Efficient Metal-free Catalyst Research Articles

Interconnected porous nitrogen-doped carbon framework: Recoverable template fabrication and excellent electrocatalytic performance for oxygen reduction reaction

The development of efficient metal-free catalysts for oxygen reduction reaction (ORR) is of great importance for accelerating the commercial application of fuel cell. Here, a 3D interconnected N-doped porous carbon skeleton (PC) was prepared by one-step pyrolysis strategy with the help of recyclable NaCl. Combining the advantages of template and pyrolysis, the obtained N-PC-900 product showed a large specific surface area (470.9 m2 g−1), porous structure and high N contents (pyridinic N 30.2% and graphitic N 40.2%). The high ID1/IG value of 1.76 suggested that N-PC-900 had more defects which could be produced at 900 °C. In alkaline condition, the prepared N-PC-900 exhibited positive onset potential (0.90 V), half-wave potential (0.75 V) and high current density (5.38 mA/cm2 at 0.2 V), which was close to the 20% Pt/C. The number of transferred electrons was 3.87–3.98 at a potential of 0.055–0.9 V, meaning a 4e− pathway towards the ORR. Meanwhile, the N-PC-900 displayed better durability and methanol tolerance for ORR in both acid and alkaline conditions than 20%Pt/C. This study provided a convenient and low-cost method to prepare high-performance metal-free ORR catalysts, and the product may have great potentials in ORR catalysis.

Revealing the dependence of active site configuration of N doped and N, S-co-doped carbon nanospheres on six-membered heterocyclic precursors for oxygen reduction reaction

Elucidating the correlation of the polymer precursors and active sites formation of multi-heteroatom doped based carbon for oxygen reduction reaction (ORR) is essential to develop molecular-level design of highly efficient metal-free carbon catalysts. To achieve this goal, herein, two series of N doped and N, S co-doped carbon nanospheres were synthesized via pyrolysis of self-polymerized compounds of three analogous precursors of 2,6-Diaminopyridine, 4, 6-Diaminopyrimidine and 1,3-Diaminobenzene as nitrogen source initiated by hydrogen peroxide and ammonium persulfate as sulfer source respectively. The results demonstrate that nitrogen atoms in the six-membered rings of three precursors play a critical role on the formation of N-doping types and synergistic effects of N and S dopants in their derived carbon nanoshperes for ORR. Among the three precursors, 2,6-Diaminopyrimidine prefers to form the Pyridinic N doped carbon, resulting in the highest activity for ORR in N doped nanospheres, in contrast, 2,6-Diaminopyridine facilitates the formation of more content of Graphitic-N dopant, which has stronger synergy with thiophenic-S dopant in carbon matrix than that of Pyridinic N dopants. This unique characteristic endows 2,6-Diaminopyrimidine derived N, S co-doped nanospheres with excellent ORR activity of higher half-wave potential of 0.87 V vs. RHE in 0.1 M KOH and Zn-Air battery power output performance than that of the same loading of commercial Pt/C catalyst. Revealing the dependence of specific active sites determined by precursors with very similar molecular structures in the work will shed light on the precise design strategy based on the precursors-selecting guidance for the development of advanced multi-heteroatom doped carbon ORR catalysts.

High nitrogen carbon material with rich defects as a highly efficient metal-free catalyst for excellent catalytic performance of acetylene hydrochlorination

In this work, we developed a simple strategy to synthesize a carbon material with high nitrogen and rich carbon defects. Our approach polymerized diaminopyridine (DAP) and ammonium persulfate (APS). Following a range of different temperature pyrolysis approaches, the resulting rough surface was shown to exhibit edge defects due to N-doping on graphite carbon. A series of catalysts were evaluated using a variety of characterization techniques and tested for catalytic performance. The catalytic performance of the N-doped carbon material enhanced alongside an increment in carbon defects. The NC-800 catalyst exhibited outstanding catalytic activity and stability in acetylene hydrochlorination (C2H2 GHSV = 30 h−1, at 220 °C, the acetylene conversion rate was 98%), with its stability reaching up to 450 h. Due to NC-800 having a nitrogen content of up to 13.46%, it had the largest specific surface area and a high defect amount, as well as strong C2H2 and HCl adsorption. NC-800 has excellent catalytic activity and stability to reflect its unlimited potential as a carbon material.

Nanostructured Carbon-Nitrogen-Sulfur-Nickel Networks Derived From Polyaniline as Bifunctional Catalysts for Water Splitting.

The development of reliable production routes for sustainable hydrogen (H2), which is an essential feedstock for industrial processes and energy carrier for fuel cells, is needed. It appears to be an unavoidable alternative to significantly reduce the dependence on conventional energy sources based on fossil fuels without increasing the atmospheric CO2 levels. Among the different power-to-X scenarios to access high purity H2, the electrochemical approach based on electrolysis looks to be a promising sustainable solution at both the small and large industrial scales. However, the practical realization of this important opportunity faces several challenges, including the efficient design of cost-effective catalytic materials to be used as a cathode with improved intrinsic and durable activity. In this contribution, we report the design and development of efficient nanostructured catalysts for the electrocatalytic hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in aqueous media, whereby noble metal-free elements are embedded in a matrix of a conducting polymer, polyaniline (PANI). To increase the electrical conductivity and further the electrocatalytic ability toward HER of the chemically polymerized PANI in the presence of nickel (II) salt (nitrate), the PANI-based materials have first been stabilized at a mild temperature of 250–350°C in air and then carbonized at 800–1,000°C under nitrogen gas to convert the chemical species into nitrogen, sulfur, nickel, and carbon nanostructured networks (CNNs). Different physicochemical (TGA-DSC, Raman spectroscopy, XRD, SEM, EDX, ICP, CHNS, BET, and XPS) and electrochemical (voltammetry and electrochemical impedance spectrometry) methods have been integrated to characterize the as-synthesized CNNs materials and interrogate the relationship of material-to-performance. It has been found that those synthesis conditions allow for the substantial increase of the electrocatalytic performance toward HER and OER in alkaline media in terms of the onset potential and charge transfer resistance and overpotential at the specific activity of 10 milliamps per square centimeter, thus ranking the present materials among the most efficient noble metal-free catalysts and making them possible candidates for integration in practical low-energy consumption alkaline electrolyzers.

Rational design of efficient metal-free catalysts for peroxymonosulfate activation: Selective degradation of organic contaminants via a dual nonradical reaction pathway

Designing efficient and low-cost catalysts to activate peroxymonosulfate (PMS) to rapidly degrade organic contaminants is important for the practical applications of the advanced oxidation process. Herein, inspired by the water absorption process of the baby diaper, we design nitrogen-doped porous carbon network catalysts (N-PCNs) for peroxymonosulfate activate to degrade recalcitrant organic pollutants. The resulting product called nitrogen-doped porous carbon networks carbonized at 800 °C (N-PCN8) exhibits enhanced adsorption and catalytic activity due to its large specific surface area (1137.7 m2 g−1), highly graphitic degree, and high graphite N content (50.3%).4-CP (0.02 g/L) was completely degraded in 30 min by using N-PCN8 (0.2 g/L) and PMS (0.2 g/L). The catalytic system is efficient over a wide pH range (3–9) and shows strong resistance to interference with inorganic anions (Cl−, HCO3−, CO32−). Several aromatic pollutants, including 4-CP, BA, NB, HBA, CBZ, and BPA, are used as target pollutants to further evaluate the oxidative capacity of the system, and the degradation rate were 100%, 19.5%, 3.5%,5 5.7%, 79% and 100%, respectively. Results suggest that the system is selective for pollutants, and singlet oxygen oxidation and mediated electron transfer effects are the main causes of 4-CP degradation.

Graphene-Modified Composites and Electrodes and Their Potential Applications in the Electro-Fenton Process.

In recent years, graphene-based materials have been identified as an emerging and promising new material in electro-Fenton, with the potential to form highly efficient metal-free catalysts that can be employed in the removal of contaminants from water, conserving precious water resources. In this review, the recent applications of graphene-based materials in electro-Fenton are described and discussed. Initially, homogenous and heterogenous electro-Fenton methods are briefly introduced, highlighting the importance of the generation of H2O2 from the two-electron reduction of dissolved oxygen and its catalysed decomposition to produce reactive and oxidising hydroxy radicals. Next, the promising applications of graphene-based electrodes in promoting this two-electron oxygen reduction reaction are considered and this is followed by an account of the various graphene-based materials that have been used successfully to give highly efficient graphene-based cathodes in electro-Fenton. In particular, graphene-based composites that have been combined with other carbonaceous materials, doped with nitrogen, formed as highly porous aerogels, three-dimensional materials and porous gas diffusion electrodes, used as supports for iron oxides and functionalised with ferrocene and employed in the more effective heterogeneous electro-Fenton, are all reviewed. It is perfectly clear that graphene-based materials have the potential to degrade and mineralise dyes, pharmaceutical compounds, antibiotics, phenolic compounds and show tremendous potential in electro-Fenton and other advanced oxidation processes.

A Highly Efficient Metal-Free Electrocatalyst of F-Doped Porous Carbon toward N2 Electroreduction.

N2 electroreduction into NH3 represents an attractive prospect for N2 utilization. Nevertheless, this process suffers from low Faraday efficiency (FE) and yield rate for NH3 . In this work, a highly efficient metal-free catalyst is developed by introducing F atoms into a 3D porous carbon framework (F-doped carbon) toward N2 electroreduction. At -0.2 V versus reversible hydrogen electrode (RHE), the F-doped carbon achieves the highest FE of 54.8% for NH3 , which is 3.0 times as high as that (18.3%) of pristine carbon frameworks. Notably, at -0.3 V versus RHE, the yield rate of F-doped carbon for NH3 reaches 197.7 µgNH3 mg-1 cat. h-1 . Such a value is more than one order of magnitude higher than those of other metal-free electrocatalysts under the near-ambient conditions for NH3 product to date. Mechanistic studies reveal that the improved performance in N2 electroreduction for F-doped carbon originates from the enhanced binding strength of N2 and the facilitated dissociation of N2 into *N2 H. F bonding to C atom creates a Lewis acid site due to the different electronegativity between the F and C atoms. As such, the repulsive interaction between the Lewis acid site and proton H suppresses the activity of H2 evolution reaction, thus enhancing the selectivity of N2 electroreduction into NH3 .

Tobacco stem-derived N-enriched active carbon: efficient metal free catalyst for reduction of nitroarene

A series of N-enriched active carbons (NACs) were prepared from carbonization of tobacco stem with KOH as activating agent. The obtained NACs were used as efficient metal free catalysts for reduction of nitroarene. The results of N2 adsorption–desorption indicate that NACs have a large number of mesopores with the increase of mass ratio of KOH to the precursor. NACs-3800 has high surface area (SBET = 2938 m2 g−1) and a large pore volume (Vtotal = 1.60 cm3 g−1) with a nitrogen content (1.32%). NACs-3800 showed a high catalytic activity (99.9% conversion, nearly 100% selectivity to aniline) in the reduction of nitrobenzene. Moreover, NACs-3800 also exhibits good catalytic activity in the reduction of substituted nitroarenes. Furthermore, the NACs-3800 presented remarkable activity and durability for nitrobenzene reduction to the corresponding aniline. The recyclability of the catalyst and its cheap feedstock makes the overall process much simple, cost-efficient and environment-friendly.

Efficient CO2 enrichment and fixation by engineering micropores of multifunctional hypercrosslinked ionic polymers

Efficient chemical fixation of carbon dioxide (CO2) into valuable fine chemicals requires porous materials with highly active catalytic centers. Herein, multifunctional imidazolium based hypercrosslinked ionic polymers with versatile functional groups (sulfonic, hydroxyl, amino, carboxyl, and alkyl group), abundant microporosity and high ionic site density were constructed in a two-step solvothermal route including free-radical copolymerization of divinylbenzene (DVB), 4-vinylbenzyl chloride (VBC) and imidazolium bromide ionic liquids (ILs) and successive Friedel-Crafts alkylation based hypercrosslinkage. The resultant hydroxyl-functional ionic polymer demonstrated promising performances in selective adsorption and conversion of CO2via cycloaddition with epoxide. High yield associating with large turnover number (TON) and turnover frequency (TOF), stable reusability and well substrate compatibility were achieved, affording an efficient metal-free heterogeneous catalyst for CO2 fixation. The full microporous structure resulted in CO2 enrichment around the robust hydroxyl-functional ionic sites, showing a synergistic effect to promote CO2 transformation.

Nitrogen and sulfur dual-doped hollow mesoporous carbon spheres as efficient metal-free catalyst for oxygen reduction reaction

Developing a cheap and efficient oxygen reduction reaction (ORR) catalyst is one of the emerging issues. The doped carbon materials have attracted much attention due to the controllable and cheap synthesis methods. In this work, the nitrogen and sulfur dual-doped hollow mesoporous carbon sphere (NS-HMCS) has been synthesized through a hard-template method. During the co-polymerization process of dopamine (DA) and cysteamine (CA), the nitrogen (N) and sulfur (S) sites are introduced into the carbon skeleton at the same time, which simplifies the tedious steps in the post-modification of carbon materials. The obtained NS-HMCS-32 exhibits high content of nitrogen about 4.8% and sulfur about 1.4%. NS-HMCS-32 has a high specific surface areas (898 m2 g−1) and a large pore volume (2.99 cm3 g−1). During the ORR measurement, the half-wave potential of NS-HMCS-32 reaches up to 0.77 V vs. RHE. Moreover, NS-HMCS-32 reveals a better durability than commercial Pt/C and 79.2% of current density remains after 9 h chronoamperometry test. This work provides a design idea for dual-doped carbon materials and the obtained NS-HMCS is a potential candidate for metal-free catalyst in fuel cells.

Filling the nitrogen vacancies with sulphur dopants in graphitic C3N4 for efficient and robust electrocatalytic nitrogen reduction

Exploring active and durable metal-free electrocatalysts represents a promising direction for electrocatalytic N2 reduction reaction (NRR). Herein, by filling the nitrogen vacancies (NVs) with S dopants in graphitic C3N4, we showed that the resultant S-NV-C3N4 could be an efficient and robust metal-free NRR catalyst in neutral solution. S-NV-C3N4 with a high S concentration of 5.2 at.% exhibited a fascinating NRR performance with an NH3 yield of 32.7 μg h−1 mg−1 and a Faradaic efficiency of 14.1 % at −0.4 V (vs. RHE), considerably outperforming pristine C3N4 and NV-containing C3N4. S-NV-C3N4 also showed exceptional durability for at least 20 h, in stark contrast to the inferior stability of NV-containing C3N4. Density functional theory calculations disclosed that the filled S dopants could break the scaling relation to effectively stabilize *N2H and destabilize *NH2 on S-NV-C3N4, leading to more optimized adsorption of NRR intermediates and a significantly reduced energy barrier. Meanwhile, the competitive HER can be effectively suppressed on S-NV-C3N4 due to the high energy barrier for water dissociation.

Graphitic carbon nitride (g-C3N4) as an efficient metal-free Fenton-like catalyst for degrading organic pollutants: the overlooked non-photocatalytic activity.

Graphitic carbon nitride (g-C3N4) has attracted a large amount of research, mainly being used as a photocatalyst, but its Fenton-like catalytic performance has been overlooked. In this paper, the dark Fenton-like catalytic performance of g-C3N4 was evaluated by degrading rhodamine B over a wide pH range. The results showed that the g-C3N4, which was synthesized by conventional urea pyrolysis without any modification, was an efficient metal-free heterogeneous Fenton-like catalyst. The highest activity occurred under a weakly alkaline condition of about pH 10. The experiment of catalyst recycling indicated that g-C3N4 had long-term stability. The reactive oxidizing species of HO·, generated by the g-C3N4 activating H2O2, was identified by EPR and further supported by a scavenging experiment of HO· using isopropanol as the scavenger. The HNO3 oxidation of g-C3N4 resulted in catalytic deactivation, implying the catalytic activity originated from the surface reduced groups of g-C3N4. The structure of synthesized g-C3N4 before and after the HNO3 oxidation was characterized by X-ray diffraction, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy, and a possible catalytic mechanism was proposed.

An efficient metal-free catalyst derived from waste lotus seedpod for oxygen reduction reaction

The sluggish kinetics of oxygen reduction reaction (ORR) has severely impeded the application of fuel cell technology. Developing economical and efficient biomass-based catalysts derived from natural waste to replace the expensive noble metal catalysts is becoming an attractive strategy. However, biomass materials often have geographical and seasonal characteristics. Thus, developing diversified and available biomass-derived catalysts is significant for different regions. In this work, N, P-dual doped porous carbon materials derived from waste lotus seedpod with tunable specific surface areas and porous structure are skillfully fabricated. The optimized sample exhibits excellent oxygen reduction reaction activity with an onset potential of 0.87 V vs. RHE and a Tafel slope of 81 mV dec−1, showing the 4e‒ electrons transfer path in alkaline solution. It also exhibits much better stability and methanol tolerance than commercial Pt/C. High ORR performance is considered to be ascribed to the abundant N, P dispersion and the hierarchical porous structure.

N,S co-doped hierarchically porous carbon materials for efficient metal-free catalysis

Polymer-derived N,S co-doped carbon materials (PDNSC-X) with a hierarchically porous structure were facilely prepared by a cost-effective and convenient strategy and were subsequently used as efficient metal-free catalysts.

Pomelo peel-derived, N-doped biochar microspheres as an efficient and durable metal-free ORR catalyst in microbial fuel cells

Naturally abundant pomelo peels were explored for the preparation of the metal-free carbon-based microspheres with high electrocatalytic activity and long-term durability toward ORR, holding potential for replacing noble metal-based catalysts.

A nitrogen-doped carbon-coated silicon carbide as a robust and highly efficient metal-free catalyst for sour gas desulfurization in the presence of aromatics as contaminants

A mesoporous N-doped carbon coating for SiC extrudates shows excellent H2S desulfurization performance along with remarkably high resistance towards deactivation/fouling in the presence of aromatics as contaminant.

Nitrogen-doped phosphorene for electrocatalytic ammonia synthesis

A facile and efficient strategy to produce nitrogen-doped (N-doped) phosphorene nanosheets that can be used as an efficient metal-free catalyst for electrochemical ammonia synthesis under ambient conditions is presented.

Confined pyrolysis of a dye pollutant for two-dimensional F,N,S tri-doped nanocarbon as a high performance oxidative coupling reaction catalyst

Two-dimensional F,N,S tri-doped nanocarbon was synthesized by confined pyrolysis of a dye pollutant and utilized as a high efficient metal-free catalyst for oxidative coupling of amines.

Stable abnormal N-heterocyclic carbenes and their applications.

Although N-heterocyclic carbenes (NHCs) have been known as ligands for organometallic complexes since the 1960s, these carbenes did not attract considerable attention until Arduengo et al. reported the isolation of a metal-free imidazol-2-ylidene in 1991. In 2001 Crabtree et al. reported a few complexes featuring an NHC isomer, namely an imidazol-5-ylidene, also termed abnormal NHC (aNHCs). In 2009, it was shown that providing to protect the C-2 position of an imidazolium salt, the deprotonation occurred at the C-5 position, affording imidazol-5-ylidenes that could be isolated. Over the last ten years, stable aNHCs have been used for designing a range of catalysts employing Pd(ii), Cu(i), Ni(ii), Fe(0), Zn(ii), Ag(i), and Au(i/iii) metal based precursors. These catalysts were utilized for different organic transformations such as the Suzuki-Miyaura cross-coupling reaction, C-H bond activation, dehydrogenative coupling, Huisgen 1,3-dipolar cycloaddition (click reaction), hydroheteroarylation, hydrosilylation reaction and migratory insertion of carbenes. Main-group metal complexes were also synthesized, including K(i), Al(iii), Zn(ii), Sn(ii), Ge(ii), and Si(ii/iv). Among them, K(i), Al(iii), and Zn(ii) complexes were used for the polymerization of caprolactone and rac-lactide at room temperature. In addition, based on the superior nucleophilicity of aNHCs, relative to that of their nNHCs isomers, they were used for small molecules activation, such as carbon dioxide (CO2), nitrous oxide (N2O), tetrahydrofuran (THF), tetrahydrothiophene and 9-borabicyclo[3.3.1]nonane (9BBN). aNHCs have also been shown to be efficient metal-free catalysts for ring opening polymerization of different cyclic esters at room temperature; they are among the most active metal-free catalysts for ε-caprolactone polymerization. Recently, aNHCs successfully accomplished the metal-free catalytic formylation of amides using CO2 and the catalytic reduction of carbon dioxide, including atmospheric CO2, into methanol, under ambient conditions. Although other transition metal complexes featuring aNHCs as ligand have been prepared and used in catalysis, this review article summarize the results obtained with the isolated aNHCs.

Phosphorus-doped carbon fibers as an efficient metal-free bifunctional catalyst for removing sulfamethoxazole and chromium (VI).

Developing an efficient and metal-free bifunctional catalyst for the simultaneous degradation of antibiotic and reduction of Cr (VI) has been regarded as increasingly attractive yet challenging objectives in the environmental catalysis field. Herein, phosphorus-doped carbon fibers (P-CFs) was innovatively prepared by doping and calcination methods, characterized by scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Sulfamethoxazole (SMX) as the target contaminant was selected to evaluate the catalytic activity of P-CFs in PMS activation, over 90% SMX removal and 82.75% mineralization were high-efficiently achieved in the P-CFs/peroxymonosulfate (PMS) system. Particularly, P-CFs/PMS system exhibited a superior catalytic oxidation performance over a wide pH range (3.5-9.5) and even in the complicated water matrix. Surprisingly, the presence of humic acid (HA) in the P-CFs/PMS system could achieve about 2 times enhancement on SMX removal, different from most reports about the inhibition of HA in PMS activation. More importantly, Brunauer-Emmett-Teller (BET) method and XPS analysis revealed that the highly toxic Cr (VI) could be reduced to Cr (III) by P-CFs. Furthermore, electron spin resonance (ESR) combined with various trapping agents demonstrated that SO4•-, •OH and 1O2. were generated and participated in the SMX degradation, while the free electron in P-CFs played a main role in Cr (VI) reduction. This finding not only provided a high-efficiency strategy in the treatment of wastewaters containing organic contaminants and heavy metals Cr (VI), but might open new insights into an innovative metal-free catalyst in environment remediation.