Hexagonal Rare Earth-Iron Mixed Oxides (REFeO3): Crystal Structure, Synthesis, and Catalytic Properties

The rare earth-iron mixed oxide (REFeO3) is an attractive material in fields such as electronic, magnetic, and catalytic research. Generally, orthorhombic REFeO3 (o-REFeO3) with a perovskite structure is better known than hexagonal REFeO3 (h-REFeO3), because o-REFeO3 is thermodynamically stable for all RE elements. However, h-REFeO3 has a very interesting crystal structure in which a RE and Fe layer are alternately stacked along the c-axis in the unit cell; nevertheless, synthesis of the h-REFeO3 belonging to metastable phase can be problematic. Fortunately, solution-based synthetic methods like solvothermal or coprecipitation synthesis have recently enabled the selective synthesis of h-REFeO3 and o-REFeO3 with comparative ease. Although the electronic and magnetic properties of h-REFeO3 have typically been evaluated, recent research has also revealed excellent catalytic properties that enable environmental cleanup reactions such as hydrocarbon or CO oxidation. This mini-review introduces a synthetic method for controlling the crystal structure between orthorhombic and hexagonal REFeO3 and the catalytic performance of h-REFeO3-based materials

Model Building of Metal Oxide Surfaces and Vibronic Coupling Density as a Reactivity Index: Regioselectivity of CO2_2 Adsorption on Ag-loaded Ga2_2O3_3

The step-by-step hydrogen-terminated (SSHT) model is proposed as a model for the surfaces of metal oxides. Using this model, it is found that the vibronic coupling density (VCD) can be employed as a reactivity index for surface reactions. As an example, the regioselectivity of CO$_2$ adsorption on the Ag-loaded Ga$_2$O$_3$ photocatalyst surface is investigated based on VCD analysis. The cluster model constructed by the SSHT approach reasonably reflects the electronic structures of the Ga$_2$O$_3$ surface. The geometry of CO$_2$ adsorbed on the Ag-loaded Ga$_2$O$_3$ cluster has a bent structure, which is favorable for its photocatalytic reduction to CO.Comment: 18 pages, 11 figure

Visible-light-assisted selective catalytic reduction of NO with NH[3] on porphyrin derivative-modified TiO[2] photocatalysts

Accepted 16 Sep 2014.Porphyrin-derivative-modified TiO[2] photocatalysts showed high photocatalytic activity for the selective catalytic reduction of NO with NH[3] in the presence of O[2] under visible-light irradiation. Tetra(p-carboxyphenyl)porphyrin (TCPP) was the most effective photosensitizer among the five porphyrin derivatives investigated. NO conversion and N[2] selectivity of 79.0% and 100%, respectively, were achieved at a gas hourly space velocity of 50 000 h[−1]. UV–Vis and photoluminescence spectroscopies revealed the presence of two species of TCPP on the TiO2surface; one was a TCPP monomer and the other was an H-aggregate of the TCPP molecules. It was concluded that the TCPP monomer is an active species for the photo-assisted selective catalytic reduction (photo-SCR). Moreover, an increase in the fraction of H-aggregates with increasing TCPP loading amount resulted in a decrease in the photocatalytic activity of the photo-SCR

Enhanced CO evolution for photocatalytic conversion of CO2 by H2O over Ca modified Ga2O3

高効率で二酸化炭素を再資源化する光触媒の合成に成功 --CO2を「ひかり」と「みず」でリサイクル--. 京都大学プレスリリース. 2020-10-14.Artificial photosynthesis is a desirable critical technology for the conversion of CO2 and H2O, which are abundant raw materials, into fuels and chemical feedstocks. Similar to plant photosynthesis, artificial photosynthesis can produce CO, CH3OH, CH4, and preferably higher hydrocarbons from CO2 using H2O as an electron donor and solar light. At present, only insufficient amounts of CO2-reduction products such as CO, CH3OH, and CH4 have been obtained using such a photocatalytic and photoelectrochemical conversion process. Here, we demonstrate that photocatalytic CO2 conversion with a Ag@Cr-decorated mixture of CaGa4O7-loaded Ga2O3 and the CaO photocatalyst leads to a satisfactory CO formation rate (>835 µmol h−1) and excellent selectivity toward CO evolution (95%), with O2 as the stoichiometric oxidation product of H2O. Our photocatalytic system can convert CO2 gas into CO at >1% CO2 conversion (>11531 ppm CO) at ambient temperatures and pressures

Role of Catalyst Support and Regioselectivity of Molecular Adsorption on a Metal Oxide Surface: NO Reduction on Cu/{\gamma}-alumina

The role of catalyst support and regioselectivity of molecular adsorption on a metal oxide surface is investigated for the NO reduction on a Cu/{\gamma}-alumina heterogeneous catalyst. For the solid surface, computational models of the {\gamma}-alumina surface are constructed based on the Step-by-Step Hydrogen Termination (SSHT) approach. Dangling bonds, which appear by cutting the crystal structure of a model, are terminated stepwise with H atoms until the model has an appropriate energy gap. The obtained SSHT models exhibit the realistic infrared (IR) and ultraviolet-visible (UV/Vis) spectra. Vibronic coupling density (VCD), as a reactivity index, is employed to elucidate the regioselectivity of the Cu adsorption on the {\gamma}-alumina and that of the NO adsorption on the Cu/{\gamma}-alumina in place of the frontier orbital theory that could not provide clear results. We discovered that the highly dispersed Cu atoms are loaded on Lewis-basic O atoms, which is known as anchoring effect, located in the tetrahedral sites of the {\gamma}-alumina surface. The role of the {\gamma}-alumina support is to raise the frontier orbital of the Cu catalyst, which in turn gives rise to the electron back-donation from the Cu/{\gamma}-alumina to NO. In addition, the penetration of the VCD distribution of the Cu/{\gamma}-alumina into the {\gamma}-alumina support indicates that the excessive reaction energies dissipate into the support after the NO adsorption and reduction. In other words, the support plays the role of a heat bath. The NO reduction on the Cu/{\gamma}-alumina proceeds even in an oxidative atmosphere because the Cu-NO bond is strongly bounded compared to the Cu-O2 bond

Dual Ag/Co cocatalyst synergism for the highly effective photocatalytic conversion of CO2 by H2O over Al-SrTiO3

金属ナノ粒子で光触媒のモチベーションを上げることに成功 --人工光合成で二酸化炭素(CO2)の再資源化の新展開--. 京都大学プレスリリース. 2021-03-11.Loading Ag and Co dual cocatalysts on Al-doped SrTiO3 (AgCo/Al-SrTiO3) led to a significantly improved CO-formation rate and extremely high selectivity toward CO evolution (99.8%) using H2O as an electron donor when irradiated with light at wavelengths above 300 nm. Furthermore, the CO-formation rate over AgCo/Al-SrTiO3 (52.7 μmol h−1) was a dozen times higher than that over Ag/Al-SrTiO3 (4.7 μmol h−1). The apparent quantum efficiency for CO evolution over AgCo/Al-SrTiO3 was about 0.03% when photoirradiated at a wavelength at 365 nm, with a CO-evolution selectivity of 98.6% (7.4 μmol h−1). The Ag and Co cocatalysts were found to function as reduction and oxidation sites for promoting the generation of CO and O2, respectively, on the Al-SrTiO3 surface

シガケン ナガハマシ ホウゲン ノ ソザイ タイグウ ケイシキ ニ カンスル キジュツテキ ケンキュウ

The isomerization of n-hexadecane over Pt–WO3 catalysts supported on TiO2–SiO2 synthesized by glycothermal reaction with various Si/Ti molar ratios was examined. The catalyst performance depended on Si/Ti molar ratio and WO3 loading. The characterization of the catalysts by XRD, XAFS, UV-vis and so on revealed that with increasing the WO3 loading, the structure of surface W species changed from monomeric species to polytungstate species, which is considered to significantly affect the isomerization selectivity of the catalysts

Hexagonal rare earth-iron mixed oxides (REFeO₃): Crystal structure, synthesis, and catalytic properties

The rare earth-iron mixed oxide (REFeO₃) is an attractive material in fields such as electronic, magnetic, and catalytic research. Generally, orthorhombic REFeO₃ (o-REFeO₃) with a perovskite structure is better known than hexagonal REFeO₃ (h-REFeO₃), because o-REFeO₃ is thermodynamically stable for all RE elements. However, h-REFeO₃ has a very interesting crystal structure in which a RE and Fe layer are alternately stacked along the c-axis in the unit cell; nevertheless, synthesis of the h-REFeO₃ belonging to metastable phase can be problematic. Fortunately, solution-based synthetic methods like solvothermal or coprecipitation synthesis have recently enabled the selective synthesis of h-REFeO₃ and o-REFeO₃ with comparative ease. Although the electronic and magnetic properties of h-REFeO₃ have typically been evaluated, recent research has also revealed excellent catalytic properties that enable environmental cleanup reactions such as hydrocarbon or CO oxidation. This mini-review introduces a synthetic method for controlling the crystal structure between orthorhombic and hexagonal REFeO₃ and the catalytic performance of h-REFeO₃-based materials

ソルボサーマル ハンノウ ニ ヨル キドルイ サンカブツ オヨビ フクゴウ サンカブツ ノ ゴウセイ

京都大学0048新制・課程博士博士(工学)甲第13071号工博第2813号新制||工||1408(附属図書館)UT51-2007-H344京都大学大学院工学研究科物質エネルギー化学専攻(主査)教授 井上 正志, 教授 江口 浩一, 教授 横尾 俊信学位規則第4条第1項該当Doctor of EngineeringKyoto UniversityDFA