结构型LaMnO3钙钛矿催化剂的催化燃烧活性和热稳定性

为降低堇青石载体对钙钛矿催化剂活性和稳定性的影响,以堇青石蜂窝陶瓷为基材,采用原位沉淀和悬浮浸渍技术分别制备了SiO₂和La₂O₃为涂层的结构型LaMnO₃催化剂,通过甲苯催化燃烧反应考察了催化剂的活性和热稳定性。结果表明,原位沉淀技术虽然可以均匀和高强度地在载体表面负载La、Mn活性组分,但无法在表面形成LaMnO₃钙钛矿的活性相。悬浮浸渍技术则可以保持LaMnO₃催化剂的结构和活性,结构催化剂与粉末LaMnO₃表现出相似的活性规律。La₂O₃涂层比SiO₂涂层可以更有效地保持LaMnO₃在蜂窝陶瓷载体表面的高活性和热稳定性。     

Study of Water–Gas-Shift Reaction over La(1−y)SryNixCo(1−x)O3 Perovskite as Precursors

The performance of La_(1?y)Sr_yNi_xCo_(1?x)O₃ perovskites for the water gas shift reaction (WGSR) was investigated. The samples were prepared by the co- precipitation method and were performed by the BET method, XRD, TPR, and XPS. The catalytic tests were performed at 300 and 400 °C and H₂O_v/CO = 2.3/1 (molar ratio). The sample with the highest surface area is La_0.70Sr_0.30NiO₃. The XRD results showed the formation of perovskite structure for all samples, and the La_0.70Sr_0.30NiO₃ sample also presented peaks corresponding to La₂NiO₄ and NiO, indicating that the solubility limit of Sr in the perovskite lattice was surpassed. The replacement of Co by Ni favored the reduction of the species at lower temperatures, and the sample containing Sr presented the highest amount of reducible species, as identified by TPR results. All samples were active, the Sr containing perovskite appearing the most active due to the highest surface area, presence of the La₂NiO₄ phase, and higher content of Cu in the surface, as detected by XPS. Among the samples containing Co, the most active one was that with x = 0.70 (60% of CO conversion).     

Communicating with wireless perovskites: cation order and zinc volatilization

The synthesis of new families of perovskites with a 1:2 ordered Ba(Zn_1/3Ta_2/3)O₃-type structure was investigated. The compound La(Li_1/3Ti_2/3)O₃ was prepared with a 1:2 ordered arrangement of Li and Ti and is the first reported titanate with this type of structure. A new family of ordered perovskites, (Sr_2/3La_1/3)(Li_1/3Ta_2/3)O₃ and (Sr_2/3La_1/3)(Li_1/3Nb_2/3)O₃, were also prepared with a 1:2 layered order of Li⁺ and B⁵⁺ cations. All three compounds exhibit dielectric constants >25 and Q.f values >20,000. Studies were also made on the phase stability of Zn-deficient compositions of BZT. The hexagonal perovskite, Ba₈ZnTa₆O₂₄, was isolated in single-phase form and was found to be the stable phase formed as a result of the loss of ZnO from BZT. Ba₈ZnTa₆O₂₄ can be sintered to high density at temperatures considerably lower than pure BZT and exhibits very good microwave properties. In particular at GHz frequencies ε=30.5, Q.f=62,000, and τf=+36 ppm/°C.     

La(1−x)SrxCo1−yFeyO3 perovskites prepared by sol–gel method: Characterization and relationships with catalytic properties for total oxidation of toluene

La_(1−x)Sr_xCo_(1−y)Fe_yO₃ samples have been prepared by sol–gel method using EDTA and citric acid as complexing agents. For the first time, Raman mappings were achieved on this type of samples especially to look for traces of Co₃O₄ that can be present as additional phase and not detect by XRD. The prepared samples were pure perovskites with good structural homogeneity. All these perovskites were very active for total oxidation of toluene above 200 °C. The ageing procedure used indicated good thermal stability of the samples. A strong improvement of catalytic properties was obtained substituting 30% of La³⁺ by Sr²⁺ cations and a slight additional improvement was observed substituting 20% of cobalt by iron. Hence, the optimized composition was La_0.7Sr_0.3Co_0.8Fe_0.2O₃. The samples were also characterized by BET measurements, SEM and XRD techniques. Iron oxidation states were determined by Mössbauer spectroscopy. Cobalt oxidation states and the amount of O⁻ electrophilic species were analyzed from XPS achieved after treatment without re-exposition to ambient air. Textural characterization revealed a strong increase in the specific surface area and a complete change of the shape of primary particles substituting La³⁺ by Sr²⁺. The strong lowering of the temperature at conversion 20% for the La_0.7Sr_0.3Co_(1−y)Fe_yO₃ samples can be explained by these changes. X photoelectron spectra obtained with our procedure evidenced very high amount of O⁻ electrophilic species for the La_0.7Sr_0.3Co_(1−y)Fe_yO₃ samples. These species able to activate hydrocarbons could be the active sites. The partial substitution of cobalt by iron has only a limited effect on the textural properties and the amount of O⁻ species. However, Raman spectroscopy revealed a strong dynamic structural distortion by Jahn–Teller effect and Mössbauer spectroscopy evidenced the presence of Fe⁴⁺ cations in the iron containing samples. These structural modifications could improve the reactivity of the active sites explaining the better specific activity rate of the La_0.7Sr_0.3Co_0.8Fe_0.2O₃ sample. Finally, an additional improvement of catalytic properties was obtained by the addition of 5% of cobalt cations in the solution of preparation. As evidenced by Raman mappings and TEM images, this method of preparation allowed to well-dispersed small Co₃O₄ particles that are very efficient for total oxidation of toluene with good thermal stability contrary to bulk Co₃O₄.     

Blood compatibility of La2O3 doped diamond-like carbon films

La₂O₃ doped diamond-like carbon films (DLC) with different concentration were deposited by using Radio-Frequency magnetron sputtering. The microstructure and surface properties of DLC films were characterized by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and contact angle test. The blood compatibility of the samples was evaluated by tests of platelet adhesion. Results show the sp²-bonded C content increases with increasing of La₂O₃ concentration doped. A remarkable decrease of platelet adhered on the surface of the La₂O₃ doped DLC films was observed comparing to the Chrono flex used in clinical application, suggesting that La₂O₃ doped DLC is able to enhance its blood compatibility. The mechanism of hemocompatibility of doped films was discussed. Our results demonstrate that La₂O₃ doped DLC films are potentially useful biomaterials with good blood compatibility.     

Ni/La<Subscript>2</Subscript>O<Subscript>3</Subscript>-ZrO<Subscript>2</Subscript> catalyst for hydrogen production from steam reforming of acetic acid as a model compound of bio-oil

Hydrogen production from steam reforming of acetic acid was investigated over Ni/La₂O₃-ZrO₂ catalyst. A series of Ni/La₂O₃-ZrO₂ catalysts were synthesized by sol-gel method coupled with wet impregnation, which was characterized by XRD, BET, TEM, EDS, TG, SEM and TPR. Catalytic activity of Ni/La₂O₃-ZrO₂ was evaluated by steam reforming of acetic acid at the temperature range of 550-750 °C. The tetragonal phase La_0.1Zr_0.9O_1.95 is formed through the doping of La₂O₃ into the ZrO₂ lattice and nickel species are highly dispersed on the support with high specific surface area. H₂ yield and CO₂ yield of Ni/La₂O₃-ZrO₂ catalyst with 15%wt Ni reaches 89.27% and 80.41% at 600 °C, respectively, which is attributed to high BET surface area and sufficient Ni active sites in strong interaction with the support. 15%wt Ni supported on La₂O₃-ZrO₂ catalyst maintains relatively stable catalytic activities for a period of 20 h.     

Studies on the redox properties of chromite perovskite catalysts for soot combustion

This paper deals with the preparation (by combustion synthesis), the characterization (by XRD, AAS, BET, SEM, TEM, TPD/R, and XPS analyses), the catalytic activity testing (in a temperature-programmed combustion microreactor and in a DSC analyzer), and the assessment of the reaction mechanism of a series of nanostructured soot combustion catalysts based on La–Cr substoichiometric or alkali-metal-substituted perovskites (La_0.9CrO₃, La_0.8CrO₃, La_0.9Na_0.1CrO₃, La_0.9K_0.1CrO₃, La_0.9Rb_0.1CrO₃, La_0.8Cr_0.9Li_0.1O₃), whose performance is compared with that of the standard LaCrO₃. Some conclusions are drawn concerning the role of each single constituting element on the activity of the most promising catalyst, La_0.8Cr_0.9Li_0.1O₃, which is already active well below 400 °C. The role of weakly chemisorbed O⁻ surface species in particular is pointed out as crucial for the soot combustion process. This indicates the way for the development of new, more active catalysts, possibly capable of delivering amounts of these oxygen species even higher than those obtained (about 700 μmol / g) for the most active Li-substituted lanthanum chromite catalyst developed.     

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.     

The Mechanical Behavior of Lanthanum Cobaltite-Based Perovskites with a Mixed Ion-Electron Conductivity at Different Temperatures

The mechanical behavior of a perovskite-type ceramic based on partially substituted lanthanum cobaltites is studied over a wide temperature range. The La_0.8Ca_0.2CoO₃ composite is shown to be the most inelastically deformable, high-strength, and crack-resistant. In perovskites, the brittleness can be considered as a measure of both inelasticity and ferroelasticity. The perovskites are tested for deformability using bending and indentation techniques, and an analogy in the results obtained by the two techniques is noted. Specific features of the fracture of perovskites tested by the Vickers indentation method are discussed. The crack resistance and stress-strain diagrams of the La_0.8Ca_0.2CoO₃ composite are studied as a function of temperature. Fractographic test data are used to analyze the results obtained.     

Surface Properties of CaO (or BaO)–La2O3–MgO Catalysts and Their Performance in Oxidative Coupling of Methane

CaO–La₂O₃–MgO and BaO–La₂O₃–MgO catalysts with different compositions have been studied for their bulk and surface properties (viz. crystal phases, surface area, acidity/acid strength distribution, basicity/base strength distribution, etc.) and catalytic activity/selectivity in the oxidative coupling of methane (OCM) at different processing conditions (reaction temperature, 700–850°C; CH₄/O₂ ratio in feed, 3·0, 4·0 and 8·0 and GHSV, 102000 and 204000 cm³ g⁻¹ h⁻¹). The surface acidity and strong basicity of La₂O₃–MgO are found to be increased due to the addition of a third component (CaO or BaO), depending upon its concentration in the catalyst. The addition of CaO or BaO to La₂O₃–MgO OCM catalyst causes a significant improvement in its performance. Both the CaO- and BaO-containing catalysts show a high activity and selectivity at 800°C, whereas, the activity and selectivity of BaO-containing catalysts at 700°C is lower than that of CaO-containing catalysts. © 1997 SCI.     

Compositional modification of Sr2TiCoO6 double perovskites by Mo and La for high temperature thermoelectric applications

Double perovskites (A₂B^/B^//O₆) are an interesting family of materials, which are amenable to compositional modification. Recently double perovskites have shown promising thermoelectric properties especially at high temperature. In this report, we investigated the combination of two different cubic double perovskites with similar lattice constant but one (Sr₂TiCoO₆) showing good Seebeck coefficient and the other (Sr₂TiMoO₆) with high electrical conductivity. Dense ceramic samples of Sr₂TiCo_0.5Mo_0.5O₆ and La_0.2Sr_1.8TiCo_0.5Mo_0.5O₆ were synthesized by solid state reaction method. Rietveld refinement of XRD data confirmed the cubic structure with Pm$\overline{3}$m space group in these ceramics. Doping of Mo in Sr₂TiCoO₆ helped in increasing its conductivity; however Seebeck coefficient showed the n-type behavior unlike the p-type conductivity observed in Sr₂TiCoO₆ ceramics. Aliovalent doping of La in Sr₂TiCo_0.5Mo_0.5O₆ further increased its conductivity and Seebeck coefficient demonstrated temperature driven p-type to n-type conduction switching behavior. Conductivity mechanism of these ceramics was found to be governed by small polaron hopping model. Temperature driven p-n switching observed in the thermopower (S) measurement of La_0.2Sr_1.8TiCo_0.5Mo_0.5O₆ was further explained by an analytical model.     

Ball milling assisted preparation of nano La–Y/ZrO2 powder ternary oxide system: Influence of doping amounts

Zirconia powder with good dispersion, fine particle size, and stability is used as high-quality raw material in many fields, such as ceramic materials and refractories. In this paper, the influence of lanthanum oxide (La₂O₃) and yttrium oxide (Y₂O₃) co-doped zirconia (ZrO₂) on its phase transformation behavior, phase stability, and microstructure were investigated. The ball milling method is applied to fabricate different amounts of La₂O₃-doped yttrium oxide stabilized zirconia oxide. Then, the powder obtained from ball milling was roasted using the microwave sintering method. The samples were characterized using XRD, FT-IR, Raman, SEM and BET to determine the optimal conditions for La₂O₃–Y₂O₃ co-doped ZrO₂ powder. The results showed that replacing part of Y₂O₃ with La₂O₃ increases zirconia powder's tetragonal and cubic phase, enhancing the fracture strength of the subsequent synthesized materials. At the same time, the stability of zirconia stabilized with La₂O₃ doping is significantly improved compared to that of Y₂O₃ alone. According to all analysis methods, when the doping amount is 2.8Y0.2La, the powder's phase composition, stability, particle size distribution, and dispersion degree are the best compared with other doping amounts in our study. The obtained powder has a smaller specific surface area, a lower surface energy, a smaller porosity, and a higher density. The samples under this condition can be better used in subsequent materials. The enhancement of various properties of zirconia can significantly prolong the service life of materials in practical applications.     

Preparation and thermal stability of oxynitride perovskite solid solution Sr1-xLaxTa1-xTixO2N

The thermal stability of oxynitride perovskites is very important for the fabrication of their dense ceramics. In this study, a solid solution of the oxynitride perovskite Sr_1-xLa_xTa_1-xTi_xO₂N was prepared via the ammonia nitridation of oxide precursors. The oxynitrides products obtained at x ≤ 0.2 and x ≥ 0.7 had tetragonal and triclinic perovskite-type structures, respectively. A neutron diffraction study on the oxynitride product obtained at x = 0.2 indicated that this tetragonal perovskite had a cis-type anion distribution similarly to SrTaO₂N. The tetragonal perovskite released a part of nitrogen at approximately 1000 °C in a nitrogen atmosphere while maintaining the perovskite structure at least to 1450 °C. The oxynitride perovskites at x ≥ 0.5 gradually decomposed to a mixture of LaTiO₂N, LaTiO₃, La(OH)₃, La₂O₃, TiN and TaO above 1100 °C.     

Evaluation of some cobalt and nickel based perovskites prepared by freeze-drying as combustion catalysts

A series of cobalt and nickel based perovskite type catalysts with high specific surface area (8–20 m² /g) was prepared by spray-freezing/freeze-drying method. The catalytic activity of all samples in methane combustion was evaluated by measuring the light-off temperature, the conversion at 823 K and the temperature of the end of the reaction. The experimental data suggest higher activity than reported in literature for similar or other perovskites, and confirm its strong dependence on the specific surface area. Among eleven tested catalysts, including seven new compositions four of which showed excellent activity, La_0.66Sr_0.34Ni_0.3Co_0.7O₃ was the best performing.     

Photoluminescence properties of La2O3 methane coupling catalysts

La₂O₃ catalysts prepared at 923 K (La₂O₃-LT) and 1073 K (La₂O₃-HT) exhibit different photoluminescence properties due to notably different concentrations of ions in position of low coordination at the surface or coordinatively unsaturated surface sites (cus). The catalyst which exhibits the higher yields of photoluminescence due to the higher concentration of cus corresponds to the one which gives the higher C₂₊ selectivity in the oxidative methane coupling reaction. On leave of absence from Laboratoire de Réactivité de Surface, Université Pierre et Marie Curie, URA 1106-CNRS, 75252 Paris Cedex, France.     

Highly Efficient Overall Water Splitting Through Optimization of Preparation and Operation Conditions of Layered Perovskite Photocatalysts

The layered perovskite materials were found to give the high photocatalytic activity in water splitting reaction under UV irradiation, where the electronic structure of perovskite slab constructing the layered structure (the total cation valency) was the most crucial factor to the high photocatalytic activity. Both the excessive cation valency and the layered structure were required for active photocatalysts, while the slab thickness of layered perovskites had an insignificant effect on water-splitting activity. In order to identify key variables that affected photocatalytic activity and to optimize the performance of (110) layered perovskite, La₂Ti₂O₇ was modified by various methods. The optimum amount of loaded nickel had a great effect and the amount depended on the surface area of the perovskite phase. When an alkaline-earth element such as Ba, Sr, and Ca was doped on La₂Ti₂O₇, the photocatalytic activity was enhanced markedly. Introduction of an alkaline hydroxide into the reaction system as an external additive enhanced the activity further showing extremely high quantum yields close to 50%.     

Effect of precursors on the morphology and the photocatalytic water-splitting activity of layered perovskite La2Ti2O7

A [110] layered perovskite, La₂Ti₂O₇, was a good photocatalyst under ultraviolet light in water splitting reaction. The material was synthesized with La₂O₃ and TiO₂ as precursors by solid-state transformation. The morphology and photocatalytic activity of La₂Ti₂O₇ depended on the preparation methods, as well as purity and morphology of the precursors. Wet-grinding of precursors in ethanol gave a product with higher crystallinity and phase purity, and thus higher photocatalytic activity, than dry-grinding without solvent. It was important to reduce the particle size of La₂O₃, as it usually had larger initial particle sizes than TiO₂. Thus, the particle size of La₂O₃ had a strong effect on the crystallinity and surface area of the product La₂Ti₂O₇. On the other hand, a severe chemical purity control was required for TiO₂, while the effect of morphology was relatively small. In all cases, a high degree of crystallinity and purity of the prepared La₂Ti₂O₇ was critical to show a high photocatalytic water-splitting activity.     

On the effect of La–Cr–O– phase composition on diesel soot catalytic combustion

A series of samples of La–Cr–O– perovskites were designed as catalysts for diesel soot combustion. They were prepared by using co-precipitation method, at ambient temperature using ammonia followed by a hydrothermal treatment (T = 200 °C, P = 20 atm, t = 24 h). All the chromium-containing precursors were then calcined at high temperature to develop the oxide catalyst. Two phase composition 86%LaCrO₃–(14%) La₂CrO₆ or 94%LaCrO₃–6%La₂O₃ were formed depending on the atmosphere of calcination (oxygen or hydrogen respectively) used. Their respective BET surface areas were 1.1 and 6.5 m² g⁻¹. Highly crystalline, pure phase of LaCrO₃ and La₂CrO₆ powders were also prepared, with BET area of 4 and 3 m² g⁻¹, respectively. The crystalline structure and properties of all samples were characterised by X-ray diffraction (XRD), using Rietveld refinement, and temperature-programmed reduction (TPR) techniques. O₂-TPD measurements performed on all samples showed the presence of suprafacial, weakly chemisorbed oxygen only for LaCrO₃, which contributes actively to soot combustion. TPR study performed on all catalysts showed that while pure LaCrO₃ and La₂O₃ samples did not reduce, the biphasic catalysts showed the presence of oxygen depletion peaks characteristic of lattice oxygen mobility in the samples at ca. 665 °C. Catalytic combustion of diesel soot was studied over all catalysts. The results showed that pure LaCrO₃ exhibited significant catalytic activity which was sensitively affected by the modifier La₂CrO₆ or La₂O₃.     

The electric and electrochemical properties of electrode perovskite La1−xSrxCoO3

The behaviour of perovskites La_1?xSr_xCoO₃ (x = 0.02?0.04) was studied at pH = 5. The oxygen evolution is controlled by the chemical reaction of adsorbed O_ads. The simultaneous presence of strong oxidants increases the concentration of active intermediates and accelerates the evolution of oxygen. La_1?xSr_xCoO₃ is a P-type semiconductor with a narrow gap (0.27–0.07 eV) and high acceptor concentration. Neither photoelectrochemical effect nor Mott-Schottky barrier were observed in electrolytes; however, rectifying effect was observed on contact of metals with low-substituted cobaltate perovskites.     

Mayenite supported perovskite monoliths for catalytic combustion of methyl methacrylate

To improve their thermal stability, La_0.8Sr_0.2MnO₃ cordierite monoliths are washcoated with mayenite, which is a novel Al-based material with the crystal structure of 12MO·7Al₂O₃ (M = Ca, Sr). The monoliths are characterized by means of nitrogen adsorption/desorption, scanning electron microscopy, and X-ray diffraction. Catalytic performances of the monoliths are tested for methyl methacrylate combustion. The results show that mayenite obviously improves both the physicchemical properties and the catalytic performance of the monoliths. Because mayenite improves the dispersity of La_0.8Sr_0.2MnO₃ and also prevents the interaction between La_0.8Sr_0.2MnO₃ and cordierite or γ-Al₂O₃, both crystal structure and surface morphology of La_0.8Sr_0.2MnO₃ phase can thereby be stable on the mayenite surface even at high temperature up to 1050 °C. Under the given reaction conditions, La_0.8Sr_0.2MnO₃ monolith washcoated with 12SrO·7Al₂O₃ shows the best catalytic activity for methyl methacrylate combustion among all the tested monoliths.