UV-Vis spectroscopy
Basic theory
Dr. Davide Ferri
Empa, Lab. for Solid State Chemistry and Catalysis
044 823 46 09
davide.ferri@empa.ch
Importance of UV-Vis in catalysis
IR
Raman
NMR
XAFS
UV-Vis
EPR
0
200
400
600
800
Number of publications
1000
1200
Number of publications containing in situ, catalysis, and respective method
Source: ISI Web of Knowledge (Sept. 2008)
The electromagnetic spectrum
VISIBLE
1015-1016 Hz
ULTRAVIOLET
1016-1017 Hz
200
source: Andor.com
The ‘energy’ unit
Typically, the wavelength (nm) is used
the distance over which the wave's shape repeats
x nm = 10’000’000 / x cm–1
y cm–1 = 10’000’000 / y nm
Is UV-vis spectroscopy popular?
pros
economic
non-invasive (fiber optics allowed)
versatile (e.g. solid, liquid, gas)
extremely sensitive (concentration)
cons
Broad signals (resolution)
Time resolution (S/N)
What is UV-vis spectroscopy?
Use of ultraviolet and visible radiation
Electron excitation to excited electronic level (electronic
transitions)
Identifies functional groups (-(C=C)n-, -C=O, -C=N, etc.)
Access to molecular structure and oxidation state
Electronic transitions
hν
Organic molecule
Organic molecule
anti-bonding
σ*
anti-bonding
empty
π*
lone pairs
e
occupied
n
e
e
e
π
e
bonding
e
bonding
e
σ
n→π* n→σ* π→π* σ→σ*
E= hν
high e- jump → high E
high E → high ν
e
n→π* n→σ* π→π* σ→σ*
λ= c/ν
high ν → low λ
Electronic transitions
anti-bonding
σ*
π*
e
n
e
π
e
bonding
e
σ
n→π* n→σ* π→π* σ→σ*
σ→σ*
high E, low λ (<200 nm)
n→σ*
150-250 nm, weak
n→π*
200-700 nm, weak
Condition to absorb light
(200-800 nm):
π and/or n orbitals
π→π*
200-700 nm, intense
CHROMOPHORE
The UV spectrum
λmax
1.0
σ*
n
e
π
σ
π→π*
0.8
absorbance
π*
217 nm
0.6
0.4
0.2
no visible light absorption
0.0
200
energy
signal envelope
E*
220
240
280
300
wavelength (nm)
vibrational
electronic levels
rotational
electronic levels
E0
260
How many signals do you
expect from CH3-CH=O?
The UV spectrum
σ*
0.10
π*
0.08
e
n (O)
e
π
σ
n→π* π→π*
absorbance
O
0.06
0.04
0.02
no visible light absorption
0.0
200
220
240
260
wavelength (nm)
280
300
The UV spectrum
Conjugation effect
λmax
delocalisation
λ
ν
171
217
258
C6H8
C4H6
C2H4
π*
e
π
e
e
E
The UV spectrum
Conjugation effect: β-carotene
white light
absorbance
300
360
420
wavelength (nm)
480
540
The UV spectrum
Complementary colours
650 - 780
580 - 595
500 - 560
380 - 435
435 - 480
480 - 490
If a colour is absorbed by white light, what the eye detects
by mixing all other wavelengths is its complementary
colour
Inorganic compounds
UV-vis spectra of transition metal complexes originate from
Electronic d-d transitions
dσ
eg
degenerate
d-orbitals
∆
+ ligand
TM
t2g
TM
dπ
…
Inorganic compounds
dx2-y2
Crystal field theory (CFT) - electrostatic model
same electronic structure of central ion as in isolated ion
perturbation only by negative charges of ligand
dx2-y2, dz2
dxy, dxz, dyz
∆
dz2
atom in
spherical field
∆
dx2-y2
∆
dxy
dx2-y2, dz2
dxy
gaseous atom
dxy, dxz, dyz
dyz, dxz
∆ = crystal field splitting
tetrahedric
field
octahedric
field
tetragonal
field
dz2
square planar
field
Inorganic compounds
d-d transitions: Cu(H2O)62+
eg
degenerate
d-orbitals
∆
+ 6H2O
light
Cu2+
t2g
Cu(H2O)62+
Yellow light is absorbed and the Cu2+ solution is coloured in blue (ca. 800 nm)
The greater ∆, the greater the E needed to promote the e-, and the shorter λ
∆ depends on the nature of ligand, ∆NH3 > ∆H2O
Inorganic compounds
TM(H2O)6n+
elec. config. TM
gas complex
t2g1
t2g1
t2g3
t2g3eg1
t2g3eg2
Ti(H2O)63+
Ti(H2O)63+
Cr(H2O)63+
Cr(H2O)62+
Mn(H2O)62+
t2g6eg3
Cu(H2O)62+
Ni2+
absorbance
3d1
3d2
3d3
3d4
3d5
3d6
3d7
3d8
3d9
Fe2+
Cu2+
Co2+
Cr3+
Ti3+
400
500
V4+
600
700
800
wavelengths (nm)
d-d transitions: εmax = 1 - 100 Lmol-1cm-1, weak
900
1000
Inorganic compounds
d-d transitions: factors governing magnitude of ∆
Oxidation state of metal ion
∆ increases with increasing ionic charge on metal ion
Nature of metal ion
∆ increases in the order 3d < 4d < 5d
Number and geometry of ligands
∆ for tetrahedral complexes is larger than for
octahedral ones
Nature of ligands
spectrochemical series
I- < Br- < S2- < SCN- < Cl- < NO3- < N3- < F- < OH- <
C2O42- < H2O < NCS- < CH3CN < py < NH3 < en <
bipy < phen < NO2- < PPh3 < CN- < CO
Inorganic compounds
d-d transitions: selection rules
spin rule:
∆S = 0
on promotion, no change of spin
Laporte‘s rule:
∆l = ±1
d-d transition of complexes with center of simmetry are forbidden
Because of selection rules, colours are faint (ε= 20 Lmol-1cm-1).
Inorganic compounds
UV-vis spectra of transition metal complexes originate from
Electronic d-d transitions
eg
degenerate
d-orbitals
∆
+ ligand
TM
t2g
TM
Charge transfer
Inorganic compounds
Charge transfer complex
no selection rules → intense colours (ε=50‘000 Lmol-1cm-1,
strong)
Association of 2 or more molecules in which a fraction of
electronic charge is transferred between the molecular
entities. The resulting electrostatic attraction provides a
stabilizing force for the molecular complex
Electron donor: source molecule
Electron acceptor: receiving species
CT much weaker than covalent forces
Ligand field theory (LFT), based on MO
Metal-to-ligand transfer (MLCT)
Ligand-to-metal transfer (LMCT)
Inorganic compounds
Ligand field theory (LFT)
involves AO of metal and ligand, therefore MO
what CFT indicates as possible electronic transitions (t2g→eg)
are now: πd→σdz2* or πd→ σdx2-y2*
πpx*, πpy*, πpz*
σp*
4p
σs *
4s
σd*
eg
∆
t2g
3d
πdxy, πdxz, πdxy
σd
AOL
AOTM
∆ = crystal field splitting
2s
σp
σs
MO(TML6n+)
Inorganic compounds
Ligand field theory (LFT)
LMCT
ligand with high energy lone pair
or, metal with low lying empty orbitals
high oxidation state (laso d0)
M-L strengthened
4σ
2π∗
O
1π
MLCT
ligands with low lying π* orbitals (CO, CN-, SCN-)
low oxidation state (high energy d orbitals)
M-L strengthened, π bond of L weakened
2π∗
2π∗
1π
3σ
C
5σ
2π∗
Metal
back donation!!!
CO adsorption on
precious metals
UV-Vis spectroscopy
Instrumentation
Examples for catalysis
Dr. Davide Ferri
Empa, Lab. for Solid State Chemistry and Catalysis
044 823 46 09
davide.ferri@empa.ch
Instrumentation
Dispersive instruments
Measurement geometry:
- transmission
- diffuse reflectance
double beam spectrometer
single beam spectrometer
In situ instrumentation
Diffuse reflectance (DRUV)
Fiber optics
to detector
gas outlet
- time resolution (CCD camera)
[spectra colleted at once]
- coupling to reactors
- 20% of light is collected
- gas flows, pressure, vacuum
- long meas. time
- no NIR (no optical fiber > 1100 nm)
- long term reproducibility (single beam)
- Limited high temperature (ca. 600°C)
- spectral collection (λ after λ)
→ different parts of spectrum do not represent same reaction time!!!
Weckhuysen, Chem. Commun. (2002) 97
In situ instrumentation
Integration sphere
White coated integration sphere
(MgO, BaSO4, Spectralon®)
integration
sphere
- > 95% light is collected
- high reflectivity
- wide range of λ
- only homemade cells
for example, for cat. synthesis
Weckhuysen, Chem. Commun. (2002) 97
Examples
Determination of oxidation state: 0.1 wt% Crn+/Al2O3
Cr6+ (250, 370 nm)
reduction in CO atmosphere
Cr3+/Cr2+
Weckhuysen et al., Catal. Today 49 (1999) 441
Examples
Determination of oxidation state: 0.1 wt% Crn+/Al2O3
calibration
distribution of Crn+
Cr3+
100
Cr6+
%
Cr6+
Cr5+
Cr3+
Cr2+
50
deconvolution
0
A
B
C
A: calc. 550°C
B: red. 200°C
C: red. 300°C
D: red. 400°C
E: red. 600°C
F: re-calc. 550°C
Weckhuysen et al., Catal. Today 49 (1999) 441
D
E
F
Examples
Determination of oxidation state: 0.2 wt% Crn+/SiO2
chemometrics * PCA, principal component analysis
FA, factor analysis
PLS, partial least squares
own routines…
* decomposition into pure components
(including noise)
Weckhuysen et al., Catal. Today 49 (1999) 441
Examples
Determination of oxidation state: 0.5 wt% Crn+/SiO2
pure components
DRUV, 350°C, 2% isobutane-N 2
360
625
450
Weckhuysen et al., Catal. Today 49 (1999) 441
360
625
Examples
Determination of oxidation state: 4 wt% Crn+/Al2O3
calc. 850°C, O 2
He
C3H8, 21 sec
C3H8, 6 min
in situ DRS
20 scans, 50 msec
ex situ DRS
K-M intensity
Cr6+
Cr3+
hydrated
calcined 550°C
reduced 550°C
26000
Cr6+, 26000 cm-1
C3H8 feed
40000
30000
20000
10000
wavenumber (cm-1)
O2 regeneration
Puurunen et al., J. Catal. 210 (2002) 418
Examples
Comparison of techniques: x wt% Crn+/support
HAADF-STEM
xCr-Al2O3
Raman
wt.%
10
5
5Cr-SBA15
1Cr-SBA15
1
0.5
1Cr-Al2O3
5Cr-Al2O3
5Cr-SBA15
0
2
4
6
8
energy (KeV)
XRD
5Cr-Al2O3
10
2
4
6
8
energy (KeV)
Santhosh Kumar et al., J. Catal. 261 (2009) 116
10
xCr-Al2O3
Examples
Reactivity of V/TiO2 after oxidative treatment
V5+Ox
VxOy
VxOy
air flow @ 450°C
O2/C3H8 @ 20°C
@ 100°C
@ 150°C
@ 200°C
V5+ → V4+
O
O
O
V
O
O
TiO2: 402 cm-1
V2O5: 996 cm-1
Brückner et al., Catal. Today 113 (2006) 16
Examples
Reactivity of V/TiO2 after oxidative treatment
UV-vis: V5+ CT (UV)
V4+ d-d transitions (vis)
V5+ → V4+
d-d
CT
Brückner et al., Catal. Today 113 (2006) 16
Examples
Determination of speciation: Fe species in Fe-ZSM5
hydrated samples
isolated Fe3+
oligomeric FexOy
extended F2O3-like clusters
calcination @ 600°C
α-Fe2O3
calcined
hydrated
Santhosh Kumar et al., J. Catal. 227 (2004) 384