Dissociation Of N2 Research Articles

Zero-dimensional modeling of unpacked and packed bed dielectric barrier discharges: the role of vibrational kinetics in ammonia synthesis

We present a zero-dimensional plasma kinetics model, including both surface and gas phase kinetics, to determine the role of vibrationally excited states in plasma-catalytic ammonia synthesis. We defined a new method to systematically capture the conditions of dielectric barrier discharges (DBDs), including those found in packed bed DBDs. We included the spatial and temporal nature of such discharges by special consideration of the number of micro-discharges in the model. We introduce a parameter that assigns only a part of the plasma power to the micro-discharges, to scale the model conditions from filamentary to uniform plasma. Because of the spatial and temporal behavior of the micro-discharges, not all micro-discharges occurring in the plasma reactor during a certain gas residence time are affecting the molecules. The fraction of power considered in the model ranges from 0.005%, for filamentary plasma, to 100%, for uniform plasma. If vibrational excitation is included in the plasma chemistry, these different conditions, however, yield an ammonia density that is only varying within one order of magnitude. At only 0.05% of the power put into the uniform plasma component, a model neglecting vibrational excitation clearly does not result in adequate amounts of ammonia. Thus, our new model, which accounts for the concept in which not all the power is deposited by the micro-discharges, but some part may also be distributed in between them, suggests that vibrational kinetic processes are really important in (packed bed) DBDs. Indeed, vibrational excitation takes place in both the uniform plasma between the micro-discharges and in the strong micro-discharges, and is responsible for an increased N2 dissociation rate. This is shown here for plasma-catalytic ammonia synthesis, but might also be valid for other gas conversion applications.

Investigation of N2 adsorption on Fe3O4(001) using ambient pressure X-ray photoelectron spectroscopy and density functional theory.

Reactions on iron oxide surfaces are prevalent in various chemical processes from heterogeneous catalysts to minerals. Nitrogen (N2) is known to dissociate on iron surfaces, a precursor for ammonia production in the Haber-Bosch process, where the dissociation of N2 is the limiting step in the reaction under equilibrium conditions. However, little is known about N2 adsorption on other iron-based materials, such as iron oxide surfaces that are ubiquitous in soils, steel pipelines, and other industrial materials. An atomistic description is reported for the binding of N2 on the Fe3O4(001) surface using first principles calculations with ambient pressure X-ray photoelectron spectroscopy. Two primary adsorption sites are experimentally identified from N2 dissociation on Fe3O4(001). The electronic signatures associated with the valence band region unambiguously show how the electronic structure of magnetite transforms near ambient pressures due to the binding of atomic nitrogen to different surface sites. Overall, the experimental and theoretical results of our study bridge the gap between ultra-high vacuum studies and reaction conditions to provide insight into other nitrogen-based chemistry on iron oxide surfaces that impact the agriculture and energy industries.

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-N3.5 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.

Wavelength-Dependent Solar N2 Fixation into Ammonia and Nitrate in Pure Water.

Solar-driven N2 fixation using a photocatalyst in water presents a promising alternative to the traditional Haber-Bosch process in terms of both energy efficiency and environmental concern. At present, the product of solar N2 fixation is either NH4+ or NO3−. Few reports described the simultaneous formation of ammonia (NH4+) and nitrate (NO3−) by a photocatalytic reaction and the related mechanism. In this work, we report a strategy to photocatalytically fix nitrogen through simultaneous reduction and oxidation to produce NH4+ and NO3− by W18O49 nanowires in pure water. The underlying mechanism of wavelength-dependent N2 fixation in the presence of surface defects is proposed, with an emphasis on oxygen vacancies that not only facilitate the activation and dissociation of N2 but also improve light absorption and the separation of the photoexcited carriers. Both NH4+ and NO3− can be produced in pure water under a simulated solar light and even till the wavelength reaching 730 nm. The maximum quantum efficiency reaches 9% at 365 nm. Theoretical calculation reveals that disproportionation reaction of the N2 molecule is more energetically favorable than either reduction or oxidation alone. It is worth noting that the molar fraction of NH4+ in the total product (NH4+ plus NO3−) shows an inverted volcano shape from 365 nm to 730 nm. The increased fraction of NO3− from 365 nm to around 427 nm results from the competition between the oxygen evolution reaction (OER) at W sites without oxygen vacancies and the N2 oxidation reaction (NOR) at oxygen vacancy sites, which is driven by the intrinsically delocalized photoexcited holes. From 427 nm to 730 nm, NOR is energetically restricted due to its higher equilibrium potential than that of OER, accompanied by the localized photoexcited holes on oxygen vacancies. Full disproportionation of N2 is achieved within a range of wavelength from ~427 nm to ~515 nm. This work presents a rational strategy to efficiently utilize the photoexcited carriers and optimize the photocatalyst for practical nitrogen fixation.

Palygorskite supported rare earth fluoride for photocatalytic nitrogen fixation under full spectrum

Nitrogen reduction to ammonia under mild conditions using cost-effective and full spectrum solar-driven photocatalyst is promising from an industrial perspective. Herein, sensitizer Yb3+ and activator Tm3+ co-doping LaF3 supported by palygorskite (Pal) nanocomposites (LaF3:Yb3+, Tm3+/Pal) are successfully prepared by the microwave hydrothermal method. The photocatalytic nitrogen fixation is performed using LaF3:Yb3+, Tm3+/Pal as photocatalyst. The impact of Tm3+ doping ratio and loading amount on the nitrogen fixation are explored. Results demonstrate that modified natural Pal nanorods have large surface area facilitating the immobilization of rare earth fluoride nanoparticles. The total amount of ammonia can reach the highest of 43.2 mg/L under solar light irradiation, while 5.7 mg/L can be achieved even under near infrared (NIR) light irradiation. The enhanced photocatalytic nitrogen fixation ability is ascribed to the upconversion capability of LaF3:Yb3+, Tm3+ which converts NIR into visible and UV light, improving the utilization of full solar spectrum. Meanwhile, LaF3:Yb3+, Tm3+ and modified Pal form an indirect Z-scheme heterostructure mediated by fluorine vacancy (FV), favoring the separation of the photogenerated electron-holes and preservation of high reduction–oxidation potentials. Additionally, the defective FV efficiently provides active sites beneficial for the adsorption and dissociation of N2.

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.

Geometric and Electronic Effects Contributing to N2 Dissociation Barriers on a Range of Active Sites on Ru Nanoparticles

Density functional theory calculations have been used to calculate activation barriers for N2 dissociation on a range of possible active sites on ruthenium (Ru) nanoparticles, including step, edge,...

Efficient Non‐dissociative Activation of Dinitrogen to Ammonia over Lithium‐Promoted Ruthenium Nanoparticles at Low Pressure

AbstractThere is an exciting possibility to decentralize ammonia synthesis for fertilizer production or energy storage without carbon emission from H2 obtained from renewables at small units operated at lower pressure. However, no suitable catalyst has yet been developed. Ru catalysts are known to be promoted by heavier alkali dopants. Instead of using heavy alkali metals, Li is herein shown to give the highest rate through surface polarisation despite its poorest electron donating ability. This exceptional promotion rate makes Ru–Li catalysts suitable for ammonia synthesis, which outclasses industrial Fe counterparts by at least 195 fold. Akin to enzyme catalysis, it is for the first time shown that Ru–Li catalysts hydrogenate end‐on adsorbed N2 stabilized by Li+ on Ru terrace sites to ammonia in a stepwise manner, in contrast to typical N2 dissociation on stepped sites adopted by Ru–Cs counterparts, giving new insights in activating N2 by metallic catalysts.

Efficient Non-dissociative Activation of Dinitrogen to Ammonia over Lithium-Promoted Ruthenium Nanoparticles at Low Pressure.

There is an exciting possibility to decentralize ammonia synthesis for fertilizer production or energy storage without carbon emission from H2 obtained from renewables at small units operated at lower pressure. However, no suitable catalyst has yet been developed. Ru catalysts are known to be promoted by heavier alkali dopants. Instead of using heavy alkali metals, Li is herein shown to give the highest rate through surface polarisation despite its poorest electron donating ability. This exceptional promotion rate makes Ru-Li catalysts suitable for ammonia synthesis, which outclasses industrial Fe counterparts by at least 195 fold. Akin to enzyme catalysis, it is for the first time shown that Ru-Li catalysts hydrogenate end-on adsorbed N2 stabilized by Li+ on Ru terrace sites to ammonia in a stepwise manner, in contrast to typical N2 dissociation on stepped sites adopted by Ru-Cs counterparts, giving new insights in activating N2 by metallic catalysts.

Power Pulsing To Maximize Vibrational Excitation Efficiency in N2 Microwave Plasma: A Combined Experimental and Computational Study

Plasma is gaining increasing interest for N2 fixation, being a flexible, electricity-driven alternative for the current conventional fossil fuel-based N2 fixation processes. As the vibrational-induced dissociation of N2 is found to be an energy-efficient pathway to acquire atomic N for the fixation processes, plasmas that are in vibrational nonequilibrium seem promising for this application. However, an important challenge in using nonequilibrium plasmas lies in preventing vibrational-translational (VT) relaxation processes, in which vibrational energy crucial for N2 dissociation is lost to gas heating. We present here both experimental and modeling results for the vibrational and gas temperature in a microsecond-pulsed microwave (MW) N2 plasma, showing how power pulsing can suppress this unfavorable VT relaxation and achieve a maximal vibrational nonequilibrium. By means of our kinetic model, we demonstrate that pulsed plasmas take advantage of the long time scale on which VT processes occur, yielding a very pronounced nonequilibrium over the whole N2 vibrational ladder. Additionally, the effect of pulse parameters like the pulse frequency and pulse width are investigated, demonstrating that the advantage of pulsing to inhibit VT relaxation diminishes for high pulse frequencies (around 7000 kHz) and long power pulses (above 400 μs). Nevertheless, all regimes studied here demonstrate a clear vibrational nonequilibrium while only requiring a limited power-on time, and thus, we may conclude that a pulsed plasma seems very interesting for energy-efficient vibrational excitation.

Beyond the Thermal Equilibrium Limit of Ammonia Synthesis with Dual Temperature Zone Catalyst Powered by Solar Light

Summary Artificial ammonia synthesis (Haber-Bosch process) is a prototypical exothermic reaction of maximum catalytic yield restricted by the unbreakable equilibrium law. This is because bottlenecked N2 dissociation necessitates high temperature, but high temperature reversely shifts the thermal equilibrium toward NH3 decomposition. To surmount this equilibrium limit, here, we propose a new scenario of dual-temperature-zone catalysis. Powered by sunlight, the apparent temperature of TiO2-xHy/Fe hybrid reaches 495°C but with local temperature difference up to 137°C between the hot zone (Fe) and “cooling” zone (TiO2-xHy) owing to the plasmonic local heating effect. The hot Fe bearing hot carriers efficiently dissociates N2, while working-in-tandem TiO2-xHy well accommodates spilled-over N from Fe via successive hydrogenation, prominently mitigating the reverse equilibrium shift and thus delivering record NH3 concentrations of 1,939 (1 atm) and 19,620 ppm (10 atm) at 495°C, 1.55 and 1.57 times the theoretical equilibrium limits of 1,249 and 12,459 ppm, respectively.

Governing factors of supports of ammonia synthesis in an electric field found using density functional theory

Efficient ammonia synthesis at low temperatures is anticipated for establishing a hydrogen carrier system. We reported earlier that application of an electric field on the Cs/Ru/SrZrO3 catalyst enhanced catalytic ammonia synthesis activity. It is now clear that N2 dissociation is activated by hopping protons in the electric field. Efficient ammonia synthesis proceeds by an “associative mechanism” in which N2 dissociates via an N2H intermediate, even at low temperatures. The governing factor of ammonia synthesis activity in an electric field for active metals differed from that in the conventional mechanism. Also, N2H formation energy played an important role. The effects of dopants (Al, Y, Ba, and Ca) on this mechanism were investigated using activity tests and density functional theory calculations to gain insights into the support role in the electric field. Ba and Ca addition showed positive effects on N2H formation energy, leading to high ammonia synthesis activity. The coexistence of proton-donating and electron-donating abilities is necessary for efficient N2H formation at the Ru–support interface.

Highly Effective Ru/BaCeO3 Catalysts on Supports with Strong Basic Sites for Ammonia Synthesis.

BaCeO3 -a and BaCeO3 -b, with strong basic sites, were synthesized by using a co-precipitation method at different calcination temperatures, and used as supports to evaluate their performance in ammonia synthesis. The ammonia synthesis rate with the 1.25 % Ru/BaCeO3 -a catalyst is 24 mmol g-1 h-1 , which is higher than that of 1.25 % Ru/BaCeO3 -b catalyst (18 mmol g-1 h-1 ) at 3 MPa and 450 °C. Moreover, the performance of the 4 % Cs-1.25 % Ru/BaCeO3 -a catalyst was further improved to 28 mmol g-1 h-1 , and no sign of deactivation was observed after a reaction time of 120 h. The XPS and H2 temperature-programmed reduction analyses indicated that the Ru/BaCeO3 -a catalyst has more oxygen vacancies than the Ru/BaCeO3 -b catalyst. In addition, the average Ru particle size of the Ru/BaCeO3 -a catalyst is closer to 2 nm than the Ru/BaCeO3 -b catalyst, which promotes the generation of B5 -type sites (the active site for N2 dissociation). The CO2 temperature-programmed desorption analysis indicates that BaCeO3 -a has a high basic density, which is beneficial for electron transfer to Ru and further facilitates the dissociation of N≡N bonds.

Deuterium Isotope Analysis of Electrochemical Promotion in Ammonia Synthesis on Iron-Based Catalyst

In this study, the deuterium isotope analysis was conducted to investigate the electrochemical promotion mechanism of ammonia synthesis at 650˚C using H2-N2, 1%W-10%Fe-BaCe0.9Y0.1O3 (BCY)|BCY|Pt, 3%H2O-10%D2-87%Ar. The performance of ammonia electrochemical synthesis was examined using high pressure liquid chromatography (HPLC) and Fourier-transform infrared spectroscopy (FTIR). The ammonia formation rate increased by around 20 times with an increase in cathodic polarization. The electrochemical synthesized ammonia of NH3 and NH2D were detected by FTIR. Furthermore, a positive correlation between the ammonia formation rate and hydrogen partial pressure in the cathode was observed. Considering the results, the electrochemical promotion mechanism of ammonia formation was probably caused by accelerated N2 dissociation via supplying D+, subsequently N* and ND* were formed from N2D*, and then ND* and N* reacted with H* which originated from H2 fed in the cathode to form NH2D and NH3.

Exploring the mechanism of alkene hydrogenation catalyzed by defined iron complex from DFT computation.

UB3LYP computation including dispersion and toluene solvation has been carried to elucidate the mechanisms of alkene hydrogenation catalyzed by bis(imino)pyridine iron dinitrogen complex (iPrPDI)Fe(N2)2, which has low stability towards N2 dissociation. The coordinatively unsaturated complexes, (iPrPDI)Fe(N2) and (iPrPDI)Fe(1-C4H8), favor open-shell singlet ground states. On the basis of our computations, we propose a new mechanism of 1-butene coordination and hydrogenation after N2 dissociation. The hydrogenation of 1-butene undergoes a concerted open-shell singlet transition state involving H2 dissociation, C-H bond formation and C=C bond elongation, as well as the subsequent C-H reductive elimination. In the whole alkene hydrogenation, the H-H bond cleavage is the rate-determining step. Graphical abstract The alkene hydrogenation catalyzed by redox-active pyridine(diimine)-chelate iron complex follows the open-shell singlet state path.

A Systematic Study on Bond Activation Energies of NO, N2, and O2 on Hexamers of Eight Transition Metals

AbstractCatalytic bond activation pathways of diatomic molecules on small metal clusters have been studied by density functional theory calculations. The focus of this study is dissociation of NO, N2, and O2 on hexamers of eight transition metals (Ni, Cu, Rh, Pd, Ag, Ir, Pt, and Au). For all the 24 cases, the lowest energy structures at the molecular‐adsorption state, bond dissociation transition state (TS), and dissociative‐adsorption state were identified by a systematic procedure. At TS of 20 cases, the transition metal hexamer moiety took a different shape from bare transition metal hexamers. The results support the importance of metastable cluster structures in catalytic activity, recently proposed in several catalytic systems. Furthermore, using the obtained dataset, a simple linear regression analysis was made to explore the applicability of the Brønsted‐Evans‐Polanyi principle to flexible metal cluster catalysts.

La2Ce2O7 supported ruthenium as a robust catalyst for ammonia synthesis

La2Ce2O7 nanoparticles were prepared by citric acid complexation method followed by calcination at varied temperatures. Then, supported with 4 wt% Ru, they were evaluated as the catalysts for ammonia synthesis under conditions similar with industry. With La2Ce2O7 being calcinated at 700 or 800 °C, the experimental results indicate that the Ru/La2Ce2O7 catalyst exhibits much higher ammonia concentration or ammonia synthesis rate than that of Ru/CeO2 and Ru/La2O3. In addition, Ru/La2Ce2O7 possesses high stability under over-heating test. In the absence of any promotor, ammonia concentration of Ru/La2Ce2O7 catalyst approaches 14% at 450 °C, GHSV of 10000 h−1 and pressure of 10 MPa. The rate-determining step of ammonia synthesis, dissociation of N2 is significantly facilitated by the strong metal–support interaction (SMSI) between Ru and La2Ce2O7. Due to the interaction, La2Ce2O7 tends to donate electrons to Ru, resulting in the high electron density over the surface of Ru active sites which is favorable for the dissociation of N2. Consequently, high activity is achieved.

Catalytic synthesis of non-carbon fuel NH3 from easily available N2 and H2O over FeO(100) surface: study of reaction mechanism using the density functional theory

The existence of –OH groups changed the controlling step from the dissociation of N2 to the formation of –NH2.

Multi-fidelity Gaussian process modeling for chemical energy surfaces

There has been a recent surge of interest in using Gaussian process (GP) regression to model chemical energy surfaces. Herein, we discuss an extension of GP modeling called autoregressive Gaussian process (ARGP) modeling, which uses an approximation to the target function to improve learning efficiency and has never been applied to high-accuracy chemical energy surfaces. Our calculations demonstrate that ARGP regression improves the prediction error of a five-point GP regression by two orders of magnitude for an N2 dissociation curve and reproduces the energy surface of H2O with sub-chemical accuracy, using only 25 training points.

The important role of N2H formation energy for low-temperature ammonia synthesis in an electric field

Abstract Development of a highly efficient ammonia synthesis process is desirable for achieving a sustainable society. Regarding conventional heterogeneous catalysts, Ru-supported catalyst exhibits higher turn-over frequency (TOF) than Fe-supported or Ni-supported catalysts. However, we found that Fe-supported and Ni-supported catalysts show higher TOF than Ru-supported catalyst in an electric field at the low temperature of 373 K. Density functional theory (DFT) calculations revealed that N2 dissociation through the “associative mechanism” plays a key role in the electric field. The ammonia synthesis activity in the electric field is determined by the N2H formation energy at the metal-support interface.