Dealloying of Cobalt from CuCo Nanoparticles under Syngas Exposure

International audienceThe structure and composition of core−shell CuCo nanoparticles were found to change as a result of cleaning pretreatments and when exposed to syngas (CO + H 2) at atmospheric pressure. In situ X-ray absorption and photoelectron spectroscopies revealed the oxidation state of the particles as well as the presence of adsorbates under syngas. Transmission electron microscopy was used for ex situ analysis of the shape, elemental composition, and structure after reaction. The original core−shell structure was found to change to a hollow CuCo alloy after pretreatment by oxidation in pure O 2 and reduction in pure H 2. After 30 min of exposure to syngas, a significant fraction (5%) of the particles was strongly depleted in cobalt giving copper-rich nanoparticles. This fraction increased with duration of syngas exposure, a phenomenon that did not occur under pure CO or pure H 2. This study suggests that Co and Cu can each individually contribute to syngas conversion with CuCo catalysts

On Minimizing Ordered Weighted Regrets in Multiobjective Markov Decision Processes

International audienceIn this paper, we propose an exact solution method to generate fair policies in Multiobjective Markov Decision Processes (MMDPs). MMDPs consider n immediate reward functions, representing either individual payoffs in a multiagent problem or rewards with respect to different objectives. In this context, we focus on the determination of a policy that fairly shares regrets among agents or objectives, the regret being defined on each dimension as the opportunity loss with respect to optimal expected rewards. To this end, we propose to minimize the ordered weighted average of regrets (OWR). The OWR criterion indeed extends the minimax regret, relaxing egalitarianism for a milder notion of fairness. After showing that OWR-optimality is state-dependent and that the Bellman principle does not hold for OWR-optimal policies, we propose a linear programming reformulation of the problem. We also provide experimental results showing the efficiency of our approach

Enhanced Electrocatalytic Activity of Copper-Cobalt Nanostructures

Novel core–shell nanostructures containing Cu and Co have been synthesized using the microemulsion method at 700 °C. The core consists of Cu–Co composite particles, whereas the shell is composed of Cu–Co alloy particles (shell thickness 12 nm). It is to be noted that in bulk Cu–Co binary system there is practically no miscibility. TEM studies show formation of spherical-shaped nanoparticles of core–shell structures. The composition of the core (Cu–Co composite) and shell (Cu–Co alloy) were confirmed by XPS studies. The formation of the Cu–Co alloy as the shell is mainly driven by surface energy considerations. We have also obtained Cu–Co nanocomposites (by controlling the concentration of reducing agent) with particle size in the range of 40–200 nm. These Cu–Co nanostructures show ferromagnetic behavior at 4 K. The saturation magnetization of the core–shell (Cu–Co composite @ Cu–Co alloy) nanostructure (125 emu/g) is found to be higher than that of pure Cu–Co nanocomposite or alloy, which may be useful for applications as a soft magnet. Electrochemical studies of these nanocrystalline Cu–Co particles show higher hydrogen evolution efficiencies (5 times) compared to bulk (micrometer-sized) Cu–Co alloy particles