NH3 H2 Research Articles

Ammonia decomposition in catalytic membrane reactors: Simulation and experimental studies

Catalytic decomposition of NH3 with H2‐selective microporous silica membranes for COx‐free hydrogen production was studied theoretically and experimentally. The simulation study shows that NH3 conversion, H2 yield and H2 purity increase with the Damköhler number (Da), and their improvement is affected by the effect of H2 extraction as well as NH3 and N2 permeation through the membranes. The experimental study of NH3 decomposition was carried out in a bimodal catalytic membrane reactor (BCMR), consisting of a bimodal catalytic support and a H2‐selective silica layer. Catalytic membranes showed H2 permeances of 6.2–9.8 × 10−7 mol m−2 s−1 Pa−1, with H2/NH3 and H2/N2 permeance ratios of 110–200 and 200–700, respectively, at 773 K. The effect of operating conditions on membrane reactor performance with respect to NH3 conversion, H2 yield and H2 purity was investigated, and the results were in agreement with those calculated by the proposed simulation model. © 2012 American Institute of Chemical Engineers AIChE J, 59: 168–179, 2013

Role of support nature (γ-Al2O3 and SiO2-Al2O3) on the performances of rhenium oxide catalysts in the metathesis of ethylene and 2-pentene

The metathesis of ethylene and 2-pentene was studied as an alternative route for propylene production over Re2O7/γ-Al2O3 and Re2O7/SiO2-Al2O3 catalysts. Both NH3 temperature-programmed desorption (NH3-TPD) and H2 temperature-programmed reduction (H2-TPR) results showed that Re2O7/SiO2-Al2O3 exhibited stronger acidity and weaker metal-support interaction than Re2O7/γ-Al2O3. At 35–60 °C, isomerization free metathesis was observed only over Re2O7/γ-Al2O3, suggesting that the formation of metal-carbene metathesis active sites required only weak acidity. Our results suggest that on the Re2O7/SiO2-Al2O3, hydrido-rhenium species ([Re]-H) were formed in addition to the metathesis active sites, resulting in the isomerization of the initial 1-butene product into 2-butenes. A subsequent secondary metathesis reaction between these 2-butenes and the excess ethylene could explain the enhanced yields of propylene observed. The results demonstrate the potential for high yield of propylene from alternative feedstocks.

Novel heavy duty engine concept for operation dual fuel H2–NH3

A prior paper has presented a novel design of a heavy duty truck engine fuelled with H2. In this design, the customary in-cylinder Diesel injector and glow plug are replaced with a main chamber fuel injector and a jet ignition pre-chamber. The jet ignition pre-chamber is a small volume that is connected to the in-cylinder through calibrated orifices accommodating another fuel injector and a glow or a spark plug that controls the start of combustion. This design permits to operate the engine in four different modes: traditional compression ignition (CI), diffusion, Diesel-like (M1); mixed gasoline/Diesel-like (M2); traditional spark ignition (SI), premixed, gasoline-like (M3); premixed, homogeneous charge compression ignition HCCI-like (M4). In the mode diffusion with jet ignition (M1), an injection occurs in the jet ignition pre-chamber before the main chamber fuel is injected and the engine operates therefore mostly Diesel-like. In the mode mixed diffusion/premixed Diesel/gasoline-like (M2) an injection occurs in the jet ignition pre-chamber after only part of the main chamber fuel is injected and mixed with air. In the mode premixed with jet ignition (M3), an injection occurs in the jet ignition pre-chamber after the main chamber fuel is injected and mixed with air and the engine operates gasoline-like. Finally, in the mode premixed without jet ignition (M4), no injection occurs in the jet ignition pre-chamber and the engine operates HCCI-like. Modelling results have already been presented and discussed with H2 as the main chamber and pre-chamber fuel. This paper considers the option to accommodate a second main chamber injector that will inject the NH3 that will then burn in air thanks to the hot combusting gases from the combustion of H2 and air using the modes M1 and M2 described above. The mode M3 also of interest is not considered here. First results of simulations show the opportunity to achieve better than Diesel fuel energy conversion efficiency thanks to the reduced heat losses of the “cold burning” NH3 and suggest to perform the experiments needed to further support the findings.

Effects of ammonia substitution on combustion stability limits and NOx emissions of premixed hydrogen–air flames

The combustion stability limits and nitrogen oxide (NOx) emissions of burner-stabilized premixed flames of ammonia (NH3)-substituted hydrogen (H2)–air mixtures at normal temperature and pressure are studied to evaluate the potential of partial NH3 substitution to improve the safety of H2 use. The effects of NH3 substitution, nitrogen (N2) coflow and mixture injection velocity on the stability limits and NOx emissions of NH3–H2–air flames are experimentally determined. Results show a reduction of stability limits with NH3 substitution and coflow, supporting the potential of NH3 as a carbon-free, green additive in H2–air flames and indicating a different tendency from that for no coflow condition. The NOx emission index is almost constant even with enhanced NH3 substitution, though the absolute value of NOx emissions increases in general. At fuel-rich conditions, the NOx emission index decreases with increasing mixture injection velocity and the existence of coflow. The thermal deNOx process in the post-flame region is involved in reducing NOx emissions for the fuel-rich flames.

High growth rate MOVPE of Al(Ga)N in planetary reactor

Possibility of AlN growth by MOVPE in a planetary reactor with high growth rate was investigated. Growth was performed on (0001) Al2O3 substrates at the reactor pressure of 100mbar. It was shown that deposition rate is close to diffusion limit at low NH3 flows and reduces abruptly above a certain threshold value of NH3 due to gas-phase parasitic reactions. At constant V/III ratio of 1.5–2, AlN growth rate dependence on TMAl flow was linear and a maximum growth rate of 8.6μm/h was achieved. Process modeling allowed predicting and explaining the trends related to the onset of parasitic chemistry for various V/III ratios and other growth conditions. Surface morphology planarization was achieved by either (1) NH3 flow rate reduction or (2) TMGa injection after the layer with thickness of 75–300nm was grown. The second approach looks more fruitful resulting in atomically flat Al(Ga)N layers with a 2μm/h growth rate. For the conditions (high temperature, low NH3 and high H2 concentration) used Ga acts mostly as surfactant (Ga content in Al(Ga)N is about 3–5%).

Technology Breakthrough by Heavy Water Board in Material Support to Indian Nuclear Power Programme

Abstract Heavy Water Board (HWB) is the sole agency in the country for production and sustained supply of heavy water, nuclear grade solvents and enriched elemental boron which are the key input materials for Indian Nuclear Power Programme. DAE has ventured to develop heavy water technology indigenously starting from scratch. H2S-H2O bi-thermal process was developed starting from laboratory studies to pilot plant and breakthrough was achieved by setting up first industrial plant at Kota and double capacity plant at Manuguru. In early 1960‟s, NH3–H2 mono-thermal process was at nascent stage of development. The rich experience from first industrial plant set up at Baroda has led to success story in second generation plants at Thal and Hazira, totally with indigenous efforts. NH3-H2O front-end process had also been developed for ammonia based Heavy Water Plants (HWPs) making them independent of fertilizer plants. Performance indicators of operating HWPs have reached the zenith level. India is the largest global producer of heavy water and is the only country using multiple technologies. HWB is also fulfilling the global demand of high quality heavy water. HWB has diversified into other activities like industrial production of other nuclear materials and development of spin off technologies.

Fabrication of Ultra High Nitrogen Austenitic Stainless Steel by NH3 Solution Nitriding

Nitrogen addition to stainless steels is very effective for improving mechanical properties and corrosion resistance. N2 solution nitriding is one of the possible methods to produce high nitrogen stainless steel. However, the nitriding rate by N2 gas is significantly small and also the attainable nitrogen contents in Fe–Cr alloys is less than 1 mass% in general. NH3 nitriding is an attractive alternative since NH3–H2 gas mixtures have high corresponding nitrogen potential than that of pure N2 gas.In the present study, aiming for the enhancement of the nitriding rate and the increase of nitrogen contents more than 1 mass%, NH3 and N2 solution nitriding of Fe–Cr alloys under various conditions has been carried out and their behaviors of nitrogen absorption in Fe–Cr alloys are investigated. It is found that the suppression of the pre-decomposition of NH3 before it reaches the sample is critical for the effective NH3 nitriding. Compared with the N contents after 1 h nitriding, NH3 nitriding is found to be about 5 times faster than that of N2 (0.1 MPa) nitriding under the present experimental conditions. In addition the N contents more than 1 mass% were easily achieved by NH3 nitriding. By using devised NH3 gas injection system to minimize the pre-decomposition of NH3 gas, austenitic Fe–23.5Cr alloys with more than 3 mass% of nitrogen (ultra high nitrogen stainless steel) that had never been achieved by solution nitriding was successfully produced.

Synthesis of Ammonia Using CH4/N2 Plasmas Based on Micro-Gap Discharge under Environmentally Friendly Condition

The synthesis of ammonia has been studied in methane-nitrogen plasmas using a micro-gap discharge under an environmentally friendly condition. The effects of some parameters such as the specific input energy, the discharge gap, the volume ratio of CH4/N2, the residence time, and the gas temperature on the yield of NH3 and conversion rate of CH4 are discussed in the paper. The results show that the highest yield of NH3 is 8000 ppm for a residence time of 1.6 s. In addition, the yield and generating rate of H2 are 9.1% (v/v) and 1879.8 μmol/min, respectively. Therefore, the micro-gap discharge is an efficient method for NH3 synthesis and H2 generation from CH4.

Selective growth of boron nitride nanotubes by plasma-enhanced chemical vapor depositionat low substrate temperature

Hexagonal boron nitride nanotubes (BNNTs) were synthesized at a low substrate temperature of800 °C on nickel (Ni) coated oxidized Si(111) wafers in a microwave plasma-enhanced chemical vapordeposition system (MPCVD) by decomposition and reaction of gas mixtures consisting ofB2H6–NH3–H2. The 1D BN nanostructures grew preferentially on Ni catalyst islands with a smallthickness only. In situ mass spectroscopic analysis and optical emission spectroscopy wereused to identify the gas reactions responsible for the BNNT formation. The morphologyand structural properties of the deposits were analyzed by SEM, TEM, EDX, SADand Raman spectroscopy. The growth mechanism of the BNNTs was identified.

CVD Synthesis of Tungsten Nitride and lts Deposition Behavior

The synthesis and deposition behavior of tungsten nitrides on a Si(400) or quartz plate were studied using a vertical hot-wall tube reactor. The preparation of the tungsten nitride by chemical vapor deposition (CVD) is predicted by the sticking probability of tungsten nitride by calculating the step coverage on the Si(400) engraved with a microtrench of different aspect ratios. The CVD deposition was performed at temperatures of 556–1063 K for deposition times up to 45 min in a gas mixture of WF6–NH3–H2 in Ar and at a total pressures of 5 and 13 Pa. From the XRD analysis, amorphous crystallites were observed at 556 and 673 K but β–W2N (111) was obtained at 790 K. The film thickness of the tungsten nitride linearly increased with the increasing deposition time at 673 and 790 K without any orientation despite the film thickness. The sticking probabilities, η, are 0.00044–0.00123 for Si(400) with different aspect ratios under the conditions of 5–13 Pa and 10–20 min.

Ammonia synthesis over the Ba-promoted ruthenium catalysts supported on boron nitride

Barium promoted ruthenium catalysts deposited on the boron nitride supports were characterised (XRD, O2 and CO chemisorption) and tested in NH3 synthesis. Prior to use, the raw BN materials marked as BNS (Starck, 96 m2/g) and HCV (Advanced Ceramics Corporation Cleveland USA, 40 m2/g) were heated in an ammonia stream at 700–800 °C for 120 h. As a result, the oxygen content was reduced from 7.0 at% (BNS) to 3.5 at% (BNSNH3) and from 3.8 to 2.7 at% (HCVNH3), as evidenced by XPS. The kinetic studies of NH3 synthesis (63 or 90 bar; H2:N2 = 3:1) revealed that the catalysts based on the modified supports were more active, respectively, than those derived from starting nitrides, the difference being especially pronounced in the case of BNS and BNSNH3. Studies of the catalysts activation have shown, in turn, that the stabilisation in a H2:N2=3:1 mixture at 1bar is very slow, i.e. the reaction rate increases slowly versus time on stream even at a high temperature of 550 – 600°C. Stabilisation is faster and the NH3 synthesis rates are higher when the activation is performed with an ammonia rich mixture (10% NH3 in H2:N2=3:1) flowing under high pressure of 90 bar. It is suggested that boron oxide (an impurity) acts as a deactivating agent for Ba–Ru/BN and that the reaction between NH3 and B2O3 (B2O3+2NH3=2BN +3H2O) is responsible for the activity increase. A poisoning mechanism of B2O3 is discussed.

Experimental and theoretical study of line mixing in NH3 spectra. II. Effect of the perturber in infrared parallel bands.

In a previous paper [J. Chem. Phys. 116, 7544 (2002) (Paper I)] a model, based on the energy corrected sudden approximation, was proposed for the construction of the line-mixing relaxation matrix. It was successfully tested by comparisons with measured infrared spectra of ammonia-helium mixtures. The present paper extends this preliminary study by considering mixtures of NH3 with H2 and Ar. Measurements have been made at room temperature in the regions of the nu2 and nu1 bands for pressures up to several hundred atmospheres. As in Paper I, the relaxation operator is constructed, within the impact approximation, using the ECS approximation. The data required are dynamical factors (which can be predicted from the NH3-X potential energy surface) and a scaling length (adjusted using line broadening data). Comparisons between measured and calculated absorptions demonstrate the quality of the model which satisfactory corrects for the large deviations with respect to the purely Lorentzian behavior. Line-mixing effects for NH3-Ar and NH3-H2 are qualitatively similar to those observed for NH3-He but quantitative differences exist, particularly when intra- and interbranch couplings are considered. Finally, the proposed model leads to very satisfactory results in the wings of both the purely rotational and nu2 bands of NH3 diluted in H2, opening promising perspectives for the remote sensing study of planetary atmospheres.

Microstructural and microtextural investigations of boron nitride deposited from BCl3–NH3–H2 gas mixtures

Abstract The structural, morphological and textural characteristics of BN coatings processed by CVD from (BCl 3 , NH 3 , H 2 ) gas mixtures, at low pressure ( P =1.3 kPa) and low temperature ( T =800 °C), with different Q NH3 / Q BCl3 gas flow rate ratios, have been investigated. Whereas the as-processed coatings are amorphous, a high degree of crystallisation can be achieved after a post-deposition heat treatment. The sole post-elaboration heat treatment does not allow the improvement of the crystallisation degree of the boron nitride. The presence of a small amount of oxygen, resulting from a simple exposure of the coating to a controlled atmosphere (temperature, moisture rate), is also necessary. For given temperature and pressure, a wide range of microstructures of the heat-treated BN coatings, from isotropic to anisotropic, can be observed by varying the Q NH3 / Q BCl3 ratio.

New possibilities for controlling gas nitriding process by simulation of growth kinetics of nitride layers

This paper presents results of computerised simulations of the gas nitriding process, demonstrating the influence of the nitriding potential of NH3–H2 atmosphere on the composition and growth kinetics of γ′ and ɛ nitride layers on pure iron. For the simulation, the recently developed KN–T phase diagram and growth kinetics models for both phases were utilised, as well as the latest results of experimental research. Simulation results indicate optimum possibilities of process control for the formation of layers of predictable phase composition, thickness of particular zones, and surface nitrogen concentrations. Also included are data on growth kinetics of the nitride layer (ɛ/γ′ and α zones) on alloy steels. The role of nitride forming and other alloying elements in process kinetics is discussed.

Crystalline carbon nitride deposition by r.f.-PECVD using a C2H4-NH3-H2 source gas mixture

Carbon nitride films have been deposited on Si(100) substrates by r.f. plasma-enhanced chemical vapour deposition (r.f.-PECVD) using an ethylene–ammonia–hydrogen (C2H4–NH3–H2) source gas mixture followed by rapid thermal annealing (RTA) at 1000 °C for 2 min. The films were characterized in terms of chemical bonding, crystallinity, N/C ratio, sp3 fraction and surface topography by diffused reflectance infrared spectroscopy (DRIFT), x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The DRIFT data revealed that, after RTA, N–H, N–C–H, C–H, CN, C–O and CN bonds were reduced but C–N, diamond C–C and N–C=O bonds were formed. The XRD data show growth of crystalline carbon nitride as well as amorphous SiC. The XPS data indicate that the sp3 fraction on the film surface is maximum at 100 sccm hydrogen flow rate. The AFM data show carbon nitride growth in clusters of sizes 60–70 nm. The DRIFT and XPS data were correlated. Copyright © 1999 John Wiley & Sons, Ltd.

Crystalline carbon nitride deposition by r.f.‐PECVD using a C2H4–NH3–H2 source gas mixture

Carbon nitride films have been deposited on Si(100) substrates by r.f. plasma-enhanced chemical vapour deposition (r.f.-PECVD) using an ethylene–ammonia–hydrogen (C2H4–NH3–H2) source gas mixture followed by rapid thermal annealing (RTA) at 1000 °C for 2 min. The films were characterized in terms of chemical bonding, crystallinity, N/C ratio, sp3 fraction and surface topography by diffused reflectance infrared spectroscopy (DRIFT), x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The DRIFT data revealed that, after RTA, N–H, N–C–H, C–H, CN, C–O and CN bonds were reduced but C–N, diamond C–C and N–C=O bonds were formed. The XRD data show growth of crystalline carbon nitride as well as amorphous SiC. The XPS data indicate that the sp3 fraction on the film surface is maximum at 100 sccm hydrogen flow rate. The AFM data show carbon nitride growth in clusters of sizes 60–70 nm. The DRIFT and XPS data were correlated. Copyright © 1999 John Wiley & Sons, Ltd.

Hardness and stiffness of amorphous SiCxNy chemical vapor deposited coatings

Abstract Thick amorphous SiCxNy coatings have been deposited by chemical vapor deposition at 1000–1200 °C from the TMS–NH3–H2 system. Hardness (H) and stiffness (E) were measured by nanoindentation with a Berkovich indenter. The relative influence on H and E have been investigated using an experimental design by varying: deposition temperature; pressure; and ammonia flow rate. The deposits exhibit a high chemical complexity in a large domain of composition (0.04≤x/x+y)≤0.67) with Si(C4−nNn) units connected by sp3–sp2 carbon configurations. Correlations between the mechanical features and the chemical bonding and local order around the different atoms have been established. Two groups of samples have been distinguished depending on the x/(x+y) ratio. Below x/(x+y)=0.1, the N-rich coatings were obtained in which the main bond was Si–N and H and E tended to decrease from 27 to 31 GPa and 224 to 289 GPa, respectively, with increasing numbers of Si–C, C–N and C–C bonds. For 0.38≤x/(x+y)≤0.67, the C-rich coatings were obtained for which the increases of H and E from 29 to 38 GPa and 240 to 360 GPa, respectively, have been correlated to the increasing proportion of SiC4 units.

Non-ideality of the system NH3–H2–N2. Comparison of equation of state and simulation predictions with experimental data

In this study, experimental PVT data of pure NH3, H2, N2 and He are used to extract parameters for a three-parameter semi-empirical equation of state (EOS) for a pure substance and interaction parameters for the exponential-6 (exp-6) potential. For ammonia, the experimental pressure and liquid density at 323 K in the liquid–vapour coexistence region and the experimental density at the same temperature and 9500 bar are taken as inputs in the fitting procedure. For the other species, supercritical data at pressures up to 10000 bar are selected. It is found that the EOS is not able to simultaneously fit both the liquid–vapour coexistence data and the high pressure region of ammonia. In contrast, the use of Monte Carlo simulations with an optimised set of exp-6 parameters leads to good agreement both at low and high pressure. The quality of the fits to H2 and N2 data using the EOS is significantly worse than that using the optimised exp-6 potential because the EOS requires physically unreasonable parameters for a good fit. Despite the higher deviations of the EOS results, their corresponding predicted equilibrium constants for the synthesis of ammonia from H2 and N2 in the industrial range agree just as well with the experimental data. Furthermore, the predicted critical point is slightly closer to the experimental value (a deviation of 10% in the critical temperature). Simulations with the exp-6 potential are performed for the system H2–He–NH3–N2 at pressures and temperatures occurring in the deep atmosphere of Jupiter. Comparison between previous ideal calculations and the simulation predictions indicates that the expected concentration of N2 at 2300 K is overestimated by about a factor of three when ideality is assumed.

Tungsten Nitrides by PECVD: Some Chemical and Structural Characterizations

Thin films of c-WN: H are synthesized by plasma enhanced chemical vapor deposition using a hot wall type reactor. These films are polycrystalline. The formation temperatures are in the 350-620°C range with WF6, NH3, as precursors and Ar and H2 as feed gas. XRD and XPS analysis show a chemical composition close to that of WN but a structure varying from tungsten form to tungsten nitride. ERDA and RBS analysis provide a hydrogen content and its; concentration profile through the thickness of the coating. AFM measurements are performed to evaluate the growth conditions.

Plasma-enhanced chemical vapor deposition of boron nitride thin films from B2H6–H2–NH3 and B2H6–N2 gas mixtures

Highly transparent and stoichiometric boron nitride (BN) films were deposited on both electrodes (anode and cathode) of a radio-frequency parallel-plate plasma reactor by the glow discharge decomposition of two gas mixtures: B2H6–H2–NH3 and B2H6–N2. The chemical, optical, and structural properties of the films, as well as their stability under long exposition to humid atmosphere, were analyzed by x-ray photoelectron, infrared, and Raman spectroscopies; scanning and transmission electron microscopies; and optical transmittance spectrophotometry. It was found that the BN films grown on the anode using the B2H6–H2–NH3 mixture were smooth, dense, adhered well to substrates, and had a textured hexagonal structure with the basal planes perpendicular to the film surface. These films were chemically stable to moisture, even after an exposition period of two years. In contrast, the films grown on the anode from the B2H6–N2 mixture showed tensile stress failure and were very unstable in the presence of moisture. However, the films grown on the cathode from B2H6–H2–NH3 gases suffered from compressive stress failure on exposure to air; whereas with B2H6–N2 gases, adherent and stable cathodic BN films were obtained with the same crystallographic texture as anodic films prepared from the B2H6–H2–NH3 mixture. These results are discussed in terms of the origin of film stress, the effects of ion bombardment on the growing films, and the surface chemical effects of hydrogen atoms present in the gas discharge.