Gold nanoparticles supported on magnesium oxide for CO oxidation
© Carabineiro et al; licensee Springer. 2011
Received: 31 August 2010
Accepted: 22 June 2011
Published: 22 June 2011
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.
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 .
However, Margitfalvi et al.  prepared Au/MgO catalysts with high activity for low temperature CO oxidation. 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 carbonate, which is responsible for deactivation . 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 treatment in flowing helium at 473 K, during which the original 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 .
Heinz et al.  showed that small clusters of gold (Au20 and 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  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.  proved also by DFT calculations that Cl was a poison for Au/MgO catalysts in CO oxidation, while Na was a promotor. Goodman and co-workers  showed a direct correlation between the concentration of F-centre 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  found that Au/MgO catalysts supported on alumina were extremely active, 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 oxidation 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-precipitation (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)  and liquid phase reductive deposition (LPRD) , 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.  dealing with TiO2 samples and our previous work on CeO2[34, 35] and ZnO  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 . As far as we know, LPRD has only been used by Sunagawa et al.  to prepare Pt and Au catalysts on Fe2O3, FeOOH, ZrO2 and TiO2 supports, and also by us for CeO2 and TiO2. US was only used by our group to prepare very active Au/ZnO catalysts .
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 applications, namely in CO removal from H2 streams for fuel cells and gas sensing [1–4, 34, 36, 37].
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) , liquid phase reductive deposition (LPRD)  and ultrasonication (US) . Briefly, the first method (DIM) consists in impregnating the support with an aqueous solution of the gold precursor (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) consists 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 solution [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 . US consists in dissolving the Au precursor in water and methanol, and sonicating for 8 h, reducing gold . In all these methods, a washing procedure is carried out to eliminate residual chloride, which is well known to cause sinterization of Au nanoparticles, turning them inactive [1–4, 37]. Further details can be found elsewhere [34–38].
The materials were analysed by adsorption of N2 at -196°C in a Quantachrom NOVA 4200e apparatus.
Temperature-programmed reduction (TPR) experiments were performed in a fully automated AMI-200 Catalyst Characterization Instrument (Altamira Instruments, Pittsburgh, PA, USA), equipped with a quadrupole 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'Celerator detector and secondary monochromator. Rietveld refinement with PowderCell software  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 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].
Results 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.
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.
Gold nanoparticles of 6 nm were reported in literature for Au/MgO catalysts prepared by CP . Smaller values of approximately 4 nm were however obtained by CP and DP on Mg(OH)2[5, 41, 42]. Sizes of approximately 4 nm were also obtained for Au on MgO prepared from a gold complex . Au nanoparticles smaller than 5 nm were obtained on MgO modified with ascorbic acid [9, 23]. Other techniques like impregnation produced gold particles of 8 nm on MgO . Values of approximately 9 nm were obtained for gold on MgO with cube morphology . Gold deposited on MgO/alumina yielded particles ranging from 2.7 to 4.6 nm [21, 22, 24, 44].
Semi-quantitative estimation of gold loading was performed by EDXS, approximately 0.9% being found for all samples.
It can be argued that there are gold catalysts that achieve full CO conversions already at room temperature, but it has to be taken into account that most studies in literature use 1% CO or less [1–4] (while we used 5% of this gas). Also, the majority of authors use higher loadings of Au [1–4] (while we used 1 wt%). Nevertheless, 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.  reported activities of 13 × 10-4 and 3.8 × 10-4 molCOgAu -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 molCOgAu -1 · s-1 at -70°C for a Au/Mg(OH)2 prepared by DP . Our values for the DIM catalyst, ranging from 1.7 × 10-4 to 3.8 × 10-4 molCOgAu -1 · s-1 at 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 . 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 ). LPRD and US showed smaller values.
Au was loaded (1 wt%) on a commercial MgO support by three different methods: double impregnation (DIM), liquid-phase reductive deposition (LPRD) and ultrasonication (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 nanoparticle 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).
energy-dispersive X-ray spectroscopy
high-resolution transmission electron microscopy
liquid-phase reductive deposition
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).
- Bond GC, Thompson DT: 'Gold-catalysed oxidation of carbon monoxide'. Gold Bull 2000, 33: 41.View Article
- Bond GC, Louis C, Thompson DT: Catalysis by Gold. London: Imperial College Press; 2006.
- Carabineiro SAC, Thompson DT: Catalytic Applications for Gold Nanotechnology, in Nanocatalysis. Berlin, Heidelberg, New York: Springer; 2007.
- Carabineiro SAC, Thompson DT: Gold catalysis. In Gold: Science and Applications. Boca Raton, London, New York: CRC Press, Taylor and Francis Group; 2010.
- Schubert MM, Hackenberg S, Van Veen AC, Muhler M, Plzak V, Behm RJ: 'CO oxidation over supported gold catalysts-"inert" and "active" support materials and their role for the oxygen supply during reaction'. J Catal 2001, 197: 113. 10.1006/jcat.2000.3069View Article
- Gasior M, Grzybowska B, Samson K, Ruszel A, Haber J: 'Oxidation of CO and C-3 hydrocarbons on gold dispersed on oxide supports'. Catal Today 2004, 91(2):131.View Article
- Ivanova S, Pitchon V, Petit C: 'Application of the direct exchange method in the preparation of gold catalysts supported on different oxide materials'. J Mol Catal a 2006, 256: 278. 10.1016/j.molcata.2006.05.006View Article
- Glaspell G, Hassan HMA, Elzatahry A, Abdalsayed V, El-Shall MS: 'Nanocatalysis on supported oxides for CO oxidation'. Topics Catal 2008, 47: 22. 10.1007/s11244-007-9036-1View Article
- Margitfalvi JL, Fasi A, Hegedus M, Lonyi F, Gobolos S, Bogdanchikova N: 'Au/MgO catalysts modified with ascorbic acid for low temperature CO oxidation'. Cataly Today 2002, 72: 157. 10.1016/S0920-5861(01)00489-8View Article
- Hao Y, Mihaylov M, Ivanova E, Hadjiivanov K, Knozinger H, Gates BC: 'CO oxidation catalyzed by gold supported on MgO: Spectroscopic identification of carbonate-like species bonded to gold during catalyst deactivation'. J Catal 2009, 261: 137. 10.1016/j.jcat.2008.11.005View Article
- Hao YL, Gates BC: '' Activation of dimethyl gold complexes on MgO for CO oxidation: Removal of methyl ligands and formation of catalytically active gold clusters. J Catal 2009, 263: 83. 10.1016/j.jcat.2009.01.015View Article
- Heiz U, Sanchez A, Abbet S, Schneider WD: 'The reactivity of gold and platinum metals in their cluster phase'. Eur Phys J D 1999, 9: 35. 10.1007/s100530050395View Article
- Heiz U, Sanchez A, Abbet S, Schneider WD: 'Tuning the oxidation of carbon monoxide using nanoassembled model catalysts'. Chem Phys 2000, 262: 189. 10.1016/S0301-0104(00)00268-8View Article
- Yoon B, Hakkinen H, Landman U, Worz AS, Antonietti JM, Abbet S, Judai K, Heiz U: 'Charging effects on bonding and catalyzed oxidation of CO on Au-8 clusters on MgO'. Science 2005, 307: 403. 10.1126/science.1104168View Article
- Guzman J, Gates BC: 'Simultaneous presence of cationic and reduced gold in functioning MgO-supported CO oxidation catalysts: Evidence from X-ray absorption spectroscopy'. J Phys Chem B 2002, 106: 7659. 10.1021/jp020584mView Article
- Guzman J, Gates BC: 'Oxidation states of gold in MgO-supported complexes and clusters: Characterization by X-ray absorption spectroscopy and temperature-programmed oxidation and reduction'. J Phys Chem B 2003, 107: 2242. 10.1021/jp026976aView Article
- Guzman J, Gates BC: 'Catalysis by supported gold: Correlation between catalytic activity for CO oxidation and oxidation states of gold'. J Am Chem Soc 2004, 126: 2672. 10.1021/ja039426eView Article
- Molina LM, Hammer B: 'Active role of oxide support during CO oxidation at Au/MgO'. Phys Rev Lett 2003, 90: 206102.1.View Article
- 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'. J Catal 2004, 227: 217. 10.1016/j.jcat.2004.07.009View Article
- Yan Z, Chinta S, Mohamed AA, Fackler JP, Goodman DW: 'The role of F-centers in catalysis by Au supported on MgO'. J Am Chem Soc 2005, 127: 1604. 10.1021/ja043652mView Article
- Grisel RJH, Nieuwenhuys BE: 'Selective oxidation of CO over supported Au catalysts'. J Catal 2001, 199: 48. 10.1006/jcat.2000.3121View Article
- Szabo EG, Hegedus M, Szegedi A, Sajo I, Margitfalvi JL: 'CO oxidation over Au/Al 2 O 3 catalysts modified by MgO'. React Kinetics Catal Lett 2005, 86: 339. 10.1007/s11144-005-0330-2View Article
- Szabo EG, Tompos A, Hegedus M, Szegedi A, Margitfalvi JL: 'The influence of cooling atmosphere after reduction on the catalytic properties of Au/Al 2 O 3 and Au/MgO catalysts in CO oxidation'. Appl Catal a 2007, 320: 114.View Article
- Szabo EG, Hegedus M, Lonyi F, Szegedi A, Datye AK, Margitfalvi JL: 'Preparation, characterization and activity of Au/Al 2 O 3 catalysts modified by MgO'. Catal Commun 2009, 10: 889. 10.1016/j.catcom.2008.12.019View Article
- Haruta M, Kobayashi T, Sano H, Yamada N: 'Novel gold catalysts for the oxidation of carbon monoxide at a temperature far below 0-degrees-C'. Chem Lett 1987., 405:
- Haruta M, Yamada N, Kobayashi T, Iijima S: 'Gold catalysts prepared by coprecipitation for low-temperature oxidation of hydrogen and of carbon monoxide. J Catal 1989, 115: 301. 10.1016/0021-9517(89)90034-1View Article
- Tanielyan SK, Augustine RL: 'Effect of catalyst pretreatment on the oxidation of carbon-monoxide over coprecipitated gold catalysts'. Appl Catal A 1992, 85: 73. 10.1016/0926-860X(92)80130-5View Article
- Hutchings GJ, Siddiqui MRH, Burrows A, Kiely CJ, Whyman R: 'High-activity Au/CuO-ZnO catalysts for the oxidation of carbon monoxide at ambient temperature'. J Chem Soc Faraday Trans 1997, 93: 187. 10.1039/a606482eView Article
- Solsona BE, Garcia T, Jones C, Taylor SH, Carley AF, Hutchings GJ: 'Supported gold catalysts for the total oxidation of alkanes and carbon monoxide'. Appl Catal A 2006, 312: 67.View Article
- Pillai UR, Deevi S: 'Room temperature oxidation of carbon monoxide over copper oxide catalyst'. Appl Catal B 2006, 64: 146. 10.1016/j.apcatb.2005.11.005View Article
- Aguilar-Guerrero V, Gates BC: 'Kinetics of CO oxidation catalyzed by supported gold: A tabular summary of the literature'. Catal Lett 2009, 130: 108. 10.1007/s10562-009-9906-1View Article
- Bowker M, Nuhu A, Soares J: 'High activity supported gold catalysts by incipient wetness impregnation'. Catal Today 2007, 122: 245. 10.1016/j.cattod.2007.01.021View Article
- Sunagawa Y, Yamamoto K, Takahashi H, Muramatsu A: 'Liquid-phase reductive deposition as a novel nanoparticle synthesis method and its application to supported noble metal catalyst preparation'. Catal Today 2008, 132: 81. 10.1016/j.cattod.2007.12.008View Article
- Carabineiro SAC, Silva AMT, Dražić G, Tavares PB, Figueiredo JL: 'Gold nanoparticles on ceria supports for the oxidation of carbon monoxide'. Catal Today 2010, 154: 21. 10.1016/j.cattod.2010.01.036View Article
- Carabineiro SAC, Bastos SST, Órfão JJM, Pereira MFR, Delgado JJ, Figueiredo JL: 'Exotemplated ceria catalysts with gold for CO oxidation'. Appl Catal A 2010, 381: 150. 10.1016/j.apcata.2010.04.001View Article
- Carabineiro SAC, Machado BF, Bacsa RR, Serp P, Dražić G, Faria JL, Figueiredo JL: 'Catalytic performance of Au/ZnO nanocatalysts for CO oxidation'. J Catal 2010, 273: 191. 10.1016/j.jcat.2010.05.011View Article
- Carabineiro SAC, Silva AMT, Dražić G, Tavares PB, Figueiredo JL: 'Effect of chloride on the sinterization of Au/CeO2 catalysts'. Catal Today 2010, 154: 293. 10.1016/j.cattod.2009.12.017View Article
- Santos VP, Carabineiro SaC, Tavares PB, Pereira MFR, Órfão JJM, Figueiredo JL: 'Oxidation of CO, ethanol and toluene over TiO2 supported noble metal catalysts'. Appl Catal B 2010, 99: 198. 10.1016/j.apcatb.2010.06.020View Article
- Kraus W, Nolze G:PowderCell for Windows. [http://ccp14.minerals.csiro.au/ccp/web-mirrors/powdcell/av/v1/powder/ecell.html]
- Radhaa AV, Vishnu Kamatha P, Subbanna GN: 'Disorder in layered hydroxides: synthesis and DIFFaX simulation studies of Mg(OH) 2 '. Mater Res Bull 2003, 38: 731. 10.1016/S0025-5408(03)00070-9View Article
- Cunningham DAH, Vogel W, Haruta M: 'Negative activation energies in CO oxidation over an icosahedral Au/Mg(OH) 2 catalyst'. Catal Lett 1999, 63: 43. 10.1023/A:1019088131252View Article
- Choudhary VR, Patil VP, Jana P, Uphade BS: 'Nano-gold supported on Fe 2 O 3 : A highly active catalyst for low temperature oxidative destruction of methane green house gas from exhaust/waste gases'. Appl Catal A 2008, 350: 186. 10.1016/j.apcata.2008.08.008View Article
- Lee JY, Schwank J: 'Infrared spectroscopy study of NO reduction by H 2 on supported gold catalysts'. J Catal 1986, 102: 207. 10.1016/0021-9517(86)90155-7View Article
- Margitfalvi JL, Hegedus M, Szegedi A, Sajo I: 'Modification of Au/Mgo catalysts used in low temperature CO oxidation with Mn and Fe'. Appl Catal a 2004, 272: 87. 10.1016/j.apcata.2004.05.035View Article
- Parmaliana A, Arena F, Frusteri F, Giordano N: 'Temperature-programmed reduction study of NiO-MgO interactions in magnesia-supported Ni catalysts and NiO-MgO physical mixture'. J Chem Soc Faraday Trans 1990, 86: 2663. 10.1039/ft9908602663View Article
- Cunningham DAH, Vogel W, Kageyama H, Tsubota S, Haruta M: 'The relationship between the structure and activity of nanometer size gold when supported on Mg(OH) 2 '. J Catal 1998, 177: 1. 10.1006/jcat.1998.2050View Article
This article is published under license to BioMed Central Ltd. 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 any medium, provided the original work is properly cited.