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 forces 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 attraction. To compensate for this, surface atoms tend to adsorb gas molecules from the surrounding environment. This process is known as gas adsorption, and it plays a crucial role in understanding the properties of solid materials. Before conducting gas adsorption experiments, it's essential to clean the solid surface thoroughly to remove contaminants like water and oil. This cleaning process, often referred to as degassing, typically involves placing the sample in a vacuum chamber and heating it to remove any unwanted substances. After pretreatment, the sample is transferred to a controlled temperature environment, such as a Dewar or a thermostatic bath, to maintain stability. Once the sample is ready, a small amount of gas—known as the adsorbate—is gradually introduced into the evacuated sample tube. These gas molecules quickly reach the surface of the solid, where they either bounce off or stick to it. The adhesion of gas molecules to a solid surface is called adsorption, and the strength of the interaction determines whether it is physical (weak force) or chemical (strong force) adsorption. Physical adsorption is the most common type, allowing molecules to move relatively freely on the surface. As more gas is added, the molecules form a thin layer across the surface. According to the BET theory, assuming a monolayer coverage, we can estimate the number of molecules required to fully cover the surface, which helps calculate the surface area. As the gas pressure increases further, multiple layers of adsorption occur, along with capillary condensation. This process can be analyzed using the Kelvin equation, which relates the equilibrium pressure to the pore size. By measuring the adsorption isotherm—the relationship between the volume of adsorbed gas and the relative pressure—we can determine the pore size distribution. At high pressures, pores become completely filled with the adsorbate. Knowing the density of the adsorbate allows us to calculate the total pore volume of the sample. Reversing the process by reducing the gas pressure gives us the desorption isotherm. These two curves usually do not overlap due to differences in adsorption and desorption mechanisms, and the hysteresis observed is related to the shape of the pores. Unlike physical adsorption, chemisorption involves the formation of strong chemical bonds between the adsorbate and active sites on the surface. This technique is widely used to determine the number of chemically active sites, which are crucial for catalytic and chemical reaction processes.

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