Shuhei Ogo

Hydrothermal and solid-state transformation of ruthenium-supported Keggin-type heteropolytungstates [XW11O39{Ru(II)(benzene)(H2O)}]n− (X = P (n = 5), Si (n = 6), Ge (n = 6)) to ruthenium-substituted Keggin-type heteropolytungstates

A new pathway for the preparation of mono-ruthenium (Ru)(iii)-substituted Keggin-type heteropolytungstates with an aqua ligand, [PW(11)O(39)Ru(iii)(H(2)O)](4-) (1a), [SiW(11)O(39)Ru(iii)(H(2)O)](5-) (1b) and [GeW(11)O(39)Ru(iii)(H(2)O)](5-) (1c), using [Ru(ii)(benzene)Cl(2)](2) as a Ru source was described. Compounds 1a-1c were prepared by reacting [XW(11)O(39)](n-) (X = P, Si and Ge) with [Ru(ii)(benzene)Cl(2)](2) under hydrothermal condition and were isolated as caesium salts. Ru(benzene)-supported heteropolytungstates, [PW(11)O(39){Ru(ii)(benzene)(H(2)O)}](5-) (2a), [SiW(11)O(39){Ru(ii)(benzene)(H(2)O)}](6-) (2b) and [GeW(11)O(39){Ru(ii)(benzene)(H(2)O)}](6-) (2c), were first produced in the reaction media, and then transformed to 1a, 1b and 1c, respectively, under hydrothermal conditions. Calcination of Ru(benzene)-supported heteropolytungstates, 2a, 2b and 2c, in the solid state produced mixtures of 1a, 1b and 1c with CO (carbon monoxide)-coordinated complexes, [PW(11)O(39)Ru(ii)(CO)](5-) (4a), [SiW(11)O(39)Ru(ii)(CO)](6-) (4b) and [GeW(11)O(39)Ru(ii)(CO)](6-) (4c), respectively. From comparison of their catalytic activities in water oxidation reaction, it was indicated that ruthenium should be incorporated in the heteropolytungstate in order to promote catalytic activity.

Surface Protonics Promotes Catalysis

Catalytic steam reforming of methane for hydrogen production proceeds even at 473 K over 1 wt% Pd/CeO2 catalyst in an electric field, thanks to the surface protonics. Kinetic analyses demonstrated the synergetic effect between catalytic reaction and electric field, revealing strengthened water pressure dependence of the reaction rate when applying an electric field, with one-third the apparent activation energy at the lower reaction temperature range. Operando–IR measurements revealed that proton conduction via adsorbed water on the catalyst surface occurred during electric field application. Methane was activated by proton collision at the Pd–CeO2 interface, based on the inverse kinetic isotope effect. Proton conduction on the catalyst surface plays an important role in methane activation at low temperature. This report is the first describing promotion of the catalytic reaction by surface protonics.

Low-temperature catalytic oxidative coupling of methane in an electric field over a Ce–W–O catalyst system

We examined oxidative coupling of methane (OCM) over various Ce–W–O catalysts at 423 K in an electric field. Ce2(WO4)3/CeO2 catalyst showed high OCM activity. In a periodic operation test over Ce2(WO4)3/CeO2 catalyst, C2 selectivity exceeded 60% during three redox cycles. However, Ce2(WO4)3/CeO2 catalyst without the electric field showed low activity, even at 1073 K: CH4 Conv., 6.0%; C2 Sel., 2.1%. A synergetic effect between the Ce2(WO4)3 structure and electric field created the reactive oxygen species for selective oxidation of methane. Results of XAFS, in-situ Raman and periodic operation tests demonstrated that OCM occurred as the lattice oxygen in Ce2(WO4)3 (short W–O bonds in distorted WO4 unit) was consumed. The consumed oxygen was reproduced by a redox mechanism in the electric field.

Catalytic Reaction Assisted by Plasma or Electric Field

Direct conversion of methane, other hydrocarbons, and alcohol at lower temperatures can be achieved using plasma or an electric field and catalysts. Non-equilibrium plasma enables activation of stable molecules including methane, carbon dioxide, and water, even at low temperatures, by virtue of high electron energy. Use of a hybrid system of plasma and catalyst provided high conversion and selectivity to products by virtue of adsorption on the catalyst. Imposing a DC electric field to the catalyst bed also promotes catalytic reactions, even at low temperatures. Two mechanisms for electro-catalytic reactions are proposed for the DC electric field imposition: reactant activation by surface protonics and production of active surface oxygen species on the catalyst. This review presents summaries of these novel processes.

Preparation and Redox Studies of α1- and α2-Isomers of Mono-Ru-Substituted Dawson-type Phosphotungstates with a DMSO Ligand: [α1/α2-P2W17O61RuII(DMSO)]8–

Both α1- and α2-isomers of mono-Ru-substituted Dawson-type heteropolytungstates with a DMSO ligand, [α1-P2W17O61Ru(II)(DMSO)](8-) and [α2-P2W17O61Ru(II)(DMSO)](8-), are prepared from the α2-isomer of a monolacunary derivative, [α2-P2W17O61](10-). Reaction of [α2-P2W17O61](10-) with Ru(DMSO)4Cl2 under hydrothermal conditions produces [α2-P2W17O61Ru(II)(DMSO)](8-) as a main product together with [α1-P2W17O61Ru(II)(DMSO)](8-), [PW11O39Ru(II)(DMSO)](5-), and [P2W18O62](6-) as byproducts. By addition of KCl to the reaction mixture, K8[α2-P2W17O61Ru(II)(DMSO)] is isolated in a moderate yield. On the other hand, reaction of [α2-P2W17O61](10-) with Ru2(benzene)2Cl4 under hydrothermal conditions produces an isomeric mixture of [P2W17O61Ru(III)(H2O)](7-) (α1-isomer/α2-isomer ratio: ca. 8/1) as a main product together with [PW11O39Ru(III)(H2O)](4-) and [P2W18O62](6-) as byproducts. By addition of acetone to the reaction mixture, K7[P2W17O61Ru(III)(H2O)] is isolated in a good yield. Reaction of [P2W17O61Ru(III)(H2O)](7-) with DMSO produces [α1-P2W17O61Ru(III)(DMSO)](7-) as a main product and [α2-P2W17O61Ru(III)(DMSO)](7-) as a minor product. By addition of KCl and acetone, the α1-isomer K8[α1-P2W17O61Ru(II)(DMSO)] is isolated in a good yield. Both compounds are fully analyzed by CV, NMR ((1)H, (13)C, (31)P, and (183)W), IR, UV-vis, elemental analysis, mass spectroscopy, and single-crystal structure analysis. Assuming that isomerization does not occur during the reaction of [P2W17O61Ru(III)(H2O)](7-) with DMSO, the isolated [P2W17O61Ru(III)(H2O)](7-) contains the α1-isomer as a main compound with the α2-isomer as a minor compound. Unusual transformation of the α2-isomer of [P2W17O61](10-) to the α1-isomer occurs. Redox behaviors of [α1-P2W17O61Ru(II)(DMSO)](8-) and [α2-P2W17O61Ru(II)(DMSO)](8-) are compared together with Ru(DMSO)-substituted α-Keggin-type heteropolytungstates, [α-XW11O39Ru(DMSO)](n-) (X = Si, Ge, and P).

Determination of α-Keggin structure of [GeW11O39RuIII(H2O)]5−. Reaction of [GeW11O39RuIII(H2O)]5−with dimethyl sulfoxide to form [GeW11O39RuIII(dmso)]5−and their structural characterization

Reaction of ruthenium((III))-substituted Keggin-type germanotungstate [GeW(11)O(39)Ru(III)(H(2)O)](5-) (1) with a dimethyl sulfoxide (dmso) produced a dmso-coordinating derivative [GeW(11)O(39)Ru(III)(dmso)](5-) (2). Structural characterization of compound 2 by using cyclic voltammetry, UV-Vis spectroscopy, IR spectroscopy, elemental analysis, (1)H-NMR spectroscopy and single crystal structure analysis (monoclinic, P2(1)/c, a = 13.461(1), b = 20.198(1), c = 18.078(1), β = 90.426(1), Z = 4) revealed that Ru(III) was incorporated in the α-Keggin-type framework and coordinated by dmso through an Ru-S bond. The IR spectra of 1 and 2 were very similar, indicating that 1 also has an α-Keggin structure. Cyclic voltammetry indicated that the incorporated Ru(III)-dmso was reversibly reduced to the Ru(II)-dmso derivative and oxidized to the Ru(IV)-dmso derivative. The redox potential in [α-XW(11)O(39)Ru(III/II)(dmso)](n-) (X = P, Ge, Si) decreased in the order of P > Ge > Si. Reaction of the complex 2 with ascorbic acid produced a one-electron-reduced compound [α-GeW(11)O(39)Ru(II)(dmso)](6-).

Lanthanoid-free perovskite oxide catalyst for dehydrogenation of ethylbenzene working with redox mechanism

For the development of highly active and robust catalysts for dehydrogenation of ethylbenzene (EBDH) to produce styrene; an important monomer for polystyrene production, perovskite-type oxides were applied to the reaction. Controlling the mobility of lattice oxygen by changing the structure of Ba1 − xSrxFeyMn1 − yO3 − δ (0 ≤ x ≤ 1, 0.2 ≤ y ≤ 0.8), perovskite catalyst showed higher activity and stability on EBDH. The optimized Ba/Sr and Fe/Mn molar ratios were 0.4/0.6 and 0.6/0.4, respectively. Comparison of the dehydrogenation activity of Ba0.4Sr0.6Fe0.6Mn0.4O3 − δ catalyst with that of an industrial potassium promoted iron (Fe–K) catalyst revealed that the Ba0.4Sr0.6Fe0.6Mn0.4O3 − δ catalyst showed higher initial activity than the industrial Fe–K oxide catalyst. Additionally, the Ba0.4Sr0.6Fe0.6Mn0.4O3 − δ catalyst showed high activity and stability under severe conditions, even at temperatures as low as 783 K, or at the low steam/EB ratio of 2, while, the Fe–K catalyst showed low activity in such conditions. Comparing reduction profiles of the Ba0.4Sr0.6Fe0.6Mn0.4O3 − δ and the Fe–K catalysts in a H2O/H2 atmosphere, reduction was suppressed by the presence of H2O over the Ba0.4Sr0.6Fe0.6Mn0.4O3 − δ catalyst while the Fe–K catalyst was reduced. In other words, Ba0.4Sr0.6Fe0.6Mn0.4O3 − δ catalyst had higher potential for activating the steam than the Fe–K catalyst. The lattice oxygen in perovskite-structure was consumed by H2, subsequently the consumed lattice oxygen was regenerated by H2O. So the catalytic performance of Ba0.4Sr0.6Fe0.6Mn0.4O3 − δ was superior to that of Fe–K catalyst thanks to the high redox property of the Ba0.4Sr0.6Fe0.6Mn0.4O3 − δ perovskite oxide.