e-Journal of Surface Science and Nanotechnology 27 December 2011-Total Oxidation of Toluene on Nano-Perovskites La1−xBxCoO3 B: Ag, Sr∗ Laboratory of Petrochemistry, Faculty of Chemistry,
Trang 1e-Journal of Surface Science and Nanotechnology 27 December 2011
-Total Oxidation of Toluene on Nano-Perovskites La1−xBxCoO3 (B: Ag, Sr)∗
Laboratory of Petrochemistry, Faculty of Chemistry, Hanoi University of Science,
Vietnam, 334 Nguyen Trai, Thanh Xuan, Hanoi, Vietnam
Nguyen Thi Ngoc Quynh
Department of Physical Chemistry, Phu Tho College of Chemistry,
Lam Thao District, Phu Tho Province, Vietnam
(Received 8 December 2009; Accepted 3 May 2010; Published 27 December 2011)
A series of nanosized perovskite oxides, LaCoO3 and La1−xBxCoO3 (x = 0.1; B: Sr, Ag), was synthesized by
citrate method The samples were characterized by XRD, IR and SEM The XRD results show that all samples have principal phase with rhombohedra perovskite structure and particles size of 30-50 nm The catalytic performance
of these nanoperovskites has been evaluated by the total combustion of 1000 ppm of toluene in air The result obtained shows the highest activity for the sample La0.9Ag0.1CoO3 The last one seems to be related with the highest quantity of oxygen released as showed by oxygen temperature program desorption results (O2-TPD)
[DOI: 10.1380/ejssnt.2011.486]
Keywords: Nano-perovskite; Total oxidation; Volatile organic compound; Toluene
The release of volatile organic compounds (VOCs) are
known to cause air pollution such as photochemical smog,
ground level ozone, ozone depletion, sick house syndrome,
and chemical sensitivity [1-3] A number of catalysts have
been used for the complete oxidation of VOCs
Gener-ally, they were classified in two groups: supported noble
metal and transition metal oxide [4-10] Supported noble
metal as Pt and Pd is well estimated as efficient
cata-lysts for the total oxidation of VOC However, they are
expensive Therefore, transition metal oxide is attired
re-searcher in recent time Because this one also shows a
good activity and, especially, it is much cheaper Among
them, mixed oxide of transition metal, as perovskite, has
more advantages Since this one not only shows a good
activity but also a high thermal and hydrothermal
stabil-ity In this word, we were prepared a series of perovskites
La1−xBxCoO3 (B: Ag, Sr) and examined their catalytic
activity in total oxidation reaction of toluene which has
been chosen as VOC probe molecule because aromatics
are present in the industrial and automotive emission [11,
12] The partial substitution of Co by Ag or Sr was
ex-pected to improve catalytic activity of LaCoO3 which is
known as the most active catalyst in perovskite group for
total oxidation of VOC
Series of perovskite La1−xBxCoO3was prepared by
cit-ric method [13] All used chemical compound have
anal-ysis purity Firstly, nitrate salt (Co(NO3)2, La(NO3)2,
Advanced Materials and Nanotechnology 2009 (IWAMN2009),
Hanoi University of Science, VNU, Hanoi, Vietnam, 24-25
Novem-ber, 2009.
AgNO3) and citric acid C6H5O2were dissolved each other
in an adequate amount of H2O Then, they were mixed with stirring for 30 minutes before being evaporated in rotary evaporator to obtain a resin This one was dried
at 80◦C in oven and then, calcined at 600◦C for 5 hour.
All prepared samples were characterized by differ-ent methods: X-ray diffraction, scanning electronic mi-croscopy (SEM), BET surface measurement, and temper-ature programmed desorption of oxygen (TPD-O2) The crystalline phases were examined by X-ray diffrac-tion using D8 Advance Brucke diffractometer with CuKa
irradiation source (λ=0.15406 nm) operated at 40 kV
and 30 mA The XRD measurement was performed with 0.03◦ step per second, from 20◦ to 70◦ (in 2θ).
SEM images of sample were taken by using
JEOSJSM-5410 LV Scanning Electron Microscope The BET sur-faces were determined by using AutochemII
(10 vol%)/He at 200◦C for 1 hour then cooled to ambient
temperature and purged in He flow The measurements were carried out from room temperature to 700◦C.
Catalytic activity of all samples was estimated by total oxidation of toluene Toluene was mixed in air to obtain
a flux at 1000 ppm of toluene concentration The organic products were determined by Chromatography HP 6280 equipped FID detector, and the formation of CO2 was detected by TCD detector
X-ray diffraction patterns are presented in Fig.1 Prin-cipal phases recognized rhombohedra prerovskite phase
Trang 2e-Journal of Surface Science and Nanotechnology Volume 9 (2011)
FIG 1(a)
3
FIG 1(b)
4
FIG 1(c)
(c)
FIG 1 XRD patterns of LaCoO3(a), La0.8Sr0.2CoO3(b) and
La0.9Ag0.1CoO3 (c)
for all samples In case La0.9Ag0.1CoO3, presence of
metallic silver was determined (2θ = 38.1, 48.2 )
Cer-tainly, this one was formed by decomposition of Ag(NO3)
It is possible that an amount of Ag+ wasn’t substituted
or/and incorporated in perovskite phase
In order to understand well their structure, these
sam-ples were characterized by IR Figure 2 showed results
obtained We observed characteristic peaks for perovskite
structure such as 593.3 562.7, 417.8 cm−1[14] In case of
sample substituted by Ag and Sr, a shoulder peak was
rec-ognized at 642cm−1 This one was characteristic for phase
Co3O4[15] Thus, it is possible that when a small amount
of Ag+ or Sr2+ was incorporated in perovskite structures
of LaCoO3, a small amount of Co2+,Co3+was pushed out
of peroskite structure and formed phase Co3O4 However,
this phase may be formed in form of cluster and/or well
dispersed on perovskite phase
Figure 3 presents the SEM image of all samples It was
noted that all samples consisted of nano particles with
diameter in range of 30-50 nm The particles were in
spherical form and quite uniform
Figure 4 presents the results TPD-O2 Generally, it was
noted that there were two type of oxygen desorbed [16]:
αO2 (oxygen desorbed at temperature below 500◦C) and
FIG 2
FIG 2 IR spectra of LaCoO3, La0.8Sr0.2CoO3 and
La0.9Ag0.1CoO
FIG 3 SEM image of of LaCoO3, La0.8Sr0.2CoO3 and
La0.9Ag0.1CoO3
6
0.00 0.05 0.10 0.15 0.20 0.25
100 200 300 400 500 600 700 800 900
Series1
Series2
Series3
Temperature ( o C)
1
2 3
LaCoO 3 1
La 0.8 Sr 0.2 CoO 3 2
La 0.8 Ag 0.1 CoO 3
FIG 4
FIG 4 TPD-O2 curves of of LaCoO3, La0.8Sr0.2CoO3 and
La0.9Ag0.1CoO3
αO2 (oxygen desorbed in range of temperature from
500-700◦C) For the sample LaCoO3, it is obvious that there
are three desorption peaks of O2 at 441◦C, 599◦C and
713◦C Among them, the peak at 441◦C was three times
more intense than two others A much more intense peak
at 426◦C was observed for La
0.8Sr0.2CoO3 In the range
of higher temperature, oxygen continued to desorbs but
no shape peak was found It seems that there were sev-eral peaks but they appeared continuously with a small difference of temperature However, it is seen that the ad-sorption of O2is shifted at lower temperature In case of
La0.9Ag0.1CoO3, a small peak at 190◦C appeared more
sharply in comparison with the precedent case The sec-ond peak was at 471◦C, but it seems that this one was
constituted by different peak, because it was obvious that there were two shoulder-peaks at 420◦C, and 600◦C The
third peak, most intense, was shifted at 774◦C This result was quite in accordance to observation of S.Ifrah et al.[17]
These curves were not normalized in weight Hence, in order to have a qualitative estimation, it is necessary to
Trang 3Volume 9 (2011) Thanh Binh, et al.
TABLE I BET surfaces and quantity of mobile oxygen per gram or square meter of sample
TPD-O2
100-200◦C 100-800◦C 100-200◦C 100-800◦C
7
0.00
20.00
40.00
60.00
80.00
100.00
C)
LaCoO3
LaAgCoO3
LaSrCoO3
FIG 5
FIG 5 Conversion of toluene versus temperature on
per-ovskite samples
report the BET surfaces and quantity of mobile oxygen
per gram or square meter of sample as showed in Table
1 It is seen that the substituted sample showed a BET
surface quite low more than non-substituted sample
The result of catalytic test was presented in Fig.5 To
compare activity of catalyst, we used two factors, T50and
T90, which are values of temperature where 50 % or 90 %
of reactive is converted respectively
classi-fied as following: La0.8Sr0.2CoO3 ≈ La 0.9Ag0.1CoO3
> LaCoO3 In case of T90, it is clearly found
La0.9Ag0.1CoO3 > La 0.8Sr0.2CoO3 > LaCoO3
Gener-ally, La0.9Ag0.1CoO3showed slightly higher activity than
La0.8Sr0.2CoO3 and activities of substituted samples
were clearly higher than pure LaCoO3 This properties
of substituted perovskites were observed by the group
of S.Kaliaguine in case of La1−xA’xBO3 (A’=Sr, Ce,
B=Co, Mn) used for the CH4 oxidation reaction [18] In
our case, basing on TPD-O2 result (Table 1), it seems that the catalytic activity was proportional with the quantity of mobile oxygen in bass temperature range (100◦C - 200◦C) The quantity of mobile oxygen released
in higher range of temperature do not have an important role for total oxidation of toluene if we were noted that, up to 200◦C, almost of toluene was converted for
all catalysts It was obvious that there was a quick diminution of catalytic activity after the beginning of toluene conversion This one is in order of LaCoO3,
La0.8Sr0.2CoO3 and La0.9Ag0.1CoO3 The diminution was possibly due to deposition of coke which blocked catalytic center on catalyst surface The substitution of
Ag or Sr seems to decrease this one
A series of perovskites La1−xBxCo3(B: Ag, x = 0.1; Sr,
x = 0.2) was synthesized by citrate method They were
constituted by nano-particles with the diameter from 30
to 50 nm Perovskite La0.9Ag0.1CoO3 showed a highest activity in total oxidation of toluene Based on TPD-O2 measurement, it seems that their activity is proportional with quantity of mobile oxygen on surface in basic range
of temperature, from 100◦C to 200◦C This result is very
potential for the total oxidation of VOC at low tempera-ture
ACKNOWLEDGMENTS
The authors gratefully acknowledges financial support from the National Foundation for Science and Technology Development of Vietnam (NAFOSTED)
[1] A Akinson and J Arey, Chem Rev 103, 4605 (2003).
[2] B J Finlayson-Pitts and J N Pitts Jr., Science 276,
1045 (1997)
[3] Z Meng, D Dabdub, and J H Seinfeld, Science 277, 116
(1997)
[4] S Krishnamoorthy, J P Baker, and M D Amiridis,
Catal Today 40, 39 (1998).
[5] S D Yim, K -H Chang, D J Koh, I -S Nam, and Y
G Kim, Catal Today 63, 215 (2000).
[6] J I Gutierrez-Ortiz, B De Rivas, R Lopez-Fonseca, and
J R Gonzalez-Velasco, Appl Catal A 269, 147, (2004).
[7] N Mizuno, H Fujii, H Igrashi, and M Misono, J Am
Chem Soc 114, 7151 (1992).
[8] R W Van den Brink, R, Louw, and P Mulder, Appl
Catal B 16, 219 (1998).
[9] J R Gonzales-Velasco, A Aranzabal, R Lopez-Fonseca,
R Ferret, and J A Gonzalez-Marcos, Appl Catal B 24,
33 (2000)
[10] H L Tidahy, S Siffert, J.-F Lamonier, R Cousin, E
A Zhilinskaya, A Aboukais, B -L Su, X Canet, G De Weireld, M Frere, J.-M Giraudon, and G Leclercq, Appl
Catal B 70, 377 (2007).
Trang 4e-Journal of Surface Science and Nanotechnology Volume 9 (2011)
[11] R G Derwent, M E Jenkin, and S M Saunders, Atmos
Environ 30, 181 (1996).
[12] R G Derwent, M E Jenkin, S M Saunders, and M J
Pilling, Atmos Environ 32, 2429 (1998).
[13] H Taguchi, S Matsu-ura, M Nagao, T Choso, and K
Tabata, J Sol State Chem 129, 60 (1997).
[14] G Sinquin, C Petit, J P Hindermann, and A
Kienne-mann, Catal Today 70, 183 (2001).
[15] J Jiu, Y Ge, X Li, and L Nie, Mater Lett 54, 260
(2002)
[16] T Seiyama, Catal Rev Sci Eng 34, 281 (1992).
[17] S Ifrah, A Kaddouri, P Gelin, and D Leonard, C R
Chimie 10 1216 (2007).
[18] S Royer, H Alamdari, D Duprez, and S Kaliaguine,
Appl Catal B 58, 273 (2005).