Synthesis and characterization of nitrogen-doped carbon xerogels

Carbon xerogels were prepared from a nitrogen-containing polymer precursor, using melamine and urea as nitrogen sources incorporated into the polymer matrix using the sol-gel process. To investigate the effects of nitrogen on the texture, morphology and surface chemistry, the carbon xerogels were characterized by nitrogen adsorption, scanning electron microscopy, X-ray photoelectron spectroscopy (XPS), and acid-base titrations. The results showed that nitrogen was incorporated onto the surface as pyridine, pyrrolic/pyridine, quaternary nitrogen, and pyridine-N-oxide. From the deconvolution of the XPS spectra, the pyrrolic/pyridine and quaternary nitrogen functionalities were found to dominate in the samples prepared from urea. All samples showed increased basicity after nitrogen incorporation.     

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.     

Hydrogen adsorption characteristics of activated carbon

Hydrogen adsorption on activated carbons was investigated in the present works up to 100 bars at 298 K. Coconut-shell was activated by potassium hydroxide, resulting in activated carbons with different porosities. All of prepared activated carbons are microporous and show the same adsorption properties. The complete reversibility and fast kinetics of hydrogen adsorption show that most of adsorbed quantity is due to physical adsorption. A linear relationship between hydrogen adsorption capacity and pressure is obtained for the all samples regardless of their porosities. Hydrogen adsorption capacities are linear function of porosities such as specific surface area, micropore surface area, total pore volume, and micropore volume. The maximum hydrogen adsorption capacity of 0.85 wt.% at 100 bars, 298 K is obtained in these materials.     

Hydrogen adsorption studies on single wall carbon nanotubes

Hydrogen adsorption data on as-grown and heat-treated single walled carbon nanotubes (SWNTs) obtained by a volumetric procedure using a Quantachrome Autosorb-1 equipment are presented. The amounts of hydrogen adsorbed at atmospheric pressure reach approximately 0.01 wt.% at 298 K and 1 wt.% at 77 K. The isosteric heat of adsorption has been calculated for both samples from H₂ equilibrium adsorption data at three temperatures, having initial values of 7.42 and 7.75 kJ mol⁻¹. Studies in porous structure by N₂ adsorption and density measurements in helium pycnometer are reported.     

Synthesis of benzxazine-based nitrogen-doped mesoporous carbon spheres for methyl orange dye adsorption

In this work, nitrogen-doped mesoporous carbon spheres (NMCS) were synthesized through a hard template method by using benzoxazine resin as precursor and ordered mesoporous silica spheres as template. The obtained N-doped mesoporous carbons were amorphous spherical nanoparticles with worm-like mesoporous channels and possessed high surface area of 789 m²/g, large pore volume of 0.49 cm³/g and high nitrogen content of 3.50 wt.%. The adsorption capacity of methyl orange (MO) by NMCS could attain 352.1 mg/g at an optimal condition, while the adsorption capacity of MO by non-doped mesoporous carbon spheres (MCS) was 251.9 mg/g at the same condition. The adsorption process fitted the pseudo-second-order kinetic model and the Langmuir isotherm well. Thermodynamic analysis indicated that the removal of MO by NMCS was spontaneous, endothermic and feasible process. In addition, the adsorption capacity of regenerated adsorbent was 89.04% of the initial level after four regeneration cycles.     

离子液体自模板合成多孔碳氮材料及其对二氧化碳的吸附

以多种氰基离子液体为前驱体,采用高温碳化法直接制备多孔碳氮材料,系统考察了离子液体前驱体阳离子结构、阴离子种类及合成条件等因素对碳化材料比表面积、氮元素含量及氮种类的影响,并研究其对CO2的吸附性能。结果表明,阴离子在聚合过程中起模板剂的作用。合成材料主要呈介孔结构,比表面积最高达732.6 m2/g,氮含量最高为9.9wt%,在温度25℃、压力1.8 MPa条件下,CO2的吸附量最高达20.9wt%。多孔碳氮材料经180℃真空加热后可完全脱附再生,再生稳定性良好。     

Synthesis of very highly dispersed platinum catalysts supported on carbon xerogels by the strong electrostatic adsorption method

Highly dispersed Pt/carbon xerogel catalysts are obtained by applying the “strong electrostatic adsorption” (SEA) of hexachloroplatinic acid to carbon xerogels (PZC = 9.4) and platinum tetraammine chloride to oxidized carbon xerogels (PZC = 2.4). After the reduction step, all these Pt/carbon xerogel catalysts display a very high level of metal dispersion: very small platinum particles (1.1–1.3 nm) are observed by TEM. Pt particle sizes obtained by CO chemisorption are in good agreement with TEM micrographs, which shows that the metal is accessible to reactants. These Pt/carbon xerogel catalysts are very active for the hydrogenation of benzene into cyclohexane.     

Preparation of nitrogen-doped porous carbon by ammonia gas treatment and the effects of N-doping on water adsorption

Nitrogen-doped porous carbons (N-RFCC) were prepared by NH₃N₂ mixture gas treatment at high temperature during the carbonization process on resorcinol–formaldehyde cryogels. To show the role of N-doping on the adsorption behavior we carried out water adsorption, and it was found that the amount of water adsorbed is directly related to the nitrogen content over the low pressure region (P/P₀ < 0.3). Applying the theoretical water adsorption model, Horikawa–Do (HD) model, to the adsorption isotherms of N-RFCCs, we could analyze the effects of nitrogen-doping on the adsorption mechanism. Although the concentration of functional groups of N-RFCC is almost equal to that of the non-doped RFCC, which was measured by Boehm titration method, the water adsorbed amounts of N-RFCCs over the low pressure region were larger. This is due to part of the doped nitrogen atoms act as functional groups, contributing to the total concentration of functional groups. The saturated concentrations depend on the packing fraction of water molecules, which in turn depends on the pore size. The packing fractions of N-RFCCs are larger than those of RFCCs, and this could be attributed to the high affinity between water clusters and N-doped surfaces, resulting in a reduced hydrophobicity of the surface.     

Hydrogen adsorption on functionalized graphene

The hydrogen adsorption on basal graphite planes functionalized by hydrogen atoms is studied by molecular modeling and numerical simulation at temperatures of 77 K and 293 K up to high pressure. At 77 K and pressure of 1 MPa, on such an adsorbing surface, the excess hydrogen physisorption is estimated equal to . At 293 K and 30 MPa, the excess physisorption reaches . A comparison between the hydrogen adsorption properties of functionalized graphite basal planes and plain graphite basal planes is presented for materials exhibiting similar porosities.     

Hydrogen adsorption on single-walled carbon nanotubes studied by core-level photoelectron spectroscopy and Raman spectroscopy

We have investigated the adsorption of atomic hydrogen on vertically aligned carbon nanotube (CNT) films using in situ synchrotron-radiation-based core-level (CL) photoelectron spectroscopy and Raman spectroscopy. From C 1s CL spectra, we identified a CL peak component due to C-H bonds of carbon atoms in single-walled carbon nanotubes (SWCNTs). We also found the suppression of π-plasmon excitation, indicating that the hydrogen adsorption deforms the bonding structure. Raman spectra of the SWCNT film indicated that the radial-breathing-mode intensities of SWCNTs decreased due to the adsorption-induced bonding-structure deformation. Moreover, the decrease for small-diameter SWCNTs was more severe than that for large-diameter SWCNTs. Our results strongly suggest that the hydrogen adsorption, which induces the structure deformation from sp² to sp³-like bonding, depends on the diameter of SWCNTs.     

Hydrogen adsorption/desorption behavior of multi-walled carbon nanotubes with different diameters

Multi-walled carbon nanotubes (MWNTs) with different mean outer diameters in the range of 13-53 nm, synthesized by the catalytic decomposition of hydrocarbons using a floating catalyst method, were purified and pretreated with the same procedure for volumetric hydrogen adsorption/desorption measurements. It was found that the hydrogen storage capacity of the purified and pretreated MWNTs was proportional to their diameter, and that hydrogen in all types of MWNTs measured could not be completely desorbed at room temperature and ambient pressure. A possible mechanism for the above behavior was proposed based on the results of cryogenic nitrogen adsorption analysis and high-resolution transmission electron microscopy observations. It was considered that small “carbon islands” might be the main hydrogen adsorption site in MWNTs. The effects of metal catalyst as well as an etched cavity on the surface of MWNTs on the hydrogen adsorption/desorption of MWNTs were also discussed.     

Efficient separation of nitrogen-doped carbon nanotube cups

Nitrogen-doped carbon nanotubes synthesized from a mixture of ferrocene, xylenes, and acetonitrile using chemical vapor deposition technique comprise compartmented hollow structures resembling stacked cups. Separating stacked nitrogen-doped carbon nanotube cups (NCNCs) into individual cups is an attractive way to further manipulate their morphology, providing additional opportunities for applications of NCNCs. Here we demonstrate an effective, simple, and low-cost separation method to obtain individual NCNCs around 180 nm in length, by sonicating as-synthesized stacked NCNCs in concentrated KCl solution. Another advantage of this technique is that it is highly selective to the separation of adjacent NCNCs and capable of preserving the surface structure and functionalities of stacked NCNCs. Here we demonstrate that the presence of potassium ions is vital for the separation and we hypothesize that the separation mechanism involves a partial intercalation that facilitates the separation of NCNCs.     

Easy and controlled synthesis of nitrogen-doped carbon

A novel synthesis of N-doped carbon with not only a high surface area (∼1000 m² g⁻¹) but also with a controlled amount of N-doping is reported from the solvothermal reduction of hexachlorobenzene (HCB) and pentachloropyridine (PCP), without the emission of harmful byproducts. In the presence of metallic sodium as a reducing agent, the N-doping amount can be regulated up to 0.12 of N/C atomic ratio by simply altering the initial HCB and PCP ratios. The mechanism is proposed where the chlorine in the HCB and PCP is reacted with metallic sodium by producing NaCl, and the N-doped carbon is synthesized as the activated carbon edges of C₅N and C₆ rings being bonded together. The surfaces of the prepared N-doped carbons are modified through heat-treatment and this dramatically improves the mechanical and electrical properties. The dominant doping phases of N are pyridinic-N and amide or amine groups; however, the amide or amine groups are eliminated and graphitic-N is newly generated through heat-treatment.     

Heteroatom-doped carbon gels from phenols and heterocyclic aldehydes: Sulfur-doped carbon xerogels

Sulfur-doped carbon xerogels were obtained through carbonization of resorcinol/2-thiophenecarboxaldehyde organic gels. The acid-catalyzed sol–gel polymerization of resorcinol and 2-thiophenecarboxaldehyde leads to organic gels whose morphology and texture is dependent on the amount of catalyst used. As a result, monolithic organic gels with sulfur content of up to 19.6 wt.% and easily tailored properties can be produced. After carbonization, a substantial amount of sulfur is retained and porous carbon xerogels with S-content of up to 10 wt.% are produced (at 800 °C). Depending on the sol–gel synthesis conditions, monolithic S-doped carbon xerogels with controllable and enhanced mesoporosity, surface areas of up to 670 m²/g and enhanced mechanical integrity were obtained. Additional KOH activation of the organic or carbon xerogels enables production of micro–mesoporous carbons with surface areas of up to 2550 m²/g while retaining over 5 wt.% of sulfur. Preliminary CO₂ adsorption measurements were performed. On the basis of resorcinol/2-thiophenecarboxaldehyde gel synthesis a more general approach towards heteroatom-doped carbon gels is proposed: sol–gel polymerization of phenols and heterocyclic aldehydes. Thus a variety of heteroatom-doped porous carbon materials with a tailored pore texture and morphology are available via this procedure.     

Water desorption from resorcinol-formaldehyde hydrogels and adsorption in the resulting xerogels

Water desorption isotherms of resorcinol-formaldehyde (RF) hydrogels and subsequent water adsorption isotherms of the resulting xerogels are determined using the static gravimetric method, at various temperatures. Isotherms obtained from samples synthesized at various pH are compared. Two different mechanisms are involved in RF hydrogels water desorption. At large relative humidity the capillary tension resulting from water removal induces a macroscopic shrinkage of the gel, whereas at low humidity water is evaporated with no network deformation. These two mechanisms are analyzed using a plastic deformation model and the Guggenheim-Anderson-de Boer (GAB) model, respectively. Adsorption isotherms of RF xerogels are analyzed using the GAB model.     

氮掺杂改性碳材料的研究进展

通过对碳材料进行氮掺杂改性,可强化碳材料的性能从而拓宽其应用领域。总结了近年来国内外对氮掺杂改性碳材料的研究进展,详细介绍了利用活化法、水热法、化学气相沉积法、模板法、溶胶-凝胶法和后处理法制备氮掺杂改性碳材料的方法,对其在催化剂、吸附材料、超级电容器、储氢等领域上的应用作了详细介绍,对氮掺杂改性碳材料的发展趋势进行了展望。     

Hydrogen adsorption and storage on porous materials

The development of safe and efficient methods of hydrogen storage is a prerequisite for the use of hydrogen with fuel cells for transport applications. In this paper, results available for adsorption of hydrogen on porous materials, ranging from activated carbons to metal organic framework materials, are discussed. The results indicate that up to 5 and 7.5 wt% of hydrogen can be stored on porous carbon and metal organic framework materials, respectively, at 77 K. The amounts of hydrogen adsorbed on porous materials at ambient temperatures and high pressures are much lower (0.5 wt%). The strong temperature dependence of hydrogen physisorption on porous materials is a limitation in the application of this method for hydrogen storage in addition to storage capacity requirements.     

One-pot synthesis and characterization of metal phosphide-doped carbon xerogels

Transition metal phosphide (Fe₂P, Co₂P and Ni₁₂P₅)-doped carbon xerogels were synthesized by a one-pot pyrolysis of the sol–gel polymer of resorcinol and formaldehyde in the presence of metal nitrates and (NH₄)₂HPO₄. Various techniques, including: X-ray diffraction, N₂ adsorption–desorption, scanning electron microscopy, transmission electron microscopy and Mössbauer spectra, were employed for characterizing the physico-chemical properties of this kind of phosphide–carbon composites. Comparing with the corresponding metal-doped carbon xerogels, metal phosphides can exert a different influence on the graphitization temperature, textural properties and surface morphology of the resultant carbon xerogels. Investigations on the mechanism of iron phosphide formation indicated that CH₄, CO and C acted probably as reducing agents in the carbothermal reduction of metal phosphates. In addition, Mössbauer studies confirmed that this one-pot synthesis method was an excellent way for preparing high purity metal phosphide-doped carbon xerogels.