Synthesis and structural investigation of new graphite intercalation compounds containing the perfluoroalkylsulfonate anions C10F21SO3, C2F5OC2F4SO3, and C2F5(C6F10)SO3

The preparation of graphite intercalation compounds (GIC’s) of three perfluorinated alkylsulfonate anions, C₁₀F₂₁SO₃⁻, C₂F₅OC₂F₄SO₃⁻ and C₂F₅(C₆F₁₀)SO₃⁻ is described for the first time. Pure stage 2 GIC’s are obtained by chemical oxidation of graphite with K₂MnF₆ in a solution containing hydrofluoric and nitric acids for 72 h. One-dimensional electron density maps derived from powder diffraction data are fit to obtain models for the intercalate interlayer regions (galleries) structures: the structure models provide details on anion concentrations, orientations, and conformations. In all cases, anion bilayers are observed with anion sulfonate headgroups oriented towards graphene sheets. Compared with structures calculated for the isolated anions, the intercalated anion conformations show changes in dihedral angles, involving rotations about C-C or C-O bonds. For the GIC containing C₂F₅(C₆F₁₀)SO₃⁻, the anion conformation change is related to the more efficient packing of anions in the intercalate gallery.     

Influence of surface properties and iron addition on the SO2 adsorption capacity of activated carbons

When iron derivatives are added to low ash activated carbons having basic surface characteristics (obtained by suitable oxidation at 800°C with 2% of O₂ in N₂), certain materials are obtained showing high SO₂ sorbent properties from gaseous mixtures having a composition close to that of the flue gases. This behaviour seems to be related to the simultaneous presence of both basic surface sites promoting the initial adsorption of SO₂ and iron promoting the transformation of the adsorbed SO₂ into other, more stable forms. The sorbent properties of these activated carbons are more stable, following consecutive cycles, in the processes of adsorption and desorption of SO₂, than those shown by similar carbons with different surface characteristics.     

Reactivity of bio-coke with CO2

Incorporation of biomass-derived materials in coal blends for cokemaking is one of the strategies that could reduce the levels of greenhouse gas emissions produced by the steelmaking process. Bio-coke refers to the resultant coke prepared with the addition of charcoal to a coal blend. In this work, characteristics of bio-coke gasification by reacting with CO₂ were examined using Thermal Gravimetric Analysis. Bio-coke samples with different levels of charcoal addition to a coal blend were prepared in the CanmetENERGY pilot-scale coke oven. These samples were heated in CO₂ for identification of the minimum gasification temperature. Sample gasification rates at 1000 °C were also measured. It was observed that mineral content plays an important role in the gasification characteristics of the bio-cokes. Those with low mineral content behave very similarly to the reference coke. Higher mineral content bio-coke reacts with CO₂ at a lower temperature. It was found that the gasification characteristics of the bio-cokes are well described by the alkalinity index.     

Impact of moisture on the thermal behavior of K2CO3-impregnated respirator carbons

The effect of moisture on the kinetics of the reaction of K₂CO₃-impregnated carbons with air at elevated temperature has been studied. Differential scanning calorimetry (DSC) measurements, isothermal DSC measurements and oven exposure tests all show that the reaction of the impregnated carbons with air is accelerated by the presence of moisture adsorbed by the impregnated carbon. Reaction kinetics extracted from isothermal DSC measurements show that this acceleration is caused by an increase in the frequency factor (γ₂) while the activation energy (E_a2) remains unchanged. For example, for an impregnated carbon containing 22.4% by weight moisture, the frequency factor was found to increase by about 65% compared to that of the dry precursor. X-ray diffraction experiments show that the increase in frequency factor is caused by a decrease in the particle size of the impregnant through a series of steps initiated by the adsorbed water. First, the deliquescent K₂CO₃ impregnant dissolves in sufficient adsorbed water; then this solution dries during heating to initially form KHCO₃, which subsequently decomposes to form small nano-particles of K₂CO₃ that apparently have a larger specific surface area and hence more catalytic activity. We suspect that the adsorbed moisture will cause such impregnant redistribution, and hence accelerated reaction kinetics with air, whenever deliquescent impregnants on hydrophilic surfaces are used.     

Heterogeneous reactions of NO2 and NO-O2 on the surface of carbons

The interactions of nitrogen oxides with carbons differing in the chemical structure of surface functional groups were studied using in situ FTIR combined with the measurements of catalytic activity. Microporous carbon samples with similar pore size distribution were prepared from cellulose. The structure and coverage of adsorbates during reactions at temperatures between 295 and 573 K are determined by FTIR. No significant changes in NO_x reaction with carbon surface were found by oxidation of the carbonized film. During the study of the reaction of NO/O₂ mixture with carbons, the infrared absorption bands for the surface species formed are similar to the IR bands observed after the reaction of carbon samples with NO₂. For both reactions, surface species, including C-NO₂, C-ONO, C-NCO and anhydride structures are formed. Catalytic NO_x reduction by carbons has been investigated in the temperature range 295-623 K in the flow reactor equipped with an FTIR gas analyzer. As the surface of carbon is exposed to NO₂ gaseous NO is formed. The reduction of NO₂ to N₂ without the use of an externally supplied reductant can be achieved with microporous carbons. Significant NO₂ conversion to N₂ occurred at 623 K on both oxidized and non-oxidized carbons.     

Comparison of DFT characterization methods based on N2, Ar, CO2, and H2 adsorption applied to carbons with various pore size distributions

Adsorption isotherms of N₂, Ar, CO₂, and H₂ were measured on selected activated carbons representing various pore size distributions. Isotherm data were analyzed using the DFT method. A good agreement was obtained between N₂ and Ar results in a wide range of pore sizes. Both N₂ and Ar results agree with CO₂ analysis at 273 K in the range of small micropores where CO₂ analysis is most applicable. Also, in this range of pores the analysis of H₂ adsorption data at 77 K gives results which are consistent with the results obtained from the more conventional vapor-phase adsorptives. However, the range of PSDs calculated from H₂ data extends to pore sizes smaller than those that can be characterized by other adsorbates.     

SO2 removal from flue gas by activated semi-cokes: 2. Effects of physical structures and chemical properties on SO2 removal activity

This paper deals with the effects of physical structures and chemical properties of the catalysts, activated semi-cokes, on SO₂ removal activity. The catalysts were characterized in terms of physical structures—specific surface area, pore volume and pore size—and chemical properties—acidity and basicity. Results show that the presence of basic groups on the catalyst surface is a precondition for SO₂ removal at 90 °C. For catalysts containing copper species, there is no relation between chemical properties and sulfur retention. For catalysts without copper species, sulfur retention shows no correlation with the content of acidic groups, but it nonlinearly increases with increasing the content of basic groups, and there is a good linear relationship between SO₂ capture capacity and Brunauer-Emmett-Teller surface area and pore volume. These results indicate that for catalysts without copper species in this work, physical structure dominates SO₂ capture capacity while chemical properties have a smaller influence on SO₂ removal efficiency.     

Positron annihilation in C60 fullerites and fullerene-like nanovoids

C₆₀ fullerites, fatty-acid triglycerides containing nanovoids, and triglycerides with dissolved fullerenes were studied by means of positron annihilation spectroscopy. Two types of nanovoids with mean radii of 0.48 and 0.34 nm were found in the fatty-acid triglycerides. The nanovoids of the latter type change size in the course of structural relaxation and ordering at room temperature. The nanovoid radius is stabilized at the value of 0.355 nm, equal to the radius of C₆₀ fullerenes, when the latter are dissolved in triglycerides. Tetrahedral interstitials in the f.c.c. lattice are shown to be the most probable positron-annihilation sites in C₆₀ fullerites. The shell of the C₆₀ molecule is a potential barrier for positron penetration to the interior of the fullerene. Vice versa, the nanovoid in triglycerides is a potential well for positively charged particles: positrons, protons and cations.     

Adsorption and reaction behavior for the simultaneous adsorption of NO-NO2 and SO2 on activated carbon impregnated with KOH

This study examined the individual and simultaneous adsorption of NO_x (NO-NO₂) and SO₂ on activated carbon impregnated with KOH (KOH-IAC). For individual component adsorption, KOH-IAC showed a higher adsorption capacity in NO-NO₂ rich air than in SO₂-air. In the simultaneous adsorption of NO-NO₂-SO₂, SO₂ showed a greater adsorption affinity than NO-NO₂. The smaller the amount of NO-NO₂ adsorbed, the more SO₂ was adsorbed. XPS analysis of the adsorption of NO-NO₂ rich SO₂-air on KOH-IAC revealed that the adsorbed SO₂ was predominantly found on the external surface, producing mainly K₂SO₄ and, additionally, H₂SO₄ and K₂SO₃. Depth profile analysis showed that the amount of SO₂ adsorbed decreased regularly away from the surface, while the amount of adsorbed NO-NO₂ increased irregularly. We confirmed that the presence of the impregnant in KOH-IAC is a determining factor in the adsorption of NO-NO₂ and SO₂ by chemical reaction, clarifying the surface chemical behavior.     

Synthesizing activated carbons from resins by chemical activation with K2CO3

We prepared activated carbons from phenol-formaldehyde (PF) and urea-formaldehyde (UF) resins by chemical activation with K₂CO₃ with impregnation during the synthesis of the resins. The influence of carbonization temperature (773-1173 K) on the pore structure (specific surface area and pore volume) and the temperature range at which K₂CO₃ worked effectively as an activation reagent, were investigated. The specific surface area and micropore volume of PF-AC and UF-AC increased with an increase of carbonization temperature in the range of 773-1173 K. We prepared activated carbon with well-developed micropores from PF, and activated carbon with high specific surface area (>3000 m²/g) and large meso-pore volume from UF. We deduced the activation mechanism with thermogravimetry and X-ray diffraction. In preparing activated carbon from PF, K₂CO₃ was reduced by carbon in the PF char. The carbon was removed as CO gas resulting in increased specific surface area and pore volume above 1000 K. In preparing AC from UF, above 900 K the carbon in UF char was consumed during the K₂CO₃ reduction step.     

Study of SO2 adsorption and thermal regeneration over activated carbon-supported copper oxide catalysts

The mechanisms of SO₂ adsorption and regeneration over activated carbon-supported copper oxide sorbent/catalysts were analyzed. Studies were carried out in a fixed-bed reactor equipped with a non-dispersive infrared gas analyzer to detect the reaction products and by using X-ray powder diffraction (XRPD) and temperature-programmed desorption (TPD) experiments to characterize the nature of the sulfate species and surface oxygen complexes. The results indicate that SO₂ was catalytically oxidized to SO₃ over a copper phase in the presence of gaseous oxygen, and then reacted with a copper site to form a sulfate linked to copper without desorption into the gas phase. The activated carbon support did not participate in this sulfation reaction. After the adsorption of SO₂, the exhausted sorbent/catalysts could be regenerated by direct heat treatment in inert gas at temperatures between 260 and 480 °C, while the neighboring surface oxygen complexes on the carbon surface were acting as the reducing agents to reduce CuSO₄ to Cu. During the subsequent adsorption process, the copper is rapidly oxidized by oxygen in the flue gas.     

Production of active carbons from waste tyres--a review

A review of the production of activated carbons from waste tyres is presented. The effects of various process parameters, particularly, temperature and heating rate, on the pyrolysis stage are reviewed. The influence of activating conditions, physical and chemical, nature of the activation chemicals, on the active carbon properties are discussed. Under certain process conditions several active carbons with BET surface areas over 1000 m²/g have been produced with extensive micropore volumes, over 40% of the total pore volume.A review is carried out of the reaction kinetic modeling applied to the pyrolysis of tyres and the chemical activation of tyres. The models cover one step and two step pyrolysis models, plus more recent models which are based on the actual chemical components such as natural rubber, SBR and other additives.     

Adsorption of H2S or SO2 on an activated carbon cloth modified by ammonia treatment

The aim of this research is to investigate how ammonia treatment of the surface can influence the activity of a viscose-based activated carbon cloth (ACC) for the oxidative retention of H₂S and SO₂ in humid air at 25 °C. Surface basic nitrogen groups were introduced either by treatment with ammonia/air at 300 °C or with ammonia/steam at 800 °C. The pore structure of the samples so prepared was examined by adsorption measurements. Changes in the surface chemistry were assessed by X-ray photoelectron spectroscopy, X-ray absorption spectroscopy and temperature programmed desorption (TPD). The change of ACC activity could not be merely attributed to surface nitrogen groups but to other changes in the support. Ammonia/steam treatment improved ACC performance the most, not only by introducing nitrogen surface groups, but also by extending the microporosity and by modifying the distribution of surface oxygen groups. Successive adsorption-regeneration cycles showed important differences between oxidative retention of H₂S and SO₂ and the subsequent catalyst/support regeneration process.     

Reaction rate coefficient of fullerene (C60) consumption by soot

The reaction of fullerene molecules with soot was studied by contacting sublimed C₆₀ fullerenes with commercially available carbon black particles at different temperatures in the range 1023-1273 K. Fullerene mass data collected both pre- and post-reaction were fit to a simple first-order kinetic model and yielded a temperature-dependent reaction rate expression. The calculated collision efficiency of the reaction is of the order 10⁻⁸ and the activation energy is ∼9.8 kcal mol⁻¹, which would be consistent with a surface diffusion reaction or a heterogeneous reaction. Simple extrapolation of the observed rate to the conditions of a fullerene forming flame would give a consumption rate six orders of magnitude too small to account for the rate of fullerene consumption observed in the post-flame zone of a fullerene-forming benzene/oxygen/argon flame. Extrapolation of the reaction rate to flame conditions also shows that the rate of consumption calculated here is too small to account for observed oscillations in the fullerene concentration profile which can be related to changes in the relative rates of consumption and formation. Calculation of activation energies required for the extrapolation of rates observed here to match those observed in flames yields significantly larger values than those obtained in the present study and are so large as to suggest that mechanisms other than those studied here control fullerenes consumption in flames. Other mechanistic possibilities for the consumption of fullerenes in flames include reactions of fullerenes with other flame species and fullerene-soot reactions in which soot reactivity depends on soot reactions with other flame species.     

Dynamic adsorption on activated carbons of SO2 traces in air: I. Adsorption capacities

Sulphur dioxide is an atmospheric pollutant which, among numerous others, has to be eliminated by habitacle filters. Breakthrough curves of low concentration SO₂ streams through beds of activated carbons have been obtained. Two carbons were studied, an activated PAN fiber (CF) and a granulated activated carbon (CN) under SO₂ concentrations lower than 100 ppm. Carbon CN used ‘as received’ is able to trap SO₂ in air at concentrations as low as 2.5 ppm. At this concentration, the adsorption of SO₂ is essentially irreversible. The fraction of reversibly adsorbed SO₂ rapidly increases when SO₂ content in air increases from 2.5 to 100 ppm. As expected, the amounts of SO₂ adsorbed per gram of carbon are much smaller than in the case of high SO₂ contents in air (>1000 ppm). The presence of water in carbon micropores enhances both reversible and irreversible adsorption of SO₂. The reversibly adsorbed part is physisorbed while the irreversibly adsorbed part results in oxidation of SO₂ at the carbon surface. This oxidation was evidenced by TPD from carbon samples after adsorption. The mechanism of SO₂ adsorption is discussed in relation to the mechanisms proposed in literature for high SO₂ contents (>1000 ppm).     

Short-range structures of poly(dicarbon monofluoride) (C2F)n and poly(carbon monofluoride) (CF)n

The short-range structures of the so-called graphite fluorides, poly(dicarbon monofluoride) ((C₂F)_n) and poly(carbon monofluoride) ((CF)_n), have been discussed, based on the neutron diffraction data. The C-C and C-F bond lengths in these compounds are determined to be 0.157-0.158 and 0.136 nm, respectively, which slightly differ from those previously evaluated and coincide with those found in polytetrafluoroethylene (PTFE). The structure models of (C₂F)_n (both AB-type and AA′-type) and (CF)_n have been refined so as to give the best fit of the atomic pair distribution functions calculated for them (G_calc(r)’s) to those experimentally observed for the compounds (G_obs(r)’s). Since the G_obs(r) of (C₂F)_n better fits to the G_calc(r)’s of AB-type model rather than those for AA′-type model, the latter model is ruled out. The a-lattice parameter and the C-C-C bond angle in the refined structure model of (CF)_n (0.260-0.261 nm and 111°, respectively) are slightly larger than those of (C₂F)_n (0.256-0.257 nm and 109-110°, respectively).     

The effect of H2 treatment on the activity of activated carbon for the oxidation of organic contaminants in water and the H2O2 decomposition

In this work, hydrogen peroxide reactions, i.e. H₂O₂ decomposition and oxidation of organics in aqueous medium, were studied in the presence of activated carbon. It was observed that the carbon pre-treatment with H₂ at 300, 500, 700 and 800 °C resulted in an increase in activity for both reactions. The carbons were characterized by BET nitrogen adsorption, thermogravimetric analyses (TG), temperature programmed reduction (TPR), electron paramagnetic resonance (EPR), iodometric titration and determination of the acid/basic sites. TPR experiments showed that activated carbon reacts with H₂ at temperatures higher than 400 °C. The treatment produces a slight increase in the surface area. EPR analyses indicate the absence of unpaired electrons in the carbon. Iodometric titrations and TG analyses suggested that the treatment with H₂ generates reduction sites in the carbon structure, with concentration of approximately 0.33, 0.53, 0.59, 0.65 and 0.60 mmol/g for carbons treated at 25, 300, 500, 700 and 800 °C, respectively. It was also observed the appearance of basic sites which might be related to the reduction sites. It is proposed that these reducing sites in the carbon can activate H₂O₂ to generate HO^* radicals which can lead to two competitive reactions, i.e. the hydrogen peroxide decomposition or the oxidation of organics in water.     

Fluorination of carbon blacks: an X-ray photoelectron spectroscopy study: IV. Reactivity of different carbon blacks in CF4 radiofrequency plasma

The effect of CF₄-plasma enhanced fluorination on the surface modification of carbon blacks has been examined using XPS. Three different types of carbon blacks have been studied: a thermal black, a furnace black and a high electrical conducting black. The analysis of the XPS spectra of fluorinated carbon black samples indicates that all fluorine atoms, fixed at the surface and in a subsuperficial zone of the particles, are covalently linked to carbon atoms. The influence of the physicochemical properties and morphology of these three types of carbon blacks on the fluorination reaction has also been investigated. The proportion of different types of fluorinated carbon atoms, i.e. on one hand CF_x surface and border groups of graphitic bulk domains for which the planar configuration of the graphene layers is preserved together with the sp² character of C, i.e. structures of type I, on the other hand polyalicyclic perfluorinated structures in which sp³C form puckered layers similar to those of covalent fluorographites, i.e. structures of type II, and also the F/C ratio of the fluorinated groups are related to the surface morphology and depend on the microstructural organization of particles. When the microstructure ordering and graphitic character of the carbon increase, the size of the ordered graphitic domain also increases. At the same time the density, the size of defects and proportion of protonated sp³C entities bridging the graphene layers decrease. As a consequence, the proportion of carbon atoms, potentially able to form perfluorinated CF₂ and CF₃ groups, decreases. The relative contribution of those groups is appreciably higher in fluorinated compounds which are derived from carbon blacks with a lower structural order.     

Structure of K2Co3-loaded activated carbon fiber and its deodorization ability against H2S gas

Coal tar pitch containing finely dispersed KOH was spun centrifugally, followed by stabilization through heating to 330°C under a (1:1) mixture of air and CO₂ and carbonization/activation by heating to 850°C under CO₂. The activated carbon fiber obtained possessed of a specific surface area of 491 m²g⁻¹ and contained ca. 2% of K as K₂CO₃ over the peripheral region of fiber. The fiber showed high deodorization ability against 30 ppm of H₂S gas in air at ambient temperature. H₂S gas did not diffuse to the most interior parts of the fiber and was oxidized around outer regions of the fiber. Elemental sulphur was deposited in the fiber after H₂S absorption. The deodorization mechanism was discussed. The role and action of the K₂CO₃ supported was explained.     

Kinetics of the oxidation of carbon black by NO2: Influence of the presence of water and oxygen

In a fixed bed reactor, the rate of carbon black oxidation by NO₂ is significant for temperatures above 300°C, leading to NO, CO and CO₂ formation. The presence of O₂ in the feed gas increases the rate of oxidation, as well as the presence of water. A cumulative effect is observed when both water and oxygen are present. An oxygen balance shows that oxygen atoms of water molecules are not consumed. Water acts as a catalyst for the C-NO₂ reaction. A kinetic mechanism in which intermediate nitro-oxygenated species are formed in the presence of NO₂ during an initial step is in agreement with all these observations. Oxygen and NO₂ are able to react with these species at 300°C. A parametric study of the effects of the temperature, NO₂, O₂ and H₂O concentrations was performed. With a one-dimensional model of NO₂ consumption along the thickness of the carbon black bed, kinetic constants were derived and a phenomenological law was proposed, accounting for the effect of the presence of oxygen and water.