N2-sorptionsmessungen zur bestimmung der spezifischen oberfläche und der porenverteilung von erhitztem normalbeton

The specific surface and pore size distribution of normal concrete is influenced by temperature effects. Therefore, the specific surface area of concrete specimens has been computed from nitrogen adsorption isotherms after a preceding temperature treatment in a thermal balance. It was found that the specific surface area increases from 2,0 m²/g at 100°C to 4,5 m²/g at 400°C. The mean pore radius decreases from 53 Å to 38 Å. A temperature treatment at 665°C is connected with a further decomposition of concrete and yields a decrease of the specific surface area to 2,4 m²/g and the pore size distribution shifts to larger radii ($\text{r}\text{= 51}\text{A}\text{?}$). Due to sintering processes, at temperatures of 1000°C the porosity of the material is decisively altered and a specific surface area of 1,1 m²/g has been calculated.     

Properties of aluminum-fluoride catalysts prepared by the fluoridation of aluminum oxide with trifluoromethane

High-purity AlF₃ has been prepared by allowing γ-Al₂O₃ to react with gaseous trifluoromethane at 670–770 K under 101 kPa total pressure. The use of gaseous trifluoromethane is a new, general method for preparing metal fluorides from metal oxides. AlF₃ prepared using this procedure retained the physical form of the starting γ-Al₂O₃. A $\text{1}\text{16}$-in. γ-Al₂O₃ extrudate, for example, yielded an AlF₃ extrudate with comparable physical dimensions and crush strengths. X-Ray diffraction, BET surface area, pore volume, and surface acidity measurements were employed to characterize various AlF₃ samples. Significant decreases in surface area and pore volume as well as surface acidity occurred upon increasing the concentration of AlF₃ from 90 to 100%. This behavior presumably results from the fluoridation of residual γ-Al₂O₃. AlF₃ extrudates were utilized as supports for Pt and Pd catalysts. Specific benzene hydrogenation activities of these catalysts are comparable to those of Pt and Pd on γ-Al₂O₃. In a unique application, $\text{Pd}\text{AlF}{}_3$ was used to hydrogenate m-diethylbenzene in superacid ($\text{HF}\text{TaF}{}_5$) solution.     

The significance of heating rate on char yield and char properties in the pyrolysis of cellulose

The carbonization of powdered cellulose was investigated in the temperature range 200–950°C by measuring weight loss, carbon and hydrogen content, BET-adsorption of nitrogen and carbon dioxide, mercury penetration and particle-size distribution. Evidence is presented in support of a kinetic model according to which cellulose decomposition is controlled by dehydration at low temperature and by cleavage/scission at high temperature. Increased char yield and lower $\text{O}\text{C}$ ratio at low heating rate, as well as kinetic investigations into the effect of potential catalysts, support this model. The difference in reaction mechanism according to the heating rate appeared to influence the char properties considerably. Yield in micropore volume and surface area of slowly carbonized cellulose is up to four times larger than that of rapidly heated cellulose. Mercury pore volume, density and particle diameter depend on the heating rate, also. By adsorption of various gases, differences in relative size of the pore openings of different chars can be discerned. Micropore volumes measured with carbon dioxide were as much as seventy times larger than the corresponding volume measured with nitrogen. Thus, it is possible to obtain chars with molecular sieve properties by simple pyrolysis heating schemes.     

Microstructures of PAN-ACF modified by catalytic benzene deposition

Benzene pyrolysis was successfully introduced to modify the pore size distributions (PSD) of polyacrylonitrile based activated carbon fibers (PAN-ACF) into a sharp distribution, at the presence of nickel catalyst. The microstructures of samples were studied by means of nitrogen adsorption, XRD, and SEM. The nitrogen isotherms were analyzed in detail using the routine BET method, α_s plot, DR equation, Horvath-Kawazoe (HK) equation, and regularized density functional theory (DFT), by which the surface area, micropore volume, and PSD were obtained. The results showed that the pore size of PAN-ACF can be effectively narrowed by catalytic benzene deposition and the PSD showed a unimodal nature, exhibiting potential behavior as a molecular sieve. A fraction residue of catalyst located in the ultramicropores can be washed by acid, resulting in increased BET surface area and micropore volume, which can also be confirmed by XRD and SEM measurements.     

Theory of adsorption of macromolecules in cylindrical pores and at surfaces of cylindrical shape

A rigorous theory of adsorption of a Gaussian chain of infinite length into cylindrically shaped pores and onto cylindrical surfaces of arbitrary diameter, D, has been developed for any energy of interaction ??. The change in the conformational free energy of a chain arriving inside the pores from an unrestricted volume is proportional to $\text{(D1)}{}^{\mathrm{?2}}$ ($\text{D1}$ is the effective pore width) over the entire molecular-sieve range (?? < ??_c). In the exclusion range (?? < 0), when the interaction of chain units with the adsorbent exhibits repulsive type forces, $\text{D1}$ is approximately equal to D. As the adsorption forces of attraction increase (0 < ?? < ??_c) the value of $\text{D1}$ increases and becomes infinite at the critical point. This makes it possible to use a single adsorbent for effective chromatographic separation of polymers over a wide range of molecular weights. In the adsorption range, where ?? > ??_c, the conformational free energy of the chain is virtually independent of the width and shape of the pores and is determined mainly by the value of ??. At the critical point, ?? = ??_c, the probability of the arrival of a polymer chain in the pore is independent of both the width and shape of the pore and is determined only by the ratio of geometrical volumes of the free volume to the pore volume. For cylindrical pores, a weak dependence of ??_c on D is observed only for narrow pores. This dependence virtually disappears on passing to adsorbents with wide pores. Consideration of the adsorption of the macromolecule on the outer surface of the cylinder showed that at any finite value of the diameter of the cylinder, D, adsorption occurs as an ‘infinite-order’ phase transition. The maximum value of the specific heat

$\text{(C}{}_{\mathrm{p}}\text{max}\text{k}\text{=}\text{25}\text{3}\text{=?}{}^2{}_{\mathrm{c}}$

does not depend on D. The degree of bonding of the macromolecule to the adsorbent increases with decreasing curvature of the surface, rapidly attaining values characteristic of chain adsorption on a plane.     

Dimensions of the average pore,the number of pores,and the surface area of hardened Portland cement paste

In the course of their small angle x-ray scattering work, Winslow and Diamond also calculated the radius of gyration of the pores. Using its value for a paste, Diamond adopted two models for the average pore: a sphere and a cylinder of equal height and diameter. This model leads to absurd values for the surface area of the paste. Using a cylindrical model, and the radius of gyration plus the hydraulic radius, the present authors calculated for a similar paste the dimensions of the average pore and obtained the values: $\text{diameter}\text{= 47.2}\text{A}\text{?}\text{,}\text{length}\text{= 466}\text{A}\text{?}$. The number of pores per gram of paste was 2.26 × 10¹⁷. The paper also discusses the surface area of hardened paste, and points out the extremely important contributions of Winslow and Diamond to the subject.     

Preparation of mesoporous activated carbon fibers by catalytic gasification

A commercial activated carbon fiber with micropores, CH700-20 (Kuraray), was reformed to a mesoporous one by catalytic gasification. The catalytic gasification was composed of two steps: CO₂ pretreatment and air oxidation. Cobalt was used as the catalyst and gasification was performed in the temperature range of 500–700 °C. BET surface area and pore volume of catalytically modified carbon fiber were analyzed by N₂ adsorption. BET surface area of the original CH700-20 was 1,711 m²/g, and the mesopore volume percentage was 11.9%. After catalytic gasification, BET surface area was similar to the original CH700-20, while mesopore volume percentage increased up to 56%. The average pore size of mesopores was 3–4 nm in diameter. The average size of mesopores could be controlled with nanometer resolution by varying the temperature and time of activation.     

The influence of different factors on the porous structure of the positive electrode in lead-acid batteries

Geometric structure measurements of positive electrodes have been determined by measuring cumulative pore volume, pore size distribution, porosity, pore specific surface area and BET specific surface area. The influence of technological parameters such as concentration of SO₄²⁻ ions in the paste, the temperature of the paste and the quality of oxides on the structure of porous positive electrode have been studied. The relation between BET specific surface area and the electrical capacity of the positive electrode is shown and interpreted.     

Changes of pore structure of cement mortars due to temperature

Changes of pore structure of cement mortars caused by high temperatures (up to 900 °C) as well as by extremely low temperatures (down to ? 170 °C) could be proved by means of mercury porosimetry. While high temperatures lead to an increase of the total volume of pores $\text{> 40}\text{A}\text{?}$ low temperatures do not. But in both cases a coarsening of the pore structure occurs. The usefulnes of the test method for detecting structural defects caused by a temperature treatment was shown exemplarily by establishing an experimental relation between the pore volume and the residual strength after low temperature cycles.     

Carbon as a catalyst in oxidation reactions and hydrogen halide elimination reactions

Carbon catalyses many reactions, mainly oxidation reactions with oxygen and with halogens, e.g. $\text{SO}{}_2\text{+}\text{1}\text{20}{}_2\text{→ SO}{}_3$, or CO + Cl₂→ COCl₂. It is known, however, that different carbons behave quite differently in the reduction of oxygen on fuel cell cathodes. Therefore the catalytic activity of carbons has been studied in other reactions. A convenient test reaction is the oxidation of dilute aqueous sulphurous acid. It became apparent that all catalytically active carbons contain small quantities of nitrogen, and inactive carbons such as wood charcoal or carbon blacks can be rendered highly active by treatment with N H₃ or HCN at elevated temperatures. Photoelectron spectra indicate that the catalytic activity increases parallel to the incorporation of a nitrogen species which is pyridine-like, i.e. incorporated in the aromatic layers. Treatment with NH₃ at 900 °C leads also to massive gasification of the carbons, increasing their surface area. Other reactions studied included the oxidation of aqueous oxalic acid and of methanol to formaldehyde. A quite different type of reaction is the elimination of hydrogen chloride from 1-chloroalkanes, e.g. 1 -chlorobutane. Again, activity changes in parallel to nitrogen content. Reaction products are olefins, dimers of the alkyl groups, and a polymer on the catalyst surface. The formation of alkyl dimers, e.g. n-octane in the case of n-butylchloride, suggests that radicals are involved in the reaction.     

氮气吸附法和压汞法测定Al2O3载体孔结构

作为催化剂载体的活性氧化铝,其孔径分布主要集中在中孔范围内,压汞法和氮气吸附法是测定多孔材料比表面积、孔径及其分布的经典方法,采甩压汞仪和比表面积、孔径测定仪对活性氧化铝载体孔结构进行测定。氮气吸附法采用BET原理进行比表面积测定,采用BJH原理进行孔径分布和孔容的测定,压汞法采用Wasburn公式测定比表面、孔径分布和孔容。对两种方法实验结果进行了讨论,认为氮气吸附法更适合氧化铝载体孔结构的测定。     

Influence of the conditions of preparation on the texture of some silica gels

Silica gels were prepared at pH values varying from 1.5 to 8, and at temperatures of 20 to 90 °C. Specific surface areas were evaluated from the nitrogen adsorption isotherms, employing the BET method, t-plots, α_s-plots, and Kiselev's integrai equation. Pore-size distributions as well as cumulative surface areas were determined by the procedure of Cranston and Inkley. An increase in pH causes a decrease in the specific area, and an increase in both the mean pore radius and pore volume of the xerogel. An increase in the BET'S C value appeared with an increase in the mean pore radius and the number of OH-groups (N_OH) of the material.     

Physical adsorption and heat of immersion studies of microporous carbons

Physisorption and heat of immersion measurements have been made with two microporous carbons—a sample of charcoal cloth (BET area, 1250 m² g^?1; pore volume, 0.59 cm³ g^?1 having predominantly narrow micropores and a sample of Amoco carbon (BET area, 3700 m² g^?1; pore volume, 1.9 cm³ g^?1), having a wider distribution of micropores extending into the supermicropore range (pore width, ca. 1–2 nm). The adsorption isotherms of nitrogen and toluene are of Type 1 and reveal that the external surface of both samples is very small. The heat of immersion measurements were carried out with the aid of a Tian-Calvet microcalorimeter and with the following liquids: n-hexane, cyclohexane, neo-hexane, toluene, mesitylene and isodurene. A sample of non-porous graphitized carbon black (Vulcan 3G: BET area, 71.1 m² g^?1) was studied. Immersion calorimetry shows that in spite of the larger adsorptive capacity of the Amoco carbon, the sample of charcoal cloth has a higher adsorption affinity. The differences in the corresponding heats of immersion and in the adsorptive properties indicate that many of the pores in charcoal cloth are slit-shaped, whereas those in the Amoco carbon appear to be more cylindrical in shape.     

Microporous activated carbon spheres prepared from resole‐type crosslinked phenolic beads by physical activation

Microporous activated carbon spheres (ACSs) with a high specific Brunauer–Emmet–Teller (BET) surface area were prepared from resole‐type spherical crosslinked phenolic beads (PBs) by physical activation. The PBs used as precursors were synthesized in our laboratory through the mixing of phenol and formaldehyde in the presence of an alkaline medium by suspension polymerization. The effects of the gasification time, temperature, and flow rate of the gasifying agent on the surface properties of ACSs were investigated. ACSs with a controllable pore structure derived from carbonized PBs were prepared by CO₂ gasification. Surface properties of ACSs, such as the BET surface area, pore volume, pore size distribution, and pore diameters, were characterized with BET and Dubinin–Reduchkevich equations based on N₂ adsorption isotherms at 77 K. The results showed that ACSs with a 32–88% extent of burn‐off with CO₂ gasification exhibited a BET surface area ranging from 574 to 3101 m²/g, with the pore volume significantly increased from 0.29 to 2.08 cm³/g. The pore size and its distribution could be tailored by the selection of suitable conditions, including the gasification time, temperature, and flow rate of the gasifying agents. The experimental results of this analysis revealed that ACSs obtained under different conditions were mainly microporous. The development of the surface morphology of ACSs was also studied with scanning electron microscopy. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

双电层电容器活性炭电极的优化

通过物理方法对双电层电容器用活性炭电极进行改性实验,探讨了活性炭电极的结构（比表面积、孔径分布、孔容）和性能（比电容、充放电特性）的优化问题.改性后活性炭电极BET比表面积从1739.77 m²&#8226;g^-1增至2215.40 m²&#8226;g^-1,其中微孔比表面积增幅22％,中孔比表面积增幅35％,孔容积也有20%～30％的增幅量,孔径分布更为合理.优化的活性炭电极结构改善了电极材料的电化学特性,比电容量可达424 F&#8226;g^-1,增幅10％.     

Preparation and characterization of polymer‐based spherical activated carbons with tailored pore structure

A series of spherical activated carbons (SACs) with different pore structure were prepared from divinylbenzene‐based polymer through CO₂ activation. The effect of activation temperature and retention time on the yield and textural properties of the resulting SACs were studied. The SACs were characterized by N₂ adsorption, X‐ray diffraction, scanning electron microscopy, and aqueous adsorption assays. Either increasing activation temperature or extending retention time decreases the yield of SACs. The BET surface area and pore volume increase with activation temperature and reach a maximum at 1000°C and then decrease at higher activation temperatures. At 1000°C, BET surface area, total pore volume, and mesopore pore volume increase with retention time from 0.5 to 2 h, and meanwhile micropore volume decreases. The micropores are gradually widened into mesopores with increasing activation temperature or extending retention time. SEM and XRD analyses of SAC10 verify the presence of developed porous structure composed of disordered micrographite stacking. Aqueous adsorption assays indicate that SACs have good adsorption capacity for phenol. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Particle structure change of CaO during high temperature sulphatization

The effects of particle size (layer thickness), BET surface area and pore size distribution on the reactivity of CaO derived from limestone were studied by using a pH-stat and the BET method. Visual analysis was obtained with SEM, and X-ray diffraction was used to determine the crystalline components present. It was found that the pore size distribution, including pore volume and pore surface distribution, affected the sulphatization behaviour. The larger pores (D > 50 Å) with their pore volume and pore surface were to some extent responsible for both initial reactivity and ultimate capacity of CaO.     

Activated carbon from sugar cane bagasse by carbonization in the presence of inorganic acids

Dried ground bagasse, impregnated with 50% inorganic acids and carbonized at 500°C, showed the sequence H₃PO₄ > H₂SO₄ > HCl > HNO₃, with respect to the efficiency of activation. Treatment with phosphoric acid of various concentrations (30–50 wt%) was followed by carbonization at 300–500°C for 3 h. Pore structure parameters were determined from the low-temperature adsorption of nitrogen, by applying the BET and α_s methods. Activated carbons obtained at low temperatures are essentially microporous with a low degree of mesoporosity. At higher temperatures products of higher surface area and total pore volume with developed mesoporosity and low microporosity are formed. An increase in the period of carbonization leads to a small decrease in both surface area and pore volume. Activated carbons with surface areas > 1000 m² g^?1 and mean pore dimensions around 2·0 nm, suitable for various purposes, are thus obtained.     

Thermal variation of the apparent specific volume of polystyrenes in solution

A new approach to the polymer interactions inside and outside the coil is discussed and we explain the experimental variation of the apparent specific volume of polystyrene in good and in θ-solvents. In cyclohexane we define two very different regions above and below the θ-point. For the region above the θ point we calculate the thermal dilation coefficient, $\text{}\text{?}$ga, subtracting the intramolecular segment density effect. We relate $\text{}\text{?}$ga to the external degrees of freedom of the chain starting from the Prigogine theory of polymer solutions. We first considered the intramolecular segment interactions of the coil in the dilute solution.     

Determination of surface area and porosity of sol–gel derived ceramic powders in the system TiO2–SiO2–Al2O3

The surface characteristics of ternary ceramic powders in the system TiO₂–SiO₂–Al₂O₃ prepared by a sol–gel technique were studied through nitrogen gas adsorption at liquid nitrogen temperature and application of the Brunauer–Emmett–Teller (BET) equation. The total surface area, total pore volume and pore radius of the powders were calculated through the construction of plots relating the amount of nitrogen gas adsorbed V₁ and the thickness of the adsorbed layer t (V₁−t plots). These results permitted the detection of the different types of pores present in the samples. In a first approximation, the V₁−t plots seem to demonstrate the correctness of the results obtained for the total specific area of the investigated samples. The influence of thermal treatment (drying, calcination) on the surface characteristics of the investigated powders was assessed.