Photocatalytic Oxidation of Aqueous Cyanide Using TiO 2 and Surface-Modified TiO Jae-Hyun Kim  Ho-In Lee Seoul National University

Sch. of Chem. Eng., SNUCatal. & Surf. Chem. Lab Experimental Conditions (I)  Chemicals  Reactant: KCN (Fluka, GR)  TPA: Tungstophosphoric acid (H 3 PW 12 O 40 )  TTIP: Titanium tetraisopropoxide (Ti(OC 3 H 7 ) 4 )  IPA: Isopropyl alcohol  Analysis  Cyanide: Standard silver nitrate titration  Products: Ion chromatography

Sch. of Chem. Eng., SNUCatal. & Surf. Chem. Lab 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)

Sch. of Chem. Eng., SNUCatal. & Surf. Chem. Lab 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)

Sch. of Chem. Eng., SNUCatal. & Surf. Chem. Lab 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)

Sch. of Chem. Eng., SNUCatal. & Surf. Chem. Lab Nomenclature of Catalysts  SG-1: 1 wt% TPA/TiO 2  method 1  SG-2: 1 wt% TPA/TiO 2  method 2  SG-3: pure TiO 2  based on method 1  SG-4: pure TiO 2  based on method 2

Sch. of Chem. Eng., SNUCatal. & Surf. Chem. Lab Effect of Dissolved Oxygen DO (ppm) k (10 -5 M min -1 )  Oxygen (DO): a rate limiting factor

Sch. of Chem. Eng., SNUCatal. & Surf. Chem. Lab Activity (TiO 2, no flow)  Conv. of cyanide depended on DO.  OCN - : first product & reaction intermediate

Sch. of Chem. Eng., SNUCatal. & Surf. Chem. Lab Activity (TiO 2, O 2 flow)  Conv. of cyanide was faster in the presence of enough DO.  Cyanate was decomposed to mainly nitrate after complete conv. of cyanide.

Sch. of Chem. Eng., SNUCatal. & Surf. Chem. Lab Activity (TiO 2, O 2 flow, OCN - ) Irradiation time (min) Conc. (mM)

Sch. of Chem. Eng., SNUCatal. & Surf. Chem. Lab 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 - + H 2 O (heterogeneous pathway)  OH  + CN - → HOCN   HOCN  → C  ONH 2 (in water)  2C  ONH 2 → HCONH 2 + HOCN (homogeneous pathway)  OCN - + 8OH - + 8h + → NO CO 2 + 4H 2 O  OCN - + 2H 2 O → NH CO 3 2- (hydrolysis)

Sch. of Chem. Eng., SNUCatal. & Surf. Chem. Lab 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  )

Sch. of Chem. Eng., SNUCatal. & Surf. Chem. Lab Effect of OH Radical Scavengers SG-1SG-2SG-3SG-4 No scavenger IPA Acetone Bromide Iodide  All values in the table are in M min -1

Sch. of Chem. Eng., SNUCatal. & Surf. Chem. Lab Silylation of the Surface of TiO 2 (I)  Surface Modification with TMCS  TMCS: Trimethylchlorosilane  Ti-OH + Cl-Si(CH 3 ) 3 → Ti-O-Si(CH 3 ) 3 + HCl TiO 2 or TPA/TiO 2 TMCS in toluene Filtering & Drying Calcination

Sch. of Chem. Eng., SNUCatal. & Surf. Chem. Lab Silylation of the Surface of TiO 2 (II) CyanideCyanate N. C.SilylatedN. C.Silylated SG SG  All values in the table are in M min -1 & O 2 flow

Sch. of Chem. Eng., SNUCatal. & Surf. Chem. Lab X-Ray Diffraction 2  (degree) Intensity (a. u.) SG-2 SG-1 SG-2 (dry) SG-1 (dry) 2  (degree) Intensity (a. u.) SG-4 SG-3 SG-3 (dry) SG-4 (dry)

Sch. of Chem. Eng., SNUCatal. & Surf. Chem. Lab Crystallite Size SG-1SG-2SG-3SG-4 No calc. (110 ℃ ) Calc. at 300 ℃  All values in the table are in Å

Sch. of Chem. Eng., SNUCatal. & Surf. Chem. Lab 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 TiO 2 or TPA modified TiO 2.  Other products such as NO 2 - and NO 3 - were detected following the complete oxidation of cyanide.

Sch. of Chem. Eng., SNUCatal. & Surf. Chem. Lab Conclusions (continued)  The activities of TPA/TiO 2 were higher than those of pure TiO 2, 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/TiO 2 than those of pure TiO 2.