Adsorption of Cs and O2 on MoS2

Adsorption of Cs on basal planes of MoS₂ has been studied with LEED, Auger and work function measurements. LEED observations show that in the 200–300 K range Cs is adsorbed as amorphous layers on MoS₂. Correlation of Auger and work function measurements indicates that the work function, sticking coefficient and the maximum density of Cs that can be deposited on the MoS₂ surface depend strongly on substrate temperature. Cesium is deposited on MoS₂ in two adsorption states. Although MoS₂ is extremely inert to O₂ adsorption, the presence of Cs causes a drastic increase in the adsorption of oxygen which in turn increases the amount of Cs that can be deposited on the surface. Lastly, it has been found that part of the Cs adatoms are diffused into the bulk of MoS₂.     

Isotope effects in electron impact desorption of CO and O2 adsorbed on the (110) plane of tungsten

Results on the isotope effect for total and ionic desorption cross sections in the electron impact desorption of various binding states of CO on the (110) plane of tungsten, and of oxygen on this plane are presented and discussed. It is shown that the observations allow a dissection of cross sections into excitation cross sections and escape probabilities, and that the latter can be used to estimate lifetimes of excited or ionic states. It is found that excitation cross sections for total desorption are of the order of 10⁻¹⁶–10⁻¹⁷ cm², but seem to be significantly smaller in some cases for excitation to ionic states, suggesting that different excitations are involved. In all cases examined here the isotope effect for total desorption is much smaller than for ion production. This can be explained by the fact that ion lifetimes are somewhat shorter than those of excited neutrals. Lifetimes are estimated, in the cases examined, to be of the order of 10⁻¹⁴s.     

Measurement of fast desorption kinetics of D2, from tungsten by laser induced thermal desorption

A laser heating technique for studying fast surface processes has been applied in an initial study to the thermal desorption of D₂ from a polycrystalline tungsten sample. This technique is a means for measuring surface reactions at rates, concentrations, and temperatures that approach conditions of technical interest, but with the high degree of definition and control made possible with an ultrahigh vacuum apparatus. The method is analogous to the fast temperature jump method used for studying reactions in condensed phases, and can sort out elementary processes that have differing activation energies. The variation of total flux desorbed with maximum surface temperature reached and initial surface coverage serves, with the aid of a model kinetic rate expression, to determine the desorption rate parameters. It is shown that the desorption of D₂ from W at rates of 5 × 10⁷ monolayers/sec is governed by the same kinetics as obtained by extrapolating previous measurements made at a rate about 10⁵ times slower. The surface is subjected to a sufficiently fast and large temperature rise to desorb surface atoms or molecules in a time short compared to the range of flight times to a mass spectrometer detector. In this way the velocity distribution of the desorbing species may be determined. This along with the surface temperature history gives additional information on the reaction rate model and also whether the species are emerging in translational thermal equilibrium with the surface. In the present experiments a significant number of desorbatedesorbate collisions occur. Corrections are made for the collision effects in the interpretation of the data. It is shown how modifications of the technique can be made to substantially eliminate these effects. The present conditions were laser pulse width of 3 × 10^?8 sec and surface temperature rise of 300 to 3000 K.     

Electron stimulated desorption of O2 from gadolinia-doped ceria surfaces

The interactions of gas phase oxygen with gadolinia-doped ceria (GDC) surfaces are investigated by electron stimulated desorption (ESD). The primary desorbed cationic species related to molecular oxygen adsorption is O₂⁺. The threshold energy for ESD of O₂⁺ is 13–14 eV, indicating electron impact ionization of molecular oxygen bound at oxygen vacancies. Dependence of O₂⁺ velocities upon incident electron energy and substrate temperature reveals the dominant influence of the effective charge of the adsorption complex. The O₂⁺ velocity distribution is bimodal, and the onset of the faster components at room temperature is related to the balance between fluxes of incident electrons and secondary electron emission, causing effective hole production and neutralization of trapped electrons at surface states.     

A computational study on nanocrystalline SnO2: Adsorption of CO and O2 onto defective nanograins

This work presents a study of the adsorption properties of defective nanostructures. The calculations have quantum mechanical detail and are based on a semi-empirical Hamiltonian, which is applied to the evaluation of both the electronic structure and of the conductance. The material considered in this study, i.e. SnO₂, has a widespread use as gas sensor and oxygen vacancies are known to act as active catalytic sites for the adsorption of small molecules. In the following calculations crystalline SnO₂ nanograins, with a size and shape comparable with the experimental ones, have been considered. The grains lattice, which has the rutile structure of the bulk material, includes oxygen vacancies and the adsorbed system is generated by depositing a gaseous molecule, either CO or O₂, above an atom on the grain surface. The calculations show that the presence of the defects enhances the grain cohesion and favors adsorption. The conductance has a functional relationship with the structure and the defective state of the nanograins and its dependence on these quantities parallels the one of the binding energy.     

Wing profile measurements of the Na-doublet lines in C2H2/O2/N2, H2/O2/N2 and H2/O2/Ar frames at 1 atm

Fluorescence-excitation (wing) profiles of the Na-D doublet lines were measured over a wavelength range extending from 0.3 to 200 Å from the line center for the red D₁ and blue D₂ wings and from 0.3 to 3 Å for the red D₂ and the blue D₁ wings, respectively. The line profiles were determined with the aid of a tunable CW dye-laser as a background source by measuring the total fluorescence intensity observed on detuning the laser wavelength. The flames were premixed, laminar, shielded flames at 1 atm, with temperatures ranging from 1860 to 2270 K; N₂ and Ar served as diluent gases. The line core and near-wing profiles (i.e. the region covering 0.3<Δλ<7 Å for the outer wings and 0.3<Δλ<3 Å for the inner ones) in all of the flames studied appeared to have the same frequency dependence, regardless of the nature and concentrations of the gases used. The blue D₂-line profile followed an unexpected (-2.2) law, while the other three profiles obeyed the theoretically expected (-2) law (the dispersion profile function). The line profile in the Δλ range between the impact and quasistatic regions was found to depend on the main perturbers involved. We found that the far blue D₂- and red D₁-wings in the Ar-diluted H₂/O₂ flame obeyed the (-$\text{5}\text{4}$) and (-$\text{3}\text{2}$) laws, respectively, as predicted by the quasi-static theory for the Lennard-Jones interaction. For the N₂-diluted C₂H₂/O₂ and H₂/O₂ flames, we did not find these wing dependences in the Δλ range investigated.     

Adsorption mechanisms of translationally-energetic O2 and N2: direct dissociation versus direct molecular chemisorption

A direct dissociation mechanism has been traditionally assigned to molecular beam data that exhibit an increase in the initial adsorption probability with increasing kinetic energy. Yet, recent experiments of nitrogen and oxygen adsorption provide support for an alternative high kinetic energy pathway in which incident energy assists in surmounting barriers to molecular chemisorption on a surface as the first step to dissociation. Moreover, systems for which the experimental evidence supports such a mechanism also demonstrate that molecularly chemisorbed intermediates can be spectroscopically observed at low temperatures and coverages from exposure to a gas in thermal equilibrium at room temperature. Likewise, such observations have not been measured for systems which are consistent with direct dissociation. A consideration of this trend regarding the existence of molecularly chemisorbed states and the implications for the dominant, dissociative chemisorption pathway at high kinetic energy is presented for a number of gas surface systems.     

Al2O3-Y2O3-ZrO2三相复合陶瓷的介电谱研究

采用传统的固相反应法,在1400–1500 ℃下烧结,制备得到Al₂O₃-Y₂O₃-ZrO₂三相复合陶瓷.样品的结构、形貌和电性能分别用X射线衍射(XRD)、扫描电子显微镜(SEM)及介电谱表征.XRD表明此三相复合体系无其他杂相,加入Y₂O₃及ZrO₂后使得Al₂O₃成瓷温度降低;SEM表明此体系晶粒直径为200–500 nm,并且样品随烧结温度的升高而变得更加致密,晶界更加清晰;介电损耗谱中出现峰值弛豫现象,根据Cole-Cole复阻抗谱得出其为非德拜弛豫. 关键词： 2O₃-Y₂O₃-ZrO₂三相陶瓷')" href="#">Al₂O₃-Y₂O₃-ZrO₂三相陶瓷 介电弛豫 阻抗谱 热导率     

Adsorption of H2S,H2O and O2 on Si(111) surfaces

Electron energy-loss spectroscopy has been applied to the study of Si(111) surfaces covered with H₂S, H₂O and O₂ at room temperature and the surfaces annealed at ～ 600°C. The experimental results strongly suggest that H₂S and H₂O adsorb in the molecular states at room temperature. It is proposed that O₂ is first adsorbed in a molecular state, then adsorbs as atoms, and finally oxidizes forming SiO₂.     

H2 and CO chemisorption on polycrystalline titanium studied by flash desorption mass spectrometry

We have used flash desorption mass spectroscopy to study the adsorption and desorption of H₂ and CO from clean titanium at room temperature. CO flash desorption occurs predominantly from a low temperature state whose binding energy is 20.3 kcal/mole. H₂ flash desorption is complex. Only one peak is observed; it is broader than flash desorption spectra normally corresponding to first or second order kinetics. The shift in the peak temperature to lower values with increasing coverage has been analysed using the isothermal desorption rate technique. The apparent order of H₂ desorption is 1.5 and is independent of temperature from 888 to 1077 K. The activation energy is 21 kcal/mole. These results will be discussed in terms of absorption of H₂ into titanium and thermal decomposition of a titanium hydride compound.     

Effect of the incident electron fluence on the electron emission yield of polycrystalline Al2O3

The electron emission yield due to electron impact on polycrystalline Al₂O₃ is measured with a technique based on the use of a Kelvin probe (KP method) and a pulsed electron beam. The KP method allows the clear discrimination between the external effects of charging and internal ones. The effect of the incident electron fluence on the yield in the region where the yield is higher than one is investigated. An overall drop of the electron emission yield with increasing the electron fluence is observed. This result is clearly associated to the internal effects of positive charging. Indeed, the recombination of the generated secondary electrons with the accumulated holes beneath the irradiated surface leads to the decrease of their mean free path and to the decay of the secondary electron emission yield.     

Adsorption studies on a stepped Pt(111) surface: O2, CO,C2H4, C2N2

A comparative study of the adsorption of several gases on a Pt(S)-[9(111) × (111)] surface was performed using LEED, Auger spectroscopy, flash desorption mass spectrometry and work function changes as surface sensitive techniques. Adsorption was found to be generally less ordered on the stepped surface than on the corresponding flat surface with the exception of the oxygen, where r well ordered overlayer in registry over many terraces was found. Absolute coverages were determined from flash desorption experiments for O₂, CO and C₂N₂. Similar values were obtained as on flat Pt surfaces. Two different surface species seem to be formed upon adsorption of C₂H₄ depending on the adsorption temperature. Contrary to reports from Pt(111) surfaces conversion between the two surface species is heavily restricted on the stepped surface. Work function changes revealed nonlinear adsorbate effects where the adsorbate is electronegative with respect to the substrate. Various adsorption models are discussed in the light of complementary experimental evidence. The results of this study are compared with data available from flat Pt surfaces and possible influences of steps are discussed. No general trends, however, emerge from this comparison and it seems that eventual influences of steps have to be considered individually for every adsorbate.     

Adsorption kinetics of alkanes on TiO2 nanotubesarray - structure-activity relationship

Presented are thermal desorption spectroscopy (TDS) measurements of iso-/n-butane adsorption on a variety of TiO₂ nanotubes (TiNTs) samples which are characterized by different crystal structures. The results are compared with a prior study on anatase(0 0 1) thin films grown on SrTiO₃(0 0 1). A distinct kinetic structure-activity relationship was present, i.e., the binding energies of the alkanes depend on the polymorph (anatase vs. mixed anatase/rutile) of TiO₂. A direct-fitting procedure of the TDS data has been applied to extract the kinetics parameters. The binding energies in the limit of zero coverage decrease as anatase thin film > amorphous-TiNTs ∼ polycrystalline anatase TiNTs > polycrystalline mixed anatase/rutile TiNTs.     

Combustion properties of H2/N2/O2/steam mixtures

The present work reports new experimental and numerical results of the combustion properties of hydrogen based mixtures diluted by nitrogen and steam. Spherical expanding flames have been studied in a spherical bomb over a large domain of equivalence ratios, initial temperatures and dilutions at an initial pressure of 100 kPa (T_ini = 296, 363, 413 K; N₂/O₂ = 3.76, 5.67, 9; %Steam = 0, 20, 30). From these experiments, the laminar flame speed $S_L^0$, the Markstein length L’, the activation energy Ea and the Zel'dovich β number have been determined. These parameters were also simulated using COSILAB^® in order to verify the validity of the Mével et al. [1] detailed kinetic mechanism. Other parameters as the laminar flame thickness δ and the effective Lewis number Le_eff were also simulated. These new results aim at providing an extended database that will be very useful in the hydrogen combustion hazard assessment for nuclear reactor power plant new design.     

Reaction kinetics of the Pt, O2(g)|c-ZrO2 system: precursor-mediated adsorption

A micro kinetic model of the Pt, O₂(g)|c-zirconia electrode/electrolyte system was developed in state space form (model M3). The oxygen adsorption/desorption process was modeled as a precursor-mediated surface reaction. The surface diffusion of atomic oxygen and the electrochemical reduction of atomic oxygen near the three-phase boundary (tpb) were considered. It was shown that the simulated charge-transfer behavior of M3 is significantly different from models with ordinary Langmuir kinetics (model M2). The electrochemical rate constant was estimated from selected experimental data as k₁₀=(6.05±0.25)·10⁶ m³/(mol·s). From experimental results it was concluded that only one adsorbed oxygen species is relevant for the dynamic behavior. In porous Pt electrodes binary gas phase diffusion of oxygen in O₂/N₂ gas mixtures becomes relevant at oxygen partial pressures below 10⁻³ atm. The general procedure for state and parameter estimation can be well adopted for the investigation of further reaction mechanisms.     

Rotation des moments magnetiques dans BaLa2 Fe2 O7

In the compound BaLa₂Fe₂O₇, below 235 K, spins are oriented along the diagonal of the quadratic cell in an antiferromagnetic configuration. Above this temperature we observe by neutron diffraction a continuous rotation of the magnetic moments in the base plane xyO, the configuration remaining antiferromagnetic; the magnetic symmetry goes from I_pm′mm′ to P2′/m Shubnikov's group. At the transition temperature we observe a discontinuity of the thermal expansion coefficient. This phenomenon can be interpreted in agreement with Landau's theory of second order transitions.     

Photoluminescence of O2 and S2 ions in synthetic sodalities

Photoluminescence spectra of oxygen-doped chloro- and bromosodalites and sulfur-doped chloro-, bromo- and iodosodalites were measured at temperatures between 4.2 and 300 °K. At 4.2 and 77 °K, the emission spectra of oxygen-doped sodalites consisted of a series of peaks in the wavelength range 400–700 nm, with an average energy separation of ∼ 1000 cm^-1. In addition, fine structure, attributed to lattice modes, was observed in each vibrational band. At 4.2 and 77 °K, the sulfur-doped samples showed a multiband spectrum in the 500–750 nm range, with an average separation of ∼ 570 cm^-1 between bands. The spectrum at 4.2 °K exhibited some asymmetry not observed at 77 °K, but no fine structure was resolved. At 300 °K weak, broad-band luminescence was observed from both oxygen- and sulfur-doped samples, with no vibrational structure evident. The results compared very favorably with those reported for oxygen- and sulfur-doped alkali halides, and by analogy the spectra were attributed to luminescence from O^-₂ and S^-₂ molecular ions.     

Associative desorption of adsorbed oxygen promoted by translational energy of incident O2 molecules

Effects of the translational energy of incident oxygen molecules on surface processes from dissociative adsorption to subsequent associative desorption on Pt(1 1 1) have been investigated by means of supersonic molecular beam scattering. The reaction rate of associative desorption increases with the incident translational energy. The enhanced associative desorption can be attributed to nonthermal diffusion of dissociated oxygen, promoted by the translational energy of incoming molecules.     

CO探针原位红外光谱研究Pt/Sn双金属重整催化剂

催化重整是生产芳烃原料和高辛烷值清洁汽油调和组分的重要工艺。以目前应用广泛的铂锡工业重整催化剂金属含量为参比,用工业剂制备方法合成了Pt含量为0.6%的一系列铂锡重整催化剂,建立CO探针原位红外的表征方法,并对其进行系统表征,首次获得了1%以下低含量助剂Sn的CO探针红外谱图。研究结果表明,系列剂的金属Pt的CO吸附特征峰主要以线式吸附状态存在。0.6%纯Pt剂上1 826 cm-1处CO桥式吸附特征峰,因添加助剂Sn后,强度下降,而CO线式吸附特征峰的强度则增加,说明Sn的加入使得Pt的分散度增加。变温CO探针吸附原位红外研究表明,对负载质量分数0.3%的纯Sn催化剂,当脱附温度升高至120 ℃时,吸附在Sn上的CO特征峰会完全消失。对负载质量分数0.6%的纯Pt催化剂,当脱附温度升高至300℃时,吸附在Pt中心上的CO特征峰会完全消失。当Pt-Sn双金属负载质量分数Pt为0.6%、Sn为0.3%时,CO的脱附温度明显提高达350 ℃。与纯Pt剂相比,随着Sn助剂的加入,使得CO的脱附温度稍有提升,Pt-Sn催化剂Pt的CO特征峰向高波数方向移动,说明Sn的加入一定程度上减弱了活性金属Pt中心上的电荷密度。因而,CO探针原位红外是表征低金属铂锡工业重整催化剂的有效手段,为阐明多金属重整催化剂的助剂作用和研究反应机理提供重要信息。