Fei Chang

Breaking scaling relations to achieve low-temperature ammonia synthesis through LiH-mediated nitrogen transfer and hydrogenation

Ammonia synthesis under mild conditions is a goal that has been long sought after. Previous investigations have shown that adsorption and transition-state energies of intermediates in this process on transition metals (TMs) scale with each other. This prevents the independent optimization of these energies that would result in the ideal catalyst: one that activates reactants well, but binds intermediates relatively weakly. Here we demonstrate that these scaling relations can be broken by intervening in the TM-mediated catalysis with a second catalytic site, LiH. The negatively charged hydrogen atoms of LiH act as strong reducing agents, which remove activated nitrogen atoms from the TM or its nitride (TMN), and as an immediate source of hydrogen, which binds nitrogen atoms to form LiNH2. LiNH2 further splits H2 heterolytically to give off NH3 and regenerate LiH. This synergy between TM (or TMN) and LiH creates a favourable pathway that allows both early and late 3d TM-LiH composites to exhibit unprecedented lower-temperature catalytic activities.

Lithium Imide Synergy with 3d Transition‐Metal Nitrides Leading to Unprecedented Catalytic Activities for Ammonia Decomposition

Alkali metals have been widely employed as catalyst promoters; however, the promoting mechanism remains essentially unclear. Li, when in the imide form, is shown to synergize with 3d transition metals or their nitrides TM(N) spreading from Ti to Cu, leading to universal and unprecedentedly high catalytic activities in NH3 decomposition, among which Li2NH-MnN has an activity superior to that of the highly active Ru/carbon nanotube catalyst. The catalysis is fulfilled via the two-step cycle comprising: 1) the reaction of Li2NH and 3d TM(N) to form ternary nitride of LiTMN and H2, and 2) the ammoniation of LiTMN to Li2NH, TM(N) and N2 resulting in the neat reaction of 2 NH3⇌N2+3 H2. Li2NH, as an NH3 transmitting agent, favors the formation of higher N-content intermediate (LiTMN), where Li executes inductive effect to stabilize the TM-N bonding and thus alters the reaction energetics.

Covalent triazine-based framework as an efficient catalyst support for ammonia decomposition

The covalent triazine-based framework (CTF), a new type of nitrogen-containing microporous polymer, was employed as a catalyst support for ammonia decomposition. Either in terms of NH3 conversion rate or turnover frequency, Ru/CTF-1 has a highly enhanced performance compared to Ru/CNTs, which rank as one of the best un-promoted catalysts reported so far. The compositional and structural information of Ru/CTF-1 and Ru/CNTs catalysts have been characterized by ICP, N2 physisorption, XRD, TEM, XPS, and NH3-TPD techniques. Ru particles on CTF-1 and CNTs are ca. 3 nm in diameter and have a similar degree of dispersion. However, the binding energy of Ru 3p electrons is ca. 0.6 eV less for Ru/CTF-1 than that for Ru/CNTs showing significant increase in electron density in the former, which is likely due to the interaction between the nitrogen-rich groups of CTF-1 and the Ru nanoparticles. Moreover, the presence of CTF-1 enhances the chemisorption of NH3, which, together with the increased electron density of Ru, may facilitate the competitive chemisorption of NH3 and recombinative desorption of adsorbed nitrogen via lowered activation energy and thus, enable faster reaction rate.

Electronic promoter or reacting species? The role of LiNH2 on Ru in catalyzing NH3 decomposition

LiNH2 decomposes to NH3 rather than N2 and H2 because of a severe kinetic barrier in NHx (x = 1, 2) coupling. In the presence of Ru, however, a drastic enhancement in N2 and H2 formation is obtained, which enables the LiNH2-Ru composite to act as a highly active catalyst for NH3 decomposition. Experimental and theoretical investigations indicate that Li creates a NHx-rich environment and Ru mediates the electron transfer facilitating NHx coupling. A strategy in catalytic material design is thus proposed.

Ammonia Decomposition with Manganese Nitride-Calcium Imide Composites as Efficient Catalysts.

Ammonia has high gravimetric and volumetric hydrogen densities and is, therefore, considered a promising carrier for the production of COx -free molecular H2 for forthcoming energy systems. Alkaline earth metals are generally regarded as structural promoters of catalysts and employed in numerous catalytic processes. Here, we report that calcium imide (CaNH) has a strong synergistic effect on Mn6 N5 in catalyzing the decomposition of NH3 , leading to a ca. 40 % drop in apparent activation energy. At 773 K, the H2 formation rate over a Mn6 N5 -11CaNH composite catalyst is about an order of magnitude higher than that of Mn6 N5 and comparable to the highly active Ni/SBA-15 and Ru/Al2 O3 catalysts. Analysis by means of temperature-programmed decomposition (TPD), X-ray diffraction (XRD), and X-ray absorption near edge spectroscopy (XANES) reveal that CaNH participates in the catalysis via forming a [Ca6 MnN5 ]-like intermediate, thus altering the reaction pathway and energetics. A two-step catalytic cycle, accounting for the synergy between CaNH and Mn6 N5 , is proposed.

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

Lithium hydride (LiH) has a strong effect on iron leading to an approximately 3 orders of magnitude increase in catalytic ammonia synthesis. The existence of lithium-iron ternary hydride species at the surface/interface of the catalyst were identified and characterized for the first time by gas-phase optical spectroscopy coupled with mass spectrometry and quantum chemical calculations. The ternary hydride species may serve as centers that readily activate and hydrogenate dinitrogen, forming Fe-(NH2 )-Li and LiNH2 moieties-possibly through a redox reaction of dinitrogen and hydridic hydrogen (LiH) that is mediated by iron-showing distinct differences from ammonia formation mediated by conventional iron or ruthenium-based catalysts. Hydrogen-associated activation and conversion of dinitrogen are discussed.

Transition and Alkali Metal Complex Ternary Amides for Ammonia Synthesis and Decomposition

A new complex ternary amide, Rb2 [Mn(NH2 )4 ], which simultaneously contains both transition and alkali metal catalytic sites, is developed. This is in line with the recently reported TM-LiH composite catalysts, which have been shown to effectively break the scaling relations and achieve ammonia synthesis under mild conditions. Rb2 [Mn(NH2 )4 ] can be facilely synthesized by mechanochemical reaction at room temperature. It exhibits two temperature-dependent polymorphs, that is, a low-temperature orthorhombic and a high-temperature monoclinic structure. Rb2 [Mn(NH2 )4 ] decomposes to N2 , H2 , NH3 , Mn3 N2 , and RbNH2 under inert atmosphere; whereas it releases NH3 at a temperature as low as 80 °C under H2 atmosphere. Those unique behaviors enable Rb2 [Mn(NH2 )4 ], and its analogue K2 [Mn(NH2 )4 ], to be excellent catalytic materials for ammonia decomposition and synthesis. Experimental results show both ammonia decomposition onset temperatures and conversion rates over Rb2 [Mn(NH2 )4 ] and K2 [Mn(NH2 )4 ] are similar to those of noble metal Ru-based catalysts. More importantly, these ternary amides exhibit superior capabilities in catalyzing NH3 synthesis, which are more than 3 orders of magnitude higher than that of Mn nitride and twice of that of Ru/MgO. The in situ SR-PXD measurement shows that manganese nitride, synergistic with Rb/KH or Rb/K(NH2 )x H1-x , are likely the active sites. The chemistry of Rb2 /K2 [Mn(NH2 )x ] and Rb/K(NH2 )x H1-x with H2 /N2 and NH3 correlates closely with the catalytic performance.

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.