The red-emitting phosphors of Mn4+-activated A2MgWO6 (A = Ba,Sr, Ca) for light emitting diodes

A series of Mn⁴⁺ ions activated A₂MgWO₆ (A = Ba, Sr, Ca) phosphors, showing bright red emission peaks appeared around 700 nm under the excitation of 355 nm, were synthesized by the solid-state reaction. The crystal structures and photo-luminescent (PL) properties of these synthesized phosphors were deeply investigated with the aids of X-ray diffraction measurement (XRD), and the temperature dependent PL/decay curves in detail. The optimum doping concentration of Mn⁴⁺ ions in A₂MgWO₆ (A = Ba, Sr, Ca) lattices were studied through the relationship between the Mn⁴⁺ ions doping concentrations and the luminescent intensities. The thermal stability of the synthesized red-emitting phosphors was checked based on the temperature-dependent PL intensities ranging from 7 to 510 K. Comparative studies of the luminescent properties for Mn⁴⁺ ions in isostructural A₂MgWO₆ (A = Ba, Sr, Ca) lattice with double perovskite structure were studied. The results indicate that the synthesized red-emitting phosphors are the ideal choice for white light emitting diodes (W-LEDs).     

Site-selective substitution and resulting magnetism in arc-melted perovskite ATiO3-δ (A = Ca,Sr, Ba)

Magnetic properties in perovskite titanates ATiO_3-δ (A = Ca, Sr, Ba) were investigated before and after arc melting. Crystal structure analysis was conducted by powder synchrotron X-ray diffraction with Rietveld refinements. Quantitative chemical element analysis was carried out by X-ray photoelectron spectroscopy. Magnetic measurements were conducted by vibrating sample magnetometer and X-ray magnetic circular dichroism (XMCD). The magnetic properties are found to be affected by impurities of 3d elements such as Fe, Co, and Ni. Depending on the composition and crystal structure, the occupation of the magnetic ions in perovskite titanates is selectively varied, which is interpreted to be the origin of the different magnetic behaviors in arc-melted perovskite titanates ATiO_3-δ (A = Ca, Sr, Ba). In addition, both formation of oxygen vacancies and the reduction of Ti⁴⁺ to Ti³⁺ during arc-melting also play a role as proven by XMCD. Nevertheless, preferential site occupation of magnetic impurities is dominant in the magnetic properties of arc-melted perovskite ATiO_3-δ (A = Ca, Sr, Ba).     

Size and Lone Pair Effects on the Multiferroic Properties of Bi0.75A0.25FeO3−δ (A = Sr,Pb, and Ba) Ceramics

The work presents a comparative study of the effects of divalent Ba, Sr, and Pb substituents on the multiferroic properties of BiFeO₃. The multiferroic properties of Bi_0.75A_0.25FeO₃ (A = Sr, Pb, Ba) solid solution have been explained taking into account the effects of size differences and electronic configuration differences between the host element (Bi) and the substituent. X‐ray diffraction studies revealed that Sr and Pb substitution at Bi‐site transforms the rhombohedral phase (R3c) to cubic phase (Pm3m), whereas the Ba‐substituted sample exhibited the presence of both rhombohedral and cubic phases (R3c + Pm3m). Electronic structure studies through XPS revealed that charge imbalance induced by divalent substitution was being compensated by the formation of oxygen vacancies, while the Fe ions exist in Fe²⁺ and Fe³⁺ states. Replacement of volatile Bi by Sr, Pb, and Ba reduces the concentration of oxygen vacancies (V_O²⁺) and helps to improve the dielectric properties. Strong magnetization enhancement was observed in the substituted compositions and was seen to be consistent with the suppression of cycloid spin structure due to structural transformation as well as possible changes in Fe–O local environment leading to local lattice distortion effects. Furthermore, the observed decrease in the values of magnetic coercivity at low temperature in all the substituted samples is explained in terms of reduced effective single ion anisotropy, originating in the magnetoelectric coupling and being a particularly stronger effect in the case of the lone pair dopant Pb, consistent with theoretical predictions. The lone pair substituent Pb leads to the largest dielectric constant, enhanced magnetization, and large effects on the low‐temperature hysteresis.     

Improved oxide-ion conductivity by substitution of Sr for Bi in Dion-Jacobson phase CsBi2Ti2NbO10

Oxide ion conducting ceramics have attracted much attention due to their various applications such as solid oxide electrolysis cells (SOECs) and solid-oxide fuel cells (SOFCs). Herein, we present the first report on the preparation, electrical and structural properties of oxide-ion conducting Dion-Jacobson phases CsBi_1.9M_0.1Ti₂NbO_9.95 (M = Mg, Ca, Sr and Ba). It was found that partial substitution of Bi³⁺ by Sr²⁺ improves the conductivity of CsBi₂Ti₂NbO₁₀. The electrical conductivity of CsBi_1.9Sr_0.1Ti₂NbO_9.95 is approximately 2 times higher than that of CsBi₂Ti₂NbO₁₀ at 600–900 °C and is 1.65 × 10^?2 S cm^?1 at 900 °C. The improvement of oxide-ion conductivity in CsBi_1.9Sr_0.1Ti₂NbO_9.95 can be attributed to the increase of the carrier (oxygen vacancy) concentration and small size mismatch between the dopant Sr and host Bi cations. The electrical conductivities of CsBi_1.9Sr_0.1Ti₂NbO_9.95 are almost constant in the oxygen partial pressure region from 10^?25 to 1 atm at 600 °C, indicating the predominant oxide-ion conduction and high chemical and electrical stability. Bond-valence-based energy landscapes of a test oxide ion in CsBi_1.9Sr_0.1Ti₂NbO_9.95 suggest that oxide ions migrate along the edges of the octahedra in the inner perovskite layers, leading to two-dimensional oxide-ion diffusion. This study demonstrates the successful improvement of oxide-ion conductivity by the substitution in Dion-Jacobson phase, which would develop the science and technology of Dion-Jacobson type oxide-ion conductors.     

Synthesis,structure, optical and magnetic properties of Nd1−xAxMn0.5Co0.5O3−δ (A = Ba,Sr and Ca; x = 0 and 0.25)

The structural, optical, and magnetic properties of polycrystalline Nd_1-xA_xMn_0.5Co_0.5O_3−δ (A = Ba, Sr and Ca; x = 0 and 0.25) perovskite oxides were investigated. The powder XRD pattern demonstrates that the unit cell volume decreases with the changing A-site dopant type. The estimated bandgap energy (Eg) from UV–vis spectroscopic for NdMn_0.5Co_0.5O_3−δ, Nd_0.75Ba_0.25Mn_0.5Co_0.5O_3−δ, Nd_0.75Sr_0.25Mn_0.5Co_0.5O_3−δ and Nd_0.75Ca_0.25Mn_0.5Co_0.5O_3−δ are 3.27, 3.82, 3.79 and 3.53 eV respectively. The substitution of divalent element alters the absorption spectrum, while the redshift optical transition was observed with an increasing ionic radius of dopant. Temperature-dependent magnetization exposes that the Curie temperature (TC) gradually decreases with the decreasing size of alkaline earth metals, and glassy nature was observed at a lower applied magnetic field. The observation of TC can be well explained by the considering of the cationic size disorder parameter in A-site than the random distribution of B-site ions.     

Theoretical investigation of anisotropic mechanical and thermal properties of ABO3 (A=Sr,Ba; B=Ti,Zr, Hf) perovskites

As promising TBC (thermal barrier coating) candidates, perovskite oxides own designable properties for their various options of cations and structural diversity, but limited comprehensions of structure‐property relationship delay their engineering applications. In this work, mechanical/thermal properties of ABO₃ (A=Sr, Ba; B=Ti, Zr, Hf) perovskites and their anisotropic nature are predicted employing density functional theory. Their theoretical minimum thermal conductivities range from 1.09 to 1.74 W·m^?1·K^?1, being lower than Y₂O₃ partially stabilized ZrO₂. Reduced thermal conductivities up to 16% along particular directions are reached after considering thermal conductivity anisotropy. All compounds own high hardness while SrZrO₃, SrHfO₃, and BaHfO₃ possess well damage tolerance. We found that small electronegativity discrepancy leads to big anisotropy of chemical bond, Young's/shear moduli and thermal conductivities, together with good damage tolerance. These results suggest that the next generation TBCs with extra low thermal conductivity should be achieved through combining material design and orientation‐growth tailoring.     

Catalytic reduction of no over perovskite-type catalysts

In the present work, we have investigated the reduction of NO by propane over perovskite-type oxides prepared by malic acid method. The catalysts were modified to enhance the activity by substitution of metal into A or B site of perovskite oxides. In addition, the reaction conditions, such as temperature, O₂ concentration, and space velocity have been varied to understand their effects on the catalytic performance. In the LaCoO₃ type catalyst, the partial substitution of Ba and Sr into A site enhanced the catalytic activity in the reduction of NO. For the La_0.6Ba (Sr)_o.4 Co_1−x Fe_xO₃ (x=0-1.0) catalyst, the partial substitution of Fe into B site enhanced the conversion of NO, but excess amount of Fe decreased the conversion of NO. The surface area and catalytic activity of perovskite catalysts prepared by malic acid method showed higher values than those of solid reaction method. The conversion of NO increased with increasing O₂ concentration and contact time. The introduction of water into reactant feed decreased the catalytic activity but the deactivation was shown to be reversible over La_0.6Ba_0.4Co_1−x ,Fe_xO₃ catalyst.     

Effect of strontium and cerium doping on the structural characteristics and catalytic activity for C3H6 combustion of perovskite LaCrO3prepared by sol–gel

Two series of Sr- or Ce-doped La_1−xM_xCrO₃ (x = 0.0, 0.1, 0.2 and 0.3) catalysts were prepared by thermal decomposition of amorphous citrate precursors followed by annealing at 800 °C in air atmosphere. The effect of Ce and Sr on the morphological/structural properties of LaCrO₃ was investigated by means of thermogravimetric/differential thermal analysis (TG/DTA) of the precursors decomposition under air, X-ray diffraction (XRD), electron paramagnetic resonance (EPR), transmission electron microscopy–X-ray energy dispersive spectroscopy (TEM–XEDS), S_BET determination, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) techniques. The characterization results are employed to explain catalytic activity results for C₃H₆ combustion. It is shown that the lanthanum chromite perovskite structure is obtained upon thermal treatment of the sol–gel derived precursors at T > ca. 800 °C. The presence of the dopant generally induces the formation of segregated oxide phases in the samples calcined at 800 °C although some introduction of the Sr in the perovskite structure is inferred from EPR measurements. The oxidation activity becomes maximised upon formation of such doped perovskite structure.     

Effect of crystal structures on nucleation mechanism in hydrothermal synthesis of MZrO3 (M=Ba,Sr, Ca)

Hydrothermal method is widely used in the synthesis of perovskite-type oxides, whereas few studies are reported for the nucleation mechanism, so that the relationship between the crystal structures and reactive activities of reactants and products is still unknown. Herein, the reaction processes are analyzed on the basis of XRD, SEM and Raman characterizations, and the nucleation mechanism is investigated for the hydrothermal synthesis of MZrO₃ (M = Ba, Sr, Ca). We propose that the negative charged cyclic tetramer complexes [Zr₄(OH)₈(OH)₁₆]^8- form in the hydrothermal reaction, which play major roles in the nucleation process. The tetramer complexes continually dehydrate and condensate to form substructural units composed of alkali-earth ions and 6-fold Zr tetramers; substructural units further dehydrate and distort to form perovskite structures. The reactive activation energy increases with the decreasing of M²⁺ (M = Ba, Sr, Ca) ionic radius because the incorporation of smaller A site ions in the perovskite structure is accompanied by greater rotation and distortion of the ZrO₆ octahedra, leading to the decrease of reactive activity accordingly. In a word, the proposed nucleation mechanism in this paper is of great significance for the study of perovskite.     

Study of Oxygen Reactivity in La<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>CoO<Subscript>3−δ</Subscript> Perovskites for Total Oxidation of Toluene

Abstract  

The total oxidation of toluene is studied over catalytic systems based on perovskite with general formula AA′CoO_3-δ (A = La, A′ = Sr). The systematic and progressive substitution of La³⁺ by Sr²⁺ cations in the series (La_1−x Sr _x CoO_3−δ system) of the perovskites have been studied to determine their influence in the final properties of these mixed oxides and their corresponding reactivity performance for the total oxidation of toluene as a model volatile organic compound with detrimental effects for health and environment. The structure and morphology of the samples before and after reaction have been characterized by XRD, BET and FE-SEM techniques. Additional experiments of temperature programmed desorption of O₂ in vacuum and reduction in H₂ were also performed to identify the main surface oxygen species and the reducibility of the different perovskites. It is remarkable that the La_1−x Sr _x CoO_3−δ series presents better catalytic performance for the oxidation of toluene, with lower values for the T₅₀ (temperature of 50 % toluene conversion) than the previously studied LaNi_1−y Co _y O₃ series.     

Lithium Ion Diffusion Mechanism in Lithium Lanthanum Titanate Solid‐State Electrolytes from Atomistic Simulations

Perovskite‐structured lithium lanthanum titanate (LLT, La_2/3–xLi_3xTiO_3, 0 <  x < 0.16) is a promising solid electrolyte with high lithium ion conductivity and a good model system to understand lithium ion diffusion behaviors in solids. Molecular dynamics (MD) and related atomistic computer simulations were used to study the diffusion behavior and diffusion mechanism as a function of composition in LLT solid‐state electrolytes. The effect of defect concentration on the structure and lithium ion diffusion behaviors in LLT was systematically studied using MD simulations and molecular static calculations with the goal to obtain fundamental understanding of the diffusion mechanism of lithium ions in these materials. The simulation results show that there exists an optimal vacancy concentration at around x = 0.067 at which lithium ions have the highest diffusion coefficient and the lowest diffusion energy barrier. The lowest energy barrier from dynamics simulations was found to be around 0.22 eV, which compared favorably with 0.19 eV from static nudged elastic band calculations. It was also found that lithium ions diffuse through bottleneck structures made of oxygen ions, which expand in dimension by 8%–10% when lithium ions pass through. By designing perovskite structures with larger bottleneck sizes can be a means of further improving lithium ion conductivities in these materials.     

Effect of Variations in Annealing Temperature and Metallic Cations on Nanostructured Molybdate Thin Films

Crystalline molybdate thin films were prepared by the complex polymerization method. The AMoO₄ (A = Ca, Sr, Ba) films were deposited onto Si wafers by the spinning technique. The Mo–O bond in the AMoO₄ structure was confirmed by FTIR spectra. X-ray diffraction revealed the presence of crystalline scheelite-type phase. The mass, size, and basicity of A²⁺ cations was found to be dependent on the intrinsic characteristics of the materials. The grain size increased in the following order: CaMoO₄ < SrMoO₄ < BaMoO₄. The emission band wavelength was detected at around 576 nm. Our findings suggest that the material’s morphology and photoluminescence were both affected by the variations in cations (Ca, Sr, or Ba) and in the thermal treatment.     

Effects of A1/A2‐Sites Occupancy upon Ferroelectric Transition in (SrxBa1−x)Nb2O6 Tungsten Bronze Ceramics

The dielectric and ferroelectric characteristics of (Sr_xBa_1?x)Nb₂O₆ unfilled tungsten bronze ceramics have been investigated together with the structure. The dielectric and ferroelectric characteristics of the present ceramics vary significantly with x, and the A1/A2‐sites occupancy has been determined as the primary parameter governing this variation tendency. Ba ions tend to occupy A2‐sites, Sr ions tend to occupy A1‐sites, and one A1‐site is empty. When the ratio of Sr/Ba is close to 1:4 (where four A2‐sites are just occupied by Ba ions, and one A1‐site is occupied by Sr ion while another A1‐site is empty), the normal ferroelectric transition is observed with one anomaly in the tanδ–T curve (x = 0.25). When the ratio of Sr/Ba is far away from 1:4, the typical relaxor behavior is indicated together with three anomalies in the tanδ–T curve (x = 0.75). The incommensurate oxygen octahedral tilting and A‐site random distribution are considered to be the structure origins for the relaxor ferroelectricity and low temperature dielectric relaxations.     

Energetic Effects of Substitution of La–Nd and Si–Ge Oxyapatite‐Type Materials

A series of lanthanide oxyapatites, neodymium silicates (Nd_9.33?xM_3x/2(SiO₄)₆O₂; x = 0.0 and 2.0 and M = Ca, Sr, and Ba), and lanthanum germanate (La₁₀(GeO₄)₆O₃) were prepared by a variety of heat treatments and characterized. High‐temperature oxide melt solution calorimetry in molten 2PbO–B₂O₃ solvent at 1078 K was performed to determine their enthalpies of formation from constituent oxides at room temperature. The energetics of these materials is discussed in terms of the effects of doping on two crystallographic sites, the lanthanide and tetrahedral sites. The enthalpy of formation from oxides becomes less exothermic by substituting La with Nd throughout the whole series, in both doped and undoped compositions, reflecting the smaller radius and lower basicity of Nd compared with La. Cation stoichiometric lanthanum germanate apatite (La₁₀(GeO₄)₆O₃) shows a more endothermic enthalpy of formation than the corresponding silicate, reflecting the larger radius and lower acidity of Ge than Si.     

Effect of sulphur poisoning on perovskite catalysts prepared by flame-pyrolysis

ABO₃ perovskite-like catalysts are known to be sensitive to sulphur-containing compounds. Possible solutions to increase resistance to sulphur are represented by either catalyst bed protection with basic guards or catalyst doping with different transition or noble metals. In the present work La_(1−x)A^′_xCoO₃, La_(1−x)A^′_xMnO₃ and La_(1−x)A^′_xFeO₃, with A′ = Ce, Sr and x = 0, 0.1, 0.2, either pure or doped with noble metals (0.5 wt% Pt or Pd), were prepared in nano-powder form by flame-pyrolysis. All the catalysts were tested for the catalytic flameless combustion of methane, monitoring the activity by on-line mass spectrometry. The catalysts were then progressively deactivated in operando with a new procedure, consisting of repeated injection of some doses of tetrahydrothiophene (THT), usually employed as odorant in the natural gas grid, with continuous analysis of the transient response of the catalyst. The activity tests were then repeated on the poisoned catalyst. Different regenerative treatments were also tried, either in oxidising or reducing atmosphere.Among the unsubstituted samples, higher activity and better resistance to poisoning have been observed in general with manganites with respect to the corresponding formulations containing Co or Fe at the B-site. The worst catalyst showed LaFeO₃, from both the points of view of activity and of resistance to sulphur poisoning. La_0.9Sr_0.1MnO₃ showed, the best results, exhibiting very high activity and good resistance even after the addition of up to 8.4 mg of THT/g of catalyst. Interesting results were attained also by adding Sr to Co-based perovskites. Sr showed a first action by forcing Mn or Co in their highest oxidation state, but, in addition, it could also act as a sulphur guard, likely forming stable sulphates due to its basicity. Among noble metals, Pt doping proved beneficial in improving the activity of both the fresh and the poisoned catalyst.     

Ferroelectric Transition and Low‐Temperature Dielectric Relaxations in Filled Tungsten Bronzes

A comprehensive review on the latest development of the ferroelectric transition and low‐temperature dielectric relaxations of filled tungsten bronze ceramics are presented together with some new issues. In the filled tungsten bronze ceramics M_6?pR_pTi_2+pNb_8?pO₃₀ (p = 1, 2; M = Ba or Sr; R = rare earth or Bi), a ferroelectric transition is generally indicated together with up to three low‐temperature dielectric relaxations. The ferroelectric transition is determined as 4/mmm → 4 mm, and the low‐temperature dielectric relaxations are deeply concerned with the structure modulations due to the order/disorder of ions in A1 and A2 sites, their random cross occupancy, and the order/disorder of B‐site ions. Both the ferroelectric transition and low‐temperature dielectric relaxations are dominated by the composition and radius difference between A1‐ and A2‐site ions, ?r. The normal ferroelectric transition might be expected if the ratio of the biggest ion and second big ion is 2:1, otherwise the diffuse or relaxor ferroelectric is expected. Meanwhile, the larger ?r generally results in the normal ferroelectric, and the smaller ?r will lead to the diffuse or relaxor ferroelectric. Moreover, the effects of A sites order/disorder and the random cross occupancy of A‐site ions are primary, and the effects of B‐site ordering/disordering are secondary. The right ratio of 2:1 for A2‐ and A1‐site ions and the large ?r should be the guidelines for designing the possible multiferroic tungsten bronzes.     

ZnBO-doped (Ba, Sr)TiO3 ceramics for the low-temperature sintering process

ZnBO-doped (Ba, Sr)TiO₃ ceramics were investigated for low-temperature co-fired ceramics (LTCCs) applications. Until now, B₂O₃ and Li₂CO₃ dopants have been commonly employed as the low-temperature sintering aids. In this paper, we suggest ZnBO as an alternative dopant to the B₂O₃ and Li₂CO₃. To reduce the sintering temperature of (Ba, Sr)TiO₃, we have added 1–5 wt.% of ZnBO to (Ba, Sr)TiO₃. ZnBO-doped (Ba, Sr)TiO₃ ceramics were respectively sintered from 750 to 1350 °C by 50 °C to confirm the sintering temperature with different dopant contents. By adding 5 wt.% of ZnBO to the (Ba, Sr)TiO₃ ceramics, the sintering temperature of (Ba, Sr)TiO₃ ceramics can be reduced to 1100 °C. From the XRD analysis, ZnBO-doped (Ba, Sr)TiO₃ has no pyro phase. By adding ZnBO dopants to (Ba, Sr)TiO₃ ceramics, both of relative dielectric permittivity and loss tangent were decreased. From the frequency dispersion of dielectric properties, the relative dielectric permittivity and loss tangent of 5 wt.% ZnBO-doped (Ba, Sr)TiO₃ were 1180 and 3.3 × 10⁻³, while those of BST were 1585 and 4.8 × 10⁻³, respectively.     

Proton conductivity and mobility in Sr-doped LaScO3 perovskites

In this work, we set out to investigate the electrical conductivity of single-phase and high-density La_1-xSr_xScO_3-δ (x = 0.05; 0.1) ceramics depending on temperature and рО₂ and рН₂О. The crystal structure of materials was characterized by XRD method. The samples show the structure of an orthorhombic perovskite with a Pnma space group. The unit cell volume increases along with the Sr concentration. The microstructure features of samples were investigated by SEM analysis. The transference numbers of protons and oxygen-ions were determined by the EMF (electromotive force) measurements in a gas concentration cell. In addition, the proton, oxygen-ion and hole conductivities were evaluated from the рО₂-dependencies of electrical conductivity at different humidity. The results obtained using both methods showed a good level of agreement. It is found that the partial conductivity of each charge carrier in La_1-xSr_xScO_3-δ increases along with an increase in the concentration of the Sr dopant from x = 0.05 to x = 0.1. The highest proton conductivity about 3 × 10^?2 S cm^?1 is achieved for La_0·9Sr_0.1ScO_3-δ at 800 °C. The mobility of proton defects increases with Sr concentration and reaches 2.5 × 10^?4 cm² V^?1 s^?1 at 800 °C for La_0·9Sr_0.1ScO_3-δ. Thus, La_0·9Sr_0.1ScO_3-δ should be considered as a promising proton-conducting electrolyte for various electrochemical devices, such as protonic ceramic fuel cells.     

Effect of strontium and cerium doping on the structural characteristics and catalytic activity for C3H6 combustion of perovskite LaCrO3 prepared by sol–gel

Two series of Sr- or Ce-doped La_1−xM_xCrO₃ (x = 0.0, 0.1, 0.2 and 0.3) catalysts were prepared by thermal decomposition of amorphous citrate precursors followed by annealing at 800 °C in air atmosphere. The effect of Ce and Sr on the morphological/structural properties of LaCrO₃ was investigated by means of thermogravimetric/differential thermal analysis (TG/DTA) of the precursors decomposition under air, X-ray diffraction (XRD), electron paramagnetic resonance (EPR), transmission electron microscopy–X-ray energy dispersive spectroscopy (TEM–XEDS), S_BET determination, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) techniques. The characterization results are employed to explain catalytic activity results for C₃H₆ combustion. It is shown that the lanthanum chromite perovskite structure is obtained upon thermal treatment of the sol–gel derived precursors at T > ca. 800 °C. The presence of the dopant generally induces the formation of segregated oxide phases in the samples calcined at 800 °C although some introduction of the Sr in the perovskite structure is inferred from EPR measurements. The oxidation activity becomes maximised upon formation of such doped perovskite structure.     

Ni–Nb-Based Mixed Oxides Precursors for the Dry Reforming of Methane

Perovskite-related mixed-oxides based on La Ni Nb and La Sr Ni Nb were synthesized by the auto combustion method to use as precursors materials for the catalytic reforming of methane at 700 ºC, atmospheric pressure, CH₄:CO₂ = 1:1. LaNiO₃ and LaNbO₄ were used as reference. XRD analysis show that the synthesis method produce a new series of precursor family formed by a mixture of oxides where Ni crystallized as part of a perovskite and Ruddlesden–Popper structure while Nb formed lanthanum orthoniobate LaNbO₄, a scheelite-type structure alternating with oxide layers, with phase distribution depending on niobium content. For Nb (x ≤ 0.3) Ni crystallizes as LaNiO₃ perovskite-type oxide while for Nb (x ≥ 0.7) it forms mainly the orthoniobate phase LaNbO₄ a scheelite-type structure. At higher calcined temperatures (~1100 °C) La₂Ni_0.8Nb_0.2O₄ was formed with a Ruddlesden–Popper structure consisting of three perovskite type layers along the c-axis alternating with a layer of the rock salt type phase. TEM analysis showed the presence of cubic particles with sizes varying between 5 and 60 nm depending on the extent of substitution of Ni by Nb. Reduction of the perovskite-related precursor oxides produced a series of Ni⁰/La₂O₃–NbOx oxides with high metallic dispersion which favors the activity and stability of the catalysts. Introduction of doping quantities of Sr into LaNi_0.8Nb_0.2O_3±λ structure produced a mixture of oxides with Sr dissolved in the lanthanum orthoniobate LaNbO₄ scheelite-type structure due to the similarity of ionic radii of La and Sr. Under the reaction conditions conversions near the thermodynamic equilibrium were attained which remains for long periods of time assessing the stability of the synthesized catalysts.