Diffuse reflectance spectroscopy UV-VIS

In situ UV-Vis Diffuse Reflectance Spectroscopy was performed under reactive atmosphere ( -butane/oxygen). These experiments confirmed that submitting the catalyst to the reaction mixture favors the development of a more oxidized active surface, and that the extent of transformation depends on the reaction temperature and on the catalyst P/V ratio. For instance, catalyst P/V 1.06 was less oxidized than catalyst P/V 1.00 at a temperature lower than 340°C. X-ray Photoelectron spectra of catalysts recorded after reaction at 380°C confirmed that catalyst P/V 1.00 was considerably more oxidized (average oxidation state for surface V 4.23) than the P/V 1.06 catalyst (average oxidation state 4.03). 

Although UV/VIS diffuse reflectance spectroscopy has not been used extensively in the study of pharmaceutical solids, its applications have been sufficiently numerous that the power of the technique is evident. The full reflectance spectra, or the derived colorimetry parameters, can be very useful in the study of solids that are characterized by color detectable by the human eye. It is evident that questions pertaining to the colorants used for identification purposes in tablet formulations can be fully answered through the use of appropriately designed diffuse reflectance spectral experiments. With the advent of newer, computer-controlled instrumentation, the utility of UV/VIS diffuse reflectance as a characterization tool for solids of pharmaceutical interest should continue to be amply demonstrated. 

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. 

A novel method has been developed for controlling the particle size of inorganic nanocrystalls. For example, a monodispersed nanocrystalline YSZ with an average particle size of 4.7 nm is obtained by using this method [10]. A very high surface area of 165 m /g and a significant band gap increase from 4.13 to 5.44 eV are observed. Smaller SnOz nanocrystals are also prepared by this method [11]. Here we report the Raman spectra and UV-vis diffuse reflectance spectroscopy (DRS) results of the SnOz nanocrystals with different particle sizes. 

In addition to the structure in the dehydrated state, the structure of supported vanadia catalysts under redox reaction conditions is directly related to the catalytic performance. Vanadia catalysts are usually reduced to some extent during a redox reaction, and the reduced vanadia species have been proposed as the active sites [4, 19-24]. Therefore, information on the valence state and molecular structure of the reduced vanadia catalysts is of great interest. A number of techniques have been applied to investigate the reduction of supported vanadia catalysts, such as temperature programmed reduction (TPR) [25-27], X-ray photoelectron spectroscopy (XPS) [21], electron spin resonance (ESR) [22], UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS) [18, 28-32], X-ray absorption fine structure spectroscopy (XAFS) [11] and Raman spectroscopy [5, 26, 33-41]. Most of these techniques give information only on the oxidation state of vanadium species. Although Raman spectroscopy is a powerful tool for characterization of the molecular structure of supported vanadia [4, 29, 42], it has been very difficult to detect reduced supported... 

Weckhuysen, B.M., Verberckmoes, A.A., Debaere, J., Ooms, K., Langhans, 1. and Schoonheydt, R.A. (2000) In situ UV-Vis diffuse reflectance spectroscopy-on line activity measurements of supported chromium oxide catalysts relating isobutane dehydrogenation activity with Cr-spedation via experimental design. Journal of Molecular Catalysis A Chemical, 151 (1-2), 115-31. 

This conclusion is further confirmed by data obtained with Co(NH3)6. In this case, amminated Co ions remain in supercages at calcination temperatures below 300°C (77). Therefore, a pronounced reduction enhancement of Co ions by Pd is found after calcination at low temperatures, e.g., 250°C. Evidence for direct interaction of Pd ions with amminated Co ions is obtained by UV-Vis diffuse reflectance spectroscopy and temperature-programmed oxidation. The oxidation of the ammine ligands of the Co ion is catalyzed by Pd, so that ammine oxidation is complete below 350°C for [Pd(NH3)4 + Co(NH3)6 ]/NaY, whereas 450°C is required in the absence of Pd. Ammine destruction of the [Co(NH3)6] ion is a stepwise process, the last step being the conversion of the tetrahedral complex [CoCOzlsNHs], where stands for framework oxygen at the SII site, to a naked Co ion inside a hexagonal prism. The locations and coordinations of Co and Pd ions in NaY are summarized in Table III. 

The platinum contents were measured using Neutron Activation Analysis. The Pt-containing materials and the parent catalyst support were characterized by Na adsorption-desorption and X-ray diffraction. Furthermore, the impregnated solids have been characterized by UV-Vis Diffuse Reflectance spectroscopy and Temperature Programmed Reduction (TPR). 

The applicability of the Pt deposition precipitation technique (DP) on mesoporous silica has been evaluated and discussed. A detailed synthesis procedure is given, and a suitable support from the SBA-15 family has been identified. The material synthesized at the conditions described here was clearly able to withstand the severe conditions of the DP treatment, indicating improved hydrothermal stability. The incorporation of the active species was accomplished without compromising the structural integrity of the parent material, as monitored by XRD and N2-sorption measurements. Using UV-Vis diffuse reflectance spectroscopy we were able to detect the platinum surface complex that coexists with platinum nanoparticles on the impregnated solid. 

The syntheses discussed above can all be scaled up by at least a factor of 3 without significantly lowering the yield. The /ac-Mo(NH3)3X3 (X = CF3SO3, Cl, Br, I) compounds are very susceptible to oxidation by the atmospheric dioxygen and should only be handled in a protective atmosphere. The halogenide complexes can, however, be handled in air for shorter (<15min) periods of time. The compounds have all been characterized by UV-vis diffuse reflectance spectroscopy./ac-Mo(NH3)3(CF3S03)3 ... 

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