3. Physisorption

Physisorption is a reversible process that allows many gaseous species (H ₂ O, CO ₂ , N ₂ , CO, C _x H _y , etc.), including noble gases, to be adsorbed by trapping onto the mostly cooled surface of an adsorbent. This is a multi-layer phenomenon in which each adsorbed particle constitutes a potential adsorption site for the next layer (figure 7 ). The forces binding the adsorbent to the adsorbate remain weak, ranging from 0.5 to 50 kJ · mol ^–1 , i.e. a tenth of the energy of a covalent bond.