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RSC Advances
test result (Fig. 3d) shows the Pd@NC nanoreactors could be
recycled up to six times without loss of catalytic activity. Aer
the sixth cycle, the Pd@NC nanoreactors were recovered for
further TEM characterization. Fig. 3e shows the dispersion state
of Pd nanoparticles in Pd@NC nanoreactors. All Pd nano-
particles partially embedded in the nitrogen-doped shells. No
Pd nanoparticles detached from the nitrogen-doped carbon
shells. Fig. 3f shows the particle size distribution of Pd nano-
particles. The Pd particle size was determined to be 12 Æ 4 nm.
Compared to the fresh Pd@NC nanoreactors, the morphology
of Pd@NC nanoreactors and dispersion state of Pd nano-
particles are almost unchanged aer the sixth cycle. Based on
the recycling test and TEM characterization results, the high
catalytic activity and stability of the Pd@NC nanoreactors are
mainly attributed to the unique nanostructures between Pd
nanoparticles and nitrogen-doped carbon shells. The presence
of a nitrogen-dropped carbon shell facilitates preventing the
migration and aggregation of Pd nanoparticles, resulting in the
high catalytic stability.
In summary, this work presents an effective strategy for
synthesizing Pd-based composite catalyst with high catalytic
performances. The Pd nanoparticles partially embedded in the
nitrogen-dropped carbon shells prevent the migration and
aggregation of Pd nanoparticles. The partially embedment
resulted in an enhanced stability of the highly dispersed Pd
nanoparticles. The catalytic activity and dispersion state of the
nanoreactors remained the same aer six cycles.
Fig. 3 (a) Time-dependent UV-Vis absorption spectra and (b) time-
dependent conversion and (c) ln(Ct/C0) versus reaction time of 4-
nitrophenol, 2-amino-4-nitrophenol and 2-chloro-4-nitrophenol
catalyzed by Pd@NC nanoreactors. (d) Catalytic conversion of 4-
nitrophenol by using recycled Pd@NC nanoreactors at different cycle
times. (e) TEM image of the recovered Pd@NC nanoreactors and (f)
particle size distribution of Pd nanoparticles partially embedded in
nitrogen-doped carbon shells after the sixth cycle.
Acknowledgements
This work was supported by the Fundamental Research Funds
for the Central Universities (2232013D3-08), DHU Distin-
guished Young Professor Program, National Natural Science
Foundation of China (51402048), and the Scientic Research
Foundation for the Returned Overseas Chinese Scholars, State
Education Ministry.
same period of time, the experiment using a commercial Pd/C
catalyst only reached 4% 4-nitrophenol conversion (see Fig
3b). In a control experiment, the hollow nitrogen-doped carbon
spheres do not show any catalytic activity. The conversion of 2-
amino-4-nitrophenol (NPNP) and 2-chloro-4-nitrophenol
(ClPNP) is similar to that of 4-nitrophenol, indicating the
functional groups have no signicant effect on the catalytic
activity of Pd@NC nanoreactors.
In this study, the concentration of NaBH4 is signicantly
higher than that of 4-nitrophenol, 2-amino-4-nitrophenol and 2-
chloro-4-nitrophenol and can be considered as constant during
the reaction period. So the pseudo rst-order kinetics can be
applied to evaluate the reaction rate constant.11,24,31 Fig. 3c
shows the plot of ln(Ct/C0) as a function of reaction time for the
catalytic reduction of 4-nitrophenol, 2-amino-4-nitrophenol and
2-chloro-4-nitrophenol, showing a linear relationship. The
reaction rate constant of 4-nitrophenol, 2-amino-4-nitrophenol
and 2-chloro-4-nitrophenol was calculated from the slope to
be 6.4 Â 10À3/s, 1.1 Â 10À2/s and 7.8 Â 10À3/s, which is similar
to the reported values.24,28,31,32 It indicates the unique embed-
ment has not obviously effect on the catalytic activity of Pd
nanoparticles.
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