# Understanding of the oxygen activation on ceria and ceria/alumina-supported gold catalysts: a study combining 18O/16O isotopic exchange and EPR spectroscopy

Pandian Lakshmanan, Frédéric Averseng, Nicolas Bion, Laurent Delannoy, Jean-Michel Tatibouet, Catherine Louis

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Pandian Lakshmanan, Frédéric Averseng, Nicolas Bion, Laurent Delannoy, Jean-Michel Tatibouet, et al.. Understanding of the oxygen activation on ceria and ceria/alumina-supported gold catalysts: a study combining 18O/16O isotopic exchange and EPR spectroscopy. Gold bulletin: a quarterly review of research on gold and its applications in industry, 2013, 46, pp.233-242. 10.1007/s13404-013-0103-z. hal-00961263

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# Understanding of the oxygen activation on ceria- and ceria/alumina-supported gold catalysts: a study combining ¹⁸O/¹⁶O isotopic exchange and EPR spectroscopy

Pandian Lakshmanan · Frédéric Averseng · Nicolas Bion · Laurent Delannoy · Jean-Michel Tatibouët · Catherine Louis

Published online: 7 September 2013

© The Author(s) 2013. This article is published with open access at SpringerLink.com

**Abstract** Gold supported on ceria or ceria–alumina mixed oxides are very active catalysts for total oxidation of a variety of molecules. The key step of the oxygen activation on such catalysts is still a matter of debate. Gold–ceria (Au/CeO₂) and gold–ceria–alumina (Au/CeO₂/Al₂O₃) catalysts were prepared by deposition–precipitation of gold precursor with urea as in former works where their efficiency to catalyze the oxidation of propene and propan-2-ol was demonstrated. To understand the phenomenon of oxygen activation over this class of catalysts, efficient techniques generally used to characterize the interaction between oxygen and cerium-based oxides were applied; the oxygen storage capacity (OSC) measurement, the ¹⁸O₂/¹⁶O₂ isotopic exchange study (OIE), as well as characterizations by in situ Raman and electron paramagnetic resonance (EPR) spectroscopies. Each of the techniques allowed showing the impact of the gold nanoparticles on the activation of dioxygen, on the kinetic governing the gas-phase/ solid oxygen atom exchange, and on the nature and the location of the adsorbed oxygen species. Gold nanoparticles were shown to increase drastically the OSC values and the rate of oxygen exchange. OIE study demonstrated the absence of pure equilibration reaction (¹⁶O₂(g) + ¹⁸O₂(g) ↔ 2 ¹⁶O¹⁸O(g)), indicating that gold did not promote the dissociation of dioxygen. Peroxo adspecies were observed by Raman spectroscopy only

in the presence of gold. On the contrary, EPR spectroscopy indicated that the concentration of superoxo adspecies was lower for oxide-supported gold samples than for bare oxides. The combination of techniques allowed reinforcing the hypothesis that the gold nanoparticles promote the activation of dioxygen by generating extremely mobile diatomic-oxygenated species at the gold/ceria interfacial perimeter. This specific gold–ceria interaction, which leads to the increase in oxygen mobility, is probably also responsible for the higher catalytic performance of Au/CeO₂ and Au/CeO₂/Al₂O₃ in oxidation reaction compared to bare supports.

**Keywords** Gold · Heterogeneous catalysis · ¹⁸O₂/¹⁶O₂ isotopic exchange · EPR · Raman · Ceria · Oxygen storage capacity · Oxygen species

## Introduction

Since the exceptional discovery of Haruta who demonstrated that, when smaller than 5 nm, the gold nanoparticles can display very high catalytic CO oxidation activity even at 203 K [1, 2], hundreds of papers on the topic of catalysis by gold have been annually published with a progression which remains exponential. Then, numerous catalytic formulations have been developed, and the efficiency of nanostructured gold deposited on reducible transition metal oxide supports was particularly reported [3]. Among the reducible oxide supports, ceria was particularly investigated because of its capability to act as an oxygen reservoir by storing/releasing oxygen through a redox process which involves the Ce⁴⁺/Ce³⁺ couple. Nanocrystalline ceria-supported gold catalysts were then studied in various catalytic reactions including the water–gas shift reaction [4, 5], the VOC combustion [6–10], the selective oxidation of alcohol [11], the selective oxidation of CO in excess of H₂ (CO-PROX) [12], and the most investigated reaction remaining the low-temperature CO oxidation [13, 14]. The morphology of

**Electronic supplementary material** The online version of this article (doi:10.1007/s13404-013-0103-z) contains supplementary material, which is available to authorized users.

P. Lakshmanan · F. Averseng · L. Delannoy · C. Louis
Laboratoire de Réactivité de Surface, UMR 7197 CNRS, Université Pierre et Marie Curie-UMPC, 4 place Jussieu, 75252 Paris Cedex 05, France

P. Lakshmanan · N. Bion (✉) · J.-M. Tatibouët
CNRS UMR 7285, Institut de Chimie des Milieux et Matériaux de Poitiers (IC2MP), University of Poitiers, 4 rue Michel Brunet, 86022 Poitiers Cedex, France
e-mail: nicolas.bion@univ-poitiers.fr
the ceria crystallites, the size of the gold clusters (particles), and the oxidation state of the active gold atoms are the parameters usually discussed because of their determining importance for the catalytic oxidation activity [15]. The nature of the intermediate species is also studied. For instance, it has been clearly demonstrated that one of the key elements for explaining the high reactivity of nanometer-sized gold nanoparticles in CO oxidation was the abundance of low-coordinated Au atoms in the small particles, where CO can be preferentially adsorbed [16]. On the contrary, the nature of the intermediate species in the oxygen activation step is still a matter of debate [17]. One reason is the fact that the adsorbed oxygen species are not experimentally easy to identify in the conditions of the reaction. Some recent papers showed that the anion photoelectron spectroscopy [18] and infrared multiple photon dissociation [19] allowed determining which oxygenated species are involved in the activation process, but these spectroscopic studies have not been applied to ceria-supported gold catalysts. Computational method is a way to get around the experimental limitations, and very impressive demonstrations in terms of reactive intermediate species and mechanism scheme were reported [17, 20]. Nevertheless, the use of the theoretical modeling did not permit to prevent controversial conclusions.

In this work, we used a combination of characterization techniques involving oxygen storage capacity measurement (OSC), oxygen isotopic exchange (OIE), and Raman and electron paramagnetic resonance (EPR) spectroscopies. These techniques were extensively used on cerium-based oxides to understand the mechanism of dioxygen activation [21–23] and to explain the property of these oxides to store and release oxygen [24, 25]. Thus, we proposed to apply the same techniques on ceria-supported gold catalysts. In addition, gold supported on mixed oxide consisting of cerium oxide supported on alumina with two CeO₂ loadings (5 to 10 wt.%) were also studied. The latter catalytic formulations showed interesting catalytic performances in the oxidation of two types of VOCs as follows: propene [7] as model of hydrocarbon and 2-propanol [26] as model of alcohol. Ceria–alumina was preferred to ceria support because of the poor resistance of the latter against thermal sintering.

# Experimental part

## Materials

Alumina, AluC Degussa (110 m² g⁻¹), was used as the support to load cerium oxide (5 and 10 wt.% with respect to alumina) by impregnation in excess of aqueous solution of Ce(NO₃)₃, 6H₂O (Aldrich, 99.9 %), followed by calcination at 500 °C. One weight percent of gold was loaded by deposition–precipitation with urea on the various CeO₂–Al₂O₃ samples as well as on pure alumina and ceria of high surface area (HSA-5 Rhodia,

200 m² g⁻¹) and low surface area, sintered HSA-5 by aging at 800 °C under a flow of humid air for 12 h (abbreviated as CeO₂ LSA 50 m² g⁻¹). The catalysts are named as Au/xCeO₂/Al₂O₃ (x=wt% of CeO₂), Au/Al₂O₃, and Au/CeO₂. The details of experimental procedures regarding catalyst preparation and characterization by elemental analyses, N₂ physisorption at 77 K, XRD, X-ray photoelectron spectroscopy (XPS), and combined TEM–energy-filtered transmission electron microscopy (EFTEM) techniques were largely described in our previous study [7].

## Oxygen storage capacity

OSC measurements were carried out at 673 K using a U-form reactor connected to a gas chromatograph equipped with a Porapak column and a thermal conductivity detector. The experimental set-up and the protocol of experiment were reported elsewhere [27]. The samples were placed into the reactor and heated under a continuous flow of helium (30 cm³ min⁻¹) up to 673 K for 30 min before pretreatment with ten pulses (0.246 cm³) of pure O₂ at atmospheric pressure followed by a purge with pure He for 10 min. Alternate pulses of CO and O₂ were undertaken three times in order to check the reproducibility of the measurement. The amounts of unconverted CO and O₂ as well as of produced CO₂ were quantified. The OSC was calculated from the average value of CO₂ production (after CO pulse) and was expressed in µmol O g⁻¹ or µmol O m⁻² taking into account the mass or the BET surface area of the samples.

The number of surface oxygen atoms involved in the OSC process was calculated considering that only ceria participated and assuming a preferential (100) orientation of the ceria surface. Calculations have been already described in a previous publication [27].

## Oxygen isotopic exchange

OIE experiments were performed in a set-up already described elsewhere [27, 28]. A U-form reactor was placed in a closed recycle system which was connected on one side to a mass spectrometer (Pfeiffer Vacuum, QMS 200) for the monitoring of the gas-phase composition and on the other side to a vacuum pump. A recycling pump removed limitations due to gas-phase diffusion. OIE experiments were undertaken on 20 mg of catalyst, subjected to a ¹⁶O₂ activation step at 873 K under atmospheric pressure for 1 h prior to cooling to the desired temperature, at which point the system was degassed and the isotopic mixture charged. The study of homoe exchange (also called equilibration reaction) was performed using an equimolar mixture of ¹⁶O₂ and ¹⁸O₂ (99.9 % purity, supplied by Isotec), whereas for the study of the heteroe exchange, the mixture was replaced by pure ¹⁸O₂. The masses 32, 34, and 36 m/z were monitored as a function of time to follow the exchange. The m/z values of 28 and 44 were also
recorded to check the absence of air or CO₂. The atomic fraction of ¹⁸O in the gas phase (αg), the rate of exchange (Re), and the number of O atoms exchanged (Ne) were calculated as described in previous references [29]. Typically:

$$ \alpha_g = \frac{P_{36} + \frac{1}{2}P_{34}}{P_{36} + P_{34} + P_{32}} \qquad (1) $$

Where P₃₆, P₃₄, and P₃₂ were the partial pressures of ¹⁸O₂, ¹⁸O¹⁶O, and ¹⁶O₂, respectively;

$$ R_e = -N_g \frac{d\alpha_g}{dt} \qquad (2) $$

Where N_g was the number of ¹⁸O atoms in gas phase at the beginning of the reaction;

$$ N_e = N_g (1 - \alpha_g) \qquad (3) $$

Finally, the number of exchangeable atoms could be calculated when equilibrium between the gas-phase and the solid was reached by using:

$$ N_s = \frac{N_e}{\alpha^*} = N_g \left[ \frac{1 - \alpha^*}{\alpha^*} \right] \qquad (4) $$

Where α* was the value of αg at equilibrium.

## Raman spectroscopy

The Raman study was performed using a KAISER RXN1 spectrometer equipped with a NIR (785 nm) laser diode (25< power <50 mW). The powder sample was put in a built-in cell that allowed heating and spectra acquisition under gas flow. Once in the cell, the sample was flushed under a flow of O₂ (50 cm³ min⁻¹) and heated up to 693 K (5 K min⁻¹) for 1 h. Then, the O₂ flow was switched to Ar (same flow rate), and the cell was cooled down to room temperature (RT), at which point a spectrum was registered. The sample was finally flushed 30 min with O₂ flow (150 cm³ min⁻¹) at RT, then possibly flushed with an Ar flow (150 cm³ min⁻¹).

## Electron paramagnetic resonance spectroscopy

The EPR spectra were recorded on a JEOL FA-300 series EPR spectrometer at ~9.3 GHz (X-band) using a 100 kHz field modulation and a 2.5 G standard modulation width. The spectra were recorded at 77 K using an insertion Dewar containing liquid nitrogen. Computer simulation of the spectra was performed using the EPRsim32 program [30].

The sample was introduced into a cell consisting of a U-shape reactor with a porous disk for thermal treatment and connected to an EPR tube. The cell can be closed using vacuum valves. After 10 min in dynamic primary vacuum (~10⁻² mbar) at RT, 250 mbar O₂ was introduced into the cell, which was then heated at 773 K (5 K min⁻¹) for 2 h. After decreasing the temperature down to 723 K, the sample was evacuated under dynamic primary then secondary vacuum (~10⁻⁴ mbar, 10 min) before rapid cooling to RT. At RT, increasing pressures of O₂ were introduced into the cell (from 1.5 up to 50 mbar). After internal transfer of the sample into the EPR tube, the spectra were recorded after 30 min cooling at 77 K (acquisition time: 8 min). The “raw” double integration of the EPR signal (arbitrary units) was performed using the cwEsr software (3.3.36E XB version) provided by JEOL. The double integration per gram of sample, which is proportional to the amount of paramagnetic species in the absence of dipolar interaction, was calculated considering the amount of sample in the EPR tube for each measurement.

## Results and discussion

The characterization of the samples studied in this work has been already described in a previous work [7]. The main physicochemical parameters are summarized in Table 1. The BET surface area of pure alumina support slightly decreased as the CeO₂ loading increased, while the high BET surface area of commercial ceria was maintained in the Au/CeO₂ catalyst to 200 m² g⁻¹. In Au/xCeO₂/Al₂O₃ (x=5 and 10 wt.%), mainly 2-D patches and 3-D nanoparticles of CeO₂ (ca 8 nm) were detected by XRD and EFTEM in accordance with similar observation made by Martínez-Arias [31]. The average sizes of the gold particles visible by EFTEM also reported in Table 1 did not depend on the support but were generally smaller after reduction than after calcination treatment (note that gold particles are visible only on alumina because of the poor contrast between gold and ceria). Finally, XPS and CO oxidation model reaction showed that whatever the mode of activation (thermal treatment under H₂ or O₂), all

<table><caption>**Table 1** BET surface area, Au loading, and Au particle size of the catalysts</caption><thead><tr><th rowspan="2">Catalysts</th><th>S<sub>BET</sub></th><th>Au</th><th colspan="2">Au particle size (nm)</th></tr><tr><th>(m<sup>2</sup> g<sup>-1</sup>)</th><th>(%)</th><th>After calcination</th><th>After reduction</th></tr></thead><tbody><tr><td>Au/Al<sub>2</sub>O<sub>3</sub></td><td>110</td><td>0.88</td><td>2.4 (0.56)<sup>a</sup></td><td>2.0 (0.49)</td></tr><tr><td>Au/5CeO<sub>2</sub>/Al<sub>2</sub>O<sub>3</sub></td><td>99</td><td>0.89</td><td>2.6 (0.43)</td><td>2.2 (0.47)</td></tr><tr><td>Au/10CeO<sub>2</sub>/Al<sub>2</sub>O<sub>3</sub></td><td>92</td><td>0.89</td><td>2.6 (0.57)</td><td>2.0 (0.45)</td></tr><tr><td>Au/CeO<sub>2</sub></td><td>200</td><td>0.97</td><td>n.m.<sup>b</sup></td><td>n.m.</td></tr></tbody></table>

<footnote>a Standard deviation is given in brackets</footnote>

<footnote>b n.m. not measurable</footnote>
gold was metallic after reduction under H₂, but gold remained unreduced on ceria or ceria patches after calcination under O₂, while it was metallic on alumina.

## Oxygen storage capacity

OSC measurements were performed at 673 K. The values obtained for the various samples are reported in Table 2, in which one can distinguish the OSC of the bare supports (denoted “without Au”) and the OSC of the samples containing gold (denoted “with Au”). As it could be anticipated, the Al₂O₃ support did not display any OSC activity at 673 K. The activity, i.e., the CO₂ production, remained null for Au/Al₂O₃, confirming the reduced state of gold when deposited on alumina after an oxidation stage at 673 K. Indeed, oxidized gold would have been reduced by the CO pulses, and CO₂ would have been produced. OSC was detectable for 5CeO₂/Al₂O₃ and 10CeO₂/Al₂O₃ but was not really different for the two samples. The OSC of bare ceria was much higher, 162 µmol O g⁻¹, corresponding to 0.81 µmol O m⁻². In order to assess the proportion of oxygen atoms involved in this process, the latter value was compared with the theoretical one. Taking into account that only one oxygen atom out of four is involved in the Ce⁴⁺/Ce³⁺ reduction step, the theoretical OSC for ceria can be estimated as 5.7 µmol O m⁻². The ratio between the experimental and theoretical OSC values gives the number of surface oxide layers (N_L) participating to the process. For the bare ceria (200 m² g⁻¹), the results in Table 2 show that at 673 K, N_L is restricted to a small fraction of the surface (N_L=0.14). The calculation performed for the ceria of low LSA area shows that N_L is the same (N_L=0.15) and therefore that the OSC process is only dependent on the surface area for a given oxide. In contrast to what was expected, the N_L values were higher when ceria was supported on alumina.

We can notice a beneficial effect of the presence of gold nanoparticles on the OSC values for the various supports

(Table 2). On CeO₂, after removing the CO₂ produced by the reduction of oxidized gold, the OSC value was three times as high in the presence of Au. It is worth noting that on CeO₂ LSA, the increase of the OSC by the presence of gold was much lower, showing that the OSC is not only dependent on the ceria surface but also on the Au/CeO₂ interaction. Flytzani-Stephanopoulos et al. also reported the improvement of the OSC property of ceria due to the presence of gold [32]. The impact of Au is more pronounced for the ceria–alumina mixed oxide supports, for which the OSC process involves the participation of bulk oxygen atoms (N_L > 1 indicates that the OSC is not limited to the surface) and for which the OSC seems to depend on the CeO₂ content (N_L=1.81 and 2.19 for Au/5CeO₂/Al₂O₃ and Au/10CeO₂/Al₂O₃, respectively).

## Isotopic oxygen exchange ($^{18}$O₂/$^{16}$O₂ IE)

We first investigated the homo-exchange reaction ($^{16}$O₂(g) + $^{18}$O₂(g) ↔ 2 $^{16}$O$^{18}$O(g)) by introducing an equimolar mixture of $^{18}$O₂/$^{16}$O₂ in the Au/CeO₂ system, and the oxygen isotopic exchange evolution was followed in a temperature-programmed experiment. When temperature increased, a decrease of the $^{18}$O₂ partial pressure and an increase of the partial pressures of $^{16}$O$^{18}$O and $^{16}$O₂ were observed (Fig. 1). A decrease of the $^{18}$O atomic fraction ($\alpha_g$) was observed in the gas phase when the exchange was taking place. This behavior is not consistent with a homo-exchange reaction during which $\alpha_g$ should remain constant. This result is therefore the indication of a hetero-exchange process in which $^{16}$O lattice oxygen atoms, supplied by ceria, participates to the reaction. A very similar evolution of the oxygen isotopomer partial pressures was observed on pure CeO₂ (Fig. S1), meaning that the hetero-exchange is due to ceria. Such a result emphasizes the inability of the gold nanoparticles to dissociatively adsorb molecular dioxygen or shows that the dissociation and the exchange with lattice oxygen atoms occur simultaneously.

In the objective to study whether gold nanoparticles influence the mobility of ceria oxygen atoms or not, we performed

<table><caption>**Table 2** OSC of xCeO₂/Al₂O₃ and CeO₂ at 673 K without or with the presence of 1 wt.% Au</caption><thead><tr><th rowspan="2">Supports</th><th colspan="2">Without Au</th><th colspan="2">With Au</th></tr><tr><th>OSC<br>(µmol O g<sup>-1</sup>)</th><th>N<sub>L</sub><sup>a</sup><br>CeO<sub>2</sub></th><th>OSC<br>(µmol O g<sup>-1</sup>)</th><th>N<sub>L</sub><br>CeO<sub>2</sub></th></tr></thead><tbody><tr><td>Al<sub>2</sub>O<sub>3</sub></td><td>0</td><td>0</td><td>0</td><td>0</td></tr><tr><td>5CeO<sub>2</sub>/Al<sub>2</sub>O<sub>3</sub></td><td>15</td><td>0.53</td><td>51</td><td>1.81</td></tr><tr><td>10CeO<sub>2</sub>/Al<sub>2</sub>O<sub>3</sub></td><td>17</td><td>0.36</td><td>115</td><td>2.19</td></tr><tr><td>CeO<sub>2</sub> (200 m<sup>2</sup> g<sup>-1</sup>)</td><td>162</td><td>0.14</td><td>474</td><td>0.42</td></tr><tr><td>CeO<sub>2</sub> LSA (49 m<sup>2</sup> g<sup>-1</sup>)</td><td>41</td><td>0.15</td><td>74</td><td>0.26</td></tr></tbody></table>

<figure><img src="image_2.png" /><figcaption>**Fig. 1** Evolution of the oxygen isotopomer partial pressures during a tempertaure-programmed homo-exchange experiment on Au/CeO₂ catalyst</figcaption></figure>

<footnote>ᵃ N_L number of layers; N_L=1 when the surface is entirely involved in the OSC process, N_L>1 when bulk oxygen atoms are involved in the OSC process</footnote>
an isothermal hetero-exchange experiment, introducing pure ¹⁸O₂ (ca 50 mbar) in the reactor cell at 723 K. The results, in terms of initial rate of exchange (Rₑ) and number of atoms exchanged (Nₑ) after 20 min reaction, are reported in Table 3. Again, in this set of experiments, we compared the bare supports (columns denoted “without Au”) with the gold catalysts (“with Au”). It clearly appears that the gold nanoparticles exalt the isotopic exchange activity. Since it is well known that Al₂O₃ support is not able to exchange oxygen at 723 K [33], these results show that the exchange is promoted by the interaction between Au nanoparticles and CeO₂. The Rₑ values, which were low for 5CeO₂/Al₂O₃ and 10CeO₂/Al₂O₃, increased in the presence of gold to a value quite similar for both, ca 6–7×10¹⁷ at O g⁻¹ s⁻¹. The similarity between both solids was confirmed in comparing the number of exchanged atoms (Ne values): 5.43 and 5.53 at O nm⁻² were exchanged for Au/5CeO₂/Al₂O₃ and Au/10CeO₂/Al₂O₃, respectively, suggesting that the proportion of gold particles in interaction with ceria was close in the two cases. The Rₑ value normalized per gram of sample was largely higher on bare CeO₂ (55.3×10¹⁷ at O g⁻¹ s⁻¹). Despite this high value, the presence of Au led to an increase of Re (77.8×10¹⁷ at O g⁻¹ s⁻¹). If one examines the Ne values, their increase was not as drastic as for the Rₑ values (13.7 at O nm⁻² and 14.7 at O nm⁻² for CeO₂ and Au/CeO₂, respectively). This can be explained by an equilibrium rapidly reached in the ¹⁸O concentration between the gas phase and the solid (depending on the mass of catalyst and the initial pressure of ¹⁸O₂), which minimized the differences that could be detected after 20 min of exchange. Finally, we studied the effect of the ceria surface area with the results obtained on CeO₂ LSA. Contrary to the conclusion previously made regarding the OSC measurement (Table 2), it clearly appears that the oxygen exchange activity is not dependent on the surface area only, since the Ne value, normalized per surface unit, was five times as small for CeO₂ LSA as compared to CeO₂. The beneficial effect of Au was also noticed on CeO₂ LSA, since the Rₑ and Nₑ values were almost four times higher in the

<table><caption>**Table 3** ¹⁸O₂/¹⁶O₂ isotopic exchange experiments on pre-oxidized samples at 723 K</caption><thead><tr><th rowspan="2">Supports</th><th colspan="2">Without Au</th><th colspan="2">With Au</th></tr><tr><th>R<sub>e</sub> (10<sup>17</sup> at O g<sup>-1</sup> s<sup>-1</sup>)</th><th>N<sub>e</sub> at 20 min (at O nm<sup>-2</sup>)</th><th>R<sub>e</sub> (10<sup>17</sup> at O g<sup>-1</sup> s<sup>-1</sup>)</th><th>N<sub>e</sub> at 20 min (at O nm<sup>-2</sup>)</th></tr></thead><tbody><tr><td>5CeO<sub>2</sub>/Al<sub>2</sub>O<sub>3</sub></td><td>1.29</td><td>1.06</td><td>7.70</td><td>5.43</td></tr><tr><td>10CeO<sub>2</sub>/Al<sub>2</sub>O<sub>3</sub></td><td>1.96</td><td>1.61</td><td>6.30 (25)<sup>a</sup></td><td>5.53 (10.50)</td></tr><tr><td>CeO<sub>2</sub></td><td>55.30</td><td>13.70</td><td>77.80 (183)</td><td>14.70 (16.40)</td></tr><tr><td>CeO<sub>2</sub> LSA</td><td>1.40</td><td>2.72</td><td>5.40</td><td>9.92</td></tr></tbody></table>

presence of the metal. Nevertheless, the activity remained smaller than that of the high surface area ceria.

In order to ascertain the conclusion that the interaction between Au particles and CeO₂ surface was the key element to explain the strong activity in oxygen exchange, we prepared two other gold catalysts with higher gold loading, i.e., containing 4 wt.% Au deposited on 10CeO₂/Al₂O₃ and CeO₂. Note that the size of the Au particles was maintained around 2.5 nm. The higher gold loading resulted in higher Rₑ and Nₑ values (see values between parentheses in Table 3). A closer inspection of the evolutions of the ¹⁶O₂ (m/z=32) and ¹⁸O¹⁶O (m/z=34) partial pressures as a function of time (Fig. 2) provided possible reasons for the beneficial impact of gold in the oxygen exchange reaction. At the beginning of the reaction, both ¹⁸O¹⁶O and ¹⁶O₂ molecules appeared simultaneously on 10CeO₂/Al₂O₃ (Fig. 2a) and on ceria support (Fig. 2c). This suggests that simple exchange (Eq. 4) and multiple exchange (Eq. 5) reactions occurred on these supports as already reported for ceria in previous studies [27, 29] as follows:

$$ ^{18}\text{O}_{2(\text{g})} + ^{16}\text{O}_{(\text{s})} \rightarrow ^{18}\text{O}^{16}\text{O}_{(\text{g})} + ^{18}\text{O}_{(\text{s})} \quad (4) $$

$$ ^{18}\text{O}_{2(\text{g})} + 2^{16}\text{O}_{(\text{s})} \rightarrow ^{16}\text{O}_{2(\text{g})} + 2^{18}\text{O}_{(\text{s})} \quad (5) $$

The presence of gold made the ¹⁶O₂ isotopomer the main molecule present at the very beginning of the experiments (Fig. 2b, d), indicating that Au favored the multiple exchange. The promotion of the multiple hetero-exchange mechanism by gold when supported on Ce-doped ordered mesoporous alumina materials has already been reported by Fonseca et al. [34]. Taking into account that the multiple exchange must involve binuclear oxygen species as intermediate, it can be inferred from Fig. 2 that the interfaces between the gold nanoparticles and the ceria crystallites are the preferential location for dioxygen activation, in the form of either diatomic peroxide or superoxide species. The correlation between multiple exchange and peroxide or superoxide intermediate species was suggested by Winter to explain the complex exchange mechanism observed over basic oxides [35]. Corra et al. provided spectroscopic evidences that superoxide species and peroxide adspecies at the one-electron defect site were the active species in the CO oxidation reaction for gold supported on nanocrystalline CeO₂ [36].

To investigate the nature of the binuclear oxygen species adsorbed on the catalytic surface, characterizations by Raman and electron paramagnetic resonance spectroscopies were performed in focusing the study on the impact of the presence of gold nanoparticles.

## Identification of oxygen active species

In an attempt to detect undissociated oxygen species by Raman spectroscopy, the catalyst pretreatments were performed in

<footnote>ᵃ Values between parentheses are the results for Au(4 wt.%) / 10CeO₂/ Al₂O₃ and Au(4 wt.%) / CeO₂</footnote>
<figure><img src="image_3.png" /><figcaption>**Fig. 2** Evolution of the oxygen isotopomer partial pressures as a function of time during an hetero-exchange experiment at 723 K on a 10CeO₂/Al₂O₃, b Au(4 wt.%)/10CeO₂/Al₂O₃, c CeO₂, and d Au(4 wt.%)/CeO₂ catalysts</figcaption></figure>

conditions as close as possible as those used for the isotopic exchange study (see Experimental part). Raman spectroscopy is a sensitive and specific technique capable to discriminate oxygen species (O₂, O₂⁻ or O₂²⁻) adsorbed on CeO₂, as reported in earlier works [22, 37]. The vibrational frequencies of these adsorbed oxygen species strongly decrease with their

electronic charge; in the 820–890 cm⁻¹ range for O₂²⁻ (peroxo), 1,120–1,140 cm⁻¹ for O₂⁻ (superoxo), and 1,480–

<figure><img src="image_4.png" /><figcaption>**Fig. 3** Raman spectra after calcination at 693 K Ar, then O₂ flow of a CeO₂, and b Au/CeO₂</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>**Fig. 4** EPR spectra of CeO₂ sample (recorded after 30 min at 77 K) under increasing PO₂</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Fig. 5** Experimental (recorded after 30 min at 77 K) and simulated EPR spectra of CeO₂ sample, under 1.5 mbar O₂</figcaption></figure>

1,570 cm⁻¹ for physically adsorbed O₂ [22, 37]. Moreover, the resulting bands are sharp compared to the ones related to ceria lattice vibrations, which make them quite easy to identify.

First of all, due to the strong fluorescence of the Al₂O₃-containing materials, only CeO₂ and Au/CeO₂ materials could be studied. Under O₂ flow at RT, apart from the vibration bands characteristic of ceria (Fig. S2), i.e., an intense band at 458 cm⁻¹ attributed to 1st order F₂g lattice vibrational mode of Oₕ and weaker bands at 253 and 1,176 cm⁻¹ attributed to second order modes, arising from a mixing of A₁g, E₉, and F₂g

lattice vibrational modes [38] (additional bands were visible at 1,357, 1,404, and 1,380 cm⁻¹, assigned to residual carbonate/carboxylate species), the spectra of CeO₂ and Au/CeO₂ (Fig. 3) revealed the presence of a sharp band at 1,551 cm⁻¹ that grew under O₂ flow. It can be reasonably attributed to physically adsorbed O₂, since it easily disappeared by flushing with inert gas. One can note that apart from the vibration bands characteristic of ceria described above, a shoulder around 590 cm⁻¹ was also visible for both CeO₂ and Au/CeO₂ (Fig. S2). It is related to the presence of oxygen vacancies [39]. The intensity of the 590 cm⁻¹ shoulder relative to the F₂g mode band (458 cm⁻¹) is about twice as high for Au/CeO₂ (9×10⁻²) as for CeO₂ (4×10⁻²), revealing a much more oxygen-defective material in the presence of gold. These additional vacancies are probably located at the perimeter of the interface between gold and ceria and would explain the exaltation of the multiple exchange mechanism observed by isotopic exchange technique for gold-containing catalysts [40]. In addition, tiny bands at 810 and 826 cm⁻¹ could be observed on Au/CeO₂ only (not on CeO₂), in the frequency range of adsorbed O₂²⁻ peroxo species. No O₂⁻ (superoxo) species could be observed. It may be noted that the observation of such superoxo species often requires low temperature (93 K), which could not be reached with our spectrometer. In order to ascertain the presence of O₂⁻ (superoxo) species, EPR experiments were also performed.

To avoid the interaction between the radical oxygen species and the gas phase O₂ which is known to induce signal broadening, it was necessary to perform the EPR experiments at low temperature, i.e., at the liquid nitrogen temperature, 77 K, which is the usual temperature used for the identification of the oxygen radicals. The EPR study was first performed over CeO₂ and Au/CeO₂. Again, the pretreatments had to be

<caption>**Table 4** Simulated EPR parameters of O₂⁻ species adsorbed on CeO₂, Au/CeO₂, 10CeO₂/Al₂O₃, and Au/10CeO₂/Al₂O₃ under 1.5 and 50 mbar O₂</caption>

<table><thead><tr><th rowspan="2">Sample</th><th rowspan="2">O₂⁻ species<br>Assignment*</th><th rowspan="2">g<sub>z</sub> (g||)</th><th rowspan="2">g<sub>x</sub> (g⊥)</th><th rowspan="2">g<sub>y</sub></th><th colspan="2">Proportion (%)</th></tr><tr><th>1.5 mbar O₂</th><th>50 mbar O₂</th></tr></thead><tbody><tr><td rowspan="5">CeO₂</td><td>OI<sub>a</sub></td><td>2.033</td><td>2.011</td><td>2.011</td><td>26.5</td><td>23.5</td></tr><tr><td>OI<sub>b</sub></td><td>2.031</td><td>2.017</td><td>2.012</td><td>30.5</td><td>20.5</td></tr><tr><td>OII<sub>a</sub></td><td>2.037</td><td>2.014</td><td>2.007</td><td>26.5</td><td>27.0</td></tr><tr><td>OII<sub>b</sub></td><td>2.047</td><td>2.011</td><td>2.001</td><td>3.5</td><td>5.5</td></tr><tr><td>OII<sub>c</sub></td><td>2.048</td><td>2.010</td><td>2.008</td><td>13.0</td><td>23.5</td></tr><tr><td rowspan="5">Au/CeO₂</td><td>OI<sub>a</sub></td><td>2.033</td><td>2.011</td><td>2.011</td><td>48.0</td><td>45.5</td></tr><tr><td>OI<sub>b</sub></td><td>2.031</td><td>2.016</td><td>2.011</td><td>14.0</td><td>16.0</td></tr><tr><td>OII<sub>a</sub></td><td>2.036</td><td>2.012</td><td>2.007</td><td>17.0</td><td>18.0</td></tr><tr><td>OII<sub>b</sub></td><td>2.048</td><td>2.011</td><td>2.001</td><td>1.5</td><td>6.5</td></tr><tr><td>OII<sub>c</sub></td><td>2.049</td><td>2.009</td><td>2.008</td><td>19.5</td><td>14.0</td></tr><tr><td>10CeO₂/Al₂O₃</td><td>OCA</td><td>2.027</td><td>2.017</td><td>2.011</td><td>≈100</td><td></td></tr><tr><td>Au/10CeO₂/Al₂O₃</td><td>OCA</td><td>2.027</td><td>2.017</td><td>2.011</td><td>≈100</td><td></td></tr></tbody></table>

<footnote>*According to notations of Martínez-Arias et al. [31]</footnote>
<table><caption>**Table 5** Overall intensity (double integration) of O₂⁻ species adsorbed on CeO₂, Au/CeO₂, 10CeO₂/Al₂O₃, and Au/10CeO₂/Al₂O₃ upon increasing O₂ pressures</caption><thead><tr><th>PO₂ (mbar)</th><th></th><th>1.5</th><th>5</th><th>8</th><th>15</th><th>50</th></tr></thead><tbody><tr><td rowspan="4">Overall intensity</td><td>CeO₂</td><td>6 590</td><td>7 140</td><td>9 200</td><td>9 060</td><td>6 740</td></tr><tr><td>Au/CeO₂</td><td>4 070</td><td>4 940</td><td>6 020</td><td>6 220</td><td>4 000</td></tr><tr><td>10CeO₂/Al₂O₃</td><td>51 300</td><td>36 000</td><td>29 500</td><td>8 350</td><td>200</td></tr><tr><td>Au/10CeO₂/Al₂O₃</td><td>22 300</td><td>8 700</td><td>3 800</td><td>550</td><td>≈0</td></tr></tbody></table>

adapted to the technical constraints, but were performed in conditions as close as possible to those of isotopic exchange (see Experimental part). After treatment under vacuum at 723 K, the EPR spectra of CeO₂ and Au/CeO₂ samples revealed a weak anisotropic signal at g≈1.97 (Figs. 4 and S3) composed of two species (g⊥₁,₂≈1.97, g‖₁=1.948, and g‖₂=1.941, insert of Fig. 4). The signal obtained for Au/CeO₂ was slightly broader because of the presence of a third species (1.98<g⊥₃<1.96 and g‖₃=1.937–1.936) (Fig. S3). Its intensity (double integration) was similar for CeO₂ and Au/CeO₂ samples. Though this signal at g≈1.97 was currently observed on ceria-based samples, its attribution is still under debate. It was tempting to assign it to Ce³⁺; however, according to the literature, such paramagnetic ion could be observed at very low temperature only (T <20 K) [41, 42]. Some authors proposed that this signal originated from Ce³⁺ in peculiar geometry/symmetry [43, 44], while other authors proposed that it arises from quasi-free electrons with some orbital mixing with the empty f-orbitals of Ce⁴⁺ ions [23, 45].

Upon addition of O₂, the signal at g≈1.97 remained unchanged, which further strengthened the hypothesis of a paramagnetic species trapped in the bulk of the CeO₂ particles (no possible dipolar interaction with O₂ that could affect the signal). Moreover, a complex new signal of high intensity appears in the 3,150 G<B<3300 G region, typical of O₂⁻ species (Fig. 4). It had almost the same shape for CeO₂ and Au/CeO₂ (Fig. S3), and the simulation of the signal (Figs. 5 and S4) indicated the presence of five different O₂⁻ species in both samples with slightly different proportions (Table 4). The OI's and OII's species have been previously observed on similar materials [46–48]. These species were attributed to different adsorption modes of O₂⁻ on the surface, OI with EPR-equivalent oxygen atoms (adsorption parallel to the surface), and OII with nonequivalent oxygen atoms (end-on or other asymmetric adsorption).

Upon increasing O₂ pressure, the shape of the O₂⁻ signal was barely affected, but its intensity increased, then decreased when PO₂>8 mbar and 15 mbar for CeO₂ and Au/CeO₂, respectively, as attested by the values of the double integration reported in Table 5. The decreasing intensity above a critical O₂ pressure probably resulted from dipolar interactions between O₂⁻ and nearby O₂ molecules [49, 50]. This points out that the O₂⁻ signal intensity depends not only on the number of paramagnetic species, but also on this broadening effect, thus comparisons of the intensities are meaningful for the smallest O₂ pressures,

only. The interesting point regarding these experiments is that the intensity (double integration) of the overall O₂⁻ signal is smaller for Au/CeO₂ than for CeO₂ (Table 4), indicating that no extra O₂⁻ species were generated by the presence of gold.

In the case of 10CeO₂/Al₂O₃ and Au/10CeO₂/Al₂O₃, upon O₂ addition, the signal of O₂⁻ was very different and much more intense at low PO₂ (more than five times) (Fig. 6) than that for CeO₂ and Au/CeO₂ samples (Figs. 4 and S3). Again, the shape of the O₂⁻ signal was similar for 10CeO₂/Al₂O₃ and Au/10CeO₂/Al₂O₃ (Fig. S5). It was also much less complex than the former ones (Figs. 4 and S3), and could be accurately simulated considering only one O₂⁻ species, thereafter referred as OCA in the literature [31] (Table 4). It has been proposed that the OI and OII species formed on 3-D CeO₂ nanoparticles, while the OCA species formed on 2-D platelets of CeO₂ [31]. This assignment fits with the fact that CeO₂ consists of 3-D particles, and that, as mentioned at the beginning of the section “Results and discussion”, previous EFTEM images of xCeO₂/Al₂O₃ revealed the presence of both 3-D and 2-D (platelets) CeO₂ particles on alumina [7]. The intensity of the OCA species strongly decreased and broadened for O₂ pressures >1.5 mbar, and almost disappeared at PO₂≈50 and 15 mbar, for 10CeO₂/Al₂O₃ and Au/10CeO₂/Al₂O₃, respectively, which

<figure><img src="image_7.png" /><figcaption>**Fig. 6** EPR spectra of 10CeO₂/Al₂O₃ sample (recorded after 30 min at 77 K) under increasing PO₂</figcaption></figure>
was not observed with CeO₂ and Au/CeO₂. Such different behavior towards O₂ dipolar broadening has been reported earlier [51] on CeO₂/SiO₂ and CeO₂/Al₂O₃ materials, and the authors assigned it to the difference in O₂ accessibility to O₂⁻ species, thus locating O₂⁻ in the bulk (CeO₂/SiO₂: little to no broadening effect) or on the surface (CeO₂/Al₂O₃: strong broadening effect) of CeO₂. Again, the double integration of the O₂⁻ signal was somewhat smaller for Au/10CeO₂/Al₂O₃ than for 10CeO₂/Al₂O₃, whatever the pressure of O₂ (Table 5).

To summarize, the comparison of the intensity of EPR signals of O₂⁻ species in CeO₂ and Au/CeO₂ pointed out a lower amount of O₂⁻ species in the presence of Au, for which the OSC property and the oxygen isotopic exchange activity was exalted, which ruled out the hypothesis that O₂⁻ species would play an active role in the corresponding processes. However, the formation of a noticeable amount of O₂²⁻ species adsorbed on Au/CeO₂ and not on CeO₂ strengthened the hypothesis of an activation of dioxygen molecule via Au nanoparticles through the formation of a peroxo species. This interpretation matches the conclusion brought by Pal et al. [18] who reported the results obtained by combining photo-electron spectroscopy and computational method, showing that the O-O bond was more activated (more elongated) in the peroxo form than that in the superoxo one, and suggesting that peroxo mode of chemisorptions plays a crucial role in the dioxygen activation.

## Conclusion

In this work, we studied the oxygen mobility and the nature of the oxygenated species responsible for the chemisorption of dioxygen on Au/CeO₂ and Au/xCeO₂/Al₂O₃ (x=5 and 10 wt.%) catalysts. We stressed the role of the Au nanoparticles in the step of dioxygen activation. To achieve this goal, oxygen storage capacity measurements and ¹⁸O/¹⁶O isotopic exchange reaction were undertaken to study the oxygen mobility, while Raman spectroscopy and electron paramagnetic resonance spectroscopies were employed to determine the nature and the location of the adsorbed oxygen species. All the characterizations were performed on catalysts previously activated in conditions as close as possible to the thermal pretreatment used prior to the reactions of oxidation studied in our previous work [7].

Au nanoparticles were shown to exalt both the OSC property of CeO₂ and the exchange rate of oxygen between the gas phase and the lattice oxygen atoms of CeO₂, whether the Au nanoparticles were supported on CeO₂ or on CeO₂/Al₂O₃. Complementary results obtained by modifying the gold loading and the ceria surface indicated that the improvement in the oxygen mobility was dependent on the Au/CeO₂ interfacial perimeter. Moreover, the close inspection of the evolutions of the isotopomer partial pressures during the isotopic exchange experiments led to the conclusion that the interfaces between

the gold nanoparticles and the ceria crystallites were the preferential location for dioxygen activation via adsorption of binuclear species.

Further characterization by Raman and EPR spectroscopies were performed to determine the nature of the dioxygen species. The study of CeO₂ and Au/CeO₂ by in situ Raman under oxygen revealed the presence of peroxo species on Au/CeO₂ and of additional vacancies probably located at the perimeter of the interface between gold and ceria. EPR revealed the presence of superoxo species on both samples, but their concentration was lower in the presence of gold. The same observation was made for 10CeO₂/Al₂O₃ and Au/10CeO₂/Al₂O₃. As a consequence, the efficiency of Au/CeO₂ and Au/xCeO₂/Al₂O₃ catalysts for oxidation reaction could be explained by the activation of dioxygen molecule at the ceria/Au nanoparticles interfacial perimeter involving peroxo species.

**Acknowledgments** The authors thank Jean-Marc Krafft, engineer at LRS for the Raman measurements, and Pantea Baripour, master student at LRS for the preliminary experiments of EPR. The authors also acknowledge the Agence Nationale pour la Recherche for financial support (ANR-BLANC07-2 183612).

**Open Access** This article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and reproduction in any medium, provided the original author(s) and the source are credited.

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