Robin S. Fletcher

Interpreting mercury porosimetry data for catalyst supports using semi-empirical alternatives to the Washburn equation

Abstract Semi-empirical alternatives to the traditional Washburn equation have been used to analyse the mercury porosimetry data for a sol–gel silica and five alumina catalyst supports. A combination of different porosimetry experiments, including the conventional primary mercury intrusion and retraction experiment conducted on both whole pellets and a fragmented sample together with a mercury re-injection curve, have demonstrated that a sol–gel silica sphere possesses a so-called “skin-effect.” This means that a thin band of narrow pore throats was located at the surface of the silica sphere guarding access to larger pores located within the interior. This particular interpretation of the raw data only became apparent when the new data analysis method was used. In addition, when an alternative expression to the Washburn equation is used to analyse the primary mercury intrusion curves for five alumina samples, the pore size distributions obtained all match a priori the corresponding distributions obtained from nitrogen desorption. This finding supports the view that both mercury intrusion and nitrogen desorption are invasion percolation processes.

Characterisation of porous solids using a synergistic combination of nitrogen sorption, mercury porosimetry, electron microscopy and micro-focus X-ray imaging techniques

Imaging methods, such as micro-focus X-ray (MFX) imaging and magnetic resonance imaging (MRI), have greatly improved our ability to characterise the highly complex internal structures of porous media. MFX imaging and MRI are now both able to provide maps of the spatial distribution of local average accessible porosity for mesoporous media over macroscopic length scales (>∼10 μm). A methodology for obtaining this type of information by MFX imaging is described here. For relatively chemically homogeneous materials, conventional 1H MRI is also able to provide a map of the spatial distribution of local average pore size. However, there are limitations on the type of materials that may be studied using 1H MRI and the range of information that may be obtained by utilising only one technique alone. For mesoporous materials, MFX imaging alone cannot currently map the spatial distribution of pore size. However, in this work it has been shown that the already extensive capabilities of MFX imaging may be even further enhanced by a combination of it with the more traditional techniques of mercury porosimetry and nitrogen sorption. The methodology described here has enabled the determination of the spatial distributions of both the local average (over length scales ∼10 μm) porosity and pore size distribution for mesoporous and macroporous materials over macroscopic length scales. The methodology is also suitable for quantitative application to interesting chemically heterogeneous materials, such as mixed oxide absorbents or coked catalysts, not amenable to conventional 1H MRI.

Improving sensitivity and accuracy of pore structural characterisation using scanning curves in integrated gas sorption and mercury porosimetry experiments

Gas sorption scanning curves are increasingly used as a means to supplement the pore structural information implicit in boundary adsorption and desorption isotherms to obtain more detailed pore space descriptors for disordered solids. However, co-operative adsorption phenomena set fundamental limits to the level of information that conventional scanning curve experiments can deliver. In this work, we use the novel integrated gas sorption and mercury porosimetry technique to show that crossing scanning curves are obtained for some through ink-bottle pores within a disordered solid, thence demonstrating that their shielded pore bodies are undetectable using conventional scanning experiments. While gas sorption alone was not sensitive enough to detect these pore features, the integrated technique was, and, thence, this synergistic method is more powerful than the two individual techniques applied separately. The integrated method also showed how the appropriate filling mechanism equation (e.g. meniscus geometry for capillary condensation equations), to use to convert filling pressure to pore size, varied with position along the adsorption branch, thereby enabling avoidance of the further systematic error introduced into PSDs by assuming a single filling mechanism for disordered solids.

Techniques for direct experimental evaluation of structure–transport relationships in disordered porous solids

Determining structure–transport relationships is critical to optimising the activity and selectivity performance of porous pellets acting as heterogeneous catalysts for diffusion-limited reactions. For amorphous porous systems determining the impact of particular aspects of the void space on mass transport often requires complex characterization and modelling steps to deconvolve the specific influence of the feature in question. These characterization and modelling steps often have limited accuracy and precision. It is the purpose of this work to present a case-study demonstrating the use of a more direct experimental evaluation of the impact of pore network features on mass transport. The case study evaluated the efficacy of the macropores of a bidisperse porous foam structure on improving mass transport over a purely mesoporous system. The method presented involved extending the novel integrated gas sorption and mercury porosimetry method to include uptake kinetics. Results for the new method were compared with those obtained by the alternative NMR cryodiffusometry technique, and found to lead to similar conclusions. It was found that the experimentally-determined degree of influence of the foam macropores was in line with expectations from a simple resistance model for a disconnected macropore network.

Simulation of mercury porosimetry using MRI images of porous media

Abstract Models of the pore structure of pellets taken from a batch of sol-gel silica spheres have been constructed from magnetic resonance images of the macroscopic (∼0.04-1 mm), spatial distribution of spin density and spin-spin relaxation time within the material. Simulations of mercury porosimetry on these models gave rise to good predictions for the point of deviation of the intrusion and retraction curves, and the level of mercury entrapment, in agreement with those found by experiment. This finding suggested that mercury intrusion and retraction within the pellets are determined by the macroscopic structure of the material, as detected using MRI.