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

Real-time screening of NixBy bifunctional electrocatalysts for overall NH3 synthesis via SG-TC SECM

Electrochemical ammonia synthesis, which couples oxygen evolution at the anode with nitrogen reduction at the cathode, holds great significance for future food and energy needs. Both of these half-cell reactions determine the overall cell potential and efficiency of the process. However, the employment of different catalysts on either side, due to discrete mechanisms, increases the complexity and material processing costs of the system, where the designing of a bifunctional catalyst active towards both the NRR and OER is of huge significance. Unfortunately, the initial screening of the designed catalysts via physical characterizations, optical methods and other techniques, does not provide details about the electrochemical activity. The scanning electrochemical microscopy (SECM) technique can be useful to screen multi-catalysts at the same time for their electrochemical activities. Herein, we employed the sample generation-tip collection (SG-TC) mode of SECM to screen the designed NixBy catalysts before half-cell investigations, which suggested that the catalyst synthesized via sonochemical reduction (SR), i.e. NixBy (SR), was a better catalyst. This inference was in accordance with the half-cell NRR and OER measurements (FE: 49% for NH3 production, OER overpotential: 300 mV). By virtue of this remarkable bifunctional activity, the NRR-OER coupled full cell was assembled, which initiated the NH3 production at just 1.7 V and produced NH3 (1.08 mg h-1 mgcat-1) at the cathode and O2 (0.81 mg h-1 mgcat-1) at the anode after 2 h of electrolysis at 1.9 V.

High-Throughput Screening of Effective Dual Atom Catalysts for the Nitric Oxide Reduction Reaction

Electrochemical NO-to-NH3 conversion has been attracting more attention in the field of green energy, which, however, imposes restrictions on the catalysts. We therefore design a family of dual atom catalysts (DACs) TM1TM2@g-CN and perform high-throughput screening to position the effective catalysts for electrocatalytic NO-to-NH3 conversion from first-principles computations. We identify that TiCr@g-CN (-0.37 V), TiMo@g-CN (-0.36 V), and MnMo@g-CN (-0.43 V) are promising candidates with low overpotentials. In particular, we find that MoMo@g-CN can spontaneously reduce NO to NH3, which makes it an excellent electrocatalyst for the NO reduction reaction (NORR) to be translated to experiments. In terms of the local geometry feature and local electronic structures, we unravel the origin of the high NORR activity and high selectivity of the DAC MoMo@g-CN.

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.

Electrocatalytic Reduction of N2 to NH3 Over Defective 1T'-WX2 (X=S, Se, Te) Monolayers

Defects in transition metal dichalcogenides (TMDs) can serve as active sites in catalytic reactions. In this work, by means of first-principles calculations, the catalytic activities of WX₂ (X=S, Se, Te) monolayers in the 1T' phase with both vacancy defects (missing chalcogen atoms, ^X V_d ) and antisite defects (replacing chalcogen atoms with W atoms, ^X A_d ) were evaluated for the nitrogen reduction reaction (NRR). Results showed that all these defective catalysts had great potential toward electrocatalytic ammonia synthesis by exhibiting low limiting potentials (U_L ). Over 1T'-WTe₂ @^Te V_d , 1T'-WS₂ @^S A_d , 1T'-WSe₂ @^Se A_d , and 1T'-WTe₂ @^Te A_d , the corresponding U_L values were -0.49, -0.21, -0.19, and -0.15 V, much smaller than that of the benchmark catalyst, the Ru (0001) surface (U_L =-0.98 V). Furthermore, the hydrogen evolution reaction (HER) was inhibited. 1T'-WX₂ monolayers with the antisite defects showed better NRR activity than those with the vacancy defects because of the smaller steric hindrance at the former. Results suggest that the steric effect at the active surface sites should be utilized to develop better catalysts.

"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.

Screening of transition metal single-atom catalysts supported by a WS2 monolayer for electrocatalytic nitrogen reduction reaction: insights from activity trend and descriptor

The electrocatalytic nitrogen reduction reaction (NRR), as an alternative green technology to the Haber-Bosch process, can efficiently synthesize ammonia under ambient conditions and has a reduced carbon footprint. Here we systematically investigate the NRR activity and selectivity of transition metal (TM) single-atom catalyst (SAC) anchored WS₂ monolayers (TM@WS₂) by means of first-principles calculations and microkinetic modeling. The construction of the reaction activity trend and the identification of an activity descriptor, namely *N₂H adsorption energy, facilitate the efficient screening and rational design of SACs with high activity. Manipulating the adsorption strength of the pivotal *N₂H intermediate is a potential strategy for enhancing NRR activity. Utilizing the limiting potential difference of NRR and the hydrogen evolution reaction (HER) as a selectivity descriptor, we screen three SACs with excellent activity and selectivity toward NRR, i.e., Re@WS₂, Os@WS₂ and Ir@WS₂ with favorable limiting potentials of -0.44 V, -0.38 V and -0.69 V. By using the explicit H₉O₄⁺ model, the kinetic barriers of the rate-determining steps (0.47 eV-1.15 eV) of the solvated proton transfer on the screened SACs are found to be moderate, indicative of a kinetically feasible process. Microkinetic modeling shows that the turnover frequencies of N₂ reduction to NH₃ on Re@WS₂, Os@WS₂ and Ir@WS₂ are 1.52 × 10⁵, 8.21 × 10² and 4.17 × 10^-4 per s per site at 400 K, achieving fast reaction rates. The coexistence of empty and occupied 5d orbitals of candidate SACs is beneficial for σ donation and π* backdonation, endowing them with extraordinary N₂ adsorption and activation. Moreover, the screened SACs possess good dispersity and thermodynamic stability. Our work provides a promising solution for the efficient screening and rational design of high-performance electrocatalysts toward the NRR.

Understanding the chemical bonding of ground and excited states of HfO and HfB with correlated wavefunction theory and density functional approximations

Knowledge of the chemical bonding of HfO and HfB ground and low-lying electronic states provides essential insights into a range of catalysts and materials that contain Hf-O or Hf-B moieties. Here, we carry out high-level multi-reference configuration interaction theory and coupled cluster quantum chemical calculations on these systems. We compute full potential energy curves, excitation energies, ionization energies, electronic configurations, and spectroscopic parameters with large quadruple-ζ and quintuple-ζ quality correlation consistent basis sets. We also investigate equilibrium chemical bonding patterns and effects of correlating core electrons on property predictions. Differences in the ground state electron configuration of HfB(X⁴Σ^-) and HfO(X¹Σ⁺) lead to a significantly stronger bond in HfO than HfB, as judged by both dissociation energies and equilibrium bond distances. We extend our analysis to the chemical bonding patterns of the isovalent HfX (X = O, S, Se, Te, and Po) series and observe similar trends. We also note a linear trend between the decreasing value of the dissociation energy (D_e) from HfO to HfPo and the singlet-triplet energy gap (ΔE_S-T) of the molecule. Finally, we compare these benchmark results to those obtained using density functional theory (DFT) with 23 exchange-correlation functionals spanning multiple rungs of "Jacob's ladder." When comparing DFT errors to coupled cluster reference values on dissociation energies, excitation energies, and ionization energies of HfB and HfO, we observe semi-local generalized gradient approximations to significantly outperform more complex and high-cost functionals.