Oxycarbonate Phase Research Articles

Stabilization of the Active Ruthenium Oxycarbonate Phase for Low-Temperature CO2 Methanation.

Interstitial carbon-doped RuO2 catalyst with the newly reported ruthenium oxycarbonate phase is a key component for low-temperature CO2 methanation. However, a crucial factor is the stability of interstitial carbon atoms, which can cause catalyst deactivation when removed during the reaction. In this work, the stabilization mechanism of the ruthenium oxycarbonate active phase under reaction conditions is studied by combining advanced operando spectroscopic tools with catalytic studies. Three sequential processes: carbon diffusion, metal oxide reduction, and decomposition of the oxycarbonate phase and their influence by the reaction conditions, are discussed. We present how the reaction variables and catalyst composition can promote carbon diffusion, stabilizing the oxycarbonate catalytically active phase under steady-state reaction conditions and maintaining catalyst activity and stability over long operation times. In addition, insights into the reaction mechanism and a detailed analysis of the catalyst composition that identifies an adequate balance between the two phases, i.e., ruthenium oxycarbonate and ruthenium metal, are provided to ensure an optimum catalytic behavior.

Low-oxidation-state Ru sites stabilized in carbon-doped RuO2 with low-temperature CO2 activation to yield methane

The generation of methane fuel using surplus renewable energy with CO2 as the carbon source enables both the decarbonization and substitution of fossil fuel feedstocks. However, high temperatures are usually required for the efficient activation of CO2. Here we present a solid catalyst synthesized using a mild, green hydrothermal synthesis that involves interstitial carbon doped into ruthenium oxide, which enables the stabilization of Ru cations in a low oxidation state and a ruthenium oxycarbonate phase to form. The catalyst shows an activity and selectivity for the conversion of CO2 into methane at lower temperatures than those of conventional catalysts, with an excellent long-term stability. Furthermore, this catalyst is able to operate under intermittent power supply conditions, which couples very well with electricity production systems based on renewable energies. The structure of the catalyst and the nature of the ruthenium species were acutely characterized by combining advanced imaging and spectroscopic tools at the macro and atomic scales, which highlighted the low-oxidation-state Ru sites (Run+, 0 < n < 4) as responsible for the high catalytic activity. This catalyst suggests alternative perspectives for materials design using interstitial dopants.

Mechanisms for texture in BaTiO3 thin films from aqueous chemical solution deposition

The prototype piezoelectric material BaTiO3 is widely used in e.g., capacitators. Chemical solution deposition (CSD) of BaTiO3 films is a simple and environmentally friendly processing route, but insight in the crystallization process is crucial to tailor the film properties. In this work, the influence of the annealing conditions on the crystallization behavior of BaTiO3 thin films from aqueous chemical solution deposition is presented. In situ synchrotron X-ray diffraction was used to reveal the phase evolution, crystallization of the films, and to study how the degree of crystallographic texture in the polycrystalline films evolved. Our results revealed that the formation of an intermediate metastable oxycarbonate phase is critical for the formation of BaTiO3 thin films prepared by aqueous CSD. The pyrolysis products present in the film before crystallization determine the degree of preferential orientation and by tuning the heating program, especially the heating rate through nucleation (<0.2 °C/s), control of the microstructure and degree of preferential orientation in the films was demonstrated.In situ X-ray diffraction patterns of a BaTiO3 film, prepared by spin coating of an aqueous chemical solution, as a function of time and temperature, showing the phase evolution and crystallization of BaTiO3.

Photocatalytic La4Ti3O12 nanoparticles fabricated by liquid-feed flame spray pyrolysis

Hexagonal plate-like nanoparticles (NPs) of the layered perovskite La4Ti3O12 were fabricated using liquid-feed flame spray pyrolysis (LF-FSP) followed by subsequent heat-treatments. Their photocatalytic activity was evaluated using decolorization of methyl orange solutions under Uv irradiation. LF-FSP combusts metalloorganic precursor aerosols to produce mixtures of cubic simple perovskite (ABO3) phase and lanthanum oxycarbonate (La2O4·846C0.846) phase with very low agglomeration and average particle sizes (APSs) of 23 nm (as-produced NPs). Rietveld refinement of synchrotron XRD powder patterns verified that the simple perovskite in the as-produced NPs is LaTiO3 (originally cubic Pm-3m-type space group) and heat-treating gives NPs of the trigonal layered perovskite La4Ti3O12 (R-3-type space group). La4Ti3O12 NPs heat-treated at 1100 °C/3-6h/air exhibits hexagonal plate-like morphology and high crystallinity offering enhanced photocatalytic degradation of methyl orange solutions compared to the as-produced NPs. The LF-FSP approach to obtaining layered perovskite La4Ti3O12 NPs provides a simple route to photocatalytic materials in reasonable quantities.

Thermal behavior and decomposition of cerium(III) butanoate, pentanoate and hexanoate salts upon heating in argon

The thermal behavior and decomposition of Ce-butanoate monohydrate (Ce(C3H7CO2)3·H2O), Ce-pentanoate (Ce(C4H9CO2)3) and Ce-hexanoate (Ce(C5H11CO2)3) were studied in a flow of argon while heating at 5°C/min. By means of several techniques such as simultaneous TG-DTA, FTIR evolved gas analysis, in-situ x-ray diffraction using a synchrotron source and hot-stage microscopy, it was found that all three compounds undergo melting transitions prior to decomposition and that decomposition involves intermediate stages including at least a Ce2O(CnH2n+1CO2)4 intermediate (n=3, 4 or 5 for Ce-butanoate, pentanoate or hexanoate respectively). The final decomposition product consists of CeO2, which is formed through a Ce-oxycarbonate. The Ce3+→Ce4+ oxidation seems to proceed via Ce2O3 that first results from the decomposition of the oxycarbonate phase. During the whole decomposition process, the evolved gas species consist of CO2 and symmetrical ketones.

Theoretical investigations of the g factors of orthorhombic Cu2+ site in oxycarbonate phase YBa2Cu2.95(CO3)0.35O6.6

The [Formula: see text] factors of orthorhombic [Formula: see text] site in oxycarbonate phase [Formula: see text] are theoretically investigated using the perturbation formulas of the [Formula: see text] factors for an orthorhombically elongated [Formula: see text] cluster. This pseudo-octahedral [Formula: see text] center is formed by two apical oxygen ligands and four coplanar oxygen ions belonging to the respective four neighboring carbonate groups. The [Formula: see text] cluster is found to suffer the relative axial elongation of about 0.04 Å and the perpendicular bond length variation of about 0.12 Å due to the Jahn–Teller effect. The theoretical [Formula: see text] factors based on the local structural parameters show good agreement with the experimental data.

In situ characterization of phase transformation and reactivity of high surface area lanthanum-based Ru catalysts for the combined reforming of methane

A lanthanum oxycarbonate phase (La2O2CO3) with high surface area was prepared as support of Ru catalysts. The metal was added by wet impregnation or incorporated during the oxycarbonate synthesis. The reactivity of the lanthanum and ruthenium species was studied through in situ Raman under dry reforming of methane conditions with and without oxygen addition. When Ru was incorporated by wet impregnation, the solid showed a higher reactivity of lanthanum oxycarbonate to the formation of La2O3 in a hydrogen flow even at low temperatures while RuOx was completely reduced at 400°C. It was also found that the reduced ruthenium species could be re-oxidized by CO2 at the reaction temperature (550°C). Under the dry reforming of methane, the metallic Ru did not change its oxidation state and the formation of graphitic carbon and oxycarbonate was observed. With the addition of oxygen, the same phase transformation occurred as in the case of the dry reforming of methane. However, the re-oxidation of Ru0 species was observed through in situ Laser Raman spectroscopy and pseudo in situ XPS measurements. These measurements allowed us to advance in the understanding of the solid transformations produced during methane reforming reactions.

Thermal stability and oxidizing properties of mixed alkaline earth-alkali molten carbonates: A focus on the lithium-sodium carbonate eutectic system with magnesium additions

A comparative study on thermal behavior and oxygen solubility properties of eutectic 52/48 lithium/sodium carbonate salt containing minor additions of magnesium up to 10mol% has been made in order to determine whether a general correlation between these two properties can be found or not. Consecutive TG/DSC heating/cooling thermal cycles carried out under alternating CO2 and N2 gas flows allowed to assign thermal events observed in the premelting region to a partial decarbonation process of the magnesium-alkali mixed carbonates. The observed decarbonation process at 460°C is believed to come from initial stage of thermal decomposition of magnesium carbonate resulting in the metastable formation of magnesium oxycarbonate-like phases MgO·2MgCO3, in a similar manner as previously reported for lanthanum. Reversible formation and decomposition of the magnesium carbonate phase has been observed under a CO2 gas atmosphere. The intensity of the decomposition process shows a maximum for a 3mol% MgO addition that gives also the highest oxygen solubility, suggesting therefore that instability thermal analysis in the premelting region can be considered as providing an effective measure of the basicity/oxidizing properties of alkali carbonate melts with magnesium or, in more general terms, with cations that are strong modifiers of the carbonate melt basicity through formation of soluble oxycarbonate phases.

Comparison of Ru/La2O2CO3 performance in two different membrane reactors for hydrogen production

Ruthenium catalysts supported on a lanthanum oxycarbonate phase (La2O2CO3) with high surface area were prepared. All solids were stable for at least 90h on stream in the combined dry reforming and oxidation of methane reactions. The catalyst with more stable metallic Ru species was selected to be studied under the more severe conditions of membrane reactors. A double tubular membrane reactor was built using a commercial self-standing membrane or a composite Pd/NaAPSS membrane. The Pd membrane reactor exhibited a higher H2 permeated/CH4 fed ratio at 450̊C that could be related to its higher hydrogen permeability. The comparison of the catalytic behavior of Ru catalysts supported on high surface area lanthanum oxycarbonate and La2O3–SiO2 binary support in a membrane reactor was performed at 550̊C. On the retentate side, the molar composition of CO2 was similar to that of CH4 when the Ru/La2O2CO3 catalyst was employed, which suggests that this solid did not favor the RWGS reaction under DRM conditions producing a higher H2 permeated/CH4 fed ratio. The Ru catalysts showed a similar performance to that of our best Rh solids when the fractions of the reaction equilibration and H2 permeated/(CH4 fed×permeation area) ratio were compared.

Thermal analysis of lanthanum hydroxide

Lanthanum oxide (La2O3) is of great interest as catalyst material. When La2O3 particles are prepared from lanthanum hydroxide (La(OH)3) by thermal processes under air, various oxycarbonate phases are formed which are resistant to thermal hydroxylation. This phenomenon has not yet been extensively investigated, even though oxycarbonate phases at the particle surfaces cause a change in lanthanum oxide’s catalytic activity. The carbonate phases formed cannot be detected by means of XRD or REM-EDX investigations due to their detection limits. Thermal analysis, particularly TG-FT-IR, allows not only for the detection of the carbonate phases in La(OH)3, but also for the tracking of the entire dehydration process from La(OH)3 via LaOOH to La2O3 as well as the correct interpretation of mass changes during the thermal transformations. Pursuant to the investigations here carried out, it was determined that carbonate-free lanthanum hydroxide compounds can only be prepared and stored in a CO2-free protective gas atmosphere (e.g., argon).

Microstructure and properties of single-crystal rare-earth oxide fibers

Single-crystal Ln2O3 (Ln = Sm, Eu, Gd) fibers are grown by the laser-heated miniature pedestal technique. The relationship between the microstructure of fibers and their plasticity is analyzed using scanning electron microscopy, energy-dispersive microanalysis, electron backscatter diffraction, and X-ray methods. It is shown that the distinction between ductile and brittle fibers is determined basically by the difference in their microstructure. The fracture of samarium oxide fibers is associated with the formation of oxycarbonate phases on their surface. The most promising are the fibers of gadolinium oxide, having the most perfect microstructure, moisture and a carbon dioxide resistance, and a wide range of transparency.

ChemInform Abstract: Sol-Gel Synthesis of Strontium-Doped Lanthanum Manganite.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

The Rapid Formation of La(OH)3 from La2O3 Powders on Exposureto Water Vapor

The reactivity of various lanthana powders in air was studied. The materials rapidly hydroxylate to form a stable hydroxide, La(OH)3, at room temperature. Smaller amounts of an oxycarbonate species (La2O2CO3) are also observed following air exposure. This oxycarbonate phase is stable to hydroxylation. All oxide materials synthesized here show rapid reactions so that 24 h of exposure to atmosphere is generally sufficient to cause complete hydroxylation at room temperature. The rate of reaction was related to the crystallite size as determined by XRD. The reaction was found to follow a two‐stage kinetic process, a relatively slow surface reaction followed by a rapid bulk reaction. The relevance of the reactivity of these powders is discussed.

Stability of lanthanum oxide-based H 2S sorbents in realistic fuel processor/fuel cell operation

We report that lanthana-based sulfur sorbents are an excellent choice as once-through chemical filters for the removal of trace amounts of H 2S and COS from any fuel gas at temperatures matching those of solid oxide fuel cells. We have examined sorbents based on lanthana and Pr-doped lanthana with up to 30 at.% praseodymium, having high desulfurization efficiency, as measured by their ability to remove H 2S from simulated reformate gas streams to below 50 ppbv with corresponding sulfur capacity exceeding 50 mg S g sorbent −1 at 800 °C. Intermittent sorbent operation with air-rich boiler exhaust-type gas mixtures and with frequent shutdowns and restarts is possible without formation of lanthanide oxycarbonate phases. Upon restart, desulfurization continues from where it left at the end of the previous cycle. These findings are important for practical applications of these sorbents as sulfur polishing units of fuel gases in the presence of small or large amounts of water vapor, and with the regular shutdown/start-up operation practiced in fuel processors/fuel cell systems, both stationary and mobile, and of any size/scale.

Disorder-dependent photoluminescence in Ba 0.8Ca 0.2TiO 3 at room temperature

The photoluminescence (PL) emission in structurally disordered Ba 0.8Ca 0.2TiO 3 (BCT20) powders was observed at room temperature with laser excitation at lines 355 and 460 nm. The structural evolution perovskite-like titanate BCT20 powders prepared by a soft chemical processing at different annealing temperatures were accompanied by X-ray diffraction (XRD) and X-ray absorption near-edge structure (XANES). Intermediate oxycarbonate phase was identified and your influence with PL emission was discarding. BCT20 annealed at 500 °C displays intense PL emission. The results indicate relationship between broad PL band and order–disorder degree.

Sustainable route to hydrogen – Design of stable catalysts for the steam gasification of biomass related oxygenates

Steam reforming of acetic acid, a biomass derived oxygenate, has been studied over Ni-based catalysts. Ni/ZrO2 showed gradual deactivation in time. Coke formation as well as a competitive adsorption of reactants is suggested to be responsible for the catalyst deactivation. Addition of potassium strongly influenced the reduction temperature of NiO. The presence of potassium and lanthanum improved catalyst stability by decreasing coke accumulation. It is proposed that the presence of lanthanum as an oxy-carbonate phase enhances coke gasification.

Formation of metastable calcite-type barium carbonate during low-temperature decomposition of (Ba,Ti)-precursor complexes

(Ba,Ti)-precursor complexes, important for the production of advanced BaTiO 3 perovskite-type materials, undergo structural transformations and complex reactions during their thermal decomposition. Based on XRD phase analysis, combined with Rietveld refinement of crystal structure data, and on IR analysis, the intermediate formation of calcite-type BaCO 3 is evidenced, which can be explained by the stabilization of this metastable modification in the form of an oxycarbonate phase down to room temperature. Two possible processes, leading to such an oxycarbonate, are discussed: (i) partial substitution of CO 3 2− by O 2− in the anionic sublattice, and (ii) topotaxial formation of calcite-type structural domains of BaCO 3 by templating with oxygen-deficient titanates, resulting in the oxide–carbonate intergrowth structures.

Phase evolution of a barium tin 1,2-ethanediolato complex to barium stannate during thermal decomposition

The thermal behaviour of [Ba(C 2H 6O 2) 4][Sn(C 2H 4O 2) 3] used as a BaSnO 3 precursor, and its phase evolution during thermal decomposition are described. The initially formed transient barium tin oxycarbonate phase disintegrates into BaCO 3 and SnO 2, reacting subsequently to BaSnO 3. The existence of the intermediate oxycarbonate phase is evidenced by Fourier transformed infrared (FT-IR), X-ray powder diffraction (XRD) and electron energy loss spectroscopy (EELS (ELNES)) investigations.

Synthesis and Crystal Structure of the Alkaline‐Earth Thallates MnTl2O3+n (M: Ca, Sr).

AbstractFor Abstract see ChemInform Abstract in Full Text.

Effects of rhodium and platinum on the reactivity of lanthanum phases

Rhodium and platinum catalysts were prepared by wet impregnation of Anedra lanthanum oxide. The polymorph forms of La 2O 2CO 3 have also been prepared by an appropriate treatment of lanthanum compounds in carbon dioxide. The characterisation of the phases present in these solids was performed by XRD, FT-IR, LRS, TPD and TGA. One of the untreated supports (Aldrich) was La(OH) 3, and the other (Anedra) consisted mainly of II-La 2O 2CO 3 with traces of type Ia oxycarbonate and lanthanum hydroxide. Catalysts with a low rhodium loading showed an increase of Ia-La 2O 2CO 3 already present on the support, whereas those with a low platinum loading would not modify the phases of the lanthanum solid. A different behaviour was observed in high metal loading catalysts. In this case, metals would catalyse the decomposition of type Ia polymorph and also prevent the hydration of the support. The treatment with CO 2 provides additional information about the effect of the gas upon the metal containing solids. Pt would favour the carbon dioxide interaction with the support to form monoclinic polymorph (type Ia), while rhodium would prevent the further formation of the oxycarbonate phase.