Splitting CO2 into CO and O2 by a single catalyst

Recent developments in the synthesis and applications of terpyridine-based metal complexes: a systematic review

Terpyridine-based metal complexes have emerged as versatile and indispensable building blocks in the realm of modern chemistry, offering a plethora of applications spanning from materials science to catalysis and beyond. This comprehensive review article delves into the multifaceted world of terpyridine complexes, presenting an overview of their synthesis, structural diversity, and coordination chemistry principles. Focusing on their diverse functionalities, we explore their pivotal roles in catalysis, supramolecular chemistry, luminescent materials, and nanoscience. Furthermore, we highlight the burgeoning applications of terpyridine complexes in sustainable energy technologies, biomimetic systems, and medicinal chemistry, underscoring their remarkable adaptability to address pressing challenges in these fields. By elucidating the pivotal role of terpyridine complexes as versatile building blocks, this review provides valuable insights into their current state-of-the-art applications and future potential, thus inspiring continued innovation and exploration in this exciting area of research.

Vacancy-Rich Bismuth-Based Nanosheets for Mitochondrial Destruction via CO Poisoning, Ca2+ Dyshomeostasis, and Oxidative Damage

Mitochondria are core regulators of tumor cell homeostasis, and their damage has become an arresting therapeutic modality against cancer. Despite the development of many mitochondrial-targeted pharmaceutical agents, the exploration of more powerful and multifunctional medications is still underway. Herein, oxygen vacancy-rich BiO2-x wrapped with CaCO3 (named BiO2-x@CaCO3/PEG, BCP) is developed for full-fledged attack on mitochondrial function. After endocytosis of BCP by tumor cells, the CaCO3 shell can be decomposed in the acidic lysosomal compartment, leading to immediate Ca2+ release and CO2 production in the cytoplasm. Near-infrared irradiation enhances the adsorption of CO2 onto BiO2-x defects, which enables highly efficient photocatalysis of CO2-to-CO. Meanwhile, such BiO2-x nanosheets possess catalase-, peroxidase- and oxidase-like catalytic activities under acidic pH conditions, allowing hypoxia relief and the accumulation of diverse reactive oxygen species (ROS) in the tumor microenvironment. Ca2+ overload-induced ion dyshomeostasis, CO-mediated respiratory chain poisoning, ROS-triggered oxidative stress aggravation, and cytosolic hyperoxia can cause severe mitochondrial disorders, which further lead to type I cell death in carcinoma. Not only does BCP cause irreversible apoptosis, but immunogenic cell death is simultaneously triggered to activate antitumor immunity for metastasis inhibition. Collectively, this platform promises high benefits in malignant tumor therapy and may expand the medical applications of bismuth-based nanoagents.

How to rationally design homogeneous catalysts for efficient CO2 electroreduction?

Electrified converting CO2 into valuable fuels and chemicals using a homogeneous electrochemical CO2 reduction (CO2ER) approach simplifies the operation, providing a potential option for decoupling energy harvesting and renewable chemical production. These merits benefit the scenarios where decentralization and intermittent power are key factors. This perspective aims to provide an overview of recent progress in homogeneous CO2ER. We introduce firstly the fundamentals chemistry of the homogeneous CO2ER, followed by a summary of the crucial factors and the important criteria broadly employed for evaluating the performance. We then highlight the recent advances in the most widely explored transition-metal coordinate complexes for the C1 and multicarbon (C2+) products from homogeneous CO2ER. Finally, we summarize the remaining challenges and opportunities for developing homogeneous electrocatalysts for efficient CO2ER. This perspective is expected to favor the rational design of efficient homogeneous electrocatalysts for selective CO2ER toward renewable fuels and feedstocks.

Assembly of highly efficient overall CO2 + H2O electrolysis cell with the matchup of CO2 reduction and water oxidation catalyst

The exploitation of highly active and stable catalysts for reduction of CO2 and water oxidation is one of the approaches to facilitate scalable and sustainable CO2 reduction potentially at the industrial scale. Herein, a feasible strategy to rationally build an overall CO2 + H2O electrocatalytic reaction device is the preparation and matchup of a high-performance CO2 reduction catalyst and low-cost and highly active oxygen anode catalyst. A heterostructured nanosheet, γ-NiOOH/NiCO3/Ni(HCOO)2, exhibited superior catalytic activity in the oxygen evolution reaction, and was integrated with CoPc/Fe-N-C to build an overall CO2 + H2O cell with a current density of 10 mA cm-2 at a very low cell voltage of 1.97 V, and the faradaic deficiency of CO2 to CO was maintained at greater than 90% at 1.9 V.

Spin State in Homoleptic Iron(II) Terpyridine Complexes Influences Mixed Valency and Electrocatalytic CO2 Reduction

Two homoleptic Fe(II) complexes in different spin states bearing superbasic terpyridine derivatives as ligands are investigated to determine the relationship between spin state and electrochemical/spectroscopic behavior. Antiferromagnetic coupling between a ligand-centered radical and the high-spin metal center leads to an anodic shift of the first reduction potential and results in a species that shows mixed valency with a moderately intense intervalence-charge-transfer band. The differences afforded by the different spin states extend to the electrochemical reactivity of the complexes: while the low-spin species is a precatalyst for electrocatalytic CO₂ reduction and leads to the preferential formation of CO with a Faradaic efficiency of 37%, the high-spin species only catalyzes proton reduction at a modest Faradaic efficiency of approximately 20%.

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⋅.

Characterizing Density and Spatial Distribution of Trap States in Ta3N5 Thin Films for Rational Defect Passivation

Tantalum nitride (Ta₃N₅) has gained significant attention as a potential photoanode material, yet it has been challenged by material quality issues. Defect-induced trap states are detrimental to the performance of any semiconductor material. Beyond influencing the performance of Ta₃N₅ films, defects can also accelerate the degradation in water during desired electrochemical applications. Defect passivation has provided an enormous boost to the development of many semiconductor materials but is currently in its infancy for Ta₃N₅. This is in part due to a lack of experimental understanding regarding the spatial and energetic distribution of trap states throughout Ta₃N₅ thin films. Here, we employ drive-level capacitance profiling (DLCP) to experimentally resolve the spatial and energetic distribution of trap states throughout Ta₃N₅ thin films. The density of deeper energetic traps is found to reach ∼2.5 to 6 × 10²² cm^-3 at the interfaces of neat Ta₃N₅ thin films, over an order of magnitude greater than the bulk. In addition to the spatial profile of deep trap states, we report neat Ta₃N₅ thin films to be highly n-type in nature, owning a free carrier density of ∼9.74 × 10¹⁷ cm^-3. This information, coupled with the present understanding of native oxide layers on Ta₃N₅, has facilitated the rational design of a targeted passivation strategy that simultaneously provides a means for catalyst immobilization. Loading catalyst via silatrane moieties suppresses the density of defects at the surface of Ta₃N₅ thin films by two orders of magnitude, while also reducing the free carrier density of films by over one order of magnitude, effectively dedoping the films to ∼2.40 × 10¹⁶ cm^-3. The surface passivation of Ta₃N₅ films translates to suppressed defect-induced trapping and recombination of photoexcited carriers, as determined through absorption, photoluminescence, and transient photovoltage. This illustrates how developing a deeper understanding of the distribution and influence of defects in Ta₃N₅ thin films has the potential to guide future works and ultimately accelerate the integration and development of high-performance Ta₃N₅ thin film devices.

Synergistic Effect in a Metal-Organic Framework Boosting the Electrochemical CO2 Overall Splitting

It is a very important but still challenging task to develop bifunctional electrocatalysts for highly efficient CO₂ overall splitting. Herein, we report a stable metal-organic framework (denoted as PcNi-Co-O), composed of (2,3,9,10,16,17,23,24-octahydroxyphthalocyaninato)nickel(II) (PcNi-(O^-)₈) ligands and the planar CoO₄ nodes, for CO₂ overall splitting. When working as both cathode and anode catalysts (i.e., PcNi-Co-O||PcNi-Co-O), PcNi-Co-O achieved a commercial-scale current density of 123 mA cm^-2 (much higher than the reported values (0.2-12 mA cm^-2)) with a Faradic efficiency (CO) of 98% at a low cell voltage of 4.4 V. Mechanism studies suggested the synergistic effects between two active sites, namely, (i) electron transfer from CoO₄ to PcNi sites under electric fields, resulting in the raised oxidizability/reducibility of CoO₄/PcNi sites, respectively; (ii) the energy-level matching of cathode and anode catalysts can reduce the energy barrier of electron transfer between them and improve the performance of CO₂ overall splitting.

Two Novel Schiff Base Manganese Complexes as Bifunctional Electrocatalysts for CO2 Reduction and Water Oxidation

One mononuclear Mn(III) complex [Mn^IIIL(H₂O)(MeCN)](ClO₄) (1) and one hetero-binuclear complex [(Cu^IILMn^II(H₂O)₃)(Cu^IIL)₂](ClO₄)₂·CH₃OH (2) have been synthesized with the Schiff base ligand (H₂L = N,N'-bis(3-methoxysalicylidene)-1,2-phenylenediamine). Single crystal X-ray structural analysis manifests that the Mn(III) ion in 1 has an octahedral coordination structure, whereas the Mn(II) ion in 2 possesses a trigonal bipyramidal configuration and the Cu(II) ion in 2 is four-coordinated with a square-planar geometry. Electrochimerical catalytic investigation demonstrates that the two complexes can electrochemically catalyze water oxidation and CO₂ reduction simultaneously. The coordination environments of the Mn(III), Mn(II), and Cu(II) ions in 1 and 2 were provided by the Schiff base ligand (L) and labile solvent molecules. The coordinately unsaturated environment of the Cu(II) center in 2 can perfectly facilitate the catalytic performance of 2. Complexes 1 and 2 display that the over potentials for water oxidation are 728 mV and 216 mV, faradaic efficiencies (FEs) are 88% and 92%, respectively, as well as the turnover frequency (TOF) values for the catalytic reduction of CO₂ to CO are 0.38 s^-1 at -1.65 V and 15.97 s^-1 at -1.60 V, respectively. Complex 2 shows much better catalytic performance for both water oxidation and CO₂ reduction than that of complex 1, which could be owing to a structural reason which is attributed to the synergistic catalytic action of the neighboring Mn(III) and Cu(II) active sites in 2. Complexes 1 and 2 are the first two compounds coordinated with Schiff base ligand for both water oxidation and CO₂ reduction. The finding in this work can offer significant inspiration for the future development of electrocatalysis in this area.

Exploiting Phase Transitions in Catalysis: Adsorption of CO on doped VO2 -Polymorphs

VO₂ is well known for its low-temperature metal-insulator transition between two phases with tetragonal rutile and monoclinic structure. The adsorption of CO on the two polymorphs of Mo-doped VO₂ is calculated to investigate the effect of a substrate phase change on the adsorption energy. The system is investigated theoretically at density-functional theory level using a hybrid functional with London dispersion correction. We establish a computational protocol applicable for the study of physisorption on open-shell transition metal oxides. The main task is to control the spin state of open-shell slab models used to model adsorption of closed-shell molecules in order to obtain numerically stable adsorption energies and to reduce spin contamination within the broken-symmetry unrestricted Kohn-Sham approximation. Applying this procedure, it is possible to identify the most stable adsorption positions of CO on both phases of VO₂ . CO adsorbs vertically with the C atom on a surface V atom in the monoclinic phase with an adsorption energy of -56 kJ/mol. The same adsorption position has an adsorption energy of only -46 kJ/mol on the rutile phase. Similar differences were obtained with multireference methods using an embedded cluster model. This effect may inspire experimental strategies exploiting the rutile ↔ ${ \leftrightarrow }$ monoclinic VO₂ phase transition in catalytic processes where CO is formed as product or as an intermediate.

Electrocatalytic CO2 Reduction by Molecular Ruthenium Complexes with Polypyridyl Ligands

Two series of ruthenium complexes with various polypyridyl ligands have been prepared. One series of complexes (5 examples) are featured with tetradentate polypyridyl ligands and two acetonitrile molecules at the axial positions of the coordination sphere; the other series (3 examples) include combinations of a tridentate polypyridyl ligand, one 2,2'-bipyridine (bpy) or two picolines, and one acetonitrile ligand. All these complexes were fully characterized by their NMR spectra as well as X-ray single crystal structures. Their electronic absorption and redox data were measured and reported. Of the 8 complexes, three candidates effectively catalyze electrochemical CO 2 reduction reaction (CO 2 RR) in wet acetonitrile medium, generating CO as the major product. All these three catalytically active complexes contain a 2,2':6',2″:6″,2‴-quaterpyridine (qpy) ligand scaffold. A maximum turnover frequency (TOF max ) of > 1000 s -1 was achieved for the electrocatalytic CO 2 reduction at a modest overpotential. On the basis of electrochemical and spectroelectrochemical evidences, the CO 2 substrate was proposed to bind with the ruthenium center at the two-electron reduced state of the complex and then enter the catalytic cycle.

Mechanism and selectivity of photocatalyzed CO2 reduction by a function-integrated Ru catalyst

The phosphine-substituted Ru(II) polypyridyl complex, [Ru^II-(tpy)(pqn)(MeCN)]²⁺ (RuP), was disclosed to be an efficient photocatalyst for the reduction of CO₂ to CO with excellent selectivity. In this work, density functional calculations were performed to elucidate the reaction mechanism and understand the origin of selectivity. The calculations showed that RuP was first excited to the singlet excited state, followed by intersystem crossing to produce a triplet species (3RuIII(L˙-)-S), which was then reduced by the sacrificial electron donor BIH to generate a Ru^II(L˙^-) intermediate. The ligand of Ru^II(L˙^-) was further reduced to produce a Ru^II(L^2-) intermediate. The redox non-innocent nature of the tpy and pqn ligands endows the Ru center with an oxidation state of +2 after two one-electron reductions. Ru^II(L^2-) nucleophilically attacks CO₂, in which two electrons are delivered from the ligands to CO₂, affording a Ru^II-COOH species after protonation. This is followed by the protonation of the hydroxyl moiety of Ru^II-COOH, coupled with the C-O bond cleavage, resulting in the formation of Ru^II-CO. Ultimately, CO is dissociated after two one-electron reductions. Protonation of Ru^II(L^2-) to generate a Ru^II-hydride, a critical intermediate for the production of formate and H₂, turns out to be kinetically less favorable, even though it is thermodynamically more favorable. This fact is due to the presence of a Ru²⁺ ion in the reduced catalyst, which disfavors its protonation.

Single-Atom (Iron-Based) Catalysts: Synthesis and Applications

Supported single-metal atom catalysts (SACs) are constituted of isolated active metal centers, which are heterogenized on inert supports such as graphene, porous carbon, and metal oxides. Their thermal stability, electronic properties, and catalytic activities can be controlled via interactions between the single-metal atom center and neighboring heteroatoms such as nitrogen, oxygen, and sulfur. Due to the atomic dispersion of the active catalytic centers, the amount of metal required for catalysis can be decreased, thus offering new possibilities to control the selectivity of a given transformation as well as to improve catalyst turnover frequencies and turnover numbers. This review aims to comprehensively summarize the synthesis of Fe-SACs with a focus on anchoring single atoms (SA) on carbon/graphene supports. The characterization of these advanced materials using various spectroscopic techniques and their applications in diverse research areas are described. When applicable, mechanistic investigations conducted to understand the specific behavior of Fe-SACs-based catalysts are highlighted, including the use of theoretical models.

Enzymes, In Vivo Biocatalysis, and Metabolic Engineering for Enabling a Circular Economy and Sustainability

Since the industrial revolution, the rapid growth and development of global industries have depended largely upon the utilization of coal-derived chemicals, and more recently, the utilization of petroleum-based chemicals. These developments have followed a linear economy model (produce, consume, and dispose). As the world is facing a serious threat from the climate change crisis, a more sustainable solution for manufacturing, i.e., circular economy in which waste from the same or different industries can be used as feedstocks or resources for production offers an attractive industrial/business model. In nature, biological systems, i.e., microorganisms routinely use their enzymes and metabolic pathways to convert organic and inorganic wastes to synthesize biochemicals and energy required for their growth. Therefore, an understanding of how selected enzymes convert biobased feedstocks into special (bio)chemicals serves as an important basis from which to build on for applications in biocatalysis, metabolic engineering, and synthetic biology to enable biobased processes that are greener and cleaner for the environment. This review article highlights the current state of knowledge regarding the enzymatic reactions used in converting biobased wastes (lignocellulosic biomass, sugar, phenolic acid, triglyceride, fatty acid, and glycerol) and greenhouse gases (CO₂ and CH₄) into value-added products and discusses the current progress made in their metabolic engineering. The commercial aspects and life cycle assessment of products from enzymatic and metabolic engineering are also discussed. Continued development in the field of metabolic engineering would offer diversified solutions which are sustainable and renewable for manufacturing valuable chemicals.

Electrocatalytic syngas generation with a redox non-innocent cobalt 2-phosphinobenzenethiolate complex

A cobalt complex supported by the 2-(diisopropylphosphaneyl)benzenethiol ligand was synthesized and its electronic structure and reactivity were explored. X-ray diffraction studies indicate a square planar geometry around the cobalt center with a trans arrangement of the phosphine ligands. Density functional theory calculations and electronic spectroscopy measurements suggest a mixed metal-ligand orbital character, in analogy to previously studied dithiolene and diselenolene systems. Electrochemical studies in the presence of 1 atm of CO₂ and Brønsted acid additives indicate that the cobalt complex generates syngas, a mixture of H₂ and CO, with faradaic efficiencies up to >99%. The ratios of H₂ : CO generated vary based on the additive. A H₂ : CO ratio of ∼3 : 1 is generated when H₂O is used as the Brønsted acid additive. Chemical reduction of the complex indicates a distortion towards a tetrahedral geometry, which is rationalized with DFT predictions as attributable to the populations of orbitals with σ*(Co-S) character. A mechanistic scheme is proposed whereby competitive binding between a proton and CO₂ dictates selectivity. This study provides insight into the development of a catalytic system incorporating non-innocent ligands with pendant base moieties for electrochemical syngas production.

Important Synthesis Parameters Affecting Crystallization of Zeolite T: A Review

Synthesis of zeolite T with a variety of desired characteristics necessitates extensive work in formulation and practical experiments, either by conventional hydrothermal methods or aided with different approaches and synthesis techniques, such as secondary growth or microwave irradiation. The objectives of this review are to adduce the potential work in zeolite T (Offretite-Erionite) synthesis, evaluating determining factors affecting the synthesis and quality of zeolite T crystals. Attention is given to extensive studies that interconnect with other significant findings.

Transition Metal Complexes as Catalysts for the Electroconversion of CO2 : An Organometallic Perspective

The electrocatalytic transformation of carbon dioxide has been a topic of interest in the field of CO2 utilization for a long time. Recently, the area has seen increasing dynamics as an alternative strategy to catalytic hydrogenation for CO2 reduction. While many studies focus on the direct electron transfer to the CO2 molecule at the electrode material, molecular transition metal complexes in solution offer the possibility to act as catalysts for the electron transfer. C1 compounds such as carbon monoxide, formate, and methanol are often targeted as the main products, but more elaborate transformations are also possible within the coordination sphere of the metal center. This perspective article will cover selected examples to illustrate and categorize the currently favored mechanisms for the electrochemically induced transformation of CO2 promoted by homogeneous transition metal complexes. The insights will be corroborated with the concepts and elementary steps of organometallic catalysis to derive potential strategies to broaden the molecular diversity of possible products.

Dye-Sensitized Nonstoichiometric Strontium Titanate Core-Shell Photocathodes for Photoelectrosynthesis Applications

A core-shell approach that utilizes a high-surface-area conducting core and an outer semiconductor shell is exploited here to prepare p-type dye-sensitized solar energy cells that operate with a minimal applied bias. Photocathodes were prepared by coating thin films of nanocrystalline indium tin oxide with a 0.8 nm Al₂O₃ seeding layer, followed by the chemical growth of nonstoichiometric strontium titanate. Films were annealed and sensitized with either a porphyrin chromophore or a chromophore-catalyst molecular assembly consisting of the porphyrin covalently tethered to the ruthenium complex. The sensitized photoelectrodes produced cathodic photocurrents of up to -315 μA/cm² under simulated sunlight (AM1.5G, 100 mW/cm²) in aqueous media, pH 5. The photocurrent was increased by the addition of regenerative hole donors to the system, consistent with slow interfacial recombination kinetics, an important property of p-type dye-sensitized electrodes.

An amide-based second coordination sphere promotes the dimer pathway of Mn-catalyzed CO2-to-CO reduction at low overpotential

The [fac-Mn(bpy)(CO)3Br] complex is capable of catalyzing the electrochemical reduction of CO2 to CO with high selectivity, moderate activity and large overpotential. Several attempts have been made to lower the overpotential and to enhance the catalytic activity of this complex by manipulating the second-coordination sphere of manganese and using relatively stronger acids to promote the protonation-first pathway. We report herein that the complex [fac-Mn(bpy-CONHMe)(CO)3(MeCN)]+ ([1-MeCN]+; bpy-CONHMe = N-methyl-(2,2'-bipyridine)-6-carboxamide) as a pre-catalyst could catalyze the electrochemical reduction of CO2 to CO with low overpotential and high activity and selectivity. Combined experimental and computational studies reveal that the amide NH group not only decreases the overpotential of the Mn catalyst by promoting the dimer and protonation-first pathways in the presence of H2O but also enhances the CO2 electroreduction activity by facilitating C-OH bond cleavage, making [1-MeCN]+ an efficient CO2 reduction pre-catalyst at low overpotential.

Powering the next industrial revolution: transitioning from nonrenewable energy to solar fuels via CO2 reduction

Solar energy has been used for decades for the direct production of electricity in various industries and devices; however, harnessing and storing this energy in the form of chemical bonds has emerged as a promising alternative to fossil fuel combustion. The common feedstocks for producing such solar fuels are carbon dioxide and water, yet only the photoconversion of carbon dioxide presents the opportunity to generate liquid fuels capable of integrating into our existing infrastructure, while simultaneously removing atmospheric greenhouse gas pollution. This review presents recent advances in photochemical solar fuel production technology. Although efforts in this field have created an incredible number of methods to convert carbon dioxide into gaseous and liquid fuels, these can generally be classified under one of four categories based on how incident sunlight is utilised: solar concentration for thermoconversion (Category 1), transformation toward electroconversion (Category 2), natural photosynthesis for bioconversion (Category 3), and artificial photosynthesis for direct photoconversion (Category 4). Select examples of developments within each of these categories is presented, showing the state-of-the-art in the use of carbon dioxide as a suitable feedstock for solar fuel production.

Solar energy has been used for decades for the direct production of electricity in various industries and devices. However, harnessing and storing this energy in the form of chemical bonds has emerged as a promising alternative to fossil fuels.

Redox-Active Manganese Pincers for Electrocatalytic CO2 Reduction

The decrease of total amount of atmospheric CO2 is an important societal challenge in which CO2 reduction has an important role to play. Electrocatalytic CO2 reduction with homogeneous catalysts is based on highly tunable catalyst design and exploits an abundant C1 source to make valuable products such as fuels and fuel precursors. These methods can also take advantage of renewable electricity as a green reductant. Mn-based catalysts offer these benefits while incorporating a relatively cheap and abundant first-row transition metal. Historically, interest in this field started with Mn(bpy-R)(CO)3X, whose performance matched that of its Re counterparts while achieving substantially lower overpotentials. This review examines an emerging class of homogeneous Mn-based electrocatalysts for CO2 reduction, Mn complexes with meridional tridentate coordination also known as Mn pincers, most of which contain redox-active ligands that enable multi-electron catalysis. Although there are relatively few examples in the literature thus far, these catalysts bring forth new catalytic mechanisms not observed for the well-established Mn(bpy-R)(CO)3X catalysts, and show promising reactivity for future studies.

Nanostructured Cobalt Doped Barium Strontium Titanate Thin Films with Potential in CO2 Detection

In this work, (Ba_0.75Sr_0.25) (Ti_0.95Co_0.05) O₃ perovskite nanostructured material, denoted subsequently as Co-doped BaSrTiO₃, was synthesized in a one-step process in hydrothermal conditions. The obtained powder was heat-treated at 800 °C and 1000 °C, respectively, in order to study nanostructured powder behavior during thermal treatment. The Co-doped BaSrTiO₃ powder was pressed into pellets of 5.08 cm (2 inches) then used for thin film deposition onto commercial Al₂O₃ substrates by RF sputtering method. The microstructural, thermal, and gas sensing properties were investigated. The electrical and thermodynamic characterization allowed the evaluation of thermodynamic stability and the correlation of structural features with the sensing properties revealed under real operating conditions. The sensing behavior with respect to the temperature range between 23 and 400 °C, for a fixed CO₂ concentration of 3000 ppm, highlighted specific differences between Co-doped BaSrTiO₃ treated at 800 °C compared to that treated at 1000 °C. The influence of the relative humidity level on the CO₂ concentrations and the other potential interfering gases was also analyzed. Two possible mechanisms for CO₂ interaction were then proposed. The simple and low-cost technology, together with the high sensitivity when operating at room temperature corresponding to low power consumption, suggests that Co-doped BaSrTiO₃ has a good potential for use in developing portable CO₂ detectors.

Ruthenium-decorated tungsten disulfide quantum dots for a CO2 gas sensor

In this work, a selective chemiresistive gas sensor for carbon dioxide gas detection at room temperature (∼25 °C) was successfully fabricated, where ruthenium-decorated tungsten disulfide (Ru@WS₂) quantum dots (QDs) have been used as the sensing material. A mixed solvent of lithium hydroxide (LiOH · H₂O) and N-methyl-2-pyrrolidone (NMP) was used to obtain the Ru-decorated WS₂ QDs from the exfoliated WS₂ nanoflakes. Then, the prepared WS₂ QDs and Ru@WS₂ QDs were confirmed using different material characterization techniques. The gas sensors were prepared by spraying the WS₂ QDs and Ru@WS₂ QDs on gold interdigitated electrodes (IDE), and were then exposed to various concentrations of CO₂ gas in dry air conditions. Also, the effect of humidity on both sensors in 5000 ppm CO₂ gas has been studied. The Ru@WS₂ QD based sensor showed superior sensitivity and good selectivity to CO₂ gas in comparison with isopropanol, acetone, ethanol, methanol and benzene at room temperature than the WS₂ QD. The sensor showed an increase in resistance when exposed to CO₂ gas ranging from 500 to 5000 ppm, indicating p-type characteristics. The Ru@WS₂ QD shows less effects at different humid conditions compared to WS₂ QD as a CO₂ gas sensor.

CO2 Reduction: From Homogeneous to Heterogeneous Electrocatalysis

Due to increasing worldwide fossil fuel consumption, carbon dioxide levels have increased in the atmosphere with increasingly important impacts on the environment. Renewable and clean sources of energy have been proposed, including wind and solar, but they are intermittent and require efficient and scalable energy storage technologies. Electrochemical CO₂ reduction reaction (CO₂RR) provides a valuable approach in this area. It combines solar- or wind-generated electrical production with energy storage in the chemical bonds of carbon-based fuels. It can provide ways to integrate carbon capture, utilization, and storage in energy cycles while maintaining controlled levels of atmospheric CO₂. Electrochemistry allows for the utilization of an electrical input to drive chemical reactions. Because CO₂ is kinetically inert, highly active catalysts are required to decrease reaction barriers sufficiently so that reaction rates can be achieved that are sufficient for electrochemical CO₂ reduction. Given the reaction barriers associated with multiple electron-proton reduction of CO₂ to CO, formaldehyde (HC(O)H), formic acid, or formate (HC(O)OH, HC(O)O^-), or more highly reduced forms of carbon, there is also a demand for high selectivity in catalysis. Catalysts that have been explored include homogeneous catalysts in solution, catalysts immobilized on surfaces, and heterogeneous catalysts. In homogeneous catalysis, reduction occurs following diffusion of the catalyst to an electrode where multiple proton coupled electron transfer reduction occurs. Useful catalysts in this area are typically transition-metal complexes with organic ligands and electron transfer properties that utilize combinations of metal and ligand redox levels. As a way to limit the amount of catalyst, in device-like configurations, catalysts are added to the surfaces of conductive substrates by surface binding, in polymeric films, or on carbon electrode surfaces with molecular structures and electronic configurations related to catalysts in solution. Immobilized, homogeneous catalysts can suffer from performance losses and even decomposition during long-term CO₂ reduction cycles, but they are amenable to detailed mechanistic investigations. In parallel efforts, heterogeneous nanocatalysts have been explored in detail with the development of facile synthetic procedures that can offer highly active catalytic surface areas. Their high activity and stability have attracted a significant level of investigation, including possible exploitation for large-scale applications. However, translation of catalytic reactivity to the surface creates a new reactivity environment and complicates the elucidation of mechanistic details and identification of the active site in exploring reaction pathways. Here, the results of previous studies based on transition-metal complex catalysts for CO₂ electroreduction are summarized. Early studies showed that transition-metal complexes of Ru, Ir, Rh, and Os, with well-defined structures, are all capable of catalyzing CO₂ reduction to CO or formate. Derivatives of the complexes were surface attached to conducting electrodes by chemical bonding, noncovalent bonding, or polymerization. The concept of surface binding has also been extended to the preparation of surface area electrodes by the chemically controlled deposition of nanostructured catalysts such as nano tin, nano copper, and nano carbon, all of which have been shown to have high selectivities and activities toward CO₂ reduction. In our presentation, we end this Account with recent advances and a perspective about the application of electrocatalysis in carbon dioxide reduction.

Steering CO2 electroreduction toward ethanol production by a surface-bound Ru polypyridyl carbene catalyst on N-doped porous carbon

Electrochemical reduction of CO₂ to multicarbon products is a significant challenge, especially for molecular complexes. We report here CO₂ reduction to multicarbon products based on a Ru(II) polypyridyl carbene complex that is immobilized on an N-doped porous carbon (RuPC/NPC) electrode. The catalyst utilizes the synergistic effects of the Ru(II) polypyridyl carbene complex and the NPC interface to steer CO₂ reduction toward C2 production at low overpotentials. In 0.5 M KHCO₃/CO₂ aqueous solutions, Faradaic efficiencies of 31.0 to 38.4% have been obtained for C2 production at -0.87 to -1.07 V (vs. normal hydrogen electrode) with 21.0 to 27.5% for ethanol and 7.1 to 12.5% for acetate. Syngas is also produced with adjustable H₂/CO mole ratios of 2.0 to 2.9. The RuPC/NPC electrocatalyst maintains its activity during 3-h CO₂-reduction periods.

Linkage Effect in the Heterogenization of Cobalt Complexes by Doped Graphene for Electrocatalytic CO2 Reduction

Immobilization of planar Co^II -2,3-naphthalocyanine (NapCo) complexes onto doped graphene resulted in a heterogeneous molecular Co electrocatalyst that was active and selective to reduce CO₂ into CO in aqueous solution. A systematic study revealed that graphitic sulfoxide and carboxyl dopants of graphene were the efficient binding sites for the immobilization of NapCo through axial coordination and resulted in active Co sites for CO₂ reduction. Compared to carboxyl dopants, the sulfoxide dopants further improved the electron communication between NapCo and graphene, which led to the increase of turnover frequency of the Co sites by about 3 times for CO production with a Faradic efficiency up to 97 %. Pristine NapCo in the absence of a graphene support did not display efficient electron communication with the electrode and thus failed to serve as the electrochemical active site for CO₂ reduction under the identical conditions.

Electrocatalytic Reduction of CO2 to Methanol by Iron Tetradentate Phosphine Complex Through Amidation Strategy

The iron complex of tetradentate tris[2-(diphenylphosphino) ethyl]phosphine (PP₃ ), [Fe(PP₃ )(MeCN)₂ ](BF₄ )₂ , was able to electrocatalytically reduce CO₂ to formate with a Faradaic efficiency (FE) of approximately 97.3 % in acetonitrile. Upon addition of diethylamine as a cocatalyst, electrocatalytic reduction to methanol was achieved with an FE of 68.5 %, and other products were formamide and formate. A mechanistic study suggested that the [FeH(PP₃ )](BF₄ ) hydride complex was the active species in the electrocatalysis. Added amine as cocatalyst could react with CO₂ to form carbamate, which could then be reduced to formamide and further to methanol. By contrast, free CO₂ could only be reduced to formate as the end-product.

The Trans Effect in Electrocatalytic CO2 Reduction: Mechanistic Studies of Asymmetric Ruthenium Pyridyl-Carbene Catalysts

A comprehensive mechanistic study of electrocatalytic CO₂ reduction by ruthenium 2,2':6',2″-terpyridine (tpy) pyridyl-carbene catalysts reveals the importance of stereochemical control to locate the strongly donating N-heterocyclic carbene ligand trans to the site of CO₂ activation. Computational studies were undertaken to predict the most stable isomer for a range of reasonable intermediates in CO₂ reduction, suggesting that the ligand trans to the reaction site plays a key role in dictating the energetic profile of the catalytic reaction. A new isomer of [Ru(tpy)(Mebim-py)(NCCH₃)]²⁺ (Mebim-py is 1-methylbenzimidazol-2-ylidene-3-(2'-pyridine)) and both isomers of the catalytic intermediate [Ru(tpy)(Mebim-py)(CO)]²⁺ were synthesized and characterized. Experimental studies demonstrate that both isomeric precatalysts facilitate electroreduction of CO₂ to CO in 95/5 MeCN/H₂O with high activity and high selectivity. Cyclic voltammetry, infrared spectroelectrochemistry, and NMR spectroscopy studies provide a detailed mechanistic picture demonstrating an essential isomerization step in which the N-trans catalyst converts in situ to the C-trans variant. Insight into molecular electrocatalyst design principles emerge from this study. First, the use of an asymmetric ligand that places a strongly electron-donating ligand trans to the site of CO₂ binding and activation is critical to high activity. Second, stereochemical control to maintain the desired isomer structure during catalysis is critical to performance. Finally, pairing the strongly donating pyridyl-carbene ligand with the redox-active tpy ligand proves to be useful in boosting activity without sacrificing overpotential. These design principles are considered in the context of surface-immobilized electrocatalysis.