Mechanisms of point defect formation and ionic conduction in divalent cation-doped lanthanum oxybromide: first-principles and experimental study.

The ionic conduction mechanism in M2+-doped (M: Mg, Ca, Zn, and Sr) lanthanum oxybromide (LaOBr) was investigated theoretically and experimentally. Formation energy calculations of point defects revealed that Br- ion vacancies and substitutional M2+ ions were the major point defects in M2+-doped LaOBr, while Br- ion vacancies and antisite O2- ions at Br sites were the major defect types in pure LaOBr. In the relaxed point defect models, doped Mg2+ and Zn2+ ions were displaced from the initial positions of the La3+ ions, and this was experimentally supported by crystal structural analysis. These significant atomic shifts were probably due to the strong interactions between Br- and the dopant ions. First-principles calculations and experimental analyses using X-ray photoelectron spectroscopy and X-ray absorption fine-structure spectroscopy also suggested the existence of strong interactions. The migration energy of Br- ions was calculated to be 0.53 eV, while the migration energy of O2- ions was 0.92 eV, implying that Br- ion migration via a vacancy system was more probable than O2- ion migration. The calculated association energies between MLa and VBr were 0.4-0.6 eV, suggesting that the association needed to be disrupted for Br- ion conduction. The sum of the association and migration energies was comparable to the experimental association energies of M2+-doped LaOBr.

Environmental catalysts advance focused on lattice oxygen for the decomposition of harmful organic compounds.

The recent industrial growth has made our lives more comfortable; however, it has led to an increase in the concentration of harmful compounds, such as carbon monoxide, volatile organic compounds (e.g., toluene), and phenolic compounds (e.g., phenol and cresol), in the environment. Catalytic oxidation using environmental catalysts is an important method for the removal of harmful compounds. To date, novel environmental catalysts have been developed from unique concepts based on solid-state ionics. In particular, the oxygen supply ability of a promoter can supply active oxygen from inside the lattice to the catalytically active site. Our catalysts exhibited high activity for the oxidation of harmful chemicals under moderate conditions in both the gaseous and liquid phases compared to conventional catalysts. This short review article describes our concepts of material design and our novel catalysts (ceria-zirconia (CeO2-ZrO2), apatite-type lanthanum silicate (La10Si6O27), and lanthanum oxyfluoride (LaOF) based catalysts).

Dihydroxyacetone production by glycerol oxidation under moderate condition using Pt loaded on La1-xBixOF solids.

Pt/La1-xBixOF/SBA-16 (SBA-16: Santa Barbara Amorphous no. 16) catalysts were prepared to produce dihydroxyacetone (DHA) from glycerol under moderate conditions. By using 7 wt% Pt/16 wt% La0.95Bi0.05OF/SBA-16, the DHA yield reached up to 78.4% (glycerol conversion: 100%) after reacting for 6 h at 30 °C in an atmospheric open-air system.

Evidence for enormous iodide anion migration in lanthanum oxyiodide-based solid.

The I− ion conduction was demonstrated and quantified in the La0.70Sr0.25Zn0.05OI0.70 solid. The I− ion is considered to be an inferior conductor because of its large ionic size compared to the previously reported conducting ion species. Using modified Tubandt electrolysis, a weight increase at the anodic pellet and a corresponding weight decrease at the cathodic pellet were observed. The weight changes were in good agreement with the theoretical values estimated by considering pure I− ion migration. Furthermore, the iodine element appeared at the anode, and the iodine concentration at the cathode decreased after electrolysis, indicating that the migrating species was only I−. This is the first study to elucidate the conduction of iodide ions in solids.

Ionic conduction mechanism in Ca-doped lanthanum oxychloride.

The mechanism of ionic conduction in Ca-doped lanthanum oxychloride (LaOCl) was investigated using first-principles calculations based on density functional theory. The calculations of the point defect formation energies suggest that Cl- ion vacancies and substituted Ca2+ ions at La sites were dominant point defects. Although the migration energy of an O2- ion is 0.95 eV, the migration energy of a Cl- ion was calculated to be 0.44 eV, which is consistent with the reported experimental value. These results imply that the main carrier in Ca-doped LaOCl is Cl- ions and ionic conduction occurs by a Cl- ion vacancy mechanism.

Catalytic Liquid-Phase Oxidation of Phenolic Compounds Using Ceria-Zirconia Based Catalysts.

Catalytic liquid-phase oxidation using a catalyst and oxygen gas (Catalytic wet air oxidation, CWAO) is one of the most promising technology to remove hazardous organic compounds in wastewater. Up to now, various heterogeneous catalysts have been reported for phenolic compounds decomposition. The CeO2-ZrO2 based catalysts have been recently studied, because CeO2-ZrO2 works as a promoter which supplies active oxygen species from inside the lattice to the active sites. Since it is difficult to dissolve oxygen gas into water, the use of the promoter is effective for realizing the high catalytic activity at moderate conditions. Also, CeO2-ZrO2 shows high resistance for the metal leaching during the catalytic reaction in the liquid-phase. This article reviews the studies of the catalytic liquid-phase oxidation of phenolic compounds using CeO2-ZrO2 based catalysts.

Novel Catalysts for Methane Combustion Based on Cobalt-Doped Lanthanum Silicates Having an Apatite-type Structure.

Novel PdO/La10Si6-xCoxO27-δ/γ-Al2O3 catalysts with applications to methane combustion were developed. These materials were based on the use of La10Si6-xCoxO27-δ as a promoter because this compound has an oxide-ion conducting apatite-type structure that allows the smooth migration of active oxygen to the PdO activator. Co3+/2+ ions were also introduced into the original La10Si6O27 lattice to enhance its redox properties. Temperature-programmed reduction measurements revealed that the oxygen supply was facilitated by introducing Co3+/2+, where the reduction was observed at 290 °C for La10Si6-xCoxO27-δ (x = 1.0), whereas no reduction was observed below 460 °C for La10Si6O27. Among the samples synthesized in this work, PdO/La10Si6-xCoxO27-δ/γ-Al2O3 (x = 1.0) exhibited the highest catalytic activity, allowing the complete oxidation of methane at 310 °C, a temperature 80 °C lower than the 390 °C required when employing PdO/La10Si6O27/γ-Al2O3.

Catalytic liquid-phase oxidation of acetaldehyde to acetic acid over a Pt/CeO2-ZrO2-SnO2/γ-alumina catalyst.

Pt/CeO2-ZrO2-SnO2/γ-Al2O3 catalysts were prepared by co-precipitation and wet impregnation methods for catalytic oxidation of acetaldehyde to acetic acid in water. In the present catalysts, Pt and CeO2-ZrO2-SnO2 were successfully dispersed on the γ-Al2O3 support. Dependences of platinum content and reaction time on the selective oxidation of acetaldehyde to acetic acid were investigated to optimize the reaction conditions for obtaining both high acetaldehyde conversion and highest selectivity to acetic acid. Among the catalysts, a Pt(6.4wt.%)/Ce0.68Zr0.17Sn0.15O2.0(16wt.%)/γ-Al2O3 catalyst showed the highest acetaldehyde oxidation activity. On this catalyst, acetaldehyde was completely oxidized after the reaction at 0°C for 8hr, and the selectivity to acetic acid reached to 95% and higher after the reaction for 4hr and longer.

Carbon monoxide oxidation at room temperature on Pt/CeO2-ZrO2-Bi2O3 catalysts.

A novel Pt/CeO(2)-ZrO(2)-Bi(2)O(3) catalyst was prepared to realize complete CO oxidation at room temperature or below even in the presence of moisture. Using this catalyst, a high CO oxidation activity and a high stability against moisture have been realized simultaneously.

Total oxidation of toluene on Pt/CeO2-ZrO2-Bi2O3/gamma-Al2O3 catalysts prepared in the presence of polyvinyl pyrrolidone.

Pt/CeO(2)-ZrO(2)-Bi(2)O(3)/gamma-Al(2)O(3) (Pt/CZB/Al(2)O(3)) catalysts for the catalytic combustion of toluene, which is one of the volatile organic compounds (VOCs), were prepared by the wet impregnation method in the presence of polyvinyl pyrrolidone (PVP). X-ray powder diffraction, transmission electron microscopy, and BET specific surface area measurement using N(2) adsorption have been used to characterize the catalysts. The catalytic test was conducted from room temperature in a flow of 900 ppm of toluene in air and gas hourly space velocity (GHSV) of 8000 h(-1). The catalytic activity was evaluated in terms of C(7)H(8) conversion and the gas composition after the reaction was analyzed using two gas chromatographs with a flame ionization detector (FID) and a thermal conductivity detector (TCD). The Pt/CZB/Al(2)O(3) catalysts are specific for the total toluene oxidation and CO and any toluene-derivative compounds were not detected as by-products. The specific surface area of the catalysts was increased by the addition of PVP in the preparation process. By the optimization of the amount of platinum, complete oxidation of toluene was realized at a temperature as low as 120 degrees C on a 7 wt%Pt/16 wt%Ce(0.64)Zr(0.15)Bi(0.21)O(1.895)/gamma-Al(2)O(3) catalyst.

Advanced materials for environmental catalysts.

Recent advances in the synthesis and characterization of materials for environmental catalysts are reported in this paper. Highly advanced environmental catalysts for decomposition of volatile organic compounds and nitrogen oxides were artificially designed based on a concept usually employed in the fields of solid-state chemistry and solid-state ionics. Catalytically active materials for complete ethylene oxidation were prepared by a citrate sol-gel method as a key process to obtain CeO(2)-ZrO(2)-Bi(2)O(3) solid solutions. On the other hand, direct NO decomposition catalysts were designed and prepared focusing on the open spaces and oxide anion vacancies in the crystal lattice. Evaluation of the materials as environmental catalysts demonstrated significant advantages over the conventional ones. The design strategy, synthetic method, and structural features of these concerto catalysts are addressed.

A new type of bromide anion conducting solid.

A new type of bromide anion conducting solid electrolyte was designed with the solid electrolyte based on oxybromide; the oxybromide lends unique characteristics of thermal stability and water insolubility (5.6 mg in 100 g of water at 25 degrees C) to the electrolyte which shows the highest Br- anion conductivity reported above 500 degrees C.

A carbon dioxide gas sensor by combination of multivalent cation and anion conductors with a water-insoluble oxycarbonate-based auxiliary electrode.

A compact and inexpensive carbon dioxide gas sensor was successfully realized by the combination of a divalent magnesium ionic conductor of Mg0.7(Zr0.85Nb0.15)4P6O24 and a divalent oxide anion conducting ZrO2-Y2O3 solid electrolyte with the water-insoluble Li- and Ba-codoped Nd2O2CO3 solid solution as the auxiliary electrode. The sensor response was continuous and reproducible, and the present sensor also demonstrated a theoretical Nernst response in the atmosphere where water vapor, nitrogen oxides, ammonia, etc., coexist. The exposure of the present sensor to water dew and variation in oxygen concentration does not interfere with the sensor response, which will be a great advantage in applying the in situ practical CO2 detection in combustion exhaust gas atmospheres.