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Catalysis Looks to the Future
oxide such as alumina or high-surface-area porous carbon. The size and morphology of metal particles can be established from transmission electron microscopy. Electron microscopy can also provide information on the nature of the interaction between the metal particles and the support. EXAFS can be used to determine the average metal-metal bond length and coordination number. For bimetallic catalysts (e.g., platinum-iridium, platinum-rhenium), the degree of interaction and the distribution of the two components can also be established.
A large body of evidence acquired in recent years has shown that the properties of metal catalysts can be modeled by using single-crystal metal surfaces. This method has proved invaluable for the advancement of catalyst science because of the large array of techniques developed by surface scientists for the characterization of single-crystal surfaces. The atomic composition of such surfaces can be analyzed by using Auger electron spectroscopy (AES), x-ray photoelectron spectroscopy (XPS), and ion scattering spectroscopy (ISS). The structure of the exposed surfaces and the presence of terraces and defects can be established by low-energy electron diffraction (LEED). An exciting recent development, scanning tunneling microscopy (STM), has made it possible to obtain atomic-resolution views of surfaces and to identify the influence of thermal and chemical pretreatment on surface structure.
Insights into structure-function relationships can be obtained through the use of model catalyst systems. Examples of such systems include metals deposited on oxides, oxides on oxides, and oxides on metals. After initial preparation by means of vapor deposition, the sample can be characterized by AES and XPS. The catalytic properties of the catalyst can be determined by transferring the sample to a high-pressure chamber. Characterization of the sample after reaction reveals changes that have occurred in the catalyst composition and structure. Model catalyst systems are also well suited for examination by means of scanning transmission spectroscopy (STEM) and controlled atmosphere electron microscopy (CAEM). These techniques can provide detailed evidence for surface reconstruction, sintering, alloying, and phase separation.
Significant advances have also been made in the characterization of enzymes, catalytic antibodies, and homogeneous catalysts. Where such catalysts can be obtained in the form of single crystals, the analysis of x-ray diffraction patterns can provide a complete atomic structure. However, although such information is quite useful, it does not indicate structural changes that may occur when the catalyst is saturated and surrounded by solvent. Recent advances in multidimensional (i.e., two-, three-, and four-dimensional) NMR spectroscopy are making it possible to obtain this information in solution, and enzyme structures containing as many as 153 amino acids have been analyzed completely.