Yoshiumi Kohno

Photoreduction of CO2 with H2 over ZrO2. A study on interaction of hydrogen with photoexcited CO2

By the analysis of the kinetic isotope effect and the effect of reaction temperature, and by EPR, the reaction between hydrogen and photoexcited CO2 species in the photoreduction of CO2 by H2 over ZrO2 was investigated. As the yield of products decreased when D2 was used as a reductant instead of H2, the rate-determining step of the reaction was associated with hydrogen. The reaction was enhanced at elevated temperature, suggesting that the reaction rate was strongly influenced by a thermal process. The EPR study showed the formation of the CO2− anion radical on irradiation of ZrO2 with adsorbed CO2. The CO2− radical had a long life and was stable in the dark for at least 60 min. However, on contact with hydrogen, the n CO2− radical was suppressed within 30 min, indicating the reaction of CO2 with hydrogen in the dark. It was concluded that n hydrogen was activated in the dark to react with the photoexcited CO2− radical.

Photoreduction of carbon dioxide by hydrogen over magnesium oxide

Magnesium oxide was found to show activity for the reduction of carbon dioxide to carbon monoxide under photoirradiation using hydrogen as a reductant. Fourier transform infrared spectroscopy was applied to a study of the reaction mechanism by detection and identification of the surface species arising during the n photoreaction. The formation of surface formate ion was observed during the photoreaction. Since CO was produced from the surface formate in the presence of CO2 under irradiation, the surface formate was a reaction intermediate which acted as a reductant and converted another CO2 molecule to CO. The correlation of the reaction activity with the amount of introduced CO2 indicated that adsorbed carbonate was reduced by H2 n to the surface formate, and that the surface formate also reduced the adsorbed carbonate to CO. The IR spectra showed the difference in the adsorption form of CO2 between the adsorbed carbonate reduced by H2 to the surface formate and that reduced by the surface formate to CO.

Reaction mechanism in the photoreduction of CO2 with CH4 over ZrO2

The n surface species arising during the photoreduction of carbon dioxide with methane over zirconium oxide is observed n by infrared spectroscopy. Two definite species have been expected to exist on the surface during n the photoreaction. One has been supposed to be a reaction intermediate and decomposed to CO at around n 623 K, and the other has not been decomposed even at 673 K which should be a carbonaceous residue. From the n resemblance of the IR spectral features, the latter species is assigned to the surface acetate ion. Several properties n of the former species are found to be quite similar to those of the surface formate ion, which is a n reaction intermediate of the photoreduction of CO2 by H2 n over ZrO2. The former species is therefore assigned to the surface formate, which is also supposed to be the reaction intermediate of the photoreaction between n CO2 and CH4. The existence of another carbonaceous residue than the surface acetate is suggested. As no n IR bands assigned to the C–H vibration are observed in the spectrum of the carbonaceous residues, the other n residue is supposed to be a highly carbonaceous species. The EPR spectrum indicates the photoexcitation of n adsorbed CO2 to the CO2− anion radical, and the interaction of the CO2− radical with CH4 in the n dark. On the basis n of these results, a n possible reaction mechanism in this reaction is proposed.

Identification and reactivity of a surface intermediate in the photoreduction of CO2 with H2 over ZrO2

Zirconium oxide is active for photoreduction of gaseous carbon dioxide to carbon monoxide with hydrogen. A stable surface species arises under the photoreduction of CO2 on zirconium oxide and is identified as surface formate by IR spectroscopy. Adsorbed CO2 is converted to formate by photoreaction with hydrogen. The surface formate is a true reaction intermediate since CO is formed by the photoreaction of formate and CO2; surface formate works as a reductant of carbon dioxide to yield carbon monoxide. The dependence on the wavelength of irradiation light shows that bulk ZrO2 is not the photoactive species. When ZrO2 adsorbs CO2 a new band appears in the photoluminescence excitation spectrum. The photoactive species in the reaction between CO2+H2 which yields HCOO- is presumably formed by the adsorption of CO2 on the ZrO2 surface.

Photostability enhancement of anionic natural dye by intercalation into hydrotalcite.

The aim of this study is the improvement of the photostability of several natural anionic dyes, carmine (CM), carthamus yellow (CY), and annatto dye (ANA), by complexation with hydrotalcite. The composite of the dyes and hydrotalcite is prepared by the coprecipitation method. CM is successfully intercalated in the hydrotalcite layer when the amount of introduced CM is large. The photostability of CM in CM/HT composites is superior to the CM adsorbed on silica surface. The effect of the stability enhancement is larger when the amount of introduced CM exceeds 0.23 g/g-host, or when the layer charge density of the hydrotalcite is larger. CY is also stabilized by complexation with hydrotalcite, whereas ANA is not stabilized by complexation with hydrotalcite. The photostability of an anionic natural dye can be improved by intercalation into the hydrotalcite layer, if the dye has a hydrophilic nature and a rather planar structure. The intercalated dye is stabilized by the protection from the attack of the atmospheric oxygen. In addition, contribution of the electrostatic interaction between the positively charged hydrotalcite layer and the intercalated anionic dye is also proposed.

Photoreduction of carbon dioxide with hydrogen over ZrO2

Among many transition-metal oxides, zirconium oxide is found to be nactive for photocatalytic reduction of carbon dioxide to carbon monoxide nwith hydrogen in the gas phase.

Application of XANES Spectra to Supported Catalysts

XANES spectroscopy is applied to the characterization of surface species of supported catalysts. In case of vanadium oxide supported on silica (V2O5/SiO2) at low loadings, comparison of the vanadium K-edge XANES spectrum with the spectra of the authentic samples allowed the determination that the surface species is a VO4 tetrahedron where three oxygen atoms of the four are linked to silica support. The coordination environment changed easily by adsorption of water molecules, indicating the direct attachment of water molecules to vanadium atoms. A platinum particle of Pt-sulfated ZrO2, known as a catalyst promoting super acidic reactions, consists of a core of platinum metal particle and surface layers of platinum oxide. The analysis of XANES spectrum of the catalyst sample by using those of Pt metal and α-PtO2 goves the fraction of α-PtO2. Fe ions in Fe, Mn-sulfated ZrO2 catalyst, knowing as a catalyst promoting super acidic reactions, were found to be present inside the ZrO2 as a solid solution and the ions does not directly participate in the super acidic reaction. Mn ions are the site of adsorption of reactant gas molecules. The in situ XANES spectra at Fe K-edge did not change throughout the reaction and those at Mn K-edge were affected by introduction of reaction gas. Europium amide complex in zeolite is a mixture of trivalent and divalent Eu species. Eu divalent species is an active species for abase-catalyzed reaction. The fraction of Eu divalent species could be determined by danalysis of XANES spectra of Eu L3-edge. The analysis applied to Pt XANES spectra can be applied to the XANES spectra of Rh/TiO2 at Rh K-edge. In the case of a Rh/TiO2 photocatalyst, the reactivity of the catalyst varies with the loading amount of Rh. XANES analysis of the Rh/TiO2 catalysts showed that surface rhodium species are a mixture of Rh metal and highly dispersed Rh2O3 and the fraction of Rh2O3 varies with the loading amount. We found that Rh atoms are present as an oxide form at low loadings even after heating the catalyst in the presence of hydrogen at 673 K.

Effect of Heat Treatment on Charge-Discharge Property of Fluoride Cathode Materials for Li Ion Secondary Batteries

ron fluoride (III) anhydrate fine particle was prepared by drying in vacuum from FeF3·3H2O, a mechanical milling process and a calcination at 473 773 K. Particle size of FeF3·3H2O was ca. 3 5 μm and that of FeF3 anhydrate was 100 500 nm after drying and milling. FeF3 sample only after drying and milling was hygroscopic and became FeF3·3H2O under atmosphere. FeF3 became stable under atmosphere after oxidation at 673 K for more than 20 minutes. It was found that Fe2O3 was produced by calcination and covered the surface of FeF3 particles. In Charge-discharge measurements, the discharge capacity of these FeF3 samples was 150 - 200 mAh/g at a discharge rate of 0.1 C. The oxidation could improve the discharge properties of FeF3 cathode.

Effects of Li-doped NiO on the charge–discharge properties of LiF–NiO composites used as cathode materials for Li-ion batteries

LiF–NiO composites are synthesized by the mechanical milling of LiF and Li-doped NiO (d_NiO) for 72xa0h. Li-d_NiO is prepared by the calcination of a mixture of Li2CO3 and NiO. The X-ray diffraction peaks of NiO are shifted by the doping and milling processes, while those of LiF disappear after milling. Rietveld analysis shows that the composites obtained by milling form solid solutions and that the Li+ and Ni2+ ions in them are disordered. LiF and Li-d_NiO samples milled individually do not exhibit a noticeable discharge capacity, while the composites show large values. The composite formed using undoped NiO without ball-milling shows a discharge capacity of 144xa0mAxa0hxa0g−1 at 0.05 C between 2.0 and 5.0xa0V, while the composite of LiF and 20% Li-d_NiO exhibits a discharge capacity of 247xa0mAxa0hxa0g−1. The rate capabilities of the composites increase with the Li content, and the discharge capacity of the LiF–Li-d_NiO (20%) is 146xa0mAxa0hxa0g−1 at 1 C for voltages of 2.0–4.8xa0V.Graphical AbstractDischarge capacity of composite of LiF and Li(20%)-doped NiO is 247 mAh g−1.

Preparation and Characterization of Mesoporous Silica and Lithium-Ion-Conductive Halocomplex Salt Composite

Composites of mesoporous SiO2 and LiAlCl4 were synthesized, and the change in ionic conductivity due to the formation of the composites was investigated. The electrical conductivity of the composite with the SiO2 : LiAlCl4 of 2 : 3 was the highest with a value of 2 x 10-6 S/cm at room temperature. A higher order of conductivity was observed for the composite compared to the non-composite LiAlCl4. In addition, it was found that the conductivity of the composites depends on the pore size of the mesoporous SiO2.