Chemisorption, or chemical adsorption, is a surface phenomenon where a chemical reaction occurs between the adsorbate (gas or vapor) and the exposed surface of a solid material. This process results in the formation of a strong bond via electron sharing and valence bonding, creating a distinct chemical species at the surface. Unlike physical adsorption, chemisorption is highly specific, involving the splitting of adsorbing gas or vapor molecules into atoms, radicals, or ions that chemically bond to specific adsorption sites on the surface. 

This interaction is typically characterized by a high binding energy and is irreversible under standard conditions. Chemisorption plays a vital role in catalyst characterization, providing key insights into properties such as the reduction temperature at which metals become catalytically active, the quantity of surface metal or active species available for reaction, the strength of specific active sites, and the material’s performance after undergoing reduction or oxidation cycles.