Determination of kinetic parameters of the oxidehydrogenation of ethane with CO2 on nanosized calcium-doped ceria under fast deactivation processes

Kinetic parameters of the oxidehydrogenation of ethane with CO₂ on nanosized Ca-doped CeO₂ have been investigated. During reaction, interaction of the reactants with the oxide catalyst causes a fast deactivation process. Overlapping of this fast deactivation with catalytic reaction makes quite difficult the reliable determination of kinetic parameters. This handicap can be overcome by getting sufficient data in a short testing time, thus reducing the degree of deactivation. The kinetics of catalytic reaction and of catalyst deactivation have been studied by conducting a series of consecutive tests at increasing temperatures in steps of 20 °C and monitoring the evolution of the reaction for periods of 30 min on stream at each temperature, with full product analysis every 3 min. At temperatures above 680 °C, catalytic rates decreased linearly with run time at isothermal operation, while deactivation rates increased with increasing temperature. Analysis of the results allows to uncouple catalyst deactivation and catalytic reaction and to obtain the kinetic parameters of both processes (i.e., steady-state and deactivation rates, and their apparent activation energies). Deactivation rate of CO formation is one order of magnitude faster than of ethene formation but both processes show the same apparent activation energy, ca. 47 kcal/mol. The apparent activation energy values for the catalytic reaction are 32 ± 4 and 26 ± 2 kcal/mol for the rates of formation of ethene and CO, respectively.     

An XPS study of metallic three-way catalysts: The effect of additives on platinum, rhodium, and cerium

The effect of Zr, Si, La, and Ba on the thermal aging of Pt, Rh, and Ce in automotive exhaust gas catalysts supported on metal foils was studied with X-ray photoelectron spectroscopy (XPS). Platinum was in metallic state under all circumstances studied in this work. Rhodium was partially (about 30%) oxidized already at room temperature. The decreases in the binding energies of the reduced noble metals due to aging was explained in terms of particle growth. The binding energy of rhodium oxide increased during aging indicating rhodium diffusion into alumina. Cerium was oxidized during aging. It was found that in the lanthanum containing catalysts the binding energies of platinum and rhodium were below the values for bulk metals, and the degree of oxidation of cerium was lower than in the other samples. It was concluded that the presence of lanthanum facilitated the reduction of the noble metals and cerium.     

Development of solid oxide fuel cells based on a Ce(Gd)O2−x electrolyte film for intermediate temperature operation

Initial tests have been carried out with the fuel cell arrangement La_0.6Sr_0.4Co_0.2Fe_0.8O₃Ce_0.9Gd_0.1O_1.95Ni/YSZ, incorporating dense film (5–10 μm) Ce_0.9Gd_0.1O_1.95 electrolyte tape cast onto the supporting anode, to investigate the feasibility of intermediate temperature operation (500–700°C). A good open circuit voltage of approx. 0.8 V was obtained at 550°C using moist hydrogen as the fuel. Slightly lower open circuit voltages were found at higher temperatures, which may have been caused by minor gas leakage and the electronic conductivity of the electrolyte. Power outputs in excess of 100 mW/cm² were obtained at 650°C, and the cell resistance was 0.8Ω cm² at this temperature. This resistance, and the greater resistance at lower temperature, was predominantly due to the cathode according to AC impedance measurements. Experiments were also carried out at 600°C using direct methanol fuels at the anode; the maximum power output was approximately half of that obtained with hydrogen.     

TPR,XRD and XPS characterisation of ceria-based materials synthesized by freeze-drying precursor method

A freeze-drying precursor method was used to obtain submicrometric powders of ceria-based materials such as Ce_1−xGd_xO_2−δ (x=0, 0.01, 0.05, 0.10 and 0.20), 80%CeO₂–20%ZrO₂, 80%CeO₂–20%Al₂O₃ and (1−y)Ce_0.99Gd_0.01O_2−δ– (y)Al₂O₃ (y=0.01, 0.02, 0.05, 0.10 and 0.30) at temperatures as low as 400 °C. The phase formation and evolution with the temperature was studied by X-ray diffraction (XRD). Also, temperature programmed reduction (TPR) was performed to investigate the reducibility of the ceramic powders. It was observed that after reduction of the ceria-based materials the fluorite structure of the samples was retained. The TPR profiles showed two peaks which are associated to the surface and bulk ceria reduction processes. Likewise, after the TPR measurements the resulting powders have showed high phase stability and reproducibility. XPS results confirmed the reduction of Ce⁴⁺ to (Ce³⁺+Ce⁴⁺) ratio with alumina doping.     

Fabrication of highly efficient heterostructured Ag-CeO2/g-C3N4 hybrid photocatalyst with enhanced visible-light photocatalytic activity

On the basis of hydrothermal synthesis of Ag-CeO₂ microspheres, Ag-CeO₂/g-C₃N₄ composite photocatalyst with heterostructure was prepared by simple solvent evaporation of Ag-CeO₂ and g-C₃N₄. To characterize the composition, structure, morphology and light absorption properties of the as-prepared Ag-CeO₂/g-C₃N₄ composites, XRD, FTIR XPS, SEM, TEM, PL, BET and UV-vis DRS were used, respectively. The as-prepared photocatalyst was subjected to photocatalytic degradation of pollutants, and the prepared composite material has excellent photocatalytic activity for photodegradation of methylene blue (MB). The research shows that the photocatalytic properties of Ag-CeO₂/g-C₃N₄ composites were related to the mass ratio of Ag-CeO₂ microspheres and g-C₃N₄ nanosheets. When the ratio of Ag-CeO₂ microspheres: g-C₃N₄ is 1:5, the composites have the highest photocatalytic activity, which was 9.6 and 3.3 times that of single Ag-CeO₂ and g-C₃N₄, respectively. The improvement of photocatalytic activity is attributed to the heterostructure between the composite materials and the addition of noble metal silver, and the degradation of methylene blue by the visible light irradiation material is greatly improved. Finally, an attempt was made to analyze the principle of photocatalytic degradation of pollutants in prepared materials.     

Ceria supported manganese(0) nanoparticle catalysts for hydrogen generation from the hydrolysis of sodium borohydride

Herein we report for the first time the preparation and catalytic use of the ceria supported manganese(0) nanoparticles in hydrogen generation from the hydrolysis of sodium borohydride. They are in situ formed from the reduction of manganese(II) ions on the surface of ceria nanopowders during the catalytic hydrolysis of sodium borohydride in aqueous solution at room temperature. Manganese(0) nanoparticles are isolated from the reaction solution by centrifugation and characterized by a combination of analytical techniques. Nanoceria supported manganese(0) nanoparticles are highly active and long-lived catalysts providing a turnover frequency of 417 h^?1 and 45,000 turnovers in hydrogen generation from the hydrolysis of sodium borohydride at 25.0 ± 0.1 °C. They also have high durability as they retain 55% of their initial catalytic activity after the fifth cycle of hydrolysis providing a release of 4 equivalent H₂ gas per mol of sodium borohydride. The noticeable activity loss in successive runs of hydrolysis is attributed to the deactivation due to agglomeration. High activity and stability of ceria supported manganese(0) nanoparticles are ascribed to the unique nature of reducible cerium oxide. The formation of cerium(III) defects under catalytic conditions provides strong binding for the manganese(0) nanoparticles to oxide surface which makes the catalytic activity and stability favorable. Our report also includes the results of kinetic study of catalytic hydrolysis of sodium borohydride depending on the temperature, catalyst and substrate concentration.     

Phase relation studies in the CeO2-Eu2O3 system at 1500 to 600 °C in air

Materials based on CeO₂-Eu₂O₃ system are promising candidates for a wide range of applications, but the phase relationship has not been studied systematically previously. To address this challenge, the subsection of the phase diagram for 600, 1100 and 1500 °C has been elucidated. Samples of different compositions have been prepared from nitrate acid solutions using conventional ceramic techniques; evaporation, drying, and calcinations. The phase relations in the binary CeO₂-Eu₂O₃ system at 600–1500 °C in air were studied from the heat treated samples using X-ray diffraction analysis, polarized light microscopy investigation and scanning electron microscopy in the overall concentration range. It was established that in the binary CeO₂-Eu₂O₃ system there exist fields of solid solutions based on monoclinic (B) modification of Eu₂O₃, cubic (C) modification of Eu₂O₃ and cubic modification of CeO₂ with fluorite-type structure (F). The systematic study that covered whole composition range excluded formation of new phases. The refined lattice parameters of the unit cells and the boundaries of the homogeneity fields for solid solutions were determined.     

Effect of process modification and presence of H2O2 in the synthesis of samaria-doped ceria powders for fuel cell applications

A fundamental step for a sustainable industrial development based on “H₂ Economy” is the implementation of fuel cell technology, in terms of new devices, materials and convenient processes for their production. Rare earth doped ceria oxides are suitable materials for the new generations of cells and their cost effective production becomes fundamental as the price of rare earths is increasing. In this view, our study investigates a modified method of co-precipitation of Ce_0.8Sm_0.2O_1.9−x (SDC) evaluating the effects of adding of H₂O₂ in the process. The parameters controlled were the molar ratio [H₂O₂]/[M³⁺], (M³⁺ = Ce³⁺, Sm³⁺ present in starting nitrate salts solutions) and the pH of precipitation; in some cases the precipitates were also treated under reflux at 373 K overnight. The powder catalysts, both as fresh precipitates and calcined oxides were analyzed via N₂ adsorption (BET), X-Ray diffraction (XRD) and temperature programmed reduction (TPR) techniques and their morphological, structural and redox properties were correlated with the synthesis parameters used. The electrical conductivity properties of these materials have also been investigated via electrochemical impedance spectroscopy (EIS) and the results compared with those of a commercial oxide. The synthesis approach was shown to be very versatile in the development of materials with properties exploitable for applications in catalysis and in intermediate temperature Solid Oxide Fuel Cell (IT-SOFC) systems.     

Study on the preparation of Ni–La–Ce oxide catalyst for steam reforming of ethanol

Two schemes for design and preparation of Ni–La–Ce oxide catalysts for steam reforming of ethanol were proposed in this work. The one via citrate complexing method was designed as NiO supported on ceria-lanthanum oxide (CL) solid solution, in which the strong interaction between NiO and CL solid solution was beneficial to inhibit the aggregation of NiO particles, and the abundant of oxygen vacancies existed in CL solid solution was in favor of carbon elimination from catalyst surface. The other was schemed as LaNiO₃ with perovskite structure loaded on CeO₂ support by using impregnation method, in which the particles of metal Ni derived from reduction of LaNiO₃ were highly dispersed, and the formation of La₂O₂CO₃ in the reaction process could act as the carbon scavenger. Both of the catalysts exhibited very good performance for steam reforming of ethanol (SRE), complete C₂H₅OH conversion was obtained with 70.3% of H₂ selectivity at 400 °C over the catalyst obtained from former method and complete C₂H₅OH conversion was achieved at 450 °C with 67% of H₂ selectivity over the catalyst from latter method. The catalyst made according to the citrate complexing method was more active for SRE and more selective for H₂ production. Both of the catalysts displayed very good anti-sintering ability which was tested at 650 °C and at a high space velocity of 180,000 ml g_cat⁻¹ h⁻¹ with reaction mixture of H₂O/C₂H₅OH = 3 in mole ratio. The results indicated that both of oxygen vacancy and La₂O₂CO₃ possessed the ability to remove the deposited carbon, and compared with La₂O₂CO₃ the oxygen vacancy could reduce one third more of the carbon deposited according to TG tests.     

Dual-templating fabrication of three-dimensionally ordered macroporous ceria with hierarchical pores and its use as a support for enhanced catalytic performance of preferential CO oxidation

In this study, several three-dimensionally ordered macroporous (3DOM) CeO₂ having hierarchical pore structure were successfully prepared via a dual ‘hard-soft’ templating strategy using Ce(NO₃)₃·6H₂O containing phenol-formaldehyde (PF) resol or/and sucrose as the ceria precursor. The resulting CeO₂ samples were characterized by N₂ adsorption-desorption analysis, scanning electron microscopy, transmission electron microscopy and X-ray diffraction, which showed that the hierarchical 3DOM CeO₂ possessed interconnected networks of the ordered macropore structures with large mesopores, and both the BET surface area and pore volume increased significantly compared with the conventional 3DOM CeO₂. The improved textural parameters should be attributed to the emergence of mesopores in the interconnected three-dimensional skeleton, which were formed by oxidative removal of carbon produced from carbonization of PF resol or sucrose. The hierarchical 3DOM CeO₂ exhibited a superior performance to the conventional 3DOM or bulk CeO₂ when used as supports for Ir catalysts in preferential CO oxidation.