Total oxidation of methane over supported CuO: Influence of the MgxAlyO support

Article abstract of DOI:10.1016/j.apcata.2017.03.012

Copper oxide supported on Al₂O₃, MgO and Mg(Al)O mixed oxides with Mg/Al atomic ratios of 1, 3 and 5 were prepared by coprecipitation followed by calcination at 750?°C. These catalysts have been characterized by XRD, EDX, N₂ adsorption, TG-DTG, NH₃-TPD, CO₂-TPD and H₂-TPR, and their catalytic properties in the total oxidation of methane were investigated. Correlations between the physico-chemical properties and the catalytic activities were established. The best activity was obtained for the catalyst presenting a distribution of CuO species going from well distributed and aggregated CuO particles to CuAl₂O₄ phase on a Mg(Al)O support composed of a mixture of γ-alumina and periclase phases. It also possesses high relative amounts of strong acid and of medium and strong basic sites in this series. All the catalysts tested exhibited very good stabilities for the reaction conditions chosen and time-on-stream up to more than 50?h.

Full text of DOI:10.1016/j.apcata.2017.03.012

Accepted Manuscript
Title: Total oxidation of methane over supported CuO:
Influence of the MgxAlyO support
Authors: Ionel Popescu, Nathalie Tanchoux, Didier Tichit,
Ioan-Cezar Marcu
PII:
S0926-860X(17)30111-4
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2017.03.012
APCATA 16171
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
18-10-2016
8-2-2017
11-3-2017
Please cite this article as: Ionel Popescu, Nathalie Tanchoux, Didier Tichit, Ioan-Cezar
Marcu, Total oxidation of methane over supported CuO: Influence of the MgxAlyO
support, Applied Catalysis A, General http://dx.doi.org/10.1016/j.apcata.2017.03.012
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Total oxidation of methane over supported CuO: influence of the MgxAlyO
support
Ionel Popescua, Nathalie Tanchouxb, Didier Tichitb and Ioan-Cezar Marcu*a,c
a Research Center for Catalysts and Catalytic Processes, Faculty of Chemistry, University of
Bucharest, 4-12, Blv. Regina Elisabeta, 030018 Bucharest, Romania
b 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, 34296, Montpellier Cedex 5, France
c
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
* Corresponding author. E-mail addresses: ioancezar.marcu@chimie.unibuc.ro ; ioancezar_marcu@yahoo.com
1

Popescu et al.
Graphical abstract
Supported CuO
CO2
H2O
CH4
CuAlO
100
CuMgAl(1)O
90
80
70
60
50
40
30
20
10
0
CuMgAl(3)O
CuMgAl(5)O
CuMgO
Increasing
basicity
Increasing
acidity
350
400
450
500
550
600
650
Temperature (°C)
Highlights
-
-
-
-
CuO supported on MgO, Al2O3 and MgAlO catalysts were compared
The most reducible CuO species were involved in the methane combustion
Both strong acid and basic sites lead to higher catalytic activity
The most active catalyst corresponds to CuO on a mixture of γ-alumina and periclase
phases
Abstract
Copper oxide supported on Al2O3, MgO and Mg(Al)O mixed oxides with Mg/Al atomic
ratios of 1, 3 and 5 were prepared by coprecipitation followed by calcination at 750 °C. These
catalysts have been characterized by XRD, EDX, N2 adsorption, TG-DTG, NH3-TPD, CO2-
TPD and H2-TPR and their catalytic properties in the total oxidation of methane were
investigated. Correlations between the physico-chemical properties and the catalytic activities
were established. The best activity was obtained for the catalyst presenting a distribution of
CuO species going from well distributed and aggregated CuO particles to CuAl2O4 phase on
2

a Mg(Al)O support composed of a mixture of -alumina and periclase phases. It also
possesses high relative amounts of strong acid and of medium and strong basic sites in this
series. All the catalysts tested exhibited very good stabilities for the reaction conditions
chosen and time-on-stream up to more than 50 h.
Keywords: Supported CuO; Layered double hydroxide; Methane; Total oxidation; Acidity;
Basicity
1. Introduction
Copper oxide is one of the most effective metal oxide-based catalysts used in the catalytic
total oxidation of methane, a key process involved in air pollution abatement [1]. However,
its catalytic performance is far below that of noble metal-based catalysts, especially Pd and
Pt, which in turn are less attractive from an economic point of view [2]. Further
improvements of the catalytic performance of CuO will certainly go through a better
understanding and control of the key parameters governing its properties, particularly using
supported CuO phases with controlled acid-base properties of the support. This latter feature
was already shown to be a key factor controlling the catalytic activity of supported noble-
metal catalysts in the total oxidation of methane [3-5]. For example, Yoshida et al. [3]
showed that palladium catalysts supported on oxides with moderate acid strength gave higher
methane conversion than palladium supported on highly acidic or basic support. Several
studies also link the acid-base properties of Cu-based catalysts to their activity in the
complete oxidation of methane. Thus, Lee et al. [6] showed that CuO-supported catalysts are
more active in methane combustion with acidic zeolites supports than with non-acidic silica
or silicalite supports. Additionally, the catalytic activity increases when the silica-to-alumina
ratio increases in zeolites corresponding to an increase of acid strength. The authors
concluded that the proton could activate the hydrocarbon allowing its easier oxidation. A
correlation was also established between the catalytic activity in both propane and methane
combustion and the acidity of copper-exchanged ZSM-5, ZSM-11 and ZSM-48 catalysts [7].
M’Ramadj et al. [8] observed the adsorption of both methane and oxygen on the acid sites of
the Cu/ZSM-5 catalysts and concluded that methane molecules could be activated on the
Brønsted acid sites, Cu2+ species acting as Lewis acid sites.
3

These studies show that the dependence of the catalytic performance on the acid-base
properties is rather complex. Nevertheless, the adsorption and activation of hydrocarbons
over oxide-based catalysts, as well as the desorption of the reaction products is related first to
the strength and distribution of the metallic cations acting as Lewis acid sites, but also to that
of the lattice oxygen anions acting as Lewis basic sites [9, 10]. For example, interaction of
methane with an acid-base pair on the catalyst surface results in heterolytic C-H bond
breaking with formation of CH3 and H+ species chemisorbed on the acid and base site,
respectively [10]. Stronger acid sites enhance the interaction with CH3 carbanions and
therefore favor the surface-catalyzed combustion [10].
Recently, we studied the total oxidation of methane over transition metal-containing
MMgAlO mixed oxides (M = Mn, Fe, Co, Ni, Cu, Zn, Ag and Pd) obtained from layered
double hydroxides (LDH) precursors [11]. We showed that among the non-noble metal-
containing catalysts the copper-based system is the most active and exhibits also a high
thermal stability. Moreover, the catalytic activity depends on the copper content, the highly
reducible copper species being more likely involved in the reaction [12]. Starting from these
results, the present study is focused on the influence of the acid-base properties of the support
on the performances of supported CuO catalysts in the total oxidation of methane. For this
purpose, Cu/Mg/Al LDH-type precursors with Mg/Al atomic ratios of 1, 3 and 5 as well as
Mg- and Al-free samples with similar Cu content have been prepared by coprecipitation. The
versatile acid-base character of Mg(Al)O mixed oxides which depend on the Mg/Al molar
ratio, and the well known acid and basic character of Al2O3 and MgO, respectively, were the
incitement to choose these materials as supports for CuO-containing catalysts.
2. Experimental
2.1. Catalysts preparation
The precursors were prepared by coprecipitation at constant pH (≈ 10) of an aqueous solution
containing suitable amounts of Mg(NO3)2 · 6 H2O and Al(NO3)3 · 9 H2O (Mg/Al atomic
ratios of 1, 3 and 5) with NaOH (2M) into a well-stirred beaker containing 200 mL of suitable
amount of Cu(NO3)2 · 3 H2O (atomic ratio Cu/(Cu+Mg+Al) = 0.05). The addition of the
alkaline solution and the pH were controlled by pH-STAT Titrino (Metrohm). 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. The dried samples were noted CuMgAl(x)-OH where x is the
4

magnesium to aluminum atomic ratio with 1, 3 and 5 values. Two Al- and Mg-free samples
were also prepared following the same protocol with Cu/(Cu+Mg) and Cu/(Cu+Al) atomic
ratios of 0.05, respectively, and noted CuMg-OH and CuAl-OH. These precursors were
calcined under air at 750 °C during 8 h in order to obtain the corresponding mixed metal
oxides catalysts. They were noted CuMgAl(x)O, CuMgO and CuAlO.
2.2. Catalysts characterization
Powder X-Ray diffraction (XRD) patterns were recorded using a Siemens D5000
Diffractometer and the monochromatic Cu-Kα1 radiation. They were recorded with 0.02°
(2θ) steps over the 3–70° 2θ angular range with 1 s counting time per step.
The chemical composition of the samples was determined by EDX microprobe on a
Cambridge Stereoscan 260 apparatus.
The textural characterization was achieved using conventional nitrogen adsorption/desorption
method, with a Micromeritics ASAP 2010 automatic analyzer. Specific surface areas were
calculated using the BET method. Prior to nitrogen adsorption, the samples were outgassed
for 8 h at 250 °C.
The thermal analysis (TG and DTG) was carried out using a Netzsch TG 209 device, in the
following conditions: linear heating rate 10 °C min-1 from room temperature to 900 °C,
dynamic air atmosphere, Al2O3 crucible, sample weight approximately 20 mg.
Temperature-programmed desorptions (TPD) of NH3 and CO2 were carried out using a
Micromeritics Autochem model 2910 instrument. Fresh calcined samples (ca. 100 mg) were
pretreated in air at 600 °C before adsorption of NH3 and CO2 at 100 °C and 50 °C,
respectively. During desorption, the sample was heated in a helium flow (30 ml min-1) at a
ramp of 10 °C min-1. The amount of the probe molecules desorbed from the sample was
estimated from the area under the peak after taking the thermal conductivity detector
response into consideration.
Hydrogen temperature-programmed reduction (H2-TPR) studies were carried out in the
temperature range from 100 to 900 °C 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 then cooled down to the room temperature. After
that, the reduction gas of 3 % H2/Ar was introduced over the sample which was heated at a
rate of 10 °C min-1. 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).
5

2.3. Catalytic testing
The total oxidation of methane was carried out in a fixed bed quartz tube down-flow reactor
operated at atmospheric pressure. The internal diameter of the reactor tube was 18 mm. 1 cm3
of catalyst powder was used. The catalyst was supported by quartz wool. The axial
temperature profile was measured using an electronic thermometer placed in a thermowell
centered in the catalyst bed. The reactor temperature was controlled using a chromel-alumel
thermocouple attached to the exterior of the reactor. 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 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 a total volume hourly space velocity (VHSV) of 20000 h-1 in the
temperature range 350 – 700 °C.
Before each activity test, the catalyst in the reactor was activated under air at 600 °C for
about 30 min. The reactor was then cooled to 350 °C, and the reactant flows were set to give
the desired reaction mixture. After the reactant feed was stabilized, it passed through the
catalyst and the temperature was increased from 350 °C to 700 °C at a rate of 1 °C min-1. The
reactor feed and 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 (a
6 ft Hayesep and a 10 ft molecular sieve 5Å). After the catalytic test the reactor was cooled
down under the flow of reactants.
The conversion was calculated as the amount of substrate transformed in the reaction divided
by the amount that was fed to the reactor. Complete selectivity to CO2 and H2O was always
observed. The carbon balance was satisfactory in all runs to within ± 2 %.
3. Results and discussion
3.1. Catalyst characterization
The XRD patterns of the as-prepared precursor samples are displayed in Fig. 1. CuMg-OH
(Fig. 1e) exhibits intense peaks corresponding to pure Mg(OH)2 brucite phase (JCPDS 82-
2453). CuAl-OH precursor (Fig. 1a) exhibits broad peaks at ~ 14.7, 28.7, 38.8, 49.6, 65.0 and
67.7° 2θ corresponding to AlOOH boehmite phase (JCPDS 83-2384) and weak peaks at ~
18.3, 20.3, 40.3, 52.1 and 54.4° 2attributed to Al(OH)3 gibbsite phase (JCPDS 74-1775).
The Cu/Mg/Al precursors (Fig. 1b, c, d) exhibit the characteristic patterns of LDH with a
6

lower crystallinity for CuMgAl(1)-OH containing as expected a small amount of Al(OH)3
gibbsite (JCPDS 74-1775) phase as pure LDH is likely obtained for 2 ≤ Mg/Al ≤ 4. We noted
the absence of diffraction lines corresponding to copper-containing additional phases
suggesting that Cu2+ cations are well distributed in the brucite-like layers of all the precursor
samples. The d003 basal spacing moves from 0.870 to 0.843 and 0.803 nm when going from
CuMgAl(1)-OH to CuMgAl(3)-OH and CuMgAl(5)-OH according, on one hand, to the
presence of mainly NO3 as compensating anions and, on the other hand, to a regular
decrease of the interlayer distance as the layer charge increases due to a stronger electrostatic
interaction with the anions.
The XRD patterns of the mixed oxides catalysts obtained after calcination at 750 °C are
displayed in Fig. 2. CuAlO catalyst (Fig. 2a) exhibits weak and broad peaks corresponding to
poorly crystallized γ-alumina (JCPDS 10-425). CuMgO catalyst (Fig. 2e) exhibits intense
peaks characteristic of periclase (MgO) structure (JCPDS 45-0946). Both CuMgAl(3)O and
CuMgAl(5)O samples (Fig. 2c, d) show the characteristic XRD patterns of the Mg-Al mixed
oxide phase with the periclase-like structure (JCPDS-ICDD 4-0829), while CuMgAl(1)O
(Fig. 2b) presents the peaks corresponding to both γ-alumina and periclase-like phases, the
former being the most intense. This is consistent with the higher aluminum content of this
sample. It is worth noting that neither CuO nor copper aluminate was detected in the calcined
catalysts. It is well known that at low loadings, Cu can form a solid solution with Mg and Al,
or give well dispersed CuO crystallites on the support not detected by XRD due to their small
size. It must also be noted that the crystallinity increases with the Mg content in this series of
mixed oxides in agreement with the highest crystallinity observed for CuMgO. The XRD
patterns of the catalysts recorded after the catalytic test (not shown) showed no modifications
even after the long-term runs.
The chemical compositions of the samples estimated by EDX analysis are reported in Table
1. It can be observed that the copper content is in all cases slightly higher than the nominal
value of 5 at %. It is close to 6 at % in the Al- and Mg-free samples and varies in the range
from 5.7 to 6.7 at % in the Cu/Mg/Al-containing samples. It is noteworthy that the Mg/Al
atomic ratios in CuMgAl(1)O, CuMgAl(3)O and CuMgAl(5)O are very close to the nominal
values.
The textural properties of the mixed oxides have been analyzed by performing the nitrogen
adsorption/desorption isotherms at liquid nitrogen temperature which are displayed in Fig. 3.
All the catalysts presented type IV nitrogen adsorption isotherms according to the IUPAC
7

classification [13]. Although the type of isotherm does not change by modifying the oxide
composition, the hysteresis loop moves from H2-type for CuAlO sample to H3-type for Mg-
containing samples [13] in agreement with a concurrent evolution toward a lamellar
morphology.
The specific surface areas of the mixed oxides are displayed in Table 1. The highest surface
area of 251 m2 g-1 is observed for CuAlO, a value twofold higher than for CuMgO (127 m2 g-
1). These results are in agreement with the respective crystallinity of these samples increasing
from the Mg-free to the Al-free one as observed by XRD (Fig. 2). Accordingly, the
CuMgAlO mixed oxides exhibit intermediate specific surface areas decreasing regularly with
the Mg content.
The decomposition of the precursors into the different mixed oxide phases is well followed
by TG-DTG analysis. Fig. 4 shows the TG-DTG curves of the as-prepared precursors. As
detailed below, they are in agreement with the crystalline phase content of the samples
previously revealed by XRD analysis. The first step in the TG curve of CuAl-OH precursor
(Fig. 4a) accounts for ca. 9 % of the weight loss with a DTG peak at 90 °C and is attributed to
the desorption of physically adsorbed water. The second step, accounting for a higher weight
loss, i.e. ca. 16 %, is attributed to the dehydroxylation of both boehmite and gibbsite phases
into -alumina. The corresponding DTG curve exhibits a peak at 245 °C associated to the
gibbsite dehydroxylation [14, 15] and a broader peak extending from 300 °C to 600 °C
accounting for complete dehydroxylation into γ-alumina. The third step above 600 °C
corresponds to less than 2 % of the weight loss, without DTG peak, and is attributed to the
elimination of residual hydroxyls.
The TG-DTG curves of CuMg-OH (Fig. 4e) correspond to the thermal decomposition of the
brucite-like material [16] with a weight loss of ca. 3 % (DTG peak at 95 °C) due to the
desorption of physically adsorbed water and a weight loss of ca. 22 % (DTG peak at 375 °C)
related to the dehydroxylation of the brucite-like phase into periclase-like phase. The last
weight loss of ca. 3 % in the temperature range from 400 to 800 °C accounts for the
elimination of residual hydroxyls.
In the CuMgAl(x) series, the TG-DTG curves of CuMgAl(3)-OH (Fig. 4c) are typical for
LDH structure [17, 18] with a first weight loss of 12 % (DTG peak at 125 °C) due to the
elimination of weakly bounded and interlayer water molecules and a second weight loss of 32
% (DTG peak at 450 °C) ascribed to the removal of hydroxyl groups in the brucite-like layers
and to the decomposition of the compensating anions.
8

The TG-DTG curves of CuMgAl(1)-OH (Fig. 4b) correspond well to the thermal
decomposition of a mixture of gibbsite and LDH phases previously identified by XRD. Thus,
the first weight loss of ca. 10 % (DTG peak at 140 °C and a shoulder at about 90 °C)
accounts for the desorption of physically adsorbed water in both phases and elimination of
interlayer water molecules in LDH. The second weight loss of about 10 % (DTG peak at 250
°C) accounts for the dehydroxylation of gibbsite into boehmite. The third weight loss
corresponds to dehydroxylation of the brucite-like layers and of boehmite into -alumina and
to the decomposition of the compensating anions of the LDH. This gives a DTG peak at 380
°C with a shoulder at 330 °C. A weight loss of less than 2 % from 600 °C until 900 °C is
attributed to the elimination of residual hydroxyls.
The TG-DTG curves of CuMgAl(5)-OH (Fig. 4d) correspond to the thermal decomposition
of a mixture of brucite-like and LDH phases observed by XRD. The first weight loss of ca.
12 % (DTG peak at 110 °C) corresponds to the dehydration of the LDH phase. The second
weight loss of ca. 33 % is in agreement with the dehydroxylation of both the brucite and the
LDH phases and the decomposition of the compensating anions in LDH. These superimposed
features give a DTG peak at 330 °C with shoulders at 375 and 410 °C.
The work is mainly focused on the influence of the acid-base properties of the support on the
performance of the Cu-supported catalysts in the total oxidation of methane. Therefore, NH3
and CO2 TPD experiments, particularly able to give useful information on the number and
strength of acid and basic sites, respectively, have been performed with the aim to establish
correlations with the catalytic activity. The NH3-TPD profiles are presented in Fig. 5. They
were decomposed using OriginPro 7.5 program in three well-defined Gaussian-shaped
desorption peaks (agreement factor higher than 0.995) with maxima at 184-190 °C, 225-275
°C and 315-375 °C corresponding to acid sites exhibiting weak (W), moderate (M) and high
(H) strengths, respectively. The amounts of NH3 desorbed in each peak are presented in Table
2. The total number of acid sites present in the weakly acidic CuMgO sample (Fig. 5e) is
about one third of that observed in the most acidic CuAlO mixed oxide (Fig. 5a) where it
reaches 4.73 mmol g-1. Accordingly, intermediate total numbers of acid sites are found in the
CuMgAlO mixed oxides (Fig. 5b, 5c and 5d) which decrease with increasing the Mg/Al ratio.
The density of sites (μmol NH3 m-2) follows the same trend and decreases of about 50 %
when going from CuAlO to CuMgAlO. The relative amount in percent of each type of sites
reveals interesting evolutions between the samples (Table 2). The relative amounts of weak
and moderate (W + M) acid sites indeed increase with the magnesium content at the expense
9

of the amount of the strong (H) acid sites. Accordingly, CuMgO possess 95.5 % of both W
and M sites and 4.5 % of H sites, while the distribution is of 49 % (W and M) and 51 % (H),
respectively, in CuAlO. Regarding the CuMgAl(x) series, the distributions of sites are, as
expected, close to those of CuAlO and CuMgO in CuMgAl(1)O and CuMgAl(5)O,
respectively. Remarkably, CuMgAl(3)O is the only sample where the different sites are in
almost similar amount ranging from 30 to 40 %.
CO2-TPD profiles of the catalysts are presented in Fig. 6. They were decomposed using
OriginPro 7.5 program in three well-defined Gaussian-shaped desorption peaks (agreement
factor higher than 0.997) with maxima at 145-160 °C, 200-250 °C, 275-315 °C and 375 °C
corresponding to basic sites arbitrary considered as exhibiting weak (W), moderate (M), high
(H) and very high (VH) strengths, respectively. The amounts of CO2 desorbed in each peak
are presented in Table 3. The CuAlO mixed oxide shows only W and M basic sites in similar
amounts (Fig. 6a) and the lower total number and density of sites. An opposite behavior is
noted for CuMgO mixed oxide. Beside W sites, only H and VH sites are observed, all of
them being in equal amounts (Fig. 6e). It is noteworthy that this is the only sample exhibiting
on one hand VH sites and on the other hand any M sites. The total number and density of
basic sites in the CuMgAlO mixed oxides are intermediate between those of CuAlO and
CuMgO (Fig. 6b, 6c and 6d). Adding Mg to CuAlO leads to an increase of the total density of
basic sites and to the appearance of H sites in CuMgAl(1)O mixed oxide (Table 3). Then,
when the Mg content increases the relative amount of H sites remains almost constant while
the relative amount of W and M sites varies in opposite way. The former increases at the
expense of the latter. CuMgAl(3)O is the only sample where the different basic sites are in
similar amounts of about 30-35 %. A similar behavior was previously observed with this
sample for the acid sites.
We can anticipate that the evolutions of the acid and basic properties will influence the
performances of the catalysts. One can note a clear evolution in the equilibrium between the
acid/base pairs of the materials going from CuAlO exhibiting mainly an acid character to
MgAlO with mainly a basic character. Remarkably, an equilibrium between the nature and
strength of the different sites was reached in CuMgAl(3)O.
The reducibility of copper is another relevant parameter influencing the catalytic activity. The
TPR profiles of the Cu-containing mixed oxides calcined at 750 °C are presented in Fig. 7.
All profiles show a main peak of H2 consumption below 400 °C and a very weak and broad
peak above 400 °C, except in the case of CuMgAl(3)O where this peak is remarkably intense
in comparison to the other mixed oxides and extends from 400 to 800 °C (Fig. 7c). All these
10

peaks correspond to the reduction of copper belonging to different crystalline phases. The
profiles were decomposed using OriginPro 7.5 program in well-defined Gaussian-shaped
components (agreement factor higher than 0.995). The temperatures of the peaks and the
corresponding H2 consumptions are reported in Table 4. The first peak below 400 °C
generally shows two distinct components. The weaker one, centered at 170-200 °C in all the
Mg-containing samples, can be attributed to the reduction of well distributed CuO species or
small CuO clusters interacting weakly with the support [19]. The H2 consumptions
corresponding to the reduction of these species represent less than 13 % of the total
consumption which confirms that they are in minor amount. The second and major
component gives a peak progressively shifted from 228 °C to 253 °C as the Al content
increases. It must be associated to large CuO particles or aggregated CuO clusters [19].
CuMgAl(1)O is the only sample exhibiting a symmetric reduction peak extending from 150
to 400 °C (Fig. 7b) which actually includes all the previous cited species and, in addition, a
small amount of a species also found in CuAlO at 302 °C. CuAlO possesses the less
reducible Cu2+ species in the temperature range below 400 °C (Fig. 7a). It shows a minor
component centered at 250 °C assigned to large CuO particles and a major component
centered at 302 °C due to the reduction of copper in bulky CuO clusters and in CuAl2O4
phase probably present after calcination at 750 °C, though not detected by XRD due to its
small amount.
To sum up, the reduction of copper below 400 °C is strongly influenced by the composition
of the samples, then by the crystalline nature of the support. Copper is more easily reduced
into small CuO aggregates present in the Mg-rich samples, likely showing a periclase-like
structure than in the large CuO aggregates and the CuAl2O4 phase present in the Al-rich
samples, likely showing a -alumina structure. The total hydrogen consumption below 400 °C
ranges as follows for the series of catalysts: CuAlO  CuMgAl(1)O > CuMgAl(3)O >
CuMgAl(5)O > CuMgO.
In the domain above 400 °C, CuMgAl(3)O (Fig. 7c) has a peculiar behavior because it is the
only sample exhibiting a broad peak with two components at 520 and 642 °C representing
about 60 % of the total H2 consumption. This peak can be assigned to the reduction of copper
into aluminate spinel phases with different cationic compositions or copper differently
distributed in octahedral and tetrahedral positions. One can suggest that such species are
specifically formed in the CuMgAl(3)O mixed oxide in relation to its composition. Its
precursor is indeed the only one corresponding to pure LDH. Therefore CuMgAl(3)O is a
11

solid solution of Cu, Mg and Al rather than a mixture of Cu into periclase and -alumina as it
is probably the case in the other CuMgAl(x)O (x = 1 and 5) mixed oxides. A higher reduction
rate is achieved into CuMgAl(3)O than in the other samples.
The H2/Cu molar ratio (Table 4) is lower than 1 for all the catalysts indicating that Cu2+ is not
quantitatively reduced. This finding has already been reported for Cu/γ-Al2O3 [20] and LDH-
derived Cu-containing catalysts [12]. It suggests that the Cu-containing particles are
decorated with the oxide species migrating from the support.
3.2. Catalytic study
The catalytic activities in methane total oxidation performed in the temperature range 350-
650 °C with 1 % vol. CH4 in air (VHSV = 20000 h-1), are shown in Fig. 8 where methane
conversions are plotted as a function of the reaction temperature. As shown in Fig. 8 and
considering the values of T10, T50 and T90 (temperature corresponding to 10 %, 50 % and 90
% conversion, respectively) presented in Table 5, the activity increases following the order
CuMgO < CuMgAl(5)O < CuMgAl(3)O < CuMgAl(1)O ≈ CuAlO, the total conversion being
achieved with the three most active catalysts in the temperature range 578-593 °C. Due to the
larger specific surface area of CuAlO (251 m2 g-1) than CuMgAl(1)O (164 m2 g-1) both
samples exhibit similar specific activities (Table 5). However, when comparing the intrinsic
activities at ca. 460 °C, CuMgAl(1)O appears to be the most active in this series of catalysts
(Table 5). This temperature was chosen as it is the lowest temperature at which all the
catalysts are active, the conversion level remaining low for the most active ones.
The specific activity of the catalysts at 460 and 485 °C was plotted (Fig. 9) in function of
both the total H2 consumption in the TPR experiments (Fig. 9.a) and the H2 consumption in
the temperature range 100-400 °C (Fig. 9.b), respectively. A linear correlation is only
observed when considering the H2 consumption below 400 °C, showing that the methane
combustion depends on the amount of the most reducible CuO species as we previously
found [12]. Based on these results, the turnover frequencies (TOF) calculated on the most
reducible CuO species for the reaction at ca. 460 °C are compared in Table 5. It can be
observed that CuMgAl(1)O and CuAlO are the most active catalysts in terms of TOF, which
increases following the order: CuMgO < CuMgAl(5)O < CuMgAl(3)O < CuAlO 
CuMgAl(1)O.
The catalysts have similar copper contents close to 6 at %, therefore the differences of
activity are more likely related to the composition and structure of the supports and to the size
12

and distribution of the CuO particles. As previously underlined, CuMgAl(1)O and CuAlO
have similar conversions but the former catalyst shows the highest intrinsic activity. Its main
peculiarity is the larger distribution of CuO species revealed by the symmetric peak
extending from 150 to 400 °C in the TPR profile. This shows that the concurrent presence of
well distributed and large CuO particles is a key parameter determining the catalytic activity.
This particular feature may be due to its unique composition and structure: it is the only
catalyst in the series with a support consisting of two phases, i.e. γ-alumina and periclase-like
(Fig. 2). It is worth noting that phase interfaces and crystal defects generated into multiphasic
structures were already shown to favor more uniform dispersion of Pt nanoparticles [21].
Therefore, one can similarly suggest that CuO particles are likely located at the phase
interfaces of the support being therefore characterized by a uniform dispersion, as confirmed
by the TPR profile (Fig. 7b).
On the other hand, the acid-base properties of the catalyst strongly influence its catalytic
behavior in methane oxidation. Indeed, methane activation involving the heterolytic C-H
bond breaking needs both surface acid and basic sites, i.e. acid-base pairs [10]. Moreover, the
total oxidation reaction is favored in the presence of acid sites of high strength which strongly
adsorb CH3 carbanions thus undergoing surface reaction with oxygen [10]. The two most
active catalysts in terms of methane conversion, i.e. CuMgAl(1)O and CuAlO, have different
acid-base equilibrium: both of them contain similar proportions of strong acid sites (ca. 50
%), but strong basic sites (ca. 30 %) are only present in CuMgAl(1)O, with also a large
proportion of M basic sites (ca. 45 %). This feature, together with the uniform distribution of
CuO species, appear optimal to obtain the most active CuMgAl(1)O catalyst in terms of
intrinsic activity (Table 5). CuMgAl(3)O catalyst contains similar relative amounts of strong
acid and basic sites (ca. 30 %) but lower relative amount of strong acid sites than
CuMgAl(1)O. This accounts for its lower activity being less able to activate methane (Table
5). CuMgAl(5)O and CuMgO catalysts contain similar relative amounts of strong basic sites
(ca. 30 %) and, furthermore, the latter contain also very strong basic sites (35 %). However,
both contain very low relative amounts of strong acid sites, i.e., ca. 10 and 5 %, respectively,
being thus the least active in the series studied (Table 5). These observations suggest that an
efficient supported CuO catalyst in the total oxidation of methane must contain well
dispersed CuO species on a support possessing both strong acid and basic sites.
We can point out that the maximum temperature of the main reduction peak below 400 °C
decreases with increasing the Mg content of the catalyst, going from 302 °C for CuAlO to
13

228 °C for CuMgO (Table 4). If the catalytic activity was only based on the reducibility of
the samples, CuMgO would be the most active catalyst. As it is not the case, this clearly
confirms that not only the reducibility, but also the acid-base properties influence the
catalytic performances being involved in the methane activation step by heterolytic C-H bond
breaking.
The apparent activation energies (Eact) on the different catalysts have been calculated for
methane conversions lower than 10 % to avoid diffusion limitations from the slope of the lnr
versus 103/T plots (Fig. S1) and are presented in Table 5. It can be observed that they increase
with the magnesium content in the mixed oxide, then with the decrease of acidity and the
increase of reducibility of the copper species. This confirms that the optimum behavior of
these mixed oxides results from a compromise between acid-base properties and reducibility.
On the other hand, the values obtained for the Eact were in agreement with those measured for
methane catalytic combustion over similar supported CuO catalysts [12, 20, 22, 23].
We have previously shown [11] that CuMgAl(3)O displayed a good stability during the
complete oxidation of methane. We also checked the stability of the different catalysts in the
series studied to see the influence of the nature of the support. Thus, Fig. 10 shows the effect
of time on stream on the catalytic activities of CuAlO, CuMgAl(1)O and CuMgO at 554 °C,
564 °C and 564 °C, respectively, with 1 % CH4 in air at VHSV of 20000 h-1. All the catalysts
display good stabilities whatever the support, at least for the reaction conditions and the
reaction times chosen. In addition, the catalysts stability and the reproducibility of the
catalytic results are confirmed by the second light-off test carried out with CuMgAl(1)O and
CuMgAl(3)O catalysts, the second sigmoid curves being practically identical with the first
ones (open symbols in Fig. 8).
Conclusion
Cu-Al, Cu-Mg and Cu-Mg-Al mixed oxides catalysts with ca. 6 at % Cu and Mg/Al atomic
ratios of 1, 3 and 5 were prepared by coprecipitation followed by calcination at 750 °C. Their
structural, textural and physico-chemical properties strongly depend on the cationic
composition. A structural evolution from pure -alumina to periclase-like phase and a regular
decrease of the surface area as well as an evolution of the acid-base equilibrium toward
higher basicity was indeed observed as the Mg content increases. The catalysts present
different CuO species reduced below 400 °C, and in the case of CuMgAl(3)O mixed oxide
14

derived from a pure LDH precursor, a significant amount of copper reduced in the
temperature range 400-900 °C belonging to poorly reducible spinel-like phases.
The catalytic activity followed the order: CuMgAl(1)O ≥ CuAlO > CuMgAl(3)O >
CuMgAl(5)O > CuMgO. The specific activity was correlated to the H2 consumption in the
domain 100-400 °C in the TPR experiments suggesting that the most reducible CuO species
were likely involved in the methane combustion. The highest specific and intrinsic activities
were observed for CuMgAl(1)O which is a mixture of -alumina and periclase phases
containing well distributed and aggregated CuO particles and high amounts of strong acid
sites and of both medium and strong basic sites in the series of catalysts. This study clearly
shows that the composition of the catalysts through the nature of the supports, the surface
areas and the acid-base properties allow adjusting the nature and the distribution of the
copper containing species and the catalytic activities in the total oxidation reaction of
methane.
Acknowledgements
This research was partially supported by the Romanian National University Research Council
(CNCSIS) under the project ‘‘IDEI’’ no. 1906/2009-2011. The authors thank Mr. Serghei
Tanasoi for performing the first light-off tests.
References
1. J. Chen, H. Arandiyan, X. Gao, J. Li, Catal. Surv. Asia 19 (2015) 140-171.
2. T.V. Choudhary, S. Banerjee, V.R. Choudhary, Appl. Catal. A 234 (2002) 1-23.
3. H. Yoshida, T. Nakajima, Y. Yazawa, T. Hattori, Appl. Catal. B 71 (2007) 70-79.
4. G. Corro, C. Cano, J.L.G. Fierro, Catal. Commun. 9 (2008) 2601-2605.
5. N.M. Kinnunen, M. Suvanto, M.A. Moreno, A. Savimaki, K. Kallinen, T.J.J. Kinnunen,
T.A. Pakkanen Appl. Catal. A 370 (2009) 78-87.
6. C.Y. Lee, K.H. Han, B.H. Ha, Micropor. Mater. 11 (1997) 227-235.
7. A.K. Neyestanaki, N. Kumar, L.E. Lindfors, Appl. Catal. B 7 (1995) 95-111.
8. O. M’Ramadj, B. Zhang, D. Li, X. Wang, G. Lu, J. Nat. Gas Chem. 16 (2007) 258-265.
9. J.C. Védrine, J.M.M. Millet, J.C. Volta, Catal. Today 32 (1996) 115-123.
10. V.R. Choudhary, V.H. Rane, J. Catal. 130 (1991) 411-422.
11. S. Tanasoi, G. Mitran, N. Tanchoux, T. Cacciaguerra, F. Fajula, I. Săndulescu, D. Tichit,
I.C. Marcu, Appl. Catal. A 395 (2011) 78-86.
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12. S. Tanasoi, N. Tanchoux, A. Urdă, D. Tichit, I. Săndulescu, F. Fajula, I.C. Marcu, Appl.
Catal. A 363 (2009) 135-142.
13. IUPAC Reporting physisorption data for gas/solid system, Pure Appl. Chem. 57 (1985)
603-619.
14. A. Boumaza, A. Djelloul, F. Guerrab, Powder Technol. 201 (2010) 177-180.
15. P. Alphonse, M. Courty, Thermochim. Acta 425 (2005) 75-89.
16. K. Nahdi, F. Rouquerol, M.T. Ayadi, Solid State Sci. 11 (2009) 1028-1034.
17. D. Tichit, N. Das, B. Coq, R. Durand, Chem. Mater. 14 (2002) 1530-1538.
18. K. M. Parida, J. Das, J. Mol. Catal. A 151 (2000) 185-192.
19. M. Jabłońska, L. Chmielarz, A. Węgrzyn, K. Guzik, Z. Piwowarska, S. Witkowski, R.I.
Walton, P.W. Dunne, F. Kovanda, J. Therm. Anal. Calorim. 114 (2013) 731-747.
20. M. Iamarino, R. Chirone, L. Lisi, R. Pirone, P. Salatino, G. Russo, Catal. Today 75
(2002) 317-324.
21. X.L. Sui, Z.B. Wang, C.Z. Li, J.J. Zhang, L. Zhao, D.M. Gu, S. Gu, J. Mater. Chem. A 3
(2015) 840-846.
22. J.W. Veldsink, G.F. Versteeg, W.P.M. van Swaaij, Chem. Eng. J. 57 (1995) 273-283.
23. Z. Jiang, Z. Hao, J. Yu, H. Hou, C. Hu, J. Su, Catal. Lett. 99 (2005) 157-163.
Figures caption
Figure 1. XRD patterns of the catalyst precursors: (a) CuAl-OH; (b) CuMgAl(1)-OH; (c)
CuMgAl(3)-OH; (d) CuMgAl(5)-OH and (e) CuMg-OH (▼ – boehmite, ♦ – gibbsite,
LDH, # – brucite).
–
*
Figure 2. XRD patterns of the catalysts calcined at 750°C: (a) CuAlO; (b) CuMgAl(1)O; (c)
CuMgAl(3)O; (d) CuMgAl(5)O and (e) CuMgO (● – periclase, ▼ – γ-alumina).
Figure 3. Adsorption/desorption isotherms of nitrogen at −196°C of CuAlO (),
CuMgAl(1)O (■), CuMgAl(3)O (), CuMgAl(5)O (●) and CuMgO ().
16

Figure 4. TG-DTG profiles of the uncalcined precursors: (a) CuAl-OH; (b) CuMgAl(1)-OH;
(c) CuMgAl(3)-OH; (d) CuMgAl(5)-OH and (e) CuMg-OH (TG – solid line; DTG – dotted
line).
Figure 5. NH3-TPD profiles of the mixed-oxide catalysts: (a) CuAlO; (b) CuMgAl(1)O; (c)
CuMgAl(3)O; (d) CuMgAl(5)O and (e) CuMgO.
Figure 6. CO2-TPD profiles of the mixed-oxide catalysts: (a) CuAlO; (b) CuMgAl(1)O; (c)
CuMgAl(3)O; (d) CuMgAl(5)O and (e) CuMgO.
Figure 7. TPR profiles of the mixed-oxide catalysts: (a) CuAlO; (b) CuMgAl(1)O; (c)
CuMgAl(3)O; (d) CuMgAl(5)O and (e) CuMgO.
Figure 8. CH4 conversion as a function of the reaction temperature for the mixed-oxide
catalysts: CuAlO (); CuMgAl(1)O (●); CuMgAl(3)O (■); CuMgAl(5)O () and CuMgO
(▲). Open symbols correspond to the second sigmoid curves. Reaction conditions: 1 % vol.
CH4 in air, total VHSV = 20000 h-1.
Figure 9. Specific activities of the catalysts at 460 and 485 °C versus total H2 consumption
(a) and H2 consumption below 400 °C (b) in the TPR experiments.
Figure 10. Effect of time on stream on the catalytic properties of CuAlO () at 554°C,
CuMgAl(1)O (●) at 564 °C and CuMgO (▲) at 564 °C. Reaction conditions: 1 % vol. CH4 in
air, total VHSV = 20000 h-1.
17

Figure 1.
#
#
#
#
#
*
#
#
(e)
*
*
*
*
*
*
*
*
(d)
(c)
*
*
*
*
*
♦
*
♦
*
♦ *
♦
♦♦
* *
*
*
(b)
(a)
▼
▼
▼
▼
♦
♦♦
▼
▼ ♦
♦
0
10
20
30
40
50
60
70
2-theta (°)
18

Figure 2.
●
●
●
(e)
(d)
(c)
(b)
(a)
▼
▼
▼
▼
0
10
20
30
40
50
60
70
2 theta (o)
19

Figure 3.
500
400
300
200
100
0
0
0.2
0.4
0.6
0.8
1
Relative pressure P/P0
20

Figure 4.
100
0
90
80
70
60
50
-0.5
-1
-1.5
-2
0
200
400
600
800
1000
Temperature (°C)
(a)
100
90
80
70
60
50
0
-0.5
-1
-1.5
-2
0
200
400
600
800
1000
Temperature (°C)
(b)
21

100
90
80
70
60
50
0
-1
-2
-3
-4
-5
0
200
400
600
800
1000
Temperature (°C)
(c)
100
90
80
70
60
50
0
-1
-2
-3
-4
0
200
400
600
800
1000
Temperature (°C)
(d)
22

100
90
80
70
60
50
0
-1
-2
-3
-4
-5
-6
0
200
400
600
800
1000
Temperature (°C)
(e)
23

Figure 5.
(e)
(d)
(c)
(b)
(a)
100 150 200 250 300 350 400 450
Temperature (oC)
24

Figure 6.
(e)
(d)
(c)
(b)
(a)
50
150
250
350
450
550
Temperature (oC)
25

Figure 7.
(e)
(d)
(c)
(b)
(a)
0 100 200 300 400 500 600 700 800 900
Temperature (oC)
26

Figure 8.
100
90
80
70
60
50
40
30
20
10
0
350
400
450
500
550
600
650
Temperature (°C)
27

Figure 9.
1.0
0.8
0.6
0.4
0.2
0.0
460 ºC
485 ºC
0
0.05
0.1
0.15
0.2
0.25
0.3
Total H2 consumption (mmol g-1)
(a)
1.0
460 ºC
485 ºC
0.8
0.6
0.4
0.2
0.0
0
0.05
0.1
0.15
0.2
H2 consumption (mmol g-1)
(b)
28

Figure 10.
100
90
80
70
60
50
40
30
20
10
0
0
10
20
30
40
50
60
Time on stream (h)
29