Mô tả:
École FRANCO-VIETNAMIENNE 2005
de CATALYSE
CINETIQUE et RAFFINAGE
Hanoi, 18-22 Avril 2005
Institut de Chimie
Industrielle
Hélène Provendier
Institut de Recherches sur la Catalyse
Université Claude Bernard Lyon 1
1
Ecole Franco-Vietnamienne 2005 de Catalyse Cinétique et Raffinage
Hanoi, 18-22 Avril 2005
Physical and chemical techniques
for catalyst characterization
Dr. Hélène Provendier
Institut de Recherches sur la Catalyse
Villeurbanne
France
Centre National
de la Recherche
Scientifique
ICI
2
Contents
Part 1 : Adsorption techniques
Part 2 : Diffraction techniques
Part 3 : Spectroscopies
3
Part 1 : Adsorption techniques
Part 1 :
Characterization of
porous solids
using adsorption
techniques
4
Outline
I. Introduction to solid surface and porosity
1) Porous solids
2) Some definitions
3) Qualitative description of a porous solid
4) Texture of a solid
5) Classification of pores
6) Characterization methods
II. Nature of Adsorption
1) Definition of adsorption
2) Two types of adsorption
3) Transition physisorption-chemisorption
4) Activation energy
III. Physisorption
1) General
2) Theoretical approach
3) Porous volume measurements
4) Pore size distribution
IV. Chemisorption
1) General
2) Chemisorption isotherms
V. Experimental part
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I. Introduction to solid surface and porosity
1) Porous solids
Most materials are to some extent porous : they contain empty cavities.
Physical properties :
- density,
- thermal conductivity
- strength
depend on the pore structure of a solid
The control of porosity is of great industrial importance for example in the design of
catalysts, industrial adsorbents, membranes and ceramics.
Porosity influences :
• the chemical reactivity (activity and
selectivity) of solids
• the physical interaction of solids with gases
and liquids
• mass and heat transfers in the solid
In catalysis, porosity determines the accessible surface to the reactant (gas or liquid)
It is thus important to characterize porous solids in catalysis
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2) Some definitions
Porous solid : a solid with pores, i.e. cavities, channels or interstices,which are deeper
than they are wide.
Pore volume Vp : volume of the pores, as measured by a given method which must be
stated, (together, for instance, with the nature of the probe-molecule, the wavelength of
the radiation used or the ultimate intrusion pressure ...).
Pore size (generally, pore width) : the distance between two opposite walls of the pore
(diameter of cylindrical pores, width of slit-shaped pores).
Porosity e : ratio of the total pore volume Vp to the apparent volume V of the particle or
powder (excluding interparticle voids).
e = VP / V
Roughness (or rugosity) factor : ratio of the external surface area to the area of the
geometrical envelope of the particles.
Surface area : extent of the total surface as determined by a given method under stated
conditions. It is essential to state the method used.
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3) Qualitative description of a porous solid
* Closed pores : (a). They influence such
macroscopic properties as bulk density,
mechanical strength and thermal conductivity, but
are inactive in such processes as fluid flow and
adsorption of gases.
* Open pores : (b) (c) (d) (e) and ( f ) . They have
a continuous channel of communication with the
external surface.
- blind (i.e. dead-end, or saccafe) pores : (b) (f)
opened only at one end
- through pores : (e)
opened at two ends
* Classification according to the shape :
- cylindrical (either open (c) or blind ( f ) ) ,
- ink-bottle shaped (b),
- funnel shaped (d) or slit-shaped.
Close to, but different from porosity is the
- roughness of the external surface (g).
f
Schematic cross-section of a porous solid
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4) Texture of a solid
Texture : detailled geometry of empty spaces in particles. It includes :
- intergranular spaces in agglomerates
- intragranular pore ditribution
- particle shape and external surface
- pore shape and porous volume
- accessibility of gases to internal surface
Parameters describing the texture of a catalyst
- specific surface area (accessible per gram of solid)
- porosity
• pore shape
• pore size distribution
• mean pore size
- granulometry
• particle size distribution
• shape and size of agglomerates
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5) Classification of pores
1) By size
- macroporous samples
> 50 nm
- mesoporous samples 2 nm < < 50 nm
- micro-porous samples < 2 nm
2) By shape
- cylindrical
- slit-shaped (parallel or not)
- funnel (entonnoir)
- spherical
- ink-bottle shaped
- waved (vague)
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6) Characterization methods
Specific surface area
Porosity
Particle size
Adsorption (BET method)
Adsorption (capillary condensation)
Hg porosimetry
Thermoporosimetry
Density measurements
Permeability (for membranes)
Transmission microscopy
Sieving
Sedimentation
Light diffusion
X-ray diffraction
Electronic Microscopies (Scanning and Transmission)
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II. Nature of Adsorption
1) Definition of adsorption
Gas
• Solid surfaces show strong affinity towards
gas molecules that it comes in contact with
and some of them are trapped on the surface
• the process of trapping or binding of
molecules to the surface is called adsorption
• Desorption is removal of these gas
molecules from the surface
Solid
2) Two types of adsorption
– Physical adsorption :
* Van der Waals forces
* bond energy is less than 50 kJ/mole
– Chemical adsorption :
* bond energy is more than 50 kJ /mole
* direct chemical bond
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Adsorption and Reaction at Surfaces
13
-5.
Exothermic like condensation
Activated desorption in original form
Rapid equilibration, transport limited
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-50…..
Energy similar to chemical reaction
Depends on reactivity of adsorbent and adsorptive
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3) Transition physisorption-chemisorption
When P increases, the volume of adsorbed gas Vads increases.
When T increases, the volume of physisorbed or chemisorbed gas globaly decreases.
Physisorption takes place at lower temperature than chemisorption and allows the
molecule to approach the surface without high energy requirement. Physisorption
predominates at low temperature and chemisorption at elevated temperature.
Vads
P = cte
Ex : O2 adsorption on Ni
At P=1 atm
Physisorption
Chemisorption
T = - 200°C physisorption
T = 25 °C chemisorption
T = 800°C oxidation (NiO)
T
If the molecule is chemically transformed before desorption, there is contact catalysis
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Example of H2 adsorption on Ni catalyst
1) Physisorption is the first step of chemisorption
Distance from surface : r (Ni…H) = 3.2Å
2) Then chemisorption must be activated to form the transition state (energy required)
H2 + 2 Ni
2 Ni-H
DHC = 120 kJ.mol-1)
Distance from surface : r (Ni-H) = 1.6Å
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4) Activation energy
Molecule A-B approaches the surface S of an adsorbent along the distance axis r.
The first interaction process is physisorption at equilibrium position rP : exothermic process
EP is evolved. Next is the endothermic stage where activation Ea is input to form the transition
state. This displaces the molecule toward equilibrium position rC and Ec is evolved during
chemisorption. Chemisorption is always an activated process.
EP = Energy evolved during
physisorption (exothermic)
EC = Energy evolved during
chemisorption (globaly
exothermic)
Ea = Activation energy
(transition state)
Ed = Energy required for
desorption
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III. Physisorption
1) General
-Physisorption is not adsorbent or adsorptive specific. This is a process similar to
condensation of gas on a surface.
-The quantity of physisorbed molecules depends on the accessible surface area and not
on its chemical nature.
-Physisorbed molecules progressively form successive layers when the gas pressure P
increases. When P reaches the vapor pressure P0, there is condensation on the surface.
-However there can be capillary condensation in the solid pores for P < P0 depending
on the pore size.
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2) Theoretical approach
a) Langmuir theory (monolayer adsorption)
1) Thermodynamic Derivation
For molecules in contact with a solid surface
at a fixed temperature, the Langmuir
Isotherm, developed by Irving Langmuir in
1916, describes the partitioning between gas
phase and adsorbed species as a function of
applied pressure.
The adsorption process between gas phase molecules, A, vacant surface sites, S, and
occupied surface sites, SA, can be represented by the equation,
assuming that there are a fixed number of surface sites present on the surface.
An equilibrium constant, K, can be written :
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