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Concept 02 / 06Surface Chemistry

Adsorption: Physisorption and Chemisorption

Adsorption and absorption, adsorbate and adsorbent, desorption, sorption, mechanism and thermodynamics of adsorption, physical versus chemical adsorption and their characteristics, factors affecting adsorption of gases on solids, and adsorption isobars.

Figure 1
Surface chemistry deals with phenomena at the interfaces between phases (solid-gas, solid-liquid, liquid-gas, liquid-liquid): adsorption, heterogeneous catalysis, colloids, corrosion, electrode processes. Adsorption. The accumulation of molecular species at the surface rather than in the bulk of a solid or liquid. The substance adsorbed is the adsorbate; the surface is the adsorbent (charcoal, silica gel, alumina, clay). Removal of the adsorbate is desorption. Absorption is uniform distribution throughout the bulk (water in a sponge, ammonia in water). Silica gel adsorbs water vapour, whereas anhydrous CaCl2\mathrm{CaCl_{2}} absorbs it. When both happen together, the term sorption is used. Examples: charcoal in a vessel of ammonia or SO2\mathrm{SO_{2}} lowers the gas pressure; animal charcoal decolourises raw sugar solution; silica gel keeps air dry. Mechanism and thermodynamics. Surface particles have unbalanced (residual) attractive forces. When molecules settle on the surface, these forces are partly satisfied and energy is released, so adsorption is always exothermic (ΔH<0\Delta H < 0). Adsorbed molecules lose freedom, so ΔS<0\Delta S < 0. For adsorption to be spontaneous, ΔG=ΔH−TΔS\Delta G = \Delta H - T\Delta S must be negative, which is possible only if ∣ΔH∣>∣TΔS∣|\Delta H| > |T\Delta S|. As adsorption proceeds, ΔH\Delta H becomes less negative and eventually ΔG=0\Delta G = 0: adsorption equilibrium. Physisorption (physical adsorption, van der Waals adsorption): due to weak van der Waals forces; not specific; reversible; forms multimolecular layers; low enthalpy of adsorption (2020 to 40 kJ mol−140\,\mathrm{kJ\,mol^{-1}}); occurs at low temperature and decreases as temperature rises; depends on how easily the gas liquefies (higher critical temperature means more adsorption); increases with surface area. Chemisorption (chemical adsorption, activated adsorption): due to chemical bond formation; highly specific; usually irreversible; forms a unimolecular layer; high enthalpy of adsorption (8080 to 240 kJ mol−1240\,\mathrm{kJ\,mol^{-1}}); requires activation energy, so it first increases with temperature and then decreases (see figure); increases with surface area. For example, H2\mathrm{H_{2}} is physisorbed on nickel at low temperature and chemisorbed at higher temperature. Factors affecting adsorption of gases on solids. Nature of gas: easily liquefiable gases (high critical temperature, strong van der Waals forces) are adsorbed more: SO2\mathrm{SO_{2}} (630 K630\,\mathrm{K}) > NH3\mathrm{NH_{3}} (406 K406\,\mathrm{K}) > CH4\mathrm{CH_{4}} (190 K190\,\mathrm{K}) > H2\mathrm{H_{2}} (33 K33\,\mathrm{K}). Surface area: finely divided metals and porous substances (activated charcoal, silica gel) are good adsorbents. Pressure: extent of adsorption increases with pressure (isotherms). Temperature: physisorption decreases with rise in temperature (Le Chatelier, exothermic). Activation of adsorbent: heating charcoal in steam or vacuum removes adsorbed gases and increases pore area. Key Takeaways: - Adsorption is a surface phenomenon; absorption is a bulk phenomenon. - Adsorption is exothermic with ΔS<0\Delta S < 0. - Physisorption: weak, non-specific, reversible, multilayer, 2020 to 40 kJ mol−140\,\mathrm{kJ\,mol^{-1}}. - Chemisorption: bonds, specific, irreversible, monolayer, 8080 to 240 kJ mol−1240\,\mathrm{kJ\,mol^{-1}}, needs activation. - Pitfall: chemisorption can increase with temperature at first, because it has an activation energy. Brain-Teaser 1: Arrange CO2\mathrm{CO_{2}} (Tc=304 KT_{c} = 304\,\mathrm{K}), N2\mathrm{N_{2}} (126 K126\,\mathrm{K}), NH3\mathrm{NH_{3}} (406 K406\,\mathrm{K}) and CH4\mathrm{CH_{4}} (190 K190\,\mathrm{K}) in order of extent of physisorption on charcoal. Solution: Higher critical temperature means stronger intermolecular forces and easier liquefaction, hence more adsorption: NH3>CO2>CH4>N2\mathrm{NH_{3} > CO_{2} > CH_{4} > N_{2}}. Brain-Teaser 2: Why is adsorption always exothermic, and why does physisorption decrease with rising temperature? Solution: Residual surface forces are partly satisfied when molecules settle, releasing energy; moreover, ΔS<0\Delta S < 0 for adsorption, so it can be spontaneous only if ΔH\Delta H is negative. Being an exothermic equilibrium, physisorption is pushed backward (desorption) by heating, according to Le Chatelier's principle. Brain-Teaser 3: How would you distinguish physisorption from chemisorption experimentally? Solution: Measure the enthalpy of adsorption (low for physisorption, high for chemisorption); test reversibility (physisorbed gas is removed easily by lowering pressure or warming; chemisorbed gas is not); check specificity (physisorption occurs on all surfaces for a given gas; chemisorption only on surfaces that can bond with it); and study the temperature dependence (chemisorption rises first, then falls).

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