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Chemistry Letters Vol.35, No.1 (2006)
Synthesis of Ni–Cu Particles by Hydrogen Reduction in Hot-compressed Water
ꢀ
Kiwamu Sue, Satoshi Tanaka, and Toshihiko Hiaki
College of Industrial Technology, Nihon University, 1-2-1 Izumi-cho, Narashino, Chiba 275-8575
(Received September 26, 2005; CL-051231; E-mail: k5sue@cit.nihon-u.ac.jp)
Synthesis of Cu and Ni–Cu particles from their aqueous
was only used as a starting material and Ni particles formed
through the following two steps: formation of NiO or Ni(OH)2
formate solution by hydrogen reduction in hot-compressed
water was carried out at 673 K with a titanium alloy autoclave.
Hydrogen was produced by thermal decomposition of the
formates. Metal particles having an average size of around
00 nm were produced without any additives. Ni–Cu metal alloy
particles having different compositions were obtained by chang-
ing Cu/Ni molar ratio in starting solutions.
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particles from Ni by hydrothermal synthesis and precipitation
of Ni on the surface of the nuclei by hydrogen reduction.
In this work, we synthesized Cu and Ni–Cu particles from
copper and nickel formates in HCW. We examined reaction time
and Cu/Ni molar ratio in starting solution on the composition of
the obtained alloys.
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Solutions were prepared by dissolving precise amounts
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of Ni(COOH)2 2H2O (Wako Pure Chemicals, Japan) and
.
Ni and Cu particles are widely used in several fields because
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Cu(COOH)2 4H2O (Wako Pure Chemicals, Japan) in distilled
of their catalytic, electronic, and magnetic properties. In addi-
water. For the synthesis of Ni–Cu alloy, Cu/Ni molar ratio
was changed from 0.33 to 3.00. Total concentration of metals
was kept constant 0.1 mol/kg in all experiment.
tion, Ni–Cu alloy particles have large potentials for the improve-
ment of the properties of pure Ni and Cu particles. A variety of
techniques have been used for producing several pure metal and
Nickel formate (NF) and/or copper formate (CF) aqueous
solutions were loaded into the batch reactor made of titanium al-
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metal alloy particles such as ball milling, spray pyrolysis, mi-
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croemulsion, and sol–gel. In some methods, the controllability
of particle size and composition is theoretically limited. Addi-
tionally, in the aspect of environmentally sustainable simple
chemical processes there exist some difficulties such as high
temperature operation above 1273 K, the use of harmful organic
solvent and high concentration of reducing agent, and multistep
operation.
loy (Ti6Al4V) of 153 cm . The temperature was measured with a
K-type thermocouple that was inserted into reactor. The air in
the reactor was replaced with nitrogen by successive purging
and then the reactor was sealed. Reactor load water density
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was 0.35 g/cm , which corresponds to about 30 MPa at a reac-
tion temperature of 673 K. The reactor was heated by immersion
into a temperature-controlled molten-salt bath. Approximately
5 min was required for the batch reactor to reach the reaction
temperature. Reaction time was 3–30 min, which includes
heating up time. The reactor was quenched in a water bath,
which was kept at room temperature. Formic acid decomposes
into hydrogen and carbon dioxide and yields of H2 and CO2
Hot-compressed water (HCW) around critical point of water
(
Tc ¼ 647 K and Pc ¼ 22:1 MPa) has significant potentials for
controlling specific solvent properties, i.e. density, dielectric
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constant, and ion product, with temperature and pressure. For
example, hot-compressed water has low dielectric constant
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(
(
about 10 to 25) compared with water at room temperature
about 80), and it forms homogeneous mixture with hydrogen
increase to ca. 90% at 673 K within 2 s.
The crystal structures of the products were analyzed by pow-
der X-ray diffractometry (XRD) (Rigaku, RAD-B system with
monochrometer), using Cu Kꢀ radiation. Observation of these
products was performed by a scanning electron microscope
(SEM) (JOEL, JSM-5400LVS). Concentrations of remaining
Ni and Cu ions in the recovered aqueous solution were measured
by atomic absorption spectroscopy (Shimadzu, AA-6300). Con-
version of metal ion to solid product was defined as ð1 ꢁ C=
C0Þ ꢂ 100, where C and C0 are molal concentrations of the metal
species in the recovered and starting solutions, respectively.
Typical SEM images are shown in Figure 1. XRD patterns
of the products from CF and NF + CF mixture are shown in
Figures 2 and 3, respectively. As shown in Figure 3, two kinds
of crystals are produced from NF + CF. The peak positions of
the crystals are slightly shifted from pure Ni and Cu. The lattice
parameter varies linearly with the amount of Cu in the lattice
according to Vegard’s low. The compositions of these crystals
were determined with the calculated value of the cell parameter
on the basis of Vegard’s low. No significant difference with
reaction temperature was observed. The compositions at 30
min as a function of Cu concentration in starting solution are
shown in Figure 4. Conversions in all experiment were more
than 98.6% and no significant dependence was observed.
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gas at this condition. From the viewpoint of reaction solvents,
the HCW has the possibility to lower the use of organic solvent
and to supply homogeneous redox reaction field. In addition, the
reaction rate near Pc and above Tc can be drastically controlled
with temperature, pressure, and also oxidizing and reducing
agents (H2 and O2). This means that particle size, crystal struc-
ture, composition, redox state of metals, and morphology can
possibly be varied with these factors.
We have reported synthesis of Ni particles by hydrogen
reduction in HCW environment from nickel acetate and iron sul-
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fate. Iron sulfate was used for producing Fe3O4 nuclei because
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Ni cannot nucleate homogeneously from a solution. In the
work, formic acid was used for producing hydrogen by thermal
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decomposition at higher temperatures and Ni particles formed
through the following two steps: formation of Fe3O4 nuclei by
hydrothermal synthesis from aqueous iron sulfate solution and
precipitation of Ni on the surfaces of the nuclei by hydrogen re-
duction from nickel acetate aqueous solution. 1,10-Phenanthro-
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line was used as a complex for preventing hydrolysis of Ni be-
cause hydrolysis promotes formation of Ni(OH)2 and/or NiO
particles at higher temperatures. Recently, we reported a simple
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method for Ni synthesis in HCW. In the method, nickel formate
Copyright ꢀ 2006 The Chemical Society of Japan
Sue, Kiwamu
