Gas adsorption principle and process - Database & Sql Blog Articles

Everything is made up of atoms. Gaseous atoms and molecules move freely. In contrast, in the solid state, atoms are in a fixed position due to electrostatic attraction between adjacent atoms. However, the outermost layer (or surface) of the solid has fewer adjacent atoms around the inner layer of atoms. To compensate for this electrostatic gravitational imbalance, surface atoms adsorb gas molecules in the surrounding air. The process of adsorbing surrounding gas molecules across the solid surface is called gas adsorption. Monitoring the gas adsorption process has proven to provide a wealth of useful information about solids characteristics. Solid surfaces must be cleaned of contaminants such as water and oil prior to gas adsorption experiments. Surface () cleaning (degassing) process, in most cases placing a solid sample in a glass sample tube and heating under vacuum. Figure 1 shows the surface of the solid particles after pretreatment, which contains cracks and pores of different sizes and shapes. Once the sample is cleaned, it is transferred to an external Dewar (or other thermostatic bath or high temperature furnace) to maintain it at a constant temperature. Then, a small amount of gas (adsorbed material, that is, adsorbate) is gradually introduced into the sample tube that is evacuated. The adsorbate molecules entering the sample tube quickly reach the surface of each well on the solid sample (ie, the adsorbent). These molecules either bounce back from the surface or stick to the solid surface. The phenomenon in which gas molecules are adhered to a solid surface is called adsorption. By the magnitude of the interaction force between the adsorbed molecules and the surface, it can be determined whether the adsorption process is essentially physical adsorption (weak force) or chemical adsorption (strong force).
Gas adsorption principle and process physical adsorption Physical adsorption is the most common type of adsorption. () The physically adsorbed molecules can move relatively freely on the surface of the sample. As more and more gas molecules are introduced into the system, the adsorbate molecules form a thin layer across the surface of the adsorbent. According to the well-known BET theory, assuming that the adsorbed molecules are monolayers, we can estimate the number of molecules required to cover the entire adsorbent surface, Nm (see Figure 2). The product of the number of adsorbed molecules Nm and the cross-sectional area of ​​the adsorbate molecules is the surface area of ​​the sample. Continued increase in the amount of gas molecules introduced will result in multiple layers of adsorption. The multilayer adsorption process and the capillary condensation process (see Figure 3) are performed simultaneously. The latter process can be fully described by the Kelvin equation. This equation quantifies the ratio of the residual (or equilibrium) gas pressure to the capillary size of the condensable gas. The aperture can be calculated from the equilibrium gas pressure using calculation methods such as Barrett, Joyner and Halenda (BJH). Therefore, we can make an experimental curve (ie isotherm) between the volume of the adsorbed gas and the relative saturation equilibrium pressure, and then convert it to obtain a cumulative or differential pore size distribution map. As the equilibrium adsorbate pressure tends to saturate, the pores are completely filled with adsorbate (see Figure 4). If you know the density of the adsorbate, you can calculate the volume it takes, and then calculate the total pore volume of the sample accordingly. If we reverse the adsorption process at this time, and gradually reduce the amount of gas from the system, the desorption isotherm can also be obtained. Adsorption and desorption isotherms rarely overlap due to differences in adsorption and desorption mechanisms. The hysteresis of the isotherms is related to the pore shape of the solid particles. Unlike physical adsorption, chemisorption is due to the formation of strong chemical bonds between the adsorbate molecules and specific locations on the surface, ie, chemically active sites. Therefore, the basic use of chemisorption is to calculate the number of surface active sites that may cause chemical and catalytic reactions.