The infrared spectrum of cyclohexane adsorbed on Pt(111)

The reflection—absorption infrared spectrum of cyclohexane adsorbed on Pt(111) has been recorded using a Fourier Transform spectrometer. The spectrum at 150 K shows the broad, low-frequency CH stretching band previously revealed by electron energy loss spectroscopy and also three sharper bands attributed to vibrational modes involving the relatively unperturbed hydrogens. At 95 K a second adsorbed phase is formed in which the perturbation of the molecule is reduced. Continued exposure leads to multilayer formation. Spectra of adsorbed monolayers of the related compounds n-hexane, cyclopentane and 2,2-dimethylpropane (neopentane) showed much less evidence for perturbation of CH bonds.     

吸附O的Cu(110)c(2×1)表面原子结构和电子态

采用第一性原理的密度泛函理论方法计算了清洁Cu(110)表面和吸附O原子的Cu(110) c(2×1)表面的原子结构, 结构弛豫和电子结构, 得到了各种表面结构参数. 分别计算了O原子在Cu(110)表面三个可能吸附位置吸附后的能量, 并给出了能量最低的吸附位置上各层原子的弛豫特性和态密度. 结果表明O吸附后的Cu(110)表面有附加列(added-row)再构的特性, O原子吸附在最表层铜原子上方, 与衬底Cu原子的垂直距离为0.016 nm, 以氧分子为能量基准的吸附能为－1.94 eV; 同时由于Cu 3d- O 2p态的杂化作用使得低于费米能级5.5~6.0 eV的范围内出现了局域的表面态. 计算得到清洁的和氧吸附的Cu(110)表面的功函数分别为4.51 eV和4.68 eV. 电子态密度的结果表明：在Cu(110) c(2×1) 表面O吸附的结构下, 吸附O原子和金属衬底之间的结合主要是由于最表层Cu原子3d态和O原子2p态的相互作用.     

Evidence for cyclohexyl as a reactive surface intermediate during high-pressure cyclohexane catalytic reactions on Pt(111) by sum frequency generation vibrational spectroscopy

Sum frequency generation (SFG) surface vibrational spectroscopy has been used to identify reactive surface intermediates in situ during catalytic dehydrogenation reactions of high-pressure cyclohexane (C(6)H(12)) on the Pt(111) crystal surface in the presence and absence of high-pressure hydrogen. These experiments provide the first spectroscopic evidence of cyclohexyl (C(6)H(11)) as a reactive surface intermediate during the cyclohexane catalytic conversion to benzene at high pressure in the presence of excess hydrogen. In addition, it was proposed from temperature-dependent SFG experiments that dehydrogenation of cyclohexyl is a rate-limiting step in the cyclohexane catalytic conversion to benzene.     

Time-resolved sum-frequency generation spectroscopy of cyclohexane adsorbed on Ni(111) under ultrashort NIR laser pulse irradiation

The adsorption of cyclohexane on Ni(111) was studied by infrared-visible sum-frequency generation (SFG) spectroscopy with and without near-infrared (NIR) pump pulse irradiation. Two adsorption states of cyclohexane were found in the monolayer region, a low-coverage state showing SFG peaks at 2740, 2815, and 2865 cm(-1), and a high-coverage state showing peaks at 2740, 2815, and 2905 cm(-1). Both states coexisted on the saturated Ni(111) surface. The broad peak at 2740 cm(-1) was due to the softened CH stretching mode of the axial CH groups of cyclohexane that point toward the Ni(111) surface. The peaks at 2815 and 2865 (or 2905) cm(-1) were due to the symmetric and asymmetric stretching modes of CH(2) groups, respectively, that were free from the surface. Irradiation with NIR pulses caused a temporary jump in temperature at the Ni(111) surface and enhanced the intensity of the 2905 cm(-1) peak, but weakened the other peaks. This indicates that the temperature jump excited the cyclohexane molecules from the low-coverage state to the high-coverage state. The dynamics of the structural change observed in the adsorbed cyclohexane on NIR irradiation is discussed.     

Two-dimensional superstructures and softened C-H stretching vibrations of cyclohexane on Rh(111): effects of preadsorbed hydrogen

Adsorption structures and interaction of cyclohexane molecules on the clean and hydrogen-preadsorbed Rh(111) surfaces were investigated using scanning tunneling microscopy, spot-profile-analysis low-energy electron diffraction, temperature-programmed desorption, and infrared reflection absorption spectroscopy (IRAS). Various ordered structures of adsorbed cyclohexane were observed as a function of hydrogen and cyclohexane coverages. When the fractional coverage (θ(H)) of preadsorbed hydrogen was below 0.8, four different commensurate or higher-order commensurate superstructures were found as a function of θ(H); whereas more densely packed incommensurate overlayers became dominant at higher θ(H). IRAS measurements showed sharp softened C-H vibrational peaks at 20 K, which originate from the electronic interaction between adsorbed cyclohexane and the Rh surface. The multiple softened C-H stretching peaks in each phase are due to the variation in the adsorption distance from the substrate. At high hydrogen coverages they became attenuated in intensity and eventually diminished at θ(H) = 1. The gradual disappearance of the soft mode correlates well with the structural phase transition from commensurate structures to incommensurate structures with increasing hydrogen coverage. The superstructure of adsorbed cyclohexane is controlled by the delicate balance between adsorbate-adsorbate and adsorbate-substrate interactions which are affected by preadsorbed hydrogen.     

High resolution electron energy loss measurements of Na/Cu(111) and H2O/Na/Cu(111): dependence of water reactivity as a function of Na coverage

Collective electronic excitations occurring in Na layers grown on Cu(111) and in H2O/Na/Cu(111) have been investigated at room temperature by high resolution electron energy loss spectroscopy. Loss spectra taken for a coverage between 0.55 and 0.70 ML of Na are characterized by a feature at 3.0 eV assigned to a Mie resonance. Further increasing the Na coverage leads to the appearance of the Na surface plasmon at 3.9 eV. Water molecules dissociate on Na layers as shown by the appearance of the OH-Na vibration. Upon water adsorption, relevant effects on both electronic excitations and vibrational modes were observed as a function of Na coverage.     

Toward photochemistry of integrated heterogeneous systems

This paper begins with describing the excitation mechanisms in surface photochemistry and nuclear dynamics of adsorbate induced by electronic excitation. An illustrative example is Cs adsorbate on a Cu(111) surface. This adsorption system shows drastic changes in the electronic structure with coverage; this allows us to examine different types of electronic excitations that stimulate nuclear motions of Cs. Remarks are made on challenges in photoinduced processes at well-defined surfaces: direct observations of adsorbate-substrate vibrational modes and photoinduced reactions between adsorbates. Then, the paper addresses some issues in more complex systems: metal-liquid interfaces and powdered photocatalysts of metal oxides. Photochemistry and photoinduced nuclear dynamics at metal-liquid interfaces have not been well explored. Studies on this subject may make it possible to bridge the gap between surface photochemistry and electrochemistry. Photocatalysis with powdered catalysts has been extensively studied and is still an active area, but our understanding of the mechanism of photocatalysis is far from satisfactory. Although complicated, the highly integrated systems provide an opportunity to extend our knowledge of surface photochemistry.     

Hot hole-induced dissociation of NO dimers on a copper surface

We use reflection-absorption infrared spectroscopy (RAIRS) to study the photochemistry of NO on Cu(110) in the UV-visible range. We observe that the only photoactive species of NO on Cu(110) is the NO dimer, which is asymmetrically bound to the surface. RAIRS shows that photoinduced dissociation proceeds via breaking of the weak N-N bond of the dimer, photodesorbing one NO(g) to the gas phase and leaving one NO(ads) adsorbed on the surface in a metastable atop position. We model the measured wavelength-dependent cross sections assuming both electron- and hole-induced processes and find that the photochemistry can be described by either electron attachment to a level 0.3 eV above the Fermi energy E(F) or hole attachment to a level 2.2 eV below E(F). While there is no experimental or theoretical evidence for an electron attachment level so close to E(F), an occupied NO-related molecular orbital is known to exist at E(F) - 2.52 eV on the Cu(111) surface [I. Kinoshita, A. Misu, and T. Munakata, J. Chem. Phys. 102, 2970 (1995)]. We, therefore, propose that photoinduced dissociation of NO dimers on Cu(110) in the visible wavelength region proceeds by the creation of hot holes at the top of the copper d-band.     

Density functional theory study of CHx (x=1-3) adsorption on clean and CO precovered Rh(111) surfaces

CH(x) (x=1-3) adsorptions on clean and CO precovered Rh(111) surfaces were studied by density functional theory calculations. It is found that CH(x) (x=1-3) radicals prefer threefold hollow sites on Rh(111) surfaces, and the bond strength between CH(x) and Rh(111) follows the order of CH(3)相似文献   

HREELS, STM, and STS study of CH(3)-terminated Si(111)-(1x1) surface

An ideally (1x1)-CH(3)(methyl)-terminated Si(111) surface was composed by Grignard reaction of photochlorinated Si(111) and the surface structure was for the first time confirmed by Auger electron spectroscopy, low energy electron diffraction, high-resolution electron energy loss spectroscopy (HREELS), scanning tunneling microscopy (STM), and scanning tunneling spectroscopy (STS). HREELS revealed the vibration modes associated to the CH(3)-group as well as the C-Si bond. STM discerned an adlattice with (1x1) periodicity on Si(111) composed of protrusions with internal features, covering all surface terraces. The surface structure was confirmed to be stable at temperatures below 600 K. STS showed that an occupied-state band exists at gap voltage of -1.57 eV, generated by the surface CH(3) adlattice. This CH(3):Si(111)-(1x1) adlayer with high stability and unique electronic property is prospective for applications such as nanoscale lithography and advanced electrochemistry.     

A Periodic Density Functional Study on Adsorption Properties of Methoxy on Au（111） Surface

1INTRODUCTION Methoxy(CH3O)has been identified as the first intermediate in the decomposition of methanol on extensive list of clean transition metal surfaces,such as Ni(100)[1],Cu(100)[2,3],Cu(111)[4],Ag(111)[5],Au(110)[6],Pd(111)[7]and Ru(0001)[8].The electronic structure of the metal is a determining factor in OH bond scission.In fact,group IB clean surfaces have shown very low activity towards this reaction,al-though there are reports on low amounts of methoxy formed on clean Cu(…     

Theoretical study of the CH2 + O photodissociation of formaldehyde adsorbed on the Ag(111) surface

Configuration interaction calculations of the ground and excited states of the H2CO molecule adsorbed on the Ag(111) surface have been carried out to study the photoinduced dissociation process leading to polymerization of formaldehyde. The metal-adsorbate system has been described by the embedded cluster and multireference configuration interaction methods. The pi electron-attachment H2CO- and n-pi* internally excited H2CO* states have been considered as possible intermediates. The calculations have shown that H2CO* is only very weakly bound on Ag(111), and thus that the dissociation of adsorbed formaldehyde due to internal excitation is unlikely. By contrast, the H2CO- anion is strongly bound to Ag(111) and gains additional vibrational energy along the C-O stretch coordinate via Franck-Condon excitation from the neutral molecule. Computed energy variations of adsorbed H2CO and H2CO- at different key geometries along the pathway for C-O bond cleavage make evident, however, that complete dissociation is very difficult to attain on the potential energy surface of either of these states. Instead, reneutralization of the vibrationally excited anion by electron transfer back to the substrate is the most promising means of breaking the C-O bond, with subsequent formation of the coadsorbed O and CH2 fragments. Furthermore, it has been demonstrated that the most stable state for both dissociation fragments on Ag(111) is a closed-shell singlet, with binding energies relative to the gas-phase products of approximately 3.2 and approximately 1.3 eV for O and CH2, respectively. Further details of the reaction mechanism for the photoinduced C-O bond cleavage of H2CO on the Ag(111) surface are also given.     

Femtosecond dynamics of electronic excitations of adsorbates studied by two-photon photoemission pulse correlation: CO/Cu(111)

An equal pulse correlation technique based on angle-resolved two-photon photoemission is employed to investigate the lifetime of electronic excitations of an adsorbate on a single crystal metal surface. Photoemission from an occupied surface state on Cu(111) by a non-resonant two-photon process via the sp-band gap is used to characterize directly at the surface the width and shape of 65 fs laser pulses at a photon energy of hv = 3.4 eV. The 2PPE correlation technique allows the establishment of an upper limit of τ < 20 fs for the lifetime of the 2π^* resonance of CO adsorbed on Cu(111), whereas from the spectral width of the 2π^* level a lower limit of ≈1 fs is estimated.     

Precursor species of carbon monoxide before its dissociation on aluminium-promoted Ni and Cu surfaces

Carbon monoxide adsorbed on Al-promoted Ni and Cu surfaces undergoes facile dissociation at 300 K or below. CO adsorbed on an Al-promoted Ni surface shows a larger separation (3.5 eV) between the 5σ and 4σ levels compared to that on a clean Ni surface (3.0 eV), besides exhibiting a low C-O stretching frequency. CO adsorbed on an Al-promoted Cu surface shows a similar large 5σ-4σ separation; this is accompanied by the disappearance of the satellites in the valence band and O(1s) regions. These features of CO adsorbed on Al-promoted Ni and Cu surfaces characterize the precursor to dissociation. Molecular orbital calculations suggest that CO bonds parallel to the Al-promnoted Cu surface.     

Interaction of coadsorbed CH3Cl and D2O layers on Pd(111) studied by sum frequency generation

Adsorption of methyl chloride and coadsorption of CH3Cl and D2O on Pd(111) surfaces at T=100 K have been studied under ultrahigh-vacuum conditions using femtosecond sum frequency generation (SFG) spectroscopy in the spectral regions of CH and OD bands. On the bare Pd(111) substrate, the CH3Cl coverage dependence of the resonant SFG signal is consistent with a progressive molecular rearrangement starting at half saturation followed by the growth of two ordered monolayers in which the molecular axes are perpendicular to the surface. When CH3Cl is adsorbed on top of predeposited D2O on Pd(111), the SFG signals as a function of the CH3Cl exposure indicate that methyl chloride is adsorbed onto D2O through hydrogen bonding. On the contrary when the adsorption order is reversed the strong decrease of the CH3 signal as a function of the D2O exposure is explained by assuming that water molecules penetrate inside the CH3Cl layers, leading to the formation of disordered CH3Cl clusters. In all cases a nonresonant contribution due to molecular adsorption is observed and it shows a dependence upon surface structure and coverage significantly different from that of the resonant vibrational bands.     

Alkyl chain conformation and the electronic structure of octyl heavy chalcogenolate monolayers adsorbed on Au(111)

Adsorption states of dioctyl dichalcogenides (dioctyl disulfide, dioctyl diselenide, and dioctyl ditelluride) arranged on Au(111) have been studied by X-ray photoelectron spectroscopy (XPS), infrared-visible sum-frequency generation (SFG), and ultraviolet photoelectron spectroscopy (UPS). XPS measurements suggest that dioctyl dichalcogenides dissociatively adsorbed on Au(111) surfaces to form the corresponding monolayers having chalcogen-gold covalent bonds. The elemental compositions of octanechalcogenolates on Au(111) indicate that the saturation coverages of the octyl heavy chalcogenolate (OcSe, OcTe) monolayers are lower than that of the octanethiolate (OcS) self-assembled monolayers (SAMs). The SFG observations of the CH(2) vibrational bands for the heavy chalcogenolate monolayers strongly suggest that a discernible amount of gauche conformation exists in the monolayers, while OcS SAMs adopt highly ordered all-trans conformation. The intensity ratio of the symmetric and asymmetric CH(3) stretching vibration modes measured by SFG shows that the average tilt angle of the methyl group of the OcSe monolayers is greater than that of the OcS SAMs. The larger tilt angle of the methyl group and the existence of a discernible amount of gauche conformation in the OcSe monolayers are due to the lower surface coverage of the OcSe monolayers compared with the OcS SAMs. The smaller polarization dependence in the angle-resolved UPS (ARUPS) spectra of the OcSe monolayers than that of the OcS SAMs is caused by the more disordered structures of the alkyl chain in the former. XPS, SFG, and ARUPS measurements indicate a similar tendency for the OcTe monolayers. The density of states (DOS) observed by UPS at around 1.3 eV for OcS adsorbed on Au(111) is considered to be the antibonding state of the Au-sulfur bond. Similar DOS is also observed by UPS at around 1.0 eV for the OcSe monolayers and at approximately 1.6 eV for the OcTe monolayers on Au(111).     

Physical properties of gamma alumina surface hydroxyls revisited through a large scale periodic quantum-chemistry approach

We have studied surface hydroxyls adsorbed onto (001), (011), and (111) gamma alumina surfaces using a quantum-chemistry approach in order to compare with empirical models proposed in the literature. Local electronic structures and geometries in the low OH coverage limit have been evaluated for both ideal and relaxed surfaces with the help of a large scale periodic quantum-chemical code. Hydroxyl groups are adsorbed onto surfaces, and a study of their local electronic structure, vibrational frequencies, charges, and adsorption energies is performed and analyzed as a function of their adsorption site geometry. Our results show that, even on ideal (nonrelaxed) surfaces, OH local environments are more complicated than those stated by empirical models and strongly influence the hydroxyl stretching vibrational mode. Large scale simulation shows that disorder takes place even at 0 K, and the analysis of the vibrational frequencies leads to a revision of Kn?zinger's empirical model. Cationic vacancies in the first surface layers have also been taken into account; they have a significant influence on the surface atomic and electronic structures, modifying the physical properties of adsorbed OH entities. This work emphasizes the necessity to perform an electronic structure calculation to better understand adsorbed OH properties on gamma alumina surfaces and reveals the difficulty to make a one-to-one correspondence between OH stretching frequencies and their other physical properties. Finally, we show that these results agree with some available experimental studies.     

First-principles study on the adhesive and electronic property of c-BN(111)/Cu(111) interface

The interface properties of c-BN/Cu composite play an important role in its application. In this work, we employed first-principles calculation to investigate the bonding properties and electronic characteristics of the c-BN(111)/Cu(111) interface. The adhesion properties, partial density of states (PDOS), charge density, and charge density difference of different interfaces were analyzed. The results show that the interface of B-termination “OT” stacking mode is the most stable one. The density of states at the c-BN(111)/Cu(111) interface is similar to that of c-BN bulk phase, indicating that the electronic states of the c-BN layer are not affected by the Cu atoms. The PDOS diagram shows that the 2p orbital of B atoms and the 2p orbital of N atoms are hybridized in the c-BN layer. Besides, 2p orbital of B(N) atoms and 3d orbital of Cu atoms are hybridized in the interface. The covalent bonds and ionic bonds in the interface of N-termination and B-termination OT stacking mode structures are stronger than that of “SL” and “TL” stacking mode. So, the OT stacking mode has larger adhesive energy. Furthermore, Cu and c-BN can form a good coherent interface, which can be used to prepare c-BN/Cu composites and functional materials with excellent mechanical properties.     

The interaction of C6H6 and C6H12 with noble metal surfaces: electronic level alignment and the origin of the interface dipole

The electronic interaction of two molecules, the aromatic benzene (C6H6) and the saturated hydrocarbon cyclohexane (C6H12) with a Cu(111) surface, have been determined using precise, ab initio electronic structure calculations. For the interaction of these adsorbates with the substrate, we present a detailed analysis and decomposition of various individual chemical mechanisms that contribute. A novel aspect of this analysis is the use of charge-density difference contour plots to graphically display the chemistry. A wave-function-based approach was used in order to avoid problems when the presently most commonly employed approach, density-functional theory, is applied to weakly chemisorbed molecules, where the interaction is dominated by van der Waals forces. The present information are not only relevant with regard to understanding the chemistry going on when molecules are adsorbed on a Cu surface but also have important consequences with regard to charge injection in molecular electronic devices, e.g., organic field-effect transistors and organic light-emitting diodes.     

Surface chemistry of CN bond formation from carbon and nitrogen atoms on Pt(111)

The mechanism of CN bond formation from CH3 and NH3 fragments adsorbed on Pt(111) was investigated with reflection absorption infrared spectroscopy (RAIRS), temperature-programmed desorption (TPD), and X-ray photoelectron spectroscopy (XPS). The surface chemistry of carbon-nitrogen coupling is of fundamental importance to catalytic processes such as the industrial-scale synthesis of HCN from CH4 and NH3 over Pt. Since neither CH4 nor NH3 thermally dissociate on Pt(111) under ultrahigh vacuum (UHV) conditions, the relevant surface intermediates were generated through the thermal decomposition of CH3I and the electron-induced dissociation of NH3. The presence of surface CN is detected with TPD through HCN desorption as well as with RAIRS through the appearance of the vibrational features characteristic of the aminocarbyne (CNH2) species, which is formed upon hydrogenation of surface CN at 300 K. The RAIRS results show that HCN desorption at approximately 500 K is kinetically limited by the formation of the CN bond at this temperature. High coverages of Cads suppress CN formation, but the results are not influenced by the coadsorbed I atoms. Cyanide formation is also observed from the reaction of adsorbed N atoms and carbon produced from the dissociation of ethylene.