Self-assembled (Ni/Cu, Ti)-YSZ with potential applications for IT-SOFCs: Catalytic and electrochemical assessment

Article abstract of DOI:10.1016/j.jallcom.2016.08.193

(Ni/Cu, Ti)-YSZ samples were synthesized by self-assembling synthesis route using Triton X as template and thermal treated at 700?°C and 900?°C in air flow. Microstructure, surface chemistry and electrochemical properties were assessed by XRD, N₂adsorption-desorption, SEM, TEM, XPS, TPR-H₂and DC conductivity measurements. The synthesis route leads to a very well incorporation of Ti into YSZ lattice. X-ray diffraction shows that (Cu, Ti)-YSZ exhibits a phase transition, from cubic fluorite phase to monoclinic, when the calcination temperature rises from 700?°C to 900?°C, while (Ni, Ti)-YSZ exhibits only two cubic phases, with a main fluorite-type phase and a secondary isostructural phase with bunsenite (NiO) for both calcination temperatures. The partial oxidation of methane was performed in the temperature range 250–900?°C, and the results can be summarized as follows: i) for the (Cu, Ti)-YSZ samples it was found that the phase transformation from cubic fluorite to monoclinic favors an increase in methane conversion as well as the selectivity to CO; ii) the samples containing Ni are more active than those containing Cu, with 90% methane conversion at 750?°C as compared to 75% conversion, respectively. Regarding the total conductivity of the material synthesized, two different behaviors were obtained depending on the composition; the (Ni, Ti-)YSZ maintained a prevailing ionic conductivity, whereas the (Cu, Ti)-YSZ exhibited mainly n-type conductivity. The ionic conductivity of Ni-based sample is beneficial for the partial oxidation of methane due to the oxygen vacancies generated on the surface of the material, which plays a fundamental role in the O₂activation mechanism and is in accordance with higher catalytic activity observed for the Ni containing sample.

Full text of DOI:10.1016/j.jallcom.2016.08.193

Journal of Alloys and Compounds 690 (2017) 873e883
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Self-assembled (Ni/Cu, Ti)-YSZ with potential applications for
IT-SOFCs: Catalytic and electrochemical assessment
,
Simona Somacescu a *, Laura Navarrete b, Mihaela Florea c, Jose Maria Calderon-Moreno a,
,
**
Jose Manuel Serra b
a “Ilie Murgulescu” Institute of Physical Chemistry, Romanian Academy, Surface Chemistry and Catalysis Laboratory, Spl. Independentei 202, 060021,
Bucharest, Romania
b
ꢀ
Instituto de Tecnología Química (Universidad Politecnica de Valencia e Consejo Superior de Investigaciones Científicas), Avenida de los Naranjos s/n,
46022, Valencia, Spain
c University of Bucharest, Faculty of Chemistry, Department of Organic Chemistry, Biochemistry and Catalysis, B-dul Regina Elisabeta 4-12, Bucharest,
Romania
a r t i c l e i n f o
a b s t r a c t
Article history:
(Ni/Cu, Ti)-YSZ samples were synthesized by self-assembling synthesis route using Triton X as template
and thermal treated at 700 ꢀC and 900 ꢀC in air flow. Microstructure, surface chemistry and electro-
chemical properties were assessed by XRD, N2 adsorption-desorption, SEM, TEM, XPS, TPR-H2 and DC
conductivity measurements. The synthesis route leads to a very well incorporation of Ti into YSZ lattice.
X-ray diffraction shows that (Cu, Ti)-YSZ exhibits a phase transition, from cubic fluorite phase to
monoclinic, when the calcination temperature rises from 700 ꢀC to 900 ꢀC, while (Ni, Ti)-YSZ exhibits
only two cubic phases, with a main fluorite-type phase and a secondary isostructural phase with bun-
senite (NiO) for both calcination temperatures. The partial oxidation of methane was performed in the
temperature range 250e900 ꢀC, and the results can be summarized as follows: i) for the (Cu, Ti)-YSZ
samples it was found that the phase transformation from cubic fluorite to monoclinic favors an in-
crease in methane conversion as well as the selectivity to CO; ii) the samples containing Ni are more
active than those containing Cu, with 90% methane conversion at 750 ꢀC as compared to 75% conversion,
respectively. Regarding the total conductivity of the material synthesized, two different behaviors were
obtained depending on the composition; the (Ni, Ti-)YSZ maintained a prevailing ionic conductivity,
whereas the (Cu, Ti)-YSZ exhibited mainly n-type conductivity. The ionic conductivity of Ni-based sample
is beneficial for the partial oxidation of methane due to the oxygen vacancies generated on the surface of
the material, which plays a fundamental role in the O2 activation mechanism and is in accordance with
higher catalytic activity observed for the Ni containing sample.
Received 12 July 2016
Received in revised form
18 August 2016
Accepted 20 August 2016
Available online 22 August 2016
Keywords:
Surfactant
Partial oxidation of methane
Ionic conductivity
Electrochemical properties
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
catalytically active for the oxidation and reduction in anode and
cathode, respectively, and to possess enough porosity to facilitate the
Solid oxide fuel cells (SOFC) offer a promising way of converting
chemical energy into electrical energy with high efficiency [1,2].
SOFCs consist of three main parts: an anode, a cathode and an
electrolyte. The electrolyte possesses high ionic or protonic conduc-
tivity, whereas electrodes should have both high ionic/protonic and
electronic conductivity. Furthermore, both electrodes have to be
gas diffusion. They operate at elevated temperatures (550e1000 ꢀC)
to ensure adequate conductivity and catalytic activity of all the
components of the device. In the fuel cell operation, air or oxygen is
fed as oxidant whereas hydrogen is usually used as fuel.
Although this is clearly a green technology, there is a huge
challenge related to competitiveness with existing technologies,
especially with respect to the cost per kWh supplied. Also, the
anode needs to operate at high temperatures and under strong
reducing atmosphere, that imposes restrictions on the anode type
material selection and requires special challenges related to its
chemical stability and materials degradation [3]. Over the last
* Corresponding author.
** Corresponding author.
E-mail
addresses:
ssimona@icf.ro,
somacescu.simona@gmail.com
(S. Somacescu), jmserra@itq.upv.es (J.M. Serra).
http://dx.doi.org/10.1016/j.jallcom.2016.08.193
0925-8388/© 2016 Elsevier B.V. All rights reserved.

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S. Somacescu et al. / Journal of Alloys and Compounds 690 (2017) 873e883
decades, a lot of work has been invested in the development of
alternative anode materials with high redox stability and longer
lifetime, but some problems still exist due to their low catalytic
activity and electronic conductivity. As a consequence, to find an
ideal candidate for anode and low-cost manufacturing routes rep-
resents challenging tasks for scientific community. It is well known
that the synthesis routes for the catalyst production influence
hugely the overall catalyst activity and the selectivity towards
different reaction products [4] The material properties which can
affect the sintering characteristics and anode microstructure, like
particle size, surface area, distribution of metal on the support,
conductivity, etc., are influenced by the synthesis adopted methods
[5]. In this regard, Jung et al. [6] showed that indeed the anode
performance depends on the synthesis route. On the other hand, it
is well known that the surfactants are used to obtain the desired
porous structure and to decrease the particles size [7e9]. The for-
mation of ordered ceramic nanostructures with tunable
morphology and composition for SOFC anode can provide high cell
performance. Different configurations are already employed for the
anode manufacture; the most common is a cermet electrode made
of a ceramic and a metal phase. In many cases, metals, as Ni or Cu,
for example, which represent the current collector, are deposited by
impregnation [10]. However, this method presents some disad-
vantages as low dispersion and increased sintering tendency, with
clusters formation as a consequence. In order to improve the
electrochemical performance for the cermets based on Ni and Cu,
different other synthesis routes were approached [11e13]. Thus,
Ni:Cu:YSZ ¼ 40:20:40 fabricated by high-energy ball-milling of Ni,
Cu, and YSZ powder [12] showed an improvement of the electrical
conductivity as results of the well-connected Ni/Cu/YSZ particles
achieved through an increased ball-milling time. Also, it has been
highlightedtheeffect of the composition on the electrical conduc-
tivity for Cu/YSZ composite [13] synthesized by the same method. A
recent report revealed an excellent SOFC performance for
Ni0.5Cu0.5Ox coated tubular YSZ prepared by hard template method
combined with wet impregnation method [11].
synthesized by a self-assembling method, using a nonionic sur-
factant e Triton X100 as template. In this synthesis route Zirconium
propoxide, yttrium acetylacetonte, titanium (IV) isopropoxide,
nickel acetyl acetonate and copper acetate were used as inorganic
precursors.1-Propanol was used as solvent. The nonionic surfactant
was dispersed in alcohol under strong stirring. The inorganic pre-
cursors were dissolved in 1-Propanol and added maintaining stir-
ring conditions at room temperature. The gels formed were dried
several days at 50 ꢀC, followed by an increase of temperature at
100 ꢀC. All samples were calcined in air at 700 ꢀC and 900 ꢀC,
respectively. The (Ni/Cu, Ti)-YSZ samples are labeled as below (see
Tabel 1).
2.2. Characterization methods
X-Ray diffraction (XRD) analyses were carried out on a Schi-
madzu XRD-7000 diffractometer using Ni-filtered Cu K
a radiation
with
l
¼ 1.5418 Å. The tube source was operated at 40 kV and 30 mA.
Scans were taken in the 2q
range of 10e80ꢀ with a step size of 0.02ꢀ
and scan time of 2ꢀ/min. Crystalline phase were identified by the
comparison of the XRD patterns with the JCPDS database. The
crystallite sizes were calculated using Scherrer's formula D ¼ k
cos , where D is the average crystallite size, k a constant equal to
0.89, the wavelength of X rays and is the corrected half width.
l/b
q
l
b
Porosity Analysis (BET) - BrunauereEmmetteTeller adsorp-
tionedesorption isotherms of N2 were obtained at ꢁ196 ꢀC over a
wide relative pressure range from 0.01 to 0.995 with a volumetric
adsorption analyzer ASAP-2020 manufactured by Micromeritics.
The sample was degassed further under vacuum at 150 ꢀC over-
night prior to nitrogen adsorption. Desorption isotherm was used
to estimate the pore-size distribution.
Scanning electron microscopy (SEM). The images were recor-
ded with a Field Emission Gun FEI Quanta 3D microscope operating
at 2 kV.
Transmission electron microscopy (TEM). TEM micrographs
were recorded with a HITACHI HT800 apparatus equipped with a
Link energy dispersive X-ray (EDX) system and operated at an
accelerating voltage of 200 kV. The powders were ultrasonicated in
ethanol and a drop of the solution dried on a carbon coated
microgrid before measurements.
In this study (Ni/Cu, Ti)-YSZ anodes were synthesized by a facile
and low-cost synthesis route. This route involves a self-assembling
of the elements using Triton X100 as template. The self-assembled
method helps to avoid the sintering phenomenon by providing a
high Ni or Cu dispersion into an oxidic matrix, providing many
oxygen vacancies. The electronic conductivity can be expected by
the presence of Cu0 species resulted from strong reducing environ-
ments, but also by reduction of Ti4þ toTi3þ rising the quantity of free
electrons. Finally, the incorporation of species with lower valence as
Y3þcan provide the ionic conductivity. The structural properties and
surface chemistry of these new materials, as well as the electro-
chemical and catalytic performance, were assessed. The surfactant
was used to obtain the desired porous structure and to decrease the
particle size in order to improve the SOFCs performance.
The reduction behavior was studied by H2-TPR in a Micro-
meritics Autochem 2910 equipment. About 50 mg of sample (par-
ticle size 0.25e0.42 mm) was initially flushed with Ar at room
temperature for 30 min. Then, the gas was switched to the reduc-
tive mixture of 10 vol% H2 in Ar (50 mL minꢁ1) and the temperature
linearly increased from RT to 900 ꢀC at a heating rate of 10 ꢀCminꢁ1
.
Water formed in the reduction was retained in a 2-propanol/N2(l)
trap and the H2 consumption rate was monitored in a thermal
conductivity detector (TCD) previously calibrated using the
reduction of CuO as reference.
X-ray photoelectron spectroscopy (XPS) - Surface analysis
performed by X-ray photoelectron spectroscopy (XPS) was carried
out on PHI Quantera equipment with a base pressure in the analysis
chamber of 10-9 Torr. The X-ray source was monochromatized Al
2. Material and methods
2.1. Materials synthesis
Ka
radiation (1486.6 eV) and the overall energy resolution is esti-
(Ni/Cu, Ti)-YSZ (20 mol. % Ni/Cu, 10 mol.% Ti) materials were
mated at 0.70 eV by the full width at half-maximum (FWHM) of the
Table 1
Compositions of the sample studied and abbreviation name.
Name
Ni content (mol. %)
Cu content (mol. %)
Ti content (mol. %)
T calcination (ꢀC)
CTST700
CTST900
NTST700
NTST900
e
20
20
e
10
10
10
10
700
900
700
900
e
20
20
e

S. Somacescu et al. / Journal of Alloys and Compounds 690 (2017) 873e883
875
Au4f7/2 photoelectron line (84eV). Although the charging effect
was minimized by using a dual beam (electrons and Ar ions) as
neutralizer, the spectra were calibrated using the C1s line
(BE ¼ 284.8 eV) of the adsorbed hydrocarbon on the sample surface
(CeC or (CH)n bondings). As this spectrum was recorded at the start
and the end of each experiment the energy calibration during ex-
periments was quite reliable.
wt.) solution in terpineol in a weight ratio of 1:2. The ink was
screenprinted on both sides of 8YSZ 0.15 mm-thick fully-dense
electrolytes provided by Kerafol (Germany). Porous electrodes
were obtained after calcining the resulting screen-printed layers at
950 ꢀC for 2 h. A top screen-printed gold mesh was applied on the
electrodes in order to ensure proper current collection.
As-fabricated symmetrical cells were tested by means of elec-
trochemical impedance spectroscopy measurements using a 0 V DC
e 20 mV AC amplitude signal using a Solartron 1470E/1455 FRA
device. Electrochemical measurements were performed in air in
the 600-900 ꢀC temperature range. Electrode microstructure was
analyzed by SEM imaging in a ULTRA 55 ZEISS microscope FESEM.
2.3. Catalytic activity measurements
The catalytic activity for partial oxidation of methane (POM) was
evaluated in a continuous fixed-bed tubular reactor (length of
300 mm and i.d. of 9 mm, Hasteloy X tube), equipped with a thermo
well in the center of the catalyst bed. The experiments were typi-
cally carried out in a downflow system at atmospheric pressure.
The catalyst powder was placed between a two layers of quartz
wool and was first activated for 2 h in air, at 800 ꢀC before the re-
action, and then the reactor was cooled down to 200 ꢀC. Reactions
were conducted at different temperatures starting from 200 ꢀC,
using a feed consisting of 13.33% CH4; 6.66% O2; 80% N2 at a total
flow of 37.5 mL minꢁ1 and GHSV of 22500 hꢁ1. The products were
analyzed online by a gas chromatograph (Shimadzu GC-2014)
equipped with a TCD and FID detectors and two capillary col-
umns Molecular Sieve 5A and Poraplot Q (length, 60 m; internal
3. Results and discussion
3.1. Physico-chemical characterization
The indexed XRD pattern of CTST700, shown in Fig. 1a, indicates
the formation of a bi-phasic nanocrystalline powder of stabilized
cubic zirconia and tenorite. The main phase corresponds to a
zirconia-based cubic phase of fluorite type (Fm3m), with lattice
parameter c ¼ 5.073 Å. XRD detects also the presence of a sec-
ondary phase (~20 mol.%, marked * in the indexed pattern in Fig. 1)
with monoclinic symmetry (C2/c), isostructural with tenorite
(CuO), with slightly enlarged lattice parameters, a ¼ 4.695 Å,
diameter, 0.32 mm; film thickness, 0.5
mm). Activity and selectivity
results were calculated according to the equations given below:
b ¼ 3.433 Å, c ¼ 5.141 Å,
a
¼
b
¼ 90ꢀ,
g
¼ 99.470ꢀ. Both phases have
small nanocrystallite size, around 8 nm for the zirconia-based
fluorite-type phase and 12 nm for the copper oxide-based tenor-
ite-type phase, respectively. In spite of the incorporation of titania
into the YSZ lattice, the high temperature cubic structure remains
stabilized at room temperature due to the small size of the
nanocrystallites.
CH4 conversion (%) ¼ mCH4in
ꢁ
mCH4out
/
mCH4in ꢂ 100
(1)
(2)
CO selectivity (%) ¼ mCO mCH4in
/
ꢁ mCH4out ꢂ 100
The XRD pattern of CTST after thermal treatment at 900 ꢀC,
CTST900, can be indexed to a monoclinic zirconia-based phase and
tenorite as secondary phase (~20 mol.%) (Fig. 1a). Thermal treat-
ment at 900 ꢀC results with thesignificant growth of crystalline
sizes for both the zirconia-based and CuO phases (30 nm and 25 nm
respectively), thus leading to the spontaneous phase trans-
formation of the cubic zirconia phase into monoclinic, caused most
likely by crystallite growth. The absence of traces of any titanium
containing phase and the spontaneous formation of the monoclinic
structure indicates that TiO2 have incorporated in YSZ forming a
solid solution phase, previous studies have demonstrated that the
solid solubility limit of TiO2 in YSZ nanoparticles is indeed much
higher than the concentration of TiO2 in the zirconia-based phase
in CTST [14]. There is no change in the lattice parameters of the CuO
phase, indicating that composition of the tenorite phase remains
unchanged.
2.4. Electrochemical characterization
2.4.1. Conductivity measurements
Rectangular probes of the powders, CTST and NTST, were uni-
axially pressed and subsequently sintered for 10 h at 1000 ꢀC and
1300 ꢀC respectively in air. Electrical conductivity measurements
were conducted by standard fourpoint DC technique. Silver paste
and wires were used to obtain the electronic contact. A constant
current was supplied by Keithley 2601 and the voltage was detec-
ted by a multimeter (Keithley 3706).
2.4.2. Electrochemical impendance spectroscopy (EIS)
measurements
Screen-printing ink made of CTST anode was prepared by mix-
ing in a three roll mill, the CTST powder and an ethylcellulose (6%
Fig. 1. XRD patterns for CTST (a) and NTST (b) samples calcined at 700 and 900 ꢀC.

876
S. Somacescu et al. / Journal of Alloys and Compounds 690 (2017) 873e883
The XRD pattern of sample NTST700 (Fig.1b) corresponds to two
typical sizes around 5 nm, which correspond to the size of single
nanocrystallites determined from XRD measurements. Porosity
arises from spaces left between nanocrystallites, seen as clearer
areas in TEM micrographs. The intercrystalline porosity measured
from N2 adsorption/desorption have somewhat bigger sizes than
single nanocrystallite size, the high resolution TEM measurements
(Fig. 3d) confirm the presence and size of intercrystalline porosity
in the mesoporous powder.
cubic phases, a main fluorite-type phase (Fm3m), and a secondary
phase (~15 mol%, also Fm3m, marked with * in Fig. 1b) isostructural
with bunsenite (NiO). The fluorite has a lattice parameter c ¼ 5.131 Å
whereas the lattice parameter of the bunsenite is c ¼ 4.184 Å, bigger
than c ¼ 4.177 Å for standard bunsenite (JCPDS Card 04-0835). In
addition, both phases have similar nanoscale crystallite size, around
5 nm. The XRD pattern of NTSTafter annealing the sample at 900 ꢀC
(NTST900, Fig. 1b) can be indexed to the same two phases, fluorite
and bunsenite, without changes in the lattice parameters. As a
result of the thermal treatment at 900 ꢀC a clear peak width
reduction is observed, evidencing the significant crystal growth of
both phases, from 5 nm to around 15 nm. Ni incorporated into YSZ
cubic matrix was found as strongly dependent on the synthesis
route and Ni relative concentration, as well. Indeed, in our recent
work on mesoporous Ni doped (CeO2-d)-YSZ (5 mol% Ni oxide,
10 mol% ceria) obtained by hydrothermal treatment [15] we high-
lighted that Ni was incorporated into the cubic structure without
any trace of NiO as secondary phase as obtained by self-assembled
method presented in this study.
The reduction behavior was investigated with H2-TPR. The TPR
profile for each sample is presented in Fig. 4 aed.
The thermal treatment at 700 ꢀC for samples containing Cu,
leads to Cu species reducible at low temperature, ~200 ꢀC, and
until300 ꢀC the Cu is completely reduced (Fig. 4a). This reduction
peak can be associated with the reduction of CuO in well-dispersed
mixed oxide species. For samples calcined at 900 ꢀC, the Cu bonded
in monoclinic zirconia-based phase is reducible at higher temper-
atures, ~320 ꢀC and the reduction process is completely finished at
around 510 ꢀC (Fig. 4b). Very small peak at ~460 ꢀC could be
attributed to the reduction of Cu2O [17]. Although the presence of
Cu2O was not evidenced by XRD nor by XPS, the presence of a very
low percentage, below the limit of detection equipment, cannot be
completely ruled out. Cu2O may be formed as a result of random
distributions of the oxygen vacancy generated by the presence of
Y3þ, Cu2þ and Ti4þ substituting Zr4þ ions. Detailed investigations
of the reduction mechanism for Cu2O demonstrated that it is less
reducible, having apparent activation energy 27.4 kcal/mol
compared to CuO 14.5 kcal/mol [18]. On the other hand, embedding
Cu2O into the matrix YSZ using our direct synthesis method, can
make Cu2Oless reducible with complete reduction at ~510 ꢀC. The
increases in the reduction temperature can be explained by the
significant growth of crystalline sizes for both the zirconia-based
main phase (from 8 to 30 nm), and CuO (from 12 to 25 nm)
(Table 2) with the calcination temperature and in agreement with
the literature data [19].
In Table 2 are gathered the structural and textural properties of
the studied samples.
As seen in Table 2, for the samples calcined at 700 ꢀC, surface
areas of 15 and 66 m2/g were observed for the CTST and NTST,
respectively. It seems that NiO acts as an inhibitor for the coars-
ening of YSZ during the calcination [16] and therefore higher sur-
face area is observed for the containing Ni sample. As expected,
after calcinations at higher temperatures, the surface area
decreased, mainly due to the thermal sintering processes. Never-
theless, the NTST900 presents higher surface area than CTST900, in
good agreement with smaller crystallite size measured from XRD
data and reported in Table 2.
BrunauereEmmetteTeller (BET) analysis of the NTST700 and
CTST700 samples isotherms confirms the presence of the meso-
porous structure (Fig. 2a, b). The shape of the N2 adsorption-
desorption isotherms, is characterized by a hysteresis loop at
higher partial pressures. This kind of isotherm (type IV according to
IUPAC) is specific to mesoporous solids. The initial part of the
isotherm is attributed to monolayer-multilayer adsorption and the
hysteresis loop is associated with capillary condensation taking
place in mesopores. The pore sizes distributions derived from the
desorption isotherm show a narrow pore size distribution for
NTST700 sample, while for the CTST700 the distribution is wider
(Fig. 2c, d). The thermal treatment has a major influence on the
porous structure, giving rise to macroporous materials, as indicated
by type III isotherm (Fig. 2a, b).
As revealed by XRD analysis, after thermal treatment, the Ni
containing samples possess mainly fluorite-type phase and a sec-
ondary phase isostructural with bunsenite (NiO). Consequently, the
reduction degree of the Ni is strongly affected by these structural
properties. The TPR profile of NTST700 and NTST900 (Fig. 4c, d)
consists of two main peaks corresponding to the reduction of the
different Ni species: i) in the temperature range 350e400 ꢀC, NiO
from the surface and Ni from the subsurface is reduced to metallic
Ni [20] ii) the peak at around 600 ꢀC is assigned to the reduction of
NiO species in intimate contact with the support [21]. Consistently
with the XRD data, the TPR profile of the calcined NTST proves the
absence of Ni2O3.
SEM micrographs of the CTST700 and NTST700 powders as
processed are shown in Fig. 3a, b. Large clusters with crystalline
framework of ultra fine particles and porous structure can be
observed in the micrographs. The samples show excellent homo-
geneity of the pores and nanocrystallite sizes in all analyzed
regions.
A shift to higher reduction temperature is observed for samples
calcined at 900 ꢀC, explained also in this case by the significant
crystal growth of both phases, from 5 to 15 nm during the thermal
treatment.
The XPS data were recorded for the CTST and NTST samples
thermal treated at 700 ꢀC and 900 ꢀC, respectively (Figs. 5 and 6).
For data processing and interpretation, we followed a critical
review of the available databases [22] and the literature.
Fig. 3c, d show TEM micrographs of NTST700. The low resolution
TEM micrograph (Fig. 3c) shows the nanocrystalline powder with
Table 2
Structural and textural properties of the anode materials after the thermal treatment at different temperatures.
Sample
Surface area (m2/g)
Average pore volume (cm3/g)
Average pore diameter (nm)
Crystallite size, da (nm)
CTST700
CTST900
NTST700
NTST900
15
1
66
8
0.077
0.03
0.173
0.120
13.4
60
7.4
47
8/12b
30/25b
5/5b
15/15b
a
Determined from XRD data.
For secondary phase.
b

S. Somacescu et al. / Journal of Alloys and Compounds 690 (2017) 873e883
877
Fig. 2. N2 adsorption/desorption isotherms (aeb) and pore size distribution (ced) for the CTST and NTST samples thermal treated at 700 and 900 ꢀC.
Additionally, standards CuO, NiO, TiO2, ZrO2 and Y2O3 samples
were measured to ensure reliable assignments to the photoelec-
tron lines, as well as to compare the Cu2p and Ni2p signature
doublets shapes. After data processing, the XPS results are sum-
marized in Figs. 5 and 6 and Table 3. While Cu2p and Ni2p tran-
sitions exhibit a complex structure, Ti2p, Zr3d and Y3d prominent
XPS transitions display typically 4þ, 4þand 3þ oxidation states,
respectively.
A close inspection of Cu2p photoelectron spectra (Fig. 5a) re-
veals the presence of Cu2þ oxidation state for CTST700 and
CTST900 samples following the assignments of the binding en-
ergies, as well as the associated shake-up satellites and their
Fig. 3. SEM micrographs of the CTST (a), NTST (b) samples calcined at 700 ꢀC and TEM micrographs (c,d) of NTST700.

878
S. Somacescu et al. / Journal of Alloys and Compounds 690 (2017) 873e883
Fig. 4. TPR profile for CTST700 (a), CTST900 (b), NTST700 (c) and NTST900 (d) samples.
relative intensities, in agreement with XRD results.
since the total number of scans over the binding energy ranges for
Ti2p and Cu2p doublets were kept.
Nevertheless, Ni2p doublet shows typical characteristics of the
2þ oxidation state (Fig. 5b) related to the binding energies, satellite
structure and the overall spectral shape. The deconvolution of the
O1s singlet highlights mainly oxygen bonded in the lattice as O2ꢁ as
well as the OH groups and H2O adsorbed on the outermost surface
layer (Fig. 6a inset). One can notice that the thermal treatment does
not produce significant changes in the oxygen chemistry, the sur-
face remaining hydroxylated even at high temperature.
The spectra collected for the CTST900 sample display an
increased level of noise as compared to the CTST700 sample sug-
gesting lower relative concentrations for Cu and Ti. A cross corre-
lation could be made after an inspection of Y3d (more pronounced)
and Zr3d (less pronounced due to relative high concentrations)
superimposed spectra (Fig. 6bed).
The quantification was made from high resolution spectra with
the sensitivity factors (SFs) checked from standard samples and
following the protocol recommended by ISO-TC201 e “Quantitative
analysis”. The data obtained are summarized in Table 3.
Regarding CTST sample, a segregation/diffusion phenomenon is
clearly observed. Thus, with increasing temperature, the surface
region is depleted in Ti and Cu and enriched in Y and Zr. A diffusion
process of Ti and Cu from the surface to the bulk/subsurface takes
place. This is clearly visible in Fig. 5a after a closer inspection of the
superimposed spectra. A direct comparison can be established,
3.2. Catalytic tests
Catalytic properties were evaluated through the partial oxida-
tion of methane which has been performed for the two composi-
tions. Firstly, blank tests were performed in the presence of quartz
wool alone, by using the same reaction parameters as during the
catalyst screening, showing no detectable occurrence of homoge-
neous reactions. To compare catalytic activity and selectivity of the
various catalysts as a function of the detected products in the re-
action stream, we assumed the following set of reactions to take
Fig. 5. The Cu2p and Ni2p photoelectron high resolution spectra for the CTST and NTST samples calcined at 700 ꢀC and 900 ꢀC, respectively.

S. Somacescu et al. / Journal of Alloys and Compounds 690 (2017) 873e883
879
Fig. 6. The O1s, Ti2p, Y3d and Zr3d photoelectron high resolution spectra for the CTST and NTST samples calcined at 700 ꢀC, 900 ꢀC.
Table 3
Binding energies and surface cation relative concentration results from the XPS measurements in CTST and NTST samples after different treatments.
Sample
Binding energies, eV
Cation relative concentrations
Cu2p3/2
Ni2p3/2
Ti3/2
Y3d5/2
Zr5/2
Cu
Ni
Ti
Y
Zr
CTST700
CTST900
NTST700
NTST900
933.3
933.4
e
e
458.2
458.2
458.2
458.2
157.2
157.3
157.2
157.2
181.8
181.9
181.9
181.9
36
12.2
e
e
14.9
6.4
12.5
9.2
7.4
41.7
61.4
57.9
62.7
e
20.1
16.8
19.1
854.8
854.8
12.8
9.0
e
place on our catalysts: syngas production Eq (3) and total oxidation
of CH4, producing CO2 and H2O Eq (4):
temperature. Products of methane oxidative coupling (C2 and
higher hydrocarbons) were not observed on the entire temperature
range studied.
CH4 þ 1/2O2 / CO þ 2H2
CH4 þ 2O2 / CO2 þ 2H2O
(3)
(4)
The conversion profile recorded for the catalytic partial oxida-
tion of methane (Fig. 7, a and b) showed that CTST materials are
active above 400 ꢀC, while NTST materials are active only above
600 ꢀC with methane completely converted at 850 ꢀC and a selec-
tivity of almost 100% in CO. On the other hand, the O2 consumption
of CTST900 sample is observed at considerably lower temperature
(200 ꢀC) with respect to the NTST900 system (450 ꢀC). This
Figs. 7 and 8 present the effect of calcination temperature on the
catalytic activity and selectivity on each sample. The methane
conversion values were taken after 30 min of reaction at each
Fig. 7. Methane conversion (full symbols) and oxygen conversion (empty symbols) on (a) CTST and (b) NTST samples; circles e for the samples calcined at 700 ꢀC and square for the
samples calcined at 900 ꢀC (CH4:O2 ¼ 2:1; total flow of 37.5 mL minꢁ1 and GHSV of 22500 hꢁ1).

880
S. Somacescu et al. / Journal of Alloys and Compounds 690 (2017) 873e883
(a)
(b)
100
100
80
60
40
20
0
80
CTST700- CO
NTST700- CO
NTST900- CO
NTST700-CO2
NTST900-CO2
60
40
20
0
CTST900- CO
CTST700-CO2
CTST900-CO2
200
400
600
800
200
400
600
800
Temperature (°C)
Temperature (°C)
Fig. 8. CO and CO2 selectivities on (a) CTST and (b) NTST samples.
consumption continues up to about 350 ꢀC and can be related to the
partial oxidation of Cu particles, due to the fact that no methane
conversion was observed. The concomitant conversions of CH4 and
O2 start above 400 ꢀC. Up to 550 ꢀC, only CO2 and H2O are detected
according with eq. (2) (Fig. 8a). At this temperature for CTST900,
complete conversion is achieved for O2 and, simultaneously, H2 and
CO sharply increases according with eq. (3).
However for NTST sample, this consumption of O2 continues up
to about 600 ꢀC and can also be related to the partial oxidation of Ni
particles, as previously reported [23]. For NTST system the associ-
ated conversions of CH4 and O2 start up at 600 ꢀC, where only CO2
and H2O are detected. Nevertheless, the complete conversion of O2
takes place above 750 ꢀC, whereas the H2 and CO production in-
creases drastically (Figs. 7 and 8 b).
in the activity between NTST700 and NTST900 can be attributed to
the modification of particle size that was reported by XRD results.
Furthermore, Cu based catalyst studied in their research, showed
the lowest performance for the partial oxidation of CH4, in agree-
ment with our results obtained for CTST samples.
Except for the CTST700 sample, on which no CO formation was
recorded, the selectivity of CO increases with the reaction tem-
perature, in detriment of CO2 formation (Fig. 8), as thermody-
namically expected. The full CH4 oxidation according with eq. (4)
giving rise to CO2 and H2O observed on CTST700, could be related
to reduction of CuO present in the initial oxidized catalyst, which
becomes fully reduced during the CH4 oxidation [26].
For the Cu containing sample CTST900, the formation of CO was
recorded at 550 ꢀC, whereas the CO production in Ni containing
samples started at around 700 ꢀC.
As can be observed from Figs. 7 and 8, the catalytic behavior in
POM of CTST system is very different from NTST samples. The
samples containing Ni are more active than those containing Cu,
with 90% conversion at 750 ꢀC for NTST900 as compared to 75%
methane conversion for CTST900, even though, for NTST system the
CH4 conversion starts at higher temperatures (600 ꢀC vs 400 ꢀC for
CTST). The situation is different in the case of CTST samples,
calcined at different temperatures; CTST900 is more active for
methane selective oxidation than CTST700. XRD analysis has
shown a phase transition of CTST when the sample is annealed at
high temperature (900 ꢀC). The monoclinic structure at 900 ꢀC al-
lows the production of CO [24], whereas CTST700, which presents
cubic phase of fluorite type, is only selective in all the operation
temperature range, to CO2. In the case of Ni containing samples, the
calcination temperature does not affect the crystalline phase, and
therefore the catalytic activity is not much affected neither.
As it has been previously reported [23,25], the CH4 combustion
takes place on NiO, while the reforming of CH4 is promoted by Ni
reduced in situ at high temperature. The CO formation at higher
temperatures for NTST system can alsobe attributed to the NiO
reduction into Ni0 by CH4, which occurs at a certain critical tem-
perature, besides the complete oxidation of methane. The fact that
nickel valence changes during the reaction, represents an impor-
tant parameter, which is responsible for the transformation of the
reaction from complete oxidation to partial oxidation of methane
(Fig. 7b). On NTST samples, in the first step of the reaction, the
active centers are NiO species, which are formed by oxidation of
metallic species at low temperature (until550 ꢀC), followed by a
second step in which the metal oxide particles are reduced at
higher temperature (750 ꢀC) and represent the active centers
responsible for the partial oxidation of methane. As already re-
ported, the Ni0 species constitute the active sites for the POM [21].
In addition, L. de Rogatis et al. [21] studied the influence of Cu and
Ni in the CH4 reforming reaction. Ni reduced in-situ showed better
results and lower initial temperature reaction, evidencing a strong
influence of the Ni particles size. This way, the difference obtained
Previous observations in the literature [27,28], showed that the
CH4 transformation on pure CuO, basically, involves only generation
of full oxidation products. The fact that only full oxidation product
CO2 is generated for CTST700 sample, suggests that mainly CuO is
involved in the process. The presence of CuO was evidenced by XPS
analysis. Also, the time needed for the CO formation for the
CTST900 sample, indicates that during the process some reduction
of CuO by CH4 is firstly achieved, with the generation of active sites
for the CO production.
Also, the catalytic stability is enhanced for the samples con-
taining nickel (see the supplementary information). Due to smaller
particle size, a higher cooking resistance and reaction stability of
catalyst is expected [29,30]. Indeed, the particle size is responsible,
Fig. 9. Arrhenius plot of conductivity as function of different temperatures in reducing
atmospheres.

S. Somacescu et al. / Journal of Alloys and Compounds 690 (2017) 873e883
881
Fig. 10. Total conductivity as function of pO2 at 780 ꢀC.
on one hand, for the catalytic stability, which is higher for NTST
samples, as compared with CTST samples, for both calcination
temperatures. On the other hand, it seems that the lower the par-
ticle size is, the higher the activity and selectivity are obtained [4].
NTST700 with the smallest crystallite size, obtained from XRD
analysis, is active to CO, with 100% CH4 conversion at 700oC,
whereas NTST900 needs 50oC more to produce CO and to reach 90%
conversion.
(where H2 and D2 (5%) are diluted in He and pH2O and pD2O are
0.025 atm) reducing atmospheres.
Figs. 9e11 shows constant conductivity activation energy (EA) in
all conditions and for the different samples in the temperature
range measured, with higher EA for NTST. However, the behavior of
the different samples in methane containing atmospheres changes
drastically. The copper containing sample presents considerably
lower total conductivity with a high EA, whereas NTST maintains
the EA obtained in H2 and D2 atmospheres, but decreasing the total
conductivity. This observation is directly related to the associated
variations in pO2 and the nature of the conduction under these
conditions.
As can be ascribed from Fig. 10, NTST shows a predominately
ionic conductivity since the conductivity is almost pO2-indepen-
dent. However, CTST has a predominately n-type conductivity
(with a pO2 dependence between ꢁ1/4 and ꢁ1/6) [31]. Further-
more, CTST exhibits a small ionic conductivity contribution since
there is a slight deviation in the theoretical pO2 dependence (ꢁ1/
4, ꢁ1/6). Additionally, in both cases the conductivity shows a non-
3.3. DC conductivity measurements
In order to check the electronic properties of the materials
synthesized, NTST and CTST were tested by means of DC conduc-
tivity measurements. Conductivity was analyzed as a function of
temperature, oxygen partial pressure, water partial pressure and H/
D isotopic effect to get insight into the nature of the conduction
processes in the material. Specifically, CTST and NTS total conduc-
tivity measurements are presented in Figs. 9 and 10 as a function of
the temperature in H2, D2, H2 pH2O, D2 pD2O and 10% of CH4 in Ar
Fig. 11. (a) Polarization resistance as function of temperature, (b) FESEM micrographs of the electrode and electrolyte (left) and electrode magnification with Ni nanoparticles
(right), (c) Nyquist and (d)Bode plot of EIS response of CTST infiltrated and not infiltrated.

882
S. Somacescu et al. / Journal of Alloys and Compounds 690 (2017) 873e883
2þ oxidation state, while Ti2p, Zr3d and Y3d prominent XPS
transitions display typically 4þ, 4þand 3þ oxidation states for
both calcination temperatures. Hydroxylated surfaces were
found for the CTST and NTST samples calcined at both
temperatures.
ꢃ The samples containing Ni are more active in POM than those
containing Cu. CTST700 has no activity for the partial oxidation
of methane, whereas the monoclinic phase CTST900 has ob-
tained good results above 700 ꢀC. Total combustion of CH4 oc-
curs on the oxidized particles, while partial oxidation of CH4
takes place on reduced particle.
ꢃ The conductivity measurements performed in H2, D2, H2 pH2O,
D2 pD2O and 10% of CH4 in Ar reducing atmospheres showed
that NTST presents prevailing ionic conductivity whereas CTST
shows mainly an n-type conductivity. The ionic conductivity of
CTST can help the partial oxidation of methane due to the oxy-
gen vacancies generated on the surface of the material, which
play a fundamental role in the O2 activation mechanism.
proton transport, since there is neither hydration effect
(
s
H2>
D2þD2O >
s
H2þH2O and
s
D2
>
s
D2þD2O) nor H/D isotopic effect
(s
s
H2þH2O) in the temperature range measurements for
both samples. NTST has maintained the conductivity behavior of
the YSZ state of the art (ionic conductor) [32] whereas for Cu
incorporated in the material the electronic conductivity prevails.
On the other hand, oxygen-deficient nonstoichiometry and ox-
ygen conducting oxides play an important role in the catalysis
process such as dehydrogenation reactions and partial oxidation of
methane, due to the lattice oxygen O2ꢁ [33]. CO and H2 can be
obtained via the partial oxidation of methane, since methane can
react with mobile lattice oxygen (O2ꢁ). The partial oxidation of
methane produces oxygen vacancies (Vo..) in the oxide that can be
regenerated with oxygen or air. That fact can also support the best
results obtained for NTST, since it has a predominately ionic con-
ductivity and oxygen vacancies are key in the reaction mechanism.
3.4. EIS analysis of CTST compound
Symmetrical cell of CTST700 powders was prepared on dense
YSZ electrolyte. Firstly, the blank electrode was tested by means of
EIS measurements in 100% wet H2. As can be ascribed from Fig. 11c
and d there is a big contribution of the resistance at low frequency
(LF) in the EIS spectra for the blank sample. This LF resistance is
usually associated with surface processes, as dissociation, adsorp-
tion, etc. [34]. In order to reduce the LF resistance associated with
the catalytic activity of the electrode, some improvement was
performed. The blank sample was infiltrated with a Ni precursor
solution after EIS measurements. New EIS measurements were
carried out in the same operation temperatures and atmospheres,
wet H2 (2.5% vol. of H2O). The LF resistance was reduced in the
whole range of temperatures measured and the Rp decreased
around five times from the initial electrode (Fig. 11 a).
From Fig. 11 b and c the huge reduction of the resistance at low
frequencies can be highlighted, showing the important role of
nanoparticles as catalyst [35] and the good performance of Ni for
the hydrogen oxidation.
The anode porosity as well as electrolyte density can be
observed in the cross-section image Fig. 11b. The cathode has
enough porosity for the gas diffusion and relatively large surface
area for the Ni deposition. Furthermore, in Fig. 11 b right it can
appreciated the Ni nanoparticles infiltrated. As can be shown in the
SEM micrograph, the whole surface of the CTST was not covered
with Ni nanoparticles, allowing the possibility of improving the
cell's performance increasing the Ni content.
Acknowledgements
This work was supported by a grant of Partnerships in priority
S&T domains Programm (PNII), MENeUEFISCDI, PCCA1, project
number 26/2012, ENE2011-24761 and Severo Ochoa SEV-2012-
0267 grants.
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