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MAX-PLANCK-GESELLSCHAFT
ACFHI
Supported Nanoparticles:Catalysis and Characterization
Friederike C. Jentoft
Department of Inorganic ChemistryFritz-Haber-Institut der
Max-Planck-Gesellschaft
Faradayweg 4-6, 14195 Berlin
IMPRS “Complex Surfaces in Materials Science”
Block Course SS 07
April 23, 2007
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Outline
1. Introduction into Heterogeneous Catalysis
2. Motivation for the Use of Nanoparticles
3. Properties of Nanoparticles
4. Preparation Methods
5. Supported Metals & Characterization
6. Supported Metal Oxides & Characterization
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Modern Catalysis Picture
Reactants
Products
G°
EA
no catalyst
Reaction coordinate
Reactants
Products
G°
EA
catalyst
Reaction coordinate
Thermodynamics of reaction remain unchangedCatalyst influences
reaction rate by changing the reaction coordinate and the nature of
transition states or intermediates
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Types of Catalysis
Biocatalysiscatalyzed reactions where the catalyst is an
enzyme
Homogeneous catalysiscatalysis with the catalyst in the same
phase as the reactant
Heterogeneous catalysis1) catalysis with the catalyst in a
different phase than the reactants, and 2) with the catalyst on
solid, porous material or impregnated in such material
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Minimal Number of Reaction Steps in Heterogeneous Catalysis
1. Adsorption of reactant on catalyst surface
AB
2. Reaction
3. Desorption of products from catalyst surface
Gas or liquid phase
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The Sabatier Principle
Catalyst and reactants form a surface-adsorbate complex
Catalyst must have affinity to reactant – should not be too
high
A+B+K
P+K
G°
Reaction coordinate
{A B}╪
AK + BAKB
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Motivation for Supported Metal and Metal Oxides: Catalytic
Applications
Cost
Modified properties
three-way converter
NOx catalyst, stationary power plants
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Three Way Converter
Pt only Rh onlyPt and Rh
Conv
ersi
on in
%
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Rh & Pt Price
1 troy ounce = 31.1 g
6200 US$/ troy ounce corresponds to about 146,586 € per kg
1200 US$/troy ounce corresponds to about 28,371 € per kg
Cost is an argument!
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How Much Catalyst is Needed?
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Bulk Catalysts
Bulk large crystallite catalystLow surface area
Fine particles catalystHigh surface area
Material used more efficiently
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Taylor’s Observations (1925)
“the amount of surface which is catalytically active is
determined by the reaction catalyzed”
active sites / centers
H.S. Taylor, Proc. Roy. Soc. A108 (1925) 105-111
Hugh Stott Taylor (1890-1974)
Professor of Physical Chemistry in Princeton
deducted from the small amount of CO adsorbed on quartz that
“only a small fraction of the surface is active”
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Taylor’s Observations (1925)
discussed the CO adsorption on Ni, “a concept of metal atoms
detached to varying extents from the normal crystal lattice“
-“capable of adsorbing several molecular reactants”
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Surface Reactivity?
A cube consisting of 27 atoms has1 atom in the bulk6 atoms at
the faces12 atoms at edges8 atoms at corners
96% of atoms at surface
A cube consistsing of 64 atoms has8 atoms in the bulk24 atoms at
the faces24 atoms at edges8 atoms at corners
87.5% of atoms at surface
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Definitions
Cluster in Inorganic Chemistry - Narrow Definition:Compounds
with metal-metal bonds with at least 3 metal atoms
Cluster – Extended DefinitionSmall groups of atoms (often used
for metal oxides)
NanoparticlesParticle with nano-dimensions
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When to Consider Interface Thermodynamics?
Interface contributions become important for particles < 1
µmSurface: A solid/liquid or solid/gas interface
0,01 0,1 1 10 100 1000-420
-400
-380
-360
-340
-320
-300
Δ fH
m /
kJ m
ol-1
Cube edge length / µm
NaCl
Graph after T. Grande, S. Stølen, Chemical Thermodynamics of
Materials, Wiley 2003, p. 157
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Surface Energy
It costs energy to produce new surface area!Depends on number
and strength of bonds that need to be broken
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Surface Energy
sdAdW γ=
dVpdW =with p the pressure in N/m2
with γ the surface energy in J/m2 = N/m
sdAVdpSdTdG γ++−=
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Examples
Compound γ / J m-2
LiF 0.34
CaF2 0.45
NaCl 0.227
MgO 1.2
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Minimization of Energy: Relaxation
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Minimization of Energy: Relaxation
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Mimimization of Energy: Reconstruction
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Does a Particle Exhibit only one Type of Surface ?
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Surface Structure
different geometry and density
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High Miller Index Planes
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Surface Energy of Different Faces
A.B. Mukhopadhyay, J.F. Sanz, C. B. Musgrave,Phys. Rev. B 73,
115330 (2006)
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Wulff Construction
Equilibrium shape
1 cm2: 4 cm x 0.32 x 25.0 µJ/cm + 4 cm x 0.59 x 22.5 µJ/cm =
85.1 µJ
(10) plane
γ = 25 µJ/cm
1 cm2: 4 cm x 25.0 µJ/cm = 100 µJ
(11) plane
γ = 22.5 µJ/cm
1 cm2: 4 cm x 22.5 µJ/cm = 90 µJ
2-dimensional crystal
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Equilibrium Shape of Crystals
total surface free energy should be at a minimum
…often not in equilibrium!
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Zeolite
MoW5O14
Defined Crystal Shapes
all images: scanning electron microscopy
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Silver IrO2
Defined Crystal Shapes
all images: scanning electron microscopy
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Structure Sensitivity
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Role in Catalysis? Structure Sensitivity!
also: ammonia synthesis (reactions involving C-C, N-N bond
breaking)
C2H6 + H2 2 CH4
rate per exposed metal surface area is a function of the metal
particle size / the exposed crystal plane
active site an ensemble of atoms
example: the hydrogenolysis of ethane
M. Boudart et al., Adv. Catal. 20 (1969) 153-177
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Structure Insensitivity (1969)
rate per exposed metal surface area is NOT a function of the
metal particle size
active site 1-2 atoms
example: the hydrogenation of cyclohexene
+ H2
M. Boudart et al., Adv. Catal. 20 (1969) 153-177
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Electronic Properties of Different Crystal Planes
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Polymorphy
One compound can form different crystal structures
TiO2: rutile, anatase, brookite
SiO2: α- or β-quartz, α- or β-trydimite, α- or
β-crystobalite
ZrO2: monoclinic, tetragonal, cubic modifications…
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Phase Stability: Al2O3 Phase Diagram
McHale et al. Science 1997
usually, α-Al2O3 is more stable than γ-Al2O3
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Influence of Particle Size on Melting Point of Au
Melting point can decrease drastically with decreasing particle
size!
Ph. Buffat, J. P. Borel, Phys. Rev. A 13 (1975) 2287-2298
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Catalytic Activity of Gold
otherwise inert elements can become catalytically active when
inthe form of small particles
Fuel cell problemFuel cells use H2 and O2 (air) as feed
H2 can be produced via reforming
removing traces of CO from H2 (CO would poison electrode)
Oxidize CO to CO2 without oxidizing H2 to H2O
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Catalytic Activity of Gold
CO
H2
Torres Sanchez, Ueda, Tanaka, Haruta, J. Catal. 168 (1997)
125-127
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Key Catalyst Properties (I)
Activity
Selectivity
AB
C
A B C
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Key Catalyst Properties (II)
Stability
200 400 600 800 10000
20
40
60
80
100
Yiel
d (%
)
Catalyst lifetime / days
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Small Particles are Instable vs. Large Particles
0,01 0,1 1 10 100 1000-420
-400
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ΔfH
m /
kJ m
ol-1
Cube edge length / µm
NaCl
Nanoparticles must be stable under activation and reaction
conditions
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Catalyst Form
Fixed bed reactor
A
B
Fluids do not pass easily through fine powder bed, large
pressure drop
Catalyst needs a form
extrudates
honeycomb
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How to Prepare Nanoparticles
molecularly dispersed species (ions or clusters in solution)
bulk species
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How to Prepare Supported Nanoparticles
molecularly dispersed species (ions or clusters in solution)
bulk solid species
SUPPORT
gas phase species
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Nature of Support
High surface area support wantedSupport often also consists of
nanoparticles!
transmission electron microscopy: Rh/SiO2
ZrO2
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Literature
B.C. Gates: Catalytic Chemistry, John Wiley & Sons, New York
1997
A.W. Adamson, A.P. Gast: Physical Chemistry of Surfaces, 6th
ed., John Wiley & Sons, New York 1997
G. Ertl, H. Knözinger, J. Weitkamp: Handbook of Heterogeneous
Catalysis, Wiley-VCH, Weinheim 1997
S. Stølen, T. Grande: Chemical Thermodynamics of Materials, John
Wiley & Sons, New York 2003
Supported Nanoparticles:�Catalysis and
Characterization��Friederike C. JentoftOutlineModern Catalysis
PictureTypes of CatalysisMinimal Number of Reaction Steps in
�Heterogeneous CatalysisThe Sabatier PrincipleMotivation for
Supported Metal and Metal Oxides: �Catalytic ApplicationsThree Way
ConverterRh & Pt PriceHow Much Catalyst is Needed?Bulk
CatalystsTaylor’s Observations (1925)Taylor’s Observations
(1925)Surface Reactivity?DefinitionsWhen to Consider Interface
Thermodynamics?Surface EnergySurface EnergyExamplesMinimization of
Energy: RelaxationMinimization of Energy: RelaxationMimimization of
Energy: ReconstructionDoes a Particle Exhibit only one Type of
Surface ?Surface StructureHigh Miller Index PlanesSurface Energy of
Different FacesWulff ConstructionEquilibrium Shape of
CrystalsDefined Crystal ShapesDefined Crystal ShapesStructure
SensitivityRole in Catalysis? Structure Sensitivity!Structure
Insensitivity (1969)Electronic Properties of Different Crystal
PlanesPolymorphyPhase Stability: Al2O3 Phase DiagramInfluence of
Particle Size on Melting Point of AuCatalytic Activity of
GoldCatalytic Activity of GoldKey Catalyst Properties (I)Key
Catalyst Properties (II)Small Particles are Instable vs. Large
ParticlesCatalyst FormHow to Prepare NanoparticlesHow to Prepare
Supported NanoparticlesNature of SupportLiterature
