Tap chi KHOA HOC OHSP TPHCM Sd 58 ndm 2014
/ \ P DUNG PHirONG PHAP DO THOfI GIAN SONG
CUA POSITRON TRONG NGHIEN CUtJ VAT LIEU ZE-6-LIT
H 6 TH! THONG*, T R A N Q U 6 C D G N G "
TOM TAT
Phuang phdp phd thdi gian sdng ciia pd-si-trdn (positron) la mdt trong nhimg ki thugt khdng phd huy him hiiu di nghiin cim vgt lieu. Ze-d-lit (zeolite) la mdt vgt lieu him ich trong nhiiu irng dung thuc tiin nhd vdo cdu true dgc biet vdi nhimg hi thdng Id rdng cd kich thudc khdc nhau. Trong nghiin cim ndy, cdc phep do phd thdi gian sdng ciia pd-si- trdn trong ze-d-Ut Fe-MFI-100, ZSM-5, Fe-FER, LDH, Fe-SBA-15 dugc tiin hdnh. Cdc kit qud ban ddu vi dgc diim cdu true bin trong ze-d-lit cho thdy: thu nhdt, cdc ze-d-lit cd bdn kinh khodng 0,254nm-0,419 nm ddi vdi Id rdng nhd vd 0, ?4?nm-l,238nm ddi vdi Id rdng trung binh, trong dd Id rdng ciia Fe-SBA-15 cd kich thudc ldn nhdt (0,419nm vd l,238nm);
thu hai, trong ZSM-5, Fe-MFI-100 vd Fe-FER mat do ciia Id rdng nhd ldn han nhiiu so vdi Id rdng trung binh, nhung trong Fe-SBA-15 thi ngugc lgi.
Tir khda: thdi gian sdng pd-si-trdn, ze-d-lit, cau true Id rong trong ze-d-lit.
ABSTRACT
Applying the positron annihilation lifetime spectroscopy to study zeolite material The positron annihilation lifetime spectroscopy (PALS) is an effective non- destructive technique for studying materials. It has been developed to find out the structures of materials and obtained results with a high accuracy in many different types of material. Because of the special structure with a network of different pores, zeolite is a very useful material used for various applications. PALS was applied to probe this structure of zeolite, particularly of Fe-MFI-100, ZSM-5, Fe-FER, LDH, Fe-SBA-15. The initial results of the pore size in zeolites show that: first, the studied zeolites have radius of pores from 0,254nm to 0,419nm for micro-pore and from 0,?4?nm-l,238nmfor meso-pore.
The pores inside Fe-SBA-15 have the largest size with 0,419nm and l,238nm; second, about the density of these pores: for ZSM-5, Fe-MFI-100 and Fe-FER, the micro-pore density is significantly higher compared with meso-pore density, conversely in Fe-SBA-15 meso-pore density is dominant.
Keywords: The positron annihilation lifetime spectroscopy, zeolite, structure of zeolite.
1. Md- dau
Phuong phap phd k l thdi gian sdng eua pd-si-trdn (PAS) la mdt cdng eu him hieu trong viec nghien cim cau triic eua vat lieu. Vdi dac diem la khdng pha buy mau,
HVCH, Trudng Dai hoc Khoa hoc Tu nhien, DHQG TPHCM
phuong phap nay giiip chiing ta tim hilu he thdng Id rdng vi md, cac tinh chat sau ben trong bl mat cua vat lieu xdp hay kilm tra sai hdng trong vat lieu. [1,6,7,13,14]
Ze-d-lit la vat lieu tinh the xdp d thang na-no ba chieu vdi thanh phan cau tao bdi nhdm (Al), Silic (Si) va Oxi (O). Ze-d-lit hip thu manh nude va cac cation nhu sat, kali, natri va nhiing ion duong khac nhau bdi kich cd ciia cac ca-ti-on (cation) nay tuong duong vdi cac lo va kenh rdng trong ze-d-lit. Cac loai ze-o-lit mang sit (sau day ggi tat la Fe-zeolite) dang dugc nghien ciiu de sir dung nhu nhirng chat xiic tac phan buy true tiep N^O trong cdng nghiep khi ddt va ddng co di-e-sel (diesel) [4]. Hon niia nd cdn cho phep phan buy va giam lugng N2O trong khi thai. Vat lieu Fe-ZSM5 - mdt loai ze-o-lit mang Fe tao ra bang phuong phap trao ddi i-on da dugc nghien ciiu nhu mdt chat xiic tac de lam giam khi thai N2O. Chat xiic tac nay cho thay tiem nang ldn trong phan buy true tiep eiia N2O [2, 3]. Ben canh dd, mdt so loai Fe-zeolite nhu Fe- BEA, Fe-Mordenite... ciing dugc nhieu phdng thi nghiem tren the gidi nghien ciiu.
Kha nang xiic tac ciia eac Fe-zeolite de xu li khi gay hieu iing nha kinh dang trd thanh mdt ITnh vuc vira mang tinh hgc thuat cao vira cd tinh iing dung cap bach nhung cac ket qua nghien ciiu chua nhieu. Van de nay dang dugc quan tam bdi cac trung tam nghien ciiu va cac nha khoa hgc trong eac ITnh vuc hda hgc, vat li, vat lieu va mdi trudng v.v...
Ddi vdi cac vat lieu, dac biet la vat lieu tinh the na-no (Fe-Zeolite la mdt minh hga rd net) cac sai hdng (vacancy, void...), cac thanh phan kim loai tap chat hay trang thai kim loai tdn tai do trao ddi ion hoac cay bdi chum hat tren may gia tdc (implantation) se anh hudng tdi phan bd eua eac e-lec-tron (electron) trong mang tinh the. Dieu nay cd thi din din thay doi rit manh tdi cac gia tri thdng sd buy thu dugc tir cac phd kl PAS [9-12]. Do vay, PAS trd thanh mdt phuong phap tin cay cho nhiing nghien ciiu ciu triic e-lec-tron, cac dang sai hdng, trang thai kim loai, tap chit ed trong vat lieu d kich thudc bac na-no ciia cac vat lieu mang tinh cdng nghe cao. Vdi chum tia po-si-tron nang lugng thip, tit ca nhimg thdng tin nay ed thi duge khao sat 0 do nhay cao lien quan tdi nhihig tinh chit be mat va viing gin bl mat, eac kenh (channel) va Idng (cage) d kich thudc tir 0,1 tdi hang tram na-no met.
Viec nghien ciiu Id rdng chii ylu dua vao su khac biet ldn cua nhirng each thiic buy pd-si-trdn trong ze-d-lit, dac biet la sir ddng gdp quan trgng eua trang thai pd-si- trd-ni-um (positronium-Ps). Trang thai nay rit dl dang hinh thanh khi pd-si-trdn di vao trong cac vat lieu xdp va su buy eua nd thi hi anh hudng bdi tinh chit li hda cua mdi trudng. Cd hai loai Ps ed thi duge tao thanh tren dudng di cua pd-si-trdn la pa-ra-Ps (p- Ps) va or-tho-Ps (o-Ps). Trong cac nghien curu vat lieu ze-d-lit, thdi gian song eua o-Ps thudng duge sir dung dh danh gia kich thudc eiia Id rdng ben trong ciu triic vat lieu.
Ben canh dd tir cudng do tuong iing vdi cac gia tri thdi gian sdng ta ed the du doan dugc mat do lo rong trong ze-d-lit, mat do cang cao thi o-Ps hi biy chiim ti le cang cao.
Tap chi KHOA HOC OHSP TPHCM Sd 58 ndm 2014
Sir tuong quan giiia kich thudc Id rdng va thdi gian sdng thdng qua md hinh the tich tu do (Free Volume Model) dugc d l xuit diu tien bdi Tao [16]. Theo md hinh nay, nhirng thanh phin thdi gian sdng dai nhit cua phd thdi gian sdng pd-si-trdn (thdi gian sdng cua o-Ps) tuong quan vdi thi tich tu do trong vat lieu dam dac dugc xac dinh bdi cdng thiic [3]:
— = /^, = 2 x R 1 .
1 sin
R + ^R 2n
/ _2nR_
R + AR
. (0
Trong dd, AR la thdng sd tinh toan tir thuc nghiem va AR=1,66 A°, TJ: thanh phin thdi gian sdng dai thir i (vdi i = 3 va 4 trong kit qua nay). X2y' hing sd phan ra 2y.
Cdng thirc tren chi dung doi vdi nhirng Id rong cd ban kinh nhd hon Inm. D I tinh toan gia tri ban kinh chinh xac hon cho Id rdng cd ban kinh ldn hon Inm , cdng thirc (1) cin dugc dua them vao su ddng gdp cua viec hiiy phat ba y vdi thdi gian song khoang 140ns va trd thanh [3]:
r, ^2y "^ ^x — 2 X
^ .. 1 . ' R 1 + — s i n
R + AR 2n V 2nR R + AR
+ •
140 (2)
Nhilu nghien ciiu xa sau hon v l ze-d-lit ciing da duge tien hanh nhu viec kiem tra anh hudng ciia Fe len ciu triic ze-d-lit [5], xac dinh do day cua nhiing thanh Id rdng ze-d-lit thdng qua su khac nhau giira thdng sd d va kich thudc Id rdng [15], hieu chinh sir khac nhau ve hieu suat cua viec ghi phan ra 27 va 3Y [8].
Bai bao nay trinh bay ket qua do phd thdi gian sdng vdi muc dich tim hieu ve kich thudc cac Id rdng trong he thdng mao quan ddng deu va rat trat tu tdn tai ben trong ze-d-lit.
2. Thuc nghiem Vat lieu
Ba loai vat lieu Id rdng dugc chgn de khao sat:
Loai A: vat lieu Id rdng nhd (microporous) ZSM-5, Fe-MFI-100, Fe-FER (dudng kinh kenh 0.5nm), cau true bao gdm eac khung Id rdng chat che va cac kenh hep.
Loai B: Hai Idp Hydro-xit (LDH): Loai ciu triic nay thi day dac hon loai tren- cac
^ f r
ion sat ket hgp chat che 0 ben trong - giiia cac Idp bru-xit eiia eac mang tao thanh -
f r
gidng cau tnic cua hidro-tanxit.
Loai C (Fe-SBA-15, SBA-15): SBA-15 la loai vat lieu ed ciu true lo rdng trung binh sd huu mdt mang ludi eua nhiing kenh va nhiing Id rong vdi kich thudc trong khoang 2-50nm, dudng kinh eua eac kenh khoang 4-6 nm, tinh thi 3D thi khdng rd rang trong cac thanh Id. Trong vat lieu Fe-SBA-15, sit duge chen vao chd phan tan ion eua ludi silie-oxit trong cau triic SBA-15.
He do
Phuong phap buy pd-si-trdn (LT) sir dung ngudn Na-22 cd boat do thip (0.5-2 MBq). Mdt may do phd triing phiing nhanh-nhanh dugc trang hi vdi bd nhan quang XP-2020-URQ va cac chat nhap nhay BaF2 va N E l l l . Cac cira sd vimg nang lugng rdng dugc thilp lap dl vira cd hieu suit cao, vin cho do phan giai thdi gian tdt (250 ps FWHM ddi vdi Co-60). Phd thdi gian sdng thu dugc nhd sir chuyin thdi gian do dugc thanh chilu cao eua xung, trong trudng hgp sir dung ddng thdi hai bd cho phep thilt lap khoang thdi gian khac nhau mgt each dong thdi. Chudi du lieu thu thap dugc tren phd buy pd-si-trdn bao gdm cac don vi tieu chuin ORTEC va TENNELEC. The nhd may tinh da kenh vi-nhanh Oxford 16k dugc sir dung dl thu thap dir lieu. Cac gia tri hieu chinh thdi gian dugc thilt lap thay ddi gia tri tir 20.1 ps/ch din 200.3 ps/ch. Thdi gian song va cudng do huy cua pd-si-trdn dugc xac dinh bing chuong trinh LT v9.
Bdt ze-d-lit dugc nen vdi ap lire 0.3 GPa trong cac dTa cd dudng kinh 8-13 mm va be day 1-1.5 mm. Trong nghien ciiu LT, cac vat lieu duge chuin hi va sip xIp sao cho ngudn Na-22 dugc dat giira hai tim Al. Vat mlu dugc dat vao mdt tim dd bing nhdm, cac thi nghiem dugc tiln hanh trong mdi trudng chan khdng d nhiet do phdng. Kha nang lap lai tdt phep tinh cac ket qua LT dat dugc bing each ghi lai phd trong nhilu ngay va lap lai eac thi nghiem.
3. Ket qua va thao luan
Phd thdi gian sdng pd-si-trdn thu nhan dugc tir viec do 5 mau dugc tien hanh phan tich bang chuong trinh lam khdp LT v.9, viec lam khdp cd the tien hanh vdi 1, 2,
> r f
3... thanh phan thdi gian sdng. Theo cac ket qua nghien ciiu vat lieu ze-d-lit da dugc cdng bo thi viec lam khdp phii hgp thudng ddi hdi it nhat 4 thanh phan thdi gian sdng trong do: 0,2-0,5ns cd the quy cho sir buy cua p-Ps va e"^ tu do, l-4ns ddi vdi o-Ps cd the bi bIy trong nhiing Id rong nhd (microvoids), 10-90ns cd the dac trung cho o-Ps buy ben trong cac kenh cua ze-d-lit, 100-13 5ns cd the tuong iing cho sir buy cua o-Ps trong nhirng Id trdng ldn hon. [8]
Kit qua phan tich dugc trinh bay trong Bang 1, cac gia tri thdi gian song nim trong pham vi tir 0,114-35,320ns. Kit qua cho thiy cd thi chia cac thanh phin thdi gian sdng thu duge thanh 3 nhdm sau: 0,114-0,336ns do sir buy eua p-Ps va e^ tu do (hai thanh phin TI, T2), l,680-3,902ns ddi vdi o-Ps bi biy trong nhirng khoang rdng nhd la nhimg khung ze-d-lit nhd (thanh phin T3) va 13,490-35,320ns cd thi tuong img vdi sir buy cua pd-si-trdn trong nhimg Id rdng trung binh (mesopores) (thanh phan T4).
Tap chi KHOA HOC OHSP TPHCM Sd 58 ndm 2014
Bdng 1. Kit qud gid tri thai gian sdng vd cuomg do tuang img ciia cdc mdu
Mau
Ti(ns) T2(ns)
tsCns)
T4(ns)
!,(%) I2(%)
h{%)
l4(%)
ZSM-5 0,114± 0,001 0,261 ±0,001 1,700 ±0,022 23,500 ±0,370 32,151 ±0,065 63,690 ±0,075 3,227 ±0,074 0,932 ±0,007
Fe-MFI-100 0,129± 0,001 0,252 ±0,001 2,217 ±0,020 18,680 ±0,530 44,860 ±0,590 53,140± 0,590 1,508 ±0,023 0,489 ±0,0095
Fe-FER 0,161 ±0,005 0,336 ±0,003 1,929 ±0,038 13,490 ±0,550 36,500± 1,200 61,400 ±1,200 1,662 ±0,048 0,460 ±0,014
LDH 0,128± 0,007 0,189 ±0,008
1,680 ±0,034 29,530 ±0,980 41,200 ±7,000 56,700 ±7,000 1,100±0,150 1,000± 0,140
Fe-SBA-15 0,149 ±0,006 0,247 ±0,005 3,902 ±0,098 35,32O±O,150 26,900 ±3,000 68,400 ±2,900 0,769 ±0,044 3,930 ±0,220 Bang 1 cho thay cudng do iing vdi thdi gian sdng thudc nhdm dau tien chiem mdt tl le rat cao so vdi hai nhdm cdn lai. Dieu nay cd nghTa la da sd pd-si-trdn khi di vao mlu se buy tu do hay buy dudi trang thai p-Ps va chi mdt phan nhd dudi 5% pd-si-trdn se buy d trang thai o-Ps. Ben canh dd, hai thanh phan dai nhit duge dimg de danh gia kich thudc va mat do Id rdng hien dien ben trong ciu true ze-d-lit.
Thdi gian sdng eua o-Ps trong chan khdng rat dai khoang 140ns trong khi gia tri thdi gian sdng ciia trang thai o-Ps thu dugc tir 5 mau ze-d-lit nay chi nam trong khoang
1,680ns - 35,320ns. Nguyen nhan lam giam thdi gian sdng ciia o-Ps la do trong vat chat dam dac, o-Ps tuong tac vdi nhirng e-lec-trdn eiia phan tir va nguyen tir xung quanh, dilu nay rut ngin thdi gian sdng ciia o-Ps va phat ra hai y thdng qua qua trinh dugc ggi la sir buy dap tit ("pick off).
Theo cdng thirc (1) va (2) ban kinh Id rdng tudng ung vdi hai thanh phin thdi gian sdng dai nhat T3 va T4 da dugc tinh va dugc trinh bay trong Bang 2.
Bdng 2. Ket qud hdn kinh Id rdng tuang img vai hai thdnh phdn TJ vd X4.
Mlu ZSM-5 Fe-MFI-100
Fe-FER LDH Fe-SBA-15
T3(ns)
1,700 2,217 1,929 1,680 3,902
r3(nm)
0,256 0,305 0,279 0,254 0,419
T4(ns)
23,500 18,680 13,490 29,530 35,320
r4(nm)
1,012 0,856 0,747 1,131 1,238
Do thj tuong quan ban kinh va thoi gian song
1,5
I •?
1 .5 1
1.5 I 0,5 0
0.05 0.1 0.15 0.2 0,25 0 .i 0.35 0,1 0.-I5 Ban kinh (nm)
Hinh 1. Dd thi tuang quan giita bdn kinh vd thai gian sdng TJ
40 35 - 30 si 25
•.g
= 20
"St- .TlS
J :
~ 10 5 0
Do thi tuong quan ban kinh va thM gian song
iTe-NB,->-l.'=) 1.2 .vS
/
^ 1 . 0 1 2 (V-SM-.^)
— - • • >/ '
*-'o.?5«,Fc0.rFi-100!
•P 0.-47,rc-ITJO
~ " ~ " • ' "
0 0.2 0.4 0.6 0.8 1 1.2 1.4 Bin kinh (nm)
Hinh 2. Dd thi tuang quan giita bdn kinh vd thdi gian sdng T4
Anh hudng eua cau triic Id rdng phan anh rd rang qua gia tri thdi gian sdng dai khac nhau ciia eac mau, ddi vdi Id rdng kich thudc cang ldn thi cang giii cho o-Ps tdn tai trong nd lau. So sanh cac gia tri T3 va T4 ta ed the nhan xet dinh tinh rang kich thudc eiia Id rdng nhd va trung binh eiia mau Fe-SBA-15 la ldn nhat trong 5 mau. Hay ndi each khac, do Fe-SBA-15 ed kich thudc Id rdng ldn nhat lam cho thdi gian sdng eiia o- Ps trong nd ciing ldn hon nhieu so vdi cac mau cdn lai. Viec xem xet gia tri cac ban kinh r3 va r4 se giiip ta dinh lucmg rd rang hon kich thudc eiia eac loai Id rdng iing vdi timg mlu.
Tap chi KHOA HOC OHSP TPHCM Sd 58 ndm 2014
Giira ban kinh Id rdng va thdi gian song cuo. pd-si-trdn cd mdi quan he tuong quan lan nhau. Mdi tuong quan giiia ban kinh va thdi gian sdng cua hai thanh phan dai nhat dugc bieu dien qua do thi d Hinh 1 va Hinh 2.
Tir do thi d Hinh 1 va 2, ta nhan thay thdi gian sdng cua o-Ps tang theo ban kinh ciia Id rdng, ban kinh cang ldn thdi gian sdng cua o-Ps trong vat chat cang dai. Va trong tat ca cac mau ze-d-lit thi SBA-15 khdng nhirng cd mat do tdn tai cac loai Id rong trong cau triic ze-d-lit cao ma kich thudc ciing ldn hon so vdi cac ze-d-lit khac. Rieng ddi vdi LDH cd ban kinh Id rdng nhd nhat nhung Id rdng trung binh ciia nd thi ldn hon so vdi cac ze-d-lit loai A.
Cac ket qua phan tich cua PAS cung chi ra sir khac biet rd ret ve do rdng cua cac ze-d-lit khac nhau ciing nhu su cd mat cua thanh phan sat trong cau triic ciia cac ze-d- lit. Tuy nhien, eac ket qua nay chi cd tinh tuong ddi khi so sanh tinh chat va cau true cua cac ze-d lit khac nhau. De cd the danh gia dinh lugng kich thudc do rdng bing phuong phap PAS can cd cac nghien ciiu sau hon tren co sd so sanh vdi eac kit qua nghien ciiu bing cac phuong phap TEM, xac dinh tinh thi bing tia X (X-ray crystallography).
4. Ket luan
Phd thdi gian sdng pd-si-trdn la mdt kT thuat cd thi sir dung cho viec nghien ciiu nhiing lo trong nhd va nhimg tinh chit ben trong be mat nhirng vat lieu xdp. Kit qua thdi gian sdng thu dugc tir phuong phap nay se phan anh kich thudc Id rdng ben trong vat lieu ze-d-lit. Ben canh dd, so lugng Id rdng trong vat lieu ze-d-lit se dugc phan anh qua cudng do tudng irng. Do dd tir cac kit qua thdi gian song va cudng do tuong ung, ta cd thi kit luan ring: eac loai ze-d-lit trong nghien ciru diu ed nhirng sai khac vl ciu tnic Id rong tir kich thudc cho din mat do. Trong dd, miu loai C (SBA-15) chiia nhimg lo rdng vdi kich thudc va mat do ldn hon nhilu so vdi miu loai A va B.
1.
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(Ngdy Ida soan nhan dwgc bdi: 26-11-2013; ngdy phan bien ddnh gid:05-5-2014;
ngdy chip nhdn dang: 16-5-2014)
