Electrocatalytic oxidation of CO on supported gold nanoparticles and submicroparticles: Support and size effects in electrochemical systems

Article abstract of DOI:10.1016/j.jcat.2007.06.013

We investigated the catalytic activity of gold particles toward CO electrooxidation by cyclic voltammetry and found that the formation of oxidic gold species on the surface of gold nanoparticles (AuNPs) is a necessary condition for electrooxidation of CO. The maximum potential in the positive-going sweep has a great effect on the formation of gold oxides as well as on the activity of AuNPs. We report the support-dependent electrooxidation of CO in aqueous solution as the first example of the contribution of the support to the high catalytic activity of AuNPs in electrochemical systems. We also show that particle size, reported to be a dominant factor in the catalytic oxidation of CO on AuNPs in solid-gas reaction systems, is no longer a critical factor controlling the activity of AuNPs in electrochemical systems. We demonstrate that indium tin oxide-supported gold submicroparticles, which are believed to be catalytically inactive due to their large size, exhibit surprisingly high activity toward electrooxidation of CO.

Full text of DOI:10.1016/j.jcat.2007.06.013

Journal of Catalysis 250 (2007) 247–253
www.elsevier.com/locate/jcat
Electrocatalytic oxidation of CO on supported gold nanoparticles and
submicroparticles: Support and size effects in electrochemical systems
a,∗
a
b
a
P. Diao , D.F. Zhang , M. Guo , Q. Zhang
a
Department of Applied Chemistry, School of Materials Science and Engineering, Beijing University of Aeronautics and Astronautics, Beijing 100083,
People’s Republic of China
b
Department of Physical Chemistry, University of Science and Technology Beijing, Beijing 100083, People’s Republic of China
Received 16 May 2007; revised 13 June 2007; accepted 13 June 2007
Available online 30 July 2007
Abstract
We investigated the catalytic activity of gold particles toward CO electrooxidation by cyclic voltammetry and found that the formation of oxidic
gold species on the surface of gold nanoparticles (AuNPs) is a necessary condition for electrooxidation of CO. The maximum potential in the
positive-going sweep has a great effect on the formation of gold oxides as well as on the activity of AuNPs. We report the support-dependent
electrooxidation of CO in aqueous solution as the first example of the contribution of the support to the high catalytic activity of AuNPs in
electrochemical systems. We also show that particle size, reported to be a dominant factor in the catalytic oxidation of CO on AuNPs in solid–gas
reaction systems, is no longer a critical factor controlling the activity of AuNPs in electrochemical systems. We demonstrate that indium tin
oxide-supported gold submicroparticles, which are believed to be catalytically inactive due to their “large” size, exhibit surprisingly high activity
toward electrooxidation of CO.
©
2007 Elsevier Inc. All rights reserved.
Keywords: Gold; CO oxidation; Electrocatalysis; Indium tin oxide
1
. Introduction
sharply as the particle size increases. For the larger AuNPs (di-
ameter >10 nm), nearly no activity is observed [3,5,14]. On
the other hand, the support on which AuNPs are dispersed is
widely accepted as another crucial factor controlling the activ-
ity of AuNPs/support catalysts. It has been demonstrated that
even with identical particle sizes, the activity of AuNPs differs
greatly when using different metal oxide supports [5].
Electrocatalytic oxidation of CO in aqueous solution also
has been observed on AuNPs supported by conductive sub-
strates [20–22]. However, only small AuNPs (diameter <6 nm)
were used to investigate the electrocatalytic properties for CO
oxidation in aqueous solution [20–22]. This may be due to the
fact that only small AuNPs show high catalytic activity in solid–
gas systems. It is self-evident that the heterogeneous catalytic
oxidation of CO at the interface of solution and AuNPs elec-
trodes should be quite different from that at the solid–gas inter-
face. In addition, the potential applied to electrode can greatly
influence the surface states of both AuNPs and the underlying
electrode and thus may change the reaction mechanism. As a
result, the factors determining the activity of AuNPs in solid–
Although bulk gold is catalytically inert, small gold nanopar-
ticles (AuNPs) dispersed on metal oxides exhibit surprisingly
high catalytic reactivity toward low-temperature oxidation of
CO in solid–gas reaction systems [1–3]. Significant efforts have
been made to elucidate the origins of the unusually high activity
of AuNPs/support systems [4–19]. Different mechanisms have
been proposed, including quantum size effect [4], support con-
tribution [5–11], effect of low-coordinated sites [12–15], and
effects of charge states or electronic structure of the AuNPs
[
16–19]. Although the exact origins of the catalytic activity of
supported AuNPs remain under debate, it has been documented
that the size of AuNPs is a critical factor influencing the activ-
ity in solid–gas reaction systems. It has been shown that only
AuNPs of diameter <10 nm exhibit extraordinary high activ-
ity toward CO oxidation [1–19], and that this activity decreases
*
Corresponding author. Fax: +86 01 82420736.
E-mail address: pdiao@buaa.edu.cn (P. Diao).
0021-9517/$ – see front matter © 2007 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2007.06.013

248
P. Diao et al. / Journal of Catalysis 250 (2007) 247–253
gas systems, such as the size of the particles and the nature of
the supports, still may not be the key factors in electrochemical
systems, although other factors that do not exist in the solid–gas
interface, such as the potential and the composition of the so-
lution, may play key roles in the oxidation of CO. The purpose
of this work is to clarify the activity-determining factors for the
electrocatalytic oxidation of CO on AuNPs in solution.
the reduction current peak was obtained from the CV curve.
This area reflects the amount of charge needed to reduce the
gold oxides formed on the particle surfaces in the positive scan
[26,27]. Therefore, the total surface area of the gold particles
can be represented by the amount of reduction charge, which is
used to normalize the catalytic oxidation current.
2
.4. Electrocatalytic oxidation of CO
2
. Experimental
The electrode kinetic measurements were carried out in a
2
.1. Preparation of gold nanoparticle assemblies on ITO and
gas-sealed box with a gas inlet tube reaching to the bottom
of the electrochemical cell to facilitate gas–liquid saturation.
The electrocatalytic oxidation measurements were conducted
by cyclic voltammetry with a potential range of −0.4–0.7 V
Pt substrates
The indium tin oxide (ITO) and the Pt substrates were first
sonicated successively in acetone, water, and methanol. After
cleaning, both the ITO and Pt substrates were immersed in the
(
vs SCE) in 0.5 M KOH after the solution was saturated with
CO for 45 min. During electrooxidation of CO, the KOH so-
lution was continuously bubbled with CO. The electrocatalytic
oxidation current was normalized by dividing by the gold ox-
ide reduction charge obtained at the same electrode in 0.5 M
KOH. All electrochemical measurements were performed on a
CHI660A electrochemical workstation (CH Instruments) using
a conventional three-electrode cell at room temperature. The
cell has a circular window on the bottom to ensure that the
−
3
1
,12-dodecanediamine (DDA) solution (5.0 × 10 M) of ab-
solute methanol for 48 h to form a DDA monolayer on their
surfaces [23,24]. The resulting film-coated ITO and Pt sub-
strates were rinsed with copious absolute methanol and then
dried under a stream of high-purity N2. The gold colloidal
nanoparticles were prepared by adding aqueous sodium citrate
−
4
(
3.5 mL, 1 wt%) into boiling HAuCl4 (100 mL, 2.4 × 10 M)
solution under vigorous stirring [25]. The average particle
size, determined by transmission electron microscopy using a
JEM-200CX (JEOL, Japan), was 13 ± 1.0 nm. Before use, the
pH of the colloid solution was adjusted to 4.0. The above-
mentioned ITO and Pt substrates, which were modified with
amino-terminated DDA monolayers, were immersed in the gold
colloid for 12 h to form nanoparticle assemblies. Micrographs
of AuNPs-modified ITO and Pt substrates were obtained using
a Philips FEI XL30 SFEG scanning electron microscope with
an accelerating potential of 10 kV.
2
area of electrode exposed to solution is 0.26 cm . A saturated
calomel electrode (SCE) and a Pt foil were used as reference
and counter electrodes, respectively. All potentials are reported
with respect to SCE.
3
. Results and discussion
3
.1. Effect of the applied potential on the activity of AuNPs
toward CO electrooxidation
To eliminate the size effect when studying the influence of
the applied potential and the support on the catalytic activity
of AuNPs, the colloidal gold particles with identical size and
shape were used. These gold particles were prepared by the
Frens method [25] and had an average diameter of 13 ± 1 nm.
The DDA was used as a linking molecule to immobilize AuNPs
on ITO [23] and Pt [24] substrates. The AuNPs are negatively
charged due to the adsorption of anions at pH > 2 [28,29].
The amino-terminated monolayers are positively charged in
aqueous solution of pH 3–6 due to the protonation of amino
groups [30]. Therefore, amino groups are usually used to im-
mobilize AuNPs by the strong electrostatic interactions [29,31].
Figs. 1a and 1b show the SEM images of AuNPs assembled
on ITO and Pt substrates, respectively. The AuNPs are uni-
formly dispersed on both substrates, with particle density of
2
.2. Electrodeposition of gold submicroparticles (AuSMPs) on
ITO substrates
The electrochemical deposition of AuSMPs on ITO sub-
−
4
strates was carried out in 10
1
M HAuCl4 solution with
M KCl as the supporting electrolyte. A deposition poten-
tial (0 V vs SCE) was applied to the ITO substrate to prepare
AuSMPs on its surface. The size of the AuSMPs was controlled
by varying the electrodeposition time. After electrodeposition,
the resulting electrodes were rinsed with copious water and then
transferred immediately to the deoxygenated 0.5 M KOH solu-
tion.
2
.3. Electrochemical determination of the total surface area of
AuNPs and AuSMPs
10
−2
10
ca. 3.8 × 10 particles cm on ITO and ca. 8.1 × 10 par-
−
2
To obtain the total surface areas of AuNPs and AuSMPs,
cyclic voltammetric (CV) measurements were first performed
on particle modified ITO or Pt electrodes in 0.5 M KOH solu-
tion that was purged of oxygen by saturating with high-purity
N2 for 25 min. The potential range of CV measurements was
ticles cm on Pt.
As shown in Fig. 2a, the unmodified ITO and Pt electrodes
are electrochemically inert within the scanned potential range
of −0.4–0.7 V in 0.5 M KOH. However, after modification
with AuNPs, both electrodes exhibit CV features of gold elec-
trodes [26] (Fig. 2b), suggesting that the AuNPs have good
electrical contact with the ITO and Pt substrates. The current
−
1
−
0.4–0.7 V (vs SCE), and the scan rate was 100 mV s . After
correction for a double-layer charging current, the area under

P. Diao et al. / Journal of Catalysis 250 (2007) 247–253
249
Fig. 2. Cyclic voltammograms of (a) bare Pt and bare ITO electrodes in 0.5 M
KOH, (b) AuNPs/Pt and AuNPs/ITO electrodes in 0.5 M KOH. Scan rate:
−
1
100 mV s
.
Fig. 4a shows the effect of applied potential on the activ-
ity of AuNPs/ITO for the electrocatalytic oxidation of CO.
From Fig. 4a, two features are immediately apparent: (1) the
catalytic oxidation peak current increases as the scan range is
extended positively, and (2) the oxidation peak cannot be ob-
served in the negative scan unless the potential exceeds 0.4 V
in the preceding positive scan. The positive extension of the
scan range results in the increase of surface gold oxides, as can
be seen from the increasing of the reduction peak of gold ox-
ides in 0.5 M KOH without CO (Fig. 4b). Moreover, Fig. 4b
also demonstrates that gold oxides cannot be formed on the
surface of AuNPs unless the maximum potential in the positive-
going sweep exceeds 0.4 V. From Figs. 4a and 4b, it can be
safely concluded that the presence of gold oxide on the sur-
face of AuNPs is a necessary condition for the catalytic oxida-
tion of CO, and that the more the amount of gold oxides, the
higher the activity of AuNPs. This is in good agreement with
recently obtained results in solid–gas systems [18,19]. To ex-
plain the action of gold oxides in the catalytic reaction, van
Bokhoven et al. [19] proposed a mechanism in which oxidic
Fig. 1. SEM images of AuNPs dispersed on (a) ITO and (b) Pt substrates.
peak at 0.35 V in the positive scan in Fig. 2b indicates the for-
mation of gold oxides on the surface of AuNPs, whereas the
peak at −0.02 V in the negative scan reflects the reduction of
gold oxides formed in the preceding positive scan [26].
After the alkali solution was saturated with CO, both the bare
ITO and Pt electrodes exhibited CV features similar to those
obtained in the solution without CO (data not shown), indicat-
ing that these electrodes have no activity for CO electrooxida-
tion. However, after modification with AuNPs, both electrodes
demonstrated reactivity toward CO. Fig. 3a shows the typical
CV behavior of AuNPs-modified ITO electrodes (AuNPs/ITO)
in 0.5 M KOH saturated with CO. Bulk gold exhibited a re-
duction peak of gold oxides at about 0 V in the negative scan
(
Fig. 3b) in CO-containing solution, whereas the AuNPs/ITO
0
exhibited an oxidation peak at nearly the same potential. Such
CV behavior of AuNPs modified electrode has been reported
previously, and the oxidation peak in the negative-going sweep
was ascribed to the electrocatalytic oxidation of CO on AuNPs
gold species act as reaction intermediates: Au +O2 → AuyOx;
0
AuyOx + CO → Au + CO2. In the electrochemical system
studied here, the gold oxides on the AuNPs surface are formed
not by activation of oxygen, but rather by electrooxidation of
0
−
−
[
20–22].
gold in alkali medium: Au + OH − e → Au O + H O.
y
x
2

250
P. Diao et al. / Journal of Catalysis 250 (2007) 247–253
−
1
Fig. 3. Cyclic voltammograms of AuNPs/ITO (a) and bulk Au (b) electrodes in 0.5 M KOH saturated with CO. Scan rate: 100 mV s
.
Fig. 4. Cyclic voltammograms of AuNPs/ITO electrode in 0.5 M KOH saturated with CO (a) and with no CO (b).
The catalytic action of gold oxides in electrooxidation of CO
can be clearly seen by comparing the charges for gold oxides
reduction and CO oxidation in the same AuNPs/ITO electrode.
As shown in the CV curve in Fig. 2b (solid line), which was
obtained in KOH solution without CO, the area under the reduc-
tion peak represents the charge needed for gold oxide reduction.
The total charge for CO oxidation in the same AuNPs/ITO elec-
trode can be obtained by measuring the area under the oxidation
peak in Fig. 3a. The calculated charges for gold oxides reduc-
tion and for CO oxidation are ca. 2.4 × 10 and 2.0 × 10 C,
respectively. The latter is more than eight times greater than the
former, suggesting that gold oxides on the surface of AuNPs act
not only as oxidants, but also as promoters for CO electrooxi-
dation. In other words, the gold oxides must take part in the
electrooxidation of CO and facilitate the electron transfer from
CO to ITO electrodes. But the exact mechanism of this process
is not quite clear and is under further study. Herein, we should
point out that the existence of oxidic gold species do not nec-
essarily result in high activity toward CO electrooxidation, as
can be seen from the CV behavior on bare gold, on which large
amount of gold oxides were formed but no catalytic oxidation
current could be observed (Fig. 3b).
The foregoing discussion demonstrates that both the electro-
chemical measurements conducted in this work and the kinetic
studies carried out in solid–gas systems [18,19] point out to the
same thing, that is, the formation of gold oxides (in solid–gas
reaction systems, usually described as the activation of oxy-
gen) on the surface of AuNPs is the prerequisite to CO oxi-
dation.
−
5
−4
3
.2. Effect of the substrate on the activity of AuNPs toward
CO electrooxidation
The support effect on the catalytic activity of AuNPs was in-
vestigated by comparing the oxidation current at AuNPs/ITO
and AuNPs/Pt electrodes. Although the AuNPs are identical
in size and shape, the particle density on the ITO and the Pt
substrates differs. Thus, the influence of particle density must
be excluded when we study the support effect. As shown in

P. Diao et al. / Journal of Catalysis 250 (2007) 247–253
251
Pt electrodes. The high activity of AuNPs/ITO suggests that
the ITO substrate participates in the reaction and plays a key
role either in stabilizing the gold oxides [21] or in lowering
the activation energy of CO electrooxidation. Therefore, it is
likely that the oxidation of CO occurs preferentially at the Au–
ITO interface rather than at sites far from the ITO substrate.
This also can explain the fact that no electrocatalytic activity is
observed on bare gold electrode, because there is no Au–ITO
interface.
3
.3. Effect of particle size on the activity of gold/support
system toward CO electrooxidation
It should be pointed out that, in this work, the high activ-
ity of ITO-supported AuNPs toward CO electrooxidation has
been obtained on relatively large (13 nm) particles, which are
believed to be far less active in solid–gas reaction systems
Fig. 5. Normalized cyclic voltammograms of AuNPs/Pt and AuNPs/ITO in
.5 M KOH saturated with CO. The AuNPs/Pt and the AuNPs/ITO electrodes
0
[
3,5,14]. As we know that in solid–gas systems, the adsorption
are the same as those used in Fig. 2b, respectively. The currents are normalized
by dividing by the corresponding charges under the reduction peaks in Fig. 2b
and activation of oxygen on AuNPs/support system is a crucial
factor in the low-temperature oxidation of CO [18,19]. It has
been proposed that, compared with the large AuNPs, the small
AuNPs (<10 nm) more readily activate oxygen (form oxidic
gold species) at their surfaces [12,14,18,19] or at the AuNPs–
support interfaces [7–11], and thus are more active toward CO
oxidation. In an electrochemical system, gold oxides can be
easily formed by applying a positive potential to the electrode;
for example, a positive scan over 0.4 V in 0.5 M KOH can gen-
erate a layer of gold oxides on the surface of AuNPs regardless
of the support and the particle size (see Figs. 2b and 4b). Thus,
particle size has little effect on the formation of gold oxides in
electrochemical system. In other words, unlike in the solid–gas
reaction systems, the size of the gold particles is no longer the
dominant factor for CO oxidation in electrochemical systems,
due to the ease of formation of gold oxide on various-sized
AuNPs. This is why such large AuNPs exhibit high catalytic
activity for CO electrooxidation. To provide further support for
this point, we prepared gold submicroparticles (AuSMPs) on
ITO electrodes and investigated their electrocatalytic activity
toward CO oxidation.
−
1
after corrected for double-layer charging current. Scan rate 100 mV s
.
Fig. 2b, the area under the reduction peak (after correction for
the double-layer charging current) reflects the amount of charge
needed to reduce the surface gold oxides. In other words, the re-
duction peak area (or reduction charge) reflects the true surface
amount of oxidic gold species. For each AuNPs modified elec-
trode used in this work, the CV response in 0.5 M KOH was
first recorded to obtain the amount of gold oxides (represented
by the reduction charge) on the AuNP surface. To eliminate
the influence of particle density, the CV curves obtained in
CO-saturated solution were normalized by dividing by the cor-
responding charge under the reduction peaks in 0.5 M KOH.
Moreover, for all samples, the potential window was fixed in
the range −0.4–0.7 V to rule out the influence of the potential
on the amount of gold oxides. The normalized CV curves on
AuNPs/Pt and AuNPs/ITO, shown in Fig. 5, demonstrate that
the catalytic oxidation current is more than 4 times greater in
AuNPs/ITO than in AuNPs/Pt.
Before ascribing this activity difference to the support ef-
fect, we need to know the average amount of gold oxides per
particle on the ITO and Pt substrates. The total charge for the
reduction of gold oxides on particle-modified electrodes is ca.
Fig. 6 shows SEM images of AuSMPs prepared on ITO
substrates by electrochemical reduction of HAuCl4 at 0 V for
different durations. The particle size distributions for each de-
position time, shown in Fig. 7, demonstrate that the average
particle size increased from ca. 150 to 525 nm as the deposition
time was increased from 15 to 1800 s. These ITO-supported
AuSMPs are too large to be catalytically active for CO oxida-
tion in solid–gas reaction systems; however, they exhibit high
activity toward CO electrooxidation in the alkali solution, as
can be seen in Fig. 8. Similar to the CV curves obtained at
AuNPs-modified electrodes (Fig. 5), the currents in Fig. 8 are
also normalized by dividing by the corresponding gold oxide
reduction charge obtained at the same electrode in 0.5 M KOH
without CO. The current peaks for catalytic oxidation of CO
can be seen in the negative scan given in Fig. 8, providing solid
evidence that high catalytic activity is not confined to small
(<10 nm) AuNPs in the electrochemical system. Fig. 8 also
shows that the catalytic oxidation current decreases when the
−5
−5
4
(
.2 × 10 C for AuNPs/Pt and 2.4 × 10 C for AuNPs/ITO
see Fig. 2b). Therefore, on the basis of particle density on the
two substrates, the average reduction charge per particle is ca.
−16
−16
C on the
5
.2 × 10
C on the Pt substrate and 6.3 × 10
ITO substrate. These two values closely agree, suggesting that
the amount of gold oxides on each gold particle is similar no
matter which substrate is used. Then it can be concluded that
the great difference in reactivity toward CO oxidation between
AuNPs/ITO and AuNPs/Pt arises not from the amount of gold
oxides, but rather from the different nature of the substrate.
This means that even in electrochemical systems, the substrate
has a great effect on the catalytic activity of AuNPs. We be-
lieve that the difference in activity arises from the different
routes through which gold oxides react with CO on ITO and

252
P. Diao et al. / Journal of Catalysis 250 (2007) 247–253
−
4
Fig. 6. SEM images of AuSMPs prepared on ITO substrates by electrodeposition at 0 V (vs SCE) in 10
electrolyte. The deposition times are (a) 15, (b) 60, (c) 300, (d) 1800 s.
M HAuCl solution with 1 M KCl as supporting
4
deposition time for AuSMPs is increased. This indicates that
the activity of AuSMPs/ITO decreases with increasing particle
size, because longer electrochemical deposition time results in
larger AuSMPs. The size-dependent activity observed on these
large AuSMPs is in line with the fact that the ratio of the Au–
ITO interface area to the whole particle surface area decreases
with increasing particle size. This provides further evidence that
the Au–ITO interface is the active location for CO electrooxida-
tion. Comparing Figs. 5 and 8 shows that for the samples with
deposition time ꢀ300 s, the catalytic oxidation currents are at
least 5 times greater on the large AuSMPs than on the small
AuNPs. This finding suggests that the Au–ITO interface pro-
duced by electrodeposition of AuSMPs on ITO is catalytically
more efficient than that produced by the assembly of AuNPs on
ITO. We believe that the relatively lower activity of AuNPs/ITO
may be due to the DDA monolayer, which acts as a linking
layer to immobilize AuNPs on ITO substrates. The DDA mole-
cules coat the ITO surface, attenuating the contribution of the
support to CO electrooxidation. As for AuSMPs-modified ITO
electrodes, AuSMPs are generated on ITO substrates by elec-
trodeposition, with no DDA molecules introduced onto the ITO
surface. As a result, the effect of the support contributes more
to CO electrooxidation in AuSMPs/ITO systems compared with
AuNPs/ITO systems.
4. Conclusion
We have shown that the formation of oxidic gold species on
the surface of AuNPs is a necessary condition for the electroox-
idation of CO. Our experimental results have identified a crucial
catalytic role of supporting electrodes in Au particle/electrode
systems. We have shown that the ITO electrode is not only a
conducting support to provide electrical contact and a means of
dispersing the gold particles on the surface, but also a promoter
of the electrooxidation of CO. We have demonstrated that par-
ticle size is no longer a critical factor controlling the activity
of AuNPs in electrochemical systems. These findings may shed
new light on the origins of the high catalytic activity of gold
particles toward CO oxidation.
Acknowledgment
Financial support from the National Natural Science Foun-
dation of China (NSFC, 20373005) is gratefully acknowledged.

P. Diao et al. / Journal of Catalysis 250 (2007) 247–253
253
Fig. 8. Normalized cyclic voltammograms of AuSMPs/ITO in CO saturated
0
.5 M KOH with different electrodeposition time for AuSMPs. Scan rate
−
1
100 mV s
.
[
[
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−
4
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4
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[
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