Homogeneous Molecular Iron Catalysts for Direct Photocatalytic Conversion of Formic Acid to Syngas (CO+H2 ).

The catalytic decomposition of formic acid to generate syngas (a mixture of H2 and CO) is a highly valuable strategy for energy conversion. Syngas can be used directly in internal combustion engines or can be converted to liquid fuels, meeting future energy challenges in a sustainable manner. Herein, we report the use of homogeneous molecular iron catalysts combined with a CdS nanorods (NRs) semiconductor to construct a highly efficient photocatalytic system for direct conversion of formic acid to syngas at room temperature and atmospheric pressure. Under optimal conditions, the photocatalytic system presents an activity of 150 mmol gcatalyst−1 h−1 towards H2, and an apparent quantum yield (AQY) of 16.8 %, making it among the most active noble‐metal‐free photocatalytic systems for H2 evolution from formic acid under visible light. Meanwhile, these iron‐based molecular catalysts also demonstrate remarkable enhancement in CO evolution with robust stability. The mechanistic role of the molecular catalyst is further investigated by using cyclic voltammetry, which suggests the formation of FeI species as the key step in the catalytic conversion of formic acid to syngas.

Integration of carbon capture with utilization technologies can lead the way to a net-zero carbon economy. Nevertheless, direct chemical conversion of chemically captured CO2 remains challenging due to its thermodynamic stability. Here, we demonstrate CO2 capture from flue gas/air and its direct conversion into syngas under solar irradiation without any externally applied voltage. The system captures CO2 with an amine/hydroxide solution and photoelectrochemically converts it into syngas (CO:H2 1:2 (concentrated CO2), 1:4 (simulated flue gas), and 1:30 (air)) using a perovskite-based photocathode with an immobilized molecular Co-phthalocyanine catalyst. At the anode, plastic-derived ethylene glycol is oxidized into glycolic acid over a Cu26Pd74 alloy catalyst. The overall process uses flue gas/air as carbon source and discarded plastic waste as electron donor, opening avenues for integrated carbon-neutral/negative solar fuel and waste upcycling technologies.

Selective electrochemical conversion of nitrate to ammonia with ammonia capture can simultaneously remediate nitrate-polluted wastewater and supplement Haber-Bosch ammonia production. Homogeneous molecular catalysts remain underexplored in wastewater treatment and, more generally, in reactive separation processes. Despite the tunable reactivity of molecular catalysts, two barriers prevent their widespread implementation for wastewater nitrate treatment and ammonia recovery. The first barrier is the lack of reports of reaction activity and selectivity of nitrate reduction molecular catalysts in real wastewaters. The second barrier is the need to separate the catalyst, catalytic product, and treated wastewater. In this study, we employ electrochemical stripping in several configurations as a reactive separation unit process to address both barriers to implementation. Using the homogeneous molecular nitrate reduction catalyst Co(DIM), we demonstrate >70% removal of nitrate from municipal wastewater treatment plant secondary effluent with >98.5% selectivity to ammonia, leading to the generation of a high-purity ammonia product (ammonium sulfate). These experiments constitute a novel demonstration of molecular catalysis for nitrate reduction in real wastewater and highlight electrochemical engineering opportunities in reactive separations to valorize wastewater resources. This work advances environmental research toward United Nations Sustainable Development Goals (SDGs) for Clean Water (SDG 6) and Responsible Consumption and Production (SDG 12).

Molecular engineering of Fe-MIL-53 electrocatalyst for effective oxygen evolution reaction

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.

Dual-Site Polymeric Heterogeneous α-Diimine Ni Catalysts with Tailored Spatial Distribution for Ethylene Polymerization

Syngas (H2 + CO) is a compatible fuel for internal combustion engines, or it can be transformed to liquid fuels, which can help overcome future energy crises sustainably. In this study, we report an inexpensive and highly active photocatalytic system for production of syngas from formic acid under ambient conditions. The photocatalytic system comprising CdS/CNT hybrids and a porphyrin-based catalyst showed remarkable H2 and CO evolution at ambient conditions. Using monochromatic light (420 nm), the highest values of apparent quantum yields reached 22.8 and 12.5% for H2 and CO, respectively. Photoluminescence spectra and photocurrent responses proved the efficient electron transfer between the hybrid photosensitizer and the molecular catalyst, which enhanced the performance of the present photocatalytic system. Mechanistic insights regarding the molecular catalyst were obtained using cyclic voltammetry, inferring the generation of Fe(I) species as a critical step in the photocatalytic decomposition of formic acid.

Carbon dioxide (CO2) is a greenhouse gas whose presence in the atmosphere significantly contributes to climate change. Developing sustainable, cost-effective pathways to convert CO2 into higher value chemicals is essential to curb its atmospheric presence. Electrochemical CO2 reduction to value-added chemicals using molecular catalysis currently attracts a lot of attention, since it provides an efficient and promising way to increase CO2 utilization. Introducing amino groups as substituents to molecular catalysts is a promising approach towards improving capture and reduction of CO2. This review explores recently developed state-of-the-art molecular catalysts with a focus on heterogeneous and homogeneous amine molecular catalysts for electroreduction of CO2. The relationship between the structural properties of the molecular catalysts and CO2 electroreduction will be highlighted in this review. We will also discuss recent advances in the heterogeneous field by examining different immobilization techniques and their relation with molecular structure and conductive effects.

The use of metal complexes as homogeneous molecular catalysts has attracted considerable attention regarding photocatalytic CO2 reduction. Enhancing these complexes with photosensitivity and photooxidation capabilities, aiming to create multifunctional molecular devices, presents significant challenges. In response to these challenges, we successfully designed and synthesized three innovative metal complexes. The complexes demonstrate a remarkable ability to perform CO2 photoreduction in tandem with methanol photooxidation, allowing for the simultaneous production of formic acid without requiring additional photosensitizers and electron sacrificial reductants. An optimal turnover number (TON) value of 855 was obtained under simulated sunlight. Even under natural sunlight, the TON can reach 207, much higher than the value of the physical mixture of the photocatalytic reductive and oxidative moieties. Spectroscopic studies and density functional theory (DFT) calculations revealed that integrating reduction and oxidation sites in one molecular catalyst can promote charge transfer kinetics and enhance activity for CO2 reduction and methanol oxidation. This is the first report that non‐noble metal homogeneous catalysts can simultaneously possess photosensitivity, photoreduction, and photo‐oxidation functions, offering new insights into designing homogeneous catalysts for artificial photosynthesis.

Observation of a potential-dependent switch of water-oxidation mechanism on Co-oxide-based catalysts

Exploring chemistry with single-molecule and -particle fluorescence microscopy

Molecular water-oxidation catalysts can deactivate by side reactions or decompose to secondary materials over time due to the harsh, oxidizing conditions required to drive oxygen evolution. Distinguishing electrode surface-bound heterogeneous catalysts (such as iridium oxide) from homogeneous molecular catalysts is often difficult. Using an electrochemical quartz crystal nanobalance (EQCN), we report a method for probing electrodeposition of metal oxide materials from molecular precursors. Using the previously reported [Cp*Ir(H(2)O)(3)](2+) complex, we monitor deposition of a heterogeneous water oxidation catalyst by measuring the electrode mass in real time with piezoelectric gravimetry. Conversely, we do not observe deposition for homogeneous catalysts, such as the water-soluble complex Cp*Ir(pyr-CMe(2)O)X reported in this work. Rotating ring-disk electrode electrochemistry and Clark-type electrode studies show that this complex is a catalyst for water oxidation with oxygen produced as the product. For the heterogeneous, surface-attached material generated from [Cp*Ir(H(2)O)(3)](2+), we can estimate the percentage of electroactive metal centers in the surface layer. We monitor electrode composition dynamically during catalytic turnover, providing new information on catalytic performance. Together, these data suggest that EQCN can directly probe the homogeneity of molecular water-oxidation catalysts over short times.

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

While homogeneous molecular catalysts are valued for their high reaction specificity, they are not as easily recoverable at industrial scales as heterogeneous catalyst powders that can be rapidly separated from the reaction solution. Here, we will discuss a new approach using atomic layer deposition (ALD) to immobilize molecular catalysts onto heterogeneous powder supports to achieve this combination of selectivity and recoverability. In this presentation, we demonstrate applicability using a non-noble metal (nickel) molecular catalyst to do Suzuki carbon-carbon cross-coupling reactions. Due to extremely short catalyst lifetimes caused by dimerization, this catalyst exhibits limited catalytic reactivity under homogeneous conditions. However, when heterogenized and immobilized, product yields of over 90% can be achieved in aqueous conditions, and the catalytic activity is preserved through over five hundred recovery and wash cycles. Following this work, we will also report on how modification of the ALD surface chemistry and number of ALD cycles affect catalytic performance.

Comprehensive SummaryThe creation of effective and inexpensive catalysts is essential for photocatalytic CO2 reduction. Homogeneous molecular catalysts, possessing definite crystal structures, are desirable to study the relationship between catalytic performance and coordination microenvironment around catalytic center. In this report, we elaborately developed three Co(II)‐based molecular catalysts with different coordination microenvironments for CO2 reduction, named [CoN3O]ClO4, [CoN4]ClO4, and [CoN3S]ClO4, respectively. The optimal [CoN3O]ClO4 photocatalyst has a maximum TON of 5652 in photocatalytic reduced CO2 reduction, which is 1.28 and 1.65 times greater than that of [CoN4]ClO4 and [CoN3S]ClO4, respectively. The high electronegativity of oxygen in L1 (N,N‐bis(2‐pyridylmethyl)‐N‐(2‐hydroxybenzyl)amine) provides the Co(II) catalytic centers with low reduction potentials and a more stable *COOH intermediate, which facilitates the CO2‐to‐CO conversion and accounts for the high photocatalytic activity of [CoN3O]ClO4. This work provides researchers new insights in development of catalysts for photocatalytic CO2 reduction.