1 / 34

Hyunwoong Park and Wonyong Choi School of Environmental Science & Engineering

Photocatalytic and Photoelectrochemical Investigations on TiO 2 Modified by Polyoxometalates and Fluorides. Hyunwoong Park and Wonyong Choi School of Environmental Science & Engineering Pohang University of Science & Technology. TiO 2 with Polyoxometalates.

hija
Download Presentation

Hyunwoong Park and Wonyong Choi School of Environmental Science & Engineering

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Photocatalytic and Photoelectrochemical Investigations on TiO2 Modified by Polyoxometalates and Fluorides Hyunwoong Park and Wonyong Choi School of Environmental Science & Engineering Pohang University of Science & Technology

  2. TiO2 with Polyoxometalates H. Park and W. Choi, J. Phys. Chem. B2003, 107, 3885.

  3. TiO2 Photocatalysis A: O2, Fe3+, Cu2+ A- A Rate Determining Step! k ~ ms e- CB Need More Efficient Electron Mediators! h UV (3.2 eV) VB h+ k ~ 100 ns D D+ D: Pollutants such as halogenated comp’d

  4. Polyoxometalates (POM, PW12O403-) • Homogeneous (photo)catalyst • Under illumination HOMO  LUMO or LMCT • Eg = 3.5 eV • Excited state, POM*, is a better oxidant than the ground state POM • E (POM/POM) = +0.22 V vs NHE

  5. Pt Collector Electrode e- POM- e- CB h POM O2 VB O2 h+ D D+ POM as an Electron Mediator (Shuttle)in TiO2 Photocatalysis

  6. POM e h D+ POM* D POM POM e h D+ POM* D POM O2 O2 POM POM-Mediated Photocurrent Generation without TiO2 Ø = 11% POM + Formate (0.2M) + N2 or Air

  7. I (= Ilight – Idark) vs. [POM]

  8. Ø = 67% e POM e POM D+ h+ D O2 POM e O2 POM O2 e POM D+ h+ Induction Period D Photocurrent Generation in POM + TiO2O2 vs. N2

  9. Dependence of the Saturated Photocurrents Isat(Ilight – Idark ),on [POM]0

  10. Current Doubling Effect of Formate 2e CO2 + H+ POM e e POM h+ COOH + H+ HCOOH Effects of Electron Donors in POM + TiO2 + N2(Formate vs. Acetate)

  11. e Fe3+ e Fe2+ h+ HCOOH + Fe3+ HCOOH COOH + H+ Effects of Electron Donors in Fe3+ + TiO2 + N2(Formate vs. Acetate) Short Circuiting Null Cycle of Fe3+ with Formate

  12. e Fe3+ Pt e Fe2+ h+ CH2COOH CH3COOH CO2 + H+ Effects of Surface Platinization of TiO2 on Electron Shuttling in Fe3+ + TiO2 + N2 Enhanced Electron Transfer Rate on Pt

  13. POM-Mediated H2 Evolution POM + 1/2H2 e H+ POM Pt e POM h+ CH2COOH CH3COOH CO2 + H+ Effects of Surface Platinization of TiO2 on Electron Shuttling in POM + TiO2 + N2

  14. e POM POM Photocurrent Decay Profiles Upon Turning Off UV Light POM-Mediated H2 Evolution POM + 1/2H2 e H+ POM Pt POM

  15. slow e Fe3+ O2 O2 Fe2+ TiO2 k2 k1 Pt/TiO2 Fe3+ fast e POM O2 O2 POM TiO2 k4 k3 Pt/TiO2 POM k2 > k1 > k3 > k4 Effect of O2

  16. Conclusions • POMs successfully mediate electron transfers from the TiO2 CB to a collector electrode. • Although POMs are less efficient than Fe3+ in mediating the photocurrent generation, they exhibit a unique behavior different from Fe3+. • In the presence of dissolved O2 and acetate as an electron donor, the POM-mediated current is completely extinguished due to the rapid reoxidation of the reduced POMs by O2. • This indicates that POMs are very efficient in transferring CB electrons on TiO2 particles to O2 molecules. • Accordingly, in air-equilibrated TiO2 suspensions, the addition of POMs that mediate the CB electron transfer to O2 enhances the overall rate of photocatalytic oxidation reaction.

  17. TiO2 Surface Fluorination H. Park and W. Choi, J. Phys. Chem. B2004, in press

  18. Photocatalytic Reaction of TiO2 OH A CB e Transfer TiIII OH O O A +H2O TiIII e Ti O O h D Direct VB h+ Transfer TiIV OH+ Ti O D+ O h+ O Ti TiIV H2O / OH O Indirect VB h+ Transfer via OH OH+ OH OH D+ D Scheme1

  19. Surface Fluorination of TiO2 (F-TiO2) TiO2 OH ? CB Electron Transfer : Not Clear F NaF at pH 3 ~ 4 Ti O O e Direct Hole Transfer : Impossible Ti F+ Ti F F-TiO2 O O h+ Ti Ti H2O / OH O F F Generation of OH : Enhanced D OH D+ Scheme 2

  20. Surface Fluorination of TiO2 (F-TiO2) [F-] = 0.1 mM; [TiO2] = 0.5 g/L M. S. Vohra, S. Kim and W. Choi, J. Photochem. Photobiol. A2003

  21. Characterization of F-TiO2 SurfaceXPS Survey Spectrum • F adsorbed on TiO2: • BE = 684.3 eV • F ions in the lattice of TiO2: • BE = 688.5 eV •  Simple Ligand Exchange • Fluorination preferentially occurs at pH 3.6 due to pKF (See Reaction 1) [NaF] = 10 mM

  22. Characterization of F-TiO2 Surface UV-Vis DRS [NaF] = 10 mM

  23. - + - - - - + - + - TiO2 F-TiO2 TiO2 F-TiO2 - + - + - - - + - - - - - + - - + Characterization of F-TiO2 Surface Electrophoretic Movement PZZP = 6.2 [NaF] = 10 mM PZZP < 6.2

  24. Photocatalytic Reactivity of F-TiO2 Acid Orange 7 hVB+ Mediated Pathway OHfree Mediated Pathway NaF

  25. Photocatalytic Reactivity of F-TiO2 Phenol OHfree mediated degradation NaF Increase

  26. Photocatalytic Reactivity of F-TiO2 Dichloroacetic Acid hVB+ mediated degradation NaF Decrease

  27. Pt collector eCB A e D+ h A D hVB+ Photoelectrochemical Investigation on F-TiO2 Photogenerated eCB Transfer D = acetate A = Fe3+ Decrease of Photocurrent

  28. Pt collector () eCB A e eCB D+ h ECB (at higher pH) (e.g., -0.5 VNHE at pH 7) A D hVB+ E (MV2+/MV+) (-0.44 VNHE)  eCB ECB (at lower pH) (e.g., -0.1 VNHE at pH 0) (+) pH-independent pH-dependent Photoelectrochemical Investigation on F-TiO2eCB Transfer: pH Dependence D = acetate A = MV2+

  29. Photoelectrochemical Investigation on F-TiO2eCB Transfer: pH Dependence Eon = 59 mV/pH  (6.40  5.58) = 31 mV CB electron transfer to MV2+ on F-TiO2 needs a higher overpotential than on naked TiO2

  30. Photoelectrochemical Investigation on F-TiO2eCB Transfer on TiO2/Ti Electrode Reduced photocurrent generation on F-TiO2 regardless of its physical forms (a suspended particle or an electrode) is due to the large electronegativity of surface fluoride spcecies

  31. Electron Transfer at F-TiO2 / Donor or Acceptor Interface Reductive Dechlorination of Trichloroacetic Acid Visible Light Sensitization of AO7 e e TiO2 Acceptor (TCA) TiO2 Donor (AO7) Electron transfer rate at TiO2 interface is reduced by addition of NaF

  32. TiIVOH+ TiIVF+  hvb+ h hvb+ h KF h OHfree TiIVF + TiIVF TiIVOH + F + OH hvb+  Dark H+ (pKa = 8.7) TiIVO + F ecb h ecb h KF* TiIIIF + OH TiIIIOH + F  UV H+ (pKa > 8.7) TiIIIO + F (kR1 > kR2) kR2 A kR1 A TiIVF TiIVOH + A + A On Naked TiO2 On F-TiO2 Elementary Reaction Steps

  33. Conclusions • (1) The generation of free OH radicals is enhanced on F-TiO2 • (2) Substrates that react mainly through OH radical-mediated pathways are more rapidly degraded in the F-TiO2 suspension, whereas substrates whose degradation is initiated by a direct hole transfer show slower kinetics due to their hindered adsorption (or complexation) on F-TiO2. • (3) Surface fluoride formation greatly lowers positive surface charges on TiO2 at acidic pH region (pH < 6) and reduces the electrostatic interaction with charged substrates. • (4) The surface Ti-F group acts as an electron-trapping site but reduces interfacial electron transfer rates by tightly holding trapped electrons due to the strong electronegativity of the fluorine. As a result, both photo-reductive dechlorination of TCA and the photocurrent generation are reduced on F-TiO2.

  34. Conclusions • (5) The use of F-TiO2 might be suggested as a simple method to test whether a photocatalytic reaction is largely hole-mediated or OH radical-mediated by comparing the photodegradation kinetics in between the pure TiO2 and F-TiO2 suspensions.

More Related