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Nanoparticles as Partners in Frustrated Lewis Pairs: Boosting the Surface Reactivity of Inorganic Nanoparticles

Periodic Reporting for period 2 - NanoFLP (Nanoparticles as Partners in Frustrated Lewis Pairs: Boosting the Surface Reactivity of Inorganic Nanoparticles)

Reporting period: 2019-07-01 to 2020-12-31

Catalytic processes are key to provide higher-value molecules, such as hydrocarbons, from abundant ones, such as CO2. For more than a century, heterogeneous catalysis has been using metal-containing nanoparticles as catalysts to perform the conversion of chemicals under fairly harsh reaction conditions (typically, several bars of pressure and a temperature higher than 150°C). In this paradigm, the catalytic process can hardly produce fragile molecules with a high degree of functionality, because these would be destroyed (or burnt) right away.
The NanoFLP project is shifting the paradigm by targeting much softer catalytic conditions, typically, with pressures below 3 bars and temperatures below 150°C. The key is to boost the reactivity of known metal or metal oxide nanoparticles.
This is attempted using the “Frustrated Lewis Pair” concept. Associating bulky and strong Lewis acid and base creates a Frustrated Lewis Pair (FLP). Traditionally, both FLP partners are molecules. Molecular FLPs have shown excellent abilities to catch and dissociate small molecules such as H2 in a heterolytic way, under mild conditions. The driving force is the destabilization of the initial acid-base adduct, sterically frustrated: it liberates a reactive pocket that catches the small molecule guest, and strongly lowers the activation energy for bond dissociation.
The pristine and challenging concept of NanoFLP consists in replacing one of the molecular FLP partner, either the acid or the base, by an inorganic nanoparticle: the other molecular partner will adsorb on the surface and boosts the reactivity of the nanoparticle by creating a frustrated active site.
Three families of inorganic nanoparticles (metals, acidic oxides, basic oxides) are being investigated, illustrating the two schemes: nanoparticle is either the Lewis acid or the Lewis base. We use probe molecules (such as CO2, H2) to investigate the nature and reactivity of the active sites. All reactions are attempted under much milder conditions (rt.-150 °C, 1-3 bars) than those required using similar nanoparticles in the absence of the molecular partner.
We also work to describe the nanoparticle surface and the dynamics of the molecular partner using benchtop and synchrotron spectroscopies with in situ cells: infrared, nuclear magnetic resonance in solution, X-ray absorption and near-ambient-pressure X-ray photoelectron spectroscopy.
In the last stage of the project, we will take advantage of the several active sites that one nanoparticle can bear to achieve combined reactions of two small molecules (reactants) on a single NanoFLP. NanoFLP proposes a new type of active site for utilizing small molecules as sources of C, N, S and O. It will open an avenue in the design of reactive interfaces, eg. for catalysis and sensors.
After 2.5 years, and with a team of six people at this point, we have produce a number of NanoFLP candidates, based on metal and metal oxide nanoparticles. We have decorated them with organic partners (the other half of the NanoFLP pair) and started to evaluate their catalytic activity in model reactions (eg. using benzaldehyde) as well as with gases (CO2 and H2). The consequence of the catalytic test on the nanoparticle’s integrity were analyzed in detail. In some cases, we found that the nanoparticles were partly decomposed and we discarded the corresponding systems. We also used X-ray spectroscopies in synchrotron Soleil to further understand the surface state of the nanoparticles before, during and after the catalytic reaction. This is providing us with key insights to tune the composition and ligand coverage of the nanoparticles, and opens the way to a better catalytic activity. Some of these results were already presented at conferences, one concept article was published, and more are being prepared for submission to materials sciences, chemistry, and catalysis.
Because our project is at the cross-over between molecular chemistry and materials sciences, we borrow concepts and tools from both fields. This translates in new methodologies, such as dedicated tools to analyzed highly air-sensitive nanoparticles dispersed in organic solvents. Moreover, we are uncovering unexpected restructuring effects, meaning that we know understand much better how the nanoparticles are transformed as a result of the catalytic process. We also worked on a better control of the surface nature of nanoparticles, which is key to optimize their catalytic activity. By the end of the project, we hope to prove the relevance of the NanoFLP concept and provide a catalytic design that can convert a couple of gases into products, under soft reaction conditions.