Kentaro Teramura

TiO2/SiO2 photocatalysts at low levels of loading: preparation, structure and photocatalysis

Abstract The effect of precursor on the dispersion and photocatalytic performance of titanium oxide supported on silica has been investigated. The catalysts were prepared by a simple impregnation method with three kinds of titanium complexes of different ligands (dipyvaroylmethanato (DPM), acetylacetonato (acac), isopropoxide (O- i Pr)) abbreviated to D-TS, A-TS, and I-TS. The UV/Vis diffuse reflectance, Raman, XAFS spectroscopic study showed that surface titanate of D-TS is isolated TiO 4 species, and that of I-TS is a mixture of rutile and anatase crystallites. The surface state of A-TS is the intermediate between D-TS and I-TS. TiO 2 /SiO 2 samples were more active than bulk TiO 2 and partial oxidation selectivity of TiO 2 /SiO 2 samples are higher than that of TiO 2 in photo-oxidation of propane. Products distribution over TiO 2 /SiO 2 was drastically varied with the surface titanates. The activity for photo-assisted selective catalytic reduction (SCR) of NO with ammonia was found for D-TS.

A Doping Technique that Suppresses Undesirable H2 Evolution Derived from Overall Water Splitting in the Highly Selective Photocatalytic Conversion of CO2 in and by Water

Photocatalytic conversion of CO2 to reduction products, such as CO, HCOOH, HCHO, CH3OH, and CH4, is one of the most attractive propositions for producing green energy by artificial photosynthesis. Herein, we found that Ga2O3 photocatalysts exhibit high conversion of CO2. Doping of Zn species into Ga2O3 suppresses the H2 evolution derived from overall water splitting and, consequently, Zn-doped, Ag-modified Ga2O3 exhibits higher selectivity toward CO evolution than bare, Ag-modified Ga2O3. We observed stoichiometric amounts of evolved O2 together with CO. Mass spectrometry clarified that the carbon source of the evolved CO is not the residual carbon species on the photocatalyst surface, but the CO2 introduced in the gas phase. Doping of the photocatalyst with Zn is expected to ease the adsorption of CO2 on the catalyst surface.

Overall water splitting using (oxy)nitride photocatalysts

Oxynitride photocatalysts with d10 electronic configuration are presented as effective non-oxide catalysts for overall water splitting. Germanium nitride (β-Ge3N4) having a band gap of 3.8-3.9 eV modified with RuO2 nanoparticles as a cocatalyst is shown to achieve stoichiometric decomposition of H2O into H2 and O2 under UV irradiation (λ > 200 nm). A novel solid solution of GaN and ZnO, (Ga1-xZnx)(N1-xOx), with a band gap of 2.4-2.8 eV (depending on composition) achieves overall water splitting under visible light (λ > 400 nm) when loaded with an appropriate cocatalyst. The narrower band gap of the solid solution is attributed to the bonding between Zn and N atoms at the top of the valence band. The photocatalytic activity of (Ga1-xZnx)(N1-xOx) for overall water splitting is strongly dependent on both the cocatalyst and the crystallinity and composition of the material. The quantum efficiency of (Ga1-xZnx)(N1-xOx) with Rh and Cr mixed-oxide nanoparticles is 2-3 % at 420-440 nm, which is the highest reported efficiency for overall water splitting in the visible-light region.

Simultaneous photodeposition of rhodium–chromium nanoparticles on a semiconductor powder: structural characterization and application to photocatalytic overall water splitting

Simultaneous photodeposition of rhodium (Rh) and chromium (Cr) nanoparticles on a semiconductor powder was examined as a means of constructing active sites for hydrogen evolution in photocatalytic overall water splitting. A solid solution of gallium nitride and zinc oxide (GaN:ZnO) powder that catalyzes overall water splitting with visible light was employed as a semiconductor support. The photodeposition was carried out in aqueous suspension containing GaN:ZnO, (NH4)3RhCl6, and K2CrO4, and used H2O as an electron donor. With increasing concentration of K2CrO4, the valence state of the codeposited Rh species varied from metallic to trivalent, while that of Cr remained trivalent. At intermediate concentrations of K2CrO4, the photodeposits were core/shell-like crystalline nanoparticles consisting of a metallic Rh core and an Rh(III)–Cr(III) mixed-oxide shell. The photocatalytic activity for visible-light-driven overall water splitting (λ > 400 nm) was strongly dependent on the structure of the photodeposits. Comparative experiments using an analogue, modified with core/shell-structured Rh/Cr2O3 nanoparticles, revealed that core/shell-structured nanoparticles consisting of a metallic Rh core were better than Rh–Cr trivalent mixed oxides for enhancing the photocatalytic activity.

Highly efficient photocatalytic conversion of CO2 into solid CO using H2O as a reductant over Ag-modified ZnGa2O4

Highly crystalline spinel phase ZnGa2O4 modified with a Ag cocatalyst exhibited high activity and selectivity toward CO evolution in the photocatalytic conversion of CO2 using H2O as a reductant under UV light irradiation. The stoichiometric evolution of CO, H2, and O2 clearly indicated that H2O worked as an electron donor for the photoreduction of CO2. Highly crystalline ZnGa2O4 was synthesized by a solid-state reaction method at a calcination temperature as low as 973 K. Upon optimizing the fabrication conditions, such as calcination temperature and duration, the photocatalytic activity of ZnGa2O4 was maximized because of the optimum balance between crystallinity and surface area in the catalyst material. Furthermore, the formation of metallic Ag particles with different sizes and dispersions on the surface of ZnGa2O4 influenced the evolution of CO. When Ag nanoparticles were loaded onto the ZnGa2O4 calcined at 1123 K for 40 h using a chemical reduction method, the highest formation rates of CO, H2, and O2 (155, 8.5, and 74.3 μmol h−1, respectively) were obtained, and the selectivity toward CO evolution reached 95.0%. An isotope-labeling experiment using 13CO2 confirmed that the origin of the evolved CO was not from organic contamination but from the CO2 gas introduced during the reaction process.

Popping of Graphite Oxide: Application in Preparing Metal Nanoparticle Catalysts

A popcorn-like transformation of graphite oxide (GO) is reported and used to synthesize metal nanoparticle catalysts. The popping step is unique and essential, not only generating a high-surface-area support but also partially decomposing the metal precursors to form well-separated metal oxide nuclei, which would further evolve into highly dispersed and uniform-sized nanoparticles in the subsequent reduction.

Narrow energy gap between triplet and singlet excited states of Sn2+ in borate glass.

Transparent inorganic luminescent materials have attracted considerable scientific and industrial attention recently because of their high chemical durability and formability. However, photoluminescence dynamics of ns2-type ions in oxide glasses has not been well examined, even though they can exhibit high quantum efficiency. We report on the emission property of Sn2+-doped strontium borate glasses. Photoluminescence dynamics studies show that the peak energy of the emission spectrum changes with time because of site distribution of emission centre in glass. It is also found that the emission decay of the present glass consists of two processes: a faster S1-S0 transition and a slower T1-S0 relaxation, and also that the energy difference between T1 and S1 states was found to be much smaller than that of (Sn, Sr)B6O10 crystals. We emphasize that the narrow energy gap between the S1 and T1 states provides the glass phosphor a high quantum efficiency, comparable to commercial crystalline phosphors.

Incarceration of (PdO)nand PdnClusters by Cage-Templated Synthesis of Hollow Silica Nanoparticles

Metal nanoclusters have recently attracted considerable interest because of their distinct chemical and physical properties, such as catalytic activities, optical properties, and magnetic behavior, which are different from those of both bulk materials and mononuclear metal species. When the number of metal centers (n) is small, the properties of the metal clusters (Mn) change dramatically with increasing n value. Therefore, intense efforts have been made to synthesize small-numbered, well-defined clusters with nondistributed n values. Gas-phase preparation by metal vaporization is a promising physical method to prepare smallnumbered Mn clusters (typically in the range of n = 10– 10000). Solution synthesis through the reduction of metal ion precursors within templates is also an efficient method to give Mn clusters of similar size. [5] With all of these methods it is still difficult to strictly control the number of metal centers particularly when n is around 10 or less. Furthermore, the resulting synthesized clusters are often chemically and physically labile and are quite prone to oxidation or fusing into larger clusters. The most controllable methods to synthesize metal clusters with small n values (n = 4–60) have been reported utilizing the voids of dendrimers, but this method is hampered by the tedious dendrimer synthesis. Herein, we report a unique approach to incarcerated metal clusters with strictly controlled n values (ca. 12) within hollow silica nanoparticles. Our method utilizes a Pd12L24 spherical complex as a template for the hollow silica synthesis. After sol-gel condensation around the sphere, the incarcerated Pd12L24 core is calcinated to give (PdO)n oxide clusters and subsequently reduced to Pdn metal clusters (Figure 1). X-ray absorption fine structure (XAFS) analysis indicated that the

Tuning the selectivity toward CO evolution in the photocatalytic conversion of CO2 with H2O through the modification of Ag-loaded Ga2O3 with a ZnGa2O4 layer

Stoichiometric evolutions of CO, H2, and O2 were achieved for the photocatalytic conversion of CO2 with H2O as an electron donor using Ag-loaded Zn-modified Ga2O3. The selectivity toward the evolution of CO over H2 can be controlled by varying the amount of Zn species added in the Ag-loaded Zn-modified Ga2O3 photocatalyst. The production of H2 gradually decreased with increasing amounts of Zn species from 0.1 to 10.0 mol%, whereas the evolution of CO was almost unchanged. The XRD, XAFS, and XPS measurements revealed that a ZnGa2O4 layer was generated on the surface of Ga2O3 by modification with Zn species. The formation of the ZnGa2O4 layer eliminated the proton reduction sites on Ga2O3, although the crystallinity, surface area, and morphology of Ga2O3 as well as the particle size and chemical state of Ag did not change. In conclusion, we designed a highly selective photocatalyst for the conversion of CO2 with H2O as an electron donor using Ag (the cocatalyst for the CO evolution), ZnGa2O4 (the inhibitor of the H2 production), and Ga2O3 (the photocatalyst).

NO Reduction with CO in the Presence of O2 over Cu/Al2O3 (3) – Structural Analysis of Active Species by Means of XAFS and UV/VIS/NIR Spectroscopy

The local structure around Cu supported on Al2O3 was determined by UV/VIS/NIR and XAFS spectroscopic techniques. The relationship between catalytic performance for NO–CO–O2 reaction and the state of supported Cu species is discussed. A new method for estimating the fraction of aggregated to isolated Cu species is proposed.