Diffuse reflectance infrared spectra vibrations

Figure 8.9 Diffuse reflectance infrared spectrum of a silica support, showing silica vibrations at frequencies below 1300 cm1, overtones and combination bands between 1700 and 2050 cm-1, and various hydroxyl groups at frequencies above 3000 cm 1. The sharp peak at 3740 cm"1 is due to isolated OH groups, the band around 3550 cm 1 to paired, H-bonded OH groups, and the band around 3660 cm 1 to hydroxyls inside the silica (courtesy of R.M. van Hardeveld, Eindhoven).

Hydrated silica gel was modified with APTS (sample 1) and studied by DRIFT (Diffuse Reflectance Infrared Fourier Transform) and CP MAS NMR. The IR spectrum of the modified silica (figure 9.4) shows silane NH, CH and Si-O-Si bands along with silica lattice and surface vibrations. Assignments of IR bands of APTS modified silica are given in table 9.2. 

Diffuse reflectance or DRIFTS (diffuse reflectance infrared Fourier-lransform spectroscopy) allows the sain)le to be analysed neat, ot diluted in a non-absorbing matrix (e.g. KCl or KBr at 1-5% w/w analyte). DRIFTS also may be used to obtain the spectrum of a solute in a volatile solvent by evaporating the solution onto KBr. When the IR radiation interacts with the powdered sample it will be absorbed, reflected and diffracted. The radiation which has been diffusely reflected contains vibrational information on the molecule. This technique allows non-destructive testing of neat materials and is suited to quantitative analysis, although care must be taken to ensure that a consistent particle size is used. 

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],... 

Vibrational spectroscopy is the method of choice for the characterizing functional groups in complex organic molecules. Infrared transmission spectroscopy has been used on dried humics pressed into KBr pellets to determine the relative carboxylate content of humic materials (14-16). However, interferences arise from the presence of water bands and possible alterations of the samples under the high pressures used to form the pellets. Diffuse-reflectance techniques can avoid some of the difficulties associated with the KBr pressed-pellet method (9,17-18). To obtain a spectrum analogous to an absorption spectrum, the data are transformed from reflectance units to Kebulka-Munk (K-M) units. However, K-M units are related to... 

As a portion of the diffusely reflected radiation must have been tfansmitted through the inside of a powder particle at least once, infrared radiation at the wavenumber positions characteristic of the molecular vibrations of the sample has been absorbed that is, a diffuse reflection spectmm of a sample contains similar information to the infrared absorption spectrum of the sample. Furthermore, as some of the diffuse-reflection radiation must have passed through powder particles more than once, at a wavenumber position where a weak absorption of the sample occurs, the infrared radiation is absorbed multiple times, thereby giving rise to an absorption band seemingly stronger than its intrinsic intensity, as would have been observed with an absorption spectrum recorded in a transmission measurement. Consequently, there are cases where the intensity of a diffuse-reflection band is not proportional to the concentration of the sample giving rise to the band. 

Vibrational spectroscopy can help us escape from this predicament due to the exquisite sensitivity of vibrational frequencies, particularly of the OH stretch, to local molecular environments. Thus, very roughly, one can think of the infrared or Raman spectrum of liquid water as reflecting the distribution of vibrational frequencies sampled by the ensemble of molecules, which reflects the distribution of local molecular environments. This picture is oversimplified, in part as a result of the phenomenon of motional narrowing The vibrational frequencies fluctuate in time (as local molecular environments rearrange), which causes the line shape to be narrower than the distribution of frequencies [3]. Thus in principle, in addition to information about liquid structure, one can obtain information about molecular dynamics from vibrational line shapes. In practice, however, it is often hard to extract this information. Recent and important advances in ultrafast vibrational spectroscopy provide much more useful methods for probing dynamic frequency fluctuations, a process often referred to as spectral diffusion. Ultrafast vibrational spectroscopy of water has also been used to probe molecular rotation and vibrational energy relaxation. The latter process, while fundamental and important, will not be discussed in this chapter, but instead will be covered in a separate review [4],... 

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