Paper
NJC
micro/nanostructures were found to have strong catalytic activity,
including metals, oxides and phosphides. One of the aromatic
nitrocompounds 4-nitroaniline can be reduced to 1,4-phenylene-
diamine (1,4-PD), which is an important intermediate to prepare
2
7–29
dyes, curing agents for epoxy resin and rubber antioxidants.
As mentioned in the previous section, nanostructural Bi
mainly used as photoresponse and field emission materials,
2 3
S was
1
0
11,12
13–15
electrochemical hydrogen storage devices,
photocatalysts,
16
and electrode materials in lithium ion battery. To date, no report
has been found to evidence its use as a catalyst for the reduction of
aromatic nitrocompounds by NaBH in aqueous solutions. In the
4
present work, experiments exhibited that the as-obtained flower-
like Bi S microspheres showed an excellent catalytic activity for
2
3
the reduction of some aromatic nitrocompounds by NaBH
aqueous systems, including 4-nitroaniline (4-NA). Fig. 5a shows
the UV-vis absorption spectra of the 4-NA–NaBH system in the
4
in
4
À1
presence of 100 mg L catalyst at various reaction duration.
A strong absorption peak at 380 nm, which originated from the
intermediate formed by 4-NA and NaBH4, gradually decreased
with increase in the reaction duration. Simultaneously, a new peak
at 306 nm appeared, which belonged to the characteristic absorp-
30
29
tion peak of 1,4-PD. After reacting for 9 min, the peak at 380 nm
almost disappeared, indicating that the reduction of 4-NA had
closely completed. It is worth noting that the reduction rate was
very rapid at the initial stage of the reaction, indicating that no
inducing period existed in the current reductive reaction in the
Fig. 4 SEM images of the products obtained at 110 1C for 10 min from the
systems with various surfactants: (a) no surfactant, (b) 5 mg of CTAB, (c) 5 mg of
SDBS, (d) 1.25 mg of PVP, (e) 2.5 mg of PVP and (f) 7.5 mg of PVP.
presence of Bi S catalyst. Subsequently, the reaction rate gradually
2 3
slowed down due to the consumption of reactants. Fig. 5b depicts
the correlation between the reaction rate and the amount of the
When 1.25 mg of PVP was employed, flowerlike structures con- catalyst. Obviously, the reductive reaction can be promoted by
À1
structed by nanorods could be obtained. However, they compactly increasing the amount of the catalyst from 50, 100 to 150 mg L
.
connected with each other and no discrete flowerlike microspheres Furthermore, the present catalyst could be recycled by centrifuga-
were formed (see Fig. 4d). However when 2.5 mg of PVP was used, tion and repeated washing. As shown in Fig. 5c, after the catalyst
flowerlike microspheres could easily be distinguished although the was used four times, its catalytic efficiency could still reach 67%
connection among flowerlike structures still existed (Fig. 4e). When within 9 min. Compared with the first catalytic efficiency, however,
the amount of PVP was increased to 5 mg, discrete and uniform the catalytic ability of the catalyst had markedly decreased. This
flowerlike microspheres were formed (Fig. 2a). Upon further should be attributed to the shape change of the catalyst. As seen
increasing the amount of PVP to 7.5 mg, however, the flowerlike from Fig. 5d, the surfaces of the flowerlike microspheres became
microspheres were destroyed (Fig. 4f). The above facts imply that compact than those shown in Fig. 2a. Namely, the shape of the
excess PVP is unfavorable for the formation of flowerlike micro- catalyst had retrograded after the first cycle. Here, the specific
spheres. In the present work, PVP had two roles in the formation of surface area and the BJH pore width of the catalyst changed to
2
À1
2 3
Bi S flowerlike microspheres: the structure-directing reagent and 28.7 m g and B10.6 nm, respectively (see Fig. 2f). Hereafter, the
surfactant. When small amounts of PVP existed in the system, they shape of the catalyst hardly changed, and so the catalyst presented
could adsorb on some specific crystal surfaces of the nuclei of a the close catalytic capacity from the 2nd to the 4th reduction. In a
preferred crystalline phase in the initial nucleation stage. Sub- catalytic system, usually, the catalytic activity of a catalyst is mainly
sequently, the nuclei grew only along the directions unoccupied determined by its active site number. More the active sites are, more
25
by PVP. Thus, discrete and uniform flowerlike microspheres the reactant molecules adsorbed and, thus, higher the catalytic
gradually formed with the increasing PVP amount to 5 mg. Here, efficiency of the catalyst is. Since the catalyst with a large specific
PVP mainly acted as the structure-directing reagent. When excess surface area often has more active sites, its catalytic efficiency will be
PVP was employed, however, the surfactant function of PVP pre- higher than that of the one with a small specific surface area. In the
dominated, causing the destruction of the flowerlike microspheres. present work, the shape retrogradation of the catalyst after the 1st
cycle caused the decrease of the specific surface area. As a result, the
3
.2 Application in the catalytic reduction of 4-nitroaniline
In recent years, the reduction of aromatic nitrocompounds catalytic efficiency of the catalyst after the 1st cycle.
by sodium borohydride (NaBH ) in the aqueous solution From Fig. 3 and 4, it is evident that the morphology of Bi
Many inorganic compounds can be affected by the reaction temperature and the surfactant.
active sites of the catalyst reduced. This led to the decrease in the
4
2 3
S
2
2,26
has made great progress.
This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2014
New J. Chem., 2014, 38, 5324--5330 | 5327