Nitrogen-containing Intermediates Research Articles

Activity of CeO2(111) supported Mnx(x = 1–6) for electrochemical N2 reduction reaction: Insights from density functional theory

It is challenging to realize an efficient nitrogen reduction reaction (NRR) under mild conditions, which suffers from low ammonia yield and low Faraday efficiency due to the extremely stable NN triple bond of N2 as well as competitive hydrogen evolution reaction. In this work, the NRR reactivity of Mnx(x= 1–6) clusters supported on CeO2(111) (Mnx(x = 1–6)/CeO2(111)) was systematically investigated using density functional theory. A volcanic relationship between the limiting potential of NRR on Mnx(x = 1–6)/CeO2(111) and the atom number of Mnx was found. Mn3/CeO2(111) shows the highest activity for NRR with a limiting potential of -0.36, -0.55 and -0.53 V along distal, alternating and enzymatic reaction pathway, respectively. Its high activity is attributed to the triangular geometry and optimal average number of electrons every Mn transferred from Mn3 to CeO2(111), which leads to the strong N2 activation and the stabilization of nitrogen-containing intermediates. Also, Mn3/CeO2(111) exhibits a high NRR selectivity by hindering H adsorption and a high thermal stability at both 298 and 773 K, suggesting its promising potential as effective NRR catalyst. This work provides new insights into the rational design of single cluster catalysts.

Supercritical hydrothermal combustion and enhanced degradation characteristics of phenol

Supercritical hydrothermal combustion is a green combustion technology that produces flames in supercritical water (SCW), which can realize efficient disposal of aqueous wastes. In this study, the typical pollutant phenol is investigated through tubular reactor experiments and mechanism analysis to explore its hydrothermal combustion properties and enhanced degradation effects on refractory species. The results show that 3.06 and 3.41 wt% phenol can be self-ignited in the tubular reactor with a critical preheating temperature of 460 °C. Methanol could improve the ignition and burnout characteristics of phenol. As the COD ratio of methanol was greater than 0.35, the ignition temperature of phenol was reduced to 420 °C, with TOC removal rate achieving 99.9%. At 460 °C, phenol positively influenced the heat release during combustion of phenol/methanol. In oxidation of phenol/ammonia, 1.61 wt% phenol resulted in the reaction temperature rising by around 62.0 °C, allowing a NH4-N removal rate of 92.8% through its synergistic kinetic and thermal effects. Whereas, the nitrogen-containing intermediates from ammonia inhibited phenol oxidation. Finally, a mechanism-based kinetics model for oxidation of phenol in SCW was developed by means of real gas properties and sensitive reaction step modifications, which was viable for simulating the ring-opening path of phenol.

Assessing Exchange-Correlation Functionals for Heterogeneous Catalysis of Nitrogen Species.

Increasing interest in the sustainable synthesis of ammonia, nitrates, and urea has led to an increase in studies of catalytic conversion between nitrogen-containing compounds using heterogeneous catalysts. Density functional theory (DFT) is commonly employed to obtain molecular-scale insight into these reactions, but there have been relatively few assessments of the exchange-correlation functionals that are best suited for heterogeneous catalysis of nitrogen compounds. Here, we assess a range of functionals ranging from the generalized gradient approximation (GGA) to the random phase approximation (RPA) for the formation energies of gas-phase nitrogen species, the lattice constants of representative solids from several common classes of catalysts (metals, oxides, and metal-organic frameworks (MOFs)), and the adsorption energies of a range of nitrogen-containing intermediates on these materials. The results reveal that the choice of exchange-correlation functional and van der Waals correction can have a surprisingly large effect and that increasing the level of theory does not always improve the accuracy for nitrogen-containing compounds. This suggests that the selection of functionals should be carefully evaluated on the basis of the specific reaction and material being studied.

Cu1-Fe Dual Sites for Superior Neutral Ammonia Electrosynthesis from Nitrate.

The electrochemical nitrate reduction reaction (NO3RR) is able to convert nitrate (NO3 -) into reusable ammonia (NH3), offering a green treatment and resource utilization strategy of nitrate wastewater and ammonia synthesis. The conversion of NO3 - to NH3 undergoes water dissociation to generate active hydrogen atoms and nitrogen-containing intermediates hydrogenation tandemly. The two relay processes compete for the same active sites, especially under pH-neutral condition, resulting in the suboptimal efficiency and selectivity in the electrosynthesis of NH3 from NO3 -. Herein, we constructed a Cu1-Fe dual-site catalyst by anchoring Cu single atoms on amorphous iron oxide shell of nanoscale zero-valent iron (nZVI) for the electrochemical NO3RR, achieving an impressive NO3 - removal efficiency of 94.8 % and NH3 selectivity of 99.2 % under neutral pH and nitrate concentration of 50 mg L-1 NO3 --N conditions, greatly surpassing the performance of nZVI counterpart. This superior performance can be attributed to the synergistic effect of enhanced NO3 - adsorption on Fe sites and strengthened water activation on single-atom Cu sites, decreasing the energy barrier for the rate-determining step of *NO-to-*NOH. This work develops a novel strategy of fabricating dual-site catalysts to enhance the electrosynthesis of NH3 from NO3 -, and presents an environmentally sustainable approach for neutral nitrate wastewater treatment.

Boosting electroreduction of nitrate to ammonia by modulating the crystalline phase of Fe2O3

Electrocatalytic nitrate reduction reaction (NO3RR) provides an alternative to the conventional Haber-Bosch process for ammonia synthesis and is an effective method for removal of nitrate ions from polluted waters, which is highly significant from both energy and environmental perspectives. However, NO3RR involves the complex eight-electron process alongside various nitrogen-containing intermediates and is also in competition with hydrogen evolution reaction, thus demanding highly active and selective electrocatalysts. In this work we prepare a Ni-doped Fe2O3 electrocatalyst via a solvent-free route. It is found that the addition of Ni induces the crystalline-phase transformation of Fe2O3 from γ-Fe2O3 to α-Fe2O3. The density functional theory (DFT) results reveal that compared to γ-Fe2O3, α-Fe2O3 gives rise to a lower potential-determining step (PDS) energy barrier, leading to the more thermodynamically favourable reaction. By modulating the crystalline phase, the optimal catalyst achieves high ammonia yield rates of > 5000 μg h−1 cm−2 and faradaic efficiencies of > 90 %, showcasing its high electrocatalytic activity and selectivity. From this perspective, this paper provides new insights and strategies for the green nitrate-to-ammonia conversion.

Synergistic impact of dual active sites in Nb-Fe2P nanoparticles on electrocatalytic nitrate reduction with high selectivity

The electrochemical transformation of nitrate into ammonia offers a promising alternative to the traditional Haber-Bosch process. The nitrate reduction reaction (NO3–RR) involves complex multi-electron/proton processes and faces competition from hydrogen evolution reactions, leading to low Faraday efficiency and diminished ammonia production. This study introduces carbon-supported niobium-doped iron phosphide nanoparticles (Nb-Fe2P) electrocatalysts, synthesized using deep eutectic solvent (DES). These electrocatalysts achieved an exceptional ammonia yield of 9.7 mg h−1 cm−2 and a Faraday efficiency of 95.8 % in the NO3–RR. Both experimental and theoretical studies reveal that the dual active sites of Fe and Nb significantly enhance the generation of active hydrogen and expedite the utilization of nitrogen-containing intermediates. This innovative approach, focusing on active hydrogen balance, offers a strategic pathway for developing highly efficient NO3–RR electrocatalysts by promoting the dissociation of H2O.

Comparative analysis of UV-initiated ARPs for degradation of the emerging substitute of perfluorinated compounds: Does defluorination mean the sole factor?

Due to the increasing attention for the residual of per- and polyfluorinated compounds in environmental water, Sodium p-Perfluorous Nonenoxybenzenesulfonate (OBS) have been considered as an alternative solution for perfluorooctane sulfonic acid (PFOS). However, recent detections of elevated OBS concentrations in oil fields and Frontal polymerization foams have raised environmental concerns leading to the decontamination exploration for this compound. In this study, three advanced reduction processes including UV-Sulfate (UV-SF), UV-Iodide (UV-KI) and UV-Nitrilotriacetic acid (UV-NTA) were selected to evaluate the removal for OBS. Results revealed that hydrated electrons (eaq−) dominated the degradation and defluorination of OBS. Remarkably, the UV-KI exhibited the highest removal rate (0.005 s−1) and defluorination efficiency (35 %) along with the highest concentration of eaq− (K = −4.651). Despite that nucleophilic attack from eaq− on sp2 carbon and H/F exchange were discovered as the general mechanism, high-performance liquid chromatography-quadrupole time-of-flight mass spectrometry (HPLC/Q-TOF-MS) analysis with density functional theory (DFT) calculations revealed the diversified products and routes. Intermediates with lowest fluorine content for UV-KI were identified, the presence nitrogen-containing intermediates were revealed in the UV-NTA. Notably, the nitrogen-containing intermediates displayed the enhanced toxicity, and the iodine poly-fluorinated intermediates could be a potential-threat compared to the superior defluorination performance for UV-KI.

Strategic regulation of nitrogen-containing intermediates for enhanced nitrate reduction over Co3O4/SiC catalyst having multiple active centers

Regulating the intermediates involved in the electrocatalytic nitrate reduction reaction (NO3RR) is crucial for the enhancement of reaction efficiency. However, it remains a great challenge to regulate the reaction intermediates through active site manipulation on the surface of the catalyst. Here, a family of n%-Co3O4/SiC (n = 5, 8, 12, 20) catalysts with a delicate percentage of Co2+ and Co3+ were prepared for NO3RR. We found that Co3+ primarily acts as the active site for NO3− reduction to NO2−, while Co2+ is responsible for the conversion of NO2− to NH3. Moreover, the conversion of these intermediates over the active sites is autonomous and separately controllable. Both processes synergistically accomplish the reduction of nitrate ions to synthesize ammonia. Combining the experimental studies and density functional theory (DFT) calculations, it is discovered the pathway (*NHO→*NHOH→*NH2OH→*NH2→*NH3) is more favorable due to the lower ΔG value (0.25 eV) for the rate-limiting step (*NO→*NHO). The NH3 yield rate of 8%-Co3O4/SiC reached 1.08 mmol/(cm2 h) with a Faradaic efficiency of 96.4% at −0.89 V versus the reversible hydrogen electrode (RHE), surpassing those of most reported non-noble NO3RR catalysts. This strategy not only provides an efficient catalyst for NO3RR but also serves as an illustrative model for the regulation of multi-step reaction intermediates through the design of distinct active sites, thereby presenting a new approach to enhance the efficiency of intricate reactions.

Unveiling the Reaction Mechanism of Nitrate Reduction to Ammonia Over Cobalt-Based Electrocatalysts.

Electrocatalytic reduction of nitrate to ammonia (NRA) has emerged as an alternative strategy for sewage treatment and ammonia generation. Despite excellent performances having been achieved over cobalt-based electrocatalysts, the reaction mechanism as well as veritable active species across a wide potential range are still full of controversy. Here, we adopt CoP, Co, and Co3O4 as model materials to solve these issues. CoP evolves into a core@shell structured CoP@Co before NRA. For CoP@Co and Co catalysts, a three-step relay mechanism is carried out over superficial dynamical Coδ+ active species under low overpotential, while a continuous hydrogenation mechanism from nitrate to ammonia is unveiled over superficial Co species under high overpotential. In comparison, Co3O4 species are stable and steadily catalyze nitrate hydrogenation to ammonia across a wide potential range. As a result, CoP@Co and Co exhibit much higher NRA activity than Co3O4 especially under a low overpotential. Moreover, the NRA performance of CoP@Co is higher than Co although they experience the same reaction mechanism. A series of characterizations clarify the reason for performance enhancement highlighting that CoP core donates abundant electrons to superficial active species, leading to the generation of more active hydrogen for the reduction of nitrogen-containing intermediates.

Exploring NH3 combustion in environments with CO2 and H2O via reactive molecular dynamics

In existing power units such as internal combustion engines and gas turbines, the combustion of ammonia mixed with hydrocarbon fuels can effectively enhance the combustion intensity of ammonia. In the duel-fuel combustion scenario, the effect mechanism of the main components of combustion exhaust gas i.e., CO2 and H2O on ammonia combustion is still unclear and needs to be studied. Therefore, this paper uses reactive molecular dynamics simulations to study the combustion and emission processes of ammonia in environments with CO2 and H2O. The effects of the diverse gas environments (N2/O2, O2/CO2, and O2/CO2/H2O environments), high CO2 levels, varying H2O proportions, and O2 concentrations on ammonia combustion and emission were systematically explored. It was found that ammonia consumption was accelerated in the O2/CO2/H2O environments compared to N2/O2 environments, resulting in more NH2, H2, and H2O production. The effect of high CO2 concentration on ammonia oxidation was mainly related to temperature. The simulations found that the high CO2 concentration inhibited ammonia consumption at T < 2800K while enhancing it at T ⩾ 2800K. In addition, the high CO2 concentration could inhibit NO production under stoichiometric conditions but promote the conversion of nitrogen-containing intermediates to NO with the oxidizing capacity of CO2 enhanced significantly under extremely high temperature conditions. It was found that the increasing content of H2O molecules in the environment promoted NO production and inhibited CO production under stoichiometric conditions. In addition, the reaction paths for the conversion of HNO and HNO2 molecules into NO molecules were significantly enhanced with increasing H2O concentration. The increase in oxygen concentration can promote OH radical production and NO formation at high temperatures.

Pyrolysis and kinetic modeling study of tetramethylethylenediamine: A potential green propellant

Tetramethylethylenediamine (TMEDA) is a promising green propellant that has been studied as a substitute for hydrazine. An in-depth study of the TMEDA thermal decomposition mechanism is important for future applications in engines. In this work, pyrolysis experiments of TMEDA were carried out in an atmospheric jet-stirred reactor (JSR) from 610 to 1000 K. Abundant intermediate species were analyzed and quantified by synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) and gas chromatography (GC). The results show that the pyrolysis of TMEDA mainly produces hydrocarbons, amines, imines, and nitriles, among which HCN and C2H5N are the main nitrogen-containing intermediates. Based on the experimental results, a detailed chemical kinetic model for describing the thermal decomposition of TMEDA was updated to improve predictions. The updated model can satisfactorily predict the consumption of TMEDA and the generation of most intermediates. Furthermore, detailed experimental measurements combined with kinetic analysis are used to clarify the decomposition reaction pathways of TMEDA. The rate of production (ROP) analysis indicates that unimolecular C-C bond dissociation is one of the leading consumption pathways of TMEDA, forming the (CH3)2NCH2 radical. In addition, the H-atom abstraction reactions involving CH3 radicals formed by the C-N bond dissociation of (CH3)2NCH2 radicals further promote the consumption of TMEDA. Sensitivity analysis highlights that the unimolecular C-C bond dissociation reaction of TMEDA and the reactions involving CH3 radicals mainly control the pyrolysis behavior of TMEDA. The detailed experimental results and kinetic analysis in this work are expected to contribute valuable references to further research on the combustion mechanism of TMEDA.

Comprehensive investigation on the slow pyrolysis product characteristics of waste tobacco stem: Pyrolysis reaction mechanism and conversion mechanism of N

The resource utilization of waste tobacco stem (WTS) is attractive but challenge for China. Slow pyrolysis is an effective method for agricultural waste while the pyrolysis performance and mechanism of WTS is still unclear. In this work, WTS was pyrolyzed at 400–800 °C. Gas chromatography-mass spectrometry (GC–MS), N2-BET method, and Raman spectrometer were used to characterize the pyrolysis products. The reaction mechanism of WTS slow pyrolysis and the reaction pathways of N conversion during WTS slow pyrolysis were proposed. The results showed that the increase in temperature reduced the biochar productivity and raised the gas product productivity, with the highest bio-oil productivity at 600 °C, which was 37.22% of the total yield. Bio-oil was mainly composed of phenols, alcohols, acids and nitrogenous compounds, the components were mainly acetic acid (average 21.18%), 1,2-ethanediol (average 9.24%) and pyridines (average 14.03%). The pyrolysis of WTS mainly contains the degradation of biomass macromolecules to small molecules (1,6-dehydroglucose, ketones, furans, etc.), re-cracking of small molecules, isomerization, dehydration, alkylation, and self-condensation. The nitrogen-containing intermediates produced by pyrolysis of amino acids, cellulose and hemicellulose, were transformed to pyrroles, pyridines and pyrimidines by condensation and recombination reactions. This research established a theoretical foundation towards the efficient utilization WTS.

Permanganate preoxidation affects the formation of disinfection byproducts from algal organic matter

During harmful algal blooms (HABs), permanganate may be used as a preoxidant to improve drinking water quality by removing algal cells and degrading algal toxins. However, permanganate also lyses algal cells, releasing intracellular algal organic matter (AOM). AOM further reacts with permanganate to alter the abundance of disinfection byproduct (DBP) precursors, which in turn affects DBP formation during disinfection. In this study, we evaluated the impacts of preoxidation by permanganate applied at commonly used doses (i.e., 1–5 mg/L) on DBP generation during chlorination and chloramination of AOM. We found that permanganate preoxidation increased trichloronitromethane (TCNM) formation by up to 3-fold and decreased dichloroacetonitrile (DCAN) formation by up to 40% during chlorination, indicating that permanganate oxidized organic amines in AOM to organic nitro compounds rather than organic nitrile compounds. To test this proposed mechanism, we demonstrated that permanganate oxidized organic amines in known DBP precursors (i.e., tyrosine, tryptophan) to favor the production of TCNM over DCAN during chlorination. Compared to the decreased formation of DCAN during chlorination, permanganate increased DCAN formation by 30–50% during chloramination of AOM. This difference likely arose from monochloramine's ability to react with non-nitrogenous precursors (e.g., organic aldehydes) that formed during permanganate preoxidation of AOM to generate nitrogen-containing intermediates that go on to form DCAN. Our results also showed that permanganate preoxidation favored the formation of dichlorobromomethane (DCBM) over trichloromethane (TCM) during chlorination and chloramination. The increased formation of DBPs, especially nitrogenous DBPs that are more toxic than carbonaceous DBPs, may increase the overall toxicity in finished drinking water when permanganate preoxidation is implemented.

Integrated flux and pool size analysis in plant central metabolism reveals unique roles of glycine and serine during photorespiration.

Photorespiration is an essential process juxtaposed between plant carbon and nitrogen metabolism that responds to dynamic environments. Photorespiration recycles inhibitory intermediates arising from oxygenation reactions catalysed by Rubisco back into the C3 cycle, but it is unclear what proportions of its nitrogen-containing intermediates (glycine and serine) are exported into other metabolisms in vivo and how these pool sizes affect net CO2 gas exchange during photorespiratory transients. Here, to address this uncertainty, we measured rates of amino acid export from photorespiration using isotopically non-stationary metabolic flux analysis. This analysis revealed that ~23-41% of the photorespiratory carbon was exported from the pathway as serine under various photorespiratory conditions. Furthermore, we determined that the build-up and relaxation of glycine pools constrained a large portion of photosynthetic acclimation during photorespiratory transients. These results reveal the unique and important roles of glycine and serine in successfully maintaining various photorespiratory fluxes that occur under environmental fluctuations in nature and providing carbon and nitrogen for metabolism.

Understanding gut-liver axis nitrogen metabolism in Fatty Liver Disease.

The homeostasis of the most important nitrogen-containing intermediates, ammonia and glutamine, is a tightly regulated process in which the gut-liver axis plays a central role. Several studies revealed that nitrogen metabolism is altered in Metabolic Dysfunction-Associated Fatty Liver Disease (MAFLD), a consensus-driven novel nomenclature for Non-Alcoholic Fatty Liver Disease (NAFLD), the most common chronic liver disease worldwide. Both increased ammonia production by gut microbiota and decreased ammonia hepatic removal due to impaired hepatic urea cycle activity or disrupted glutamine synthetase activity may contribute to hepatic ammonia accumulation underlying steatosis, which can eventually progress to hyperammonemia in more advanced stages of steatohepatitis and overt liver fibrosis. Furthermore, our group recently showed that augmented hepatic ammoniagenesis via increased glutaminase activity and overexpression of the high activity glutaminase 1 isoenzyme occurs in Fatty Liver Disease. Overall, the improved knowledge of disrupted nitrogen metabolism and metabolic miscommunication between the gut and the liver suggests that the reestablishment of altered gut-liver axis nitrogenous balance is an appealing and attractive therapeutic approach to tackle Fatty Liver Disease, a growing and unmet health problem.

Phytate-based transparent and waterproof intumescent flame-retardant coating for protection of wood

Phytic acid-based phosphate esters (PAPEGs) were prepared through the reaction between phytic acid (PA) and different proportions of polyethylene glycol 200 (PEG-200). Intumescent flame-retardant coating (IFRC) with PAPEG3, PAPEG6, PAPEG9 and PAPEG12 were obtained via thermal curing of PAPEGs and melamine formaldehyde resin (MFR), respectively. The backside temperature of steel with MFR/PAPEGs coating was much lower than that of MFR/PA coating. MFR/PAPEG6 coating showed the best flame retardancy, and the char thickness reached 48 mm from initial thickness of 0.4 mm, and the expansion ratio was 120 times. Cone calorimeter confirmed that the incorporation of PAPEG6 could minimize the heat release rates and total heat release of materials during combustion. Moreover, the MFR/PAPEGs coating possessed both excellent transparency and water resistance. The transmittance of MFR/PAPEG6 and MFR/PAPEG9 was above 87% at 600 nm, and transmittance of MFR/PAPEG12 even reached 90%. The proposed flame-retardant mechanism was that MFR/PAPEGs produced phosphorus-containing acids to dehydrate and carbonize PEG, leading to the formation of graphitized chars. Meanwhile, the nitrogen-containing intermediates simultaneously released noncombustible gases to expand the char and interrupt the combustion reaction.

Boosting electrochemical nitrate-ammonia conversion via organic ligands-tuned proton transfer

Electrochemical nitrate (NO3-) reduction reaction (NO3-RR) offers an ideal route to harvest ammonia (NH3) under ambient conditions. Despite recent advances in Cu-based NO3-RR electrocatalysts, their synthesis heavily relies on the regulation of adsorption strength towards nitrogen-containing intermediates, and other important factors are ignored (i.e., the proton transfer rate). Here, we select Cu nanoparticles (NPs) as model catalysts to investigate whether and how the proton transfer rate impacts the NO3-RR kinetics. The results indicate that the proton transfer is involved in the rate-determining step (RDS) of NO3-RR, and the weak water dissociation ability of Cu leads to slow proton transfer rate and consequently sluggish NO3-RR kinetics. To this end, we enhance the water dissociation ability of Cu NPs by incorporating uncoordinated carboxylate ligands to enable rapid proton transfer, which in turn boosts the hydrogenation of key intermediates for reducing the overall energy barrier of NO3-RR. As a result, Cu NPs with the ligands display a maximum NH3 yield rate of 496.4 mmol h−1 gcat−1, outperforming counterpart without ligands. This work not only deepens our knowledge on the NO3-RR mechanism, but also offers new guidelines for the smart design of efficient electrocatalysts.

Realizing efficient C-N coupling via electrochemical co-reduction of CO2 and NO3- on AuPd nanoalloy to form urea: Key C-N coupling intermediates

Electrochemical C-N coupling of carbon dioxide and oxynitride under ambient environment is an emerging approach which promisingly enables sustainable production of valuable industrial chemicals, such as amines and their derivatives. However, the understanding of the C-N coupling is still in its infancy. Herein, we reported highly efficient electrochemical co-reduction of carbon dioxide and nitrate to form urea catalyzed by AuPd nanoalloy. Faradic efficiency and urea formation rate achieved 15.6% and 204.2 μg·mg−1·h−1, respectively, in a gas-tight H-type cell under ambient environment. The DFT studies on the C-N coupling mechanism showed that hydroxylamine was the most likely C-N coupling N-intermediate among many nitrogen-containing intermediates and that *NH2OH and *CO, rather than *NH2 and *CO, were confirmed to realize the C-N coupling to form urea through a one-step synergistic coupling which is a thermodynamically spontaneous process with a low activation barrier. This work not only provides new insights into the C-N coupling of oxynitride and carbon dioxide under ambient environment, but also may pave a way for promoting sustainable production of C-N coupling products.

Combustion kinetics of [formula omitted]-propylamine: Theoretical calculations and ignition delay time measurements

To better understand the combustion chemistry and improve the model performance of aliphatic amines, the kinetics of H-abstraction reactions from n-propylamine (NPA) by H, CH3, OH, and NH2 radicals and the ignition characteristics of NPA are investigated. The CCSD(T)-F12/cc-pVTZ-F12 method is selected as the benchmark, and the performances of several density functional theory methods are evaluated by comparing the calculated reaction energies and barrier heights against the benchmark values. The M06-2X/jun-cc-pVTZ method is identified as the best one with a mean unsigned deviation of 0.59 kcalmol−1. With the selected method, the rate constants are calculated using the variational transition-state theory combined with the small-curvature tunneling and multi-structural torsional anharmonicity (MS-CVT/SCT) options at 298–2000 K. The H-abstraction reactions at the α-C site are the dominant channels for NPA + H/CH3/NH2, but this channel is less important for NPA + OH due to the significant variational effect. The ignition characteristics of NPA is studied in a shock tube at 1105–1441 K, 4–20 atm, and different equivalence ratios (0.5, 1.0, and 2.0). The ignition delay time of NPA decreases noticeably with increasing temperature and pressure, while it is insensitive to equivalence ratio. Based on our calculations, a literature model is updated and then assessed against the measured ignition delay times. The updated model shows a better agreement with the measurements but fails to predict the ignition behavior of NPA under certain conditions. The reaction pathway and sensitivity analyses demonstrate that the unsaturated aliphatic amines and imines (such as CH3CHCHNH2 and CH2NH) are the important nitrogen-containing intermediates, and further investigation of the subset of these species is needed to improve the combustion model of aliphatic amines.

TM3 (TM = V, Fe, Mo, W) single-cluster catalyst confined on porous BN for electrocatalytic nitrogen reduction

• Confined metal clusters in the triplet form as sub-nanometer reactors for NRR. • Mo 3 @p-BN exhibits excellent catalytic activity and selectivity. • A rather low limiting potential (−0.34 V) was obtained. • Mo atoms act as “cache” to accelerate electron transfer. • Synergistic effect makes the “donation–backdonation” mechanism more efficient. Confined metal clusters as sub-nanometer reactors for electrocatalytic N 2 reduction reaction (eNRR) have received increasing attention due to the unique metal-metal interaction and higher activity than single-atom catalysts. Herein, the inspiration of the superior capacitance and unique microenvironment with regular surface cavities of the porous boron nitride (p-BN) nanosheets, we systematically studied the catalytic activity for NRR of transition-metal single-clusters in the triplet form (V 3 , Fe 3 , Mo 3 and W 3 ) confined in the surface cavities of the p-BN sheets by spin-polarized density functional theory (DFT) calculations. After a two-step screening strategy, Mo 3 @p-BN was found to have high catalytic activity and selectivity with a rather low limiting potential (–0.34 V) for the NRR. The anchored Mo 3 single-cluster can be stably embedded on the surface cavities of the substrate preventing the diffusion of the active Mo atoms. More importantly, the Mo atoms in the Mo 3 single-cluster would act as “cache” to accelerate electron transfer between active metal centers and nitrogen-containing intermediates via the intimate Mo-Mo interactions. The cooperation of Mo atoms can also provide a large number of occupied and unoccupied d orbitals to make the "donation–backdonation" mechanism more effective. This work not only provides a quite promising electrocatalyst for NRR, but also brings new insights into the rational design of triple-atom NRR catalysts.