Reversible hydrogen storage using CO2 and a proton-switchable iridium catalyst in aqueous media under mild temperatures and pressures

Efficient reduction of CO2 and inhibition of hydrogen precipitation by polyoxometalate photocatalyst modified with the metal Mn

Photocatalytic reduction of CO2 to chemical fuels is attractive for solving both the greenhouse effect and the energy crisis, but the key challenge is to design and synthesize photocatalysts with remarkable performance under visible light irradiation. Efficient catalytic carbon dioxide reduction (CO2RR) with light is considered a promising sustainable and clean approach to solve environmental problems. Herein, we found a new photocatalyst ([Mn(en)2]6[V12B18O54(OH)6]) (abbreviated as Mn6V12) based on the modifiability of polyoxometalates, in which Mn acts as a modifying unit to efficiently reduce CO2 to CO and effectively inhibit the hydrogen precipitation reaction. This Mn modified polyoxometalate catalyst has a maximum CO generation rate of 4625.0 μmol g-1 h-1 and a maximum H2 generation rate of 499.6 μmol g-1 h-1, with a selectivity of 90.3% for CO generation and 9.7% for H2 generation. This polyoxometalate photocatalyst can effectively reduce CO and inhibit the hydrogen precipitation reaction. It provides a new idea for the efficient photocatalytic carbon dioxide reduction (CO2RR) with polyoxometalate catalysts.

A mechanistic study on conversion of carbon dioxide into formic acid promoted by 1-ethyl-2, 3-dimethyl-imidazolium nitrite

CONTEXT

The conversion of carbon dioxide (CO2) to formic acid (FA) through hydrogenation using 1-ethyl-2,3- dimethyl imidazolium nitrite (EDIN) ionic liquid was studied to understand the catalytic roles within EDIN. CO2 hydrogenation in various solvents has been explored, but achieving high efficiency and selectivity remains challenging due to the thermodynamic stability and kinetic inertness of CO2. This study explored two mechanistic pathways through theoretical calculations, revealing that the nitrite (NO2-) group is the most active site. The oxygen site on nitrite favorably activates H2, while the nitrogen site shows a minor activation barrier of 108.90 kJ/mol. The Gibbs energy variation indicates stable FA formation via EDIN, suggesting effective hydrogen (H2) activation and subsequent CO2 conversion. These insights are crucial for developing improved catalytic sites and processes in ionic liquid catalysts for CO2 hydrogenation.

METHODS

Quantum chemical calculations were conducted using the ORCA software package at the Restricted Hartree-Fock (RHF) and density functional theory (DFT) levels. The RHF method, known for its predictive abilities in simpler systems, provided a baseline description of electronic structures. In contrast, DFT was employed for its effectiveness in complex interactions involving significant electron correlation. A valence triple-zeta polarization (def2-TZVPP) basis set was employed for both RHF and DFT, ensuring accurate and correlated calculations. The B3LYP functional was utilized for its rapid convergence and cost-efficiency in larger molecules. Dispersion corrected functionals (DFT-D) addressed significant dispersion forces in ionic liquids, incorporating Grimme's D2, D3, and D4 corrections. Geometry optimizations, kinetics, and thermodynamic calculations were performed in the gas phase. The Nudged Elastic Band Transition State (NEB-TS) approach, combining Climbing Image-NEB (CINEB) and Eigenvector-Following (EF) methods, was used to find the minimum energy path (MEP) between reactants and products. Thermochemical analyses based on vibrational frequency calculations evaluated properties such as Enthalpy, Entropy, and Gibbs energy using ideal gas statistical mechanics.

Selective N-Alkylation of Aminobenzenesulfonamides with Alcohols for the Synthesis of Amino-(N-alkyl)benzenesulfonamides Catalyzed by a Metal-Ligand Bifunctional Ruthenium Catalyst

[(p-Cymene)Ru(2,2'-bpyO)(H2O)] was proven to be an efficient catalyst for the synthesis of amino-(N-alkyl)benzenesulfonamides via selective N-alkylation of aminobenzenesulfonamides with alcohols. It was confirmed that functional groups in the bpy ligand are crucial for the activity of catalysts. Furthermore, the utilization of this catalytic system for the preparation of a biologically active compound was presented.

Enhancing Long-Term Durability of Electrochemical Reactors Producing Formate from CO2 and Water Designed for Integration with Solar Cells

Artificial photosynthetic cells producing organic matter from CO2 and water have been extensively studied for carbon neutrality, and the research trend is currently transitioning from proof of concept using small-sized cells to large-scale demonstrations for practical applications. We previously demonstrated a 1 m2 size cell in which an electrochemical (EC) reactor featuring a ruthenium (Ru)-complex polymer (RuCP) cathode catalyst was integrated with photovoltaic cells. In this study, we tackled the remaining issue to improve the long-term durability of cathode electrodes used in the EC reactors, demonstrating high Faradaic efficiencies exceeding 80% and around 60% electricity-to-chemical energy-conversion efficiencies of a 75 cm2 sized EC reactor after continuous operation for 3000 h under practical conditions. Introduction of a pyrrole derivative containing an amino group in the RuCP coupled with UV-ozone treatment to create carboxyl groups on the carbon supports effectively reduced the detachment of the RuCP catalyst by forming a strong amide linkage. A newly developed chemically resistant graphite adhesive prevented the carbon supports from peeling off of the conductive substrates. In addition, highly durable anodes composed of IrOx-TaOy/Pt-metal oxide/Ti were adopted. Even though the EC reactor was installed at an inclined angle of 30°, which is approximately the optimal angle for receiving more solar energy, the crossover reactions were sufficiently suppressed because the porous separator film impeded the transfer of oxygen gas bubbles from the anode to the cathode. The intermittent operation improved the energy-conversion efficiency because the accumulated bubbles were removed at night.

Feeble Single-Atom Pd Catalysts for H2 Production from Formic Acid

Single-atom catalysts are thought to be the pinnacle of catalysis. However, for many reactions, their suitability has yet to be unequivocally proven. Here, we demonstrate why single Pd atoms (PdSA) are not catalytically ideal for generating H2 from formic acid as a H2 carrier. We loaded PdSA on three silica substrates, mesoporous silicas functionalized with thiol, amine, and dithiocarbamate functional groups. The Pd catalytic activity on amino-functionalized silica (SiO2-NH2/PdSA) was far higher than that of the thiol-based catalysts (SiO2-S-PdSA and SiO2-NHCS2-PdSA), while the single-atom stability of SiO2-NH2/PdSA against aggregation after the first catalytic cycle was the weakest. In this case, Pd aggregation boosted the reaction yield. Our experiments and calculations demonstrate that PdSA in SiO2-NH2/PdSA loosely binds with amine groups. This leads to a limited charge transfer from Pd to the amine groups and causes high aggregability and catalytic activity. According to the density functional calculations, the loose binding between Pd and N causes most of Pd's 4d electrons in amino-functionalized SiO2 to remain close to the Fermi level and labile for catalysis. However, PdSA chemically binds to the thiol group, resulting in strong hybridization between Pd and S, pulling Pd's 4d states deeper into the conduction band and away from the Fermi level. Consequently, fewer 4d electrons were available for catalysis.

Ruthenium(II)-Catalyzed Hydrogenation and Tandem (De)Hydrogenation via Metal-Ligand Cooperation: Base- and Solvent-Assisted Switchable Selectivity

A versatile, selective, solvent (methanol vs ethanol)- and base (potassium vs lithium carbonate)-assisted switchable synthesis of saturated ketone and α-methyl saturated ketone from α,β-unsaturated ketone is developed. Mechanistic aspects, evaluated from spectroscopic studies, in situ monitoring of the reaction progress, control studies, and labeling studies, further indicate the involvement of a tandem dehydrogenation-condensation-hydrogenation sequence in the reaction, in which the interconvertible coordination mode (imino N → Ru and amido N-Ru) of coordinated imidazole with Ru(II)-para-cymene is crucial, without which the efficiency and selectivity of the catalyst are completely lost. The catalyst demonstrates good efficiency, selectivity, and functional group tolerance and displays a broad scope (69 examples) for monomethylation and hydrogenation of unsaturated chalcones, double methylation of ketones, and N-methylation of amines.

An Aqueous Redox Flow Battery Using CO2 as an Active Material with a Homogeneous Ir Catalyst

For the application of CO2 as an energy storage material, a H2 storage system has been proposed based on the interconversion of CO2 and formic acid (or formate). However, energy losses are inevitable in the conversion of electrical energy to H2 as chemical energy (≈70 % electrical efficiency) and H2 to electrical energy (≈40 % electrical efficiency). To overcome these significant energy losses, we developed a system based on the interconversion of CO2 and formate for the direct storage and generation of electricity. In this paper, we report an aqueous redox flow battery system using homogeneous Ir catalysts with CO2 -formate redox pair. The system exhibited a maximum discharge capacity of 10.5 mAh (1.5 Ah L-1 ), capacity decay of 0.2 % per cycle, and total turnover number of 2550 after 50 cycles. During charging-discharging, in situ fluorescence X-ray absorption fine structure spectroscopy based on an online setup indicated that the active species was in a high valence state of IrIV .

Formic Acid Dehydrogenation through Ligand Design Strategy of Amidation in Half-Sandwich Ir Complexes

A series of Cp*Ir (Cp* = pentamethylcyclopentadienyl) complexes with amidated 8-aminoquinoline ligands were synthesized and tested for formic acid (FA) dehydrogenation. These complexes showed improved activities compared to pristine 8-anminquinoline (L1). Specially, amidation changed the outer coordination sphere of the complex (3) bearing N-8-quinolinylformamide (L3), and 3 was proved to be a proton-responsive catalyst. Our experimental results and DFT calculations demonstrated that the deprotonated carbanion in L3 could interact with a water molecule to stabilize the transition states and lower the reaction energy barrier, which improved the reaction activity. A turnover frequency of 206250 h-1 was achieved by 3 under optimized conditions. This study presents a method to develop new ligands and modify the existing ligands for efficient FA dehydrogenation.

N-Site Regulation of Pyridyltriazole in Cp*Ir(N̂N)(H2O) Complexes Achieving Catalytic FA Dehydrogenation

A series of novel Cp*Ir complexes with nitrogen-rich N̂N bidentate ligands were developed for the catalytic dehydrogenation of formic acid in water under base-free conditions. These complexes were synthesized by using pyridyl 1,2,4-triazole, methylated species, or pyridyl 1,2,3-triazole as a N-site regulation ligand and were fully characterized. Complex 1-H2O bearing 1,2,4-triazole achieved a high turnover frequency of 14192 h-1 at 90 °C in 4 M FA aqueous solution. The terminal and bridged Ir-H intermediates of 1-H2O were successfully detected by 1H NMR and mass spectrometry measurements. Kinetic isotope effect experiments and density functional theory (DFT) calculations were performed; then a plausible mechanism was proposed involving the β-hydride elimination and formation of H2. Water-assisted H2 release was proven to be the rate-determining step of the reaction. The distribution of Mulliken charges on N atoms of triazole ligand internally revealed that the ortho site N2 of 1-H2O with a higher electron density was conducive to efficient proton transfer. Additionally, the advantage of water-assisted short-range bridge of 1,2,4-triazole moieties led to a higher catalytic activity of 1-H2O. This study demonstrated the effectiveness of nitrogen-rich ligands on FA dehydrogenation and revealed a good strategy for N site regulation in the development of new homogeneous catalysts.

Recent Developments in Reversible CO2 Hydrogenation and Formic Acid Dehydrogenation over Molecular Catalysts

Carbon dioxide (CO2), a valuable feedstock, can be reutilized as a hydrogen carrier by hydrogenating CO2 to formic acid (FA) and releasing hydrogen by FA dehydrogenation in a reversible manner. Notably, FA is liquid at room temperature and can be stored and transported considerably more safely than hydrogen gas. Herein, we extensively reviewed transition-metal-based molecular catalysts explored for reversible CO2 hydrogenation and FA dehydrogenation. This Review describes different approaches explored for carbon-neutral hydrogen storage and release by applying CO2 hydrogenation to FA/formate and the subsequent release of H2 by the dehydrogenation of FA over a wide range of molecular catalysts based on noble and non-noble metals. Emphasis is also placed on the specific catalyst-to-substrate interaction by highlighting the specific role of the catalyst in the CO2 hydrogenation-FA dehydrogenation pathway.

Recent advances of Cp*Ir complexes for transfer hydrogenation: focus on formic acid/formate as hydrogen donors

Transfer hydrogenation reactions offer synthetically powerful strategies to deliver various hydrogenated compounds with the advantages of efficiency, atom economy, and practicability. On one hand, formic acid/formate function as promising hydrogen sources owing to their readily obtainable, inexpensive, and easy to handle nature. On the other hand, Cp*Ir complexes show high activities in transfer hydrogenation. This review highlights progress achieved for transfer hydrogenation of CO, CC, and CN bonds of a variety of unsaturated substrates, as well as amides focusing on Cp*Ir complexes as catalysts and formic acid/formate as hydrogen sources.

Renewable Hydrogen Production and Storage Via Enzymatic Interconversion of CO2 and Formate with Electrochemical Cofactor Regeneration

The urgent need to reduce CO2 emissions has motivated the development of CO2 capture and utilization technologies. An emerging application is CO2 transformation into storage chemicals for clean energy carriers. Formic acid (FA), a valuable product of CO2 reduction, is an excellent hydrogen carrier. CO2 conversion to FA, followed by H2 release from FA, are conventionally chemically catalyzed. Biocatalysts offer a highly specific and less energy-intensive alternative. CO2 conversion to formate is catalyzed by formate dehydrogenase (FDH), which usually requires a cofactor to function. Several FDHs have been incorporated in bioelectrochemical systems where formate is produced by the biocathode and the cofactor is electrochemically regenerated. H2 production from formate is also catalyzed by several microorganisms possessing either formate hydrogenlyase or hydrogen-dependent CO2 reductase complexes. Combination of these two processes can lead to a CO2 -recycling cycle for H2 production, storage, and release with potentially lower environmental impact than conventional methods.

Reaction and separation system for CO2 hydrogenation to formic acid catalyzed by iridium immobilized on solid phosphines under base-free condition

Hydrogenation of carbon dioxide (CO₂) to produce formic acid (HCOOH) in base-free condition can avoid waste producing and simplify product separation process. However, it remains a big challenge because of the unfavorable energy in both thermodynamics and dynamics. Herein, we report the selective and efficient hydrogenation of CO₂ to HCOOH under neutral conditions with imidazolium chloride ionic liquid as the solvent, catalyzed by a heterogeneous Ir/PPh₃ compound. The heterogeneous catalyst is more effective than the homogeneous one because it is inert in catalyzing the decomposition of product. A turnover number (TON) of 12700 can be achieved, and HCOOH with a purity of 99.5% can be isolated by distillation because of the non-volatility of the solvent. Both the catalyst and imidazolium chloride can be recycled at least 5 times with stable reactivity.

Diruthenium Catalyst for Hydrogen Production from Aqueous Formic Acid

Diruthenium complexes [{(η⁶-arene)RuCl}₂(μ-κ²:κ²-benztetraimd)]²⁺ containing the bridging bis-imidazole methane-based ligand {1,4-bis(bis(2-ethyl-5-methyl-1H-imidazol-4-yl)methyl)benzene} (benztetraimd) are synthesized for catalytic formic acid dehydrogenation in water at 90 °C. Catalyst [{(η⁶-p-cymene)RuCl}₂(μ-κ²:κ²-benztetraimd)]²⁺ [1-Cl₂] exhibited a remarkably high turnover frequency (1993 h^-1 per Ru atom) and long-term stability over 60 days for formic acid dehydrogenation, while the analogous (η⁶-benzene)diruthenium and mononuclear catalysts displayed low activity with poor long-term stability. Notably, catalyst [1-Cl₂] also displayed an appreciably high turnover number of 93 200 for the bulk-scale reaction. In addition, the in-depth mass and nuclear magnetic resonance investigations under the catalytic and control experimental conditions revealed the active involvement of several crucial catalytic intermediate species, such as Ru-aqua species [{(η⁶-p-cymene)Ru(H₂O)}₂(μ-L)]²⁺ [1-(OH₂)₂], Ru-formato species [{(η⁶-p-cymene)Ru(HCOO)}₂(μ-L)] [1-(HCOO)₂], and Ru-hydrido species [{(η⁶-p-cymene)Ru(H)}₂(μ-L)] [1-(H)₂], in the catalytic formic acid dehydrogenation reaction.

Iridium-(κ2-NSi) catalyzed dehydrogenation of formic acid: effect of auxiliary ligands on the catalytic performance

The iridium(III) complexes [Ir(H)(Cl)(κ²-NSi^tBu₂)(κ²-bipy^Me₂)] (2) and [Ir(H)(OTf)(κ²-NSi^tBu₂)(κ²-bipy^Me₂)] (3) (NSi^tBu₂ = {4-methylpyridine-2-yloxy}ditertbutylsilyl) have been synthesized and characterized including X-ray studies of 3. A comparative study of the catalytic activity of complexes 2, 3, [Ir(H)(OTf)(κ²-NSi^tBu₂)(coe)] (4), and [Ir(H)(OTf)(κ²-NSi^tBu₂)(PCy₃)] (5) (0.1 mol%) as catalysts precursors for the solventless formic acid dehydrogenation (FADH) in the presence of Et₃N (40 mol%) at 353 K has been performed. The highest activity (TOF_{5 min} ≈ 3260 h^-1) has been obtained with 3 at 373 K. However, at that temperature the FTIR spectra show traces of CO together with the desired products (H₂ and CO₂). Thus, the best performance was achieved at 353 K (TOF_{5 min} ≈ 1210 h^-1 and no observable CO). Kinetic studies at variable temperature show that the activation energy of the 3-catalyzed FADH process is 16.76 kcal mol^-1. Kinetic isotopic effect (5 min) values of 1.6, 4.5, and 4.2 were obtained for the 3-catalyzed dehydrogenation of HCOOD, DCOOH, and DCOOD, respectively, at 353 K. The strong KIE found for DCOOH and DCOOD evidenced that the hydride transfer from the C-H bond of formic acid to the metal is the rate-determining step of the process.

Photoinduced Catalytic Organic-Hydride Transfer to CO2 Mediated with Ruthenium Complexes as NAD+ /NADH Redox Couple Models

The catalytic organic-hydride transfer to CO₂ was first achieved through the photoinduced two-electron reduction of the [Ru(bpy)₂ (pbn)]²⁺ /[Ru(bpy)₂ (pbnHH)]²⁺ (bpy=2,2'-bipyridine, pbn=2-(pyridin-2-yl)benzo[b]-1,5-naphthyridine, and pbnHH=2-(pyridin-2-yl)-5,10-dihydrobenzo[b]-1,5-naphthyridine) redox couple in the presence of 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH). The active species for the catalytic hydride transfer to carbon dioxide giving formate is [Ru(bpy)(bpy⋅^- )(pbnHH)]⁺ formed by one-electron reduction of [Ru(bpy)₂ (pbnHH)]²⁺ with BI⋅.

Versatile CO2 Hydrogenation-Dehydrogenation Catalysis with a Ru-PNP/Ionic Liquid System

High catalytic activities of Ru-PNP [Ru = ruthenium; PNP = bis alkyl- or aryl ethylphosphinoamine complexes in ionic liquids (ILs) were obtained for the reversible hydrogenation of CO₂ and dehydrogenation of formic acid (FA) under exceedingly mild conditions and without sacrificial additives. The novel catalytic system relies on the synergic combination of Ru-PNP and IL and proceeds with CO₂ hydrogenation already at 25 °C under a continuous flow of 1 bar of CO₂/H₂ (1:5), leading to 14 mol % FA with respect to the IL. A pressure of 40 bar of CO₂/H₂ (1:1) provides 126 mol % of FA/IL corresponding to a space-time yield (STY) of FA of 0.15 mol L^-1 h^-1. The conversion of CO₂ contained in imitated biogas was also achieved at 25 °C. Furthermore, the Ru-PNP/IL system catalyzes FA dehydrogenation with average turnover frequencies up to 11,000 h^-1 under heat-integrated conditions for proton-exchange membrane fuel cell applications (<100 °C). Thus, 4 mL of a 0.005 M Ru-PNP/IL system converted 14.5 L FA over 4 months with a turnover number exceeding 18,000,000 and a STY of CO₂ and H₂ of 35.7 mol L^-1 h^-1. Finally, 13 hydrogenation/dehydrogenation cycles were achieved with no sign of deactivation. These results demonstrate the potential of the Ru-PNP/IL system to serve as a FA/CO₂ battery, a H₂ releaser, and a hydrogenative CO₂ converter.

A Recent Review of Primary Hydrogen Carriers, Hydrogen Production Methods, and Applications

Hydrogen is a promising energy carrier, especially for transportation, owing to its unique physical and chemical properties. Moreover, the combustion of hydrogen gas generates only pure water; thus, its wide utilization can positively affect human society to achieve global net zero CO2 emissions by 2050. This review summarizes the characteristics of the primary hydrogen carriers, such as water, methane, methanol, ammonia, and formic acid, and their corresponding hydrogen production methods. Additionally, state-of-the-art studies and hydrogen energy applications in recent years are also included in this review. In addition, in the conclusion section, we summarize the advantages and disadvantages of hydrogen carriers and hydrogen production techniques and suggest the challenging tasks for future research.

Second Sphere Effects Promote Formic Acid Dehydrogenation by a Single-Atom Gold Catalyst Supported on Amino-Substituted Graphdiyne

Regulating the second sphere of homogeneous molecular catalysts is a common and effective method to boost their catalytic activities, while the second sphere effects have rarely been investigated for heterogeneous single-atom catalysts primarily due to the synthetic challenge for installing functional groups in their second spheres. Benefiting from the well-defined and readily tailorable structure of graphdiyne (GDY), an Au single-atom catalyst on amino-substituted GDY is constructed, where the amino group is located in the second sphere of the Au center. The Au atoms on amino-decorated GDY displayed superior activity for formic acid dehydrogenation compared with those on unfunctionalized GDY. The experimental studies, particularly the proton inventory studies, and theoretical calculations revealed that the amino groups adjacent to an Au atom could serve as proton relays and thus facilitate the protonation of an intermediate Au-H to generate H₂ . Our study paves the way to precisely constructing the functional second sphere on single-atom catalysts.

Does the Spin State and Oriented External Electric Field Boost the Efficiency of Fe(II) Pincer Catalyst toward CO2 Hydrogenation Reaction?

In this study, we have explored the catalytic reactivities of four PNP-pincer supported Fe(II) complexes, namely, [(^iPrPN^MeP)FeH₂(CO)] (1), [(^iPrPN^MeP)FeH(CO)(BH₄)] (2), [(^iPrPN^HP)FeH₂(CO)] (3), and [(^iPrPN^MeP)FeH(BH₄)] (4) (^iPrPN^MeP = MeN{CH₂CH₂(PⁱPr₂)}₂ and ^iPrPN^HP = HN{CH₂CH₂(PⁱPr₂)}₂) toward reductive CO₂ hydrogenation for formate production. Our density functional theory and ab initio complete active space self-consistent field study have identified three fundamental steps in this catalytic transformation: (i) anchoring of the CO₂ molecule in the vicinity of the metal using noncovalent interactions, (ii) catalyst regeneration via H₂ cleavage, and (iii) formate rebound step leading to catalytic poisoning. The variations in the catalytic efficiency observed among these catalysts were attributed to either easing of steps (i) and (ii) or the hampering step (iii). This can be achieved in various chemical/non-chemical ways, for instance, (a) incorporation of strong-field ligands such as CO facilitating single-state reactivity and eliminating two-state reactivity that generally enhances the rate and (b) inclusion of Lewis acids such as LiOTf and strong bases found to either avoid catalytic poisoning or ease the H-H cleavages, to enhance the rate of reaction (c) evading mixing of excited open-shell singlet states to the ground closed-shell singlet state that hampers the catalytic regeneration. We have probed the role of oriented external electric fields (OEEFs) in the entire mechanistic profile for the best and worst catalyst, and our study suggests that imposing OEEFs opposite to the reaction axis (z-axis) fastens the catalytic regeneration step and, at the same time, hampers catalytic poisoning. The application of OEEFs is found to regulate the energetics of various spin states and can hamper two-state reactivity, therefore increasing the efficiency. Thus, this study provides insights into the CO₂ hydrogenation mechanism where the role of bases/Lewis acid, ligand design, spin states, and electric field in a particular direction has been established and is, therefore, likely to pave the way forward for a new generation of catalysts.

Interheteromolecular Hyperconjugation Boosts (De)hydrogenation for Reversible H2 Storage

Interheteromolecular hyperconjugation is ubiquitous in organic systems, affecting bond length, dipole moments, conformations and so on, while its effect on (de)hydrogenation reactivity in a heterogeneous thermo-catalytic system has rarely been explored. Herein, the N-heterocycles containing a benzene ring and aliphatic chain [N-ethylcarbazole (NEC) and N-propylcarbazole (NPC)] were utilized to study the correlation between interheteromolecular hyperconjugation and catalytic (de)hydrogenation. Density functional theory calculations, variable-temperature ¹ H nuclear magnetic resonance spectroscopy, and catalytic experiments showed that the presented hyperconjugation between NEC and NPC weakened the electron cloud density of aromatic rings and thus facilitated the reactivity with hydrogen featuring unpaired electrons. Therefore, an extremely low temperature of 80 °C was enough for the hydrogenation. Moreover, this interheteromolecular hyperconjugation was general in other N-heterocycles (e. g., N-methyindole and NPC) and was also effective to (de)deuterate as revealed by isotope experiments. This work expands the application of interheteromolecular hyperconjugation to heterogeneous thermocatalysis for reversible H₂ storage.

State of Formic Acid Dissolved in Tar According to Infrared Spectroscopy

Formic acid is considered as a promising hydrogen carrier and can be used as a source of hydrogen in the processing of heavy oil fractions such as tar. The interaction of formic acid with tar was studied by infrared Fourier transform spectroscopy via special technique using a mirror substrate. The infrared (IR) spectra were interpreted considering density functional theory (DFT) calculations. It was shown that formic acid dissolved in tar in three forms, as dimers, monomers of cis- and trans-configurations, hydrogen-bonded to the aromatic rings of the tar compounds, and as free-rotating gas molecules (microbubbles in the tar bulk). The research performed provides an opportunity and methodological base for studying the process of tar conversion in the presence of formic acid into gasoline fractions at temperatures up to 300 ^oC.

Advancements and Challenges in Reductive Conversion of Carbon Dioxide via Thermo-/Photocatalysis

Carbon dioxide (CO₂) is the major greenhouse gas and also an abundant and renewable carbon resource. Therefore, its chemical conversion and utilization are of great attraction for sustainable development. Especially, reductive conversion of CO₂ with energy input has become a current hotspot due to its ability to access fuels and various important chemicals. Nowadays, the controllable CO₂ hydrogenation to formic acid and alcohols using sustainable H₂ resources has been regarded as an appealing solution to hydrogen storage and CO₂ accumulation. In addition, photocatalytic CO₂ reduction to CO also provides a potential way to utilize this greenhouse gas efficiently. Besides direct CO₂ hydrogenation, CO₂ reductive functionalization integrates CO₂ reduction with subsequent C-X (X = N, S, C, O) bond formation and indirect transformation strategies, enlarging the diverse products derived from CO₂ and promoting CO₂ reductive conversion into a new stage. In this Perspective, the progress and challenges of CO₂ reductive conversion, including hydrogenation, reductive functionalization, photocatalytic reduction, and photocatalytic reductive functionalization are summarized and discussed along with the key issues and future trends/directions in this field. We hope this Perspective can evoke intense interest and inspire much innovation in the promise of CO₂ valorization.

Single-Site Heterogeneous Organometallic Ir Catalysts Embedded on Graphdiyne: Structural Manipulation Beyond the Carbon Support

Accurate control over the coordination circumstances of single-atom catalysts (SACs) is decisive to their intrinsic activity. Here, two single-site heterogeneous organometallic catalysts (SHOCs), Cp*Ir-L/GDY (L = OH^- and Cl^- ; Cp* = pentamethylcyclopentadienyl), with the fine-tuned local coordination and electronic structure of Ir sites, are constructed by anchoring Cp*Ir complexes on graphdiyne (GDY) matrix via a one-pot procedure. The spectroscopic studies and theoretical calculations indicate that the Ir atoms in Cp*Ir-Cl/GDY and Cp*Ir-OH/GDY have a much higher oxidation state than Ir in the SAC Ir/GDY. As a proof-of-principle demonstration, the GDY-supported SHOCs are used for formic acid dehydrogenation, which display a fivefold enhancement of catalytic activity compared with SAC Ir/GDY. The kinetic isotope effect and in situ Fourier-transform infrared studies reveal that the rate-limiting step is the β-hydride elimination process, and Cp* on the Ir site accelerates the β-hydride elimination reaction. The GDY-supported SHOCs integrate the merits of both SACs and molecular catalysts, wherein the isolated Ir anchored on GDY echoes with SACs' behavior, and the Cp* ligand enables precise structural and electronic regulation like molecular catalysts. The scheme of SHOCs adds a degree of freedom in accurate regulation of the local structure, the electronic property, and therefore the catalytic performance of single-atom catalysts.

Manganese Promoted (Bi)carbonate Hydrogenation and Formate Dehydrogenation: Toward a Circular Carbon and Hydrogen Economy

We report here a feasible hydrogen storage and release process by interconversion of readily available (bi)carbonate and formate salts in the presence of naturally occurring α-amino acids. These transformations are of interest for the concept of a circular carbon economy. The use of inorganic carbonate salts for hydrogen storage and release is also described for the first time. Hydrogenation of these substrates proceeds with high formate yields in the presence of specific manganese pincer catalysts and glutamic acid. Based on this, cyclic hydrogen storage and release processes with carbonate salts succeed with good H₂ yields.

Liquid Sunshine: Formic Acid

"Liquid sunshine" is the conceptual green liquid fuel that is produced by a combination of solar energy, CO₂, and H₂O. Alcohols are commonly regarded as the preferred candidates for liquid sunshine because of their advantages of high energy density and extensive industrial applications. However, both the alcohol synthesis and H₂ release processes require harsh reaction conditions, resulting in large external energy input. Unlike alcohols, the synthesis and dehydrogenation of formic acid (FA)/formate can be performed under mild conditions. Herein, we propose liquid sunshine FA/formate as a promising supplement to alcohol. First, we outline the vision of using FA/formate as liquid sunshine and discuss its feasibility. Then, we concentrate on the application of FA/formate as liquid organic hydrogen carrier and summarize the recent developments of CO₂ hydrogenation to FA/formate and FA/formate dehydrogenation under mild conditions. Finally, we discuss the current applications, challenges, and opportunities surrounding the use of FA/formate as liquid sunshine.

Cyclometalated iridium complexes based on monodentate aminophosphanes

Monodentate aminophosphanes HNP [NH(4-tolyl)PPh₂] and SiMe₃NP [SiMe₃N(4-tolyl)PPh₂] react with [Ir(μ-Cl)(cod)]₂ affording tetra- or pentacoordinate complexes of formula [IrCl(L)_n(cod)] (L = HNP, n = 1, 2; L = SiMe₃NP, n = 1). The reaction of [IrCl(SiMe₃NP)(cod)] with carbon monoxide smoothly renders [Ir(CO)₃(SiMe₃NP)₂][IrCl₂(CO)₂]. The reaction of HNP or SiMe₃NP with [Ir(CH₃CN)₂(cod)][PF₆] yields the cyclometalated iridium(III)-hydride derivatives [IrH{κ²C,P-NR(4-C₆H₃CH₃)PPh₂}(cod)(CH₃CN)][PF₆] (R = H, SiMe₃) as a result of the intramolecular oxidative addition of the tolyl C²-H bond to iridium. The straighforward formation of [IrH{κ²C,P-SiMe₃N(4-C₆H₃CH₃)PPh₂}(cod)(CH₃CN)]⁺ was observed when the reaction was monitored by NMR spectroscopy at 233 K, whereas a more complex reaction sequence was observed in the formation of [IrH{κ²C,P-NH(4-C₆H₃CH₃)PPh₂}(cod)(CH₃CN)]⁺, including the formation of [IrH{κ²C,P-NH(4-C₆H₃CH₃)PPh₂}(HNP)(cod)]⁺ and [Ir(cod)(HNP)₂]⁺. The "mixed" complex [IrH{κ²C,P-SiMe₃N(4-C₆H₃CH₃)PPh₂}(HNP)(cod)]⁺ was obtained upon reaction of [IrH{κ²C,P-NH(4-C₆H₃CH₃)PPh₂}(cod)(CH₃CN)][PF₆] with SiMe₃NP at 233 K. Finally, the reaction of [Ir(CH₃CN)₂(coe)₂][PF₆] with SiMe₃NP or HNP resulted in the formation of [Ir(CH₃CN)₂(SiMe₃NP)₂][PF₆] and [IrH{κ²C,P-NH(4-C₆H₃CH₃)PPh₂}(HNP)₂(CH₃CN)][PF₆], respectively. Both the OC-6-35 and the OC-6-52 isomers of [IrH{κ²C,P-NH(4-C₆H₃CH₃)PPh₂}(HNP)₂(CH₃CN)]⁺ - featuring facial and meridional dispositions of the phosphorus atoms, respectively - were isolated depending on the reaction solvent. Several compounds described herein catalyse the dehydrogenation of formic acid in DMF, [IrCl(HNP)₂(cod)] being the most active, with TOF_{1 min} of about 2300 h^-1 (5 mol% catalyst, 50 mol% sodium formate, DMF, 80 °C).

Single-Site Iridium Picolinamide Catalyst Immobilized onto Silica for the Hydrogenation of CO2 and the Dehydrogenation of Formic Acid

The development of an efficient heterogeneous catalyst for storing H₂ into CO₂ and releasing it from the produced formic acid, when needed, is a crucial target for overcoming some intrinsic criticalities of green hydrogen exploitation, such as high flammability, low density, and handling. Herein, we report an efficient heterogeneous catalyst for both reactions prepared by immobilizing a molecular iridium organometallic catalyst onto a high-surface mesoporous silica, through a sol–gel methodology. The presence of tailored single-metal catalytic sites, derived by a suitable choice of ligands with desired steric and electronic characteristics, in combination with optimized support features, makes the immobilized catalyst highly active. Furthermore, the information derived from multinuclear DNP-enhanced NMR spectroscopy, elemental analysis, and Ir L₃-edge XAS indicates the formation of cationic iridium sites. It is quite remarkable to note that the immobilized catalyst shows essentially the same catalytic activity as its molecular analogue in the hydrogenation of CO₂. In the reverse reaction of HCOOH dehydrogenation, it is approximately twice less active but has no induction period.

We report the synthesis of a heterogeneous immobilized catalyst (Ir_PicaSi_SiO₂) and its successful application in aqueous CO₂ hydrogenation and FA dehydrogenation. The information derived from multinuclear DNP-enhanced NMR spectroscopy, elemental analysis, and XAS indicates the presence of cationic iridium sites in Ir_PicaSi_SiO₂. The latter shows essentially the same catalytic activity as its molecular analogue in the hydrogenation of CO₂. In the reverse reaction of HCOOH dehydrogenation, it is approximately twice less active but has no induction period.

Alumina‐Supported Alpha‐Iron(III) Oxyhydroxide as a Recyclable Solid Catalyst for CO 2 Photoreduction under Visible Light

Photocatalytic conversion of CO₂ into transportable fuels such as formic acid (HCOOH) under sunlight is an attractive solution to the shortage of energy and carbon resources as well as to the increase in Earth's atmospheric CO₂ concentration. The use of abundant elements as the components of a photocatalytic CO₂ reduction system is important, and a solid catalyst that is active, recyclable, nontoxic, and inexpensive is strongly demanded. Here, we show that a widespread soil mineral, alpha‐iron(III) oxyhydroxide (α‐FeOOH; goethite), loaded onto an Al₂O₃ support, functions as a recyclable catalyst for a photocatalytic CO₂ reduction system under visible light (λ>400 nm) in the presence of a Ru^II photosensitizer and an electron donor. This system gave HCOOH as the main product with 80–90 % selectivity and an apparent quantum yield of 4.3 % at 460 nm, as confirmed by isotope tracer experiments with ¹³CO₂. The present work shows that the use of a proper support material is another method of catalyst activation toward the selective reduction of CO₂.

Cluster self-assembly and anion binding by metal complexes of non-innocent thiazolidinyl-thiolate ligands

Zn^II and Fe^II chloride complexes of a di(methylthiazolidinyl)pyridine ligand were deprotonated to form the corresponding thiolate complexes supported by redox-active iminopyridine moieties. The thiolate donor groups are nucleophilic and reactive toward oxidants, electrophiles, and protons, while the pendant thiazolidine rings are available for hydrogen bonding. Anion exchange with the weakly-coordinating triflate anion resulted in self-assembly of the iminopyridine complexes to form a trimeric [M₃S₃] cluster. Hydrogen bonding closely associates anions with this trimetallic core.

Recent developments in first-row transition metal complex-catalyzed CO2 hydrogenation

Our modern civilization is currently standing at a crossroads due to excessive emission of anthropogenic CO₂ leading to adverse climate change effects. Hence, a proper CO₂ management strategy, including appropriate CO₂ capture, utilization, and storage (CCUS), has become a prime concern globally. On the other hand, C₁ chemicals such as methanol (CH₃OH) and formic acid (HCOOH) have emerged as leading materials for a wide range of applications in various industries, including chemical, biochemical, pharmaceutical, agrochemical, and even energy sectors. Hence, there is a concerted effort to bridge the gap between CO₂ management and methanol/formic acid production by employing CO₂ as a C₁-synthon. CO₂ hydrogenation to methanol and formic acid has emerged as one of the primary routes for directly converting CO₂ to a copious amount of methanol and formate, which is typically catalyzed by transition metal complexes. In this frontier article, we have primarily discussed the abundant first-row transition metal-driven hydrogenation reaction that has exhibited a significant surge in activity over the past few years. We have also highlighted the potential future direction of the research while incorporating a comparative analysis for the competitive second and third-row transition metal-based hydrogenation.

Homogeneous Carbon Capture and Catalytic Hydrogenation: Toward a Chemical Hydrogen Battery System

Recent developments of CO₂ capture and subsequent catalytic hydrogenation to C1 products are discussed and evaluated in this Perspective. Such processes can become a crucial part of a more sustainable energy economy in the future. The individual steps of this catalytic carbon capture and usage (CCU) approach also provide the basis for chemical hydrogen batteries. Here, specifically the reversible CO₂/formic acid (or bicarbonate/formate salts) system is presented, and the utilized catalysts are discussed.

Iridium-Catalyzed Dehydrogenation in a Continuous Flow Reactor for Practical On-Board Hydrogen Generation From Liquid Organic Hydrogen Carriers

To enable the large-scale use of hydrogen fuel cells for mobility applications, convenient methods for on-board hydrogen storage and release are required. A promising approach is liquid organic hydrogen carriers (LOHCs), since these are safe, available on a large scale, and compatible with existing refueling infrastructure. Usually, LOHC dehydrogenation is carried out in batch-type reactors by transition metals and their complexes and suffers from slow H₂ release kinetics and/or inability to reach high energy density by weight, owing to low conversion or the need to dilute the reaction mixture. In this study, a continuous flow reactor is used in combination with a heterogenized iridium pincer complex, which enables a tremendous increase in LOHC dehydrogenation rates. Thus, dehydrogenation of isopropanol is performed in a regime that, in terms of gravimetric energy density, hydrogen generation rate, and precious metal content, is potentially compatible with applications in a fuel-cell powered car.

Utilization of Low-Concentration CO2 with Molecular Catalysts Assisted by CO2-Capturing Ability of Catalysts, Additives, or Reaction Media

Increasing concentration of atmospheric CO₂ is a worldwide concern and continues to trigger various environmental problems. Photo- or electrocatalytic CO₂ reduction (CO₂-Red) using solar energy, i.e., artificial photosynthesis, is a prospective technique owing to its sustainability and the usefulness of the reaction products. Concentrations of CO₂ in exhaust gases from industries are several % to 20%, and that in the atmosphere is about 400 ppm. Although condensation processes of CO₂ require high energy consumption and cost, pure CO₂ has been used in most of the reported studies for photo- and electrocatalytic CO₂-Red because the reaction between CO₂ and the catalyst could be one of the rate-limiting steps. To address these issues and provide a repository of potential techniques for other researchers, this perspective summarizes the catalytic systems reported for the reduction of low-concentration CO₂, which utilize a combination of catalytic CO₂-Red and CO₂-capturing reactions (or CO₂ adsorption). First, we describe CO₂ insertions into M-X bonds of the catalysts, which increase the rate constants and/or equilibrium constants for CO₂ binding on the catalysts, and modifications of the second coordination sphere to stabilize the CO₂-bound catalysts. Furthermore, we discuss the reaction media used for catalytic CO₂-Red that have the unique effect of increasing CO₂ concentrations around the catalysts. These reaction media include typical CO₂-capturing additives, ionic liquids, and metal-organic frameworks.