The electrochemical CO2 reduction reaction (CO2RR) into valuable chemicals has the potential to realize a carbon-neutral energy cycle. Developing catalysts that can achieve high selectivity towards one of the possible reduction products is one of the biggest challenges in this field. Catalysts possessing one single type of active sites cannot fulfil this need, yet to establish design rules for bifunctional catalysts is not trivial. To have material platforms which allow to study the sensitivity of the catalytic behaviour to structural and compositional features of the catalyst is of the uttermost importance to inform catalyst design. This thesis aims at combining well-defined colloidal nanocrystals (NCs) and porous reticular frameworks as hybrid materials to explore their synergistic interactions as CO2RR catalysts. These classes of materials were chosen because they possess complementary chemical properties which are appealing towards eventually construct an ideal catalyst with high performance. However, one of the challenges is to obtain these hybrids with tunable structural features in an unrestricted compositional range. The overall objective is to design and explore synthetic strategies for the formation of new multifunctional composites consisting of colloidal NCs and metal-organic/covalent organic frameworks (MOFs/COFs). First, a synthetic approach for the formation of NC/MOF hybrid thin films is proposed, which enables their application as a novel catalytic platform for CO2RR. We demonstrate that a combination of colloidal chemistry, atomic-layer deposition and solvothermal synthesis allows to form an intimate contact between Ag NCs and an Al-PMOF matrix. Our tuneable Ag@Al-PMOF hybrid catalysts show promoted CO2RR which is attributed to electronic changes in the Ag NC surface and minor mass-transport effects. Moreover, the hybrids improve the morphological stability of the Ag NCs under CO2RR conditions. We show that the synthetic approach can be further extended to form hybrids with other metallic NCs, thereby representing a new tool for the preparation of composite catalysts for CO2RR. One of the limitations of thin film composites is that synthesis conditions might need to be adapted for different substrates. To prepare hybrid materials in the form of dispensible colloidal solutions is highly appealing and better suited for mechanistic studies. The latter are important because understanding the chemistry driving the formation of such composites is eventually the key for achieving compositional and structural tunability. In the second part, we introduce a new strategy to encapsulate various colloidal NCs in the imine-linked COF LZU1. The synthetic approach allows us to tune the shell thickness and crystallization of the COF in the presence of the NCs. After understanding the nucleation and growth mechanisms of the hybrids, we extend the developed synthesis to obtain multi-layered structures with controlled spatial distribution of various NCs with different compositions. Altogether, we demonstrate that our approach can be applied to prepare hybrids with new functionalities that derive from the encapsulated NCs. Overall, this thesis proposes two different approaches which combine colloidal and reticular chemistry to synthesize tunable material platforms which might aid the design of CO2RR catalysts as well as contributing to other applications which require multifunctional porous materials.
EPFL2021
