View
1
Download
0
Category
Preview:
Citation preview
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 24, 2007
© F
.C. J
ento
ft, F
ritz
Hab
erIn
stitu
te, M
ax P
lanc
k So
ciet
y, 2
007
Three Way Converter
Pt only Rh onlyPt and Rh
Conv
ersi
on in
%
© F
.C. J
ento
ft, F
ritz
Hab
erIn
stitu
te, M
ax P
lanc
k So
ciet
y, 2
007
Precious Metal Content in Three Way ConverterCo
nver
sion
in %
Pt: 41.7 g/ft3
Rh: 8.3 g/ft3Pt: 40 g/ft3
Rh: 0 g/ft3Pt: 0 g/ft3
Rh: 8.3 g/ft3
© F
.C. J
ento
ft, F
ritz
Hab
erIn
stitu
te, M
ax P
lanc
k So
ciet
y, 2
007
Outline
1. Introduction into Heterogeneous Catalysis
2. Motivation for the Use of Nanoparticles
3. Properties of Nanoparticles
4. Catalyst Preparation Methods
5. Characterization of Supported Metals
6. Characterization of Supported Metal Oxides
© F
.C. J
ento
ft, F
ritz
Hab
erIn
stitu
te, M
ax P
lanc
k So
ciet
y, 2
007
How to Prepare Supported Nanoparticles
molecularly dispersed species (ions or clusters in solution)
bulk solid species
SUPPORT
gas phase species
© F
.C. J
ento
ft, F
ritz
Hab
erIn
stitu
te, M
ax P
lanc
k So
ciet
y, 2
007
Steps of Catalyst Preparation (IUPAC)
1. Preparation of the primary solidassociating all useful components (impregnation or coprecipitation, or, in the case of zeolites, crystallization)
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, deammoniation
Pure and Applied Chemistry 67 (1995) 1257-1306
© F
.C. J
ento
ft, F
ritz
Hab
erIn
stitu
te, M
ax P
lanc
k So
ciet
y, 2
007
Supports & Reactants
SupportsHigh surface area oxidic compounds: zeolites, SiO2, Al2O3, TiO2, ZrO2, CeO2, ZnOCarbon materials
Goal: disperse an active phase on an inexpensive and inert (?) support
Dispersed phaseIncreased surface area
StabilizedSUPPORT
© F
.C. J
ento
ft, F
ritz
Hab
erIn
stitu
te, M
ax P
lanc
k So
ciet
y, 2
007
Active Species Sources
Liquid Phase - Solid Reactions:Soluble metal complexes, e.g. [Pt(NH3)4]2+
Solid-Solid-Reactions:Oxides
SUPPORT
Gas Phase - Solid Reactions:Volatile metal compounds, e.g. Ni(CO)4, SiCl4
© F
.C. J
ento
ft, F
ritz
Hab
erIn
stitu
te, M
ax P
lanc
k So
ciet
y, 2
007
Termination of Oxides
Brønstedsitesproton orOH donor
O-H
Mx+/
Mx+
Lewisacid siteselectron pairacceptor
HO
Mx+
HO
Mx+ Mx+
HO
Mx+ Mx+ Mx+
type I type II type III
© F
.C. J
ento
ft, F
ritz
Hab
erIn
stitu
te, M
ax P
lanc
k So
ciet
y, 2
007
Solid Surface in Solution
surfaces can be charged in solution
changes with pH
Point of zero charge (pzc): surface charge σ0 is zero
Metal oxide
-OH
-OH-OH
-OH
-OH-OH
-OHM
etal oxide
-OH
-OH2+
-OH
-OH
-O-
-OH2+
OH-
OH-
OH-
-OH2+
H3O+
σ0
© F
.C. J
ento
ft, F
ritz
Hab
erIn
stitu
te, M
ax P
lanc
k So
ciet
y, 2
007
Solid Surface in Solution
Isoelectric point: no mobility of particle in electric field(ζ-potential zero)
any ion present in solution can interact with surface
Metal oxide
A-
A-
A-
C+
+
+
+
+
+σ0 σ1
C+
A-
A-
C+
© F
.C. J
ento
ft, F
ritz
Hab
erIn
stitu
te, M
ax P
lanc
k So
ciet
y, 2
007
Influence of pH: Example for Change in Zeta-Potential
Silica does easily not adsorb cationsAlumina amphoteric
Preparation of Solid Catalysts, Eds. Ertl, Knözinger, Weitkamp, Wiley-VCh 1999
© F
.C. J
ento
ft, F
ritz
Hab
erIn
stitu
te, M
ax P
lanc
k So
ciet
y, 2
007
Precursor Complex and Support
Bulk Interface Solution
H2OLl
L- M2+-LlL
H2O
H2O
H2O
H2O
A-
A-
H2O
-OH
-OH
pH Effects
Surface charge
Solvation of complexes
Degree of condensation of precursor species (e.g. from MoO4
2- at high pH to Mo8O26
4- at low pH)
Solubility of Support
Foreign ions
Can compete for adsorption sites
© F
.C. J
ento
ft, F
ritz
Hab
erIn
stitu
te, M
ax P
lanc
k So
ciet
y, 2
007
Interaction of Precursor Complex and Support
Bulk Interface Solution
H2OLl
L- M2+-LlL
H2O
H2O
H2O
H2O
A-
A-
H2O
H-O
-OH
© F
.C. J
ento
ft, F
ritz
Hab
erIn
stitu
te, M
ax P
lanc
k So
ciet
y, 2
007
Interaction of Precursor Complex and Support
Bulk Interface Solution
H3O+
M2+ - L
lL
H3O+H2O
H2O
H2O
A-
A-
H2OO
OL
L
© F
.C. J
ento
ft, F
ritz
Hab
erIn
stitu
te, M
ax P
lanc
k So
ciet
y, 2
007
Main Categories of Distribution
homogeneous egg shell egg yolkegg white
MC and A same affinity
MC high affinity, no competitor
High affinity of A, low affinity of MC, low A/MC ratio
High affinity of A, low affinity of MC, high A/MC ratio
MC: Metal complex, A: Competitor
© F
.C. J
ento
ft, F
ritz
Hab
erIn
stitu
te, M
ax P
lanc
k So
ciet
y, 2
007
Capillary vs. Diffusional Impregnation
Pore space filled with same solvent Concentration gradient driving force
Predried support Capillary forces driving forceExothermicPressure in pores
© F
.C. J
ento
ft, F
ritz
Hab
erIn
stitu
te, M
ax P
lanc
k So
ciet
y, 2
007
Solvent Removal
Filtration: only ion exchanged / strongly adsorbed complexes on supportEvaporation: species in solution will settle on support
© F
.C. J
ento
ft, F
ritz
Hab
erIn
stitu
te, M
ax P
lanc
k So
ciet
y, 2
007
Gas Phase Reactions
Metal halides, e.g. MoCl5, TiCl4,
Metal alkoxides, e.g. Ti(OC2H5)4
Metal carbonyls, e.g. Ni(CO)4
Metal oxide
-OH
-OH-OH
-OH-OH
+ MoCl5
Metal oxide
-OH
-OH-OH
-O-OH
MoCl4
-HCl
473 K
O2
773 KM
etal oxide-OH
-OH-OH-O
-OMo
O
O
© F
.C. J
ento
ft, F
ritz
Hab
erIn
stitu
te, M
ax P
lanc
k So
ciet
y, 2
007
Preparation of SCR Catalyst by Solid-Solid-Wetting
MoO3/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
start out with physical mixture: small particles, intimate mixture
evoke spreading through thermal treatment
© F
.C. J
ento
ft, F
ritz
Hab
erIn
stitu
te, M
ax P
lanc
k So
ciet
y, 2
007
Interaction Between Active Phase and Support
© F
.C. J
ento
ft, F
ritz
Hab
erIn
stitu
te, M
ax P
lanc
k So
ciet
y, 2
007
Effect of Different Preparation Methods
Okumura, Nakamura, Tsubota, Nakamura, Azuma, Haruta, Catal. Lett. 51 (1998) 53
© F
.C. J
ento
ft, F
ritz
Hab
erIn
stitu
te, M
ax P
lanc
k So
ciet
y, 2
007
Rates in Heterogeneous Catalysis
Rate with respect to mass or surface area
⎥⎥⎦
⎤
⎢⎢⎣
⎡
catalystgmol
min
⎥⎦
⎤⎢⎣
⎡surfacecatalystm
mol2min
© F
.C. J
ento
ft, F
ritz
Hab
erIn
stitu
te, M
ax P
lanc
k So
ciet
y, 2
007
Turn Over Frequency (1966)
Rate with respect to number of active sites
low site density high site density
Turnover frequency (TOF) is the number of molecules formed per active site per second (in a stage of saturation with reactant, i.e. a zero order reaction with respect to the reactant)
[ ]1−=⎥⎦
⎤⎢⎣
⎡s
ssitemolecules
M. Boudart et al., J. Catal. 6 (1966) 92
© F
.C. J
ento
ft, F
ritz
Hab
erIn
stitu
te, M
ax P
lanc
k So
ciet
y, 2
007
TOF, TON, Catalysis
TONTotal number of product formed molecules per active siteTON= TOF*catalyst life time
TON = 1 stoichiometric reactionTON ≥ 102 catalytic reactionTON = 106-107 industrial application
TON origins from enzyme kinetics, definitions vary
© F
.C. J
ento
ft, F
ritz
Hab
erIn
stitu
te, M
ax P
lanc
k So
ciet
y, 2
007
Methods to Characterize Supported Catalysts
DiffractionXRD (shows ill-dispersed particles)
MicroscopySEM, TEM
Chemisorption
SpectroscopyIR and Raman spectroscopy
X-ray absorption
X-ray photoelectron spectroscopy
Ion scattering spectroscopy
UV-vis spectroscopy
© F
.C. J
ento
ft, F
ritz
Hab
erIn
stitu
te, M
ax P
lanc
k So
ciet
y, 2
007
Powder X-ray Diffraction
1 wt% Pt / H-Mordenite
10 20 30 40 500
200
400
600
800
1000
1200
1400In
tens
ity (a
.u.)
2 θ (°)
Pt/HM. calcined at 500°C Pt, PDF 4-802
© F
.C. J
ento
ft, F
ritz
Hab
erIn
stitu
te, M
ax P
lanc
k So
ciet
y, 2
007
Scanning Electron Microscopy & Energy Dispersive X-ray Analysis
011044a BSE HV 15kV011044a BSE HV 15kV xy01l LS HV 15kVxy01l LS HV 15kV
011034 BSE HV 15kV011034 BSE HV 15kV
© F
.C. J
ento
ft, F
ritz
Hab
erIn
stitu
te, M
ax P
lanc
k So
ciet
y, 2
007
Transmission Electron Microscopy
© F
.C. J
ento
ft, F
ritz
Hab
erIn
stitu
te, M
ax P
lanc
k So
ciet
y, 2
007
Dispersion of Metal Particles
Dispersion: fraction of metal atoms exposed
Total amount of metal known from synthesis
Exposed metal surface area can be determined from chemisorption,provided the ratio of probe molecules : metal atoms is known and there is no spill-overCO, H2, N2O, O2
Further information from TEM and EXAFS
© F
.C. J
ento
ft, F
ritz
Hab
erIn
stitu
te, M
ax P
lanc
k So
ciet
y, 2
007
Metal Surface Sites
specific adsorption of probe on only one type of site (e.g. on metal and not on support): depends on strength of interaction of probewith metal/support sites, conditions (T, p) can be optimized
Metal particles
Support (e.g. oxide) surface
CO molecules
© F
.C. J
ento
ft, F
ritz
Hab
erIn
stitu
te, M
ax P
lanc
k So
ciet
y, 2
007
Measuring Metal Surface Area with H2 or CO
if adsorption is specific, number of sites can be derived from isotherm(no site heterogeneity, no spill-over)
© F
.C. J
ento
ft, F
ritz
Hab
erIn
stitu
te, M
ax P
lanc
k So
ciet
y, 2
007
The Ratio of H to M
For very small particles, the stoichiometry H:M may change (verified by independent EXAFS measurement)
© F
.C. J
ento
ft, F
ritz
Hab
erIn
stitu
te, M
ax P
lanc
k So
ciet
y, 2
007
Spillover
Although the probe molecule itself may not adsorb on the pure support, molecules or atoms can spillover from metal particles
Metal particles
Support (e.g. oxide) surface
H2 molecules
H atoms
© F
.C. J
ento
ft, F
ritz
Hab
erIn
stitu
te, M
ax P
lanc
k So
ciet
y, 2
007
Cu Surface Area
2 CuS + N2O → (CuS-O-CuS) + N2
O. Hinrichsen, T. Genger, M. Muhler, Chem. Ing. Techn. 72 (2000) 94
© F
.C. J
ento
ft, F
ritz
Hab
erIn
stitu
te, M
ax P
lanc
k So
ciet
y, 2
007
Ag Surface Area
O:Ag = 1:1
A. Gavriilidis, B. Sinno, A. Varma, J. Catal. 139 (1993) 41
© F
.C. J
ento
ft, F
ritz
Hab
erIn
stitu
te, M
ax P
lanc
k So
ciet
y, 2
007
Chemisorption & IR Spectroscopy
H. Knözinger, Fund. Aspects of Heterogeneous Catalysis, Eds. H.H. Brongersma, R. Van Santen, Plenum Press New York, 1991, p 167-189
© F
.C. J
ento
ft, F
ritz
Hab
erIn
stitu
te, M
ax P
lanc
k So
ciet
y, 2
007
Chemisorption & IR Spectroscopy
© F
.C. J
ento
ft, F
ritz
Hab
erIn
stitu
te, M
ax P
lanc
k So
ciet
y, 2
007
Metal Particle Size and TOF
Xu, Gates, et al. Nature 1994
© F
.C. J
ento
ft, F
ritz
Hab
erIn
stitu
te, M
ax P
lanc
k So
ciet
y, 2
007
Supported Metal Catalysts: Metal-Support Interaction
© F
.C. J
ento
ft, F
ritz
Hab
erIn
stitu
te, M
ax P
lanc
k So
ciet
y, 2
007
He+ Ekin (~kV) He+
E'kin
Ion Scattering Spectroscopy
topmost layer of surface is probed with ion beam (He+)
also: LEIS = low energy ion scattering
kinetic energy after interaction depends on mass of scattering atom
highly surface sensitive, destructive
© F
.C. J
ento
ft, F
ritz
Hab
erIn
stitu
te, M
ax P
lanc
k So
ciet
y, 2
007
Depth Profiling of Supported Noble Metal Catalyst
Rh/TiO2: a system with strong metal support interaction (SMSI)
© F
.C. J
ento
ft, F
ritz
Hab
erIn
stitu
te, M
ax P
lanc
k So
ciet
y, 2
007
Literature
Gabor A. Somorjai, Introduction to Surface Chemistry and Catalysis, John Wiley, New York, 1994
Bruce C. Gates, Catalytic Chemistry, John Wiley, New York, 1992
G. Ertl, H. Knözinger, J. Weitkamp, Handbook of HeterogeneousCatalysis, Wiley-VCH, Weinheim 1997
G. Ertl, H. Knözinger, J. Weitkamp, Preparation of Solid Catalysts, Wiley-VCH, Weinheim 1999
Recommended