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Adsorption and Catalysis Dr. King Lun Yeung Department of Chemical Engineering Hong Kong University of Science and Technology CENG 511 Lecture 3

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

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Pore Size and Shape Pore Diameter –micropores (< 2 nm) –mesopores (2 – 50 nm) –macropores (> 50 nm) Pore Shape –cylinder –slit –ink-bottle –wedge

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Pore Size and Shape Pore Structure Silica Carbon Zeolite

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Pore Size and Shape Why is it important? it dictates the diffusion process through the material.

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Pore Size and Shape Why is it important? directly affect the selectivity of the catalytic reaction.

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Pore Size and Shape Measurement Techniques Pore diameter (nm) Micro Meso Macro 2 50 N 2 capillary condensation Hgporosimetry

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N 2 Physisorption Adsorption and Desorption Isotherms Adsorption Desorption

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Adsorption and Desorption 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 N 2 Physisorption

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Isotherms 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 Type I Langmuir Adsorption Isotherm

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

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

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Physisorption Surface area measurement

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Physisorption Different Adsorbates Used in Physisorption Studies

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N 2 Physisorption Adsorption and Desorption Isotherms Langmuir Adsorption? No: strong adsorption at low p due to condensation in micropores at higher p saturation due to finite (micro)pore volume

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BET Isotherm

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Nonporous 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

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Pore Size and Surface Area

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Pore Size Distribution Kelvin Equation

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Pore Size Distribution Kelvin Equation Cylindrical pore Ink-bottle pore Pore with shape of interstice between close-packed particles Adsorbed layer t dpdp dmdm

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Kelvin Equation

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Pore filling Model Cylindrical Pore Channel

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Hysteresis Loop Information on pore shape

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Pore Size Distribution t-Method n ad t Proportional to S t Note: n ad is experimental result t is calculated from correlation t versus p

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Kelvin Equation t-Method

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Kelvin Equation Shape of t-plots

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Kelvin Equation Interpretation of t-Plot -alumina

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Kelvin Equation Pore Size Distribution -alumina r = t + 2 V RTIn P0P0 P

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Mercury Porosimetry Pore Size Distribution

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Mercury Porosimetry Pore Size Distribution -alumina

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N 2 Physisorption versus Hg Porosimetry Hg cannot penetrate small (micro)pores, N 2 can Uncertainty of contact angle and surface tension values Cracking or deforming of samples

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Texture Data on Common Catalysts

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N 2 Adsorption Isotherms & Pore Volume Distributions wide-pore silica -alumina N 2 Adsorption Isotherms & Pore Volume Distributions

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-alumina activated carbon

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Raney NiZSM-5 N 2 Adsorption Isotherms & Pore Volume Distributions

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wide-pore silica -alumina Hg Intrusion Curves & Pore Volume Distributions

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-alumina activated carbon

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Raney NiZSM-5 Hg Intrusion Curves & Pore Volume Distributions

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BET- & t-plots wide-pore silica -alumina

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-alumina activated carbon BET- & t-plots

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Raney NiZSM-5 BET- & t-plots

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Chemisorption Surface Characterization Specific surface area of phases Types of active sites Number of active sites Reactivity of active sites Stability of active sites

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Chemisorption Metal Dispersion

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Adsorption Mode

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Adsorption Stoichiometry

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Particle Size and Dispersion

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Supported Metal Particles

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Number of Surface Atoms

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Pulse Chemisorption

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Monolayer capacity: 0.06 mmol / g Pt On-line Thermoconductivity Detector

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Step Chemisorption On-line Mass Spectrometer

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Temperature Programmed Desorption Adsorption Site Differentiation NH 3 desorption from HZSM-5

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Temperature Programmed Desorption Adsorption Energetics After ammonia saturation the sample is degassed at 120 °C for 60 minutes Heating Rate of 5, 10, 15 and 20 °C/min

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Temperature Programmed Desorption Adsorption Energetics Beta = heating rate [K / min] Tp = maximum desorption peak temperature Ed = Desorption energy [Kj / mole] A = Arrhenius factor R = [J / mol K] 12.49A factor 24.51Ed (kJ/mole) Intercept Slope

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Temperature Programmed Reduction –characterisation of oxidic catalysts and other reducible catalysts –qualitative information on oxidation state –quantitative kinetic data –optimisation of catalyst pretreatment

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Temperature Programmed Reduction Fe 2 O 3

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Temperature Programmed Reduction Fe 2 O 3 Dry H 2 /Ar

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Wet H 2 /Ar (3% H 2 O) Temperature Programmed Reduction Fe 2 O 3

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Temperature Programmed Reduction Fe 2 O 3

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Kinetic Models for Reduction

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Infrared Spectroscopy

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Reactor Cell DRIFTS Transmittance

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Analysis of Catalyst Preparation Surface Hydroxyl Groups NH 4 ReO 4 Alumina Dry impregnation Drying 383 K, 16 h Calcination 323 K, 2 h Re 2 O 7 / Alumina

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Analysis of Catalyst Preparation

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IR Probe Molecule Acidity Measurement

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IR Probe Molecule Acidity Measurement

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Kelvin Equation Pore Size Distribution

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Kelvin Equation Pore Size Distribution

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In-Situ Reaction Study TCE Photocatalytic Oxidation

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In-Situ Reaction Study PCO of Ethylene

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In-Situ Reaction Study PCO of 1,1-DCE

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In-Situ Reaction Study PCO of cis-1,2-DCE

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In-Situ Reaction Study PCO of trans-1,2-DCE

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In-Situ Reaction Study PCO of TCE

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In-Situ Reaction Study PCO of Tetrachloroethylene

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