Design and Polymerization Behavior of Carbocyclic-Fused (Imino)Pyridine-Ligated Late-Transition-Metal Catalysts

A major roadblock in realizing the large-scale production of hydrogen via electrochemical water splitting is the lack of cost-effective and highly efficient catalysts for the oxygen evolution reaction (OER). In this regard, computational research has driven important developments in the understanding and the design of heterogeneous OER catalysts by establishing linear scaling relations. These relations are of paramount importance since they drastically reduce the amount of time required to traverse the vast chemical search space of potential OER materials. In this work, we interrogate 17 of the most active molecular OER catalysts known to date based on different transition metals (M= Ru, Mn, Fe, Co, Ni, and Cu), and show that they obey the linear scaling relations established for metal oxides. This demonstrates that the conventional OER descriptor established for heterogeneous systems can also be applied to rapidly screen novel molecular catalysts. However, we find that this descriptor underestimates the activity of some of the most active OER complexes as it does not consider the additional one-electron oxidation that these undergo prior to O–O bond formation. Importantly, we show that this additional step allows certain molecular catalysts to circumvent the “overpotential wall” observed for heterogeneous systems (i.e. 370 mV), leading to an enhanced performance in agreement with experimental observations. To describe the activity of such highly active catalysts, we propose a new OER descriptor that opens up the possibility of designing molecular catalysts exhibiting zero theoretical overpotential. With all this knowledge, we establish the fundamental principles for the rational design of ideal OER catalysts to advance the development of water splitting technologies.

Molecular single iron site catalysts for electrochemical nitrogen fixation under ambient conditions

ConspectusThe conversion of CO2 into reduced carbon products by valorizing sunlight as the energy source is a highly attractive strategy to simultaneously mitigate CO2 emissions and generate renewable fuels. Metal complexes can serve as versatile molecular catalysts for constructing high-performance light-driven systems for CO2 reduction owing to their well-defined structures for facile mechanism-based synthetic optimization. To drive the CO2 reduction reaction mediated by molecular catalysts, suitable light absorbers, such as molecular photosensitizers (PSs) or solid-state semiconductors are desirable. Although considerable attention has been dedicated to the synthetic modifications in both molecular catalysts and light absorbers, further improvement using these mature components has reached a plateau. This limitation underscores the need for new design strategies. In this regard, fine-tuning interactions between catalysts and light absorbers holds great promise, as it offers the potential to substantially improve electron transfer kinetics beyond those observed in noninteracting systems, thereby enhancing overall photocatalytic efficiency.We introduce this Account first with an overview comprised of advantages and limitations of molecular systems for photocatalytic CO2 reduction. We then describe our strategies for modulating charge transfer processes between molecular catalysts and light absorbers by installing additional intermolecular or interfacial interactions, tailored for homogeneous and heterogeneous photocatalytic systems, respectively. For homogeneous systems, we highlight the use of dynamic interactions in supramolecular preassemblies to enhance electron transfer between molecular catalysts and PSs. Representative examples illustrate how such dynamic interactions significantly improve electron transfer efficiency, resulting in state-of-the-art photocatalytic performance. We also describe methods for probing the existence, strength, and functional roles of these interactions in CO2 photoreduction. For heterogeneous systems, we will discuss the immobilization of molecular catalysts on semiconductor surfaces as molecular hybrid photocatalysts in CO2 reduction. This section focuses on the correlation among anchoring interactions, interfacial electron transfer dynamics, and overall photocatalytic performance. Finally, we highlight the current challenges and outline future directions for the advancement of interaction-driven molecular systems in CO2 photoreduction. Overall, this Account is intended to provide strategies on rational design and optimization of CO2 photoreduction systems, while offering mechanistic insights into interaction-dependent charge transfer pathways.

Molecular copper catalysts for electro-reductive homocoupling of CO2 towards C2 compounds

Devising cost effective methods for efficiently capturing and storing solar energy is among the grand challenges of science (1). We are using insights from studies of natural photosynthetic systems (2) to develop bioinspired materials for photo-electrochemical water oxidation and solar fuel production by using molecular catalysts and dyes attached to mesoporous metal oxide photoanodes. Molecular catalysts are known for their high activity and tunability, but their solubility and limited stability often restrict their use in practical applications. We are developing anchoring chemistry to attach molecular water-oxidation catalysts to metal oxide surfaces (3), which not only greatly increases the stability of the molecular catalyst but also improves the catalytic performance of the oxide material. Our progress on the development and characterization of molecular iridium, copper and manganese water-oxidation catalysts (4-5), along with their application for photoelectrochemical water oxidation (6) and solar fuel production, will be discussed. Directing Matter and Energy: Five Challenges for Science and the Imagination, U.S. Department of Energy, Washington, DC, December 2007.D.J. Vinyard and G.W. Brudvig, Annu. Rev. Phys. Chem. (2017) 68, 101.J.L. Troiano, R.H. Crabtree and G.W. Brudvig, ACS Appl. Mater. Interfaces (2022) 14, 6582.S.W. Sheehan et al., Nature Comm. (2015) 6, 6469.K.J. Fisher et al., ACS Catalysis (2017) 7, 3384.Y. Zhao et al., Proc. Natl. Acad. Sci. U.S.A. (2018) 115, 2902.

A major roadblock in realizing large-scale production of hydrogen via electrochemical water splitting is the cost and inefficiency of current catalysts for the oxygen evolution reaction (OER). Computational research has driven important developments in understanding and designing heterogeneous OER catalysts using linear scaling relationships derived from computed binding energies. Herein, we interrogate 17 of the most active molecular OER catalysts, based on different transition metals (Ru, Mn, Fe, Co, Ni, and Cu), and show they obey similar scaling relations to those established for heterogeneous systems. However, we find that the conventional OER descriptor underestimates the activity for very active OER complexes as the standard approach neglects a crucial one-electron oxidation that many molecular catalysts undergo prior to O–O bond formation. Importantly, this additional step allows certain molecular catalysts to circumvent the “overpotential wall”, leading to enhanced performance. With this knowledge, we establish fundamental principles for the design of ideal molecular OER catalysts.

Exploring chemistry with single-molecule and -particle fluorescence microscopy

The demanding multi-electron transfer process renders the oxygen evolution reaction (OER) a bottleneck for achieving efficient clean hydrogen generation via water electrolysis.[1] Over the past decades, two main categories of catalysts, namely, homogeneous molecular and heterogeneous catalysts, have been implemented for the OER. However, due to the sluggish reaction kinetics and the aggressive reaction media of the OER, the structural integrity of both homogeneous molecular and heterogeneous catalysts faces the dramatic challenges. This calls for a thorough understanding and close monitoring of OER catalysts under their operando reaction conditions.Over the last years, we have been combining a variety of in situ/operando spectroscopy approaches with computational studies toward the comprehensive understanding of our designed catalysts at the atomic level. With this information in hand, we established a full identification of catalytically active species and sites for several systems, some of which are discussed here.First, inspired by nature’s {Mn4CaOx} OER complexes, we recently reported on the design of a tetramer Cu-bipyridyl complex for the OER.[2] Structural characterizations demonstrated a new defect-cubane structure of our designed complex, [Cu4(pyalk)4(OAc)4](ClO4)(HNEt3). We found that this Cu-bipyridyl complex can further undergo structural transformations into two unique complexes under different solution conditions, namely Cu-dimer and Cu-monomers, as revealed by in situ UV-vis and ERP characterizations as well as electrospray ionization mass spectrometry. Specifically, the Cu-monomers can be only formed in presence of carbonate buffer (pH 10.5). Otherwise, the structural transformation into a Cu-dimer complex [Cu2(pyalk)2(OAc)2(H2O)] is dominant under solution conditions. Furthermore, electrochemical characterizations revealed an overpotential of 960 mV to reach a current density of 0.1 mA/cm2 of our designed Cu-dimer catalysts, which is comparable with significant Cu-based OER electrocatalysts. To gain in-depth insights into their conversion processes, postcatalytic characterizations of Cu-based molecular catalysts were carried out based on X-ray photoelectron/absorption (XAS/XPS) spectroscopy appraoches. The results showed that nanosized Cu-oxide-based species were formed in situ in Cu-based molecular catalysts after the OER. Our study highlights the crucial role of the structural integrity of molecular catalysts in solutions for their efficient design.In parallel, we explored the structural transformations of heterogeneous electrocatalysts during the OER. As a typical example, we developed a cost-effective and high-performance NiFe-based coordination polymer (referred to as NiFe-CP) as OER electrocatalyst, which is being investigated as the best-known bimetallic combination for the OER.[3] A central element of our study is the monitoring of true catalytically active species. Results from spectroscopic characterizations revealed a kinetic restructuring of NiFe-CPs into NiFe (oxy)hydroxides during the OER. To further improve the OER activity, we introduced a facile NaBH4-assisted reduction strategy to prepare low-crystalline reduced NiFe-CP (denoted as R-NiFe-CP) OER electrocatalysts with rich structural deficiencies. These catalysts can maintain a very low overpotential of 225 mV at 10 mA/cm2 for over 120 h without any performance decline, outperforming many recent reported bimetallic OER electrocatalysts. As revealed by XAS characterizations and density functional theory (DFT) calculations, engineering of structural deficiencies not only tunes the local electronic structure but also optimizes the rate-determining step towards facilitated OH- adsorption. Noteworthy, the true OER active sites of R-NiFe-CPs originate from the in situ reconstructed Ni-O-Fe motifs. However, fundamental questions, as to (a) the role of engineered structural deficiencies in the generation of active species and (b) facilitating the formation of catalytically active dual oxygen-bridged moieties, need to be answered. Combination of time-resolved operando XAS monitoring and DFT calculations enables the tracking and understanding of the kinetic changes of active species and sites under the operando reaction conditions. We found that the OER of R-NiFe-CPs relies on the in situ formation of crucial high-valent NiIV-O-FeIVO moieties.[4] Furthermore, an anionic engineering strategy through heteroatom sulfur incorporation was carried out to obtain S-R-NiFe-CP showing faster OER kinetics. Importantly, engineered sulfur content promotes the generation of catalytically active S-NiIVO-FeIVO motifs prior to the OER. This offers a lower onset potential to trigger the OER of S-R-NiFe-CPs compared to sulfur-free R-NiFe-CPs. Moreover, our results also suggest a dual-site mechanism pathway of S-R-NiFe-CPs during the OER, in which the O-O bond formed atop the S-NiIVO-FeIVO moieties. Such an anionic modulation strategy for promoting the formation of catalytically active structural moieties and for optimizing the OER kinetics opens an avenue to optimize a wide range of heterogeneous catalysts for the OER.[1] Zhao, Y. et al. Chem. Rev. 2023 , doi.org/10.1021/acs.chemrev.2c00515.[2] Adiyeri Saseendran, D. P. et al. Chem. Comm. 2023, In Revision.[3] Zhao, Y. et al. Adv. Energy Mater. 2020, 10, 2002228.[4] Zhao, Y. et al. ACS Nano 2022, 16, 15318-15327.

Molecular catalysts for CO2 Electroreduction: Progress and prospects with pincer type complexes

Atomic Fe/Co–N–C materials represent one promising type of noble-metal-free oxygen reduction reaction (ORR) catalyst for metal–air batteries and fuel cells, but their inherent features of complex and indistinct active sites derived from high-temperature treatment impede further development in terms of high-performance catalyst design. Herein, molecular model catalysts with defined active sites are developed to understand the structure–performance relationship. Specifically, cobalt phthalocyanine and cobalt porphyrin with identical Co–N4 moieties but completely different three-shell coordination structure are respectively anchored on reduced graphene oxide to synthesize molecular catalysts of CoPc@rGO and CoPr@rGO by π–π coupling interactions. Experimental results show that CoPc@rGO with three-shell N-modified Co–N4 sites holds a fairly good ORR activity, which is more than 3 times that for CoPr@rGO (7.36 vs 2.34 e–1 site–1 s–1 at 0.85 V). Density functional theory calculations reveal that three-shell N can modulate the electron structure of the Co–N4 site and reduce the ORR energy barrier. Furthermore, when used in a Zn–O2 battery, CoPc@rGO also exhibits an ultrahigh peak power density of 425 mW cm–2. This study offers molecular model catalysts to reveal the structure–performance relationships of Co–N–C and beyond.

Molecularly defined organometallic rhodium phosphine complexes were efficiently heterogenized within a MOF structure without affecting neither their molecular nature nor their catalytic behavior. Phosphine-functionalized MOF-808 served as a solid ligand in a series of eight rhodium phosphine catalysts. These MOF-heterogenized molecular catalysts showed activity up to 2100 h–1 for ethylene hydroformylation toward propionaldehyde as the sole carbon-containing product. The combined experimental and computational methods applied to this unique MOF-based molecular system allowed unraveling the structure and evolution of the active Rh species within the MOF under catalytic conditions, in line with the molecular mechanisms at play during the hydroformylation reaction. The MOF-based Rh catalyst also successfully catalyzed the hydroformylation of longer and bulkier alkenes with similar activity and selectivity than that obtained with its molecular homogeneous counterpart. MOF-808, designed as a porous crystalline macroligand for well-defined molecular catalysts, allows benefiting from the molecular scale understanding of interactions and mechanisms as well as from stabilization through site isolation and recycling ability.

Homogeneous reactions in general are relatively easy to study with respect to heterogeneous systems since all catalytic sites are uniform and can be addressed simultaneously. The latter feature is fully out of the window in an electrochemical context, where only the few catalytic species that are sufficiently close to the electrode undergo redox reactions. Especially in the water oxidation reaction where harsh reaction conditions are employed, a clear picture of what is the active species, what products are formed, how one can steer this, and how it all depends on the exact reaction conditions is important to be able to fully unravel the key reaction paths. The combination of electrochemical experiments with on-line detection of the catalytic species and reaction products is a powerful approach to successfully address these questions. Recently, a significant progress has been made in on-line studies on molecular water oxidation catalysts during electrochemical experiments. These are reviewed here.

Electrocatalysts that start a reaction as molecules do not always end the reaction as molecules, and even when they do, they might not be molecules during catalysis. In this Perspective, we discuss knowledge learned from the study of Cu‐based molecularly structured electrocatalysts––including metal coordination complexes, metal‐organic frameworks, single‐atom catalysts, and polymeric materials––that restructure under electrochemical CO2 reduction reactions. Recent reports are summarized with an emphasis on the nature and significance of post‐mortem and in situ characterization for the proper identification of active sites. We demonstrate that molecular and material structures determine whether electrocatalysts restructure and how they restructure, that understanding of restructuring processes can help us identify active sites for catalysis, and that this knowledge can be leveraged to design precatalysts that generate highly active catalysts under reaction conditions. In addition, we provide recommended practices for studying the integrity of heterogeneous molecular catalysts during and after CO2 reduction reactions.Key Points Heterogeneous molecular Cu catalysts have been observed to restructure to metallic Cu clusters under reaction conditions. Dynamic or reversible restructuring confounds identification of real active sites. Both ex situ and in situ techniques are recommended for robust catalyst characterization.

Rapidly increasing levels of atmospheric carbon dioxide and their damaging impact on the global climate system raise doubts about the sustainability of the fossil resource based energy system. Meanwhile, raising living standards and increasing global population lead to an ever growing need for energy. Renewable energy sources are believed to present a solution to these problems with the sheer abundance of solar energy showing particular promise to fulfill the world's energy needs. However, for large scale application of solar energy to be possible, the problem of its storage has to be addressed. The insufficient flexibility of present-day storage technologies has led to the quest for producing solar fuels, centering on hydrogen as a fuel in a prospective hydrogen economy. Nevertheless, the gaseous state, low volumetric energy density and explosive nature of hydrogen makes it a challenging fuel for practical applications. Using solar energy to produce carbon-based liquid fuels solves these challenges, closes the anthropogenic carbon cycle and allows for the continued utilization of existing infrastructures. A promising method for the production of such fuels consists in the photoelectrochemical and electrochemical conversion of carbon dioxide. In this thesis, both methods are investigated using molecular homogeneous catalysts and heterogeneous systems. The photoelectrochemical reduction of carbon dioxide on TiO2-protected Cu2O photocathodes was investigated using a rhenium bipyridyl catalyst in solution. Important charge transport limitations were encountered, which could be overcome by the addition of protic additives to the electrolyte. Improving on this result, the molecular catalyst was covalently immobilized on the TiO2 surface of the photocathode by modifying the bipyridyl ligand with a phosphonate binding group. A nanostructure of TiO2 was needed to support sufficient catalyst to sustain the photocurrent generated by the Cu2O photoelectrode. The complete device showed photocurrents exceeding 2.5 mA cm-2 and large faradaic efficiency for the production of CO. Moving toward heterogeneous catalysis, the promotion of the CO2 to CO conversion reaction on silver surfaces by imidazolium cations was investigated. Replacing the imidazolium C2 proton with a phenyl substituent led to an enhancement of the co-catalytic effect. Replacing the C4 and C5 protons with methyl groups, however, suppressed the catalysis-promoting effect of the imidazolium salt for different C2 substituents and led to new insights into the role of imidazolium. The unassisted solar-driven splitting of CO2 into CO and O2 was demonstrated using water as electron source. This was achieved by the use of a porous gold cathode and an IrO2 anode, driven by three methylammonium lead iodide perovskite solar cells in series. Extended operation over 18 h was shown, achieving a solar to CO efficiency exceeding 6.5 %. Atomic layer deposition (ALD) modification of CuO nanowire cathodes with SnO2 was investigated, leading to striking impacts on the catalytic selectivity of this system. In an aqueous electrolyte, bare CuO led to the production of a wide spectrum of products, which was modified to the production of CO with high selectivity upon ALD modification. By exploiting the oxygen evolving activity of SnO2-coated CuO, a low cost bifunctional system was constructed, achieving sustained solar-driven production of CO with up to 13.4% efficiency.

The electrochemical reduction of nitrate is an emerging field offering a sustainable pathway toward both nitrate waste mitigation and ammonia production. To ensure the advancement of electrochemical nitrate reduction technologies, a fundamental understanding of the reaction mechanisms required to convert nitrate selectively to ammonia is critical. This insight can be provided through extensive testing of structurally well-defined catalysts to understand the electrochemical performance of various active site structures and their coordinating environments.In this work, we synthesized and tested molecular catalysts with well-defined structures containing tetra pyridinic coordinated metal sites for the electrochemical nitrate reduction reaction. The selectivity, stability, and activity of the catalysts, containing various first row transition metal centers, for the nitrate reduction reaction have been evaluated. The catalysts dispersed onto multi–walled carbon nanotubes displayed high selectivity toward ammonia production of up to 98.6 % faradic efficiency at an electrode potential of -0.5 V vs RHE. The development of such catalysts is critical for understanding the role of tetra pyridinic coordination sites needed to guide the design of future metal–nitrogen coordinated catalysts. In this presentation, we will discuss the characterization and electrochemical performance of the synthesized tetra pyridinic molecular catalysts to understand the role of coordination site structure for the electrochemical nitrate reduction reaction.