EXAFS microstructure

Even when this target is reached, it must be kept in mind that XRD can, by the very nature of its basic physics, fall short of describing the structure of a catalyst at all length scales. It misses out on variations of the local structure that are better addressed by EXAFS spectroscopy (Clausen et al., 1993, 1998). Furthermore, XRD is insensitive to the texture and microstructure with dimensions larger than about 10 nm, which are better investigated by electron microscopy or gas adsorption techniques—and the surface structure of a working polycrystalline catalyst is in most cases inaccessible by XRD. 

Sinfelt has greatly contributed to the catalyses of bimetallic nanoparticles [18]. His group has thoroughly studied inorganic oxide-supported bimetallic nanoparticles for catalyses and analyzed their microstructures by an EXAFS technique [19-22]. Nuzzo and co-workers have also studied the structural characterization of carbon-supported Pt/Ru bimetallic nanoparticles by using physical techniques, such as EXAFS, XANES, STEM, and EDX [23-25]. These supported bimetallic nanoparticles have already been used as effective catalysts for the hydrogenation of olefins and carbon-skeleton rearrangement of hydrocarbons. The alloy structure can be carefully examined to understand their catalytic properties. Catalysis of supported nanoparticles has been studied for many years and is practically important but is not considered further here. 

Synchrotron radiation is incredibly versatile allowing a wide variety of experimental techniques to be developed. Of direct interest to solid oxide fuel-cell science and technology are both diffraction and spectroscopy. Diffraction beamlines offer the potential to access complex structural information that is not revealed by competing diffraction techniques, while the spectroscopic techniques (X-ray Absorption Near Edge Spectroscopy [XANES]) and Extended X-ray Absorption Fine Structure [EXAFS]) allow users to access chemical information, such as redox kinetics. The latest developments include 3D tomography of complete cells which give unparalled information about the microstructure of composite electrodes [73, 74]. 

In this section, the nature of the iron material resulting from the activation process will be described. Its bulk properties as revealed by Mossbauer spectroscopy, X-ray diffraction, and EXAFS will be discussed, followed by a description of the typical morphologies of activated particles as seen in the SEM, and finally the complex microstructure of the material as seen in the high-resolution TEM will also be demonstrated. The main purpose of the section is to illustrate the fundamental differences between the active catalyst material and pure iron powder. 

In summary, the EXAFS results show that there is no difference between pure iron and the catalyst in the average local structure. Furthermore, there are no iron atoms present in the catalyst in significant quantities which exhibit a different geometric environment to the average, i.e., there is only one type of iron atom present. This is strong evidence against the paracrystallinity theory and is more consistent with a macroscopic distribution of the promoter phases, which affects fewer iron atoms than can be detected with the X-ray absorption technique. The porosity of the reduced catalyst may account for a certain lack of EXAFS amplitude. Any significant reduction in the coordination number of the iron in the catalyst would require a cluster-like microstructure for which no evidence has yet been found. 

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