Silica spectrum

The initial APTS modified silica spectrum (figure 9.27 a) shows the three peaks of the alkyl carbon atoms. Upon hydration of the sample, all peaks narrow and the BC peak shifts to a lower position (figure 9.27 b). Heating the hydrated sample again to 474 K under vacuum restores the spectrum to that of the initial sample. From this experiment and other analogous results,21 the shift of the BC atom is correlated with... 

K or 673 K (figure 9.34 a,b, resp.) no OHf band is present in the spectrum, either with or without predeuteration. The maximum at 3743 cm 1 corresponds to the position of the free surface silanol peak in the original silica spectrum. This peak may therefore be assigned to unreacted surface hydroxyls, as discussed above. 

IR spectra of aerogels and xerogels prepared with the different Zr/Si molar ratios are shown in Fig. 4. In the spectrum of pure silica (spectrum a), five features are observed Asymetric Si-O-Si network stretches at 1089 and 1228 cm , the corresponding symmetric stretch at 801 cm , and Si-O-Si or Si-OH at 980 cm [5,10,12]. The band at 459 cm corresponds to the bending mode of silica. 

Silicon-29 CP-MAS NMR is probably most useful for the study of oxide surfaces used in chemically modified stationary phases for HPLC and catalytic materials which contain a considerable fraction of silica. The spectrum of silica in a variety of physical forms has been well-documented in many studies. An example of a typical silica spectrum is shown in Figure 7 [28]. The three peaks observed are readily understandable in terms of the expected surface structure of... 

Fig. 17 IH spectra of A) isotactic PMMA in CDCI3, B) after silica addition and D) after subtraction of the silica spectrum C). Spectra taken from [59] with permission.

The Si02 framework vibrations that occur at about 430, 800, 1070, and 1180cm in the fused silica spectrum (discussed in Section 1.4.2) can be explained by a vibrational calculation on a continuous random network (CRN) [36,37]. The 1070 and 1180 cm bands are assigned to the TO and LO modes of the Si-O asymmetric stretching vibration, respectively. Due to the selection rules, Raman-active modes involve symmetric vibrations, which... 

Still another type of adsorption system is that in which either a proton transfer occurs between the adsorbent site and the adsorbate or a Lewis acid-base type of reaction occurs. An important group of solids having acid sites is that of the various silica-aluminas, widely used as cracking catalysts. The sites center on surface aluminum ions but could be either proton donor (Brpnsted acid) or Lewis acid in type. The type of site can be distinguished by infrared spectroscopy, since an adsorbed base, such as ammonia or pyridine, should be either in the ammonium or pyridinium ion form or in coordinated form. The type of data obtainable is illustrated in Fig. XVIII-20, which shows a portion of the infrared spectrum of pyridine adsorbed on a Mo(IV)-Al203 catalyst. In the presence of some surface water both Lewis and Brpnsted types of adsorbed pyridine are seen, as marked in the figure. Thus the features at 1450 and 1620 cm are attributed to pyridine bound to Lewis acid sites, while those at 1540... 

Figure Bl.25.9(a) shows the positive SIMS spectrum of a silica-supported zirconium oxide catalyst precursor, freshly prepared by a condensation reaction between zirconium ethoxide and the hydroxyl groups of the support [17]. Note the simultaneous occurrence of single ions (Ff, Si, Zr and molecular ions (SiO, SiOFf, ZrO, ZrOFf, ZrtK. Also, the isotope pattern of zirconium is clearly visible. Isotopes are important in the identification of peaks, because all peak intensity ratios must agree with the natural abundance. In addition to the peaks expected from zirconia on silica mounted on an indium foil, the spectrum in figure Bl. 25.9(a)...

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]. 

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. 

High-pressure pumps operating at up to 6000 psi are required to force solvent through a tightly packed HPLC column, and electronic detectors are used to monitor the appearance of material eluting from the column. Alternatively, the column can be interfaced to a mass spectrometer to determine the mass spectrum of every substance as it elutes. Figure 12.18 shows the results of HPLC analysis of a mixture of 10 fat-soluble vitamins on 5 jam silica spheres with acetonitrile as solvent. 

The focusing of radiation within the instrument was formerly done by means of lenses, but these suffer from chromatic aberration and particularly in respect of the relationship between the visible and ultraviolet parts of the spectrum. Focusing is now usually carried out by means of suitably curved mirrors having a reflecting surface coated with aluminium which is protected by a silica film. 

This review will endeavor to outline some of the advantages of Raman Spectroscopy and so stimulate interest among workers in the field of surface chemistry to utilize Raman Spectroscopy in the study of surface phenomena. Up to the present time, most of the work has been directed to adsorption on oxide surfaces such as silicas and aluminas. An examination of the spectrum of a molecule adsorbed on such a surface may reveal information as to whether the molecule is physically or chemically adsorbed and whether the adsorption site is a Lewis acid site (an electron deficient site which can accept electrons from the adsorbate molecule) or a Bronsted acid site (a site which can donate a proton to an adsorbate molecule). A specific example of a surface having both Lewis and Bronsted acid sites is provided by silica-aluminas which are used as cracking catalysts. 

Fig. 25. Interference of plasma lines from the Ar+ emiasion in the Raman spectrum of a Cab-O-Sil silica sample.

There are, at present, two overriding reasons an experimentalist would choose to employ laser Raman spectroscopy as a means of studying adsorbed molecules on oxide surfaces. Firstly, the weakness of the typical oxide spectrum permits the adsorbate spectrum to be obtained over the complete fundamental vibrational region (200 to 4000 cm-1). Secondly, the technique of laser Raman spectroscopy is an inherently sensitive method for studying the vibrations of symmetrical molecules. In the following sections, we will discuss spectra of pyridine on silica and other surfaces to illustrate an application of the first type and spectra of various symmetrical adsorbate molecules to illustrate the second. 

Kagel (29) found that whereas pyridine is hydrogen bonded to a silica gel surface 2-chloropyridine is not, the spectrum of 2-chloropyridine adsorbed on silica gel being identical with that of the liquid, and concluded that steric hindrance probably prevents hydrogen bond formation in this case. 

From the Raman spectrum of acetaldehyde adsorbed on silica gel... 

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... 

For complex, wide-spectrum deposit problems, consider Acumer 3100 (high iron) at up to 900 psig, or 5,000 (high silica) if over 650 psig, remove silica at source. All-polymer/all-organic product feed rates vary, depending on these factors. 

The main problem in developing fibers for IR transmission is that silica glass is not transparent in that area of the spectrum. Suitable materials include the selenides and other chalcogenide glasses (particularly for the CO2 laser light source) and the fluorohafnate glasses. In addition, materials, such as zirconium fluo-... 

In view of the accessibility of zeolite A (only linear molecules adsorb) the coupling will take place at the outer surface of the zeolite crystals. Indeed, Ag-Y and especially a Ag-loaded amorphous silica-alumina, containing a spectrum of wider pores, mrned out to be much better promoter-agents (ref. 28). The silica-alumina is etched with aqueous NaOH and subsequently exchanged with Ag(I). 

The spectrum of Fe-K/y-Al203 itself (Fig. 3(a)) displays a double Al resonance with S -50, 0, +45 ppm. In the case of Fe-K/y-Al203 with silica, the amount of sixfold coordinate non-firamework aluminum increases. However, in Fig 3(c), the a1 MAS NMR spectra of Fe-K/y-Al203 with alumina binder clearly reveal a narrow peak at +45 ppm. It has been... 

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