R. Puskás

Synthesis of tungsten carbide and tungsten disulfide on vertically aligned multi-walled carbon nanotube forests and their application as non-Pt electrocatalysts for the hydrogen evolution reaction

Direct growth of tungsten carbide and sulfide on vertically aligned carbon nanotubes (CNTs) is demonstrated for a facile construction of three-dimensional electrocatalyst nanostructures. A thin layer of tungsten was first deposited on free-standing CNT forests followed by annealing in an inert atmosphere or in the vapor of sulfur for obtaining tungsten carbide or disulfide, respectively. Besides structural characterization of the composites by means of scanning and transmission electron microscopy, Raman spectroscopy and X-ray diffraction, the produced CNT–catalyst composites are also evaluated in the hydrogen evolution reaction and compared to Pt as well as to the original and W coated CNT forests. The results indicate that the template growth of catalyst nanostructures on vertically aligned CNT forests is a suitable approach for catalytic electrode fabrication, which for example may find use in the hydrogen evolution reaction.

Palladium Nanoparticle–Graphene Catalysts for Asymmetric Hydrogenation

We report for the first time the application of palladium nanoparticle-graphene (Pd/Gn) catalysts in the asymmetric hydrogenation of aliphatic α,β-unsaturated carboxylic acids using cinchonidine as chiral modifier. Pd/Gns were prepared by deposition–precipitation from the aqueous phase over graphite oxide and subsequent simultaneous reduction of both the support and the metal precursor with NaBH4. The materials obtained were characterized by ICP optical emission spectroscopy, X-ray diffraction spectroscopy, Raman spectroscopy, transmission electron microscopy and X-ray photoelectron spectroscopy. We demonstrate that the Pd/Gns modified by cinchonidine can act as efficient catalysts in the asymmetric hydrogenation of α,β-unsaturated carboxylic acids for producing optically enriched saturated carboxylic acids.Graphical Abstract

Influence of gold additives on the stability and phase transformation of titanate nanostructures

Gold nanoparticles were prepared and characterized on protonated (H-form) titanate nanotubes (TiONTs) and nanowires (TiONWs). The chemical nature and morphology of gold particles were monitored by X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, X-ray diffraction (XRD) and high resolution electron microscopy (HRTEM). The optical properties of Au-containing titanate nanowires were explored by means of ultraviolet-visible diffuse reflectance spectroscopy. The size distribution and homogeneity of gold particles depend on the reduction mode from the corresponding gold salt to metal particles. Smaller clusters (3-8 nm) were obtained with the NaBH4 reactant at 293 K than with molecular hydrogen reduction. An unexpectedly high binding energy gold state was found by XPS in gold-loaded titanate nanostructures. This state was absent from the spectra of gold-loaded TiO2(110). A likely explanation for this phenomenon, supported also by the characteristic decrease of band gap energy from 3.10 eV to 2.74 eV with increasing Au content, is that depending on the metal loading and titanate structure, Au is stabilized on titanate nanowires partially in positively charged gold form by ion exchange and also as Au clusters. Our important new finding is that the thermal annealing behavior of Au-loaded titanate nanotubes and nanowires is different. The former lose their tubular morphology and are readily transformed into anatase even at a very low temperature of 473 K. On the other hand, gold stabilizes the layered structure of titanate nanowires up to 873 K.

Propionic Acid Produced by Propionibacterium acnes Strains Contri-butes to Their Pathogenicity.

Propionibacterium acnes is an important member of the skin microbiome. The bacterium can initiate signalling events and changes in cellular properties in keratinocytes. The aim of this study was to analyse the effect of the bacterium on an immortalized human keratinocyte cell line. The results show that various P. acnes strains affect the cell-growth properties of these cells differentially, inducing cytotoxicity in a strain-specific and dose-dependent manner. We propose that bacterially secreted propionic acid may contribute to the cytotoxic effect. This acid has a role in maintaining skin pH and exhibits antimicrobial properties, but may also have deleterious effects when the local concentration rises due to excessive bacterial growth and metabolism. These results, together with available data from the literature, may provide insight into the dual role of P. acnes in healthy skin and during pathogenic conditions, as well as the key molecules involved in these functions.

Comparison of Nanoscaled Palladium Catalysts Supported on Various Carbon Allotropes

Today there is a need for inexpensive and reliable catalysts due to the increasing demand for “green” catalytic processes of high conversion and selectivity. We now report on the performance of palladium nanoparticles supported on various carbon allotropes in the model reaction of cyclohexene hydrogenation. Palladium catalysts supported on pristine and oxidatively functionalized multiwalled carbon nanotubes and activated carbon were synthesized and characterized by transmission electron microscopy, X-ray diffraction, temperature programmed desorption and nitrogen adsorption measurements. Catalyst based on multiwalled carbon nanotube proved to be more active and less prone to deactivation than the activated carbon supported one. This finding could be interpreted on the basis of morphological differences between the supports.

A novel WS2 nanowire-nanoflake hybrid material synthesized from WO3 nanowires in sulfur vapor

In this work, WS2 nanowire-nanoflake hybrids are synthesized by the sulfurization of hydrothermally grown WO3 nanowires. The influence of temperature on the formation of products is optimized to grow WS2 nanowires covered with nanoflakes. Current-voltage and resistance-temperature measurements carried out on random networks of the nanostructures show nonlinear characteristics and negative temperature coefficient of resistance indicating that the hybrids are of semiconducting nature. Bottom gated field effect transistor structures based on random networks of the hybrids show only minor modulation of the channel conductance upon applied gate voltage, which indicates poor electrical transport between the nanowires in the random films. On the other hand, the photo response of channel current holds promise for cost-efficient solution process fabrication of photodetector devices working in the visible spectral range.

Electrocatalytic Properties of Carbon Nanotubes Decorated with Copper and Bimetallic CuPd Nanoparticles

Copper particles were deposited on the surface of palladium decorated buckypapers by the means of chemical plating, in which the palladium nanoparticles act as catalyst for initiating the reduction of complexed Cu2+ ions in the presence of formaldehyde. By adjusting the plating time, it is possible to tune the size of the highly crystalline copper particles and at the same time control the coverage of the metal on the surface of carbon nanotubes. In a subsequent step, the copper particles were partially exchanged with palladium to obtain bimetallic CuPd nanoparticles on the nanotubes by applying galvanic replacement reactions in aqueous solution of Pd2+ ions. Sufficiently high electrical conductivity of both Cu and CuPd/buckypaper composites makes them suitable to be used as electrocatalytic electrodes. The electrochemical properties of the different electrode materials were also evaluated by the model reaction of methanol electrooxidation. The degradation mechanism of copper and CuPd bimetallic catalysts were systematically studied by employing surface characterization techniques on the composite films after electrocatalytic testing in alkaline solution in the presence and absence of methanol. Chronoamperometric test of the catalysts/buckypaper composites had revealed that palladium plays a protecting role in CuPd bimetallic structure during methanol electrooxidation.

Effects of carbon nanotube functionalization on the agglomeration and sintering of supported Pd nanoparticles

The size of carbon nanotube supported Pd and PdO nanoparticles was investigated on oxidatively functionalized multiwall carbon nanotubes. All samples were characterized by transmission electron microscopy, X-ray diffractometry, X-ray photoelectron spectroscopy and Raman spectroscopy. The average particle diameter calculated from TEM image analysis was found to be inversely proportional with the duration of the oxidation in nitric acid. Crystallite sizes determined from XRD patterns confirmed this general tendency.

Understanding the role of post-CCVD synthetic impurities, functional groups and functionalization-based oxidation debris on the behaviour of carbon nanotubes as a catalyst support in cyclohexene hydrogenation over Pd nanoparticles

A new method has been developed to separately study the effects of (i) impurities resulting from the catalytic chemical vapor deposition synthesis, (ii) the attached functional groups and (iii) the oxidation debris on the properties of carboxyl-functionalized multiwall carbon nanotubes (CNTs) by incorporating a Soxhlet-extractor enhanced acetone washing process into the synthesis method. Pd nanoparticles supported on the carbon nanotubes were investigated in the hydrogenation of cyclohexene to cyclohexane. Despite the fact that the specific surface area and the Pd dispersion were both low, the Pd/CNT catalyst with post-synthetic impurities showed ∼20 times higher catalytic activity compared to functional group free, acetone-washed samples. Meanwhile oxidation debris originating from the functionalization was found to affect both the specific surface area and the G/D ratio obtained from Raman spectra, it had slight effect on the size of the supported Pd nanoparticles or the catalytic activity. On the other hand, functional groups have a significant effect on the catalytic activity without influencing the specific surface area, the G/D ratio or the Pd nanoparticle dispersion.