H2 Production via Ammonia Decomposition Using Non-Noble Metal Catalysts: A Review

Enhanced oxygen vacancies on yttrium oxide with cobalt oxide loaded for accelerating hydrogen production from ammonia decomposition

Ammonia decomposition for hydrogen production has become a critical process in the development of hydrogen economy. Non-noble based material possesses great potential of application in thermal catalysis due to its low cost and appreciable theoretical catalytic activity. However, the durability of non-noble metal catalysts under prolonged operation remains a significant challenge. Herein, the cobalt oxide supported on yttrium oxide catalysts with enhanced oxygen vacancies via co-precipitation was reported by using urea serving as a pH stabilizer, followed by annealing process in an inert atmosphere. The formation of enhanced oxygen vacancies was demonstrated accelerated the NH bond division and nitrogen recombination-desorption, thereby enhancing the efficiency of thermal catalytic decomposition of ammonia. Based on such advantages, the synthesized 50CoO/Y2O3 catalyst exhibited optimal NH3 decomposition performance, achieving an ammonia conversion rate of 98.6 % under a high gas hourly space velocity of 20,000 mL·gcat-1·h-1 with a hydrogen production rate of 22.3 mmol·gcat-1·min-1 at 550 °C. Moreover, the catalyst maintained a stable activity for 120 h without significantly degradation, suggesting non-noble based catalyst a potential competitor in hydrogen production from thermal catalytic ammonia decomposition.

Green ammonia as hydrogen carrier: current status, barriers, and strategies to achieve sustainable development goals

Hydrogen, a carbon-free fuel, has the potential to aid global nations in achieving eight of the 17 Sustainable Development Goals (SDG). The shortcomings associated with H2 transportation and storage can be mitigated by using NH3 as hydrogen carrier because of its better safety, physical, and environmental properties. However, to achieve the global climate target, green ammonia production must be incremented by four times (688 MT) from the current level. Hence, understanding of advanced green NH3 production and storage technologies, along with the factors that influence them becomes necessary. It also aids in identifying the factors hindering green H2 and NH3 production, which can be resolved by promoting research. At the same time, drafting policies that encourage green H2 and NH3 production can abet in overcoming the bottleneck faced by the industry. Presently, green ammonia production can be made feasible only when the renewable electricity cost is less than $20/MWh and carbon price of $150/t of CO2 emissions is levied. Approximately 80 % of the energy consumed during NH3 is spent on H2 generation; therefore, it is necessary to enact policies that promote green H2 production globally. Producing green H2 can aid in mitigating ∼90 % of the greenhouse gases emitted during NH3 manufacturing thereby facilitating to reduce the carbon footprint of H2 carrier and decarbonize NH3 industry.

Recent progress in the decomposition of ammonia as a potential hydrogen-carrier using green technologies

To meet the global carbon neutrality target set by the United Nations, finding alternative and cost-effective energy sources has become prominent while enhancing energy conversion methods' efficiency. The versatile applications of hydrogen (H2) as an energy vector have been highly valued over the past decades due to its significantly lower greenhouse gas emissions compared to conventional fossil fuels. However, challenges related to H2 generation and storage for portable applications have increasingly called attention to ammonia (NH3) decomposition as an effective method for on-site hydrogen production due to its high hydrogen content, high energy density, and affordability. This review highlights recent developments in green decomposition techniques for ammonia, including photocatalysis, electrocatalysis, non-thermal plasma, and other techniques, with a focus on the latest developments in new methods and materials (catalysts, electrodes, and sorbents) employed in these processes. Moreover, technical challenges and recommendations are discussed to assess the future potential of ammonia in the energy sector. The role of machine learning and artificial intelligence in ammonia decomposition is also emphasized, as these tools open up ways of simulating reaction mechanisms for the exploration of a new generation of high-performance catalysts and reducing trial-and-error approaches.

Strong Metal-Support Interaction-Induced Hydrogen Spillover Boosts Thermocatalytic Ammonia Decomposition

Efficient non-noble-metal catalysts are essential for the industrial application of ammonia decomposition reaction (ADR). This work reports a Ni-Co bimetallic thermocatalyst supported by BaCe0.8Y0.2O3-δ (BCY), a proton-conducting cermet, demonstrating state-of-the-art ADR performance. This catalyst achieves 100% NH3 conversion at 550 °C with a gas hourly space velocity of 9000 mL h-1 gcat -1 and 95.1% conversion at 520 °C, notably lowering ADR temperature requirements. It also demonstrates outstanding stability under diverse conditions. The superior ADR activity arises from Ni-Co synergies, Ce's strong basicity, as well as extensive hydrogen spillover facilitated by Y doping-induced high oxygen vacancies. Specifically, BCY's strong adsorption of H* promotes the migration of H* from the active Ni/Co surface to oxygen sites of the support and subsequent rapid diffusion as OH* in oxygen vacancy channels. This metal-support-interaction-induced hydrogen spillover effect further liberates the original active sites and boosts ammonia activation.

Efficient Hydrogen Production from Ammonia Using Ru Nanoparticles on Ce-Based Metal-Organic Framework (MOF)-Derived CeO2 with Oxygen Vacancies

Ammonia is a promising hydrogen storage material because it is easy to store and decompose into COX-free hydrogen. A Ru-based catalyst exhibits good catalytic performance in ammonia decomposition, and enhancing the interaction between the Ru atoms and the support is an important way to further improve its catalytic activity. In this study, CeO2 was prepared by calcination using a cerium-based metal-organic framework (MOF) as the precursor, and the number of oxygen vacancies on the surface of CeO2 was regulated by hydrogen reduction. The XPS and Raman results showed that abundant oxygen vacancies were formed on the surface of these CeO2, and their number increased with an increase in the reduction time. The Ru/CeO2-4 h catalyst, using CeO2 reduced for 4 h as the support, exhibited good catalytic activity in ammonia decomposition, reaching 98.9% ammonia conversion and 39.74 mmol gcat-1 min-1 hydrogen yield under the condition of GHSV = 36,000 mL gcat-1 h-1 at 500 °C. The XAFS results demonstrated that Ru was stably anchored with oxygen vacancies on the surface of CeO2 via Ru-O-Ce bonds. Density functional theory calculations further showed that these bondings lower the reaction energy barrier for N-H bond cleavage, thereby significantly enhancing the catalytic activity.

Ammonia decomposition for H2 production over an iron catalyst in a molten barium amide

We report on an unconventional and highly active iron catalyst embedded within molten barium amide (Ba(NH2)2) for NH3 decomposition. A ternary barium iron amide (Ba-NHx-Fe) species plays a crucial role in NH3 decomposition catalysis. The synergy among multiple Fe atoms, Ba atoms, and the NH2 moiety is responsible for its high catalytic activity under low temperatures.

The critical role of Fe 3d-N 2p orbital hybridization in ammonia decomposition on graphene-supported Fe6Nx clusters: a DFT study

Ammonia (NH3) is a promising carbon-free hydrogen carrier, but lowering the temperature required for its catalytic decomposition to produce H2 remains a challenge. The main obstacle is the strong adsorption of nitrogen (N) on the active sites, which can remain on the catalysts' surface and lead to poisoning. Using first-principles calculations, we investigate the effects of N accumulation on Fe6 clusters during NH3 decomposition and aim to develop strategies to mitigate N poisoning. Graphene-supported Fe6 clusters mitigate N poisoning by reducing Fe-N interaction strength, thereby improving NH3 decomposition efficiency. The energy barriers of the graphene-supported Fe6Nx (x = 1, 2) clusters' rate-limiting step have been reduced below 2 eV, compared to those calculated for the pure Fe6 cluster (2.08 eV) and the graphene-supported Fe6 cluster (2.53 eV). The rate-limiting step involves the Fe 3d-N 2p hybridization, during which an adsorbed N atom migrates across the Fe-Fe bond and combines with another N atom to form N2. This study provides new insights into the potential application of graphene-supported metal catalysts for NH3 decomposition.

Electrified Dynamically Responsive Ammonia Decomposition to Hydrogen Based on Magnetic Heating of a Ru Nanocatalyst

Storing and transporting pressurized or liquid hydrogen is expensive and hazardous. As a result, safer methods, such as chemical storage in ammonia, are becoming increasingly important. However, the instantaneous start of a conventionally heated decomposition reactor is challenging. Here we report on the electrified and dynamically responsive decomposition of ammonia as a means of releasing on-demand chemically bonded hydrogen based on the rapid magnetic heating of a well-designed Ru-based nanocomposite catalyst. Under relatively mild conditions (400 °C, 1 bar) a rapid decomposition rate of 5.33 molNH3 gRu -1 h-1 was achieved. Experimental observations under non-isothermal, dynamic conditions coupled with modelling at the level of density functional theory and micro-kinetic modeling confirmed the minute-scale response of the H2 release. The rapid response of our catalytic system would, at least in principle, enable the utilization of intermittent, renewable electricity and a tunable H2/NH3 ratio in the reactor's effluent.

Ammonia Decomposition via MOF-Derived Photothermal Catalysts

Three cobalt-based metal-organic framework (MOF)-derived catalysts were developed for photothermal hydrogen production via ammonia decomposition. The selected MOFs were from distinct families, featuring carboxylate and imidazole linkers, and diverse in terms of porosity. The resulting catalysts consisted of uniform and homogeneously dispersed cobalt nanoparticles embedded within a carbon matrix. The carboxylate-based MOF-74 derived catalyst showed the highest initial activity, but gradually deactivated. ZIF-67 derived catalyst, however, demonstrated stable performance. The synergy between photo and thermal effects was confirmed. Additionally, this catalyst was found to be also effective in ammonia synthesis, potentially closing the loop for sustainable ammonia utilization.

Unlocking Low-Temperature Ammonia Decomposition via an Iron Metal-Organic Framework-Derived Catalyst Under Photo-Thermal Conditions

Photo-thermal catalysis, leveraging both thermal and non-thermal solar contributions, emerges as a sustainable approach for fuel and chemical synthesis. In this study, an Fe-based catalyst derived from a metal-organic framework is presented for efficient photo-thermal ammonia (NH3) decomposition. Optimal conditions, under light irradiation without external heating, result in a notable 55% NH3 conversion. Mechanistic investigations reveal that the enhanced catalytic activity arises from the synergistic interplay between light-induced hot carriers and elevated temperatures during irradiation. Supported by density functional theory calculations, the findings elucidate the dual role of the catalyst. At lower temperatures, photo-generated electrons in the Fe3O4 phase serve to raise the energies of reaction intermediates, preventing the formation of a thermodynamic sink. Conversely, at higher temperatures, metallic Fe emerges as the predominant active phase, with thermal contributions prevailing. Overall, this work advances the understanding of the cooperative effects between light and heat in photo-thermal systems, paving the way for innovative applications in sustainable energy conversion.

Oxygen-Doped γ-Mo2N as High-Performance Catalyst for Ammonia Decomposition

γ-Mo2N catalysts exhibit excellent activity and stability in ammonia decomposition reactions. However, the influence of oxygen on its activity is still unknown. In this work, two γ-Mo2N catalysts with different oxygen content are synthesized using temperature-programmed nitridation of α-MoO3. The γ-Mo2N catalysts are highly oxidized and their ammonia decomposition performance is closely related to their oxygen content. The activity of γ-Mo2N with high oxygen content (HO-γ-Mo2N) is much higher, whose H2 formation rate at 550 °C is 3.3 times higher than the γ-Mo2N with low oxygen content (LO-γ-Mo2N). This is mainly attributed to two aspects: on the one hand, the higher valence state of Mo in the HO-γ-Mo2N leads to stronger Mo─NH3 bonds, which promotes the adsorption and activation of NH3. On the other hand, the H generated by N─H bond breaking is more easily migrated to O, which avoids excessive H occupying the γ-Mo2N active sites and alleviates the negative effect of hydrogen poisoning.

Boron Nitride-Supported Metal Catalysts for the Synthesis and Decomposition of Ammonia and Formic Acid

This review explores the recent advancements in the application of boron nitride (BN) as a support material for metallic nanoparticles, highlighting its potential in fostering sustainable chemical reactions when employed as a heterogeneous catalyst. Two key processes, both critical to hydrogen storage and transport, are examined in detail. First, the reversible synthesis and decomposition of ammonia using BN-supported metallic catalysts has emerged as a promising technology. This approach facilitates the preparation of Ru nanoparticles with precisely structured surface atomic ensembles, such as B5 sites, which are critical for maximizing catalytic efficiency. Second, the review emphasizes the role of BN-supported catalysts in the production of formic acid (FA), a process intrinsically linked to the reuse of carbon dioxide. In this context, hydrogen and carbon dioxide-potentially sourced from atmospheric capture-serve as reactants. BN's high CO2 adsorption capacity makes it an ideal support material for such applications. Moreover, FA can serve as a source of hydrogen through decomposition or as a precursor to alternative chemicals like carbon monoxide (CO) via dehydration, further underscoring its versatility in sustainable catalysis.

Ammonia Decomposition Catalyzed by Co Nanoparticles Encapsulated in Rare Earth Oxide

We fabricated Co-based catalysts by the low-temperature thermal decomposition of R-Co intermetallics (R = Y, La, or Ce) to reduce the temperature of ammonia cracking for hydrogen production. The catalysts synthesized are nanocomposites of Co/ROx with a metal-rich composition. In the Co13/LaO1.5 catalyst derived from LaCo13, Co nanoparticles of 10-30 nm size are enclosed by the LaO1.5 matrix. The nanocomposite exhibited superior catalytic activity (91% at 500 °C), which was attributed to dual advantages; the low workfunction of the supporter, O-deficient LaO1.5-x nanoparticles, promotes electron donation to the Co catalyst in the interface, which leads to enhanced N-H bond dissociation. Moreover, such a composite structure is effective in suppressing the grain growth of Co nanoparticles because the LaO1.5 layer works as a diffusion barrier against Co. The thermal decomposition of intermetallics is a new route for the facile synthesis of catalysts having an electronically active support.

Hydrogen production by NH3 decomposition at low temperatures assisted by surface protonics

Ammonia, which can be decomposed on-site to produce CO2-free H2, is regarded as a promising hydrogen carrier because of its high hydrogen density, wide availability, and ease of transport. Unfortunately, ammonia decomposition requires high temperatures (>773 K) to achieve complete conversion, thereby hindering its practical applicability. Here, we demonstrate that high conversion can be achieved at markedly lower temperatures using an applied electric field along with a highly active and readily producible Ru/CeO2 catalyst. Applying an electric field lowers the apparent activation energies, promotes low-temperature conversion, and even surpasses equilibrium conversion at 398 K, thereby providing a feasible route to economically attractive hydrogen production. Experimentally obtained results and neural network potential studies revealed that this reaction proceeds via HN-NH intermediate formation by virtue of surface protonics.

Active nitrogen sites on nitrogen doped carbon for highly efficient associative ammonia decomposition

Nitrogen doped carbon materials have been studied as catalyst support for ammonia decomposition. There are 4 different types of nitrogen environments (graphitic, pyrrolic, pyridinic and nitrogen oxide) on the amorphous support identified. In this paper, we report a 5%Ru on MgCO3 pre-treated nitrogen doped carbon catalyst with high content of edge nitrogen-containing sites which displays an ammonia conversion rate of over 90% at 500°C and WHSV = 30,000 mL gcat -1 h-1. It also gives an impressive hydrogen production rate of 31.3 mmol/(min gcat) with low apparent activation energy of 43 kJ mol-1. Fundamental studies indicate that the distinct average Ru-N4 coordination site on edge regions is responsible for such high catalytic activity. Ammonia is stepwise decomposed via a Ru-N(H)-N(H)-Ru intermediate. This associative mechanism circumvents the direct cleavage of energetic surface nitrogen from metal to form N2 hence lowering the activation barrier for the decomposition over this catalyst.

The Huge Role of Tiny Impurities in Nanoscale Synthesis

Nanotechnology is vital to many current industries, including electronics, energy, textiles, agriculture, and theranostics. Understanding the chemical mechanisms of nanomaterial synthesis has contributed to the tunability of their unique properties, although studies frequently overlook the potential impact of impurities. Impurities can show adverse effects, clouding the interpretation of results or limiting the practical utility of the nanomaterial. On the other hand, as successful doping has demonstrated, the intentional introduction of impurities can be a powerful tool for enhancing the properties of a nanomaterial. This Review examines the complex role of impurities, unintentionally or intentionally added, during nanoscale synthesis and their effects on the performance and usefulness of the most common classes of nanomaterials: nanocarbons, noble metal and metal oxide nanoparticles, semiconductor quantum dots, thermoelectrics, and perovskites.

Highly loaded bimetallic iron-cobalt catalysts for hydrogen release from ammonia

Ammonia is a storage molecule for hydrogen, which can be released by catalytic decomposition. Inexpensive iron catalysts suffer from a low activity due to a too strong iron-nitrogen binding energy compared to more active metals such as ruthenium. Here, we show that this limitation can be overcome by combining iron with cobalt resulting in a Fe-Co bimetallic catalyst. Theoretical calculations confirm a lower metal-nitrogen binding energy for the bimetallic catalyst resulting in higher activity. Operando spectroscopy reveals that the role of cobalt in the bimetallic catalyst is to suppress the bulk-nitridation of iron and to stabilize this active state. Such catalysts are obtained from Mg(Fe,Co)2O4 spinel pre-catalysts with variable Fe:Co ratios by facile co-precipitation, calcination and reduction. The resulting Fe-Co/MgO catalysts, characterized by an extraordinary high metal loading reaching 74 wt.%, combine the advantages of a ruthenium-like electronic structure with a bulk catalyst-like microstructure typical for base metal catalysts.

Emerging trends in research and development on earth abundant materials for ammonia degradation coupled with H2 generation

Ammonia, as an essential and economical fuel, is a key intermediate for the production of innumerable nitrogen-based compounds. Such compounds have found vast applications in the agricultural world, biological world (amino acids, proteins, and DNA), and various other chemical transformations. However, unlike other compounds, the decomposition of ammonia is widely recognized as an important step towards a safe and sustainable environment. Ammonia has been popularly recommended as a viable candidate for chemical storage because of its high hydrogen content. Although ruthenium (Ru) is considered an excellent catalyst for ammonia oxidation; however, its high cost and low abundance demand the utilization of cheaper, robust, and earth abundant catalyst. The present review article underlines the various ammonia decomposition methods with emphasis on the use of non-noble metals, such as iron, nickel, cobalt, molybdenum, and several other carbides as well as nitride species. In this review, we have highlighted various advances in ammonia decomposition catalysts. The major challenges that persist in designing such catalysts and the future developments in the production of efficient materials for ammonia decomposition are also discussed.

Plasma-Promoted Ammonia Decomposition over Supported Ruthenium Catalysts for COx -Free H2 Production

The efficient decomposition of ammonia to produce COx -free hydrogen at low temperatures has been extensively investigated as a potential method for supplying hydrogen to mobile devices based on fuel cells. In this study, we employed dielectric barrier discharge (DBD) plasma, a non-thermal plasma, to enhance the catalytic ammonia decomposition over supported Ru catalysts (Ru/Y2 O3 , Ru/La2 O3 , Ru/CeO2 and Ru/SiO2 ). The plasma-catalytic reactivity of Ru/La2 O3 was found to be superior to that of the other three catalysts. It was observed that both the physicochemical properties of the catalyst (such as support acidity) and the plasma discharge behaviours exerted significant influence on plasma-catalytic reactivity. Combining plasma with a Ru catalyst significantly enhanced ammonia conversion at low temperatures, achieving near complete NH3 conversion over the 1.5 %-Ru/La2 O3 catalyst at temperatures as low as 380 °C. Under a weight gas hourly space velocity of 2400 mL gcat -1  h-1 and an AC supply power of 20 W, the H2 formation rate and energy efficiency achieved were 10.7 mol gRu -1  h-1 and 535 mol gRu -1 (kWh)-1 , respectively, using a 1.5 %-Ru/La2 O3 catalyst.

Ammonia Cracking Catalyzed by Ni Nanoparticles Confined in the Framework of CeO2 Support

For the extraction of hydrogen from ammonia at low temperatures, we investigated Ni-based catalysts fabricated by the thermal decomposition of RNi5 intermetallics (R = Ce or Y). The interconnected microstructure formed via phase separation between the Ni catalyst and the resulting oxide support was observed to evolve via low-temperature thermal decomposition of RNi5. The resulting Ni/CeO2 nanocomposite exhibited superior catalytic activity of ∼25% at 400 °C for NH3 cracking. The high catalytic activity was attributed to the interlocking of Ni nanoparticles with the CeO2 framework. The growth of Ni nanoparticles was prevented by this interconnected microstructure, in which the Ni nanoparticles incorporated nitrogen owing to the size effect, whereas Ni does not commonly form nitrides. To the best of our knowledge, this is a unique example of a microstructure that enhances catalytic NH3 cracking.

Self-Construction of Efficient Interfaces Ensures High-Performance Direct Ammonia Protonic Ceramic Fuel Cells

Direct ammonia protonic ceramic fuel cells (PCFCs) are highly efficient energy conversion devices since ammonia as a carbon-neutral hydrogen-rich carrier shows great potential for storage and long-distance transportation when compared with hydrogen fuel. However, traditional Ni-based anodes readily suffer from severe structural destruction and dramatic deactivation after long-time exposure to ammonia. Here a Sr2 Fe1.35 Mo0.45 Cu0.2 O6-δ (SFMC) anode catalytic layer (ACL) painted onto a Ni-BaZr0.1 Ce0.7 Y0.1 Yb0.1 O3- δ (BZCYYb) anode with enhanced catalytic activity and durability toward the direct utilization of ammonia is reported. A tubular Ni-BZCYYb anode-supported cells with the SFMC ACL show excellent peak power densities of 1.77 W cm-2 in wet H2 (3% H2 O) and 1.02 W cm-2 in NH3 at 650 °C. A relatively stable operation of the cells is obtained at 650 °C for 200 h in ammonia fuel. Such achieved improvements in the activity and durability are attributed to the self-constructed interfaces with the phases of NiCu or/and NiFe for efficient NH3 decomposition, resulting in a strong NH3 adsorption strength of the SFMC, as confirmed by NH3 thermal conversion and NH3 -temperature programmed desorption. This research offers a valuable strategy of applying an internal catalytic layer for highly active and durable ammonia PCFCs.

Simple Rate Expression for Catalyzed Ammonia Decomposition for Fuel Cells

This paper examines NH3 decomposition rates based on a literature-proven six-step elementary catalytic (Ni-BaZrO3) mechanism valid for 1 × 105 Pa pressure in a 650-950 K range. The rates are generated using a hypothetical continuous stirred tank catalytic reactor model running the literature mechanism. Excellent correlations are then obtained by fitting these rates to a simple overall kinetic expression based on an assumed slow step, with the remaining steps in fast pseudo-equilibria. The robust overall simple rate expression is then successfully demonstrated in various packed bed reactor applications. This expression facilitates engineering calculations without the need for a complex, detailed mechanism solver package. The methodology used in this work is independent of the choice of catalyst. It relies on the availability of a previously published and validated elementary reaction mechanism.

Advances and Perspectives of H2 Production from NH3 Decomposition in Membrane Reactors

Hydrogen is often regarded as an ideal energy carrier. Its use in energy conversion devices does in fact not produce any pollutants. However, due to challenges related to its transportation and storage, liquid hydrogen carriers are being investigated. Among the liquid hydrogen carriers, ammonia is considered very promising because it is easy to store and transport, and its conversion to hydrogen has only nitrogen as a byproduct. This work focuses on a review of the latest results of studies dealing with ammonia decomposition for hydrogen production. After a general introduction to the topic, this review specifically focuses on works presenting results of membrane reactors for ammonia decomposition, particularly describing the different reactor configurations and operating conditions, membrane properties, catalysts, and purification steps that are required to achieve pure hydrogen for fuel cell applications.

Recent Progress on Hydrogen Production from Ammonia Decomposition: Technical Roadmap and Catalytic Mechanism

Ammonia decomposition has attracted significant attention in recent years due to its ability to produce hydrogen without emitting carbon dioxide and the ease of ammonia storage. This paper reviews the recent developments in ammonia decomposition technologies for hydrogen production, focusing on the latest advances in catalytic materials and catalyst design, as well as the research progress in the catalytic reaction mechanism. Additionally, the paper discusses the advantages and disadvantages of each method and the importance of finding non-precious metals to reduce costs and improve efficiency. Overall, this paper provides a valuable reference for further research on ammonia decomposition for hydrogen production.

Strategies To Improve the Performance of Hydrogen Storage Systems by Liquefaction Methods: A Comprehensive Review

The main challenges of liquid hydrogen (H2) storage as one of the most promising techniques for large-scale transport and long-term storage include its high specific energy consumption (SEC), low exergy efficiency, high total expenses, and boil-off gas losses. This article reviews different approaches to improving H2 liquefaction methods, including the implementation of absorption cooling cycles (ACCs), ejector cooling units, liquid nitrogen/liquid natural gas (LNG)/liquid air cold energy recovery, cascade liquefaction processes, mixed refrigerant systems, integration with other structures, optimization algorithms, combined with renewable energy sources, and the pinch strategy. This review discusses the economic, safety, and environmental aspects of various improvement techniques for H2 liquefaction systems in more detail. Standards and codes for H2 liquefaction technologies are presented, and the current status and future potentials of H2 liquefaction processes are investigated. The cost-efficient H2 liquefaction systems are those with higher production rates (>100 tonne/day), higher efficiency (>40%), lower SEC (<6 kWh/kgLH2), and lower investment costs (1-2 $/kgLH2). Increasing the stages in the conversion of ortho- to para-H2 lowers the SEC and increases the investment costs. Moreover, using low-temperature waste heat from various industries and renewable energy in the ACC for precooling is significantly more efficient than electricity generation in power generation cycles to be utilized in H2 liquefaction cycles. In addition, the substitution of LNG cold recovery for the precooling cycle is associated with the lower SEC and cost compared to its combination with the precooling cycle.

Energy Transfer to Molecular Adsorbates by Transient Hot Electron Spillover

Hot electron (HE) photocatalysis is one of the most intriguing fields of nanoscience, with a clear potential for technological impact. Despite much effort, the mechanisms of HE photocatalysis are not fully understood. Here we investigate a mechanism based on transient electron spillover on a molecule and subsequent energy release into vibrational modes. We use state-of-the-art real-time Time Dependent Density Functional Theory (rt-TDDFT), simulating the dynamics of a HE moving within linear chains of Ag or Au atoms, on which CO, N₂, or H₂O are adsorbed. We estimate the energy a HE can release into adsorbate vibrational modes and show that certain modes are selectively activated. The energy transfer strongly depends on the adsorbate, the metal, and the HE energy. Considering a cumulative effect from multiple HEs, we estimate this mechanism can transfer tenths of an eV to molecular vibrations and could play an important role in HE photocatalysis.

Non-Noble FeCrOx Bimetallic Nanoparticles for Efficient NH3 Decomposition

Ammonia has the advantages of being easy to liquefy, easy to store, and having a high hydrogen content of 17.3 wt%, which can be produced without CO_x through an ammonia decomposition using an appropriate catalyst. In this paper, a series of FeCr bimetallic oxide nanocatalysts with a uniform morphology and regulated composition were synthesized by the urea two-step hydrolysis method, which exhibited the high-performance decomposition of ammonia. The effects of different FeCr metal ratios on the catalyst particle size, morphology, and crystal phase were investigated. The Fe_0.75Cr_0.25 sample exhibited the highest catalytic activity, with an ammonia conversion of nearly 100% at 650 °C. The dual metal catalysts clearly outperformed the single metal samples in terms of their catalytic performance. Besides XRD, XPS, and SEM being used as the means of the conventional characterization, the local structural changes of the FeCr metal oxide catalysts in the catalytic ammonia decomposition were investigated by XAFS. It was determined that the Fe metal and FeN_x of the bcc structure were the active species of the ammonia-decomposing catalyst. The addition of Cr successfully prevented the Fe from sintering at high temperatures, which is more favorable for the formation of stable metal nitrides, promoting the continuous decomposition of ammonia and improving the decomposition activity of the ammonia. This work reveals the internal relationship between the phase and structural changes and their catalytic activity, identifies the active catalytic phase, thus guiding the design and synthesis of catalysts for ammonia decomposition, and excavates the application value of transition-metal-based nanocomposites in industrial catalysis.

Plate-Like Colloidal Metal Nanoparticles

The pseudo-two-dimensional (2D) morphology of plate-like metal nanoparticles makes them one of the most anisotropic, mechanistically understood, and tunable structures available. Although well-known for their superior plasmonic properties, recent progress in the 2D growth of various other materials has led to an increasingly diverse family of plate-like metal nanoparticles, giving rise to numerous appealing properties and applications. In this review, we summarize recent progress on the solution-phase growth of colloidal plate-like metal nanoparticles, including plasmonic and other metals, with an emphasis on mechanistic insights for different synthetic strategies, the crystallographic habits of different metals, and the use of nanoplates as scaffolds for the synthesis of other derivative structures. We additionally highlight representative self-assembly techniques and provide a brief overview on the attractive properties and unique versatility benefiting from the 2D morphology. Finally, we share our opinions on the existing challenges and future perspectives for plate-like metal nanomaterials.

Incorporation of a Metal Catalyst for the Ammonia Synthesis in a Ferroelectric Packed-Bed Plasma Reactor: Does It Really Matter?

Plasma-catalysis has been proposed as a potential alternative for the synthesis of ammonia. Studies in this area focus on the reaction mechanisms and the apparent synergy existing between processes occurring in the plasma phase and on the surface of the catalytic material. In the present study, we approach this problem using a parallel-plate packed-bed reactor with the gap between the electrodes filled with pellets of lead zirconate titanate (PZT), with this ferroelectric material modified with a coating layer of alumina (i.e., Al₂O₃/PZT) and the same alumina layer incorporating ruthenium nanoparticles (i.e., Ru-Al₂O₃/PZT). At ambient temperature, the electrical behavior of the ferroelectric packed-bed reactor differed for these three types of barriers, with the plasma current reaching a maximum when using Ru-Al₂O₃/PZT pellets. A systematic analysis of the reaction yield and energy efficiency for the ammonia synthesis reaction, at ambient temperature and at 190 °C and various electrical operating conditions, has demonstrated that the yield and the energy efficiency for the ammonia synthesis do not significantly improve when including ruthenium particles, even at temperatures at which an incipient catalytic activity could be inferred. Besides disregarding a net plasma-catalysis effect, reaction results highlight the positive role of the ferroelectric PZT as moderator of the discharge, that of Ru particles as plasma hot points, and that of the Al₂O₃ coating as a plasma cooling dielectric layer.

Nickel Nanoparticles Anchored on Activated Attapulgite Clay for Ammonia Decomposition to Hydrogen

Ammonia decomposition to hydrogen technique is an effectively way to solve the problems associated with the storage and transportation of hydrogen, but the development of a high-performance catalyst for ammonia decomposition is a great challenge. Ni species supported on activated attapulgite clay (AATP) is prepared by a homogeneous precipitation method for ammonia decomposition to COx-free H2. The structural properties of the Ni/AATP catalysts are characterized by thermogravimetric analysis, X-ray diffraction, scanning and transmission electron microscopy, H2 temperature-programmed reduction, and N2 sorption technique. It is revealed that the porous structure and high surface area of rod-like symmetric AATP results in highly dispersed NiO particles because the presence of a strong interaction between AATP and NiO particles. In particular, the Si-OH in AATP can react with Ni species, forming Si-O-Ni species at the interface between Ni and AATP. The Ni/AAPT catalysts are used for ammonia decomposition, the 20%-Ni/ATTP catalyst shows a 95.3% NH3 conversion with 31.9 mmol min−1 gcat−1 H2 formation rate at 650 °C. This study opens a new way to utilize natural minerals as an efficient support of catalysts towards ammonia decomposition reaction.

Catalytic ammonia reforming: alternative routes to net-zero-carbon hydrogen and fuel

Ammonia is an energy-dense liquid hydrogen carrier and fuel whose accessible dissociation chemistries offer promising alternatives to hydrogen electrolysis, compression and dispensing at scale. Catalytic ammonia reforming has thus emerged as an area of renewed focus within the ammonia and hydrogen energy research & development communities. However, a majority of studies emphasize the discovery of new catalytic materials and their evaluation under idealized laboratory conditions. This Perspective highlights recent advances in ammonia reforming catalysts and their demonstrations in realistic application scenarios. Key knowledge gaps and technical needs for real reformer devices are emphasized and presented alongside enabling catalyst and reaction engineering fundamentals to spur future investigations into catalytic ammonia reforming.

Hydrogenolysis of Polyethylene and Polypropylene into Propane over Cobalt-Based Catalysts

The development of technologies to recycle polyethylene (PE) and polypropylene (PP), globally the two most produced polymers, is critical to increase plastic circularity. Here, we show that 5 wt % cobalt supported on ZSM-5 zeolite catalyzes the solvent-free hydrogenolysis of PE and PP into propane with weight-based selectivity in the gas phase over 80 wt % after 20 h at 523 K and 40 bar H2. This catalyst significantly reduces the formation of undesired CH4 (≤5 wt %), a product which is favored when using bulk cobalt oxide or cobalt nanoparticles supported on other carriers (selectivity ≤95 wt %). The superior performance of Co/ZSM-5 is attributed to the stabilization of dispersed oxidic cobalt nanoparticles by the zeolite support, preventing further reduction to metallic species that appear to catalyze CH4 generation. While ZSM-5 is also active for propane formation at 523 K, the presence of Co promotes stability and selectivity. After optimizing the metal loading, it was demonstrated that 10 wt % Co/ZSM-5 can selectively catalyze the hydrogenolysis of low-density PE (LDPE), mixtures of LDPE and PP, as well as postconsumer PE, showcasing the effectiveness of this technology to upcycle realistic plastic waste. Cobalt supported on zeolites FAU, MOR, and BEA were also effective catalysts for C2-C4 hydrocarbon formation and revealed that the framework topology provides a handle to tune gas-phase selectivity.

Ammonia Decomposition over Ru/SiO2 Catalysts

Ammonia decomposition is a key step in hydrogen production and is considered a promising practical intercontinental hydrogen carrier. In this study, 1 wt.% Ru/SiO2 catalysts were prepared via wet impregnation and subjected to calcination in air at different temperatures to control the particle size of Ru. Furthermore, silica supports with different surface areas were prepared after calcination at different temperatures and utilized to support a change in the Ru particle size distribution of Ru/SiO2. N2 physisorption and transmission electron microscopy were used to probe the textural properties and Ru particle size distribution of the catalysts, respectively. These results show that the Ru/SiO2 catalyst with a high-surface area achieved the highest ammonia conversion among catalysts at 400 °C. Notably, this is closely related to the Ru particle sizes ranging between 5 and 6 nm, which supports the notion that ammonia decomposition is a structure-sensitive reaction.

Catalysis of Alloys: Classification, Principles, and Design for a Variety of Materials and Reactions

Alloying has long been used as a promising methodology to improve the catalytic performance of metallic materials. In recent years, the field of alloy catalysis has made remarkable progress with the emergence of a variety of novel alloy materials and their functions. Therefore, a comprehensive disciplinary framework for catalytic chemistry of alloys that provides a cross-sectional understanding of the broad research field is in high demand. In this review, we provide a comprehensive classification of various alloy materials based on metallurgy, thermodynamics, and inorganic chemistry and summarize the roles of alloying in catalysis and its principles with a brief introduction of the historical background of this research field. Furthermore, we explain how each type of alloy can be used as a catalyst material and how to design a functional catalyst for the target reaction by introducing representative case studies. This review includes two approaches, namely, from materials and reactions, to provide a better understanding of the catalytic chemistry of alloys. Our review offers a perspective on this research field and can be used encyclopedically according to the readers' individual interests.

Hydrogen production from urea in human urine using segregated systems

Removal of nitrogen compounds through biological processes represents the highest energy consumption in conventional centralised wastewater treatment facilities. Alternatively, segregated systems, where wastewater is treated at its source, present the potential to provide value to nitrogen-rich compounds contained in wastewater like urea. This paper demonstrates the feasibility of a novel process to recover energy from human urine based on the pre-isolation of urea to decrease the energy requirements for its thermal decomposition compared to the conventional thermal treatment when in solution, followed by its decomposition into hydrogen. Herein, urea is separated from an aqueous solution by adsorption onto activated carbon. Thermal urea desorption and decomposition into ammonia and CO₂ at 250 °C leads to full regeneration of the carbon, showing a constant adsorption capacity for at least 5 consecutive adsorption/desorption cycles. Finally, when the regeneration and urea decomposition step is coupled to an ammonia decomposition catalyst, hydrogen is produced to be used as an energy fuel. This process opens the door to a new way of circular economy by energy recovery from hydrogen-rich components in segregated wastewater streams. Preliminary energy balances show that the adoption of this energy recovery system in a city of 160,000 inhabitants would lead to a daily hydrogen production of 430 kg, with a net energy production of 2,500 kWh/day. In addition, such waste-to-energy process would lead to energy savings of 4,600 kWh/day in a conventional wastewater treatment plant reducing its energy consumption by around 35%.

Recent Advances in Bimetallic Catalysts for Hydrogen Production from Ammonia

The emerging concept of the hydrogen economy is facing challenges associated with hydrogen storage and transport. The utilization of ammonia as an energy (hydrogen) carrier for the on-site generation of hydrogen via ammonia decomposition has gained attraction among the scientific community. Ruthenium-based catalysts are highly active but their high cost and less abundance are limitations for scale-up application. Therefore, combining ruthenium with cheaper transition metals such as nickel, cobalt, iron, molybdenum, etc., to generate metal-metal (bimetallic) surfaces suitable for ammonia decomposition has been investigated in recent years. Herein, the recent trends in developing bimetallic catalyst systems, the role of metal type, support materials, promoter, synthesis techniques, and the investigations of the reaction kinetics and mechanism for ammonia decomposition have been reviewed.

Nanomaterials enhancing the solid-state storage and decomposition of ammonia

Hydrogen is ideal for producing carbon-free and clean-green energy with which to save the world from climate change. Proton exchange membrane fuel cells use to hydrogen to produce 100% clean energy, with water the only by-product. Apart from generating electricity, hydrogen plays a crucial role in hydrogen-powered vehicles. Unfortunately, the practical uses of hydrogen energy face many technical and safety barriers. Research into hydrogen generation and storage and reversibility transportation are still in its very early stages. Ammonia (NH₃) has several attractive attributes, with a high gravimetric hydrogen density of 17.8 wt% and theoretical hydrogen conversion efficiency of 89.3%. Ammonia storage and transport are well-established technologies, making the decomposition of ammonia to hydrogen the safest and most carbon-free option for using hydrogen in various real-time applications. However, several key challenges must be addressed to ensure its feasibility. Current ammonia decomposition technologies require high temperatures, pressures and non-recyclable catalysts, and a sustainable decomposition mechanism is urgently needed. This review article comprehensively summarises current knowledge about and challenges facing solid-state storage of ammonia and decomposition. It provides potential strategic solutions for developing a scalable process with which to produce clean hydrogen by eliminating possible economic and technical barriers.

Emerging Materials and Methods toward Ammonia-Based Energy Storage and Conversion

Efficient storage and conversion of renewable energies is of critical importance to the sustainable growth of human society. With its distinguishing features of high hydrogen content, high energy density, facile storage/transportation, and zero-carbon emission, ammonia has been recently considered as a promising energy carrier for long-term and large-scale energy storage. Under this scenario, the synthesis, storage, and utilization of ammonia are key components for the implementation of ammonia-mediated energy system. Being different from fossil fuels, renewable energies normally have intermittent and variable nature, and thus pose demands on the improvement of existing technologies and simultaneously the development of alternative methods and materials for ammonia synthesis and storage. The energy release from ammonia in an efficient manner, on the other hand, is vital to achieve a sustainable energy supply and complete the nitrogen circle. Herein, recent advances in the thermal-, electro-, plasma-, and photocatalytic ammonia synthesis, ammonia storage or separation, ammonia thermal/electrochemical decomposition and conversion are summarized with the emphasis on the latest developments of new methods and materials (catalysts, electrodes, and sorbents) for these processes. The challenges and potential solutions are discussed.