Formation of nitric oxide dimers on MgO-supported gold particles

We present density functional theory (DFT) calculations on the formation of nitric oxide dimers (N₂O₂) on Au atoms, dimers and trimers adsorbed on regular O^2 ? sites and neutral oxygen vacancies (F_s sites) of the MgO(100) surface. The study of the N₂O₂ species is of great interest since it has been detected in the NO reduction reaction as an intermediate towards the formation of N₂O. We found that the coupling of a NO molecule with a previously adsorbed one on Au/MgO is energetically favorable on Au₁ and Au₃, but unfavorable on Au₂. The stability of N₂O₂ is in direct relation with the amount of charge taken from the support. Furthermore, one of the N―O bonds can be activated as a result of the attraction between the negatively charged NO dimer and the ionic oxide surface. In fact, for Au₁ anchored on the F_s site a barrierless reaction occurs between N₂O₂ and a third NO molecule, forming adsorbed N₂O and NO₂.     

Oxygen-exchange reaction between O2 and NO coadsorbed on a Pt(111) surface: reactivity of molecularly adsorbed oxygen

The oxygen-exchange reaction between N¹⁶O and ¹⁸O₂ coadsorbed on Pt(111) has been studied by temperature-programmed desorption (TPD). Reaction products of N¹⁸O and ¹⁸O¹⁶O are desorbed from Pt(111) initially saturated with ¹⁸O₂ at 94 K followed by exposure of N¹⁶O. Three distinct desorption peaks are observed in N¹⁸O TPD spectra at 145, 310, and 340 K, and two peaks in ¹⁸O¹⁶O at 155 K and between 600 and 1000 K. In contrast, the exchange reaction is greatly suppressed when oxygen molecules are replaced with oxygen adatoms at three-fold hollow sites of Pt(111). These results strongly suggest that adsorbed oxygen molecules are responsible for the exchange reaction. NO₂ or NO₃ is postulated as a reaction intermediate. However, since desorption signals corresponding to these species are not detected, the oxygen-exchanged products are not due to the cracking processes of the higher order nitrogen oxides in the mass spectrometer. Thus, the reaction proceeds via the intermediate that is dissociated during the elevation of surface temperature.     

Hybrid-DFT study for the initial oxidation steps on silicon cluster surface

We performed a hybrid density functional theory calculation for the successive adsorption of nitrous oxide (N₂O) on Si(1 0 0)-Si₉H₁₂O_x (x = 0 and 1) cluster surfaces to elucidate N₂O decomposition and the subsequent surface oxidation processes. N₂O decomposed into N₂ and O fragments, and the latter fragment inserted into either surface-dimer bonds or back-bonds with similar activation barriers on both the clean and partially oxidized Si surfaces. The Si₉H₁₂ cluster surface was eventually oxidized to five distinct structures of Si₉H₁₂O₂.     

A density functional theory study on the adsorption and dissociation of N2O on Cu2O(1 1 1) surface

Density functional theory has been employed to investigate the adsorption and the dissociation of an N₂O at different sites on perfect and defective Cu₂O(1 1 1) surfaces. The calculations are performed on periodic systems using slab model. The Lewis acid site, Cu_CUS, and Lewis base site, O_SUF are considered for adsorption. Adsorption energies and the energies of the dissociation reaction N₂O → N₂ + O(s) at different sites are calculated. The calculations show that adsorption of N₂O is more favorable on Cu_CUS adsorption site energetically. Cu_CUS site exhibits a very high activity. The Cu_CUS-N₂O reaction is exothermic with a reaction energy of 77.45 kJ mol⁻¹ and an activation energy of 88.82 kJ mol⁻¹, whereas the O_SUF-N₂O reaction is endothermic with a reaction energy of 205.21 kJ mol⁻¹ and an activation energy of 256.19 kJ mol⁻¹. The calculations for defective surface indicate that O vacancy cannot obviously improve the catalytic activity of Cu₂O.     

Electrode reaction at platinum / ceria surface modified YSZ interface

The electrode reaction was examined on ceria coated YSZ by a platinum point electrode in H₂-H₂O atmosphere at 973 K- 1173 K. The thickness of the ceria coating layer was altered from 0 to 2.5 μm, fabricated by a laser ablation and by a vacuum vapor deposition method on YSZ single crystals. The electrode / electrolyte interface conductivity increased with 1/4 powers ofp(H₂) andp(H₂O) on both ceria coated and non-coated YSZ. The interface conductivity was significantly improved on a thicker ceria coating surface than 1 μm. The effective electrode reaction radius also increased in a thick ceria coating. The¹⁸O/¹⁶O exchange experiment at low oxygen partial pressure revealed that the oxygen surface exchange rate of ceria is not high compared with that of YSZ. It can be concluded that the bulk ionic conduction of ceria makes a more effective contribution to the electrode reaction than the surface catalytic activity in H₂-H₂O atmosphere. Paper presented at the 4th Euroconference on Solid State Ionics, Renvyle, Galway, Ireland, Sept. 13–19, 1997     

Additional information from the measurements by Hushfar,Rogers and Stair of the infrared chemiluminescence from the reaction N+O2 → NO+O

Hushfar et al. measured the chemiluminescent first-overtone radiation at 2.7 μm from the reaction N+O₂ → NO+O, which is followed by the fast quenching reaction N+O → N₂+O. From the rate of overtone photons per O₂ molecule and the spectrum approximated by a Boltzmann distribution, they computed ?₂, the number of overtone photons per NO-forming reaction in the low-density limit when no quenching occurs. We show ?₂ is essentially independent of the spectrum, infer the limits on the fundamental-band photon efficiency, ?₁, and obtain the initial population distribution of NO(2 ? χ ? 6).     

Theoretical vibrational terms and rotational constants for the 15N substituted isotopologues of N2O calculated using normal hyperspherical coordinates

Variational calculations of the vibrational terms G_v and rotational constants B_v of the ¹⁴N¹⁵N¹⁶O, ¹⁵N¹⁴N¹⁶O and ¹⁵N¹⁵N¹⁶O isotopologues of nitrous oxide are carried out using normal hyperspherical coordinates. The Morse-cosine potential energy surface for N₂O previously determined by the authors by fitting to a set of experimental vibrational frequencies is employed. The G_v and B_v spectroscopic constants calculated for the ¹⁵N substituted isotopologues show an satisfactory agreement with those experimentally observed for a large number of vibrational bands of these isotopologues recently measured. Predicted calculated values of these spectroscopic constants for unobserved vibrational bands of the ¹⁵N substituted isotopologues are given in order to be of help in the identification and characterization of such bands, as a complement to the use of global effective Hamiltonians.     

The oxidation of CO by O2, NO and N2O on polycrystalline platinum under knudsen conditions: A comparison

The oxidation of CO by O₂, NO and N₂O can be described by quite similar elementary reaction sequences which differ only in the way atomic oxygen is generated on the surface. This difference is sufficient to explain the observed experimental results.     

Comparison of fractal analysis methods in the study of fractals with independent branching

The notion of dimension as a quantitative characteristic of space geometry is discussed. It is supposed that hadrons created in interactions between particles and nuclei can be considered sets of points possessing fractal properties in the three-dimensional phase space (p _T , η, ?). The Hausdorff-Besicovich dimension D _F is considered the most natural characteristic for determining the fractal dimension. Different methods for determining the fractal dimension are compared: box counting (BC), P-adic coverage (PaC), and system of equations of P-adic coverage (SePaC). A procedure for choosing optimum values of parameters of the considered methods is presented. These parameters are shown to be able to reconstruct the fractal dimension D _F , number of levels N _lev, and fractal structure with maximal efficiency. The features of the PaC- and SePaC-methods in the analysis of fractals with independent branching are noted.     

Rovibrational spectrum and potential energy surface of the N2-N2O van der Waals complex

The rovibrational spectrum of the N₂-N₂O van der Waals complex has been recorded in the N₂O ν₁ region (∼1285 cm⁻¹) using a tunable diode laser spectrometer to probe a pulsed supersonic slit jet. The observed transitions together with the data observed previously in the N₂O ν₃ region are analyzed using a Watson S-reduced asymmetric rotor Hamiltonian. The rotational and centrifugal distortion constants for the ground and excited vibrational states are accurately determined. The band-origin of the spectrum is determined to be 1285.73964(14) cm⁻¹. A restricted two-dimensional intermolecular potential energy surface for a planar structure of N₂-N₂O has been calculated at the CCSD(T) level of theory with the aug-cc-pVDZ basis sets and a set of mid-bond functions. With the intermolecular distance fixed at the ground state value R = 3.6926 Å, the potential has a global minimum with a well depth of 326.64 cm⁻¹ at θ_N2 = 11.0° and θ_N2O = 84.3° and has a saddle point with a barrier height of 204.61 cm⁻¹ at θ_N2 = 97.4° and θ_N2O = 92.2°, where θ_N2(θ_N2O) is the enclosed angle between the N-N axis (N-N-O axis) and the intermolecular axis.     

Nitrogen release during reaction of coal char with O2, CO2, and H2O

The chemistry of char-N release and conversion to nitrogen-containing products has been probed by studying its release and reactions with O₂, CO₂, and H₂O. The experiments were performed in a fixed bed flow reactor at pressures of up to 1.0 MPa. The results show that the major nitrogen-containing products observed depend on the reactant gas; with O₂, NO, and N₂ being the major species observed. Char-N reaction with CO₂ produces N₂ with very high selectivity over a broad range of pressures and CO₂ concentrations, and reaction with H₂O gives rise to HCN, NH₃, and N₂. Observed distributions of nitrogen-containing products are little affected by pressure when O₂ and CO₂ are the reactant gases, but increasing pressures in the reaction with H₂O results in the formation of increasing proportions of NH₃. Formation of NH₃ is also promoted by increasing concentrations of H₂O in the feed gas. The results suggest that NO and HCN are primary products when O₂ and H₂O, respectively, are used as the reactant gases, and that the other observed products arise from interactions of these primary products with the char surface.     

The adsorption and decomposition of N2O on nickel (100)

The adsorption and decomposition of N₂O on clean and oxygen covered Ni(100) surfaces has been studied using a combination of Auger electron spectroscopy (AES) and molecular beam relaxation spectroscopy (MBRS) techniques. As observed in a previous study of this reaction on the Ni(110) surface, N₂O decomposes to yield N₂ gas and adsorbed O at temperatures between 200 and 800 K. Measurements at temperatures below 200 K led to the identification of two weakly adsorbed precursor species, one on clean surfaces and the other on surfaces covered with 0.25 ML of adsorbed O. The adsorption rate constants measured for these two species are consistent with values inferred indirectly in the previous study.     

Adsorption of N2O on Ru(001)

The adsorption of N₂O on Ru(001) at ~ 100K has been studied using X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and thermal desorption spectroscopy (TDS). At low exposures, N₂O partly dissociates leaving atomic oxygen on the surface and desorbing N₂. With increasing N₂O exposures, molecular adsorption becomes dominant. He II UPS of the gas phase, solid and monolayer adsorbed molecular N₂O are compared. To within experimental error, the peak spacings in all three are the same. The distributions of intensities in the gas and solid phase spectra are the same. In the monolayer spectra, the 7~σ (terminal nitrogen lone pair) orbital intensity is decreased significantly indicating that it is more strongly coupled to the surface than the other valence orbitals. No molecular N₂O remains after heating to above 180 K and no detectable amount of dissociated nitrogen appears. Molecularly adsorbed N₂O is easily dissociated by an electron beam to give N₂(g), NO(g) and O(a).     

Analysis of fractals with dependent branching by box counting, P-adic coverages, and systems of equations of P-adic coverages

Concept of the dimension of space-time in the general relativity theory and quantum theory is discussed. It is emphasized that the dimension of a discrete space can be defined based on the Hausdorff measure. The noninteger dimension is a typical characteristic of a fractal. The process of hadron formation in interactions between high-energy particles and nuclei is supposed to possess fractal properties. The following methods for analyzing fractals are considered: box counting (BC), method of P-adic coverages (PaC), and method of systems of equations of P-adic coverages (SePaC), for determining the fractal dimension. A comparative analysis of fractals with dependent branching is performed using these methods. We determine the optimum values of parameters permitting one to determine the fractal dimension D _F , number of levels N _lev, and the fractal structure with maximal efficiency. It is noted that the SePaC method has advantages in analyzing fractals with dependent branching.     

An X-ray photoelectron spectroscopic study of the adsorption of N2, NH3, NO,and N2O on dysprosium

The adsorption of N₂, NH₃, NO, and N₂O onto clean polycrystalline dysprosium at 300 and 115 K and the changes undergone by the adsorbed species upon heating from 115 K have been investigated using X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). At 115 K, N₂ adsorbs dissociatively, vielding two peaks in the N 1s region at 396.2 and 398.2 eV corresponding respectively to a nitride and to chemisorbed nitrogen N(a). No peaks corresponding to molecularly adsorbed N₂ (BE 400.2 eV [10]) were observed. Upon heating the sample the N(a) is converted into the nitride species, as evidenced by a decrease in the 398.2 eV peak and a corresponding increase in the 396.2 eV peak. At a warm-up temperature of 300 K, the N(a) species accounts for only ～10% of the total nitrogen on the surface. Ammonia adsorbed at 115 K shows three distinct peaks, at 401.7, 399.3 and 396.2 eV, corresponding to molecular, partly dissociated, and completely dissociated (nitride) ammonia. Upon heating multilayer ammonia to 175 K, it desorbs to leave predominantly the peak corresponding to the partly dissociated species. Upon further heating the molecular and partly dissociated ammonia is converted into the nitride species. At 400 K only nitride-type nitrogen remains on the surface. The adsorption of NO and N₂O at 115 K is predominantly dissociative. NO has N 1s peaks at 403.1 and 396.3 eV corresponding possibly to molecularly adsorbed NO, and to nitride species. After N₂O adsorption there is very little nitrogen on the surface. Adsorption of N₂ and NO at 300 K yields only the peak at 396.2 eV, whereas NH₃ yields, in addition to this peak, a small intensity (～20%) of the peak at 398.2 eV (partly dissociated ammonia).     

Adsorption and interaction of CO and NO on Pt(410): I. TPD studies

NO and CO adsorption and the NO/CO reaction on Pt(410) are studied by TPD. NO is found to dissociate on Pt(410) at 120 K, but it reacts to form N₂O at higher exposures. The N₂O desorbs in two peaks at 135 and 150 K. CO adsorbs molecularly, and desorbs in 5 peaks at 550, 500, 450, 380 and about 130 K. CO is also found to dissociate upon heating, leaving a carbon residue on the surface which changes the TPD spectra. The NO/CO reaction shows a surface explosion at 360 K. These results provide additional evidence that Pt(410) has unusual reactivity, as predicted by Banholzer, Park, Mak and Masel, Surface Sci. 128 (1983) 176.     

The gas-phase pyrolysis of methyl azidoformate in the absence and presence of water: a theoretical study

The potential energy surface profiles for the gas-phase pyrolysis of methyl azidoformate (MA, CH₃OC(O)N₃) in the absence and presence of one water molecule have been investigated by ab initio methods at CCSD(T)/6-311++G(2df,2pd)//MP2(full)/6-311++G(d,p)+0.95×ZPE levels of theory. Three types of mechanisms are discussed for the gas-phase decomposition of CH₃OC(O)N₃. Ab initio calculations show that a four-membered-ring intermediate can be formed by the stepwise routes. The resulting intermediate can undergo two competitive decomposition channels to generate the major products CO₂?+?CH₂?=?NH and HNCO?+?HC(O)H. The calculated results are in qualitative agreement with the observed experimental data. However, CH₃ONCO can be produced from the Curtius-type rearrangement route. This is an intriguing finding in this study. Moreover, the effect of one water molecule on the gas-phase pyrolysis of MA has been also explored. We find that the relative energy of the hydrated transition states is effectively lowered when water is added to the reaction. However, the estimated rate constant at 625?K for the naked reaction is about 30 times faster than the reaction with water. Thus, a single water molecule cannot play an important role in the thermal decomposition of MA.     

Experimental and numerical study of product gas and N2O emission characteristics of ammonia/hydrogen/air premixed laminar flames stabilized in a stagnation flow

In order to achieve carbon neutrality, the use of ammonia as a fuel for power generation is highly anticipated. The utilization of a binary fuel consisting of ammonia and hydrogen can address the weak flame characteristics of ammonia. In this study, the product gas characteristics of ammonia/hydrogen/air premixed laminar flames stabilized in a stagnation flow were experimentally and numerically investigated for various equivalence ratios for the first time. A trade-off relationship between NO and unburnt ammonia was observed at slightly rich conditions. At lean conditions, NO reached a maximum value of 8,700 ppm, which was larger than that of pure ammonia/air flames. The mole fraction of nitrous oxide (N₂O) which has large global warming potential rapidly increased around the equivalence ratio of 0.6, which was attributed to the effect of a decrease in flame temperature downstream of the reaction zone owing to heat loss to the stagnation wall. To understand this effect further, numerical simulations of ammonia/hydrogen/air flames were conducted using the stagnation flame model for various equivalence ratios and stagnation wall temperatures. The results show that the important reactions for N₂O production and reductions are NH +NO = N₂O + H, N₂O + H = N₂ + OH, and N₂O (+M) = N₂ + O (+M). A decrease in flame temperature in the post flame region inhibited N₂O reduction through N₂O (+M) = N₂ + O (+M) because this reaction has a large temperature dependence, and thus N₂O was detected as a product gas. N₂O is reduced through N₂O (+M) = N₂ + O (+M) in the post flame region if the stagnation wall temperature is sufficiently high. On the other hand, it was clarified that an increase in equivalence ratio enhances H radical production and promotes N₂O reduction by H radical through the reaction of N₂O + H = N₂ + OH.     

The reduction of NO with D2 over the (110) surface of iridium

The heterogeneously catalyzed reaction between NO and D₂ to produce N₂, ND₃ and D₂O over Ir(110) was investigated under ultra-high vacuum conditions for partial pressures of the reactants between 5 × 10^?8 and 1 × 10^?6Torr, total pressures between 10^?7 and 10^?6 Torr, and surface temperatures between 300 and 1000 K. Mass spectrometry, LEED, UPS, XPS and AES measurements were used to study this reacting system. In addition, the competitive coadsorption of NO and deuterium was investigated via thermal desorption mass spectrometry and contact potential difference measurements to gain further insight into the observed steady state rates of reaction. Depending on the ratio of partial pressures ($\text{R }\text{P}{}_{\mathrm{<mtext>D</mtext>}}{}_2\text{P}{}_{\mathrm{<mtext>NO</mtext>}}$), the rate of reduction of NO to N₂ shows a pronounced enhancement when the surface is heated above a critical temperature. As the surface is cooled, the rate maintains a high value independent of temperature until a lower critical temperature is reached, where the rate drops precipitously. This hysteresis is due to a change in the structure and composition of the surface. For sufficiently large values of R and for an “activated” surface, N₂ and ND₃ are produced competitively between 470 and 630 K. Empirical models of the different regimes of the steady state reaction are presented with interpretations of these models.