Adsorption Hysteresis in Porous Solids

Area of Science:

• Physical Chemistry
• Materials Science
• Adsorption Science

Background:

• Hysteresis in gas-solid adsorption isotherms is commonly observed in mesoporous materials.
• This phenomenon is typically attributed to capillary condensation and classified as IUPAC Type IV or V.
• Existing models often rely on macroscopic concepts like the Kelvin equation.

Purpose of the Study:

• To investigate the molecular origins of hysteresis in fluid adsorption using lattice theory.
• To determine conditions under which equilibrium hysteresis occurs in slit-like pores.
• To model the change in interface geometry during adsorption and desorption without macroscopic assumptions.

Main Methods:

• Application of lattice theory and the Ono-Kondo theory for fluid adsorption.
• Simulation of adsorption in infinite, semi-finite, and finite slit-like pores.
• Analysis of phase transitions within the adsorbed phase and interface behavior.

Main Results:

• Phase transitions in the adsorbed phase can lead to hysteresis in kinetically controlled experiments.
• Equilibrium hysteresis is predicted only for pores of finite length.
• The molecular model demonstrates different interface geometries during adsorption and desorption in finite pores.

Conclusions:

• Finite pore length is a critical factor for equilibrium hysteresis in adsorption.
• The Ono-Kondo theory provides a molecular explanation for hysteresis by predicting changes in interface geometry.
• This model offers a microscopic approach to understanding adsorption hysteresis, bypassing macroscopic concepts.