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 1000 1200

Number of publications

Number of publications containing in situ, catalysis, and respective method

Source: ISI Web of Knowledge (Sept. 2008)
The electromagnetic spectrum

VISIBLE ULTRAVIOLET

1015-1016 Hz 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 anti-bonding
σ*
empty
π*
lone pairs e e n e e
occupied e π e
bonding bonding
e σ e

n→π* n→σ* π→π* σ→σ* n→π* n→σ* π→π* σ→σ*

E= hν λ= c/ν

high e- jump → high E

high E → high ν high ν → low λ
 Electronic transitions

anti-bonding
σ*

π*
e e n
e π
bonding
e σ
n→π* n→σ* π→π* σ→σ*

σ→σ* n→π* Condition to absorb light

high E, low λ (<200 nm) 200-700 nm, weak (200-800 nm):

n→σ* π and/or n orbitals

150-250 nm, weak π→π*
200-700 nm, intense CHROMOPHORE
 The UV spectrum
1.0 λmax
217 nm

σ* 0.8

π*

absorbance
0.6
n
0.4
e π
0.2
σ
0.0 no visible light absorption
π→π*
200 220 240 260 280 300
wavelength (nm)
signal envelope

vibrational
E*
energy

electronic levels
How many signals do you
expect from CH3-CH=O?
rotational
electronic levels
E0
 The UV spectrum

σ* 0.10
O
π* 0.08

absorbance
e n (O) 0.06

e π 0.04

0.02
σ
0.0 no visible light absorption
n→π* π→π*
200 220 240 260 280 300
wavelength (nm)
 The UV spectrum
 Conjugation effect
delocalisation λmax λ ν E

171

217

258

C2H4 C4H6 C6H8

π*

e
e
e π
 The UV spectrum
 Conjugation effect: β-carotene
white light

absorbance

300 360 420 480 540

wavelength (nm)
 The UV spectrum
 Complementary colours

650 - 780 380 - 435

580 - 595 435 - 480

500 - 560 480 - 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
dx2-y2

dxy, dxz, dyz

dz2
∆
atom in
spherical field
∆ ∆

dx2-y2, dz2 dxy

dxy
gaseous atom
dxy, dxz, dyz

dyz, dxz
∆ = crystal field splitting dz2

tetrahedric octahedric tetragonal square planar

field field field field
 Inorganic compounds
 d-d transitions: Cu(H2O)62+
eg
degenerate
d-orbitals

∆ light
+ 6H2O

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 Cu2+ Fe2+
3d1 t2g1 Ti(H2O)63+ Ni2+ Co2+
3d2 t2g1 Ti(H2O)63+

absorbance
3d3 t2g3 Cr(H2O)63+
3d4 t2g3eg1 Cr(H2O)62+ Cr3+
3d5 t2g3eg2 Mn(H2O)62+
Ti3+ V4+
3d6
3d7
3d8
3d9 t2g6eg3 Cu(H2O)62+

400 500 600 700 800 900 1000

wavelengths (nm)

d-d transitions: εmax = 1 - 100 Lmol-1cm-1, weak

 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 *

σd*
4s eg
∆
t2g
πdxy, πdxz, πdxy 2s
3d
σd

AOTM AOL
σp
∆ = crystal field splitting
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) 4σ

M-L strengthened 2π∗ 2π∗
O
1π 1π
3σ
C

MLCT
2π∗ 5σ 2π∗

ligands with low lying π* orbitals (CO, CN-, SCN-)

low oxidation state (high energy d orbitals)
Metal

M-L strengthened, π bond of L weakened
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

 - no NIR (no optical fiber > 1100 nm)
- gas flows, pressure, vacuum
- long term reproducibility (single beam)
- Limited high temperature (ca. 600°C)
 - long meas. time
- 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
- > 95% light is collected
sphere
- 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

Cr3+ distribution of Crn+

100

Cr6+
Cr6+
% 50
Cr5+
Cr3+
Cr2+

deconvolution
0
A B C D E F
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

 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

360 360 625

625

450

Weckhuysen et al., Catal. Today 49 (1999) 441

 Examples
 Determination of oxidation state: 4 wt% Crn+/Al2O3
20 scans, 50 msec
in situ DRS
calc. 850°C, O 2
He
C3H8, 21 sec
C3H8, 6 min

ex situ DRS
Cr6+
hydrated
K-M intensity

calcined 550°C 26000

Cr3+ reduced 550°C

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

1Cr-SBA15 5Cr-SBA15 5

1
0.5

1Cr-Al2O3 5Cr-Al2O3

5Cr-SBA15 5Cr-Al2O3
XRD

0 2 4 6 8 10 2 4 6 8 10
xCr-Al2O3
energy (KeV) energy (KeV)

Santhosh Kumar et al., J. Catal. 261 (2009) 116

 Examples
 Reactivity of V/TiO2 after oxidative treatment
V5+Ox

VxOy VxOy

air flow @ 450°C

O2/C3H8 @ 20°C V5+ → V4+
@ 100°C
@ 150°C O
@ 200°C
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