Hydroxyl-Boosted Nitrogen Reduction Reaction: The Essential Role of Surface Hydrogen in Functionalized MXenes

Impact of Vacancy Defects on Electrochemical Nitrogen Reduction Reaction Performance of MXenes

We investigated electrochemical nitrogen reduction reaction (eNRR) on MXenes consisting of the vacancy defects in the functional layer using density functional theory calculations. We considered Mo2C, W2C, Mo2N, and W2N MXenes with F, N, and O functionalization and investigated distal and alternative associative pathways. We analyzed these MXenes for eNRR based on N2 adsorption energy, NH3 desorption energy, NRR selectivity, and electrochemical limiting potential. While we find that most of the considered MXenes surfaces are more favorable for eNRR compared to hydrogen evolution, these surfaces also have strong NH3 binding (>-1.0 eV) and thus will be covered with NH3 during operating conditions. Amongst all considered MXenes, only W2NF2 is found to have a low NH3 desorption energy along with low eNRR overpotential and selectivity towards eNRR. The obtained eNRR overpotential and NH3 desorption energy on W2NF2 are superior to those reported for pristine W2N3 as well as functionalized MXenes.

Theoretical exploration of the nitrogen fixation mechanism of two-dimensional dual-metal FeTM@GY (TM = Fe, Mo, Co, and V) electrocatalysts

In contrast to the energy-consuming Haber-Bosch process, ammonia synthesis by electrocatalysis under ambient conditions is an efficient and environmentally friendly method. In this work, through first principles calculations, the potential of four dual-atom FeTM (TM = Fe, Mo, Co, and V) anchored graphyne (FeTM@GY) as efficient nitrogen reduction reaction (NRR) catalysts is systematically investigated. Among them, FeMo@GY is the most promising, with excellent NRR catalytic activity, high ability to suppress the competing hydrogen evolution reaction (HER), and good stability. Moreover, NRR prefers the maximum pathway with the calculated onset potentials of -0.27 V for FeMo@GY. This work not only suggests that FeMo@GY holds great promise as an efficient, low-cost, and stable dual-atom catalyst for NRR but also further provides a guiding idea for the design of efficient NRR catalysts.

Carbon doped hexagonal boron nitride as an efficient metal-free catalyst for NO capture and reduction

The electrochemical NO reduction reaction (NORR) towards NH3 is considered a promising strategy to cope with both NO removal and NH3 production. Currently, the research on NORR electrocatalysts mainly focuses on metal-based catalysts, while metal-free catalysts are quite scarce. In this work, we have systematically investigated the properties of pristine and C/O doped h-BN for efficient NO capture and reduction. Our results reveal that the basal plane of pristine h-BN is inert to the adsorption of NO, while doping C or O can significantly enhance the NO capture abilities of h-BN. Then, we highlight that C-doped h-BN exhibits excellent NORR catalytic performance with a relatively low limiting potential of -0.28 V. Further analysis shows that the suitable adsorption strength of NO on the C-doped h-BN surface is the prime reason for its excellent NO reduction activity, which is shown to be due to appropriate electronic interactions between the active site and NO. Last but not least, the catalytic selectivity of h-BN towards the NORR is confirmed by inhibiting the competing hydrogen evolution reaction. Our findings not only provide deeper insight into the essential effect of element doping on the catalytic activities of h-BN, but also propose general design principles for high-performance metal-free NORR electrocatalysts.

Oxidation engineering triggered peroxidase-like activity of VOxC for detection of dopamine and glutathione

MXenes, two-dimensional nanomaterials, are gaining traction in catalysis and biomedicine. Yet, their oxidation instability poses significant functional constraints. Gaining insight into this oxidation dynamic is pivotal for designing MXenes with tailored functionalities. Herein, we crafted VOxC nanosheets by oxidatively engineering V4C3 MXene. Interestingly, while pristine V4C3 displays pronounced antioxidant behavior, its derived VOxC showcases enhanced peroxidase-like activity, suggesting the crossover between antioxidant and pro-oxidant capability. The mixed valence states and balanced composition of V in VOxC drive the Fenton reaction through multiple pathways to continually generate hydroxyl radicals, which was proposed as the mechanism underlying the peroxidase-like activity. Furthermore, this unique activity rendered VOxC effective in dopamine and glutathione detection. These findings underscore the potential of modulating MXenes' oxidation state to elicit varied catalytic attributes, providing an avenue for the judicious design of MXenes and derivatives for bespoke applications.

Theoretical insight into the essential role of charged surface for ammonia synthesis: Si-decorated carbon nitride electrode

We report a new Si-decorated carbon nitride (C5N2H2) electrode for the sustainable generation of a hydrogen storage medium, ammonia (NH3), which not only possesses sound electrical conductivity, dynamic stability, and electrochemical activity for the nitric oxide/nitrogen reduction reaction (NORR/NRR), but also provides an option for designing metal-free electrodes. Most importantly, it is found that the charged surface is of great significance to the improved catalytic performance compared to the neutral condition, but this has always been overlooked. Herein, by means of DFT computations, the stubborn chemical bonds of NO and N2 can be entirely activated under an electron density of -2.15 × 10-2 e Å-2 on the Si-C5N2H2 material with an inconsiderable kinetic energy barrier (0.28 eV) along the protonation path. In brief, this finding paves a way for understanding false results by theoretical calculations compared to experiments.

Efficient asymmetrical silicon-metal dimer electrocatalysts for the nitrogen reduction reaction

The electrocatalytic nitrogen reduction reaction (ENRR) has been regarded as an eco-friendly and feasible substitute for the Haber-Bosch method. Identifying the effective catalysts for the ENRR is an extremely important prerequisite but challenging. Herein, asymmetrical silicon-metal dimer catalysts doped into g-C₃N₄ nanosheets with nitrogen vacancies (SiM@C₃N₄) were designed to address nitrogen activation and reduction. The concept catalysts of SiM@C₃N₄ can combine the advantages of silicon-based and metal-based catalysts during the ENRR. Among the catalysts investigated, SiMo@C₃N₄ and SiRu@C₃N₄ exhibited the highest activities towards the ENRR with ultra-low onset potentials of -0.20 and -0.39 V; meanwhile, they suppressed the competing hydrogen evolution reaction (HER) due to the relative difficulty in releasing hydrogen. Additionally, SiRu@C₃N₄ is demonstrated to possess strong hydrophobicity, which is greatly beneficial to the production of ammonia. This research provides insights into asymmetrical silicon-metal dimer catalysts and reveals a new method for developing dual-atom electrocatalysts. This asymmetrical dimer strategy can be applied in other electrocatalytic reactions for energy conversion.

Photothermal enhanced fluorescence quenching of Tb-norfloxacin for ultrasensitive human epididymal 4 detection

A fluorescence quenching enhanced immunoassay has been developed to achieve ultrasensitive recognition of human epididymal 4 (HE4) modifying the fluorescence quencher. The carboxymethyl cellulose sodium-functionalized Nb₂C MXene nanocomposite (CMC@MXene) was firstly introduced to quench the fluorescence signal of the luminophore Tb-Norfloxacin coordination polymer nanoparticles (Tb-NFX CPNPs). The Nb₂C MXene nanocomposite as fluorescent nanoquencher inhibits the electron transfer between Tb and NFX to quench the fluorescent signal by coordinating the strongly electronegative carboxyl group on CMC with Tb (III) of Tb-NFX complex. Simultaneously, due to the superior photothermal conversion capability of CMC@MXene, the fluorescence signal has been further weakened by the photothermal effect driven non-radiative decay of the excited state under near-infrared laser irradiation. The constructed fluorescent biosensor based on CMC@MXene probe finally realized the enhanced fluorescence quenching effect, and achieved ultra-high sensitivity and selective detection of HE4, exhibiting a wide linear relationship with HE4 concentration on the logarithmic axis in the range of 10^-5 to 10 ng/mL and a low detection limit of 3.3 fg/mL (S/N = 3). This work not only provides an enhanced fluorescent signal quenching method for the detection of HE4, but also provides novel insights for the design of fluorescent sensor toward different biomolecules.

Nickel Nanoflowers with Controllable Cation Vacancy for Enhanced Electrochemical Nitrogen Reduction

The key to the design of electrochemical nitrogen reduction (NRR) catalysts is that the reaction sites can not only activate the N≡N bond but also have high catalytic selectivity. Vacancy engineering is an effective way to modulate active sites, and cation vacancies are considered to have enormous potential in tuning catalytic selectivity. However, research on NRR activity is still at an early stage due to the difficulty in preparation and precise regulation. Here, we provided an adjusted method of cation vacancy through topotactic transformation, which combines solvothermal reduction with etching via lattice confinement effect to accomplish precursor reduction and vacancy construction while maintaining consistent material morphologies. Based on the topotactic transformation, NiAl-LDH precursor was reduced to Ni metal nanoflower, while Al is simultaneously etched by alkali, thus the precise tunability of the cation vacancy can be achieved by adjusting the Al content in the LDH. The Ni nanoflower achieved excellent stability and high ammonia yield by adjusting the vacancy concentration. In addition, the insight into the selectivity and intrinsic activity of cation vacancies on NRR process has been revealed. For the reaction selectivity, the cation vacancy is beneficial to activate N≡N but not conducive to the HER process. For the intrinsic NRR activity, the generation of cation vacancies can also significantly reduce the energy barrier of NRR process and accelerate the reaction kinetics.

"Capture-Backdonation-Recapture" Mechanism for Promoting N2 Reduction by Heteronuclear Metal-Free Double-Atom Catalysts

Facing the increasingly serious energy and environmental crisis, the development of heteronuclear metal-free double-atom catalysts is a potential strategy to realize efficient catalytic nitrogen reduction with low energy consumption and no pollution because it could combine the advantages of flexible active sites in double-atom catalysts while also being pollution-free and have high Faraday efficiency in metal-free catalysts simultaneously. However, according to the existing mechanism, the finite orbits of other nonmetallic atoms, except the boron atom, reduce the possibility of metal-free catalysis and hinder the development of heteronuclear metal-free double-atom catalysts. Herein, we propose a new "capture-backdonation-recapture" mechanism, which skillfully uses the electron capture-backdonation-recapture between boron, the substrate, and other nonmetallic elements to solve the above problems. Based on this mechanism, by means of the first-principle calculations, the material structure, adsorption energy, catalytic activity, and selectivity of 36 catalysts are systematically investigated to evaluate their catalytic performance. B-Si@BP1 and B-Si@BP3 are selected for their good catalytic performance and low limiting potentials of -0.14 and -0.10 V, respectively. Meanwhile, the "capture-backdonation-recapture" mechanism is also verified by analyzing the results of adsorption energy and electron transfer. Our work broadens the ideas and lays the theoretical foundation for the development of heteronuclear metal-free double-atom catalysts in the future.

Steric effects in the hydrogen evolution reaction based on the TMX4 active center: Fe-BHT as a case study

In this work, Fe-BHT is identified as the most efficient catalyst for the hydrogen evolution reaction (HER) among the TM-BHTs (TM = Sc, Ti, V, Cr, Mn, Fe, Co, and Ni), with an overpotential as low as 0.09 V. It is found that Fe d_z² orbitals do not participate in the bonding with surrounding S/N atoms in the FeX₄ active center but are bonding states for hydrogen adsorption. In accordance with our results, a steric effect determined energy gap acts as an efficient descriptor for the HER activity, which has never been discussed in previous studies. In addition, strain engineering proves the proposed steric effects, which also highlights the importance of the point group symmetry of active centers.

Two-dimensional transition metal borides as high activity and selectivity catalysts for ammonia synthesis

In comparison to defect/doping induced activity in materials, transition metal borides with exposed metal atoms, large specific surface area, and high active site density show advantages as durable and efficient catalysts for specific electrochemical reactions. In this work, ReB₂ for N₂ reduction reaction (NRR) for ammonia (NH₃) with a record-low limiting potential of U_L = -0.05 V and high Faraday efficiency (FE) of 100% is screened out from a new class of TMB₂. It is concluded that high pressure/temperature is favorable to N₂ adsorption and kinetic barrier minimization; the maximal turnover frequency (TOF) at 700 K and 100 bar is 1.24 × 10^-2 per s per site, which is comparable to that of the benchmark Fe₃/Al₂O₃ catalysts, achieving an extremely fast reaction rate. In addition, crystal orbital Hamilton population (COHP) of *N₂ reveals the intrinsic origin of N₂ activation by analyzing the d-2π* interactions, and integrated COHP could be a quantitative descriptor to describe the N₂ activation degree. It is evident that our results not only identify an efficient NRR electrocatalyst in particular, paving the way for sustainable NH₃ production, but also explain the chemical and physical origin of the activity, advancing the design principle for catalysts for various reactions in general.

Novel Design Strategy of High Activity Electrocatalysts toward Nitrogen Reduction Reaction via Boron-Transition-Metal Hybrid Double-Atom Catalysts

Electrocatalytic nitrogen reduction reaction (NRR) is a promising method for sustainable production of NH₃, which provides an alternative to the traditional Haber-Bosch process. However, the poor Faraday efficiency caused by N≡N triple bond activation and competitive hydrogen evolution reaction (HER) have seriously hindered the application of NRR. In this work, a novel strategy to promote NRR through boron-transition-metal (TM) hybrid double-atom catalysts (HDACs) has been proposed. The excellent catalytic activity of HDACs is attributed to a significant difference of valence electron distribution between boron and TMs, which could better activate N≡N bonds and promote the conversion of NH₂ to NH₃ compared with boron or metal single-atom catalysts and traditional double-atom catalysts (DACs). Hence, by means of DFT computations, the stability, activity, and selectivity of 29 HDACs are systematically investigated to evaluate their catalytic performance. B-Ti@g-CN and B-Ta@g-CN are screened as excellent nitrogen-fixing catalysts with particularly low limiting potentials of 0.13 and 0.11 V for NRR and rather high potentials of 0.54 and 0.82 V for HER, respectively. This work provides a new idea for the rational design of efficient nitrogen-fixing catalysts and could also be widely used in other catalytic reactions.

Synergistic Enhancement of Electrocatalytic Nitrogen Reduction Over Boron Nitride Quantum Dots Decorated Nb2 CTx -MXene

Electrochemical N₂ fixation represents a promising strategy toward sustainable NH₃ synthesis, whereas the rational design of high-performance catalysts for the nitrogen reduction reaction (NRR) is urgently required but remains challenging. Herein, a novel hexagonal BN quantum dots (BNQDs) decorated Nb₂ CT_x -MXene (BNQDs@Nb₂ CT_x ) is explored as an efficient NRR catalyst. BNQDs@Nb₂ CT_x presents the optimum NRR activity with an NH₃ yield rate of 66.3 µg h^-1 mg^-1 (-0.4 V) and a Faradaic efficiency of 16.7% (-0.3 V), outperforming most of the state-of-the-art NRR catalysts, together with an excellent stability. Theoretical calculations revealed that the synergistic interplay of BNQDs and Nb₂ CT_x enabled the creation of unique interfacial B sites serving as NRR catalytic centers capable of enhancing the N₂ activation, lowering the reaction energy barrier and impeding the H₂ evolution.