Catalysts2014, 4(4), 375-396; doi:10.3390/catal4040375 (registering DOI) - published 28 November 2014 Show/Hide Abstract
Abstract: Nanometer-sized materials have significantly different chemical and physical properties compared to bulk material. However, these properties do not only depend on the elemental composition but also on the structure, shape, size and arrangement. Hence, it is not only of great importance to develop synthesis routes that enable control over the final structure but also characterization strategies that verify the exact nature of the nanoparticles obtained. Here, we consider the verification of contemporary synthesis strategies for the preparation of bimetallic core-shell particles in particular in relation to potential particle structures, such as partial absence of core, alloying and raspberry-like surface. It is discussed what properties must be investigated in order to fully confirm a covering, pin-hole free shell and which characterization techniques can provide such information. Not uncommonly, characterization strategies of core-shell particles rely heavily on visual imaging like transmission electron microscopy. The strengths and weaknesses of various techniques based on scattering, diffraction, transmission and absorption for investigating core-shell particles are discussed and, in particular, cases where structural ambiguities still remain will be highlighted. Our main conclusion is that for particles with extremely thin or mono-layered shells—i.e., structures outside the limitation of most imaging techniques—other strategies, not involving spectroscopy or imaging, are to be employed. We will provide a specific example of Fe-Pt core-shell particles prepared in bicontinuous microemulsion and point out the difficulties that arise in the characterization process of such particles.
Catalysts2014, 4(4), 356-374; doi:10.3390/catal4040356 - published 3 November 2014 Show/Hide Abstract
Abstract: Reducing the size from the bulk material to nanoparticles produces a scaling behavior in physical properties in the later ones, due to the large surface-to-volume fraction. By further size reduction, entering into the subnanometric cluster region, physical properties are largely affected by strong quantum confinement. These quantum size effects (HOMO-LUMO gap), the small size and the specific geometry award subnanometric clusters with totally new and fascinating properties, including cluster photoluminescence, enhanced catalytic activity, etc. In this review, we report an introduction to the physical properties of clusters based on the jellium model; the controlled synthesis by microemulsion methods and the catalytic properties in different areas as heterogeneous catalysis, photocatalysis or electrocatalysis among others.
Catalysts2014, 4(4), 346-355; doi:10.3390/catal4040346 - published 27 October 2014 Show/Hide Abstract
Abstract: We present the chemical synthesis of hexaniobate nanotubes using two routes, (1) starting material K4Nb6O17 and (2) parent material of H4Nb6O17 via ion exchange. The as-synthesized materials were exfoliated by adjusting the pH to 9–10 using tetra-n-butylammonioum hydroxide (TBA+OH−), leading to a formation of hexaniobate nanotubes. In order to understand morphology a full characterization was conducted using SEM, HRTEM, BET and powder-XRD. The photocatalytic activity was evaluated using photolysis method using Bromocresol Green (BG) and Methyl Orange (MO) as model contaminants. Results indicate a nanotube porous oxide with large porous and surface area; the photocatalytic activity is about 95% efficient when comparing with commercial TiO2.
Catalysts2014, 4(3), 321-345; doi:10.3390/catal4030321 - published 10 September 2014 Show/Hide Abstract
Abstract: In this review, we summarize our recent development of palladium(0)-catalyzed cross-coupling reactions of perfluoro organic compounds with organometallic reagents. The oxidative addition of a C–F bond of tetrafluoroethylene (TFE) to palladium(0) was promoted by the addition of lithium iodide, affording a trifluorovinyl palladium(II) iodide. Based on this finding, the first palladium-catalyzed cross-coupling reaction of TFE with diarylzinc was developed in the presence of lithium iodide, affording α,β,β-trifluorostyrene derivatives in excellent yield. This coupling reaction was expanded to the novel Pd(0)/PR3-catalyzed cross-coupling reaction of TFE with arylboronates. In this reaction, the trifluorovinyl palladium(II) fluoride was a key reaction intermediate that required neither an extraneous base to enhance the reactivity of organoboronates nor a Lewis acid additive to promote the oxidative addition of a C–F bond. In addition, our strategy utilizing the synergetic effect of Pd(0) and lithium iodide could be applied to the C–F bond cleavage of unreactive hexafluorobenzene (C6F6), leading to the first Pd(0)-catalyzed cross-coupling reaction of C6F6 with diarylzinc compounds.
Catalysts2014, 4(3), 307-320; doi:10.3390/catal4030307 - published 20 August 2014 Show/Hide Abstract
Abstract: Pd-catalyzed, site-selective mono-cross-coupling of substrates with two identical halo groups is a useful method for synthesizing substituted monohalogenated arenes. Such arenes constitute an important class of compounds, which are commonly identified as drug components and synthetic intermediates. Traditionally, these site-selective reactions have been realized in a “substrate-controlled” manner, which is based on the steric and electronic differences between the two carbon-halogen bonds of the substrate. Recently, an alternative strategy, “catalyst-controlled” site-selective cross-coupling, has emerged. In this strategy, the preferred reaction site of a dihaloarene can be switched, merely by changing the catalyst used. This type of selective reaction further expands the utility of Pd-catalyzed cross-coupling. In this review, we summarize the reported examples of catalyst-controlled site-selectivity switching in Pd-catalyzed cross-coupling of dihaloarenes.
Abstract: Perovskite-type catalysts have been prominent oxide catalysts for many years due to attributes such as flexibility in choosing cations, significant thermal stability, and the unique nature of lattice oxygen. Nearly 90% metallic elements of the Periodic Table can be stabilized in perovskite’s crystalline framework . Moreover, by following the Goldschmidt rule , the A- and/or B-site elements can be partially substituted, making perovskites extremely flexible in catalyst design. One successful example is the commercialization of noble metal-incorporated perovskites (e.g., LaFe0.57Co0.38Pd0.05O3) for automotive emission control used by Daihatsu Motor Co. Ltd. . Thus, growing interest in, and application of perovskites in the fields of material sciences, heterogeneous catalysis, and energy storage have prompted this Special Issue on perovskite catalysts. [...]