Titanium-Based Hydrides as Heterogeneous Catalysts for Ammonia Synthesis

High-Pressure and High-Temperature Synthesis of Anion-Disordered Vanadium Perovskite Oxyhydrides

Crystallographic order-disorder phenomena in solid state compounds are of fundamental interest due to intimate relationship between the structure and properties. Here, by using high-pressure and high-temperature synthesis, we obtained vanadium perovskite oxyhydrides Sr_1-xNa_xVO_3-yH_y (x = 0, 0.05, 0.1, 0.2) with an anion-disordered structure, which is different from anion-ordered SrVO₂H synthesized by topochemical reduction. High-pressure and high-temperature synthesis from nominal composition SrVO₂H yielded the anion-disordered perovskite SrVO_3-yH_y (y ∼ 0.4) with a significant amount of byproducts, while Na substitution resulted in the almost pure anion-disordered perovskite Sr_1-xNa_xVO_3-yH_y with an increased amount of hydride anion (y ∼ 0.7 for x = 0.2). The obtained disordered phases for x = 0.1 and 0.2 are paramagnetic with almost temperature-independent electronic conductivity, whereas anion-ordered SrVO₂H is an antiferromagnetic insulator. Although we obtained the anion-disordered perovskite under high pressure, a first-principles calculation revealed that the application of pressure stabilizes the ordered phase due to a reduced volume in the ordered structure, suggesting that a further increase of the pressure or reduction of the reaction temperature leads to the anion ordering. This study shows that anion ordering in oxyhydrides can be controlled by changing synthetic pressure and temperature.

Shedding Light on the Role of Chemical Bond in Catalysis of Nitrogen Fixation

Ammonia (NH₃ ) and nitrates are essential for human society because of their widespread utilization for producing medicines, fibers, fertilizers, etc. In recent years, the development on nitrogen fixation under mild reaction conditions has attracted much attention. However, the very low conversion efficiency and ambiguous catalytic mechanism remain the major hurdles for the research of nitrogen fixation. This review aims to clarify the role of chemical bond in catalytic nitrogen fixation by summarizing and analyzing the recent development of nitrogen fixation research. In detail, the atomic-scale mechanism of nitrogen fixation reaction, the various methods to improve the nitrogen fixation performance, and the computational investigation of nitrogen fixation are discussed, all from a chemical bond perspective. It is hoped that this review could trigger more profound pondering and deeper exploration in the field of catalytic nitrogen fixation.

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).

Barium Titanium Oxynitride from Ammonia-Free Nitridation of Reduced BaTiO3

We investigated the nitridation of reduced BaTiO3, BaTiO2.60H0.08, corresponding to an oxyhydride with a large concentration of O defects (>10%). The material is readily nitrided under flowing N2 gas at temperatures between 400 and 450 °C to yield oxynitrides BaTiO2.6Nx (x = 0.2−0.22) with a slightly tetragonally distorted perovskite structure, a ≈ 4.01 and c ≈ 4.02 Å, and Ti partially remaining in the oxidation state III. The tetragonal structure was confirmed from Raman spectroscopy. 14N MAS NMR spectroscopy shows a single resonance at 270 ppm, which is typical for perovskite transition metal oxynitrides. However, largely different signal intensity for materials with very similar N content suggests N/O/vacancy ordering when prolonging nitridation times to hours. Diffuse reflectance UV-VIS spectroscopy shows a reduction of the intrinsic band gap to 2.4–2.45 eV compared to BaTiO3 (~3.2 eV). Mott-Schottky measurements confirm n-type conductivity and reveal a slight negative shift of the conduction band edge from –0.59 V (BaTiO3) to ~–0.65 eV.

Ultrathin 2D Fe-Nanosheets Stabilized by 2D Mesoporous Silica: Synthesis and Application in Ammonia Synthesis

Developing high-performance Fe-based ammonia catalysts through simple and cost-efficient methods has received an increased level of attention. Herein, we report for the first time, the synthesis of two-dimensional (2D) FeOOH nanoflakes encapsulated by mesoporous SiO₂ (mSiO₂) via a simple solution-based method for ammonia synthesis. Due to the sticking of the mSiO₂ coating layers and the limited spaces in between, the Fe after reduction retains the 2D morphology, showing high resistance against the sintering in the harsh Haber-Bosch process. Compared to supported Fe particles dispersed on mSiO₂ spheres, the coated catalyst shows a significantly improved catalytic activity by 50% at 425 °C. Thermal desorption spectroscopy (TDS) reveals the existence of a higher density of reactive sites for N₂ activation in the 2D Fe catalyst, which is possibly coupled to a larger density of surface defect sites (kinks, steps, point defects) that are generally considered as active centers in ammonia synthesis. Besides the structural impact of the coating on the 2D Fe, the electronic one is elucidated by partially substituting Si with Al in the coating, confirmed by ²⁹Si and ²⁷Al magic-angle spinning nuclear magnetic resonance (MAS NMR). An increased apparent activation energy (E_a) of the Al-containing catalyst evidences an influence on the nature of the active site. The herein-developed stable 2D Fe nanostructures can serve as an example of a 2D material applied in catalysis, offering the chance of a rational catalyst design based on a stepwise introduction of various promoters, in the coating and on the metal, maintaining the spatial control of the active centers.

Computational Chemistry-Guided Syntheses and Crystal Structures of the Heavier Lanthanide Hydride Oxides DyHO, ErHO, and LuHO

Heteroanionic hydrides offer great possibilities in the design of functional materials. For ternary rare earth hydride oxide REHO, several modifications were reported with indications for a significant phase width with respect to H and O of the cubic representatives. We obtained DyHO and ErHO as well as the thus far elusive LuHO from solid-state reactions of RE2O3 and REH3 or LuH3 with CaO and investigated their crystal structures by neutron and X-ray powder diffraction. While DyHO, ErHO, and LuHO adopted the cubic anion-ordered half-Heusler LiAlSi structure type (F4¯3m, a(DyHO) = 5.30945(10) Å, a(ErHO) = 5.24615(7) Å, a(LuHO) = 5.171591(13) Å), LuHO additionally formed the orthorhombic anti-LiMgN structure type (Pnma; LuHO: a = 7.3493(7) Å, b = 3.6747(4) Å, c = 5.1985(3) Å; LuDO: a = 7.3116(16) Å, b = 3.6492(8) Å, c = 5.2021(7) Å). A comparison of the cubic compounds’ lattice parameters enabled a significant distinction between REHO and REH1+2xO1−x (x < 0 or x > 0). Furthermore, a computational chemistry study revealed the formation of REHO compounds of the smallest rare earth elements to be disfavored in comparison to the sesquioxides, which is why they may only be obtained by mild synthesis conditions.

Formation of PbCl2-type AHF (A = Ca, Sr, Ba) with partial anion order at high pressure

The high-pressure structures of alkaline earth metal hydride-fluorides (AHFs) (A = Ca, Sr, Ba) were investigated up to 8 GPa. While AHF adopts the fluorite-type structure (Fm3[combining macron]m) at ambient pressure without anion ordering, the PbCl2-type (cotunnite-type) structure (Pnma) is formed by pressurization, with a declining trend of critical pressure as the ionic radius of the A2+ cation increases. In contrast to PbCl2-type LaHO and LaOF whose anions are fully ordered, the H-/F- anions in the high-pressure polymorph of SrHF and BaHF are partially ordered, with a preferential occupation of H- at the square-pyramidal site (vs. tetrahedral site). First-principles calculations partially support the preferential anion occupation and suggest occupation switching at higher pressure. These results provide a strategy for controlling the anion ordering and local structure in mixed-anion compounds.

Anion ordering enables fast H- conduction at low temperatures

The introduction of chemical disorder by substitutional chemistry into ionic conductors is the most commonly used strategy to stabilize high-symmetric phases while maintaining ionic conductivity at lower temperatures. In recent years, hydride materials have received much attention owing to their potential for new energy applications, but there remains room for development in ionic conductivity below 300°C. Here, we show that layered anion-ordered Ba_2-δH_3-2δ X (X = Cl, Br, and I) exhibit a remarkable conductivity, reaching 1 mS cm^-1 at 200°C, with low activation barriers allowing H^- conduction even at room temperature. In contrast to structurally related BaH₂ (i.e., Ba₂H₄), the layered anion order in Ba_2-δH_3-2δ X, along with Schottky defects, likely suppresses a structural transition, rather than the traditional chemical disorder, while retaining a highly symmetric hexagonal lattice. This discovery could open a new direction in electrochemical use of hydrogen in synthetic processes and energy devices.

Interplay of Alkali, Transition Metals, Nitrogen, and Hydrogen in Ammonia Synthesis and Decomposition Reactions

ConspectusThe fixation of dinitrogen to ammonia is critically important for the biogeochemical cycle on earth. Ammonia also holds promise as a sustainable energy carrier. Tremendous effort has been devoted to the development of green processes and advanced materials for ammonia synthesis and decomposition under milder conditions, and encouraging progress has been made.The reduction of dinitrogen to ammonia needs electrons and protons, which hydridic hydrogen H^- could supply. Polarized, electron-rich N_xH_y intermediates, on the other hand, can be stabilized by alkali or alkaline earth metal cations to lower kinetic barriers in the transformation. The inherent properties of alkali/alkaline earth metal hydrides (denoted as AH) endow them with a unique function in ammonia synthesis.In this Account, recent efforts in the exploration of alkali or alkaline earth metal hydrides (denoted as AH), amides, and imides (denoted as ANH hereafter) for ammonia synthesis and decomposition reactions will be summarized and discussed. We begin with an introduction to the chemistry of A with N₂, NH₃, and H₂, highlighting the interconversion between AH and ANH that has profound implications on the formation and decomposition of NH₃. We then present our finding on the strong synergistic effect between ANH and transition metals (TM) in ammonia decomposition catalysis, which stimulated our subsequent research on AH for ammonia synthesis. We discuss the effect and function mechanism of AH in the thermocatalytic and chemical looping ammonia synthesis processes. In the thermocatalytic process, AH cooperates with both early and late TM forming either composite catalysts with two active centers or complex metal hydride catalysts with electron- and hydrogen-rich ionic centers facilitating ammonia synthesis with high activities at lower temperatures. Very interestingly, AH levels the catalytic performances of TMs and intervenes in the energy-scaling relations of TM-only catalysts. Moreover, ANH serves as a new type nitrogen carrier effectively mediating ammonia synthesis via a low-temperature chemical looping process, in which N₂ is fixed by AH forming ANH. Subsequently, ANH is hydrogenated to ammonia and AH. Late TMs have a strong catalytic effect on the chemical looping process. The unique interplay of A, N, TM, and H^- offers plenty of opportunities for achieving dinitrogen conversion under mild conditions, while further efforts are needed to address the challenges in the fundamental understanding and practical application.

Highly efficient ammonia synthesis at low temperature over a Ru-Co catalyst with dual atomically dispersed active centers

The desire for a carbon-free society and the continuously increasing demand for clean energy make it valuable to exploit green ammonia (NH3) synthesis that proceeds via the electrolysis driven Haber-Bosch (eHB) process. The key for successful operation is to develop advanced catalysts that can operate under mild conditions with efficacy. The main bottleneck of NH3 synthesis under mild conditions is the known scaling relation in which the feasibility of N2 dissociative adsorption of a catalyst is inversely related to that of the desorption of surface N-containing intermediate species, which leads to the dilemma that NH3 synthesis could not be catalyzed effectively under mild conditions. The present work offers a new strategy via introducing atomically dispersed Ru onto a single Co atom coordinated with pyrrolic N, which forms RuCo dual single-atom active sites. In this system the d-band centers of Ru and Co were both regulated to decouple the scaling relation. Detailed experimental and theoretical investigations demonstrate that the d-bands of Ru and Co both become narrow, and there is a significant overlapping of t2g and eg orbitals as well as the formation of a nearly uniform Co 3d ligand field, making the electronic structure of the Co atom resemble that of a "free-atom". The "free-Co-atom" acts as a bridge to facilitate electron transfer from pyrrolic N to surface Ru single atoms, which enables the Ru atom to donate electrons to the antibonding π* orbitals of N2, thus resulting in promoted N2 adsorption and activation. Meanwhile, H2 adsorbs dissociatively on the Co center to form a hydride, which can transfer to the Ru site to cause the hydrogenation of the activated N2 to generate N2H x (x = 1-4) intermediates. The narrow d-band centers of this RuCo catalyst facilitate desorption of surface *NH3 intermediates even at 50 °C. The cooperativity of the RuCo system decouples the sites for the activation of N2 from those for the desorption of *NH3 and *N2H x intermediates, giving rise to a favorable pathway for efficient NH3 synthesis under mild conditions.

HoHO: A Paramagnetic Air-Resistant Ionic Hydride with Ordered Anions

The substitution of hydrogen for oxygen atoms in metal oxides provides opportunities for influencing the solid-state properties. Such hydride oxides (or oxyhydrides) are potential functional materials and scarce. Here, we present the synthesis and characterization of holmium hydride oxide with the stoichiometric composition HoHO. It was prepared by the reaction of Ho₂O₃ with either HoH₃ or CaH₂ as a powder of light-yellow color in sunlight and pink color in artificial light (Alexandrite effect), which is commonly observed for ionic Ho(III) compounds. HoHO crystallizes with an ordered fluorite superstructure (F4̅3m, a = 5.27550(13) Å, half-Heusler LiAlSi type), as evidenced by powder X-ray and neutron powder diffraction on both hydride and deuteride and supported by quantum-mechanical calculations. HoHO is the first representative with considerable ionic bonding for this structure type. The thermal stability and inertness toward air are remarkably high for a hydride because it reacts only above 540 K to form Ho₂O₃. At 294(1) K and 25(3)% relative humidity, HoHO is stable for at least 3 months. HoHO is paramagnetic with μ_eff(Ho³⁺) = 10.41(2) μ_B without any sign of magnetic ordering down to 2 K.

The sequential activation of H2 and N2 mediated by the gas-phase Sc3N+ clusters: Formation of amido unit

The activation and hydrogenation of nitrogen are central in industry and in nature. Through a combination of mass spectrometry and quantum chemical calculations, this work reports an interesting result that scandium nitride cations Sc₃N⁺ can activate sequentially H₂ and N₂, and an amido unit (NH₂) is formed based on density functional theory calculations, which is one of the inevitable intermediates in the N₂ reduction reactions. If the activation step is reversed, i.e., sequential activation of first N₂ and then H₂, the reactivity decreases dramatically. An association mechanism, prevalent in some homogeneous catalysis and enzymatic mechanisms, is adopted in these gas-phase H₂ and N₂ activation reactions mediated by Sc₃N⁺ cations. The mechanistic insights are important to understand the mechanism of the conversion of H₂ and N₂ to NH₃ synthesis under ambient conditions.

Surface activation by electron scavenger metal nanorod adsorption on TiH2, TiC, TiN, and Ti2O3

Metal/oxide support perimeter sites are known to provide unique properties because the nearby metal changes the local environment on the support surface. In particular, the electron scavenger effect reduces the energy necessary for surface anion desorption, and thereby contributes to activation of the (reverse) Mars-van Krevelen mechanism. This study investigated the possibility of such activation in hydrides, carbides, nitrides, and sulfides. The work functions (WFs) of known hydrides, carbides, nitrides, oxides, and sulfides with group 3, 4, or 5 cations (Sc, Y, La, Ti, Zr, Hf, V, Nb, and Ta) were calculated. The WFs of most hydrides, carbides, and nitrides are smaller than the WF of Ag, implying that the electron scavenger effect may occur when late transition metal nanoparticles are adsorbed on the surface. The WF of oxides and sulfides decreases when reduced. The surface anion vacancy formation energy correlates well with the bulk formation energy in carbides and nitrides, while almost no correlation is found in hydrides because of the small range of surface hydrogen vacancy formation energy values. The electron scavenger effect is explicitly observed in nanorods adsorbed on TiH2 and Ti2O3; the surface vacancy formation energy decreases at anion sites near the nanorod, and charge transfer to the nanorod happens when an anion is removed at such sites. Activation of hydrides, carbides, and nitrides by nanorod adsorption and screening support materials through WF calculation are expected to open up a new category of supported catalysts.

Trapping of different stages of BaTiO3 reduction with LiH

We investigated the hydride reduction of tetragonal BaTiO₃ using LiH. The reactions employed molar H : BaTiO₃ ratios of 1.2, 3, and 10 and variable temperatures up to 700 °C. The air-stable reduced products were characterized by powder X-ray diffraction (PXRD), scanning electron microscopy, thermogravimetric analysis (TGA), X-ray fluorescence (XRF), and ¹H magic-angle spinning (MAS) NMR spectroscopy. Effective reduction, as indicated by the formation of dark blue to black colored, cubic-phased, products was observed at temperatures as low as 300 °C. The product obtained at 300 °C corresponded to oxyhydride BaTiO_∼2.9H_∼0.1, whereas reduction at higher temperatures resulted in simultaneous O defect formation, BaTiO_2.9-x H_0.1□ _x , and eventually - at temperatures above 450 °C - to samples void of hydridic H. Concomitantly, the particles of samples reduced at high temperatures (500-600 °C) display substantial surface alteration, which is interpreted as the formation of a TiO _x (OH) _y shell, and sintering. Diffuse reflectance UV-VIS spectroscopy shows broad absorption in the VIS-NIR region, which is indicative of the presence of n-type free charge carriers. The size of the intrinsic band gap (∼3.2 eV) appears only slightly altered. Mott-Schottky measurements confirm the n-type conductivity and reveal shifts of the conduction band edge in the LiH reduced samples. Thus LiH appears as a versatile reagent to produce various distinct forms of reduced BaTiO₃ with tailored electronic properties.

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 (E_NV) dominates the catalytic performance. The relatively low E_NV (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 (E_NH3) of 0.45 vol %, reaching the thermodynamic equilibrium (E_NH3 = 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.

Zeolite-seed-directed Ru nanoparticles highly resistant against sintering for efficient nitrogen activation to ammonia

To stabilize Ru nanoparticles against sintering is an urgent problem in the utilization of Ru-based catalysts for NH₃ synthesis. In the present study, we used Ru-containing ZSM-5 as seeds to crystallize ZSM-5, and the resulted Ru@ZSM-5 catalyst is highly resistant against Ru sintering. According to the results of diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) and transmission electron microscopy (TEM) analyses, the average size of Ru nanoparticles is around 3.6 nm, which is smaller than that of Ru/ZSM-5-IWI prepared by incipient wetness impregnation. In NH₃ synthesis (N₂:H₂ = 1:3) at 400 °C and 1 MPa, Ru@ZSM-5 displays a formation rate of 5.84 mmol_NH3 g_cat^-1 h^-1, which is much higher than that of Ru/ZSM-5-IWI (2.13 mmol_NH3 g_cat^-1 h^-1). According to the results of TEM, N₂-temperature-programmed desorption (N₂-TPD), X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS) studies, it is deduced that the superior performance of Ru@ZSM-5 is attributable to the small particle size and the ample existence of metallic Ru⁰ sites. This method of zeolite encapsulation is a feasible way to stabilize Ru nanoparticles for NH₃ synthesis.

Palladium supported on triazolyl-functionalized hypercrosslinked polymers as a recyclable catalyst for Suzuki-Miyaura coupling reactions

A novel hypercrosslinked polymers-palladium (HCPs-Pd) catalyst was successfully prepared via the external cross-linking reactions of substituted 1,2,3-triazoles with benzene and formaldehyde dimethyl acetal. The preparation of HCPs-Pd has the advantages of low cost, mild conditions, simple procedure, easy separation and high yield. The catalyst structure and composition were characterized by N₂ sorption, TGA, FT-IR, SEM, EDX, TEM, XPS and ICP-AES. The HCPs were found to possess high specific surface area, large micropore volume, chemical and thermal stability, low skeletal bone density and good dispersion for palladium chloride. The catalytic performance of HCPs-Pd was evaluated in Suzuki-Miyaura coupling reactions. The results show that HCPs-Pd is a highly active catalyst for the Suzuki-Miyaura coupling reaction in H₂O/EtOH solvent with TON numbers up to 1.66 × 10⁴. The yield of biaryls reached 99%. In this reaction, the catalyst was easily recovered and reused six times without a significant decrease in activity.

Insight into dynamic and steady-state active sites for nitrogen activation to ammonia by cobalt-based catalyst

The industrial synthesis of ammonia (NH₃) using iron-based Haber-Bosch catalyst requires harsh reaction conditions. Developing advanced catalysts that perform well at mild conditions (<400 °C, <2 MPa) for industrial application is a long-term goal. Here we report a Co-N-C catalyst with high NH₃ synthesis rate that simultaneously exhibits dynamic and steady-state active sites. Our studies demonstrate that the atomically dispersed cobalt weakly coordinated with pyridine N reacts with surface H₂ to produce NH₃ via a chemical looping pathway. Pyrrolic N serves as an anchor to stabilize the single cobalt atom in the form of Co₁-N_3.5 that facilitates N₂ adsorption and step-by-step hydrogenation of N₂ to *HNNH, *NH-NH₃ and *NH₂-NH₄. Finally, NH₃ is facilely generated via the breaking of the *NH₂-NH₄ bond. With the co-existence of dynamic and steady-state single atom active sites, the Co-N-C catalyst circumvents the bottleneck of N₂ dissociation, making the synthesis of NH₃ at mild conditions possible.

Complex Hydrides for Energy Storage, Conversion, and Utilization

Functional materials are the key enabling factor in the development of clean energy technologies. Materials of particular interest, which are reviewed herein, are a class of hydrogenous compound having the general formula of M(XH_n )_m , where M is usually a metal cation and X can be Al, B, C, N, O, transition metal (TM), or a mixture of them, which sets up an iono-covalent or covalent bonding with H. M(XH_n )_m is generally termed as a complex hydride by the hydrogen storage community. The rich chemistry between H and B/C/N/O/Al/TM allows complex hydrides of diverse composition and electronic configuration, and thus tunable physical and chemical properties, for applications in hydrogen storage, thermal energy storage, ion conduction in electrochemical devices, and catalysis in fuel processing. The recent progress is reviewed here and strategic approaches for the design and optimization of complex hydrides for the abovementioned applications are highlighted.

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, BaCeO_3-xN_yH_z, can be directly synthesized by the reaction of CeO₂ with Ba(NH₂)₂ at low temperatures (300-600 °C). BaCeO_3-xN_yH_z, with and without transition metal nanoparticles, functions as an efficient catalyst for ammonia synthesis through the lattice N^3- 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.

YHO, an Air-Stable Ionic Hydride

Metal hydride oxides are an emerging field in solid-state research. While some lanthanide hydride oxides (LnHO) were known, YHO has only been found in thin films so far. Yttrium hydride oxide, YHO, can be synthesized as bulk samples by a reaction of Y₂O₃ with hydrides (YH₃, CaH₂), by a reaction of YH₃ with CaO, or by a metathesis of YOF with LiH or NaH. X-ray and neutron powder diffraction reveal an anti-LiMgN type structure for YHO (Pnma, a = 7.5367(3) Å, b = 3.7578(2) Å, and c = 5.3249(3) Å) and YDO (Pnma, a = 7.5309(3) Å, b = 3.75349(13) Å, and c = 5.3192(2) Å); in other words, a distorted fluorite type with ordered hydride and oxide anions was observed. Bond lengths (average 2.267 Å (Y-O), 2.352 Å (Y-H), 2.363 Å (Y-D), >2.4 Å (H-H and D-D), >2.6 Å (H-O and D-O), and >2.8 Å (O-O)) and quantum-mechanical calculations on density functional theory level (band gap 2.8 eV) suggest yttrium hydride oxide to be a semiconductor and to have considerable ionic bonding character. Nonetheless, YHO exhibits a surprising stability in air. An in situ X-ray diffraction experiment shows that decomposition of YHO to Y₂O₃ starts at only above 500 K and is still not complete after 14 h of heating to a final temperature of 1000 K. YHO hydrolyzes in water very slowly. The inertness of YHO in air is very beneficial for its potential use as a functional material.

Single and double boron atoms doped nanoporous C2N-h2D electrocatalysts for highly efficient N2 reduction reaction: a density functional theory study

The electrocatalytical process is the most efficient way to produce ammonia (NH₃) under ambient conditions, but developing a highly efficient and low-cost metal-free electrocatalysts remains a major scientific challenge. Hence, single atom and double boron (B) atoms doped 2D graphene-like carbon nitride (C₂N-h2D) electrocatalysts have been designed (B@C₂N and B₂@C₂N), and the efficiency of N₂ reduction reaction (NRR) is examined by density functional theory calculation. The results show that the single and double B atoms can both be strongly embedded in natural nanoporous C₂N with superior catalytic activity for N₂ activation. The reaction mechanisms of NRR on the B@C₂N and B₂@C₂N are both following an enzymatic pathway, and B₂@C₂N is a more efficient electrocatalyst with extremely low overpotential of 0.19 eV comparing to B@C₂N (0.29 eV). In the low energy region, the hydrogenation of N₂ is thermodynamically more favorable than the hydrogen production, thereby improving the selectivity for NRR. Based on these results, a new double-atom strategy may help guiding the experimental synthesis of highly efficient NRR electrocatalysts.

Plasmon Sensitized Heterojunction 2D Ultrathin Ag/AgI-δ-Bi2O3 for Enhanced Photocatalytic Nitrogen Fixation

A novel 2D ultrathin Ag/AgI-δ-Bi₂O₃ photocatalyst was constructed by a facile hydrothermal and in situ photodeposition method, which presented a uniform nanosheet structure with an average height of 6 nm. Its composition, morphology and light-harvesting properties were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV-vis spectrophotometer (UV-vis) and photoluminescence (PL) measurements in detail. The Ag/AgI-δ-Bi₂O₃ nanocomposites showed an excellent photocatalytic nitrogen fixation performance of 420 μmol L^-1 g^-1 h^-1 in water without any sacrificial agent. The introduction of Ag/AgI nanoparticles caused the morphology modification of δ-Bi₂O₃, a higher concentration of oxygen vacancy, and the construction of a plasmon sensitized heterojunction, resulting in enhanced light absorption, improved separation efficiency of charge carriers and strong N₂ absorption and activation ability, which are responsible for the superior photocatalytic performance of Ag/AgI-δ-Bi₂O₃.

Tailoring widely used ammonia synthesis catalysts for H and N poisoning resistance

Despite many advancements, an inexpensive ammonia synthesis catalyst free from hydrogen and nitrogen poisoning, and capable of synthesizing ammonia under mild conditions is still unknown and is long sought-after. Here we present an active nanoalloy catalyst, RuFe, formed by alloying highly active Ru and inexpensive Fe, capable of activating both N2 and H2 without blocking the surface active sites and thereby overcoming the major hurdle faced by the current best performing pure metal catalysts. This novel RuFe nanoalloy catalyst operates under milder conditions than the conventional Fe catalyst and is less expensive than the so far best performing Ru-based catalysts providing additional advantages. Most importantly, by integrating theory and experiments, we identified the underlying mechanisms responsible for lower surface poisoning of this catalyst, which will provide directions for fabricating poison-free efficient NH3 synthesis catalysts in future.