Microstructural characterization of Ruddlesden-Popper ... - cnpem

Microstructural characterization of Ruddlesden-Popper nickelates for use in catalysis
Pimentel, P. M., 1 Oliveira, R. M. P. B., 1 Oliveira, F. S., 1 Melo, D. M. A., 1 and Melo,M.A.F. 1
INTRODUCTION
Several catalysts containing noble metals supported (Rh,
Pt, Ru, Pd) have shown high conversion and high selectivity
for production of syngas (mixture of CO and H2) [1, 2].
However, the high cost of noble metals has motivated the development
of nickel catalysts for industrial use and one of
the challenges is to make it more resistant to carbon deposition.
The most widely used commercial catalyst is nickel
supported on alumina, with or without promoters. In recent
years, much attention has been given to the study of oxide
systems with perovskite structure and its application as catalysts,
mainly oxides, which presents the Ruddelsden-Popper
(RP) phase. The RP phases with general formula of A2BO4
consist of n ABO3 perovskite blocks, separated by a rock saltlike
layer of composition AO. These materials are usually obtained
through routes which make use of high pressures and
high temperatures [3]. The aim of this work was to characterize
the microstructure of the RP phases of nickelates in order
to the understanding of their catalytic properties.
EXPERIMENT
The systems were synthesized by using gelatin powder
(GELITA R○ ) as organic precursor and metallic nitrates as
starting materials. Gelatin was added to a becker containing
deionized water under stirring for 30 minutes at 50 o C.
M(NO3)2.6H2O (M = Ni, Nd or Sm, Sr) were added to the
solution at 70 o C for some minutes. The temperature was
slowly increased to 90 o C and the solution was stirred on
a hot plate until a gel formed. The gel was then calcined
at 350 o C for 2 hours. This resulted in a precursor powder,
which was calcined at 900 o C for 4 hours. The system containing
neodymium was calcinated at 900 o C for 6 hours to
evaluate the behavior of the structure. The X-ray patterns
were acquired from samples calcined at different temperatures.
Measurements were recorded on a Shimadzu diffractometer
model XRD-6000 with monochromatic radiation of
CuK α (λ = 1.5406 Å). The crystallites size was obtained
from the Scherrer equation. The FEG-SEM images of the
powders were examined in a JEOL JSM 6330F microscopy
at the LNLS - National Synchrotron Light Source (Campinas-
Brazil).
RESULTS AND DISCUSSION
Figure 1 shows the XRD patterns of the Nd2−xSrxNiO4
(0, 0.4, 1.2) powders. A comparison with ICDD database
1 Universidade Federal do Rio Grande do Norte - Natal RN Brazil
revealed Nd1.6Sr0.4NiO4 (PDF 80-2324) and Nd0.8Sr1.2NiO4
(PDF 80-2326) RP phases as being tetragonal symmetry. In
addition were observed peaks belongs to Nd2O3 (PDF 83-
1346) and NiO (PDF 73-1523) phases. Figure 2 shows the
XRD patterns of the Sm2−xSrxNiO4 (0, 0.4, 1.2) powders.
Peaks indexation led to the identification of the the Sm1.6Sr
0.4NiO4 (PDF 88-0116) and Sm0.8Sr1.2NiO4 (PDF 88-0121)
as being orthorhombic and tetragonal symmetry, respectively.
The Sm2O3 (PDF 15-0813) and NiO phases were also observed.
For systems containing Nd and Sm, the RP phases
increase with the substitution of Ln 3+ ions by the Sr 2+ ions.
It is also observed that the crystallinity increases with the calcination
temperature. The average crystallite sizes for the materials
calcinated for 4 hours ranged from 31 to 37 nm for
nickelates of samarium and 37 to 43 nm for neodymium nickelates.
The powder calcinated at 900 o C for six hours had
crystallite size of 73.93 nm.
FIG. 1: XRD pattern of Nd2−xSrxNiO4(x= 0, 0.4, 1.2) powders
The images obtained by FEG-SEM for the Nd2NiO4,
LNLS 2009 Activity Report 1 Brazilian Synchrotron Light Laboratory

FIG. 2: XRD pattern of Sm2−xSrxNiO4 (x= 0, 0.4, 1.2) powders
Nd0.8Sr1.2NiO4 and Sm0.8Sr1.2NiO4 samples are shown in
Fig. 3 and 4, respectively. As can be seen, the particles
have a round shape, uniform distribution and do not show noticeable
agglomeration. The porous surface of the material is
caused by the evolution of high gas content during synthesis
and powder production. The gelatin provides the system with
a large amount of organic matter, which during calcination is
removed and favors the appearance of pores in the material.
The materials calcinated for 4 h showed nanometric particle
size, but in the material calcined for 6 h, there was a significant
increase in the particle size.
FIG. 3: SEM-FEG images of a) Nd2NiO4 calcinated for 6 h and b)
Nd1.6Sr0.4NiO4 calcinated for 4 h
FIG. 4: SEM-FEG images of Sm0.8Sr 1.2NiO4 calcinated for 4 h
CONCLUSION
In this work, perovskite type RP was obtained successfully
through a simple and fast method and makes use of lower temperatures
and pressures compared to other work in literature.
In addition, gelatin is a low-cost and non-toxic organic precursor,
making it a promising alternative to the usual synthesis
methods. Powders produced by using this method were
nanometric, homogeneous and porous. These characteristics
are important for technological application such as catalysts.
ACKNOWLEDGEMENTS
The authors are grateful to CNPq for the financial support,
GELITA R○ for supplying the gelatin and Brazilian Synchrotron
Light Laboratory (LNLS) for SEM-FEG images
(SEM-FEG-9042/2009).
[1] E. Boehm, J.M. Bassat, P. Dordor, F. Mauvy, J.C. Grenier, P.
Stevens, Solid State Ionics 176, 2717 (2005)
[2] G.Wu, J.J. Neumeier, M.F. Hundley, Phys. Rev. B 63, 1 (2001)
[3] M. Zinkevich , N. Solak, H. Nitsche, M. Ahrens, F. Aldinger.
Journal of Alloys and Compounds 438, 92 (2007)
LNLS 2009 Activity Report 2 Brazilian Synchrotron Light Laboratory