Periodic Reporting for period 4 - HY-CAT (Multifunctional Hybrid Platforms based on Colloidal Nanocrystals to Advance CO2 Conversion Studies )
Reporting period: 2021-07-01 to 2022-06-30
In reimagining the world’s energy future, while researchers are seeking alternative ways to produce energy, our current dependence on fossil fuels requires us to capture and store the CO2 to prevent reaching unacceptable CO2 levels in the atmosphere. In this scenario, recycling CO2 by converting it into useful chemicals represents an important research area as it will eventually lead to independence from fossil fuels and petroleum. While much progress has been made, this emerging field is still challenged by many technical and scientific questions.
The reaction of CO2 with protons from water can generate a variety of valuable products, such as basic chemicals for materials and liquid fuels for transportation. However, CO2 is a very stable molecule, thus efficient catalysts to break its bonds are needed. These catalysts need to be selective when forming new bonds between carbon, oxygen and hydrogen in order to target one specific combination of these and, thus, one specific final product. The rules of design for efficient and selective CO2 reduction electrocatalysts are yet to be known.
The goal of Hy-Cat was to develop the synthesis of tunable hybrid material platforms comprising different functionalities which could serve as platforms to discover new guiding principles for the design of more selective CO2 reduction catalysts
The reaction of CO2 with protons from water can generate a variety of valuable products, such as basic chemicals for materials and liquid fuels for transportation. However, CO2 is a very stable molecule, thus efficient catalysts to break its bonds are needed. These catalysts need to be selective when forming new bonds between carbon, oxygen and hydrogen in order to target one specific combination of these and, thus, one specific final product. The rules of design for efficient and selective CO2 reduction electrocatalysts are yet to be known.
The goal of Hy-Cat was to develop the synthesis of tunable hybrid material platforms comprising different functionalities which could serve as platforms to discover new guiding principles for the design of more selective CO2 reduction catalysts
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)]
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)]
Hy-Cat had the ambitious goal to contribute to defining the rules for more active, selective and stable catalysts for the electrochemical conversion of CO2. At the start of the project, state-of-the-art was represented by copper foil, which generates 16 different products from CO2. HY-CAT demonstrated that tuning the size and the shape of copper at the nanoscale and constructing interfaces with materials of different chemical nature makes copper more selective towards one specific product. Looking to the future, we hope to demonstrate that long term stability can be achieved so that a technology based on electrochemical CO2 recycling will become closer to be a reality.