The HY-CAT hybrid catalysts included size and shape-controlled metal nanocrystals (NCs) interfaced with porous coordination polymers (WP1), metal oxides (WP2) and organic moieties (WP3).
WP-1: Herein, the first example were Ag@Al-TCCP NC@MOF hybrids (TCCP is a porphyrin linker) [Guntern et al Angew. Chemie Int. Ed. (2019)]. We embedded Ag NCs in the MOF while still preserving electrical contact with a conductive substrate. This key feature allowed the investigation of the Ag@Al-PMOFs as CO2RR electrocatalysts. We showed that the pristine interface between the NCs and the MOFs accounts for electronic changes in the Ag, which suppress the hydrogen evolution reaction (HER) and promote the CO2RR. Furthermore, we found an increased morphological stability of the Ag NCs when combined with the Al-PMOF.
Because of the scarce long-term stability of the Al-TCCP MOF during electrocatalysis, we moved from MOFs to COFs, which are more resilient to degradation CO2RR conditions. We prepared tunable NC@LZU1 (LZU1 is an imine-linked COF) core@shell hybrids by combining colloidal chemistry and microwave-assisted syntheses, an approach which allows the tailoring of the shell thickness while ensuring COF crystallinity in the presence of the NCs. The synthetic route is general, thus revealing a new way to impart functionalities to COFs [Guntern et al Chem. Mater. (2021)].
Unfortunately, the Cu NC surface oxidize during the formation of the LZU1, which challenges the retention of its morphology, important for CO2RR selectivity. Therefore, we moved to a different system, synthesizable under milder conditions. We successfully prepared Cu@CoTPyP (TpyP is another porphyrin linker) core@shell with retained Cu NC morphology through controlled self-assembly of the TPyP at room temperature. However, we found that the hybrids were less selective towards CO2RR than the pristine Cu NCs, which we correlated to the amorphous nature of the CoTPyP shell decreases the accessible Cu active sites to CO2. This result inspires further synthetic development
WP-2: Herein, the first example were Cu/CeO2-x NCs [Varandili et al. ACS Catal. (2019)]. A colloidal seeded-growth synthesis was developed to connect the two highly mismatched domains (Cu and CeO2-x) through an interface. The Cu/CeO2-x NCs exhibited state-of-the-art selectivity toward CO2RR and were exceptionally selective for methane, ∼5 times more than a Cu and CeO2-x physical mixture. Operando spectroscopies and theory revealed the active site motif to be oxygen vacancies at the interface.
Following this first success, we further developed the seeded growth approach to tune the number of heterojunctions and to access Cu/(CeO2)n hybrids, with n=1, 2, 6 representing the number of Cu/CeO2 interfaces in each particle. Surprisingly, we found that increasing n inhibits CO2RR, which highlights that a balance between interfacial area and accessibility to the active catalytic sites must be sought after to exploit synergies arising at the interface between different domains [Varandili et al. Chem. Sci. (2020)].
We then used Cu/ZnO hybrid NCs as pre-catalysts of CuZn to elucidate the features promoting methane and ethanol in CO2RR. We found that the Zn content in the surface alloy dictates the selectivity, with 5% Zn forming methane and 19% Zn forming ethanol. Theory explained that the combination of electronic effects, dominating in the former, and tandem effects, dominating in the latter, accounts for such selectivity. These findings marked a step ahead towards understanding structure–property relationships in bimetallic catalysts for the CO2RR and their rational tuning to increase selectivity towards target products, especially alcohols [Varandili et al. Chem. Sci. (2021)].
WP-3: Herein, the first study was dedicated to rationally designed imidazolium-based ligands functionalizing the surface of Ag NCs. This study allowed us to learn about the importance of anchoring groups and length of the aliphatic chains in organic functionalizing moieties [Pankhurst et al Chem. Sci. (2019)].
Continuing along this new direction, we investigated the impact and fate of organic ligands on Cu NCs during CO2RR. We showed that the latter desorb from the surface at a cathodic potential that depends on their binding strength with the metal surface, rather than their own electroreduction potentials. This work provides a criterion to select labile ligands versus ligands that will persist on the surface, thus offering opportunity for interface design [Pankhurst et al Chem. Sci. (2020)].
While this was not included in the initial planning, the fast progress in the field required us to start testing our catalysts under more realistic conditions. We successfully demonstrated that the selectivity of our shape-controlled Cu NCs towards CO2RR detected in the low current density device can be translated into the high current density gas-fed electrochemical cell [De Gregorio et al. ACS Catal. (2020)]
The results of this work inspired us to continue tuning the shape of the Cu NCs as it offers alone a great knob to tune selectivity. Thus, we discovered that short-chain phosphonate ligands generate intermediate polymer lamellae that template anisotropic Cu NCs, being primarily triangular plates. In contrast, lamellae formed from long-chain ligands lose their structure and form spherical Cu NCs. [Pankhurst et al. J. Am. Chem. Soc. (2022)]
