B-cation partial substitution of double perovskite La2NiTiO6 by Co2 +: Effect on crystal structure, reduction behavior and catalytic activity

Article abstract of DOI:10.1016/j.catcom.2017.04.030

La₂NiTi_0.9Co_0.1O₆ and La₂Ni_0.9Co_0.1TiO₆ perovskites were obtained by partial substitution of either Ti^{4 +} or Ni^{2 +} from double perovskite La₂NiTiO₆ by Co^{2 +}. The Rietveld analysis demonstrated that all as-synthesized samples have orthorhombic symmetry and space group Pbnm. Quantitative phase analysis using Rietveld refinement of XRD profiles after temperature programmed reduction showed that the reduction of La₂NiTiO₆ was modified by partial substitution of either Ni^{2 +} or Ti^{4 +} by Co^{2 +}, being more evident for the La₂NiTi_0.9Co_0.1O₆. The activity of all catalysts for steam reforming of methane was attributed to both the reduction degree of two-valence metals and the metal-support interaction.

Full text of DOI:10.1016/j.catcom.2017.04.030

Accepted Manuscript
B-cation partial substitution of double perovskite La2NiTiO6 by
Co2+: Effect on crystal structure, reduction behavior and catalytic
activity
Pablo V. Tuza, Mariana M.V.M. Souza
PII:
S1566-7367(17)30165-6
doi: 10.1016/j.catcom.2017.04.030
CATCOM 5015
DOI:
Reference:
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
5 January 2017
12 April 2017
15 April 2017
Please cite this article as: Pablo V. Tuza, Mariana M.V.M. Souza , B-cation partial
substitution of double perovskite La2NiTiO6 by Co2+: Effect on crystal structure,
reduction behavior and catalytic activity. The address for the corresponding author was
captured as affiliation for all authors. Please check if appropriate. Catcom(2016), doi:
1
0.1016/j.catcom.2017.04.030
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ACCEPTED MANUSCRIPT
2
+
B-cation partial substitution of double perovskite La2NiTiO6 by Co : effect on crystal
structure, reduction behavior and catalytic activity
1
†
1*
Pablo V. Tuza and Mariana M.V.M. Souza
1
Escola de Química, Universidade Federal do Rio de Janeiro (UFRJ), Centro de Tecnologia, Bloco
E, Sala 206, CEP 21941-909, Rio de Janeiro/RJ, Brazil
†
Present address: Universidad Nacional de Loja, Carrera Químico Biológicas, Av. Pío Jaramillo
Alvarado y Reinaldo Espinoza, sector La Argelia, Loja, Ecuador
*
Corresponding author (Tel: 5521-39387598, Email: mmattos@eq.ufrj.br)
Abstract

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La2NiTi0.9Co0.1O6 and La2Ni0.9Co0.1TiO6 perovskites were obtained by partial substitution of either
4
+
2+
2+
Ti or Ni from double perovskite La2NiTiO6 by Co . The Rietveld analysis demonstrated that all
as-synthesized samples have orthorhombic symmetry and space group Pbnm. Quantitative phase
analysis using Rietveld refinement of XRD profiles after temperature programmed reduction
2
+
4+
showed that the reduction of La2NiTiO6 was modified by partial substitution of either Ni or Ti
2
+
by Co , being more evident for the La2NiTi0.9Co0.1O6. The activity of all catalysts for steam
reforming of methane was attributed to both the reduction degree of two-valence metals and the
metal-support interaction.
Keywords: double perovskite, cobalt, nickel, quantitative phase analysis, reduction.

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1
. Introduction
Perovskites are oxides that can be represented by the formula ABO3 [1], where A is a rare
earth, alkali or alkaline earth metal and B is a transition metal [2]. Perovskite structure can be
illustrated by B cations in the center of octahedra sharing corners, whose vertices are occupied by
oxygen atoms (BO6), and thus forming a framework that contains the A cations in the center of the
cavities between layers of BO6 octahedra. Double perovskites (DPs) are represented by the formula
A2B’B’’O6 [3] or A’A’’B’B’’O6 [4] and the B-cation sublattice types known for DPs are rock salt,
layered and random [4]. In rock salt ordering, B’O6 and B’’O6 octahedra are ordered like Na and Cl
atoms in NaCl structure. Layered ordering occurs when layers composed of either B’O6 or B’’O6
octahedra are placed one above other. Random ordering is characterized by the absence of B’O6 and
B’’O6 octahedra sequence in the structure formed by octahedra sharing corners. Therefore, the rock-
salt and layered present an ordered arrangement [4], which characterizes compounds known as
ordered DPs.
DPs show interesting physical and chemical properties such as superconductivity,
ferroelectricity, antiferroelectricity, piezoelectricity, magnetic properties and optical properties or a
combination of both magnetic and electric interactions [5,6]. These materials have been used as
electrode in fuel cells [7,8] as well as catalysts [9] and catalyst precursors [10].
The reduction of catalyst precursors is important for catalytic reactions because it is
associated with the formation of metal active sites and the crystalline phases composing the support.
Reducibility of cations varies according to its respective redox potential, e.g., copper, nickel or
cobalt are more difficult to reduce than iridium, rhodium or ruthenium, and can be modified by the
neighboring anions in the crystalline structure since it is also influenced by the coordination number
and precise location of the cations [11]. Concerning perovskites, the reduction of LaNiO3 was well

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detailed in the literature [12,13]. The reduction of the La2NiTiO6 DP was reported in our previous
work, as well as its catalytic activity for steam reforming of methane [14].
To the best of our knowledge, the influence of partial substitution of B-cations of the
La2NiTiO6 DP has not been studied yet. Therefore, the main objective of this work is to partially
4
+
2+
2+
substitute either Ti or Ni from La2NiTiO6 by Co
for obtaining La2NiTi0.9Co0.1O6 and
La2Ni0.9Co0.1TiO6 perovskites, respectively, and then to evaluate the crystal structure after
synthesis, crystalline phases after reduction and the catalytic activity of the prepared samples.
2
. Experimental
2
.1. Synthesis
La NiTi Co O (LNT Co O), La Ni Co TiO (LN Co TO) and La NiTiO
6
2
0.9
0.1
6
0.9
0.1
2
0.9
0.1
6
0.9
0.1
2
(LNTO) were prepared by modified Pechini method [15]. Steps related to citrate and the
preparation of the polyester solution with subsequent resin formation and its calcination were
detailed in our previous work [14]. The weak sponge-like structure obtained was mixed and ground
-
1
in agate mortar, calcined in air at 800 °C for 17 h with a heating rate of 10 °C min , and finally
milled again.
2
.2. Characterization
Chemical composition of the materials was determined by X-ray fluorescence (XRF) using a
Rigaku Primini spectrometer and expressed on the basis of the amount of cations in each sample.
Crystalline phases of as-synthesized samples and the materials after temperature programmed
reduction were determined by means of X-ray diffraction (XRD), using a Rigaku Miniflex II X-ray
diffractometer equipped with a graphite monochromator and CuKα radiation (30 kV and 15 mA).
The measurements were carried out over the 2θ range of 5-90°, step size equal to 0.05°, and a
counting time of 1 s per step. The identification of the crystalline phases was performed using

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crystallographic information files from Inorganic Crystal Structure Database (ICSD) and the
collection code.
Average crystallite size was determined by Scherrer equation, using the full width at half
maximum of the (112) or (111) diffraction peaks for the as-synthesized materials (2θ equal to 32.3°)
or after temperature programmed reduction (2θ equal to 44.5°), respectively.
In order to refine the structure of the as-synthesized samples and to perform quantitative
phase analysis (QPA) for each perovskite after temperature programmed reduction, it was carried
out a Rietveld refinement of the corresponding XRD patterns using the Fullprof Suite Program 3.0
(
version Febraury-2015) [16]. Details of the Rietveld refinement are provided on Supplementary
Material.
Magnetic measurements as function of temperature for the LNTO sample, either in the zero-
field cooled or in field cooled mode, were conducted using a cryogenic superconducting quantum
interference device (SQUID) magnetometer, MPMS Quantum design model, in the temperature
range 2-250 K, applying a magnetic field of 0.01 T. The molar susceptibility χm was calculated on
the basis of the sample mass and the molecular weight.
Temperature programmed reduction (TPR) was performed in a microflow quartz-tube
reactor operating at atmospheric pressure, using a sample amount equal to 1400 mg. This great
amount of sample was used in order to distinguish the TPR peaks and variations of the baseline
during isothermal period. The first step consisted in dehydration at 150 °C for 60 min under flowing
-
1
Ar. The another step was the sample reducing with a flow rate of 55 mL min of 10 % H2/Ar, from
room temperature to 1000 °C, coupled to an isothermal period at 1000 °C until the stabilization of
the baseline, which occurred at 330 min. The outflowing gases were detected by thermal
conductivity detector (TCD).
2
.3 Catalytic tests

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Catalytic activity for steam reforming of methane was evaluated in a fixed bed continuous
flow reactor operated at atmospheric pressure. Initially, 50 mg of catalyst was reduced in situ with
-
1
1
50 mL min of 10 % H2/N2 at 1000 °C for 2 h. The reaction was carried out with a total flow rate
-
1
equal to 100 mL min of 9.8 % CH4/He and steam, which was introduced into the reactor through a
water saturator at 43.7 °C under flowing CH4/He. The reaction was performed in the temperature
range 450-950 °C and H2O/CH4 molar ratio of 1:1. Analysis of reaction products was carried out in
a Shimadzu gas chromatograph, with Carboxen 1010 Plot column, thermal conductivity and flame
ionization detectors. CO and CO2 were separated in the column and converted to CH4 with a
methanizer operating at 400 °C previous to detection by the flame ionization detector.
3
. Results and discussion
Table 1 indicates the chemical composition of the as-synthesized materials determined by
XRF. These values are close to nominal values, indicating appropriate application of the synthesis
method.
Diffraction patterns and Rietveld analysis and of the as-prepared samples are presented in
Fig. S1 of the Supplementary Material. The structural parameters, cell volumes, R-factors and chi
squared values are shown in Table S1. The average crystallite sizes are smaller than 36 nm, which
shows nanostructured materials. The lattice parameters of these materials are in accordance with the
values reported for LaNi0.5Ti0.5O3. In the case of the LN0.9Co0.1TO and LNTO, metal to oxygen
bond distances are similar to the corresponding from LaNi0.5Ti0.5O3. In relation to the LNT0.9Co0.1O,
although Ni/Ti/Co-O2 and Ni/Ti/Co-Oaverage are close to values reported for LaNi0.5Ti0.5O3,
Ni/Ti/Co-O1 bond distances are different to the corresponding calculated from the crystallographic
data of LaNi0.5Ti0.5O3.
4
+
2+
The larger difference between atomic radii of Ti (0.605 Å, [17]) and Co (0.745 Å, [17])
2
+
2+
in LNT0.9Co0.1O when compared to the corresponding from Ni (0.69 Å, [17]) and Co in the

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LN0.9Co0.1TiO could be related to the variation of the Ni/Ti/Co-O1 bond distances (Table S1). In
4
+
fact, the exchange of Ti from ordered DP La2NiTiO6 by a cation with significantly different
atomic radii can affect the titanium to oxygen bond distances. For example, Yang et al. [18]
4
+
4+
reported that after substitution of Ti from La2NiTiO6 by Mn (atomic radius: 0.530 Å, [18]) in 0.2
mol, the six Ti-O bond distances of La2NiTiO6 (ICSD: 186433) were different to the corresponding
of La2NiMn0.2Ti0.8O6 (ICSD: 186432).
2
+
According to the XRD patterns of Co -containing materials, the absence of segregated
2
+
4+
2+
phases points that an isomorphic partial substitution of either Ni or Ti from LNTO by Co in
0
.1 mol was achieved, which was confirmed by Rietveld refinement (Fig. S1a and S1b of
2
+
Supplementary Material). Therefore, Co atoms are included in the LNTO structure by partial
4
+
2+
substitution of either Ti or Ni . Moreover, all as-synthesized samples showed orthorhombic
symmetry and space group Pbnm, and for the LNTO a random type arrangement of B-cations
octahedra was obtained, according to the classification of DPs reported by Anderson et al. [4].
The evolution of the χmT values with temperature for the LNTO sample (Fig. S2 of
Supplementary Material) indicates an antiferromagnetic behavior [19,20], with Néel temperature
around 91 K, which agrees with the value of 100 K reported by Rodríguez et al. [20] for the same
material with orthorhombic symmetry and space group Pbnm. Therefore, magnetic measurements
corroborate the symmetry and space group of LNTO found by the Rietveld method.
Fig. 1 shows TPR profiles of the as-prepared samples performed from room temperature up
3
+
2+
to 1000 °C. Peaks at low temperature are related both to the reduction of Ni to Ni and the
3
+
reaction between hydrogen from the mixture and adsorbed oxygen. The presence of Ni species
and adsorbed oxygen was found from XPS studies reported by Yang et al. [21] for LaNi0.5Ti0.5O3
2
+
2+
perovskite. Peaks at high temperature are associated with the reduction of Ni and Co in the
0
0
0
2+
0
crystalline structure to either Ni -Co or Ni (for the Co -free DP). It is worth mentioning that Co
0
0
0
atoms were assumed to enter in the crystal lattice of Ni (ICSD: 53807), and thus forming a Ni -Co
alloy (Fig. 2a and 2b). The reduction proceeds by reaction between lattice oxygen, bonded with

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2
+
2+
0
0
either Ni or Co , and hydrogen from the mixture. Migration of either Ni -Co for cobalt-
0
containing materials or Ni for LNTO from the structure is coupled to segregation of A-site species,
like La2TiO5 and La2O3. These crystalline phases are in accordance with XRD patterns of the
materials either after TPR (Fig. 2a-2c) or after reduction before reaction (RBR) (Fig. S3a-S3c of
Supplementary Material), and with the reduction pathway reported by Arrivé et al. [22].
The isothermal period of TPR (Fig. 1) indicates different reduction behavior of each sample,
2
+
2+
0
0
0
as observed in the variation of the baseline. Ni and Co reduction degree to Ni -Co or Ni (in the
case of LNTO), related to hydrogen consumption from the peak at high temperature and the
isothermal period at 1000 °C, is shown in Table 1 for each sample. Reduction behavior for each
sample was established by calculating the reduction degree which directly depends on the area of
both the second reduction peak and the isothermal period. The integration method (IM), which
calculates the area from any region under the TPR profile, was used instead of the deconvolution
method for two reasons: (i) as shown in our last work for the case of the TPR of the La2NiTiO6
sample carried out with 10 % H2/Ar [14], the reduction degree obtained from the area calculated by
IM of the peak at 992 °C agrees with the reduction degree obtained from Rietveld refinement of the
XRD pattern of the same sample after the TPR peak at 992 °C, indicating that the IM is a valid
method to obtain the area of each TPR peak, and, (ii) considering that deconvolution method is
more appropriate to obtain a peak area, the IM is adequate to obtain the area under the curve for the
isothermal period because this area tends to be a rectangle rather than a peak. These values of
reduction degree (more than 100 %) indicate additional hydrogen consumption in relation to the
required quantity to reduce all the nickel and cobalt species contained in the samples. This increase
in the expected hydrogen uptake could be related to the reduction of NiO and CoO formed by
0
0
2+
oxidation of either Ni -Co with water vapor in the case of Co -containing perovskites or NiO from
0
oxidation of Ni for LNTO. The presence of steam through the bed is associated with the low gas
flow rates used for TPR [23], which could be favored by the large amount of sample used in this
analysis.

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Fig. 2 shows the Rietveld refinement of XRD profiles of the materials after TPR.
2
+
0
0
Concerning Co -containing perovskites, the calculated XRD profile of Ni -Co was obtained by
0
modifying the crystallographic information file of Ni (ICSD: 53807) with occupation number
0
0
2+
2+
values for Ni and Co proportional to Ni and Co content in the precursor.
0
0
0
After TPR, mass content of La2TiO5, Ni -Co (or Ni in the case of LNTO), La2O3 and the
non-stoichiometric precursor are different for each sample, as shown in Table 1. The reduction
2
+
2+
0
0
2+
0
degree of Ni and Co to Ni -Co or Ni to Ni (for the case of LNTO) of the samples after TPR
was 80.8 %, 86.7 % and 97.1 % for LNT0.9Co0.1O, LN0.9Co0.1TO and LNTO, respectively (Table 1).
This is according to the reduction degree of the same cations for each perovskite after RBR (Table
S2 of supplementary data). Thus, the reduction of the LNTO is modified by substitution of either
2
+
4+
2+
0
0
0
Ni or Ti by Co . Also, the average crystallite sizes of Ni -Co or Ni obtained after TPR (42.3-
3
1.1 nm, Table 1) are in accordance with the values of Ni crystallites sizes in the range 5-50 nm
obtained by reduction of the La0.5Sr0.5Ti0.75Ni0.25O3-δ perovskite [22].
The activity of these materials for steam reforming of methane (CH4 + H2O  3H2 + CO)
was investigated, and the results are displayed in Fig. 3. LN0.9Co0.1TO and LNTO presented similar
o
methane conversions over the whole temperature range, with activity from 550 C. On the other
o
hand, LNT0.9Co0.1O showed much lower activity, with initial methane conversion only at 750 C.
The activity of LN Co TO and LNTO catalysts can be related to their similar reduction degree
0
.9
0.1
2
+
2+
after RBR (87.7 % and around 100 % for the perovskite where Ni was substituted by Co and the
unsubstituted perovskite, respectively, Table S2 of Supplementary Material). On the other hand, the
low activity of LNT0.9Co0.1O can be attributed to the low reduction degree after RBR (39.1 %,
Table S2 of Supplementary Material). It is worth noting that the reduction degrees for the
perovskites after RBR can be correlated to those observed after TPR, with the same tendency (Table
1
). The performance of each catalyst for the reaction can also be attributed to metal-support
0
0
0
interaction, since Ni -Co from LN0.9Co0.1TO or Ni from LNTO are mainly supported on La2TiO5,
0
0
while Ni -Co sites are principally supported on LNT0.9Co0.1O non-s and La2O3. Related to the

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LNT0.9Co0.1O, similar activity was found when compared to La2NiTiO6 reduced with 1.8 % H2/Ar,
whose performance was attributed to the reduction degree equal to 37.6 %, being the Ni sites
supported on non-stoichiometric La2NiTiO6 and La2O3 [14]. The stability of these catalysts during
o
o
heating (from 450 to 950 C) and cooling (from 950 to 450 C) cycle is reported on Supplementary
Material. H2/CO ratio close to stoichiometric value of 3 was obtained for all three catalysts at
o
temperatures higher than 850 C. At lower temperatures the H2/CO ratio is higher than that expected
for steam reforming of methane, due to the influence of shift reaction (CO + H2O  CO2 + H2),
which is favored at low temperatures [24].
4
. Conclusions
La2NiTi0.9Co0.1O6, La2Ni0.9Co0.1TiO6 and La2NiTiO6 were synthesized by modified Pechini
method. Rietveld refinement of XRD patterns of the samples indicated orthorhombic symmetry and
space group Pbnm for all as-synthesized samples, as well as the isomorphic partial substitution of
2
+
4+
2+
Ni or Ti from La2NiTiO6 by Co , and the random ordering of B-cations of the La2NiTiO6. Also,
magnetic measurements corroborate the symmetry and space group of the double perovskite where
2
+
4+
B-cations are Ni and Ti . Quantitative phase analysis using the Rietveld refinement of XRD
2
+
4+
2+
patterns of the perovskites after TPR showed that partial substitution of Ni or Ti by Co
2
+
modified the reduction of the Co -free material. Moreover, La2Ni0.9Co0.1TiO6 and La2NiTiO6
presented higher catalytic activity for steam reforming of methane at temperatures between 450-750
°C. Furthermore, a higher onset temperature of 750 °C was observed for the reaction catalyzed with
La2NiTi0.9Co0.1O6. The performance for the steam reforming of methane of all catalysts was
correlated with the reduction degree and the metal-support interaction.
Acknowledgments

ACCEPTED MANUSCRIPT
The authors thank Carlos André de Castro Perez from NUCAT/PEQ/COPPE/UFRJ for
suggestions of Rietveld analysis. Also, they are grateful to Professor Alexandre Novak and Jilder
Peña Serna from Laboratory of Low Temperatures at Physics Institute of the Federal University of
Rio de Janeiro and Geraldo Cernicchiaro from Brazilian Center for Research in Physics for the
magnetic measurements and data treatment. Pablo V. Tuza acknowledges to Secretaría de
Educación Superior, Ciencia, Tecnología e Innovación SENESCYT-Ecuador for the financial
support of this work.
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operating conditions for production of hydrogen in three different reforming technologies, J. Power
Sources 108 (2002) 213–225.

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2
+
2+
0
0
2+
0
Table 1. Hydrogen uptake, QPA, reduction degree of Ni and Co to Ni -Co or Ni to Ni
(LNTO) and average crystallite size of the prepared samples after TPR.
Reduction
degree (%)
c
0
0
L
(
Ni -Co
H
2
uptake
Sample
Crystalline phases
QPA (%)
nm)
-
1
a
b
(
mol H
2
g )
TPR
QPA
d
-3
d
LNT0.9Co0.1
O
1.249 x 10
La
Ni -Co
La
2
TiO
5
22.60(0.65)
10.88(0.54)
18.82(0.54)
54.7
67.3
80.8
86.7
97.1
41
e
-3
0
0
e
1.538 x 10
2
O
3
La
2
NiTi0.9Co0.1
O
6
non-s 47.70(1.03)
d
e
-4
d
e
LN0.9Co0.1TO
9.219 x 10
1
La
Ni -Co
La
2
TiO
5
53.04(1.18)
10.59(0.52)
4.24(0.39)
44.3
56.0
42.3
31.1
-3
0
0
.165 x 10
2
O
3
La
2
Ni0.9Co0.1TiO
6
non-s 32.13(0.86)
d
e
-4
d
LNTO
8.962 x 10
1
La
Ni
La
NiTiO
2
T0iO
5
63.38(1.21)
11.87(0.53)
1.82(0.26)
43
-3
e
.376 x 10
66.1
2
O
3
La
2
6
non-s
22.93(0.78)
a
b
Reduction degree calculated from TPR results and QPA obtained by means of Rietveld
refinement.
c
0
0
0
In the case of LNTO, the average crystallite size LNi -Co is changed by LNi .
d
The hydrogen uptake and the reduction degree are calculated by integration of the second reduction
e
peak (Fig. 2a) and total isothermal period at 1000 °C (Fig. 2b) from TPR profiles.
The numbers in parentheses represent the standard deviation of the respective value.

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Figure Captions
Figure 1. TPR of the as-synthesized materials from room temperature up to 1000 °C (the isothermal
period at 1000 °C for 330 min is shown in the upper part).
Figure 2. Observed (orange symbols), calculated (black line) and difference (blue line) XRD
profiles of (a) LNT0.9Co0.1O, (b) LN0.9Co0.1TO and (c) LNTO after TPR. Vertical green bars
indicate reflections of each crystalline phase.
Figure 3. (a) Methane conversion and (b) H2/CO molar ratio evolution during steam reforming of
methane over (■) LNT0.9Co0.1O, (○) LN0.9Co0.1TO and (▲) LNTO.

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Figure 1
9
9
30 C LNT0.9Co0.1
87 C LN0.9Co0.1TO
92 C LNTO
O
9
Isothermal
200
0
50
100
150
250
300
350
400
Time (min)
409 C
930 C
417 C
987 C
LNT0.9Co0.1
O
400 C
992C
LN0.9Co0.1TO
LNTO
0
200
400
600
800 1000 1200Iso 1t h4 e0 r 0m al1600 1800
Temperature (C)

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Figure 2

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Figure 3

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Graphical abstract

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Highlight
4
+
2+
2+

Ti or Ni from La2NiTiO6 double perovskite was partially substituted by Co .
2
+



Co was included in the crystal structure of La2NiTiO6.
After reduction at 1000°C the perovskite structure is partially decomposed.
Activity on steamreforming of methane depends on reduction degree.