N2 Reduction Reaction Research Articles

Embedding Double Transition Metal Atoms in B-Modified Two-Dimensional Carbon-Rich Conjugated Frameworks for Efficient Ammonia Synthesis.

It is important to develop an efficient green catalyst for electrochemical nitrogen reduction under environmental conditions to replace the Haber-Bosch process with high energy consumption, limited economic scale of production, and high CO2 emissions, but there are still some difficulties in its development and design. In this work, based on 2D CCFs, a B-modified rectangular-shaped expanded phthalocyanine bimetallic atomic catalyst is proposed. Because both the TM-d orbital and the B-sp3 hybrid orbital can accept the lone pair and feedback electrons of N2, and there are significantly different valence electron distributions between the two different metals and boron, this catalyst not only keep the advantages of metal catalysts but also play the role of the B atom to further increase the polarity of nitrogen and activate N2. In order to evaluate the catalytic performance of this catalyst, its stability, activity, selectivity, reactivity, and other aspects are screened and analyzed by density functional theory calculation. Among them, the FeNb-BPc CCF catalyst has better N2 adsorption capacity under ambient conditions, exhibits excellent catalytic activity through a continuous mechanism, and has a lower limiting potential of -0.24 V. We believe that this work opens a new path for the rational design of efficient N2 reduction reaction catalysts and also provides a theoretical basis for experimental development.

Fe-doped Mo2C for boosting electrocatalytic N2 reduction

• Our work designed the synthesis of 5 %Fe-doped Mo 2 C a and its catalytic performance for the NRR. • 5 %Fe-doped Mo 2 C achieves a high Faradic efficiency of 10.3 % and NH 3 yield 36.6 μg∙h −1 ∙mg −1 at 0.3 V vs RHE. • Density functional (DFT) calculations of the rate-determining step energy barriers for these catalysts further show that the 5 % Fe-doped Mo 2 C catalyst has a lower rate-determining step energy barrier (0.84 eV). • Mo 2 C, 5% Fe-doped Mo 2 C and 11.1% Fe-doped Mo 2 C catalysts for ammonia synthesis were studied theoretically and experimentally. Electrocatalytic fixation of N 2 is highlighted as a carbon-free route to generate NH 3 , which strongly demands highly efficient and durable electrocatalysts. We explored the performance of Fe-doped-Mo 2 C catalysts for ammonia nitrogen reduction synthesis. We demonstrate that 5 %Fe-doped Mo 2 C can be an effective and durable catalyst for the electrocatalytic N 2 reduction reaction (NRR). 5 %Fe-doped Mo 2 C achieves a remarkable NH 3 yield of 36.6 μg∙h −1 ∙mg −1 and high faradaic efficiency of 10.3 % at 0.3 V vs RHE, far superior to Mo 2 C nanosheets and outperforming most reported NRR catalysts. Density functional theory (DFT) shows that 5 %Fe-doped Mo 2 C activates N 2 molecule better than Mo 2 C and 11.1 %Fe-doped Mo 2 C, which is reflected by its lower energy barrier (0.84 eV). This research provides an attractive non-noble-metal catalyst for electrocatalytic NH 3 synthesis for efficient electrocatalytic N 2 fixation.

Recent progress in electrochemical synthesis of carbon-free hydrogen carrier ammonia and ammonia fuel cells: A review

Ammonia (NH3) is a cornerstone widely used in the modern agriculture and industry, the annual global production gradually increases to almost 200 million tons. Nearly 80% of the produced NH3 is used in the fertilizer industry and is essential for the development of global agriculture and consequently for maintaining population growth. Furthermore, NH3 can power hydrogen (H2) fueled devices, such as H2 fuel cells (FC), to use the interconversion between chemical energy and electric energy of nitrogen (N2) cycle, which can effectively alleviate the intermittent problems of renewable energy. However, the problems faced by NH3 in storage and release still restrict its development. Herein, this review introduces the latest research and development of electrochemical NH3 synthesis and direct NH3 FC, as well as outlines the technical challenges, possible improvement measures and development perspectives. N2 reduction reaction (NRR) and nitrate reduction reaction (NO3−RR) are two potential approaches for electrochemical NH3 synthesis. However, the existing research foundation still faces challenges in achieving high selectivity and efficiency. Direct NH3 FC are easy to transport and are expected to be widely used in mobile energy consuming equipment, but also limited by the lack of highly active and stable NH3 oxidation electrocatalysts. The perspectives of ammonia fuel cells as an alternative green energy are discussed.

V (Nb) Single Atoms Anchored by the Edge of a Graphene Armchair Nanoribbon for Efficient Electrocatalytic Nitrogen Reduction: A Theoretical Study.

Efficient and low-cost electrocatalysts are urgently required for the electrocatalytic N2 reduction reaction (NRR) to produce valuable NH3. Single-atom catalysts (SACs) represent one class of promising candidates. Besides the defects on the basal plane, very recently, the one-dimensional edge universally existing in the finite graphene or carbon sheet has gained attention as the anchoring site for SACs, which may enable unique catalytic mechanism. Herein, using first-principles calculations, we systematically investigated the NRR over SACs of transition metals (Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Ru, Rh, Pd, Nb, Mo, and W) anchored by the N-modified edge of the graphene armchair nanoribbon (denoted as TM@GNR). Three criteria were employed to screen the best candidate from all the TM@GNR, including the high stability of TM@GNR, the preferable adsorption of N2 compared with H, and the lower applied potential for the first protonation of N2 compared with that of the active site. Accordingly, V(Nb)@GNR were theoretically demonstrated to be promising NRR electrocatalyst toward NH3 with low limiting potentials of -0.65 (-0.52) V, excellent selectivity of ∼100% (97%), and good stability. Particularly, NRR on the V@GNR and Nb@GNR precedes through a novel reaction mechanism with three spectator N2 molecules. Further analysis reveals that the strong capture and activation of N2 molecules by the edge-anchored V (Nb) atoms derives from their localized spin moment and atomic orbitals. Our studies emphasize the great potential of the edge of carbon materials to synthesize SACs for NRR and other reactions, and further reveal a novel NRR reaction mechanism on SACs.

A Brief Assessment on Recent Developments in Efficient Electrocatalytic Nitrogen Reduction with 2D Non-Metallic Nanomaterials.

In recent years, the synthesis of ammonia (NH3) has been developed by electrocatalytic technology that is a potential way to effectively replace the Haber–Bosch process, which is an industrial synthesis of NH3. Industrial ammonia has caused a series of problems for the population and environment. In the face of sustainable green synthesis methods, the advantages of electrocatalytic nitrogen reduction for synthesis of NH3 in aqueous media have attracted a great amount of attention from researchers. This review summarizes the recent progress on the highly efficient electrocatalysts based on 2D non-metallic nanomaterial and provides a brief overview of the synthesis principle of electrocatalysis and the performance measurement indicators of electrocatalysts. Moreover, the current development of N2 reduction reaction (NRR) electrocatalyst is discussed and prospected.

A‐Site Substitution Motivating the Phase Transformation of LaNiO3 Perovskite for High‐Performance Artificial N2 Fixation

AbstractElectrocatalytic N2 reduction reaction (NRR) to NH3 under ambient conditions is an attractive strategy compared to the traditional Haber‐Bosch process. However, there still remains a great challenge due to the high activation barrier of N2 and consequent sluggish kinetics. Herein, A‐site substitution of LaNiO3 perovskites by Sr‐doping is proposed which can lead to phase transformation and to the formation of oxygen vacancies, significantly improving the NRR performance of La1‐xSrxNiO3 (LSNO). The appropriate oxygen vacancy concentration can be modulated by changing the stoichiometric ratio of La and Sr elements. LSNO‐III (x=0.5) delivers the highest ammonia production rate of 55.9 μg h−1 mg−1cat. at −0.7 V and Faraday efficiency of 23.17 % at −0.5 V versus the reversible hydrogen electrode in 0.1 M Na2SO4. Density functional theory (DFT) calculations further demonstrate that introduction of oxygen vacancies decreases the distance of the d‐band center to the Fermi energy level, which not only facilitates adsorption and activation of N2 molecules and subsequent hydrogenation reactions, but also further optimizes reaction pathways in NRR.

Synergize curvature and confinement effects for Fe-, Co-, Ni- N2 sites on graphene nanobuds towards eNRR

Exploring efficient and highly selective ammonia synthesis electrocatalysts can advance the development of both modern agriculture and industry. Graphene nanobuds (GN) feature both fullerene and graphene characteristics which inspires new design ideas of electrocatalysts for N2 reduction reaction (NRR) to ammonia based on the novel GN with traditional Fe-, Co-, Ni-N2 sites. Among these structures, CoN2-GN is investigated as the most promising candidate with a limiting potential of -0.83 V through a distal pathway. The excellent activity and selectivity for the eNRR can be attributed to synergistic effects originated from both confinement space of buds (fullerene fragment) and curvature of the junctions. Especially, acting as an H-activator to N2, the ‘neck’ C atoms in the GNs improve the selectivity significantly for eNRR and facilitate the subsequent processes. This research would greatly contribute to develop the novel, efficient, multi-component graphene-based eNRR catalysts.

Theoretical Study on the Efficient Electrocatalytic N2 Reduction Reaction of Bimetallic Single Atom Embedded in Phthalocyanine

Theoretical Study on the Efficient Electrocatalytic N2 Reduction Reaction of Bimetallic Single Atom Embedded in Phthalocyanine

Atomic-Scale Homogeneous RuCu Alloy Nanoparticles for Highly Efficient Electrocatalytic Nitrogen Reduction.

Ruthenium (Ru) is the most widely used metal as an electrocatalyst for nitrogen (N2 ) reduction reaction (NRR) because of the relatively high N2 adsorption strength for successive reaction. Recently, it has been well reported that the homogeneous Ru-based metal alloys such as RuRh, RuPt, and RuCo significantly enhance the selectivity and formation rate of ammonia (NH3 ). However, the metal combinations for NRR have been limited to several miscible combinations of metals with Ru, although various immiscible combinations have immense potential to show high NRR performance. In this study, an immiscible combination of Ru and copper (Cu) is first utilized, and homogeneous alloy nanoparticles (RuCu NPs) are fabricated by the carbothermal shock method. The RuCu homogeneous NP alloys on cellulose/carbon nanotube sponge exhibit the highest selectivity and NH3 formation rate of ≈31% and -73μmol h-1 cm-2 , respectively. These are the highest values of the selectivity and NH3 formation rates among existing Ru-based alloy metal combinations.

Regulating the interfacial charge transfer and constructing symmetry‐breaking sites for the enhanced N2 electroreduction activity

AbstractThe Haber–Bosch process for industrial NH3 production is energy‐intensive with heavy CO2 emissions. Electrochemical N2 reduction reaction (NRR) is an attractive carbon‐neutral alternative for NH3 synthesis, while the challenge associated with N2 activation highlights the demand for efficient electrocatalysts. Herein, we demonstrate that PdCu nanoparticles with different Pd/Cu ratios anchored on boron nanosheet (PdCu/B) behave as efficient NRR electrocatalysts toward NH3 synthesis. Theoretical and experimental results confirm that the highly efficient NH3 synthesis can be achieved by regulating the charge transfer between interfaces and forming a symmetry‐breaking site, which not only alleviates the hydrogen evolution but also changes the adsorption configuration of N2 and thus optimizes the reaction pathway of NRR over the separated Pd sites. Compared with monometallic Pd/B and Cu/B, the PdCu/B with the optimized Pd/Cu ratio of 1 exhibits superior activity and selectivity for NH3 synthesis. This study provides new insight into developing efficient catalysts for small energy molecule catalytic conversion via regulating the charge transfer between interfaces and constructing symmetry‐breaking sites.

Controlled etching to immobilize highly dispersed Fe in MXene for electrochemical ammonia production

AbstractElectrocatalytic N2 reduction reaction (NRR), as an efficient and low‐carbon NH3 production method, is highly desired to replace the traditional Haber–Bosch process under ambient conditions. Nevertheless, its industrial application remains hampered by the low Faradaic efficiency (FE) and an inferior ammonia yield mainly owing to the sluggish kinetics and the competing hydrogen evolution reaction (HER). Here, highly dispersed Fe immobilized in Ti3C2Tx MXene (HD‐Fe‐MXene) is for the first time developed by a controlled etching strategy to act as an efficient electrochemical ammonia production catalyst. In this process, the Al phase of MAX is firstly replaced by Fe through Lewis‐acidic‐melt selective etching, then the HD‐Fe‐MXene is obtained by etching with an HCl solution. The as‐prepared HD‐Fe‐MXene delivers outstanding NRR activity with an FE of 21.8% at −0.1 V versus reversible hydrogen electrode (RHE) and a favorable NH3 yield of 18.25 µg h−1 mg−1 (−0.25 V vs. RHE) in 0.1 M Na2SO4 electrolyte. Notably, HD‐Fe‐MXene exhibits superior stability during the six times recycling tests and the 24 h chronoamperometric test. This excellent NRR electrocatalytic activity is mainly ascribed to the highly dispersed Fe immobilized in the fluorine‐free Ti3C2Tx MXene, which inhibits Fe centers from aggregation for providing abundant active sites for NRR. Moreover, the two‐dimensional structure and the fluorine‐free property of Ti3C2Tx MXene benefit to provide abundant sites for N2 adsorption and promote the electron transfer thus boosting the NRR process. This study delivers a feasible strategy for adding transition metal into the MXene phase to enhance the electrocatalytic activity of MXene‐based catalysts for nitrogen electroreduction to ammonia.

Point‐to‐face contact heterojunctions: Interfacial design of 0D nanomaterials on 2D g‐C3N4 towards photocatalytic energy applications

AbstractGreen energy generation is an indispensable task to concurrently resolve fossil fuel depletion and environmental issues to align with the global goals of achieving carbon neutrality. Photocatalysis, a process that transforms solar energy into clean fuels through a photocatalyst, represents a felicitous direction toward sustainability. Eco‐rich metal‐free graphitic carbon nitride (g‐C3N4) is profiled as an attractive photocatalyst due to its fascinating properties, including excellent chemical and thermal stability, moderate band gap, visible light‐active nature, and ease of fabrication. Nonetheless, the shortcomings of g‐C3N4 include fast charge recombination and limited surface‐active sites, which adversely affect photocatalytic reactions. Among the modification strategies, point‐to‐face contact engineering of 2D g‐C3N4 with 0D nanomaterials represents an innovative and promising synergy owing to several intriguing attributes such as the high specific surface area, short effective charge‐transfer pathways, and quantum confinement effects. This review introduces recent advances achieved in experimental and computational studies on the interfacial design of 0D nanostructures on 2D g‐C3N4 in the construction of point‐to‐face heterojunction interfaces. Notably, 0D materials such as metals, metal oxides, metal sulfides, metal selenides, metal phosphides, and nonmetals on g‐C3N4 with different charge‐transfer mechanisms are systematically discussed along with controllable synthesis strategies. The applications of 0D/2D g‐C3N4‐based photocatalysts are focused on solar‐to‐energy conversion via the hydrogen evolution reaction, the CO2 reduction reaction, and the N2 reduction reaction to evaluate the photocatalyst activity and elucidate reaction pathways. Finally, future perspectives for developing high‐efficiency 0D/2D photocatalysts are proposed to explore potential emerging carbon nitride allotropes, large‐scale production, machine learning integration, and multidisciplinary advances for technological breakthroughs.

Transition metal atoms anchored on nitrogen-doped α-arsenene as efficient electrocatalysts for nitrogen electroreduction reaction

Electrocatalytic reduction of N2 to NH3 under ambient conditions, inspired by biological nitrogen fixation, is a new approach to address the current energy shortage crisis. As a result, developing efficient and low-cost catalysts is critical. The catalytic activity, catalytic mechanism, and selectivity of α-arsenene (α-Ars) catalysts anchored with various transition metal atoms and doped with different numbers of N atom were investigated for N2 reduction reaction (NRR) in this paper. Results reveal that compared with WN3-α-Ars which is coordinated with three N atoms, asym-WN2As-α-Ars that coordinated with two N atoms not only exhibits high catalytic activity (UL = −0.36 V), but can also successfully suppress the hydrogen evolution reaction (HER). It is manifested that reducing the number of coordination atoms can promote the selectivity of the transition metal (TM) loaded N-doped arsenene catalysts. Furthermore, activity origin analyses show both the charge on ∗N–NH and φ form volcano-type relationship with the limiting potential. The active center of the catalyst, which acts as the charge transporter and has the moderate ability to retrieve charges, is the most efficient in NRR. Overall, this research creates high performance NRR catalysts by varying the number of coordinating N atoms, which provides a novel idea for the development of new NRR catalysts.

An integrated Si photocathode with lithiation-activated molybdenum oxide nanosheets for efficient ammonia synthesis

As an alternative to the conventional industrial Haber-Bosch process, photoelectrochemical (PEC) routes that are powered by renewable solar energy hold great promise for N2 reduction reaction (NRR) towards NH3 synthesis at ambient conditions. However, great challenges remain in promoting NH3 production rate for the PEC NRR devices, especially with the earth-abundant catalysts. Here we report an integrated LixMoO3/n+np+-Si photocathode could achieve an unprecedented PEC NH3 yield rate of 8.7 μg cm−2 h−1, which is among the highest PEC NRR systems ever reported. With an optically and electrocatalytically decoupled configuration, the integrated PEC photocathode could harvest the sunlight sufficiently and simultaneously promote the catalytic kinetics, thus leading to the improved NH3 synthesis. More importantly, such high PEC NRR performance is derived from earth-abundant elements without precious noble metals. Verified by the electrochemical experiments and density functional theory (DFT) calculations, the lithiation strategy gives rise to dramatic structural distortion accompanying the abundant oxygen vacancies and Mo5+ ions, which results in faster NRR kinetics and activates inert MoO3 into efficient LixMoO3 electrocatalyst towards NH3 synthesis. This work holds great promise in constructing monolithic PEC device to directly harvest solar light for artificial ammonia photosynthesis.

Single Nb atom modified anatase TiO2(110) for efficient electrocatalytic nitrogen reduction reaction

We report the theory-guided design of anatase-supported Nb catalysts for electrochemical N 2 reduction reaction (NRR). Theoretical calculations predict that Nb atoms deliver multi-functional enhancement toward the NRR when incorporated in an anatase TiO 2 (110) catalyst: (1) decreasing the band gap and inducing electrons to promote the conductivity of TiO 2 (110); (2) suppressing the undesired competitive hydrogen evolution reaction; (3) activating the inert Ti sites for N 2 adsorption; (4) enabling fast charge transfer between ∗NNH and the TiO 2 (110) surface; and (5) reducing the energy barrier of the potential-determining ∗N 2 → ∗NNH step, further facilitating NH 3 formation. As a result, our Nb-TiO 2 (110) catalyst exhibits superior activity and selectivity for the NRR, which affords an NH 3 production rate of about 21.3 μg h −1 mg cat −1 and NH 3 faradaic efficiency of ∼9.2% at −0.5 V (versus reversible hydrogen electrode). This study provides insights for the rational design of efficient electrocatalysts for the NRR. • Nb single-atom-decorated TiO 2 (110) facilitates nitrogen reduction reaction (NRR) • TiO 2 (110) is more active than TiO 2 (101) for NRR • Nb-TiO 2 (110) delivers an NH 3 production rate of ∼21.3 μg h −1 mg cat −1 • Single Nb atom modified anatase TiO 2 (110) for efficient electrocatalytic nitrogen reduction reaction An electrochemical N 2 reduction reaction (NRR) that can be powered by electricity generated from renewable sources has recently gained heightened research interest, providing a promising alternative to the conventional Haber-Bosch process. Current major research efforts are being devoted to the design and development of advanced electrocatalysts to enhance the efficiency of NRR. Herein, guided by density functional theory calculations that predict the cooperative effect of Nb and anatase TiO 2 (110) in promoting electrocatalytic NRR performance, we successfully synthesize TiO 2 single crystals with selectively exposed (110) facets, supplemented with Nb atomic loading, which exhibited remarkable performance for NRR, achieving an NH 3 production rate of ∼21.3 μg h −1 mg cat −1 at −0.5 V (versus reversible hydrogen electrode). Ammonia is extensively used in industry, agriculture, and energy storage. Currently, the industrial synthesis of ammonia predominantly still relies on the energy- and capital-intensive Haber-Bosch (HB) process. Electrochemical N 2 reduction reaction (NRR) provides a renewable and distributed route for ammonia production. To boost the NRR, the development of electrocatalysts is key. Herein, we design and synthesize Nb single-atom-decorated anatase TiO 2 (110) for enhanced NRR, affording an NH 3 production rate of ∼21.3 μg h −1 mg cat −1 .

Nano-CaCO3 templated porous carbons anchored with Fe single atoms enable high-efficiency N2 electroreduction to NH3

The traditional Haber-Bosch N2 fixation process suffers from rigorous reaction conditions and massive CO2 emission. By contrast, N2 electroreduction by water to the direct production of NH3 can be conducted at room temperature and atmospheric pressure, and thus has been attracting increasingly attention. Herein, we develop a nano-CaCO3 template route for the synthesis of Fe, N co-doped porous carbon materials from zeolitic imidazolate framework-8 (ZIF-8). The Fe atoms are atomically dispersed in the skeleton of carbon and bonded with four nitrogen atoms. Interestingly, the morphology and pore structure of these carbons have notable effects on their performance for the electrocatalytic reduction of N2. With an optimized morphology and pore structure tuned by the addition of nano-CaCO3 template, the ZIF-8 derived Fe, N co-doped carbon catalyst shows a Faradaic efficiency of 13.14% and a NH3 yield as high as 13.01 µg h−1 mgcat−1 (−0.2 vs. reversible hydrogen electrode) for N2 reduction reaction in 0.1 M KOH aqueous electrolyte. Morphology engineering strategy may open a new path to designing and synthesizing high-performance carbon-based catalysts for the electroreduction of N2 to NH3.

Fe‐VS2 Electrocatalyst with Organic Matrix‐Mediated Electron Transfer for Highly Efficient Nitrogen Fixation

Electrochemical N2 fixation is considered to be a promising alternative to Haber-Bosch technology. Inspired by the composition and structure of natural nitrogenase, Fe-doped VS2 nanosheets were prepared via one-step solvothermal method. The electron transfer system mediated by organic conductive polymer (1-AAQ-PA) was constructed to promote the electron transfer between Fe-VS2 nanosheets and the electrode in electrocatalytic N2 reduction reaction (NRR). The obtained 1-AAQ-PA-Fe-VS2 electrode converted N2 to NH3 with a yield of 31.6 μg h-1 mg-1 at -0.35 V vs. reversible hydrogen electrode and high faradaic efficiency of 23.5 %. The introduction of Fe dopants favored N2 adsorption and activation, while the Li-S bond between Fe-VS2 and Li2 SO4 effectively inhibited hydrogen evolution. The highly efficient electron utilization in the electrocatalytic NRR process was realized using the 1-AAQ-PA as the electron transfer medium. Density functional theory calculations showed that N2 was preferentially adsorbed on Fe and reduced to NH3 via both distal and alternating mechanism.

Hydrophobicity modulation on a ferriporphyrin-based metal–organic framework for enhanced ambient electrocatalytic nitrogen fixation

Electrocatalytic N2 fixation represents an energy-efficient and long-term sustainable approach, which can convert N2 to NO3– or NH3 via the electrochemical N2 oxidation reaction (NOR) or N2 reduction reaction (NRR). However, the inert N2 molecule, low activity of electrocatalysts, and predisposed competitive reactions result in the poor yields and Faradaic efficiencies of N2 fixation reactions, which greatly restrict the application of such green synthesis technology. In this work, a molecular-level post-modification strategy has been explored to integrate diverse alkyl chains on a ferriporphyrin-based metal–organic framework (MOF) PCN-222(Fe), which provides adjustable hydrophobicity and highly dispersed active sites. The increased lengths of alkyl groups can gradually improve the hydrophobicity of decorated MOFs, which effectively suppress the competitive reactions and boost the electrocatalytic NOR and NRR performances. Significantly, the highest Faradaic efficiency of 70.7% so far and a state-of-the-art NO3− yield of 110.9 μg h–1 mgcat.–1 can be achieved for NOR, which are attributed to the synergistic effect of FeN4 active sites, high porosity, and strong hydrophobicity for n-octadecylphosphonic acid (OPA) decorated PCN-222(Fe).

Mechanistic Study on Enhanced Electrocatalytic Nitrogen Reduction Reaction by Mo Single Clusters Supported on MoS2.

The electrocatalytic N2 reduction reaction (eNRR) at ambient conditions is an appealing method for NH3 synthesis. It has attracted broad research interest in eNRR catalysts. In this work, by a theoretical study based on density functional calculations, we attributed the higher eNRR activity of defective MoS2 than pure MoS2 to the exposed Mo atom with unsaturated coordination sites in the interlayer of defective MoS2. The finding inspired us to explore the eNRR performance of Mo single atom/clusters with one/more active Mo sites supported on MoS2 [Mon@MoS2 (n = 1∼11)] and the corresponding catalytic mechanism. All considered Mon@MoS2 irrespective of N2 or H adsorption selectivity can achieve higher eNRR activity with lower overpotential and lower NH3 desorption free energy than defective MoS2. The competitive hydrogen evolution reaction can be well suppressed on Mon@MoS2 when n = 2∼10. In particular, Mo9@MoS2 with N2 adsorption selectivity exhibits excellent eNRR activity (η = 0.19 V) and high eNRR selectivity, and it can efficiently desorb the produced NH3 with a low desorption free energy (0.50 eV) to achieve a high ammonia yield with the aid of the produced ammonia molecule in the first eNRR process, which is coadsorbed on the Mo9 single cluster during the later eNRR process. The high eNRR activity of Mon@MoS2 can be attributed to its inherent properties of excellent electrical conductivity, electron accessibility, and multiple exposed Mo active sites available for N-containing species coadsorption. The results demonstrate the significance of H preadsorption, the additional N2 adsorption, and the adsorbed product ammonia in the prior eNRR process in enhancing the overall eNRR performance of different-size single-cluster catalysts. Our work provides a guidance for future study of single-cluster catalysts.

Recent Advances in Application of Graphitic Carbon Nitride-Based Catalysts for Photocatalytic Nitrogen Fixation.

Ammonia, the second most-produced chemical, is widely used in agricultural and industrial applications. However, traditional industrial ammonia production dominated by the Haber-Bosch process presents huge resource and environment issues due to the massive energy consumption and CO2 emission. The newly emerged nitrogen fixation technology, photocatalytic N2 reduction reaction (p-NRR), uses clean solar energy with zero-emission, holding great prospect to achieve sustainable ammonia synthesis. Although great efforts are made, the p-NRR catalysts still suffer from poor N2 adsorption and activation, inferior light absorption, and fast recombination of photocarriers. Due to the tunable electronic structure of the metal-free polymeric graphitic carbon nitride (g-C3 N4 ), the above-mentioned issues can be significantly alleviated, making it the most promising p-NRR photocatalyst. This review summarizes the recent development of g-C3 N4 -based catalysts for p-NRR, including the working principle of p-NRR catalysts, the challenges of developing p-NRR catalysts, and corresponding solutions. Particularly, the roles of defect engineering and heterojunction construction on g-C3 N4 to the enhancement of photocatalytic performances are emphasized. In addition, computational studies are introduced to deepen the understanding of reaction pathways. At last, perspectives are provided on the development of p-NRR catalysts.