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

Efficient solar-driven: Photothermal catalytic reduction of atmospheric CO2 at the gas-solid interface by CuTCPP/MXene/TiO2

Directly capturing atmospheric CO2 and converting it into valuable fuel through photothermal synergy is an effective way to mitigate the greenhouse effect. This study developed a gas-solid interface photothermal catalytic system for atmospheric CO2 reduction, utilizing the innovative photothermal catalyst (Cu porphyrin) CuTCPP/MXene/TiO2. The catalyst demonstrated a photothermal catalytic performance of 124 μmol·g-1·h-1 for CO and 106 μmol·g-1·h-1 for CH4, significantly outperforming individual components. Density functional theory (DFT) results indicate that the enhanced catalytic performance is attributed to the internal electric field between the components, which significantly enhances carrier utilization. The introduction of CuTCPP reduces free energy of the photothermal catalytic reaction. Additionally, the local surface plasmon resonance (LSPR) effect and high-speed electron transfer properties of MXene further boost the catalytic reaction rate. This well-designed catalyst and catalytic system offer a simple method for capturing atmospheric CO2 and converting it in-situ through photothermal catalysis.

Hydride Transfer-Based CO2 Reduction Catalysis: Navigating Metal Hydride to Organic Hydride in the Catalytic Loop

ConspectusThe reductive conversion of carbon dioxide (CO2) into value-added products is a process of immense importance. In the context of rising CO2 concentration in the atmosphere and the detrimental effects it is having on the biosphere, use of alternative fuels which can offer a low-carbon or carbon-neutral pathway for storage and utilization of low-carbon energy by maintaining the net atmospheric CO2 concentration might be a prospective solution. Among the wide variety of reduced products that can be obtained from CO2, formic acid and formate salts are particularly important due to their ability to be used as an alternative fuel or a reversible hydrogen storage material. Utilization of molecular catalysts for CO2 conversion offers several advantages such as high selectivity, mechanistic clarity, versatility, and stability, making them attractive for thermochemical and electro/photochemical CO2 reduction processes. The presence of N-heterocyclic carbene (NHC) ligands in transition-metal-based molecular catalysts enhances the stability of the catalysts under harsh reaction conditions, such as high pressure, high temperature, and reductive environments, providing crucial benefits for sustained catalytic activity and longevity. Though the development of metal complex-based catalysts is essential to addressing the challenge of CO2 reduction, the possibility of using purely organic compounds as catalysts for this transformation is lucrative from the aspect of developing a truly sustainable protocol with photosynthesis being its biggest inspiration. We begin this Account by examining our systematic development of molecular metal complexes based on NHC ligands for the chemical upgradation of CO2 to formic acid/formate salt. In such cases, the ability of NHCs to act as strong σ-donor ligands for a greater hydride transfer propensity is discussed and analyzed. The reports range from catalytic ambient- and high-pressure CO2 hydrogenation to CO2 transfer-hydrogenation. Coupling of CO2 capture methodologies with CO2 conversion is also discussed. A case is made for the heterogenization of one of the highly efficient metal-NHC catalysts to develop a self-supported single-site catalyst for practical applications. Finally, our recent success of developing a novel organic catalyst system inspired from the natural NADP+/NADPH-based hydride-transfer redox couple that is active in photosynthetic CO2 reduction has been discussed. This catalyst is designed based on a bis-imidazolium-embedded heterohelicene with a central pyridine ring and is capable of electrocatalytically converting CO2 to HCO2H with TON values 100-1000 times greater than the existing reported values achieved so far by organic catalysts. Overall, we believe that the results of hydride transfer-based CO2 reduction catalysis presented in this Account hold significant implications beyond our work and have the potential for motivating future research toward further development in this important field.

Transition metal pincer catalysts for formic acid dehydrogenation: a mechanistic perspective

The storage and transportation of hydrogen gas, a non-polluting alternative to carbon-based fuels, have always been challenging due to its extreme flammability. In this regard, formic acid (FA) is a promising liquid organic hydrogen carrier (LOHC), and over the past decades, significant progress has been made in dehydrogenating FA through transition metal catalysis. In this review, our goal is to provide a detailed insight into the existing processes to expose various mechanistic challenges associated with FA dehydrogenation (FAD). Specifically, methodologies catalyzed by pincer-ligated metal complexes were chosen. Pincer ligands are preferred as they provide structural rigidity to the complexes, making the isolation and analysis of reaction intermediates less challenging and consequently providing a better mechanistic understanding. In this perspective, the catalytic activity of the reported pincer complexes in FAD was overviewed, and more importantly, the catalytic cycles were examined in detail. Further attention was given to the structural modifications, role of additives, reaction medium, and their crucial effects on the outcome.

Hydroformylation of Olefins with CO2/H2 and Hydrosilane by Copper/Cobalt Tandem Catalysis

A Cu/Co tandem catalysis protocol was developed to conduct the hydroformylation of olefins using CO2/H2 and PMHS (polymethylhydrosiloxane) as a readily available and environmentally friendly hydride source. This methodology was performed via a two-step approach consisting of the copper-catalyzed reduction of CO2 by hydrosilane and subsequent cobalt-promoted hydroformylation with H2 and the in situ formed CO. The optimized triphos oxide ligand, which presumably facilitates the migratory insertion of CO gives moderate to excellent yields for both terminal and internal alkenes. This earth-abundant metal catalysis provides a reliable and efficient way to afford useful aldehydes in industry using silicon by-product PMHS as hydrogen source and renewable CO2 as carbonyl source.

Development of a practical formate/bicarbonate energy system

Liquid (organic) hydrogen carriers ([18H]-dibenzyltoluene, MeOH, formic acid, etc.) form a toolbox for the storage and transport of green hydrogen, which is crucial for the implementation of renewable energy technologies. Simple organic salts have been scarcely investigated for this purpose, despite many advantages such as low cost and minor toxicity, as well as easy handling. Here, we present a potassium formate/potassium bicarbonate hydrogen storage and release energy system, that is applicable and shows high stability (6 months). Utilizing ppm amounts of the molecularly defined Ru-5 complex, hydrogen release rates of up to 9.3 L h-1 were achieved. The same catalyst system promoted the hydrogenation of KHCO3 to HCOOK with a TON of 9650. In this way, combined hydrogen storage-release cycles can be performed for 40 times.

S-scheme heterojunction Cu-porphyrin/TiO2 nanosheets with highly efficient photocatalytic reduction of CO2 in ambient air

Directly capturing CO2 in ambient air and converting it into value-added fuels using photocatalysis is a potentially valuable technology. In this study, Cu-porphyrin (tetrakis-carboxyphenyl porphyrin copper, CuTCPP) was innovatively anchored on the surface of TiO2 (titanium dioxide) nanosheets to form an S-scheme heterojunction. Based on this, a photocatalytic reaction system for stably converting CO2 in ambient air into value-added fuels at the gas-solid interface was constructed without addition of sacrificial agents and alkaline liquids. Under the illumination of visible light and sunlight, the evolution rate of CO is 56 μmol·g-1·h-1 and 73 μmol·g-1·h-1, respectively, with a potential CO2 conversion rate of 35.8 % and 50.4 %. The enhanced of photocatalytic performance is attributed to the introduction of CuTCPP, which provides additional active sites, significantly improves capture capacity of CO2 and the utilization of electrons. Additionally, the formation of S-scheme heterojunction expands the redox range and improves the separation efficiency of photo-generated charges.

CO2 Reduction by an Iron(I) Porphyrinate System: Effect of Hydrogen Bonding on the Second Coordination Sphere

Transforming CO2 into valuable materials is an important reaction in catalysis, especially because CO2 concentrations in the atmosphere have been growing steadily due to extensive fossil fuel usage. From an environmental perspective, reduction of CO2 to valuable materials should be catalyzed by an environmentally benign catalyst and avoid the use of heavy transition-metal ions. In this work, we present a computational study into a novel iron(I) porphyrin catalyst for CO2 reduction, namely, with a tetraphenylporphyrin ligand and analogues. In particular, we investigated iron(I) tetraphenylporphyrin with one of the meso-phenyl groups substituted with o-urea, p-urea, or o-2-amide groups. These substituents can provide hydrogen-bonding interactions in the second coordination sphere with bound ligands and assist with proton relay. Furthermore, our studies investigated bicarbonate and phenol as stabilizers and proton donors in the reaction mechanism. Potential energy landscapes for double protonation of iron(I) porphyrinate with bound CO2 are reported. The work shows that the bicarbonate bridges the urea/amide groups to the CO2 and iron center and provides a tight bonding pattern with strong hydrogen-bonding interactions that facilitates easy proton delivery and reduction of CO2. Specifically, bicarbonate provides a low-energy proton shuttle mechanism to form CO and water efficiently. Furthermore, the o-urea group locks bicarbonate and CO2 in a tight orientation and helps with ideal proton transfer, while there is more mobility and lesser stability with an o-amide group in that position instead. Our calculations show that the o-urea group leads to reduction in proton-transfer barriers, in line with experimental observation. We then applied electric-field-effect calculations to estimate the environmental effects on the two proton-transfer steps in the reaction. These calculations describe the perturbations that enhance the driving forces for the proton-transfer steps and have been used to make predictions about how the catalysts can be further engineered for more enhanced CO2 reduction processes.

Synergistic Interplay of Dual-Active-Sites on Metallic Ni-MOFs Loaded with Pt for Thermal-Photocatalytic Conversion of Atmospheric CO2 under Infrared Light Irradiation

Infrared light driven photocatalytic reduction of atmospheric CO2 is challenging due to the ultralow concentration of CO2 (0.04 %) and the low energy of infrared light. Herein, we develop a metallic nickel-based metal-organic framework loaded with Pt (Pt/Ni-MOF), which shows excellent activity for thermal-photocatalytic conversion of atmospheric CO2 with H2 even under infrared light irradiation. The open Ni sites are beneficial to capture and activate atmospheric CO2 , while the photogenerated electrons dominate H2 dissociation on the Pt sites. Simultaneously, thermal energy results in spilling of the dissociated H2 to Ni sites, where the adsorbed CO2 is thermally reduced to CO and CH4 . The synergistic interplay of dual-active-sites renders Pt/Ni-MOF a record efficiency of 9.57 % at 940 nm for converting atmospheric CO2 , enables the procurement of CO2 to be independent of the emission sources, and improves the energy efficiency for trace CO2 conversion by eliminating the capture media regeneration and molecular CO2 release.

Carbon neutral hydrogen storage and release cycles based on dual-functional roles of formamides

The development of alternative clean energy carriers is a key challenge for our society. Carbon-based hydrogen storage materials are well-suited to undergo reversible (de)hydrogenation reactions and the development of catalysts for the individual process steps is crucial. In the current state, noble metal-based catalysts still dominate this field. Here, a system for partially reversible and carbon-neutral hydrogen storage and release is reported. It is based on the dual-functional roles of formamides and uses a small molecule Fe-pincer complex as the catalyst, showing good stability and reusability with high productivity. Starting from formamides, quantitative production of CO-free hydrogen is achieved at high selectivity ( > 99.9%). This system works at modest temperatures of 90 °C, which can be easily supplied by the waste heat from e.g., proton-exchange membrane fuel cells. Employing such system, we achieve >70% H₂ evolution efficiency and >99% H₂ selectivity in 10 charge-discharge cycles, avoiding undesired carbon emission between cycles.

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.

pH-Universal Decoupled Water Electrolysis Enabled by Electrocatalytic Hydrogen Gas Capacitive Chemistry

In conventional water electrolysis (CWE), the H₂ and O₂ evolution reactions (HER/OER) are tightly coupled, making the generated H₂ and O₂ difficult to separate, thus resulting in complex separation technology and potential safety issues. Previous efforts on the design of decoupled water electrolysis mainly concentrated on multi-electrode or multi-cell configurations; however, these strategies have the limitation of involving complicated operations. Here, we propose and demonstrate a pH-universal, two-electrode capacitive decoupled water electrolyzer (referred to as all-pH-CDWE) in a single-cell configuration by utilizing a low-cost capacitive electrode and a bifunctional HER/OER electrode to separate H₂ and O₂ generation for decoupling water electrolysis. In the all-pH-CDWE, high-purity H₂ and O₂ generation alternately occur at the electrocatalytic gas electrode only by reversing the current polarity. The designed all-pH-CDWE can maintain a continuous round-trip water electrolysis for over 800 consecutive cycles with an electrolyte utilization ratio of nearly 100%. As compared to CWE, the all-pH-CDWE achieves energy efficiencies of 94% in acidic electrolytes and 97% in alkaline electrolytes at a current density of 5 mA cm^-2. Further, the designed all-pH-CDWE can be scaled up to a capacity of 720 C in a high current of 1 A for each cycle with a stable HER average voltage of 0.99 V. This work provides a new strategy toward the mass production of H₂ in a facilely rechargeable process with high efficiency, good robustness, and large-scale applications.

Hydrogenation of Carboxylic Acids, Esters, and Related Compounds over Heterogeneous Catalysts: A Step toward Sustainable and Carbon-Neutral Processes

The catalytic hydrogenation of esters and carboxylic acids represents a fundamental and important class of organic transformations, which is widely applied in energy, environmental, agricultural, and pharmaceutical industries. Due to the low reactivity of the carbonyl group in carboxylic acids and esters, this type of reaction is, however, rather challenging. Hence, specifically active catalysts are required to achieve a satisfactory yield. Nevertheless, in recent years, remarkable progress has been made on the development of catalysts for this type of reaction, especially heterogeneous catalysts, which are generally dominating in industry. Here in this review, we discuss the recent breakthroughs as well as milestone achievements for the hydrogenation of industrially important carboxylic acids and esters utilizing heterogeneous catalysts. In addition, related catalytic hydrogenations that are considered of importance for the development of cleaner energy technologies and a circular chemical industry will be discussed in detail. Special attention is paid to the insights into the structure-activity relationship, which will help the readers to develop rational design strategies for the synthesis of more efficient heterogeneous catalysts.

Improved CO2 Capture and Catalytic Hydrogenation Using Amino Acid Based Ionic Liquids

A series of alkyl ammonium (or imidazolium) based ionic liquids was tested as novel and potentially green absorbent for CO₂ capture and utilization. By exploiting various amino acids as counter ions for ionic liquids, CO₂ capture and hydrogenation to formate occur with high activity and excellent productivity utilizing arginine. The reaction was easily scalable without any significant drop in formate production, and the catalyst was reused for five consecutive runs leading to an overall TON of 12,741 for the formation of formate salt.

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.

Homogeneous Hydrogenation of CO2 and CO to Methanol: The Renaissance of Low-Temperature Catalysis in the Context of the Methanol Economy

The traditional economy based on carbon-intensive fuels and materials has led to an exponential rise in anthropogenic CO2 emissions. Outpacing the natural carbon cycle, atmospheric CO2 levels increased by 50 % since the pre-industrial age and can be directly linked to global warming. Being at the core of the proposed methanol economy pioneered by the late George A. Olah, the chemical recycling of CO2 to produce methanol, a green fuel and feedstock, is a prime channel to achieve carbon neutrality. In this direction, homogeneous catalytic systems have lately been a major focus for methanol synthesis from CO2 , CO and their derivatives as potential low-temperature alternatives to the commercial processes. This Review provides an account of this rapidly growing field over the past decade, since its resurgence in 2011. Based on the critical assessment of the progress thus far, the present key challenges in this field have been highlighted and potential directions have been suggested for practically viable applications.

Challenges and recent advancements in the transformation of CO2 into carboxylic acids: straightforward assembly with homogeneous 3d metals

Transformation of carbon dioxide (CO2) into valuable organic carboxylic acids is essential for maintaining sustainability. In this review, such CO2 thermo-, photo- and electrochemical transformations under 3d-transition metal catalysis are described from 2017 until 2022.