NANO EXPRESS Open Access
Gold nanoparticles supported on magnesium
oxide for CO oxidation
Sónia AC Carabineiro
1*
, Nina Bogdanchikova
2
, Alexey Pestryakov
3
, Pedro B Tavares
4
, Lisete SG Fernandes
4
and
José L Figueiredo
1
Abstract
Au was loaded (1 wt%) on a commercial MgO support by three diffe rent methods: double impregnation, liquid-
phase reductive deposition and ultrasonication. Samples were characterised by adsorption of N
2
at -96°C,
temperature-programmed reduction, high-resolution transmission electron microscopy, energy-dispe rsive X-ray
spectroscopy and X-ray diffraction. Upon loading with Au, MgO changed into Mg(OH)
2
(the hydroxide was most
likely formed by reaction with water, in which the gold precursor was dissolved). The size range for gold
nanoparticles was 2-12 nm for the DIM method and 3-15 nm for LPRD and US. The average size of gold particles
was 5.4 nm for DIM and larger than 6.5 for the other methods. CO oxidation was used as a test reaction to
compare the catalytic activity. The best results were obtained with the DIM method, followed by LPRD and US.
This can be explained in terms of the nanoparticle size, well known to determine the catalytic activity of gold
catalysts.

Introduction
It is well known from the literature that for gold to be
active as a catalyst, a careful preparation is needed to
obtain nanoparticles well dispersed on the support [1-4].
Compared with other supports, MgO is considered as
“inactive” [5-8] since it is basically an irreducible oxide,
such as Al
2
O
3
. These materials have low ability to
adsorb or store oxygen at low temperatures [5].
However, Margitfalvi et al. [9] prepared Au/MgO cata-
lysts with h igh activity for low temperature CO oxida-
tion. The activity of these catalyst s was further increased
by modification with ascorbic acid in a relatively narrow
concentration range. These authors suggested that the
addition of ascorbic acid slightly changes the ionic/
metallic gold ratio and suppresses formation of carbo-
nate, which is responsible for deactivation [9]. Gates and
co-workers [10,11] also managed to produce a Au/MgO
catalyst that was active for CO oxidation at 30°C by
bringing Au(CH
3
)
2
(acac) (acac is acetylacetonate) in
contact with partially dehydroxylated MgO and by treat-
ment in flowing helium at 473 K, during which the ori-
ginal mononuclear Au(III) species decomposed, gold

being reduced and aggregated. The catalyst unde rwent
rapid deactivation due to the formation of carbonate-
like species on t he support and on gold, but could be
reactivated by treatment in flowing helium, which le d to
the removal of the carbonate-like species [10].
Heinz et al. [12] showed that small clusters of gold
(Au
20
and Au
8
) are active towards CO oxidation. In fact,
for Au
8
clusters,itwasfoundthattheoxidationofCO
at -33°C is activated after deposition on defect sites of
the MgO support [13,14]. Guzman and Gates [15-17]
showed, by X-ray absorption spectroscopy, the presence
of both cationic and reduced gold in MgO-supported
gold clusters during CO oxidation. Molina and Hammer
[18] showed by DFT calculations that O
2
can bind
simultan eously to both metal cent res (Au and Mg) with
CO bonded to another nearby Au centre. Broqvist et al.
[19] proved also by DFT calculations that Cl was a poi-
son for Au/MgO catalysts in CO oxidation, while Na
was a promotor. Goodman and co-workers [20] showed
a direct correlation between the concentration of F-cen-
tre surface defects in the MgO support and the catalytic
activity for CO oxidation of the subsequently deposited

Au, implying a critical role of surface F-centres in the
activation of Au in Au/MgO catalysts.
Grisel and Nieuwenhuys [21] found that Au/MgO cat-
alysts supported on alumina were extremely active,
* Correspondence:
1
Laboratório de Catálise e Materiais, Departamento de Engenharia Química,
Faculdade de Engenharia, Universidade do Porto, 4200-465 Porto, Portugal
Full list of author information is available at the end of the article
Carabineiro et al. Nanoscale Research Letters 2011, 6:435
/>© 2011 Carabineiro et al; licensee Springer. This is an Open Acc ess article distribu ted under the terms of the Creative Commons
Attribution License ( which permits unre stricted use, distribution, and reproduction in
any medium, provided the original work is properly cited.
achieving 50% CO conversion at room temperature and
full conversion at approximately 250°C. It is, however,
worth to note that those materials had 5% Au loading,
while 1% Au was used in this study. Moreover, these
authors used 2% CO in the gas feed for the CO oxida-
tion experiments, while we used 5% CO. Szabó et al.
[22-24] also reported that Au/Al
2
O
3
catalysts modified
by MgO exhibited high activity in the sub-ambient and
ambient temperature ranges for CO oxidation.
Co-precipitation (CP) [1-5,25-31] and deposition-pre-
cipitation (DP) [1-4,6,21,22,29,31] are the most common
methods to prepare oxide-supported gold catalysts. In
this study, less usual Au loading methods were used,

such as double impregnation (DIM) [32] and liquid
phase reductive de position (LPRD) [33], to prepare Au
nanoparticles. To the best of our knowledge, the only
reports on the use of DIM is the work of Bowker et al.
[32] dealing with TiO
2
samples and our previous work
on CeO
2
[34,35] and ZnO [ 36] catalysts. This m ethod
represents an environmentally and economically more
favourab le route to the preparation of high activity gold
catalysts, in comparison to the traditional deposition-
precipitation (DP) method [32]. As far as we know,
LPRD has only been used by Sunagawa et al. [33] to
prepare Pt and Au catalysts on Fe
2
O
3
,FeOOH,ZrO
2
and TiO
2
supports, and also by us for CeO
2
[37] and
TiO
2
[38]. US was only used by our group to prepare
very active Au/ZnO catalysts [36].

The aim of this study is to compare the activity for
CO oxidation of Au/MgO catalysts prepared by these
unusual methods. This is a simple model reaction to
evaluate gold ca talysts that has many potential applica-
tions, namely in CO removal from H
2
streams for fuel
cells and gas sensing [1-4,34,36,37].
Experimental
Commercial MgO (p.a., Merck) was used as received
and after a treatment at 400°C, in N
2
, for 2 h.
Preparation of Au catalysts
Au was loaded on the MgO support by the double
impregnation method (DIM) [32], liquid phase reductive
deposition (LPRD) [33] and ultrasonication (US) [36].
Briefly, the first method (DIM) consists in impregnating
the support with an aqueous solution of the gold pre-
cursor (HAuCl
4
) and then with a solution of Na
2
CO
3
that precipitates gold hydroxide within the pores of the
catalyst [32,34-36]. The second procedure (LPRD) con-
sists of mixing a solution of HAuCl
4
with a soluti on of

NaOH (with a ratio of 1:4 in weight) that hydroxylates
the Au
3+
ions, before the support is added to the solu-
tion [33,37,38]. Au
3+
ions are reduced to metallic Au
0
by electron tr ansfer from coordinated OH
-
ions on the
surfaces of support particles through their catalytic
action [33]. US consists i n dissolving the Au precursor
in water and methanol, and sonica ting for 8 h, reducing
gold [36]. In all these methods, a washing procedure is
carried out to eliminate residual chloride, which is well
known to cause sinterization of Au nanoparticles, turn-
ing them inactive [1-4,37]. Further details can be found
elsewhere [34-38].
Characterization techniques
The materials were analysed by adsorption of N
2
at
-196°C in a Quantachrom NOVA 4200e apparatus.
Temperature-programmed reduction (TPR) experi-
ments were performed in a fully automate d AMI-200
Catalyst Characterization Instrument ( Altamira Instru-
ments, Pittsburgh, PA, USA), equipped with a quadru-
pole mass spectrometer (Dymaxion 200 amu, Ametek).
Further details can be found elsewhere [34-38].

High-resolution transmission electron microscopy
(HRTEM) measurements were performed with a JEOL
2010 microscope with a point- to-point resolution better
than 0.19 nm. The sample was mounted on a carbon
polymer-supported copper micro-grid. A few droplets of
a suspension of the ground catalyst in isopropyl alcohol
were placed on the grid, followed by drying at ambient
conditions. The average gold particles and the particle
size distribution were determined from a count of at
least 250-300 particles. Semi-quantitative estimation of
gold loading was performed by energy-dispersive X-ray
spectroscopy (EDXS).
X-ray diffraction (XRD) analysis was carried out in
aPAN’alytical X’Pert MPD equipped with a X’ Celera-
tor detector and secondary monochromator. Rietveld
refinement with PowderCell software [39] was used to
identify the crystallographic phases present and to
calculate the crystallite size from the XRD diffraction
patterns. Further details can be found elsewhere
[34-38].
Catalytic tests
Catalytic activity measurements for CO oxidatio n were
performed using a conti nuous-flow reactor. The catalys t
sample (0.2 g) was placed on a quartz wool plug in a
45-cm long silica tube with 2.7 cm i.d., inserted into a
vertical furnac e equipped with a temperature controller.
Feed gas (5% CO, 10% O
2
in He) was passed through
the catalytic bed at a total flow rate of 50 ml · min

-1
(in
contras t with most literature studies that use 1% CO or
less [1-4,31]). The composition of the outgoing gas
stream was determined using a gas chromatograph
equipped with a capillary column (Carboxen 1010 Plot,
Supelco) and a thermal conductivity detector. Further
details can be found elsewhere [34-38].
Carabineiro et al. Nanoscale Research Letters 2011, 6:435
/>Page 2 of 6
Results and discussion
Characterization of samples
BET surface area
TheBETsurfaceareaobtainedfortheMgOsampleby
N
2
adsorption at -196°C was 32 m
2
·g
-1
. This value is
smaller than those reported in the literature [9,23]. Both
the thermal treatment of the support at 400°C and/or
addition of gold by any of the methods described did
not produce significant changes in the BET surface area.
XRD
Figure 1 shows the XRD spectra of the oxide supports
alone, and loaded with 1 wt% Au by DIM. T he identi-
fied phase for the unloaded material is the respective
oxide (cubic, Fm-3m, 01-078-0430), with a crystallite

size of 42 nm; however, when gold is loaded, a new Mg
(OH)
2
phase (hexagonal, P-3m1, 01-076-0667) was
formed (Figure 1). 99% of this hydroxide phase was
detected along with 1% MgO. It was not possible to cal-
culate the particle size of the Mg(OH)
2
phase due to
interstratification of hydrated phases, as also found by
other authors [40], which makes it very difficult to
simulate the spectra, so the results obtained (in this case
approximately 25 nm) are not reliable. The hydroxide is
most likely formed by reaction with water, in which the
gold precursor is dissolved (MgO + H
2
O ® Mg(OH)
2
).
Similar results were obtained for the other loading
methods.
TheAuparticlesizecouldnotbedeterminedforany
of the gold-loaded samples through XRD analysis, since
the characteristic XRD reflection was absent in these
materials. This can be due to the low loading (1 wt%)
and small siz e of Au particles present in these catalysts,
as it will be seen by HRTEM.
HRTEM
Figure 2a shows a HRTEM image of the MgO support
which is quite different from what is obser ved in Figure

2b,c,d(MgOwithAuloadedbyDIM,LPRDandUS,
respectively), as the suppo rt changes from large crystals
(Figure 2a) into a different structure (Figure 2b, c, d).
Figure 3 shows the Au nanoparticle size distributions on
MgO, prepared by the different meth ods. Gold particles
are also observed with sizes ranging from 2 t o 12 nm
for DIM (Figures 2b, 3a). Other methods showed larger
gold nanoparticle sizes betwee n 3 and 15 nm (Figures
2c, 3b for LPRD and Figures 2d, 3c for US). The average
size of gold particles is 5.4 nm for DIM and 6.6 nm for
LPRD. US showed a slig htly larger average gold size (6.7
nm), however the particles were closer to each other
(Figure 2d).
Gold nanoparticles of 6 nm were reported in literature
for Au/MgO catalysts prepared by CP [5]. Smaller
values of approximately 4 nm were however obtained by
CP and DP on Mg(OH)
2
[5,41,42]. Sizes of approxi-
mately 4 nm were also obtained for Au on MgO pre-
pared from a gold complex [20]. Au nanoparticles
smaller than 5 nm were obtai ned on MgO modified
with ascorbic acid [9,23]. Other techn iques like impr eg-
nation produced gold particles of 8 nm on MgO [43].
Values of approximately 9 nm were obtained for gold
on MgO with cube morphology [8]. Gold deposited on
MgO/alumina yielded particles ranging from 2.7 to 4.6
nm [21,22,24,44].
EDXS
Semi-quantitative estimation of gold loading was per-

formed by EDXS, a pproximately 0.9% being found for
all samples.
TPR
TPRresultsareshowninFigure4forthepureMgO
and MgO loaded with gold by DIM. It can be seen that
pure MgO does not show any significant reduction peak
in the studied range of temperatures (t hin line), as
expected from the literature [16,45]. When Au is loaded
Figure 1 X-ray diffraction spectra of commercial MgO, pure
(thin line) and loaded with 1% Au wt (thicker line) by DIM,
with phases and respective crystal planes (Miller indexes)
identified.
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





a
b
c
d
Figure 2 HRTEM image s of the commercial MgO, pure (a) and
loaded with 1% Au wt by DIM (b), LPRD (c) and US (d).

Carabineiro et al. Nanoscale Research Letters 2011, 6:435
/>Page 3 of 6
into MgO, as discussed abo ve, the support is trans-
formed into Mg(OH)
2
, most likely by reaction with
water. As can be seen in Figure 4 (thick line), a large
negative peak is observed on the TPR spectrum between
approximately 300 and approximately 600°C. This
means that hydrogen is not being consumed. H owever,
water release was detected by mass spectrometry, most
likely meaning that MgO is being formed (Mg(OH)
2
®
MgO + H
2
O). In fact, a second TPR run produced a
spectrum with no peaks, as for the oxide, as expected
from the literature [16,45]. Similar results were obtained
for samples loaded by the other methods.
Catalytic tests
It was found that the activity for CO oxidation (with or
without Au) of the heat-treated MgO did not improve
when compared with the as-received oxide; therefore,
only the results of the untreated samples are shown in
Figure 5a. Loading MgO with Au causes total CO con-
version to o ccur at much lower temperatures than with
the support alone, as expected. DIM showed to be the
best gold-loading method, followed by LPRD and US.
It can be argued that there are gold catalysts that

achieve full CO conversions already at room tempera-
ture, but it has t o be taken into account that most stu-
dies in literature use 1% CO or less [1-4] (while we used
5% of this gas). Also, the majori ty of authors use higher
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
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

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
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
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








c
b
a
Figure 3 Size distribution histograms of Au nanopar ticle s on
MgO, prepared by DIM (b), LPRD (c) and US (d), with
respective average sizes.
Figure 4 H
2
-TPR profiles of the commercial MgO, pure (thin
line) and loaded with 1% Au wt (thicker line) by DIM.

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a
b
Figure 5 CO conversion (%): CO conversion (%) versus
temperature for MgO supports alone and with Au loaded by

different methods (a). Specific activities for the Au/MgO catalysts
determined at 25 and at 100°C (b).
Carabineiro et al. Nanoscale Research Letters 2011, 6:435
/>Page 4 of 6
loadings of Au [1-4] (while we used 1 wt%). Neverthe-
less, it is possible to see, in our case, that CO conversion
increases up to four times by addition of gold (for MgO
with Au loaded by DIM), when compared to the
unloaded samples.
Schubert et al. [5] repo rted activi ties of 13 × 10
-4
and
3.8 × 10
-4
mol
CO
g
Au
-1
·s
-1
at 80°C for Au/Mg(OH)
2
and
Au/MgO catalysts, respectively, both prepared by CP,
while Haruta’s group obtained 1.2 × 10
-4
mol
CO
g

Au
-1
·s
-
1
at-70°CforaAu/Mg(OH)
2
prepare d by DP [46]. Our
values for the DIM catalyst, ranging from 1.7 × 10
-4
to
3.8 × 10
-4
mol
CO
g
Au
-1
·s
-1
at 25 and 1 00°C (Figure 5b),
respectively, are similar to the literature value obtained
with Au/MgO catalyst, but below the value obtained for
the Au/Mg(OH)
2
material [5]. Nevertheless, it was
shown that the heat-treated samples (that have MgO
instead of Mg(OH)
2
) have similar activity, meaning that

the here reported DIM materials have similar catalytic
activity to those reported in the literature, although with
doubl e Au content (1% Au, instead of 0.5% Au reported
in [5]). LPRD and US showed smaller values.
Conclusions
Au was loaded (1 wt%) on a commercial MgO support
by three different methods: double impregnation (DIM),
liquid-phase reductive deposition (LPRD) and ultrasoni-
cation (US). CO oxidation was used as a test reaction to
compare the catalytic activity. The best results were
obtained with the DIM method, which showed activities
of 1.7 × 10
-4
to 3.8 × 10
-4
mol
CO
g
Au
-1 ·
s
-1
at 25 and
100°C. This can be explained in terms of t he nanoparti-
cle size, well known to be related with the catalytic
activity of gold catalysts . This sample had the narrowest
size range (2-12 nm) and the lowest average size (5.4
nm). Samples prepared by other methods (LPRD and
US) showed broader size ranges (3-12 nm) and larger
average gold sizes (> 6.6 nm).

Abbreviations
CP: co-precipitation; DP: deposition-precipitation; DIM: double impregnation;
EDXS: energy-dispersive X-ray spectroscopy; HRTEM: high-resolution
transmission electron microscopy; LPRD: liquid-phase reductive deposition;
TPR: temperature-programmed reduction; US: ultrasonication; XRD: X-ray
diffraction.
Acknowledgements
Authors thank Fundação para a Ciência e a Tecnologia (FCT), Portugal, for
financial support (CIENCIA 2007 program for SAC), and project PTDC/EQU-
ERQ/101456/2008, financed by FCT and FEDER in the context of Programme
COMPETE. We also acknowledge CONACYT Grant No 79062, PAPIT-UNAM
IN100908 (Mexico) and by RFBR grant 09-03-00347-a (Russia).
Author details
1
Laboratório de Catálise e Materiais, Departamento de Engenharia Química,
Faculdade de Engenharia, Universidade do Porto, 4200-465 Porto, Portugal
2
Universidad Nacional Autónoma de México, Centro de Nanociencias y
Nanotecnología, Carretera Tijuana-Ensenada, 22800 Ensenada, Baja California,
Mexico
3
Tomsk Polytechnic University, 30, Lenin Avenue, Tomsk 634050,
Russia
4
Universidade de Trás-os-Montes e Alto Douro, CQVR Centro de
Química-Vila Real, Departamento de Química, 5001-911 Vila Real, Portugal
Authors’ contributions
SACC conceived the research work, prepared the catalysts, performed the
activity tests, carried out the analysis and interpretation of the experimental
results and drafted the manuscript. J.L. Figueiredo provided the means for

the realization of this work and contributed to the writing. N.B. and A.P.
performed the HRTEM experiments, while P.B.T. and L.S.G.F. carried out the
XRD analyses. All authors read and approved the final manuscript.
Competing interests
The authors declare that they have no competing interests.
Received: 31 August 2010 Accepted: 22 June 2011
Published: 22 June 2011
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doi:10.1186/1556-276X-6-435
Cite this article as: Carabineiro et al.: Gold nanoparticles supported on
magnesium oxide for CO oxidation. Nanoscale Research Letters 2011
6:435.
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