Periodic Reporting for period 1 - TRUSol (Towards Rational Understanding of the Fe-quarterpyridine-mediated CO2 Reduction to Solar Fuels)
Reporting period: 2022-08-01 to 2024-07-31
Summary of the context and overall objectives of the project
The search for environmentally benign renewable energy sources as alternatives to fossil fuels is essential given increasing energy demands and the consequences of the associated greenhouse gas emissions. Natural catastrophes derived from global warming threaten billions of living beings, with a disproportionate effect on the poorest and most vulnerable people. In addition, we still source about 85% of our energy from combustion of fossil fuels, which not only makes us dependent on a limited resource, but also continues to drive global warming. This may ultimately lead to a point of no return in terms of the balance of our ecosystem as we know it. For all these reasons, there is an urgent need to shift from fossil fuels to sustainable energy sources. On this basis, European initiatives such as the “2030 climate & energy framework” aim to fight climate change by decreasing greenhouse gas emissions by 40% towards a carbon-neutral Europe by 2050 (European Green Deal). A promising direction in renewable energy research applies artificial photosynthesis to generate energy vectors from water, sunlight and harvested atmospheric CO2. The CO2 reduction reaction (CO2RR) is a highly attractive target since it can reduce environmental CO2 levels, while simultaneously providing a wide variety of solar fuels and useful chemicals. To overcome the kinetic and thermodynamic energy barriers of CO2 reduction, the design of efficient, selective and robust catalysts is indispensable for innovative sustainable technologies. Molecular catalysts based on Co phthalocyanines, Fe porphyrins and Ni macrocycles have shown to be highly active and selective towards CO production. They prove the opportunity to study mechanistic pathways through spectroscopic techniques and their catalytic activity can be tuned via specific ligand design. Moreover, the possibility of their immobilization on (photo)electrodes makes molecular catalysts promising candidates for future use in photoelectrochemical (PEC) devices. In this context, understanding the activity, selectivity, mechanism and deactivation pathways of molecular catalysts is fundamental for guiding rational catalytic design. A wide range of experimental techniques, such as X-ray spectroscopy, (spectro)electrochemistry, Mössbauer and electron paramagnetic resonance (EPR) spectroscopies have recently emerged as powerful tools for experimentally revealing the mechanisms of catalytic CO2 reduction.
The TRUSol project explores the mechanisms and the factors that control catalytic performance through the rational design of ligands and spectroscopic studies. With this goal in mind, we developed a strategy to synthesize a novel family of Fe-based quaterpyridine-based CO2 reduction catalysts. The new complexes were tested electro- and photocatalytically, combining the molecular catalysts with light absorbing materials. Afterwards, the reaction mechanisms was investigated under steady-state and operando conditions using a spectroscopic toolkit that includes EPR, Mössbauer, X-ray absorption, and emission spectroscopy to shed light on the electronic structures, geometries of key reactive intermediates. Then, taking advantage of the ability of synchrotron techniques to selectively irradiate the metal center, laser/X-ray pump/probe time-resolved X-ray absorption spectroscopy was employed to investigate the highly reactive intermediates on the nanosecond to microsecond time scales.
The TRUSol project explores the mechanisms and the factors that control catalytic performance through the rational design of ligands and spectroscopic studies. With this goal in mind, we developed a strategy to synthesize a novel family of Fe-based quaterpyridine-based CO2 reduction catalysts. The new complexes were tested electro- and photocatalytically, combining the molecular catalysts with light absorbing materials. Afterwards, the reaction mechanisms was investigated under steady-state and operando conditions using a spectroscopic toolkit that includes EPR, Mössbauer, X-ray absorption, and emission spectroscopy to shed light on the electronic structures, geometries of key reactive intermediates. Then, taking advantage of the ability of synchrotron techniques to selectively irradiate the metal center, laser/X-ray pump/probe time-resolved X-ray absorption spectroscopy was employed to investigate the highly reactive intermediates on the nanosecond to microsecond time scales.
Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far
TRUSol aims to rationalize the factors controlling the electro- and photo-catalytic performance of a new family of qpy-based CO2 reduction catalysts by combining rational ligand design with catalytic and operando spectroscopic investigations. This combination has the general objective of ultimately revealing the key electronic and geometric features that optimize these molecular catalysts for facile CO2 reduction. The list of objectives, results and main achievements along with the general progress is outlined below:
1. Synthesis and characterization of a new family of quarterpyridine-based ligands complexed with iron.
In line with this objective, the previously reported [Fe(qpy)(H2O)2]2+ CO2R catalyst was prepared by modifying the preparation of the qpy (where qpy is quaterpyridine) ligand. Additionally, a series of derivatives were prepared using synthetic techniques and fully characterized. The family of qpy-based catalysts was extended with the preparation of the Fe complexes containing different electronic substituents to modify the electronic structures. Also, a pyrene-functionalized ligand was prepared, which allowed the immobilization of the system on graphitic supports for heterogeneous CO2R. Finally, the newly prepared ligands were used to prepare the corresponding Co-based derivatives, thereby extending the family to other metals.
2. Study of their redox properties and photo- as well as electro-catalytic performance.
The Fe- and Co-based systems prepared have been studied by cyclic voltammetry (CV), differential pulse voltammetry (DPV), and controlled potential electrolysis (CPE) to investigate their redox properties and activity towards CO2RR in organic/aqueous solutions in the absence and presence of the substrate (CO2). Additionally, the light-driven activity of the complexes has been investigated in ternary mixtures of Catalyst/PS/SD (where PS is photosensitizer, and SD is sacrificial electron donor. These experiments were used to study the conditions used in our spectroscopic measurements and select an appropriate candidate. The selected candidate for the spectroscopic studies proposed in TRUSol was the [FeII(qpy)(L)2]2+ CO2R catalyst.
3. Characterization of the electronic structures of the reaction intermediates by steady state, operando and time-resolved spectroscopic techniques.
In order to gain mechanistic insights, the electronic structures of the selected candidate along with its corresponding reduced species and resting states were explored in WP3 and WP4 using a combination of steady-state and time-resolved spectroscopic techniques. This involved the use of Fe K-edge X-ray absorption spectroscopy to analyze the 1s to 3d pre-edge and 1s to 4p rising edge features sensitive to the oxidation state of the metal center, the coordination environment, and the local symmetry. Additionally, Mössbauer, EPR, and Kβ (metal 3p to 1s transition) XES spectroscopy were applied to further probe the electronic structure of these complexes by monitoring changes in the spin configuration. Overall, these studies concluded that the prepared [FeII(qpy)(L)2]2+ contains two aquo ligands in the solid state with an electronic configuration of S = 2. Upon dissolution, the system partially substitutes the aquo ligands for the corresponding solvent (e.g. MeCN), and a change in the spin configuration occurs from high spin (HS) to low spin (LS). Additionally, the one- and two-electron reduced steady-state intermediates were prepared and characterized as well, resulting in [FeI(qpy)(L)]+ and [FeI(qpy●-)] systems, with a mixed electronic configuration for the first (S = 3/2 and S = 1/2) and LS (S = 0) for the second intermediate.
Subsequently, laser/X-ray pump/probe time-resolved X-ray absorption spectroscopy was used to study the photocatalytic intermediates. We successfully identified two processes. The first was the reduction from [FeII(qpy)(L)2]2+ to [FeI(qpy)(L)]+, which takes place on the microsecond time scale (τ = 4.4 µs). During the CO2R reaction, [FeI(qpy)CO] species accumulate in the medium, and a second transient process, tentatively assigned to the [FeI(qpy)CO] to [FeI(qpy)] reaction, was monitored on the microsecond time scale (τ = 53 µs). Time-resolved Fourier transform infrared spectroscopy is currently being carried out to further confirm the nature of the second light-triggered process observed by time-resolved X-ray absorption spectroscopy.
1. Synthesis and characterization of a new family of quarterpyridine-based ligands complexed with iron.
In line with this objective, the previously reported [Fe(qpy)(H2O)2]2+ CO2R catalyst was prepared by modifying the preparation of the qpy (where qpy is quaterpyridine) ligand. Additionally, a series of derivatives were prepared using synthetic techniques and fully characterized. The family of qpy-based catalysts was extended with the preparation of the Fe complexes containing different electronic substituents to modify the electronic structures. Also, a pyrene-functionalized ligand was prepared, which allowed the immobilization of the system on graphitic supports for heterogeneous CO2R. Finally, the newly prepared ligands were used to prepare the corresponding Co-based derivatives, thereby extending the family to other metals.
2. Study of their redox properties and photo- as well as electro-catalytic performance.
The Fe- and Co-based systems prepared have been studied by cyclic voltammetry (CV), differential pulse voltammetry (DPV), and controlled potential electrolysis (CPE) to investigate their redox properties and activity towards CO2RR in organic/aqueous solutions in the absence and presence of the substrate (CO2). Additionally, the light-driven activity of the complexes has been investigated in ternary mixtures of Catalyst/PS/SD (where PS is photosensitizer, and SD is sacrificial electron donor. These experiments were used to study the conditions used in our spectroscopic measurements and select an appropriate candidate. The selected candidate for the spectroscopic studies proposed in TRUSol was the [FeII(qpy)(L)2]2+ CO2R catalyst.
3. Characterization of the electronic structures of the reaction intermediates by steady state, operando and time-resolved spectroscopic techniques.
In order to gain mechanistic insights, the electronic structures of the selected candidate along with its corresponding reduced species and resting states were explored in WP3 and WP4 using a combination of steady-state and time-resolved spectroscopic techniques. This involved the use of Fe K-edge X-ray absorption spectroscopy to analyze the 1s to 3d pre-edge and 1s to 4p rising edge features sensitive to the oxidation state of the metal center, the coordination environment, and the local symmetry. Additionally, Mössbauer, EPR, and Kβ (metal 3p to 1s transition) XES spectroscopy were applied to further probe the electronic structure of these complexes by monitoring changes in the spin configuration. Overall, these studies concluded that the prepared [FeII(qpy)(L)2]2+ contains two aquo ligands in the solid state with an electronic configuration of S = 2. Upon dissolution, the system partially substitutes the aquo ligands for the corresponding solvent (e.g. MeCN), and a change in the spin configuration occurs from high spin (HS) to low spin (LS). Additionally, the one- and two-electron reduced steady-state intermediates were prepared and characterized as well, resulting in [FeI(qpy)(L)]+ and [FeI(qpy●-)] systems, with a mixed electronic configuration for the first (S = 3/2 and S = 1/2) and LS (S = 0) for the second intermediate.
Subsequently, laser/X-ray pump/probe time-resolved X-ray absorption spectroscopy was used to study the photocatalytic intermediates. We successfully identified two processes. The first was the reduction from [FeII(qpy)(L)2]2+ to [FeI(qpy)(L)]+, which takes place on the microsecond time scale (τ = 4.4 µs). During the CO2R reaction, [FeI(qpy)CO] species accumulate in the medium, and a second transient process, tentatively assigned to the [FeI(qpy)CO] to [FeI(qpy)] reaction, was monitored on the microsecond time scale (τ = 53 µs). Time-resolved Fourier transform infrared spectroscopy is currently being carried out to further confirm the nature of the second light-triggered process observed by time-resolved X-ray absorption spectroscopy.
Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)
A fundamental scientific challenge is the pursuit of clean and sustainable fuel sources. The TRUSol project has been dedicated to a comprehensive understanding of the mechanistic factors governing catalytic performance in CO2 reduction reactions (CO2RR) driven by molecular catalysts, particularly those with qpy-based ligands. Although the findings from the TRUSol project at MPI-CEC have not yet been published, they are expected to significantly advance the field by optimizing catalysts to enhance their effectiveness. This progress is crucial for developing new CO2RR catalysts that, when immobilized on (photo)electrodes, can work synergistically with water oxidation catalysts or other components, utilizing sunlight as an energy source to produce solar fuels and valuable chemicals.
TRUSol project has successfully developed a new family of Fe qpy-based molecular CO2R catalysts. The [Fe(qpy)(H2O)2]2+ was used to investigate the reaction mechanism, where the steady state intermediates [FeI(qpy)(L)]+ and [FeI(qpy●-)] have been fully characterized by spectroscopic techniques, which remained unraveled in the literature. Additionally, the highly reactive intermediates were studied by time-resolved techniques understanding the kinetics for the FeII-to-FeI formation, and a second process that is currently being investigated by TR-FTIR spectroscopy. Additionally, the newly prepared ligands were used to prepare the corresponding Co-based derivatives, thereby delivering a new family of complexes. The systems were employed to understand the factors that control the catalytic performance. A electrochemical studied identified the nature of the real active species under CO2R to CO catalytic conditions, where the [CoII(QPYMe2)(PhO)(ClO4)]+ is the responsible of the catalysis, and used to propose a catalytic cycle. Finally, TRUSol project has successfully developed a novel CO2RR hybrid material, Copyn@CNT, which demonstrates high current densities (ca. 50 mA cm-2) at low potentials (-1.2 V vs NHE) after immobilization on conductive supports. This innovative hybrid material improved achieved current densities and holds promise for the future development of solid-state photocathodes in photoelectrochemical (PEC) cells.
TRUSol project has successfully developed a new family of Fe qpy-based molecular CO2R catalysts. The [Fe(qpy)(H2O)2]2+ was used to investigate the reaction mechanism, where the steady state intermediates [FeI(qpy)(L)]+ and [FeI(qpy●-)] have been fully characterized by spectroscopic techniques, which remained unraveled in the literature. Additionally, the highly reactive intermediates were studied by time-resolved techniques understanding the kinetics for the FeII-to-FeI formation, and a second process that is currently being investigated by TR-FTIR spectroscopy. Additionally, the newly prepared ligands were used to prepare the corresponding Co-based derivatives, thereby delivering a new family of complexes. The systems were employed to understand the factors that control the catalytic performance. A electrochemical studied identified the nature of the real active species under CO2R to CO catalytic conditions, where the [CoII(QPYMe2)(PhO)(ClO4)]+ is the responsible of the catalysis, and used to propose a catalytic cycle. Finally, TRUSol project has successfully developed a novel CO2RR hybrid material, Copyn@CNT, which demonstrates high current densities (ca. 50 mA cm-2) at low potentials (-1.2 V vs NHE) after immobilization on conductive supports. This innovative hybrid material improved achieved current densities and holds promise for the future development of solid-state photocathodes in photoelectrochemical (PEC) cells.