Catalytic performance of Ru, Os, and Rh nanoparticles for ammonia synthesis: A density functional theory analysis

Hybrid Quantum Mechanical, Molecular Mechanical, and Machine Learning Potential for Computing Aqueous-Phase Adsorption Free Energies on Metal Surfaces

Performing reliable computer simulations of elementary processes occurring at metal-water interfaces is pivotal for novel catalyst design in sustainable energy applications. Computational catalyst design hinges on the ability to reliably and efficiently compute the potential energy surface (PES) of the system. Due to the large system sizes needed for studying processes at liquid water-metal interfaces, these systems can currently not be described using density functional theory (DFT). In this work, we used a hybrid quantum mechanical, molecular mechanical, and machine learning potential for studying the adsorption behavior of phenol, atomic hydrogen, 2-butanol, and 2-butanone on the (0001) facet of Ru under reducing conditions when Ru is not oxidized. Specifically, we describe the adsorbate and the surrounding metal atoms at the DFT level of theory. Here, we also considered the electrostatic field effect of the water molecules on adsorbate-metal interactions. Next, for the water-water and water-adsorbate interactions, we used established classical force fields. Finally, for the water-Ru surface interaction, for which no reliable force fields have been published, we used Behler-Parrinello high-dimensional neural network potentials (HDNNPs). Employing this setup, we used our explicit solvation for metal surface (eSMS) approach to compute the aqueous-phase effect on the low-coverage adsorption of selected molecules and atoms on the (0001) facet of Ru. In agreement with previous experimental and computational studies of oxygenated molecules over transition metal facets, we found that liquid water destabilizes the tested adsorbates on Ru(0001). Interestingly, our findings indicate that adsorbates on Ru are less affected by the presence of an aqueous phase than on other transition metals (e.g., Pt), highlighting the necessity of experimental investigations of Ru-based catalytic systems in liquid water.

Ammonolysis-Driven Exsolution of Ru Nanoparticle Embedded in Conductive Metal Nitride Matrix to Boost Electrocatalyst Activity

Exsolution is an effective method for synthesizing robust nanostructured metal-based functional materials. However, no studies have investigated the exsolution of metal nanoparticles into metal nitride substrates. In this study, a versatile nitridation-driven exsolution method is developed for embedding catalytically active metal nanoparticles in conductive metal nitride substrates via the ammonolysis of multimetallic oxides. Using this approach, Ti1-xRuxO2 nanowires are phase-transformed into holey TiN nanotubes embedded with exsolved Ru nanoparticles. These Ru-exsolved holey TiN nanotubes exhibit outstanding electrocatalytic activity for the hydrogen evolution reaction with excellent durability, which is significantly higher than that of Ru-deposited TiN nanotubes. The enhanced stability of the Ru-exsolved TiN nanotubes can be attributed to the Ru nanoparticles embedded in the robust metal nitride matrix and the formation of interfacial Ti3+─N─Ru4+ bonds. Density functional theory calculations reveal that the exsolved Ru nanoparticles have a lower d-band center position and optimized hydrogen affinity than deposited Ru nanoparticles, indicating the superior electrocatalyst performance of the former. In situ Raman spectroscopic analysis reveals that the electron transfer from TiN to Ru nanoparticles is enhanced during the electrocatalytic process. The proposed approach opens a new avenue for stabilizing diverse metal nanostructures in many conductive matrices like metal phosphides and chalcogenides.

Controllable Exfoliation of MOF-Derived Van Der Waals Superstructure into Ultrathin 2D B/N Co-Doped Porous Carbon Nanosheets: A Superior Catalyst for Ambient Ammonia Electrosynthesis

The electrocatalytic nitrogen reduction reaction (NRR) to synthesize NH₃ under ambient conditions is a promising alternative route to the conventional Haber-Bosch process, but it is still a great challenge to develop electrocatalysts' high Faraday efficiency and ammonia yield. Herein, a facile and efficient exfoliation strategy to synthesize ultrathin 2D boron and nitrogen co-doped porous carbon nanosheets (B/NC NS) via a metal-organic framework (MOF)-derived van der Waals superstructure, is reported. The results of experiments and theoretical calculations show that the doping of boron and nitrogen can modulate the electronic structure of the adjacent carbon atoms; which thus, promotes the competitive adsorption of nitrogen and reduces the energy required for ammonia synthesis. The B/NC NS exhibits excellent catalytic performance and stability in electrocatalytic NRR, with a yield rate of 153.4 µg·h^-1 ·mg^-1 cat and a Faraday efficiency of 33.1%, which is better than most of the reported NRR electrocatalysts. The ammonia yield of B/NC NS can maintain 92.7% of the initial NRR activity after 48 h stability test. The authors' controllable exfoliation strategy using MOF-derived van der Waals superstructure can provide a new insight for the synthesis of other 2D materials.

Favorable Role of the Metal-Support Perimeter Region in Electrochemical NH3 Synthesis: A Density Functional Theory Study on Ru/BaCeO3

The catalytic electrochemical synthesis of NH₃ on Ru/BaCeO₃ was investigated using density functional theory. The competition between NH₃ formation and the hydrogen evolution reaction (HER) is a key for a high NH₃ formation rate. Our calculations show that H adsorbs more strongly than N₂ at the Ru particle moiety, while the adsorption of N₂ is stronger than the H adsorption at the Ru/BaCeO₃ perimeter, a model for the triple-phase boundary that is proposed to be an active site by experimental studies. This indicates that, while the HER is more favorable at the Ru particle moiety, it should be suppressed at the Ru/BaCeO₃ perimeter. We also calculated the Gibbs free energy changes along the NH₃ formation and found that the N₂H formation, the NHNH₂ formation, and the NH₃ formation steps have a relatively large Gibbs energy change. Therefore, these are possible candidates for the potential-determining step. The calculated equilibrium potential (U = -0.70 V, vs RHE) is in reasonable agreement with experiments. We also evaluated the reaction energy (ΔE) and the activation barrier (E _a) of the N₂H formation at several sites. ΔE and E _a were high at the Ru particle moiety (ΔE = 1.18 eV and E _a = 1.38 eV) but became low (ΔE = 0.32 eV and E _a = 1.31 eV) at the Ru/BaCeO₃ perimeter. These provide the atomic-scale mechanism how the proton conduction in BaCeO₃ assists the electrochemical NH₃ synthesis.

Osmium and OsO x nanoparticles: an overview of syntheses and applications

Precious metal nanoparticles are key for a range of applications ranging from catalysis and sensing to medicine. While gold (Au), silver (Ag), platinum (Pt), palladium (Pd) or ruthenium (Ru) nanoparticles have been widely studied, other precious metals are less investigated. Osmium (Os) is one of the least studied of the precious metals. However, Os nanoparticles are interesting materials since they present unique features compared to other precious metals and Os nanomaterials have been reported to be useful for a range of applications, catalysis or sensing for instance. With the increasing availability of advanced characterization techniques, investigating the properties of relatively small Os nanoparticles and clusters has become easier and it can be expected that our knowledge on Os nanomaterials will increase in the coming years. This review aims to give an overview on Os and Os oxide materials syntheses and applications.

Heterogeneous catalyst design by generative adversarial network and first-principles based microkinetics

Microkinetic analysis based on density functional theory (DFT) was combined with a generative adversarial network (GAN) to enable the artificial proposal of heterogeneous catalysts based on the DFT-calculated dataset. The approach was applied to the NH₃ formation reaction on Rh−Ru alloy surfaces as an example. The NH₃ formation turnover frequency (TOF) was calculated by DFT-based microkinetics. Six elementary reactions, namely, N₂ dissociation, H₂ dissociation, NH_x (x = 1–3) formation, and NH₃ desorption, were explicitly considered, and their reaction energies were evaluated by DFT calculations. Based on the TOF values and atomic compositions, new alloy surfaces were generated using the GAN. This approach successfully generated the surfaces that were not included in the initial dataset but exhibited higher TOF values. The N₂ dissociation reaction was more exothermic for the generated surfaces, leading to higher TOF. The present study demonstrates that the automatic improvement of catalyst materials is possible using DFT calculations and GAN sample generation.

Uncovering the Mechanism of the Hydrogen Poisoning on Ru Nanoparticles via Density Functional Theory Calculations

Although hydrogen plays a crucial role in ammonia synthesis, very little is known about its poisoning of Ru catalysts. In this study, density functional theory calculations of H2 and N2 dissociations, and H atom binding on Ru153 were performed to provide a fundamental understanding of hydrogen poisoning. Because of the kinetic dominance of the H2 dissociation over N2 (vertically or horizontally adsorbed) splitting, the dissociated H atoms block the active sites required for horizontal (less energetically demanding dissociation) N2 adsorption to occur either from the gas phase or after its geometrical transformation from being adsorbed vertically. Additionally, the dissociated H atoms withdraw electrons from the surface, which reduces the ability of the neighboring Ru atoms to donate electrons for N2 activation, hindering its dissociation and suppressing ammonia synthesis.

Adsorption States of N2/H2 Activated on Ru Nanoparticles Uncovered by Modulation-Excitation Infrared Spectroscopy and Density Functional Theory Calculations

The adsorption states of N₂ and H₂ on MgO-supported Ru nanoparticles under conditions close to those of ammonia synthesis (AS; 1 atm, 250 °C) were uncovered by modulation-excitation infrared spectroscopy and density functional theory calculations using a nanoscale Ru particle model. The two most intense N₂ adsorption peaks corresponded to the vertical chemisorption of N₂ on the nanoparticle's top and bridge sites, while the remaining peaks were assigned to horizontally adsorbed N₂ in view of the site heterogeneity of Ru nanoparticles. Long-term observations showed that vertically adsorbed N₂ molecules gradually migrated from the top sites to the bridge sites. Compared to those adsorbed vertically, N₂ molecules adsorbed horizontally exhibited a lower dipole moment, an increased N─N bond distance, and a decreased N─N bond order (i.e., were activated), which was ascribed to enhanced Ru-to-N charge transfer. H₂ molecules were preferentially adsorbed horizontally on top sites and then rapidly dissociated to afford strongly surface-bound H atoms and thus block the active sites of Ru nanoparticles. Our results clarify the controversial adsorption/desorption behavior of N₂ and H₂ on AS catalysts and facilitate their further development.

Comprehensive Understanding of the Thriving Ambient Electrochemical Nitrogen Reduction Reaction

The electrochemical method of combining N₂ and H₂ O to produce ammonia (i.e., the electrochemical nitrogen reduction reaction [E-NRR]) continues to draw attention as it is both environmentally friendly and well suited for a progressively distributed farm economy. Despite the multitude of recent works on the E-NRR, further progress in this field faces a bottleneck. On the one hand, despite the extensive exploration and trial-and-error evaluation of E-NRR catalysts, no study has stood out to become the stage protagonist. On the other hand, the current level of ammonia production (microgram-scale) is an almost insurmountable obstacle for its qualitative and quantitative determination, hindering the discrimination between true activity and contamination. Herein i) the popular theory and mechanism of the NRR are introduced; ii) a comprehensive summary of the recent progress in the field of the E-NRR and related catalysts is provided; iii) the operational procedures of the E-NRR are addressed, including the acquisition of key metrics, the challenges faced, and the most suitable solutions; iv) the guiding principles and standardized recommendations for the E-NRR are emphasized and future research directions and prospects are provided.

Efficient Electrocatalytic N2 Reduction on Three-Phase Interface Coupled in a Three-Compartment Flow Reactor for the Ambient NH3 Synthesis

The electrochemical N₂ reduction reaction (eNRR) represents a carbon-free alternative to the Haber-Bosch process for a sustainable NH₃ synthesis powered by renewable energy under ambient conditions. Despite significant efforts to develop catalyst activity and selectivity toward eNRR, an appropriate electrochemical system to obstruct the drawback of low N₂ solubility remains broadly unexplored. Here, we demonstrate an electrocatalytic system combining a ruthenium/carbon black gas diffusion electrode (Ru/CB GDE) with a three-compartment flow cell, enabling solid-liquid-gas catalytic interfaces for the highly efficient Ru-catalyzed eNRR. The electrolyte optimization and the Ru/CB GDE development through the hydrophobicity, the Ru/CB loading, and the post-treatment have revealed the crucial contribution of interfacial N₂ transportation and local pH environment. The optimized hydrophobic Ru/CB GDE generated excellent eNRR performance, achieving a high NH₃ yield rate of 9.9 × 10^-10 mol/cm² s at -0.1 V vs RHE, corresponding to the highest faradaic efficiency of 64.8% and a specific energy efficiency of 40.7%, exceeding the most reported system. This work highlights the critical role of design and optimization of the GDE-flow cell combination and provides a valuable practicable solution to enhance the electrochemical reaction involving gas-phase reactants with low solubility.

Fingerprinting the Ammonia Synthesis Pathway Using Spatiotemporal Electrostatic Potential Distribution of Intermediates

It remains a research challenge in determining the catalytic reaction mechanisms primarily caused by the difficulty to experimentally identify active intermediates with current analytic characterizations. Although computational chemistry has provided an alternative approach to simulate the catalysis process and achieve insights into the reaction pathways, the simulation results would not be conclusive without experimental evidence. Herein, we investigate spatiotemporal electrostatic potential (ESP) distribution surrounding reacting molecules during the catalysis process and suggest its use as a fingerprint to help differentiate and identify active intermediates. Our ESP study of ammonia synthesis on the Ru surface shows a high spatial sensitivity of ESP distribution to molecular configuration and structure of intermediate species and only minor temporal ESP oscillation throughout the lifetime of the intermediates, which provides strong theoretical support to use ESP distribution as a new approach to characterize intermediates. With the ESP measurements at the microscale and in real-time, turning feasible, experimental identification of active intermediates and determination of reaction pathways would become possible by measuring the ESP surrounding the reacting molecules. We suggest developing ESP measurement tools to experimentally explore and unveil reaction mechanisms.

Surface reactions of ammonia on ruthenium nanoparticles revealed by 15N and 13C solid-state NMR

Ruthenium nanoparticles (Ru NPs) stabilized by bis-diphenylphosphinobutane (dppb) and surface-saturated with hydrogen have been exposed to gaseous ¹⁵NH₃ and ¹³CO and studied using solid-state NMR and DFT calculations.

Recent advances in catalysts, electrolytes and electrode engineering for the nitrogen reduction reaction under ambient conditions

With the conventional Haber-Bosch NH3 synthesis in industry requiring harsh pressures and high temperatures, artificial N2 fixation has been long sought after. The electrochemical nitrogen reduction reaction (NRR) could offer a solution by allowing NH3 production under ambient conditions. In this review, important recent findings on theoretical calculations and experimental exploration on the NRR at room temperature are systematically reviewed. Firstly, we discuss the mechanism of electrochemical heterogeneous catalysis for the NRR. The NRR is a multi-proton coupled electron transfer (PCET) process which implies that in addition to catalyst surface size effects, ligand and strain effects will also significantly influence the binding energy of the adsorbed N atoms, reaction intermediates and product species. Electrocatalysts including metals, metal nitrides, metal oxides and carbon-based materials will also be discussed at length. A linear scaling relationship seems to limit the NRR activity on most metals and metal oxides. Metal nitrides, however, follow the Mars-van Krevelen (MvK) mechanism which usually shows a lower potential energy barrier compared to the associative mechanism. Carbon-based materials and some single atom catalysts exhibit improved activity and selectivity due to ligand effects. Thus, electrolytes containing a proton donor might play a crucial role in the NRR. The limiting concentration of proton donors and the rate of proton transport to the active sites might be effective factors in boosting the selectivity of the NRR. Specifically, ionic liquids with high N2 solubility demonstrate much larger faradaic efficiency and would be promising candidates for use in NRR processes. Inspired by the characteristics of PCET, four strategies of electrode engineering were introduced including limiting protons, tuning the electron transport, modifying the electrode structure facilitating mass transport, and completely changing the NRR mechanism inspired by bio-nitrogenase and Li mediated N2 fixation.

Atomic Modulation, Structural Design, and Systematic Optimization for Efficient Electrochemical Nitrogen Reduction

Ammonia (NH3) is a pivotal precursor in fertilizer production and a potential energy carrier. Currently, ammonia production worldwide relies on the traditional Haber-Bosch process, which consumes massive energy and has a large carbon footprint. Recently, electrochemical dinitrogen reduction to ammonia under ambient conditions has attracted considerable interest owing to its advantages of flexibility and environmental friendliness. However, the biggest challenge in dinitrogen electroreduction, i.e., the low efficiency and selectivity caused by poor specificity of electrocatalysts/electrolytic systems, still needs to be overcome. Although substantial progress has been made in recent years, acquiring most available electrocatalysts still relies on low efficiency trial-and-error methods. It is thus imperative to establish some critical guiding principles for nitrogen electroreduction toward a rational design and accelerated development of this field. Herein, a basic understanding of dinitrogen electroreduction processes and the inherent relationships between adsorbates and catalysts from fundamental theory are described, followed by an outline of the crucial principles for designing efficient electrocatalysts/electrocatalytic systems derived from a systematic evaluation of the latest significant achievements. Finally, the future research directions and prospects of this field are given, with a special emphasis on the opportunities available by following the guiding principles.

Doping strain induced bi-Ti3+ pairs for efficient N2 activation and electrocatalytic fixation

The electrochemical N₂ fixation to produce ammonia is attractive but significantly challenging with low yield and poor selectivity. Herein, we first used density function theory calculations to reveal adjacent bi-Ti³⁺ pairs formed on anatase TiO₂ as the most active electrocatalytic centers for efficient N₂ lying-down chemisorption and activation. Then, by doping of anatase TiO₂ with Zr⁴⁺ that has similar d-electron configuration and oxide structure but relatively larger ionic size, the adjacent bi-Ti³⁺ sites were induced and enriched via a strained effect, which in turn enhanced the formation of oxygen vacancies. The Zr⁴⁺-doped anatase TiO₂ exhibited excellent electrocatalytic N₂ fixation performances, with an ammonia production rate (8.90 µg·h⁻¹·cm⁻²) and a Faradaic efficiency of 17.3% at −0.45 V versus reversible hydrogen electrode under ambient aqueous conditions. Moreover, our work suggests a viewpoint to understand and apply the same-valance dopants in heterogeneous catalysis, which is generally useful but still poorly understood.

Electrocatalytic N₂ conversion to ammonia offers a low-carbon alternative to industrial ammonia production, although the rational design of active, efficient catalysts remains limited. Here, authors show Ti³⁺-pair sites in anatase TiO₂ to be active for aqueous N₂ electroreduction to ammonia.

Catalytic synthesis of non-carbon fuel NH3 from easily available N2 and H2O over FeO(100) surface: study of reaction mechanism using the density functional theory

The existence of –OH groups changed the controlling step from the dissociation of N₂ to the formation of –NH₂.