Sean P. Rigby

Determination of the electrochemically active surface area of Pt/C PEM fuel cell electrodes using different adsorbates

The characterization of fuel cell electrodes is paramount to the optimum design and performance of fuel cell systems. The specific electrochemical surface area and structural properties of two Pt/C fuel cell electrodes have been determined. Three different electrochemical adsorbates, namely, H2 (adsorption and desorption), CO and Ag (underpotentially deposited, UPD) have shown that the estimated surface areas and roughness factors vary. These variations were shown to be ca 100% between Ag and H2 adsorption for the high-loaded sample, whilst for the low-loaded sample the difference is ca 30%. The surface areas obtained show that the low-loaded Pt electrode possesses a higher specific surface area than the higher-loaded sample. The results illustrate the importance of using multiple adsorbates to accurately determine and understand the structure of complex, porous fuel cell electrodes.

NMR and fractal modelling studies of transport in porous media

Abstract Magnetic Resonance Imaging (MRI) and PGSE NMR studies have shown that it is necessary to consider heterogeneities in the porous structure over different lengthscales in order to be able to understand the relationship between structure and transport in porous solids. Moreover, the spatial distribution in pore structure is seen to influence strongly mass transfer processes occurring within the porous medium. The MRI studies described here have suggested that a fractal representation might be appropriate in describing heterogeneous porous systems. Numerical simulations have been performed of transport within Cluster-Cluster Aggregate (CCA) structures. A comparison of methods of simulating the diffusion process is presented. The results of the simulations are compared with MRI and PGSE NMR measurements of the tortuosity of commercial catalyst pellets. In the light of complementary nitrogen desorption and mercury porosimetry data, a multifractal description of porous media is also proposed in the form of a Composite CCA structure in which both the macroscopic heterogeneity associated with the pore-size distribution and the fractal characteristics of the microscopic pore structure itself are represented.

Liquid intrusion and alternative methods for the characterization of macroporous materials (IUPAC Technical Report)

This document deals with the characterization of porous materials having pore widths in the macropore range of 50 nm to 500 μm. In recent years, the development of advanced adsorbents and catalysts (e.g., monoliths having hierarchical pore networks) has brought about a renewed interest in macropore structures. Mercury intrusion–extrusion porosimetry is a well-established method, which is at present the most widely used for determining the macropore size distribution. However, because of the reservations raised by the use of mercury, it is now evident that the principles involved in the application of mercury porosimetry require reappraisal and that alternative methods are worth being listed and evaluated. The reliability of mercury porosimetry is discussed in the first part of the report along with the conditions required for its safe use. Other procedures for macropore size analysis, which are critically examined, include the intrusion of other non-wetting liquids and certain wetting liquids, capillary condensation, liquid permeation, imaging, and image analysis. The statistical reconstruction of porous materials and the use of macroporous reference materials (RMs) are also examined. Finally, the future of macropore analysis is discussed.

Engineering Silica Particles as Oral Drug Delivery Vehicles

Porous silica particles are emerging as complementary systems to polyester microspheres for the encapsulation and controlled delivery of small-organic drugs. Their recent application in pharmaceutics is strengthened by well-established characterization and synthetic routes from the chemical engineering sciences. Silica is an interesting scaffold material for the encapsulation of organic molecules. It can be formed into hierarchical structures over a wide range of length scales and interconnectivities. Encapsulation can therefore be tailored not only to the drug but the desired release properties. In addition to surfactant-templating of hierarchical silica structures, polypeptides from marine organisms may offer biological routes to novel silica materials. Silica sol-gels have also been evaluated as delivery vehicles, particularly with regard to generating hybrid systems with mesoporous silica or composite xerogels. This review will first focus on the detailed characterisation of pore size and structure of mesoporous silica with regards water penetration and drug diffusion. We then describe the pharmaceutical applications of silica materials with regard to improving oral bioavailability, multiparticulate systems for gastroretention or sustained release, composite xerogels and in vivo biocompatibility.

Selective incorporation of functional dicarboxylates into zinc metal–organic frameworks

Zinc(II) nitrate reacts with different ratios of 1,4-benzenedicarboxylic acid (H(2)bdc) and 2-halo-1,4-benzenedicarboxylic acid (H(2)bdc-X, X = Br or I) to give [Zn(4)O(bdc)(3-x)(bdc-X)(x)], in which preferential incorporation of bdc is observed. The selective incorporation is related to crystal growth rates, and the proportion of incorporated bdc-X rises with increasing reaction time.

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.

NMR imaging studies of transport heterogeneity and anisotropic diffusion in porous alumina pellets

Abstract One- and two-dimensional NMR imaging techniques have been used to investigate the influence of pore structure on mass transfer through porous alumina catalyst pellets. Studies are reported for two batches of pellets. In the first study it is shown that radial variations in voidage and pore size directly influence mass transfer. In the second study, a relatively homogeneous cylindrical tablet is studied. Effective diffusivity, and hence tortuosity, is observed to differ significantly with respect to axial and radial transient diffusion processes. This observation is correlated with a similar difference in fractal dimension describing the pore structure in these two directions. NMR imaging is shown to be a useful technique for gaining insight into the effect of manufacturing process on the mass transfer properties of porous catalyst pellets.

A statistical model for the heterogeneous structure of porous catalyst pellets

The complex structures of the void space of porous media are often characterised by parameters such as pore network connectivity and lattice size. This paper presents a comparison of the estimates of these parameters obtained from two previous methods based on nitrogen sorption and mercury porosimetry, and also from a new, completely independent approach based on pulsed-gradient spin-echo nuclear magnetic resonance (PGSE NMR). It was found that the new PGSE NMR technique obtains estimates of connectivity and lattice size in agreement with nitrogen sorption but different to mercury porosimetry. This difference was attributed to the various physical processes involved actually probing different aspects of the pore space geometry. It was further suggested that the representation of the pore structure derived from either nitrogen sorption or PGSE NMR is really a mapping of the real pore space onto an equivalent abstract, random pore bond network. However, it has been shown that this mapping does capture some of the characteristic properties of the pore space that control transport over mesoscopic ( < 10 microm) length scales. For materials which additionally possessed macroscopic (> 10 microm) structural heterogeneity, it was found that the model could also be adapted to predict the macroscopic transport properties of the porous medium.

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.