In-situ electrochemical quantification of active sites in Fe-N/C non-precious metal catalysts

The economic viability of low temperature fuel cells as clean energy devices is enhanced by the development of inexpensive oxygen reduction reaction catalysts. Heat treated iron and nitrogen containing carbon based materials (Fe–N/C) have shown potential to replace expensive precious metals. Although significant improvements have recently been made, their activity and durability is still unsatisfactory. The further development and a rational design of these materials has stalled due to the lack of an in situ methodology to easily probe and quantify the active site. Here we demonstrate a protocol that allows the quantification of active centres, which operate under acidic conditions, by means of nitrite adsorption followed by reductive stripping, and show direct correlation to the catalytic activity. The method is demonstrated for two differently prepared materials. This approach may allow researchers to easily assess the active site density and turnover frequency of Fe–N/C catalysts

Competition for Graphene: Graphynes with Direction-Dependent Dirac Cones

The existence of Dirac cones in the band structure of two-dimensional materials accompanied by unprecedented electronic properties is considered to be a unique feature of graphene related to its hexagonal symmetry. Here, we present other two-dimensional carbon materials, graphynes, that also possess Dirac cones according to first-principles electronic structure calculations. One of these materials, 6,6,12-graphyne, does not have hexagonal symmetry and features two self-doped nonequivalent distorted Dirac cones suggesting electronic properties even more amazing than that of graphene

Enhanced relativistic-electron beam collimation using two consecutive laser pulses

The double laser pulse approach to relativistic electron beam (REB) collimation has been investigated at the LULI-ELFIE facility. In this scheme, the magnetic field generated by the first laser-driven REB is used to guide a second delayed REB. We show how electron beam collimation can be controlled by properly adjusting laser parameters. By changing the ratio of focus size and the delay time between the two pulses we found a maximum of electron beam collimation clearly dependent on the focal spot size ratio of the two laser pulses and related to the magnetic field dynamics. Cu-K alpha and CTR imaging diagnostics were implemented to evaluate the collimation effects on the respectively low energy ( MeV) components of the REB

Kinetic isotope effect in the oxygen reduction reaction (ORR) over Fe-N/C catalysts under acidic and alkaline conditions.

Heat treated Fe-N/C materials which are highly effective oxygen reduction catalysts in alkaline and acid, show a significant kinetic isotope effect (KIE). The values in acid (~3.4) and alkaline (~2.5) are much larger than the value for the metal free catalyst in acid (~1.8) suggesting that the rate determining step (RDS) is a proton coupled electron transfer in acid with a significant pathway involving a proton independent step under an alkaline environmen

Time evolution of stimulated Raman scattering and two-plasmon decay at laser intensities relevant for shock ignition in a hot plasma

Laser–plasma interaction (LPI) at intensities 1015–1016 W cm2 is dominated by parametric instabilities which can be responsible for a significant amount of non-collisional absorption and generate large fluxes of high-energy nonthermal electrons. Such a regime is of paramount importance for inertial confinement fusion (ICF) and in particular for the shock ignition scheme. In this paper we report on an experiment carried out at the Prague Asterix Laser System (PALS) facility to investigate the extent and time history of stimulated Raman scattering (SRS) and two-plasmon decay (TPD) instabilities, driven by the interaction of an infrared laser pulse at an intensity 1:2 1016 W cm2 with a 100 mm scalelength plasma produced from irradiation of a flat plastic target. The laser pulse duration (300 ps) and the high value of plasma temperature (4 keV) expected from hydrodynamic simulations make these results interesting for a deeper understanding of LPI in shock ignition conditions. Experimental results show that absolute TPD/SRS, driven at a quarter of the critical density, and convective SRS, driven at lower plasma densities, are well separated in time, with absolute instabilities driven at early times of interaction and convective backward SRS emerging at the laser peak and persisting all over the tail of the pulse. Side-scattering SRS, driven at low plasma densities, is also clearly observed. Experimental results are compared to fully kinetic large-scale, two-dimensional simulations. Particle-in-cell results, beyond reproducing the framework delineated by the experimental measurements, reveal the importance of filamentation instability in ruling the onset of SRS and stimulated Brillouin scattering instabilities and confirm the crucial role of collisionless absorption in the LPI energy balance

Resonant Structure in the π⁺ π⁺π‾ π‾ System between 1.5 and 1.9 GeV

Resonant structure is studied in the 1.5-to-1.9-GeV mass region of the neutral four-pion system in the reaction Ï€+n→pÏ€+Ï€+Ï€-Ï€- at 6 GeV/c incident momentum. The Ï0 and A2± content are determined as a function of the four-pion mass. The 4Ï€ mass spectrum is best fitted by two resonances, both of which decay predominantly into Ï0Ï€+Ï€-, each with evidence for an appreciable A2Ï€ component

Effects of beam polarization on Λ and K0 inclusive production in pp interactions at 12 GeV/c

Inclusive production of Λ and K0 in pp interactions at 12 GeV/c has been studied in an exposure of the Argonne National Laboratory 12-foot bubble chamber to a 50% transversely polarized proton beam. Beam-polarization-induced asymmetries are observed for both K0 and Λ production in the forward direction

Study of the reaction p↑p→∆⁺⁺ at 6 GeV/c with polarized beam

We have studied the reaction p↑p→nΔ++ at 6 GeV/c incident momentum in which the incident protons are 60% transversely polarized. The experiment used the Argonne National Laboratory Zero Gradient Synchrotron polarized beam and the 12-foot hydrogen bubble chamber. We report on about 6000 Δ++ events, with the Δ++ produced in the backward hemisphere in the c.m. system. Spin-density matrix elements as a function of momentum transfer are presented and small beam-polarization-induced effects are described

Evidence for the ωππ Decay Modes of the A₂ and ω(1675)

We present evidence for the decay A20→ωπ+Ï€0 with a branching ratio Γ(A2→ωππ)/Γ(A2→ÏÏ€)=0.28±0.09 and for the decay ω(1675)→ωπ+Ï€- with a branching ratio Γ(ω(1975)→ωπ+Ï€-)/Γ(ω(1675)→ÏÏ€)=0.47±0.18. Evidence is given for an intermediate B(1235)Ï€ state in the ω(1675) decay