La2O2CO3 Phase Research Articles

Influence of catalyst synthesis methods on anti-coking strength of perovskites derived catalysts in biogas dry reforming for syngas production

This study examined biogas dry reforming using perovskite-derived catalysts over the temperature range of 650–850 °C. Citrate-gel combustion and co-precipitation methods were used to prepare perovskites (LaCoO3 and LaNiO3). These perovskites were converted into metal-supported catalysts by reduction process. Also, impregnation was applied for the synthesis of 30 wt% Ni/La2O3 and 10 wt% Ni/Al2O3. X-ray diffraction and high-resolution transmission electron microscopy confirmed that the average particle size of Ni or Co was smaller in the combustion method over co-precipitated method. Citrate-gel combustion method favors La2O2CO3 phase, whereas co-precipitation method prefers La(OH)3 phase in the support. Although the biogas dry reforming condition used in this study (CH4/CO2 = 1.5) is much severe for coke deposition than conventional dry reforming condition (CH4/CO2 = 1.0), the catalyst derived from perovskite LaNiO3 by citrate-gel combustion method showed outstanding performance among examined catalysts. The La2O2CO3 phase self-regenerates and oxidizes carbon from the nickel sites continuously. The high anti-coking strength was attributed to the highly dispersed nickel nanoparticles, La2O2CO3 species, and low acidity confirmed by NH3-TPD. Therefore, the citrate-gel combustion method is an alternative to conventional methods for biogas dry reforming.

Ni/La2O3 catalysts for dry reforming of methane: Effect of La2O3 synthesis conditions on the structural properties and catalytic performances

10 wt%Ni/La2O3 catalysts for dry reforming of methane (DRM) were synthesized by wetness impregnation of lanthana supports prepared using sol-gel citric method with and without NH3 addition (Ni–La CA-NH3 and Ni–La CA, respectively). The support preparation conditions affect the nature, phase composition, and distribution of Ni phases (LaNiO3, NiO and La3Ni2O6). The gradient temperature DRM tests (400–800 °C) reveal higher catalytic activity of Ni–La CA (at 650 °C, X(CO2) = 65.7%, X(CH4) = 54.6%, H2/CO = 0.71). The Ni–La CA-NH3 shows higher stability (at 650 °C and 24 h, X(CO2): 73.7% => 76.4%, X(CH4): 64.7% => 64.6%, H2/CO: 0.77 => 0.72). For both catalysts, La2O2CO3 phase is formed after long run tests at 650 °C 24 h, with the greater TGA weight loss and stronger deactivation being observed for Ni–La CA. The H2-reduced Ni La CA-NH3 features ultrasmall (1–2 nm) Ni NPs strongly interacting with the support. Catalyst nature affects the amount of carbon coke formed.

Hexagonal and Monoclinic Phases of La2O2CO3 Nanoparticles and Their Phase-Related CO2 Behavior.

In this study, we prepared hexagonal and monoclinic phases of La2O2CO3 nanoparticles by different wet preparation methods and investigated their phase-related CO2 behavior through field-emission scanning microscopy, high-resolution transmission electron microscopy, Fourier transform infrared, thermogravimetric analysis, CO2-temperature programmed desorption, and linear sweeping voltammetry of CO2 electrochemical reduction. The monoclinic La2O2CO3 phase was synthesized by a conventional precipitation method via La(OH)CO3 when the precipitation time was longer than 12 h. In contrast, the hydrothermal method produced only the hexagonal La2O2CO3 phase, irrespective of the hydrothermal reaction time. The La(OH)3 phase was determined to be the initial phase in both preparation methods. During the precipitation, the La(OH)3 phase was transformed into La(OH)CO3 owing to the continuous supply of CO2 from air whereas the hydrothermal method of a closed system crystallized only the La(OH)3 phase. Based on the CO2-temperature programmed desorption and thermogravimetric analysis, the hexagonal La2O2CO3 nanoparticles (HL-12h) showed a higher surface CO2 adsorption and thermal stability than those of the monoclinic La2O2CO3 (PL-12h). The crystalline structures of both La2O2CO3 phases predicted by the density functional theory calculation explained the difference in the CO2 behavior on each phase. Consequently, HL-12h showed a higher current density and a more positive onset potential than PL-12h in CO2 electrochemical reduction.

Synthesis and optical properties of La2O2CO3:Eu3+ hollow sphere phosphors by hydrothermal synthesis assisting with double-crucible carbonization method

Lanthanum oxide, europium oxide, hydrochloric acid, ammonia water, and carbon powder were used as the starting materials. Detailed characterizations were attained by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), thermogravimetry/differential thermal analysis (TG-DTA), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), and photoluminescence (PL) spectroscopy. The La(OH)3 precursor was synthesized by an hydrothermal synthesis method at 120 °C for 24 h, which was then converted into La2O2CO3 phase through calcining the precursor at 800 °C for 2 h in a carbon atmosphere. It was demonstrated that the as-obtained La2O2CO3 is a hollow sphere with uniform size about 200 nm. The formation mechanism of the La2O2CO3 phase has also been proposed. Photoluminescence analysis showed that the La2O2CO3:Eu3+ phosphors have the strongest red emissions at 614 nm under 281 nm ultraviolet (UV) light excitation, which correspond to the 5D0 → 7F2 transition of Eu3+ ions. The La2O2CO3:Eu3+ phosphors have double-exponential decay behavior and the calculated lifetime is determined to be t1 = 0.205 μs and t2 = 1.177 μs.

Auto-combustion synthesis and characterization of perovskite-type LaFeO3 nanocrystals prepared via different routes

Abstract LaFeO3 was prepared via simple, economical and environmentally friendly auto-combustion methods using different fuels, including citrate, urea, sucrose, egg whites, gelatin and chitosan. The prepared ferrites were systematically characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, high resolution transmission electron microscopy (HRTEM), vibrating sample magnetometry (VSM) and electrical measurements to study the impact of the synthetic routes on the various properties. XRD showed the formation of a single orthorhombic phase, except for the samples prepared via egg white, gelatin and urea methods, which indicated a secondary La2O2CO3 phase. The FT-IR and TEM studies confirmed the formation of the phase. Differential thermal analysis-thermogravimetry techniques (DTA-TG) were used to characterize the auto-combustion process up to ferrite formation. The obtained unsaturated magnetization for all of the samples suggested the presence of anti-ferromagnetic ordering in addition to weak ferromagnetic ordering. The egg white method showed higher magnetization (1.78 emu/g), while the urea and chitosan methods indicated lower magnetization (0.13 emu/g). The coercivity measurements confirmed the presence of ferromagnetic/anti-ferromagnetic interfaces with higher values than those reported in the literature. The AC-conductivity measurements indicated semiconducting characteristics and exhibited a change in the conduction mechanism from hopping to polaron conduction by increasing the temperature. LaFeO3 prepared via the egg white method indicated an anti-ferromagnetic/paramagnetic transition at approximately 715 K. The dielectric measurements confirmed the AC-conductivity results and exhibited values higher than those reported in the literature.

Carbon-Based Nanostructures Vertically Arrayed on Layered Lanthanum Oxycarbonate as Highly Efficient Catalysts for Oxygen Reduction Reactions.

Controllable pyrolysis of collapsible metal-organic frameworks (MOFs) into carbon-based nanostructures without obvious collapse and aggregation is of importance for the fabrication of well catalytic active and durable carbon-based catalysts for the oxygen reduction reaction (ORR). Herein, we fabricate morphology-controlled carbon-based nanostructures derived from the Co-based zeolitic imidazolate framework (ZIF-67) that epitaxially grows on layered lanthanum oxycarbonate (La2O2CO3) as a structure-oriented template, followed by pyrolysis at 800 °C. These synthesized carbon-based nanostructures show a well-defined dodecahedron morphology and vertical array on the template surface. In 0.1 M KOH solution, the ORR activity and durability of the carbon-based nanostructures are not only much higher than those obtained by pyrolytic carbons derived from pure ZIF-67 but also exceed commercial Pt/C (20 wt %, Pt). The significantly improved ORR performance can be ascribed to the increased Co-N x level, high specific surface area, and graphitization of the pyrolytic carbon, caused by the introduction of the La2O2CO3 phase into the composite catalyst. Therefore, using La2O2CO3 as the template may be a smart synthetic strategy for MOF-derived nanocarbons with a controlled morphology and composition for energy storages and conversions.

Structural properties of disordered macroporous La2O2CO3/ZnO materials prepared by a solution combusion method

Disordered macroporous La2O2CO3/ZnO materials were prepared by a solution combustion method, and then their microstructures and La2O2CO3 phases were investigated as functions of La/Zn ratios and calcination temperatures. All of the materials prepared by the solution combustion method clearly showed disordered macroporous morphology whose framework was composed of ZnO and La2O2CO3 nanoparticles. A monoclinic La2O2CO3 structure was mainly formed in the disordered macroporous materials at La/Zn=1 and 2. In contrast, the conventional coprecipitation of La2O2CO3/ZnO materials dominantly formed a hexagonal La2O2CO3 phase with aggregating morphology of ZnO and La2O2CO3 nanoparticles. However, nanocrystalline sizes of ZnO (7–10 nm) and monoclinic La2O2CO3 (12–14 nm) in the disordered macroporous structure were much smaller than those (29–36 nm for ZnO and 44–58 nm for hexagonal La2O2CO3) by conventional co-precipitation. In addition, the high temperature calcination at 700 °C increased the ZnO nanocrystallite size (24 nm) in the disordered macroporous framework, with transforming La2O2CO3 into La2O3. This result implies that the interaction between monoclinic La2O2CO3 and ZnO in the disordered macroporous structure inhibited ZnO nanoparticle agglomeration.

Influence of Preparation Conditions on the Performance of Ni-Based Catalysts for Glycerol Steam Reforming.

The effect of preparation conditions on the performance of Ni-based catalysts was investigated for glycerol steam reforming. La0.7Ce0.3NiO3 mixed oxides were synthesized using different solution concentrations and calcination temperatures by a co-precipitation method. Brunauer–Emmett–Teller, inductively coupled plasma, X-ray diffraction, temperature-programmed reduction, field emission scanning electron microscopy, CO2-temperature-programmed desorption, thermal gravity analysis, and Raman spectroscopy were performed to characterize the catalysts. With an La2NiO4 phase at lowest solution concentration, a catalyst precursor included an LaNiO3 phase instead of La2NiO4 at other solution concentrations. At a low calcination temperature of 700 °C, it was found that the smaller particle size of CeO2 incorporated more lanthanum and higher surface basicity, and the La2O2CO3 phase could effectively inhibit and eliminate coking leading to the better performance of catalysts.

Glycerol carbonation with CO2 and La2O2CO3/ZnO catalysts prepared by two different methods: Preferred reaction route depending on crystalline structure

La2O2CO3/ZnO catalysts for glycerol carbonation with CO2 to produce glycerol carbonate were prepared by two different methods – co-precipitation (conventional method) and ethylene glycol combustion (novel method) – and investigated their catalytic properties and performance through characterization and reaction tests. La2O2CO3/ZnO prepared by the novel method (NLZ) showed higher yields of glycerol carbonate than La2O2CO3/ZnO prepared by the conventional method (CLZ) due to extremely high selectivity of glycerol carbonate by NLZ, even though NLZ exhibited lower glycerol conversions. Depending on the preparation method, predominant La2O2CO3 phases were different: monoclinic La2O2CO3 for NLZ and hexagonal La2O2CO3 for CLZ. From characterization of the spent catalysts, zinc glycerolate, an intermediate in this reaction, was formed in a large aggregate of nanoparticles over the spent CLZ, while the spent NLZ contained a trace amount of zinc glycerolate. These different catalytic behaviors were explained on the basis of catalytic reaction routes and La2O2CO3 structures.

Lanthanum Oxide-catalyzed Transesterification of Dimethyl Carbonate with Glycerol: Effect of Surfactant

Three kinds of lanthanum oxides (La2O3) were synthesized from different methods and used as a catalyst in the transesterification of dimethyl carbonate (DMC) with glycerol for the synthesis of glycerol carbonate (GLC). Lanthanum oxide synthesized using a surfactant (S-La) showed a much higher GLC yield of 89.9% compared to other lanthanum oxides synthesized by calcination (C-La) and precipitation (P-La) at the reaction conditions of 90 °C, DMC/glycerol = 2, and catalyst/glycerol = 5 wt %. The best catalyst was obtained when the surfactant/La weight ratio was 12. XRD study revealed that S-La has large amount of monoclinic and hexagonal La2O2CO3 phases, which are assumed as active sites of the catalyst for the reaction.

Modifying perovskite-type oxide catalyst LaNiO3 with Ce for carbon dioxide reforming of methane

Perovskite-type oxide catalysts LaNiO3 and La1−xCexNiO3 (x ≤ 0.5) were prepared by the Pechini method and used as catalysts for carbon dioxide reforming of methane to form synthesis gas (H2 + CO). The gaseous reactants consisted of CO2 and CH4 in a molar ratio of 1:1. At a GHSV of 10,000 hr−1, CH4 conversion over LaNiO3 catalyst increased from 66% at 600 °C to 94% at 800 °C, while CO2 conversion increased from 51% to 92%. The achieved selectivities of CO and H2 were 33% and 57%, respectively, at 600 °C. To prevent the deposition of carbon and the sintering nickel species, some of the Ni in perovskite-type oxide catalyst was substituted by Ce. Ce provided lattice oxygen vacancies, which activated C–H bonds, and increased the selectivity of H2 to 61% at 600 °C. XRD analysis indicates that the catalyst exhibited a typical perovskite spinel structure and formed La2O2CO3 phases after CO2 reforming. The FE-SEM results reveal carbon whisker of the LaNiO3 catalyst and the BET analysis indicates that the specific surface area increases after the reforming reaction. The H2-TPR results confirm that Ce metals can store and provide oxygen.

Observation of exceptional strong emission transitions 5Dj (j=1−3) to 7Fj (j=1−3): Multicolor from single Eu3+ ion doped La2O3 nanoparticles

Eu3+ ion doped La2O2CO3 and La2O3 nanoparticles have been synthesized through hydrothermal route with subsequent heat-treatment. It is found that La2O2CO3 phase is highly stable at ambient atmosphere and decomposes to La2O3 at or above 700°C. Exceptional stronger emissions from 5Dj (j=1−3) to 7Fj (j=1−3) transitions are observed compared to most probable 5D0 to 7Fj (j=1−3) transitions in 1 and 3at% Eu3+ doped samples heated at 900°C under 278nm excitation. This may be ascribed due to minimum cross-relaxation among 5Dj (j=1−3) levels. Significant decrease of emission intensity from 5D1 or 5D2 level and subsequent increase of emission intensity from 5D0 level on increasing Eu3+concentration up to 10at% indicate relaxation of excited electrons from 5Dj=1, 2 to 5D0 level. Excitation at 394nm shows strong background emission intensity in 425–600nm due to different defect states/intermediate bands present within host electronic band. Colors including blue, green, yellow, pink and red to white could be produced by changing concentration of Eu3+, excitation wavelength and annealing temperature. Samples are dispersible and could be incorporated in polyvinyl alcohol (PVA) films. Materials will also be potential candidates for light emitting diodes (LEDs) and bio-labeling.

Carbon dioxide reforming of methane for syngas production over La-promoted NiMgAl catalysts derived from hydrotalcites

La-promoted NiMgAl mixed-oxide catalysts derived from hydrotalcite were synthesized by coprecipitation for the carbon dioxide reforming of methane. X-ray diffraction, thermogravimetric analysis, temperature-programmed reduction, X-ray photoelectron spectroscopy, temperature-programmed desorption of carbon dioxide, hydrogen chemisorption and transmission electron microscopy were used to investigate the structure and deactivation characteristics of the catalysts. The catalysts exhibited a narrow Ni-metal particle-size distribution between 5 and 11nm. The presence of lanthanum evidently improved reforming activity in the temperature range of 600–700°C, because the introduction of lanthanum into the NiMgAl catalysts increased the total amount of basic sites and the surface Ni content. The activity of the Ni0.5Mg2.5Al0.9La0.1O4.5 catalyst was slightly higher than those of the other lanthanum-containing catalysts. The addition of lanthanum improved the stability of the NiMgAl catalysts significantly because of the suppression of coke deposition. Lanthanum effectively inhibited coking by increasing the basicity of the catalysts, forming the La2O2CO3 phase and improving nickel metal dispersion.

Properties and phase composition of Ni–MgO–La2O3 systems submitted to different thermal treatments

This work investigated the properties and phase composition of nickel-based systems supported on mixed oxides of lanthanum and magnesium (in different proportions) that were submitted to different thermal treatments. Catalysts were obtained by co-precipitation and were characterised using different techniques. The diffractograms of the nickel and lanthanum oxide catalysts show more intense peaks associated with a perovskite-type phase. The presence of magnesium leads to a decrease in the intensity of the peaks that correspond to the perovskite phase. The mixed system showed a higher specific surface area compared with that of the system formed by nickel and lanthanum oxides. The specific surface area of the mixed system was proportional to the magnesium content. The reduction profiles showed interactions in these systems. Metallic nickel is formed at temperatures greater than 873 K after reduction with hydrogen. The La2O2CO3 phase is formed under hydrogen-production reaction conditions. In conclusion, the systems formed by nickel, lanthanum and magnesium exhibit properties that render them suitable for application in hydrogen production reactions.

MOCVD of Lanthanum Oxides from La(tmhd)3 and La(tmod)3 Precursors: A Thermal and Kinetic Investigation

AbstractMetal‐organic (MO)CVD processes using two different precursors, (La(tmhd)3 and La(tmod)3), have been investigated. In‐situ Fourier transform infrared (FTIR) spectroscopy investigations and thermal analysis have provided information on their thermal robustness during the sublimation processes and mass‐transport properties during MOCVD experiments. Both precursors can be efficiently vaporized in the temperature range 160–230 °C even though La(tmod)3 exhibits higher volatility. Lanthanum oxides have been efficiently deposited using a direct liquid injection (DLI) equipped MOCVD reactor in the temperature range 350–500 °C for both precursors. The effect of the addition of tetraglyme to the precursor solution has been evaluated. Films mainly consist of different phases of La2O3 and LaO(OH), while the carbon‐containing La2O2CO3 phase was observed only on the surface, with no relevant carbon contamination being present in the bulk of the film.