Journal of Natural Gas Chemistry 182009 –catalysts for low-temperature CO oxidation Kairong Li, Yaojie Wang, Shurong Wang∗, Baolin Zhu, Department of Chemistry, Nankai University, Tianji
Trang 1Journal of Natural Gas Chemistry 18(2009) –
catalysts for low-temperature CO oxidation
Kairong Li, Yaojie Wang, Shurong Wang∗, Baolin Zhu,
Department of Chemistry, Nankai University, Tianjin 300071, China
[ Received April 22, 2009; Revised May 11, 2009; Available online November 22, 2009 ]
Abstract
Nanometer SnO2 particles were synthesized by sol-gel dialytic processes and used as a support to prepare CuO supported catalysts via a deposition-precipitation method The samples were characterized by means of TG-DTA, XRD, H2-TPR and XPS The loading of CuO in the CuO/TiO2-SnO2 catalysts markedly influenced the catalytic activity, and the optimum CuO loading was 8 wt.% (T100 = 80◦C) The CuO/TiO2-SnO2catalysts exhibited much higher catalytic activity than the CuO/TiO2and CuO/SnO2catalysts H2-TPR result indicated that
a large amount of CuO formed the active site for CO oxidation in 8 wt.% CuO/TiO2-SnO2catalyst
Key words
sol-gel dialytic processes; CuO/TiO2-SnO2catalyst; low-temperature CO oxidation
1 Introduction
Carbon monoxide, emitted from automated vehicles,
air-craft, natural gas emission, industrial wastage, sewage
leak-ing, mines, etc, is one of the most common and dangerous
pollutants present in the environment Catalytic oxidation is
an efficient way to convert CO to CO2 at low-temperature
Many precious metal supported catalysts, such as Au/TiO2,
Au/Fe2O3, Au/CeO2, Pt/SnO2, Pd/CeO2, Ir/TiO2, have been
demonstrated to be very efficient for low-temperature CO
ox-idation [1−5] However, due to the high cost and the scarcity
of precious metal, attention has been given to search
alterna-tive catalysts, especially for copper oxide [6]
Tin dioxide is one of the most widely applied
materi-als in solid-state gas sensor devices detecting toxic and
com-bustible gases in air [7] Moreover, tin dioxide as CO
ox-idation catalyst has attracted particular attention in the past
[8] The preparation of nanometer SnO2powders has been
widely studied, and different methods have been proposed to
synthesize pure or doped SnO2 At present, the main methods
used for preparing nanometer SnO2 powders are the sol-gel
method, the alkoxide method, and the hydrothermal method
The sol-gel method is one of the best methods used to prepare
nanometer powders However, this method has two
restric-tions, one is time-consuming, generally 15−30 days, the other
is the difficulty in removing Cl−
In this paper, tin (IV) chloride and alcohol were used as the start materials Ammonia gas as catalyst for forming a colloidal solution and as agent for removing Cl−, was intro-duced to the dialytic processes to improve and accelerate the formation of gels It took about 18 h to form SnO2wet-gels, which did not need washing CuO/TiO2-SnO2catalysts were prepared via a deposition-precipitation method and their cat-alytic activities in CO oxidation were studied The catcat-alytic activity of CuO supported on TiO2-SnO2was compared with that of CuO supported on TiO2 or SnO2 The samples were characterized by means of TG-DTA, XRD, H2-TPR and XPS
2 Experimental
2.1 Catalyst preparation
Nanometer tin dioxide particles were prepared by sol-gel dialytic processes SnCl4(A R.) was mixed with anhydrous alcohol (A R.) to obtain SnCl4 alcohol solution The pre-cursor Sn(OC2H5)4colloidal solution was prepared by adding ammonia gas to SnCl4alcohol solution at 0◦C under vigorous
∗ Corresponding author Tel: +86-22-23505896; Fax: +86-22-23502458; E-mail: shrwang@nankai.edu.cn (S R Wang)
This work was supported by the National Natural Science Foundation of China (20771061 and 20871071), the 973 Program (2005CB623607) and Science and Technology Commission Foundation of Tianjin (08JCYBJC00100 and 09JCYBJC03600)
Copyright©2009, Dalian Institute of Chemical Physics, Chinese Academy of Sciences All rights reserved.
doi:10.1016/S1003-9953(08)60144-9
Trang 22 Kairong Li et al./ Journal of Natural Gas Chemistry Vol 18 No 4 2009
stirring After white precipitation filtered, colloidal solution
was inputted a dialytic membrane, and then put in the distilled
water When no Cl− was examined by 0.1 mol/L AgNO3
aqueous solution, and the pH of dialysate was 7, the wet-gels
formed in the process of dialysis were dried at 80◦C for 24 h
Based on the result of TG-DTA analyses, the dried samples
were calcined at 400◦C in air for 3 h
TiO2/SnO2nano-composite was prepared by
deposition-precipitation method with nominal 50 wt.% TiO2contents A
suitable amount of SnO2 powders were dispersed in alcohol
solution, and the slurry was constantly stirred for 2 h Then the
Ti(OBu)4alcohol solution was added drop-wise to the above
resulting slurry The mixture was stirred for another 30 min
The samples were dried at 80◦C in water bath, and then dried
in oven at 80◦C for 1 h The as-made material was calcined
at 400◦C in air for 3 h TiO2 nanopowders were prepared
by hydrolyzation-precipitation from Ti(OBu)4 alcohol
solu-tion, and the preparation procedure was similar to that of the
preparation of the TiO2/SnO2samples
CuO/SnO2, CuO/TiO2 and CuO/TiO2-SnO2 catalysts
were prepared by a deposition-precipitation method Under
vigorous stirring, a suitable amount of SnO2, TiO2or TiO2
-SnO2powders were dispersed in Cu(NO3)2solution, and the
slurry was constantly stirred for 1 h 0.25 mol/L Na2CO3
so-lution was added drop-wise to the above slurry under vigorous
stirring until the pH was 10 The mixture was stirred for
an-other an hour, then filtered and washed with distilled water
The samples were dried at 80◦C in air in an oven, followed
by calcination at 400◦C in air for 3 h
2.2 Measurement of catalytic activity
Catalytic activity tests were performed in a continuous
flow fixed bed microreactor, using 100 mg catalyst powder A
stainless steel tube with an inner diameter of 8 mm was
cho-sen as the reactor tube The reaction gas mixture consisting
of 1 vol.% CO balanced by air passed through the catalyst bed
at a total flow rate of 33.4 ml/min A typical weight hourly
space velocity (WHSV) was 20040 ml·h−1
·g−1 After 30 min under reaction conditions, the effluent gases were analyzed
online by GC-508A gas chromatography equipped with a
ther-mal conductivity detector (TCD) and a TDX 01 column The
activity was expressed by the degree of CO conversion
2.3 Catalyst characterization
Thermogravimetry and differential thermal analyses
(TG-DTA) were performed by ZRY-2P thermal analyzer at a linear
heating rate of 10◦C/min α-Al2O3was used as reference
X-ray diffraction (XRD) analyses were performed on
D/MAX-RAX diffractometer operating at 40 kV and 100 mA, using
Cu Kαradiation (scanning range 2θ: 20o– 75o) X-ray
pho-toelectron spectroscopy (XPS) measurements were performed
with a Perkin-Elmer PHI 5600 spectrophotometer with the
Mg Kα radiation The operating conditions were kept
con-stant at 187.85 eV and 250.0 W In order to subtract the
sur-face charging effect, the C 1s peak has been fixed, in
agree-ment with the literature, at a binding energy of 284.6 eV The reduction properties of CuO supported catalysts were mea-sured by means of temperature-programmed reduction (TPR) techniques under a mixture of 5% H2in N2 100 mg catalyst was placed in a quartz reactor which was connected to a con-ventional TPR apparatus, and the heating rate is 10◦C/min
3 Results and discussion
3.1 Catalyst characterization
Figure 1 shows thermogravimetry and differential ther-mal analyses (TG-DTA) curves of as-prepared SnO2dried at
80◦C The TG curve can be divided into three stages The first stage is from room temperature to 97◦C The weight loss is about 3%, which is caused by dehydration and evap-oration of alcohol existing in the gel The second stage is from 97 to 257◦C The weight loss is about 3.1%, which is attributed to the removal of the strongly bound water or the surface hydroxyl groups in the gel The third stage is 257 to
377◦C The weight loss is about 3.5%, which results from the decomposition of the remained ammonium chlorate The existence of a small amount of water, alcohol and ammonium chlorate in samples is a normal phenomenon, and they can be removed by heating to above 377◦C In the DTA curve, the broad weak endothermic peak at 97◦C comes from desorption
of water and alcohol Between 257 and 377◦C, a small en-dothermic peak is observed due to the removal of the strongly bound water or the surface hydroxyl groups The endother-mic peak centered at 340◦C is assigned to the decomposition
of the remained ammonium chlorate No any exothermal peak exists in the range of room temperature to 800◦C, which indi-cates that there is no phase transformation in this temperature range From the result of the above TG-DTA, therefore, the as-prepared SnO2was calcined at 400◦C in air for 3 h for the subsequent use
Figure 1 TG-DTA curves of the as-prepared SnO2 dried at 80 ◦ C
Figure 2 shows the X-ray patterns of nanosized SnO2, TiO2and TiO2-SnO2calcined at 400◦C Compared to JCPDS standard pattern, these XRD results indicate that SnO in both
Trang 3Journal of Natural Gas Chemistry Vol 18 No 4 2009 3
pure SnO2and TiO2-SnO2samples is tetragonal rutile
struc-ture, and TiO2 in pure TiO2 and TiO2-SnO2 samples is all
tetragonal anatase structure Average particle size of SnO2
powders in pure SnO2and TiO2/SnO2is 6.7 and 6.3 nm,
re-spectively, obtained by Scherrer’s equation from SnO2(101)
Average particle size of TiO2in pure TiO2and TiO2/SnO2,
calculated by Scherrer’s equation from TiO2(110), is 12.7 nm
and 12.4 nm, respectively
Figure 2 X-ray diffraction patterns of nanosized SnO2 (1), TiO 2 (2), and
TiO 2 -SnO 2 (3) calcined at 400 ◦ C for 3 h
Figure 3 XPS spectra of Sn3d (a), Ti2p (b) and Cu2p (c) region for 8 wt.%
CuO/TiO -SnO catalyst calcined at 400 ◦ C
Figure 3 presents the XPS spectra in the Sn 3d, Ti 2p and
Cu 2p region for 8 wt.% CuO/TiO2-SnO2 catalyst Two dis-tinguished Sn 3d peaks, centered at 486.5eV and 495.0 eV, re-spectively, can be observed and were attributed to Sn4+[9]
In Ti 2p spectrum, two shoulder peaks at 458.4 and 464.0 eV indicated the presence of Ti4+[10] The Cu 2p3 2binding en-ergy was 934.5 eV for the CuO/TiO2-SnO2catalyst, which is similar to that of pure CuO [11]
3.2 Catalytic performance
Figure 4 shows the activity of CuO/TiO2-SnO2catalysts with various CuO loadings in CO oxidation The loading of CuO in the CuO/TiO2-SnO2catalysts markedly influences the catalytic activity The optimum CuO loading is 8 wt.%, which has an appreciably high catalytic activity [the temperature of 100% CO conversation (T100) is 80◦C] When the CuO load-ing is below 8 wt.%, T100decreases with the increase of the CuO loading When the CuO loading is above 8 wt.%, T100 increases with the increased CuO loading This indicates that
a proper CuO loading is necessary to gain high activity for CuO catalysts
Figure 4 Influence of CuO loading on the activity of CuO/TiO2 -SnO 2 cata-lysts for CO oxidation
Figure 5 shows the CO oxidation activity of 8 wt.% CuO/SnO2, 8 wt.% CuO/TiO2 and 8 wt.% CuO/TiO2-SnO2 catalysts It can be seen that the 100% CO conversion over CuO/SnO2, CuO/TiO2 and CuO/TiO2-SnO2 catalysts takes place at 130, 110, and 80◦C, respectively This shows that the CuO/TiO2-SnO2 catalyst exhibits much higher catalytic activity in CO oxidation than CuO/SnO2and CuO/TiO2 cat-alysts We also measured the activity of the supports in CO oxidation TiO2 support displayed no catalytic activity be-low 310◦C T100 on SnO2 support was 300◦C However, the TiO2/SnO2support exhibited the higher catalytic activity than SnO2support, and T100decreased to 270◦C Compared
to the single oxides, the particle sizes of SnO2 and TiO2 in the TiO2/SnO2composite have no obvious difference It was suggested that the high catalytic activity of CuO/TiO2-SnO2 catalyst was related to the synergistic effect between Ti and
Sn in the TiO -SnO support
Trang 44 Kairong Li et al./ Journal of Natural Gas Chemistry Vol 18 No 4 2009
Figure 5 Activity of CuO/SnO2 , CuO/TiO 2 and CuO/TiO 2 -SnO 2 catalysts
for CO oxidation
3.3 H 2 -TPR of CuO supported on dif ferent supports
H2-TPR has been extensively used to characterize the
re-ducibility of oxygen species in metal oxide containing
ma-terials To study the reduction ability of CuO supported
on different supports, we compare the H2-TPR profiles of
CuO/SnO2, CuO/TiO2, and CuO/TiO2-SnO2 catalysts
(Fig-ure 6) H2-TPR profile of the CuO/SnO2catalyst shows one
broad reduction peak centered at 227◦C (β) with two
shoul-der peaks at 217 and 258◦C (α and γ) There are two
reduc-tion peaks (α and γ) at around 209 and 258◦C, respectively,
in the H2-TPR profile of CuO/TiO2 catalyst H2-TPR
pro-file of CuO/TiO2-SnO2catalyst exhibits one sharp reduction
peak centered at 193◦C (α) and a strong broad shoulder peak
at 227◦C followed a very weak peak at around 258◦C It
is likely that the α peak results from the reduction of
well-dispersed CuO species on support [12], and the β peak is
as-signed to the reduction peak of the surface-capping oxygen
of SnO2, and the γ peak is ascribed to the reduction of larger
CuO species on the surface Due to the TiO2reduction is very
difficult at low temperature, there are no peaks related to the
reduction of TiO2 are observed during the TPR from 24 to
400◦C The strong broad β peaks in the H2-TPR profile of
the CuO/SnO2and CuO/TiO2-SnO2catalysts suggested that a
Figure 6 H2 -TPR profiles of 8 wt.% CuO/SnO 2 (1), 8 wt.% CuO/TiO 2 (2),
and 8 wt.% CuO/TiO 2 -SnO 2 (3)
significant amount of SnOxcan be reduced even at low tem-perature In the CuO/TiO2-SnO2catalyst, the intensity of α peak is stronger than that in the CuO/SnO2 and CuO/TiO2 catalysts, and α peak shifted to lower temperature, while the
γ peak becomes pretty weak The result indicates the amount
of well-dispersed CuO is large and the reduction temperature
of well-dispersed CuO is low in the CuO/TiO2-SnO2 cata-lyst, compared with those in the CuO/SnO2 and CuO/TiO2 catalysts The reduction of CuO depends strongly on the sort of support, and the interaction of SnO2and TiO2 might largely promote CuO well-dispersion on TiO2-SnO2support and make CuO more easily to be reduced Therefore, it is sug-gested that CuO species with high dispersion is the active site
of CO oxidation, and the improvement in catalytic activities is probably related to highly dispersion of CuO species [13,14]
4 Conclusions
The loading of CuO in the CuO/TiO2-SnO2 catalysts markedly influences the catalytic activity, and the opti-mum CuO loading is 8 wt.% CuO/TiO2-SnO2catalyst ex-hibits much higher catalytic activity for CO oxidation than CuO/SnO2and CuO/TiO2catalysts H2-TPR result shows that there is a large amount of CuO to form the active site for CO oxidation in CuO/TiO2-SnO2catalyst
Acknowledgements
We gratefully acknowledge the financial support from the National Natural Science Foundation of China (20771061 and 20871071), the 973 Program (2005CB623607) and Science and Technology Commission Foundation of Tianjin (08JCYBJC00100 and 09JCYBJC03600)
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