Kyoichi Sawabe

Oxidant‐Free Dehydrogenation of Alcohols Heterogeneously Catalyzed by Cooperation of Silver Clusters and Acid–Base Sites on Alumina

A gamma-alumina-supported silver cluster catalyst--Ag/Al(2)O(3)--has been shown to act as an efficient heterogeneous catalyst for oxidant-free alcohol dehydrogenation to carbonyl compounds at 373 K. The catalyst shows higher activity than conventional heterogeneous catalysts based on platinum group metals (PGMs) and can be recycled. A systematic study on the influence of the particle size and oxidation state of silver species, combined with characterization by Ag K-edge XAFS (X-ray absorption fine structure) has established that silver clusters of sizes below 1 nm are responsible for the higher specific rate. The reaction mechanism has been investigated by kinetic studies (Hammett correlation, kinetic isotope effect) and by in situ FTIR (kinetic isotope effect for hydride elimination reaction from surface alkoxide species), and the following mechanism is proposed: 1) reaction between the alcohol and a basic OH group on the alumina to yield alkoxide on alumina and an adsorbed water molecule, 2) C-H activation of the alkoxide species by the silver cluster to form a silver hydride species and a carbonyl compound, and 3) H(2) desorption promoted by an acid site in the alumina. The proposed mechanism provides fundamental reasons for the higher activities of silver clusters on acid-base bifunctional support (Al(2)O(3)) than on basic (MgO and CeO(2)) and acidic to neutral (SiO(2)) ones. This example demonstrates that catalysts analogous to those based on of platinum group metals can be designed with use of a less expensive d(10) element--silver--through optimization of metal particle size and the acid-base natures of inorganic supports.

Unique catalytic features of Ag nanoclusters for selective NOx reduction and green chemical reactions

The size control of Ag into nanoclusters generates unique catalytic features of Ag on automotive emission control and environmentally friendly organic reactions. Hydrogen-assisted selective catalytic reduction of NO by hydrocarbons (H2–HC–SCR), which is one of the promising technologies for removal of NO in the diesel engine exhausts, results in the formation of Ag clusters on alumina or zeolites and enhancement of the conversion of NO into N2. A mechanistic study using in situUV-Vis, EXAFS, and FT/IR revealed that the promotion of partial oxidation of hydrocarbons by active oxygen species is the essential role of Ag clusters. Hydrogen is suggested to be indispensable for the formation of the Ag cluster and the active oxygen. The important role of hydride on the formation of H2O2-like active chemical species on Ag clusters was confirmed by density functional theory (DFT) calculation. Knowing the high activity of Ag nanoclusters for the formation of Ag-hydride, supported Ag clusters were applied for various organic reactions. Supported Ag clusters on appropriate metal oxide, especially Al2O3, are recyclable and show very high activity and selectivity for atom-economic reactions, i.e., alcohol dehydrogenation, C–C coupling reaction of alcohols, direct amide synthesis from alcohols and amines, N-benzylation of anilines with alcohols, and selective hydrogenation of nitroaromatics.

Laser‐induced photochemistry of methane on Pt(111): Excitation mechanism and dissociation dynamics

Adsorption states and photochemistry of methane and deuterated methane on a Pt(111) surface have been investigated by measuring temperature‐programmed desorption spectra, x‐ray photoelectron spectra, work function changes, and angle‐resolved time‐of‐flight distributions of desorbed species. Methane weakly adsorbed on the Pt(111) surface at 40 K is dissociated to methyl and hydrogen fragments with laser irradiation at 193 nm. This is remarkably different from the photochemistry of methane in the gas phase where photodissociation takes place only at λ<145 nm. While the photofragments mostly remain on the surface, some fraction of methyl desorbs with average translational energy of 0.27 eV. Photodesorption of methane is a minor channel. Desorbed methane is sharply collimated along the surface normal and shows two hyperthermal velocity components. Among the two, the faster component is attributed to associative recombination between a methyl adsorbate and a hydrogen atom produced by the photodissociation of a...

An ab initio molecular orbital study on adsorption at the MgO surface. I. H2 chemisorption on the (MgO)4 cluster

The hydrogen molecule chemisorption on the low coordination or corner site of the MgO surface has been studied with the ab initio method, adopting the (MgO)4 cluster as the surface site model. The effect of basis functions and correlation have been examined. Two physisorbed complexes have been found; one is an end‐on complex on the oxygen site and the other a side‐on complex on the Mg site. These complexes are led to a common transition state (TS) and then to a dissociative chemisorption product. A substantial polarization of charges in the hydrogen molecule takes place at the TS, from where the dissociative chemisorption proceeds ionically. The best estimates of energies of the physisorbed complexes, the TS and the product, relative to the isolated reactants are −2, +2, and −21 kcal/mol, respectively, for this cubic cluster model. When the effect of the Madelung potential is taken into account, the chemisorbed product is further stabilized by 4 kcal/mol.

Photochemical CH bond activation of methane on a Pt(111) surface

Abstract Methane adsorbed on a Pt(111) surface simply desorbs at ≈70 K. However, after the methane-precovered surface is irradiated with 193 nm photons, the intensity of the thermal desorption peak decreases and a new peak of methane at ≈260 K grows with the accumulated number of photons. This new desorption peak is attributed to recombination of CH 3 and H on Pt(111). It clearly indicates that methane on Pt(111) is photodissociated to yield a chemisorbed CH 3 radical and an adsorbed H atom as products. The total cross section of photochemical processes is estimated to be 1.5 × 10 −19 cm 2 .

Structure of adsorbed N2O on a Pt(111) surface and photodissociation at 193 nm: effective formation of oxygen adatoms

Abstract The structure of adsorbed N 2 O on a Pt(111) surface and its photochemistry at 193 nm have been studied by low energy electron diffraction and temperature-programmed desorption. N 2 O adsorbs on the Pt(111) surface molecularly at 82 K and is not dissociated thermally. The observed LEED pattern indicates that one monolayer of N 2 O has a (3 × 3) structure with a fractional coverage of 4 9 ML referenced to the Pt substrate. Adsorbed N 2 O molecules are dissociated with 193 nm photons and produce oxygen adatoms on the surface. The cross sections of total depletion and photodissociation of N 2 O are determined to be (1.4 ± 0.2) × 10 −18 cm 2 and (7 ± 1) × 10 −19 cm 2 , respectively. Atomic oxygen coverages of up to 0.75 ML are generated with photolysis of adsorbed N 2 O at 193 nm.

Laser-induced photochemistry of nitrous oxide on a Pt(111) surface

Photochemistry of N2O molecularly adsorbed on clean and oxygen-covered Pt(111) surfaces has been studied using pulsed excimer laser light with 6.4 eV (193 nm) photon energy. From thermal desorption spectra before and after photon irradiation, it is found that adsorbed N2O molecules are dissociated with 193 nm photons to produce oxygen adatoms. In addition, NO is generated by secondary reactions of photofragments with N2O. The cross sections of total depletion and photodissociation of N2O adsorbed on O-covered Pt(111) are not much different from those on clean Pt(111). The dependence of the photodissociation yield on photon polarization and incident angle is in good agreement with that of the bulk absorption predicted from classical electrodynamics. This strongly indicates that the substrate excitation is mostly responsible for the primary step of the photochemistry of N2O on Pt(111).

Self-Regenerative Silver Nanocluster Catalyst for CO Oxidation

As compared to their bulk counterparts, nanoparticles and nanoclusters (NCs) possess unique surface and catalytic properties. Au-NCs catalysts with excellent oxidation activity are some of the most successful examples. Principally, the high reactivity of metal NCs originates from their excess free energy and the dynamic natures of their surface, as well as the surface area exposed to the reactant. However, these properties accelerate the particle growth and catalyst deactivation. After heating at 600 8C, the CO oxidation activity of the Au-NCs catalyst was completely lost as a result of Au agglomeration. Recent experimental 7] and theoretical works have indicated that a less expensive analogue, Ag-NCs, can also show high catalytic activity. Owing to the low melting point of Ag ( 962 8C) and its tendency to sinter, however, disables its practical use in oxidation catalysis. A recent strategy, in which metal NCs are coated with a metal oxide host, can prevent sintering. 5e, 6c] Confining Ag-NCs by mesoporous silica succeeded in stabilization of Ag-NCs up to 500 8C, which is higher than the Tammann temperature of bulk Ag (344 8C), but agglomeration through Ostwald ripening occurred at 600 8C. Conventional catalysts for automobile pollution control, alumina-supported platinum group metals (PGM), significantly deactivate after aging at 700–800 8C under the cyclic oxidative and reductive atmospheres typically encountered in automotive exhaust gas. Recently, highly durable PGM-based catalysts, such as perovskite-supported Pd and ceria-supported Pt have been developed and commercialized. In these catalysts, the strong metal–support interaction is shown to reduce the mobility of the cationic metal species under oxidative atmospheres at high temperature and prevent metal-particle growth. For the worldwide spread of less toxic cars, the development of sintering-resistant PGM-free oxidation catalysts is indispensable. Here, we present an inexpensive alternative, alumina-supported Ag-NCs with high oxidation activity and stability. Contrary to the conventional synthetic strategy for nanocatalysts, Ag-NCs were prepared by a simple high-temperature solid– solid reaction of non-nano Ag powder and alumina. As expected from the synthetic method, Ag particles once agglomerated during the H2-reduction (900 8C) are atomically redispersed by re-calcination, and show two orders of magnitude higher activity than the aged Pt catalysts. Experimental and DFT studies were performed to investigate the origin of the high activity of Ag-NCs. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), X-ray diffraction (XRD), and X-ray absorption fine structure (EXAFS) experiments were performed to identify the catalyst structure at each preparation step (Figure 1 a–c). Structure of catalysts (Table S1 in the Supporting Information) and EXAFS data (Table S2), Ag Ag and Ag O coordination numbers (NAg Ag, NAg O) and interatomic distances (R), are shown in the Supporting Information. The starting material of our catalyst, denoted Ag5+Al2O3, is a physical mixture of commercial Ag powder (5 wt %) and g-Al2O3. The Ag powder is large ( 600 nm) polycrystalline consisting of Ag crystallites of 25 nm in size (from the XRD line broadening). The as-mixed sample was completely inactive for CO oxidation, the XRD experiment revealed a line at 38.18, and EXAFS (NAg Ag = 12, R = 0.289 nm) revealed features characteristic of large Ag particles. These features in the XRD and EXAFS spectra disappeared after calcination of the mixture in air at 1000 8C. The Ag metal particles were converted to atomically dispersed Ag surrounded by oxygen atoms of alumina, as evident by examining the EXAFS features of Ag O coordination. X-Ray absorption nearedge structure (XANES) spectrum (Figure S3 in the Supporting Information) indicates the valence state of Ag. Additionally, the crystal phase of alumina changed to q-phase (Figure S4). These results indicate that the solid–solid reaction of Ag and gAl2O3 in air at 1000 8C yields an aluminate-like phase, in which Ag ions are atomically dispersed on q-Al2O3. Comparative experiments show that the atmosphere during heating and Lewis acidity of support oxide (Figure S5) affect the ability of metallic Ag to disperse; metallic Ag particles remained when the as-mixed Ag5+Al2O3 was heated at 1000 8C in He or when mixtures of Ag powder and non-Lewis acidic oxides (a-Al2O3, SiO2, MgO, or 10 wt % Na-doped g-Al2O3) were heated at 1000 8C in air (Figure S6). Considering the recent observation that coordinatively unsaturated Al + centers act as PtO cluster binding sites of g-Al2O3, [11] the following reaction should account for the atomic-level dispersion of Ag on q-Al2O3 under oxidizing conditions;

An ab initio molecular orbital study on adsorption at the MgO surface. II. Site dependence of hydrogen chemisorption on the (MgO)4,6,8 clusters

Using (MgO)4, (MgO)6, and (MgO)8 clusters as models of the MgO surface, the structures and energies of H2 chemisorption on various pair sites are studied with ab initio molecular orbital calculations. The effect of Madelung potential on the chemisorption energetics is also studied. The chemisorption energy is not very sensitive to the cluster size used in the calculation. It however depends strongly on the coordination numbers of Mg and O at the chemisorption site, and decreases in the following order: the valley site ≫Mg3c–O3c≫Mg4c–O3c≳Mg3c–O4c site. The valley and the Mg3c–O3c pair sites are strongly bonding, whereas the Mg4c–O3c and the Mg3c–O4c sites are only marginally bonding. All other higher coordination sites are nonbonding. The coordination number of the nearest neighbor atoms only secondarily affects the stability of hydrogen chemisorption. The contribution of electron correlation to the chemisorption energy is essentially determined only by the localized electrons near the chemisorption site.

THE REACTIVITY OF MOLECULAR AND ATOMIC OXYGEN IN OXYGEN-EXCHANGE REACTION BETWEEN NO AND O2 COADSORBED ON A PT(111) SURFACE

Surface reactions between N16O and 18O2 coadsorbed on Pt(111) have been studied by temperature‐programmed desorption (TPD), low energy electron diffraction and infrared reflection absorption spectroscopy (IRAS). When the surface covered with N16O and 18O2 is annealed, 18O16O desorbs at 155 K and N18O at 145, 310, and 340 K. In addition, a new absorption line at 1658 cm−1 due to N18O appears above 265 K. However, there is no indication of oxidation products of NO in the TPD and IRAS measurements. Thus, only oxygen‐exchange reactions take place on the coadsorbed surface. Reaction yields and mechanisms of the oxygen‐exchange strongly depend on the adsorption states of oxygen. At ∼145 K, molecularly adsorbed oxygen and N16O (νNO=∼1930 cm−1) directly interacting with the oxygen molecule are involved in the exchange reaction. Above 270 K the exchange reaction takes place between atop N16O (νNO=∼1720 cm−1) and the metastable oxygen adatoms that do not possess a long‐range order. On the contrary, the oxygen‐excha...