Experimental and kinetic modeling of the reduction of NO by isobutane in a Jsr at 1 atm

A kinetic study of the reduction of nitric oxide (NO) by isobutane in simulated conditions of the reburning zone was carried out in a fused silica jet‐stirred reactor operating at 1 atm, at temperatures ranging from 1100 to 1450 K. In this new series of experiments, the initial mole fraction of NO was 1000 ppm, that of isobutane was 2200 ppm, and the equivalence ratio was varied from 0.75 to 2. It was demonstrated that for a given temperature, the reduction of NO is favored when the temperature is increased and a maximum NO reduction occurs slightly above stoichiometric conditions. The present results generally follow those reported in previous studies of the reduction of NO by C₁ to C₃ hydrocarbons or natural gas as reburn fuel. A detailed chemical kinetic modeling of the present experiments was performed using an updated and improved kinetic scheme (979 reversible reactions and 130 species). An overall reasonable agreement between the present data and the modeling was obtained. Furthermore, the proposed kinetic mechanism can be successfully used to model the reduction of NO by ethylene, ethane, acetylene, a natural gas blend (methane‐ethane 10:1), propene, and HCN. According to this study, the main route to NO reduction by isobutane involves ketenyl radical. The model indicates that the reduction of NO proceeds through the reaction path: iC₄H₁₀ → C₃H₆ → C₂H₄ → C₂H₃ → C₂H₂ → HCCO; HCCO + NO → HCNO + CO and HCN + CO₂; HCNO + H → HCN → NCO → NH; NH + NO → N₂ and NH + H → followed by N + NO → N₂; NH + NO → N₂O followed by N₂O + H → N₂. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 365–377, 2000     

Theoretical study of reactions of N2O with NO and OH radicals

The reactions of N₂O with NO and OH radicals have been studied using ab initio molecular orbital theory. The energetics and molecular parameters, calculated by the modified Gaussian-2 method (G2M), have been used to compute the reaction rate constants on the basis of the TST and RRKM theories. The reaction N₂O + NO → N₂ + NO₂ (1) was found to proceed by direct oxygen abstraction and to have a barrier of 47 kcal/mol. The theoretical rate constant, k₁ = 8.74 × 10⁻¹⁹ × T^2.23 exp (−23,292/T) cm³ molecule⁻¹ s⁻¹, is in close agreement with earlier estimates. The reaction of N₂O with OH at low temperatures and atmospheric pressure is slow and dominated by association, resulting in the HONNO intermediate. The calculated rate constant for 300 K ≤ T ≤ 500 K is lower by a few orders than the upper limits previously reported in the literature. At temperatures higher than 1000 K, the N₂O + OH reaction is dominated by the N₂ + O₂H channel, while the HNO + NO channel is slower by 2–3 orders of magnitude. The calculated rate constants at the temperature range of 1000–5000 K for N₂O + OH → N₂ + O₂H (2A) and N₂O + OH → HNO + NO (2B) are fitted by the following expressions: in units of cm³ molecule ⁻¹s⁻¹. Both N₂O + NO and N₂O + OH reactions are confirmed to enhance, albeit inefficiently, the N₂O decomposition by reducing its activation energy. © 1996 John Wiley & Sons, Inc.     

A theoretical ab initio study on the H(2)NO + O(3) reaction

The deviation of the NH(2) pseudo-first-order decay Arrhenius plots of the NH(2) + O(3) reaction at high ozone pressures measured by experimentalists, has been attributed to the regeneration of NH(2) radicals due to the subsequent reactions of the products of this reaction with ozone. Although these products have not yet been characterized experimentally, the radical H(2)NO has been postulated, because it can regenerate NH(2) radicals through the reactions: H(2)NO + O(3) --> NH(2) + O(2) and H(2)NO + O(3) --> HNO + OH + O(2). With the purpose of providing a reasonable explanation from a theoretical point of view to the kinetic observed behaviour of the NH(2) + O(3) system, we have carried ab initio electronic structure calculations on both H(2)NO + O(3) possible reactions. The results obtained in this article, however, predict that of both reactions proposed, only the H(2)NO + O(3) --> NH(2) + O(2) reaction would regenerate indeed NH(2) radicals, explaining thus the deviation of the NH(2) pseudo-first-order decay observed experimentally.     

O + NNH: A possible new route for NOX formation in flames

We propose a new high temperature pathway for NO formation that involves the reaction of NNH with oxygen atoms. This reaction forms the HNNO* energized adduct via a rapid combination reaction; HNNO* then rapidly dissociates to NH + NO. The rate constant for O + NNH ? NH + NO is calculated via a QRRK chemical activation analysis to be 3.3 × 10¹⁴ T^?0.23exp(+510/T) cm³ mol^?1 s^?1. This reaction sequence can be an important or even major route to NO formation under certain combustion conditions. The presence of significant quantities of NNH results from the reaction of H with N₂. The H + N₂ ? NNH reaction is only ca. 6 kcal/mol endothermic with a relatively low barrier. The reverse reaction, NNH dissociation, has been reported in the literature to be enhanced by tunneling. Our analysis of NNH dissociation indicates that tunneling dominates. We report a two-term rate constant for NNH dissociation: 3.0 × 10⁸ + [M] {1.0 × 10¹³T^0.5exp(?1540/T)} s^?1. The first term accounts for pressure-independent tunneling from the ground vibrational state, while the second term accounts for collisional activation to higher vibration states from which tunneling can also occur. ([M] is the total concentration in units of mol cm^?3.) Use of this dissociation rate constant and microscopic reversibility results in a large rate constant for the H + N₂ reaction. As a result, we find that NNH ? H + N₂ can be partially equilibrated under typical combustion conditions, resulting in NNH concentrations large enough for it to be important in bimolecular reactions. Our analysis of such reactions suggests that the reaction with oxygen atoms is especially important. © 1995 John Wiley & Sons, Inc.     

Kinetic studies of NHx radical reactions

Combined experimental and theoretical studies of the reactions

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and

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were carried out in the present work. The rate constant of reaction (1) was investigated in the temperature range 293 K T612 K using the laser photolysis continuous-wave laser-induced fluorescence technique for the production and detection of NH₂. The results are well described by k₁(T)=5.43×T^−4.02 exp(−1034K/T) cm³ molecule⁻¹ s⁻¹. Stationary points on the potential energy surface were characterized using the gaussian-2 (G2) ab initio method. The surface is complex, with hydrogen-atom transfers and cis—trans isomerization connecting five stable adduct species.The product distribution of reaction (2) was studied at room temperature using the discharge flow technique with mass spectrometric detection of the reaction products. Measured branching fractions for the production of N₂O+H(D) were k_2a/k₂ = 0.84±0.4 for NH+NO and k_2a/k₂ = 0.87±0.17 for ND+NO. Stationary points on the ground ²A′ surface were calculated using the G2 method. The transition state energy for the dissociation of the cis isomer into H+N₂O was found to be lower than the transition state energy for dissociation into OH+N₂. Additionally, trans-HNNO was found to isomerize to cis-HNNO before dissociation.For reaction (3), the molecular properties of all relevant intermediates and transition states on the ground state potential energy surface were determined using the G2 method. The results predict the formation of three intermediates, H₂NO, trans-HNOH and cis-HNOH, all exothermic relative to the reactants. The transition states separating these intermediates from one another and their products (H₂+NO, H+HNO(¹A′) or NH+OH) were also characterized, several for the first time.     

Kinetic modeling of the reduction of nitric oxide in combustion products by isocyanic acid

The RAPRENO_x process for NO reduction in combustion products involves reaction of nitric oxide with isocyanic acid. We have developed a mechanism for the gas-phase reaction of isocyanic acid with nitric oxide in the presence of various amounts of O₂, H₂O, and CO. Kinetics calculations using the mechanism are compared with the experimental data of Siebers and Caton, and the model reproduces all trends of these data. Sensitivity and rate-of-production analyses show that the reactions of HNCO with OH, O, and H play a major role in the NO-removal process and that NO removal occurs primarily by reaction of NO with NCO to form N₂O, which subsequently reacts slowly to form N₂. The overall reaction is critically dependent on production of radicals. When O₂, H₂O, and CO are present, the radicals are supplied by the moist-CO chain branching sequence. When any of these species is absent, radicals must be supplied by other reactions, principally the N₂O decomposition reaction and the reaction of the NH₂ radical with NO.     

The double H‐atom acceptability of the P=O group in new XP(O)(NHCH2C6H4‐2‐Cl)2 phosphoramidates [X = C6H5O– and CF3C(O)NH–]: a database analysis of compounds having a P(O)(NHR) group

In the hydrogen‐bond patterns of phenyl bis(2‐chlorobenzylamido)phosphinate, C₂₀H₁₉Cl₂N₂O₂P, (I), and N,N′‐bis(2‐chlorobenzyl)‐N′′‐(2,2,2‐trifluoroacetyl)phosphoric triamide, C₁₆H₁₅Cl₂F₃N₃O₂P, (II), the O atoms of the related phosphoryl groups act as double H‐atom acceptors, so that the P=O...(H—N)₂ hydrogen bond in (I) and the P=O...(H—N_amide)₂ and C=O...H—N_C(O)NHP(O) hydrogen bonds in (II) are responsible for the aggregation of the molecules in the crystal packing. The presence of a double H‐atom acceptor centre is a result of the involvement of a greater number of H‐atom donor sites with a smaller number of H‐atom acceptor sites in the hydrogen‐bonding interactions. This article also reviews structures having a P(O)NH group, with the aim of finding similar three‐centre hydrogen bonds in the packing of phosphoramidate compounds. This analysis shows that the factors affecting the preference of the above‐mentioned O atom to act as a double H‐atom acceptor are: (i) a higher number of H‐atom donor sites relative to H‐atom acceptor centres in molecules with P(=O)(NH)₃, (N)P(=O)(NH)₂, C(=O)NHP(=O)(NH)₂ and (NH)₂P(=O)OP(=O)(NH)₂ groups, and (ii) the remarkable H‐atom acceptability of this atom relative to the other acceptor centre(s) in molecules containing an OP(=O)(NH)₂ group, with the explanation that the N atom bound to the P atom in almost all of the structures found does not take part in hydrogen bonding as an acceptor. Moreover, the differences in the H‐atom acceptability of the phosphoryl O atom relative to the O atom of the alkoxy or phenoxy groups in amidophosphoric acid esters may be illustrated by considering the molecular packing of compounds having (O)₂P(=O)(NH) and (O)P(=O)(NH)(N)groups, in which the unique N—H unit in the above‐mentioned molecules almost always selects the phosphoryl O atom as a partner in forming hydrogen‐bond interactions. The P atoms in (I) and (II) are in tetrahedral coordination environments, and the phosphoryl and carbonyl groups in (II) are anti with respect to each other (the P and C groups are separated by one N atom). In the crystal structures of (I) and (II), adjacent molecules are linked via the above‐mentioned hydrogen bonds into a linear arrangement parallel to [100] in both cases, in (I) by forming R₂²(8) rings and in (II) through a combination of R₂²(10) and R₂¹(6) rings.     

Study of the HNO + HNO and HNO + NO reactions by intracavity laser spectroscopy

Flash photolysis of CH₃CHO and H₂CO in the presence of NO has been investigated by the intracavity laser spectroscopy technique. The decay of HNO formed by the reaction HCO + NO → HNO + CO was studied at NO pressures of 6.8–380 torr. At low NO pressure HNO was found to decay by the reaction HNO + HNO → N₂O + H₂O. The rate constant of this reaction was determined to be k₁ = (1.5 ± 0.8) × 10^?15 cm³/s. At high NO pressure the reaction HNO + NO → products was more important, and its rate constant was measured to be k₂ = (5 ± 1.5) × 10^?19 cm³/s. NO₂ was detected as one of the products of this reaction. Alternative mechanisms for this reaction are discussed.     

Modeling the thermal DENOx process in flow reactors. Surface effects and Nitrous Oxide formation

We have investigated the impact of surface reactions such as NH₃ decomposition and radical adsorption on quartz flow reactor data for Thermal DeNO_x using a model that accounts for surface chemistry as well as molecular transport. Our calculations support experimental observations that surface effects are not important for experiments carried out in low surface to volume quartz reactors. The reaction mechanism for Thermal DeNO_x has been revised in order to reflect recent experimental results. Among the important changes are a smaller chain branching ratio for the NH₂ + NO reaction and a shorter NNH lifetime than previously used in modeling. The revised mechanism has been tested against a range of experimental flow reactor data for Thermal DeNO_x with reasonable results. The formation of N₂O in Thermal DeNO_x has been modelled and calculations show good agreement with experimental data. The important reactions in formation and destruction of N₂O have been identified. Our calculations indicate that N₂O is formed primarily from the reaction between NH and NO, even though the NH₂ + NO₂ reaction possibly contributes at lower temperatures. At higher temperatures N₂O concentrations are limited by thermal dissociation of N₂O and by reaction with radicals, primarily OH. © 1994 John Wiley & Sons, Inc.     

The reaction of the hydroxyl radical with acetylene

The reaction of OH with acetylene was studied in a discharge flow system at room temperature. OH was generated by the reaction of atomic hydrogen with NO₂ and was monitored throughout the reaction using ESR spectroscopy. Mass-spectrometric analysis of the reaction products yielded the following results: (1) less than 3 molecules of OH were consumed, and less than 2 molecules of H₂O were formed for every molecule of acetylene that reacted; (2) CO was identified as the major carbon-containing product; (3) NO, formed in the generation of OH, reacted with a reaction intermediate to give among other products N₂O. These observations placed severe limitations on the choice of a reaction mechanism. A mechanism containing the reaction OH + C₂H₂ → HC₂O + H₂ better accounted for the experimental results than one involving the abstraction reaction OH + C₂H₂ → C₂H + H₂O. The rate constant for the initial reaction was measured as 1.9 ± 0.6 × 10^?13 cm³ molecule^?1 sec^?1.     

Homogeneous catalytic reduction of NO in the presence of Co(III) complexes

Several Co(III) complexes have been found to be active catalysts in the reaction of NO with n-butylamine. N₂ and N₂O were identified as gaseous products of the reduction. Isotope scrambling in N₂ and N₂O in the experiments with N-15 enriched NO suggests two types of stoichiometric processes. The catalysis is interpreted by formation of a NO-complex as intermediate.     

Modelling reduction of nitric oxide by hydrogen over supported catalysts

A mean field model for NO oxidation with H₂ over supported catalysts is proposed and solved numerically. The model is composed of a system of PDEs subject to nonclassical conjugate conditions at the catalyst–support interface and includes the bulk diffusion of reactants and reaction products and surface diffusion of all intermediate products. The influence of the particle jumping rate constants via the catalyst–support interface and reaction rate constants on the evolution of the reactivity of the catalyst surface is investigated. It is shown that the conversion rates (turnover frequencies) of NO and H₂ into products, N₂, H₂O, NH₃, and N₂O, are nonmonotonous functions of time. The conversion rates of NO and H₂ into N₂ and N₂O can have one or two local maxima, while their conversion rates into H₂O and NH₃ can possess one, two, or three local maxima. The mechanism and conditions for arising of the second maximum are discussed and reaction steps that essentially increase the surface reactivity are indicated.

     

Experimental and kinetic modeling study of the effect of sulfur dioxide on the mutual sensitization of the oxidation of nitric oxide and methane

New experimental results were obtained for the mutual sensitization of the oxidation of NO and methane in a fused silica jet‐stirred reactor operating at 10⁵ Pa, over the temperature range 800–1150 K. The effect of the addition of sulfur dioxide was studied. Probe sampling followed by online FTIR analyses and off‐line GC‐TCD/FID analyses allowed the measurement of concentration profiles for the reactants, stable intermediates, and final products. A detailed chemical kinetic modeling of the present experiments was performed. An overall reasonable agreement between the present data and modeling was obtained. According to the present modeling, the mutual sensitization of the oxidation of methane and NO proceeds via the NO to NO₂ conversion by HO₂ and CH₃O₂. The conversion of NO to NO₂ by CH₃O₂ is more important at low temperatures (800 K) than at higher temperatures (850–900 K) where the production of NO₂ is mostly due to the reaction of NO with HO₂. The NO to NO₂ conversion is favored by the production of the HO₂ and CH₃O₂ radicals yielded from the oxidation of the fuel. The production of OH resulting from the oxidation of NO accelerates the oxidation of the fuel: NO + HO₂ → OH+ NO₂ followed by OH + CH₄→ CH₃. In the lower temperature range of this study, the reaction further proceeds via CH₃ + O₂→ CH₃O₂; CH₃O₂+ NO → CH₃O + NO₂. At higher temperatures, the production of CH₃O involves NO₂: CH₃+ NO₂→ CH₃O. This sequence of reactions is followed by CH₃O → CH₂O + H; CH₂O +OH → HCO; HCO + O₂ → HO₂ and H + O₂ → HO₂ → CH₂O + H; CH₂O +OH → HCO; HCO + O₂ → HO₂ and H + O₂ → HO₂. The data and the modeling show that unexpectedly, SO₂ has no measurable effect on the kinetics of the mutual sensitization of the oxidation of NO and methane in the present conditions, whereas it frequently acts as an inhibitor in combustion. This result was rationalized via a detailed kinetic analysis indicating that the inhibiting effect of SO₂ via the sequence of reactions SO₂+H → HOSO, HOSO+O₂ → SO₂+HO₂, equivalent to H+O₂?HO₂, is balanced by the reaction promoting step NO+HO₂ → NO₂+OH. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 406–413, 2005     

Two new XP(O)[NHC(CH3)3]2 phosphoramidates,with X = (CH3)2N and [(CH3)3CNH]2P(O)(O)

In N,N′‐di‐tert‐butyl‐N′′,N′′‐dimethylphosphoric triamide, C₁₀H₂₆N₃OP, (I), and N,N′,N′′,N′′′‐tetra‐tert‐butoxybis(phosphonic diamide), C₁₆H₄₀N₄O₃P₂, (II), the extended structures are mediated by P(O)...(H—N)₂ interactions. The asymmetric unit of (I) consists of six independent molecules which aggregate through P(O)...(H—N)₂ hydrogen bonds, giving R₂¹(6) loops and forming two independent chains parallel to the a axis. Of the 12 independent tert‐butyl groups, five are disordered over two different positions with occupancies ranging from to . In the structure of (II), the asymmetric unit contains one molecule. P(O)...(H—N)₂ hydrogen bonds give S(6) and R₂²(8) rings, and the molecules form extended chains parallel to the c axis. The structures of (I) and (II), along with similar structures having (N)P(O)(NH)₂ and (NH)₂P(O)(O)P(O)(NH)₂ skeletons extracted from the Cambridge Structural Database, are used to compare hydrogen‐bond patterns in these families of phosphoramidates. The strengths of P(O)[...H—N]_x (x = 1, 2 or 3) hydrogen bonds are also analysed, using these compounds and previously reported structures with (N)₂P(O)(NH) and P(O)(NH)₃ fragments.     

Local structure of highly dispersed lead species incorporated within zeolite: experimental and theoretical studies

XAFS (both XANES and FT-EXAFS) measurements revealed that the Pb²⁺ /ZSM-5 catalyst prepared from precursor H-ZSM-5 by a conventional ion-exchange method includes a highly dispersed 3-fold coordinated Pb²⁺ ion species within the zeolite framework. UV-irradiation of Pb²⁺ /ZSM-5 led to effective decomposition of NO and N₂O producing N₂. The photocatalytic decomposition of NO is found to be slightly preferable than that of N₂O. The isolated Pb²⁺ ions play a significant role in the decomposition of pollutant NO _x . Ab initio and DFT quantum chemical studies at the HF/Lanl2dz and B3PW91/Lanl2dz levels further shed light on local structures of the Pb²⁺ active site of lead-containing zeolites, as well as on their interactions with pollutant NO and N₂O molecules. In agreement with experiments, 3-fold coordination was found to be the most favorable state for the Pb²⁺ site within the zeolite framework.     

Effects of nitric oxide on the thermal decomposition of methyl nitrite: Overall kinetics and rate constants for the HNO + HNO and HNO + 2NO reactions

The effects of NO on the decomposition of CH₃ONO have been investigated in the temperature range 450–520 K at a constant pressure of 710 torr using He as buffer gas. The measured time-dependent concentration profiles of CH₃ONO, NO, N₂O, and CH₂O can be quantitatively accounted for with a general mechanism consisting of various reactions of CH₃O, HNO, and (HNO)₂. The results of kinetic modeling with sensitivity analyses indicate that the disappearance rate of CH₃ONO is weakly affected by NO addition, whereas that of the HNO intermediate strongly altered by the added NO. In the presence of low NO concentrations, the modeling of N₂O yields leads to the rate constant for the bimolecular reaction, HNO + HNO → N₂O + H₂O (25): In the presence of high NO concentrations (P_NO > 50 torr), the modeling of CH₂O yields gives the rate constant for the termolecular radical formation channel, HNO + 2NO → HN₂O + NO₂ (35): Discussion on the mechanisms for reactions (25) and (35), and the alkyl homolog of (35), RNO + 2NO, is presented herein. © John Wiley & Sons, Inc.     

Thermal reduction of NO by H2: Kinetic measurement and computer modeling of the HNO + NO reaction

The kinetics and mechanism of the thermal reduction of NO by H₂ have been investigated by FTIR spectrometry in the temperature range of 900 to 1225 K at a constant pressure of 700 torr using mixtures of varying NO/H₂ ratios. In about half of our experimental runs, CO was introduced to capture the OH radical formed in the system with the well-known, fast reaction, OH + CO → H + CO₂. The rates of NO decay and CO₂ formation were kinetically modeled to extract the rate constant for the rate-controlling step, (2) HNO + NO → N₂O + OH. Combining the modeled values with those from the computer simulation of earlier kinetic data reported by Hinshelwood and co-workers (refs. [3] and [4]), Graven (ref.[5]), and Kaufman and Decker (ref. [6]) gives rise to the following expression: . This encompasses 45 data points and covers the temperature range of 900 to 1425 K. RRKM calculations based on the latest ab initio MO results indicate that the reaction is controlled by the addition/stabilization processes forming the HN(O)NO intermediate at low temperatures and by the addition/isomerization/decomposition processes producing N₂O + OH above 900 K. The calculated value of k₂ agrees satisfactorily with the experimental result. © 1995 John Wiley & Sons, Inc.     

A theoretical study of the reaction of N(4 S) with nitrogen dioxide on the N2O2 potential energy surface

The reaction of N(⁴ S) radical with NO₂ molecule has been studied theoretically using density functional theory and ab initio quantum chemistry method. Both singlet and triplet electronic state [N₂O₂] potential energy surfaces (PESs) are calculated at the G3B3 level of theory. Also, the highly cost-expensive coupled-cluster theory including single and double excitations and perturbative inclusion of triple excitations CCSD(T)/cc-pVTZ single-point energy calculation is performed on the basis of the geometries obtained at the Becke??s three parameter Lee-Yang-Parr B3LYP/6-311++G(d, p) level. On the singlet PES of the title reaction, it is shown that the most feasible pathway should be as follows. The atomic radical N attacking the NO bond of the NO₂ molecule first to form the adduct 1 N(NO)O, followed by one of the NO bond broken to give intermediate 2 ONNO, and then to the major products P1 (2NO). On the triplet PES of the title reaction, it is shown that the most favorable pathway should be the atomic radical N attacking the N-atom of NO₂ firstly to form the adduct 7 NN(OO), followed by one of the NO bonds breaking to give intermediate 8 NNOO, and then leading to the major products P2 (O₂ + N₂). As efficient routes to the reduction of NO₂ to form N₂ and O₂ are sought, both kinetic and thermodynamic considerations support the viability of this channel. All the involved transition states for generation of (2NO), (³O + N₂O), and (O₂ + N₂) lie much lower than the reactants. Thus, the novel reaction N + NO₂ can proceed effectively even at low temperatures and it is expected to play a role in both combustion and interstellar processes. The other reaction pathways are less competitive due to thermodynamical or kinetic factors. On the basis of the analysis of the kinetics of all path-ways through which the reactions proceed, we expect that the competitive power of reaction pathways may vary with experimental conditions for the title reaction. The calculated reaction heats of formation are in good agreement with that obtained experimentally.     

The reaction of [Ru(bpy)2(NO)Cl]2+ and [Fe(CN)5NO]2− with benzylamine: coordinated nitrosyl as an oxidizing agent

[Fe(CN)₅NO]^2–, (1) and [Ru(bpy)₂(NO)Cl]²⁺, (2) react with PhCH₂NH₂ to produce mainly N-benzylphenyl-methanimine and PhCN as oxidation products. (PhCH₂)₂NH, PhCH₂Cl and PhCH₂OH are formed as diazotization products. Products derived from the benzyl radical (such as PhMe), are also formed. Since oxidation products are generated even in the absence of oxygen, a mechanism in which the nitrosyl ligand acts as an oxidant is proposed.     

DFT quantum-chemical calculations of nitrogen oxide chemisorption and reactivity on the Cu(100) surface

A DFT quantum-chemical study of NO adsorption and reactivity on the Cu₂₀ and Cu₁₆ metal clusters showed that only the molecular form of NO is stabilized on the copper surface. The heat of monomolecular adsorption was calculated to be ΔH _m = ?49.9 kJ/mol, while dissociative adsorption of NO is energetically unfavorable, ΔH _d = + 15.7 kJ/mol, and dissociation demands a very high activation energy, E _a = + 125.4 kJ/mol. Because of the absence of NO dissociation on the copper surface, the formation mechanism of the reduction products, N₂ and N₂O, is debatable since the surface reaction ultimately leads to N-O bond cleavage. As the reaction occurs with a very low activation energy, E _a = 7.3 kJ/mol, interpretation of the NO direct reduction mechanism is both an important and intriguing problem because the binding energy in the NO molecule is high (630 kJ/mol) and the experimental studies revealed only physically adsorbed forms on the copper surface. It was found that the formation mechanism of the N₂ and N₂O reduction products involves formation (on the copper surface) of the (O_adN-NO_ad) dimer intermediate that is chemisorbed via the oxygen atoms and characterized by a stable N-N bond (r _N-N ～1.3 Å). The N-N binding between the adsorbed NO molecules occurs through electron-accepting interaction between the oxygen atoms in NO and the metal atoms on the “defective” copper surface. The electronic structure of the (O_adN-NO_ad) surface dimer is characterized by excess electron density (ON-NO)^δ? and high reactivity in N-O_ad bond dissociation. The calculated activation energy of the destruction of the chemisorbed intermediate (O_adN-NO_ad) is very low (E _a = 5–10 kJ/mol), which shows that it is kinetically unstable against the instantaneous release of the N₂ and N₂O reduction products into the gas phase and cannot be identified by modern experimental methods of metal surface studies. At the same time, on the MgO surface and in the individual (Ph₃P)₂Pt(O₂N₂) complex, a stable (O_adN-NO_ad) dimer was revealed experimentally.