Mercury porosimetry, simulation

For the detailed study of reaction-transport interactions in the porous catalytic layer, the spatially 3D model computer-reconstructed washcoat section can be employed (Koci et al., 2006, 2007a). The structure of porous catalyst support is controlled in the course of washcoat preparation on two levels (i) the level of macropores, influenced by mixing of wet supporting material particles with different sizes followed by specific thermal treatment and (ii) the level of meso-/ micropores, determined by the internal nanostructure of the used materials (e.g. alumina, zeolites) and sizes of noble metal crystallites. Information about the porous structure (pore size distribution, typical sizes of particles, etc.) on the micro- and nanoscale levels can be obtained from scanning electron microscopy (SEM), transmission electron microscopy ( ), or other high-resolution imaging techniques in combination with mercury porosimetry and BET adsorption isotherm data. This information can be used in computer reconstruction of porous catalytic medium. In the reconstructed catalyst, transport (diffusion, permeation, heat conduction) and combined reaction-transport processes can be simulated on detailed level (Kosek et al., 2005). 

Experimental techniques commonly used to measure pore size distribution, such as mercury porosimetry or BET analysis (Gregg and Sing, 1982), yield pore size distribution data that are not uniquely related to the pore space morphology. They are generated by interpreting mercury intrusion-extrusion or sorption hysteresis curves on the basis of an equivalent cylindrical pore assumption. To make direct comparison with digitally reconstructed porous media possible, morphology characterization methods based on simulated mercury porosimetry or simulated capillary condensation (Stepanek et al., 1999) should be used. 

A new correlation for the product y.cosd for retreating mercury menisci in alumina samples has been developed. This eorrelation has been used, together with simulations of mercury porosimetry on suitable models, to obtain statistics that characterise the pattern of the spatial distribution of macropore sizes in a bidisperse alumina tablet. 

Several authors have investigated the differences in deactivation for small or for large pores within the catalysts. In general, the researchers have employed mercury porosimetry for the characterization of the actual catalysts and they have employed mostly models of parallel pores of differing dimensions (as contrasted with an interconnected network) in their simulations. 

We review a recently developed molecular-based approach for modeling mercury porosimetry. This approach is built on the use of a lattice model of the porous material microstructure and the use of mean-field density fiuictional theory (MF-DFT) calculations and Monte Carlo simulations to calculate the three-dimensional density distribution in the system. The lattice model exhibits a symmetry between the adsorption/desorption of a wetting fluids and intnision/extrusion of a nonwetting fiuid. In consequence, macroscopic approaches used previously to transform mercury porosimetry curves into gas adsorption iso erms are essentially exact in the context of the model. We illustrate the approach with some sample results for intrusion and extrusion in Vycor and controlled pore glass (CPG). 

In recent years it has been recognized that a more accurate method of pore analysis should consist of an appropriate combination of techniques of which mercury porosimetry is but one of the components (refs. 18,30,31). First, serial sectioning analysis of pore casts (refs. 32-35) can be used to determine the chamber-size distribution, the correlation between the sizes of adjacent chambers, and information pertaining to the interconnectivity of the network (e.g. specific genus and coordination number). Then, the capillary pressure curves can be used to determine the throat-size distribution, and the correlation between the sizes of contiguous throats and chambers. In order to deconvolve these curves a reliable simulator of intrusion and retraction of mercury in evacuated chamber-and-throat networks must be developed. 

The commercial sample, spherical bead activated carbon, was supplied by Kureha Chemical Industry. This activated carbon is referred to as Kureha carbon, which has a total micropore volume of 0.56 cm g" and a BET surface area of 1300 m g . The detailed textural properties of Kureha carbon are reported elsewhere [9]. The pore size distribution was evaluated in terms of the simulation of the density hmctional theory (DFT) using the isotherm data of nitrogen adsorption at 77 K and relative pressures up to 0.2. Only micropores contribute to the total pore volume and surface area. This was further confirmed by mercury intrusion porosimetry, no significantly additional porosity was observed in the pore size range from 2 nm to 100 pm. So, the investigated adsorbent is a purely microporous material and its pore size distribution covers the range from 0.4 to 1.9 nm [9]. 

These statistical correlation functions were used as the reference systems in the annealing micro-structural reconstruction. Each sample was reconstructed and simulated five times with a different random series. In this reconstruction, relative porosities (O/) were determined by the statistical analysis of SEM micrographs and verified by the total experimental porosity obtained by mercury intrusion porosimetry. CL thickness (e) was also determined by SEM. CLs were studied by SEM at different resolutions. 

A new theoretical simulator of mercury intrusion in and retraction from three-dimensional chamber-and-throat networks is developed. Stepwise porosimetry is modelled as a sequence of flow events occurring at each new external pressure value. The main conclusions resulting from the study of the effects of geometrical, topological and statistical parameters on the capillary pressure curves are listed below. 

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