Controlled Generation of Hydrogen from Formic Acid Amine Adducts at Room Temperature and Application in H2/O2Fuel Cells

Mechanistic Investigation into Copper(I) Hydride Catalyzed Formic Acid Dehydrogenation

Copper(I) hydride complexes are typically known to react with CO2 to form their corresponding copper formate counterparts. However, recently it has been observed that some multinuclear copper hydrides can feature the opposite reactivity and catalyze the dehydrogenation of formic acid. Here we report the use of a multinuclear PNNP copper hydride complex as an active (pre)catalyst for this reaction. Mechanistic investigations provide insights into the catalyst resting state and the rate-determining step and identify an off-cycle species that is responsible for the unexpected substrate inhibition in this reaction.

Hydrogen Production through Distinctive C-C Cleavage during Acetic Acid Reforming at Low Temperature

Acetic acid reforming is a green method for sustainable hydrogen production owing to its renewable source from biomass conversion. However, conventional acetic acid reforming would produce various byproducts, including CO, CH4 and so on. Here, we develop a distinctive method for selective hydrogen production from C-C directional cleavage during acetic acid reforming. Completely different from conventional acetic acid reforming process, acetic acid would react with water over organoruthenium catalyst during its C-C cleavage at low temperature, then produce methanol and formic acid (CH3COOH+H2O→CH3OH+HCOOH). Lastly, methanol and formic acid could further decompose into hydrogen and carbon dioxide over organoruthenium selectively. As a result, there is little CO and CH4 produced in the first step of C-C bond cleavage during acetic acid reforming at 100 °C. Hydrogen production rate is up to 26.8 molH2/(h-1*mol-1 Ru) at 150 °C through a tandem catalysis. A mechanism for C-C cleavage of acetic acid is proposed based on intermediate product analysis and density functional theory (DFT) calculation. Firstly, the C-C single bond was transformed into C=C double bond by dropping one H atom to organoruthenium. Then the coming H2O molecule reacted with the C=C bond by an addition reaction, forming methanol and formic acid. This research not only proposes distinctive reaction pathway for hydrogen production from acetic acid reforming, but also provides some inspiration for selective C-C bond cleavage during ethanol reforming.

Copper Single Atoms Chelated on Ligand-Modified Carbon for Ullmann-type C-O Coupling

Cross-coupling reactions are of great importance in chemistry due to their ability to facilitate the construction of complex organic molecules. Among these reactions, the Ullmann-type C-O coupling between phenols and aryl halides is particularly noteworthy and useful for preparing diarylethers. However, this reaction typically relies on homogeneous catalysts that rapidly deactivate under harsh reaction conditions. In this study, we introduce a novel heterogeneous catalyst for the Ullmann-type C-O coupling reaction, comprised of isolated Cu atoms chelated to a tetraethylenepentamine-pyrrole ligand that is immobilized on graphite nanoplatelets. The catalytic study reveals the recyclability of the material, and demonstrates the crucial role of the pyrrole linker in stabilizing the Cu sites. The work expands the potential of single-atom catalyst nanoarchitectures and underscores the significance of ligands in stabilizing metals in cationic forms, providing a novel, tailored catalyst for cross-coupling chemistries.

Catalytic Dehydrogenation of Formic Acid Promoted by Triphos-Co Complexes: Two Competing Pathways for H2 Production

In this study, we reported the synthesis and structural characterization of a triphos-CoII complex [(κ3-triphos)CoII(CH3CN)2]2+ (1) and a triphos-CoI-H complex [(κ2-triphos)HCoI(CO)2] (4). The facile synthetic pathways from 1 to [(κ3-triphos)CoII(κ2-O2CH)]+ (1') and [(κ3-triphos)CoI(CH3CN)]+ (2), respectively, as well as the interconversion between [(κ3-triphos)CoI(CO)2]+ (3) and 4 have been established. The activation energy barrier, associated with the dehydrogenation of a coordinated formate fragment in 1' yielding the corresponding 2 accompanied by the formation of H2 and CO2, was experimentally determined as 23.9 kcal/mol. With 0.01 mol % loading of 1, a maximum TON ∼ 1735 within 18 h and TOF ∼ 483 h-1 for the first 3 h could be achieved. Kinetic isotope effect (KIE) values of 2.25 (kHCOOH/kDCOOH) and 1.36 (kHCOOH/kHCOOD) for the dehydrogenation of formic acid and its deuterated derivatives, respectively, implicate that the H-COOH bond cleavage is likely the rate-determining step. The catalytic mechanism proposed by density functional theory (DFT) calculations coupled with experimental 1H NMR and gas chromatography-mass spectrometry (GC-MS) analysis unveils two competing pathways for H2 production; specifically, deprotonating a HCOO-H bond by a proposed Co-H intermediate C and homolytic cleavage of the CoII-H moiety of C, presumably via a dimeric Co intermediate D containing a [Co2(μ-H)2]2+ core, to yield the corresponding 2 and H2.

Iridium Catalyst Immobilized on Crosslinked Polyethyleneimine for Continuous Hydrogen Production Using Formic Acid

Hydrogen is an alternative fuel that can play a critical role in achieving net zero emissions, leading to global environment sustainability. An iridium-immobilized catalyst based on polyethyleneimine (PEI) was synthesized and utilized for hydrogen production via formic acid dehydrogenation (FADH). Iridium complex is cross-linked with its ligand and PEI to form the immobilized catalyst, where the iridium content could be easily varied in the range of 1-10 %. The structure of the iridium-immobilized catalyst was confirmed using solid-state NMR, DNP NMR, and FTIR spectroscopies. The iridium-immobilized catalyst with PEI showed excellent catalytic activity for FADH, exhibiting the catalyst's highest turnover frequency (TOF) value of 73 200 h-1 and a large turnover number (TON) value of over 1 130 000. The catalyst could be used for continuous hydrogen production via FADH, exhibiting high durability for over 2 000 h with TON value of 332 889 without any degradation in catalytic activity. The obtained hydrogen gas was evaluated for power generation using a standard fuel cell, as well as achieved 5 h of stable power generation.

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.

Defect-driven nanostructuring of low-nuclearity Pt-Mo ensembles for continuous gas-phase formic acid dehydrogenation

Supported metal clusters comprising of well-tailored low-nuclearity heteroatoms have great potentials in catalysis owing to the maximized exposure of active sites and metal synergy. However, atomically precise design of these architectures is still challenging for the lack of practical approaches. Here, we report a defect-driven nanostructuring strategy through combining defect engineering of nitrogen-doped carbons and sequential metal depositions to prepare a series of Pt and Mo ensembles ranging from single atoms to sub-nanoclusters. When applied in continuous gas-phase decomposition of formic acid, the low-nuclearity ensembles with unique Pt3Mo1N3 configuration deliver high-purity hydrogen at full conversion with unexpected high activity of 0.62 molHCOOH molPt-1 s-1 and remarkable stability, significantly outperforming the previously reported catalysts. The remarkable performance is rationalized by a joint operando dual-beam Fourier transformed infrared spectroscopy and density functional theory modeling study, pointing to the Pt-Mo synergy in creating a new reaction path for consecutive HCOOH dissociations.

Real flue gas CO2 hydrogenation to formate by an enzymatic reactor using O2- and CO-tolerant hydrogenase and formate dehydrogenase

It is challenging to capture carbon dioxide (CO2), a major greenhouse gas in the atmosphere, due to its high chemical stability. One potential practical solution to eliminate CO2 is to convert CO2 into formate using hydrogen (H2) (CO2 hydrogenation), which can be accomplished with inexpensive hydrogen from sustainable sources. While industrial flue gas could provide an adequate source of hydrogen, a suitable catalyst is needed that can tolerate other gas components, such as carbon monoxide (CO) and oxygen (O2), potential inhibitors. Our proposed CO2 hydrogenation system uses the hydrogenase derived from Ralstonia eutropha H16 (ReSH) and formate dehydrogenase derived from Methylobacterium extorquens AM1 (MeFDH1). Both enzymes are tolerant to CO and O2, which are typical inhibitors of metalloenzymes found in flue gas. We have successfully demonstrated that combining ReSH- and MeFDH1-immobilized resins can convert H2 and CO2 in real flue gas to formate via a nicotinamide adenine dinucleotide-dependent cascade reaction. We anticipated that this enzyme system would enable the utilization of diverse H2 and CO2 sources, including waste gases, biomass, and gasified plastics.

Efficient additive-free formic acid dehydrogenation with a NNN-ruthenium complex

A new ruthenium complex containing a pyridylidene amine-based NNN ligand was developed as a catalyst precursor for formic acid dehydrogenation, which, as a rare example, does not require basic additives to display high activity (TOF ∼10 000 h-1). Conveniently, the complex is air-stable, but sensitive to light. Mechanistic investigations using UV-vis and NMR spectroscopic monitoring correlated with gas evolution profiles indicate rapid and reversible protonation of the central nitrogen of the NNN ligand as key step of catalyst activation, followed by an associative step for formic acid dehydrogenation.

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.

The role of oxygen-vacancy in bifunctional indium oxyhydroxide catalysts for electrochemical coupling of biomass valorization with CO2 conversion

Electrochemical coupling of biomass valorization with carbon dioxide (CO₂) conversion provides a promising approach to generate value-added chemicals on both sides of the electrolyzer. Herein, oxygen-vacancy-rich indium oxyhydroxide (InOOH-O_V) is developed as a bifunctional catalyst for CO₂ reduction to formate and 5-hydroxymethylfurfural electrooxidation to 2,5-furandicarboxylic acid with faradaic efficiencies for both over 90.0% at optimized potentials. Atomic-scale electron microscopy images and density functional theory calculations reveal that the introduction of oxygen vacancy sites causes lattice distortion and charge redistribution. Operando Raman spectra indicate oxygen vacancies could protect the InOOH-O_V from being further reduced during CO₂ conversion and increase the adsorption competitiveness for 5-hydroxymethylfurfural over hydroxide ions in alkaline electrolytes, making InOOH-O_V a main-group p-block metal oxide electrocatalyst with bifunctional activities. Based on the catalytic performance of InOOH-O_V, a pH-asymmetric integrated cell is fabricated by combining the CO₂ reduction and 5-hydroxymethylfurfural oxidation together in a single electrochemical cell to produce 2,5-furandicarboxylic acid and formate with high yields (both around 90.0%), providing a promising approach to generate valuable commodity chemicals simultaneously on both electrodes.

A ligand design strategy to enhance catalyst stability for efficient formic acid dehydrogenation

New Ir complexes bearing N-(methylsulfonyl)-2-pyridinecarboxamide (C1) and N-(phenylsulfonyl)-2-pyridinecarboxamide (C2) were employed as catalysts for aqueous formic acid dehydrogenation (FADH). The ligands were designed to maintain the picolinamide skeleton and introduce strong sigma sulfonamide moieties. C1 and C2 exhibited good stability towards air and concentrated formic acid (FA). During 20 continuous cycles, C1 and C2 could achieve the complete conversion of FA with TONs of 172 916 and 172 187, respectively. C1 achieved a high TOF of 19 500 h^-1 at 90 °C and an air-stable Ir-H species was observed by ¹H NMR spectroscopy.

Hydrogen-fueled CO2 reduction using oxygen-tolerant oxidoreductases

Hydrogen gas obtained from cheap or sustainable sources has been investigated as an alternative to fossil fuels. By using hydrogenase (H₂ase) and formate dehydrogenase (FDH), H₂ and CO₂ gases can be converted to formate, which can be conveniently stored and transported. However, developing an enzymatic process that converts H₂ and CO₂ obtained from cheap sources into formate is challenging because even a very small amount of O₂ included in the cheap sources damages most H₂ases and FDHs. In order to overcome this limitation, we investigated a pair of oxygen-tolerant H₂ase and FDH. We achieved the cascade reaction between H₂ase from Ralstonia eutropha H16 (ReSH) and FDH from Rhodobacter capsulatus (RcFDH) to convert H₂ and CO₂ to formate using in situ regeneration of NAD⁺/NADH in the presence of O₂.

Base-Free Catalytic Hydrogen Production from Formic Acid Mediated by a Cubane-Type Mo3S4 Cluster Hydride

Formic acid (FA) dehydrogenation is an attractive process in the implementation of a hydrogen economy. To make this process greener and less costly, the interest nowadays is moving toward non-noble metal catalysts and additive-free protocols. Efficient protocols using earth abundant first row transition metals, mostly iron, have been developed, but other metals, such as molybdenum, remain practically unexplored. Herein, we present the transformation of FA to form H2 and CO2 through a cluster catalysis mechanism mediated by a cuboidal [Mo3S4H3(dmpe)3]+ hydride cluster in the absence of base or any other additive. Our catalyst has proved to be more active and selective than the other molybdenum compounds reported to date for this purpose. Kinetic studies, reaction monitoring, and isolation of the [Mo3S4(OCHO)3(dmpe)3]+ formate reaction intermediate, in combination with DFT calculations, have allowed us to formulate an unambiguous mechanism of FA dehydrogenation. Kinetic studies indicate that the reaction at temperatures up to 60 °C ends at the triformate complex and occurs in a single kinetic step, which can be interpreted in terms of statistical kinetics at the three metal centers. The process starts with the formation of a dihydrogen-bonded species with Mo-H···HOOCH bonds, detected by NMR techniques, followed by hydrogen release and formate coordination. Whereas this process is favored at temperatures up to 60 °C, the subsequent β-hydride elimination that allows for the CO2 release and closes the catalytic cycle is only completed at higher temperatures. The cycle also operates starting from the [Mo3S4(OCHO)3(dmpe)3]+ formate intermediate, again with preservation of the cluster integrity, which adds our proposal to the list of the infrequent cluster catalysis reaction mechanisms.

Formic Acid Dehydrogenation Using Noble-Metal Nanoheterogeneous Catalysts: Towards Sustainable Hydrogen-Based Energy

The need for sustainable energy sources is now more urgent than ever, and hydrogen is significant in the future of energy. However, several obstacles remain in the way of widespread hydrogen use, most of which are related to transport and storage. Dilute formic acid (FA) is recognized asa a safe fuel for low-temperature fuel cells. This review examines FA as a potential hydrogen storage molecule that can be dehydrogenated to yield highly pure hydrogen (H2) and carbon dioxide (CO2) with very little carbon monoxide (CO) gas produced via nanoheterogeneous catalysts. It also present the use of Au and Pd as nanoheterogeneous catalysts for formic acid liquid phase decomposition, focusing on the influence of noble metals in monometallic, bimetallic, and trimetallic compositions on the catalytic dehydrogenation of FA under mild temperatures (20–50 °C). The review shows that FA production from CO2 without a base by direct catalytic carbon dioxide hydrogenation is far more sustainable than existing techniques. Finally, using FA as an energy carrier to selectively release hydrogen for fuel cell power generation appears to be a potential technique.

Recent Progress in Homogeneous Catalytic Dehydrogenation of Formic Acid

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.

Dehydrogenation of Formic Acid Using Iridium-NSi Species as Catalyst Precursors

Using a low loading of the iridium(III) complexes [Ir(CF3SO3)(κ2-NSiiPr)2] (1) (NSiiPr = (4-methylpyridin-2-iloxy)diisopropylsilyl and [{Ir(κ2-NSiMe)2}2(µ-CF3SO3)2] (2) (NSiMe = (4-methylpyridin-2-iloxy)dimethylsilyl) in presence of Et3N, it has been possible to achieve the...

Water-induced gaseous formaldehyde decomposition using ruthenium organic crystalline particles

Novel ruthenium organic crystalline particles are prepared for providing two distinctive approaches for formaldehyde decomposition: catalytic oxidation or water-induced formaldehyde decomposition.

Formic Acid as H2 Storage System: Hydrogenation of CO2 and Decomposition of Formic Acid by Solid Molecular Phosphine Catalysts

The synthesis and decomposition of formic acid (FA) in aqueous triethylamine (NEt3) with solid molecular phosphine catalysts is demonstrated. Ru-catalyst based on the polymeric analog of 1,2-bis(diphenylphosphino)ethane presented the highest...

Impact of Green Cosolvents on the Catalytic Dehydrogenation of Formic Acid: The Case of Iridium Catalysts Bearing NHC-phosphane Ligands

The catalysts [Ir(COD)(κ3-P,C,P'-PCNHCP)]BF4 and [Ir(COD)(κ2-P,C-PCNHCO)]BF4 proved to be active in the solventless dehydrogenation of formic acid. The impact of various cosolvents on the activity was evaluated, showing an outstanding improvement of the catalytic performance of [Ir(COD)(κ2-P,C-PCNHCO)]BF4] in "green" organic carbonates: namely, dimethyl carbonate (DMC) and propylene carbonate (PC). The TOF1h value for [Ir(COD)(κ2-P,C-PCNHCO)]BF4 increases from 61 to 988 h-1 upon changing from solventless conditions to a 1/1 (v/v) DMC/HCOOH mixture. However, in the case of [Ir(COD)(PCNHCP)]BF4, only a marginal improvement from 156 to 172 h-1 was observed under analogous conditions. Stoichiometric experiments allowed the identification of various key reaction intermediates, providing valuable information on their reactivity. Experimental data and DFT calculations point to the formation of dinuclear species as the catalyst deactivation pathway, which is prevented in the presence of DMC and PC.

Bis-Imidazole Methane Ligated Ruthenium(II) Complexes: Synthesis, Characterization, and Catalytic Activity for Hydrogen Production from Formic Acid in Water

A series of half sandwich arene-ruthenium complexes [(η⁶-arene)RuCl(κ²-L)]⁺ ([Ru]-1-[Ru]-10) containing bis-imidazole methane-based ligands {4,4'-(phenylmethylene)bis(2-ethyl-5-methyl-1H-imidazole)} (L1), {4,4'-((4-methoxyphenyl)methylene)bis(2-ethyl-5-methyl-1H-imidazole)} (L2), {4,4'-((2-methoxyphenyl)methylene)bis(2-ethyl-5-methyl-1H-imidazole)} (L3), {4,4'-((4-chlorophenyl)methylene)bis(2-ethyl-5-methyl-1H-imidazole)} (L4), and {4,4'-((2-chlorophenyl)methylene)bis(2-ethyl-5-methyl-1H-imidazole)} (L5) are synthesized. The synthesized and purified complexes ([Ru]-1-[Ru]-10) are further employed for hydrogen production from formic acid in aqueous medium. Among the investigated complexes, [(η⁶-p-cymene)RuCl(κ²-L2)]⁺ [Ru]-2, having Ru(II) coordinated 4-methoxy phenyl substituted bis-imidazole methane ligand (L2), outperformed over others, displaying a higher catalytic turnover of 8830 and high efficiency (TOF = 1545 h^-1) with appreciably high long-term stability for formic acid dehydrogenation in water.

Formic Acid as a Potential On-Board Hydrogen Storage Method: Development of Homogeneous Noble Metal Catalysts for Dehydrogenation Reactions

Hydrogen can be used as an energy carrier for renewable energy to overcome the deficiency of its intrinsically intermittent supply. One of the most promising application of hydrogen energy is on-board hydrogen fuel cells. However, the lack of a safe, efficient, convenient, and low-cost storage and transportation method for hydrogen limits their application. The feasibility of mainstream hydrogen storage techniques for application in vehicles is briefly discussed in this Review. Formic acid (FA), which can reversibly be converted into hydrogen and carbon dioxide through catalysis, has significant potential for practical application. Historic developments and recent examples of homogeneous noble metal catalysts for FA dehydrogenation are covered, and the catalysts are classified based on their ligand types. The Review primarily focuses on the structure-function relationship between the ligands and their reactivity and aims to provide suggestions for designing new and efficient catalysts for H₂ generation from FA.

A Process for Hydrogen Production from the Catalytic Decomposition of Formic Acid over Iridium-Palladium Nanoparticles

The present study investigates a process for the selective production of hydrogen from the catalytic decomposition of formic acid in the presence of iridium and iridium-palladium nanoparticles under various conditions. It was found that a loading of 1 wt.% of 2% palladium in the presence of 1% iridium over activated charcoal led to a 43% conversion of formic acid to hydrogen at room temperature after 4 h. Increasing the temperature to 60 °C led to further decomposition and an improvement in conversion yield to 63%. Dilution of formic acid from 0.5 to 0.2 M improved the decomposition, reaching conversion to 81%. The reported process could potentially be used in commercial applications.

In Situ Bismuth Nanosheet Assembly for Highly Selective Electrocatalytic CO2 Reduction to Formate

The reduction of carbon dioxide (CO₂ ) into value-added fuels using an electrochemical method has been regarded as a compelling sustainable energy conversion technology. However, high-performance electrocatalysts for CO₂ reduction reaction (CO₂ RR) with high formate selectivity and good stability need to be improved. Earth-abundant Bi has been demonstrated to be active for CO₂ RR to formate. Herein, we fabricated an extremely active and selective bismuth nanosheet (Bi-NSs) assembly via an in situ electrochemical transformation of (BiO)₂ CO₃ nanostructures. The as-prepared material exhibits high activity and selectivity for CO₂ RR to formate, with nearly 94% faradaic efficiency at -1.03 V (versus reversible hydrogen electrode (vs. RHE)) and stable selectivity (>90%) in a large potential window ranging from -0.83 to -1.18 V (vs. RHE) and excellent durability during 12 h continuous electrolysis. In addition, the Bi-NSs based CO₂ RR/methanol oxidation reaction (CO₂ RR/MOR) electrolytic system for overall CO₂ splitting was constructed, evidencing the feasibility of its practical implementation.

Formate-Bicarbonate Cycle as a Vehicle for Hydrogen and Energy Storage

In recent years, hydrogen has been considered a promising energy carrier for a sustainable energy economy in the future. An easy solution for the safer storage of hydrogen is challenging and efficient methods are still being explored in this direction. Despite having some progress in this area, no cost-effective and easily applicable solutions that fulfill the requirements of industry are yet to be claimed. Currently, the storage of hydrogen is largely limited to high-pressure compression and liquefaction or in the form of metal hydrides. Formic acid is a good source of hydrogen that also generates CO₂ along with hydrogen on decomposition. Moreover, the hydrogenation of CO₂ is thermodynamically unfavorable and requires high energy input. Alkali metal formates are alternative mild and noncorrosive sources of hydrogen. On decomposition, these metal formates release hydrogen and generate bicarbonates. The generated bicarbonates can be catalytically charged back to alkali formates under optimized hydrogen pressure. Hence, the formate-bicarbonate-based systems being carbon neutral at ambient condition has certain advantages over formic acid. The formate-bicarbonate cycle can be considered as a vehicle for hydrogen and energy storage. The whole process is carbon-neutral, reversible, and sustainable. This Review emphasizes the various catalytic systems employed for reversible formate-bicarbonate conversion. Moreover, a mechanistic investigation, the effect of temperature, pH, kinetics of reversible formate-bicarbonate conversion, and new insights in the field are also discussed in detail.

Efficient Iridium Catalysts for Formic Acid Dehydrogenation: Investigating the Electronic Effect on the Elementary β-Hydride Elimination and Hydrogen Formation Steps

We report herein a series of Cp*Ir complexes containing a rigid 8-aminoquinolinesulfonamide moiety as highly efficient catalysts for the dehydrogenation of formic acid (FA). The complex [Cp*Ir(L)Cl] (HL = N-(quinolin-8-yl)benzenesulfonamide) displayed a high turnover frequency (TOF) of 2.97 × 10⁴ h^-1 and a good stability (>100 h) at 60 °C. Comparative studies of [Cp*Ir(L)Cl] with the rigid ligand and [Cp*Ir(L')Cl] (HL' = N-propylpypridine-2-sulfonamide) without the rigid aminoquinoline moiety demonstrated that the 8-aminoquinoline moiety could dramatically enhance the stability of the catalyst. The electron-donating ability of the N,N'-chelating ligand was tuned by functionalizing the phenyl group of the L ligand with OMe, Cl, and CF₃ to have a systematical perturbation of the electronic structure of [Cp*Ir(L)Cl]. Experimental kinetic studies and density functional theory (DFT) calculations on this series of Cp*Ir complexes revealed that (i) the electron-donating groups enhance the hydrogen formation step while slowing down the β-hydride elimination and (ii) the electron-withdrawing groups display the opposite effect on these reaction steps, which in turn leads to lower optimum pH for catalytic activity compared to the electron-donating groups.

Highly efficient additive-free dehydrogenation of neat formic acid

Formic acid (FA) is a promising hydrogen carrier which can play an instrumental role in the overall implementation of a hydrogen economy. In this regard, it is important to generate H₂ gas from neat FA without any solvent/additive, for which existing systems are scarce. Here we report the remarkable catalytic activity of a ruthenium 9H-acridine pincer complex for this process. The catalyst is unusually stable and robust in FA even at high temperatures and can catalyse neat FA dehydrogenation for over a month, with a total turnover number of 1,701,150, while also generating high H₂/CO₂ gas pressures (tested up to 100 bars). Mechanistic investigations and DFT studies are conducted to fully understand the molecular mechanism to the process. Overall, the high activity, stability, selectivity, simplicity and versatility of the system to generate a CO-free H₂/CO₂ gas stream and high pressure from neat FA makes it promising for large-scale implementation.

A Use-Store-Reuse (USR) Concept in Catalytic HCOOH Dehydrogenation: Case-Study of a Ru-Based Catalytic System for Long-Term USR under Ambient O2

Commercial use of H2 production catalysts requires a repeated use/stop/store and reuse of the catalyst. Ideally, this cycle should be possible under ambient O2. Herein we exemplify the concept of Use-Store-Reuse (USR) of a (Ru-phosphine) catalyst in a biphasic catalytic system, for H2 production via dehydrogenation of HCOOH. The catalytic system can operate uninterrupted for at least four weeks, including storage and reuse cycles, with negligible loss of its catalytic efficiency. The catalytic system consisted of a RuP(CH2CH2PPh2)3 (i.e. RuPP3) in (tri-glyme/water) system, using KOH as a cocatalyst, to promote HCOOH deprotonation. In a USR cycle of 1 week, followed by storage for three weeks under ambient air and reuse, the system achieved in total TONs > 90,000 and TOFs > 4000 h−1. Thus, for the first time, a USR concept with a readily available stable ruthenium catalyst is presented, operating without any protection from O2 or light, and able to retain its catalytic performance.

Efficient carbon dioxide hydrogenation to formic acid with buffering ionic liquids

The efficient transformation of CO₂ into chemicals and fuels is a key challenge for the decarbonisation of the synthetic production chain. Formic acid (FA) represents the first product of CO₂ hydrogenation and can be a precursor of higher added value products or employed as a hydrogen storage vector. Bases are typically required to overcome thermodynamic barriers in the synthesis of FA, generating waste and requiring post-processing of the formate salts. The employment of buffers can overcome these limitations, but their catalytic performance has so far been modest. Here, we present a methodology utilising IL as buffers to catalytically transform CO₂ into FA with very high efficiency and comparable performance to the base-assisted systems. The combination of multifunctional basic ionic liquids and catalyst design enables the synthesis of FA with very high catalytic efficiency in TONs of >8*10⁵ and TOFs > 2.1*10⁴ h⁻¹.

Basic ionic liquids provide a buffering effect that enables the efficient synthesis of free formic acid from CO₂ hydrogenation. Here, a highly efficient catalytic system that transforms CO₂ to formic acid without the need of strong bases is demonstrated, avoiding the formation of formate salts.

Effects of rare earth metal doping on Au/ReZrO2 catalysts for efficient hydrogen generation from formic acid

Rare earth metal doped ZrO₂ can promote the formation of oxygen vacancies in zirconia, which enhances the metal–support interaction, finally promoting catalytic activity of FA dehydrogenation.

Cobalt Single-Atom Catalysts with High Stability for Selective Dehydrogenation of Formic Acid

Metal-organic framework (MOF)-derived Co-N-C catalysts with isolated single cobalt atoms have been synthesized and compared with cobalt nanoparticles for formic acid dehydrogenation. The atomically dispersed Co-N-C catalyst achieves superior activity, better acid resistance, and improved long-term stability compared with nanoparticles synthesized by a similar route. High-angle annular dark-field-scanning transmission electron microscopy, X-ray photoelectron spectroscopy, electron paramagnetic resonance, and X-ray absorption fine structure characterizations reveal the formation of CoII Nx centers as active sites. The optimal low-cost catalyst is a promising candidate for liquid H2 generation.

Hydrogen Generation from Catalytic Reforming of Paraformaldehyde and Water by Polymeric Bifunctional Catalysts Comprising Ruthenium and Sulfonic Acid Units

As a clean and sustainable source of energy, hydrogen shows great potential to be the ultimate energy source in future. In this research, paraformaldehyde is used as hydrogen carrier. Several bifunctional catalysts are prepared for the hydrogen generation from paraformaldehyde. The bifunctional catalysts contain two catalytically active sites. One is a sulfonic acid group for paraformaldehyde hydrolysis, and the other is an organometallic group that catalyzes the hydrogen release from formaldehyde. Bifunctional iridium catalysts and bifunctional rhodium catalysts could only generate traces of hydrogen in the initial phase of paraformaldehyde decomposition. Only the bifunctional ruthenium catalyst shows high activity due to its bifunctional catalytically active sites, thus more than 98.0 % of the initially produced gas contains hydrogen. The initial TOF is 685 h^-1 at 363 K when the paraformaldehyde concentration is 20 wt%. A reaction mechanism is proposed for the hydrogen generation from paraformaldehyde in which formaldehyde and formic acid are intermediates Formic acid decomposition is the rate-determining step in the later phase of paraformaldehyde decomposition.