H2O和OH在UO(100)表面吸附的密度泛函研究

运用密度泛函理论中的广义梯度近似(GCA)的PW91方法结合周期性平板模型,研究了H2O分子和OH在UO(100)表面上的吸附.通过对不同吸附位的吸附能和几何结构参数的计算和比较发现:水分子在UO(100)表面的吸附为化学吸附,水分子平面与UO(100)表面夹角为15°的吸附构型最稳定,吸附能最大,近89 kJ·mol-1.对H2O吸附前后的态密度分析可知,H2O通过其O原子的P轨道与底物U原子的d轨道作用.本文还进一步探讨H2O在UO(100)表面的解离机理.     

H2S splitting on Cu(110): Insight from combined periodic density functional theory calculations and microkinetic simulation

Practical copper (Cu)‐based catalysts for the water–gas shift (WGS) reaction was long believed to expose a large proportion of Cu(110) planes. In this work, as an important first step toward addressing sulfur poisoning of these catalysts, the detailed mechanism for the splitting of hydrogen sulfide (H₂S) on the open Cu(110) facet has been investigated in the framework of periodic, self‐consistent density functional theory (DFT‐GGA). The microkinetic model based on the first‐principles calculations has also been developed to quantitatively evaluate the two considered decomposition routes for yielding surface atomic sulfur (S*): (1) H₂S → H₂S* → SH* → S* and (2) 2H₂S → 2H₂S* → 2SH* → S* + H₂S* → S* + H₂S. The first pathway proceeding through unimolecular SH* dissociation was identified to be feasible, whereas the second pathway involving bimolecular SH* disproportionation made no contribution to S* formation. The molecular adsorption of H₂S is the slowest elementary step of its full decomposition, being related with the large entropy term of the gas‐phase reactant under realistic reaction conditions. A comparison of thermodynamic and kinetic reactivity between the substrate and the close‐packed Cu(111) surface further shows that a loosely packed facet can promote the S* formation from H₂S on Cu, thus revealing that the reaction process is structure sensitive. The present DFT and microkinetic modeling results provide a reasonably complete picture for the chemistry of H₂S on the Cu(110) surface, which is a necessary basis for the design of new sulfur‐tolerant WGS catalysts. © 2013 Wiley Periodicals, Inc.     

The chemisorption of hydrogen on Cu(111): A dynamical study

Ab initio band-structure calculations within a density functional formalism were performed to compute the binding energy curves of atomic hydrogen with the high-symmetry adsorption sites of the (111) surface of copper. For a two-layer slab of Cu atoms and H coverage equal to 0.25, the binding energies are 2.25, 3.12, and 3.24 eV, for on-top, bridge, and threefold sites, so that the chemisorption of H₂ on Cu(111) is exothermic for threefold and bridge sites, but endothermic for on-top sites. Starting from these results, an LEPS potential for the interaction of H₂ with the Cu(111) surface was built. In this model potential, the most favored approaches correspond to a H₂ molecule parallel to the Cu surface, and for them, the activation barrier is located at the corner between the entrance and the exit channels of the reaction, and its lowest value is 0.6 eV. The LEPS potential was used in quasi-classical trajectories calculations to simulate the adsorption of a beam of H₂ molecules on Cu(111). The results show that (a) when H₂ is in the ground vibrational state the dissociative adsorption probability P_a increases from 0 to .90 along a roughly sigmoidal curve by increasing the collision kinetic energy from 0.4 to 1.3 eV, and (b) the vibrational energy can be as effective as the translational one in promoting dissociative chemisorption, in agreement with the experimental results. © 1994 John Wiley & Sons, Inc.     

Nb(110)表面氧原子覆盖度对氧分子解离的影响

用基于密度泛函理论的第一性原理方法研究了Nb(110)表面氧原子覆盖度分别为0.25、0.50、0.75 和1.00单层时对氧分子解离的影响. 结果表明, 在氧原子覆盖度不大于0.50单层时, 由于氧分子和表面铌原子的较强相互作用, 使它们能够自发解离. 然而在氧原子覆盖度为0.75单层时, 氧分子只能够在未占据的洞位附近解离, 同时发生严重的晶格畸变. 在形成一个氧原子单层后(1.00 单层), 氧分子只能弱吸附在Nb(100)表面上,此时氧原子向内扩散成为氧分子继续解离的速率决定步骤. 这些结果从理论上解释了在形成一个氧原子单层后, Nb(110)表面氧分子吸收速率迅速下降的原因.     

氢原子在Be(0001)表面吸附的密度泛函理论研究

采用第一性原理的密度泛函理论研究单个氢原子和多个氢原子在Be(0001)表面吸附性质.给出了氢吸附Be(0001)薄膜表面的原子结构、吸附能、饱和度、功函数、偶极修正等特性参数.同时也讨论了相关吸附性质与氢原子覆盖度(0.06-1.33ML)的关系.计算结果表明:氢原子的吸附位置与覆盖度之间有强烈的依赖关系,覆盖度低于0.67ML时,氢原子能量上易于占据fcc或hcp的中空位置;覆盖度为0.78ML时,中空位与桥位为氢原子的最佳吸附位;覆盖度在0.89到1.00ML时,桥位是氢原子吸附能量最有利的位置;以上覆盖度中Be(0001)表面最外层铍原子的结构均没有发生明显变化.当覆盖度为1.11-1.33ML,高覆盖度下Be(0001)表面的最外层铍原子部分发生膨胀,近邻氢原子渗入到铍表面次层,氢原子易于占据在hcp和桥位.吸附结构中的氢原子比氢分子中的原子稳定.当覆盖度大1.33ML时,计算结果没有发现相对于氢分子更稳定的吸氢结构.同时从分析偶极修正和氢原子吸附垂直高度随覆盖度的变化关系判断氢覆盖度为1.33ML时,在Be(0001)表面吸附达到饱和.     

Modification of Surface Reactivity by CO: Effects on Decomposition and Polymerization of Acetaldehyde on Ru(0001)

The adsorption and reaction of acetaldehyde on the clean and CO pre-covered Ru(0001) surfaces have been investigated using temperature programmed desorption method. On the clean Ru(0001) surface, the decomposition of acetaldehyde is the main reaction channel, with little polymerization occurring. However, on the CO pre-covered Ru(0001) surface, the decomposition of acetaldehyde is inhibited considerably with increasing CO coverage. Whereas, the polymerization occurs efficiently, especially at high CO coverage (θ_CO>0.5 ML), which is strongly CO coverage dependent. Combined with previous studies, the well-ordered hexagonal structure of CO layer formed on the Ru(0001) surface at high CO coverage that matches the configuration of paraldehyde is likely to be the origin of this remarkable phenomenon.     

Theoretical Study of Adsorption and Dehydrogenation of C2H4 on Cu(410)?

Adsorption and dehydrogenation of ethylene on Cu(410) surface are investigated with firstprinciples calculations and micro-kinetics analysis. Ethylene dehydrogenation is found to start from the most stable π-bonded state instead of the previously proposed di-σ-bonded state. Our vibrational frequencies calculations verify the π-bonded adsorption at step sites at low coverage and low surface temperature and di-σ-bonded ethylene on C-C dimer (C₂H₄-CC) is proposed to be the species contributing to the vibrational peaks experimentally observed at high coverage at 193 K. The presence of C₂H₄-CC indicates that the dehydrogenation of ethylene on Cu(410) can proceed at temperature as low as 193 K.     

Mass spectrometric studies on the decomposition of trialkylgallium on gaas surfaces

The thermal decomposition of trimethylgallium [(CH₃)₃Ga] and triethylgallium [(C₂H₅)₃Ga] on gallium arsenide (GaAs) surfaces was studied under an ultra-high vacuum using mass spectrometry. It was observed that the decomposition process of (CH₃)₃Ga and (C₂H₅)₃Ga depends on the arsenic coverage of the substrate surface. On a (100)-oriented surface, increasing the arsenic coverage basically enhances the decomposition of (CH₃)₃Ga and (C₂H₅)₃Ga to gallium atoms above 350 and 300°C, respectively. The decomposition of (CH₃)₃Ga proceeds by emitting CH₃ radicals. On a surface with low arsenic coverage, the decomposition of (CH₃)₃Ga is imperfect and fewer than three methyl groups of alkylgallium are desorbed. On a (111)B-oriented surface, however, an increase in the surface arsenic coverage suppresses the decomposition of alkylgallium, which is different from the case for a (100) surface.     

CO2在Fe3O4(111)表面的吸附结构及能量研究

利用密度泛函理论研究了CO₂在Fe₃O₄（111）表面Fe_tet1和Fe_oct2两种终结的吸附行为。在Fe_tet1终结表面,当覆盖度为1/5 ML时,CO₂倾向于线性吸附;而在高覆盖度下,弯曲的CO₂与表面O作用形成CO₃^2-结构。在Fe_oct2终结表面,CO₂倾向于弯曲吸附,在1/6 ML和1/3 ML覆盖度时都可以形成CO₃^2-和-COO结构。覆盖度对Fe_tet1终结的表面影响很弱,但是对Fe_oct2终结的表面影响很大。从热力学上来说,CO₂在Fe_oct2终结表面的吸附要比Fe_tet1终结表面更有利。     

Oxygen Atom Adsorption and Diffusion on Pd Low-index Surfaces and （311） Stepped Surface

Introduction Atom adsorption on transition metal surfaces has attracted special attention as a base for understanding the fundamental processes of oxidative catalysis. Particularly interesting is the adsorption and diffusion of oxygen on well-defined metal surfaces. An oxygen covered palladium surface, for example, plays a central role in several important reactions such as oxidation of carbon monoxide and ammonia. In particular, the (100), (111), (110) surfaces and the interactions with oxyge…     

Electron spectroscopic studies of the adsorption and decomposition of methanol on transition metals. A review

Results of investigations of the adsorption and decomposition of methanol on the surface of transition metals such as Fe, Ni, Cu, Pd, Ag, Mo, W and Pt byuv and x-ray photoelectron spectroscopy, electron energy loss spectroscopy, Auger electron spectroscopy and thermal desorption spectroscopy have been reviewed. The first step in the decomposition of CH₃OH on these metal surfaces is the formation of the methoxy species, OCH₃ radical. In the case of Fe, Mo and W, complete decomposition of CH₃OH occurs leaving CO(β), H₂ and CH₄ on the surface. Dissociation proceeds upto CO(α) and H₂ on the surface of Ni, Pd and Pt whereas on Ag and Cu, selective oxidation of CH₃OH to H₂CO is preferred. The difference in the reactivity of metals towards CH₃OH is rationalised from the heats of adsorption of O₂, CO and H₂ on these metals. Contribution No. 253 from the Solid State and Structural Chemistry Unit.     

Adsorption Sequence of Multifunctional Groups: A Study on the Reaction Pathway and the Adsorption Structure of Homocysteine on the Ge(100) Surface

We investigated the adsorption mechanism of homocysteine (HS? CH₂? CH₂? CH(NH₂)? COOH) on the Ge(100) surface along with its electronic structures and adsorption geometries to determine the sequence of adsorption of this amino acid′s functional groups using core‐level photoemission spectroscopy (CLPES) in conjunction with density functional theory (DFT) calculations. We found that the “SH‐dissociated OH‐dissociated N‐dative‐bonded structure” and the “SH‐dissociated OH‐dissociation‐bonded structure” were preferred at a monolayer (ML) coverage of 0.30 (lower coverage) and 0.60 (higher coverage), respectively. The “SH‐dissociated OH‐dissociated N‐dative‐bonded structure” was the most stable structure. Moreover, we systematically confirmed the sequence of adsorption of the functional groups of the homocysteine molecule on the Ge(100) surface, which is thiol group (? SH), carboxyl group (? COOH), and amine group (? NH₂).     

Adsorption of Water on an MgSO4(100) Surface: A First‐Principles Investigation

The adsorption properties of water molecules on an MgSO₄(100) surface were investigated by using density functional theory (DFT) and supercell models. Optimized stable geometries of one and more than one water molecules adsorbed on an ideal MgSO₄(100) surface were obtained. The configurations with water molecules adsorbed on atoms of the second and third atomic layers of the MgSO₄(100) surface are quite stable. After adsorption, the separations between both the adjacent Mg atoms (R_{Mg? Mg}) and the adjacent O atoms of the surface (R_{O? O}) increase, which indicates that the MgSO₄(100) surface starts to deliquesce. In addition, water molecules are more likely to adsorb onto a defective surface rather than an ideal surface. Mulliken population analysis suggests that fewer charges transfer to the water molecule from the Mg atom of a defective substrate. Finally, Raman spectra were calculated for 0.5, 1, and 2 ML (ML=monolayer) water adsorbed on an MgSO₄(100) surface, which is helpful for further related experiments.     

First-principles study on the effect of arsenic impurity on oxidation of pyrite surfaces

Arsenic (As) frequently exists in pyrite (FeS₂) in the form of impurities. The oxidation behavior of As in FeS₂ is important in environmental science, mineral processing, and other related fields. The adsorption behaviors of H₂O and O₂ molecules on the As-bearing pyrite (100) surface (As FeS₂(100)) are studied using the density functional theory (DFT). The results show that As prefers the S site on the pyrite (100) surface (FeS₂(100)). In the absence of O₂, an isolated H₂O molecule does not dissociate when adsorbed at an iron (Fe) site and is repelled at an As site. Furthermore, the surface area around the As atoms exhibits a hydrophobic behavior. Adsorption energy analysis reveals that the presence of As atoms is unfavorable for the adsorption of H₂O molecules on the pure FeS₂ surface, and that the adsorption of H₂O molecules on the As FeS₂(100) is physical adsorption. In the absence of H₂O, it is suggested that the O₂ molecule easily dissociates on both the pure FeS₂(100) and As FeS₂(100). The adsorption of O₂ on the As-bearing surface is weaker than that on the pure FeS₂(100). For the co-adsorption of H₂O and O₂, the adsorption energy on the As-bearing surface is more negative than that on the pure surface. This indicates that the presence of As promotes surface oxidation. Additionally, two  OH and O (AsO or SO) or  O (Fe O) species are formed on the surface of pyrite when the H₂O molecule is dissociated.     

Theoretical Study on Adsorption and Diffusion of N Atoms on Cu Low-index Surface

The adsorption and diffusion of N atoms on the three low-index Cu planes were studied using 5-parameter Morse potential (5-MP) method, and the best theory-experiment agreement was obtained. N atoms of Cu(100) surface sit on the fourfold hollow site with the vertical height of 0.018 nm closely coplanar with the topmost copper layer, and the four Cu-N bond lengths are 0.182 nm and the fifth Cu-N distance is 0.199 nm. For Cu(111) system, the existence of aberrant Cu(100) reconstructed structure is approved at higher coverage, and at low coverage the structure is almost an ideal Cu(111) surface structure. With respect to Cu(110) system, the N atoms are adsorbed at LB and H3 sites, not at SB site. The diffusion passage and diffusion barrier of adsorbed N atoms were also studied.     

Density Functional Theoretical Studies on the Methanol Adsorption and Decomposition on Ru(0001) Surfaces

Periodic density functional theory(DFT) calculations are presented to describe the adsorption and decomposition of CH₃OH on Ru(0001) surfaces with different coverages, including p(3×2), p(2×2), and p(2×1) unit cells, corresponding to monolayer(ML) coverages of 1/6, 1/4, and 1/2, respectively. The geometries and energies of all species involved in methanol dissociation were analyzed, and the initial decomposition reactions of methanol and the subsequent dehydrogenations reactions of CH₃O and CH₂OH were all computed at 1/2, 1/4, and 1/6 ML coverage on the Ru(0001) surface. The results show that coverage exerts some effects on the stable adsorption of CH₃O, CH₂OH, and CH₃, that is, the lower the coverage, the stronger the adsorption. Coverage also exerts effects on the initial decomposition of methanol. C-H bond breakage is favored at 1/2 ML, whereas C-H and O-H bond cleavages are preferred at 1/4 and 1/6 ML on the Ru(0001) surface, respectively. At 1/4 ML coverage on the Ru(0001) surface, the overall reaction mechanism can be written as 9CH₃OH→3CH₃O+6CH₂OH+9H→6CH₂O+3CHOH+18H→7CHO+COH+CH+OH+26H→8CO+C+O+36H.     

Pt adsorption on the PbTiO3(110) polar surface: a density functional theory study

The monolayer (ML) and submonolayer Pt on both terminations of PbTiO₃(110) polar surface have been studied by using density functional theory (DFT) with projector‐augmented wave(PAW) potential and a supercell approach. The most favored ML Pt arrangements on PbTiO and O₂ terminations are the hollow site and the short‐bridge site, respectively. By examining the geometries of different ML arrangements, we know that the dominant impetus for stability of the favored adsorption site for PbTiO termination is the Pt–Ti interaction (mainly from covalent bonding), while that for O₂ termination is the Pt–O interaction (mainly from ionic bonding). In addition, the appearance of the gap electronic states in the outermost layers of each termination indicates that a channel for charge transfer between adsorbed layer and substrate is formed. Moreover, the interface hybridization between Pt 5d and O 2p orbitals is also observed, especially for ML Pt on O₂ termination. The stability sequences for various arrangements of 1/2 ML Pt adsorption conform well with those of ML Pt adsorption, and the most stable arrangement is energetically more favorable than the corresponding ML coverage in the view of adsorption energy maximization. The behavior, i.e. the increase in adsorption energy with decrease in coverage, indicates that Pt? Pt interactions weaken those between Pt and the substrate. Copyright © 2009 John Wiley & Sons, Ltd.     

二维TiC 层表面H2的化学吸附与物理吸附

第一性原理计算研究发现由于二维TiC 单原子层具有高的比表面积与大量的暴露在表面的Ti 原子,其是一种非常有潜力的储氢材料. 计算结果显示H₂可以在二维TiC 单原子层表面进行物理吸附与化学吸附. 其中化学吸附能为每个氢分子0.36 eV,物理吸附能是每个氢分子0.09 eV. 覆盖度为1和1/4层（ML）时,H₂分子在二维TiC 单原子层表面的离解势垒分别为1.12 和0.33 eV. 因此,除了物理吸附与化学吸附,TiC 表面还存在H单原子吸附. 最大的H₂储存率可以达到7.69%（质量分数）. 其中,离解的H原子、化学吸附的H₂、物理吸附的H₂的储存率分别为1.54%、3.07%、3.07%. 符合Kubas吸附特征的储存率为3.07%. 化学吸附能随覆盖度的变化非常小,这有利于H₂分子的吸附与释放.     

Formation of sulfhydryl (SH) species on the clean and (2 × 2)-S covered Pt(111) surfaces by H2S decomposition

H₂S decomposition on the clean and (2 × 2)-S covered Pt(111) surfaces has been characterized using high-resolution electron energy loss (HREELS) and temperature-programmed desorption (TPD) spectroscopies. On the Pt(111)-(2 × 2)-S surface, a mixture of molecular H₂S and sulfhydryl (SH) species forms following H₂S adsorption at 110 K. The molecular H₂S desorbs at 140 K leaving a (2 × 2)-S overlayer saturated with SH; the SH is stable up to 190 K. On the clean Pt(111) surface, a mixture of atomic sulfur, SH and chemisorbed molecular H₂S is formed following H₂S adsorption at 110 K. On the clean surface, adsorbed SH decomposes near 150 K. We report here the first definitive observation of an adsorbed sulfhydryl species on a metal surface.     

First-principles study of O2 adsorption and dissociation on the CuCr2O4 (100) surface

The adsorption and dissociation of molecular oxygen on spinel CuCr₂O₄ (100) surface were carried out by first-principles calculations based on density functional theory (DFT). The calculated results indicate that the Cr site is most favorable for atomic oxygen adsorption, with an adsorption energy of 402.8 kJ/mol. For molecular oxygen adsorption, there are three types of favorable interaction modes: O₂ forms bonds with the Cu site or O₂ binds to two Cr sites or O₂ interacts with both Cu and Cr sites simultaneously. The lowest activation energy (E_a = 35.4 kJ/mol) was found through exploring possible reaction pathways for O₂ dissociation. The relationship between E_a and reaction enthalpy (ΔH) for O₂ dissociation adsorption reactions fits Brønsted-Evans-Polanyi (BEP) behavior.