Joint reduction of Fe(III), Ni(II), and Co(II) ions in solution in contact with aluminum

Article abstract of DOI:10.1134/S1070427208110013

Joint reduction of iron group metals in aqueous solutions on a dispersed aluminum matrix was studied. The powders obtained were examined by X-ray phase analysis and electron scanning microscopy.

Full text of DOI:10.1134/S1070427208110013

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2008, Vol. 81, No. 11, pp. 1879–1884. © Pleiades Publishing, Ltd., 2008.
Original Russian Text © A.F. Dresvyannikov, M.E. Kolpakov, E.V. Pronina, 2008, published in Zhurnal Prikladnoi Khimii, 2008, Vol. 81, No. 11,
pp. 1761–1766.
INORGANIC SYNTHESIS
AND INDUSTRIAL INORGANIC CHEMISTRY
Joint Reduction of Fе(III), Ni(II), and Co(II) Ions in Solution
in Contact with Aluminum
A. F. Dresvyannikov, M. E. Kolpakov, and E. V. Pronina
Kazan State University of Technology, Kazan, Tatarstan, Russia
Received February 5, 2008
Abstract—Joint reduction of iron group metals in aqueous solutions on a dispersed aluminum matrix was stud-
ied. The powders obtained were examined by X-ray phase analysis and electron scanning microscopy.
DOI: 10.1134/S1070427208110013
the strongly defective state of the surface oxide–
hydroxide coatings of the particles, and also to intense
heating of the solid phase in the course of the oxidation
[10]. These factors can strongly affect the extent and
character of the interaction of compact and dispersed
aluminum matrices with an aqueous solution contain-
ing metal ions. Of particular interest is contact deposi-
tion of iron group metals on a dispersed aluminum ma-
trix. This process may lead to the development of pow-
dered systems with variable composition and proper-
ties, showing promise as precursors of compact materi-
als and coatings.
The development of new generations of structural
and functional materials is a topical problem. A possi-
ble way to solve this problem is variation of the com-
position of a system used a precursor.
It is well known that evolution of any physico-
chemical systems occurring under nonequilibrium con-
ditions can lead to formation of fractal structures and
nanocomposites [1]. For example, dendritic structures
of frequently formed, e.g., in the course of nonequilib-
rium electrolytic deposition of metals and solidifica-
tion of alloys under the conditions of strong supercool-
ing, or in the course of phase decomposition. These
methods can be considered as formation of nanostruc-
tures in the bulk of a matrix under the conditions of its
chemical modification. This process allows the pa-
rameters of the particles in the matrix to be controlled
in the step of their formation and varied in the course
of material operation.
Alloys and composites based on iron and aluminum
occupy a particular place among intermetallic systems
promising from both theoretical and applied stand-
point. They exhibit a unique set of physicochemical,
mechanical, magnetic, and corrosion properties. How-
ever, iron aluminides are not still demanded by the
industry because of the lack of a relatively simple and
cheap procedure for their preparation. Today Fe–Al
systems are prepared by mechanochemical methods
which are not always efficient and are power-con-
suming.
Much researchers’ attention is attracted today by
synthesis of metallic composites with elemental alumi-
num or its compounds as a matrix [2–6]. Owing to
unique combination of the protective properties of sur-
face oxide–hydroxide layers and intrinsic chemical
activity, aluminum under definite conditions chemi-
cally reacts with the environment to form products ex-
hibiting a set of specific properties [7, 8]. According to
the results of numerous studies, the chemical activity
of finely dispersed aluminum powders toward water
and aqueous solutions of chemical compounds consid-
erably exceeds the chemical activity of the compact
metal [9, 10]. Many authors believe [7, 9, 10] that this
is due to the developed surface of the powders and to
It was proposed previously to prepare alloys and
composites based on the Fe–Al binary system from
aqueous solutions by contact exchange of metals on
aluminum microparticles [11]. This process occurs in
solution under normal conditions and is accompanied
by the release of a large amount of heat, mainly due to
ionization of aluminum. The deposition of Fe(0) from
such solutions occurs in two steps: Fe(III) → Fe(II) →
Fe(0), with simultaneous ionization of the aluminum
1879

1880
DRESVYANNIKOV et al.
base and formation of an α-Fe layer on its surface. As
a result, dendritic spherical particles with large internal
cavities and pores are formed, which determines the
low bulk density of the powder [11]. In the process,
threadlike nuclei of submicrometer cross section can
be formed in particle pores.
10 keV (0–500), acquisition time Т = 100 s, Еprobe
30 keV. The size of the microanalysis area was varied
from 20 × 30 to 50 × 75 μm.
=
Electrolytic codeposition of two or several metals is
of particular interest because of the mutual influence of
the jointly reduced ions on the rate of electrochemical
processes [14]. It is known [14–17] that, in joint reduc-
tion of Fe(II) and Ni(II) ions, the electrochemical reac-
tion is conjugate, i.e., the rate of discharge of their ions
differs from the rate at which these ions are reduced
separately. It was shown in a number of papers [14,
17] that the rate of reduction of Fe(II) ions increases by
a factor of almost 3 in joint electrodeposition with
Ni(II). On the contrary, the rate of reduction of Ni(II)
in joint deposition with Fe(II) decreases by a factor of
approximately 8. In the course of nickel electrodeposi-
tion, hydrogen is released at a considerably higher rate
than in joint deposition of iron and nickel, because of
the lower overvoltage of hydrogen on nickel.
EXPERIMENTAL
Experiments were performed with close-cut frac-
tions of dispersed aluminum (85±15 μm, purity no less
than 99.0%), with a specific surface area of 260 cm2 g–1.
The chemicals we used were FeCl3·6H2O, NiCl2·6H2O,
and CoCl2 (analytically pure grade). The molar ratio
of the starting aluminum powder and precursor was
1.5 : 1 in all the experiments.
To study the process kinetics, we took samples at
regular intervals of time and examined them by X-ray
fluorescence analysis on a VRA-20L installation (Carl
Zeiss) under the following conditions: LiF 200 ana-
lyzer crystal, voltage 55 kV, current 35 mA, X-ray
tube with a tungsten anode, collimator 2, SZ counter,
discriminator 1.0 V, and sensitivity 3 × 104–1 ×
105 counts s–1 cm–2. The degree of precursor reduction
to metal was estimated from variation of the concentra-
tion of ions of the corresponding elements in solution.
The experimental error was governed by the methods
used and amounted to 5%.
The experiments performed [18] suggest that the
reactions of Fe(III) and Ni(II) reduction when these
ions are present in solution simultaneously are conju-
gate, i.e., the successively occurring steps Fe(III) →
Fe(II) → Fe(0) initiate the reaction Ni(II) → Ni(0).
Both conjugated reduction reactions can be ade-
quately described by first-order rate equations
c(Fe3+) = c0(Fe3+) exp (–kt) ,
c(Ni2+) = c0(Ni2+) exp (–kt) .
The chronopotentiograms in a suspension of dis-
persed aluminum were recorded with an EPV-1 plati-
num electrode and an EVL-1MZ-1 silver chloride ref-
erence electrode.
Previously, in studying joint deposition of Fe and
Ni on an Al surface, we showed [18] that Ni metal is
mainly deposited on the surface of the iron formed,
with a certain part of iron undergoing oxidation and
passing into the solution. In joint deposition of Fe(III)
and Ni(II), a dark gray ferromagnetic powder is
formed. The reaction is accompanied by considerable
(to 373 K) heat-up of the reaction mixture. In joint
deposition of Fe and Ni on Al, the powder is enriched
in the less noble component [18]. The similar effect
observed in electrodeposition is attributed in [19] to a
shift of the metal deposition potentials when the metals
are present simultaneously and to possible formation
of passive films on the iron surface, preventing the
deposition of Ni. Hydrogen evolution occurs simulta-
neously with the metal codeposition.
The samples we obtained were analyzed with a
DRON-2 X-ray diffractometer to determine their
chemical and phase composition. To reduce the back-
ground from scattering of the primary X-ray beam in
air, we used long-wave FeKα radiation with a β-filter.
The diffraction patterns were recorded in the following
mode: voltage 30 kV, current 15 mA, recording range
2θ 10°–160°. The diffraction patterns were processed
with MAUD 1.999 multifunctional software [12, 13].
As a reference for comparing the line profiles (to de-
termine the size of coherent scattering regions and mi-
crostresses), we used annealed copper foil.
Analysis and recording of microscopic images of
the initial aluminum and powders obtained were per-
formed with a Philips SM XL-30 TMP scanning elec-
tron microscope at the following parameters: ΔE scale
Similar process is observed in the Fe(III)–Co(II)
system. When the cobalt and iron ions are present si-
multaneously, they are reduced somewhat more slowly
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JOINT REDUCTION OF Fе(III), Ni(II), AND Co(II) IONS
α, %
1881
compared to the separate reduction. At equal content
of the Fe(III) and Co(II) ions in the initial solution, the
deposition of both metals is virtually simultaneous
(electrolytic alloy formation), and high degrees of iron
and cobalt deposition are attained (Fig. 1). Apparently,
electrolytic alloy formation may take place in the
course of contact exchange when the metals are depos-
ited simultaneously. Karbasov et al. [20, 21] reported
on the alloy formation in the course of contact ex-
change and proved the existence of various kinds of
alloys by X-ray diffraction analysis and electron mi-
croscopy.
Both conjugate reactions in joint reduction of iron
and cobalt can be adequately described by first-order
rate equations
t, s
Fig. 1. Kinetic curves of joint deposition of (1) Fe and
(2) Co from a 0.5 M FeCl3 + 0.5 M CoCl2 solution. Points
refer to experimental data, and lines, to calculation; the
same for Fig. 2. (α) Degree of deposition and (t) deposition
time; the same for Fig. 2.
c(Fe3+) = c0(Fe3+) exp (–kt) ,
c(Co2+) = c0(Co2+) exp (–kt) .
According to reference data [22], joint reduction of
Fe(III), Co(II), and Ni(II) with Al to the metals is ther-
modynamically possible, because the redox potential
of the Al(III)/Al(0) couple is more negative than those
of the corresponding couples involving Fe, Co, and Ni.
However, the process is stongly complicated by the
presence of the natural oxide film on Al and high over-
voltage of the reduction of iron ions.
α, %
Joint Fe–Ni–Co deposition is a still more complex
process and is determined both by the state of the sur-
face of dispersed Al and by the concentration of the
ions of these metals in solution. Iron and cobalt are
deposited at approximately equal rate, and nickel, at a
somewhat lower rate (Fig. 2). In ternary solutions, iron
and cobalt are deposited virtually synchronously (Fig. 2,
curves 1 and 3), but cobalt is deposited more com-
pletely (98%, against 90% for Fe). In the case of Ni,
the deposition rate increases after the lapse of 90 s, at
the time by which iron and cobalt have been 50% re-
duced. This result is consistent with the previous data
on the joint deposition of iron and nickel [18].
t, s
Fig. 2. Kinetic curves of joint deposition of (1) Fe, (2) Ni,
and (3) Co from a 0.7 M FeCl3 + 0.6 M CoCl2 + 0.7 M
NiCl2 solution.
adsorption of hydroxy compounds on the alloy surface,
compared to deposition of the individual metals. The
observed deceleration of the reduction of Ni ions indi-
cates that the discharge of the ions is not facilitated by
the alloy formation. Deceleration of the ion discharge
in the course of the joint reduction can be largely at-
tributed to a change in the concentration of the ions
being discharged in the electrical double layer.
The results of studying joint deposition of metallic
iron, nickel, and cobalt showed that the reactions of the
Fe(III), Ni(II), and Co(II) reduction are conjugate, i.e.,
the consecutive reactions with iron ions initiate the
reactions with the cobalt [Co(II) → Co(0)] and nickel
[Ni(II) → Ni(0)] ions. The conjugate reduction reac-
tions can be described by first-order rate equations
(Fig. 2).
The reduction process can be illustrated by the
chronopotentiograms obtained for dispersed aluminum
suspended in the working solution (Fig. 3). Compari-
son of the chronopotentiogram (Fig. 3) obtained with a
platinum electrode in the reaction mixture containing
Al with the kinetic curves (Fig. 2) shows that, in the
Deceleration of the reduction of Ni ions in joint
deposition with Fe and Co may be due to increased
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1882
E, mV
DRESVYANNIKOV et al.
that the deposit fully blocks the aluminum surface. Ac-
cording to [20, 21], plateaus in the chronopoten-
tiograms correspond to the formation of solid solutions
or intermetallic compounds in the surface layer.
According to the results of X-ray phase analysis
(Fig. 4) and local microanalysis, the powders obtained
are mechanical mixtures of crystallites of iron group
metals and aluminum, like the Fe–Al samples obtained
previously [11, 24]. The amount of residual Al can be
readily controlled by treating the powder with an alkali
solution and can be reduced to tenth fractions of per-
cent. When the powder is not kept in an alkali, an
amorphous phase (probably pseudoboehmite) is de-
tected, and with time bayerite crystals formed from
metastable products of aluminum dissolution also ap-
pear [25]. In accordance with the results of X-ray
structural and phase analysis and of local microanaly-
sis, it can be concluded that the systems under consid-
eration can be classed with substitution solid solutions
with atoms of the major components (iron group ele-
ments) partially substituted in the crystal lattices by
aluminum atoms. Furthermore, intermetallic phases of
iron group elements are also detected. The process ki-
netics is strongly affected by the size and state of the
surface of the aluminum particles.
t, s
Fig. 3. Redox potential E as a function of time t in deposi-
tion of iron group metals (0.7 M FeCl3 + 0.6 M CoCl2 +
0.7 M NiCl2).
Electron microscopic examination of particles taken
from the solution after the metal deposition shows that
they are virtually always spherical with the size similar
to that of the starting Al particles (Fig. 5). This fact
allows control of the properties of the synthesized par-
ticles by varying the parameters of the initial Al ma-
trix. The presence of a large number of nuclei on the
surface of coarse particles, in combination with the
systems of cracks and pores, promotes the develop-
ment of the metal surface, which may be very impor-
tant in the possible use of such powders as heterogene-
ous catalysts. Furthermore, an important feature of the
powders obtained is the presence of internal cavities.
2θ, deg
Fig. 4. Diffraction patterns of dispersed (1) Fe–Al, (2) Fe–
Ni–Al, and (3) Fe–Ni–Co–Al samples. (2θ) Bragg angle.
course of the first 90 s, the potential smoothly de-
creases and the degree of metal deposition grows, also
smoothly. The length of this portion of the curves
strongly depends on the concentration of the starting
reactants and is primarily caused by the presence of an
oxide film on the aluminum particle surface. Further-
more, the potential does not decrease significantly until
the nickel deposition starts. After the drop toward the
negative region, the potential fairly actively shifts to-
ward more positive values and then gets stabilized. It is
known [23] that a metal being reduced can fully plug
the pores of the powder, which leads to termination of
the contact exchange. In the process, the electrode po-
tential becomes close to the equilibrium potential of
the more electropositive components (Fe, Ni, Co) in
the solution. The chronopotentiogram (Fig. 3) indicates
It should be noted that, virtually in all the experi-
ments, we clearly observed the formation of nanostruc-
tures in the form of whiskers (Fig. 5). However, on the
surface of particles obtained in the Fe–Co system,
there are also conical flaky individuals with a mean
size of 100–200 nm (Fig. 5). Such a structure of the
surface, in combination with the phase composition of
the objects, is responsible for the specific properties of
the Al–M systems (M = Fe, Ni, Co) obtained from
aqueous solutions.
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JOINT REDUCTION OF Fе(III), Ni(II), AND Co(II) IONS
1883
(a)
(b)
1 μm
0.5 μm
(c)
(d)
1 μm
1 μm
(e)
30 μm
Fig. 5. Electron micrographs of disperse systems (a) Fe–Co, (b) Fe–Co–Al, (c) Fe–Co–Ni, (d) Fe–Co–Ni–Al, and (e) Al.
(2) The reactions of Fe(III), Ni(II), and Co(II) re-
duction are conjugate and are adequately described by
first-order rate equations with close rates of iron and
cobalt deposition and lower rate of nickel deposition.
CONCLUSIONS
(1) Kinetic features of the formation of Fe–Co and
Fe–Co–Ni deposits on dispersed aluminum matrix
were elucidated.
(3) It was found by X-ray phase analysis that the
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DRESVYANNIKOV et al.
13. The Rietveld Method, Young, R.A., Ed., New York: Int.
precipitates obtained contain phases of aluminum and
of individual iron group metals.
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14. Elektroliticheskoe osazhdenie splavov (Electrolytic
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15. Elektroliticheskoe osazhdenie zheleza (Electrolytic
Deposition of Iron), Zaidman, G.N., Ed., Chisinau: Shti-
intsa, 1990.
(4) Electron microscopic examination revealed
nanosized whiskers and conical formations on the de-
posit surface.
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