Photocatalytic Oxidation ofAqueous Cyanide Using TiO2 and Surface-

Modified TiO2

2003. 1. 24

Jae-Hyun Kim Ho-In LeeSeoul National University

Catal. & Surf. Chem. Lab Sch. of Chem. Eng., SNU

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 Sch. of Chem. Eng., SNU

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 Sch. of Chem. Eng., SNU

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 Sch. of Chem. Eng., SNU

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 Sch. of Chem. Eng., SNU

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 Sch. of Chem. Eng., SNU

Effect of Dissolved Oxygen

0

1

2

3

4

5

6

0 2 4 6 8 10

DO (ppm)

k (1

0-5 M

min

-1) Oxygen

(DO): a rate limiting factor

Catal. & Surf. Chem. Lab Sch. of Chem. Eng., SNU

Activity (TiO2, no flow)

0 50 100 150 200 250 3000.0

0.5

1.0

1.5

2.0

Co

nc

. (m

M)

Irradiation time (min)

cyanide nitrite cyanate nitrate

0

3

6

9

Co

nc

. o

f D

O (

pp

m)

Conv. of cyanide depended on DO.

OCN-: first product & reaction intermediate

Catal. & Surf. Chem. Lab Sch. of Chem. Eng., SNU

Activity (TiO2, O2 flow)

0 30 60 90 120 1500.0

0.5

1.0

1.5

2.0

Co

nc

. (m

M)

Irradiation time (min)

cyanide nitrite cyanate nitrate

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 Sch. of Chem. Eng., SNU

Activity (TiO2, O2 flow, OCN-)

0.0

0.3

0.6

0.9

1.2

0 30 60 90 120 150 180 210 240

nitrite

cyanate

nitrate

Irradiation time (min)

Conc

. (m

M)

Catal. & Surf. Chem. Lab Sch. of Chem. Eng., SNU

Reaction Pathways of Cyanide (I)

CN-ads + h+

TR → CNads

CN-ads + Ti-O → CNads + Ti-O-

2CNads → (CN)2

(CN)2 + 2OH- → OCN- + CN- + H2O (heterogeneous pathway)

OH + CN- → HOCN HOCN → CONH2 (in water) 2CONH2 → HCONH2 + HOCN (homogeneous pathway)

OCN- + 8OH- + 8h+ → NO3- + CO2 + 4H2O

OCN- + 2H2O → NH4+ + CO3

2- (hydrolysis)

Catal. & Surf. Chem. Lab Sch. of Chem. Eng., SNU

Reaction Pathways of Cyanide (II)

CN-ads + h+

TR → CNads (shallow trap)

vs.

CN-ads + Ti-O → CNads + Ti-O- (deep trap)

OH-ads + h+

TR → OH

OH-ads + Ti-O → OH + Ti-O- (diffuse/free OH)

Catal. & Surf. Chem. Lab Sch. of Chem. Eng., SNU

Effect of OH Radical Scavengers

SG-1 SG-2 SG-3 SG-4

No scavenger 4.93 5.20 4.71 4.95

IPA 3.95 4.11 1.88 1.95

Acetone 4.11 4.40 2.15 2.33

Bromide 3.83 4.02 1.43 1.49

Iodide 3.60 3.69 0.73 0.77

All values in the table are in 10-5 M min-1

Catal. & Surf. Chem. Lab Sch. of Chem. Eng., SNU

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

TiO2 or TPA/TiO2

TMCS in

toluene

Filtering &

DryingCalcination

Catal. & Surf. Chem. Lab Sch. of Chem. Eng., SNU

Silylation of the Surface of TiO2 (II)

Cyanide Cyanate

N. C. Silylated N. C. Silylated

SG-2 5.20 4.73 0.46 0.43

SG-4 4.95 3.83 0.39 0.34

All values in the table are in 10-5 M min-1 & O2 flow

Catal. & Surf. Chem. Lab Sch. of Chem. Eng., SNU

X-Ray Diffraction

0 10 20 30 40 50 60

2 (degree)

Inte

nsi

ty (

a.

u.) SG-2

SG-1

SG-2 (dry)

SG-1 (dry)

20 30 40 50 60

2 (degree)

Inte

nsi

ty (

a.

u.)

SG-4

SG-3

SG-3 (dry)

SG-4 (dry)

Catal. & Surf. Chem. Lab Sch. of Chem. Eng., SNU

Crystallite Size

SG-1 SG-2 SG-3 SG-4

No calc. (110 ℃) 72 72 71 63

Calc. at 300 ℃ 79 75 82 77

All values in the table are in Å

Catal. & Surf. Chem. Lab Sch. of Chem. Eng., SNU

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 Sch. of Chem. Eng., SNU

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.