Adsorption and Catalysis

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Adsorption and Catalysis Dr. King Lun
Yeung Department of Chemical Engineering Hong
Kong University of Science and Technology
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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 (lt 2 nm)
• mesopores (2 50 nm)
• macropores (gt 50 nm)

• Pore Shape
• cylinder
• slit
• ink-bottle
• wedge

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Pore Size and Shape
Pore Structure
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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
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N2 Physisorption
Adsorption and Desorption Isotherms
Desorption
Adsorption
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N2 Physisorption
Adsorption and Desorption Isotherms
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Isotherms
Type I Langmuir Adsorption Isotherm

• Assumptions
• homogeneous surface
• (all adsorption sites energetically identical)
• monolayer adsorption (so no multilayer
  adsorption)
• no interaction between adsorbed molecules

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Isotherms
Type II
Multilayer adsorption (starting at B) Common for
pore-free materials
Type IV
Similar to II at low p Pore condensation at high p
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Isotherms
Type III
Strong cohesion force between adsorbed molecules,
e.g. when water adsorbs on hydrophobic activated
carbon
Type IV
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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N2 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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BET Isotherm
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BET Isotherm
Nonporous Silica and Alumina

• Low p/p0
• filling of micropores
• favoured adsorption at most reactive sites
  (heterogeneity)
• High p/p0
• capillary condensation

BET equation
Range 0.05 lt p/p0 lt 0.3 is used to determine SBET
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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
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Kelvin Equation
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Kelvin Equation
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
Note nad 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 g-alumina
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Kelvin Equation
Pore Size Distribution g-alumina
r t 2sV
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Mercury Porosimetry
Pore Size Distribution
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Mercury Porosimetry
Pore Size Distribution g-alumina
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N2 Physisorption versus Hg Porosimetry

• Hg cannot penetrate small (micro)pores, N2 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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N2 Adsorption Isotherms Pore Volume
Distributions
N2 Adsorption Isotherms Pore Volume
Distributions
wide-pore silica
?-alumina
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N2 Adsorption Isotherms Pore Volume
Distributions
?-alumina
activated carbon
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N2 Adsorption Isotherms Pore Volume
Distributions
Raney Ni
ZSM-5
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Hg Intrusion Curves Pore Volume Distributions
wide-pore silica
?-alumina
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Hg Intrusion Curves Pore Volume Distributions
?-alumina
activated carbon
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Hg Intrusion Curves Pore Volume Distributions
Raney Ni
ZSM-5
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BET- t-plots
wide-pore silica
?-alumina
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BET- t-plots
?-alumina
activated carbon
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BET- t-plots
Raney Ni
ZSM-5
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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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Pulse Chemisorption
On-line Thermoconductivity Detector
Monolayer capacity 0.06 mmol / g Pt
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Step Chemisorption
On-line Mass Spectrometer
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Temperature Programmed Desorption
Adsorption Site Differentiation NH3 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
8.314451 J / mol K
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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
Fe2O3
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Temperature Programmed Reduction
Fe2O3
Dry H2/Ar
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Temperature Programmed Reduction
Fe2O3
Wet H2/Ar (3 H2O)
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Temperature Programmed Reduction
Fe2O3
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Kinetic Models for Reduction
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Infrared Spectroscopy
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Infrared Spectroscopy
Reactor Cell
Transmittance
DRIFTS
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Analysis of Catalyst Preparation
Surface Hydroxyl Groups
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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