Effect of Heat Treatment on the Formation of Copper Active Centers Obtained by the Interaction of Copper Chloride with H-Mordenite

Physics of the Solid State - The effect of temperature conditions on the formation of copper active sites obtained by solid state ion exchange between copper chloride and H-mordenite zeolite using...     

Surface interaction between H2 and CO2 over silicalite-supported platinum

Infrared spectroscopic evidence is presented for the formation of linearly bonded CO species, as a result of surface interaction between H₂ and CO₂ at room temperature over silicalite-supported Pt. Comparison with direct CO adsorption results suggests that the active sites for this CO₂ reaction are the corner or step sites on platinum particles. The CO formed on these active sites then migrates to other sites on the surface of Pt particles. Co-adsorbed hydrogen and water make the linearly bonded CO species more strongly adsorbed on Pt particles. However, exposure to oxygen or air at room temperature effectively removes these CO species.     

Oxidation during the production of FGH4095 superalloy powders by electrode induction-melt inert gas atomization

Super-clean and super-spherical FGH4095 superalloy powder is produced by the ceramic-free electrode inductionmelt inert gas atomization(EIGA) technique.A continuous and steady-state liquid metal flow is achieved at high-frequency(350 k Hz) alternating current and high electric power(100 k W).The superalloy is immersed in a high-frequency induction coil,and the liquid metal falling into a supersonic nozzle is atomized by an Ar gas of high kinetic gas energy.Numerical calculations are performed to optimize the structure parameters for the nozzle tip.The undesired oxidation reaction of alloying elements starts at 1000?C with the reaction originating from the active sites on the powder surfaces,leading to the formation of oxides,Me_xO_y.The role of active sites and kinetic factors associated with the diffusion of oxygen present in the atomization gas streams are also examined.The observed results reveal that the oxidation process occurring at the surface of the produced powders gradually moves toward the core,and that there exists a clear interface between the product layer and the reactant.The present study lays a theoretical foundation for controlling the oxidation of nickel-based superalloy powders from the powder process step.     

Catalyst design using an inverse strategy: From mechanistic studies on inverted model catalysts to applications of oxide-coated metal nanoparticles

Metal-oxide interfaces are of great importance in catalytic applications since each material can provide a distinct functionality that is necessary for efficient catalysis in complex reaction pathways. Moreover, the synergy between two materials can yield properties that exceed the superposition of single sites. While interfaces between metals and metal oxides can play a key role in the reactivity of traditional supported catalysts, significant attention has recently been focused on using “inverted” oxide/metal catalysts to prepare catalytic interfaces with unique properties. In the inverted systems, metal surfaces or nanoparticles are covered by oxide layers ranging from submonolayer patches to continuous films with thickness at the nanometer scale. Inverse catalysts provide an alternative approach for catalyst design that emphasizes control over interfacial sites, including inverted model catalysts that provide an important tool for elucidation of mechanisms of interfacial catalytic reactions and oxide-coated metal nanoparticles that can yield improved stability, activity and selectivity for practical catalysts.This review begins by providing a summary of recent progress in the use of inverted model catalysts in surface science studies, where oxides are usually deposited onto the surface of metal single crystals under ultra-high vacuum conditions. Surface-level studies of inverse systems have yielded key insights into interfacial catalysis and facilitated active site identification for important reactions such as CO oxidation, the water-gas shift reaction, and CO₂ reduction using well-defined model systems, informing strategies for designing improved technical catalysts. We then expand the scope of inverted catalysts, using the “inverse” strategy for preparation of higher-surface area practical catalysts, chiefly through the deposition of metal oxide films or particles onto metal nanoparticles. The synthesis techniques include encapsulation of metal nanoparticles within porous oxide shells to generate core-shell type catalysts using wet chemical techniques, the application of oxide overcoat layers through atomic layer deposition or similar techniques, and spontaneous formation of metal oxide coatings from more conventional catalyst geometries under reaction or pretreatment conditions. Oxide-coated metal nanoparticles have been applied for improvement of catalyst stability, control over transport or binding to active sites, direct modification of the active site structure, and formation of bifunctional sites. Following a survey of recent studies in each of these areas, future directions of inverted catalytic systems are discussed.     

氧原子在Zr(0001)表面附近的扩散

在密度泛函理论计算的基础上,利用微动弹性带（nudged elastic band）方法研究了氧原子在Zr（0001）表面附近的扩散.首先计算了氧原子从稳定的表面面心立方（SFCC）位置向表面六角密排位置的扩散激活能（0.77 eV）;然后计算了氧原子从稳定的SFCC位置扩散到表面下第1层与第2层之间的八面体间隙位置,再继续向表面下第2层与第3层之间的八面体间隙位置扩散的激活能,在此过程中氧原子需克服两个能垒,其激活能分别为2.14和2.57 eV.结果表明,氧原子在Zr（0001）表面上方的扩散比较容易,而氧原子向Zr（0001）表面下的扩散相对较难. 关键词： Zr（0001）表面 微动弹性带 氧的扩散     

Mn助剂对K-Co-Mo催化剂结构及CO加氢合成低碳醇性能的影响

通过溶胶-凝胶法合成了一系列Mn掺杂K-Co-Mo催化剂,并利用X射线衍射、N₂吸脱附、NH₃程序升温脱附、原位漫反射红外光谱以及X射线吸收谱等技术对催化剂的结构进行了表征.活性测试结果显示Mn掺杂催化剂比未掺杂催化剂表现更高的合成低碳醇的催化活性,尤其是C₂₊醇的选择性得到了明显的提高.醇产物分布偏离了ASF分布规律,甲醇的含量显著减少,乙醇成为主要醇产物.表征结果表明Mn助剂的加入增强了Co和Mo之间的相互作用,促进了醇生成活性中心Co-Mo-O物种的生成.显著减少了催化剂强酸性位的数量,促进了弱酸性位的产生,有利于醇产物的生成.助剂的加入有利于催化剂对CO的线性和桥式吸附,促进了醇产物的生成和碳链的增长,提高了催化剂对C₂₊醇的选择性.     

混合悬浮液中纳米颗粒对核化形态的影响

均相沸腾活化核心的形成靠液体分子(或密度)脉动形成,低过热温度下临界活化核心比纳米颗粒本身粒径要大很多,颗粒添加的影响相对很小,依旧保持为均相沸腾。随着液体过热温度的增加至临界活化核心与颗粒尺寸可比时,颗粒才有可能成为新的活化核心,液体内部的均相沸腾也转变为非均相沸腾,但在考虑颗粒吸附的情况下,颗粒的吸附会使表面部分核化中心失去活性,弱化了原有非均相核化。     

C-radiolabeling study of methanol decomposition on copper oxide modified mesoporous SBA-15 silica

¹¹C-radiolabeling technique is applied to investigate methanol decomposition on copper oxide modified SBA-15. Nitrogen physisorption, XRD, FTIR, UV-vis and TPR techniques are used for catalyst characterization. Selective adsorption coverage of the catalytic active sites with ¹¹C- and ¹²C-methanol molecules is carried out and the products of their conversion are followed. The mechanism of methyl formate, methylal and CO₂ formation from methanol is discussed.     

Separation of chiral molecules by temperature programmed desorption

The Monte Carlo simulation method was used to model thermal desorption of a pair of enantiomers from a solid surface with a chiral periodic pattern of active sites. The main objective of the study was to determine the optimal number of the active sites and their spatial distribution within the unit cell of the surface to achieve the most efficient separation of the enantiomers. For that purpose we tested the series of chiral patterns which were found previously for the equilibrium adsorption. Temperature programmed desorption spectra were calculated using a square lattice of adsorption sites in which the active sites were distributed spatially according to the candidate patterns. Additionally, influence of relaxation of the adsorbed layer on the relative shift of the TPD peaks of the enantiomers was assessed and the key factors affecting the chiral separation were identified.     

A model calculation on adsorption of CO on the flat,stepped and kinked nickel surfaces

The adsorption of CO on Ni was investigated by quantum chemical calculations using the CNDO/2 tight binding method. The surfaces used as models are the (111), 4(111) × (111), 3(111) × (110) and 3(111) × (100) surfaces. The CO bond is weakened in this sequence of surfaces. The active sites for the CO bond fission are the trench regions of the step and kink structures. The Ni 3d orbitals play an important role for the weakening of the CO bond, though their contribution is small for the Ni-C bond formation.     

Two-step reaction on a strained, nanoscale segmented surface

By means of scanning tunneling microscopy and density functional theory calculations we demonstrate that on the Rh(110)-(10 x 2)-O surface, a prototypical multiphase surface of an oxidized transition metal model catalyst, water formation upon H2 exposure is a two-step reaction, with each step requiring special active sites. The 1st step initiates at (2 x 1)p2mg-O defect islands in the (10 x 2) structure and propagates across the surface as a reaction front, removing half of the adsorbed oxygen. The oxygen decorated Rh ridges of the (10 x 2) structure lose their tensile strain upon this reduction step, whereby nanoscale patches of clean Rh become exposed and act as special reaction sites in the 2nd reaction step, which therefore initiates homogeneously over the entire surface.     

The Formation of Defect Complexes in a ZnO Grain Boundary

The microscopic properties a ZnO grain boundary containing extrinsic point defects are studied using a density functional computational approach. The results show that the grain boundary acts as a sink for native defects, such as the zinc vacancy and the oxygen interstitial, and also for bismuth substitutional impurities. The defects tend to accumulate at under-coordinated sites in the boundary core and prefer to form small clusters. In particular the segregation of Bi promotes the formation of the other native defects by lowering their formation energies in the boundary. Individually, the native defects and the Bi impurity do not produce deep interface states in the band gap which are electrically active. However, when the defects cluster to form a Bi_Zn-V_Zn-O_i complex, new gap states are created of acceptor type. It is suggested that these new states are caused by defect interactions which compensate one another resulting in the depletion of an occupied impurity state and new bond formation. The results are discussed in terms of the Schottky barrier model commonly used to describe the electrical characteristics of ZnO varistors.     

The stoichiometry and kinetics of carbon combustion at low temperature: A surface complex approach

The stoichiometry and rate of carbon combustion at low temperature (673 K) were investigated. Oxidation and TPD experimental data provide quantification of gaseous products and stable surface complexes over a broad range of conversion. Our analysis distinguishes between surface complexes forming CO and CO₂ and has assumed a certain fraction of each complex type decomposes instantaneously upon formation, leaving the remainder on the surface as stable complexes, C(O) and C(O₂). This analysis suggests that a maximum of 25% of CO-complexes and 89% of CO₂-complexes are unstable upon formation. At low conversion, unstable complex formation is the dominant pathway for the CO product. As conversion increases, decomposition of stable CO-complexes eventually becomes the main source of CO. Formation of unstable CO₂-complexes is the dominant pathway for the CO₂ product at all times. The combustion rate is initially high due to a high availability of vacant active sites, decreases sharply as these sites are filled with stable complexes, and gradually increases as the stable complexes promote CO₂-complex formation, in turn, driving their decomposition. The dynamics of formation and decomposition of C(O) and C(O₂) dictates their ratio on the carbon surface at any moment, which may be measured by TPD. This work may help in developing new kinetic models of carbon combustion which can predict the stoichiometry as well as the rate.     

1H and 13C MAS NMR studies of light alkanes activation over MFI zeolite modified by Zn vapour

The early stages of methane, ethane and propane conversion were studied by in situ ¹H and ¹³C MAS NMR techniques over fully exchanged Zn²⁺/MFI catalyst obtained by the reaction with zinc vapour. The in situ techniques revealed strong interaction of alkanes with Zn²⁺ cations evidenced by significant shift of the corresponding NMR lines. Besides that, the formation of methyl zinc, ethyl zinc and n-propyl zinc species along with bridging and silanol surface OH-groups was detected already at the ambient temperature. These results pointed to dissociative adsorption of alkanes over (ZO)–Zn²⁺–(OZ) and (ZO)–Zn²⁺–(OSi) active sites of the catalyst. The dissociative adsorption was shown to be a dead-end surface reaction in the case of methane starting reactant, while in the case of ethane and propane, it appeared to be responsible for the initiation of the catalytic cycle leading to alkenes and dihydrogen formation and regeneration of zinc containing catalytic sites.     

Burning structures and propagation mechanisms of nanothermites

Nanothermites demonstrate attractive combustion characteristics such as tunable reactivity and high energy density. There is however a lack of fundamental understanding on their burning structures and reaction mechanisms due to the multi-scale complexity associated with the material and reaction heterogeneities. This gap in turn hinders the optimization of nanothermite design with desirable microstructures and controllable burning properties. In this work, a high-speed microscopy imaging system was used to reveal the burning structure of Al/CuO nanothermites and to investigate the propagation mechanism of its flame front at micron and sub-millimeter scales which have not been studied. An Al/CuO nanothermite film was fabricated as a model structure. First, the previously proposed reactive sintering was confirmed as a micron-scale burning characteristic. Then, at the sub-millimeter scale, it was demonstrated that the non-uniform burning propagation of nanothermite films is featured with distinguishable roles of the active burning sites and the pre-ignition sites. The active burning sites are clusters of reactive sintering particles and the pre-ignition sites appear in the preheating regions where Al and CuO particles have not yet participated in the reaction due to insufficient ignition energy. These pre-ignition sites form randomly and are subsequently ignited by heat transferred from the adjacent active burning sites, resulting in an active burning propagation tangentially along the propagation front. At the same time, as the thermite reaction of nanoparticles in the unburnt region is initiated, the propagation front advances in the normal direction. This experimental work reveals that the burning propagation mechanism of nanothermite films is governed by active burning propagation in both tangential and normal directions of the propagation front. Although the rates of these two modes are on the same order of magnitude, the tangential propagation of active burning is slightly faster, implying that pre-ignition sites are readily ignited with lower ignition energy.     

放射性核素铀在针铁矿中的占位研究

用第一性原理研究放射性同位素铀在针铁矿(α-FeOOH)中的占位情况, 分别考虑铀原子替代针铁矿中的铁的替位缺陷和铀的多种八面体和多种四面体间隙缺陷. 计算发现了三个最稳定的缺陷构型, 它们分别对应于一个铀替位缺陷(S) 及其中的一个铀的八面体(O)和四面体(T)间隙缺陷, 其形成能分别为-13.49, -3.86, -1.60 eV. 也研究了两个相邻的铀原子在针铁矿中的占位情况, 发现双铀原子很容易掺入到相邻的SS或OS位, 它们的形成能分别为-27.392和-16.214 eV, 结合能分别为-0.417和1.131 eV. 表明双原子铀在针铁矿中会以SS形式发生偏聚而较难以OS形式偏聚. 关键词： 铀 针铁矿 占位 第一性原理     

Surface studies of novel hydrophobic active carbons

The efficient adsorption of toxic organic species from humid airstreams by active carbons is impeded by the competitive adsorption of water vapour which, at low values of p/p^s, occurs at specific (polar) adsorption sites located at the edges of the carbon layer-planes and at in-plane defects. At higher pressures, adsorption in micropores and mesopores also occurs. The concentration of polar adsorption sites therefore determines the hydrophilicity of the carbon structure and their accelerated formation, by exposure to air and water vapour, is also responsible for the ‘ageing’ of active carbons. Overall, the adsorption of water reduces the volume of porosity available for the adsorption of target species and the hydrophilic nature of active carbons is recognized as a major barrier to their effective use in many applications.We present here results for the adsorption of nitrogen, organic and water vapours by a hydrophobic respirator granular active carbon produced by the thermal treatment of a base carbon, to desorb polar oxygen groups, followed by use of a plasma enhanced chemical vapour deposition (PECVD) treatment to apply a hydrophobic, fluorine containing, surface nanolayer. We show that at equivalent %RH values the treated carbon adsorbs significantly less water compared to an untreated (control) carbon and that the treatment does not reduce the levels of open porosity or impede the adsorption of a range of organic vapours at ambient temperatures. Preliminary evidence for the presence, after treatment, of constrictions at pore entrances which act as molecular gates is also presented. The treated carbon (after ageing for 6 weeks at 80%RH) is shown to have greater adsorptivity than an untreated base carbon toward hexane present in a humid (80%RH) airstream. This results in a 39% increase in break-through time. These hydrophobic properties persist one year after manufacture. The mechanism leading to the modified water adsorption properties is the partial desorption of polar oxygen sites followed by deposition at the external carbon surfaces of hydrophobic plasma polymer species. This reduces the polar surface free energy of the carbon and hence the amount of water adsorption occurring by the primary mechanism. This in turn retards the diffusion of water molecules into the micropores and leads to lower adsorption volumes at higher pressures.     

Can one predict DNA transcription start sites by studying bubbles?

It has been speculated that bubble formation of several base pairs due to thermal fluctuations is indicatory for biologically active sites. Recent evidence, based on experiments and molecular dynamics simulations using the Peyrard-Bishop-Dauxois model, seems to point in this direction. However, sufficiently large bubbles appear only seldom, which makes an accurate calculation difficult even for minimal models. In this Letter, we introduce a new method that is orders of magnitude faster than molecular dynamics. Using this method, we show that the present evidence is unsubstantiated.     

Investigation of the Cu2+ and VO2+states in synthetic ion-exchange resins of varying sorts by E.P.R. method

The interaction of Cu²⁺ and VO²⁺ ions with high-molecular ligands of the sulpho, carboxyl, phosphor, amino and aminophosphorus resins were studied by E.P.R. The influence of an active resin group on the formation of the intra complex compounds with metal ions and, hence, on the sorption selectivity has been investigated; the influence of moisture content and temperature on the state of the paramagnetic ions in the resin has been studied. Inhomogeneous distribution of metal ions in the resin matrix which leads to the formation of both individual and exchange-coupled systems has been established. The influence of changes in polymer conformation on the stability of ion-exchange complexes is shown.

The data obtained from the E.P.R. spectra of Cu²⁺ and VO²⁺ complexes in a high polymeric matrix were compared with the corresponding monomeric systems having identical structures for the complex sites.     

Probing of local dissolution of Al-alloys in chloride solutions by AFM and SECM

Local dissolution of Al alloys was probed in situ in chloride solutions by using atomic force microscopy (AFM) and scanning electrochemical microscopy (SECM). Preferential dissolution in the boundary region between some intermetallic particles (IMPs) and alloy matrix, and trench formation around large IMPs during free immersion and under electrochemical anodic polarization were observed, which indicate different dissolution behavior associated to different types of IMPs. Moreover, by using an integrated AFM/SECM system with a dual mode cantilever/microelectrode probe, simultaneous probing of electrochemical active sites and topographic changes over the same area was performed with sub-micron resolution. This allowed the ongoing localized corrosion processes related to the IMP to be revealed.