Interpretation of mercury porosimetry data

A., Lane, Interpretation of Mercury Porosimetry Data, PhD Thesis, University of Massachusetts, Amherst, 1984. 

Some materials, among the most porous, show a large volume variation due to mechanical compaction when submitted to mercury porosimetry. High dispersive precipitated silica shows, as low density xerogels and carbon black previously experimented, two successive volume variation mechanisms, compaction and intrusion. The position of the transition point between the two mechanisms allows to compute the buckling constant used to determine the pore size distribution in the compaction part of the experiment. The mercury porosimetry data of a high dispersive precipitated silica sample wrapped in a tight membrane are compared with the data obtained with the same sample without memlM ane. Both experiments interpreted by equations appropriate to the mechanisms lead to the same pore size distribution. 

In spite of the growing popularity of mercury porosimetry and the ready availability of excellent automated equipment, the interpretation of the mercury intrusion-extrusion data is still far from clear. The values of surface tension and contact angle which must be inserted in the Washburn equation are still uncertain - as are the limits of applicability of the equation itself. Other problems include the reversible or irreversible deformation of the pore structure, which undoubtedly occurs with some corpuscular or weakly agglomerated systems. 

Although it is not possible to arrive at an unambigious interpretation of the mercury porosimetry data, there is little doubt that on heat treatment the pore size distribution of the HAC-MDF became much broader than that of the OPC-MDF. It seems that the development of the wider pores in the HAC samples was responsible for the appearance of their very high permeabilities. On the other handi the complete removal of the polymer by heat treatment of the HAC-MDF at 450°C evidently led to the formation of a highly reactive material and an unstable pore structure. The curious result obtained on rehydration of the 150°C sample of HAC-MDF was probably due to the leaching out of residual polymer which in turn led to pore widening and increased permeability. 

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. 

We are not going to deal with all these examples of application of percolation theory to catalysis in this paper. Although the physics of these problems are different the basic numerical and mathematical techniques are very similar. For the deactivation problem discussed here, for example, one starts with a three-dimensional network representation of the catalyst porous structure. Systematic procedures of how to map any disordered porous medium onto an equivalent random network of pore bodies and throats have been developed and detailed accounts can be found in a number of publications ( 8). For the purposes of this discussion it suffices to say that the success of the mapping techniques strongly depends on the availability of quality structural data, such as mercury porosimetry, BET and direct microscopic observations. Of equal importance, however, is the correct interpretation of this data. It serves no purpose to perform careful mercury porosimetry and BET experiments and then use the wrong model (like the bundle of pores) for data analysis and interpretation. 

Figure 9.7. In mercury intrusion porosimetry, the volume of mercury entering the pores of a body at various applied pressures can be interpreted generally as data on the size distribution of the pores within a porous body. However the presence of large pores with small access throats can lead to misinterpretation of the true structure of the porous body.

Despite these limitations mercury porosimetry has proved to be a useful tool with which to investigate the internal structure of solids. It should not be considered an absolute method and care should be used in interpreting data. 

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