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 δ-Al2O3 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, X-ray diffraction, energy-dispersive X-ray spectrometry and transmission electron microscopy. Both samples prepared by PM and IC showed a mixture of the δ-alumina 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 0.62 and 1.34 mol CO h−1 g −1Au , respectively, which are larger than those found in the literature for doped γ-alumina samples.
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Introduction
It 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/γ-Al2O3 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 O-vacancies 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 Au-support interface (Bond and Thompson 1999, 2000, 2009; Grisel and Nieuwenhuys 2001b, 2007b; Bond et al. 2006).
Most 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 1:15 (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), fuel-cells, 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, 2010).
Materials and methods
Commercial alumina (labelled as Al2O3 gamma phase) from Sigma-Aldrich was used (however, as it will be explained later, strangely, the phase present was δ-Al2O3 and not γ-Al2O3). 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), Riedel-de 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 Sigma-Aldrich), 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 cases.
Gold 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 1:4 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 supports.
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) using around 100 mg of sample with a heating rate of 10°C/min. X-ray 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 = 0.154 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. Energy-dispersive X-ray spectrometry (EDXS) was also used. X-ray photoelectron spectroscopy (XPS) analysis was performed with a VG Scientific ESCALAB 200A spectrometer using Al Kα radiation (1,486.6 eV) to determine the Au oxidation state of samples.
Catalytic activity measurements for CO oxidation were performed using a continuous-flow 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 −1Au ) 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).
Results and discussion
X-ray diffraction
Figure 1a shows the XRD results obtained for the commercial alumina sample. It was shown that, strangely, the phase present was δ-alumina (PDF 00-046-1131, Tetragonal, P-4m2; Repelin and Husson 1990; De Souza Santos et al. 1996, 2000), instead of the expected γ-alumina mentioned on the label. According to De Souza Santos et al. (1996, 2000), δ-Al2O3 is produced from γ-Al2O3, being the transformation pseudomorphic either from hexagonal laths of boehmite or from fibrillar pseudoboehmite as precursors of γ-Al2O3.
XRD spectra of commercial Al2O3 with crystal planes (Miller indexes) identified for δ-alumina phase (a) and of the alumina samples doped with Ce (b), Co (c), Cu (d), Fe (e), La (f), Mg (g), Mn (h), Ni (i) and Zn (j) prepared by IC. The arrows in figures (b–j) indicate peaks of CeO2 (b), Co3O4 (c), CuO (d), Fe3O4 (e), La2O3 (f), MgO (g), MnO2 (h), NiO2 (i) and ZnO (j); other peaks are from δ-alumina
The oxide/alumina materials were also analysed by XRD (Figs. 1, 2). Samples prepared by PM were a mixture of the alumina phase with the respective metal oxide, as seen in Table 1 and Fig. 2. For the IC samples (Fig. 1), oxide phases were also detected, in most cases similar to those of PM (exceptions are samples loaded with Ni, La and Fe, where CuO, NiO2, La2O3 and Fe3O4 were found instead of NiO, La(OH)3 and Fe2O3), so this method also originated aluminas doped with metal oxides. In general, the particle sizes of the oxide phases were larger in the PM samples, when compared with the IC materials. The particle size of the alumina phase increased slightly upon loading by IC, and remained the same when PM method was used. In some cases, the particle size could not be determined. No aluminate phases were detected. Unlike what was found by other authors, that could not unambiguously identify the formal oxidation state of the oxides doped on alumina and concluded that presumably they had a mixture of oxides (Grisel and Nieuwenhuys 2001b; Grisel et al. 2001; Gluhoi et al. 2003), we detected only one single oxide phase per sample. As for the Au/metal oxide/alumina samples, gold was not detected by XRD, possibly due to the low loading and low particle size, as happened in previous works dealing with other supports, that had similar values of loading and particle sizes (Carabineiro et al. 2010d, h, 2011b).
XRD spectra of the alumina samples doped with Fe (a), La (b) and Ni (c) prepared by PM. The arrows indicate peaks of Fe2O3 (a), La(OH)3 (b) and NiO (c); other peaks are from δ-alumina
Energy-dispersive X-ray spectrometry
Energy-dispersive X-ray spectrometry confirmed the ~7% metal loading of the several metals on the alumina samples (Table 2). Both methods were efficient in terms of metal addition to Al2O3. Gold loading did not change the Ce, Co, Cu, Fe, La, Mg, Mn, Ni or Zn amounts, as expected. For simplicity, those similar values are not shown in Table 2, instead the gold loading results obtained by the same technique are presented between parentheses.
Textural characterisation
The characterisation results obtained by N2 adsorption at −196°C for the PM and IC alumina samples are displayed in Table 2. It can be seen that the surface areas of the IC samples are higher than those of the PM materials, with the exception of samples containing Ce and Ni. Loading the alumina samples with gold did not significantly change the surface area (those results are shown between parentheses in Table 2). The values reported in the literature for γ-alumina are 275 and 260 m2/g for Au/γ-alumina (prepared by deposition-precipitation) (Gluhoi et al. 2003, 2004, 2005a, 2006ab; Gluhoi and Nieuwenhuys 2007a, b), which are higher than the values of 171 and 169 m2/g found in this work for δ-alumina. A larger decrease in the surface area after gold addition was found for those authors, most likely due to the higher gold loading used (5% Au, while we used 1% Au). The values reported in the literature for the γ-alumina samples doped with Ce, Co, Cu, Fe, Mg and Mn by IC with gold, were 218, 207, 212, 234, 224, and 222 m2/g, respectively (Gluhoi et al. 2003, 2005a, b, 2006a; Lin et al. 2004). Again those values are larger than ours, even considering the higher gold loading used by these authors. That can be due to the use of γ-alumina instead of δ-alumina.
Temperature programmed reduction
Temperature programmed reduction results showed that the commercial alumina had no significant reduction peaks, as expected. Some small negative peaks were found for this sample, which appeared also in the doped aluminas spectra. H2 consumption peaks were found for aluminas doped with metal oxides. A summary of their maximum temperatures can be found in Table 2, and graphics are displayed in Figures S1 and S2 of Electronic Supplementary Material. Two peaks related with CeO2 reduction were found for the Ce doped samples. As shown in previous works dealing with ceria supports (Andreeva et al. 2002; Fu et al. 2003, 2005; Jacobs et al. 2004; Lai et al. 2006; Carabineiro et al. 2010b, f, h, g), the two major peaks seen in the TPR profiles correspond to the reduction of bulk oxygen and the formation of cerium oxides of lower oxidation state (high temperature peak at ~750°C) and to the reduction of surface oxygen species (lower temperature peak at ~450°C), respectively. These peaks were observed around 720 and 440°C, in our case (Table 2).
According to the literature (Potoczna-Petru and Kepinski 2001), the peaks (Table 2) observed in the TPR spectra of Co3O4 are due to the reduction of this species to CoO and the further reduction of the latter to Co. One single peak (Table 2) corresponding to the reduction of CuO to Cu (Solsona et al. 2006; Carabineiro et al. 2011b) was found for samples doped with this metal.
According to the literature (Ilieva et al. 1997; Neri et al. 1999; Boccuzzi et al. 1999; Minico et al. 2000; Venugopal et al. 2003; Hua et al. 2004; Khoudiakov et al. 2005; PalDey et al. 2005; Milone et al. 2005, 2007; Silberova et al. 2006; Albonetti et al. 2010), the peak at ~400°C for Fe doped sample can be attributed to the reduction of hematite (Fe2O3) to magnetite (Fe3O4), while the peak at ~600°C arises from the reduction of Fe3O4 to FeO (wustite) and finally the peak at ~850°C is assigned to the reduction to Fe. Three main peaks corresponding to the those reductions were obtained at similar temperatures (Table 2), being the first more intense. Sample prepared by IC, that contains Fe3O4, according to the XRD results (Table 1), showed a very small peak at 410°C, while the second at 536°C was more intense, which clearly shows a larger amount of Fe3O4 (although Fe2O3 seems to be also present in a small amount, as shown by the smaller peak at lower temperature, although it was not detected by XRD).
Two small negative peaks (Table 2) were observed in the TPR profile of sample doped with La by PM, coming from La(OH)3 present on the sample, according to XRD results (Table 1), showing that no consumption of hydrogen occurred. Water release was also detected by MS at those same temperatures, most likely meaning that La2O3 is being formed through the reaction La(OH)3 → La2O3 + H2O, as shown in a previous work (Russo et al. 2006). The sample prepared by IC contains La2O3 (Table 1), which explains the positive peaks observed for this sample (Table 2), as expected from a previous work (Carabineiro et al. 2011b). As the reduction of sesquioxides is not easy at temperatures below 1,000°C (Adachi and Imanaka 1998), the H2 consumption peak measured can be attributed to the reduction of carbonate species to oxide (Haneda et al. 2002), as represented by the equation: La2O2CO3 + H2 → La2O3 + H2CO3. The Mg/alumina samples showed one reduction peak (Table 2). As MgO is not easily reducible (Fruehan and Martonik 1976; Parmaliana et al. 1990), maybe a similar mechanism to that of La sample is occurring. The TPR profile for samples doped with Mn showed two peaks (Table 2). According to what was seen in previous works (Hoflund et al. 1995; Ferrandon et al. 1999; Stobbe et al. 1999; Arena et al. 2001; PalDey et al. 2005; Xu et al. 2006; Ramesh et al. 2008; Liang et al. 2008; Wang et al. 2008, 2009; Carabineiro et al. 2010a), the reduction of MnO2 to Mn3O4 generates a peak at ~350°C, whereas reduction of Mn3O4 to MnO produces a peak at ~450°C, sometimes with some overlapping between the two. This matches well with the results obtained for the Mn/alumina samples. The characteristic green colour of MnO was observed after the TPR experiments confirming the presence of this oxide, as reported in other works (Ferrandon et al. 1999; Stobbe et al. 1999; Arena et al. 2001; Ramesh et al. 2008; Carabineiro et al. 2010a).
According to XRD, sample prepared by PM contains NiO (Table 1) that generates a peak between 200 and 400°C, corresponding to the reduction of NiO to Ni (Arena et al. 1990; Parmaliana et al. 1990; Kotsev and Ilieva 1993; Li and Chen 1995; Carabineiro et al. 2011b), explaining the result obtained (Table 2). Sample prepared by IC contains NiO2 (Table 1) therefore the extra peak at lower temperatures might be due to the reduction of NiO2 to NiO, explaining the two TPR peaks obtained. Finally the profiles of Zn/Al2O3 samples did not show any significant peaks, as expected (Carabineiro et al. 2010c).
Addition of gold made most peaks shift to lower temperature (Table 2), showing that its presence facilitated the reduction of the oxides, as expected (Carabineiro et al. 2010b, e, f, g, h). As expected from literature, addition of gold caused the peaks of Ce (Andreeva et al. 2002; Fu et al. 2003, 2005; Lai et al. 2006; Carabineiro et al. 2010a, b, e, g, h,, 2011b) and Fe (Ilieva et al. 1997; Neri et al. 1999; Boccuzzi et al. 1999; Minico et al. 2000; Hua et al. 2004; Khoudiakov et al. 2005; Milone et al. 2005, 2007; Solsona et al. 2006; Silberova et al. 2006; Albonetti et al. 2010) oxides to shift to much lower temperatures.
The presence of gold also shifted the first two peaks of Fe/alumina to lower temperatures (to ~100 and 290°C, respectively) as expected from literature (Ilieva et al. 1997; Neri et al. 1999; Minico et al. 2000; Venugopal et al. 2003; Hua et al. 2004; Khoudiakov et al. 2005; Milone et al. 2005, 2007; Solsona et al. 2006; Silberova et al. 2006; ). The reduction temperature of the first peaks is reported to decrease upon gold loading (Neri et al. 1999; Boccuzzi et al. 1999; Milone et al. 2007). The reduction to metallic iron, at higher temperatures, was less influenced by the presence of gold, as expected from literature (Neri et al. 1999; Minico et al. 2000; Solsona et al. 2006).
As the peaks area and shape with and without gold is nearly the same (as confirmed by area measurements by integration of curves, carried out with the program Origin), it seems that gold is in the metallic state (Carabineiro et al. 2010b, h).
X-ray photoelectron spectroscopy
In order to confirm the oxidation state of gold, selected samples were analysed by XPS (those which had the most interesting results for CO oxidation). Table 3 shows the results obtained, which were similar for all samples. Two peaks were obtained (Fig. 3), which are characteristic of gold in the reduced (Au0) state. Similar results were obtained for the other analysed samples (images not shown). Other authors also found gold in the reduced state in doped alumina materials, as reported in the literature (Grisel et al. 2000, 2001, 2002a, b; Grisel and Nieuwenhuys 2001a, b; Gluhoi et al. 2003, 2004, 2005a, b, 2006a, b; Lin et al. 2004; Gavril et al. 2006; Gluhoi and Nieuwenhuys 2007a, b; Lippits et al. 2007, 2008, 2009).
Au 4f XPS spectra with peak deconvolution of Zn- (top) and Fe- (bottom) doped alumina samples prepared by PM with gold
Transmission electron spectroscopy
Selected samples with gold were analyzed by TEM (those which had the most interesting results for CO oxidation). Some images are show in Fig. 4, with the gold nanoparticles seen as dark spots. As not many particles were found (around 20–30 particles, depending on samples), possibly due to the low loading, it was not possible to calculate accurate size distribution histograms, as also happened in earlier works of our group (Carabineiro et al. 2010a, b, h, g), however, the ranges found are displayed in Table 3.
TEM image of Fe/Al2O3 (a), La/Al2O3 (b), Ni/Al2O3 (c), Ce/Al2O3 (d), Cu/Al2O3 (e) and Zn/Al2O3 (f) prepared by PM with gold
It can be seen that the IC samples showed larger Au nanoparticles than PM. The possible reasons for this difference are not fully understood since, as it can be seen in Table 1, the phases detected were in most cases similar between PM and IC samples.
Comparing the size of gold particles on Fe/δ-alumina prepared by IC (11–14 nm) with the values reported in the literature (3–10 nm for Fe/γ-alumina) (Grisel and Nieuwenhuys 2001a; Grisel et al. 2001; Gluhoi et al. 2005b; Gluhoi and Nieuwenhuys 2007a), and gold on Zn/δ-alumina prepared by IC (13–16 nm) with literature (4–5 nm with Zn/γ-alumina) (Grisel and Nieuwenhuys 2001a; Grisel et al. 2001), it can be seen that our values are larger. Also the values of samples prepared by PM (the smallest obtained in this work) are larger than those reported in the literature for γ-alumina doped samples prepared by IC: the Fe/δ-alumina PM sample showed gold particle sizes of 8–12 nm [compared with 3–10 nm from literature for a Fe/γ-alumina IC sample (Grisel and Nieuwenhuys 2001a; Grisel et al. 2001; Gluhoi et al. 2005b; Gluhoi and Nieuwenhuys 2007a)], Ni/alumina had 11–15 nm [4–5 nm was reported in literature (Grisel and Nieuwenhuys 2001a; Grisel et al. 2001)], Cu/alumina had 12–16 nm [literature refers 3–7 nm (Grisel et al. 2001; Lin et al. 2004) and 18 nm (Grisel and Nieuwenhuys 2001a)], Zn/alumina had 7–13 nm [literature reports 4–5 nm (Grisel and Nieuwenhuys 2001a; Grisel et al. 2001)], Ce/alumina had 11–15 nm [literature: 1.7–5. 8 nm (Grisel et al. 2002a; Gluhoi et al. 2003, 2004, 2005b; Lin et al. 2004; Gluhoi and Nieuwenhuys 2007b; Lippits et al. 2007, 2008)]. Again, works from other authors used γ-alumina, while we had δ-alumina.
In this work, we did not analyse samples loaded with Co, Mg or Mn by TEM, but we believe that the gold size will be larger in these samples (since they were less active for CO oxidation as it will be seen ahead). Values in literature for γ-alumina samples doped with Co, Mg and Mn prepared by IC range from 5 to 10 nm (Grisel and Nieuwenhuys 2001a; Grisel et al. 2001; Gluhoi et al. 2005b, 2006a; Gluhoi and Nieuwenhuys 2007a), 2 to 4 nm (Grisel and Nieuwenhuys 2001b; Grisel et al. 2002a, b; Gluhoi et al. 2003, 2005a; Szabo et al. 2005), and 1.4–10 nm (Grisel and Nieuwenhuys 2001a, b; Grisel et al. 2001, 2002a; Gluhoi et al. 2003, b), respectively. To the best of our knowledge, La was never doped on alumina samples. Also, as far as we know, no work was performed using the δ-alumina phase.
Catalytic tests for CO oxidation
The catalysts with and without gold were tested for CO conversion. The results for the alumina sample were negligible. Those obtained for the doped aluminas were also very low with full conversions achieved only at ~500°C (not shown). Loading gold onto the samples caused the full CO conversion to occur at much lower temperatures, as expected from previous works (Santos et al. 2010; Carabineiro et al. 2010b, c, d, e, g, h, 2011a, b). These results can be found in Fig. 5. It can be seen that samples prepared by PM are much more active than those synthesised by IC. The best results were obtained for aluminas doped with Zn and Fe (Fig. 5a).
CO conversion (%) versus temperature for the doped alumina samples prepared by PM (a) and IC (b) with gold
Results from literature show that Mn is the metal that once doped into γ-alumina by IC, gives the best results, followed by Fe, Ni, Co, Zn and Cu as dopants (Grisel and Nieuwenhuys 2001a). However, in the present work, doping with Mn did not produce good results, when compared to doping with other metals. Nevertheless, the activity for the δ-alumina sample with Au doped with Mn by IC was 0.09 mol CO h−1 g −1Au (Table 4), which is very similar to the values calculated from literature (0.1–0.15 mol CO h−1 g −1Au ) for Au/MnO x /γ-alumina (Grisel and Nieuwenhuys 2001a; Grisel et al. 2002a). This is a good result, specially taking into consideration that those authors used 5% Au loading (while we used 1% Au) and that they had 2–3% CO in the feed (while we had 5% CO). Also the values obtained in literature for gold on γ-alumina only and doped with Co and Ni (0.02, 0.06 and 0.06 mol CO h−1 g −1Au , respectively) (Grisel and Nieuwenhuys 2001a) are similar to our values for Co and Ni doped δ-alumina (Table 3). Our activities for samples doped with Fe, Cu and Zn by IC (1.34, 0.22 and 0.62 mol CO h−1 g −1Au , as seen in Table 4), are higher than those calculated from literature (0.07, 0.03 and 0.04 mol CO h−1 g −1Au , respectively) (Grisel and Nieuwenhuys 2001a). The values obtained for samples prepared with PM are even higher, ranging from 0.12 to 2.34 mol CO h−1 g −1Au (Table 4).
The smaller gold nanoparticle size of samples prepared by PM, compared with those prepared by IC, obtained in this work (Table 3), explains the higher performance of the former, as activity is well known to be related with gold particle size (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, 2010; Carabineiro et al. 2010a, b, c, d, e, f, g, h, 2011a, b). Also the most active samples (Zn and Fe doped aluminas prepared by PM) were those that showed smaller gold nanoparticle sizes. Even with the larger values of gold nanoparticle size, compared to what was reported in the literature (as seen above), the catalysts obtained in this work are more active than similar materials reported in the literature. In an earlier work, it was found that smaller gold nanoparticles were found on a ceria support with larger particle size, when compared with other materials that had smaller size of CeO2 particles (Carabineiro et al. 2010h).
Since the addition of MO x , to Au/Al2O3 stabilizes the Au nanoparticles (Grisel and Nieuwenhuys 2001a, b; Grisel et al. 2002b; Gluhoi et al. 2005a, 2006b), it seems that in samples prepared by PM, the oxide has a better role of structural promoter than in those prepared by IC. Also since O2 activation proceeds via a Mars and van Krevelen mechanism (Grisel and Nieuwenhuys 2001a, b; Grisel et al. 2002b; Gluhoi et al. 2005a, 2006b), it seems that the commercial oxides that are simply mixed with the alumina support are more able to perform that role than those being formed by calcinations of the impregnated samples. IC procedure likely involves the formation of an oxide surface layer which is in a close interaction with the active phase. On the other hand, the PM samples might have a higher concentration of surface hydroxyl groups which imply the preferential interaction of the active phase with the alumina. This might explain the smaller size of Au particles in PM samples and their consequent higher catalytic activity.
Conclusions
This work showed that a simpler PM procedure is able to produce Ce-, Co-, Cu-, Fe-, La-, Mg-, Mn-, Ni- and Zn-doped δ-alumina samples with larger surface areas than the traditional IC method. When loaded with Au, PM materials are also more active for CO oxidation than IC samples. The best results were obtained with aluminas containing Zn and Fe prepared by PM, that showed the smallest gold nanoparticles sizes (7–13 nm and 8–12 nm, respectively). Those samples had room temperature activities for CO conversion of 0.62 and 1.34 mol CO h−1 g Au−1, respectively, which are larger than those found in the literature for doped γ-alumina samples. Gold was found to be in the metallic state. As the PM samples have a higher concentration of surface hydroxyl groups which might imply the preferential interaction of the active phase with the alumina, it might explain their smaller size of Au nanoparticles and consequent higher catalytic activity.
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Acknowledgments
Authors thank Fundação para a Ciência e a Tecnologia (FCT), Portugal, for financial support (CIENCIA 2007 program for SACC and project PTDC/EQU-ERQ/101456/2008, financed by FCT and FEDER in the context of Programme COMPETE). Authors are grateful to Evonik Spain (Mr. Juán M. Flores) for the free ZnO (VP AdNano 20) sample provided.
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Carabineiro, S.A.C., Tavares, P.B. & Figueiredo, J.L. Gold on oxide-doped alumina supports as catalysts for CO oxidation. Appl Nanosci 2, 35–46 (2012). https://doi.org/10.1007/s13204-011-0037-9
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DOI: https://doi.org/10.1007/s13204-011-0037-9