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Inorganic nanocrystals (NCs) of uniform size are of eminent importance for the fields of catalysis, energy storage and conversion, and electrochemical devices. Current efforts in science of nanoparticles have focused on the propensity of discrete colloidal NCs to assemble into two- or three-dimensional porous architectures. This aspect becomes important, especially, in applications where mass transport-related effects are a concern, such as in catalysis, separation, and chemical sensing. In this perspective, we describe how colloidal nanocrystals can be used as functional building blocks to construct highly porous networks with large and accessible surface area. The synthesis of these mesostructured assemblies, however, is not a simple process and often requires more sophisticated and elegant processing steps. We focused particularly on the potential of a polymer templating technique for the construction of ordered mesostructured assemblies of metal oxide and metal chalcogenide NCs and concentrate on the ...

In this article, the catalytic activity of titania-supported gold nanoparticles (Au/TiO2) was studied for the selective oxidation of amines into nitroso compounds using hydrogen peroxide (H2O2). Gold nanoparticles deposited on Degussa P25 polymorphs of titania (TiO2) have been found to promote the selective formation of a variety of nitroso arenes in high yields and selectivities, even in a large-scale synthesis. In contrast, alkyl amines are oxidized to the corresponding oximes under the examined conditions. Kinetic studies indicated that aryl amines substituted with electron-donating groups are oxidized faster than the corresponding amines bearing an electron-withdrawing functionality. A Hammett-type kinetic analysis of a range of para-X-substituted aryl amines implicates an electron transfer (ET) mechanism (ρ=−1.15) for oxidation reactions with concomitant formation of the corresponding N-aryl hydroxylamine as possible intermediate. We also show that the oxidation protocol of aryl amines in the presence of 1,3-cyclohexadiene leads in excellent yields to the corresponding hetero Diels–Alder adducts between the diene and the in situ formed nitrosoarenes.

Photocatalytic water splitting to produce hydrogen using solar energy is a particularly attractive solution to increasing energy demands. However, to be of practical use, semiconductor electrodes need to be made of inexpensive, abundant elements and have a high, yet stable, photocatalytic H2-production activity. Here we report the first demonstration of 3D mesoporous networks of Cu2O and TiO2 nanoparticles as highly efficient photocatalysts for hydrogen generation from water. These assembled structures feature a highly accessible pore surface that exposes a large fraction of anatase TiO2 and Cu2O nanoparticles to electrolytes, and has a small grain size of the constituent nanocrystals, which lead to excellent activity for H2 evolution via a UV-visible light-driven reduction of protons. Catalytic results associated with optical UV-vis/NIR absorption and photoluminescence (PL) data indicated that the large separation of photogenerated electrons and holes at the Cu2O–TiO2 p–n junctions was the main reason attributed to the improved photochemical performance. Consequently, the mesoporous Cu2O/TiO2 catalyst containing ∼1.5 wt% Cu reaches an average H2 evolution rate of ∼542 μmol h−1 (or ∼36 133 μmol h−1 g−1) with an apparent quantum efficiency (QE) of 13.5% at 365 nm and an incident photon conversion efficiency of ∼4.1% under UV-visible light illumination (360–780 nm), which is one of the best HER activities among TiO2-based semiconductor systems reported to date.

Mesoporous TiO2-based semiconductors with visible-light response are promising materials for photocatalytic and photoelectrochemistry applications. In this work, we have used surfactant-assisted aggregating assembly of CdS and TiO2 nanocrystals to assemble mesoporous binary CdS-TiO2 heterostructure. The product features a three-dimensional network of interconnected CdS quantum dots and anatase TiO2 nanoparticles and exhibits large internal BET surface area (157 m2 g−1) and uniform pores (ca. 7.5 nm). Catalytic experiments showed an exceptionally high catalytic activity of these mesophases under UV–visible light oxidation of various para-substituted aryl alcohols, using molecular oxygen as oxidant. Moreover, product analysis and kinetic results indicated that these photooxidation reactions proceed via an electron transfer route from alcohol substrate to the excited states of the catalyst.

Band edge engineering of semiconductor nanostructures is one of the most appealing approaches to enhance light absorption, carrier separation and, ultimately, solar to fuel conversion efficiency. In this study, we devise a facile polymer-assisted sol-gel chemical method to prepare highly porous, crystalline implanted SrTiO3 (STO) nanoparticles and demonstrate their performance for photocatalytic hydrogen generation from water. X-ray scattering, electron microscopy, and nitrogen physisorption data corroborate that the as-made catalysts comprise 100-nm-sized nanocuboid particles containing a highly internal porous structure (BET surface area ∼176 m2 g−1) with uniform mesopores (ca. 5.8 nm in diameter). Interestingly, a partial substitution of N and C for O is attained in STO lattice with this synthetic protocol, according to the elemental analysis, and infrared (IR) and X-ray photoelectron spectroscopy (XPS) studies. Compared to STO:C,N, the STO:C,N mesoporous decorated with Pt nanoparticles (ca. 3 nm) present unique attributes that allow for an impressive improvement of up to 74-fold in photocatalytic H2-production activity. By combining UV–vis/NIR optical absorption, photoluminescence, Raman and electrochemical impedance spectroscopy, we show that this improved performance arises from the unique nanostructure, which provides massive surface active sites, and the proper alignment of defect states and conduction band-edge position of the STO:C,N semiconductor with respect to the interband transitions of metal, which permit efficient plasmon-induced interfacial electron transfer between the Pt–STO:C,N junction.

In this study, the catalytic activity of decatungstate (W10O32(4-)) supported on mesoporous TiO2 nanoparticle assemblies (DT-MTA) was compared with that of homogeneous [Bu4N]4W10O32 catalysts under mild conditions. Our experiments showed that both catalytic systems achieve exceptionally high activity and selectivity under UV-visible light oxidation of various para-substituted aryl alcohols, using molecular oxygen as a "green" oxidant. The chemoselective transformation of aryl alcohols into the corresponding ketones was investigated with gas chromatography (GC) and NMR spectroscopy. Product analysis and kinetic results also indicated that these photooxidation reactions proceed via both electron transfer (ET) and hydrogen atom transfer (HAT) mechanisms over the DT-MTA catalyst, with the former one as the predominant, whereas a HAT route was adopted to account for the decatungstate homogeneous catalyzed reactions.

Herein, we show that mesoporous titania-supported gold nanoparticle assemblies (Au/MTA) catalyze the activation of NaBH4 and 1,1,3,3-tetramethyl disiloxane (TMDS) compounds, which act as transfer hydrogenation agents for the reduction of nitroarenes to the corresponding anilines in moderate to high yields. On the other hand, nitroalkanes are reduced to the corresponding diazo and hydrazo compounds under the studied conditions. The substantial measured primary kinetic isotope effects found here suggested that B–H bond cleavage occurs in a rate-determining step and [Au]–H active hybrids are formed, which are responsible for the reduction of nitroarenes to the corresponding amines. Formal Hammett-type kinetic analysis of a range of para-X-substituted nitroarenes lends support to this hypothesis. Nitro compounds substituted with electron-withdrawing groups were reduced faster than the corresponding compounds with electron-donating groups. The presence of water enhanced the catalytic activity of Au/MTA in apro...

In this article, the catalytic activity of titania-supported gold nanoparticles (Au/TiO2) was studied for the selective oxidation of amines into nitroso compounds using hydrogen peroxide (H2O2). Gold nanoparticles deposited on Degussa P25 polymorphs of titania (TiO2) have been found to promote the selective formation of a variety of nitroso arenes in high yields and selectivities, even in a large-scale synthesis. In contrast, alkyl amines are oxidized to the corresponding oximes under the examined conditions. Kinetic studies indicated that aryl amines substituted with electron-donating groups are oxidized faster than the corresponding amines bearing an electron-withdrawing functionality. A Hammett-type kinetic analysis of a range of para-X-substituted aryl amines implicates an electron transfer (ET) mechanism (ρ=−1.15) for oxidation reactions with concomitant formation of the corresponding N-aryl hydroxylamine as possible intermediate. We also show that the oxidation protocol of aryl amines in the presence of 1,3-cyclohexadiene leads in excellent yields to the corresponding hetero Diels–Alder adducts between the diene and the in situ formed nitrosoarenes.

Here, we propose novel mesoporous Au-loaded TiO2 nanoparticle assemblies (Au-MTA) as highly effective catalysts for the reduction of nitroaromatic compounds into the corresponding aryl amine products. The obtained materials possess a continuous network of interconnected gold and anatase TiO2 (ca. 9 nm in size) nanoparticles with controllable gold particle size (i.e. ranging from ∼3.2 to ∼9.4 nm) and exhibit large and accessible pore surface area (ca. 100–160 m2/g), as evidenced by SAXS, XRD, TEM and N2 physisorption measurements. Interestingly, the Au-MTA mesophases have exhibited remarkable activity and selectivity for the reduction of nitro into amine groups using NaBH4 as reducing agent. Indeed, the Au loading and particle size have a key effect on hydrogenation reactions, affecting significantly the yield and product composition.

Mesoporous cobalt oxide-tungstophosphoric acid composite frameworks have been synthesized by structure replication from cubic mesoporous KIT-6 silica. The products possess a regular structure with uniform wall thickness (~7 nm) and large internal BET surface area (~87-141 m2/g). The pore walls of these materials consist of nanocrystalline Co3O4 and 12-tungstophosphoric acid (HPW) components with different HPW content, i.e. 6, 15 and 36 wt. %. Total X-ray scattering analysis and UV/vis spectroscopy confirmed the Keggin structure of HPW into the mesoporous frameworks. Preliminary catalytic experiments found that Co3O4-HPW composites exhibited a remarkable activity in the direct decomposition of nitrous oxide into N2 and O2.

In this article, we demonstrate novel mesoporous Au-loaded TiO2 nanoparticle assemblies (Au–MTA) as high-effective catalysts for the selective transformation of nitroaromatics into the corresponding aryl amine products. These materials feature a three-dimensional open porous structure consisting of interconnected uniform gold and TiO2 nanoparticles, large internal surface area (ca. 104–120 m2 g−1), and narrow mesopores (ca. 7.3–7.6 nm). Au–MTA displays outstanding performance for the selective reduction of nitro into amine groups using sodium borohydride as a reducing agent under ambient conditions. We show that both chemoselectivity and hydrogenation activity of the Au–MTA catalysts are highly related to the Au loading and particle size. As a result the 2% Au–MTA associated with 5 nm sized Au particles was found to be a prominent catalyst for hydrogenation reactions, providing exceptionally high selectivity (>96%) and conversion yield (>92%) to the corresponding amines. The implication of these results is that mesoporous ensembles of gold and TiO2 nanoparticles should be considered for the synthesis of fine amine chemicals.

We report an unprecedented formation of mesoporous titania via a surfactant-assisted aggregating assembly of TiO(2) nanoparticles. These mesostructures possess a highly crystalline anatase phase, large and accessible pore surface area, and exhibit superior photocatalytic performance.

Ordered mesoporous chromium(III) oxide–phosphotungstic acid (PWA) nanocomposite structures with controllable composition (∼17 to 49 wt% in PWA) have been successfully prepared via an ultrasound-assisted nanocasting route, using mesoporous SBA-15 silica as a rigid mold. These materials possess 3D hexagonal mesostructure, large internal BET surface areas of ∼67 to 80 m2 g−1, and uniform pores of ∼3 to 4 nm size according to small-angle X-ray scattering, high resolution transmission electron microscopy and N2 physisorption. The Keggin-type structure of [PW12O40]3− anions is preserved intact into the Cr2O3 framework, as confirmed by total X-ray diffuse scattering and pair distribution function analysis and infrared and diffuse reflectance ultraviolet-visible (UV-vis) spectroscopy. The integration of regular porosity, large internal surface area, and Cr2O3–PWA composition makes these materials highly promising for applications in oxidation catalysis. Although pure mesoporous Cr2O3 and PWA compounds exhibit low catalytic activity, the mesoporous Cr2O3–PWA composites showed superior activity and selectivity for the oxidation of selected secondary benzyl alcohols, giving good-to-high yields within a short reaction time. Furthermore, the Cr2O3–PWA composite frameworks demonstrated remarkable durability and reusability upon multiple usages without leaching or decomposition of the incorporated PWA clusters. This enhancement is attributed to the synergistic interactions between the PWA and Cr2O3 components as well as the well-ordered open-pore structure and large catalytically active surface area.

The use of 2-amino-isobutyric acid in Co/Ln chemistry has led to the isolation of two unique [Co(II)(6)Ln(III)] 3d-4f metallic cages in which the Ln(III) centre (Ln = Eu, Dy) is encapsulated within a Co(II)(6) trigonal prism.