Understanding the Origin of Highly Selective CO2 Electroreduction to CO on Ni,N‐doped Carbon Catalysts

Ni,N‐doped carbon catalysts have shown promising catalytic performance for CO₂ electroreduction (CO₂R) to CO; this activity has often been attributed to the presence of nitrogen‐coordinated, single Ni atom active sites. However, experimentally confirming Ni?N bonding and correlating CO₂ reduction (CO₂R) activity to these species has remained a fundamental challenge. We synthesized polyacrylonitrile‐derived Ni,N‐doped carbon electrocatalysts (Ni‐PACN) with a range of pyrolysis temperatures and Ni loadings and correlated their electrochemical activity with extensive physiochemical characterization to rigorously address the origin of activity in these materials. We found that the CO₂R to CO partial current density increased with increased Ni content before plateauing at 2 wt % which suggests a dispersed Ni active site. These dispersed active sites were investigated by hard and soft X‐ray spectroscopy, which revealed that pyrrolic nitrogen ligands selectively bind Ni atoms in a distorted square‐planar geometry that strongly resembles the active sites of molecular metal–porphyrin catalysts.     

Exclusive Formation of Formic Acid from CO2 Electroreduction by a Tunable Pd‐Sn Alloy

Conversion of carbon dioxide (CO₂) into fuels and chemicals by electroreduction has attracted significant interest, although it suffers from a large overpotential and low selectivity. A Pd‐Sn alloy electrocatalyst was developed for the exclusive conversion of CO₂ into formic acid in an aqueous solution. This catalyst showed a nearly perfect faradaic efficiency toward formic acid formation at the very low overpotential of −0.26 V, where both CO formation and hydrogen evolution were completely suppressed. Density functional theory (DFT) calculations suggested that the formation of the key reaction intermediate HCOO* as well as the product formic acid was the most favorable over the Pd‐Sn alloy catalyst surface with an atomic composition of PdSnO₂, consistent with experiments.     

Selective CO2 Electroreduction to Ethylene and Multicarbon Alcohols via Electrolyte‐Driven Nanostructuring

Production of multicarbon products (C₂₊) from CO₂ electroreduction reaction (CO₂RR) is highly desirable for storing renewable energy and reducing carbon emission. The electrochemical synthesis of CO₂RR catalysts that are highly selective for C₂₊ products via electrolyte‐driven nanostructuring is presented. Nanostructured Cu catalysts synthesized in the presence of specific anions selectively convert CO₂ into ethylene and multicarbon alcohols in aqueous 0.1 m KHCO₃ solution, with the iodine‐modified catalyst displaying the highest Faradaic efficiency of 80 % and a partial geometric current density of ca. 31.2 mA cm^?2 for C₂₊ products at ?0.9 V vs. RHE. Operando X‐ray absorption spectroscopy and quasi in situ X‐ray photoelectron spectroscopy measurements revealed that the high C₂₊ selectivity of these nanostructured Cu catalysts can be attributed to the highly roughened surface morphology induced by the synthesis, presence of subsurface oxygen and Cu⁺ species, and the adsorbed halides.     

Selective and Efficient Reduction of Carbon Dioxide to Carbon Monoxide on Oxide‐Derived Nanostructured Silver Electrocatalysts

In this work, the selective electrocatalytic reduction of carbon dioxide to carbon monoxide on oxide‐derived silver electrocatalysts is presented. By a simple synthesis technique, the overall high faradaic efficiency for CO production on the oxide‐derived Ag was shifted by more than 400 mV towards a lower overpotential compared to that of untreated Ag. Notably, the Ag resulting from Ag oxide is capable of electrochemically reducing CO₂ to CO with approximately 80 % catalytic selectivity at a moderate overpotential of 0.49 V, which is much higher than that (ca. 4 %) of untreated Ag under identical conditions. Electrokinetic studies show that the improved catalytic activity is ascribed to the enhanced stabilization of COOH^. intermediate. Furthermore, highly nanostructured Ag is likely able to create a high local pH near the catalyst surface, which may also facilitate the catalytic activity for the reduction of CO₂ with suppressed H₂ evolution.     

Anodic silver (II) oxides investigated by combined electrochemistry,ex situ XPS and in situ X‐ray absorption spectroscopy

Silver (II) oxide layers (AgO) were prepared by anodic oxidation of pre‐oxidized, Ag₂O‐covered silver electrodes in 1 M NaOH (pH 13.8). The oxidized electrodes were investigated using a combination of electrochemical techniques, ex situ X‐ray photoelectron spectroscopy (XPS) and in situ surface‐sensitive grazing incidence X‐ray absorption spectroscopy (EXAFS) under full potential control. The application of these different techniques leads to a detailed, consistent picture of the anodic silver (II) oxide layer formation. The experiments have shown that the chemical composition of the AgO layer varies significantly with oxidation potential, revealing a decreasing oxygen deficiency with increasing anodization potential and oxidation time. XPS as well as EXAFS experiments support the interpretation of the oxide as a mixed valence Ag ⁺ Ag^{3 +} O₂ with different contributions of Ag ⁺ and Ag^{3 +} species, depending on potential and anodization time. Copyright © 2009 John Wiley & Sons, Ltd.     

Low‐Coordinated Edge Sites on Ultrathin Palladium Nanosheets Boost Carbon Dioxide Electroreduction Performance

Electrochemical conversion of carbon dioxide (CO₂) to value‐added products is a possible way to decrease the problems resulting from CO₂ emission. Thanks to the eminent conductivity and proper adsorption to intermediates, Pd has become a promising candidate for CO₂ electroreduction (CO₂ER). However, Pd‐based nanocatalysts generally need a large overpotential. Herein we describe that ultrathin Pd nanosheets effectively reduce the onset potential for CO by exposing abundant atoms with comparatively low generalized coordination number. Hexagonal Pd nanosheets with 5 atomic thickness and 5.1 nm edge length reached CO faradaic efficiency of 94 % at ?0.5 V, without any decay after a stability test of 8 h. It appears to be the most efficient among all of Pd‐based catalysts toward CO₂ER. Uniform hexagonal morphology made it reasonable to build models and take DFT calculations. The enhanced activity originates from mainly edge sites on palladium nanosheets.     

Metal‐Doped Nitrogenated Carbon as an Efficient Catalyst for Direct CO2 Electroreduction to CO and Hydrocarbons

This study explores the kinetics, mechanism, and active sites of the CO₂ electroreduction reaction (CO2RR) to syngas and hydrocarbons on a class of functionalized solid carbon‐based catalysts. Commercial carbon blacks were functionalized with nitrogen and Fe and/or Mn ions using pyrolysis and acid leaching. The resulting solid powder catalysts were found to be active and highly CO selective electrocatalysts in the electroreduction of CO₂ to CO/H₂ mixtures outperforming a low‐area polycrystalline gold benchmark. Unspecific with respect to the nature of the metal, CO production is believed to occur on nitrogen functionalities in competition with hydrogen evolution. Evidence is provided that sufficiently strong interaction between CO and the metal enables the protonation of CO and the formation of hydrocarbons. Our results highlight a promising new class of low‐cost, abundant electrocatalysts for synthetic fuel production from CO₂.     

2‐Aminobenzenethiol‐Functionalized Silver‐Decorated Nanoporous Silicon Photoelectrodes for Selective CO2 Reduction

A molecularly thin layer of 2‐aminobenzenethiol (2‐ABT) was adsorbed onto nanoporous p‐type silicon (b‐Si) photocathodes decorated with Ag nanoparticles (Ag NPs). The addition of 2‐ABT alters the balance of the CO₂ reduction and hydrogen evolution reactions, resulting in more selective and efficient reduction of CO₂ to CO. The 2‐ABT adsorbate layer was characterized by Fourier transform infrared (FTIR) spectroscopy and modeled by density functional theory calculations. Ex situ X‐ray photoelectron spectroscopy (XPS) of the 2‐ABT modified electrodes suggests that surface Ag atoms are in the +1 oxidation state and coordinated to 2‐ABT via Ag?S bonds. Under visible light illumination, the onset potential for CO₂ reduction was ?50 mV vs. RHE, an anodic shift of about 150 mV relative to a sample without 2‐ABT. The adsorption of 2‐ABT lowers the overpotentials for both CO₂ reduction and hydrogen evolution. A comparison of electrodes functionalized with different aromatic thiols and amines suggests that the primary role of the thiol group in 2‐ABT is to anchor the NH₂ group near the Ag surface, where it serves to bind CO₂ and also to assist in proton transfer.     

Enhanced CO2 Electroreduction on Neighboring Zn/Co Monomers by Electronic Effect

It is of great significance to reveal the detailed mechanism of neighboring effects between monomers, as they could not only affect the intermediate bonding but also change the reaction pathway. This paper describes the electronic effect between neighboring Zn/Co monomers effectively promoting CO₂ electroreduction to CO. Zn and Co atoms coordinated on N doped carbon (ZnCoNC) show a CO faradaic efficiency of 93.2 % at ?0.5 V versus RHE during a 30‐hours test. Extended X‐ray absorption fine structure measurements (EXAFS) indicated no direct metal–metal bonding and X‐ray absorption near‐edge structure (XANES) showed the electronic effect between Zn/Co monomers. In situ attenuated total reflection‐infrared spectroscopy (ATR‐IR) and density functional theory (DFT) calculations further revealed that the electronic effect between Zn/Co enhanced the *COOH intermediate bonding on Zn sites and thus promoted CO production. This work could act as a promising way to reveal the mechanism of neighboring monomers and to influence catalysis.     

Iridium Complexes of 2,2′‐Bidipyrrins

The reaction of [{Ir(cod)(μ‐Cl)}₂] and K₂CO₃ or of [{Ir(cod)(μ‐OMe)}₂] alone with the non‐natural tetrapyrrole 2,2′‐bidipyrrin (H₂BDP) yields, depending on the stoichiometry, the mononuclear complex [Ir(cod)(HBDP)] or the homodinuclear complex [{Ir(cod)}₂(BDP)]. Both complexes react readily with carbon monoxide to yield the species [Ir(CO)₂(HBDP)] and [{Ir(CO)₂}₂(BDP)], respectively. The results from NMR spectroscopy and X‐ray diffraction reveal different conformations for the tetrapyrrolic ligand in both complexes. The reaction of [{Ir(coe)₂(μ‐Cl)}₂] with H₂BDP proceeds differently and yields the macrocyclic [4e^?,2H⁺]‐oxidized product [IrCl₂(9‐Meic)] (9‐Meic = monoanion of 9‐methyl‐9,10‐isocorrole), which can be addressed as an iridium analog of cobalamin.     

Metal‐Free Nitrogen‐Doped Mesoporous Carbon for Electroreduction of CO2 to Ethanol

CO₂ electroreduction is a promising technique for satisfying both renewable energy storage and a negative carbon cycle. However, it remains a challenge to convert CO₂ into C2 products with high efficiency and selectivity. Herein, we report a nitrogen‐doped ordered cylindrical mesoporous carbon as a robust metal‐free catalyst for CO₂ electroreduction, enabling the efficient production of ethanol with nearly 100 % selectivity and high faradaic efficiency of 77 % at −0.56 V versus the reversible hydrogen electrode. Experiments and density functional theory calculations demonstrate that the synergetic effect of the nitrogen heteroatoms and the cylindrical channel configurations facilitate the dimerization of key CO* intermediates and the subsequent proton–electron transfers, resulting in superior electrocatalytic performance for synthesizing ethanol from CO₂.     

Microwave‐Hydrothermal Preparation and Visible‐Light Photoactivity of Plasmonic Photocatalyst Ag‐TiO2 Nanocomposite Hollow Spheres

Visible‐light‐driven plasmonic photocatalyst Ag‐TiO₂ nanocomposite hollow spheres are prepared by a template‐free chemically‐induced self‐transformation strategy under microwave‐hydrothermal conditions, followed by a photochemical reduction process under xenon lamp irradiation. The prepared samples are characterized by using scanning electron microscopy, transmission electron microscopy, X‐ray diffraction, N₂ adsorption‐desorption isotherms, X‐ray photoelectron spectroscopy, UV/Vis and Raman spectroscopy. Production of ?OH radicals on the surface of visible‐light illuminated TiO₂ was detected by using a photoluminescence method with terephthalic acid as the probe molecule. The photocatalytic activity of as‐prepared samples was evaluated by photocatalytic decolorization of Rhodamine B (RhB) aqueous solution at ambient temperature under visible‐light irradiation. The results show that the surface plasmon absorption band of the silver nanoparticles supported on the TiO₂ hollow spheres was red shifted, and a strong surface enhanced Raman scattering effect for the Ag‐TiO₂ nanocomposite sample was observed. The prepared nanocomposite hollow spheres exhibits a highly visible‐light photocatalytic activity for photocatalytic degradation of RhB in water, and their photocatalytic activity is higher than that of pure TiO₂ and commercial Degussa P25 (P25) powders. Especially, the as‐prepared Ag‐TiO₂ nanocomposite hollow spheres at the nominal atomic ratio of silver to titanium ( R ) of 2 showed the highest photocatalytic activity, which exceeds that of P25 by a factor of more than 2.     

Silver‐Doped TiO2‐Coated Cotton Fabric as an Effective Photocatalytic System for Dye Decolorization in UV and Visible Light

In this study, titanium tetra‐isopropoxide was used as a precursor of TiO₂ for in situ coating on cotton fabric by sol–gel method. Subsequently, silver nitrate was used as doping agent to prepare silver‐doped TiO₂‐coated cotton fabric during hydrothermal treatment. The treated samples were characterized through field‐emission scanning electron microscopy, energy‐dispersive X‐ray analysis, inductively coupled plasma‐mass spectroscopy and UV–visible absorption spectroscopy to study morphology, composition of deposited elements and light absorption behavior of treated samples. X‐ray photoelectron spectroscopy was carried out to analyze the electronic state of silver in TiO₂‐coated fabric after hydrothermal treatment. Doping of silver on TiO₂‐coated fabric and subsequent hydrothermal treatment was found to enhance dye decolorization rate of rhodamine B dye solution in both UV and visible light radiations with respect to undoped TiO₂. The study shows that an optimal level of silver‐doped TiO₂‐coated fabric can be used repeatedly for dye decolorization without significant loss in its photocatalytic activity. The self‐cleaning properties of samples were also studied using methylene blue as a staining agent. It was observed that the presence of 1.8% silver on the weight of titanium in doped samples provides almost 82% of stain degradation.     

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.     

Ternary Sn‐Ti‐O Electrocatalyst Boosts the Stability and Energy Efficiency of CO2 Reduction

Simultaneously improving energy efficiency (EE) and material stability in electrochemical CO₂ conversion remains an unsolved challenge. Among a series of ternary Sn‐Ti‐O electrocatalysts, 3D ordered mesoporous (3DOM) Sn_0.3Ti_0.7O₂ achieves a trade‐off between active‐site exposure and structural stability, demonstrating up to 71.5 % half‐cell EE over 200 hours, and a 94.5 % Faradaic efficiency for CO at an overpotential as low as 430 mV. DFT and X‐ray absorption fine structure analyses reveal an electron density reconfiguration in the Sn‐Ti‐O system. A downshift of the orbital band center of Sn and a charge depletion of Ti collectively facilitate the dissociative adsorption of the desired intermediate COOH* for CO formation. It is also beneficial in maintaining a local alkaline environment to suppress H₂ and formate formation, and in stabilizing oxygen atoms to prolong durability. These findings provide a new strategy in materials design for efficient CO₂ conversion and beyond.     

Electrocatalytic reduction of carbon dioxide over reduced nanoporous zinc oxide

Nanoporous zinc oxide (ZnO) is prepared by a hydrothermal method followed by thermal decomposition for electrocatalytic reduction of CO₂. In situ X-ray absorption spectroscopy results indicate that ZnO is reduced to Zn under the electrolysis conditions for catalyzing CO₂ electroreduction. The reduced nanoporous ZnO exhibits obviously higher CO Faradaic efficiency and current density than commercial Zn foil with a maximum CO Faradaic efficiency of 92.0%, suggesting that the nanoporous structure facilitates electrocatalytic reduction of CO₂ over reduced nanoporous ZnO, probably due to increased surface area and more coordination unsaturated surface atoms.     

Modification of porous poly(ether sulfone) membranes by low‐temperature CO2‐plasma treatment

A general method of modifying the entire cross section of porous poly(ether sulfone) membranes with a low‐temperature CO₂‐plasma treatment is reported. Both surfaces of the membranes are highly hydrophilic, with a water drop on the surface disappearing in less than 1 s, even 6 months after plasma treatment. This high hydrophilicity of both membrane surfaces results from the incorporation of hydrophilic functionalities, as evidenced by Fourier transform infrared spectroscopy and X‐ray photoelectron spectroscopy. The incorporation of these hydrophilic functionalities takes place primarily during plasma treatment, with some incorporation of atmospheric oxygen and nitrogen immediately upon exposure to air. Scanning electron microscopy shows that the membrane surface is covered by a thin, white layer that is likely the result of etching and redeposition of sputtered surface fragments. An increase in the water bubble point and glass‐transition temperature is also observed for CO₂‐plasma‐treated membranes. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2473–2488, 2002     

Reflective and conductive surface‐silvered polyimide films prepared by surface graft copolymerization and electroless plating

Argon plasma‐pretreated polyimide (PI) films were subjected to UV‐induced surface graft copolymerization with 4‐vinylpyridine(4VP) under atmospheric conditions. Electroless plating of silver was carried out effectively on the 4VP graft copolymerized PI (PI‐g‐P4VP) surface after PdCl₂ activation and in the absence of SnCl₂ sensitization (the Sn‐free process). The surface compositions of the modified PI films were studied by X‐ray photoelectron spectroscopy (XPS). XPS results showed that the PI‐g‐P4VP surface is ready for electroless deposition of silver via the Sn‐free process. The grafted 4VP layer with well‐preserved pyridine groups was used not only as the chemisorption sites for the palladium complexes (without the need for prior sensitization by SnCl₂) during the electroless plating of silver, but also as an adhesion promotion layer for the electrolessly deposited silver. The silver metallized PI films show high reflectivity and conductivity with a surface resistance of 1.5 Ω and a reflectivity of 91.3%, respectively. Copyright © 2007 John Wiley & Sons, Ltd.     

Encapsulation of Silver Nanoparticles in an Amine‐Functionalized Porphyrin Metal–Organic Framework and Its Use as a Heterogeneous Catalyst for CO2 Fixation under Atmospheric Pressure

A new porphyrin‐based compound, [Zn₃(C₄₀H₂₄N₈)(C₂₀H₈N₂O₄)₂(DEF)₂](DEF)₃ ( 1 ; DEF=N,N‐diethylformamide), has been synthesized by employing 5,10,15,20‐tetrakis(4‐pyridyl)porphyrin, 1,2‐diamino‐3,6‐bis(4‐carboxyphenyl)benzene, and Zn²⁺ salt at 100 °C under solvothermal conditions. The structure, as determined by single‐crystal XRD studies, is three‐dimensional with threefold interpenetration. The usefulness of free −NH₂ groups in the ligand was exploited for anchoring silver nanoparticles through a simple solution‐based route. The silver‐loaded sample, Ag@ 1 , was characterized by powder XRD, energy‐dispersive X‐ray spectroscopy, high‐resolution TEM, SEM, X‐ray photoelectron spectroscopy, and inductively coupled plasma MS analysis, which clearly indicated that silver nanoparticles with a size of 3.83 nm were uniformly distributed within the metal–organic framework (MOF). The Ag@ 1 sample was evaluated for possible catalytic activity for the carboxylation of a terminal alkyne by employing CO₂ under atmospheric pressure; this gave excellent results. The Ag@ 1 catalyst was found to be robust, active, and recyclable. The present studies suggest that porphyrin MOFs not only exhibit interesting structures, but also show good heterogeneous catalytic activity towards the fixation of CO₂.     

A Coordination Polymer from HAMTTO with Silver(I) (HAMTTO = 4‐Amino‐6‐methyl‐1,2,4‐triazine‐3‐thione‐5‐one)

The reaction of 4‐amino‐6‐methyl‐1,2,4‐triazine‐3‐thione‐5‐one, HAMTTO, with silver (I) nitrate in methanol led under deprotonation to the polymeric compound [(AMTTO)Ag]_n. The coordination polymer {[Ag(HAMTTO)]ClO₄}_n ( 1 ) is synthesized from the reaction of the latter polymeric compound with perchloric acid. Both compounds were characterized by elemental analysis and IR spectroscopy. Single‐crystal X‐ray diffraction studies on compound 1 showed that HAMTTO acts as a bidentate ligand and chelates the silver atom via its hydrazine nitrogen atom and its sulfur atom. Crystal data for 1 at ?90 °C: space group P2₁, Z = 2, a = 629.3(1), b = 748.7(1), c = 1071.7(1) pm, β = 98.28(1)°, R₁ = 0.0533.