Effect of ozone treatment on ammonia removal of activated carbons

In this work, activated carbons (ACs) were modified by ozone treatment to enhance the efficiency of removal of ammonia gas over the ACs. Surface properties of the ACs were confirmed by X-ray photoelectron spectroscopy (XPS) analysis and N2 adsorption isotherms at 77 K were investigated by BET and D-A methods to characterize the specific surface area, total pore volume, and micropore volume. The ammonia removal efficiency was confirmed by the gas-detecting tube technique. The results showed that the specific surface area and micropore volume of ACs were slightly destroyed as the ozone treatment time increased. However, the ozone treatment led to an increase in ammonia removal efficiency of ACs, mainly due to an increase of acid functional groups, such as carbonyl and ether groups, on carbon surfaces. It was revealed that the improvement of ammonia removal efficiency of ACs was greatly affected by the interfacial acid-base interactions between modified ACs and basic ammonia adsorbate.     

Plasma Catalytic Synthesis of Ammonia Using Functionalized-Carbon Coatings in an Atmospheric-Pressure Non-equilibrium Discharge

We investigate the synthesis of ammonia in a non-equilibrium atmospheric-pressure plasma using functionalized-nanodiamond and diamond-like-carbon coatings on α-Al₂O₃ spheres as catalysts. Oxygenated nanodiamonds were found to increase the production yield of ammonia, while hydrogenated nanodiamonds decreased the yield. Neither type of nanodiamond affected the plasma properties significantly. Using diffuse-reflectance FT-IR and XPS, the role of different functional groups on the catalyst surface was investigated. Evidence is presented that the carbonyl group is associated with an efficient surface adsorption and desorption of hydrogen in ammonia synthesis on the surface of the nanodiamonds, and an increased production of ammonia. Conformal diamond-like-carbon coatings, deposited by plasma-enhanced chemical vapour deposition, led to a plasma with a higher electron density, and increased the production of ammonia.     

Ozonation of naphthalenesulphonic acid in the aqueous phase in the presence of basic activated carbons

The present study aimed to explore the possibility of increasing the purification efficacy of ozone in the removal of high-toxicity contaminants by using carbons of basic character and to analyze the mechanism involved in this process. These carbons were prepared by treating a commercial activated carbon (Witco, W) with ammonia (W-A), ammonium carbonate (W-C), or urea (W-U), under high pressure and temperature. The ammonia and carbonate treatments slightly increased the mesoporosity and, to a greater degree, the macroporosity of carbon W, whereas the urea treatment produced an increase in the porosity across the whole range of pore sizes. In addition, treatment of the activated carbon with these nitrogenating agents produced a marked change in the chemical nature of its surface. Thus, according to the pH of the point of zero charge (pHPZC) values obtained for each sample, carbon W was neutral (pHPZC = 7.12), but the treated carbons were basic, especially carbon W-U (pHPZC = 8.85). This basicity results from an increased concentration of basic oxygenated and nitrogenated surface functional groups, as confirmed by the results of elemental and XPS analyses. An increase in the degradation of 1,3,6-naphthalenetrisulfonic acid was observed when the activated carbon samples were added to the system. This degradation was especially enhanced in the presence of carbon W-U. The increased NTS degradation rate in the presence of the activated carbon is due to an increased concentration of highly reactive radicals in the system. When the catalytic activity of the activated carbon samples was related to their chemical and textural characteristics, it was found that: (i) The catalytic activity increased with an increase in the surface basicity. Interestingly, in the sample with greatest catalytic activity in NTS ozonation, carbon W-U, most of the nitrogenated surface groups introduced were pyrrol groups. These groups increase the electronic density of the basal plane of the activated carbon, thereby enhancing the reduction of ozone on the surface and the generation of highly reactive radicals in the system. (ii) The greater catalytic activity of carbon W-U may also be partly related to its greater surface area and higher volume of mesopores and macropores; these large pores facilitate access of the ozone to the surface active centers of the carbon, increasing its catalytic activity. The presence of the activated carbon samples during NTS ozonation also favored the removal of total organic carbon present in the solution, due to (a) transformation of organic matter into CO2 through the generation of highly reactive species catalyzed by the presence of the activated carbons (catalytic contribution) and (b) adsorption of NTS oxidation byproducts on the activated carbon (adsorptive contribution). The results obtained show that activated carbons treated with nitrogenating agents are very promising catalysts for application in the ozonation of aromatic compounds.     

Liquid-phase adsorption of multi-ring thiophenic sulfur compounds on carbon materials with different surface properties

This work examines the effects of structural and surface properties of carbon materials on the adsorption of benzothiophene (BT), dibenzothiophene (DBT), 4-methyldibenzothiophene (4-MDBT) and 4,6-dimethyl-dibenzothiophene (4,6-DMDBT) in the presence of 10 wt % of aromatics in liquid alkanes that simulate sulfur compounds in diesel fuels. The equilibrium-adsorption capacity varies significantly, from 1.7 to 7.0 mg-S/g-A. The results show that different carbon materials have significantly different sulfur-adsorption capacities and selectivities that depend not only on textural structure but also on surface functional groups. The adsorption of multi-ring sulfur compounds on carbon materials was found to obey the Langmuir isotherm. On the basis of adsorption tests and the characterization of carbon materials by BET and XPS, the oxygen-containing functional groups on the surface appear to play an important role in increasing sulfur-adsorption capacity. The adsorption-selectivity trend of the carbon materials for various compounds increases in the order of BT < naphthalene < 2-methylnaphthalene < DBT < 4-MDBT < 4,6-DMDBT, regardless of carbon material types. This selectivity trend for sulfur compounds is dramatically different and almost opposite from that previously observed for adsorption over nickel-based adsorbents. The regeneration of spent activated carbons was also conducted by solvent washing. The high-adsorption capacity and selectivity for methyl DBTs indicate that certain activated carbons are promising adsorbents for selective adsorption for removing sulfur (SARS) as a new approach to ultra deep desulfurization of diesel fuels.     

Pore structure and surface properties of chemically modified activated carbons for adsorption mechanism and rate of Cr(VI)

Effects of hydrochloric acid and sodium hydroxide treatments of activated carbons (ACs) on chromium(VI) reduction were studied. The surface properties were determined by pH, acid-base values, FT-IR, and X-ray photoelectron spectrometer (XPS). And the porous structure of the activated carbons was characterized by adsorption of N(2)/77 K. The Cr(VI) adsorption experiments were carried out to analyze the influence of porous texture and surface properties changed by the chemical surface treatments of ACs on adsorption rate with carbon-solution contact time. From the experimental results, it was observed that the extent of adsorption and reduction processes depends on both microporous structure and functional groups. And the adsorption of Cr(VI) ion was more effective in the case of acidic treatment on activated carbons, resulting from the increases of acid value (or acidic functional group) of activated carbon surfaces. However, basic treatment on activated carbons was not significantly effective on the adsorption of Cr(VI) ion, probably due to the effects of the decrease of specific surface area and basic Cr(VI) in nature.     

Influence of anodic treatment on heavy metal ion removal by activated carbon fibers

In this work, the effect of electrochemical oxidation treatment on activated carbon fibers (ACFs) was studied in the context of Cr(VI), Cu(II), and Ni(II) adsorption behavior. Ten weight percent phosphoric acid (A-ACFs) and ammonia (B-ACFs) were used for acidic and basic electrolytes, respectively. Surface properties of ACFs were determined by X-ray photoelectron spectroscopy (XPS). The specific surface area and the pore structure were evaluated from nitrogen adsorption data at 77 K. As a result, the electrochemical oxidation treatment led to an increase in the amount of oxygen-containing functional groups. Also, the adsorption capacity of the electrochemically oxidized ACFs was improved in the order B-ACFs > A-ACFs > untreated-ACFs, in spite of a decrease in specific surface area which resulted from pore blocking by functional groups and pore destruction by acidic electrolyte. It was clearly found that the heavy metal ions were largely influenced by the functional groups on the ACF surfaces.     

Assessing surface chemistry and pore structure of active carbons by a combination of physisorption (H2O, Ar, N2, CO2), XPS and TPD-MS

In order to address open questions concerning the surface chemistry and pore structure characterization of nanoporous carbons, we performed extensive experiments by combining various experimental techniques on a series of commercially available activated carbons which exhibit diverse surface chemistry characteristics. Pore size analysis was performed on Ar (87 K), N₂ (77 K) and CO₂ (273 K) adsorption isotherms using state-of-the art methods based on density functional theory, including the recently developed quenched solid density functional theory (QSDFT). A detailed study of the surface chemistry was obtained by applying temperature programmed desorption coupled with mass spectrometry (TPD-MS) as well as XPS (X-Ray-Photoelectron Scattering). This information together with the pore structure information leads to a reliable interpretation of systematic water adsorption measurements obtained on these materials. Our results clearly suggest that water adsorption is indeed a sensitive tool for detecting differences in surface chemistry between chemically and physically activated active carbon materials with comparable ultramicropore structure. The occurrence of sorption hysteresis associated with the filling of micro- and narrow mesopores (in a range where nitrogen and argon isotherms are reversible) provides additional structural information, complementary to the insights from argon/nitrogen/carbon dioxide adsorption.     

几种植物基活性炭材料的表面结构与吸附性能比较——(II)表面化学结构与吸附性能研究

用X-射线光电子能谱对3种植物基活性炭材料:椰壳活性炭 (CAC4)、剑麻茎基活性炭(SSAC)和剑麻基活性碳纤维 (SACF) 的表面化学结构进行了表征,并研究和对比了它们的吸附性能,包括对碘、苯酚和亚甲基蓝的液相吸附性能,对有机蒸汽的吸附性能以及对Au3+的还原吸附性能等。结果表明,3个样品表面均含有多种含氧官能团,吸附能力SACF>SSAC> CAC4。样品的吸附性能主要取决于自身孔结构,与其表面化学结构也有密切的关系。     

Water vapor adsorption and microporous structure of carbon adsorbents. 18. Effect of surface chemistry of activated carbons on water vapor adsorption

This investigation has been devoted to a study of the chemical composition of the surfaces of activated carbons. A study has been made of the way in which changes in the surface chemistry of a series of carbons, as a result of heat treatment, affects the nature of their adsorption of water vapor. A differentiation has been made between oxygen-containing groups found on the surface of activated carbons before and after their heat treatment. It has been established that the original adsorption centers, which play a determining role in water vapor adsorption by activated carbons, comprise functional groups like strongly acidic free hydrogen ions, carboxylic and phenolic groups, situated on on the pore surface of the activated carbons. The number of these functional groups on the pore surface of the activated carbons has been correlated with the parametera ₀ (the number of original adsorption centers) in the isotherm equation for water vapor adsorption. The relative pressure corresponding to the formation of an adsorption layer on the surface of the activated carbons has been shown to depend on the number of original adsorption centers, the acidic functional groups.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 1, pp. 35–40, January, 1991.     

Surface functionalization of ordered mesoporous carbons--a comparative study

Hexagonally structured mesoporous carbons C15 and CMK-5 and cubically structured carbon C48 were synthesized using ordered silica SBA-15 and MCM-48 as templates and carbon precursors of different structures. The surfaces of these ordered carbons were chemically functionalized by employing an approach, in which the selected diazonium compounds were in situ generated and reacted with the carbon frameworks of the mesoporous carbons. The aromatic organic molecules containing chlorine, ester, and alkyl groups were covalently attached to the surface of these ordered mesoporous carbons. The presence of functional groups on the modified carbons was confirmed with Fourier transform infrared spectroscopy, thermogravimetric analysis, and nitrogen adsorption. The BET-specific surface area and the pore width of ordered carbons were significantly reduced, whereas the primary structure of these ordered carbons and their unit cells were intact. Basically, the density of grafted functional groups is related to the specific surface area of the sample, particularly the surface area of mesopores. The surface functionalization reaction takes place only on the external surface of carbon C15, while it occurs on both of the internal and external surface of CMK-5 carbon with the nanopipe structure. The presence of the micropores in CMK-5 carbon should be responsible for its lower grafting density because the small micropores are inaccessible in the reaction. It was also proposed that the preferred adsorption/reaction in C48 may be related to the observed unsymmetrical degradation of the XRD patterns for the functionalized C48 samples. The chemical modification process considerably reduced the primary mesopores in these ordered carbons by approximately 1-1.5 nm, affording carbons with micropores in the cases of C15 and C48, and mixed micropores and small mesopores in the case of CMK-5. A grafting density of approximately 0.9-1.5 micromol/m(2) was achieved under current research.     

活性炭电极材料的表面改性和性能

以硝酸、双氧水、氨水三种化学试剂分别对活性炭进行表面改性, 用N2吸附法和FTIR表征炭材料改性前后孔结构和表面官能团的变化. 制备了以改性活性炭为电极材料, KOH溶液为电解质的模拟双电层电容器. 用恒流充放电、循环伏安、交流阻抗等方法考察了双电层电容器的电化学性能. 结果表明, 改性活性炭比表面积和平均孔径有所降低, 并且在炭材料表面引入了含氧或含氮官能团, 如—OH、>CO、—NH2等, 使炭材料的润湿性增强、电阻减小、电化学性能显著提高. 用65%硝酸改性后炭材料的比容量最高达到250 F·g-1, 比原样炭提高了72.4%; 实验电容器的漏电流急剧下降, 只有3-18 μA, 为原来电容器的漏电流(371 μA)的0.8%-4.9%.     

Adsorption properties of activated carbon from waste newspaper prepared by chemical and physical activation

Adsorption properties of activated carbons prepared from waste newspaper by chemical and physical activation were investigated using water vapor, ammonia, methane, and methylene blue (MB) as adsorbents. The water vapor adsorption isotherms show type V behavior and the maximum vapor adsorption of the chemically and physically activated products is about 1050 and 450 ml/g, respectively. The higher water vapor adsorption of the chemically activated products is attributed to the higher specific surface area (S(BET)) and greater hydrophilic activity (arising from the surface oxygen-containing functional groups) than in the physically activated products. The adsorption of ammonia and methane was measured by temperature-programmed desorption (TPD). NH(3) adsorption is found to be higher in the chemically activated product than in the physically activated product while methane adsorption is slightly higher in the physically activated products even though these have lower S(BET) values. In the MB adsorption, the chemically activated products show higher adsorption (390 mg/g) than the physically activated product. These results are suggested to be related to the surface characteristics.     

Hierarchical modeling of ammonia adsorption in functionalized metal-organic frameworks

The adsorption of ammonia in four metal-organic frameworks modified with different functional groups (-OH, -C=O, -Cl, -COOH) was investigated using a hierarchical molecular modeling approach. To describe the hydrogen bonding and other strong interactions between NH(3) and the surface functional groups, a set of Morse potential parameters were obtained by fitting to energies from quantum chemical calculations at the MP2 level of theory. We describe a systematic force field parameterization process, in which the Morse parameters were fitted using simulated annealing to match a large number of single-point MP2 energies at various distances and angles. The fitted potentials were then used in grand canonical Monte Carlo simulations to predict ammonia adsorption isotherms and heats of adsorption in functionalized MIL-47, IRMOF-1, IRMOF-10, and IRMOF-16. The results show that ammonia adsorption can be significantly enhanced by using materials with appropriate pore size, strongly interacting functional groups, and high density of functional groups.     

Adsorption of ammonia on graphite oxide/aluminium polycation and graphite oxide/zirconium-aluminium polyoxycation composites

Graphite oxide (GO) synthesized from commercial graphite was modified with aluminium or zirconium-aluminium polyoxycations and then calcined at 350 degrees C. On the samples obtained adsorption of ammonia from moist air was investigated. The surface of materials before and after exposure to ammonia was characterized using adsorption of nitrogen, XRD, SEM, FTIR, TA, CHN analysis, and potentiometric titration. The results showed that in spite of the fact that graphite composites/pillared graphites (PG) have Keggin-like ions located between the layers, that space blocked for nitrogen molecules used to determine the specific surface area. During calcinations, the deflagration of layers occurred as a result of decomposition of epoxy groups. This results in formation of disordered graphitic carbons with some mesoporosity. Even though these materials were not porous, the significant amount of ammonia was retained on the surface. Since ammonia molecule is able to specifically interact with oxygen groups of graphite oxide and Br?nsted centers of inorganic pillars, it is likely intercalated between the composite layers. While the best performance was found for GO modified with aluminium-zirconium species, after calcinations the samples containing Keggin Al(13) like cations revealed the high capacity which is linked to the high acidity of incorporated inorganic compounds.     

Surface Modification and Performance of Activated Carbon Electrode Material

A commercial activated carbon was modified by surface treatment using three chemicals, nitric acid, hydrogen peroxide, and ammonia, respectively. The modified carbons were characterized by N₂ adsorption-desorption isotherms and FTIR spectroscopy. The resultant carbon electrode-based electric double-layer capacitors (EDLCs) were assembled with 6 mol·L⁻¹ KOH as the electrolyte. The influence of surface modification on the performance of EDLCs was studied by galvanostatic charge-discharge, cyclic voltammetry, and alternating current impedance. The surface modification resulted in no big decrease in specific surface area and little decrease in average pore size, and introduced functional groups, such as hydroxyl, carbonyl, and amidogen, on the carbon surface. These functional groups significantly improved the wettability and reduced the resistance of the activated carbon. As a result, the specific capacitance of the carbon modified with 65% HNO₃ reached 250 F·g⁻¹, 72.4% higher than that of original carbon. The leakage current of testing EDLCs decreased unexpectedly to 3-18 μA, only 0.8%-4.9% that of the original carbon electrode-based EDLC (371 μA).     

Adsorption of NH3 onto activated carbon prepared from palm shells impregnated with H2SO4

Adsorption of ammonia (NH3) onto activated carbons prepared from palm shells impregnated with sulfuric acid (H2SO4) was investigated. The effects of activation temperature and acid concentration on pore surface area development were studied. The relatively large micropore surface areas of the palm-shell activated carbons prepared by H2SO4 activation suggest their potential applications in gas adsorption. Adsorption experiments at a fixed temperature showed that the amounts of NH3 adsorbed onto the chemically activated carbons, unlike those prepared by CO2 thermal activation, were not solely dependent on the specific pore surface areas of the adsorbents. Further adsorption tests for a wide range of temperatures suggested combined physisorption and chemisorption of NH3. Desorption tests at the same temperature as adsorption and at an elevated temperature were carried out to confirm the occurrence of chemisorption due to the interaction between NH3 and some oxygen functional groups via hydrogen bonding. The surface functional groups on the adsorbent surface were detected by Fourier transform infrared spectroscopy. The amounts of NH3 adsorbed by chemisorption were correlated with the contents of elemental oxygen present in the adsorbents. Mechanisms for chemical activation and adsorption processes are proposed based on the observed phenomena.     

Ammonia removal of activated carbon fibers produced by oxyfluorination

In this study, activated carbon fibers (ACFs) were produced by an oxyfluorination treatment to enhance the capacity of ammonia gas removal. The introduction of polar groups, such as CF, CO, and COOH, on the ACFs was confirmed by a XPS analysis, and N2/77 K adsorption isotherm characteristics including specific surface area and total and micropore volumes were studied by the BET and t-plot methods. The ammonia-removal efficiency was confirmed by a gas-detecting tube technique. As a result, the specific surface area and micropore volume of ACFs were slightly destroyed as the surface treatment time was increased. However, the oxyfluorinated ACFs led to an increase of fluorine and oxygen-containing polar functional groups in ACF surfaces, resulting in an increase in the ammonia-removal efficiency of the ACFs produced.     

Phenobarbital interactions with derivatized activated carbon surfaces

The interactions between phenobarbital and activated carbon surfaces were studied in detail in this work. This was accomplished by utilizing different reagents to manipulate the surface polar functional group compositions of different activated carbons, and determining how those modifications changed phenobarbital adsorption. Oxidation of an activated carbon surface caused a systematic decrease in the basal carbon surface, resulting in a concurrent systematic decrease in the non-specific adsorption of phenobarbital. Even more interesting, it was shown for the first time that chemical reduction of some of the carbonyl-containing functional groups on the activated carbon surface caused a significant increase in the specific adsorption of phenobarbital without any significant effect on the non-specific adsorption. These results support the notion that the OH groups on activated carbon surfaces are the specific adsorption sites for phenobarbital from aqueous solutions, and that the basal carbon surface is the region where non-specific adsorption takes place.     

Effect of KOH activation on the formation of oxygen structure in activated carbons synthesized from polymeric precursor

In this work, the influence of KOH activation on the surface chemistry of activated carbons (ACs) synthesized from polystyrene-based cation exchangeable resin (PSI) has been investigated. The surface chemistry of ACs has been characterized by using Fourier transformed infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), pH measurement, and Boehm's titration method. As a result, PSI can be successfully converted into ACs with high porosities. The total oxygen content on the ACs studied increases with increasing the KOH-to-PSI ratio. FT-IR and XPS analyses show that the resulting carbons possess a number of oxygen surface functional groups, such as carbonyl, quinone, phenol, ether, and carboxylic acid groups. The highest oxygen content and acid value are observed at a KOH-to-PSI ratio of 4 (KPS-4). However, its pH and surface basicity are higher than those of a KOH-to-PSI ratio of 2 (KPS-2), indicating the formation of basic species, such as quinone and pyrone groups. Although the oxygen-containing groups with basic character exist in the resulting carbons, all the samples are still acidic in character.     

The physical and surface chemical characteristics of activated carbons and the adsorption of methylene blue from wastewater

Adsorption of a basic dye, methylene blue, from aqueous solutions onto as-received activated carbons and acid-treated carbons was investigated. The physical and surface chemical properties of the activated carbons were characterized using BET-N(2) adsorption, X-ray photoelectron spectroscopy (XPS), and mass titration. It was found that acid treatment had little effect on carbon textural characteristics but significantly changed the surface chemical properties, resulting in an adverse effect on dye adsorption. The physical properties of activated carbon, such as surface area and pore volume, have little effect on dye adsorption, while the pore size distribution and the surface chemical characteristics play important roles in dye adsorption. The pH value of the solution also influences the adsorption capacity significantly. For methylene blue, a higher pH of solution favors the adsorption capacity. The kinetic adsorption of methylene blue on all carbons follows a pseudo-second-order equation.