International audienceno abstrac

We present here a complete study on a synthesis of nickel nanoparticles involving the reaction of [Ni(acac)2] with oleylamine (OA) and trioctylphosphine (TOP) reactants, whose simultaneous presence and relative amounts are paramount. The role of every reactant in the nucleation and growth of the nanoparticles has been delineated: OA is the reductant and thus controls the nucleation rate, meanwhile TOP provides a tunable surface stabilization through coordination on the Ni(0) surface. This result leads us to a design synthesis providing tailored monodispersed nanoparticles in a wide range (2−30 nm), which allows self-consistent studies of size-induced changes in catalytic and magnetic properties. Additionally, the growth mechanism is demonstrated to include an aggregation step which thus correlates with the polycrystalline feature of the nanoparticles obtained through this process. Moreover, the critical influence of the phosphine in this system was demonstrated a) for the outcome of the ripening mechanisms: defocusing effect and/or reshaping of the particles and b) for the surface properties: binding of the phosphine to the surface and its behavior toward oxidation was investigated by FTIR. Preliminary SQUID measurements show the impact of crystallites size on the magnetic properties.

In situ soft X-ray absorption spectroscopy (XAS) was employed to study the adsorption and dissociation of carbon monoxide molecules on cobalt nanoparticles with sizes ranging from 4 to 15 nm. The majority of CO molecules adsorb molecularly on the surface of the nanoparticles, but some undergo dissociative adsorption, leading to oxide species on the surface of the nanoparticles. We found that the tendency of CO to undergo dissociation depends critically on the size of the Co nanoparticles. Indeed, CO molecules dissociate much more efficiently on the larger nanoparticles (15 nm) than on the smaller particles (4 nm). We further observed a strong increase in the dissociation rate of adsorbed CO upon exposure to hydrogen, clearly demonstrating that the CO dissociation on cobalt nanoparticles is assisted by hydrogen. Our results suggest that the ability of cobalt nanoparticles to dissociate hydrogen is the main parameter determining the reactivity of cobalt nanoparticles in Fischer-Tropsch synthesis.

Unveiling the mechanism of electrocatalytic processes is fundamental for the search of more efficient and stable electrode materials for clean energy conversion devices. Although several in situ techniques are now available to track structural changes during electrocatalysis, especially of water oxidation, a direct observation, in real space, of morphological changes of nanostructured electrocatalysts is missing. Herein, we implement an in situ electrochemical Transmission Electron Microscopy (in situ EC-TEM) methodology for studying electrocatalysts of the oxygen evolution reaction (OER) during operation, by using model cobalt oxide Co3O4 nanoparticles. The observation conditions were optimized to mimic standard electrochemistry experiments in a regular electrochemical cell, allowing to perform cyclic voltammetry and chronopotentiometry in similar conditions in situ and ex situ. This in situ EC-TEM method enables us to observe the chemical, morphological and structural evolutions occurring in the initial nanoparticle-based electrode exposed to different aqueous electrolytes and under OER conditions. The results show that surface amorphization occurs, yielding a nanometric cobalt (oxyhydr)oxide-like phase during OER. This process is irreversible and occurs to an extent that has not been described before. Furthermore, we show that the pH and counter-ions of the electrolytes impact this restructuration, shedding light on the materials properties in neutral phosphate electrolytes. In addition to the structural changes followed in situ during the electrochemical measurements, this study demonstrates that it is possible to rely on in situ electrochemical TEM to reveal processes in electrocatalysts while preserving a good correlation with ex situ regular electrochemistry.

The interplay between crystallization and phosphorus diffusion in the versatile synthesis of metal phosphide nanoparticles from well-defined metal nanoparticles is studied by using a favorable "P(0)" source for mechanistic studies: white phosphorus. In this study, the reaction of Ni, Fe, Pd, and Cu nanoparticles with P 4 was quantitative even at relatively low temperatures thanks to the high reactivity of this soluble "P" source. Intermediate amorphous alloys could be identified for the first time in the case of Fe and Pd, while the quantitative character of the reaction provided a selective and controlled access to Pd 5 P 4 versus PdP 2 and Cu 3 P versus CuP 2 . Morphological evolution of the nanoparticles with temperature and M/ P stoichiometry was also discussed and provided new insights in the kinetics of the reaction in each case. Hollow Ni2P and FeP nanoparticles were finally obtained while the particularly high stability of the amorphous plain Pd3P nanoparticles was uncovered.

Nanoscaling of the nickel phosphides profoundly alters their domains of stability. An original mechanism of "nanoscale-induced phase segregation" was uncovered in the present study: the appearance of two crystallized phases inside each single nanoparticle (here, Ni 2 P and Ni fcc), where the single-phase product (Ni 3 P) would have been preferred at the bulk scale. This behavior was obtained by reacting at low temperature (<220 °C) substoichiometric amounts of white phosphorus (P 4 ) on well-defined monodisperse Ni nanoparticles in solution. Phosphorus insertion inside the Ni fcc nanoparticles triggers the crystallization of a Ni 2 P core surrounded by a Ni shell. The crystallization process was monitored by HRTEM, EFTEM, XRD, and SQUID analyses and revealed a direct transformation of Ni fcc to a coreÀshell structure without any other Ni x P y crystallized intermediate. This coreÀshell NiÀP system was tuned by adjusting the amount of P 4 added, providing tunable magnetic shells supported on monodisperse nanoparticles.

Elsevier Carenco, S.; Leyva Perez, A.; Concepción Heydorn, P.; Boissiere, C.; Mezailles, N.; Corma Canós, A. (2012) Well-defined 25 nm nickel phosphide nanoparticles act as a colloidal catalyst for the chemoselective hydrogenation of terminal and internal alkynes. Cis-alkenes are obtained in mild conditions (85°C, a few hours) with good conversion and selectivity. The phosphorus inserted in the Ni-P nanoparticles is critical for the selectivity of the nanocatalyst. Mechanistic investigations support a pre-reduction of the catalyst for its activation. They pinpoint the occurrence of C-H bond cleavage in terminal alkynes during the reaction.

White phosphorus, P4, was reacted in a stoichiometric manner with Ni(0) complexes or particles to produce nanoparticles of nickel phosphide.