Dinitrogen Dissociation Research Articles

Regulating the Scaling Relations in Ammonia Synthesis through a Light-Driven Bendable Seesaw Effect on Tailored Iron Catalyst.

Advancing the energy-intensive Haber-Bosch process faces significant challenges due to the intrinsic constraints of scaling relations in heterogeneous catalysis. Herein, we reported an approach of bending the "seesaw effect" to regulate the scaling relations over a tailored α-Fe metallic material (α-Fe-110s), realizing highly efficient light-driven thermal catalytic ammonia synthesis with a rate of 1260 μmol gcatalyst -1 h-1 without additional heating. Specifically, the thermal catalytic activity of α-Fe-110s was significantly enhanced by the novel stepped {110} surface, exhibiting a 3.8-fold increase compared to the commercial fused-iron catalyst with promoters at 350 °C. The photo-induced hot electron transfer further accelerates the dinitrogen dissociation and hydrogenation simultaneously, effectively overcoming the limitation of scaling relation over identical sites. Consequently, the ammonia production rate of α-Fe-110s was further enhanced by 30 times at the same temperature with irradiation. This work designs an efficient and sustainable system for ammonia synthesis and provides a novel approach for regulating the scaling relations in heterogeneous catalysis.

Regulating the Scaling Relations in Ammonia Synthesis through a Light‐Driven Bendable Seesaw Effect on Tailored Iron Catalyst

AbstractAdvancing the energy‐intensive Haber–Bosch process faces significant challenges due to the intrinsic constraints of scaling relations in heterogeneous catalysis. Herein, we reported an approach of bending the “seesaw effect” to regulate the scaling relations over a tailored α‐Fe metallic material (α‐Fe‐110s), realizing highly efficient light‐driven thermal catalytic ammonia synthesis with a rate of 1260 μmol gcatalyst−1 h−1 without additional heating. Specifically, the thermal catalytic activity of α‐Fe‐110s was significantly enhanced by the novel stepped {110} surface, exhibiting a 3.8‐fold increase compared to the commercial fused‐iron catalyst with promoters at 350 °C. The photo‐induced hot electron transfer further accelerates the dinitrogen dissociation and hydrogenation simultaneously, effectively overcoming the limitation of scaling relation over identical sites. Consequently, the ammonia production rate of α‐Fe‐110s was further enhanced by 30 times at the same temperature with irradiation. This work designs an efficient and sustainable system for ammonia synthesis and provides a novel approach for regulating the scaling relations in heterogeneous catalysis.

Terminal N2 Dissociation in [(PNN)Fe(N2 )]2 (μ-N2 ) Leads to Local Spin-State Changes and Augmented Bridging N2 Activation.

Nitrogen fixation at iron centres is a fundamental catalytic step for N2 utilisation, relevant to biological (nitrogenase) and industrial (Haber-Bosch) processes. This step is coupled with important electronic structure changes which are currently poorly understood. We show here for the first time that terminal dinitrogen dissociation from iron complexes that coordinate N2 in a terminal and bridging fashion leaves the Fe-N2 -Fe unit intact but significantly enhances the degree of N2 activation (Δν≈180 cm-1 , Raman spectroscopy) through charge redistribution. The transformation proceeds with local spin state change at the iron centre (S= →S=3 /2 ). Further dissociation of the bridging N2 can be induced under thermolytic conditions, triggering a disproportionation reaction, from which the tetrahedral (PNN)2 Fe could be isolated. This work shows that dinitrogen activation can be induced in the absence of external chemical stimuli such as reducing agents or Lewis acids.

Plasma-Assisted Dinitrogen Activation via Dual Platinum Cluster Catalysis: A Strategy for Ammonia Synthesis under Mild Conditions

Plasma-Assisted Dinitrogen Activation via Dual Platinum Cluster Catalysis: A Strategy for Ammonia Synthesis under Mild Conditions

Ternary ruthenium complex hydrides for ammonia synthesis via the associative mechanism

Ammonia is the feedstock for nitrogen fertilizers and a potential carbon-free energy carrier; however, its production is highly energy intensive. Conventional heterogeneous catalysts based on metallic iron or ruthenium mediate dinitrogen dissociation and hydrogenation through a relatively energy-expensive pathway. Here we report the ternary ruthenium complex hydrides Li4RuH6 and Ba2RuH6 as an alternative class of catalysts, composed of electron- and hydrogen-rich [RuH6] anionic centres, for non-dissociative dinitrogen reduction, where hydridic hydrogen transports electrons and protons between the centres, and the Li/Ba cations stabilize NxHy (x = 0–2, y = 0–3) intermediates. The dynamic and synergistic involvement of all the components of the ternary complex hydrides facilitates an associative reaction mechanism with a narrow energy span and perfectly balanced kinetic barriers for the multistep process, leading to ammonia production from N2 + H2 with superior kinetics under mild conditions.

Adsorption and dissociation of dinitrogen on mono-metallic and bimetallic dimers

This paper deals with Density Functions Theory (DFT) based theoretical investigation of the dissociation of strong N–N triple bond of dinitrogen (N2). Mono-metallic and bimetallic dimers of selected transition metals (Zr, Nb, Hf, and Ta) are used as adsorbent. The dissociation of N–N bond is found to be strongly dependent on orientation of N–N axis with respect to the axis of the adsorbent dimer. N–N axis perpendicular to dimer axis has been found to be suitable for dissociative adsorption of dinitrogen. Apart from orientation the combination of two different transition metal atoms in the dimer also has significant effect in the elongation and dissociation of N–N bond. Our study shows even 2-atom clusters (dimers) of these transition metals are capable of breaking N–N triple bond.

Microwave-driven heterogeneous catalysis for activation of dinitrogen to ammonia under atmospheric pressure

This paper presents an innovative approach to producing energy-dense, carbon–neutral liquid ammonia as a means for carrying energy. This approach synergistically integrates microwave reaction chemistry with novel heterogeneous catalysis that decouples dinitrogen activation from high-temperature and high-pressure reactions, altering reaction pathways and increasing ammonia formation rate. Results presented here demonstrate that ammonia synthesis can be conducted at 280 ℃ and ambient pressure to achieve ~1 mmol ammonia/g cat/h over supported ruthenium catalyst systems utilizing microwave irradiation. It is further shown that adding promoter ions such as potassium, cerium, and barium significantly improves the ammonia production rate over undoped ruthenium-based catalysts. This effect could be attributed to enhanced dielectric loss processes that lead to stronger microwave absorption by the catalyst. Measurement of the equilibrium constant under microwave conditions showed a higher ammonia yield than under thermal equilibrium conditions for both the iron- and ruthenium-based catalysts. Finally, this study also illustrates the advantages of using a variable-frequency microwave reactor for ambient-pressure ammonia synthesis. Mechanistically, investigators believed that the oscillating electric fields of the radiation can couple with adsorbed nitrogen on the surface and accelerate its dissociation. Since dinitrogen dissociation on the surface is rate limiting, this effectively accelerates the reaction. Overall, the study provides an in-depth analysis of the parameters affecting the use of microwaves in catalyzed ammonia synthesis. This process is fundamentally different from the commercial Haber–Bosch process and provides an alternative method of ammonia synthesis for certain applications, as it is tolerant to intermittent supplies of renewable energy, therefore effectively operating at variable rates of production.

Artificial Neural Networks Applied as Molecular Wave Function Solvers.

We use artificial neural networks (ANNs) based on the Boltzmann machine (BM) architectures as an encoder of ab initio molecular many-electron wave functions represented with the complete active space configuration interaction (CAS-CI) model. As first introduced by the work of Carleo and Troyer for physical systems, the coefficients of the electronic configurations in the CI expansion are parametrized with the BMs as a function of their occupancies that act as descriptors. This ANN-based wave function ansatz is referred to as the neural-network quantum state (NQS). The machine learning is used for training the BMs in terms of finding a variationally optimal form of the ground-state wave function on the basis of the energy minimization. It is relevant to reinforcement learning and does not use any reference data nor prior knowledge of the wave function, while the Hamiltonian is given based on a user-specified chemical structure in the first-principles manner. Carleo and Troyer used the restricted Boltzmann machine (RBM), which has hidden units, for the neural network architecture of NQS, while, in this study, we further introduce its replacement with the BM that has only visible units but with different orders of connectivity. For this hidden-node free BM, the second- and third-order BMs based on quadratic and cubic energy functions, respectively, were implemented. We denote these second- and third-order BMs as BM2 and BM3, respectively. The pilot implementation of the NQS solver into an exact diagonalization module of the quantum chemistry program was made to assess the capability of variants of the BM-based NQS. The test calculations were performed by determining the CAS-CI wave functions of illustrative molecular systems, indocyanine green, and dinitrogen dissociation. The simulated energies have been shown to converge to CAS-CI energy in most cases by improving RBM with an increasing number of hidden nodes. BM3 systematically yields lower energies than BM2, reproducing the CAS-CI energies of dinitrogen across potential energy curves within an error of 50 μEh.

Adsorption and dissociation of dinitrogen on transition metal (Ta, W and Re) doped MgO surface

The adsorption and dissociation of dinitrogen on transition metal (Ta, W and Re) doped MgO(100) surface has been studied employing density functional theory. It is found that all these transition metals (TM) on MgO(100) surface are capable of adsorbing dinitrogen (N2), however there is no dissociative adsorption of N2 on single transition metal dopant. When two TM atoms are doped on MgO(100) surface, dissociative adsorption of dinitrogen occurs in all the three cases. Whether the dissociation is spontaneous or is it associated with activation barrier depends on the orientation of N2 molecule approaching the dopant site.

Highly effective catalysis of the double-icosahedral Ru19 cluster for dinitrogen dissociation – a first-principles investigation

The N2 bond cleavage is the rate-limiting step in the synthesis of ammonia, and ruthenium is a catalyst well known for this reaction. The double-icosahedral (D5h) Ru19 cluster is famous as an active catalyst, and has a remarkable stability towards the adsorption of H2, N2 and CO. Using first-principles calculations, we have investigated the adsorption and dissociation of dinitrogen on a double-icosahedral Ru19 cluster. Our results show that the hollow site in the rhombus region (BHB site) of the Ru19 cluster possesses the greatest catalytic activity to dissociate N2, with the reaction barrier of 0.89 eV and an exothermicity of -1.45 eV. Multiple coadsorption of N2 on the cluster (i.e. coadsorption of 2N2 and 3N2 on a single Ru19 cluster) causes the barrier to dissociate N2 to be less on a BHB site than for adsorption of a single N2. To understand the catalytic properties of a Ru19 cluster towards N2 bond cleavage, we calculated the electron population, vibrational wavenumbers and local densities of states; the results are explicable.

Dramatic reduction in the activation barrier for dinitrogen splitting using amine–borane as a hydrogen carrier: insights from the DFT study

DFT based mechanistic investigations on a silica surface supported Tantalum system reveal that there is a significant reduction in the free energy activation barrier for N≡N bond dissociation upon using a suitable amine-borane as a hydrogenating agent as compared to that of using molecular H2 for the same purpose, which suggests that dinitrogen dissociation can be achieved in surface chemistry at much lower temperatures than those in the presently used systems.

Design and Preparation of Molybdenum–Dinitrogen Complexes with Ferrocenyldiphosphine and Pentamethylcyclopentadienyl Moieties as Auxiliary Ligands

Fix me! Molybdenum-dinitrogen complexes containing ferrocenyldiphosphine and pentamethylcyclopentadienyl moieties as auxiliary ligands have been designed, prepared, and characterized. The ferrocenyldiphosphine works as a unique ligand to inhibit dissociation of dinitrogen, as well as to make the coordinated molecular dinitrogen reactive toward electrophiles.

Dinitrogen Dissociation on an Isolated Surface Tantalum Atom

Both industrial and biochemical ammonia syntheses are thought to rely on the cooperation of multiple metals in breaking the strong triple bond of dinitrogen. Such multimetallic cooperation for dinitrogen cleavage is also the general rule for dinitrogen reductive cleavage with molecular systems and surfaces. We have observed cleavage of dinitrogen at 250 degrees C and atmospheric pressure by dihydrogen on isolated silica surface-supported tantalum(III) and tantalum(V) hydride centers [(identical with Si-O)2Ta(III)-H] and [(identical with Si-O)2Ta(V)H3], leading to the Ta(V) amido imido product [(identical with SiO)2Ta(=NH)(NH2)]: We assigned the product structure based on extensive characterization by infrared and solid-state nuclear magnetic resonance spectroscopy, isotopic labeling studies, and supporting data from x-ray absorption and theoretical simulations. Reaction intermediates revealed by in situ monitoring of the reaction with infrared spectroscopy support a mechanism highly distinct from those previously observed in enzymatic, organometallic, and heterogeneous N2 activating systems.

Structure and reactivity of Ru nanoparticles supported on modified graphite surfaces: a study of the model catalysts for ammonia synthesis.

Supported ruthenium metal catalysts have higher activity than traditional iron-based catalysts used industrially for ammonia synthesis. A study of a model Ru/C catalyst was carried out to advance the understanding of structure and reactivity correlations in this structure-sensitive reaction where dinitrogen dissociation is the rate-limiting step. Ru particles were grown by chemical vapor deposition (CVD) via a Ru(3)(CO)(12) precursor on a highly oriented pyrolytic graphite (HOPG) surface modified with one-atomic-layer-deep holes mimicking activated carbon support. Scanning tunneling microscopy (STM) has been used to investigate the growth, structure, and morphology of the Ru particles. Thermal desorption of dissociatively adsorbed nitrogen has been used to evaluate the reactivity of the Ru/HOPG model catalysts. Two different Ru particle structures with different reactivities to N(2) dissociation have been identified. The initial sticking coefficient for N(2) dissociative adsorption on the Ru/HOPG model catalysts is approximately 10(-6), 4 orders larger compared to that of Ru single-crystal surfaces.

Use of kinetic models to explore the role of base promoters on Ru/MgO ammonia synthesis catalysts

The kinetics of ammonia synthesis over unpromoted and Cs-, Ba-, and La-promoted Ru/MgO (∼2 wt%) were studied in a tubular reactor at 20.7 atm. The reaction over all of the catalysts was nearly first order in N 2 and nearly zero order in NH 3. However, the order of reaction in H 2 was largely dependent on the choice of promoter, with Ba and La significantly reducing the inhibition by dihydrogen seen on Cs-promoted Ru/MgO. The turnover frequencies on Ba- and La-promoted Ru/MgO under stoichiometric conditions and 673 K were almost an order of magnitude greater than that on Cs-promoted Ru/MgO. Two kinetic models with optimized parameters were used to describe the outlet ammonia pressures determined experimentally both close to and far from equilibrium. Although Cs promotion of Ru lowered the activation barrier for N 2 dissociation, the enthalpy of dihydrogen adsorption and therefore the H atom surface coverage increased. Thus, promotion of Ru catalysts with bases cannot be attributed solely to an effect on dinitrogen dissociation, which is the rate-determining step. Base promotion is a trade-off between lowering the activation barrier for N 2 dissociation and increasing the competitive adsorption of H 2. The coverages of nitrogen-containing species determined by an optimized kinetic model matched well those determined experimentally by isotopic transient analysis over the same catalysts.

Use of kinetic models to explore the role of base promoters on Ru/MgO ammonia synthesis catalysts

The kinetics of ammonia synthesis over unpromoted and Cs-, Ba-, and La-promoted Ru/MgO (∼2 wt%) were studied in a tubular reactor at 20.7 atm. The reaction over all of the catalysts was nearly first order in N2 and nearly zero order in NH3. However, the order of reaction in H2 was largely dependent on the choice of promoter, with Ba and La significantly reducing the inhibition by dihydrogen seen on Cs-promoted Ru/MgO. The turnover frequencies on Ba- and La-promoted Ru/MgO under stoichiometric conditions and 673 K were almost an order of magnitude greater than that on Cs-promoted Ru/MgO. Two kinetic models with optimized parameters were used to describe the outlet ammonia pressures determined experimentally both close to and far from equilibrium. Although Cs promotion of Ru lowered the activation barrier for N2 dissociation, the enthalpy of dihydrogen adsorption and therefore the H atom surface coverage increased. Thus, promotion of Ru catalysts with bases cannot be attributed solely to an effect on dinitrogen dissociation, which is the rate-determining step. Base promotion is a trade-off between lowering the activation barrier for N2 dissociation and increasing the competitive adsorption of H2. The coverages of nitrogen-containing species determined by an optimized kinetic model matched well those determined experimentally by isotopic transient analysis over the same catalysts.

Isotopic Transient Analysis of Ammonia Synthesis over Ba or Cs-Promoted Ru/Carbon Catalysts

The effects of Cs or Ba promoters on the global and intrinsic turnover frequency of ammonia synthesis over ruthenium supported on carbon (9.1 wt% Ru) were examined at 3 atm and stoichiometric conditions by steady state and isotopic transient measurements at 573 K. Although the global turnover frequency based on total surface Ru atoms was nearly independent of promoter, Cs was more effective than Ba at promoting the intrinsic turnover frequency determined by the isotopic switching experiment. The similarity of the global turnover frequency over the two samples resulted from the coverage of nitrogen-containing intermediates being greater on the Ba-promoted catalyst. The promoters presumably affect the global activity of ammonia synthesis by modifying both the inhibition by dihydrogen and the activation barrier for dinitrogen dissociation.

Isotopic transient analysis of ammonia synthesis over Ru/MgO catalysts promoted by cesium, barium, or lanthanum

The global and intrinsic kinetics of ammonia synthesis over ruthenium supported on MgO (∼2 wt% Ru) and promoted with Cs, Ba, or La were examined at 3 atm and various dihydrogen pressures with steady-state and isotopic transient measurements. Steady-state, global measurements revealed that Cs–Ru/MgO was strongly inhibited by dihydrogen whereas Ba–Ru/MgO and La–Ru/MgO were weakly inhibited by the reactant. However, the isotopic transient measurements showed that the intrinsic turnover frequency was a weak positive function of dihydrogen pressure regardless of the added base. Promotion by these compounds appeared to be electronic in nature because their presence increased the intrinsic turnover frequency of an active site, with Cs being the most effective. However, the coverage of nitrogen-containing intermediates was greater on Ba- and La-promoted catalysts than on Cs–Ru/MgO at 673 K and stoichiometric conditions. Therefore, the global activity of a promoted catalyst is a competition between inhibition by dihydrogen and enhancement of dinitrogen dissociation. Furthermore, promotion with Cs, Ba, and La introduced a new and highly active class of sites to the Ru/MgO system.

Isotopic Transient Kinetic Analysis of Cs-Promoted Ru/MgO during Ammonia Synthesis

Steady-state isotopic transient kinetic analysis was used to examine a cesium-promoted Ru/MgO catalyst (1.75 wt% Ru) for ammonia synthesis. For comparison, an unpromoted Ru/SiO 2 (22 wt% Ru) catalyst was also studied. Steady-state and isotopic transient measurements were conducted at temperatures between 603 and 673 K with a stoichiometric ratio of N 2 and H 2 at a total pressure of 3 atm. The fractional surface coverage of nitrogen-containing intermediates (θ NH x ) based on total H 2 chemisorption was found to range from 0.02 to 0.05 on both catalysts under the conditions used in the current study. However, the intrinsic turnover frequency of ammonia synthesis over Cs–Ru/MgO was two orders of magnitude greater than that of the unpromoted Ru/SiO 2. The combination of the Cs promoter and MgO support enhanced the intrinsic activity of Ru, presumably by lowering the barrier for dinitrogen dissociation.

Evidence for DirecttransInsertion in a Hydrido‐Olefin Rhodium Complex—Free Nitrogen as a Trap in a Migratory Insertion Process

Reduction of the hydrido chloride complex [Rh(H)Cl{CH3 C(CH2 CH2 -P(tBu)2 )2 }] (4) with NaH under a nitrogen atmosphere results in formation of two products: the dinitrogen complex [Rh(N2 )-{CH3 C(CH2 P(tBu)2 )2 }] (2) and the unusual low-valent hydrido-olefin complex, [RhH{CH2 C(CH2 CH2 P(tBu)2 )2 }] (3). In the presence of N2 , complexes 2 and 3 are in equilibrium in solution; 2 is about 2.9kcalmol(-1) more stable than 3 + N2 . Both complexes co-crystallize in the solid state; they occupy the same crystallographic site in the crystal lattice (P2-(1)/c; Z = 4; a = 12.173(2), b = 14.121 (3), c = 15.367 (3); α = 90, β = 106.50(3), γ = 90°). The mechanism of the reversible interconversion of 2 and 3 has been studied in detail. Complex 3 undergoes rapid olefin insertion/β-hydrogen elimination processes. The insertion rates were measured at different temperatures by saturation transfer NMR experiments, providing evidence for a highly organized late transition state (δS≠≈︂ - 40 e.u.), which can be caused by a concerted "trans migration". This theoretically unfavorable process is assisted by a distortion from the ideal square-planar configuration, including a decrease of the P-Rh-P angle and some bias of the double bond toward the hydride as indicated by the X-ray crystal structure of 3. Under a nitrogen atmosphere, the intermediate formed upon olefin insertion is slowly trapped by free dinitrogen to form complex 2. The dinitrogen dissociation from 2 was found to be the rate-determining step for the overall interconversion of 2 and 3 (δG≠298 = 24.1 kcalmol(-1) ).