Environmental cell

With the gas reaction cell or an environmental cell (ECELL), controlled chemically reducing atmospheres and oxidizing atmospheres can be maintained in the EM, and a wide range of gases and vapors can be used. The development of the methods is described in the following sections. 

The design of in situ atomic-resolution environmental cell TEM under controlled reaction conditions pioneered by Gai and Boyes (87,89) has been adopted by commercial TEM manufacturers, and latter versions of this in situ instrument have been installed in a number of laboratories. In situ atomic resolution-ETEM data demonstrated by Gai et al. (85-90) have now been reproduced by researchers in laboratories using commercial instruments examples include investigations of promoted ruthenium and copper catalysts in various gas environments (93) and detailed investigations of Ziegler-Natta catalysts (94). 

Following early ETEM investigations using environmental cells, environmental scanning electron microscopy (ESEM) has been developed for characterization of surface effects of bulk SEM samples in the presence of gaseous or wet environments (111-114). The method has been applied to the examination of food, wool fibers (111), and polymers (112) and in the conservation of cultural properties (113). Recently, fuel cell catalysts have been characterized using a low-voltage ESEM with a resolution capability of 2 nm (114). 

Several general conclusions are drawn concerning the status of EM as a supremely versatile tool in the study of the materials chemistry of catalysts. First, it is no longer necessary to regard EM as a tool for model studies (131-133). The triumphant exploitation of the environmental cell in HRTEM marks the dawn of a new era in probing dynamic catalysis (4,87—95). Second, EM techniques, as has... 

The reduction of hematite with H2 at 387-610 °C has been followed in situ using TEM and an environmental cell (Rau et al., 1987). The reduction reaction started at nudeation sites on the edge of the sample and as the reaction proceeded, a particle showed four reaction zones consisting of umeacted hematite, lamellar magnetite, porous magnetite and finally porous iron (the temperature was too low for wiistite). 

The primary difference between optical and electron microscopy is that the latter uses an electron beam as the probe. Since 10- to 500-keV electron beams have much lower wavelengths than light, the resolution is greater. At the same time, the electron beam requires completely different instrumentation (source, collimator, detector, magnification control, etc.). Moreover, electrons are very readily absorbed by matter. Therefore, the entire path of the beam, from source to specimen to detector, has to be in vacuum. From the sample preparation point of view, this is of major significance. For specimens that may change in vacuum, biological tissues, for instance, this can be a major concern, and newly developed accessories such as environmental cells [8] need to be added to the microscope. 

Fig. 5.52. Environmental cells for degradation measurements under (a) light and...

A sample of catalyst was placed in the environmental cell within the FTIR spectrometer, reduced as outlined in the Experimental section, and subjected to a 1 3 propyneidhihydrogen reaction mixture. The bands observed in the various spectra are reported in Table L At no time were any bands observed which indicated the presence of propane and the pressure fall was consistent with the conversion of propyne to propene. 

FIGURE 25 All-silica environmental cell for XAS measurements of fluorescence yield at temperatures up to 1273 K with sample in an atmosphere of corrosive gas (Moggridge et al, 1995). Reprinted from (Moggridge et al., 1995), Copyright 1995, with permission from Elsevier. 

The in-situ electron microscopy studies presented here were performed either in a JEM 120 instrument fitted with an AGI gas reaction cell or a modified JEOL 200CX TEM/STEM microscope equipped with a custom designed environmental cell which accommodates a heating stage. With this latter instrument it is possible to obtain detailed information concerning the chemical state of the specimen under reaction conditions via in-situ electron diffraction analysis. 

The infrared spectra were obtained using a commercial FTIR spectrometer (Mattson Centauri). The studies were performed in transmission mode, with the catalyst in the form of a pressed disc, using an environmental cell where the temperature, pressure, and gas composition could all be controlled [7]. All spectra were recorded at 4 cm resolution with the co-addition of 300 scans using an MCT detector. 

Samples of the support and catalyst were subjected to propane (2020 Pa) at various temperatures in the FTIR environmental cell. By 673 K the principal bands observable are those due to carboxylate and aromatic species (Table 3). Bands due to C-H stretch were not observed, and the intensity of the bands obtained from the catalyst sample was greater than those from the alumina by approximately a factor of 2.5. Once again there is a similarity between the species present on the alumina and those on the catalyst but with the amounts being significantly higher on the catalyst. 

These specimens were then examined in a JEOL 200 CX electron microscope equipped with an environmental cell and heating stage, which allowed specimens to be heated in 1 Torr hydrogen up to 1100 K under these conditions the spatial resolution was 0.8 nm. In situ chemical analysis of the reacting specimen could be obtained using electron diffraction, energy-dispersive X-ray analysis, and electron energy loss spectroscopy. 

Figure 1. Modes of operation of the cerl field emission source SEM with environmental cell. Key top, scanning electron microscopy, Auger electron spectroscopy, and argon ion etching and bottom, gas reaction cell configuration.

Raman and UV-Vis spectra were recorded on the bisected pellets at several points in time after impregnation. UV-Vis spectra were obtained using a house-built set-up, which is schematically depicted in Fig. 4. The use of optical fibers with different diameter allows one to record a UV-Vis spectrum from a small spot of 50- 100 pm on the sample. Manipulation of the sample can be carried out with great precision with the aid of an automated X-Y-Z-table. The sample is placed in an environmental cell under water-saturated air to prevent dehydration of the wet pellets during measurements. 

Because many surface probes require high vacuum during their application, most surface science instruments are also equipped with high-pressure or environmental cells. The sample to be analyzed is first subjected to the usual high-pressure and/or high-temperature conditions encountered during reactions in the environmental cell. Then it is transferred into the evacuated chamber where the surface probe is located for surface analysis. One such apparatus is shown in Figure 1.13. 

There are many different designs available for combined high-pressure reaction studies and ultrahigh-vacuum surface science investigations. Transfer rods that move the sample from the environmental cells to the UHV chamber and reaction cells that permit liquid-phase or gas-phase reaction studies have been described in the literature. 

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