Hydrogen adsorption on nitrogen-doped carbon xerogels

Nitrogen-doped (1.2-4.5 wt%) carbon xerogels were synthesized from carbonization of resorcinol-formaldehyde polymer in an ammonia atmosphere at various temperatures. The textural properties and the chemical nature of nitrogen in the nitrogen-doped carbon xerogels were analyzed by Ar adsorption/desorption isotherms and X-ray photoelectron spectroscopy, respectively. The maximum hydrogen uptakes were measured to be 3.2 wt% at −196 °C and 0.28 wt% at 35 °C. Hydrogen adsorption had a stronger correlation with specific surface area than nitrogen content at the low temperature of −196 °C. At the higher temperature of 35 °C, optimal nitrogen doping enhanced hydrogen adsorption by electronic modification of carbon in agreement with previous theoretical predictions.     

Nitrogen adsorption on carbonaceous materials: A comparison between static and dynamic methods

The purpose of this study was to investigate the influence of the method of adsorption of N₂ at − 196 °C on the isotherm obtained for, and hence derived textural parameters of, a wide series of carbonaceous materials (CM). Two pyrolyzed products, six activated carbons and two carbon blacks were used. The carbonized products were prepared by pyrolysis of cherry stones at 600 or 900 °C in nitrogen atmosphere (P-600, P-900). Three activated carbons were made by activation of P-600 at 275 °C in air and of P-900 at 850 °C in carbon dioxide or steam, whereas the remaining CM were commercial products. The adsorption isotherms for N₂ at − 196 °C were determined by static and dynamic methods in Quantachrome equipments. The CM were further characterized texturally by means of mercury porosimetry and helium and mercury density measurements. Because of the presence of helium in the adsorptive gas stream, the adsorption of nitrogen noticeably decreases for the CM containing micropores obstructed with tarry products (i.e. P-600 and the activated carbon prepared from it by air activation). For the rest of the activated carbons the adsorption increases, as they must possess narrow micropores having easier access to N₂ at − 196 °C. Helium causes a decrease in the degree of interaction between the nitrogen molecules in the gas stream and as a result the diffusion of nitrogen in pores of the adsorbent increases. For the carbon blacks, however, helium hardly affects the adsorption of nitrogen, except for at high relative pressures of this gas. Helium also influences the capillary condensation phenomenon occurring in mesopores. The variation percentages in the micro- and mesopore volumes are as high as 20 and 50, respectively. Such percentages as a rule are higher for the activated carbons.     

The effects of multi-walled carbon nanotubes graphitization treated with different atmospheres and electrolyte temperatures on electrochemical hydrogen storage

Using multi-walled carbon nanotubes (MWCNTs), the present study focuses on their electrochemical hydrogen storage capacities. The results showed that the hydrogen desorption process is composed of two steps with voltages around −0.75 and −0.15 V. Hydrogen adsorption at −0.15 V took place at temperatures above 30 °C, and the amount of energy required for adsorbing hydrogen was 1.68 eV. The hydrogen storage capacity increased with increasing electrolyte temperature from 30 to 60 °C in both steps. The hydrogen storage capacity of the MWCNTs treated at different atmospheres showed that the decrease in the graphitization of MWCNTs led to the increase in hydrogen adsorption. The results also showed that the MWCNTs treated in a CO₂ atmosphere had the highest hydrogen storage capacity at −0.15 V.     

Catalytic filamentous carbon: Structural and textural properties

Catalytic filamentous carbon (CFC) synthesized by the decomposition of methane over iron subgroup metal catalysts (Ni, Co, Fe or their alloys) is a new family of mesoporous carbon materials possessing the unique structural and textural properties. Microstructural properties of CFC (arrangement of the graphite planes in filaments) are shown to depend on the nature of catalyst for methane decomposition. These properties widely vary for different catalysts: the angle between graphite planes and the filament axis can be 0° (Fe-Co-Al₂O₃), 15° (Co-Al₂O₃), 45° (Ni-Al₂O₃), 90° (Ni-Cu-Al₂O₃). The textural properties of CFC depend both on the catalyst nature and the conditions of methane decomposition (T, °C). The micropore volume in CFC is very low, 0.001-0.022 cm³ g⁻¹ at the total pore volume of 0.26-0.59 cm³ g⁻¹. Nevertheless, the BET surface area may reach 318 m² g⁻¹. Results of the TEM (HRTEM), XRD, Raman spectroscopic, SEM and adsorption studies of the structural and textural properties of CFC are discussed.     

Catalytic hydrogen adsorption of nano-crystalline hydrotalcite derived mixed oxides

Nano-crytalline hydrotalcite derived reduced mixed oxides containing magnesium, nickel and aluminium (MNAM) have been synthesized using coprecipitation and showed successfully nickel catalysed reversible hydrogen adsorption using the temperature programmed technique under near ambient conditions. ICP-MS and XRD analysis ensured the adsorbent homogeneity and different crystalline phases of mixed oxides. Morphology and textural properties of mixed oxides have been explored using the FESEM, BET and HRTEM analysis techniques. Nano-crystalline and mesporous reduced mixed oxides exhibited a 3.9 wt% H₂ adsorption capacity in where desorption capacity was 1.9 wt% H₂. Hydrogen adsorbed surface and different phases were analysed by XPS, Raman and FTIR analysis techniques. The hydrogen adsorption enthalpy (ΔH) and entropy (ΔS) changes of reduced mixed oxides were −47.58 kJ/mol and −120.98 J/mol K, respectively, and the promising desorption activation energy of 65 kJ/mol correspond its reversibility as potential energy storage material.     

Synthesis, characterization and energy-related applications of carbide-derived carbons obtained by the chlorination of boron carbide

A nanoporous carbide-derived carbon (CDC) was synthesized by chlorination of boron carbide powder using hydrogen chloride as the reactive gas. The structure and texture of the CDCs were characterized by X-ray diffraction, high-resolution transmission electron microcopy and nitrogen adsorption at 77 K, which confirmed a structural and textural dependence on chlorination temperature and reaction time. The CDC technique to produce porous carbons is very attractive because it can obtain carbons with desired structure and porosity and the CDCs produced here show great potential for energy-related applications. Used as hydrogen storage materials, the hydrogen uptake capacity could reach 1.06 wt.% at 77K and 1 bar. When tested as electrodes for supercapacitors, specific surface capacitance value up to 0.403 F m⁻² and a capacitance retention ratio up to 86% (at a voltage scan rate of 50 mV s⁻¹) could be obtained.     

Improvements to the sintering of yttria-stabilized zirconia by the addition of Ni

We have studied the effect of nickel oxide (NiO) on the sintering of yttria-stabilized zirconia (YSZ) at temperatures from 1100 to 1400 °C. Differences in the densification behaviour were observed between the direct use of NiO powders and Ni metal as precursor. Our results show that with the addition of Ni into YSZ, sintering was completed at 1300 °C instead of 1400 °C, a 100 °C reduction. The addition of Ni also increased the shrinkage rate at 1200 °C from −0.29×10⁻⁶ s⁻¹ to −0.46×10⁻⁶ s⁻¹. Young's modulus of the samples heat treated at 1200 °C measured by microindentation also increased from 26 GPa for YSZ to 65 or 191 GPa for YSZ plus NiO or Ni, respectively. Addition of NiO or Ni also stabilised the cubic phase and promoted grain growth in YSZ during sintering.     

Adsorption of carbon dioxide on hydrotalcite-like compounds of different compositions

The adsorption of carbon dioxide on hydrotalcite-like compounds was investigated. Two different powdered hydrotalcites were used containing the cations nickel and iron. The powdered materials were screened for carbon dioxide adsorption using a thermogravimetric method and it was found that NiMgAl (Sample 1) hydrotalcite has the largest capacity for CO₂, adsorbing 1.58 mmol g⁻¹ at 20 °C, and highest rate of adsorption of up to 0.17 mmol g⁻¹ min⁻¹. This represented an increase of 53% in adsorption capacity, compared with NiMgAlFe (Sample 2). In order to improve the rheological behaviour of hydrotalcite paste for extrusion, hydrotalcite powders were combined with boehmite alumina (70:30 and 50:50 ratios of hydrotalcite:boehmite) before extrusion into pellets suitable for use in a fixed bed adsorber. These pellets were then re-crushed and further tested by thermogravimetric methods. The effects of temperature, composition and pre-treatment of the hydrotalcites on the adsorption of carbon dioxide and nitrogen are reported. At 20 °C, the amount of carbon dioxide adsorbed was between 2.0 and 2.5 mmol g⁻¹ for all the hydrotalcite/alumina samples in this study, although this decayed rapidly with increasing temperature. The results are compared with silica gel as a common sorbent reference, and with literature values. Hydrotalcite/alumina samples have thermal stability and a high adsorption capacity for carbon dioxide over a wide range of temperatures. The composition of the hydrotalcite/alumina pellets investigated in this study has less effect upon the adsorption behaviour compared with the non-calcined hydrotalcite powder, thus allowing a wide choice of pellet compositions to be used.     

Characterisation of activated nanoporous carbon for supercapacitor electrode materials

Commercial nanoporous carbon RP-20 was activated with water vapor in the temperature range from 950 °C to 1150 °C. The XRD analysis was carried out on nanoporous carbon powder samples to investigate the structural changes (graphitisation) in modified carbon that occurred at activation temperatures T ? 1150 °C. The first-order Raman spectra showed the absorption peak at 1582 cm⁻¹ and the disorder (D) peak at 1350 cm⁻¹. The low-temperature N₂ adsorption experiments were performed at −196 °C and a specific surface area up to 2240 m²g⁻¹ for carbon activated at T = 1050 °C was measured. The cell capacitance for two electrode activated nanoporous carbon system advanced up to 60 F g⁻¹ giving the specific capacitance ∼240 F g⁻¹ to one electrode nanoporous carbon ∣1.2 M (C₂H₅)₃CH₃NBF₄ + acetonitrile solution interface. A very wide region of ideal polarisability for two electrode system (∼3.2 V) was achieved. The low frequency limiting specific capacitance very weakly increases with the rise of specific area explained by the mass transfer limitations in the nanoporous carbon electrodes. The electrochemical characteristics obtained show that some of these materials under discussion can be used for compilation of high energy density and power density non-aqueous electrolyte supercapacitors with higher power densities than aqueous supercapacitors.     

The use of exhausted olive cake ash (EOCA) as a low cost adsorbent for the removal of toxic metal ions from aqueous solutions

The removal characteristics of cadmium (Cd(II)) and nickel (Ni(II)) ions from aqueous solution by exhausted olive cake ash (EOCA) were investigated under various conditions of contact time, pH, initial metal concentration and temperature. Batch kinetic studies showed that an equilibrium time of 2 h was required for the adsorption of Ni(II) and Cd(II) onto EOCA. Equilibrium adsorption is affected by the initial pH (pH₀) of the solution. The pH₀ 6.0 is found to be the optimum for the individual removal of Cd(II) and Ni(II) ions by EOCA. The adsorption test of applying EOCA into synthetic wastewater revealed that the adsorption data of this material for nickel and cadmium ions were better fitted to the Langmuir isotherm since the correlation coefficients for the Langmuir isotherm were higher than that for the Freundlich isotherm. The estimated maximum capacities of nickel and cadmium ions adsorbed by EOCA were 8.38 and 7.32 mg g⁻¹, respectively. The thermodynamic parameters for the adsorption process data were evaluated using Langmuir isotherm. The free energy change (ΔG°) and the enthalpy change (ΔH°) showed that the process was feasible and endothermic respectively. As the exhausted olive cake is discarded as waste from olive processing, the adsorbent derived from this material is expected to be an economical product for metal ion remediation from water and wastewater.     

Fabrication and microwave properties of Ni hollow powders by electroless plating and template removing method

Using activated carbon as templates, Ni coated activated carbon powders were prepared by electroless nickel-plating method. Removing the activated carbon templates, nickel hollow powders were prepared. The prepared Ni coated activated carbon powders and nickel hollow powders were characterized by means of X-ray diffraction (XRD), scanning electron microscope (SEM), and Field emission scanning electron microscope (FESEM). The results show that most of the activated carbon template could be eliminated at 210 °C in air or at 700 °C in nitrogen without oxidation of most of the coated nickel. The Ni hollow powders heated in air showed more porous than that of annealed in nitrogen. The dielectric and magnetic loss of Ni hollow powders -paraffin wax composites were measured in the frequency range of 2-15 GHz by reflection/transmission way using network analyzer. The reflection coefficient of Ni hollow powders-paraffin wax absorbers with 2 mm thickness was calculated by transmission line theory, the maximum reflection loss can be up to − 16 dB.     

Effect of NiO content on structural, surface and catalytic characteristics of nano-crystalline NiO/CeO2 system

The NiO/CeO₂ nano-composite catalysts containing different nickel content prepared by impregnation method have been characterized by XRD and TEM. The surface and catalytic properties of Ni/Ce mixed oxide solids were determined by nitrogen adsorption at −196 °C and catalytic conversion of isopropanol at different temperatures. These composites can be described as a mixture of nickel oxide and ceria modified by the insertion of a part of nickel in the ceria lattice. The size of the nickel oxide varies considerably from clusters to a crystallized material, depending on the amount of nickel oxide. From the characterization of the composites, it was concluded: at low Ni loading, the ceria surface is gradually covered with the dispersed NiO species. At higher loading, highly dispersed NiO, well crystalline nickel oxide and Ni-Ce-O solid solution coexist.It was verified that the structural, morphological, surface and catalytic properties could be influenced by nickel loading. This treatment led to a slightly increase in the crystallite size of ceria particles. On the other hand, the augmentation in the nickel content brought about an increase in the crystallite size, lattice constant and unit cell volume of nickel oxide. The nickel loading brought about an increase in the formation of Ni-Ce-O solid solution with subsequent creation of oxygen vacancies.     

The production of a homogeneous and well-attached layer of carbon nanofibers on metal foils

Carbon nanofibers (CNFs) were deposited on metal foils including nickel (Ni), iron (Fe), cobalt (Co), stainless steel (Fe:Ni; 70:11 wt.%) and mumetal (Ni:Fe; 77:14 wt.%) by the decomposition of C₂H₄ at 600 °C. The effect of pretreatment and the addition of H₂ on the rate of carbon formation, as well the morphology and attachment of the resulting carbon layer were explored. Ni and mumetal show higher carbon deposition rates than the other metals, with stainless steel and Fe the least active. Pretreatment including an oxidation step normally leads to higher deposition rates, especially for Ni and mumetal. Enhanced formation of small Ni particles by in situ reduction of NiO, compared to formation using a Ni carbide, is probably responsible for higher carbon deposition rates after oxidation pretreatment. The addition of H₂ during the CNF growth leads to higher carbon deposition rates, especially for oxidized Ni and mumetal, thus enhancing the effect of the reduction of NiO. The diameters of CNFs grown on metal alloys are generally larger compared to those grown on pure metals. Homogenously deposited and well-attached layers of nanotubes are formed when the carbon deposition rate is as low as 0.1-1 mg cm⁻² h⁻¹, as mainly occurs on stainless steel.     

Preparation of Ni/MgO catalyst for CO2 reforming of methane by dielectric-barrier discharge plasma

A novel plasma-treated Ni/MgO catalyst was prepared by treating coprecipitated NiCO₃–MgCO₃ with dielectric-barrier discharge plasma. The results by XRD, TEM and N₂ adsorption analyses showed that the plasma-prepared Ni/MgO catalyst possessed smaller particle size, enhanced nickel dispersion, and higher specific surface area than a conventionally reduced Ni/MgO catalyst. The plasma-prepared Ni/MgO catalyst also exhibited better catalytic activity for carbon dioxide reforming of methane. More than 20% higher conversions of methane and carbon dioxide were obtained than those over the conventional Ni/MgO catalyst at 700 °C and a space velocity of 96,000 mL/(h?g_cat).     

Acid properties of phosphoric acid activated carbons and their catalytic behavior in ethyl-tert-butyl ether synthesis

Carbon catalysts were prepared by phosphoric acid activation of styrene-divinylbenzene copolymer followed by liquid phase oxidation with nitric acid. Their surface properties were modified by heat treatment at 300-600 °C in an argon atmosphere, and their acid properties were characterized by quasi-equilibrium temperature desorption of ammonia and by acid-base titration. The pore structure was characterized by nitrogen adsorption at −196 °C. Their catalytic activity in the synthesis of ethyl-tert-butyl ether (ETBE) from isobutene and ethanol was investigated. It has been shown that the carbon catalysts are active and selective in ETBE synthesis. The main factor determining the catalytic activity of carbon catalysts is the total number of acid surface sites. A linear correlation between total amount of surface sites and catalytic activity was found.     

Magnetically separable bimodal mesoporous carbons with a large capacity for the immobilization of biomolecules

A method for the fabrication of carbon-based mesoporous magnetic composites with a large capacity for the adsorption/immobilization of biomolecules is presented. The composites consist of iron oxide spinel nanoparticles inserted into the pores of templated unimodal or bimodal mesoporous carbons. The deposition of the magnetic iron oxide nanoparticles was carried out following two synthetic routes: (1) the direct incorporation of nanoparticles into the pores of the templated carbons and (2) the insertion of nanoparticles into the mesopores of the carbon-silica composite followed by the selective removal of silica framework. The carbon-iron oxide magnetic composites prepared according to route 2 were found to have better textural properties (larger BET surface areas and pore volumes) and significantly higher capacity for the adsorption of hemoglobin and immobilization of lysozyme. The amounts of hemoglobin or lysozyme adsorbed/immobilized by these materials were 176 mg hemoglobin g⁻¹ support and 131 mg lysozyme g⁻¹ support using route 1 and 430 mg hemoglobin g⁻¹ support and 322 mg lysozyme g⁻¹ support by route 2. Furthermore, we have demonstrated that, when no inorganic nanoparticles are deposited, the bimodal mesoporous carbon shows exceptionally a large immobilization capacity for hemoglobin (830 mg g⁻¹ support) and lysozyme (510 mg g⁻¹).     

Continuous extraction of glycerol acetates from their mixture using supercritical carbon dioxide

Supercritical carbon dioxide was used for partially selective extraction of triacetin from a mixture of triacetin, diacetin, and monoacetin with a molar ratio of 1:2:1. The extraction was carried out in two stages. In the first stage, a central composite design was used to optimize the four variables of pressure, temperature, liquid CO₂ flow rate, and extraction time at three levels using a semi-continuous, supercritical carbon dioxide extraction setup. The composition of the extract under the predicted optimum conditions (i.e., 109 bar, 56 °C, 0.86 mL min⁻¹, and 61 min) was about 69% triacetin accompanied by only 30% diacetin and no detectable monoacetin. In the second stage, the effect of the two factors, pressure (100, 109, and 140 bar) and liquid CO₂ flow rates of 0.86 and 1.5 mL min⁻¹ measured at average laboratory temperature (27 °C) and pressure (0.89 bar), were studied using a continuous, supercritical carbon dioxide fractionation setup equipped with a glass-bead packed column kept under a thermal gradient of 56-70 °C. The experimental design was organized as a 3 × 2 general factorial design. Under the best conditions (i.e., 140 bar and 1.5 mL min⁻¹), the extraction yield of triacetin and diacetin were 41.8 and 3.0%, respectively, without any detectable monoacetin as verified by GC-FID.     

Measurement of through-plane effective thermal conductivity and contact resistance in PEM fuel cell diffusion media

An experimental study was performed to determine the through-plane thermal conductivity of various gas diffusion layer materials and thermal contact resistance between the gas diffusion layer (GDL) materials and an electrolytic iron surface as a function of compression load and PTFE content at 70 °C. The effective thermal conductivity of commercially available SpectraCarb untreated GDL was found to vary from 0.26 to 0.7 W/(m °C) as the compression load was increased from 0.7 to 13.8 bar. The contact resistance was reduced from 2.4×10⁻⁴ m²°C/W at 0.7 bar to 0.6×10⁻⁴ m²°C/W at 13.8 bar. The PTFE coating seemed to enhance the effective thermal conductivity at low compression loads and degrade effective thermal conductivity at higher compression loads. The presence of microporous layer and PTFE on SolviCore diffusion material reduced the effective thermal conductivity and increased thermal contact resistance as compared with the pure carbon fibers. The effective thermal conductivity was measured to be 0.25 W/(m °C) and 0.52 W/(m °C) at 70 °C, respectively at 0.7 and 13.8 bar for 30%-coated SolviCore GDL with microporous layer. The corresponding thermal contact resistance reduced from 3.6×10⁻⁴ m²°C/W at 0.7 bar to 0.9×10⁻⁴ m²°C/W at 13.8 bar. All GDL materials studied showed non-linear deformation under compression loads. The thermal properties characterized should be useful to help modelers accurately predict the temperature distribution in a fuel cell.     

Hydrothermal stability of HZSM-5 catalysts modified with Ni for the transformation of bioethanol into hydrocarbons

The hydrothermal stability of catalysts prepared from HZSM-5 zeolites doped with Ni (by impregnation) has been studied in the transformation of bioethanol into hydrocarbons, in order to remove the main barrier for the use of HZSM-5 zeolite catalysts in this process, which is the irreversible deactivation by dealumination of the zeolite above 400 °C with water in the reaction medium. The main effect of doping is the attenuation of the zeolite acid strength from 135 to 125 kJ (mol of NH₃)⁻¹ for a Ni content of 1 wt.%. The catalysts maintain a high level of activity and a high selectivity of propene and butenes, and Ni doping significantly attenuates irreversible deactivation of the catalyst by dealumination of the zeolite. The zeolite catalyst doped with 1 wt.% of Ni maintains its kinetic behaviour in reaction-regeneration cycles when the reaction step is carried out at 500 °C and with 5 wt.% of water in the feed. This catalyst allows operating at 400 °C without irreversible deactivation with bioethanol containing 75 wt.% of water.     

Hydrogen adsorption on KOH activated carbons from mesophase pitch containing Si, B, Ti or Fe

Different activated carbons with large micropore volume (0.78-0.99 cm³/g) have been prepared by KOH activation of mesophase pitch obtained by co-pyrolysis of a petroleum residue and small amounts of different compounds, triphenylsilane, borane pyridine complex, tetrabutyl orthotitanate or ferrocene. During the preparation, the Ti introduced in the petroleum residue concentrate into the activated carbon, whereas some loss of Si and Fe occurs. The compounds modify the size of mesophase structure formed during the co-pyrolysis process, as well as the apparent height of lamelae stack, L_c, both having an important effect in the development of the porosity of the activated carbon. However, there is a scarce influence of all heteroatoms in the adsorption capacity of H₂ at −196 °C and at 25 °C, which seems to be mainly influenced by the volume and size of micropores of the activated carbon. Only the activated carbon containing Fe adsorbs a higher amount of hydrogen at 25 °C and 10 MPa than the expected one according to its micropore volume.