The Formation of Surface Lithium-Iron Ternary Hydride and its Function on Catalytic Ammonia Synthesis at Low Temperatures

Single-atom and cluster catalysts for thermocatalytic ammonia synthesis at mild conditions

Ammonia (NH3) is closely related to the fields of food and energy that humans depend on. The exploitation of advanced catalysts for NH3 synthesis has been a research hotspot for more than one hundred years. Previous studies have shown that the Ru B5 sites (step sites on the Ru (0001) surface uniquely arranged with five Ru atoms) and Fe C7 sites (iron atoms with seven nearest neighbors) over nanoparticle catalysts are highly reactive for N2-to-NH3 conversion. In recent years, single-atom and cluster catalysts, where the B5 sites and C7 sites are absent, have emerged as promising catalysts for efficient NH3 synthesis. In this review, we focus on the recent advances in single-atom and cluster catalysts, including single-atom catalysts (SACs), single-cluster catalysts (SCCs), and bimetallic-cluster catalysts (BCCs), for thermocatalytic NH3 synthesis at mild conditions. In addition, we discussed and summarized the unique structural properties and reaction performance as well as reaction mechanisms over single-atom and cluster catalysts in comparison with traditional nanoparticle catalysts. Finally, the challenges and prospects in the rational design of efficient single-atom and cluster catalysts for NH3 synthesis were provided.

Zn Promotes Chemical Looping Ammonia Synthesis Mediated by LiH-Li2 NH Couple

Chemical looping ammonia synthesis (CLAS) is a promising alternative route to ammonia production because of its advantages of avoiding competitive adsorption of N2 and hydrogen source (H2 O or H2 ) and intervening the scaling relations in the catalytic process. Our previous studies showed that NH3 can be synthesized at low temperatures via a CLAS mediated by an alkali or alkaline earth metal hydride-imide couple with the aid of transition metal catalysts. Herein, we demonstrate that a group-IIB metal Zn, which has rarely been studied in the thermal-catalytic process, can significantly promote the performance of the lithium hydride-lithium imide (LiH-Li2 NH)-mediated CLAS process (denoted as Zn-LiH-Li2 NH). The addition of Zn dramatically changes the reaction pathway of the LiH-Li2 NH mediated loop by forming a series of intermediates including Li2 NH, lithium zinc intermetallic compounds (LiZnx ), and a ternary metal nitride (LiZnN). LiZnN together with Li2 NH functions as nitrogen carrier in the Zn-LiH-Li2 NH-mediated CLAS. Because of these properties, the kinetics of N2 fixation is significantly enhanced with a reduction in apparent activation energy from 102 kJ mol-1 to 50 kJ mol-1 . The ammonia production rate reaches 956 μmol g-1  h-1 at 350 °C, which is 19 times higher than that of the neat LiH-Li2 NH-mediated CLAS.

Facilitating green ammonia manufacture under milder conditions: what do heterogeneous catalyst formulations have to offer?

Ammonia production is one of the largest industrial processes, and is currently responsible for over 1.5% of global greenhouse gas emissions. Decarbonising this process, yielding 'green ammonia', is critical not only for sustainable fertilizer production, but also to unlocking ammonia's potential as a zero-carbon fuel and hydrogen store. In this perspective, we critically assess the role of cutting-edge heterogeneous catalysts to facilitate milder ammonia synthesis conditions that will help unlock cheaper, smaller-scale, renewables-coupled ammonia production. The highly-optimised performance of catalysts under the high temperatures and pressures of the Haber-Bosch process stands in contrast to the largely mediocre activity levels reported at lower temperatures and pressures. We identify the recent advances in catalyst design that help overcome the sluggish kinetics of nitrogen activation under these conditions and undertake a categorized analysis of improved activity achieved in a range of heterogeneous catalysts. Building on these observations, we develop a 'catalyst efficiency' analysis which helps uncover the success of a holistic approach - one that addresses the issues of nitrogen activation, hydrogenation of adsorbed nitrogen species, and engineering of materials to maximize the utilization of active sites - for achieving the elusive combination of high-activity, low-temperature formulations. Furthermore, we present a discussion on the industrial considerations to catalyst development, emphasising the importance of catalyst lifetime in addition to catalyst activity. This assessment is critical to ensuring that high productivities can translate into real advances in commercial ammonia synthesis.

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.