Techniques for Porosity Analysis Mercury y Provide information egarding pore connectivity Pore size can be measured if the materials contain ordered pores Rarely used for pore Small angle analysis http://nano.materials.drexel.edu Techniques for Porosity Analysis SEM Pore size 5nm Rarely used for pore analysis
6 http://nano.materials.drexel.edu Techniques for Porosity Analysis Techniques for Porosity Analysis Mercury porosimetry TEM SEM Small angle X-ray scattering Small Angle Neutron scattering Gas adsorption Techniques ¾ Provide information regarding pore connectivity ¾ Pore size can be measured if the materials contains ordered pores ¾ Rarely used for pore analysis http://nano.materials.drexel.edu Techniques for Porosity Analysis Techniques for Porosity Analysis Mercury porosimetry TEM SEM Small angle X-ray scattering Small Angle Neutron scattering Gas adsorption Techniques ¾ Pore size > 5nm ¾ Rarely used for pore analysis
Techniques for Porosity Analysis Mercury scattering Small angle, Any pore size catering Open CI porosity http://nano.materials.drexel.edu Techniques for Porosity Analysis Any pore size Mercury porosimetry Neutron porosity scatterin
7 http://nano.materials.drexel.edu Techniques for Porosity Analysis Techniques for Porosity Analysis Mercury porosimetry TEM SEM Small angle X-ray scattering Small Angle Neutron scattering Gas adsorption Techniques ¾ Any pore size ¾ Open + Close porosity http://nano.materials.drexel.edu Techniques for Porosity Analysis Techniques for Porosity Analysis Mercury porosimetry TEM SEM Small angle X-ray scattering Small Angle Neutron scattering Gas adsorption Techniques ¾ Any pore size ¾ Open & Close porosity ¾ Costly
Theory of Adsorption http://nano.materials.drexel.edu Adsorption Process Adsorptive. adsorbate before being adsorbed on the surface Adsorbate- the gas adsorbed on the surface of solids Adsorbent- the solid where adsorption takes place Adsorption is brought by the forces acting between the solid and the molecules of the gas. These forces are of two kinds: physical (physiosorption)and chemical (chemisorption
8 http://nano.materials.drexel.edu Theory of Adsorption Theory of Adsorption http://nano.materials.drexel.edu Adsorption Process Adsorption Process Adsorption is brought by the forces acting between the solid and the molecules of the gas. These forces are of two kinds: physical (physiosorption) and chemical (chemisorption) Adsorbent - the solid where adsorption takes place Adsorbate - the gas adsorbed on the surface of solids Adsorptive - adsorbate before being adsorbed on the surface
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 Hall
9 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
Adsorption Process J。=f(W,T,,P) Adsorbate P=pressure of the adsorbate I=interaction between adsorbate and adsorbent If w, T, and I are made constant, the above equation can be written as J ere Adsorbent pressure of adsorbate S. Lowell J. E. Shields, Powder Surface Area and P saturated pressure of adsorptive Porosity, 3rd Ed. Chapman&Hall,New York, 1991 Gas Sorption: Isotherm 。 Desorption isotherm P pressure of adsorbate p saturated pressure of adsorptive Adsorption isotherm >Isotherm is a measure of the volume of gas adsorbed at a constant temperature function pressure >Isotherms can be grouped into SIX lasse
10 http://nano.materials.drexel.edu Adsorption Process Adsorption Process Adsorbent Adsorbate saturated pressure of adsorptive pressure of adsorbate where equation can be written as: If W, T, and I are made constant, the above interaction between adsorbate and adsorbent. temperature; pressure of the adsorbate; weight of adsorbent; volume of gas adsorbed; where ( , , , ) = ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ = = = = = = = p p p p f I T P W f W T I P o V o V V a a a Equation of adsorption isotherm S. Lowell & J. E. Shields, Powder Surface Area and Porosity, 3rd Ed. Chapman & Hall, New York, 1991 http://nano.materials.drexel.edu Gas Sorption: Isotherm Adsorption isotherm ¾Isotherm is a measure of the volume of gas adsorbed at a constant temperature as a function of gas pressure. ¾Isotherms can be grouped into six classes. saturated pressure of adsorptive pressure of adsorbate where = ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ = p p p p f o V o a Va Desorption isotherm p po