Nitrogen Co-doped Carbon Catalysts Research Articles

Iron and nitrogen co-doped carbon catalysts derived from ZIF-8 towards enhanced peroxymonosulfate activation and contaminants removal: The synergistic effects of FeNx and α/γ-Fe species

The performance upgrading of iron and nitrogen co-doped carbon (Fe-N-C) for peroxymonosulfate (PMS) activation towards contaminants removal and elucidation of the activation mechanism still remains a challenge. In this study, novel iron and nitrogen co-doped carbon (Fe-N-C) catalysts are fabricated through pyrolysis of Fe-modified zeolitic imidazolate framework-8 (ZIF-8) nanocrystals. Catalytic experiments prove that synergistic effects of FeNx and α/γ-Fe species can boost peroxymonosulfate activation and pollutants degradation. Density functional theory simulations disclose that α/γ-Fe species can optimize the d-band center of FeNx and adsorption energy for PMS activation, and thus achieve the enhanced catalytic activities for degradation process. Mechanism studies testify that Fe-N-C-PMS* complex and 1O2 are main contributors for the catalytic process. This work paves a new way for elaborate design of high-performance Fe-N-C materials and understanding about the non-radical mechanism during environmental applications.

Directional Construction of Low-Coordination Fe-N3 Coupled with Intrinsic Carbon Defects for High-Efficiency Oxygen Reduction.

Regulating the coordination environment of Fe-Nx sites is an efficient but challenging approach for promoting the intrinsic catalytic activity of single-atom Fe/N-codoped carbon (Fe-N-C) toward the oxygen reduction reaction (ORR). Herein, low-coordination Fe-N3 sites coupled with carbon vacancies (Fe-N3/CV) are directionally constructed in Fe-N-C via pyrolysis of a metal-organic framework (MOF) precursor with N3-Zn-O-Fe moieties, which are delicately prefabricated by chemically anchoring Fe3+ onto a H2O-etching induced linker-missing Zn-N3 site in the MOF precursor. The optimized Fe-N-C with the Fe-N3/CV sites displays a high ORR half-wave potential of 0.92 V (vs RHE), which is attributed to the optimized electronic structure and binding strengths of the active Fe center toward the ORR intermediates stemming from the synergy of the asymmetric configuration of Fe-N3 as well as the adjacent carbon vacancies. This work could be enlightening for the design and construction of high-activity coupling sites in metal and nitrogen-codoped carbon catalysts.

Reversible metal cluster formation on Nitrogen-doped carbon controlling electrocatalyst particle size with subnanometer accuracy

Copper and nitrogen co-doped carbon catalysts exhibit a remarkable behavior during the electrocatalytic CO2 reduction (CO2RR), namely, the formation of metal nanoparticles from Cu single atoms, and their subsequent reversible redispersion. Here we show that the switchable nature of these species holds the key for the on-demand control over the distribution of CO2RR products, a lack of which has thus far hindered the wide-spread practical adoption of CO2RR. By intermitting pulses of a working cathodic potential with pulses of anodic potential, we were able to achieve a controlled fragmentation of the Cu particles and partial regeneration of single atom sites. By tuning the pulse durations, and by tracking the catalyst’s evolution using operando quick X-ray absorption spectroscopy, the speciation of the catalyst can be steered toward single atom sites, ultrasmall metal clusters or large metal nanoparticles, each exhibiting unique CO2RR functionalities.

Cobalt and nitrogen co-modified hollow periodic mesoporous organosilica spheres for selective oxidation of aryl alkanes

Hollow periodic mesoporous organosilica spheres (HPMOs) have stimulated great research interest owing to its unique organic–inorganic hybrid skeleton. In this work, HPMOs were synthesized by hydrothermal method, and then cobalt and nitrogen co-doped carbon catalyst was prepared by using HPMOs as supports. The optimal catalyst (Co-Nx/HPMOs-D) performed excellent activity in the selective oxidation reaction of aryl alkanes, with the conversion of ethylbenzene up to 97.0% and the selectivity of acetophenone was 98.9%, which was attributed to the Co-Nx active species and the unique HPMOs supports. The unique amphiphilic property of Co-Nx/HPMOs-D could better facilitate the adsorption and diffusion of organic substrates in the aqueous phase. The addition of dicyandiamide not only enhanced the nitrogen content in the catalyst, but also promoted the conversion of pyrrolic-N to pyridinic-N during the pyrolysis process, thus strengthened the interaction between the substrate and cobalt and improved the stability of the catalyst. This work presented a green and environmentally friendly method for the preparation of HPMOs and a facile strategy to obtain high performance catalysts for selective oxidation of aryl alkanes.

Advances in Atomically Dispersed Metal and Nitrogen Co-Doped Carbon Catalysts for Advanced Oxidation Technologies and Water Remediation: From Microenvironment Modulation to Non-Radical Mechanisms.

Atomically dispersed metal and nitrogen co-doped carbon catalysts (M-N-C) have been attracting tremendous attentions thanks to their unique MNx active sites and fantastic catalytic activities in advanced oxidation technologies (AOTs) for water remediation. However, precisely tailoring the microenvironment of active sites at atomic level is still an intricate challenge so far, and understanding of the non-radical mechanisms in persulfate activation exists many uncertainties. In this review, latest developments on the microenvironment modulation strategies of atomically dispersed M-N-C catalysts including regulation of central metal atoms, regulation of coordination numbers, regulation of coordination heteroatoms, and synergy between single-atom catalysts (SACs) with metal species are systematically highlighted and discussed. Afterwards, progress and underlying limitations about the typical non-radical pathways from production of singlet oxygen, electron transfer mechanism to generation of high-valent metal species are well demonstrated to inspire intrinsic insights about the mechanisms of M-N-C/persulfate systems. Lastly, perspectives for the remaining challenges and opportunities about the further development of carbon-based SACs in environment remediation are also pointed out. It is believed that this review will be much valuable for the further design of active sites in M-N-C/persulfate catalytic systems and promote the wide application of SACs in various fields.

Molten salt-assisted carbonization and unfolding of Fe, Co-codoped ZIF-8 to engineer ultrathin graphite flakes for bifunctional oxygen electrocatalysis

Zeolitic imidazolate frameworks (ZIFs) are already utilized to synthesis precursors for transition metal and nitrogen co-doped carbon catalysts. However, there are few reports on deliberately tailoring catalyst structure except for the hierarchical pores formed from Zn evaporation during the pyrolysis. Herein, NaCl salt-assisted carbonization and unfolding of Fe, Co-codoped ZIF-8 polyhedron (FeCo/ZIF-8) result in ultrathin Fe, Co, N-codoped graphite flake (FeCo/NG), while FeCo/ZIF-8 pyrolysis without NaCl yields Fe, Co, N-codoped carbon spheres (FeCo/NC). Because of the high specific surface area, porosity, and degree of defects, the active site is further exposed. The synthesized FeCo/NG demonstrates excellent oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) properties with half-wave potential (E 1/2 ) at 0.88 V and overpotential at 460 mV and good cycling stability, superior to those of FeCo/NC. In addition, the assembled Zn-air batteries with FeCo/NG catalysts deliver high peak power density at 109 mW cm −2 , specific capacity at 783 mAh g −1 and an operating time for 100 h. The study will provide a feasible synthesis method to design high-performance electrocatalysts by deliberate tailoring structures with molten salt-assisted carbonization and unfolding of ZIFs. • Molten salt-assisted pyrolysis tailors ZIF-8 polyhedron into ultrathin graphite flake. • Molten NaCl assists carbonization and unfolding of FeCo/ZIF-8 to form ultrathin FeCo/NG. • Evaporation of Zn in pyrolysis and removal of NaCl result in porous FeCo/NG. • FeCo/NG demonstrates excellent bifunctional oxygen electrocatalytic performances. • Zn-air battery with FeCo/NG delivers high peak power density and specific capacity.

Cu-Nx active sites derived from copper phthalocyanine in porous carbon promoting oxygen reduction reaction

Transition metal-nitrogen-carbon materials have been reported to be a promising candidate for cathodic oxygen reduction reaction in fuel cell and metal air battery. In this paper, a copper and nitrogen co-doped carbon catalyst (Cu/PC) was designed and successfully synthesized through hydrothermal method and two high temperature calcinations process. The new designed Cu/PC catalyst exhibited a superior catalytic activity with the half-wave potential and limited diffusion current density of 0.847 V and 5.52 mA cm−2, respectively. Mechanisms insight from electrochemical experiments and density functional calculations revealed that the Cu-Nx active sites on the surface of porous carbon and large pore volume played key roles on the excellent electrochemical performance. The results reported in this work provide a new perspective for the design of transition metal catalysts and lay a solid foundation for the related mechanism studies.

Efficient electrochemical CO2 reduction to CO by metal and nitrogen co-doped carbon catalysts derived from pharmaceutical wastes adsorbed on commercial carbon nanotubes

Transition metal and nitrogen doped carbon catalysts (MNC) are effective in electrochemical reduction of CO2 to CO with a high selectivity. However, scalable and cost-effective synthesis of active metal-nitrogen catalysts is yet to be developed. Herein, we report a simple and sustainable method that utilizes commercial carbon nanotubes (CNTs) to adsorb a pharmaceutical waste, sulfamethoxazole (SMX), followed by moderate pyrolysis to prepare an efficient MNC catalyst. The intrinsic metal impurities from CNTs are essential to form active metal sites, and it requires significantly less nitrogen precursor than methods using most widely nitrogen precursors such as melamine and urea. The CNT-SMX catalyst delivers high CO2RR performance with 91.5 % CO Faradaic efficiency and 14 mA/cm2 CO partial current density at −0.76 V vs RHE in a traditional H-Cell. The catalyst is also efficient in a scalable flow cell, exhibiting 97.5 % CO selectivity at 300 mA/cm2, plus stable CO2RR performance for more than 24 h at 100 mA/cm2. The scanning transmission electron microscopy (STEM) and X-ray absorption spectroscopy (XAS) analyses confirm the existence of single atomic sites primarily in the form of Fe-N bonds that are active sites for CO2RR. Density functional theory (DFT) calculations suggest a synergy between the single atomic FeNC sites and Ni nanoparticles embedded in the CNTs, which enhances CO production rate and selectivity by lowering the desorption energy of *CO intermediate. To the best of our knowledge, the results in this work are among the top performing carbon-based catalysts. Furthermore, catalysts developed in this work are synthesized at a moderate temperature without pre-oxidation or post-acid-washing and utilize cheap or waste materials, presenting a simple, sustainable, and cost-effective way to synthesize highly active catalysts.

Performance enhancement and degradation mechanism identification of a single-atom Co–N–C catalyst for proton exchange membrane fuel cells

The development of catalysts free of platinum-group metals and with both a high activity and durability for the oxygen reduction reaction in proton exchange membrane fuel cells is a grand challenge. Here we report an atomically dispersed Co and N co-doped carbon (Co–N–C) catalyst with a high catalytic oxygen reduction reaction activity comparable to that of a similarly synthesized Fe–N–C catalyst but with a four-time enhanced durability. The Co–N–C catalyst achieved a current density of 0.022 A cm−2 at 0.9 ViR-free (internal resistance-compensated voltage) and peak power density of 0.64 W cm−2 in 1.0 bar H2/O2 fuel cells, higher than that of non-iron platinum-group-metal-free catalysts reported in the literature. Importantly, we identified two main degradation mechanisms for metal (M)–N–C catalysts: catalyst oxidation by radicals and active-site demetallation. The enhanced durability of Co–N–C relative to Fe–N–C is attributed to the lower activity of Co ions for Fenton reactions that produce radicals from the main oxygen reduction reaction by-product, H2O2, and the significantly enhanced resistance to demetallation of Co–N–C. Platinum-group-metal-free, non-iron catalysts are highly desirable for the oxygen reduction reaction at proton exchange membrane (PEM) fuel cell cathodes, as they avoid the detrimental Fenton reactions. Now, a cobalt and nitrogen co-doped carbon catalyst with atomically dispersed porphyrin-like CoN4C12 sites is reported with an improved activity and durability in PEM fuel cell conditions.

Recycling the Catalyst of Atom Transfer Radical Polymerization to Prepare a Cu, N Codoped Mesoporous Carbon Electrocatalyst for Oxygen Reduction

Non-noble metal and nitrogen codoped carbon catalysts are considered as the alternative options for Pt-based materials for the cathodic oxygen reduction reaction (ORR). Atom transfer radical polyme...

Kinetic Analysis of Electrochemical Oxygen Reduction over a Fe/N/C Catalyst Considering the Chemical Decomposition of H2O2

Iron and nitrogen codoped carbon catalysts (Fe/N/C catalysts) are promising nonprecious-metal catalysts for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells. Understanding the ORR kinetics over Fe/N/C catalysts is crucial for active catalyst development. Here, we propose an analytical method for rotating ring–disk electrode (RRDE) voltammetry to separate the ORR for O2 reduction to H2O via the 4-e pathway (k1), O2 reduction to H2O2 via the 2-e pathway (k2), H2O2 reduction to H2O via the 2-e pathway (k3), H2O2 chemical decomposition to O2 and H2O (k4), and H2O2 oxidation to O2 by the 2-e pathway (k–2). First, RRDE voltammetry in an H2O2 solution was performed, yielding k3, k4, and k–2. The obtained parameters were used to analyze the ORR voltammogram to calculate I1′, I2′, I3′, I4′, k1′, and k2′. These currents and kinetic constants were corrected by studying the effect of the catalyst loading density to obtain I10, I20, I30, I40, k10, and k20, thus avoiding the overestimation of I10 and k10 caused by the quasi-four-electron reduction of O2. The contribution of k4 during the ORR is negligible at considerable anodic and cathodic overpotentials but was detected at 0.8 V, for which k3 and k–2 are relatively small.

Highly Efficient Nickel, Iron, and Nitrogen Codoped Carbon Catalysts Derived from Industrial Waste Petroleum Coke for Electrochemical CO2 Reduction

Electrochemical CO2 reduction reaction (ECO2RR) is a potentially promising way of producing sustainable energy by converting CO2 into fuels or useful chemicals using alternative power sources such ...

An efficient Co-N/C electrocatalyst for oxygen reduction facilely prepared by tuning cobalt species content

Transition metal and nitrogen co-doped carbon catalysts for the oxygen reduction reaction (ORR) have emerged as promising candidates to replace the expensive platinum catalysts but still remain a great challenge. Herein, a novel and efficient nitrogen-doped carbon material with metal cobalt co-dopant (Co–N/C) has been prepared by pyrolyzing porphyrin-based covalent organic polymer where Co is anchored. The optimized 10%-Co-N/C catalyst through facilely and efficiently tuning the cobalt content is carefully characterized by XRD, Raman, XPS, SEM and TEM for composition and microstructure analysis. This catalyst with only 0.56% Co exhibits an excellent ORR catalytic activity with a positive half-wave potential of 0.816 V (vs. RHE) in 0.1 M KOH solution, which is comparable to that of commercial Pt/C (20 wt%). Notably, the 10%-Co-N/C catalyst displays better electrochemical stability with only a loss of 5.1% of its initial current density in chronoamperometric measurement and also gives rise to stronger methanol tolerance than Pt/C. The good ORR catalytic behaviour for this catalyst may be attributed to the dispersion of the Co-NX active sites via adjusting the contents of cobalt species in porous organic framework.

Solvent-Free Melting-Assisted Pyrolysis Strategy Applied on the Co/N Codoped Porous Carbon Catalyst

From the current demand for aromatic ketones, economical and environmentally friendly renewable catalysts are highly economical for the efficient selection of aralkyl oxides to form the corresponding aromatic ketone compounds. Yet, since the preparation of the Co–N–C catalyst is complicated at present, it is difficult to be widely applied to the oxidation of arylalkanes. In this study, a one-step solvent-free melting-assisted heat-treatment strategy is adopted to synthesize cobalt and nitrogen codoped carbon catalysts from sucrose, 5-(4-carboxyphenyl)-10,15,20-triphenylcobalt porphyrin (CoCPTPP), and KIT-6. Thanks to the interaction between the silicon hydroxyl of KIT-6 and the carboxyl hydroxyl of CoCPTPP as well as the distinctive properties of CoCPTPP with Co–N4 structures, the Co species in the as-synthesized catalysts are uniformly dispersed on carbon materials. Hereafter, the catalyst showed a high efficiency for the oxidation of arylalkane, e.g., the conversion rate of ethylbenzene is 96% and the selectivity of acetophenone is 99%. According to the characterizations including transmission electron microscopy etc, the results show that a porous special structure and thriving Co–Nx active sites in the catalysts is the main reason for the catalyst’s good catalytic activity and stability. In addition, the catalyst can also be used for the oxidation reaction of various aromatic hydrocarbon substrates with high catalytic activity.

Iron and nitrogen codoped carbon catalyst with excellent stability and methanol tolerance for oxygen reduction reaction

A series of non-precious metal FexNC electrocatalysts for oxygen reduction reaction (ORR) were successfully synthesized using Fe(NO3)3, glucose, and melamine as the Fe, C, and N sources, respectively. The effects of the pyrolysis temperature and Fe/N contents on the catalytic performances are comprehensively investigated. Electrochemical results reveal that among the FexNC catalysts, Fe1.5NC-900-2 pyrolyzed at 900°C with the mass ratio of FeC to melamine being 1:10 proves the highest catalytic performance. The half-wave potential (E1/2) of ORR was 821 mV (vs reversible hydrogen electrode (RHE)) and only 36 mV lower than that on commercial Pt/C catalyst (857 mV). More importantly, Fe1.5NC-900-2 catalyst shows excellent stability and methanol tolerance. After 1000 sequential cycles, the E1/2 on Pt/C catalyst shifts negatively by approximately 60 mV, while for Fe1.5NC-900-2 catalyst, this shift is only 28 mV although the number of sequential cycles is increased to 8000. In the presence of methanol, the current decay in the chronoamperometric response at 1000 seconds is only 8% and also much lower than that on Pt/C catalyst (46%). The high catalytic performances arise from the abundant Fe3N active sites embedded in the carbon matrix of the FexNC catalysts. These findings can be used to discuss the catalytic mechanism of ORR on the FexNC catalysts and design the nonprecious metal carbon-based electrocatalysts for ORR.

A Bi-functional Cobalt and Nitrogen Co-doped Carbon Catalyst for Aerobic Oxidative Esterification of Benzyl Alcohol with Methanol and Oxygen Reduction Reaction

A bi-functional cobalt and nitrogen co-doped carbon catalyst was fabricated by calcinating the chelate compound containing citric acid, melamine and CoCl2·6H2O in N2, with the mixed salts of LiCl and KCl as the template. Co and N species are highly dispersed in the target catalyst, which has a high surface area of 1011 m2 g−1. Such structure enables it to efficiently catalyze the direct aerobic oxidative esterification of benzyl alcohol with methanol to the ester of methyl benzoate; meanwhile, it exhibits superior potential ability for the application of oxygen reduction reaction, which is a vital process happening on the cathode of fuel cells and metal-air batteries.

Understanding Oxygen Activation on Metal- and Nitrogen-Codoped Carbon Catalysts

Multidoped carbons are often used for oxygen activation catalysis, both in heterogeneous catalysis and electrocatalysis. Identifying their catalytic sites is crucial for developing better catalysts. We now report an in-depth study into O2 activation on an important class of materials: carbons codoped by nitrogen and 10 different metals (V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, and Pb). To identify catalytic sites, we studied their composition and structure (both bulk and surface) by a wide range of techniques, including temperature-programmed reduction, X-ray diffraction, electron microscopy, X-ray photoelectron spectroscopy, and N2 sorption porosimetry. The O2 activation step was studied by electrochemical oxygen reduction. To assign active sites, the electrocatalytic activity, selectivity, and stability were correlated to material composition and to known reactivity pathways. Two types of sites for O2 activation were identified and assigned for each multidoped material: (1) particles of partially reduced meta...

Slat templated formation of efficient oxygen reduction electrocatalyst with a fluidic precursor

Development of cost-effective and efficient oxygen reduction catalyst is critical for the commercialization of proton exchange membrane fuel cell. Metal and nitrogen co-doped carbon is recognized as a promising alternative to traditional platinum-based oxygen reduction catalyst. Herein, we report a novel metal and nitrogen co-doped carbon catalyst with an ionic liquid precursor. Salt template, which can be easily removed with mild treatment after the synthesis, is used to generate abundant mesopores in the resulting catalyst. We show that the novel catalyst shows a superior activity comparable to commercial Pt/C catalyst. Furthermore, the important role of the mesopore for the activity of the catalyst is demonstrated.

Identification of champion transition metals centers in metal and nitrogen-codoped carbon catalysts for CO2 reduction

Transition metal is known to influence electrochemical activities over transition metals (M) and nitrogen (N)-codoped carbon (MNC) catalysts. However, champion transition metals centers in MNC for catalyzing CO2 reduction reaction (CO2RR) remain unclear, hindering further catalyst development with enhanced performance. Herein, we report the investigation of effects of five transition metals (Cr, Mn, Fe, Co, Ni) on CO2RR activities and mechanisms using metal-doped nitrogenated carbon nanosheets as model catalysts fabricated via a novel space-confinement-assisted molecular-level complexing approach. Analyzing N 1s XPS spectra confirmed the formation of M−N complexes via the coordination of metals atoms with pyridinic N, which was identified as the active species in CO2RR. According to activity descriptors including overpotentials, Faradaic efficiency (FE) and Turnover Frequency (TOF) per metal site, we here established that Fe and Ni are more active than Co, Mn, and Cr in MNC for the reduction of CO2 to CO. The main role of Fe is to reduce overpotentials, exhibiting the lowest onset overpotential of 0.19 V to yield CO on FeNC. Ni can drastically improve CO selectivity and reaction rates, yielding the highest CO Faradaic efficiency of 96%, partial current density of −8.2 mA cm−2, and TOF of 1060 h−1 at a moderate overpotential of 0.65 V. Mechanism explorations reveal that CO2RR on MNC (M = Fe, Cr, Mn) undergoes the formation of a *COOH intermediate as the rate-determining step, whereas MNC (M = Ni, Co) catalyzes CO2RR via the transfer of the first electron to form a *CO2− species. On the basis of the findings, we suggest doping Fe and/or Ni to design advanced MNC for CO2 electroreduction.

Role of Local Carbon Structure Surrounding FeN4Sites in Boosting the Catalytic Activity for Oxygen Reduction

Development of effective nonprecious metal and nitrogen codoped carbon catalysts for the oxygen reduction reaction (ORR) requires a fundamental understanding of the mechanisms underlying their catalytic activity. In this study, we employed the first-principles density functional theory calculations to predict some key parameters (such as activation energy for O–O bond breaking and free-energy evolution as a function of electrode potential) of ORR on three FeN4-type active sites with different local carbon structures. We find that the FeN4 site surrounded by eight carbon atoms and at the edge of micropores has the lowest activation energy (about 0.20 eV) for O–O bond breaking among the three FeN4-type active sites for promoting a direct four-electron ORR. Consequently, our computational results suggest that introduction of micropores in the nonprecious metal catalysts could enhance their catalytic activity for ORR through facilitating the formation of FeN4–C8 active sites with high specific activity.