Catalyst Characterization
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Transcript of Catalyst Characterization
12/6/2008 112/6/2008 1Chemical Engineering 126University of the Philippines
Catalyst Characterization
CHEMICAL ENGINEERING 126:
KINETICS OF HETEROGENEOUS REACTION
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Properties to be investigated1. Surface area2. Void Volume3. Density of solid material in the particle4. Distribution of void volume according to void size (pore‐volume
distribution)
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Schematic representation of a typical catalyst pellet comprised of smallporous particles.
Properties to be investigated1. Surface area2. Void Volume3. Density of solid material in the
particle4. Distribution of void volume
according to void size (pore‐volume distribution)
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The main terms for describing physical catalyst properties are as follows:
• Morphology: steric conditions and topology of the surface• Porosity: share of the hollow space (pore volume) of a catalyst pellet
• Texture: generally refers to the pore structure of the particles (pore size, pore size distribution, pore shape)
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The BET Surface Area
Adsorption isotherms for various gases on a 0.606‐g sample of silica gel.
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The BET Surface Area
P/P0
“Point B”
submonolayer
monolayer
multilayer
V
VM
Type IIisotherm
BET area of a catalyst or a catalyst support is one of the first properties one wants to know in catalyst development
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The BET Surface Area
Typical isotherm for physisorption
volume of a gas (usually N2) that givesmonomolecular coverage is measured
Capillary condensation
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The BET Surface Area• BET equation, was derived from a model that extended the Langmuir
isotherm and included the following assumptions
1. Each adsorbed molecule in the 1st layer serves as a site for the 2nd layer (lateral interactions are ignored).
2. The rate of adsorption (condensation) on any layer (x) equals the rate of desorption (evaporation) from the layer above it (x + 1).
3. The heat of adsorption of the 2nd layer and all those above it equals the heat of liquefaction of the adsorbate.
( )0 0
1 1
m m
P C PV P p V C V C P
−= + ⋅
−BET Model
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The BET Surface Area
( )3
0
10PV P p
×−
0
PP
( )0 0
1 1
m m
P C PV P p V C V C P
−= + ⋅
−
BET Isotherm
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The BET Surface Area
Specific surface areas of catalysts and support materials
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Pore Volume and Pore Size Distribution
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Pore Volume and Pore Size Distribution
• Hg Porosimetry Method
– is used to determine pore volumes and the pore size distribution of larger pores, i.e., those with radii larger than about 10 nm
• N2 Desorption Method
– Is used to determine the distribution of pores with diameters smaller than 20 nm,
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Pore Volume and Pore Size Distribution
100 1 103× 1 104×0
0.1
0.2
0
0.2
0.4
0.6
CumPen cm 3− gm⋅( )⋅ Dist cm 3− gm⋅( )⋅
a P( ) A 1−( )⋅
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Pore Volume and Pore Size Distribution
10 100 1 103× 1 104×0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
Yhi cm3gm 1−( ) 1−⋅
Ylo cm3gm 1−( ) 1−⋅
Xhi A1−⋅ Xlo A 1−⋅,
Cumulative Pore Volum
e, cm
3 /g
Pore radius a, A
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Pore Volume and Pore Distribution
10 100 1 103× 1 104× 1 105×0
2
4
Dist hi cm3gm 1−( ) 1−⋅
Dist lo cm3gm 1−( ) 1−⋅
Xhi A1−⋅ Xlo A 1−⋅,
Pore
Distribution
Pore Radius
micropores macropores