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Infrared Spectroscopy

Infrared Spectroscopy. Applications: Catalyst characterisation direct measurement of catalyst IR spectrum measurement of interaction with “probe” molecules: NH 3 , pyridine: acidity CO, NO: nature of active sites (e.g. Pt on alumina) Mechanistic studies adsorbed reaction intermediates

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Infrared Spectroscopy

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  1. Infrared Spectroscopy Applications: • Catalyst characterisation • direct measurement of catalyst IR spectrum • measurement of interaction with “probe” molecules: • NH3, pyridine: acidity • CO, NO: nature of active sites (e.g. Pt on alumina) • Mechanistic studies • adsorbed reaction intermediates • deactivation by strongly adsorbing species • Analysis of reactants and products (in situ reaction monitoring Catalysis and Catalysts - Infrared Spectroscopy

  2. IRS - General Concepts Electromagnetic Spectrum Frequency () = c/ Wavenumber () = 1/  Energy (E) = h  = h c  UV Visible IR 4000 - 400 cm-1 Catalysis and Catalysts - Infrared Spectroscopy

  3. Solid/Gas-Phase Applications Catalysis and Catalysts - Infrared Spectroscopy

  4. Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) Catalysis and Catalysts - Infrared Spectroscopy

  5. Liquid Phase IR Source ATR Crystal Detector Liquid-Phase Applications Attenuated Total Reflection (ATR) Catalysis and Catalysts - Infrared Spectroscopy

  6. Re2O7/Al2O3 - Preparation NH4ReO4 Dry impregnation on -Al2O3 Drying Calcination (825 K, 2h) Structure??? Re2O7/Al2O3 Catalysis and Catalysts - Infrared Spectroscopy

  7. NH4ReO4 Alumina O H H O A l A l A l Dry impregnation A l Intensity decreases Re-loading increases Drying 383 K, 16 h Calcination 323 K, 2 h Re2O7/Alumina Re2O7/-Al2O3 - IR Spectrum in OH stretching region Neutral Acidic Basic Re2O7 loading 0% 3% Absorbance 6% 12% 18% 3900 3800 3700 3600 3500 Wavenumber (cm-1) Catalysis and Catalysts - Infrared Spectroscopy

  8. Metathesis of Propylene on Re2O7/Al2O3 2 CH3CH=CH2 CH2=CH2 + CH3CH=CHCH3 Very active catalyst (already at room temperature) N: mol converted/(mol Re-atoms s) Catalysis and Catalysts - Infrared Spectroscopy

  9. Model for Re-sites based on IRS ReO4 on Lewis site not active Basic -OH substituted by ReO4 slightly active Acidic -OH substituted by ReO4 active Catalysis and Catalysts - Infrared Spectroscopy

  10. Summary IRS Re/Al • Alumina contains Lewis and Brönsted sites • OH-spectrum different acid sites • Impregnation • OH + HOReO3 -OReO3 + H2O • Al3+ + HOReO3 coordination complex • Low-loading Re/Al not effective • IRS gives detailed picture of surface Catalysis and Catalysts - Infrared Spectroscopy

  11. HF Al2O3 F/Al2O3 F-salt F/Al2O3 Determination of Nature and Number of Active Sites for F/Al2O3 F/Al2O3 very active in acid-catalysed reactions Structure of F/Al2O3 ??? Acid sites? Bronsted, Lewis???, How many?? Catalysis and Catalysts - Infrared Spectroscopy

  12. N N N Brönsted acid Lewis acid IR Spectra “Probe” Molecule Pyridine adsorbs on acid sites Spectrum changes Different IR Spectra Catalysis and Catalysts - Infrared Spectroscopy

  13. IR Absorption Spectra of Fluorinated Alumina L 1452 H2O Lewis site Brönsted site L 1619 B 1490 L 1497 After adsorption of pyridine at 330 K Transmission b L 1579 After addition of H2O at 330 K and evacuation at 330 K B 1639 B 1542 c a Background spectrum F/Al2O3 1300 1500 1700 Catalysis and Catalysts - Infrared Spectroscopy Wavenumber (cm-1)

  14. Reference Spectra Catalysis and Catalysts - Infrared Spectroscopy

  15. IR results versus Catalytic Activity If Brönsted sites are active sites, DMP is an irreversible poison Conv. Number of active sites Amount DMP added Example: Oligomerisation of Isobutylene Catalysis and Catalysts - Infrared Spectroscopy

  16. Number of Brönsted sites vs. F content Catalysis and Catalysts - Infrared Spectroscopy

  17. Correlation IR - DMP Poisoning Catalysis and Catalysts - Infrared Spectroscopy

  18. Summary IR F/Al2O3 • Al2O3 • Lewis sites: weak adsorption of Py and DMP • F/Al2O3 • Lewis sites: weakly adsorbed DMP • Brönsted sites: strongly adsorbed DMP • DMP specific poison number of Brönsted sites • Oligomerisation of isobutylene occurs at Brönsted sites Catalysis and Catalysts - Infrared Spectroscopy

  19. NO Adsorption on Fe-ZSM5 Catalyst Fe-based zeolites have high activity for: • deNOx-SCR • N2O-mediated selective oxidation of benzene to phenol • Catalytic N2O decomposition NO acts as reactant and has been used as probe molecule Preparation of Fe-ZSM5: • liquid ion exchange • solid ion exchange • special route: • incorporation of Fe into zeolite structure during synthesis • extraction of Fe (and Al and Si) to non-framework positions by steaming Catalysis and Catalysts - Infrared Spectroscopy

  20. Fe extracted from the framework MFI: class of zeolites, e.g. ZSM-5, silicalite • Ex-[Fe,Al]MFI: • Si/Al: 31.3 • Si/Fe: 121.7 • Fe (wt%): 0.67 • Fe species: • (FeO)n; n < 5: “oligonuclear clusters” • FeAlOx: “nano-particles” Catalysis and Catalysts - Infrared Spectroscopy

  21. Catalysis and Catalysts - Infrared Spectroscopy

  22. Assignments of Absorption Bands of NO on Fe-zeolite Catalysis and Catalysts - Infrared Spectroscopy

  23. Fe-containing sites in MFI Catalysis and Catalysts - Infrared Spectroscopy

  24. Fe AlO -NO II x 1874 g Iso Fe -NO ( ) II FeAlO -NO x 2 a ( Fe O ) -NO ( ) II n 1635 1886 0.05 Absorbance NO + 2133 2200 2100 2000 1900 1800 1700 1600 Wavenumber (cm-1) IR Absorption Spectra of ex-[Fe,Al]MFI Catalysis and Catalysts - Infrared Spectroscopy

  25. Au/TiO2 Catalysed Oxidation of Propylene to Propylene Oxide Catalysis and Catalysts - Infrared Spectroscopy

  26. Desorption of PO as a Function of Time Catalysis and Catalysts - Infrared Spectroscopy

  27. C=O O-C=O C-H H-C=O 1383 1716 1456 1336 Exposure to H /O /C = 2 2 3 PO desorption Au/TiO /SiO 2 2 1375 1355 1443 1568 1549 1254 0.1 Absorbance Exposure to 1682 H /O /C = 1716 2 2 3 1625 Au/TiO 2 PO desorption 1800 1600 1400 1200 Wavenumber (cm-1) IR Spectra of Au/TiO2 and Au/TiO2/SiO2 Catalysis and Catalysts - Infrared Spectroscopy

  28. Chemical Interaction of PO with Au Catalysts Catalysis and Catalysts - Infrared Spectroscopy

  29. ATR Spectroscopy Nafion Catalysed Esterification Catalysis and Catalysts - Infrared Spectroscopy

  30. Drying tube Hastelloy housing ZnSe Support/focusing IR energy in Reflux cooler element IR energy out Diamond crystal IR-probe Gold seal ATR Diamond crystal IR energy out Thermometer IR energy in Reaction medium Heater Catalyst particle Solvent vapour Magnetic stirrer bubble Equipment - Glass Reactor with Dicomp Probe Catalysis and Catalysts - Infrared Spectroscopy

  31. “Waterfall Graph” of Esterification1800 - 1000 cm-1 n - Decane Ester Ester Abs 0.20 0.16 0.12 0.08 0.04 0.00 5.0 4.0 3.0 2.0 1.0 Hexanoic acid Time (h) 1-Octanol (shoulder) 1800 1600 1400 1200 1000 Wavenumber (cm-1) Catalysis and Catalysts - Infrared Spectroscopy

  32. Transient Spectra of Hexanoic acid and Ester 1800 - 1700 cm-1 Ester Abs 0.08 0.06 0.04 0.02 0.00 5.0 4.0 3.0 2.0 1.0 Hexanoic acid Time (h) 1780 1760 1740 1720 1700 Wavenumber (cm-1) Catalysis and Catalysts - Infrared Spectroscopy

  33. Concentration Profiles Ester Hexanoic acid Catalysis and Catalysts - Infrared Spectroscopy

  34. 0.35 0.30 0.25 Hexanoic acid (GC) Hexanoic acid (IR) Ester (GC) Ester (IR) 0.20 Concentration (mol/l) 0.15 0.10 0.05 0.00 0.00 30.00 60.00 90.00 120.00 150.00 180.00 210.00 240.00 Time (min) Esterification in -decane n Catalysis and Catalysts - Infrared Spectroscopy

  35. Transient Spectra 1300 - 1000 cm-1 Ester Abs 0.03 0.02 0.01 0.00 5.0 4.0 3.0 2.0 1.0 Hexanoic acid 1-Octanol (shoulder) Time (h) 1250 1200 1150 1100 1050 1000 Wavenumber (cm-1) Catalysis and Catalysts - Infrared Spectroscopy

  36. Subtracted Transient Spectra1300 - 1000 cm-1 Ester Abs 0.05 0.04 0.03 0.02 0.01 0.00 5.0 4.0 3.0 2.0 1.0 1-Octanol Time (h) 1250 1200 1150 1100 1050 1000 Wavenumber (cm-1) Catalysis and Catalysts - Infrared Spectroscopy

  37. 1-Octanol Concentration Profile Catalysis and Catalysts - Infrared Spectroscopy

  38. 0.40 0.35 0.30 0.25 Concentration (mol/l) 0.20 0.15 0.10 0.05 0.00 0.00 30.00 60.00 90.00 120.00 150.00 180.00 210.00 240.00 Time (min) Esterification in n-decane 1-Octanol (GC) 1-Octanol (IR) Catalysis and Catalysts - Infrared Spectroscopy

  39. Concluding Remarks • IR spectroscopy very useful in heterogeneous catalysis • ex-situ • in-situ • Simple technique • Study of catalytic sites on catalyst surface, both qualitatively and quantitatively • Information on reaction mechanism and reaction intermediates • Analysis of liquid-phase catalytic reactions Catalysis and Catalysts - Infrared Spectroscopy

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