We used 93 mg of Cu-BTC for our measurements. Prior to the start of the experiments, the sample was placed in a stainless steel cell and outgassed at 100 oC for 6 hours under vacuum. The cell was then transported, without breaching vacuum integrity, to the adsorption setup, placed to on the closed-cycle refrigerator and connected to the gas handling system.
Microporous Cu3(BTC)2(H2O)3 (BTC = Benzene-1,3,5-tricarboxylate) was first reported in 1999 by Chui et al. There are two types of pores present in this material: square-shaped main channels (~9 angstroms in diameter) and tetrahedron-shaped side pockets with windows that are ~3.5 angstroms in diameter. We observed three different substeps in the isotherm data. The first two substeps correspond to adsorption in the two different sites in the sample: side pockets with a triangular windows, and main channels, respectively. The third substep was attributed, in a previous computer simulation study of this system, to the solidification of argon on the Cu-BTC metal-organic framework.
Our experimental adsorption data is in excellent agreement with our Grand Canonical Monte Carlo (GCMC) simulation results as well as with results from previous theoretical and experimental studies done by Vishnyakov et al.[1].
By assuming the atomic area of argon to be 13.2 Å2/atom, we calculated the effective specific area of the Cu-BTC sample to be 886 m2/g. This value is lower than the 1500 m2/g determined in the previous studies done by Vishnyakov et al.[1].
We also obtained the total pore volume for this sample. Our measured value of 0.32 cm3/g is 86 % of the than the total pore volume reported by Vishnyakov et al[1].
We performed a detailed investigation of the isosteric heat dependence on the coverage. We observed three peaks in the isosteric heat, which correspond to the 3 substeps in the isotherm data. The values of the isosteric heat for argon adsorbed in the tetrahedron side pockets and main channels are 146 and 112 meV respectively. These values are in good agreement with those obtained from our GCMC simulations (165 and 96 meV respectively). The value for the isosteric heat corresponding to the first substep is also in very good agreement with the one obtained by Vishnyakov et al.[1] (of 150.8 meV).
[1] Vishnyakov, A.; Ravikovitch, P. I.; Neimark, A. V.; Bulow, M.; Wang, Q. M. “Nanopore Structure and Sorption Properties of Cu-BTC Metal-Organic Framework” Nano Letters 3 (2003) 713.