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: 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 Carabineiro et al. Nanoscale Research Letters 2011, 6:435 http://www.nanoscalereslett.com/content/6/1/435 © 2011 Carabineiro et al; licensee Springer. This is an Open Acc ess article distribu ted under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), 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 http://www.nanoscalereslett.com/content/6/1/435 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.             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 http://www.nanoscalereslett.com/content/6/1/435 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                               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.             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 http://www.nanoscalereslett.com/content/6/1/435 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. 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Submit your manuscript to a journal and benefi t from: 7 Convenient online submission 7 Rigorous peer review 7 Immediate publication on acceptance 7 Open access: articles freely available online 7 High visibility within the fi eld 7 Retaining the copyright to your article Submit your next manuscript at 7 springeropen.com Carabineiro et al. Nanoscale Research Letters 2011, 6:435 http://www.nanoscalereslett.com/content/6/1/435 Page 6 of 6 . 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 6:435 http://www.nanoscalereslett.com/content/6/1/435 Page 5 of 6 19. Broqvist P, Molina LM, Gronbeck H, Hammer B: ’Promoting and poisoning effects of Na and Cl coadsorption on CO oxidation over MgO -supported Au nanoparticles Gold nanoparticles supported on magnesium oxide for CO oxidation. Nanoscale Research Letters 2011 6:435. Submit your manuscript to a journal and benefi t from: 7 Convenient online submission 7