Fundamental study of adsorption and desorption process in porous materials with functional groups

Grand Canonical Monte Carlo (GCMC) simulation was used to determine the isotherms and isosteric heats of argon and the strongly polar molecule, sulphur dioxide (SO2), adsorbed at 78 and 273 K on a graphitized thermal carbon black (GTCB) surface with functional groups. The functional group, was modelled as oxygen atoms bonded to a C-atom in the graphene surface, since these have been shown to be retained after thermal treatment of GTCB. The simulated adsorption isotherms and isosteric heats of argon and SO2 were compared with the experimental data. It is shown that, while functional groups do not affect the adsorption of argon, adsorption of SO2 is very sensitive to their concentration, especially at low loadings, where the adsorption is dominated by the electrostatic interaction between SO2 and the functional group. This is confirmed by analysis of the various contributions to the isosteric heat: (1) fluid-functional group interactions, (2) fluid-basal plane interactions, and (3) fluid–fluid interactions. Finally, we investigated the orientation of SO2 in the first and second layers depends on loading as well as on the distance of the molecule from the surface.

Adsorption of argon on graphitized carbon black preloaded with methanol, ammonia and water: The role of adsorption regions and adsorbates

Effects of surface mediation on the adsorption isotherm and heat of adsorption of argon on graphitized thermal carbon black

The performance of intermolecular potential models on the adsorption of benzene on graphitized thermal carbon black at various temperatures is investigated. Two models contain only dispersive sites, whereas the other two models account explicitly for the dispersive and electrostatic sites. Using numerous data in the literature on benzene adsorption on graphitized thermal carbon black at various temperatures, we have found that the effect of surface mediation on interaction between adsorbed benzene molecules must be accounted for to describe correctly the adsorption isotherm as well as the isosteric heat. Among the two models with partial charges tested, the WSKS model of Wick et al. that has only six dispersive sites and three discrete partial charges is better than the very expensive all-atom model of Jorgensen and Severance. Adsorbed benzene molecules on graphitized thermal carbon black have a complex orientation with respect to distance from the surface and also with respect to loading. At low loadings, they adopt the parallel configuration relative to the graphene surface, whereas at higher loadings (still less than monolayer coverage) some molecules adopt a slant orientation to maximize the fluid-fluid interaction. For loadings in the multilayer region, the orientation of molecules in the first layer is influenced by the presence of molecules in the second layer. The data that are used in this article come from the work of Isirikyan and Kiselev, Pierotti and Smallwood, Pierce and Ewing, Belyakova, Kiselev, and Kovaleva, and Carrott et al.

Adsorption of water and methanol on highly graphitized thermal carbon black: The effects of functional group and temperature on the isosteric heat at low loadings

Tailoring Zirconium-based metal organic frameworks for enhancing Hydrophilic/Hydrophobic Characteristics: Simulation and experimental investigation

In this paper, we study the effect of solid surface mediation on the intermolecular potential energy of nitrogen, and its impact on the adsorption of nitrogen on a graphitized carbon black surface and in carbon slit-shaped pores. This effect arises from the lower effective interaction potential energy between two particles close to the surface compared to the potential energy of the same two particles when they are far away from the surface. A simple equation is proposed to calculate the reduction factor and this is used in the Grand Canonical Monte Carlo (GCMC) simulation of nitrogen adsorption on graphitized thermal carbon black. With this modification, the GCMC simulation results agree extremely well with the experimental data over a wide range of pressure; the simulation results with the original potential energy (i.e. no surface mediation) give rise to a shoulder in the neighbourhood of monolayer coverage and a significant over-prediction of the second and higher layer coverages. The influence of this surface mediation on the dependence of the pore-filling pressure on the pore width is also studied. It is shown that such surface mediation has a significant effect on the pore-filling pressure. This implies that the use of the local isotherms obtained from the potential model without surface mediation could give rise to a serious error in the determination of the pore-size distribution.

In this paper, we study the surface heterogeneity and the surface mediation on the intermolecular potential energy for nitrogen adsorption on graphitized thermal carbon black (GTCB). The surface heterogeneity is modeled as the random distribution of “effective” carbonyl functional groups on the graphite surface. The molecular parameters and the discrete charges of this carbonyl group are taken from Jorgensen, et al. (J. Am. Chem. Soc., (1984) 106, 6638) while those for nitrogen (dispersive parameters and discrete charges) are taken from Murthy et al. (Mol. Phys., (1983) 50, 531) in our Grand Canonical Monte Carlo (GCMC) simulation. The solid surface mediation in the reduction of intermolecular potential energy between two fluid molecules was taken from a recent work by Do et al. (Langmuir, (2004) 20, 7623). Our simulation results accounting for the surface heterogeneity and surface mediation on intermolecular potential energy were compared with the experimental data of nitrogen at 77 and 90 K. The solid-fluid dispersive parameters are determined from the Lorentz-Berthelot (LB) rule. The fraction of the graphite surface covered with carbonyl functional groups was then derived from the consideration of the Henry constant, and for the data of Kruk et al. (Langmuir, (1999) 15, 1435) we have found that 1% of their GTCB surface is covered with “effective” carbonyl functional groups. The damping constant, due to surface mediation, was determined from the consideration of the portion of the adsorption isotherm where the first layer is being completed, and it was found to take a value of 0.0075. With these parameters, we have found that the GCMC simulation results describe the data over the complete range of pressure substantially better than any other MC models in the literature. The implication of this work is demonstrated with local adsorption isotherms of 10 and 20 A slit pores. One was obtained without allowance for surface mediation, while the other correctly accounts for these factors. The two local isotherms differ substantially, and the implication is that if we used incorrect local isotherms (i.e. without the surface mediation) the pore size distribution would be incorrectly derived.

Study of heat of adsorption across the capillary condensation in cylindrical pores

Adsorption phenomena are increasingly studied by computer molecular simulation because it can be used to minimize the experimentation effort or to predict adsorption isotherms at temperatures other than those used in experiments. Activated carbon represents an important group of adsorbents because of its desirable properties for a broad range of applications, including purification of gases and liquids, membrane technology, catalysis, energy storage and environmental technology. Traditionally molecular simulations of activated carbons rely on the modelling of pore as a parallel pair of infinite graphite layers. In reality pores are neither infinite nor uniform and they contain functional groups, chemical impurities and defects on the basal graphene layers. Therefore to describe correctly the physical adsorption on activated carbon, a real carbon pore of finite length and carbon surfaces as graphene layers with structural and chemical heterogeneities should be taken into account. The aim of this thesis is to extend the understanding of the effects of pore geometries and surface heterogeneities on the adsorption of polar and non-polar fluids in porous carbons. This thesis investigates the adsorption behaviour of finite length pores using various Monte Carlo simulation techniques (a Canonical (NVT), a Gibbs (GEMC) and a Grand Canonical (GCMC), ensembles). First, the behaviour of non-polar molecules such as argon and methane is investigated. The sub-critical adsorption of these species in a finite pore is significantly different from that in an infinite slit pore, in terms of capacity and hysteresis loop. The slant hysteresis loop observed for finite pores is very similar to hysteresis loops typically observed for activated carbons. The placement of high energy sites and pore constrictions also alters the adsorption behaviour. Basically it shifts the onset of adsorption to lower pressure and the adsorption isotherms for these heterogeneous pores are generally greater than that for corresponding homogeneous pores. In the case of pore constriction, the adsorbed phase is generally started by forming the initial two contact layers at the constriction then at the larger cavity and subsequently filling the inner cores. To model non-graphitized surface, a defective surface model is used and it is found to be an excellent model to describe argon and nitrogen adsorption on Cabot Non-Graphitized Carbon Blacks. In addition to non-polar molecules, the adsorption of water in finite pores has also been studied. The adsorption of water on activated carbon is very complex, due to the strong hydrogen bonding, and it depends on the concentration and position of the functional group. The onset of adsorption shifts to lower pressure when the concentration of functional group increases or when the functional group is positioned at the centre of the graphene surface. In all cases investigated, the hysteresis loop always exists, and the loop size depends on the concentration of functional group and its position. Similar to the chemical heterogeneity due to functional group, the structural defects also have significant effects on the adsorption isotherm in shifting the pore filling to a lower pressure when it is located at a position away from the pore entrance. We found that the molecular simulation results agree well with the experimental data of a commercial activated carbon when the model porous solid is composed of pores having widths in the range between 8 and 30 A and the functional groups positioned at the centre of the graphitic wall, and the simulated isotherm of water on a heterogeneous surface can describe the behaviour of water on Graphitized Thermal Carbon Black (GTCB) satisfactorily. For both cases of non-polar and polar fluids, we particularly investigated the effects of curvature on the behaviour of adsorption isotherms. For the homogeneous cylinder, the pore filling occurs at a pressure lower than the saturation pressure while it is greater in the case of homogeneous slit pore of a comparable size. The size of hysteresis loop is more sensitive to the length of cylinder than that of slit, and it increases with a decrease in pore length. In the case of argon adsorption in homogeneous cylinder, the curvature effects lead to an early onset of adsorption isotherm and a lower amount at saturation (P → P0).

The adsorption of Lennard-Jones fluids (argon and nitrogen) onto a graphitized thermal carbon black surface was studied with a Grand Canonical Monte Carlo Simulation (GCMC). The surface was assumed to be finite in length and composed of three graphene layers. When the GCMC simulation was used to describe adsorption on a graphite surface, an over-prediction of the isotherm was consistently observed in the pressure regions where the first and second layers are formed. To remove this over-prediction, surface mediation was accounted for to reduce the fluid–fluid interaction. Do and co-workers have introduced the so-called surface-mediation damping factor to correct the over-prediction for the case of a graphite surface of infinite extent, and this approach has yielded a good description of the adsorption isotherm. In this paper, the effects of the finite size of the graphene layer on the adsorption isotherm and how these would affect the extent of the surface mediation were studied. It was found that this finite-surface model provides a better description of the experimental data for graphitized thermal carbon black of high surface area (i.e. small crystallite size) while the infinite-surface model describes data for carbon black of very low surface area (i.e. large crystallite size).

Percolation theory and the associated conductance networks have provided deep insights into the flow and transport properties of a vast number of heterogeneous materials and media. In practically all cases, however, the conductance of the networks' bonds remains constant throughout the entire process. There are, however, many important problems in which the conductance of the bonds evolves over time and does not remain constant. Examples include clogging, dissolution and precipitation, and catalytic processes in porous materials, as well as the deformation of a porous medium by applying an external pressure or stress to it that reduces the size of its pores. We introduce two percolation models to study the evolution of the conductivity of such networks. The two models are related to natural and industrial processes involving clogging, precipitation, and dissolution processes in porous media and materials. The effective conductivity of the models is shown to follow known power laws near the percolation threshold, despite radically different behavior both away from and even close to the percolation threshold. The behavior of the networks close to the percolation threshold is described by critical exponents, yielding bounds for traditional percolation exponents. We show that one of the two models belongs to the traditional universality class of percolation conductivity, while the second model yields nonuniversal scaling exponents.

Adsorption of ethylene and ethane on graphitized thermal carbon black and in slit pores whose walls are composed of graphene layers is studied in detail to investigate the packing efficiency, the two-dimensional critical temperature, and the variation of the isosteric heat of adsorption with loading and temperature. Here we used a Monte Carlo simulation method with a grand canonical Monte Carlo ensemble. A number of two-center Lennard-Jones (LJ) potential models are investigated to study the impact of the choice of potential models in the description of adsorption behavior. We chose two 2C-LJ potential models in our investigation of the (i) UA-TraPPE-LJ model of Martin and Siepmann for ethane and Wick et al. for ethylene and (ii) AUA4-LJ model of Ungerer et al. for ethane and Bourasseau et al. for ethylene. These models are used to study the adsorption of ethane and ethylene on graphitized thermal carbon black. It is found that the solid-fluid binary interaction parameter is a function of adsorbate and temperature, and the adsorption isotherms and heat of adsorption are well described by both the UA-TraPPE and AUA models, although the UA-TraPPE model performs slightly better. However, the local distributions predicted by these two models are slightly different. These two models are used to explore the two-dimensional condensation for the graphitized thermal carbon black, and these values are 110 K for ethylene and 120 K for ethane.

Underlying mechanism of CO2 uptake onto biomass-based porous carbons: Do adsorbents capture CO2 chiefly through narrow micropores?

To gain a deeper understanding as to the nature of the adsorption hysteresis due to capillary condensation of nitrogen in ordered mesoporous silicas, we calculated the temperature dependences of the activated condensation, equilibrium transition, and activated desorption pressures for nitrogen in spherical and cylindrical silica pores with several different pore sizes on the basis of semimacroscopic continuum models. The results clearly indicate that the models capture the exact nature of capillary condensation and evaporation phenomena of a fluid in cagelike and cylindrical mesopores. The temperature dependences of the adsorption hysteresis of nitrogen measured confirm previous theoretical predictions for cylindrical pores: for the ordered mesoporous silicas with cylindrical mesopores at least greater than ∼7 nm in diameter, the capillary condensation takes place via a nucleation process followed by a growth process of a bridging meniscus at pressures higher than the equilibrium transition, while the capillary evaporation takes place via a receding meniscus from pore ends at the equilibrium. For SBA-15 and MCM-41 with smaller mesopore sizes, on the other hand, the capillary condensation takes place close to the equilibrium transition pressures, while the capillary evaporation takes place at pressures lower than the equilibrium, owing to single pore blocking due to corrugation of the cylindrical pores. We discuss the effect of curvature on surface tension in capillary condensation, as well as the relation between a change in the mechanisms of adsorption and desorption and the pore corrugation in the cylindrical pores.