Catalysis and Catalysts - Physical Adsorption Physical Adsorption  Texture and morphology –pore size –pore shape –pore-size distribution (same size or various sizes?) –pore volume –specific surface area of catalyst

Catalysis and Catalysts - Physical Adsorption Internal Diffusion  Types of diffusion –Molecular –Knudsen –Surface (also called “configurational diffusion)”  Knudsen number: K n = /l = molecular free path length l = characteristic pore diameter K n << 1bulk diffusion K n >> 1Knudsen diffusion

Catalysis and Catalysts - Physical Adsorption Pore Diameters, Shapes??  Pore diameters –micropores (< 2 nm) –mesopores ( nm) –macropores (> 50 nm)  Experimental techniques –capillary condensation –Hg intrusion –microscopy  Shapes –cylinder, slit, ink-bottle, wedge,...

Catalysis and Catalysts - Physical Adsorption Pore Size and Diffusion Regimes Configurational diffusion Surface migration E a (kJ/mol) D (m 2 /s) Pore diameter (nm) Molecular diffusion Knudsen diffusion Surface migration

Catalysis and Catalysts - Physical Adsorption Pore Diameters and Measurement Techniques Pore diameter (nm) Micro Meso Macro 2 50 N 2 capillary condensation Hgporosimetry

Catalysis and Catalysts - Physical Adsorption Shape Selectivity Reactant selectivity + Product selectivity CH 3 OH + Restricted transition-state selectivity

Catalysis and Catalysts - Physical Adsorption Pore Shapes Slit Ink-bottle Cylindrical Wedge ab dc

Catalysis and Catalysts - Physical Adsorption Pore Structures of Zeolites ZSM-5Mordenite

Catalysis and Catalysts - Physical Adsorption Volumetric Adsorption Measurement N 2 (77.3 K) or Ar, He, CH 4, CO 2, Kr adsorbate adsorbent pressure gauge P V 1 V 2 high vacuum

Catalysis and Catalysts - Physical Adsorption Adsorption Isotherms Adsorption Desorption

Catalysis and Catalysts - Physical Adsorption Adsorption Isotherms III n ad p / p 0 VI n ad p / p 0 V n p / p 0 I n p / p 0 p / p II n ad 0 B IV n ad p / p 0 B

Catalysis and Catalysts - Physical Adsorption Langmuir Adsorption Isotherm (Type I) Assumptions: homogeneous surface (all adsorption sites energetically identical) monolayer adsorption (so no multilayer adsorption) no interaction between adsorbed molecules I n ad p / p 0

Catalysis and Catalysts - Physical Adsorption Type II and IV Isotherms Multilayer adsorption (starting at B) Common for pore-free materials Similar to II at low p Pore condensation at high p p / p II n ad 0 B IV n ad p / p 0 B

Catalysis and Catalysts - Physical Adsorption Type III and V Isotherms III n ad p / p 0 V n p / p 0 Strong cohesion force between adsorbed molecules, e.g. when water adsorbs on hydrophobic activated carbon Similar to III at low p Pore condensation at high p

Catalysis and Catalysts - Physical Adsorption Surface Area & Monolayer Capacity S = n m A m N monolayer capacity (mol/g) specific surface area (m 2 /g) area occupied by one molecule (m 2 /molecule) Avogadro’s number (molecules/mol) BET model: S BET t model: S t

Catalysis and Catalysts - Physical Adsorption Properties of Adsorbates for Physisorption Measurements

Catalysis and Catalysts - Physical Adsorption N 2 Adsorption Isotherm in ZSM-5 Langmuir Adsorption? No: strong adsorption at low p due to condensation in micropores at higher p saturation due to finite (micro)pore volume

Catalysis and Catalysts - Physical Adsorption BET (Brunauer, Emmett, Teller) Method  Modification of Langmuir isotherm  Both monolayer and multilayer adsorption  Layers of adsorbed molecules divided in: –First layer with heat of adsorption  H ad,1 –Second and subsequent layers with H ad,2 =  H cond BET isotherm:  BET equation does not fit entire adsorption isotherm –different mechanisms play a role at low and at high p

Catalysis and Catalysts - Physical Adsorption BET Model reality model 55 44 33 22 11 00 1 st layer n th layer For every layer Langmuir model Assume with

Catalysis and Catalysts - Physical Adsorption Non-Porous Silica and Alumina Low p/p 0 : filling of micropores favoured adsorption at most reactive sites (heterogeneity) High p/p 0 : capillary condensation Range 0.05 < p/p 0 < 0.3 is used to determine S BET BET equation

Catalysis and Catalysts - Physical Adsorption Texture Data of Commercial Catalysts

Catalysis and Catalysts - Physical Adsorption Adsorption at Pore Wall Cylindrical pore Ink-bottle pore Pore with shape of interstice between close-packed particles Adsorbed layer t dpdp dmdm

Catalysis and Catalysts - Physical Adsorption Kelvin Equation for Nitrogen micromeso macro V L =  m 3 /mol  = 8.88 mN/m d m (nm) Relative pressure p/p 0

Catalysis and Catalysts - Physical Adsorption Hysteresis Loops Information on pore shape

Catalysis and Catalysts - Physical Adsorption t-method n ad t Proportional to S t Note: n ad is experimental result t is calculated from correlation t versus p

Catalysis and Catalysts - Physical Adsorption t-method  BET –only valid in small pressure interval –interpretation not very easy  thickness (t) of adsorbed layer can be calculated  plot of t versus p for non-porous materials is the same (has been checked experimentally)  t-plot helps in interpretation nm

Catalysis and Catalysts - Physical Adsorption p/p0p/p0 Adsorbed-layer thickness t (nm) t-curves a b Halsey Harkins-Jura-de Boer

Catalysis and Catalysts - Physical Adsorption t-plot of  -alumina mesopores macropores S t,micro = 0 m 2 /g V t,micro = 0 ml/g S t = 200 m 2 /g

Catalysis and Catalysts - Physical Adsorption Shape of t-plots t n ad t t Non-porousMicroporous Micro- and mesoporous StSt S mesopores p n ad Adsorption isotherm t = f(p)

Catalysis and Catalysts - Physical Adsorption t-plot of N 2 Physisorption on ZSM-5

Catalysis and Catalysts - Physical Adsorption Pore-Size Distribution of  -Alumina d p (nm) dV/dd (ml/g/nm)

Catalysis and Catalysts - Physical Adsorption Mercury Intrusion Porosimetry  Hg does not wet surfaces; pressure is needed to force intrusion  From a force balance: (d in nm, p in bar)  Convenient method for determining pore volume versus pore size

Catalysis and Catalysts - Physical Adsorption Mercury Intrusion Curve of  -Alumina

Catalysis and Catalysts - Physical Adsorption Surface Area’s - S Hg and S BET

Catalysis and Catalysts - Physical Adsorption Discrepancy S Hg and S BET for Microporous Materials  Hg cannot penetrate small (micro)pores, N 2 can  Uncertainty of contact angle and surface tension values  Cracking or deforming of samples

Catalysis and Catalysts - Physical Adsorption Texture Properties

Catalysis and Catalysts - Physical Adsorption N 2 Adsorption Isotherms & Pore Volume Distributions wide-pore silica  -alumina

Catalysis and Catalysts - Physical Adsorption N 2 Adsorption Isotherms & Pore Volume Distributions  -alumina activated carbon

Catalysis and Catalysts - Physical Adsorption N 2 Adsorption Isotherms & Pore Volume Distributions Raney NiZSM-5

Catalysis and Catalysts - Physical Adsorption Hg Intrusion Curves & Pore Volume Distributions wide-pore silica  -alumina

Catalysis and Catalysts - Physical Adsorption Hg Intrusion Curves & Pore Volume Distributions  -alumina activated carbon

Catalysis and Catalysts - Physical Adsorption Hg Intrusion Curves & Pore Volume Distributions Raney NiZSM-5

Catalysis and Catalysts - Physical Adsorption BET- & t-plots wide-pore silica  -alumina

Catalysis and Catalysts - Physical Adsorption BET- & t-plots  -alumina activated carbon

Catalysis and Catalysts - Physical Adsorption BET- & t-plots Raney NiZSM-5