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Gold supported on metal oxides for volatile organic compounds total oxidation S.A.C. Carabineiro a,∗ , X. Chen a , O. Martynyuk b , N. Bogdanchikova b , M. Avalos-Borja b,1 , A. Pestryakov c , P.B. Tavares d , J.J.M. Órfão a , M.F.R. Pereira a , J.L. Figueiredo a a Laboratório de Catálise e Materiais, Laboratório Associado LSRE/LCM, Departamento de Engenharia Química, Faculdade de Engenharia, Universidade do Porto, 4200-465 Porto, Portugal b Universidad Nacional Autónoma de México, Centro de Nanociencias y Nanotecnología, Carretera Tijuana-Ensenada, 22800 Ensenada, Baja California, México c Tomsk Polytechnic University, 30, Lenin Avenue, Tomsk 634050, Russia d Universidade de Trás-os-Montes e Alto Douro, CQVR Centro de Química—Vila Real, Departamento de Química, 5001-911 Vila Real, Portugal

a r t i c l e

i n f o

Article history: Received 23 May 2014 Received in revised form 13 June 2014 Accepted 26 June 2014 Available online xxx Keywords: Gold Volatile organic compounds Oxidation Ethyl acetate Toluene

a b s t r a c t Au was loaded (1 wt.%) on different commercial oxide supports (CuO, Fe2 O3 , La2 O3 , MgO, NiO, Y2 O3 ) by a double impregnation method (DIM). Samples were characterised by N2 adsorption at −196 ◦ C, scanning electron microscopy, high-resolution transmission electron microscopy, selected area electron diffraction, energy dispersive X-ray spectrometry, high-angle annular dark-field imaging (Z-contrast), X-ray diffraction and temperature programmed reduction. The materials were tested for the total oxidation of two volatile organic compounds: ethyl acetate and toluene. Toluene was more difficult to oxidise than ethyl acetate. Gold loaded on CuO and NiO yielded the best catalysts, although there was a larger increase in catalytic activity upon gold loading for less active oxides like MgO and Y2 O3 . The oxidation state of gold does not seem to play a significant role in the catalytic activity, with the exception of Au/Y2 O3 where Au3+ was detected, leading to a decrease in the activity. The catalytic activity seems to be related with the reducibility of the support and the gold nanoparticle size, following a Mars-van Krevelen type of mechanism. The role of gold is to enhance the reducibility and reactivity of the surface of the oxide support and to increase the exchange rate between lattice and surface oxygen. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Volatile organic compounds (VOCs) are pollutants originated from painting, printing, petroleum refineries, fuel storage and motor vehicles that are dangerous to human health and the environment [1,2]. Catalytic oxidation is an environment friendly technology for VOC abatement that requires lower temperatures (around 250–500 ◦ C) and causes less NOx formation, compared to conventional thermal oxidation that requires high operation temperatures (650–1100 ◦ C) [3,4]. Metal oxides (like manganese dioxide [4,5], copper oxide [6–8], nickel oxide [6,8,9], iron oxide [8,10], cobalt oxide [8,10–12], among others [10,13]) have been used as heterogeneous catalysts for VOC total oxidation. Mixed

∗ Corresponding author. Tel.: +351 220414907; fax: +351 225081449. E-mail address: [email protected] (S.A.C. Carabineiro). 1 On leave at Instituto Potosino de Investigación Científica y Tecnológica (IPICyT), San Luis Potosi, S.L.P., México.

metal oxides [9,14–20], including perovskite [21–24] and cryptomelane type materials [25–27] have also been reported, with good results. It has been reported that, although metal oxide based catalysts are more resistant to poisoning phenomena, they generally are less active than supported noble metals in oxidizing VOC streams [25,28]. Common supports for metals include alumina [29], titania [14], zirconia [30], zeolites [6] and carbon based materials [7]. Platinum and palladium are the precious metals mostly used in such applications [4,31]. It is known that gold nanoparticles [1,32–35] and gold complexes [32,36–38] are excellent catalysts for oxidation reactions. Recent reviews analysed the growing literature dealing with the use of gold in the catalytic total oxidation of several VOC [1,2,33]. Other studies dealing with gold on oxide-based supports [1,5,34,39] and zeolites [40] were published recently. It is known that VOC oxidation over Au/metal oxide catalysts is governed by both the properties of the support and the (small) size of gold nanoparticles, which often interact in synergy [1,2,33].

http://dx.doi.org/10.1016/j.cattod.2014.06.034 0920-5861/© 2014 Elsevier B.V. All rights reserved.

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In previous works, we reported on the loading of gold on metal oxides and the performance of these catalysts for the oxidation of CO [13,41–48], which is an important reaction in pollution control, fuel-cells, and gas sensing [1,32–34]. Moreover, the study of CO oxidation is also important in this context, CO being a possible by-product in VOC oxidation (since the complete oxidation of the pollutant to CO2 does not always take place [13]). In the present work, six metal oxides, pure or loaded with gold, were tested in the total oxidation of ethyl acetate and toluene. To the best of our knowledge, no similar work has been published so far, apart from a comparison between gold on exotemplated CeO2 and Mnx Oy catalysts performed by our group [5]. Moreover, only a few studies in the literature compare the activities of gold on different supports for VOC oxidation, propene oxidation being often used as model reaction [49–51].

Energy-dispersive X-ray spectroscopy (EDS) confirmed the elemental composition and allowed the semi-quantitative estimation of gold loading. Surface species were studied by in situ FT-IR experiments performed on a Nicolet model Impact 400 FT-IR apparatus equipped with a deuterated triglycine sulfate (DTGS) detector. 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 K␣  = 0.154 nm, 50 kV, 40 mA; data recorded at a 0.017◦ step size, 100 s/step). Rietveld refinement with PowderCell software [52] 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 [5,8,19,24,41–48]. 2.4. Catalytic tests

2. Experimental 2.1. Oxide supports The following commercial supports were tested: CuO (Riedel-de Haën), Fe2 O3 (Sigma Aldrich), La2 O3 (Aldrich), MgO (Merck), NiO (Aldrich) and Y2 O3 (Merck). 2.2. Preparation of Au catalysts The supports were impregnated by the double impregnation method (DIM) [5,41–43,46–48] with an aqueous solution (5 × 10−3 M) of HAuCl4 ·3H2 O (Alfa Aesar) and then with an aqueous solution of Na2 CO3 (10−2 M) for chloride removal, under constant ultrasonic stirring. The slurry was then thoroughly washed with distilled water and dried in an oven at 120 ◦ C overnight. 2.3. Characterization techniques The materials were analysed by adsorption of N2 at −196 ◦ C, in a Quantachrome NOVA 4200e apparatus. Temperature programmed reduction (TPR) and temperature programmed desorption (TPD) experiments were performed in a fully automated AMI-200 Catalyst Characterization Instrument (Altamira Instruments), equipped with a quadrupole mass spectrometer (Dymaxion 200 amu, Ametek). Conventional transmission electron microscopy (TEM) measurements were performed with a JEOL 2010 microscope with a point-to-point resolution better than 0.19 nm, operated at 200 keV. High resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectroscopy (EDS) and Z-contrast measurements were done on a FEI Tecnai F30 instrument, operated at 300 keV. The Z-contrast images were collected using a high-angle annular dark-field detector (HAADF), in scanning transmission mode (STEM). The sample was mounted on a carbon 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 particle size and the particle size distribution were determined from measurements made on ∼100–300 particles, depending on the sample. X-ray photoelectron spectroscopy (XPS) analysis was performed with a VG Scientific ESCALAB 200A spectrometer using Al K␣ radiation (1486.6 eV) to determine Au oxidation states. The charge effect was corrected taking the C1s peak as a reference (binding energy of 285 eV). Surface analysis for morphological characterisation was carried out by SEM, using a FEI Quanta 400 FEG ESEM (15 keV) electron microscope. The sample powders were mounted on a double-sided adhesive tape and observed at different magnifications under two different detection modes, secondary and back-scattered electrons.

The catalytic reactions were performed in a U-shaped quartz tube fixed-bed reactor with 6 mm internal diameter, placed inside a temperature controlled electrical furnace, with a total air flow rate of 500 cm3 /min (measured at room temperature and atmospheric pressure), corresponding to a space velocity of 60,000 h−1 (determined in terms of total bed volume), having a composition of 1000 mgCarbon /m3 (∼466.7 ppmV) of ethyl acetate or ∼226 ppmV of toluene) [5,8,19,24,25]. About 50 mg of catalyst (with particle sizes between 0.2 and 0.5 mm) were used for each experiment. The catalyst was mixed thoroughly with an inert (SiC, carborundum) with particle sizes between 0.2 and 0.5 mm. The total volume of the mixture of catalyst sample and inert was about 0.5 cm3 . A pre-treatment in air was carried out before the catalytic reaction by heating from room temperature up to 350 ◦ C at 10 ◦ C/min. Two cycles of increasing (at a heating rate of 2 ◦ C/min) and decreasing temperature were performed for each catalyst. The extent of VOC oxidation was evaluated by continuously monitoring CO2 formation with a non-dispersive infrared (NDIR) sensor (Vaisala GMP222). The concentration of VOC in the effluent was also measured with a total VOC analyzer MiniRAE2000. In case of incomplete conversion of VOC into CO2 and H2 O, a portable CO sensor was also used to obtain information on CO formation at the maximum reaction temperature during experiments. The catalytic performance is presented as conversion into CO2 , XCO2 , obtained by the following equation: XCO2 =

FCO2 Fin,VOC

,

where Fin ,VOC is the inlet molar flow rate of VOC, FCO2 the outlet molar flow rate of CO2 , and  is the number of carbon atoms in the VOC molecule (for ethyl acetate,  = 4; for toluene,  = 7). 3. Results and discussion 3.1. Characterization of samples 3.1.1. SEM Fig. 1 shows SEM images of the metal oxide supports (gold nanoparticles on the Au/oxide materials were not observed by this technique). The CuO sample (Fig. 1a) has a heterogeneous appearance, consisting of larger dark pieces and smaller light “needles”. EDS performed in both structure types gave similar results. Fig. 1b shows the Fe2 O3 material. This sample is also heterogeneous in morphology since smaller pieces are observed along with larger “stone-blocks” with a porous structure. On the other hand, La2 O3 has a very homogeneous structure (Fig. 1c), with a “cotton-like” appearance. The MgO sample presents some “veils” on the surface, but the sample seems to have a rougher part underneath

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Fig. 1. SEM images of CuO (a), Fe2 O3 (b), La2 O3 (c), MgO (d), NiO (e) and Y2 O3 (f). The scale bar shown represents 2 ␮m.

(Fig. 1d). Again, EDS of both parts revealed similar results. NiO (Fig. 1e) has a more compact and porous structure resembling a “cauliflower”. Y2 O3 has a smoother surface with some defects (Fig. 1e). Other images showed pieces with various dimensions (not shown).

3.1.2. BET surface area Table 1 shows the BET surface areas obtained for the oxide supports by N2 adsorption at −196 ◦ C. The values for oxides of Cu and Ni are within those reported in literature [53,54]. Smaller values were obtained in this work for Y2 O3 , La2 O3 and Fe2 O3 supports, when compared with the literature [55–58]. Addition of gold to the oxides did not produce significant changes in the BET surface areas, as shown in Table 1.

3.1.3. XRD Fig. 2 and Table 1 show the XRD results obtained for the oxide supports (as received). Gold was not detected on the Au/oxide materials (the characteristic XRD reflection was absent). This can be due to the low loading (1 wt.%) and small size of Au particles present in these catalysts. It was shown that the identified phases for the supports are the respective oxides, as expected, with the exception of La2 O3 in which La(OH)3 was found (Fig. 2c). This phase was also detected in unsupported and supported La2 O3 catalysts by other authors and is formed mostly by the hydrolysis of La2 O3 during the exposure of the catalysts to atmospheric moisture [59]. The crystallite size of CuO is similar to what was found in literature by other authors [53,60], while the values of Y2 O3 and La2 O3 are larger than those found by Corma and co-workers for Y2 O3

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Table 1 BET surface areas of the oxide samples obtained by adsorption of N2 at −196 ◦ C, phases and crystallite sizes detected by XRD, and TPR peak onset (bold) and maxima (plain text) temperatures (data from [46–48]). Sample

BET area (m2 /g)

Phase detected (crystal system, space group, reference code in database)

Crystallite size (nm)

TPR peaks (◦ C)

CuO Au/CuO Fe2 O3 Au/Fe2 O3 La2 O3 Au/La2 O3 MgO Au/MgO

11 10 6 5 11 10 32 31

CuO (monoclinic C2/c, 01-080-0076) CuO (monoclinic C2/c, 01-080-0076) (hematite, ␣-Fe2 O3 , rhombohedral, R-3c, 01-073-2234) (hematite, ␣-Fe2 O3 , rhombohedral, R-3c, 01-073-2234) La(OH)3 (hexagonal, P63/m, 01-083-2034) La(OH)3 (hexagonal, P63/m, 01-083-2034) MgO (cubic, Fm-3m, 01-078-0430) Mg(OH)2 phase (hexagonal, P-3m1, 01-076-0667)—99% MgO (cubic, Fm-3m, 01-078-0430)—1% NiO (cubic Fm-3m, 00-047-1049) NiO (cubic Fm-3m, 00-047-1049) Y2 O3 (cubic, Ia-3, 01-083-0927) Y2 O3 (cubic, Ia-3, 01-083-0927)

25 24 63 63 45 42 42

170, 302 170, 295 220, 390, 650, 900, 1060 181, 380, 600, 950

NiO Au/NiO Y2 O3 Au/Y2 O3 a b

79 75 9 8

a

28 26 24 43 43

b b b b

240, 340 175, 297 ∼300, ∼500 ∼300, ∼500

Value not reliable (see text for details). Negative peaks.

powder [55,61] and by Mihaylov et al. for La2 O3 synthesized by base hydrolysis of lanthanum nitrate with NaOH [58], respectively. The identified phase for the unloaded MgO 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 (Fig. 2d) and 99% of this hydroxide phase was detected along with 1% MgO (28 nm). It was not possible to calculate the particle size of the Mg(OH)2 phase due to interstratification of hydrated phases, as also found by other authors [62], which makes it very difficult to simulate

Fig. 2. X-ray diffraction patterns of commercial CuO (a), Fe2 O3 (b), La2 O3 (c), MgO (d), NiO (e), Y2 O3 (f), as received (lower pattern) and loaded with 1 wt.% Au (upper pattern), with phases and respective crystal planes (Miller indexes) identified (data from [46–48]).

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Fig. 3. H2 -TPR profiles of commercial CuO (a), Fe2 O3 (b), La2 O3 (c), MgO (d), NiO (e), Y2 O3 (f), as received (thin lines) and loaded with 1 wt.% Au (solid lines). Data from [46–48].

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 + H2 O → Mg(OH)2 ) [47]. Hematite, ␣-Fe2 O3 (rhombohedral, R-3c, 01-073-2234) was identified for the commercial iron oxide material, according to other authors [63–65]. Addition of gold produced no structural changes in the iron oxide phase with respect to that of the parent support, as also found by other authors [64]. 3.1.4. TPR TPR results are shown in Fig. 3a–f, for as received commercial supports, and loaded with gold by DIM. One major peak is seen in the TPR profile of the CuO sample at 302 ◦ C, with a shoulder at 244 ◦ C (Fig. 3a). With the addition of gold, those temperatures decrease slightly to 295 and 238 ◦ C. According to literature, two reduction peaks are expected around these temperatures for CuO (with or without Au) which correspond to the CuO → Cu2 O and Cu2 O → Cu transitions [66]. TPR results shown in Fig. 3b indicate that iron oxide was reduced in several stages. According to the literature, the peak at ∼300 ◦ C can be attributed to the reduction of the hydroxylated iron oxide species [64,67,68] and the peak at ∼400 ◦ C to the reduction of hematite (Fe2 O3 ) to magnetite (Fe3 O4 ) [64,67–69]. At ∼600 ◦ C the

reduction of Fe3 O4 to FeO (wustite) occurs [67–69] and finally the reduction to Fe, above 800 ◦ C [68], but in some cases this last peak overlaps with the previous one [67]. In our case, we observe two peaks above 800 ◦ C, indicating that maybe the reduction of Fe3 O4 to FeO (or of FeO to Fe) possibly occurs in two stages. The presence of gold shifted the first two peaks to lower temperatures (Fig. 3b), as expected from literature [64,66–69]. The subsequent reduction to metallic iron, at higher temperatures, was less influenced by the presence of gold (Fig. 3b), as expected from literature [66,68], although some authors also report that this last peak can be shifted to lower temperatures by addition of gold [69] and, in case there is some overlapping with the FeO peak, it will disappear upon gold loading [67]. Negative TPR peaks were found for the La(OH)3 materials (since lanthanum hydroxide was detected instead of the expected oxide) with or without gold, as can be seen in Fig. 3c, indicating no consumption of hydrogen. Water release was detected by MS, most likely meaning that La2 O3 is being formed (La(OH)3 → La2 O3 + H2 O). In fact, a second TPR run produced the characteristic La2 O3 profile found in the literature for the oxide [70]. Pure MgO does not show any significant reduction peak in the studied range of temperatures (Fig. 3d), as expected from the literature [71]. When Au is supported on MgO, as discussed before, the

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Fig. 4. Images of gold nanoparticles supported on CuO (a-TEM), Fe2 O3 (b-TEM), La2 O3 (c-TEM), MgO (d-TEM), NiO (e-HAADF), Y2 O3 (f-HRTEM).

support is transformed into Mg(OH)2 , most likely by reaction with water. As can be seen in Fig. 3d (thick line), a large negative peak is observed in the TPR profile between ∼300 and ∼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 + H2 O). In fact, a sequent TPR run produced a profile with no peaks, as for the oxide, as expected from the literature [71]. Typical TPR profiles of NiO samples exhibit a main peak in the 220–370 ◦ C range, which is due to the Ni2+ → Ni0 transition [72,73]. In our case (Fig. 3e), a major peak is observed at ∼340 ◦ C, with a shoulder at ∼250 ◦ C. With the addition of gold, these temperatures decrease to ∼297 and ∼215 ◦ C, respectively. Some authors attributed the peak at the lowest temperature to the reduction of non-stoichiometric surface oxygen and the other one to the reduction of non-stoichiometric bulk oxygen in the oxide [72].

Fig. 3f shows the TPR profile of Y2 O3 , with or without Au. A very small peak is observed at ∼500 ◦ C, in both cases. A small peak at ∼630 ◦ C was also observed by other authors for Y2 O3 prepared from yttrium nitrate, which was attributed to the reduction of the oxide [74]. 3.1.5. Electron microscopy Samples loaded with Au were analysed by HRTEM and HAADF. EDS was used to confirm the presence of gold (∼1 wt.%) for all samples. HAADF and HRTEM results of supports loaded with Au are shown in Fig. 4. Gold particles are seen as darker spots in HRTEM and as bright dots in HAADF images. The size distribution histograms are displayed in Fig. 5, and Table 2 shows the respective average gold nanoparticle sizes and size ranges. Fig. 4a shows a representative image of the CuO support with gold nanoparticles. Analysing the size distribution histogram of

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Fig. 5. Size distribution histograms of gold nanoparticles on CuO (a), Fe2 O3 (b), La2 O3 (c), MgO (d), NiO (e) and Y2 O3 (f). Data from [46–48].

gold nanoparticles on this support, taken from several images (Fig. 5a) and Table 2, the gold size range is the same: 2–12 nm, with an average of 5.8 nm. Larger Au particles were observed by Hutchings et al. for Au/CuO catalysts prepared by co-precipitation (CP), where the mean Au particle size was 20–30 nm and there were many particles as large as 50 nm [75]. In contrast, Ko et al. obtained Au/CuO prepared by deposition-precipitation (DP) from Cu(OH)2 with an average gold particle size of 3.8 ± 0.5 nm [76].

Table 2 Average gold nanoparticle sizes and size ranges for the Au/oxides samples, obtained from measurements made on ∼100–300 particles (data from [46–48]). Au/oxide

Gold size range (nm)

Average gold particle size (nm)

Au/CuO Au/Fe2 O3 Au/La2 O3 Au/MgO Au/NiO Au/Y2 O3

2–12 4–20 2–12 2–12 2–8 2–10

5.8 11.8 5.9 5.4 4.8 5.5

Larger nanoparticles were found in the Au/Fe2 O3 catalyst (Fig. 4b) with an average of 11.8 nm. Gold on iron oxide prepared by CP, by several authors, showed mean diameters of the Au nanoparticles ranging from 2 to 15 nm [66,77–80]. Sizes ranging from 2.8 to 11 nm were reported for catalysts prepared by other authors by DP [69,81]. Au supported on the La2 O3 sample (Fig. 4c) yielded a size range from 2 to 12 nm. Similar results were obtained with MgO (Fig. 4d) with a smaller average particle size (5.4 nm, compared to 5.9 nm, respectively). In this image the “veils” (seen in Fig. 1d dealing with a SEM image of the support) are also visible. Haruta and co-workers reported sizes of 8 nm on La(OH3 ) using CP [82]. Gold nanoparticles of 6 nm were reported in literature for Au/MgO catalysts prepared by CP [81]. However, smaller values of ∼4 nm were obtained by CP and DP on Mg(OH)2 [81,83]. Other authors reported larger values [84]. Gold nanoparticles on NiO were imaged by HAADF (Fig. 4e). This technique is highly sensitive to variations in the atomic number of atoms in the sample, allowing Z-contrast images to be formed. Gold particles supported on NiO are seen as bright spots, as confirmed

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Table 3 Temperatures (◦ C) corresponding to 100% (T100 ), 90% (T90 ), 50% (T50 ), 20% (T20 ) and 10% (T10 ) conversion of ethyl acetate to CO2 over metal oxides alone and loaded with gold. Sample

T10 (◦ C)

T20 (◦ C)

T50 (◦ C)

T90 (◦ C)

T100 (◦ C)

CuO Au/CuO Fe2 O3 Au/Fe2 O3 La2 O3 Au/La2 O3 MgO Au/MgO NiO Au/NiO Y2 O3 Au/Y2 O3

229 229 294 280 295 269 296 246 270 245 335 307

240 240 308 293 312 278 319 256 282 256 359 322

259 255 328 315 338 295 349 267 309 278 400 345

290 272 355 341 367 315 >400 273 344 315 >400 363

311 289 370 354 384 325 >400 290 365 345 >400 379

by EDS (not shown). The Au/NiO sample shows a narrower size distribution (2-8 nm) and a smaller average gold particle size of 4.8 nm (Figs. 4e and 5e and Table 2). These are smaller than those found by Haruta et al. for Au/NiO catalysts prepared by CP and DP (∼8 nm) [77,82], but closer to the value of 3.2 ± 1 nm obtained by Behm and co-workers in samples prepared by the same method [81]. Gold particles are visible on the Y2 O3 support (Fig. 4f shows a closer detail of a gold nanoparticle imaged by HRTEM). An average of 5.5 nm was obtained. Gold nanoparticles of smaller size (4 nm) were found by Guzman and Corma on nanocrystalline Y2 O3 [55]. 3.1.6. XPS In order to determine the oxidation state of gold, Au/oxide samples were analysed by XPS. Fig. 6 compares the XPS Au 4f spectra of all gold containing catalysts, showing that the Au 4f XPS peak appears as a doublet, 4f7/2 and 4f5/2 , for Au/Fe2 O3 , Au/La2 O3 , Au/NiO and Au/CuO. The peak positions show that gold is in the reduced form (Au0 ) in Au/La2 O3 , Au/NiO and Au/CuO, and in the Au+ state on Au/Fe2 O3 . Au/Y2 O3 revealed three peaks, which seems to show that Au3+ is also present. Evaluation of Au/MgO was not possible due to superimposition of the Mg 2s peak. Fe 3s and Cu 3p peaks are also near the range expected for gold; therefore, Au 4d spectra were also measured and results are shown in Fig. 7. As this line is less intense than the Au 4f, a not very good spectrum was obtained for Au/Fe2 O3 , which was mostly noise. However for La2 O3 , NiO and CuO, it was confirmed that gold was in the reduced state. For Au/Y2 O3 , the spectrum obtained was also noisy, seeming to indicate also the presence of Au+ . For Au/MgO, again it was not possible to determine the oxidation state of gold, as there is also another superimposition with another Mg peak (Mg KLL). We tried to measure Au 4p, but as this line is even less intense than Au 4d, only noise was detected.

Table 4 Temperatures (◦ C) corresponding to 100% (T100 ), 90% (T90 ), 50% (T50 ), 20% (T20 ) and 10% (T10 ) conversion of toluene to CO2 over metal oxides alone and loaded with gold. Sample

T10 (◦ C)

T20 (◦ C)

T50 (◦ C)

T90 (◦ C)

T100 (◦ C)

CuO Au/CuO Fe2 O3 Au/Fe2 O3 La2 O3 Au/La2 O3 MgO Au/MgO NiO Au/NiO Y2 O3 Au/Y2 O3

245 239 315 273 375 330 >400 310 285 273 400 330

254 247 329 285 >400 353 >400 323 299 285 >400 343

272 264 358 307 >400 >400 >400 352 315 305 >400 367

309 293 394 330 >400 >400 >400 379 324 313 >400 394

330 315 345 >400 >400 >400 387 330 320 >400 400

Au/NiO, Au/La2 O3 , Au/Fe2 O3 and Au/Y2 O3 , which is also related with the reducibility of the samples, since it follows the order of the onset TPR peak for CuO, NiO and Fe2 O3 ; the remaining samples have negative peaks, as mentioned before. Au/MgO has a better performance than Au/La2 O3 . Certainly, comparisons between gold nanoparticle sizes need to be performed with caution when different supports are used (although they have been done in the past for CO [1,34,47,85] and VOC oxidation [5,49–51]). However, it is possible that the lower average particle size of Au/MgO (5.4 nm, cf. Table 2) plays a role, as it is known that lower Au sizes usually mean more active gold catalysts [1,5,32,34,42–47,49–51,85]. Au/Y2 O3 had the worst behaviour, although its gold particle size is not so high (5.5 nm, Table 2) and is in between that of Au/La2 O3 and Au/MgO. Since addition of gold improves catalytic activity to

3.2. Catalytic tests The catalytic performance of oxide and Au/oxide samples is shown in Figs. 8 and 9, Tables 3 and 4 and in Figures S1-S12 of Supporting Information (SI). To what concerns ethyl acetate conversion (Fig. 8 and Table 3), CuO was the most active support of the series, followed by NiO, Fe2 O3 , La2 O3 , MgO and Y2 O3 . This seems to follow the order of the TPR peak maxima (Table 1) for the first three oxides (others showed negative peaks). It can be seen that addition of gold improves catalytic activity, full conversion being achieved at lower temperatures. The exception is Au/CuO below 250 ◦ C, in which addition of gold does not affect the activity (see T10 and T20 , in Table 3 and Figure S1 from SI). The best improvements were observed for Au/MgO and Au/Y2 O3 . However, Au/CuO was the most active sample (in spite of its performance being similar to that of bare CuO), followed by Au/MgO,

Fig. 6. Au 4f XPS spectra of Au/oxides samples.

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Fig. 8. Catalytic performances of oxide (a) and Au/oxide (b) samples for the conversion of ethyl acetate. The curves are continuous; symbols are only to facilitate identification.

Fig. 7. Au 4d XPS spectra of Au/oxides samples.

a lower extent on Y2 O3 than it did on other supports of similar behavior (La2 O3 and MgO), we suggest that the difference comes from the presence of Au3+ on Y2 O3 , which was not seen in other samples. Gold on CeO2 and MnOx materials prepared in a previous work by exotemplating showed better results for ethyl acetate conversion [5]. The reason might be the “extra” oxygen content of those exotemplated oxides, compared to the commercial supports used now, since they had gold nanoparticles with sizes larger than those reported in the present work. To what concerns toluene conversion (Fig. 9 and Table 4), it can be seen that this VOC is more difficult to oxidise, as shown by the higher temperatures needed for full conversion, and as also shown in previous works [5,13]. In terms of supports without gold, CuO, NiO and Fe2 O3 showed the best results, and that is the same order as the TPR peak maxima (Table 1) of these materials, showing that toluene oxidation is very much related to reducibility of the samples. The results obtained with the Au/CuO catalyst are similar to those obtained in a previous work using Au/CeO2 and Au/MnOx catalysts [5]. La2 O3 , Y2 O3 and MgO showed insufficient activity in the present work. Once gold is loaded, the tendency is maintained, showing that reducibility of the support also plays an important role. After that, it seems that particle size matters, as the sequence is related to the particle size (Table 2). Au/Y2 O3 is more active than Au/La2 O3 , showing that the oxidation state of gold is not

Fig. 9. Catalytic performances of oxide (a) and Au/oxide (b) samples for the conversion of toluene. The curves are continuous; symbols are only to facilitate identification.

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so important for toluene oxidation, as also pointed out by several authors [2]. Au/Fe2 O3 was the only catalyst that had gold in the +1 state (although it was not possible to determine that of Au/MgO), and it seems that this is not so important for VOC conversion, but reducibility of the support is, along with gold nanoparticle size. The relationship between catalyst activity and reducibility has been drawn for several oxides used in VOC total oxidation [5,13,19,24]. A Mars-van Krevelen mechanism, involving the participation of lattice/surface oxygen species in the oxidation process can be accounted for this activity–reducibility correlation. This mechanism has been accepted by several authors, as stated in a recent review [2]. The participation of the support lattice oxygen on the process was pointed out by Scirè and co-workers [2,86]. In the particular case of toluene, a recent study using a temporal-analysisof-products set-up, showed that there was adsorption of toluene on the oxide catalyst surface; activation of toluene by dehydrogenation with adsorbed oxygen; oxidation of activated toluene mainly by the lattice oxygen, and re-oxidation of the reduced catalyst by dioxygen [87]. It was shown that gold enhances the reducibility and reactivity of the surface of the support oxide [2,88,89] and increases the exchange rate between lattice and surface oxygen [90]. The existence of a synergetic effect between gold and the partially reducible support was reported by several authors [91,92]. The role of the support, strictly associated to that of the gold nanoparticle size was also claimed [93,94]. In our work, CO formation was only observed at low temperatures, i.e., during the incomplete conversion. When full conversion was achieved, conversion to CO2 is total for both VOCs and no other products are observed. 4. Conclusions Gold was loaded on several oxide materials, which were tested for the total oxidation of ethyl acetate and toluene. Toluene was more difficult to oxidise. Gold loaded on CuO and NiO yielded the best catalysts. The catalytic activity increased upon gold loading for all samples, which was more important for less active oxides like MgO and Y2 O3 . The catalytic activity seems to be related with the reducibility of the support and the gold nanoparticle size, following a Mars-van Krevelen type of mechanism. The role of gold is to enhance the reducibility and reactivity of the surface of the support oxide and increase the exchange rate between lattice and surface oxygen. Gold is active in both Au+ (on Fe2 O3 ) and Au0 (on La2 O3 , NiO and CuO) oxidation states; however the presence of Au3+ is a possible explanation for the reduced catalytic activity of Au/Y2 O3 . Acknowledgements Funding from International Association for the Exchange of Students for Technical Experience (IAESTE) (PT/2011/59), Portuguese Association for International Exchance (APIET) and Helsinki Metropolia University of Applied Sciences (XC), Foundation for Science and Technology (FCT) and European Fund for Regional Development (FEDER) in the framework of Program COMPETE (Projects PEst-C/EQB/LA0020/2013, PEst-C/QUI/UI0616/2011, and Project QREN-I&D in co-promotion no. 21616 (GASCLEAN)) and CIENCIA 2007 and Investigador FCT programs (SACC). This work was co-financed by Board of National strategic Reference (QREN), New North program (ON2) and FEDER (Project NORTE-07-0124FEDER-0000015). Funding from Program of Support to Projects of Research and Technological Innovation (PAPIIT) of UNAM Project and IT200114CONACYT grant 00232624 (Mexico), Russian Science Foundation and RF Government Program Science of TPU is also acknowledged. The authors are indebted to Dr. Carlos

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Please cite this article in press as: S.A.C. Carabineiro, et al., Gold supported on metal oxides for volatile organic compounds total oxidation, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.06.034