Effect of calcination temperature on structural properties and catalytic activity in oxidation reactions of LaNiO3 perovskite prepared by Pechini method

JOURNAL OF RARE EARTHS, Vol. 30, No. 3, Mar. 2012, P. 210

Effect of calcination temperature on structural properties and catalytic activity in oxidation reactions of LaNiO3 perovskite prepared by Pechini method K. Rida1, M.A. Peña2, E. Sastre2, A. Martínez-Arias2 (1. Laboratory for the Study of Interactions Between Materials and Their Environment, University of Jijel, Algeria; 2. Institute of Catalysis and Petroleum-Chemistry, CSIC , C/Marie Curie 2, Campus de Cantoblanco, 28049 Madrid, Spain) Received 15 July 2011; revised 28 September 2011

Abstract: The study presented the preparation of the perovskite oxide LaNiO3 by the complex citrate method, paying particular attention to evolution of its formation from the amorphous precursor with varied calcination temperatures. The products obtained after heat treatment under air between 200 and 800 °C were characterized by X-ray diffraction (XRD), thermogravimetric and differential thermal analysis (TG-DTA), Fourier transform infrared spectroscopy (FTIR), SBET measurements and X-ray photoelectron spectroscopy (XPS). The results showed the formation of a single phase with perovskite structure from ca. 550 °C. Tests on the two catalytic oxidation reactions of C3H6 and CO over the system calcined between mentioned temperatures were examined on the basis of characterization results and showed that optimum catalytic properties for such reactions were achieved for the perovskite calcined at 600 °C. In turn, correlations between redox and catalytic properties were established on the basis of thermogravimetric temperature programmed reduction (TPR) analysis. Keywords: LaNiO3; perovskite; Pechini; SOFC; C3H6 and CO combustion; rare earths

Perovskite mixed oxides with the general formula ABO3 containing both rare earth elements and 3d transition metals have received considerable attention in recent years. These can be considered as strategic materials as a consequence of their interesting electrical, magnetic, optical and catalytic properties[1–6]. Among these materials, lanthanum nickel oxide LaNiO3 with perovskite structure is considered of great interest because of its electronic and catalytic properties which make it a promising base material for its use as electrode material for storage and conversion of energy or electrolytic synthesis[7–10], as well as a catalyst for the methane reforming reaction[11–14], for redox reactions involving NO, CO or soot[15,16] or for VOC’s combustion reactions[17,18]. Within this context, as already examined for other perovskite oxides of this type[19], it is of relevance to examine preparation parameters of perovskite lanthanum nickelate that could provide optimum catalytic properties in correlation with analysis of structural, morphological and electronic characteristics for this type of compound. For this purpose, LaNiO3 was synthesized by a modified sol-gel method (using the Pechini method[20,21]). This technique allows the preparation of crystalline materials with excellent control of its stoichiometry and homogeneous composition along with relatively large specific surface area after thermal treatment[22]. The effect of calcination temperature on the formation of the LaNiO3 oxide was studied to establish correlations between the catalytic activity in two combustion reactions (of C3H6 and CO) and morphological/structural/electronic prop-

erties of the systems in this work.

1 Experimental 1.1 Catalyst preparation A modified sol-gel (Pechini) method was employed to prepare the samples. Powders of La(NO3)·6H2O (Aldrich, 99.99% purity) and Ni(NO3)2·6H2O (Aldrich, 99.0% purity) were weighed to achieve equimolar amounts of Ni and La and dissolved in a small amount of distilled water. Citric acid (from Aldrich, 99.5% purity) was used to prepare the gel. Gelation was induced by heating the solution at 90 °C; this temperature was maintained for 10 h, after which a dried powder precursor was obtained. Thus obtained portions of such precursor were then fired in a muffle oven at 200, 400, 600 and 750 °C for 5 h under air, using a heating rate of 10 °C/min. 1.2 Catalyst characterization Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) of the precursor decomposition were performed on a Perkin-Elmer TGA7 and a Perkin-Elmer DTA7 device, respectively, from 25 to 950 °C at a heating rate of 10 °C/min and under an air flow of ca. 60 ml/min. Fourier transform infrared (FTIR) spectra of the samples were recorded in the 4000–400 cm–1 range with a Perkin-Elmer 1730 FT-IR, using the KBr pellet technique

Foundation item: Project supported by Comunidad de Madrid Project (DIVERCEL S2009/ENE-1475) Corresponding authors: K. Rida, A. Martínez-Arias (E-mail: [email protected], [email protected]; Tel.: +34-915854940) DOI: 10.1016/S1002-0721(12)60025-8

K. Rida et al., Effect of calcination temperature on the structural properties and catalytic activity in oxidation reactions of …

(about 1 mg of sample and 300 mg of KBr were used for the preparation of the pellets). Powder XRD patterns were recorded in the 10°–80° 2 range in the scan mode (0.02° step size, 2 s counting time) using a D8-Advance de Bruker-AXS powder diffractometer employing Cu K radiation. The unit cell parameters were obtained by refining the peak positions of the XRD pattern with a least square method using the CELREF program (unit-cell refinement software). For the determination of peak positions, the peak profiles were fitted with the WINPLOTR program. The crystallite size was calculated from the full width at half maximum of the most intense diffraction peaks using Scherrer’s equation. The specific surface area of the samples was determined by applying the BET method to nitrogen adsorption isotherms recorded at –196 °C, using a micromeritics apparatus model ASAP-2000. Prior to adsorption, the samples were degassed for 2 h at 180 °C. X-ray photoelectron spectroscopy studies were performed with a VG Escalab 200R spectrometer employing a Al K (1486.6 eV) X-ray source. The sample was first placed in a stainless steel holder mounted on a sample-rod in the pretreatement chamber of the spectrometer and then outgassed (ca. 1.33×10–3 Pa) at room temperature for 1 h before being transferred to the analysis chamber. A selected region of the XP spectrum (La 3d, Ni 2p, O 1s and C 1s) was then scanned for a determinate number of times such as to obtain a good signal to noise ratio. The binding energies (BE) were referenced to the spurious C1s peak (284.6 eV) used as internal standard to take into account charging effects. The areas of the peaks were computed by fitting the experimental spectra to Gaussian/Lorentzian curves after removal of the background (Shirley function). Surface atom ratios were calculated from peak area ratios normalized by the corresponding atomic sensitivity factors[23]. Thermogravimetric analysis for the study of the redox behaviour of the sample calcined at different temperatures was carried out using a Mettler Toledo TGA/SDTA 851e equipment, with 200 ml/min of N2 as carrier gas, 50 cm3 /min of hydrogen (temperature programmed reduction - TPR - tests) or oxygen (air treatment) as reactive gas, and a heating rate of 10 qC/min.

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products of the reaction were analyzed by infrared spectroscopy using a Perkin-Elmer 1725X FTIR spectrometer fitted with a multiple reflection transmission cell for gas analysis (infrared analysis). Propene bands appearing in the 3200–2700 cm–1 range and carbon dioxide bands in the 2400–2200 or 750–600 cm–1 range (depending on the degree of saturation of the most intense former ones) and the band of CO in the range of 2250–2000 cm–1 were employed to determine conversion levels. In all cases, the samples were heated to the measurement temperature under the reaction atmosphere with a ramp of 5 °C/min and stabilized for 45 min prior to analysis, in order to ensure stationary conditions.

2 Results and discussion 2.1 Analysis of thermal decomposition of the precursor complex Fig. 1 shows the TG-DTA curves for the decomposition of the solid precursor obtained at 90 °C. The decomposition reaction apparently takes place in three steps, the system becoming fairly stabilized above ca. 650 °C. The first part (I) between 20 and 150 °C corresponds to a mass loss of about 23%, with an inflection at a temperature of ca. 100 °C, related to a broadband endothermic process. This mainly corresponds most likely to desorption of adsorbed or hydration water that may remain in the precursor[24–26]. The second part (II) represents a significant mass loss (47%) which is produced between 150 and 300 °C, basically related to an exothermic process displaying a sharp shape with the maximum at 170 °C. The experimental mass loss (47%) seems close to the value expected if the metals were complexed with citrate species in the initial precursor. It can be accordingly concluded that this second stage is basically related to the decomposition of citrate in such complexes. The third part (III) represents a mass loss of 14% produced between 300 and 550 °C and involves two main exothermic processes at 375 and 500 °C. These processes most likely involve the decomposition of metal carbonate or carboxylate entities to yield the final oxide. Only a small DTA peak and a residual

1.3 Catalytic activity tests The catalysts were tested in propene (C3H6) and CO oxidation reactions at atmospheric pressure. Powder catalyst particles (obtained by sieving to 0.125–0.250 mm; 500 mg being employed in every test) were mixed homogeneously with SiC to obtain a total volume of 2 ml. The mixture was then loaded in a cylindrical Pyrex reactor tube (5 mm i.d.; ca. 10 mm catalyst bed height). The flow rate employed was in all cases of 40000 ml/(h·g) and a feed composition of 0.3% C3H6 and 5.4% O2 for the oxidation of propene, and 1% CO and 2% O2 for the oxidation of CO (volume percentages balanced with N2 employed as carrier gas) was employed (mass flow controllers being used for this purpose). The

Fig. 1 TG/DTA curves during heating under air of the powder precursor

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weight loss is detected at/above 500 ºC, suggesting that the LaNiO3 perovskite expected to be formed upon decomposition of the precursors becomes finally formed at that temperature, in agreement also with literature results[27]. It must be noted in this sense that expected further high temperature decomposition of LaNiO3 into Lan+1NinO3n+1+NiO does not apparently take place below 800 ºC, in agreement also with literature results[27–29]. The results interpretation is also consistent with FTIR and XRD data, as discussed below. 2.2 Infrared spectroscopy of the precursor calcined at selected temperatures On the basis of the TG-DTA results, different calcination temperatures were selected in order to isolate to a maximum possible different specific composition during lanthanum nickelate formation. Infrared spectra obtained for these samples are displayed in Fig. 2. The infrared spectrum obtained for the precursor sample (after drying at 90 ºC) appears basically constituted by bands related to the citric acid/citrate precursor, according to previous works[30–32]. Thus, a broad band appearing in the 3500–2500 cm–1 range identifies the presence of hydroxyl from citric acid or citrate, and/or from structural water. Weak bands at 3000–2800 cm–1 overlapped on the tail of the broad hydroxyl band are also characteristic of those species and related to C–H stretching modes. In turn, intense bands in the 1800–1500 cm–1 range along with a shoulder at 1350–1300 cm–1 are related to antisymmetric and symmetric stretching modes in the carboxylate groups, either protonated, as in citric acid, or not, as in citrate; the main difference in this sense is reflected by bands related to the antisymmetric stretching which appears at ca. 1710 cm–1 for the protonated carboxylate compared to ca. 1635 cm–1 in the citrate complex. The bands at 1500–1400 cm–1 indicate the presence of ionic nitrate, with a sharp band at 1385 cm–1 due to N–O stretching, and shoulders at ca. 1350 and 1430 cm–1 due to splitting of this vibration mode in coordinated nitrates, as well as less intense bands at 1100–1000 and ca. 850 cm–1[33–35]. Chemisorbed or structural water could contribute to bands at ca. 3300 and 1620 cm–1 [36]. Most of these bands apparently disappear for samples calcined at 200 ºC or above. Indeed, the spectrum of the sample calcined at 200 ºC only displays bands at ca. 1540 and 1395 cm–1 that can be related to the

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presence of some carboxylates, likely bidentated onto Ni3+ and La3+ cations[30], as well as ionic nitrate; these latter somewhat shift and decrease at 400 ºC, which could be related to the presence of some residual carbonate- or nitrate-type complexes at that temperature[37]. The infrared spectrum of samples calcined at 600 ºC or above shows that total decomposition of residues from the organic phase or nitrate precursors has been achieved while a significant increase in absorption bands below 650 cm–1, attributable to Ni–O stretching and O–Ni–O deformation in the LaNiO3 perovskite[30], is observed, being therefore indicative of the formation of this compound. 2.3 Structural XRD examination Fig. 3 shows the X-ray diffraction patterns of the samples calcined at selected temperatures. The results reveal the basically amorphous nature of the material after calcination at 400 °C, in correlation with incomplete decomposition of the organic phase, evidenced by FTIR data described above (Fig. 2). Indeed, most intense peaks are consistent with the presence of poorly crystalline lanthanum oxy-carbonate entities, according to previous investigation[27]. The features of the LaNiO3 perovskite-type phase with rhombohedral symmetry, space group R3m (corresponding lattice parameters are summarized in Table 1), appear in the XRD spectra when the annealing temperature is raised to 600 °C[22,29,30], in agreement with FTIR and TG-DTA results (Figs. 1 and 2). Such phase is also the only one apparently present for the sample calcined at 750 ºC, although a significant sample sin-

Fig. 3 X-ray diffractograms of the samples calcined at indicated temperatures Table 1 Structural characteristics extracted from analysis of the X-ray diffraction for the LaNiO3 perovskite phase and specific surface area of the samples determined from N2 adsorption isotherms as a function of the calcination temperature applied to the La-Ni oxide precursor

Fig. 2 FTIR spectra of the sample calcined at indicated temperatures

Calcination

Unit cell parameters/

Crystal

SBET/

temperature/°C

nm

size/nm

(m2/g)

400

–

–

9.0

600

a=0.5457, c=0.6549

11.7

11.8

750

a=0.5459, c=0.6513

22.9

6.3

K. Rida et al., Effect of calcination temperature on the structural properties and catalytic activity in oxidation reactions of …

tering is produced at this temperature[38], according to crystal sizes summarized in Table 1. This is also consistent with apparent decrease in specific surface area produced at this temperature (Table 1). 2.4 XPS surface analysis The surface characteristics of the samples calcined at selected temperatures were examined by XPS. A relevant atomic ratio value as well as binding energies determined from the fittings for the main peaks in La 3d, Ni 2p, O 1s and C 1s zones are collected in Table 2. The O 1s profile is basically constituted by two peaks at 528.2–529.1 eV and 530.9–531.1 eV, which can be attributed to lattice oxygen (O2) and surface oxygen species (in the form of carbonates or nitrates, hydroxyls or adsorbed water), respectively[18,34,39]. Similarly, basically two peaks are detected in the C 1s zone. A first one at around 284.5 eV can be attributed to surface contamination from atmospheric hydrocarbons while a second one around 288.8 eV is typical of carbonate species[18,39]. It must be considered in this sense that since La-based perovskites are basic materials, they can be easily carbonated or hydrated upon exposure to ambient atmosphere, which is also reflected in the mentioned O 1s components. In any case, the relatively higher contribution of the peak from carbonate species in the sample calcined at 400 ºC is in agreement with TG-DTA and infrared analysis above showing precursors decomposition has not been accomplished at that temperature yet. No significant differences were detected between the spectra observed for the samples in the La 3d zone. These displayed the typical two peaks of La 3d3/2 located at ca. 854.7 and 851.1 eV and those of La 3d5/2 at ca. 837.5 and 834.0 eV, close to those expected for La3+ ions in an oxidic environment[40,41]. Analysis of the Ni 2p profile results, however, is much more complex due to overlapping of Ni 2p3/2 and La 3d3/2 peaks[18]. The most intense Ni 2p3/2 peak appears at around 855.1 eV which is characteristic of Ni2+/Ni3+ ions in an oxygen environment[42]. This peak is accompanied by a satellite line positioned at ca. 7 eV higher binding energy BE (similarly, the Ni 2p1/2 component at ca. 872 eV displays a satellite peak at about 7 eV higher binding energy BE). As this satellite peak comes only from Ni2+ ions, and it can be concluded that the surface of all the perovskite oxides con-

213

tains a certain proportion of Ni2+ together with Ni3+ ions. We cannot however extract quantitative conclusions for the nickel components since afore-mentioned overlapping may mask not only the accurate measure of the BE of nickel but also its intensity (basically determined by considering corresponding intensity link between 2p1/2 and 2p3/2 components, the former being practically unaffected by overlapping with other peaks). In any case, analysis of Ni/La atomic ratio suggests an important surface enrichment in Ni for any of the three samples, apparently decreasing with the calcination temperature. 2.5 Redox behaviour Thermogravimetric TPR profiles of the sample calcined at different temperatures are displayed in Fig. 4. Before TPR, the sample calcined at 400 ºC was pre-treated at 400 ºC in air while samples calcined at 600 and 750 ºC were pre-treated at 600 ºC in air. It must be noted it is not possible an appropriate quantitative analysis of the perovskite calcined at 400 ºC since an important mass loss during the reduction must be ascribed to the decomposition of remaining carbonate and nitrate residues. Nevertheless, the start of the reduction for this sample (produced below 400 ºC) has been included in Fig. 4 in order to show that reduction of Ni3+/Ni2+ in this catalyst is hindered, probably due to the presence of mentioned carbonate or nitrate residues. In contrast, for the sample calcined at 600 ºC, nickel appears easily reduced in comparison with the sample calcined at 400 ºC as well as with that calcined at 750 ºC, for which a well crystallized perovskite was produced. Besides, the first step of reduction (Ni3+Ni2+) for the perovskite calcined at 600 ºC presents at least two stages, indicating the heterogeneity of the Ni3+ sites in this catalyst. On the other hand, it may be noted that quantification of the concentration of Ni2+ (detected by XPS) cannot be made in a precise way upon analysis of the TPR profiles, possibly due to its relatively low concentration in the bulk of the sample. Nevertheless, thermogravimetric thermal treatment of the calcined perovskites under air at high temperature provides information of the oxygen content. Thus, as shown in Fig. 5, after an initial mass loss due to decomposition of different surface carbonate and hydroxides up to 600 ºC

Table 2 XPS-derived surface characteristics and basic data for the main peaks detected in the spectra as a function of the calcination temperature applied Calcination

XPS binding energies of main peaks/eV

temperature/

(relative percentages in parentheses)

ºC

La 3d5/2

Ni 2p3/2

O 1s

C 1s

ratio

400

834.3

855.0

529.1 (28)

288.8 (69)

2.7

531.1 (72)

284.5 (31)

528.2 (21)

288.7 (47)

530.9 (79)

284.5 (53)

528.4 (20)

288.9 (55)

531.0 (80)

284.5 (45)

837.8 600

833.9

855.1

837.5 750

833.8 837.4

855.3

Ni/La atomic

2.4

2.2

Fig. 4 Thermogravimetric TPR profiles of the catalysts calcined at indicated temperature after pre-treatment under air at 400 ºC (catalyst calcined at 400 ºC) and 600 ºC (catalysts calcined at 600 or 750 ºC)

214

Fig. 5 Thermogravimetric profiles under air at high temperature of the perovskite calcined at indicated temperature

(not shown in this figure but in accordance to TG/DTA analysis, Fig. 1), the perovskite calcined at 600 ºC shows a mass increase corresponding to an oxygen uptake from which a stoichiometry of LaNiO2.95 can be estimated; in contrast, such mass increase is not observed for the perovskite calcined at 700 ºC which must therefore be basically fully oxidised. It can also be noted in Fig. 5 that above ca. 1000 ºC an important decomposition process of the perovskite takes place in any case. 2.6 Catalytic activity Fig. 6 shows the catalytic activity results obtained during light-off tests of oxidation of C3H6 and CO over the samples calcined at different temperatures. Partial oxidation products

Fig. 6 Catalytic activity data for C3H6 (a) and CO (b) combustion reactions over the sample calcined at indicated temperatures

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(CO or oxygenated hydrocarbons) were not observed in the gas infrared detector during any of the tests, thus revealing that full combustion is produced over any of the samples. The catalytic activity shows a maximum for the sample calcined at 600 °C, also in terms of the activity normalized per surface area, as illustrated in Fig. 7; note that, although only two representative temperatures have been selected for these plots, this holds for all the reaction temperature range. Analysis of these results in the light of the characterization ones and in particular taking into account the surface area values of the samples (Table 1) reveals that the maximum activity within the examined series is achieved in the presence of relatively small nanosized crystals of the LaNiO3 perovskite. In any of the cases, it must be considered that the catalytic differences must be related to differences in the oxygen handling properties (adsorption, transport) in each case within a suprafacial or intrafacial catalytic mechanism which, according to the results obtained, becomes most favoured in the presence of the LaNiO3 perovskite structure at the sample surface; on the contrary, the presence of amorphous carbonate- or nitrate-type precursor complexes (present in the sample calcined at 400 ºC) apparently results detrimentally to the activity in any of the reactions[4,19]. The highest activity achieved for the sample calcined at 600 ºC in comparison with the sample calcined at 750 ºC suggests that surface morphological or chemical differences between LaNiO3 crystals present in both cases could play an important role in this difference, as observed for other oxide materials[43]. Involvement of oxygen handling properties in the catalytic activity is supported by thermogravimetric analysis of redox properties since the activity can be correlated with the facility for nickel reduction in each case (Fig. 4). Besides, the heterogeneity of nickel species in the sample calcined at 600 ºC (according to TPR analysis, Fig. 4) could play an interesting role in the catalytic activity. Another important factor to be taken into account is the surface composition revealed by XPS. In this sense, the Ni/La surface atomic ratio shows a highest enrichment of nickel at the surface of the sample calcined at 600 °C compared with that calcined at 750 ºC (Fig. 7). In addition, thermogravimetic experiments have shown a

Fig. 7 Surface normalized specific activity for C3H6 (at 250 °C) and CO (at 150 °C) combustion and Ni/La atomic ratio as a function of calcination temperature applied to the sample

K. Rida et al., Effect of calcination temperature on the structural properties and catalytic activity in oxidation reactions of …

bulk stoichiometry of LaNiO2.95 when the calcination temperature is 600 ºC (Fig. 5), and considering the mentioned surface Ni enrichment, this oxygen deficiency could be higher at the sample surface. In any case, since oxygen handling and redox changes must basically be associated with the nickel component, both nickel enrichment at the sample surface and the presence of chemically defective surface structure (as inferred for the sample calcined at 600 ºC) can apparently play most relevant roles for the combustion activity of this type of materials, also in agreement with previous findings for other systems of this type[18].

3 Conclusions Analysis by various techniques of synthesized LaNiO3 by the modified sol-gel Pechini method showed that this compound was formed as a single phase from about 550 °C being stable at least up to 800 °C. The study of the thermal decomposition of the precursor revealed that the LaNiO3 phase nucleated after the complete decomposition of the organic phase and nitrate precursors. The maximum catalytic activity for propene and CO combustion reactions was reached in the presence of such lanthanum nickelate perovskite phase as soon as all organic or nitrate precursors were decomposed. Apparently, the combustion activity was improved in the presence of relatively small nanocrystals of such phase (after calcination at 600 °C) in which surface enrichment in nickel appeared somewhat higher than for bigger crystals (present in the sample calcined at 750 °C), which suggested an important role for surface morphological details on the catalytic properties of this type of perovskite oxide. In accordance with this hypothesis, a certain oxygen deficiency was inferred from thermogravimetric analysis for the sample with the highest catalytic activity (calcined at 600 °C) while redox/catalytic correlations could be established in any case on the basis of TPR analysis. Acknowledgements: K. Rida thanks the Ministry of Higher Education and Scientific Research of Algeria for a grant under which part of this work was performed. Thanks are also due to the ICP-CSIC Support Unit staff, Thermal Analysis Laboratory and Mr E. Pardo for performing experimental work.

References: [1] Lacorre P, Torrance J B, Pannetier J, Nazzal A I, Wang P W, Huang T C. Synthesis, crystal structure, and properties of metallic PrNiO3: Comparison with metallic NdNiO3 and semiconducting SmNiO3. J. Solid State Chem., 1991, 91: 225. [2] Chen M S, Wu T B, Wu J M. Effect of textured LaNiO3 electrode on the fatigue improvement of Pb(Zr0.53Ti0.47)O3 thin films. Appl. Phys. Lett., 1996, 68: 1430. [3] Singh R N, Tiwari S K, Singh S P, Jain A N, Singh N K. Electrocatalytic activity of high specific surface area perovskite-type LaNiO3 via sol-gel route for electrolytic oxygen evolution in alkaline solution. Int. J. Hydrogen Energy, 1997, 22: 557.

215

[4] Peña M A, Fierro J L G. Chemical structures and performance of perovskite oxides. Chem. Rev. 2001, 101: 1981. [5] Yoshii, K, Nakamura A, Mizumaki M, Ikeda N, Mizuki J. Magnetic behavior of some rare-earth transition-metal perovskite oxide systems. J. Rare Earths, 2004, 22: 733. [6] Rida K, Benabbas A, Bouremmad F, Peña M A, Sastre E, Martínez-Arias A. Effect of strontium and cerium doping on the structural characteristics and catalytic activity for C3H6 combustion of perovskite LaCrO3 prepared by sol-gel. Appl. Catal. B, 2008, 84: 457. [7] Lin J Y, Zhao M K, Chen K N. Synthesis and electrocatalytic properties of complex oxides containing lanthanum. J. Rare Earths, 1999, 17: 231. [8] Ling T R, Chen Z B, Lee M D. Catalytic behavior and electrical conductivity of LaNiO3 in ethanol oxidation. Appl. Catal. A, 1996, 136: 191. [9] Burselle M, Pirjamali M, Kiros Y. La0.6Ca0.4CoO3, La0.1Ca0.9MnO3 and LaNiO3 as bifunctional oxygen electrodes. Electrochim. Acta, 2002, 47: 1651. [10] Shodai, Y, Inaba, M, Onoda, K, Tasaka, A. Preparation of lanthanum nickel oxide-coated Ni sheet anodes and their application to electrolytic production of (CF3)(3)N in (CH3)(4)NF center dot 4.0HF melt. J. Rare Earths, 2006, 24: 1. [11] Lima S M, Assaf J M, Peña M A, Fierro J L G. Structural features of La1xCexNiO3 mixed oxides and performance for the dry reforming of methane. Appl. Catal. A, 2006, 311: 94. [12] Valderrama G, Kiennemann A, Goldwasser M R. Dry reforming of CH4 over solid solutions of LaNi1xCoxO3. Catal. Today, 2008, 133-135: 142. [13] Valderrama G, Goldwasser M R, Urbina de Navarro C, Tatibouët J M, Barrault J, Batiot-Dupeyrat C, Martnez F. Dry reforming of methane over Ni perovskite type oxides. Catal. Today, 2005, 107-108: 785. [14] de Lima S M, da Silva A M, da Costa L O O, Assaf J M, Jacobs G, Davis B H, Mattos L V, Noronha F B. Evaluation of the performance of Ni/La2O3 catalyst prepared from LaNiO3 perovskite-type oxides for the production of hydrogen through steam reforming and oxidative steam reforming of ethanol. Appl. Catal. A, 2010, 377: 181. [15] Norman A K, Morris M A. The preparation of the single-phase perovskite LaNiO3. J. Mater. Proc. Technol., 1999, 92-93: 91. [16] Russo N, Fino D, Saracco G, Specchia V. Promotion effect of Au on perovskite catalysts for the regeneration of diesel particulate filters. Catal. Today, 2008, 137: 306. [17] Pecchi G, Reyes P, Zamora R, Cads L E, Fierro J L G. Surface properties and performance for VOCs combustion of LaFe1yNiyO3 perovskite oxides. J. Solid State Chem., 2008, 181: 905. [18] García de la Cruz R M, Falcón H, Peña M A, Fierro J L G. Role of bulk and surface structures of La1xSrxNiO3 perovskite-type oxides in methane combustion. Appl. Catal. B, 2001, 33: 45. [19] Rida K, Benabbas A, Bouremmad F, Peña Sastre E, Martínez-Arias A. Effect of calcination temperature on the structural characteristics and catalytic activity for propene combustion of sol-gel derived lanthanum chromite perovskite. Appl. Catal. A, 2007, 327: 173. [20] Pechini M P. Method of preparing lead and alkaline earth titanates and niobates and coating method using the same to form a capacitor. US Patent No. 3.330.697. July 1, 1967.

216 [21] Chen Z H, Xing X R, Dai J, Huang X W, Li H W. Sol-gel synthesis of LaMO3 (M=Cr, Mn, Fe, Co, Ni) nanocrystalline powders. J. Rare Earths, 2004, 22: 55. [22] Le N T H, Calderon-Moreno J M, Popa M, Crespo D, Hong J V, Phuc N X. LaNiO3 nanopowder prepared by an ‘amorphous citrate’ route. J. Eur. Ceram. Soc., 2006, 26: 403. [23] Wagner C D, Davis L E, Zeller M V, Taylor J A, Raymond R H. Empirical atomic sensitivity factors for quantitative analysis by electron spectroscopy for chemical analysis. Surf. Interf. Anal., 1981 3: 211. [24] Duran P, Tartaj J, Capel F, Moure C. Formation, sintering and thermal expansion behaviour of Sr- and Mg-doped LaCrO3 as SOFC interconnector prepared by the ethylene glycol polymerized complex solution synthesis method. J. Eur. Ceram. Soc., 2004, 24: 2619. [25] Bilger S, Blab G, Frthman R. Sol-gel synthesis of lanthanum chromite powder. J. Eur. Ceram. Soc., 1997, 17: 1027. [26] Devi P S, Rao M S. Study of the thermal decomposition of lanthanum and chromium citrate hydrates. J. Anal. Appl. Pyrol., 1992, 22: 187. [27] Fjellvag H, Hansteen O H, Tilset B G, Olafsen A, Sakai N, Seim H. Thermal analysis as an aid in the synthesis of nonstoichiometric perovskite type oxides. Thermochim. Acta, 1995, 256: 75. [28] Weng X, Boldrin P, Abrahams I, Skinner S J, Kellici S, Darr J A. Direct syntheses of Lan+1NinO3n+1 phases (n=1, 2, 3 and ) from nanosized co-crystallites. J. Solid State Chem., 2008, 181: 1123. [29] Hofer H E, Kock W F. Crystal chemistry and thermal behavior in the La(Cr,Ni)O3 perovskite system. J. Electrochem. Soc., 1993, 140: 2889. [30] Fernandes J D G, Melo D M A, Zinner L B, Salustiano C M, Silva Z R, Martinelli A E, Cerqueira M, Alves Junior C, Longo E, Bernardi M I B. Low-temperature synthesis of single-phase crystalline LaNiO3 perovskite via Pechini method. Mater. Lett., 2002, 53: 122. [31] Djordjevic C, Lee M, Sinn E. Oxoperoxo(citrato)- and dioxo (citrato)vanadates(V): synthesis, spectra, and structure of a hydroxyl oxygen bridged dimer K2[VO(O2)(C6H6O7)]2·2H2O

JOURNAL OF RARE EARTHS, Vol. 30, No. 3, Mar. 2012 Inorg. Chem., 1989, 28: 719. [32] Wang W, Zhang X, Chen F, Ma C, Chen C, Liu Q, Liao D, Li L. Homo- and hetero-metallic manganese citrate complexes: Syntheses, crystal structures and magnetic properties. Polyhedron, 2005, 24: 1656. [33] Tas A C, Majewski P J, Aldinger F. Chemical preparation of pure and strontium- and/or magnesium-doped lanthanum gallate powders. J. Am. Ceram. Soc., 2000, 83: 2954. [34] Ponce S, Peña M A, Fierro J L G. Surface properties and catalytic performance in methane combustion of Sr-substituted lanthanum manganites. Appl. Catal. B, 2000, 24: 193. [35] Zhang H M, Teraoka Y, Yamazoe N. Preparation of perovskite oxides with large surface-area by citrate process. Chem. Lett., 1987, 16: 665. [36] Gamarra D, Martínez-Arias A. Preferential oxidation of CO in rich H2 over CuO/CeO2: Operando-DRIFTS analysis of deactivating effect of CO2 and H2O. J. Catal., 2009, 263: 189. [37] Kumar S, Messing G L, White W B. Metal organic resin derived barium titanate: I, formation of barium titanium oxycarbonate intermediate. J. Am. Ceram. Soc., 1993, 76: 617. [38] Wang Y, Zhu J, Yang X, Lu L, Wang X. Preparation and characterization of LaNiO3 nanocrystals. Mater. Res. Bull., 2006, 41: 1565. [39] Fierro J L G. Structure and composition of perovskite surface in relation to adsorption and catalytic properties. Catal. Today, 1990, 8: 153. [40] Uwamino Y, Ishizuka T, Yamatera H. X-ray photoelectron spectroscopy of rare-earth compounds. J. Elect. Spect. Rel. Phenom., 1984, 34: 67. [41] Rida K, Benabbas A, Bouremmad F, Peña M A, MartínezArias A. Surface properties and catalytic performance of La1–xSrxCrO3 perovskite-type oxides for CO and C3H6 combustion. Catal. Commun., 2006, 7: 963. [42] Damyanova S, Daza L, Fierro J L G. Surface and catalytic properties of lanthanum-promoted Ni/sepiolite catalysts for styrene hydrogenation. J. Catal., 1996, 159: 150. [43] Fernández-García M, Martínez-Arias A, Hanson J C, Rodriguez J A. Nanostructured oxides in chemistry: Characterization and properties. Chem. Rev., 2004, 104: 4063.