Intensive Edge Effects of Nanographenes in Molecular Adsorptions

Graphene has become a primary material in nanotechnology and has a wide range of potential applications in electronics. Fabricated graphenes are generally nanosized and composed of stacked graphene layers. The edges of nanographenes predominantly influence the chemical and physical properties because nanographene layers have a large number of edges. We demonstrated the edge effects of nanographenes and discrimination against basal planes in molecular adsorption using grand canonical Monte Carlo simulations. The edge sites of nanographene layers have relatively strong Coulombic interactions as a result of the partial charges at the edges, but the basal planes rarely have Coulombic interactions. CO₂ and N₂ prefer to be adsorbed on the edge sites and basal planes, respectively. As a result of these different preferences, the separation ability of CO₂ is higher than that of N₂ in the low-pressure region, thereby offering selective adsorptions, reactions, and separations on nanographene edges

Facilitation of Water Penetration through Zero-Dimensional Gates on Rolled-up Graphene by Cluster–Chain–Cluster Transformations

We demonstrate a water penetration mechanism through zero-dimensional nanogates of a single-walled carbon nanohorn. Water vapor adsorption via the nanogates is delayed in the initial adsorption stage but then proceeds at a certain rate. The mechanism is proposed to be a water cluster–chain–cluster transformation via the nanogates. The growth of water clusters in internal nanospaces facilitates water penetration into these nanospaces, providing an intrinsic mechanism for zero-dimensional water

Significant Hydration Shell Formation Instead of Hydrogen Bonds in Nanoconfined Aqueous Electrolyte Solutions

Nanoscale confined electrolyte solutions are frequently observed, specifically in electrochemistry and biochemistry. However, the mechanism and structure of such electrolyte solutions are not well understood. We investigated the structure of aqueous electrolyte solutions in the internal nanospaces of single-walled carbon nanotubes, using synchrotron X-ray diffraction. The intermolecular distance between the water molecules in the electrolyte solution was increased because of anomalously strong hydration shell formation. Water correlation was further weakened at second-neighbor or longer distances. The anomalous hydrogen-bonding structure improves our understanding of electrolyte solutions in nanoenvironments

Faster Sorption of Propylene Compared to Propane Using an Elastic Layer-Structured Metal–Organic Framework (ELM-11)

The separation of propane and propylene is the most energy-consuming and difficult separation process in the petrochemical industry because of their extremely similar physical properties. Separating propylene from propane using sorption can considerably reduce the energy consumed by current cryogenic distillation techniques. However, sorption involves several major challenges. An elastic layer-structured metal–organic framework (ELM-11) exhibited a highly efficient propane/propylene sorption separation, owing to its kinetic properties. Under equilibrium conditions, propane and propylene exhibited similar sorption capacities, gate opening pressures, and heats of sorption. Thus, their separation under equilibrium conditions is impractical. However, the sorption rates of the two gases were considerably different, showing different diffusion coefficients, resulting in a high kinetic selectivity (214 at 298 K) of propylene over propane on ELM-11. This kinetic selectivity is considerably higher than those obtained in previous studies. Thus, ELM-11 is a promising sorbent for separation technologies

Mechanism of Sequential Water Transportation by Water Loading and Release in Single-Walled Carbon Nanotubes

Water in carbon nanotubes (CNTs) displays unique behaviors such as ring-like structure formation, anomalous hydrogen bonds, and fast transportation. We demonstrated the structures and stability of water in loading and release processes using a combination of X-ray diffraction analysis and hybrid reverse Monte Carlo simulations. Water formed nanoclusters in water loading, whereas layered structures were formed in water release. The water nanoclusters formed in water loading were well stabilized in CNTs. In contrast, in water release, the water layers were less stable than the water nanoclusters. The significant stabilization of nanoclusters in water loading and the relatively low stability of water layers in water release suggest easy water loading and release through CNTs, providing sequential water transportation through CNTs

Rapid Water Transportation through Narrow One-Dimensional Channels by Restricted Hydrogen Bonds

Water plays an important role in controlling chemical reactions and bioactivities. For example, water transportation through water channels in a biomembrane is a key factor in bioactivities. However, molecular-level mechanisms of water transportation are as yet unknown. Here, we investigate water transportation through narrow and wide one-dimensional (1D) channels on the basis of water-vapor adsorption rates and those determined by molecular dynamics simulations. We observed that water in narrow 1D channels was transported 3–5 times faster than that in wide 1D channels, although the narrow 1D channels provide fewer free nanospaces for water transportation. This rapid transportation is attributed to the formation of fewer hydrogen bonds between water molecules adsorbed in narrow 1D channels. The water-transportation mechanism provides the possibility of rapid communication through 1D channels and will be useful in controlling reactions and activities in water systems

Kinetics and Structural Changes in CO₂ Capture of K₂CO₃ under a Moist Condition

The capacity and kinetics of CO₂ capture of K₂CO₃ were studied to determine the mechanism for CO₂ sequestration under ambient conditions. Bicarbonate formation of K₂CO₃ was examined by thermogravimetric analysis under various CO₂ concentrations in the presence of water vapor, and the accompanying structural changes of K₂CO₃ were demonstrated by X-ray diffraction (XRD). Morphological variations were observed during the reaction in the presence of different CO₂ concentrations through scanning electron microscopy (SEM). Structural changes and morphological variations, which occurred during the course of the reaction, were then connected to the kinetic and exothermic properties of the CO₂ capture process from XRD and SEM measurements. The XRD results showed that the bicarbonate formation process of K₂CO₃ could be divided into three reactions, such as the formation of K₂CO₃·1.5H₂O from K₂CO₃, the subsequent formation of K₄H₂(CO₃)₃·1.5H₂O from K₂CO₃·1.5H₂O, and the slow formation of KHCO₃ from K₄H₂(CO₃)₃·1.5H₂O. The SEM observations showed that the morphology of the particles at all three stages played a crucial role in the kinetic behavior for CO₂ sorptivity of K₂CO₃. CO₂ capture of K₂CO₃ was inhibited under a concentrated CO₂ atmosphere during the initial stage, consisting of the first and second reactions, but the formation of KHCO₃ from K₄H₂(CO₃)₃·1.5H₂O was thermodynamically favorable upon the increase of the CO₂ concentration

Cooperative Adsorption of Supercritical CH₄ in Single-Walled Carbon Nanohorns for Compensation of Nanopore Potential

High-density CH₄ storage using adsorption techniques is an important issue in the use of CH₄ as a clean energy source. The CH₄ adsorption mechanism has to be understood to enable innovative improvements in CH₄ adsorption storage. Here, we describe the adsorption mechanism, based on CH₄ structure, and stabilities in the internal and external nanopores of single-walled carbon nanohorns, which have wide and narrow diameters, respectively. The adsorption of larger amounts of CH₄ in the narrow nanopores at pressures lower than 3 MPa was the result of strong adsorption potential fields; in contrast, the wider nanopores achieve higher-density adsorption above 3 MPa, despite the relatively weak adsorption potential fields. In the wider nanopores, CH₄ molecules were stabilized by trimer formation. Formation of CH₄ clusters therefore compensates for the weak potential fields in the wider nanopores and enables high-density adsorption and adsorption of large amounts of CH₄

Double-Step Gate Phenomenon in CO₂ Sorption of an Elastic Layer-Structured MOF

A double-step CO₂ sorption by [Cu­(4,4′-bpy)₂(BF₄)₂] (ELM-11) was observed during isothermal measurements at 195, 253, 273, and 298 K and was accompanied by interlayer expansion in the layered structure of ELM-11. The first step occurred in the range of the relative pressure (<i>P</i>/<i>P</i>₀) from 10^–3 to 10^–2. The second step was observed at <i>P</i>/<i>P</i>₀ ≈ 0.3 at the four temperatures. Structural changes in ELM-11 during the CO₂ sorption process were examined by X-ray diffraction (XRD) measurements. The structural change for the first step was well understood from a detailed structural analysis, as reported previously. The XRD results showed further expansion of the layers during the second step as compared to the already expanded structure in the first step, and both steps were found to be caused by the gate phenomenon. The energy for the expansion of the layer structure was estimated from experimental and simulated data

Gas Adsorption Mechanism and Kinetics of an Elastic Layer-Structured Metal–Organic Framework

The gate adsorption mechanism and kinetics of an elastic layer-structured metal–organic framework (ELM), [Cu­(bpy)₂(BF₄)₂]_<i>n</i> (ELM-11), that shows typical single-step CO₂ gate adsorption/desorption isotherms accompanied with dynamic structural transformation in a wide temperature range were investigated. Adsorption of quite a small amount of CO₂ on the external surface of ELM-11 crystals was observed at the pressure just below a gate adsorption pressure and induced a slight structural change in ELM-11. The structural change should start occurring at the outer parts of ELM-11 and transmit to more inner parts with rising pressure. The adsorption provides the stabilization of the framework through the interaction between fluid–solid and fluid–fluid and enables the framework to expand largely along the stacking direction. The CO₂ adsorption rate of ELM-11 is almost comparable to that of Zeolite 5A at around ambient temperatures and shows temperature dependence with an anti-Arrhenius trend: higher adsorption rate with lower temperature