锂原子修饰B6团簇的储氢性能研究

利用密度泛函理论研究B₆和Li_mB₆ (m= 1–2)团簇的结构及其储氢性能. 结果表明, 氢分子在B₆团簇的三种可能结构中均发生解离吸附, Li原子在B₆团簇表面不发生团聚,每一个Li原子均吸附几个氢分子. 其中以两个Li原子修饰笼形B₆团簇吸附完整氢分子数最多,储氢质量分数为20.38%, 氢分子的平均吸附能为1.683 kcal/mol,表明了它在常温常压条件下作为储氢材料的可行性. 关键词： mB₆ (m=1-2)团簇')" href="#">Li_mB₆ (m=1-2)团簇 密度泛函理论(DFT) 吸附能 储氢性能     

碱金属修饰的多孔石墨烯的储氢性能

基于第一性原理深入研究了碱金属原子(Li,Na,K)修饰的多孔石墨烯(PG)体系的储氢性能,并且通过从头算分子动力学模拟了温度对Li-PG吸附的H2分子稳定性的影响.研究结果表明,PG结构的碳环中心是碱金属原子最稳定的吸附位置,PG单胞最多可以吸附4个碱金属原子,Li原子被束缚最强,金属原子间无团聚的倾向;H2分子通过极化机制吸附在碱金属修饰的PG结构上,每个金属原子周围最多可以稳定地吸附3个H2分子;Li-PG对H2分子的吸附最强(平均吸附能为-0.246 eV/H2),Na-PG对H2分子的吸附较弱(平均吸附能为-0.129 eV/H2),K-PG对H2分子的吸附最弱(平均吸附能为-0.056 eV/H2),不适合用做储氢材料;在不考虑外界压强且温度为300 K的情况下,Li-PG结构可稳定地吸附9个H2分子,储氢量为9.25 wt.%;在400 K时,有7个吸附H2分子脱离Li-PG的束缚,在600-700 K的范围内,吸附H2分子全部脱离了Li-PG体系的束缚.     

Hydrogen isotope replacement reactions on rhenium

The existence of a mobile equilibrium at room temperature between part of the hydrogen adsorbed on rhenium and gaseous hydrogen is demonstrated by the easy exchange of isotopes between the adsorbed layer and the gas phase. The adsorbed gas is desorbed as a mixture of homonuclear molecules (of H₂ or D₂)and of the isotopically mixed species (HD). However, the replacement reactions are not symmetrical; there is a greater proportion of HD in the desorbed gas when deuterium is replaced by hydrogen than in the converse reaction. This kinetic isotope effect is attributed to differences between the zero-point energies of the various hydrogen containing species.Quantatitive agreement between the shapes of the experimentally observed desorption curves and calculated curves is obtained if the zero-point energy of the bond between a surface rhenium atom and deuterium is assigned the value 2.6 kcal mole^?1.     

Self-oscillatory heterogeneous recombination of hydrogen atoms and nonequilibrium desorption of molecules from the surface (Teflon)

The channel of the accommodation of the energy of a heterogeneous chemical reaction (recombination of hydrogen atoms) related to vibrational V-V exchange between excited chemical reaction products and adsorption layer molecules (H₂O, HDO, D₂O, and H₂) was studied by the method of modulated molecular beams. The chemical reaction was found to proceed in an oscillatory mode caused by the nonequilibrium character of its elementary steps. The participation of adsorbed molecules in accommodation was studied by analyzing nonequilibrium desorption of these molecules. An isotope effect was observed in nonequilibrium desorption. The kinetic mechanism of the reaction and the micromechanism of elementary reaction events, which determine the “physical” mechanism of catalysis in the system under study, are discussed.     

UV desorption and photochemistry of dimethylgold hexafluoroacetylacetonate adsorbed on a quartz substrate

The photodesorption and photodecomposition pathways of dimethylgold hexafluoroacetylacetonate, DMG (hfac), adsorbed on a cooled quartz substrate is reported for 222-nm KrCl excimer laser radiation. The time-of-flight (TOF) of neutral photoproducts, desorbed from the surface of the gold film formed during the experiment, were analyzed under collisionless conditions by a differentially-pumped mass spectrometer. Extensive dissociation of adsorbed DMG (hfac) into DMG and the hfac ligand was observed. The ligand was found to recombine with a CH₃ radical on the surface. Translational energy distributions for the detected species were obtained by deconvoluting the TOF curves into a self-consistent set of Maxwell-Boltzmann distributions for the desorbed parent molecule, laser-induced decomposition products, and surface recombination reaction products. The implications of these results for the mechanistic details of the low-pressure, laser-assisted organometallic deposition of DMG(hfac) are discussed.ONT/NRL Research Associate (Nov. 1987-Oct. 1988)NRC/NRL Cooperative Research Associate     

Coadsorption of silicon and chalcogen (S, O) atoms on heated Re(10\bar 10) substrate

The coadsorption of silicon and Group VI elements on the Re $(10\bar 10)$ surface is investigated by the 1 high-resolution Auger spectroscopy. It is demonstrated that, upon deposition of silicon on the surface oxide or surface sulfide, a part of silicon atoms deposited interacts with chalcogen atoms to be desorbed in the form of SiO or SiS molecules. The rest of silicon atoms occupy the becoming free adsorption sites, thus forming surface silicide. The silicon atoms incorporated into the surface silicide loose their reactivity and coexist on the surface together with adsorbed chalcogen atoms.     

碱金属钠原子修饰硅原子团簇的结构及储氢性能研究

提出碱金属钠原子修饰笼形Si_6团簇的结构模型,采用密度泛函理论(DFT)研究钠原子修饰笼形Si_6团簇的结构及储氢性能.研究结果表明,氢分子与笼形Si_6团簇表面相互作用很弱,氢分子在其表面容易脱附.采用钠原子修饰笼形Si_6团簇后可有效避免氢分子的脱附,并且钠原子在笼形Si_6团簇的表面不发生团聚,有利于氢分子在其表面吸附和循环利用.研究发现在两个钠原子修饰笼形Si_6团簇的结构中,每个钠原子可以有效吸附六个氢分子.计算得到Na2Si_6团簇结构储氢的质量分数高达10.08 wt%,且氢分子的平均吸附能约为0.837 kcal/mol.可见,实现钠原子修饰笼形Si_6团簇结构在常温常压条件下储氢是有可能的.     

Charging and stabilization of Pd atoms and clusters on an electron-rich MgO surface

We report density functional theory calculations on the interaction of Pd atoms and small Pd clusters with an electron-rich MgO surface. This surface can be generated by forming a specific kind of defects, named (H⁺)(e⁻) centers, using well known chemical recipes. By deposition of gas-phase Pd atoms on the properly functionalized MgO surface, one can generate collections of small Pd cluster anions with peculiar chemical properties. The (H⁺)(e⁻) centers act as nucleation sites for diffusing Pd atoms and favor the formation of small, thermally stable clusters. The presence of an extra charge on the metal cluster results in a large vibrational red-shift of adsorbed CO molecules. The present results intend to stimulate experimental work to produce stable metal cluster anions on the surface of an ionic oxide.     

Adsorption and thermal decomposition of diethylaluminum hydride on Si(1 0 0)-2 × 1

Chemistry of organoaluminum compounds on silicon surfaces forms a foundation of chemical vapor deposition (CVD) for the formation of metal-semiconductor interconnects. We have applied multiple internal reflection Fourier-transform infrared spectroscopy and thermal desorption mass spectrometry to analyze the chemistry of one of the promising Al-CVD precursors, diethylaluminum hydride, on a Si(1 0 0)-2 × 1 surface. Diethylaluminum hydride adsorbs molecularly on this surface both at room temperature and at 100 K. Thermally induced surface reaction consumes the monolayer of adsorbed organoaluminum molecule. The only hydrocarbon product is ethylene desorbing from the silicon surface around 600 K. Despite a clean reaction that removes carbon from the surface, aluminum deposition is not significant because of the formation of alane products.     

含锰钢表面氢原子/氢分子吸附行为的第一性原理研究

采用基于密度泛函理论的第一性原理方法研究了氢原子和氢分子在纯铁表面和锰原子掺杂表面的吸附与解离行为.研究结果表明,氢原子可在纯铁(001)表面稳定吸附,吸附能按照顶位,桥位和心位依次增强;而溶质原子锰降低了氢原子距离表面的位置并强化了氢原子的吸附行为.氢分子在纯铁表面的吸附解离行为取决于氢分子距离模型表面的初始距离和初始空间构型.氢分子平行于纯铁(001)表面时,距离心位1.2?发生解离,而桥位、顶位均不会发生解离;氢分子垂直放置时,距离桥位0.6?、顶位1.0?发生解离,心位不会发生解离.氢分子平行于锰掺杂纯铁(001)表面时,距离桥位0.6?、顶位0.7?、心位1.2?发生解离;氢分子垂直放置时,距离桥位、心位0.8?发生解离,而顶位放置氢分子不发生解离.归纳可知,锰溶质原子掺杂会增加铁基体表面氢原子和氢分子的吸附作用并促进氢分子发生分解.     

Reactions and structures of water on clean and oxygen covered Pt(111) and Fe(100)

An atom superposition and election delocalization (ASED) technique applied to water adsorption on a small cluster model of Pt(111) shows weak and reversible chemisorption and facile and reversible hydrogen transfer to preadsorbed oxygen atoms as observed by Fisher, Sexton and Gland in EELS and UPS studies. Our theory predicts much stronger adsorption of water to Fe(100) with low barriers to dehydrogenation, in agreement with high temperature LEED-Auger results of Dwyer, Simmons, and Wei and wide temperature range XPS studies of Akimov. We predict a low barrier to transfer of hydrogen from water to adsorbed oxygen atoms, forming hydroxyl groups on the iron surface, and a fairly low barrier to the reverse reaction. On both metals we find hydroxyl groups are strongly held. Our calculations produce a trend toward greater negativity on going from adsorbed water to hydroxyl groups, and to hydroxyl dissociation products on these surfaces. We present reaction mechanisms, transition state geometries, and analysis in terms of molecular orbital theory and the total energy. It is found that the platinum is generally less reactive than iron toward water and hydroxyl species because platinum orbitals are less diffuse and the platinum s-d band lies lower, closer to adsorbate energy levels such that adsorbate-platinum antibonding orbitals are filled.     

Spatial and speed distributions of H2 and D2 desorbed from a polycrystalline nickel surface

The desorption of hydrogen (H₂ and D₂) from a polycrystalline nickel surface has been investigated by measuring the spatial and speed distributions of the desorbed molecules. The Ni specimen was constructed as a membrane with one side exposed to hydrogen at ~ 1 atm pressure and the other side exposed to vacuum, thereby enabling us to supply hydrogen to the test surface via permeation of atoms through the membrane. These atoms recombine on the surface to form molecules that desorb into the evacuated chamber. The spatial distribution of the desorbed molecules was measured with a rotatable ionization gauge, whereas the speed distribution of molecules desorbed along the surface normal was determined by means of a time-of-flight detector in a second apparatus.     

Influence of Pb adatom on adsorption of oxygen molecules on Pb(111) surface: a first-principles study

Using first-principles calculations, we systematically study the influence of Pb adatom on the adsorption and the dissociation of oxygen molecules on Pb(111) surface, to explore the effect of a point defect on the oxidation of the Pb(111) surface. We find that when an oxygen molecule is adsorbed near an adatom on the Pb surface, the molecule will be dissociated without any obvious barriers, and the dissociated O atoms bond with both the adatom and the surface Pb atoms. The adsorption energy in this situation is much larger than that on a clean Pb surface. Besides, for an adsorbed oxygen molecule on a clean Pb surface, a diffusing Pb adatom can also change its adsorption state and enlarge the adsorption energy for O, but it does not make the oxygen molecule dissociated. And in this situation, there is a molecule-like PbO2 cluster formed on the Pb surface.     

Li修饰的C24团簇的储氢性能

利用密度泛函理论研究了Li原子修饰的C₂₄团簇的储氢性能. Li原子在C₂₄团簇表面的最佳结合位是五元环. Li原子与C₂₄团簇之间的作用强于Li原子之间的相互作用, 能阻止它们在团簇表面发生聚集. 当Li原子结合到C₂₄表面时, 它们向C原子转移电子后带正电荷. 当氢分子接近这些Li原子时, 在电场作用下发生极化, 通过静电相互作用吸附在Li原子周围. 在Li修饰的C₂₄复合物中, 每个Li原子能吸附两到三个氢分子, 平均吸附能处于0.08到0.13 eV/H₂范围内. C₂₄Li₆能吸附12个氢分子, 储氢密度达到6.8 wt%.     

Dissociation of H_2 on Mg-coated B_(12)C_6N_6

The dissociation of H_2 molecule is the first step for chemical storage of hydrogen, and the energy barrier of the dissociation is the key factor to determine the kinetics of the regeneration of the storage material. In this paper, we investigate the hydrogen adsorption and dissociation on Mg-coated B_(12)C_6N_6. The B_(12)C_6N_6 is an electron deficient fullerene, and Mg atoms can be strongly bound to this cage by donating their valance electrons to the virtual 2p orbitals of carbon in the cluster. The preferred binding sites for Mg atoms are the B_2C_2 tetragonal rings. The positive charge quantity on the Mg atom is 1.50 when a single Mg atom is coated on a B_2C_2 ring. The stable dissociation products are determined and the dissociation processes are traced. Strong orbital interaction between the hydrogen and the cluster occurs in the process of dissociation, and H_2 molecule can be easily dissociated. We present four dissociation paths, and the lowest energy barrier is only 0.11 eV, which means that the dissociation can take place at ambient temperature.     

Two-dimensional hydrogen behaviors on solid surfaces studied by hydrogen microscope

Recently, much work has been done to study hydrogen behavior on solid surfaces for applications in fuel cells, semiconductor devices, and diamond-like carbon films. We have developed a hydrogen microscope making use of electron stimulated desorption (ESD) spectroscopy. A thermal-field emission type electron gun set to a low-energy range (<1 keV) is used to obtain a beam size less than 100 nm in diameter. A pulsed beam has been used to measure the time-of-flight (TOF) to detect desorbed ions from specimen surfaces. Scanning the pulsed beams across solid surfaces, a two-dimensional distribution image of hydrogen atoms can be obtained. This paper reviews some capabilities of the hydrogen microscope and a chemical state analysis for H and O adsorbed by different elements on a surface.     

Low energy electron diffraction and auger electron spectroscopy studies of the structure of adsorbed gases on solid surfaces

Low energy electron diffraction (LEED) studies of the structure of adsorbed molecules on crystal surfaces revealed that ordered surface structures predominate under most conditions of the experiments. In the absence of chemical reactions with the substrate, the degree of ordering depends on the heats of adsorption, ΔH_ads, and the activation energies for surface diffusion, ΔE_D1. Since ΔH_ads is usually markedly larger than ΔE_D1, small changes of substrate temperature facilitate ordering without appreciable increase in desorption rates. The surface structures of adsorbed gases that have been reported so far have been tabulated. For molecules whose size is compatible with the interatomic distance of the substrate, rules of ordering can be proposed that permit prediction of the structure of the adsorbed layer that is likely to form. These rules indicate close packing due to attractive interactions in the adsorbed layer, and that the rotational multiplicity of the substrate is likely to be maintained by the adsorbate structure. When molecules whose dimensions are larger than the substrate interatomic distance are adsorbed, the conditions that control ordering are more complex and simple rules may not be readily applicable.The surface structures of adsorbed gases have also been studied on high Miller Index substrate surfaces. These surfaces are characterized by ordered steps separated by terraces of low index surface orientation. Many gases have different ordering characteristics on stepped surfaces than on low index crystal faces due to the stronger substrate-adsorbate interactions in these surfaces. The dissociation of diatomic molecules at steps induces the formation of new types of surface structures (frequently one-dimensional) and the dehydrogenation of hydrocarbons at steps induces the formation of ordered carbonaceous surface structures that would not nucleate on low index substrate planes.So far, mostly work function changes upon adsorption gave indication of the magnitude of charge transfer upon adsorption and on forming of new surface chemical bonds. Most recently, chemical shifts of the Auger transitions of the substrate atoms and of the adsorbed molecules upon chemisorption, have been found to provide additional information on charge redistribution during adsorption.     

硼掺杂富勒烯储氢-从物理吸附到化学吸附的转变

本文采用密度泛函理论(DFT)中的局域密度近似(LDA)方法对硼(B)掺杂富勒烯(C_(35)B)储氢问题在前人的基础上做了进一步研究,结果表明被C_(35)B吸附的氢分子很容易解离,经历从物理吸附到化学吸附的转变,并且发现解离产物C_(34)BHCH有分子内氢转移反应发生,这时B原子仍能与氢分子有很强的相互作用,最终导致B位置以及与B最邻近的三个C原子上都有氢原子吸附.并利用过渡态理论从热力学上分析了这种反应的发生趋势.     

Formaldehyde decomposition and oxidation on Pt(110)

The decomposition reactions of formaldehyde on clean and oxygen dosed Pt(110) have been studied by LEED, XPS and TPRS. Formaldehyde is adsorbed in two states, a monolayer phase and a multilayer phase which were distinguishable by both TPRS and XPS. The saturated monolayer (corresponding to 8.06 × 10¹⁴ molecules cm⁻²) desorbed at 134 K and the multilayer phase (which could not be saturated) desorbed at 112 K. The only other reaction products observed at higher temperatures were CO and H₂ produced in desorption limited processes and these reached a maximum upon saturation of the formaldehyde monolayer. The desorption spectrum of hydrogen was found to be perturbed by the presence of CO as reported by Weinberg and coworkers. It is proposed that local lifting of the clean surface (1 × 2) reconstruction is responsible for this behaviour. Analysis of the TPRS and XPS peak areas demonstrated that on the clean surface approximately 50% of the adsorbed monolayer dissociated with the remainder desorbing intact. Reaction of formaldehyde with preadsorbed oxygen resulted in the formation of H₂O (hydroxyl recombination) and CO₂ (decomposition of formate) desorbing at 200 and 262 K, respectively. The CO and H₂ desorption peaks were both smaller relative to formaldehyde decomposition on the clean surface and in particular, H₂ desorbed in a reaction limited process associated with decomposition of the formate species. No evidence was found for methane or hydrocarbon evolution in the present study under any circumstances. The results of this investigation are discussed in the light of our earlier work on the decomposition of methanol on the same platinum surface.     

The adsorption of water on clean and oxygen covered Cu(110)

The water adsorption on clean and oxygen precovered Cu(110) surfaces is studied by means of UPS, LEED, work function measurements and ELS. At 90 K on the clean surface molecular water adsorption is indicated by UPS. The H₂O molecules are bonded at the oxygen end and the H-O-H angle is increased as compared with the free molecule. In the temperature range between 90 and 300 K distorted H₂O molecules and adsorbed hydroxyl species (OH) are detected, which are desorbed at room temperature. On an oxygen covered surface hydroxyl groups are formed by dissociation of adsorbed water molecules at a lower temperature than on the clean surface. Multilayers of condensed water are found below 140 K in both cases.