First Molecular Dynamics simulation insight into the mechanism of organics adsorption from aqueous solutions on microporous carbons (original) (raw)
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Physical Chemistry Chemical Physics, 2010
MD simulation studies showing the influence of porosity and carbon surface oxidation on phenol adsorption from aqueous solutions on carbons are reported. Based on a realistic model of activated carbon, three carbon structures with gradually changed microporosity were created. Next, a different number of surface oxygen groups was introduced. The pores with diameters around 0.6 nm are optimal for phenol adsorption and after the introduction of surface oxygen functionalities, adsorption of phenol decreases (in accordance with experimental data) for all studied models. This decrease is caused by a pore blocking effect due to the saturation of surface oxygen groups by highly hydrogen-bounded water molecules.
The use of organic vapour preadsorption to understand water adsorption on activated carbons
Microporous and Mesoporous Materials, 2017
In this work, n-octane preadsorption coupled with water vapour sorption has been assessed as a useful technique for the advanced characterization of porous carbon materials. First, two microporous activated carbons with different oxygen contents were gradually preloaded with the n-alkane and then characterized by different adsorbates. As the micropore system is being filled, a narrowing of the hysteresis loop and a decrease in the slope of the water isotherms at low and medium relative pressures was observed. The latter result indicates that the available surface oxygen content gradually decreased with the blockage of the micropores, thus suggesting that the primary water adsorption centers of activated carbons are located in the narrow pores. Moreover, correlations between the available surface oxygen content and the micropore volumes measured by N 2 and CO 2 adsorption were found. Then, the microporous system of two activated carbons with a mainly mesoporous structure was nearly completely blocked. The surface chemistry of these two carbons was modified in order to have a deeper analysis of the role played by the surface oxygen groups on the water adsorption mechanism. The obtained results showed that the water sorption in the mesopores is highly dependent on the available surface oxygen content.
Journal of physics. Condensed matter : an Institute of Physics journal, 2014
The results of systematic studies of organics adsorption from aqueous solutions (at the neutral pH level) in a system of slit-like carbon pores having different sizes and oxygen groups located at the pore mouth are reported. Using molecular dynamics simulations (GROMACS package) the properties of adsorbent-adsorbate (benzene, phenol or paracetamol) as well as adsorbent-water systems are discussed. After the introduction of surface oxygen functionalities, adsorption of organic compounds decreases (in accordance with experimental data) and this is caused by the accumulation of water molecules at pore entrances. The pore blocking effect decreases with the diameter of slits and practically vanishes for widths larger than approx. 0.68 nm. We observed the increase in phenol adsorption with the rise in temperature. Moreover, adsorbed molecules occupy the external surface of the slit pores (the entrances) in the case of oxidized adsorbents. Among the studied molecules benzene, phenol and pa...
Carbon, 2004
The present paper examines the adsorption of water by microporous carbons containing various amounts of surface oxygen and a smaller proportion of basic centres. The modelling of water adsorption for 293 and 310 K, using variable pore size distributions (PSD), confirms that the overall type IV isotherm is the sum of a type I isotherm associated with the specific interactions, and a type V isotherm reflecting the non-specific interactions. The principle of temperature invariance is followed by these isotherms, which indicates that modelling leads to the Dubinin-Astakhov equation.
Molecular modeling and adsorption properties of porous carbons
Carbon, 2006
In this work, we calculate the adsorption isotherms and isosteric heat of argon in molecular models of saccharose coke obtained via the Hybrid Reverse Monte Carlo method. In the first route (method A), the molecular models were built by considering only carbon atoms, and all other heteroatoms present were neglected. In the second route (method B), the molecular models were built by considering carbon and hydrogen atoms. We find that the models obtained via method B have smaller pores as compared to the models obtained via method A. This is reflected in the adsorption properties. The amount adsorbed is less in models obtained via method B as compared to method A. We also find that the isosteric heat calculated in the models obtained via method B match the experimental data more closely as compared to models obtained from method A.
Adsorption, 2005
We present a study on the effects of activation on a saccharose-based carbon using molecular simulation. A constrained Reverse Monte Carlo method is used to build molecular models that match the experimental structure factors of both activated and unactivated carbon, using appropriate constraints for bond angle and coordination number to describe the three body correlation. The semi-coke sample, that is named CS1000, is obtained by pyrolyzing pure saccharose at 1000 • C under nitrogen flow. An activated form of this carbon, CS1000a, was obtained by heating CS1000 in an atmosphere of CO 2 for 20 hours. We built molecular models for CS1000 and CS1000a and also simulated the TEM images of the model. We perform GCMC simulation of a Lennard-Jones model of Argon on the resulting models to obtain the adsorption isotherms. We then study the difference in the morphology of CS1000 and CS1000a that lead to different adsorption properties in carbon upon activation.
Predicting adsorption of water/organic mixtures using molecular simulation
AIChE Journal, 2003
The use of Monte Carlo simulation to predict the adsorption of mixtures of polar and nonpolar species on acti®ated carbon was in®estigated using water and ethane on BPL carbon as a prototype system. The structure of the adsorbent was modeled by an array of slit-shaped pores, characterized by a pore-size distribution. The chemical heterogeneity of the carbon was taken into account by including oxygen-containing sites on the surface of the pores. The pore-size distribution was obtained from pure-ethane adsorption on the same carbon sample, while the concentration and distribution of surface sites were determined by analyzing pure-water adsorption. Model predictions agree well with experimental multicomponent data.
Simulation study of the effect of the chemical heterogeneity of activated carbon on water adsorption
Langmuir, 2002
In this paper we present results from the molecular simulation of water adsorption in slit-shaped activated carbon pores. We calculate adsorption isotherms by grand canonical Monte Carlo (GCMC) simulation, Henry's constants by Monte Carlo integration, and vapor-liquid equilibrium data by the gauge-cell Monte Carlo method, to investigate the chemical heterogeneity of activated carbon adsorbents. Several types of polar oxygen-containing sites are placed on the surface of the carbon with different densities and local distributions, in order to determine the individual effects of each of these factors on the adsorption of water. Our results confirm the role of surface sites in the enhancement of water adsorption. Furthermore, we show that the local distribution of these sites has a strong effect on low-pressure adsorption, while the overall site density affects mainly the vapor-liquid phase transition. The type of oxygen-containing group is shown not to be of critical importance, since more complex groups can effectively be represented by simpler sites. This study forms the basis for the development of a model for activated carbon that is able to represent the chemical heterogeneity of this type of material.
Atomistic calculation of adsorption in activated carbon with pore-size distribution
Journal of Colloid and Interface Science, 2010
We use molecular mechanics universal force field parameters to calculate single-and multicomponent adsorption of phenol, para-bromophenol, and m-cresol from the gas or liquid phase on activated carbon (AC). The carbon pores are modeled by shallow carbon nanotubes of various pore diameters. The effect of pore length is studied for the case of phenol. This calculation yields the Gibbs free energy change (DG) of adsorption which is used to predict the adsorption isotherm following reaction rate theory. The singlepore adsorption can be integrated to predict the overall adsorption if pore-size distribution is known. We show that the Freundlich adsorption isotherm may result from pore-size distribution coupled with a linear decline of DG with pore diameter. When the pore-size distribution of the carbon under study is not known, the adsorption of one species can be used to predict that of another on the same AC. We extend this methodology for the case of a bisolute adsorption system. Reasonable prediction of measured adsorption isotherms is demonstrated for single solute adsorption when both pore-size distribution and adsorbate-adsorbate interaction are taken into accounted. The suggested methodology is highly sensitive to the numerous parameters it requires.
We employ a previously developed model of a high surface area activated carbon, based on a random packing of small fragments of a carbon sheet, functionalized with hydroxyl surface groups, to explore adsorption of water and multicomponent mixtures under conditions representing typical carbon capture processes. Adsorption of water is initialized and proceeds through the growth of clusters around the surface groups, in a process predominantly governed by hydrogen bond interactions. In contrast, energetically favorable locations for carbon dioxide molecules are different from that for water, with the main contribution coming from the Lennard-Jones interactions with the extended surfaces of the fragments. This explains why over a broad range of conditions small amounts of water do not have any substantial impact on adsorption of carbon dioxide and other species in activated carbons. From the studies of various carbon capture processes, the model material shows promising properties for pre-combustion capture due to large capacity at high pressures and other favorable characteristics.