Download - 2 Structure Characterization

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Structure

■ X-Ray Diffraction (XRD)− Crystalline materials (long range order)

■ Neutron scatteringH/D containing molecules− H/D containing molecules

■ X-Ray absorption spectroscopy (XAS)− Amorphous materials (short range order)p ( g )− Composite materials

Principle of X-Ray Diffraction

θ θ

Interference of photons scattered by ordered structures

θλ sin2dn =

Point lattice with crystalline long range (min. 10 unit cells)

Interference positive for:

Bragg’s LawLong range order

X-Ray Powder Diffraction

a ba. b.

111200

220

200

220

A fraction of the crystallites will be orientated to satisfy the Bragg condition for each set of planes (hkl) These

KCl NaCl311222400331420422511333

222400420422

440600442for each set of planes (hkl). These

crystallites will be randomly oriented around the incoming beam, so the diffracted beams forms a cone around the

333440531600442

533620

622444

442620622444640642

diffracted beams forms a cone around the incident beam at the angle of 2θ.

444711551640

Experimental Setup XRDPowder DiffractionPowder Diffraction

Position of XRD lines

Powder XRD of α-Quarz

XRD: Identification of structure Miller indicesMiller indices

Neighboring atoms/Free valencesFree valences

12/0 8/4 7/5 9/3

Features of a powder XRD and their origin Application of X-ray diffractionIn situ characterization of catalystsIn situ characterization of catalysts

M O f 60 minMnO reference40 min

60 min

Reduced Fe-MnOcatalyst

20 min

After CO hydro-genation

Fe (bcc) converts into Fe-carbides

Formation of Pd hydride during Benzene hydrogenation

Formation of metal oxide phasesin situ XRDin situ XRD

Reduction of a supported Cu catalysts

Properties of neutrons

− Mass m=1.675 x 10-24g− Charge = 0− Spin =½− Magnetic moment µ = 1.913 nuclear magnetons

− Neutron wavelength range: 0.2 - 20 Å, 1 meV=8.065 cm-1

kmvh πλ 2

− Wave vector (mag.)

kmv

mvk ==λπ2

− Neutron energy kmvE 1 222 ==

− Neutron momentum

mmvE

kmvp

Interaction of neutrons with matter

Momentum Transfer

2221 kmvE ==

( )fkkQkmvp

−=

Energy Transfer

( )222

kkEE

mmvE

−=−=

( )f

( )00 2 ff kkm

EE −=−= ωk0

mvk ==λπ2

■ Elastic scattering− kf = k0 ћω=0

■ Inelastic scattering− kf < k0 (energy loss)kf k0 ћω 0

only momentum (Q) is transferred

kf k0 (energy loss)

Incoherent and coherent scattering cross-sections

Incoherent scattering cross-sectionD N Ni

Coherent scattering cross-section

Absorption cross-section

■ Coherent scattering interference effects between waves scattered from different nucleifrom different nuclei− Structure and motion of atoms relative to each other

■ Incoherent scattering deviation of individual atom positions from the g pmean potential− Motion of single atoms

Elastic neutron scattering

■ Elastic Neutron scattering is a coherent scattering process analogous to X-ray diffraction.− kf = k0 only momentum is transferred (no f 0 y (

energy analysis) − Incoherent scattering increases

background background use deuterated substances

■ Neutrons are scattered by the nuclei, X-b th l trays by the electrons

■ Light nuclei (e.g., H, C) are easier to locate in structures with heavy atoms bylocate in structures with heavy atoms by neutron diffraction

Neutron scattering densities for Neutron scattering densities for C6D6 adsorbed on ZSM5, (top) 4 mol/UC, (bottom) 8 mol/UC

Experimental setupElastic neutron scatteringElastic neutron scattering

Forschungsneutronenquelle Heinz Maier-Leibnitz (FRM II)(FRM II)

http://www.frm2.tum.de

Secondary SourcesFRM-IIFRM-II

Ø=243 mm, l=700 mm

− Thermal neutrons: from D2O moderator − Hot source: Block of graphite (20 cm diameter, 30 cm high) heated by theHot source: Block of graphite (20 cm diameter, 30 cm high) heated by the

gamma radiation to a temperature of ~ 2900 K. Spectrum from 100 meV to 1eV

− Cold source: Liquid D2 moderator (20 l) temperature about 25 K at a distanceCold source: Liquid D2 moderator (20 l) temperature about 25 K at a distance of 40 cm from the core axis. The cold neutron spectrum peaked around 5 meV

Neutron energies at the FRM-IIat the FRM-II

Energy of neutrons and their application in a research reactorresearch reactor. The size of the colorized area is proportional to the amount of neutrons available for the application.

Spallation source ISIS

X-Ray absorption spectroscopy X-Ray absorption edge

Absorption of X-ray's and promotion of a corelevel electron to continuum

X-Ray absorption near edge structureXANESXANES

X-Ray absorption near edge structureDensity of states (DOS)Density of states (DOS)

TiO2 Fe2O32 2 3

Extended X-ray absorption fine structureEXAFSEXAFS

Constructive(in phase)

Destructive(out of phase)

EXAFSSingle scattering plane wave approximationSingle scattering plane wave approximation

2 22( )( ) sin (2 ( )) kF k Nk kr k e σχ φ −⋅= + ⋅∑ 2( ) sin (2 ( ))k kr k e

r kχ φ= + ⋅

⋅∑

0.4 Due to φ(k) the

0.3

0.4 Due to φ(k) the distances are shifted to smaller values !!!

|FFT

0.1

0.2values !!!

outgoing electron wave

0 2 4 6 8 100.0backscattered electron wave

r[Å]Short range order

Cluster size and scattering contributions

0.08NiO i t l

0.080.080.08NiO i t l

0.06

NiO experimental

0.06

NiO experimental NiO 1 shell

0.06

NiO experimental NiO 1 shell NiO 2 shells

0.06

NiO experimental NiO 1 shell NiO 2 shells NiO 3 shells

0.06

NiO experimental NiO 1 shell NiO 2 shells NiO 3 shells NiO 7 shells

FT m

0 02

0.04

FT m

0.04

FT m

0.04

FT m

0.04

FT m

0.04

0 1 2 3 4 5 6 7 80.00

0.02

0 1 2 3 4 5 6 7 80.00

0.02

0 1 2 3 4 5 6 7 80.00

0.02

0 1 2 3 4 5 6 7 80.00

0.02

0 1 2 3 4 5 6 7 80.00

0.02

r[A]

0 1 2 3 4 5 6 7 8

r[A]

0 1 2 3 4 5 6 7 8

r[A]

0 1 2 3 4 5 6 7 8

r[A]

0 1 2 3 4 5 6 7 8

r[A]

0 1 2 3 4 5 6 7 8

Location of Zn2+ cations in zeolites

C f 2

2.1

Coordination sites for Zn2+

in zeolite Beta

2.1 2.0

■ 6-membered rings■ 5-membered rings

2.3 2. 3

2.5

3.4

2.0 2.0

■ 4-membered rings

3.0 2.3

3 4

2.0

3.4

Preferential location of Zn2+ in BEA

Sample N r

Z BEA

Zn- BEA 4.281.75

1.963.00

Zn-BEA

mag

Zn 6MR 42

1.973.43

ZnO

FFT Zn 5MR 3

22.302.77

Metall Zn 4MR 22

1.982.05

P f ti l l ti f Z 2+ t 6 MR iti

0 2 4 6 8r [Å]

Preferential location of Zn2+ at 6-MR positions

Bimetallic Ni-Rh catalysts (Ni-K edge)

NiO

FFT

Ni/HTC

NiRh/HTC Bimetallic catalyst 25% Ni, 0.89 wt% Rh

R (Å)0 1 2 3 4 5 6

Ni foil

R (Å)

Coordination parameters for Ni containing samples

sampleN R (Å) Δσ2(Å2) N R (Å) Δσ2(Å2)

Ni-O Ni-Ni

Ni foil 12 2.49NiO 6 2.07 12 2.94Ni/HTC 1.3 2.05 0.0081 9.4 2.49 0.0043NiRh/HTC 1.3 2.05 0.0095 9.2 2.48 0.0050

Bimetallic Ni-Rh catalysts (Rh-K edge)Rh2O3

Rh/HTC Bimetallic catalyst 25% Ni, 0.89 wt% Rh

FFT

NiRh/HTC

R (Å)0 1 2 3 4 5 6

Rh foil

R (Å)

Coordination parameters for Rh containing samples

Sample Rh-O Rh-Rh Rh-NiN R (Å) Δσ2(Å2) N R (Å) Δσ2(Å2) N R (Å) Δσ2(Å2)N R (Å) Δσ (Å ) N R (Å) Δσ (Å ) N R (Å) Δσ (Å )

Rh foil 12 2.68Rh2O3 6 2.06Rh/HTC 5.3 2.11 0.0087 2.5 2.68 0.0017NiRh/HTC 2.9 2.05 0.0029 0.1 2.61 0.0090 7.1 2.52 0.0041

X-Ray absorption near edge structure Electronic properties of d-metalsElectronic properties of d-metals

Fermi levelFermi level

5d5/2 5d 3/2Electron deficientElectron deficient

Pt particles

LIII

2p 3/2

IIL

LIII2p 1/2

Electron deficient particles show a higher peak above the absorption edge

Determination of oxidation state by XANES Characterization of S-species (XANES S K-edge)(XANES S K-edge)

XANES for ZnS, ZnSO3 and ZnSO4

Comparison EXAFS and XANESEXAFS and XANES

■ EXAFS Information level− Single scattering dominates− Mathematical description using

phase shifts and amplitudes form experiment or theory

− Structural environment• Number and kind of Neighbors• Distance• Disorderexperiment or theory

■ XANES

• Disorder

− Oxidation state− Electronic information

− Electronic transitions− Multiple scattering− Exact description based on

t h i l l l ti

Electronic information− DOS in the final state − Geometry, distortions

quantum-mechanical calculations− Interpretation of characteristic

spectral features using references (peak fitting PCA correlation(peak fitting, PCA, correlation spectroscopy)

Experimental setup XAS

Sample Cell

Reference

Slits

Ionization Chambers

Synchrotron Monochromator

The first accelerators (cyclotrons) were built by particle physicists in the 1930s. The nucleus of the atom was split using the collision of high energy particles. From the results of these collisions the physicists tried to deduce the laws of fundamentalresults of these collisions the physicists tried to deduce the laws of fundamental physics that govern our world and the whole of the universe.

Synchrotron radiation was seen for the first time at the General Electric in the USA inSynchrotron radiation was seen for the first time at the General Electric in the USA in 1947 in a different type of accelerator (synchrotron). It was first considered a nuisance because it caused the particles to lose energy, but in the 1960s exceptional properties as light source were recognized.

2 GeV20 mAPulslänge: 0.17 nsPulsbastand 20 nsRadius 15.3m

600 MeV20 maRadius 5 m

10-15 MeV20 mA

SRs Daresbry UK