Transcript The Preparation of Catalytic Materials: Carries, Active

The Preparation of Catalytic Materials
朱信
Hsin Chu
Professor
Dept. of Environmental Eng.
National Cheng Kung University
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1. Introduction
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Catalysts
Metals: Pt, Pd, Rh
Metal oxides: V2O5
Carriers
Al2O3, SiO2, TiO2, or crystalline alumina-silicates
(zeolites)
Supports
usually monolithic
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2. Carriers
Play a critical role in maintaining the activity, selectivity, and durability
of the finished catalyst
2.1 Al2O3
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The most common carrier used in commercial environmental
applications
Properties
Surface area
Pore size distribution
Surface acidic property
Crystal structure
Preparation of alumina hydrates
Precipitation either from acid or basic solutions
An amphoteric oxide soluble at pHs above about 12 and below about 6
e.g., A trihydrate species, Al2O3‧3H2O, called bayerite at a pH of 11
A monohydrate species, Al2O3‧H2O, called pseudo-boehmite at a pH of
9
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At a pH of 6
The precipitate lacks any definite long-range crystal structure
(amorphous).
The high surface area is created by heat treating or calcining in
air, typically about 500℃.
Al2O3 particles: 20~50Å in diameter bond together to form
polymer-type chains
Next slide (Fig. 2.1)
Scanning electron micrographs (SEM) of γ-Al2O3
andα-Al2O3.
γ-Al2O3 particle has parallel pores 10Å in diameter and a
surface area of about 100-200 m2/g.
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Once precipitated, Al2O3 is thoroughly washed to remove any
impurities, e.g., if the acidic solution of Al+3 is neutralized with
NaOH, the Na+ should be removed by washing.
Drying is usually performed at about 110℃ to remove excess H2O
and other volatile species such as NH3.
Calcination
delta 
theta 
 alpha Al O
boehmite  gamma 
2 3
(monohydrate)
theta 
 alpha Al O
bayerite eta 
2 3
(trihydrate)
→ Causes an irreversible loss in physical surface area and a loss in
its surface OH– or Bronsted acid sites (donate a hydrogen ion)
500850o C
300500o C
8501050o C|
8501050o C
10501150o C
1150o C
1150o C
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Boehmite loses the bulk of its water below about 300℃, during and after
which it begins to sinter or lose surface area.
α-Al2O3: 1-5m2/g
With pore diameters of 100Å
The surface becomes progressively more dehydrated or more hydrophobic.
Small amounts of Na2O present in the Al2O3 can enhance the sintering of
the Al2O3 and thus act as a flux.
Na2O: low melting point
The presence of a few percent of a stabilizer such as La2O3 can greatly
retard the sintering of the γ-Al2O3.
To develop high-temperature durable catalytic converters for the
automobile
Other examples: CeO2, BaO, and SiO2
Mechanisms: A solid solution of the stabilizing ion in the Al2O3
structure decreases the mobility of the Al and O ions, resulting in a
reduction in the rate of sintering.
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2.2 SiO2
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The inertness of SiO2 toward reacting with sulfur-bearing
compounds in exhaust streams makes it a suitable catalyst carrier.
Al2O3 is highly reactive with SO3 and forms compounds that alter
the internal surface of the carrier.
Alkaline solutions of silicate (pH > 12) can be neutralized with acid,
resulting in the formation of silicic acid.
H
SiO4 
  Si (OH ) 4 x  SiO2 H 2O
2
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Similar to Al2O3, it is then washed, dried, and calcined.
Surface area of SiO2: 300 - 400 m2/g
With a small amount of chemically held water, giving rise to some
surface acidic hydroxyl groups
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2.3 TiO2
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Because of its inertness to sulfate formation, TiO2 is a
preferred carrier for vanadia (V2O5) in selective catalytic
reduction of NOX.
Two important crystal structures
Anatase
Rutile
The anatase form is the most important
High surface area (50 – 80 m2/g)
Thermally stable up to about 500℃
The rutile form
Low surface area (< 10 m2/g)
Formed at about 550℃
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2.4 Zeolites
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The Al2O3 and SiO2 are bound in a tetrahedral structure with each Al and Si
cation bonded to four oxygen anions.
Si+4, Al+3, O-2
To maintain charge neutrality, an extra Na+ or H+ must be bonded to the
AlO–, giving rise to an exchangeable cation site.
These sites are acidic when the cation is H+.
The pore structure dimensions of zeolites are between about 3 and 8 Å.
Therefore, zeolites are often referred to as molecular sieves.
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The empirical formula for Na+ exchanged mordenite:
Na8 ( AlO2 )8 (SiO2 )40 4H2O
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In dilute acid → the H+ exchanged state
Mid-point of each line: An -o- bonded to a cation of either Si or Al at the
intersection.
Two nonintersecting pore structures: The main pore contains 12 oxygens
with size 6.7×7.0 Å, the minor pore has 8 oxygens with size 2.9×5.7 Å.
Within the channel or pore, the framework AlO–H+ or AlO–M＋, provides
the active sites for the desired catalytic reactions.
Synthetic zeolites are generally prepared from aqueous solutions of alkali
salts of aluminum and silicon, and sometimes an organic amine, called a
template, which aids in establishing a particular crystalline structure.
Reaction is usually carried out in autoclaves at temperatures between 150
and 180℃.
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3. Making the Finished Catalyst
3.1 Impregnation
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The most common commercial procedure for dispersing the catalytic
species within the carrier is by impregnating an aqueous solution
containing a salt of the catalytic element or elements.
e.g., pt(NH3)2+2 cation salts can ion exchange with the H+ present on the
hydroxy containing surfaces of Al2O3.
Anions such as PtCl4-2 will be electrostatically adsorbed to the H+ sites.
3.1.1 Incipient Wetness or Capillary Impregnation
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The maximum water uptake by the carrier is referred to as the water
pore volume.
This is determined by slowly adding water to a carrier until it is
saturated, as evident by the beading of the excess H2O.
The precursor salt is then dissolved in an amount of water equal to the
water pore volume.
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3.1.2 Electrostatic Adsorption
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It is customary to use a precursor salt that generates a charge opposite
to that of the carrier.
In weakly alkaline solutions, the surface charge on Al2O3 or SiO2 is
generally negative.
Cations such as pd+2, pt(NH3)2+2, and others derived from nitrates or
oxalate salts should adsorb over the carrier surface.
anions are generated from chloride precursor salts, e.g., PdCl4-2 from
Na2PdCl4.
3.1.3 Ion Exchange
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Most commonly used for zeolite catalysts
Highly disperse
Treat the acid form of the zeolite (H+Z-) with an aqueous solution
containing NH4+(NH4NO3) to form the ammonium exchanged zeolite
(NH4+Z-)
Then, treat with a salt solution containing a catalytic cation forming the
metal exchanged zeolite (MZ)
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3.2 Fixing the Catalytic Species
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Following impregnation, it is often desirable to fix the catalytic species so
subsequent processing steps such as washing, drying, and high-temperature
calcinations will not cause significant movement or agglomeration of the
well dispersed catalytic species.
Adjusting pH to precipitate the catalytic species
e.g., presoaking Al2O3 in a solution of NH4OH, the addition of an acidic Pd
salt, such as Pd(NO3)2, will precipitate hydrated PdO on the surface of the
pores within the carrier.
H2S gas can be used as a precipitating agent
e.g., fixing of Rh onto Al2O3 in automobile exhaust catalysts
Rh2O3  H2 S  Rh2 S3  H 
600 C
Rh2 S3  O2 
 Rh  SO2
o
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Addition of reducing agents to precipitate catalytic species as metals
HCOOH  Pd 2  Pd  2H   CO2
The advantage of the reducing agents is that, upon subsequent heat
treatment, they leave no residue.
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3.3 Drying
110℃→excess water and other volatile species
3.4 Calcination
Calcine the catalyst in forced air to about
400-500℃ to remove all traces of decomposable
salts
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4. Nomenclature for Dispersed Catalysts
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Amount and specific catalytic material/carrier
e.g., 0.5% Pt/SiO2, 1% Pd/Al2O3, 3% V2O5/TiO2
5. Monolithic Materials as Catalyst Substrates
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Next slide (Fig. 2.2)
Monolithic honeycomb: parallel channels (ceramic or
metallic)
400 cells per square inch square channels: the most
commonly used for automotive applications.
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Monoliths have large pores and low surface area
(0.3 m2/g), so it is necessary to deposit a high surface area
carrier, which is subsequently catalyzed.
Last slide (Fig. 2.2)
Catalyzed washcoat deposited over the ceramic monolith
The monolith has a large open frontal area and with
straight parallel channels offers less resistance to flow
than that of a pellet type catalyst.
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5.1 Ceramic Monoliths
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Synthetic Cordierite, 2MgO‧2Al2O3‧5SiO2, is by far the most commonly
used ceramic for monolithic catalyst support applications.
Kaolin, talc, alumina, aluminum hydroxide, and silica are blended into a
paste and extruded and calcined. It is possible to produce sizes up to about
11 inches in diameter and 7 inches long, with cell densities from about 9 to
600 cells per square inch.
Several important properties that makes cordierite preferable for use as a
support:
(1) Thermal shock resistance (critical for automotive
applications)
Low thermal expansion coefficient: 10-6/℃
Thus, it resists cracking due to thermal shock.
Mullite, zirconyl mullite, and alpha alumina have higher
melting points but from 5 to 10 times the thermal expansion
coefficients.
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(2) Mechanical strength
Monoliths are made with axial strengths of over 3000
pounds per square inch.
(3) Melting point
The melting point of cordierite is over 1300℃, far
greater than temperatures expected for modern
environmental applications.
(4) Catalyst compatibility
Automotive ceramic monoliths have well designed
pore structures (3-4 μm) that allow good chemical
and mechanical bonding to the washcoat.
Next slide (Table 2.1)
physical properties of ceramic monoliths
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5.2 Metal Monoliths
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Aluminum-containing steel monoliths: thinner walls than a ceramic
Only 25% thickness compared to ceramic monoliths
Higher cell densities with lower pressure drop
The open frontal area of the metal monoliths is typically about 90%
verses 70% for the ceramic.
The thermal conductivity is also considerably higher (15-20 times) than
the ceramic: resulting in faster heat-up.
Adhesion of the oxide based washcoat to the metallic surface and
corrosion of the steel in high-temperature steam environments were
early problems.
Surface pretreatment of the metal has reduced the adherence problems
and new corrosion-resistant steels are allowing metals to penetrate the
automotive markets.
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6. Preparing Monolithic Catalysts
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The catalyzed carrier is made into an acidified aqueous slurry with a
solids content of from 30-50 percent. The mixture is ball milled for at
least two hours to reduce the particle size (10-25μm).
The preparation of the finished catalyst involves dipping the monolith
into the slurry. The excess slurry is air blown to clear the channels and
dried at about 110℃.
The final step is calcinations, performed in air at temperatures between
300 and 500℃.
Great care must be taken to avoid rapid heat-up since H2O trapped in
the micropores can build up sufficient pressure to crack the monolith.
An alternative approach: coat the monolithic honeycomb with the
uncatalyzed carrier, followed by drying and calcining. It is then dipped
into a solution containing the catalytic salts → drying →calcining.
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7. Catalytic Monoliths
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Recently developed technology allows the vanadia, titania plus
additives such as silica, to be extruded directly into a low cell density
honeycomb. Organic additives such as polyvinyl alcohol are sometimes
added as plasticizers to aid in the extrusion process.
Since the entire monolith is catalyst, a higher concentration of active
component is present than would be for a similar washcoated
honeycomb.
For some cases, a paste of TiO2 powder is first extruded, calcined at
500℃ and impregnated with ammonium vanadate/oxalic acid, and
calcined to the finished product. The first applications are with
V2O5/TiO2 and zeolites for selective catalytic reduction of NOx.
The major disadvantage of these monoliths is the inability to produce
high cell density material of sufficient strength to maintain mechanical
integrity in operation.
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8. Catalyzed Monolith Nomenclature
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Washcoat loading
g/in3 of monolith
Catalytic component loading
g/ft3 of monolith
9. Precious Metal Recovery from Monolithic Catalysts
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The hydrometallurgical procedure
Crushing the spent catalyst
Treating it with acid to dissolve only the ceramic components
Leaving an insoluble precious metal rich residue
Purifing by chemical procedures
The pyrometallurgical method
Smelting: the ceramic floats to the top as a slag
The highly dense precious metals alloy with an added metal (Cu or Fe) at
the bottom of the smelter.
Chemically removed and purified
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