Impact of Chemical Heterogeneity on the Accuracy of Pore Size Distributions in Disordered Solids

Abstract

Disordered porous solids, such as heterogeneous catalysts, possess internal void spaces with high degrees of both chemical and geometric heterogeneity. For pore size distributions (PSDs) to be useful void space descriptors for aiding understanding of phenomena such as catalyst activity, their level of accuracy must be known. Due to differences in specific interactions and wetting behavior, the choice of adsorbate can severely impact the accuracy of PSDs derived for disordered materials using gas sorption. Wetting effects in the capillary condensation region have recently been studied extensively by simulation, but there have been no complementary, definitive experimental studies of sorption mechanisms. However, in this work, the novel integrated gas sorption and mercury porosimetry technique has been used to prepare a particular subset of pores, located within the disordered network of a given amorphous material (eg silica or alumina), that had dead-ends with different chemical properties to the walls, and then definitively compare the individual sorption behavior of two different, common adsorbates (nitrogen and argon) within just these pores. The presence of a heavy metal surface was found to lead to a shift to higher pressure in the positions of adsorption and desorption branches and a widening of the hysteresis for argon isotherms for silica pores, but not for alumina pores. In contrast, no such effect was observed for nitrogen. Hence, using argon adsorption rather than nitrogen adsorption was found likely to lead to pore size inaccuracies of at least ∼200% in characterizing silica-supported platinum catalysts, for example. The experimental findings on effects of wetting and pore length were found to be consistent with findings from recent Monte Carlo and MFDFT simulations. Further, analysis of hysteresis widths also suggested that the standard approach using NLDFT would be unsuitable for the type of samples studied here. The results highlight particular issues for the accuracy of PSDs for catalysts consisting of metal nanoparticles supported on oxide materials.

Detection of the delayed condensation effect and determination of its impact on the accuracy of gas adsorption pore size distributions

“…This is just what was observed for nitrogen and argon adsorption for sol-gel silicas in previous work [8].…”

“…In previous work [8] it was found that, while the surface fractal dimension obtained from the adsorption isotherm declined following mercury entrapment in a mesoporous silica when nitrogen was used as the adsorbate, there was no such significant change in fractal dimension when argon was used as the adsorbate. It was suggested that this effect occurred because nitrogen wetted the smooth mercury metal surface significantly more than argon.…”

“…Previous work with this technique has demonstrated the existence of advanced adsorption of nitrogen in disordered silica gel materials [5], but has not studied delayed condensation. More recently the integrated gas sorption and mercury porosimetry technique has shown that there is a difference in the wetting of solid mercury relative to silica for nitrogen and argon adsorbates at their boiling points [8]. It was found that nitrogen will readily wet solid mercury surfaces, while argon, relatively, does not very much.…”

Connecting theory and simulation with experiment for the study of diffusion in nanoporous solids

“…Nevertheless, these shortcomings can be considerable, as the interpretation of adsorption measurements using nitrogen or other probes, as well as (mercury) porosimetry, again requires a model, which involves modeling assumptions as well. The interpretation of porosimetry measurements is a mathematical "inverse problem", thus inferring a pore size distribution and the pore connectivity is far from easy for a material with limited additional information on the pore shape [320,[345][346][347][348][349][350][351][352][353][354][355]. The danger in this is conflicting or circular argumentations.…”

Structural and chemical heterogeneity in ancient glass probed using gas overcondensation, X-ray tomography, and solid-state NMR

“…On heterogeneous polar surfaces, such as silica, the most commonly used adsorptive, nitrogen, can show some specific adsorption because the nitrogen molecule quadrupole moment is preferentially attracted to polar adsorption sites, such as ions and hydroxyl groups [4]. In previous work, the surface fractal dimensions for a range of controlled pore glasses, and sol-gel and fumed silicas, were obtained by two sets of completely independent physical processes and theories for data analysis, where the first was gas adsorption and the fractal BET model, and the second was small-angle X-ray scattering (SAXS) and Porod analysis [3,7]. It was found that, while argon and butane adsorption gave fractal dimensions that agreed with SAXS measurements, thereby mutually validating each result, the fractal dimension obtained from nitrogen adsorption tended to be always larger than that from SAXS.…”