were made from the oxidized BCNT, after which time the
spherical carbon composites were calcinated. These beads
were used as supports for the preparation of palladium-,
nickel- and rhodium-containing hydrogenation catalysts. The
activity of the BCNT-supported catalysts was tested in CO2 hy-
drogenation reactions. Based on these results, we can con-
clude that the oxide forms of the nickel catalyst promote
methane formation. Application of reduced metallic palladium
and rhodium does not lead to a greater amount of methane
formed, especially at higher temperature. Compared to the
two noble metal catalysts, it can be concluded that the appli-
cation of palladium led to the formation of CO in larger quanti-
ties; in this sense the Pd was more efficient than the Rh. A
striking difference can be observed between the size distribu-
tion of the rhodium and the palladium particles. This difference
likely resulted in different catalytic activity during the CO2 hy-
drogenation to CO. The formation rate of the CO was higher at
723 K compared to the other two catalysts. Based on these re-
sults, we conclude that the final product can be influenced by
means of a careful selection of the catalyst. Accordingly, on the
Pd/BCNT catalyst the generation of CO was favored and in the
case of the Ni/BCNT catalyst, the major pathway was the for-
mation of CH4. We assume that the surface of the Ni/BCNT cat-
alyst was mainly covered by elemental Ni. However, under the
reaction conditions employed, the Ni/NiOx form was still pres-
ent which was responsible for the high activity of the catalyst.
Functionalization of BCNTs
The nanotubes were functionalized with a mixture of nitric acid
and sulfuric acid (1:3 v/v ratio) over 24 h at room temperature with
continuous stirring. The samples were washed with distilled water
and dried at 380 K overnight. The success of the functionalization
was confirmed by Fourier transform infrared spectroscopy (FTIR).
Preparation of Calcium Alginate BCNT Spheres
Two different mixtures were used to prepare calcium alginate
BCNT spheres.[35] One of them contained 100 mL distilled water, 1 g
Na-alginate and 1 g oxidized BCNT. The other mixture contained
200 mL distilled water and 6 g CaCl2. The Na alginate was dis-
persed by using a Hielscher homogenizer. Subsequently, using a
syringe pump the Na–alginate–BCNT mixture was added dropwise
to the CaCl2 solution. During this process the formed beads had
the same size as the droplets. After the preparation, the remaining
solution was effused, the spheres were washed with distilled water
then dried at 350 K for 48 h. The prepared spherically shaped cata-
lyst was calcinated at 1073 K under a nitrogen flow for 20 minutes.
Decoration of Spheres with Pd, Rh and Ni Nanoparticles
The prepared BCNT spheres were impregnated with a palladium
chloro complex. 50 mL distilled water was added to 475 mg beads,
then 43.1 mg PdCl2 was dissolved in 5 mL distilled water and 1 mL
hydrochloric acid with continuous heating. The solution was added
to the spheres, and then the water was removed by a rotary evap-
orator. Finally, the beads were dried at circa 380 K for 12 h and re-
duced at 673 K in a hydrogen flow. The rhodium-containing cata-
lyst was prepared from 74.3 mg rhodium-acetate dimer
[(CH3CO2)2Rh]2 and 657 mg BCNT spheres, similarly to the previous
method. In the case of the nickel catalyst, 495 mg Ni(NO3)2*6H2O
was dissolved in 100 mL distilled water and 1.9 g granulated BCNT
was added to the nickel solution. After 30 min agitation, the water
was evaporated from the impregnated carbon support and the
sample was dried at 380 K. The reduction step was identical to
that described previously.
Experimental Section
Materials
For the CCVD synthesis the following materials were used: n-butyl-
amine (Sigma Aldrich; as the carbon source), Ni(NO3)26H2O
(Merck) and MgO (Sigma Aldrich) for preparation of CCVD catalyst.
The purification of nanotubes was performed with 37 wt% cc. HCl
(VWR). Nitric acid, 67 wt% (VWR) and sulfuric acid, 98 wt% (VWR)
were used for functionalization of BCNTs. During of application
gelled BCNT spheres sodium alginate (Sigma Aldrich) and CaCl2
(Merck) were used. The hydrogenation catalysts were synthetized
from the following precursors: PdCl2 (Alfa Aesar), [(CH3CO2)2Rh]2
(Sigma Aldrich), Ni(NO3)26H2O (Merck).
Characterization Techniques
Transmission Electron Microscopy (TEM)
Imaging of the different BCNT supports, metal nanoparticles and
derived catalysts were characterized by FEI TECNAI G2 20 X-Twin
high-resolution transmission electron microscope, (equipped with
electron diffraction) operating at 200 kV accelerating voltage. The
samples were dropcast onto 300 Mesh Copper grids from an aque-
ous suspension.
Synthesis of N-Doped Carbon Nanotubes
N-doped BCNTs were synthesized from butylamine by a catalytic
chemical vapor deposition (CCVD) method. The preparation was
performed in a quartz reactor. The nitrogen-containing carbon
source (butylamine) was injected into the furnace with an infusion
pump. The applied carrier gas was nitrogen and the CNTs were
synthesized at 973 K. The reaction took place on a catalyst bed,
placed in a quartz-bowl, containing 5 wt% of Ni/MgO catalyst. The
desired temperature was controlled by the tube furnace and the
sample was positioned in it. After CNT synthesis, the catalyst was
removed by boiling the nanotubes in concentrated hydrochloric
acid for 6 h. Then the BCNTs were washed with distilled water and
dried at 380 K overnight. The purity of the product was checked
with thermogravimetric tests (TG).
Thermogravimetric Measurements (TG)
The purity of the BCNTs was checked by thermogravimetric analy-
sis. The measurements were applied with a MOM Derivatograph-C
device, using aluminum oxide as the reference in air. The heat-up
rate of the furnace was 5 KminÀ1 from ambient temperature up to
1173 K. A differential thermogravimetric curve (DTG) was used to
evaluate the TG curve with the MOM WinderC program.
ChemistryOpen 2018, 7, 789 –796
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ꢀ 2018 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim