Fabrication of nanoparticle (NP)-based materials have attracted much attention in the field of catalysis, because their special properties lie between those of single metal species and bulks. In this review article, we highlight our recent progress in the development of promising NPs-based catalysts designed by precise architecture that enable efficient and selective chemical reactions and can be easily separated and recyclable. Firstly, novel strategies to fabricate well-controlled catalytically active metal NPs on solid supports are described. A single-site photocatalyst comprising tetrahedrally coordinated Ti-oxide moieties within a silica framework (Ti-MCM-41) enabled a high dispersion of metal NPs via the photo-excitation. Both the deposition of Pd and Au to form PdAu bimetallic NPs was also accomplished under UV-light irradiation. These bimetallic NPs showed an improvement in the catalytic efficiency of H2O2 formation. Based on pH-induced assembly-dispersion properties of Ag NPs stabilized with 3-mercaptopropionic acid (3-MPA), the deposition of Ag NPs on Al2O3 support was successfully attained by electrostatic attraction while keeping their inherent dispersion state. In the second topic, new multifunctional nanocomposites exhibiting magnetic properties and catalytic activities are presented. Magnetic NPs were encapsulated with silica layer containing isolated and tetrahedrally-coordinated Ti-oxide species, which offered simple and efficient catalyst systems for the selective liquid-phase oxidations. The FecorePtshell NPs stabilized by oleic acid and oleylamine was drastically improved by the formation of an inclusion complex with γ-cyclodextrin (γ-CD), which showed an enhanced catalytic activity in water rather than in organic solvents. Similarly, the FecorePdshell NPs were subsequently treated with 2,2’-bis(diphenylphosphino)-1,1’-binaphthene (BINAP) as chiral modifiers, which were shown to catalyze the asymmetric Suzuki-Miyaura coupling reaction. Hollow γ-Fe2O3 nanoshperes with diameter of 400-500 nm have been synthesized by a simple templating method combined with subsequent thermal treatments and calcinations. It exhibited higher catalytic activity than its bulk counterparts and also showed a satisfactory selectivity compared to the nano-γ-Fe2O3. The synthetic strategies described here are simple and general for practical catalyst design; thus allowing a strong protocol for creating various nanostructured catalysts.
Distearyl hydroxylamine has been developed as a polymer stabilizer, but the mechanism of action has not yet been clarified. The mechanism was investigated by evaluating the radical-scavenging potential. Distearyl hydroxylamine could better scavenge oxygen-centered radicals such as an alkoxy radical than carbon-centered radicals. The α,α-dimethylbenzyloxy radical scavenging ability (kinh = 5.8 × 104 dm3 · mol−1 · s−1) of distearyl hydroxylamine was considerably higher than that of BHT (kinh = 1.7 × 104 dm3 · mol−1 · s−1) or HALS NOH, but the radical scavenging number (n) of distearyl hydroxylamine was not so high as that reported previously. The high radical-scavenging ability of distearyl hydroxylamine seems to be based on the easier cleavage of the C–N bond of N,N-distearyl nitroxide radical derived from hydrogen abstraction by an alkoxy radical, compared with that of HALS NOH. A new radical scavenging mechanism of distearyl hydroxylamine is proposed based on the observations of electron spin resonance spectroscopy.
Combinations of two types of phenolic antioxidants were examined to assess synergistic or antagonistic interaction from both radical scavenging rate constant (kinh) and radical scavenging number (n). The synergism of two types of phenols was observed especially with comparatively lower and similar oxidation potentials. In synergism of kinh, a phenol with lower oxidation potential simply acts as a peroxy radical scavenger, whereas the other phenol with higher oxidation potential acts as a hydrogen donor in the regeneration of the former phenol from its phenoxy radical. On the other hand, synergism of n is seen for less or non-hindered phenols, especially with 4-methoxy substituent, and is controlled by the reaction conditions, such as solvent polarity. This is interpreted as a molecular association of phenols formed through hydrogen bonds and hydrophilic interactions, resulting in obstruction of coupling of the phenoxy radicals. As a result, 3,5-di-t -butyl-4-hydroxy-toluene (BHT) and 2-t -butyl-4-methoxyphenol (3 : 2 by mole ratio), and BHT and 4-methoxyphenol (3 : 2 by mole ratio) were estimated to show total synergism of 1.41 and 1.35 times, respectively, compared with the simple sum of the antioxidant activities in a hydrophobic system, such as chlorobenzene. These results are important for reduction of costs as well as environmental pollution by allowing decreased use of phenols.
The Friedel-Crafts reaction is a very important process with many uses in the synthesis of fine chemicals, intermediates and petrochemicals. Solid catalysts should be developed for improved environmental compatibility of these chemical processes. A high-throughput screening (HTS) system was developed using test tubes for the screening and optimizing of solid catalysts for the benzylation of anisole by benzyl alcohol. To eliminate the time consuming steps in the screening, microwave heating was applied instead of the conventional ohmic heating of the reactant liquid. Temperature of the liquid phase was 145°C after 3 min microwave irradiation. Experiments could be conducted safely under the boiling points of the reactants. Many types of solid catalysts were screened using this microwave HTS, and the order of activity was as follows: Sn, Fe cation exchanged montmorillonite > montmorillonite > Nafion >> zeolite ≈ silica alumina > titania > alumina, silica, magnesia.
The effects of manganese salts on Ru/Mn/Al2O3 catalysts prepared by the impregnation method for Fischer-Tropsch synthesis were investigated. The catalysts were named Ru/Mn(N)/Al2O3, Ru/Mn(A)/Al2O3, Ru/Mn(S)/Al2O3 and Ru/Mn(Cl)/Al2O3 according to the manganese nitrate, manganese acetate, manganese sulfate and manganese chloride salts. In the slurry phase Fischer-Tropsch reaction, Ru/Mn(N)/Al2O3 showed high and stable catalytic performance for CO conversion and space time yield under the reaction conditions, whereas other catalysts were deactivated. The order of catalytic performance was Ru/Mn(N)/Al2O3 >> Ru/Mn(A)/Al2O3 > Ru/Mn(Cl)/Al2O3 ≅ Ru/Mn(S)/Al2O3. The catalyst porosity, CO chemisorption, XRD, TEM, TPR and XPS were observed, which indicated that the similar size of Ru particle and pore diameter at 8 nm on Ru/Mn(N)/Al2O3 may increase the number of active Ru atoms on the catalyst surface due to the formation of manganese chloride, resulting in the high catalytic activity and stability for Fischer-Tropsch synthesis.
Bioethanol production is based on the cultivation of ethanol fermentation microorganisms. Nutrients such as nitrogen, phosphorus, and potassium are loaded into the cultivation process, and these nutrients are absorbed into the microorganisms. Finally, these microorganisms become waste after the fermentation process. Therefore, recycling of these nutrients is desirable, because the elemental composition of the microorganisms is basically the same before and after ethanol fermentation. Wet oxidation can be applied to nutrient recovery. Nutrient recycling in the fermentation system by wet oxidation was evaluated. High economic efficiency can be achieved by reduction of substrate consumption.
Recently, it was reported that 1 wt%Ru/Ni-Al mixed oxides showed excellent catalytic activity for selective CO methanation in the fundamental experiment by our group. In this work, the realistic catalytic performance of this catalyst for a fuel processor of residential polymer electrolyte fuel cells was investigated. The catalyst was coated into metallic monoliths with external diameter of 100 mm. High CO removal activity and selectivity was obtained by realistic test containing 0.2-0.8% CO in the feed gas, the same feed gas composition used for the basic experiments. The catalyst was placed at the last stage in 1 kW fuel processor, which achieved 8 ppm-levels CO (99.9% CO removal) with long-time stability from 0.8% CO (GHSV = 2400 h−1) level, flowed out from water-gas-shift reactor.
A compact fuel processor for 1 kW class PEFCs was designed with dramatically reduced volume and catalyst amount by employing a monolithic catalyst compared with those for existing conventional ones. Only 0.7 g of precious metal was used for all catalysts. A high thermal efficiency, 82.3% HHV, was achieved at 100% load, with a low pressure drop, 3.6 kPa. The outlet gas, with a composition of 75.0 vol% H2, 19.2 vol% CO2, 2.2 vol% CH4, and 20 ppm CO, was obtained under normal conditions.