Ken-ichi Maruya

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.

Carbon monoxide and carbon dioxide adsorption on cerium oxide studied by Fourier-transform infrared spectroscopy. Part 1.—Formation of carbonate species on dehydroxylated CeO2, at room temperature

The adsorption of CO and CO2 on cerium oxide has been studied by Fourier-transform infrared spectroscopy (F.t.i.r.). For CO adsorption at room temperature, in addition to linearly adsorbed CO (2177 and 2156 cm–1), two kinds of carbonate (unidentate: 854, 1062, 1348 and 1454 cm–1 and bidentate: 854, 1028, 1286 and 1562 cm–1) and inorganic carboxylate (1310 and 15⊙0 cm–1) species were identified spectroscopically. As for CO2 adsorption, apart from weak bands at 1728, 1396, 1219, and 1132 cm–1 attributed to bridged carbonate species, bands due to unidentate carbonate, bidentate carbonate and inorganic carboxylate species, similar to those aising from CO adsorption, were observed. Except for the linearly adsorbed-CO, all species arising from CO and CO2 are stable at room temperature in vacuo. The desorption of these species at elevated temperatures shows that the order of thermal stability is bridged carbonate <bidentate carbonate < inorganic carboxylate < unidentate carbonate species, and the residual of unidentate carbonate species can remain on the surface up to 773 K under evacuation. Forming carbonte and inorganic carboxylate species proved that the CeO2 surface could be partially reduced by CO even at room temperature. No bands in the region 2300–800 cm–1 were detected below 373 K for CO adsorption on hydroxylated CeO2. This indicates that CO adsorption depends on the degree off dehydroxylation of the surface. The mechanism of CO adsorption is also discussed.

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.

Spectroscopic identification of adsorbed species derived from adsorption and decomposition of formic acid, methanol, and formaldehyde on cerium oxide

Abstract The surface species formed from adsorption and decomposition of HCOOH, CH 3 OH, and HCHO over dehydroxylated cerium oxide have been identified using in situ Fourier transform infrared spectroscopy at temperatures from 300 to 673 K. Bidentate and unidendate formate species were formed upon adsorption of HCOOH at 300 K. It is suggested that the two types of formate species originated from dehydroxylation and deprotonation of the formic acid on the surface. The formate species decompose above 473 K in uacuo and subsequently leave a residue of carbonate and isolated OH groups on the surface. At least two kinds of surface methoxy species with characteristic IR bands in the regions 2950-2780 and 1100-1030 cm −1 were detected when cerium oxide was exposed to CH 3 OH at 300 K. These methoxy species were partly oxidized to form formate species by oxygenation of cerium oxide at 473 K. Dioxymethylene species together with formate species were produced from adsorption of HCHO on cerium oxide at 300 K. Upon warming the samples to 373 K, IR bands due to the dioxymethylene species nearly disappeared while bands of the methoxy species emerged. Band intensities of both dioxymethylene and methoxy species increased markedly when cerium oxide was partially reduced prior to the adsorption. It is concluded that the methoxy and formate species are produced mainly via Cannizzaro reaction, particularly on partially reduced cerium oxide.

Adsorption of carbon monoxide and carbon dioxide on cerium oxide studied by Fourier-transform infrared spectroscopy. Part 2.—Formation of formate species on partially reduced CeO2 at room temperature

The adsorption of CO on partially reduced CeO2[CeO2(573-H) and CeO2(673-H), CeO2 reduced in H2 at 573 and 673 K for 1 h, respectively] has been studied by Fourier-transform infrared spectroscopy (F.t.i.r.). The observation of formate species (771, 1329, 1369, 1558, 1587, 2852 and 2945 cm–1) formed by the reaction of CO with the surface of CeO2(673-H) at room temperature is reported. Two weak bands at 2796 and 2706 cm–1, tentatively attributed to formyl species, were also detected at room temperature when the bands of the formate species no longer appeared to grow. No band was observed when CO was dosed on CeO2(573-H) or CeO2(673-H2O)(CeO2 hydrated at 673 K for 1 h) at room temperature and 373 K. Two sharp bands at 3685 and 3643 cm–1, due to isolated hydroxyl groups, and a broad band centred at 3421 cm–1 were generated on CeO2(673-H) through H2 reduction. Among these only the OH groups giving the 3685 cm–1 band manifest activity to CO at room temperature. A mechanism involving a formyl intermediate was proposed to explain the formation of formate species on partially reduced CeO2. The partial reduction of CeO2 causes pronounced inhibition of the formation of carbonate-like species from CO at room temperature. This is consistent with the conclusions reported in Part 1 of this work.

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.

Photocatalytic decomposition of water over a Ni-Loaded Rb4Nb6O17 catalyst

Nickel-loaded Rb4Nb6O17, which has the same layered structure of niobium oxide sheets with interlayers as that of K4Nb6O17, exhibits a high activity for photocatalytic water splitting to form H2 and O2 under band gap irradiation. In order to obtain an efficient catalyst, it was essential to treat the Rb4Nb6O17 powder before nickel-loading by washing the Rb4Nb6O17 with deionized water and then recalcining this powder in air in the temperature range 1153–1173 K for more than 10 h. The maximum activity was obtained when the reaction was carried out in distilled water where the pH was ca. 9.7 and the quantum efficiency at 330 nm was ca. 10% at the initial stage of the reaction over NiO(0.1 wt%)-Rb4Nb6O17.

Effect of the particle size for photocatalytic decomposition of water on Ni-loaded K4Nb6O17

Abstract The photocatalytic activities of water splitting by small particles (0.1–2 μm) of K 4 Nb 6 O 17 were studied. The milled catalysts were prepared by two kinds of ball-milling methods from the powder whose particles were several tens of micrometers in diameter. The surface area of the milled catalyst measured by N 2 adsorption at 77 K increased by an order of magnitude. The crystal structure was almost retained as K 4 Nb 6 O 17 , as evidenced by the results of X-ray diffraction and scanning electron micrography. The photocatalytic activity for an overall decomposition of water of the Ni-loaded K 4 Nb 6 O 17 was twice as high as that for the original catalyst.

Oxygen exchange reactions over cerium oxide: An FT-IR study

Abstract Oxygen (site and isotope) exchange reactions over well-outgassed and partially reduced cerium oxide were examined in the temperature range 200–373 K by means of Fourier-transform infrared (FT-IR) spectroscopy. Isotopic exchange between gaseous O 2 and lattice oxygen of the cerium oxide does not occur at the temperature below 373 K. Site exchange via gaseous O 2 with adsorbed superoxide (O 2 − ads ) species was found to be very fast on both the outgassed and partially reduced surfaces even at 200 K, but site exchange between gaseous O 2 and adsorbed peroxide (O 2 2− ads ) species does not take place in the temperature range 200-373 K. The isotopically exchanged superoxide species ( 16 O 18 O ads − ) with characteristic IR bands at 2175 and 1095 cm −1 are readily formed from a mixture of 1602 + 1

Infrared studies of adsorbed species of H2, CO and CO2 over ZrO2

02 adsorption over well-outgassed and partially reduced cerium oxides. It is concluded that isotope exchange reactions proceed via adsorbed superoxide species and the exchange reaction is presumed to involve a tetraoxygen intermediate as the result of the reaction of gaseous O 2 with adsorbed superoxide species.