Understanding Heteroatom-Mediated Metal–Support Interactions in Functionalized Carbons: A Perspective Review

Carbon-based materials show unique chemicophysical properties, and they have been successfully used in many catalytic processes, including the production of chemicals and energy. The introduction of heteroatoms (N, B, P, S) alters the electronic properties, often increasing the reactivity of the surface of nanocarbons. The functional groups on the carbons have been reported to be effective for anchoring metal nanoparticles. Although the interaction between functional groups and metal has been studied by various characterization techniques, theoretical models, and catalytic results, the role and nature of heteroatoms is still an object of discussion. The aim of this review is to elucidate the metal–heteroatoms interaction, providing an overview of the main experimental and theoretical outcomes about heteroatom-mediated metal–support interactions. Selected studies showing the effect of heteroatom–metal interaction in the liquid-phase alcohol oxidation will be also presented.

The chemical identity, state and structure of catalytically active centers during the electrochemical CO2 reduction on porous Fe-nitrogen-carbon (Fe-N-C) materials

We report novel structure–activity relationships and explore the chemical state and structure of catalytically active sites under operando conditions during the electrochemical CO₂ reduction reaction (CO₂RR) catalyzed by a series of porous iron–nitrogen–carbon (FeNC) catalysts.

We report novel structure–activity relationships and explore the chemical state and structure of catalytically active sites under operando conditions during the electrochemical CO₂ reduction reaction (CO₂RR) catalyzed by a series of porous iron–nitrogen–carbon (FeNC) catalysts. The FeNC catalysts were synthesized from different nitrogen precursors and, as a result of this, exhibited quite distinct physical properties, such as BET surface areas and distinct chemical N-functionalities in varying ratios. The chemical diversity of the FeNC catalysts was harnessed to set up correlations between the catalytic CO₂RR activity and their chemical nitrogen-functionalities, which provided a deeper understanding between catalyst chemistry and function. XPS measurements revealed a dominant role of porphyrin-like Fe–N_x motifs and pyridinic nitrogen species in catalyzing the overall reaction process. Operando EXAFS measurements revealed an unexpected change in the Fe oxidation state and associated coordination from Fe²⁺ to Fe¹⁺. This redox change coincides with the onset of catalytic CH₄ production around –0.9 V_RHE. The ability of the solid state coordinative Fe¹⁺–N_x moiety to form hydrocarbons from CO₂ is remarkable, as it represents the solid-state analogue to molecular Fe¹⁺ coordination compounds with the same catalytic capability under homogeneous catalytic environments. This finding highlights a conceptual bridge between heterogeneous and homogenous catalysis and contributes significantly to our fundamental understanding of the FeNC catalyst function in the CO₂RR.

Three-Dimensional Networks of S-Doped Fe/N/C with Hierarchical Porosity for Efficient Oxygen Reduction in Polymer Electrolyte Membrane Fuel Cells

Reasonable design and synthesis of Fe/N/C-based catalysts is one of the most promising way for developing precious metal-free oxygen reduction reaction (ORR) catalysts in acidic mediums. Herein, we developed a highly active metal-organic framework-derived S-doped Fe/N/C catalyst [S-Fe/Z8/2-aminothiazole (2-AT)] prepared by thermal treatment. The S-Fe/Z8/2-AT catalyst with uniform S-doping possesses a three-dimensional macro-meso-micro hierarchically porous structure. Moreover, the chemical composition and structural features have been well-optimized and characterized for such S-Fe/Z8/2-AT catalysts; and their formation mechanism was also revealed. Significantly, applying the optimal S-Fe/Z8/2-AT catalysts into electrocatalytic test exhibits remarkable ORR catalytic activity with a half-wave potential of 0.82 V (vs reversible hydrogen electrode) and a mass activity of 18.3 A g^-1 at 0.8 V in 0.1 M H₂SO₄ solution; the polymer electrolyte membrane fuel cell test also confirmed their excellent catalytic activity, which gives a maximal power density as high as 800 mW cm^-2 at 1 bar. A series of designed experiments disclosed that the favorable structural merits and desirable chemical compositions of S-Fe/Z8/2-AT catalysts are critical factors for efficient electrocatalytic performance. The work provides a new approach to open an avenue for accurately controlling the composition and structure of Fe/N/C catalysts with highly activity for ORR.

Nanoscale mapping of catalytic hotspots on Fe, N-modified HOPG by scanning electrochemical microscopy-atomic force microscopy

The scanning electrochemical microscopy-atomic force microscopy (SECM-AFM) technique is used to map catalytic currents post Fe and N surface modification of graphitic carbon with an ultra-high resolution of 50 nm. The oxidation current of the partial reduction product, hydrogen peroxide, was also mapped in the same location in the graphitic carbon. The current mapping and ex situ spectroscopic evidence revealed that Fe-coordinated nitrogen sites formed both in the edge and basal planes of highly ordered pyrolytic graphite (HOPG) constitute the primary oxygen reduction catalytic sites in acid solutions of this important yet insufficiently understood class of catalysts.

Role of Nitrogen Moieties in N-Doped 3D-Graphene Nanosheets for Oxygen Electroreduction in Acidic and Alkaline Media

This study elucidates the synthesis-structure-property correlations of nitrogen moieties present in nitrogen-functionalized graphene nanomaterials toward oxygen reduction reactions (ORRs) and their electrochemical pathways in acidic and alkaline electrolytes. Porous three-dimensional nitrogen-doped graphene nanosheets (N/3D-GNSs) were fabricated using the sacrificial support method and doped with nitrogen using 10 atom % NH₃ under thermal pyrolysis at T = 650, 850, and 1050 °C for evaluating the nitrogen species formed under different temperatures. The abundances of the various nitrogen species formed under pyrolytic conditions were evaluated with X-ray photoelectron spectroscopy. Using rotating ring-disk electrode, we analyzed the role played by the nitrogen moieties influencing the electrochemical activity of the N/3D-GNS supports for oxygen reduction reactions (ORRs) in both acidic and alkaline media. It was demonstrated that the concentrations of the nitrogen moieties: graphitic-N, quaternary, hydrogenated-N (hydrogenated nitrogen combined pyrrolic nitrogen and hydrogenated pyridine) and pyridinic-N varied considerably with pyrolysis temperatures. A decrease in graphitic-N content and an increase in the ratio of hydrogenated-N/pyridinic-N significantly improved the activity of the material. The half-wave and onset potentials as well as the current densities and hydrogen peroxide (H₂O₂)/(HO₂^-) yields of the N/3D-GNS materials also varied between acidic and alkaline electrolytes but followed the general trend in terms of pyrolysis temperatures and abundance of the nitrogen moieties. Among the synthesized materials, the 3D-graphene nanosheets that were doped with nitrogen at 850 °C, optimized to have the highest hydrogenated-N and lowest pyridinic-N as well as better catalyst-ionomer integration, showed the highest ORR performance. This strategy for the tunable synthesis of nitrogen-doped graphene materials with controlled nitrogen functionalization offers a platform for developing active supports or catalytic nanomaterials for fuel cell applications.

Engineering crystalline CoOOH anchored on an N-doped carbon support as a durable electrocatalyst for the oxygen reduction reaction

A novel electrocatalyst of crystalline CoOOH anchored on an N-doped carbon support is constructed via a facile approach. CoOOH@NC-175/300 demonstrates much higher ORR activity than those with either nitrogen-doping or CoOOH anchoring in an N-undoped carbon support, which can be attributed to Co–N_x, C–N species and even the CoOOH species.

Core electron binding energies of adsorbates on Cu(111) from first-principles calculations

C1s and O1s core level binding energy shifts have been calculated for various adsorbates on Cu(111) using the ΔSCF method.

Polymorphism and metal-induced structural transformation in 5,5′-bis(4-pyridyl)(2,2′-bispyrimidine) adlayers on Au(111)

Addition of iron to a self-assembled molecular network can lift polymorphism and leads to the expression of one single metal–organic structure on a surface.

Tungsten nitride/carbide nanocomposite encapsulated in nitrogen-doped carbon shell as an effective and durable catalyst for hydrogen evolution reaction

In situ carbonized WO_X/aniline hybrid nanoparticles were prepared and used to obtain WN–W₂C nanocomposites encapsulated in nitrogen-doped carbon shell, which demonstrated excellent HER performance.

Understanding activity and selectivity of metal-nitrogen-doped carbon catalysts for electrochemical reduction of CO2

Direct electrochemical reduction of CO₂ to fuels and chemicals using renewable electricity has attracted significant attention partly due to the fundamental challenges related to reactivity and selectivity, and partly due to its importance for industrial CO₂-consuming gas diffusion cathodes. Here, we present advances in the understanding of trends in the CO₂ to CO electrocatalysis of metal- and nitrogen-doped porous carbons containing catalytically active M–N_x moieties (M = Mn, Fe, Co, Ni, Cu). We investigate their intrinsic catalytic reactivity, CO turnover frequencies, CO faradaic efficiencies and demonstrate that Fe–N–C and especially Ni–N–C catalysts rival Au- and Ag-based catalysts. We model the catalytically active M–N_x moieties using density functional theory and correlate the theoretical binding energies with the experiments to give reactivity-selectivity descriptors. This gives an atomic-scale mechanistic understanding of potential-dependent CO and hydrocarbon selectivity from the M–N_x moieties and it provides predictive guidelines for the rational design of selective carbon-based CO₂ reduction catalysts.

Inexpensive and selective electrocatalysts for CO₂ reduction hold promise for sustainable fuel production. Here, the authors report N-coordinated, non-noble metal-doped porous carbons as efficient and selective electrocatalysts for CO₂ to CO conversion.

Defect Engineering toward Atomic Co-Nx -C in Hierarchical Graphene for Rechargeable Flexible Solid Zn-Air Batteries

Rechargeable flexible solid Zn-air battery, with a high theoretical energy density of 1086 Wh kg^-1 , is among the most attractive energy technologies for future flexible and wearable electronics; nevertheless, the practical application is greatly hindered by the sluggish oxygen reduction reaction/oxygen evolution reaction (ORR/OER) kinetics on the air electrode. Precious metal-free functionalized carbon materials are widely demonstrated as the most promising candidates, while it still lacks effective synthetic methodology to controllably synthesize carbocatalysts with targeted active sites. This work demonstrates the direct utilization of the intrinsic structural defects in nanocarbon to generate atomically dispersed Co-N_x -C active sites via defect engineering. As-fabricated Co/N/O tri-doped graphene catalysts with highly active sites and hierarchical porous scaffolds exhibit superior ORR/OER bifunctional activities and impressive applications in rechargeable Zn-air batteries. Specifically, when integrated into a rechargeable and flexible solid Zn-air battery, a high open-circuit voltage of 1.44 V, a stable discharge voltage of 1.19 V, and a high energy efficiency of 63% at 1.0 mA cm^-2 are achieved even under bending. The defect engineering strategy provides a new concept and effective methodology for the full utilization of nanocarbon materials with various structural features and further development of advanced energy materials.

Ruthenium-cobalt nanoalloys encapsulated in nitrogen-doped graphene as active electrocatalysts for producing hydrogen in alkaline media

The scalable production of hydrogen could conveniently be realized by alkaline water electrolysis. Currently, the major challenge confronting hydrogen evolution reaction (HER) is lacking inexpensive alternatives to platinum-based electrocatalysts. Here we report a high-efficient and stable electrocatalyst composed of ruthenium and cobalt bimetallic nanoalloy encapsulated in nitrogen-doped graphene layers. The catalysts display remarkable performance with low overpotentials of only 28 and 218 mV at 10 and 100 mA cm⁻², respectively, and excellent stability of 10,000 cycles. Ruthenium is the cheapest platinum-group metal and its amount in the catalyst is only 3.58 wt.%, showing the catalyst high activity at a very competitive price. Density functional theory calculations reveal that the introduction of ruthenium atoms into cobalt core can improve the efficiency of electron transfer from alloy core to graphene shell, beneficial for enhancing carbon–hydrogen bond, thereby lowing ΔG_H* of HER.

Ruthenium is the cheapest platinum-group metal, yet active hydrogen evolution catalysts with low amounts of ruthenium have yet to be designed. Here, the authors report the preparation of a ruthenium–cobalt nanoalloy and demonstrate its potential as an effective hydrogen evolution catalyst in basic media.

Air Breathing Cathodes for Microbial Fuel Cell using Mn-, Fe-, Co- and Ni-containing Platinum Group Metal-free Catalysts

The oxygen reduction reaction (ORR) is one of the major factors that is limiting the overall performance output of microbial fuel cells (MFC). In this study, Platinum Group Metal-free (PGM-free) ORR catalysts based on Fe, Co, Ni, Mn and the same precursor (Aminoantipyrine, AAPyr) were synthesized using identical sacrificial support method (SSM). The catalysts were investigated for their electrochemical performance, and then integrated into an air-breathing cathode to be tested in "clean" environment and in a working microbial fuel cell (MFC). Their performances were also compared to activated carbon (AC) based cathode under similar conditions. Results showed that the addition of Mn, Fe, Co and Ni to AAPyr increased the performances compared to AC. Fe-AAPyr showed the highest open circuit potential (OCP) that was 0.307 ± 0.001 V (vs. Ag/AgCl) and the highest electrocatalytic activity at pH 7.5. On the contrary, AC had an OCP of 0.203 ± 0.002 V (vs. Ag/AgCl) and had the lowest electrochemical activity. In MFC, Fe-AAPyr also had the highest output of 251 ± 2.3 μWcm^-2, followed by Co-AAPyr with 196 ± 1.5 μWcm^-2, Ni-AAPyr with 171 ± 3.6 μWcm^-2, Mn-AAPyr with 160 ± 2.8 μWcm^-2 and AC 129 ± 4.2 μWcm^-2. The best performing catalyst (Fe-AAPyr) was then tested in MFC with increasing solution conductivity from 12.4 mScm^-1 to 63.1 mScm^-1. A maximum power density of 482 ± 5 μWcm^-2 was obtained with increasing solution conductivity, which is one of the highest values reported in the field.

Enhanced Activity for Hydrogen Evolution Reaction over CoFe Catalysts by Alloying with Small Amount of Pt

The hydrogen evolution reaction highly relied on Pt electrocatalysts, with high activity and stability. In the past few years, a host of efforts have been made in the development of novel platinum nanostructures with a low amount of Pt because the scarcity and high price of Pt hinder its practical applications. Here, we report the preparation of PtCoFe@CN electrocatalysts with a remarkably reduced Pt loading amount of 4.60% by annealing Pt-doped metal-organic frameworks (MOFs). The electrocatalyst demonstrated an outstanding performance with only 45 mV overpotential to achieve the 10 mA cm^-2 current density, which is quite close to that of the commercial 20% Pt/C catalyst. The enhanced catalytic capability is originated from the modification of the electronic structures of CoFe by alloying with Pt. The results indicate that robust and superstable alloy electrocatalysts which contain a very small amount of noble metal could be prepared by annealing noble metal-doped MOFs.

Improved oxygen reduction activity of carbon nanotubes and graphene through adenine functionalization

The SWNT or graphene can draw electrons from the nitrogen species in adenine to form pyrrolic/graphitic-N-type groups as ORR active sites, similar with that of N-doped carbon materials, offering an opportunity to develop a class of new ORR catalysts.

The water-based synthesis of chemically stable Zr-based MOFs using pyridine-containing ligands and their exceptionally high adsorption capacity for iodine

Zr-MOFs with inherent pyridine moieties were synthesized by a water-based approach, and exhibited exceptionally high adsorption capacity for I₂.

Active Site Structures in Nitrogen-Doped Carbon-Supported Cobalt Catalysts for the Oxygen Reduction Reaction

The catalytic mechanism and the nature of active sites are revealed for the oxygen reduction reaction (ORR) with new non-noble-metal nitrogen-doped carbon-supported transition-metal catalysts (metal-N-C catalyst). Specifically, new nitrogen-doped carbon-supported cobalt catalysts (Co-N-C catalysts) are made by pyrolyzing various ratios of the nitrogen-atom rich heterocycle compound, 1-ethyl-3-methyl imidazolium dicyanamide (EMIM-dca) and cobalt salt (Co(NO₃)₂). The ORR activity (J_K at 0.8 V vs RHE, in 0.1 M KOH solution) of a typical catalyst in this family, Co₁₅-N-C800, is 8.25 mA/mg, which is much higher than the ORR activity values of N-C catalysts (0.41 mA/mg). The active site in the catalyst is found to be the Co-N species, which is most likely in the form of Co₂N. Metallic cobalt (Co) particles, Co₃C species, and N-C species are not catalytically active sites, nor do these moieties interact with the Co-N active sites during the catalysis of the ORR. Increasing the Co salt content during the synthesis favors the formation of Co-N active sites in the final catalyst. Higher pyrolysis temperatures (e.g., a temperature higher than 800 °C) do not favor the formation of the Co-N active sites, but cause the formed Co-N active sites to decompose, which, therefore, leads to a lower catalytic activity. This reveals that the control of the parameters that affect the final structure is critical to catalyst performance and, therefore, the effective development of high-performance heteroatom-doped non-noble-metal ORR catalysts.

Graphene Oxide-Based Sensor for Ultrasensitive Visual Detection of Fluoride

Visual fluoride ion detection with a detection limit down to pmol L-1 is achieved through quenching/reactivating the fluorescence of N-doped graphene oxide.

Sulfur-Doped Fe/N/C Nanosheets as Highly Efficient Electrocatalysts for Oxygen Reduction Reaction

The high-temperature pyrolyzed Fe/N/C is one of the tremendous potential nonprecious metal electrocatalysts for oxygen reduction reaction (ORR). Metal-free carbon materials doped with heteroatoms have also demonstrated prominent electrocatalytic performance for ORR. The previous work of S-doping Fe/N/C was produced by pyrolyzing melamine and iron thiocyanate, in which S was from iron thiocyanate. Here, for the first time, we realized an in situ S-doping in Fe/N/C (denoted as S-Fe/N/C) electrocatalyst with S doping reaching 4.76 at % by pyrolyzing thiourea as the S source and iron acetate showing high-performance ORR activity in both alkaline and acid solution. The catalyst pyrolyzed at 700 °C and the mass ratio 1:0.2 shows the best activity for ORR. The optimal catalyst displays much greater durability and higher tolerance to methanol than Pt/C (20 wt %). Results of electrochemical measurements show that the S-Fe/N/C follows 4e pathway in alkaline and acid conditions.

Active and Durable Hydrogen Evolution Reaction Catalyst Derived from Pd-Doped Metal-Organic Frameworks

The water electrolysis is of critical importance for sustainable hydrogen production. In this work, a highly efficient and stable PdCo alloy catalyst (PdCo@CN) was synthesized by direct annealing of Pd-doped metal-organic frameworks (MOFs) under N2 atmosphere. In 0.5 M H2SO4 solution, PdCo@CN displays remarkable electrocatalytic performance with overpotential of 80 mV, a Tafel slope of 31 mV dec(-1), and excellent stability of 10 000 cycles. Our studies reveal that noble metal doped MOFs are ideal precursors for preparing highly active alloy electrocatalysts with low content of noble metal.

One-step synthesis of Au nanoparticle–graphene composites using tyrosine: electrocatalytic and catalytic properties

The in situ synthesis of a reduced graphene oxide/Au nanoparticle composite for nonenzymatic H₂O₂ detection and nitrophenol reduction.

A multifunctional nanoplatform for lysosome targeted delivery of nitric oxide and photothermal therapy under 808 nm near-infrared light

NIR light induced spatiotemporal delivery of NO to lysosome accompanied by hyperthermia was realized.

Facile synthesis of MOF-derived ultrafine Co nanocrystals embedded in a nitrogen-doped carbon matrix for the hydrogen evolution reaction

Non-precious metal-based catalysts with low cost and rich reserves are emerging as promising alternatives for Pt-based catalysts for the hydrogen evolution reaction (HER).

Twin-like ternary PtCoFe alloy in nitrogen-doped graphene nanopores as a highly effective electrocatalyst for oxygen reduction

A novel twin-like ternary PtCoFe alloy in nitrogen-doped graphene nanopores (PtCoFe/NPG) was fabricated, serving as a high-performance electrocatalyst for ORR.

Quantifying the density and utilization of active sites in non-precious metal oxygen electroreduction catalysts

Carbon materials doped with transition metal and nitrogen are highly active, non-precious metal catalysts for the electrochemical conversion of molecular oxygen in fuel cells, metal air batteries, and electrolytic processes. However, accurate measurement of their intrinsic turn-over frequency and active-site density based on metal centres in bulk and surface has remained difficult to date, which has hampered a more rational catalyst design. Here we report a successful quantification of bulk and surface-based active-site density and associated turn-over frequency values of mono- and bimetallic Fe/N-doped carbons using a combination of chemisorption, desorption and ⁵⁷Fe Mössbauer spectroscopy techniques. Our general approach yields an experimental descriptor for the intrinsic activity and the active-site utilization, aiding in the catalyst development process and enabling a previously unachieved level of understanding of reactivity trends owing to a deconvolution of site density and intrinsic activity.

Iron and nitrogen doped carbon materials are widely studied electrocatalysts, however measurement of features such as intrinsic turn-over frequency and active site utilization has proved difficult. Here, the authors use a combination of chemisorption and spectroscopy techniques to determine these properties.

Tuning of ZIF-Derived Carbon with High Activity, Nitrogen Functionality, and Yield - A Case for Superior CO2 Capture

A highly effective and facile synthesis route is developed to create and tailor metal-decorated and nitrogen-functionalized active microporous carbon materials from ZIF-8. Clear metal- and pyrrolic-N-induced enhancements of the cyclic CO2 uptake capacities and binding energies are achieved, particularly at a much lower carbonization temperature of 700 °C than those often reported (1000 °C). The high-temperature carbonization can enhance the porosity but only at the expense of considerable losses of sample yield and metal and N functional sites. The findings are comparatively discussed with carbons derived from metal-organic frameworks (MOFs) reported previously. Furthermore, the porosity of the MOF-derived carbon is critically dependent on the structure of the precursor MOF and the crystal growth. The current strategy offers a new and effective route for the creation and tuning of highly active and functionalized carbon structures in high yields and with low energy consumption.

Computational chemistry for graphene-based energy applications: progress and challenges

Research in graphene-based energy materials is a rapidly growing area. Many graphene-based energy applications involve interfacial processes. To enable advances in the design of these energy materials, such that their operation, economy, efficiency and durability is at least comparable with fossil-fuel based alternatives, connections between the molecular-scale structure and function of these interfaces are needed. While it is experimentally challenging to resolve this interfacial structure, molecular simulation and computational chemistry can help bridge these gaps. In this Review, we summarise recent progress in the application of computational chemistry to graphene-based materials for fuel cells, batteries, photovoltaics and supercapacitors. We also outline both the bright prospects and emerging challenges these techniques face for application to graphene-based energy materials in future.

Room temperature synthesis of an amorphous MoS2 based composite stabilized by N-donor ligands and its light-driven photocatalytic hydrogen production

An amorphous molybdenum sulfide based composite has been synthesized at room temperature applying a kinetically controlled reaction. The new material exhibits an extraordinary performance in the visible light driven hydrogen evolution reaction.

Chiral modification of copper exchanged zeolite-Y with cinchonidine and its application in the asymmetric Henry reaction

(−)-Cinchonidine is being encapsulated inside copper exchanged zeolite-Y and used as heterogeneous catalyst for the asymmetric Henry reaction.