Vibrational spectroscopy diffuse reflectance mode

Clearly, both vibrational and UV-visible spectroscopy also play an important role in materials chemistry, and are well described in other texts. The principles involved in carrying out these experiments on solids rather than in solution are similar, but often experimental methods vary. For example, an IR spectrum of a zeolite would be carried out by dispersing the solid in a matrix of potassium bromide and pressing into a disk, rather than in solution. Typically, a UV-visible spectrum of a solid would be carried out in diffuse reflectance mode, where the solid is dispersed in a white matrix (such as barium carbonate) and the UV light is reflected off the surface rather than passing through a solution. 

Diffuse reflectance spectroscopy (DRS) has been frequently employed in UV-Vis spectroscopy of zeolites (cf. Volume 4, Chapter 4 of the present series). More recently, it became also popular in IR investigations (cf., e.g., [160, 161]). The DRIFT technique is advantageous because it is successfully applicable not only in the mid infrared but also in the near infrared (NIR) region of 4000-10,000 cm where the transmission technique usually fails because of severe scattering through the absorbent particles. The NIR range, however, is very important with respect to the study of overtone and combination modes of vibrations. Moreover, diffuse reflectance IR experiments can be carried out not only with pressed wafers as in transmission spectroscopy, but also with zeolite pow-... 

Measurements of supported catalysts in diffuse reflection and transmission mode are in practice limited to frequencies above those where the support absorbs (below about 1250 cm-1). Infrared Emission Spectroscopy (IRES) offers an alternative in this case. When a material is heated to about 100 °C or higher, it emits a spectrum of infrared radiation in which all the characteristic vibrations appear as clearly recognizable peaks. Although measuring in this mode offers the attractive advantage that low frequencies such as those of metal-oxygen or sulfur-sulfur bonds are easily accessible, the technique has hardly been explored for the purpose of catalyst characterization. An in situ cell for IRES measurements and some experiments on Mo-O-S clusters of interest for hydrodesulfurization catalysts have been described by Weber etal. [11],... 

Conventionally, in IR spectroscopy the measured absorptions are caused by the fundamental vibrations of the atoms within a molecule. In many cases, important information can be obtained by examining the overtones of these fundamental vibrations. This can be done in the transmission mode, but elegant work using diffuse reflectance spectroscopy is discussed by Klier. 

SnOz nanocrystals with a particle size ranging from 3.5 to 17.9 nm are prepared by heating a smaller SnOz nanocrystal at different temperatures. The samples are characterized by powder X-ray diffraction, Raman spectroscopy and UV-vis diffuse reflectance spectroscopy. The Raman spectra show a surface-related vibration mode, which is dependent on the crystallite size of the SnOz nanocrystals. The quantum confinement effect is observed. The UV-vis diffuse reflectance spectroscopic results indicate that the band gap of the SnOz nanocrystals increases from 3.65 eV to 3.95 eV when the particle size decreases from 4.0 nm to 3.6 nm. 

In NIR reflectance spectroscopy, the finely ground. solid sample is irradiated with one or more narrow bands of radiation ranging in wavelength from 1 to 2.5 pm or lO.(KK) U> 4tXK) cm Diffuse reflectance occurs in which the radiation penetrates the surface layer of the particles, excites vibrational modes of the analyte molecule, and is then scattered in all directions. A rclleclancc s K ctrum Ls thus produced (hat is depen-... 

Near-IR spectroscopy arises from transitions in which a photon excites a normal mode of vibration from the ground state to the second or higher excited vibrational state (overtones, vide infra) or transitions in which one photon simultaneously excites two or more vibrational modes (combinations bands, vide infra). The use of the near-IR, especially diffuse reflection spectroscopy, in both quantitative and qualitative analysis has increased significantly due to better instrumentation and the development of chemometrics to better handle the effect of seriously overlapping bands. 

Another spectroscopic investigation was carried out by Borovkov et al. [103], who investigated by diffuse reflectance IR spectroscopy the vibration modes of CO adsorbed on Cu(I)-Y prepared via SSIE of NH4-Y and CuCl. The various fundamental and 1st overtone bands were compared with those observed with conventionally ion-exchanged Cu-Y, Cu-MOR and Cu-L, which were subsequently auto-reduced at 673 K in vacuum. 

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