Search

The efficiency of CO2 methanation was estimated through gas chromatography in the presence of Co–Fe catalysts. Scanning electron microscopy, X-ray powder diffraction, X-ray photoelectron spectroscopy, and Mossbauer spectroscopy were applied for ex-situ analysis of the catalysts after their test in the methanation reaction. Thermal programmed desorption mass spectroscopy experiments were performed to identify gaseous species adsorbed at the catalyst surface. Based on the experimental results, surface reaction model of CO2 methanation on Co–Fe catalysts was proposed to specify active ensemble of metallic atoms at the catalyst surface, orientation of adsorbed CO2 molecule on the ensemble and detailed reaction mechanism of CO2→CH4 conversion. The reaction step when OH group in the FeOOH complex recombined with the H atom adsorbed at the active ensemble to form H2O molecule was considered as the rate-limiting step.

Multi-walled carbon nanotubes (MWCNTs) have an ordered geometric structure, large specific surface area, prominent electrical conductivity, and unique mechanical properties. They are nontoxic and c...

Because of the low content of chelating groups onto carbon fibers (CFs), their adsorptive parameters are poor, and this has negative effects on their applications as lightweight sorbents. In this work, we established a modification method to incorporate amine groups into carbon fiber surfaces by bromination followed by ammonolysis to create an interfacial layer which can adsorb heavy metal ions from solutions. The changed chemical composition, surface morphology, and thermal stability were investigated. Thermoprogrammed desorption mass-spectrometry and thermal analysis showed thermal transformation and interplay between forms of the grafted bromine groups of 0.5 mmol/g and the resulting amino groups of 0.44–0.56 mmol/g. After grafting, the surface chemistry parameters were improved due to the covalent bonding and grafting of the amine groups as interface modifier. Scanning electron microscopy observation also confirmed that the surface morphology maintains the same, without impairment of fiber properties. This work is therefore a beneficial approach towards enhancing the adsorption parameters by controlling the interface layer of CFs.

The functionalization of the carbon fiber based on polyacrylonitrile with sulfur-containing groups of high acidity was carried out in order to obtain the acid-base processes catalysts. Fibers were treated with sulfur vapors in the temperature range of 400-800°C, followed by surface oxidation with 30% hydrogen peroxide solution. Modified samples were investigated by chemical analysis, thermo-programmed desorption with mass spectrometric registration of products, IR spectroscopy and thermogravimetry. It is shown that the obtained materials contain SO3H-functional groups and oxygen-containing groups (carboxyl, lactone, phenolic, etc.) formed in the surface layer during the oxidation of the fiber surface. The chemical analysis showed that the concentration of sulfur in the samples of the modified fiber is 1.6-6.5 mmol/g. The synthesized samples have a satisfactory thermal stability. The synthesized catalysts were investigated in the model reaction - gas phase dehydration of isopropyl alcohol. It was found that obtained SO3H-containing carbon fibers were catalytically active and had high propylene selectivity. For all the samples obtained there is a complete conversion of alcohol into propylene. The activity of modified carbon fiber samples in the reaction indicated is a fairly high, temperatures of the total conversion of alcohol into propylene are in the range of 160-190°C. During the study of synthesized catalysts in several cycles of catalysis it have been shown that within repeated use (3 cycles) of all modified fiber samples, the yield of propylene does not decrease, the activity remains stable - the temperature of the dehydration reaction remains unchanged or increases insignificantly (by 5-10ºС). The temperatures of complete conversion of isopropyl alcohol in propylene for synthesized catalysts are lower than the temperatures of destruction maxima of surface sulfogroups. Thus, modified carbon fibers can be used as low-temperature catalysts of acid-base processes, in particular dehydration of alcohols.

Here, we have examined the nanoporous activated carbon fibers (ACFs) sulfonated using the direct sulfonation and the staged method that included bromination, followed by sulfidation and oxidation. TEM confirmed the nanoporous structure of the prepared sulfonated ACFs. Nitrogen porometry and 2D nonlocal DFT simulations showed the nanoporosity reduction and variations of the pore size distribution because of the functionalization. Comparison of parameters of the SO3H groups confined in nanopores, e.g., the thermal stability and catalytic potential, showed that the most efficient acid sites, in the catalytic 2-propanol dehydration to propylene, are the SO3H groups grafted by the staged Houben–Weil methods. From the productivity of reactions used at the preparation stage, and in contrast to the one-staged aromatic substitution, the bromine addition to π sites of the edges of carbon matrix supplies enough active sites and is a reason for further high yields of the grafted thermostable SO3H groups. Hydrolysis of the grafted bromine and the surface oxidation of nanopores walls are parallel reactions that lowered the SO3H-related acidity, increasing the total acidity to 1.5 mmol g−1. The reported nanoporous sulfonated ACFs are effective to be used in the dehydration reactions catalyzed by solid acids.

An electrophilic bromination is used as a probe reaction to assess the reactivity of a carbon surface, as well as to prepare versatile precursors for multifunctional modifications. In this chapter, we consider the bromination of nanoporous carbon materials with liquid bromine and KBr3. This bromination was performed under different reaction protocols. Here, we report the parallel oxidation and hydrolysis during bromination and show their effect on surface chemistry. Also, the pros and cons of the usage of the proposed reaction are considered. The surface oxidation and modification rules are discussed, and the amount of introduced bromine that can be substituted by a nucleophile is presented. In general, the potential for obtaining chemically uniform surfaces covered with amino groups by amination of the surface of brominated carbon materials has been shown.

The production of propylene by dehydration of 2-propanol at solid-vapor interfaces, as an example of the acidcatalyzed surface reaction, was explored by using sulfonated polyacrylonitrile-derived activated carbon fibers (PAN-ACFs). To prepare these catalysts, the PAN-ACFs obtained by onestep carbonation-activation were modified with a surface coverage of high acidity. The surface of PAN-ACFs was sulfurated with sulfur vapors at high temperatures followed by oxidation, to yield the corresponding sulfonates. The chemical analysis showed that the carbon surface layer contains from 1.28 to 6.10 mmol$\mathrm{g}^{-1}$ of sulfur, including the contribution of sulfonic $(\mathrm{S}\mathrm{O}{}_{3}\mathrm{H})$ groups. In addition to sulfonic groups, the obtained catalysts contain carboxyl, lactone, and phenolic groups, which are formed as a result of oxidation treatment. In order to understand the changes in surface chemistry and the results of sulfuration and oxidation, the sulfonated PAN-ACFs were characterized by a variety of techniques including TPD MS, TGA, SEM, and FTIR ATR. During a typical temperature mode screening at catalyst testing, it was found that the sulfonated PAN-ACFs prepared by treatment with sulfur vapor at low temperature, at $400\circ \mathrm{c}$ and $500\circ \mathrm{c}$, are very efficient at dehydrating 2-propanol. A weighed mass of 100 mg of each of these catalysts was operated at 165-175°C in 12 hours, and the catalysts were characterized by high conversion of 2-propanol and high selectivity to propylene, with a propylene yield of about 100%. Furthermore, the solid acid catalysts have high stability and strong acidity and can be reused with no significant loss in the activity after the third cycle in the subsequent 20 cycles. The preparation of solid acid catalysts from PAN-ACFs affords a novel strategy for producing propylene through alternative green and sustainable technologies.

Fluorination of nanoporous activated carbon and its subsequent modification with sodium sulfide, which leads to the introduction of sulfur-containing groups in the carbon matrix containing fluorine, was carried out. After oxidation with hydrogen peroxide, sulfo groups are formed in the surface layer of activated carbon, i.e., in this way, it is possible to obtain samples that have both fluorine-and sulfur-containing functional groups. As a result of this work, active and stable catalysts of the reactions catalyzed by acids were obtained. They have a fairly high catalytic activity and thermal stability. When testing the obtained samples in the temperature range up to 250°C under the catalytic 2-propanol dehydration, the yield of propylene remains unchanged for each cycle, the temperature of the dehydration reaction does not change, the activity remains stable for at the least three catalysis cycles, with no deactivation of catalysts.

We proposed the brominated carbon cloth that made of polyacrylonitrile-based activated carbon fibers (PAN-ACFs) as a precursor to chemically and uniformly graft SO3H groups to prepare the solid acid catalyst. The thermal and catalytic properties of the sulfonated PAN-ACFs were examined by IR controlled catalytic measurements and thermal analysis. The catalytic test results showed that the sulfonated surface remarkably improved the operating efficiency in isopropanol dehydration by decreasing the reaction temperature. All PAN-ACFs with grafted SO3H groups prepared through brominated precursors can converse 100% of isopropanol into propylene at moderate temperature. They showed the highest catalytic activity compared to PAN-ACFs sulfonated with oleum and chlorosulfonic acid, which conversed only 40% and 70% of isopropanol into propylene and deactivated at the higher temperatures in the reaction medium.

Multi-walled carbon nanotubes (MWCNTs) were functionalized with residues of either 3-sulfolene or diethyl acetylenedicarboxylate through the Diels-Alder reaction. The structure of the functionalized MWCNTs was characterized by Fourier-transform infrared spectroscopy and by nitrogen adsorption with pore size analysis using the Density functional theory models. Thermogravimetric analysis in an inert atmosphere demonstrates the presence of the grafted pendant groups, which can be active centers for future effective functionalization.

Nanoporous activated carbon fibers (BusofitTM, BACF) showed effective adsorption of carbon tetrachloride (CTC) vapors. About 2.50 mmol × g−1 of chlorine, in the form of trichloromethyl and ...

Adsorption is currently the most promising capture technology to shorten atmospheric emissions of carbon dioxide (CO2). In this article, we report on the adsorption of CO2 onto pristine, oxidized, and aminated activated carbon (AC) sorbents. From our findings, some functionalized AC sorbents have shown very promising results in the CO2 capture process. Their maximum adsorption capacity measured by the thermogravimetric method at 20 °C varies between 2.2 and 3.9 mmol CO2/g depending on the content of diethylamino and oxygen-containing groups. The functionalization of the carbon surface with diethylamino groups improves the adsorption capacity by 30–40%. The CO2 adsorption little depends on the texture parameters of the pristine AC sorbents. In the range from 20 to 100 °C, the CO2 thermodesorption showed the effective regeneration of the sorbents. The aminated carbon surface demonstrates the best CO2 adsorption but binds the adsorbed molecules stronger than the oxidized surface, which limits the sorbent regeneration.

Here we report on the phosphorylation of activated carbon (AC) at 500–800 °C. From thermal, titrimetric, and catalytic data, high surface acidity of the prepared solids is attributed to the total concentration of the protogenic groups. Transmission electron microscopy and nitrogen physisorption measurements showed a chemisorption-induced contraction of microporosity. The most active catalysts prepared at 700 °C contain 1.04 mmol g−1 of the phosphonic groups and supports dehydration of isopropanol with about 100% conversion and 100% selectivity to propylene at 170–180 °C.

The thermal resistivity method, which is a convenient and sensitive to electrical resistance changes, was proposed for control of chemical modification of nanoporous activated carbon fibers (NACF) with hydrofluorocarbons (HFC). The surface state changes could be monitored by measuring Ohmic resistance during heating, physisorption, and chemical reaction between the activated carbon material and HFC. Thus, we found a correlation between the fluorine content in the resulted carbon material and electrical resistance, measured from the room temperature to 900 °C. It was shown that in this way the temperature of the beginning of a chemical reaction and the temperature of the most effective chemical modification could be determined. The thermal grafting of fluororganic groups onto NACF is assumed to be the reason for the changed electrical resistivity of the initial NACF.

Activated carbon fibers originated from viscose was brominated in the dibromine solution. An active surface precursor with 1.0 mmol of bromine per gram of carbon fibers was obtained. The bromine groups are capable for a further replacement, e.g. for amines residues. The thermochemical properties of the synthesized samples were studied. The rules of the surface layer bromination were discussed within the results of the thermal and chemical analysis. Two types of bromine groups on the carbon fiber surface were determined by the thermal desorption. These groups were associated with different centers situated on the accessible surface or placed in tight nanoscale pores. The reason of such distribution was the bromination that producing groups which binding strength varies and the effect of configuration factor. This factor limits the thermal desorption of various gaseous molecules which were formed and released during the thermal decomposition of grafted functionalities and sorbed by the samples with small nanosized pores. The use of the precursor route shows that the interface covered with from 0.62 to 0.95 mmol/g of the residual amine functional groups can be prepared.

Activated carbon fibers (ACFs) made from polyacrylonitrile and viscose were treated with the sulfur vapor at the temperatures of 500–800 °C in an argon flow. Obtained materials were oxidized by 30% H 2 O 2 water solution in order to convert S-containing surface moieties into SO 3 H-groups. The samples were studied using Boehm titration, thermogravimetric analysis and temperature-programmed desorption with a mass spectrometric registration of gaseous desorption products. According to obtained data, the surface layer of synthesized samples contains grafted SO 3 H-groups and different functional groups formed at the carbon oxidation. The decomposition of SO 3 H-groups occurs in the temperature range of 75–590 °C. Two peak maxima at 200±30 °C and 300±30 °C were registered on all SO 2 desorption profiles. These maxima are assigned to two forms of surface SO 3 H groups having a different chemical environment. Testing of the catalytic activity shows that SO 3 H-functionalized carbon fibers are high active in the model reaction of propan-2-ol dehydration. The total conversion of propan-2-ol to propene is observed at 145–220 °C. The lowest temperature of total conversion was registered for the ACFs prepared from polyacrylonitrile being treated with the sulfur vapor at 500 °C. All SO 3 H-functionalized samples maintain their activity after repeated use.

The present study is objected to develop an analytical remote optical diagnostics of the functionalized carbons surface. Carbon composites with up to 1 mmol g−1 of irreversibly adsorbed bromine were produced by the room temperature plasma treatment of an activated carbon fabric (ACF) derived from polyacrylonitrile textile. The brominated ACF (BrACF) was studied by elastic optical scattering indicatrix analysis at wavelength 532 nm. The obtained data were interpreted within results of the thermogravimetric analysis, X-ray photoelectron spectroscopy and temperature programmed desorption mass spectrometry. The bromination dramatically reduces the microporosity producing practically non-porous material, while the incorporated into the micropores bromine induces the dielectric and structural impact on surface polarizability and conductivity due to the charging effect. We have found that the elastic optical scattering in proper solid angles in the forward and the backward hemispheres is sensitive to the kind of the bromine bonding, e.g., physical adsorption or chemisorption, and the bromination level, respectively, that can be utilized for the express remote fabrication control of the nanoscale carbons with given interfaces. Electronic supplementary material The online version of this article (doi:10.1186/s11671-017-1873-7) contains supplementary material, which is available to authorized users.