Quantum-chemical modeling of ethylene and acetylene adsorption on gold clusters

The interaction of ethylene and acetylene molecules with planar (2D) and nonplanar (3D) gold clusters Au _n (n = 10, 12, 20) was studied by the density functional theory (DFT) method. The coordination of hydrocarbons at the vertices, edges, and fragments of the Au₃ cluster was shown to form π, di-σ, and μ type complexes, respectively. The standard Gibbs energy and the C-C bond length of the hydrocarbon change during its adsorption in the series μ > di-σ > π complexes. The highest selectivity in adsorption of acetylene relative to that of ethylene was achieved on Au₁₂ (3D) and Au₂₀ (2D) clusters.     

Study of Structures and Electronic Properties of Pdn-1Pb and Pdn(n≤8) Clusters

Geometric and electronic properties of Pdn-1Pb and Pdn(n≤8) clusters have been studied by using density functional theory with effective core potentials, focusing on the differences between mono- and bimetallic clusters. The average bond length of Pdn-1Pb (n≤8) bimetallic clusters is longer than that of pure palladium clusters except for n = 2 and 3. The most stable structure of Pdn-1Pb (n≤7) is the singlet where there is at least a Pd or Pb atom on its excited state. The energy gaps of Pd-Pb binary clusters are narrower than those of Pdn clusters, and then the chemical activity is strengthened when Pdn clusters are doped with Pb.     

First principles investigations of Pd-on-Au nanostructures for trichloroethene catalytic removal from groundwater

Catalytic groundwater remediation from chlorinated organic solvents like trichloroethene (TCE) has been found to be more effective and sustainable than traditional non-destructive methods. Among the experimentally studied catalyst materials, Pd-decorated Au nanoparticles show the highest activity and selectivity combined with the best resistance towards poisoning by chemicals present in groundwater. In this study the thermochemistry and adsorption geometries of TCE and its hydrodechlorination products are investigated via density functional theory calculations. Various model systems for Pd-supported Au nanoparticles are addressed. The adsorption of TCE is endothermic on bare Au(111), almost thermoneutral or slightly exothermic on Pd-Au surface alloys and clearly exothermic on Pd overlayer structures on Au(111). The strongest chemisorption is on the di-σ configuration between Pd atoms over the smallest 2D Pd clusters containing only a few Pd atoms. These are not, however, the best catalysts as they are too small to co-adsorb hydrogen needed for hydrodechlorination reaction. We demonstrate good correlation between adsorption energies and the d-band center of the system. The variation of adsorption energy from the one Pd-Au composition to the other can be tentatively assigned to be due to the ligand and coordination effects. Also, the ensemble effects are important; without the right ensemble the adsorption is weak or endothermic.     

苯在Si(111)-7*7表面化学吸附的理论研究

采用两种大小不同的原子簇模型Si_(30)H_(28)和Si_(13)H_(16),分别用两层 ONIOM方法（对较大原子簇）和普通量子化学方法（对较小原子族）考察了苯分子 在Si(111)-7 * 7表面的化学吸附。对三种可能的吸附物种分别用DFT或HF方法进行 了计算。通过大小原子簇吸附物种的吸附能以及几何构型优化参数的比较发现,对 于稳定的吸附物种,较小的原子簇基本上可以代替较大的原子簇进行计算,而对于 不太稳定的吸附物种,就不得不考虑周边原子的影响。计算结果表明苯在Si(111)- 7 * 7表面的主要吸附种是双σ成键的1,4加成产物,不稳定的单吸附物种可能是 1,4加成物种的前驱态。     

金属簇X (X=Pt-Au,Au-Au)负载在(3×2) TiO2(110)完整表面上的覆盖度效应

采用自旋极化密度泛函和广义梯度近似的方法并结合周期平板模型, 探讨了不同覆盖度(θ)下双金 属簇X (X=Pt-Au, Au-Au)在(3×2)TiO₂(110)完整表面上的吸附行为. 另外, 在本文给出的所有覆盖度模式下(θ= 1/6-1 ML), 我们仅研究其基态构型. 计算结果表明: 当θ<1/2 ML时, 金属簇X在TiO₂(110)表面上吸附能随覆盖 度的增加而增加; 当θ>1/2 ML时, 除了饱和覆盖度下, 吸附能随覆盖度的增加而减小; 当θ=1/2 ML时, 吸附能最 大. 即使Pt-Au/TiO₂体系的吸附能比Au-Au/TiO₂体系的小, 但相对于Au-Au 簇, Pt-Au 簇更容易在TiO₂(110)表 面上形成双金属单分子层. 在半覆盖和全覆盖下, X簇的峰与TiO₂的峰在-3.0 eV到费米能级之间产生明显重 叠, 表明簇与底物之间存在化学作用. 且当覆盖度小时, X-TiO₂相互作用是成簇的主要因素; 随着覆盖度的增 大, X-X原子间相互作用就逐渐变成了成簇的主要动力.     

Theoretical investigation of the adsorption of oxygen on small bimetallic LimCun (m,n ≤ 4) clusters

A theoretical study of the adsorption of molecular oxygen on small bimetallic Li_mCu_n (m, n ≤ 4) clusters was carried out using density functional methods, and it was compared with the adsorption of O₂ on copper (Cu_n, n ≤ 8) clusters. The study of O₂‐Li_mCu_n system is important to understand the promotion effects of the alkali atoms on the copper surface participating in the catalytic processes. Adsorption energies ranging from 7.9 to 51 kcal/mol were found, which represented values over 30% to those calculated for the adsorption of O₂ on copper clusters in a previous study. Thus, the reactivity of molecular oxygen on bimetallic clusters is more favorable with high tendency being in favor of the dissociation of the O₂ molecule. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Pt3Ru6 clusters supported on gamma-Al2O3: synthesis from Pt3Ru6(CO)21(mu3-H)(mu-H)3, structural characterization, and catalysis of ethylene hydrogenation and n-butane hydrogenolysis

The supported clusters Pt-Ru/gamma-Al2O3 were prepared by adsorption of the bimetallic precursor Pt3Ru6(CO)21(mu3-H)(mu-H)3 from CH2Cl2 solution onto gamma-Al2O3 followed by decarbonylation in He at 300 degrees C. The resultant supported clusters were characterized by infrared (IR) and extended X-ray absorption fine structure (EXAFS) spectroscopies and as catalysts for ethylene hydrogenation and n-butane hydrogenolysis. After adsorption, the nu(CO) peaks characterizing the precursor shifted to lower wavenumbers, and some of the hydroxyl bands of the support disappeared or changed, indicating that the CO ligands of the precursor interacted with support hydroxyl groups. The EXAFS results show that the metal core of the precursor remained essentially unchanged upon adsorption, but there were distortions of the metal core indicated by changes in the metal-metal distances. After decarbonylation of the supported clusters, the EXAFS data indicated that Pt and Ru atoms interacted with support oxygen atoms and that about half of the Pt-Ru bonds were maintained, with the composition of the metal frame remaining almost unchanged. The decarbonylated supported bimetallic clusters reported here are the first having essentially the same metal core composition as that of a precursor metal carbonyl, and they appear to be the best-defined supported bimetallic clusters. The material was found to be an active catalyst for ethylene hydrogenation and n-butane hydrogenolysis under conditions mild enough to prevent substantial cluster disruption.     

Reactivity and stability of bimetallic clusters

Using two independent vaporization lasers, bimetallic clusters composed of transition elements and A1 were generated by the laser vaporization method. Reactivity toward hydrogen adsorption of bimetallic clusters was compared with genuine clusters. It was found that A1 which has no reactivity toward hydrogen plays a role of either inhibitor or accelerator of the reaction when A1 is mixed with Nb or Co. Unusual stability of Co₁₂ V₁ in contrast to the high reactivity of Co_12–13 is attributed to the rigid geometric structure where V occupies the central position.     

有机化学基本理论问题新的研究方法、新的观念和新 的思维模式

有机化学基本理论研究的总结和回顾。15年来,在国家自然科学基金委员会的支持下,为了认识电子离域的本质,在量子化学领域,我们建立和发展了新的作用能分解方法和大型计算程序,发展和完善了轨道定域化程序。我们的方法可以为任何一个共轭分子(无论是平面的还是非平面的,是含共轭双键的还是含累积双键的),提供一个π与σ体系彻底分离的片断分子轨道基组。这个轨道基组不仅满足分子特殊的对称性,而且还具有确切的电子占据数。与Hückel理论完全不同,我们强调：π电子的离域除了对它原先的定域π体系有强烈的失稳定作用外,它还可通过π-σ空间作用,对σ构架产生强烈的稳定作用。据此,我们提出了芳环化合物新的分类准则,揭示了芳香环流起因的必要条件,定义环的刚度为芳香性一个新判据。发现,分子内基团间的局部作用(CT和EX)同它们对分子整体性能的影响是完全相反的。就构象而言,稳定的CT作用是相斥的,失稳定的EX作用是相吸的;就电子转移而言,大的EX作用是电荷转移的助动力。其助动性在于,它能降低因CT作用而产生的给体自身对电荷转移的阻力。论证了,在二苯乙烯类分子中,π-π共轭,π-σ超共轭和σ-σ非键轨道作用都是失稳定的。与σ-σ和π-σ作用相比,π-π作用对于分子构像的影响是非常微弱的。与经典的思维模式相反,有机分子总是倾向于较小的失稳定,而不是较大的稳定。为了维持尽可能最稳定的电子总能量,在σ-σ作用的驱动下,共轭基团应该尽量地偏离共平面。阻止分子扭曲的(非电子作用力)是核排斥力。因此,一个空间拥挤的构象可以是能量有利的构象。在我们的研究中,经典有机结构理论的整体因果关系已经全面地被颠倒。     

Structural, electronic, bonding, magnetic, and optical properties of bimetallic [Ru(n)Au(m)](0/+) (n + m ≤ 3) clusters

The structural, electronic, bonding, magnetic, and optical properties of bimetallic [Ru(n)Au(m)](0/+) (n + m ≤ 3; n, m = 0-3) clusters were computed in the framework of the density functional theory (DFT) and time-dependent DFT (TD-DFT) using the full-range PBE0 non local hybrid GGA functional combined with the Def2-QZVPP basis sets. Several low-lying states have been investigated and the stability of the ground state spinomers was estimated with respect to all possible fragmentation schemes. Molecular orbital and population analysis schemes along with computed electronic parameters illustrated the details of the bonding mechanisms in the [Ru(n)Au(m)](0/+) clusters. The TD-DFT computed UV-visible absorption spectra of the bimetallic clusters have been fully analyzed and compared to those of pure gold and ruthenium clusters. Assignments of all principal electronic transitions are given and interpreted in terms of contribution from specific molecular orbital excitations.     

Recent advances in the homogeneous polymerisation of olefins mediated by nickel complexes

A brief overview is presented on recent advances in the application of nickel coordination complexes as mediators of olefin polymerisation; in some cases involving stereo-regular polymer formation where appropriate. Emphasis is also given on recent findings concerning di- and tri-nuclear Ni clusters with details on ethylene, methyl methacrylate or styrene-based monomer polymerisations.     

Effects of toluene on thiophene adsorption over NaY and Ce(IV)Y zeolites

Zeolites NaY and Ce(IV)Y were employed as adsorbents to remove organic sulfur compounds from model gasoline(MG) solutions with and without toluene in static adsorption experiments at room temperature(RT) and atmospheric pressure.The adsorbents were characterized by XRD,XRF and pyridine infrared spectrum(IR).The adsorption experiments show that the desulfurization performance of Ce(IV)Y is much better than that of NaY.The sulfur removal over both NaY and Ce(IV)Y decreases with the increase of toluene concentration in MG,however,the decline tendency on Ce(IV)Y is smooth,and it is steep on NaY.FT-IR spectra of thiophene adsorption indicate that thiophene molecules are mainly adsorbed on NaY via π electron interaction,but on Ce(IV)Y,in addition to the π electron interaction,both Ce4+-S direct interaction and protonation of thiophene also play important roles.Toluene molecules are adsorbed on NaY also via π electron interaction.Although the amount of Brnsted acid sites is increased due to the introduction of Ce4+ ions into NaY zeolite,it is not found to influence the adsorption mode of toluene over Ce(IV)Y.Compared with NaY zeolite,the improved desulfurization performance over Ce(IV)Y for removing organic sulfur compounds from MG solution,especially those containing large amount of aromatics,may be ascribed to the direct Ce(IV)-S interaction,which is much resistant to the influence resulted from toluene adsorption.     

Geometry optimization of bimetallic clusters using an efficient heuristic method

In this paper, an efficient heuristic algorithm for geometry optimization of bimetallic clusters is proposed. The algorithm is mainly composed of three ingredients: the monotonic basin-hopping method with guided perturbation (MBH-GP), surface optimization method, and iterated local search (ILS) method, where MBH-GP and surface optimization method are used to optimize the geometric structure of a cluster, and the ILS method is used to search the optimal homotop for a fixed geometric structure. The proposed method is applied to Cu(38-n)Au(n) (0 ≤ n ≤ 38), Ag(55-n)Au(n) (0 ≤ n ≤ 55), and Cu(55-n)Au(n) (0 ≤ n ≤ 55) clusters modeled by the many-body Gupta potential. Comparison with the results reported in the literature indicates that the present method is highly efficient and a number of new putative global minima missed in the previous papers are found. The present method should be a promising tool for the theoretical determination of ground-state structure of bimetallic clusters. Additionally, some key elements and properties of the present method are also analyzed.     

DFT能量分解和氮苄叉基苯胺分子π电子离域能的研究

为了探索密度泛函理论(DFT)方法中氮苄叉基苯胺分子π电子离域的本质, 介绍了将非平面分子氮苄叉基苯胺分子的DFT能量分成π和σ的方法, 并将π和σ电子能量分成单电子能部分: 动能ΔEπK(θ), ΔEσK(θ)和位能ΔEπP(θ), ΔEσP(θ); 双电子相互作用部分: 库仑作用ΔEππJ(θ), ΔEσσJ(θ), ΔEπσJ(θ)和交换相关作用ΔEππXC(θ), ΔEσσXC(Δ)以及ΔEπσXC(θ), 分析了垂直离域能ΔEV的稳定性及π电子离域对π和σ体系的影响. 在B3LYP/6-31G*, 6-311G*, 6-31G(2d), 6-311G(2d)水平下的计算结果表明, 与经典观点不同, π电子的离域是失稳定的, 且平面时失稳定性最强; 分析各个能量分量表明, 在π电子的离域过程中, π和σ体系均对基组较敏感, π体系本身单电子能的影响大于σ体系, π电子离域对双电子部分作用的影响主要体现在π-σ的耦合作用上.     

Density Functional Theory Study of HCN Adsorption on Small Gold Clusters

A theoretical study was carried out on the adsorption of hydrocyanic acid on small Aun （n ≤ 7） clusters using density functional methods. For HCN adsorption on gold clusters, no dependence was found with respect to the even-odd alternation in relation to the number of gold atoms in the cluster. The HCN molecule is adsorbed at simple adsorption sites （1-fold coordination）, perpendicular to the adsorption site. The largest adsorption energy is only about 74.61 kJ·mol^-1, which indicates that the HCN molecule does not decompose and the C-N bond retains triple bond, and that the C-H and C-N stretching frequencies are only weakly perturbed. The adsorbed C-N and C-H stretching frequencies are blue- and red-shifted compared with the values of free HCN, respectively.     

Hydrogen chemisorption on supported platinum, gold, and platinum-gold-alloy catalysts

Alloyed catalysts receive considerable attention, because of their unique catalytic properties; they often show higher selectivity, activity, and stability compared to the pure metal particles. To provide insights in the origins of these features, we report the structure and the interaction of hydrogen with each of the metals in an intimately mixed platinum-gold catalyst and compare these characteristics to those in the single metal particles. X-Ray absorption spectroscopy (XAS) and electron microscopy analysis showed that the structure of the mixed particle differed from the single metal particles. The interaction of platinum with hydrogen is stronger than the H-Au interaction and the adsorption sites were different. EXAFS analysis showed that the structure of the platinum clusters changes with increasing hydrogen coverage, observed as a relaxation of the contracted Pt-Pt distance and an increase in the Pt-Pt coordination number. No such changes were observed for gold clusters. Well-mixed PtAu-alloy clusters, with a bulk Au-to-Pt ratio of two, supported on SiO(2), adsorb hydrogen on both platinum and gold atoms, which indicates that gold cannot be regarded as an inert metal. The heat of adsorption on the platinum ensembles does not decrease upon alloying; the weakening of the overall hydrogen adsorption strength when alloying platinum with gold is an ensemble-size effect.     

SCF-α Scattered wave model cluster calculations for the adsorption systems CO/Ni and N2/Ni

SCF-α-SW calculations have been performed on model clusters which can be taken as representations of chemisorbed CO and N₂ on Ni(100). As a check of the influence of the cluster size, NiCO/NiN₂ and Ni₉N₂ clusters have been studied as well as the isolated molecules CO and N₂ . In order to recognize and eliminate errors due to the differing muffintin geometries, other reference molecular clusters (CO/N₂ in an enlarge outer sphere, and on 9 empty spheres corresponding in size to Ni₉) have also been calculated. The discussion of results is based on (Xα ground state and transition state) energy shifts and on charge distributions. The main conclusions reached about the surface bond in the two cases (dative 5 σ bond and π backbond for CO; for N₂ main dative bond through 4σ with a smaller 5σ contribution, and a weaker π backbond than for CO) agree with those reached by other methods. Additional information (e.g σ backbond contribution for the large clusters; a strong localization of the adsorbate bond for N₂, but noticeable delocalization for CO) is derived.     

A Study of Polynuclear Aromatic Hydrocarbons on an Amino Bonded Phase Liquid Chromatographic Column in the Normal and Reversed Phase

Abstract

Experiments were run using an n-propyl amine polar bonded phase (Chromosorb LC-9) liquid chromatographic column in both the normal and reversed phase mode. Results confirm that the mechanism of separation in the normal phase is due mainly to a charge transfer interaction between the lone pair electrons on the stationary phase nitrogen and the π electron cloud of the solute PNAs. Elution order seems to depend upon a combination of π energy, and type of ring condensation of the solute. Plots of log I versus number of aromatic carbons for catacondensed PNAs suggest that while the specific interaction is different than that seen in silica chromatography, the overall adsorption effect is comparable. In the reversed phase there may be two types of separation mechanisms: 1) a pure partitioning effect in highly polar mobile phases (methanol/water), or 2) a mixture of liquid-solid adsorption and liquid-liquid partition in less polar solvent systems (acetonitrile/water).