The conduction and valence band edges for electronic band gaps and Fermi levels are determined for Ta2O5, TaON, and Ta3N5 by ultraviolet photoelectron spectroscopy (UPS) and electrochemical analyses. Reasonable agreement between the results of the two methods is obtained at the pH at which the ζ potentials of the particles are zero. The tops of the valence bands are found to be shifted to higher potential energies on the order Ta2O5 < TaON < Ta3N5, whereas the bottoms of the conduction bands are very similar in the range −0.3 to −0.5 V (vs NHE at pH = 0). From the results, it is concluded that TaON and Ta3N5 are promixing catalysts for the reduction and oxidation of water using visible light in the ranges λ < 520 nm and λ < 600 nm, respectively. It is also demonstrated that the proposed UPS technique is a reliable alternative to electrochemical analyses for determining the absolute band gap positions for materials in aqueous solutions that would otherwise be difficult to measure using electrochemical methods.

The production of diesel from vegetable oil calls for an efficient solid catalyst to make the process fully ecologically friendly. Here we describe the preparation of such a catalyst from common, inexpensive sugars. This high-performance catalyst, which consists of stable sulphonated amorphous carbon, is recyclable and its activity markedly exceeds that of other solid acid catalysts tested for 'biodiesel' production.

A Ti-based oxysulfide, Sm(2)Ti(2)S(2)O(5), was studied as a visible light-driven photocatalyst. Under visible light (440 nm < or = lambda < or = 650 nm) irradiation, Sm(2)Ti(2)S(2)O(5) with a band gap of approximately 2 eV evolved H(2) or O(2) from aqueous solutions containing a sacrificial electron donor (Na(2)S-Na(2)SO(3) or methanol) or acceptor (Ag(+)) without any noticeable degradation. This oxysulfide is, therefore, a stable photocatalyst with strong reduction and oxidation abilities under visible-light irradiation. The electronic band structure of Sm(2)Ti(2)S(2)O(5) was calculated using the plane-wave-based density functional theory (DFT) program. It was elucidated that the S3p orbitals constitute the upper part of the valence band and these orbitals make an essential contribution to the small band gap energy. The conduction and valence bands' positions of Sm(2)Ti(2)S(2)O(5) were also determined by electrochemical measurements. It indicated that conduction and valence bands were found to have satisfactory potentials for the reduction of H(+) to H(2) and the oxidation of H(2)O to O(2) at pH = 8. This is consistent with the results of the photocatalytic reactions.

Under visible light irradiation (lambda = 420-500 nm), a tantalum oxynitride, TaON, functions as a stable and very efficient photocatalyst for oxidation of water into O2 with a sacrificial electron acceptor (Ag+).

Niobic acid, Nb(2)O(5)·nH(2)O, has been studied as a heterogeneous Lewis acid catalyst. NbO(4) tetrahedra, Lewis acid sites, on Nb(2)O(5)·nH(2)O surface immediately form NbO(4)-H(2)O adducts in the presence of water. However, a part of the adducts can still function as effective Lewis acid sites, catalyzing the allylation of benzaldehyde with tetraallyl tin and the conversion of glucose into 5-(hydroxymethyl)furfural in water.

Carbonization of d-glucose at 573−723 K followed by sulfonation produces a functionalized amorphous carbon material with acid catalytic activity as a solid-acid replacement for sulfuric acid. The carbon material contains phenolic hydroxyl, carboxylic acid, and sulfonic acid groups and exhibits high catalytic performance for liquid-phase acid-catalyzed reactions. Carbonization at higher temperature followed by sulfonation also results in amorphous carbon, but the resultant does not exhibit catalytic activity although the amorphous carbon has sufficient amount of sulfonic acid groups. Structural and active site analyses suggest that the marked difference in catalytic activity is due to the accessibility of reactants to sulfonic acid groups in the carbon structure.

Solid acids are conventional materials that have wide applications in chemical production, separation/purification, and polymer-electrolyte fuel-cell (PEFC) technologies, and the chemical industry is currently searching for a highly active and stable solid acid to improve the environmental safety of the production of chemicals and energy. Over 15 million tons of sulfuric acid is annually consumed as "an unrecyclable catalyst"-which requires costly and inefficient separation of the catalyst from homogeneous reaction mixtures-for the production of industrially important chemicals, thus resulting in a huge waste of energy and large amounts of waste products. The "green" approach to chemical processes has stimulated the use of recyclable strong solid acids as replacements for such unrecyclable "liquid acid" catalysts. [1][2][3][4] Thermostable strong solid acids would have genuine applications in PEFCs as proton conductors, for improving fuel efficiency, and for reducing the use of noble-metal catalysts by increasing the working temperature.[5] However, a major obstacle to such progress is the lack of a solid acid that is as active, stable, and inexpensive as sulfuric acid.An ideal solid material for the applications considered here should have high stability and numerous strong protonic acid sites. It is essential for the solid acid to maintain strong acidity even in water since water participates in fuel-cell reactions and many industrially important acid-catalyzed reactions. While organic acid/inorganic solid oxide hybrids and strong acidic cation-exchangeable resins, including perfluorosulfonated ionomers (for example, nafion), have been studied extensively as promising approaches for the construction of desired solid acids or proton conductors, [6] such materials are expensive and the acid activities are still much lower than that of sulfuric acid.[3] These drawbacks have limited their practical utility. Herein, we report the synthesis of a carbon-based solid acid with a high density of sulfonic acid groups (SO 3 H) and discuss its performance as a novel strong and stable solid acid. Here, a new strategy is adopted for the development of new types of solid acid: a carbon material is obtained by incomplete carbonization of sulfoaromatic hydrocarbons and consists of small polycyclic aromatic carbon sheets with attached SO 3 H groups. This approach is simple and allows for the use of sulfoaromatic hydrocarbons-strong, stable solvent-soluble acids (for example, benzene sulfonic acid and naphthalene sulfonic acid)-as insoluble solid acids.Such carbon-based solid acids can be readily prepared by heating aromatic compounds such as naphthalene in sulfuric acid at 473-573 K. [7] In this synthesis, the sulfonation of the aromatic compounds is the first stage of the reaction. The resulting sulfonated aromatic compounds are incompletely carbonized, which results in the formation of a solid with a nominal sample composition of CH 0.35 O 0.35 S 0.14 . The total yield of the product based on carbon is about 55 % by this metho...

Photochemical reactions on LaTiO 2 N, a perovskite-type oxynitride, were examined. Under visible-light irradiation (420 nm < λ < 600 nm), LaTiO 2 N reduced H + into H 2 and oxidized H 2 O into O 2 in the presence of a sacrificial electron donor (methanol) or acceptor (Ag + ) by the band gap transition (2.1 eV). Oxidation of water proceeded with little degradation of the oxynitride, whereas partial substitution of Ca 2+ for La 3+ of LaTiO 2 N and modification by IrO 2 colloid markedly suppressed degradation of the oxynitride and increased O 2 evolution efficiency.