Ammonia Pyrolysis Research Articles

Direct conversion of cellulose and raw biomass to acetonitrile by catalytic fast pyrolysis in ammonia

Concentrated acetonitrile was produced directly from biomass via a CFP-A process and the mechanism was elucidated by experiments and DFT calculations.

Improving CO2 electroreduction over ZIF-derived carbon doped with Fe-N sites by an additional ammonia treatment

Iron-nitrogen-carbon (Fe-N-C) composite materials show considerable Faradaic efficiency for CO production in electrochemical CO2 reduction reaction (CO2RR), which are promising alternatives to noble metal catalysts. However, reported results exhibit limited CO current densities. Herein, an additional ammonia treatment process is explored to futher improve the activity of a ZIF-derived carbon material doped with Fe-N sites. Physicochemical characterizations reveal that ammonia treatment facilitates the sublimation of residual Zn species and etching of unstable carbon mioeties, thus increasing the loading of Fe-N active sites and specific surface area. The increased specific surface area effectively maximizes the exposure of Fe-N active sites and facilitates mass transfer during the CO2RR. With an additional pyrolysis in ammonia, the optimal catalyst shows higher CO Faradaic efficiency and much improved CO current density, outperforming other reported Fe-N-C catalysts.

High Efficiency Direct Ammonia Type Fuel Cells based on BaZr x Ce0.8-X Y0.2O3/Pd Oxide-Metal Junctions

Recently, ammonia has attracted a considerable interest as a prospective carbon-free fuel source for fuel cells due to the low cost, ease in liquefaction and transportation at ambient temperatures, and high volumetric energy density. The proton conducting ceramic fuel cells (PCFCs) using proton-conducting oxides, such as BaZr x Ce0.8-x Y0.2O3- d , as an electrolyte is promising as a direct ammonia type fuel cells. The water formation in PCFCs is mainly progressive at the cathode side, unlike solid oxide fuel cells (SOFCs), and therefore, the contamination of the ammonia fuels into the exhaust water is precluded, thereby eliminating re-purification of water exhaust and simplifying total systems. Hydrogen membrane fuel cell (HMFC), which was firstly proposed by Ito et al., consists of an ultrathin proton conducting ceramic electrolyte supported on a dense hydrogen permeable metal anode. The cell facilitates efficient power generation, giving 900 mW cm-2 at 400°C, since reduced electrolyte thickness can significantly decreases the ohmic loss at lower temperatures. This unique cell configuration could be advantageous for ammonia fuel cells because 1) the NO x formation at the anode is perfectly prevented because NH3 gases are isolated from the oxide electrolytes by Pd solid anode and 2) Pd is expected to capture efficiently the hydrogen gases produced by the pyrolysis of ammonia due to the large ahydrogen sorption ability. Herein, we demonstrated the high efficiency power generation of the direct ammonia type HMFC due to the hydrogen pumping effects realized by BaZr0.1Ce0.7Y0.2O3- d (BZCY) and Pd heterointerface. Ohmic characteristics of the heterojunctions, altogether with hydrogen solubility of Pd, can facilitate the hole proton exchange between Pd and BZCY regardless of the lowered hydrogen concnetartion in NH3-fed anodes. The cell gained the maximum power density of 0.52 W cm-2 at 600°C, which was much higher than the champion data of ammonia PCFCs.

Identification of catalytic sites for oxygen reduction in iron- and nitrogen-doped graphene materials

While platinum has hitherto been the element of choice for catalysing oxygen electroreduction in acidic polymer fuel cells, tremendous progress has been reported for pyrolysed Fe-N-C materials. However, the structure of their active sites has remained elusive, delaying further advance. Here, we synthesized Fe-N-C materials quasi-free of crystallographic iron structures after argon or ammonia pyrolysis. These materials exhibit nearly identical Mössbauer spectra and identical X-ray absorption near-edge spectroscopy (XANES) spectra, revealing the same Fe-centred moieties. However, the much higher activity and basicity of NH3-pyrolysed Fe-N-C materials demonstrates that the turnover frequency of Fe-centred moieties depends on the physico-chemical properties of the support. Following a thorough XANES analysis, the detailed structures of two FeN4 porphyrinic architectures with different O2 adsorption modes were then identified. These porphyrinic moieties are not easily integrated in graphene sheets, in contrast with Fe-centred moieties assumed hitherto for pyrolysed Fe-N-C materials. These new insights open the path to bottom-up synthesis approaches and studies on site-support interactions.

Active Sites for Oxygen Reduction Reaction in Tantalum Oxynitride Nanoparticles on Multi-Walled Carbon Nanotubes Synthesized Using Ammonia Pyrolysis

Group 4 or group 5 metal oxide compounds have attracted attention as an oxygen reduction reaction (ORR) catalyst in acidic media where they displayed excellent stability.1 Because of their short development period, discussions on the active sites in these oxide compounds are lacking compared with those in other non-platinum group metal catalysts, e.g., compounds of iron/cobalt, nitrogen and carbon and these metal-free nitrogen-doped carbons (CN x ). Besides, recent progress in the activity of oxide compounds has attracted suspicious eyes for the active sites as it was achieved primarily by changing the oxide precursor from the metal carbonitrides to oxy-metal phathalocyanine, which could produce both oxides and CN x . We recently synthesized oxide catalyst containing one of the group 5 metals, tantalum via the decomposition of oxy-tantalum phthalocyanine (TaOPc) in melted salicylic acid powders to control the size of tantalum oxide (TaO x ) particles to the order of nanometers and to connect TaO x to a support, multi-walled carbon nanotube (MWCNT). Graphitic carbons built from TaOPc successfully provided conductive paths to TaO x insulation particles.2 The activity of TaO x -MWCNT was far higher than tantalum-free phthalocyanine-adsorbed MWCNTs after pyrolysis under identical conditions. This work thus demonstrated that the active site was TaO x , not CN x species originating from TaOPc. Substitution of oxygen atoms in these oxide catalysts by nitrogen is an excellent technique to stabilize oxygen defects which have been acknowledged to be an ORR active sites. The nitrogen-doping has often been performed after oxide particles were supported on carbon supports3 or simaltaneously.4 Therefore, ORR activity of CN x species synthesized by the nitrogen doping process and that of metal-oxynitrides have been carefully compared to discuss active sites.3,4 In any of these previous works,3,4 the oxynitrides showed higher activity than CN x . In this study, two different TaO x -MWCNT catalysts were synthesized by changing the preparation of TaOPc precursors. Then they were pyrolyzed under ammonia gas at various temperatures to obtain TaO x N y -MWCNTs. The ORR activity of these two catalyst types in acidic media have been carefully compared to clarify the active sites. For the first time, slightly oxidized Ta3N5was proposed to be an ORR active site. References (1) A. Ishihara, Y. Ohgi, K. Matsuzawa, S. Mitsushima and K. Ota, Electrochim. Acta, 55, 8005 (2010). (2) A. Ishihara, M. Chisaka, Y. Ohgi, K. Matsuzawa, S. Mitsushima and K. Ota, Phys. Chem. Chem. Phys., 17, 7643 (2015). (3) M. Chisaka, T. Iijima, T. Yaguchi and Y. Sakurai, Electrochim. Acta, 56, 4581 (2011). (4) M. Chisaka, H. Sasaki and H. Muramoto, Phys. Chem. Chem. Phys., 16, 20419 (2014). Acknowledgment The authors gratefully acknowledge Dainichiseika Color & Chemicals Mfg. Co. and Showa Denko K. K. for supplying TaOPc and MWCNT powders, respectively. This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) in Japan.

Nano-TaOxNy particles synthesized from oxy-tantalum phthalocyanine: how to prepare precursors to enhance the oxygen reduction reaction activity after ammonia pyrolysis?

Nano-TaOxNy oxygen reduction reaction catalysts with a new active site, slightly oxidized-Ta3N5, were synthesized for use in acidic media.

Synthesis of Tetragonal and Orthorhombic Polymorphs of Hf3N4 by High-Pressure Annealing of a Prestructured Nanocrystalline Precursor

Hf3N4 in nanocrystalline form is produced by solution phase reaction of Hf(NEtMe)4 with ammonia followed by low-temperature pyrolysis in ammonia. Understanding of phase behavior in these systems is important because early transition-metal nitrides with the metal in maximum oxidation state are potential visible light photocatalysts. A combination of synchrotron powder X-ray diffraction and pair distribution function studies has been used to show this phase to have a tetragonally distorted fluorite structure with 1/3 vacancies on the anion sites. Laser heating nanocrystalline Hf3N4 at 12 GPa and 1500 K in a diamond anvil cell results in its crystallization with the same structure type, an interesting example of prestructuring of the phase during preparation of the precursor compound. This metastable pathway could provide a route to other new polymorphs of metal nitrides and to nitrogen-rich phases where they do not currently exist. Importantly it leads to bulk formation of the material rather than surface conversion as often occurs in elemental combination reactions at high pressure. Laser heating at 2000 K at a higher pressure of 19 GPa results in a further new polymorph of Hf3N4 that adopts an anion deficient cottunite-type (orthorhombic) structure. The orthorhombic Hf3N4 phase is recoverable to ambient pressure and the tetragonal phase is at least partially recoverable.

Nitrogen-enriched ordered mesoporous carbons through direct pyrolysis in ammonia with enhanced capacitive performance

Download options Please wait... Supplementary files Supplementary information PDF (240K) Article information DOI https://doi.org/10.1039/C3TA11342F Article type Paper Submitted 03 Apr 2013 Accepted 01 May 2013 First published 02 May 2013 Download Citation J. Mater. Chem. A, 2013,1, 7920-7926 BibTex EndNote MEDLINE ProCite ReferenceManager RefWorks RIS Permissions Request permissions Nitrogen-enriched ordered mesoporous carbons through direct pyrolysis in ammonia with enhanced capacitive performance X. Wang, C. Liu, D. Neff, P. F. Fulvio, R. T. Mayes, A. Zhamu, Q. Fang, G. Chen, H. M. Meyer, B. Z. Jang and S. Dai, J. Mater. Chem. A, 2013, 1, 7920 DOI: 10.1039/C3TA11342F To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page. If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given. If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page. Read more about how to correctly acknowledge RSC content. Social activity Tweet Share Search articles by author Xiqing Wang Chen-Guang Liu David Neff Pasquale F. Fulvio Richard T. Mayes Aruna Zhamu Qing Fang Guorong Chen Harry M. Meyer Bor Z. Jang Sheng Dai

Growth kinetics of Al xGa 1– xN grown via ammonia-based metal-organic molecular beam epitaxy

The structural characteristics and growth regimes of AlGaN grown by ammonia-based metal-organic molecular beam epitaxy (NH 3-MOMBE) on GaN templates were investigated. The NH 3 utilization efficiency for the growth of AlGaN was estimated to be 2–2.5 times greater than the growth of GaN. Increasing the Al gas phase composition lead to an increase in the utilization efficiency as a result of increased NH 3 catalyzation. Despite the increased pyrolysis of ammonia, AlGaN films grown at 860 °C had significant active species desorption, leading to slower growth rates as well as lower calculated utilization efficiencies. AlGaN films grown with constant Al gas phase compositions showed an increase in the solid Al composition when grown more metal-rich, because of the preferential Al incorporation over Ga in N-limited growth environments. AlGaN surface morphologies became smoother with higher V/III ratios since surface pitting, which is attributed to decoration of threading dislocations, was reduced with increasing NH 3 flux.

Electrochemical Evidence of Two Types of Active Sites for Oxygen Reduction in Fe-based Catalysts

Fe-based catalysts for the reduction of oxygen were synthesized by wet-impregnation of FeII acetate or FeIII-porphyrin on pristine carbon black followed by pyrolysis in ammonia or argon. The catalysts were characterized using RDE and RRDE voltammetry in both acid and alkaline electrolytes. The results support the presence of two types of active sites for oxygen reduction in these catalysts: (i) the Fe-N4/C type, with a structure resembling the central porphyrin moiety, and (ii) the Fe-N2+2/C type, hosted in the micropores of the carbon support. Fe-N4/C sites are poorly active in acid but highly active in alkaline, whereas Fe-N2+2/C sites are highly active in both electrolytes.

NH2 radical formation by ammonia pyrolysis in a temperature range of 800?1000�K

The NH2 radical absolute concentrations were measured using intracavity laser absorption spectroscopy during the process of ammonia pyrolysis at relatively low temperatures of 800–1000 K. The NH2 spectra have been recorded at pressures of 12–380 Torr. The observed absolute concentrations (in the order of ∼1011 molecules/cm3) were higher than was predicted by the gas-phase pyrolysis mechanisms. These mechanisms also overestimate the activation energy of the process. The experimental observations can be explained by the existence of an additional channel of NH2 formation – probably on the surface of the reaction vessel. The addition of oxygen to the reaction mixture causes a considerable (∼ four-fold) increase in the NH2 radical concentration.

Chemical solution deposition derived buffer layers for MOCVD-grown GaN films

A polymeric Ga-containing liquid precursor was used for the first time to prepare a buffer layer for the growth of GaN by metalorganic chemical vapor deposition. The buffer layer was prepared by chemical solution deposition (CSD), i.e. spincoating the precursor on a sapphire substrate followed by pyrolysis in ammonia. The GaN films were characterized by optical and electron microscopy, XRD rocking curves, photoluminescence and Hall measurements. The CSD-buffered GaN films showed a significantly better crystal quality than unbuffered films and came close to the quality of conventionally buffered ones.

Ammonia Pyrolysis and Oxidation in the Claus Furnace

The modified Claus process is commonly used in oil refining and gas processing to recover sulfur and destroy contaminants formed in upstream processing. In oil refining, in addition to the typical modified Claus plant feed of H2S and CO2, NH3 is also often present. NH3 is a process contaminant and must be destroyed in the front-end furnace of the modified Claus plant, otherwise, it poses a risk of poisoning the catalyst beds and plugging off downstream equipment because of the formation of ammonium salts. In this paper, the pyrolysis and oxidation of NH3 was studied under Claus furnace temperatures and residence times. Experimental data was taken, and new reaction rate expressions were developed for NH3 pyrolysis and oxidation. The derived rate expressions are outlined as follows: the NH3 pyrolysis rate expression r = A exp(−Ea/RT)PNH31.25, where A is 0.00421 mol s-1 atm-1.25 cm-3 and Ea is 16.5 kcal mol-1, and the NH3 oxidation rate expression r = A exp(−Ea/RT)PNH3PO20.75, where A is 4430 mol s-1 atm-1....

Kinetic Modeling of the Thermal Decomposition of Ammonia

A detailed N/H reaction mechanism has been developed and validated in comparison with experimental data for ammonia pyrolysis in shock waves (Davidson et al., Int. J. Chem. Kinet, 1990, 22:513). It has been shown that incorporation of the reactions with N2H3 and N2H4 into the mechanism significantly influences calculated rise-time and peak concentrations of the NH and NH2 radicals if the currently adopted rate constant of the reaction is employed. A sensitivity analysis reveals which reactions are critical for the quality of the modeling in particular experimental conditions. The choice of the rate constants for these reactions is discussed. It has been found that only significant decrease of the reaction (22) rate constant can improve the agreement between the modeling and experimental data. The best fit in the range 2200 - 2800 K is met with the rate constant k22 = 1.0E+11 T0.5 exp(-21600/RT).

Electrochemical synthesis of TiN precursors and their conversion to fine particles

TiN precursors were synthesized at room temperature by galvanostatic electrolysis of a titanium plate andn-butylamine. The resulting liquid was black in color, and was converted to viscous liquid by polymerization, then to black solids by heat treatment at 200 °C in vacuum. The solid samples were characterized by IR and 1H NMR measurements, suggesting formation of butylamidotitanium.The black solids were pyrolyzed both in vacuum and under an ammonia atmosphere at 200 to 1000 °C. The samples were characterized by XRD, TEM, and chemical analysis. Fine TiN crystallites of up to 30 nm in size were produced with the retention of free carbon and/or hydrocarbons at 800 to 1000 °C in vacuum. These impurities were drastically reduced by increasing the TiN content by pyrolysis in ammonia. Two step pyrolysis, consisting of heating at 400 °C for 12 h and successively at a fixed temperature (600, 800 and 1000 °C) for 1 h, was found to be effective in reducing impurities and producing the TiN powders at temperatures as low as 600 °C.

Hot-press sintering studies of amorphous silicon nitride powders

The hot-press sintering behaviour of amorphous Si3N4 powders prepared from the ammonia pyrolysis of polycarbosilane or hydridopolysilazane polymers was studied. In the presence of yttria and alumina, the amorphous powders sintered to > 98% of theoretical density at 2023 K. Both the microstructure and average four-point MOR bend strengths, of test bars machined from the sintered compacts were comparable to those obtained from UBE SN-E-10 Si3N4 powder processed under the same conditions. However, in contrast to commercial crystalline powders, between approximately 1690 and 1700 K the amorphous Si3N4 powders underwent a rapid shrinkage corresponding to 50–60% of the total densification. In this narrow temperature regime, a radical change in morphology and phase composition of the amorphous powder occurred. Prior to 1690 K, the Si3N4 powders were totally amorphous as determined by X-ray diffraction analysis and consisted of angular shards with an average particle size of 2–3 μm. Samples quickly cooled after heating to 1700 K, consisted of a 53/47 mixture of equiaxed α and β Si3N4 crystallites with an average particle size of 0.1–0.3 μm. Thus, the rapid densification at R~ 1700 K is identified with the amorphous to (α + β) transition. Beyond ∼ 1700 K, these samples gradually densified to the maximum density as mentioned. The fully densified samples consisted of 100% β-Si3N4 phase.

A pyrolysis mechanism for ammonia

AbstractThe mechanism of NH3 pyrolysis was investigated over a wide range of conditions behind reflected shock waves. Quantitative time‐history measurements of the species NH and NH2 were made using narrow‐linewidth laser absorption. These records were used to establish an improved model mechanism for ammonia pyrolysis. The risetime and peak concentrations of NH and NH2 in this experimental database have also been summarized graphically.Rate coefficients for several reactions which influence the NH and NH2 profiles were fitted in the temperature range 2200 K to 2800 K. The reaction and the corresponding best fit rate coefficients are as follows: with a rate coefficient of 4.0 × 1013 exp(−3650/RT) cm3 mol−1 s−1, with a rate coefficient of 1.5 × 1015T−0.5 cm3 mol−1 s−1 and with a rate coefficient of 5.0 × 1013 exp(−10000/RT) cm3 mol−1 s−1. The uncertainty in rate coefficient magnitude in each case is estimated to be ±50%. The temperature dependences of these rate coefficients are based on previous estimates.The experimental data from four earlier measurements of the dissociation reaction were reanalyzed in light of recent data for the rate of NH3 + H → NH21 + H2, and an improved rate coefficient of 2.2 × 1016 exp(−93470/RT) cm3 mol−1 s−1 in the temperature range 1740 to 3300 K was obtained. The uncertainty in the rate coefficient magnitude is estimated to be ± 15%.

Rate constants of chemical reactions in the high temperature pyrolysis of ammonia

The thermal decomposition of ammonia in shock waves was studied by means of vacuum ultra-violet absorption spectroscopy, monitoring the concentration of atomic hydrogen. Rate parameters of the initiation reaction, NH3+Ar→NH2+H+Ar (I) and the consecutive reactions, NH3+H→NH2+H2 (II) and NH2+H→NH+H2 (III) were determined. Under the experimental conditions studied, the profile of the concentration of atomic hydrogen formed by decomposition of ammonia was considered to result from these three reactions and the reaction 2NH→N2+2H (IV)

NOx formation in flat, laminar, opposed jet methane diffusion flames

It is shown that the detailed structure of a flat laminar opposed jet diffusion flame can be modeled, by properly coupling the momentum and energy conservation equatins and by using detailed finite rate combustion kinetics. Such a flame is one dimensional (for a given stretching rate) provided that the proper velocity boundary conditions, which are a result of the model are employed. A laboratory CH 4 /N 2 /O 2 opposed jet diffusion flame was realized and gave good agreement with predictions with respect to both, one dimensinality and the temperature and species profiles within the reaction zone. However, the actual location of the reaction zone was displaced by a small distance. The theoretical/experimental tool developed was utilized to test kinetic mechanisms of NO formation from fuel nitrogen. In this case, anhydrous ammonia was introduced first with the fuel and then with the oxidizer. The formation of NO was well predicted, using a detailed kinetic mechanism developed by others, when ammonia was introduced with the fuel, but agreement was poor when ammonia was injected in the air side. This indicates that there are some deficiencies in the ammonia pyrolysis kinetic mechanism when it is utilized in the absence of hydrocarbon fragments. The qualitative dependence of the NO profile on flame stretching was correctly predicted by the model. This also allowed the prediction of reaction zone thickness and the rate of formation of NO as a function of stretching rate and these results can be related to NO formation in strained laminar diffusion flamelets as they occur in turbulent diffusion flames.

A shock-tube study of ammonia pyrolysis

The pyrolysis of NH3 was studied behind reflected shock waves at temperatures 2500–3000 K, using mass spectrometric analysis of dynamically sampled gas. The initial mixtures contained 0.14% to 6% NH3, with Kr as diluent, at total gas concentrations of about 2 × 10−6 mol cm−3. Concentration profiles of NH3, NH2, NH, and N2 were measured. It was found that the apparent rate coefficient for overall removal of NH3 is increased by increasing the initial NH3 concentration, but is decreased by addition of H2. Addition of H2 also suppressed NH, but left the NH2 concentration relatively unchanged. A close correlation was found between NH2 concentration and N2 formation rate, indicating that NH2 participates in the reaction which produces N2.The experimental results are consistent with a chain mechanism in which NH3 is removed by unimolecular decomposition and by attack by H, NH, and NH2. Computer analysis yields a rate constant of 1.2 × 1016 exp (−91 000 cal mol−1/RT) cm3 mol−1 s−1 for the unimolecular process.