Physisorption vs Chemisorption PHYSISORPTION CHEMISORPTION WEAK. LONG RANGE BONDING STRONG SHORT RANGE BONDING Van der Waals interactions Chemical bonding involved. NOT SURFACE SPECIFI SURFACE SPECIFIC Physisorption takes place between all E.g. Chemisorption of hydrogen takes place on molecules on any surface providing the transition metals but not on gold or mercury. temperature is low enough. △Ha=5……50 kJ mol-1 △H=50…500 kJ mol-t vated in which t relatively quickly. Increasing temperature be slow and increasing temperature can favour always reduces surface coverage adsorption No surface reactions Surface reactions may take place: Dissociation, reconstruction, catalysis. MULTILAYER ADSORPTION MONOLAYER ADSORPTION BET Isotherm used to model adsorption Langmuir Isotherm is used to model adsorption equilibrium. quilibrium http://nano.materials.drexel.edu Adsorption Process >Gas molecules admitted under increasing pressure to a clean, cold surface >Data treatment techniques find the quantity of gas that forms the first layer. 1. Diffusion to adsorbent surface 2. Migration into pores of adsorbent S Lowell & J E Shields, Powder Surface 3. Monolayer builds up of adsorbate Area and Porosity, 3rd Ed. Chapman Hall9 http://nano.materials.drexel.edu MONOLAYER ADSORPTION Langmuir Isotherm is used to model adsorption equilibrium. MULTILAYER ADSORPTION BET Isotherm used to model adsorption equilibrium. Surface reactions may take place:- Dissociation, reconstruction, catalysis. No surface reactions. Can be activated, in which case equilibrium can be slow and increasing temperature can favour adsorption. Non activated with equilibrium achieved relatively quickly. Increasing temperature always reduces surface coverage. ∆Hads = 50 ….. 500 kJ mol ∆H -1 ads = 5 ….. 50 kJ mol-1 SURFACE SPECIFIC E.g. Chemisorption of hydrogen takes place on transition metals but not on gold or mercury. NOT SURFACE SPECIFIC Physisorption takes place between all molecules on any surface providing the temperature is low enough. STRONG, SHORT RANGE BONDING Chemical bonding involved. WEAK, LONG RANGE BONDING Van der Waals interactions PHYSISORPTION CHEMISORPTION Physisorption Physisorption vs Chemisorption http://www.soton.ac.uk http://nano.materials.drexel.edu Adsorption Process Adsorption Process 1. Diffusion to adsorbent surface 2. Migration into pores of adsorbent 3. Monolayer builds up of adsorbate 1 2 3 ¾Gas molecules admitted under increasing pressure to a clean, cold surface. ¾Data treatment techniques find the quantity of gas that 1 2 3 forms the first layer. S. Lowell & J. E. Shields, Powder Surface Area and Porosity, 3rd Ed. Chapman & Hall, New York, 1991