Electrochemical versus gas-phase oxidation of ru single-crystal surfaces

The electrochemical uptake of oxygen on a Ru(0001) electrode was investigated by electron diffraction, Auger spectroscopy, and cyclic voltammetry. An ordered (2 × 2)-O overlayer forms at a potential close to the hydrogen region. At +0.42 and +1.12 V vs Ag/AgCl, a (3 × 1) phase and a (1 × 1)-O phase, respectively, emerge. When the Ru electrode potential is maintained at +1.12 V for 2 min, RuO2 grows epitaxially with its (100) plane parallel to the Ru(0001) surface. In contrast to the RuO domains, the non-oxidized regions of the Ru electrode surface are flat. If, however, the electrode potential is increased to +1.98 V for 2 min, the remaining non-oxidized Ru area also becomes rough. These findings are compared with O overlayers and oxides on the Ru(0001) and Ru(101¯1) surfaces created by exposure to gaseous O under UHV conditions. On the other hand, gas-phase oxidation of the Ru(101¯0) surface leads to the formation of RuO with a (100) orientation. It is concluded that the difference in surface energy between RuO(110) and RuO(100) is quite small. RuO again grows epitaxially on Ru(0001), but with the (110) face oriented parallel to the Ru(0001) surface. The electrochemical oxidation of the Ru(0001) electrode surface proceeds via a 3-dimensional growth mechanism with a mean cluster size of 1.6 nm, whereas under UHV conditions, a 2-dimensional oxide film (1-2 nm thick) is epitaxially formed with an average domain size of 20 µm. © 2000 American Chemical Society.

The host galaxy of the super-luminous SN 2010gx and limits on explosive nickel-56 production

Super-luminous supernovae have a tendency to occur in faint host galaxies which are likely to have low mass and low metallicity. While these extremely luminous explosions have been observed from z=0.1 to 1.55, the closest explosions allow more detailed investigations of their host galaxies. We present a detailed analysis of the host galaxy of SN 2010gx (z=0.23), one of the best studied super-luminous type Ic supernovae. The host is a dwarf galaxy (M_g=-17.42+/-0.17) with a high specific star formation rate. It has a remarkably low metallicity of 12+log(O/H)=7.5+/-0.1 dex as determined from the detection of the [OIII] 4363 Angs line. This is the first reliable metallicity determination of a super-luminous stripped-envelope supernova host. We collected deep multi-epoch imaging with Gemini + GMOS between 240-560 days after explosion to search for any sign of radioactive nickel-56, which might provide further insights on the explosion mechanism and the progenitor's nature. We reach griz magnitudes of m_AB~26, but do not detect SN 2010gx at these epochs. The limit implies that any nickel-56 production was similar to or below that of SN 1998bw (a luminous type Ic SN that produced around 0.4 M_sun of nickel-56). The low volumetric rates of these supernovae (~10^-4 of the core-collapse population) could be qualitatively matched if the explosion mechanism requires a combination of low-metallicity (below 0.2 Z_sun), high progenitor mass (>60 M_sun) and high rotation rate (fastest 10% of rotators).

Slowly fading super-luminous supernovae that are not pair-instability explosions

Super-luminous supernovae that radiate more than 10 44 ergs per second at their peak luminosity have recently been discovered in faint galaxies at redshifts of 0.1-4. Some evolve slowly, resembling models of 'pair-instability' supernovae. Such models involve stars with original masses 140-260 times that of the Sun that now have carbon-oxygen cores of 65-130 solar masses. In these stars, the photons that prevent gravitational collapse are converted to electron-positron pairs, causing rapid contraction and thermonuclear explosions. Many solar masses of 56 Ni are synthesized; this isotope decays to 56 Fe via 56 Co, powering bright light curves. Such massive progenitors are expected to have formed from metal-poor gas in the early Universe. Recently, supernova 2007bi in a galaxy at redshift 0.127 (about 12 billion years after the Big Bang) with a metallicity one-third that of the Sun was observed to look like a fading pair-instability supernova. Here we report observations of two slow-to-fade super-luminous supernovae that show relatively fast rise times and blue colours, which are incompatible with pair-instability models. Their late-time light-curve and spectral similarities to supernova 2007bi call the nature of that event into question. Our early spectra closely resemble typical fast-declining super-luminous supernovae, which are not powered by radioactivity. Modelling our observations with 10-16 solar masses of magnetar-energized ejecta demonstrates the possibility of a common explosion mechanism. The lack of unambiguous nearby pair-instability events suggests that their local rate of occurrence is less than 6 × 10 -6 times that of the core-collapse rate. © 2013 Macmillan Publishers Limited. All rights reserved.

Influence of surface structures, subsurface carbon and hydrogen, and surface alloying on the activity and selectivity of acetylene hydrogenation on Pd surfaces: A density functional theory study

The selective hydrogenation of acetylene to ethylene on several Pd surfaces (Pd(111), Pd(100), Pd(211), and Pd(211)-defect) and Pd surfaces with subsurface species (carbon and hydrogen) as well as a number of Pd-based alloys (Pd-M/Pd(111) and Pd-M/Pd(211) (M = Cu, Ag and Au)) are investigated using density functional theory calculations to understand both the acetylene hydrogenation activity and the selectivity of ethylene formation. All the hydrogenation barriers are calculated, and the reaction rates on these surfaces are obtained using a two-step model. Pd(211) is found to have the highest activity for acetylene hydrogenation while Pd(100) gives rise to the lowest activity. In addition, more open surfaces result in over-hydrogenation to form ethane, while the close-packed surface (Pd(111)) is the most selective. However, we also find that the presence of subsurface carbon and hydrogen significantly changes the reactivity and selectivity of acetylene toward hydrogenation on Pd surfaces. On forming surface alloys of Pd with Cu, Ag and Au, the selectivity for ethylene is also found to be changed. A new energy decomposition method is used to quantitatively analyze the factors in determining the changes in selectivity. These surface modifiers are found to block low coordination unselective sites, leading to a decreased ethane production. (C) 2013 The Authors. Published by Elsevier Inc. All rights reserved.

'Super-Chandrasekhar' Type Ia Supernovae at nebular epochs

We present a first systematic comparison of superluminous Type Ia supernovae (SNe Ia) at late epochs, including previously unpublished photometric and spectroscopic observations of SN 2007if, SN 2009dc and SNF20080723-012. Photometrically, the objects of our sample show a diverse late-time behaviour, some of them fading quite rapidly after a light-curve break at ∼ 150-200 d. The latter is likely the result of flux redistribution into the infrared, possibly caused by dust formation, rather than a true bolometric effect. Nebular spectra of superluminous SNe Ia are characterized by weak or absent [Fe III] emission, pointing at a low ejecta ionization state as a result of high densities. To constrain the ejecta and Ni masses of superluminous SNe Ia, we compare the observed bolometric light curve of SN 2009dc with synthetic model light curves, focusing on the radioactive tail after ∼60 d. Models with enough Ni to explain the light-curve peak by radioactive decay, and at the same time sufficient mass to keep the ejecta velocities low, fail to reproduce the observed light-curve tail of SN 2009dc because of too much γ -ray trapping.We instead propose a model with ∼1M of Ni and ∼2 M of ejecta, which may be interpreted as the explosion of a Chandrasekhar-mass white dwarf (WD) enshrouded by 0.6-0.7 M of C/O-rich material, as it could result from a merger of two massive C/O WDs. This model reproduces the late light curve of SN 2009dc well. A flux deficit at peak may be compensated by light from the interaction of the ejecta with the surrounding material.

Enhanced laccase stability through mediator partitioning into hydrophobic ionic liquids

Laccase-mediator systems have numerous potential uses for green oxidations, but their practical use may be limited because the reactive, oxidised mediators deactivate the enzyme. TEMPO, 4-hydroxybenzyl alcohol, phenothiazine and 2-hydroxybiphenyl caused almost complete deactivation of laccase from Trametes versicolor within 24-140 h. By contrast, 18% activity was retained after 188 h in controls without mediator, and 15% in the presence of ABTS. A biphasic reaction system was developed to protect the laccase, by partitioning the mediator into water-immiscible ionic liquids. In the presence of [C mim][AOT], laccase retained 54, 35, 35 and 41% activity after 188 h in the presence of 4-hydroxybenzyl alcohol, phenothiazine and 2-hydroxybiphenyl and ABTS, respectively, whilst 30% activity was retained in the presence of [N][Sac] and TEMPO. The protection against deactivation by the mediators correlated strongly with the distribution coefficients of the mediators between ionic liquids and water. © 2014 The Royal Society of Chemistry.

More bang for your buck: Super-adiabatic quantum engines

The practical untenability of the quasi-static assumption makes any realistic engine intrinsically irreversible and its operating time finite, thus implying friction effects at short cycle times. An important technological goal is thus the design of maximally efficient engines working at the maximum possible power. We show that, by utilising shortcuts to adiabaticity in a quantum engine cycle, one can engineer a thermodynamic cycle working at finite power and zero friction. Our findings are illustrated using a harmonic oscillator undergoing a quantum Otto cycle.

The effects of stepped sites and ruthenium adatom decoration on methanol dehydrogenation over platinum-based catalyst surfaces

A density functional theory study of methanol dehydrogenation over stepped Pt(2 1 1) surfaces without and with Ru modification was carried out to understand fuel catalytic reactions on Pt-based catalysts. Two main pathways of the CH3OH dehydrogenation were examined: the O–H pathway which was initiated by O–H bond scission to form the methoxy (CH3O) intermediate followed by sequential cleavage of C–H bonds to CO, and the C–H pathway which was initiated by C–H bond scission to form the hydroxymethyl (CH2OH) followed by two C–H bond cleavages to COH and then CO. Possible crossover reactions between the O–H and C–H pathways were also computed. Compared to flat Pt(1 1 1), stepped Pt(2 1 1) increases the adsorption energies of intermediates, making no significant contribution to decreasing the reaction barriers of most elementary steps involved, except in the first hydrogen scission. However, on the Ru-modified surface, a significant reduction was found in reaction barriers for the first step of the C–H bond scission and a number of further dehydrogenation steps crossing over to the O–H pathway, with the most facile paths identified. Our data reveals the complexity of methanol catalytic reaction processes at the atomic level and contributes to a fundamental understanding of fuel reactions on Pt-based catalysts.

Chaotropic and hydrophobic stress mechanisms of antifungal substances

Growth and metabolism of fungi can be curtailed by chaotropic solutes and hydrophobic substances, both of which can weaken or inhibit non-covalent interactions within and between macromolecular systems. Here we explore the potential to utilize the fungistatic and fungicidal activities of such stressors as the basis for commercial formulations. A method was developed for the quantification of chaotropicity, which can be used for chemically diverse substances, in order elucidate roles of chaotropicity and hydrophobicity in microbial ecology (both of which are sufficiently potent to limit the Earth’s microbial biosphere). A large number of naturally occurring substances act as chaotropic or hydrophobic stressors including aliphatic alcohols, salts such as MgCl2, aromatics such as phenol, and hydrocarbons such as hexane and octene. We suggest that these stress parameters provide the (hitherto unidentified) modes-of-action for some extant antifungal products. The findings are discussed in relation to the development of a new generation of antifungals.

Universal tight binding model for chemical reactions in solution and at surfaces. II. Water

A revised water model intended for use in condensed phase simulations in the framework of the self consistent polarizable ion tight binding theory is constructed. The model is applied to water monomer, dimer, hexamers, ice, and liquid, where it demonstrates good agreement with theoretical results obtained by more accurate methods, such as DFT and CCSD(T), and with experiment. In particular, the temperature dependence of the self diffusion coefficient in liquid water predicted by the model, closely reproduces experimental curves in the temperature interval between 230 K and 350 K. In addition, and in contrast to standard DFT, the model properly orders the relative densities of liquid water and ice. A notable, but inevitable, shortcoming of the model is underestimation of the static dielectric constant by a factor of two. We demonstrate that the description of inter and intramolecular forces embodied in the tight binding approximation in quantum mechanics leads to a number of valuable insights which can be missing from ab initio quantum chemistry and classical force fields. These include a discussion of the origin of the enhanced molecular electric dipole moment in the condensed phases, and a detailed explanation for the increase of coordination number in liquid water as a function of temperature and compared with ice-leading to insights into the anomalous expansion on freezing. The theory holds out the prospect of an understanding of the currently unexplained density maximum of water near the freezing point.