Oscillatory behaviour of the NOH2 reaction over Rh(533)

Waves moving over the surface of a Rh field emitter tip in an oscillatory way during the NO-H₂ reaction have been visualized earlier by field electron microscopy (FEM). An autocatalysis model has been proposed to describe the oscillatory behaviour of the reduction of NO by H₂. To study the oscillatory behaviour and the effect of the surface structure in more detail a large Rh(100) surface and a large stepped Rh(533) surface, Rh[4(111)¹(100)], have been selected. The first results show that, in correspondence with the FEM experiments, rate oscillations could be observed over the Rh(533) surface in the 10⁻⁶ mbar pressure regime around 470 K. No oscillatory behaviour was obtained on the Rh(100) surface under these conditions. The structure-sensitivity of the process is related to the large dependence of the Rh-N bond strength on the surface structure. Nitrogen desorbs at a much higher temperature from Rh(100) than from Rh(533).     

Compatibility of field emitter studies of oscillating surface reactions with single crystal measurements: catalytic CO oxidation on Pt

Kinetic oscillations in catalytic CO oxidation on Pt have been studied on large (millimeter size) single crystal planes of Pt as well as on a Pt field emitter tip that exposes different crystal facets of nanometer size. In order to examine the compatibility of results from the two types of experiments, the regions of different dynamical behavior (bifurcation diagram) have been mapped out in p_CO,T-parameter space using a field electron microscope (FEM) and a field ion microscope (FIM). The comparison with the results of single crystal measurements shows that in the case of applied electrostatic fields less than 5 V nm⁻¹ (FEM), the field-induced effects are negligible, but they are significant for fields exceeding 12 V nm⁻¹ (FIM). The field-induced shift of the bifurcation diagram toward lower p_CO values, observed with FIM, is explained in terms of a field-modified interaction of CO and O₂ with Pt studied here with field ion appearance energy spectroscopy. With coadsorbed lithium (submonolayer coverage), the existence range for rate oscillations is shifted toward higher p_CO values. This shift is attributed to a redistribution of the electron density at the surface induced by alkali metal co-adsorption.     

Li-modified oscillations in CO oxidation on a Pt field emitter

The first observations of Li-modified isothermal kinetic oscillations during CO oxidation on a Pt field emitter tip are reported. The oscillations are realized on the surface of a [111]-oriented Pt tip in the presence of a Li submonolayer coverage of about 2.5 × 10¹⁴cm⁻² and are compared to the corresponding Li-free oscillations occurring on the same tip. In both cases the oscillatory process is visualized using the field electron emission (FEM) mode of the lithium field desorption microscope (Li-FDM). The origin of the FEM imaging contrast of coadsorbed LiCO and LiO layers on Pt is clarified. The influence of the coadsorbed Li submonolayer on the oscillating behavior of CO oxidation on Pt is discussed.     

Field emission and field ionization studies and their impact on the formulation of elementary steps in the heterogeneous activation of gas molecules by transition metal surfaces

The FIM and FEM evidence for the existence of the localized orbitals on transition metal surfaces and the problem of charge density distribution at the “stepped” surfaces (FEM and FIM tips) are reviewed. The role of these localized orbitals and of the nearly free electrons in the activation of gas molecules by transition metal surfaces is discussed.     

激光等离子体相互作用中Weibel不稳定性

利用粒子模拟法对超强激光与等离子体相互作用中产生的Weibel不稳定性及其产生机制进行了详细的研究。给出不稳定性的线性色散关系和饱和磁场与各向异性参数之间的函数关系,发现Weibel不稳定性的存在使超强激光在等离子体中激发的自生磁场饱和,饱和自生磁场的存在使粒子速度分布在激光传播方向上表现出各向异性。讨论了Weibel不稳定性的线性和非线性饱和过程,对更好地理解快点火物理中自生磁场的产生、快电子输运等过程有重要意义。     

Surface structure effects in the reactions of nitric oxide with hydrogen and carbon monoxide on rhodium: A comparison with the surface structure effects observed for the hydrogen-oxygen and carbon monoxide-oxygen reactions

Using field electron microscopy (FEM) and thermal desorption and reaction spectroscopy (TDS) the behaviour of various Rh single crystal surfaces towards reactions involving NO has been studied. If, after NO adsorption up to saturation at 77 K, the temperature is slowly raised the FEM results suggest that dissociation of NO starts at the (321), (331) and (533) surfaces. The reaction of NO_ads with hydrogen starts also at these surfaces (at about 360 K) suggesting that NO bond scission initiates the reaction. After initiation a surface explosion is observed. Depending on the heating rate either a clean surface or a N_ads covered surface is obtained after completion of the reaction. Apparently, the reduction of adsorbed N_ads by hydrogen can occur at a significant rate at this temperature. At a higher heating rate the formed N adatoms do not react with hydrogen and are readily desorbed as N₂ at 600 K. The reaction of NO_ads with CO starts again on the (321) and (331) surfaces. The rate of the reaction with CO is, however, much lower than that with hydrogen. For the reaction of CO_ads with NO, desorption of CO is the initiation step. The mechanisms of the reactions and the dependence of the reaction on the surface structure are discussed in relation to literature data.     

Finite element simulation and analytical analysis for nano field emission sources that terminate with a single atom: A new perspective on nanotips

Nanotips are highly demanded for various applications in nanotechnology. For instance, nanotips with a single atom end can be used as a source of self collimated electron or ion beams. Such tips are usually characterized in the field ion microscope (FIM) or the field emission microscope (FEM), where only a top view can be captured and analyzed. We have noticed that single atom tips fabricated by different methods produce electrons in FEM mode, or ions in FIM mode, at a wide range of applied voltages for the respective mode. In this work we present numerical and analytical analyses to the distribution of the electric field in the vicinity of the nanotip apex that holds the topmost single atom. We demonstrate that although the electric field is relatively enhanced by the nano protrusion it is still significantly dominated by the tip base. The analyses explicitly show that nanotips with broad bases produce even less field than some modest tips, at the same applied voltage. This pronounced effect of the tip base accounts for the relatively high voltages needed at the imaging threshold field. The results reveal that single atom tips are not necessarily sharp at a mesoscopic scale and the tip sharpness has to be determined from the combination of the nanotip apex (FIM or FEM) image and the applied voltage.     

Interaction of hydrogen with palladium surface: FIM and FEM studies

In the present paper, we focus on the geometrical and electronic changes in palladium surface structure which appeared during its interaction with hydrogen in the presence of an external electric field. The interaction process was examined by using the field ion microscopy (FIM) as well as the field emission microscopy (FEM) techniques. In order to study the geometrical changes in substrate surface structure, the distance distribution function (DDF) was constructed on the basis of FIM patterns of both a clean and hydrogen-covered palladium surface. The electronic changes were examined by the measurement of the total energy distribution (TED) of electrons emitted from the palladium tip surface. The most pronounce examples of such changes are an expansion of the equilibrium interatomic distance in palladium surface and a shift of the Fermi level of the metal. These changes may be explained among others by palladium hydrides formation. This process is the most efficient if the field strength exceeds 23 V/nm.     

Oscillations and chaos in CO+O(2) combustion

The gas-phase reaction between carbon monoxide and oxygen (in the presence of small amounts of hydrogen) shows bistability and oscillatory behavior. Typically, the oscillatory ignition has a period-1 relaxation waveform. The limit cycle is born at a saddle-node loop and terminates via a supercritical Hopf bifurcation. For a mean residence time of 8 s there is a period-doubling to a period-2 solution followed by period-halving to quasisinusoidal period-1 oscillations. At longer residence times, more period-doublings forming a full cascade to chaos with subsequent periodic windows are observed. The chaotic attractor has an underlying single-humped next maximum map.     

Pulsed power modeling of very-large-aperture,transverse-discharge CO2 lasers

Discharge behavior of large-aperture (10–70 cm) CO₂ lasers is computed using a coupled particle kinetics-equivalent circuit model. Pulsed power systems that produce a single excitation pulse are only able to satisfy the preionization, ignition, and impedance matching conditions for interelectrode gaps 40–50 cm. Double-pulse systems are required for larger gaps, not only to optimize electrical efficiency, but to avoid oscillatory behavior which can lead to arc formation during the second cycle.     

The study of radiation damage in metals with the field-ion and atom-probe microscopes

The application of the field-ion microscope (FIM) and the atom-probe FIM techniques to the study of radiation damage in metals is reviewed. The FIM research in the following three areas is discussed: (1) the defect structure of the primary state of irradiated metals; (2) the recovery behavior in Stages I, II and III of irradiated body-centered and face-centered cubic metals (tungsten and platinum); and (3) the recovery behavior in Stages I, II and III of irradiated body-centered and face-centered cubic alloys [tungsten (carbon), tungsten (rhenium) and platinum (gold)]. The data obtained in each of these three categories provide fundamental structural, kinetic and thermodynamic information about the properties of point defects and point-defect clusters. The atom-probe FIM research on the direct measurement of the range of 300 eV He⁺ ions and the mobility of interstitial helium atoms in tungsten is also presented and compared with the theoretical predictions of Wilson, Haggmark and Bisson.     

Field ion microscopy of germanium: Field ionization and surface states

Field evaporated Ge tips were imaged with Ar and Ne at 80° K. The ions of these gases display identical field ion images. A striking correlation between the regional brightness of FIM and FEM patterns was found. Electron shadow microscopy and the behavior of FIM images above BIV indicate no local field enhancements, thus this correlation must be explained by the electronic structure of the surface. Explanation is furnished by a model, where electrons of the image gas tunnel preferentially into surface states. This model, based on the known fact that surface states of Ge contribute in field emission, is supported by the observation that in regions of low index poles, oxygen adsorption decreases the surface density of centers above which field ionization occurs.     

Nonlinear effects in the convective instability of a binary mixture in a porous medium near threshold

An amplitude equation is derived near threshold for both the stationary and the oscillatory instabilities and an oscillatory behavior in time for the Nusselt and the Froude number is predicted for the oscillatory instability.     

Surface structure effects in the adsorption and desorption of nitric oxide on rhodium

The adsorption, desorption and decomposition of NO on Rh surfaces have been investigated using field electron microscopy (FEM) and thermal desorption spectroscopy (TDS). At 77 K NO is molecularly adsorbed on all surfaces of Rh. At room temperature, however, about 30% of NO adsorbed on the rough surfaces is dissociated. The work function change Δφ due to NO adsorption increases as the surface becomes rougher. The results suggest the following order in Δφ: 0.93 eV = (100) < (111) < (511) < (410) < (331), (533) < (321) < (110) < (650) < (531), (210) = 1.4 eV. Upon heating the tip covered with molecular NO the FEM results suggest that the (321) surface is most active in the NO bond scission. The smooth surfaces are least effective in NO dissociation. The most likely interpretation of the FEM results is that the activity in NO bond scission increases in the following order: (111), (110) < (100), (511) < (650) < (410) < (210) < (331), (533) < (321). These results are discussed in relation to literature data concerning the dissociation of NO on the noble metals of Group VIII.     

O2/CO2气氛下CO对煤焦异相还原NO影响的研究

为揭示O₂/CO₂燃烧过程中高浓度的CO对煤焦异相还原NO的影响,在1073 K温度下使用山西褐煤在卧式炉上进行了实验。分别对O₂/CO₂浓度比及CO浓度下NO的还原特性进行详细实验研究。研究结果表明:在O₂/CO₂气氛下,O₂浓度为30%时具有较高的还原率;相同O₂浓度下O₂/CO₂气氛较空气气氛NO还原率高,表明在CO存在的条件下,高浓度的CO₂会促进NO的还原;当CO浓度从1.5%逐渐升高时,NO的还原率逐渐降低,到CO浓度为5%时,NO还原率比没有加入CO时还要低,而在空气气氛下CO浓度的变化对NO的还原率影响较小。     

In situ DRIFTS研究NO在Cu-ZSM-5上的表面吸附及选择性催化还原

Cu-ZSM-5分子筛催化剂选择性催化还原NO具有较好的低温活性,在613 K时NO还原成N2的转化率达70.6%。原位漫反射红外光谱(In situ DRIFTS)是研究催化剂表面吸附物种及催化机理的重要方法,应用该方法在298～773 K范围原位考察了以C₃H₆为还原剂及富O₂ 条件下,NO在Cu-ZSM-5催化剂上的表面吸附及选择性催化还原。认为NO在Cu-ZSM-5催化剂上还原为N₂的过程中,NO以一系列NO_x吸附态形式与丙稀的活化物种(C_xH_y或C_xH_iyO_z)反应,生成有机中间体,再进一步反应,最终生成N₂。有机中间体存在一个明显的从有机胺物种到腈(或—CN)再到有机氮氧物种(R—NO₂或R—ONO)的过程,催化剂表面形成有机中间物种是关键步骤,Cu的作用是促进NO_x形成,O₂ 的作用是促进C₃H₆活化,并且是有效产生有机-氮氧化物不可缺少的条件。     

An improved high-order scheme for DNS of low Mach number turbulent reacting flows based on stiff chemistry solver

We present an improved numerical scheme for numerical simulations of low Mach number turbulent reacting flows with detailed chemistry and transport. The method is based on a semi-implicit operator-splitting scheme with a stiff solver for integration of the chemical kinetic rates, developed by Knio et al. [O.M. Knio, H.N. Najm, P.S. Wyckoff, A semi-implicit numerical scheme for reacting flow II. Stiff, operator-split formulation, Journal of Computational Physics 154 (2) (1999) 428–467]. Using the material derivative form of continuity equation, we enhance the scheme to allow for large density ratio in the flow field. The scheme is developed for direct numerical simulation of turbulent reacting flow by employing high-order discretization for the spatial terms. The accuracy of the scheme in space and time is verified by examining the grid/time-step dependency on one-dimensional benchmark cases: a freely propagating premixed flame in an open environment and in an enclosure related to spark-ignition engines. The scheme is then examined in simulations of a two-dimensional laminar flame/vortex-pair interaction. Furthermore, we apply the scheme to direct numerical simulation of a homogeneous charge compression ignition (HCCI) process in an enclosure studied previously in the literature. Satisfactory agreement is found in terms of the overall ignition behavior, local reaction zone structures and statistical quantities. Finally, the scheme is used to study the development of intrinsic flame instabilities in a lean H₂/air premixed flame, where it is shown that the spatial and temporary accuracies of numerical schemes can have great impact on the prediction of the sensitive nonlinear evolution process of flame instability.     

Low-temperature Catalytic Reduction of Nitrogen Monoxide with Carbon Monoxide on Copper Iron and Copper Cobalt Composite Oxides

通过共沉淀法, 在低温下制备了铜铁复合氧化物CuO/Fe2O3和铜钴复合氧化物CuO/Co3O4．利用GC微反应器评价了这些复合氧化物的催化活性和热稳定性,结果表明, CuO/Fe2O3和CuO/Co3O4的NO100%转化温度分别为80和90 ℃,该催化活性和热稳定性在较宽的温度和较长的时间范围内都能得以保持．此外,还系统研究了试剂的摩尔比率、NaOH的体积、陈化时间、煅烧温度和煅烧时间对该复合氧化物催化活性的影响．     

Intrinsic kinetics mechanisms for the catalytic reduction of NO by Na-loaded char

An insight into the interaction between NO and Na-loaded char is essential to improve the catalytic ability of Na to NO reduction, which will be useful to lower NO emissions during thermal utilization of sodium-containing fuels. Here, the intrinsic kinetics mechanisms for the catalytic reduction of NO by Na-loaded char were discussed in details. Using density functional theory (DFT) calculations, possible reaction pathways were first obtained, followed by evaluation of the rate coefficients through transition state theory (TST) calculations. On this basis, the analyses of both sensitivity and rate of products (ROP) were performed to illustrate the intrinsic kinetic mechanism for the NO reduction by Na-loaded char in a certain combustion condition, with an emphasis on the effects of temperature and NO-to-CO stoichiometric ratio. Results indicated that the catalytic active center –ONa plays an important role in the catalytic reduction of NO by Na-loaded char. Specifically, in most cases, the interaction of NO with Na-loaded char largely depends on the elementary reaction of CNO-Na+NO+CO→21-IM3+CO₂. As the stoichiometric ratio of NO to CO increases, the CO-Na+2NO→8-IM4+N₂ becomes increasingly dominant. Moreover, higher temperature causes the CNO-Na+NO→20-P + N₂O as the dominant reaction. Nonetheless, one thing that these reactions have in common is that they are all related to the catalytic active center –ONa. Therefore, the NO reduction Na-loaded char largely depends on the interaction of NO with the carbonaceous surface containing –ONa. Inspired by this, a conceptual approach was proposed to improve the catalytic performance of Na on NO reduction, and it has been shown to be theoretically feasible. To summarize, the combination of DFT, TST and kinetic calculations is useful to clarify the interaction between NO with Na-loaded char, and it gives a basis for the development of micro-kinetic model.     

Treating NOx emission of hydrogen fueled combustion engines by NOx storage and reduction catalysts: A transient kinetic study including PLIF measurements

NOx storage and reduction (NSR) catalysts are a well-known and broadly used technology to reduce NOx emissions from combustion engines, which may also be applied for hydrogen fueled engines in the future. In this study, Pt- and Pd-based NSR-catalysts were investigated in the absence and presence of water to understand how NO oxidation as well as the storage and reduction phases are influenced by the gaseous environment with H₂ as a reductant. A planar channel configuration was chosen for conducting planar laser-induced fluorescence experiments during the storage phase in addition to steady-state oxidation measurements and transient lean/rich cycles in a packed bed reactor. The presence of steam significantly decreases the NO oxidation activity of both noble metal catalysts. The Pt/BaO/Al₂O₃ catalyst is more active during transient lean/rich cycles, however, it suffers an activity loss during repeated cycles, whereas the activity of the Pd/BaO/Al₂O₃ sample is slightly more stable in the wet gas feed over time. All experiments showed a strong correlation between the NO₂ formation over the catalyst and its storage capability. The influence of water in the exhaust gas on the NSR-catalysts shows a strong temperature dependency on storage and reduction of NO for both catalysts containing Pt and Pd. The storage behavior is also strongly influenced by both the experimental configurations chosen revealing the significance of the interaction of intrinsic catalytic kinetics and mass transfer in the surrounding flow field.