Silica pore radius

A platinum on silica gel catalyst was prepared by impregnation of silica gel (BDH, for chromatographic adsorption) by a solution containing 0.5% (wt.) of sodium hydroxide and 0.5% (wt.) of chloroplatinic acid (both of analytical grade). The dried catalyst contained 1% (wt.) of platinum and a corresponding amount of the alkaline component. The BET surface area of the catalyst was 40 m2/g, the mean pore radius 150 A. The catalyst was always reduced directly in the reactor in a stream of hydrogen at 200°C for 2 hr. 

An interesting application in porous silica glass employs deuterium spin-lattice relaxation rates in selectively deuterated pyridine (50). The pore-size dependence shown in Fig. 20 is linear in the reciprocal of the pore radius for both the para and ortho positions, but the significant difference in the relaxation at these two positions precludes analysis by an isotropic... 

Fig. 20. The spin-lattice relaxation rates of liquid pyridine deuterated in ortho and para positions as a function of the pore radius reciprocal in porous sol-gel silica glasses (50).

Si02 gel DDAB/toluene/water (48.7% DDAB, 19.5% decane, 31.8% aqueous silica sol) TMOS (partially hydroiyzed)/H20 (+ 0.4-10 wt%HF) Bicontinuous p,Es used as templates for microporous silica gels monodisperse pores (2 nm pore radius) large specific surface area (—103 m2/g) (51)... 

The pores in these materials can be so small that they exert constraining effects on chains contained within them. For example, no glass transition was observed for PS in a mesoporous hexagonal silica [243] with a pore radius of 28 A [144]. 

Figure 2 shows the cumulative pore volume vs. pore radius for AC-ref SC-100 and SC-155 obtained by mercury intrusion technique. The curve corresponding to AC-ref shows a wide pore radius distribution instead, the curves assigned to SC-100 and SC-155 showed sharpened zones with maximum slope in 459A and 524A respectively, denoting a small increase of these values with the increase of the synthesis temperature. This phenomenon is probably produced by the growing of the big pores of the silica network at the expense of the... 

Kieselgel 60 a silica gel with an average pore radius 3.6 nm, obtained from Merck. 

Three mesoporous silica gels, with variable mean pore radius and specific surface area, have been studied. The substrates are named according to their approximated mean pore diameter. Measured values appeared to differ somewhat from the product names.30 The Kieselgels 40, 60 and 100 have a mean pore diameter of 4.2, 7.0 and 12.0 nm, respectively. Specific surface area increases with decreasing pore radius. Measured values, using the BET method, are given in table 9.3. 

A pore-radius that is sufficiently small to enable the application of a silica layer. 

Bartholomew and Sorensen [23] also measured loss of nickel surface area, BET surface area, and pore radius/volume after sintering of 15% Ni/alumina and 13.5% Ni/silica in H2 at 923, 973, and 1023 K. Their results for Ni/alumina were generally consistent with those of Bartholomew et al. [27] that is, percentage losses in nickel surface area of 5-13% at 923 and 973 K were comparable with observed losses in BET surface area and pore volume (e.g., 14% at 973 1C), while a 25% observed decrease in nickel area at 1023 K was twice as large as the... 

Figure 14. Characteristics of interfacial water in aqueous suspensions of A-300 sonicated (US) or treated in a ball-mill (MCA) at different concentration of silica (a) amounts of unfrozen water as a function of temperature at T < 273 K (b) relationship between the thickness of unfrozen water layer and temperature and changes in Gibbs free energy of interfacial water versus (c) pore radius, (d) pore volume, and (e) amounts of water unfrozen in these pores (f) interfacial Gibbs free energy as a function of silica concentration in suspensions differently treated.

Catalyst Impregnation. The adsorption and desorption isotherm record, and the essential pore size distribution data for a small pore silica-alumina catalyst, are shown in Figure 4. The sample pore size distribution was fairly sharp, in the 15-and 30-A. radii range, and the median pore radius was 27 A. 

Catalyst Fluorination. The adsorption and desorption record and calculated data for a silica-alumina catalyst of low alumina content and a median pore radius of 78 A. were obtained. This indicated a fairly wide distribution of pore sizes in this material. 

Figure 7 shows the influence of the initial pore radius, in the case of a wide- and narrow-pore silica catalyst, on the vanadium deposition profiles at an average axial position in the reactor. Both cases show the presence of deposition maxima, indicating that the deposition process is diffusion rate-limited. In the case of the narrow-pore silica the core volume of the pellet potentially available for vanadium deposition cannot be reached by reactant and intermediates and is lost for vanadium deposition. 

Figure 5. Influence of the bulk difftision coefficient, D, on the vanadium deposition profile in a wide pore silica catalyst pellet at reactor inlet conditions. (Model compound VO-TPP, 673 K, 10 MPa H2, initial pore radius 30 nm, catalystpellet radius 0.85 mm)...

Figure 7. Influence of the initial pore radius on the vanadium deposition profile (wide- and narrow pore silica catalyst pellet) at an average axial position in the reactor.

The pore condensation hysteresis of two fluids (CHF3 and C2F6) in mesoporous silicas with open cylindrical pores of uniform size (MCM-41 and SBA-15), and in a silica with large cellular mesopores which are accessible only via micropores or narrow mesopores, has been studied over a wide temperature range up to the critical point of the fluids. From the sorption isotherms in MCM-41 and SBA-15 the hysteresis onset-temperapore 7h and the corresponding pore condensation pressure plpo)H was determined for several materials of different pore radius R. 

The primary aim of this work has been to compare the limits of pore condensation hysteresis of fluids in silica materials of grossly different pore morphologies. For materials with open ended cylindrical pores of uniform pore width (MCM-41 and SBA-I5) we have determined the hysteresis temperatures 7h of the two fluids in samples with pore widths from 4 to 10 nm. Our results yield a linear dependence of the hysteresis temperature increment, AZh = Tc-Th, on the inverse pore radius, MR, in agreement with similar results reported in the literature. 

It is generally accepted that thermal and especially hydrothermal treatment of aluminas and other catalytic materials results in deterioration of porous structure, i.e. increase in average pore radius and diminishing in specific surface area [1-4]. It is very important that such alumina materials as some catalyst washcoats and membranes have to be exploited at higher temperatures and at atmosphere of large humidity. Therefore it is necessary to improve their thermal and hydrothermal stabilization by application of new binder materials or additives. Such additives as silica, ceria or zirconia are known as thermal stabilizers. The aim of this work was to determine the influence of addition of the selected stabilizers on hydrothermal stability of alumina material in the temperature range 150 - 225 °C and time up to 72 hours. 

Fig.3. The theoretical dependence of positronium lifetime on pore radius for - RP-2 -RP 8 - RP-18. Full points - LiChrosorbs RP, open points - LiChrosorbs RP after burning off Triangle - lifetime and pore radius for amorphous silica gel Si-100. The size of symbols is longer than experimental error.

The simulations conducted model the flow of Lennard-Jones (LJ) methane at ISO K and 170 K in a cylindrical silica pore of radius 1.919 nm, having infinitely thick pore walls comprising spherical LJ sites. For methane we use the established LJ parameter values... 

Kf is a function of properties of the liquid enclosed in the pores (constant). The tf term is tentatively proposed to be identical to the thickness of a surface layer of nonfteezing water, which effectively reduces the actual pore radius frrom rp to rp - tf. The pore size distribution of amoiphous silica determined independently by NMR and nitrogen adsorption agreed well. However, the application of this method to microporous materials (rp < 10 A) may be limited. By establishing a correlation between the freezing point depression AT and the pore size rp of mesoporous mat ials, a new method for the determination of pore size distributions was created (81). 

In the case of SiC12T sample and benzene as a wetting liquid the desorption curve is smooth without characteristic step. It may be explained by not satisfactory wetting of this silica by benzene and restricted penetration of narrow pores. Observed effect is confirmed by small pore volume for the same sample, derived from benzene desorption data. The localization of desorption steps on temperature axis corresponds to emptying of pores with dominant share in total pore volume. Converting the temperature into pore radius, by using the Kelvin equation, the dimensions of pores and pore size distributions PSD, AV Affp vs. R, may be calculated in the manner described earher [9]. 

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