Adsorption of gases on layered silicates at high pressures

Specific and nonspecific interactions in the adsorption of argon, nitrogen, and water vapor on oxides

High-resolution isotherms of argon and nitrogen adsorption on macroporous silica have been simulated with universal Langmuir and fractal models. A four-parameter, fractal universal Langmuir equation is a good fit to the data at low pressures. Standard Gibbs energy changes calculated from equilibrium adsorption coefficients show a series of broad peaks that indicate adsorbate structural transformations as a function of pressure and coverage. The Freundlich equation or mean fractal model is also a good fit to isotherms at low pressures. Pressure-varying fractals are accurate fits to the data. Fractal exponents provide information on adsorbate coverage and surface access. Broad peaks in pressure-varying exponents are indicators of adsorbate structure. From adsorptive gas amounts, mean and pressure-varying fractal exponents provide details of adsorbate fractal dimensions and surface roughness. Both Ar and N2 adsorption cause increases in mean surface roughness when compared with pure silica. Surface roughness fluctuations from pressure-dependent adsorptive gas fractal dimensions are associated with adsorbate structure. At one trough, the surface is smooth and is linked to close-packed Ar or N2. For Ar adsorption at 87 K, this structure is a complete monolayer (1.00(4)), while for Ar (77 K), 1.15(4) layers and for N2 (87 K), 2.02(10) layers. The universal Langmuir specific area of the silica is 10.1(4) m2 g-1. Pressure- and coverage-dependent adsorbate structures range from filling defects and holes on the surface to cluster formation to adsorbed Ar or N2 evenly distributed or packed across the surface. The Ar (87 K) isotherm is most sensitive to adsorbate structural transformations.

An ordered mesoporous MFI zeolite material (Meso-MFI) was prepared by using CMK-type mesoporous carbons as a hard template. The Meso-MFI exhibits both structural and adsorption differences compared to the conventional bulk MFI zeolite. To study the argon (Ar) adsorption process in Meso-MFI, an in situ gas adsorption powder X-ray diffraction (XRD) analysis was performed using synchrotron X-ray source. Structural rearrangement of the mesoporous MFI zeolite upon Ar adsorption at low temperature (83 K) was intensively studied together with Ar adsorption process in Meso-MFI. We observed that a structural transition of the Meso-MFI zeolite framework from monoclinic (P21/n) to orthorhombic (Pnma) occurred at around 126 Pa at 83 K. Positions of Ar atoms are determined as a function of the Ar gas pressure through Rietveld refinement of powder XRD data. Ar atoms are observed at straight channels, sinusoidal channels, and the intersection of these channels at low pressure. As gas pressure increases, Ar atoms in the pore intersection are pulled off from the intersection toward the straight and sinusoidal channels. The pore shape of the straight channel is changed accordingly with the amount of adsorbed Ar atoms within the pores from circular to oval. These results indicate that Ar adsorption induces not only continuous rearrangement of framework atoms but also symmetry change in the Meso-MFI. A molecular simulation study combined with Rietveld refinement of in situ XRD data provided a full understanding of the adsorption process of Ar in Meso-MFI.

Physical characterization of activated carbons with narrow microporosity by nitrogen (77.4 K), carbon dioxide (273 K) and argon (87.3 K) adsorption in combination with immersion calorimetry

Characterization of modified mesoporous silicas using argon and nitrogen adsorption

An integrated model for adsorption-induced strain in microporous solids

Modeling of Gas Adsorption Equilibrium over a Wide Range of Pressure: A Thermodynamic Approach Based on Equation of State

Adsorption isotherms of nitrogen and argon on an “agot” grade artificial nuclear graphite at 77 and 90 K

Surface properties of organic pigments: I. Adsorption of argon and nitrogen on copper phthalocyanines

The Dubinin–Stoeckli equation and the Horvath–Kawazoe equation have been used for the determination of the pore size distribution (PSD) for three fractions of activated carbon separated by elutriation due to their different degrees of activation (burn-off). A comparison of the PSD functions on the basis of nitrogen, argon and benzene adsorption isotherms has been undertaken. The general form of the Dubinin–Stoeckli equation has been developed for nitrogen and argon as an adsorbate. For all adsorbates studied, correction factors including adsorption in mesopores were introduced. New formulae for this correction factor have been proposed for nitrogen and argon. Quantitative evaluations of PSD functions for these three different adsorbates lead to the conclusion that it is possible to use the benzene isotherm as well as the nitrogen or argon isotherms to characterise the microporous structure of the adsorbent. Comparison of the porous structure parameters was extended by a fractal analysis.

Comprehensive approach to determining the physical properties of granular activated carbons

We present a new approach accounting for the non‐additivity of attractive parts of solid–fluid and fluid–fluid potentials to improve the quality of the description of nitrogen and argon adsorption isotherms on graphitized carbon black in the framework of non‐local density functional theory. We show that the strong solid–fluid interaction in the first monolayer decreases the fluid–fluid interaction, which prevents the two‐dimensional phase transition to occur. This results in smoother isotherm, which agrees much better with experimental data. In the region of multi‐layer coverage the conventional non‐local density functional theory and grand canonical Monte Carlo simulations are known to over‐predict the amount adsorbed against experimental isotherms. Accounting for the non‐additivity factor decreases the solid–fluid interaction with the increase of intermolecular interactions in the dense adsorbed fluid, preventing the over‐prediction of loading in the region of multi‐layer adsorption. Such an improvement of the non‐local density functional theory allows us to describe experimental nitrogen and argon isotherms on carbon black quite accurately with mean error of 2.5 to 5.8% instead of 17 to 26% in the conventional technique. With this approach, the local isotherms of model pores can be derived, and consequently a more reliable pore size distribution can be obtained. We illustrate this by applying our theory against nitrogen and argon isotherms on a number of activated carbons. The fitting between our model and the data is much better than the conventional NLDFT, suggesting the more reliable PSD obtained with our approach.

Recently (Nguyen and Bhatia, J. Phys. Chem. C 111:2212-2222, 2007) we have proposed a new algorithm utilising cluster analysis principles to determine pore network accessibility of a disordered material. The algorithm was applied to determine pore accessibility of the reconstructed molecular structure of a saccharose char, obtained in our recent work using hybrid reverse Monte Carlo simulation (Nguyen et al., Mol. Simul. 32:567-577, 2006). The method also identifies kinetically closed pores not accessed by adsorbate molecules at low temperature, when their low kinetic energy cannot overcome the potential barrier at the mouths of pores that can otherwise accommodate them. In the current work, the results are validated by transition state theory calculations for N-2 and Ar adsorption, showing that N-2 can equilibrate in narrow micropores at practical time scales at 300 K, but not at 77 K. Large differences between time scales for micropore entry and exit are predicted at low temperature for N-2, the latter being smaller by over three orders of magnitude. For N-2 at 77 K the time constant for pore entry exceeds 3 hr., while for exit it is 134 days. At 300 K these values are smaller than 1 mu s, indicating good accessibility at this temperature. These results are verified by molecular dynamics simulations, which reveal that while N-2 molecules enter and leave all pores frequently at 300 K, entry and exit events for apparently inaccessible pores are absent at 77 K. For Ar at 87 K better accessibility is evident for the saccharose char compared to N-2 at 77 K. This finding is now experimentally shown in this work by comparison of pore size distributions obtained from experimental nitrogen adsorption isotherms of nitrogen and argon at 77 K and 87 K.

Amorphous materials are usually characterized using nitrogen adsorption isotherms at 77 K taken at pressures up to 1 bar to obtain pore size distributions. Activated carbons are amorphous microporous graphitic materials containing pores which can range from nanometers to microns in width and which can, in principle, be tailored to adsorb specific molecules or classes of molecule by changing the method of preparation (the activation process). For the physical chemist, they pose the challenge of understanding how gases adsorb in graphitic nanopores, that is, in restricted geometries, and of using that understanding to improve their characterization. In this paper, we compare pore size distributions of an ultrahigh surface area activated carbon (AX21) determined from nitrogen adsorption measurements up to 0.6 bar at 77 K with those determined from carbon dioxide adsorption measurements up to 20 bar at 298 K. Our analysis employs grand canonical and Gibbs ensemble Monte Carlo simulations together with accurate site−site interaction models of the adsorbates. We find that the calculated pore size distributions for each adsorbate are quite different, and the adsorption of one gas can be estimated from the adsorption of the other gas to within an error of 25% at the highest pressures only. At lower pressures, we speculate that large errors are due to the behavior of nitrogen in carbon micropores in which diffusion is severely limited. To substantiate this speculation, we have calculated the self-diffusion coefficient for nitrogen at 77 K and carbon dioxide at 298 K in carbon slit pores using equilibrium molecular dynamics. The results suggest that nitrogen is diffusionally limited, and possibly frozen, in such pores whereas carbon dioxide remains mobile. We conclude that room-temperature carbon dioxide adsorption isotherms up to the saturation pressure could provide a more accurate characterization of carbon microstructure than nitrogen isotherms at 77 K up to 1 bar.

A computational study of the adsorption of n-perfluorohexane in zeolite BCR-704