Transition Metals Embedded Two-Dimensional Square Tetrafluorotetracyanoquinodimethane Monolayers as a Class of Novel Electrocatalysts for Nitrogen Reduction Reaction

Diatomic Active Sites Embedded Graphyne as Electrocatalysts for Ammonia Synthesis

Ammonia (NH3) is a vital chemical compound in industry and agriculture, and the electrochemical nitrogen reduction reaction (eNRR) is considered a promising approach for NH3 synthesis. However, the development of eNRR faces the challenge of high overpotential and low Faradaic efficiency. In this work, graphyne (GY) is anchored by 3d, 4d, and 5d dual transition metal atoms to form diatomic catalysts (DACs) and is roundly investigated as an electrocatalyst for eNRR via density functional theory calculations. Due to the protrusion of anchored metal atoms, the active sites of GY are better exposed compared to other substrates, exhibiting higher activity. Through four-step hierarchical high-throughput screening (ΔG*N2 < 0 eV, ΔG*N2 → *N2H < 0.4 eV, ΔG*NH2 → *NH3 < 0.4 eV, and ΔG*N2 < ΔG*H), the number of selected catalysts in each step is 325, 240, 145, and 20, respectively. Considering a series of factors, including stability, initial potential, and selectivity, 13 kinds of eligible catalysts are identified. Optimal eNRR paths studies show that the best catalyst Mn2@GY features no onset potential. For the three catalysts (Mn2@GY, Ir2@GY, and RhOs@GY), the onset potentials of the most favorable eNRR pathways are -0.07, -0.12, and -0.17 V, respectively. The excellent catalytic activity can be credited to the effective charge transfer and orbital interaction between the active site and adsorbed N2. Our work demonstrates the significance of DACs for ammonia synthesis and provides a paradigm for the study of DACs even for other important reactions.

Charge-Transfer Complexes: Fundamentals and Advances in Catalysis, Sensing, and Optoelectronic Applications

Supramolecular assemblies, formed through electronic charge transfer between two or more entities, represent a rich class of compounds dubbed as charge-transfer complexes (CTCs). Their distinctive formation pathway, rooted in charge-transfer processes at the interface of CTC-forming components, results in the delocalization of electronic charge along molecular stacks, rendering CTCs intrinsic molecular conductors. Since the discovery of CTCs, intensive research has explored their unique properties including magnetism, conductivity, and superconductivity. Their more recently recognized semiconducting functionality has inspired recent developments in applications requiring organic semiconductors. In this context, CTCs offer a tuneable energy gap, unique charge-transport properties, tailorable physicochemical interactions, photoresponsiveness, and the potential for scalable manufacturing. Here, an updated viewpoint on CTCs is provided, presenting them as emerging organic semiconductors. To this end, their electronic and chemical properties alongside their synthesis methods are reviewed. The unique properties of CTCs that benefit various related applications in the realms of organic optoelectronics, catalysts, and gas sensors are discussed. Insights for future developments and existing limitations are described.

Density Functional Theory Investigation on the Nitrogen Reduction Mechanism in Two-Dimensional Transition-Metal Boride with Ordered Metal Vacancies

The creation of ordered collective vacancies in experiment proves challenging within a two-dimensional lattice, resulting in a limited understanding of their impact on catalyst performance. Motivated by the successful experimental synthesis of monolayer molybdenum borides with precisely ordered metal vacancies [Zhou et al. Science 2021, 373, 801-805] through dealloying, the nitrogen reduction reaction (NRR) in monolayer borides was systematically investigated to elucidate the influence of such ordered metal vacancies on catalytic reactions and the underlying mechanisms. The results reveal that the N-containing intermediates tend to dissociate, facilitating the NRR process with reduced UL. The emergence of ordered metal vacancies modulates the electronic properties of the catalyst and partially facilitates the decomposition of N-containing intermediates. However, the UL for NRR in Mo4/3B2 and W4/3B2 exhibits a significant increase. The compromised electrochemical performance is explained through the development of a simple electronic descriptor of the d-p band center (ΔdM-pB). Among these materials, Mo4/3Sc2/3B2 exhibits the most superior catalytic activity with a UL of -0.5 V and favorable NRR selectivity over the HER. Our results provide mechanistic insights into the role of ordered metal vacancies in transition-metal boride for the NRR and highlight a novel avenue toward the rational design of superior NRR catalysts.

The novel π-d conjugated TM2B3N3S6 (TM = Mo, Ti and W) monolayers as highly active single-atom catalysts for electrocatalytic synthesis of ammonia

Recently, single-atom catalysts (SACs) are receiving significant attention in electrocatalysis fields due to their excellent specific activities and extremely high atomic utilization ratio. Effective loading of metal atoms and high stability of SACs increase the number of exposed active sites, thus significantly improving their catalytic efficiency. Herein, we proposed a series (29 in total) of two-dimensional (2D) conjugated structures of TM2B3N3S6 (TM means those 3d to 5d transition metals) and studied the performance as single-atom catalysts for nitrogen reduction reaction (NRR) using density functional theory (DFT). Results show that TM2B3N3S6 (TM = Mo, Ti and W) monolayers have superior performance for ammonia synthesis with low limiting potentials of -0.38, -0.53 and -0.68 V, respectively. Among them, the Mo2B3N3S6 monolayer shows the best catalytic performance of NRR. Meanwhile, the π conjugated B3N3S6 rings undergo coordinated electron transfer with the d orbitals of TM to exhibit good chargeability, and these TM2B3N3S6 monolayers activate isolated N2 according to the "acceptance-donation" mechanism. We have also verified the good stability (i.e., Ef < 0, and Udiss > 0) and high selectivity (Ud = -0.03, 0.01 and 0.10 V, respectively) of the above four types of monolayers for NRR over hydrogen evolution reaction (HER). The NRR activities have been clarified by multiple-level descriptors (ΔG*N2H, ICOHP, and Ɛd) in the terms of basic characteristics, electronic property, and energy. Moreover, the aqueous solution can promote the NRR process, leading to the reduction of ΔGPDS from 0.38 eV to 0.27 eV for the Mo2B3N3S6 monolayer. However, the TM2B3N3S6 (TM = Mo, Ti and W) also showed excellent stability in aqueous phase. This study proves that the π-d conjugated monolayers of TM2B3N3S6 (TM = Mo, Ti and W) as electrocatalysts show great potentials for the nitrogen reduction.

Electrochemical ammonia synthesis under ambient conditions using TM-embedded porphine-fused sheets as single-atom catalysts

In this research, we systematically investigated the reaction mechanism and electrocatalytic properties of transition metal anchored two-dimensional (2D) porphine-fused sheets (TM-Por) as novel single-atom catalysts (SACs) for the electrochemical nitrogen reduction reaction (eNRR) under ambient conditions. Using high-throughput screening and first-principles calculations based on the density functional theory (DFT) method, three eNRR catalyst candidates, i.e. Mo-Por, Tc-Por, and Nb-Por, were screened out, with the eNRR onset potentials on them being -0.36, -0.53, and -0.74 V, respectively. Furthermore, these catalyst candidates all have good stability and selectivity. Analyzing the band structures found that these catalyst candidates all are metallic, which is needed for good electrocatalysts. Ab initio molecular dynamics (AIMD) simulations show that these catalyst candidates have good stability at 500 K. It is hoped that our work will open up new possibilities for the experimental synthesis of electrochemical ammonia catalysts.

Magnetic Moment Is an Effective Descriptor for Electrocatalytic Nitrogen Reduction Reaction on Two-Dimensional Organometallic Nanosheets

Electrocatalytic reduction of nitrogen to ammonia (eNRR) under ambient condition is a potential sustainable and promising alternative to the traditional Haber-Bosch process. However, this electrochemical transformation is limited by the high overpotential, poor selectivity, low efficiency, and low yield. Herein, a new class of two-dimensional (2D) organometallic nanosheets c-TM-TCNE (c = cross motif, TM = 3d/4d/5d transition metals, TCNE = tetracyanoethylene) were comprehensively investigated as potential electrocatalysts for eNRR through high-throughput screening combined with spin-polarized density functional theory computations. After a multistep screening and follow-up systematic evaluation, c-Mo-TCNE and c-Nb-TCNE were selected as eligible catalysts, and c-Mo-TCNE showed the lowest limiting potential of -0.35 V via a distal pathway, displaying high catalytic performance. In addition, the desorption of NH₃ from the surface of c-Mo-TCNE catalyst is also easy, with the free energy being 0.34 eV. Furthermore, the stability, metallicity, and eNRR selectivity are preeminent, making c-Mo-TCNE a promising catalyst. Unexpectedly, the magnetic moment of the transition metal shows a strong correlation with the catalytic activity (limiting potential), i.e., the larger the magnetic moment of the transition metal, the smaller the limiting potential of the electrocatalyst. The Mo atom has the largest magnetic moment and the c-Mo-TCNE catalyst features the smallest magnitude of limiting potential. Thus, the magnetic moment can be used as an effective descriptor for eNRR on c-TM-TCNE catalysts. The present study opens a way toward the rational design of highly efficient electrocatalysts for eNRR with novel two-dimensional functional materials. This work will promote further experimental efforts in this field.