Dr. Davide FerriEmpa, Lab. for Solid State Chemistry and Catalysis� 044 823 46 09� davide.ferri@empa.ch

UV-Vis spectroscopy

Basic theory

0 200 400 600 800 1000 1200

EPR

UV-Vis

XAFS

NMR

Raman

IR

Number of publications

Number of publications containing in situ, catalysis, and respective methodSource: ISI Web of Knowledge (Sept. 2008)

Importance of UV -Vis in catalysis

source: Andor.com

The electromagnetic spectrum

VISIBLE

1016-1017 Hz

ULTRAVIOLET

1015-1016 Hz

200

� Typically, the wavelength (nm) is used� the distance over which the wave's shape repeats

The ‘energy’ unit

x nm = 10’000’000 / x cm–1

y cm–1 = 10’000’000 / y nm

pros� economic� non-invasive (fiber optics allowed)� versatile (e.g. solid, liquid, gas)� extremely sensitive (concentration)

cons� Broad signals (resolution)� Time resolution (S/N)

Is UV-vis spectroscopy popular?

� 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

What is UV -vis spectroscopy?

e

e

e

e

n→σ*n→π* π→π* σ→σ*

bonding

anti-bonding

Electronic transitions

E= hν

e

e

e

e

σ*

π*

n

π

σ

n→σ*n→π* π→π* σ→σ*

bonding

anti-bonding

empty

occupied

lone pairs

hν

λ= c/ν

high ν→ low λhigh e- jump → high E

high E→ high ν

Organic molecule Organic molecule

e

e

e

e

σ*

π*

n

π

σ

n→σ*n→π* π→π* σ→σ*

bonding

anti-bonding

σ→σ*high E, low λ (<200 nm)

n→σ*150-250 nm, weak

n→π* 200-700 nm , weak

π→π*200-700 nm , intense

Electronic transitions

Condition to absorb light(200-800 nm):

π and/or n orbitals

CHROMOPHORE

1.0

0.8

0.6

0.4

0.2

0.0200 300280260240220

λmax217 nm

wavelength (nm)

abso

rban

ce

The UV spectrum

no visible light absorption

e

σ*

π*

n

π

σ

π→π*

signal envelope

ener

gy

vibrational electronic levels

rotational electronic levels

E0

E*How many signals do you expect from CH3-CH=O?

0.10

0.08

0.06

0.04

0.02

0.0200 300280260240220

wavelength (nm)

abso

rban

ce

The UV spectrum

no visible light absorption

e

σ*

π*

n (O)

π

σ

π→π*

e

n→π*

O

The UV spectrum

λmax λ ν E

171

217

258

delocalisation

e

π*

πe

e

� Conjugation effect

C2H4 C4H6 C6H8

The UV spectrum

� Conjugation effect: β-carotenewhite light

300 360 420 480 540

wavelength (nm)

abso

rban

ce

The UV spectrum

380 - 435

435 - 480

480 - 490500 - 560

580 - 595

650 - 780

If a colour is absorbed by white light, what the eye detects by mixing all other wavelengths is its complementary colour

� Complementary colours

Inorganic compounds

� UV-vis spectra of transition metal complexes originate from

� Electronic d-d transitions

� …

TM

degenerated-orbitals

+ ligand

TM

∆

eg

t2g

dσ

dπ

� Crystal field theory (CFT) - electrostatic model� same electronic structure of central ion as in isolated ion� perturbation only by negative charges of ligand

Inorganic compounds

tetrahedricfield

octahedricfield

tetragonal field

square planarfield

gaseous atom

atom in spherical field

∆

∆∆

dxy, dxz, dyz

dx2-y2, dz2

dx2-y2, dz2

dxy, dxz, dyz

dyz, dxz

dxy

dz2

dx2-y2

dx2-y2

dxy

dz2∆ = crystal field splitting

Inorganic compounds

� d-d transitions: 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

Cu2+

degenerated-orbitals

+ 6H2O∆

eg

t2g

Cu(H2O)62+

light

400 500 600 700 800 900 1000

wavelengths (nm)

abso

rban

ce

Inorganic compounds

Ni2+ Co2+

Ti3+

Cr3+

Cu2+

V4+

Fe2+

� TM(H2O)6n+

3d1

3d2

3d3

3d4

3d5

3d6

3d7

3d8

3d9

t2g1

t2g1

t2g3

t2g3eg

1

t2g3eg

2

t2g6eg

3

Ti(H2O)63+

Ti(H2O)63+

Cr(H2O)63+

Cr(H2O)62+

Mn(H2O)62+

Cu(H2O)62+

gaselec. config. TM

complex

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- < C2O4

2- < 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

� Charge transfer

TM

degenerated-orbitals

+ ligand

TM

∆

eg

t2g

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 ofelectronic charge is transferred between the molecularentities. 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*

3d

∆ = crystal field splitting

∆

4s

4p

AOLAOTM

MO(TML6n+)

σs

2s

σp

σd

σd*

σp*

σs*

πpx*, πpy

*, πpz*

πdxy, πdxz, πdxy

eg

t2g

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

� 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

back donation!!!

C

O

4σ

1π 1π3σ

2π∗2π∗

2π∗2π∗5σ

Metal

CO adsorption on precious metals

Dr. Davide FerriEmpa, Lab. for Solid State Chemistry and Catalysis� 044 823 46 09� davide.ferri@empa.ch

UV-Vis spectroscopy

InstrumentationExamples for catalysis

Instrumentation

double beam spectrometer

single beam spectrometer

� Dispersive instruments

Measurement geometry:- transmission- diffuse reflectance

In situ instrumentation

� Diffuse reflectance (DRUV) � Fiber optics

gas outlet

to detector

Weckhuysen, Chem. Commun. (2002) 97

- time resolution (CCD camera)[spectra colleted at once]

- coupling to reactors

- no NIR (no optical fiber > 1100 nm)- long term reproducibility (single beam)- Limited high temperature (ca. 600°C)

- 20% of light is collected- gas flows, pressure, vacuum

- long meas. time- spectral collection (λ after λ)→ different parts of spectrum do not represent same reaction time!!!

�

�

�

�

In situ instrumentation

� Integration sphere

- > 95% light is collected- high reflectivity- wide range of λ

- only homemade cells

White coated integration sphere(MgO, BaSO4, Spectralon®)

integrationsphere

Weckhuysen, Chem. Commun. (2002) 97

for example, for cat. synthesis

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

Examples

� Determination of oxidation state: 0.1 wt% Cr n+/Al2O3

reduction in CO atmosphereCr6+ (250, 370 nm)

Cr3+/Cr2+

� Determination of oxidation state: 0.1 wt% Cr n+/Al2O3

Examples

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

Cr6+

Cr5+

Cr3+

Cr2+

A B C D E F

%

0

50

100

distribution of Crn+

A: calc. 550°CB: red. 200°CC: red. 300°CD: red. 400°CE: red. 600°CF: re-calc. 550°C

calibration

deconvolution

Cr6+

Cr3+

� Determination of oxidation state: 0.2 wt% Cr n+/SiO2

Examples

chemometrics PCA, principal component analysisFA, factor analysisPLS, partial least squaresown routines…

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

* decomposition into pure components(including noise)

*

Examples

DRUV, 350°C, 2% isobutane-N 2

� Determination of oxidation state: 0.5 wt% Cr n+/SiO2

625

625360360

450

pure components

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

Puurunen et al., J. Catal. 210 (2002) 418

Examples

� Determination of oxidation state: 4 wt% Cr n+/Al2O3

10000200003000040000

wavenumber (cm-1)

K-M

inte

nsity

ex situ DRS

hydratedcalcined 550°Creduced 550°C

Cr6+

Cr3+ 26000

calc. 850°C, O 2HeC3H8, 21 secC3H8, 6 min

Cr6+, 26000 cm-1

C3H8 feed

O2 regeneration

20 scans, 50 msecin situ DRS

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

Examples

� Comparison of techniques: x wt% Cr n+/support

RamanxCr-Al2O3

xCr-Al2O3

10

5

10.5

wt.%

XRD

0 2 4 6 8 10 2 4 6 8 10

HAADF-STEM

1Cr-Al2O3 5Cr-Al2O3

1Cr-SBA15 5Cr-SBA15

energy (KeV) energy (KeV)

5Cr-SBA15 5Cr-Al2O3

Examples

V2O5: 996 cm-1

TiO2: 402 cm-1

Brückner et al., Catal. Today 113 (2006) 16

VxOy

air flow @ 450°CO2/C3H8 @ 20°C@ 100°C@ 150°C@ 200°C

V5+Ox

VxOy

V5+→ V4+

O OOO

O

V

� Reactivity of V/TiO2 after oxidative treatment

Examples

� Reactivity of V/TiO2 after oxidative treatment

UV-vis: V5+ CT (UV)V4+ d-d transitions (vis)

CT

d-d

V5+→ V4+

Brückner et al., Catal. Today 113 (2006) 16

Examples

Santhosh Kumar et al., J. Catal. 227 (2004) 384

� Determination of speciation: Fe species in Fe-ZSM5

α-Fe2O3

hydrated samplesisolated Fe3+

oligomeric FexOyextended F2O3-like clusters

calcination @ 600°C

calcined

hydrated