Vacancy-enabled N2 activation for ammonia synthesis on an Ni-loaded catalyst.

Ammonia (NH3) is pivotal to the fertilizer industry and one of the most commonly produced chemicals1. The direct use of atmospheric nitrogen (N2) had been challenging, owing to its large bond energy (945 kilojoules per mole)2,3, until the development of the Haber-Bosch process. Subsequently, many strategies have been explored to reduce the activation barrier of the N≡N bond and make the process more efficient. These include using alkali and alkaline earth metal oxides as promoters to boost the performance of traditional iron- and ruthenium-based catalysts4-6 via electron transfer from the promoters to the antibonding bonds of N2 through transition metals7,8. An electride support further lowers the activation barrier because its low work function and high electron density enhance electron transfer to transition metals9,10. This strategy has facilitated ammonia synthesis from N2 dissociation11 and enabled catalytic operation under mild conditions; however, it requires the use of ruthenium, which is expensive. Alternatively, it has been shown that nitrides containing surface nitrogen vacancies can activate N2 (refs. 12-15). Here we report that nickel-loaded lanthanum nitride (LaN) enables stable and highly efficient ammonia synthesis, owing to a dual-site mechanism that avoids commonly encountered scaling relations. Kinetic and isotope-labelling experiments, as well as density functional theory calculations, confirm that nitrogen vacancies are generated on LaN with low formation energy, and efficiently bind and activate N2. In addition, the nickel metal loaded onto the nitride dissociates H2. The use of distinct sites for activating the two reactants, and the synergy between them, results in the nickel-loaded LaN catalyst exhibiting an activity that far exceeds that of more conventional cobalt- and nickel-based catalysts, and that is comparable to that of ruthenium-based catalysts. Our results illustrate the potential of using vacancy sites in reaction cycles, and introduce a design concept for catalysts for ammonia synthesis, using naturally abundant elements.

Discovery of Self-Assembled 2D Ru/Si Superlattices Boosting Hydrogen Evolution.

2D heterostructuring is a versatile methodology for designing nanoarchitecture catalytic systems that allow for reconstruction and modulation of interfaces and electronic structures. However, catalysts with such structures are extremely scarce due to limited synthetic strategies. Here, a highly ordered 2D Ru/Si/Ru/Si… nano-heterostructures (RSHS) is reported by acid etching of the LaRuSi electride. RSHS shows a superior electrocatalytic activity for hydrogen evolution with an overpotential of 14 mV at 10 mA cm-2 in alkaline media. Both experimental analysis and first-principles calculations demonstrate that the electronic states of Ru can be tuned by strong interactions of the interfacial Ru-Si, leading to an optimized hydrogen adsorption energy. Moreover, due to the synergistic effect of Ru and Si, the energy barrier of water dissociation is significantly reduced. The well-organized superlattice structure will provide a paradigm for construction of efficient catalysts with tunable electronic states and dual active sites.

Room-Temperature CO2 Hydrogenation to Methanol over Air-Stable hcp-PdMo Intermetallic Catalyst.

CO2 hydrogenation to methanol is one of the most promising routes to CO2 utilization. However, difficulty in CO2 activation at low temperature, catalyst stability, catalyst preparation, and product separation are obstacles to the realization of a practical hydrogenation process under mild conditions. Here, we report a PdMo intermetallic catalyst for low-temperature CO2 hydrogenation. This catalyst can be synthesized by the facile ammonolysis of an oxide precursor and exhibits excellent stability in air and the reaction atmosphere and significantly enhances the catalytic activity for CO2 hydrogenation to methanol and CO compared with a Pd catalyst. A turnover frequency of 0.15 h-1 was achieved for methanol synthesis at 0.9 MPa and 25 °C, which is comparable to or higher than that of the state-of-the-art heterogeneous catalysts under higher-pressure conditions (4-5 MPa).

BaTiO3-xNy: Highly Basic Oxide Catalyst Exhibiting Coupling of Electrons at Oxygen Vacancies with Substituted Nitride Ions.

The base strength of oxide catalysts is controlled by the electron charge distribution between cations and anions, with unsaturated oxygen ions that have lone pair electrons typically acting as basic sites. Substitution of oxide ions with anions that have different valences, such as nitride and hydride ions, can often generate basic sites. It is plausible that electrons trapped at oxygen vacancy sites could provide increased electron density and shift the highest occupied molecular orbital energy levels of anions upward in the case that the oxygen vacancies couple with surface-substituted anions. The present work demonstrates that high catalytic basicity can be obtained via site-selective doping of anions at face-sharing Ti2O9 dimer sites with oxygen vacancies in BaTiO3-x. This improved basicity stems from the coupling of substituted nitride ions to electrons at oxygen vacancies. The oxynitride BaTiO3-xNy was found to contain nitride ions that have increased electronic charge density on the basis of such interactions. Enhanced surface basicity following doping with nitride ion was also confirmed by CO2 temperature-programmed desorption and infrared spectroscopy in conjunction with the adsorption of CHCl3. The strong Lewis base sites resulting from the formation of the oxynitride evidently facilitated the catalytic activation of C-H bonds to promote Knoevenagel condensation reactions between aldehydes and active methylene compounds with pKa values of up to 28.9.

A 2D Ba2N Electride for Transition Metal-Free N2 Dissociation under Mild Conditions.

N2 activation is a key step in the industrial synthesis of ammonia and other high-value-added N-containing chemicals, and typically is heavily reliant on transition metal (TM) sites as active centers to reduce the large activation energy barrier for N2 dissociation. In the present work, we report that a 2D electride of Ba2N with anionic electrons in the interlayer spacings works efficiently for TM-free N2 dissociation under mild conditions. The interlayer electrons significantly boost N2 dissociation with a very small activation energy of 35 kJ mol-1, as confirmed by the N2 isotopic exchange reaction. The reaction of anionic electrons with N2 molecules stabilizes (N2)2- anions, the so-called diazenide, in the large interlayer space (∼4.5 Å) sandwiched by 2 cationic slabs of Ba2N as the main intermediate.

Boosted Activity of Cobalt Catalysts for Ammonia Synthesis with BaAl2O4-xHy Electrides.

Electrides are promising support materials to promote transition metal catalysts for ammonia synthesis due to their strong electron-donating ability. Cobalt (Co) is an alternative non-noble metal catalyst to ruthenium in ammonia synthesis; however, it is difficult to achieve acceptable activity at low temperatures due to the weak Co-N interaction. Here, we report a novel oxyhydride electride, BaAl2O4-xHy, that can significantly promote ammonia synthesis over Co (500 mmol gCo-1 h-1 at 340 °C and 0.90 MPa) with a very low activation energy (49.6 kJ mol-1; 260-360 °C), which outperforms the state-of-the-art Co-based catalysts, being comparable to the latest Ru catalyst at 300 °C. BaAl2O4-xHy with a stuffed tridymite structure has interstitial cage sites where anionic electrons are accommodated. The surface of BaAl2O4-xHy with very low work functions (1.7-2.6 eV) can donate electrons strongly to Co, which largely facilitates N2 reduction into ammonia with the aid of the lattice H- ions. The stuffed tridymite structure of BaAl2O4-xHy with a three-dimensional AlO4-based tetrahedral framework has great chemical stability and protects the accommodated electrons and H- ions from oxidation, leading to robustness toward the ambient atmosphere and good reusability, which is a significant advantage over the reported hydride-based catalysts.

Advances in Materials and Applications of Inorganic Electrides.

Electrides are materials in which electrons serve as anions. Here, the concept of inorganic electrides is extended in several respects: from ionic crystals to intermetallic compounds in host materials, from crystalline to amorphous solids, and from 0-dimensional to 1- and 2-dimensional materials in electron-confined spaces. In particular, 2D electrides, in which anionic electrons are sandwiched by cationic slabs, can form a bulk crystal of a 2-dimensional electron gas, thus exhibiting a large electron mobility and providing a platform for topological materials. Exploration of new electrides by computation and high pressure has advanced, revealing that an electride is a stable equilibrium phase of many elements and compounds under high pressure. This review describes the history and current status of electride research and next summarizes the chemical application of electrides and relevant materials. An emphasis is placed on catalysts for ammonia synthesis from N2 and H2 at mild conditions. This subject is accelerated by a demand for on-site ammonia synthesis using hydrogen produced by renewable energy sources. A wide applicability of electride for chemical reactions such selective hydrogenation and carbon-carbon coupling is shown by extending the concept of electrides. Finally, a view for the relationship between electrides and crystallographic voids and current issues are described.

Hexagonal BaTiO(3-x)Hx Oxyhydride as a Water-Durable Catalyst Support for Chemoselective Hydrogenation.

We present heavily H--doped BaTiO(3-x)Hx (x ≈ 1) as an efficient and water-durable catalyst support for Pd nanoparticles applicable to liquid-phase hydrogenation reactions. The BaTiO(3-x)Hx oxyhydride with a hexagonal crystal structure (P63/mmc) was synthesized by the direct reaction of BaH2 and TiO2 at 800 °C under a stream of hydrogen, and the estimated chemical composition was BaTiO2.01H0.96. Density functional theory calculations and magnetic measurements indicated that such heavy H- doping results in a metallic nature with delocalized electrons and a low work function. The potential of BaTiO(3-x)Hx as a catalyst support was examined for the selective hydrogenation of unsaturated C-C bonds by Pd nanoparticles deposited on BaTiO(3-x)Hx. We found that the turnover frequency for phenylacetylene hydrogenation per total amount of Pd in Pd/BaTiO(3-x)Hx was the highest among the supported Pd catalysts reported to date. The strong electronic charge transfer between Pd and the support, as confirmed by X-ray photoelectron spectroscopy measurements, can be attributed to be responsible for such high catalytic activity. The combination of the BaTiO(3-x)Hx support and Pd nanoparticles provides for the selective hydrogenation of unsaturated C-C bonds and highlights the validity of catalyst design that integrates H- in support materials.

Unique Catalytic Mechanism for Ru-Loaded Ternary Intermetallic Electrides for Ammonia Synthesis.

Intermetallic electrides have recently shown their priority as catalyst components in ammonia synthesis and CO2 activation. However, their function mechanism has been elusive since its inception, which hinders the further development of such catalysts. In this work, ternary intermetallic electrides La-TM-Si (TM = Co, Fe, and Mn) were synthesized as hosts of ruthenium (Ru) particles for ammonia synthesis catalysis. Although they have the same crystal structure and possess low work functions commonly, the promotion effects on Ru particles rather differ from each other. The catalytic activity follows the sequence of Ru/LaCoSi > Ru/LaFeSi > Ru/LaMnSi. Furthermore, Ru/LaCoSi exhibits much better catalytic durability than the other two. A combination of experiments and first-principles calculations shows that apparent N2 activation energy on each catalyst is much lower than that over conventional Ru-based catalysts, which suggests that N2 dissociation can be conspicuously promoted by the concerted actions of the specific electronic structure and atomic configuration of intermetallic electride-supported catalysts. The NHx formations proceeded on La are energetically favored, which makes it possible to bypass the scaling relations based on only Ru as the active site. The rate-determining step of Ru/La-TM-Si was identified to be NH2 formation. The transition metal (TM) in La-TM-Si electrides has a significant influence on the metal-support interaction of Ru and La-TM-Si. These findings provide a guide for the development of new and effective catalyst hosts for ammonia synthesis and other hydrogenation reactions.

Approach to Chemically Durable Nickel and Cobalt Lanthanum-Nitride-Based Catalysts for Ammonia Synthesis.

Metal nitride complexes have recently been proposed as an efficient noble-metal-free catalyst for ammonia synthesis utilizing a dual active site concept. However, their high sensitivity to air and moisture has restricted potential applications. We report that their chemical sensitivity can be improved by introducing Al into the LaN lattice, thereby forming La-Al metallic bonds (La-Al-N). The catalytic activity and mechanism of the resulting TM/La-Al-N (TM=Ni, Co) are comparable to the previously reported TM/LaN catalyst. Notably, the catalytic activity did not degrade after exposure to air and moisture. Kinetic analysis and isotopic experiment showed that La-Al-N is responsible for N2 absorption and activation despite substantial Al being introduced into its lattice because the local coordination of the lattice N remained largely unchanged. These findings show the effectiveness of metallic bond formation, which can support the chemical stability of rare-earth nitrides with retention of catalytic functionality.

Dissociative and Associative Concerted Mechanism for Ammonia Synthesis over Co-Based Catalyst.

The current catalytic reaction mechanism for ammonia synthesis relies on either dissociative or associative routes, in which adsorbed N2 dissociates directly or is hydrogenated step-by-step until it is broken upon the release of NH3 through associative adsorption. Here, we propose a concerted mechanism of associative and dissociative routes for ammonia synthesis over a cobalt-loaded nitride catalyst. Isotope exchange experiments reveal that the adsorbed N2 can be activated on both Co metal and the nitride support, which leads to superior low-temperature catalytic performance. The cooperation of the surface low work function (2.6 eV) feature and the formation of surface nitrogen vacancies on the CeN support gives rise to a dual pathway for N2 activation with much reduced activation energy (45 kJ·mol-1) over that of Co-based catalysts reported so far, which results in efficient ammonia synthesis under mild conditions.

Contribution of Nitrogen Vacancies to Ammonia Synthesis over Metal Nitride Catalysts.

Ammonia is one of the most important feedstocks for the production of fertilizer and as a potential energy carrier. Nitride compounds such as LaN have recently attracted considerable attention due to their nitrogen vacancy sites that can activate N2 for ammonia synthesis. Here, we propose a general rule for the design of nitride-based catalysts for ammonia synthesis, in which the nitrogen vacancy formation energy (ENV) dominates the catalytic performance. The relatively low ENV (ca. 1.3 eV) of CeN means it can serve as an efficient and stable catalyst upon Ni loading. The catalytic activity of Ni/CeN reached 6.5 mmol·g-1·h-1 with an effluent NH3 concentration (ENH3) of 0.45 vol %, reaching the thermodynamic equilibrium (ENH3 = 0.45 vol %) at 400 °C and 0.1 MPa, thereby circumventing the bottleneck for N2 activation on Ni metal with an extremely weak nitrogen binding energy. The activity far exceeds those for other Co- and Ni-based catalysts, and is even comparable to those for Ru-based catalysts. It was determined that CeN itself can produce ammonia without Ni-loading at almost the same activation energy. Kinetic analysis and isotope experiments combined with density functional theory (DFT) calculations indicate that the nitrogen vacancies in CeN can activate both N2 and H2 during the reaction, which accounts for the much higher catalytic performance than other reported nonloaded catalysts for ammonia synthesis.

Low-Temperature Synthesis of Perovskite Oxynitride-Hydrides as Ammonia Synthesis Catalysts.

Mixed anionic materials such as oxyhydrides and oxynitrides have recently attracted significant attention due to their unique properties, such as fast hydride ion conduction, enhanced ferroelectrics, and catalytic activity. However, high temperature (≥800 °C) and/or complicated processes are required for the synthesis of these compounds. Here we report that a novel perovskite oxynitride-hydride, BaCeO3-xNyHz, can be directly synthesized by the reaction of CeO2 with Ba(NH2)2 at low temperatures (300-600 °C). BaCeO3-xNyHz, with and without transition metal nanoparticles, functions as an efficient catalyst for ammonia synthesis through the lattice N3- and H- ion-mediated Mars-van Krevelen mechanism, while ammonia synthesis occurs over conventional catalysts through a Langmuir-Hinshelwood mechanism with high energy barriers (85-121 kJ mol-1). As a consequence, the unique reaction mechanism leads to enhancement of the activity of BaCeO3-based catalysts by a factor of 8-218 and lowers the activation energy (46-62 kJ mol-1) for ammonia synthesis. Furthermore, isotopic experiments reveal that this catalyst shifts the rate-determining step for ammonia synthesis from N2 dissociation to N-H bond formation.

Zeolitic Intermetallics: LnNiSi (Ln = La-Nd).

LnNiSi (Ln = La-Nd) comprising a three-dimensional NiSi framework has electrons in the crystallographic cavity space. In the temperature region 473-773 K, it accepts the insertion/de-insertion of hydrogen topotactically without a change in unitcell volume. The insertion of hydrogens into the cavity space is accompanied by a redox reaction with the orbitals of atoms constituting the cavity wall. Having small work functions, such intermetallic electrides exhibit metallic electrical and magnetic properties. Owing to a high electron-donating power and reversible exchange between hydrogen and the electrons, Ru5wt%-loaded LaNiSi powder worked as an efficient catalyst for ammonia synthesis under ambient pressure.

Pseudogap Control of Physical and Chemical Properties in CeFeSi-Type Intermetallics.

We describe the synthesis of the new ternary compound CaRuSi whose chemical and physical properties help draw a clear picture of how electronic structure controls the behavior of an isostructural series of intermetallics. DFT calculations reveal that an electronic pseudogap arises near the Fermi level ( EF), corresponding to 14 valence electrons per RuSi unit. The closed-shell-like character is further investigated by comparisons with the electronic structures of CaCoSi (15 electrons), where the EF lies above the corresponding pseudogap, and its hydride CaCoSiH, where formation of H anions restores the 14-electron count on the metal sublattice, returning the EF to the pseudogap. The chemical origin of the 14-electron pseudogap is interpreted with a reversed approximation Molecular Orbital analysis. Here, the pseudogap is shown to coincide with the filling of Ru 16 electron configurations isolobal to the d8 square planar complexes of coordination chemistry (but where 4 electron pairs are shared covalently between Ru atoms such that only 12 electrons are required), and the occupation of Si lone pairs (2 electrons). Experimentally, the pseudogap is confirmed with heat capacity measurements, which indicate that the 14-electron systems CaRuSi and CaCoSiH each exhibit  a smaller electronic density of states at the EF than the 15-electron system CaCoSi. Importantly, the 14-electron pseudogap also significantly affects the chemical properties of the compounds, as evidenced by the difference in the stabilities of CaCoSiH and CaRuSiH observed in hydrogen desorption measurements. These results may support the design of functional materials for superconductivity, hydrogen storage, and catalysis involving hydrogenation.

Intermetallic Electride Catalyst as a Platform for Ammonia Synthesis.

Electrides loaded with transition-metal (TM) nanoparticles have recently attracted attention as emerging materials for catalytic NH3 synthesis. However, they suffer from disadvantages associated with the growth and aggregation of nanoparticles. TM-containing intermetallic electrides appear to be promising catalysts with the advantages of both electrides and transition metals in a single phase. LaRuSi is reported here to be an intermetallic electride with superior activity for NH3 synthesis, and direct evidence is provided supporting its electride-character-induced catalytic performance. The discussion is made mainly based on the contrasting synthesis rates over the isostructural compounds LaRuSi, CaRuSi, and LaRu2 Si2 , and the N2 isotope-exchange reactions over these compounds. Lattice hydride ions, which can reversibly exchange with anionic electrons, are shown to be indispensable in the promotion of NHx formation. The mechanism derived from the present findings provides new guidelines for NH3 synthesis.

Large Oblate Hemispheroidal Ruthenium Particles Supported on Calcium Amide as Efficient Catalysts for Ammonia Decomposition.

Ammonia decomposition is an important technology for extracting hydrogen from ammonia toward the realization of a hydrogen economy. Herein, it is reported that large oblate hemispheroidal Ru particles on Ca(NH2 )2 function as efficient catalysts for ammonia decomposition. The turnover frequency of Ru/Ca(NH2 )2 increased by two orders of magnitude when the Ru particle size was increased from 1.5 to 8.4 nm. More than 90 % ammonia decomposition was achieved over Ru/Ca(NH2 )2 with large oblate hemispheroidal Ru particles at 360 °C, which is comparable to that of alkali-promoted Ru catalysts with small Ru particle sizes. XAFS analyses revealed that Ru particles are immobilized on Ca(NH2 )2 by Ru-N bonds formed at the metal/support interface, which lead to oblate hemispheroidal Ru particles. Such a strong metal-support interaction in Ru/Ca(NH2 )2 is also substantiated by DFT calculations. The high activity of Ru/Ca(NH2 )2 with large Ru particles primarily originates from the shape and appropriate size of the Ru particles with a high density of active sites rather than the electron-donating ability of Ca(NH2 )2 .

Self-organized Ruthenium-Barium Core-Shell Nanoparticles on a Mesoporous Calcium Amide Matrix for Efficient Low-Temperature Ammonia Synthesis.

A low-temperature ammonia synthesis process is required for on-site synthesis. Barium-doped calcium amide (Ba-Ca(NH2 )2 ) enhances the efficacy of ammonia synthesis mediated by Ru and Co by 2 orders of magnitude more than that of a conventional Ru catalyst at temperatures below 300 °C. Furthermore, the presented catalysts are superior to the wüstite-based Fe catalyst, which is known as a highly active industrial catalyst at low temperatures and pressures. Nanosized Ru-Ba core-shell structures are self-organized on the Ba-Ca(NH2 )2 support during H2 pretreatment, and the support material is simultaneously converted into a mesoporous structure with a high surface area (>100 m2 g-1 ). These self-organized nanostructures account for the high catalytic performance in low-temperature ammonia synthesis.

Copper-Based Intermetallic Electride Catalyst for Chemoselective Hydrogenation Reactions.

The development of transition metal intermetallic compounds, in which active sites are incorporated in lattice frameworks, has great potential for modulating the local structure and the electronic properties of active sites, and enhancing the catalytic activity and stability. Here we report that a new copper-based intermetallic electride catalyst, LaCu0.67Si1.33, in which Cu sites activated by anionic electrons with low work function are atomically dispersed in the lattice framework and affords selective hydrogenation of nitroarenes with above 40-times higher turnover frequencies (TOFs up to 5084 h-1) than well-studied metal-loaded catalysts. Kinetic analysis utilizing isotope effect reveals that the cleavage of the H-H bond is the rate-determining step. Surprisingly, the high carrier density and low work function (LWF) properties of LaCu0.67Si1.33 enable the activation of hydrogen molecules with extreme low activation energy (Ea = 14.8 kJ·mol-1). Furthermore, preferential adsorption of nitroarenes via a nitro group is achieved by high oxygen affinity of LaCu0.67Si1.33 surface, resulting in high chemoselectivity. The present efficient catalyst can further trigger the hydrogenation of other oxygen-containing functional groups such as aldehydes and ketones with high activities. These findings demonstrate that the transition metals incorporated in the specific lattice site function as catalytically active centers and surpass the conventional metal-loaded catalysts in activity and stability.

A meso-meso ß-ß ß-ß Triply Linked Subporphyrin Dimer.

A meso-meso ß-ß ß-ß triply linked subporphyrin dimer 6 was synthesized by stepwise reductive elimination of ß-to-ß doubly PtII -bridged subporphyrin dimer 9. Dimer 6 was characterized by spectroscopic and electrochemical measurements, theoretical calculations, and picosecond time-resolved transient absorption spectroscopy. X-ray diffraction analysis reveals that 6 has a bowl-shaped structure with a positive Gaussian curvature. Despite the curved structure, 6 exhibits a remarkably red-shifted absorption band at 942 nm and a small electrochemical HOMO-LUMO gap (1.35 eV), indicating an effectively conjugated π-electronic network.