Chemisorption and diffusion of hydrogen on surface and subsurface sites of flat and stepped nickel surfaces

Plane-wave density functional theory calculations were performed to investigate the binding and diffusion of hydrogen on three flat Ni surfaces, Ni(100), Ni(110), and Ni(111), and two stepped Ni surfaces, Ni(210) and Ni(531). On each surface, the favored adsorption sites were identified by considering the energy and stability of various binding sites and zero-point energy corrections were computed. Binding energies are compared with experimental and theoretical results from the literature. Good agreement with experimental and previous theoretical data is found. At surface coverages where adsorbate-adsorbate interactions are relatively weak, the binding energy of H is similar on the five Ni surfaces studied. Favorable binding energies are observed for stable surface sites, while subsurface sites have unfavorable values relative to the gas phase molecular hydrogen. Minimum energy paths for hydrogen diffusion on Ni surfaces and into subsurface sites were constructed.     

CO在无序二元合金表面上化学吸附研究

在紧束缚近似下，用相关势近似（CPA）和Einstein-Schrieffer（ES）单电子化学吸附理论讨论了CO在无序二元合金NixCu1-x,PtxNi1-x和PtxCu1-x表面上的化学吸附特性,结果表明：CO在NixCu1-x和PtxCu1-x表面上化学吸附稳定性随Ni和Pt含量的增加而增强；CO在PtxNi1-x表面上化学吸附时，当Pt与Ni的含量比例（摩尔比）为2：8时，吸附最稳定，在Pt与Ni的含量比例（摩尔比）为7:3时最不稳定     

Photocatalytic Oxidation of CO on TiO2: Chemisorption of O2, CO,and H2

On the surface : Adsorption of O₂ at the surface oxygen vacancy (SOV) sites of TiO₂ reconstructs the lattice oxygen (healing SOVs), resulting in a decrease of the photocatalytic activity of oxidizing CO over vacuum‐pretreated TiO₂ with increasing temperature (see scheme). Adsorption of H₂ produces new SOVs at the TiO₂ surface and stabilizes the photocatalytic activity.

     

CO2在Cu-Ni/ZrO2-SiO2催化剂上的吸附与反应

采用表面反应改性法,制备了ＺｒＯ２－ＳｉＯ２（ＺｒＳｉＯ）表面复合物,用等体积浸渍法制备了ＺｒＳｉＯ担载的Ｃｕ－Ｎｉ双金属催化剂,用ＩＲ和ＴＰＤ技术,研究了ＣＯ２在其表面上的化学吸附与反应性能。实验结果表明：在Ｃｕ－Ｎｉ／ＺｒＳｉＯ催化剂ＣＯ２可形成线式吸附态、剪式吸附态和卧式吸附态;催化剂表面金属位Ｍ上的剪式吸附态ＣＯ２可与邻近的ｌｅｗｉｓ酸位Ｚｒ＾ｎ＋作用,形成ＣＯ２卧式吸附态Ｍ－（ＣＯ）－     

Chemisorption of CO and mechanism of CO oxidation on supported platinum nanoclusters

Kinetic, isotopic, and infrared studies on well-defined dispersed Pt clusters are combined here with first-principle theoretical methods on model cluster surfaces to probe the mechanism and structural requirements for CO oxidation catalysis at conditions typical of its industrial practice. CO oxidation turnover rates and the dynamics and thermodynamics of adsorption-desorption processes on cluster surfaces saturated with chemisorbed CO were measured on 1-20 nm Pt clusters under conditions of strict kinetic control. Turnover rates are proportional to O(2) pressure and inversely proportional to CO pressure, consistent with kinetically relevant irreversible O(2) activation steps on vacant sites present within saturated CO monolayers. These conclusions are consistent with the lack of isotopic scrambling in C(16)O-(18)O(2)-(16)O(2) reactions, and with infrared bands for chemisorbed CO that did not change within a CO pressure range that strongly influenced CO oxidation turnover rates. Density functional theory estimates of rate and equilibrium constants show that the kinetically relevant O(2) activation steps involve direct O(2)* (or O(2)) reactions with CO* to form reactive O*-O-C*=O intermediates that decompose to form CO(2) and chemisorbed O*, instead of unassisted activation steps involving molecular adsorption and subsequent dissociation of O(2). These CO-assisted O(2) dissociation pathways avoid the higher barriers imposed by the spin-forbidden transitions required for unassisted O(2) dissociation on surfaces saturated with chemisorbed CO. Measured rate parameters for CO oxidation were independent of Pt cluster size; these parameters depend on the ratio of rate constants for O(2) reactions with CO* and CO adsorption equilibrium constants, which reflect the respective activation barriers and reaction enthalpies for these two steps. Infrared spectra during isotopic displacement and thermal desorption with (12)CO-(13)CO mixtures showed that the binding, dynamics, and thermodynamics of CO chemisorbed at saturation coverages do not depend on Pt cluster size in a range that strongly affects the coordination of Pt atoms exposed at cluster surfaces. These data and their theoretical and mechanistic interpretations indicate that the remarkable structure insensitivity observed for CO oxidation reactions reflects average CO binding properties that are essentially independent of cluster size. Theoretical estimates of rate and equilibrium constants for surface reactions and CO adsorption show that both parameters increase as the coordination of exposed Pt atoms decreases in Pt(201) cluster surfaces; such compensation dampens but does not eliminate coordination and cluster size effects on measured rate constants. The structural features and intrinsic non-uniformity of cluster surfaces weaken when CO forms saturated monolayers on such surfaces, apparently because surfaces and adsorbates restructure to balance CO surface binding and CO-CO interaction energies.     

CO在Cu/ZnO上吸附的簇模型研究

采用DFT方法和HF方法对CO在Cu／ZnO催化剂上的Zn（Ⅱ）和Cu（Ⅰ）表面位上的吸附行为进行了比较研究.结果表明：HF方法给出了较弱的M-CO（M＝Zn（Ⅱ），Cu（Ⅰ））表面吸附键描述，但无法正确预测其强弱顺序，MP2方法与DFT方法则给出与实验事实一致的描述.文章还对CO／铜基催化剂吸附体系的IR光谱进行了合理的理论预测.     

Chemisorption and physisorption of CO2 on cation exchanged zeolitesA,X and mor

Calorimetric measurements of the heat of adsorption of CO₂ on zeolites with variable content of mono- and divalent cations lead to common conclusions. High initial heats (up to 120 kJ·mol^–1 for NaA), generally associated with a slow and activated rate of adsorption, are found for high contents of Na⁺, Li⁺ or Ca²⁺. They are attributed to a limited number of chemisorption sites (0.3 per cage in NaA).Physisorption results in lower heats (25–50 kJ·mol^–1). The lowest values are obtained with partially or totally decationized zeolites. Transition metal cations induce frequently weaker interactions than IA or IIA. Finally the stronger the energy of adsorption is, the larger the adsorbed amount is.

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Zusammenfassung Kalorimetrische Messungen der Adsorptionswärme von CO₂ an Zeolithen mit einem unterschiedlichen Gehalt an ein- und zweiwertigen Kationen führen zu allgemeinen Schlußfolgerungen. Für einen hohen Gehalt an Na⁺, Li⁺ oder Ca²⁺ werden hohe Initiierungswärmen (bis zu 120 kJ·mol^–1 für NaA) gefunden, die immer in Verbindung mit langsamen und aktivierten Adsorptionsgeschwindigkeiten auftreten. Dies wird einer begrenzten Anzahl an Stellen für die Chemisorption zugeschrieben (0.3 pro -Cage in NaA). Physisorption verursacht niedrigere Wärmen (25–50 kJ·mol^–1). Die niedrigsten Werte erhält man mit teilweise oder total entkationisierten Zeolithen. Kationen von Übergangs-metallen verursachen häufig schwächere Wechselwirkungen als IA-oder IIA-Kationen. Je höher die Adsorptionsenergie, um so größer ist die adsorbierte Menge.

     

Rh2CO2/Al2O3上H2吸附及CO和H2共吸附

CO加氢反应机理一直是许多化学工作者感兴趣的课题．Rh催化剂因其优良的性能而被用于 CO加氢机理研     

CO在Pt低指数面上吸附行为的理论研究

用密度泛函理论(DFT)中的广义梯度近似(GGA)方法对CO-Pt低指数面吸附体系进行了结构优化, 并对吸附体系的吸附热、C—O键和C—Pt键的键长、布居数分析、电子态密度进行了研究. 计算结果表明, 在0.25 ML(monolayer)的覆盖率下, CO最容易在Pt(100)晶面的桥位、Pt(110)晶面的短桥位、Pt(111)晶面的hcp三重位吸附, 吸附热分别达到了2.11、2.37、1.96 eV; CO在吸附成键过程中伴有电子在CO分子和Pt之间的转移. 吸附后, C—O键被削弱, 键长变长, 金属内部的作用亦被削弱, 其表层Pt 原子的布居数明显降低; 态密度分析表明, CO在吸附过程中, 其4σ、1π、5σ、2π轨道均参与成键.     

CO2 decomposition over Pd membrane surfaces

The interaction of pure CO 2 with a 3 microm thin, supported Pd membrane has been investigated between 473 and 773 K. Diagnostic H 2 permeation measurements indicate a reduction of the H 2 flux after CO 2 exposure at the lower and upper ends of this temperature range. Temperature-programmed oxidation and desorption in combination with scanning electron microscopy analyses reveal the dissociation of CO 2 above 523 K, yielding molecularly and/or dissociatively adsorbed CO below 623 K and nanoscopic carbon deposits above 723 K. CO 2 is obviously not inert over Pd surfaces at practical Pd membrane operation temperatures but could be kinetically stabilized in a narrow temperature window around 673 K.     

Rh／SiO_2上H_2的化学吸附

Ｒｈ／ＳｉＯ_２上Ｈ_２的化学吸附陈耀强，龚茂初，明虹，周建略，杜宗英，祝小红，陈豫（四川大学化学系，成都，６１００６４）关键词Ｒｈ／ＳｉＯ_２催化剂，Ｈ_２化学吸附，ＩＲＨ２在Ｒｈ上的化学吸附被认为是解离化学吸附［１］．而Ｒｈ－Ｈ的红外振动谱带是Ｈ２在...     

Explicit study on the oxidation mechanism of nickel in molten Li2CO3-K2CO3

The oxidation behavior of nickel in Li+K carbonate melt is followed by measuring the open-circuit potential and by electrochemical impedance spectroscopy under an O₂+CO₂ gas mixture in the ratio 90/10 at a total pressure of 1 atm at 650 °C. X-ray diffraction (XRD) and energy-dispersive spectroscopy are employed for qualitative and quantitative analyses of the different compounds involved during the oxidation of nickel. Atomic force microscopy is used for both imaging the evolution of the oxide layer and determining its surface roughness. The in situ oxidation process of nickel demonstrates three stages: rapid formation of a compact surface oxide (first stage), thicker oxide layer (second stage), and a porous oxide structure (third stage). The lithiation reaction has been identified to occur during the second stage. Formation of an intermediate and unstable compound, namely NiCO₃, has been confirmed by XRD. Electronic Publication     

Chemisorption of SO2 and Cl2 on indium oxide

The mutual influence of SO₂ and Cl₂ during their consecutive chemisorption on the In₂O₃ surface has been investigated. It was found that SO₂ is chemisorbed in the uncharged form, and the amount of chemisorption decreases as the temperature increases. The preliminary chemisorption of Cl₂ results in a dramatic decrease in the amount of SO₂ sorbed and in a change in the character of the bonding of SO₂ with the oxide surface. When SO₂ is sorbed first, the temperature of the formation of chlorides during subsequent chemisorption of Cl₂ decreases.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1718–1721, October, 1994.     

Unexpected Chemisorption State of CO2 on the Si(100)-2×1 Surface Predicted by Theory

Li the past decade, the reactivity of carbon containing confounds with single crystal silicon surfaces has received extensive attention from both experimental and theoretical sides owing to its compact relevance to the growth of silicon carbide and diamond films. While considerable interest has been focused on the adsorptions of alkenes, alkanes and some other small organic molecules^[1], very few experiments^[2,3] can be found in the literature regarding the adsorption of CO₂,a greenhouse gas, on Si surfaces. We report herein the results of our recent theoretical study concerning CO₂ adsorption on the reconstructed Si(100)-2×l surface. The methods employed in our calculations are the hybrid density functional B3LYP method and a two-layer ONIOM (CCSD(T):B3LYP) approach^[4].     

Chemisorption of CO on Pd(100): An lcgto-lsd cluster study

LCGTO-LSD model potential calculations have been performed for CO interacting with two-, four-, and eight-atom clusters of Pd, chosen to model the bridge site of the (100) surface. The geometry and vibrational frequencies are not very sensitive to the cluster size. For Pd₈ + CO we obtain d_C—O = 1.18 Å (1.13 ± 0.1 exp.), d_Pd—C = 1.87 Å (1.93 ± 0.07 exp.), and (uncoupled) estimates for ω_C—O = 1828 cm^?1 (1895 exp.) and ω_Pd—CO = 454 cm^?1 (339 exp.) Binding energies of 4.8, 3.8, and 2.6 eV are calculated, respectively, for Pd₂ + CO, Pd₄ + CO, and Pd₈ + CO which may be compared with the experimental initial heat of adsorption of 1.6 eV. Ionization potentials for CO-derived levels are in excellent agreement with experiment (relative to ?_F: 4σ (-11.0 eV, -11.2 exp.); 5σ (-8.0, ?8.2 exp.); 1π [?7.5 (b₁), ?7.3 (b₂), ?7.5 exp.]). The main negative ion states of 2π* character are calculated at 2.8 eV (b₁) and 2.7 eV (b₂) above E_F. Other states with appreciable 2π* character are found near 5 eV. These may be compared with inverse photoemission results which show a broad peak centered at 4.8 eV. Interactions of the 4σ, 5σ, 1π, and 2π* orbitals of CO with the metal are discussed. The 4σ and 5σ levels are highly mixed, each receiving appreciable contributions from the 4σ and 5σ orbitals of isolated CO. This is discussed in relation to the dispersion of the 4σ and 5σ levels observed in UPS and to the photon-energy dependent intensities of the 4σ and 5σ resonances. The 2π* component of the backbonding comes through several levels in the upper part of the d band which contain small 2π* contributions in bonding combination with Pd d orbitals. The main 2π* orbitals (contaminated by small antibonding contributions from the metal) are empty (see above).     

Structure and growth of oxides on polycrystalline nickel surfaces

The oxidation of polycrystalline nickel (Ni) metal surfaces after exposure to oxygen gas (O₂) at 25 and 300 °C and pressures near 130 Pa, was studied using X‐ray photoelectron spectroscopy (XPS). Oxide structures involving both divalent (Ni²⁺) and trivalent (Ni³⁺) species could be distinguished using Ni 2p spectra, while surface adsorbed O₂ and atomic oxygen (O) species could be differentiated from bulk oxide (O^2?) using O 1s spectra. Oxide thicknesses and distributions were determined using QUASES^?, and the average oxide thickness was verified using the Strohmeier formula. The reaction kinetics for oxide films grown at 300 °C followed a parabolic mechanism, with an oxide thickness of greater than 4 nm having formed after 60 min. Exposure at 25 °C followed a direct logarithmic mechanism with an oxide growth rate about four to five times slower than at 300 °C. Reaction of a Ni (100) single crystal under comparable conditions showed much slower reaction rates compared to polycrystalline specimens. The higher reaction rate of the polycrystalline materials is attributed to grain boundary transport of Ni cations. Oxide thickness was measured on a microscopic scale for polycrystalline Ni exposed to large doses of O₂ at 25 and 300 °C. The thickness of oxide was not strongly localized on this scale. However, the QUASES^? analysis suggests that there is localized growth on a nanometric scale—the result of island formation. Copyright © 2007 John Wiley & Sons, Ltd.     

Exploring the reactions of CO2 with PCP supported nickel complexes

The reactions of PCP supported Ni hydride, methyl and allyl species with CO(2) to generate Ni carboxylates are described. Computational studies suggest that all three reactions follow different pathways.     

Chemisorption on metals

The field of chemisorption has recently seen great advances both in experimental spectroscopies for examining the geometry and electronic structures of adsorbed species and in numerical calculational techniques. The aim of this article is a critical analysis of these results, especially theoretical ones, with emphasis on the relationship to simple models of chemisorption. An important conclusion is the strong coupling nature of many chemisorption systems, i.e. that there exists something like a chemical bond between the adsorbate and the nearest-neighbour substrate atoms with their associated bonding and antibonding electronic levels.We start by outlining the phenomenological fundamentals, such as the structure of the adsorbed layer. Basic theoretical concepts, such as local density functional theory, on which much of the modern discussion of the electronic chemisorption problem depends, and the electronic structure of the clean surface, are then outlined. There follows a fuller discussion of the Anderson model, which is regarded as the underlying canonical model for considering chemisorption. A discussion, in detail, of the theoretical and some experimental aspects of a number of systems is then given. The principal systems considered, which are of contemporary interest, include hydrogen, oxygen and alkalis on free-electron and transition metals, chalcogenides on nickel, CO adsorption on various transition metals, and oxygen on silver.     

Chemisorption of (CHx and C2Hy) hydrocarbons on Pt(111) clusters and surfaces from DFT studies

We used the B3LYP flavor of density functional theory (DFT) to study the chemisorption of all CH(x) and C(2)H(y) intermediates on the Pt(111) surface. The surface was modeled with the 35 atom Pt(14.13.8) cluster, which was found to be reliable for describing all adsorption sites. We find that these hydrocarbons all bind covalently (sigma-bonds) to the surface, in agreement with the studies by Kua and Goddard on small Pt clusters. In nearly every case the structure of the adsorbed hydrocarbon achieves a saturated configuration in which each C is almost tetrahedral with the missing H atoms replaced by covalent bonds to the surface Pt atoms. Thus, (Pt(3))CH prefers a mu(3) hollow site (fcc), (Pt(2))CH(2) prefers a mu(2) bridge site, and PtCH(3) prefers mu(1) on-top sites. Vinyl leads to (Pt(2))CH-CH(2)(Pt), which prefers a mu(3) hollow site (fcc). The only exceptions to this model are ethynyl (CCH), which binds as (Pt(2))C=CH(Pt), retaining a CC pi-bond while binding at a mu(3) hollow site (fcc), and HCCH, which binds as (Pt)HC=CH(Pt), retaining a pi bond that coordinates to a third atom of a mu(3) hollow site (fcc) to form an off center structure. These structures are in good agreement with available experimental data. For all species we calculated heats of formation (DeltaH(f)) to be used for considering various reaction pathways on Pt(111). For conditions of low coverage, the most strongly bound CH(x) species is methylidyne (CH, BE = 146.61 kcal/mol), and ethylidyne (CCH(3), BE = 134.83 kcal/mol) among the C(2)H(y) molecules. We find that the net bond energy is nearly proportional to the number of C-Pt bonds (48.80 kcal/mol per bond) with the average bond energy decreasing slightly with the number of C ligands.     

Chemisorption sites of CO on small gold clusters and transitions from chemisorption to physisorption

Gold clusters adsorbed with CO, Au(m)(CO)(n) (-) (m=2-5; n=0-7), were studied by photoelectron spectroscopy (PES). The first few CO adsorptions were observed to induce significant redshifts to the PES spectra relative to pure gold clusters. For each Au cluster, a critical CO number (n(c)) was observed, beyond which the PES spectra of Au(m)(CO)(n) (-) change very little with increasing n. n(c) was shown to correspond exactly to the available low coordination apex sites in each Au cluster. CO first chemisorbs to these sites and additional CO then only physisorbs to the chemisorption-sautrated Au(m)(CO)(n) (-) complexes.