I. INTRODUCTION
Perovskites oxides are compounds with the formula ABO3, where A is a lanthanide or alkali earth metal and B is a transition metal (Mefford et al., Reference Mefford, Hardin, Dai, Johnston and Stevenson2014). They can be represented by B-cations placed in the center of octahedra connected through the vertices, creating octahedra layers and cavities between them. The octahedra vertices are occupied by oxygen atoms, while A cations are located at the center of the cavities.
Double perovskites (DPs), mostly represented by the formula A′A″B′B″O6, where the number of primes indicate distinct possible metal ions with the same amount (Anderson et al., Reference Anderson, Greenwood, Taylor and Poeppelmeier1993), present various physical and chemical properties (Anderson et al., Reference Anderson, Greenwood, Taylor and Poeppelmeier1993; Shaheen and Bashir, Reference Shaheen and Bashir2010; Ezzahi et al., Reference Ezzahi, Manoun, Ider, Bih, Benmokhtar, Azrour, Azdouz, Igartua and Lazor2011). B-cation sublattice types known for DPs are rock-salt, layered, and random (Anderson et al., Reference Anderson, Greenwood, Taylor and Poeppelmeier1993). The rock-salt and layered sublattices characterize ordered DPs. Random-like DPs can be assumed to be perovskites with equal number of two different B-cations. They have been applied as electrode materials for fuel cells (Deng et al., Reference Deng, Smit, Niu, Evans, Li, Xu, Claridge and Rosseinsky2009; Li et al., Reference Li, Wang, Zhao, Liu, He, Hu and Chen2011) or a catalyst for some reactions (Campbell, Reference Campbell1992; Huang et al., Reference Huang, Liang, Croft, Lehtimäki, Karppinen and Goodenough2009; Tuza and Souza, Reference Tuza and Souza2016).
Hydrogen is an important raw material for chemical industries and oil refineries. Because of environmental regulations, low quality of crude oil, and China and India's growing markets, the increase of H2 consumption is expected (Zohuri, Reference Zohuri2019). H2 is mainly produced by steam reforming of methane (Iulianelli et al., Reference Iulianelli, Liguori, Wilcox and Basile2016), which has been carried out over various catalysts, including perovskites. For instance, Ni0/La2O3, a catalyst obtained from LaNiO3 perovskite, showed the catalytic activity for steam reforming of methane. However, catalyst deactivation was observed due to coking. For instance, Provendier et al. (Reference Provendier, Petit and Kiennemann2001) prepared an LaNiO3 perovskite catalyst for steam reforming of methane. The authors observed catalyst deactivation and coke formation over the catalyst surface.
Titanium inclusion in LaNiO3 perovskite can mitigate coke deposition on Ni0/La2O3. For instance, Hayakawa et al. (Reference Hayakawa, Harihara, Andersen, Suzuki, Yasuda, Tsunoda, Hamakawa, York, Yoon, Shimizu and Takehira1997) synthesized Ni/Ca0.8Sr0.2TiO3 as a catalyst for CO2 reforming of CH4. Catalyst performance was related to Ni dispersion over the perovskite and strong interaction between nickel and perovskite containing titanium. Urasaki et al. (Reference Urasaki, Sekine, Kawabe, Kikuchi and Matsukata2005) used an Ni/SrTiO3 catalyst for steam reforming of methane. Hindering production of carbon species due to oxygen mobility in perovskite containing titanium was related to high catalytic activity of an Ni/SrTiO3 catalyst.
By substituting Ni3+ from LaNiO3 by Ti4+ such that equal amount of B-cations is attained, either the random (LaNi0.5Ti0.5O3, orthorhombic symmetry, and space group Pbnm (Rodríguez et al., Reference Rodríguez, Álvarez, López, Veiga and Pico1999)) or the rock-salt (La2NiTiO6, monoclinic symmetry, and space group P21/n (Rodríguez et al., Reference Rodríguez, López, Campo, Veiga and Pico2002; Pérez-Flores et al., Reference Pérez-Flores, Ritter, Pérez-Coll, Mather, García-Alvarado and Amador2011; Yang et al., Reference Yang, Liu, Lin and Chen2012)) sublattice was achieved. Ni0/La2TiO5, which is obtained after reduction of LaNi0.5Ti0.5O3 perovskite, showed both catalytic activity and stability for steam reforming of methane because of the metal–support interaction (Tuza and Souza, Reference Tuza and Souza2016).
Partial substitution of either NiO6 or TiO6 from double perovskite LaNi0.5Ti0.5O3 by CoO6, i.e., LaNi0.5Ti0.45Co0.05O3 and LaNi0.45Co0.05Ti0.5O3, and the corresponding influence on crystal structure, reductive behavior, and catalytic activity for steam reforming of methane were reported in a previous work (Tuza and Souza, Reference Tuza and Souza2017). These properties were completely different for LaNi0.5Ti0.45Co0.05O3 when compared with the other materials, which was attributed to different metal–support interaction between Ni0 (for the LaNi0.5Ti0.5O3) or Ni0–Co0 (for cobalt-containing perovskites) and the corresponding supports, as confirmed from quantitative phase analysis for these perovskites after reduction with 10% H2/N2. It is worth noting that LaNi0.5Ti0.5O3 can be a double perovskite with the random ordering of B-cations (Anderson et al., Reference Anderson, Greenwood, Taylor and Poeppelmeier1993). Moreover, only the catalyst obtained from the perovskite synthesized by partial substitution of Ni2+ from LaNi0.5Ti0.5O3 by Co2+ in 0.05 mol did not present catalytic stability for the investigated reaction.
In this work, we report powder X-ray diffraction (XRD) data for LaNi0.5Ti0.45Co0.05O3, LaNi0.45Co0.05Ti0.5O3, and LaNi0.5Ti0.5O3 materials, which were published in the Powder Diffraction File™ with the PDF numbers 00-69-0417, 00-69-0418, and 00-69-0419, respectively.
II. MATERIALS AND METHODS
A. Synthesis
LaNi0.5Ti0.45Co0.05O3 and LaNi0.45Co0.05Ti0.5O3 materials were prepared by the modified Pechini method and reported in a previous work (Tuza and Souza, Reference Tuza and Souza2016, Reference Tuza and Souza2017). Briefly, adequate amounts of precursors (lanthanum nitrate hexahydrate (La(NiO3)3·6H2O, 99.5%), nickel nitrate hexahydrate (Ni(NiO3)2·6H2O, 97%), cobalt nitrate hexahydrate (Co(NiO3)2·6H2O, 100%), and titanium isopropoxide (C12H28O4Ti, 97%) from Sigma-Aldrich) were dissolved in water. Metal citrates were prepared by heating metal solutions to 60 °C, to which citric acid was added. The polyester solution was achieved after adding ethylene glycol at 90 °C to a solution composed of lanthanum, nickel, cobalt, and titanium citrates. The resin obtained by evaporation of the polyester solution was calcined at 240 °C for 1 h and 450 °C for 4 h. The material was milled and then calcined in air at 800 °C for 17 h. Moreover, LaNi0.5Ti0.5O3 perovskite was synthesized by using the same method. The perovskites were obtained in the form of black powders.
B. Characterization
The chemical composition of the as-prepared materials was determined by X-ray fluorescence analysis, which was carried out using a Rigaku Primini Spectrometer equipped with a Pd X-ray tube operating at 50 W (40 kV, 1.25 mA), and a ZSX software package. A sample amount around 200 mg was packed on a polyethylene sample cup, covered with polypropylene thin-film, and then fixed with a ring. Chemical composition was expressed on the weight percentage basis of the cation amount for each sample. Sample images were recorded by means of scanning electron microscopy (Model Quanta™ 450 FEG, FEI Co) operating with an accelerating voltage of 20 kV. Previously, samples were sputter-coated with a gold-based layer for enhancing image quality.
Powder XRD patterns were recorded at room temperature using the Rigaku Miniflex II X-ray diffractometer equipped with a graphite monochromator and CuKα radiation (30 kV and 15 mA), 2θ range 5–90°, step size and counting time per step equal to 0.02° and 6 s (for LaNi0.5Ti0.5O3 perovskite) or 0.05° and 1 s (for cobalt-containing perovskites). A mass amount approximately equal to 300 mg was packed on a quartz specimen holder as a thin layer of powdered compound. The d-values were calculated using CuKα radiation.
The Rietveld method of powder XRD patterns was performed using the Fullprof Program (Rodríguez-Carvajal, Reference Rodríguez-Carvajal1993). The structural model of LaNi0.5Ti0.5O3 perovskite (space group Pbnm) (ICSD, 2017) was used as a starting point to refine the corresponding structures for the as-synthesized samples. The background (fourth-degree polynomial), the scale factor, the unit-cell parameters, three halfwidth parameters, La1 at (x, y, 0.25), O1 at (x, y, z), and O2 at (y, 0.25) atomic coordinates, and the sample displacement, Sycos, were refined. Sycos is equal to the ratio between a correction parameter attributed to sample displacement error, in degrees, and cosine of θ angle, where θ is measured in radians (Rodríguez-Carvajal, Reference Rodríguez-Carvajal2001). Since a satisfactory fit could not be achieved, “x” atomic coordinate for O2 was not refined. The isotropic displacement parameter was maintained at 0.5 Å2, as suggested for atoms in a metal oxide (Attfield et al., Reference Attfield, Barnes, Cockcroft and Driessen2004). Peak shapes of powder XRD patterns were described using the pseudo-Voigt function. The fraction of site occupancy for all elements was not refined. To achieve the electroneutrality for LaNi0.5Ti0.45Co0.05O3 perovskite, the fraction of site occupancy for B-cations was divided and fixed into four parts: 0.225 for Ni2+ or Ti4+ and 0.025 for Ni3+ or Co3+. For LaNi0.45Co0.05Ti0.5O3 perovskite, this value was divided and fixed into three parts: 0.25, 0.225, and 0.025 for Ti4+, Ni2+, and Co2+, respectively. For LaNi0.5Ti0.5O3 perovskite, the fraction of site occupancy for B-cations was equal and fixed to 0.25 for Ti4+ and Ni2+ (ICSD, 2017).
III. RESULTS AND DISCUSSION
Chemical composition (weight percentage basis) obtained from X-ray fluorescence, unit-cell parameters, X-ray densities, atomic coordinates, B-cation to metal bond distances, and agreement factors from Rietveld refinement of powder XRD data for as-synthesized materials are indicated in Table I.
Chemical composition determined from X-ray fluorescence, Sycos, X-ray density, atomic coordinates, unit-cell parameters, bond lengths, bonding angles, and conventional discrepancy factors from Rietveld refinement of powder XRD data for LaNi0.5Ti0.45Co0.05O3, LaNi0.45Co0.05Ti0.5O3, and LaNi0.5Ti0.5O3.
Theoretical chemical composition of La (wt%)/Ni (wt%)/Ti (wt%)/Co (wt%).
a 72.07%/15.22%/11.18%/1.53%.
b 72.27%/13.74%/12.45%/1.54%.
c 72.28%/15.27%/12.45%/0.
d Reference: ICSD (2017).
e Bond lengths (Å), in which B is equal to Ni/Ti or Ni/Ti/Co, according to B-cations.
f Bond lengths (Å), in which B is equal to Ni/Ti or Ni/Ti/Co, according to B-cations.
g Bonding angles (°), in which B is equal to Ni/Ti or Ni/Ti/Co, according to B-cations.
h Bonding angles (°), in which B is equal to Ni/Ti or Ni/Ti/Co, according to B-cations
The chemical composition was close to their corresponding nominal values, whose variation was attributed to experimental error. Therefore, it supports the expected composition. Experimental, calculated, and difference XRD patterns are shown in Figure 1. Impurity phases were not detected in XRD patterns of the as-synthesized materials. The unit-cell parameters for as-synthesized materials are similar to the respective for LaNi0.5Ti0.5O3 (ICSD, 2017). X-ray density for LaNi0.5Ti0.5O3 perovskite is similar to the reported literature value (6.643 vs. 6.631 g cm−3; Table I). For LaNi0.5Ti0.45Co0.05O3 (6.685 vs. 6.631 g cm−3; Table I) and LaNi0.45Co0.05Ti0.5O3 (6.661 vs. 6.631 g cm−3; Table I), the variations of X-ray density are expected due to chemical composition. Metal-to-oxygen bond distances and bonding angles are close to values reported in the literature, except the B–O1 and B–O2 bond distances for the perovskite obtained from the partial substitution of Ti4+ by Co2+ in 0.05 mol. These results are attributed to different values of metal-to-oxygen bond distances for B-cations with two different valences (Ni2+, Ni3+, Ti4+, and Co3+) assumed for the achievement of electroneutrality. Since R factors from Rietveld refinement can be improved by increasing the counting time per step and decreasing the step size of powder XRD data acquisition, agreement factors were acceptable for the used experimental conditions. It is worth mentioning that we reported magnetic measurements for LaNi0.5Ti0.5O3 perovskite and temperature-programmed reduction for the as-prepared materials in a previous work (Tuza and Souza, Reference Tuza and Souza2017). Magnetic measurements support the assumed symmetry for LaNi0.5Ti0.5O3 perovskite. Also, LaNi0.5Ti0.45Co0.05O3 is difficult to reduce, when compared with the other as-synthesized perovskites, which can be attributed to some nickel and all the cobalt with a valence equal to 3+. Therefore, the symmetry of as-prepared perovskites is orthorhombic, with space group Pbnm, and glazer notation a−a−c+ (Martin and Parise, Reference Martin and Parise2008; Fowlie, Reference Fowlie2019). The powder diffraction data of as-synthesized perovskites are provided in Tables II–IV. The indexation of the powder XRD pattern from each sample was carried out using the McMaille program (Le Bail, Reference Le Bail2004). Moreover, the crystal structure of the as-prepared samples is indicated in Figure 1, which was drawn using VESTA software (Momma and Izumi, Reference Momma and Izumi2011).
Observed (red symbols), calculated (black line), and difference (blue line) XRD profiles for (a) LaNi0.5Ti0.45Co0.05O3, (c) LaNi0.45Co0.05Ti0.5O3, and (e) LaNi0.5Ti0.5O3. Crystal structure of (b) LaNi0.5Ti0.45Co0.05O3, (d) LaNi0.45Co0.05Ti0.5O3, and (f) LaNi0.5Ti0.5O3.
Powder diffraction data of LaNi0.5Ti0.45Co0.05O3.
F(20) = 16.13 (0.0090, 138) (Smith and Snyder, Reference Smith and Snyder1979).
a Corrected observed peak positions.
b Peak positions calculated using the McMaille program.
c Peak positions calculated using the Fullprof program.
d Interplanar distance calculated using the Fullprof program.
e Δ2θ (°) = a2θ obs (°) –b2θ calc (°).
Powder diffraction data of LaNi0.45Co0.05Ti0.5O3.
F(20) = 19.87 (0.0082, 123) (Smith and Snyder, Reference Smith and Snyder1979).
a Corrected observed peak positions.
b Peak positions calculated using the McMaille program.
c Peak positions calculated using the Fullprof program.
d Interplanar distance calculated using the Fullprof program.
e Δ2θ (°) = a2θ obs (°) –b2θ calc (°).
Powder diffraction data of LaNi0.5Ti0.5O3.
F(20) = 17.50 (0.0085, 134) (Smith and Snyder, Reference Smith and Snyder1979)
a Corrected observed peak positions.
b Peak positions calculated using the McMaille program.
c Peak positions calculated using the Fullprof program.
d Interplanar distance calculated using the Fullprof program.
e Δ2θ (°) = a2θ obs (°) –b2θ calc (°).
A representative FEG-SEM image of LaNi0.5Ti0.5O3 is shown in Supplementary Figure S1. Related to this perovskite, grains are composing elongated and spherical particles at the range 15.5–128.1 nm, with mean particle size equal to 57.3 nm. Grain population with particle size less than 100 nm was equal to 87.3%. This result is in accordance with the average crystallite size determined by the Scherrer equation (27.1 nm; Table I). It is worth noting that the average crystallite size for LaNi0.5Ti0.45Co0.05O3 (35.8 nm; Table I) and LaNi0.45Co0.05Ti0.5O3 (32.7 nm; Table I) was similar to the corresponding for LaNi0.5Ti0.5O3. As the synthesis method was the same for all the perovskites, i.e., the same calcination temperature was employed to synthesize all the perovskites, we concluded that LaNi0.5Ti0.45Co0.05O3, LaNi0.45Co0.05Ti0.5O3, and LaNi0.5Ti0.5O3 materials are composed mainly by nanoparticles.
IV. CONCLUSIONS
LaNi0.5Ti0.45Co0.05O3, LaNi0.45Co0.05Ti0.5O3, and LaNi0.5Ti0.5O3 perovskites were synthesized by the modified Pechini method. The chemical composition of these materials was close to nominal values, which indicated the appropriate application of the synthesis method. The as-prepared perovskites are composed largely by nanoparticles, with orthorhombic crystal structure, space group Pbnm, and glazer notation a−a−c+. XRD data affirmed the single-phase of each as-prepared material.
V. DEPOSITED DATA
CIF files with information related to as-prepared perovskites, RAW, and DAT files with XRD data of these materials were deposited with the ICDD. You may request this data from ICDD at info@icdd.com.
SUPPLEMENTARY MATERIAL
The supplementary material for this article can be found at https://doi.org/10.1017/S0885715620000767.
ACKNOWLEDGEMENTS
The authors thank Carlos André de Castro Perez from Instituto Federal de Ciência e Tecnologia do Rio de Janeiro – IFRJ, Brazil, for Rietveld analysis suggestions. Also, they are grateful to Andréa Maria Duarte de Farias and Francisco Luiz Correa Rangel from Nanotechnology Characterization Center (CENANO)/National Institute of Technology, Brazil for SEM images used to determine the particle size of as-synthesized perovskites.
CONFLICTS OF INTEREST
The authors have no conflicts of interest to declare.
