Transient diffusion and cluster formation of water molecules on Rh(111) at 20 K

The authors investigated the initial stage of water adsorption on Rh(111) at 20 K, using infrared reflection absorption spectroscopy. In this low coverage region, isolated water molecules and small water clusters are observed. Since thermal diffusion is suppressed at 20 K, the formation of water clusters at low coverage is controlled by both coverage and transient diffusion on the surface. Within a simple random walk model of the transient diffusion and clustering process, the authors estimate the mean lateral displacement from the first impact point to the final adsorption site to be 7.6 A; an incoming water molecule on Rh(111) is trapped with eight postcollision hops on the average.     

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.     

Dewetting growth of crystalline water ice on a hydrogen saturated Rh(111) surface at 135 K

We investigated the water (D(2)O) adsorption at 135?K on a hydrogen pre-adsorbed Rh(111) surface using temperature programmed desorption and infrared reflection absorption spectroscopy (IRAS) in ultrahigh vacuum. With increasing the hydrogen coverage, the desorption temperature of water decreases. At the saturation coverage of hydrogen, dewetting growth of water ice was observed: large three-dimensional ice grains are formed. The activation energy of water desorption from the hydrogen-saturated Rh(111) surface is estimated to be 51 kJ/mol. The initial sticking probability of water decreases from 0.46 on the clean surface to 0.35 on the hydrogen-saturated surface. In IRAS measurements, D-down species were not observed on the hydrogen saturated surface. The present experimental results clearly show that a hydrophilic Rh(111) clean surface changes into a hydrophobic surface as a result of hydrogen adsorption.     

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.     

A reflection absorption infrared spectroscopy and density-functional theory investigation of methanol dehydrogenation on Rh(111)V alloy surfaces

The dehydrogenation reaction of methanol on a Rh(111) surface, a Rh(111)V subsurface alloy, and on a Rh(111)V islands surface has been studied by thermal-desorption spectroscopy, reflection absorption infrared spectroscopy, and density-functional theory calculations. The full monolayer of methanol forms a structure with a special geometry with methanol rows, where two neighboring molecules have different oxygen-rhodium distances. They are close enough to form a H-bonded bilayer structure, with such a configuration, where every second methanol C-O bond is perpendicular to the surface on both Rh(111) and on the Rh(111)V subsurface alloy. The Rh(111)V subsurface alloy is slightly more reactive than the Rh(111) surface which is due to the changes in the electronic structure of the surface leading to slightly different methanol species on the surface. The Rh(111)V islands surface is the most reactive surface which is due to a new reaction mechanism that involves a methanol species stabilized up to about 245 K, partial opening of the methanol C-O bond, and dissociation of the product carbon monoxide. The latter two reactions also lead to a deactivation of the Rh(111)V islands surface.     

Coverage-dependent Orientations of Dy@C82 Molecules on Au(111) Surface

The adsorption and molecular orientation of Dy@C82 isomer I on Au(111) has been investi-gated using ultrahigh-vacuum scanning tunneling microscopy at 80 K. At low coverages, the Dy@C82 molecules tend to grow along the step edges of Au(111), forming small clusters and molecular chains. Adsorption of Dy@C82 on the edges is dominated by the fullerene-substrate interaction and presents various molecular orientations. At higher coverages, the Dy@C82 is found to form ordered islands consisting of small domains of equally oriented molecules. The Dy@C82 molecules in the islands prefer the adsorption configurations with the major C2 axis being approximately parallel to the surface of the substrate. Three preferable orientations of the Dy@C82 molecules are found in a two-dimensional hexagonal close packed overlayer. These observations are attributed to the interplay of the fullerene-substrate interaction and dipole-dipole interaction between the metallofullerenes.     

The first layer of water on Rh(111): microscopic structure and desorption kinetics

The adsorption states and growth process of the first water (D2O) layer on Rh(111) were investigated using infrared reflection absorption spectroscopy, temperature programed desorption, and spot-profile-analysis low energy electron diffraction. Water molecules wet the Rh(111) surface intact. At the early stage of first layer growth, a (square root 3 x square root 3)R30 degrees commensurate water layer grows where "up" and "down" species coexist; the up and down species represent water molecules which have free OD, pointing to a vacuum and the substrate, respectively. The up domain was a flatter structure than an icelike bilayer. Water desorption from Rh(111) was a half-order process. The activation energy and the preexponential factor of desorption are estimated to be 60 kJ/mol and 4.8 x 10(16) ML(1/2)/s at submonolayer coverage, respectively. With an increase in water coverage, the flat up domain becomes a zigzag layer, like an ice bilayer. At the saturation coverage, the amount of down species is 1.3 times larger than that of the up species. In addition, the activation energy and the preexponential factor of desorption decrease to 51 kJ/mol and 1.3 x 10(14) ML(1/2)/s, respectively.     

Coverage-dependent sticking probability and desorption kinetics of water molecules on Rh(111)

The adsorption and desorption kinetics of water molecules on Rh(111) were investigated using temperature programed desorption (TPD). Water molecules on Rh(111) show coverage-dependent sticking probability; the initial sticking probability is estimated to be 0.46. In the desorption process, a dilute gaslike phase and two-dimensional islands of water coexist on the surface. Based on the model proposed by Kreuzer and Payne [Surf. Sci.200, L433 (1988)], the apparent fractional-order TPD spectra can be interpreted as first-order desorption from the coexistence of two phases on which the sticking probabilities are different. Based on this, the previous estimation of pre-exponential factors assuming half-order desorption [A. Beniya et al., J. Chem. Phys.125, 054717 (2006)] should be revised.     

Initial oxidation of the Rh(110) surface: ordered adsorption and surface oxide structures

The initial oxidation of the Rh(110) surface was studied by scanning tunneling microscopy, core level spectroscopy, and density functional theory. The experiments were carried out exposing the Rh(110) surface to molecular or atomic oxygen at temperatures in the 500-700 K range. In molecular oxygen ambient, the oxidation terminates at oxygen coverage close to a monolayer with the formation of alternating islands of the (10x2) one-dimensional surface oxide and (2x1)p2mg adsorption phases. The use of atomic oxygen facilitates further oxidation until a structure with a c(2x4) periodicity develops. The experimental and theoretical results reveal that the c(2x4) structure is a "surface oxide" very similar to the hexagonal O-Rh-O trilayer structures formed on the Rh(111) and Rh(100) substrates. Some of the experimentally found adsorption phases appear unstable in the phase diagram predicted by thermodynamics, which might reflect kinetic hindrance. The structural details, core level spectra, and stability of the surface oxides formed on the three basal planes are compared with those of the bulk RhO2 and Rh2O3.     

Co-adsorption of water and hydrogen on Ni(111)

We have studied the surface coverage dependence of the co-adsorption of D and D(2)O on the Ni(111) surface under UHV conditions. We use detailed temperature-programmed desorption studies and high resolution electron energy loss spectroscopy to show how pre-covering the surface with various amounts of D affects adsorption and desorption of D(2)O. Our results show that the effects of co-adsorption are strongly dependent on D-coverage. In the deuterium pre-coverage range of 0-0.3 ML, adsorption of deuterium leaves a fraction of the available surface area bare for D(2)O adsorption, which shows no significant changes compared to adsorption on the bare surface. Our data indicate phase segregation of hydrogen and water into islands. At low post-coverages, D(2)O forms a two-phase system on the remaining bare surface that shows zero-order desorption kinetics. This two phase system likely consists of a 2-D solid phase of extended islands of hexamer rings and a 2-D water gas phase. Increasing the water post-dose leads at first to 'freezing' of the 2-D gas and is followed by formation of ordered, multilayered water islands in-between the deuterium islands. For deuterium pre-coverages between 0.3 and 0.5 ML, our data may be interpreted that the water hexamer ring structure, (D(2)O)(6), required for the formation of an ordered multilayer, does not form anymore. Instead, more disordered linear and branched chains of water molecules grow in-between the extended, hydrophobic deuterium islands. These deuterium islands have a D-atom density in agreement with a (2x2)-2D structure. The disordered water structures adsorbed in-between form nucleation sites for growth of 3-D water structures. Loss of regular lateral hydrogen bonding and weakened interaction with the substrate reduces the binding energy of water significantly in this regime and results in lowering of the desorption temperature. At deuterium pre-coverages greater than 0.5 ML, the saturated (2x2)-2D structure mixes with (1x1)-1D patches. The mixed structures are also hydrophobic. On such surfaces, submonolayer doses of water lead to formation of 3-D water structures well before wetting the entire hydrogen-covered surface.     

CO oxidation on Pt-modified Rh(111) electrodes.

The CO electro-oxidation reaction was studied on platinum-modified Rh(111) electrodes in 0.5 M H2SO4 using cyclic voltammetry and chronoamperometry. The Pt-Rh(111) electrodes were generated during voltammetric cycles at 50 mV s(-1) in a 30 microM H2PtCl6 and 0.5 M H2SO4 solution. Surfaces generated by n deposition cycles were investigated (Ptn-Rh(111) with n=2, 4, 6, 8, 10, and 16). The blank cyclic voltammograms of these surfaces are characterized by a pronounced sharpening of the hydrogen/(bi)sulfate adsorption/desorption peaks, typical for Rh(111), and the appearance of contributions between 0.1 and 0.4 V, which were ascribed to hydrogen/(bi)sulfate adsorption/desorption on the deposited platinum. At higher potentials, the surface oxidation of Rh(111) is enhanced by the presence of platinum. The structure of the Pt-modified electrodes was investigated by STM imaging. At low Pt coverages (Pt2-Rh(111)), monoatomically high islands are formed, which grow three dimensionally as the number of deposition cycles increases. After eight cycles, the monolayer islands have grown in diameter and range from mono- to multiatomic height. At even higher Pt coverage (Pt16-Rh(111)), the islands grow to particles of approx. 10 nm in diameter, which are 5-6 atoms high. The CO stripping voltammetry on these surfaces is characterized by two peaks: A low-potential, structure-insensitive peak, ascribed to CO reacting at the platinum monolayer islands, whose onset is shifted 150, 250, and 100 mV negatively with respect to pure Rh(111), Pt(111), and polycrystalline Pt, respectively, indicating the enhanced CO electro-oxidation properties of the Pt overlayer system. A peak at higher potentials displays strong structure sensitivity (particle-size effect) and was ascribed to CO reacting on the islands of multiatomic height. Current-time transients recorded on the surface with the highest amount of monolayer islands (Pt4-Rh(111)) also indicate enhanced CO-oxidation kinetics. Comparison of the Pt4-Rh(111) current-time transients recorded at 0.635, 0.675, and 0.750 V versus RHE (reversible hydrogen electrode) with those of pure Rh(111) and Pt(111) shows greatly reduced reaction times. A Cottrellian decay at long times indicates surface-diffusion-limited CO oxidation on the bare Rh(111) surface, while the peak visible at short times is indicative of CO reacting at the monolayer platinum islands. The results presented here show that, as indicated by density functional theory (DFT) calculations, the CO-adlayer oxidation for this system is enhanced compared to both pure Rh and Pt.     

Balance of nanostructure and bimetallic interactions in Pt model fuel cell catalysts: in situ XAS and DFT study

We have studied the effect of nanostructuring in Pt monolayer model electrocatalysts on a Rh(111) single-crystal substrate on the adsorption strength of chemisorbed species. In situ high energy resolution fluorescence detection X-ray absorption spectroscopy at the Pt L(3) edge reveals characteristic changes of the shape and intensity of the "white-line" due to chemisorption of atomic hydrogen (H(ad)) at low potentials and oxygen-containing species (O/OH(ad)) at high potentials. On a uniform, two-dimensional Pt monolayer grown by Pt evaporation in ultrahigh vacuum, we observe a significant destabilization of both H(ad) and O/OH(ad) due to strain and ligand effects induced by the underlying Rh(111) substrate. When Pt is deposited via a wet-chemical route, by contrast, three-dimensional Pt islands are formed. In this case, strain and Rh ligand effects are balanced with higher local thickness of the Pt islands as well as higher defect density, shifting H and OH adsorption energies back toward pure Pt. Using density functional theory, we calculate O adsorption energies and corresponding local ORR activities for fcc 3-fold hollow sites with various local geometries that are present in the three-dimensional Pt islands.     

Infrared spectroscopy of physisorbed and chemisorbed O2 on Pt(111)

The adsorption of O2 on the Pt(111) surface, with particular emphasis on the influence of substrate temperature, has been studied by infrared reflection absorption spectroscopy (IRAS). In the temperature range 30-90 K the IRAS spectra reveal three different molecular adsorption states. A physisorbed state appears below 40 K while chemisorbed peroxo- and superoxo-like states are observed in the whole temperature range, the characteristic vibrational frequencies are at full coverages of 16O2, 1543 cm(-1) and around 700 cm(-1) and 870 cm(-1), respectively. Flash heating from 30 K to 45 K reveal that the physisorbed state acts as a precursor to the superoxo chemisorption. Theoretical calculations suggest that peroxo molecules may occupy both fcc and hcp threefold sites on the Pt(111) surface. However, within the high resolution of the IRAS measurements we only observe one peroxo state in the temperature range 45-90 K, assigned to occupy the fcc site. The peroxo adsorption probability is significantly lower at 45 K than at 90 K, presumably due to reduced thermal activation from the physisorbed precursor state. A longer lifetime in this precursor state at the low temperature results in formation of larger superoxo islands already at low oxygen coverage.     

Water adsorption, desorption, and clustering on FeO(111)

The adsorption of water on FeO(111) is investigated using temperature programmed desorption (TPD) and infrared reflection absorption spectroscopy (IRAS). Well-ordered 2 ML thick FeO(111) films are grown epitaxially on a Pt(111) substrate. Water adsorbs molecularly on FeO(111) and desorbs with a well resolved monolayer peak. IRAS measurements as a function of coverage are performed for water deposited at 30 and 135 K. For all coverages (0.2 ML and greater), the adsorbed water exhibits significant hydrogen bonding. Differences in IRAS spectra for water adsorbed at 30 and 135 K are subtle but suggest that water adsorbed at 135 K is well ordered. Monolayer nitrogen TPD spectra from water covered FeO(111) surfaces are used to investigate the clustering of the water as a function of deposition or annealing temperature. Temperature dependent water overlayer structures result from differences in water diffusion rates on bare FeO(111) and on water adsorbed on FeO(111). Features in the nitrogen TPD spectra allow the monolayer wetting and 2-dimensional (2D) ordering of water on FeO(111) to be followed. Voids in a partially disordered first water layer exist for water deposited below 120 K and ordered 2D islands are found when depositing water above 120 K.     

Selective Oxygen and Hydrogen Functionalization of the h-BN/Rh(111) Nanomesh

We present detailed studies on the covalent adsorption of molecular oxygen and atomic hydrogen on the hexagonal boron nitride (h-BN) nanomesh on Rh(111). The functionalization of this two-dimensional (2D) material was investigated under ultra-high vacuum conditions using synchrotron radiation-based in situ high-resolution X-ray photoelectron spectroscopy, temperature-programmed X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy. We are able to provide a deep insight into the adsorption behavior and thermal stability of oxygen and hydrogen on h-BN/Rh(111). Oxygen functionalization was achieved via a supersonic molecular beam while hydrogen functionalization was realized using an atomic hydrogen source. Adsorption of the respective species was observed to occur selectively in the pores of h-BN leading to spatially defined modification of the 2D layer. The adsorption of the observed molecular oxygen species was found to be an activated process that requires high-energy oxygen molecules. Upon heating to 700 K, oxygen functionalization was observed to be almost reversible except for small amounts of boron oxides evolving due to the reaction of oxygen with the 2D material. Hydrogen functionalization of h-BN/Rh(111) was fully reversed upon heating to about 640 K.     

Infrared spectroscopy of oxygen adsorbed on hydrogen covered Pt(111)

Using infrared reflection absorption spectroscopy we have investigated how preadsorbed hydrogen affects the adsorption of O(2) on the Pt(111) surface at temperatures below the onset of the water formation reaction. On the fully hydrogen covered surface, Theta(H)=1, O(2) physisorbs at temperatures below 45 K, the weakly dipole active internal stretch vibration is observed at 1548 cm(-1). Unlike on the clean Pt(111) surface, this adsorption state does not act as a precursor for O(2) chemisorption. The physisorbed molecules simply desorb above 45 K and no chemisorbed O(2) state is populated directly from the gas phase in the temperature range 45-90 K. When the surface is approximately half covered, Theta(H) approximately 0.4, with preadsorbed hydrogen, O(2) chemisorbs on the clean Pt(111) surface regions in the characteristic peroxolike and superoxolike states with vibration frequencies around 700 cm(-1) and 870 cm(-1). These values correspond to dense O(2) islands which develop already at low O(2) coverages. At this hydrogen coverage, we find that the initial sticking probability of chemisorbed O(2) is drastically reduced at 90 K and the general uptake also proceeds slowly when compared with observations for the clean surface. We suggest that this is due to a change in the behavior of the physisorbed O(2) precursor.     

Adsorption and thermal decomposition of 2-octylthieno[3,4-b]thiophene on Au(111)

The adsorption and thermal stability of 2-octylthieno[3,4-b]thiophene (OTTP) on the Au(111) surfaces have been studied using scanning tunneling microscopy (STM), temperature programmed desorption (TPD), and X-ray photoelectron spectroscopy (XPS). UHV-STM studies revealed that the vapor-deposited OTTP on Au(111) generated disordered adlayers with monolayer thickness even at saturation coverage. XPS and TPD studies indicated that OTTP molecules on Au(111) are stable up to 450K and further heating of the sample resulted in thermal decomposition to produce H(2) and H(2)S via C-S bond scission in the thieno-thiophene rings. Dehydrogenation continues to occur above 600K and the molecules were ultimately transformed to carbon clusters at 900K. Highly resolved air-STM images showed that OTTP adlayers on Au(111) prepared from solution are composed of a well-ordered and low-coverage phase where the molecules lie flat on the surface, which can be assigned as a (9×2√33)R5° structure. Finally, based on analysis of STM, TPD, and XPS results, we propose a thermal decomposition mechanism of OTTP on Au(111) as a function of annealing temperature.     

Growth of nanocrystalline MoO3 on Au(111) studied by in situ scanning tunneling microscopy

The growth of nanocrystalline MoO3 islands on Au(111) using physical vapor deposition of Mo has been studied by scanning tunneling microscopy and low energy electron diffraction. The growth conditions affect the shape and distribution of the MoO3 nanostructures, providing a means of preparing materials with different percentages of edge sites that may have different chemical and physical properties than atoms in the interior of the nanostructures. MoO3 islands were prepared by physical vapor deposition of Mo and subsequent oxidation by NO2 exposure at temperatures between 450 K and 600 K. They exhibit a crystalline structure with a c(4 x 2) periodicity relative to unreconstructed Au(111). While the atomic-scale structure is identical to that of MoO3 islands prepared by chemical vapor deposition, we demonstrate that the distribution of MoO3 islands on the Au(111) surface reflects the distribution of Mo clusters prior to oxidation although the growth of MoO3 involves long-range mass transport via volatile MoO3 precursor species. The island morphology is kinetically controlled at 450 K, whereas an equilibrium shape is approached at higher preparation temperatures or after prolonged annealing at the elevated temperature. Mo deposition at or above 525 K leads to the formation of a Mo-Au surface alloy as indicated by the observation of embedded MoO3 islands after oxidation by NO2. Au vacancy islands, formed when Mo and Au dealloy to produce vacancies, are observed for these growth conditions.     

Adsorption and reaction of methanol on clean and oxygen modified rhodium/vanadium surface alloys

The dehydrogenation of methanol on Rh(111), on a Rh(111)/V subsurface alloy and on Rh(111) with V islands has been studied with and without preadsorbed oxygen using a supersonic molecular beam and temperature programmed desorption. The reactivity is highest for the V islands surface without oxygen. But this surface is deactivated due to CO dissociation. The subsurface alloy is less reactive than the islands, but still more active than the Rh(111) surface. The reaction products are carbon monoxide and hydrogen only. With preadsorbed oxygen Rh(111) is the most active surface, but a strong dependence of the activity on the amount of preadsorbed oxygen is found for all three surfaces. The reaction products with preadsorbed oxygen are water, hydrogen, carbon dioxide and carbon monoxide. The reactions follow the same mechanism on all surfaces, but the activation energy of the individual reaction steps is different leading to significant changes in the thermal desorption spectra and in King and Wells-type experiments.     

吸附氢气后的钯团簇在氧化亚铜表面上的稳定性研究

为了探究吸附H2后的Pdn团簇在Cu2O（111）完整表面和铜缺陷表面上的稳定性,计算了负载在Cu2O（111）完整表面和铜缺陷表面上的Pdn（n=1-4）对H2分子的最稳定吸附结构;利用在给定H2压力和温度下Pdn / Cu2O表面吸附H2的相图揭示了Pdn团簇在Cu2O（111）两个表面的变化情况。结果表明,在吸附了H2分子以后,Pdn团簇更倾向于保持原有的结构,且随着Pd团簇的增大,吸附H2的数量也逐渐增长。