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Photocatalytic Oxidation of Aqueous Cyanide Using TiO 2 and Surface-Modified TiO 2

Photocatalytic Oxidation of Aqueous Cyanide Using TiO 2 and Surface-Modified TiO 2. 2003. 1. 24 Jae-Hyun Kim  Ho-In Lee Seoul National University. Experimental Conditions (I). Chemicals Reactant: KCN (Fluka, GR) TPA : Tungstophosphoric acid (H 3 PW 12 O 40 )

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Photocatalytic Oxidation of Aqueous Cyanide Using TiO 2 and Surface-Modified TiO 2

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  1. Photocatalytic Oxidation ofAqueous Cyanide Using TiO2 and Surface-Modified TiO2 2003. 1. 24 Jae-Hyun Kim  Ho-In Lee Seoul National University

  2. Experimental Conditions (I) • Chemicals • Reactant: KCN (Fluka, GR) • TPA: Tungstophosphoric acid (H3PW12O40) • TTIP: Titanium tetraisopropoxide (Ti(OC3H7)4) • IPA: Isopropyl alcohol • Analysis • Cyanide: Standard silver nitrate titration • Products: Ion chromatography Catal. & Surf. Chem. Lab

  3. Experimental Conditions (II) • Activity test • Initial conc.: CN- 50 ppm (ca. 1.9 mM) • Reactor: Pyrex cylindrical reactor • Light source: 450 W high-pressure Hg lamp • Amount of catalyst: 0.67 g/L • Sampling filter: 0.2 m Nylon syringe filter • pH: pH meter 520A (Orion) • Dissolved oxygen: DO meter 810 (Orion) Catal. & Surf. Chem. Lab

  4. Preparation of Catalysts (I) Dissolution of TPA in water-IPA mixture  Dropwise addition of TTIP-IPA into the mixture  Aging for 6 hrs  Dry at 110℃ for 12 hrs  Calcination at 300℃ (SG-1) Catal. & Surf. Chem. Lab

  5. Preparation of Catalysts (II) Dissolution of TPA in water-IPA mixture  Dropwise addition of the mixture into TTIP-IPA  Aging for 6 hrs  Dry at 110℃ for 12 hrs  Calcination at 300℃ (SG-2) Catal. & Surf. Chem. Lab

  6. Nomenclature of Catalysts • SG-1: 1 wt% TPA/TiO2method 1 • SG-2: 1 wt% TPA/TiO2 method 2 • SG-3: pure TiO2 based on method 1 • SG-4: pure TiO2based on method 2 Catal. & Surf. Chem. Lab

  7. k (10-5 M min-1) DO (ppm) Effect of Dissolved Oxygen • Oxygen (DO): a rate limiting factor Catal. & Surf. Chem. Lab

  8. Activity (TiO2, no flow) • Conv. of cyanide depended on DO. • OCN-: first product & reaction intermediate Catal. & Surf. Chem. Lab

  9. Activity (TiO2, O2 flow) • Conv. of cyanide was faster in the presence of enough DO. • Cyanate was decomposed to mainly nitrate after complete conv. of cyanide. Catal. & Surf. Chem. Lab

  10. Activity (TiO2, O2 flow, OCN-) Conc. (mM) Irradiation time (min) Catal. & Surf. Chem. Lab

  11. Reaction Pathways of Cyanide (I) • CN-ads + h+TR → CNads • CN-ads + Ti-O → CNads + Ti-O- • 2CNads → (CN)2 • (CN)2 + 2OH- → OCN- + CN- + H2O (heterogeneous pathway) • OH + CN- → HOCN • HOCN → CONH2 (in water) • 2CONH2 → HCONH2 + HOCN (homogeneous pathway) • OCN- + 8OH- + 8h+ → NO3- + CO2 + 4H2O • OCN- + 2H2O → NH4+ + CO32- (hydrolysis) Catal. & Surf. Chem. Lab

  12. Reaction Pathways of Cyanide (II) • CN-ads + h+TR → CNads (shallow trap) vs. • CN-ads + Ti-O → CNads + Ti-O- (deep trap) • OH-ads + h+TR → OH • OH-ads + Ti-O → OH + Ti-O- (diffuse/free OH) Catal. & Surf. Chem. Lab

  13. Effect of OH Radical Scavengers  All values in the table are in 10-5 M min-1 Catal. & Surf. Chem. Lab

  14. TMCS in toluene Filtering & Drying TiO2 or TPA/TiO2 Calcination Silylation of the Surface of TiO2 (I) • Surface Modification with TMCS • TMCS: Trimethylchlorosilane • Ti-OH + Cl-Si(CH3)3 → Ti-O-Si(CH3)3 + HCl Catal. & Surf. Chem. Lab

  15. Silylation of the Surface of TiO2 (II)  All values in the table are in 10-5 M min-1 & O2 flow Catal. & Surf. Chem. Lab

  16. SG-2 SG-4 SG-2 (dry) SG-4 (dry) Intensity (a. u.) Intensity (a. u.) SG-1 SG-3 SG-1 (dry) SG-3 (dry) 2  (degree) 2  (degree) X-Ray Diffraction Catal. & Surf. Chem. Lab

  17. Crystallite Size  All values in the table are in Å Catal. & Surf. Chem. Lab

  18. Conclusions • Dissolved oxygen became a rate-limiting factor when its concentration went below 6 ppm. • CNO- was the first product of photocatalytic oxidation of cyanide in the presence of TiO2 or TPA modified TiO2. • Other products such as NO2- and NO3- were detected following the complete oxidation of cyanide. Catal. & Surf. Chem. Lab

  19. Conclusions (continued) • The activities of TPA/TiO2 were higher than those of pure TiO2, and the tendency became clearer in the presence of OH radical scavengers. • The amount of surface hydroxyl groups was decreased due to silylation, and consequently, the oxidation rate of both cyanide and cyanate was retarded. • Introduction of TPA brought about the increase of surface roughness due to smaller crystallite sizes of TPA/TiO2 than those of pure TiO2. Catal. & Surf. Chem. Lab

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