In situ aberration corrected-transmission electron microscopy of magnesium oxide nanocatalysts for biodiesels

Biofuels are promising renewable energy sources and can be derived from vegetable oil feedstocks. Although solid catalysts show great promise in plant oil triglyceride transesterification to biodiesel, the identification of active sites and operating surface nanostructures created during their processing is essential for the development of efficient heterogeneous catalysts. Systematic, direct observations of dynamic MgO nanocatalysts from a magnesium hydroxide-methoxide precursor were performed under controlled calcination conditions using novel in situ aberration corrected-transmission electron microscopy at the 0.1 nm level and quantified with catalytic reactivity and physico-chemical studies. Surface structural modifications and the evolution of extended atomic scale glide defects implicate coplanar anion vacancies in active sites in the transesterification of triglycerides to biodiesel. The linear correlation between surface defect density (and therefore polarisability) and activity affords a simple means to fine tune new, energy efficient nanocatalysts for biofuel synthesis. © 2009 Springer Science+Business Media, LLC.

In-situ XPS study on the reducibility of Pd-promoted Cu/CeO2 catalysts for the oxygen-assisted water-gas-shift reaction

Cu/CeO2, Pd/CeO2, and CuPd/CeO2 catalysts were prepared and their reduction followed by in-situ XPS in order to explore promoter and support interactions in a bimetallic CuPd/CeO2 catalyst effective for the oxygen-assisted water-gas-shift (OWGS) reaction. Mutual interactions between Cu, Pd, and CeO2 components all affect the reduction process. Addition of only 1 wt% Pd to 30 wt% Cu/CeO2 greatly enhances the reducibility of both dispersed CuO and ceria support. In-vacuo reduction (inside XPS chamber) up to 400 °C results in a continuous growth of metallic copper and Ce3+ surface species, although higher temperatures results in support reoxidation. Supported copper in turn destabilizes metallic palladium metal with respect to PdO, this mutual perturbation indicating a strong intimate interaction between the Cu–Pd components. Despite its lower intrinsic reactivity towards OWGS, palladium addition at only 1 wt% loading significantly improved CO conversion in OWGS reaction over a monometallic 30 wt% Cu/CeO2 catalysts, possibly by helping to maintain Cu in a reduced state during reaction.

Ag alloyed Pd single-atom catalysts for efficient selective hydrogenation of acetylene to ethylene in excess ethylene

Semihydrogenation of acetylene in an ethylene-rich stream is an industrially important process. Conventional supported monometallic Pd catalysts offer high acetylene conversion, but they suffer from very low selectivity to ethylene due to overhydrogenation and the formation of carbonaceous deposits. Herein, a series of Ag alloyed Pd single-atom catalysts, possessing only ppm levels of Pd, supported on silica gel were prepared by a simple incipient wetness coimpregnation method and applied to the selective hydrogenation of acetylene in an ethylene-rich stream under conditions close to the front-end employed by industry. High acetylene conversion and simultaneous selectivity to ethylene was attained over a wide temperature window, surpassing an analogous Au alloyed Pd single-atom system we previously reported. Restructuring of AgPd nanoparticles and electron transfer from Ag to Pd were evidenced by in situ FTIR and in situ XPS as a function of increasing reduction temperature. Microcalorimetry and XANES measurements support both geometric and electronic synergetic effects between the alloyed Pd and Ag. Kinetic studies provide valuable insight into the nature of the active sites within these AgPd/SiO2 catalysts, and hence, they provide evidence for the key factors underpinning the excellent performance of these bimetallic catalysts toward the selective hydrogenation of acetylene under ethylene-rich conditions while minimizing precious metal usage.

On the impact of Cu dispersion on CO2 photoreduction over Cu/TiO2

A family of Cu/TiO2 catalysts was prepared using a refined sol–gel method, and tested in the photocatalytic reduction of CO2 by H2O to CH4 using a stirred batch, annular reactor. The resulting photoactivity was benchmarked against pure TiO2 nanoparticles (synthesised by an identical sol–gel route). CO2 photoreduction exhibited a strong volcano dependence on Cu loading, reflecting the transition from 2-dimensional CuOx nanostructures to 3-dimensional crystallites, with optimum CH4 production observed for 0.03 wt.% Cu/TiO2.

Synergistic synthesis of quasi-monocrystal CdS nanoboxes with high-energy facets

Hollow nanostructures with a highly oriented lattice structure and active facets are promising for catalytic applications, while their preparation via traditional approaches contains multiple steps and is time and energy consuming. Here, we demonstrate a new one-step strategy involving two complementary reactions which promote each other; it is capable of producing unique hollow nanoparticles. Specifically, we apply synergic cooperation of cation exchange and chemical etching to attack PbS nanosized cubes (NCs) and produce CdS quasi-monocrystal nanoboxes (QMNBs) which possess the smallest dimensions reported so far, a metastable zinc-blende phase, a large specific surface area, and particularly high-energy {100} facets directly visualized by aberration-corrected scanning transmission electron microscopy. These properties in combination allow the nanoboxes to acquire exceptional photocatalytic activities. As an extension of the approach, we use the same strategy to prepare Co9S8 and Cu7.2S4 single-crystal hollow nanooctahedrons (SCHNOs) successfully. Hence, the synergic reaction synthesis strategy exhibits great potential in engineering unique nanostructures with superior properties.

Association behavior of the synthetic protein-lipid complex analogues

Hypercoiling poly(styrene-ALT-maleic anhydride) (PSMA) is known to undergo conformational transition in response to environmental stimuli. This behavior allows it to associate with the phospholipid, 2-dilauryl-SN-glycero-3- phosphocholine (DLPC) to produce nanostructures analogous to lipoproteins. The complex represents a new bio-mimetic delivery vehicle with applications in the cosmetic and pharmaceutical industries. This study investigates, for the first time, the association behavior of PSMA and DLPC through the combination of different analytical techniques. The results indicate that the association is primarily driven by hydrophobic interactions and depends on various factors including the polymer/lipid ratio, the polymer molecular weight and the pH of the aqueous environment. The conformational transition of PSMA leads to the formation of discrete micellar complexes involving anisotropic-to-isotropic lipid phase transformation. As the number of hydrophobic moieties in the polymer is increased, the pH-dependent conformational transition of the polymer plays less important part in achieving this phase transition of the lipid. © (2012) Trans Tech Publications.

Ferroelectric-like behaviour of melanin:humidity effect on current-voltage characteristics

The influence of low vacuum on quasistatic current-voltage (I–V) dependences and the impact of wet air pulse on dynamic bipolar I-V-loops and unipolar I-V-curves of fungal melanin thin layers have been studied for the first time. The threshold hysteresis voltages of I–V dependences are near to the standard electrode potentials of anodic water decomposition. Short wet air pulse impact leads to sharp increase of the current and appearance of “hump”-like and “knee”-like features of I-V-loops and I-V-curves, respectively. By treatment of I-V-loop allowing for I-V-curve shape the maxima of displacement current are revealed. The peculiarities of I-V-characteristics were modelled by series-parallel RC-circuit with Zener diodes as nonlinear elements. As a reason of appearance of temporal polar media with reversible ferroelectric-like polarization and ionic space charge transfer is considered the water-assisted dissociation of some ionic groups of melanin monomers that significantly influences electrophysical parameters of melanin nanostructures.

Catalyst studies on the ring opening of tetrahydrofuran-dimethanol to 1,2,6-hexanetriol

The metal catalyzed hydrogenolysis of the biomass-derived THF-dimethanol to 1,2,6-hexanetriol using heterogeneous catalysts was investigated. Bimetallic Rh-Re catalysts (4 wt% Rh and a Re/Rh (mol. ratio of 0.5) on a silica support gave the best performance and 1,2,6-hexanetriol was obtained in 84% selectivity at 31% conversion (120 C, 80 bar, 4 h); the selectivity reaches a maximum of 92% at 80 C. The product distribution at prolonged reaction times or higher temperatures or both shows the formation of diols and mono-alcohols, indicating that the 1,2,6-hexanetriol is prone to subsequent hydrodeoxygenation reactions. Different silica supports were investigated and optimal results were obtained with an amorphous silica featuring an intermediate surface area and an average mesopore size of about 6 nm. TPR and XPS surface analysis support the presence of mixed Rh and Re particles. The redox Reδ+/ReTotal surface ratio correlates with the conversion in a volcano type dependency. Both gas phase as well as Rh200Re1OH cluster DFT calculations support an acid-metal bifunctional mechanism and explain the products distribution. © 2013 Elsevier B.V. All rights reserved.

Uniform design for the optimization of Al2O3 nanofilms produced by electrophoretic deposition

Surface modification by means of nanostructures is of interest to enhance boiling heat transfer in various applications including the organic Rankine cycle (ORC). With the goal of obtaining rough and dense aluminum oxide (Al2O3) nanofilms, the optimal combination of process parameters for electrophoretic deposition (EPD) based on the uniform design (UD) method is explored in this paper. The detailed procedures for the EPD process and UD method are presented. Four main influencing conditions controlling the EPD process were identified as nanofluid concentration, deposition time, applied voltage and suspension pH. A series of tests were carried out based on the UD experimental design. A regression model and statistical analysis were applied to the results. Sensitivity analyses of the effect of the four main parameters on the roughness and deposited mass of Al2O3 films were also carried out. The results showed that Al2O3 nanofilms were deposited compactly and uniformly on the substrate. Within the range of the experiments, the preferred combination of process parameters was determined to be nanofluid concentration of 2 wt.%, deposition time of 15 min, applied voltage of 23 V and suspension pH of 3, yielding roughness and deposited mass of 520.9 nm and 161.6 × 10− 4 g/cm2, respectively. A verification experiment was carried out at these conditions and gave values of roughness and deposited mass within 8% error of the expected ones as determined from the UD approach. It is concluded that uniform design is useful for the optimization of electrophoretic deposition requiring only 7 tests compared to 49 using the orthogonal design method.

ZnO nanorods prepared via ablation of Zn with millisecond laser in liquid media

ZnO nanomaterials with controlled size, shape and surface chemistry are required for applications in diverse areas, such as optoelectronics, photocatalysis, biomedicine and so on. Here, we report on ZnO nanostructures with rod-like and spherical shapes prepared via laser ablation in liquid using a laser with millisecond-long pulses. By changing laser parameters (such as pulse width and peak power), the size or aspect ratio of such nanostructures could be tuned. The surface chemistry and defects of the products were also strongly affected by applied laser conditions. The preparation of different structures is explained by the intense heating of liquid media caused by millisecond-long pulses and secondary irradiation of already-formed nanostructures.