Ken-ichi Aika

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.

Activation of nitrogen by alkali metal promoted transition metal I. Ammonia synthesis over ruthenium promoted by alkali metal

Abstract The rate of ammonia synthesis over ruthenium was found to be remarkably promoted by addition of alkali metal, particularly when ruthenium is supported by active carbon (AC) or alumina. The synthesis rate over the 5% RuACK catalyst can be raised to about 10 times that of conventional doubly promoted iron (Fe Al 2 O 3 K 2 O) catalyst at 250 °C. RuACCs gives even greater activity than RuACK. The catalytic activity increases with decrease in ionization potential of added alkali (Cs > K > Na) and with increase in the added amount of alkali metal, approaching a plateau value at around 3–4 mg-atom alkali/g-catalyst. Unsupported ruthenium is also remarkably promoted by addition of alkali metal. It is accordingly concluded that the promoter action is provided by a charge transfer from alkali metal to transition metal which brings about a higher electron density in the transition metal, a favorable state for the activation of nitrogen. The specific synthesis rates per surface ruthenium are higher over active carbon and alumina than other supports, suggesting a role of these supports as a medium of electron transfer.

Preparation and characterization of chlorine-free ruthenium catalysts and the promoter effect in ammonia synthesis: 3. A magnesia-supported ruthenium catalyst

The support and promoter effects of Ru in ammonia synthesis were studied systematically using Ru carbonyl, Ru3(CO)12, as a precursor of the Ru catalyst. MgO was found to be the most effective support among several pure oxides. This high activity is thought to be due to the high Ru dispersion, the Cl-free system, and the basicity of the support. Modifying MgO by adding other oxides was also tried. Na2OMgO and Al2O3MgO supports were found to be as effective as the pure MgO. Fourteen kinds of metal nitrates, including alkali metals, alkali earth metals, and lanthanides, were tested as promoters of Ru/MgO prepared from Ru3(CO)12. Alkali metal nitrates (Cs+, Rb+, and K+), especially CsNO3, were most effective. The synthesis rate was greatest when the Cs/Ru ratio was as low as 1.0, where the activity was 20 times as high as that without CsNO3 at 588 K and under 80 kPa of N2 + 3H2. CsNO3 was considered to be disproportionated to CsOH during the hydrogen treatment. The alkali promoter proved to act not only as a Cl scavenger in the case of a Cl-containing Ru catalyst but also as an electronic surface modifier to Ru. Hydrogen chemisorption studies disclosed that alkali covered mostly the Ru surface but had a weak interaction with the MgO surface. The importance of surface morphology in the combination of Ru and promoter was noted. RuCsOH/MgO (5 wt%; Cs/Ru = 1) was even more active than 4.7 wt% RuK/AC and was comparable to the Raney RuCsNO3 catalyst, which had previously been reported to be the most active catalyst at 573 K and under 80 kPa of N2 + 3H2.

Support and promoter effect of ruthenium catalyst. II: Ruthenium/alkaline earth catalyst for activation of dinitrogen

The turnover frequency (TOP) of the ammonia synthesis on Ru was promoted by using alkaline earths as supports. The TOF of this reaction on RuMgO and RuCaO was as high as those on RuCsOHAl2O3 catalyst. The activity was shown to be correlated with the electronegativity of oxide support. XPS results seem to support the electron transfer from alkaline earth to Ru. Activity of RuMgO was further promoted with CsOH, which increased not only TOF but also Ru dispersion. On the other hand, Ru/alkaline earth with K, which may be called “superbase-supported Ru catalysts,” proved to have excellent activities for N2 activation. TOF of ammonia synthesis over those catalyst was as high as that on RuKAC (activated carbon) which has been known to be the best catalyst under these conditions. Furthermore, TOF of N2 isotopic equilibration reaction on these catalyst were several times as high as those on RuKAl2O3 or RuKAC which have been believed to be the most active catalysts for this reaction.

The electronic structure of oxygen atom vacancy and hydroxyl impurity defects on titanium dioxide (110) surface

Introducing a charge into a solid such as a metal oxide through chemical, electrical, or optical means can dramatically change its chemical or physical properties. To minimize its free energy, a lattice will distort in a material specific way to accommodate (screen) the Coulomb and exchange interactions presented by the excess charge. The carrier-lattice correlation in response to these interactions defines the spatial extent of the perturbing charge and can impart extraordinary physical and chemical properties such as superconductivity and catalytic activity. Here we investigate by experiment and theory the atomically resolved distribution of the excess charge created by a single oxygen atom vacancy and a hydroxyl (OH) impurity defects on rutile TiO(2)(110) surface. Contrary to the conventional model where the charge remains localized at the defect, scanning tunneling microscopy and density functional theory show it to be delocalized over multiple surrounding titanium atoms. The characteristic charge distribution controls the chemical, photocatalytic, and electronic properties of TiO(2) surfaces.

Support and promoter effect of ruthenium catalyst: I. Characterization of alkali-promoted ruthenium/alumina catalysts for ammonia synthesis

RuAl2O3 catalysts promoted with various alkali precursors were prepared and characterized by various methods including XPS and TPR (temperature-programmed reduction) techniques. The activation of the promoted catalyst was studied in detail. The addition of an alkali precursor, especially CsNO3, promoted the turnover frequency (TOF) of ammonia synthesis, and also increased þe Ru dispersion. It is suggested that the alkali precursor became converted to hydroxide after reduction at 400 °C. The promotion of TOF by alkali corresponded to the electron-donating tendency (CsOH > KOH). Ru-alkali(KOH)Al2O3 was found to be tolerant to CO and/or H2O poisoning, and treatment in H2 led to restoration of the original catalytic activity.

Preparation and characterization of chlorine-free ruthenium catalysts and the promoter effect in ammonia synthesis: 2. A lanthanide oxide-promoted Ru/Al2O3 catalyst

La(NO3)3, Ce(NO3)3, and Sm(NO3)3 were proved to be more effective promoters of ammonia synthesis than CsNO3 when they were applied to a chlorine-free Ru/Al2O3 catalyst. Small amounts of lanthanide nitrates (M/Ru = 1) were as effective as a much greater amount of CsNO3 (CS/RU = 10) for 2 wt% Ru/Al2O3. A chemisorption study disclosed that a smaller amount of lanthanide oxide could cover the Ru surface, whereas much more cesium oxide was necessary to cover the Ru surface. Lanthanide oxide was suggested to have a stronger interaction with the Ru surface than Cs2O and/or CsOH, which are strongly basic and have a greater interaction with the Al2O3 Surface. A model of promoter action is proposed. The apparent activation energies of this reaction were lower (11 to 15 kcal mol−1) than those using a Cs+ promoter (24 kcal mol−1) under 101 kPa of N2 + 3H2.

Oxidative coupling of methane over Na+- and Rb+-doped MgO catalysts

Abstract The promoter effect of Na + and Rb − on MgO catalysts was studied for the oxidative coupling of methane. The maximum yield and selectivity obtained for the formation of C 2 hydrocarbons (C 2 H 6 + C 2 H 4 ) from CH 4 and O 2 were 22.4% (1073 K) and 57% (1023 K), respectively, over 2 g of 15 mol% Na + MgO catalyst. Physical factors of the promoted MgO catalysts were investigated with BET, SEM, and XRD techniques and were compared with the activities. Alkali metal ions, the promoters, caused structural changes in MgO. Those changes, which are related to the activity of C 2 hydrocarbon formation, are classified in two categories: (1) surface activation through the lattice distortion, and (2) specific surface area reduction. When catalysts with the same specific surface areas were compared, alkali ion-doped MgO in which extensive line broadening is observed by XRD was more active than pure MgO. On the other hand, a sintered pure MgO with low specific surface area showed higher C 2 hydrocarbon yield than unsintered pure MgO. The two factors are discussed in relation to the reaction mechanism.

Support and promoter effect of ruthenium catalyst. III. Kinetics of ammonia synthesis over various Ru catalysts

Abstract Ammonia synthesis over Ru powder, Ru/Al2O3, Ru-CsOH/Al2O3, Ru-CsOH, Ru/MgO, and Ru-CsOH/MgO was studied kinetically under 79 kPa (600 torr) at around 598 K. The kinetic expression was found to be affected markedly by the kind of support or promoter. The reaction order in N2 was unity for all the catalysts. However, the order in NH3 was negative and that in H2 was positive for Ru/Al2O3 and Ru/MgO, while the orders were near zero and negative respectively for other catalysts. It is discussed that N2 adsorption is the rate determining step, and that the reaction is retarded by the different adsorbed species the nature of which depends upon the kind of support and promoter. It is suggested that the retarding species are both N (or NH) and H over Ru/Al2O3 and Ru/MgO, and H alone over Ru powder and CsOH promoted catalysts. Although promoted Ru catalysts are more active than iron catalysts at atmospheric pressure, the activity of the former would be exceeded by that of the latter at high pressure because of a hydrogen retardation.

Kinetic analysis of the oxidative coupling of methane over Na+-doped MgO

The reaction rate of oxidative coupling of methane was studied kinetically using 0.0165 or 0.05 g of 15% Na+-MgO catalyst at 923, 973, and 1023 K in a flow reactor under a CH4 pressure of 1.36 to 13 kPa and an O2 pressure of 0.36 to 4.7 kPa. The reaction rate was well expressed by the following mechanism. The kinetic data were analyzed by the Rideal-redox-type rate equation assuming methyl radical and active surface oxygen as the steady-state intermediates: (1) O2 + site active oxygen, (2) CH4 + active oxygen · CH3 + OH(a) + site, (3) · CH3 + xO CO, CO2, (4) 2 · CH3 C2H6. The constants k1, k2, x, andk4k32 were obtained for every temperature. The activity is related to k1and k2, while the selectivity (C2C1) is related to k4k23. The activation energies are 18 and 36 kcal/mol for k1and k2, respectively. By considering the negative activation energy of k3 (−7 kcal/mol), step 3 is inferred to contain the equilibrium reaction in which ·CH3 and O2 form methyl peroxide if the value of x is 2.0. The actual smaller value of x suggests that part of the methyl radical is oxidized by the surface oxygen. The specific surface area effect is also explained by this reaction mechanism if we assume that k3 occurs on the surface (methyl peroxide decomposition) and k4 occurs in the gas phase. The Langmuir-Hinshelwood mechanism can also be applied; however, it leaves several ambiguous points. These conclusions are only valid at high temperatures (923 to 1023 K) and under the low conversion (both XCH4and XO2 are lower than 10%), whereas the consecutive oxidation of C2 compounds should be taken into account under the high conversion condition.