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HETEROGENEOUS CATALYSTS
Sergio RojasICP-CSIC
January 2013
1. WHAT IS A CATALYST? RELEVANCE IN TODAYS SOCIETY
2. RELEVANT PARAMETERS DURING THE SYNTHESIS OF
HETEROGENEOUS CATALYTS
1. PREPARATION METHODS
2. SIZE AND STRUCTURE CONTROL
3. CONCLUSIONS
Conversion can not be higher than thermodynamic value
Catalysts is a substance that
accelerates the rate of a chemical
reaction by lowering the
activation energy of the reaction
It is not consumed during the course
of the reaction
Solids (NaOH, Pt/C, V2O5)
Liquids (H2SO4)
Conversion can not be higher than thermodynamic value
Chemical reactions results from
collisions with a certain minimum
energy (Activation Energy)
A catalyst provides an alternative
route for the reaction with a lower
activation energy."
It does not strictly "lower the
activation energy of the reaction0
10
20
30
40
50
60
70
80
90
0 2 4 6 8 10
Num
ber o
f par
ticle
sEnergy
Maxwell Boltzman distribution
Molecules that wont react because they lack of energy
Activation Energy
Conversion can not be higher than thermodynamic value
Chemical reactions results from
collisions with a certain minimum
energy (Activation Energy)
A catalyst provides an alternative
route for the reaction with a lower
activation energy."
It does not strictly "lower the
activation energy of the reaction0
10
20
30
40
50
60
70
80
90
0 2 4 6 8 10
Num
ber o
f par
ticle
sEnergy
Maxwell Boltzman distribution
Molecules that wont react because they lack of energy
Activation Energy
Activation Energy with catalyst
Extra molecules to react
Catalysts are present in key industrial/society sectorssuch as production of chemicals, energy transformationand environmental processes
Most industrial process rely on the use of solid catalysts(heterogeneous catalysis) Petrochemistry Fine chemicals
85% of all chemical processes have use catalystsduring, at least, one step during their preparation.
Catalysis in industry is dominated by heterogeneous processes World catalysts sales in 2004 amounted to 15 billion US$/a (growth rate
about 5% year) The ratio of product margin divided by catalyst cost is around 100-300
Heterogeneous80%
Homogeneous17%
Biocatalysis3%
Catalysis in Industry
Synthesis of solid catalysts, Ed. K.P. de Jong, 2009, Wiley
Heterogeneous catalysis is a surface process
Simplified mechanism heterogeneous catalysis
1. Adsorption of reactants
on to the surface of the
catalyst (active site)
2. Reaction
3. Desorption of products
Two phase process
Solid phase (catalyst)
Gas/Liquid phase
(reactants)
Heterogeneous catalysis is a surface phenomenon The performance of heterogeneous catalysts is determined
by the exposed surface area
Exposed (specific) area increases by decreasing particle size
Stabilization of particles by deposition onto supports
Solid support
Adapted from: G. Mul and J.A. Moulijn. Preparation of supported metal catalysts
What are the components of a heterogeneous catalysts?
Support; stabilize the catalytic particles Catalytic particles; (oxide, metal or sulphide) hold the active
sites Promoters; enhance the catalytic performance or structural
effects
Solid support
Extrudedcylindrical
ceramic
Low manufacturing costsRelatively high pressure dropLarge diffusion length HDS, Methanation
Not common Low-surface-area catalystsAmmonia Synthesis, Fromaldehyde
Regular shape; Most commonGood strength. CO shift Hydrogenation
Low pressure drop; Poor strength HDS
High strength; Low pressure dropSmall diffusion length. Steam Reforming
Low pressure drop; Insensitive to dust; Small diffusion length Exhaust gas cleaning
Low-surface-area catalysts Ammonia oxidationHigh Temperature reactions
Spheres
Granules
Pellets
Extrudates
Rings
Monoliths
Gauzes
Sphere Pellet
Ring Minilith Wagon wheel
Monolith
metallic
Irregulargranule
1-2 mm
Porous support body
Metal particles1-10 nm
Support particles20-50 nm
Adapted from: Synthesis of solid catalysts, Ed. K.P. de Jong, 2009, Wiley
Macroscopic scale
10 nm
Micros(nanos)copic scale< 2 nm
Microscopic scale involves the structure of the active sites. It determines
the intrinsic activity of the catalyst
The mesoscopic scale the pore system and the sizes of the support
particles as well as catalyst particles of the active phase. It affects
intraparticle mass transfer of the catalyst
The macroscopic length scale involves the size and shape of the catalyst
body. Relevant for properties such as pressure drop, mechanical
strength and attrition resistance
Criteria for a good catalyst
Activity
Selectivity
Thermal and Mechanical Properties
Stability
Morphology
Cost
A catalytic process is the combination of a catalyst a
reactor and reaction conditions (P, T, space velocity)
The shape of the catalyst body (macroscopic scale) is
paramount to determine its performance at the
industrial level
Catalysts preparationRelevant aspects and strategies for the synthesis of solid catalysts for
heterogeneous applications
CATALYST
T, P, pH, templates, size control
agents
SupportOrganic or inorganic
Modified
PrecursorMetal salt or coordination complex
The preparation of supported catalysts aims to attach the
active phase onto the support
Impregnation, co-precipitation (controlled pH or not),
homogeneous deposition, deposition of surfactant (organic
agent) stabilized metal particles
The support is either a powder or a pre-shaped solid the
most common ones being -Al2O3, -Al2O3, SiO2, TiO2 or
carbons
Mixing solutions/solids
Equilibration or aging
Solid liquid separation
Drying Calcination activation
Precipitation is in principle a crystallization process and can occur in the bulk of the liquid or on a relatively inert surface. The support particles act as crystallization nuclei for the active site precursor
Impregnation is related to ion-exchange / adsorption processes and the interaction with the support is dominant
Coprecipitation One or more metals are precipitated
together with the support or precursor
Chemical phases dispersion surface areas, porous structure
and particle size and shape are created in a single steep
It can reach very high metal loading of up to 80%
Low solubility of hydroxides alkaline media
Careful with counter ions
nucleation crystallization
CuOx ZnOx
Zn2+
Cu2+
OH-
CuO//ZnO
Liquid mixing
Nucleation rate growth rate
High supersaturation promotes nucleation Very concentrated solutions of highly soluble precursors
Carbonates or hydroxides are intended due to their low solubility [Ni][OH]2=5.4710-6 moll-1; [Ni][CO3]=5.4710-7 moll-1
Based upon solubility constants the precipitation order of the hydroxides is as follows Fe3+, Cr3+, Cu2+, Zn2+ and Ni2+
Forward precipitation Adding the base solution to
the acid (metal containing) solution (pH increases)
Reverse precipitation Adding the metal solution to
the base (pH decreases) Simultaneous precipitation
Base and acid are added simultaneously to a base solution and pH is carefully controlled
Ce
Fe
NH4OH
343 K
pH=8.0
pH, temperature, stirring, precursors and recovery
and thermal treatments are key features in the final
material morphology, structure and performance.
Mixed Cu/Zn/Al2O3 (methanol synthesis) are usually
prepared by coprecipitation from nitrate precursors
If pH = 7.0 very active catalyst is obtained as compared to
the solid obtained at pH < 6
Impregnation is the simplest method to preparing supported catalysts.
(Water) solution containing the metal precursors is contacted with a porous support Dry impregnation (pore volume impregnation) the exact
amount of liquid to fill the pore volume of the support is used vs Wet impregnation the amount of liquid is only controlled by the solubility of the metal precursor
Electrostatic forces control the adsorption mechanism Depending of the process conditions different profiles
of the active phase are obtained
The Point of Zero Charge is the
pH at which the surface is
electrically neutral
The surface has OH groups PZC is the pH where the surface overall is electrically neutral
The catalyst precursor in the
solution becomes fixed to the
support by different means;
reaction, exchange with surface
OH groups or by adsorption
The support has -OH groups
depending on the thermal and
chemical history
Si OOH
Si O SiOHOH
Protonated surface pH< PZC Deprotonated surface pH > PZC
The charge of the surface hydroxyl groups varies with the pH
Electrostatic forces will lead to the preferential adsorption of
anions or cations onto the charged surface
Si OO
Si O SiOO
Si OOH2
+
Si O SiOH2
+OH2
+
pH>pzcpH
OH2+
OH
O-
[PtCl6]2-
[(NH3)4Pt]2+
pHZPC
The maximum loading obtained is a monolayer
In reality it is much lower than a monolayer 1
complex/nm2 vs 8 OH/nm2 for alumina
CAREFUL: Solutions that are mild acid or basic do not
contain sufficient proton cations/hydroxyl anions to
protonate/deprotonate the surface so the pH of the
solution reaches the ZPC of the support
Support PZC ComplexMoO3 < 1 CationsNb2O5 2-2.5 CationsSiO2 4 CationsOxidized carbon 2-4 CationsTiO2 4-6 Cations or anionsCeO2 7 Cations or anionsZrO2 8 Cations or anionsCo3O4 7-9 Cations or anionsAl2O3 8.5 Anions or cationsCarbon black 8-10 Anions
Adapted from: Synthesis of solid catalysts, Ed. K.P. de Jong, 2009, Wiley
The distribution of the solute is governed by the balance between diffusion of solute into the pores and adsorption onto the support. Concentration, viscosity and
contact time
Drying. Elimination of solvent. Precipitation of ions. Fast drying lead better dispersions by creating higher supersaturation
Adapted from: Synthesis of solid catalysts, Ed. K.P. de Jong, 2009, Wiley
Uniform distribution. Weakly interacting precursors + mild drying
Egg-shell: Strong adsorption during impregnation. Viscous solution. Slow drying regime.
Egg-yolk. Fast drying regime. Preferential adsorption of other species (citric acid)
Deposition-precipitation (DP). DP is possible by the presence of the support which
provides nucleation sites for the metal precursor after addition of a nucleation agent (base)
It is important to control base addition to avoid concentration gradients during precipitation
Urea is an optimum precipitating agent since it gently varies the pH leading to marginal concentration gradients
Controlling atomic arrangement, size and shape
Size control (metal dispersion) by thermal treatments
Incipient wetness impregnation
Dry at room temperature for 12 h
Ru(NO)(NO3)3
-Al2O3
H2
523 K / 1 h
773 K / 1 h
873 K / 1 h
923 K / 1 h
973 K / 1 h
973 K / 2 h
973 K / 3 h
4Ru
5Ru7Ru
8Ru12Ru16Ru
23Ru
1.5 wt.% Ru
0 5 10 15 20 250
10
20
30
Freq
uenc
y
Rusize(nm)
0
50
100
150
0
100
200
4Ru
8Ru
12Ru
2.00.3 nm
6.01.9 nm
12.04.0 nm0 5 10 15 20 25
0.1
0.2
TOF C
O(s
1)
Rusize(nm)
Size determines catalytic performances
TOF for CO dissociation vs size
microemulsion
0.00 0.25 0.50 0.75 1.00
0.00
0.25
0.50
0.75
1.000.00
0.25
0.50
0.75
1.00
Orgnico
Agu
a
Surfactante
H2O
Orgnico
Particle size
Metal precursor
oil
Surfactant/Oil
Surfactant
Temperature
Ratio0
Rh0
RhCl3
H2O
RhCl3
H2N2
AuHCl4
H2O
HAuCl4H2PtCl6
PtAu
H2PtCl6
H2N2 /NaBH4
H2O
Fe(NO3)3
Fe(NO3)3
NaOH/NH4OH
Metallic particles Bimetallic particles xides
Fe[Oy(OH)x]
Catal.
H2O %
Org.%
Surf.%
oa s/ob nm
RhA 9,6 71,4 19,0 9,18 0,27 n.d.
RhB 1,8 85,4 12,8 2,46 0,15 5
RhC 2,6 84,7 12,7 3,69 0,15 10
RhD 0,7 78,4 20,9 0,53 0,27 22
RhE 2,6 76,9 20,5 2,26 0,27 30
3wt% Rh/Al2O3
Size control of Rh particles by using MEMCareful control of water/surfacant ratio determines the size of the
micelles
0 10 20 30 40 50 60 70 80 90 100
110hcp
101hcp 102hcp002hcp
100hcp
222fcc220
fcc
200fcc331fcc
PR6
PR4
PR2
PR1
2 ()
Pt1
111fcc PtRu/C from MEM Positive effect on methanol
electrooxidation
Alloyed Pt-Ru (DRX)
Preferential growht directions (HRTEM)
H2PtCl6 + Ru(NO)(NO3)3Berol O50; isooctano
Polyol method. Easy method to prepare reduced
particles by taking advantage of the reducing power
of the polyol which acts both as solvent and reducing
agent.
CH2OH-CH2OH(l) + 14 OH- 2 CO32- + 10 H2O + 10 e- 0 = 1,65 V
OHF
RTEE log9212.0
Narrow distribution of nanosized Pt particles Does it work with PtSn?
Pt particles in the range 1-5 nm can be obtained
The presence of protecting agents or the support in the reaction medium impedes agglomeration of primary particles
pH of 11 and higher are needed to reduce Ru, Mo or other metals
SnCl2H2O
H2PtCl6EG/NaOH
Carbon
The polyol method does not work for all metallic combinations
Temperature Cell constant TXRFPt/Sn
190C a = 3,937 14
140C a = 3,916 42
140C /5%H2O
a = 3,916 42
aPt3Sn=4,01 Pt/Sn=3
Sn is loss during the synthesis !
45
H2PtCl6SnCl2H2OH2O/HClCarbon
2,0 2,5 3,0 3,5 4,0 4,5 5,0 5,5 6,0 6,5 7,0
SnCl
Inte
nsid
ad /
u.a.
Energa / keV
Pt EDS
1
46
Thermal treatments
EDSEDS
2,0 2,5 3,0 3,5 4,0 4,5 5,0 5,5 6,0 6,5 7,0
SnCl
Inte
nsid
ad /
u.a.
Energa / keV
Pt
EDS32
1
Sn??
SnClx is volatile during thermal treatments Sn(OH)Cl remains in the solid
SnClx species are volatile at ca. 160-200 C
They can be hydrolized in water to Sn(OH)Cl
47
Cl2HClH2O Cl2
SnClx
PtImp
SnCl3- + H2O Sn(OH)Cl+ HCl + Cl-
48
EDS
20 30 40 50 60 70 80 90fc
c 220
fcc 2
20
fcc 3
11
fcc 2
00
fcc 1
11
u.a.
2 Grados
C00
2
Fresch 1
DRXPt3Sn PtSnx
Pt/Sn=3
Tratamiento con agua
Repeated wasing in water until pH = 7
S. Garca-Rodrguez y col. J. Power Sources 195 (2010) 5564
Encapsulating metal precursors with in dendrimers renders precursors for the formation of very small particles (few atoms)
It is possible to produce alloys and core@shellstructures
Simultaneous reduction of Pt&Ru
precursors
0123456789
3.53.02.52.01.51.00.50.0012345678
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5Position / nm
EDX
cou
nts
(Ru
L
1) / a
.u.
/
EDX
cou
nts
(Pt M
1)
/ a.u
.
1.-Reduction of Pt2.-Addition and Ru of Ru
It is possible to control the structure, shape and metal
content when preparing heterogeneous catalysts
Reaction conditions strongly affect the final catalyst
and as a consequence its final performance
There are many aspect yet to be rationalized
Nucleation by addition of adequate agents
Above critical supersaturation nucleation commences
Below critical supersaturation aggregation of particles occurs
The larger the area the more nuclei, the smaller the particle
In microemulsion growing is constrained within the micelles
H. B nemman et al and electrocatalysis at nanoparticles surfaces; A.Wieckowski (Ed)
Dekker, 2003
Nucleacin
Ncleo estable irreversible
Interaccintomos
metlicosInteraccinMetal-ion
M. Lade et al, Colloids and Surf. A 163 (2000) 3