Revealing a Double-Volcano-Like Structure-Activity Relationship for Substitution-Functionalized Metal-Phthalocyanine Catalysts toward Electrochemical CO2 Reduction.

Electron-donating/-withdrawing groups (EDGs/EWGs) substitution is widely used to regulate the catalytic performance of transition-metal phthalocyanine (MPc) toward electrochemical CO2 reduction, but the corresponding structure-activity relationships and regulation mechanisms are still ambiguous. Herein, by investigating a series of substitution-functionalized MPc (MPc-X), this work reveals a double-volcano-like relationship between the electron-donating/-withdrawing abilities of the substituents and the catalytic activities of MPc-X. The weak-EDG/-EWG substitution enhances whereas the strong-EDG/-EWG substitution mostly lowers the CO selectivity of MPc. Experimental and calculation results demonstrate that the electronic properties of the substituents influence the symmetry and energy of the highest occupied molecular orbitals of MPc-X, which in turn determine the CO2 adsorption/activation and lead to diverse CO2 reduction pathways on the EWG or EDG substituted MPc via different CO2 adsorption modes. This work provides mechanism insights that could be guidance for the design and regulation of molecular catalysts.

Electrochemical Synthesis of Urea: Co-Reduction of Nitrite and Carbon Dioxide on Binuclear Cobalt Phthalocyanine.

Exploration of molecular catalysts with the atomic-level tunability of molecular structures offers promising avenues for developing high-performance catalysts for the electrochemical co-reduction reaction of carbon dioxide (CO2) and nitrite (NO2 -) into value-added urea. In this work, a binuclear cobalt phthalocyanine (biCoPc) catalyst is prepared through chemical synthesis and applied as a C─N coupling catalyst toward urea. Achieving a remarkable Faradaic efficiency of 47.4% for urea production at -0.5 V versus reversible hydrogen electrode (RHE), this biCoPc outperforms many known molecular catalysts in this specific application. Its unique planar macromolecular structure and the increased valence state of cobalt promote the adsorption of nitrogenous and carbonaceous species, a critical factor in facilitating the multi-electron C─N coupling. Combining highly sensitive in situ attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) with density functional theory (DFT) calculations, the linear adsorbed CO (COL) and bridge adsorbed CO (COB) is captured on biCoPc catalyst during the co-reduction reaction. COB, a pivotal intermediate in the co-reduction from CO2 and nitrite to urea, is evidenced to be labile and may be attacked by nitrite, promoting urea production. This work demonstrates the importance of designing molecular catalysts for efficient co-reduction of CO2 and nitrite to urea.

Photocatalytic CO2 Reduction Using Homogeneous Carbon Dots with a Molecular Cobalt Catalyst.

A simple and precious-metal free photosystem for the reduction of aqueous CO2 to syngas (CO and H2) is reported consisting of carbon dots (CDs) as the sole light harvester together with a molecular cobalt bis(terpyridine) CO2 reduction co-catalyst. This homogeneous photocatalytic system operates in the presence of a sacrificial electron donor (triethanolamine) in DMSO/H2O solution at ambient temperature. The photocatalytic system exhibits an activity of 7.7 ± 0.2 mmolsyngas gCDs -1 (3.6 ± 0.2 mmolCO gCDs -1 and 4.1 ± 0.1 mmolH2 gCDs -1) after 24 hours of full solar spectrum irradiation (AM 1.5G). Spectroscopic and electrochemical characterization supports that this photocatalytic performance is attributed to a favorable association between CDs and the molecular cobalt catalyst, which results in improved interfacial photoelectron transfer and catalytic mechanism. This work provides a scalable and inexpensive platform for the development of CO2 photoreduction systems using CDs.

Modulating the Electronic Structure of Cobalt in Molecular Catalysts via Coordination Environment Regulation for Highly Efficient Heterogeneous Nitrate Reduction.

Ammonia (NH3) is pivotal in modern industry and represents a promising next-generation carbon-free energy carrier. Electrocatalytic nitrate reduction reaction (eNO3RR) presents viable solutions for NH3 production and removal of ambient nitrate pollutants. However, the development of eNO3RR is hindered by lacking the efficient electrocatalysts. To address this challenge, we synthesized a series of macrocyclic molecular catalysts for the heterogeneous eNO3RR. These materials possess different coordination environments around metal centers by surrounding subunits. Consequently, electronic structures of the active centers can be altered, enabling tunable activity towards eNO3RR. Our investigation reveals that metal center with an N2(pyrrole)-N2(pyridine) configuration demonstrates superior activity over the others and achieves a high NH3 Faradaic efficiency (FE) of over 90 % within the tested range, where the highest FE of approximately 94 % is obtained. Furthermore, it achieves a production rate of 11.28 mg mgcat -1 h-1, and a turnover frequency of up to 3.28 s-1. Further tests disclose that these molecular catalysts with diverse coordination environments showed different magnetic moments. Theoretical calculation results indicate that variated coordination environments can result in a d-band center variation which eventually affects rate-determining step energy and calculated magnetic moments, thus establishing a correlation between electronic structure, experimental activity, and computational parameters.

Well-defined N3 C1 -anchored Single-Metal-Sites for Oxygen Reduction Reaction.

N-, C-, O-, S-coordinated single-metal-sites (SMSs) have garnered significant attention due to the potential for significantly enhanced catalytic capabilities resulting from charge redistribution. However, significant challenges persist in the precise design of well-defined such SMSs, and the fundamental comprehension has long been impeded in case-by-case reports using carbon materials as investigation targets. In this work, the well-defined molecular catalysts with N3 C1 -anchored SMSs, i.e., N-confused metalloporphyrins (NCPor-Ms), are calculated for their catalytic oxygen reduction activity. Then, NCPor-Ms with corresponding N4 -anchored SMSs (metalloporphyrins, Por-Ms), are synthesized for catalytic activity evaluation. Among all, NCPor-Co reaches the top in established volcano plots. NCPor-Co also shows the highest half-wave potential of 0.83 V vs. RHE, which is much better than that of Por-Co (0.77 V vs. RHE). Electron-rich, low band gap and regulated d-band center contribute to the high activity of NCPor-Co. This study delves into the examination of well-defined asymmetric SMS molecular catalysts, encompassing both theoretical and experimental facets. It serves as a pioneering step towards enhancing the fundamental comprehension and facilitating the development of high-performance asymmetric SMS catalysts.

Decoupling Redox Hopping and Catalysis in Metal-Organic Frameworks -based Electrocatalytic CO2 Reduction.

Traditional MOF e-CRR, constructed from catalytic linkers, manifest a kinetic bottleneck during their multi-electron activation. Decoupling catalysis and charge transport can address such issues. Here, we build two MOF/e-CRR systems, CoPc@NU-1000 and TPP(Co)@NU-1000, by installing cobalt metalated phthalocyanine and tetraphenylporphyrin electrocatalysts within the redox active NU-1000 MOF. For CoPc@NU-1000, the e-CRR responsive CoI/0 potential is close to that of NU-1000 reduction compared to the TPP(Co)@NU-1000. Efficient charge delivery, defined by a higher diffusion (Dhop =4.1×10-12  cm2 s-1 ) and low charge-transport resistance ( R C T M O F ${{R}_{{\rm C}{\rm T}}^{{\rm M}{\rm O}{\rm F}}}$ =59.5 Ω) in CoPC@NU-1000 led FECO =80 %. In contrast, TPP(Co)@NU-1000 fared a poor FECO =24 % (Dhop =1.4×10-12  cm2 s-1 and R C T M O F ${{R}_{{\rm C}{\rm T}}^{{\rm M}{\rm O}{\rm F}}}$ =91.4 Ω). For such a decoupling strategy, careful choice of the host framework is critical in pairing up with the underlying electrochemical properties of the catalysts to facilitate the charge delivery for its activation.

Coupling Cobalt Phthalocyanine Molecules on 3D Nitrogen-Doped Vertical Graphene Arrays for Highly Efficient and Robust CO2 Electroreduction.

Metallic phthalocyanines (MePcs) have shown their potential as catalysts for CO2 reduction reactions (CO2 RR). However, their low conductivity, easy agglomeration, and poor stability enslave the further progress of their CO2 RR applications. Herein, an integrated heterogeneous molecular catalyst through anchoring CoPc molecules on 3D nitrogen-doped vertical graphene arrays (NVG) on carbon cloth (CC) is reported. The CoPc-NVG/CC electrodes exhibit superior performance for reducing CO2 to CO with a Faradic efficiency of above 97.5% over a wide potential range (99% at an optimal potential), a very high turnover frequency of 35800 h-1 , and decent stability. It is revealed that NVG interacts with CoPc to form highly efficient channels for electron transfer from NVG to CoPc, facilitating the Co(II)/Co(I) redox of CO2 reduction. The strong coupling effect between NVG and CoPc molecules not only endows CoPc with high intrinsic activity for CO2 RR, but also enhances the stability of electrocatalysts under high potentials. This work paves an efficient approach for developing high-performance heterogeneous catalysts by using rationally designed 3D integrated graphene arrays to host molecular metallic phthalocyanines so as to ameliorate their electronic structures and engineer stable active sites.

Electrocatalytic CO2 Reduction: from Discrete Molecular Catalysts to Their Integrated Catalytic Materials.

Molecular catalysts (metal complexes), with molecularly defined uniform active sites and atomically precise structural tailorability allowing for regulating catalytic performance through metal- and ligand-centered engineering and elucidating reaction mechanisms via routine photoelectrochemical characterizations, have been increasingly explored for electrocatalytic CO2 reduction (ECR). However, their poor stability and low catalytic current density are undesirable for practical applications. Heterogenizing discrete molecular catalysts can potentially surmount these issues, and the resulting integrated catalysts largely share catalytical properties with their discrete molecular counterparts, which bridge the gap between heterogeneous and homogeneous catalysis and combine their advantages. This minireview surveys advances in design and regulation of molecular catalysts such as porphyrin, phthalocyanine, and bipyridine-based metal complexes and their integrated catalytic materials for selective ECR.

Sunlight-Driven Highly Selective Catalytic Oxidation of 5-Hydroxymethylfurfural Towards Tunable Products.

Owing to the easy over-oxidation, it is a promising yet challenging task to explore renewable carbon resources to control the sunlight-driven selective catalytic oxidation of biomass-derived 5-hydroxymethylfurfural (HMF), producing important chemical feedstocks, namely, less-oxidized 2,5-diformylfuran (DFF) and 5-hydroxymethyl-2-furancarboxylic acid (HMFCA). Herein, we have developed a photocatalyst by anchoring a Ru complex on CdS quantum dots, which achieves selective oxidation of HMF toward DFF or HMFCA with high conversion (>81 %) and selectivity (>90 %), based on the controllable generation of two oxygen radicals under different atmospheres. Such selective conversion can also work well outside the laboratory by using natural sunlight. In particular, the selective production of HMFCA through photocatalytic HMF oxidation is achieved for the first time. More importantly, our photocatalyst is applicable for the selective oxidation of other compounds with hydroxyl and aldehyde groups.

A High-Throughput Approach to Repurposing Olefin Polymerization Catalysts for Polymer Upcycling.

Efficient and economical plastic waste upcycling relies on the development of catalysts capable of polymer degradation. A systematic high-throughput screening of twenty-eight polymerization catalyst precursors, belonging to the catalyst families of metallocenes, ansa-metallocenes, and hemi- and post-metallocenes, in cis-1,4-polybutadiene (PB) degradation reveals, for the first time, important structure-activity correlations. The upcycling conditions involve activation of the catalysts (at 0.18 % catalyst loading) with tri-iso-butyl aluminum at 50 °C in toluene. The data indicate the ability to degrade PB is a general reactivity profile of neutral group 4 metal hydrides. A simple quantitative-structure activity relationship (QSAR) model utilizing two descriptors for the distribution of steric bulk in the active pocket and one measuring the metal ion electrophilicity reveals the degradation ability improves with increased but not overbearing steric congestion and lower electrophilicity.

Heterogenised Molecular Catalysts for Sustainable Electrochemical CO2 Reduction.

There has been a rapid rise in interest regarding the advantages of support materials to protect and immobilise molecular catalysts for the carbon dioxide reduction reaction (CO2 RR) in order to overcome the weaknesses of many well-known catalysts in terms of their stability and selectivity. In this Review, the state of the art of different catalyst-support systems for the CO2 RR is discussed with the intention of leading towards standard benchmarking for comparison of such systems across the most relevant supports and immobilisation strategies, taking into account these multiple pertinent metrics, and also enabling clearer consideration of the necessary steps for further progress. The most promising support systems are described, along with a final note on the need for developing more advanced experimental and computational techniques to aid the rational design principles that are prerequisite to prospective industrial upscaling.

Versatile Construction of Biomimetic Nanosensors for Electrochemical Monitoring of Intracellular Glutathione.

The current strategies for nanoelectrode functionalization usually involve sophisticated modification procedures, uncontrollable and unstable modifier assembly, as well as a limited variety of modifiers. To address this issue, we propose a versatile strategy for large-scale synthesis of biomimetic molecular catalysts (BMCs) modified nanowires (NWs) to construct functionalized electrochemical nanosensors. This design protocol employs an easy, controllable and stable assembly of diverse BMCs-poly(3,4-ethylenedioxythiophene) (PEDOT) composites on conductive NWs. The intrinsic catalytic activity of BMCs combined with outstanding electron transfer ability of conductive polymer enables the nanosensors to sensitively and selectively detect various biomolecules. Further application of sulfonated cobalt phthalocyanine functionalized nanosensors achieves real-time electrochemical monitoring of intracellular glutathione levels and its redox homeostasis in single living cells for the first time.

Ligand Controls the Activity of Light-Driven Water Oxidation Catalyzed by Nickel(II) Porphyrin Complexes in Neutral Homogeneous Aqueous Solutions.

Finding photostable, first-row transition metal-based molecular systems for photocatalytic water oxidation is a step towards sustainable solar fuel production. Herein, we discovered that nickel(II) hydrophilic porphyrins are molecular catalysts for photocatalytic water oxidation in neutral to acidic aqueous solutions using [Ru(bpy)3 ]2+ as photosensitizer and [S2 O8 ]2- as sacrificial electron acceptor. Electron-poorer Ni-porphyrins bearing 8 fluorine or 4 methylpyridinium substituents as electron-poorer porphyrins afforded 6-fold higher turnover frequencies (TOFs; ca. 0.65 min-1 ) than electron-richer analogues. However, the electron-poorest Ni-porphyrin bearing 16 fluorine substituents was photocatalytically inactive under such conditions, because the potential at which catalytic O2 evolution starts was too high (+1.23 V vs. NHE) to be driven by the photochemically generated [Ru(bpy)3 ]3+ . Critically, these Ni-porphyrin catalysts showed excellent stability in photocatalytic conditions, as a second photocatalytic run replenished with a new dose of photosensitizer, afforded only 1-3 % less O2 than during the first photocatalytic run.

Lewis-Basic EDTA as a Highly Active Molecular Electrocatalyst for CO2 Reduction to CH4.

The most active catalysts so far successful in hydrogenation reduction of CO2 are mainly heterogeneous Cu-based catalysts. The complex coordination environments and multiple active sites in heterogeneous catalysts result in low selectivity of target product, while molecular catalysts with well-defined active sites and tailorable structures allow mechanism-based performance optimization. Herein, we firstly report a single ethylenediaminetetraacetic acid (EDTA) molecular-level immobilized on the surface of carbon nanotube as a catalyst for transferring CO2 to CH4 with an excellent performance. This catalyst exhibits a high Faradaic efficiency of 61.6 % toward CH4 , a partial current density of -16.5 mA cm-2 at a potential of -1.3 V versus reversible hydrogen electrode. Density functional theory calculations reveal that the Lewis basic COO- groups in EDTA molecule are the active sites for CO2 reduction reaction (CO2 RR). The energy barrier for the generation of CO from *CO intermediate is as high as 0.52 eV, while the further protonation of *CO to *CHO follows an energetic downhill path (-1.57 eV), resulting in the high selectivity of CH4 . This work makes it possible to control the product selectivity for CO2 RR according to the relationship between the energy barrier of *CO intermediate and molecular structures in the future.

Predesign of Catalytically Active Sites via Stable Coordination Cluster Model System for Electroreduction of CO2 to Ethylene.

Purposefully designing the well-defined catalysts for the selective electroreduction of CO2 to C2 H4 is an extremely important but challenging work. In this work, three crystalline trinuclear copper clusters (Cu3 -X, X=Cl- , Br- , NO3 - ) have been designed, containing three active Cu sites with the identical coordination environment and appropriate spatial distance, delivering high selectivity for the electrocatalytic reduction of CO2 to C2 H4 . The highest faradaic efficiency of Cu3 -X for CO2 -to-C2 H4 conversion can be adjusted from 31.90 % to 55.01 % by simply replacing the counter anions (NO3 - , Cl- , Br- ). The DFT calculation results verify that Cu3 -X can facilitate the C-C coupling of identical *CHO intermediates, subsequently forming molecular symmetrical C2 H4 product. This work provides an important molecular model system and a new design perspective for electroreduction of CO2 to C2 products with symmetrical molecular structure.

Organic-Inorganic Hybrid Nanomaterials for Electrocatalytic CO2 Reduction.

Electrochemical CO2 reduction (ECR) to value-added chemicals and fuels is regarded as an effective strategy to mitigate climate change caused by CO2 from excess consumption of fossil fuels. To achieve CO2 conversion with high faradaic efficiency, low overpotential, and excellent product selectivity, rational design and synthesis of efficient electrocatalysts is of significant importance, which dominates the development of ECR field. Individual organic molecules or inorganic catalysts have encountered a bottleneck in performance improvement owing to their intrinsic shortcomings. Very recently, organic-inorganic hybrid nanomaterials as electrocatalysts have exhibited high performance and interesting reaction processes for ECR due to the integration of the advantages of both heterogeneous and homogeneous catalytic processes, attracting widespread interest. In this work, the recent advances in designing various organic-inorganic hybrid nanomaterials at the atomic and molecular level for ECR are systematically summarized. Particularly, the reaction mechanism and structure-performance relationship of organic-inorganic hybrid nanomaterials toward ECR are discussed in detail. Finally, the challenges and opportunities toward controlled synthesis of advanced electrocatalysts are proposed for paving the development of the ECR field.

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.

Kinetics of the Same Reaction Monitored over Nine Orders of Magnitude in Concentration: When Are Unique Subensemble and Single-Turnover Reactivity Displayed?

Essentially no information is known about the behavior of individual molecular catalysts under reaction conditions. This is a result of the averaging inherent to traditional analytical techniques. Herein, a combined fluorescence microscopy and 1 H NMR spectroscopy study reveals that unique (that is, non-ensemble averaged) distributions and time-variable kinetics from molecular ruthenium catalysts within growing polynorbornene occur and are detectable between 10-9 m and 10-6 m of substrate, surprisingly just 1000-fold less concentrated than a typical laboratory bench-scale reaction. The kinetic states governing single-turnover events are determinable by overlay of the signal arising from individual monomer insertion reactions with that from polymer growth from neighboring catalysts.

Single Turnover at Molecular Polymerization Catalysts Reveals Spatiotemporally Resolved Reactions.

Multiple active individual molecular ruthenium catalysts have been pinpointed within growing polynorbornene, thereby revealing information on the reaction dynamics and location that is unavailable through traditional ensemble experiments. This is the first single-turnover imaging of a molecular catalyst by fluorescence microscopy and allows detection of individual monomer reactions at an industrially important molecular ruthenium ring-opening metathesis polymerization (ROMP) catalyst under synthetically relevant conditions (e.g. unmodified industrial catalyst, ambient pressure, condensed phase, ca. 0.03 m monomer). These results further establish the key fundamentals of this imaging technique for characterizing the reactivity and location of active molecular catalysts even when they are the minor components.

Photoelectrochemical CO2 Reduction by a Molecular Cobalt(II) Catalyst on Planar and Nanostructured Si Surfaces.

In the presence of a molecular Co(II) catalyst, CO2 reduction occurred at much less negative potentials on Si photoelectrodes than on an Au electrode. The addition of 1 % H2 O significantly improved the performance of the Co(II) catalyst. Photovoltages of 580 and 320 mV were obtained on Si nanowires and a planar Si photoelectrode, respectively. This difference likely originated from the fact that the multifaceted Si nanowires are better in light harvesting and charge transfer than the planar Si surface.