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Journal of Materials Chemistry A
Paper
stability by recycling 10 times without any signicant loss of
The catalytic reduction of 4-nitrophenol (4-NP) was also
activity, which is superior to that of the commercial catalysts performed using Pd@C and Pt@C as the catalysts, respectively.
(active metals supported on the surface of active carbon). ICP- In a typical run, 20 mL of 4-NP aqueous solution (0.1 mM) was
AES suggested that the leaching of noble metals in the reaction mixed with 10 mg of catalyst. The suspension was then purged
solvent is negligible. Based on TEM analysis, the morphology of with N2 for 30 min to remove the dissolved O2. Under constant
noble metal nanoparticles was still retained inside the carbon stirring, 10 mM NaBH4 was added to start the reaction. The
matrix aer the cycling, and no serious aggregation of the Pd reduction process was monitored by monitoring the absorbance
nanoparticles was observed, which demonstrates the feasibility at 400 nm using a UV-vis spectrophotometer. When the reaction
of our strategy.
was complete, the catalyst was recovered by centrifugation and
subsequently reused. Reduction kinetics of 4-NP over the Pd@C
catalyst was conducted under the following conditions: 5 mL of
4-NP aqueous solution (0.13 mM), 5 mg of Pd@C catalyst and 13
mM NaBH4.
Experimental
Preparation and characterization
1.0 g of BWT was dissolved in deionized water, and then 1.0 g of
g-Al2O3 was added in the above solution under constant stir-
ring. Aer that, the mixture was dried. The obtained materials
were then added into desired amounts of noble metal salt
solution and kept under constant stirring for 24 h, allowing the
chelating interaction of noble metal ions with BWT. The
mixture was completely dried in an oven and transferred to a
tube furnace for thermal treatment under an Ar/H2 (5%) stream
at 700 ꢀC for 2 h. Aer cooling down to room temperature, the
samples were collected and treated with 1.0 M/L NaOH solution,
followed by washing with deionized water and vacuum drying.
The as-prepared catalysts were denoted as mesoporous carbon
encapsulated Pd nanoparticles (Pd@C). The Pt@C catalyst was
also prepared by similar procedures.
Results and discussion
As shown in Fig. 1a, BWT consists of a large number of adjacent
phenolic hydroxyls in its B rings, and its molecular backbone is
built from aromatic rings. When BWT was impregnated onto g-
Al2O3 (Fig. 1b), the hydroxyls on the surface of g-Al2O3 inter-
acted with the free phenolic hydroxyls of BWT via the formation
of multiple hydrogen bonds, promoting the distribution of BWT
in the porous channel network of g-Al2O3. Aer the impregna-
tion of BWT, Al2O3-BWT showed a dark brown color in contrast
with the white g-Al2O3 (ESI, S1†). Subsequently, the BWT
located on the inner surface of g-Al2O3 was able to chelate with
Pd2+ via its adjacent phenolic hydroxyls, forming very stable
ve-membered chelating rings (BWT-Pd2+). According to UV-vis
analysis, the characteristic peak of BWT, located at 278 nm,
showed a decrease in its intensity aer reacting with Pd2+,
conrming the chelating interactions between BWT and Pd2+
(ESI, S2†). In a subsequent thermal treatment in an Ar/H2
stream at 700 ꢀC for 2 h (Fig. 1c and d), Pd2+ was in situ reduced
to Pd nanoparticles. Meanwhile, the BWT molecules chelated
with Pd2+ were decomposed to amorphous carbon that effec-
tively stabilized the Pd nanoparticles. The dosage of used BWT
has been optimized in order to ensure that the carbon matrices
can stabilize the formed Pd nanoparticles and also adopt the
porous structure of g-Al2O3. Based on TGA analysis (ESI, S3†),
Thermogravimetric analysis (TGA, Q500) was carried out
under theÀt1emperature range 33–850 ꢀC at a heating rate of
ꢀ
20 C min in N2 ow. Raman spectra were recorded using a
WITec CRM200 confocal Raman microscopy system with a laser
wavelength of 488 nm and a laser spot size of 0.5 mm. The Si
peak at 520 cmÀ1 was used as a reference for wavenumber
calibration. TEM characterization and elemental composition
analysis were performed with a JEOL 2100F operated at 200 kV
with energy dispersive X-ray spectroscopy (EDS) attachment.
The analyses of the substrate and product were performed with
a GC (Agilent, 7890A) equipped with a chromatographic column
HP-5. N2 adsorption/desorption experiments were conducted
using a Micromeritics ASAP 2010 instrument at 77 K.
Catalytic reactions
The catalytic hydrogenation was conducted in a stainless
reactor equipped with a stirring bar. A suitable amount of
substrate (styrene, 1-pentene, 1-hexene, 1-heptene, cyclohexene,
cyclooctene, quinoline and cinnamaldehyde), 15.0 mL of ethyl
acetate as the solvent and the desired amount of catalyst were
added into the reactor. Subsequently, the reactor was heated to
100 ꢀC and kept at 2 MPa H2 under constant stirring. When the
reaction was complete, the products were analyzed by gas
chromatography (GC) and the catalysts were collected by
centrifugation, followed by thorough washing with ethyl
acetate. For comparison, the catalyst reaction was also con-
ducted using commercial Pd/carbon (5%, Sigma) as the catalyst
under the same experimental conditions.
Fig. 1 (a) Molecular structure of BWT, and its chelating interaction
with Pd2+. (b–d) Preparation strategy of the Pd@C catalyst.
5848 | J. Mater. Chem. A, 2014, 2, 5847–5851
This journal is © The Royal Society of Chemistry 2014