泉 康雄

The geometrical order around divalent Co ion of Co-MgO solid solution was found to control the catalytic reactivity of simple decomposition of nitrous oxide as demonstrated by extended X-ray absorption fine structure spectroscopy (EXAFS).

The active site structures and electronic states were studied for the [Rh10Se]/TiO2 catalyst by means of edge spectra and EXAFS. The rate and selectivity of ethanol synthesis from CO2 on [Rh10Se]/TiO2 had strong dependence on the heating temperature in vacuum (Tevac) (1). Corresponding to the maximum of rate and selectivity when the Tevac was 623 K, the distance rSe-Rh reached a minimum (2.41 Å) by Se K- and Rh K-edge EXAFS analyses. The contracted [Rh10Se] cluster was found to have an electronic state similar to that of Rh3Se8 or RhSe2 rather than of metallic Rh. A new reaction path control is proposed by regulating the distance between the interstitial Se atom and the metal framework [Rh10]. A strong peak due to 1s → np transition was observed around the Se K absorption edge. The peak intensity did not exhibit significant change when the Tevac was varied for [Rh10Se]/TiO2. On the other hand, the area of peak observed around 23,230 eV in Rh K-edge spectra gradually decreased when the Tevac was elevated for [Rh10Se]/TiO2. Hence, the Se atom surrounded by [Rh10] framework was always kept in anionic state, while the electronic state of Rh atoms gradually changed by the interaction with TiO2 surface. The change for Rh atoms was also supported by the gradual increase of rRh-Rh obtained by EXAFS with the temperature increase. In the case of Rh sites with lower total coordination number around Rh ([Rh10Se]/SiO2, [Rh10Se]/Al2O3, and [Rh10Se]/MgO), the reaction CO2 → CO(a) + O(a) occurred predominantly and formed CO(a) poisoned the catalysis. © 1998 Academic Press.

The N-N stretching frequency of adsorbed dinitrogen on the actual (supported or promoted) Ru catalysts has been shown to be extremely sensitive to the electronic state of the active surface which is usually hard to detect even by XPS. The wavenumber was also a good index of the N-2 activation and ammonia synthesis on these Ru catalysts. Atomically adsorbed hydrogen and its effect on N-2 activation was also observed by FTIR. The addition of alkali promotes N-2 activation on Ru although it causes strong hydrogen poisoning. These various techniques have been discussed with regard to the development of the 'second generation ammonia catalyst'. (C) 1997 Elsevier Science B.V.

Work on the synthesis of ethanol from carbon dioxide over a rhodiumselenium catalyst is reported, and related reactions and characterisation studies are briefly reviewed. In order to inhibit the formation of methane (complete reduction of carbon dioxide) and simultaneously activate carbon-carbon bond formation by the reaction of CHxwith carbonyl derivatives, it is necessary to control the active rhodium sites. Based on a study of single crystal rhodium surfaces it is proposed that acetyl and acetate intermediates are formed. Recently it has been discovered that supported Rh/TiO2, promoted by selenium from inside the rhodium metal sites, is a potential catalyst for ethanol synthesis from carbon dioxide. The action of this catalyst is compared to related studies.

Catalysis on Ru clusters supported on CeO2 or Ni-doped CeO2 was investigated. The Ru3(CO)12 was reacted with CeO2, followed by heating in vacuum at 673 or 813 K, then in H2 at 588 - 1073 K (T(H2)). Ammonia synthesis activities of both catalysts had the T(H2) dependence, which has a maximum at T(H2) = 873 K. On a sample in which Ru3(CO)12 was supported on previously reduced Ni/CeO2, the highest synthesis rate was 1.5×10**-3 mol h-1g•cat(-1) at T(H2) = 588 K. The activity order can be understood in terms of two factors: (A) reduction extent of support, and (B) number of active Ru sites. The two factors conflicted with each other when the treatment temperature in H2 increased. By heating the samples in H2 up to 873 K to satisfy factor (A), the aggregation of Ru clusters or physical blocking of surface Ru sites by CeO(2-x) occurred: factor (B) was not satisfied. The two factors should be optimized in catalyst Ru3-Ni/CeO2, where the support cerium oxide was thoroughly reduced through the doped Ni. On reduced Ni/CeO2, the Ru cluster implantation can be done at low temperature (588 K). Obtained values of r(Ru-Ru) at 0.262 nm (N = 7.1) and r(Ru-O(s) )(O(s): oxygen atom at surface) at 0.212 nm (N = 1.2) by EXAFS for Ru3-Ni/CeO2 suggested a flat Ru cluster model comprised of several Ru atoms on reduced Ni/CeO(2-x) surface. The H(a)/Ru(total) ratio exceeded unity, suggesting new H adsorption sites. The temperature programmed desorption for hydrogen (simultaneous desorption of HD and D2 at 330 - 430 K) suggested that the H at the new site and H on Ru surface were exchangeable above 330 K. The "reservoir" effect of the new site for H on catalysis is discussed in relation to new kinetic design of hydrogenation catalyst.

Supported [Rh10Se] catalyst on TiO2 converted carbon dioxide into ethanol faster (60 × 10-3 mol h-1 g cat-1) and more selectively ( ≈ 83%) than other supported Rh clusters or [Rh10Se] on other inorganic oxides, presumably due to the effects of interstitial Se and the interface of the [Rh10Se] cluster and TiO2.

Nitrido-ruthenium cluster [Ru6N] catalysts were prepared by reacting [Ru6N(CO)(16)](-) cluster with MgO, K+-doped MgO or Cs+-doped MgO as a potential ruthenium nitride catalyst. The [Ru6N] unit remained in reaction conditions, and exhibited higher activities in ammonia synthesis than conventional Ru catalysts or Ru clusters prepared from [Ru6C(CO)(16)Me](-) or [Ru-6(CO)(18)](2-).

Catalysis (ammonia synthesis) on supported nitrido clusters [Ru6N] was investigated as a potential catalysis system of transition metal + main group element in relation to the framework structure (change) of [Ru6N] clusters in H2 or N2 as found in the accompanying paper. The [Ru6N(μ-Osu)3] (Osu, oxygen atom at surface) clusters were prepared from the [Ru6N(CO)16]- cluster on MgO, K+-doped MgO, and Cs+-doped MgO, and the stability in reaction conditions of ammonia synthesis was probed by EXAFS (extended X-ray absorption fine structure). The reaction rates on these nitrido clusters were found to be faster than non-nitrido [Ru6] clusters prepared from [Ru6(CO)18]2-, degraded [Ru3(μ2-Osu)3] clusters or aggregated Ru clusters (NRu-Ru = 6.2-6.6) prepared from [Ru6N(CO)16]-, or conventional Ru catalysts. Also, the H2-D2 exchange reactions (in the presence/absence of N2) proceeded faster on supported [Ru6N] clusters than the other catalysts. The Ru wt % dependence of ammonia synthesis activities on [Ru6N]/MgO suggested the importance of the Ru-hexamer ensemble and cluster/support interface for the catalysis. Related to the coordination structures of H or N2 and structure changes of the [Ru6N] framework in H2 or N2 in the accompanying paper, the promoted reaction mechanism of ammonia synthesis on supported [Ru6N] clusters is discussed in terms of (1) nuclearity of Ru, (2) cluster/support interface, (3) structural effect through expansion/contraction of the [Ru6N] framework, and (4) electron donation by nitrido nitrogen, based on in-situ EXAFS, in-situ IR, H2-D2 exchange reactions, reaction orders, and H/D isotope effects. © 1995 American Chemical Society.

The supported ruthenium clusters [Ru6N] were prepared on MgO, K+/MgO, and Cs+/MgO from [Ru6N(CO)16] cluster as hydrogenation catalysts, stabilized, and chemically modified by nitrido nitrogen. The characterization and reactivities with H2 and N2 were investigated by EXAFS (extended X-ray absorption fine structure) in relation to their catalysis promoted by nitrido nitrogen. The [Ru6N] cluster unit was found to remain on MgO after heating in vacuum at 813 K (decarbonylated) and in H2 at 588 K (reaction condition of N2 hydrogenation), whereas the [Ru6N(CO)16]- cluster strongly interacted with the Al2O3 surface to degrade to [Ru3] by heating in vacuum at 813 K. The decarbonylated [Ru6N] framework also remained in H2 at 588 K on K+/MgO and Cs+/MgO without aggregation or degradation. By changing the Ru loading from 0.48 to 3.9 wt % on MgO, the coordination number NRU-Osu (Osu, oxygen atom at surface) decreased from 1.2 to 0.3, while the [Ru6N] cluster unit remained unchanged for samples with Ru loading up to ∼2.5 wt %. The preferable reaction of [Ru6N] clusters with MgO(001) flat surfaces was suggested for the sample at 2.5 wt % Ru, but the cluster should have reacted mainly with lower coordination sites of MgO for the sample with lower Ru loading (∼0.5 wt %). The rRu-Osu was shorter for [Ru6N] on K+/MgO and Cs+/MgO (2.00-2.03 Å) than on MgO (2.13 Å), implying that the [Ru6N] was interacted with Osu atoms bonded to K+ or Cs+ ions to have a direct support effect of basic oxide K+/MgO or Cs+/MgO on catalysis. H-induced structure changes were observed for [Ru6N]/MgO and [Ru6N]-Cs+/MgO as the reversible changes on bonding distance rRu-Ru of 0.03 and 0.08 Å, respectively, by the adsorption/desorption of H2. The adsorption of N2 was simple adsorption on [Ru6N] without structural change of [Ru6N] on MgO or Cs+/MgO. © 1995 American Chemical Society.

The effects of adsorbed hydrogen on the N-2 adsorption and N-2 isotope displacement rate on Ru/MgO and Ru-Cs+/MgO were investigated by FTIR. The H-2 was found to be adsorbed dissociatively on on-top sites [1880w(sh), 1801w(sh), and 1717s cm(-1)], bridging sites [1550w and 1330w cm(-1)], and threefold sites [1120m and 933m cm(-1)] on Ru/MgO, and similarly on three kinds of adsorption sites on Ru-Cs+/MgO. The bridging hydrogen on Ru/MgO was thermally more stable than the other two. Molecular N-2 could be adsorbed as an on-top form on Ru. By the small amount of preadsorbed H(a) [(a)/Ru-surf=14%], the N-2 isotope displacement rate N-15(2)(a)-->N-14(2)(a) in N-14(2) On Ru/MgO was largely reduced to 12%, and the reduction on Ru-Cs+/MgO was more serious (to 4%). The main factors of these reductions were interpreted as direct repulsion of H(a) and N-2 on Ru/MgO and Ru-hydride dipole effect enhanced by doped Cs+ on Ru-Cs+/MgO. The common factors in N-2 displacement reaction and the catalysis from N-2 are discussed in terms of the hydrogen effect.

Catalysis and structures of supported ruthenium carbido clusters [Ru6C(CO)16/(CH3)]-/oxides were investigated in comparison with those of supported non-carbido clusters prepared from [Ru-6(CO)(18)](2-) and traditional ruthenium catalysts prepared from Ru(NO)(NO3)(3). Oxygenate synthesis (methanol, dimethyl ether, and formaldehyde) in CO/H-2 reaction was observed on the supported carbido clusters in contrast to the preferential formation of methane and hydrocarbons on the conventional Ru catalysts and the supported non-carbido clusters. The active structure for oxygenate synthesis crucially depended on the kind of oxides; on basic oxides (MgO and La2O3) the cluster of framework was incorporated with surface oxygen atoms and expanded or shrunk under CO/H-2 reaction conditions depending on the CO pressure. The reversible expansion-shrinking of the cluster framework was correlated with the activated CO adsorption (CO breathing-induced structural change). On TiO2 the [Ru6C] framework always held a shrunk structure as proved by EXAFS. The expanded clusters showed high selectivities in oxygenate synthesis. The shrunk [Ru6C]/TiO2 also showed much higher activity than the non-carbido cluster or the traditional catalyst. IR,and hydrogen isotope effects suggested the formation of oxygenates through a mu 2-formyl intermediate. The switchover of reaction path from the formation of methane and hydrocarbons to oxygenate synthesis is ascribed to the interstitial carbido carbon which has structural effect like a central spring and electronic effect as a four-electron donor on the behavior of the cluster framework.

Stoichiometric acetaldehyde formation by insertion of CO into the methyl ligand and catalytic ethene hydroformylation on the cluster, [Ru6C(CO)16Me]- supported on silica at 373-473 K have been investigated to understand the effects of the catalysis on the metal cluster framework and also to develop new catalytic systems on a molecular scale. Two elementary steps for stoichiometric acetaldehyde formation, (i) from methyl to acetyl and (ii) from acetyl to acetaldehyde, were observed by Fourier-transform IR spectroscopy. The rate of (i) in CO + H-2 was faster than that in CO, suggesting a hydride-promoted mechanism for carbonyl insertion (acetyl formation). The hydride promotion and hydrogen pressure dependence suggested dissociative adsorption of H-2 so as to bridge a Ru-Ru and the incorporation of the multi-Ru sites in the acetaldehyde formation mechanism. The reductive elimination of hydride and methyl ligands upon methane formation was much slower than the reductive elimination of H and MeO for acetaldehyde formation as well as the insertion of CO (methyl migration) for acetyl formation. In terms of this specific feature the catalytic hydroformylation of ethene was found to proceed on the catalyst with nearly 100% selectivity at 398 K in the case of highly dehydrated SiO2 (823 K). The retention of the cluster framework under the reaction conditions was confirmed by extended X-ray adsorption fine structure curve-fitting analysis. On the contrary, [Ru6C(CO)16Me]- in a homogeneous system did not catalyse this reaction and conventional impregnation Ru-SiO2 catalysts showed only 0-0.09% selectivities. A reaction mechanism is presented.

Carbido ruthenium carbonyl cluster [Ru6C(CO)16(CH3)]-(dRu-Ru = 0.290 nm) was supported on MgO and La2O3, and transformed to decarbonylated clusters [Ru6C)] by evacuation at 623 K. The framework of the [Ru6C] has the Ru-Ru distance of 0.261 nm (MgO) or 0.262 nm (La2O3) by means of EXAFS. When the clusters were interacted with a mixture of CO and H2 at 523 K. The cluster framework expanded to have the Ru-Ru distance of 0.287 nm (MgO) and 0.277 nm (La2O3). The expanded clusters were shrunk by CO desorption. This CO-breathing cycle is also monitored with the Ru(-C-) Ru multiple scattering peak (0.410 nm) by EXAFS. The CO-breathing carbido clusters were selective for the catalytic synthesis of oxygenates (methanol, dimethyl ether, and formaldehyde) from CO/H2.

The promoting effect of Se additive and the catalysis including hydrogen spillover for ethene hydroformylation on supported Rh6 cluster were investigated. Se enhanced propanal formation by 1.9 times at Se/Rh6 = 0.6 and the selectivity toward propanal formation increased from 17% to 50% at Se/Rh6 = 1.0. The retention of the cluster framework after Se doping and catalytic reactions was proved by EXAFS and FT-IR. The observed non-integer ratio of Se/Rh6 (=0.6) for the maximum activity implies that hydrogen atoms adsorbed on Rh6 clusters without Se spilled over to Rh6 clusters doped with Se on which propanal was selectively formed. In this case the hydrogen supply to Se-doped Rh6 clusters on MgO is rate-datermining. The addition of Pt (0.15 wt%) enhanced the rate of propanal formation on SeRh6/MgO (Se/Rh = 1.0). This observation provides evidence on the important contribution of hydrogen spillover from metallic Pt to Rh6 with Se atom on MgO to ethene hydroformylation catalysis.

Selenium-modified Rh6(CO)16/MgO catalysts were prepared by the reaction of Rh6(CO)16/MgO with (CH3)2Se in order to examine the promoting effect of an electronegative additive on Rh catalysis for ethene hydroformylation. The deposited Se (Se/Rh6 = 0.6) enhanced the rate of propanal formation 1.9 times as compared with the case of an undeposited catalyst. The selectivity of the hydroformylation was improved from 20 to 50% by the Se addition. To the contrary, the monotonous suppression of ethane formation by increasing the Se amount was observed. Further doping of Se reduced the hydroformylation activity. TPD, IR, and Se K-edge and Rh K-edge EXAFS revealed that (CH3)2Se reacted with the Rh atoms of partially-decarbonylated Rh6 species on the MgO surface, forming Se-Rh bonds at a distance of 0.244 nm. XPS data suggested that the oxidation state of Se in the catalyst is Se-, while Rh is in a nearly metallic state. CO adsorbs on the rhodium atoms bonded to Se, which is contrasted to blocking of the neighboring Rh sites by Se atom observed with usual impregnated Rh catalysts. The structures of the Se-undoped and -doped Rh6 clusters on MgO are presented in relation to the active site of the cluster catalyst. The data indicate an advantage of molecular clusters over impregnated particles in adsorption capability and catalysis.

Supported ruthenium carbido-cluster catalysts ([Ru6C(CO)16Me]-/oxide) selectively produced methanol, dimethyl ether, and formaldehyde in CO-H-2, in contrast to the preferential formation of methane and hydrocarbons on supported ruthenium cluster catalysts {[Ru6(CO)18]2-/oxide} without the interstitial carbon and conventional metallic Ru catalysts.

The clusters [Ru6C(CO)16Me]- supported on SiO2, Al2O3 and TiO2 were characterized by means of extended X-ray absorption fine structure (EXAFS), temperature-programmed desorption (TPD) and Fourier-transform IR spectroscopy in relation to developments of new catalytic systems. The EXAFS analysis revealed that the Ru6C cluster framework which has an interstitial carbon atom was stable at 473 (SiO2), 523 (Al2O3) and 473 K (TiO2) under CO or CO + H-2. The supported clusters produced methane upon heating in vacuum by the reaction with surface hydrogen-bonded OH groups, whereas in the presence of CO + H-2 acetaldehyde was preferably produced, unlike [Ru6C(CO)16Me]- in a homogeneous system where only methane was formed. The selectivity to acetaldehyde was larger with SiO2 (77%) than with Al2O3 (15) or TiO2 (10%).

The positive effects of Se added to Rh/ZrO2 catalysts in ethene hydroformylation were studied from the viewpoints of promoter effects of electronegative atoms usually regarded as poisoning atoms as well as electronic interactions with both substrate metal and adsorbed species. Se-modified Rh/ZrO2 catalysts prepared by the reactive deposition of (CH3)2Se or coimpregnation method using SeO2 were found to be active and selective for ethene hydroformylation. The Se addition promoted the formation of propanol rather than propanal, the propanol formation being increased more than 70 times at an optimumSe/Rh ratio. The propanoyl intermediates were observed at ca. 1770 cm-1 converted reversibly to propionate species (bridge and bidentate) observed at 1425-1575 cm-1. The ensemble of Se2-/Rh was suggested to promote CO adsorption and the insertion of CO to ethyl species, where there is an electronic interaction between Se2- and CO or propanoyl species. The isotope effects of D2 and 13C18O showed the switchover of the rate-determining step for hydroformylation from the CO insertion on the Rh/ZrO2 catalysts to the dissociative adsorption of H2 on the SeRh/ZrO2 catalysts. The role of Se is discussed in relation with the reaction mechanism. © 1991.

Experimental evidence for effects of through-space/through-bond interactions on electron distributions of molecular orbitals was observed by Penning ionization electron spectroscopy. For norbornadiene, the next highest occupied molecular orbital, which has high electron densities in the endo face augmented by the through-space interactions between π electrons on the double bonds, showed a large reactivity upon electrophilic attacks by metastable helium atoms. In the case of l, 4-diazabicyclo[2.2.2]octane, the highest occupied MO composed of a symmetric coupling of the lone-pairs on nitrogen atoms, which has a smaller exterior electron density than the next HOMO because of the strong through-bond interactions, was found to give the smaller reactivity in Penning ionization. © 1985, American Chemical Society. All rights reserved.