Yasuaki Okamoto

Active sites of molybdenum sulfide catalysts supported on Al2O3 and TiO2 for hydrodesulfurization and hydrogenation

A comparative characterization of sulfided MoO3Al2O3 and MoO3TiO2 catalysts was conducted by using LRS, XPS, IR, and TDS of NO. On the basis of the characterization, the active sites were estimated for the hydrodesulfurization of thiophene and hydrogenation of butadiene. The LRS and XPS results indicated that sulflded MoO3Al2O3 catalysts consisted of MoS2-like phases and Mo(V) species, while sulfided solMoO3TiO2 catalysts was fully sulfided to MoS2-like phases. The TDS of NO demonstrated that there were at least two distinctly different NO adsorption sites on sulfided molybdenum catalysts. They were assigned to triply (α) and doubly (β) cus Mo-sites on the basis of the H2Sue5f8NO coadsorption. The fractions of α- and β-sites depended on both molybdenum loading and support, suggesting a variation in the morphology of MoS2-like phases. The number of α-sites was linearly correlated to the activity for the hydrogenation of butadiene at 273 K, being independent of the support. This suggests that α-sites are responsible for the hydrogenation. Hydrogenation cycles involving α-sites are proposed at low and high reaction temperatures and in the presence and absence of H2S. With the HDS reaction, separate parabolic correlations were obtained for the MoO3Al2O3 and MoO3TiO2 catalysts between the activity and NO adsorption, suggesting dual sites as the HDS active centers and negative effects of molybdenum sulfide-support interactions.

Electronic structure of zeolites studied by X-Ray photoelectron spectroscopy

Abstract The electronic structure of zeolites was systematically investigated by utilizing XPS. Zeolites A, X, Y, and mordenites in their sodium and decationized forms as well as the series of alkali metal cation-exchanged X and Y zeolites were examined. The electronic structure of zeolites was demonstrated to depend strongly on both the composition and the cation involved. It is suggested that the basic strength of framework oxygen increases with increasing Al content regardless of the crystal structure and with the decreasing electronegativity of the counter cation. These results explain well certain catalytic properties of alkali metal cation-exchanged zeolites. On the basis of the XPS results, the nature of chemical bonding in zeolites is discussed.

Surface state and catalytic activity and selectivity of nickel catalysts in hydrogenation reactions: III. Electronic and catalytic properties of nickel catalysts

Abstract Various nickel catalysts (nickel-boride, nickel-phosphide, Raney-nickel, Urushibara-nickel, and decomposed-nickel) were investigated to examine the relationships between catalytic and electronic properties of nickel catalysts modified by component elements (boron, phosphorus, aluminum, and zinc) in the catalysts. Based on the X-ray photoelectron spectroscopic results, a parameter Δ q was tentatively proposed to characterize the electronic properties of the catalysts. The specific activity (activity per surface area of nickel metal) for hydrogenation reaction, the adsorption equilibrium constant of acetophenone, the resistivity against poisoning, and the characteristic selectivities in hydrogenation of 1,2-butylene oxide were found to be summarized in terms of the parameter Δ q . It is suggested that Δ q is a useful parameter to reflect the electronic properties of the nickel catalysts.

Surface structure of CoOMoO3Al2O3 catalysts studied by X-ray photoelectron spectroscopy

X-Ray photoelectron spectroscopic studies of oxidic and sulfided CoOue5f8MoO3Al2O3 catalysts revealed the chemical species, the surface structure of the catalysts, and the promoting effect of Co. It was found from the Mo(3d)Al(2s) and Co(2p)Al(2s) intensity ratios that the surface structure of the oxidic catalysts was highly sensitive to the preparation method. Bilayer structures are proposed for the catalysts prepared by sequential impregnations, while a separate phase structure is suggested to be plausible for the catalysts prepared by a simultaneous impregnation. On sulfidation the surface structure of the CoOue5f8MoO3Al2O3 catalysts was not essentially altered under atmospheric pressure, compared to that of the oxidic precursor catalysts, although Co and Mo were sulfided. The oxidic catalysts mainly consist of Co3O4 and pseudo-CoAl2O4, the fraction of Co3O4 increasing with Co content and depending on the preparation method. On the basis of the observation that both the extent of sintering and the sulfidation degree of Mo are depressed by Co, it is suggested that the stabilization effect of Co (most likely pseudo-CoAl2O4) for Mo monolayers is operative during hydrodesulfurizations, thus holding the Mo effective for the reactions.

Active states of rhodium in rhodium exchanged Y zeolite catalysts for hydrogenation of ethylene and acetylene and dimerization of ethylene studied with X-ray photoelectron spectroscopy

Abstract Rh-Y zeolite catalysts were investigated with X-ray photoelectron spectroscopy (XPS) techniques. It was found, for the first time, that Rh (I) was formed in Rh-Y zeolites as a significantly stable intermediate during the reduction of Rh(III) to Rh metal by heat treatment in vacuum. It was revealed that Rh(I) in Rh-Y zeolites was active for the hydrogenation and the dimerization of ethylene, whereas Rh metal was active for the hydrogenation of both ethylene and acetylene. Strong correlations were established between homogeneous Rh complex catalysts and Rh-Y catalysts both in the active oxidation states of Rh and the effect of additives on the reactions. Furthermore, XPS was successfully applied to define not only the chemical nature but also the structure of Rh in zeolites. The structure of Rh in Rh-Y zeolite was suggested to correlate strongly with the chemical nature of Rh.

Copper-zirconia catalysts for methanol synthesis from carbon dioxide : effect of ZnO addition to Cu-ZrO2 catalysts

Coprecipitated Cu-ZrO2 catalysts were found to show higher selectivity to methanol in CO2 hydrogenation than conventional Cu-ZnO catalysts. Addition of ZnO to Cu-ZrO2 catalysts of Cu/ZrO2 = 1 (weight ratio) greatly enhanced the activity at lower temperatures, while keeping the high methanol selectivity of Cu-ZrO2 catalysts. A remarkable increase in the Cu dispersion with increased amount of added ZnO explains the increased activity at lower temperatures, while the reforming of methanol to CO is accelerated by ZnO at higher temperatures, leading to a lowered yield of methanol. It is suggested that ZrO2 rather than ZnO in the ternary systems plays a more effective role for the selective formation of methanol.

Stabilization effect of Co for Mo phase in CoMoAl2O3 hydrodesulfurization catalysts studied with X-Ray photoelectron spectroscopy

Abstract The promotional effects of Co in Coue5f8Mo Al 2 O 3 hydrodesulfurization (HDS) catalysts were studied by means of X-ray photoelectron spectroscopy. The higher MoO 3 -content Mo Al 2 O 3 catalysts (10 and 20 wt% MoO 3 ) contain mobile Mo, which migrates from the pores to the outermost surface layers of the catalysts and segregates to form less active crystalline MoS 2 during the HDS reaction, while in the case of Mo Al 2 O 3 (5 wt% MoO 3 ) catalyst: no migration of Mo was observed. It is revealed that the Co in Coue5f8Mo Al 2 O 3 catalyst inhibits the migration and segregation of Mo and that it keeps Mo effective for the HDS reaction, since no surface enrichment of Mo was observed. It is concluded that stabilization of the Mo monomolecular layer is the main role of Co. The active species of Mo is suggested to have the composition of S/Mo(IV) = 1 on the basis of the sulfur contents of the catalysts under the mild HDS reaction conditions.

Surface state and catalytic activity and selectivity of nickel catalysts in hydrogenation reactions: IV. Electronic effects on the selectivity in the hydrogenation of 1,3-butadiene

Abstract The hydrogenation of 1,3-butadiene was carried out over various unsupported Ni catalysts: nickel-boride, Raney-nickel, decomposed-nickel, nickel-phosphide, and nickel treated with H2S. It was found that the butene distributions characteristic of the catalysts were explainable in terms of the electron density of Ni metal and the reaction mechanism proposed by Wells and co-workers. It is suggested that there are critical electron densities of Ni metal at which catalytic properties change significantly.

Surface structure and catalytic activity of sulfided MoO3Al2O3 catalysts: Hydrodesulfurization and hydrogenation activities

Abstract The hydrogenation activities of sulfided MoO 3 Al 2 O 3 catalysts for the butenes produced in the hydrodesulfurization of thiophene were studied in connection with the surface structure of the catalysts. The hydrodesulfurization of thiophene was carried out at 400 °C and atmospheric pressure. It was found that the catalytic activity for the hydrogenation depended simply on the sulfidation degree of molybdenum ( S Mo by XPS) regardless of the molybdenum content, calcination temperature, and sulfidation conditions as well as for the hydrodesulfurization. In the low-sulfur-content catalysts ( S Mo ), the relative activity for the hydrogenation to the hydrodesulfurization was constant, implying that only one kind of molybdenum species, which is active for both reactions, is present. On the basis of the relationships between the relative activity and the sulfidation degree of molybdenum, it is suggested that two-dimensional MoS2 (monolayer structure) is initially formed at the expense of the above species and subsequently transformed to three-dimensional MoS2 microcrystals by further sulfidation of the catalysts, accompanying the sintering of molybdenum.

Effect of starting salt on catalytic behaviour of Cu-ZrO2 catalysts in methanol synthesis from carbon dioxide

Starting salts used in the co-precipitation of Cu-ZrO2 precursors had marked influence on both activity and selectivity of resulting catalysts for methanol synthesis: chlorides increased the selectivity, suggesting the structure sensitivity of methanol formation reaction, while sulfates much enhanced the activity by affecting the dispersion of Zr species. A simultaneous use of copper chloride and zirconium sulfate greatly improved the activity and selectivity of the catalyst for methanol formation.