Harnessing halogen bond donors for enhanced nitrogen reduction: a case study on metal-free boron nitride single-atom catalysts.

Developing efficient catalysts for ammonia synthesis is increasingly crucial but remains a formidable challenge due to the lack of robust design criteria, particularly in addressing the activity and selectivity issues, especially in electrochemical nitrogen reduction reactions (NRR). In this study, we systematically investigated the catalytic potential of hexagonal boron nitride (BN) embedded with non-metal (C, Si, P and S) atoms as an electrocatalyst for the nitrogen reduction reaction using density functional theory (DFT) computations. The preference for non-metal-doped BN nanomaterials stems from their ability to suppress hydrogen evolution and their environmentally friendly nature, in contrast to transition metals. Among the designed single-atom catalysts (SACs), Si-doped boron nitride (SiBBN) exhibits a favorable inclination toward activating nitrogen, which is determined by the combination of advantageous molecular orbital coupling and formation of a covalent bond with the N2 molecule. Under thermal conditions, the first protonation step emerges as the rate-determining step (22.66 kcal mol-1) for SiBBN. Conversely, under electrochemical conditions, the final elementary step becomes the potential-determining step (PDS) with 2.38 eV. We explored the impact of the exogenous addition of Lewis acids (alkali metal ions, neutral boron Lewis acids, and halogen bond donors) on modulating the electrochemical NRR activity. Our results highlight the pivotal role of halogen bond donors as catalytic promoters in facilitating electron density transfer through activated N2, establishing a push-pull charge transfer mechanism that populates the distal nitrogen more than the proximal nitrogen. This facilitates the potential requirements for the first reduction step. The synergistic effect of both halogen bonding and hydrogen bonding interactions in the final reduction step was proven to be the main determinant for a significant reduction in the PDS from 2.38 to 0.10 V. Notably, this study unveils the pioneering role of halogen bond donors as promoters for NRR, providing valuable insights into the development of robust metal-free catalysts and promoters in experimental research.

Acetylene-Mediated Borophosphene Dirac Materials as Efficient Anode Materials for Lithium-Ion Batteries.

Generally, graphynes have been generated by the insertion of acetylenic content (-C≡C-) in the graphene network in different ratios. Also, several aesthetically pleasing architectures of two-dimensional (2D) flatlands have been reported with the incorporation of acetylenic linkers between the heteroatomic constituents. Prompted by the experimental realization of boron phosphide, which has provided new insights on the boron-pnictogen family, we have modelled novel forms of acetylene-mediated borophosphene nanosheets by joining the orthorhombic borophosphene stripes with different widths and with different atomic constituents using acetylenic linkers. Structural stabilities and properties of these novel forms have been assessed using first-principles calculations. Investigation of electronic band structure elucidates that all the novel forms show the linear band crossing closer to the Fermi level at Dirac point with distorted Dirac cones. The linearity in the hole and electronic bands impose the high Fermi velocity to the charge carriers close to that of graphene. Finally, we have also unravelled the propitious features of acetylene-mediated borophosphene nanosheets as anodes in Li-ion batteries.

Defect Engineered Ternary Spinel: An Efficient Cathode for an Aqueous Rechargeable Zinc-Ion Battery of Long-Term Cyclability.

The rational defect engineering of Mn-based spinel cathode materials by metal-ion doping and vacancy creation fosters reversible intercalation/deintercalation of charge carriers and boosts the charge storage performance of an aqueous rechargeable zinc-ion battery (ZIB). Herein, we demonstrate the Zn2+ ion storage performance of a defect-engineered ternary spinel cathode based on Zn, Ni, and Mn. The defect engineering of ZnMn2O4 is achieved by Ni2+ doping and creating a cation (Mn3+ and Zn2+) deficiency. The engineered cathode material has cubic spinel structure in contrast to the defect-free ZnMn2O4. The DFT studies show that the defect engineering modifies the electronic structure and improves the electronic conductivity. An aqueous rechargeable ZIB is fabricated by using the spinel cathode, and its performance is evaluated in terms of charge-discharge cycling stability, specific capacity, and so on. The ternary spinel-based ZIB has a very long charge-discharge cycling stability with a specific capacity as high as 265 mAh g-1 (at 100 mA g-1). A 2-fold enhancement in the specific capacity is observed after 5000 cycles. Ni doping affords ultralong cycling stability. The self-discharge studies for a year show that the device retains 63% of the initial performance.

Cascade aryne insertion/vinylogous aldol reaction of vinyl-substituted ß-keto/enol carbonyls.

Cyclic and acyclic vinyl substituted ß-keto/enol carbonyl substrates, on reaction with arynes, result in differentially substituted naphthyl carbocycles, hitherto difficult to synthesize with existing protocols. While the substitutions on the arynes have no role, the ring size of the cyclic ß-keto/enol esters has a profound influence on the product formation.

Diastereoselective access to [4,4]-carbospirocycles: governance of thermodynamic enolates with an organocatalyst in vinylogous cascade annulation.

A vinylogy concept driven annulation strategy is developed to access [4,4]-carbospirocycles from alkylidene malononitriles and cyclopentene-1,3-diones. The reaction is catalyzed by an inexpensive organocatalyst and products with three stereocenters were obtained as a single diastereomer in high yields. The spiro-selectivity originates from the reaction of the thermodynamic enolate intermediate which is fundamentally intriguing.

Unorthodox cascade reaction of arynes and N-nitrosamides leading to indazole scaffolds.

An unusual cascade annulation of arynes with N-alkyl-N-nitrosamides is developed by leveraging aryne σ-insertion and C(sp3)-H bond functionalization strategies under transition-metal-free conditions at ambient temperature, offering functionalized indazoles in high yields and regioselectivity. The protocol is scalable and exhibits a broad substrate scope. The reaction mechanism is also studied with DFT calculations.

Nitrogen Fixation at the Edges of Boron Nitride Nanomaterials: Synergy of Doping.

Synthesis of ammonia at ambient conditions is very demanding yet challenging to achieve due to the production of ammonia fuel, which is considered to be a future fuel for sustainable energy. In this context, computational studies on the catalytic activity of the edge sites of boron nitride nanomaterials for possible nitrogen reduction into ammonia have been investigated. Geometrical and electronic properties of zigzag and armchair B-open edges of BN sheet (BOE) models have been unraveled to substantiate their catalytic nature. Results reveal that BOE sites exhibit very high potential determining steps (PDS) of 2.0 eV. Doping of carbon (C) at the nitrogen center, which is vicinal to the BOE site reduces the PDS of the N2 reduction reaction (NRR) (to 1.18-1.33 eV) due to the regulation of charge distribution around the active BOE site. Further, the NRR at the C doped at various edge sites of a boron nitride sheet (BNS) has also been studied in detail. Among the 12 new C-doped defective BNS models, 9 model catalysts are useful for nitrogen activation through either chemisorption or physisorption. Among these, ZC N , AC N , and ZC BV models are efficient in catalyzing NRR with lower PDS of 0.86, 0.88, and 0.86 eV, respectively. The effect of carbon doping in tuning the potential requirements of NRR has been analyzed by comparing the relative stability of intermediates on the catalyst with and without carbon doping. Results reveal that C-doping destabilizes the intermediates compared to non-doped systems, thereby reducing the possibility of catalyst poisoning. However, their interactions with catalysts are good enough so that the NRR activity of the catalyst does not decrease due to C-doping.

Electrocatalytic Reduction of CO2 to Ethylene by Molecular Cu-Complex Immobilized on Graphitized Mesoporous Carbon.

The electrochemical reduction of carbon dioxide (CO2 ) to hydrocarbons is a challenging task because of the issues in controlling the efficiency and selectivity of the products. Among the various transition metals, copper has attracted attention as it yields more reduced and C2 products even while using mononuclear copper center as catalysts. In addition, it is found that reversible formation of copper nanoparticle acts as the real catalytically active site for the conversion of CO2 to reduced products. Here, it is demonstrated that the dinuclear molecular copper complex immobilized over graphitized mesoporous carbon can act as catalysts for the conversion of CO2 to hydrocarbons (methane and ethylene) up to 60%. Interestingly, high selectivity toward C2 product (40% faradaic efficiency) is achieved by a molecular complex based hybrid material from CO2 in 0.1 m KCl. In addition, the role of local pH, porous structure, and carbon support in limiting the mass transport to achieve the highly reduced products is demonstrated. Although the spectroscopic analysis of the catalysts exhibits molecular nature of the complex after 2 h bulk electrolysis, morphological study reveals that the newly generated copper cluster is the real active site during the catalytic reactions.

Tris(2-benzimidazolylmethyl)amine-Directed Synthesis of Single-Atom Nickel Catalysts for Electrochemical CO Production from CO2.

The electrochemical reduction of carbon dioxide (CO2 ) to value-added products is a promising approach to reducing excess CO2 in the atmosphere. However, the development of electrocatalysts for highly selective and efficient electrochemical CO2 reduction has been challenging because protons are usually easier to reduce than CO2 in an aqueous electrolyte. Recently, single-atom catalysts (SACs) have been suggested as candidate CO2 reduction catalysts due to their unique catalytic properties. To prepare single-atom metal active sites, the stabilization of metal atoms over conductive supports such as graphene sheets to prevent metal aggregation is crucial. To address this issue, a facile method was developed to prepare single-atom nickel active sites on reduced graphene oxide (RGO) sheets for the selective production of carbon monoxide (CO) from CO2 . The tris(2-benzimidazolylmethyl)amine (NTB) ligand was introduced as a linker that can homogeneously disperse nickel atoms on the graphene oxide (GO) sheets. Because the NTB ligands form strong interactions with the GO sheets by π-π interactions and with nickel ions by ligation, they can effectively stabilize nickel ions on GO sheets by forming Ni(NTB)-GO complexes. High-temperature annealing of Ni(NTB)-GO under inert atmosphere produces nickel- and nitrogen-doped reduced graphene oxide sheets (Ni-N-RGO) with single-atom Ni-N4 active sites. Ni-N-RGO shows high CO2 reduction selectivity in the reduction of CO2 to CO with 97 % faradaic efficiency at -0.8 V vs. RHE (reversible hydrogen electrode).