Porous Nanocrystalline Silicon Supported Bimetallic Pd-Au Catalysts: Preparation, Characterization, and Direct Hydrogen Peroxide Synthesis.

Bimetallic Pd-Au catalysts were prepared on the porous nanocrystalline silicon (PSi) for the first time. The catalysts were tested in the reaction of direct hydrogen peroxide synthesis and characterized by standard structural and chemical techniques. It was shown that the Pd-Au/PSi catalyst prepared from conventional H2[PdCl4] and H[AuCl4] precursors contains monometallic Pd and a range of different Pd-Au alloy nanoparticles over the oxidized PSi surface. The PdAu2/PSi catalyst prepared from the [Pd(NH3)4][AuCl4]2 double complex salt (DCS) single-source precursor predominantly contains bimetallic Pd-Au alloy nanoparticles. For both catalysts the surface of bimetallic nanoparticles is Pd-enriched and contains palladium in Pd0 and Pd2+ states. Among the catalysts studied, the PdAu2/PSi catalyst was the most active and selective in the direct H2O2 synthesis with H2O2 productivity of 0.5 [Formula: see text] at selectivity of 50% and H2O2 concentration of 0.023 M in 0.03 M H2SO4-methanol solution after 5 h on stream at -10°C and atmospheric pressure. This performance is due to high activity in the H2O2 synthesis reaction and low activities in the undesirable H2O2 decomposition and hydrogenation reactions. Good performance of the PdAu2/PSi catalyst was associated with the major part of Pd in the catalyst being in the form of the bimetallic Pd-Au nanoparticles. Porous silicon was concluded to be a promising catalytic support for direct hydrogen peroxide synthesis due to its inertness with respect to undesirable side reactions, high thermal stability, and conductivity, possibility of safe operation at high temperatures and pressures and a well-established manufacturing process

Planar and 3D fibrous polyaniline-based materials for memristive elements

We discuss the effect of structure formation of Langmuir polyaniline layers on the performance of memristive thin-film elements as well as the morphology and conductivity of electrospinned PANI–PEO nonwovens

Structure-transport correlation reveals anisotropic charge transport in coupled PbS nanocrystal superlattices

Semiconductive nanocrystals (NCs) can be self-assembled into ordered superlattices (SLs) to create artificial solids with emerging collective properties. Computational studies have predicted that properties such as electronic coupling or charge transport are determined not only by the individual NCs but also by the degree of their organization and structure. However, experimental proof for a correlation between structure and charge transport in NC SLs is still pending. Here, we perform X-ray nano-diffraction and apply Angular X-ray Cross-Correlation Analysis (AXCCA) to characterize the structures of coupled PbS NC SLs, which are directly correlated with the electronic properties of the same SL microdomains. We find strong evidence for the effect of SL crystallinity on charge transport and reveal anisotropic charge transport in highly ordered monocrystalline hexagonal close-packed PbS NC SLs, caused by the dominant effect of shortest interparticle distance. This implies that transport anisotropy should be a general feature of weakly coupled NC SLs.Comment: 49 pages, 20 Figure

Efficiency of porous silicon photosensitizer in the singlet oxygen-mediated oxidation of organic compounds

The efficiency of singlet oxygen photosensitization by porous silicon is compared with that of a conventional dye photosensitizer tetraphenylporphine by means of singlet oxygen-mediated photooxidation of a-terpinene. Based on photoluminescence measurements it was concluded that the efficiency of porous silicon as a sensitizer is much lower than that of the conventional organic dye. The main reasons for this are the low quantum yield of long living photoinduced electronic excitations (excitones) confined in silicon nanocrystals and deactivation of singlet oxygen by H-terminated internal surface of porous silicon powder. Efficient quenching of excitons in porous silicon by 1,3-diphenylisobenzofuran via the direct energy transfer was demonstrated. This process is believed to be responsible for photobleaching of 1,3-diphenylisobenzofuran at its millimolar concentrations. (C) 2010 Elsevier B.V. All rights reserved

The effect of hydrothermal conditions on the mesoporous structure of TiO2 nanotubes

A systematic analysis of the influence of preparation conditions in the alkali hydrothermal synthesis on the morphology of TiO2 nanotubes is performed using HRTEM and low temperature nitrogen adsorption. The possible mechanisms of nanotube formation are reviewed and a mechanism based on the key stage of wrapping of intermediate multilayered titanate nanosheets is suggested. The driving force for wrapping is considered to be the mechanical stress arising during crystallisation/dissolution. The average diameter of the nanotubes was found to depend on the temperature and on the ratio of weight of TiO2 to the volume of sodium hydroxide solution. An increase in the temperature from 120 to 150 °C results in an increase in the average nanotube diameter. Subsequent increases in the temperature result in the formation of non-hollow TiO2 nanofibers with an average diameter of 75 nm, a wide distribution in diameter and a length in excess of 10 µm. The increase of the TiO2 : NaOH molar ratio results in an increase in the average diameter of nanotubes and a decrease of surface area. The average inner diameter of TiO2 nanotubes varied between 2 and 10 nm. The pore-size distribution was evaluated from TEM, and low-temperature nitrogen adsorption data using the BJH method. It was shown that nitrogen adsorption is a suitable method for characterisation of the pore morphology of nanotubes

In situ synthesis and catalytic activity in CO oxidation of metal nanoparticles supported on porous nanocrystalline silicon

Reactive surface of mesoporous nanocrystalline silicon was used to synthesise noble metal nanoparticles via in situ reduction of the precursor salt solutions. The synthetic methodology for metal nanoparticle formation was systematically developed, and reaction conditions of metal salts reduction were optimised to prepare nanoparticles of controlled size distribution in the order 5-10 nm inside the mesoporous silicon template. CO oxidation was used as a test reaction for the synthesised Pt/porous silicon catalysts. Sharp reaction light-off was observed at about 120 degrees C on the optimised catalysts. The catalysts were shown to be stable in the extended steady-state runs and in the catalysts re-use experiments. Metal nanoparticles were shown to be stable to sintering at elevated temperatures up to 1000 degrees C. However, after thermal treatment on air, Pt nanoparticles were covered by a SiOx layer and were less active in CO oxidation. (C) 2010 Elsevier Inc. All rights reserved

Liquid-phase oxidation of organic feedstock in a compact multichannel reactor

A millimeter-scale compact multifunctional reactor has been developed and tested for the selective oxidation of benzyl alcohol, using molecular oxygen as the oxidant. The reactor consists of parallel reaction channels that are packed with catalyst particles. Within the structured assembly are an integrated heat-exchange system, a gas/liquid mixing zone, and a provision for reactant injection. Experiments were performed in square channels that are 10 cm long with a cross-sectional area of 9 mm2. These were packed either with Ru/Al2O3 (0.9 wt % of Ru) or Ru/TiO2 (2 wt % of Rt) catalysts. Hydrodynamic characteristics of the system were evaluated to establish pressure drop and effectiveness of gas/liquid mixing and to confirm the nature of the gas-liquid flow regime. Operating the reactor at 8 bar and 388 K with a liquid flow L = 3.2 kg m-2 s-1 and gas flows G > 2.5 × 10-2 kg m-2 s-1, it was shown that, even in a short 10 cm length of channel, a product yield of up to 55% (with 99.7% selectivity) could be obtained. Although the adiabatic temperature rise at 55% yield is estimated to be ca. 180 K, the reactor was shown to operate isothermally, because of the efficient removal of heat through the integrated micro-heat-exchange system. It was concluded that this structured design of a reactor showed considerable promise for the development of cleaner oxidation processes

Selective oxidation of alcohols in a continuous multifunctional reactor: ruthenium oxide catalysed oxidation of benzyl alcohol

A multifunctional compact reactor was designed, built, and tested, using as a model reaction the selective oxidation of benzyl alcohol to benzaldehyde with molecular oxygen. The reactor contains static mixers, heat-transfer channels and mm-scale packed-bed reaction channels, i.e., integrating mixing, heat transfer and reaction functionalities. Integrated compact reactor technology should be particularly attractive to the fine chemistry and pharmaceutical industries. The reactor was shown to operate in the kinetic regime over a broad range of operating conditions due to intensified mass transfer. The reactor was also shown to operate isothermally despite high reaction rate, an appreciable heat effect and high reactant concentration (TOF = 300 h-1, ΔH°r = -183.7 kJ mol-1). Staged injection of oxygen along the length of the reactor was investigated as a method of increasing the selectivity of oxidation reactions. Staged injection was shown to be beneficial; however, this was likely to be due to the development of a more uniform hydrodynamic regime of two-phase flow along the packed reaction channel. Experiments were performed with the ruthenium(III) hydrated oxide catalyst supported on alumina (0.9 wt% Ru/Al2O3). High activity and selectivity were observed even when working with reactant concentrations approaching industrial conditions (1 mol L-1)