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.

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.

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.

Relevance between the Bulk Density and Li+-Ion Conductivity in a Porous Electrolyte: The Case of Li[Li1/3Ti5/3]O4.

The Li+-ion conductivity (σLi) in an electrolyte is an important parameter with respect to the performance of all-solid-state lithium-ion batteries (LIBs). However, little is known about how σLi in a porous electrolyte differs from that in a highly dense electrolyte. In this study, the relationship between the bulk density (dbulk) and apparent σLi (σLiapp) in a porous electrolyte of Li[Li1/3Ti5/3]O4 (LTO) was examined by theoretical and experimental approaches. The theoretical calculations demonstrated that dbulk and σLi have a simple relationship irrespective of the radius of the spherical pores in the electrolyte; i.e., σLi increases almost linearly with increasing ζ,where ζ is the ratio of d bulk to the theoretical density. In fact, the observed σLiapp of LTO, which was determined by four-probe alternating-current impedance measurements, increased with increasing ζ. Hence, with this relationship, σLiapp can be estimated by ζ and intrinsic σLi (σLiint) and vice versa; such estimations provide critical information for determining the optimum compositions of composite electrodes for all-solid-state LIBs. The temperature dependence of σLiapp in LTO and differences between the calculated and experimental results are also discussed.