in hydrogen (for hydrogen fuel cells) or from the intermediate product of the electro-oxidation of methanol (for direct methanol fuel cells, DMFCs), leads to a sluggish electrooxidation of fuel and a high overpotential being required for the oxidation.3 (3) The severe conditions at the cathode and anode, such as low pH (required in a fuel cell using acid-based polymer electrolyte), high potential, and oxygen or reducing atmosphere, cause a rapid deactivation of these catalysts because Pt metal r 2011 American Chemical Society
particles could grow through the dissolution and subsequent deposition of Pt metal (Ostwald ripening).4,5 An effective way to solve these problems is modifying Pt/C catalysts with appropriate oxide promoters. An oxide promoter that has received considerable attention is ceria (cerium oxide). Pt-ceria is a unique system as compared with other Pt-oxides because of the alterable valence of the Ce cations and the high oxygen storage capacity of ceria. Due to the interaction between Pt and ceria supports, the growth and sintering of Pt metal particles can be considerably inhibited.6,7 Moreover, Takahashi et al. reported that the ORR activity on Pt CeOx/C (1.5 e x e 2) cathode was higher than that of a commercially available Pt/C cathode.8 Lim et al. suggested that the active oxygen supplied by ceria supports could transfer to Pt surface and contribute to the improvement of the ORR activity of the Pt CeOx/C cathode.9,10 They also examined the performance of fuel cell using Pt CeOx/C cathode and demonstrated the higher performance and much better durability compared with Pt/C catalysts. When Pt CeOx/C catalyst was applied as anode materials in Received: June 16, 2011 Revised: August 19, 2011 Published: August 24, 2011 19239
dx.doi.org/10.1021/jp205640k | J. Phys. Chem. C 2011, 115, 19239–19245
The Journal of Physical Chemistry C DMFCs, it was found that the Pt CeOx/C catalyst has a higher activity than Pt/C catalyst.11 15 In a few studies, such as those by Takahashi et al.14 and Huang et al.,15 the catalytic activity of Pt CeOx/C has exceeded that of PtRu/C catalyst. In literature, the mechanism of the promotion effect of ceria supports has been discussed based on the properties of ceria. It has been suggested that the promotion effects for ORR could be attributed to the high oxygen storage capacity of ceria at low temperature,8 10 while the effect on CO tolerance of anode materials could be associated with the reaction between CO absorbed on Pt and OH groups formed on the surfaces of ceria (a bifunctional mechanism13,16). Furthermore, the high reducibility of ceria, especially in the presence of Pt, and the prevention of sintering of Pt metal particles also contribute to the improved activity and durability of the catalysts.15 However, so far few studies were concerned about the detailed microstructural features of these catalysts (especially those features after the electro-catalytic reactions). Since the electro-catalytic process could be dramatically dependent on the detailed microstructures, it is significantly important to clarify the microstructural features of the catalysts for further understanding the fundamentals of the catalysis and the promotion effects of the ceria supports. For this reason, in the present study, a comparison study of the Pt CeOx/C catalysts before and after the catalytic reactions (ORR and electro-oxidation of methanol) was performed. The chemical and microstructural features of the catalysts after catalytic reaction were carefully examined and then compared with those of the as-prepared catalysts. In our previous studies, we successfully prepared Pt CeOx/C catalysts by impregnating Pt onto ceria particles and demonstrated the promotion effect of ceria supports in these catalysts.8,14,17 In addition, we have examined the metal support interaction, the chemical and microstructural features of the as-prepared catalysts.18 On the basis of these studies, the emphasis of this article will be laid on the ceria supports in the catalysts, especially their chemical and microstructural features, the stability in evaluation condition, and the possible influence on promotion effects.
2. EXPERIMENTAL SECTION Pt CeOx/C catalysts with 20 and 30 wt % Pt (denoted as 20PtCe and 30PtCe, in which the content of ceria supports was 18.9 wt % CeO2) were synthesized using a combined process of precipitation and impregnation methods.19 First, pure CeO2 particles were synthesized by the ammonium carbonate precipitation method. The dried precursors were subsequently calcined at 400 C for 2 h in an oxygen environment to yield oxide powders. To impregnate Pt on ceria, pure CeO2 particles and the required amount of H2PtCl6 3 6H2O (purity >98.5%, Kishida Chemical Co., Japan) were mixed and well-dispersed in ethanol. The mixture was dried at room temperature and reduced at 400 C for 2 h in a mixed flow of 10% H2 and 90% He to form metallic Pt. Then the Pt and ceria particles were mixed with the required amount of carbon black (Vulcan XC-72R, Cabot Co.) in ethanol, and this mixture was also dried at room temperature under a flow of nitrogen. Cyclic voltammetry (CV) studies of the catalysts were carried out at a temperature of 28 C, using a scan rate of 50 mV/s. The samples for the electrochemical measurement were prepared by spreading a suspension of the catalyst materials in an aqueous ethanol solution onto the surface of an Au electrode (area:
ARTICLE
0.20 cm2) by using a micropipet. In our previous studies, the catalysts 20PtCe and 30PtCe have been synthesized and used as cathode materials in PEMFCs8,17 and anode materials in DMFCs,14 respectively. In this study, they were tested in the same way; that is, the CV measurement for 20PtCe was performed in an aqueous solution of 0.5 M H2SO4, using a standard three-electrode glass cell with a rotating disk electrode (the test for a cathode material), while the CV measurement of 30PtCe was performed in an aqueous solution of 0.5 M H2SO4 and 0.5 M CH3OH (with the assumption that 30PtCe is anode materials in DMFCs). In both cases, Pt foil and Ag/AgCl were used as counter and reference electrodes, respectively. The measured potentials were converted to the reversible hydrogen electrode (RHE) scale. More details of the CV measurement can be found in literature.14,17 To study the chemical and microstructural features of Pt CeOx/C catalysts after ORR or electro-oxidation of methanol, the catalysts on Au electrode were collected by scraping the catalysts off Au electrode with a small knife after designed potential sweeps. Then the chemical composition of the catalysts before and after the catalytic reactions was analyzed by an inductively coupled plasma-mass spectrometry (ICP-MAS) experiment. The detailed microstructural features of the catalysts were investigated by transmission electron microscopy (TEM), using a JEM-2000EX electron microscope operating at 200 kV. The TEM specimens were prepared by dispersing the catalyst in ethanol and then collecting it on a carbon film supported by a Cu grid. Selected-area electron diffraction (SAED) and high-resolution TEM (HRTEM) were applied to identify the crystal structure and to investigate the atomic-scale microstructure of the catalysts, respectively. To identify oxidation states of ceria in the catalysts, an electron energy loss spectroscopy (EELS) study was performed in a FEI Tecnai-F30 electron microscope (operating at 300 kV) equipped with a Gatan Imaging Filtering system. The power-law technique20 was used to subtract the background. Since a high density (I) or a high dose of electron will damage the ceria supports and cause the reduction of Ce cations,21,22 a spread beam (I ∼ 104 105 e Å 2 s 1 on sample) and a short acquisition time (∼ 4 s) were applied to examine the redox state of ceria. The literature,21,22 as well as our previous study,23 have proved that no considerable increase in Ce3+ cations was induced under these conditions.
3. RESULTS Figure 1 shows the TEM images and SAED pattern of pure CeO2 particles synthesized by the ammonium carbonate precipitation method. The particles are rod-shaped, about 10 30 nm in diameter and tens to hundreds of nanometers in length (as shown in the inset of Figure 1). In the HRTEM image, it can be clearly seen that the CeO2 particles are polycrystalline and the grains are about 5 10 nm in size. The SAED pattern indicated that the CeO2 particles are solely fluorite-structured. In the as-prepared Pt CeOx/C catalysts with a Pt content of 20 and 30 wt % (20PtCe and 30PtCe), the structure and morphology of the ceria supports were quite different from their original state. As shown in Figure 2, the morphologies of two catalysts are analogous. Probably due to the metal support interaction,23 26 Pt particles invaded into the ceria supports, and then the original morphology of ceria was destroyed in both catalysts. As a result, the formation of more grain boundaries and 19240
dx.doi.org/10.1021/jp205640k |J. Phys. Chem. C 2011, 115, 19239–19245
The Journal of Physical Chemistry C the appearance of shorter range ordered materials gave rise to materials with sizes in the range 2 5 nm. Such destroyed ceria appear to be intimately dispersed around/between Pt particles. The EELS study revealed that the microstructural modification of the ceria supports is accompanied with the changes in the redox state of ceria. Figure 3 presents the energy-loss near edge structure (ELNES) of the Ce M4,5-edge of the catalysts. The spectra of the pure CeO2 particles and the reduce ceria (Ce2O3) were also given for comparison. On all spectra, two sharp peaks close to the ionization threshold are clearly seen (M4 and M5). It has been demonstrated that the relative intensity of the sharp peaks, IM4/IM5, can reflect the valent state of Ce.27 In our experiments, this value was 1.23 for pure CeO2 particles and 0.91 for reduced ceria. In the case of Pt CeOx/C catalysts, the average value of IM4/IM5, which was calculated from four to five spectra collected in different regions of a sample, was 1.18 for 20PtCe and 1.10 for 30PtCe. These results highlight the fact that the ceria supports in the catalysts are partly reduced. For the catalysts 20PtCe and 30 PtCe, the CV measurement was performed in an aqueous solution of 0.5 M H2SO4 (cathode test) and in an aqueous solution of 0.5 M H2SO4 and 0.5 M
Figure 1. HRTEM image of the CeO2 nanorods synthesized by the ammonium carbonate precipitation method. Insets are the low-MAG TEM image and SAED pattern.
ARTICLE
CH3OH (anode test), respectively. Then the catalysts after CV measurements were collected and also examined by TEM. Figures 4 and 5 show the TEM images and SAED patterns of 20PtCe and 30PtCe catalysts after 30 cycles of potential sweeps. On SAED patterns, besides the strong diffraction rings of Pt, the rings that belong to ceria supports can be noticed. Compared with those of as-prepared catalysts, the diffraction rings of the supports become very dim and broad, and their positions shift to those of Ce2O3, indicating that a great amount of ceria was lost during the potential sweeps and that the remaining oxides are mainly amorphous reduced ceria. Correspondingly, on HRTEM images, only a small amount of the residual oxide supports can be noticed between and around the Pt particles (Figures 4 and 5b). Instead of a mixture of nanocrystalline ceria and shorter range ordered materials in the as-prepared catalysts (Figure 2), the oxide supports remaining after CV measurement were mainly amorphous, which is very consistent with the appearance of broad diffraction rings on SAED patterns. These amorphous oxides (Figures 4 and 5b) can be easily distinguished from amorphous CB (as shown in Figure 6) because their morphologies are quite different. Owing to the loss of the ceria supports, some Pt particles were dispersed around CB particles (Figure 6). On the surface of these dispersed Pt particles, the residual oxides are also faintly seen.
Figure 3. EELS spectra of the CeM4,5-edge of the as-prepared Pt CeOx/C catalysts (20PtCe and 30PtCe), pure CeO2, and reduced ceria.
Figure 2. HRTEM images of the as-prepared Pt CeOx/C catalysts: (a) 20PtCe and (b) 30PtCe. Insets are low-MAG TEM images and SAED patterns. 19241
dx.doi.org/10.1021/jp205640k |J. Phys. Chem. C 2011, 115, 19239–19245
The Journal of Physical Chemistry C In the local area, the remains of the supports are considerable, and then we have the chance to examine the detailed structures of the remaining oxides. As shown in Figure 7, the remaining oxides contain short-range orderings and a few of nanocrystallites. Besides small Pt particles, the careful analysis of SAED patterns revealed that the short-range ordered materials and nanocrystallites in the residual oxides could include fluorite-structured ceria (CeO2, d(111) ≈ 3.1 Å), reduced ceria (CeO2 δ, d(111) ≈ 3.2 3.4 Å), PtO, and other compounds that were difficult to identify at this stage. We believe that the amorphous materials around the Pt particles could be composed of these compounds as well. To quantitatively describe the loss of ceria in the catalysts, the ICP-MAS study was performed.17 It was found that the ceria supports in the catalysts were highly unstable in the sulfuric acid
ARTICLE
solution. As the 20PtCe catalyst was put into an aqueous solution of 0.5 M H2SO4, most of the ceria was lost after 1 h even without any potential sweeps, and the atomic ratio of Ce/Pt determined by ICP-MAS dramatically decreased from ∼1.0 to ∼0.11. This ratio further decreased to about 0.03 after five sweeping cycles and remained as this value during 5 30 cycles. A similar phenomenon was also observed during the CV measurement for 30PtCe. It means that, though the ceria support in the catalysts is easily dissolved into the sulfuric acid solution, the small amount of oxides remaining around/between the Pt particles is quite stable. We suspected that the remaining amount
Figure 6. Pt particles dispersed around CB particles in the 20PtCe catalyst after 30 cycles of potential sweeps.
Figure 4. TEM images and SAED patterns of the 20PtCe catalyst after 30 cycles of potential sweeps.
Figure 7. Residual oxides in 20PtCe catalysts with larger Pt particles gone.
Figure 5. TEM images and SAED patterns of the 30PtCe catalyst after 30 cycles of potential sweeps. 19242
dx.doi.org/10.1021/jp205640k |J. Phys. Chem. C 2011, 115, 19239–19245
The Journal of Physical Chemistry C
ARTICLE
Figure 9. Cyclic voltammograms of 1st, 5th, 30th, 100th, and 1000th cycles for the 20PtCe catalyst. The experiment was carried out in an aqueous solution of 0.5 M H2SO4 at a temperature of 28 C, using a scan rate of 50 mV/s.
Figure 8. (a) TEM image of 20PtCe after 1000 cycles of potential sweeps and (b) HRTEM image of the residual oxides remaining after 1000 cycles of potential sweeps.
of these small amounts of oxides and their high stability could be due to the strong metal support interaction existing in the catalysts,26 which might restrain the dissolution of these oxides. Although only a small amount of oxides remains in the catalysts, it can effectively prevent the growth and sintering of Pt particles during the CV measurement. Figure 8 presents the distribution of Pt particles in 20PtCe catalyst after 1000 potential sweeps. From the 30th cycle (Figure 4) to the 1000th cycle (Figure 8a), the diameter of the Pt particles in the catalyst slightly increases from about 5 15 nm to about 20 nm (Nevertheless, the small Pt particles dispersed on or in the oxide, as can be seen by the TEM image and SAED pattern in Figure 7, are difficult to be counted). On most of the areas, it can be noticed that the oxides around/between the Pt particles remain in the catalysts after 1000 potential sweeps (Figure 8b). It is known that generally the Pt particles in the catalysts grow by the dissolution and subsequent deposition of Pt under the severe conditions at the cathode and anode. Therefore, it can be concluded that the oxides covering on the metal surface can effectively restrain the dissolution and redeposition of metal species. Furthermore, the slight increase in the size of Pt particles reveals that the oxide layers covering on Pt particles could be porous or discontinuous. In this case, the dissolution and redeposition of the metal species could take place in spite of the coverage, but this process becomes much slower compared with that in Pt/C catalysts. Simultaneously, because of the steric effects, these oxides could effectively prevent the direct contact between Pt particles. Thus, the possibility of metal particle sintering is small though the distance between Pt particles decreases due to the dramatic loss of the ceria supports, which could be the key to the high durability of Pt CeOx/C reported in literature.
Figure 10. Electrochemically active surface area (ESA) of the 20PtCe catalyst during potential sweeps, measured using the Coulombic charge for oxidation of the adsorbed atomic hydrogen. The inset compares ESA with the atomic ratio of Ce/Pt measured by ICP-MAS. The value of Ce/ Pt for the zero cycle is of the sample treated in 0.5 M H2SO4 for 1 h without any sweep.
Because CV has a big advantage in the detection of surface processes, the changes in chemical and microstructural features revealed by TEM and ICP-MAS studies can also be reflected by cyclic voltammograms during potential sweeps. Figure 9 presents the cyclic voltammograms of 1st, 5th, 30th, 100th, and 1000th cycles for the 20PtCe catalyst. The experiment was carried out in an aqueous solution of 0.5 M H2SO4 at a temperature of 28 C, using a scan rate of 50 mV/s. From the 30th cycle, the peaks owing to the surface of Pt can be clearly seen on the cyclic voltammograms, including the peaks for the hydrogen adsorption and oxidation (0 0.3 V) and those for the reduction and oxidation of Pt (0.7 1.1 V). Then the electrochemically active surface area (ESA) during potential sweeps was measured using the Coulombic charge for oxidation of the adsorbed atomic hydrogen, assuming a Coulombic charge of 220 mC/cm2 for the oxidation of absorbed atomic hydrogen on smooth Pt,28 and the result is shown in Figure 10. As can be seen from the inset of Figure 10, besides the activation of the catalyst surface, the dramatic increase in ESA during the 1st to 30th could be related 19243
dx.doi.org/10.1021/jp205640k |J. Phys. Chem. C 2011, 115, 19239–19245
The Journal of Physical Chemistry C
Figure 11. Schematics showing the changes in microstructural features of the oxide supports: (a) pure CeO2, (b) as-prepared Pt CeOx/C catalyst, and (c) Pt CeOx/C catalyst after electro-catalytic reactions. (d) The enlarged schematic illustrates the discontinuous coverage of Pt particle with residual oxides and the resultant triple-phase interface of Pt, oxide, and the atmosphere/solution.
with the fast dissolution of ceria supports. After 100 cycles, the ESA slightly decreases from 47.4 to 38.9 m2/g (the average diameter of Pt particles calculated from ESA increases from 5.9 to 7.2 nm), which might be owing to the slow growth of Pt particles revealed by TEM (Figure 8). Moreover, Figures 9 and 10 revealed that the Pt particles covered with porous or discontinuous oxide layers are totally electrochemically active.
4. DISCUSSION On the basis of the experimental results, the morphology changes of ceria supports and the catalysts can be imaged as shown in Figure 11. First, owing to the interaction between Pt and ceria, Pt could invade into ceria supports and break the original morphology of the ceria supports. Then, at low PH conditions, most of the ceria supports will dissolve into sulfuric acid, and as a result, the Pt particles in the catalysts were only surrounded with a small amount of residual oxides. It is known that Pt in the catalysts (e.g., Pt/C) will also dissolve from the catalysts under the severe conditions at the cathode and anode, such as low pH (required in a fuel cell using acid-based polymer electrolyte), high potential, and oxygen or reducing atmosphere, and then the metal species deposit on other metal particles which increase the particle size of the metals.4,5 Compared with the reversible dissolution of Pt from the catalysts, the dissolution of CeOx seems more thorough, and the redeposition of the dissolved ceria species is not considerable. The dramatic dissolution of ceria supports into the sulfuric acid is somewhat surprising because it is known that the solubility of CeO2 is subtle in sulfuric acid. To clarify this problem, the dissolution of pure CeO2 was examined. First, commercial CeO2 powders with a particle size of 1 2 μm and the homemade nano CeO2 particles (as shown Figure 1) were dispersed in 0.5 M H2SO4 solution, respectively. The mole ratio of CeO2 to H2SO4 in the suspension was 1:20 to ensure superfluous H2SO4. Then the suspension was stirred at 28 C for 48 h. After that, the remains of the oxide powders were washed with distilled water, collected by filtration, and dried in 60 C for 48 h. The total loss of CeO2 (including the loss in acid solution and those during
ARTICLE
washing and filtration) is ∼3% for commercial CeO2 powders and ∼4% for homemade nano CeO2 powder. It indicates that the solubility of CeO2 powder in the 0.5 M H2SO4 solution is small and that the large surface area of nanoparticles has no considerable contribution to the dissolution. Therefore, we suspect that the poor stability of the ceria supports in the catalysts could be due to their chemical and microstructural features. As mentioned above, owing to the interaction between Pt and ceria, the ceria supports in the as-prepared catalysts will be partially reduced (Figure 3). Compared with tetravalent cerium oxides, trivalent cerium oxides present higher dissolubility in acidic solutions.29 Thus, the dissolution of the oxide supports can be described as follows: Ce2O3 + 6H+ f 2Ce3+ + 3H2O. On the other hand, the interaction (e.g., physical or chemical bonding) between Pt and ceria can restrain the dissolution of ceria oxides adjacent to the surface of Pt particles, which can explain the formation of oxide coverage on Pt particles as shown in the HRTEM images (Figures 4 and 5). In a previous study, we had suggested that the appearance of trivalent Ce cations could be mainly attributed to the metal oxide interactions.18 Additionally, the reduction of chlorinecontaining precursor/ceria systems for the preparation of the ceria supported catalyst could induce the incorporation of chloride ions into the support structure, which also contributes to the reduction of the ceria support.30 In our study, the content of chlorine in anions (Cl/(O + Cl) 100%) is about 4 5 wt % for 20PtCe and 30PtCe,18 indicating that the chloride ions in the oxide supports are not the main reason for the reduction of ceria support. However, we suspect the chloride ions in the oxide supports might further decrease the stability of ceria lattice and therefore also promote the dissolution of ceria supports into the sulfuric acid. As suggested in previous studies, the contribution of the ceria supports in the catalyst is multifold: First, the metal support interaction can restrain the growth and sintering of metal particles;6,7 second, the ceria supports can promote methanol electro-oxidation at anode and the ORR at the cathode.8 15,17 The experimental results in this study have shown that the small amount of oxide supports remaining around/between the Pt particles could effectively restrain the growth and sintering of Pt particles in Pt CeOx/C catalysts. On the other hand, in our previous studies, the promotion effect of ceria supports on methanol electro-oxidation and on ORR has been demonstrated.8,14,19 We believed that the residual oxides can also contribute to the promotion effects of the ceria supports, which could be understood as follows. In literature, it has been suggested that the promotion effect of ceria is closely related with the interface between the Pt and the supports. When Pt-CeOx/C catalysts are used as anode materials, at the interfacial region, OH groups generated and supported by ceria could react with CO absorbed on Pt particles (a bifunctional mechanism13,18) and then increase the CO tolerance of the catalysts. In the case of Pt CeOx/C used as cathode materials, the active oxygen supplied from CeO2 could transfer to Pt surface and contribute to the improvement of the ORR activity of the Pt CeOx cathode.8 10 Therefore, it can be concluded that in both cases the triple-phase interfaces of Pt, oxide, and the atmosphere/solution play a very important role in the catalytic reactions. As discussed earlier, though most of the ceria supports can be dissolved into the sulfuric acid solution, a small amount of oxides could remain on the surface of Pt particles because the strong metal oxide interaction and form porous or discontinuous oxide coverage of the metal particles. As illustrated in 19244
dx.doi.org/10.1021/jp205640k |J. Phys. Chem. C 2011, 115, 19239–19245
The Journal of Physical Chemistry C Figure 9, such porous or discontinuous oxide coverage greatly increases the amount of the triple-phase interfaces and guarantees the considerable promotion effect of the oxide supports.
5. CONCLUSIONS Ceria supports in Pt CeOx/C catalysts are instable. During the preparation of catalysts, Pt could invade into the ceria supports because of the Pt-ceria interaction, and consequently, a great amount of Pt/ceria interface is formed. In an acidic condition that is required in a fuel cell based on acid-based polymer electrolyte, the ceria supports are easily dissolved into the sulfuric acid solution, and only a small amount of oxides remains around/between the Pt particles and may form a porous or discontinuous coverage. Although only a small amount ceria remains in the catalysts, it can effectively inhibit the growth and sintering of Pt particles and contribute to promotion effect of ceria on the methanol electro-oxidation and the oxygen reduction reaction. ’ AUTHOR INFORMATION Corresponding Author
ARTICLE
(18) Ou, D. R.; Mori, T.; Togasaki, H.; Takahashi, M.; Ye, F.; Drennan, J. Langmuir 2011, 27, 3859–3866. (19) Togasaki, H.; Mori, T.; Takahashi, M.; Tada, A.; Matolin, V.; Drennan, J. Trans. Mater. Res. Soc. Jpn. 2008, 33, 1097–1100. (20) Egerton, R. F. Electron Energy Loss Spectroscopy in the Electron Microscope, 2nd ed.; Plenum Press: New York, 1996; pp 269 276. (21) Garvie, L. A. J.; Buseck, P. R. J. Phys. Chem. Solids 1999, 60, 1943–1947. (22) Bentley, J.; Gilliss, S. R.; Carter, C. B.; Al-Sharab, J. F.; Cosandey, F.; Anderson, I. M. J. Phys.: Conf. Ser. 2006, 26, 69–72. (23) Ou, D. R.; Mori, T.; Ye, F.; Zou, J.; Auchterlonie, G.; Drennan, J. Phys. Rev. B 2008, 77, 024108. (24) Tauster, S. J. Acc. Chem. Res. 1987, 20, 389–394. (25) Koningsberger, D. C.; Gates, B. C. Catal. Lett. 1992, 14, 271–277. (26) Yang, Z.; Lu, Z.; Luo, G. Phys. Rev. B 2007, 76, 075421. (27) Arai, S.; Muto, S.; Murai, J.; Sasaki, T.; Ukyo, Y.; Kuroda, K.; Saka, H. Mater. Trans. 2004, 45, 2951–2955. (28) Lee, S. J.; Mukerjee, S.; McBreen, J.; Rho, Y. W.; Kho, Y. T.; Lee, T. H. Electrochim. Acta 1998, 43, 3693–3701. (29) Stoyanova, E.; Guergova, D.; Stoychev, D.; Avramova, I.; Stefanov, P. Electrochim. Acta 2010, 55, 1725–1732. (30) Bernal, S.; Calvino, J. J.; Cifredo, G. A.; Rodríguez-Izquierdo, J. M. J. Phys. Chem. 1995, 99, 11794–11796.
*E-mail:
[email protected]. Tel.: +86-411-84379028. Fax: +86-411-84379028.
’ ACKNOWLEDGMENT The financial support of the Japan Society for the Promotion of Science (JSPS) is gratefully acknowledged. ’ REFERENCES (1) Steele, B. C. H.; Heinzel, A. Nature 2001, 414, 345–352. (2) Anderson, A. B. Electrochim. Acta 2002, 47, 3759–3763. (3) Ren, X.; Wilson, M. S.; Gottesfeld, S. J. Electrochem. Soc. 1996, 143, L12–L14. (4) Tseung, A. C. C.; Dhara, S. C. Electrochim. Acta 1975, 20, 681–683. (5) Honji, A.; Mori, T.; Tamura, K.; Hishimura, Y. J. Electrochem. Soc. 1988, 135, 355–359. (6) Huang, S.-Y.; Chang, C.-M.; Yeh, C.-T. J. Catal. 2006, 241, 400–406. (7) Bera, P.; Gayen, A.; Hegde, M. S.; Lalla, N. P.; Spadaro, L.; Frusteri, F.; Arena, F. J. Phys. Chem. B 2003, 107, 6122–6130. (8) Takahashi, M.; Mori, T.; Yoshikawa, H.; Togasaki, H.; Fugane, K.; Tada, A.; Matolin, V.; Drennan, J. Trans. Mater. Res. Soc. Jpn. 2008, 33, 1101–1104. (9) Lim, D.-H.; Lee, W.-D.; Choi, D.-H.; Lee, H.-I. Appl. Catal., B 2010, 94, 85–96. (10) Lim, D.-H.; Lee, W.-D.; Choi, D.-H.; Kwon, H.-H.; Lee, H.-I. Electrochem. Commun. 2008, 10, 592–596. (11) Xu, C.; Shen, P. K. Chem. Commun. 2004, 2238–2239. (12) Xi, J.; Wang, J.; Yu, L.; Qiu, X.; Chen, L. Chem. Commun. 2007, 1656–1658. (13) Scibioh, M. A.; Kim, S.-K.; Cho, E. A.; Lim, T.-H.; Hong, S.-A.; Ha, H. Y. Appl. Catal., B 2008, 84, 773–782. (14) Takahashi, M.; Mori, T.; Ye, F.; Vinu, A.; Kobayshi, H.; Drennan, J. J. Am. Ceram. Soc. 2007, 90, 1291–1294. (15) Huang, S.-Y.; Chang, C.-M.; Yeh, C.-T. J. Catal. 2006, 241, 400–406. (16) Hamnett, A. Catal. Today 1997, 38, 445–457. (17) Fugane, K.; Mori, T.; Ou, D. R.; Suzuki, A.; Yoshikawa, H.; Masuda, T.; Uosaki, K.; Yamashita, Y.; Ueda, S.; Kobayashi, K.; Okazaki, N.; Matolinova, I.; Matolin, V. Electrochim. Acta 2011, 56, 3874–3883. 19245
dx.doi.org/10.1021/jp205640k |J. Phys. Chem. C 2011, 115, 19239–19245
