Gold on oxidedoped alumina supports as catalysts

Authors: Sónia A C Carabineiro Pedro B Tavares José L Figueiredo

Publish Date: 2011/10/18

Volume: 2, Issue: 1, Pages: 35-46

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Abstract

The effect of doping a commercial alumina support with metal oxides of Ce Co Cu Fe La Mg Mn Ni and Zn was investigated Doped δAl2O3 samples were obtained by simple physical mixture PM of the alumina with the desired commercial oxide and by traditional impregnation of alumina with precursor salts of the same metals followed by calcination IC The metal load 7 wt was the same in both cases Gold 1 wt was loaded using a liquid phase reductive deposition method The obtained materials were characterized by adsorption of N2 at −196°C temperature programmed reduction Xray diffraction energydispersive Xray spectrometry and transmission electron microscopy Both samples prepared by PM and IC showed a mixture of the δalumina phase with the respective metal oxide but the BET surface areas of the IC samples were in general higher than those of the PM materials The particle size of the oxide phases were larger for the PM samples than for the IC materials Nevertheless catalytic experiments for CO oxidation showed that PM samples were much more active than IC That could be explained by the size of gold nanoparticles well known to be related with catalytic activity that was lower in samples prepared by PM 7–16 nm than by IC 11–17 nm Gold was found to be in the metallic state The most active samples were aluminas containing Zn and Fe prepared by PM that had the smallest gold nanoparticles sizes 7–13 and 8–12 nm respectively and had room temperature activities for CO conversion of 062 and 134 mol CO h−1 g  Au −1 respectively which are larger than those found in the literature for doped γalumina samplesIt is well known from the literature that a careful preparation is crucial for gold to be active as a catalyst in order to obtain it in the form of nanoparticles well dispersed on the support Bond and Thompson 1999 2000 2009 Haruta 2003 2004 Hutchings and Haruta 2005 Bond et al 2006 Hashmi and Hutchings 2006 Carabineiro and Thompson 2007 2010 Hutchings et al 2008 Many supports have been used so far including doped aluminas Grisel et al 2000 2001 2002a b Grisel and Nieuwenhuys 2001a b Gluhoi et al 2003 2004 Lin et al 2004 Szabo et al 2005 2007 2008 2009 Gluhoi et al 2005a b 2006a b Gluhoi and Nieuwenhuys 2007a b Lippits et al 2007 2008 2009 Tornpos et al 2008 2010 Detailed studies on the effects of addition of earth alkali metals to Au/γAl2O3 catalysts reveal that the earth alkali metal oxide MO x where M is the metal acts as structural promoter stabilizing the Au nanoparticles preventing them from sintering under mild conditions and facilitating the O2 activation via Ovacancies or surface OH groups on oxide as it supplies active O possibly via Mars and van Krevelen redox cycles Grisel and Nieuwenhuys 2001b Grisel et al 2001 2002b Gluhoi et al 2005a 2006b Gluhoi and Nieuwenhuys 2007b The Au/MO x perimeter defined as the boundary between Au MO x and the gas phase may be crucial for O2 activation Bamwenda et al 1997 Tsubota et al 1998 Grisel and Nieuwenhuys 2001a Grisel et al 2002b It has also been suggested that the reaction solely takes place on the Au/MO x perimeter with CO adsorbed on Au and oxygen coming from MO x Nieuwenhuys 1993 Grisel and Nieuwenhuys 2001a Grisel et al 2002b or CO being activated on Au and on the Ausupport interface Bond and Thompson 1999 2000 2009 Grisel and Nieuwenhuys 2001b 2007b Bond et al 2006Most doped aluminas described in literature were obtained by impregnation of a metal oxide precursor solution into the alumina support followed by calcination between 350 and 450°C Grisel et al 2000 2001 2002a b Grisel and Nieuwenhuys 2001a b Gluhoi et al 2004 2005a 2006b Lin et al 2004 Szabo et al 2005 2007 2008 2009 Gluhoi and Nieuwenhuys 2007a b Lippits et al 2007 2008 2009 Tornpos et al 2008 2010 and were prepared using a metal/Al ratio of 115 Gluhoi et al 2005a Since this procedure is quite lengthy involves a calcination step and the results obtained are aluminas doped with metal oxides we decided to compare the results obtained with the “classical” doping method of impregnation and calcination IC with those obtained by a simpler method directly adding the metal oxide to the alumina support by physical mixture PM and test both types of materials for CO oxidation before and after gold addition Although simple and intensively studied CO oxidation reaction is still poorly understood and its mechanistic pathways are still uncertain therefore it is worth to be further investigated as it is an extremely important in pollution control CO removal fuelcells and gas sensing Bond and Thompson 1999 2000 2009 Haruta 2003 2004 Hutchings and Haruta 2005 Bond et al 2006 Hashmi and Hutchings 2006 Hutchings et al 2008 Carabineiro and Thompson 2007 2010Commercial alumina labelled as Al2O3 gamma phase from SigmaAldrich was used however as it will be explained later strangely the phase present was δAl2O3 and not γAl2O3 The effect of doping this support with metal oxides of Ce Co Cu Fe La Mg Mn Ni and Zn was investigated Doped alumina samples were obtained by simple physical mixture PM of the alumina with the desired oxide Commercial oxide samples were used from Fluka CeO2 Sigma Aldrich Co2O3 Fe2O3 La2O3 and NiO Riedelde Haën CuO Merck MgO MnO2 and Evonik Degussa ZnO Doped alumina samples were also obtained by traditional impregnation of Al2O3 with precursor salts nitrates of the same metals supplied by SigmaAldrich followed by calcination IC at 350°C as described in the literature Grisel et al 2000 2001 2002a b Grisel and Nieuwenhuys 2001a b 2007b Gluhoi et al 2004 2005a 2006b Lin et al 2004 Lippits et al 2007 2008 2009 The metal load 7 wt was the same in both casesGold was loaded onto all samples by using HAuCl4·3H2O as the gold precursor Alfa Aesar in order to achieve 1 wt Au A liquid phase reductive deposition LPRD method was used to load gold Sunagawa et al 2008 Santos et al 2010 Carabineiro et al 2011a b Briefly this procedure consists in mixing a solution of HAuCl4 with a solution of NaOH in a ratio of 14 in weight with stirring at room temperature The resulting solution was aged for 24 h in the dark at room temperature to complete the hydroxylation of Au3+ ions Then the appropriate amount of support was added to the solution After ultrasonic dispersion for 30 min the suspension was aged in the oven at ~100°C overnight The resulting solid was washed repeatedly with distilled water for chloride removal which is well known to cause sinterization of Au nanoparticles thus turning them inactive Bond and Thompson 1999 Bond et al 2006 Carabineiro and Thompson 2007 2010 Carabineiro et al 2010g Santos et al 2010 and again dried in the oven at ~100°C overnight and used without any further treatment As far as we know LPRD has only been used by Sunagawa et al 2008 to prepare Pt and Au catalysts on Fe2O3 FeOOH ZrO2 and TiO2 supports and by us for Au/CeO2 Carabineiro et al 2010g Au/CuO Carabineiro et al 2011b Au/La2O3 Carabineiro et al 2011b Au/MgO Carabineiro et al 2011a Au/NiO Carabineiro et al 2011b Au/TiO2 Santos et al 2010 and Au/Y2O3 Carabineiro et al 2011b catalysts To the best of our knowledge this is the first report on its use for alumina and doped alumina based supportsThe 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 AMI200 Catalyst Characterization Instrument Altamira Instruments using around 100 mg of sample with a heating rate of 10°C/min Xray diffraction XRD analysis was carried out in a PAN’alytical X’Pert MPD equipped with a X’Celerator detector and secondary monochromator Cu Kα l = 0154 nm 50 kV 40 mA Further details can be found elsewhere Carabineiro et al 2010a b c d e f h g Carabineiro et al 2011a b Santos et al 2010 Transmission electron microscopy TEM analyses were performed on a Leo 906 E apparatus at 100 kV Samples were prepared by ultrasonic dispersion in ethanol and placed on a copper grid for TEM analysis Energydispersive Xray spectrometry EDXS was also used Xray photoelectron spectroscopy XPS analysis was performed with a VG Scientific ESCALAB 200A spectrometer using Al Kα radiation 14866 eV to determine the Au oxidation state of samplesCatalytic activity measurements for CO oxidation were performed using a continuousflow reactor The catalyst sample weight was 200 mg and the feed gas 5 CO 10 O2 in He was passed through the catalytic bed at a flow rate of 50 cm3/min The composition of the outgoing gas stream was determined by gas chromatography Activities mol CO h−1 g  Au −1 were determined at room temperature after the steady state was reached Further details can be found elsewhere Carabineiro et al 2010a b c d e f h g 2011a b Santos et al 2010

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