Methylthiolate on Au(111): adsorption and desorption kinetics

Low energy electron diffraction, Auger electron spectroscopy, X-ray photoelectron spectroscopy and line of sight mass spectrometry have been used to study the adsorption and desorption of dimethyldisulfide (DMDS) on Au(111). At 300 K adsorption is dissociative, forming a chemisorbed adlayer of methylthiolate with a 1/3 ML, (sq rt 3 x sq rt 3)R30 degrees, structure. At 100 K adsorption is molecular, with dissociation to form the 1/3 ML (sq rt 3 x sq rt 3)R30 degrees methylthiolate structure occurring at 138-160 K. A physisorbed DMDS layer, with a coverage of 1/6 ML of DMDS, forms on top of the (sq rt 3 x sq rt 3)R30 degrees chemisorbed MT surface for T < or = 180 K, with multilayers forming for T < or = 150 K. In temperature programmed desorption, multilayers of DMDS desorbed with zero order kinetics and an activation energy of 41 kJ mol(-1); the physisorbed layer desorbed with first order kinetics, exhibiting repulsive lateral interactions with an activation energy which varied from 63 kJ mol(-1) (theta = 0) to 51 kJ mol(-1) (theta = 1); the chemisorbed methylthiolate layer desorbed associatively as DMDS via the physisorbed layer, the activation energy for the reaction, 2 methylthiolate --> physisorbed DMDS, exhibiting repulsive lateral interactions with an activation energy which varied from 65 kJ mol(-1) (theta = 0) to 61 kJ mol(-1) (theta = 1). The physisorbed disulfide layer explains the pre-cursor state adsorption kinetics observed in sticking probability measurement, while its relatively facile formation provides a mechanism by which thiolate self-assembled monolayers can become mobile at room temperature.     

Adsorption, desorption, and clustering of H2O on Pt111

The adsorption, desorption, and clustering behavior of H2O on Pt111 has been investigated by specular He scattering. The data show that water adsorbed on a clean Pt111 surface undergoes a structural transition from a random distribution to clustered islands near 60 K. The initial helium scattering cross sections as a function of temperature are found to be insensitive to the incident H2O flux over a range of 0.005 monolayers (ML)/s-0.55 ML/s indicating that the clustering process is more complex than simple surface diffusion. The coarsening process of an initially random distribution of water deposited at 25 K is found to occur over a broad temperature range, 60相似文献   

Water desorption from an oxygen covered Pt(111) surface: multichannel desorption

Mixed OH/H2O structures, formed by the reaction of O and water on Pt(111), decompose near 200 K as water desorbs. With an apparent activation barrier that varies between 0.42 and 0.86 eV depending on the composition, coverage, and heating rate of the film, water desorption does not follow a simple kinetic form. The adsorbate is stabilized by the formation of a complete hydrogen bonding network between equivalent amounts of OH and H2O, island edges, and defects in the structure enhancing the decomposition rate. Monte Carlo simulations of water desorption were made using a model potential fitted to first-principles calculations. We find that desorption occurs via several distinct pathways, including direct or proton-transfer mediated desorption and OH recombination. Hence, no single rate determining step has been found. Desorption occurs preferentially from low coordination defect or edge sites, leading to complex kinetics which are sensitive to both the temperature, composition, and history of the sample.     

CO诱导的FeO(111)/Ru(0001)负载Au原子吸附位和电荷的改变

采用密度泛函理论研究了CO气氛对FeO(111)/Ru(0001)负载Au原子吸附位、电荷及其稳定性的影响. 首先考察了FeO(111)单层薄膜在Ru(0001)表面上的界面结构. 研究发现,表面莫尔超晶胞内的HCP区域有最小的Fe-O层间距(rumpling),且Fe和O原子均与衬底Ru形成化学键. Au原子在FeO/Ru(0001)上最稳定的吸附在HCP区域的Fe-bridge位. 其中,Au原子诱导两个Fe原子从O原子层的下面翻转到其上面,形成两个Au-Fe键,且Au带负电. 当把体系暴露在CO气氛下后,CO能诱导Au原子从原来最稳定的Fe-bridge位转移到其邻近的O-top位,伴随着Au的电荷从负变到正,形成非常稳定的Au⁺-CO羰基物. 结果表明,反应气氛对负载金属催化剂的化学状态及其稳定性的影响很大; 同时也强调了反应条件下催化剂原位表征的重要性.     

Adsorption and desorption of HCl on Pt(111)

The adsorption and desorption of HCl on Pt(111) is investigated by temperature programmed desorption, infrared reflection absorption spectroscopy, and low energy electron diffraction. Five peaks are identified in the desorption spectra prior to the onset of multilayer desorption. At low coverage ( < 0.25 monolayers (ML)), desorption peaks at approximately 135 and 200 K are observed and assigned to recombinative desorption of dissociated HCl. At higher coverages, desorption peaks at 70, 77, and 84 K are observed. These peaks are assigned to the desorption of molecularly adsorbed HCl. The infrared spectra are in agreement with these assignments and show that HCl deposited at 20 K is amorphous but crystallizes when heated above 60 K. Kinetic analysis of the desorption spectra reveals a strong repulsive coverage dependence for the desorption energy of the low coverage features ( < 0.25 ML). The diffraction data indicate that at low temperature the adsorbed HCl clusters into ordered islands with a (3 x 3) structure and a local coverage of 4/9 with respect to the Pt(111) substrate.     

Water adsorption and desorption on microporous solids at elevated temperature

Summary An accurate gravimetric method was used to explore water adsorption/desorption isotherms between 105 and to 250°C for a number of synthetic and natural porous solids including controlled pore glass, activated carbon fiber monoliths, natural zeolites, pillared clay, and geothermal reservoir rocks. The main goal of this work was to evaluate water adsorption results, in particular temperature dependence of hysteresis, for relatively uniform, nano-structured solids, in the context of other state-of-the-art experimental and modeling methods including nitrogen adsorption, spectroscopy, neutron scattering, and molecular simulation. Since no single method is able to provide a complete characterization of porous materials, a combination of approaches is needed to achieve progress in understanding the fluid-solid interactions on the way to developing a predictive capability.     

n-alkanes on Pt(111) and on C(0001)Pt(111): chain length dependence of kinetic desorption parameters

We have measured the desorption of seven small n-alkanes (C(N)H(2N+2), N=1-4,6,8,10) from the Pt(111) and C(0001) surfaces by temperature programed desorption. We compare these results to our recent study of the desorption kinetics of these molecules on MgO(100) [J. Chem. Phys. 122, 164708 (2005)]. There we showed an increase in the desorption preexponential factor by several orders of magnitude with increasing n-alkane chain length and a linear desorption energy scaling with a small y-intercept value. We suggest that the significant increase in desorption prefactor with chain length is not particular to the MgO(100) surface, but is a general effect for desorption of the small n-alkanes. This argument is supported by statistical mechanical arguments for the increase in the entropy gain of the molecules upon desorption. In this work, we demonstrate that this hypothesis holds true on both a metal surface and a graphite surface. We observe an increase in prefactor by five orders of magnitude over the range of n-alkane chain lengths studied here. On each surface, the desorption energies of the n-alkanes are found to increase linearly with the molecule chain length and have a small y-intercept value. Prior results of other groups have yielded a linear desorption energy scaling with chain length that has unphysically large y-intercept values. We demonstrate that by allowing the prefactor to increase according to our model, a reanalysis of their data resolves this y-intercept problem to some degree.     

Decoupling surface reconstruction and perchlorate adsorption on Au(111)

On Au(111) electrodes, the investigation of ClO₄⁻ adsorption is hampered by a simultaneous surface reconstruction. We demonstrate that these two processes can be decoupled in cyclic voltammograms by a proper choice of the scan rate and of the initial potential. Our approach allowed the establishment of a relation between potentials of zero charge for the reconstructed and unreconstructed Au(111) surfaces.     

Pt(111) 表面上表层 Fe (FeO) 结构和次表层 Fe 结构的可控转变

 利用扫描隧道显微镜 (STM) 和 X 射线光电子能谱 (XPS) 对 Pt(111) 表面制备的 Fe 单层薄膜及其在不同环境气氛条件下的多种结构进行了研究. 在温度为 487 K 的 Pt(111) 表面制备出了完整的 Fe 单层薄膜Fe/Pt(111). 对 Fe/Pt(111) 依次升高温度进行超高真空退火, STM 和 XPS 结果表明退火温度高于 800 K 时, 表面 Fe 原子扩散到次表层区域, 形成次表层 Fe 结构Pt/Fe/Pt(111). Pt/Fe/Pt(111) 在 O2 氧化气氛中经 850 K 退火可转变成表面 FeO 薄膜FeO/Pt(111). FeO/Pt(111) 结构在温和的 H2 还原气氛中 (600 K) 转变成表面 Fe 结构, 进一步的还原处理 (800 K) 则可以重新生成 Pt/Fe/Pt(111). 控制样品的环境气氛在 O2 和 H2 之间切换, 使得表面 Fe (FeO) 和次表面 Fe 可以重复地转变. 本研究实现了多种 Fe-Pt 表面结构的可控制备, 可为合理地设计高效、价廉的催化剂提供借鉴.     

Iron adsorption on SiO2/Si(111)

The adsorption of ferric and ferrous iron onto the native oxide of the SiO₂/Si(111) surface has been evaluated using X‐ray photoelectron spectroscopy (XPS). Through a series of immersion experiments, performed at room temperature and pH 1, it has been shown that the ferric species is strongly adsorbed onto the hydrophilic surface, while ferrous iron remains in solution. Dehydroxylation of the silica surface by etching with hydrofluoric acid reduces the concentration of receptive Si‐OH groups, thereby limiting iron adsorption. The experiments were reproduced in a combined ultrahigh vacuum‐electrochemical system (UHV‐EC), which allowed a carbon‐free surface to be prepared before contacting the iron solutions, and confirmed the strong affinity of ferric iron towards the SiO₂/Si(111) surface. Copyright © 2007 John Wiley & Sons, Ltd.     

Coverage-dependent adsorption geometry of octithiophene on Au(111)

The adsorption behavior of α-octithiophene (8T) on the Au(111) surface as a function of 8T coverage has been studied with low-temperature scanning tunneling microscopy, high resolution electron energy loss spectroscopy as well as with angle-resolved two-photon photoemission and ultraviolet photoemission spectroscopy. In the sub-monolayer regime 8T adopts a flat-lying adsorption geometry. Upon reaching the monolayer coverage the orientation of 8T molecules changes towards a tilted configuration, with the long molecular axis parallel to the surface plane, facilitating attractive intermolecular π-π-interactions. The photoemission intensity from the highest occupied molecular orbitals (HOMO and HOMO - 1) possesses a strong dependence on the adsorption geometry due to the direction of the involved transition dipole moment for the respective photoemission process. The change in molecular orientation as a function of coverage in the first molecular layer mirrors the delicate balance between intermolecular and molecule/substrate interactions. Fine tuning of these interactions opens up the possibility to control the molecular structure and accordingly the desirable functionality.     

CO在Pt(111)和Pt-M(111)(M=Ni,Mg)表面吸附的理论研究

采用密度泛函理论与周期性平板模型相结合的方法,对CO在Pt(111)表面top,fcc,hcp和bridge 4个吸附位和Pt-M(111)(M=Ni,Mg)表面h-top,M-top,Pt(M)Pt-bridge,Pt(M)M-bridge,Pt(Pt)M-bridge,M(Pt)M-bridge,Pt1M2-hcp...     

Diffusion-limited thiol adsorption on the gold(111) surface

An optical second harmonic generation measurement of the kinetics of self-assembly of a monolayer of thiols on the Au(111) surface reveals a marked dependence of the adsorption rate upon the solution flow rate. The nature of this dependence indicates that at low concentration and low flow rate the monolayer growth is limited by the existence of a Nernst diffusion layer, not by surface reaction rate kinetics.     

Tuning the Reactivity of Ultrathin Oxides: NO Adsorption on Monolayer FeO(111)

Ultrathin metal oxides exhibit unique chemical properties and show promise for applications in heterogeneous catalysis. Monolayer FeO films supported on metal surfaces show large differences in reactivity depending on the metal substrate, potentially enabling tuning of the catalytic properties of these materials. Nitric oxide (NO) adsorption is facile on silver‐supported FeO, whereas a similar film grown on platinum is inert to NO under similar conditions. Ab initio calculations link this substrate‐dependent behavior to steric hindrance caused by substrate‐induced rumpling of the FeO surface, which is stronger for the platinum‐supported film. Calculations show that the size of the activation barrier to adsorption caused by the rumpling is dictated by the strength of the metal–oxide interaction, offering a straightforward method for tailoring the adsorption properties of ultrathin films.     

Heat of adsorption of naphthalene on Pt(111) measured by adsorption calorimetry

The heat of adsorption of naphthalene on Pt(111) at 300 K was measured with single-crystal adsorption calorimetry. The heat of adsorption on the ideal, defect-free surface is estimated to be (300 - 34 - 199(2)) kJ/mol. From this, a C-Pt bond energy for aromatic hydrocarbons on Pt(111) of approximately 30 kJ/mol is estimated, consistent with earlier results for benzene on Pt(111). There is higher heat of adsorption at very low coverage, attributed to step sites where the adsorption heat is >/=330 kJ/mol. Saturation coverage, = 1 ML, corresponds to 1.55 x 10(14) molecules/cm(2). Sticking probability measurements of naphthalene on Pt(111) give a high initial value of 1.0 and a Kisliuk-type coverage dependence that implies precursor-mediated sticking. The ratio of the hopping rate to the desorption rate of this precursor is approximately 51. Naphthalene adsorbs transiently on top of chemisorbed naphthalene molecules with a heat of adsorption of 83-87 kJ/mol.     

Water adsorption on Rh(111) at 20 K: from monomer to bulk amorphous ice

The adsorption of water (D(2)O) molecules on Rh(111) at 20 K was investigated using infrared reflection absorption spectroscopy (IRAS). At the initial stage of adsorption, water molecules exist as monomers on Rh(111). With increasing water coverage, monomers aggregate into dimers, larger clusters (n = 3-6), and two-dimensional (2D) islands. Further exposure of water molecules leads to the formation of three-dimensional (3D) water islands and finally to a bulk amorphous ice layer. Upon heating, the monomer and dimer species thermally migrate on the surface and aggregate to form larger clusters and 2D islands. Based on the temperature dependence of OD stretching peaks, we succeeded in distinguishing water molecules inside 2D islands from those at the edge of 2D islands. From the comparison with the previous vibrational spectra of water clusters on other metal surfaces, we conclude that the number of water molecules at the edge of 2D islands is comparable with that of water molecules inside 2D islands on the Rh(111) surface at 20 K. This indicates that the surface migration of water molecules on Rh(111) is hindered as compared with the cases on Pt(111) and Ni(111) and thus the size of 2D islands on Rh(111) is relatively small.     

Density functional theory for adsorption of HCHO on the FeO（100） surface

The density functional theory (DFT) and periodic slab model were used to get information concerning the adsorption of HCHO on the FeO(100) surface. A preferred η2-(C,O)-di-σ four-membered ring adsorption conformation on the Fe-top site was found to be the most favorable structure with the predicted adsorption energy of 210.7 kJ/mol. The analysis of density of states, Mulliken population, and vibrational frequencies before and after adsorption showed clear weakening of the carbonyl bond, and high sp3 charact...     

NO adsorption and dissociation on Rh(111): PM-IRAS study

We have used in situ polarization-modulation infrared reflection absorption spectroscopy to study the adsorption/dissociation of NO on Rh(111). While these studies have not been conclusive regarding the detailed surface structures formed during adsorption, they have provided important new information on the dissociation of NO on Rh(111). At moderate pressures (< or =10(-6) Torr) and temperatures (<275 K), a transition from 3-fold hollow to atop bonding is apparent. Data indicate that this transition is not due to the migration of the 3-fold hollow NO but rather to the adsorption of gas-phase NO that is directed toward the atop position due to the presence of NO decomposition products, particularly chemisorbed atomic O species at the hollow sites. These results indicate that NO dissociation occurs at temperatures well below the temperature previously reported. Additionally, high pressure (1 Torr) NO exposure at 300 K results in only atop NO, calling into question the surface structures previously proposed at these adsorption conditions consisting of atop and 3-fold hollow sites.     

TNT adsorption on Au(111): electrochemistry and adlayer structure

Electrochemistry and adlayer structure of trinitrotoluene (TNT) on an Au(111) electrode were investigated using cyclic voltammetry and in situ electrochemical scanning tunneling microscopy (ECSTM).     

Low-temperature adsorption of H2S on Ag(111)

H(2)S forms a rich variety of structures on Ag(111) at low temperature and submonolayer coverage. The molecules decorate step edges, exist as isolated entities on terraces, and aggregate into clusters and islands, under various conditions. One type of island exhibits a (√37×√37)R25.3° unit cell. Typically, molecules in the clusters and islands are separated by about 0.4 nm, the same as the S-S separation in crystalline H(2)S. Density functional theory indicates that hydrogen-bonded clusters contain two types of molecules. One is very similar to an isolated adsorbed H(2)S molecule, with both S-H bonds nearly parallel to the surface. The other has a S-H bond pointed toward the surface. The potential energy surface for adsorption and diffusion is very smooth.