Templated mesoporous silica materials MCM-41 and SBA-15 consists of hexagonal arrays of cylindrical pores with diameters between 1.5 and 20 nm, narrow pore size distributions and negligible pore networking. These and other properties make these materials suitable for a number of applications in catalysis, adsorption, optics and electronics [1]. Detailed knowledge of the pore size and pore volume is crucial in order to optimize these applications. Argon adsorption is a key standard measurement for characterization of porous materials [2]. Due to its low vapor pressure, krypton adsorption at 77 K is often used to measure very low surface areas [2,3], and is currently also being explored for the pore size analysis of thin silica films [4].

In this work we report molecular simulations of argon and krypton adsorption on atomistic models of templated mesoporous silicas. These models [5] add atomistic levels of detail to mesoscale representations [6] of these porous materials; the latter were originally generated from lattice Monte Carlo simulations mimicking the synthesis process of templated mesoporous silicas. Three atomistic porous samples were considered: (A) a model of SBA-15 including both mesopores and micropores with mean diameters of 5.4 nm and 1.1 nm, respectively; (B) a model of MCM-41, which is similar to (A) but without microporosity; and (C) a regular cylindrical pore model. Our results [7] for Ar and Kr adsorption indicate that, among the three pore models considered, model A provides the best agreement with experimental data of gas adsorption on SBA-15 samples. Our calculations also suggest that fine-tuning the microporosity and the surface chemistry of model A can lead to better agreement with experiments. Marked differences were observed in the simulated adsorption isotherms, isosteric heat curves, pore filling mechanisms, and structure of the confined phases inside the three pore models. These results suggest that pore surface roughness, microporosity and other morphological features such as constrictions, play an important role in the mechanism of gas adsorption and filling of the mesopores.

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