In situ characterization of iron-promoted ceria–alumina gold catalysts during the water-gas shift reaction

Catalysis Today 205 (2013) 41–48

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In situ characterization of iron-promoted ceria–alumina gold catalysts during the water-gas shift reaction Tomás Ramírez Reina a,∗ , Wenqian Xu b , Svetlana Ivanova a , Miguel Ángel Centeno a , Jonathan Hanson b , José A. Rodriguez b , José Antonio Odriozola a a b

Departamento de Química Inorgánica – Instituto de Ciencia de Materiales de Sevilla, Centro Mixto Universidad de Sevilla-CSIC, Avda. Américo Vespucio 49, 41092 Sevilla, Spain Chemistry Department, Brookhaven National Laboratory, Upton, NY 11973, United States

a r t i c l e

i n f o

Article history: Received 25 May 2012 Received in revised form 26 July 2012 Accepted 1 August 2012 Available online 7 September 2012 Keywords: Gold catalyst Iron oxide Cerium oxide In situ TR-XRD In situ TR-XAS Water-gas shift reaction

a b s t r a c t In this work an in situ XRD and XANES study of two gold catalysts supported on iron-promoted ceria–alumina carriers was carried out during the water-gas shift reaction (WGS). The first catalyst, Au/CeO2 –FeOx /Al2 O3 , was prepared using a commercial alumina support in order to obtain a Ce–Fe oxide solid solution and in the second one, Au/FeOx /CeO2 –Al2 O3 , an iron oxide monolayer was deposited onto a ceria–alumina commercial support to promote its redox properties. Catalytic activities in the WGS were remarkably different for both systems. The catalytic activity of the Au/CeO2 –FeOx /Al2 O3 catalyst was higher than the one shown by the Au/FeOx /CeO2 –Al2 O3 catalyst that resulted active at much higher temperatures. In situ XRD demonstrates the formation of magnetite (Fe3 O4 ) during the WGS reaction and the presence of big gold particles, ca. 21 nm in diameter, in the low-activity system. This in contrast to the high-activity system that shows undetectable gold nanoparticles and the absence of diffraction peaks corresponding to magnetite during the WGS. The data obtained using in situ XANES states that Ce4+ species undergo reduction to Ce3+ during the WGS for both catalysts, and also confirms that in the highactivity catalyst iron is just present as Fe3+ species while in the low-activity catalyst Fe3+ and Fe2+ coexist, resulting in iron spinel observed by XRD. These results allow us conclude that the Au/CeO2 –Fe2 O3 /Al2 O3 catalyst is a suitable catalyst for WGS when avoiding the formation of magnetite, in such a case Fe3+ species favors reduction and water splitting increasing the catalytic activity in the WGS reaction. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The water-gas shift reaction (WGSR) is one of the oldest catalytic processes used in the chemical industry for hydrogen production [1]: CO + H2 O ↔ H2 + CO2 (H = −41 kJ/mol) Recently, the interest of the scientific community in this process has exponentially increased due to the clean-up requirements of H2 -rich reformate gas streams for feeding polymer electrolyte fuel cells (PEMFC). In contrast with preferential oxidation (PROX), the WGSR not only reduces CO concentration but also produces hydrogen. Nevertheless, the WGSR is an equilibrium-limited reaction making mandatory further clean-up processes for achieving the desirable levels of carbon monoxide in the gases feeding PEMFCs (<50 ppm) [2].

∗ Corresponding author. E-mail address: [email protected] (T.R. Reina). 0920-5861/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cattod.2012.08.004

In the last decade, Au nanoparticles supported on CeO2 have been widely studied as a very efficient system for the WGS reaction [3–6]. The promotion of the WGS activity by ceria is due to its ability for undergoing fast reduction/oxidation cycles since its intrinsic redox behavior (Ce4+ /Ce3+ ). This ability results in the formation of oxygen vacancies whose concentration has been directly related to the catalytic activity in the WGSR [7]. In principle, the presence of Ce3+ in the oxide phase of the catalyst can help in the dissociation of water and in the activation of the metal phase. Several strategies are used for improving the activity of gold catalysts when supported on cerium oxide based solids, most of them directed towards the increase in the number of oxygen vacancies. Mixed ceria–alumina supports results in an increased number of oxygen vacancies with respect to pure ceria, in avoiding ceria crystallites to sinter and gold particles agglomeration during the WGS reaction, thus maintaining a high catalytic activity in the steady state [8]. In general, doping CeO2 with elements having different ionic radii and/or oxidation state than cerium enhances the redox properties of ceria [9–11]. Redox reactions involving cerium species, vacancies and the nature of the dopant ions have been proposed for explaining the enhanced reducibility of doped ceria surfaces [9]. However, either the formation of CeO2 –MOx solid

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solutions or CeO2 –MOx segregated phases depends on the preparation method [12,13]. Thus the solubility limit of iron in CeO2 –FeOx solid solutions is higher for solids prepared by the microemulsion method [12] than for solids prepared by the conventional impregnation route [9]. This later system seems particularly interesting since iron has its own redox behavior (Fe3+ /Fe2+ ) and is active in the WGSR [14]. In fact, we have shown in a recent paper the positive effect of Fe2 O3 as a ceria promoter for CO oxidation on gold catalysts supported on ceria–alumina [15]. Supporting CeO2 –FeOx system on alumina increases the catalyst surface area, may maximize the oxygen vacancies concentration [8] and, therefore, may increase the catalytic activity in the WGSR. On the other hand, the presence of iron in CeO2 –FeOx solid solutions results in altering the ceria band structure leading to a decrease of the energy barrier for oxygen migration [10], which results in favoring redox reactions such as WGS; however, for segregated Fe2 O3 phases on ceria the catalytic properties depends on the rich electron sites created in the perimeter of contact between the solid solution and the segregated metal oxide [12]. Concerning WGSR pathways, it is established that in Au/CeO2 catalysts (even for the inverse system CeO2 /Au) water splitting occurs on ceria oxygen vacancies, CO adsorbs on Au nanoparticles and subsequent reaction steps take place at the metal-oxide interface [16]. On the other hand, in operando DRIFTS studies have revealed that water addition provokes oxidation of the ceria surface and improves the CO oxidation activity [17]. However, further studies are required to establish the role of iron as a doping agent and the nature of the active sites in Au/CeO2 –FeOx catalysts during the WGSR. In order to address these questions an in situ characterization of iron-promoted gold ceria catalyst under reaction conditions is performed. Synchrotron-based in situ time-resolved X-ray absorption spectroscopy (TR-XAS) and time-resolved X-ray diffraction (TRXRD) at different temperatures relevant in the WGSR are employed to study water-gas shift reaction over two different iron-promoted gold ceria catalysts designed to maximize either ceria–iron oxide solid solution or iron oxide segregated phases. The remarkably different catalytic behavior obtained in these two cases will help in obtaining clues on the nature of the active sites and the role of iron in the WGS catalytic activity of iron promoted Au/CeO2 catalysts.

2. Experimental 2.1. Catalyst preparation Two supports were prepared in order to obtain either FeOx –CeO2 solid solutions or segregated Fe2 O3 phases onto the ceria surface, in this later case the chemical composition was chosen in such a way that an iron oxide theoretical monolayer were achieved, a detailed description of this procedure is given elsewhere [15]. The FeOx /CeO2 –Al2 O3 support, intended for being constituted by segregated iron oxide particles on the ceria surface, was synthesized by impregnation. The required amount of the iron oxide precursor (Fe(NO3 )3 ·9H2 O (Aldrich) for obtaining a Fe2 O3 monolayer on the CeO2 –Al2 O3 support, ca. 7.9 wt% of iron oxide, was dissolved in excess ethanol and submitted to a round bottom flask were a commercial CeO2 –Al2 O3 support (20% CeO2 –80% Al2 O3 ; Puralox, Sasol) was added. The excess solution was eliminated at 50 ◦ C under reduced pressure. The obtained solid was then calcined at 500 ◦ C for 4 h. The CeO2 –FeOx /Al2 O3 support, intended for supporting a ceria–iron oxide solid solution on alumina, was synthesized by a procedure similar to the one used for the FeOx /CeO2 –Al2 O3 support but impregnating simultaneously iron and cerium species

Table 1 Chemical composition the prepared solids.

a

Samplea

Al2 O3 (wt%)

CeO2 (wt%)

FeOx (wt%)

Au (wt%)

Au/CeO2 –FeOx /Al2 O3 Au/FeOx /CeO2 –Al2 O3

80.9 (81) 73.1 (72)

14.3 (15) 19.1 (18)

1.4 (2) 6.59 (8)

2.2 (2) 1.26 (2)

Nominal values in parenthesis.

on a commercial ␥-alumina powder (Sasol). Ce(NO3 )3 ·6H2 O and Fe(NO3 )3 ·9H2 O (Aldrich) were used as metal precursors. After eliminating the excess of solvent the solid was submitted for 30 min to a 10 mol L−1 NH3 solution to assure the conversion of the nitrates precursors to hydroxides. The solid was then filtered, dried and calcined at 500 ◦ C for 4 h. The initial precursor quantities were calculated in such a way that the total amount of CeO2 , Fe2 O3 and Au should represent ca. 20 wt% of the final solid, as expressed in Table 1. The iron oxide content was chosen following the solubility limits of Fe3+ to obtain a solid solution with ceria as stated by Laguna et al. [11]. Gold catalysts (2 wt% nominal loading) were prepared using these two supports by the direct anionic exchange method, assisted by NH3 (30% Aldrich) as described elsewhere [18]. The obtained solids were dried overnight at 70 ◦ C and finally calcined at 400 ◦ C during 4 h with a heating rate of 10 ◦ C min−1 . 2.2. Characterization The chemical composition of the samples was determined by X-ray microfluorescence spectrometry (XRMF) in an EDAX Eagle III spectrophotometer with a rhodium source of radiation working at 40 kV. The textural properties were studied by N2 adsorption–desorption measurements at liquid nitrogen temperature. The experiments were carried out on a Micromeritics ASAP 2010 instrument. Before analysis, the samples were degassed for 2 h at 150 ◦ C in vacuum. The Barrett–Joyner–Halenda (BJH) method was used for determining the pore size distributions, and in every case desorption isotherm was used. 2.2.1. Time-resolved X-ray diffraction The time-resolved X-ray diffraction experiments were carried ˚ of the National Synchrotron out on beamline X7B ( = 0.3196 A) Light Source (NSLS) at Brookhaven National Laboratory (BNL). The powder sample (3 mg) was loaded into a glass capillary cell (ID = 0.7 mm) which was attached to a flow system [19–21]. A small resistance heating wire was installed right below the capillary and the temperature was monitored with 0.5 mm chromel–alumel thermocouple that was placed in the capillary near the sample. The samples were first activated in oxygen/helium (21% of oxygen) mixture at 350 ◦ C for 1 h, then cooled and step-wise heated from room temperature to 350 ◦ C in a WGS environment, 3% H2 O and 5% CO in He with a flow rate of 10 ml/min., A Perkin-Elmer detector was used to record X-ray patterns. Two dimensional powder patterns were collected with an image-plate detector and the powder rings were integrated using the FIT2D code [22]. Time-resolved Au L3 -edge, Ce L3 -edge, and Fe K-edge XANES spectra were collected in situ during the WGS at beamline X18B of the NSLS at BNL. The XANES spectra were taken in the “fluorescence-yield mode” for iron and in “transmission-yield mode” for gold and cerium. The catalysts were loaded into a 3.5 mm ID kapton capillary cell that was attached to the same flow system used in time-resolved XRD experiments [19–21]. The temperature was monitored with a 0.1 mm chromel–alumel thermocouple placed in the capillary near the catalyst. For every experiment a fresh sample was activated in an oxidant flow (21% O2 balanced in He) at 350 ◦ C for 1 h. After activation the sample was cooled and

T.R. Reina et al. / Catalysis Today 205 (2013) 41–48

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Au/CeO2-FeOx/Al2O3 Au/CeO2/Al2O3

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Au/FeOx/CeO2-Al2O3

Au/CeO2-Al2O3

Au/CeO2-Al2O3

16

5,2

5,4

5,6

2 theta

5,8

6,0

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6,6

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Fig. 1. (A) XRD patterns of the studied catalysts compared to their corresponding iron-unpromoted systems. (B) Main ceria diffraction peak.

step-wise heated from room temperature to 350 ◦ C in a WGS environment, 3% H2 O and 5% CO in He with a flow rate of 10 ml/min. Spectra were collected for 1 h at each of the following temperature steps: room temperature, 150, 250, 300, and 350 ◦ C. 2.3. Catalytic activity Water-gas shift reaction tests were performed in a flow reactor at atmospheric pressure and 140 to 350 ◦ C temperature range. To reproduce as far as possible the conditions in the XANES and TR-XRD flow cells a high temperature catalytic DRIFTS chamber (Spectra-Tech) was employed. The catalytic cell has 0.1 cm3 bed volume accounting for ca. 30 mg catalyst, which results in a GHSV of 18,000 h−1 and WHSV of 1 L g−1 min−1 when working at a water vapor partial pressure of 17.3 kPa and the reactant gas mixture containing 5 vol.% CO in helium. The catalytic activity measured in percentage of CO conversion was determined through quantitative gas analysis carried out on a Blazers Omnistar Bentchop mass spectrometer instrument calibrated using certified gas mixtures of CO and CO2 in helium. 3. Results and discussion 3.1. Catalysts characterization 3.1.1. Chemical compositions and textural properties The chemical composition of the catalysts is shown in Table 1. The iron oxide content is slightly below the target value in both systems. The estimate for a Fe2 O3 theoretical monolayer over the commercial ceria–alumina support is 7.9 wt% [15], and a 2 wt% was the targeted iron oxide loading for the Au/CeO2 –FeOx /Al2 O3 catalyst; therefore, the relatively small iron oxide loss occurring in both catalysts must be associated to the highly basic media employed in the gold deposition process. On the other hand, the synthesized supports show different behaviors with respect to gold; the gold content of the Au/CeO2 –FeOx /Al2 O3 catalyst (2.17 wt%) is quite close to the nominal value, however, the gold content of the Au/FeOx /CeO2 –Al2 O3 catalyst is ca. 37% below the targeted one. If the formation of an iron oxide monolayer on the CeO2 –Al2 O3 support is assumed, the poorer capacity of this support for holding gold anchored on its surface may be associated to the strong interaction of the support with the modifying monolayer, since this contact limits the support ability to create new interactions with other species, gold particles in particular [15,23–25]. Besides this, CeO2 –FeOx supported on alumina forms Ce–Fe oxide solid solution that maximizes the oxygen vacancy concentration while

Table 2 Ceria lattice parameter of the studied Au/CeO2 –FeOx /Al2 O3 catalysts compared to their corresponding systems without iron oxide. Sample

Ceria lattice parameter (Å)

Au/CeO2 –FeOx /Al2 O3 Au/CeO2 /Al2 O3 Au/FeOx /CeO2 –Al2 O3 Au/CeO2 –Al2 O3

5.38 5.48 5.41 5.41

supporting FeOx on CeO2 –Al2 O3 generates segregated iron oxide phases resulting in a lower concentration of oxygen vacancies [8,12]. Therefore, as oxygen vacancies act as nucleation centers for gold deposition the amount of deposited gold and its dispersion may be improved [9–11]. Both catalysts are mesoporous materials having specific surface areas of ca. 175 m2 g−1 governed by the presence of the primary ␥-Al2 O3 support. A slight increase of the specific surface area of the gold catalysts compared to their corresponding support is observed. As for example, the measured specific surface area of CeO2 –FeOx /Al2 O3 and FeOx /CeO2 –Al2 O3 of 175 and 160 m2 g−1 respectively increase to 184 and 167 m2 g−1 after gold deposition. This effect has been early reported [26] and attributed to the increase of the pore volume caused by the presence of gold nanoparticles inside the pore structure.

3.1.2. In situ XRD In situ XRD patterns of the prepared systems are shown in Fig. 1. Fig. 1A presents XRD patterns of the studied Au/CeO2 –FeOx /Al2 O3 and Au/FeOx /CeO2 –Al2 O3 catalysts compared with their parent catalysts in the absence of iron. Both Au/CeO2 –FeOx /Al2 O3 and Au/FeOx /CeO2 –Al2 O3 solids showed the diffraction patterns of the cubic CeO2 fluorite type structure (JCPDS# 00-004-0593) and ␥Al2 O3 (JCPDS# 00-048-0367). However, the presence of crystalline iron oxides, hematite Fe2 O3 (JCPDS# 00-033-0664) or magnetite (JCPDS# 00-65-3107) was not detected. The formation of a mixed oxide phase, a rather high dispersion, or the amorphous character of the iron oxide may account for this fact. Actually, for the Au/CeO2 –FeOx /Al2 O3 catalyst a Ce–Fe solid solution is detected through the shift of the cerianite (1 1 1) diffraction line towards higher diffraction angles (Fig. 1B). The cerianite lattice parameter is calculated (Table 2), using the conventional expression for a FCC structure. ˛=



h2 + k2 + l2



 2 sin 



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A

B CeO2 Au

25ºC 350ºC 300ºC

intensity (a.u.)

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CeO2 Fe2O3 Au

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150ºC

25ºC

6

8

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2 theta

25ºC

6

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Fig. 2. In situ XRD patterns: (A) Au/FeOx /CeO2 –Al2 O3 activation in oxygen/helium, (B) Au/CeO2 –FeOx /Al2 O3 activation in oxygen/helium, (C) Au/FeOx /CeO2 –Al2 O3 during WGS, (D) Au/CeO2 –FeOx /Al2 O3 during WGS.

The calculated ceria lattice parameter for the CeO2 –Al2 O3 support, 0.541 nm, matches the experimental value of bulk cerium oxide. By adding iron oxide to this support the lattice parameter remains unchanged. However, the ceria lattice parameter for the prepared CeO2 /Al2 O3 support is slightly higher, 0.548 nm, which may indicate a smaller cerium oxide particle size. Zhang et al. have shown a cerium oxide lattice expansion on decreasing ceria particle size mainly due to an increase in oxygen defect concentration [27]. The coprecipitation of cerium and iron oxides results in a support with a ceria lattice parameter well below the one observed after precipitating cerium alone clearly indicating the formation of a CeO2 –FeOx solid solution. Ce–Fe solid solution results ˚ in ceria lattice contraction due to the substitution of Ce4+ (0.97 A) ˚ On the contrary no indications of ceria–iron solid by Fe3+ (0.64 A). solution were observed for the monolayer-doped system. Finally, the main and a very relevant difference between both systems was the presence of diffraction lines characteristic of metallic gold (JCPDS# 7440-57-5) in the Au/FeOx /CeO2 –Al2 O3 system compared to Au/CeO2 –FeOx /Al2 O3 where gold diffraction peaks were not found. Gold particle size was calculated by the Scherrer method [28] using the (1 1 1) diffraction line. The gold particle size resulted to be 21 nm for the Au/FeOx /CeO2 –Al2 O3 system meanwhile gold particle size for the Au/CeO2 –FeOx /Al2 O3 system must be considered lower than the limit of detection of the instrument (<2 nm).

Catalyst activation in oxidative atmosphere does not alter the XRD patterns of both catalysts except for a slight shift of the diffraction peaks due to a thermal expansion (Fig. 2A and B). Fig. 2C and D presents in situ temperature resolved XRD experiments obtained during the WGS over the Au/FeOx /CeO2 –Al2 O3 and Au/CeO2 –FeOx /Al2 O3 catalysts, respectively. The Au/CeO2 –FeOx /Al2 O3 XRD pattern remains unaltered whatever the reaction temperature, neither new phase appears nor gold particle agglomerates. Within the detection limits of XRD, the Au/CeO2 –FeOx /Al2 O3 catalyst seems to be stable in terms of dispersion and nature of present phases during the WGS process. However, the Au/FeOx /CeO2 –Al2 O3 catalyst underwent important modifications in the dispersion and nature of the present phases. For reaction temperatures above 200 ◦ C, the formation of an iron spinel (magnetite) Fe3 O4 (JCPDS# 65-3107) is evident, which accounts for a decrease in the dispersion of the iron oxide phase and possibly for a partial reduction of the iron phase. Upon cooling this new generated phase remains on the catalyst surface resulting in an irreversible modification of the Au/FeOx /CeO2 –Al2 O3 catalyst. 3.1.3. In situ XANES To fully understand the catalytic cycle not only the size and crystalline structure of the active phases but also their electronic structures have to be identified. A complete in situ XANES study

T.R. Reina et al. / Catalysis Today 205 (2013) 41–48

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Au L3 edge during WGS

0

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Fig. 3. Au L3 XANES spectra collected during WGS: (A) Au/CeO2 –FeOx /Al2 O3 catalyst, (B) Au/FeOx /CeO2 –Al2 O3 catalyst.

during the WGS was carried out to accomplish this goal for both catalysts. Normalized and background-corrected XANES Au L3 edge spectra for the Au/CeO2 –FeOx /Al2 O3 and Au/FeOx /CeO2 –Al2 O3 catalysts are presented in Fig. 3A and B, respectively. Four peaks can be distinguished in the XANES profiles for both systems. The first resonance labeled 1, just past the edge (3.8 eV), is the white line, arising from 2p3/2,1/2 → 5d dipole transitions, the intensity of which is very strong for the majority of the transition metals with a partially occupied d-band and it is related to the unoccupied density of d-states [29]. Although the 5d orbitals in gold are nominally filled, a s–p–d hybridation allows a small white line in XANES spectra of bulk Au [30]. The other three peaks contain information of the extended local structure around the absorbing atom and their shifting provides information about the gold cluster size [31]. The higher the energy of these features the smaller the Au–Au interatomic distances as well as a higher oxidation state of the metal [32,33]. In our case a clear shift of the highest energy peak towards lower energies during WGS in both catalysts suggests that gold agglomeration occurs during the reaction. It is well known that gold particles tend to nucleate in the support defects (typically oxygen vacancies) in such a way that during water gas shift, gold particles are in competition with water to be situated on these electron rich places [34]. In this sense, the activation of water on the support vacancies together with the thermal instability of gold nanoparticles may explain the observed gold agglomeration. In situ Ce-L3 XANES spectra of the samples are shown in Fig. 4A and B. The XANES line shape of Ce3+ and Ce4+ are quite different [35]. Typically three peaks are identified in the XANES spectra of cerium compounds, peak labeled 3 of Ce4+ is usually assigned to Ce4+ absorption into the 5d level where there is no occupancy of 4f orbitals for either initial or final state and it is used to detect CeO2 presence. This is the situation at the beginning of the WGS for both catalysts. However when Ce4+ is reduced to Ce3+ , this band disappears [36]. XANES profiles reveal this reduction process in our gold catalysts under WGS conditions. The peak identified as 2 is split into at least, two separate assignments accounting for different final state configurations [36,37]. Moreover during ceria reduction peaks 2 and 3 decrease and a new peak labeled 1, related to Ce3+ species increases. This transition accounts for an absorption into the 5d level with 4f occupancy in the final state. In our experiments this peak develops progressively with temperature indicating CeO2 reduction during the reaction. The reduction process started at 150 ◦ C where both Ce4+ /Ce3+ XANES peaks are present and seems to be finished at 250 ◦ C where the peak labeled 1 is predominant. On the other hand, small differences are observed in the relative intensities of peaks labeled 2 and 3 depending on the catalyst nature, the

intensity ratio of these two peaks correlates with the covalency of the Ce O bond that is affected by the formation of the solid solution altering this way the Lewis basicity of the oxide ions involved in the Ce O Fe bonds. This electronic density modification may affect the stability of the proposed intermediate species in the WGS reaction affecting the catalyst activity. The XANES Fe K-edge spectra shown in Fig. 5A and B corresponds to ex situ experiments prior and after reaction of the Au/CeO2 –FeOx /Al2 O3 and Au/FeOx /CeO2 –Al2 O3 respectively. Apparently, both post reaction samples exhibit similar profiles. Nevertheless, the first derivate of the Fe K-edge spectrum of the Au/CeO2 –FeOx /Al2 O3 system (Fig. 5C) correlates with Fe2 O3 hematite structure while subtle modifications in the first derivate of the Au/FeOx /CeO2 –Al2 O3 catalyst Fe K-edge spectrum points to a system gaining in Fe3 O4 character [38]. In addition to this, minute but critical differences were found in the Fe K pre-edge region. Pre-edge features are always observed on the low-energy side of K-absorption edges of first row transition elements. Although these electric dipole transitions are forbidden in a centro-symmetric environment, they can be observed due to quadropole coupling effects [39]. We would then expect that tetrahedrally coordinated iron compounds show more intense pre-edge than octahedrally coordinated ones [40]. Our Au/FeOx /CeO2 –Al2 O3 sample showed a slightly more intense pre-edge (Fig. 5D). In addition, a spinel structure in this catalyst is expected due to the pre-edge broadening and shifting to lower absorption energies pointing the presence of additional octahedrally Fe2+ species of the inverse Fe3 O4 spinel in good agreement with in situ XRD data. In summary XANES data reveal that Au/CeO2 –FeOx /Al2 O3 catalyst is composed by metallic gold, Ce4+ /Ce3+ and Fe3+ (Fe2 O3 ) during the WGS meanwhile the Au/FeOx /CeO2 –Al2 O3 sample is composed by metallic gold, Ce4+ /Ce3+ and Fe3+ –Fe2+ (Fe3 O4 ) during the catalytic process. 3.1.4. Water gas shift catalytic activity The catalytic activity of the studied systems is compared in Fig. 6. Each catalyst behaves differently. The Au/CeO2 –FeOx /Al2 O3 catalyst approaches the equilibrium conversion at 350 ◦ C while the Au/FeOx /CeO2 –Al2 O3 catalyst is considerably less active reaching just a 40% CO conversion at 350 ◦ C. Reaction rates for the Au/CeO2 –FeOx /Al2 O3 catalyst are 10.6 × 10−4 and 3.6 × s−1 at 350 and 180 ◦ C, respectively, whereas for 10−4 molCO g−1 Au the Au/FeOx /CeO2 –Al2 O3 catalyst the reaction rates are 9.1 × 10−4 and 0.5 × 10−4 molCO g−1 s−1 at 350 and 180 ◦ C, respecAu tively. Although the parent gold catalysts without iron oxide promoter have not been measured in the same conditions an exhaustive catalytic study on the effect of iron oxide content of

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150 ºC 25 ºC

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60

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Fig. 4. Ce L3 XANES spectra collected during WGS: (A) Au/CeO2 –FeOx /Al2 O3 , (B) Au/FeOx /CeO2 –Al2 O3 catalyst.

these catalysts have shown that iron promotion enhances the catalytic activity of the Au/CeO2 /Al2 O3 catalyst [41]. It has been demonstrated that for Au/CeO2 model catalysts the activation of CO takes place on gold nanoparticles while water dissociation, which is the rate determinant step in the WGS process, occurs on the support, further steps leading to the formation of carbon dioxide and hydrogen takes place at the metal-support interface [16]. Therefore, in our catalysts the activation of CO will occur on gold particles while water dissociation is going to happen on CeO2 promoted by iron oxide (hematite or spinel).

A

The catalytic behavior exhibited by the catalysts is in good agreement with the in situ characterization data discussed above. First, the XRD patterns of the catalysts show that gold nanoparticles are smaller for the Au/CeO2 –FeOx /Al2 O3 catalyst than for the Au/FeOx /CeO2 –Al2 O3 one. Kung et al. [42] postulate that CO oxidation occurs with the participation of Au–OH species formed at a perimeter of the gold nanoparticles by transferring hydroxide ions from the support. These hydroxyl species further react with CO adsorbed on low-coordination sites of gold, therefore, as the smaller the size of gold particles the higher the catalyst activity.

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8

E- E0 (eV)

Fig. 5. Fe K XANES spectra collected during WGS: (A) Au/CeO2 –FeOx /Al2 O3 , (B) Au/FeOx /CeO2 –Al2 O3 catalyst, (C) first derivate of the post reaction XANES profile, (D) pre-edge region.

T.R. Reina et al. / Catalysis Today 205 (2013) 41–48

of oxygen vacancy formation and therefore of the oxygen transfer reaction rate with respect to pure ceria, this improves the redox properties of the former solid [9,12]. Cerium oxide reduction results in the formation of oxygen vacancies, this may favor water dissociation that typically takes place on the support. Our XRD data points to the formation of Ce–Fe oxide solid solution for the Au/CeO2 –FeOx /Al2 O3 catalyst while the Au/FeOx /CeO2 –Al2 O3 one results during WGS in segregated iron oxide phases, the reducibility of the former may be higher than the reducibility of the latter and so does the oxygen transfer reaction rate, which may affect the catalytic activity. Upon increasing reaction temperature the feature labeled 1 in the Ce LIII edge spectra of the studied catalysts increases indicating partial reduction of Ce4+ to Ce3+ species during WGS. At the same time, the ratio between the intensity of peaks labeled 2 and 3 changes indicating a change in the number of vacancies and the covalency of the system, As previously reported in XAS studies of the LIII -edge of cerium oxides [43,44] the cerium average valence is empirically defined using the area under the deconvoluted curves assigned to the different final state transitions. Fig. 7 shows an example of deconvolution of the LIII -edge of cerium corresponding to the Au/CeO2 –FeOx /Al2 O3 catalyst during the WGS at 350 ◦ C. After deconvoluting all the cerium L3 -edge spectra, the average valence of cerium species in both catalysts can be estimated (Fig. 8A). The reducibility of cerium species in the Ce–Fe oxide solid solution is higher than in the case where the prepared catalyst consisted in cerium oxide and iron oxide segregated phases in all the temperature range. At the same time the energy difference between the peaks labeled 2 and 3, E(2, 3), that accounts for the covalency of the system follows the same trend that the average oxidation state (Fig. 8B). It has been shown that the energy difference, E(2, 3), systematically increase on increasing ceria particle size. This increase is related, in covalent compounds like CeO2 , to an enhancement of covalence between Ce(4f) and O(p) [43]. The different covalency between the Ce O bonds for both catalysts is generated by the different synergy between cerium and iron as a result of the catalyst structure. This justifies the different stability of the surface hydroxide ions generated upon dissociation of water molecules in the support oxygen vacancies and, therefore, the different reactivity of both catalysts. XANES data (Figs. 7 and 8A, B), clearly demonstrate that reduction takes place during water gas shift favoring water dissociation by generating surface oxygen vacancies. Moreover, the modification of the Ce O bond covalent character alters the stability of the hydroxide ions on the support. Cerium–iron oxide solid solution

100

Au/CeO2-FeOx/AlO3 Au/FeOx/CeO2-Al2O3 equilibrium

60

40

20

0 150

200

250

300

350

temperature (ºC) Fig. 6. WGS activity of the gold catalysts.

Fig. 7. Deconvolution of the L3 edge of cerium corresponding to the Au/CeO2 –FeOx /Al2 O3 catalyst during the WGS at 350 ◦ C.

The Au/CeO2 –FeOx /Al2 O3 catalyst consisting of very small and welldispersed gold nanoparticles does show a higher catalytic activity in the WGSR than the Au/FeOx /CeO2 –Al2 O3 one. In the presence of supports with redox properties, such as the Ce–Fe mixed oxides, CO oxidation may also occur by active oxide ions transferred from the support. CeO2 –FeOx solid solutions result in an enhancement

4,4

45

4,4

B

A 40

E (2,3) (eV)

Ce (III) (%)

35 30 25

4,2

4,2

4,0

4,0

3,8

3,8

3,6

3,6

3,4

3,4

Ce Valence

CO conversion (%)

80

47

20 15

3,2

10 50

100

150

200

Temperature (ºC)

250

300

350

0

10

20

30

40

50

60

70

80

90

3,2 100

CO conversion (%)

Fig. 8. (A) Estimate of the average valence of cerium during the WGS as a function of reaction temperature for the studied catalysts. (B) Influence of the covalency of the Ce O bond and the average oxidation state of cerium species on the CO conversion during WGS for the studied catalysts.

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favors the reducibility through the reduction of Ce4+ ions by Fe2+ species resulting from the reduction of Fe3+ species during WGS as proposed for Ce–Ni oxide solid solutions [45] or CeO2 –ZnO mixed oxides [46]. If iron oxide is present as a segregated phase Fe3+ reduction during WGS result in divalent iron ions that stabilizes by crystallizing as Fe3 O4 spinel structures. Thus the cerium–iron synergy is restricted to the perimeter of the spinel structure. In this way, the reducibility of the support is smaller than in the case of the solid solution. 4. Conclusions The activity of gold catalysts when supported on cerium oxide based solids is generally improved by increasing the number of oxygen vacancies. Promoting cerium oxide with trivalent or divalent transition metal ions allows reaching this. However, the catalyst synthesis method greatly influences the catalyst performances since the resulting microstructure affects to the catalyst reducibility. In this study we have prepared two gold catalysts directed to form CeO2 –FeOx solid solutions supported on alumina, Au/CeO2 –FeOx /Al2 O3 , or to form segregated iron oxide phases on top of the ceria phase, Au/FeOx /CeO2 –Al2 O3 . The activity in the WGSR of the Au/CeO2 –FeOx /Al2 O3 is remarkably higher than the one of the Au/FeOx /CeO2 –Al2 O3 catalyst. In situ XRD and XANES studies have clarified the superior activity of the Au/FeOx –CeO2 /Al2 O3 catalyst in terms of gold particle size and oxidation state of the species involved in the reaction. Moreover, the cerium–iron synergy alters the covalent character of the Ce O bonds affecting the stability of the surface hydroxyl groups and hence the catalytic activity in the WGSR. Acknowledgments T. Ramírez acknowledges CSIC for his JAE-Predoc fellowship and S. Ivanova MEC for her Ramon y Cajal contract. The Spanish Ministerio de Ciencia e Innovación under Contract ENE2009-14522-C05-01) and Junta de Andalucía under Contract P09-TEP-5454 provide financial support for this work, both programs are co-funded by the European Union FEDER Program. All the authors acknowledge US Department of Energy for supporting part of this work carried out at X7B and X18B of the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory (BNL). The work done at the Chemistry Department of BNL was supported by the U.S. Department of Energy, Division of Chemical Sciences under Contract DE-AC02-98CH10886. References [1] D.L. Trimm, Z.I. Onsan, Catalysis Review 43 (2001) 31–84. [2] R. Burch, Physical Chemistry Chemical Physics 8 (2006) 5483–5500. [3] J.A. Rodriguez, P. Liu, J. Hrbek, J. Evans, M. Pérez, Angewandte Chemie International Edition 46 (2007) 1329–1332. [4] D. Andreeva, I. Ivanov, L. Ilieva, J.W. Sobezak, G. Avdeev, K. Petrov, Topics in Catalysis 44 (1–2) (2007) 173–182. [5] W. Deng, J. De Jesus, H. Saltsburg, M. Flytzani-Stephanopoulos, Applied Catalysis A 291 (2005) 126–135. [6] A. Abd El-Moemen, G. Kucerova, R.J. Behm, Applied Catalysis B 95 (2010) 57–70. [7] T. Tabakova, F. Boccuzzi, M. Manzoli, D. Andreeva, Applied Catalysis A 252 (2003) 385–397. [8] D. Andreeva, I. Ivanov, L. Ilieva, M.V. Abrashev, Applied Catalysis A 21 (2006) 127–132.

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