Nitrous oxide decomposition over Fe-ZSM-5 in the presence of nitric oxide: a comprehensive DFT study

A number of experimental studies have shown recently that ppm-level additions of nitric oxide (NO) enhance the rate of nitrous oxide (N(2)O) decomposition catalyzed by Fe-ZSM-5 at low temperatures. In the present work, the NO-assisted N(2)O decomposition over mononuclear iron sites in Fe-ZSM-5 was studied on a molecular level using density functional theory (DFT) and transition-state theory. A reaction network consisting of over 100 elementary reactions was considered. The structure and energies of potential-energy minima were determined for all stable species, as were the structures and energies of all transition states. Reactions involving changes in spin potential-energy surfaces were also taken into account. In the absence of NO and at temperatures below 690 K, most active single iron sites (Z(-)[FeO](+)) are poisoned by small concentrations of water in the gas phase; however, in the presence of NO, these poisoned sites are converted into a novel active iron center (Z(-)[FeOH](+)). These latter sites are capable of promoting the dissociation of N(2)O into a surface oxygen atom and gas-phase N(2). The surface oxygen atom is removed by reaction with NO or nitrogen dioxide (NO(2)). N(2)O dissociation is the rate-limiting step in the reaction mechanism. At higher temperatures, water desorbs from inactive iron sites and the reaction mechanism for N(2)O decomposition becomes independent of NO, reverting to the reaction mechanism previously reported by Heyden et al. [J. Phys. Chem. B 2005, 109, 1857].     

Kinetics and dynamics of N2 formation in a steady-state N2O + CO reaction on Pd(110)

The N(2)O decomposition kinetics and the product (N(2) and CO(2)) desorption dynamics were studied in the course of a catalyzed N(2)O+CO reaction on Pd(110) by angle-resolved mass spectroscopy combined with cross-correlation time-of-flight techniques. The reaction proceeded steadily above 400 K, and the kinetics was switched at a critical CO/N(2)O pressure ratio. The ratio was about 0.03 at 450 K and reached approximately 0.08 at higher temperatures. Below it, the reaction was first order in CO, and negative orders above it. Throughout the surveyed conditions, the N(2) desorption sharply collimated along about 45 degrees off the normal toward the [001] direction. Desorbing N(2) showed translational temperatures in the range of 2000-5000 K. It is proposed that the decomposition proceeds in N(2)O(a) oriented along the [001] direction. On the other hand, the CO(2) desorption sharply collimated along the surface normal, showing a translational temperature of about 1600 K.     

N2 emission in NO and N2O reduction on Rh(100) and Rh(110)

The angular distribution of desorbing N(2) was studied in both the thermal decomposition of N(2)O(a) on Rh(100) at 60-140 K and the steady-state NO (or N(2)O) + D(2) reaction on Rh(100) and Rh(110) at 280-900 K. In the former, N(2) desorption shows two peaks at around 85 and 110 K. At low N(2)O coverage, the desorption at 85 K collimates at about 66 degrees off normal towards the [001] direction, whereas at high coverage, it sharply collimates along the surface normal. In the NO reduction on Rh(100), the N(2) desorption preferentially collimates at around 71 degrees off normal towards the [001] direction below about 700 K, whereas it collimates predominantly along the surface normal at higher temperatures. At lower temperatures, the surface nitrogen removal in the NO reduction is due to the process of NO(a) + N(a) --> N(2)O(a) --> N(2)(g) + O(a). On the other hand, in the steady-state N(2)O + D(2) reaction on Rh(110), the N(2) desorption collimates closely along the [001] direction (close to the surface parallel) below 340 K and shifts to ca. 65 degrees off normal at higher temperatures. In the reduction with CO, the N(2) desorption collimates along around 65 degrees off normal towards the [001] direction above 520 K, and shifts to 45 degrees at 445 K with decreasing surface temperature. It is proposed that N(2)O is oriented along the [001] direction on both surfaces before dissociation and the emitted N(2) is not scattered by adsorbed hydrogen.     

Role of adsorbed NO in N2O decomposition over iron-containing ZSM-5 catalysts at low temperatures

Transient response and temperature-programmed desorption/reaction (TPD/TPR) methods were used to study the formation of adsorbed NO(x) from N2O and its effect during N2O decomposition to O2 and N2 over FeZSM-5 catalysts at temperatures below 653 K. The reaction proceeds via the atomic oxygen (O)(Fe) loading from N2O on extraframework active Fe(II) sites followed by its recombination/desorption as the rate-limiting step. The slow formation of surface NO(x,ads) species was observed from N2O catalyzing the N2O decomposition. This autocatalytic effect was assigned to the formation of NO(2,ads) species from NO(ads) and (O)(Fe) leading to facilitation of (O)(Fe) recombination/desorption. Mononitrosyl Fe2+(NO) and nitro (NO(2,ads)) species were found by diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) in situ at 603 K when N2O was introduced into NO-containing flow passing through the catalyst. The presence of NO(x,ads) does not inhibit the surface oxygen loading from N2O at 523 K as observed by transient response. However, the reactivity of (O)(Fe) toward CO oxidation at low temperatures (<523 K) is drastically diminished. Surface NO(x) species probably block the sites necessary for CO activation, which are in the vicinity of the loaded atomic oxygen.     

N2O分解反应的蒙特卡罗模拟

N2O作为大气污染物之一早已受到人们的注意，许多人研究了N2O在金属氧化物上的分解反应得到了一些基本的实验事实[1]：（1）反应速率与原料气中N2O的分压近似成比例．(2)在反应初期，N2的生成速率由大变小，O2的生成速率由小变大，达到平衡时，N2生成速率是O2生成速率的二倍．(3)过渡应答法研究表明，把原料气由N2O和He的混合气体切换成纯He后，N2立即停止发生，而O2的生成仍能维持一段时间．基于上述实验事实，人们对Nzo的催化分解反应提出了一些可能的机理，主要有下面两种看法：另一方面，人们还发现几O的催化分解反应是一个振荡…     

N2O分解反应的动力学-Monte Carlo模拟

N2O是一种大气污染物，许多金属氧化物能使它发生分解反应，生成N2和O2．因此，N2O的催化分解反应很早就引起了研究者的兴趣[1]．传统的动力学着重去研究反应速率和一些反应参数，如温度、压力、组成等的关系，往往忽视催化表面的性质，从而使速率方程变得十分复杂．Kobayashi等人利用过渡应答法研究了N2O分解反应的动力学，计算了两个基元步骤的速率常数，但没有给出表面物种浓度变化的更进一步的信息[2]．近年来，MonteCarlo方法在研究表面反应机理和动力学方面已经取得了一些成功[3，4]．我们曾用MonteCarlo方法研究了N2O催化分解…     

N2O分解反应中复杂动力学行为的模拟

在多相催化反应中，自ZO多年前第一次在实验中观察到反应的振荡现象以来，现已发现在许多催化反应中都存在振荡现象山．N。O分解是一个振荡反应问．我们在用MonteCarloMC）方法研究N20分解反应机理时，曾发现反应速率和表面覆盖度在一定条件下都会出现振荡现象＊．非统性理论的研究表明，体系内部只要存在非线性的相互作用就可能表现出一些复杂的动力学行为，如振荡和混饨等．N。O分解反应中的振荡行为，也是由于体系内部非线性相互作用的结果·本文用非线性动力学的方法研究N。O分解反应中表面氧覆盖度的振荡行为，发现振荡在一定条…     

The collimation angle shift of desorbing product N2 in a steady-state N2O+CO reaction on Rh(110)

The angular distribution of desorbing product N2 was studied in N2O decompositions on Rh(110) in the temperature range of 60-700 K. The N2 desorption collimates along 62 degrees -68 degrees off normal toward either the [001] or [001] direction in a transient N2O decomposition below ca. 470 K or in the steady-state N2O+CO reaction above 540 K. In the steady-state reaction at the temperature from ca. 470 to 540 K, however, the collimation angle shifts from 62 degrees to 45 degrees with decreasing surface temperature. This angle shift is ascribed to the steric hindrance by coadsorbed CO because the N2 collimation in transient N2O decomposition at around 65 degrees is recovered in the range of 380-500 K by an abrupt CO pressure drop followed by the decrease in CO coverage. N2O is oriented along the [001] direction before dissociation. A scattering model of the nascent N2 by adsorbed CO is proposed, yielding smaller collimation angles.     

Thermal Decomposition of Ni(II) and Fe(III) Nitrates and their Mixture

The thermal decompositions of nickel(II) nitrate hexahydrate and iron(III) nitrate nonahydrate were followed. It was found that the final decomposition products were NiO at 623 K and Fe2O3 at 523 K, respectively. The two salts exhibited only endothermic peaks and a loss in mass until constant mass was attained. The decomposition reactions and the compounds corresponding to each reaction were established. A heating rate of 1 K min-1 revealed several intermediates; higher heating rates shifted the peaks to higher temperatures. The use of an air flow during decomposition shifted the reactions to lower temperatures. The DTA for the mixed salts was found to be an overlap and the TG a summation of the results for the two individual salts. At 773 K, the decomposition products were composed of three phases: NiO, Fe2O3 and NiFe2O4. When these products were heated to 1773 K, only NiFe2O4 was identified by X-ray diffraction. This revised version was published online in July 2006 with corrections to the Cover Date.     

Formation of the surface NO during N2O interaction at low temperature with iron-containing ZSM-5

Interaction of N2O at low temperatures (473-603 K) with Fe-ZSM-5 zeolites (Fe, 0.01-2.1 wt %) activated by steaming and/or thermal treatment in He at 1323 K was studied by the transient response method and temperature-programmed desorption (TPD). Diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) of NO adsorbed at room temperature as a probe molecule indicated heterogeneity of surface Fe(II) sites. The most intensive bands were found at 1878 and 1891 cm(-1), characteristic of two types mononitrosyl species assigned to Fe2+(NO) involved in bi- and oligonuclear species. Fast loading of atomic oxygen from N2O on the surface and slower formation of adsorbed NO species were observed. The initial rate of adsorbed NO formation was linearly dependent on the concentration of active Fe sites assigned to bi- and oligonuclear species, evolving oxygen in the TPD at around 630-670 K. The maximal coverage of a zeolite surface by NO was estimated from the TPD of NO at approximately 700 K. This allowed the simulation of the dynamics of the adsorbed NO formation at 523 K, which was consistent with the experiments. The adsorbed NO facilitated the atomic oxygen recombination/desorption, the rate determining step during N2O decomposition to O2 and N2, taking place at temperatures > or =563 K.     

N_2O在Cu_xFe_(1-x)Fe_2O_4和Ni_xFe_(1-x)Fe_2O_4复合氧化物催化剂上的分解反应

用共沉淀法制备了一组具有尖晶石结构的Cu-Fe和Ni-Fe复合氧化物,用于有氧条件下催化分解N2O,考察了催化剂组成对催化活性的影响.用N2物理吸附(BET)、X射线衍射(XRD)、H2程序升温还原(H2-TPR)等技术对催化剂进行了结构表征.结果表明:在不同组成的Cu-Fe、Ni-Fe系列复合氧化物催化剂中,Cu Fe2O4和Ni Fe2O4对于N2O分解反应的初活性较高,这是因为Cu Fe2O4和Ni Fe2O4的比表面积较高、晶粒较小,而且其表面氧物种与金属(Cu2+、Fe3+)的化学作用较弱,氧物种易脱除、脱氧量较高.相比较而言,Ni Fe2O4催化剂上的N2O分解活化能低于Cu Fe2O4,Ni Fe2O4的初活性优于Cu Fe2O4.500℃连续反应100 h,Cu Fe2O4上的N2O转化率降至84.9%,而Ni Fe2O4上的N2O转化率一直保持99%,Ni Fe2O4有较高的催化稳定性.     

Protonated clathrate cages enclosing neutral water molecules: (H+)(H2O)21 and (H+)(H2O)28

This paper describes a systematic study on the clathrate structure of (H+)(H2O)21 using tandem mass spectrometry, vibrational predissociation spectroscopy, Monte Carlo simulations, and density functional theory calculations. We produced (H+)(H2O)n from a continuous corona-discharged supersonic expansion and observed three anomalies simultaneously at the cluster temperature near 150 K, including (1) the peak at n=21 is more intense than its neighboring ions in the mass spectrum, (2) the size-dependent dissociation fractions show a distinct drop for the 21-mer, and (3) the infrared spectrum of (H+)(H2O)21 exhibits only a single feature at 3699 cm(-1), corresponding to the free-OH stretching of three-coordinated water molecules. Interestingly, the anomalies appear or disappear together with cluster temperature, indicating close correlation of these three observations. The observations, together with Monte Carlo simulations and density functional theory calculations, corroborate the notion for the formation of a distorted pentagonal dodecahedral (5(12)) cage with a H2O molecule in the cage and a H3O+ ion on the surface for this "magic number" water cluster ion. The dodecahedral cage melts at higher temperatures, as evidenced by the emergence of a free-OH stretching feature at 3717 cm(-1) for the two-coordinated water in (H+)(H2O)21 produced in a warmer molecular beam. Extension of this study to larger clusters strongly suggests that the experimentally observed isomer of (H+)(H2O)28 is most likely to consist of a distorted protonated pentakaidecahedral (5(12)6(3)) cage enclosing two neutral water molecules.     

Experimental and modeling study of the ion-molecule association reaction H3O+ + H2O (+M) --> H5O2(+) (+M)

Experimental results for the rate of the association reaction H3O+ + H2O (+M) --> H5O2(+) (+M) obtained with the Cinetique de Reactions en Ecoulement Supersonique Uniforme flow technique are reported. The reaction was studied in the bath gases M=He and N2, over the temperature range of 23-170 K, and at pressures between 0.16 and 3.1 mbar. At the highest temperatures, the reaction was found to be close to the limiting low-pressure termolecular range, whereas the limiting high-pressure bimolecular range was approached at the lowest temperatures. Whereas the low-pressure rate coefficients can satisfactorily be reproduced by standard unimolecular rate theory, the derived high-pressure rate coefficients in the bath gas He at the lowest temperatures are found to be markedly smaller than given by simple ion-dipole capture theory. This result differs from previous observations on the related reaction NH4(+) + NH3 (+M) --> N2H7(+) (+M). This observation is tentatively attributed to more pronounced contributions of the valence part of the potential-energy surface to the reaction in H5O2(+) than in N2H7(+). Falloff curves of the reaction H3O+ + H2O (+M) --> H5O2(+) (+M) are constructed over wide ranges of conditions and represented in compact analytical form.     

Quantum features of a barely bound molecular dopant: Cs2(3Σu) in bosonic helium droplets of variable size

We present in this work the study of small (4)He(N)-Cs(2)((3)Σ(u)) aggregates (2 ≤ N ≤ 30) through combined variational, diffusion Monte Carlo (DMC), and path integral Monte Carlo (PIMC) calculations. The full surface is modeled as an addition of He-Cs(2) interactions and He-He potentials. Given the negligible strength and large range of the He-Cs(2) interaction as compared with the one for He-He, a propensity of the helium atoms to pack themselves together, leaving outside the molecular dopant is to be expected. DMC calculations determine the onset of helium gathering at N = 3. To analyze energetic and structural properties as a function of N, PIMC calculations with no bosonic exchange, i.e., Boltzmann statistics, at low temperatures are carried out. At T = 0.1 K, although acceptable one-particle He-Cs(2) distributions are obtained, two-particle He-He distributions are not well described, indicating that the proper symmetry should be taken into account. PIMC distributions at T = 1 K already compare well with DMC ones and show minor exchange effects, although binding energies are still far from having converged in terms of the number of quantum beads. As N increases, the He-He PIMC pair correlation function shows a clear tendency to coincide with the experimental boson-liquid helium one at that temperature. It supports the picture of a helium droplet which carries the molecular impurity on its surface, as found earlier for other triplet dimers.     

Effect of co-adsorbed CO and reaction temperature on the dynamics of N2 desorption under steady-state N2O-CO reaction on Rh(110)

We have investigated the effect of co-absorbed CO and reaction temperature on the angular distribution of N(2) desorption by N(2)O decomposition under the steady state of N(2)O-CO reaction on Rh(110). Spatial distributions of desorbing product N(2) emission have been measured at various surface temperatures and CO coverages. The decomposed N(2) collimates at 48°-61° off normal in the parallel plane to [001] and [110] directions, indicating that adsorbed N(2)O just before the decomposition is oriented along the [001] direction. Although the inclined and collimated N(2) desorption is always observed at any steady-state CO coverage and reaction temperature, the shape of the collimated N(2) distribution varied dependent on the co-adsorbed CO coverage. The distribution becomes sharp and shifts toward the surface normal direction with increasing CO coverage. These effects of adsorbed CO on the angular distribution of N(2) are interpreted by the collision of desorbed N(2) with co-adsorbed CO.     

Mn_xCo_(3-x)O_4复合氧化物及改性催化剂催化分解N_2O

用共沉淀法制备了一组不同组成的MnxCo3-xO4尖晶石型复合氧化物,表面负载碱金属助剂制备改性催化剂,用于催化分解N2O.用X射线衍射(XRD)、N2物理吸附(BET)、红外光谱(FTIR)、扫描电镜(SEM)、H2程序升温还原(H2-TPR)、X射线光电子能谱(XPS)等技术表征催化剂结构.考察了复合氧化物组成、碱金属助剂类型、钾前驱物等制备参数对催化剂结构和催化活性的影响.结果表明:添加助剂K、Cs降低了催化剂表面Co、Mn元素的电子结合能,弱化了Co—O和Mn—O键,有利于氧物种的脱除,提高了催化剂活性.优化出了活性较高的催化剂K/Mn0.4Co2.6O4(K2CO3),有氧无水、有氧有水气氛400℃连续反应50 h,N2O转化率分别保持100%和74.2%,催化剂稳定性较高.     

A molecular beam study of the NO + CO reaction on Pd(111) surfaces

Nitric oxide (NO) reduction with carbon monoxide (CO) on the Pd(111) surface was studied under isothermal conditions by molecular beam techniques as a function of temperature, NO:CO beam composition, and beam flux. Systematic experiments were performed under transient and steady state conditions. Displacement of adsorbed CO by NO in the transient state of the reaction was observed at temperatures between 375 and 475 K for all the NO:CO compositions studied. NO accumulation occurs on Pd(111) surface under steady state conditions, below 475 K, due to stronger chemisorption of NO. The steady state reaction rates attain a maximum at about 475 K, nearly independent of beam composition. N2 was found to be the major product of the reduction, along with a minor production of N2O. The production of N2 and N2O indicates molecular and dissociative adsorption of NO on Pd(111) at temperatures up to 525 K. Postreaction TPD measurements were performed in order to determine the nitrogen coverage under steady-state conditions. Finally, the results are discussed with respect to the rate-controlling character of the different elementary steps of the reaction system.     

Direct measurements of the high temperature rate constants of the reactions NCN + O, NCN + NCN, and NCN + M

The rate constant of the reaction NCN + O has been directly measured for the first time. According to the revised Fenimore mechanism, which is initiated by the NCN forming reaction CH + N(2)→ NCN + H, this reaction plays a key role for prompt NO(x) formation in flames. NCN radicals and O atoms have been quantitatively generated by the pyrolysis of NCN(3) and N(2)O, respectively. NCN concentration-time profiles have been monitored behind shock waves using narrow-bandwidth laser absorption at a wavelength of λ = 329.1302 nm. Whereas no pressure dependence was discernible at pressures between 709 mbar < p < 1861 mbar, a barely significant temperature dependence corresponding to an activation energy of 5.8 ± 6.0 kJ mol(-1) was found. Overall, at temperatures of 1826 K < T < 2783 K, the rate constant can be expressed as k(NCN + O) = 9.6 × 10(13)× exp(-5.8 kJ mol(-1)/RT) cm(3) mol(-1) s(-1) (±40%). As a requirement for accurate high temperature rate constant measurements, a consistent NCN background mechanism has been derived from pyrolysis experiments of pure NCN(3)/Ar gas mixtures, beforehand. Presumably, the bimolecular secondary reaction NCN + NCN yields CN radicals hence triggering a chain reaction cycle that efficiently removes NCN. A temperature independent value of k(NCN + NCN) = (3.7 ± 1.5) × 10(12) cm(3) mol(-1) s(-1) has been determined from measurements at pressures ranging from 143 mbar to 1884 mbar and temperatures ranging from 966 K to 1900 K. At higher temperatures, the unimolecular decomposition of NCN, NCN + M → C + N(2) + M, prevails. Measurements at temperatures of 2012 K < T < 3248 K and at total pressures of 703 mbar < p < 2204 mbar reveal a unimolecular decomposition close to its low pressure limit. The corresponding rate constants can be expressed as k(NCN + M) = 8.9 × 10(14)× exp(-260 kJ mol(-1)/RT) cm(3) mol(-1) s(-1)(±20%).     

Interaction of oxygen with TiN(001): N<-->O exchange and oxidation process

This work presents a detailed experimental and theoretical study of the oxidation of TiN(001) using a combination of synchrotron-based photoemission and density functional theory (DFT). Experimentally, the adsorption of O2 on TiN(001) was investigated at temperatures between 250 and 450 K. At the lowest temperature, there was chemisorption of oxygen (O(2,gas)-->2O(ads)) without significant surface oxidation. In contrast, at 450 K the amount of O2 adsorbed increased continuously, there was no evidence for an oxygen saturation coverage, a clear signal in the Ti 2p core level spectra denoted the presence of TiOx species, and desorption of both N2 and NO was detected. The DFT calculations show that the adsorption/dissociation of O2 is highly exothermic on a TiN(001) substrate and is carried out mainly by the Ti centers. A high oxygen coverage (larger than 0.5 ML) may induce some structural reconstructions of the surface. The exchange of a surface N atom by an O adatom is a highly endothermic process (DeltaE=2.84 eV). However, the overall oxidation of the surface layer is thermodynamically favored due to the energy released by the dissociative adsorption of O2 and the formation of N2 or NO. Both experimental and theoretical results lead to conclude that a TiN+mO2 -->TiOx + NO reaction is an important exit channel for nitrogen in the oxidation process.