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.

Electrocatalysis is a promising tool for utilizing carbon dioxide as a feedstock in the chemical industry. However, controlling the selectivity for different CO2 reduction products remains a major challenge. We report a series of manganese carbonyl complexes with elaborated bipyridine or phenanthroline ligands that can reduce CO2 to either formic acid, if the ligand structure contains strategically positioned tertiary amines, or CO, if the amine groups are absent in the ligand or are placed far from the metal center. The amine-modified complexes are benchmarked to be among the most active catalysts for reducing CO2 to formic acid, with a maximum turnover frequency of up to 5500 s–1 at an overpotential of 630 mV. The conversion even works at overpotentials as low as 300 mV, although through an alternative mechanism. Mechanistically, the formation of a Mn–hydride species aided by in situ protonated amine groups was determined to be a key intermediate by cyclic voltammetry, 1H NMR, DFT calculations, and infrared spectroelectrochemistry.

Recently, a large number of nanostructured metal-containing materials have been developed for the electrochemical CO 2 reduction reaction (eCO 2 RR). 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 eCO 2 RR 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.

Carbon dioxide utilization through electrocatalysis is a promising pathway toward a more sustainable future. In this work the electrocatalytic reduction of carbon dioxide by ReI and RuII bipyridine complexes bearing pendant amines (N,N′-(([2,2′-bipyridine]-6,6′-diylbis­(2,1-phenylene))­bis­(methylene))­bis­(N-ethylethanamine) (dEAbpy)) is evaluated. In both cases, the major reduction product is carbon monoxide accompanied by some formic acid, although the yield of the latter never reaches the predominant level known from the corresponding Mn­(dEAbpy)­(CO)3Br complex. This demonstrates the profound effect of the identity of the metal center, in addition to the ligand, for the product distribution. In this work, we report the synthesis procedures and X-ray diffraction studies along with electrochemical and infrared spectroelectrochemical studies of Re­(dEAbpy)­(CO)3Cl and Ru­(dEAbpy)­(CO)2Cl2 to propose a mechanism for the CO2 reduction reaction.

Development of new catalytic approaches for reduction of small molecules is an aspiring technology to alleviate increasing energy demands without use of fossil fuels. One of the most promising molecular catalysts for reduction of CO 2 is fac-Mn(bpy)(CO) 3 Br (bpy = 2,2'-bipyridine). In this work, the mechanism of electrochemical reduction of this complex is elucidated using cyclic voltammetry along with digital simulations. It is revealed that the dimer complex, Mn 2 (bpy) 2 (CO) 6 , is not, as often assumed, formed by dimerization of the singly reduced manganese complex, [Mn(bpy)(CO) 3 ] * , but instead from its reduction to [Mn(bpy)(CO) 3 ] À , followed by further reaction with Mn(bpy)(CO) 3 Br (rate constant = 1.75 × 10 5 M À 1 s À 1 ). On the basis of cyclic voltammetry, infrared-spectroelectrochemistry, and 1 H NMR spectroscopy, we establish that this parent-child reaction involving Mn(bpy)(CO) 3 Br as electrophile can be diverted by adding either acids or iodomethane as substituting electrophiles. This demonstrates that [Mn(bpy)(CO) 3 ] À , which is the central catalyst for CO 2 reduction, can be formed at less extreme potentials than previously believed by suppressing the parent-child reaction.

Urgent solutions are needed to efficiently convert the greenhouse gas CO2 into higher-value products. In this work, fac-Mn­(bpy)­(CO)3Br (bpy = 2,2′-bipyridine) is employed as electrocatalyst in reductive CO2 conversion. It is shown that product selectivity can be shifted from CO toward HCOOH using appropriate additives, i.e., Et3N along with iPrOH. A crucial aspect of the strategy is to outrun the dimer-generating parent-child reaction involving fac-Mn­(bpy)­(CO)3Br and [Mn­(bpy)­(CO)3]− and instead produce the Mn hydride intermediate. Preferentially, this is done at the first reduction wave to enable formation of HCOOH at an overpotential as low as 260 mV and with faradaic efficiency of 59 ± 1%. The latter may be increased to 71 ± 3% at an overpotential of 560 mV, using 2 M concentrations of both Et3N and iPrOH. The nature of the amine additive is crucial for product selectivity, as the faradaic efficiency for HCOOH formation decreases to 13 ± 4% if Et3N is replaced with Et2NH. The origin of this difference lies in the ability of Et3N/iPrOH to establish an equilibrium solution of isopropyl carbonate and CO2, while with Et2NH/iPrOH, formation of the diethylcarbamic acid is favored. According to density-functional theory calculations, CO2 in the former case can take part favorably in the catalytic cycle, while this is less opportune in the latter case because of the CO2-to-carbamic acid conversion. This work presents a straightforward procedure for electrochemical reduction of CO2 to HCOOH by combining an easily synthesized manganese catalyst with commercially available additives.

Selective reduction of CO 2 is an efficient solution for producing nonfossil-based chemical feedstocks and simultaneously alleviating the increasing atmospheric concentration of this greenhouse gas. With this aim, molecular electrocatalysts are being extensively studied, although selectivity remains an issue. In this work, a combined experimental−computational study explores how the molecular structure of Mn-based complexes determines the dominant product in the reduction of CO 2 to HCOOH, CO, and H 2 . In contrast to previous Mn(bpy-R)(CO) 3 Br catalysts containing alkyl amines in the vicinity of the Br ligand, here, we report that bpybased macrocycles locking these amines at the side opposite to the Br ligand change the product selectivity from HCOOH to H 2 . Ab initio molecular dynamics simulations of the active species showed that free rotation of the Mn(CO) 3 moiety allows for the approach of the protonated amine to the reactive center yielding a Mn-hydride intermediate, which is the key in the formation of H 2 and HCOOH. Additional studies with DFT methods showed that the macrocyclic moiety hinders the insertion of CO 2 to the metal hydride favoring the formation of H 2 over HCOOH. Further, our results suggest that the minor CO product observed experimentally is formed when CO 2 adds to Mn on the side opposite to the amine ligand before protonation. These results show how product selectivity can be modulated by ligand design in Mn-based catalysts, providing atomistic details that can be leveraged in the development of a fully selective system.

A general and simple solvent-free procedure using direct heating of a ball-milled mixture of L-histidine−Fe 2 O 3 −FeCl 3 is developed for the synthesis of iron-and nitrogen-doped porous carbon electrocatalysts. Through adjustment of the reactant ratios and the pyrolysis temperature, a series of electrocatalysts are easily obtained with varying activities for electrochemical CO 2 reduction reaction (CO 2 RR). The electrocatalyst synthesized from L-histidine−Fe 2 O 3 − FeCl 3 at a 4:1:0.25 component ratio at 1000 °C exhibits the highest Faradaic efficiency of 83% for CO 2 -to-CO conversion at a small overpotential (360 mV) in aqueous media. The use of a number of characterization techniques, including X-ray photoelectron spectroscopy, X-ray diffraction, electron microscopy, and nitrogen sorption experiments, reveals that both Fe 2 O 3 and FeCl 3 contribute to the iron doping and formation of porosity. As a result, they are both crucial to produce the optimal CO 2 RR electrocatalyst. Correlation of the CO 2 RR activity with the carbon structure suggests that the degree of graphitization of the carbon electrocatalysts plays an important role in their CO 2 RR performance.