Reducibility and catalytic behavior of La0.25Sr1.75MnO4 Ruddlesden- Popper phase prepared by a modified Pechini method Reducibilidad y comportamiento catalítico de la fase

Few studies have been reported concerning the use of manganite compounds (La,Sr)2MnO4 of Ruddlesden-Popper structure type as Solid Oxide Fuel Cells (SOFCs) electrode materials; in particular, on the anode side. In the present work La0.25Sr1.75MnO4 compound was synthesized by a modified Pechini method. This prepared manganite was characterized by X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and its structure was analyzed into detail by refinement of recorded XRD profiles. La0.25Sr1.75MnO4 was also subjected to a reducibility study through Temperature Programmed Reduction (TPR) and the catalytic properties for total and/or partial oxidation of methane were evaluated. Estimated values of lattice parameters in the refinement are in good agreement with those reported in the literature for various neighbored compositions of the series. The reduction of the material in diluted H2 occurs in several stages and the decomposition starts above 800°C. In the catalytic studies, the material acts as a catalyst for total oxidation of methane even in oxygen deficient atmospheres.


Introduction
Solid Oxide Fuel Cells (SOFCs) major issues are related to the degradation of electrode materials as well as the electrode-electrolyte and electrodeinterconnect interfaces during long-term operation or transient conditions. It makes heating or cooling procedures sometimes heavy and complex. The most widely used SOFC anode is a cermet (ceramicmetal composite) of nickel and Yttria-Stabilized Zirconia (YSZ). Ni/YSZ cermet is commonly chosen due to low cost and high chemical stability in the reducing atmosphere of the anode; furthermore, its thermal expansion coefficient is compatible with that of the YSZ electrolyte [1][2][3]. Nickel in the cermet anode acts as an electrocatalyst for the electrochemical oxidation of hydrogen and provides high electronic conductivity. YSZ phase provides the ionic conductivity to the anode. The material must be highly porous in order to help increasing the number of Triple-Phase Boundaries (TPB) where the reaction can take place. At the same time, the YSZ network maintains the dispersion of nickel particles and acts as a growth inhibitor of metal grains during elaboration and operation of the cell. Unfortunately, the activity of the classical cermet and the cell performance can be affected due to kinetically favored reactions such as carbon deposition when hydrocarbons are directly fed to the anode, especially at low current density or in transient conditions. Carbon formation on the nickel surface can block the TPB [4], leading to a decrease in the power efficiency of the cell. Most research aiming at overcoming the limitations of nickel-based anodes has focused on the development of alternative materials that are catalytically active for the reforming or the oxidation of hydrocarbons (directly or indirectly) and inactive for cracking reactions that lead to carbon deposition. Other problems of Ni-YSZ cermets are the irreversible losses of activity stemming from sulphur poisoning in the case of realistic fuels like Natural Gas as well as coarsening of Ni-particles in operation and redox cycling, the latter being true even in H 2 /H 2 O or H 2 /CO atmospheres [3,5,6]. Due to their high resistance toward reducing and sulfur-containing atmospheres, perovskite compounds (of general formula ABO 3 ) have attracted great interest in the last decade as a possible replacement of Ni-cermets [6][7][8]. Most of the solids that have been considered as anode materials, like doped-SrTiO 3 , present the classical tridimensional perovskite structure, with La 3+ (LST) or Y 3+ (YST) principally doping the A-site [9][10][11][12]. To improve the electrochemical performance of those materials, which is still too low to be used as a single phase anode, cation doping at the B-site of the perovskite has been considered; among the possible cations, Mn 3+ doping [13,14] and Ga 3+ /Mn n+ co-doping [15] are worth mentioning, but with limited success [16]. Actually, for the anode side, no real attention has been played to new structure types, especially layered materials that have revealed interesting performances as a cathode component, e.g. oxygen vacancy ordered REBaCo 2 O 5+ δ [17] or Ruddlesden-Popper (RP) type nickelates [18][19][20]. Among the RP series (AO)(ABO 3 ) n , one of great interest is the A 2 BO 4 (K 2 NiF 4 -type) structure, which consists of alternate layers along the [001] direction of ABO 3 perovskite and AO rock salt compounds [21], as can be seen in Figure 1. Lanthanum-strontium manganites (La,Sr) 2 MnO 4 have the K 2 NiF 4 structure type with tetragonal I4/ mmm space group [22]. In this structure, every manganese atom is surrounded by six oxygen atoms forming an octahedral and the Mn-O bonds have the direction of the crystallographic axes. According to literature, very few studies have been reported concerning the use of (La,Sr) 2 MnO 4 compounds as SOFC electrode materials; in particular, on the anode side, only redox stability considerations and oxygen stoichiometry in the series are described [19,[23][24][25][26][27]. On the other hand, and in contrast with perovskite oxides, these RP manganites have not been widely studied as oxidation catalysts (reducing atmosphere), one of the main properties to fulfil for SOFC anodes [28,29]. Considering the latter, this work was focused not only on the study of reducibility and the structural characterization of the La 0.25 Sr 1.75 MnO 4 composition, but also on the evaluation of its catalytic properties for methane oxidation.

Material and methods
A powder sample with the nominal composition La 0.25 Sr 1.75 MnO 4 was synthesized by a modified Pechini method [30], using Sr(NO 3 ) 2 (99.9965%, Alfa Aesar), La(NO 3 ) 2 (99.99%,Aldrich) and Mn(NO 3 ) 2 (99.99%, Merck) as precursors. Initially, 0.06 mole of citric acid (99%, Prolabo) were dissolved in 50ml of distilled water. Later, 0.0088 mole of Sr(NO 3 ) 2 , 0.0013 mole of La(NO 3 ) 2 and 0.0050 mole of Mn(NO 3 ) 2 were added to the solution and the temperature was raised to 60°C. After getting a homogeneous solution, 0.09 mole of ethylene glycol (99%, Aldrich) was added and the temperature adjusted to 70°C, for which the polyesterification reaction occurred. Finally, the temperature was raised to 90°C to remove solvent excess and form a xerogel. The xerogel was initially treated at 350°C for 5h and then the powders were heated at 1300°C in air for 10h, at this later temperature the pure phase was formed.
Powder X-Ray Diffraction (XRD) data were collected at Room Temperature (RT) using a Bruker D8 ADVANCE X-ray Diffractometer working in Bragg-Brentano geometry with Cu Kα 1,2 radiations (DAVINCI design). XRD data were collected from 10º to 140º (2θ) with a step size of 0.015º. Structural refinements using the Lebail method were performed with the Fullprof Suite program [31,32]. A pseudo-Voigt function was used to model profile shapes, including the Cagliotti function variables U, V, W, the Gaussian-Lorentzian mixing parameter η and two asymmetry parameters. The values of standard deviations were corrected according to Berar and Lelann´s description [33]. The Temperature-Programmed Reduction (TPR) technique was performed using a Micromeritics ChemiSorb 2720 chemisorption system. In each test, about 35mg of sample was probed with an H 2 /Ar (5/95) reducing mixture (50cm 3 min -1 at 25°C, 1atm). The temperature in the sample was increased from 30°C to 900°C using a heating ramp of 10°Cmin -1 , with a dwell time at 900°C of 20min. Before each TPR test, the catalyst was subjected to a degassing process in order to remove adsorbed substances. This process consisted in passing a helium gas flow of 50cm 3 min -1 through the sample while heated from 30°C to 300°C, followed by a dwell time of 30min at 300°C before cooling to room temperature. For an optimal reduction profile, the weight of catalyst was selected in agreement with the recommended values of the characteristic numbers K (55-140s) and P (P<20°C) proposed by Monti et al. [34] and Malet et al. [35]. These characteristic numbers allow obtaining narrow peaks being easier to determine the position of the maximum rate and the H 2 consumption amount. The surface analysis of the sample was examined with a Quanta 650 FEG Scanning Electron Microscope (SEM), operating in high vacuum mode using a secondary electron detector (Everhart-Thornley). A small fraction of the sample was fixed on a single specimen holder using carbon adhesive tape that was coated with gold using a Quorum Q150 TES metallizer. The sample analysis was performed with the following analytical conditions: 5−10kV of accelerating voltage and 182μA of beam current. The purpose of the catalytic evaluation was to determine the activity of La 0.25 Sr 1.75 MnO 4 for partial and/or complete oxidation of methane as described by Equations 1 and 2.
Catalytic experiments were performed in a fixed bed lab-scale reactor, operated isothermally and at atmospheric pressure. The reactor consisted of a quartz glass tube of 12mm outer diameter and 11.2mm of inner diameter, filled with the catalytic bed. The catalyst was diluted with inert material in order to avoid the developing of hot spots, forming a bed of ~15mm in length. The reactor was placed in an oven provided with a temperature controller. The reaction temperature was monitored with a K-type thermocouple axially located in the center of the catalyst bed. Previous catalytic experiments were performed to select the operation conditions that ensure chemical reaction control [36][37][38]. In order to satisfy these conditions, a flow of 170mLmin -1 of a CH 4 , O 2 and N 2 mixture was fed to a fixed-bed reactor containing 52mg of catalyst dissolved in 552mg of inert phase (catalyst/inert ratio=1/10). The inert material was a ceramic with composition unavailable. This material was previously tested in operating conditions to verify the absence of catalytic activity in the whole temperature range. The outlet and feed stream compositions were analyzed by on-line gas chromatography using a Clarus 500 Chromatograph (Perkin Elmer), equipped with a concentric packed column (Porapak // Molecular Sieve), a thermal conductivity detector and an automatic injection valve. The amount of reaction gases was determined by calibration curves. For each data collection, several (generally 3) measurements were recorded and the final value was taken as an average. The catalyst was subjected to three different tests corresponding to the following conditions: • Catalytic evaluation varying the reaction temperature every 50°C from 500 to 800°C to assess the catalyst activity at different temperatures.

Structural characterization
The pure-phase formation of the material was confirmed by comparison with the PDF card number #54-1279 using the Search/Match program [39]. A Full Pattern Matching refinement of the structural parameters was carried out by least-squares method using XRD data, according to the Le Bail procedure. The graphical result of the refinement is shown in Figure 2, in good agreement with a tetragonal cell of I4/mmm space group. The refinement quality was estimated not only by the reliability factors R p , R wp and c 2 , but also by the difference between experimental and calculated patterns. In addition to reliability factors, refined cell parameters are given in Table 1 [26][27][28][29][30][31][32][33][34][35][36][37][38][39][40]. The morphology of La 0.25 Sr 1.75 MnO 4 sample, prepared by the modified Pechini method, has been observed by SEM imaging and is shown in Figure 3. The material presents an average grain size of about 500nm, despite the high calcination temperature (1300°C). The porous microstructure is relatively homogeneous and the grains are strongly aggregated.

Temperature-Programmed Reduction (TPR)
The reduction profile of La 0.25 Sr 1.75 MnO 4 obtained from TPR is shown in Figure 4; it presents a twosteps reduction profile, i.e. a low temperature peak located in the 400-600°C range and centered around 552°C, and a high temperature peak between 600°C and 800°C, centered on 697°C. TPR curve shows also an additional step beyond 850°C that is not completely finished at 900°C. Such reduction profile gives information about the reducibility of Mn n+ species in the material, since the A-site La 3+ and Sr 2+ are both non-reducible cations under the conditions of H 2 -TPR [41]. TPR studies conducted on parent perovskite-type oxides La 1-x Sr x MnO 3 (x=0-0.5) have also shown two clear reduction regions (150-530 and 550-930°C), where the first region is primarily associated with reduction of Mn 4+ →Mn 3+ and the second region with Mn 3+ →Mn 2+ with a subsequent structure decomposition [42][43][44][45]. In our case, the first and second peak can be reasonably associated to Mn 4+ →Mn 3+ and Mn 3+ →Mn 2+ reduction steps, respectively, just like it proceeds for perovskite-type (La,Sr)MnO 3 .
The hydrogen uptake in each peak and total H 2 consumption were determined by comparing the TPR profile with a calibration function that relates H 2 consumption as a function of the reduction curve area. The results are displayed in  [46][47]. Reduction peaks would thus be described by Equations 3 and 4:   46 . Finally, XRD analysis of the TPR residue, shown in the inset of Figure 4, confirms that La 0.25 Sr 1.75 MnO 4-δ sample began to decompose with formation of MnO, with the conclusion that the RP manganite becomes unstable in a reducing atmosphere for temperatures higher than 800°C (during the last TPR event), what is in agreement with literature [46]. Another explanation to the reduction steps observed by TPR would be a symmetry-(and not Mn valence-) driven transition from original tetragonal I4/mmm space group to a monoclinic P2 1 /c structure with ordered oxygen vacancies, as recently described in the case of non-doped Sr 2 MnO 4-δ [48]. In this work, the authors demonstrate that tetragonal Sr 2 MnO 4 material, prepared in air and heated under dilute hydrogen flow, loses oxygen from the "MnO 2 " equatorial layer above T∼ 470°C with retention of tetragonal symmetry up to Sr 2 MnO 3.70 (1) . Further oxygen loss induces ordering of the oxygen vacancies within the equatorial layers transforming the tetragonal cell into a P2 1 /c monoclinic supercell. When the phase transition is complete, the refined composition of the singlephase P2 1 /c material is found to be Sr 2 MnO 3.55 (1) and does not vary on extended heating. If the same behavior is occurring in La-doped Sr 2 MnO 4 , it would be interesting to understand if the origin of the second TPR peak around 700°C is not also related to the structure decomposition that was initially associated to the last H 2 consumption beyond 800°C.

Catalytic Evaluation
The catalytic tests were carried out with molar ratios of O 2 :CH 4 in the range 0.4:1 to 1:1. In such conditions, the feed presents an oxygen deficiency with respect to the O 2 :CH 4 =2:1 stoichiometric ratio corresponding to the complete methane oxidation (Equation 2), making oxygen the limiting reactant.
In Figures 5a and 5b, the oxygen conversion profiles were plotted as a function of temperature and the molar ratio O 2 :CH 4 , respectively. The catalytic activity of La 0.25 Sr 1.75 MnO 4 begins to become important at temperatures above 500°C (Figure 5a). This is consistent with the temperature at which the material begins to be reduced in H 2 -Ar atmosphere ( Figure 4). It can be observed that oxygen is completely consumed at a temperature of 800°C, which is the highest temperature of reaction being probed. According to Figure 5b, the O 2 consumption increases to near 100% when the O 2 /CH 4 ratio decreases below 0.4.
The only carbonaceous product that is formed is CO 2 in all catalytic tests; neither CO nor H 2 is detected at any temperature or reaction conditions. It is therefore concluded that La 0.25 Sr 1.75 MnO 4 catalyst is selective to the total methane oxidation even in oxygen-defi cient conditions. Moreover, considering the carbon balance analysis that is obtained comparing the consumed amount of CH 4 to produced CO 2 , a difference of less than 5% is found. Such result, coupled with the catalyst stability attributed to the absence of carbon (see Figure 5c), confi rms the excellent capacity of the RP manganites for methane oxidation. As the water that is produced through the complete oxidation reaction is retained in a desiccator placed before the GC device, it was not possible to carry out any hydrogen or oxygen mass balance analysis.  Figure 5b), is different from the third test (stability), as observed in Figure 5c. This difference could be associated with a small deactivation caused by catalyst reduction at the beginning of reaction, then presenting a stable behavior. The stability behavior during 5h of reaction indicates that the catalyst does not promote the formation of carbonaceous deposits. It means that in oxygen conversion conditions, the solid is in a dynamic equilibrium between the processes of reduction (when it delivers lattice oxygen atoms to the fuel) and re-oxidation (when it reintegrates oxygen atoms transferred from the gas phase).

Conclusions
La 0.25 Sr 1.75 MnO 4 has been synthesized by a modified Pechini method and the K 2 NiF 4 -type phase was obtained at a temperature of 1300°C in air. The material shows a reduction process in several steps with two hypotheses: (i) a first strong reduction peak of manganese from Mn 4+ to Mn 3+ below 600°C, followed by a second reduction peak to Mn 2+ , weaker in intensity, between 600ºC and 800°C, before a high Mn 3+ to Mn 2+ reduction that leads to a decomposition of the RP manganite beyond 800°C. (ii) A first reduction from tetragonal I4/mmm symmetry to monoclinic P2 1 /c ordered oxygen deficient compound, as in Sr 2 MnO 4-δ followed by a material decomposition. More work is needed, including a structural study as a function of temperature in reducing and oxidizing atmosphere, to clear up the observed behavior. Catalytic evaluation of La 0.25 Sr 1.75 MnO 4 indicates that such material is selective towards total methane oxidation even in oxygen-deficient atmosphere, the catalytic activity being high for temperatures above 500°C. The catalytic behavior at 745°C is stable during the studied reaction time of 5h, without apparent formation of carbonaceous residues on the material. The RP manganite characteristics make it promising as anode material for intermediate temperature SOFC with operating temperatures below 800°C.