Krassimir L. Kostov

Observation of a novel high density 3O(2 × 2) structure on Ru(001)

Abstract High-resolution electron energy loss spectroscopy and scanning tunneling microscopy have been applied to identify and characterize a novel high density phase of oxygen adsorbed on Ru(001). Three oxygen atoms per (2 × 2) unit cell are arranged such that a (2 × 2) vacancy superstructure is formed. Atomically resolved STM images clearly establish the existence of such a phase. The ordered layer is characterized by a sharp (2 × 2) superstructure as seen by low energy electron diffraction. Two dipole active energy losses were found for the 3O(2 × 2) layer at 0.75 ML and interpreted as the in-phase oxygen to metal stretching vibration and the totally symmetric eigenmode consisting of frustrated translations of three next-neighbor oxygen atoms towards each other. The vibrational modes of this novel structure are compared to data for oxygen coverages in the range 0.5–1 ML.

Structural features of the NO/Ru(001) adsorption complexes: A linear combination of Gaussian‐type orbitals local density functional model cluster analysis of high‐resolution electron energy loss spectroscopy data

High‐resolution electron energy loss spectra (HREELS) of NO adsorbed at low temperature on the Ru(001) surface are reported with particular emphasis on the low coverage regime. The improved resolution compared to earlier studies allowed one to clearly separate the various vibrational bands and to establish correlations among them. The experimental data are analyzed with the help of linear combination of Gaussian‐type orbitals local density functional model cluster calculations. We conclude that the loss peak of the low coverage samples at 1130 cm−1 can be attributed to the stretching vibrations of upright oriented μ3‐bridge nitrosyl species which are bound in an unusual configuration, i.e., via the oxygen atom. The dominating feature at small coverages around 1400 cm−1, corresponding to the intramolecular vibration of NO moieties at the same site but bound via nitrogen, is calculated in agreement with experiment. Bent structures of the adsorbate are energetically less favored both for the μ3‐ON and μ3‐NO ...

Interaction of oxygen with a clean Ir(111) surface

HREELS and XPS methods were used for the investigation of the interaction of oxygen with Ir(111). At temperatures of 180 and 300 K oxygen is adsorbed dissociatively, 0.52±0.08 oxygen atoms per one Ir atom corresponding to a saturated coverage. Adsorbed oxygen atoms are characterized by a v (Ir-O) mode at 550 cm −1 . With rising temperature (above 300 K) diffusion of oxygen atoms into the subsurface Ir layer begins. At 600 K and an oxygen pressure of 1×10 −5 Pa, formation of a surface oxide begins which characterizes the initial stage of Ir(111) oxidation. A v (Ir-O) peak at about 800 cm −1 in the HREEL spectrum corresponds to the surface oxide. More intensive oxidation of Ir(111) proceeds at 850 K and an oxygen pressure of 0.1 MPa, the surface oxide formed differing in composition from IrO 2 .

Efficient mesoporous silica–titania catalysts from colloidal self-assembly

Mesoporous silica-titania materials of tunable composition and texture, which present a high catalytic activity in the mild oxidation of sulfur compounds, have been obtained by combining the spray-drying process with the colloidal self-assembly of α-chitin nanorods (biopolymer acting as a template) and organometallic oligomers.

The vibrational structure of benzene adsorbed on Si(001)

High resolution electron energy loss spectroscopy (HREELS) measurements and density functional model cluster calculations are presented to clarify the vibrational structure of the adsorption system C6H6/Si(001). All vibrational modes of the adsorption complex, which previously was identified to exhibit a cyclohexadiene-like structure, have been calculated and characterized according to the motion of the different atoms of the adsorption complex. Special emphasis is placed on the low-frequency modes. The coupling between the adsorbate and the substrate modes is analyzed with the help of a model that represents various limiting situations. Different coupling variants are found to apply to different collective modes of the adsorbate. The A1 and B1 modes can be described rather well by a model that only encompasses the adsorbate and the Si dimer underneath; for the A2 and B2 modes a frozen substrate description of the adsorption complex is more appropriate.

A new high density COoxygen coadsorbate layer on Pt(111) and its role in CO oxidation

A high density CO/oxygen coadsorbate layer with maximal composition 2CO + O can be prepared by exposing the O(2 x 2 )/Pt (111) layer to large CO doses (up to 10 5 L). It has been studied by HREELS, TPD and LEED. At 120 K all CO molecules are adsorbed in linear form, irrespective of the coverage, showing another case of noncorrelation of adsorption site and internal CO stretch frequency. Heating to 260 K, i.e. in the range of incipient CO oxidation, leads to partial conversion of CO into two higher coordinated states with v(C-O) at 1750 and 1850 cm -1 . While only one well known CO oxidation channel between 330 and 360 K is observed for CO coverages below 8(CO) = 0.25 ML, a new CO oxidation channel at 290-300 K opens up for higher Φ(CO).

Improved silica–titania catalysts by chitin biotemplating

Silica–titania materials with improved catalytic performance were elaborated as mesoporous microparticles by combining sol–gel and spray-drying processes with the self-assembly properties of α-chitin nanorods acting as biotemplates. Three different synthesis approaches are discussed, leading to materials with varied textural and chemical characteristics studied by SEM, N2 volumetry, TEM, XPS and DR-UV techniques. The use of water or ethanol as initial solvent for chitin nanorod suspensions, as well as the mixing conditions of the precursors, has been shown to have a significant impact on the final properties. Materials of specific surface areas of up to 590 m2 g−1 and porous volumes of up to 0.84 mL g−1, with low surface Si/Ti ratio, could be disclosed. Properties were further investigated by employing the silica–titania materials as heterogeneous catalysts for the sulfoxidation of bulky model compounds. The location of Ti active sites at the pore surface has been maximized and allows for improved productivity.

Vibrational characterization of ethylene adsorption and its thermal evolution on Si(001)-(2×1): Identification of majority and minority species

The vibrational and structural properties of a single-domain Si(001)-(2 x 1) surface upon ethylene adsorption have been studied by density functional cluster calculations and high-resolution electron energy loss spectroscopy. The detailed analysis of the theoretically and the experimentally determined vibrational frequencies reveals two coexisting adsorbate configurations. The majority species consist of ethylene molecules which are di-sigma bonded to the two Si atoms of a single Si-Si dimer. The local symmetry of this adsorption complex is reduced to C(2) for ethylene saturation coverage as determined by surface selection rules for the vibrational excitation process. The symmetry reduction includes the rotation of the C-C bond around the surface normal and the twist of the methylene groups around the C-C axis. Experimentally, 17 ethylene-derived modes are found and assigned for the majority and the minority species based on a comparison with calculated vibrational frequencies. The minority species which can account up to 14% of the total ethylene coverage is spectroscopically identified for the first time. It is assigned to ethylene molecules di-sigma bonded to two adjacent Si-Si dimers (in an end-bridge configuration). One part of the minority species desorbs molecularly at 665 K, about 50 K higher than the majority species, whereas the remaining part dissociates to adsorbed acetylene at temperatures around 630 K. For the latter, a di-sigma end-bridge like bonding configuration is proposed based on a comparison with vibrational data for adsorbed acetylene on Si(100)-(2 x 1).

Interaction of ethylene and acetylene with oxygen on An Ir(111) surface

Investigations have been carried out using HREELS and XPS to study the interaction of C 2 H 4 and C 2 H 2 with oxygen on Ir(111). In the presence of atomically adsorbed oxygen, ethylene is adsorbed on Ir(111) at 180 K molecularly and is bonded to the surface by a π-bond. At this temperature acetylene is adsorbed on an oxygen-covered Ir(111) surface in a similar way as on a clean surface. The oxygen coverage hinders the dehydrogenation of ethylene to a higher degree than in the case of acetylene. At 300≤ T ≤400 K, C 2 H 2 and C 2 H 4 react with adsorbed oxygen atoms forming CO, CO 2 and H 2 O. Decomposition products of C 2 H 2 and C 2 H 4 , which are observed on a clean surface, are also found on the oxygen-covered iridium surface. An oxidation mechanism of C 2 H 2 and C 2 H 4 via an intermediate product, CCH, is proposed. Above 450 K both hydrocarbons are completely oxidized to CO 2 and H 2 O. Atomically adsorbed oxygen is needed for the oxidation of the hydrocarbon species on Ir(111).

Two bonding configurations of acetylene on Si(001)-(2×1): A combined high-resolution electron energy loss spectroscopy and density functional theory study

Two coexisting adsorption states of molecularly adsorbed acetylene on the Si(001)-(2 x 1) surface have been identified by a combined study based on the high-resolution electron energy loss spectroscopy and density functional computations. Seven possible adsorbate-substrate structures are considered theoretically including their full vibrational analysis. Based on a significantly enhanced experimental resolution, the assignment of 15 C2H2- and C2D2-derived vibrational modes identifies a dominant di-sigma bonded molecule adsorbed on top of a single Si-Si dimer. Additionally there is clear evidence for a second minority species which is di-sigma bonded between two Si-Si dimers within the same dimer row (end-bridge geometry). The possible symmetries of the adsorbate complexes are discussed based on the specular and off-specular vibrational measurements. They suggest lower than ideal C(2v) and C(s) symmetries for on-top and end-bridge species, respectively. At low coverages the symmetry reductions might be lifted.