Modern Methods in Heterogeneous Catalysis Research:
Theory and Experiment
Photons:
In situ spectroscopy in the soft X-ray energy range
Axel Knop-Gericke
knop@fhi-berlin.mpg.de
Outline
Photons for the investigation of heterogeneous catalytic processes
Synchrotron radiation
diffraction : single slit, double slit, grating
in situ XAS in the soft energy range
examples: methanol oxidation over copper
n-butane oxidation over VPO catalysts
Spectrum of electromagnetic radiation
Other lectures in this field:
UV-vis : A. Brückner 6.12.2002
Vibrational spectroscopy (IR spectroscopy, Raman:
Rupprechter 13.12.2002 )
G.
Electron spectroscopy: R. Schlögl 20.12.2002
X-ray diffraktion: I. Erran 10.1.2003
EXFAS/NEXAFS in the hard X-ray range:
17.1.2003
T.Ressler
Synchrotron radiation
Synchrotron radiation
Synchrotron radiation
Synchrotron radiation
Synchrotron radiation
Synchrotron radiation
Synchrotron radiation
Synchrotron radiation
Synchrotron radiation
Undulator
Synchrotron radiation
Synchrotron radiation
Single Slit Diffraction
Princip of Huygens
Minimum: g= k k=1,2,3,...<a/
sin k= g/a ; tan k =dk/l
Diffraction Pattern
Double Slit Diffraction
Expected diffraction pattern:
Actually observed
diffraction pattern
Double Slit Diffraction
l>>a : waves normals related to Pk on the screen are parallel,
sin k = g/a, tan k = dk/l
k < 8 : sin k = tan k = dk/l = g/a  dk= gl/a
Maximum (g= 0, , 2, 3,...) dk= kl/a k= 0,1,2,3..
Minimum (g= /2, 3/2 , 5/2 ,...) dk=(2k+1) l/2a k=0,1,2,3,..
From single slit to grating: Diffraction
pattern
XAS in the soft energy range
Soft energy range: 250 - 1000 eV
which elements: C(1s), N(1s), O(1s), transition metal (2p)....
•
L edge / 2p XPS peaks of transition metal are sensitive to details of
chemical
•XAS is a local process not restricted to material with long range order
•surface sensitive when applied in electron yield mode
pellets can be investigated under reaction conditions
•orientation of molecules on single crystal surfaces can be estimated
Spektroskopische Methode
Prinzip der
Röntgenabsorptionsspektroskopie (XAS)
Auger-Elektron
•Anregung von
Rumpfniveauelektronen mittels
Röntgenstrahlung
•Relaxation mittels
Fluoreszenzstrahlung oder durch
Aussenden eines AugerElektrons (TEY, PEY, AEY)
•Untersuchung der unbesetzten
Zustände
Fluoreszenz

I  f pi
2
Mean free path of electron in solids
Experimental Technique
In situ methods are required to investigate heterogeneous
catalytic reactions since the structure of a catalyst estimated
ex situ might differ from the structure revealed by in situ
studies
XAS in the soft energy
range represent surface
sensitive spectroscopic
methods, which can be
applied in the mbar
pressure range
Material gap
single crystal vs
real catalyst
Pressure gap
UHV vs p > 1bar
Transmission of 20 cm air
1.0
Transmission
0.8
0.6
0.4
0.2
0.0
0
2000
4000
6000
Energy (eV)
8000
10000
Experimental Set-Up
manipulator
properties of the
set-up
mass spektrometer
150 mm
butterfly-valve
•pressure up to 20 mbar
150 mm
•batch- and flow-through-mode
process pump
sample
gas inlet by MFC
UHV-valve
Plattenventil
turbo pump
f 100 mm
•heating up to 900 K
•angular dependent measurements
In situ XAS Detector system
Simultaneous detection
of gas phase- and sample
signal
Gas phase subtraction
Analysis of the Near Edge X-ray Absorption Fine Structure (NEXAFS)
O K-edge
2.5
*
530.8 eV
*
539.2 eV
541.4 eV
2
Intensity (a. u.)

NEXAFS of the O K-edge
O2
1.5
Idet
1
CH3OH + O2 Igas

537.3 eV
0.5
534.0 eV
0
Cu2O
520
CH3OH
( Idet- Igas )* 3
532.8 eV
540
Photon Energy / eV
560
• Total electron yield of the gas
phase dominates all signals,
therefore only small
differences in the detector
signals
•Substraction allows to
separate the absorption signal
of the surface of the catalyst
Methanol oxidation
2 CH3OH + O2
CH3OH
2 CH3OH + 3 O2
2 CH2O + 2 H2O oxidative dehydrogenation
CH2O + H2
dehydrogenation
2 CO2 + 4 H2O
total oxidation
Cu L3- NEXAFS
NEXAFS at the Cu L3-edge
Catalytic Activity
Conversion CH 3OH (%)
100
80
60
40
0.2
0.4
0.6
Flow ratio O
2
0.8
1
/ CH3OH
Increased activity for
gas flow ratios:
O2 / CH3OH  0.5
Transition from an
oxidic copper-phase to
the metallic state
Electronic Structure, Dept. AC, Fritz-Haber-Institut (MPG), Berlin, Germany
NEXAFS at the O K-edge
O K-edge
Flow ratio O2 / CH3OH
531.6 eV
Temperature
10
536.6 eV
0.6
• 2 oxidic- and 1
suboxidic species can be
distinguished
670 K
Total Electron Yield (a. u.)
•NEXAFS of the active
state is completly
different from the
NEXAFS of the known
copper-oxides
less active
8
0.2
670 K
6
0.2
570 K
4
2
Reference Cu2O
532.8 eV
0
300 K
520
530
540
550
Photon Energy / eV
560
570
very active
Correlation between
the SuboxideSpecies and CH2O
Variation of temperature at
O2 / CH3OH = 0.2
•Intensity of the suboxide
species increases with
increasing temperature
•Intensity of the suboxide
species is positively
correlated to the yield of
CH2O and CO
Correlations between oxidic species and CO2
•Intensity of the oxidic species Oxsurf decreases with increasing CO2-yield
•2 areas of activity can be distinguished
Model
Proposed model of the copper surface under reaction conditions for methanol oxidation
CO+ H2
CO2+ H2 O
CH3OH
CH3OH
CH3 OH
CH2O+ H2
Subox
Oxsurf
CH2O+ H2O
Cu 0
O+ O
lk
u
Ox b
Ovol
O2
n-Butane Oxidation to MA
by Vanadium Phosphorus Catalysts
O
+
3,5 O2
VPO
O
400 °C, 1 bar
+
4 H2O
O
1,5 Vol%
Maleic Anhydride (MA)
air
C4H10
+
6,5 O2
4 CO2
+
5 H2O
C4H10
+
4,5 O2
4 CO
+
5 H2O
Active phase: highly ordered vanadyl pyrophosphate (VO)2P2O7) ?
Electronic Structure, Dept. AC, Fritz-Haber-Institut (MPG), Berlin, Germany
The VPO V L3-NEXAFS
Total Electron Yield (norm. u.)
Analysis of spectral shape by unconstrained least squares fit
6
V5
V L3-edge
V valence
V6
V4
Details of the local
chemical bonding
4
V3
V2
2
Local geometric
structure
V7
V1
0
512
514
516
518
520
Photon Energy / eV
Electronic Structure, Dept. AC, Fritz-Haber-Institut (MPG), Berlin, Germany
Interpretation of V L3
NEXAFS
 NEXAFS resonances
appear in a sequence of
V-O bond lengths
3
2
Relative Resonance Position / eV
Relative Resonance Position / eV
Experimental finding:
*
V6
V5*
V4*
1
V4
3
V2O5
V3
2
V2
1
V1
0
V6
-1
1.6
1.8
2
2.2
2.4
2.6
2.8
Bond Length / Å
V3*
0
V2*
VPO
V1*
-1
1.6
1.8
2
2.2
2.4
Bond Length / †Å
2.6
2.8
Relative spectral Intensity of V5 at V L3-edge
RT
400°C
RT
400°C
Proportion of int. intensity of V5
0,55
Total Electron Yield (norm. u.)
Changes of NEXAFS
while heating
6
V L3-edge
4
RT
2
0
512
514
516
518
Photon Energy / eV
0,50
0,45
V5
0,40
0,35
0,30
Spectra number
Electronic Structure, Dept. AC, Fritz-Haber-Institut (MPG), Berlin, Germany
520
Interpretation of V L3
NEXAFS
Identification of resonances (V5, V6):
O(1a)
V2O5 as model substance for VPO
DFT calculation of DOS (V2O5 !)*:
O(3)
O(2)
V
V2O5: Close relationship between geometric
and electronic structure at V L3-absorption edge
O(3)
O(2)
O(1b)
 main contributions to
NEXAFS resonances
appear in a sequence of
V-O bond length
 V6: O(1a)
 V5: ? (estimated value of bond length
between O(2) and O(1a): 1.72 Å)
Electronic Structure, Dept. AC, Fritz-Haber-Institut (MPG), Berlin, Germany