Catalytic reduction of NOx by biomass-derived activated carbon supported metals

In this study, to prepare a series of activated carbon-supported metals for the catalytic reduction of NO_x to N₂ in excess O₂, activated carbons derived from lignocellulosic and herbaceous biomasses were selected as the reducing agents, and alkali and transition metals were used as the catalytic active phases. The effects of the type of biomass, carbonization temperature and catalyst composition on NO_x reduction efficiency were analyzed in a fixed-bed flow reactor. The results showed that two temperature regimes are present for the NO_x-carbon reaction:at temperatures below 250℃, the NO_x adsorption process on the carbon surface was predominant, whereas true NO_x reduction by carbon occurred at temperatures above 250℃, producing N₂, CO₂ and CO. The influence of the carbonization temperature on carbon reactivity depended on the effect of the carbonization temperature on the carbon surface area and the reduction of the metal species on carbon. All studied metals catalyzed both NO_x and O₂ reduction by carbon, and potassium could strongly enhance the C-NO_x reaction without substantial carbon consumption by O₂. Moreover, the potassium supported by sawdust-derived activated carbon exhibited higher selectivity and capacity towards NO_x reduction than did its previously reported coal-derived counterparts. These properties were ascribed to the high dispersion of the active potassium species on the carbon surface, as observed through the comparison of X-ray photoelectron spectroscopy and powder X-ray diffraction results for the carbons made from biomass and coal-based precursors.     

SO2 retention on CaO/activated carbon sorbents. Part II: Effect of the activated carbon support

The present paper analyses the role of the activated carbon (AC) properties on the SO₂ uptake capacity of CaO/AC sorbents prepared by AC impregnation or ionic exchange with calcium acetate water solutions. Gas adsorption and mercury porosimetry have been used for textural characterization of the AC and surface oxygen groups have been characterized by temperature programmed desorption (TPD). Thermogravimetry has been used for SO₂ retention tests and CO₂ chemisorption at 300 °C for CaO dispersion (d) determinations. The results show that the surface calcium on CaO/AC samples, determined as “Ca loading · CaO dispersion” (parameter Ca(%) · d), governs the SO₂ uptake. The surface oxygen content is the AC property that mainly controls both the calcium loading and surface calcium on CaO/AC samples, which could be explained by the fact that the surface oxygen lowers the hydrophobic character of the AC supports therefore favouring the interaction with the calcium acetate water solutions. The combination of high calcium loading and dispersion leads to SO₂ uptakes up to 123 mg SO₂/g. The textural properties of the supports have some influence in the calcium loading. However, the effect is masked by the blockage of AC porosity by the calcium loaded.     

Hierarchical pore structure of activated carbon fabricated by CO2/microwave for volatile organic compounds adsorption

An activated carbon pore-expanding technique was achieved through innovative reactivation by CO_2/microwave.The original and modified activated carbons were characterized by nitrogen adsorption–desorption,scanning electron microscopy,transmission electron microcopy,and Fourier transform infrared spectroscopy.The mesopore volume increased from 0.122 cm~3·g~(-1) to 0.270 cm~3·g~(-1),and a hierarchical pore structure was formed.A gradual decrease in the phenolic hydroxyl and carboxyl groups on the surface of activated carbon enhanced the surface inertia of granular activated carbon(GAC).The toluene desorption rate of the modified sample increased by 8.81% compared with that of the original GAC.Adsorption isotherm fittings revealed that the Langmuir model was applicable for the original and modified activated carbons.The isosteric adsorption heat of toluene on the activated carbon decreased by approximately 50%,which endowed the modified sample with excellent stability in application.The modified samples showed an enhanced desorption performance of toluene,thereby opening a way to extend the cycle life and improve the economic performance of carbon adsorbent in practical engineering applications.     

Estimation of activated carbon adsorption efficiency for organic vapours: I. A strategy for selecting test compounds

Strategies for developing quantitative structure–affinity relationships (QSAfR) for the prediction of break-through performance of 31 chlorinated hydrocarbons on activated carbon have been studied. Two different approaches for the selection of a limited set of compounds for modelling were evaluated through the predictive power of the resulting QSAfR models. When the model was based on a training-set selected without a rational strategy, the developed QSAfR model showed poor predictive performance. Accordingly, such models have a limited capability to produce information concerning the important adsorbate related parameters influencing adsorption. By using a strategy where multivariate data analytical techniques are used in conjunction with statistical experimental design to select a balanced set of compounds for break-through performance evaluation, it was possible to develop QSAfR models with high predictive capability.     

Adsorption of volatile organic compounds by means of activated carbon fibre-based monoliths

Activated carbon fibre monoliths (ACFMs) were prepared from the rejects of polymeric fibres (Nomex™). These were carbonised, agglomerated with a phenolic resin and steam activated at burnoff degrees between 0 and 40%. Adsorption experiments with n-butane at 30 °C show that, at high adsorbate concentrations, the amount adsorbed is a function of pore volume, but at low concentrations this mainly depends on pore size distribution. The porosity of Nomex-based ACFMs is formed by narrow micropores, which permit higher amounts of vapour to be adsorbed in low concentrations compared to monoliths prepared from different commercial activated fibres and a commercial granular activated carbon, which exhibits wider pores. The agglomeration of Nomex-fibres to form ACFMs does not cause any loss in adsorption properties with respect to non-agglomerated activated fibres. From the adsorption experiments of different vapours on a Nomex-based ACFM (40% burnoff) it was found that at high concentrations (p/p_o=1) the adsorbed volume was independent of the nature of the adsorbate and depended only on pore volume. However, at low vapor concentrations (p/p_o=0.004), the amount adsorbed depended on the adsorbate being well correlated to the molecular parachor and the polarizability of the adsorbates     

Catalytic effects of inorganic compounds on the development of surface areas of fly ash carbon during steam activation

This study was to investigate the catalytic effect of inorganic compositions present in fly ash carbons, high-unburned-carbon content fly ashes from coal-fired power plants, on the development of surface area during steam activation. Through this work, the relationships between the concentrations of alkali metals (Na and K) and a mixture of metals, Na-K-Ca and Na-K-Fe, in fly ash carbon and the surface areas of the produced activated carbon were studied.Six fly ash carbons were selected as feedstocks for activated carbon. SEM/EDS studies showed that there are two groups of inorganic particles present in fly ash carbons, in terms of the way they are associated with carbon particles: inorganic particles that exist as free single particles, and inorganic particles that are combined with the carbon particles. A series of froth flotation tests was conducted to separate the individual inorganic particles from the fly ash carbons. The concentration of the inorganic particles was analyzed using ICP-AES before and after froth flotation separation. Studies showed that the Na-K, Na-K-Ca, and Na-K-Fe which are combined with carbon particles had catalytic effects on the development of the surface areas of activated carbons. The higher the concentration of these catalytic particles, the more significant effects they had on the development of the surface areas with increasing carbon burn-off levels.     

Adsorption of chlorinated volatile organic compounds in a fixed bed of activated carbon

Adsorption isotherms of dichloromethane and 1,1,2-trichloro-1,2,2-trifluoroethane on an activated carbon pellet, Norit B4, were studied. For these chemicals, the Sips equation gave the best fit for the single component adsorption isotherm. The adsorption affinity on activated carbon was greater for dichloromethane than that of 1,1,2-trichloro-1,2,2-trifluoroethane. An experimental and theoretical study was made for the adsorption of dichloromethane and 1,1,2-trichloro-1,2,2-trifluoroethane in a fixed bed. Experimental results were used to examine the effect of operation variables, such as feed concentration, flow rate and bed height. Intraparticle diffusion was able to be explained by a surface diffusion mechanism. An adsorption model based on the linear driving force approximation (LDFA) was found to be applicable to fit the experimental data.     

Catalytic properties of activated carbon surface in the process of adsorption/oxidation of methyl mercaptan

Activated carbons of different origins were used as adsorbents of methyl mercaptan (MM). Before the MM breakthrough capacity tests were carried out, the surface of carbons was characterized from the point of view of its chemistry (Boehm titration) and porosity (adsorption of nitrogen at its boiling point). The results showed that the ability of carbon to adsorb methyl mercaptan depends strongly on its surface chemistry, particularly on the presence of basic oxygen-containing groups and ash content. Catalytic effect of one metal, iron, was studied in more details. It was found that introduction of iron enhances the removal capacity significantly as a result of electron transfer reaction in which thiolate ion is oxidized to dimethyl disulfide (DMDS). This reaction likely involves the reduction of iron sites, which are regenerated after further re-exposure to oxygen. DMDS as a main reaction product is strongly adsorbed in small pores. Water is required for the formation of DMDS since it facilitates the dissociation of MM. That dissociation occurs in water film when the pH of the local system is greater than the apparent pK_a of MM in the confined pore space.     

Growth of carbon nanofibers on activated carbon fiber fabrics

Activated carbon fiber fabrics, an excellent adsorbent, were used as catalyst supports to grow carbon nanofibers. Because of the microporous structure of the activated carbon fibers, the catalysts could be distributed uniformly on the carbon surface. Based on this concept, the carbon nanofibers can be grown directly on the activated carbon fiber fabrics. We demonstrate that carbon nanofibers with a diameter between 20 and 50 nm for most of the fibers can be synthesized uniformly and densely on activated carbon fiber fabrics, impregnated by nickel nitrate catalyst precursor, using catalytic chemical vapor deposition. Although the carbon nanofibers are not straight with a crooked morphology, they form a three-dimensional network structure. Structure characterizations by TEM and XRD indicate that the carbon nanofibers have a turbostratic graphite structure and the graphite layers are stacked with a herringbone structure.     

Preparation of hydrophilic activated carbon through alkaline hydrolysis of ester for effective water-vapor adsorption

Hydrophilic activated carbon was prepared by a novel ethyl acetate hydrolysis method. Surface area analysis, spectroscopies, and microscopy were used to examine characteristic differences between raw and hydrophilic activated carbons. Results showed that hydrophilic groups were introduced onto the surface of activated carbon through ethyl acetate hydrolysis in an alkaline environment. The modified activated carbon exhibited higher water vapor adsorption capacity because the hydrophilic groups on its surface bound with water molecules through H-bonding. Adsorption isotherms of water vapor were well fitted by an adsorption model. The optimal temperature for thermal regeneration was determined to be 398 K.     

Removal of thiophenic compounds from liquid fuel by different modified activated carbon cloths

The adsorption behavior of benzothiophene (BT), dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (DMDBT) from n-heptane was investigated onto activated carbon cloth (ACC) and its modified forms at 30 °C in batch condition. ACC was modified by HNO₃, (NH₄)₂S₂O₈, H₂SO₄, HCl and NaOH at ambient temperature. The adsorbents were characterized using nitrogen adsorption/desorption. It was found that the adsorbents are mainly microporous but differ in their surface chemistry, which is related to the effect of oxidizing agent. The adsorption process was studied from both equilibrium and kinetics point of view. The equilibrium experimental data were fitted to the Langmuir, Freundlich and Langmuir-Freundlich by non-linear method. Among the tested adsorbents, the modified ACC with HNO₃ (ACC-HNO₃) had the highest capacity for adsorption of DBT. Kinetic characterization of the adsorption process indicated that the mixed-order and modified pseudo-n-order models can describe the kinetics of adsorption of thiophenic compounds onto ACCs. The ACC and ACC-HNO₃ were used to test the removal efficiency of total sulfur contents (BT, DBT and DMDBT, 150 ppmw for each of them), too. The effect of shaking and ultrasound methods and also temperature and time on the regeneration of saturated ACC-HNO₃ with DBT was studied.     

A review of the effects of covapors on adsorption rate coefficients of organic vapors adsorbed onto activated carbon from flowing gases

Published breakthrough time, adsorption rate, and capacity data for components of organic vapor mixtures adsorbed from flows through fixed activated carbon beds have been analyzed. Capacities (as stoichiometric centers of constant pattern breakthrough curves) yielded stoichiometric times τ, which are useful for determining elution orders of mixture components. Where authors did not report calculated adsorption rate coefficients k_v of the Wheeler (or, more general, Reaction Kinetic) breakthrough curve equation, we calculated them from breakthrough times and τ. Ninety-five k_v (in mixture)/k_v (single vapor) ratios at similar vapor concentrations were calculated and averaged for elution order categories. For 43 first-eluting vapors the average ratio (1.07) was statistically no different (standard deviation 0.21) than unity, so that we recommend using the single-vapor k_v for such. Forty-seven second-eluting vapor ratios averaged 0.85 (standard deviation 0.24), also not significantly different from unity; however, other evidence and considerations lead us to recommend using k_v (in mixture)=0.85k_v (single vapor). Five third- and fourth-eluting vapors gave an average of 0.56 (standard deviation 0.16) for a recommended k_v (in mixture)=0.56k_v (single vapor) for such.     

Bimetallic Pt–Sn catalysts supported on activated carbon. II. CO oxidation

A commercial activated carbon (AC) was used as a catalyst support either in its original form or after two different oxidation treatments, namely air oxidation and HNO₃ oxidation, aiming at the enhancement of its textural and surface chemical characteristics. These properties were determined by N₂ adsorption and temperature programmed desorption (TPD), respectively. Monometallic Pt and bimetallic Pt–Sn catalysts were prepared on the AC supports. Impregnation was used in the preparation of the monometallic samples. For the bimetallic samples, coimpregnation and a sequential impregnation procedure, in which the Sn precursor is introduced prior to Pt, were used. The Pt load was kept fixed as 1 wt.% for all monometallic and bimetallic samples. Two different Sn loads, 0.25 and 0.50 wt.%, were used for the bimetallic samples in order to investigate the effects of Sn load on the catalytic properties. The catalyst samples were characterized by H₂ adsorption, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and structure insensitive benzene hydrogenation. The activities of all samples were measured in CO oxidation. The results indicate the strong effects of the surface chemistry of the AC supports, the Pt:Sn ratio, the preparation procedure and the reduction procedure, on the CO oxidation activities of the catalysts.     

不同有机化合物在TiO2负载活性炭纤维上的

以环氧树脂为炭质前驱体将TiO₂粉体粘附于ACF表面，然后在N2气氛460 ℃焙烧，树脂热解、逸失使剩余残炭成为二者之间的连接体，获得TiO₂/ACF复合产品。ACF负载TiO₂后，其比表面积降低，但仍很好地维持了ACF的孔隙结构。由SEM可知，纳米级单体TiO₂颗粒以微小团簇结构分布于ACF表面。以对氨基苯酚、亚甲基兰和β-环状糊精三种不同分子尺寸的有机化合物为探针，通过TiO₂/ACF对它们的光催化降解研究发现，化合物自身分子尺寸对其降解程度有重要影响。具有适宜分子尺寸亚甲基兰的去除程度最高，TiO₂/ACF的降解速率高于悬浮态P25，在重复利用过程中未见降低。通过对比三种有机分子在有、无紫外光照下的去除程度，可确认有机分子是被降解而非吸附于TiO₂/ACF上。     

Removal of volatile organic compound by activated carbon fiber

Experiments were carried out to study adsorption/desorption of volatile organic compound (VOC) on the activated carbon fiber (ACF) under dynamic conditions. The primary objective was to experimentally demonstrate the suitability of ACF in effectively adsorbing VOCs from inert gaseous stream under varying operating conditions, and compare its performance vis-à-vis that of the other commercially available adsorbents, such as granular activated carbon (GAC), silica gel, and zeolites. The adsorption experiments were carried out in a fixed tubular packed bed reactor under various operating conditions including temperature (35-100 °C), gas concentration (2000-10,000 ppm), gas flow rate (0.2-1.0 slpm) and weight of the adsorbent (2-10 g). A mathematical model was developed to predict the VOC breakthrough characteristics on ACF. The model incorporated the effects of the gas-particle film mass transfer resistance, adsorbent pore diffusion and the adsorption/desorption rates within the pore. The experimental data and the corresponding model simulated results were compared and found to be in good agreement. The ACF repeatedly showed a good regeneration capability following desorption by DC electrical heating.     

Influence of solvent type on dibenzothiophene adsorption onto activated carbon fiber and granular coconut-shell activated carbon

The adsorption behavior of dibenzothiophene (DBT) on an activated carbon fiber (ACF) and a granular coconut-shell activated carbon (GCSAC) in the solvents n-hexane, n-decane, toluene, and mixture of n-decane and toluene was investigated. The DBT adsorption onto both samples was more active in n-hexane than in n-decane. The lowest DBT adsorption was observed in toluene. Regardless of the type of activated carbons and solvents, all the isotherms fit the Freundlich equation better than the Langmuir equation. At low equilibrium concentrations of <2 mass ppm-S, GCSAC displayed greater capacity for DBT adsorption than did ACF in all the tested solvents. The adsorption kinetics of ACF and GCSAC in all the tested solvents were governed by a pseudo-second-order model.