Diffuse Reflectance UV-Vis

A diffuse reflectance measurement begins with the collection of a reference scan. For samples deposited on reflective substrates, such as metallic molybdenum, the bare metal serves as a reference to account for any absorption in the metal itself. Samples deposited onto transparent substrates, such as a transparent conducting oxide deposited on glass (e.g., indium-tin oxide, or ITO), require the use of a white diffuse reflectance standard. This standard is often made from Ba2S04 or PTFE based material similar to, if not the same as, that used to coat the interior surface of the integrating sphere. The user places the standard against the open 

Following the reference and dark scans, the working sample is placed against the aperture of the integrating sphere. The spectrometer then determines the amount of light reflected by the sample by comparison against the reference standard. 

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Fig. 3. Diffuse reflectance UV-Vis spectra of catalysts Curves 1-5 represent Na-Y, CuCl Pc, CuCl Pc-Na-Y (0.26), Cu(N02)4Pc and Cu(NOj)4Pc-Na-Y (0.16), respectively.

The most notable feature of these intrazeolite photooxygenations (Fig. 30) is that the oxygen CT band experiences a dramatic bathochromic shift in comparison to solution. This was detected initially by recording the product growth as a function of irradiation wavelength (laser reaction excitation spectrum)98,110 and was later verified by direct observation using diffuse reflectance UV-Vis spectroscopy.111 For example, 2,3-dimethyl-2-butene CT-absorbance is shifted to lower energy by more than 300 nm... 

Figure 1.15 Diffuse reflectance UV-Vis spectra from a series of chromia/alumina catalysts after various treatments [115], All these spectra display a shoulder at about 16,700 cm-1 corresponding to the first d-d transition of Cr3+, but the main feature seen in the hydrated and calcined samples at about 26,000 cm-1 due to a Cr6+ charge transition is absent in the data for the reduced sample. This points to a loss of the catalytically active Cr6+ phase upon reduction. (Reproduced with permission from Elsevier.)...

Figure 8 in Ref. 213, reproduced on the right, displays diffuse-reflectance UV-Vis spectroscopic data obtained for titanium-substituted mesoporous (Ti-SBA-15) catalysts as a function of titanium content. Assign the main absorption feature observed at 200 to 220 mu and the shoulder seen at about 300 nm. What can we learn from this figure in terms of the different titanium species present on the solid ... 

The progressive increase in the extent of POM reduction, as evidenced by the ultraviolet visible diffuse reflectance (UV-vis DR) spectra of catalysts downloaded after different reaction times during catalyst equilibration. 

XANES and EXAFS were conducted at BL-lOB in the Photon Factory of the National Laboratory for High Energy Physics (KEK-PF)[12]. s Fe Nttssbauer spectra were recorded with a Shimadzu MEG-2 spectrometer(13]. Isomer shifts were given relative to a-Fe. Infrared spectra were recorded by a Shimadzu Fourier-transform infrared spectrometer(FTIR-4100) with a resolution of 2 cm i. Diffuse reflectance UV-VIS spectra were obtained on a Hitachi 330 spectrophotometer. 

The diffuse-reflectance UV-vis spectra of the powdered saaples showed as in Pig 2 the draaatic changes of the relative peak intensities at 280 nn(B-band) and 550,580 nn(Q-band) between the NaY intrazeolite and external FePc(t-Bu)4, where the Q-hand(7T-7r transition of Pc ring) is Markedly suppressed in the FePc(t-Bu>4 inside NaY, whereas those of B-band for peripheral benzene ring... 

The Ti environment in trimethylsilylated Ti-MCM-41 catalysts with different degrees of silylation was studied by means of diffuse reflectance UV-vis spectroscopy. The UV-vis spectra of trimethylsilylated samples are compared with the calcined material in figure 6. 

Figure 4. Diffuse reflectance UV/vis spectra of MCM-41 a) with adsorbed rhodamine dye (0.0075-mmol/g MCM-41), b) with covalently grafted rhodamine dye (0.01 m mol/g MCM-41 0.5 mmol APTES/g MCM-41) compared to rhodamine B sulfonylchloride c) in ethanol and d) in water.

The following gives a brief compilation of procedures to determine hexenuronic acids in cellulosic pulp samples. The common methods are based on hydrolysis of HexA moieties from pulp, either enzymatically or chemically, with subsequent quantification of the hydrolysis products either directly or after chemical conversion into UV active compounds. A comparison of these three methods is given by Tenkanen et al. [136]. For comparison rather than exact determination of HexA, e.g., during bleaching stages, the diffuse reflection UV VIS method can be applied [137]. A photoacoustic FTIR procedure based on chemometric analysis has been described as well [138]. In Table 3, the available methods to analyze HexA moieties in cellulosic material are summarized. 

The anchorcd/grafted metal species may be characterized by different spectroscopic methods 1R, UV-Vis diffuse reflectance (UV-Vis DRS), electron spin reso-... 

FIGURE 5 Diffuse reflectance UV-vis spectra recorded at 298 K of C-Hyd-V, 5Si/i and C-Hyd-V o osSi/ samples with the parent V-free Si/ zeolite as reference. (Reproduced with permission from Dzwigaj et al. (2001).)... 

Optical Accessory Design for Diffuse Reflectance UV-vis-NIR Spectroscopy... 

Lezna and coworkers (Juanto et al., 1994 Lezna et al., 2003) combined diffuse reflectance UV-vis spectroscopy with cyclovoltammetry, using a specially designed spectrometer with an optical multichannel analyzer. 

Diffuse reflectance UV-vis spectroscopy was applied in electrocatalysis by El Mouahid et al. (1998), who followed the electropolymerization of a cobalt porphyrin complex on a vitreous carbon electrode. The thin polymer... 

FIGURE 15 (A) Diffuse reflectance UV-vis spectra of H-mordenite during n-butane... 

The adsorption property was measured by a static method at 30 °C with a conventional volumetric apparatus as well as by the temperature programmed desorption (TPD) method. The details of the pretreatment and adsorption procedures were shown in Results and Discussion section. Metal-loaded zeolite samples were characterized by XRD, diffuse reflectance UV-Vis spectroscopy (DRS) and electron spin resonance (ESR). 

Figure 20. Diffuse reflectance UV-VIS spectrum of dehydrated CuNa-chabazite. Reproduced with permission from reference 59.

Moreover, no adsorption band near 600 nm typical of VO ions is observed in the spectra of the two as-made samples, although vanadyl cations in a square pyramidal geometry have been detected by ESR in V-MCM-41. This is probably due to the fact that d-d transitions in VO are generally 10-30 times weaker than those of CT transitions and therefore are undetectable by diffuse reflectance UV-vis [34]. However, a small band at ca 605 nm has been detected by Chao et al. [24] in the as-synthesized V-MCM-41 samples having Si/V= 30, This band is responsible for the pale violet colour of the as-made sample. Nevertheless, our as-made samples are white in colour, suggesting that VO species should occur, if any, in very small concentration... 

The diffuse reflectance UV-Vis spectra have been obtained between 190 to 900 nm using a Perkin Elmer UVA IS/NIR Spectrometer Lambda 19. 

Diffuse reflectance UV-Vis spectroscopy (DREAS) and EPR provide useful information regarding the oxidation state of the metal. Other techniques that are regularly applied are MAS-NMR, X-ray photoelectron spectroscopy (XPS) and scanning and transmission electron microscopy (SEM and TEM). Finally, BET surface area measurements and adsorption experiments are indispensible for checking the stractural integrity of the molecular sieve. 

Diffuse Reflectance UV-Vis DR UV-Vis spectra of differently loaded V20s/Si02 samples, calcined in situ in O2 at 600°C, were obtained by a Perkin Elmer Lambda 19 spectrophotometer, equipped with an integrating sphere. 

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