The decomposition of methanol at low pressures by rhenium

A heated rhenium wire is an effective catalyst for the decomposition of methanol at low pressures, the mass-spectrometrically recorded products of decomposition being carbon monoxide and hydrogen. When newly cleaned, more than 30% of c led to decomposition, for filament temperatures between 900 and 1300 K. Under steady-state conditions, or when the filament was pre-saturated with oxygen, the temperature dependence of catalytic activity was significantly differe though the maximum efficiency was still as much as 20% at 900 K.     

The chemisorption of O2, CO,D2 and C2H4 over epitaxially grown rhenium crystalline films

The adsorption and desorption of oxygen, carbon monoxide, deuterium and ethylene has been studied over rhenium films using thermal desorption spectroscopy, low energy electron diffraction and Auger electron spectroscopy. The films, obtained by evaporating rhenium onto a platinum (111) single crystal, grow over the substrate forming (0001) basal plane rhenium surfaces. Oxygen chemisorbs on this film, forming an ordered structure, consisting of three (2 × 1) overlayer domains and giving a saturation coverage of half a monolayer of atomic oxygen. CO chemisorption is mainly molecular, although some dissociation occurs at temperatures above about 700 K. A complicated LEED pattern is obtained when saturating the surface at 150 K with CO, but it changes to a (2 × 2) or (2 × 1) structure upon heating. Also, CO chemisorption can be modified by predissociated CO or preadsorbed oxygen on the rhenium surface. Deuterium desorbs in three peaks, starting at temperatures as low as 150 K. Ethylene desorbs partially intact at around 250 K, the rest decomposing and yielding hydrogen, that appears as two main peaks at 357 K and 460 K during thermal desorption. We conclude that epitaxially grown films may be an alternative to single crystals for studying chemisorption over well ordered surfaces.     

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The catalytic activity of the substoichiometric tungsten trioxide electrodes for the oxidation of hydrogen has been investigated, using simultaneous EPR spectroscopy and linear-sweep voltammetry measurements. The parameters of the EPR spectral line observed for all oxides except stoichiometric WO₃ were determined in relation to the electronic structure of the paramagnetic centers associated with the oxygen anion vacancies. Hydrogen adsorption was followed by means of changes both in the EPR line parameters and in the electrode potential. The intensity-versus-potential curves for suboxide electrodes under hydrogen were analyzed in terms of the occurrence of two processes. It was shown that one process is associated with the hydrogen oxidation and that the other is connected with oxidation of the WO_3?x surface. The mechanisms of both reactions were interpreted as involving the formation and oxidation of an hydrogen tungsten bronze.     

The adsorption and decomposition of formaldehyde by tungsten

A hot, clean tungsten surface was highly effective in decomposing formaldehyde vapour to carbon monoxide and hydrogen. At temperatures above 1300 K about 30% of the impinging molecules decomposed. At lower temperatures the reactivity was reduced, probably as the result of inhibition by the adsorbed carbon monoxide. Pre-adsorbed oxygen affected the reactivity in an unusual way. At low coverages of oxygen (θ < 0.4) and intermediate filament temperatures (T < 1000 K) no effect was observed, whereas under more extreme conditions (θ > 0.5 or T > 1200 K) a nearly linear decline in activity was recorded. These results are attributed to a similarity between the inhibiting effects of adsorbed oxygen and carbon monoxide.     

The high temperature and field treatment of the tungsten tip in a field emission microscope and the adsorption of oxygen and hydrogen

The adsorption of hydrogen and oxygen on the clean surface of tungsten obtained by thermal desorption of impurities from the tip in FEM and on the surface of a tungsten tip formed at a high temperature under the influence of a high voltage was studied. It could be deduced from the experiments that at 78K even the adsorption heat of oxygen is not sufficient for the surface layers of tungsten to be rearranged. The influence of temperature and an electric field on the emission pattern of the tip and its shape was also studied.     

Interactions du rhénium à haute température avec N2O sous basse pression

Starting at room temperature, N20 adsorption on rhenium proceeds dissociatively. Oxygen atoms remain on the surface while nitrogen molecules are desorbed. The overall process is characterized by an initial sticking coefficient value equal to 0.3 at 298 °K. In stationary conditions, and in a higher temperature range (> 1200°K) rhenium trioxide and oxygen atoms are the reaction products, depending on oxygen coverage on the surface. When the oxygen coverage is low, atomization, characterized by a reactive sticking probability of 0.2 is the only observable process. All the results are consistent with a model, previously proposed for the system oxygen-rhenium and oxygen-transition metals. The main differences in reaction rates between rhenium and oxygen or N₂O are interpreted in terms of saturation coverages.     

Study of adsorption of titanium on tungsten and rhenium by field electron emission

The deposition and layer growth of titanium, evaporated on to tungsten and rhenium field emitter tips has been studied, and field emission average work-functions measured at varying titanium layer thicknesses from less than a monolayer to several monolayers. Titanium evaporated on to a clean tungsten tip at 293 K in the submonolayer region tended to collect preferentially on atomically rough planes. With further deposition, a “pseudo-clean” emission pattern was obtained at about a monolayer coverage when the titanium atoms were concluded to be in epitaxial registry with the BCC substrate; at about this coverage the work-function for the system passed through a minimum. Further titanium deposition in excess of three monolayers resulted in a progressive roughening of the surface due to microcrystallinity: brief heating (~ 1 min, 1300 K) of such a surface resulted in a well ordered layer identified as ß-titanium (BCC) presumably epitaxed to the tungsten. This epitaxed layer remained indefinitely stable at 293 K.     

Hydrogen promoted corrosion of tungsten by oxygen in an electric field: A field ion microscope study

The extensive changes in surface topography observed to occur on tungsten field ion emitters as a result of exposure to oxygen in presence of hydrogen at 78°K and at fields of 2 V/Å have been studied in detail. Field promoted diffusion of gas from the low field region of the emitter shank over the imaged area of the surface removes kink site metal atoms and subsequently deposits them on either side of the well defined diffusion paths along close packed zones; field evaporation of such atoms may also occur when the change in local surface contour causes sufficient field enhancement. Although oxygen is primarily responsible for the corrosion effects, its diffusion at such temperatures is promoted by the presence of hydrogen, and the rate of reaction is dependent, inter alia, on hydrogen pressure.     

Kinetic modelling of the H2---O2 reaction on Pd and of its influence on the hydrogen response of a hydrogen sensitive Pd metal-oxide-semiconductor device

We have established a model for the water forming reaction on Pd in the temperature range 350–475 K. Importantly, the model takes into account the possibility that hydrogen may absorb and adsorb at interface sites on supported Pd catalysts. It is shown that already at modest conditions interface adsorption may significantly affect reaction rates. The model may also be used to quantify the response of a hydrogen sensitive Pd-MOS device during hydrogen sensing in oxygen. In the case of Pd supported on SiO₂, the concentration of interface sites is so low that interface hydrogen adsorption will have only a minor influence on a catalytic reaction. The fact that a Pd-MOS device may be used as a very sensitive hydrogen detector at atmospheric oxygen conditions, despite a steric oxygen blocking of hydrogen dissociation sites, is predicted by the model.     

The water-forming reaction on palladium

The water-forming reaction on Pd has been studied on a PdSiO₂Si (Pd-MOS) structure in the temperature range 323–473 K. The reaction is found to be of the Langmuir-Hinshelwood type with the formation of OH beeing rate limiting. Since the Pd-MOS structure works as a sensitive hydrogen detector unique information on the behaviour of hydrogen during this catalytic reaction has been obtained. The reaction can be described in a model where the hydrogen atoms on the Pd surface have a large temperature activated lateral mobility and with no evidence of beeing in hot precursor states. At T = 473 K this means that for oxygen coverages ? 0.01 monolayers all hydrogen adsorbed will also react with oxygen. For smaller oxygen coverages unreacted hydrogen will not initially desorb towards the vacuum but towards the internal Pd surface of the Pd-MOS structure. Futhermore, hydrogen adsorption is blocked by adsorbed oxygen. The sticking coefficient for hydrogen on the bare Pd surface is, however, close to one and only weakly temperature dependent. An effect giving rise to a hysteresis in the work function versus oxygen coverage curve during oxygen adsorption - desorption is also discussed.     

Surface composition of prepared tungsten carbide and its catalytic activity

The surface composition and electronic structure of carburized tungsten trioxide, which is prepared by heating WO₃ in carbon monoxide atmosphere at 700°C, are investigated using X-ray photoelectron spectroscopy (XPS). The relationship between the surface composition and the catalytic activity for methanol electro-oxidation is clarified. The formation of tungsten carbide at the surface enhances the catalytic activity. On the other hand, the presence of free carbon or tungsten trioxide in the surface layer reduces the activity remarkably. It is also shown that, the higher the electronic density of states near the Fermi level, the higher the catalytic activity. Additionally, the catalytic behavior of tungsten carbide for methanol electro-oxidation is mentioned.     

Results on low pressure oxygen adsorption on a Pt60Re40 alloy sample: A SIMS and XPS study

Low pressure oxygen adsorption on Pt₆₀Re₄₀ was investigated at 300, 570 and 770 K employing XPS and SIMS. At 300 K oxygen chemisorption on both Re and Pt surface atoms was observed. At 570 and 770 K. a rhenium surface oxide covers the alloy surface after oxygen treatment. It is suggested that this surface oxide equals “ReO” as observed by Alnot et al. on pure Re.     

Application of Black Pearl carbon-supported WO3 nanostructures as hybrid carriers for electrocatalytic RuSex nanoparticles

RuSe_x electrocatalytic nanoparticles were deposited onto hybrid carriers composed of Black Pearl carbon-supported tungsten oxide; and the resulting system's electrochemical activity was investigated during oxygen reduction reaction. The tungsten oxide-utilizing and RuSe_x nanoparticle-containing materials were characterized using transmission electron microscopy, X-ray diffraction and electrochemical diagnostic techniques such as cyclic voltammetry and rotating ring-disk voltammetry. Application of Black Pearl carbon carriers modified with ultra-thin films of WO₃ as matrices (supports) for RuSe_x catalytic centers results during electroreduction of oxygen in 0.5 mol dm⁻³ H₂SO₄ (under rotating disk voltammetric conditions) in the potential shift of ca. 70 mV towards more positive values relative to the behavior of the analogous WO₃-free system. Also the percent formation (at ring in the rotating ring-disk voltammetry) of the undesirable hydrogen peroxide has been decreased approximately twice by utilizing WO₃-modified carbon carriers. The results are consistent with the bifunctional mechanism in which oxygen reduction is initiated at RuSe_x centers and the hydrogen peroxide intermediate is reductively decomposed at reactive WO₃-modified Black Pearl supports. The electrocatalytic activity of the system utilizing WO₃-modified Black Pearl supports has been basically unchanged upon addition of acetic acid, formic acid or methyl formate to the sulfuric acid supporting electrolyte.     

Adsorption studies of aluminum oxide on rhenium by means of field emission microscopy

Aluminum oxide deposited on a clean rhenium field emission microscope tip at room temperature starts to migrate on the metal surface at about 600° C. Various face specific layer formations occur above this temperature. They either increase or decrease the local electron emission. Complete desorption of the aluminum oxide is obtained at about 1350°C. The remaining oxygen can be removed by heating at 1900°C. Average work function changes are discussed together with the corresponding FEM patterns of the various adsorption stages. Comparisons are made with the adsorption system aluminum oxide on tungsten.     

钨中氢同位素热脱附实验的速率理论模拟研究

本文采用基于速率理论的模拟方法研究钨材料中氢同位素氘的热脱附谱. 热脱附数据来源于520 K下受等离子体辐照的多晶钨, 入射离子能量为40 eV, 剂量为1× 10²⁶ D/m². 通过调节速率理论中的俘获能、俘获率等参数, 最终获得与实验相符合的热脱附拟合谱. 拟合结果表明, 钨中俘获的氘存在于三种俘获态, 俘获能分别为1.14 eV, 1.40 eV和1.70 eV, 相应脱附温度峰值为500 K, 600 K和730 K. 这三个俘获能分别应对应于第一原理计算得到的空位俘获第3–5个氢原子的俘获能(含零点振动能修正)、空位俘获第1–2个氢原子的俘获能, 空位团簇对氢原子的俘获能. 模拟结果表明, 在本辐照实验条件下, 钨中空位及空位团簇是氘在钨中的主要俘获态.     

Field ion microscope observations of surface rearrangement and oxide formation at clean rhenium surfaces heated in oxygen

The interaction between oxygen and clean, well-ordered rhenium surfaces has been investigated in the field ion microscope over a wide range of temperatures. The surface structure of vacuum annealed specimens was extensively modified by the presence of adsorbed oxygen. At partial monolayer coverages, surface metal atom rearrangement was inhibited, particularly at {112?2}, but at coverages near saturation, facets were produced at {101?0} at a lower temperature than on the clean surface, resulting in the exposure of an increased proportion of closepacked surface structures and suggesting a change in the rearrangement mechanism at high coverages. The production of facets at the major poles was temperature and coverage dependent as a result of the varying influence of adsorbed oxygen on surface free energy. A decrease in oxygen coverage was observed at 1200–1300 K on near-saturated surfaces, which may be associated with the formation of volatile ReO₃ from regions where oxygen concentration exceeds a critical value. There was little evidence of the production of a discrete surface oxide phase formed on specimens heated in the presence of gas phase oxygen, but a reduction in specimen radius as a result of volatile oxide formation was observed at pressures greater than 10^?4 Torr and temperatures above 1400 K. It is concluded that the final structure of rhenium surfaces heated in oxygen is dependent upon the rate of oxide formation, the effective oxygen coverage during oxidation, and the extent of surface rearrangement.     

Electron induced ion desorption of oxygen adsorbed on rhenium

The behavior of low energy electron-stimulated O⁺ ions desorbing from oxygen adsorbed on polycrystalline rhenium samples has been found to be significantly different from those observed desorbing from other oxygen ad layers. Oxygen initially adsorbed at 300°K has an ionic desorption cross section below the level of detectability, (< 10⁻⁹ ions/electron) in these experiments, a result in accordance with that previously observed on other metals. On continued oxygen exposure, the O⁺ signal first increases to a maximum value and then decays to a lower level; the rate of both these processes is directly proportional to the oxygen pressure in the gas phase. If the temperature of the ad layer is raised above 300°K, the adsorbed species with high ionic desorption cross section are removed at about 500°K. At higher temperatures, O⁺ ions which can be related to the oxygen initially adsorbed are detected. The possible reasons for the observed O⁺ desorption characteristics for oxygen on rhenium are discussed.     

过渡金属锆铁对钨中氢氦行为的影响

钨被广泛地认为是最具潜力的面向等离子体材料。钨在聚变堆中的服役过程中,会受到强束低能的氢氦粒子流的影响,其结果是钨的性能下降。本工作通过第一性原理计算的方法研究了过渡金属锆铁对钨中氢氦行为的影响。研究结果表明,锆或铁的掺入会使钨的机械强度降低,延展性增加;锆铁的掺入不会改变氢氦在钨中的最佳占位,但是它们对氢氦在钨中的稳定性有一定影响;锆和铁对钨中氢氦的捕获作用存在一个最佳作用半径;铁原子在短距离（< 3.626 Å）时对氦有捕获作用,在长距离（> 3.626 Å）时存在排斥作用,而锆对钨中氢氦均具有捕获作用;钨中氢表现为亲电子的性质,而氦表现出疏电子的特性。总体上讲,锆对钨中氢氦的捕获作用要强于铁对钨中氢氦的捕获作用。本研究工作能够为钨基面向等离子体材料制备提供理论指导。Tungsten was widely considered as a highly promising candidate of plasma facing material, while the presence of hydrogen and helium has an adverse effect on the performance of the tungsten. The effects of transition metals (zirconium, iron) on the behavior of hydrogen and helium in tungsten were investigated by using the first-principles calculation method. The results show that doping of zirconium and iron decreases the mechanical strength of tungsten a little, but they increase the ductility of tungsten; zirconium and iron can't change the best occupied site of hydrogen and helium in tungsten, but they have some effect on the stability of the point defects formed by hydrogen and helium in tungsten; there is the best attraction radius between the transition metals (zirconium, iron) and hydrogen or helium in tungsten; there is an attractive interaction between iron and helium in a short distance (<3.626 Å), but a repulsion interaction in a long distance (>3.626 Å). An attractive interaction exists between zirconium and helium or hydrogen in tungsten whatever the distance is; the hydrogen that in tungsten has an electrophilic nature, while the helium has opposite features. The attraction interaction between zirconium and hydrogen or helium in tungsten is stronger than that of iron. Our works in this paper might provide a theory guide for the selection and preparation of the tungsten based alloy that is used as the plasma facing materials.     

The co-adsorption of oxygen and hydrogen on Rh(111)

The co-adsorption of oxygen and hydrogen on Rh(111) at temperatures below 140 K has been studied by thermal desorption mass spectrometry, Auger electron spectroscopy, and lowenergy electron diffraction. The co-adsorption phenomena observed were dependent upon the sequence of adsorption in preparing the co-adsorbed overlayer. It has been found that oxygen extensively blocks sites for subsequent hydrogen adsorption and that the interaction splits the hydrogen thermal desorption into two states. The capacity of the oxygenated Rh(111) surface for hydrogen adsorption is very sensitive to the structure of the oxygen overlayer, with a disordered oxygen layer exhibiting the lowest capacity for hydrogen chemisorption. Studies with hydrogen pre-adsorption indicate that a hydrogen layer suppresses completely the formation of ordered oxygen superstructures as well as O₂ desorption above 800 K. This occurs with only a 20% reduction in total oxygen coverage as measured by Auger spectroscopy.