One-dimensional confinement in heterogeneous catalysis: Trapped oxygen on RuO2(110) model catalysts

With temperature programmed reaction (TPR) experiments and kinetic Monte Carlo (kMC) simulations of coadsorbed oxygen and HCl on the RuO₂(110) surface we studied the thermal stabilization of dissociatively adsorbed oxygen. Due to one-dimensional confinement single surface O atoms can be trapped by surface chlorine atoms so that surface oxygen is not able to desorb from the RuO₂(110) surface at the expected temperature of 420 K. Trapped oxygen needs desorption temperatures as high as 700 K where it recombines with bridging O from RuO₂(110) to form O₂. Kinetic modeling of catalytic reactions with dimensional confinement of their reaction intermediates on the catalyst's surface requires the application of kinetic Monte Carlo simulations which are beyond the mean field approach.     

One-dimensional spin-polarized quantum-wire states in Au on Ni(110)

Au chain structures have been prepared on Ni(110). Au6 s,p-derived features in photoemission spectra are identified as quantum-wire states due to their strong dispersion along the chains and absence of dispersion perpendicular to the chains in agreement with our ab initio calculation of the electronic structure. Spin analysis reveals that the states have minority-spin character showing that the confinement of electrons in the chain structure depends on the electron spin.     

Tetraoxygen on reduced TiO2(110): oxygen adsorption and reactions with bridging oxygen vacancies

Oxygen adsorption on reduced TiO2(110) is investigated using temperature programmed desorption and electron-stimulated desorption. At low temperatures, 2 O(2) molecules can be chemisorbed in each oxygen vacancy. These molecules do not desorb upon annealing to 700 K. Instead, for 200 K相似文献   

Imaging intrinsic diffusion of bridge-bonded oxygen vacancies on TiO2(110)

We report the first measurements and calculations of the intrinsic mobility of bridge-bonded oxygen (BBO) vacancies on a rutile TiO2(110). The sequences of isothermal (340-420 K) scanning tunneling microscope images show that BBO vacancies migrate along BBO rows. The hopping rate increases exponentially with increasing temperature with an experimental activation energy of 1.15 eV. Density functional theory calculations are in very good agreement giving an energy barrier for hopping of 1.03 eV. Both theory and experiment indicate repulsive interactions between vacancies on a given BBO row.     

Adsorption of oxygen on W(110): I. The p(2 × 1) structure

The adsorption of oxygen on W(110) for ccverages below 0.5 monolayer has been studied with a number of complementary techniques. Data on adsorption kinetics, LEED intensities, work function changes and desorption kinetics have been used to propose a model combining statistical adsorption and island growth for the formation of the p(2 × 1) structure. On the basis of the measurements it is concluded that the surface is reconstructed for θ < 0.3 monolayer after heating to T > 2000 K, and for θ < 0.1 monolayer for adsorption at 300 K.     

Bonding of gold nanoclusters to oxygen vacancies on rutile TiO2(110)

Through an interplay between scanning tunneling microscopy (STM) and density functional theory (DFT) calculations, we show that bridging oxygen vacancies are the active nucleation sites for Au clusters on the rutile TiO2(110) surface. We find that a direct correlation exists between a decrease in density of vacancies and the amount of Au deposited. From the DFT calculations we find that the oxygen vacancy is indeed the strongest Au binding site. We show both experimentally and theoretically that a single oxygen vacancy can bind 3 Au atoms on average. In view of the presented results, a new growth model for the TiO2(110) system involving vacancy-cluster complex diffusion is presented.     

Interaction of cesium and oxygen on W(110): I. Cesium adsorption on oxygenated and oxidized W(110)

Cesium adsorption on oxygenated and oxidized W(110) is studied by Auger electron spectroscopy, LEED, thermal desorption and work function measurements. For oxygen coverages up to 1.5 × 10¹⁵ cm^?2 (oxygenated surface), preadsorbed oxygen lowers the cesiated work function minimum, the lowest (～1 eV) being obtained on a two-dimensional oxide structure with 1.4 × 10¹⁵ oxygen atoms per cm². Thermal desorption spectra of neutral cesium show that the oxygen adlayer increases the cesium desorption energy in the limit of small cesium coverages, by the same amount as it increases the substrate work function. Cesium adsorption destroys the p(2 × 1) and p(2 × 2) oxygen structures, but the 2D-oxide structure is left nearly unchanged. Beyond 1.5 × 10¹⁵ cm^?2 (oxidized surface), the work function minimum rises very rapidly with the oxygen coverage, as tungsten oxides begin to form. On bulk tungsten oxide layers, cesium appears to diffuse into the oxide, possibly forming a cesium tungsten bronze, characterized by a new desorption state. The thermal stability of the 2D-oxide structure on W(110) and the facetting of less dense tungsten planes suggest a way to achieve stable low work functions of interest in thermionic energy conversion applications.     

The structure of the c(2 × 2) oxygen overlayer on the unreconstructed (110) surface of iridium

An Ir(110)-c(2 × 2)O structure has been prepared by adsorbing a half-monolayer of oxygen at room temperature on an unreconstructed (1 × 1)Ir surface stabilized by a quarter-monolayer of randomly adsorbed oxygen. Results of the low energy electron diffraction structural analysis indicate that the ordered oxygen atoms are residing on the short-bridged sites on the (110) surface. The Ir-O interlayer spacing is 1.37 ± 0.05 Å, and the bond length is 1.93 ± 0.07 Å. The topmost substrate interlayer spacing is found to be 1.33 ± 0.07 Å rather than 1.26 ± 0.07 Å which is the topmost interlayer spacing of the unreconstructed (1× 1)Ir surface.     

Effect of surface nanostructure on temperature programmed reaction spectroscopy: first-principles kinetic monte Carlo simulations of CO oxidation at RuO2(110)

Using the catalytic CO oxidation at RuO2(110) as a showcase, we employ first-principles kinetic Monte Carlo simulations to illustrate the intricate effects on temperature programmed reaction spectroscopy data brought about by the mere correlations between the locations of the active sites at a nanostructured surface. Even in the absence of lateral interactions, this nanostructure alone can cause inhomogeneities that cannot be grasped by prevalent mean-field data analysis procedures, which thus lead to wrong conclusions on the reactivity of the different surface species.     

Anomalous spin-density distribution on oxygen and Ru in Ca(1.5)Sr(0.5)RuO(4): polarized neutron diffraction study

By means of polarized neutron diffraction in a magnetic field of 7.0 T at 1.6 K an anomalously large magnetization density is observed on the in-plane oxygen in Ca(1.5)Sr(0.5)RuO(4). Field-induced moments of different ions are determined by refinement on the flipping ratios, yielding micro(Ru)=0.346(11)micro(B), micro(O1)=0.076(6)micro(B), and micro(O2)=0.009(6)micro(B). The moment on the oxygen arises from the strong hybridization between the Ru-4d and O-2p orbitals. The maximum entropy magnetization density reconstruction reveals a strongly anisotropic density at the Ru site, consistent with the distribution of the xy (t(2g) band) d orbitals.     

Electronic excitations at oxygen deficient TiO2(110) surfaces: A study by EELS

TiO₂(110) surfaces with controlled oxygen deficiency introduced by 160 eV electron bombardment have been studied by XPS and EELS. Stoichiometry was monitored by the growth of core peaks due to Ti³⁺ states in XPS. Oxygen desorption is characterised by an initial cross section of 3 × 10⁻²¹ cm² that decreases with increasing oxygen loss, tending toward a limiting composition Ti₄O₇. The oxygen deficient surfaces display sub-bandgap excitations in electronic EELS, whilst in the vibrational region there is a selective downward shift and attenuation of the highest energy phonon loss. This is attributed to modification of the effective background dielectric constant by the defect excitations. Quantitative consideration of the changes in HREELS leads to an estimate of 0.1 for the oscillator strength of the defect electronic excitations. The high value supports the idea that electrons at oxygen deficient TiO₂ surfaces occupy states that are pulled down below the conduction band by polaronic self trapping.     

Angle-resolved photoemission of the c(2 × 2) and c(3 × 1) oxygen overlayers on Fe(110)

Angle-resolved photoemission spectroscopy utilizing synchrotron radiation has been used to study the band structure of the c(2×2) and (3×1) oxygen overlayers on Fe(110). The symmetries of the O-2p-derived states at the center of the surface Brillouin zone $\text{(}\text{Γ}\text{)}$ were identified using polarized light. At $\text{Γ}$ the p_xp_y- and p_z-derived levels are at about 5.5 and 7.0 eV below the Fermi level, respectively, for both ordered overlayers. The p-states of the c(2×2)-O structure show very little dispersion (?0.1 eV) with $\text{k}{}_{\mathrm{}}$. On the other hand, the c(3×1)-O overlayer exhibits considerable dispersion of ～1.6 eV. The essential features of the measured dispersion are reproduced well by the dispersion predicted in a qualitative way for an isolated c(3×1) oxygen monolayer.     

Experimental verification of a zero order desorption model: D2 on Ni(110)

The rate of desorption of deuterium from the low temperature (α) state on Ni(110) follows nearly zero order kinetics. Simultaneous measurements of intensities and profiles of half-order diffraction beams and changes in work function during isothermal desorptionof D₂, have shown that the zero order region is associated with the presence of islands of the (1 × 2) phase which act as reservoirs of deuterium. These results are discussed in terms of a recent model for zero order desorption which requires the presence of 3 deuterium surface phases in equilibrium. The activation energy for desorption of the α-state is 67 ± 6 kJ mol⁻¹.     

The reaction of H2S with surface oxygen on Ni(110)

The reactions of H₂S with predosed surface oxygen on Ni(110) surfaces were studied for a variety of coverage conditions. The primary reaction product is H₂O, but the details of the water formation and desorption depends on the coverage of both O and H₂S.

For high coverages of oxygen (p(2 × 1)−O; 0.5 ML), the reaction to form water is quantitative. The loss of oxygen from the surface (as measured by AES) is equal to the increase in sulfur coverage. XPS and HREELS measurements indicate the presence of chemisorbed H₂O immediately following large exposures of H₂S on the oxygen predosed surface at 110 K. Deuterium incorporation results suggest that the primary mechanism for these coverage conditions involves direct transfer of hydrogen from SH or H₂S moieties to the oxygen.

A second mechanism involving reaction of surface hydroxyl groups with surface hydrogen was also identified. This mechanism is particularly important for high coverages of oxygen (0.5 ML) and low coverages of H₂S (0.15 ML), where water desorption was observed at 235 K, but was not observed spectroscopically at 110 K. The sequential addition of two surface hydrogen atoms to surface oxygen is not an important mechanism in this system.

These reactions were modeled using a bond-order conservation method, and the model successfully reproduced the important mechanistic conclusions.     

Interplay between external strain and oxygen vacancies on a rutile TiO2(110) Surface

Comprehensive first-principle calculations on strained rutile TiO2(110) indicate that the formation energy of different types of oxygen vacancies depends on the external strain. For the unstrained state, the energetically favorable oxygen vacancy (EFOV) appears on the bridging site of the first layer; when 3% tensile strain along [11[over ]0] is applied, EFOV moves to the in-plane site, while 2% compressive strain along either [001] or [11[over ]0] shifts EFOV to the subbridging site. We therefore suggest that the distribution of oxygen vacancies can be engineered by external strain, which may help to improve the applications of a TiO2 surface where oxygen vacancy plays an important role.     

Oxygen chemisorption on W(110): A displacement reconstruction model

The system W(110) p(2 × 1)-O is studied. A bond stretching and bending model which predicts that the ordering of the adsorbed oxygen reconstructs the W(110) surface, is developed. The magnitude of the lateral displacement is 0.076 Å. The implications of the model in the formation of the (2 × 1) overlayer structure are also examined.     

Thermal desorption of molecular oxygen from SnO2 (110) surface: Insights from first-principles calculations

First-principles density functional theory calculations in the generalized gradient approximation, with plane wave basis set and pseudopotentials, have been used to investigate the desorption pathways of molecular oxygen species adsorbed on the SnO₂ (110) surface. Energetics of the thermodynamically favored precursors is studied in dependence on the surface charge provided either by surface defects or by donor type impurities from the near-surface region. The resonant desorption modes of O₂ molecules are examined in the framework of ab initio atomic thermodynamics and relationship of these results to experimental observations is discussed.