TY - JOUR
AU - Kamino,, Takeo
AB - Abstract To clarify the influence of moisture on the structural changes of heated nano materials, in situ high temperature transmission electron microscopy (TEM) has been carried out using a conventional analytical TEM combined with a gas injection-specimen heating holder. Air with high moisture content, above 94% relative humidity (RH), from a humidifier was directly injected onto the heated platinum catalyst dispersed on carbon black (Pt/CB), and the morphological changes of the specimens were observed at high magnification dynamically. The result of the experiment was compared with a result obtained from an experiment using air with a low moisture content, 34% RH. Active movement of the Pt particles, leading agglomeration and grain growth, occurred prior to degradation of the CB support at high moisture content. In contrast, the degradation of the CB support leading agglomeration and grain growth of the Pt particles occurred before the displacement of the Pt particles on the CB supports in a low humidity environment. in situ observation, analytical TEM, humid atmosphere, gas injection, specimen heating holder, catalyst Introduction Improvement in the durability of the material used in the cell is one of the most important criteria, while considering the added applications of polymer electrolyte membrane fuel cells. Among the materials used in the cell, the electrocatalyst is the most studied material by the transmission electron microscope (TEM), and several experimental studies have been reported. Mayrhofer et al. [1] have observed Pt/C catalysts before and after a potential cycle treatment in argon-saturated 0.1 mol l−1 HClO4 solution and confirmed the movement and agglomeration of the Pt particles. Hartl et al. [2] have also observed almost the same structural changes of the Pt/C catalyst after 2 h accelerated degradation treatment and clearly demonstrated the morphological changes of the carbon support and related movement and agglomeration of the Pt particles. Xu et al. [3] have carried out an ex situ experiment on Pt/C catalysts in a vapor steam environment at 800°C and obtained information on the structural changes of several types of carbon supports and related Pt particle size changes. Recently, Perez-Alonso et al. [4] have carried out an oxygen reduction reaction corrosion experiment on a Pt/C catalyst and studied the relation between the number of potential cycles and structural changes of the carbon support and particle sizes of the Pt catalyst. All of these studies were carried out in ex situ experiments and did not provide us with information on the reaction process. A TEM equipped with an environment-controlled specimen chamber (ETEM) is essential to directly study catalysts under reaction conditions at nanoscale dimensions [5–7]. We have developed an ETEM technique based on a conventional TEM. In this technique, the microscope column, specimen chamber and pumping system remained unchanged from a standard microscope, and most of the functional capability for carrying out the in situ observation was fitted into specimen holders. We have developed a specimen heating holder with a gas injection nozzle [8]. Saka [9] have applied this specimen heating holder for the observation of oxidation, reduction and re-oxidation of Si nanoparticles. Kishita et al. [10] have applied this for the in situ observation of Pt particles on alumina and Pt on carbon under the oxygen atmosphere using an environmental TEM system with an improved differential pumping capacity and mounting a new gas injection mechanism. To improve the technical capabilities of our method, we have added a second heating element and applied it to the in situ synthesis of an AuPd/Al2O3 catalyst. In this method, the Al2O3 support was synthesized in situ by oxidation of Al particles, then AuPd nanoparticles were deposited on the Al2O3 support by using the second heating element as a metal evaporator. After the synthesis, the morphological stability of the synthesized catalyst under various gaseous atmospheres was examined continuously using the single specimen heating holder [11]. The latter work was our trial application of the developed techniques to the field of catalysis nano-particles, and through the experiment we found that the process of metal oxidation can be followed relatively at high magnification if the oxidation speed is decelerated by lowering the gas pressure in the specimen area. However, all of our in situ observations of solid–gas reactions had been carried out using dry gases, and the capability of the technique in the in situ experiment at high humidity had not yet been clarified. So far, several papers have reported the in situ observation of nanoparticles in the presence of water vapor using ETEM techniques [12–15]. Regarding the field of the fuel cell, Hansen et al. reported atomic resolution imaging of dynamic shape changes in Cu/ZnO and Cu/silica catalysts in various gas environments for methanol synthesis and hydrocarbon conversion processes. The result of the in situ observation shows that the addition of water to the hydrogen gas transforms the Cu crystals into a more spherical morphology [12]. In the present study, highly moisturized air was introduced to the specimen area, and morphological changes of a platinum catalyst dispersed on carbon black (Pt/CB) were dynamically observed. To study the dependence of morphological changes of the catalyst on the air humidity, experimental results at high moisture content were compared with results obtained with lower moisture content. Materials and methods A Hitachi H-9500 high resolution 300 kV analytical TEM equipped with an AMT high-resolution charge-coupled device camera system and a specimen heating holder with gas injection nozzle [8] were used in the study. To introduce humidity-controlled air into the specimen chamber, we developed a moisturized air supply system. Figure 1 shows an external view of the moisturized air supply unit developed for the H-9500 in situ TEM. The unit consists of a humidifier, a duct hose and a built-in thermo-hygrometer. In this method, the moisturized air from the humidifier is introduced into the specimen chamber of the microscope via a gas injection nozzle built into the specimen heating holder. The flow rate of the air to be introduced into the specimen chamber is controlled by a needle valve of the gas injection unit of the specimen heating holder. A schematic diagram of the moisturized air supply unit combined with the specimen heating holder is depicted in Fig. 2. The moisture level and temperature of the air from the humidifier are continuously measured in the vicinity of the needle valve of the specimen heating holder during the in situ observation by the thermo-hygrometer and the measured values are logged automatically. Fig. 1. Open in new tabDownload slide External view of a moisturized air supply unit developed for the H-9500 in situ TEM. Fig. 1. Open in new tabDownload slide External view of a moisturized air supply unit developed for the H-9500 in situ TEM. Fig. 2. Open in new tabDownload slide Schematic diagram of the moisturized air supply unit combined with the specimen heating holder. Fig. 2. Open in new tabDownload slide Schematic diagram of the moisturized air supply unit combined with the specimen heating holder. The specimen used in the experiment was a commercially available Pt/CB catalyst (Tanaka Kikinzoku Kogyo K.K., TEC10E50E). The specimen was wetted with ethanol and mounted on the heating element of the specimen heating holder directly using a small paint brush. When the Pt/CB catalyst is used as the electrocatalyst of a polymer electrolyte fuel cell (PEFC), the operating temperature is approximately 80oC. However, if the specimen is kept heated at this temperature during an in situ TEM study, morphological changes of the catalyst will take place over an unrealistically long time period. In this study, the specimen was heated to 300°C for in situ TEM observation of morphological changes of the catalyst and to 100°C for high-resolution TEM observation of the structural changes of the Pt particles and carbon support. Before the in situ high temperature TEM observation in highly moisturized air atmosphere, the fine structure of the Pt/CB catalyst was investigated by high-resolution TEM in a vacuum of around 10−5 Pa at room temperature. As shown in Fig. 3, a Pt particle showed a typical crystallized metal nanoparticle structure with 0.23 nm crystal lattice fringes of the Pt (111) plane and, no other substances are observed, either on the Pt particle or on the carbon support. Fig. 3. Open in new tabDownload slide High-resolution TEM image of the Pt/CB catalyst observed in vacuum of around 10−5 Pa at room temperature. Fig. 3. Open in new tabDownload slide High-resolution TEM image of the Pt/CB catalyst observed in vacuum of around 10−5 Pa at room temperature. In addition, to understand the maximum allowable electron dose in the in situ observation of the catalyst, we carried out preliminary TEM image observation at various electron beam densities and found that electron beam damage does not arise when the electron beam density is kept below 1 A cm−2. Accordingly, we carried out the in situ observation at elevated temperatures with an electron beam density of 0.5 A cm−2. Figure 4 shows TEM images observed at 300°C in a vacuum of 5 × 10−4 Pa. It is evident that the catalyst maintains its original structure during the in situ TEM observation for 500 s. Fig. 4. Open in new tabDownload slide Pt/CB catalyst heated to 300°C in a vacuum of 5 × 10−4 Pa. (a) Before heating and (b) after heating for 500 s. Fig. 4. Open in new tabDownload slide Pt/CB catalyst heated to 300°C in a vacuum of 5 × 10−4 Pa. (a) Before heating and (b) after heating for 500 s. Figure 5 shows the procedure of in situ TEM observation of a heated Pt/CB catalyst in a moisturized air atmosphere. First, the specimen was heated to 300°C or 100°C in vacuum. Then, the humidifier was switched on to fill the duct hose with moisturized air of 94% relative humidity (RH) at 40oC. After measuring the humidity and temperature of the air inside the duct hose, the air was introduced into a specimen chamber via the needle valve of the specimen heating holder. Fig. 5. Open in new tabDownload slide Procedure of in situ TEM observation of a heated Pt/CB catalyst in a moisturized air atmosphere. Fig. 5. Open in new tabDownload slide Procedure of in situ TEM observation of a heated Pt/CB catalyst in a moisturized air atmosphere. The pressure in the specimen area was kept at 0.2 Pa throughout the experiment. The method to measure the pressure in the specimen area and the method for measuring the specimen heating temperature in a gaseous atmosphere are described elsewhere [10]. Results and discussion In order to clarify the specific behavior of the catalyst in a highly moisturized air atmosphere, in situ TEM observation in a low humidity air atmosphere had been carried out in advance. TEM images demonstrating the morphological changes of Pt particles and carbon support observed at 300°C for 160 s in the 0.2 Pa air atmosphere of 34% RH are shown in Fig. 6. In this environment, rapid morphological changes of the carbon support occurred immediately after the air introduction. Several Pt particles have grown and agglomerated as a consequence of the morphological changes of the carbon support. Fig. 6. Open in new tabDownload slide TEM images demonstrating the morphological changes of Pt particles and carbon support observed at 300°C for 160 s in the 0.2 Pa air atmosphere of 34% RH. Fig. 6. Open in new tabDownload slide TEM images demonstrating the morphological changes of Pt particles and carbon support observed at 300°C for 160 s in the 0.2 Pa air atmosphere of 34% RH. Figure 7 depicts sequential TEM images demonstrating the behavior of the Pt particles observed at 300°C for 190 s in the 0.2 Pa air atmosphere of 94% RH. Under this condition, the morphological changes of the carbon support were smaller than they were in the air atmosphere of 34% RH. However, the Pt particles moved and agglomerated in a relatively short time, and the structure of the carbon support appeared to have changed locally. Figure 8 includes partially enlarged TEM images of Fig. 7, showing the structure of the Pt/CB catalyst observed (a) before and (b) after heating at 300°C in the 0.2 Pa air atmosphere of 94% RH. As demonstrated in the figures, after movement, the Pt particles (arrows in a) appeared to have created structures such as etch pits (arrows in b) in the carbon support. Fig. 7. Open in new tabDownload slide TEM images demonstrating the behavior of the Pt particles observed at 300°C for 190 s in the 0.2 Pa air atmosphere of 94% RH. Fig. 7. Open in new tabDownload slide TEM images demonstrating the behavior of the Pt particles observed at 300°C for 190 s in the 0.2 Pa air atmosphere of 94% RH. Fig. 8. Open in new tabDownload slide Partially enlarged TEM images of Fig. 7, showing the structure of the Pt/CB catalyst observed (a) before and (b) after the heating at 300°C in the 0.2 Pa air atmosphere of 94% RH. After movement, Pt particles (arrows in a) appeared to have created structures such as etch pits (arrows in b) in the carbon support. Fig. 8. Open in new tabDownload slide Partially enlarged TEM images of Fig. 7, showing the structure of the Pt/CB catalyst observed (a) before and (b) after the heating at 300°C in the 0.2 Pa air atmosphere of 94% RH. After movement, Pt particles (arrows in a) appeared to have created structures such as etch pits (arrows in b) in the carbon support. Figure 9 shows selected area electron diffraction patterns obtained (a) before and (b) after heating of the specimen at 300°C in the 0.2 Pa air atmosphere. Electron diffraction rings of Pt (111), Pt (200) and Pt (220) planes were obtained in both cases, and several electron diffraction spots of Pt (111), Pt (200) and Pt (220) planes appeared in (b). From the difference of the electron diffraction patterns between (a) and (b), it is evident that some of the Pt particles have grown, but no oxidation of the Pt particles occurred during the heat treatment in the highly moisturized air atmosphere. Fig. 9. Open in new tabDownload slide Selected area electron diffraction patterns obtained from the Pt/CB catalyst; (a) before the heating of the specimen; (b) after the heating of the specimen at 300°C in the 0.2 Pa air atmosphere of 94% RH. Electron diffraction rings of Pt (111), Pt (200) and Pt (220) planes were obtained in both cases, and several electron diffraction spots of Pt (111), Pt (200) and Pt (220) planes appeared in (b). Fig. 9. Open in new tabDownload slide Selected area electron diffraction patterns obtained from the Pt/CB catalyst; (a) before the heating of the specimen; (b) after the heating of the specimen at 300°C in the 0.2 Pa air atmosphere of 94% RH. Electron diffraction rings of Pt (111), Pt (200) and Pt (220) planes were obtained in both cases, and several electron diffraction spots of Pt (111), Pt (200) and Pt (220) planes appeared in (b). To investigate the mechanism of the formation of etch pit-like structures in the carbon support beneath the Pt particles that had moved, we carried out high resolution in situ TEM observation. Since the structural changes of the specimen at 300°C are too fast to record high-resolution TEM images, the experiment was carried out at 100oC. Figure 10 shows high-resolution TEM images observed at 100°C in the 0.2 Pa air atmosphere of 94% RH. In this environment, most of the Pt particles were covered with amorphous substances. Since the original Pt particles have no substances on their surfaces, as shown in Fig. 3, we assume that the substance observed in Fig. 10 is made up of water molecules or is an adsorbate containing water molecules. Fig. 10. Open in new tabDownload slide High-resolution TEM images of Pt particles of the Pt/CB catalyst observed at 100°C in the 0.2 Pa air atmosphere of 94% RH. Most of the Pt particles are covered with amorphous substances. Fig. 10. Open in new tabDownload slide High-resolution TEM images of Pt particles of the Pt/CB catalyst observed at 100°C in the 0.2 Pa air atmosphere of 94% RH. Most of the Pt particles are covered with amorphous substances. Figure 11 is a sequence of high-resolution TEM images demonstrating the structural changes of the Pt particles and the carbon support of the Pt/CB catalyst observed at 100°C in the 0.2 Pa air atmosphere of 94% RH. The Pt particle appeared to create a depression in the carbon support, and settle into the depression, due to accelerated degradation of the carbon support. After just 50 s, a pit was created of nearly the same diameter as the Pt particle. We assume that the instability of the Pt particle at higher moisture content is caused by a rapid, local degradation at the interface between the Pt particle and the carbon support. To clarify the degradation mechanism of the carbon support at the interface with the Pt particles, a high resolution in situ TEM observation using a graphitized-carbon supported Pt catalyst has been carried out in the same environment as the Pt/CB catalyst observation. Figure 12 shows video images demonstrating a profile view of the structural change of the carbon support at the interface with a Pt particle. At the beginning (Fig. 12a), the Pt particle appears to be covered with the same adsorbate as shown in Fig. 10. After 27 s, a clean facet emerged at the right side of the particle, and the adsorbate accordingly moved to the other side (Fig. 12b). Then, the adsorbate aggregated at the interface and behaved as if it accelerated the degradation of the carbon support beneath the Pt particle (Fig. 12c, d). Fig. 11. Open in new tabDownload slide High-resolution TEM images demonstrating the structural changes of the Pt/CB catalyst observed at 100°C in the 0.2 Pa air atmosphere of 94% RH. The Pt particle appeared to create a depression in the carbon support, and settle into the depression, due to accelerated degradation of the carbon support. Fig. 11. Open in new tabDownload slide High-resolution TEM images demonstrating the structural changes of the Pt/CB catalyst observed at 100°C in the 0.2 Pa air atmosphere of 94% RH. The Pt particle appeared to create a depression in the carbon support, and settle into the depression, due to accelerated degradation of the carbon support. Fig. 12. Open in new tabDownload slide Video images demonstrating a profile view of the structural change of the carbon support at the interface with a Pt particle. At the beginning (Fig. 12a), the Pt particle appears to be covered with the same adsorbate as shown in Fig. 10. After 27 s, a clean facet emerged at the right side of the particle, and the adsorbate accordingly moved to the other side (Fig. 12b). Then, the adsorbate aggregated at the interface and behaved as if it accelerated the degradation of the carbon support beneath the Pt particle (Fig. 12c, d). Fig. 12. Open in new tabDownload slide Video images demonstrating a profile view of the structural change of the carbon support at the interface with a Pt particle. At the beginning (Fig. 12a), the Pt particle appears to be covered with the same adsorbate as shown in Fig. 10. After 27 s, a clean facet emerged at the right side of the particle, and the adsorbate accordingly moved to the other side (Fig. 12b). Then, the adsorbate aggregated at the interface and behaved as if it accelerated the degradation of the carbon support beneath the Pt particle (Fig. 12c, d). The result of the experiment suggested the following mechanism. In the air atmosphere of low moisture content, degradation of the carbon takes place irrespective of the position of the loaded Pt particles. In contrast, in the highly moisturized air atmosphere, degradation of the carbon support takes place at the interface between the carbon and the Pt particle, and it acts as a trigger of degradation of the catalyst. The influence of water on catalysts through ex situ microscopic and spectroscopic analyses has been reported by Yamada et al. [15]. They showed that catalyst deactivation readily occurs due to carbon coating and that water acts to remove this coating and revive the catalytic activity. In our study, it appears that the Pt particle is activated by the water molecules or adsorbate containing water molecules. Then, the activated water-containing adsorbate reacted with the carbon support at the interface. Thus, the carbon support was locally degraded at the site of the Pt particle. The observed degradation mechanism of the carbon support at the interface with the Pt particle is demonstrated in Fig. 12 and is schematically depicted in Fig. 13. Fig. 13. Open in new tabDownload slide Schematic diagram of the degradation mechanism observed in Fig. 12. The carbon support is locally degraded at the interface with the surface of the Pt particle covered with the adsorbate. Fig. 13. Open in new tabDownload slide Schematic diagram of the degradation mechanism observed in Fig. 12. The carbon support is locally degraded at the interface with the surface of the Pt particle covered with the adsorbate. Concluding remarks We have developed facile new technique for in situ TEM observation of structural changes of nanomaterials being exposed to humidity-controlled air at high temperatures. The first application of this technique to the study of the influence of moisture content on the degradation mechanism of a Pt/CB catalyst of a PEFC has been successfully carried out. In this technique, since the introduction of gas is confined to the specimen mounted on a heating element via a fine injection nozzle built into the specimen heating holder, the atmosphere in the specimen area can be controlled more quickly than inside the standard environmental TEM, which requires control of the whole specimen chamber. With this new capability, we are beginning to study the further application of this technique to support the development of a wide range of new nanomaterials. Funding This work was partly supported by Grant-in-Aid for Scientific Research (C) 23510136 from the Japan Society for the Promotion of Science (JSPS). References 1 Mayrhofer K J J , Ashton S J , Meier J C , Wiberg G K H , Hanzik M , Arenz M . Non-destructive transmission electron microscopy study of catalyst degradation under electrochemical treatment , J. Power Sources , 2008 , vol. 185 (pg. 734 - 739 ) Google Scholar Crossref Search ADS WorldCat 2 Hartl K , Hanzik M , Arenz M . IL-TEM investigations on the degradation mechanism of Pt/C electrocatalysts with different carbon supports , Energy Environ. Sci. , 2011 , vol. 4 (pg. 234 - 238 ) Google Scholar Crossref Search ADS WorldCat 3 Xu F , Wan M , Liu Q , Sun H , Simonson S , Ogbeifun N , Stach E A , Xie J . 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TI - Development of a technique for in situ high temperature TEM observation of catalysts in a highly moisturized air atmosphere
JO - Journal of Electron Microscopy
DO - 10.1093/jmicro/dfs041
DA - 2012-08-01
UR - https://www.deepdyve.com/lp/oxford-university-press/development-of-a-technique-for-in-situ-high-temperature-tem-hDg07LBs63
SP - 199
EP - 206
VL - 61
IS - 4
DP - DeepDyve
ER -