What information does water adsorption provide for pore structure?
Water vapor adsorption analysis is part of routine basic research and industrial basic materials research. Water can be absorbed by most solid surfaces, to varying degrees. Desiccants, battery oxides, the unique functions of catalysts, and the stability of pharmaceuticals and foods are all related to their water adsorption behavior. The amount of water adsorbed is a function of the following factors: the affinity of the surface with water molecules; temperature; water vapor concentration (ie pressure, also expressed as partial pressure, relative pressure, relative humidity or water activity); absolute amount of exposed surface area; pore volume / aperture (affinity). In addition to molecules that are directly adsorbed on the solid surface, Other water molecules will coalesce in channels of appropriate size. ?
Water adsorption studies can distinguish chemical differences on the surface of carbon materials. For example, both PBL and F400 carbon materials have nearly identical microporous and ultramicroporous structures, but according to chemical adsorption experiments of XPS and TPD-MS, they have different surface chemistry. The action of water and surface has both physical adsorption and chemical adsorption. Therefore, the adsorption of water in the micropores occurs at a relatively high relative pressure (see Figure 95). The nitrogen adsorption ultramicroporous pore size distribution maps of the above two types of materials are almost the same, but they are quite different in the low pressure section of water adsorption. Experiments have shown that PBL can adsorb more water, which means that there are a large number of oxidized groups on the surface. ??
Figure 95 Comparison of nitrogen adsorption isotherms of three carbon materials (left, at 77K) and water adsorption isotherms (right, at 298K)
Layered zeolites have very superior properties in catalysis and adsorption. In order to explain its outstanding performance, it is essential to use high-end physicochemical adsorption techniques to characterize its pore structure, so that we can grasp the precise knowledge of its pore size, interoperability and surface properties. ?
The mesoporous ZSM-5 zeolite compositions have the same porosity but differ in composition. Water adsorption is sensitive to the Si/Al ratio and the distribution of aluminum in the zeolite molecular sieve and is also sensitive to polar groups present on the mesoporous surface. Evaluation of the mesoporous ZSM-5 zeolite composition by argon adsorption (87.3 K) and water adsorption (298.5 K) is a very good technical means. ?
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