Nanoparticles & Surface Ligands... A Love/Hate Story

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When thinking of nanoparticles, one typically imagines what transmission electron microscopy shows us: a nice assembly of a few thousands metal atoms, all clean and standing amidst the world without a care for the outside environment. This ideal view has little to do with the actual aspect of a nanoparticle in the beaker, on the bench or inside a composite material.

Because nanoparticles have a significant part of their atoms in the outer shells, exposed to the outside, they have to cope with a high surface energy. To stand alive and not coalesce with its neighbours or be dislocated, a nanoparticle has to cope with this surface energy: most of the time, this means having a shell of organic species around them ("ligands" or "surfactants", depending on the context), forming a protective layer and lowering the surface energy. These ligands play a crucial role in the nanoparticle existence, properties, and interaction with the outside world: friends or foes, depends on how you choose them...

Using a set of dedicated tools, which include synthesis&coordination as well as in situ spectroscopy and microscopy, we study the role of the ligands shell on a nanoparticle properties. For example, we examine the reactivity of a nanoparticle in a chosen chemical transformation, eg. Fischer-Tropsch synthesis, as a function of the nature of the ligand shell.

Related papers:

Influence of Copper Precursor on the Catalytic Transformation of Oleylamine during Cu Nanoparticles Synthesis

Copper nanoparticle synthesis was studied by thorough characterization of the organic reactions happening during the synthesis. The reduction of copper(II) acetate by oleylamine resulted in a high amount of water and few by-products while the reduction of copper(II) acetylacetonate resulted in a low amount of water and many products. The nanoparticles showed different abilities to further dehydrogenate and transaminate oleylamine in the synthesis reaction pot. This was explained by the presence of a copper oxide phase in the nanoparticles prepared from copper acetate.

CatSciTech2021

A. Pesesse, S. Carenco, Cat. Sci. Tech.., 2021, 10.1039/D1CY00639H.

Direct Synthesis of N-Heterocyclic Carbene-Stabilized Copper Nanoparticles from a N-Heterocyclic Carbene-Borane

N-Heterocyclic carbene-stabilized copper nanoparticles are synthesized using a NHC-borane and mesitylcopper(I) in thermal conditions (refluxing toluene for 2.5 h). Nanoparticles with a size distribution of 11.6 ± 1.8 nm were obtained. The interaction between Cu NPs and NHC ligands was probed by XPS, showing the covalent binding of the NHC to the surface of the nanoparticles.

Mechanistic studies suggest that the NHC-borane plays two roles: contributing to the reduction of [CuMes]2 to release Cu0 species and providing NHC ligands to stabilize the copper nanoparticles.

ChemEurJ2019-2

X. Frogneux, L. Hippolyte, D. Mercier, D. Portehault, C. Chaneac, C. Sanchez, P. Marcus, F. Ribot, L. Fensterbank, S. Carenco, Chem. Eur. J. 2019, 25, 11481-85.

Surprisingly high sensitivity of copper nanoparticles toward coordinating ligands: consequences for the hydride reduction of benzaldehyde

Functionalized copper nanoparticles are widely used as catalysts. In this study we investigate hydride-assisted reduction reactions with special focus on the structural evolution of copper nanoparticles in the presence of phosphine and nitrogen-based ligands. Ultrasmall nano-objects are formed as key intermediates to produce catalytically active species in the hydrosilylation of benzaldehyde. Moreover, we found that the strength of the hydride donor is essential for the formation of the active catalysts.

CatSciTech

X. Frogneux, F. Borondics, S. Lefrançois, F. D’Accriscio, C. Sanchez, S. Carenco, Catal. Sci. Technol. 2018, 8, 5073-80.

Organometallic Ruthenium Nanoparticles as Model Catalysts for CO Hydrogenation: A Nuclear Magnetic Resonance and Ambient-Pressure X-ray Photoelectron Spectroscopy Study

We present a study of the structure and reactivity of Ru nanoparticles of different sizes for CO hydrogenation using gas phase NMR and mass spectroscopy. In addition the nanoparticles were characterized under reaction mixtures in situ by ambient-pressure X-ray photoelectron spectroscopy. We found that during reaction the Ru is in the metallic state and that the diphosphine ligands (dppb) on the surface of nanoparticles of 1.9 and 3.1 nm, act not only as capping and protecting agents, but stay on the surface during reaction and improve their activity and selectivity towards C2-C4 hydrocarbons.

ACSCatal2014

L. M. Martínez-Prieto, S. Carenco, C. H. Wu, E. Bonnefille, S. Axnanda, Z. Liu, P. F. Fazzini, K. Philippot, M. Salmeron, B. Chaudret, ACS Catal. 2014, 4, 3160

Synthesis and Structural Evolution of Nickel-Cobalt Nanoparticles Under H2 and CO2

CO2 can be converted into hydrocarbon using cobalt-based catalysts. However and to our surprise, core-shell nickel-cobalt nanoparticles produced oxygenated molecules instead (CO, methanol, formaldehyde).

We showed that, as a result of the heating and the presence of reactive gas, nickel was able to migrate to the surface of the nanoparticle. Alongside, small amounts of phosphorus were also found to get exposed to the surface. This phosphorus actually comes from the ligands (TOP) that were required in the first step of the nanoparticles synthesis but partially decomposed upon heating.

The active catalyst should not be seen as a cobalt surface, but rather as a nickel-cobalt alloy containing significant level of phosphide species.

Small2015

S. Carenco, C.-H. Wu, A. Shavorskiy, S. Alayoglu, G. A. Somorjai, H. Bluhm, M. Salmeron, Small 2015, 11, 3045

Reviews papers:

Describing inorganic nanoparticles in the context of surface reactivity and catalysis

Surface and core of inorganic nanoparticles may undergo profound transformations in their environment of use. Accurate description is key to understand and control surface reactivity.

Through a selection of case studies, this feature article proposes a journey from surface science to nanoparticle design, while illustrating state-of-the-art spectroscopies that help provide a relevant description of inorganic nanoparticles in the context of surface reactivity.

ChemComm2018

S. Carenco, Chem. Commun. 2018, 54, 6719-6727

 

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