Effect of the carbon surface layer chemistry on benzene adsorption from the vapor phase and from dilute aqueous solutions

We present a complex study of benzene adsorption on chemically modified commercial activated carbons. The porosity of studied carbons is almost the same, whereas the chemical composition and the acid-base properties of surface layers differ drastically from amphoteric (initial de-ashed carbon D43/1, Carbo-Tech, Essen, Germany) and acidic (carbon modified with concentrated HNO3 and fuming H2SO4) to strongly basic (carbon modified with gaseous NH3). Benzene adsorption isotherms measured from aqueous solution at three temperatures (298, 313, and 323 K) and at the neutral pH level are reported. They are supported by studies of water and benzene adsorption from the gaseous phase (volumetric and calorimetric data) and the data of benzene temperature-programmed desorption (TPD). Moreover, the data of the enthalpy of immersion in water and benzene are also presented. Obtained data of benzene adsorption from the gaseous phase are approximated by applying the method of Nguyen and Do (ND) and the Dubinin-Astakhov (DA) equation. The data of adsorption from solution are described by the hybrid DA-Freundlich (DA-F) model. We show that there are similarities in the mechanisms of benzene adsorption from the gaseous phase and from aqueous solutions and that the pore-blocking effect is the main stage of the adsorption mechanism. This effect strongly depends on the polarity of the carbon surface. The larger the ratio of the enthalpy of carbon immersion in water to the enthalpy of immersion in benzene, the larger the reduction in adsorption from solution, compared to that in the gaseous phase, that is observed.     

Heterogeneity on high-resolution alpha(s) plots for carbon nanotubes-GCMC study

We present the first study showing the influence of heterogeneity of the internal wall of carbon nanotubes on the shape of high resolution alpha(s)-plots.     

Carbon surface chemical composition in para-nitrophenol adsorption determined under real oxic and anoxic conditions

A series of commercial unmodified and modified activated carbons was studied. The surface chemical composition was characterized using X-ray photoelectron spectroscopy and Boehm titration methods. Data on p-nitrophenol (pnp) adsorption isotherms determined under real oxic and anoxic conditions (at 310 K) are presented and described using bimodal Langmuir and lattice density functional theory models. The applicability of the pnp molecule for determination of surface area using adsorption from solution data is discussed. It is shown that under anoxic conditions adsorption and relative enthalpy of this process depend on the value of BET apparent surface area and DA micropore volumes. The differences between adsorption levels under both conditions increase with rise in solute equilibrium concentration. Moreover, the average difference between adsorption values under both conditions increases and next decreases with rise in the concentration of surface acidic groups. Applying quantum chemical calculations, we show that under anoxic conditions the influence of surface oxygen groups on pnp adsorption is small, whereas under oxic conditions the reverse situation is observed. Obtained theoretical results show very good correspondence to the experimental data and the origin of the relationships observed experimentally is explained and discussed.     

BET surface area of carbonaceous adsorbents—Verification using geometric considerations and GCMC simulations on virtual porous carbon models

The applicability of BET model for calculation of surface area of activated carbons is checked by using molecular simulations. By calculation of geometric surface areas for the simple model carbon slit-like pore with the increasing width, and by comparison of the obtained values with those for the same systems from the VEGA ZZ package (adsorbate-accessible molecular surface), it is shown that the latter methods provide correct values. For the system where a monolayer inside a pore is created the ASA approach (GCMC, Ar, T = 87 K) underestimates the value of surface area for micropores (especially, where only one layer is observed and/or two layers of adsorbed Ar are formed). Therefore, we propose the modification of this method based on searching the relationship between the pore diameter and the number of layers in a pore. Finally BET; original and modified ASA; and A, B and C-point surface areas are calculated for a series of virtual porous carbons using simulated Ar adsorption isotherms (GCMC and T = 87 K). The comparison of results shows that the BET method underestimates and not, as it was usually postulated, overestimates the surface areas of microporous carbons.     

Simulating the changes in carbon structure during the burn-off process

Using a simple energetic criterion, we modelled the process of activation of 'soft' activated carbons. Eighteen carbon samples, differing in degree of graphitisation, and obtained using Molecular Dynamics annealing of an amorphous carbon precursor were studied. For all samples, the geometric pore size distribution was calculated using the method proposed by Bhattacharya and Gubbins. Adsorption isotherms for Ar at 87 K were simulated and analysed using different approaches widely applied in adsorption science (α(s), DA, APD, ND, BET). It is shown that our approach leads to similar changes in microporosity (with the rise in carbon burn-off) to those observed in real experiments. Moreover, the conclusions about the reality of popular methods of carbon porosity characterisation are given.     

EFFECT OF COMPOSITION AND PROCESSING CONDITION ON MICROSTRUCTURAL PROPERTIES AND DURABILITY OF FLUOROPOLYMER/ACRYLIC BLENDS

Fluoropolymer blends have been widely used as binders for exterior coatings because of their excellent resistance to ultra-violet (UV) radiation as well as to many corrosive chemical agents.It is known that the fluorinated component usually has a lower glass transition temperature and easily crystallizes in the final structure depending upon the blend composition and sample annealing condition.We investigated the effect of blend composition and annealing process (slow and fast cooling) on the surface mor...     

The HSAB principle as a means to interpret the reactivity of carbon nanotubes

Polycyclic curved aromatic fragments (C₂₄H₁₂) have been employed as models of the single-walled carbon nanotubes (n,0), where n varies from 4 up to 30. Those structures were chosen on the basis of the analysis of the strain energy values calculated for the models possessing various sizes. The flat coronene structure has been chosen as a molecular fragment topologically resembling the honeycomb lattice in order to investigate the relation between the curvature and reactivity of the sidewalls of SWNTs. In the current study we took into account the interaction of CO and NH₂ (treated as probe molecules) with the exterior surface of nanotubes. Obtained results illustrate that both total as well as local hardness and/or molecular electrostatic potential (MEP) can be a good measure for the reactivity if the influence of geometrical changes is considered. The systematic theoretical studies also show that the calculated interaction energies of sorbed CO on those models are related to the both types of hardness. On the other hand, in the case of amidogen sorbed on the nanotube surface the correlation between the binding energy and MEP is visible. Those differences can be explained by various kinds of the adsorption mechanism, i.e. physical or chemical adsorption, respectively.