Oxide-free oxygen incorporation into Ru(0001)

A smooth Ru(0001) surface prepared under ultra-high vacuum conditions has been loaded with oxygen under high-pressure (p approximately 1 bar) and low-temperature (T < 600 K) conditions. Oxygen phases created in this way have been investigated by means of thermal desorption spectroscopy, low-energy electron diffraction, and ultraviolet photoelectron spectroscopy. The exposure procedures applied lead to oxygen incorporation into the subsurface region without creation of RuO2 domains. For oxygen exposures ranging from 10(11) to 10(14) L oxygen contents up to about 4 monolayer equivalent could be achieved. The oxygen incorporation is thermally activated. The CO oxidation reaction conducted at mild temperatures (T < 500 K) at a sample loaded with subsurface oxygen reaches CO --> CO2 conversion probabilities of 10(-3).     

Ultrathin Rh films on Ru(0001): oxidation in confinement

Ultrathin rhodium films with a thickness ranging from 1 to a few monolayers were deposited on a single-crystal Ru(0001) surface in order to investigate the oxidation behavior of ultrathin epitaxial films on a dissimilar substrate. It is found that rhodium grows on Ru(0001) initially layer by layer, adapting the in-plane lattice parameters of Ru(0001). When exposing Rh films to oxygen environment (approximately 4.8 x 10(6) L O2 exposure) at 660 K, 2-4 ML Rh films form a surface oxide composed of (9 x 9) O-Rh-O trilayers. Quite in contrast, oxidation of the 1 ML RhRu(0001) film leads to a poorly ordered oxide with a rutile structure reminiscent of RuO2(110) on Ru(0001). The oxidized 1 ML RhRu(0001) film contains much more oxygen than the oxidized thicker Rh films. Lower temperatures (535 K) and high doses of oxygen lead to a (1 x 1)-O overlayer on the 1 ML RhRu(0001) surface, whose atomic geometry resembles closely that of the (1 x 1)-O phase on clean Ru(0001).     

The growth of perylene on Ru(0001)

The structure of perylene adsorbed on Ru(0001) surface has been studied by ultraviolet photoemission spectroscopy (UPS) and low-energy electron diffraction. An ordered p(4x4) structure is observed from a monolayer (about 4 A thickness) of the perylene on Ru(0001) surface. UPS measurements show the molecular features, from the perylene multiplayer, between 2 and 10 eV below the Fermi level. Angle-resolved ultraviolet photoemission spectroscopy measurements suggest that the perylene molecular plane is parallel to the substrate. Temperature dependent UPS measurements show that the perylene multilayer is stable on Ru(0001) surface up to 125 degrees C. The desorption of the multilayer and the decomposition of the monolayer are observed above 125 degrees C.     

Deposition and crystallization studies of thin amorphous solid water films on Ru(0001) and on CO-precovered Ru(0001)

The deposition and the isothermal crystallization kinetics of thin amorphous solid water (ASW) films on both Ru(0001) and CO-precovered Ru(0001) have been investigated in real time by simultaneously employing helium atom scattering, infrared reflection absorption spectroscopy, and isothermal temperature-programmed desorption. During ASW deposition, the interaction between water and the substrate depends critically on the amount of preadsorbed CO. However, the mechanism and kinetics of the crystallization of approximately 50 layers thick ASW film were found to be independent of the amount of preadsorbed CO. We demonstrate that crystallization occurs through random nucleation events in the bulk of the material, followed by homogeneous growth, for solid water on both substrates. The morphological change involving the formation of three-dimensional grains of crystalline ice results in the exposure of the water monolayer just above the substrate to the vacuum during the crystallization process on both substrates.     

氢原子在Ru(0001)表面的化学吸附

用密度泛函理论研究了氢原子的污染对于Ru(0001)表面结构的影响. 通过PAW(projector-augmented wave)总能计算研究了p(1×1)、p(1×2)、(3^(1/2)×3^(1/2))R30°和p(2×2)等几种氢原子覆盖度下的吸附结构, 以及在上述结构下Ru(0001)面fcc(面心立方)格点和hcp(六方密堆)格点的氢原子吸附. 所得结果表明, 在p(1×1)-H、p(1×2)-H、(3^(1/2)×3^(1/2))R30°-H和p(2×2)-H几种H原子覆盖度下, 以p(1×1)-H结构单个氢原子吸附能为最大. 在p(1×1)-H吸附结构下,由于氢原子吸附导致的Ru(0001) 表面第一层Ru 原子收缩的理论计算数值分别为-1.11%(hcp 吸附)和-1.55%(fcc 吸附), 因此实际上最有可能的情况是两种吸附方式都有一定的几率. 而实验中观察到的“清洁”Ru(0001)表面实际上是有少量氢原子污染的表面. 不同覆盖度和氢分压下氢原子吸附的污染对Ru(0001)表面结构有极大的影响,其表面的各种特性都会随覆盖度的不同而产生相应的变化.     

Reactivity of oxide precursor states on Ru(0001)

Smooth and defect-rich Ru(0001) surfaces prepared under ultrahigh-vacuum (UHV) conditions have been loaded with oxygen under high-pressure (p 相似文献   

Formation of identical-size graphene nanoclusters on Ru(0001)

Identical-size graphene nanoclusters (GNCs) form on Ru(0001) mediated by the substrate-induced clustering effect. The two kinds of uniform GNCs were identified as the seven C6-ring (noted as 7-C6) and three C6-ring (3-C6) structures with a dome-shape by using scanning tunneling microscopy.     

Initial growth of the water layer on (1 x 1)-oxygen-covered Ru(0001) in comparison with that on bare Ru(0001)

The initial growth of a water (D2O) layer on (1 x 1)-oxygen-covered Ru(0001) has been studied in comparison with that on bare Ru(0001) by means of temperature-programmed desorption (TPD) and infrared reflection absorption spectroscopy (IRAS). Although water molecules adsorbed on both bare and (1 x 1)-oxygen-covered Ru(0001) commonly tend to form hydrogen bonds with each other when mobility occurs upon heating, the TPD and IRAS measurements for the two surfaces exhibit distinct differences. On (1 x 1)-oxygen-covered Ru(0001), most of the D2O molecules were desorbed with a peak at 160 K, even at submonolayer coverage, as condensed water desorption. The vibration spectra of adsorbed D2O also showed broad peaks such as a condensed water phase, from the beginning of low coverage. For submonolayer coverage, in addition, we found a characteristic O-D stretching mode at around 2650 cm(-1), which is never clearly observed for D2O on bare Ru(0001). Thus, we propose a distinctive water adsorption structure on (1 x 1)-oxygen-covered Ru(0001) and discuss its influence on water layer growth in comparison with the case of D2O on bare Ru(0001).     

Morphological change during crystallization of thin amorphous solid water films on Ru(0001)

The isothermal crystallization process of thin amorphous solid water (ASW) films on Ru(0001) has been investigated in real time by simultaneously employing helium atom scattering, infrared reflection absorption spectroscopy, and isothermal temperature-programmed desorption. The measurements reveal that the crystallization mechanism consists of random nucleation events in the bulk of the ASW films, followed by homogeneous growth. Morphological changes of the solid water film during crystallization expose the water monolayer just above the substrate to the vacuum during the crystallization process.     

Pb intercalation underneath a graphene layer on Ru(0001) and its effect on graphene oxidation

Intercalation of Pb at graphene/Ru(0001) interfaces has been dynamically observed using in situ low energy electron microscopy and photoemission electron microscopy. A comparative study of Pb intercalation on the submonolayer and complete monolayer graphene surfaces suggest that the Pb intercalation happens through the open edges of graphene islands, starting at around 150 °C. Spatially-resolved low energy electron diffraction measurements reveal that the Pb-intercalated graphene overlayers are quasi-free-standing. The intercalated graphene sheets show lower reactivity to oxidation in O(2).     

Pt(x)Ru(1-x)/Ru(0001) surface alloys-formation and atom distribution

The formation of PtRu surface alloys by deposition of submonolayer Pt films on a Ru(0001) substrate and subsequent annealing to about 1350 K and the distribution of the Pt atoms in the surface layer were investigated by scanning tunneling microscopy. Quantitative statistical analysis reveals (i) negligible losses of Pt into subsurface regions up to coverages close below 1 monolayer, (ii) a homogeneous distribution of the Pt atoms over the surface, and (iii) the absence of a distinct long-range or short-range order in the surface layer. In addition, the density of specific adsorption ensembles is analyzed as a function of Pt surface content. Possible conclusions on the process for surface alloy formation are discussed. The results are compared with the properties of PtRu bulk alloys and the findings in previous adsorption studies on similar surface alloys (H. Rauscher, T. Hager, T. Diemant, H. Hoster, F. Bautier de Mongeot and R. J. Behm, Surf. Sci., 2007, 601, 4608; T. Diemant, H Rauscher and R. J. Behm, J. Phys. Chem. C, in press).     

Oxygen adsorption on Pt/Ru(0001) layers

Chemical properties of epitaxially grown bimetallic layers may deviate substantially from the behavior of their constituents. Strain in conjunction with electronic effects due to the nearby interface represent the dominant contribution to this modification. One of the simplest surface processes to characterize reactivity of these substrates is the dissociative adsorption of an incoming homo-nuclear diatomic molecule. In this study, the adsorption of O(2) on various epitaxially grown Pt films on Ru(0001) has been investigated using infrared absorption spectroscopy and thermal desorption spectroscopy. Pt/Ru(0001) has been chosen as a model system to analyze the individual influences of lateral strain and of the residual substrate interaction on the energetics of a dissociative adsorption system. It is found that adsorption and dissociative sticking depends dramatically on Pt film thickness. Even though oxygen adsorption proceeds in a straightforward manner on Pt(111) and Ru(0001), molecular chemisorption of oxygen on Pt/Ru(0001) is entirely suppressed for the Pt/Ru(0001) monolayer. For two Pt layers chemisorbed molecular oxygen on Pt terraces is produced, albeit at a very slow rate; however, no (thermally induced) dissociation occurs. Only for Pt layer thicknesses N(Pt) ≥ 3 sticking gradually speeds up and annealing leads to dissociation of O(2), thereby approaching the behavior for oxygen adsorption on genuine Pt(111). For Pt monolayer films a novel state of chemisorbed O(2), most likely located at step edges of Pt monolayer islands is identified. This state is readily populated which precludes an activation barrier towards adsorption, in contrast to adsorption on terrace sites of the Pt/Ru(0001) monolayer.     

Highly proton-ordered water structures on oxygen precovered Ru{0001}

We present helium scattering measurements of a water ad-layer grown on a O(2 × 1)/Ru(0001) surface. The adsorbed water layer results in a well ordered helium diffraction pattern with systematic extinctions of diffraction spots due to glide line symmetries. The data reflects a well-defined surface structure that maintains proton order even at surprisingly high temperatures of 140 K. The diffraction data we measure is consistent with a structure recently derived from STM measurements performed at 6 K. Comparison with recent DFT calculation is in partial agreement, suggesting that these calculations might be underestimating the contribution of relative water molecule orientations to the binding energy.     

Calculations of scattering of N2 molecules from Ru(0001)

Recent experimental measurements of state resolved scattering of nitrogen molecules from a Ru(0001) surface are discussed in comparison with a mixed quantum-classical theory that has given reasonable explanations for similar data on other systems. Acceptable agreement between data and calculations is obtained, but only upon assuming an effective mass of the surface equal to 2.3 times the mass of a single Ru atom.     

Self-assembly of a hexagonal boron nitride nanomesh on Ru(0001)

Hexagonal boron nitride (h-BN) nanostructures were grown on Ru(0001), and are very similar to those previously reported on Rh(111). They show a highly regular 12 x 12 superstructure, comprising 2 nm wide apertures with a depth of about 0.1 nm. Valence band photoemission reveals two distinctly bonded h-BN species, and X-ray photoelectron spectroscopy indicates an h-BN monolayer film. The functionality of the h-BN/Ru(0001) nanomesh is demonstrated by using this structure for the assembly of gold nanoclusters.     

Adsorption and thermal evolution of SO2 on Ru(0001)

Using high resolution S 2p and O 1s x-ray photoelectron spectroscopies, the adsorption of SO2 and its surface bound reaction products on Ru(0001) have been investigated simultaneously while dosing SO2 and while heating the adsorbed species. SO2 is found to adsorb on Ru(0001) at 100 K molecularly in two variants as well as dissociatively and to react to SO3, SO4, SO, and S with increasing coverage. After the monolayer has been saturated, SO2 adsorbs molecularly in multilayers. When heating adsorbed SO2 from 100 K, SO, SO2, and SO4 decompose in a wide temperature range up to 305 K. In contrast SO3 is found to be stable bound to Ru(0001) up to 300 K and to disappear from the surface to below 325 K. At 550 K the surface remains with a saturated atomic sulfur and oxygen layer and some sulfur species in a second layer. Our quantitative analysis of the sulfur amount bound to the surface supports a simple desorption process only for SO4. All other species mainly or partly decompose on the surface.     

Structure of Oxygen and CO Adsorption on Ru(0001) Electrode

It is important to understand the chemisorption of oxygen and CO on Ru(0001) surface. CO oxidation at oxygen precovered Ru(0001) surface at low oxygen coverages gave an extremely low CO oxidation rate, and it was also observed that, with a nominal oxygen coverage exceeding ca. 3 mL, rather high CO/CO2 conversion probabilities were achieved1. In the case of coadsorption of CO and oxygen on Ru(0001) surface under UHV conditions, a model comprising two CO molecules in an (22)-O unit cel…     

CO Blocking of D2 Dissociative Adsorption on Ru(0001)

The influence of pre‐adsorbed CO on the dissociative adsorption of D₂ on Ru(0001) is studied by molecular‐beam techniques. We determine the initial dissociation probability of D₂ as a function of its kinetic energy for various CO pre‐coverages between 0.00 and 0.67 monolayers (ML) at a surface temperature of 180 K. The results indicate that CO blocks D₂ dissociation and perturbs the local surface reactivity up to the nearest‐neighbour Ru atoms. Non‐activated sticking and dissociation become less important with increasing CO coverage, and vanish at θ_CO≈0.33 ML. In addition, at high D₂ kinetic energy (>35 kJ mol^?1) the site‐blocking capability of CO decreases rapidly. These observations are attributed to a CO‐induced activation barrier for D₂ dissociation in the vicinity of CO molecules.     

Six-dimensional potential energy surface for H2 at Ru(0001)

The six-dimensional (6D) potential energy surface (PES) for the H(2) molecule interacting with a clean Ru(0001) surface has been computed accurately for the first time. Density functional theory (DFT) and a pseudopotential based periodic plane-wave approach have been used to calculate the electronic interactions between the molecule and the surface. Two different generalized gradient approximation (GGA) exchange-correlation functionals, PW91 and RPBE, have been adopted. Based on the DFT/GGA calculated potential energies, an analytical 6D PES has been constructed using the corrugation reducing procedure. A very accurate representation of the DFT/GGA data has been achieved, with an average error in the interpolation of about 3 meV and a maximum error not larger than about 30 meV. The top site is found to be the most reactive site for both functionals used, but PW91 predicts a higher reactivity than RPBE, with lower-energy and earlier-located dissociation barriers. The energetic corrugation displayed by the RPBE PES is larger than the PW91 PES while the geometric corrugation is smaller. The differences between the two PESs increase as the distance of the molecular center of mass to the surface decreases. A direct comparison with experimental investigations on H(2)/Ru(0001) could shed light on the suitability of these XC potentials often used in DFT calculations.     

Surface Defects Activating New Reaction Paths: Formation of Formate during Methanol Oxidation on Ru(0001)

Methanol was co‐adsorbed with oxygen on Ru(0001) under conditions approaching those of real catalysts: at room temperature and at relatively high pressures and exposures, together with a comparative analysis of flat and defective surfaces. To clarify reaction routes, parallel exposures to formaldehyde and oxygen have also been analyzed. It is found that for both mixtures of gases, a new reaction path is activated on defective surfaces, in which methanol is oxidized to formate. Furthermore, at variance with pure methanol adsorption, apart from CO, various intermediates are observed in both flat and defective surfaces. On flat surfaces, formaldehyde and formyl are recognized whereas on defective ones methoxy and formate are detected. A model involving steering effects is presented, which accounts for the activity of surface defects towards the synthesis of formate.