CO2‐Speicherung. Chancen und Risiken

Anthropogenic emissions of carbon dioxide (CO₂) into the atmosphere have had a significant impact on the Earth's carbon cycle. As part of the global effort to reduce climate change, the geological storage of CO₂ has been accepted as a method that may provide up to 25 % of the total reduction of emissions, although this figure is still subject to change. In Germany and worldwide, geological storage capacities are expected to be sufficient for several decades. Carbon dioxide can be captured from sources such as large‐scale industrial (energy, steel, cement or chemical) facilities and transported to long‐term storage sites in deep saltwater‐bearing aquifers. Above the porous sandstone reservoirs in which the CO₂ is to be stored, an impermeable cap rock is required to provide a barrier for the upward‐migrating gas. In time, a significant quantity of the CO₂ can be retained within the reservoir pore space by capillary forces, dissolved in water to form carbonic acid, or deposited as carbonate minerals. The storage site must be free of potential leakage pathways. To this end, extensive monitoring programs need to be carried out. The Ketzin pilot site, an example of such a program, has shown CO₂ storage on a research scale to be safe and reliable.     

Single‐ and double‐electron reductions of CO2 by using superalkalis: An ab initio study

CO₂, a major contributor to global warming, can be balanced by converting it into fuels. The reduction of CO₂ has been difficult due to its extremely high stability. Recently, single‐electron reduction of CO₂ by superalkalis has been proposed using quantum chemical methods. Herein, we report a systematic study on the single‐reduction of CO₂ by using typical superalkalis. Superalkalis are hypervalent species possessing lower ionization energies than alkali atoms. We have studied the interaction of CO₂ with FLi₂, OLi₃, and NLi₄ superalkalis using ab initio MP2 calculations. We notice that this interaction leads to stable superalkali‐CO₂ complexes in which the structure of CO₂ is bent due to electron transfer from superalkalis. This clearly reveals that the CO₂ can successfully be reduced to the anion. It has been also noticed that the size of superalkalis plays a crucial in the single‐electron reduction of CO₂. For instance, the binding energy of superalkali‐CO₂ complex and charge transfer to CO₂ decreases monotonically with the increase in the size of superalkali. We have also proposed that CO₂ can be further reduced to in case of the anionic complex such as (FLi₂ CO₂)‾. Thus, FLi₂ superalkali is also capable of double‐electron reduction of CO₂. These findings should provide new insights into CO₂‐activation as well as motivate further research in this direction.     

CO2 Electroreduction in Ionic Liquids: A Review

The increasing emission of carbon dioxide (CO₂) caused by the unrestrained consumption of fossil fuels in recent hundreds of years, has caused global environmental and social problems. Meanwhile, CO₂ is a cheap, abundant and renewable C1‐feedstock, which can be converted into alcohols, ethers, acids and other value‐added chemicals. Compared with the thermal reactions, electrochemical reduction of CO₂ is more attractive because of its advantages by using the seasonal, geographical and intermittent energy (tide, wind and solar) under mild conditions. In recent years, taking ionic liquids (ILs) as electrolytes in the CO₂ electrochemical reduction reaction has been paid much more attention due to the advantages of lowering the overpotential of CO₂ electroreduction and improving the Faradaic efficiency. In this paper, we summarized the recent progresses of electrochemical reduction of CO₂ in ILs electrolytes, and analyzed the reaction mechanism of CO₂ reaction in the electrode‐electrolyte interface region by experimental and simulation methods. Finally, the research which needs to be highlighted in this area was proposed.     

Atomistic modeling toward high‐efficiency carbon capture: A brief survey with a few illustrative examples

With the negative environmental implications of the anthropogenic emission of greenhouse gases like CO₂ having been scientifically established, emphasis is being placed on a concerted global effort to prevent such gases from reaching the atmosphere. Especially important are capture efforts at large point emission sources like fossil fuel power generation, natural gas processing, and various industrial plants. Given the importance and scale of such activities, it is a significant priority to optimize the capture process in terms of speed, energy requirements, and cost efficiency. For CO₂ capture, in particular, multiple systems are being pursued both with near‐term retrofitting and medium‐ to long‐term designs in mind, including: (1) liquid solvents like amines, carbonates, and ionic liquids (ILs); (2) microporous sorbents like zeolites, activated carbon, and metal‐organic frameworks; (3) solid sorbents like metal‐oxides and ionic clays; and (4) polymeric and inorganic membrane separators. Each system is unique in its molecular‐level guest–host interactions, chemistry, heats of adsorption/desorption, and equilibrium thermodynamic and transport properties as a function of loading, temperature, and pressure. This opens up exciting opportunities for molecular modeling in the design and optimization of materials systems. Here, we offer a brief survey of molecular modeling applications in the field of carbon capture, with a few illustrative examples from our own work primarily involving amine solutions and ILs. Important molecular dynamics, Monte Carlo, and correlations‐based work in the literature relevant to CO₂ capture in other systems are also discussed. © 2013 Wiley Periodicals, Inc.     

Electrocatalytic Production of C3‐C4 Compounds by Conversion of CO2 on a Chloride‐Induced Bi‐Phasic Cu2O‐Cu Catalyst

Electrocatalytic conversion of carbon dioxide (CO₂) has recently received considerable attention as one of the most feasible CO₂ utilization techniques. In particular, copper and copper‐derived catalysts have exhibited the ability to produce a number of organic molecules from CO₂. Herein, we report a chloride (Cl)‐induced bi‐phasic cuprous oxide (Cu₂O) and metallic copper (Cu) electrode (Cu₂O_Cl) as an efficient catalyst for the formation of high‐carbon organic molecules by CO₂ conversion, and identify the origin of electroselectivity toward the formation of high‐carbon organic compounds. The Cu₂O_Cl electrocatalyst results in the preferential formation of multi‐carbon fuels, including n‐propanol and n‐butane C3–C4 compounds. We propose that the remarkable electrocatalytic conversion behavior is due to the favorable affinity between the reaction intermediates and the catalytic surface.     

Poly(Ionic Liquid)‐Derived Carbon with Site‐Specific N‐Doping and Biphasic Heterojunction for Enhanced CO2 Capture and Sensing

CO₂ capture is a pressing global environmental issue that drives scientists to develop creative strategies for tackling this challenge. The concept in this contribution is to produce site‐specific nitrogen doping in microporous carbon fibers. Following this approach a carbon/carbon heterojunction is created by using a poly(ionic liquid) (PIL) as a “soft” activation agent that deposits nitrogen species exclusively on the surface of commercial microporous carbon fibers. This type of carbon‐based biphasic heterojunction amplifies the interaction between carbon fiber and CO₂ molecule for unusually high CO₂ uptake and resistive sensing.     

Polyethylenimine‐assisted Synthesis of Au Nanoparticles for Efficient Syngas Production

The ability to capture, store, and use CO₂ is important for remediating greenhouse‐gas emissions and combatting global warming. Herein, Au nanoparticles (Au‐NPs) are synthesized for effective electrochemical CO₂ reduction and syngas production, using polyethylenimine (PEI) as a ligand molecule. The PEI‐assisted synthesis provides uniformly sized 3‐nm Au NPs, whereas larger irregularly shaped NPs are formed in the absence of PEI in the synthesis solution. The Au‐NPs synthesized with PEI (PEI?Au/C, average PEI Mw=2000) exhibit improved CO₂ reduction activities compared to Au‐NPs formed in the absence of PEI (bare Au NPs/C). PEI?Au/C displays a 34 % higher activity toward CO₂ reduction than bare Au NPs/C; for example, PEI?Au/C exhibits a CO partial current density (j_CO) of 28.6 mA cm^?2 at ?1.13 V_RHE, while the value for bare Au NPs/C is 21.7 mA cm^?2; the enhanced j_CO is mainly due to the larger surface area of PEI?Au/C. Furthermore, the PEI?Au/C electrode exhibits stable performance over 64 h, with an hourly current degradation rate of 0.25 %. The developed PEI?Au/C is employed in a CO₂‐reduction device coupled with an IrO₂ water‐oxidation catalyst and a proton‐conducting perfluorinated membrane to form a PEI?Au/C|Nafion|IrO₂ membrane‐electrode assembly. The device using PEI?Au/C as the CO₂‐reduction catalyst exhibits a j_CO of 4.47 mA/cm² at 2.0 V_cell. Importantly, the resulted PEI?Au/C is appropriate for efficient syngas production with a CO ratio of around 30–50 %.     

Electrocatalytic and Photocatalytic CO2 Methanation: From Reaction Fundamentals to Catalyst Developments

Transitioning from fossil fuels to renewable energy sources is demanded due to the gradual depletion of petroleum oil/gas and the environmental impact of carbon dioxide (CO₂) emissions into the atmosphere. Electrocatalytic and photocatalytic CO₂ reduction to methane (CH₄) using renewable energy sources is crucial for sustainable chemical/fuel production and greenhouse gas reduction. In recent years, extensive research has focused on understanding the fundamental aspects of the two approaches, such as reaction mechanisms and active sites, and exploring/designing novel catalytic materials. This review initially discusses the reaction fundamentals, including performance evaluation indexes, reactors, and mechanisms, to understand the catalytic reactions. Subsequently, various catalyst preparation strategies and characterization methods are summarized, trying to outline the catalyst design principle based on the obtained understanding of the reaction mechanisms. Finally, research challenges and perspectives for future development in this area are discussed and presented. It is expected to provide a comprehensive understanding of the photo/electrocatalytic CO₂ methanation, valuable knowledge to novice researchers, and a helpful reference for future research endeavors.     

Micro and Nanoporous Membrane Platforms for Carbon Neutrality: Membrane Gas Separation Prospects

Recently, carbon neutrality has been promoted as a potentially practical solution to global CO₂ emissions and increasing energy-consumption challenges. Many attempts have been made to remove CO₂ from the environment to address climate change and rising sea levels owing to anthropogenic CO₂ emissions. Herein, membrane technology is proposed as a suitable solution for carbon neutrality. This review aims to comprehensively evaluate the currently available scientific research on membranes for carbon capture, focusing on innovative microporous material membranes used for CO₂ separation and considering their material, chemical, and physical characteristics and permeability factors. Membranes from such materials comprise metal-organic frameworks, zeolites, silica, porous organic frameworks, and microporous polymers. The critical obstacles related to membrane design, growth, and CO₂ capture and usage processes are summarized to establish novel membranes and strategies and accelerate their scaleup.     

2D Catalysts for CO2 Photoreduction: Discussing Structure Efficiency Strategies and Prospects for Scaled Production Based on Current Progress

Currently, the excessive consumption of fossil fuels is accompanied by massive emissions of CO₂, leading to severe energy shortages and intensified global warming. It is of great significance to develop and use renewable clean energy while reducing the concentration of CO₂ in the atmosphere. Photocatalytic technology is a promising strategy for carbon dioxide conversion. Clearly, the achievement of the above goals largely depends on the design and construction of catalysts. This review is mainly focused on the application of 2D materials for photocatalytic CO₂ reduction. The contribution of synthetic strategies to their structure and performance is emphasized. Finally, the current challenges, and prospects of 2D materials for photoreduction of CO₂ with high efficiency, even for practical applications are discussed. It is hoped that this review can provide some guidance for the rational design, controllable synthesis of 2D materials, and their application for efficient photocatalytic CO₂ reduction.     

Recent Advances in Photocatalytic CO2 Reduction Using Earth‐Abundant Metal Complexes‐Derived Photocatalysts

Photochemical reduction of CO₂ with H₂O into energy‐rich chemicals using inexhaustible solar energy is an appealing strategy to simultaneously address the global energy and environmental issues. Earth‐abundant metal complexes show promising application in this field due to their easy availability, rich redox valence and tunable property. Great progress has been seen on catalytic reduction of CO₂ under visible light illumination employing earth‐abundant metal complexes and their hybrids as key contributors, especially for producing CO and HCOOH via the two‐electron reduction process. In this minireview, we will summarize and update advances on earth‐abundant metal complex‐derived photocatalytic system for visible‐light driven CO₂ photoreduction over the last 5 years. Homogeneous earth‐abundant metal complex photocatalysts and earth‐abundant metal complex derived hybrid photocatalysts were both presented with focus on efficient improvement strategy.     

Der globale Kohlenstoffkreislauf im Anthropozän. Betrachtung aus meereschemischer Perspektive

By burning of fossil fuels humankind emits more than 8 billion tons of carbon (Gt C) in the form of CO₂ to the atmosphere. Since the onset of the industrial revolution the cumulative emissions have led to an increase of the atmospheric CO₂ concentration which corresponds to an additional radiative forcing in the atmosphere. Of the three reservoirs which exchange carbon on the time scale of centuries – atmosphere, terrestrial biosphere, and ocean – the ocean is by far the largest. The marine CO₂ system comprises the chemical species HCO₃^–, CO₃^2–, and CO₂(aq). This gives rise to the pH‐buffering nature of seawater as well as its high uptake capacity for anthropogenic CO₂. Four measurement parameters of the marine CO₂ system are available for an accurate analytical characterization. These parameters also provide a means of sensing the role of physical, chemical, and biological drivers for the marine carbon cycle. The marine carbon cycle features major natural processes that exchange carbon with the atmosphere and re‐distribute it throughout the ocean. These are known as “pumps” and driven by physical and biological factors. While the “physical pump” is inevitably enhanced by the oceanic uptake of anthropogenic CO₂, even the sign of the response is currently not clear for the “biological pumps”. A host of potential consequences of global change (temperature rise, ocean carbonation, ocean acidification) have been identified. These are currently studied intensively with respect to their climate sensitivity as well as the climate feedback potential.     

Sorbents for the Direct Capture of CO2 from Ambient Air

The urgency to address global climate change induced by greenhouse gas emissions is increasing. In particular, the rise in atmospheric CO₂ levels is generating alarm. Technologies to remove CO₂ from ambient air, or “direct air capture” (DAC), have recently demonstrated that they can contribute to “negative carbon emission.” Recent advances in surface chemistry and material synthesis have resulted in new generations of CO₂ sorbents, which may drive the future of DAC and its large‐scale deployment. This Review describes major types of sorbents designed to capture CO₂ from ambient air and they are categorized by the sorption mechanism: physisorption, chemisorption, and moisture‐swing sorption.     

Metal‐Free Fluorine‐Doped Carbon Electrocatalyst for CO2 Reduction Outcompeting Hydrogen Evolution

The electrochemical CO₂ reduction (ECDRR), as a key reaction in artificial photosynthesis to implement renewable energy conversion/storage, has been inhibited by the low efficiency and high costs of the electrocatalysts. Herein, we synthesize a fluorine‐doped carbon (FC) catalyst by pyrolyzing commercial BP 2000 with a fluorine source, enabling a highly selective CO₂‐to‐CO conversion with a maximum Faradaic efficiency of 90 % at a low overpotential of 510 mV and a small Tafel slope of 81 mV dec^?1, outcompeting current metal‐free catalysts. Moreover, the higher partial current density of CO and lower partial current density of H₂ on FC relative to pristine carbon suggest an enhanced inherent activity towards ECDRR as well as a suppressed hydrogen evolution by fluorine doping. Fluorine doping activates the neighbor carbon atoms and facilitates the stabilization of the key intermediate COOH* on the fluorine‐doped carbon material, which are also blocked for competing hydrogen evolution, resulting in superior CO₂‐to‐CO conversion.     

Rare‐Earth Single Erbium Atoms for Enhanced Photocatalytic CO2 Reduction

The solar‐driven photocatalytic reduction of CO₂ (CO₂RR) into chemical fuels is a promising route to enrich energy supplies and mitigate CO₂ emissions. However, low catalytic efficiency and poor selectivity, especially in a pure‐water system, hinder the development of photocatalytic CO₂RR owing to the lack of effective catalysts. Herein, we report a novel atom‐confinement and coordination (ACC) strategy to achieve the synthesis of rare‐earth single erbium (Er) atoms supported on carbon nitride nanotubes (Er₁/CN‐NT) with a tunable dispersion density of single atoms. Er₁/CN‐NT is a highly efficient and robust photocatalyst that exhibits outstanding CO₂RR performance in a pure‐water system. Experimental results and density functional theory calculations reveal the crucial role of single Er atoms in promoting photocatalytic CO₂RR.     

Nanostructured Materials for Heterogeneous Electrocatalytic CO2 Reduction and their Related Reaction Mechanisms

The gradually increased concentration of carbon dioxide (CO₂) in the atmosphere has been recognized as the primary culprit for the rise of the global mean temperature. In recent years, development of routes for highly efficient conversion of CO₂ has received much attention. This Review describes recent progress on the design and synthesis of solid‐state catalysts for the electrochemical reduction of CO₂. The significance of this catalytic conversion is presented, followed by the general parameters for CO₂ electroreduction and a summary of the reaction apparatus. We also discuss various types of solid catalysts based on their CO₂ conversion mechanisms. We summarize the crucial factors (particle size, surface structure, composition, etc.) determining the performance for electroreduction.     

Hydrophosphination of CO2 and Subsequent Formate Transfer in the 1,3,2‐Diazaphospholene‐Catalyzed N‐Formylation of Amines

Hydrophosphination of CO₂ with 1,3,2‐Diazaphospholene (NHP‐H; 1 ) afforded phosphorus formate (NHP‐OCOH; 2 ) through the formation of a bond between the electrophilic phosphorus atom in 1 and the oxygen atom from CO₂, along with hydride transfer to the carbon atom of CO₂. Transfer of the formate from 2 to Ph₂SiH₂ produced Ph₂Si(OCHO)₂ ( 3 ) in a reaction that could be carried out in a catalytic manner by using 5 mol % of 1 . These elementary reactions were applied to the metal‐free catalytic N‐formylation of amine derivatives with CO₂ in one pot under ambient conditions.     

Strategies for Designing Nanoparticles for Electro‐ and Photocatalytic CO2 Reduction

Conversion of carbon dioxide (CO₂) into value‐added chemicals has attracted much attention because it can not only resolve global warming issues by reducing CO₂ accumulation in the atmosphere, but also produce renewable hydrocarbon fuels that are important feedstocks for the chemical industry. Among the diverse approaches reported, CO₂ reduction via electro‐ and photocatalytic methods is at the center of topics due to potential engineering of reaction performance through rational design of catalyst features. In this Minireview, we highlight recent strategies for designing nanoparticles to maximize the reaction efficiency and selectivity; from a materials viewpoint, these strategies can provide critical information to guide future research directions.     

An Abnormal N‐Heterocyclic Carbene–Carbon Dioxide Adduct from Imidazolium Acetate Ionic Liquids: The Importance of Basicity

In the reaction of 1‐ethyl‐3‐methylimidazolium acetate [C₂C₁Im][OAc] ionic liquid with carbon dioxide at 125 °C and 10 MPa, not only the known N‐heterocyclic carbene (NHC)–CO₂ adduct I , but also isomeric aNHC‐CO₂ adducts II and III were obtained. The abnormal NHC‐CO₂ adducts are stabilized by the presence of the polarizing basic acetate anion, according to static DFT calculations and ab initio molecular dynamics studies. A further possible reaction pathway is facilitated by the high basicity of the system, deprotonating the initially formed NHC‐CO₂ adduct I , which can then be converted in the presence of the excess of CO₂ to the more stable 2‐deprotonated anionic abnormal NHC–CO₂ adduct via the anionic imidazolium‐2,4‐dicarboxylate according to DFT calculations on model compounds. This suggests a generalizable pathway to abnormal NHC complex formation.