A nitrogen‐stabilized single‐atom catalyst containing low‐valence zinc atoms (Znδ+‐NC) is reported. It contains saturated four‐coordinate (Zn‐N4) and unsaturated three‐coordinate (Zn‐N3) sites. The ...latter makes Zn a low‐valence state, as deduced from X‐ray photoelectron spectroscopy, X‐ray absorption spectroscopy, electron paramagnetic resonance, and density functional theory. Znδ+‐NC catalyzes electrochemical reduction of CO2 to CO with near‐unity selectivity in water at an overpotential as low as 310 mV. A current density up to 1 A cm−2 can be achieved together with high CO selectivity of >95 % using Znδ+‐NC in a flow cell. Calculations suggest that the unsaturated Zn‐N3 could dramatically reduce the energy barrier by stabilizing the COOH* intermediate owing to the electron‐rich environment of Zn. This work sheds light on the relationship among coordination number, valence state, and catalytic performance and achieves high current densities relevant for industrial applications.
A nitrogen‐anchored low‐valence Zn single‐atom catalyst, containing coordinately unsaturated Zn‐N3 active sites, can boost electrochemical CO2 reduction to industrial application levels.
In a comparative study of the electrocatalytic CO2 reduction, cobalt meso‐tetraphenylporphyrin (CoTPP) is used as a model molecular catalyst under both homogeneous and heterogeneous conditions. In ...the former case, employing N,N‐dimethylformamide as solvent, CoTPP performs poorly as an electrocatalyst giving low product selectivity in a slow reaction at a high overpotential. However, upon straightforward immobilization of CoTPP onto carbon nanotubes, a remarkable enhancement of the electrocatalytic abilities is seen with CO2 becoming selectively reduced to CO (>90 %) at a low overpotential in aqueous medium. This effect is ascribed to the particular environment created by the aqueous medium at the catalytic site of the immobilized catalyst that facilitates the adsorption and further reaction of CO2. This work highlights the significance of assessing an immobilized molecular catalyst from more than homogeneous measurements alone.
Heterogeneous vs. homogeneous: When cobalt meso‐tetraphenylporphyrin (CoTPP) is immobilized on carbon nanotubes, a remarkably enhanced catalytic activity in CO2 electroreduction is observed, with CoITPP− serving as the active species. The simple approach for heterogenization enables facile screening and evaluation of molecular catalysts under heterogeneous conditions.
Recently, a large number of nanostructured metal‐containing materials have been developed for the electrochemical CO2 reduction reaction (eCO2RR). However, it remains a challenge to achieve high ...activity and selectivity with respect to the metal load due to the limited concentration of surface metal atoms. Here, it is reported that the bismuth‐based metal–organic framework Bi(1,3,5‐tris(4‐carboxyphenyl)benzene), herein denoted Bi(btb), works as a precatalyst and undergoes a structural rearrangement at reducing potentials to form highly active and selective catalytic Bi‐based nanoparticles dispersed in a porous organic matrix. The structural change is investigated by electron microscopy, X‐ray diffraction, total scattering, and spectroscopic techniques. Due to the periodic arrangement of Bi cations in highly porous Bi(btb), the in situ formed Bi nanoparticles are well‐dispersed and hence highly exposed for surface catalytic reactions. As a result, high selectivity over a broad potential range in the eCO2RR toward formate production with a Faradaic efficiency up to 95(3)% is achieved. Moreover, a large current density with respect to the Bi load, i.e., a mass activity, up to 261(13) A g−1 is achieved, thereby outperforming most other nanostructured Bi materials.
A bismuth‐containing metal–organic framework transforms to an ensemble of bismuth nanoparticles dispersed in a porous organic matrix under electrochemical conditions in aqueous solution. When used as a catalyst in the electroreduction of carbon dioxide, it selectively produces formate (≈95%) with large activity relative to the metal content (≈261 A g−1), outperforming previously reported bismuth‐based materials.
Formaldehyde (HCHO) is a crucial C1 building block for daily‐life commodities in a wide range of industrial processes. Industrial production of HCHO today is based on energy‐ and cost‐intensive ...gas‐phase catalytic oxidation of methanol, which calls for exploring other and more sustainable ways of carrying out this process. Utilization of carbon dioxide (CO2) as precursor presents a promising strategy to simultaneously mitigate the carbon footprint and alleviate environmental issues. This Minireview summarizes recent progress in CO2‐to‐HCHO conversion using hydrogenation, hydroboration/hydrosilylation as well as photochemical, electrochemical, photoelectrochemical, and enzymatic approaches. The active species, reaction intermediates, and mechanistic pathways are discussed to deepen the understanding of HCHO selectivity issues. Finally, shortcomings and prospects of the various strategies for sustainable reduction of CO2 to HCHO are discussed.
This Minireview summarizes recent progress in the production of HCHO from CO2, including chemical catalysis (hydrogenation using H2 and hydroboration/hydrosilylation), photo/electrocatalysis, and biocatalysis (enzymatic reduction). From analysis of advantages and deficits of each methodology, we present viewpoints and potential strategies for optimizing the CO2‐to‐HCHO conversion.
Earth-abundant transition metal (Fe, Co, or Ni) and nitrogen-doped porous carbon electrocatalysts (M-N-C, where M denotes the metal) were synthesized from cheap precursors via silica-templated ...pyrolysis. The effect of the material composition and structure (i.e., porosity, nitrogen doping, metal identity, and oxygen functionalization) on the activity for the electrochemical CO2 reduction reaction (CO2RR) was investigated. The metal-free N-C exhibits a high selectivity but low activity for CO2RR. Incorporation of the Fe and Ni, but not Co, sites in the N-C material is able to significantly enhance the activity. The general selectivity order for CO2-to-CO conversion in water is found to be Ni > Fe ≫ Co with respect to the metal in M-N-C, while the activity follows Ni, Fe ≫ Co. Notably, the Ni-doped carbon exhibits a high selectivity with a faradaic efficiency of 93% for CO production. Tafel analysis shows a change of the rate-determining step as the metal overtakes the role of the nitrogen as the most active site. Recording the X-ray photoelectron spectra and extended X-ray absorption fine structure demonstrates that the metals are atomically dispersed in the carbon matrix, most likely coordinated to four nitrogen atoms and with carbon atoms serving as a second coordination shell. Presumably, the carbon atoms in the second coordination shell of the metal sites in M-N-C significantly affect the CO2RR activity because the opposite reactivity order is found for carbon supported metal meso-tetraphenylporphyrin complexes. From a better understanding of the relationship between the CO2RR activity and the material structure, it becomes possible to rationally design high-performance porous carbon electrocatalysts involving earth-abundant metals for CO2 valorization.
Significant efforts have been devoted over the last few years to develop efficient molecular electrocatalysts for the electrochemical reduction of carbon dioxide to carbon monoxide, the latter being ...an industrially important feedstock for the synthesis of bulk and fine chemicals. Whereas these efforts primarily focus on this formal oxygen abstraction step, there are no reports on the exploitation of the chemistry for scalable applications in carbonylation reactions. Here we describe the design and application of an inexpensive and user-friendly electrochemical set-up combined with the two-chamber technology for performing Pd-catalysed carbonylation reactions including amino- and alkoxycarbonylations, as well as carbonylative Sonogashira and Suzuki couplings with near stoichiometric carbon monoxide. The combined two-reaction process allows for milligram to gram synthesis of pharmaceutically relevant compounds. Moreover, this technology can be adapted to the use of atmospheric carbon dioxide.Electroreduction of CO
to CO is a potential valorisation pathway of carbon dioxide for fine chemicals production. Here, the authors show a user-friendly device that couples CO
electroreduction with carbonylation chemistry for up to gram scale synthesis of pharmaceuticals even under atmospheric CO
.
Organic micro‐ and nanostructures are expected to be promising candidates for micro‐ and nanophotonic materials with desirable properties owing to their low cost, flexible molecular design, and ...tunable self‐assembly. Among these candidates, well‐known squaraine dyes (SQs) have rarely been investigated because of their nonfluorescent properties in the solid state and because their optical behavior varies with changes in morphology. In this contribution, two novel 1,2‐SQs, SQM and SQB, with strong bright‐yellow to red fluorescence emission in the crystalline state, were designed and structured at the molecular level and by solvent adjustment. Their self‐assembly behavior was studied, and it was revealed that the SQM assembly provided 1D microrods, whereas 1D microrods (Z‐SQB⋅CH2Cl2) and 2D microplates (E‐SQB⋅2 CH3OH) could be obtained from SQB assemblies through a solution‐based self‐assembly method. The varied assembly behaviors of these SQs were attributed to different π–π stacking interactions that resulted in different molecular conformations and packing modes. These assemblies exhibited distinct optical properties, and in particular, SQB⋅solvent assemblies showed multiple thermo‐ and vapochromic effects. Thus, the SQB assemblies are potential fluorescent sensors for organic solvent vapors. More importantly, favorable optical‐waveguide properties were observed in these SQ‐based microstructures.
Order, please! Well‐ordered 1,2‐squaraine assemblies, SQM and SQB, with strong bright‐yellow to red colored emission are successfully constructed. Multiple reversible thermo‐ and vapochromic behavior is observed for the first time in these assemblies; thus, they could be potential “naked‐eye” fluorescent sensors for organic solvent vapors (see figure).
It is still a great challenge to explore hydrogen evolution reaction (HER) electrocatalysts with both lower overpotential and higher stability in acidic electrolytes. In this work, an efficient HER ...catalyst, Ru@COF‐1, is prepared by complexation of triazine‐cored sp2 carbon‐conjugated covalent organic frameworks (COFs) with ruthenium ion. Ru@COF‐1 possesses high crystallinity and porosity, which are beneficial for electrocatalysis. The large specific surface area and regular porous channels of Ru@COF‐1 facilitate full contact between reactants and catalytic sites. The nitrogen atoms of triazines are protonated in the acidic media, which greatly improve the conductivity of Ru@COF‐1. This synergistic effect makes the overpotential of Ru@COF‐1 about 200 mV at 10 mA cm–2, which is lower than other reported COFs‐based electrocatalysts. Moreover, Ru@COF‐1 exhibits exceptionally electrocatalytic durability in the acidic electrolytes. It is particularly stable and remains highly active after 1000 cyclic voltammetry cycles. Density functional theory calculations demonstrate that tetracoordinated Ru‐N2Cl2 moieties are the major contributors to the outstanding HER performance. This work provides a new idea for developing protonated HER electrocatalysts in acidic media.
An efficient electrocatalyst Ru@COF‐1 is prepared by complexation of triazine‐cored sp2 carbon‐conjugated covalent organic frameworks (COFs) with ruthenium ion for hydrogen evolution. The regular porous channels of Ru@COF‐1 with high porosities facilitate full contact between reactants and catalytic sites. The triazine moities can be protonated in the acidic media, which greatly improve the conductivity. Ru@COF‐1 shows excellent electrocatalytic performance and exceptional electrochemical stability in the acidic electrolytes.
The thorough understanding of homogeneous catalysis has triggered intense research activities on the immobilization of molecular catalysts for the heterogeneous CO2 electroreduction. Herein, we ...discuss recent advances in the heterogeneous field with focus on the intrinsic effect coming from the catalyst structure and the extrinsic effect exerted by the catalyst immobilization strategy and support material on the catalytic performance.
•Molecular catalysts are immobilized on support material by chemisorption or physisorption.•Immobilized catalysts exhibit good performance for CO2 electroreduction.•Modification of catalyst and support material improves the catalytic performance.
A straightforward procedure has been developed to prepare a porous carbon material decorated with iron by direct pyrolysis of a mixture of a porous polymer and iron chloride. Characterization of the ...material with X‐ray diffraction, X‐ray absorption spectroscopy, and electron microscopy indicates the presence of iron carbide nanoparticles encapsulated inside the carbon matrix, and elemental mapping and cyanide poisoning experiments demonstrate the presence of atomic Fe centers, albeit in trace amounts, which are active sites for electrochemical CO2 reduction. The encapsulated iron carbide nanoparticles are found to boost the catalytic activity of atomic Fe sites in the outer carbon layers, rendering the material highly active and selective for CO2 reduction, although these atomic Fe sites are only present in trace amounts. The target material exhibits near‐unity selectivity (98 %) for CO2‐to‐CO conversion at a small overpotential (410 mV) in water. Furthermore, the material holds potential for practical application, as a current density over 30 mA cm−2 and a selectivity of 93 % can be achieved in a flow cell.
Just encase: A porous carbon material, with atomic iron sites embedded and iron carbide nanoparticles encased, is prepared directly from a porous polymer. The atomic iron sites are active for electrocatalytic CO2 reduction, whereas the iron carbide – although not in direct contact with the electrolyte and CO2 – promotes CO2 reduction. Thus, the material exhibits near‐unity selectivity for CO2‐to‐CO conversion at a small overpotential in water.
PDF
