Ammonia Precursors Research Articles

Numerical analysis of ammonia homogenization for selective catalytic reduction application

Selective catalytic reduction based on urea water solution as ammonia precursor is a promising method for the NOx abatement form exhaust gasses of mobile diesel engine units. It consists of injecting the urea-water solution in the hot flue gas stream and reaction of its products with the NOx over the catalyst surface. During this process flue gas enthalpy is used for the urea-water droplet heating and for the evaporation of water content. After water evaporates, thermolysis of urea occurs, during which ammonia, a known NOx reductant, and isocyanic acid are generated. The uniformity of the ammonia before the catalyst as well as ammonia slip to the environment are important counteracting design requirements, optimization of which is crucial for development of efficient deNOx systems.The aim of this paper is to show capabilities of the developed mathematical framework implemented in the commercial CFD code AVL FIRE®, to simulate physical processes of all relevant phenomena occurring during the SCR process including chemical reactions taking part in the catalyst. First, mathematical models for description of SCR process are presented and afterwards, models are used on the 3D geometry of a real SCR reactor in order to predict ammonia generation, NOx reduction and resulting ammonia slip. Influence of the injection direction and droplet sizes was also investigated on the same geometry. The performed study indicates importance of droplet sizes on the SCR process and shows that counterflow injection is beneficial, especially in terms of minimizing harmful ammonia slip to environment.

Water-assisted oxygen activation during gold-catalyzed formic acid decomposition under SCR-relevant conditions

Owing to the problems associated with the use of urea as an ammonia precursor compound in the selective catalytic reduction process in automobiles, alternative compounds such as ammonium formate, methanamide, and guanidinium formate have been suggested as more efficient and cleaner candidates. For the application of such formate-based ammonia precursors, a fundamental understanding of catalytic formic acid decomposition in the presence of the diesel exhaust components water and oxygen is required. Oxygen activation is a reaction step common to many catalytic processes. We observed that oxygen activation over titania and base-modified titania-supported gold catalysts is greatly enhanced in the presence of water, resulting in a significant increase in carbon dioxide production from the decomposition of formic acid. Unlike the supports, gold catalyzed the selective production of carbon dioxide. Monodentate and bidentate formates are the kinetically relevant surface species for carbon monoxide and carbon dioxide production, respectively. The support acts as a reservoir, storing bidentate formates that do not react in the steady state when formic acid and oxygen (and water) are co-fed. However, during transient experiments, when the feed is switched from formic acid to oxygen (and water), they are reactivated upon reverse spillover to the active site associated with gold, where they decompose to carbon dioxide. In the presence of oxygen and water, carbon monoxide oxidation and the water gas shift reaction do not produce carbon dioxide. Instead, a direct oxidative-dehydrogenation-type pathway proceeds, which strongly differs from stoichiometric formic acid decomposition. A kinetically consistent mechanism is proposed in which the hydroperoxy species facilitate the C–H bond cleavage of formates to release carbon dioxide and water in the rate-determining step.

Ultrathin, wafer-scale hexagonal boron nitride on dielectric surfaces by diffusion and segregation mechanism

Chemical vapor deposition (CVD) of two-dimensional (2D) hexagonal boron nitride (h-BN) is at the center of numerous studies for its applications in novel electronic devices. However, a clear understanding of the growth mechanism is lacking for its wider industrial adoption on technologically relevant substrates such as SiO2. Here, we demonstrate a controllable growth method of thin, wafer scale h-BN films on arbitrary substrates. We also clarify the growth mechanism to be diffusion and surface segregation (D-SS) of boron (B) and nitrogen (N) in Ni and Co thin films on SiO2/Si substrates after exposure to diborane and ammonia precursors at high temperature. The segregation was found to be independent of the cooling rates employed in this report, and to our knowledge has not been found nor reported for 2D h-BN growth so far, and thus provides an important direction for controlled growth of h-BN. This unique segregation behavior is a result of a combined effect of high diffusivity, small film thickness and the inability to achieve extremely high cooling rates in CVD systems. The resulting D-SS h-BN films exhibit excellent electrical insulating behavior with an optical bandgap of about 5.8 eV. Moreover, graphene-on-h-BN field effect transistors using the as-grown D-SS h-BN films show a mobility of about 6000 cm2 V−1 s−1 at room temperature.

Simultaneous removal of ammonia and N-nitrosamine precursors from high ammonia water by zeolite and powdered activated carbon

When adding sufficient chlorine to achieve breakpoint chlorination to source water containing high concentration of ammonia during drinking water treatment, high concentrations of disinfection by-products (DBPs) may form. If N-nitrosamine precursors are present, highly toxic N-nitrosamines, primarily N-nitrosodimethylamine (NDMA), may also form. Removing their precursors before disinfection should be a more effective way to minimize these DBPs formation. In this study, zeolites and activated carbon were examined for ammonia and N-nitrosamine precursor removal when incorporated into drinking water treatment processes. The test results indicate that Mordenite zeolite can remove ammonia and five of seven N-nitrosamine precursors efficiently by single step adsorption test. The practical applicability was evaluated by simulation of typical drinking water treatment processes using six-gang stirring system. The Mordenite zeolite was applied at the steps of lime softening, alum coagulation, and alum coagulation with powdered activated carbon (PAC) sorption. While the lime softening process resulted in poor zeolite performance, alum coagulation did not impact ammonia and N-nitrosamine precursor removal. During alum coagulation, more than 67% ammonia and 70%–100% N-nitrosamine precursors were removed by Mordenite zeolite (except 3-(dimethylaminomethyl)indole (DMAI) and 4-dimethylaminoantipyrine (DMAP)). PAC effectively removed DMAI and DMAP when added during alum coagulation. A combination of the zeolite and PAC selected efficiently removed ammonia and all tested seven N-nitrosamine precursors (dimethylamine (DMA), ethylmethylamine (EMA), diethylamine (DEA), dipropylamine (DPA), trimethylamine (TMA), DMAP, and DMAI) during the alum coagulation process.

Metal‐organic vapor phase epitaxy of GaPN alloys via surface nitridation using ammonia

The present work is intended to pilot the possible use of ammonia precursors for the growth of III–V–N alloys with metal‐organic vapor phase epitaxy (MOVPE) as opposed to typical growth procedures using dimethylhydrazine. The GaPN alloys are grown by repeated cycles of surface nitridation and growth of a thin GaP layer with an interval separating the nitridation and growth. The effects of conditions and duration of each step on the N composition and crystallinity are systematically clarified. An N composition is controlled up to 1.6% in the current conditions, maintaining a high crystalline quality comparable to the GaP substrates, while a continuous‐supply method yields a much lower value.

Crystalline growth of AlN thin films by atomic layer deposition

Aluminum nitride (AlN) thin film was grown by plasma enhanced atomic layer deposition using trimethylaluminum and ammonia precursors. A method was found to have crystalline thin film AlN with almost zero thickness variation and a truly one layer deposition of atoms per each cycle of the process. The growth rate saturated at ∼ 1 Å/cycle, and the thickness was proportional to the number of reaction cycles. The preferred crystal orientation, uniformity of the nucleation and the surface roughness of the grown AlN were investigated. X-ray diffraction (XRD), atomic focused microscopy (AFM) and scanning electron microscopy (SEM) were carried out to analyze the crystallinity and properties of the films.

Direct Comparison of Urea-SCR and NH3-SCR Activities Over Acidic Oxide and Exchanged Zeolite Prototype Powdered Catalysts

In order to develop new NOx selective catalytic reduction (SCR) catalysts for automotive application, the DeNOx catalytic activity is commonly evaluate at the laboratory scale using NH3 as reductant. However, NH3 is not directly used on board: an ammonia precursor based on urea aqueous solution is injected in the exhaust pipe upstream the SCR catalyst. It is admitted that ammonia is then obtained by two successive reactions: the thermal decomposition of urea, leading to HNCO and NH3, and the HNCO hydrolysis, providing the second molecule of NH3. However, the complete availability of ammonia from urea could be not achieved before the SCR catalyst. Then, the influence of the SCR catalyst on these reactions may impact the NOx reduction efficiency. With the aim to study the possible role of the SCR catalyst on the ammonia availability, an innovative synthetic gas bench adjusted to powdered material was developed, allowing the direct comparison of the use of gaseous NH3 or urea (injected aqueous solution) for the NOx conversion, depending on the temperature (200–500 °C). This work presents results obtained with an oxide based prototype SCR catalyst in comparison with a patented Fe-exchanged zeolite, evaluated in both standard and fast SCR stoichiometry. This study points out that, in contrast with the exchanged zeolite, the evaluated oxide based catalyst may not allow an optimal NOx conversion because of a lack in ammonia availability, attributed to insufficient activity in HNCO hydrolysis.

Investigations of GaN growth on the sapphire substrate by MOCVD method with different AlN buffer deposition temperatures

The effects of different AlN buffer deposition temperatures on the GaN material properties grown on sapphire substrate was investigated. At relatively higher AlN buffer growth temperature, the surface morphology of subsequent grown GaN layer was decorated with island-like structure and revealed the mixed-polarity characteristics. In addition, the density of screw TD and leakage current in the GaN film was also increased. The occurrence of mixed-polarity GaN material result could be from unintentional nitridation of the sapphire substrate by ammonia (NH3) precursor at the beginning of the AlN buffer layer growth. By using two-step temperature growth process for the buffer layer, the unintentional nitridation could be effectively suppressed. The GaN film grown on this buffer layer exhibited a smooth surface, single polarity, high crystalline quality and high resistivity. AlGaN/GaN high electron-mobility transistor (HEMT) devices were also successfully fabricated by using the two-step AlN buffer layer.

Chemical lift-off and direct wafer bonding of GaN/InGaN P–I–N structures grown on ZnO

p-GaN/i-InGaN/n-GaN (PIN) structures were grown epitaxially on ZnO-buffered c-sapphire substrates by metal organic vapor phase epitaxy using the industry standard ammonia precursor for nitrogen. Scanning electron microscopy revealed continuous layers with a smooth interface between GaN and ZnO and no evidence of ZnO back-etching. Energy Dispersive X-ray Spectroscopy revealed a peak indium content of just under 5at% in the active layers. The PIN structure was lifted off the sapphire by selectively etching away the ZnO buffer in an acid and then direct bonded onto a glass substrate. Detailed high resolution transmission electron microscoy and grazing incidence X-ray diffraction studies revealed that the structural quality of the PIN structures was preserved during the transfer process.

Metal-Free Low-Temperature Water–Gas Shift Catalysis over Small, Hydroxylated Ceria Nanoparticles

The water–gas shift (WGS) reaction is an important process for the production of H2 either for industrial use as, e.g., an ammonia precursor or to produce low-CO concentration H2 gas streams for use in fuel cells. Hybrid metal/metal oxide catalysts have permitted low-temperature WGS reaction by promoting associative reaction mechanisms in which -OH groups adsorbed at the metal oxide surface combine with reactant CO to form metastable intermediates. Here we show that sufficiently small, hydroxylated ceria nanoparticles (CNPs) can directly catalyze the WGS reaction without requiring the presence of a metal cocatalyst. Hydroxyl groups intrinsically present at the surfaces of such CNPs allow associative mechanisms with activation barriers on the order of 0.5 eV, low enough to allow metal-free low-temperature catalysis of the WGS reaction.

Use of a µ-Scale Synthetic Gas Bench for Direct Comparison of Urea-SCR and NH3-SCR Reactions over an Oxide Based Powdered Catalyst

The selective catalytic reduction (SCR) of NOx by NH3 has been extensively studied in the literature, mainly because of its high potential to remediate the pollution of diesel exhaust gases. The implementation of the NH3-SCR process into passenger cars requires the use of an ammonia precursor, provided by a urea aqueous solution in the conventional process. Although the thermal decomposition and hydrolysis mechanisms of urea are well documented in the literature, the influence of the direct use of urea on the NOx reduction over SCR catalysts may be problematic. With the aim to evaluate prototype powdered catalysts, a specific synthetic gas bench adjusted to powdered material was developed, allowing the use of NH3 or urea as reductant for direct comparison. The design of the experimental setup allows vaporization of liquid urea at 200 °C under 10 bar using an HPLC pump and a micro injector of 50 μm diameter. This work presents the experimental setup of the catalytic test and some remarkable catalytic results towards further development of new catalytic formulations specifically dedicated to urea-SCR. Indeed, a possible divergence in terms of DeNOx efficiency is evidenced depending on the nature of the reductant, NH3 or urea solution. Particularly, the evaluated catalyst may not allow an optimal NOx conversion because of a lack in ammonia availability when the urea residence time is shortened. This is attributed to insufficient activity of isocyanic acid (HNCO) hydrolysis, which can be improved by addition upstream of an active solid for the hydrolysis reaction such as ZrO2. Thus, this µ-scale synthetic gas bench adjusted to powdered materials enables the specific behaviour of urea use for NOx reduction to be demonstrated.

AlN capping layer inserted between Cu and SiCN dielectric barrier layer for enhancing reliability of 28 nm technological node and beyond

The improvement of the interface adhesion between Cu and silicon carbonitride (SiCN) dielectric barrier layer is required in the back end of the line for 28nm technological node and beyond. The main purpose of this paper was to investigate the reliability of breakdown voltage (VBD) and electromigration (EM) improved by inserting the AlN capping layer enhancing the above mentioned interfacial adhesion. The AlN capping layer was deposited by the ALD process using trimethylaluminum and ammonia precursors. The AlN capping layer was characterized by a cross-sectional transmission electron microscopy (XTEM) and energy dispersive X-ray spectrum (EDX) for the thickness and interfacial composition, electrical test for resistance–capacitance examination, reliability test for VBD and EM lifetime. XTEM observation indicated that the thickness of the AlN capping layer was about 8.5nm, and the EDX examination indicated the Al and N elements of the AlN capping layer were distributed in the interface between Cu and SiCN dielectric barrier layer. Reliability test indicated that the VBD and EM performances were improved obviously by inserting the AlN capping layer for enhancing the interfacial adhesion between Cu and SiCN dielectric barrier layer.

Characterization of annealed zinc oxide nanotip array prepared by chemical bath deposition

We investigate the characteristics of thermally annealed ZnO nanotip in various ambient. AZO seed layer was first prepared on glass substrate by RF sputtering. A vertically oriented ZnO nanotip array was grown on AZO seed layer/glass by chemical bath deposition with precursors of zinc nitrate and ammonia at 70 °C. By thermal annealing in N2O ambient at 300 °C for 1 hour, the characteristics of ZnO nanotip array were improved and the ultraviolet sensor shows the best rise time of 50 s, decay time of 70 s and on/off current ratio of 26.04.

Borohydride Ionic Liquids as Hypergolic Fuels: A Quest for Improved Stability.

Hydrazine and its derivatives are used as fuels in rocket propellant systems; however, due to high vapor pressure, toxicity, and carcinogenicity, handling of such compounds is extremely hazardous. Hypergolic ionic liquids have shown great promise to become viable replacements for hydrazines as fuels. Borohydride-containing ionic liquids have now been synthesized using a more efficient synthetic pathway that does not require liquid ammonia and halide precursors. Among the eight new compounds, 1-allyl-3-n-butyl-imidazolium borohydride (1) and 1, 3-diallylimidazolium borohydride (5) exhibit very short ignition-delay times (ID) of 8 and 3 ms, respectively. The hydrolytic stability of borohydride compounds has been greatly improved by attaching long-chain alkyl substituents to the imidazole ring. 1,3-Di-(n-octyl)-imidazolium borohydride (3) is a water stable borohydride-containing ionic liquid. 1,3-Di-(n-butyl)-imidazolium borohydride (2) is a unique example of a borohydride liquid crystal. These ionic liquids have some unusual advantages, including negligible vapor pressures, good ignition delay (ID) times, and reduced synthetic and storage costs, thereby showing good application potential as environmentally friendly fuels in bipropellant formulations. In addition, they also have potential applications in the form of reducing agents and hydrogen storage materials.

Promotion of Ammonium Formate and Formic Acid Decomposition over Au/TiO2 by Support Basicity under SCR-Relevant Conditions

This work demonstrates the rational design of a dedicated hydrolysis catalyst for application in the selective catalytic reduction (SCR) of NOx. Modification of titania by lanthanum prior to gold deposition entailed highly improved catalytic activities for ammonium formate (AmFo) and formic acid decomposition under SCR-relevant conditions stemming from dual phenomena: particle size effect and base effect. Smaller gold particles were stabilized, and there was higher uptake of CO2 and formic acid, as demonstrated by HAADF-STEM and in situ DRIFT analyses, respectively. The difference in the activities between the lanthanum-modified, unmodified, and tungsten-modified catalysts was implicitly dictated by the formic acid coverage, which was in turn greatly increased in the presence of base. In situ DRIFT studies under reaction conditions identified formate as a relevant reaction intermediate, under reaction conditions. Higher E-a,E-app alongside a higher pre-exponential factor (A), describe an underlying compensation effect originating from the contribution of enthalpy associated with the desorption of the strongly adsorbed formate, which is consistent with the highly negative formic acid orders observed in the case of the lanthanum-modified catalysts. Gold is essential to achieve selectivity to CO2; its absence yields CO. The introduction of lanthanum to the catalytic system preferentially promoted the CO2 formation mechanism, enabling complete decomposition of formic acid selectively to CO2 at significantly lower gold loading and lower contact times, making it a promising candidate for decomposition of formate-based ammonia precursors in the SCR process.

Pyrolysis of urea and guanidinium salts to be used as ammonia precursors for selective catalytic reduction of NOx

In this work, the thermal decomposition (pyrolysis) of urea, ammonium carbamate, and guanidinium acetate, carbonate and chloride was performed, using thermogravimetric and differential thermal analysis coupled to gas chromatography and mass spectrometry (TGA/DTA–GC/MS, from 25 to 500°C). Solid products from pyrolysis were characterized with infrared spectroscopy (FT-IR) and X-ray diffraction (XRD). Triazines and heptazines were found as solid products of pyrolysis for the tested solids. Crystallite sizes were assessed using the Scherrer equation; it was found that guanidinium acetate and guanidinium chloride had the biggest crystallites of all solids analyzed (maximums of 113 and 109nm for guanidinium acetate and guanidinium chloride, respectively) up to 350°C. These compounds also showed the highest decomposition temperatures, with thermal decomposition starting at 246°C for guanidinium acetate and 300°C for guanidinium chloride. This results show the composition of the solid and gaseous products for guanidinium acetate, carbonate and chloride, along with identification of ammonia production of guanidinium salts, which offer an important comparative with the thermal decomposition of well-known ammonia precursor compounds like urea and ammonium carbamate.

On the feasibility of silicene encapsulation by AlN deposited using an atomic layer deposition process.

Since epitaxial silicene is not chemically inert under ambient conditions, its application in devices and the ex-situ characterization outside of ultrahigh vacuum environments require the use of an insulating capping layer. Here, we report on a study of the feasibility of encapsulating epitaxial silicene on ZrB2(0001) thin films grown on Si(111) substrates by aluminum nitride (AlN) deposited using trimethylaluminum (TMA) and ammonia (NH3) precursors. By in-situ high-resolution core-level photoelectron spectroscopy, the chemical modifications of the surface due to subsequent exposure to TMA and NH3 molecules, at temperatures of 300 °C and 400 °C, respectively, have been investigated. While an AlN-related layer can indeed be grown, silicene reacts strongly with both precursor molecules resulting in the formation of Si-C and Si-N bonds such that the use of these precursors does not allow for the protective AlN encapsulation that leaves the electronic properties of silicene intact.

Anatase titania nanorods by pseudo-inorganic templating

A novel, facile and cost-effective single step aqueous sol–gel method for the synthesis of anatase TiO2 nanorods without the assistance of structure-directing organic/inorganic templates is reported. We specifically demonstrate a pseudo-inorganic templating method using ammonium iron (III) sulfate. Highly thermal stable anatase TiO2 nanorods were obtained using titanium oxysulfate and ammonium iron (III) sulfate as precursors. The structural, microstructural and chemical analyses of the nanorods synthesized, strongly supported the pseudo inorganic templating role of ammonium iron (III) sulfate on the formation of nanorod morphology. The materials have been characterized using different techniques such as TEM, XRD, BET surface area measurement, diffuse reflectance spectra and XPS. TEM study confirmed the rod shape of nano-anatase TiO2, having a diameter in the range of 20–40nm and a length of 100nm. XPS investigation showed that along with Fe3+, nitrogen and sulfur were also been doped into the TiO2 lattice from the single source dopant precursor ammonium iron (III) sulfate. Moreover, UV/vis diffuse reflectance spectra of nanorods showed red-shifts towards visible light and a corresponding decrease in band-gap energies consistent with an n-type doping of the anatase TiO2 matrix. This aqueous sol–gel synthesis of anatase nanorod with pseudo inorganic templating explores the advantages of inexpensive precursors, control over the powder morphology and optical properties, and distribution of the dopants over TiO2 at nano level due to homogeneous mixing of the precursors. Finally the photocatalytic analysis showed that the TiO2 nanorod have two times higher activity than the commercially available Degussa P 25.

MOVPE growth of semipolar ([formula omitted]) [formula omitted] across the alloy composition range ([formula omitted

We report the synthesis and characterization of semipolar (112¯2) oriented Al1−xInxN alloys over a wide composition range (0≤x≤0.55) using metalorganic vapour phase epitaxy (MOVPE) in a close-coupled showerhead reactor system. AlInN layers were deposited on AlN buffer layers on m-plane (101¯0) sapphire substrates using trimethylaluminium (TMAl), trimethylindium (TMIn), and ammonia (NH3) precursors and the alloy composition tuned by changing the growth temperature from 860°C to 625°C, corresponding to an indium content from 2.9% to 54.6%. High resolution X-ray diffraction (HRXRD) measurements were used to determine the microstructure and anisotropic biaxial strain arising from the intrinsic tricilinic distortion of the unit cell in semipolar nitrides. The epilayers exhibit an overall tilt about the [11¯00] direction and phase separation was observed for samples with indium content over 40%. The ternary alloy compositions evaluated from HRXRD were compared with those obtained from optical transmission measurements.

Effect of ammonia on the decomposition of ammonium formate over Au/TiO2 under oxidizing conditions relevant to SCR: Enhancement of formic acid decomposition rate and CO2 production

Ammonium formate (AmFo) and formic acid decomposition were carried out in the presence of excess O2 and H2O over 0.5wt% Au/TiO2 anatase monolithic catalysts using various contact times and temperatures between 160°C and 300°C, under spray conditions in a dedicated setup. A systematic investigation of ammonia influence on formic acid decomposition revealed a highly beneficial influence on the reaction rate and the CO2 yield in the temperature range 160–300°C. Ammonia oxidation did not occur at any of the studied temperatures and space velocities. Both AmFo and a stoichiometric ammonia–formic acid mixture exhibited identical homogeneous gas phase as well as heterogeneous catalytic decomposition behavior. With the introduction of ammonia at a concentration of only 0.25 molar equivalents, the pseudo-first-order rate constants for formic acid decomposition experienced close to 110% and 15% increase at 160°C and 260°C, respectively and dosing 12 molar equivalents of ammonia in the gas phase, the rate constants underwent nearly 8-fold increase at 160°C, while at 260°C, only two times increase was achieved. Increasing the ammonia to formic acid molar ratio from 0 to 12 lead to a steep increase in the CO2 yield from 18% to 75% at 160°C, while a relatively smaller rise from 60% to 75% was observed at 260°C. Activity testing of bare titania revealed an inhibitory effect of ammonia on formic acid decomposition to CO. Overall, it can be concluded that the presence of gold is critical for the realization of such an ammonia-induced enhancement of rate and CO2 yield. The obtained results are relevant for the application of formate-based ammonia precursor compounds in the selective catalytic reduction of NOx in Diesel exhaust gases.