Total oxidation of methane over rare earth cation-containing mixed oxides derived from LDH precursors

Total oxidation of methane over rare earth cation-containing mixed oxides derived from LDH precursors

Accepted Manuscript Title: Total oxidation of methane over rare earth cation-containing mixed oxides derived from LDH precursors Author: Adriana Urd˘a...

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Accepted Manuscript Title: Total oxidation of methane over rare earth cation-containing mixed oxides derived from LDH precursors Author: Adriana Urd˘a Ionel Popescu Thomas Cacciaguerra Nathalie Tanchoux Didier Tichit Ioan-Cezar Marcu PII: DOI: Reference:

S0926-860X(13)00281-0 http://dx.doi.org/doi:10.1016/j.apcata.2013.05.012 APCATA 14224

To appear in:

Applied Catalysis A: General

Received date: Revised date: Accepted date:

26-11-2012 7-5-2013 10-5-2013

Please cite this article as: A. Urd˘a, I. Popescu, T. Cacciaguerra, N. Tanchoux, D. Tichit, I.-C. Marcu, Total oxidation of methane over rare earth cation-containing mixed oxides derived from LDH precursors, Applied Catalysis A, General (2013), http://dx.doi.org/10.1016/j.apcata.2013.05.012 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Total oxidation of methane over rare earth cation-containing mixed oxides derived from LDH precursors Adriana Urdăa, Ionel Popescub, Thomas Cacciaguerrac, Nathalie Tanchouxc, Didier Tichitc and

Laboratory of Chemical Technology and Catalysis, Department of Organic Chemistry,

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Ioan-Cezar Marcua,b*

Biochemistry and Catalysis, Faculty of Chemistry, University of Bucharest, 4-12, Blv. Regina b

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Elisabeta, 030018, Bucharest, Romania

Research Center for Catalysts and Catalytic Processes, Faculty of Chemistry, University of

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Bucharest, 4-12, Blv. Regina Elisabeta, 030018, Bucharest, Romania

Institut Charles Gerhardt, UMR 5253 CNRS/ENSCM/UM2/UM1, Matériaux Avancés pour la

Catalyse et la Santé (MACS), Ecole Nationale Supérieure de Chimie, 8, rue de l’Ecole Normale,

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Abstract

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34296, Montpellier Cedex 5, France

Ln(x)MgAlO (Ln = Ce, Sm, Dy and Yb) (x = 5 % and also 7, 10 and 20 % in the case of Ce)

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mixed oxide catalysts obtained from layered double hydroxide (LDH) precursors calcined at 750 °C were used in the complete oxidation of methane. The catalysts were characterized by XRD, N2 adsorption, EDX, XPS and H2-TPR experiments. Their catalytic activity in the complete oxidation of methane indicated by the temperature for 50 % conversion (T50) followed the order MgAlO < Dy(5)MgAlO < Sm(5)MgAlO < Yb(5)MgAlO < Ce(5)MgAlO for Ln(5)MgAlO catalysts and CeO2 < Ce(20)MgAlO < Ce(5)MgAlO < Ce(7)MgAlO < Ce(10)MgAlO for Ce(x)MgAlO catalysts. Total conversion was achieved at ca. 700 °C with Ce(10)MgAlO, the most active and highly stable catalyst. Nevertheless, the intrinsic reaction rates of the Ce(x)MgAlO catalysts followed the order: Ce(5)MgAlO < Ce(7)MgAlO < Ce(10)MgAlO < Ce(20)MgAlO and were correlated to the reducibility of the cerium-containing species below 600 °C showing that their oxido-reduction ability is involved in the catalytic combustion of *

Corresponding author. Tel.: +40 214103178x138; fax: +40 213159249. E-mail addresses: [email protected] ; [email protected]

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methane. This was confirmed by the existence of Ce(IV)/Ce(III) redox couple on the catalyst surface. The oxido-reduction of the catalyst was also involved in the reaction over Sm(5)MgAlO, while for MgAlO support as well as for Yb(5)MgAlO and Dy(5)MgAlO catalysts the surface adsorbed oxygen species were mostly involved. The apparent activation energy of methane

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oxidation calculated based on a pseudo first order kinetics was in the range from 72.0 kJ mol-1 to

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100.9 kJ mol-1 for Ce-based catalysts and much higher for the other catalysts.

Keywords: rare earth cations, ceria, mixed oxides, layered double hydroxides, methane

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combustion

1. Introduction

Catalytic combustion represents an efficient method for the removal of a wide range of organic

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pollutants, and has several advantages compared to traditional flame combustion. It indeed needs lower temperatures for the complete oxidation and control of NOx formation while avoiding the

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apparition of incomplete oxidation products [1-5].

Methane is the most difficult hydrocarbon molecule to oxidize due to its high stability [3], being

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then often used as a test molecule in catalytic combustion processes. Although the mechanism of its combustion on the catalyst surface is not well established, it is accepted that the activation

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step proceeds through the homolytic abstraction of a hydrogen atom from the hydrocarbon (dissociative chemisorption of the hydrocarbon), followed by a fast sequence of reactions between the alkyl group and oxygen species coordinated on the solid surface [6, 7]. Two different mechanisms were hypothesized for methane catalytic combustion: one at low temperatures involving chemisorbed oxygen, and the other at high temperatures involving lattice oxygen [8].

For combustion applications, very active and highly stable catalysts are needed. Two groups of catalysts were extensively studied: supported noble metals and transition metal oxides. In the first group, Pd is the most active for methane combustion above 400 °C, with deactivation occuring above 700 °C due to PdO decomposition [1, 9]. Although the highest activity is displayed by noble metals dispersed on high surface-area oxide supports, their high cost, low thermal stability and high poisoning ability stimulated the research for alternative catalysts [9].

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Transition metal oxides are less active but cheaper and more resistant to poisoning. Several simple oxides of Cu [9], Mn [10] and Co [11], have shown high activity, but low stability. Among mixed oxides, perovskites [2, 3, 8, 12] or pyrochlore-type ones [13, 14], as well as transition metal-containing mixed oxides obtained from LDH precursors [15-19] proved high

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activity and good stability in methane combustion.

Promotion by cerium of lanthanum oxide [20] and lanthanum-based perovskites [3], Co3O4 or

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Co3O4-ZrO2 [21] increases the activity of these catalysts.

As support for noble metals (mainly Pd) [22], as simple oxide [23, 24], as mixed oxides (e.g.

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with ZrO2) [23, 25-27] or as supported nanoparticles [5] ceria enhances the catalytic properties in combustion processes. Activation of oxygen on ceria gives rise to at least two species, the

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superoxide ( O 2 ) and the peroxide ( O 22  ) ions, both known to be involved in the total oxidation of hydrocarbons [23]. Other rare earth cations like Nd [8], Sm [8, 13, 14, 28, 29], Eu [14], Gd [13, 14] and Tb [14] have been proven to increase the activity and/or the thermal stability of the

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combustion catalysts.

The above-mentioned results led us to study in this work the total oxidation reaction of methane

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on a series of LnMgAlO mixed oxides (Ln = Ce, Sm, Dy and Yb) prepared from LDH

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precursors. The aim was to investigate the influence of the nature of the rare earth cations on the activity of catalysts obtained by a preparation route known to lead to highly dispersed mixed

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oxides. Moreover, the cerium content of the Ce-containing catalysts has been varied, in order to find the optimum concentration for the above-mentioned reaction.

2. Experimental

2.1. Catalysts preparation

LDH precursors of the LnMgAlO (Ln = Ce, Sm, Dy and Yb) mixed oxides were prepared by coprecipitation. For this purpose an aqueous solution of Mg(NO3)2 · 6 H2O and Al(NO3)3 · 9 H2O (Mg/Al = 3) was added dropwise at a rate of 2 mL/min and room temperature into a well-stirred beaker containing 200 mL of rare earth cation nitrate solution (Ce(NO3)3 · 6 H2O; Sm(NO3)3 · 6 H2O; Dy(NO3)3 · 5 H2O; Yb(NO3)3 · 5 H2O). The rare earth cation content, as atomic percent with respect to cations, was calculated to be 5 % (Ln/(Ln+Mg+Al) = 0.05) and, in the case of Cecontaining sample it was varied in the range from 5 to 20 % (0.05 ≤ Ce/(Ce+Mg+Al) ≤ 0.2). Simultaneously, appropriate volume of NaOH (2 M) was added at a controlled rate to maintain

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the pH close to 10 using a pH-STAT Titrino (Metrohm) apparatus. The precipitates formed were aged in their mother liquor overnight at 80 °C under stirring, separated by centrifugation, washed with deionized water until a pH of 7 was reached and dried at 80 °C overnight. These samples were hereafter labeled as Ln(x)MgAl-LDH (Ln = Ce, Sm, Dy and Yb) where x is the rare earth

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cation content. The LnMgAlO (Ln = Ce, Sm, Dy and Yb) mixed oxide catalysts were obtained by calcination of the previously dried samples in air at 750 °C for 8 h. They were labeled as

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Ln(x)MgAlO (Ln = Ce, Sm, Dy and Yb). The Ln-free Mg-Al mixed oxide and the corresponding precursor were obtained following the same protocol and were noted MgAlO and MgAl-LDH,

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respectively. The same method was used for preparing pure ceria. 2.2. Catalysts characterization

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Powder X-Ray diffraction (XRD) patterns were recorded using a Siemens D5000 Diffractometer and the monochromatic Cu-Kα radiation. They were recorded with 0.02° (2) steps over the 3– 2

angular range with 1 s counting time per step.

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70°

The textural characterization was achieved using the conventional nitrogen adsorption/desorption

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method, with a Micromeritics ASAP 2010 automatic analyzer. Specific surface areas were h at 250 °C.

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calculated using the BET method. Prior to nitrogen adsorption, the samples were outgassed for 8

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The chemical composition of the mixed oxide samples was determined by EDX microprobe with a Cambridge Stereoscan 260 apparatus. The surface composition of the Ce-containing mixed oxides was monitored by X-ray photoelectron spectroscopy (XPS) on an ESCALAB 250 (Thermo Electron). The X-ray excitation was provided by a monochromatic Al Kα (1486.6 eV) source. The analyzed surface has a 400 µm diameter. The background signal was removed using the Shirley method [30]. The surface atomic concentrations were determined from photoelectron peaks areas using the atomic sensitivity factors reported by Scofield [31]. Binding energies (BE) of all core levels were referred to the C1s core level binding energy at 285 eV. Hydrogen temperature-programmed reduction (H2-TPR) studies were carried out using a Micromeritics Autochem model 2910 instrument. Fresh calcined samples (100 mg), placed in a U-shaped quartz reactor, were pretreated in air at 750 °C before reduction. After cooling down to room temperature and introducing the 3 % H2/Ar reduction gas, the sample was heated at a rate

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of 10 °C/min. The hydrogen consumption was estimated from the area under the peak after taking the thermal conductivity detector response into consideration. Calibration of TCD signal has been done with an Ag2O standard (Merck, reagent grade).

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2.3. Catalytic complete oxidation

The total oxidation of methane was carried out in a fixed bed quartz tube down-flow reactor

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operating at atmospheric pressure. The internal diameter of the reactor tube was 18 mm. 1 cm3 (0.8 – 1 g) of catalyst supported by quartz wool was always used. The axial temperature profile

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was measured using an electronic thermometer placed in a thermowell centered in the catalyst bed. Quartz chips were used to fill the dead volumes before and after the catalyst bed to minimize potential gas-phase reactions at higher reaction temperatures. The reaction mixture

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consisted of 1 % (vol.) methane in air. Flow rates were controlled by fine needle valves and were measured by capillary flow-meters. Catalytic tests were performed at total volume hourly space

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velocity (VHSV) of 16000 h-1, in the temperature range from 450 to 740 °C. Before each activity test, the catalyst in the reactor was heated under air at 600 °C for about 30

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min to clean its surface [32]. The reactor was then cooled to the desired temperature, and the reactant flows were set to give the desired reaction mixture. The system was allowed to stabilize

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for about 30 min at the reaction temperature before the first product analysis was made. Each run was carried out over a period of 1-2 hours, until two consecutive analyses were identical. The

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feed and the reactor outflow gases were analyzed on-line by a Clarus 500 Gas-Chromatograph equipped with a thermal conductivity detector (TCD), using two packed columns in series (6 ft Hayesep and 10 ft molecular sieve 5Å). After the catalytic test the reactor was cooled down under the flow of reactants.

The amount of raw material transformed in reaction divided by the amount that was fed to the reactor gave the conversion. Complete selectivity to CO2 and H2O was always observed. The carbon balance was satisfactory in all runs with an error margin of ± 2 %.

3. Results and discussion 3.1. Textural and structural properties of the catalysts The XRD patterns of the Ln(5)MgAl-LDH precursors and of the corresponding mixed oxides were presented and discussed elsewhere [32]. Poorly crystallized layered hydrotalcite-type phase

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(JCPDS 37-0630) was detected in all the precursors. CeO2 (JCPDS 75-0076), Dy2O3 (JCPDS 220612) and Sm(OH)3 (JCPDS 83-2036) phases were also detected in Ce-, Dy- and Sm-containing precursors, respectively. In the case of all Ln(x)MgAlO catalysts, Mg(Al)O mixed oxide phase with the periclase-like structure (JCPDS-ICDD 4-0829) and the oxide phase corresponding to the

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rare earth cation involved, i.e. CeO2, Sm2O3 (JCPDS 15-0813), Dy2O3 and Yb2O3 (JCPDS 411106), were identified. In the diffractograms of the Ce(x)MgAlO catalysts the intensity of the

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peaks corresponding to CeO2 phase increased, as expected, with increasing cerium content (Fig. 1). More unexpected was the concurrent increase of the intensity of the peaks corresponding to

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Mg(Al)O phase up to Ce(10)MgAlO. Pure CeO2 was highly crystallized. It is noteworthy that the peaks corresponding to CeO2 in the Ce(x)MgAlO mixed oxides shifted to higher 2-theta values with respect to pure ceria suggesting insertion of smaller Mg2+ ions in the lattice of CeO2 to form

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a solid solution [33]. This was confirmed by the values of the lattice parameters of the fluorite structure of the ceria phase calculated from the three most intense lines in the diffractograms

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(Table 1) which are slightly lower for all Ce(x)MgAlO samples than for CeO2. The full-width at half-maximum (FWHM) of the three most intense reflections of CeO2 phase allow estimating the

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average crystallite size using the Debye-Scherrer equation. They are presented in Table 1. The pure ceria showed the largest crystallite size while it varied irregularly for Ce(x)MgAlO samples.

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Nevertheless, samples with higher Ce content showed a larger crystallite size. The chemical compositions of the mixed oxides reported in Table 2 show that the rare-earth

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cation content was in all cases slightly higher than the nominal value and that the Mg/Al atomic ratio varied in a large range, i.e. between 2.5 for Ce(20)MgAlO and 3.9 for Yb(5)MgAlO. The specific surface areas of the Ln(5)MgAlO catalysts in the range 102 – 160 m2/g were lower than that of MgAlO (188 m2/g) (Table 2) due to the presence of well crystallized rare earth cation oxide-side phases. For the Ce(x)MgAlO catalysts the specific surface area decreased regularly with increasing the cerium content in agreement with the concurrent increase of crystallinity of the CeO2 phase detected by XRD. Accordingly, the specific surface area of Ce(20)MgAlO sample was almost similar (22 m2/g) to that of pure ceria (16 m2/g). The series of Ce-containing catalysts was characterized by XPS analysis. All the expected elements, i.e. Ce, Mg, Al, O and C are present on the surface. Notably, the O 1s core level XPS spectra (Fig. 2) showed for all the samples two well defined peaks which could be related to oxygen in oxide (BE around 529 eV) and in hydroxyl and/or carbonate species (BE around 532

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eV) [34]. The peak at 532 eV accounts for the hydroxylation and carbonatation of the catalyst surface. Accordingly, its intensity increased with the specific surface area of the catalyst. Similarly, the C 1s core level (Fig. 3) shows two main contributions: the adventitious hydrocarbon species (BE 285 eV) and carbon in carbonate (BE around 290 eV) [34]. These

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results confirm the need of pretreating the catalysts in the reactor under air at 600 °C before each activity test.

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The XPS spectra of Ce 3d3/2 and Ce 3d5/2 core levels of the Ce-containing catalysts are presented in Fig. 4. It is well known [35] that the XPS spectrum of pure Ce(IV) oxide presents six peaks

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(three pairs of spin-orbit doublets) which are labeled in order of decreasing energy U```, U``, U (corresponding to the Ce 3d3/2 level) and V```, V``, V (corresponding to the Ce 3d5/2 level). The XPS spectrum of Ce(III) oxide presents four peaks (two pairs of spin-orbit doublets) which are

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labeled, in order of decreasing energy U`, U0 (corresponding to the 3d3/2 level) and V`, V0 (corresponding to the 3d5/2 level). When both Ce(III) and Ce(IV) species are present, the

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resulting spectrum is a superposition of all these ten features. They were all evidenced in the XPS spectra in Fig. 4 suggesting that both Ce(IV) and Ce(III) species are present on the surface

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of the Ce-MgAlO catalysts as already observed for Ce-containing LDH-derived mixed oxides [36].

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The surface composition of the Ce-containing samples, obtained from the XPS analysis, is summarized in Table 3. It can be observed that the surface Ce content was much lower than that

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of the bulk for all the catalysts. The Ce(IV)/Ce atomic ratio, based on the decomposition of the XPS spectra presented in Fig. 4, was in all cases higher than 0.8 suggesting the presence of a low quantity of Ce(III). It is noteworthy that the reduction of Ce(IV) under the X-ray beam during the XPS analysis is, at least partly, responsible for the presence of surface Ce(III) species. The Mg/Al surface atomic ratio was for all Ce-MgAlO samples lower than the bulk ratio indicating the Al enrichment of the surface.

Different reducibility profiles were already reported for the Ln(x)MgAlO catalysts with 5 at % Ln (Ln = Ce, Sm, Dy and Yb) [32]. Briefly, Sm(5)MgAlO, Dy(5)MgAlO and Yb(5)MgAlO samples displayed a large peak in the temperature range from 550 to 800 °C accounting for the reduction of trivalent Ln3+ cations in Ln2O3 particles with, in addition, a weakly intense peak at 450 °C for Sm(5)MgAlO sample likely due to the reduction of highly dispersed samarium oxide species. The total H2 consumption and amount of reduced Ln3+ of 23.3 %, 35.7 % and 38.2 % for

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Sm(5)MgAlO, Dy(5)MgAlO and Yb(5)MgAlO, respectively, were reported in Table 2. It was also checked that MgAlO sample does not consume any H2. The TPR profiles of the series of CeO2-containing samples are shown in Fig. 5 and the corresponding H2 consumptions are reported in Table 2. CeO2 showed a profile with a very

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broad and weakly intense peak at 480 °C and an intense peak extending from 550 to 800 °C with a maximum at 720 °C corresponding to surface and bulk reduction of Ce4+ in CeO2 particles,

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respectively [37, 38]. The total amount of Ce4+ reduced was 19.7 % in good agreement with previous results [37, 39].

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For Ce(5)MgAlO a large asymmetric peak was observed in the 400-650 °C range. This TPR profile is similar to that previously reported for CeMgO solid solution [33]. It suggests that Mg(Al)O whose reducibility is increased [33].

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besides bulk CeO2 detected in the XRD pattern, an amount formed a solid solution with In the case of the other Ce(x)MgAlO catalysts (x = 7, 10 and 20) three reduction peaks were

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observed in the temperature ranges 230-330 °C, 350-620 °C and 620-800 °C, respectively. They accounted for ceria particles having different sizes or interactions with the Mg-Al mixed oxide

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matrix. Thus, the low temperature peak, which shows a shoulder at lower temperature and a maximum at about 270 °C, can be attributed to highly reducible ceria particles dispersed on the

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surface of the Mg(Al)O support. The medium temperature peak was observed in the same range than that reported in the case of Ce(5)MgAlO. However, this peak became broader and its

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maximum was shifted toward lower temperature as the ceria content increased. It can be attributed to ceria particles interacting with Mg(Al)O and to Ce4+ forming CeMgO solid solution. The asymmetric peak above 600 °C, therefore in the same temperature range than for bulk CeO2, can be attributed to less reducible large ceria crystallites dispersed in the Mg(Al)O matrix. The shoulders or the asymmetric shape observed for the three reduction peaks suggest that the surface and subsurface reduction of different ceria particles takes place first, followed by their bulk reduction. Note that the total amount of Ce4+ reduced was comparable for the Ce(5)MgAlO, Ce(7)MgAlO and Ce(10)MgAlO samples, in the range 25.4-27.9 %. For the Ce(20)MgAlO sample it decreased to 19.1 % being similar to that corresponding to the CeO2 sample. No correlation was observed between the catalyst reducibility and the particle size of ceria phase estimated using the Debye-Scherrer equation.

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3.2. Catalytic total oxidation The MgAlO and Ln(5)MgAlO mixed oxides (Ln = Ce, Sm, Dy and Yb) were tested in the total oxidation reaction of methane in the temperature range from 450 to 740 °C and 1 % vol. CH4 in air (VHSV = 16000 h-1). The methane conversions as a function of the reaction temperature were

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plotted in Fig. 6.a. It can be observed that the Ce(5)MgAlO catalyst was the most active, while the MgAlO support was the least active in the series studied. Moreover, in the temperature range

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considered, only the Ce-containing catalyst gave a complete conversion of methane at 700 °C. The activity of the different catalysts and of the support in the total oxidation of methane ranged

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as follows: MgAlO < Dy(5)MgAlO < Sm(5)MgAlO < Yb(5)MgAlO < Ce(5)MgAlO. This order was confirmed by comparison of the temperatures corresponding to 50 % methane conversion (T50) and of the specific reaction rates of the different catalysts measured at ca. 620 °C, i.e. above

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20 % conversion, presented in Table 4. Nevertheless, at lower conversions in the kinetic domain, this order was slightly modified and became: MgAlO < Dy(5)MgAlO < Yb(5)MgAlO <

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Sm(5)MgAlO < Ce(5)MgAlO, as confirmed by comparison of the temperatures corresponding to 10 % methane conversion (T10) and of the specific reaction rates of the different catalysts

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measured at ca. 580 °C (Table 4). The presence of the rare-earth cation in the mixed oxide as well as its nature determines the catalytic activity.

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It is well known that methane total oxidation follows a pseudo first order kinetics [4, 17, 40] and by combining the integrated equation of first order kinetics for the plug-flow reactor

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1 1   ln k 1 x

with the Arrhenius equation k  A e



Ea RT

the following relationship can be established: ln[  ln(1  x )]  f (1000 / T )

where x is the degree of methane conversion at the reaction temperature T in Kelvin, τ the reaction time, k the reaction rate constant, Ea the apparent activation energy, R the gas constant and A the preexponential factor. The ln[–ln(1 – x)] versus 1000/T plots corresponding to MgAlO and Ln(5)MgAlO catalysts are shown in Fig. S.1. Only methane conversions lower than ca. 40 % were considered to avoid diffusion limitations. For all the catalysts studied a linear dependency was observed confirming the pseudo first order kinetics for methane catalytic total oxidation.

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The apparent activation energies (Ea) on the different catalysts have been calculated from the slope of the ln[–ln(1 – x)] versus 1000/T plots and are presented in Table 4. They were in the range from 100.9 kJ mol-1 to 200 kJ mol-1. The highest apparent activation energy observed for MgAlO support was considerably lower than that corresponding to thermal combustion [17, 41].

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For Yb(5)MgAlO and Dy(5)MgAlO similar and relatively high apparent activation energies were observed, only slightly lower than that corresponding to MgAlO. This suggested that on

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these oxides similar catalytic sites were involved in the methane total oxidation. Taking into account that these oxides are not reducible below 600 °C, as shown in TPR experiments, the

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surface adsorbed oxygen rather than lattice oxygen species must be involved in the reaction. For Ce(5)MgAlO and Sm(5)MgAlO much lower activation energies were observed, similar to those reported for methane catalytic combustion over reducible oxide-based catalysts and kinetic

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regimes free of mass transport limitations [5, 15, 21, 40-42]. This is in line with their higher reducibility than the Yb- and Dy-containing catalysts below 600 °C observed in H2-TPR

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experiments. It was indeed shown above that in this domain the highly dispersed Sm2O3 species were reduced in Sm(5)MgAlO and that the reducible Ce4+ species in Ce(5)MgAlO were almost

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totally reduced. This behavior suggested that on Ce(5)MgAlO and Sm(5)MgAlO the oxidoreduction of the catalyst is involved in the methane total oxidation in the temperature range

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considered. Nevertheless, taking into consideration that the H2-TPR experiments allow us to determine the order of reducibility of the different catalysts but not the exact reduction

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temperature of the catalyst in the reaction conditions, the involvement of the surface adsorbed oxygen species in the catalytic reaction over Ce(5)MgAlO and Sm(5)MgAlO catalysts cannot be totally ruled out. Similarly, the involvement of the lattice oxygen species in the catalytic reaction over Yb(5)MgAlO and Dy(5)MgAlO catalysts cannot be totally ruled out as well. The lowest activation energy measured for Ce(5)MgAlO accounts for its highest activity in the series studied.

Taking into account that the Ce-based catalyst was the most active, the influence of the cerium content has been studied. The results obtained in the total oxidation reaction of methane with the series of Ce(x)MgAlO mixed oxides (x = 5, 7, 10 and 20) and pure CeO2 were presented in Fig. 6.b. It can be observed that pure CeO2 was the least active catalyst while the Ce(10)MgAlO mixed oxide gave the highest activity with total conversion being achieved at temperatures lower than 700 °C. The activity of the Ce(x)MgAlO catalysts increased with the Ce content up to 10 %.

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It then decreased for Cu(20)MgAlO, probably due to the significant decrease of the specific surface area caused by the excess of ceria. Therefore the activity ranges as follows: Ce(10)MgAlO > Ce(7)MgAlO > Ce(5)MgAlO > Cu(20)MgAlO > CeO2. This order was confirmed by comparison of the values of T10, T50 and T90 (temperatures corresponding to 10 %,

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50 % and 90 % conversion, respectively) and of the specific reaction rates of the different catalysts presented in Table 4. Thus, according to the XPS results, the optimum surface Ce-

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content with respect to cations is around 4.5 at. % (Table 3) and CeO2 crystallite size is of 15 nm (Table 1). Nevertheless, when the intrinsic rate of methane conversion is considered (Fig. S.2),

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the most active catalyst was Ce(20)MgAlO and the activity of the Ce(x)MgAlO catalysts decreased with the cerium content as follows: Ce(20)MgAlO > Ce(10)MgAlO > Ce(7)MgAlO > Ce(5)MgAlO. This can be assigned, on one hand, to the sevenfold decrease of surface area when

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going from Ce(5)MgAlO to Ce(20)MgAlO and, on the other hand, to the increased reducibility of the cerium-containing species below 600 °C observed by TPR. The involvement of the

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reducibility was confirmed by the good linear correlation between the intrinsic activity of the Ce(x)MgAlO catalysts at 600 °C and the H2 consumption below 600 °C (Fig. 7). This shows that

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the ceria species reduced below 600 °C are involved in the catalytic reaction and confirm the oxido-reduction mechanism. This is also supported by the presence of the Ce(IV)/Ce(III) redox

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couple on the catalyst surface revealed by XPS analysis. It is interesting to note that comparing T10, T50 and T90 values, the Ce(x)MgAlO catalysts show similar activities with CeO2-MgO [39]

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and CeO2-ZrO2 [38] solid solutions.

Fig. S.3 shows the ln[–ln(1 – x)] versus 1000/T plots corresponding to Ce(x)MgAlO catalysts for methane conversions lower than 40 %. It can be observed that the results fitted well with the pseudo first order model. The apparent activation energies (Ea) on the different catalysts have been presented in Table 4. They were in the range from 72.0 kJ mol-1 to 100.9 kJ mol-1 and similar to those reported for methane catalytic combustion over ceria-based catalysts in the kinetic regimes free of mass transport limitations [4, 21, 40]. The slightly lower activation energy measured for Ce(10)MgAlO is in agreement with the higher activity of this system. Fig. 8 shows the effect of time on stream on the catalytic activities of Ce(5)MgAlO and Ce(10)MgAlO at 600 °C with 1 % CH4 in air and VHSV = 16000 h-1. Both Ce(5)MgAlO and Ce(10)MgAlO catalysts display a very good stability for more than 100 h reaction time.

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Conclusion Among the LnMgAlO mixed oxide catalysts (Ln = Ce, Sm, Dy and Yb) containing 5 at. % Ln with respect to cations, obtained by thermal decomposition of LDH precursors, the Cecontaining one was the most active giving complete conversion of methane at 700 °C. For

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MgAlO support as well as for Yb(5)MgAlO and Dy(5)MgAlO catalysts the reaction involves mainly the surface adsorbed oxygen species. The catalytic behavior of Ce(5)MgAlO and

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Sm(5)MgAlO is mainly governed by their oxido-reduction ability.

The catalytic activities of the Ce(x)MgAlO mixed oxides with Ce contents between 5 and 20 at.

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% with respect to cations, ranged as follows: Ce(10)MgAlO > Ce(7)MgAlO > Ce(5)MgAlO > Cu(20)MgAlO > CeO2. The optimum surface Ce content with respect to cations is around 4.5 at. % and the optimum CeO2 crystallite size is of 15 nm. The correlation observed between their

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intrinsic activities and H2 consumptions in the TPR experiments confirmed the involvement of an oxido-reduction mechanism in the catalytic reaction. This is also supported by the presence of

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the Ce(IV)/Ce(III) redox couple on the catalyst surface as suggested by the XPS analysis. The catalysts displayed a very good stability for the reaction at 600 °C during ca. 100 h. It was

te

Acknowledgements

d

confirmed that methane total oxidation follows a pseudo first order kinetics in all cases.

The authors are grateful to Valérie Flaud for her expert technical assistance with XPS analysis.

Ac ce p

This research was partially supported during the year 2011 by the Executive Agency for Higher Education, Research, Development and Innovation Funding (UEFISCDI) under the project ‘‘IDEI’’ no. 1906/2009.

References

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5. A.Y. Zarur, J.Y. Ying, Nature 403 (2000) 65-67. 6. P.A. Carlsson, M. Skoglundh, Appl. Catal. B 101 (2011) 669-675. 7. M. Alifanti, J. Kirchnerova, B. Delmon, D. Klvana, Appl. Catal. A 262 (2004) 167-176. 8. P. Ciambelli, S. Cimino, S. De Rossi, M. Faticanti, L. Lisi, G. Minelli, I. Pettiti, P. Porta, G.

ip t

Russo, M. Turco, Appl. Catal. B 24 (2000) 243-253.

9. D.P. Dissanayake, in: J.L.G. Fierro (Ed.), Metal Oxides Chemistry and Applications, CRC

cr

Press, Boca Raton FL, 2006, pp. 543-568.

10. A.R. Gandhe, J.S. Rebello, J.L. Figueiredo, J.B. Fernandes, Appl. Catal. B 72 (2007) 129-

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135.

11. B. Solsona, T.E. Davies, T. Garcia, I. Vazquez, A. Dejoz, S. Taylor, Appl. Catal. B 84 (2008) 176-184.

an

12. E. Campagnoli, A. Tavares, L. Fabbrini, I. Rossetti, Yu. A. Dubitsky, A. Zaopo, L. Forni, Appl. Catal. B 55 (2005) 133-139.

M

13. S. Park, H.J. Hwang, J. Moon, Catal. Lett. 87 (2003) 219-223.

14. J.M. Sohn, M.R. Kim, S.I. Woo, Catal. Today 83 (2003) 289-297.

d

15. S. Tanasoi, G. Mitran, N. Tanchoux, T. Cacciaguerra, F. Fajula, I. Săndulescu, D. Tichit, I.C. Marcu, Appl. Catal. A 395 (2011) 78-86.

te

16. S. Tanasoi, N. Tanchoux, A. Urdă, D. Tichit, I. Săndulescu, F. Fajula, I.C. Marcu, Appl. Catal. A 363 (2009), 135-142.

Ac ce p

17. Z. Jiang, J. Yu, J. Cheng, T. Xiao, M.O. Jones, Z. Hao, P.P. Edwards, Fuel Proc. Technol. 91 (2010) 97-102.

18. Z. Jiang, Z. Hao, J. Yu, H. Hou, C. Hu, J. Su, Catal. Lett. 99 (2005) 157-163. 19. K. Jirátová, P. Čuba, F. Kovanda, L. Hilaire, V. Pitchon, Catal. Today 76 (2002) 43-53. 20. M.F. Wilkes, P. Hayden, A.K. Bhattacharya, J. Catal. 219 (2003) 286-294. 21. L.F. Liotta, G. Di Carlo, G. Pantaleo, G. Deganello, Catal. Commun. 6 (2005) 329-336. 22. A.D. Mayernick, M.J. Janik, J. Catal. 278 (2011) 16-25. 23. M. Primet, E. Garbowski, in: A. Trovarelli (Ed.), Catalysis by Ceria and Related Materials (Catalytic Science Series, vol. 2, Series Editor G.J. Hutchings), Imperial College Press, London, 2005, pp. 407-429. 24. G.C. Pontelli, R.P. Reolon, A. Kopp Alves, F.A. Berutti, C. Perez Bergmann, Appl. Catal. A 405 (2011), 79-83.

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25. P.O. Thevenin, A. Alcade, L.J. Pettersson, S.G. Jaras, J.L.G. Fierro, J. Catal. 215 (2003) 7886. 26. S. Specchia, E. Finocchio, G. Busca, P. Palmisano, V. Specchia, J. Catal. 263 (2009) 134145. 28. A. Gil, L.M. Gandia, S.A. Korili, Appl. Catal. A 274 (2004) 229-235.

ip t

27. S. Colussi, A. Trovarelli, C. Cristiani, L. Lietti, G. Groppi, Catal. Today 180 (2012) 124-130.

cr

29. B. Yue, R. Zhou, Y. Wang, X. Zheng, Appl. Catal. A 295 (2005) 31-39.

30. D. A. Shirley, Phys. Rev. B: Condens. Matter Mater. Phys. 5 (1972) 4709-4714.

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31. J. H. Scofield, J. Electron Spectrosc. Relat. Phenom. 8 (1976) 129-137.

32. G. Mitran, A. Urda, N. Tanchoux, F. Fajula, I.-C. Marcu, Catal. Lett. 131 (2009) 250-257. 33. M. Chen, H. Zheng, C. Shi, R. Zhou, X. Zheng, J. Mol. Catal. A 237 (2005) 132-136. Védrine, Appl. Catal. A 359 (2009) 47-54.

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36. G. Carja, S. Dranca, G. Ciobanu, E. Husanu, I. Balasanian, Mater. Sci. Poland, 27 (2009)

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Page 14 of 30

Figures caption Figure 1. XRD patterns of the Ce(x)MgAlO mixed oxide catalysts after calcination at 750 °C: (a) MgAlO, (b) Ce(5)MgAlO, (c) Ce(7)MgAlO, (d) Ce(10)MgAlO, (e) Ce(20)MgAlO, (f) CeO2

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(MgAlO mixed oxide phase (▼)).

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Figure 2. O1s core level XPS spectra of the Ce(x)MgAlO mixed oxide catalysts: (a)

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Ce(5)MgAlO, (b) Ce(7)MgAlO, (c) Ce(10)MgAlO, (d) Ce(20)MgAlO, (e) CeO2.

Figure 3. C1s core level XPS spectra of the Ce(x)MgAlO mixed oxide catalysts: (a)

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Ce(5)MgAlO, (b) Ce(7)MgAlO, (c) Ce(10)MgAlO, (d) Ce(20)MgAlO, (e) CeO2. Figure 4. Ce 3d core levels XPS spectra of the Ce(x)MgAlO mixed oxide catalysts: (a)

M

Ce(5)MgAlO, (b) Ce(7)MgAlO, (c) Ce(10)MgAlO, (d) Ce(20)MgAlO, (e) CeO2.

d

Figure 5. H2-TPR profiles of the mixed oxide samples: (a) Ce(5)MgAlO, (b) Ce(7)MgAlO, (c)

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Ce(10)MgAlO, (d) Ce(20)MgAlO, (e) CeO2.

Figure 6. CH4 conversion as a function of the reaction temperature for (a) the Ln(5)MgAlO and

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MgAlO catalysts: Ce(5)MgAlO (▲), Sm(5)MgAlO (◆), Dy(5)MgAlO (■), Yb(5)MgAlO (●), MgAlO () and (b) the Ce(x)MgAlO and CeO2 catalysts: Ce(5)MgAlO (▲), Ce(7)MgAlO (●), Ce(10)MgAlO (■), Ce(20)MgAlO (◆), CeO2 () (1 % vol. CH4 in air, total VHSV = 16000 h-1). Figure 7. Intrinsic rate of methane conversion on the Ce(x)MgAlO catalysts at 600 °C vs. H2 consumption in TPR experiments at temperatures lower than 600 °C: Ce(5)MgAlO (▲), Ce(7)MgAlO (●), Ce(10)MgAlO (■), Ce(20)MgAlO (◆). Figure 8. Effect of time on stream on the catalytic activities of Ce(5)MgAlO (▲) and Ce(10)MgAlO (■) at 600 °C with 1 % CH4 in air and VHSV = 16000 h-1.

Page 15 of 30

Table 1. Lattice parameter and mean crystallite size of ceria phase in the Ce(x)MgAlO and CeO2 catalysts. Lattice parameter (nm)

Crystallite size (nm)

Ce(5)MgAlO

0.5391

10.9

Ce(7)MgAlO

0.5403

9.9

Ce(10)MgAlO

0.5375

15.2

Ce(20)MgAlO

0.5377

14.4

CeO2

0.5411

26.7

an

us

cr

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Catalyst

SSA a

Atomic content b (%)

Mg/Al

Total H2 consumption

(m2/g)

Ln

Al

ratio

(mmol/g)

(%) c

MgAlO

188

-

-

-

-

-

-

Yb(5)MgAlO

142

5.6

75.1

19.3

3.9

0.166

38.2

Dy(5)MgAlO

102

5.1

71.4

23.5

3.0

0.130

35.7

Ac ce p

M

Table 2. Physico-chemical characteristics of the catalysts.

d

Mg

te

Catalyst

Sm(5)MgAlO

160

5.2

74.5

20.3

3.7

0.099

23.3

Ce(5)MgAlO

157

5.6

70.4

24.0

2.9

0.104

25.4

Ce(7)MgAlO

101

7.6

69.9

22.5

3.1

0.175

27.9

Ce(10)MgAlO

52

11.0

66.5

22.5

3.0

0.188

26.7

Ce(20)MgAlO

22 (24) d

20.4

57.0

22.6

2.5

0.224

19.1

CeO2

16

-

-

-

-

0.573

19.7

a

Specific surface area.

b

Determined by EDX.

c

Calculated considering the reduction Lnn+ → Ln(n-1)+ of the total amount of rare earth cation in the catalyst.

d

After the catalytic test.

Page 16 of 30

a

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Mg

Al

O

At.

B. E.

At.

B. E.

At.

(eV)

(%)

(eV)

(%)

(eV)

(%)

Ce(5)MgAlO

882.2

0.74

1304.6

13.97

74.7

6.53

Ce(7)MgAlO

883.0

0.80

1304.0

15.88

74.3

Ce(10)MgAlO

882.1

0.89

1303.8

13.12

Ce(20)MgAlO

882.8

2.85

1303.9

12.74

CeO2

882.3

19.36

-

-

Surface atomic ratios

At.

At.

Ce/(Ce+Mg+Al)

Mg/Al

Ce(IV)/Ce

(eV)

(%)

(%)

%

531.9

52.73

26.04

3.48

2.14

0.82

7.90

531.5

51.57

23.86

3.25

2.01

0.80

74.1

5.86

531.6

49.91

30.22

4.48

2.24

0.80

74.3

6.50

531.2

49.06

28.85

12.9

1.96

0.80

-

-

529.3

52.01

28.63

100

-

0.84

ce pt

ed

Binding Energy. Reference binding energy: C1s = 285 eV.

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a

B. E.

M an

B. E.

C

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Ce

Sample

cr

Table 3. XPS analysis of the Ce-containing catalysts.

Page 17 of 30

Table 4. Catalytic performances in methane total oxidation of the catalysts studied. Catalyst

T10 (°C) T50 (°C) T90 (°C) Specific rate (107 mol g-1 s-1) At 580 °C At 600 °C At 620 °C -

1.3

-

Yb(5)MgAlO

565

640

-

3.3

-

Dy(5)MgAlO

585

703

-

1.9

-

Sm(5)MgAlO

550

680

-

3.9

-

Ce(5)MgAlO

505

593

652

8.2

12.4

Ce(7)MgAlO

479

575

652

-

Ce(10)MgAlO

450

561

637

-

Ce(20)MgAlO

500

620

714

-

CeO2

510

635

740

-

(kJ mol-1)

4.4

200.0

10.0

177.0

5.3

177.6

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725

cr

590

8.0

125.7

16.5

100.9

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MgAlO

Ea

-

83.1

16.4

-

72.0

7.9

-

79.9

4.8

-

74.2

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te

d

M

an

14.3

Page 18 of 30

Urdă et al.

cr

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Figure 1.

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Intensity (a. u.)

(f)

M

(e)

d

(d)

te

(c) (b)

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(a)

0





10

20

30

40

50

60

70

2-theta (°)

Page 19 of 30

Urdă et al.

cr

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Figure 2.

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(d)

M

(c)

te

d

(b)

(a)

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Counts per second (a. u.)

(e)

538

536

534

532

530

528

526

Binding energy (eV)

Page 20 of 30

Urdă et al.

cr

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Figure 3.

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(d)

M

(c)

d

(b)

te

(a)

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Counts per second (a. u.)

(e)

294

292

290

288

286

284

282

280

Binding energy (eV)

Page 21 of 30

Urdă et al.

0

U' U U V'''

U''

V'

0

V V

an

us

(e)

Counts per second (a.u.)

V''

cr

U'''

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Figure 4.

(d)

M

(c)

te

d

(b)

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(a)

925

920

915

910

905

900

895

890

885

880

875

Binding energy (eV)

Page 22 of 30

Urdă et al.

cr

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Figure 5.

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Intensity (a. u.)

(e)

Ac ce p

(b)

te

(c)

d

M

(d)

(a)

0

100 200 300 400 500 600 700 800 900 Temperature (°C)

Page 23 of 30

Urdă et al.

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Figure 6.

100

cr

90

us

70

an

60 50 40

M

Methane conversion (%)

80

30

te

d

20 10

Ac ce p

0 450

500

550

600

650

700

750

Temperature (°C)

(a)

Page 24 of 30

100 90

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70

cr

60

us

50 40 30

an

Methane conversion (%)

80

20

450

500

550

600

d

0 400

M

10

650

700

750

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(b)

te

Temperature (°C)

Page 25 of 30

Urdă et al.

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Figure 7.

cr

40

us

-2

-1

Intrinsic rate (10 mol m s )

50

an

-9

30

M

20

0 0.12

0.14

0.16

0.18

0.2

-1

H2 consumption (mmol g )

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0.1

te

d

10

Page 26 of 30

Urdă et al. Figure 8.

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100

cr

90

us

70 60

an

50 40

M

30 20

0 0

10 20

te

d

10

30 40 50 60 70 80 90 100 Time on stream (h)

Ac ce p

Methane conversion (%)

80

Page 27 of 30

Urdă et al.

Research highlights > Rare earth cation-containing layered double hydroxides as catalyst precursors. > Effective catalysts for methane

Ac ce p

te

d

M

an

us

cr

content. > The catalyst reducibility played a key role in the catalytic reaction.

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total oxidation. > The Ce-containing catalyst was the most active. > The catalytic activity depended on the Ce

Page 28 of 30

Urdă et al.

Graphical abstract Ce(5) > Yb(5) > Sm(5) > Dy(5) > MgAlO

70

ip t

60

an

50 40

M

30 20

500

te

0 450

d

10

550

600

650

700

750

Temperature (°C)

Ac ce p

Methane conversion (%)

80

cr

90

Ce(5)MgAlO Sm(5)MgAlO Dy(5)MgAlO Yb(5)MgAlO MgAlO

us

100

Page 29 of 30

Ce(10) > Ce(7) > Ce(5) > Ce(20) > CeO2 90

70

cr

60

us

50 40

an

30 20

450

500

550

600

d

0 400

M

10

650

700

750

te

Temperature (°C)

Ac ce p

Methane conversion (%)

80

Ce(5)MgAlO Ce(7)MgAlO Ce(10)MgAlO Ce(20)MgAlO CeO2

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100

Page 30 of 30