Heat of adsorption - Kyuchakunetsu

The enthalpy change that occurs during adsorption. It is expressed as the amount per 1 mole of adsorbed molecules, but there are two types: the differential heat of adsorption, which is the partial molar amount at a given pressure and temperature, and the integral heat of adsorption, which is the total enthalpy change assigned to the total molar amount of adsorbed molecules. The heat of adsorption is generally heat generated and varies with the amount of adsorption or the surface coverage, so the two do not coincide. The magnitude of the heat of adsorption varies depending on the properties of the adsorption bond; in physical adsorption, the heat generated is about 10 kJ mol ^-1 , while in chemical adsorption it can reach 50-500 kJ mol ^-1 . To calculate the heat of adsorption, an adiabatic calorimeter is used to directly measure the heat generated for a certain amount of adsorption, and the Clapeyron-Clausius equation is used to evaluate the isosteric differential heat of adsorption from two adsorption isotherms with slightly different temperatures. This value is larger than the true differential heat of adsorption by RT .

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

吸着に際して生じるエンタルピー変化．吸着分子1 mol 当たりの量として表されるが，与えられた圧および温度のもとでの部分モル量である微分吸着熱と，全エンタルピー変化を吸着分子の全モル量に割りつけた積分吸着熱の2種類がある．吸着熱は，一般に発熱であり，吸着量あるいは表面被覆率につれてかわるから，両者は一致しない．吸着結合の性質によって吸着熱の大きさは異なり，物理吸着では10 kJ mol^－1 程度の発熱であるが，化学吸着では50～500 kJ mol^－1 に達する．吸着熱を求めるには，断熱型熱量計を用いて一定の吸着量に対する発熱を直接測定するほか，温度がわずかに異なる二つの吸着等温線からクラペイロン-クラウジウスの式を利用して等量微分吸着熱を評価する．この値は真の微分吸着熱よりRTだけ大きい．

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