Development and Recent Progress on Ammonia Synthesis Catalysts for Haber–Bosch Process

Plasma catalytic ammonia synthesis in a fluidized-bed DBD reactor over M/Al2O3: Enhancement of plasma-catalyst surface interaction

Ammonia is a promising alternative energy to fossil fuels for reducing CO2 emissions. Plasma catalysis technology for ammonia production using clean energy is gaining attention. Introducing catalysts to the plasma increases ammonia synthesis rates, but the effect of catalyst particle movement in the plasma region, such as in a fluidized-bed reactor, is less explored. The study investigates plasma catalytic ammonia synthesis using fluidized-bed and fixed-bed dielectric barrier discharge (DBD) reactors with M/Al2O3 (M = Co, Ru, Ru-Co) catalysts. Results show the ammonia synthesis rate in the fluidized-bed DBD reactor was significantly higher than in the fixed-bed DBD reactor. This intensified plasma discharge substantially increased ammonia production. The dynamic movement of catalyst particles in the fluidized-bed reactor enhanced the plasma-surface interaction, thereby accelerating the interaction with reactive species and promoting ammonia synthesis. This research provides insights into reactor design and catalyst packing for plasma catalytic ammonia synthesis.

Photodriven Ammonia Synthesis from N2 and H2: Recycling of a Molecular Molybdenum Nitride

The synthesis of ammonia from its elements, N2 and H2, is the most atom-economical and thermodynamically preferred route but presents a high kinetic barrier and thus is rare using molecular compounds. Irradiation of a molecular molybdenum nitride prepared from N2 cleavage with visible light in the presence of an iridium photocatalyst and 1-4 atm of H2 produced high yields of ammonia along with the formation of a cationic, formally molybdenum(VI) pentahydride as the major molybdenum-containing product. Continued irradiation of the molybdenum hydride under an N2 atmosphere resulted in regeneration of the molybdenum nitride that was recycled and used for additional hydrogenation to generate more ammonia, demonstrating superstoichiometric batch ammonia synthesis using only N2 and H2 with molecular compounds under ambient conditions.

In vivo synthesis of semiconductor nanoparticles in Azotobacter vinelandii for light-driven ammonia production

Ammonia (NH3) is an important commodity chemical used as an agricultural fertilizer and hydrogen-storage material. There has recently been much interest in developing an environmentally benign process for NH3 synthesis. Here, we report enhanced production of ammonia from diazotrophs under light irradiation using hybrid composites of inorganic nanoparticles (NPs) and bacterial cells. The primary focus of this study lies in the intracellular biosynthesis of semiconductor NPs within Azotobacter vinelandii, a diazotroph, when bacterial cells are cultured in a medium containing precursor molecules. For example, enzymes in bacterial cells, such as cysteine desulfurase, convert cysteine (Cys) into precursors for cadmium sulfide (CdS) synthesis when supplied with CdCl2. Photoexcited charge carriers in the biosynthesized NPs are transferred to nitrogen fixation enzymes, e.g., nitrogenase, facilitating the production of ammonium ions. Notably, the intracellular biosynthesis approach minimizes cell toxicity compared to extracellular synthesis due to the diminished generation of reactive oxygen species. The biohybrid system based on the in vivo approach results in a fivefold increase in ammonia production (0.45 mg gDCW-1 h-1) compared to the case of diazotroph cells only (0.09 mg gDCW-1 h-1).

Multi-Layer Kesterite-Based Photocathodes for NH3 Photosynthesis from N2 Reduction Reaction

The necessity of new methods to substitute the Haber-Bosch process in the NH3 synthesis, generating fewer greenhouse gases, and dispensing less energy, drove the investigation of the photoelectrocatalytic approach in the N2 reduction reaction (N2RR). For that, this work presents the synthesis and characterization of the layered CZTSSe/CdS/TiO2 photocathode decorated with Pt nanoparticles for application in NH3 production using the photoelectrocatalysis technique. The CZTSSe/CdS/TiO2-Pt characterization showed a well-designed and stable photocatalyst synthesized layer by layer with an important contribution of the Pt nanoparticles for the catalyst performance, improving the photocurrent density and the charge transfer. The N2RR in a two-compartment photochemical cell with 0.1 mol L-1 Na2SO3 and 0.05 mol L-1 H2SO4 in the cathodic and anodic chamber, respectively, using CZTSSe/CdS/TiO2-Pt and under 1 sun of light incidence and applied potential of -0.4 VAg/AgCl reached 0.22 mmol L-1 cm-2 NH3, a value 28 folds higher than using the catalyst without Pt modification. The superiority of N2RR under the photoelectrocatalysis technique was demonstrated compared to photocatalytic and electrocatalytic techniques, together with the investigation of the supporting electrolyte influence in the cathodic compartment. Additionally, that is the first time a kesterite-based photocathode has been applied to NH3 photosynthesis, showing excellent photoconversion capability.

Unveiling the electrochemical nitrogen reduction reaction mechanism in heteroatom-decorated-Mo2CS2-MXene: the synergistic effect of single-atom Fe and heteroatom

Conversion of nitrogen (N2) to ammonia (NH3) is a significant process that occurs in environment and in the field of chemistry, but the traditional NH3 synthesis method requires high energy and pollutes the environment. In this work, the charge, orbital and spin order of the single-atom Fe loaded on heteroatom (X) doped-Mo2CS2 (X = B, N, O, F, P and Se) and its synergistic effect on electrochemical nitrogen reduction reaction (eNRR) were investigated using well-defined density functional theory (DFT) calculations. Results revealed that the X-element modified the charge loss capability of Fe atoms and thereby introduced a net spin through heteroatom doping, resulting in the magnetic moment modulation of Fe. Upon incorporating N2 molecule vertically into Fe@P-doped-Mo2CS2, the strongest eNRR performance and activation ability for the NN were achieved. This was due to the 1πu antibonding orbitals being filled with extra charges from Fe atoms and the σ2s bonding orbitals experiencing a splitting phenomenon as a result of net spin injection from P atoms. Thus, this work provides rational design principles for the development of non-noble metal eNRR electrocatalysts by ingeniously manipulating their spin order and local environments.

Surface Oxygen Vacancies on Copper-Doped Titanium Dioxide for Photocatalytic Nitrate-to-Ammonia Reduction

Photocatalytic transformation of nitrate (NO3-) in wastewater into ammonia (NH3) is a challenge in the detoxification and recycling of limited nitrogen resources. In particular, previously reported photocatalysts cannot promote the reaction using water as an electron donor. Herein, we report that copper-doped titanium dioxide (Cu-TiO2) powders, prepared via the sol-gel method and subsequent calcination, promote NO3--to-NH3 reduction in water. The Cu2+ doping into TiO2 creates a large number of surface oxygen vacancies (OVsurf), which are stable even under aerated conditions. The Ti3+ and Cu2+ atoms adjacent to OVsurf behave as active sites for the NO3--to-NH3 reduction. Doping with an appropriate amount of Cu2+ and calcination at an appropriate temperature produce the catalysts with a large number of OVsurf, while maintaining a high conductivity, and exhibit a high photocatalytic activity.

Design of Iron-Based Multifunctional Alloys Electrodeposited from Complexing Electrolytes

There is a growing focus on sustainability, characterized by making changes that anticipate future needs and adapting them to present requirements. Sustainability is reflected in various areas of materials science as well. Thus, more research is focused on the fabrication of advanced materials based on earth-abundant metals. The role of iron and its alloys is particularly significant as iron is the second most abundant metal on our planet. Additionally, the electrochemical method offers an environmentally friendly approach for synthesizing multifunctional alloys. Thus, iron can be successfully codeposited with a targeted metal from complexing electrolytes, opening a large horizon for a smart tuning of properties and enabling various applications. In this review, we discuss the practical aspects of the electrodeposition of iron-based alloys from complexing electrolytes, with a focus on refractory metals as multifunctional materials having magnetic, catalytic, mechanical, and antimicrobial/antibacterial properties with advanced thermal, wear, and corrosion resistance. Peculiarities of electrodeposition from complexing electrolytes are practically significant as they can greatly influence the final structure, composition, and designed properties by adjusting the electroactive complexes in the solution. Moreover, these alloys can be further upgraded into composites, multi-layered, hybrid/recovered materials, or high-entropy alloys.

Chemical looping ammonia synthesis based on the self-carried active N and Al from aluminum dross as low-cost nitrogen carrier

Ammonia (NH3), as a hydrogen energy carrier, is advantageous for mitigating energy consumption and carbon emission; however, the current Haber process for NH3 synthesis requires harsh conditions with a low conversion rate. To address these challenges, chemical looping ammonia synthesis (CLAS) has been proposed for utilizing aluminum dross (AD)-a hazardous waste containing sufficient active Al and N as a nitrogen carrier-involving carbon conversion without emissions. AD samples were employed to investigate the performance of CLAS, and the evolution of nitrogen carrier. During the NH3 production step, the amount of reacted AlN increased with hydrolysis temperature and steam concentration, with the highest NH3 yield reaching 2.32 × 10-3 mol/g at 700 °C, achieving the highest NH3 production rate. The efficiency was comparable to commercial AlN, with a yield 10% higher. The by-product H2 was attributed to NH3 decomposition and lattice N transformation at high temperatures, which significantly reduced NH3 yield. The contents of heavy metals, Cl-, and F- all decreased after NH3 production, indicating a reduction in environmental risk. Sorting small particles (0.075-0.15 mm) from SA increased NH₃ yield by 2.45 times compared to raw materials and 3.95 times compared to coarse materials (0.355-0.5 mm). Furthermore, OH- and Al-O-N were detected, describing the NH3 production process of AD, with the hydrolysis products tending to nucleate and grow around the parent crystal particles before elongating. Moreover, the NH3 yield of regenerated nitrogen carrier decreased to 10% when the hydrolyzed AD was nitridated at 1600 °C, but increased by 2.4 times when using Na-modified AD for the second hydrolysis and reducing the nitridation temperature to 1400 °C. Na-modified AD exhibited good cyclic stability in short-term operation. This work provides a novel approach to waste-to-energy/chemical by utilizing AD while simultaneously reducing the cost of NH₃ production through the CLAS process.

Life cycle assessment on the role of H2S-based hydrogen via H2S-methane reforming for the production of sustainable fuels

Meeting current decarbonization targets requires a shift to a hydrogen energy nexus, yet, water is a valuable resource for hydrogen production, shifting the perspective to the use of H2S instead within the context of circular economy. A comprehensive understanding of the environmental impacts, using a cradle-to-gate life cycle assessment (LCA), was developed focusing on the operation of hydrogen sulfide-methane reforming (H2SMR) for H2 production benchmarked to conventional technologies, steam methane reforming (SMR) and SMR + carbon capture (CC), as feedstock to produce sustainable fuels (i.e., methanol and ammonia). The environmental impact of the different application routes was evaluated in terms of normalized impact categories and monetized indicators by calculating the environmental damage cost. The results indicated that the environmental impact increased when moving from H2SMR < SMR + CC < SMR, and ammonia compared to methanol production. Across all the processing schemes, the impact on human health is the largest based on the normalized values, representing 63.0-85.0 % of endpoint level impacts. Within the scope of climate change, the use of H2SMR is indeed more supportive of climate mitigation efforts, reducing environmental costs related to GWP by 58.0 % from SMR and 12.0 % from SMR + CC. Addressing these concerns demands a comprehensive overhaul of existing practices within the oil and gas sector concerning raw material extraction, coupled with the implementation of effective waste management strategies to significantly minimize adverse environmental effects.

Onsite ammonia synthesis from water vapor and nitrogen in the air

An innovative method for onsite ammonia synthesis under ambient conditions has been developed using a catalyst mesh composed of magnetite (Fe3O4) and Nafion polymer. We pass air through the catalyst, which condenses microdroplets from atmospheric water vapor and uses nitrogen from the air, resulting in ammonia concentrations ranging from 25 to 120 μM in 1 hour, depending on local relative humidity. Operated at room temperature and atmospheric pressure, this technique eliminates the need for additional electricity or radiation, thereby substantially reducing carbon dioxide emissions compared to the traditional Haber-Bosch process. In laboratory experiments, we further optimized the reaction conditions and scaled up the process. After 2 hours of spraying, the ammonia concentration increased to 270.2 ± 25.1 μM. In addition, we present a portable device designed for onsite ammonia production which consistently produces an ammonia concentration that is adequate for some agricultural irrigation purposes.

ESG assessment methodology for emerging technologies: plasma versus conventional technology for ammonia production

Environmental, social and governance (ESG) criteria demand that enterprises should not be assessed solely on their financial performance, but also on their environmental, social, and governance performance. This numerical assessment of ESG criteria enables them to be evaluated with the consideration of other financial issues of enterprises' performance and thereby guides financial investments into environmentally and socially responsible firms. ESG, however, solidifies the continuance of conventional technologies but can potentially disadvantage emerging technologies. This study is the first to forecast the ESG potential of emerging chemical technologies. The Morgan Stanley Capital International (MSCI) rating system is applied to one of the top 3 global chemical processes. Ammonia (NH3) is produced via the Haber-Bosch (HB) process, which needs a huge fossil fuel input and high energy consumption, leading to a significant contribution to carbon dioxide (CO2) emissions. In contrast, the ESG assessment rates emerging plasma technology and its spearhead companies that lead innovation and development in this field, which provide the benefits of being a clean, sustainable alternative for green NH3 production. Five different plasma-technology companies are considered, with the technology readiness level (TRL) ranging from 3 to 9. These are compared to five different conventional HB companies. We examine the final ESG result of the plasma technology companies, exploring their environmental advances and social viability. In this study, five different themes were selected, including eleven issues, to measure the plasma-technology company's management related to ESG risks and opportunities.

Silicon Clathrate-Supported Catalysts with Low Work Functions for Ammonia Synthesis

Diamond-type silicon has a work function of ≈4.8 eV, and conventional n- or p-type doping modifies the value only between 4.6 and 5.05 eV. Here, it is shown that the alkali clathrates AxSi46 have substantially lower work functions approaching 2.6 eV, with clear trends between alkali electropositivity and clathrate cage size. The low work function enables alkali clathrates such as K8Si46 to be effective Haber-Bosch catalyst supports for NH3 synthesis. The catalytic properties of Si, Ge, and Sn-based clathrates are investigated while supporting Fe and Ru on the surface. The activity largely scales with the work function, and low activation energies below 60 kJ mol-1 are observed due to strong electron donation effects from the support. Ru metal and Sn clathrates seem to be unsuitable for stability issues. Compared to other similar hydride/electride catalysts, the simple structure and composition combined with stability in air/water make a systematic study of these clathrates possible and open the door to other electron-rich Zintl phases and related intermetallics as low-work function materials suitable for catalysis. The observed low work function may also have implications for other existing electronic applications.

Atomic Hydrogen Interaction with Transition Metal Surfaces: A High-Throughput Computational Study

Hydrogen adatoms are involved in many reactions catalyzed by Transition Metal (TM) surfaces, such as the Haber-Bosch process or the reverse water gas shift reaction, key to our modern society. Any rational improvement on such a catalyst requires an atomistic knowledge of the metal↔hydrogen interaction, only attainable from first-principles calculations on suited, realistic models. The present thorough density functional theory study evaluates such H interaction at a low coverage on most stable surfaces of bcc, fcc, and hcp TMs. These are (001), (011), and (111) for bcc and fcc TMs and (0001), (101̅0), and (112̅0) for hcp, covering 27 TMs and 81 different TM surfaces in total. In general terms, the results validate, while expanding, previous assessments, revealing that TM surfaces can be divided into two main groups, one in the majority where H2 would be thermodynamically driven to dissociate into H adatoms, located at heights of ∼0.5 or ∼1.0 Å, and another for late TMs, generally with a d 10 electronic configuration, where H2 adsorption with no dissociation would be preferred. No trends in H adsorption energies are found down the groups, but yes along the d series, with a best linear adjustment found for the d-band center descriptor, especially suited for close-packed fcc and hcp TMs surfaces, with a mean absolute error of 0.15 eV. Gibbs free adsorption energies reveal a theoretical volcano plot where fcc TMs are best suited, but with peak Pt performance displaced due to dispersive force inclusion in the method. Still, the volcano plot with respect to the experimental logarithm of the exchanged current density polycrystalline data is far from being valid for a quantitative assessment, although useful for a qualitative screening and to confirm the trends computationally observed.

Light-driven nitrogen fixation routes for green ammonia production

The global goal for decarbonization of the energy sector and the chemical industry could become a reality by a massive increase in renewable-based technologies. For this clean energy transition, the versatile green ammonia may play a key role in the future as a fossil-free fertilizer, long-term energy storage medium, chemical feedstock, and clean burning fuel for transportation and decentralized power generation. The high energy-intensive industrial ammonia production has triggered researchers to look for a step change in new synthetic approaches powered by renewable energies. This review provides a comprehensive comparison of light-mediated N2 fixation technologies for green ammonia production, including photocatalytic, photoelectrocatalytic, PV-electrocatalytic and photothermocatalytic routes. Since these approaches are still at laboratory scale, we examine the most recent developments and discuss the open challenges for future improvements. Last, we offer a technoeconomic comparison of current and emerging ammonia production technologies, highlighting costs, barriers, recommendations, and potential opportunities for the real development of the next generation of green ammonia solutions.

Controlled nucleation of ultrasmall MoO3 nanoparticles on defective graphene for enhanced ammonia efficiency

Ultrasmall nanoparticles on nanocarbons enhance the electrocatalytic nitrogen reduction (NRR) efficiency. Herein, we demonstrate a novel method for depositing MoO3 nanoparticles on defective graphene, achieving a high faradaic efficiency (FE) of 43.1% at -0.2 V vs. RHE.

Design and Application of Joule Heating Processes for Decarbonized Chemical and Advanced Material Synthesis

Atmospheric CO2 concentrations keep increasing at intensifying rates due to rising energy and material demands. The chemical production industry is a large energy consumer, responsible for up to 935 Mt of CO2 emissions per year, and decarbonization is its major goal moving forward. One of the primary sources of energy consumption and CO2 emissions in the chemical sector is associated with the production and use of heat for material synthesis, which conventionally was generated through the combustion of fossil fuels. To address this grand challenge, Joule heating has emerged as an alternative heating method that greatly increases process efficiency, reducing both energy consumption and greenhouse gas emissions. In this Review, we discuss the key concepts that govern these Joule heating processes including material selection and reactor design, as well as the current state-of-the-art in the literature for employing these processes to synthesize commodity chemicals along with advanced materials such as graphene, metal species, and metal carbides. Finally, we provide a perspective on future research avenues within this field, which can facilitate the widespread adoption of Joule heating for decarbonizing industrial processes.

Tandem Electroreduction of Nitrate to Ammonia Using a Cobalt-Copper Mixed Single-Atom/Cluster Catalyst with Synergistic Effects

Electrochemical conversion of waste nitrate (NO3 -) to ammonia (NH3) for environmental applications, such as carbon-neutral energy sources and hydrogen carriers, is a promising alternative to the energy-intensive Haber-Bosch process. However, increasing the energy efficiency is limited by the high overpotential and selectivity. Herein, a Co─Cu mixed single-atom/cluster catalyst (Co─Cu SCC) is demonstrated-with well-dispersed Co and Cu active sites anchored on a carbon support-that delivers high NH3 Faradaic efficiency of 91.2% at low potential (-0.3 V vs. RHE) due to synergism between the heterogenous active sites. Electrochemical analyses reveal that Cu in Co─Cu SCC preferentially catalyzes the NO3 --to-NO2 - pathway, whereupon Co catalyzes the NO2 --to-NH3 pathway. This tandem electroreduction bypasses the rate-determining steps (RDSs) for Co and Cu to lower the reaction energy barrier and surpass scaling relationship limitations. The electrocatalytic performance is amplified by the subnanoscale catalyst to increase the partial current density of NH3 by 2.3 and 5.4 times compared to those of individual Co, Cu single-atom/cluster catalysts (Co SCC, Cu SCC), respectively. This Co─Cu SCC is operated stably for 32 h in a long-term bipolar membrane (BPM)-based membrane electrode assembly (MEA) system for high-concentration NH3 synthesis to produce over 1 m NH3 for conversion into high-purity NH4Cl at 2.1 g day-1.

Heteromultimetallic Platform for Enhanced C-H Bond Activation: Aluminum-Incorporated Dicopper Complex Mimicking Cu-ZSM-5 Structure and Oxidative Reactivity

Bimetallic complexes have sparked interest across various chemical disciplines, driving advancements in research. Recent advancements in this field have shed light on complex reactions in metalloenzymes and unveiled new chemical transformations. Two primary types of bimetallic platforms have emerged: (1) systems where both metals actively participate in reactivity, and (2) systems where one metal mediates the reaction while the other regulates reactivity. This study introduces a novel multinucleating ligand platform capable of integrating both types of bimetallic systems. To demonstrate the significance of this platform, we synthesized a unique dicopper complex incorporating aluminum in its coordination environment. This complex serves as the first structural model for the active site in copper-based zeolites, highlighting the role of aluminum in hydrogen atom abstraction reactivity. Comparative studies of oxidative C-H bond activation revealed that the inclusion of aluminum significantly alters the thermodynamic driving force (by -11 kcal mol-1) for bond activation and modifies the proton-coupled electron-transfer reaction mechanism, resulting in a 14-fold rate increase. Both computational and experimental data support the high modularity of this multinucleating ligand platform, offering a new approach to fine-tune the reactivity of bimetallic complexes.

Structures and ammonia synthesis activity of hexagonal ruthenium iron nitride phases

A series of ruthenium iron nitride phases with Ru:Fe ratios of ca. 1:3 were synthesized by ammonolysis. When the ammonolysis temperature was above 500°C, the obtained RuxFe3Ny materials had a ε-Fe3N (P6322) structure, while two similar phases were present when the ammonolysis was lower than 500°C. Powder neutron diffraction identified one phase as relating to the ε-Fe3N structure, while the other had a disordered NiAs-type (P63/mmc) structure. These ternary metal nitrides show ammonia synthesis activity at low temperature (200°C-300°C) and ambient pressure, which can be related to the loss of lattice nitrogen. Steady state catalytic performance at 400°C is associated with ruthenium-iron alloy. Additionally, density functional theory calculations were performed using an approximate model for the disordered hexagonal phase, revealing that this phase is more stable than a cubic anti-perovskite phase which has been previously investigated computationally, and corroborating the experimental findings of the present work.

Sulfur-regulated CoSe2 nanowires with high-charge active centers for electrochemical nitrate reduction to ammonium

Developing high-efficiency electrocatalysts for nitrate-to-ammonia transformation holds significant promise for the production of ammonia, a crucial component in agricultural fertilizers and as a carbon-free energy carrier. In this study, we propose a viable strategy involving sulfur doping to modulate both the microstructure and electronic properties of CoSe2 for nitrate reduction. This approach remarkably enhances the conversion of nitrate to ammonia by effectively regulating the adsorption capability of nitrogenous intermediates. Specifically, sulfur-doped CoSe2 nanowires (S-CoSe2 NWs) exhibit a peak faradaic efficiency of 93.1% at -0.6 V vs. RHE and achieve the highest NH3 yield rate of 11.6 mg h-1 cm-2. Mechanistic investigations reveal that sulfur doping facilitates the creation of highly charged active sites, which enhance the adsorption of nitrite and subsequent hydrogenation, leading to improved selectivity towards ammonia production.

Catalytic Nanoparticles in Biomedical Applications: Exploiting Advanced Nanozymes for Therapeutics and Diagnostics

Catalytic nanoparticles (CNPs) as heterogeneous catalyst reveals superior activity due to their physio-chemical features, such as high surface-to-volume ratio and unique optical, electric, and magnetic properties. The CNPs, based on their physio-chemical nature, can either increase the reactive oxygen species (ROS) level for tumor and antibacterial therapy or eliminate the ROS for cytoprotection, anti-inflammation, and anti-aging. In addition, the catalytic activity of nanozymes can specifically trigger a specific reaction accompanied by the optical feature change, presenting the feasibility of biosensor and bioimaging applications. Undoubtedly, CNPs play a pivotal role in pushing the evolution of technologies in medical and clinical fields, and advanced strategies and nanomaterials rely on the input of chemical experts to develop. Herein, a systematic and comprehensive review of the challenges and recent development of CNPs for biomedical applications is presented from the viewpoint of advanced nanomaterial with unique catalytic activity and additional functions. Furthermore, the biosafety issue of applying biodegradable and non-biodegradable nanozymes and future perspectives are critically discussed to guide a promising direction in developing span-new nanozymes and more intelligent strategies for overcoming the current clinical limitations.

Hydrogen energy systems: Technologies, trends, and future prospects

This review critically examines hydrogen energy systems, highlighting their capacity to transform the global energy framework and mitigate climate change. Hydrogen showcases a high energy density of 120 MJ/kg, providing a robust alternative to fossil fuels. Adoption at scale could decrease global CO2 emissions by up to 830 million tonnes annually. Despite its potential, the expansion of hydrogen technology is curtailed by the inefficiency of current electrolysis methods and high production costs. Presently, electrolysis efficiencies range between 60 % and 80 %, with hydrogen production costs around $5 per kilogram. Strategic advancements are necessary to reduce these costs below $2 per kilogram and push efficiencies above 80 %. Additionally, hydrogen storage poses its own challenges, requiring conditions of up to 700 bar or temperatures below -253 °C. These storage conditions necessitate the development of advanced materials and infrastructure improvements. The findings of this study emphasize the need for comprehensive strategic planning and interdisciplinary efforts to maximize hydrogen's role as a sustainable energy source. Enhancing the economic viability and market integration of hydrogen will depend critically on overcoming these technological and infrastructural challenges, supported by robust regulatory frameworks. This comprehensive approach will ensure that hydrogen energy can significantly contribute to a sustainable and low-carbon future.

Fast and Sensitive Detection of Ammonia from Electrochemical Nitrogen Reduction Reactions by 1H NMR with Radiation Damping

A facile NMR method is reported for analysis of ammonia from the electrochemical reduction of nitrogen, which compares a calibrated colorimetric method, a calibrated 1H NMR method and two 1H NMR direct measurements using external reference materials. Unlike spectrophotometric methods, 1H NMR requires less bench time and does not require separation of ammonia from the electrolyte. A novel approach to the problem of radiation damping in NMR measurements considered the specific role of hardware tuning. Radiation damping is suppressed improving signal-to-noise ratio and detection limit (1.5 µg L-1). The method is demonstrated to be effective for the analysis of ammonia from direct electrochemical nitrogen reduction in KOH, and from lithium-mediated nitrogen reduction in a non-aqueous solution. An uncertainty budget is prepared for the measurement of ammonia.

Challenges and Advancements in the Electrochemical Utilization of Ammonia Using Solid Oxide Fuel Cells

Solid oxide fuel cells utilized with NH3 (NH3-SOFCs) have great potential to be environmentally friendly devices with high efficiency and energy density. The advancement of this technology is hindered by the sluggish kinetics of chemical or electrochemical processes occurring on anodes/catalysts. Extensive efforts have been devoted to developing efficient and durable anode/catalysts in recent decades. Although modifications to the structure, composition, and morphology of anodes or catalysts are effective, the mechanistic understandings of performance improvements or degradations remain incompletely understood. This review informatively commences by summarizing existing reports on the progress of NH3-SOFCs. It subsequently outlines the influence of factors on the performance of NH3-SOFCs. The degradation mechanisms of the cells/systems are also reviewed. Lastly, the persistent challenges in designing highly efficient electrodes/catalysts for low-temperature NH3-SOFCs, and future perspectives derived from SOFCs are discussed. Notably, durability, thermal cycling stability, and power density are identified as crucial indicators for enhancing low-temperature (550 °C or below) NH3-SOFCs. This review aims to offer an updated overview of how catalysts/electrodes affect electrochemical activity and durability, offering critical insights for improving performance and mechanistic understanding, as well as establishing the scientific foundation for the design of electrodes for NH3-SOFCs.

Elucidating the role of potassium addition on the surface chemistry and catalytic properties of cobalt catalysts for ammonia synthesis

The ammonia synthesis process produces millions of tons of ammonia annually needed for the production of fertilisers, making it the second most produced chemical worldwide. Although this process has been optimised extensively, it still consumes large amounts of energy (around 2% of global energy consumption), making it essential to improve its efficiency. To accelerate this improvement, research on catalysts is necessary. Here, we studied the role of potassium in ammonia synthesis on cobalt catalysts and found that it was detrimental to the catalytic activity. It was shown that, regardless of the amount of introduced K, the activity of the K-modified catalysts was much lower than that of the undoped catalyst. K was found to be in the form of oxide; however, it was unstable and reducible to metallic K, which easily volatilised from the catalyst surface under activation conditions. In addition, potassium doping resulted in the sintering of the catalyst, the decrease in the surface basicity, and contributed to the loss of the active sites, mainly due to the coverage of Co surface by residual K species.

Density Functional Theory Study of Triple Transition Metal Cluster Anchored on the C2N Monolayer for Nitrogen Reduction Reactions

The electrochemical nitrogen reduction reaction (NRR) is an attractive pathway for producing ammonia under ambient conditions. The development of efficient catalysts for nitrogen fixation in electrochemical NRRs has become increasingly important, but it remains challenging due to the need to address the issues of activity and selectivity. Herein, using density functional theory (DFT), we explore ten kinds of triple transition metal atoms (M3 = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) anchored on the C2N monolayer (M3-C2N) as NRR electrocatalysts. The negative binding energies of M3 clusters on C2N mean that the triple transition metal clusters can be stably anchored on the N6 cavity of the C2N structure. As the first step of the NRR, the adsorption configurations of N2 show that the N2 on M3-C2N catalysts can be stably adsorbed in a side-on mode, except for Zn3-C2N. Moreover, the extended N-N bond length and electronic structure indicate that the N2 molecule has been fully activated on the M3-C2N surface. The results of limiting potential screen out the four M3-C2N catalysts (Co3-C2N, Cr3-C2N, Fe3-C2N, and Ni3-C2N) that have a superior electrochemical NRR performance, and the corresponding values are -0.61 V, -0.67 V, -0.63 V, and -0.66 V, respectively, which are smaller than those on Ru(0001). In addition, the detailed NRR mechanism studied shows that the alternating and enzymatic mechanisms of association pathways on Co3-C2N, Cr3-C2N, Fe3-C2N, and Ni3-C2N are more energetically favorable. In the end, the catalytic selectivity for NRR on M3-C2N is investigated through the performance of a hydrogen evolution reaction (HER) on them. We find that Co3-C2N, Cr3-C2N, Fe3-C2N, and Ni3-C2N catalysts possess a high catalytic activity for NRR and exhibit a strong capability of suppressing the competitive HER. Our findings provide a new strategy for designing NRR catalysts with high catalytic activity and selectivity.

Electrocatalysts for Inorganic and Organic Waste Nitrogen Conversion

Anthropogenic activities have disrupted the natural nitrogen cycle, increasing the level of nitrogen contaminants in water. Nitrogen contaminants are harmful to humans and the environment. This motivates research on advanced and decarbonized treatment technologies that are capable of removing or valorizing nitrogen waste found in water. In this context, the electrocatalytic conversion of inorganic- and organic-based nitrogen compounds has emerged as an important approach that is capable of upconverting waste nitrogen into valuable compounds. This approach differs from state-of-the-art wastewater treatment, which primarily converts inorganic nitrogen to dinitrogen, and organic nitrogen is sent to landfills. Here, we review recent efforts related to electrocatalytic conversion of inorganic- and organic-based nitrogen waste. Specifically, we detail the role that electrocatalyst design (alloys, defects, morphology, and faceting) plays in the promotion of high-activity and high-selectivity electrocatalysts. We also discuss the impact of wastewater constituents. Finally, we discuss the critical product analyses required to ensure that the reported performance is accurate.

Deciphering Indirect Nitrite Reduction to Ammonia in High-Entropy Electrocatalysts Using In Situ Raman and X-ray Absorption Spectroscopies

This research adopts a new method combining calcination and pulsed laser irradiation in liquids to induce a controlled phase transformation of Fe, Co, Ni, Cu, and Mn transition-metal-based high-entropy Prussian blue analogs into single-phase spinel high-entropy oxide and face-centered cubic high-entropy alloy (HEA). The synthesized HEA, characterized by its highly conductive nature and reactive surface, demonstrates exceptional performance in capturing low-level nitrite (NO2 -) in an electrolyte, which leads to its efficient conversion into ammonium (NH4 +) with a Faradaic efficiency of 79.77% and N selectivity of 61.49% at -0.8 V versus Ag/AgCl. In addition, the HEA exhibits remarkable durability in the continuous nitrite reduction reaction (NO2 -RR), converting 79.35% of the initial NO2 - into NH4 + with an impressive yield of 1101.48 µm h-1 cm-2. By employing advanced X-ray absorption and in situ electrochemical Raman techniques, this study provides insights into the indirect NO2 -RR, highlighting the versatility and efficacy of HEA in sustainable electrochemical applications.

Laser regulated mixed-phase TiO2 for electrochemical overall nitrogen fixation

Traditional oxide electrocatalytic materials encounter significant challenges associated with sluggish reaction kinetics and formidable energy barriers for NH intermediates formation in electrocatalytic nitrogen fixation. The implementation of phase control emerges as an effective strategy to address these challenges. Herein, leveraging the energy localization of laser, this work achieved precise phase control of TiO2. In the optimized material system, the rutile phase TiO2 facilitates nitrogen adsorption, while the anatase phase TiO2 provides proton sources and active oxygen species. The synergistic effect of the two phases effectively enhances the electrocatalytic activity for nitrogen reduction and oxidation, with an ammonia yield reaching ∼22.3 μg h-1 cm-2 and a nitrate yield reaching ∼60.9 μg h-1 cm-2. Furthermore, a coupled dual-electrode system with mixed-phase titanium dioxide as both the anode and cathode successfully achieved a breakthrough in electrochemical overall nitrogen fixation. This laser precision control strategy for manipulating phase sites lays the groundwork for designing efficient catalysts for energy conversion and even energy storage nanomaterials.

Integrating climate policies in the sustainability analysis of green chemicals

New and enhanced processes will not be the only drivers toward a sustainable chemical industry. Implementing climate policies will impact all components of the chemical supply chain over the following decades, making improvements in energy generation, material extraction, or transportation contribute to reducing the overall impacts of chemical technologies. Including this synergistic effect when comparing technologies offers a clearer vision of their future potential and may allow researchers to support their sustainability propositions more strongly. Ammonia and methanol production account for more than fifty percent of the CO2 emissions in this industry and are, therefore, excellent case studies. This work performs a prospective life cycle assessment until 2050 for fossil, blue, wind, and solar-based technologies under climate policies aiming to limit the global temperature rise to 1.5 °C, 2 °C, or 3.5 °C. The first finding is the inability of fossil-based routes to reduce their CO2 emissions beyond 10% by 2050 without tailored decarbonisation strategies, regardless of the chemical and climate policy considered. In contrast, green routes may produce chemicals with around 90% fewer emissions than today and even with net negative emissions (on a cradle-to-gate basis), as in the case of methanol (up to -1.4 kg CO2-eq per kg), mainly due to the contributions of technology development and increasing penetration of renewable energies. Overall, the combined production of these chemicals could be net-zero by 2050 despite their predicted two to fivefold increase in demand. Lastly, we propose a roadmap for progressive implementation by 2050 of green routes in 26 regions worldwide, applying the criterion of at least 80% reduction in climate change impacts when compared to their fossil alternatives. Furthermore, an exploratory prospective techno-economic assessment showed that by 2050, green routes could become more economically attractive. This work offers quantitative arguments to reinforce research, development, and policymaking efforts on green chemical routes reliant on renewable energies.

Cluster dynamics of heterometallic trinuclear clusters during ligand substitution, redox chemistry, and group transfer processes

Stepwise metalation of the hexadentate ligand tbsLH6 (tbsLH6 = 1,3,5-C6H9(NHC6H4-o-NHSiMe2tBu)3) affords bimetallic trinuclear clusters (tbsL)Fe2Zn(thf) and (tbsL)Fe2Zn(py). Reactivity studies were pursued to understand metal atom lability as the clusters undergo ligand substitution, redox chemistry, and group transfer processes. Chloride addition to (tbsL)Fe2Zn(thf) resulted in a mixture of species including both all-zinc and all-iron products. Addition of ArN3 (Ar = Ph, 3,5-(CF3)2C6H3) to (tbsL)Fe2Zn(py) yielded a mixture of two trinuclear products: (tbsL)Fe3(μ3-NAr) and (tbsL)Fe2Zn(μ3-NAr)(py). The two imido species were separated via crystallization, and outer sphere reduction of (tbsL)Fe2Zn(μ3-NAr)(py) resulted in the formation of a single product, [2,2,2-crypt(K)][(tbsL)Fe2Zn(μ3-NAr)]. These results provide insight into the relationship between heterometallic cluster structure and substitutional lability and could help inform both future catalyst design and our understanding of metal atom lability in bioinorganic systems.

Sunlight-Driven Nitrate-to-Ammonia Reduction with Water by Iron Oxyhydroxide Photocatalysts

The photocatalytic reduction of harmful nitrates (NO3-) in strongly acidic wastewater to ammonia (NH3) under sunlight is crucial for the recycling of limited nitrogen resources. This study reports that a naturally occurring Cl--containing iron oxyhydroxide (akaganeite) powder with surface oxygen vacancies (β-FeOOH(Cl)-OVs) facilitates this transformation. Ultraviolet light irradiation of the catalyst suspended in a Cl--containing solution promoted quantitative NO3--to-NH3 reduction with water under ambient conditions. The photogenerated conduction band electrons promoted the reduction of NO3--to-NH3 over the OVs. The valence band holes promoted self-oxidation of Cl- as the direct electron donor and eliminated Cl- was compensated from the solution. Photodecomposition of the generated hypochlorous acid (HClO) produced O2, facilitating catalytic reduction of NO3--to-NH3 with water as the electron donor in the entire system. Simulated sunlight irradiation of the catalyst in a strongly acidic nitric acid (HNO3) solution (pH ∼ 1) containing Cl- stably generated NH3 with a solar-to-chemical conversion efficiency of ∼0.025%. This strategy paves the way for sustainable NH3 production from wastewater.

Design of Single-Atom Catalysts for E lectrocatalytic Nitrogen Fixation

The Electrochemical nitrogen reduction reaction (ENRR) can be used to solve environmental problems as well as energy shortage. However, ENRR still faces the problems of low NH3 yield and low selectivity. The NH3 yield and selectivity in ENRR are affected by multiple factors such as electrolytic cells, electrolytes, and catalysts, etc. Among these catalysts are at the core of ENRR research. Single-atom catalysts (SACs) with intrinsic activity have become an emerging technology for numerous energy regeneration, including ENRR. In particular, regulating the microenvironment of SACs (hydrogen evolution reaction inhibition, carrier engineering, metal-carrier interaction, etc.) can break through the limitation of intrinsic activity of SACs. Therefore, this Review first introduces the basic principles of NRR and outlines the key factors affecting ENRR. Then a comprehensive summary is given of the progress of SACs (precious metals, non-precious metals, non-metallic) and diatomic catalysts (DACs) in ENRR. The impact of SACs microenvironmental regulation on ENRR is highlighted. Finally, further research directions for SACs in ENRR are discussed.

Catalytic ammonia synthesis on HY-zeolite-supported angstrom-size molybdenum cluster

The development of new catalysts with high N2 activation ability is an effective approach for low-temperature ammonia synthesis. Herein, we report a novel angstrom-size molybdenum metal cluster catalyst for efficient ammonia synthesis. This catalyst is prepared by the impregnation of a molybdenum halide cluster complex with an octahedral Mo6 metal core on HY zeolite, followed by the removal of all the halide ligands by activation with hydrogen. In this activation, the size of the Mo6 cluster (ca. 7 Å) is almost retained. The resulting angstrom-size cluster shows catalytic activity for ammonia synthesis from N2 and H2, and the reaction proceeds continuously even at 200 °C under 5.0 MPa. DFT calculations suggest that N[triple bond, length as m-dash]N bond cleavage is promoted by the cooperation of the multiple molybdenum sites.

Theoretical Insights into Amido Group-Mediated Enhancement of CO2 Hydrogenation to Methanol on Cobalt Catalysts

Catalytic reduction of carbon dioxide into high-value-added products, such as methanol, is an effective approach to mitigate the greenhouse effect, and improving Co-based catalysts is anticipated to yield potential catalysts with high performance and low cost. In this study, based on first-principles calculations, we elucidate the promotion effects of surface *NHx (x = 1, 2, and 3) on the carbon dioxide hydrogenation to methanol from both activity and selectivity perspectives on Co-based catalysts. The presence of *NHx reduced the energy barrier of each elementary step on Co(100) by regulating the electronic structure to alter the binding strength of intermediates or by forming a hydrogen bond between surface oxygen-containing species and *NHx to stabilize transition states. The best promotion effect for different steps corresponds to different *NHx. The energy barrier of the rate-determining step of CO2 hydrogenation to methanol is lowered from 1.55 to 0.88 eV, and the product selectivity shifts from methane to methanol with the assistance of *NHx on the Co(100) surface. A similar phenomenon is observed on the Co(111) surface. The promotion effect of *NHx on Co-based catalysts is superior to that of water, indicating that the introduction of *NHx on a Co-based catalyst is an effective strategy to enhance the catalytic performance of CO2 hydrogenation to methanol.

Effect of a QTL on wheat chromosome 5B associated with enhanced root dry mass on transpiration and nitrogen uptake under contrasting drought scenarios in wheat

BACKGROUND

A sufficient nitrogen supply is crucial for high-quality wheat yields. However, the use of nitrogen fertilization can also negatively influence ecosystems due to leaching or volatile atmospheric emissions. Drought events, increasingly prevalent in many crop production areas, significantly impact nitrogen uptake. Breeding more efficient wheat varieties is necessary to achieve acceptable yields with limited nitrogen and water. Crop root systems play a crucial role as the primary organ for absorbing water and nutrients. To investigate the impact of an enhanced root system on nitrogen and water use efficiency in wheat under various irrigation conditions, this study conducted two experiments using precision phenotyping platforms for controlled drought stress treatment. Experiment 1 involved four contrasting winter wheat genotypes. It included the Chinese variety Ning0604, carrying a quantitative trait locus (QTL) on chromosome 5B associated with a higher root dry biomass, and three elite German varieties, Elixer, Genius, and Leandrus. Experiment 2 compared near-isogenic lines (NIL) of the three elite varieties, each containing introgressions of the QTL on chromosome 5B linked to root dry mass. In both experiments, nitrogen partitioning was tracked via isotope discrimination after fertilization with 5 Atom % 15N-labeled KNO3-.

RESULTS

In experiment 1 the quantification by 15N isotope discrimination revealed significantly (p < 0.05) higher nitrogen derived from fertilizer in the root organ for Ning0604 than those of the three German varieties. In experiment 2, two out of three NILs showed a significantly (p < 0.05) higher uptake of N derived from fertilizer than their respective recipient line under well-watered conditions. Furthermore, significantly lower transpiration rates (p < 0.1) were observed in one NIL compared to its respective recipient.

CONCLUSIONS

The combination of the DroughtSpotter facility coupled with 15N tracer-based tracking of N uptake and remobilization extends the insight into the impact of genetically altered root biomass on wheat NUE and WUE under different water availability scenarios. The study shows the potential for how a modified genetic constitution of the locus on wheat chromosome 5B can reduce transpiration and enhance N uptake. The dependence of the observations on the recipient and water availability suggests a need for further research to investigate the interaction with genetic background traits.

Membranes Matter: Preventing Ammonia Crossover during Electrochemical Ammonia Synthesis

The electrochemical nitrogen and nitrate reduction reactions (E-NRR and E-NO3RR) promise to provide decentralized and fossil-fuel-free ammonia synthesis, and as a result, E-NRR and E-NO3RR research has surged in recent years. Membrane NH3/NH4+ crossover during E-NRR and E-NO3RR decreases Faradaic efficiency and thus the overall yield. During catalyst evaluation, such unaccounted-for crossover results in measurement error. Herein, several commercially available membranes were screened and evaluated for use in ammonia-generating electrolyzers. NH3/NH4+ crossover of the commonly used cation-exchange membrane (CEM) Nafion 212 was measured in an H-cell architecture and found to be significant. Interestingly, some anion exchange membranes (AEMs) show negligible NH4+ crossover, addressing the problem of measurement error due to NH4+ crossover. Further investigation of select membranes in a zero-gap gas diffusion electrode (GDE)-cell determines that most membranes show significant NH3 crossover when the cell is in an open circuit. However, uptake and crossover of NH3 are mitigated when -1.6 V is applied across the GDE-cell. The results of this study present AEMs as a useful alternative to CEMs for H-cell E-NRR and E-NO3RR electrolyzer studies and present critical insight into membrane crossover in zero-gap GDE-cell E-NRR and E-NO3RR electrolyzers.

Niobium Carbide and Tantalum Carbide as Nitrogen Reduction Electrocatalysts: Catalytic Activity, Carbophilicity, and the Importance of Intermediate Oxidation States

Significant interest in the electrocatalytic reduction of molecular nitrogen to ammonia (the nitrogen reduction reaction: NRR) has focused attention on transition metal carbides as possible electrocatalysts. However, a fundamental understanding of carbide surface structure/NRR reactivity relationships is sparse. Herein, electrochemistry, DFT-based calculations, and in situ photoemission studies demonstrate that NbC, deposited by magnetron sputter deposition, is active for NRR at pH 3.2 but only after immersion of an ambient-induced Nb2O5 surface layer in 0.3 M NaOH, which leaves Nb suboxides with niobium in intermediate formal oxidation states. Photoemission data, however, show that polarization to -1.3 V vs Ag/AgCl restores the Nb2O5 overlayer, correlating with electrochemical measurements showing inhibition of NRR activity under these conditions. In contrast, a similar treatment of a sputter-deposited TaC sample in 0.3 M NaOH fails to reduce the ambient-induced Ta2O5 surface layer, and TaC is inactive for NRR at potentials more positive than -1.0 V even though a significant cathodic current is observed. A TaC sample with surface oxide partially reduced by Ar ion sputtering in UHV prior to in situ transfer to UHV exhibits a restored Ta2O5 surface layer after electrochemical polarization to -1.0 V vs Ag/AgCl. The electrochemical and photoemission results are in accord with DFT-based calculations indicating greater N≡N bond activation for N2 bound end-on to Nb(IV) and Nb(III) sites than for N2 bound end-on to Nb(V) sites. Thus, theory and experiment demonstrate that with respect to NbC, the formation and stabilization of intermediate (non-d0) oxidation states for surface transition metal ions is critical for N≡N bond activation and NRR activity. Additionally, the Nb suboxide surface, formed by immersion in 0.3 M NaOH of ambient-exposed NbC, is shown to undergo reoxidation to catalytically inactive Nb2O5 at -1.3 V vs Ag/AgCl, possibly due to hydrolysis or other, as yet not understood, phenomena.

Biomonitoring of dairy farm emitted ammonia in surface waters using phytoplankton and periphyton

The increasing environmental abundance of reactive N ('Nr') entails many adverse effects for society such as soil degradation and eutrophication. In addressing the global surplus of N, there is a pressing need to quantify local sources and dynamics of Nr. Although quantified as an important anthropogenic source of Nr, the spatiotemporal patterns of ammonia ('NH3') emitted by dairy farming and its resulting pressure on local surface waters lacks quantification. Quantification could optimize farm management with minimized losses of valuable nitrogen and protection of freshwater ecology. This study aimed to unravel spatiotemporal dynamics of ammonia nitrogen emitted by a dairy farm in the atmospheric and aquatic geo-ecosphere. Atmospheric NH3 and aqueous ammonium ('NH4+') were determined over time, together with meteorological variables. Aquatic biomonitors (periphyton and phytoplankton) were employed to monitor the spatial impacts of cattle-stable emitted NH3. Atmospheric NH3 on the farm was significantly regulated by wind, sharply declining over increasing distances from the stable (average decrease in the dominant wind direction from 55.5 μg/m3 at 20 m to 5.8 μg/m3 at 500 m, in the other wind directions values decreased from 38.3 μg/m3 to 6.0 μg/m3). This was also reflected in local surface water concentrations of NH4+, with average concentrations decreasing from 37.0 mg [NH4+-N]/L at 65 m to 4.8 mg [NH4+-N]/L in the dominant wind direction, and from 1.2 to 0.7 in other directions. Periphyton biomass, total N ("TN") and δ15N all significantly reflected spatiotemporal dynamics of atmospheric NH3 and aqueous NH4+, as did phytoplankton TN. The cattle stable significantly influenced local water quality through atmospheric spreading of NH3, and both aquatic biomonitors were influenced by and reflected dairy farm emitted NH3 with a sharp dilution over distance. This study strongly underlines the importance of atmospheric transport of dairy farm emitted NH3 and its effects on local water quality.

Crystal-Phase Engineering in Heterogeneous Catalysis

The performance of a chemical reaction is critically dependent on the electronic and/or geometric structures of a material in heterogeneous catalysis. Over the past century, the Sabatier principle has already provided a conceptual framework for optimal catalyst design by adjusting the electronic structure of the catalytic material via a change in composition. Beyond composition, it is essential to recognize that the geometric atomic structures of a catalyst, encompassing terraces, edges, steps, kinks, and corners, have a substantial impact on the activity and selectivity of a chemical reaction. Crystal-phase engineering has the capacity to bring about substantial alterations in the electronic and geometric configurations of a catalyst, enabling control over coordination numbers, morphological features, and the arrangement of surface atoms. Modulating the crystallographic phase is therefore an important strategy for improving the stability, activity, and selectivity of catalytic materials. Nonetheless, a complete understanding of how the performance depends on the crystal phase of a catalyst remains elusive, primarily due to the absence of a molecular-level view of active sites across various crystal phases. In this review, we primarily focus on assessing the dependence of catalytic performance on crystal phases to elucidate the challenges and complexities inherent in heterogeneous catalysis, ultimately aiming for improved catalyst design.

Operando probing of the surface chemistry during the Haber-Bosch process

The large-scale conversion of N2 and H2 into NH3 (refs. 1,2) over Fe and Ru catalysts3 for fertilizer production occurs through the Haber-Bosch process, which has been considered the most important scientific invention of the twentieth century4. The active component of the catalyst enabling the conversion was variously considered to be the oxide5, nitride2, metallic phase or surface nitride6, and the rate-limiting step has been associated with N2 dissociation7-9, reaction of the adsorbed nitrogen10 and also NH3 desorption11. This range of views reflects that the Haber-Bosch process operates at high temperatures and pressures, whereas surface-sensitive techniques that might differentiate between different mechanistic proposals require vacuum conditions. Mechanistic studies have accordingly long been limited to theoretical calculations12. Here we use X-ray photoelectron spectroscopy-capable of revealing the chemical state of catalytic surfaces and recently adapted to operando investigations13 of methanol14 and Fischer-Tropsch synthesis15-to determine the surface composition of Fe and Ru catalysts during NH3 production at pressures up to 1 bar and temperatures as high as 723 K. We find that, although flat and stepped Fe surfaces and Ru single-crystal surfaces all remain metallic, the latter are almost adsorbate free, whereas Fe catalysts retain a small amount of adsorbed N and develop at lower temperatures high amine (NHx) coverages on the stepped surfaces. These observations indicate that the rate-limiting step on Ru is always N2 dissociation. On Fe catalysts, by contrast and as predicted by theory16, hydrogenation of adsorbed N atoms is less efficient to the extent that the rate-limiting step switches following temperature lowering from N2 dissociation to the hydrogenation of surface species.

Ammonia Synthesis via an Associative Mechanism on Alkaline Earth Metal Sites of Ca3 CrN3 H

Typically, transition metals are considered as the centers for the activation of dinitrogen. Here we demonstrate that the nitride hydride compound Ca3 CrN3 H, with robust ammonia synthesis activity, can activate dinitrogen through active sites where calcium provides the primary coordination environment. DFT calculations also reveal that an associative mechanism is favorable, distinct from the dissociative mechanism found in traditional Ru or Fe catalysts. This work shows the potential of alkaline earth metal hydride catalysts and other related 1 D hydride/electrides for ammonia synthesis.

A Conceptual Approach for the Design of New Catalysts for Ammonia Synthesis: A Metal-Support Interactions Review

The growing interest in green ammonia production has spurred the development of new catalysts with the potential to carry out the Haber-Bosch process under mild pressure and temperature conditions. While there is a wide experimental background on new catalysts involving transition metals, supports and additives, the fundamentals behind ammonia synthesis performance on these catalysts remained partially unsolved. Here, we review the most important works developed to date and analyze the traditional catalysts for ammonia synthesis, as well as the influence of the electron transfer properties of the so-called 3rd-generation catalysts. Finally, the importance of metal-support interactions is highlighted as an effective pathway for the design of new materials with potential to carry out ammonia synthesis at low temperatures and pressures.

Understanding the Synergy between Fe and Mo Sites in the Nitrate Reduction Reaction on a Bio-Inspired Bimetallic MXene Electrocatalyst

Mo- and Fe-containing enzymes catalyze the reduction of nitrate and nitrite ions in nature. Inspired by this activity, we study here the nitrate reduction reaction (NO3 RR) catalyzed by an Fe-substituted two-dimensional molybdenum carbide of the MXene family, viz., Mo2 CTx  : Fe (Tx are oxo, hydroxy and fluoro surface termination groups). Mo2 CTx  : Fe contains isolated Fe sites in Mo positions of the host MXene (Mo2 CTx ) and features a Faradaic efficiency (FE) and an NH3 yield rate of 41 % and 3.2 μmol h-1  mg-1 , respectively, for the reduction of NO3 - to NH4 + in acidic media and 70 % and 12.9 μmol h-1  mg-1 in neutral media. Regardless of the media, Mo2 CTx  : Fe outperforms monometallic Mo2 CTx owing to a more facile reductive defunctionalization of Tx groups, as evidenced by in situ X-ray absorption spectroscopy (Mo K-edge). After surface reduction, a Tx vacancy site binds a nitrate ion that subsequently fills the vacancy site with O* via oxygen transfer. Density function theory calculations provide further evidence that Fe sites promote the formation of surface O vacancies, which are identified as active sites and that function in NO3 RR in close analogy to the prevailing mechanism of the natural Mo-based nitrate reductase enzymes.

Fe-MOF Catalytic Nanoarchitectonic toward Electrochemical Ammonia Production

Electrochemical reduction of nitrate into ammonia has lately been identified as one among the promising solutions to address the challenges triggered by the growing global energy demand. Exploring newer electrocatalyst materials is vital to make this process effective and feasible. Recently, metal-organic framework (MOF)-based catalysts are being well investigated for electrocatalytic ammonia synthesis, accounting for their enhanced structural and compositional integrity during catalytic reduction reactions. In this study, we investigate the ability of the PCN-250-Fe3 MOF toward ammonia production in its pristine and activated forms. The activated MOF catalyst delivered a faradaic efficiency of about 90% at -1 V vs RHE and a yield rate of 2.5 × 10-4 mol cm-2 h-1, while the pristine catalyst delivered a 60% faradaic efficiency at the same potential. Theoretical studies further provide insights into the nitrate reduction reaction mechanism catalyzed by the PCN-250-Fe3 MOF catalyst. In short, simpler and cost-effective strategies such as pretreatment of electrocatalysts have an upper hand in aggravating the intrinsic material properties, for catalytic applications, when compared to conventional material modification approaches.

A Broad Light-Harvesting Conjugated Oligoelectrolyte Enables Photocatalytic Nitrogen Fixation in a Bacterial Biohybrid

We report a rationally designed membrane-intercalating conjugated oligoelectrolyte (COE), namely COE-IC, which endows aerobic N2 -fixing bacteria Azotobacter vinelandii with a light-harvesting ability that enables photosynthetic ammonia production. COE-IC possesses an acceptor-donor-acceptor (A-D-A) type conjugated core, which promotes visible light absorption with a high molar extinction coefficient. Furthermore, COE-IC spontaneously associates with A. vinelandii to form a biohybrid in which the COE is intercalated within the lipid bilayer membrane. In the presence of L-ascorbate as a sacrificial electron donor, the resulting COE-IC/A. vinelandii biohybrid showed a 2.4-fold increase in light-driven ammonia production, as compared to the control. Photoinduced enhancement of bacterial biomass and production of L-amino acids is also observed. Introduction of isotopically enriched 15 N2 atmosphere led to the enrichment of 15 N-containing intracellular metabolites, consistent with the products being generated from atmospheric N2 .

Energy crisis in Europe enhances the sustainability of green chemicals

Ammonia and methanol are essential to modern societies, but their production has been heavily reliant on natural gas, which contributes to supply disruptions and significant CO2 emissions. While low-carbon or green production routes have been extensively researched, their adoption has been hindered by higher costs, making them unsustainable. However, a recent energy crisis in Europe has created a unique opportunity to shift towards greener production technologies. Here we show that, green ammonia, produced through wind-powered water electrolysis, had the potential to outperform its fossil counterpart for six months as of December 2021, while methanol produced through CO2 capture and wind-based water electrolysis became an economically appealing alternative. With a coordinated effort from academia, industry, and policymakers, Europe can lead the grand transition towards more sustainable practices in the chemical industry.

Adsorbate-Induced Adatom Formation on Lithium, Iron, Cobalt, Ruthenium, and Rhenium Surfaces

Recent experimental and theoretical studies have demonstrated the reaction-driven metal-metal bond breaking in metal catalytic surfaces even under relatively mild conditions. Here, we construct a density functional theory (DFT) database for the adsorbate-induced adatom formation energy on the close-packed facets of three hexagonal close-packed metals (Co, Ru, and Re) and two body-centered cubic metals (Li and Fe), where the source of the ejected metal atom is either a step edge or a close-packed surface. For Co and Ru, we also considered their metastable face-centered cubic structures. We studied 18 different adsorbates relevant to catalytic processes and predicted noticeably easier adatom formation on Li and Fe compared to the other three metals. The NH3- and CO-induced adatom formation on Fe(110) is possible at room temperature, a result relevant to NH3 synthesis and Fischer-Tropsch synthesis, respectively. There also exist other systems with favorable adsorbate effects for adatom formation relevant to catalytic processes at elevated temperatures (500-700 K). Our results offer insight into the reaction-driven formation of metal clusters, which could play the role of active sites in reactions catalyzed by Li, Fe, Co, Ru, and Re catalysts.

Intrinsic and external active sites of single-atom catalysts

Active components with suitable supports are the common paradigm for industrial catalysis, and the catalytic activity usually increases with minimizing the active component size, generating a new frontier in catalysis, single-atom catalysts (SACs). However, further improvement of SACs activity is limited by the relatively low loading of single atoms (SAs, which are heteroatoms for most SACs, i.e., external active sites) because of the highly favorable aggregation of single heteroatoms during preparation. Research interest should be shifted to investigate SACs with intrinsic SAs, which could circumvent the aggregation of external SAs and consequently increase the SAs loading while maintaining them individual to further improve the activity. In this review, SACs with external or intrinsic SAs are discussed and, at last, the perspectives and challenges for obtaining high-loading SACs with intrinsic SAs are outlined.