Role of support-nanoalloy interactions in the atomic-scale structural and chemical ordering for tuning catalytic sites

The understanding of the atomic-scale structural and chemical ordering in supported nanosized alloy particles is fundamental for achieving active catalysts by design. This report shows how such knowledge can be obtained by a combination of techniques including X-ray photoelectron spectroscopy and synchrotron radiation based X-ray fine structure absorption spectroscopy and high-energy X-ray diffraction coupled to atomic pair distribution function analysis, and how the support-nanoalloy interaction influences the catalytic activity of ternary nanoalloy (platinum-nickel-cobalt) particles on three different supports: carbon, silica, and titania. The reaction of carbon monoxide with oxygen is employed as a probe to the catalytic activity. The thermochemical processing of this ternary composition, in combination with the different support materials, is demonstrated to be capable of fine-tuning the catalytic activity and stability. The support-nanoalloy interaction is shown to influence structural and chemical ordering in the nanoparticles, leading to support-tunable active sites on the nanoalloys for oxygen activation in the catalytic oxidation of carbon monoxide. A nickel/cobalt-tuned catalytic site on the surface of nanoalloy is revealed for oxygen activation, which differs from the traditional oxygen-activation sites known for oxide-supported noble metal catalysts. The discovery of such support-nanoalloy interaction-enabled oxygen-activation sites introduces a very promising strategy for designing active catalysts in heterogeneous catalysis.     

On efficient stopping times

This paper is devoted to efficient sequential estimation in stochastic processes whose corresponding sufficient statistics are processes with stationary independent increments. It is proved that a stopping time is efficient if and only if it represents a time of the first attaining of a hyperplane., which cannot ‘be passed’, in the sense which is made precise below. The problem of determining the explicit form of the hyperplanes which cannot ‘be passed’ is also discussed.     

A note on the relativistic covariance of the B-cyclic relations

It is shown that the Evans-Vigier modified electrodynamics is compatible with relativity theory.     

The law of large numbers for weakly correlated random elements in the spaces lp, 1 ⩽ p < ∞

Lithuanian Mathematical Journal - We prove an analogue of Khinchin’s theorem for weakly correlated random elements with values in the spaces lp, 1 ? p &lt; ∞.     

Acyclic tautomers in crystalline carbohydrates: the keto forms of 1-deoxy-1-carboxymethylamino-D-2-pentuloses (pentulose-glycines)

NMR and X-ray diffraction studies on acyclic carbohydrate keto tautomers of N-(1-deoxy-d-erythro-2-pentulos-1-yl)- and N-(1-deoxy-d-threo-2-pentulos-1-yl)glycines (ribulose-glycine, 1 and xylulose-glycine, 2), are reported. In aqueous solutions, both 1 and 2 exist as a tautomeric mixture, with the beta-furanose as a major form, followed by the alpha-furanose and the acyclic keto tautomer. Both 1 and 2 crystallize as zwitterions in the acyclic keto form. Solid-state 13C NMR spectra reveal the existence of two crystallographically independent molecules in 2 but only one in crystalline 1. The structure of 2 was solved by X-ray diffraction; in the crystal, two conformational isomers, 2a and 2b in a 1:1 ratio, were identified. The conformers are packed in stacks of similar molecules, with a system of intermolecular hydrogen bonds involving all hydroxyl, ammonium, and carboxyl groups. The structural data indicate that the sugar portion of acyclic 2 adopts conformations similar to those proposed for the parent d-xylulose.     

Fluorescence detection of DNA by the catalytic activation of an aptamer/thrombin complex

A conjugate consisting of a thrombin aptamer tethered to the thrombin, Th, with a sensing nucleic acid (1) is used for the optical detection of DNA. The thrombin/aptamer complex blocks the biocatalytic functions of Th. Hybridization of the analyte DNA (2) to the sensing nucleic acid 1 yields a rigid duplex that detaches the aptamer from Th, a process that activates the protein toward the hydrolysis of bis(p-tosyl-Gly-Pro-Arg)-R110 (3) to the rhodamine 110 fluorophore (4). The system allows the DNA sensing with a sensitivity limit of 1 x 10-8 M. The aptamer/Th conjugate is also immobilized on glass slides for the optical detection of DNA. The dissociation of the aptamer/Th complex upon hybridization and the subsequent dehybridization of the duplex and the regeneration of the catalytically inactive Th/aptamer complex duplicate machinery functions.     

Spin polarized photoemission from molecular beam epitaxy-grown be-doped GaAs

We have made a comparative study of the performance of spin polarized photocathodes based on molecular beam epitaxy (MBE) grown GaAs with Be-doping and on liquid phase epitaxy (LPE) grown GaAs with Zn-doping. The experiments were performed on GaAs (100), (110), and (111) surfaces atT-300 K. The photoelectron spin polarization (P) of the MBE-photocathodes is face dependent reaching 49% for the (111) surface, a value close to P=50% predicted by theory. In contrast for the LPE-photocathodes P is significantly lower (19P36%). Possible causes for the higher polarization of the MBE-photocathodes are investigated. The influence of the Zn- and Be-dopant is elucidated by theoretical model calculations which shows that replacing the Zn-dopant by Be reduces significantly the depolarization at very low temperatures, but not at room temperature. It is therefore concluded that theinterface region between the GaAs substrate and the MBE-GaAs layer grown on top of it strongly influences the performance of the source.On leave from the Istituto di Fisica Universitá di Modena I-41100 Modena Italy.     

Temperature dependent deuterium quadrupole coupling constants of short hydrogen bonds

Very short hydrogen bonds universally show large positive dependences of the deuterium NMR quadrupolar coupling constant with temperature. We present temperature dependent NMR data for eight such systems, with OO distances of between 238 and 250 pm, and show we can model the temperature dependences by density functional methods, as long as proper attention is paid to intermolecular effects and intermode couplings.