ARTICLE IN PRESS
L. Gao et al. / Journal of Solid State Chemistry 181 (2008) 2804–2807
2805
publication [18]. Briefly, the SWNTs were made by cracking of CH4
(CH4/H2/He ¼ 1/1/8) at 800 1C over mixed-oxide catalyst
electron microscope (TEM) (JEOL, JEM 2010) and field emission
scanning electron microscope (FESEM) (JEOL, JSM 7600F).
a
Mg0.8Mo0.05Ni0.10Co0.05Ox. The SWNTs sample was purified by
nitric-acid washing repeatedly in an ultrasonic bath. The carbon
nanotubes were single walled with a 2 nm average inner diameter.
For hydrothermal synthesis of La2CuO4 single-crystal nanofibers,
by using SWNTs as templates, the mixed solution of the surfactant
poly(ethylene glycol)-block-poly(propylene glycol)-block-poly
(ethylene glycol) (0.1 wt%), La(NO3)3 ꢁ 6H2O and Cu(NO3)2 ꢁ 6H2O
(according to the stoichiometric composition of La2CuO4), SWNTs
(0.005 wt%) and H2O2 (2 mL) was dispersed ultrasonically and was
put into an autoclave for hydrothermal synthesis at 60 1C for
20 h. The precipitation obtained from hydrothermal synthesis was
filtered and washed with distilled water repeatedly and then was
heated at 110 1C for 1 h. Thus La2CuO4 nanofibers were synthe-
sized. Roughly, 0.01 g CNTs templates could produce 1 g La2CuO4
nanofibers. The CNTs remaining in La2CuO4 nanofibers were
removed by the method of temperature-programmed oxidation
(TPO). We know that carbon nanotubes could be burnt out
at temperatures around 650 1C in air [19]. Therefore, the as-
synthesized La2CuO4 nanofibers were exposed to air at 700 1C for
1 h to remove SWNTs before they were used for catalytic reaction.
La2CuO4 bulk powder was prepared by heating the mixture of
La(NO3)2 and Cu(NO3)2 at 800 1C.
The contents of copper in different oxidation states were
measured by means of iodometry according to the procedures
adopted by Hairis and Hewston [20] and Wu et al. [21]. The
oxygen non-stoichiometry values were calculated from the
amount of Cu2+, Cu+ or Cu3+ present, assuming that La3+ was in
its stable oxidation state.
Oxygen/NO temperature programmed desorption (O2/NO-TPD)
was performed to study O2/NO adsorption/desorption behaviors.
The sample was first exposed to oxygen (or NO) at 150 1C for 2 h
for adsorption, and then cooled down to room temperature. The
oxygen (or NO) desorbed gradually from room temperature to
800 1C at the rate of 21/min in the He stream.
3. Results and discussion
The FESEM image of the as-synthesized La2CuO4 nanofiber
shown in Fig. 1A indicates that the diameter of fibers was around
60 nm and the lengths of fibers were estimated to be nearly 3 mm.
After being treated at 700 1C in air, diameter turned to be around
30 nm (Fig. 1B). XRD pattern confirmed that this nanofiber
material was of La2CuO4 crystal structure [18]. We were not sure
whether the nanofibers were hollow tubes or not. The composi-
tion and BET-specific surface area of the La2CuO4 nanofiber and its
bulk powder counterpart were La2Cu Cu+0.12O3.94:105.0 and
2+
0.88
La2Cu Cu O4.04:2.7 m2/g, respectively. The BET specific sur-
2+
0.92
3+
0.08
face areas of Pd-Al2O3, Cu-ZSM-5 and Pd/MCM-41 NO decom-
position catalysts were about 150 m2/g [1], 420 m2/g [14] and
622 m2/g [4], respectively. The specific surface area of the La2CuO4
nanofiber was less than the mesoporous materials but was much
higher than the unsupported mixed oxide powders prepared by
traditional methods. So far the highest specific area reported for
La2CuO4 catalyst was 13.0 m2/g made by sol–gel technique [22]. In
the La2CuO4 nanofiber there were Cu2+/Cu+ and oxygen vacancies
while in the bulk powder, there were Cu3+/Cu2+ and excess
oxygen. The Cu-ZSM-5 was a well known NO decomposition
catalyst and has been deeply studied. In Cu-ZSM-5, there were Cu+
and Cu2+ and when it was exposed to NO, the Cu+ ions were
oxidized to Cu2+. Whereas the desorption of oxygen could result in
the reduction of Cu2+ species to the Cu+ species, over-exchanged
Cu-ZSM-5 showed higher NO decomposition activity and pro-
duced more Cu+ sites at lower temperatures than the unex-
changed sample during the N2 formation accompanied by the
Cu+(NO). Cu+ initiated the NO decomposition process. Adsorbed
oxygen from dissociated NO changed the oxidation state of Cu+
ion, causing the formation of Cu2+(NO3ꢀ), which decomposed to
N2, N2O, NO2 and O2 [23]. So the existence of the pair of Cu+–Cu2+
was the premise for the NO decomposition reaction. Cu+ was
crucially important for leading the reaction to the products N2 and
O2. The La2CuO4 nanofiber satisfied these requirements. The NO
decomposition conversion and the selectivity to each product over
the nanofiber and powder La2CuO4 catalysts at different tem-
peratures are listed in Tables 1 and 2, respectively. Overall, NO
decomposition temperatures over the nanofiber La2CuO4 were
much lower than those over the usual powder La2CuO4 catalyst.
The steady-state catalytic activities for NO direct decomposi-
tions were measured at atmospheric pressure, 1 h after perfor-
mance stabilization over a fixed bed quartz micro-reactor. The
sample (0.5 g) was placed in the quartz tube between two quartz
wool plugs. The quartz tube was placed in a vertical tubular
furnace. The total space velocity was 60,000 hꢀ1 and the NO feed
concentration was 1% with helium being the balance. After
steady-state activity was reached, the effluent gas was analyzed
by gas chromatography using a molecular sieve 5A column
(for the analysis of N2 and O2) and a Porapak Q column (for
N2O). The concentration of NOx (NO+NO2) was monitored with a
chemi-luminescence NOx analyzer. The morphologies of La2CuO4
nanofibers and bulk powder were observed under transmission
100% NO conversion (turn over frequency-TOF: 0.17 gNO/gcatalyst
s
or 0.17 sꢀ1) was obtained over the nanofiber catalyst at tempera-
tures above 300 1C with products being only N2 and O2. When the
NO conversion was below 100%, apart from N2 and O2, the
Fig. 1. Field emission scanning electron microscopy (FESEM) images of La2CuO4 nanofibers (A): ( ꢂ 75,000) obtained after hydrothermal synthesis lasting 20 h.
(B) ( ꢂ 50,000) of La2CuO4 fiber after being treated in air at 700 1C for 1 h.