Chemical adsorption

Adsorption occurs due to chemical bonds between a solid surface and molecules in a homogeneous phase adjacent to it. In chemical adsorption, the bonds within the adsorbed molecule are broken or double bonds are cleaved, followed by the formation of new bonds between the surface atoms or the transfer of electrons to vacant levels in the adsorbed molecule or solid. Therefore, the combination of the adsorbed molecule and the surface is selective, and adsorption does not exceed a monolayer. Because the energy of the adsorption bond is large, the heat generated by adsorption is large, showing values ​​of 50 to 500 kJ mol ^-1 , even after subtracting the energy loss due to the dissociation of the adsorbed molecule. Depending on the adsorption system, it becomes activated adsorption, which requires activation energy. In this case, the adsorption rate is significantly slower than physical adsorption, but the rate increases as the temperature increases. For simple molecules, the adsorption energy is approximately calculated by quantum chemical calculations using the valence bond method or molecular orbital method. It is also considered that the initial process of a catalytic reaction requires the generation of moderately strong chemical adsorption.

Source: Morikita Publishing "Chemical Dictionary (2nd Edition)" Information about the Chemical Dictionary 2nd Edition

固体表面とこれに接する均一相内の分子との化学的結合によって生じる吸着．化学吸着では，吸着分子内の結合の切断や二重結合の開裂と，これに続く表面原子との間の新しい結合生成，あるいは吸着分子，固体の空準位への電子移動が行われるものと考えられる．したがって，吸着分子と表面との組合せは選択的であり，吸着は単分子層を越えない．吸着結合のエネルギーが大きいため，吸着分子の解離などによるエネルギー損失を差し引いても吸着による発熱は大きく，50～500 kJ mol^－1 の値を示す．吸着系によっては，活性化エネルギーを必要とする活性化吸着となる．この場合，吸着速度は物理吸着よりいちじるしく遅いが，高温となるにつれて速度は増大する．単純な分子について，吸着エネルギーが原子価結合法あるいは分子軌道法を用いた量子化学的計算で近似的に求められている．また，触媒反応の初期過程として，適度の強さの化学吸着を生じることが必要であると考えられている．

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