Mesoporous silicas as versatile supports to tune the palladium-catalyzed selective aerobic oxidation of allylic alcohols

Surfactant templating offers a simple route to synthesize high-surface area silicas with ordered, tunable mesopore architectures. The use of these materials as versatile catalyst supports for palladium nanoparticles has been explored in the aerobic selective oxidation (selox) of allylic alcohols under mild conditions. Families of Pd/mesoporous silicas, synthesized through incipient wetness impregnation of SBA-15, SBA-16, and KIT-6, have been characterized by using nitrogen porosimetry, CO chemisorption, diffuse reflection infrared Fourier transform spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and high-resolution TEM and benchmarked in liquid phase allylic alcohol selox against a Pd/amorphous SiO2 standard. The transition from amorphous to two-dimensional parallel and three-dimensional interpenetrating porous silica networks conferred significant selox rate enhancements associated with higher surface densities of active palladium oxide sites. Dissolved oxygen was essential for insitu stabilization of palladium oxide, and thus maintenance of high activity on-stream, whereas selectivity to the desired aldehyde selox product over competing hydrogenolysis pathways was directed by using palladium metal. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Efficient alkyne homocoupling catalysed by copper immobilized on functionalized silica

Copper immobilized on a functionalized silica support is a good catalyst for the homocoupling of terminal alkynes. The so-called Glaser-Hay coupling reaction can be run in air with catalytic amounts of base. The copper catalyst is active for multiple substituted alkynes, in both polar and non-polar solvents, with good to excellent yields (75-95%). Depending on the alkyne, full conversion can be achieved within 3-24 h. The catalyst was characterized by TGA, inductively coupled plasma and X-ray photoelectron spectroscopy. Leaching tests confirm that the catalyst is and remains heterogeneous. Importantly, the overall reaction requires only alkyne and oxygen (in this case, air) as reagents, making this a clean catalytic oxidative coupling reaction. © 2012 John Wiley & Sons, Ltd.

Optimising catalytic properties of supported sulfonic acid catalysts

Siliceous mesoporous molecular sieves (SBA-15) have been functionalised with propylsulfonic acid groups by both co-condensing 3-mercaptopropyltrimethoxysilane with the solid at the synthesis (sol-gel) stage and by grafting the same compound to pre-prepared SBA-15, followed, in both cases, by oxidation to sulfonic acid. The acidic and catalytic properties of the supported sulfonic acids prepared in the two ways have been compared, using ammonia adsorption calorimetry and the benzylation reaction between benzyl alcohol and toluene. Using a combination of X-ray photoelectron spectroscopy and other analytical techniques, the level of functionalisation and the extent of subsequent oxidation of tethered thiol to sulfonic acid, both in the bulk and close to the surface of SBA-15 particles, have been assessed. The research shows that the co-condensing route leads to higher levels of functionalisation than the grafting route. The extent of oxidation of added thiol to acid groups is similar using the two routes, about 70% near the surface and only 50% in the bulk. Comparison is made with polymer supported sulfonic acid catalysts, Amberlysts 15 and 35, and Nafion. Nafion shows the highest acid strength and the highest specific catalytic activity of all materials studied. Amongst the other materials, average acid strengths are broadly similar but there appears to be a relationship between the concentration of acid sites on the catalysts and their specific activity in the benzylation reaction. A model is proposed to explain this, in which clustering of sulfonic acid groups, even to a small extent, leads to disproportionately enhanced catalytic activity. © 2009 Elsevier B.V. All rights reserved.

Radical intermediates in chloroform reactions over triphenylphosphine-protected Au nanoparticles

Reactions of chloroform over triphenylphosphine-protected Au nanoparticles have been studied using electron paramagnetic resonance (EPR) spectroscopy and a spin trapping technique. Two competing reactions, abstraction of hydrogen and halogen atoms, were identified. The hydrogen abstraction reaction showed an inverse kinetic isotope effect. Treatment of nanoparticles with oxidizing or reducing reagents made it possible to tune the selectivity of radical formation from halogen to hydrogen (deuterium) abstraction. Treatment with PbO2 promoted the deuterium abstraction reaction followed by the loss of nanoparticle activity, whereas treatment with NaBH4 regenerated the nanoparticle activity towards Cl atom abstraction. X-ray photoelectron spectroscopy showed an increased Au:P ratio upon treatment with oxidizing reagents. This is likely due to the oxidation of some phosphine ligands to phosphine oxides which then desorb from the nanoparticle surface. © 2009 The Royal Societ of Chemistry.

Through-thickness plasma modification of biodegradable and nonbiodegradable porous polymer constructs

Pure poly(lactide-co-glycolide) and polystyrene surfaces are not very suitable to support cell adhesion/ spreading owing to their hydrophobic nature and low surface energy. The interior surfaces of large porous 3D scaffolds were modified and activated using radio-frequency, low-pressure air plasma. An increase in the wettability of the surface was observed after exposure to air plasma, as indicated by the decrease in the contact angles of the wet porous system. The surface composition of the plasma-treated polymers was studied using X-ray photoelectron spectroscopy. pH-dependent zeta-potential measurements confirm the presence of an increased number of functional groups. However, the plasma-treated surfaces have a less acidic character than the original polymer surfaces as seen by a shift in their isoelectric point. Zeta-potential, as well as contact angle measurements, on 3D scaffolds confirm that plasma treatment is a useful tool to modify the surface properties throughout the interior of large scaffolds. © 2008 Wiley Periodicals, Inc.

Surface modification of natural fibers using bacteria:depositing bacterial cellulose onto natural fibers to create hierarchical fiber reinforced nanocomposites

Triggered biodegradable composites made entirely from renewable resources are urgently sought after to improve material recyclability or be able to divert materials from waste streams. Many biobased polymers and natural fibers usually display poor interfacial adhesion when combined in a composite material. Here we propose a way to modify the surfaces of natural fibers by utilizing bacteria (Acetobacter xylinum) to deposit nanosized bacterial cellulose around natural fibers, which enhances their adhesion to renewable polymers. This paper describes the process of modifying large quantities of natural fibers with bacterial cellulose through their use as substrates for bacteria during fermentation. The modified fibers were characterized by scanning electron microscopy, single fiber tensile tests, X-ray photoelectron spectroscopy, and inverse gas chromatography to determine their surface and mechanical properties. The practical adhesion between the modified fibers and the renewable polymers cellulose acetate butyrate and poly(L-lactic acid) was quantified using the single fiber pullout test. © 2008 American Chemical Society.

Sulfate-enhanced catalytic destruction of 1,1,1-trichlorethane over Pt(111)

The catalytic destruction of 1,1,1-trichloroethane (TCA) over model sulfated Pt(111) surfaces has been investigated by fast X-ray photoelectron spectroscopy and mass spectrometry. TCA adsorbs molecularly over SO4 precovered Pt(111) at 100 K, with a saturation coverage of 0.4 monolayer (ML) comparable to that on the bare surface. Surface crowding perturbs both TCA and SO4 species within the mixed adlayer, evidenced by strong, coverage-dependent C 1s and Cl and S 2p core-level shifts. TCA undergoes complete dechlorination above 170 K, accompanied by C−C bond cleavage to form surface CH3, CO, and Cl moieties. These in turn react between 170 and 350 K to evolve gaseous CO2, C2H6, and H2O. Subsequent CH3 dehydrogenation and combustion occurs between 350 and 450 K, again liberating CO2 and water. Combustion is accompanied by SO4 reduction, with the coincident evolution of gas phase SO2 and CO2 suggesting the formation of a CO−SOx surface complex. Reactively formed HCl desorbs in a single state at 400 K. Only trace (<0.06 ML) residual atomic carbon and chlorine remain on the surface by 500 K.

Analysis of femtosecond laser surface patterning on bulk single-crystalline diamond

Preliminary work is reported on 2-D and 3-D microstructures written directly with a Yb:YAG 1026 nm femtosecond (fs) laser on bulk chemical vapour deposition (CVD) single-crystalline diamond. Smooth graphitic lines and other structures were written on the surface of a CVD diamond sample with a thickness of 0.7mm under low laser fluences. This capability opens up the opportunity for making electronic devices and micro-electromechanical structures on diamond substrates. The fabrication process was optimised through testing a range of laser energies at a 100 kHz repetition rate with sub-500fs pulses. These graphitic lines and structures have been characterised using optical microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy and atomic force microscopy. Using these analysis techniques, the formation of sp2 and sp3 bonds is explored and the ratio between sp2 and sp3 bonds after fs laser patterning is quantified. We present the early findings from this study and characterise the relationship between the graphitic line formation and the different fs laser exposure conditions. © 2012 Taylor & Francis.

Physical and biological properties of a novel siloxane adhesive for soft tissue applications

The aim of this study was to investigate the adhesive properties of an in-house amino-propyltrimethoxysilane-methylenebisacrylamide (APTMS-MBA) siloxane system and compare them with a commercially available adhesive, n-butyl cyanoacrylate (nBCA). The ability of the material to perform as a soft tissue adhesive was established by measuring the physical (bond strength, curing time) and biological (cytotoxicity) properties of the adhesives on cartilage. Complementary physical techniques, X-ray photoelectron spectroscopy, Raman and infrared imaging, enabled the mode of action of the adhesive to the cartilage surface to be determined. Adhesion strength to cartilage was measured using a simple butt joint test after storage in phosphate-buffered saline solution at 37°C for periods up to 1 month. The adhesives were also characterised using two in vitro biological techniques. A live/dead stain assay enabled a measure of the viability of chondrocytes attached to the two adhesives to be made. A water-soluble tetrazolium assay was carried out using two different cell types, human dermal fibroblasts and ovine meniscal chondrocytes, in order to measure material cytotoxicity as a function of both supernatant concentration and time. IR imaging of the surface of cartilage treated with APTMS-MBA siloxane adhesive indicated that the adhesive penetrated the tissue surface marginally compared to nBCA which showed a greater depth of penetration. The curing time and adhesion strength values for APTMS-MBA siloxane and nBCA adhesives were measured to be 60 s/0.23 MPa and 38 min/0.62 MPa, respectively. These materials were found to be significantly stronger than either commercially available fibrin (0.02 MPa) or gelatin resorcinol formaldehyde (GRF) adhesives (0.1 MPa) (P <0.01). Cell culture experiments revealed that APTMS-MBA siloxane adhesive induced 2% cell death compared to 95% for the nBCA adhesive, which extended to a depth of approximately 100-150 μm into the cartilage surface. The WST-1 assay demonstrated that APTMS-MBA siloxane was significantly less cytotoxic than nBCA adhesive as an undiluted conditioned supernatant (P <0.001). These results suggest that the APTMS-MBA siloxane may be a useful adhesive for medical applications. © VSP 2005.

Direct observation of extremely low temperature catalytic dehydrochlorination of 1,1,1-trichloroethane over platinum

Fast X-ray photoelectron spectroscopy reveals that the efficient catalytic destruction of 1,1,1-trichloroethane occurs over Pt{111} surfaces at temperatures as low as 150 K. Decomposition occurs via rapid, sequential C-Cl bond scission to form an alkylidyne surface intermediate that in turn dehydrogenates above room temperature. Atomic chlorine liberated during dehydrochlorination undergoes efficient reaction with surface hydrogen, resulting in the evolution of gaseous HCl and small amounts of ethane, presumably via ethylidyne hydrogenation. Irreversible dehydrogenation of residual hydrocarbon fragments results in significant surface coking above 500 K.

Epitaxial growth and magnetic properties of half-metallic Fe3O4 on GaAs(100)

The growth and magnetic properties of epitaxial magnetite Fe3O4 on GaAs(100) have been studied by reflection high-energy electron diffraction, x-ray photoelectron spectroscopy, magneto-optical Kerr effect, and x-ray magnetic circular dichroism. The epitaxial Fe3O4 films were synthesized by in situ post growth annealing of ultrathin epitaxial Fe films at 500K in an oxygen partial pressure of 5×10−5mbar. The XMCD measurements show characteristic contributions from different sites of the ferrimagnetic magnetite unit cell, namely, Fetd3+, Feoh2+, and Feoh3+. The epitaxial relationship was found to be Fe3O4(100)⟨011⟩∕∕GaAs(100)⟨010⟩ with the unit cell of Fe3O4 rotated by 45° to match that of GaAs(100) substrate. The films show a uniaxial magnetic anisotropy in a thickness range of about 2.0–6.0nm with the easy axes along the [011] direction of the GaAs(100) substrate.

Low temperature 1,1,1-trichloroethane dehydrochlorination over Pt catalysts:from model surfaces to the real world

Fast X-ray photoelectron spectroscopy reveals efficient C–Cl activation of 1,1,1-trichloroethane occurs over platinum surfaces at 150 K, and in the presence of hydrogen, sustained ambient temperature dehydrochlorination to HCl and ethane is possible over supported Pt/Al2O3 catalysts.

Ferroelectric and dielectric properties of Nd2−xCexTi2O7 ceramics

The solid solution system Nd2−xCexTi2O7 has been investigated. The solubility limit of Ce in Nd2−xCexTi2O7 was found to be 0·5–0·75 according to X-ray diffraction and X-ray photoelectron spectroscopy results. Ce substitution increases the b and c axes and the volume of the unit cell due to its larger ionic radius. Nd2−xCexTi2O7 (x  = 0·05, 0·25, 0·5, 0·75) textured ceramics were fabricated using spark plasma sintering. The ferroelectric and dielectric properties of the ceramics were studied. Ce substitution decreases the Curie point Tc of Nd2−xCexTi2O7 compounds. The results suggest that the Tc of Ce2Ti2O7 is <1445°C.

Fast x-ray spectroscopy study of ethene on clean and SO4 precovered Pt{111}

The adsorption and decomposition of ethylene over a Pt{111} single crystalsurface has been investigated by fast x-ray spectroscopy. At 100 K ethene displays precursor-mediated adsorption kinetics, adopting a single environment with a saturation C2H4 coverage of 0.25 ML and binding energy of 283.2 eV. Thermal decomposition proceeds above 240 K via dehydrogenation to ethylidyne with an activation barrier of 57±3 kJ mol−1 and preexponential factor ν=1×1010±0.5 s−1. Site-blocking by preadsorbed SO4 reduces the saturation ethene coverage but induces a new, less reactive π-bonded ethene species centered around 283.9 eV, which in turn decomposes to ethylidyne at 350 K.

In-situ X-ray studies of clean catalytic technologies

The rational design of new heterogeneous catalysts for clean chemical technologies can be accelerated by molecular level insight into surface chemical processes. In-situ methodologies, able to provide time-resolved and/or pressure dependent information on the evolution of reacting adsorbed layers over catalytically relevant surfaces, are therefore of especial interest. Here we discuss the application of in-situ XPS and in-situ, synchronous DRIFTS/MS/XAS methodologies to elucidate the active site in Pd-catalyzed, selective aerobic oxidation of allylic alcohols.