Screening of Transition-Metal Single-Atom Catalysts Anchored on Covalent-Organic Frameworks for Efficient Nitrogen Fixation

Stable, while Still Active? A DFT Study of Cu, Ag, and Au Single Atoms at the C3N4/TiO2 Interface

Hybrid DFT calculations are employed to compare the adsorption and stabilization of Cu, Ag, and Au atoms on graphitic C3N4 and on the heterojunction formed by g- C3N4 and TiO2. While Cu and Ag can be strongly chemisorbed in form of cations on g- C3N4, Au is only weakly physisorbed. On g- C3N4/TiO2, all coinage metal adatoms can be strongly chemisorbed, but, while Cu and Ag forms cations, Au form an Au- species. Ab Initio Molecular Dynamics simulations confirm that the metal adatoms on g-C3N4 are highly mobile at room temperature, while they remain confined in the interfacial spacing between C3N4 and TiO2 on the heterojunction, being both stably bound and reachable for the reactants in a catalytic cycle. Doping g- C3N4/TiO2 with metal single atoms permits thus to generate catalytic systems with tunable charge and chemical properties and improved stability with respect to bare C3N4. Moreover, the changes in the electronic structure of g- C3N4/TiO2 induced by the presence of the metal single atoms are beneficial also for photocatalytic applications.

Advanced dual-atom catalysts on graphitic carbon nitride for enhanced hydrogen evolution via water splitting

In this comprehensive investigation, we explore the effectiveness of 55 dual-atom catalysts (DACs) supported on graphitic carbon nitride (gCN) for both alkaline and acidic hydrogen evolution reactions (HER). Employing density functional theory (DFT), we scrutinize the thermodynamic and kinetic profiles of these DACs, revealing their considerable potential across a diverse pH spectrum. For acidic HER, our results identify catalysts such as FePd-gCN, CrCr-gCN, and NiPd-gCN, displaying promising ΔGH* values of 0.0, 0.0, and -0.15 eV, respectively. This highlights their potential effectiveness in acidic environments, thereby expanding the scope of their applicability. Within the domain of alkaline HER, this study delves into the thermodynamic and kinetic profiles of DACs supported on gCN, utilizing DFT to illuminate their efficacy in alkaline HER. Through systematic evaluation, we identify that DACs such as CrCo-gCN, FeRu-gCN, and FeIr-gCN not only demonstrate favorable Gibbs free energy change (ΔGmax) for the overall water splitting reaction of 0.02, 0.27, and 0.38 eV, respectively, but also feature low activation energies (Ea) for water dissociation, with CrCo-gCN, FeRu-gCN, and FeIr-gCN notably exhibiting the Ea of just 0.42, 0.33, and 0.42 eV, respectively. The introduction of an electronic descriptor (φ), derived from d electron count (Nd) and electronegativity (ETM), provides a quantifiable relationship with catalytic activity, where a lower φ corresponds to enhanced reaction kinetics. Specifically, φ values between 4.0-4.6 correlate with the lowest kinetic barriers, signifying a streamlined HER process. Our findings suggest that DACs with optimized φ values present a robust approach for the development of high-performance alkaline HER electrocatalysts, offering a pathway towards the rational design of energy-efficient catalytic systems.

Single Atom Catalysts Based on Earth-Abundant Metals for Energy-Related Applications

Anthropogenic activities related to population growth, economic development, technological advances, and changes in lifestyle and climate patterns result in a continuous increase in energy consumption. At the same time, the rare metal elements frequently deployed as catalysts in energy related processes are not only costly in view of their low natural abundance, but their availability is often further limited due to geopolitical reasons. Thus, electrochemical energy storage and conversion with earth-abundant metals, mainly in the form of single-atom catalysts (SACs), are highly relevant and timely technologies. In this review the application of earth-abundant SACs in electrochemical energy storage and electrocatalytic conversion of chemicals to fuels or products with high energy content is discussed. The oxygen reduction reaction is also appraised, which is primarily harnessed in fuel cell technologies and metal-air batteries. The coordination, active sites, and mechanistic aspects of transition metal SACs are analyzed for two-electron and four-electron reaction pathways. Further, the electrochemical water splitting with SACs toward green hydrogen fuel is discussed in terms of not only hydrogen evolution reaction but also oxygen evolution reaction. Similarly, the production of ammonia as a clean fuel via electrocatalytic nitrogen reduction reaction is portrayed, highlighting the potential of earth-abundant single metal species.

Cobalt-based Polymerized Porphyrinic Network for Visible-light-driven CO2 Reduction

Visible-light-driven conversion of carbon dioxide to valuable compounds and fuels is an important but challenging task due to the inherent stability of the CO2 molecules. Herein, we report a series of cobalt-based polymerized porphyrinic network (PPN) photocatalysts for CO2 reduction with high activity. The introduction of organic groups results in the addition of more conjugated electrons to the networks, thereby altering the molecular orbital levels within the networks. This integration of functional groups effectively adjusts the levels of the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO). The PPN(Co)-NO2 exhibits outstanding performance, with a CO evolution rate of 12 268 μmol/g/h and 85.8% selectivity, surpassing most similar photocatalyst systems. The performance of PPN(Co)-NO2 is also excellent in terms of apparent quantum yield (AQY) for CO production (5.7% at 420 nm). Density functional theory (DFT) calculations, time-resolved photoluminescence (TRPL), and electrochemical tests reveal that the introduction of methyl and nitro groups leads to a narrower energy gap, facilitating a faster charge transfer. The coupling reaction in this study enables the formation of stable C-C bonds, enhancing the structural regulation, active site diversity, and stability of the catalysts for photocatalytic CO2 reduction. This work offers a facile strategy to develop reliable catalysts for efficient CO2 conversion.

Structural design of thiadiazole-based donor-acceptor COF/Fe-doped N vacancy g-C3Nx nanosheets for photocatalytic nitrogen fixation under visible light

The rational design of efficient photocatalysts to achieve artificial nitrogen fixation is an urgent challenge. Herein, we combined donor-acceptor covalent organic framework with iron-doped nitrogen vacancy graphitized carbon nitride (D-A COF/Fe-g-C3Nx) for photocatalytic nitrogen fixation. The photocatalyst exhibited good crystallinity, high porosity, and a large specific surface area. Without a sacrificial agent, the optimal 40 % D-A COF/Fe-g-C3Nx exhibited an excellent rate of ammonia production (646 μmol h-1 g-1) at 420 nm, and durable stability after successive cycling. Exhaustive experimental research and theory calculations verified that the D-A unit and Fe doping redistributed the distribution of the charge, which enhanced the visible light utilization and provided chemisorption sites for further polarization. Besides N-vacancies can serve as electron-trapping active sites to promote the directional migration of carriers. The reaction mechanism demonstrated that superoxide radical and hydrogen peroxide were formed by electron and hole, respectively, which promote the reduction of nitrogen to ammonia. This work provides a new idea for the rationalizing design of efficient catalysts for photocatalytic nitrogen fixation under mild conditions.

CO2 electroreduction on single atom catalysts: the role of the DFT functional

One key process involving single atom catalysts (SACs) is the electroreduction of CO2 to fuels. The chemistry of SACs differs largely from that of extended catalytic surfaces, presenting an opportunity to improve the ability to activate very stable molecules, such as CO2. In this work, we performed a density functional theory (DFT) study of CO2 activation on a series of SACs, focusing on the role played by the adopted functional in activity predictions. The role of the exchange-correlation functional has been widely investigated in heterogenous catalysts, but it is less explored in SACs. We tested the widely used PBE and the PBE+U corrected functionals against the more robust hybrid PBE0 functional. The results show that PBE is reliable if one is interested in qualitative predictions, but it leads to some inaccuracies in other cases. A possible way to attenuate this effect is by adopting the PBE+U framework, as it gives results that are very similar to PBE0 at an acceptable computational cost. The results of this study further underline the importance of the computational framework adopted in predicting the activity of SACs. The work suggests that one needs to go beyond PBE for quantitative estimates, an important consideration when performing screening and high-throughput calculations.

Mo-X4 (X = O, NH and S)-mediated triphenylene-based two-dimensional carbon-rich conjugate frameworks for an efficient nitrogen reduction reaction

The electrocatalytic nitrogen reduction reaction (NRR) is a highly competitive approach for the ammonia synthesis to overcome the problems of high energy consumption and air pollution by the traditional Haber-Bosch process. However, the challenges of inert N2 molecule activation and the competitive hydrogen evolution reaction (HER) restrict the real utilization of the NRR. Herein, by means of density functional theory (DFT) calculations, we proposed three two-dimensional carbon-rich conjugate frameworks (2D-CCFs) with hexa-substituted triphenylene organic linkers with a metal atom Mo and functional groups X (X = O, NH, and S), namely Mo3(HOTP)2, Mo3(HITP)2 and Mo3(THT)2, to investigate their NRR performance. Our theoretical calculations reveal that Mo atoms in 2D-CCFs can efficiently capture and activate N2 molecules. Among the three structures, Mo3(HOTP)2 exhibited the most superior performance toward the NRR with a favorable limiting potential of -0.41 V and good selectivity for the HER. Furthermore, the catalytic efficiency of 2D-CCFs can be regulated by changing the atoms X in Mo-X4 motifs, providing a new scenario for the development of highly efficient NRR catalysts.

A DFT Investigation of B-Doped C3 N as Single Atom Electrocatalysts for N2 -to-NH3 Conversion

The NH3 synthesis from N2 plays an important role in the ecological cycle and industrial production. Different from industrial NH3 synthesis with high pollution and energy consumption, electrocatalytic NH3 synthesis is favored because of its environmental protection, energy saving, ambient reaction conditions and other characteristics. However, due to the low efficiency and poor reaction selectivity of the existing electrocatalysts, which can not be used actually, the development of new electrocatalysts for nitrogen reduction reaction (NRR) is particularly urgent. Herein, we designed a series of transition metal atoms anchored B-doped defective C3 N surface (TM@B2 C3 N) as single-atom catalysts. Through the screening process of N2 adsorption activation, N2 H formation and NH3 desorption, finally the excellent electrocatalysts with strong stability and high activity (Cr@B2 C3 N and Mn@B2 C3 N) were obtained. After simulating the entire pathway, it was found that the NRR process on Cr@B2 C3 N and Mn@B2 C3 N via consecutive and distal pathways with the lowest limiting potential of -0.42 and -0.52 V, which have the good ability to inhibit hydrogen evolution reaction. Finally, the electronic properties were analyzed, and the reason for their high catalytic activity was summarized. This work provides a new idea for the rational design of NRR electrocatalysts and promotes the practical application of electrocatalysts.

A High-Throughput Screening toward Efficient Nitrogen Fixation: Transition Metal Single-Atom Catalysts Anchored on an Emerging π-π Conjugated Graphitic Carbon Nitride (g-C10N3) Substrate with Dirac Dispersion

TM-N_x is becoming a comforting catalytic center for sustainable and green ammonia synthesis under ambient conditions, resulting in increasing interest in single-atom catalysts (SACs) for the electrochemical nitrogen reduction reaction (NRR). However, given the poor activity and unsatisfactory selectivity of existing catalysts, it remains a long-standing challenge to design efficient catalysts for nitrogen fixation. Currently, the two-dimensional (2D) graphitic carbon-nitride substrate provides abundant and evenly distributed holes for stably supporting transition-metal atoms, which presents a fascinating prospect for overcoming this challenge and promoting single-atom NRR. An emerging holey graphitic carbon-nitride skeleton with a C₁₀N₃ stoichiometric ratio (g-C₁₀N₃) from a supercell of graphene is constructed, which provides outstanding electric conductivity for achieving high-efficiency NRR due to the Dirac band dispersion. Herein, a high-throughput first-principles calculation is carried out to evaluate the feasibility of π-d conjugated SACs resulting from a single TM atom anchored on g-C₁₀N₃ (TM = Sc-Au) for NRR. We find that W metal embedded in g-C₁₀N₃ (W@g-C₁₀N₃) can compromise the ability to adsorb the key target reaction species (N₂H and NH₂), hence acquiring an optimal NRR behavior among 27 TM-candidates. Our calculations demonstrate that W@g-C₁₀N₃ shows a well-suppressed HER ability and, impressively, a low energy cost of -0.46 V. Additionally, all-around descriptors are proposed to uncover the fundamental mechanism of NRR activity, among which a 3D volcano plot (limiting potential, screening strategy, and electron origin) uncovers the NRR activity trend, achieving a quick and high-efficiency prescreening for numerous candidates. Overall, the strategy of the structure- and activity-based TM-N_x-containing unit design will offer useful insight for further theoretical and experimental attempts.

Metalated covalent organic frameworks: from synthetic strategies to diverse applications

Covalent organic frameworks (COFs) are a class of organic crystalline porous materials discovered in the early 21st century that have become an attractive class of emerging materials due to their high crystallinity, intrinsic porosity, structural regularity, diverse functionality, design flexibility, and outstanding stability. However, many chemical and physical properties strongly depend on the presence of metal ions in materials for advanced applications, but metal-free COFs do not have these properties and are therefore excluded from such applications. Metalated COFs formed by combining COFs with metal ions, while retaining the advantages of COFs, have additional intriguing properties and applications, and have attracted considerable attention over the past decade. This review presents all aspects of metalated COFs, from synthetic strategies to various applications, in the hope of promoting the continued development of this young field.