Y.V. Shubin, Y.I. Bauman, P.E. Plyusnin et al.
Journal of Alloys and Compounds 866 (2021) 158778
2
developed surface area of 200–400 m /g and a unique segmented
structure called a feathery-like structure. It was found that the cat-
alytic activity depends non-linearly on the molybdenum con-
centration, and the maximum corresponds to 7–9 wt%. An
introduction of the third component in the composition of Ni-Mo
alloy is expected to open new possibilities for the creation of an even
more active catalyst. In order to choose the third component, an
analysis of the phase diagrams of the binary and triple systems was
performed. It is important to note that the metal-carbon phase
diagrams should be taken into account as well. The process of the
CNF formation over iron subgroup metals (Fe, Co, Ni) proceeds in
accordance with the so-called carbide cycle mechanism when the
formation of intermediate non-stoichiometric carbides on the metal
surface is considered [31]. Therefore, the ability of the doping metal
to form the interstitial solid solution with carbon seems to be one of
the necessary conditions to obtain the active catalyst. The Ni-Mo-W
system meets these requirements.
In the present research, specially prepared sponge-like porous
nanoscale Ni-W-Mo alloys were used as catalysts for the CCVD
process of 1,2-dichloroethane (DCE). Such alloys attract a growing
interest of researchers since they provide a developed porous
structure along with considerable resistance to sintering at elevated
temperatures that make it possible to use them under high-
temperature reaction conditions [41]. The prepared Ni-W-Mo
samples were studied by scanning electron microscopy and X-ray
diffraction analysis, and their catalytic activity was examined in the
hydrogen-assisted catalytic decomposition of DCE, resulting in the
formation of carbon nanofibers (CNF). The effect of W and Mo
addition on the catalytic activity of the alloys has been elucidated.
The resulting CNF materials were investigated by low-temperature
nitrogen adsorption, Raman spectroscopy, and transmission electron
microscopy.
2.2. Characterization techniques
The elemental analysis of metal content in the alloy samples was
performed by an atomic absorption method using Thermo
Scientific iCAP-6500 spectrometer. The sample was dissolved in an
aqua regis at heating and then evaporated with hydrochloric acid up
to the complete removal of nitric acid. The relative standard devia-
tion of the Mo and W determination was 0.1.
Simultaneous thermal analysis (STA) involved concurrent ther-
mogravimetry (TG), differential thermal analysis (DTA), and mass
spectral (MS) analysis of the evolved gas (EGA). The STA measure-
ments were performed in an apparatus consisting of an STA 449 F1
Jupiter thermal analyzer and a 403D Aëolos quadrupole mass spec-
trometer (NETZSCH, Germany). The spectrometer was connected
online to a thermal analyzer (STA) instrument by a quartz capillary
heated to 280 °C. The mass spectrometer operated with an electron
impact ionizer with an energy of 70 eV. The ion currents of the se-
lected mass/charge (m/z) numbers were monitored in a multiple
ions detection mode with a collection time of 0.1 s for each channel.
a
2 3
The measurements were made using Al O crucible in a hydrogen-
helium mixture (10 vol% H ) in a temperature range of 30–600 °C, a
2
flow rate of 30 ml/min, and a heating rate of 1–10 °C/min.
The powder X-ray diffraction (XRD) analysis of the samples was
performed at room temperature on
a Shimadzu XRD-7000
diffractometer (CuKα radiation, nickel filter in reflected beam, scin-
tillation detector with amplitude discrimination). The data were
collected step-by-step in 2θ ranges of 20–100° with a step of 0.1°
(for the phase identification) and 140–148° with a step of 0.05°
(for the precise determination of lattice parameter). The lattice
parameters of the alloys were refined by an angular position of the
(331) reflection using the PowderCell 2.4 software Kraus [43].
Porous structure was characterized by means of the adsorption
porosimetry using N
Micromeritics, USA) instrument. Before the adsorption experi-
ments, the samples were stepwise degassed under fore-vacuum at
2
as adsorbate at 77.4 K on an ASAP-2400
(
2. Experimental
90 °C for 4 h and at 350 °C overnight.
2.1. Synthesis of the alloys
The scanning electron microscopy (SEM) study of the initial al-
loys and produced carbon nanomaterials was carried out using a
JSM-6460 scanning electron microscope (JEOL, Japan). The magnifi-
cation factors were in a range of 8–300,000. The elemental mapping
of the samples was performed by means of energy-dispersive X-ray
(EDX) analysis.
The transmission electron microscopy (TEM) images were ob-
tained using a Hitachi HT7700 microscope equipped with an EDX
detector Bruker X-flash 6 T/60. The accelerating voltage was 100 kV.
Raman spectra of the samples were collected on a Horiba Jobin
Yvon LabRAM HR UV–VIS–NIR Evolution Raman spectrometer
equipped with an Olympus BX41 microscope and 514.5 nm line of
Ar-ion laser. The power of light focused in a spot with a diameter of
~2 µm was less than 0.8 mW to avoid the thermal decomposition
of the sample.
The starting reagent used for the preparation of alloys were
NiCl
NH
2
·6H
2
O
(Reachem, Russia),
H
2
WO
4
(Vekton, Russia), and
]Cl was synthe-
(
4
6
) Mo
7
O
24·4H O (Reachem, Russia). [Ni(NH
2
3
)
6
2
sized in accordance with the procedure described elsewhere [42].
Acetone was used as a precipitating agent. All the reagents were of
chemical purity grade and were used without any preliminary
purification.
In the case of Ni(92)Mo(4)W(4) preparation, H
and (NH Mo 24·4H O (0.073 g) were dissolved in a concentrated
ammonia solution (50 ml) at heating. Thus obtained solution was
evaporated to a volume of 30 ml. Then, 3.633 g of [Ni(NH ]Cl was
2 4
WO (0.054 g)
4
)
6
7
O
2
)
3 6
2
added to the obtained solution at continuous stirring until complete
dissolution. Thus obtained joint solution was added at intensive
stirring to the 15-fold volume of acetone (450 ml) cooled to 0 °C. The
sediment was filtered, dried at room temperature for 5 h, and cal-
cined at 800 °C for 1 h in a hydrogen atmosphere. Similar procedures
were applied for the preparation of all other precursors. The che-
mical composition and purity of the Ni-Mo-W alloys were controlled
by an inductively coupled plasma atomic emission spectroscopy
2.3. Catalytic performance measurements
The catalytic chemical vapor deposition (CCVD) of 1,2-
dichloroethane (DCE) over the prepared triple Ni-W-Mo alloys was
performed in a flow-bed quartz reactor equipped with MacBain
balances. Therefore, the mass changes during the carbon product
deposition were controlled in a real-time regime. The specimen of
the alloy (2.0 ± 0.03 mg) was loaded in a quartz basket and heated
in argon flow up to 600 °C. The next stage was reductive pretreat-
ment in a hydrogen stream for 15 min, which allowed us to eliminate
(
ICP-AES) method. The samples were labeled as Ni(x)Mo(y)W(z),
where x, y, and z is content (wt%) of Ni, Mo, and W, correspondingly.
The monometallic sample Ni(100) and bimetallic reference
samples Ni-W(x) and Ni-Mo(x), where x is content (wt%) of W or Mo,
were obtained by similar procedures.
2