NHx Species Research Articles

The Mechanism of Chemical Vapor Deposition of Cubic Boron Nitride Films from Fluorine‐Containing Species

Growing pains: A new surface reaction mechanism for the growth of films of cubic BN (cBN) in an electron cyclotron resonance plasma fed with a reactive gas mixture (BF3/H2/N2) is revealed. The excited plasma contains reactive H, F, BFx, and NHx species which are responsible for the etching of hexagonal BN (mainly through F atoms) and for the growth of the cBN crystals (through BFx and NHx species; see picture).

Analysis of carbon nitride growth in pedestal reactors by chemical vapor deposition

The chemical vapor deposition of polycrystalline carbon nitride in stagnation flow reactors is simulated. A model is used to predict the gas phase chemistry, temperature and velocity profiles, potential gaseous film growth precursors, and to evaluate the likelihood of bond rearrangement occurring in the bulk phase or on the deposition surface once the gaseous precursors are adsorbed. Numerical studies are carried out to predict the effects of inlet and substrate temperatures, reactor pressure, and inlet gas composition on the gas phase chemistry. Potential gaseous film growth precursors of carbon nitride are determined by quantitatively comparing the calculated results against existing experimental data. Results of the model indicate that the gas phase chemistry, including the gas composition at the deposition surface, is strongly affected by reactor pressure and inlet gas composition. However, the gas composition at the deposition surface does not depend strongly on the inlet temperature, while it is found to be strongly dependent on the substrate temperature. Since no correlation is found between the predicted near-surface concentrations of potential film growth precursors and experimentally measured bond types in the carbon nitride films, the experimentally measured bond types in the films must therefore result from chemical bond rearrangement occurring on the deposition surface or in the bulk phase once the gaseous precursors are adsorbed. Comparison between the calculated film growth rate using potential growth precursors and experimental data indicates that the CHx (x=0,2,3), C2H2, NHx (x=0,1,2), and HxCN (x=1,2) species are the most probable crystalline carbon nitride growth species. Among these, C and CH3 dominate the carbon contribution to the film growth, and N is the primary nitrogen bearing species responsible for the film growth. The sum of predicted film growth rates for carbon bearing species is comparable to the experimentally determined film growth rate.

Catalytic Decomposition of Hydrazine over α-Mo2C/γ-Al2O3 Catalysts

A series of α-Mo2C/γ-Al2O3 catalysts with different Mo loadings were prepared by temperature-programmed reaction and characterized by X-ray diffraction, BET specific surface area determination, and NH3 temperature-programmed desorption. Their ability to decompose hydrazine was tested using a thruster and a microreactor. The catalytic activity for hydrazine decomposition of the fresh α-Mo2C/γ-Al2O3 catalyst is higher than that of the passivated α-Mo2C/γ-Al2O3 catalyst. The catalytic activity also decreased with the reaction time at 303 K, and the deactivation is mainly due to strongly adsorbed NHx species. Flushing with N2 at 773 K recovers the activity of the catalyst. It was shown that hydrazine decomposes to NH3 and N2 at low temperatures, and the produced NH3 dissociates into N2 and H2 at elevated temperatures.

Electrostatic probe measurements in an expanding microwave discharge sustained in an Ar–N2–H2 gas mixture: Investigation on plasma parameters

This work is devoted to the study of plasma parameters in the expansion of a microwave discharge sustained within an Ar–N2–H2 gas mixture. The plasma expansion in the reactor depends on the wave propagation conditions.We show that in a pure argon discharge the electron energy distribution function (EEDF) is identical to a Maxwell energy distribution function whatever the distance between the discharge exit and the substrate holder is and whatever the incident power is, ranging between 100 and 400 W. However, results show that the electron density is larger close to the discharge exit than to the substrate holder.When N2 or H2 is mixed with Ar, the discharge shrinks. The EEDF is displaced towards a low electron energy, and this behaviour increases with increasing N2 or H2 percentage. The EEDF cannot be compared with a Maxwell energy distribution function for a gas mixture containing more than 10% N2. Beyond this value, the plasma is unstable 3 cm below the discharge exit, and for larger N2 concentrations, the wave propagation is not observed in these regions.When the three gases H2, N2 and Ar are all mixed together in the gas mixture, the plasma is more stable and the expansion is detected even for gas mixtures containing a large concentration of (N2, H2) mixed with argon, in a 80% Ar–20% (N2, H2) or 48% Ar–52% (N2, H2) gas mixture. However, when the concentration of H2 is too low (49% Ar–50% N2–1% H2), the plasma shrinks and wave propagation is not observed. This behaviour could be explained assuming that these gas mixtures containing Ar–H2–N2 are producing NHx species in plasma that are low ionization energy species. However, numerical models are necessary in order to study such a complex plasma and to confirm the accuracy of this last assumption.

Ordered mesoporous MCM-41 silicon oxynitride solid base materials with high nitrogen content: synthesis, characterisation and catalytic evaluation

We present here the synthesis, physicochemical characterisation and catalytic evaluation of mesoporous silicon oxynitride materials. The solid base catalytic materials were prepared by subjecting pure silica MCM-41 mesoporous molecular sieves to nitridation, i.e., treatment with ammonia at high temperature. The mesoporous MCM-41 silicon oxynitride materials are well ordered and possess high surface area and pore volume. The nitrogen content was controlled within the range 8.6 to 25.1 wt% by simply changing the nitridation temperatures (between 850 and 1150 °C) or the time allowed for nitridation (from 20 to 80 hours). The nitrogen is incorporated onto the MCM-41 framework in the form of various NHx species, including terminal NH2 and bridging NH groups as well as adsorbed ammonium ions. The oxynitride materials were found to exhibit considerable catalytic activity for the Knoevenagel condensation reaction. The catalytic activity was correlated to the nitrogen content and was therefore highest for materials nitrided at high (1150 °C) temperature. In all cases, the nitrided materials were found to be generally stable with respect to storage and they suffered no significant structural degradation over a period of a few months. Oxynitride materials with the highest nitrogen content (i.e., those nitrided at temperatures ≥1050 °C) were found to be more susceptible to loss of surface area and pore volume after storage under ambient conditions for 6 months. The ease of preparation and control of the nitrogen content of mesoporous oxynitride materials and their stability makes them attractive as alternative solid base catalysts especially for large molecule transformations.

Selective NOxReduction During the H2+ NO + O2Reaction Under Oxygen-Rich Conditions Over Pd/V2O5/Al2O3: Evidence for In Situ Ammonia Generation

Under oxygen-rich conditions in H2 + NO + O2 mixtures, Pd/V2O5/Al2O3 catalysts are active and highly selective (∼80%) for NOx reduction to N2. In situ DRIFT spectroscopy and reactor data show that the system operates via formation of NHx species on the highly dispersed V2O5 component. Both NH3 and NH4+ are formed, with the latter dominant. The role of the palladium component is also discussed.

Surface Chemistry of Azomethane Adsorbed on Si(111)-7 × 7 Surface Studied by SR-Photoemission, HREELS, and STM

The adsorption and thermal decomposition behavior of azomethane on the Si(111)-7 × 7 surface has been studied by high-resolution electron energy loss spectroscopy (HREELS), synchrotron radiation photoelectron spectroscopy (SR-PES) in the soft X-ray and UV photon energy ranges, and scanning tunneling microscopy (STM). It is found that at 90 K substrate temperature, azomethane is adsorbed largely intact in both trans and cis forms. The decomposition of the molecules starts at an annealing temperature around 200 K with NN bond breaking, and at 300 K, a mixture of remaining intact azomethane, NCHx, and Si−NHx species is present on the surface with three distinct N 1s photoemission peaks. At this temperature, the Si−N species begins to appear as nitride, but no Si−C bonding is detected. Carbon dissociates from nitrogen and reacts directly with the Si surface above 400 K with CH3 adsorption as an intermediate adspecies. The CH3 adsorption is strongly enhanced if azomethane is thermally precracked prior to the exposure. At 700 K, all surface carbon and nitrogen are bonded to Si. The remaining hydrogen is desorbed at 1000 K, and Si−C and Si−N phases are formed. After it is annealed to 1500 K, photoemission features typical for the clean Si(111)-7 × 7 surface are immediately recovered. Si−C and Si−N species are desorbed, leaving a homogeneous but disordered surface with an underlying 1 × 1 structure, which transforms back into the 7 × 7 structure by forming characteristic triangular-shaped patterns as intermediate steps in the surface reconstruction process.

Semi-Empirical Calculations on the Stability and Reactivity of NHx Species on Metal Surfaces

The semi-empirical method of interacting bonds was used to elucidate the mechanism of oscillation phenomena in the NO + H2 reaction on metal surfaces. Basic single-crystal planes of Pt, Rh, Ir, Fe, Ru, and Re were examined with respect to the stability of adsorbed NH n species (n = 0, 1, 2, 3); to the reactivity of NH n (n = 0, 1, 2) species toward adsorbed hydrogen atoms; and to the possibility of combination reactions between two NH or two NH2 species resulting in the formation of gaseous N2 molecules. All studied surfaces were found to form readily stable NH species. The principal difference between Pt, Rh, and Ir single-crystal planes exhibiting reaction rate oscillations, and Fe, Ru, and Re surfaces, which do not show an oscillatory behavior, is that the combination reaction of NH species can easily proceed in the former case, whereas this reaction is not allowed thermodynamically in the latter. This result is consistent with an earlier suggested model that attributes the oscillatory surface wave propagation to the intermediate formation of NH species. Stable NH2 species can be formed on Ru, Re, and Fe surfaces, whereas the noble metal surfaces of Pt, Rh, and Ir can only form weakly stable NH2 species at the very edge of their existence region. The combination reaction between two NH2 species is endothermic in all cases.

Selective Photocatalytic Oxidation of NH3 to N2 on Platinized TiO2 in Water

Selective photocatalytic oxidation of NH3 to N2 is proposed as a new treatment method for controlling the levels of ammonia in water. The photocatalytic oxidation of ammonia on naked and metallized TiO2 in water saturated with air, nitrogen, or NO2 gas was investigated. While the slow photocatalytic oxidation of NH3 to NO2-/NO3- is the only pathway for decomposition of NH3 on naked TiO2 and Au/TiO2, a new pathway, that of selective oxidation of ammonia to dinitrogen, opens up on Pt/TiO2. The formation of dinitrogen from the oxidation of 15NH3 was confirmed by mass spectrometric detection of 15N2. The photocatalytic conversion of NH3 to N2 greatly increases when the Pt/TiO2 suspension is saturated with NO2 gas, whereas NO2 itself shows little reactivity with naked TiO2 and Au/TiO2. Over 80% of the total nitrogen available in ammonia (0.1 mM) is converted into N2 within 40 min illumination of the N2O-saturated Pt/TiO2 suspension. The ability of N2O to accept the conduction band electrons of Pt/TiO2 was verified by photoelectrochemical measurements. NO2 reductively decomposes to generate OH radicals on Pt/TiO2; the rate of ammonia degradation in the NO2-saturated Pt/TiO2 suspension significantly decreases in the presence of excess tert-butyl alcohol, an OH radical scavenger. The presence of Pt deposits on the TiO2 particles changes the photocatalytic pathway of ammonia conversion by both enhancing OH radical production from NO2 and stabilizing intermediate NHx (x=0, 1, 2) species to facilitate their recombination into N2.

Comparison of surface interactions for NH and NH2 on polymer and metal substrates during NH3 plasma processing

How different plasma species influence the reactions and reactivity of each other during plasma processing is not fully understood, especially with respect to surface interactions. The goal of this study is to provide insight into the relationships between gas-phase NHx species in NH3 plasmas. Specifically, formation of NHx, their relationship with the surface during plasma processing, and the effects of charged species on these interactions are discussed. The surface reactivities of NH and NH2 radicals with different substrate materials during NH3 plasma processing were investigated using the imaging of radicals interacting with surfaces technique. Scatter coefficients, S, for NHx species were obtained as a function of applied rf power for polyimide, polytetrafluoroethylene, and platinum substrates. To investigate the role of ions on the formation of NHx radicals in the gas phase and at the surface, ions were removed (>98%) from the plasma molecular beam. Results from the “ion-free” conditions suggest that ions enhance NH2 surface generation, but suppress NH-forming mechanisms at high rf powers. Surface interaction results for NH and NH2 with and without ions provide the basis for a discussion of possible surface interaction mechanisms for the different substrates examined.

Unique Performance and Characterization of a Crystalline SbRe2O6Catalyst for Selective Ammoxidation of Isobutane

The catalytic performances of a new family of crystalline Re−Sb−O compounds SbRe2O6, SbOReO4·2H2O and Sb4Re2O13 in selective ammoxidation of isobutane (i-C4H10) to methacrylonitrile (MAN) have been studied and compared with those of a mechanical mixture of Sb2O3 + Re2O7, coprecipitated SbRe2Ox, Sb2O3-supported Re2O7, bulk Re oxides, and Sb oxides. SbRe2O6 efficiently catalyzed the i-C4H10 ammoxidation to MAN at 673 K with the good selectivity to MAN (44.9%) and to the sum of MAN + i-C4H8 (84.3%) at a steady-state conversion of 4.4%, while significantly no MAN activity was observed on the other catalysts. No structural change in the bulk and surface of SbRe2O6 after the i-C4H10 ammoxidation was observed by means of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and in situ confocal laser Raman microscopic spectroscopy (LRM). The good performance of SbRe2O6 may be ascribed to its specific crystal structure composed of alternate octahedral (Re2O6)3- and (SbO)+ layers. It was found that the presence of NH3 was prerequisite to the C−H bond breaking of i-C4H10, and the oxidation/dehydrogenation of i-C4H10 never proceeded in the absence of NH3. The presence of NH3 was also prerequisite to maintain the crystal structure of SbRe2O6 under the reaction conditions. Fourier transformed infrared (FT-IR) spectra showed that NHx species irreversibly adsorbed on the SbRe2O6 catalyst. The i-C4H10 ammoxidation proceeded on SbRe2O6 by a redox mechanism, in which the oxidative dehydrogenation of i-C4H10 to i-C4H8 was the rate-determining step. Increasing reaction temperature and decreasing GHSV did not give rise to increase in the formation of byproducts CO2 and acetonitrile. Thus the crystalline SbRe2O6 compound may be regarded to be a new promising catalyst for the ammoxidation of light alkanes.

A Novel Application of Trapping Catalysts for the Selective Low-Temperature Oxidation of NH3 to N2 in Simulated Biogas

The low-temperature selective oxidation of NH3 to N2 in simulated biogas containing a large excess of CO and H2 has been examined using a novel NH3 and a standard NOx trapping catalyst. The N2 selectivity during NH3 oxidation at 200°C for a 1%Pt–20%BaO–Al2O3 NOx trapping material, with typical lean/rich switches, was initially good (>90%) but decreased markedly over a small number of cycles. In contrast, the N2 yield obtained using a novel NH3 trapping material (1%Pt–20%CuO–Al2O3) with rich/lean fuel switching exceeded 95% and was stable over many switching cycles, while an unmodified 1%Pt–Al2O3 catalyst displayed poor N2 selectivity under all conditions. The data obtained from probe reactions between the various potential adsorbates and gaseous species of the reaction indicate that the N2 yields obtained from the 1% Pt–20% CuO–Al2O3 catalyst are formed via an Internal Selective Catalytic Reduction (iSCR) between NHx species adsorbed on the trapping component and NO formed from NH3 total oxidation on the Pt during the lean cycle of operation. For the 1%Pt–20%BaO–Al2O3 catalyst, NOx, formed during lean operation, is reduced to N2 in the rich cycle by a combination of reactions with NH3, CO, and H2. The use of a hybrid catalyst, based upon a combination of iSCR and NOx trapping processes, gave a peak N2 yield of >95% and an integrated N2 production over the entire rich/lean cycle of 75%. These results reflect the potentially dramatic improvements possible by rational design of catalyst systems based upon a fundamental knowledge of the processes involved.

X-Ray Photoelectron Spectroscopy and Temperature-Programmed Desorption Studies of Nitrided Molybdena-Alumina Catalyst

The relationship between the Mo oxidation states measured by X-ray photoelectron spectroscopy (XPS) and the NHX species was discussed based on results of temperature-programmed desorption (TPD). From the XPS analysis, the major Mo oxidation states were found to be Mo3+ and Mo4+ for 1.0–18.7 wt% Mo/Al2O3 nitrided at 973 K. The TPD results indicated that the desorption of nitrogen and hydrogen from the nitrided Mo/Al2O3were deconvoluted to those of NHX {αN (NHX, X=0), βN and γN (NHX, X=0.6 to 1.5)} adsorbed on molybdenum species. The nitrogen species (NHX, X=0) adsorb only on Mo3+ ions of the 973 K-nitrided Mo/Al2O3, but NHX species (X=0.6–1.5) adsorb on either Mo3+ or Mo4+ ions.

Platinum aluminophosphate oxynitride (Pt-AIPON) catalysts for the dehydrogenation of isobutane to isobutene

The catalytic dehydrogenation of isobutane to isobutene over two series of platinum-supported aluminophosphate oxynitride (Pt-AlPON) catalysts, containing different amounts of nitrogen and synthesized using two different platinum precursors, was studied. The impregnation and platinum reduction processes result in a partial loss of nitrogen and a decrease in the specific surface area of the AlPON precursors. The surface and bulk structures of the catalysts are functions of the nitrogen content of the solid. At low nitrogen contents (less than or equal to 6% w/w), the nitrogen is found mainly on the surface of the catalyst as an -NHx species. Above 6% w/w, nitridation of the bulk begins. The basicity of the solids increases with increasing nitrogen content. The conditions under which synthesis takes place influence the final nitrogen content and the surface structure of the catalysts. Dehydrogenation of isobutane to isobutene of the Pt-AlPON catalyst correlates with the basicity of the catalysts. The surface structure of the solids also influences the yield of isobutene. (C) 2000 Academic Press.

Production of stable and metastable phases of zirconium nitrides by NH3 plasma nitridation and by double ion beam sputtering of zirconium films

Nitrided surfaces and nitrogen composition gradients in thin films exhibit interesting mechanical, electrical and optical properties. Metal, semiconductor or oxide surfaces can be transformed into a nitrided compound via interactions of nitrogen species issued from a plasma or an ion beam. The thermal activation is a key factor in both cases to ensure chemical reactions and short/long-range diffusion necessary to allow the growth of stable or metastable structures. In this work, we focus our attention on zirconium nitrides prepared under controlled temperature through reaction and diffusion, in Zr films, of low energy NHx species produced in NH3 plasma and through the implantation–diffusion of energetic N+2 ions during Zr deposition by using double ion beam sputtering. Zirconium nitrides show optical and electrical properties that depend on the conditions and on kinetics of the nitrogen take-up; the material exhibits a transition from the stable metallic ZrN to a metastable phase Zr3N4 that appears transparent and insulating. The influence of the energy of the nitriding species and of the temperature on nitride compositions and phases are addressed. A model using coupled implantation and thermal diffusion mechanisms is proposed to explain the phases produced. In relation with the described phenomena, a temperature-controlled plasma-immersion ion-implantation system is proposed for tailoring in-depth stable/metastable ceramic structures such as nitrides, oxides and carbides.

XPS and TPR Studies of Nitrided Molybdena−Alumina

The relationship between the surface molybdenum species and adsorbed nitrogen species on nitrided 1.0−18.7% MoO3/Al2O3 was elucidated by XPS and temperature-programmed reduction (TPR). The MoO3/Al2O3 samples were nitrided by temperature-programmed reaction with NH3. From the XPS analysis, Mo3+ and Mo4+ ions were predominant on the surface of the nitrided Mo/Al2O3 samples. From the TPR measurement, the ammonia desorption was due to nitrogen species adsorbed on alumina. The nitrogen desorption was due to two kinds of nitrogen desorption from the structures of γ-Mo2N and β-Mo2N0.78 and four kinds of nitrogen desorption from NHX species adsorbed on MoO2, Mo3+ ion (γ-Mo2N), Mo2+ ion (molybdenum nitride on alumina such as highly dispersed molybdenum nitride), and alumina.

Temperature-Programmed Reduction and XRD Studies of Ammonia-Treated Molybdenum Oxide and Its Activity for Carbazole Hydrodenitrogenation

The change structure and composition of molybdenum nitride catalysts with cooling in a stream of ammonia or helium gas after NH3 treating was determined using temperature-programmed reduction (TPR) and X-ray powder diffraction analyses. The relationship between the molybdenum species and the catalytic activities of the molybdenum nitride catalysts for the hydrodenitrogenation (HDN) of carbazole was discussed. MoO2, γ-Mo2N, and Mo metal were mainly formed during the temperature-programmed reaction of MoO3 with ammonia at 773, 973, and 1173 K, respectively. During the TPR experiment, a portion of the adsorbed NHx (x=0–3) species caused further nitriding of the catalyst at higher temperatures. It was found that nitrogen desorption during TPR could be assigned to four types of nitrogen species: (1) NHx adsorbed on MoO2, (2) NHx adsorbed on γ-Mo2N, (3) N2 during the transformation of γ-Mo2N to β-Mo2N0.78, and (4) N2 during the reduction of β-Mo2N0.78 to molybdenum metal. Purging the NH3-treated catalyst with helium at 973 K not only removed the adsorbed NHx and diffused nitrogen but also altered the structure of the molybdenum compounds, i.e., from γ-Mo2N to β-Mo2N0.78. During the HDN of carbazole, γ-Mo2N was the most active, followed by β-Mo2N0.78 for C–N hydrogenolysis, while molybdenum metal had the highest activity for hydrogenation. NH3-treated MoO2 was much less active for both C–N hydrogenolysis and hydrogenation during carbazole HDN. From these results, the C–N hydrogenolysis sites were most likely located on small nitrogen deficient particles and crystallites of the molybdenum nitrides. The hydrogenation sites were located on the surface grain boundary of molybdenum metals.

Adsorbed NH x species on Pt(111) and Pt(100) surfaces studied by the semi-empirical method of interacting bonds

The properties of the adsorbed NHx species (x=0,1,2,3) on platinum(111) and (100)‐(1×1) single‐crystal planes are studied by the semi‐empirical method of interacting bonds. Both surfaces reveal similar features. The adsorbed species NH and NH2 are stable on the surface, and stable NH3(ads) species cannot form. The NH2(ads) species is favourable in adsorbed hydrogen excess, but lack of the latter results in NHads becoming dominant. Both NH and NH2 species are expected to diffuse easily over the surface due to the small difference between their bond strengths to various adsorption sites.

A fast x-ray photoelectron spectroscopy study of the NO-H2 reaction over Rh(533): Identifying surface species

The behavior of the NO-H2 reaction over Rh(533) was studied in situ using fast x-ray photoelectron spectroscopy. The high energy resolution achieved by using synchrotron light allowed us to distinguish five surface species by analyzing the line shape and energy positions of the N 1s and O 1s photoelectron spectra. It was established that there are three nitrogen species, labeled NO, N(I) and N(II). The species NO and N(I) are attributed to adsorbed NO and atomic nitrogen, respectively. The nitrogen species N(II) might be attributed to atomic nitrogen adsorbed at step sites or to an NHx species. The short measurement time for the spectra opens the possibility of studying the reaction more dynamically.

Surface species and the stability of γ-Mo 2N

The surface species of passivated and nonpassivated Mo 2N and their stability at elevated temperature in various environments have been investigated by thermal programmed desorption with mass spectrometry (TPD-MS), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The results demonstrate that two types of NHx and H species remain on fresh Mo 2N, and a small amount of weakly adsorbed NHx species is present on passivated Mo 2N. During heating at 1000 °C in He, fresh Mo 2N transforms first from γ-Mo 2N to β-Mo 2N and then decomposes to metallic Mo. No obvious change of the crystalline phase occurs for passivated Mo 2N before its decomposition or reduction. The XRD spectra showed a high stability of the Mo 2N bulk structure against sulphidation. XPS data indicated that some changes of the surface composition occurred in sulphided Mo 2N. Oxygen incorporated during passivation reacted with sulphur and formed a thin layer molybdenum of sulphide covering the nitride surface.