Why XAS? What can we learn from XAS?

Transcription

Why XAS? What can we learn from XAS?
Why XAS?
What can we learn from XAS?
• Element specific
• Metal valence during synthesis and reaction
• Sensitive to low concentrations
• Metal coordination
(0.01-0.1 %)
• Applicable under extreme conditions
(high-pressure, high temperature, operando)
• Applicable to gasses, liquids and solids
(+ surfaces, buried interfaces, impurities, etc.)
Binuclear centers, (very small) cluster sizes
• Metal site symmetry
• d-band occupation
(3d, 4d or 5d; metal versus oxide, valence)
• 20 nm microscopy, 50 ps time-resolved
• Local geometric information
• Local electronic information
2p edges
3d
Normalized Intensity
1
4
3d
2
3d
3
3d
4
3d
2
3d
6
3d
1
3d
8
3d
3
5
CrIII in Al2O3
SrTiO3
0
Calculated pre-edges
3dN→2p53dN+1
• Single configuration
• Atomic values for
intra-atomic interactions
• Cubic crystal field of 1.2 eV
7
3d
0
9
MnO
1
q
PRB 42,
5459 (1990)
Normalised Intensity
5
Metal L edges
0
-6 -4 -2 0 2 4 6 8 10 12 14 16
635
640
645
650
Energy (eV)
Relative Energy (eV)
Fe/ZSM5
• proposed for selective oxidation of hydrocarbons
¾ Soft X-ray absorption
• proposed for deNOx
in Heterogeneous Catalysis
• soft X-ray absorption
• STEM-EELS
Cat. Today
100, 228 (2005)
1
Soft X-rays: bulk and surface probe
Fe/ZSM5
Soft X-rays:
Penetration depth:
Fluorescence yield:
Electron yield (escape depth):
100 – 1000 eV
10 µm
> 10 µm
4 nm
+ 4. 5 V
InIn-situ Soft XX-ray Absorption
InIn-situ Soft XX-ray Absorption
manipulator
150 m m
ter
hea
mass spectrometer
Fe2O3
butterfly-valve
hυ
150 mm
sample
foreline pump
gas inlet from MFC
Fe/ZSM5 (O2)
Plattenventil
UHV
gate valve
turbo pump
φ 100 mm
J. Phys. Chem. B.
107, 13069 (2003)
Redox behavior of Fe/ZSM-5
J. Phys. Chem. B.
107, 13069 (2003)
Redox behavior of Fe/ZSM-5
J. Phys. Chem. B.
107, 13069 (2003)
2
Conclusions Fe/ZSM5
TEMTEM-EELS of calcined Fe/ZSM5
1.0
fe4
1. Fe/ZSM5 in O2: FeIII, octahedral
Y Axis Title
0.8
2. Weaker Fe-O bonding than Fe2O3: 10Dq ~ 1.0 eV
0.6
0.4
3. Fast auto-reduction in He at 25 °C
0.2
0.0
700
705
710
715
720
725
730
X Axis Title
4. Reversible oxidation/reduction at 350 °C.
B
>> Extremely active iron redox system
180000
Y Axis Title
160000
5. Calcined Fe/ZSM5 has Fe2O3 nanoparticles
140000
120000
100000
80000
60000
530
535
540
545
550
555
560
565
X Axis Title
570
Cat. Today
100, 228 (2005)
5. No spectra for 0.25 wt% FeZSM5
Cat. Today
100, 228 (2005)
InIn-situ XX-ray Absorption in Catalysis
Co/TiO2 FischerFischer-Tropsch catalysts
Production of hydrocarbons
from CO + H2 (Syngas)
ƒ 1926: Fischer and Tropsch
Charcoal
Gas
Biomass
• used for Fischer-Tropsch synthesis
• transformation of CO + H2 to hydrocarbons
Partial oxidation
Steam reforming
Co-based catalyst
Syngas
(Clean liquid)
Synfuels
220°C / ↑ pressure
• 5B$ plant being build at Qatar
• make clean fuels from natural gas or biomass
• Study Mn-doped Co/TiO2 surface
• soft X-ray absorption
• Follow Co and Mn L2,3 edge
• STEM-EELS
• Reduction:
temperature from RT to 425 º
under 5 mbar H2
(Fernando Morales, UU)
Co/TiO2 FischerFischer-Tropsch catalysts
Co/TiO2 FischerFischer-Tropsch catalysts
MnOx
HDP-CoMn/TiO2 catalyst
Bulk Co3O4
L3
Co0
CoO
425°C
425°C
385°C
350°C
350°C
300°C
300°C
250°C
250°C
Co3O4
Bulk Co3O4 reduces to pure Co0
RT
RT
Co/TiO2 catalysts reduce to CoO + Co0
Cobalt average valency
3
Co3O4
Co3O4
2.5
Co3O4
TiO2
CoO
2
Reduction
1.5
δ+
MnO
1
Co0
Bulk Co3O4
0.5
HDP-CoMn
Co0
0
0
100
200
300
400
Reduction temperature (Celsius)
500
Co0
TiO2
Mn promoted catalyst
Mn is preventing CoO reduction
to Co0
3
Co/TiO2 FischerFischer-Tropsch catalysts
Co/TiO2 FischerFischer-Tropsch catalysts
Quantify EELS spectra of Ti, Mn and Co
10nm
Co/TiO2 FischerFischer-Tropsch catalysts
Co/TiO2 FischerFischer-Tropsch catalysts
Before reduction
(Co,Mn)3O4 particles
After reduction
Co metal +
MnO particles
PCCP 7, 568 (2005)
J. Catal.
Catal. 230, 310 (2005)
Transmission X-ray Microscopy
icroscopy (TXM)
Transmission X-ray Microscopy
icroscopy (TXM)
20 nm
soon 10 nm
200 nm
1
1 bar
250°
250°C
0.2 eV
0
780
785
790
Energy (eV)
795
4
Co/TiO2 FischerFischer-Tropsch catalysts
STEMSTEM-EELS
soft XAS
UHV
Co/TiO2 FischerFischer-Tropsch catalysts
STEMSTEM-EELS
10 mbar
0.5 nm / 0.3 eV
Bulk / 0.2 eV
TXM
1 atm / 450 °C
UHV
15x15 nm / 0.2 eV
0.5 nm / 0.3 eV
1
10 nm
10 nm
Conclusions Co/TiO2
0
780
785
790
Energy (eV)
795
Charge Transfer in EELS
CuIII
1. TiO2 and Mn prevent complete reduction
30% 3d8
2. Mn and Co segregate (depending on preparation
method)
3. TXM-XAS allows to track single particle during
1A
1
3d8
∆<0
catalysis
30% 3d8
3d9L
3A
2
Chem. Phys. Lett.
Lett. 297, 321 (1998)
EELS and Electronic Structure
• Directional variations of 3d-electrons:
symmetry: 2S+1LJ, crystal field, charge transfer
interplay spin-orbit, exchange, distortions
• Hybridization of correlated electrons
DFT + Multiplet approaches
FeIII(tacn)2: Ground state: 3d5 + 3d6L
Energy of 3d6L: Charge transfer energy ∆
3d6L
superconductivity, magnetism, Kondo
mixed valence, orbital orderings
∆
3d5
2p53d7L
∆+U-Q ≈ ∆
2p53d6
5
DFT + Multiplet approaches
DFT + Multiplet approaches
¾ Projection of charge transfer simulation to ‘constituents’
FeIII(tacn)2: Ground state: 3d5 + 3d6L
Different mixing for different symmetries
¾ Comparison to DOS from DFT calculation
FeIII(tacn)2
• low-spin 3d5 [t2g5]
• admixture of 3d6L [t2g5eg1] (eg mixing)
• admixture of 3d6L [t2g6] (t2g mixing)
JACS 125,
12894 (2003)
JACS 125,
12894 (2003)
LigandLigand-metal Charge Transfer
DFT + Multiplet approaches
¾ Projection of charge transfer simulation to ‘constituents’
FeIII: Ground state: 3d5 + 3d6L
¾ Comparison to DOS from DFT (ADF) calculations
M
C
filled σfilled
emptyempty
d or p
3d6L
2p53d7L
∆
∆+U-Q ≈ ∆
2p53d6
3d5
JACS 125,
12894 (2003)
JACS, 128,
10442 (2006)
π-bonding
+ MetalMetal-ligand Charge Transfer
FeIII: Ground state: 3d5 + 3d6L + 3d4L
3d4L
∆π-U+Q ≈ ∆π + 2
∆π
3d6L
∆
3d5
2p53d7L
∆+U-Q ≈ ∆ - 2
2p53d6
JACS, 128,
10442 (2006)
FeIII(tacn)2
10
Normalized Absorption
2p53d5L
N
C
M
N
8
Fit X
Series2
FeIII(CN)6
6
4
2
0
700
-2
705
710
715
Energy (eV)
720
725
730
JACS, 128,
10442 (2006)
6
Conclusions: Soft XX-ray absorption
Conclusions: Metal L edges
8
t2g
Fe L edge of FeIII(CN)6
eg
?
eg
LS+
D3D
?
0
705
710
715
720
Energy (eV)
725
low-spin FeIII
3d5 [2T2]
t2g one hole
eg empty
¾
¾
¾
¾
¾
DFT works well for oxygen K edge
LFM works well for metal L edge
CTM needed for XPS
CTM needed for Cu3+
Add MLCT for π-bonding
JACS, 128,
10442 (2006)
7