Pseudo-topochemical synthesis of α-Fe in aqueous solution using an aluminum matrix

Article abstract of DOI:10.1007/s11176-005-0190-5

Processes occurring in synthesis of α-Fe from a Fe(III) solution using an aluminum matrix were studied. The reaction kinetics strongly depends on the state of the matrix. 2005 Pleiades Publishing, Inc.

Full text of DOI:10.1007/s11176-005-0190-5

Russian Journal of General Chemistry, Vol. 75, No. 2, 2005, pp. 155 161. Translated from Zhurnal Obshchei Khimii, Vol. 75, No. 2, 2005,
pp. 177 183.
Original Russian Text Copyright
2005 by Dresvyannikov, Kolpakov.
Pseudo-Topochemical Synthesis of -Fe in Aqueous Solution
Using an Aluminum Matrix
A. F. Dresvyannikov and M. E. Kolpakov
Kazan State University of Technology, Kazan, Tatarstan, Russia
Received May 20, 2003
Abstract Processes occurring in synthesis of -Fe from a Fe(III) solution using an aluminum matrix were
studied. The reaction kinetics strongly depends on the state of the matrix.
Dispersed and ultradispersed iron metal and its
alloys are widely used in various branches of science
and engineering thanks to their unique magnetic,
mechanical, catalytic, and other properties. The prop-
erties of these materials largely depend on the proce-
dure of their preparation. It should be noted that the
known processes for reduction of Fe(III) from aqueous
solutions are very few and often do not ensure high
yield of iron metal under normal conditions. Also, it
is known that the history of iron metal preparation
appreciably affects its physical and chemical proper-
ties, which should be taken into account in preparation
of materials with preset characteristics.
reaction of compact and dispersed (with different
particle size) aluminum with an aqueous solution of
Fe(III) chloride and to reveal characteristic kinetic
features of separate steps of the process.
We found previously that Fe(III) in aqueous solu-
tion is reduced on a dispersed aluminum matrix in two
steps: Fe(III)
Fe(II)
Fe(0) [10]. Since the reduc-
tion of Fe(III) yields two products, Fe(II) and Fe(0),
a question arises: What is the sequence of transforma-
tions in the system? The tentative scheme of redox
transformations of Fe(III) in its reaction with alumi-
num is as follows.
Fe(III)
Fe(II)
Metal formulations with an aluminum matrix at-
tract much researchers’ attention [1 5]. Thanks to a
unique combination of protective properties of surface
oxide hydroxide layers and intrinsic chemical activ-
ity, aluminum under definite conditions chemically
reacts with the surrounding medium to form com-
pounds exhibiting a series of specific properties [6, 7].
According to numerous papers, finely dispersed alu-
minum powder considerably surpasses compact alu-
minum in the chemical activity toward water and
aqueous solutions of various compounds [8, 9]. It is
believed [6, 8, 9] that this is due to the developed
surface of the powders, to strongly defective state of
the surface oxide hydroxide films on fine particles,
and to strong heat-up of the solid phase in the course
of oxidation [9]. These factors can appreciably affect
the course and extent of the reaction of finely dis-
persed and compact aluminum with an aqueous solu-
tion containing metal ions. It should be noted, how-
ever, that we found no detailed information on the
reduction of Fe(III) ions with aluminum in solution.
In this connection, it seems important to study how
the aluminum matrix affects the redox process involv-
ing reduction of Fe(III).
Fe(0)
The reactions in such solutions are complicated by
complexation of the oxidized and reduced iron species
and their blocking with ligands and solvent molecules.
The results of our preliminary experiments, in combi-
nation with published data on redox reactions involv-
ing iron ions [11, 12], suggest the occurrence of the
following steps (at pH 0.5):
Fe(H2O)36+ + e
Fe(H2O)26+ + 2e
Fe0 + 6H2O
Fe(H2O)26+
Fe(H2O)36+ + 3e
Al0 + aq
Fe(H2O)26+,
Fe0 + 6H2O,
Fe(H2O)26+ + 2e,
Fe(H2O)36+ + e,
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
Fe0 + 6H2O,
3+
Al
+ 3e,
aq
1/2H2 + H2O
H3O+ + e
e
H3O+,
1/2H2 + H2O,
O2 + 4H3O+,
6H2O.
6H2O
4e
4H3O+ + O2 + 4e
Our goal was to elucidate the mechanism of the
Since the equilibrium potential of the reduced iron
1070-3632/05/7502-0155 2005 Pleiades Publishing, Inc.

156
DRESVYANNIKOV, KOLPAKOV
is more negative than the potential of the equilibrium
or
[Fe(II)] = [Fe(III)]0
hydrogen electrode at this pH, the metal can dissolve
2+
by the reaction Fe0 + 2H3O+ = Fe + H2 + 2H2O
1
exp[ (k1 + k2)t]
aq
[12]. The presence of dissolved oxygen in the electro-
lyte makes probable the occurrence of reaction (10),
which, when conjugated with reaction (3), will cause
appreciable dissolution of the deposited iron. Along
with reaction (2), several other reactions (e.g., redox
processes of liberation ionization of hydrogen or
reduction liberation of oxygen) are also possible on
contact of the deposited iron with a solution of iron
salts. However, in this case the rates of reactions (7)
(10) will be very low compared to the rates of reac-
tions (2) and (3).
k2
k1 + k2
{1 exp[ (k1 + k2)t]}
k3
k3 k1
k1
1
exp( k1t) +
exp( k3t)
,
(13)
k3 k1
where [Fe(III)]0 is the initial Fe(III) concentration, M;
[Fe(III)], [Fe(II)], and [Fe(0)] are the running concen-
trations of Fe(III), Fe(II), and Fe(0), respectively, M;
and t is the time, s.
Since Fe(0) occurs in the nonionic state, [Fe(0)]
can be replaced by m[Fe(0)]:
To elucidate the mechanism of Fe(III) reduction,
we used the method of relative selectivity [13, 14].
The relative selectivity is the ratio of the selectivities
S with respect to reaction products, or the ratio of the
degrees of conversion of the starting reactant into the
products [13]. To determine the relative contributions
of the parallel and consecutive pathways of formation
of reaction products, it is sufficient to determine the
relative selectivity at a short reaction time.
k2
m[Fe(0)]
m0[Fe(III)]
=
=
{1 exp[ (k1 + k2)t]}
k1 + k2
k3
k1
+ 1
exp( k1t) +
exp( k3t)
.
(14)
k3 k1
k3 k1
where
is the yield of Fe(0); m[Fe(0)], weight of
We showed previously [15] that, with aluminum
foil as reductant, the relative selectivity of the process
0, which suggests concurrent formation of Fe(II)
and Fe(0) from Fe(III). A similar calculation for the
case of reduction of Fe(III) with dispersed aluminum
showed that tends to zero (evidence for the con-
secutive reaction).
Fe(0), g; m0[Fe(III)], weight of the starting Fe(III), g.
Two limiting cases can be considered.
(1) k3
0. In this case, from Eqs. (13) and (14)
we obtain the following expressions:
k1
[Fe(II)] = [Fe(III)]0
{1 exp[ (k1 + k2)t]},(15)
k1 + k2
These results allow us to suggest a general scheme
of the reaction of Fe(III) ions with aluminum, both
compact and dispersed with various particle size:
k2
k1 + k2
m[Fe(0)]
(16)
{1 exp[ (k1 + k2)t]}.
=
=
m0[Fe(III)]
These equations correspond to formation of Fe(II)
and Fe(0), i.e., Fe(0) is formed concurrently with
Fe(II) [Eq. (5)], with the contribution of the consecu-
tive pathway being negligible. This is the case at a
small specific surface area of the aluminum matrix,
k1
Fe(III)
Fe(II)
k3
k2
Fe(0)
1
characteristic of the compact metal (Ssp < 100 cm2 g ,
where k1, k2, and k3 are the rate constants of the cor-
responding reactions, s .
1
Fig. 1a).
(2) k2
0. For this case, from Eqs. (11), (13),
The overall process can be described by a system
of kinetic equations (11) (13):
and (14) we obtain Eqs. (17) (19), respectively.
[Fe(III)] = [Fe(III)]0exp[ (k1t)],
(17)
[Fe(III)] = [Fe(III)]0exp[ (k1 + k2)t],
(11)
k1
k1 + k2
k2
k1 + k2
[Fe(II)] = [Fe(III)]0
[exp( k1t) exp( k3t)],
(18)
[Fe(0)] = [Fe(III)]0
{1 exp[ k1 + k2)t]}
k3
k3 k1
m[Fe(0)]
k1
k3 k1
=
= 1
exp( k1t) +
exp( k3t).
k3
k1
m0[Fe(III)]
+
exp( k1t) +
exp( k3t)
,
(12)
(19)
k3 k1
k3 k1
At a sufficiently large specific surface area (Ssp
>
[Fe(II)] =[Fe(III)]0 [Fe(III)] [Fe(0)]
1
200 cm2 g ), Fe(0) is formed by the consecutive
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 75 No. 2 2005

PSEUDO-TOPOCHEMICAL SYNTHESIS OF -Fe IN AQUEOUS SOLUTION
157
pathway [Eq. (2)], with the contribution of the parallel
pathway being negligible. This case is illustrated
graphically in Fig. 1b.
(a)
C, M
1.0
0.8
0.6
1.0
In a certain transition region (tentative range
0.8
0.6
2
1
100 < Ssp < 200 cm3 g ), k2
k3, i.e., Fe(0) is
formed by a mixed parallel-consecutive mechanism
with a comparable contribution of the two alternative
pathways (Fig. 1c).
3
0.4
0.2
0.0
0.4
0.2
0.0
In all these cases, the results of calculations with
the suggested model reasonably agree with the experi-
mental data (Figs. 1a 1c). The kinetic curves of Fe(0)
1
deposition,
, have a shape typical of topochemical
0
2
4
6
8
10 12
reactions with a pronounced induction period and
steps of reaction development and deceleration.
The reaction is accompanied by a fairly strong heat
evolution. Lyashko et al. [9] considered the heat-up
of aluminum particles and their aggregates upon their
reaction with water. According to their estimates,
under the adiabatic conditions the heat release from
the reaction of two or three Al monolayers is suffi-
cient to heat a particle 0.1 m in diameter to the melt-
ing point (according to the experimental data, the
reaction rate is 0.5 1.5 monolayers per second). With
the heat exchange taken into account, the estimated
heat-up is of the order of hundreds degrees. This con-
siderable increase in the particle temperature causes
apparent rate constant of the reaction to increase,
which results in progressing heat-up. The reaction
becomes controlled by external diffusion, and the
temperature of the particles being oxidized reaches a
maximum. A characteristic evidence for this transition
is a portion in the kinetic curve, in which the reaction
2
10 , s
(b)
C, M
1.0
0.8
0.6
1.0
3
0.8
0.6
0.4
0.2
0.0
0.4
0.2
0.0
2
1
0
2
4
6
8
10 12
2
10 , s
(c)
C, M
1.0
0.8
0.6
1.0
0.8
0.6
rate is maximal and constant in a wide range of
.
3
The processes under consideration can be described
taking into account differences in the structure and
physicochemical properties of the surface oxide layer
in compact and dispersed aluminum [16, 17]. Experi-
ments performed in various media show that alumi-
num occurs in the passive state even in aggressive
aqueous solutions [16, 18]. The oxide film on alumi-
num microparticles, compared to that on the compact
metal, is nonuniform and rough, with a fine relief in
the form of folds, pits, and prominences; it is an
amorphous matrix of aluminum oxide with inclusions
of very fine -Al2O3 crystallites; traces of crystalline
mono- and trihydrates of aluminum oxide were re-
vealed [17]. The surface layers also contain ultrami-
croamounts of a foreign phase (e.g., Si, Fe, Zn, K,
Mg, etc.).
0.4
0.2
0.0
0.4
0.2
0.0
2
1
0
2
4
6
8
10 12
2
10 , s
Fig. 1. Kinetic curves of reduction of Fe(III) ions in
aqueous solution: (1) Fe(III), (2) Fe(II), and (3) Fe(0).
Data for (a) A-999 Al foil, (b) Al particles of size
85 15 m, and (c) Al particles of size 175 55 m;
298 K, 1.0 M FeCl , pH 0.5; points are experimental
3
data, and lines are calculation results.
It is also known that the crystalline oxide forming
in dry air or after high-temperature heating of Al
powders extends the metal lattice at room temperature
and contracts it on heating [19]. The arising tangential
stresses, which are more are less uniformly distributed
over the particle volume, are due to differences in the
structure and thermal expansion of aluminum metal
and its oxide [20]. As a result, the microstructure of
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 75 No. 2 2005

158
DRESVYANNIKOV, KOLPAKOV
Characteristics of the crystal lattice of dispersed Al and
reduced Fe, determined by X-ray diffraction analysis
layer [23]. Oxidation of aluminum is accompanied by
reduction of proton donors (hydrogen depolarization)
and of the oxidant (iron ions). The latter process, as
shown above, is intricate (it occurs in one or two
steps depending on the extent of dispersion of the
aluminum matrix) and yields a Fe(0) precipitate as
the final product. After appearance of a layer of iron
metal, hydrogen liberation becomes more intense.
However, the rate of this process is appreciably (by
approximately three orders of magnitude) lower than
the rate of the deposition of iron metal [15].
Miller indices
Interplanar Crystallite
Micro-
stress
spacing,
size,
h
k
l
Dispersed Al
2.3375
2.0244
1.4314
1.2207
1
2
2
3
2
4
1
0
2
1
2
0
1
0
0
1
2
0
1092
2.0863
5.9230
1.7823
1.3656
2.0863
5.9230
1616
1223
1369
1092
1616
The particle-size distribution of iron powders ob-
tained using a close-cut fraction of dispersed Al (71
100 m) is shown in Fig. 2. It is seen that the narrow
partricle-size distribution of the powder is preserved.
The Fe(0) deposit is formed on the surface of Al
particles with preservation of the shape and size of the
initial matrix, i.e., dissolution of Al and formation of
spherical nuclei of Fe metal occur simultaneously.
With time, Al is mostly dissolved, and the forming Fe
particle is virtually a polar sphere with a large number
of spherical subparticles on the surface, of pores, and
of thread-like nuclei inside the pores (Fig. 3). This is
also indirectly suggested by the bulk density, which is
1.1688
1.0122
Reduced Fe
2.0268
1.4332
1.1702
1.0134
1
2
2
2
1
0
1
2
0
0
1
0
534
370
534
534
0.0029
0.0042
0.0029
0.0029
the surface of the oxide layer on Al particles is charac-
terized by numerous microcracks whose formation is
caused by high stresses in the oxide shells, due to
considerable differences in the molar volumes of the
metal and oxide and to different densities of the amor-
phous and crystalline oxides [16, 20]. Furthermore,
the oxide shells on particles have numerous structural
defects. It should be noted also that powders whose
particles are coated with a crystalline surface film
are oxidized more intensely than those coated with the
amorphous oxide [21]. These features of compact and
dispersed aluminum determine the specific features of
their reaction with a solution containing Fe(III) ions.
3
abnormally low for iron powders (1.25 g cm ). It
should be noted that the deposition of Fe on the Al
particles is nonuniform, i.e., in any stage of the reac-
tion, particles fully covered with the deposit coexist
with unchanged particles.
Since deposition of Fe may be accompanied by
formation of its basic compounds, it is appropriate to
determine the qualitative and quantitative composition
of the deposits formed on the Al matrix. According
to X-ray phase analysis and local microanalysis, the
samples obtained from dispersed Al (85 15- m frac-
tion) are mechanical mixtures of -Fe and Al crystals
(Fig. 4, pattern 1). After keeping the deposit in 3.0 M
NaOH, only the -Fe phase was detected (Fig. 4, pat-
tern 2). Without alkali treatment, an amorphous phase
(presumably pseudoboehmite) gradually appears,
which is followed by formation of bayerite crystals
formed from metastable products of aluminum dis-
solution.
The Cl ions released in dissolution of FeCl3 are
depassivators displacing or partially substituting, by
the adsorption mechanism, the passivating oxygen in
the surface film to form a surface complex [22] cap-
able of subsequently passing into the solution. Also,
aggressive ions can directly contact the metal surface
as a result of mechanical degradation of the oxide
q,
Q, %
rel. units
80
60
40
20
0
8
6
4
2
0
From the X-ray diffraction data, using the MAUD
software, we determined the Miller indices of the
initial Al and of the synthesized Fe and calculated
the sizes of the coherent scattering domains and the
microstresses (see table). Also, with the MAUD soft-
ware, we obtained an image of an -Fe crystallite
(Fig. 5). It has the shape of a cube with smoothed
edges; axis (100) is directed toward the reader, (010)
to the right, and (100) upwards (the crystallographic
directions are indicated by thin black lines). The
0.1 0.5 1
5 10 50 100 500
d, m
Fig. 2. Particle-size distribution of -Fe: (Q) integral
(curve) and (q) differential (histogram).
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 75 No. 2 2005

PSEUDO-TOPOCHEMICAL SYNTHESIS OF -Fe IN AQUEOUS SOLUTION
(b)
159
(a)
Al
Al
Al
Al
40 m
40 m
Fig. 3. Electron micrographs of particles of (a) initial Al (85 15- m fraction) and (b) -Fe.
Al
Fe
1
2
50.0
100.0
150.0
2 , deg
Fig. 4. Diffraction patterns of powdered iron-containing deposits obtained on a dispersed Al matrix (85 15- m fraction):
(1) immediately after Fe deposition and (2) after leaching of the residual Al. (2 ) Bragg angle.
observed deviation of the -Fe crystallite shape from
the ideal cube is due to microstresses caused by spe-
cific features of formation and growth of microcrys-
tals of the deposit on the Al matrix.
Thus, in the reaction of a relatively concentrated
( 1.0 M) solution of iron(III) chloride with Al, Fe(III)
is reduced to -Fe. The reaction occurs by different
mechanisms depending on the dispersity of the Al
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 75 No. 2 2005

160
DRESVYANNIKOV, KOLPAKOV
in [24]}. The relative error of the indirect potentio-
metric determinations did not exceed 1%. The total Fe
content in the solution was additionally monitored by
X-ray fluorescence analysis (VRA-20L installation,
Carl Zeiss). Also, samples of the initial Al and the
reaction products were analyzed by X-ray phase anal-
ysis (DRON-3M diffractometer, CoK radiation,
1.79021 ) and electron microscopy (REMMA-
202M installation). The diffraction patterns were
identified using the JCPDS file [25]. Prior to analysis,
some deposit samples were kept in aqueous alkali to
remove excess Al, washed with double-distilled water
to neutral reaction and then with ethanol, and vacuum-
dried at 60 C.
Also, the diffraction patterns were processed with
the MAUD software [26].
The particle-size distribution of Al and Fe was
determined with an Analysette C-22 laser analyzer.
The measurements were performed using monochro-
Fig. 5. Image of an -Fe crystallite, obtained by re-
construction from X-ray diffraction data using the
MAUD software.
matic radiation with
638 nm.
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