Insight into hydroxides-activated coals: chemical or physical activation?

The objective of this paper is to get an insight into the chemical activation mechanism using KOH and NaOH as activated agents. Three coals have been selected as carbon precursors. It was found that KOH and NaOH develop a similar narrow microporosity, independently of the coal rank, whereas only KOH generates supermicroporosity. Temperature-programmed desorption experiments, carried out with impregnated anthracite, show differences on the gas evolved during the activated carbon preparation using the two activating agents. Thus, whereas hydrogen profiles are quite similar for both activated agents, the CO and H(2)O profiles are different. It is remarkable the high amount of H(2)O evolved at the maximum treatment temperature for both activating agents. The results obtained allow concluding that the chemical activation is due to a combination of different process driving the development of material porosity.     

High resolution N2 adsorption isotherms at 77.4 K and 87.3 K by carbon blacks and activated carbon fibers--analysis of porous texture of activated carbon fibers by αs-method

The standard α(s)-data of N(2) at 87.3 K by graphitized and nongraphitized carbon black samples (GCB-I and NGCB) (cf.Figs. 3 and 4) have been determined on the basis of the high resolution adsorption isotherms of N(2) at 87.3 K, which were repeatedly measured in the pressure range of p/p(o)=5×10(-8)-0.4. The high resolution adsorption isotherms of N(2) by two kinds of activated carbon fibers (ACF-I and ACF-II) were measured from p/p(o)=10(-7) to p/p(o)=0.995 at 77.4 K and from p/p(o)=10(-7) to p/p(o)=0.4 at 87.3 K. Combination of the adsorption isotherms by ACF-I and ACF-II with the standard α(s)-data by NGCB at 77.4 K and 87.3 K make it possible to construct the high resolution α(s)-plots from very low filling (1%) to complete filling (100%). The high resolution α(s)-plots of N(2) at 77.4 K and 87.3 K were analyzed. On the basis of the analyzed result, the porous textures of ACF-I and ACF-II will be discussed.     

Enhanced physical and chemical adsorption of carbon dioxide using bimetallic copper–magnesium oxide/carbon nanocomposite

Mixed Cu and Mg oxides on nitrogen-rich activated carbon (AC) from Nypha fruticans biomass were characterized and their CO₂ adsorption performance was measured. Highly dispersed CuO and MgO nanoparticles on AC was obtained using an ultrasonic-assisted impregnation method. The optimum adsorbent is 5%CuO–25%MgO/AC having good surface properties of high surface area, pores volume and low particles agglomeration. The higher content of MgO of 5%CuO–25%MgO/AC adsorbent contributes to less metal carbide formation which increases their porosity, surface area and surface basicity. XPS analysis showed some amount of nitrogen content on the surface of the adsorbent which increased their surface basicity towards selective CO₂ adsorption. The presence of moisture accelerated the CO₂ chemisorption to form a hydroxyl layer on the surfaces. The 5%CuO–25%MgO/AC adsorbent successfully adsorbed CO₂ via physisorption and chemisorption of 14.8 and 36.2 wt%, respectively. It was significantly higher than fresh AC with better selectivity to CO₂.     

Surface modification of activated carbon and fullerene black by Diels–Alder reaction

Activated carbon and fullerene black react with cyclopentadiene at room temperature or slightly elevated temperature. At higher temperature a retro-Diels–Alder reaction takes place. The reaction with the diene and the retro-Diels–Alder reaction could be repeated. As a consequence of the reaction with cyclopentadiene or other suitable dienes and the retro reaction, the surface structure of different carbons changed considerably. The surface area of micropores on fullerene black was much higher than for the original sample. The influence on the surface area of porosity is reported for two different types of carbon.     

Adsorption of methane in activated carbons obtained from coconut shells using H3PO4 chemical activation

Activated carbon samples from coconut shells (Brazilian coconut species “Coco da Baía”) were prepared by chemical activation with phosphoric acid as the activating agent. Samples were characterized by nitrogen adsorption isotherms at 77 K. Some samples were randomly chosen in order to perform methane adsorption experiments under pressures between 1 and 60 bar at 303 K. A close relationship between surface area, micropore volume and methane adsorption capacity for carbons prepared from the same starting material was observed. The highest methane storage capacity in the tested samples was found to be 95 v/v at 303 K and 35 bar, which is comparable to results obtained for commercial samples indicated for this application. A moderate concentration of phosphoric acid (around 35%) seems to favor high surface areas, micropore volumes and, hence, gas storage capacity. The inclusion of an acid wash step before carbonization and the use of inert gas flow during carbonization also seem to enhance the development of porosity. This result suggests that activated carbons prepared from “Coco da Baía” by chemical activation with phosphoric acid have potential to be used as a storage media for natural gas.     

Enhanced removal of hydrogen sulfide from a gas stream by 3-aminopropyltriethoxysilane-surface-functionalized activated carbon

A commercial activated carbon was functionally modified by silylation with 3-aminopropyltriethoxysilane (APTES). The silylation led to the fixation of weakly basic functional groups, –NH₂, on the surface as indicated by pH titration, Boehm titration, N $_{2^{-}}$ BET analysis and X-ray photoelectron spectroscopic (XPS) analysis. Despite reducing the specific BET area and the pore volume, silylation improved the H₂S removal capacity so that APTES modified activated carbon (APTES-AC) was 3.55 times more effective than the original activated carbon. XPS results indicate that H₂S removal may be associated with the amino (–NH₂) group and the presence of sulfur in the four oxidation states S^2?, S⁰, S⁴⁺ and S⁶⁺. The effects of moisture, oxygen content and temperature on the performance of APTES-AC for H₂S removal were investigated. The presence of moisture in the gas stream was found to have an adverse effect on the H₂S removal, whilst the presence of oxygen favored the removal of H₂S by APTES-AC. The higher removal capacity of APTES-AC relative to the original activated carbon indicates that APTES-AC is a potential candidate for the removal of H₂S from gas streams. The H₂S removal efficiency of APTES-AC was proved be superior to that of Na₂CO₃-impregnated AC by the pilot-scale test of purification H₂S containing industrial waste gas, yellow phosphorus off-gas.     

Have photosynthetic pigments been formulated for chemical stability? A cursory insight into the reactivity of magnesium porphyrinoids

Abstract

Magnesium complexes with reduced tetrapyrrolic ligands are active compounds of plant and bacteria photosystems. However, also the porphyrin complex appears as an intermediate on the biosynthetic pathway of the photosynthetic pigments. Its transformations, in particular the reduction of pyrrole rings, lead to the acquisition of the properties that are primary for activity in antenna systems and reaction centers. On the other hand, modifications of the porphyrin system must affect the resistance to destructive processes, such as loss of metal ion and its substitution. In order to compare the stability of three natural Mg complexes, namely Mg protoporphyrin IX, chlorophyll a, and bacteriochlorophyll a, spectroscopic studies in solution were performed. The difference in the electronic structure of the macrocyclic ligand was the basic variable in testing the action against d-electron metal salts and acetic acid. The spectroscopic studies were supplemented with calculations using the Density Functional Theory which provided insight into the stability of M(II)-N bonds depending on the dimension of the delocalized electron system. The results indicate the decreasing stability of Mg(II) complexes on the biosynthetic pathway, thereby providing an additional justification for incorporation of the metal ion into porphyrin prior to the electronic modifications of the tetrapyrrolic system.     

Radiation-induced chemical reactions of carbon monoxide and hydrogen mixture—IV. On the water produced by the addition of small amounts of ammonia

The isotope content in water obtained from irradiated CO-H₂ mixture with added small amounts of NH₃ or their deuterated equivalents was compared. Compared with that in water obtained from CO-D₂ mixture with added NH₃, D content in water produced from the mixture with the substitution of ND₃ for NH₃ was increased remarkably. The NH₃ added to the mixture contributes to the water formation under irradiation.     

Adsorption studies of β-naphthol by untreated and treated activated carbon [optimizing of adsorption by central composite design (CCD)]

In the present work the adsorption of aromatic compound, namely ??-naphthol (BN) by two granular activated carbons, one untreated and the other treated with HNO₃ carried out under controlled conditions. The effects of experimental parameters on adsorption process such as pH, contact time and adsorbent dosage have been investigated. Experimental design methodology was applied to optimize the removal of ??-naphthol. The effect of various experimental parameters was investigated using five-level three-factorial central composite design (CCD). The relationship between the parameters and the response for model optimization was found and optimum conditions were obtained by CCD. In the optimum conditions obtained by response surface modeling, 100% BN was adsorbed on the carbons. Treatment with HNO₃ led to a decrease in the point of zero charge and the adsorption capacity (Q _max) of the activated carbon. The adsorption capacity of the carbons was determined using Freundlich and Langmuir homogenous equation. The variation of the model parameters with the solution pH was also studied. The fitted parameters obtained from both models showed the Q _max value decrease with increasing of pH.     

Is a catalyst always beneficial in plasma catalysis? Insights from the many physical and chemical interactions

Plasma-catalytic dry reforming of CH₄(DRM) is promising to convert the greenhouse gasses CH₄ and CO₂ into value-added chemicals, thus simultaneously providing an alternative to fossil resources as feedstock for the chemical industry. However, while many experiments have been dedicated to plasma-catalytic DRM, there is no consensus yet in literature on the optimal choice of catalyst for targeted products,because the underlying mechanisms are far from understood. I...     

A comparative study of claisen and cope rearrangements catalyzed by chorismate mutase. An insight into enzymatic efficiency: transition state stabilization or substrate preorganization?

In this work we present a detailed analysis of the activation free energies and averaged interactions for the Claisen and Cope rearrangements of chorismate and carbachorismate catalyzed by Bacillus subtilischorismate mutase (BsCM) using quantum mechanics/molecular mechanics (QM/MM) simulation methods. In gas phase, both reactions are described as concerted processes, with the activation free energy for carbachorismate being about 10-15 kcal mol(-)(1) larger than for chorismate, at the AM1 and B3LYP/6-31G levels. Aqueous solution and BsCM active site environments reduce the free energy barriers for both reactions, due to the fact that in these media the two carboxylate groups can be approached more easily than in the gas phase. The enzyme specifically reduces the activation free energy of the Claisen rearrangement about 3 kcal mol(-)(1) more than that for the Cope reaction. This result is due to a larger transition state stabilization associated to the formation of a hydrogen bond between Arg90 and the ether oxygen. When this oxygen atom is changed by a methylene group, the interaction is lost and Arg90 moves inside the active site establishing stronger interactions with one of the carboxylate groups. This fact yields a more intense rearrangement of the substrate structure. Comparing two reactions in the same enzyme, we have been able to obtain conclusions about the relative magnitude of the substrate preorganization and transition state stabilization effects. Transition state stabilization seems to be the dominant effect in this case.     

The removal and kinetic study of Mn,Fe, Ni and Cu ions from wastewater onto activated carbon from coconut shells

Activated carbon from coconut shells (ACCS) was synthesised and used for the removal of metal ions (manganese, iron, nickel and copper) from aqueous solutions. Two different adsorption models were used for analysing the data. Adsorption capacities were determined: copper ions exhibited the greatest adsorption on activated carbon obtained from coconut shells because of their size and pH conditions. Adsorption capacity varied as a function of the pH. Adsorption isotherms from aqueous solutions of heavy metals on ACCS were determined and were found to be consistent with Langmuir’s adsorption model. Adsorbent quantity and immersion enthalpy were studied. The results were compared with other adsorbents used in a prior study.     

Structures and mechanisms in chemical reactions: George A. Olah’s life-long search of chemistry

The Hungarian-born American chemistry Nobel laureate George A. Olah used superacids to give longer life to carbocations. He resolved a long-standing debate on reaction mechanism in organic chemistry and, more importantly, opened new vistas in hydrocarbon chemistry to produce hosts of new compounds. The concerted utilization of organic synthesis, physical techniques, and computational methods led to spectacular achievements in hydrocarbon chemistry. Olah has always been on the lookout for the practical applications of his discoveries in fundamental chemistry. He continued his research after his Nobel award and has worked out the idea, which he labeled “the methanol economy.” Olah’s example shows that a great researcher can also be a devoted and caring human being.     

Preparation and characterization of a high-efficiency radon adsorbing material based on activated carbon modified by water immersion and freeze–thaw

Journal of Radioanalytical and Nuclear Chemistry - In this paper, activated carbon has been modified by water immersion and freeze–thaw cycles to further improve its radon adsorption...     

Synthesis and modification of activated carbon originated from Indonesian local Orange peel for lithium ion Capacitor’s cathode

Lithium ion capacitor (LIC) is a novel hybrid capacitor technology that possesses a better performance than the conventional supercapacitor, such as EDLC and lithium ion battery. However, the overall performance of LIC is limited by activated carbon’s capacity as its cathode. This work is focused on synthesize of activated carbon from Indonesian local orange peel waste for LIC cathode and its modification to perform a good electrochemical performances. Activated carbon was synthesized by ZnCl₂ as impregnant with mass ratio of orange peel to ZnCl₂ 1:2 for 1 h at 500 °C. The obtained activated carbon possessed large surface area (1200 m² g ^?1), higher than commercial activated carbon (Merck-p.a.) with surface area of 775.6 m² g^?1. Modification was done using wet oxidation method by utilizing HNO₃ 65 %, H₂SO₄ 98 %, and H₂O₂ 30 %, with ratio 1:1 (w/w). Electrochemical properties were studied by using half cell test pouch with Li metal as reference electrode and 1 M LiPF₆ (EC:EMC:DMC = 1:1:1) as electrolyte. Modification had successfully increased oxygen atoms in activated carbon samples. Surface-modified cells demonstrated a higher capacitance (almost twice in the galvanostatic test) than that of the unmodifed one due to the pseudocapacitance effect of oxygen functional groups. However, its electrochemical performance was unstable. The unmodified cell showed a stable performance with 56.3 F g^?1 at current density of 0.3 mA g^?1 and voltage window 2.5–4 V.

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Investigation of the mechanisms of the action of chemical modifiers for electrothermal atomic absorption spectrometry: what for and how?

Modern trends in the research of the action of chemical modifiers for electrothermal atomic absorption spectrometry (ETAAS) are discussed critically. The most prolific approach is that of investigation of processes occurring during the drying and pyrolysis stages with wide application of data from different fields of chemistry and physics.     

Lanthanum ion–impregnated granular activated carbon for the removal of phenol from aqueous solution: Equilibrium and kinetic study

The current study discusses application of the lanthanum ions (La³⁺) as an activating agent incorporated /immobilized into coconut shell–based granular activated carbon (GAC) for porosity development; subsequently, the carbon material is used for the adsorption of phenol from aqueous solutions. The new carbons were characterized using FTIR, XRD, CHNO, burn off, and carbon yield. The surface functional groups were determined by Boehm titration. The Brunauer–Emmett–Teller (BET) surface area of the carbons is 953 m² g⁻¹ (GACLa1073), 997 m² g⁻¹ (GAC383), and 973 m² g⁻¹ (GACO383). Langmuir, Freundlich, Dubinin–Radushkevich, and John–Sivanandan Achari (J-SA) isotherm models on the equilibrium isotherm data were examined for the new carbon-phenol system. It is found that the Langmuir isotherm fits better with a monolayer adsorption capacity, highest for GACLa1073 (387.59 mg g⁻¹) followed by GAC383 (303.03 mg g⁻¹) and GACO383 (197.62 mg g⁻¹). Kinetic studies reveal that the adsorption system follows the pseudo–second-order kinetic model. Isotherm analysis by the phase change method of John-Sivanandan Achari (J-SA) isotherm gives a better insight into adsorption phenomena, which is accompanied by regeneration studies of carbon with >75% for GACLa1073 after three cycles.