Introduction Of Ammonia Research Articles

Effects of ammonia energy ratio and PODE injection timing on combustion and emissions in a PODE/ammonia RCCI engine

The application of ammonia as an alternative carbon-free fuel has attracted increasing attention. In this paper, the effects and mechanisms of ammonia energy ratio (AER) and polyoxymethylene dimethyl ethers (PODE) injection timing on the combustion and emissions characteristics of a PODE/ammonia RCCI engine are investigated by both experimental and numerical methods. Results show that increasing AER from 10% to 40% leads to the delayed low temperature heat release (LTHR) due to reduced OH radical generated by PODE and the competition between PODE and ammonia for OH radical consumption. The larger AER increases the indicated thermal efficiency (ITE) by lowering combustion temperature and reducing heat transfer losses at the cost of higher hydrocarbon (HC), carbon monoxide (CO), and unburned NH3 emissions. The reduction of fuel-based nitrogen oxide (NOx) emissions with a larger AER is attributed to the enhanced in-cylinder thermal denitrification reactions. The delayed start of injection (SOI) from −60 °CA ATDC to −45 °CA ATDC leads to the higher peak in-cylinder pressure and temperature, resulting in the larger heat transfer losses and lower ITE. The impact of retarding SOI on NOx is insignificant under the combined effect of more thermal NO formation, less fuel-based NO generation, and weakened denitrification reactions. Additionally, nitrous oxide (N2O) shows dominant effects on the total greenhouse gas (GHG) emissions, and increasing AER or advancing injection timing produces more GHG. Under low loads and low AERs, although the introduction of ammonia helps to improve ITE, more researches are required to further reduce exhaust emissions.

Effects of different ammonia energy ratio on soot formation and oxidation in an ammonia diesel dual-fuel engine

Due to environmental pollution and energy crises, zero‑carbon fuel ammonia (NH3) has attracted extensive attention as an alternative fuel for engines. In this paper, the effects of ammonia energy ratio (AER) and injection strategy on particulate emission characteristics of an ammonia diesel dual-fuel engine were examined by merging experimental and simulation results; additionally, soot formation and oxidation mechanism were investigated. Results showed that the reduction in particulate emission was substantially higher than the increase in AER. When AER increased to 60 %, the reduction in particulate mass concentration reached 97.5 %. The initial soot formation area gradually moved to the bottom of the piston bowl with increasing AER. When the piston reached the top dead center, the high-soot-concentration area was shifted to the center of the piston bowl as AER increased. The contents of acetylene (C2H2) and methyl (CH3) reduced considerably, which restricted the formation of soot precursors. With AER increasing, the contents of nitric oxides (NOx) and other nitrogen-containing species increased and reacted with CH3 and other carbon-containing species, which effectively reduced the number of C in soot formation pathway, thereby lowering particulate emissions. As AER increased, hydroxyl (OH) involved in soot formation gradually decreased, and only 14 % of OH was involved in the oxidation of n-heptane at 60 % AER, which was favorable for reducing the soot formation rate. Furthermore, OH is a substantial species in soot oxidation. The introduction of ammonia caused an increase in OH, which facilitated the removal of soot. The decrease in hydrogenium (H) hindered the hydrogen–abstraction–acetylene–addition (HACA) reaction, further limiting the soot surface growth. By optimizing the injection timing and AER, particulate emission was lowered to 4.31 × 10−5 μg/cm3, and particle size was reduced by 64.2 % when AER was 60 %, injection timing was −20° CA ATDC, and injection pressure was 60 MPa.

Calculating the penetration depth of reaction in chemical gas-phase deposition of boron nitride within porous bodies

The thermodynamic calculations conducted using the TERRA software package for the B–Cl–N–H system revealed that the inclusion of hydrogen into the B–Cl system significantly diminishes the thermodynamic stability of BCl3 with the possibility of boron formation in the condensed phase. On the other hand, the introduction of ammonia, which includes hydrogen, results in the synthesis of boron nitride across a broad temperature spectrum. The analysis of kinetic relationships uncovered three distinct regions in the boron nitride deposition process: K – kinetic region (up to 1400 K), D – diffusion region (above 1800 K) and T – transition region. The activation energy for the kinetic region was calculated as Ea = 134 kJ/mol. Within the temperature range of 1023–1123 K, linear dependences were observed. The computation of the penetration depth for the boron nitride deposition process assumed a gas mixture of boron trichloride, ammonia, and argon (BCl3 + NH3 + 30Ar). The results indicated that boron trichloride governs the extent of penetration. The depths of penetration for the chemical vapor infiltration boron nitride (CVI-BN) process, conducted at 0.1 kPa within the temperature range of 1100–1400 K, were determined for pore diameters of 1, 10, 30, 100, 200 and 300 µm. When porosimetry data for a specific preform is available, the acquired penetration depth relationships for the CVI-BN process under specific parameters and process temperatures facilitate the estimation of essential parameters for interphase formation using pyrolytic boron nitride.

A computational comparison of NH3/O2 and CH4/O2 non-premixed laminar flames

The structure of non-premixed, laminar, counterflow NH3/O2 flames was studied and compared with the structure of CH4/O2 flames. A commercially available computational tool was utilized through the introduction of ammonia and methane chemistry in order to compute the flow fields of strained flames. The tool was validated by comparing with previously published results for CH4 flames and by employing two different mechanisms for NH3 oxidation kinetics. It was shown that NH3 flames achieve lower maximum temperature and narrower high-temperature area compared to CH4 flames, which was attributed to much less heat release from the NH3 oxidization process. This is due to the fact that NH3 oxidation proceeds through a chemical path drastically different than the one of CH4 and is completed with the formation of N2 as an equilibrium product, without substantial formation of nitrogen oxides. The structure of the CH4 and NH3 flames were compared for mildly strained flames and it was shown that, despite its much slower kinetics, ammonia can sustain near-equilibrium flames, even for relatively small values of the Damköhler number.

Novel investigation of perovskite membrane based electrochemical nitric oxide control phenomenon

The combustion of hydrocarbon fuels within the automotive industry results in harmful and reactive incomplete combustion byproducts. Specifically, nitric oxide emissions (NO) lead to increased smog, acid rain, climate change, and respiratory inflammation within the population [Nitrogen Dioxide | American Lung Association]. Current methods for treating combustion exhaust include the catalytic converter in conjunction with nitrogen oxide traps. However, there is no active, continuous reduction method that does not require restrictions on the combustion environment (Hirata in Catal Surv Asia 18:128–133, 2014). Here, a small voltage potential oscillation across a newly designed electro-chemical catalytic membrane significantly reduces NO emissions. A ceramic membrane consisting of two dissimilar metal electrodes, sandwiching a dielectric layer, is able to achieve an NO reduction in excess of 2X that of a platinum group metal (PGM) three way catalytic converter. An analysis of the exhaust effluent from the membranes indicates N2O as a precursor to N2 and O2 formation, without the introduction of ammonia (NH3), during the reaction of NO indicating a divergence from current literature. Our results demonstrate how an oscillatory electric potential on a catalytic surface may alter anticipated reaction chemistry and interaction between the catalytic surface and fluid flow.

Microbial Nitrogen Metabolism in Chloraminated Drinking Water Reservoirs.

Ammonia availability due to chloramination can promote the growth of nitrifying organisms, which can deplete chloramine residuals and result in operational problems for drinking water utilities. In this study, we used a metagenomic approach to determine the identity and functional potential of microorganisms involved in nitrogen biotransformation within chloraminated drinking water reservoirs. Spatial changes in the nitrogen species included an increase in nitrate concentrations accompanied by a decrease in ammonium concentrations with increasing distance from the site of chloramination. This nitrifying activity was likely driven by canonical ammonia-oxidizing bacteria (i.e., Nitrosomonas) and nitrite-oxidizing bacteria (i.e., Nitrospira) as well as by complete-ammonia-oxidizing (i.e., comammox) Nitrospira-like bacteria. Functional annotation was used to evaluate genes associated with nitrogen metabolism, and the community gene catalogue contained mostly genes involved in nitrification, nitrate and nitrite reduction, and nitric oxide reduction. Furthermore, we assembled 47 high-quality metagenome-assembled genomes (MAGs) representing a highly diverse assemblage of bacteria. Of these, five MAGs showed high coverage across all samples, which included two Nitrosomonas, Nitrospira, Sphingomonas, and Rhizobiales-like MAGs. Systematic genome-level analyses of these MAGs in relation to nitrogen metabolism suggest that under ammonia-limited conditions, nitrate may be also reduced back to ammonia for assimilation. Alternatively, nitrate may be reduced to nitric oxide and may potentially play a role in regulating biofilm formation. Overall, this study provides insight into the microbial communities and their nitrogen metabolism and, together with the water chemistry data, improves our understanding of nitrogen biotransformation in chloraminated drinking water distribution systems.IMPORTANCE Chloramines are often used as a secondary disinfectant when free chlorine residuals are difficult to maintain. However, chloramination is often associated with the undesirable effect of nitrification, which results in operational problems for many drinking water utilities. The introduction of ammonia during chloramination provides a potential source of nitrogen either through the addition of excess ammonia or through chloramine decay. This promotes the growth of nitrifying microorganisms and provides a nitrogen source (i.e., nitrate) for the growth for other organisms. While the roles of canonical ammonia-oxidizing and nitrite-oxidizing bacteria in chloraminated drinking water systems have been extensively investigated, those studies have largely adopted a targeted gene-centered approach. Further, little is known about the potential long-term cooccurrence of complete-ammonia-oxidizing (i.e., comammox) bacteria and the potential metabolic synergies of nitrifying organisms with their heterotrophic counterparts that are capable of denitrification and nitrogen assimilation. This study leveraged data obtained for genome-resolved metagenomics over a time series to show that while nitrifying bacteria are dominant and likely to play a major role in nitrification, their cooccurrence with heterotrophic organisms suggests that nitric oxide production and nitrate reduction to ammonia may also occur in chloraminated drinking water systems.

A selective and sensitive optical sensor for dissolved ammonia detection via agglomeration of fluorescent Ag nanoclusters and temperature gradient headspace single drop microextraction

In this paper, a simple sensor platform is presented for highly selective and sensitive detection of dissolved ammonia in aqueous solutions without pretreatment based on temperature gradient headspace single drop microextraction (HS-SDME) technique, and fluorescence and UV–vis spectrophotometry are utilized with the Ag nanoclusters (Ag NCs) functioned by citrate and glutathione as the probe. The sensing mechanism is based on the volatility of ammonia gas and the active response of Ag NCs to pH change caused by the introduction of ammonia. High pH can make the Ag NCs agglomerate and lead to the obvious decrease of fluorescence intensity and absorbance of Ag NCs solution. Moreover, the presented method exhibits a remarkably high selectivity toward dissolved ammonia over most of inorganic ions and amino acid, and shows a good linear range of 10–350μM (0.14–4.9mgNL−1) with a low detection limit of 336nM (4.70μgNL−1) at a signal-to-noise ratio of 3. In addition, the practical applications of the sensor have been successfully demonstrated by detecting dissolved ammonia in real samples.

Experimental Studies on A New Fluorescent Ensemble of Calix[4]pyrrole and Its Sensing Performance in the Film State

The supramolecular approach plays a pivotal role in the construction of smart and functional materials due to the reversible nature of noncovalent interactions. In the present work, two compounds, cholesterol-functionalized calix[4]pyrrole (CCP) and perylene bisimide diacid (PDA), were synthesized. Little fluorescence is observed in the ethanol solution of the mixture of CCP and PDA, while the solution turns fluorescent upon introduction of ammonia, which is attributed to the formation of a supramolecular ensemble, PDA/(CCP)2/NH3. The fluorescence emission of the as-formed ensemble is sensitive to the presence of phenol, an electron-rich analyte. Interestingly, the sensing can also be observed in the film state, and the relevant detection limit (DL) is lower than 1 ppb. Moreover, the sensing could also be performed in a visualized manner. Upon the basis of the findings, a sensor device with instant response and good reversibility was developed. Further studies revealed that the as-developed fluorescent ensemble is also sensitive to the presence of TNT, an electron-poor compound. The DL for this sensing is ∼80 nM. To our knowledge, this is the first report that a fluorescent sensor could be used for phenol sensing in the vapor state, and for sensing of both electron-rich and electron-poor analytes in solution state. It is believed that the present study presents a distinctive example that demonstrates how smart sensing is realized via combination of the host-guest chemistry of calix[4]pyrrole and the aggregation and disaggregation property of PBI derivatives.

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.

AMMONIA AS A MODIFIER IN ION MOBILITY SPECTROMETRY: EFFECTS ON ION MOBILITIES AND POTENTIAL AS A SEPARATION TOOL

A commercial ammonia solution was introduced in the buffer gas of an ion mobility spectrometer and the mobilities of tetramethylammonium (TMA), tetraethylammonium (TEA), tetrapropylammonium (TPA), and tetrabutylammonium (TBA) ions, tribenzylamine (TBzA), tributylamine (TBtA), 2,4-dimethyl pyridine (2,4-lutidine), 2,6-di-tert-butyl pyridine (DTBP), serine, atenolol, and valinol decreased depending on their structures. Electrospray ionization-ion mobility spectrometry-quadrupole mass spectrometry was used in these experiments. Analyte ion mobilities decreased to different extents with the amount of ammonia introduced in the mobility spectrometer. When the amount of ammonia increased from 0.0 to 22 mmol m-3 (with a concomitant concentration of water of 106 mmol m-3), percentage reductions in mobilities were: -4.8% (serine), -1.9% (reactant ions), -1.1% (TBtA), -0.9% (TBzA), -0.5% (TEA), -0.4% (valinol and TBA), -0.3% (TEA, TMA, and 2,4-lutidine), and 0% (atenolol and DTBP). These selective variations in mobilities were due to formation of large analyte ion-ammonia-water clusters. The small change in mobility of tetraalkylammonium ions, TBzA, TBtA, atenolol, and DTBP with the introduction of ammonia into the buffer gas was explained by steric hindrance of bulky substituents which shielded the positive charge of the ion from the attachment of ammonia and water molecules, and delocalized the positive charge. Ammonia in the buffer gas produced ion clusters with one or two ammonia molecules in compounds with little steric hindrance such as serine. At concentrations of ammonia of 4.4 mmol m-3 or higher (at 150°C), ligand-saturation with ammonia and water molecules occurred on the positive charge of some ions; when the positive charge saturated, no significant reductions in ion mobility occurred when increasing the concentration of ammonia in the buffer gas.

Morphological Investigation of Calcium Carbonate during Ammonification‐Carbonization Process of Low Concentration Calcium Solution

Ultrafine calcium carbonate is a widely used cheap additive. The research is conducted in low degree supersaturation solution in order to study the polymorphic phases’ change and its factors of the calcium carbonate precipitate in the ammonification‐carbonization process of the solution with calcium. Fine particles of calcium carbonate are made in the solution containing 0.015 mol/L of Ca2+. Over 98% of the calcium carbonate precipitate without ammonification resembles the morphology of calcite, while the introduction of ammonia can benefit the formation of vaterite. It was inferred that the main cause should be serious partial oversaturation or steric effects. Ammonia also helps to form the twin spherical calcium carbonate. However, particles formed in the process of ammonification‐carbonization in solution with low concentration degree of calcium are not even with a scale of the particle diameter from 5 to 12 μm. Inorganic salts, alcohol, or organic acid salts have significant controlling effect on the particle diameter of calcium carbonate and can help to decrease the particle diameter to about 3 μm. Anionic surfactants can prevent the conglobation of calcium carbonate particles and shrink its diameter to 500 nm–1 μm.

Modeling serpentinization: Applied to the early evolution of Enceladus and Mimas

Serpentinization is an exothermic reaction which involves the metamorphic transition of ultramafic rocks, and liquid water, into hydrated serpentine minerals. It has been suggested that serpentinization may be a fundamental process in the development of icy planetary objects, prompting a reaction runaway, in which heat from serpentinization in a localized region of the body could result, in principle, in the melting of ice throughout the body. We use a 1-dimensional adaptive-grid thermal evolution code suited for small and medium sized icy bodies of the Solar System, which we apply to Enceladus and Mimas, the satellites of Saturn. The code is used to investigate the multiphase flow of water through a porous rocky medium, thus giving us a detailed look on the internal distribution of mass and energy. We consider heating by serpentinization reactions, accounting for the phase transition of the chemically altered rock and the water lost in the process; short-lived and long-lived radionuclides; gravitational potential energy; and the introduction of ammonia, to modify the thermodynamical properties of water–ammonia solution. We test several cases with different initial compositions. Our results show, that in a short term radioactive heating scenario, Enceladus will fully differentiate into a rocky inner core, topped by a very thin icy crust. Considering only long-term radioactive heating, in the absence of any short-term radionuclides, serpentinization in Enceladus can be triggered, providing ammonia is present in the water. However, long-term triggered serpentinization is confined only to the inner core. We conclude that serpentinization may have been triggered in Enceladus during its early evolution, and not in Mimas, leading to the dichotomy between them. Based on the assumptions that went into our model, we may conclude that the formation time of Saturn’s satellites was around 3Myr after the formation of Calcium–Aluminum inclusions.

Influence of the composition of the inverse micelle solution on the formation of nickel nanoparticles under the effect of γ-rays of 60Co

The influence of additions on the formation of nickel nanoparticles in inverse micelle solutions under the effect of γ-irradiation is shown. The dependence of intensity of absorption bands of optical spectra of inverse micelle solutions on the degree of hydration is revealed. The formation of nickel nanoparticles is accompanied by the appearance and growth of the absorption band peaked at 263 nm. It is found that the introduction of ammonia in the inverse micelle system leads to the appearance of a new absorption band peaked at 312 nm. Introduction of ethanol leads to an increase in intensity of the absorption band without the appearance of new bands.

Effect of Ammonia on the Growth of Carbon Nanotubes

Carbon nanotubes were grown on hydrogen-treated Fe catalyst at 700 degrees C using a thermal chemical vapor deposition method. During the growth, acetylene was used as the carbon source, which is balanced by hydrogen and/or ammonia. Raman analysis shows that the introduction of ammonia to the gaseous carbon source can lead to defect structures in the carbon nanotubes due to the incorporation of nitrogen atoms into the carbon nanotubes. Furthermore, the growth rate of carbon nanotubes was also affected by the introduction of ammonia into the gaseous carbon source. We show that the dependence of the growth rate on the gaseous source composition is better described in terms of the ratio of ammonia to acetylene than the overall ammonia concentration. It is proposed that there is a competition between the ammonia and the acetylene during the growth of CNTs. At low ammonia/acetylene ratios the growth increases with ammonia concentration; while at high ammonia/acetylene ratios the growth decreases with the ammonia concentration. A critical ammonia/acetylene ratio of 4.4, at which the growth peaks, was found and discussed.

Introduction of preheated ammonia during GaN growth on Si by compound-source MBE at low temperature

Preheated ammonia was introduced during GaN growth on Si by compound-source molecular-beam epitaxy (CS-MBE) at a low temperature. The use of preheated ammonia increased the surface smoothness and the intensity of cathodoluminescence (CL). Although the introduction of ammonia leads to the limited migration of the Ga surface atoms during the growth, the thermal energy supplied by the preheated ammonia assisted the surface migration. Furthermore, it was demonstrated that the introduction of preheated ammonia during the growth was effective for improving the CL intensity of the GaN layers at low temperatures. This is due to the reduction of the hydrogen concentration in the layers.

Synthesis of SiCNAl(O) pre-alloyed nanopowders by pyrolysis of an aluminosilazane aerosol

A liquid aluminosilazane precursor, with various Al contents, was synthesized by direct reaction between hexamethyldisilazane and trimethylaluminum at room temperature. Mass spectrometry and Fourier-transform infrared spectroscopy showed respectively a methane evolution and the formation of Al N bonds. Thermal-spray pyrolysis of this precursor was realized under different conditions: temperature (1200–1400 °C), pyrolysis atmosphere (Ar, Ar/NH 3) and gaseous flow rate (1 and 3 L min −1). SEM micrographs revealed spherical SiCNAl(O) nanopowders which size (20–180 nm in diameter) depending on the residence time duration of the product in the furnace. Pyrolysis under argon led to a high C content, and then to the presence of free carbon and amorphous SiC phase detected by XRD and NMR analysis. Introduction of ammonia in the pyrolysis atmosphere induced an important decrease of C content in the pre-alloyed nanopowders, correlated with an increase of N and Al. With this process, multielement nanopowders with an adjustable composition exhibit a higher thermal stability than powders processed by laser-spray pyrolysis with an equivalent precursor.

Evolution of AlN buffer layers on silicon and effects on the properties of epitaxial GaN films

The morphological evolution of AlN buffer layers grown on (111) silicon at high-temperature has been studied using atomic force microscopy (AFM) and transmission electron microscopy (TEM). The structure and morphology of subsequently grown GaN films were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), and photoluminescence (PL) measurements. It was found that even though thicker AlN buffer layers are the poor single crystal with some defects like mis-oriented domains, stacking faults, they provide better templates for subsequent growth of GaN films with better crystalline and optical properties, compared to the GaN grown on a thin AlN buffer layer. To prevent the formation of SiNx layers at the interface, TMA was flowed for several seconds prior to the introduction of ammonia into the chamber. The AFM result showed that quasi 2-dimensional growth was quickly achieved for the AlN buffer layer with TMA pre-treatment. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Dissolution behavior of gold in ammoniacal solutions with iodine as an oxidant

The leaching behavior of gold in ammonical solutions with iodine as an oxidant was investigated using cyclic voltammogram and Tafel polarization. The majority of the experiments were carried out at 75 °C, and the results were compared with consideraton given to the equilibrium of the iodine-ammonia system. The distribution of nine iodide-containing species at various pH values in the absence and presence of ammonia were considered at three different temperatures. It was found that, with the introduction of ammonia into the system, the concentration of iodide (I−), iodine (I2) and triiodide (I3−) is increased substantially, which in turn helps the rate of gold dissolution. The apparent activation energy was found to be about 46 kJ/mole, indicating a chemical-controlled mechanism.

A new route to high-T c superconducting Bi–Sr–Ca–Cu–O thin films: Improved deposition efficiency and film morphology using ammonia–argon mixtures as the carrier gas

Highly oriented films of the high-Tc superconductor Bi2Sr2CaCu2Ox have been prepared by a low-pressure organometallic chemical vapor deposition process using a mixture of ammonia and argon as the carrier gas together with Sr(dpm)2 (dpm-dipivaloylmethanate), Ca(dpm)2, Cu(acac)2 (acac-acetylacetonate), and triphenylbismuth as the organometallic precursors. By introducing ammonia into the carrier gas, a significant improvement in the volatility and thermal stability of both Sr(dpm)2 and Ca(dpm)2 is observed. Typical required source temperatures for Sr(dpm)2 and Ca(dpm)2 with the introduction of ammonia are about 40–50 °C lower than the source temperatures of the precursors without the introduction of ammonia. Enhancement of source volatility for Cu(acac)2 is also observed. After annealing at 865 °C in flowing oxygen, the films consist predominantly of the Bi2Sr2CaCu2Ox phase and exhibit high preferential orientation of the crystallite c axes perpendicular to the substrate surface. Four-probe resistivity measurements indicate the onset of film superconductivity at ∼110 K and zero resistivity achieved at 74 K.

Modifications of surface properties of nickel/silica catalysts by nitrogen-containing compounds II. Piperidine, 3,5 dimethyl piperidine and pyridine

The interactions of piperidine, 3,5-dimethylpiperidine and pyridine with a reduced Ni/SiO 2 sample have been studied as a function of the temperature, using magnetic measurements, infrared spectroscopy and catalytic activity changes in the hydrogenation of 1,3 butadiene. The three cyclic nitrogen-containing compounds (CNCC) are adsorbed at room temperature without cracking of the molecule: the bond number (number of nickel atoms demagnetized by the adsorption of one adsorbate molecule) is equal to 2,4 and 6 for piperidine, 3,5-dimethylpiperidine and pyridine respectively. For the first two CNCC, the increase of the adsorption temperature considerably increases the bond number; at 373 K, it reaches values of 12 for piperidine and 24 for 3,5-dimethylpiperidine. These values show that a complete cracking of the two molecules does not occur. In the case of pyridine adsorption, a constant bond number close to 7 was observed in the 298–373 K temperature range. Regeneration of the poisoned catalysts was performed by vacuum treatments at increasing temperature. During this process the insertion of nitrogen and carbon atoms in the metal lattice has been evidenced. Regeneration under flowing hydrogen was more efficient than treatment under vacuum. Concerning the hydrogenation of 1,3-butadiene, the addition of CNCC leads to a decrease of the catalytic activity associated with an increase of the selectivity for partial hydrogenation. By contrast with the introduction of ammonia, these effects are permanent, i.e., changes in catalytic activity and selectivity are maintained even after the removal of the CNCC from the feed. The increase in the selectivity in partial hydrogenation of 1,3-butadiene was explained by an electron enrichment of the metallic sites in the presence of cyclic nitrogen-containing compounds.