Raman cells

In-sltu Raman experiments were performed on a Spex 1401 double monochrometer Raman spectrometer, using a Spectra-Physlcs Model 165 argon Ion laser with an exciting wavelength of 5145 A. The In-sltu Raman cell consists of a quartz tube situated In a temperature controlled heating block. The Raman spectra were collected In the 180° backscatterlng mode. 

With single-mode lasers the resolution can be considerably improved since the laser line width is then reduced below lO cm". Rotational Raman spectra of gases could be resolved using a multiple-pass Raman cell and a single-mode argon laser I85a) development of tunable... 

The evolution of the structure of four vanadyl phosphate hemihydrates has been studied and then used as catalysts for n-butane oxidation using an in-situ Raman cell and MAS-NMR. The catalytic performance for maleic anhydride... 

Figure 3 X-Ray diffraction patterns of the VPO catalysts at room temperature after the in situ Raman cell run.

This study has resulted in interesting informations concerning the active sites of the VPO catalysts for n-butane oxidation to maleic anhydride being obtained. The study of VPO catalysts in the course of n-butane oxidation by an in-situ Raman cell has shown... 

The T-jump pulse is generated by Raman shifting the fundamental output of a Q-switched Nd YAG laser (1 = 1.064 pm) in a 1-m-long Raman cell filled with H2 at 500 psi. The laser operates at a repetition rate of 10 Hz and has a pulse temporal width of <10 ns (FWHM). The Stokes shift in H2 is 4155 cm-1. As a result, the first Stokes line is at a wavelength of 1.9 pm that is partially absorbed by the D20 solvent, and thus serves as the T-jump pulse. A Pellin-Broca prism is used to separate the... 

Gorbaty YE, Bondarenka GV. High-pressure high-temperature Raman cell for corrosive liquids. Rev Sci Instrum 1995 66(8) 4347-4349. 

Our laser system consists of three stages A XeCl excimer laser, a combination of dye and Ti Sa laser, and a H2 Raman cell for conversion to 6 pm (Fig. 8). 

Fig. 8. The components of the laser system. The high power XeCl excimer laser pulse triggered my the muon entrance detector is converted in two steps to a high quality 7 ns long pulse of 708 nm which is shifted to the desired 6 pm light inside the multipass Raman cell. The light is injected into a multipass cavity to effectively illuminate the muon stop volume inside the PSC solenoid. High resolution frequency selection is provided by injection of a cw Ti Sa laser...

Figure 2-24 High-temperature Raman cell. (Reproduced with permission from Ref. 52.)...

The future will see an increasing number of Raman investigations that aim at a better understanding of the processes occurring during catalyst preparation. In this respect, Raman cells or vials for the characterization of... 

Raman spectroscopy has been used frequently to investigate the chemisorption of probe molecules (Cooney et al., 1975 Weber, 2000). Several groups reported variable Raman cells in which the temperature of the sample and the environment can be controlled so that catalytic reaction conditions can be simulated (Abdelouahab et al., 1992 Brown et al., 1977 Chan and Bell, 1984 Cheng et al., 1980 Lunsford et al., 1993 Mestl et al., 1997a Vedrine and Derouane, 2000). In these investigations, conversion and selectivity values were not measured simultaneously with the spectra. The developments of these Raman experiments have been reviewed elsewhere (Banares, 2004 Knozinger and Mestl, 1999 Vedrine and Derouane, 2000). 

FIGURE 3 Designs of Raman cells for investigations during treatment or catalysis,... 

Figure 3 illustrates the concepts of Raman cells that can be used for experiments under reaction conditions. Several commercial cells are suitable for use in combination with Raman microscopy. The rotating sample design was modified by Wachs s group (Figure 3A, Banares et al., 1994) and used to investigate supported oxides during selective alkane oxidation (Banares et al., 2000C Guliants et al., 1995 Sim et al., 1997) and various catalysts...

Designs of Raman cells that seek to meet above-stated criterion are presented in Figure 4. Figure 4A shows the reactor cell used in an early... 

FIGURE 4 Designs of Raman cells that, for selected reactions, deliver catalyst performance data corresponding to those of an ideal reactor, (A) C.G. Hill [Adapted from Snyder T.P., and Hill C.G.,/ Catal. 132, 536 (1991) Stability of Bismuth Molybdate Catalysts at Elevated-Temperatures in Air and under Reaction Conditions, copyright (1991), with permission from Elsevier (377)], (B) G. Mestl [source, M.A. Banares],... 

Xie and Bell (2000) used a Raman cell that was a reactor to identify the carbon-containing species on the surface of zirconia during the synthesis of dimethyl carbonate from C02 and methanol. The results showed that surface methoxides and surface carbonates formed from monomethyl carbonate species that yield dimethyl carbonate upon reaction with methanol. 

Lunsford and coworkers (Xie et al., 1999) reported kinetics data characterizing the catalytic NO decomposition measured in their Raman cell. Other attempts were made by Volta and coworkers (Abdelouahab et al., 1992), but only low conversions were reached for alkane oxidation because of significant temperature gradients (Volta et al., 1992). The cell designed by Stair (2001) appears to be suitable for simultaneous determination of spectra and catalytic activity. Several groups have addressed the issue of obtaining accurate catalytic performance data when the reactor is a spectroscopic cell (Banares and Khatib, 2004 Guerrero-Perez and Banares, 2002 Kerkhof et al., 1979 Mestl, 2002). 

As a further check on this mechanism the decomposition of N02C104 was monitored by Raman spectroscopy, employing the Raman cell as the reactor. The sample was analyzed by Raman periodically throughout the course of the decomposition at 65° C. A plot of the absorptions for NC>2+ (1400 cm.-1) and NO+ (2300 cm.-1) is shown in Figure 4. As would be expected from the proposed mechanism, the rapid drop in the NC>2+ absorption coincides with the appearance and increase in the NO+ absorption. Figure 5 shows the oxygen production for the sample over the... 

You will note some structure in the Stokes band near 460 which is due to chlorine isotopic frequency shifts. (See Exp. 37 for a discussion of isotope effects in diatomic molecules.) Rescan this region at higher resolution (1 cm or less) with an expansion of the chart display and measure the frequencies of each of the components. Record the ambient temperature near the Raman cell. 

As a result, cuvettes for Raman spectroscopy should be carefully selected. They may, due to their impurities, add a background to the spectrum of the sample. In addition, all cuvette materials produce their own Raman spectra, which have to be considered, when the Raman spectra of the sample are evaluated. Fig. 3.5-17 a shows a Raman spectrum of a typical optical glass BK7, Fig. 3.5-17 b that of quartz glass suprasil, and Fig. 3.5-17 c of sapphire. Suprasil is a synthetic quartz which does not normally contain impurities. Therefore, Suprasil of ESR quality is highly recommended as Raman cuvette material. Also, sapphire is a good cuvette material, as it is very hard, inert, has a good thermal conductance, and shows only weak but sharp Raman lines (Porto and Krishnan, 1967). It is used for the production of the universal Raman cell (Schrader, 1987). The sharp Raman lines of sapphire observed in the spectra of the sample may be subtracted from the spectrum or used as internal standard for quantitative analyses (Mattioli et al, 1991). 

Following the same principles as those which govern the designing of cells for absorption experiments, Raman cells for high-pressure, high- temperature studies have also been constructed by Lindner and Franck (see Tddheide, 1972). A Raman cell for use up to 7 kbar and 250 °C (Eckel et al., 1981) is shown in Fig. 6.7-8. 

The formation of peroxide and superoxide on Fe,H/MFI compared with Fe/MFI also shows two distinguishing features. First, the amount of peroxide on Fe,H/MFI at room temperature is significantly greater than on Fe/MFI, as determined by the peroxide peak intensity relative to the intensities of the zeolite bands (52). Second, on Fe,H/MFI, peroxide is converted to superoxide when the sample temperature is lowered to 93 K, and then restored when the temperature is returned to 300 K. Figure 8 shows an overlay of Raman spectra characterizing Fe,H/MFI measured at 300 and at 93 K using 2. The band at 703 cm ( 02 ) decreases at 93 K relative to its intensity at 300 K, whereas the intensity near 1090cm ( Oj) shows the opposite behavior with temperature. In the spectrum of Fe/MFI, the relative peak intensities of peroxide and superoxide remain constant with temperature between 93 and 300 K. (A specialized variable-temperature fluidized-bed Raman cell was constructed for these experiments.)... 

A near-infrared laser pulse at 1.54 pm was used to rapidly heat the aqueous peptide solution 10-20 °C, while a cw ultraviolet probe beam excited the fluorescence of the labeled peptide and monitored the relaxation kinetics. A schematic of the instrument is shown in Figure 1. To produce the temperature jump pulse the fundamental (1064 nm) of a Nd YAG laser (Continuum Surelite I), operating at 1.67 Hz, was focused with a 0.75 m lens into a one meter Raman cell (Princeton Optics, Inc.). The Raman cell contained 600 psi of CH4 and 500 psi of He and had a conversion efficiency of up to 20% for the first Stokes line (1.54 pm). The 1.54 pm wavelength... 

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