Mechanism of the reaction between the aqua complexes of palladium(II) and iron(II) in a solution of HClO4

Article abstract of DOI:10.1023/B:KICA.0000032172.04936.f4

The kinetics of the reaction of an aqua complex of iron(II) with a tetraaqua complex of palladium(II) in a perchloric acid solution was studied over the temperature range 2055°C, and a reaction mechanism was proposed. It was found that the reaction is autocatalytic, and it occurs by a two-step one-electron transfer mechanism with the formation of iron(III) and palladium black. Based on the kinetic data, the nature of the autocatalysis in the test reaction was hypothesized.

Full text of DOI:10.1023/B:KICA.0000032172.04936.f4

Kinetics and Catalysis, Vol. 45, No. 3, 2004, pp. 381–384. Translated from Kinetika i Kataliz, Vol. 45, No. 3, 2004, pp. 407–410.
Original Russian Text Copyright © 2004 by V. V. Potekhin, Matsura, Solov’eva, V. M. Potekhin.
Mechanism of the Reaction between the Aqua Complexes
of Palladium(II) and Iron(II) in a Solution of HClO4
V. V. Potekhin, V. A. Matsura, S. N. Solov’eva, and V. M. Potekhin
St. Petersburg State Technological Institute (Technical University), St. Petersburg, 198013 Russia
Received June 20, 2003
Abstract—The kinetics of the reaction of an aqua complex of iron(II) with a tetraaqua complex of palla-
dium(II) in a perchloric acid solution was studied over the temperature range 20–55°C, and a reaction mecha-
nism was proposed. It was found that the reaction is autocatalytic, and it occurs by a two-step one-electron
transfer mechanism with the formation of iron(III) and palladium black. Based on the kinetic data, the nature
of the autocatalysis in the test reaction was hypothesized.
INTRODUCTION
EXPERIMENTAL
The reaction between a tetraaqua complex of palla-
dium(II) and an aqua complex of iron(II) was per-
formed in an aqueous HëlO4 solution with intense stir-
ring in an argon atmosphere in a thermostatted reactor
equipped with a sampler.
Electron transfer in redox reactions between d-metal
complexes has been considered in a great number of
publications [1–4].
Previously [5], we studied the oxidation of alcohols
by iron(III) in the presence of a tetraaqua complex of
palladium(II):
The [Pd(H2O)4](ClO4)2 complex was prepared in
accordance with a published procedure [9]. Mohr’s salt
((NH4)2SO4 · FeSO4 · 6H2O) served as a source of
iron(II). Iron(III) was introduced into the reaction solu-
tion as an Fe2(SO4)3 · 9H2O salt solution in perchloric
acid.
R1R2CHOH + 2Fe3+
aq
(I)
2+
R1R2CHO + 2Fe2+ + 2H+.
Pd
aq
aq
We found that the rate of reaction (I) is independent
of the initial concentration of Fe(III). At a degree of
conversion of iron(III) approximately equal to 30%,
palladium black rapidly precipitated from a homoge-
neous solution; this led to an increase in the concentra-
tion of Fe(III) and to the rupture of a catalytic cycle of
alcohol oxidation (Fig. 1).
3
3
3+
2+
CFe × 10 , mol/l
60
CPd × 10 , mol/l
6
55
50
45
5
4
The shape of kinetic curves plotted as changes in the
concentrations of Fe(III) and Pd(II) with time allowed
us to assume that the following reaction rapidly occurs
between the aqua ions of iron(II), which are accumu-
lated in the course of reaction (I), and a tetraaqua com-
plex of palladium(II):
1
3
2
1
0
25
20
2[Fe(H2O) ]2+ + [Pd(H2O) ]2+
2'
1'
6
4
2
(II)
2[Fe(H2O) ]3+ + Pd0.
6
0
50
100 150 200 250 300 350
The use of the Pd(II)–Fe(III) catalytic system in the
oxidation of organic and inorganic substances was
reported [6–8]; however, we failed to find published
data on the kinetics and mechanism of reaction (II).
This work was devoted to studying the mechanism of
Fe(II) oxidation by a tetraaqua complex of palla-
dium(II).
Time, min
3+
2+
Fig. 1. Kinetic curves of (1, 2) Fe and (1', 2') Pd concen-
tration changes in reaction (I): [Fe
3+
] = (1, 1') 0.06 or
0
–3
2+
(2, 2') 0.03 mol/l; [Pd
4 mol/l; [HClO ] = 0.7 mol/l; T = 55°C.
] = 5 × 10 mol/l; [MeOH] =
0
4
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382
POTEKHIN et al.
3+
CFe3+ × 103, mol/l
(CFe3+–[Fe ] ) × 103, mol/l
aq 0
12
12
1
2
3
10
1
2
3
4
10
8
8
6
4
2
6
4
4
0
50
100
350
400
Time, min
2
0
50
100
150
Time, min
3+
Fig. 3. Effect of the initial concentration of Fe on the
2+
shape of the kinetic curves of Fe formation according to
the potentiometric data. [Fe ] , mol/l: (1) 0, (2) 0.004,
3+
Fig. 2. Kinetic curves of Fe formation in reaction (II) at
various initial Fe concentrations, mol/l: (1) 0.03, (2) 0.02,
(3) 0.01, and (4) 0.005. [Pd ] = 5 × 10 mol/l; [HClO ] =
0.7 mol/l; T = 36°C.
3+
2+
0
2+
–2
2+
(3) 0.008, and (4) 0.025. [Pd ] = 5 × 10 mol/l; [Fe ] =
2+
–3
0
0
0
4
–2
2.5 × 10 mol/l; [HClO ] = 0.7 mol/l; T = 36°C.
4
kinetic curves suggest the autocatalytic character of the
reaction. After an induction period, the rate of reaction
remained constant up to the complete conversion of
Pd(II) into palladium black. The duration of the induc-
tion period decreased with increasing initial concentra-
2+
The concentrations of the starting complexes and
reagents were varied over the following ranges: from
0.001 to 0.005 mol/l for Pd(II), from 0.01 to 0.04 mol/l
for Fe(II), from 0 to 0.04 mol/l for Fe(III), and from 0.3
to 3.6 mol/l for HëlO4. The concentration of Pd(II) was
determined by spectrophotometry after the addition of
an excess of an SnCl2 solution to the test sample [10].
Iron(III) was determined by spectrophotometry as a
complex with sulfosalicylic acid [11].
tion of palladium(II) ([Pd ]0), whereas the steady-
aq
state rate of Fe3+ formation in the developed step of oxi-
dation increased.
Freshly precipitated palladium black was prepared
by the reduction of [Pd(H2O)4](ClO4)2 with potassium
borohydride.
As can be seen in Figs. 3 and 4, the duration of the
induction period was directly proportional to the con-
centration of iron(III) added at the beginning of the
reaction. At the same time, a change in the initial con-
3+
Because palladium(0) losses were unavoidable on
sampling in the course of the reaction and the concen-
tration of palladium(0) in solution could affect the rate
of reaction, simultaneous kinetic measurements by
potentiometry were performed in some cases without
sampling the reaction solution. The potentiometric
experiments were performed in a thermostatted reactor
with a platinum electrode. A hydrogen electrode with
an aqueous HëlO4 solution was used as a reference
electrode. The concentrations of Fe(II) and Fe(III) were
calculated from the Nernst equation.
centration of iron(III) ([Fe ]0) had an insignificant
aq
effect on the steady-state rate of the catalytic step of
Fe(II) oxidation.
An increase in the duration of the induction period
with the initial concentration of Fe3+ suggests that in the
course of the reaction electron transfer takes place at
the reversible step of the one-electron reduction of pal-
ladium(II) by iron(II):
k1
2+
aq
2+
aq
Pd + Fe
Pda+q + Fe3+.
(III)
As judged from the constants of hydrolysis [12],
Pd(II), Fe(II), and Fe(III) occur as the corresponding
aqua complexes in solutions over the perchloric acid
concentration range tested (0.3–3.0 mol/l).
aq
k–1
The resulting Pda+q ion oxidizes the second iron(II)
aqua ion:
Pda+q + Fe2+
Pd0 + Fea3q+.
(IV)
k2
RESULTS AND DISCUSSION
aq
Figure 2 demonstrates the kinetic curves of Fe(III)
buildup in the course of oxidation of the iron(II) aqua
However, reactions (III) and (IV) cannot explain the
complex by the tetraaqua complex of palladium(II) at autocatalytic character of the oxidation of iron(II) by
various initial concentrations of Fe(II). The S-shaped the tetraaqua complex of palladium(II), and the occur-
KINETICS AND CATALYSIS Vol. 45 No. 3 2004

MECHANISM OF THE REACTION BETWEEN THE AQUA COMPLEXES
383
rence of palladium(I) as a mononuclear complex is
unlikely [13].
τ, min
The role of Pd(0) in redox processes with the partic-
ipation of Pd(II) is well known. For example, the auto-
catalytic character of hydrogen [14, 15] and carbon
monoxide [16] oxidation by palladium(II) complexes
with anionic ligands are related to the above species.
It is well known that the colloidal particles of noble
metals, such as gold, in contrast to bulk samples,
exhibit enhanced reducing properties in aqueous solu-
tions [17]. Therefore, we assumed that catalytically
active colloidal particles of Pd(0) are formed in the
course of reaction (IV). They react with the tetraaqua
complex of palladium(II) to form a palladium(I) com-
pound, the [Pd2]2+ cluster:
300
250
200
150
100
50
Pd0 + Pd2+
[Pd2]2+.
(V)
aq
aq
0
5
10
15
20
25
[Fe ] × 103, mol/l
To test this hypothesis, we performed experiments
with the introduction of freshly prepared palladium
black at the beginning of the reaction. In this case, the
rate of the test reaction remained unchanged. Consis-
tent kinetic data obtained using a sampling method and
potentiometry are indicative of the absence of catalytic
activity from palladium black formed in the course of
the test reaction and of the homogeneity of the reaction
phase.
3+
aq 0
Fig. 4. Dependence of the length (τ) of an induction period on
the initial concentration of Fe at 36°C. [Pd ] = × 10 mol/l;
[Fe ] = 2.5 × 10 mol/l; [HClO ] = 0.7 mol/l.
3+
2+
–3
0
2+
–2
0
4
of the formation of catalytically active species, reac-
tions (VI)–(XI) are the steps of a chain reaction, and
reaction (XII) is the formation of palladium black with
By the end of the induction period, a quasi-steady-
stage concentration of [Pd2]2+ ions was attained. It is the regeneration of the catalyst and the cluster species
aq
in a catalytic cycle. This reaction scheme explains auto-
catalysis in the test reaction.
likely that these ions were converted into other species
in the course of iron(II) oxidation in a steady-state
region (reactions (VI)–(XII)).
Based on the experimental data, we can propose the
following kinetic model for the test reaction: The rate
of uncatalyzed Fe2+ oxidation by the tetraaqua complex
of Pd2+ via reactions (III) and (IV) is equal to
[Pd2]2+ + Fe2+
[Pd2]+ + Fe3+,
(VI)
(VII)
(VIII)
(IX)
k6
aq
aq
aq
aq
[Pd2]+ + Fe2+
[Pd2]0 + Fe3+,
aq
aq
aq
2
2+
aq
2+
k1k2[Pd ][Fe ]
aq
[Pd2]0 + Pd2+
[Pd3]2+,
-------------------------------------------------
wind = 1/2wFe3+
=
.
(1)
aq
aq
aq
3+
2+
k–1[Fe ] + k2[Fe ]
aq
aq
[Pd3]2+ + Fe2+
[Pd3]+ + Fe3+,
aq
aq
aq
aq
2+
aq
3+
aq
With the constraint k2[Fe ] ꢀ k–1[Fe ], we obtain
[Pd3]+ + Fe2+
[Pd3]0 + Fe3+,
(X)
aq
aq
aq
aq
2+
aq
2+
aq
wFe3+ = 2k1[Pd ][Fe ].
(2)
[Pd3]0 + Pd2+
[Pd4]2+ , etc.,
(XI)
aq
aq
aq
The rate of reaction in the catalytic (steady-state)
process developed, which depends on the rate of reac-
tion (VI), can be represented in the form
[Pdn]2+
[Pd2]2+ + [Pdn – 2
]
Pdb0lack . (XII)
0
aq
k12
aq
aq
In general, the concentration of active Pda0q and
[Pd2]2+ species remained unchanged, as indicated by
wcat = k6[Pd22+][Fe2+].
(3)
aq
aq
On the assumption that the rate of formation of the
[Pdn]2+ species is equal to the rate of its decay
the steady-state rate of the reaction.
In special experiments, we found that the introduc-
tion of iron(III) into the reaction solution after an induc-
tion period did not affect the rate of the reaction; that is,
iron(III) did not react with the catalytically active pal-
ladium species. This allowed us to consider the set of
reactions (III)–(XII) as a homogeneous catalytic chain
process, in which reaction (III) is the rate-limiting step
aq
k1[Pd2+][Fe2+] = k12[Pdn2+],
aq
aq
2+
aq
2+
k1[Pd ][Fe ]
2+
2+
aq
[Pdn ] = [Pd2 ] = ------------------------------------
k12
KINETICS AND CATALYSIS Vol. 45 No. 3 2004

384
POTEKHIN et al.
tional to the squared concentration of Fe2+. The experi-
2+
[Pd ] × 103, mol/l
aq 0
mental data provide support for our previous hypothe-
sis [5] on the participation of palladium in an
intermediate oxidation state in the catalysis of alcohol
oxidation by the palladium(II) tetraaqua complex–
iron(III) aqua complex system.
0
2
4
6
8
10
12
10
0
9
8
7
6
5
4
3
2
1
1
2
1
2
3
4
5
6
7
8
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10
15
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Fig. 5. Dependence of the apparent rate of Fe formation
2+
2+
on the initial concentrations of (1) Fe and (2) Pd at T =
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–3
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2
2+
2+
wcat = kcat[Pd ][Fe ] ,
(4)
aq
aq
where kcat = k1k6/k12.
Then, the rate of reaction (II) is equal to
2
2+
aq
2+
aq
2+
aq
2+
aq
wFe3+ = 2k1[Pd ][Fe ] + 2kcat[Pd ][Fe ] . (5)
Upon substituting experimental data in Eq. (5), the
dependence of wFe3+ /[Fe2+]0 on [Fe2+]0 is expressed as
a straight line (Fig. 5, line 1). The slope of this straight
line is proportional to the rate constant kcat, and the
intercept on the ordinate axis makes it possible to find
the rate constant k1. The rate constants at 36°C were calcu-
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Nauk SSSR, Ser. Khim. Nauk., 1968, no. 3, p. 144.
lated to be equal to kcat = (3.0 0.5) l2 mol–2 s–1 and k1 =
(2.0 0.3) × 10–2 l mol–1 s–1.
The above data allowed us to conclude that the
redox reaction between the Fe2+ aqua ion and the tet-
raaqua complex of Pd2+ in an HClO4 solution occurs by
a chain one-electron mechanism with palladium auto-
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KINETICS AND CATALYSIS Vol. 45 No. 3 2004