Preparation Strategies
Preparation
of Heterogeneous
catalysts
Preparation
of heterogeneous
catalysts
Strategies
Formation of Solid
Modification of Solid
Precipitation
(template assisted)
Sol-Gel
Evaporation
Flame hydrolysis, fusion
Ion exchange
Incipient wetness
Grafting
Conditionning of Solid
post-precipitation processing
Calcination
Reduction
Activation
Preparation of Heterogeneous catalysts
Precipitation
unit operations
three modes
three kinetic situations
in all cases variable conditions for
precipitation and ppp in mother liquid
Preparation of Heterogeneous catalysts
Precipitation at const. pH
Cu(OH)2
2
(+NO3)
+CO32- 1
Zn(OH)2
georgeite
Cu(OH)2/Zn(OH)2 + CO32[M (OH)x]# + NO3-
2
3, 4, 5
6
am. hydroxid
precipitation
1
3, 4, 5
?
#
#
rosasite
NO3
`HCO3-` OH H2O
M
M
6 [M (HCO3- ) (OH)x (H2O)]
Preparation of Heterogeneous catalysts
aurichalcite
ageing
time
(CuxZn1-x)2(OH)2CO3
[M (OH)x-1 (NO3)]#
NO3
M
OH
M NO3 OH-
[M (CO3 )(OH)x+1 ] -
H
O
M
M
[M (CO3 )(OH)x ]
Solid Phases
constant pH
Malachite (M)
Rosasite (RS)
decreasing pH
extended
multiphase region
(green) in
(CuxZn1-x)2(OH)2CO3decreasing series
Cu2(OH)2CO3
better
Aurichalcite (AU) (CuxZn1-x)5(OH)6(CO3)2
crystallinity in
constant series
Hydrozincite (HZ) Zn5(OH)6(CO3)2
Preparation of Heterogeneous catalysts
Nucleation
• homogeneous nucleation: concentration of
species above solubility limit plus
supersaturation (kinetics)
– Lp = [cation]n x [anion]m (mass law)
• heterogeneous nucleation: concentration of
species above solubility limit plus solid
particle (surface) for chemisorption
Preparation of Heterogeneous catalysts
Nucleation
Co-precipitation of two species (Cu/Zno, Mo/V) only possible in
the never existing situation of equal boundary conditions
(solubility, nuclation supersaturation)
Preparation of Heterogeneous catalysts
Incongruent
solidification/precipitation
The standard result is a mixture of species
(phases, if crystalline)
Preparation of Heterogeneous catalysts
Control variables
A multitude of variables
affects the kinetically
determined process of
precipitation.
For co-precipitation all
variables for each
component
Poorly defined but reactive
(metastable initial products:
method of choice for catalysts).
Preparation of Heterogeneous catalysts
Sequence of products during typical precipitation
slow
fast
10
pH
8
6
4
2
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
mol Soda/mol Cu
Preparation of Heterogeneous catalysts
ppp: post precipitation processes
reduction
ageing,
filtering
precipitation
Cu2+
Zn2+ Al3+
precipitate
Active catalyst
washing,
drying
altered
material
dry precursor
oxide
calcination
/ NO3-
Rosasite (CuxZn1-x)2(OH)2CO3
Aurichalcite (CuxZn1-x)5(OH)6(CO3)2
Preparation of Heterogeneous catalysts
Model experiment:
ageing of a single precipitate
Cu(OH)2
Malachite
Amorphous
Phase??
rapid transformation
gerhardtite > malachite
Gerhardtite
+ small amounts CO32-
Simple binary system (Cu-O): all of these steps exhibit
strong kinetic effects (pH, temperature) and often do not
complete during the precipitation process: phase mixture
Preparation of Heterogeneous catalysts
Kinetics and product identity
constant-pH
rosasite
georgeite
modified rosasite
aurichalcite
Cu(OH)2 (+NO3)
decreasing-pH
Cu(OH)2
gerhardtite
rosasite
rosasite
georgeite
precipitation
Preparation of Heterogeneous catalysts
aurichalcite
ageing
modified rosasite
Example: washing of the filtered precipitate
-7
1,0x10
-8
9,0x10
-8
8,0x10
1
H2O
2
-8
7,0x10
400
FHI: washing
T = 493 K
p = 1 bar
after reduction
3 350
45
300
-8
-1
5,0x10
-8
-1
4,0x10
-8
3,0x10
-8
2,0x10
-8
1,0x10
100
200
300
T [°C]
-8
7,0x10
-8
6,0x10
-8
5,0x10
-8
I [A]
4,0x10
Production rate / µmolMeOH·gcat ·h
I [A]
-8
6,0x10
CO2
250
400
6
C7030b_6wk
500
C7030b_5wk
C7030b_4wk
600
200
C7030b_3wk
150
C7030b_2wk
100
50
0
-8
3,0x10
14
16
18
20
22
-8
2,0x10
Cu surface area / m²·g
-8
1,0x10
0,0
100
200
300
T [°C]
400
Preparation of Heterogeneous catalysts
500
600
-1
24
26
Well-defined precipitates
• Ultrafast, continuous precipitation yielding a
metastable homogeneous (amorphous) initial
product: microreactor, spraydrying, solvent
extraction.
• Ultraslow, equlibrated precipitate with excessive
ppp in well-controlled environments (crystalline,
low reactive, homogeneous): conventional
methods.
• Ultimate goal: kinetically homogeneous precataylsts.
Preparation of Heterogeneous catalysts
Realisation of a microreactor
Co-operation with a Germanybased start-up company holding
technology for metal-based
microreactors
Preparation of Heterogeneous catalysts
Proof of Principle
Cu/Zn 70:30 single pass, no ppp.
Conventional: 8 m²/g Cu, 22.81 µmol/g/h
Microreactor: 21.4 m²/g Cu, 334,5 µmol/g/h
Conventional: 2.88 µmol/m²/h
Microreactor: 15.63 µmol/m²/h
Preparation of Heterogeneous catalysts
From solution to solid
• Molecular chemistry of solid formation:
• concentrated solutions, never bare ions, always
solvate complexes with different structures
preventing atomic contact of constituents of solid:
• Co-ordination chemistry required for
crystallisation,
• Auto-reactions according to polarisation of solvate
(solvent effects in solidfication).
Preparation of Heterogeneous catalysts
From solution to solid
Solvatation geometries in water: destruction of water structure in first
coordination shell (hydrophilic) or incorporation of weakly polarizing
ions (organic, ammonium) in water structure (hydrophobic)
Preparation of Heterogeneous catalysts
From solution to solid
Solvation is a very energetic effect (more than cohesion energis in
elements) exhibiting a very variable structural dynamics depending on
shape of hydration complex (slow for well-defined complex, rapid for illdefined structures).
Preparation of Heterogeneous catalysts
From solution to solid
(M-OH2)n+ aq (M-OH)n-1+ + H3O+ aq (M-O)n-2+ + H3O+
Polarisation acidfies protons in solvation shell and leads to –ols or –oxos
depending on the partial charge on the cation:
Note that multiple redox states have drastically differing partial charges
and hence acdification properties:
3+
Cr2O3 + 15 H2O  2[Cr(OH2)6] + 6OH
CrO3 + H2O  [CrO4]2- + 2 H3O+
Preparation of Heterogeneous catalysts
Hydrothermal synthesis
• Conditions above 373 K and 1 bar pressure for
water as solvent.
• Mild conditions: 573 K 10 bar: strong effect of
thermal mobility breaks up complex structures and
decrease kinetic barriers.
• Rigid conditions: 773 K, kbar: totally different
properies of the solvent, low viscosity, extremely
acidic (corrosive), low dielectric constant:
complexation favoured.
Preparation of Heterogeneous catalysts
Solidfication by complexation
M-OH + M-OH2  M-OH-M + H2O
base
acid
olation condensation
M-OH + M-OH  M-OH-M-OH  M-O-M + H2O
acid base
oxolation condensation
control variables: polarisation, ionic
strength, pH, temperature, redox reactions
Preparation of Heterogeneous catalysts
Sol-gel synthesis
variant of precipitation with
kinetically slow steps of
initial condensation (sol)
and final condensation (gel)
that can occur through
mixed olation-oxolation
processes.
Preparation of Heterogeneous catalysts
Geometry of condensates
true shape of an
„OH“ group is
[H3O2]-
giving rise to ring
structures due to ist
stick-shape
Preparation of Heterogeneous catalysts
Geometry of condensates
control variables: polarisation , charge, oxidation state
connectivity maximum is 3, usual
1,2 through non-bonding states
translates into corner-edge-face
sharing polyhedra
control variables: charge-radius
relation, oxidation states
Preparation of Heterogeneous catalysts
Growth control
• Olation leads in dilute cases to large hydroxides
precipitationg as gels provided that the inital
dimer is electrically neutral
• Olation of charged species ends at polyions
terminated in growth by a relaxation of the
charge/radius ratio violating the polarisation
required for a leaving group at the metal.
• Often spontaneous dehydration (oxolation) occurs
with initial hydroxides.
Preparation of Heterogeneous catalysts
Iron oxide formation
Preparation of Heterogeneous catalysts
Ein Modellsystem ist
strukturell wohldefiniert
Preparation of Heterogeneous catalysts
Preparation of Heterogeneous catalysts
Grafting, Immobilisation
Co-ordination chemistry with support as ligand
Beware of side reactions and note the dilute reaction limit
Preparation of Heterogeneous catalysts
Impregnation
Complex heterogeneous
process
requiring change in
molecular chemistry of
„ion“
highly demading kinetics
(many control variables)
very slow (24 h or so)
beware of side reactions
like hydrolysis,
restructuring and –OH
incorporation
Preparation of Heterogeneous catalysts
Conditioning of Solids
• Gas-Solid reactions at elevated temperatures
–
–
–
–
drying
calcination
reduction
oxidation
• Usually poorly controlled, inhomogeneous conditions
(gradients in T and gasphase)
• Formulation of real catalysts affects initial solid
• Activation is treatment in feed gas converting
prepared pre-cataylsts into active state.
Preparation of Heterogeneous catalysts
Structures: Precursor (Rosasite) and CuO (Tenorite)
Rosasite
(Cu)(ZnO0.82Cu0.18)(CO3)(OH)2
• monoclinic, P21/a (14),a = 12.88,
b = 9.36, c = 3.164 Å,  = 110.42°
CuO
• monoclinic, PtS-type,
4+2 coordination of Cu
• chemically well defined mineral specie
• two non-equivalent cation sites => ordered structure
Preparation of Heterogeneous catalysts
TEM (CuO/ZnO): Crystallite sizes and morphology
• calcined samples consist of a
highly interdispersed network
of single crystalline particles
• for samples prepared at constant
pH smaller agglomerates which is
in agreement with the smaller CuO
and ZnO crystallite sizes
determined by XRD
Preparation of Heterogeneous catalysts
In situ XRD: TPR (Cu / Zn - 70/30)
750
250
225
200
500
250
42
44
2 [°]
46
T ramp: RT-250 °C at 5 K/min
7 min/step
3 vol. % H2
Preparation of Heterogeneous catalysts
250
Cu
0.5
200
175
0.0
H2
H2O
150 200 250 300 350
Time [min]
 Evolution of Cu (111) peak  onset and extent of
reduction
Peak Profile of Cu (111)  evaluation of
crystallite size and growth upon heating
Temperature [°C]
1.0
MS
1000
Int. Intensity Cu(111)
Intensity
1250
Temperature [° C]
Evolution of Cu (111) peak
In-situ XAFS: Kinetic Analysis
Reduction proceeds
via nucleation-growth
mechanism
Extent of reduction 
T-ramp: 160 -250 °C at 5 K/min
 measuring time: 15 s/step
8 vol. % H2
1.0
0.75
60/40
40/60
70/30
0.5
0.25
T-ramp 5 K/min
0.0
0
150 300 450 600 750
Time [s]
Preparation of Heterogeneous catalysts
 sigmoidal shape of Cu
evolution
kinetic model according
to Avrami-Erofeev
 reaction rate reduced
for sample with maximum
deviation from ideal
precursor stoichiometry
(60/40)
Metastable Cu
ZnO
0.015
70/30
40/60
60/40
Δ a [Å]
0.01
Measurements for the fresh reduced samples C70/30,
C60/40, and C40/60 under methanol steam reforming,
and after two successive oxygen addition cycles.
0.005
0.0
ZnO
 continuous increase in lattice parameter
Δc [Å]
0.02
of Cu and ZnO indicates an expansion of
the unit cell
0.01
0.0
0.015
1
Cu
2
3
treatment
4
 strain contribution to the peak broadening (0.7%)
0.01
Δ a [Å]
 increase in crystallite size of Cu and ZnO
(e.g. Cu: 76 to 110 Å and then to 120 Å 
loss of Cu surface area)
was unchanged, indicating that there was no
accompanying annealing of defects.
0.005
0.0
fresh reduced
under reaction 1st O2 cycle
2nd
O2 cycle
Preparation of Heterogeneous catalysts
Catalytic performance of Mo9V3W1.2Ox in acrolein
oxidation
35000
30000
activated in the reaction of
acrolein oxidation at 603 K for 2 h
Intensity
30
25000
25
T = 544 K
2
W*10 , mol/m *s
20000
20
15000
Thermally activated in
inert gas at 818
7
15
Thermally activated
in inert gas at 813 K
10
initial material
10000
10
20
30
40
50
60
70
2 theta
Same
catalyst
structure?
5
0
Crystalline phase of the Mo5O14
operation time in the reaction, h
type,
Preparation of Heterogeneous
catalysts traces of MoO3
0
5
10
15
20
25
30
Science of Preparation ?
Preparation of Heterogeneous catalysts