characterisation of catalysts at their working · pdf file ·...

$: Characterisation of catalysts at their working · PDF file · 2015-01-09Characterisation of catalysts at their working conditions ... The most direct measurement of fraction of surface$
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Magnus Rønning, Summer School NTNU 2009

Characterisation of catalysts at their working conditions

Magnus RønningDepartment of Chemical Engineering

NTNU

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Outline

•

Why study catalysts in situ•

Choice of in situ techniques

•

Combining techniques•

Some in situ techniques:

–

Infrared spectroscopy–

Raman spectroscopy–

X-ray diffraction–

X-ray absorption spectroscopy

•

Examples

3


Catalyst characterisation methods (not a complete list)

PhD course at Department of Chemical Engineering, NTNU:Characterisation of heterogeneous catalysts-Emphasis on in situ techniques

•

Temperature programmed techniques (TPX)•

X-ray diffraction (XRD)•

X-ray absorption (EXAFS)•

X-ray scattering (SAXS, WAXS)•

Vibrational spectroscopies (FTIR, EELS, Raman, UV-vis) •

Scanning probe microscopy (SPM)•

XPS and Auger spectroscopy (AES)•

Measurements of surface acidity•

Dispersion measurements (chemisorption) •

Electron microscopy/tomography (TEM/SEM) •

Nuclear Magnetic Resonance (NMR)•

Tapered element oscillating Microbalance (TEOM)•

Transient kinetic analysis (SSITKA)•

Thermal analysis (TGA-DSC)•

Mössbauer spectroscopy•

Ion spectroscopies (SIMS, RBS)

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Measurement of particle size and size distribution•

X-ray diffraction (XRD)–

Averaging technique–

No visible peaks for particles smaller than approximately 5 nm–

Problems with overlapping peaks from various components•

X-ray absorption spectroscopy (XAS)–

Averaging technique–

Only valid for particle sizes below approx 5 nm–

Underestimation of particle size compared to TEM •

Transmission electron microscopy (TEM)–

Local probe–

Overestimation of particle size (Clusters appear as particles)•

Selective chemisorption (H2 , CO, N2 O etc.)–

The most direct measurement of fraction of surface atoms–

Fraction of surface atoms not necessarily reflecting size–

Adsorption stoichiometry?–

Particle shape?•

X-ray photoelectron spectroscopy (XPS)–

Sensitive to inhomogeneity•

Small angle scattering (SAXS/SANS)–

Problems with multi-component systems /contrast

Not at working conditions

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Small is better ?

Figures from Richardson, J. T. Richardson, "Principles of Catalyst Development," Plenum Press, NewYork NY, 1989, and E. McCash, “Surface Chemistry” Oxford Univ. Press 2001

•

Small particles expose a large fraction of the surface as edge and corner atoms•

Catalytic activity is often attributed to atoms with deficient coordination

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The catalytic activity of gold particles is highly dependent on particle size…

Are the Au corner atoms the active sites for CO oxidation?

N. Lopez, J. Nørskov et.al, Journal of Catalysis, 223 (2004) 232

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Coordination number7 8 9 10 11 12

Rea

ctio

n ra

te [

mol

/gca

t*s]

2,0

2,5

3,0

3,5

4,0

4,5

5,0

5wt%AuTiO2CNF

3wt%AuTiO2CNF

1.5wt%AuTiO2CNF

1.5wt%AuTiO2

5wt%AuTiO2

Sample Au dispersion [%] TOF [s-1] at 310oCAuTiO2 -Col 11.0 0.2AuTiO2 -DP 11.7 1.6

Deposition-precipitation gives a much more active catalyst for the water-gas shift reaction than colloid deposition (for comparable Au particle sizes)

…but certainly not on size alone

Au C.N. = 8

N. Hammer, I. Kvande, X. Xu, V. Gunnarsson, B. Tødal, D. Chen, M. Rønning; Catalysis Today 123 (2007) 245

N. Hammer, I. Kvande, W. van Beek, D. Chen, M. Rønning, Topics in Catalysis 45 (2007) 25

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Choice of technique…

-Identify the problem and the conditions, then choose the technique

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Why in situ characterisation?

Phenomena such as particle size, shape, oxidation state, adsorption, selectivity, deactivation etc. are highly depending on reaction conditions

Should be measured at the relevant conditions

In situ: Characterisation of catalysts at relevant working conditionsOperando: In situ studies with simultaneous activity measurements

Characterisation of real catalysts at real working conditions

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Reactants

Products

X-rays, -rays, light, electrons…

Absorption Vibration DiffractionReflectanceScatteringTomographyImagingetc.

What is in situ characterisation?

•

We build our reactor into the measuring unit

•

Measurements are being carried out while the catalyst is doing catalysis

•

Simultaneous monitoring of catalytic activity

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Combining techniques

Advantages:–

Measurements at identical conditions•

Can exclude experimental effects–

Time-saving (?)•

All the information you need in one shot

Disadvantages…–

Compromises•

One shot but useless information–

Complicated•

Murphy’s Law•

Cell design•

Window materials•

Space constraints

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Compromises

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Example:Combined XAS-XRD-Raman FT synthesis at ESRF:

–

Experimental conditions:•

20 bar pressure•

210ºC•

H2

/CO = 2.1•

MS for product analysis

In situ set-up at SNBL

XAS XRD Raman

Activity data

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Vibrational Spectroscopies

Vibrations in molecules or in solid lattices are excited by:

–

Absorption of photons in the

IR spectroscopyInfrared region (1-1000 μm)

–

Absorption of photons in the

UV-Visible spectroscopyUV-Visible region (160-780 nm)

–

Scattering of photons

Raman spectroscopy

–

Scattering of electrons

Electron energy loss spectroscopy (EELS)

–

Scattering of neutrons

Inelastic Neutron Scattering

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Molecular Vibrations•

The potential energy V(r) can be approximated by the Harmonic oscillator:

V(r) = ½

k ( r-req

)2

V(r) = interatomic potentialr = distance between vibrating atomsreq

= equilibrium distance between atomsk = force constant of vibrating bond

•

The harmonic approximation only valid for small deviations from equilibrium, the Morse potential physically more realistic:

V(r) = D (1-ea(r-req))2

-D

D = dissociation energy of the vibrating bonda = parameter which controls the steepness of the potential well

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Molecular Vibrations and Infrared Spectroscopy

•

Molecules have discrete levels of rotational and vibrational energy•

Absorption of an IR photon causes transitions between vibrational levels•

A chemical functional group absorbs IR photons of a specific wavenumber. E.g. the C-O stretch appears at around 2143 cm-1

•

IR spectroscopy detects the vibration characteristics of chemical functional groups

•

For a molecule to be IR active the vibration must be associated with changes in the dipole moment (H2 , O2 , N2 are IR inactive)

Stretching BendingSymmetric Asymmetric Scissoring Rocking Wagging Twisting

Vibrations of -CH2

-groups Source: Wikipedia

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Modes of IR Spectroscopy

•

Transmission IR:–

catalyst powder pressed into a disk (diluted)

•

Diffusive reflectance IR (DRIFTS): –

catalyst powder

•

Reflection-absorption IR (RAIRS):–

adsorbates

on a flat metal surface

•

Attenuated total reflection (ATR):–

probes the media in contact with the probe crystal

•

Electron energy loss spectroscopy (EELS):

–

dipole scattering (1) or impact scattering (2) Spectroscopy in Catalysis by J. W. Niemantsverdriet

ATR. Alfons

Baiker

et al. ETH Zürich

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Monochromatic light (from a LASER) is scattered by a sample:• without loosing energy (Rayleigh)• by loosing energy (Stokes) • a vibrationally

excited mode in the sample is de-excited (anti-Stokes)

Raman spectroscopy

C.V. RamanNoble prize 1930

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•

Number of peaks related to degrees of freedom

3N - 6 (bent) or 3N - 5 (linear) for N atoms

•

Energy related to harmonic oscillator

•

Selection rules related to symmetry:

Rule of thumb: symmetric Raman active, asymmetric IR active

Raman: 1335 cm–1

IR: 2349 cm–1

IR: 667 cm–1

CO2

or c

2k(m1m2)

m1m2

Raman + IR: 3657 cm–1

Raman + IR: 3756 cm–1

IR: 1594 cm–1

H2 O

Raman spectroscopy

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Excitation Energy, σ

(cm–1)

Inte

nsity

11,000 13,000 15,000 17,000 19,000 21,000

Near IR785 nm

Visible514 nm

–∆σ +∆σ –∆σ +∆σStokes Anti-Stokes

Stokes Anti-Stokes

785 nm: Fluorescence less probable;Lower Raman signal

514 nm: Fluorescence more probable;Resonance more likely; Higher signal

Raman spectroscopy; Choice of LASER

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X-ray Absorption Spectroscopy (XAS)XAS is a structure determination method

–

Is element-specific, i.e. determines the local environment of a chosen element in the sample, within a distance of 5-6 Å

–

Requires no long-range order–

Bulk method

Two modes:

EXAFS–

Extended X-ray Absorption Fine Structure–

Bond distances to neighbouring atoms–

Number of neighbouring atoms–

Type of neighbouring atoms

XANES–

X-ray Absorption Near Edge Spectroscopy–

Can be used as fingerprint–

Oxidation state of central atom–

Site symmetry–

Disorder

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EXAFS - Basic principle:

P.Behrens, Trends in Analytical Chemistry, 11(7) (1992) 237

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Measuring EXAFS

Ca. 1 metre

Optical benchMonochromator

Entrance slits Exit slitsI0 detector It detector

Sample

Beam

UHVchamber

sampleincoming beam

transmitted beam

Io

xI

o Il n xI

m=

where μ

is the absorption coefficient

Absorbing atom

Scattering atom

X-ray photon

Outgoing wave

Backscattered wave

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EXAFS data processing

2

)

R [Å]

4.0

6.0

8.0

10.0

10.5 10.75 11 11.25 11.5

abso

rptio

n [a

.u.]

photon energy [keV]

0.0

0.5

1.0

1.5

2.0

0.0

10.5 10.75 11 11.25 11.5

norm

. abs

orpt

ion

[a.u

.]

photon energy [keV]

0.0

2.0

0.0

-2.0

5 10 15

(k)

*k2

k [Å -1 ]

0.01

0.02

0.03

0.04

0 2 4 6 8

FT(

(k)*

k

a) b)

c) d)

The peaks in the Fourier transform represent the different backscattering shells

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X-ray diffraction

•

From Peak Position:Unit Cell Dimensions –

d-spacing

Qualitative phase identification

•

From Peak Intensity:Unit cell contents Quantitative phase fractions

•

From Peak Shapes and Widths:Crystallite size (2-200 nm)Non-uniform micro strainExtended defects

2 2 2

arcsin( )2

,

dad N h k lN

Scherrer equation

coshklL

2

exp 2 ( )hkl i i i ii

hkl hkl

F f i hx ky lz

I F

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27


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Synchrotron radiation as a tool

Main assets: High flux and tunable source-The Swiss-Norwegian Beam Lines at ESRF

•

Multi-purpose beamline–

Crystallography–

Powder XRD–

XAS–

Raman spectroscopy•

Recently installed gas distribution system (and MS)

–

Up to 20 bar

http://uit.no/getfile.php?SiteId=100&PageId=2678&FileId=7

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Example I: The Fischer-Tropsch synthesis

Finalproducts

Natural gas/steam

Synthesis gasproduction

Fischer-Tropschsynthesis

Productupgrading

H2COCO2

Oil and wax

Catalyst: Co, Fe, Ru og NiTemperature: 180-350°CPressure: 1-50 bar

•

Gas to liquids (GTL)•

Production of liquid fuels from natural gas

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Why in situ measurements at actual FT conditions?•

From the Anderson-Schultz-Flory product distribution:–

Only methane and wax (C20+ ) can be produced with high selectivity–

We need to be sure that we have high C5+ selectivity:•

Operation at 20 bar, 210ºC and H2 /CO = 2.1 –

High conversion (~50%) and wax production lead to relevant partial pressures of products (hydrocarbons, oxygenates, water)

C 1

C 2

C 3

C 4

C 5 - C 1 1

C 1 2 - C 2 0

C 2 0 +

0

0 , 2

0 , 4

0 , 6

0 , 8

0 0 , 2 0 , 4 0 , 6 0 , 8 1

P r o b a b i li ty o f c h a i n g r o w th ,

Wei

ght f

ract

ion

of p

rodu

ct, W

n

ASF product distribution:

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Proposed deactivation mechanisms for Cobalt FT catalysts

•

Sintering–

Particle growth/agglomeration

–

Loss of active surface

•

Reoxidation–

Less cobalt in the active state

–

More pronounced for small Co particles and at high PH2O

/PH2

ratios

•

Solid state reactions–

Formation of inactive cobalt phases (CoAl2

O4

)

•

Coke deposition–

Blocking of active sites by polymeric carbon deposits

A.M. Hilmen et al., Appl. Catal. A 186 (1999) 169G. Jacobs et al., Appl. Catal. A 233 (2002) 215J. van de Loosdrecht et al. Catal. Today,123 (2007) 293A.M. Saib, et al Appl. Catal. A 312 (2006) 12

G. Jacobs et al. Appl. Catal. A 233 (2002) 215T.K. Das et al. Fuel82 (2003) 805M.J. Overett et al. Prepr. Pap.-Am. Chem. Soc., Div. Petr. Chem. 53 (2008) 126

D.J. Moodley et al. Prepr. Pap.-Am. Chem. Soc., Div. Petr. Chem. 53 (2008) 122D.J. Moodley et al. Appl. Catal. A. (2008) doi:10.1016/j.apcata.2008.11.015

X.X Gao et al. Catal. Today 131 (2008) 211A. Moen et al. J. Mater. Chem., 1998, 8(11), 2533

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Deactivation mechanisms: -Sintering and reoxidation

0

2000

4000

6000

8000

10000

16 17 18 19 20 21 22

20

40

60

80

100

120

140

160

Inte

nsity

2è M

easu

rem

ent

Co3 O4CoO

Co0Co3 O4

CoO

Co0

XANES during reduction

0

2000

4000

6000

8000

1015

2025

100

120

140

160

180

200

Inte

nsity

2è

Mea

sure

men

t

Co0

XANES during FT reaction (210ºC, 18 bar) 6 hrs

XRD during reduction

XRD during FT reaction (210ºC, 10 bar) 4 hours

CoO

Co0

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Example II: Au/TiO2 catalysts on carbon nanofibres

Au/Oxide/CNFCNF Oxide/CNF

TiO2 Au

N. Hammer, I. Kvande, X. Xu, V. Gunnarsson, B. Tødal, D. Chen, M. Rønning, Catal. Today 123 (2007) 245

Need for different characterisation techniques for obtaining nanoscale structural information

–

(XRD), TEM, XAS

Looking at changes in particle sizes and morphology during reaction–

In situ XAS

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X-ray Absorption Spectroscopy, Ti K-edge XANES

Energy [keV]4,966 4,968 4,970 4,972 4,974 4,976 4,978 4,980

Nor

m. A

bs

0,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7AsprepCAuTiO2AuCTiO2 CAuCTiO2 AnataseRutile

B

A3A2

A1

The intensity of the pre-edge transitions are sensitive to the symmetry of the surrounding atoms

N. Hammer, I. Kvande, X. Xu, V. Gunnarsson, B. Tødal, D. Chen, M. Rønning, Catal. Today 123 (2007) 245

•Four pre-edge peaks are present

•A1 and B sensitive to distortion and crystallinity

•A2, A3 sensitive to particle size

•Same particle size for unsupported and supported TiO2

•A more distorted environment for supported TiO2

•TiO2

is predominantly present as anatase

•The CNF stabilise the titania and hence the gold particles from sintering

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X-ray absorption spectroscopy

In situ XASData collected in fluorescence mode for the Au-L3 absorption edgeWGS conditions:Temperature range: 190 ºC – 310 ºC CO:H2 O ratio 1:1

aAu-Au coordination number

Sample Before WGS reaction After WGS reaction

Na dp (nm) Na dp (nm)AuCNF 10.2 3.2 10.8 4.3

1.5wt%AuTiO2 CNF 7.8 1.6 9.5 2.4

1.5wt%AuTiO2 9.2 2.1 11.4 8.4

5wt%AuTiO2 CNF 7.5 1.5 7.5 1.5

5wt%AuTiO2 6.7 1.1 8.3 1.8

Increased coordination number after reaction–

Particle size increases more without CNF

N. Hammer, I. Kvande, W. van Beek, D. Chen, M. Rønning, Topics in Catalysis 45 (2007) 25

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Example III:Dynamic behaviour of Cu/ZnO-based methanol catalysts

Top panel: Combined in situ EXAFS coordination numbers and on-line methanol synthesis activities for a Cu/ZnO catalyst exposed to different synthesis gases.Bottom panel: Illustration of the wetting/non-wetting changes occurring in different reaction environments.

• Simultaneous in situ EXAFS andon-line activity measurements

• State and activity of the catalystis sensitive to the reduction potential of the synthesis gas

changing morphology of Cu particles with changing reduction potential (= different conversion levels)

• In situ studies allowed formulationof dynamic micro kinetic modelsfor industrial performance

Reference: Halldor Topsøe AS

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armchair graphite surface zigzag

Example IV: Atomistic simulations of binding of Pt clusters to carbon nanostructures

isolated nanocone

C.F. Sanz-Navarro, P.-O. Åstrand, D. Chen, M. Rønning, A.C.T. van Duin, T. Jacob, W.A. Goddard III: J. Phys. Chem. A 112 (7), (2008) 1392

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Flattening of Pt100 when adsorbed•Some Pt atoms are detached from the cluster•The gaps left by the detached atoms are filled by new adsorbed atoms•Increase in the number of Pt atoms having low bond order (red)•Thus the number of adsorbed atoms increases and the structure of

the overall

cluster changes considerably

tMD = 25 ps

tMD = 0 ps


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Flattening of Pt100 when adsorbed•Red atoms at Pt-C interface. Blue further away•Tendency towards longer Pt-C bonds


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Identification of favorable Pt sites

Schematic presentation of the restructuring of the C-Pt interface

Armchair position

Zig-zag I position

Basal plane position


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MD and experiments?

Sample NPt-O/C RPt-O/C NPt--Pt RPt-Pt

Pt foil - - 12 2.77

Pt/CNF 0.6 2.08 5.2 2.76

Molecular dynamics X-ray absorption spectroscopy

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Finally: Catalyst characterisation using chemical reactions -Reactions over Cu-Ni catalysts of various compositions

Ethane hydrogenolysis:-Structure sensitive (demanding)-Ni is the only active catalyst

Cyklohexane dehydrogenation:-Structure insensitive (facile)-Both Ni and Cu are active catalysts

Ammonia synthesis, Fe crystal planes:-Structure sensitive (demanding)

Reviews by:-G.A. Somorjai: Catal. Lett. 7 (1990) 169-R.A. van Santen: Acc. Chem. Res. 42 (2009) 57

G. Somorjai

et al. J.Catal. (1987)

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Acknowledgements•

Catalysis group, NTNU: Alexey Voronov, Nikolaos

Tsakoumis, Elisabeth Windstad, De Chen, Anders Holmen

•

SINTEF: Ingvar Kvande, John C. Walmsley (TEM), Pascal Dietzel

•

Dept of Chemistry, UiO: Poul

Norby, Rune E. Johnsen, David Wragg

•

Dept of Chemistry, NTNU: Carlos Sanz-Navarro, Per-Olof Åstrand

•

StatoilHydro: Øyvind Borg, Nina Hammer, Erling

Rytter

•

Swiss-Norwegian Beamlines: Olga Safonova, Wouter van Beek, Hermann Emerich

•

ELETTRA, Trieste: Luca Olivi

•

Funding: Statoilhydro, NFR, NTNU

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