UV-Vis spectroscopy Basic theory

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1 UV-Vis spectroscopy Basic theory Dr. Davide Ferri Empa, Lab. for Solid State Chemistry and Catalysis

2 Importance of UV-Vis in catalysis IR Raman NMR XAFS UV-Vis EPR Number of publications Number of publications containing in situ, catalysis, and respective method Source: ISI Web of Knowledge (Sept. 2008)

3 The electromagnetic spectrum VISIBLE ULTRAVIOLET Hz Hz 200 source: Andor.com

4 The energy unit Typically, the wavelength (nm) is used the distance over which the wave's shape repeats x nm = / x cm 1 y cm 1 = / y nm

5 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)

6 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

7 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

8 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* nm, weak n π* nm, weak π π* nm, intense Condition to absorb light ( nm): π and/or n orbitals CHROMOPHORE

9 The UV spectrum σ* π* n e π σ π π* signal envelope absorbance λ max 217 nm 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

10 The UV spectrum e e σ* π* n (O) π absorbance O σ 0.02 n π* π π* 0.0 no visible light absorption wavelength (nm)

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

12 The UV spectrum Conjugation effect: β-carotene white light absorbance wavelength (nm)

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

15 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

16 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+

17 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 wavelengths (nm) d-d transitions: ε max = Lmol -1 cm -1, weak

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 4 2- < 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 Because of selection rules, colours are faint (ε= 20 Lmol -1 cm -1 ).

20 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

21 Charge transfer complex Inorganic compounds no selection rules intense colours (ε= 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 * 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+ )

23 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

24 UV-Vis spectroscopy Instrumentation Examples for catalysis Dr. Davide Ferri Empa, Lab. for Solid State Chemistry and Catalysis

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

26 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

27 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

28 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

29 Examples Determination of oxidation state: 0.1 wt% Cr n+ /Al 2 O 3 calibration Cr 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

30 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

31 Examples Determination of oxidation state: 0.5 wt% Cr n+ /SiO 2 DRUV, 350 C, 2% isobutane-n 2 pure components Weckhuysen et al., Catal. Today 49 (1999) 441

32 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 Cr 6+, cm -1 C 3 H 8 feed wavenumber (cm -1 ) O 2 regeneration Puurunen et al., J. Catal. 210 (2002) 418

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

34 Examples Reactivity of V/TiO 2 after oxidative treatment V 5+ O x V x O y V x O y air 450 C O 2 /C 3 H 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

35 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

36 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 600 C α-fe 2 O 3 calcined hydrated Santhosh Kumar et al., J. Catal. 227 (2004) 384