The mechanism of oxygen catalytic reduction in the presence of platinum and silver nanoparticles

Article abstract of DOI:10.1007/s11172-010-0268-z

The reaction of oxygen reduction on the mercury electrode in the solution of the reversed micelles and in the presence of platinum and silver nanoparticles was studied. The data of inverse voltammetry show that in the presence of platinum nanoparticles th

Full text of DOI:10.1007/s11172-010-0268-z

1
488
Russian Chemical Bulletin, International Edition, Vol. 59, No. 8, pp. 1488—1494, August, 2010
The mechanism of oxygen catalytic reduction
in the presence of platinum and silver nanoparticles
N. A. Yashtulov,a A. A. Revina, and V. R. Flida
b
aM. V. Lomonosov Moscow State Academy of Fine Chemical Technology,
8
6 Vernadskogo prosp., 119571 Moscow, Russian Federation.
Fax: +7 (495) 434 7111. Eꢀmail: YashtulovNA@rambler.ru
A. N. Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences,
b
3
1 Leninsky prosp., 119991 Moscow, Russian Federation
Fax: +7 (495) 955 4017
The reaction of oxygen reduction on the mercury electrode in the solution of the reversed
micelles and in the presence of platinum and silver nanoparticles was studied. The data of
inverse voltammetry show that in the presence of platinum nanoparticles the reaction can
proceed via both twoꢀelectron and fourꢀelectron reaction mechanisms. In the case of silver
nanoparticles it proceeds in accordance with the twoꢀelectron mechanism. Cumulative effect
of catalytic action of platinum and silver nanoparticles on the molecular oxygen reduction was
found.
Key words: oxygen reduction, platinum and silver nanoparticles, catalytic mechanism.
The reaction of oxygen reduction (OR) is one of the
ucts such as peroxides and radicals capable to decrease
catalyst activity. To suppress the peroxide action on platiꢀ
num, the silver additives promoting the peroxides decomꢀ
position are usually used.
most important multistage processes in the kinetics and
catalysis. It is of the great theoretical and practical value.
The studies of OR mechanisms in the electrochemical
energy generators (ECEG) are the most topical. In the
ECEG the molecular oxygen or some oxygenꢀcontaining
substances are used as the oxidants of different kinds of
fuel. Nanomaterials such as carbon nanotubes, carbon
fibers, fulleroids, porous silicon, polymer composites, and
nanocatalysts received increasing application in these
generators.
The overall process of OR is described by following
equations:9
—12
O + 4 H + 4 e–
+
2 H O,
2
2
–
4 OH–.
O + 2 H O + 4 e
2
2
i. Acidic medium; ii. Basic medium.
In the series of our works1—4 and in the other publicaꢀ
tions5—8 the efficiency of nanoparticles of silver and of
Two mechanisms of the overall process are possible.
The first mechanism is the fourꢀelectron route and it proꢀ
ceeds with the direct cleavage of the double bond in the
oxygen molecule dissociatively adsorbed on the surface at
the first process step which is the rateꢀlimiting one:
platinum group metals as electrocatalyst of OR on the
porous supports such as porous silicon and carbon nanoꢀ
tubes was shown. Such nanocatalysts are produced by the
radiation chemical synthesis in the reversed micelles of
9
,10
the stable metal nanoparticles.
By using this method
O2
2 Oads
.
(1)
the nanosized metal particles can be prepared. When
incorporated into solid nanocomposites they retain
activity for a long period of time upon the contact with
the liquid phase.
Then follows the second, a more rapid step:
Oads + 4 H+ + 4 e–
2
2 H O.
2
(2)
Platinum and the platinumꢀbased composites are conꢀ
sidered as the best catalysts for the OR in ECEG. The cost
of the catalysts can be considerably reduced due to the
introduction of nanostructured catalyst such as platinum
nanoparticles on the surface of stabilizing support. As OR
proceeds at high values of cathode potential the oxygen
reduction is accompanied by the formation of side prodꢀ
The second mechanism of OR is the twoꢀelectron
route and it consists of two twoꢀelectron steps and acꢀ
companied with the formation of peroxides. The first step
is described by the equation:
O + 2 H + 2 e–
+
H O .
(3)
2
2
2
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1456—1461, August, 2010.
066ꢀ5285/10/5908ꢀ1488 © 2010 Springer Science+Business Media, Inc.
1

Mechanism of oxygen catalytic reduction
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 8, August, 2010
1489
This step consists of two subꢀsteps: first subꢀstep (3а)
is the rateꢀlimiting one, the second subꢀstep (3b) results
in formation of peroxides.
reference electrode was the saturated silverꢀchloride Ag/AgCl/
KCl (Е = +0.20 V) electrode. The polarograph allows one to
–
8
–1
achieve the sensitivity of analysis up to 10 mol L at the
background/substance ratio up to 10– , what makes it possible
to analyse any of redox pairs independent of their extent of
reversibility. The device operation mode was one with alternating
current, the potential sweep interval was up to 1600 mV, and the
sweep rate was from 1 to 200 mV s–1 with a 1 mV step.
The mercury electrode is the one with a high degree of
polarization. The interval of potentials of DME applications for
the electroꢀanalytical purposes is substantially extended from
the cathode region, which is typical of OR. Therefore DME is
suitable for the study of OR mechanisms without competitive
influence of the other reactions.
4
O + H + e–
+
HO2•
2
–
O2–, O2– + H+
HO2•).
(
O + e
(3a)
2
HO2• + H+ + e–
H O
2
2
HO2• + e–
HO2–,
H O2•).
(
HO2– + H+
(3b)
2
The second step (4) which also involves two subꢀsteps
(
4a), (4b), is the step of the product formation (H O)
The platinum and silver nanoparticles were synthesized
2
1
3,14
according to the procedure described earlier.
The reversed
during the reduction of hydrogen peroxide:
micelles are microdrops of aqueous solution (pools stabilized
with surfactant in the organic solvent — isooctane). The metal
H O + 2 H + 2 e–
+
2 H O,
(4)
2
2
2
6
0
nanoparticles form in the pool of micelle at radiofrequency Co
γꢀirradiation. By varying conditions of synthesis as well as reagent
concentrations it is possible to control the size range of the
particles formed. For the reversed micelles preparation 0.15 М
solution of surfactant (sodium bis(2ꢀethylhexyl)sulfosuccinate
–
•
–
H O + e
HO + HO ,
(4a)
2
2
HO• + HO + e + 2 H+
–
–
2 H O.
(4b)
2
(
SES) 99%, Sigma) in isooctane was used. The molar ratio
water : surfactant was varied from 1.5 to 5, that corresponds to
solubilization extent ω = [H O]/[SES] = 1.5—5.
During the reduction of molecular oxygen on the platiꢀ
num group metals and on the silver the only one wave,
corresponding to the fourꢀelectron mechanism (reactions
0
2
The intervals of Pt nanoparticles concentration in the
(
1), (2)) is detected on voltammetric curves. On the merꢀ
cury and carbon electrodes only twoꢀelectron mechanism
with the step of intermediate H O formation is possible.
solutions of reversed micelles dependent on ω are listed below.
0
–1
ω0
C(Pt)/mol L
1.40•10–5—1.12•10–4
2
2
1.5
Two broad waves corresponding to two steps (formation
of H O (reaction (3)) and reduction of H O to the product
2.80•10–5—2.80•10–4
4.66•10–5—3.26•10–4
3
.0
5.0
For the Ag nanoparticles at ω = 1.5 the concentration
2
2
2
2
H O (reaction (4)) are observed on voltammeric curves.
2
0
increased from 1.2•10– to 1.2•10 mol L
4
–3
–1
.
The limiting reaction of the overall twoꢀelectron mechaꢀ
nism is the addition of the first electron to the oxygen
molecule according to the first subꢀstep of the first stage
According to fotonꢀcorrelation, HPLC and highꢀresolution
scanning electron microscopy data
2
—4
the size range for the
synthesized nanoparticles is within 2—14 nm, whereas for Ag
nanoparticles it is within 4.5—11 nm. When the solubilization
(
(
reaction (3a)). On the step of H O reduction to H O
reaction (4)) the rate of the first subꢀstep (reaction (4а))
2 2 2
extent ω increases the share of fraction with the larger sizes also
0
is considerably lower than the rate of the second (reaction
(
increases.
9
4b)) . The rate decrease at any step causes the polarizaꢀ
The waterꢀethanol (1 : 1) solution of phosphate buffer with
pH 6.86 was used as a background solution. The molecular
oxygen concentration was maintained constant in the waterꢀ
tion (the process of the change of the potential of elecꢀ
trode reaction with respect to the equilibrium value). The
reason of polarization is determined by the nature of
limiting step. The value of the potential shift is considered
as the overcurrent.
The aim of the present study is the refinement of the
mechanism of catalytic action of Pt and Ag nanoparticles
in OR on dropping mercury electrode (DME) in the soluꢀ
tions of reversed micelles by the method of alternatingꢀ
current inverse voltammetry (ACIV).
ethanol solution (8.5—9•10– mol L ) by bubbling the air
through the system before the each measurement. Such value
corresponds to the oxygen solubility in the solution. Voltammetry
curves were registered in a 20 mL electrochemical cell. Each
measurement was performed on the renewed surface of the
mercury drop. The same background solution was applied during
the measurements.
4
–1
Results and Discussion
Experimental
The values of potentials (Е) the peak maxima of OR
on DME with respect to saturated silverꢀchloride elecꢀ
The ACIV measurements were conducted on a ABC 1.1
voltammetry polarograph with the buildꢀin Modul ЕМꢀ04 sensor
trode Ag|AgCl|KCl and corresponding to them currents
—
(
I ), read from the baseline are listed in Table 1 as a
(
Certificate of Russian Federation No. 7936), providing the
function of the Pt nanoparticles concentrations at the
squareꢀwave modulation of a potential sweep. The working
electrode was DME, the secondary electrode was platinum, the
solubilization extent ω = 1.5.
0

1
490
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 8, August, 2010
Yashtulov et al.
—
Table 1. The influence of concentration of platinum nanoꢀ
and H O , as well as the increase of current I (O )
2
2
2
particles on the electrochemical parameters of OR at ω = 1.5
0
suggest that in solutions with the small content of Pt
nanoparticles, OR proceeds in accordance with the
twoꢀelectron mechanism with the lower kinetic limitaꢀ
tions and a higher rate, than in the solutions in which Pt
nanoparticles are absent.
The substantial changes are observed on the voltamꢀ
metric curve (see Fig. 1, curves 2 and 3) upon the increase
in the content of Pt nanoparticles. The new peak appears
—
—
—
–
С(Pt)•10 I (O ) –E(O ) I (O2–) –E(O ) I (H O ) –E(H O )
5
2
2
2
2
2
2
2
–
1
/mol L
/μA
/mV
/μA
/mV
/μA
/mV
0
1
2
5
8
1
—
—
—
—
—
139
161
1.27
1.35
1.30
1.06
0.99
0.81
375
306
340
370
528
541
1.56
1.32
1.19
1.84
2.09
1.70
1300
1225
1220
1231
1240
1248
.4
.8
.6
.4
—
—
—
0.12
0.98
–
at E(O ) = –139 (curve 2) and –161 mV (curve 3). The
main peak E(O ) broadens and shifts to the region of
2
1.2
2
negative cathode potentials to –528 and –541 mV,
—
The voltammetric curve of the background solution in
whereas the current of oxygen reduction I (O ) decreases.
2
the absence of Pt nanoparticles (Fig. 1) has two typical
broadened peaks, characteristic of the twoꢀelectron OR
mechanism on the mercury electrode. The first peak with
The peak of hydrogen peroxide reduction virtually does
not change its potential, whereas the current of hydrogen
—
peroxide reduction I (H O ) increases (see Table 1).
2
2
Е(О ) = –375 mV is related to the reaction (3), which is
The presence of the strong peak of H O reduction
2
2 2
the reduction of molecular oxygen to H O . The second
unambigously suggests that OR follows the twoꢀelectron
mechanism. The first peak of volammetric curve (for the
2
2
peak with Е(H O ) = –1300 mV is related to the reaction
2
2
(
4), which is the reduction of H O to H O. The peaks on
background solution at E(O ) = –375 mV) is the overall
2
2
2
2
OR voltammetric curves are usually substantially broadꢀ
ened because of the complicated multistep process with
the presence of the limiting steps. The voltammetric
curves for the rapid processes have the shape of the narꢀ
row peaks; the slower the process, the stronger the peaks
broadening and the lower the value of their reduction
currents.
wave associated with the first step of O reduction (reacꢀ
2
tion (3)). The first subꢀstep (reaction (3a)) with the interꢀ
–
mediate formation of the dioxygen anion O2 is the rateꢀ
–
9
limiting one. The anion O2 stabilization was found for
OR in the presence of some organic substances.
The presence of weakly broadened distinct peaks in
–
the anode region E(O ) ≈ –150 mV can be explained by
2
The voltammetric curve for the solution with the miniꢀ
mal Pt nanoparticles content (see Fig. 1, curve 1) also
consists of the two peaks, but the parameters of the peaks
are substantially varied with respect to the background
solution (see Fig. 1, curve 4). The oxygen peak related to
the increase of the reaction (3a) rate caused by the presꢀ
ence of Pt nanoparticles. The increase in the rate of reacꢀ
tion (3a) rate results in the increase in the concentration
of HO2• radicals and, therefore, in the increase in the rate
of reaction (3b): HO2• + H + e
+
–
H O , that
2
2
the molecular oxygen reduction (Е(O ) = –306 mV) is
is reflected in the increase in the H O reduction current.
2
2
2
shifted to the anode region by 69 mV. The similar anode
shift is also observed for the peak related to the reduction
of hydrogen peroxide: ΔE(H O ) = 75 mV. The reduction
The results of analysis of inversed voltammetry on
DME of micellar solutions of Pt nanoparticles with the
solubilization extent ω = 3 are listed in Table 2 and
2
2
0
—
current of oxygen I (O ) increases from 1.27 to 1.35 mA
shown in Fig. 2. Two strong waves of OR are observed on
voltammetry curves. The peaks at cathode potentials from
2
(
see Table 1). The anode shift of the reduction peaks of O2
—
I /μA
—
I /μA
4
3
4
3
2
2
1
3
2
2
4
1
1
3
1
–
1200
–800
–400
E/mV
–1200
–800
–400
E/mV
Fig. 1. The electrocheimical characteristics of OR in the presence
Fig. 2. The electrocheimical characteristics of OR in the presꢀ
–
5
–5
of platinum nanoparticles at ω = 1.5: C(Pt) = 1.4•10 (1),
ence of platinum nanoparticles at ω = 3: C(Pt) = 5.6•10 (1),
0
0
.4•10– (2), 11.2•10 (3), and 0 mol L (4).
5
–5
–1
16.8•10 (2), and 28•10 mol L (3).
–5
–5
–1
8

Mechanism of oxygen catalytic reduction
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 8, August, 2010
1491
Table 2. The influence of concentration of platinum nanoꢀ
—
I /μA
particles on the electrochemical parameters of OR at ω = 3
0
4
3
—
—
—
С(Pt)•10 I (O ) –E(O ) I (O2–) –E(O2–) I (H O ) –E(H O )
5
2
2
2
2
2
2
–
1
/mol L
/μA
/mV
/μA
/mV
/μA
/mV
2
5
1
1
2
2
.8
.6
1.2
6.8
2.4
8
0.08
0.22
0.12
+
+
+
150
145
150
150
150
150
1.15
0.97
0.82
0.80
0.82
0.55
413
358
447
500
450
653
1.55
1.51
1.70
1.40
1.53
1.25
1239
1228
1246
1246
1246
1256
3
2
2
1
1
–
1200
–800
–400
E/mV
–
1225 to –1256 mV correspond to the reduction of H O
2 2
Fig. 3. The electrocheimical characteristics of OR in the presence
of platinum nanoparticles at ω = 3 and C(Pt) = 2.8•10 mol L ,
0
–4
–1
to H O in accordance with the twoꢀelectron mechanism.
2
obtained at the different duration of contact on the mercury
electrode: τ = 1 (1), 36 (2), and 46 min (3).
Additional shoulder in a cathode region at Е = –1500 mV
is observed along with the main intensive peaks due to
H O reduction. The character of H O reduction peaks
2
2
2
2
curves. The increase of reduction peak E(O
–) = –150 mV
2
remains virtually unchanged at all concentration of the
Pt nanoparticles. The only exception is the peak correꢀ
sponding to the sample with the maximal Pt content
suggests the catalytic action of nanoparticles on the step
(3a) when the twoꢀelectron OR mechanism is operative.
With the increasing duration of the contact the broadꢀ
ened peak of oxygen reduction (see Fig. 3, curve 3) is
divided to two components: –660 and –400 mV. The
E(Oads) = –660 mV potential is typical of the fourꢀelectron
–
4
–1
(
(
C = 2.8•10 mol L ); its reduction current deacreases
—
I = 1.25 μA).
–
In the region of potentials of E(O ) = –150 mV weak
2
peaks are observed for all samples with ω = 3, but maxiꢀ
0
9
—11
mal reduction current is typical of the solutions with a low
concentration of nanoparticles. Let us analyse the data
mechanism OR (steps (1) and (2)) on Pt catalysts.
The reduction potentials in the region near 400 mV were
registered in our work for the electrode step (3) when OR
proceeds via the twoꢀelectron mechanism in the absence
or at a low content of Pt nanoparticles (see Tables 1 and 2).
Therefore, for ω0 = 3 at low Pt concentration the twoꢀ
electron mechanism is prefered for the catalytic step (3).
That is indicated by the lower overvoltage of the reducꢀ
(
see Table 2 and Fig. 2), concerning the main wave of
molecular dioxygen reduction in OR. With increasing conꢀ
centration of Pt nanoparticles the currents of oxygen reꢀ
–
duction I (O ) decrease from 1.15 to 0.55 μA, the reducꢀ
2
–
tion potentials E(O ) shift to the cathode region from
2
–
413 to –653 mV, the substantial peak broadening is
observed. Similar results in the frames of twoꢀelectron
mechanism of OR catalysis should suggest the decrease of
the process rate in the presence of Pt nanoparticles.
Characteristics of voltammetric curves as a function of
the duration of contact of mercury electrode with the
solution of nanoparticles were evaluated for the futher
inquiry about OR mechanism in the presence of Pt
tion of both O
at E(O
2
, and H
) = –150 mV. When the content of Pt nanoꢀ
2
2 2
O and the appearance of the peak
–
particles increases in the solution the OR can proceed in
accordance with the fourꢀelectron mechanism typical of
platinum group metals. This is confirmed by both the
splitting of peak of oxygen reduction into two peaks
a–nd the reduction of hydrogen peroxide reduction current
nanoparticles. The voltammetric curves at ω = 3 with Pt
I (H O ) (see Figs 2 and 3).
2 2
0
concentration С(Pt) = 2.8•10–4 mol L at different duꢀ
ration of the contact with the solution are shown in Fig. 3.
At τ = 31 and 46 min four typical peaks (–150, –420,
–1
In solutions with ω0 = 1.5 at increased Pt nanoparticles
concentrations, OR, probably, can proceed in accordance
with the twoꢀelectron as well as with the fourꢀelectron
mechanism. This is suggested by the shift of the peaks of
–
660 and –1250 mV) are observed on the voltammetric
Table 3. The influence of concentration of platinum nanoparticles on the electrochemical parameters
of OR at ω = 5
0
—
—
—
5
С(Pt)•10
V
/mL
I (O )
–E(O2)
/mV
I (H O )
/μA
–E(H O ) I (H O )´ –E(H O )´
2
2
2
2
2
2
2
2
2
–
1
/mol L
/μA
/mV
/μA
/mV
4
9
1
3
.66
.32
3.98
2.62
0.1
0.2
0.3
0.7
1.44
1.26
1.27
1.07
350
380
379
410
1.59
1.49
1.60
1.40
1225
1235
1235
1241
+
+
+
+
1500
1500
1500
1500

1
492
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 8, August, 2010
Yashtulov et al.
—
I /μA
subꢀsteps (reactions (4a) and (4b)). The increase of the
current of cathode shoulder (see Fig. 4) suggests the inꢀ
crease in the rate of the limiting step (4a) upon the
increase in the concentration of Pt nanoparticles:
4
3
2
1
–
•
–
H O + e
HO + HO .
2
2
2
1
Such catalytic influence on H O reduction in OR is
2
—12
2
not typical of Pt macroparticles.9
The presence of perꢀ
3
oxides results in the sharp decrease in the catalytic activꢀ
ity of Pt because of the formation of oxides. As indicated
above, the silver is considered as the most effective cataꢀ
lyst of hydrogen peroxide reduction. In order to elaborate
the results of catalytic action of Pt nanoparticles on OR,
the influence of nanoparticles of Ag on OR was studied.
To evaluate some features of catalytic action of Ag
nanoparticles on OR, the voltammetric analysis of
aqueousꢀalcohol solutions with the solubilization extent
–
1200
–800
–400
E/mV
Fig. 4. The electrocheimical characteristics of OR in the presence
of platinum nanoparticles at ω = 5: C(Pt) = 4.66•10 (1),
.32•10– (2), and 32.62•10 mol L (3).
–
5
0
5
–5
–1
9
oxygen reduction E(O ) to the cathode region and their
broadening (see Table 1 and Fig. 1) observed when the
concentration of Pt nanoparticles increases.
The voltammetric parameters of OR of solutions conꢀ
taining Pt nanoparticles at ω = 5 on DME are listed at
2
ω = 1.5 was performed. The results (Table 4, Fig. 5)
0
allow one to conclude that the influence of Ag and Pt
nanoparticle on OR is essentially different (see Figs 1
and 5). In addition, the presence of the strong peak of
0
H O reduction indicates that OR proceeds on DME in
Table 3 and shown in Fig. 4. The strong peak of H O
reduction and the absence of the substantial changes in
2
2
2
2
accordance with the twoꢀelectron mechanism.
the pattern of the peak of O reduction relatively to the
2
background solution indicate that at ω = 5 OR proceeds
via the twoꢀelectron mechanism.
0
—
I /μA
How can the catalytic action of Pt nanoparticles be
interpreted? Let us compare the data obtained for the
6
2
solution with ω = 5 with the result of analysis of the
3
0
background solution and the solutions with another valꢀ
4
ues of ω . The wave parameters of H O reduction are
changed; in this case, not only anode shift of the main peak
0
2
2
1
E(H O ) = –(1225—1241) mV relative to the background
2
2
2
is observed, but the increase of reduction current of the
cathode shoulder in the region with E(H O *) = –1500 mV
2
2
is also registered. This is distinctly clear for the sample
with the highest Pt content.
–
1200
–800
–400
E/mV
2
The observable splitting of the overall wave of H O2
Fig. 5. The electrocheimical characteristics of OR in the presꢀ
–
4
reduction into two peaks can be interpreted due to the
fact, that the mechanism of H O reduction includes two
ence of silver nanoparticles at ω = 1.5: C(Ag) = 12•10 (1),
0
4.8•10– (2), and 7.2•10 mol L (3).
4
–4
–1
2
2
Table 4. The influence of concentration of silver nanoparticles on the electrochemical
parameters of OR at ω = 1.5
0
—
4
—
—
С(Ag)•10
I (O )
–E(O2)
/mV
I (H O ) –E(H O ) I (H O )´ –E(H O )´
2
2
2
2
2
2
2
2
2
–
1
/
mol L
/μA
/μA
/mV
/μA
/mV
1
2
4
7
8
9
1
1
.2
.4
.8
.2
.4
.6
0.8
2
1.27
1.45
1.11
1.44
1.42
1.68
1.56
1.44
362
376
553
410
428
408
415
415
2.12
2.47
+
1225
1281
1280
1280
1224
1217
1210
1220
+
+
3.56
3.80
+
+
1500
1500
1423
1322
1500
1500
1500
1500
+
2.54
2.48
2.33
2.48
+
+

Mechanism of oxygen catalytic reduction
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 8, August, 2010
1493
There are notable differences in characteristics of the
voltammetric curves between solutions with concentraꢀ
nanoparticles fraction with the larger (up to 10—14 nm)
sizes (ω = 5) increases, the catalytic action leads primaꢀ
0
tions С(Ag) = 4.8•10–4 and 7.2•10 mol L (see Table 4
and Fig. 5, curves 2 and 3) and solutions containing Ag in
another concentrations. A particular evidence is the splitꢀ
ting of the wave of H O reduction into two components.
–4
–1
rily to the increase in the rate of H O electroreduction in
2
2
accordance with the twoꢀelectron mechanism.
From the data described above, the catalytic action of
Ag nanoparticles in OR is explained by the cleavage of the
oxygen—oxygen bond in H O molecule upon its reducꢀ
2
2
But, while for the majority of the samples the cathode
component is a hardly perceptible shoulder, for the samples
2
2
tion in accordance with the twoꢀelectron mechanism,
whereas for catalytic Ag macroparticles the fourꢀelectron
mechanism is typical.9
with silver concentrations 4.8•10–4 and 7.2•10 – mol L
–4
–1
,10
it is represented by the high values of current I (H O *)
2
2
and with the distinct narrow peaks. Both the broadened
anode peak and the novel narrow peaks on the voltamꢀ
metric curve are characterized by a lower overvoltage of
reduction if the anode is estimated in relation to the backꢀ
ground (Е = –1300 mV), and the novel peaks – to the
cathode component in another samples (Е = –1480 mV)).
According to the twoꢀelectron mechanism the wave of
H O reduction is the superposition of two peaks associꢀ
Catalytic action of both Pt and Ag in OR results in
formation of complexes with the oxygen and the oxygenꢀ
containing intermediates caused by specific adsorpꢀ
1
1,15
tion.
The selectivity of this reaction is determined by
the type of the oxygen atom coordination. For the smallꢀ
1
5
sized Pt nanoparticles oneꢀsite coordination of oxygen
molecule polarized without dissociation and the subseꢀ
quent formation of the molecular anion of dioxygen on
the mercury electrode may occur in accordance with the
twoꢀelectron mechanism (reaction (3a)):
2
2
9
,10
ated with the rateꢀlimiting reaction
(4a) that involves
reduction of H O molecule with the cleavage of the single
2
2
oxygen—oxygen bond and the final reaction (4b) of H O
reduction.
2
–
Pt + O2– + (Hg).
Pt—O—O + (e —Hg)
Accordingly, the first and the second subꢀsteps of H O
2
2
Coordination15 of H O molecule to two sites on the
reduction can be descriminated as a result of selective
catalytic action of Ag nanoparticles. This offers the possiꢀ
bility of using electrochemical data to estimate catalytic
activity. The increase of reduction currents of H O and
2
2
two Pt atoms in the case of larger nanoparticles (ω = 5)
0
results in the cleavage of the single bond in H O molꢀ
2
2
ecule and the reduction on the cathode (reaction (4a)):
2
2
the substantial decrease of the width of cathode peak sugꢀ
gest the substantial increase in the rate of the rateꢀlimiting
step (reaction (4a)), which results in the increase of the
rate of the subsequent reaction (4b) of H O reduction. It
–
+ (e —Hg)
•
–
2
Pt + HO + HO + (Hg).
2
2
can be assumed that because of selective electrocatalytic
action of Ag nanoparticles on the first subꢀstep of oneꢀ
electron H O decomposition via the twoꢀelectron mechaꢀ
nism of catalysis the rates of the first and the second steps
of the second stage of OR are levelled off.
Therefore, when the molecular oxygen is reduced
on the electrodes even with the extremely low oxygen
adsorption (such as mercury) the platinum and silver
nanoparticles can form the sites that are active in catalytic
reactions proceeding via both the twoꢀelectron and fourꢀ
electron mechanisms.
2
2
The results on the selective action of Ag nanoparticles
help to explain the bifurcation of the wave of H O reducꢀ
2
2
tion under the influence of Pt nanoparticles. They conꢀ
firm the previously made assumption, that the peak with
the lower value of potential can be ascribed to reaction (4a),
whereas the peak with the higher value of potential is
The authors appreciate P. M. Zaytsev for the assistꢀ
ance in conducting of the electrochemical experiments.
The work was financially supported by the Russian
Foundation for Basic Research (Project Nos 09ꢀ08ꢀ00547
and 09ꢀ08ꢀ00758).
attributable to reaction (4b) for Pt nanoparticles at ω = 5.
0
At the high concentrations of lowꢀsized fractions
(
2—5 nm) with ω = 1.5 in the presence of Pt nanoparticles
0
the cumulative catalysis occurs. It proceeds via the fourꢀ
electron mechanism that is typical for the bulky platinum
catalysts and follows twoꢀelectron mechanism of OR not
typical for Pt. Apparently, prior to the reduction step, the
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Received March 10, 2010;
in revised form July 8, 2010