Real surface area measurements in electrochemistry

Silica aerogel; synthesis, properties and characterization

In the alkoxy-derived sol-gel system, various macroporous morphologies can be obtained by inducing the phase separation parallel to the sol-gel transition. This principle of macroporous morphology control can be best applied to pure silica and silica-based multicomponent oxide systems. The earlier the phase separation takes place than the sol-gel transition, the larger the characteristic sizes of pores and gel skeletons become. The time resolved light scattering measurements revealed that the morphology formation process exhibits the features of spinodal decomposition and that the final gel morphology is determined by the competitive kinetics between the domain coarsening and the structure freezing by sol-gel transition. The mesopore structure of such macroporous gel skeletons could be easily tailored by the solvent exchange procedures. Silica gels with controlled macropores and mesopores were successfully applied as a material for the continuous rod type column for high performance liquid chromatography.

Pore Structure Control of Silica Gels Based on Phase Separation

Progress in the synthesis and engineering of advanced porous materials demands better pore structure characterization. The analysis of pore structure is complicated by (1) the wide range in pore sizes observed, from molecular ( 1 mm) dimensions, (2) complex pore shapes and connectivities, (3) chemical and physical heterogeneities, and (4) pore structure changes that can occur during characterization.The required pore structure information varies with application. Bulk density and the pore-size distribution are needed for thermal insulation. In this case, the dimension of interest is the so-called hydraulic radius since, for small pores, the gas-phase conductivity is proportional to the mean hydraulic radius to the mean free path. A few large but isolated pores will significantly affect conductivity but will go undetected in typical gas-absorption methods. In contrast, for separations, bottlenecks control performance. For transport, such as migration through geologic formations, both the pore-size distribution and pore connectivity are important. For adsorption, surface area and pore size are the relevant factors. Finally, the conventional concepts of pore structure lose meaning as the pore size approaches molecular dimensions, typical of adsorbents and gas-separation membranes.

Characterization of Porous Solids

The effect of pore structure and gas pressure upon the transport properties of coal: a laboratory and modeling study. 1. Isotherms and pore volume distributions

Low field NMR spin-lattice relaxation measurements of pore fluids contained in silica gels are employed as a pore structure probe to ascertain the changes that occur in a two-step acid/base-catalyzed silica gel during aging in its mother liquor. It has been shown that both a narrowing of pore size distribution and an increase in mean pore size is obtained for a gel aged at 303 K over a time period of several weeks. By increasing the aging temperature from 303 to 333 K, the time required for this same degree of pore size distribution narrowing is decreased by an order of magnitude. Increasing the gel solids content from 15 to 30 wt% SiO 2 results in smaller pore sizes and pore volumes (a factor of ∼ 2) but a similar pore size narrowing with comparable kinetics. In contrast, little change in pore structure is observed in a two-step acid-catalyzed gel aged in its mother liquor at 303 K for similar time periods.

Pore structure evolution in silica gel during aging/drying I. Temporal and thermal aging

A two-step acid-base-catalyzed silica gel has been aged in a series of alcohol and water baths and some chemical and physical structures of the gel were measured using several techniques (low-field NMR, 29Si and 13C MAS-NMR, IR, Raman, nitrogen adsorption) on gels in wet and dry states. When the gel was placed in alcohol, the surface area increased as a result of esterification and depolymerization to 1500–2000 m2/g in the wet state which then decreased to 1000 m2/g after drying. In water, hydrolysis and condensation decreased surface area to 1000 m2/g (wet) and 500 m2/g (dry). This process was reversible. Alcohol-aging led to small pore size distributions (in much shorter times than previous studies of aging in the gel's mother liquor, ∼ 90% ethanol, 10% H2O). Surface areas calculated from 29Si MAS-NMR-derived Q distributions were in good agreement with adsorption-derived values for the samples dried from alcohol but water-dried samples appeared to contain some surface area which was inaccessible to adsorbing nitrogen after drying. These results indicate that the pore structure of silica gels may be dramatically altered during the aging process and that their wet gel features may be preserved (at least partially) upon drying.

Pore structure evolution in silica gel during aging/drying II. Effect of pore fluids

In an effort to understand the various phenomena which occur during aging of silica gels at different pH, silica particles prepared by three routes distinguished prinipally by their silicon coordination (Qn distributions) Stober silica spheres, Ludox and Cab-O-Sil) were employed. In situ techniques were used including proton spin-lattice relaxation, SAXS and zeta potential for the wet state and 29Si MAS-NMR, SEM, TEM and N2 sorption for dry samples. In solution, the surface area increased in the following order after aging at pH 10 as compared with that at pH 7 and the initial dry surface area: Cab-O-Sil < Ludox ⪡ Stober which is inversely proportional to the degree of silicon coordination. This same trend was observed for the surface charge measurements. Surface areas calculated from 29Si MAS-NMR for four different Stober sphere sizes were of order 1000 m2/g as compared with nitrogen/BET surface areas which varied from 9 to 370 m2/g, indicating significant internal surface which is inaccessible to nitrogen. The 29Si NMR results for the Stober spheres were consistent with a close packed aggregate of ∼3 nm primary particles. In addition to particulate silica, aging at varying pH of a two-step acid/base-catalyzed silica gel was studied. At all pH values, the surface area was higher in the wet gel as compared with the xerogel. Aging at higher pH was found to yield lower surface area, larger pore volume and a narrower pore size distribution depending upon the pH, aging time and surface tension of the final pore fluid.

Pore structure evolution in silica gel during aging/drying. IV. Varying pore fluid pH☆

Low-field spin-lattice relaxation (20 MHz proton NMR) measurements of ethanol contained in two base-catalyzed silica xerogels aged under different conditions have been applied as an in situ probe of pore structure development during xerogel drying. Two base-catalyzed silica gels were prepared and aged under different conditions. One sample (designated E) was aged in pure ethanol for 1 week and the second (designated B) in an ethanol-KOH mixture for the same time period prior to drying. 180{degree}-{tau}-90{degree} relaxation experiments were performed at 20 MHz and 303 K as each gel dried. In addition, gel shrinkage and weight loss were monitored. After complete drying, the surface area and pore size distribution of each sample were obtained from nitrogen adsorption/condensation. During drying, a gradual pore shrinkage for both samples was observed until the structures stiffened, causing the fluid meniscus to penetrate the pores. For sample E, this caused the surface area to decrease from {approx} 1000 to 600 m{sup 2}/g and microporosity development in the pore size distribution. For sample B, the surface area decreased from {approx} 1500 to 200 m{sup 2}/g and microporosity disappeared. The final NMR-derived pore size distributions were in good agreement with that obtained from nitrogen condensation.

Pamela J. Davis

Papers

Pore structure evolution in silica gel during aging/drying I. Temporal and thermal aging

Pore structure evolution in silica gel during aging/drying II. Effect of pore fluids

Pore structure evolution in silica gel during aging/drying. IV. Varying pore fluid pH☆

In situ pore structure studies of xerogel drying

Rapid surface area determination via NMR spin-lattice relaxation measurements