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Fe2O3 catalysts in CO-PROX
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Effect of calcination temperature on catalytic properties of Au/Fe2O3 catalysts in CO-PROX J.L. Ayastuy*, A. Gurbani, M.A. Gutierrez-Ortiz Department of Chemical Engineering, Faculty of Science and Technology, University of the Basque Country UPV/ EHU, P.O. Box 644, E-48080, Bilbao, Spain
article info
abstract
Article history:
Au/a-Fe2O3 catalysts have been prepared by deposition-precipitation. The support a-Fe2O3
Received 18 September 2015
has been previously synthesized by precipitation technique. The effect of thermal treat-
Received in revised form
ment on structural, morphological and redox properties of Au/a-Fe2O3 samples have been
2 February 2016
thoroughly studied for the samples calcined at 150, 200 and 300 C, together with a non-
Accepted 10 February 2016
calcined sample. Their catalytic activity toward CO oxidation in the presence of
Available online xxx
hydrogen (CO-PROX) has been tested.
Keywords:
with the calcination temperature, being the minimum for the sample calcined at 200 C.
Gold
There was not detected cationic gold by X-ray photoelectron spectroscopy (XPS) in none of
Iron oxide
the samples. The presence of gold promoted notably the reduction of the iron oxide. The
CO-PROX
highest activity for CO-PROX was achieved by the non-calcined catalysts (simply dried at
Reducibility
110 C), while the sample calcined at 200 C was the least active. The catalysts activity was
The results showed that the gold particle size and surface adsorbed oxygen changed
assigned to the gold particle size and surface adsorbed oxygen. Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Polymer electrolyte membrane fuel cells (PEMFC) have become a powerful alternative to internal combustion engines, either for automotive or residential applications [1]. One of the most efficient ways to obtain hydrogen is by hydrocarbon reforming. The as-obtained hydrogen contains CO at levels which can poison the Pt-containing anode in the PEMFC, thus further removal of CO must be attained (with the goal to decrease CO to trace levels). This is carried out by means of two-stage Water Gas Shift reaction (WGS) followed by the Selective Oxidation of CO (CO-PROX) [2]. Catalysts used in the later stage must fulfill a number of requirements,
such as high activity, high selectivity towards CO2, tolerance to the presence of CO2 and H2O in the feedstream, and stability in time. Several supported metals and oxides have been investigated as catalysts for the CO-PROX reaction, such as noble metals [3], copper oxide [4] and gold supported on reducible oxides [5e8,18]. Supported gold catalysts are considered particularly attractive for CO-PROX, as at low temperature they are much more active for oxidation CO than for H2 oxidation, leading to high selectivity towards CO2 [8,9,18]. There is not a consensus about the origin of the high CO oxidation activity of supported gold catalysts. Among others, the source of the high catalytic activity of supported Au nanoparticles has been related to the presence of low
* Corresponding author. Tel.: þ34 94 601 2619; fax: þ34 94 601 5963. E-mail address: [email protected] (J.L. Ayastuy). http://dx.doi.org/10.1016/j.ijhydene.2016.02.080 0360-3199/Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Ayastuy JL, et al., Effect of calcination temperature on catalytic properties of Au/Fe2O3 catalysts in COPROX, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.02.080
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coordinated corner sites [10], cationic gold [11] and electronic effect of the support on Au nanoparticle [12]. The chemical nature of the support also plays a crucial role in the overall performance of the gold-based catalysts [13]. Many different reducible supports have been studied for gold, such as Mn2O3 [18], doped ceria [14], SnO2 [15] or Fe2O3 [8,16,17]. Iron oxide in its hematite phase (a-Fe2O3) leads to Au/ a-Fe2O3 catalysts with high activity for CO oxidation [13,17e20]. The use as support of reducible oxides provides reactive oxygen and, thereby, eliminates the oxygen dissociation as the rate-limiting step. Some discrepancy has been found in the literature about the effects of thermal treatments over gold-based catalysts. Some authors reported that gold-based catalysts were active for CO-PROX only after its calcination at above 400 C [13,21]. Wolf and co-workers [22] investigated the effect of calcination temperature on the Au/MOx (M ¼ Ti, Co, Zr, Al) catalysts used for CO oxidation, and found that the optimal temperature for all of them is 200 C. Other authors [23,24] found that gold supported on reducible oxides performed well in CO-PROX without any thermal treatment. For example, Park et al. [25] reported an inferior performance of gold-based catalysts after calcination (even at low temperatures), because the gold particles sinter. On the contrary, Haruta et al. [26] suggested that the strong gold-support interaction, responsible of the high activity of gold catalysts, is lead by the calcination treatment. The aim of this work is to study the effect of thermal treatment of Au/a-Fe2O3 catalysts on the catalytic performance in CO-PROX reaction, which will be discussed in terms of their structural, morphological and redox properties. For this purpose, the catalysts were calcined between 150 and 300 C, and for comparison, a sample without calcination has also been studied.
Experimental Support and catalysts preparation An aqueous solution of 1 M Fe(NO3)3$9H2O (Carlo Erba) was used to synthesize the iron oxide support. The above solution was added dropwise to a continuously stirred aqueous solution € en) at room temperature and pH 12, of 1 M Na2CO3 (Riedel-de Ha and the resultant suspension was aged for 1 h under stirring at room temperature, filtered and thoroughly washed with distilled water to eliminate impurities of the synthesis process, and finally dried at 110 C for 12 h. The as-prepared solid was divided into fractions which were calcined at different temperatures (between 400 and 600 C) for 5 h. The samples were labeled as Fe-T (T is the calcination temperature, in C). Au/a-Fe2O3 samples were prepared by depositionprecipitation (DP) with urea (Fluka) as the precipitation agent. An aqueous solution containing HAuCl4$3H2O (Fluka, purity > 99.9%) and excess of urea were simultaneously added to an aqueous suspension of the support under vigorous stirring at 80 C to obtain samples with 1 wt.% of gold. The pH of the solution increased gradually as a consequence of urea decomposition. After 2 h at constant pH 7.5, the resultant slurry was filtered, thoroughly washed with distilled water
until chloride ions were removed, and dried overnight at 110 C. The resultant solid was divided into four parts: three were subjected to different calcination temperatures (150, 200 and 300 C) for 2 h, and the fourth was left without calcination. The as-prepared samples were labeled as AuFe-T, being T the calcination temperature (in C). The sample which has not been calcined was labeled as AuFe-d.
Support and catalysts characterization The actual gold content was evaluated by inductively coupled plasma spectroscopy (ICP-AES Horiba). The BET specific surface area of the catalysts was determined by N2 adsorptionedesorption isotherms at 78 K (Micromeritics ASAP 2010). The crystalline structure of the prepared supports and catalysts was analyzed by means of XRD (Philips PW1710) with the sample in a finely grounded powder. PANalytical X'pert HighScore specific software was used for data treatment. The crystallite size and the lattice strain of Fe2O3 were estimated by X-ray broadening technique using the WilliamsoneHall plot. The size and morphology of the gold and iron oxide particles were characterized by TEM (Philips CM200), being the samples supported on a copper mesh with a carbon microgrid. The average diameter of gold and support particles, as well as their size distribution, was determined by measuring the size of large number of particles (>100 particles) from the TEM micrographs, using a number-averaged diameter. XPS measurements were carried out in SPECS equipment with Phoibos 150 1D-DLD analyzer and Al Ka (1486.6 eV) monochromatic source. All the spectra were fitted by GaussLorentzian contributions with CasaXPS 2.3.16 software, after a Shirley background subtraction. All binding energies were adjusted by setting C 1s at 284.6 eV. The reducibility of the samples was investigated by means of temperature programmed reduction with hydrogen (H2TPR). Before the H2-TPR experiment, the samples were treated in 5%O2/He flow at 110 C for 15 min. Then, the reactor was cooled down to 20 C into He flow. H2-TPR profiles of the samples were recorded during heating into 60 mL/min (STP) flow of 5%H2/Ar, from 20 to 400 C (up to 850 C in the case of bare support) with a temperature ramp of 20 C/min. For CO-TPD experiments the samples were heated from room temperature up to 150 C in 5%H2eAr flow, and then cooled into He flow to 0 C. Then, the flow was switched to 30 mL/min (STP) of 5%CO/He flow for 30 min, at constant temperature of 0 C, and then purged into He flow for 2 min. Finally, the sample was heated into He flow of 60 mL/min (STP) at 20 C/min up to 200 C (375 C in the case of bare support). Before O2-TPD experiments the samples were heated from room temperature up to 110 C in 5%H2eAr flow, and then cooled down to 0 C into He flow. This pre-treatment leaded to partially reduced catalyst. Then, the flow was switched to 30 mL/min (STP) of 5%O2/He flow for 30 min, at constant temperature of 0 C, and then purged into He flow for 15 min. Finally, the sample was heated into He flow of 60 mL/min (STP) at 20 C/min up to 300 C. H2-TPR, O2-TPD, and CO-TPD measurements were carried out in the same experimental equipment (Micromeritics Autochem 2910) with about 0.1 g of sample loaded in a quartz reactor. In H2-TPR and O2-TPD experiments TCD detector was
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used, while in CO-TPD experiment a mass spectrometer (MKS Cirrus 300) coupled to NDIR selective detector for CO (Siemens Ultramat 23) was used, following the m/z signals of 28 (CO) and 44 (CO2).
Catalytic tests Catalytic tests were carried out in a plug flow reactor at atmospheric pressure. About 0.1 g of catalyst (particles of 0.16e0.25 mm diameter) was loaded and diluted with 0.5 g of g-alumina (it has nil activity in CO-PROX) to reach a total volume of 1 mL. Total flow rate of 200 mL/min (STP), equivalent to a weight hourly space velocity WHSV ¼ 110 NL/h$g was fed to the reactor with the following composition (in vol. %): CO/O2/H2 ¼ 1/0.5/60 (l ¼ 1), and He to balance. All the flows were controlled by mass flow controllers (Bronkhorst). Feed and exhaust gases were continuously analyzed by QMS (MKS Cirrus 300) coupled to NDIR selective detector for CO (Siemens Ultramat 23). Prior to the reaction, the catalyst placed in the reactor was dried at 100 C into He flow for 1 h. The CO and O2 conversions and the selectivity towards CO2 were obtained as described elsewhere [27]. Neither methanation nor WGS was observed during the activity experiments.
Results and discussion Structural characterization of the support Firstly, the effect of the calcination temperature on the support structure was studied. The XRD patterns of the Fe-T samples are shown in Fig. 1A. The non-calcined support (labeled “as-prepared” in Fig. 1A) was identified as lowcrystalline goethite (PDF 00-29-0713) in agreement with literature [28]. For all the calcined supports, typical diffraction peaks of hematite phase (PDF 00-033-0664) were observed in Fig. 1A. Additional low intensity peak was observed at 2q ¼ 43.5 , which corresponds to (400) peak of maghemite phase (PDF 00-0190629) of iron oxide. The integration of both peaks allowed us to estimate the relative amount of each phase, resulting in 3.0e5.2 wt.% maghemite, depending of the calcination temperature (Table 1). According to Schimanke et al. [29] the phase transition from nanocrystalline maghemite to hematite takes place around 300 C. Therefore, it can be concluded that the iron oxide may be composed of a physical mixture of hematite (mainly) and maghemite. The diffraction peaks became sharper with the increase of the calcination temperature, suggesting the increase of crystallinity with the increase of temperature. The crystal size and the lattice microstrain of the iron oxide, calculated by the WilliamsoneHall approach, are summarized in Table 1. The lattice strain is associated to the intrinsic defects such as oxygen vacancies, which are the key factor for an active support in oxidation catalysts [30]. The crystal size slightly increased with the calcination temperature; on the opposite, the lattice microstrain remarkably decreased with the calcination temperature, pointing that the amount of structural defects inversely correlated with the solid crystallinity. Additionally, the hematite phase in the sample
Fig. 1 e (A) XRD spectra for the iron oxide calcined at different temperatures. (B) XRD spectra for AuFe-T catalysts.
calcined at 400 C was close to stoichiometric but slightly oxygen deficient, in agreement to [31]. All of the diffraction peaks can be indexed to the hexagonal structure of a-Fe2O3 (spatial group R3c) and the obtained lattice parameters, given in Table 1, were in good agreement with the reference ICDD PDF file. The iron oxide calcined at 400 C was chosen as support for gold due to its highest oxygen defective structure.
Structural and morphological characterization of the AuFe-T catalysts Some of the textural and structural properties of the AuFe-T samples are listed in Table 2. Both the support and the goldcontaining samples present type IV adsorptionedesorption isotherms and H1 hysteresis characteristic of mesoporous solids with uniform pore sizes (not displayed). The BET surface area of the support calcined at 400 C was 47.2 m2/g and slightly decreased after deposition of gold. The pore volume and mean diameter decreased after addition of gold without marked trend with the calcination temperature. The actual gold content of all the samples is somewhat lower than the nominal value, probably due to the loss of gold in form of complexes which could be re-dissolved during the preparation step [32].
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Table 1 e Structural properties and composition of the supports. Sample
Microstrain (%)
Crystal size (nm)
Lattice Parameters (nm)
Phases (in brackets the relative amount of maghemite)
Fe-400
0.086
25.6
hematite (94.8%) þ maghemite (5.2%)
Fe-500
0.065
29.2
Fe-600
0.045
31.6
a ¼ b ¼ 5.0376 ± 0.0010 c ¼ 13.7609 ± 0.0097 a ¼ b ¼ 90 g ¼ 120 a ¼ b ¼ 5.0377 ± 0.0009 c ¼ 13.7608 ± 0.0086 a ¼ b ¼ 90 g ¼ 120 a ¼ b ¼ 5.0370 ± 0.0038 c ¼ 13.7536 ± 0.0357 a ¼ b ¼ 90 g ¼ 120
hematite (96.4%) þ maghemite (3.6%)
hematite (97.0%) þ maghemite (3.0%)
Table 2 e Textural, structural and morphological properties of AuFe-T samples. Sample Bare support AuFe-d AuFe-150 AuFe-200 AuFe-300
SBET (m2/g) Vpore (cm3/g) dpore (nm) Au (wt%) 47.2 47.0 45.2 45.5 39.8
0.28 0.18 0.18 0.16 0.19
19.2 12.2 12.5 11.7 14.2
0 0.7 0.7 0.7 0.8
Phases (from XRD) Hematite (94.8%) þ maghemite (5.2%) hematite (96.6%) þ maghemite (3.4%) hematite (96.9%) þ maghemite (3.1%) Hematite (97.1%) þ maghemite (2.9%) hematite (97.7%) þ maghemite (2.3%)
dFe2 O3 (nm) dAu (nm) DAu (%) 25.6 30.1 25.6 29.2 31.6
n.a. 6.7 ± 1.8 6.2 ± 1.8 3.8 ± 2.3 4.6 ± 1.9
n.a. 13.4 14.5 23.7 19.6
dFe2 O3 : iron oxide particle size (by TEM). dAu: gold particle size (by TEM). DAu: gold dispersion (by TEM).
XRD pattern of the Au/a-Fe2O3 samples are shown in Fig. 1B. No diffraction peaks from any of the gold species were detected, probably due to the low gold content in our samples. In addition, the XRD patterns of the gold-containing samples also showed a mixture of hematite and maghemite phases, the latter with lower fraction than in the bare support. As expected, the relative amount of the maghemite phase decreased with the increase of the calcination temperature (Table 2). Independent of the calcination temperature, neither the crystallite size nor lattice parameters of the iron oxide were appreciably modified after deposition of gold, suggesting that gold remained on the surface of the iron oxide. Fig. 2 shows the TEM images for AuFe-T samples, as well as the size distributions of the gold nanoparticles. The gold particles are seen as dark contrasts on the surface of Fe2O3 particles. As a general trend, the support particles show distorted spherical shape with mean diameter in the 20e35 nm range for all the samples, in conformity with XRD. On the other hand, gold particles were mostly hemispherical. Mean gold particle size was smaller than 7 nm for all the samples, and substantially changed with the calcination temperature, as shown in the histograms of Fig. 2. Dispersion of gold (Table 2) varied from 13.4% (dried sample) to 23.7% (sample calcined at 200 C). Surprisingly, the dispersion of gold in the sample AuFe-300 was higher than in the samples AuFed and AuFe-150. It was found that the calcination temperature of 200 C leaded the maximum dispersion for gold in AuFe-T samples, while more severe calcination treatment (300 C) enlarged the mean gold particles size (which increased from 3.8 to 4.6 nm). It has been suggested [33] that a mild thermal treatment leads to some re-dispersion, likely re-distribution, of the gold particles over the support surface. Nevertheless,
after calcination at 300 C the mean gold particle size still was smaller than in the uncalcined sample.
XPS analysis The analysis of the high-resolution Fe 2p and Au 4f spectra are shown in Fig. 3. For all the AuFe-T samples it is visible the satellite peak at about 719 eV, which points to the presence of Fe3þ. For all the samples, the main Fe 2p3/2 peak was detected at 711.1 eV, which shows the presence either of Fe2O3 or FeO(OH), whose binding energies are very close to each other. The satellite peak of Fe 2p3/2 is located approximately 8 eV higher than the main Fe 2p3/2 peak, which is also characteristic of Fe3þ [34]. These results points that the iron of the support is mainly fully oxidized as Fe3þ, although the small shoulder at about 709.6 eV suggests that a small fraction of Fe2þ could also be present. The Au 4f7/2 photoelectron peak is located at BE value between 83.8 and 84.1 eV for all the samples, which are typical values of pure metallic Au0 species [35]. A slight positive shift in the core binding energy of the Au0 for AuFe-200 (84.1 eV) is related to the smaller particle size of gold in this sample [36]. It may be noted that the high vacuum applied to the XPS chamber could lead to the reduction of a fraction of cationic gold species [37]. Therefore, one cannot discard the presence of small fraction of cationic gold in the samples. Therefore, from XPS is concluded that the oxidation state of iron and gold was independent of the calcination temperature.
Reducibility of the support and catalysts H2-TPR profiles for the bare support and gold-containing samples are shown in Fig. 4. The H2-TPR profile for the bare
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Fig. 2 e TEM images of AuFe-T catalysts and the corresponding gold particle size distributions. (A) AuFe-d; (B) AuFe-150; (C) AuFe-200; (D) AuFe-300.
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Fig. 3 e XPS core level spectra of (A) Fe 2p, and (B) Au 4f.
support (a-Fe2O3) shows two not well-resolved reduction peaks and another, unfinished reduction process at 850 C. Peak assignation for the support reduction was done according to Pineau et al. [38]. The low temperature peak showed a maximum at 440 C and it was associated to the reduction of Fe2O3 to Fe3O4, according to the reaction:
3Fe2O3 þ H2 / 2Fe3O4 þ H2O
(1)
The intermediate temperature reduction peak has a maximum at 692 C and it was related to the reduction of Fe3O4 to FeO:
Fe3O4 þ 4H2 / 3FeO þ 4H2O
(2)
The increase of the hydrogen consumption signal at above 750 C was related to the reduction of FeO to Fe0 which at 850 C was an unfinished process:
FeO þ H2 / Fe0 þ H2O
(3)
The FeO / Fe0 reduction (reaction (3)) is characterized for the slow diffusion of hydrogen through the solid which explains the high temperature profile obtained in our sample [39]. The total hydrogen consumption of bare support up to 850 C was 1.79$104 mmolH2/g while assuming the stoichiometry for the complete reduction of hematite to metallic iron the theoretical consumption was 1.88$104 mmolH2/g. In accordance, the uncompleted recovery of the baseline suggested that reduction of about 5% of iron species is unfinished at 850 C. Deposition of gold on the support substantially modified the TPR profile, promoting the reduction of iron species to lower temperatures, which indicates a strong interaction between gold and support.
Fig. 4 e H2-TPR of support (upper figure) and AuFe-T catalysts.
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For all the gold-containing samples, mainly two peaks were observed along the reduction profile; the low temperature peak was observed between 100 and 150 C while the second, large and broad peak was observed for all the goldcontaining samples between 180 and 400 C. According to literature, the low temperature peak corresponds to the reduction of different cationic gold species [40]. Au3þ is known to be readily reduced to Au1þ and subsequently to Au0 in reducing atmospheres [41]. Indeed, the method employed for the incorporation of gold leads the formation of Au3þ species rather than Au0 [32]. Bulk gold oxide reduction was reported to occur at about 195 C [42], which indicates that the close interaction with iron oxide promotes its reduction, as the temperature of the first reduction peak ranged between 115 and 148 C (Table 3). The low temperature peak of reduction became less intense with the increase of calcination temperature, which suggested that the increase of the calcination temperature decreased the amount of weakly bounded oxygen. Assuming that Au2O3 was the only gold species, the hydrogen uptake corresponding to the low temperature peak was calculated, and those values are given in Table 3. Noticeably, for all the samples, the actual hydrogen consumption exceeds the theoretical one (which ranged between 51 and 57 mmolH2/g, depending on the actual gold content), suggesting the simultaneous reduction of easily reducible gold species and the iron species in the vicinity of gold particles. Huang et al. [43] suggested that promotion in the reduction temperature of the support was caused by the polarization of the FeeO ionic bond by cationic gold. The total hydrogen uptake during the reduction of the catalysts correlated with the gold dispersion, i.e., the higher the dispersion was, higher the hydrogen uptake was, because of the increase of the FeeO polarized bonds. The reduction in the 180e400 C range observed for all gold-containing samples showed a maximum consumption at about 300 C, and was related to a-Fe2O3 / Fe3O4 / FeO multiple reduction process [42]. The temperature for this reduction process was substantially lowered compared to the bare support, as also found in the literature [44]. For the samples calcined at 200 and 300 C these reduction processes could not be resolved, as they coalesced into a single peak, while for sample AuFe-150, and in higher extent for sample
Table 3 e H2 uptake (mmolH2/g) during H2-TPR for all catalysts; CO2 released (in mmolCO2/g) in CO-TPD up to 200 C. Sample
H2-TPR Low temperature peak ( C)
Uptake (low temperature)
Total uptake (✕104)
COTPD
n.d. 115 126
0 187 273
1.79a 0.454 0.473
0.1 10.7 7.9
144
188
0.562
4.6
148
129
0.545
6.1
Fe2O3 AuFe-d AuFe150 AuFe200 AuFe300 a
Up to 850 C.
7
AuFe-d, multiple peaks were distinguished. The onset of the reduction peak in the 180e400 C range increased with the calcination temperature (from 175 C for AuFe-d to 207 C for AuFe-300) which suggests that the oxygen defective lattice promoted the reduction of the support. Taking into account the theoretical hydrogen consumptions (2087 mmolH2/g for Fe2O3 / Fe3O4 and 4320 mmolH2/g for Fe3O4 / FeO), it was suggested that FeO and metallic gold were the main iron and gold species, respectively, at 400 C.
CO-TPD and O2-TPD To study the source of the oxygen for the formation of CO2, COTPD experiment was carried out, and the resulting evolution of released CO2 is shown in Fig. 5A. No desorption of CO was found for any of the samples. During the CO-TPD, desorption of CO2 started at temperatures higher than 300 C in the case of the bare support. It is reported that upon CO adsorption on Fe2O3 surface, mainly carbonates were formed, which were decomposed to CO2 at high temperature [45]. Thus, the CO2 release at about 300 C corresponded to the decomposition of such carbonates. On the contrary, all the gold-containing samples showed a release of CO2 at low temperature (about 50e60 C), with the absence of CO desorption. From the literature it is well known that CO can be adsorbed on gold nanoparticles [46]. Thus, it is clear that CO2 was formed by reaction of CO molecule adsorbed on gold with oxygen from the iron oxide framework in the vicinity of gold, likely in the proximity of the gold-support interface, demonstrating that even at low temperature as 50 C the lattice oxygen was activated. The last column of Table 3 shows that the most and the least CO2 released correspond to AuFe-d and AuFe-200 samples, respectively, which is in agreement with the O2-TPD. It is proposed that oxygen activation occurs at the Ausupport interface while CO adsorption occurs on the lowcoordination gold atoms [47]. On the other hand, oxygen molecule can be adsorbed on the vacancy defects on the oxide supports [48,49] and then diffuse towards Au-support interface, where reacts with the adsorbed CO. Fig. 5B shows the O2-TPD profiles of the AuFe-T samples, and bare Fe2O3 support. There are three kinds of desorption peaks at about 100 C, 220 C and 290 C, which may be assigned to O2,ads, O 2 (superoxide), and O , respectively [50]. 2 corresponding to lattice oxygen should appear at The O higher temperatures, at about 600 C. The physically adsorbed oxygen O2 is the easiest to desorb. It can be activated as different oxygen species according to following path: / O2, their desorption temperature O2,ads / O 2 / O increasing in the same sense [51]. These species are intermediates in the activation of oxygen on the oxide surface (storage) or oxygen carriers throughout the oxygen surface migration process (diffusion). After adding gold to Fe2O3 the increase intensity of the desorption peaks of O 2 , and O remarkably, that is, the oxygen species on the AuFe-T catalyst, should be more active than on the Fe2O3 support, which is very important for the catalyst when used in the CO oxidation. Among gold-containing samples AuFe-d shows the easiest desorption of surface oxygen, with onset temperature for oxygen desorption at 70 C, which is associated to oxygen adsorbed on surface defects. On the other hand, desorption of
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(and H2, in CO-PROX) weakly adsorbed on gold. The oxygen transfer to the Au perimeter generates oxygen defect sites in the support surface, which can be restored by the gas-phase oxygen [52].
Catalytic performance CO conversion and selectivity towards CO2 of the catalysts in the CO-PROX reaction at l ¼ 1 are presented in Fig. 6. It must be said that the activity of the bare support at the studied temperatures is almost null. For all the reaction temperatures studied, the CO conversion in CO-PROX follows the same order as in CO total oxidation (not shown), being AuFe-d and AuFe-200 catalysts the most and least active, respectively. None of the catalysts achieved the complete conversion of CO, AuFe-d catalyst achieving the maximum conversion of 81% at 125 C. Noticeably, the CO conversion correlated with the oxygen released from the solid phase in CO-TPD experiment. The scheme of the reaction mechanism is likely reaction occurs at the interface between Au particle and iron oxide. At the interface, CO is adsorbed on the Au atoms, while the support supplies the oxygen. Then the adsorbed oxygen species originates from iron oxide spillover across the interface and reacts with the adsorbed CO to form CO2.
Fig. 5 e (A) CO-TPD, and (B) O2-TPD of AuFe-T catalysts.
the adsorbed oxygen in AuFe-200 sample started at the highest temperature among all the samples, at about 120 C. Clearly, a correlation exists between this desorption temperature and the amount of CO2 released in CO-TPD. The lattice oxygen of Fe2O3 is involved in the CO oxidation by migration from the bulk to the Au-iron oxide perimeter. Along the Au-iron oxide perimeter, oxygen reacts with CO
Fig. 6 e (A) CO conversion in CO-PROX; (B) Selectivity toward CO2. Feed composition: CO/O2/H2 ¼ 1/0.5/60, and He to balance.
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AuFe-d and AuF-300 catalysts showed a maximum CO conversion at 125 C. A further increase of the reaction temperature to 175 C decreased the CO conversion for both catalysts, accompanied by the decrease in selectivity, which indicates that the oxidation of hydrogen occurred at higher temperatures. Several studies indicate that the rate of CO oxidation over supported Au catalysts exceeds that of H2 oxidation [18,53] which leads to high selectivity toward CO2. Indeed, supported Au catalysts are much less effective than the traditional supported Pd and Pt catalysts for H2 oxidation [54]. For all AuFe-T catalysts, the selectivity toward CO2 is strongly influenced by the reaction temperature, the higher the reaction temperature the lower the selectivity. This behavior can be ascribed to the temperature dependence of the ratio of the surface coverage between CO and H on gold particles [55]. In line with this, Lin et al. [56] found that hydrogen is hardly chemisorbed on gold particles. Both the CO conversion and selectivity towards CO2 in COPROX for the studied AuFe-T catalysts greatly varied with the calcination temperature of the samples. Finch et al. [57] € ssbauer spectroscopy studied gold-iron oxide catalysts by Mo and reported that the poorly ordered not calcined samples were more active than the calcined samples, which consisted of Au particles supported on a-Fe2O3, with strong synergistic interaction between gold and support.
Conclusions The effect of the calcination temperature of Au/Fe2O3 catalytic system on the structural, redox and catalytic properties for CO-PROX reaction was investigated. The characterization results showed that the support consisted mainly in hematite phase of iron oxide, while the gold particle size and surface adsorbed oxygen changed upon the calcination temperature. XPS analysis on the calcined catalysts showed the absence of cationic gold species, which, for some authors, are responsible of the high CO oxidation activity. By H2-TPR it was seen that the reducibility of the support was notably promoted by the presence of gold. The activity in CO oxidation in the presence of H2 strongly depends on the calcination temperature of the Au/Fe2O3 catalysts, as the ability to adsorb oxygen is greatly modified. It is found that the amount of adsorbed oxygen species are much more critical that the gold particle size. The activity and selectivity differences among the AuFe-T catalysts are ascribed to the varying ability of the supports to supply oxygen to facilitate the CO oxidation. The non-calcined catalysts (simply dried at 110 C) has been found to be the most active in CO-PROX, while the sample calcined at 200 C was the least active among all the samples.
Acknowledgments The authors wish to thank Spanish MEC (Projects ENE200767975 and ENE2013-41187-R) and Basque Government (UFI 11/39 UPV/EHU) for the financial support, and technical and human support provided by SGIker of UPV/EHU.
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Effect of calcination temperature on catalytic properties of Au/Fe2O3 catalysts in CO-PROX J.L. Ayastuy*, A. Gurbani, M.A. Gutierrez-Ortiz Department of Chemical Engineering, Faculty of Science and Technology, University of the Basque Country UPV/ EHU, P.O. Box 644, E-48080, Bilbao, Spain
article info
abstract
Article history:
Au/a-Fe2O3 catalysts have been prepared by deposition-precipitation. The support a-Fe2O3
Received 18 September 2015
has been previously synthesized by precipitation technique. The effect of thermal treat-
Received in revised form
ment on structural, morphological and redox properties of Au/a-Fe2O3 samples have been
2 February 2016
thoroughly studied for the samples calcined at 150, 200 and 300 C, together with a non-
Accepted 10 February 2016
calcined sample. Their catalytic activity toward CO oxidation in the presence of
Available online xxx
hydrogen (CO-PROX) has been tested.
Keywords:
with the calcination temperature, being the minimum for the sample calcined at 200 C.
Gold
There was not detected cationic gold by X-ray photoelectron spectroscopy (XPS) in none of
Iron oxide
the samples. The presence of gold promoted notably the reduction of the iron oxide. The
CO-PROX
highest activity for CO-PROX was achieved by the non-calcined catalysts (simply dried at
Reducibility
110 C), while the sample calcined at 200 C was the least active. The catalysts activity was
The results showed that the gold particle size and surface adsorbed oxygen changed
assigned to the gold particle size and surface adsorbed oxygen. Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Polymer electrolyte membrane fuel cells (PEMFC) have become a powerful alternative to internal combustion engines, either for automotive or residential applications [1]. One of the most efficient ways to obtain hydrogen is by hydrocarbon reforming. The as-obtained hydrogen contains CO at levels which can poison the Pt-containing anode in the PEMFC, thus further removal of CO must be attained (with the goal to decrease CO to trace levels). This is carried out by means of two-stage Water Gas Shift reaction (WGS) followed by the Selective Oxidation of CO (CO-PROX) [2]. Catalysts used in the later stage must fulfill a number of requirements,
such as high activity, high selectivity towards CO2, tolerance to the presence of CO2 and H2O in the feedstream, and stability in time. Several supported metals and oxides have been investigated as catalysts for the CO-PROX reaction, such as noble metals [3], copper oxide [4] and gold supported on reducible oxides [5e8,18]. Supported gold catalysts are considered particularly attractive for CO-PROX, as at low temperature they are much more active for oxidation CO than for H2 oxidation, leading to high selectivity towards CO2 [8,9,18]. There is not a consensus about the origin of the high CO oxidation activity of supported gold catalysts. Among others, the source of the high catalytic activity of supported Au nanoparticles has been related to the presence of low
* Corresponding author. Tel.: þ34 94 601 2619; fax: þ34 94 601 5963. E-mail address: [email protected] (J.L. Ayastuy). http://dx.doi.org/10.1016/j.ijhydene.2016.02.080 0360-3199/Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Ayastuy JL, et al., Effect of calcination temperature on catalytic properties of Au/Fe2O3 catalysts in COPROX, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.02.080
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coordinated corner sites [10], cationic gold [11] and electronic effect of the support on Au nanoparticle [12]. The chemical nature of the support also plays a crucial role in the overall performance of the gold-based catalysts [13]. Many different reducible supports have been studied for gold, such as Mn2O3 [18], doped ceria [14], SnO2 [15] or Fe2O3 [8,16,17]. Iron oxide in its hematite phase (a-Fe2O3) leads to Au/ a-Fe2O3 catalysts with high activity for CO oxidation [13,17e20]. The use as support of reducible oxides provides reactive oxygen and, thereby, eliminates the oxygen dissociation as the rate-limiting step. Some discrepancy has been found in the literature about the effects of thermal treatments over gold-based catalysts. Some authors reported that gold-based catalysts were active for CO-PROX only after its calcination at above 400 C [13,21]. Wolf and co-workers [22] investigated the effect of calcination temperature on the Au/MOx (M ¼ Ti, Co, Zr, Al) catalysts used for CO oxidation, and found that the optimal temperature for all of them is 200 C. Other authors [23,24] found that gold supported on reducible oxides performed well in CO-PROX without any thermal treatment. For example, Park et al. [25] reported an inferior performance of gold-based catalysts after calcination (even at low temperatures), because the gold particles sinter. On the contrary, Haruta et al. [26] suggested that the strong gold-support interaction, responsible of the high activity of gold catalysts, is lead by the calcination treatment. The aim of this work is to study the effect of thermal treatment of Au/a-Fe2O3 catalysts on the catalytic performance in CO-PROX reaction, which will be discussed in terms of their structural, morphological and redox properties. For this purpose, the catalysts were calcined between 150 and 300 C, and for comparison, a sample without calcination has also been studied.
Experimental Support and catalysts preparation An aqueous solution of 1 M Fe(NO3)3$9H2O (Carlo Erba) was used to synthesize the iron oxide support. The above solution was added dropwise to a continuously stirred aqueous solution € en) at room temperature and pH 12, of 1 M Na2CO3 (Riedel-de Ha and the resultant suspension was aged for 1 h under stirring at room temperature, filtered and thoroughly washed with distilled water to eliminate impurities of the synthesis process, and finally dried at 110 C for 12 h. The as-prepared solid was divided into fractions which were calcined at different temperatures (between 400 and 600 C) for 5 h. The samples were labeled as Fe-T (T is the calcination temperature, in C). Au/a-Fe2O3 samples were prepared by depositionprecipitation (DP) with urea (Fluka) as the precipitation agent. An aqueous solution containing HAuCl4$3H2O (Fluka, purity > 99.9%) and excess of urea were simultaneously added to an aqueous suspension of the support under vigorous stirring at 80 C to obtain samples with 1 wt.% of gold. The pH of the solution increased gradually as a consequence of urea decomposition. After 2 h at constant pH 7.5, the resultant slurry was filtered, thoroughly washed with distilled water
until chloride ions were removed, and dried overnight at 110 C. The resultant solid was divided into four parts: three were subjected to different calcination temperatures (150, 200 and 300 C) for 2 h, and the fourth was left without calcination. The as-prepared samples were labeled as AuFe-T, being T the calcination temperature (in C). The sample which has not been calcined was labeled as AuFe-d.
Support and catalysts characterization The actual gold content was evaluated by inductively coupled plasma spectroscopy (ICP-AES Horiba). The BET specific surface area of the catalysts was determined by N2 adsorptionedesorption isotherms at 78 K (Micromeritics ASAP 2010). The crystalline structure of the prepared supports and catalysts was analyzed by means of XRD (Philips PW1710) with the sample in a finely grounded powder. PANalytical X'pert HighScore specific software was used for data treatment. The crystallite size and the lattice strain of Fe2O3 were estimated by X-ray broadening technique using the WilliamsoneHall plot. The size and morphology of the gold and iron oxide particles were characterized by TEM (Philips CM200), being the samples supported on a copper mesh with a carbon microgrid. The average diameter of gold and support particles, as well as their size distribution, was determined by measuring the size of large number of particles (>100 particles) from the TEM micrographs, using a number-averaged diameter. XPS measurements were carried out in SPECS equipment with Phoibos 150 1D-DLD analyzer and Al Ka (1486.6 eV) monochromatic source. All the spectra were fitted by GaussLorentzian contributions with CasaXPS 2.3.16 software, after a Shirley background subtraction. All binding energies were adjusted by setting C 1s at 284.6 eV. The reducibility of the samples was investigated by means of temperature programmed reduction with hydrogen (H2TPR). Before the H2-TPR experiment, the samples were treated in 5%O2/He flow at 110 C for 15 min. Then, the reactor was cooled down to 20 C into He flow. H2-TPR profiles of the samples were recorded during heating into 60 mL/min (STP) flow of 5%H2/Ar, from 20 to 400 C (up to 850 C in the case of bare support) with a temperature ramp of 20 C/min. For CO-TPD experiments the samples were heated from room temperature up to 150 C in 5%H2eAr flow, and then cooled into He flow to 0 C. Then, the flow was switched to 30 mL/min (STP) of 5%CO/He flow for 30 min, at constant temperature of 0 C, and then purged into He flow for 2 min. Finally, the sample was heated into He flow of 60 mL/min (STP) at 20 C/min up to 200 C (375 C in the case of bare support). Before O2-TPD experiments the samples were heated from room temperature up to 110 C in 5%H2eAr flow, and then cooled down to 0 C into He flow. This pre-treatment leaded to partially reduced catalyst. Then, the flow was switched to 30 mL/min (STP) of 5%O2/He flow for 30 min, at constant temperature of 0 C, and then purged into He flow for 15 min. Finally, the sample was heated into He flow of 60 mL/min (STP) at 20 C/min up to 300 C. H2-TPR, O2-TPD, and CO-TPD measurements were carried out in the same experimental equipment (Micromeritics Autochem 2910) with about 0.1 g of sample loaded in a quartz reactor. In H2-TPR and O2-TPD experiments TCD detector was
Please cite this article in press as: Ayastuy JL, et al., Effect of calcination temperature on catalytic properties of Au/Fe2O3 catalysts in COPROX, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.02.080
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3
used, while in CO-TPD experiment a mass spectrometer (MKS Cirrus 300) coupled to NDIR selective detector for CO (Siemens Ultramat 23) was used, following the m/z signals of 28 (CO) and 44 (CO2).
Catalytic tests Catalytic tests were carried out in a plug flow reactor at atmospheric pressure. About 0.1 g of catalyst (particles of 0.16e0.25 mm diameter) was loaded and diluted with 0.5 g of g-alumina (it has nil activity in CO-PROX) to reach a total volume of 1 mL. Total flow rate of 200 mL/min (STP), equivalent to a weight hourly space velocity WHSV ¼ 110 NL/h$g was fed to the reactor with the following composition (in vol. %): CO/O2/H2 ¼ 1/0.5/60 (l ¼ 1), and He to balance. All the flows were controlled by mass flow controllers (Bronkhorst). Feed and exhaust gases were continuously analyzed by QMS (MKS Cirrus 300) coupled to NDIR selective detector for CO (Siemens Ultramat 23). Prior to the reaction, the catalyst placed in the reactor was dried at 100 C into He flow for 1 h. The CO and O2 conversions and the selectivity towards CO2 were obtained as described elsewhere [27]. Neither methanation nor WGS was observed during the activity experiments.
Results and discussion Structural characterization of the support Firstly, the effect of the calcination temperature on the support structure was studied. The XRD patterns of the Fe-T samples are shown in Fig. 1A. The non-calcined support (labeled “as-prepared” in Fig. 1A) was identified as lowcrystalline goethite (PDF 00-29-0713) in agreement with literature [28]. For all the calcined supports, typical diffraction peaks of hematite phase (PDF 00-033-0664) were observed in Fig. 1A. Additional low intensity peak was observed at 2q ¼ 43.5 , which corresponds to (400) peak of maghemite phase (PDF 00-0190629) of iron oxide. The integration of both peaks allowed us to estimate the relative amount of each phase, resulting in 3.0e5.2 wt.% maghemite, depending of the calcination temperature (Table 1). According to Schimanke et al. [29] the phase transition from nanocrystalline maghemite to hematite takes place around 300 C. Therefore, it can be concluded that the iron oxide may be composed of a physical mixture of hematite (mainly) and maghemite. The diffraction peaks became sharper with the increase of the calcination temperature, suggesting the increase of crystallinity with the increase of temperature. The crystal size and the lattice microstrain of the iron oxide, calculated by the WilliamsoneHall approach, are summarized in Table 1. The lattice strain is associated to the intrinsic defects such as oxygen vacancies, which are the key factor for an active support in oxidation catalysts [30]. The crystal size slightly increased with the calcination temperature; on the opposite, the lattice microstrain remarkably decreased with the calcination temperature, pointing that the amount of structural defects inversely correlated with the solid crystallinity. Additionally, the hematite phase in the sample
Fig. 1 e (A) XRD spectra for the iron oxide calcined at different temperatures. (B) XRD spectra for AuFe-T catalysts.
calcined at 400 C was close to stoichiometric but slightly oxygen deficient, in agreement to [31]. All of the diffraction peaks can be indexed to the hexagonal structure of a-Fe2O3 (spatial group R3c) and the obtained lattice parameters, given in Table 1, were in good agreement with the reference ICDD PDF file. The iron oxide calcined at 400 C was chosen as support for gold due to its highest oxygen defective structure.
Structural and morphological characterization of the AuFe-T catalysts Some of the textural and structural properties of the AuFe-T samples are listed in Table 2. Both the support and the goldcontaining samples present type IV adsorptionedesorption isotherms and H1 hysteresis characteristic of mesoporous solids with uniform pore sizes (not displayed). The BET surface area of the support calcined at 400 C was 47.2 m2/g and slightly decreased after deposition of gold. The pore volume and mean diameter decreased after addition of gold without marked trend with the calcination temperature. The actual gold content of all the samples is somewhat lower than the nominal value, probably due to the loss of gold in form of complexes which could be re-dissolved during the preparation step [32].
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Table 1 e Structural properties and composition of the supports. Sample
Microstrain (%)
Crystal size (nm)
Lattice Parameters (nm)
Phases (in brackets the relative amount of maghemite)
Fe-400
0.086
25.6
hematite (94.8%) þ maghemite (5.2%)
Fe-500
0.065
29.2
Fe-600
0.045
31.6
a ¼ b ¼ 5.0376 ± 0.0010 c ¼ 13.7609 ± 0.0097 a ¼ b ¼ 90 g ¼ 120 a ¼ b ¼ 5.0377 ± 0.0009 c ¼ 13.7608 ± 0.0086 a ¼ b ¼ 90 g ¼ 120 a ¼ b ¼ 5.0370 ± 0.0038 c ¼ 13.7536 ± 0.0357 a ¼ b ¼ 90 g ¼ 120
hematite (96.4%) þ maghemite (3.6%)
hematite (97.0%) þ maghemite (3.0%)
Table 2 e Textural, structural and morphological properties of AuFe-T samples. Sample Bare support AuFe-d AuFe-150 AuFe-200 AuFe-300
SBET (m2/g) Vpore (cm3/g) dpore (nm) Au (wt%) 47.2 47.0 45.2 45.5 39.8
0.28 0.18 0.18 0.16 0.19
19.2 12.2 12.5 11.7 14.2
0 0.7 0.7 0.7 0.8
Phases (from XRD) Hematite (94.8%) þ maghemite (5.2%) hematite (96.6%) þ maghemite (3.4%) hematite (96.9%) þ maghemite (3.1%) Hematite (97.1%) þ maghemite (2.9%) hematite (97.7%) þ maghemite (2.3%)
dFe2 O3 (nm) dAu (nm) DAu (%) 25.6 30.1 25.6 29.2 31.6
n.a. 6.7 ± 1.8 6.2 ± 1.8 3.8 ± 2.3 4.6 ± 1.9
n.a. 13.4 14.5 23.7 19.6
dFe2 O3 : iron oxide particle size (by TEM). dAu: gold particle size (by TEM). DAu: gold dispersion (by TEM).
XRD pattern of the Au/a-Fe2O3 samples are shown in Fig. 1B. No diffraction peaks from any of the gold species were detected, probably due to the low gold content in our samples. In addition, the XRD patterns of the gold-containing samples also showed a mixture of hematite and maghemite phases, the latter with lower fraction than in the bare support. As expected, the relative amount of the maghemite phase decreased with the increase of the calcination temperature (Table 2). Independent of the calcination temperature, neither the crystallite size nor lattice parameters of the iron oxide were appreciably modified after deposition of gold, suggesting that gold remained on the surface of the iron oxide. Fig. 2 shows the TEM images for AuFe-T samples, as well as the size distributions of the gold nanoparticles. The gold particles are seen as dark contrasts on the surface of Fe2O3 particles. As a general trend, the support particles show distorted spherical shape with mean diameter in the 20e35 nm range for all the samples, in conformity with XRD. On the other hand, gold particles were mostly hemispherical. Mean gold particle size was smaller than 7 nm for all the samples, and substantially changed with the calcination temperature, as shown in the histograms of Fig. 2. Dispersion of gold (Table 2) varied from 13.4% (dried sample) to 23.7% (sample calcined at 200 C). Surprisingly, the dispersion of gold in the sample AuFe-300 was higher than in the samples AuFed and AuFe-150. It was found that the calcination temperature of 200 C leaded the maximum dispersion for gold in AuFe-T samples, while more severe calcination treatment (300 C) enlarged the mean gold particles size (which increased from 3.8 to 4.6 nm). It has been suggested [33] that a mild thermal treatment leads to some re-dispersion, likely re-distribution, of the gold particles over the support surface. Nevertheless,
after calcination at 300 C the mean gold particle size still was smaller than in the uncalcined sample.
XPS analysis The analysis of the high-resolution Fe 2p and Au 4f spectra are shown in Fig. 3. For all the AuFe-T samples it is visible the satellite peak at about 719 eV, which points to the presence of Fe3þ. For all the samples, the main Fe 2p3/2 peak was detected at 711.1 eV, which shows the presence either of Fe2O3 or FeO(OH), whose binding energies are very close to each other. The satellite peak of Fe 2p3/2 is located approximately 8 eV higher than the main Fe 2p3/2 peak, which is also characteristic of Fe3þ [34]. These results points that the iron of the support is mainly fully oxidized as Fe3þ, although the small shoulder at about 709.6 eV suggests that a small fraction of Fe2þ could also be present. The Au 4f7/2 photoelectron peak is located at BE value between 83.8 and 84.1 eV for all the samples, which are typical values of pure metallic Au0 species [35]. A slight positive shift in the core binding energy of the Au0 for AuFe-200 (84.1 eV) is related to the smaller particle size of gold in this sample [36]. It may be noted that the high vacuum applied to the XPS chamber could lead to the reduction of a fraction of cationic gold species [37]. Therefore, one cannot discard the presence of small fraction of cationic gold in the samples. Therefore, from XPS is concluded that the oxidation state of iron and gold was independent of the calcination temperature.
Reducibility of the support and catalysts H2-TPR profiles for the bare support and gold-containing samples are shown in Fig. 4. The H2-TPR profile for the bare
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5
Fig. 2 e TEM images of AuFe-T catalysts and the corresponding gold particle size distributions. (A) AuFe-d; (B) AuFe-150; (C) AuFe-200; (D) AuFe-300.
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Fig. 3 e XPS core level spectra of (A) Fe 2p, and (B) Au 4f.
support (a-Fe2O3) shows two not well-resolved reduction peaks and another, unfinished reduction process at 850 C. Peak assignation for the support reduction was done according to Pineau et al. [38]. The low temperature peak showed a maximum at 440 C and it was associated to the reduction of Fe2O3 to Fe3O4, according to the reaction:
3Fe2O3 þ H2 / 2Fe3O4 þ H2O
(1)
The intermediate temperature reduction peak has a maximum at 692 C and it was related to the reduction of Fe3O4 to FeO:
Fe3O4 þ 4H2 / 3FeO þ 4H2O
(2)
The increase of the hydrogen consumption signal at above 750 C was related to the reduction of FeO to Fe0 which at 850 C was an unfinished process:
FeO þ H2 / Fe0 þ H2O
(3)
The FeO / Fe0 reduction (reaction (3)) is characterized for the slow diffusion of hydrogen through the solid which explains the high temperature profile obtained in our sample [39]. The total hydrogen consumption of bare support up to 850 C was 1.79$104 mmolH2/g while assuming the stoichiometry for the complete reduction of hematite to metallic iron the theoretical consumption was 1.88$104 mmolH2/g. In accordance, the uncompleted recovery of the baseline suggested that reduction of about 5% of iron species is unfinished at 850 C. Deposition of gold on the support substantially modified the TPR profile, promoting the reduction of iron species to lower temperatures, which indicates a strong interaction between gold and support.
Fig. 4 e H2-TPR of support (upper figure) and AuFe-T catalysts.
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For all the gold-containing samples, mainly two peaks were observed along the reduction profile; the low temperature peak was observed between 100 and 150 C while the second, large and broad peak was observed for all the goldcontaining samples between 180 and 400 C. According to literature, the low temperature peak corresponds to the reduction of different cationic gold species [40]. Au3þ is known to be readily reduced to Au1þ and subsequently to Au0 in reducing atmospheres [41]. Indeed, the method employed for the incorporation of gold leads the formation of Au3þ species rather than Au0 [32]. Bulk gold oxide reduction was reported to occur at about 195 C [42], which indicates that the close interaction with iron oxide promotes its reduction, as the temperature of the first reduction peak ranged between 115 and 148 C (Table 3). The low temperature peak of reduction became less intense with the increase of calcination temperature, which suggested that the increase of the calcination temperature decreased the amount of weakly bounded oxygen. Assuming that Au2O3 was the only gold species, the hydrogen uptake corresponding to the low temperature peak was calculated, and those values are given in Table 3. Noticeably, for all the samples, the actual hydrogen consumption exceeds the theoretical one (which ranged between 51 and 57 mmolH2/g, depending on the actual gold content), suggesting the simultaneous reduction of easily reducible gold species and the iron species in the vicinity of gold particles. Huang et al. [43] suggested that promotion in the reduction temperature of the support was caused by the polarization of the FeeO ionic bond by cationic gold. The total hydrogen uptake during the reduction of the catalysts correlated with the gold dispersion, i.e., the higher the dispersion was, higher the hydrogen uptake was, because of the increase of the FeeO polarized bonds. The reduction in the 180e400 C range observed for all gold-containing samples showed a maximum consumption at about 300 C, and was related to a-Fe2O3 / Fe3O4 / FeO multiple reduction process [42]. The temperature for this reduction process was substantially lowered compared to the bare support, as also found in the literature [44]. For the samples calcined at 200 and 300 C these reduction processes could not be resolved, as they coalesced into a single peak, while for sample AuFe-150, and in higher extent for sample
Table 3 e H2 uptake (mmolH2/g) during H2-TPR for all catalysts; CO2 released (in mmolCO2/g) in CO-TPD up to 200 C. Sample
H2-TPR Low temperature peak ( C)
Uptake (low temperature)
Total uptake (✕104)
COTPD
n.d. 115 126
0 187 273
1.79a 0.454 0.473
0.1 10.7 7.9
144
188
0.562
4.6
148
129
0.545
6.1
Fe2O3 AuFe-d AuFe150 AuFe200 AuFe300 a
Up to 850 C.
7
AuFe-d, multiple peaks were distinguished. The onset of the reduction peak in the 180e400 C range increased with the calcination temperature (from 175 C for AuFe-d to 207 C for AuFe-300) which suggests that the oxygen defective lattice promoted the reduction of the support. Taking into account the theoretical hydrogen consumptions (2087 mmolH2/g for Fe2O3 / Fe3O4 and 4320 mmolH2/g for Fe3O4 / FeO), it was suggested that FeO and metallic gold were the main iron and gold species, respectively, at 400 C.
CO-TPD and O2-TPD To study the source of the oxygen for the formation of CO2, COTPD experiment was carried out, and the resulting evolution of released CO2 is shown in Fig. 5A. No desorption of CO was found for any of the samples. During the CO-TPD, desorption of CO2 started at temperatures higher than 300 C in the case of the bare support. It is reported that upon CO adsorption on Fe2O3 surface, mainly carbonates were formed, which were decomposed to CO2 at high temperature [45]. Thus, the CO2 release at about 300 C corresponded to the decomposition of such carbonates. On the contrary, all the gold-containing samples showed a release of CO2 at low temperature (about 50e60 C), with the absence of CO desorption. From the literature it is well known that CO can be adsorbed on gold nanoparticles [46]. Thus, it is clear that CO2 was formed by reaction of CO molecule adsorbed on gold with oxygen from the iron oxide framework in the vicinity of gold, likely in the proximity of the gold-support interface, demonstrating that even at low temperature as 50 C the lattice oxygen was activated. The last column of Table 3 shows that the most and the least CO2 released correspond to AuFe-d and AuFe-200 samples, respectively, which is in agreement with the O2-TPD. It is proposed that oxygen activation occurs at the Ausupport interface while CO adsorption occurs on the lowcoordination gold atoms [47]. On the other hand, oxygen molecule can be adsorbed on the vacancy defects on the oxide supports [48,49] and then diffuse towards Au-support interface, where reacts with the adsorbed CO. Fig. 5B shows the O2-TPD profiles of the AuFe-T samples, and bare Fe2O3 support. There are three kinds of desorption peaks at about 100 C, 220 C and 290 C, which may be assigned to O2,ads, O 2 (superoxide), and O , respectively [50]. 2 corresponding to lattice oxygen should appear at The O higher temperatures, at about 600 C. The physically adsorbed oxygen O2 is the easiest to desorb. It can be activated as different oxygen species according to following path: / O2, their desorption temperature O2,ads / O 2 / O increasing in the same sense [51]. These species are intermediates in the activation of oxygen on the oxide surface (storage) or oxygen carriers throughout the oxygen surface migration process (diffusion). After adding gold to Fe2O3 the increase intensity of the desorption peaks of O 2 , and O remarkably, that is, the oxygen species on the AuFe-T catalyst, should be more active than on the Fe2O3 support, which is very important for the catalyst when used in the CO oxidation. Among gold-containing samples AuFe-d shows the easiest desorption of surface oxygen, with onset temperature for oxygen desorption at 70 C, which is associated to oxygen adsorbed on surface defects. On the other hand, desorption of
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(and H2, in CO-PROX) weakly adsorbed on gold. The oxygen transfer to the Au perimeter generates oxygen defect sites in the support surface, which can be restored by the gas-phase oxygen [52].
Catalytic performance CO conversion and selectivity towards CO2 of the catalysts in the CO-PROX reaction at l ¼ 1 are presented in Fig. 6. It must be said that the activity of the bare support at the studied temperatures is almost null. For all the reaction temperatures studied, the CO conversion in CO-PROX follows the same order as in CO total oxidation (not shown), being AuFe-d and AuFe-200 catalysts the most and least active, respectively. None of the catalysts achieved the complete conversion of CO, AuFe-d catalyst achieving the maximum conversion of 81% at 125 C. Noticeably, the CO conversion correlated with the oxygen released from the solid phase in CO-TPD experiment. The scheme of the reaction mechanism is likely reaction occurs at the interface between Au particle and iron oxide. At the interface, CO is adsorbed on the Au atoms, while the support supplies the oxygen. Then the adsorbed oxygen species originates from iron oxide spillover across the interface and reacts with the adsorbed CO to form CO2.
Fig. 5 e (A) CO-TPD, and (B) O2-TPD of AuFe-T catalysts.
the adsorbed oxygen in AuFe-200 sample started at the highest temperature among all the samples, at about 120 C. Clearly, a correlation exists between this desorption temperature and the amount of CO2 released in CO-TPD. The lattice oxygen of Fe2O3 is involved in the CO oxidation by migration from the bulk to the Au-iron oxide perimeter. Along the Au-iron oxide perimeter, oxygen reacts with CO
Fig. 6 e (A) CO conversion in CO-PROX; (B) Selectivity toward CO2. Feed composition: CO/O2/H2 ¼ 1/0.5/60, and He to balance.
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AuFe-d and AuF-300 catalysts showed a maximum CO conversion at 125 C. A further increase of the reaction temperature to 175 C decreased the CO conversion for both catalysts, accompanied by the decrease in selectivity, which indicates that the oxidation of hydrogen occurred at higher temperatures. Several studies indicate that the rate of CO oxidation over supported Au catalysts exceeds that of H2 oxidation [18,53] which leads to high selectivity toward CO2. Indeed, supported Au catalysts are much less effective than the traditional supported Pd and Pt catalysts for H2 oxidation [54]. For all AuFe-T catalysts, the selectivity toward CO2 is strongly influenced by the reaction temperature, the higher the reaction temperature the lower the selectivity. This behavior can be ascribed to the temperature dependence of the ratio of the surface coverage between CO and H on gold particles [55]. In line with this, Lin et al. [56] found that hydrogen is hardly chemisorbed on gold particles. Both the CO conversion and selectivity towards CO2 in COPROX for the studied AuFe-T catalysts greatly varied with the calcination temperature of the samples. Finch et al. [57] € ssbauer spectroscopy studied gold-iron oxide catalysts by Mo and reported that the poorly ordered not calcined samples were more active than the calcined samples, which consisted of Au particles supported on a-Fe2O3, with strong synergistic interaction between gold and support.
Conclusions The effect of the calcination temperature of Au/Fe2O3 catalytic system on the structural, redox and catalytic properties for CO-PROX reaction was investigated. The characterization results showed that the support consisted mainly in hematite phase of iron oxide, while the gold particle size and surface adsorbed oxygen changed upon the calcination temperature. XPS analysis on the calcined catalysts showed the absence of cationic gold species, which, for some authors, are responsible of the high CO oxidation activity. By H2-TPR it was seen that the reducibility of the support was notably promoted by the presence of gold. The activity in CO oxidation in the presence of H2 strongly depends on the calcination temperature of the Au/Fe2O3 catalysts, as the ability to adsorb oxygen is greatly modified. It is found that the amount of adsorbed oxygen species are much more critical that the gold particle size. The activity and selectivity differences among the AuFe-T catalysts are ascribed to the varying ability of the supports to supply oxygen to facilitate the CO oxidation. The non-calcined catalysts (simply dried at 110 C) has been found to be the most active in CO-PROX, while the sample calcined at 200 C was the least active among all the samples.
Acknowledgments The authors wish to thank Spanish MEC (Projects ENE200767975 and ENE2013-41187-R) and Basque Government (UFI 11/39 UPV/EHU) for the financial support, and technical and human support provided by SGIker of UPV/EHU.
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Please cite this article in press as: Ayastuy JL, et al., Effect of calcination temperature on catalytic properties of Au/Fe2O3 catalysts in COPROX, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.02.080