Silica comparison, surface

Methyl radicals formed on a silica gel surface are apparently less mobile and less stable than on porous glass (56, 57). The spectral intensity is noticeably reduced if the samples are heated to —130° for 5 min. The line shape is not symmetric, and the linewidth is a function of the nuclear spin quantum number. Hence, the amplitude of the derivative spectrum does not follow the binomial distribution 1 3 3 1 which would be expected for a rapidly tumbling molecule. A quantitative comparison of the spectrum with that predicted by relaxation theory has indicated a tumbling frequency of 2 X 107 and 1.3 X 107 sec-1 for CHr and CD3-, respectively (57). 

In comparison to the values obtained for toluene, the ET (30) values calculated from measurements carried out in 1,2-dichloroethane are always about 3 kcal mol 1 larger. The difference is close to the expected value resulting from the polarity of the two solvents where ET (30) of 1,2-dichloroethane is 41.3 kcal mol 1 and Ej (30) of toluene is 33.9 kcal mol 1 [81]. The decrease of Ej (30) values with increasing the content of amino groups in the adsorbed copolymer layer can be explained by a decrease of the HBD capacity and/or the dipolarity/dipolarizability of the former silica particle surface. Because the interaction between silanol groups and amino groups of PVFA-co-PVAm is really responsible for the decrease of the Ej (30) values, a relationship between the Ej (30) values and IEP of the PVFA-co-PVAm/silica hy-... 

Figure 4. A comparison of the experimental (filled circles) and theoretical phase diagram of n-octadecanol monolayer on silica gel surface. Aq is the minimal and A - the real surface area occupied by a single molecule in monolayer.

Stationary phases. They prepared 2,3-di-0-methyl-6-0-ferf-butyldimethylsilyl-p-CD (TBCD) for comparison to the more common permethyl-P-CD (PMCD) CSP. Their idea was to narrow the secondary opening of the CD cavity and to block the opening on the primary side of the CD giving rise to a structurally unique CSP. The TBCD was dissolved in a lipophilic polysiloxane and coated on treated and nonpretreated silica capillary surfaces. The test compounds studied were limonene and 1-phenylethanol. S-limonene is eluted before R and a = 1.078 on TBCD and a = 1.029 on PMCD. The R enantiomer emerges before S for the polar alcohol with a = 1.044 on TBCD and 1.048 on PMCD. 

This comparison suggests that a proton is transferred from the silica gel surface into a cluster of adsorbed CH OH molecules and that it jumps very rapidly from one molecule to another before recombining with a = SiO species. 

Such modified surface was then used as a substrate for the adsorption of the luminescent complex Eu(fbd)3.2H20 (fod is the anion of the 2,2-dimethyl-6,6,7,7,8,8,8-heptafluoro-3,5-octanedione). Eu(fod)3.2H20 and the sUicon hybrid were mixed in stoichiometric amormts to produce samples with 1%, 5%, and 10% (m/m) of adsorbed complex. The adsorbed complex was then studied from the spectroscopic point of view. The measured lifetimes for the emission process are 476, 542, and 566 )Is for the 1%, 5%, and 10% samples, respectively. Furthermore, comparison with experimental data for -[3(trimethoxysilyl) propyl]-ethylenediamine-modified silica gel surfaces [8] emphasizes the prominent role of the chemical composition of the silica-modified surface in the spectroscopic properties of the adsorbed complex. 

The increase in relative intensity of the D2 Raman band correlates well with a reduction in the average Si-O-Si bond angle of Q sites, 0, determined from the Q peak positions in the NMR data. D2 is practically absent at SO and 200 C (0 = 148°), intense at 600°C (0 = 138 and 148°), and quite low in relative intensity after heating to 1100°C (0 = 149°). The relative intensity of D2 is also quite low for the sample exposed to water vapor after heating to 600°C (0 = 148°). By comparison, although the D1 species can form by condensation reactions on the silica gel surface, its formation is not correlated with a reduction in the average value of 0. 

Another method to study the contact mechanics and adhesion behavior of soft solids is the so-called JKR test using elastomeric poly(dimethylsiloxane) (PDMS) lenses that are brought in contact with flat surfaces or with each other [884]. The soft PDMS ensures almost ideal JKR behavior of the contacting surfaces. Apphed load, indentation and contact radius, and neck shape can be determined simultaneously, which allows comparison with the JKR predictions. The surfaces of the lenses can easily be modified by treatment with an oxygen plasma to induce a silica-like surface that can then be modified using silane chemistry. As long as these layers are kept thin, the mechanical properties will still be dominated by the bulk PDMS. This type of experiments have been used extensively to study the influence of separation rate on adhesion (for a review, see Ref. [885]). 

The catalyst used was zinc triflate supported on silica, which was glued onto the surface of the SDR. Although total conversion could be achieved in a single pass, selectivity was highly dependent on residence time, increasing with shorter times. In a direct comparison with a batch process using the same catalyst at the same conversion and residence time a 200 times increase in throughput could be obtained. 

In order to verify the presence of bimetallic particles having mixed metal surface sites (i.e., true bimetallic clusters), the methanation reaction was used as a surface probe. Because Ru is an excellent methanation catalyst in comparison to Pt, Ir or Rh, the incorporation of mixed metal surface sites into the structure of a supported Ru catalyst should have the effect of drastically reducing the methanation activity. This observation has been attributed to an ensemble effect and has been previously reported for a series of silica-supported Pt-Ru bimetallic clusters ( ). 

The surface-catalyst composition data for the silica-supported Ru-Rh cuid Ru-Ir catalyst are shown in Figure 1. A similcir plot for the series of silica-supported Pt-Ru bimetallic catalysts taken from ref. P) is included for comparison purposes. Enthalpies of sublimation for Pt, Ru, Rh and Ir are 552, 627, 543, and 648 KJ/mole. Differences in enthalpies of sublimation (a<75 KJ/mole) between Pt and Ru cind between Rh and Ru are virtually identical, with Pt euid Rh having the lower enthalpies of sublimation. For this reason surface enrichment in Pt for the case of the Pt-Ru/Si02 bimetallic clusters cannot be attributed solely to the lower heat of sublimation of Pt. Other possibilities must also be considered. 

The Raman spectra (0-1400 cm l) shown in Fig re 6 illustrate the structural changes which accompany the consolidation of silica gels. The 1100°C sample is fully dense, whereas the 50 and 600°C samples have high surface areas (1050 and 890 m2/g), respectively. The important features of the Raman spectra attributable to siloxane bond formation are the broad band at about 430 cm 1 and the sharp bands at 490 and 608 cm 1(which in the literature have been ascribed to defects denoted as D1 and D2, respectively). The D2 band is absent in the dried gel. It appears at about 200°C and becomes very intense at intermediate temperatures, 600-800°C. Its relative intensity in the fully consolidated gel is low and comparable to that in conventional vitreous silica. By comparison the intensities of the 430 and 490 cm 1 bands are much more constant. Both bands are present at each temperature, and the relative intensity of the 430 cm 1 band increases only slightly with respect to D1 as the temperature is increased. Figure 7 shows that in addition to elevated temperatures the relative intensity of D2 also decreases upon exposure to water vapor. 

Platinum is an important example of a metal where, even on an uncontaminated surface such as is offered by an evaporated film, there is a strong tendency for only one C—C bond to be ruptured in any particular reacting molecule. On this basis, one may express the distribution of reaction products in terms of relative C—C bond rupture probabilities. Some data of this sort are contained in Table XI for thick and ultrathin film catalysts, and for comparison there are included some data for reactions on a silica-supported catalyst containing 0.8% platinum. These data all refer to reactions carried out in the presence of a large excess of hydrogen, although the results of Kikuchi et al. (128) indicate that on platinum catalysts the position of C—C bond rupture (in n-pentane) is very little dependent on hydrogen pressure. The data in Table XI show that, on the whole, the 0.8% platinum/silica catalyst used by Matsumoto et al. (110) was inter-... 

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