Teresa Siemieniewska

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 8C. Surface basic nitrogen groups were introduced either by treatment with ammonia/air at 300 8C or with ammonia/steam at 800 8C. 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.

Transport and sorption of water vapour in activated carbons

The adsorption of water vapour on microporous carbons derived from the carbonization of coconut shell has been studied. The adsorption and desorption characteristics of water vapour on the activated carbons were investigated over the pressure range p/p0 0–0.95 in a static water vapour system. In these experiments the process of water adsorption/desorption was investigated by both kinetic and equilibrium experimental data. Activated carbons differing by the degree of burn-off have shown the importance of the microstructure. A carbon enriched with nitrogen functions underlined the influence of the surface chemistry.

Formation of porous structures in activated brown-coal chars using O2, CO2 and H2O as activating agents

Two brown coals, xylitic and earthy, carbonized at 1173 K were activated with water vapour, carbon dioxide and oxygen, each producing a different distribution of porosity. In the xylitic coke, activated in the range of burn-offs from 1 to 70%, the action of water vapour results in the development of pores of all dimensions. At the highest burn-off the product has an effective surface area of 920 m2 g−1 and a total sorptive pore volume of 0.83 cm3 g−1, 33% of which is in micropores. Carbon dioxide creates, from the xylitic coke at the burn-off of 70%, a highly microporous adsorbent with about the same surface area (890 m2 g−1) as the corresponding water-vapour activated product. The pore volume of the carbon dioxide sample is lower (0.49 cm3 g−1) but these contain 63% of micropores, which amounts to a contribution of 92% of these pores to the effective total surface area. The activation of the xylitic coke with oxygen leads to a high development of porosity at low burn-offs, but becomes ineffective on continuation of the process to medium and high burn-offs. This is thought to be due to a blocking of the entrances of the micropores by surface oxygen complexes formed on the surface of the coke. Oxygen gives, at a high burn-off, a product with the lowest total adsorptive volume (0.45 cm3 g−1) and surface area (650 m2 g−1). All the activated products obtained from the xylitic coke can be regarded, when effective surface areas are considered, as microporous adsorbents. With the earthy coke a total adsorptive pore volume (consisting mainly of wide mesopores) is developed which is higher than with the corresponding xylitic coke, but this result is difficult to reproduce, because the earthy coke samples are easily influenced by temperature in the process of activation, especially that by oxygen.

Development of porosity in brown-coal chars on activation with carbon dioxide

Abstract Two petrographic types of Tertiary brown coals, xylitic and earthy, were carbonized, and activated with carbon dioxide between 1123 and 1273 K. The development of porosity in the activated chars was studied by adsorption of benzene and carbon dioxide at 298 K and by mercury porosimetry. The type of brown coal exerts a dominant influence on the properties of the activated chars. The xylitic brown-coal, when compared with the earthy brown-coal, yields products with a higher pore volume and better sorptive properties. Activated chars from the xylitic brown-coal reach a surface area of 800 m 2 g −1 , contained principally in micropores and very narrow mesopores (radius below 3.0 nm). Dimensions of pores in the activated chars from the earthy brown-coal are less uniform, the mesopores are broader (an important part of them has a radius between 5.0 and 100.0 nm), and micropores are present to a smaller extent; the surface area of these products is between 200 and 350 m 2 g −1 . Activated chars from both types of brown coals have a well developed system of macropores.

The pore structure of activated chars of brown coal humic acids obtained at increased rate of carbonization

Brown coal humic acids, after pyrolysis at a heating rate of 600°C/min, have been activated in steam to different burn-offs. Carbonaceous adsorbents were obtained with well developed micro-, meso-, and macroporous structures, and nitrogen BET surface areas of about 1000–1500 m2/g. An increase in the heating rate during pyrolysis to 600°C/min (from the standard rate of 5°C/min) strongly enhances macropore development during subsequent activation, although micro- and mesopore volumes remain similar. The pore structure parameters of the activated chars based on the adsorption of nitrogen have been compared with those obtained from benzene and cyclohexane adsorption. The differences observed are less pronounced for benzene than for cyclohexane. The kind of nonporous carbonaceous standard employed, whether ungraphitized (Elftex 120) or graphitized (Spheron 6–2700), has little effect upon the results of calculations of the mesopore size distribution, but a marked effect on parameters derived from the α5 plot. For those chars that, in addition to their microporous system, were characterised by a significant contribution of mesoporosity, the values of Vm (from the BET plot) are visibly lower than the respective values of VB (from the position of point B). An attempt was made to use the existence of these differences in calculations of the surface area of the mesopores.

Cheap adsorbent. Part 1: Active cokes from lignites and improvement of their adsorptive properties by mild oxidation

In this article, cheap adsorbents were produced starting from two different lignites. About 500 kg of each coal was pyrolyzed in a rotary kiln at semi-pilot scale. Characterization of the obtained chars are made in terms of porosity development, surface functional groups and adsorptive properties determined for both 4-nitrophenol and lead. A post-oxidative treatment at low temperature is proposed in order to improve the adsorptive capacities of the initial chars by the introduction of oxygen containing functional groups, which are widely involved in chemisorption. The conditions of the post-treatment were chosen to be easily exported in an industrial process, for example during the cooling step. The importance of both lignite characteristics (ashes content, water content) and pyrolysis conditions is demonstrated. The positive effect of a simple post-oxidative treatment is shown by a strong increase in the adsorptive capacities, and correlated to the evolution of the porosities and surface oxygen functionalities. Considering unit area, the properties of our active cokes are similar to those of commercial active carbon, but their specific areas are less developed (about 400 m2 g−1).

Activation of brown-coal chars with oxygen

Abstract Semicokes and cokes prepared respectively at 773 and 1173 K from brown-coals, xylitic and earthy, from Polish coal seams, were activated with gaseous oxygen (10% oxygen and 90% argon) in a thermogravimetric apparatus to different burn-offs. With increasing temperature of oxygen activation a constant decrease of the sum of micropores and mesopores is observed, but probably as a result of chemisorption of oxygen the micropore volume passes through a maximum at 663 K. There is a strong influence of the temperature of carbonization of the char on the formation of porosity in the products of oxygen activation: activated cokes have better adsorptive properties than activated semicokes. The highest value of surface areas (benzene adsorption) are, for semicokes and cokes respectively, 520 and 700 m2 g−1. These differences can be attributed to the uniform microporosity in the non-activated coke as distinct from the wide range of the micropore diameters in the non-activated semicoke, and also to the lack of ultramicropores in the former sample. The earthy type of brown coal yields products with a less developed porosity than the corresponding products from the xylitic coal. For the xylitic semicoke as well as for the coke, after continuing the process of activation to burn-offs higher than 50%, a lowering of adsorptive properties is observed.

Influence of the oxygen content of low-rank coals on the development of porosity during carbonization

Abstract The development of porosity in the course of carbonization of a flame coal, original and pre-oxidized, was studied by means of the adsorption of benzene and carbon dioxide. The results were compared with corresponding data for cokes from a xylitic brown coal. The influence of coal oxygen content on the formation of coke porosity and its thermal dependence is discussed.

Following changes in the porous texture of Nomex-derived activated carbon fibres with the molecular probe technique

A series of Nomex-derived activated carbon fibres (ACFs) activated to different burn-offs (BOs) together with two series derived from the previous ACFs through chemical vapour deposition (CVD) treatments were used as adsorbents. The adsorption of molecules of different size, namely, dichloromethane, benzene, cyclohexane and carbon tetrachloride was used to monitor the textural evolution of microporous carbon fibres owing to these treatments. The analysis of the results showed that carbon deposit has taken place mainly at the entrance of the pores rather than in the interior. The information obtained about the porous structure and the molecular sieve behaviour of the samples in the different series was also useful for establishing the possible uses of these materials: thus, the obtained ACFs could be used for adsorbing volatile organic compounds (VOCs) in the field of environmental control while CVD-treated ACFs could be used as adsorbents for gas separations. In particular, the combined use of CH2Cl2 and C6H6 could be used to determine whether an adsorbent is a good carbon molecular sieve (CMS) for CO2/CH4 separation.

Capillary structure and reactivity of chars from brown coal humic acids containing Ca, Fe, Mn and Na

Abstract Salts of Ca, Fe, Mn and Na were added to humic acids extracted from brown coal. The influence of the added inorganic substances on the course of carbonization and gasification with H 2 O, CO 2 and 10% O 2 was studied.