CuxCeMgAlO mixed oxide catalysts derived from multicationic LDH precursors for methane total oxidation

Article abstract of DOI:10.1016/j.apcata.2019.117215

A series of five Cu(x)CeMgAlO mixed oxides with different copper contents (x) ranging from 6 to 18 at. % with respect to cations, but with fixed 10 at. % Ce and Mg/Al atomic ratio of 3, were prepared by thermal decomposition of layered double hydroxide (LDH) precursors at 750 °C. The solid containing 15 at. % Cu, i.e. Cu(15)CeMgAlO, was also calcined at 550 and 650 °C. Powder XRD was used to characterize the crystalline structure and SEM-EDX was used to monitor the morphology and chemical composition of both as prepared and calcined materials. Additionally, the textural properties and the reducibility of the mixed oxide catalysts were studied by nitrogen adsorption/desorption and temperature programmed reduction with hydrogen (H₂-TPR) techniques, respectively. X-ray photoelectron spectroscopy (XPS) was used to determine the chemical state of the elements on the catalyst surface and the diffuse reflectance UV–vis spectroscopy, to obtain information about the stereochemistry and aggregation of copper in the Cu-containing mixed oxides. Their catalytic properties in the total oxidation of methane, used as a volatile organic compound (VOC) model molecule, were evaluated and compared with those of an industrial Pd/Al₂O₃ catalyst. Their catalytic behavior was explained in correlation with their physicochemical properties. Cu(15)CeMgAlO mixed oxide was shown to be the most active catalyst in this series, with a T₅₀ (temperature corresponding to 50% methane conversion) value of only ca. 45 °C higher than that of a commercial Pd/Al₂O₃ catalyst. This difference becomes as low as ca. 25 °C for the Cu(15)CeMgAlO system calcined at 550 °C. The influences of the contact time and of the methane concentration in the feed gas on the catalytic performances of the Cu(15)CeMgAlO catalyst have been investigated and its good stability on stream was evidenced.

Full text of DOI:10.1016/j.apcata.2019.117215

Journal Pre-proof
CuxCeMgAlO mixed oxide catalysts derived from multicationic LDH
precursors for methane total oxidation
Hussein Mahdi S. Al-Aani, Emmanuel Iro, Pramodh Chirra, Ioana
Fechete, Mihaela Badea, C a˘ t a˘ lin Negril a˘ , Ionel Popescu, Maria
Olea, Ioan-Cezar Marcu
PII:
S0926-860X(19)30370-9
https://doi.org/10.1016/j.apcata.2019.117215
117215
DOI:
Article Number:
Reference:
APCATA 117215
To appear in:
Applied Catalysis A: General
Received Date:
Revised Date:
Accepted Date:
1 February 2019
9 August 2019
24 August 2019
Please cite this article as: Al-Aani HMS, Iro E, Chirra P, Fechete I, Badea M, Negril a˘ C,
Popescu I, Olea M, Marcu I-Cezar, CuxCeMgAlO mixed oxide catalysts derived from
multicationic LDH precursors for methane total oxidation, Applied Catalysis A, General
(2019), doi: https://doi.org/10.1016/j.apcata.2019.117215

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AL-AANI et al.
Cu
x
CeMgAlO mixed oxide catalysts derived from multicationic LDH
precursors for methane total oxidation
1
2
2
3
Hussein Mahdi S. AL-AANI , Emmanuel IRO , Pramodh CHIRRA , Ioana FECHETE ,
4
5
6
2
Mihaela BADEA , Cătălin NEGRILĂ , Ionel POPESCU , Maria OLEA , Ioan-Cezar
MARCU*, 1, 6
1
Laboratory of Chemical Technology and Catalysis, Department of Organic Chemistry,
Biochemistry and Catalysis, Faculty of Chemistry, University of Bucharest, 4-12, Blv.
Regina Elisabeta, 030018, Bucharest, Romania
2
School of Science, Engineering & Design, Teesside University, Middlesbrough TS1
3
BX, United Kingdom
3
International Center for CVD Innovation-Nogent, Pole Technologique Sud Champagne,
Université de Troyes-Antenne de Nogent, 26, rue Lavoisier, 52800 Nogent, France
4
Department of Inorganic Chemistry, Faculty of Chemistry, University of Bucharest, 90-
9
2 Panduri Str., 050663, Bucharest, Romania
5
National Institute of Material Physics, P. O. Box MG 7, Măgurele, Romania
Research Center for Catalysts and Catalytic Processes, Faculty of Chemistry, University
6
of Bucharest, 4-12 Blv Regina Elisabeta, 030018, Bucharest, Romania
*
Corresponding author. Phone: +40213051464; Fax: +40213159249; E-mail addresses:
ioancezar.marcu@chimie.unibuc.ro, ioancezar_marcu@yahoo.com
Al-Aani et al.
Graphical abstract

AL-AANI et al.
Methane combustion: CH + 2O → CO + 2H O
4
2
2
2
100
90
80
70
60
50
40
30
20
10
0
ΔT50 ≈ 25 ꢀC
CeMgAlO
Cu(6)CeMgAlO
Cu(9)CeMgAlO
Cu(12)CeMgAlO
Cu(15)CeMgAlO
Cu(18)CeMgAlO
Cu(15)CeMgAlO-550
Industrial Pd/Al O
2
3
300
350
400
450
500
550
600
650
700
750
Temperature (°C)
Highlights





Complex Cu(x)CeMgAl mixed oxides have been synthesized from LDH-based precursors.
Cu(15)CeMgAlO is the most active and highly stable catalyst.
T
50 value is only ca. 25 °C higher than that of an industrial Pd/Al
2
O catalyst.
3
Cu dispersion and Cu-Ce interaction determine the catalytic activity.
Increasing calcination temperature strengthens the Cu-Ce interaction.

AL-AANI et al.
Abstract
A series of five Cu(x)CeMgAlO mixed oxides with different copper contents (x) ranging
from 6 to 18 at. % with respect to cations, but with fixed 10 at. % Ce and Mg/Al atomic
ratio of 3, were prepared by thermal decomposition of layered double hydroxide (LDH)
precursors at 750 °C. The solid containing 15 at. % Cu, i.e. Cu(15)CeMgAlO, was also
calcined at 550 and 650 °C. Powder XRD was used to characterize the crystalline
structure and SEM-EDX was used to monitor the morphology and chemical composition
of both as prepared and calcined materials. Additionally, the textural properties and the
reducibility of the mixed oxide catalysts were studied by nitrogen adsorption/desorption
and temperature programmed reduction with hydrogen (H -TPR) techniques,
2
respectively. X-ray photoelectron spectroscopy (XPS) was used to determine the
chemical state of the elements on the catalyst surface and the diffuse reflectance UV-Vis
spectroscopy, to obtain information about the stereochemistry and aggregation of copper
in the Cu-containing mixed oxides. Their catalytic properties in the total oxidation of
methane, used as a volatile organic compound (VOC) model molecule, were evaluated
and compared with those of an industrial Pd/Al
2
3
O catalyst. Their catalytic behavior was
explained in correlation with their physicochemical properties. Cu(15)CeMgAlO mixed
oxide was shown to be the most active catalyst in this series, with a T50 (temperature
corresponding to 50 % methane conversion) value of only ca. 45 °C higher than that of a
commercial Pd/Al
2
3
O catalyst. This difference becomes as low as ca. 25 °C for the
Cu(15)CeMgAlO system calcined at 550 °C. The influences of the contact time and of
the methane concentration in the feed gas on the catalytic performances of the
Cu(15)CeMgAlO catalyst have been investigated and its good stability on stream was
evidenced.
Keywords: layered double hydroxides, mixed oxides catalysts, copper, cerium, methane
combustion
1
. Introduction

AL-AANI et al.
Volatile organic compounds (VOC), resulting from various industrial processes, but also
from transportation and other activities, represent an important class of air pollutants [1].
Besides the direct harmful effects on human health, these compounds also contribute to
the increasing of photochemical smog, formation of ground-level ozone and atmospheric
ozone destruction [2-4]. Methane is a major hydrocarbon air pollutant of particular
importance in global climate change, occupying the second place after carbon dioxide
among the anthropogenic greenhouse gases (GHG) emitted at the global scale [5]. The
Global Warming Potential (GWP) is used to compare the ability of GHG to trap heat and
their persistence in the atmosphere, and methane has a GWP 25 times higher than CO
2
over a 100-year time horizon [6]. On the other hand, being the most difficult organic
molecule to oxidize due to its high stability, methane is often used as a test molecule in
catalytic combustion of VOC [7].
VOC recovery from the residual gaseous effluents from different industrial processes is
economically disadvantageous due to their very low concentrations and, therefore, their
destruction is the only viable alternative [1, 8]. One of the main processes for the
destruction of VOC is the catalytic combustion, which have some important advantages
compared to traditional flame combustion [9, 10]. Thus, this process uses lower
temperatures for the complete oxidation, which leads to lower energy consumption and
also to the control of NO formation, avoiding, at the same time, the appearance of
x
incomplete oxidation products [11-13].
The most active catalysts for VOC destruction belong to two main categories: supported
noble metals and transition metal oxides [14, 15]. The supported noble metals, like Pd or
Pt, are the most active catalysts for this process, which is their main advantage compared
to the oxide-based catalysts [1, 16, 17]. Nevertheless, the noble metal-based catalysts
have some major disadvantages: they are expensive and, because of their volatility and
high sintering rates, they are easily deactivated at elevated temperatures [18].
Additionally, they may be deactivated by poisoning under the operating conditions [19].
Therefore, much effort has been made in the last years toward the design of new single
and mixed transition-metal oxides catalysts to replace the noble metals for VOC
abatement, a high number of research papers being published on this subject, which
engendered several review papers and book chapters focusing either on catalytic

AL-AANI et al.
combustion of VOC [20-23] and methane [24], or on different types of oxide catalysts,
such as cobalt oxides [25], mesoporous silica-supported catalysts [26], ordered porous
transition metal oxides [27], pillared clays [28, 29] and layered double hydroxide-derived
mixed oxides [30].
Among the transition-metal mixed oxides, those obtained from layered double
hydroxides (LDH) precursors were proven to have great potential as combustion
catalysts, as they have high specific surface areas, high thermal stability and tunable
redox and acid-base properties [30]. Indeed, it has been shown that mesoporous
MMgAlO mixed oxide catalysts (M = Mn, Fe, Co, Ni, Cu, Zn, Ag and Pd), obtained by
thermal decomposition of LDH precursors, are promising catalysts for the total oxidation
of short-chain hydrocarbons [31]. Among the non-noble metal-containing catalysts, the
Cu-containing system has been shown to be the most active in methane combustion,
being also highly stable in the reaction conditions. Its catalytic activity depends on the Cu
content, the optimum being of ca. 12 at. % Cu and corresponds to the best dispersion of
the copper-containing species in the MgAlO matrix [32]. On the other hand, from a series
of lanthanide-containing LDH-derived mixed oxides catalysts, LnMgAlO (Ln = Ce, Sm,
Dy and Yb), the Ce-containing one turned out to be the most active in methane
combustion, the optimum content being ca. 10 at. % Ce [33]. Therefore, in an attempt to
converge the benefits of preparation of mixed oxides from LDH precursors with the high
activity of Cu-based systems in methane combustion and taking into consideration an
expected Cu-Ce synergistic effect [34, 35], the present work investigates a series of new
LDH-derived Cu-Ce-MgAl mixed oxides with 10 at. % Ce and different Cu content
ranging from 6 to 18 at. %.
2
. Experimental section
2
.1. Catalysts preparation
A series of five Cu(x)CeMgAl LDH precursors with different copper contents ranging
from 6 to 18 at. % with respect to cations, but with fixed 10 at. % Ce and Mg/Al atomic
ratio of 3, were prepared by coprecipitation under ambient atmosphere. In a typical
procedure, a mixed salts solution (200 mL) of Mg(NO
3
)
2
·6H
2
O, Al(NO
3
)
3
·9H O and
2
Cu(NO ·3H O and an alkaline solution of NaOH (2 M) were simultaneously added
3
)
2
2

AL-AANI et al.
dropwise into a beaker containing 200 mL of cerium nitrate solution (Ce(NO
3
)
3
·6H O) at
2
room temperature with controlled rate to maintain the pH close to 10. After complete
precipitation, the slurry was aged at 80 °C overnight under vigorous stirring. The
suspension was then separated by centrifugation, washed with deionized water and finally
dried overnight at 80 °C. The Cu(x)CeMgAlO catalysts (with x = 6, 9, 12, 15 and 18 at.
%, respectively) were obtained by calcination of their corresponding LDH precursors in
air at 750 °C for 8 h. The Cu-free CeMgAl LDH precursor and the corresponding
CeMgAlO mixed oxide were obtained following the same protocol.
In order to investigate the influence of the calcination temperature on the catalytic
properties of the mixed oxides, the Cu(15)CeMgAl LDH precursor was also calcined at
5
50 and 650 °C, the resulting mixed oxides being noted Cu(15)CeMgAlO-550 and
Cu(15)CeMgAlO-650, respectively.
2
.2. Catalysts characterization
Powder X-ray diffraction (XRD) patterns of both precursors and mixed oxides were
recorded on a Siemens D500 powder X-ray diffractometer with Bragg-Brentano
geometry using nickel-filtered Cu-Kα radiation (λ = 0.15406 nm). They were recorded
-1
over the 5-70° 2θ angular range at a scanning rate of 1° min . Crystalline phases were
identified using standard JCPDS files.
Scanning electron microscopy (SEM) together with X-ray energy dispersion analysis
(
EDX) were used to monitor the morphology and chemical composition of both
precursors and mixed oxides. SEM/EDX examination was performed using a Hitachi S-
400N microscope operated at an accelerating voltage of 20 kV. Four different points
3
were analyzed on each sample.
X-ray photoelectron spectroscopy (XPS) was employed to determine the chemical state
of the elements on the catalyst surface with a SPECS spectrometer equipped with a
PHOIBOS 150 analyzer using a monochromatic Al Kα radiation source (1486.7 eV). The
acquisition was operated at a pass energy of 20 eV for the individual spectral lines and 50
eV for the extended spectra. The analysis of the spectra has been performed with the
Spectral Data Processor v2.3 software using Voigt functions and usual sensitivity factors.

AL-AANI et al.
The textural characterization was performed using the conventional nitrogen
adsorption/desorption method, with a Micromeritics ASAP 2010 automatic equipment.
The surface areas were calculated using the BET method in the relative pressure, P/P ,
0
region 0.065-0.2, while the pore sizes were determined by the BJH method from the
nitrogen desorption branch. Prior to nitrogen adsorption, the oxide samples were
degassed at 300 °C for 12 h.
The reducibility of the catalysts was studied by temperature programmed reduction under
hydrogen (H
2
-TPR). Experiments were performed using a CATLAB microreactor – MS
in Ar mixture (flowrate 30 mL
min ) through the CATLAB’s packed micro-reactor (4 mm internal diameter and length
8.5 cm) containing about 35 mg of sample, which was heated at a constant rate of 10 °C
system (Hiden Analytical, UK) under a flow of 5 % H
2
-1
1
-1
min up to 800 °C. The system was maintained for 1 h at 800 °C under H
complete the reduction.
2
/Ar flow to
Diffuse reflectance UV-Vis spectra were recorded in the range 200-1300 nm, using
Spectralon as a standard, in a Jasco V 670 spectrophotometer. The obtained reflectance
spectra were converted
into the dependencies of Kubelka-Munk function on the absorption energy.
2
.3. Catalytic test
The catalytic tests for the methane combustion over the mixed oxide catalysts were
carried out in a fixed bed quartz tube down-flow reactor at atmospheric pressure. If not
otherwise specified, a mixture of CH
4
and air containing 1 vol. % methane was passed
3
-1
through 1 cm (ca. 0.9 g) catalyst bed with a total flow rate of 267 mL min
-1
corresponding to a gas hourly space velocity (GHSV) of 16000 h . For comparison, an
industrially used Pd/Al catalyst (supplied by ARPECHIM Piteşti, Romania) was
2
O
3
-1
tested in methane combustion in similar conditions, with a GHSV of 20000 h . Before
testing, the catalyst was pre-treated for 30 min in a stream of nitrogen at 600 °C (500°C
for the sample calcined at 550 °C) for cleaning its surface. After pre-treatment, the
catalyst was cooled down to 300 °C and the reaction was started by introducing the
reaction mixture. Activity measurements were performed by increasing the reaction
temperature from 300 to 650 °C at regular intervals. The reactants and product gases

AL-AANI et al.
were analyzed on-line by a Clarus 500 Gas-Chromatograph equipped with a thermal
conductivity detector, using two packed columns in series (6 ft Hayesep and 10 ft
molecular sieve 5 Å). The catalysts activity was characterized by T10, T50 and T90
representing the temperatures of methane conversions of 10, 50 and 90 %, respectively.
The conversion was calculated as the amount of methane transformed in the reaction
divided by the amount that was fed to the reactor by using the following formula:
퐶
−퐶
퐶퐻 ,표푢푡
퐶퐻 ,푖푛
4
4
퐶표푛푣푒푟푠푖표푛 %ꢀ =
× 100
퐶
퐶퐻 ,푖푛
4
where CCH ,in and CCH ,out represent the methane concentration (v/v) in the feed and
4
4
effluent gases, respectively.
Complete selectivity to CO
2
and H O was always observed. The carbon balance was
2
calculated based on the following equation:
= 퐶 + 퐶
퐶
퐶퐻 ,푖푛
퐶퐻 ,표푢푡
퐶푂 ,표푢푡
2
4
4
where CCO ,out is the concentration of carbon dioxide (v/v) in the effluent gas. It was
2
satisfactory in all runs to within ± 2 %.
3
. Results and discussion
3
.1. Catalysts characterization
The XRD patterns of the as-prepared precursor samples are displayed in Fig. 1. It can be
observed that all the precursors consist of poorly crystallized LDH (JCPDS 37-0630) and
boehmite AlOOH (JCPDS 83-2384) phases. The absence of diffraction lines
corresponding to copper- or cerium-containing additional phases can be noted suggesting
that these cations are well dispersed in the precursor samples.
For all the Cu(x)CeMgAlO catalysts, Mg(Al)O mixed oxide phase with the periclase-like
structure (JCPDS-ICDD4-0829) and CeO
2
fluorite phase (JCPDS 75-0076) were
identified (Fig. 2a). Except for Cu(18)CeMgAlO system, no diffraction lines
corresponding to CuO phase were observed suggesting that copper is well dispersed in
the Cu(x)CeMgAlO catalysts with x < 18.
Unexpectedly, the value of the lattice parameter, d, of the fluorite structure of the ceria
phase calculated using the Bragg’s law, from the three most intense lines in the
diffractograms, i.e. (111), (220) and (311), increases with increasing Cu content in the

AL-AANI et al.
Cu(x)CeMgAlO catalysts (Table 1). Indeed, taking into consideration that the radius of
2
+
4+
+
4+
Cu ion is lower than that of Ce , and those of Cu and Ce ions are similar, the lattice
parameter should decrease with increasing Cu content [36, 37]. Compared to the
theoretical value of the ceria lattice parameter of 0.5411 nm [38], that of the samples with
x < 12 is lower, and that of the samples with x ≥ 12 is even higher. This suggests the
incorporation of copper into the ceria lattice at low Cu content, while at high Cu content
ceria crystallites coexist with separate CuO particles, the latter being well developed in
the Cu(18)CeMgAlO mixed oxide, in line with its XRD pattern. This is in agreement
with the observed decrease of Cu solubility in ceria with increasing the grain size of the
latter, resulting in the segregation of copper to the grain boundaries [39].
The full-width at half-maximum (FWHM) of the three most intense reflections of CeO
phase allows estimating the average crystallite size using the Debye-Scherrer equation:
2
1
.08
D 
(
2)FWHM cos
where D is the crystallite size, λ is the wavelength of the Cu K-alpha radiation (0.15406
nm), θ is the Bragg diffraction angle. They are presented in Table 1. It can be observed
that the crystallite size of ceria continuously increases with increasing the Cu content in
the catalyst from 6.3 nm for Cu(6)CeMgAlO to 15.1 nm for Cu(18)CeMgAlO. A similar
behavior has already been observed for coprecipitated CuO-CeO mixed oxides [40].
2
Taking into consideration that the Ce content is almost constant in all the mixed oxide
samples, it can be concluded that at low Cu content highly dispersed Cu-doped ceria
crystallites are formed in the Mg(Al)O matrix, while at high Cu content larger ceria
crystallites coexist with CuO particles that form a separate phase identified by XRD in
the Cu(18)CeMgAlO mixed oxide. This reflects the low solubility of Cu ions in ceria
[
41].
The effect of calcination temperature on the XRD pattern of the Cu(15)CeMgAlO mixed
oxide is shown on Fig. 2b. It can be observed that the diffraction lines of both Mg(Al)O
mixed oxide and CeO fluorite phases become significantly larger when the calcination
2
temperature decreases from 750 to 650 °C suggesting a strong decrease of the crystallite
size. Indeed, the average crystallite size of ceria decreases from 12.2 nm for the sample
calcined at 750 °C to 4.1 nm for that calcined at 650 °C (Table 1). Further decreasing the

AL-AANI et al.
calcination temperature from 650 to 550 °C results in slightly larger and less intense
diffraction lines accounting for a lower crystallinity of the sample Cu(15)CeMgAlO-550.
The average crystallite size of ceria slightly decreases from 4.1 nm for Cu(15)CeMgAlO-
6
50 to 3.1 nm for Cu(15)CeMgAlO-550 (Table 1).
It is worth noting that the XRD patterns of the catalysts remain unchanged after the
catalytic tests as it can be observed in Fig. S1 for the Cu(x)CeMgAlO samples with x = 6,
1
2 and 18, respectively.
The cationic composition of both LDH precursors and calcined oxide catalysts has been
determined by EDX spectroscopy and it is tabulated in Table 2. It can be observed that in
both as-prepared and calcined samples the cationic content is close to the theoretical
values within the limits of experimental error of the method used. The Mg/Al atomic
ratio was very close to the fixed value of 3, while the Cu/Ce atomic ratio was slightly
higher than the fixed value.
The SEM micrographs of both as-prepared and calcined Cu(x)CeMgAl samples are
shown in Fig. S2. It can be observed that there are no significant differences between the
particle morphologies of the precursors, on one hand, and of the calcined oxides, on the
other hand. More unexpected, no significant differences can be observed between the
particle morphologies of the precursors and of the corresponding mixed oxides. This
indicates that neither the Cu content within the range 6-18 at. %, nor the calcination
temperature up to 750 °C influence the particle morphology of the studied materials.
X-ray photoelectron spectroscopy (XPS) has been used to investigate the oxidation states
of the different elements and the surface composition of the Cu(x)CeMgAlO mixed
oxides. All the expected elements, i.e. Cu, Ce, Mg, Al, O and C are present on their
surface (Table 3). The O 1s core level XPS spectra (Fig. 3a) showed for all the samples a
peak deconvoluted into two components which could be related to lattice oxygen in oxide
(BE ca. 529.7 eV) and oxygen in the lateral structure (BE ca. 531.6 eV), respectively
[42]. The oxygen in the lateral structure corresponds to hydroxyl and/or carbonate species
[43] and also to subsurface oxygen ions with particular coordination and lower electron
density than the lattice oxygen [42]. The component at 531.6 eV obviously accounts for
the hydroxylation and carbonatation of the catalyst surface, as its relative intensity
increased linearly with the specific surface area of the catalyst (Fig. S3), except for

AL-AANI et al.
Cu(18)CeMgAlO sample. For the latter, the subsurface oxygen ions with particular
coordination, probably located at the interface of the crystalline CuO and CeO
also have an important contribution to the XPS peak at 531.6 eV. The C 1s core level
Fig. 3b) shows two main contributions: the adventitious hydrocarbon species (BE 284.8
2
phases,
(
eV) and carbon in carbonate (BE ca. 289.2 eV) [43]. These results confirm the need of
pretreating the catalysts in the reactor under air at 600 °C before each activity test [33].
The photoelectron profile of Cu 2p region for all the Cu(x)CeMgAlO mixed oxides are
displayed in Fig. 4a and the surface concentrations of copper together with Cu(II)/Cu
atomic ratios are tabulated in Table 3. The observation of Cu 2p3/2 with binding energies
+
2+
centered at 932.2 and 934.2 eV indicates the presence of Cu and Cu species,
respectively, in all the samples [44, 45]. The shake-up satellites of Cu 2p3/2 emission line
2
+
centered at 941.2 and 943.7 eV also confirm the presence of Cu [45]. The existence of
+
Cu species in the Cu(x)CeMgAlO mixed oxide catalysts suggests that, at least at the
3
+
4+
interface, CuO phase could be doped with higher valence cations, i.e. Al and/or Ce ,
existing in the mixed oxide. The data in Table 3 show that the surface of the catalysts is
enriched in Cu compared to the bulk composition (Table 2). The surface Cu(II)/Cu
atomic ratio was calculated by the method developed in Ref. [45] using the following
equation:
퐵(1 + (퐴1 /퐵 )
푠
푠
%
퐶푢 퐼퐼ꢀ =
× 100
퐴+ 퐵
where A is the total area of the main Cu 2p3/2 emission line, B is the area of the shake-up
peak and A1 /B is a factor representing the ratio of the main peak/shake-up peak areas
s
s
for a 100 % pure Cu(II) sample equal to 1.89 ± 0.08 for 20 eV pass energy [45]. The
surface Cu(II)/Cu atomic ratio passes through a maximum for the Cu(15)CeMgAlO
mixed oxide with increasing the Cu content, then it decreases for Cu(18)CeMgAlO
sample. For the Cu(15)CeMgAlO sample calcined at different temperatures, the
Cu(II)/Cu atomic ratio increases linearly with increasing the calcination temperature (Fig.
S4).
The X-ray photoelectron spectra of the Ce 3d3/2 and Ce 3d5/2 core levels of the
Cu(x)CeMgAlO mixed oxides are presented in Fig. 4b. The XPS spectrum of pure
Ce(IV) oxide should present six peaks (three pairs of spin-orbit doublets) which are

AL-AANI et al.
conventionally 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) [46]. The XPS spectrum
of Ce(III) oxide should present four peaks (two pairs of spin-orbit doublets) which are
labeled in order of decreasing energy U’, U (corresponding to the 3d3/2 level) and V’, V0
0
(corresponding to the 3d5/2 level) [46]. When both Ce(III) and Ce(IV) species are present
on the surface of an oxide, the resulting spectrum is a superposition of all these ten
features, as it can be seen in Fig. 4b for the Cu(x)CeMgAlO mixed oxides. The values of
the characteristic binding energies of these ten features present in the XPS spectra of
Ce(III) and Ce(IV) were taken from Ref. [47]. It has been reported that the U”’ peak is
exclusively attributed to Ce(IV), being absent from the Ce3d spectrum of pure Ce(III)
oxide, and, hence, it is used as quantitative measure of the amount of Ce(IV) [48]. Thus,
taking into consideration that for pure Ce(IV) oxide the U”’ peak represents ca. 14 % of
total integral intensity [48], the percentage of surface Ce(IV) can be calculated using the
following equation:
%
푈′′′
%
퐶푒 퐼푉ꢀ =
× 100
1
4
where % U’’’ represents the percentage of U”’ peak area with respect to the total Ce 3d
area. Thus, the Ce(IV)/Ce surface atomic ratio (Table 3) is higher than 0.8 for all the
mixed oxides, suggesting the presence of low quantities of surface Ce(III) that vary
irrespective to the Cu content. It is worth noting 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. However, for the Cu(15)CeMgAlO sample calcined at different
temperatures, the Ce(IV)/Ce atomic ratio increases linearly with increasing the
calcination temperature (Fig. S4). The data in Table 3 also shows that the surface Ce
content is significantly lower than that of the bulk (Table 2) for all the Cu(x)CeMgAlO
mixed oxides. Taking this into consideration and the surface Cu enrichment observed, the
Cu/Ce surface atomic ratio (Table 3) is significantly higher than the bulk ratio (Table 2)
for all the mixed oxide samples.
The X-ray photoelectron spectra of the Al 2p and Mg 2p of the Cu(x)CeMgAlO mixed
oxides are shown in Figs. S5 and S6, respectively. Fig. S6 also shows the Auger Mg KLL
spectra of the Cu(x)CeMgAlO mixed oxides. The Al 2p peak appears at almost the same

AL-AANI et al.
binding energy as Cu 3p. The relative peak positions of both Al and Mg are very stable
(
Table S1) and account for Al3+ and Mg2+ in their corresponding oxides. This was
confirmed for Mg by calculating the modified Auger parameter (m-AP) using the
following formula:
푚-AP = 퐵퐸푀푔2푝 + 퐾퐸푀푔퐾퐿퐿
where BEMg2p is the binding energy of Mg 2p and KEMgKLL, the kinetic energy of the
MgKLL peak. Indeed, the values obtained for m-AP (Table S1) are specific for MgO [49]
in all the mixed oxide samples. The Mg/Al surface atomic ratio (Table 3) is lower than
the bulk ratio for all the Cu(x)CeMgAlO mixed oxides indicating the Al enrichment of
the surface.
The specific surface area, the pore volume and the pore size of the Cu(x)CeMgAlO
mixed oxides are listed in Table 1 and their corresponding adsorption-desorption
isotherms and pore size distributions are shown in Figs. S7 and S8, respectively. The
specific surface area of the Cu-containing samples calcined at 750 °C are significantly
larger than that of the CeMgAlO support and decreased with increasing Cu content from
2
-1
2
-1
1
69 m g for Cu(6)CeMgAlO to 108 m g for Cu(18)CeMgAlO, in line with the
concurrent increase of the crystallinity of these oxides (Fig. 2a). On the other hand, as
expected, the surface area of the Cu(15)CeMgAlO mixed oxide calcined at different
2
-1
temperatures decreases from 156 to 120 m g when the calcination temperature
increases from 550 to 750 °C due to the sintering of the particles. All the mixed oxide
catalysts reveal type IV isotherms according to the IUPAC classification, with H3-type
hysteresis loops characteristic of mesoporous materials with slit-shaped pores [50]. For
both series of mixed oxides, the pore volume decreases with decreasing the surface area
(
Table 1). The pore size distributions for the oxides calcined at 750 °C obtained from the
desorption branch of isotherms (Fig. S8) indicate bimodal pore structures extending from
to ca. 20 nm, with well-defined maxima at 3.7 – 3.9 nm and 7.4 – 11.5 nm,
3
respectively. For the Cu(15)CeMgAlO samples calcined at lower temperatures larger and
less well-defined bimodal pore size distributions are observed, extending from 2 to ca. 30
nm. Thus, well-defined maxima at 3.7 and 4.1 nm are observed for the samples calcined
at 650 and 550 °C, respectively, together with shoulders at ca. 9 nm for both samples
(Fig. S8). This indicates that increasing the calcination temperature of the Cu(x)CeMgAl

AL-AANI et al.
precursors from 550 to 650 °C no major textural changes take place, while increasing
from 650 to 750 °C results in narrower bimodal pore structure with an increase of the
large pores.
H
2
-TPR measurements were carried out to investigate the reduction behavior of the
different Cu(x)CeMgAlO catalysts. The TPR profiles of the mixed oxides are displayed
in Fig. 5 and the corresponding H consumptions are presented in Table 4. For CeMgAlO
support three weak asymmetric reduction peaks were observed in the temperature ranges
30-330 °C, 330-580 °C and 580-750 °C, respectively. They account for ceria particles
2
2
having different sizes or interactions with the Mg-Al mixed oxide matrix, as described
elsewhere [33]. Thus, the low temperature peak was attributed to highly reducible ceria
particles dispersed on the surface of the Mg(Al)O support. The medium temperature peak
4
+
was attributed to ceria particles interacting with Mg(Al)O support and to Ce species
forming CeMgO solid solution. The high temperature peak was attributed to less
reducible large ceria crystallites dispersed in the Mg(Al)O matrix. The total amount of
4
+
Ce species reduced was ca. 27 %. For Cu(x)CeMgAlO catalysts only one broad and
intense reduction peak was observed between ca. 100 and 390 °C, with a queue extending
up to a temperature going increasingly from 500 °C for Cu(6)CeMgAlO to 570 °C for
Cu(18)CeMgAlO. The intensity of the peak increased with the Cu content of the
catalysts. This TPR profile accounts for the reduction of both Cu2+ and Ce4+ species.
Thus, most likely the broad and intense reduction peak can be attributed to the successive
2
+
reduction of Cu species doped in the ceria particles [51] and from well-dispersed and
sintered CuO particles interacting more or less strongly with the CeMgAlO support [32]
4
+
as well as to the reduction of Ce species from highly reducible smaller ceria particles
[
52]. The weak and extended reduction signal above 390 °C can be attributed to the
reduction of Ce4+ species from less reducible larger ceria particles whose reduction
extends to higher and higher temperatures with increasing the ceria crystallite size, which
increases with Cu content. Indeed, it has been shown that the high temperature reduction
of ceria strongly depends on its crystallinity [53]. On the other hand, it is well known that
the reducibility of ceria is enhanced by copper in the CuO-CeO
2
system [54, 55]. The
reduction of ceria at lower temperature in CuCeMgAlO mixed oxides compared to Cu-

AL-AANI et al.
free CeMgAlO support is obviously due to the fact that ceria phase is reduced by atomic
hydrogen formed by the dissociation of dihydrogen on the reduced copper particles [51].
The hydrogen consumption of the Cu(x)CeMgAlO catalysts is much higher compared to
that of the CeMgAlO support and increases with increasing the Cu content. Assuming the
total reduction of all the Cu2+ species in the mixed oxide, the amount of Ce species
reduced was calculated and is presented in Table 4. It can be observed that the amount of
Ce4+ species reduced strongly decreases with increasing the Cu content in the mixed
oxide. Moreover, the amount of Ce4+ species reduced exponentially decreases with
increasing the ceria particle size (Fig. 6a), on one hand, and with increasing the Cu/Ce
surface atomic ratio (Fig. 6b), on the other hand. These results confirm the incorporation
of copper into the ceria lattice at low Cu content thus favoring the reduction of Ce4+
species, while at high Cu content, it strongly tends to agglomerate leading to separate
CuO particles enriching the surface of the solid which coexist with large crystallites of
ceria that are less exposed on the surface and more difficult to be reduced. Notably, the
segregation of copper oxide onto ceria nanoparticles has been shown to take place for Cu
4+
x
molar fractions (1-x) as low as 0.1 in Ce Cu1-x nanocrystalline oxides [39].
The H -TPR patterns of the Cu(15)CeMgAlO systems calcined at three different
2
temperatures, i.e. 550, 650 and 750 °C, are shown in Fig. 5b. It can be observed that the
increase of the calcination temperature from 550 to 750 °C leads to a shift of the TPR
peaks to lower temperature which corresponds to an increased reducibility of the samples
in terms of easiness of reduction, in line with previously reported studies [56]. Regarding
the hydrogen consumption (Table 4), it corresponds to the total reduction of all and of ca.
9
5 % Cu2+ species in the Cu(15)CeMgAlO samples calcined at 550 and 650 °C,
respectively, therefore suggesting that no reduction of cerium takes place. Calcination at
2
+
7
7
50 °C leads not only to the total reduction of all Cu species in the Cu(15)CeMgAlO-
50 sample, but also to the reduction of a fraction of Ce4+ species. This suggests a
stronger Cu-Ce interaction in the Cu(15)CeMgAlO sample calcined at 750 °C compared
to the samples calcined at lower temperatures.
To obtain information about the stereochemistry and aggregation of copper in the Cu-
containing mixed oxides, they were analyzed by DR-UV-Vis spectroscopy. The DR-UV-
Vis spectra of all the Cu(x)CeMgAlO samples (Figure 7) show two absorption bands.

AL-AANI et al.
The first one in the ultraviolet region can be assigned to charge transfer transitions
between copper and oxygen ions [57]. It evidences the presence of mononuclear Cu2+
2
+
2-
2+ 2+
centers (the shoulder at ca. 260 nm) as well as oligomeric (Cu – O – Cu )
n
species
(band at ca. 330 nm) [53]. The second band centered at 680-700 nm in the visible region
accounts for d-d transitions characteristic for copper (II) ion in an octahedral
stereochemistry [57]. The different shape of the UV-Vis spectrum of the
Cu(18)CeMgAlO mixed oxide, i.e. higher intensity and a supplementary peak at ca. 450
nm, can be explained by the presence of copper (II) ions with two different
stereochemistries, i.e. octahedral and square-planar, the latter corresponding to crystalline
CuO, in line with the XRD data. Notably, the peak at ca. 450 nm, absent in the spectra of
Cu(15)CeMgAlO-550 and Cu(15)CeMgAlO-650 samples, is also visible in the UV-Vis
spectrum of the Cu(15)CeMgAlO mixed oxide calcined at 750 °C, suggesting that small
CuO crystallites (with copper (II) ions in a square planar configuration) are also present
in this sample, although not detected by XRD. They are obviously formed with
increasing the calcination temperature.
3
.2. Catalytic properties
Methane was used as a model molecule to evaluate the oxidation ability of the catalysts.
Fig. 8a shows the conversion – reaction temperature plots for the Cu(x)CeMgAlO
catalysts. The T10, T50 and T90 temperatures, which correspond to 10, 50 and 90 %
methane conversion, respectively, and both the intrinsic and specific activities at 380 °C,
where the conversion level remains low for the most active catalysts, are listed in Table
5
. The total oxidation activity of the Cu(x)CeMgAlO catalysts was compared to that of a
reference Pd/Al catalyst supplied by ARPECHIM, Piteşti, Romania, previously
reported in Ref. [58]. The conversion – reaction temperature sigmoid for Pd/Al and
2
O
3
2
O
3
the corresponding T90 value of 484 °C clearly show that this catalyst is highly active for
methane combustion. In contrast, the conversion – reaction temperature sigmoid for
CeMgAlO support is significantly shifted to higher temperatures (T90 = 637 °C)
indicating its lowest activity in the series studied. The light-off sigmoids for
Cu(x)CeMgAlO catalysts are between that of Pd/Al
2
3
O and CeMgAlO, indicating that
copper is a key active component in methane combustion [59]. It appears that the copper

AL-AANI et al.
content strongly influences, in a complex manner, the catalytic activity of the
Cu(x)CeMgAlO catalysts. Indeed, in terms of T10 the catalytic activity follows the order:
CeMgAlO < Cu(12)CeMgAlO < Cu(9)CeMgAlO < Cu(6)CeMgAlO ≈ Cu(18)CeMgAlO
<
Cu(15)CeMgAlO. This suggests that when Cu content ranges between 6 and 12 at. %,
the effect of Cu dispersion on the catalytic activity is stronger than the effect of Cu
content. This behavior has already been observed for Pd/Al catalysts [60]. However,
2
O
3
regarding the T90 values the order of activity appears inversed for the low Cu-content
systems (6 ≤ x ≤ 12). This is obviously due to increased mass transfer limitations with
increasing the surface area and decreasing the pore size when x decreases from 12 to 6
(Table 1). The most active catalyst in this series is the Cu(15)CeMgAlO mixed oxide
2
+
likely due to the excellent dispersion of Cu in this material for which oligomeric (Cu –
2
-
2+ 2+
O – Cu )
n
species together with tiny not XRD-visible CuO crystallites were evidenced
by UV-Vis spectroscopy (Fig. 7). Indeed, Cu(18)CeMgAlO, which contains XRD-visible
crystalline CuO phase (Fig. 2), is less active than Cu(15)CeMgAlO within all the
temperature range. For the latter, the T50 value is ca. 100 °C lower than that of the Cu-
free catalyst and remained only ca. 45 °C higher than that of the commercial Pd/Al
2
O
3
catalyst. The complete conversion of methane was achieved at 600 °C (T100) for both
Cu(15)CeMgAlO and Cu(18)CeMgAlO systems, temperature which is only 30 °C higher
than that corresponding to the Pd/Al
2
3
O catalyst. Both specific and intrinsic activity
values calculated at 380 °C (Table 5) show a marked superiority of the Cu(15)CeMgAlO
system. This behavior can be attributed not only to the excellent dispersion of copper in
this catalyst, but also to a synergy effect between Cu and Ce. Indeed, the CuO-CeO
2
2
interactions are known to play a key role on the catalytic performance of the CuO-CeO
catalysts, a review paper focused on this subject being recently published [34]. Notably,
this synergy effect leading to an enhanced catalytic activity in methane combustion was
shown to correspond to an optimum composition in the Cu-Ce system [59]. This
optimum corresponds to the Cu(15)CeMgAlO system in the studied Cu(x)CeMgAlO
series.
Linear correlations between the rate of methane conversion and the hydrogen
consumption in H
2
-TPR experiments for mixed oxides catalysts obtained from LDH
precursors containing Cu [32, 58, 61] and Ce [33] have been previously evidenced.

AL-AANI et al.
However, in the case of Cu(x)CeMgAlO systems the rate of methane transformation
passes through a maximum for the Cu(15)CeMgAlO (Fig. S9) clearly suggesting that not
only Cu, but also Ce is involved in catalysis, in line with the data in Table 4, with an
enhanced synergy effect between these two elements for the Cu(15)CeMgAlO sample.
On the other hand, the intrinsic methane conversion rate at 380 and 400 °C and the
Cu(II)/Cu surface atomic ratio follow the same trend as a function of the Cu content in
the mixed oxide catalysts, with a maximum of both of them for the Cu(15)CeMgAlO
system (Fig. 9). This clearly suggests that Cu(II)/Cu surface atomic ratio is a key factor
controlling the catalytic activity of the Cu(x)CeMgAlO catalysts.
The presence of diffusion limitations, indicated by the decreasing slope in the Arrhenius
plots (Fig. S10), can be observed at higher reaction temperatures. As a consequence, only
low temperature conversion data have been used to obtain the apparent activation
energies (E
a
) on the different catalysts. Thus, they have been calculated from the slope of
3
the linear part of the lnr
i
versus 10 /T plots in Fig. S10 and are presented in Table 5. It
can be observed that the activation energy decreases when Cu is added to the CeMgAlO
support to obtain Cu(6)CeMgAlO composition, then it continuously increases with
increasing the Cu content in the mixed oxide catalyst. This suggests an evolution of the
nature of the catalytic site, in line with the XRD and TPR results showing, on one hand,
the incorporation of copper into the ceria lattice at low Cu content determining the
4
+
simultaneous reduction of an important amount of Ce species, which results in more
reactive catalytic sites (lower activation energies). However, their surface density is
obviously low to explain the weak catalytic activity of the catalysts with small Cu
content. On the other hand, at high Cu content, separate CuO particles coexist with larger
less reducible ceria crystallites and, the most likely, the less reactive catalytic site (higher
activation energies) can be associated with CuO particles less interacting with CeO . The
2
high catalytic activity of the catalysts with high Cu content is due to their high density of
active sites. The enhanced activity of the Cu(15)CeMgAlO system is obviously due to an
excellent dispersion of CuO particles favoring their interaction with ceria, the CuO
species involved in catalysis being located at CuO-CeO
2
particles interface [55, 59]. It is
noteworthy that the values obtained for the E are in agreement with those measured for
a

AL-AANI et al.
methane catalytic combustion over both similar LDH-derived CuO catalysts [32, 58, 62]
and CuO-CeO
2
and CuO-ZrO [55].
2
The calcination temperature of the precursor is known to influence the catalytic activity
of the catalysts [56, 63]. Figure 8b shows the light-off curves for the total oxidation of
methane over Cu(15)CeMgAlO mixed oxides catalysts calcined at 550, 650 and 750 °C.
It can be observed that the sigmoid shifts to lower temperature by decreasing the
calcination temperature of the catalyst, the T50 value ranging as follows:
Cu(15)CeMgAlO-750 > Cu(15)CeMgAlO-650 > Cu(15)CeMgAlO-550. This is in line
with the surface areas of the catalysts, which decrease as the calcination temperature
increases (Table 1). It is worth noting that the T50 and T100 values for the
Cu(15)CeMgAlO-550 catalyst are only ca. 25 and 5 °C, respectively, higher than those of
the commercial Pd/Al
2
O
3
catalyst. However, in terms of intrinsic rate of CH conversion
4
(Table 5), the most active catalyst is that calcined at 750 °C in line with its lowest
activation energy, the intrinsic activity ranging as follows: Cu(15)CeMgAlO-750 >
Cu(15)CeMgAlO-550 > Cu(15)CeMgAlO-650. Notably, the apparent activation energy
decreases with increasing the calcination temperature (Table 5), suggesting that higher
calcination temperature gives rise to more reactive catalytic sites in Cu(15)CeMgAlO
system within the calcination temperature range studied. This could be attributed to a
strengthened Cu-Ce interaction with increasing calcination temperature as suggested by
the observed increase of both Cu(II)/Cu and Ce(IV)/Ce surface atomic ratios (Fig. S4)
and by the evolution of the easiness of reduction in the H -TPR studies. Indeed, it has
2
been shown that the calcination process favors the Cu-Ce interaction in the mixed oxides
obtained from LDH precursors [64]. This interaction results in an easier reduction of Cu2+
4
+
3+
species due to synergistic involvement of the Ce /Ce redox couple according to the
equilibrium:
3
+
2+
4+
+
Ce + Cu  Ce + Cu
with beneficial consequences on the catalytic ability of the mixed oxide functioning via a
heterogeneous redox mechanism to oxidize methane. Notably, for the catalysts calcined
at different temperatures, a linear increase of the intrinsic activity with the hydrogen
consumption in H
2
-TPR experiments has been observed (Fig. 10). This suggests that all
-TPR experiments are involved in the catalytic
the reducible species evidenced in the H
2

AL-AANI et al.
process and confirms the redox mechanism. It is noteworthy that in the CuO-CeO
2
system the optimum of the calcination temperature, studied in the range from 500 to 900
C, was found to be 700 °C where the most stable state of Cu-Ce-O solid solution was
°
formed [65].
Fig. 11a shows the effect of GHSV on the catalytic activity of the Cu(15)CeMgAlO
mixed oxide. It can be observed that the light-off curve slightly shifts toward higher
temperatures with increasing the GHSV while keeping constant the concentration of
methane in the feed gas at 1 vol. %. Thus, T50 increases from 462 to 466 and 469 °C
-1
when the GHSV has been increased from 16000 to 20000 and 30000 h , respectively. A
shift of the light-off curve toward higher temperatures is also observed when the
concentration of methane in the feed gas has been increased from 1 to 3 vol. %, for the
-1
reaction performed over Cu(15)CeMgAlO catalyst at a constant GHSV of 16000 h , as
shown in Fig. 11b. Indeed, T10 and T50 increase from 380 to 385 and 390 °C and from
4
62 to 466 and 468 °C when the concentration of methane has been increased from 1 to 2
and 3 vol. %, respectively. This behavior has already been observed for a Pd/Al
catalyst and was attributed to methane reaction orders lower than 1 [66].
2
O
3
It has been previously shown that both CuMgAlO [32] and CeMgAlO [33] catalysts
obtained from LDH precursors display good stabilities during the complete oxidation of
methane. Nevertheless, the stability of the Cu(15)CeMgAlO catalyst, the most active one
in the series studied, was checked by maintaining it on stream at 550 °C for more than 50
h. Fig. 12 shows no effect of time on stream on the catalytic activity of the
Cu(15)CeMgAlO catalyst suggesting its good stability, at least for the reaction conditions
and the reaction time chosen.
4
. Conclusion
A series of Cu(x)CeMgAlO mixed oxides with fixed Ce content of 10 at. % with respect
to cations and Mg/Al mol ratio of 3, but with different copper loadings x in the range
from 6 to 18 at. % were prepared by thermal decomposition at 750 °C of precursors
consisting of poorly crystallized LDH and boehmite AlOOH phases. They have slit-like
bimodal mesopores and relatively high surface areas, which regularly decrease from 169
2
-1
to 108 m g with increasing the Cu content, and consist of periclase-like Mg(Al)O

AL-AANI et al.
mixed oxide and CeO fluorite phases, except for Cu(18)CeMgAlO, which also contains
2
well developed CuO crystallites. At low Cu content highly dispersed Cu-doped ceria
2
+
2-
2+ 2+
crystallites coexist with oligomeric (Cu – O – Cu )
n
species in the Mg(Al)O matrix,
while at high Cu content larger ceria crystallites less exposed on the catalyst surface
coexist with separate CuO particles enriching the surface. The Cu(II)/Cu surface atomic
ratio was shown to be a key factor controlling the catalytic activity of the
Cu(x)CeMgAlO catalysts. With the highest Cu(II)/Cu surface atomic ratio, the
Cu(15)CeMgAlO mixed oxide is the most active catalyst in this series, with a T50 value
of only ca. 45 °C higher than that of an industrial Pd/Al
2
O
3
catalyst. Its enhanced
2
+
2-
catalytic activity is attributed to an excellent dispersion of CuO, oligomeric (Cu – O –
2
+ 2+
Cu )
interacting with ceria, which leads to a strong synergy effect between Cu and Ce. The
catalytic sites are located at the CuO-CeO particles interface. Decreasing the calcination
n
species coexisting with tiny not XRD-visible CuO crystallites strongly
2
temperature of the Cu(15)CeMgAl precursor from 750 to 650 and 550 °C results in
mixed oxides with higher surface areas and, hence, higher catalytic activities in terms of
T50, which for the Cu(15)CeMgAlO-550 system is only ca. 25 °C higher than that of the
industrial Pd/Al catalyst. However, in terms of intrinsic reaction rates the most active
2
O
3
catalyst is that calcined at 750 °C accounting for a strengthened Cu-Ce interaction with
increasing calcination temperature. A shift of the light-off curve obtained over
Cu(15)CeMgAlO catalyst toward higher temperatures with increasing the methane
content in the feed gas is observed accounting for methane reaction orders lower than 1.
A good stability on stream of the Cu(15)CeMgAlO catalyst calcined at 750 °C was noted.
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AL-AANI et al.
FIGURES CAPTION
Figure 1. XRD patterns of the Cu(x)CeMgAl precursors as prepared. Symbols: # - LDH
phase;  - boehmite (AlOOH) phase.
Figure 2. XRD patterns of the Cu(x)CeMgAlO mixed oxides calcined at 750 °C (a) and
of the Cu(15)CeMgAlO mixed oxides calcined at different temperatures (b). Symbols: x -
CuO tenorite phase; ♦ - CeO fluorite phase;  - Mg(Al)O mixed oxide periclase-like
2
phase.
Figure 3. O 1s core level (a) and C 1s core level (b) XPS spectra of the Cu(x)CeMgAlO
mixed oxide catalysts: CeMgAlO (a), Cu(6)CeMgAlO (b), Cu(9)CeMgAlO (c),
Cu(12)CeMgAlO (d), Cu(15)CeMgAlO (e), Cu(15)CeMgAlO-650 (f), Cu(15)CeMgAlO-
5
50 (g), Cu(18)CeMgAlO (h).
Figure 4. Cu 2p core level (a) and Ce 3d core levels (b) XPS spectra of the
Cu(x)CeMgAlO mixed oxide catalysts: Cu(6)CeMgAlO (a), Cu(9)CeMgAlO (b),
Cu(12)CeMgAlO
(c),
Cu(15)CeMgAlO
(d),
Cu(15)CeMgAlO-650
(e),
Cu(15)CeMgAlO-550 (f), Cu(18)CeMgAlO (g), CeMgAlO (h).
Figure 5. H
2
-TPR profiles of the Cu(x)CeMgAlO mixed oxides calcined at 750 °C (a)
and of the Cu(15)CeMgAlO mixed oxides calcined at different temperatures (b).
4
+
Figure 6. Amount of Ce species reduced in H -TPR experiments vs. ceria particle size
2
in the Cu(x)CeMgAlO mixed oxides calcined at 750 °C.
Figure 7. DR-UV-Vis spectra of the Cu(x)CeMgAlO mixed oxides: Cu(6)CeMgAlO (a),
Cu(9)CeMgAlO (b), Cu(12)CeMgAlO (c), Cu(15)CeMgAlO (d), Cu(15)CeMgAlO-650
(e), Cu(15)CeMgAlO-550 (f), Cu(18)CeMgAlO (g).

AL-AANI et al.
Figure 8. The light-off curves for the combustion of methane over Cu(x)CeMgAlO
catalysts calcined at 750 °C (a) and over Cu(15)CeMgAlO catalysts calcined at different
temperatures (b) compared with that of an industrial reference Pd/Al
2
3
O catalyst.
-1
3
Reaction conditions: 1 vol. % methane in air, GHSV of 16000 h , 1 cm of catalyst.
Figure 9. Variation of the Cu(II)/Cu surface atomic ratio and the intrinsic methane
conversion rate at 380 and 400 °C as a function of the Cu content in the Cu(x)CeMgAlO
mixed oxide catalysts.
Figure 10. Intrinsic activities measured at 380 °C vs. hydrogen consumption in the H -
2
TPR experiments for the Cu(15)CeMgAlO catalysts calcined at different temperatures.
Figure 11. Effects of gas hourly space velocity at constant concentration of methane in
the feed gas of 1 vol. % (a) and of methane concentration in the feed gas at constant
-1
GHSV of 16000 h (b) on methane conversion over Cu(15)CeMgAlO catalyst.
Figure 12. Conversion of methane versus time on stream for the reaction at 550 °C over
Cu(15)CeMgAlO catalyst. Reaction conditions: 1 vol. % CH in air and GHSV of 16000
4
-1
3
h , 1 cm of catalyst.

AL-AANI et al.
Figure 1.
Cu(18)CeMgAl
Cu(15)CeMgAl
Cu(12)CeMgAl
#
*
Cu(9)CeMgAl
Cu(6)CeMgAl
#
*
#
*
#
# *
#
#
*
*
0
10
20
30
40
50
60
70
2θ (°)

AL-AANI et al.
Figure 2.
(
a)
Cu(18)CeMgAlO
Cu(15)CeMgAlO
x x
x
xx
Cu(12)CeMgAlO
Cu(9)CeMgAlO
Cu(6)CeMgAlO
ꢀ
ꢀ
ꢀ
ꢀ
CeMgAlO
ꢀ
0
10 20 30 40 50 60 70
θ (°)
2