The important role of N2H formation energy for low-temperature ammonia synthesis in an electric field

Stabilisation of solid-state cubic ammonia confined in a glass substance at ambient temperature under atmospheric pressure

Ammonia, a widely available compound, exhibits structural transitions from solid to liquid to gas depending on temperature, pressure, and chemical interactions with adjacent atoms, offering valuable insights into planetary science. It serves as a significant hydrogen storage medium in environmental science, mitigating carbon dioxide emissions from fossil fuels. However, its gaseous form, NH3(g), poses health risks, potentially leading to fatalities. The sublimation pressure (psub) of solid cubic ammonia, NH3(cr), below 195.5 K is minimal. In this study, we endeavoured to stabilise NH3(cr) at room temperature for the first time. Through confinement within a boric acid glass matrix, we successfully synthesised and stabilised cubic crystal NH3(cr) with a lattice constant of 0.5165 nm under atmospheric pressure. Thermodynamic simulations affirmed the stabilisation of NH3(cr), indicating its quasi-equilibrium state based on the estimated standard Gibbs energy of formation, . Despite these advancements, the extraction of H2(g) from NH3(cr) within the boric acid glass matrix remains unresolved. The quest for an external matrix with catalytic capabilities to decompose inner NH3(cr) into H2(g) and N2(g) presents a promising avenue for future research. Achieving stability of the low-temperature phase at ambient conditions could significantly propel exploration in this field.

Electrochemical ammonia synthesis under ambient conditions using TM-embedded porphine-fused sheets as single-atom catalysts

In this research, we systematically investigated the reaction mechanism and electrocatalytic properties of transition metal anchored two-dimensional (2D) porphine-fused sheets (TM-Por) as novel single-atom catalysts (SACs) for the electrochemical nitrogen reduction reaction (eNRR) under ambient conditions. Using high-throughput screening and first-principles calculations based on the density functional theory (DFT) method, three eNRR catalyst candidates, i.e. Mo-Por, Tc-Por, and Nb-Por, were screened out, with the eNRR onset potentials on them being -0.36, -0.53, and -0.74 V, respectively. Furthermore, these catalyst candidates all have good stability and selectivity. Analyzing the band structures found that these catalyst candidates all are metallic, which is needed for good electrocatalysts. Ab initio molecular dynamics (AIMD) simulations show that these catalyst candidates have good stability at 500 K. It is hoped that our work will open up new possibilities for the experimental synthesis of electrochemical ammonia catalysts.

Synergistic Effects of the Ni3B Cocatalyst and N Vacancy on g-C3N4 for Effectively Enhanced Photocatalytic N2 Fixation

The photocatalytic fixation of N₂ is a promising technology for sustainable production of ammonia, while the unsatisfactory efficiency resulting from the low electron-transfer rate, narrow light absorption range, and limited active sites of the photocatalyst seriously hinder its application. Herein, we designed a noble metal-free Schottky junction photocatalyst constructed by g-C₃N₄ nanosheets with N vacancies (V_N-CN) and metallic Ni₃B nanoparticles (Ni₃B/V_N-CN) for N₂ reduction to ammonia. The ammonia yield rate over the optimized Ni₃B/V_N-CN is 7.68 mM g^-1 h^-1, which is 6.7 times higher than that of pristine CN (1.15 mM g^-1 h^-1). The superior photocatalytic N₂ fixation performance of Ni₃B/V_N-CN can be attributed not only to the formation of Schottky junctions between Ni₃B and V_N-CN, which facilitates the migration and separation of photogenerated electrons, but also to the incorporation of V_N into g-C₃N₄, which enhances visible light absorption and improves electrical conductivity. More importantly, Ni₃B nanoparticles can act as the cocatalyst, which provide more active sites for the adsorption and activation of N₂, thereby improving the N₂ reduction activity. This work provides an effective strategy of designing noble metal-free-based cocatalyst photocatalyst for sustainable and economic N₂ fixation.

A DFT study of plasma-catalytic ammonia synthesis: the effect of electric fields, excess electrons and catalyst surfaces on N2 dissociation

Plasma catalytic synthesis of ammonia has the advantages of flexible on-off and environmental friendliness, making ammonia a potential vector for renewable energy storage. The synergistic interaction between plasmas and catalyst surfaces remains unclear. In this work, we develop a quantum chemical model based on density functional theory where the plasma environment is simplified. The effect of electric fields and surface electrons on N₂ adsorption and dissociation is studied on the typical catalysts (Ru and Ni) with different surface morphologies. The combined effect of the electric fields and excess electrons will promote the adsorption of N₂ and the weakening of the NN triple bond. It is shown that the electron distribution on the surface is optimized, and the electrostatic interaction between surface atoms and adsorbates is strengthened. The marginal effect has been observed, and the promotion effect on the catalysts with better performance in thermal-catalytic N₂ dissociation is weaker.

Ultrathin 2D Fe-Nanosheets Stabilized by 2D Mesoporous Silica: Synthesis and Application in Ammonia Synthesis

Developing high-performance Fe-based ammonia catalysts through simple and cost-efficient methods has received an increased level of attention. Herein, we report for the first time, the synthesis of two-dimensional (2D) FeOOH nanoflakes encapsulated by mesoporous SiO₂ (mSiO₂) via a simple solution-based method for ammonia synthesis. Due to the sticking of the mSiO₂ coating layers and the limited spaces in between, the Fe after reduction retains the 2D morphology, showing high resistance against the sintering in the harsh Haber-Bosch process. Compared to supported Fe particles dispersed on mSiO₂ spheres, the coated catalyst shows a significantly improved catalytic activity by 50% at 425 °C. Thermal desorption spectroscopy (TDS) reveals the existence of a higher density of reactive sites for N₂ activation in the 2D Fe catalyst, which is possibly coupled to a larger density of surface defect sites (kinks, steps, point defects) that are generally considered as active centers in ammonia synthesis. Besides the structural impact of the coating on the 2D Fe, the electronic one is elucidated by partially substituting Si with Al in the coating, confirmed by ²⁹Si and ²⁷Al magic-angle spinning nuclear magnetic resonance (MAS NMR). An increased apparent activation energy (E_a) of the Al-containing catalyst evidences an influence on the nature of the active site. The herein-developed stable 2D Fe nanostructures can serve as an example of a 2D material applied in catalysis, offering the chance of a rational catalyst design based on a stepwise introduction of various promoters, in the coating and on the metal, maintaining the spatial control of the active centers.

Enhanced activity of catalysts on substrates with surface protonic current in an electrical field - a review

It has over the last few years been reported that the application of a DC electric field and resulting current over a bed of certain catalyst-support systems enhances catalytic activity for several reactions involving hydrogen-containing reactants, and the effect has been attributed to surface protonic conductivity on the porous ceramic support (typically ZrO2, CeO2, SrZrO3). Models for the nature of the interaction between the protonic current, the catalyst particle (typically Ru, Ni, Co, Fe), and adsorbed reactants such as NH3 and CH4 have developed as experimental evidence has emerged. Here, we summarize the electrical enhancement and how it enhances yield and lowers reaction temperatures of industrially important chemical processes. We also review the nature of the relevant catalysts, support materials, as well as essentials and recent progress in surface protonics. It is easily suspected that the effect is merely an increase in local vs. nominal set temperature due to the ohmic heating of the electrical field and current. We address this and add data from recent studies of ours that indicate that the heating effect is minor, and that the novel catalytic effect of a surface protonic current must have additional causes.

Theoretical prediction by DFT and experimental observation of heterocation-doping effects on hydrogen adsorption and migration over the CeO2(111) surface

Hydrogen (H) atom adsorption and migration over the CeO₂-based materials surface are of great importance because of its wide applications to catalytic reactions and electrochemical devices. Therefore, comprehensive knowledge for controlling the H atom adsorption and migration over CeO₂-based materials is crucially important. For controlling H atom adsorption and migration, we investigated irreducible divalent, trivalent, and quadrivalent heterocation-doping effects on H atom adsorption and migration over the CeO₂(111) surface using density functional theory (DFT) calculations. Results revealed that the electron-deficient lattice oxygen (O_lat) and the flexible CeO₂ matrix played key roles in strong adsorption of H atoms. Heterocations with smaller valence and smaller ionic radius induced the electron-deficient O_lat. In addition, smaller cation doping enhanced the CeO₂ matrix flexibility. Moreover, we confirmed the influence of H atom adsorption controlled by doping on surface proton migration (i.e. surface protonics) and catalytic reaction involving surface protonics (NH₃ synthesis in an electric field). Results confirmed clear correlation between H atom adsorption energy and surface protonics.

Effects of A-site composition of perovskite (Sr1-x Ba x ZrO3) oxides on H atom adsorption, migration, and reaction

Hydrogen (H) atomic migration over a metal oxide is an important surface process in various catalytic reactions. Control of the interaction between H atoms and the oxide surfaces is therefore important for better catalytic performance. For this investigation, we evaluated the adsorption energies of the H atoms over perovskite-type oxides (Sr1-x Ba x ZrO3; 0.00 ≤ x ≤ 0.50) using DFT (Density Functional Theory) calculations, then clarified the effects of cation-substitution in the A-site of perovskite oxides on H atom adsorption, migration, and reaction. Results indicated local distortion at the oxide surface as a key factor governing H atom adsorption. Subtle Ba2+ substitution for Sr2+ sites provoked local distortion at the Sr1-x Ba x ZrO3 oxide surface, which led to a decrement in the H atom adsorption energy. Furthermore, the effect of Sr2+/Ba2+ ratio on the H atoms' reactivities was examined experimentally using a catalytic reaction, which was promoted by activated surface H atoms. Results show that the surface H atoms activated by the substitution of Sr2+ sites with a small amount of Ba2+ (x = 0.125) contributed to enhancement of ammonia synthesis rate in an electric field, which showed good agreement with predictions made using DFT calculations.

Agglomeration Suppression of a Fe-Supported Catalyst and its Utilization for Low-Temperature Ammonia Synthesis in an Electric Field

Fe-supported heterogeneous catalysts are used for various reactions, including ammonia synthesis, Fischer-Tropsch synthesis, and exhaust gas cleaning. For the practical use of Fe-supported catalysts, suppression of Fe particle agglomeration is the most important issue to be resolved. As described herein, we found that Al doping in an oxide support suppresses agglomeration of the supported Fe particle. Experimental and computational studies revealed two tradeoff Al doping effects: the Fe particle size decreased and remained without agglomeration by virtue of the anchoring effect of doped Al. Also, some Fe atoms anchored by Al cannot function as an active site because of bonding with oxygen atoms. Using an appropriate amount of Al doping is effective for increasing the number of active Fe sites and catalytic activity. This optimized catalyst showed high practical activity and stability for low-temperature ammonia synthesis in an electric field. The optimized catalyst of 12.5 wt % Fe/Ce_0.4Al_0.1Zr_0.5O_2-δ showed the highest ammonia synthesis rate (2.3 mmol g^-1 h^-1) achieved to date under mild conditions (464 K, 0.9 MPa) in an electric field among the Fe catalysts reported.

Recent progress in use and observation of surface hydrogen migration over metal oxides

Hydrogen migration over a metal oxide surface is an extremely important factor governing the activity and selectivity of various heterogeneous catalytic reactions. Passive migration of hydrogen governed by a concentration gradient is called hydrogen spillover, which has been investigated broadly for a long time. Recently, well-fabricated samples and state-of-the-art measurement techniques such as operando spectroscopy and electrochemical analysis have been developed, yielding findings that have elucidated the migration mechanism and novel utilisation of hydrogen spillover. Furthermore, great attention has been devoted to surface protonics, which is hydrogen migration activated by an electric field, as applicable for novel low-temperature catalysis. This article presents an overview of catalysis related to hydrogen hopping, sophisticated analysis techniques for hydrogen migration, and low-temperature catalysis using surface protonics.

First observation of surface protonics on SrZrO3 perovskite under a H2 atmosphere

This is the first direct observation that surface proton hopping occurs on SrZrO₃ perovskite even under a H₂ (i.e. dry) atmosphere.

Key factor for the anti-Arrhenius low-temperature heterogeneous catalysis induced by H+ migration: H+ coverage over support

Low-temperature heterogeneous catalytic reaction in an electric field is anticipated as a novel approach for on-demand and small-scale catalytic processes.