Vacuum vs. Flow Outgassing for Physisorption Characterization of Microporous Materials
Vacuum outgassing represents a reliable pre-treatment method for microporous materials such as zeolites. Existing standard outgas protocols can be applied to sensitive materials without causing irreversible changes to the sample structure. In contrast, flow outgassing is less suitable of the outgassing of microporous materials, in particular in the presence of ultramicropores (pores of width < 0.7 nm) from which pre-adsorbed species are difficult to remove. For hydrophilic zeolites, flow outgassing is inefficient, may lead to irreproducible states of the adsorbent surface, and can potentially alter the pore structure of the solid.
Prior to a physisorption experiment, it is required to remove all physically adsorbed material from the adsorbent surface while avoiding irreversible changes to the surface and texture of the material [1, 2]. This can be accomplished by vacuum pumping (vacuum outgassing) or purging with an inert gas at elevated temperatures (flow outgassing). An advantage of flow outgassing is that its setup is very easy (i.e., no expensive vacuum system is required) and it efficiently removes larger amounts of loosely-bound adsorbates such as water from nonporous or mesoporous materials. However, flow outgassing is generally considered to be less suitable as compared to vacuum outgassing for microporous materials, in particular in the presence of ultramicropores (pores of width < 0.7 nm). In contrast to flow outgassing, vacuum outgassing is attractive because it prepares the surface under the same conditions that are required to start a vacuum volumetric adsorption experiment. In fact, the advantages of vacuum outgassing have been well known for many decades. For example, Gregg and Sing  state in their 1982 textbook: “Prior to the determination of an isotherm, all physisorbed material has to be removed from the surface to high vacuum, the exact conditions required (temperature and residual pressure) being dependent on particular gas-solid system.”
- S. Lowell, J. Shields, M.A. Thomas, M. Thommes. Characterization of Porous Solids and Powders: Surface Area, Pore Size and Density, Springer, The Netherlands, 2004.
- S.J. Gregg, K.S.W. Sing. Adsorption, Surface Area and Porosity, Academic Press, London, 1982.
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