Effect of vacuum and of strong adsorbed water films on micropore formation in aluminum hydroxide xerogel powders

Aluminum hydroxide gels were washed with water, ethanol, methanol and isopropanol to obtain new gels with different liquid phases that were dried either in air at 120 degrees C or under vacuum at 80 degrees C. Drying in air leads to alcoholic xerogels with BET surface areas larger than the aqueous ones. The effect of the alcoholic groups as substitutes of the hydroxyl ones has been discussed to account for the final size of xerogel crystallites. Drying under vacuum decreases the BET surface of the methanol xerogels, but no micropores are formed in all the alcoholic xerogel matrixes. On the contrary, the vacuum drying process changes significantly the microstructure of the aqueous xerogels. Their BET surface increases by 34 m(2)/g, and micropores are formed within their crystallite aggregates. It has been experimentally shown that these changes are due to a shear transformation that occurs in the boehmite xerogels obtained under vacuum. To discuss these data, the existence of chemical compounds such as AlOOHnH(2)O was postulated. On this ground, a neat analogy between vacuum drying process and vacuum interfacial decomposition reactions of inorganic salts can be drawn. This analogy explains how a state of stresses forms in aqueous xerogel matrix during vacuum drying process.     

Porous silicon oxycarbide glasses from organically modified silica gels of high surface area

High surface area alkyl-substituted silica aerogels and xerogels were successfully prepared by sol-gel processing and supercritical drying. The gels were further heat treated in inert atmosphere to temperatures as high as 1000°C. Surface areas and pore structure of the gels and gels pyrolyzed at high temperatures were determined by multipoint BET surface area measurement. The aerogels and xerogels exhibited surface areas of about 1100 m²/g. No significant effect of pH was found on the surface areas of gels in the two step sol-gel process, but gels of low pH showed smaller pore diameter and higher density. Xerogels showed smaller surface area, pore size, and pore volume compared to aerogels. Upon pyrolyzing in inert atmosphere, the surface areas of all the gels decreased with temperature as a result of collapse of micropores and shrinkage of mesopores. Unlike pure silica gel, which loses almost all surface area and densifies at 1000°C, the advantage of the alkyl-substituted gels is that they maintained a high surface area of 400 m²/g at 1000°C.Also with the Department of Agronomy.     

Contribution to the evaluation of density of methane adsorbed on activated carbon

The adsorption isotherms of N(2) at -196 degrees C, CO(2) at 0 degrees C, and CH(4) at 25 degrees C on 35 activated carbons with a wide range of micropore volumes and pore size distributions have been compared to evaluate the density of adsorbed methane. Results indicate that methane is adsorbed in the micropores of the activated carbon with a density that is a function of the carbon porosity because methane is packed more compactly in narrow than in wide micropores. An experimental procedure is proposed to evaluate the density in both types of micropores as a function of pressure. Its application to these porous carbons indicates that density of adsorbed methane increases rapidly with pressure on narrow micropores, the increase becoming slower above 1.5 MPa. The value reached at 3 MPa is 0.21 g/cm(3), near that estimated as the limiting value, 0.23 g/cm(3). Density in wide micropores is low, 0.09 g/cm(3) at 3 MPa, but it continuously increases with pressure.     

在蒸气相中合成中孔分子筛MCM—41及其孔结构参数的表征

在水蒸气中，由含表面活性剂十六烷基三甲基溴化铵的无定形凝胶合成出Ｓｉ－ＭＣＭ－４１和Ａｌ－Ｓｉ－ＭＣＭ－４１分子筛纯相，研究了它们的合成条件。     

Preparation and structural characterization of activated carbons based on polymeric resin

In this work, activated carbons (ACs) with high porosity were synthesized from polystyrene-based cation-exchangeable resin (PSI) by chemical activation with KOH as the activating agent. And the influence of the KOH-to-PSI ratio on the porosity of the ACs studied was investigated by using nitrogen adsorption isotherms at 77 K and a scanning electron microscope (SEM). As a result, PSI could be successfully converted into ACs with well-developed micro- and mesopores. The specific surface area and pore volumes increased with an increase in the KOH-to-PSI ratio. However, it was found that the addition of KOH did lead to the transformation of the micropores to the meso- and macropores. From the results of pore size analysis, quite different pore size distributions were observed, resulting from the formation of new pores and the widening of the existing micropores during KOH activation. A SEM study showed that the resulting carbons possessed a well-developed pore structure and the pore size of the ACs studied increased with the KOH-to-PSI ratio.     

Characterization of Alumina Gel Catalysts by Emanation Thermal Analysis (ETA)

Alumina gels made from the metal alkoxide is known to have high catalyst activity for the selective reduction of NO _x by hydrocarbons. It is also reported that the fine structure of the gels effects the activity. In this study, the effect of the preparation method on the fine structure and catalyst activity of the gels was investigated. Monolithic gels were obtained by hydrolysis of Al(sec-C₄H₉O)₃. The wet gels were dried at 90°C (xerogels), supercritically dried (aerogels), or dried after immersion in an ethanol solution of methyltrimethoxysilane (modified xerogels). The changes in the microstructure during heating were discussed using the results of TG-DTA, ETA and N₂ adsorption. The ETA curves show the ²²⁰Rn-release rate, E, of the samples, previously labelled with ²²⁸Th and ²²⁴Ra, during heating. The decrease in E of the xerogel at temperatures higher than 400°C indicates a gradual decrease in the surface area and porosity. A remarkable decrease in the BET surface area of the xerogel was found after heat-treating at 500°C. On the other hand, constant E of the aerogels and modified xerogels above 450°C suggests high thermal stability. The pore radii, estimated by BJH method, and the catalyst activities at 500°C of the aerogels and the modified xerogels were higher than those of the xerogels. The temperature range in which the alumina gels are applicable as catalysts was determined.     

Multiscale Porosity from Thermoreversible Poly(vinylidene fluoride) Gels in Diethyl Azelate

Poly(vinylidene fluoride) (PVF₂) produces thermoreversible gels in a series of diesters. The polymer-solvent complexation occurred for intermittent number of carbon atoms n ⩾ 2 and the enthalpy of complexation increased with increasing n. The gels were dried by replacing the diesters with low boiling solvent like cyclohexane (bp. 80 °C) and methylcyclohexane (bp. 99 °C). The porosity of the dried gels was measured using Poremaster-60. For PVF₂-DEAZ gel meso and macro porosity have been observed. The former pore dimensions have been attributed for polymer-solvent complexation while the macroporosity has been attributed for caging of solvent between the PVF₂ fibrils The porosity measured from nitrogen adsorption isotherms using BJH method indicate presence of minimum pore diameter of 3.8 nm for the 10% dried gel of PVF₂.     

The microstructure of hybrid silica gels and its modification by evaporative and supercritical dryings

The effect of evaporative drying on the pore size of two series of hybrid silica gels is investigated by combining beam bending on the gels and mercury porosimetry on the aerogels and xerogels with transmission electron microscopy on the xerogels. It is shown that the shrinkage of the pores during drying is different in the two series: in one series the pores shrink proportionally to their volume, in the other the pores collapse until they all reach the same size. The experimental data enable us to discuss the relation between these two different behaviours and the different morphology of the two series of gels.     

Synthesis of novel Schiff base doped sol–gel silicas

Sulfonamide Schiff bases were doped uniformly in silica sol–gels prepared from liquid precursors by a fast and easy way at room temperature and processed to form xerogels. Schiff bases are efficient chelating agents, bioactive and catalytically active compounds. The structures of the newly synthesized Schiff base doped xerogels were elucidated by their physical (morphology, surface area, porosity), spectral (FTIR) and analytical (CHNSO/Si) data. The powder X-ray diffraction studies were carried out to confirm the formation of single phase. Characterization confirmed that Schiff base molecules are entrapped inside the pores as well as physically bound onto the silica surface. All Schiff base doped xerogels are stable mesoporous materials showing hydrophilic properties. Loadings of Schiff bases from 0.10 to 0.23 g/g of xerogel were obtained resulting amorphous materials. The doping of Schiff bases with xerogel caused change in surface area, pore volume and pore diameter of xerogel without damaging the main framework of siliceous skeleton. Morphology and colour of xerogel was also changed after doping. The entrapment of Schiff bases in xerogel caused increase in their decomposition temperatures. The final Schiff base doped xerogels show remarkable thermal stability.     

Cluster-Mediated Water Adsorption on Carbon Nanopores

The adsorption isotherms of water at 303 K and N2 at 77 K on various kinds of porous carbons were compared with each other. The saturated amounts of water adsorbed on carbons almost coincided with amounts of N2 adsorption in micropores. Although carbon aerogel samples have mesopores of the great pore volume, the saturated amount of adsorbed water was close to the micropore volume which is much small than the mesopore volume. These adsorption data on carbon aerogels indicated that the water molecules are not adsorbed in mesopores, but in micropores only. The adsorption isotherms of water on activated carbon having micropores of smaller than 0.7 nm in width had no clear adsorption hysteresis, while the water adsorption isotherms on micropores of greater than 0.7 nm had a remarkable adsorption hysteresis above P/P0 = 0.5. The disappearance of the clear hysteresis for smaller micropores suggested that the cluster of water molecules of about 0.7 nm in size gives rise to the water adsorption on the hydrophobic micropores; the formation and the structure of clusters of water molecules were associated with the adsorption mechanism. The cluster-mediated pore filling mechanism was proposed with a special relevance to the evidence on the formation of the ordered water molecular assembly in the carbon micropores by in situ X-ray diffraction.     

Tailoring of nanoscale porosity in carbide-derived carbons for hydrogen storage

The poor performance of hydrogen storage materials continues to hinder development of fuel cell-powered automobiles. Nanoscale carbons, in particular (activated carbon, exfoliated graphite, fullerenes, nanotubes, nanofibers, and nanohorns), have not fulfilled their initial promise. Here we show that carbon materials can be rationally designed for H2 storage. Carbide-derived carbons (CDC), a largely unknown class of porous carbons, are produced by high-temperature chlorination of carbides. Metals and metalloids are removed as chlorides, leaving behind a collapsed noncrystalline carbon with up to 80% open pore volume. The detailed nature of the porosity-average size and size distribution, shape, and total specific surface area (SSA)-can be tuned with high sensitivity by selection of precursor carbide (composition, lattice type) and chlorination temperature. The optimum temperature is bounded from below by thermodynamics and kinetics of chlorination reactions and from above by graphitization, which decreases SSA and introduces H2-sorbing surfaces with binding energies too low to be useful. Intuitively, pores of different size and shape should not contribute equally to hydrogen storage. By correlating pore properties with 77 K H2 isotherms from a wide variety of CDCs, we experimentally confirm that gravimetric hydrogen storage capacity normalized to total pore volume is optimized in materials with primarily micropores ( approximately 1 nm) rather than mesopores. Thus, in agreement with theoretical predictions, a narrow size distribution of small pores is desirable for storing hydrogen, while large pores merely degrade the volumetric storage capacity.     

The role of pore size and structure on the thermal stability of gold nanoparticles within mesoporous silica

Highly dispersed gold particles (<2 nm) were synthesized within the pores of mesoporous silica with pore sizes ranging from 2.2 to 6.5 nm and different pore structures (2D-hexagonal, 3D-hexagonal, and cubic). The catalysts were reduced in flowing H2 at 200 degrees C and then used for CO oxidation at temperatures ranging from 25 to 400 degrees C. The objective of this study was to investigate the role of pore size and structure in controlling the thermal sintering of Au nanoparticles. Our study shows that sintering of Au particles is dependent on pore size, pore wall thickness (strength of pores), and pore connectivity. A combination of high-resolution TEM/STEM and SEM was used to measure the particle size distribution and to determine whether the Au particles were located within the pores or had migrated to the external silica surface.     

Enhanced hydrogen storage capacity of high surface area zeolite-like carbon materials

We report the synthesis of zeolite-like carbon materials that exhibit well-resolved powder XRD patterns and very high surface area. The zeolite-like carbons are prepared via chemical vapor deposition (CVD) at 800 or 850 degrees C using zeolite beta as solid template and acetonitrile as carbon precursor. The zeolite-like structural ordering of the carbon materials is indicated by powder XRD patterns with at least two well-resolved diffraction peaks and TEM images that reveal well-ordered micropore channels. The carbons possess surface area of up to 3200 m2/g and pore volume of up to 2.41 cm3/g. A significant proportion of the porosity in the carbons (up to 76% and 56% for surface area and pore volume, respectively) is from micropores. Both TEM and nitrogen sorption data indicate that porosity is dominated by pores of size 0.6-0.8 nm. The carbon materials exhibit enhanced (and reversible) hydrogen storage capacity, with measured uptake of up to 6.9 wt % and estimated maximum of 8.33 wt % at -196 degrees C and 20 bar. At 1 bar, hydrogen uptake capacity as high as 2.6 wt % is achieved. Isosteric heat of adsorption of 8.2 kJ/mol indicates a favorable interaction between hydrogen and the surface of the carbons. The hydrogen uptake capacity observed for the zeolite-like carbon materials is among the highest ever reported for carbon (activated carbon, mesoporous carbon, CNTs) or any other (MOFs, zeolites) porous material.     

改性Y沸石的孔结构与催化性能

测定了不同方法改性的Y沸石样品的N_2吸附和脱附等温线, 并计算了样品的微孔、大孔和二次孔的孔容和表面积, 以及样品的二次孔分布, 证实改性方法对样品的孔结构有显著的影响。同时, 还考察了不同尺码探针分子在改性Y沸石样品上的酸催化反应活性, 将所得数据与样品的酸量、酸强度和二次孔容相关联, 取得了满意的结果。说明对大尺码反应分子, 改性过程中生成的大孔径二次孔, 对提高沸石催化剂的反应活性是有利的。     

29Si MAS-NMR Study of Silica Gels and Xerogels: Influence of the Catalyst

Four silica gels were prepared by hydrolysis of tetraethoxysilane (TEOS) in ethanol, using different catalysts: HCl, NaOH, NH₃, and NBu₄F. Nitrogen adsorption-desorption isotherms indicated that the HCl-catalyzed xerogel was purely microporous, whereas the other samples exhibited a very broad distribution of mesopores and a variable amount of micropores. ²⁹Si MAS NMR spectroscopy of the wet gels (before drying) pointed to a low degree of condensation for the HCl-catalyzed gel, and to the presence of unhydrolyzed TEOS monomer in the NaOH-catalyzed gel. Comparison with the ²⁹Si MAS NMR spectra of the xerogels indicated a significant increase of the degree of condensation during the drying procedure (3 hrs at 120°C under vacuum) for the HCl-catalyzed gel.     

The influence of the Si(OC2H5)4/(CH3O)3Si(CH2)3SH ratio on the structure-adsorption characteristics of xerogels formed and accessibility of functional groups in their surface layers

It was shown for the example of the Si(OC₂H₅)₄/(CH₃O)₃Si(CH₂)₃SH system that successively increasing the fraction of tetraethoxysilane in it (from 1: 1 to 5: 1 (mol)) successively decreased the content of 3-mercaptopropyl groups in xerogels synthesized by the sol-gel method (in the presence of methanol as a solvent and fluorine ions as a catalyst) from 5.0 to 1.9 mmol/g, whereas the specific surface area of such xerogels simultaneously increased from 13 to 631 m²/g. The sorption volume of pores also increased, their mean diameter varying insignificantly. The mean diameter of pores (2.2–2.5 nm) was close to the boundary between meso-and micropores, which was in agreement with the form of nitrogen adsorption isotherms (type I according to the IUPAC classification). It was shown by scanning electron microscopy that virtually nonporous xerogels formed at a 1: 1 ratio between alkoxysilanes consisted of spherical partially united particles 2.5–3 μm in diameter. All the 3-mercaptopropyl groups of this and other samples were, however, accessible to silver(I) ions. It follows that these groups are situated in the surface layer of xerogels. The number of thiol groups per 1 nm² of the surface of nonporous xerogels was 1.7–7.0 groups/nm² and depended on the ratio between reacting alkoxysilanes and s _sp.     

The Pore Structure Determination of Carbon Aerogels

The detailed adsorption isotherms of nitrogen on carbon aerogels at 77 K were measured. The N2 adsorption isotherm had a marked hysteresis. The adsorption isotherms were analyzed by high resolution s-plots to evaluate their porosity. The s-plots showed an explicit upward deviation from the linearity below s = 0.5, suggesting the presence of micropores. The mesoporosity and microporosity were separately determined from the s-plot. The predominant pores in carbon aerogels were mesopores and the percentage of micropores was in the range of 5 to 10% of the total pore volume. The N2 adsorption hysteresis was analyzed with the Saam-Cole theory under the assumption of the cylindrical pore shape. The parameters determined from the Saam-Cole method were associated with the carbon aerogel structure.     

Porous structure of the catalyst layers of electrodes in a proton-exchange membrane fuel cell: A stage-by-stage study

Porous structure is studied by standard contact porosimetry after each stage in the preparation of a catalyst layer, which contains a carbon substrate (CS), an ionomer in the form of Nafion resin, and a platinum catalyst. The influence of the ionomer on the porous structure of ten different CS is investigated. The structure of these samples is studied over the maximum range of their pore radii r ∼ 0.3–10⁵ nm. Pores of main volume within particles of the CS under investigation are mainly distributed over the maximum range of their radii from r ≤ 1 to ∼ 50 nm. Ionomer introduction into all the CS under investigation leads to an increase in the integral porosity due to the porosity of the intergranular structure. The change in porosity of the intragranular structure is caused by ionomer blocking small pores in the CS. In most CS, ionomer blocks pores of different sizes, from micropores with radii r ≤ 1 nm and up to r ∼ 1000 nm. It is concluded that the extent of blockage of CS pores is largely determined by the surface properties of the CS and Nafion resin and, more precisely, by the difference in resin adhesion to the CS surface because of the presence of different surface groups on the CS surface. When platinum is applied to CS, this leads to an increase in the specific volume of the micropores. The smallest dimensions of platinum particles are estimated to be on the order of 1 nm.     

多喷嘴对置式水煤浆气化炉内固体颗粒微观特性研究

气化炉内固体颗粒微观结构特性对气流床气化过程中熔渣、粗渣和细渣的形成具有重要影响。基于多喷嘴对置式水煤浆气化实验,对典型工况(O/C原子比为1.0)下气化炉轴向不同位置的固体颗粒进行取样,利用氮气等温吸附法和扫描电子显微镜对颗粒孔隙结构和微观形态进行研究。结果表明,气化炉内固体颗粒典型形态为不规则多孔状和规则球状,喷嘴平面有少量致密性不规则颗粒和中空颗粒。从喷嘴平面沿气化炉轴向向下,随着气化反应的进行,颗粒表面愈加粗糙,孔隙结构愈加发达。颗粒吸附曲线属于II型等温线,迟滞回线属于H3型回线,表明颗粒具有大量裂缝形孔和较连续的完整孔系统。比表面积和孔容积均随着与喷嘴平面距离的增加而增大,而平均孔径逐渐减小,在喷嘴平面附近变化幅度较大。孔结构以孔径小于10 nm的孔为主,随着气化反应的进行颗粒中小于10 nm的孔逐渐增多,而大于10 nm的孔分布状态变化不大。     

Preparation and Characterization of N-4-Nitrophenyl-L-prolinol Nanocrystals in Sol-Gel Matrices

Organic nanocrystals of N-4-nitrophenyl-l-prolinol (NPP) have been grown in sol-gel matrices prepared from silicon alkoxide precursors. Our process is based on the control of the nucleation and growth kinetics of the dye in the pores of dense gels. Nanocomposites gel-glasses are obtained with a high optical quality due to the small size of the nanocrystals (10-20 nm). Differential scanning calorimetry experiments evidenced clearly the melting point of NPP nanocrystals, which is registered 51°C above that of NPP powder. Micro-Raman and solid state NMR spectroscopies allowed us to demonstrate that our nanocrystallization process does not chemically modify NPP molecules. We specified also the nature of interactions existing between the NPP nanocrystals and the xerogels. These strong interactions, which explain the important increase of the melting point of the nanocrystals in comparison to the NPP powder, are hydrogen bonds between nitro groups of NPP and uncondensed silanol functions of the silicate network.