催化剂表征5
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Measurement of pore size
Total pore volume – measurement (1) • Physical adsorption • Mercury penetration • Liquid absorption (used when the pore volume is the only target) • Density measurement (used when the pore volume is the only target)
Porosity - Definition
Pore volume of a particle Porosity = Total volume of aຫໍສະໝຸດ Baiduparticle
Total volume = Pore volume + Particle volume Typically, porosity = 0.5 indicating that about a half of the particle is void space
Average pore radius
• Assumptions - All the pores are cylindrical Average pore radius r =
2Vg Sg
Vg – pore volume Sg – surface area • Just a rough estimate of pore radius • No meaning for a bidisperse pellet
8.74 × 105 r (Å) = P ( Psi )
• Can be used to pressure as high as 400 MPa (3 nm pore width)
Pore size distribution – Mercury penetration (3)
• Mercury intrusion into pore space as pressure increases; black areas indicate mercury. • Mercury envelops the mass. • Mercury fills the interparticle voids. • Mercury penetrates into the pores of the individual particles.
Pore size distribution – measurement methods
• • • • • Physical adsorption Mercury porosimetry (Mercury penetration) Small angle X-ray scattering Small angle neutron scattering All methods give results in terms of idealized models for pore shapes: spherical, cylindrical or slit-shaped pores
Pore size distribution – Mercury penetration (4)
Pore size distribution – Mercury penetration (5)
• Cumulative penetration and pore size distribution curve
∆V
Vi+1-Vi
∆r
ri+1-ri
∆ V/∆r
Pore size distribution – Mercury penetration (6)
The intrusion volume points on a mercury intrusion plot that are critical in the determination of volume and density.
Pore volume – physical adsorption (3)
N2 adsorption at P/P0=0.97 Vg ml/g = 0.0011567Va/W Vg --- Pore volume Va --- N2 adsorption volume at P/P0 = 0.97 (STP) W --- Weight of the sample
Pore volume – measurement (2) • Helium – Mercury method
– Measure the amount of He displaced by a catalyst sample (at room temperature He will not adsorb on catalyst) – Measure the amount of Hg displaced by a catalyst sample – Since Hg will not fill the pores of most catalysts at atmospheric pressure, the difference between (a) and (b) is the pore volume
• Pore size – definition (IUPAC)
Macropores Mesopores Micropores Pore width > 50 nm 2~50 nm <2 nm
Pore size (volume) distribution
• A plot of the derivative of pore volume per unit weight with respect to pore radius (dV/dr) as a function of pore radius (r) • Two types of distributions
• Method based upon behavior of nonwetting liquids in capillaries • Assumes pores are cylinders • If the contact angle between liquid and solid, θ, is greater than 90 º, the interfacial tension opposes the entrance of liquid into the pore. This can be overcame by external pressure. • For a cylindrical pore: pressure force = force due to surface tension πr 2 P = −2πrσ cosθ ⇒ r = − 2σ cosθ
Pore volume – Liquid absorption (4)
• Principle: fill the liquid into the pore volume without going into interparticle space • Liquid: water, tetrachlorethylene (CCl4) • End-point judgment: the sample cakes and there is a large change in speed of magnetic stirrer bar
• Electron microscopy can also be used to measure the pore size
Anodized alumina
Sintered stainless steel
Pore size distribution – Mercury penetration (1)
P
Pore size distribution – Mercury penetration (2)
• For mercury, θ varies with the nature of the solid • 140 º appears to be a good average value for Hg (now 130 º is recommended) • The working equation for evaluating the radius corresponding to a given pressure
– Very few situations where these models are accurate description of reality – Data are conventionally presented on the equivalent model basis
Pore size distribution – measurement methods (Cont.)
– Point A is used to determine bulk or envelope volume, points A and B: determine interparticle void volume, and points A and C: determine skeletal volume.
– Monodiperse – Bidisperse
Pore size distribution - monodisperse
Pore size distribution - bidisperse
Dual pore system
– Pellets or tablets are often compressed together from powders – Agglomeration of these porous powders give a pellet containing two types of void regions
Mercury penetration - Problems
• Compressibility of Hg. • Damage of the sample at high pressure • Pore shape may be quite different from reality • Contact angle is different with different sample
Pore Structure - Introduction
• Importance of pore structure on catalysis
– – – – Reactant accessibility Product selectivity Provide enough surface for active components to balance between the diffusion and reaction processes (high surface area and optimum diffusion rate of reactants and products).
pore radium r ~ cumulative pore volume V pore radium r ~ dV/ d(log r)
Pore size distribution – Mercury penetration example
No. 1 2 3 4 5 6 7 8 P kg/cm2 33.5 83.8 151 168 201 296 452 614 0.0321 0.0815 0.191 0.330 0.430 0.503 0.515 V ml/g r nm 224 89.5 49.7 44.8 37.3 28.7 16.6 12.2 0.0321 0.0494 0.1095 0.1390 0.1000 0.0730 0.0120 134.5 0.24x10-3 39.8 4.9 7.5 8.6 12.1 4.4 1.24x10-3 22.3x10-3 18.5x10-3 11.1x10-3 6.0x10-3 2.2x10-3
Total pore volume – measurement (1) • Physical adsorption • Mercury penetration • Liquid absorption (used when the pore volume is the only target) • Density measurement (used when the pore volume is the only target)
Porosity - Definition
Pore volume of a particle Porosity = Total volume of aຫໍສະໝຸດ Baiduparticle
Total volume = Pore volume + Particle volume Typically, porosity = 0.5 indicating that about a half of the particle is void space
Average pore radius
• Assumptions - All the pores are cylindrical Average pore radius r =
2Vg Sg
Vg – pore volume Sg – surface area • Just a rough estimate of pore radius • No meaning for a bidisperse pellet
8.74 × 105 r (Å) = P ( Psi )
• Can be used to pressure as high as 400 MPa (3 nm pore width)
Pore size distribution – Mercury penetration (3)
• Mercury intrusion into pore space as pressure increases; black areas indicate mercury. • Mercury envelops the mass. • Mercury fills the interparticle voids. • Mercury penetrates into the pores of the individual particles.
Pore size distribution – measurement methods
• • • • • Physical adsorption Mercury porosimetry (Mercury penetration) Small angle X-ray scattering Small angle neutron scattering All methods give results in terms of idealized models for pore shapes: spherical, cylindrical or slit-shaped pores
Pore size distribution – Mercury penetration (4)
Pore size distribution – Mercury penetration (5)
• Cumulative penetration and pore size distribution curve
∆V
Vi+1-Vi
∆r
ri+1-ri
∆ V/∆r
Pore size distribution – Mercury penetration (6)
The intrusion volume points on a mercury intrusion plot that are critical in the determination of volume and density.
Pore volume – physical adsorption (3)
N2 adsorption at P/P0=0.97 Vg ml/g = 0.0011567Va/W Vg --- Pore volume Va --- N2 adsorption volume at P/P0 = 0.97 (STP) W --- Weight of the sample
Pore volume – measurement (2) • Helium – Mercury method
– Measure the amount of He displaced by a catalyst sample (at room temperature He will not adsorb on catalyst) – Measure the amount of Hg displaced by a catalyst sample – Since Hg will not fill the pores of most catalysts at atmospheric pressure, the difference between (a) and (b) is the pore volume
• Pore size – definition (IUPAC)
Macropores Mesopores Micropores Pore width > 50 nm 2~50 nm <2 nm
Pore size (volume) distribution
• A plot of the derivative of pore volume per unit weight with respect to pore radius (dV/dr) as a function of pore radius (r) • Two types of distributions
• Method based upon behavior of nonwetting liquids in capillaries • Assumes pores are cylinders • If the contact angle between liquid and solid, θ, is greater than 90 º, the interfacial tension opposes the entrance of liquid into the pore. This can be overcame by external pressure. • For a cylindrical pore: pressure force = force due to surface tension πr 2 P = −2πrσ cosθ ⇒ r = − 2σ cosθ
Pore volume – Liquid absorption (4)
• Principle: fill the liquid into the pore volume without going into interparticle space • Liquid: water, tetrachlorethylene (CCl4) • End-point judgment: the sample cakes and there is a large change in speed of magnetic stirrer bar
• Electron microscopy can also be used to measure the pore size
Anodized alumina
Sintered stainless steel
Pore size distribution – Mercury penetration (1)
P
Pore size distribution – Mercury penetration (2)
• For mercury, θ varies with the nature of the solid • 140 º appears to be a good average value for Hg (now 130 º is recommended) • The working equation for evaluating the radius corresponding to a given pressure
– Very few situations where these models are accurate description of reality – Data are conventionally presented on the equivalent model basis
Pore size distribution – measurement methods (Cont.)
– Point A is used to determine bulk or envelope volume, points A and B: determine interparticle void volume, and points A and C: determine skeletal volume.
– Monodiperse – Bidisperse
Pore size distribution - monodisperse
Pore size distribution - bidisperse
Dual pore system
– Pellets or tablets are often compressed together from powders – Agglomeration of these porous powders give a pellet containing two types of void regions
Mercury penetration - Problems
• Compressibility of Hg. • Damage of the sample at high pressure • Pore shape may be quite different from reality • Contact angle is different with different sample
Pore Structure - Introduction
• Importance of pore structure on catalysis
– – – – Reactant accessibility Product selectivity Provide enough surface for active components to balance between the diffusion and reaction processes (high surface area and optimum diffusion rate of reactants and products).
pore radium r ~ cumulative pore volume V pore radium r ~ dV/ d(log r)
Pore size distribution – Mercury penetration example
No. 1 2 3 4 5 6 7 8 P kg/cm2 33.5 83.8 151 168 201 296 452 614 0.0321 0.0815 0.191 0.330 0.430 0.503 0.515 V ml/g r nm 224 89.5 49.7 44.8 37.3 28.7 16.6 12.2 0.0321 0.0494 0.1095 0.1390 0.1000 0.0730 0.0120 134.5 0.24x10-3 39.8 4.9 7.5 8.6 12.1 4.4 1.24x10-3 22.3x10-3 18.5x10-3 11.1x10-3 6.0x10-3 2.2x10-3