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ConspectusThe Paradigm shift in considering CO 2 as an alternative carbon feedstock as opposed to a waste product has recently prompted intense research activities. The implementation of CO 2 utilization may be achieved by designing highly efficient catalysts, exploring processes that minimize energy consumption and simplifying product purification and separation. Among possible target products derived from CO 2 , methanol is highly valuable because it can be used in various chemical feedstocks and as a fuel. Although it is currently produced on a plant scale by heterogeneous catalysis using a Cu/ZnO-based catalyst, a limited theoretical conversion ratio at high reaction temperatures remains an issue. In addition, a catalytic system that can be adjusted to accommodate a variable renewable energy source for the synthesis of methanol is more desirable than current continuous-operation systems, which require a reliable energy supply. Recently, significant progress has been made in the field of homogeneous catalysis, which primarily relies on an indirect route to synthesize methanol via the hydrogenation of carbonate or formate derivatives in the presence of additives and solvents. However, homogeneous catalysis is inappropriate for industrial-scale methanol production because of the inefficient separation and purification processes involved.In this Account, we demonstrate a novel approach for methanol production under mild reaction conditions by CO 2 hydrogenation catalyzed by multinuclear iridium complexes under heterogeneous gas-solid phase conditions without any additives and solvents. One of the aims of this Account provides insights for overcoming the barriers for efficient CO 2 hydrogenation by focusing on catalyst design, specifically by incorporating varying functionalities into the ligand. The fundamental strategy entails activating hydrogen molecule and enhancing the hydricity of the resulting metal-hydride species, which is based on the following two concepts of catalyst design: (i) Activating a metal-hydride by electronic effects; and (ii) accelerating H 2 heterolysis. We have elucidated the mechanism for accelerating H 2 heterolysis using a state-of-the-art catalyst that contains an actor-ligand that responds to or participates in catalysis as opposed to a classical spectator-ligand .We have also demonstrated a novel heterogeneous catalysis using a molecular catalyst as a key step for the hydrogenation of CO 2 to methanol beyond formic acid formation. The dehydrogenation of formic acid as a reverse reaction of formic acid hydrogenation is strongly favored in acidic aqueous solution. To circumvent the equilibrium limitation, we have envisioned an alternative route that both prevents the liberation of formic acid into the reaction medium, and develops a multinuclear complex to facilitate the transfer of multiple reactive hydrides. The unconventional gas-solid phase catalysis is capable of preventing the liberation of formate species and promoting further hydrogenation of formic acid through multihydride transfer.This novel catalytic system, which is the fusion of a molecular catalyst in heterogeneous catalysis, provides high performance for methanol synthesis through a sophisticated catalyst design and straightforward separation processes. A detailed mechanistic analysis of molecular catalysts in the gas phase would lead to significant progress in the field of Surface Organometallic Chemistry (SOMC).

The catalytic hydrogenation of carbon dioxide to formate is of great interest due to its significant role in CO 2 utilization. In this study, a novel heterogeneous Ru(III) catalyst was prepared by immobilizing RuCl 3 on a porous organic polymer (POP) obtained from 1,4-phthalaldehyde (PTA) and 4,4'-biphenyldicarboxaldehyde (BPDA) with melamine. A copolymerization strategy utilizing monomers of varying lengths was employed to prepare the POP-supported Ru catalyst with adjustable porosity. The optimization of the framework porosity resulted in enhanced CO 2 affinity, accelerated mass transfer, and a remarkable enhancement in catalytic activity. A high turnover number (TON) of 2458 was achieved for the CO 2 hydrogenation to formate in 2 h with catalyst Cat-3 under 3 MPa (CO 2 /H 2 = 1 : 1) at 120 °C in 1 M Et 3 N aqueous solution. Moreover, the Cat-3 demonstrated good recyclability and was able to be reused for five consecutive runs, resulting in a high total TON of 9971.

Covalent triazine frameworks (CTFs) are emerging as a promising molecular platform for photocatalysis. Nevertheless, the construction of highly effective charge transfer pathways in CTFs for oriented delivery of photoexcited electrons to enhance photocatalytic performance remains highly challenging. Herein, a molecular engineering strategy is presented to achieve highly efficient charge separation and transport in both the lateral and vertical directions for solar-to-formate conversion. Specifically, a large π-delocalized and π-stacked Schottky junction (Ru-Th-CTF/RGO) that synergistically knits a rebuilt extended π-delocalized network of the D-A 1 -A 2 system (multiple donor or acceptor units, Ru-Th-CTF) with reduced graphene oxide (RGO) is developed. It is verified that the single-site Ru units in Ru-Th-CTF/RGO act as effective secondary electron acceptors in the lateral direction for multistage charge separation/transport. Simultaneously, the π-stacked and covalently bonded graphene is regarded as a hole extraction layer, accelerating the separation/transport of the photogenerated charges in the vertical direction over the Ru-Th-CTF/RGO Schottky junction with full use of photogenerated electrons for the reduction reaction. Thus, the obtained photocatalyst has an excellent CO 2 -to-formate conversion rate (≈11050 µmol g -1 h -1 ) and selectivity (≈99%), producing a state-of-the-art catalyst for the heterogeneous conversion of CO 2 to formate without an extra photosensitizer., (© 2023 Wiley-VCH GmbH.)

For the application of CO 2 as an energy storage material, a H 2 storage system has been proposed based on the interconversion of CO 2 and formic acid (or formate). However, energy losses are inevitable in the conversion of electrical energy to H 2 as chemical energy (≈70 % electrical efficiency) and H 2 to electrical energy (≈40 % electrical efficiency). To overcome these significant energy losses, we developed a system based on the interconversion of CO 2 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 CO 2 -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 Ir IV ., (© 2023 Wiley-VCH GmbH.)

Recently, there has been a strong demand for technologies that use hydrogen as an energy carrier, instead of fossil fuels. Hence, new and effective hydrogen storage technologies are attracting increasing attention. Formic acid (FA) is considered an effective liquid chemical for hydrogen storage because it is easier to handle than solid or gaseous materials. This review presents recent advances in research into the development of homogeneous catalysts, primarily focusing on hydrogen generation by FA dehydrogenation. Notably, this review will aid in the development of useful catalysts, thereby accelerating the transition to a hydrogen-based society.

Earth-abundant metal-catalyzed oxidative conversion of ammonia into dinitrogen is a promising process to utilize ammonia as a transportation fuel. Herein, we report the manganese-catalyzed ammonia oxidation under chemical or electrochemical conditions using a manganese complex bearing (1S,2S)-N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamine. Under chemical conditions using oxidant, up to 17.1 equivalents of N 2 per catalyst are generated. Also, mechanistic studies by stoichiometric reactions reveal that a nucleophilic attack of ammonia on manganese nitrogenous species occurs to form a nitrogen-nitrogen bond leading to dinitrogen. Moreover, we conduct density functional theory (DFT) calculations to confirm the plausible reaction mechanism. In addition, this reaction system is applicable under electrochemical conditions. The catalytic reaction proceeds with 96 % faradaic efficiency (FE) in bulk electrolysis to give up to 6.56 equivalents of N 2 per catalyst., (© 2021 Wiley-VCH GmbH.)

We report a novel approach toward the catalytic hydrogenation of CO 2 to methanol performed in the gas-solid phase using multinuclear iridium complexes at low temperature (30-80 °C). Although homogeneous CO 2 hydrogenation in water catalyzed by amide-based iridium catalysts provided only a negligible amount of methanol, the combination of a multinuclear catalyst and gas-solid phase reaction conditions led to the effective production of methanol from CO 2 . The catalytic activities of the multinuclear catalyst were dependent on the relative configuration of each active species. Conveniently, methanol obtained from the gas phase could be easily isolated from the catalyst without contamination with CO, CH 4 , or formic acid (FA). The catalyst can be recycled in a batchwise manner via gas release and filling. A final turnover number of 113 was obtained upon reusing the catalyst at 60 °C and 4 MPa of H 2 /CO 2 (3:1). The high reactivity of this system has been attributed to hydride complex formation upon exposure to H 2 gas, suppression of the liberation of FA under gas-solid phase reaction conditions, and intramolecular multiple hydride transfer to CO 2 by the multinuclear catalyst.

Formic acid (FA) is among the most promising hydrogen storage materials. The development of efficient catalysts for the dehydrogenation of FA via molecular-level control and precise tuning remains challenging. A series of biomimetic Ir complexes was developed for the efficient dehydrogenation of FA in an aqueous solution without base addition. A high turnover frequency of 46510 h -1 was achieved at 90 °C in 1 m FA solution with complex 1 bearing pendant pyridine. Experimental and mechanistic studies revealed that the integrated pendant pyridine and pyrazole moieties of complex 1 could act as proton relay and facilitate proton shuttling in the outer coordination sphere. This study provides a new strategy to control proton transfer accurately and a new principle for the design of efficient catalysts for FA dehydrogenation., (© 2020 Wiley-VCH GmbH.)

Novel heterodinuclear Ir III -M II complexes (M = Co, Ni, or Cu) with two adjacent reaction sites were synthesized by using 3,5-bis(2-pyridyl)-pyrazole (Hbpp) as a structure-directing ligand and employed as catalysts for H 2 evolution through formic acid dehydrogenation in water. A cooperative effect of the hetero-metal centers was observed in the H 2 evolution in comparison with the corresponding mononuclear Ir III and M II complexes as the components of the Ir III -M II complexes. The H 2 evolution rate for the Ir III -M II complexes was at most 350-fold higher than that of the mononuclear Ir III complex. The catalytic activity increased in the following order: Ir III -Cu II complex < Ir III -Co II complex < Ir III -Ni II complex . The Ir III -H intermediates of the Ir III -M II complexes were successfully detected by ultraviolet-visible, 1 H nuclear magnetic resonance, and ESI-TOF-MS spectra. The catalytic enhancement of H 2 evolution by the Ir III -M II complexes indicates that the Ir III -H species formed in the Ir III moiety act as reactive species and the M II moieties act as acceleration sites by the electronic effect from the M II center to the Ir III center through the bridging bpp - ligand. The Ir III -M II complexes may also activate H 2 O at the 3d M II centers as a proton source to facilitate H 2 evolution. In addition, the affinity of formate for the Ir III -M II complexes was investigated on the basis of Michaelis-Menten plots; the Ir III -Co II and Ir III -Ni II complexes exhibited affinities that were relatively higher than that of the Ir III -Cu II complex. The catalytic mechanism of H 2 evolution by the Ir III -M II complexes was revealed on the basis of spectroscopic detection of reaction intermediates, kinetic analysis, and isotope labeling experiments.

A series of Cp*Ir (Cp* = pentamethylcyclopentadienyl anion) complexes with amino-functionalized ligands were developed for the production of high-pressure H 2 via catalytic dehydrogenation of formic acid (DFA) in water under base-free conditions. The Ir complexes with 2,2'-bipyridine (bpy) ligands bearing amino or alkylamino groups at the para positions exhibited high activity and stability for DFA compared with complexes containing bpy ligands bearing para -hydroxyl groups. In addition, para -amino groups afforded superior catalytic stability under high-pressure conditions compared with ortho -amino groups. By exploiting these amino-functionalized Cp*Ir complexes, it was possible to continuously produce high-pressure CO-free H 2 via selective DFA in water upon the addition of concentrated FA (>99.5 wt %) to the base-free solution. Systematic investigation of the ligand effects on DFA revealed that the presence of alkylamino groups on the bpy ligand enhanced the catalytic activity (initial turnover frequency, TOF), although the stability decreased with increasing alkyl chain length on the amino groups. According to a Hammett plot, the increased catalytic activity of the Ir complexes after the introduction of amino-functionalized ligands may be attributable to the electron-donating effect of para -amino groups on the bpy ligand. Based on the experimental results, a reaction mechanism is proposed that involves a hydride intermediate whose stability is affected by the position of the amino groups on the bpy ligand, as confirmed through NMR studies.

Aiming to develop a highly effective and durable catalyst for high-pressure H 2 production from dehydrogenation of formic acid (DFA), the ligand effect on the catalytic activity and stability of Cp*Ir (Cp*:pentamethylcyclopentadienyl anion) complexes were investigated using 5 different kinds of N,N'-bidentate ligands (bipyridine, biimidazoline, pyridyl-imidazoline, pyridyl-pyrazole and picolinamide). The Ir complex with biimidazoline ligand exhibited the highest catalytic activity, but deactivation occurred readily at high pressure. The pyridine moiety in the ligand can enhance the stability of Ir complex catalysts for the high-pressure reaction. The Ir complex catalyst containing pyridyl-imidazoline ligand showed the high activity and best stability under the high-pressure conditions., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

To develop highly efficient catalysts for dehydrogenation of formic acid in water, we investigated several Cp*Ir catalysts with various amide ligands. The catalyst with an N-phenylpicolinamide ligand exhibited a TOF of 118 000 h -1 at 60 °C. A constant rate (TOF>35 000 h -1 ) was maintained for six hours, and a TON of 1 000 000 was achieved at 50 °C., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

For harvesting energy from waste heat, the power generation densities and fabrication costs of thermoelectric generators (TEGs) are considered more important than their conversion efficiency because waste heat energy is essentially obtained free of charge. In this study, we propose a miniaturized planar Si-nanowire micro-thermoelectric generator (SiNW-μTEG) architecture, which could be simply fabricated using the complementary metal-oxide-semiconductor-compatible process. Compared with the conventional nanowire μTEGs, this SiNW-μTEG features the use of an exuded thermal field for power generation. Thus, there is no need to etch away the substrate to form suspended SiNWs, which leads to a low fabrication cost and well-protected SiNWs. We experimentally demonstrate that the power generation density of the SiNW-μTEGs was enhanced by four orders of magnitude when the SiNWs were shortened from 280 to 8 μm. Furthermore, we reduced the parasitic thermal resistance, which becomes significant in the shortened SiNW-μTEGs, by optimizing the fabrication process of AlN films as a thermally conductive layer. As a result, the power generation density of the SiNW-μTEGs was enhanced by an order of magnitude for reactive sputtering as compared to non-reactive sputtering process. A power density of 27.9 nW/cm 2 has been achieved. By measuring the thermal conductivities of the two AlN films, we found that the reduction in the parasitic thermal resistance was caused by an increase in the thermal conductivity of the AlN film and a decrease in the thermal boundary resistance.

The hydroxyl groups of a 2,2'-bipyridine (bpy) ligand near the metal center activated the catalytic performance of the Ir complex for the dehydrogenation of formic acid at high pressure. The position of the hydroxyl groups on the ligand affected the catalytic durability for the high-pressure H 2 generation through the decomposition of formic acid. The Ir complex with a bipyridine ligand functionalized with para-hydroxyl groups shows a good durability with a constant catalytic activity during the reaction even under high-pressure conditions, whereas deactivation was observed for an Ir complex with a bipyridine ligand with ortho-hydroxyl groups (2). In the presence of high-pressure H 2 , complex 2 decomposed into the ligand and an Ir trihydride complex through the isomerization of the bpy ligand. This work provides the development of a durable catalyst for the high-pressure H 2 production from formic acid., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Kinetic studies on the catalytic reaction mechanism of formic acid (FA) dehydrogenation were performed in the presence of a water-soluble iridium complex bearing a 4,4'-dihydroxy-2,2'-bipyridine ligand. Determination of kinetic isotope effects revealed that a shift of the rate-limiting step at low and high concentrations of FA can be caused by the pH dependence of the reaction steps. The proposed equation for the reaction rate corresponds well with the experimental results concerning the shift phenomena. Towards industrial application in future hydrogen fueling stations, this will able the design of a dehydrogenation system catalyzed by the iridium complex., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Pentamethylcyclopentadienyl iridium (Cp*Ir) complexes with bidentate ligands consisting of a pyridine ring and an electron-rich diazole ring were prepared. Their catalytic activity toward CO 2 hydrogenation in 2.0 m KHCO 3 aqueous solutions (pH 8.5) at 50 °C, under 1.0 MPa CO 2 /H 2 (1:1) have been reported as an alternative to photo- and electrochemical CO 2 reduction. Bidentate ligands incorporating an electron-rich diazole ring improved the catalytic performance of the Ir complexes compared to the bipyridine ligand. Complexes 2, 4, and 6, possessing both a hydroxy group and an uncoordinated NH group, which are proton-responsive and capable of generating pendent bases in basic media, recorded high initial turnover frequency values of 1300, 1550, and 2000 h -1 , respectively. Spectroscopic and computational investigations revealed that the reversible deprotonation changes the electronic properties of the complexes and causes interactions between pendent base and substrate and/or solvent water molecules, resulting in high catalytic performance in basic media., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)