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CHARACTERIZATION METHODS. 64 catalysts: (1) x-ray emission spectroscopy (XES) for elemental analysis and elemental distribution images (Lyman, 1986); (2) electron energy loss spectroscopy (EELS) for analysis of light elements and chemical shift effects (Lyman et al., 1984); and (3) convergent beam electron diffraction (CBED) for identification and structure determination of small particles (Cowley, 1984). Unfortunately, high-or ultrahigh-vacuum environments are generally required for these techniques. The morphology of the particles can limit the characterization; for example, catalysts prepared using nonporous oxide particles as model supports permit more detailed characterization of the structure of the metal crystallites and the metal support interface (edge-on views) than is easily possible with porous supports (Datye and Logan, 1986). SPECTROSCOPIES Optical Spectroscopy The literature on the application of infrared (IR) spectroscopy to the study of adsorbed molecules on supported metals is extensive. The frequency bands for adsorbed molecules such as NO and CO provides information on the state of the metal on the support (e.g., dispersion). The wide availability of Fourier transform infrared spectroscopy (FTIR) has renewed interest in this technique. For example, in situ studies of catalytic reactions are possible using cells that can be pressurized with reactants (or flow through) and heated to high temperatures. Diffuse reflectance spectroscopy is a characterization technique particularly well suited for the study of materials that strongly scatter light (and poorly transmit light) in the 7 to 0.05 eV energy range (UV-VIS-IR). When applied in the infrared region, this technique is used to study vibrational modes of molecules adsorbed on surfaces. Thus, information is gained about how molecules interact with surfaces (Kortum, 1969; Klier, 1980). Raman spectroscopy has recently been used to probe the grain and grain-boundary structures in nanophase TiO2 (Melendres et al., 1989). It was found that the strong Raman-active lines representative of the rutile structure dominated all of the observed spectra, independent of average grain size (10 to 100 nm) and annealing treatment. These Raman data gave no indication of grain-boundary structures in nanophase TiO2 that are significantly different from those in conventional polycrystals. This method can be usefully applied to the study of other nanoscale materials with strong Raman-active features.
