- Email: [email protected]
Surface and Texture Properties of Tb-Doped Ceria-Zirconia Solid Solution Prepared by Sol-Gel Method
Available online at www.sciencedirect.com
ScienceDirect JOURNAL OF RARE EARTHS 25 (2007) 42 - 47
www .elsei-ier.codlocateljre
Surface and Texture Properties of Tb-Doped Ceria-Zirconia Solid Solution Prepared by Sol-Gel Method
ah$)'
Fan Guodong ( % * , Feng Changgen ( 4%$P)', Zhang Zhao ( $fi @ )' ( 1 , Institute of Chemistry and Chemical Engineering, Shaanxi University of Science and Technology, Xianyang 712081 , China ; 2 . State Key Laboratory of Prevention and Control of Explosion Disasters, Beijing Institute of Technology , Beijing 100081 , China ) Receivzd 22 May 2006; revised 21 June 2006
Abstract: The three-way catalysts (TWCs) promoters Ceo.6Zro.4-.Tb,yOz- were prepared by sol-gel method. BET surface areas analysis indicated that an increase of the dopant Tb content from x = 0.05 to x = 0.15 favors an increase of surface area from 6 6 . 8 to 8 0 . 4 m' * g compared with the undoped sample Ceo.60 Zr0.4o 0 2 65. 1 m2 * g - after calcination at 650 "c . Transmission electron microscopy (TEM) observation indicated that the doped samples have a higher thermal stability. The XRD and Raman spectra confirmed that the Ceo,6ZrO.4-,TbTO,_, cubic solid solution is formed. XPS analysis revealed that Ce and Tb mainly existed in the form of Ce4' and Tb3 , and Zr existed in the form of Zr4' on the surface of
'
'
+
the samples. The doped samples were homogenous in composition ; the introduction of Tb into the CeOP-ZrOz promoters resulted in the formation of a solid solution, and the concentration of surface lattice oxygen was increased.
Key words: Ceo.sZro.4 - .Tb,OZ earths CLC number : 0 6 4 3 . 3
-
solid solution ; three-way catalysts ; sol-gel method ; surface and texture structure ; rare
Document code : A
Article ID : 1002 - 0721(2007 )01 - 0042 - 06
The most effective three-way catalysts( TWCs) are the systems containing Ce02i1'21 because of the buffering capacity of oxygen under conditions of the rich/ lean oscillations of composition of exhaust gases. However, the use of the catalysts has certain drawbacks in that. The oxygen storage capacity ( 0 % 1 of pure Ce02 is deactivated when the exhaust temperature exceeds 850 "c , due to sintering of the Ce02 particles and decrease in surface area. Therefore, it is very important to stabilize Ce02 and to avoid sintering when it is used at high temperature. Generally, doping of different cations significantly stabilize the cerium oxide against sintering. Among many different cations, the addition of zirconium, es-
-
pecially the formation of ceria-zirconia solid solution, is very effective for the inhibition of the sintering. The doping of rare earth elements with variable oxidation states strongly affects the redox properties of TWCs. For example, incorporation of terbium or praseodymium increases both oxygen desorption at lower temperatures and the creation of oxygen vacancies, compared with pure ceria. The former is due to the lower binding energy of a lattice oxygen in the mixed oxides and the latter is due to the existence of trivalent terbium and praseodymium ions'31. In this study, the ternary oxides ceo6 Zro4-* ' I I I , O ~ -were ~ prepared by the sol-gel method. These mixed oxides were characterized by BET surface area,
Corresponding author (E-mail : fangd @ sust .edu .cn ) Foundation item: Project supported by the Natural Science Basic Research Plan in Shaanxi Province of China (2004B13) Biography : Fan Gucdong ( 1964 - ) , Male, Doctor, -4ssociate professor; Research direction: purification of automotive exhaust Copyright 0 2 0 0 7 , by Editorial Committee of Journal of the Chinese Rarr Earths Society . Published by Elsevier B . V . All rights reserved.
43
Fan G D et a1 . Tb-Doped Ceria-Zirconia Solid Solution Prepared by Sol-Gel Method
XRD, Raman :.pectra, TEM, and XPS. The surface and texture properties of the samples were also discussed.
1 Experimental 1 . 1 Catalyst preparation The conqiosition Ceo,mZro.40 O2 ( denoted as CZ) and Ceo 6ZroI ,‘Tb,02- , with x = 0.05, 0.10, and 0 . 1 5 (denote,] as CZTl CZT2, and CZT3, respectively), were prepared by sol-gel method to obtain mixed Ce/Zr ‘Tb nitrate solutions containing the required ratio The starting materials containing Ce, Zr, and Tb such as Ce ( NO,), * 6 H 2 0 , Zr ( NO, ) 4 5 H 2 0 , and Tl),O, were all of A K grade. To obtaiii a mixed Ce/Zr/Tb nitrate solution, a calculated amount of d c - Tb,O, powder was dissolved in nitric acid. To this solution, a calculated amount of Ce( KO3 ), 6H20 and Zr ( NO, )I * 5H20 powder was added. This :elution was then diluted with ethanol solvent to adjust the metal content to 0 . 2 mo1.L-I. 4 total of 0 . 4 mo * L - of citric acid-ethanol solution was added into 0 . 2 mo1.L-l of the mixed metal nitrate solutions at an equal volumetric ratio, and the solution was desolventized at 80 “c until the gel appeared. The wet gel was ciried overnight at 80 T and finally calcined in air k t 650 T for 4 h . Some portions of the calcined rnix,d oxides were once again heated at 900 T for 4 11.
XPS data were acquired using a VG Scientific CALab220i-XL spectrometer. This system was equipped with an A1 Ka X-ray source operated at 300 W . The base pressure was about 3 x lo-’ P a . The binding energies were referenced to the C l s line at 284.6 eV from the adventitious carbon.
2 Results and Discussion
I
.
.
-
’
1 . 2 Catalyst characterization The surf.ice areas of the sample were measured by BET method ]sing N2 adsorption. X-ray powder diffraction patterns were recorded on a Rigaku D/inax-2200PC diffractometer using Cu Ka ( A = 0.154 1 3 nm) radiation as X-ray source, operated at 40 k\; and 40 mA. Data acquisition was realized in the 20 of 20“ 6.5’ with a scan step size of 0.03”. Raman > pectra were recorded at room temperature using a Bruk1.r RFS- 100 spectrometer. The surface morphology and corresponding microstructures wwe obsened using H-700 transmission electron micr,mopy .
-
2 . 1 Analysis of BET surface area The doping composition, the denotation, and the BET surface area of the mixed oxides are listed in Table 1 . From Table 1 , it can be seen that the samples calcined at 650 “c exhibit reasonably high specific surface area than those calcined at 900 T . ‘The observed decrease in the surface area with the increase in calcination temperature could be due to various reasons such as better crystallization of ceria and sintering and phase separation of the mixed oxide. The CZT samples exhibit larger surface area than that of the CZ sample. An increase of the dopant Tb content from x = 0.05 to x = 0.15 favors an increase of surface area from 66.8 to 80.4 m’ g - compared with the undoped CZ sample with a surface area of 65.1 m * - g - ’ after calcination at 650 “c. The results suggested that the addition of Tb enhanced the stability of the surface areas of the catalysts. This might be due to the fact that the crystallite growth process was retarded by the incorporation of Tb ions into the mixed oxide catalysts or the by the high dispersion of Tb on surface of the CZ sample. However, when the calcination temperature rose up to 900 “c: , the effect of Tb ions was found to be minimized. The surface areas of all the samples were found to decrease drastically after calcination at 900 “c.
- ’
2.2 XRD and Raman spectra analysis The powder XRD patterns of CZ and CZT samples after calcination at 650 “c are shown in Fig. 1 . It could be seen that the patterns consisted of four main reflections, typical spectrum of a fluorite-structured material corresponding to the ( 111) , (200) , (220) ,
Table 1 Surface area, crystallite size, and lattice parameters of doped samples Surface a r e a / ( r n 2 * g - ’ )
Danotarioii
I +iy
cz
1 Rl 6&1
CZTl
CZT2 CZT3
P,,
(
viiiiipuaition
65.1
401
mZr0 i5Tho
e,, MZro ,i,Tho eo &n
650 ‘t:
zsTbo
~
1002.
IS&
~
,
66.8 68.0 80.4
(4 h )
Crystallite size
900 ‘C (4 11) 10.4 13.7 12.5 14. I
Lattice parameter
n /inn 9.6 9.8 9.5
9.8
0.5310 0.5315 0,5328 0.5326
JOURNAL OF RARE EARTHS, Vol. 2 5 , N o . I , Feb . 2007
44
and (311) planes. The average crystallite sizes and lattice parameters of the samples, calculated from the ( 111) , ( 2 0 0 ) , ( 2 2 0 ) , and (311) planes, are listed in Table 1. Lattice parameters of Tb-doped samples are close to those of CZ, which means that Zr4+ and Tb3+(or Tb4+) ions dissolved in CeOz and formed the solid solution. Crystallite sizes of doped samples changed only slightly, but the BET surface areas increased dramatically with the introduction of T b . The drastic increase of BET surface areas is a combined result of enhancement of pore structure and inhibition of crystal aggregation. To further support XRD results, the Raman study on doped samples was undertaken. Raman spectra of Ceo mZro30TbO l o 0 2 - are shown in Fig. 2. The Raman spectra display a single sharp intense peak at 462 cm-' due to the T2g Raman active mode of a cubic fluorite structure'41. The absence of characteristic peaks around 177, 190, and 260, 330 cm-' for monoclinic and tetragonal phase of ZrOz, respectively, indicating that Zr4' ions dissolved into the CeO, lattice and formed the solid solution. ~
2.3 TEM analysis of morphology Fig. 3 shows the transmission electron micrograph
20
30
40
50
60
70
2 U/(")
Fig. 1 XRD patterns of samples synthesized by sol-gel method
I
200
300
400 500
600 700 ROO 900
Wavcnumbericin I
Fig. 2
Raman spectra of Cr, 6oZro30Ti1O
~
of Ceo mZro 30Tb0 Oz-, surface, after calcination at 650 and 900 T , respectively. The micrograph shows that the samples exhibit irregular shape. especially in the portion of the particles that is aggregated. The difference of average particle size between 650 and 900 "c are not obvious, which implies that the doped samples have a higher thermal stability than pure CeOz.
2 . 4 XPS study Fig.4 shows the Ce 3d spectra of the doped saniples. From Fig.4, it can be observed that two sets of spin-orbit multiplets: u and v correspond to the features of 3d3/2 and 3ds/2, respectively; the Ce 3d spectrum correspond to three main features of 3d5,, at c a . 882.5 ( v ) , 889.0 ( v z ) , and 898.5 eV ( v , ) and three main features of 3d3,, at c a . 900.8 ( u ) , 9 0 7 . 7 (u,) , and 91 6 . 6 eV ( ul) . The six peaks correspond to unique photoelectron features of the Ce4+ state'" . Specific Ce3+ features are present at approximately 885.6 eV ( v , ) and 9 0 3 . 9 eV ( u1) in the Ce 3d region, and these signals are considered as fingerprints indicating the existence of Ce3+ ion'". The absence of v1 and u1 peaks in the Ce 3d specura( Fig. 4) indicated that Ce mainly existed in the form of Ce'" on the surface of samples. (b)
Fig.3 ' E M image of Ceo.soZro.30Tbo.mOz-, ( a ) Calcination at 650 "c for 4 h ; (b) Calcination at 900 T for 4 h
45
Fan G D et a l . Tb-Doped Ceria-Zirconia Solid Solution Prepared by Sol-Gel Method
-’
Accordi ig to He et a1 . , signals at ca . 149.3 eLT were assignecl to the characteristic features of Tb3’ and the signal at ca . 156.9 and 164.5 eV to Tb4+. Fig. 5 shows the Tli 4d spectra of the doped samples. From the relative intensit!- of the signals, it can be seen that Tb are rnainli present in the Th“-oxidation state on the surface of samples, and w-ith the increase in Tb content, the intensities of the Tb3+ signals increases. Fig. 6 shows Tb3d ipectra of the doped samples, and it can be seen that the signal at ca. 1241.6 el- correspond to the characteristic feature of Tb3+. Fig. 7 chows the hinding energy of the Zr 3d photoelectron peaks at c a . 181. 6 and 184. 0 eV for Zr 3d5,?and Zr 3d3,?lines, respectivel?.. The binding energies of Zr 3d3,, and its full-width at half-maximum ( FX’HM 1 d u e s are listed in Table 2 . From Table 2 , it can be s’en that the constant FWHM values are around 1 . 5 , implying that one type of doublet is present . “his provides evidence for the presence of a single type. of zirconium oxide with an oxidation state of + 4 in the doped samples. The binding energies of Zr 3dSI2and Zr 3d3,, lines in pure ZrO, are 182.2 and 1 8 4 . 6 eV , respectively’ . However, compared with pure ZrO,. B shift of ca . - 0 . 6 eV toward lower binding e n e r u ( a n be noted in the Zr 3d spectra of Ceo,60
’
I
Z r o ~ 3 0 T b o , , 0 0. 2This ~ ? is mainly due to the formation of the solid solution and the longer Zr-0 bond distance in the particular mixed oxides‘”. Fig. 8 shows the 01s spectra of the doped samples treated in air flow at 650 “c . There are two 01s speaks at c a . 5 2 9 . 4 and 5 3 1 . 6 eV for all the samples. The signal at lower binding energy (529.4 eV ) was assigned to surface lattice oxygen and the signal at higher binding energy to adsorbed oxygen species such as 0 - , 0;- , or 02-r5’. It can be found that with the increase in Tb content, the intensity of the component at ca. 529.4 eV increases. The atomic percent of surface lattice oxygen (low BE of O l d O l s ) of the doped samples are listed in Table 2 , and the results indicate that the incorporation of Tb3’ into CeO2-ZnOz lattice led to the formation of defective structure and increases the concentration of lattice oxygen. ’Ihble 2 Binding energies (eV) and FWHM of Zr 3dg2,Ols Position and
Samples k W H M of Zr 3dy2
Position and FWHM of 01s ___--__---Lower BE Higher BE
CZTl WT2 WT3
529.4(1.85)” 531.6(1.98)* 86.4 529.4(1.88) 531.6(1.42) 87.8 529.4(2.13) 531.6(1.36) 88.8
*
184.0(1.46)‘
184.0(1.51) 183.9(1.48)
Atomic percent of surface lattice oxygen/%
Numhem in parentheses are FWHM values
1
Cd3dl
Tb4d
Th3’
871
880
890
900
910
920
130
140
Binding encrgy:cV Fig . 4
150
160
170
Binding energy/cV
Ce3d YPS of Ceu 6oZro ,Th, OZ.
Fig.5
I
Tb4d XPS of Ceo boZro40-.Tb,0z.y
I
I
Zr 3d
F.
z
v
0
.e 0)
2 3
1220
1230
1240
1250
1260
170
Binding cnergyieV
FI: 6 Tb3d XPS of Ceo &r0
175
180
185
190
I!
Binding cncrgy/eV
,Tb,02.
~
Fig.7
Zr3d XPS of Ceu60Zro40-rTbx02-)
.
46
JOURNAL OF RARE EARTHS, Vol. 2 5 , N o . 1 Feb
520
525
Binding energyieV
Fig.8
530
535
. 2007
541
Binding energylev
01s XPS of CeoMZr~40-~TbrOz.3 (a) 0 1 s of TC1; (b) 0 1s
An important effect of the cation doping is increased in the number of defect such as oxygen vacancies. The creation of oxygen vacancies improves the mobility of oxygen in the bulk sample, increases the efficiency of CeO, as oxygen materials in the automotive three-way catalysts, and produces a material with a higher OSC"ol . For hydrocarbon oxidation reaction, the stabilization of RE the catalyst would result in an enhanced activity of catalyst because a number of oxygen vacancies become available due to KE3+ formation. Tb02 is easily reduced than Ce02 at lower temperature, which means that the stability of RE3+ is Tb3 > Ce3' ; so the element Tb is mainly present in the Tb3+ oxidation state on the surface of the doped samples. Trovarelli A et al."] pointed out that the doping of trivalent into CeO, lattice could generate anion vacancies, leading to an increase in ability of the materials to take up and release oxygen. Similarly, the incorporation of Tb3+ into Ce02-Zn02lattice would also enhance the redox activity and lower the light-off temperature of TWCs. Such a behavior was also confirmed by the results of previous studies'"'. In order to investigate the surface composition of the doped samples, Ce 3d5,, ( v ) , Zr 3d5,2, Tb 3d512, and 0 1 s are selected as the peaks for the quantitative analysis of element. The XPS quantitative analysis results of Ceo 60Zr0 3oTbo 02-?are listed in Table 3 . With the increase of the dopant Tb content from x = 0.05 to x = 0.15, the values of surface concentration of Ce4+ and oxygen species (including lattice oxygen +
0'- and adsorbed oxygen species such as 0 - , Oz2- , or 02-) are constant, the surface concentration of
Th3' increases from 1.0% to 3 . 0 % , and the surface concentration of Z$+ decreases from 14. 3 % to 10.8 % . The surface atomic ratio obtained from XPS is close to the composition of the samples, which means that the surface and bulk of the doped samples are homogenous in composition. These results indicate that the incorporation of Tb into the CeO,-%rO, leads to a higher dispersion of Tb itself on the surface and promotes the formation of a solid solution.
3 Conclusions 1. The results of BET surface areas analysis suggested that doping a small amount of Tb into Ce02ZrO, could increase the surface areas of catalysts. 2 . From XRD and Raman results, it was found that all the Tb-doped catalysts possessed a cubic fluorite lattice, which means that Tb4+and Zr4+ ions dissolved into the CeO, lattice and formed the Ceo Jr0 Tb,Oz_ cubic solid solution. 3 . XPS analysis demonstrated that Ce and Tb mainly existed in the form of Ce4+ and Tb3+, Ceo 60 Zro ,,Tho x = 0.05, 0.10 and 0.15) and Zr existed in the form of Zr4+ on the surface of the doped samples. The doped samples were homogenous in composition, and the incorporation of Tb into Ce02ZrOz promoted the formation of a solid solution and increased the concentration of surface lattice oxygen. ~
Table 3 WS quantitative results of Ceo.mZro.JoTbo.1002_, Surface atomic percent/%
Samples
Ce 3d5/2(u)(882.6 eV)
CZTl
11.9
CZT2
12.1
CZT3
11.8
Tb/Ce atomic
%rice atomic rat in
Tb 3dm(1241.6 rV) 01s (529.4 and 531.6 eV)
ratio
1.1
72.7
0.09
12.8
2.1
12.9
0.17
1.1
10.8
3.0
72.4
0.25
0.9
Zr 3d5,2(181.6 eV)
____ 14.3
____ 1.3
47
Fan G D et a1 , Tb-Doped Ceria-Zirconia Solid Solution Prepared by Sol-Gel Method
tion of Ce02-Zr0, mixed oxides comparison of thr citrate and sol-gel preparation methods [ J ] . Chem . Mater . ,
References : 1
h n g C h: , Ozawa h i .
The role of rarr earth dopants in
nanophsar zirconia catal!-sts for automoti\ e emission control J . Journal of A l l o ~ s and Compounds , 2000, ~
.2
303 - 301: 60. Trot arell i A . Boaro M , Rocrhini E , et a1 .
Some recent
dex elopi~ierits in the characterization of ceria-based catalysts .J . . Journal of A l l o ? ~and Compounds, 2001,
323 - 323 : 584. Troxarel i A . Catal!sis b!- Ceria and Related Materials :)I . 1, indon : Imperial College Press, 2002. 73. 4. Potdar II 5, Deshpande S R , Deshpande A S, et al. Preparat cin of crria-zirconia ( Ceo ..%r0 a O ? ) powders hy micro~~-a,t-h!drothermal ( MH route J j . Waterials Chemistrr and PhJsirs. 2002. 7 4 ( 3 ) : 306. 5 He H . Dai H X , R ong I\ a, et a1 . SrCI2-promoted REO, ( F'E = Ce . Pr , Th ) catalysts for the selectivr oxi-
.3
I
dation t f ethane: a study on performance and defect , structure- for ethene formation [ J 1 . Journul of Catal>~is
2001. 19912): 177.
6
.4lifanti \ I , Baps B . Blangrnois N . rt a l .
characteriza-
2003, lS(2): 395. [ 7 ] Chary K V R , Sagar G V , Naresh D ,
et
al.
Character-
ization and reartivity of copper oxide catalysts supported on TiO,-ZrOz [ J ] . J . Phys. Chem. H . . 2005, 109
(19) : 9437. [ S ] Ardizzone S , Bianchi C L , Signoretto M . Zr( ) surface chemical state and acid features of sulphated-zirconia samples [ J 1 . Applied Surjiace Science, 1998, 136 (3): 213. [ 9 ] Kim Y J , Thevuthasan S , Shutthananadan V , et al. Growth and structure of epitaxial Cel-.Zr,02 thin films on yttria-stabilized zirconia ( 1 I1 ) [ J ] . Journal of Electron Spectroscop and Related Phenomena , 2002, 126 ( 1 - 3 ) : 177. [ 101 Feng Changgen , Fan guodong . Recent development in doping and structural modification of ceria-based catalysts [ J] . Journal of Rare Earths , 2005, 23( 3 ) : 309. [ 11] Feng Changgen , Fan Guodong , Liu Xia . Study of ceria-zirconia solid solution promoters modified by Tb [ J]. China Journal of Applied Chemistry, 2005, 22( 7 ) : 703.
n'
ScienceDirect JOURNAL OF RARE EARTHS 25 (2007) 42 - 47
www .elsei-ier.codlocateljre
Surface and Texture Properties of Tb-Doped Ceria-Zirconia Solid Solution Prepared by Sol-Gel Method
ah$)'
Fan Guodong ( % * , Feng Changgen ( 4%$P)', Zhang Zhao ( $fi @ )' ( 1 , Institute of Chemistry and Chemical Engineering, Shaanxi University of Science and Technology, Xianyang 712081 , China ; 2 . State Key Laboratory of Prevention and Control of Explosion Disasters, Beijing Institute of Technology , Beijing 100081 , China ) Receivzd 22 May 2006; revised 21 June 2006
Abstract: The three-way catalysts (TWCs) promoters Ceo.6Zro.4-.Tb,yOz- were prepared by sol-gel method. BET surface areas analysis indicated that an increase of the dopant Tb content from x = 0.05 to x = 0.15 favors an increase of surface area from 6 6 . 8 to 8 0 . 4 m' * g compared with the undoped sample Ceo.60 Zr0.4o 0 2 65. 1 m2 * g - after calcination at 650 "c . Transmission electron microscopy (TEM) observation indicated that the doped samples have a higher thermal stability. The XRD and Raman spectra confirmed that the Ceo,6ZrO.4-,TbTO,_, cubic solid solution is formed. XPS analysis revealed that Ce and Tb mainly existed in the form of Ce4' and Tb3 , and Zr existed in the form of Zr4' on the surface of
'
'
+
the samples. The doped samples were homogenous in composition ; the introduction of Tb into the CeOP-ZrOz promoters resulted in the formation of a solid solution, and the concentration of surface lattice oxygen was increased.
Key words: Ceo.sZro.4 - .Tb,OZ earths CLC number : 0 6 4 3 . 3
-
solid solution ; three-way catalysts ; sol-gel method ; surface and texture structure ; rare
Document code : A
Article ID : 1002 - 0721(2007 )01 - 0042 - 06
The most effective three-way catalysts( TWCs) are the systems containing Ce02i1'21 because of the buffering capacity of oxygen under conditions of the rich/ lean oscillations of composition of exhaust gases. However, the use of the catalysts has certain drawbacks in that. The oxygen storage capacity ( 0 % 1 of pure Ce02 is deactivated when the exhaust temperature exceeds 850 "c , due to sintering of the Ce02 particles and decrease in surface area. Therefore, it is very important to stabilize Ce02 and to avoid sintering when it is used at high temperature. Generally, doping of different cations significantly stabilize the cerium oxide against sintering. Among many different cations, the addition of zirconium, es-
-
pecially the formation of ceria-zirconia solid solution, is very effective for the inhibition of the sintering. The doping of rare earth elements with variable oxidation states strongly affects the redox properties of TWCs. For example, incorporation of terbium or praseodymium increases both oxygen desorption at lower temperatures and the creation of oxygen vacancies, compared with pure ceria. The former is due to the lower binding energy of a lattice oxygen in the mixed oxides and the latter is due to the existence of trivalent terbium and praseodymium ions'31. In this study, the ternary oxides ceo6 Zro4-* ' I I I , O ~ -were ~ prepared by the sol-gel method. These mixed oxides were characterized by BET surface area,
Corresponding author (E-mail : fangd @ sust .edu .cn ) Foundation item: Project supported by the Natural Science Basic Research Plan in Shaanxi Province of China (2004B13) Biography : Fan Gucdong ( 1964 - ) , Male, Doctor, -4ssociate professor; Research direction: purification of automotive exhaust Copyright 0 2 0 0 7 , by Editorial Committee of Journal of the Chinese Rarr Earths Society . Published by Elsevier B . V . All rights reserved.
43
Fan G D et a1 . Tb-Doped Ceria-Zirconia Solid Solution Prepared by Sol-Gel Method
XRD, Raman :.pectra, TEM, and XPS. The surface and texture properties of the samples were also discussed.
1 Experimental 1 . 1 Catalyst preparation The conqiosition Ceo,mZro.40 O2 ( denoted as CZ) and Ceo 6ZroI ,‘Tb,02- , with x = 0.05, 0.10, and 0 . 1 5 (denote,] as CZTl CZT2, and CZT3, respectively), were prepared by sol-gel method to obtain mixed Ce/Zr ‘Tb nitrate solutions containing the required ratio The starting materials containing Ce, Zr, and Tb such as Ce ( NO,), * 6 H 2 0 , Zr ( NO, ) 4 5 H 2 0 , and Tl),O, were all of A K grade. To obtaiii a mixed Ce/Zr/Tb nitrate solution, a calculated amount of d c - Tb,O, powder was dissolved in nitric acid. To this solution, a calculated amount of Ce( KO3 ), 6H20 and Zr ( NO, )I * 5H20 powder was added. This :elution was then diluted with ethanol solvent to adjust the metal content to 0 . 2 mo1.L-I. 4 total of 0 . 4 mo * L - of citric acid-ethanol solution was added into 0 . 2 mo1.L-l of the mixed metal nitrate solutions at an equal volumetric ratio, and the solution was desolventized at 80 “c until the gel appeared. The wet gel was ciried overnight at 80 T and finally calcined in air k t 650 T for 4 h . Some portions of the calcined rnix,d oxides were once again heated at 900 T for 4 11.
XPS data were acquired using a VG Scientific CALab220i-XL spectrometer. This system was equipped with an A1 Ka X-ray source operated at 300 W . The base pressure was about 3 x lo-’ P a . The binding energies were referenced to the C l s line at 284.6 eV from the adventitious carbon.
2 Results and Discussion
I
.
.
-
’
1 . 2 Catalyst characterization The surf.ice areas of the sample were measured by BET method ]sing N2 adsorption. X-ray powder diffraction patterns were recorded on a Rigaku D/inax-2200PC diffractometer using Cu Ka ( A = 0.154 1 3 nm) radiation as X-ray source, operated at 40 k\; and 40 mA. Data acquisition was realized in the 20 of 20“ 6.5’ with a scan step size of 0.03”. Raman > pectra were recorded at room temperature using a Bruk1.r RFS- 100 spectrometer. The surface morphology and corresponding microstructures wwe obsened using H-700 transmission electron micr,mopy .
-
2 . 1 Analysis of BET surface area The doping composition, the denotation, and the BET surface area of the mixed oxides are listed in Table 1 . From Table 1 , it can be seen that the samples calcined at 650 “c exhibit reasonably high specific surface area than those calcined at 900 T . ‘The observed decrease in the surface area with the increase in calcination temperature could be due to various reasons such as better crystallization of ceria and sintering and phase separation of the mixed oxide. The CZT samples exhibit larger surface area than that of the CZ sample. An increase of the dopant Tb content from x = 0.05 to x = 0.15 favors an increase of surface area from 66.8 to 80.4 m’ g - compared with the undoped CZ sample with a surface area of 65.1 m * - g - ’ after calcination at 650 “c. The results suggested that the addition of Tb enhanced the stability of the surface areas of the catalysts. This might be due to the fact that the crystallite growth process was retarded by the incorporation of Tb ions into the mixed oxide catalysts or the by the high dispersion of Tb on surface of the CZ sample. However, when the calcination temperature rose up to 900 “c: , the effect of Tb ions was found to be minimized. The surface areas of all the samples were found to decrease drastically after calcination at 900 “c.
- ’
2.2 XRD and Raman spectra analysis The powder XRD patterns of CZ and CZT samples after calcination at 650 “c are shown in Fig. 1 . It could be seen that the patterns consisted of four main reflections, typical spectrum of a fluorite-structured material corresponding to the ( 111) , (200) , (220) ,
Table 1 Surface area, crystallite size, and lattice parameters of doped samples Surface a r e a / ( r n 2 * g - ’ )
Danotarioii
I +iy
cz
1 Rl 6&1
CZTl
CZT2 CZT3
P,,
(
viiiiipuaition
65.1
401
mZr0 i5Tho
e,, MZro ,i,Tho eo &n
650 ‘t:
zsTbo
~
1002.
IS&
~
,
66.8 68.0 80.4
(4 h )
Crystallite size
900 ‘C (4 11) 10.4 13.7 12.5 14. I
Lattice parameter
n /inn 9.6 9.8 9.5
9.8
0.5310 0.5315 0,5328 0.5326
JOURNAL OF RARE EARTHS, Vol. 2 5 , N o . I , Feb . 2007
44
and (311) planes. The average crystallite sizes and lattice parameters of the samples, calculated from the ( 111) , ( 2 0 0 ) , ( 2 2 0 ) , and (311) planes, are listed in Table 1. Lattice parameters of Tb-doped samples are close to those of CZ, which means that Zr4+ and Tb3+(or Tb4+) ions dissolved in CeOz and formed the solid solution. Crystallite sizes of doped samples changed only slightly, but the BET surface areas increased dramatically with the introduction of T b . The drastic increase of BET surface areas is a combined result of enhancement of pore structure and inhibition of crystal aggregation. To further support XRD results, the Raman study on doped samples was undertaken. Raman spectra of Ceo mZro30TbO l o 0 2 - are shown in Fig. 2. The Raman spectra display a single sharp intense peak at 462 cm-' due to the T2g Raman active mode of a cubic fluorite structure'41. The absence of characteristic peaks around 177, 190, and 260, 330 cm-' for monoclinic and tetragonal phase of ZrOz, respectively, indicating that Zr4' ions dissolved into the CeO, lattice and formed the solid solution. ~
2.3 TEM analysis of morphology Fig. 3 shows the transmission electron micrograph
20
30
40
50
60
70
2 U/(")
Fig. 1 XRD patterns of samples synthesized by sol-gel method
I
200
300
400 500
600 700 ROO 900
Wavcnumbericin I
Fig. 2
Raman spectra of Cr, 6oZro30Ti1O
~
of Ceo mZro 30Tb0 Oz-, surface, after calcination at 650 and 900 T , respectively. The micrograph shows that the samples exhibit irregular shape. especially in the portion of the particles that is aggregated. The difference of average particle size between 650 and 900 "c are not obvious, which implies that the doped samples have a higher thermal stability than pure CeOz.
2 . 4 XPS study Fig.4 shows the Ce 3d spectra of the doped saniples. From Fig.4, it can be observed that two sets of spin-orbit multiplets: u and v correspond to the features of 3d3/2 and 3ds/2, respectively; the Ce 3d spectrum correspond to three main features of 3d5,, at c a . 882.5 ( v ) , 889.0 ( v z ) , and 898.5 eV ( v , ) and three main features of 3d3,, at c a . 900.8 ( u ) , 9 0 7 . 7 (u,) , and 91 6 . 6 eV ( ul) . The six peaks correspond to unique photoelectron features of the Ce4+ state'" . Specific Ce3+ features are present at approximately 885.6 eV ( v , ) and 9 0 3 . 9 eV ( u1) in the Ce 3d region, and these signals are considered as fingerprints indicating the existence of Ce3+ ion'". The absence of v1 and u1 peaks in the Ce 3d specura( Fig. 4) indicated that Ce mainly existed in the form of Ce'" on the surface of samples. (b)
Fig.3 ' E M image of Ceo.soZro.30Tbo.mOz-, ( a ) Calcination at 650 "c for 4 h ; (b) Calcination at 900 T for 4 h
45
Fan G D et a l . Tb-Doped Ceria-Zirconia Solid Solution Prepared by Sol-Gel Method
-’
Accordi ig to He et a1 . , signals at ca . 149.3 eLT were assignecl to the characteristic features of Tb3’ and the signal at ca . 156.9 and 164.5 eV to Tb4+. Fig. 5 shows the Tli 4d spectra of the doped samples. From the relative intensit!- of the signals, it can be seen that Tb are rnainli present in the Th“-oxidation state on the surface of samples, and w-ith the increase in Tb content, the intensities of the Tb3+ signals increases. Fig. 6 shows Tb3d ipectra of the doped samples, and it can be seen that the signal at ca. 1241.6 el- correspond to the characteristic feature of Tb3+. Fig. 7 chows the hinding energy of the Zr 3d photoelectron peaks at c a . 181. 6 and 184. 0 eV for Zr 3d5,?and Zr 3d3,?lines, respectivel?.. The binding energies of Zr 3d3,, and its full-width at half-maximum ( FX’HM 1 d u e s are listed in Table 2 . From Table 2 , it can be s’en that the constant FWHM values are around 1 . 5 , implying that one type of doublet is present . “his provides evidence for the presence of a single type. of zirconium oxide with an oxidation state of + 4 in the doped samples. The binding energies of Zr 3dSI2and Zr 3d3,, lines in pure ZrO, are 182.2 and 1 8 4 . 6 eV , respectively’ . However, compared with pure ZrO,. B shift of ca . - 0 . 6 eV toward lower binding e n e r u ( a n be noted in the Zr 3d spectra of Ceo,60
’
I
Z r o ~ 3 0 T b o , , 0 0. 2This ~ ? is mainly due to the formation of the solid solution and the longer Zr-0 bond distance in the particular mixed oxides‘”. Fig. 8 shows the 01s spectra of the doped samples treated in air flow at 650 “c . There are two 01s speaks at c a . 5 2 9 . 4 and 5 3 1 . 6 eV for all the samples. The signal at lower binding energy (529.4 eV ) was assigned to surface lattice oxygen and the signal at higher binding energy to adsorbed oxygen species such as 0 - , 0;- , or 02-r5’. It can be found that with the increase in Tb content, the intensity of the component at ca. 529.4 eV increases. The atomic percent of surface lattice oxygen (low BE of O l d O l s ) of the doped samples are listed in Table 2 , and the results indicate that the incorporation of Tb3’ into CeO2-ZnOz lattice led to the formation of defective structure and increases the concentration of lattice oxygen. ’Ihble 2 Binding energies (eV) and FWHM of Zr 3dg2,Ols Position and
Samples k W H M of Zr 3dy2
Position and FWHM of 01s ___--__---Lower BE Higher BE
CZTl WT2 WT3
529.4(1.85)” 531.6(1.98)* 86.4 529.4(1.88) 531.6(1.42) 87.8 529.4(2.13) 531.6(1.36) 88.8
*
184.0(1.46)‘
184.0(1.51) 183.9(1.48)
Atomic percent of surface lattice oxygen/%
Numhem in parentheses are FWHM values
1
Cd3dl
Tb4d
Th3’
871
880
890
900
910
920
130
140
Binding encrgy:cV Fig . 4
150
160
170
Binding energy/cV
Ce3d YPS of Ceu 6oZro ,Th, OZ.
Fig.5
I
Tb4d XPS of Ceo boZro40-.Tb,0z.y
I
I
Zr 3d
F.
z
v
0
.e 0)
2 3
1220
1230
1240
1250
1260
170
Binding cnergyieV
FI: 6 Tb3d XPS of Ceo &r0
175
180
185
190
I!
Binding cncrgy/eV
,Tb,02.
~
Fig.7
Zr3d XPS of Ceu60Zro40-rTbx02-)
.
46
JOURNAL OF RARE EARTHS, Vol. 2 5 , N o . 1 Feb
520
525
Binding energyieV
Fig.8
530
535
. 2007
541
Binding energylev
01s XPS of CeoMZr~40-~TbrOz.3 (a) 0 1 s of TC1; (b) 0 1s
An important effect of the cation doping is increased in the number of defect such as oxygen vacancies. The creation of oxygen vacancies improves the mobility of oxygen in the bulk sample, increases the efficiency of CeO, as oxygen materials in the automotive three-way catalysts, and produces a material with a higher OSC"ol . For hydrocarbon oxidation reaction, the stabilization of RE the catalyst would result in an enhanced activity of catalyst because a number of oxygen vacancies become available due to KE3+ formation. Tb02 is easily reduced than Ce02 at lower temperature, which means that the stability of RE3+ is Tb3 > Ce3' ; so the element Tb is mainly present in the Tb3+ oxidation state on the surface of the doped samples. Trovarelli A et al."] pointed out that the doping of trivalent into CeO, lattice could generate anion vacancies, leading to an increase in ability of the materials to take up and release oxygen. Similarly, the incorporation of Tb3+ into Ce02-Zn02lattice would also enhance the redox activity and lower the light-off temperature of TWCs. Such a behavior was also confirmed by the results of previous studies'"'. In order to investigate the surface composition of the doped samples, Ce 3d5,, ( v ) , Zr 3d5,2, Tb 3d512, and 0 1 s are selected as the peaks for the quantitative analysis of element. The XPS quantitative analysis results of Ceo 60Zr0 3oTbo 02-?are listed in Table 3 . With the increase of the dopant Tb content from x = 0.05 to x = 0.15, the values of surface concentration of Ce4+ and oxygen species (including lattice oxygen +
0'- and adsorbed oxygen species such as 0 - , Oz2- , or 02-) are constant, the surface concentration of
Th3' increases from 1.0% to 3 . 0 % , and the surface concentration of Z$+ decreases from 14. 3 % to 10.8 % . The surface atomic ratio obtained from XPS is close to the composition of the samples, which means that the surface and bulk of the doped samples are homogenous in composition. These results indicate that the incorporation of Tb into the CeO,-%rO, leads to a higher dispersion of Tb itself on the surface and promotes the formation of a solid solution.
3 Conclusions 1. The results of BET surface areas analysis suggested that doping a small amount of Tb into Ce02ZrO, could increase the surface areas of catalysts. 2 . From XRD and Raman results, it was found that all the Tb-doped catalysts possessed a cubic fluorite lattice, which means that Tb4+and Zr4+ ions dissolved into the CeO, lattice and formed the Ceo Jr0 Tb,Oz_ cubic solid solution. 3 . XPS analysis demonstrated that Ce and Tb mainly existed in the form of Ce4+ and Tb3+, Ceo 60 Zro ,,Tho x = 0.05, 0.10 and 0.15) and Zr existed in the form of Zr4+ on the surface of the doped samples. The doped samples were homogenous in composition, and the incorporation of Tb into Ce02ZrOz promoted the formation of a solid solution and increased the concentration of surface lattice oxygen. ~
Table 3 WS quantitative results of Ceo.mZro.JoTbo.1002_, Surface atomic percent/%
Samples
Ce 3d5/2(u)(882.6 eV)
CZTl
11.9
CZT2
12.1
CZT3
11.8
Tb/Ce atomic
%rice atomic rat in
Tb 3dm(1241.6 rV) 01s (529.4 and 531.6 eV)
ratio
1.1
72.7
0.09
12.8
2.1
12.9
0.17
1.1
10.8
3.0
72.4
0.25
0.9
Zr 3d5,2(181.6 eV)
____ 14.3
____ 1.3
47
Fan G D et a1 , Tb-Doped Ceria-Zirconia Solid Solution Prepared by Sol-Gel Method
tion of Ce02-Zr0, mixed oxides comparison of thr citrate and sol-gel preparation methods [ J ] . Chem . Mater . ,
References : 1
h n g C h: , Ozawa h i .
The role of rarr earth dopants in
nanophsar zirconia catal!-sts for automoti\ e emission control J . Journal of A l l o ~ s and Compounds , 2000, ~
.2
303 - 301: 60. Trot arell i A . Boaro M , Rocrhini E , et a1 .
Some recent
dex elopi~ierits in the characterization of ceria-based catalysts .J . . Journal of A l l o ? ~and Compounds, 2001,
323 - 323 : 584. Troxarel i A . Catal!sis b!- Ceria and Related Materials :)I . 1, indon : Imperial College Press, 2002. 73. 4. Potdar II 5, Deshpande S R , Deshpande A S, et al. Preparat cin of crria-zirconia ( Ceo ..%r0 a O ? ) powders hy micro~~-a,t-h!drothermal ( MH route J j . Waterials Chemistrr and PhJsirs. 2002. 7 4 ( 3 ) : 306. 5 He H . Dai H X , R ong I\ a, et a1 . SrCI2-promoted REO, ( F'E = Ce . Pr , Th ) catalysts for the selectivr oxi-
.3
I
dation t f ethane: a study on performance and defect , structure- for ethene formation [ J 1 . Journul of Catal>~is
2001. 19912): 177.
6
.4lifanti \ I , Baps B . Blangrnois N . rt a l .
characteriza-
2003, lS(2): 395. [ 7 ] Chary K V R , Sagar G V , Naresh D ,
et
al.
Character-
ization and reartivity of copper oxide catalysts supported on TiO,-ZrOz [ J ] . J . Phys. Chem. H . . 2005, 109
(19) : 9437. [ S ] Ardizzone S , Bianchi C L , Signoretto M . Zr( ) surface chemical state and acid features of sulphated-zirconia samples [ J 1 . Applied Surjiace Science, 1998, 136 (3): 213. [ 9 ] Kim Y J , Thevuthasan S , Shutthananadan V , et al. Growth and structure of epitaxial Cel-.Zr,02 thin films on yttria-stabilized zirconia ( 1 I1 ) [ J ] . Journal of Electron Spectroscop and Related Phenomena , 2002, 126 ( 1 - 3 ) : 177. [ 101 Feng Changgen , Fan guodong . Recent development in doping and structural modification of ceria-based catalysts [ J] . Journal of Rare Earths , 2005, 23( 3 ) : 309. [ 11] Feng Changgen , Fan Guodong , Liu Xia . Study of ceria-zirconia solid solution promoters modified by Tb [ J]. China Journal of Applied Chemistry, 2005, 22( 7 ) : 703.
n'