87

TP CH KHOA HC, i hc Hu, S 65, 2011

NGHIN CU PHN NG QUANG OXY HA XANH METHYLEN TRN XC TC TiO2 NANO DI TC DNG CA NH SNG MT TRI

Trn Thi Ho, L Th Ho, inh Quang Khiu

Trng i hc Khoa hc, i hc Hu

Trn Quc Vit, S Khoa hc v Cng ngh, tnh Tha Thin Hu

L Cng Sn, Trung tm Bo tn Di tch C Hu

TM TT

Bi bo ny trnh by hot tnh xc tc quang oxi ha xanh methylen trn xc tc TiO2

di tc dng ca nh sng mt tri. Hnh thi ca TiO2 nano nh hng n hot tnh xc tc

ca n. TiO2 nano kch thc 20 - 70 nm th hin hot tnh xc tc cao hn cc ht TiO2 hnh

cu ng knh c micromet hnh thnh t s kt cm ca mt s ht kch thc khong 25 nm,

do cc ht bn trong t c kch hot. S oxi ha xanh methylen trn xc tc quang TiO2 nano

tun theo ng hc Langmuir Hinshelwood.

1. t vn

Vt liu TiO2 nano c nhiu ng dng lin quan n tnh cht quang ca n chng hn nh qu trnh quang xc tc phn hy rt nhiu loi hp cht hu c nhim [1,2] v dit khun c bit l vi vi khun E.Coli [3] v.v Tuy nhin, nhng ng dng ny li b cn tr bi vng cm nng lng ca n. Chnh do vng cm nng lng ca TiO2 nm trong vng nh sng t ngoi chim khong 10% nng lng mt tri (3.0 eV i vi pha rutile v 3,2 eV i vi pha anatase). V vy, cn phi ci thin hot tnh ca TiO2 bng cch chuyn vng sng kch thch t t ngoi sang kh kin. C hai hng chnh gii quyt vn ny: th nht, iu ch vt liu oxit titan kch thch nano s dng trit hn nh sng UV ca mt tri [4]; th hai pha tp vo TiO2 mt nguyn t kim loi hay phi kim loi nhm thu hp khong cch vng cm [5].

Vit Nam, vt liu TiO2 nano c nhiu nh khoa hc quan tm. Tc gi Nguyn Vn Dng v cng s [6] cng b nghin cu iu ch vt liu xc tc quang ha TiO2 t sa khong ilmenite v th hot tnh xc tc ca TiO2 trong phn ng quang phn hu axt orange 10. Kt qu nghin cu cho thy, c mi tng quan gia tinh th ho ca pha anatase vi hot tnh quang ho xc tc ca TiO2. Hot tnh quang ha tt nht thu c vi mu c tinh th ho cao vi kch thc tinh th trung bnh ca pha anatase khong 20 nm, tng ng vi mu c nung nhit bt u xy ra s chuyn pha cu trc anatase-rutile. Tc gi Trn Quang Trung v ng s [7] cng b nghin cu tng hp thnh cng ht TiO2 ch to bng phng php sol-gel ng dng trong quang xc tc. Kh nng quang xc tc ca ht TiO2 c kho st

88

thng qua s phn hu ca dung dch methylene blue (MB) c cha ht di ng sng ca n thu ngn kh nc. Vi nhng thng s ch to ti u, tc phn hu MB t n 89,7% sau 1 gi chiu.

Trong nghin cu trc y, chng ti trnh by phng php tng hp nano oxide titanium c hnh thi khc nhau bng phng php to h vi nh tng o [8]. Trong nghin cu ny, phn ng quang ho oxy ho xanh methylen trn cc xc tc c tng hp s c trnh by. ng hc phn hu theo m hnh phn hu Langmur-Hinshelwood cng s c tho lun.

2. Thc nghim

Trong nghin cu ny chng ti tin hnh kho st nh gi hot tnh xc tc cc vt liu nano oxit titan c iu ch bng phng php nh tng o [8] c hnh thi khc nhau c trnh by hnh 1. Vt liu xc tc c k hiu T1 (nh SEM v TEM hnh 1a) ng vi trng hp cc ht TiO2 nano nm ri rc c kch thc vo khong 20-50 nm v T2 ng vi trng hp cc ht nano ng knh khong 25 nm kt t li vi nhau thnh khi cu ng knh vi m (nh SEM v TEM hnh 1b). Thnh phn pha nghin cu bng phng php nhiu x tia X cho thy c hai mu ch yu tn ti l pha anatase c ln mt t rutile.

Hnh 1. nh SEM v TEM ca cc mu c thy nhit cc nhit : a. T1 v b. T2

Cho 0,2 gam lng xc tc xc nh vo 200 ml dung dch xanh metylen, khuy u bng my khuy t thi gian l 60 pht trong phng ti t cn bng hp ph,

T1

T2

89

200 300 400 500 600 700

0.0

0.5

1.0

1.5

2.0

2.5

Mu xc tc T3

Bc sng (nm)

Mt

q

uan

g

C Xanh metylen ban u

=20 ppm

Cn bng hp ph 30 pht 60 pht 90 pht 120 pht 150 pht 180 pht 210 pht 240 pht

sau thc hin qu trnh phn ng di nh sng mt tri. iu kin thc hin phn ng l tri nng, khng my, nhit ngoi tri vo khong 32oC 34oC. Chng ti thc hin phng php gin on c sau 15 pht, mu c ly ra 5 ml v tt c u c lc ly tm loi b cht xc tc, sau o mt quang cc dung dch v qut ph UV-Vis (200 nm 700 nm) .

3. Kt qu v tho lun

Hnh 2 trnh by s thay i ph hp th ca dung dch xanh metylen theo thi gian phn ng khi s dng cc mu xc tc T1. Cng hp th trong vng UV-VIS (200-700nm) ca dung dch gim theo thi gian chiu sng ca nh sng mt tri. gim cng vn hp th di bc sng c trng ca xanh metylen (max = 665 -666 nm) nh ch ra trong hnh tng ng vi s kh mu ca dung dch xanh metylen trong qu trnh phn ng. Cc di ph hp th ca cc hp cht hu c mch nhnh hoc mch vng xut hin trong vng bc sng 200 - 300 nm. Do , s gim cng hp th trong vng bc sng ny cung cp thng tin nh tnh v qu trnh khong ho ca cc sn phm trung gian ca xanh metylen trong qu trnh phn ng. Nh vy, vic s dng mu xc tc TiO2 nano c tng hp bng phng php trn. Bn cnh vic kh mu hon ton qu trnh quang ho xc tc cn c kh nng khong ho dung dch xanh metylen n cc sn phm v c.

Hnh 2. S thay i ph UVVis ca dung dch xanh metylen 20ppm

trong qu trnh quang ho xc tc vi mu T1

TiO2 l mt oxit kim loi bn dn in hnh. Khi c kch thch bi nh sng c bc sng thch hp, cc electron ha tr s tch ra khi lin kt, chuyn ln vng dn, to ra mt l trng mang in tch dng vng ha tr. Cc electron khc c th nhy vo v tr ny bo ha in tch ti , ng thi to ra mt l trng mi ngay ti v tr m n va i khi. Nh vy, l trng mang in tch dng c th t do chuyn ng trong vng ha tr. Cc l trng v electron ny c chuyn n b mt v tng

90

tc vi cc ha cht hp ph trn b mt. Cc l trng c tnh oxi ha mnh v c kh nng oxi ha nc thnh HO:

++ ++ HHOOHh 2VB (1)

+ + HOOHhVB (2)

Cc electron chuyn ln vng dn c kh nng kh O2 hp ph trn b mt to ra 2O :

+ 22CB OOe (3)

22222 O2OHOHO2HO2 +++ (4)

++ OHHOeOH CB22 (5)

Chnh cc gc v sn phm trung gian nh HO, O2 -, H2O2, O2 ng vai tr

quan trng trong c ch quang phn hy cc hp cht hu c. Theo Fu v ng nghip [9], th gc OH l tc nhn oxi ha chnh ca qu trnh quang phn hy xanh metylen v cc sn phm trung gian. V xanh metylen l mt loi phm nhum cation nn khng c kh nng nhng electron. Giai on khi mo ca phn ng chnh l phn ng b gy lin kt ca C-S+=C ca xanh metylen

R-S+=R' + OH R-S(=O)-R' + H+ (6)

Gc OH th hai tip tc tn cng gc sulfoxide to ra hp cht sulfone lm phn tch 2 vng benzen theo phn ng (7) hoc (8):

NH2-C6H3(R)-S(=O)-C6H4-R + OH NH2-C6H3(R)-SO2 + C6H5-R (7)

hoc NH2-C6H3(R)-S(=O)-C6H4- R + OH NH2-C6H4-R + SO2-C6H4-R. (8)

Sau khi c to thnh, hp cht sunfone tip tc b tn cng bi gc OH th ba to ra axit sunfonic

SO2-C6H4-R + OH R-C6H4-SO3H (9)

Cui cng gc OH th t s tn cng vo axit sunfonic gii phng gc SO42

R-C6H4-SO3H + OH R-C6H4 + SO42 + 2H+ (10)

Ngoi ra gc OHcn thay th nhm amin trong phn t ca xanh metylen to thnh phenol v gii phng gc NH2

R-C6H4-NH2 + OH R-C6H4-OH + NH2

(11)

NH2 + H NH3 (12)

NH3 + H+ NH4

+ (13)

91

Hai nhm dimethyl-phenyl-amino i xng ca xanh metylen b tn cng bi gc OH ln lt to ra ancol, tip n l aldehyde, hnh thnh axit v cui cng l tch nhm cacboxyl to ra CO2 theo phn ng photo-Kolbe [9].

Hnh 3 trnh by kt qu ph UV-Vis ca sn phm xanh methylen b oxy ho quang ho trn hai loi xc tc T1 v T2. C th thy rng, tuy mu T2 c kh nng hp ph tt hn nhng kh nng quang xc tc li km hn nhiu mu so vi T1, c th l i vi mu T1 thi gian mt mu hon ton l 60 pht, trong khi vi mu T2 l 90 pht.

Khi hp th photon nh sng th TiO2 s gii phng l trng quang sinh, mt phn l trng quang sinh ny s tip xc dung dch thc hin qu trnh to gc OH nh cp trn, mt phn s ti kt hp tr li vi electron quang sinh. Ngoi ra, y l phn ng quang xc tc. V vy, ch c b mt no ca vt liu nhn c nh sng kch thch th mi xy ra qu trnh sinh electron - l trng quang sinh thc hin phn ng, cn phn b mt khng tip nhn c nh sng th khng xy ra qu trnh trn. T hnh 1b cho thy, i vi mu T2 l gm nhiu ht nano kt hp vi nhau to thnh nhng khi cu ng nht vi kch thc 1 m. Nh vy, khi chiu nh sng kch thch th ch nhng ht nm bn ngoi mi nhn c nh sng kch thch thc hin qu trnh phn ng. Do , hiu sut hp th photon nh sng trn mu T2 s km hn so vi mu T1. Chnh hai yu t trn dn n hiu qu xc tc cho phn ng quang ha ca mu T2 thp hn so vi mu T1.

Qu trnh quang xc tc phn hy xanh metylen trn nn vt liu nano TiO2 l mt qu trnh d th v ng hc ca qu trnh c th c m t theo phng trnh ng hc biu kin bc mt theo m hnh Langmuir-Hinshelwood [10].

KCo

KCok

dt

dCR to

+=

=

1 (14)

350 400 450 500 550 600 650 700

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6 (a)

Mt

qu

ang

Bc sng (nm)

C Xanh metylen ban u

=10 ppm

Cn bng hp ph 15 pht 30 pht 45 pht 60 pht

350 400 450 500 550 600 650 700

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6 (b)

C Xanh metylen ban u

=10 ppm

Cn bng hp ph 15 pht 30 pht 45 pht 60 pht 75 pht 90 pht

Mt

qu

ang

Bc sng (nm)

Hnh 3. S thay i ph UVVis trong vng kh kin (350 nm 700 nm) ca dung

dch xanh metylen 10ppm trong qu trnh quang ho xc tc (a) mu T1; (b) mu T2

92

Trong , R l tc ban u ca qu trnh phn hy xanh metylen. Co l nng ban u, K l hng s cn bng hp ph ca xanh metylen trn TiO2 v kt phn nh tc gii hn ca phn ng trong iu kin th nghim.

T phng trnh trn ta c:

tt k

CtCo

C

Co

Kkt

+

= ln

1 (15)

Trong , t l thi gian cn thit phn hy xanh metylen t nng ban u (Co) n nng ti thi im t (Ct). nng xanh metylen thp, th phng trnh (15) tr thnh.

tktKkC

Cot

t

'...ln ==

(16)

Nh vy, ng biu din lnCo/Ct theo thi gian t l ng thng v h s gc chnh l hng s tc biu kin k.

Hnh 4. th m t ng hc qu trnh quang xc tc trn nn xc tc l mu T1 v T2

theo m hnh Langmuir-Hinshelwood

Bng 1. Cc tham s ng hc gi bc mt ca m hnh Langmuir-Hinshelwood khi s dng

cc mu xc tc ng vi nhit nung khc nhau

K hiu mu

Nng xanh metylen

H s tng quan ( R2)

Hng s tc phn ng biu kin k (pht-1)

T1 10 ppm 0,99663 0,06891

T2 10 ppm 0,99401 0,04615

0 10 20 30 40 50 60 70 80 90 100

0

1

2

3

4

5

Thi gian chiu sng (pht)

ln(C

o/C

)

T1

T2

93

0 50 100 150 200 250 300

0

20

40

60

80

100

M

c

ph

n h

y (

%)

Thi gian phn hy (pht)

10 ppm

15 ppm

20 ppm

T phng trnh ng hc m t qu trnh quang xc tc phn hy xanh metylen ca mu T1 v T2 ta c th thy rng mi tng quan bc nht xy ra vi h s tng quan cao gn bng n v. Do , n ph hp vi m hnh Langmuir-Hinshelwood, l dng m hnh p dng cho phn ng lng phn t. Theo m hnh ny A tc dng vi cht xc tc to thnh dng trung gian hot tnh ho hc, dng trung gian ny s tc dng vi tc cht ho hc B to ra sn phm (y l giai on chm quyt nh tc phn ng). Trong trng hp ny, A l nc s tng tc vi cht xc tc to ra nhng sn phm trung gian (giai on ny nhanh). Sau , cc gc t do ny s tc dng vi xanh methylen to ra sn phm (y l giai on chm quyt nh tc phn ng). Ngoi ra, c th thy rng hng s tc phn ng biu kin ca mu T1 cao hn hn so vi mu T2, tc l phn ng xy ra nhanh hn, qu trnh quang xc tc hiu qu hn.

Hnh 5. nh hng ca nng ban u ca xanh metylen n mc phn hy vi lng

cht xc tc T1 thm vo l 0,2g/200ml dung dch

Hnh 5 biu din lng xanh metylen b phn hy theo thi gian chiu sng cng lng cht xc tc T1 l 0,2g/200ml dung dch xanh metylen nhng vi nng tng ng l 10ppm, 15ppm, v 20ppm. C th thy r rng, khi nng ca xanh metylen cng cao th thi gian phn hy ht lng xanh metylen cng tng, i vi nng xanh metylen l 10 ppm th thi gian phn hy hon ton l 75 pht (99,2%), trong khi ng vi nng ca xanh metylen l 15 ppm th phi cn chiu sng n 240 pht (98,0%) v nng l 20 ppm th phi cn n 285 pht (90,1%). iu ny c th gii thch l khi nng ca xanh metylen tng th kh nng cn tr nh sng chiu vo dung dch cng tng do kh nng hp th photon nh sng ca xanh metylen, lm nh hng n kh nng hp th photon nh sng thc hin qu trnh quang xc tc ca TiO2 , hiu qu thc hin qu trnh quang xc tc ph thuc rt ln vo s lng photon hp thu trn b mt ca TiO2 bi v chnh nhng photon kch thch cc electron trn b mt ca TiO2 sinh ra cc l trng quang sinh thc hin qu trnh quang xc tc. Chnh v vy, khi nng ca xanh metylen tng ln s lm gim s photon hp thu trn b mt ca TiO2 dn n gim hot tnh xc tc. ng thi, khi nng ca xanh metylen tng ln th mt lng ln phm nhum ny s hp th ln b mt TiO2 lm

94

cn tr l trng quang sinh tip xc dung dch, chnh l trng s phn ng vi ion OH- trong dung dch to ra gc OH, tc nhn oxi ha chnh ca qu trnh oxi ha nng cao phn hy cht hu c trong nc.

Bng 2. Cc tham s ng hc gi bc mt ca m hnh Langmuir-Hinshelwood

khi s dng mu xc tc T1 ng vi nng ban u ca xanh metylen khc nhau

Nng dung dch xanh metylen

H s tng quan ( R2)

Hng s tc phn ng biu kin k (pht-1)

10 ppm 0,99663 0,06891

15 ppm 0,9922 0,01807

20 ppm 0,96756 0,01248

Kt qu hng s tc v h s tng quan theo m hnh ng hc lng phn t Langmui-Hindshelwood trnh by bng 2. T y c th thy rng, khi nng xanh metylen 10 ppm th qu trnh phn ng tun theo m hnh ng hc ca Langmuir-Hinshelwood vi h s tng quan cao vi gi tr k cng cao, nhng khi nng tng th c h s tng quan R2 v k u gim, iu ny gii thch v mt ton hc v sao tng nng xanh metylen th thi gian phn hy cng cng tng.

4. Kt lun

Bi bo ny nghin cu kh nng xc tc quang ho trn xc tc TiO2 nano vi phn ng phn hy phm nhum xanh metylen di nh sng mt tri. Cc vt liu TiO2 nano tng hp bng phng php vi nh tng o c hot tnh quang ho cao vi kch thch ca nh sng mt tri. Hnh thi nh hng nhiu n hot tnh xc tc. ng hc qu trnh quang xc tc tun theo m hnh phn ng lng phn t Langmuir-Hinshelwood.

TI LIU THAM KHO

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THE STUDY ON THE VISIBLE LIGHT ACTIVED PHOTOCATALYTIC

OXIDATION OF METHYLEN GREEN OVER NANO TiO2

Tran Thai Hoa, Le The Hoa, Dinh Quang Khieu

College of Sciences, Hue University

Tran Quoc Viet, Department of Science and Technology Thua Thien Hue Province

Le Cong Son, Hue Monuments Conservation Centre

SUMMARY

In this paper, catalytic activity of nano titanium oxides prepared by microemulsion

through the visiable light active photocatalytic oxidation of methylen green was investigated.

The nano TiO2 obtained exhibited excellent catalytic activity using sun light. However, the

morphologies of nano TiO2 have an effect on the photocatalyitc activity. The nano TiO2 with size

around 20-70 nm exhibited higher activity than that of some micromer spherical particles of

nano TiO2 containing very fine particles around 25 nm due to its less activated ultra violet-inner

particles. The photocatalytic oxidation of methylen green over nano TiO2 followed Langmuir

Hinshelwood kinetics.