Ultramicropore Characterization of Microporous Carbons by Low-Temperature Helium Adsorption

Structural mechanism of reactivation with steam of pitch-based activated carbon fibers

Customized micro- and mesoporous carbons are in high demand for ecofriendly technologies. Reactivation of the well-characterized pitch-based activated carbon fiber (ACF) can provide a clear understanding of the structural mechanism of steam activation, which would be helpful for designing better micro- and mesoporous carbons. ACFs were reactivated with steam at 973-1173 K. X-ray diffraction and Raman spectroscopy indicated that the stacking number of graphene-like layers of the pore wall decreased with an increase in the reactivation temperature. The average fiber diameter of the ACFs, which was measured via scanning electron microscopy, decreased with the increase in the reactivation temperature. The relationship between the decrease in the fiber diameter and the burn-off suggested that reactivation above 1023 K produced micropores inside the fiber. A deconvolution analysis of the pore-size distribution revealed the variation of the distribution. The peak difference was approximately 0.3 nm, depending on the reactivation temperature. These results indicate that reactivation with steam proceeds via the preferential one-by-one gasification of less-crystalline graphene-like units.

Activated carbon as catalyst support: precursors, preparation, modification and characterization

The preparation of activated carbon materials is discussed along selected examples of precursor materials, of available production and modification methods and possible characterization techniques. We evaluate the preparation methods for activated carbon materials with respect to its use as catalyst support and identify important parameters for metal loading. The considered carbon sources include coal, wood, agricultural wastes or biomass as well as ionic liquids, deep eutectic solvents or precursor solutions. The preparation of the activated carbon usually involves pre-treatment steps followed by physical or chemical activation and application dependent modification. In addition, highly porous materials can also be produced by salt templating or ultrasonic spray pyrolysis as well as by microwave irradiation. The resulting activated carbon materials are characterized by a variety of techniques such as SEM, FTIR, nitrogen adsorption, Boehm titrations, adsorption of phenol, methylene blue and iodine, TPD, CHNS/O elemental analysis, EDX, XPS, XRD and TGA.

Asphaltene-Derived Metal-Free Carbons for Electrocatalytic Hydrogen Evolution

The design of new and improved catalysts is an exciting field and is being constantly improved for the development of economically, highly efficient material and for the possible replacement of platinum (Pt)-based catalysts. In this, carbon-based materials play a pivotal role due to their easy availability and environment friendliness. Herein, we report a simple technique to synthesize layered, nitrogen-doped, porous carbon and activated carbons from an abundant petroleum asphaltene. The derived nitrogen-doped carbons were found to possess a graphene-like nanosheet (N-GNS) texture with a significant percentage of nitrogen embedded into the porous carbon skeleton. On the other hand, the activated porous carbon displayed a surface area (SA) of 2824 m²/g, which is significantly higher when compared to the nitrogen-doped carbons (SA of ∼243 m²/g). However, the nonactivated N-GNS were considered as an attractive candidate due to their high electrochemical active surface area, the presence of a mixture of porous structures, uniform layers, and effective doping of nitrogen atoms within the carbon matrix. Importantly, the hydrogen evolution reaction activity of the derived N-GNS sample illustrates a significant catalytic performance when compared to that of other nonfunctionalized carbons. Our current finding demonstrates the possibility of converting the asphaltene wastes into a high-value-functionalized porous carbon for catalytic applications.

Ion-Transport Design for High-Performance Na+-Based Electrochromics

Sodium ion (Na⁺)-based electrochemical systems have been extensively investigated in batteries and supercapacitors and also can be quality candidates for electrochromic (EC) devices. However, poor diffusion kinetics and severe EC performance degradation occur during the intercalation/deintercalation processes because the ionic radii of Na⁺ are larger than those of conventional intercalation ions. Here, through intentional design of ion-transport channels in metal-organic frameworks (MOFs), Na⁺ serves as an efficient intercalation ion for incorporation into a nanostructured electrode with a high diffusion coefficient of approximately 10^-8 cm² s^-1. As a result, the well-designed MOF-based EC device demonstrates desirable Na⁺ EC performance, including fast switching speed, multicolor switching, and high stability. A smart "quick response code" display is fabricated using a mask-free laser writing method for application in the "Internet of Things". In addition, the concept of ion transport pathway design can be widely adopted for fabricating high-performance ion intercalation materials and devices for consumer electronics.

Accurate Characterization of the Pore Volume in Microporous Crystalline Materials

Pore volume is one of the main properties for the characterization of microporous crystals. It is experimentally measurable, and it can also be obtained from the refined unit cell by a number of computational techniques. In this work, we assess the accuracy and the discrepancies between the different computational methods which are commonly used for this purpose, i.e, geometric, helium, and probe center pore volumes, by studying a database of more than 5000 frameworks. We developed a new technique to fully characterize the internal void of a microporous material and to compute the probe-accessible and -occupiable pore volume. We show that, unlike the other definitions of pore volume, the occupiable pore volume can be directly related to the experimentally measured pore volumes from nitrogen isotherms.

Helium-3 gas self-diffusion in a nematically ordered aerogel at low temperatures: enhanced role of adsorption

We performed ³He gas diffusion measurements for the first time in a highly porous ordered Al₂O₃ aerogel sample at a temperature of 4.2 K using a nuclear magnetic resonance field gradient technique. A strong influence of ³He adsorption in the aerogel on self-diffusion is observed. The classical consideration of adsorptive gas diffusion in mesopores leads to anomalously high tortuosity factors. The application of a more sophisticated model than the simple combination of empirical two-phase diffusion and the Knudsen gas diffusion models is required to explain our results. Anisotropic properties of the aerogel are not reflected in the observed gas diffusion even at low gas densities where the anisotropic Knudsen regime of diffusion is expected. The observed gas densification indicates the influence of the aerogel attractive potential on the molecular dynamics, which probably explains the reduced diffusion process. Perhaps this behavior is common for any adsorptive gases in nanopores.

Use of Carbon Fibre Composite Molecular Sieves for Air Separation

The adsorption of oxygen, nitrogen and carbon dioxide onto a carbon fibre composite was investigated using static and dynamic techniques. Molecular-sieving effects in the composite were highlighted by the adsorption of carbon dioxide, a more sensitive probe molecule for the presence of micro-porosity in adsorbents. The kinetic studies revealed that oxygen was more rapidly adsorbed on the composite than nitrogen and with a higher uptake under equilibrium conditions. Preliminary experiments indicated that the carbon fibre composite was capable of separating oxygen and nitrogen from air on the basis of the different diffusion rates of the two molecules in the micropore network of the composite. It is proposed that the relatively high electrical conductivity of the carbon fibre composite material could be exploited for air separation by facilitating the production of O2 and N2 through electrical swing adsorption rather than the depressurization of adsorber beds.

2H NMR study of 2D melting and dynamic behaviour of CDCl3 confined in ACF nanospace

Two-dimensional melting of trichloromethane (chloroform) confined in activated carbon fibre was investigated using differential thermal analysis and (2)H NMR techniques. Differential thermal analysis revealed a thermal anomaly with an endothermic peak at 269 K, which was distributed from 250 K to 287 K on the heating direction. This anomaly was also observed upon cooling at the same temperature. Furthermore, (2)H NMR revealed that slow motion such as molecular hopping and/or diffusion of CDCl(3) in ACF affected the spectral line width. The temperature dependence (Arrhenius plot) of the spectral line width showed an inflection point at 227 K. The activation energy of molecular motion of CDCl(3) in ACF was 4 kJ mol(-1) at temperatures greater than 227 K and 7.7 kJ mol(-1) at temperatures less than 227 K. Reduction of the activation energy suggests that the average intermolecular distance between CDCl(3) molecules enlarges above the inflection point. The difference of activation energy (3.7 kJ mol(-1)) is close to the enthalpy of fusion in typical plastic crystals. These results reveal that the thermal anomaly and the transition of dynamic process correspond respectively to melting of CHCl(3) in ACF and the pre-melting phenomenon.

Porous Silica Particles Prepared from Silicon Tetrachloride Using Ultrasonic Spray Method

Silica particles having the median diameter of 1 to 3 µm were prepared from silicon tetrachloride vapor by a reaction with water droplets using the ultrasonic spray method. The particle sizes of silicas were controlled by changing the composition of silicon tetrachloride and water. These silica particles had microporous structures from nitrogen and water adsorption measurements. The microporous and mesoporous particles were prepared from the reaction of water droplets including sodium and potassium carbonates with silicon tetrachloride vapor. Copyright 1999 Academic Press.