Diffuse reflectance UV-visible spectroscopy

Several authors used diffuse reflectance UV-Visible spectroscopy to probe the local environment of Ti sites [136-138,140,141,143,147]. Sayari et al. [137,141,147] found that both Ti-HMS and Ti-MCM-41 exhibit an absorption band at 220 ran with no indication of a band at 330 ran characteristic of anatase [148]. There was however a weak shoulder at 270 nm, particularly for Ti-iich samples. Conna et al. [140] also found a band at 205-220 nm and a shoulder at 270 nm. The absence of anatase was also confirmed by the absence of the characteristic 140 cm Raman band [138,149]. The band at 220 nm was attributed to isolated framework Ti species in interaction with water molecules [150]. The 270 nm band was assigned to partially condensed hexacoordinated Ti species belonging to a silicon rich amorphous phase [155]. 

J Schmidt and C Heimer. Use of Diffuse Reflectance UV-Visible Spectroscopy to Characterize Chromophores in Wood Fibers. In D Argyropoulos, ed. Progress in Lignocellulosics Characterization. Atlanta TAPPI Press, 1999, pp. 187-209. International Standards Organization. ISO 2470 Measurement of Diffuse Blue Reflectance Factor (ISO Brightness). 

Deposition of the redox-active polymers onto high-surface-area silica was monitored using diffuse reflectance UV-visible spectroscopy. Shown in Figure 4a are the spectra for the PS-MV /Cab-O-Sil composites, both coiled and... 

We report in this study the hydrothermal synthesis of Co-silicate of MFI structure in alkaline media. The samples were characterized by XRD, chemical analysis, thermal analysis, A1-NMR, diffuse reflectance UV-visible spectroscopy and XPS spectroscopy. 

Mesoporous vanadosilicate molecular sieves with MCM-48 structure and atomic SiA ratio 30-200 have been synthesized using vanadyl sulphate as the source of vanadium. The product was characterized using X-ray diffraction (XRD), Nj adsorption analysis, transmision electron microscopy (TEM), electron spin resonance (ESR), Fourier-transform Infrared (FTIR), Diffuse reflectance UV-visible spectroscopy (UV-vis) and V solid state NMR. A noticeable decrease in unit cell parameters and main pore diameter was observed. Thus, a strong interaction between vanadium and mesoporous wall can be suggested. 

Diffuse reflectance UV-Visible spectroscopy was carried out to further identify the transformations of the cationic species in the exchanged zeolite after reduction, at different temperatures (Figure 3). This allows a better comprehension of the silver reduetion mechanism. In freshly calcined Ag/13X(E), cationic silver (band at 235mn) and silver clusters (246-280 nm) with a slight contribution of metallic silver were detected (>290 nm) [11]. The decrease of the cationic silver band contribution when the temperature increases to 700°C, and the simultaneous increase of the metallic and clusters bands, suggest a two step formation of the silver particles in the exchanged silver zeolite as follows Ag" Agn Ag. 

The quantification of the extra-framework titanium species in titanium silicalites of MFI structure, TS-1, was performed using either XANES at the Ti K-edge or XPS Ti (2p) photolines. In addition, two different framework sites, [Ti(OH)(OSi)3] and [Ti(OSi)4], were characterized in dehydrated samples using Diffuse Reflectance UV-visible, multiple scattering analysis of EXAFS, H and Si NMR spectroscopies. 

The mere exposure of diphenyl-polyenes (DPP) to medium pore acidic ZSM-5 was found to induce spontaneous ionization with radical cation formation and subsequent charge transfer to stabilize electron-hole pair. Diffuse reflectance UV-visible absorption and EPR spectroscopies provide evidence of the sorption process and point out charge separation with ultra stable electron hole pair formation. The tight fit between DPP and zeolite pore size combined with efficient polarizing effect of proton and aluminium electron trapping sites appear to be the most important factors responsible for the stabilization of charge separated state that hinder efficiently the charge recombination. 

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. 

The shifts in wavenumbers indicate a change in the electronic structure of aromatics adsorbed compared to the liquid or gas phase. Such modiflcadons in adsorbates have been rather extensively studied in the case of benzene using diffuse reflectance UV spectroscopy (77), UV-visible transmission microscope spectrophotometry (78) or laser raman spectroscopy (79). All the results suggest the formation of a complex between the cation and the it electrons of the aromatic ring. Hindrance in mobility compared to that of benzene in the solid state have been observed (78). 

The metal content analysis of the samples was effected by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES Varian Liberty II Instrument) after microwaves assisted mineralisation in hydrofluoric/hydrochloric acid mixture. Ultraviolet and visible diffuse reflectance spectroscopy (UV-Vis DRS) was carried out in the 200-900 nm range with a Lambda 40 Perkin Elmer spectrophotometer with a BaS04 reflection sphere. HF was used as a reference. Data processing was carried out with Microcal Origin 7.1 software. 

With the exception of single-crystal transmission work, most solids are too opaque to permit the conventional use of ultraviolet/visible (UV/VIS) electronic spectroscopy. As a result, such work must be performed through the use of diffuse reflection techniques [8-10]. Important work has been conducted in which UV/VIS spectroscopy has been used to study the reaction pathways of various solid state reactions. Other applications have been made in the fields of color measurement and color matching, areas which can be of considerable importance when applied to the coloring agents used in formulations. 

The role of adsorbed oxygen species in the mechanism of alkane transformation, on the contrary, is more questionable. The effect induced by the substitution of O2 with N2O and IR indications are in agreement with this interpretation, but, on the other hand, activated electrophilic oxygen species form on reduced sites, preferably in tetrahedral coordination (79). The partial reduction of tetrahedral V =0 with formation of tetrahedral v after propane oxidative dehydrogenation can be observed using UV-Visible diffuse reflectance, ESR and V-NMR spectroscopies. It is thus not possible to assign unequivocally the active species in propane selective activation to a tetrahedral V =0 species or to or V -0-0 species formed in the... 

M(lI)AlSn-LDHs with M(II) being Mg, Ni or Co were synthesized by a coprecipitation method. The influence of Sn on the thermal transformations and redox properties were investigated in detail using XRD, TG/DTA, SEM, TPR, 1 l9Sn-MAS NMR and UV-visible diffuse-reflectance (DR) spectroscopy methods. Some of these samples calcined at 450 °C were tested as catalysts in the partial oxidation of methanol (POM) reaction. In this paper we discuss briefly the effect of Sn-incorporation on the structural features and reducibility of CoAI-LDH. The catalytic performance of Co-spinel microcrystallites derived from CoAl-, and CoAlSn-LDHs was also evaluated. 

In this study, we focus on the encapsulation of [Re(l)(CO)3(bpy)(py)] into mesopore of A1MCM-41 and its photophysical characterization using XRD, FTIR, Xe-NMR, diffuse reflectance (DR) UV-visible, electron spin resonance (ESR), and photoluminescence spectroscopy with photoirradiation and C02 adsorption. 

Stone F.S., Garrone E., Zecchina A. Surface properties of alkaline earth oxides as studied by UV-visible diffuse reflectance spectroscopy. Mater. Chem. Phys. 1985 13 331-46. 

The simultaneous application of EPR/ultraviolet-visible diffuse reflectance spectroscopy (UV-vis-DRS)/online gas chromatography (GC) to characterize working catalysts was realized a few years ago 14), and laser-Raman spectroscopy has recently been coupled with these to provide the first such simultaneous application of three techniques 15). 

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