Heteroatom-doped Carbon Catalysts Research Articles

One-step microwave-assisted synthesis of metal-free heteroatom-doped carbon catalyst for H2O2 electrosynthesis

This study reports on a one-step conversion of polyaniline into a metal-free heteroatom-doped carbon electrocatalyst through microwave heating. A high surface area carbonaceous structure forms just after a total synthesis time of 140 s, with the presence of nitrogen and oxygen functional groups, as confirmed by thorough spectroscopic analysis. This catalyst exhibits high activity (onset potential of 0.73 V vs. RHE), selectivity (82%), and stability (over a 7.5-hour test period) for the electrochemical oxygen reduction reaction towards hydrogen peroxide in alkaline media. Microwave synthesis reduces heating time by 29-fold and energy consumption by 77-fold, while producing materials with high electrocatalytic efficiency comparable to those conventionally prepared at 700°C. The microwave and the conventionally synthesized heteroatom-doped carbon catalysts show similar electrochemical performances, which can be attributed to the presence of nearly identical nitrogen functional groups and surface area in the two samples. In contrast, the microwave and conventionally synthesized samples exhibit significant variations in their oxygen functional groups. These results suggest that nitrogen functional groups are the main active sites for alkaline hydrogen peroxide formation, while oxygen functional groups play a minor role in the catalytic activity. Our work brings a solid contribution to the debate regarding the active centers for hydrogen peroxide formation.

In-situ construction of ionic liquid salt-derived graphene-like dual-heteroatoms doping engineering as efficient metal-free electrocatalyst for oxygen reduction reaction

Metal-free heteroatom-doped carbon catalysts can effectively eliminate the degradation and pollution problems caused by metal dissolution. However, single heteroatom doping often restricts the regulation of geometric and electronic structures for oxygen reduction reaction (ORR) electrocatalysts. Herein, a dual atomic doping engineering to optimize the electronic structure and local coordination environment of ionic liquid salt-derived graphene-like host material as an efficient metal-free electrocatalyst for ORR. The resultant PNG catalyst with a typical nanosheets morphology with a high specific surface area and unique hierarchical porous structure. Benefiting from the uniform doping of P, N heteroatoms, graphene-like structure, optimized electronic structure and coordination environment, thus-prepared PNG-2 exhibits outstanding ORR activity, which the onset potential and half-potential are negatively shifted by around 70 mV and 46 mV, respectively, as compared to the commercial Pt/C (20 wt.%) for ORR. Furthermore, the PNG-2 exhibits the higher diffusion limiting current density, superior long-term stability as well as excellent resistance to methanol oxidation than Pt/C. Experimental studies and density functional theory (DFT) calculations reveal that the enhanced chemisorption of oxygen species (*OOH, *O and *OH) onto the P and N sites of the PNG catalysts can effectively accelerate the charge transfer kinetics and promote a favorable ORR performance.

In situ N-doping engineered biochar catalysts for oxidation degradation of sulfadiazine via nonradical pathways: Singlet oxygen and electron transfer

Understanding the structure of non-metallic heteroatom-doped carbon catalysts and the subsequent degradation of new pollutants is crucial for designing more efficient carbon catalysts. Environmentally friendly in situ N-doped biochar catalysts were prepared for peroxymonosulfate (PMS) activation and sulfadiazine (SDZ) degradation. The acid washing process and calcination temperature of catalyst increased π-π* shake up, graphitic N percentage, specific surface area and defects, promoting the transformation of pollutant degradation mechanism from radical pathway to non-radical pathway. 100 % of the SDZ with the initial concentration of 10 mg/L was quickly degraded within 60 min using 0.2 g/L catalysts and 0.5 mM PMS. Excellent catalytic performance was attributed to singlet oxygen and electron transfer-dominated non-radical pathways. The four potential degradation pathways of SDZ were proposed, and toxicity predication indicated that overall biotoxicity of the intermediates during SDZ degradation was decreased. This research deepens our understanding of the mechanisms of non-radical pathways and guides the synthesis of carbon-based catalysts.

Cutting-edge nitrogen, boron, and fluorine triply doped chain-like porous carbon nanofibers: a versatile solution for high-performance zinc–air batteries and self-powered water splitting

Heteroatom-doped carbon catalysts are a class of materials that have been widely studied for their potential to improve the performance and kinetics of electrochemical reactions such as the oxygen reduction/evolution reaction (ORR/OER).

In Situ S-Doping Engineering for Highly Efficient NH3-SCR over Metal-Free Carbon Catalysts: A Novel Synergetic Promotional Mechanism.

Cyclic desulfurization-regeneration-denitrification based on metal-free carbon materials is one of the most promising ways to remove NOx and SO2 simultaneously. However, the impact of S-doping induced by the cyclic desulfurization and regeneration (C-S-R) process on the selective catalytic reduction (SCR) is not well understood. Herein, it is demonstrated that the C-S-R process at 500 °C induces in situ S-doping with a significant accumulation of C-S-C structures. NOx conversion was dramatically enhanced from 18.95% of the original sample to 84.55% of the S-doped sample. Density functional theory calculations revealed that the C-S-C structure significantly regulates the electronic structure of the C atom adjacent to the ketonic carbonyl group, thereby significantly altering the NH3 adsorption configuration with superior adsorption capacity. Moreover, S-doping induces an extra electron transfer between the N atom of the NH3 molecule and the C atom of the carbon plane, thereby promoting the activation of NH3 over the ketonic carbonyl group with a reduced energy barrier. This study elucidates a synergetic promotional mechanism between the ketonic carbonyl group and the C-S-C structure for SCR, offering a novel design strategy for high-performance heteroatom-doped carbon catalysts in industrial applications.

Remediation of atrazine in environment by persulfate activation via N/B co-doped Si-rich biochar: Performance, mechanisms, degradation pathways and phytotoxicity

Biochar activated with peroxydisulfate (PDS) has been widely applied in the removal of recalcitrant organic pollutants in the environment, but its wide application is limited by expensive and complex preparation process. In this study, a nitrogen and boron-doped silicon-rich biochar (NBC) was prepared by one-pot synthesis using fungus chaff of Auricularia auricula, diatomite, urea and boric acid as raw materials. The results showed that the specific surface area of NBC was 9 times that of the original, the adsorption capacity of ATZ was increased by 8 times, PDS could be effectively activated, and the removal efficiency of ATZ was as high as 98%. Density functional theory analysis proved that NBC had a lower energy barrier and a higher probability of activating PDS, thereby degrading ATZ through a non-radical pathway dominated by electron transfer. Hydroponic and pot experiments showed that NBC/PDS system effectively restored the growth status, leaf and root morphology, and chlorophyll content of plants, demonstrating the ability to efficiently remediate the toxicity of ATZ on soybean seedlings. This study not only proposed a novel biochar material as an efficient PDS activator but also provided new insights into the practical application of heteroatom-doped carbon catalysts for activating advanced oxidation processes in pollutant degradation.

Renewable biomass-derived, P-doped granular activated carbon for efficient oxidation of 5-hydroxymethylfurfural to 2,5-diformylfuran: Insights into the crucial role of P and N functionality

Heteroatom-doped carbon materials for eco-friendly metal-free catalysis have gained increasing attention, but they nevertheless face technological challenges relating to the difficult recycling and high cost. In this work, P-doped granular activated carbons with intrinsic N functionality (PN-GAC-T) were successfully fabricated from low-cost and sustainable waste peanut shells using a simple self-bonding strategy assisted by phosphoric acid. The as-prepared PN-GAC-T were found to have abundant N, P functional groups and large specific surface areas, making them potentially efficient metal-free catalysts for the selective oxidation of 5-hydroxymethylfurfural (HMF). Catalytic results showed that an excellent HMF conversion (98.4%) and 2,5-diformylfuran (DFF) selectivity (99.7%) could be achieved under mild conditions via employing PN-GAC-800 as a catalyst. The good catalytic performance of PN-GAC-800 is highly reliant on the synergistic effect between doped P and intrinsic N functionalities as verified by the control experiments. Furthermore, the density functional theory (DFT) calculations demonstrated that the introduction of both P and N functional groups would aid in the activation of HMF and the improvement of oxygen adsorption capability. This work has emphasized the possibility of high-value exploitation of biomass waste, and it is predicted to provide new inspiration for the development of readily recyclable heteroatom-doped carbon catalysts.

Effect of pre-annealing on chemical configuration and heteroatom doping of highly active carbon catalysts for the oxygen reduction reaction

Heteroatom-doped carbon catalysts are promising alternatives to platinum group metal-free catalysts for oxygen reduction reaction (ORR). To enhance the ORR activity of carbon-based catalysts, the break of electroneutrality and number of active sites must be maximized. In this study, an annealing process at a mild temperature was introduced before heteroatom doping to control the content and configuration of the oxygen functional groups on the graphene oxide (GO). Pre-annealing affects the chemical configuration and the porosity of GO such that the epoxy groups are converted into carbonyl groups accompanying the defect formation. Interestingly, The carbonyl-enriched environment allowed ammonia borane, as a dopant with nitrogen and boron atoms, the enhanced doping performance, inducing the formation of higher number of heteroatom doping sites and additional defects in the graphene lattice. The aBNG-150 catalyst demonstrated outstanding ORR activity in both half- and single-cell evaluations because it had the highest heteroatom content and meso/macropores derived from the elimination of oxygen functional groups, which can facilitate ORR catalytic activity and O2 diffusion pathway. Therefore, we believe that this approach provides an effective route for synthesizing carbon-based catalysts with remarkable ORR performances.

Covalent organic framework-derived fluorine, nitrogen dual-doped carbon as metal-free bifunctional oxygen electrocatalysts

The construction of heteroatom-doped metal-free carbon catalysts with bifunctional catalytic activity for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is highly desired for Zn-air batteries, but remains a great challenge owing to the sluggish kinetics of OER and ORR. Herein, a self-sacrificing template engineering strategy was employed to fabricate fluorine (F), nitrogen (N) co-doped porous carbon (F-NPC) catalyst by direct pyrolysis of F, N containing covalent organic framework (F-COF). The predesigned F and N elements were integrated into the skeletons of COF precursor, thus achieving uniformly distributed heteroatom active sites. The introduction of F is beneficial for the formation of edge-defects, contributing to the enhancement of the electrocatalytic activity. Attributing to the porous feature, abundant defect sites induced by F doping, as well as the strong synergistic effect between N and F atoms to afford a high intrinsic catalytic activity, the resulting F-NPC catalyst exhibits excellent bifunctional catalytic activities for both ORR and OER in alkaline mediums. Furthermore, the assembled Zn-air battery with F-NPC catalyst shows a high peak power density of 206.3 mW cm−2 and great stability, surpassing the commercial Pt/C + RuO2 catalysts.

Conductivity-enhanced porous N/P co-doped metal-free carbon significantly enhances oxygen reduction kinetics for aqueous/flexible zinc-air batteries

Heteroatom-doped metal-free carbon catalysts for oxygen reduction reactions have gained significant attention because of their unusual activity and economic cost. Here, a novel N/P co-doped porous carbon catalyst (NPPC) with a high surface area for oxygen reduction reaction (ORR) is constructed by a facile high-temperature calcination method employing ZIF-8 as the precursor and red phosphorus as the phosphorus source. In particular, ZIF-8 is firstly calcined to obtain N-doped carbon (NC) followed by further calcination with red phosphorus to obtain NPPC. Ultraviolet photoelectron spectroscopy (UPS) analysis shows that the ultra-low amount of P doping could significantly decrease the work function from 4.32 to 3.86 eV. The resultant catalyst exhibits a promising electrocatalytic activity with a half-wave potential (E1/2) of 0.87 V and a limiting current density (JL) of 5.15 mA cm−2. Besides, it also shows improved catalytic efficiency and excellent durability with a negligible decay of JL after 2000 CV cycles. Moreover, aqueous and solid-state flexible zinc-air batteries (ZAB) using the catalyst show a promising application potential. This work provides new insight into developing P/N-doped metal-free carbon ORR catalysts.

Selective Aerobic Oxidation of Csp3-H Bonds Catalyzed by Yeast-Derived Nitrogen, Phosphorus, and Oxygen Codoped Carbon Materials.

Nitrogen, phosphorus, and oxygen codoped carbon catalysts were successfully synthesized using dried yeast powder as a pyrolysis precursor. The yeast-derived heteroatom-doped carbon (yeast@C) catalysts exhibited outstanding performance in the oxidation of Csp3-H bonds to ketones and esters, giving excellent product yields (of up to 98% yield) without organic solvents at low O2 pressure (0.1 MPa). The catalytic oxidation protocol exhibited a broad range of substrates (38 examples) with good functional group tolerance, excellent regioselectivity, and synthetic utility. The yeast-derived heteroatom-doped carbon catalysts showed good reusability and stability after recycling six times without any significant loss of activity. Experimental results and DFT calculations proved the important role of N-oxide (N+-O-) on the surface of yeast@C and a reasonable carbon radical mechanism.

Very efficient dehydrogenation of methanolysis reaction with nitrogen doped Chlorella Vulgaris microalgae carbon as metal-free catalysts

In this study, nitrogen (N) doped metal-free catalysts were obtained as a result of nitric acid (HNO3) activation of carbon sample (C–KOH–N), which was obtained based on Chlorella Vulgaris microalgae by KOH activation (C–KOH). These catalysts have been effectively used to produce hydrogen (H2) from the sodium borohydride (NaBH4) methanolysis reaction. Compared to the C–KOH catalyst, the catalytic activity for C–KOH–N showed a seven-fold improvement. Hydrogen generation rate (HGR) values obtained for the NaBH4 methanolysis reaction for C–KOH and C–KOH–N metal-free catalysts were 3250 and 20,100 mL min−1 g−1. The catalysts were characterized using various analytical techniques such as XPS, XRD, SEM, FTIR, BET, and elemental analysis. This work can provide a new alternative strategy to produce specific heteroatom-doped metal-free carbon catalysts for environmentally friendly conversion to produce H2 efficiently.

Pre-leaching strategy for tuning porosity and composition to generate Co2P/Co@P/N-doped carbon towards highly efficient bifunctional oxygen electrocatalysis

Constructing porous nonprecious metal heteroatom-doped carbon catalysts with efficient and robust electrocatalytic properties for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is identified as a pivotal part for propelling practical application of diverse energy-related devices. This study reports a facile and eco-friendly method to synthesize micro-/mesoporous Co2P/Co-embedded P/N co-doped carbon (Co/P/N–CNP) material through H3PO4-selective-etching ZIF-67 precursor and subsequent pyrolysis. H3PO4 not only acts as an etching agent to engineer ZIF’s porosity but also is an ideal phosphorus source for P-doping in carbon matrix and the formation of Co2P, which greatly enriches the electrocatalytic active sites for obtained carbon catalysts. Pyrolyzed hybrid carbon nanomaterial (Co/P/N–CNP) is decorated by multiwalled carbon nanotubes and possesses micro- and mesopores simultaneously, which facilitate the exposure for more active sites and the diffusion of reactive species in the ORR/OER electrocatalytic process. Thanks to the promising physicochemical characteristics, finely-tuned Co/P/N–CNP catalyst manifests excellent bifunctional electrocatalytic activity compared with commercial Pt/C (for ORR) and RuO2 (for OER) in alkaline media. This simple strategy paves a new avenue for its further application in catalyst fabrication for energy storage and conversion.

Heteroatom-doped carbon catalysts for zinc–air batteries: progress, mechanism, and opportunities

This review presents the recent progress in advanced bifunctional heteroatom-doped carbon catalysts for rechargeable aqueous and all-solid-state Zn–air batteries, along with current challenges and future perspectives in the field.

Hollow Spherical (Co, Zn)/N, S-Doped Carbons: Efficient Catalysts for Oxygen Reduction in Both Alkaline and Acidic Media

Platinum (Pt)-based catalysts are widely used for the oxygen reduction reaction (ORR) in many energy conversion and storage devices, such as fuel cells and metal–air batteries. However, Pt is among the least earth abundant and most expensive elements; as a result, these energy devices are currently difficult to scale up. This has motivated researchers to try to develop sustainable and alternative catalysts that can replace Pt as well as other noble metal-based catalysts. Herein, novel Co nanocrystals-loaded, Zn, N, and S codoped carbon materials, dubbed (Co, Zn)/NSC, which have hollow spherical structures and which can serve as efficient ORR catalysts, are developed via a facile synthetic method involving microemulsion and pyrolysis. The key in the synthesis of these nonprecious metal-containing heteroatom-doped nanostructured carbon catalysts is the preparation of nanosized, mixed metal–organic frameworks (MOFs) in solution using microemulsion, and the exploitation of their morphology to produce hollow spherical structures via pyrolysis. Elemental mapping shows the presence of Co, Zn, N, and S atoms throughout the structures of the materials. The material obtained at pyrolysis temperature of 800 °C, denoted (Co, Zn)/NSC-800, exhibits excellent electrocatalytic activity for ORR in alkaline and acidic media, with onset potentials of 1.000 and 0.802 V vs RHE, respectively. In addition, the catalyst is stable and tolerates the methanol crossover reaction in both cases. Thus, this material has the potential to replace Pt catalysts in fuel cells. Moreover, the synthetic approach used to make it can be extended to produce other robust, nonprecious transition metal-based catalysts for ORR.

Bio-Derived Co2 P Nanoparticles Supported on Nitrogen-Doped Carbon as Promising Oxygen Reduction Reaction Electrocatalyst for Anion Exchange Membrane Fuel Cells.

Recently, nonnoble-metal catalysts such as a metal coordinated to nitrogen doped in a carbon matrix have been reported to exhibit superior oxygen reduction reaction (ORR) activity in alkaline media. In this work, Co2 P nanoparticles supported on heteroatom-doped carbon catalysts (NBSCP) are developed with an eco-friendly synthesis method using bean sprouts. NBSCP can be easily synthesized through metal precursor absorption and carbonization at a high temperature. It shows a very large specific surface area with various dopants such as nitrogen, phosphorus, and sulfur derived from small organic molecules. The catalyst can exhibit activity in various electrochemical reactions. In particular, excellent performance is noted for the ORR. Compared to the commercial Pt/C, NBSCP exhibits a lower onset potential, higher current density, and superior durability. This excellent ORR activity and durability is attributable to the synergistic effect between Co2 P nanoparticles and nitrogen-doped carbon. In addition, superior performance is noted on applying NBSCP to a practical anion exchange membrane fuel cell system. Through this work, the possibility of applying an easily obtained bio-derived material to energy conversion and storage systems is demonstrated.

Facile synthesis of nitrogen, sulfur dual-doped porous carbon via carbonization of coal tar pitch and MgCl2·6H2O for oxygen reduction reaction

Developing heteroatom-doped carbon catalysts toward oxygen reduction reaction (ORR) to replace platinum (Pt)-based catalysts is a promising strategy. Herein, nitrogen (N), sulfur (S) dual-doped porous carbons (NS@C-x-y) were prepared via a one-step carbonization of the mixture of acid treatment coal tar pitch (CTP) and MgCl2·6H2O, through in situ self-generating MgO template route. N and S were successfully introduced into the carbon framework of CTP by mixed acid treatment, leading to N, S co-doping especially pyridinic-N, graphitic-N, and C-S-C as ORR active sites. Benefiting from the fluffy lump graphitic structure, large surface areas, and synergistic effect between N and S electrocatalytic sites, the resulting samples demonstrate superior ORR performances with the highest diffusion-limited current density of 5.97 mA cm−2 and half-wave potential of 0.82 V (vs. RHE), long-term stability, and excellent methanol tolerance in alkaline medium, which are competitive with Pt/C catalyst.

Intrinsic Carbon-Defect-Driven Electrocatalytic Reduction of Carbon Dioxide.

Heteroatom-doped carbon catalysts are currently attracting enormous attention due to their excellent performance for the electrocatalytic carbon dioxide reduction reaction (ECRR). However, the origin of the high catalytic activities of doped-carbon materials remains obscure with the role of intrinsic carbon defects in promoting the ECRR receiving little attention despite the abundance of carbon defects in all carbon-based catalytic materials. Herein, a positive correlation is reported between the ECRR performance of carbon-based catalysts and the content of intrinsic carbon defects contained within these catalysts. Further, it is demonstrated that defective porous carbon catalysts containing no active heteroatom dopants also show excellent catalytic performance for ECRR. C K-edge near edge X-ray absorption fine structure measurements and density functional theory calculations reveal that sp2 defects (octagonal and pentagonal) rather than edge defects are key to the excellent ECRR activity of the defective porous carbon catalysts. This work thus makes an important contribution to the understanding of the origin of the ECRR activity of carbon-based catalysts, with heteroatom doping perhaps being less important than previously envisaged for achieving a high ECRR performance.

N-, P-, and S-doped graphene-like carbon catalysts derived from onium salts with enhanced oxygen chemisorption for Zn-air battery cathodes

Compared to currently studied metal-based catalysts, metal-free heteroatom-doped carbon catalysts have many advantages including no issues of degradation and contamination from metal dissolution. Relying on single type of doping usually cannot yield optimal electronic and geometric structures favorable for the oxygen reduction reaction (ORR). Herein, heteroatom N, P, and S simultaneously doped graphene-like carbon (NPS-G) was successfully synthesized from onium salts by a facile one-step pyrolysis method. The resulting metal-free NPS-G catalyst with optimized N, P, and S contents exhibits enhanced catalytic activity towards the ORR in alkaline media, relative to any single doping. In particular, this metal-free catalyst shows an encouraging half-wave potential (E1/2 = 0.857 V) comparable to that of metal-based catalysts. It also demonstrates excellent electrochemical stability and methanol tolerance. This catalyst was further studied as a cathode in a primary Zn-air battery, showing exceptional open-circuit voltage (1.372 V) and power density (0.151 W cm−2). The NPS-G cathode delivers a specific capacity of 686 mA h gZn-1 at a current density of 10 mA cm−2 while utilizing 82.2% of the theoretical capacity (835 mA h gZn-1). The origin of high activity associated with various heteroatom dopings is elucidated through X-ray photoelectron spectroscopy analysis and density functional theory studies. The enhanced chemisorption of oxygen species (*OOH, *O and *OH) onto the dopants of the NPS-G catalysts reduces charge transfer resistance and facilitate the ORR. The porous 2D structure also contributes to the increase of active site density and facile mass transport.

Simple synthesis of nitrogen-doped porous carbon from Chinese steamed bread flour and its catalytic application for hydrogen evolution reaction

A cost-effective green synthesis method for nitrogen-doped porous carbon from Chinese steamed bread flour by successive hydrothermal treatment and pyrolysis is reported. The hydrothermal treatment remarkably enhances the strength of the resulting pyrolyzed carbon materials, enabling the facile fabrication of robust monolithic nitrogen-doped porous carbon with the yield of 44.33%. This nitrogen-doped porous carbon exhibits 3-dimensional porous structure which is mainly mesoporous and high surface area. The thin and 3-dimensional self-supported working electrode is made from the synthesized nitrogen-doped porous carbon without using any other current collector or polymeric binder. The hydrogen evolution reaction electrocatalytic activity in 0.5 M H2SO4 reveals the achievement of the low overpotential of ∼220 mV at 10 mA cm−2, a Tafel slope of ∼77 mV dec−1, a nearly 100% Faradaic yield, an excellent stability, and a low charge transfer resistance. The self-supported electrode exhibits higher electrocatalytic activity than the powder-state supported on graphite felt electrode. Its hydrogen evolution reaction electrocatalytic activity is better than that of the reported non-metal heteroatom-doped carbon catalysts and comparable to that of some metal catalysts in acidic media. The use of flour enables the cheap and facile fabrication of carbon electrode with tunable pore sizes and surface areas, making flour-based carbon electrodes highly promising.