Fritz-Haber-Institut der Max-Planck-Gesellschaft ...

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

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Three Way Converter

Pt only Rh onlyPt and Rh

Conv

ersi

on in

%

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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

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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

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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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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

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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

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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

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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

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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

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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+

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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

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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

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Interaction of Precursor Complex and Support

Bulk Interface Solution

H2OLl

L- M2+-LlL

H2O

H2O

H2O

H2O

A-

A-

H2O

H-O

-OH

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Interaction of Precursor Complex and Support

Bulk Interface Solution

H3O+

M2+ - L

lL

H3O+H2O

H2O

H2O

A-

A-

H2OO

OL

L

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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

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Capillary vs. Diffusional Impregnation

Pore space filled with same solvent Concentration gradient driving force

Predried support Capillary forces driving forceExothermicPressure in pores

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Solvent Removal

Filtration: only ion exchanged / strongly adsorbed complexes on supportEvaporation: species in solution will settle on support

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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

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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

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Interaction Between Active Phase and Support

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Effect of Different Preparation Methods

Okumura, Nakamura, Tsubota, Nakamura, Azuma, Haruta, Catal. Lett. 51 (1998) 53

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Rates in Heterogeneous Catalysis

Rate with respect to mass or surface area

⎥⎥⎦

⎤

⎢⎢⎣

⎡

catalystgmol

min

⎥⎦

⎤⎢⎣

⎡surfacecatalystm

mol2min

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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

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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

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Examples for TOFs

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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

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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

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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

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Transmission Electron Microscopy

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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

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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

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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)

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The Ratio of H to M

For very small particles, the stoichiometry H:M may change (verified by independent EXAFS measurement)

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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

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Cu Surface Area

2 CuS + N2O → (CuS-O-CuS) + N2

O. Hinrichsen, T. Genger, M. Muhler, Chem. Ing. Techn. 72 (2000) 94

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Ag Surface Area

O:Ag = 1:1

A. Gavriilidis, B. Sinno, A. Varma, J. Catal. 139 (1993) 41

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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

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Chemisorption & IR Spectroscopy

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Metal Particle Size and TOF

Xu, Gates, et al. Nature 1994

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Supported Metal Catalysts: Metal-Support Interaction

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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

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Depth Profiling of Supported Noble Metal Catalyst

Rh/TiO2: a system with strong metal support interaction (SMSI)

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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