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Thermal Treatment of Catalysts
Modern Methods in Heterogeneous Catalysis Research Friederike C. Jentoft, October 31, 2003
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Outline
1. Terminology (calcination)2. Sample vs. oven set temperature3. Self-generated atmosphere & self-steaming of zeolites4. Combustion5. Glow phenomenon & zirconia catalysts6. Crystallization7. Loss of surface area8. Effect of additives9. Solid-solid wetting10.Reductive treatments & SMSI
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Steps of Catalyst Preparation
v IUPAC defines 3 steps of catalyst preparation
1. Preparation of primary solid, associating all the useful compounds
2. Processing of that primary solid to obtain the catalyst precursor for example by heat treatment
3. Activation of the precursor to give the active catalyst (reduction to metal, formation of sulfides, deammoniationof zeolites)
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Heat Treatment of Intermediate Solids or Precursors
v drying
v thermal decomposition of salts (nitrates, ammonium salts)
v calcination
v product is a "reasonably inert solid" which can be stored easily
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Annealing
v in a general meaning: a heating of a material over a long time span; strain and cracks in a crystalline solid can be removed
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Origin of the Term "Calcination"
v latin "calx" = playstone limestone, (greek chálix)
v burning of calcium carbonate (limestone) to calcium oxide (quicklime)
CaCO3 → CaO + CO2 ΔH(900°C)=3010 kJ mol-1
v used to construct Giza pyramids (ca. 2800 A.C.), burning of limestone ("Kalkbrennen") mentioned by Cato 184 A.C.
v performed in kilns (ovens) at 900°C
v addition of air to sustain combustion + cool product
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Examples for Kilns for Calcination
v Schematic of a vertical shaft kiln.a) Preheating zone; b) Calcining zone; c) Cooling zone
v Schematic of a rotary kiln a) Burner; b) Combustion air; c) Pre-heater; d) Kiln; e) Cooler
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General Definition of "Calcination"
v decomposition of a substance through heating, transformation in lime-like substance – Duden
v to heat (as inorganic materials) to a high temperature but without fusing in order to drive off volatile matter or to effect changes (as oxidation or pulverization) – Websters
v heating (burning) of solids to a certain degree of decomposition, whereby with e.g. soda, gypsum the crystal water is completely or partially removed – Römpp's Chemielexikon
v the heating of a solid to a high temperature, below its melting point, to create a condition of thermal decomposition or phase transition other than melting or fusing – Hüsing, Synthesis of Inorganic Materials
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Definition of "Calcination" in Catalysis Research
v thermal treatment (of a catalyst) in oxidizing atmosphere. The calcination temperature is usually slightly higher than that of the catalyst operating temperature – Ullmann's Encyclopedia of Industrial Chemistry
v a heat treatment of catalyst precursor in an oxidizing atmosphere for a couple of hours - Catalysis from A to Z, Eds. Cornil et al.
v heating in air or oxygen; the term is most likely to be applied to a step in the preparation of a catalyst - IUPAC Compendium of Chemical Terminology
v often, with respect to catalysts, an oxidizing treatment is meant
v however, you will find statements such as "calcined in inert atmosphere"
v sample-generated atmosphere may be oxidative (nitrate decomposition)
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Example from Patent Literature
v extremely vague! unimportant?
v no, there is a secret to it!
v often only temperature + holding time given
E.J. Hollstein, J.T. Wei, C.-Y. Hsu, US Patent 4,918,041
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Calcination Procedure: Temperature Program
v heating rate, holding time, cooling rate
0 100 200 300 400 500300
400
500
600
700
800
900
Tem
pera
ture
/ K
Time / min
v cooling usually uncontrolled below certain T, slower
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Actual Temperature Program
v oven may not be able to perform selected program (lack in power): temperature lag of actual temperature behind set temperature
v poorly tuned controller may give temperature oscillations
v heat needs to be transferred from oven to sample, needs a gradient: temperature lag of sample temperature vs. oven temperature
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Role of Sample
v strongly endo- or exothermic events may interfere with the heating program
v endothermic events: solvent evaporation
v exothermic events: combustion of organics, crystallization
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Evaporation of Water: Thermal Effects
v example: a 10 g sample containing 18 % water (0.1 mol)
v ΔHevap(H2O, 373 K) = 41 kJ mol-1
v to evaporate in 1 minute: ≈ 70 Watt
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Example: Calcination of Zirconium Hydroxide
v with a 10 g sample, deviations can occur already at moderate heating rates
0.0 0.5 1.0 1.5 2.0 2.5 3.0
300
400
500
600
70010 K min-1
1 K min-1
3 K min-1
Sam
ple
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tem
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ture
/ K
Time / h0.0 0.5 1.0 1.5 2.0 2.5 3.0
300
400
500
600
70010 K min-1
1 K min-1
3 K min-1
Sam
ple
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/ K
Time / h
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Evaporation of Water: Gas Formation
v example: a 10 g sample containing 18 % water (0.1 mol)
v at 400 K: corresponds to ≈ 3.3 l of water vapor
v depending on the - form of the bed- the type of furnace (tubular / muffle)- static / dynamic atmosphere (no flow / flow)
the sample will be exposed to vapor for minutes!
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Zeolite Y as a Cracking Catalyst
v zeolite Y is used as a cracking catalyst (FCC)
v first synthesize NaY, then exchange Na+ by NH4+ (liquid
phase)
v obtain active HY through thermal decomposition of NH4Y
v regular HY not very stable
Faujasite structure
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Ultrastable Y zeolite
vMcDaniel and Maher 1967 report „new ultra-stable form of faujasite“
v worked with 100 g of zeolite, first exchange then heat treatment
C.V. McDaniel, P.K. Maher in Molecular Sieves, Soc. Chem. Ind. London, 1968, p. 186
v „keeping the elapsed time between the exchange step and the heating step at 815°C to a minimum is quite critical“
0 100 200 300 400 500300
400
500
600
700
800
900
Tem
pera
ture
/ K
Time / min
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Ultrastable Y Zeolite
v Kerr 1967: treatment of HY at 700-800°C in inert static atmosphere
v „any technique keeping this water in the system during the heating process will result in a stable product“
v published comparison of heating in „deep bed“ or „shallow bed“: deep bed produces stable product
v ascribes success of McDaniel & Maher to the large amount that they used
G.T. Kerr, J. Phys. Chem. 1967, 71, 4155 and J. Catal. 1969, 15, 200.
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Influence of Packing
v Packing of a solid influences evolution of gas (water vapor)
MS analysism/e = 18 (H2O)
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Evaporation – Autogeneous Pressure
v boiling point of water is determined properly only in crucible with lid + hole in lid!
6.2 mg H2Ono lid
2.09 mg H2O, 50 µm hole in lidexplosion
onset
5.8 mg H2O, sealed
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Stabilization Through Dealumination
v water vapor removes aluminum from zeolite framework (extra-framework aluminum)
v leads to stabilization
v today, ultrastable Y or USY is obtained through „steaming“, treatment of NH4-Y 600-800°C in rotary kilns
v USY is used in „fluid catalytic cracking“ and „hydrocracking“ and „hydroprocessing“
v „steaming“ is a general method for dealumination of zeolites
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Exothermic Reactions: Combustion
v organic matter may be present, e.g. from sol-gel process, surfactant-assisted synthesis
v will combust upon thermal treatment in oxygen-containing environment
v look at thermochemical dataCRC Handbook of Thermophysical and Thermochemical DataEds. David R. Lide, Henry V. Kehiaian, CRC Press Boca Raton New York 1994 FHI library 50 E 55D'Ans Lax, Taschenbuch für Chemiker und PhysikerEd. C. Synowietz, Springer Verlag 1983, FHI library 50 E 54
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Example: Pentane Combustion
C5H12 (g) + 8 O2 (g) → 5 CO2 (g) + 6 H2O (g)
∑=
Θ °∆=∆L
iifir HH
1
ν
12522165 HCfOHfCOfr HHHH °∆−°∆+°∆=∆ Θ
)44.146(1)82.241(6)51.393(5 111 −−−Θ −−−+−=∆ kJmolkJmolkJmolHr
10.3272 −Θ =∆ kJmolHr
v combustion is strongly exothermic!
v oxygenates have higher enthalpies of formation, i.e. enthalpy of combustion becomes smaller
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Example: Surfactant-Assisted Synthesis Mesoporous Zirconia
v surfactants (hexadecyl-trimethyl-ammonium bromide) form micelles
v inorganic matter forms around micelles
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Example: TG/DTA Analysis of ZrO2-Precursor
v ZrO2/CTAB composite synthesized with Zr(O-nPr)4 in the presence of sulfate ions at Zr:S:CTAB = 2:2:1, measured with 10 K/min in an air stream
200 400 600 800 1000 1200
40
50
60
70
80
endo
exo
TG (m
g)
Temperature (K)
-10
0
10
20
30
40
50
60
DTA
(mV
)
200 400 600 800 1000 1200
40
50
60
70
80
endo
exo
TG (m
g)
Temperature (K)
-10
0
10
20
30
40
50
60
DTA
(mV
)
H2O loss
SO42- decomposition
combustion of organics
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Other Exothermic Reactions
v Example: calcination of X-ray amorphous zirconium hydroxide
v "ZrO2 * 2.5 H2O"
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640 660 680 700 720 740 760550
600
650
700
750
800
850
900
950
Sam
ple
bed
tem
pera
ture
/ K
Oven temperature / K
115 120 125 130 135 140 145 150 155 Heating time / min
Heating of Zirconium Hydroxide
640 660 680 700 720 740 760550
600
650
700
750
800
850
900
950
2.2 ml boat
Sam
ple
bed
tem
pera
ture
/ K
Oven temperature / K
115 120 125 130 135 140 145 150 155 Heating time / min
2.2 ml
8.4 ml
17.1 ml
v strong influence of batch size / heat transfer
v rapid overheating (up to 40-50 K/s)
v overshoot of up to 300 K
640 660 680 700 720 740 760550
600
650
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750
800
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950
2.2 ml boat
8.4 ml boat
17.1 ml boat
Sam
ple
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tem
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ture
/ K
Oven temperature / K
115 120 125 130 135 140 145 150 155 Heating time / min
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History of Glow
vOverheating is so violent, it is accompanied by emission of
visible light ("glow")Berzelius 1812 (antimonates, antimonites)
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The Glow Phenomenon
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Oxides Showing a Glow
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Origin of Glow
v combustion of organic contaminants
v heat of crystallization
v loss of surface energy through sintering
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Effect of Combustion?
v atmosphere little influence on overheating effect
v heat not caused by combustion of organic contaminants
780 790 800 810 820 830 840
760
780
800
820
840
860
880
900Oxygen
AirArgon
Sam
ple
bed
tem
pera
ture
/ K
Oven temperature / K
165 170 175 180 Heating time /min
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Origin of Glow
v combustion of organic contaminants
v heat of crystallization
v loss of surface energy through formation of larger particles
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Heat of Crystallization of ZrO2 (kJ mol-1)
?-ZrO2 –28.7 Keshavarajaany ZrO2 –23.2 Srinivasant- or m-ZrO2 –4.3 to –22.5 Chuaht- or m-ZrO2 –12.9 Tatsumit-ZrO2 –29.3 to –33.4 Livaget-ZrO2 –19.6 Mercerat-ZrO2 –30.1 ± 0.8 Haberkot-ZrO2 –53 Molodetskyt-ZrO2 –13 Xiem-ZrO2 –58.6 ± 3.3 Molodetsky
Change in surface energy upon a-ZrO2 → t-ZrO2(assumed ABET = 100 m2•g-1): +14.6 Molodetsky
Transition t-ZrO2 to m-ZrO2: –5 Molodetsky–6 Xie–6 Coughlin
heat of crystallization of t-ZrO2:
according to literature between -4.3 and -53.6 kJ mol-1
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Estimation of Temperature Rise through Crystallization
v assume a medium heat of 25 kJ mol-1
v process assumed quasi-adiabatic (δQ = 0)
vmolar heat of precursor / intermediate assumed similar to that of ZrO2: 82.3 J mol-1 K-1
KcHTp
300≈∆
=∆
v corresponds approximately to observation
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Does Crystallization Happen During Glow?
v use method that allows structure-determination and good time resolution
v X-ray absorption spectroscopy at Zr K edge at ESRF(1 spectrum per s; 10 K min-1 heating rate)
v allows observation of local environment around Zr4+ ions
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5mm5mm5 mm
In situ X-ray Absorption Spectroscopy
v use large pellet to create a sufficiently large glow effect
v put small pellet inside that is transparent for X-rays
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XAS Spectra
0.0
0.25
0.5
0.75
1.0
18 18.25 18.5 18.75 19
50
100
150
200
Nor
m. a
bsor
ptio
n
Photon energy [keV]
Time, seconds
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110 112 114 116 118 120 122 124 126 128 130350
400
450
500
550
600
Tem
pera
ture
, °C
Time, seconds
Sample Temperature vs. Time
Heating rate≈ 50 K s-1
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2.0
4.0
6.0
0 2 4 6 8
FT( χ(
k)*k2 )
R [Å]
436 °C
447 °C
547 °C499 °C
439 °C
Structural Evolution
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Origin of Glow
v combustion of organic contaminants
v heat of crystallization - yes!
v loss of surface energy through formation of larger particles
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Sintering
v a heat treatment at 2/3 to 3/4 of the melting point to solidify shaped bodies from pressed metal powders, occurs in 3 steps
v 1. increase of particle contacts through "sinter bridges"2. formation of a contiguous backbone, original particles lose their identity, shrinkage, formation of new grain boundaries3. rounding and elimination of pores, further shrinkage, closed pores
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Tammann and Hüttig Temperature
v Tammann temperaturetemperature necessary for lattice (bulk) recrystallizationfor metal oxides TTammann ≈ 0.52 TF
v Hüttig temperaturetemperature necessary for surface recrystallizationfor metal oxides THüttig ≈ 0.26 TF
with TF the absolute melting temperature
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Influence of Particle Size
v smaller particles react at lower temperature
50-80 µm
1 µm
0.2 µm
v decomposition of Al(OH)3,
thermogravimetric analysis
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Influence of Particle Size on Melting Point of Au
vMelting point can decrease drastically with decreasing particle size!
Ph. Buffat, J. P. Borel, Phys. Rev. A 13 (1975) 2287-2298
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Surface Energy
v surface energy of t-ZrO2(101) ≈ 1.1 J m-2
v surface area shrinks from 250 to 150 m2 g-1 during calcination → 110 J g-1 or 13.5 kJ mol-1 (ZrO2)
v is not negligible in comparison to crystallization
v loss of surface area through formation of larger crystals
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Formation of a Crystalline Solid from Amorphous Precursor
v nature of amorphous precursor usually not well-known
v ΔH depends on nature of precursor & product
v if, through variation of the treatment conditions, for the same precursor different ΔHs are obtained, the products will be different
ΔH???
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Effect of Additives
v additives shift glow to higher temperatures and reduce overshoot
650 700 750 800 850600
650
700
750
800
850
900
950
1000
Rampe 3°/min17,1 ml-Schiffchen
ZH
Prob
ente
mpe
ratu
r / K
Ofentemperatur / K
120 130 140 150 160 170 180 190Zeit / min
650 700 750 800 850600
650
700
750
800
850
900
950
1000
Rampe 3°/min17,1 ml-Schiffchen
SZH
ZH
Prob
ente
mpe
ratu
r / K
Ofentemperatur / K
120 130 140 150 160 170 180 190Zeit / min
650 700 750 800 850600
650
700
750
800
850
900
950
1000
Rampe 3°/min17,1 ml-Schiffchen
FeSZH
SZH
ZH
Prob
ente
mpe
ratu
r / K
Ofentemperatur / K
120 130 140 150 160 170 180 190Zeit / min
650 700 750 800 850600
650
700
750
800
850
900
950
1000
Rampe 3°/min17,1 ml-Schiffchen
MnSZHFeSZH
SZH
ZH
Prob
ente
mpe
ratu
r / K
Ofentemperatur / K
120 130 140 150 160 170 180 190Zeit / minTime / min
Oven set temperature / K
Sam
ple
bed
tem
pera
ture
/ K
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0 100 200 300 400 500300
400
500
600
700
800
900
1000
Tem
pera
ture
/ K
Time / min
Glow Phenomenon: MnSZ and FeSZ
v max. calcination T may be exceeded
v promoters influence calcination chemistry (systemic), Fe and Mn different
v strong batch size dependence
780 800 820 840 860
750
800
850
900
950
1000
2%MnSZH
25 g
12 g
3 g
Sam
ple
tem
pera
ture
/ K
Oven temperature / K
165 170 175 180 185 190Heating time / min
780 800 820 840 860
750
800
850
900
950
1000
2%FeSZH
2%MnSZ
25 g
12 g
3 g3 g12 g
25 g
Sam
ple
tem
pera
ture
/ K
Oven temperature / K
165 170 175 180 185 190Heating time / min
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samples calcined in larger batches are more active (1 vol% n-butane at 338 K)
Influence on Catalytic Activity?!
780 800 820 840 860
750
800
850
900
950
1000 25 g
12 g
3 g3 g12 g
25 g
Prob
ente
mpe
ratu
r / K
Ofentemperatur / K
165 170 175 180 185 190Zeit / min
0 120 240 360 480
02468
10121416 17.1 ml boat
8.4 ml boat 2.2 ml boat
Yiel
d i-b
utan
e (%
)
Time on stream / min
2%FeSZ
0 120 240 360 480
0
2
4
6
8
10
12
14
17.1 ml boat 8.4 ml boat 2.2 ml boat
Yiel
d i-b
utan
e (%
)
Time on stream / min
2%MnSZ
characterize catalysts
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Surface Area & Calcination Batch Size
v surface area increases with calcination batch size
v differences in activity exceed differences in surface area
0 2 4 6 8 10 12 14 16 18 2080
85
90
95
100
105
110
115
120
FeSZ MnSZ
Surf
ace
area
/ m2 g-1
Boat size / ml
0 120 240 360 480
02468
10121416 17.1 ml boat
8.4 ml boat 2.2 ml boat
Yiel
d i-b
utan
e (%
)
Time on stream / min0 120 240 360 480
0
2
4
6
8
10
12
14
17.1 ml boat 8.4 ml boat 2.2 ml boat
Yiel
d i-b
utan
e (%
)
Time on stream / min
2%FeSZ 2%MnSZ
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0 120 240 360 480
02468
10121416 17.1 ml boat
8.4 ml boat 2.2 ml boat
Yiel
d i-b
utan
e (%
)
Time on stream / min0 120 240 360 480
0
2
4
6
8
10
12
14
17.1 ml boat 8.4 ml boat 2.2 ml boat
Yiel
d i-b
utan
e (%
)
Time on stream / min
Activity and Bulk Structure
v lattice parameters of tetragonal ZrO2 change
0 5 10 15 201.438
1.439
1.440
1.441
1.442
1.443
1.444
1.445
c/a
Boat size / ml0 5 10 15 20
1.438
1.439
1.440
1.441
1.442
1.443
1.444
1.445
c/a
Boat size / ml
2%MnSZ 2%FeSZ
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Porosity of Iron-Promoted Sulfated Zirconia
v large batch calcination: formation of mesopores, 1-4 nm
0.0 0.2 0.4 0.6 0.8 1.010
20
30
40
50
60
70
p/p0
Ads
orbe
d vo
lum
e / c
m3 *g-1
2.2 ml boat 8.4 ml boat 17.1 ml boat
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Effect of Calcination Batch Size
v samples from the same raw material (precursor) are converted into different products by variation of the batch size during calcination
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Why Does Overheating Occur?
v heat is generated faster than transferred away
v look at heat transfer
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Heat Transfer Modes
v all of them play role during calcination
v estimations can be made!
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Heat Transfer by Convection
v free vs. forced convection makes a considerable difference
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Thermal Conductivity
Kingery 1955
ZrO2
v data are for solids, conduction worse in loose powders
v exact material during calcination unknown
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Radiation
v conduction and convection are proportional to difference between the temperatures (T gradient , ΔT) of the body of interest and the surrounding
v radiation is proportional to the difference between the temperatures to the forth power
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IUPAC Recommendations on Calcination
v all particles of catalyst should be subjected (..) to exactly the same (..) conditionsonly possible in moving beds (fluid beds, rotating furnaces, spray drying)
v supply a sufficient quantity of gas or liquid to the reactor to ensure complete reaction (..); special consideration should be given to mass and heat transfer
J. Haber, J.H. Block, B. Delmon, Pure & Appl. Chem. 67 (1995) 1257-1306
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Preparation of Supported Catalysts
v goal: disperse an active phase on an inexpensive and inert (?) support
increase surface area
support
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Interaction Between Active Phase and Support
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Spreading Mechanisms
v transport via gas phase also possible
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Preparation of SCR Catalyst by Solid-Solid-Wetting
vMoO3/TiO2: typical catalyst for selective catalytic reduction (reaction of NOx with NH3 to give N2 and H2O)
720 K heating in O2,saturated with H2O
v start out with physical mixture: small particles, intimate mixture
v evoke spreading through thermal treatment
v analysis of surface composition!
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He+ Ekin (~kV) He+
E'kin
Ion Scattering Spectroscopy
v topmost layer of surface is probed with ion beam (He+)
v also: LEIS = low energy ion scattering
v kinetic energy after interaction depends on mass of scattering atom
v highly surface sensitive, destructive
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Spreading
v Spreading of MoO3
on TiO2 can be achieved by thermal treatment in O2/H2O
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Supported Metal Catalysts: Metal-Support Interaction
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Depth Profiling of Supported Noble Metal Catalyst
v Rh/TiO2: a system with strong metal support interaction (SMSI)
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Conclusion
vMany things can happen during a thermal treatment!