Gas adsorption principle and process - Database & Sql Blog Articles

Everything is made up of atoms. Gaseous atoms and molecules move freely, while in the solid state, atoms are held in fixed positions due to electrostatic attraction between neighboring atoms. However, the surface layer of a solid has fewer adjacent atoms compared to the inner layers, leading to an imbalance in electrostatic forces. To compensate for this, surface atoms tend to adsorb gas molecules from the surrounding environment. This process, known as gas adsorption, provides valuable insights into the properties of solid materials. Before conducting gas adsorption experiments, it is essential to clean the solid surface to remove contaminants like water and oil. This is typically done by placing the sample in a glass tube under vacuum and heating it, a process called degassing. After cleaning, the sample is transferred to a thermostatic bath or furnace to maintain a constant temperature. Then, a small amount of gas (adsorbate) is gradually introduced into the evacuated sample tube. The gas molecules quickly reach the surface of the solid, where they either bounce off or stick to it, depending on the strength of the interaction. Adsorption can be classified into two main types: physical adsorption and chemical adsorption. Physical adsorption involves weak intermolecular forces, allowing adsorbed molecules to move freely on the surface. As more gas is introduced, these molecules form a thin layer across the surface. According to the BET theory, we can estimate the number of molecules needed to cover the entire surface, which helps determine the surface area. Further introduction of gas leads to multilayer adsorption and capillary condensation, processes that can be analyzed using the Kelvin equation. By plotting the adsorbed gas volume against the relative pressure, we obtain an adsorption isotherm. From this, we can derive pore size distribution maps using methods like BJH. As the pressure approaches saturation, the pores become completely filled with adsorbate. Knowing the adsorbate's density allows us to calculate the total pore volume. Desorption isotherms can also be obtained by reducing the gas pressure, but they often differ from adsorption isotherms due to hysteresis, which is influenced by the pore structure. Chemisorption, on the other hand, involves strong chemical bonds between the adsorbate and active sites on the surface. It is commonly used to determine the number of chemically active sites available for reactions and catalysis. Understanding both physical and chemical adsorption mechanisms is crucial in material science, especially in applications such as gas storage, catalysis, and sensor development.

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