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Surface Area and Pore Size Determination 19/10/2007 A Modern Methods in Heterogeneous Catalysis Research Surface area and pore size determination 19/10/2007 A. Trunschke Further reading S. Lowell, J.E. Shields, M.A. Thomas, M. Thommes, Characterization of Porous Solids and Powders: Surface Area, Pore Size and Density, Kluwer Academic Publisher, Dordrecht, 2004. R. Brdička, Grundlagen der physikalischen Chemie, Deutscher Verlag der Wissenschaften, Berlin 1982. F. Schüth, K.S.W. Sing, J. Weitkamp (Eds.), Handbook of Porous Solids, Vol. 1, Wiley-VCH, Weinheim 2002. G. Ertl, H. Knözinger, J. Weitkamp (Eds.), Handbook of Heterogeneous Catalysis, VCH, Weinheim, 1997. Outline 1. Introduction 2. Adsorption 3. Surface area measurements 4. Adsorption in micropores 5. Capillary condensation in mesopores 6. Experimental Modern Methods in Heterogeneous Catalysis Research; 19/10/2007; Surface Area and Pore Size Determination; A. Trunschke 2 Heterogeneous catalysis 1 600-800°C (NH4)2[PtCl6] → Pt with high surface area 2 H2 + O2 → 2 H2O Johann Wolfgang Döbereiner‘s lighter (1823) adsorbent surface area [m2/g] Some catalysts and support charcoal 300 - 2500 materials in heterogeneous silica gel 300 -350 catalysis γ-alumina 200 - 500 zeolites 500 - 1100 Modern Methods in Heterogeneous Catalysis Research; 19/10/2007; Surface Area and Pore Size Determination; A. Trunschke 3 Factors affecting surface area 1 1 particle of edge 1018 particles of edge lenght = 1 m lenght = 10-6 m size 2 -12 2 S=6 m Si=6x10 m 6 2 Stotal=6x10 m shape MoVTeNbOx porosity Aerosil 300 S = 300 m2/g www.aerosil com Structure of ZSM-5 Modern Methods in Heterogeneous Catalysis Research; 19/10/2007; Surface Area and Pore Size Determination; A. Trunschke 4 Surface area measurements 1 •Surface area from particle size distribution Dynamic light scattering 2 Measures Brownian motion and relates this to the size of the particles by using Q= scattering vector a correlation function and the Stokes-Einstein equation Non-spherical particles will be measured as equivalent spheres •Microscopy 0.5 1761AR Shape analysis 0.4 D= Diffusion coefficient 939 k= Boltzmann constant 0.3 1650AR T= absolute temperature η= dynamic viscosity of the solvent 0.2 R= radius of the particle Fraction 0.1 0.0 0 100 200 300 400 Diameter [nm] •Small angle X-ray scattering Scattering of X-rays by small domains of uniform matter (crystalline or amorphous), for which the electron density ρe is different from the continuous medium The central peak of scattered intensity gets broader as the domain size (particles, voids) decreases SAXS parameters (mean size / size distribution / specific surface area) are derived from analysis of the profile of the SAXS curve •Mercury porosimetry Modern Methods in Heterogeneous Catalysis Research; 19/10/2007; Surface Area and Pore Size Determination; A. Trunschke 5 Surface area measurements 1 • Gas adsorption •Surface area •Pore volume •Texture •Pore size distribution •Pore geometry •Connectivity Modern Methods in Heterogeneous Catalysis Research; 19/10/2007; Surface Area and Pore Size Determination; A. Trunschke 6 Outline 1. Introduction 2. Adsorption 3. Surface area measurements 4. Adsorption in micropores 5. Capillary condensation in mesopores 6. Experimental Modern Methods in Heterogeneous Catalysis Research; 19/10/2007; Surface Area and Pore Size Determination; A. Trunschke 7 Driving force of adsorption 2 Surface atom – unbalanced forces Δ= Surface energy Bulk atom – balanced forces Modern Methods in Heterogeneous Catalysis Research; 19/10/2007; Surface Area and Pore Size Determination; A. Trunschke 8 Adsorption 2 solid solid gas chemisorption physisorption Modern Methods in Heterogeneous Catalysis Research; 19/10/2007; Surface Area and Pore Size Determination; A. Trunschke 9 Adsorption 2 activated chemisorption solid gas chemisorption physisorption Heterogeneous catalysis Surface area/pore size determination Modern Methods in Heterogeneous Catalysis Research; 19/10/2007; Surface Area and Pore Size Determination; A. Trunschke 10 Physisorption forces 2 Van der Waal´s forces 1. Dispersion forces* (major part of the interaction potential) The electron motion in an atom or molecule leads to a rapidly oscillating dipole moment coupling of two neighboring moments into phase leads to a net attractive potential 2. Ion-dipole (ionic solid/polar gas molecule) 3. Ion-induced dipole (polar solid/polarizable gas molecule) 4. Dipole-dipole (polar solid/polar gas molecule) 5. Quadrupole interactions (e.g. –O-C++-O-) * F. London, Z. Phys. 63 (1930) 245. Similar to forces that lead to liquifidation of vapors Modern Methods in Heterogeneous Catalysis Research; 19/10/2007; Surface Area and Pore Size Determination; A. Trunschke 11 Adsorption 2 Physisorption Chemisorption -1 -1 ΔaH < 50 kJ mol > 500 kJ mol Coverage Multilayer → Monolayer or less, site restriction → Measurement of surface area Titration of active sites Interaction No structural changes, reversible Disruption of chemicalbonds may happen Kinetics Fast Activation required Pores Pores may be filled → - Pore volume measurements Modern Methods in Heterogeneous Catalysis Research; 19/10/2007; Surface Area and Pore Size Determination; A. Trunschke 12 Adsorption 2 gas phase (adsorptive) adsorbed molecule/atom adsorbate (adsorption complex) solid phase = adsorbent Modern Methods in Heterogeneous Catalysis Research; 19/10/2007; Surface Area and Pore Size Determination; A. Trunschke 13 Description of adsorption 2 Relation between T, p, adsorbed amount (surface concentration) Γ = ns/sa γ = ns/ma Γ = γ/Sa,sp. Ws Vs adsorpt (s) adsorbent (a) θθ0 =1- θ 1 Fraction of occupied surface (coverage) θ = Γ/ Γm = γ/ γm = N/Nm = W/Wm Γ = f (p) <eq., T = const.> adsorption isotherm Γ = f (T) <eq., p = const.> adsorption isobar p = f(T) <eq., Γ = const.> adsorption isostere Modern Methods in Heterogeneous Catalysis Research; 19/10/2007; Surface Area and Pore Size Determination; A. Trunschke 14 Adsorption isotherm 2 Γ = f (p)T T1 < T2 T1 T2 Adsorption is favored at lower temperatures Amount adsorbed ΔG = ΔH – TΔS Pressure P Decrease in translation freedom by adsorption: ΔS<0 Adsorption is a spontaneous process: ΔG<0 ΔH<0 Exothermic process Modern Methods in Heterogeneous Catalysis Research; 19/10/2007; Surface Area and Pore Size Determination; A. Trunschke 15 Adsorption isotherm 2 Γ = f (p)T The shape of the isotherm of pure fluids depends on •Interplay between the strength of fluid-wall and fluid-fluid interaction •Pore space Classification by the International Union of Pure and Applied Chemistry* Width* [nm] • Ultramicropores below 0.7nm Micropores < 2 • Supramicropores 0.7~2nm Mesopores 2 ~ 50 Macropores > 50 Width* •Diameter of a cylindrical pore •Distance between opposite walls in case of slit pores * K.S.W. Sing et al., Pure Appl. Chem. 57 (1985) 603. Modern Methods in Heterogeneous Catalysis Research; 19/10/2007; Surface Area and Pore Size Determination; A. Trunschke 16 Adsorption isotherm 2 Planar, mesoporous microporous nonporous, macropores Schematic illustration of adsorption potential Modern Methods in Heterogeneous Catalysis Research; 19/10/2007; Surface Area and Pore Size Determination; A. Trunschke 17 IUPAC classification of adsorption isotherms* 2 Adorption in micropores Unrestricted monolayer- I II multilayer adsorption Adsorption limited to a few molecular layers Nonporous or macroporous true chemisorption B adsorbent Zeolites, active carbon B monolayer coverage complete Weak adsorbtive- Monolayer-multilayer adsorbent interactions III IV adsorption and capillary condensation Nitrogen on polyethylene Complete pore filling Amount adsorbed Weak interactions and Stepwise multilayer capillary condensation V VI adsorption on a nonporous non-uniform surface Ar or Kr on graphitized carbon Relative pressure * K.S.W. Sing et al., Pure Appl. Chem. 57 (1985) 603. Modern Methods in Heterogeneous Catalysis Research; 19/10/2007; Surface Area and Pore Size Determination; A. Trunschke 18 Outline 1. Introduction 2. Adsorption 3. Surface area measurements 4. Adsorption in micropores 5. Capillary condensation in mesopores 6. Experimental Modern Methods in Heterogeneous Catalysis Research; 19/10/2007; Surface Area and Pore Size Determination; A. Trunschke 19 Surface area measurements 3 S = Ax Nm Ax cross-sectional area of the adsorbed molecule •Nitrogen 0.162 nm2 •Argon 0.166 nm2 •Krypton 0.210 nm2 Nm number of adsorbate molecules required to cover the solid with a single monolayer •Theories that give access to the monolayer capacity using the isotherm •Langmuir •BET Modern Methods in Heterogeneous Catalysis Research; 19/10/2007; Surface Area and Pore Size Determination; A. Trunschke 20 Langmuir isotherm* 3 Description of Type I Isotherm Assumptions • Monolayer adsorption • Energetically uniform surface • No interactions between adsorbed species (heat of adsorption independent of coverage) kinetic expression of the adsorption equilibrium rads = rdes dNads = dNdes * I. Langmuir, J. Am. Chem. Soc. 40 (1918) 1361. Modern Methods in Heterogeneous Catalysis Research; 19/10/2007; Surface Area and Pore Size Determination; A. Trunschke 21 Langmuir isotherm 3 dN/dt = k p θ0 N number of collisions with the surface θ0 fraction of surface unoccupied Kinetic gas theory: θ fraction of surface occupied k = N / (2π M R T)1/2 A condensation coefficient A (probability of a molecule of being adsorbed upon collision) Nads = k p θ0 A Nm number of molecules in a complete monolayer N = N θ v e-E/RT v vibr. frequency of adsorbate normal to the surface des m 1 23 −1 NA = 6,022 141 79 (30) × 10 mol -E/RT k p θ0 A = Nm θ v e -E/RT Nm
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