UV-Vis spectroscopy Basic theory

(1)

Dr. Davide Ferri

Empa, Lab. for Solid State Chemistry and Catalysis

044 823 46 09

[email protected]

UV-Vis spectroscopy

(2)

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 method

Source: ISI Web of Knowledge (Sept. 2008)

(3)

source: Andor.com

The electromagnetic spectrum

VISIBLE

1016_-1017 _Hz

ULTRAVIOLET

1015_-1016_Hz

(4)

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}

(5)

pros

economic

non-invasive (fiber optics allowed)

versatile (e.g. solid, liquid, gas)

extremely sensitive (concentration)

cons

Broad signals (resolution)

Time resolution (S/N)

(6)

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

(7)

e e e e n→σ_* n→π_* π→π_* σ→σ_* bonding anti-bonding

Electronic transitions

E= h

ν

e e e e σ_* π_* n π σ n→σ_* n→π_* π→π_* σ→σ_* bonding anti-bonding empty occupied lone pairs h

ν

λ

= c/

ν

high

ν

_→

low

λ

high e

-

_jump

→

high E

_→

high

ν

(8)

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}

(9)

1.0 0.8 0.6 0.4 0.2 0.0 200 220 240 260 280 300 λmax 217 nm wavelength (nm) a b s o rb a n c e

The UV spectrum

no visible light absorption

e σ_* π_* n π σ π→π_* signal envelope e n e rg

y vibrational _{electronic levels}

rotational

electronic levels

E

₀

E*

How many signals do you

expect from CH

₃

-CH=O?

(10)

0.10 0.08 0.06 0.04 0.02 0.0 200 220 240 260 280 300 wavelength (nm) a b s o rb a n c e

The UV spectrum

no visible light absorption

e σ_* π_* n (O) π σ π→π_* e n→π_*

O

(11)

The UV spectrum

λ

max

λ

ν

E

171

217

258 delocalisation

e

π

_*

π

e e

Conjugation effect

C

₂

H

₄

C

₄

H

₆

C

₆

H

₈

(12)

The UV spectrum

Conjugation effect:

β

-carotene

white light 300 360 420 480 540 wavelength (nm) a b s o rb a n c e

(13)

The UV spectrum

380 - 435 435 - 480 480 - 490 500 - 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

(14)

Inorganic compounds

UV-vis spectra of transition metal complexes originate from

Electronic d-d transitions

…

TM degenerate d-orbitals + ligand TM ∆ e_g t_2g dσ dπ

(15)

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

tetrahedric field octahedric field tetragonal field square planar field gaseous atom atom in spherical field ∆ ∆ ∆ d_xy, d_xz, d_yz dx2-y2, dz2 d_x2-y2, d_z2 dxy, dxz, dyz dyz, dxz d_xy dz2 dx2-y2 d_x2-y2 dxy dz2

(16)

Inorganic compounds

d-d transitions: Cu(H

2

O)

62+

Yellow light is absorbed and the Cu

2+

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+ degenerate d-orbitals + 6H₂O ∆ e_g t_2g Cu(H₂O)₆2+ light

(17)

400 500 600 700 800 900 1000 wavelengths (nm) a b s o rb a n c e

Inorganic compounds

Ni

2+

_Co

2+

Ti

3+

Cr

3+

Cu

2+

V

4+

Fe

2+

TM(H

2

O)

6n+ 3d1 3d2 3d3 3d4 3d5 3d6 3d7 3d8 3d9 t_2g1 t_2g1 t_2g3 t_2g3_e g1 t_2g3_e g2 t_2g6_e g3 Ti(H₂O)₆3+ Ti(H₂O)₆3+ Cr(H₂O)₆3+ Cr(H₂O)₆2+ Mn(H₂O)₆2+ Cu(H₂O)₆2+

gas

elec. config. TM

complex

d-d transitions:

ε

(18)

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}

-

_{< S}

2-

_{< SCN}

-

_{< Cl}

-

_{< NO}

3-

< N

3-

< F

-

< OH

-

<

C

₂

O

₄2-

_{< H}

2

O < NCS

-

< CH

3

CN < py < NH

3

< en <

bipy < phen < NO

₂-

_{< PPh}

3

< CN

-

< CO

(19)

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

(20)

Inorganic compounds

UV-vis spectra of transition metal complexes originate from

Electronic d-d transitions

Charge transfer

TM degenerate d-orbitals + ligand TM ∆ e_g t_2g

(21)

Inorganic compounds

Charge transfer complex

no selection rules

→

intense colours (

ε

=50‘000 Lmol

-1

cm

-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)

(22)

Inorganic compounds

Ligand field theory (LFT)

involves AO of metal and ligand, therefore MO

what CFT indicates as possible electronic transitions (t

_2g

→

e

_g

)

are now:

π

d

→σ

dz2*

or

π

d

→ σ

dx2-y2*

3d

∆ _{= crystal field splitting}

∆ 4s 4p

AO

_L

AO

_TM

MO(TML

₆n+

₎

σ_s 2s σ_p σ_d σ_d* σ_p* σ_s* π_px*, π_py*, π_pz* π_dxy, π_dxz, π_dxy e_g t_2g

(23)

Inorganic compounds

Ligand field theory (LFT)

LMCT

ligand with high energy lone pair

or, metal with low lying empty orbitals

high oxidation state (laso d

0

)

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 1π 3σ 2π∗ 2π∗ 2π∗ 2π∗ 5σ Metal

CO adsorption on

precious metals

(24)

Dr. Davide Ferri

Empa, Lab. for Solid State Chemistry and Catalysis

044 823 46 09

[email protected]

UV-Vis spectroscopy

Instrumentation

(25)

Instrumentation

double beam spectrometer

single beam spectrometer

Dispersive instruments

Measurement geometry:

- transmission

(26)

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!!!

(27)

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®)

integration sphere

Weckhuysen, Chem. Commun. (2002) 97

(28)

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

Examples

Determination of oxidation state: 0.1 wt% Cr

n+

/Al

₂

O

₃

reduction in CO atmosphere

Cr

6+

_{(250, 370 nm)}

(29)

Determination of oxidation state: 0.1 wt% Cr

n+

/Al

₂

O

₃

Examples

Cr6+ Cr5+ Cr3+ Cr2+

A

B

C

D

E

F

%

0 50 100

distribution of Cr

n+ 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

calibration

deconvolution

Cr

6+

Cr

3+

(30)

Determination of oxidation state: 0.2 wt% Cr

n+

/SiO

₂

Examples

chemometrics

PCA, principal component analysis FA, factor analysis

PLS, partial least squares own routines…

* decomposition into pure components (including noise)

(31)

Examples

DRUV, 350°C, 2% isobutane-N₂

Determination of oxidation state: 0.5 wt% Cr

n+

/SiO

₂

625 625 360 360 450 pure components

(32)

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

Examples

Determination of oxidation state: 4 wt% Cr

n+

/Al

₂

O

₃

10000 20000 30000 40000 wavenumber (cm-1₎ K -M i n te n s it y ex situ DRS hydrated calcined 550°C reduced 550°C

Cr

6+

Cr

3+ 26000 calc. 850°C, O₂ He C₃H₈, 21 sec C₃H₈, 6 min Cr6+_{, 26000 cm}-1 C3H8feed O₂regeneration 20 scans, 50 msec in situ DRS

(33)

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

Examples

Comparison of techniques: x wt% Cr

n+

/support

Raman xCr-Al2O3 xCr-Al2O3 10 5 1 0.5 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-Al₂_O₃

(34)

Examples

V₂O₅: 996 cm-1

TiO₂: 402 cm-1

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

V_xO_y air flow @ 450°C O2/C3H8 @ 20°C @ 100°C @ 150°C @ 200°C V5+_O x V_xO_y

V

5+

→

V

4+ O O O O O V

(35)

Examples

Reactivity of V/TiO

₂

after oxidative treatment

UV-vis: V

5+

_{CT (UV)}

V

4+

_{d-d transitions (vis)}

CT

d-d

V

5+

→

V

4+

(36)

Examples

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

Determination of speciation: Fe species in Fe-ZSM5

α_-Fe

2O3 hydrated samples

isolated Fe3+

oligomeric Fe_xO_y

extended F₂O₃-like clusters

calcination @ 600°C

calcined