In situ ETEM

We describe the development of in situ (dynamic) ETEM for direct imaging of CS defects in dynamic catalytic oxides in chapter 3. These studies have recently led to better insights into the formation of CS planes (leading to further developments in the dislocation model) and their role in oxidation catalysis. By directly probing the formation of CS planes and their growth by in situ ETEM... 

Before discussing novel in situ ETEM developments for heterogeneous catalysis, we present some background information about various in situ developments in the materials sciences. A number of notable in situ experiments have relied upon modifications to the standard TEM operations. The main electron optical functions of the TEM, especially the electron gun, depend on a high vacuum environment. The typical 10 -10 mbar TEM environment is mildly reducing. With an ECELL, controlled chemically reducing atmospheres, such as... 

Figure 2.9. The importance of dynamic (in situ) ETEM studies (a) Cu/C catalyst (particles are, e.g., at B) reacting at 200°C in 10% He/He gas after 30 min (b) time-resolved studies of the growth of catalyst particles (e.g. at B) indicative of Ostwald ripening, after 45 min (c) static studies of the same sample (the sample is cooled to room temperature and the gas is turned off) the image is not the same as in the dynamic studies in (b)—particles (e.g. at B) have moved (have faint contrast around them) and more features are present.

On M0O3, methanol (CH3OH or MeOH) chemisorbs at a low temperature of 100°C, which suggests that some defects (or dangling bonds) are necessary for chemisorption. In situ ETEM experiments in methanol show the formation of misfit defects at 100°C (and surface domains) accommodating the shape... 

Here we present thermodynamic discussions and developments based on recent in situ ETEM studies. These are important in predicting the enthalpy of formation of vacancies in oxide catalysts, the probability for CS planes to form and catalyst performance. They are also important in the design of new or improved oxide catalysts. 

For high performance industrial catalysts capable of activating butane, organic media are used for the synthesis of VPO, which results in a large area of active (010) orientations. VPO catalysts synthesized in organic media are therefore used in the present chapter to elucidate dynamic butane catalysis by in situ ETEM. The other methods of preparation can be multi-phasic, which can result in the modification of the reactivity of the catalyst (Centi 1993, Kiely et al 1996). 

As described earlier, many EM techniques including in situ ETEM, STEM, HRTEM, image analysis and EEELS are being pushed to their limits by researchers to study surfaces and ultrafine particles (or clusters) which are thought to be active species. Eor catalysis, metal particles are, in general, supported on ceramic substrates. Detection of supported particles less than 10 A (1 nm) in size is possible by HRTEM however, caution must be exercised in interpreting the size and surface structure of the metal particles since the surface structure is obscured by the support contrast, as demonstrated by calculations. 

In situ ETEM studies of metal-irreducible ceramic support interactions... 

In situ ETEM permits direct probing of particle sintering mechanisms and the effect of gas environments on supported metal-particle catalysts under reaction conditions. Here we present some examples of metals supported on non-wetting or irreducible ceramic supports, such as alumina and silica. The experiments are important in understanding metal-support interactions on irreducibe ceramics. 

Figure 5.14. In situ ETEM studies directly probing a dynamic Cu/alumina catalyst in different gas environments (scale bar is 100 nm). Dynamic images are recorded from the same area of the sample at 200 in 0.2 atm gas pressnre (a) in CO gas and (b) in O2 gas. Complex wetting and spreading is observed in (b). (After Gai et at Nature 348 430.)...

---