UV-Vis spectroscopy Basic theory

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 10 15-10 16 Hz 10 16-10 17 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 Organic molecule hν Organic molecule empty anti-bonding σ* π* anti-bonding lone pairs occupied e bonding e n π* n σ* π π* σ σ* e e n π σ e bonding e n π* n σ* π π* σ σ* e e E= hν high e - jump high E high E high ν λ= c/ν high ν low λ

Electronic transitions anti-bonding σ* e e π* n bonding n π* n σ* π π* σ σ* e e π σ 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 Condition to absorb light (200-800 nm): π and/or n orbitals CHROMOPHORE

The UV spectrum σ* π* n e π σ π π* signal envelope absorbance 1.0 0.8 0.6 0.4 0.2 λ max 217 nm 0.0 200 220 240 260 280 300 wavelength (nm) no visible light absorption energy E* vibrational electronic levels rotational electronic levels How many signals do you expect from CH 3 -CH=O? E 0

The UV spectrum e e σ* π* n (O) π absorbance 0.10 0.08 0.06 0.04 O σ 0.02 n π* π π* 0.0 no visible light absorption 200 220 240 260 280 300 wavelength (nm)

Conjugation effect The UV spectrum delocalisation λ max λ ν E 171 217 258 C 2 H 4 C 4 H 6 C 6 H 8 π* e π e e

The UV spectrum Conjugation effect: β-carotene white light absorbance 300 360 420 480 540 wavelength (nm)

Complementary colours The UV spectrum 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 d σ e g degenerate d-orbitals + ligand TM TM t 2g d π

Inorganic compounds Crystal field theory (CFT) - electrostatic model same electronic structure of central ion as in isolated ion perturbation only by negative charges of ligand d x2-y2 d x2-y2, d z2 d x2-y2 atom in spherical field d xy, d xz, d yz d z2 d x2-y2, d z2 d xy gaseous atom d xy d xy, d xz, d yz = crystal field splitting d yz, d xz d z2 tetrahedric field octahedric field tetragonal field square planar field

Inorganic compounds d-d transitions: Cu(H 2 O) 6 2+ degenerate d-orbitals e g + 6H 2 O light Cu 2+ 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 t 2g Cu(H 2 O) 6 2+

Inorganic compounds TM(H 2 O) 6 n+ elec. config. TM gas complex Cu 2+ Fe 2+ 3d 1 3d 2 3d 3 3d 4 3d 5 3d 6 3d 7 3d 8 3d 9 t 2g 1 t 2g 1 t 2g 3 t 2g3 e g 1 t 2g3 e g 2 t 2g6 e g 3 Ti(H 2 O) 6 3+ Ti(H 2 O) 6 3+ Cr(H 2 O) 6 3+ Cr(H 2 O) 6 2+ Mn(H 2 O) 6 2+ Cu(H 2 O) 6 2+ absorbance Ni 2+ Co 2+ Ti 3+ Cr 3+ V 4+ 400 500 600 700 800 900 1000 wavelengths (nm) d-d transitions: ε max = 1-100 Lmol -1 cm -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 - < S 2- < SCN - < Cl - < NO 3- < N 3- < F - < OH - < C 2 O 4 2- < H 2 O < NCS - < CH 3 CN < py < NH 3 < en < bipy < phen < NO 2- < PPh 3 < 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 -1 cm -1 ).

Inorganic compounds UV-vis spectra of transition metal complexes originate from Electronic d-d transitions e g degenerate d-orbitals + ligand TM TM t 2g Charge transfer

Charge transfer complex Inorganic compounds 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)

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 * π px*, π py*, π pz * 4p 4s e g σ * p σ * s σ * d 3d AO TM σ d t 2g π dxy, π dxz, π dxy 2s AO L = crystal field splitting σ p σ s MO(TML 6 n+ )

Ligand field theory (LFT) Inorganic compounds 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 4σ 2π O 2π 1π 1π 3σ C 2π 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

Dispersive instruments Measurement geometry: - transmission - diffuse reflectance Instrumentation double beam spectrometer single beam spectrometer

In situ instrumentation Diffuse reflectance (DRUV) Fiber optics to detector - 20% of light is collected - gas flows, pressure, vacuum gas outlet - 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) - long meas. time - spectral collection (λ after λ) different parts of spectrum do not represent same reaction time!!! Weckhuysen, Chem. Commun. (2002) 97

Integration sphere In situ instrumentation White coated integration sphere (MgO, BaSO 4, Spectralon ) integration sphere - > 95% light is collected - high reflectivity - wide range ofλ - only homemade cells for example, for cat. synthesis Weckhuysen, Chem. Commun. (2002) 97

Examples Determination of oxidation state: 0.1 wt% Cr n+ /Al 2 O 3 Cr 6+ (250, 370 nm) Cr 3+ /Cr 2+ reduction in CO atmosphere Weckhuysen et al., Catal. Today 49 (1999) 441

Examples Determination of oxidation state: 0.1 wt% Cr n+ /Al 2 O 3 calibration Cr 3+ 100 distribution of Cr n+ Cr 6+ % 50 Cr 6+ Cr 5+ Cr 3+ Cr 2+ 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% Cr n+ /SiO 2 * 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% Cr n+ /SiO 2 DRUV, 350 C, 2% isobutane-n 2 pure components 360 360 625 625 450 Weckhuysen et al., Catal. Today 49 (1999) 441

Examples Determination of oxidation state: 4 wt% Cr n+ /Al 2 O 3 calc. 850 C, O 2 He C 3 H 8, 21 sec C 3 H 8, 6 min in situ DRS 20 scans, 50 msec K-M intensity ex situ DRS Cr 6+ hydrated Cr 3+ calcined 550 C reduced 550 C 26000 Cr 6+, 26000 cm -1 C 3 H 8 feed 40000 30000 20000 wavenumber (cm -1 ) 10000 O 2 regeneration Puurunen et al., J. Catal. 210 (2002) 418

Examples Comparison of techniques: x wt% Cr n+ /support HAADF-STEM xcr-al 2 O 3 Raman wt.% 10 1Cr-SBA15 5Cr-SBA15 5 1 0.5 1Cr-Al 2 O 3 5Cr-Al 2 O 3 5Cr-SBA15 5Cr-Al 2 O 3 XRD 0 2 4 6 8 10 2 4 6 8 10 energy (KeV) energy (KeV) xcr-al 2 O 3 Santhosh Kumar et al., J. Catal. 261 (2009) 116

Examples Reactivity of V/TiO 2 after oxidative treatment V 5+ O x V x O y V x O y air flow @ 450 C O 2 /C 3 H 8 @ 20 C @ 100 C @ 150 C @ 200 C V 5+ V 4+ O O O V O O TiO 2 : 402 cm -1 V 2 O 5 : 996 cm -1 Brückner et al., Catal. Today 113 (2006) 16

Examples Reactivity of V/TiO 2 after oxidative treatment UV-vis: V 5+ CT (UV) V 4+ d-d transitions (vis) V 5+ V 4+ d-d CT Brückner et al., Catal. Today 113 (2006) 16

Examples Determination of speciation: Fe species in Fe-ZSM5 hydrated samples isolated Fe 3+ oligomeric Fe x O y extended F 2 O 3 -like clusters calcination @ 600 C α-fe 2 O 3 calcined hydrated Santhosh Kumar et al., J. Catal. 227 (2004) 384