This can be explained in terms of the nanoparticle size, well known to determine the catalytic activity of gold catalysts.. Gates and co-workers [10,11] also managed to produce a Au/MgO
Trang 1N A N O E X P R E S S Open Access
Gold nanoparticles supported on magnesium
oxide for CO oxidation
Sónia AC Carabineiro1*, Nina Bogdanchikova2, Alexey Pestryakov3, Pedro B Tavares4, Lisete SG Fernandes4and José L Figueiredo1
Abstract
Au was loaded (1 wt%) on a commercial MgO support by three different methods: double impregnation, liquid-phase reductive deposition and ultrasonication Samples were characterised by adsorption of N2 at -96°C,
temperature-programmed reduction, high-resolution transmission electron microscopy, energy-dispersive 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 Al2O3 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 high activity for low temperature CO
oxida-tion The activity of these catalysts 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(CH3)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 underwent rapid deactivation due to the formation of carbonate-like species on the support and on gold, but could be reactivated by treatment in flowing helium, which led to the removal of the carbonate-like species [10]
Heinz et al [12] showed that small clusters of gold (Au20and Au8) are active towards CO oxidation In fact, for Au8 clusters, it was found that the oxidation of CO
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 O2 can bind simultaneously to both metal centres (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: scarabin@fe.up.pt
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
© 2011 Carabineiro et al; licensee Springer This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in
Trang 2achieving 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/Al2O3 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 deposition (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 TiO2 samples and our previous work
on CeO2 [34,35] and ZnO [36] catalysts This method
represents an environmentally and economically more
favourable 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 Fe2O3, FeOOH, ZrO2
and TiO2 supports, and also by us for CeO2 [37] and
TiO2 [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 catalysts that has many potential
applica-tions, namely in CO removal from H2 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 N2, 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 (HAuCl4) and then with a solution of Na2CO3
that precipitates gold hydroxide within the pores of the
catalyst [32,34-36] The second procedure (LPRD)
con-sists of mixing a solution of HAuCl4 with a solution of
NaOH (with a ratio of 1:4 in weight) that hydroxylates
the Au3+ions, before the support is added to the
solu-tion [33,37,38] Au3+ions are reduced to metallic Au0
by electron transfer from coordinated OH- ions on the
surfaces of support particles through their catalytic action [33] US consists in dissolving the Au precursor
in water and methanol, and sonicating 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 N2 at -196°C in a Quantachrom NOVA 4200e apparatus Temperature-programmed reduction (TPR) experi-ments were performed in a fully automated 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
a PAN’alytical X’Pert MPD equipped with a X’Celera-tor detecX’Celera-tor and secondary monochromaX’Celera-tor 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 oxidation were performed using a continuous-flow reactor The catalyst 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 furnace equipped with a temperature controller Feed gas (5% CO, 10% O2 in He) was passed through the catalytic bed at a total flow rate of 50 ml · min-1 (in contrast 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]
Trang 3Results and discussion
Characterization of samples
BET surface area
The BET surface area obtained for the MgO sample by
N2 adsorption at -196°C was 32 m2 · 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 The
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 + H2O ® Mg(OH)2)
Similar results were obtained for the other loading
methods
The Au particle size could not be determined for any
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 size 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 observed in Figure
2b, c, d (MgO with Au loaded by DIM, LPRD and US, respectively), as the support 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 methods Gold particles are also observed with sizes ranging from 2 to 12 nm for DIM (Figures 2b, 3a) Other methods showed larger gold nanoparticle sizes between 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 slightly 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 obtained on MgO modified with ascorbic acid [9,23] Other techniques like impreg-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, approximately 0.9% being found for all samples
TPR TPR results are shown in Figure 4 for the pure MgO 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 (thin 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.
Figure 2 HRTEM images of the commercial MgO, pure (a) and loaded with 1% Au wt by DIM (b), LPRD (c) and US (d).
Trang 4into MgO, as discussed above, 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 However, water release was detected by mass spectrometry, most likely meaning that MgO is being formed (Mg(OH)2 ® MgO + H2O) 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 occur 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 to 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 majority of authors use higher
c
b
a
Figure 3 Size distribution histograms of Au nanoparticles 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.
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).
Trang 5loadings 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] reported activities of 13 × 10-4 and
3.8 × 10-4 molCOgAu-1 · s-1 at 80°C for Au/Mg(OH)2and
Au/MgO catalysts, respectively, both prepared by CP,
while Haruta’s group obtained 1.2 × 10-4
molCOgAu-1· s
-1
at -70°C for a Au/Mg(OH)2prepared by DP [46] Our
values for the DIM catalyst, ranging from 1.7 × 10-4to
3.8 × 10-4molCOgAu-1 · s-1at 25 and 100°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
double 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 molCOgAu-1 · s-1 at 25 and
100°C This can be explained in terms of the
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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