Effects of nanostructure on catalytic degradation of ethanol on SrCO 3 catalysts

Article abstract of DOI:10.1021/jp045884d

Degradation of ethanol over SrCO₃ nanowires and nanoparticles was used as a model reaction to investigate the effect of nanostructure on chemical property. Differences in catalytic degradation activity with nanostructure are evaluated. The results indicated that catalytic activity of SrCO₃ particles increases with decreasing of particle size due to high surface area. But this conclusion cannot be applicable to evaluating SrCO₃ nanowires and nanoparticles. SrCO₃ nanowires have lower ignition temperatures and wider working temperature ranges than SrCO ₃ nanoparticles, though nanowires had lower surface areas. Besides, ethanol degraded over nanowires in three ways, and the dominating reaction changes with reaction temperature. Consequently, the main degradation products of nanowires differed with temperature. But for nanoparticles, acetaldehyde is the only main product. Since transmission electron microscopy, X-ray diffraction, and bond equilibrium theory analysis demonstrated that nanowires and nanoparticles had similar crystal structure, surface area, and grain size, the differences in catalytic degradation activity between SrCO₃ nanowires and nanoparticles can be attributed to different distributions of active sites, as proven by CO₂ and ethanol temperature programmed desorption. ? 2005 American Chemical Society.

Full text of DOI:10.1021/jp045884d

5118
J. Phys. Chem. B 2005, 109, 5118-5123
Effects of Nanostructure on Catalytic Degradation of Ethanol on SrCO3 Catalysts
Li Wang and Yongfa Zhu*
Key Lab of Organic Optoelectronics and Molecular Engineering, Beijing 100084, People’s Republic of China,
and Department of Chemistry, Tsinghua UniVersity, Beijing 100084, People’s Republic of China
ReceiVed: September 12, 2004; In Final Form: NoVember 25, 2004
Degradation of ethanol over SrCO
the effect of nanostructure on chemical property. Differences in catalytic degradation activity with nanostructure
are evaluated. The results indicated that catalytic activity of SrCO particles increases with decreasing of
particle size due to high surface area. But this conclusion cannot be applicable to evaluating SrCO nanowires
and nanoparticles. SrCO nanowires have lower ignition temperatures and wider working temperature ranges
than SrCO nanoparticles, though nanowires had lower surface areas. Besides, ethanol degraded over nanowires
3
nanowires and nanoparticles was used as a model reaction to investigate
3
3
3
3
in three ways, and the dominating reaction changes with reaction temperature. Consequently, the main
degradation products of nanowires differed with temperature. But for nanoparticles, acetaldehyde is the only
main product. Since transmission electron microscopy, X-ray diffraction, and bond equilibrium theory analysis
demonstrated that nanowires and nanoparticles had similar crystal structure, surface area, and grain size, the
3
differences in catalytic degradation activity between SrCO nanowires and nanoparticles can be attributed to
different distributions of active sites, as proven by CO and ethanol temperature programmed desorption.
2
1
. Introduction
activity. Further investigation on CO2 and ethanol temperature
programmed desorption (TPD) proved that the variation of
catalytic activity with nanostructure could be attributed to
different distribution of catalytic active sites.
Nanosized materials have attracted widespread attention for
their special features that differ from bulk materials since the
990s. Catalysts and chemical sensors based on nanosized
1
materials have been developed rapidly. For example, ABO3 type
perovskite oxide composites and noble metal supported oxides
with nanostructure are widely used in catalytic combustion of
volatile organic compounds, greenhouse gases, methane, and
so on. The individual semiconducting single-walled carbon
nanotube (SWNT)-based chemical sensors that are capable of
2
. Experimental Section
.1. Preparation and Characterization of SrCO3 Nano-
2
1
2
3
wires and Nanoparticles. SrCO nanowires were prepared by
reacting a Sr(OH)2 solution with CO2 in air. Fresh saturated
Sr(OH)2 solution absorbed CO2 in air for 1 h and then was dried
at 60 °C for 4 h to form SrCO3 nanowires, and then the product
was washed with distilled water and dried at 60 °C.
3
9
detecting small concentrations of gas molecules have been
realized,4 boron-doped silicon nanowire nanosensors have
SrCO3 nanoparticles were prepared by the rapid precipitation
showed highly sensitive and selective detection of biological
and chemical species,5 and palladium mesowire arrays for
method. After 0.1 M Na CO solution was dropped into 0.1 M
2
3
SrCl2 solution while being stirred by a magnetic stirrer, white
deposition formed. The deposition was stirred for 10 h to prepare
detection of hydrogen have been described based upon resistivity
changes caused by the changes in the structure of the wire itself.
6
4
0 nm particles and 24 h to prepare 80 nm particles. And then
It is well-known that nanostructure generally has great effect
7
the obtained SrCO nanoparticles were washed with distilled
on properties of nanomaterials, but few investigations in this
3
water 3-5 times and dried at 60 °C. Commercial SrCO3 was
field are focused on variation of chemical property with
nanostructure. Catalysts are one of the important applications
for many nanomaterials. Since a catalytic reaction generally
occurs at a surface and it strongly depends on the distribution
of the active site, which is greatly effected by structure,
investigation on the relationship between nanostructure and
catalytic activity has valuable importance.
used as bulk material to evaluate the catalytic activity of
nanomaterials. It was analytical reagent grade with a purity of
greater than 99.0%.
The particle size of SrCO3 materials were measured using
transmission electron microscopy (TEM, Hitachi H-800) with
a 200 kV accelerating voltage for the electron beam. Their
crystal structure was measured by X-ray powder diffraction
Here, we report the significant effect of nanostructure on
oxidation activity of SrCO3 nanomaterials. On the basis of the
(
XRD) on a Rigaku DMAX-2400 with Cu KR radiation. Before
8
CO -TPD and ethanol-TPD analyses, about 0.1 g of catalyst
work reported by Zhang et al., in which SrCO3 was proven to
2
was first heated at 350 °C for 2 h in flowing He gas to remove
be a potential chemical sensor, catalytic degradation of ethanol
was used as the model reaction. The results indicated that
compared with a SrCO3 nanoparticle, though SrCO3 nanowire
had the same crystal structure, similar grain size, and lower
surface-to-volume ratio, the nanowire had higher catalytic
contaminations adsorbed on the surface, then saturated with CO2
or ethanol at room temperature for 1 h. Mass spectroscopy was
used as a detector in TPD analysis.
2.2 Evaluation of Catalytic Oxidation Properties of SrCO3
Nanowires and Nanoparticles. Evaluation of catalytic oxida-
tion properties was carried out in a thermal reactor along with
a gas chromatography (GC) detector. As shown in Figure 1,
*
Author to whom correspondent should be addressed. Phone: (+8610)-
6
2783586. Fax: (+8610)-62787601. E-mail: zhuyf@chem.tsinghua.edu.cn.
1
0.1021/jp045884d CCC: $30.25 © 2005 American Chemical Society
Published on Web 03/01/2005

Catalytic Degradation of Ethanol on SrCO3 Catalyst
J. Phys. Chem. B, Vol. 109, No. 11, 2005 5119
Figure 1. Schematic diagram of the thermal reactor.
0.1 g of SrCO3 powder was put into an U-shaped quartz tube,
which was 10 mm in diameter, and 1 g of quartz powder covered
the SrCO3 catalyst at the end of the tube to prevent the catalyst
from washing away by the flow. Thermal catalytic oxidations
at different temperatures were realized with an oven, which was
controlled by a heater controller and a thermal couple. Air and
ethanol vapor were accurately mixed by a mass flow meter,
and the flow rate was examined by a bubble flow meter. The
concentration of ethanol was determined by GC, with a GDX-
4
03 GC-column (1.5 mm × 4 mm, 373 K) and hydrogen flame
detector.
3
. Results and Discussion
.1 Characterization of SrCO3 Nanowires and Nanopar-
3
ticles. Figure 2 showed the TEM photographs of SrCO3
nanowires and nanoparticles. All of these SrCO3 nanomaterials
are well-dispersed, and their morphology and grain size can be
easily evaluated. SrCO3 nanowires were uniform, and their
average diameter and length were about 30 nm and 2.5 µm,
respectively, as shown in Figure 2A. These nanowires are rough,
but no particle boundaries can be clearly found in every
nanowire, indicating that the nanowires are continuous and
holistic. In addition, the electron diffraction in the upper-left
corner showed that only (001) appeared along the length
direction of nanowires, indicating that the SrCO3 nanowires were
Figure 2. TEM photographs of the SrCO
nanowires; (B) 40 nm SrCO3 nanoparticles; (C) 80 nm SrCO3
nanoparticles; (D) 250 nm commercial SrCO nanoparticles.
3 3
catalyst. (A) SrCO
3
9
,10
crystalline and grown preferentially along the c-axis.
In
Figures 2B and 2D, SrCO3 particles are not very uniform, and
their average grain sizes were about 40 and 80 nm, respectively.
Electron diffraction patterns of both nanoparticle samples
indicated that the nanoparticles were all crystalline, as shown
by the typical electron diffraction in the upper-right corner of
Figure 2B. Figure 2D shows the TEM micrograph of com-
mercial SrCO3. It showed that these particles were not uniform
with an average particles size of about 250 nm. Similar to 40
and 80 nm particles, electron diffraction analysis showed that
every particle was a single crystal.
Figure 3. Variation of ethanol degradation ratio over SrCO
and nanoparticles with reaction temperature. Ethanol concentration )
10 000 ppm; O ) 20%; flow rate ) 80 mL/min.
3
nanowires
The X-ray diffraction (XRD) patterns of nanowires and
nanoparticles were similar both in peak position and in relative
intensity, indicating that nanowires and nanoparticles had the
same crystal structure. According to the Joint Committee on
Powder Diffraction Standards (JCPDS 84-1778), these nano-
structured materials were both orthorhombic SrCO3. Besides,
from the broadened diffraction peaks of nanowires, 40 nm
nanoparticles, and 80 nm nanoparticles, it can be concluded that
the crystal sizes are 40 nm nanoparticles < nanowires < 80
2
equilibrium theory (BET) analysis. This result implied that the
grain size of the nanowires may be larger than 40 nm but less
than 80 nm.
3
.2. Catalytic Oxidation of Ethanol by SrCO3 Nanowires
and Nanoparticles. 3.2.1. Effect of Temperature on Ethanol
Degradation oVer SrCO3 Nanowires and Nanoparticles. Varia-
tions of the ethanol degradation ratio over different SrCO3
nanomaterials with temperature were shown in Figure 3. Three
conclusions can be drawn from this result. First, SrCO3
nanomaterials can greatly lower the degradation temperature
of ethanol. This result is consistent with reported work, in which
it was demonstrated that SrCO3 material showed good catalytic
1
1
nm nanoparticles, according to the Scherrer equation.
In addition, the specific surface areas of SrCO3 nanowires,
0 nm nanoparticles, and 40 nm nanoparticles were about 22,
6, and 42 m /g, respectively, as determined by using bond
8
1
2

5120 J. Phys. Chem. B, Vol. 109, No. 11, 2005
Wang and Zhu
shown in Figure 5. The working conditions were ethanol
concentration ) 10 000 ppm, O2 ) 20%, and flow rate ) 80
mL/min.
Figure 5A showed the variation of CH3CHO formation with
temperature over SrCO3 nanowires and nanoparticles. It can be
shown from this figure that CH3CHO formed throughout the
working temperature range, and it showed a peak distribution
at about 670 K on both SrCO3 catalysts, but its yield was
different over the two catalysts. For example, the maximal yield
of the nanoparticle was about 2.5 times of that of the nanowire.
According to Figure 3, CH3CHO formed from ignition tem-
perature and decreased greatly when the degradation ratio was
above 90%. In addition, these two curves were similar to the
degradation ratio curve at temperatures under 600 K. This can
be explained by the fact that CH3CH2OH f CH3CHO domi-
nated ethanol conversion at low temperatures and CH3CHO was
also degraded at high temperature.
Figure 4. GC chromatograms of the oxidation products of ethanol on
SrCO nanowires and nanoparticles at 673 K. Ethanol concentration
10 000 ppm; O ) 20%; flow rate ) 80 mL/min.
3
)
2
Figure 5B shows the comparison of CH2O formation with
temperature on nanowires and nanoparticles. Both of the curves
showed peak distributions, and they reached the maximal values
around 550 and 610 K on nanowires and nanoparticles,
respectively. Furthermore, the maximum yield over the nano-
wires was 4 times higher than that over the nanoparticles.
According to Figure 3, all of the above indicated that CH2O
was mainly formed at low temperatures and that the degradation
ratio of ethanol was only about 20%.
Figure 5D showed the variation of C2H4 formation with
temperature over nanowires and nanoparticles. Similar to the
formation of CH2O, the maximal yield of C2H4 on nanowires
was 9 times higher than that on nanoparticles. But C2H4 could
be observed only when the temperature was higher than 525
and 620 K for nanowires and nanoparticles, respectively. Though
the temperatures corresponding to the maximal yields were
different for the two catalysts, they were all approximately total
conversion temperature, indicating that C2H4 was mainly formed
at high temperatures at which ethanol was totally degraded.
It can be concluded from Figures 5A-C that the three
products had different formation temperatures. For the SrCO3
nanowire catalyst, the main product changed with temperature.
At low temperature, CH3CHO and CH2O formed with ap-
proximate yield, but CH3CHO increased greatly and became
the only main product with the temperature rising. When the
temperature was greater than 730 K, C2H4 emerged and became
the main product. However, CH3CHO was the only main
product all through the working temperature range for the SrCO3
nanoparticles.
activity due to its basicity.8,12 The ignition temperature of ethanol
was decreased by approximately 150 °C for nanoparticles, and
it decreased by approximately 270 °C for nanowires. Second,
the catalytic activity of nanoparticles increased with decreasing
particle size. As shown in Figure 3, 40 nm particles exhibited
slightly higher degradation ability than that of 80 nm particles
in all working temperature ranges, and 80 nm particles showed
obviously higher catalytic activity than that of 250 nm particles.
Since nanoparticles were similar in morphology and crystal
structure, the weak advantage of 40 nm nanoparticles can be
attributed to their higher surface area. The third conclusion
seemed contrary with the second conclusion. Though nanowire
catalysts had lower surface areas than nanoparticles with a
particle size of 40 nm, nanowire catalytic activity was signifi-
cantly higher. For example, the nanowire ignition temperature
was about 120 °C lower than that of the nanoparticle, and the
degradation ratio was observably higher at temperatures below
650 K. In comparison with nanoparticles, nanowires preferen-
tially grew in a certain direction. Therefore, this remarkable
difference between nanowires and nanoparticles may be related
with their nanostructures, which generally determined the
distribution of the active sites.
However, though three catalysts showed different catalytic
activities, they had almost the same total conversion temperature.
This result indicated that nanowires had a wider working
temperature range. Is the degraded ethanol all totally oxidized,
and how does nanostructure affect ethanol catalytic oxidation?
Further investigation was undertaken, and 40 nm nanoparticles
and nanowires were chosen as the model system.
Since various products in a catalytic reaction are generally
related to different catalytic active sites, the variation of CH3-
CHO, CH2O, and C2H4 formation with temperature can be
attributed to different distributions of three kinds of catalytic
active centers on SrCO3 nanowires and nanoparticles. Further
investigations on catalytic active sites were performed using
TPD, which will be discussed below.
3.2.3. Effects of Flow Rate on Catalytic Oxidation of Ethanol
oVer SrCO3 Nanowires and Nanoparticles. Effects of flow rate
on the catalytic degradation of ethanol over SrCO3 nanowires
and nanoparticles was investigated, as shown in Figures 6A and
6B, respectively. A temperature of 623 K was chosen as the
working temperature, since the degradation ratio of both
catalysts was near 50% (the degradation ratio was about 55%
for nanowires and 45% for nanoparticles) at this temperature
and the degradation reaction may be easily affected by changes
in the reaction condition. It can be observed that for ethanol
degradation little changed with the flow rate over the nanowires.
3
.2.2. Effects of Temperature on Products of SrCO3 Nanow-
ires and Nanoparticles. A typical chromatogram of the gas
catalyzed by SrCO3 nanowires and 40 nm particles at 673 K is
shown in Figure 4. A temperature of 673 K was chosen to
illustrate the reaction products, since the degradation ratios of
the two catalysts were both near 80% at this temperature. Except
ethanol, C2H4, CH3CHO, and CH2O can be observed as
degradation products, implying that ethanol was catalyzed in
different manner on these two SrCO3 nanomaterials. In addition,
the three products showed different yields over each catalyst.
For SrCO3 nanoparticles, CH3CHO was obviously the main
product. But for SrCO3 nanowires, C2H4 increased greatly, and
CH3CHO decreased in the products. This result indicated that
different reactions dominated the degradation of ethanol over
nanowires and nanoparticles, respectively, even under the same
conditions.
Formation of CH3CHO, CH2O, and C2H4 over SrCO3
nanowires and nanoparticles with different temperatures are

Catalytic Degradation of Ethanol on SrCO3 Catalyst
J. Phys. Chem. B, Vol. 109, No. 11, 2005 5121
Figure 5. Formation of (A) CH CHO, (B) CH O, and (C) C H
3 2 2 4
on SrCO
3
nanowires and nanoparticles with reaction temperature. Ethanol concentration
)
10 000 ppm; O ) 20%; flow rate ) 80 mL/min.
2
flow rate for both catalysts, but CH3CHO increased the most
sharply. For nanowires, CH3CHO increase 1.25 times, but the
reactant increased only 0.12 times. This can be explained as
follows. Besides being mainly degraded into C2H4, CH2O, and
CH3CHO, part of the ethanol was totally converted into CO2
and H2O, which cannot be detected by a hydrogen flame detector
but has been proven by mass spectroscopy. In ethanol degrada-
tion, CH3CH2OH f CH3CHO was the main conversion, and
other products were produced by further oxidation of
13
CH3CHO. Fast flow greatly reduced degradation of CH3CHO,
so more CH3CHO was formed, but less was consumed, causing
a faster increase than ethanol.
3.2.4. Effects of Ethanol Concentration on Catalytic Oxidation
of Ethanol oVer SrCO3 Nanowires and Nanoparticles. The
effects of concentration on ethanol oxidation over SrCO3
nanowires and nanoparticles were investigated, as shown in
Figures 7A and 7B. It was observed that all substances increased
with concentration to a different extent. For example, products
increased more than ethanol in Figure 7A, while they increased
less than ethanol in Figure 7B. This indicated that ethanol
concentration had weak effect on degradation over nanowires
but a great effect over nanoparticles, demonstrating that nano-
wires had a higher catalytic activity, which was in agreement
with the result shown in Figure 3.
Figure 6. Effect of flow rate on every species in ethanol oxidation at
23 K over (A) SrCO nanowires and (B) SrCO nanoparticles.
In Figure 7A, CH2O increased flatly, but C2H4 and CH3CHO
increased sharply with concentration, indicating that C2H4 and
CH3CHO were the main products at this temperature. This
results were consistent with those shown in Figure 5. However,
in Figure 7B, the phenomena are different. Both ethanol and
CH3CHO increase greatly with concentration, while C2H4 and
CH2O showed weak changes with concentration, implying that
CH3CHO is the main product no matter whether oxidation
proceeded completely or not.
6
3
3
For example, the peak area of ethanol increased by only 20 for
nanowires but 100 for nanoparticles when the flow rate
decreased from 60 to 80 mL/min. This was because the catalytic
activity of the nanowires is about 2 times higher than that of
the nanoparticles, as shown in Figure 3.
Another phenomenon that can be observed was that the
oxidation products and residual reactant all increased with the

5122 J. Phys. Chem. B, Vol. 109, No. 11, 2005
Wang and Zhu
3
Figure 9. TPD profile of ethanol absorbed on SrCO nanowires and
SrCO nanoparticles.
3
Therefore, it can be deduced that for nanowire the basic sites
at 460, 583, and 779 K were related to the formation of
CH3CHO, CH2O, and C2H4 and only the basic site at 670 K
was related to the formation of CH3CHO.
Ethanol-TPD over nanowires and nanoparticles was also
studied. As shown in Figure 9, nanowires showed two desorption
peaks at 467 and 605 K, respectively, and the former was much
stronger than the latter. Different from nanowires, nanoparticles
showed only one weak desorption peak at 676 K, whose
intensity was comparable with the weak one of nanowires. These
results were in agreement with Figure 8. In comparison of Figure
8
with Figure 3, it can be observed that the first desorption
Figure 7. Effect of concentration on ethanol oxidation over (A) SrCO
nanowires and (B) SrCO nanoparticles at 623 K. a ) 10 000 ppm; b
13 000 ppm; c ) 16 000 ppm.
3
temperature of ethanol on nanowires was quite consistent with
the ignition temperature. Furthermore, the second desorption
temperature of the nanowires and the only desorption temper-
ature of nanoparticles were in agreement with the temperatures
at which the degradation of ethanol was about 65%. As shown
in Figure 3, though nanowires had a much lower ignition
temperature than that of nanoparticles, its total conversion
temperature was almost the same as that of the nanoparticles.
So the lower ignition temperature of the nanowires can be
attributed to the active centers related to desorption at 467 K,
while the total conversion of ethanol was mainly determined
by the active center related to desorption over 600 K.
As mentioned above, the nanowires had a different distribu-
tion of active sites compared with that of nanoparticles. This
may be because active sites were mainly determined by the
crystal face. To decrease surface energy, particles tend to expose
the most stable crystal face. However, since nanowires grew
preferentially along the c-axis, the exposed crystal face was
determined by the structure, and thus it may not be the most
stable.
3
)
2 3 3
Figure 8. TPD profile of CO absorbed on SrCO nanowires and SrCO
nanoparticles.
3
.3. Mechanism for Variation of Catalytic Activity with
4
. Conlusions
Nanostructure. The TPD profiles of CO2 absorbed on nano-
wires and nanoparticles were studied to illustrate the different
basic sites on these two catalysts, as shown in Figure 8.
Nanowires showed three peaks at 460, 583, and 779 K,
respectively, and the latter two were much weaker than the one
at 460 K. These peaks can be attributed to three kinds of basic
sites with different basicity. Different from nanowires, nano-
particles showed only one peak at 670 K, indicating that only
one kind of strong basic site on the nanoparticle catalyst.
As shown Figure 4, the nanowire catalyst produced three
products that existed at different temperature ranges, while the
nanoparticle had only one main product at all temperatures.
According to Figures 4 and 8, the basic sites were in agreement
with the main product in both number and temperature.
SrCO nanowires had lower ignition temperatures and wider
3
working temperature ranges than SrCO nanoparticles, though
3
nanowires had lower surface areas than those of nanoparticles.
Besides, nanowires have different products than those of
nanoparticles, and the products can change with reaction
temperature, concentration, and flow rate. TPD analysis proved
that differences of catalytic activity between nanowires and
nanoparticles can be attributed to a different distribution of active
sites with nanostructure, and the products correspond well with
the enabled sites.
Acknowledgment. This work was partly supported by the
Chinese National Science Foundation (20071021), the Trans-

Catalytic Degradation of Ethanol on SrCO3 Catalyst
J. Phys. Chem. B, Vol. 109, No. 11, 2005 5123
Century Training Program Foundation for the Talents by the
Ministry of Education, People’s Republic of China, and the
Excellent Young Teacher Program of the Ministry of Education,
People’s Republic of China.
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