Behavior of Lithium Amide Under Argon Plasma.

Alkali and alkaline earth metal amides are a type of functional materials for hydrogen storage, thermal energy storage, ion conduction, and chemical transformations such as ammonia synthesis and decomposition. The thermal chemistry of lithium amide (LiNH2), as a simple but representative alkali or alkaline earth metal amide, has been well studied previously encouraged by its potentials in hydrogen storage. In comparison, little is known about the interaction of plasma and LiNH2. Herein, we report that the plasma treatment of LiNH2 in an Ar flow under ambient temperature and pressure gives rise to distinctly different reaction products and reaction pathway from that of the thermal process. We found that plasma treatment of LiNH2 leads to the formation of Li colloids, N2, and H2 as observed by UV-vis absorption, EPR, and gas products analysis. Inspired by this very unique interaction between plasma and LiNH2, a chemical loop for ammonia decomposition to N2 and H2 mediated by LiNH2 was proposed and demonstrated.

Light-driven ammonia synthesis under mild conditions using lithium hydride.

Photon-driven chemical processes are usually mediated by oxides, nitrides and sulfides whose photo-conversion efficiency is limited by charge carrier recombination. Here we show that lithium hydride undergoes photolysis upon ultraviolet illumination to yield long-lived photon-generated electrons residing in hydrogen vacancies, known as F centres. We demonstrate that photon-driven dehydrogenation and dark rehydrogenation over lithium hydride can be fulfilled reversibly at room temperature, which is about 600 K lower than the corresponding thermal process. As light-driven F centre generation could provide an alternative approach to charge carrier separation to favour chemical transformations that are kinetically or thermodynamically challenging, we show that light-activated lithium hydride cleaves the N≡N triple bond to form a N-H bond under mild conditions. Co-feeding a N2/H2 mixture with low H2 partial pressure leads to photocatalytic ammonia formation at near ambient conditions. This work provides insights into the development of advanced materials and processes for light harvesting and conversion.

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.

Barium hydride activates Ni for ammonia synthesis catalysis.

Nickel (Ni) metal has long been considered to be far less active for catalytic ammonia synthesis as compared to iron, cobalt, and ruthenium. Herein, we show that Ni metal synergized with barium hydride (BaH2) can catalyse ammonia synthesis with an activity comparable to that of an active Cs-Ru/MgO catalyst typically below 300 °C. Kinetic analyses show that the addition of BaH2 makes the apparent activation energy for the Ni catalyst decrease dramatically from 150 kJ mol-1 to 87 kJ mol-1. This result together with N2-TPR experiments suggests a strong synergistic effect between Ni and BaH2 for promoting N2 activation and hydrogenation to NH3. It is suggested that an intermediate [N-H] species is generated upon N2 fixation and then is hydrogenated to NH3 with the regeneration of hydride species, forming a catalytic cycle.

Transition-Metal-Free Barium Hydride Mediates Dinitrogen Fixation and Ammonia Synthesis.

Transition-metal-mediated dinitrogen fixation has been intensively investigated. The employment of main group elements for this vital reaction has recently sparked interest because of new dinitrogen reaction chemistry. We report ammonia synthesis via a chemical looping process mediated by a transition-metal-free barium hydride (BaH2 ). Experimental and computational studies reveal that the introduction of hydrogen vacancies is essential for creating multiple coordinatively unsaturated Ba sites for N2 activation. The adjacent lattice hydridic hydrogen (H- ) then undergoes both reductive elimination and reductive protonation to convert N2 to NHx . The ammonia production rate supports this hydride-vacancy mechanism via a chemical looping route that far exceeds that of a catalytic process. The BaH2 -mediated chemical looping process has prospects in future technologies for ammonia synthesis using transition-metal-free materials.

In situ formed Co from a Co-Mg-O solid solution synergizing with LiH for efficient ammonia synthesis.

A cobalt magnesium oxide solid solution (Co-Mg-O) supported LiH catalyst has been synthesized, in which LiH functions both as a strong reductant for the in situ formation of Co metal nanoparticles and a key active component for ammonia synthesis catalysis. Dispersion of the Co-LiH composite on the Co-Mg-O support results in a significantly higher ammonia synthesis rate under mild reaction conditions (19 mmol g-1 h-1 at 300 °C, 10 bar).

Thermodynamic Properties of Ammonia Production from Hydrogenation of Alkali and Alkaline Earth Metal Amides.

Thermodynamic properties of alkali and alkaline earth metal amides are critical for their performance in hydrogen storage as well as catalytic ammonia synthesis. In this work, the ammonia equilibrium concentrations of LiNH2 , KNH2 and Ba(NH2 )2 at ca.10 bar of hydrogen pressure and different temperatures were measured by using a high-pressure gas-solid reaction system equipped with a conductivity meter. Hydrogenation of KNH2 gives the highest ammonia equilibrium concentration, followed by Ba(NH2 )2 and LiNH2 . Based on these data, the entropy and enthalpy changes of the reaction of ANH2 +H2 →AH+NH3 (A=Li, K, and Ba) were obtained from the van't Hoff equation. These thermodynamic parameters provide important information on the understanding of metal amides in catalytic ammonia synthesis reaction.

Alkali and Alkaline Earth Hydrides-Driven N2 Activation and Transformation over Mn Nitride Catalyst.

Early 3d transition metals are not focal catalytic candidates for many chemical processes because they have strong affinities to O, N, C, or H, etc., which would hinder the conversion of those species to products. Metallic Mn, as a representative, undergoes nitridation under ammonia synthesis conditions forming bulk phase nitride and unfortunately exhibits negligible catalytic activity. Here we show that alkali or alkaline earth metal hydrides (i.e., LiH, NaH, KH, CaH2 and BaH2, AHs for short) promotes the catalytic activity of Mn nitride by orders of magnitude. The sequence of promotion is BaH2 > LiH > KH > CaH2 > NaH, which is different from the order observed in conventional oxide or hydroxide promoters. AHs, featured by bearing negatively charged hydrogen atoms, have chemical potentials in removing N from Mn nitride and thus lead to significant enhancement of N2 activation and subsequent conversion to NH3. Detailed investigations on Mn-LiH catalytic system disclosed that the active phase and kinetic behavior depend strongly on reaction conditions. Based on the understanding of the synergy between AHs and Mn nitride, a strategy in the design and development of early transition metals as effective catalysts for ammonia synthesis and other chemical processes is proposed.