Hetero-atoms as activation centers for hydrogen absorption in carbon nanotubes

Carbon materials should have specific centers for hydrogen adsorption/absorption. The role of heteroatom substitution in carbon nanotubes as an activator has been identified by Density Functional Theory. The effect of various hetero-atoms like nitrogen, phosphorus, sulphur and boron for hydrogen activation and their geometrical positions has been recognized as the one of the possible reasons for easy hydrogenation. Experimentally, nitrogen and boron containing carbon nanotubes have been synthesized by using template method. The hydrogen absorption capacity of these materials has been evaluated. It is shown that, there is a need to stabilize nitrogen in the carbon nanotube framework for reproducible hydrogen uptake. In the case of boron containing carbon nanotubes, two different chemical environment of boron facilitates hydrogen interaction. They exhibit a maximum of 2 wt.% of hydrogen storage capacity at 80 bar and 300 K. This configuration has a bearing in hydrogen sorption characteristics.     

Hydrogen storage on chemically activated carbons and carbon nanomaterials at high pressures

Hydrogen adsorption measurements have been carried out at different temperatures (298 K and 77 K) and high pressure on a series of chemically activated carbons with a wide range of porosities and also on other types of carbon materials, such as activated carbon fibers, carbon nanotubes and carbon nanofibers. This paper provides a useful interpretation of hydrogen adsorption data according to the porosity of the materials and to the adsorption conditions, using the fundamentals of adsorption. At 298 K, the hydrogen adsorption capacity depends on both the micropore volume and the micropore size distribution. Values of hydrogen adsorption capacities at 298 K of 1.2 wt.% and 2.7 wt.% have been obtained at 20 MPa and 50 MPa, respectively, for a chemically activated carbon. At 77 K, hydrogen adsorption depends on the surface area and the total micropore volume of the activated carbon. Hydrogen adsorption capacity of 5.6 wt.% at 4 MPa and 77 K have been reached by a chemically activated carbon. The total hydrogen storage on the best activated carbon at 298 K is 16.7 g H₂/l and 37.2 g H₂/l at 20 MPa and 50 MPa, respectively (which correspond to 3.2 wt.% and 6.8 wt.%, excluding the tank weight) and 38.8 g H₂/l at 77 K and 4 MPa (8 wt.% excluding the tank weight).     

Study of hydrogen storage by carbonaceous material at room temperature

Recently, many studies have been reported about a variety of carbon materials in adsorbing hydrogen. Regarding that, hydrogen adsorption in different carbonaceous materials was investigated at room temperature, 298 K and three different pressures which were 6.5, 8.5 and 9.5 bar. Pressure drop of hydrogen was measured and the amount it adsorbed was calculated by using ideal gas law and it was presented in weight percent, wt.%. In this paper, the effect of a purification process on hydrogen adsorption was also discussed. Along with that, pretreatment also gave a major influence in hydrogen adsorption because it affected the adsorption behavior of the carbon nanotubes surfaces. The highest result obtained during this work was 0.195 wt.% for purified carbon nanotubes.     

Hydrogen adsorption in different carbon nanostructures

Hydrogen adsorption in different carbonaceous materials with optimized structure was investigated at room temperature and 77 K. Activated carbon, amorphous carbon nanotubes, SWCNTs and porous carbon samples all show the same adsorption properties. The fast kinetics and complete reversibility of the process indicate that the interaction between hydrogen molecules and the carbon nanostructure is due to physisorption. At 77 K the adsorption isotherm of all samples can be explained with the Langmuir model, while at room temperature the storage capacity is a linear function of the pressure. The surface area and pore size of the carbon materials were characterized by N₂ adsorption at 77 K and correlated to their hydrogen storage capacity. A linear relation between hydrogen uptake and specific surface area (SSA) is obtained for all samples independent of the nature of the carbon material. The best material with a SSA of 2560 m²/g shows a storage capacity of 4.5 wt% at 77 K.     

Characterization and adsorption performance of waste-based porous open-cell geopolymer with one-pot preparation

Porous geopolymer foams are promising lightweight materials combining strong strength and adsorption properties. A waste-based porous open-cell geopolymer (POG) was synthesized by one-pot method and investigated in terms of unconfined compressive strength (UCS), pore distribution and adsorption ability. This paper investigates the effect of preparation conditions (raw materials and stabilizing/foaming agents proportion, modulus, curing temperature) on the performance of POG. Results indicated that POG was successfully prepared by industrial wastes (blast furnace slag, BFS) and municipal wastes (water treatment residue, WTR). The appropriate range of conditions were determined for the preparation of POG (H₂O₂ = 1.50 ~ 2.50 wt%, K12 = 1.50 ~ 2.00 wt%, modulus = 1.25 ~ 2.00, and temperature = 60 ~ 70 °C). Under these conditions, the UCS in the range 1.77 ~ 4.77 MPa, and the total porosity in the range 35.19 ~ 69.97 vol%. The extreme environments resulted in the form of instable structure and discontinuous pore structure. The statistical results demonstrated that the total porosity, mean diameter, and max diameter of POG are significantly negative correlated with UCS, and the relationship of total porosity and UCS can be described by Ryshkevith (R²=0.8459) and Schiller model (R²=0.8689). Compared to the geopolymer bulk, POG showed significant adsorption advantage for heavy metal cations and cationic dyes, and the adsorption removal rates of POG for Cd²⁺, Cu²⁺, Pb²⁺, and MB rising to 92.25%, 119.80%, 110.77%, and 163.98%, respectively. The adsorption mechanisms are mainly based on the negative charge of [AlO₄]^- tetrahedron and cation exchange between heavy cations and Na⁺ or Ca²⁺ in internal matrix. This study indicated that the BFS and WTR are feasible solid wastes for the fabrication of POG, which can be applied in the filtration and adsorption fields for contaminants removal.     

Enhancing the thermoelectric performance of cold sintered calcium cobaltite ceramics through optimised heat-treatment

Cold sintering is a promising technology for preparing electronic materials, enabling densification at low temperature, but rarely employed for thermoelectrics. Herein, high-quality Ca_2.7Bi_0.3Co_3.92O_9+δ ceramics were synthesised by a combination of cold sintering and annealing processes. Stoichiometric mixtures of raw materials were calcined once or twice at 1203 K for 12 h in air, and then cold sintered at 673 K for 60 min under a pressure of 85 MPa, followed by annealing at 1203 K for 12 h or 24 h in air. The effects of the calcination processes and annealing conditions on the thermoelectric performance of cold sintered samples were investigated. By optimising heat-treatment, the formation of secondary phases, texture development and porosity were controlled, leading to enhanced electrical conductivity and reduced thermal conductivity. Consequently, at 800 K there was 85% increase in power factor and 35% increase in ZT (value of 0.15) compared to previous studies.     

Grain size effect in conductive phosphate / carbon nanotube ceramics

Composite materials based on aluminium phosphate matrix with different grain sizes and small inclusions of carbon nanotubes were studied by means of broadband dielectric spectroscopy in a wide frequency (20 Hz to 36 GHz) and temperature (25–600 K) ranges. The highest electrical percolation threshold was observed for ceramics with the grain size of 0.8 µm, which is higher than the carbon nanotubes cluster size. The electrical transport in ceramics occurs due to the thermal activation at higher temperatures (above room temperature) and the tunneling at lower temperatures. The potential barrier for electron hopping is the lowest in nanosized ceramics. The distance for electron tunneling is also lowest in nanosized ceramics. The electrical properties of ceramics are stable up to 560 K.     

Investigation of photocatalytic and luminescence properties of Na0.5Ce0.5WO4 self-assembled photocatalysts under solar light irradiation: Morphological,temperature and pH role

Tungstate-based scheelite structures have attracted much attention for the photocatalytic, adsorption and luminescence. To improve their performance, several ways have been considered, such as morphology control, thermal treatment and nanostructuring materials. In this work, three uniform and homogeneous morphologies, such as spindles, spheres and flowers, of self-assembled three-dimensional Na_0.5Ce_0.5WO₄ were used as photocatalysts for methylene blue dye photodegradation under solar irradiation. Depending on morphology, they required different temperatures to reach crystallization. Thermal treatments at 500 °C and 800 °C resulted in changes in crystallite size, porosity, surface state, but also in bandgap and emission properties. Thus, the crystallite sizes are about 50 nm for samples (spindles and flowers) treated at 500°Cand 87–167 nm for those treated at 800 °C. Their respective bandgap values measured by diffuse reflectance were 2.85 eV beyond 3.15 eV. The samples treated at 500 °C showed a lower emission and a longer charge carrier lifetime. A strong trend to adsorption was revealed, especially at low pH value and for the samples treated at 500 °C, reaching 100% at a pH value of 2.5. With decreasing pH, the photocatalysis activity increases (up to 50%), being also more efficient with catalysts treated at low temperature. It follows that the degradation efficiency of spindles treated at 500 °C is clearly higher compared to other morphologies treated at different temperature, and suitable for solar photocatalysis.     

Cyclic voltammetric and FTIR studies of powdered carbon electrodes in the electrosorption of 4-chlorophenols from aqueous electrolytes

Several types of carbon materials (activated carbon, carbon black, multiwalled carbon nanotubes) differing in porosity and surface chemistry were used to prepare powdered electrodes. Activated carbon (Norit R3-ex) was demineralized and modified by oxidation with conc. HNO₃, heat treatment in NH₃ at 900 °C or heat treatment in argon at 1800 °C. Carbon black (Vulcan XC72) was flushed with an organic solvent, while the MWCNTs were functionalized to the hydroxyl and carboxyl forms. Nitrogen adsorption isotherms were used to characterize the pore structure of these materials. Their surface chemistry was assessed using thermogravimetry (TG), elemental analysis, FTIR, EDS and XPS. The ability to adsorb (isotherms) 4-chlorophenol (4-CP) in aqueous solution was determined. Cyclovoltammetric (CV) measurements of powdered carbon electrodes were carried out for blank electrolyte solution (0.1 M Na₂SO₄) and with different concentrations of 4-CP. Changes in the electric double layer capacity and other electrochemical parameters were estimated from the CV curves. The dependence of the electrochemical behavior of a powdered carbon bed on porosity and surface chemistry is analyzed and discussed. The electrochemical properties were related to chlorophenol adsorption ability and FTIR spectral analysis of the adsorption layer.     

Usefulness of CO2 adsorption at 273 K for the characterization of porous carbons

Characterization of microporous solids (activated carbons and carbon molecular sieves) has been carried out by N₂ (subatmospheric pressures) and CO₂ adsorption (at subatmospheric and high pressures) at 77 and 273 K, respectively. Because the relative fugacity range covered by our CO₂ study is similar to the relative pressure range covered with N₂, a suitable comparison of both adsorptives can be made. The results of such comparison show that both adsorptives give the same micropore size distribution (MPSD) for open porosity activated carbons. This observation confirms that the adsorption mechanism of both adsorptives is similar. However, carbon molecular sieves, with very narrow microporosity, cannot be characterized by N₂ at 77 K, due to the existence of diffusional problems. This is also extensive to many other carbon materials, such as carbon fibers and activated carbons with low degree of activation. As a consequence, in this type of samples, N₂ adsorption at 77 K is useless to determine neither the micropore volumes of the narrowest porosity nor their micropore size distributions (MPSD). In this work, the usefulness of CO₂ for the characterization of carbon molecular sieves and activated carbons with different activation degrees is demonstrated. In addition, examples of applications that cannot be explained from N₂ adsorption but yes by CO₂ are presented. As a result, we strongly encourage the use of CO₂ (i.e. at 273 K) as a complement to N₂ adsorption at 77 K.     

Characterization of single wall carbon nanotubes by nonane preadsorption

The preferential blocking of the interior adsorption sites of single walled carbon nanotubes (SWNTs) by n-nonane is demonstrated. Following adsorption of n-nonane and evacuation for 24 h at 323 K, it was found that interior sites with diameters less than ∼14 Å remained filled with n-nonane, blocking the physical adsorption of N₂ on these sites at 77.3 K. We demonstrate that “nonane blocking” is a very useful technique for nanotube porosity characterization.     

Low-temperature catalytic oxidation of multi-walled carbon nanotubes

When in a pure form, carbon nanotubes are known to be stable in air up to ∼800 K making them attractive for a large variety of applications. In this work, we report a significant decrease of ignition temperature (in some cases occurring at ∼500 K) and a reduction in the apparent activation energy for oxidation in air as a result of impregnation with nanoparticles (<2 nm) of metal (Pt, Pd, Ni and Co) acetylacetonates or by decoration with corresponding oxides. Surprisingly, defects introduced by partial oxidation of the carbon nanotubes do not in practice have any influence on the enhancement of further oxidation. Reduction temperatures of metal oxides with H₂ were close to those of other carbon supported catalyst materials. However, the carbon nanotubes showed a tendency for low temperature gasification in the presence of hydrogenation catalyst metals (Pt, Pd).     

Eggshells as agro-industrial waste substitute for CaCO3 in glass foams: A study on obtaining lower thermal conductivity

In this work, discarded glass bottles (GB) and eggshells (ES) were used to produce foam glass designed for thermal insulation. The literature on the thermal conductivity of foam glasses produced with eggshells is sparse. This material was used as pore-forming agent at 3% and 5% weight fractions to obtain a foam glass with low thermal conductivity. Homogenized powders were uniaxially pressed, and the compacts were fired at three temperatures (800, 850, and 900°C). Raw materials were characterized by chemical analysis and particle size distribution. The foam glasses were characterized by their porosity, phases, compressive strength, and thermal conductivity. The best insulating properties were obtained for the composition containing 5 wt% ES fired at 800°C. This sample displayed a porosity of 91.4% while its thermal conductivity was of 0.037 W/m.K, with a compressive strength of 1.12 ± 0.38 MPa. Crystalline phases were observed in samples fired at 850 and 900°C as a result of the devitrification process. The final properties of the materials are comparable to those of commercial foam glasses obtained from non-renewable, more expensive raw materials, a great indicator that the studied compositions could be used as an environmentally friendly substitute.     

A facile route for the preparation of silica foams using rice husk ash

This work aims to synthesize silica foam with around 75 vol. % open porosity without using any additive or pore forming agents, in order to prevent the generation of greenhouse gases during pore formation in the silica matrix. Waste rice husk ash (RHA) derived silica is used as a silica source, which is extracted through the alkali extraction synthesis route. Several physical characterizations such as X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM), differential thermal-thermogravimetric (DTA-TGA) and FTIR analysis have been done for RHA extracted silica. Silica foam specimens are fabricated through control compaction pressure and at low foaming temperature. Samples which are fired at 550°C for 30 minutes exhibit both a adequate apparent porosity (AP; ~75.82%) and significant compressive strength (~1.54 MPa). It can also be observed that the porosity and strength values are changed with the variation in compaction pressure and foaming temperature.     

Aerogel-like ceramic foams with super-high porosity and nanoscale cell wall from sol nanoparticles stabilized foams

Ultralight ceramic foam materials with high porosity play an important role in increasingly hi-tech areas due to the combinative merit of ceramic material and highly porous structure. So far, it remains challenging to fabricate alumina ceramic foams with extremely high porosity and high specific surface area that are comparable to aerogel materials by employing a low cost, eco-friendly and convenient approach. For the first time, we propose the preparation of aerogel-like ceramic foams with nanoscale cell wall and unprecedentedly high porosity using boehmite sol as both ceramic source and bubble interface stabilizer, based on sol nanoparticles stabilized foams using sodium lauryl sulfate (SDS) as modifier. The obtained ultra-stable sol foams allow for the achievement of bulk foams with ultrathin cell wall with thickness in the range of 30-90 nm, super-high porosity up to 99%, and large specific surface area of 280 m²/g, which is attributed to the well-organized assembly of nanoparticles at the liquid/air interfaces. This novel foam material demonstrates excellent adsorption ability for polar volatile organic gases (VOCs) due to its extremely high porosity and large specific surface area.     

Reinforcement of epoxy resins with multi-walled carbon nanotubes for enhancing cryogenic mechanical properties

Epoxy resins are widely applied in cryogenic engineering and their cryogenic mechanical properties as important parameters have to be improved to meet the high requirements by cryogenic engineering applications. Carbon nanotubes (CNTs) are regarded as exceptional reinforcements for polymers. However, poor carbon nanotube (CNT)–polymer interfacial bonding leads to the unexpected low reinforcing efficiency. This paper presents a study on the cryogenic mechanical properties of multi-walled carbon nanotube reinforced epoxy nanocomposites, which are prepared by adding multi-walled carbon nanotubes (MWCNTs) to diglycidyl ether of bisphenol-F epoxy via the ultrasonic technique. When the temperature decreases from room temperature to liquid nitrogen temperature (77 K), a strong CNT–epoxy interfacial bonding is observed due to the thermal contraction of epoxy matrix because of the big differences in thermal expansion coefficients of epoxy and MWCNTs, resulting in a higher reinforcing efficiency. Moreover, synthetic sequence leads to selective dispersion of MWCNTs in the brittle primary phase but not in the soft second phase in the two phase epoxy matrix. Consequently, the cryogenic tensile strength, Young's modulus, failure strain and impact strength at 77 K are all enhanced by the addition of MWCNTs at appropriate contents. The results suggest that CNTs are promising reinforcements for enhancing the cryogenic mechanical properties of epoxy resins that have potential applications in cryogenic engineering areas.     

Low-temperature preparation of high-performance porous ceramics composed of anorthite platelets

Platelet-like anorthite based porous ceramics with improved mechanical strength were fabricated via direct gelcasting and firing at 1223-1473 K using CaCO₃, Al(OH)₃, and SiO₂ powders as the raw materials, along with H₃BO₃ and melamine sintering/crosslinking agents. Based on density functional theory calculations, H₃BO₃ promoted the formation of platelet-like anorthite at a relatively low temperature via covering the {130} facet of anorthite and reducing the corresponding adsorption energy, which led to the preferential growth along the a- and b-axes. The optimal amount of H₃BO₃ for the anorthite platelet formation was 0.9 wt%. The porous anorthite sample with an original solid content of 22.0 wt%, after firing at 1373 K, contained 71.0% porosity and exhibited a compressive strength as high as 5.7 MPa, which were comparable or even superior to those of porous anorthite ceramics prepared previously at a much higher temperature (1573-1723 K), indicating that the preparation strategy reported in this paper is feasible in fabricating high-performance porous anorthite ceramics at a much milder condition. The thermal conductivity of the porous anorthite sample at 1073 K was as low as 0.266 W/(m·K), much lower than that (0.645 W/(m·K)) of the control sample, suggesting that the former could be potentially used for thermal insulation at high temperatures.     

Supercritical CO2 and polycarbonate based nanocomposites: A critical issue for foaming

Supercritical carbon dioxide readily induced foaming of various polymers. In that context, supercritical CO₂ was applied to carbon nanotubes based polycarbonate nanocomposites to ensure their foaming. Surprisingly, efficient foaming only occurs when low pressure is applied while at high pressure, no expansion of the samples was observed. This is related to the ability of supercritical carbon dioxide to induce crystallization of amorphous polycarbonate. Moreover, this behaviour is amplified by the presence of carbon nanotubes that act as nucleating agents for crystals birth. The thermal behaviour of the composites was analysed by DSC and DMA and was related to the foaming observations. The uniformity of the cellular structure was analysed by scanning electron microscopy (SEM). By saturating the polycarbonate nanocomposites reinforced with 1 wt% of MWNTs at 100 bar and 100 °C during 16 h, microcellular foams were generated, with a density of 0.62, a cell size ranging from 0.6 to 4 μm, and a cellular density of 4.1 × 10¹¹ cells cm⁻³. The high ability of these polymeric foams to absorb electromagnetic radiation was demonstrated at low MWNT content as the result of the high affinity of the polycarbonate matrix for MWNTs, and therefore to the good MWNTs dispersion.     

Low temperature CO pulse adsorption for the determination of Pt particle size in a Pt/cerium-based oxide catalyst

Unexpectedly large amounts of CO adsorption have resulted from a pulse adsorption experiment at 323 K, giving about 300% Pt dispersion in a Pt/cerium-based oxide catalyst. An in situ diffuse reflectance infrared Fourier transform spectroscopic investigation on a Pt/cerium-based oxide during CO adsorption has revealed that carbonate species on the cerium oxide surface are responsible for the unrealistically large CO adsorption at 323 K, as a result of CO spillover. Lowering the temperature to 195 K considerably diminished the amount of CO adsorption. The size of the Pt particles in the Pt/cerium-based oxide catalyst was determined by CO pulse adsorption at 195 K and showed good agreement with the particle size determined by X-ray diffraction and low energy ion scattering. This indicates that CO pulse adsorption at 195 K is a useful technique to reliably estimate the Pt particle size in a Pt/cerium-based oxide catalyst.