Fundamental studies of gas sorption within mesopores situated amidst an inter-connected, irregular network

Abstract

There has been little, or no, direct testing of theories of gas sorption within particular pores situated amidst a highly inter-connected pore network. The concept of thermodynamically independent pores within networks has also been challenged. In this work, a novel integrated nitrogen sorption and mercury porosimetry technique has been used to deconvolve the condensation and evaporation processes within a specific subset of pores contained within a larger, irregular network. The sizes and geometry of these pores were obtained completely independently of gas sorption, using mercury porosimetry and NMR cryoporometry, respectively. Hence, various theories of capillary condensation, such as the Kelvin equation, the Broeckhoff-de Boer method, Saam-Cole theory, and NLDFT could be directly tested, and the potential influence of any collective network phenomena detected. It was found that, even for a shielded pore, the Cohan equation for a cylindrical meniscus gave rise to the best prediction for the relative pressure of capillary condensation, once the effects of surface chemical heterogeneity on multi-layer build-up had been taken into account. The results were also found to be incompatible with the presence of particular collective adsorption effects, such as advanced condensation.

Interpretation of integrated gas sorption and mercury porosimetry studies of adsorption in disordered networks using mean-field DFT

“…The key finding from these previous studies (Rigby et al 2008) was that the relative pressure for capillary condensation, in the abstracted pores, was well predicted by the Cohan (1938) equation for a cylindrical meniscus, once the correct t-layer correction, appropriate to the particular degree of geometric and chemical heterogeneity of the surface of S1, had been made. Previous work (Rigby 2007) had suggested that the surface of sol-gel silicas is both chemically and geometrically heterogeneous.…”

“…Since, the heterogeneities were macroscopic the 'neck' can be removed from the 'bottle' by fragmentation of the S1 pellets. In previous studies (Rigby et al 2008) of this disordered system, using integrated gas sorption and porosimetry, it was shown (independently of gas sorption) that mercury becomes entrapped within S1, only in those pores with a very narrow range of radii, centered around 7.32 nm. In the integrated experiments, the subtraction of the nitrogen isotherms obtained following mercury entrapment, from the corresponding isotherms obtained beforehand, allowed deconvolution of the changes to the capillary condensation and evaporation processes occurring because of the presence, or lack thereof, of the sub-set of pores that entrap mercury.…”

“…Studies of adsorption in slightly more complex geometries are achieved by adding pore interconnections, such as with SBA-16, MCM-48 or KIT-5 (Morishige and Tateishi 2003;Morishige and Tarui 2007). However, recently (Rigby and Fletcher 2004;Rigby et al 2008) a novel method to test adsorption theories, including the notion of independent pores, even in disordered, inter-connected pore network systems, has been proposed. This alternative approach involves taking a (potentially) much more complex, inter-connected pore network and removing simple elements, such as pores of a particular characteristic dimension.…”

“…Data from mercury porosimetry (Rigby et al 2008) and magnetic resonance imaging (Hollewand and Gladden 1995) have demonstrated that a ∼3 mm diameter, mesoporous, sol-gel silica sphere, denoted S1 possesses macroscopic (∼100 µm) heterogeneities in the spatial distribution of pore size, such that the distribution is a macroscopic analogue of an 'ink-bottle' pore geometry. Spatially extensive, but isolated, regions of similarly-sized, larger pores are shielded by smaller pores.…”

“…1. The new approach has been implemented using the novel integrated gas sorption and mercury porosimetry method (Rigby et al 2008). This technique consists of a sequence of alternating gas sorption and mercury porosimetry experiments conducted in series on the same sample.…”