Vacuum vs. Flow Degassing for Physisorption Characterization of Microporous Materials
Vacuum degassing represents a reliable pre-treatment method for microporous materials such as zeolites. Existing standard degassing protocols can be applied to sensitive materials without causing irreversible changes to the sample structure. In contrast, flow degassing is less suitable for degassing 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 degassing 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, all physically adsorbed material should be removed from the adsorbent surface while avoiding irreversible changes to the surface and texture of the material. This can be accomplished by vacuum pumping (vacuum degassing) or purging with an inert gas at elevated temperatures (flow degassing). An advantage of flow degassing 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 degassing is generally considered to be less suitable than vacuum degassing for microporous materials, in particular in the presence of ultramicropores (pores of width < 0.7 nm). In contrast, vacuum degassing is attractive because it prepares the surface under the same conditions that are required to start a vacuum volumetric adsorption experiment.
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