Akihiko Kudo

Mechanism of photocatalytic decomposition of water into H2 and O2 over NiOSrTiO3

The mechanism of the photocatalytic decomposition of H2O into H2 and O2 over NiOSrTiO3 powder was studied on the basis of the structure of the catalyst. It was elucidated that H2 evolution occurs over NiO while O2 evolution takes place over SrTiO3. The importance of the existence of Ni metal between the interface of NiO and SrTiO3 suggested the transfer of an electron between both materials. Two possible mechanisms, a one-photon process and a two-photon process, were discussed.

Photocatalytic decomposition of water over NiOK4Nb6O17 catalyst

Photocatalytic decomposition of H2O to form H2 and O2 over NiOK4Nb6O17 powder (1–10 μm, band gap = 3.3 eV), which is an ion-exchangeable layered compound, proceeds steadily more than 50 h under the bandgap irradiation. Maximum activity was obtained when the reaction was carried out in distilled water where the pH was ca. 11 by elution of K+ and the quantum efficiency at 330 nm was 3.5 ± 0.5% at the initial stage of the reaction over pretreated NiO(0.1 wt%)K4Nb6O17. Several characteristic features of NiOK4Nb6O17 were discussed and compared with those of NiOSrTiO3.

Nickel-loaded K4Nb6O17 photocatalyst in the decomposition of H2O into H2 and O2: Structure and reaction mechanism

The structure of nickel-loaded K4Nb6O17 pholocatalyst in an overall water splitting reaction was studied by means of XPS, EXAFS, TEM, and XRD. K4Nb6O17 has an ion-exchangeable layered structure which possesses two different kinds of alternating interlayer spaces, i.e. interlayers I and II, where K+ ions are located. The interlayers are hydrated in an aqueous solution. It was revealed that in the active catalyst which was pretreated by H2 at 773 K for 2 h and reoxidized by O2 at 473 K for 1 h, loaded nickel is predominantly located in interlayer I as ultrafine metal particles (ca. 5 A). In contrast, only a very small amount of nickel was observed over the external surface of K4Nb6O17. On the basis of the structure, a novel mechanism for the photodecomposition of H2O into H2 and O2 is proposed; i.e., intercalated water is reduced to H2 in interlayer 1 and is oxidized to O2 in interlayer II. Therefore, each niobate macroanion sheet is regarded as a “two-dimensional” photocatalyst where H2 and O2 evolve at different sides of the layer.

Electrochemical reduction of carbon dioxide under high pressure on various electrodes in an aqueous electrolyte

Abstract The electrochemical reduction of CO 2 at a pressure of 30 atm on various electrodes at large current densities (163–700 mA cm −2 ) in an aqueous KHCO 3 solution and the cathodic voltammograms obtained were investigated. The electrocatalytic activities of various electrodes for the electrochemical reduction of CO 2 under high pressure conditions were clarified. The main products on Ag, Au, Zn, Pb and In electrodes were formic acid and/or CO, as at CO 2 pressures of 1 atm. The total current density on the cathodic voltammograms of those metal electrodes increased with increasing CO 2 pressure. On group 8–10 metals, such as Fe, Co, Rh, Ni, Pd and Pt, the main reduction products were formic acid and/or CO under high pressure, whereas under 1 atm CO 2 hydrogen was formed by reduction of water. In these cases, the total current density changed only slightly with increasing CO 2 pressure. By comparing the electrocatalytic activities of various electrodes at a CO 2 pressure of 30 atm with those at a CO 2 pressure of 1 atm, the electrodes were classified into four groups. On Rh, Ag and Pd electrodes, the maximum partial current densities for CO 2 reduction under 30 atm were 237 mA cm −2 , 383 mA cm −2 and 397 mA cm −2 respectively. The reduction of CO 2 at large current densities could be accomplished at high pressures.

Spectral sensitization of a TiO2 semiconductor electrode by CdS microcrystals and its photoelectrochemical properties

Abstract CdS microcrystals were deposited on TiO2 and ZnO semiconductors and photoelectrochemical properties were studied from the viewpoint of semiconductor sensitization. The photocurrent of a CdSTiO2 photoelectrode was caused by photoinduced electron transfer in the direction from CdS to TiO2. The photoelectrochemical properties of CdSTiO2, such as a flat band potential, were found to be strongly influenced by those of CdS as sensitizer. The quantum yield of the photocurrent amounted to 30%. A photoelectrochemical solar cell composed of CdSZnO and polysulphide solution was studied.

Photocatalytic activities of TiO2 loaded with NiO

Abstract A pretreated NiO-TiO 2 powder system is an active catalyst for photocatalytic decomposition of H 2 O into H 2 and O 2 in aqueous alkaline solution (3 N NaOH) as well as under NaOH coating conditions.

Room‐temperature preparation of the highly crystallized luminescent CaWO4 film by an electrochemical method

Highly crystallized polycrystalline film of single‐phase CaWO4 has been prepared on a tungsten substrate at room temperature in an alkaline solution containing calcium ions by an electrochemical method with the current density of 1 mA/cm2. This film showed blue emission (456 nm wavelength) with excitation light (254 nm wavelength) at room temperature.

Electrochemical reduction of high pressure CO2 at Pb, Hg and In electrodes in an aqueous KHCO3 solution

The electrochemical reduction of CO2 under high pressure in an aqueous solution at metal electrodes with high overpotentials for hydrogen evolution such as Pb, Hg and In was studied. When the electrolysis was conducted under high pressure CO2, HCOOH was formed with faradaic efficiencies of almost 100%, even at high current densities (∼ 200 mA cm−2). The maximum partial current density of HCOOH formation amounted to 560 mA cm−2 at the In electrodes under 60 atm of CO2. On the other hand, it was found that CO could form as the main reduction product of CO2 at Pb and In electrodes, at which HCOOH has previously been believed to be the main product under 1 atm of CO2. The selectivity for CO formation depended on the electrode potential. The less negative were the potentials, the higher was the faradaic efficiency of CO formation.

New aspects of heterogeneous photocatalysts for water decomposition

Several new photocatalysts for overall water splitting are described. Under UV light irradiation (270 nm), La-doped NaTaO3 modified with NiO decomposed water into H2 and O2 with extremely high quantum efficiency. Under an optimized condition, the apparent quantum efficiency, which was estimated with numbers of irradiated photons and evolved H2 molecules, reached 56%. New stable photocatalytic materials containing elements with d10 electronic configuration such as In3+ Sn4+ and Sb5+ were developed for overall water splitting. Some mesoporous oxides were proved to be effective photocatalysts. (Oxy)nitrides of some early transition metals, i.e., Ta, Nb and Ti, were found to be stable materials having potentials for H2 and O2 evolutions under visible light irradiation (⪯600 nm). The electronic structures of these photocatalysts are also discussed based on DFT calculation.

The effects of the calcination temperature of SrTiO3 powder on photocatalytic activities

Photocatalytic activity of SrTiO3 powder prepared by the alkoxide method was examined on the photodecomposition of water, the evolution of H2 from aqueous methanol solution, and the evolution of O2 from aqueous silver nitrate solution. The activity depends strongly on the calcination temperature of SrTiO3 powder. The optimum calcination temperatures of SrTiO3 for these reactions were different from each other. Several factors affecting the photocatalytic activity were discussed on the basis of the characterization of SrTiO3 powder.