Simple model of adsorption on external surface of carbon nanotubes: a new analytical approach basing on molecular simulation data

Nitrogen adsorption on carbon nanotubes is wide- ly studied because nitrogen adsorption isotherm measurement is a standard method applied for porosity characterization. A further reason is that carbon nanotubes are potential adsorbents for separation of nitrogen from oxygen in air. The study presented here describes the results of GCMC simulations of nitrogen (three site model) adsorption on single and multi walled closed nanotubes. The results obtained are described by a new adsorption isotherm model proposed in this study. The model can be treated as the tube analogue of the GAB isotherm taking into account the lateral adsorbate-adsorbate interactions. We show that the model describes the simulated data satisfactorily. Next this new approach is applied for a description of experimental data measured on different commercially available (and characterized using HRTEM) carbon nanotubes. We show that generally a quite good fit is observed and therefore it is suggested that the observed mechanism of adsorption in the studied materials is mainly determined by adsorption on tubes separated at large distances, so the tubes behave almost independently

Porosity of closed carbon nanotubes compressed using hydraulic pressure

Experimental data of nitrogen adsorption (T = 77.3 K) from gaseous phase measured on commercial closed carbon nanotubes are presented. Additionally, we show the results of N2 adsorption on compressed (using hydraulic press) CNTs. In order to explain the experimental observations the results of GCMC simulations of N2 adsorption on isolated or bundled multi-walled closed nanotubes (four models of bundles) are discussed. We show that the changes of the experimental adsorption isotherms are related to the compression of the investigated adsorbents. They are qualitatively similar to the theoretical observations. Taking into account all results it is concluded that in the "architecture" of nanotubes very important role has been played by isolated nanotubes

The effects of curvature and surface heterogeneity on the adsorption of water in finite length carbon nanopores: A computer simulation study

The effects of pore curvature and surface heterogeneity on the adsorption of water on a graphitic surface at 298 K were investigated using a Grand Canonical Monte Carlo (GCMC) simulation. Slit and cylindrical pores are used to study the curvature effects. To investigate the surface heterogeneity the functional group and the structural defect on the surface were specifically considered. The hydroxyl group (OH) is used as a model for the functional group and the water potential model proposed by Müller et al. is used to calculate the water interaction. For the homogeneous cylinder, the pore filling occurs at a pressure lower than the saturation pressure of the water model, while it is greater in the case of homogeneous slit pore. The size of hysteresis loop is more sensitive to the length of cylinder than that of the slit, and it increases with decreasing pore length. The isotherms of water in cylindrical pores are found to depend on the position and the concentration of the functional group. The pore filling pressure is lower with an increased number and/or with the position of the functional group. The structural defect shows significant effects on the adsorption isotherm in shifting to a lower pore filling pressure when it is located at a position away from the pore entrance. The adsorption of water on the heterogeneous surface was studied and it was found that the simulated isotherm can describe the behaviour of water on Graphitized Thermal Carbon Black (GTCB) satisfactorily. The water cluster grows mostly along the surface for the case of finite extent surface, while for the slit the pore grows in all directions but the preference is a direction perpendicular to the pore wall. Reasons for the direction of growth will be discussed

Effect of pore constriction on adsorption behaviour in nanoporous carbon slit pore: a computer simulation

A Grand Canonical Monte Carlo simulation (GCMC) method is used to study the effects of pore constriction on the adsorption of argon at 87.3 K in carbon slit pores of infinite and finite lengths. It is shown that the pore constriction affects the pattern of adsorption isotherm. First, the isotherm of the composite pore is greater than that of the uniform pore having the same width as the larger cavity of the composite pore. Secondly, the hysteresis loop of the composite pore is smaller than and falls between those of uniform pores. Two types of hysteresis loops have been observed, irrespective of the absence or presence of constriction and their presence depend on pore width. One hysteresis loop is associated with the compression of adsorbed particles and this phenomenon occurs after pore has been filled with particles. The second hysteresis loop is the classical condensation-evaporation loop. The hysteresis loop of a composite pore depends on the sizes of the larger cavity and the constriction. Generally, it is found that the pore blocking effect is not manifested in composite slit pores, and this result does not support the traditional irkbottle pore hypothesis

Characterization of Cabot non-graphitized carbon blacks with a defective surface model: Adsorption of argon and nitrogen

A grand canonical Monte Carlo simulation (GCMC) is used to study the adsorption of argon and nitrogen on non-graphitized carbon black. The surface is assumed to be finite in length and composed of three graphene layers, the top layer of which contains defects. The isotherm obtained for the non-graphitized carbon shows a smooth S-shaped type while that obtained for the perfect graphene layer shows a wavy type. The isosteric heat is also affected by the defect; its behaviour versus loading exhibits a decrease at the beginning and then slightly increases once the first layer has been formed. The decreasing behaviour of isosteric heat at low loadings is not observed in the case of graphitized carbon black. The simulated results are compared against the experimental data of argon and nitrogen at 77 and 87.3 K on the Cabot carbon black BP 280, 460 and 2000. It is found that the defected finite surface describes well the data of these blacks. For the case of BP 2000 we have found that besides the defects of the surface, this sample contains a small population of small micropores having a width of 8.2 A and its specific pore volume of 0.08 cm(3)/g. (c) 2007 Elsevier Ltd. All rights reserved

Adsorption of polar and non-polar fluids in carbon nanotube bundles: Computer simulation and experimental studies

The effects of adsorbate on the adsorption in a bundle of carbon nanotubes are investigated to explore the preferential adsorption over various adsorption sites: inside the tube, in the cusp interstices and in the square interstices outside the tubes. This is carried Out with the Grand Canonical Monte Carlo simulation and the Simulation results are tested against the experimental results of bundles of single wall carbon nanotubes (SWCN). With regard to adsorbate, we choose argon and nitrogen to represent simple fluids and water to represent strong associating fluids with strong orientation interaction. The preferential adsorption of argon and nitrogen depends on the tube size. For tube size smaller than 10.8 angstrom, adsorption inside the tube is preferred because the solid-fluid potential is greatest in the tube interiors. While for larger tubes adsorption occurs initially in the small Cusp interstices between the tubes, and as adsorption is progressed adsorption occurs inside the tube as well as the larger square interstices. At higher pressures capillary condensation occurs in the square interstices. For water, however, the adsorption mechanism is different. Its adsorption occurs dominantly inside the tube, irrespective of the tube size. This is due to the requirement of appropriate geometry to allow hydrogen bonding among water Molecules to occur. The small cusp interstices do not provide proper space for clusters of hydrogen bonded waters, while the larger square interstices are too large and hence the solid-fluid potential is not strong enough to induce adsorption unless the partial pressure is sufficiently high. Finally the model of these fluids and carbon nanotube is tested with the experimental data of a commercial SWCN, and the Simulation results are in agreement with the data. (C) 2008 Elsevier Inc. All rights reserved

Explanation of the unusual peak of calorimetric heat in the adsorption of nitrogen, argon and methane on graphitized thermal carbon black

Heats of adsorption and adsorption isotherms of argon, nitrogen and methane on a perfect graphitic surface and a defective graphitic surface are studied with a Grand Canonical Monte Carlo Simulation (GCMC). For the perfect surface, the isosteric heat versus loading shows a typical pattern of adsorption of simple fluids on graphite. Depending on adsorbate, degree of graphitization and temperature, a spike in the heat curve versus loading is observed when the first layer is mostly covered with adsorbate molecules. The heat spike is observed for argon and nitrogen at 77 K while for argon at 87.3 K it is no longer present. These simulation results are consistent with the experimental data of J. Rouquerol, S. Partyka and F. Rouquerol, J. Chem. Soc., Faraday Trans. 1, 1977, 73, 306. In the case of methane we observe heat spikes at low temperatures, 84.5, 92.5 and 104 K. The heat spike shifts to higher loading with temperature and it then disappears at high temperatures. These observations are in qualitative agreement with the experimental data of A. Inaba, Y. Koga and J. A. Morrison, J. Chem. Soc., Faraday Trans. 2, 1986, 82, 1635. In all cases where heat spikes are observed, the GCMC simulation results indicate that the heat spike is associated with the squeezing of molecules into the already dense first layer, and the rearrangement of molecules to form a highly structured fluid of this layer. While this squeezing into the first layer is happening, molecules continue to adsorb onto the relatively sparse second layer