Resorcinol-formaldehyde Cryogels Research Articles

Composition, structure and potential energy application of nitrogen doped carbon cryogels

Resorcinol–formaldehyde (RF) cryogels were synthesized by sol–gel polycondensation of resorcinol with formaldehyde and freeze-drying was carried out with t-butanol. Carbon cryogel (CC) was obtained by pyrolyzing RF cryogels in an inert atmosphere to 950 °C. Nitrogen doped CCs (CCN) were synthesized by introducing melamine into RF precursor mixture solution to obtain nitrogen concentration 2, 6 and 10 wt.%. Material was characterized by elemental analysis, nitrogen adsorption– desorption measurements, scanning electron microscopy (SEM), Raman spectroscopy, FT-IR Spectroscopy. Cyclic voltammetry (CV) was used to investigate capacitive and electrocatalytic properties. Conductivity measurement was also performed. Elemental analysis results confirmed presence of nitrogen in CCN samples in the range from 0.45 to 1.15 wt.%. Raman spectroscopy of the samples showed increase of D and G peak integrated intensity ratio (ID/IG) with nitrogen doping suggesting that the structural disorder as well as edge plane density increase, but according to similar ID/IG values for CCN samples, their share is not directly related to the amount of incorporated N. Characterization by nitrogen adsorption showed that overall specific surface and maximum mesopores are achieved in CCN sample with medium nitrogen concentration. Results of cyclic voltammetry experiments demonstrated maximum capacitance for CCN sample with smallest N wt.% indicating that narrow pore size distribution and high specific surface area are dominant factors to achieve good capacitive behavior. The relatively low doping level of nitrogen reached in CCN samples may be the reason for the incomplete reduction of oxygen to hydroxide and furthermore it turned out that presence of N in the structure of CC had a negligible effect on the otherwise relatively high conductivity of CC.

Effect of pyrolysis temperature and thermal oxidation on the adsorption properties of carbon cryogels

The effect of pyrolysis temperature (500°C and 930°C) and partial oxidation by oxygen (250°C–450°C) of resorcinol–formaldehyde cryogels (RF) and their graphene derivatives (RFGO) on adsorption capacity for Pb(II) and methylene blue were investigated. Increasing temperature of partial oxidation causes increase in adsorption capacity for Pb(II) from 0.09 to 0.20 (RF) and 0.10 to 0.29mmolg−1 (RFGO) for samples pyrolysed at 500°C resp. from 0.20 to 0.23 (RF) and 0.16 to 0.27mmolg−1 (RFGO) for samples pyrolysed at 930°C. Partial oxidation does not affect adsorption capacity for methylene blue. The adsorption capacity of carbon cryogels for both adsorbates increased with higher pyrolysis temperature about 90% for RF and 60% for RFGO. The presence of graphene in the structure does not improve the adsorption properties of prepared carbon cryogels.

Surface characterization of mesoporous carbon cryogel and its application in arsenic (III) adsorption from aqueous solutions

Resorcinol–formaldehyde (RF) cryogels were synthesized by sol–gel polycondensation of resorcinol with formaldehyde and freeze-drying was carried out with t-butanol. Carbon cryogel (CC) was obtained by pyrolyzing RF cryogels in an inert atmosphere. Material was characterized by nitrogen adsorption–desorption measurements, field emission scanning electron microscopy (FESEM), determination of isoelectric point (IEP) and point of zero charge (PZC). The As(III) removal from aqueous solutions using CC as adsorbent was studied. The adsorption of As(III) on CC was investigated as a function of As(III) concentration and solution pH. Kinetic and equilibrium behavior were defined. The amount of adsorbed As(III) remained similar within the investigated pH of the solution at equilibrium (pH range 2–11). The adsorption mechanism of As(III) ions was discussed in view of arsenic speciation and cryogel PZC. It mainly involved physical adsorption of neutral arsenic acid molecule and H2AsO3− anion on the CC surface with some electrostatic contribution.

Raman spectroscopy study of carbon-doped resorcinol-formaldehyde thin films

In this work, we investigated the properties of carbon resorcinol-formaldehyde (RF) cryogel thin films. RF cryogels were doped by single-wall carbon nanotubes, graphene and graphene quantum dots. The structure of the deposited films was investigated by Raman spectroscopy, and optical, transmission electron and atomic force microscopy. Raman spectroscopy was performed using three excitation laser energies in the visible range. The effect of glass substrate on the Raman features of investigated carbon cryogel thin films was determined as well. The particle size of the carbon-doped RF cryogel thin films was determined by atomic force microscopy.

Preparation of Nitrogen-Doped Porous Carbon and its Water Adsorption Behaviour

In this study, nitrogen-doped porous carbons were prepared by high-temperature carbonization of resorcinol–formaldehyde cryogels, which were synthesized with nitrogen precursor (either urea or melamine) added during the synthesis. The pore structure of the resulting carbon was found to depend on the nitrogen precursor added and its concentration. To quantify the extent of the surface hydrophilicity, which is due to the effect of nitrogen doping, we carried out water adsorption at 298 K, and found that the adsorbed amount of water is proportional to the nitrogen content over the low-pressure region (P/P0 < 0.3). In addition, this amount correlated with the pore structure in the intermediate and high-pressure regions. The adsorption isotherms were analyzed with the Horikawa–Do model to quantify the amount of nitrogen doped during water adsorption.

Effects of temperature on water adsorption on controlled microporous and mesoporous carbonaceous solids

Water adsorption isotherms in porous carbons were measured at temperatures in the range 263 and 298K. To investigate the role of porous structure, we used RF-50, RF-100, RF-200, derived from resorcinol–formaldehyde cryogels, and an activated carbon (B-AC) derived from bamboo. RF-50 is microporous while RF-100 and RF-200 are micro- and mesoporous with non-overlapping distributions of micropores and mesopores. The isotherms of RF-50 exhibit only one step at all temperatures, and they are independent of temperature when plotted against the reduced pressure. On the other hand, the isotherms of RF-100 and RF-200 have two-step uptake at 298K, while they exhibit only one-step uptake at 263K; the first step of the 298K-isotherms is practically the same as the 263K-isotherm. We attributed the first step to adsorption in micropores and the second one to mesopores. The isotherms of B-AC, showing a single broad hysteresis loop, could be viewed as the merging of the two steps that we observed with RF-100 and RF-200 because of the broad pore size distribution of B-AC. From the analysis of all isotherms we developed a universal mechanism for water adsorption in porous carbon, which is based on the temperature dependence of the clustering of water molecules.

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.

Bimodal activated carbons derived from resorcinol-formaldehyde cryogels

Resorcinol-formaldehyde cryogels prepared at different dilution ratios have been activated with phosphoric acid at 450 °C and compared with their carbonaceous counterparts obtained by pyrolysis at 900 °C. Whereas the latter were, as expected, highly mesoporous carbons, the former cryogels had very different pore textures. Highly diluted cryogels allowed preparation of microporous materials with high surface areas, but activation of initially dense cryogels led to almost non-porous carbons, with much lower surface areas than those obtained by pyrolysis. The optimal acid concentration for activation, corresponding to stoichiometry between molecules of acid and hydroxyl groups, was 2 M l−1, and the acid–cryogel contact time also had an optimal value. Such optimization allowed us to achieve surface areas and micropore volumes among the highest ever obtained by activation with H3PO4, close to 2200 m2 g−1 and 0.7 cm3 g−1, respectively. Activation of diluted cryogels with a lower acid concentration of 1.2 M l−1 led to authentic bimodal activated carbons, having a surface area as high as 1780 m2 g−1 and 0.6 cm3 g−1 of microporous volume easily accessible through a widely developed macroporosity.

Characterization of Carbon Cryogel Synthesized by Sol-Gel Polycondensation and Freeze-Drying

Resorcinol-formaldehyde (RF) cryogels were synthesized by sol-gel polycondensation of resorcinol with formaldehyde and freeze-drying carried out with t-butanol. Carbon cryogels were obtained by pyrolyzing RF cryogels in an inert atmosphere. Characterization by nitrogen adsorption showed that carbon cryogels were micro- and mesoporous materials with high surface areas (500 m2/g&lt; SBET &lt;700 m2/g). Cyclic voltammetry experiments at various scan rates (2 to 200 mV s-1) were carried out to study the electrical double-layer charging of carbon cryogel electrodes in 0.5 mol dm-3 HClO4 solution. It has been demonstrated that the total specific capacitance can further be divided into mesoporous and microporous specific capacitance by analyzing the linear dependence of the specific capacitance (C) on the reciprocal of the square root of the potential scan rate ( ν -1/2), as well as linear dependence of the reciprocal specific charge (1/C) on the square root of the potential scan rate ( ν 1/2).

Characterization of carbon cryogels synthesized by sol-gel polycondensation

Resorcinol-formaldehyde (RF) cryogels were synthesized by the sol-gel polycondensation of resorcinol (R) with formaldehyde (F) and freeze-drying was carried out with t-butanol. Carbon cryogels were obtained by pyrolyzing RF cryogels in an inert atmosphere. Characterization by nitrogen adsorption showed that the carbon cryogels were micro and mesoporous materials with high specific surface areas (SBET ? 550 m2/g). Cyclic voltammeter experiments at various scan rates (2 to 200 mV s-1) were performed to study the electrical double-layer charging of carbon cryogel electrodes in 0.5 mol dm-3 HClO4 solution. It has been demonstrated that it is possible to sub-divide the total specific capacitance into the mesoporous and the microporous specific capacitance by analyzing the linear dependence of the charge (q) on the reciprocal of the square root of the potential scan rate (?-1/2), and the linear dependence of the reciprocal charge (1/q) on the square root of the potential scan rate (?-1/2). The specific capacitance was found to be constant over a wide range of sample weight (12.5 to 50.0 _g) and a very promising specific capacitance value of 150 F/g, was found for this material operating in an acidic 0.5 mol dm-3 HClO4 solution at room temperature.

Characterization of carbon cryogel synthesized by sol–gel polycondensation and freeze-drying

Resorcinol-formaldehyde (RF) cryogels were synthesized by sol–gel polycondensation of resorcinol with formaldehyde and freeze-drying was carried out with t-butanol. Carbon cryogels were obtained by pyrolyzing RF cryogels in an inert atmosphere. Characterization by nitrogen adsorption showed that carbon cryogels were micro and mesoporous materials with high surface areas ( 500 m 2/ g<S BET <750 m 2/ g ). Cyclic voltammetry experiments at various scan rates (2–200 mV s −1) have been performed to study the electrical double-layer charging of carbon cryogel electrodes in 0.5 mol dm −3 HClO 4 solution. It has been demonstrated that it is possible to divide further total specific capacitance into mesoporous and microporous specific capacitance by analyzing the linear dependence of the specific capacitance ( C) on the reciprocal of the square root of the potential scan rate ( ν −1/2), and linear dependence of the reciprocal specific charge (1/ C) on the square root of the potential scan rate ( ν 1/2).

Control of mesoporosity of carbon gels prepared by sol–gel polycondensation and freeze drying

Carbon cryogels were prepared by pyrolyzing resorcinol–formaldehyde (RF) cryogels, which were synthesized via the sol–gel polycondensation of resorcinol with formaldehyde in a slightly basic aqueous solution followed by freeze drying. Various RF cryogels were synthesized by changing the amount of reactant, catalyst and water used in the sol–gel polycondensation. The porous properties of RF cryogels were evaluated by nitrogen adsorption method and scanning electron microscopy (SEM). The mesoporosity of RF cryogels was related to the ratio of catalyst to water (C/W). Although the mesopores of RF cryogels were reduced in size to some extent by pyrolysis, the mesoporosity of carbon cryogels was also related to C/W. For the carbon cryogels synthesized under the condition of C/W⩽70 mol/m 3 , the peak radius of the mesopore size distribution, the BET surface area and the mesopore volume were decreased with the increase in C/W. On the other hand, for the carbon cryogels synthesized under the condition of C/W>70 mol/m 3 , it was difficult to prepare mesoporous carbon.

Influence of freeze-drying conditions on the mesoporosity of organic gels as carbon precursors

Resorcinol–formaldehyde (RF) gels were synthesized by sol-gel polycondensation of resorcinol with formaldehyde to prepare mesoporous carbon precursors. RF cryogels were prepared by freeze drying of RF gels, where t-butanol and water were used as solvents. The freezing temperature, the number of repetitive rinse times with t-butanol, and the sample size were changed in the preparation of RF cryogels. Porous properties of the RF cryogels were estimated by N2 adsorption. The influence of freeze-drying conditions was elucidated on the mesoporosity of RF cryogels. Freeze drying of RF gels with t-butanol developed more mesopores than freeze drying with water. The freezing temperature of the gels did not greatly influence the porous properties of the cryogels. If the gels were repeatedly rinsed with t-butanol and freeze-dried in smaller sizes, the cryogels were favorable for use as mesoporous carbon precursors.

PREPARATION OF ORGANIC MESOPOROUS GEL BY SUPERCRITICAL/FREEZE DRYING

ABSTRACT Resorcinol-formaldehyde (RF) aerogels were synthesized by sol-gel polycondensation of resorcinol with formaldehyde in a slightly basic aqueous solution and supercritical drying with carbon dioxide. The control of mesoporous structure of the aerogels was studied by changing the amounts of resorcinol, formaldehyde, distilled water, and sodium carbonate (basic catalyst) used in the polycondensation. As a result of characterization by nitrogen adsorption, the mesopore radius of the RF aerogel was controlled in the range of 2.5 – 9.2 nm. After the hydrogels were immersed in excess of t-butanol, RF cryogels were prepared by freeze drying. The cryogels prepared were mesoporous materials with high surface areas > 500 m2/g and large mesopore volumes > 0.8 cm3/g. Although the surface areas and mesopore volumes of RF cryogels were smaller than those of RF aerogels, the cryogels were useful precursors of mesoporous carbons.

Preparation of mesoporous carbon by freeze drying

Resorcinol–formaldehyde (RF) cryogels were synthesized by sol-gel polycondensation of resorcinol with formaldehyde and freeze drying with t-butanol. The cryogels were characterized by nitrogen adsorption and density measurements. Their porous properties were compared with those of RF aerogels prepared by supercritical drying with carbon dioxide. RF cryogels were mesoporous materials with large mesopore volumes >0.58 cm3/g. Although surface areas and mesopore volumes of the cryogels were smaller than those of the aerogels, the cryogels were useful precursors of mesoporous carbons. Carbon cryogels were obtained by pyrolyzing RF cryogels in an inert atmosphere. Carbon cryogels were mesoporous materials with high surface areas >800 m2/g and large mesopore volumes >0.55 cm3/g. When pyrolyzed, micropores are formed inside the cryogels more easily than inside the aerogels.