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

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

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

Pore Size and Shape Pore Structure Silica Carbon Zeolite

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

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

Pore Size and Shape Measurement Techniques 1 10 100 1000 10000 10 100 1000 10000 Pore diameter (nm) Micro Meso Macro 2 50 N 2 capillary condensation Hg porosimetry

N2 Physisorption Adsorption and Desorption Isotherms Desorption

N2 Physisorption Adsorption and Desorption Isotherms III n p / VI V I VI V I II B IV

Isotherms Type I Langmuir Adsorption Isotherm Assumptions: / Assumptions: homogeneous surface (all adsorption sites energetically identical) monolayer adsorption (so no multilayer adsorption) no interaction between adsorbed molecules

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

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

Physisorption Surface area measurement

Physisorption Different Adsorbates Used in Physisorption Studies

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

BET Isotherm

BET Isotherm

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 < p/p0 < 0.3 is used to determine SBET

Pore Size and Surface Area

Pore Size Distribution Kelvin Equation

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

Kelvin Equation

Kelvin Equation Pore filling Model Cylindrical Pore Channel

Hysteresis Loop Information on pore shape

Pore Size Distribution t-Method nad t Proportional to St Note: nad is experimental result t is calculated from correlation t versus p

Kelvin Equation t-Method

Kelvin Equation Shape of t-plots

Kelvin Equation Interpretation of t-Plot -alumina

Kelvin Equation Pore Size Distribution -alumina r = t + 2V RTIn P0 P

Mercury Porosimetry Pore Size Distribution

Mercury Porosimetry Pore Size Distribution -alumina

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

Texture Data on Common Catalysts

N2 Adsorption Isotherms & Pore Volume Distributions wide-pore silica -alumina

N2 Adsorption Isotherms & Pore Volume Distributions -alumina activated carbon

N2 Adsorption Isotherms & Pore Volume Distributions Raney Ni ZSM-5

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

Hg Intrusion Curves & Pore Volume Distributions -alumina activated carbon

Hg Intrusion Curves & Pore Volume Distributions Raney Ni ZSM-5

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

BET- & t-plots -alumina activated carbon

BET- & t-plots Raney Ni ZSM-5

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

Chemisorption Metal Dispersion

Adsorption Mode

Adsorption Stoichiometry

Particle Size and Dispersion

Supported Metal Particles

Number of Surface Atoms

Pulse Chemisorption

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

Step Chemisorption On-line Mass Spectrometer

Temperature Programmed Desorption Adsorption Site Differentiation NH3 desorption from HZSM-5

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

Temperature Programmed Desorption Adsorption Energetics 12.49 A factor 24.51 Ed (kJ/mole) 5.4639 Intercept 2948.07 Slope Beta = heating rate [K / min] Tp = maximum desorption peak temperature Ed = Desorption energy [Kj / mole] A = Arrhenius factor R = 8.314451 [J / mol K]

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

Temperature Programmed Reduction Fe2O3

Temperature Programmed Reduction Fe2O3 Dry H2/Ar

Temperature Programmed Reduction Fe2O3 Wet H2/Ar (3% H2O)

Temperature Programmed Reduction Fe2O3

Kinetic Models for Reduction

Infrared Spectroscopy

Infrared Spectroscopy Reactor Cell Transmittance DRIFTS

Analysis of Catalyst Preparation Surface Hydroxyl Groups NH4ReO4 Alumina Dry impregnation Drying 383 K, 16 h Calcination 323 K, 2 h Re2O7/ Alumina

Analysis of Catalyst Preparation

IR Probe Molecule Acidity Measurement

IR Probe Molecule Acidity Measurement

Kelvin Equation Pore Size Distribution

Kelvin Equation Pore Size Distribution

In-Situ Reaction Study TCE Photocatalytic Oxidation

In-Situ Reaction Study PCO of Ethylene

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

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

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

In-Situ Reaction Study PCO of TCE

In-Situ Reaction Study PCO of Tetrachloroethylene