Silica difference spectra

The observation of the spectrum for styrene polymerized on the surface of silane-treated silica and of the difference spectrum of polystyrene adsorbed on the surface of silica have revealed that there are absorption bands of atactic polystyrene at 1602, 1493, 1453, 756, and 698 cm. The absorption bands at 1411 and 1010 cm are related to vinyl trimethoxy silane, and C of the difference spectrum is below the base line. This indicates that the vinyl groups of silane react with styrene to form a copolymer. 

An aerosil sample was methoxylated at 400 C to examine the effect of surface composition on the infrared spectrum. The difference spectrum between the methoxylated silica and the dried silica is shown in Figure 2b. Comparing this with the difference spectrum for hydroxylated silica (2a) several changes are apparent. First, the band due to the hydroxyl stretches at 3744 cm is diminished and replaced by bands at 2958 and 2856 cm" due to the asymmetric and symmetric CH stretching modes of the adsorbed methoxy. 

Figure 10.28 AES for silica-supported Pd nanoparticles before and after 141 torr CO exposure. Inset Difference spectrum between the AES for clean and poisoned Pd particles. (Reprinted from Ozensoy, E. et al., J. Phys. Chem. B, 108, 4351-4357, 2004. Copyright 2004. With permission from American Chemical Society.)...

Figure 10.1 FTIR-PA spectrum of the hydroxyl stretching bands of silica gel, (a) pretreated at 473 K, (b) reacted with BCl at 293 K. Spectrum (c) is the difference spectrum of (a) and (b).

When in situ dosing onto two different samples of Pt/silica (45, 48) and onto Cu/MgO was used (49), no evidence for spillover was found from NMR. Only one detailed study based on fully relaxed spectra led to observation of a non zero spillover (4T). In a Ru/Si02 catalyst, the silanol protons w ere exchanged for deuterons, the sam.ple was evacuated at 623 K, and a reference NMR spectrum w as taken at room, temperature. The sample was then exposed to 20 Torr of H2, an NMR spectrum was taken, and the difference with respect to the reference was calculated (line in Fig. 14). This represents the sum of reversible and irreversible hydrogen on the metal (resonating at -65 ppm) and spilled over on the support (at about 3 ppm). Then the sample was pumped out at room temperature for 10 min, and again a difference spectrum with the reference state was obtained (dashed line in Fig. 14) this represents irreversible hydrogen both on the support and on the metal. Similar in situ NMR techniques were used... 

In the upper half of the figure, the left-hand section compares the resonance for a silica-supported osmium catalyst containing 1 wt% osmium with that for pure metallic osmium. The magnitude of the resonance is higher for the osmium dispersed on the support, the extent of increase being indicated by the difference spectrum in the lower left-hand section of the figure. This effect is similar to the results we reported for iridium and platinum dispersed on an alumina support (39). 

Figure 3 (top) shows the spectra (1500-750 cm ) of silica before and after the reaction, respectively. Figure 25.3 (bottom) shows the difference spectrum at about ten times the original absorbance scale. All of the peaks are better resolved and the peaks at 1244 cm (peak A) and 1045 cm (peak B), which previously could not be clearly seen, are characteristic of P=0 and SiOP vibrational modes respectively. These bands also underwent the expected 0 shift when the reactant was adsorbed on an 0 exchanged silica. The remaining modes, which are due to CH3 deformation and rocking modes, did not exhibit any shift after 0 exchange. 

FIGURE 25.3 Top thin film spectra of silica before (solid line) and after (dotted line) the chemisorption of P(CH3)2C1. Bottom difference spectrum (dotted minus solid) scaled about 10-fold relative to the top spectra. 

On the contrary, stishovite, the only crystalline allotropic variety of Si02 in which Si has a coordination number equal to 6, gives an entirely different spectrum (see Fig. 16 and Ref. 20). One can thus conclude that the coordination number of Si in vitreous silica is equal to 4 and not to 6. 

Fig. 6 UV-vis spectra of LbL films of C54 P450 enz5unes on aminosilane-functionalized fused silica slides a high spin ferric form of purified human cyt P450 1A2 assembled with polyions b CO difference spectrum of human C54 P450 1A2 assembled with polyions after reducing to the ferrous form by sodium dithionite and purging pH 7.0 buffer with CO. c Low spin ferric form of purified bacterial cyt P450cam assembled with pol5dons. Reprinted with permission from the American Chemical Society, Ref. [47], Cop5urght 2009...

Figure 8. Comparison of sucrose Raman spectra obtained using PCF and solid-core silica pump fibers, (a) spectra obtained using about 30 mW excitation at 532 nm (lO-s exposure) each spectrum was normalized to the intensity of the large sucrose peak at 850 cm, (b) difference spectrum (solid-core - PCF) showing residual silica Raman excited by stray 532 nm light within solid-core silica collection fibers the inset shows a (smoothed) silica Raman spectrum obtained using a different instrument with excitation at 257.2 nm.

Observation of absorption bands due to LO phonons in RAIR spectra of thin, silica-like films deposited onto reflecting substrates demonstrates an important difference between RAIR and transmission spectra. Berreman has shown that absorption bands related to transverse optical (TO) phonons are observed in transmission infrared spectra of thin films obtained at normal incidence [17]. However, bands related to LO phonons are observed in transmission spectra of the same films obtained at non-normal incidence and in RAIR spectra. Thus, it is possible for RAIR and transmission spectra of thin films of some materials to appear very different for reasons that are purely optical in nature. For example, when the transmission infrared spectrum of a thin, silica-like film on a KBr disc was obtained at normal incidence, bands due to TO phonons were observed near 1060,790,and450cm [18]. 

Loader 38) studied the Raman spectra of styrene adsorbed on different silicas—chromatographic grade silica gel, Cab-O-Sil, and Aerosil 380. The author utilized the fact that chemisorption will bring about marked changes in the spectrum whereas physical adsorption will cause only a broadening of the Raman lines accompanied in some cases by a frequency... 

Figure 7. Optical absorption spectra of Au-implanted silica samples atmealed in air (a) or Ar (b) for 1 h at different temperatures, (c) Nonlinear fit (empty circles) to the optical absorption spectrum of the sample annealed at 900 °C in air, from which the average cluster diameter is obtained and compared to the TEM measured one, (d) Evolution of the optical spectra of Au-...

HYSCORE spectra of zeaxanthin radicals photo-generated on silica-alumina were taken at two different magnetic fields B0=3450G and B0=3422G, respectively. In order to combine the data from the two spectra, the field correction was applied (Dikanov and Bowman 1998). The correction consists of a set of equations that allow transformation of spectra to a common nuclear Zeeman frequency. The set of new frequencies was added to that of the former spectrum and plotted as the squares of the frequencies v2a and v2p. Examples of these plots can be found in Focsan et al. 2008. 

The Cr5+ ion has only one unpaired electron hence, no zero-field splitting is expected. Indeed, a well-resolved spectrum has been observed for the ion on alumina 147, 148), silica gel 149-151), silica-alumina 152-154), and magnesium oxide 155). The line may be resolved into parallel and perpendicular g values. As van Reijen and Cossee (151) have shown, the values of g range from 1.970 to 1.975 whereas, the values of g range from 1.898 to 2.002, depending on the treatment of a Cr/SiCh sample. These authors have suggested that Cr5+ in two different symmetries is present one has a long relaxation time and can be observed at room temperature, but the other has a very short relaxation time and can be observed only at very low temperatures (—253°). 

The effect from the top is behind the differences in IR spectra of CO adsorbed on various Na-zeolites (Fig. 1). The IR spectrum of CO adsorbed on the high-silica Na-FER shows only one band (centred at 2175 cm 1) that is due to the carbonyl complexes formed on isolated Na+ sites. When the content of Na+ in the sample increases (Na-FER with Si/Al=8), in addition to the band at 2175 cm 1 a new band at 2158 cm"1 appears due to the formation of linearly bridged carbonyl complexes on dual cation sites. The IR spectrum of CO adsorbed on Na-A,which has a large concentration of Na+ cations, shows bands centred at 2163, 2145, and 2129 cm 1 the band at 2163 cm"1 is due to the carbonyl species formed on dual cation sites, while bands at 2145 and 2129 cm"1 are due to carbonyls formed on multiple cation sites (Table 1), i.e., on adsorption sites involving more than two cations. 

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