Giorgio Zgrablich

Mechanistic and Experimental Aspects of the Structural Characterization of Some Model and Real Systems by Nitrogen Sorption and Mercury Porosimetry

Several mechanistic and phenomenological aspects of mercury intrusion and nitrogen sorption processes involving some model and real mesoporous systems were studied. The experimental pore systems examined consisted of two substrates: (a) a globular solid composed of monodisperse silica spheres in a perfect rhombohedral arrangement and (b) a controlled pore glass solid. Comparisons between the experimental nitrogen sorption and mercury porosimetry pore-size distributions demonstrated: (i) the existence of several mechanistic effects responsible of irreversible capillary behaviour that influences the calculation of pore structure parameters; (ii) the choice of the right sorption process (i.e. condensation or evaporation) suitable for comparison with either intrusion or extrusion results; (iii) the types of porous structures capable of convenient pore-size characterization by either nitrogen sorption and/or mercury porosimetry methods; and (iv) the nature of the pore entities (i.e. chambers or necks) that control the incumbent capillary process.

Percolation in correlated site-bond Bethe lattices

Percolation probabilities are obtained for site-bond Bethe lattices where the interconnection of sites and bonds is restricted by a correlation function which depends on the overlap, Omega , between site and bond probability distributions. The behaviour of the percolation phase diagram and percolation probabilities as Omega changes is discussed. The model presents interesting and qualitatively new features in comparison with a classical (non-correlated) site-bond percolation process.

High-pressure methane adsorption in 5A zeolite and the nature of gas-solid interactions

Experimental adsorption isotherms for methane in 5A zeolite over the pressure range 0–5 MPa have been obtained and analyzed using a statistical thermodynamical formulation which relates the observed macroscopic thermodynamic quantities to microscopic gas–solid interactions. The efficiency of the zeolite as a methane storage system has thus been evaluated.

Pore-blocking and pore-assisting factors during capillary condensation and evaporation

Abstract Thirty-four years ago Everett [The Solid–Gas Interface, Vol. 2, Marcel Dekker, New York, 1967, p. 1055] proposed a pore-blocking factor when establishing the foundations of a non-independent domain theory (IDT) of sorption hysteresis. Such pore-blocking factor was defined as the ratio between two desorbed volumes within the same pressure range. The first volume arose from a non-independent pore structure. The second quantity was a virtual one since it represented the volume desorbed if the pores of the substrate had acted as independent domains. In fact, Everett calculated the ratio between pore-blocking factors, while not their absolute values, from experimental data proceeding from sorption results on porous glasses. The astonishing conclusion of all this preliminary work, was that blocking factors depended upon the total amount of condensate at a certain stage of a desorption process rather than on the distribution of it within the porous network. In this way, a unique pore-blocking factor curve ensued from different sorption processes such as boundary and scanning curves. Now, through the aid of simulated heterogeneous 3-D porous networks and the sorption curves thereon developed, an assessment of the above mentioned important assertion has been undertaken. Besides, a pore-assisting factor that may arise during an ascending sorption process has been treated under a similar context.

Effective states approximation (ESA) for adsorption on homogeneous and heterogeneous surfaces

Abstract A simple approximation for the local and overall adsorption isotherms of monoatomic species on heterogeneous surfaces with different topographies is presented. The model relies on the lattice-gas approach where the site adsorption energies generated by the gas–solid potential, split into a set of independent energy substates because of the ads–ads interactions. The distinctive feature of the approximation is that it preserves in a simple manner the energy states for single particles by making more crude assumptions in counting the total number of configurations compared to standard approaches (mean-field, quasi-chemical) in which the energetics of single particles are oversimplified while the counting of lattice configurations is carried out as accurately as possible. At a comparable degree of approximation this approach, which is denoted as ESA leads to appreciably better results than mean-field. ESA can also be applied to adsorption on heterogeneous surfaces with distinct energy topographies. The effect of lateral interactions and surface topography on the local isotherm at fixed overall coverage and temperature (the so-called energetic structure of the adsorbate) is analyzed and the results are compared with Monte Carlo simulations, the Bragg–Williams and the quasi-chemical approximations. The assumption of an intermediate energy topography characterized by nearest-neighbor site energy correlation, rather than random or patch-wise distributions, is shown to have a significant effect on the distribution of molecules over adsorption sites.

Determination of pore size distributions using the Dual Site-Bond Model: experimental evidence

Abstract In previous papers, we have described a suitable method to obtain pore size distributions for voids and necks using the Dual Site-Bond Model (DSBM) and Monte Carlo simulations. This method basically consists in the determination of the corresponding size distributions by using adsorption–desorption hysteresis data. Void size frequency functions are featured from the ascending curve. From the descending curve, we obtain a characteristic pressure value that will give us, via a quasi-universal curve, information about the neck size distribution function. In this work, we use our method to predict, using experimental hysteresis loops, the size distributions of several mesoporous samples. Once these functions are determined, we simulate the adsorption–desorption isotherms on a simple cubic network of voids and necks whose radii are sampled from the obtained size distributions. Comparison with experimental data is performed, drawing out fruitful conclusions and future perspectives based on the simplicity and predictive capability of the method.

Capillary hysteresis in porous media

The capillary hysteresis for the drainage-imbibition process in correlated site-bond porous networks is studied. The degree of correlations between pore radii is shown to produce important modifications on the hysteresis loop which is wider for non-correlated networks and narrower for correlated ones. A relation between the degree of correlation and the type of solid is suggested.

Determination of the pore size distribution of correlated mesoporous networks

Abstract In the present work we study how the adsorption desorption hysteresis loop of a mesoporous disordered medium represented by a 3-dimensional Dual Site-Bond Model (DSBM) is affected by percolation. Site and bond distributions are assumed to be gaussians. The behavior of the threshold pressure for the evaporation processe suggests a method to determine the site and bond distributions from experimental adsorption-desorption hysteresis curves. Traditional methods developed for non-correlated networks are tested and evaluated against our simulation results showing the discrepancy mainly for highly correlated networks. Results of the prediction capability of our method are shown.

A model for correlated porous surfaces: Applications to hysteresis phenomena in adsorption‐desorption and drainage‐imbibition processes

A model for correlated porous surfaces is developed and applied for explaining the hysteresis loops occurring in adsorption‐desorption and drainage‐imbibition processes. Different degrees of correlation of the surface model different types of hysteresis loops. This is explained by a percolation model developed on those surfaces and comparisons are made for the case of the square lattice and the Bethe one.

Energetic Topography Effects

The role of the adsorptive surface characteristics in many processes of practical importance is a topic of increasing interest in surface science. Adsorption, surface diffusion, and reactions on catalysts are some of the phenomena that are strongly dependent upon surface structure. Most materials have heterogeneous surfaces that, when interacting with gas molecules, present a complex spatial dependence of the adsorptive energy. This is specially the case for activated carbons, where many defects and impurity atoms and molecules are incorporated in the graphitic pore walls. It is of substantial interest to attempt a complete characterization of such heterogeneity. Through the last 50 years physical adsorption has been used for determining energetic properties of heterogeneous substrates, but this still remains an open problem in many aspects [1–4].