Yasutaka Kuwahara

Efficient photocatalytic degradation of organics diluted in water and air using TiO2 designed with zeolites and mesoporous silica materials

Titanium dioxide (TiO2) is a promising photocatalyst for degradation of organic compounds ideally under environmentally benign conditions. This paper reviews recent developments in designing TiO2-sorbent hybrid photocatalysts, especially those supported on ordered nano-porous silica materials including zeolites and mesoporous silicas, with the objective of fabricating efficient photodegradation systems toward organic compounds diluted in water and air. This review also describes the basic features of zeolites and mesoporous silica, adsorption kinetics on their surfaces, and the principles of surface modification techniques, and highlights that the hydrophobic nature of support materials can offer significant enhancement in photodegradation. Additionally, we note the several emerging synthetic approaches for related zeolitic materials from siliceous industrial wastes, which indicate the realization of energy-saving and recycling-oriented manufacturing processes in this field.

Design and functionalization of photocatalytic systems within mesoporous silica.

In the past decades, various photocatalysts such as TiO2, transition-metal-oxide moieties within cavities and frameworks, or metal complexes have attracted considerable attention in light-excited catalytic processes. Owing to high surface areas, transparency to UV and visible light as well as easily modified surfaces, mesoporous silica-based materials have been widely used as excellent hosts for designing efficient photocatalytic systems under the background of environmental remediation and solar-energy utilization. This Minireview mainly focuses on the surface-chemistry engineering of TiO2/mesoporous silica photocatalytic systems and fabrication of binary oxides and nanocatalysts in mesoporous single-site-photocatalyst frameworks. Recently, metallic nanostructures with localized surface plasmon resonance (LSPR) have been widely studied in catalytic applications harvesting light irradiation. Accordingly, silver and gold nanostructures confined in mesoporous silica and their corresponding catalytic activity enhanced by the LSPR effect will be introduced. In addition, the integration of metal complexes within mesoporous silica materials for the construction of functional inorganic-organic supramolecular photocatalysts will be briefly described.

Enhanced Catalytic Activity on Titanosilicate Molecular Sieves Controlled by Cation−π Interactions

A new class of heterogeneous catalytic systems utilizing cation-guest interactions was designed based on microporous titanosilicate molecular sieves. Introducing heavier alkali metal cations on ion-exchange sites of the framework resulted in a significant enhancement of the catalytic activity for oxidation of cyclohexene and styrene, whereas such an enhancement was not observed in oxidation of cyclohexane without π systems. Distinct relationships between the catalytic activities and intermolecular interaction energies which were determined by IR spectroscopic and computational approaches clearly evidenced the predominance of the cation-π interaction in this catalytic system.

Hydrogen Doped Metal Oxide Semiconductors with Exceptional and Tunable Localized Surface Plasmon Resonances

Heavily doped semiconductors have recently emerged as a remarkable class of plasmonic alternative to conventional noble metals; however, controlled manipulation of their surface plasmon bands toward short wavelengths, especially in the visible light spectrum, still remains a challenge. Here we demonstrate that hydrogen doped given MoO3 and WO3 via a facile H-spillover approach, namely, hydrogen bronzes, exhibit strong localized surface plasmon resonances in the visible light region. Through variation of their stoichiometric compositions, tunable plasmon resonances could be observed in a wide range, which hinge upon the reduction temperatures, metal species, the nature and the size of metal oxide supports in the synthetic H2 reduction process as well as oxidation treatment in the postsynthetic process. Density functional theory calculations unravel that the intercalation of hydrogen atoms into the given host structures yields appreciable delocalized electrons, enabling their plasmonic properties. The plasmonic hybrids show potentials in heterogeneous catalysis, in which visible light irradiation enhanced catalytic performance toward p-nitrophenol reduction relative to dark condition. Our findings provide direct evidence for achieving plasmon resonances in hydrogen doped metal oxide semiconductors, and may allow large-scale applications with low-price and earth-abundant elements.

A novel conversion process for waste slag: synthesis of a hydrotalcite-like compound and zeolite from blast furnace slag and evaluation of adsorption capacities

The main components of blast furnace slag (BFS) are CaO, SiO2, A12O3, MgO and slight amounts of transition metals such as Fe, Ti and Mn. We successfully synthesized a hydrotalcite-like compound from BFS via a convenient chemical process: acid-leaching and precipitation. After the HCl acid-leaching process, BFS was separated into hydrated silica with 92 wt% SiO2 content and leaching solution including other components, which afforded a hydrotalcite-like compound after subsequent NaOH addition in high yield (6.2 g from 10.0 g of BFS). By means of XRD and chemical analysis, the product synthesized at 100 °C was identified to be a Ca-Al-based hydrocalumite compound with the stoichiometric molar ratio of Cau2006:u2006Alu2006:u2006Cl = 2u2006:u20061u2006:u20061, incorporating other metal cations in its structure. The phosphate adsorption capacity of the raw slag was 1.5 mg P/g but increased to over 40 mg P/g when converted into the hydrotalcite-like compound, which was more than three times greater than that of conventional Mg-Al-based hydrotalcite. Furthermore, single-phase A- and X-type zeolites with high crystallinities and excellent water adsorption capacities (247 and 333 mg g−1, respectively) were successfully synthesized using the residual silica through a hydrothermal treatment for 6 h at 100 °C. This conversion process, which enables us to fabricate two different kinds of valuable materials from BFS at low cost and through convenient preparation steps, is surely beneficial from the viewpoint of economical use of BFS.

Transesterifications using a hydrocalumite synthesized from waste slag: an economical and ecological route for biofuel production

Blast furnace slag (BFS), a high volume byproduct resulting from iron-making processes, was used as a low-cost and abundant precursor for preparing a hydrocalumite, and the thus prepared slag-made hydrocalumite and its derivatives were applied for transesterifications of esters including triglycerides. In the transesterification of n-ethyl butyrate, calcined samples provided higher catalytic activities than those of the as-synthesized hydrocalumite due to the interfusion of slag-derived impurity elements, such as Fe and Mn, which act as catalyst promoters. Furthermore, the catalyst calcined at 800 °C in air worked as an efficient catalyst for the transesterification of soybean oil with methanol, affording up to 97% yield of fatty acid methyl esters (FAME) after 6 h under relatively moderate reaction conditions (i.e., methanol/soybean oil = 12, reaction temperature = 60 °C, use of 1 wt% catalyst), and its catalytic performance was reproduced even after air-exposure for 1 day. It is believed that the slag-made hydrocalumite can replace existing solid base catalysts as a low-cost alternative for biodiesel production and potentially contribute to the sustainable chemical processes in an economical and ecological way.

Non-Noble-Metal Nanoparticle Supported on Metal–Organic Framework as an Efficient and Durable Catalyst for Promoting H2 Production from Ammonia Borane under Visible Light Irradiation

In this work, we propose a straightforward method to enhance the catalytic activity of AB dehydrogenation by using non-noble-metal nanoparticle supported on chromium-based metal-organic framework (MIL-101). It was demonstrated to be effective for hydrogen generation from ammonia borane under assistance of visible light irradiation as a noble-metal-free catalyst. The catalytic activity of metal nanoparticles supported on MIL-101 under visible light irradiation is remarkably higher than that without light irradiation. The TOFs of Cu/MIL-101, Co/MIL-101, and Ni/MIL-101 are 1693, 1571, and 3238 h(-1), respectively. The enhanced activity of catalysts can be primarily attributed to the cooperative promoting effects from both non-noble-metal nanoparticles and photoactive metal-organic framework in activating the ammonia borane molecule and strong ability in the photocatalytic production of hydroxyl radicals, superoxide anions, and electron-rich non-noble-metal nanoparticle. This work sheds light on the exploration of active non-noble metals supported on photoactive porous materials for achieving high catalytic activity of various redox reactions under visible light irradiation.

Shape and Composition Effects on Photocatalytic Hydrogen Production for Pt-Pd Alloy Cocatalysts.

The shape and composition effects of platinum-palladium (Pt-Pd) alloy nanoparticle cocatalysts on visible-light photocatalytic hydrogen evolution from an aqueous ammonium sulphite solution have been reported and discussed. The activity of Pt-Pd nanoparticles loaded Pt-Pd/CdS photocatalysts are affected based on both the Pt-Pd alloy nanoparticles shape and their compositions. In this research, two shapes of Pt-Pd nanoparticles have been studied. One is Pt-Pd nanocubes enclosed by {100} crystal planes and the other is nano-octahedra covered with {111} crystal facets. Results show that the photocatalytic turnover frequency (TOF), defined as moles of hydrogen produced per surface mole of Pt-Pd metal atom per second, for Pt-Pd nanocubes/CdS (Pt-Pd NCs/CdS) photocatalyst can be 3.4 times more effective than Pt-Pd nano-octahedra/CdS (Pt-Pd NOTa/CdS) nanocomposite photocatalyst. Along with the shape effect, the atomic ratio of Pt to Pd can also impact the efficiency of Pt-Pd/CdS photocatalysts. When the Pt to Pd atomic ratio changes from 1:0 to about 2:1, the rate of hydrogen production increases from 900 μmol/h for Pt NCs/CdS catalyst to 1837 μmol/h for Pt-Pd (2:1) NCs/CdS photocatalyst-a 104% rate increase. This result suggests that the 33 mol % of more expensive Pt can be replaced with less costly Pd, resulting in a more than 100% hydrogen production rate increase. The finding of this research will lead to the research and development of highly effective catalysts for photocatalytic hydrogen production using solar photonic energy.

Catalytic transfer hydrogenation of levulinate esters to γ-valerolactone over supported ruthenium hydroxide catalysts

Production of γ-valerolactone (GVL) from levulinate esters and several alcohols as hydrogen donors via a catalytic transfer hydrogenation (CTH) process was performed over supported ruthenium hydroxide catalysts. Among the catalysts examined, Ru(OH)x supported on high-surface-area, anatase TiO2 containing highly-dispersed ultrasmall Ru(OH)x nano-clusters was found to be the most active catalyst, which converted levulinate esters to GVL in an almost quantitative yield under mild reaction conditions with low catalyst loading. Addition of heterogeneous bases could afford a faster reaction rate in GVL production by accelerating the following intramolecular dealcoholation step. The catalyst was reusable over repeated catalytic cycles without loss of catalytic performance, making this material a potential candidate for efficient GVL production under ambient reaction conditions.

A novel synthetic route to hydroxyapatite–zeolite composite material from steel slag: investigation of synthesis mechanism and evaluation of physicochemical properties

Steel slag is a commercial waste material mainly consisting of SiO2, Al2O3 and CaO, the former two chemicals being major components of zeolites and the latter a major component of hydroxyapatite (HAP). A hydroxyapatite–zeolite composite material (HAP-ZE) was successfully synthesized from steel slag by adding appropriate chemical reagents, H3PO4 and NaOH, viaaging at 363 K for approximately 2 days. The synthesis mechanism and structural properties were clarified by detailed analysis using XRD, FT-IR, SEM, EDX, elemental mapping, and N2 adsorption–desorption measurements. The Ca and Mg components were chemically reacted with phosphate in the early stages of aging, being precipitated as Mg-substituted HAP with (Ca + Mg)/P = 1.67. After 2 days of aging, well-crystallized HAP and faujasite-type zeolite (Na type X-zeolite with SiO2/Al2O3 = 2.4) were separately formed via a non-simultaneous crystallization process. The over-run in aging time led to phase transformation from FAU-zeolite to Pl-zeolite. The minor components in steel slag such as Fe and Mn had little effect on the synthesis of HAP-ZE; however, the inherent SiO2/Al2O3 ratio in steel slag led to a lower yield from the zeolite phase. Furthermore, from the adsorption assessment of volatile organic compounds (VOCs), fatty acid and protein, the HAP-ZE synthesized under optimum conditions was found to have adsorption properties comparable to those of pure zeolite and HAP.