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Dr. Davide FerriEmpa, Lab. for Solid State Chemistry and Catalysis� 044 823 46 09� [email protected]
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
� 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
� 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� [email protected]
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