Jesper Nerlov

A photoemission study of the coadsorption of CO2 and Na on TiO2(110)-(1 × 1) and -(1 × 2) surfaces: adsorption geometry and reactivity

The coadsorption of CO2 and Na on TiO2(110)-(1 × 1) and -(1 × 2) surfaces have been investigated by synchrotron-radiation based core-level and valence band photoemission. We find that the initially adsorbed Na exhibits a core-level shift of 1.15 eV when the two surfaces are compared. From a simple adsorption model this binding energy shift is understood in terms of a difference in initial Na adsorption site on these surfaces. While the (1 × 1) surface seems to favor Na adsorption in a hollow site “between” bridging surface oxygen atoms, it is found that the (1 × 2) surface facilitates a chemically more advantageous Na adsorption “adjacent to” the bridging oxygen atoms. Valence band measurements support this model since Na adsorption on the (1 × 2) surface leads to emission characteristic of alkali-oxygen-like compounds while this is not the case for the NaTiO2(110)-(1 × 1) system. Finally, the relatively high resolution of the core-level emission allows in a direct way the various features contributing to the Na 2p core-level emission to be determined. With respect to adsorption of CO2 we find for the (1 × 2) surface that CO2 uptake saturates around 0.5 ML Na coverage compared to 1 ML for the (1 × 1) surface, indicating that the Na coverage required for saturation of CO2 uptake is proportional to the density of protruded oxygen rows present at the surface. The CO2 uptake, however, increases as the density of the oxygen rows decreases. Valence band photoemission data obtained from both interfaces show that a surface carbonate species is formed. At lower coverages/exposures there are, however, indications of the presence of a CO−2 species rather than carbonate, thereby suggesting that the carbonate species is formed through the surface reaction: 2CO−2→CO2−3 + CO.

Resonant photoemission from TiO2(110) surfaces: implications on surface bonding and hybridization

Valence band photoemission obtained near the Ti 3p absorption edge has been used to investigate the surface electronic structure of TiO2(110)-(1 × 1), −(1 × 2) and Na/TiO2(110). The variation of the energy distribution curves with photon energy suggests that the observed resonant emission should be divided into two parts. A low-energy part is attributed to Ti 3p → 3d initial exitations, while a high-energy part is assigned to Ti 3p → 4s exitations. We find that both the intensity and the shape of the resonance profile as well as the spectral shape of the resonant emission vary substantially with surface conditions, the largest changes being associated with Ti 4s initial excitations. By applying a molecular orbital bonding scheme developed for rutile compounds and by considering the surface-induced perturbations to this, we are able to relate our experimental findings to recent structural models for the (1 × 1) and (1 × 2) surfaces. Particularly for the (1 × 1) surface the existence of bonding Ti 4s-O 2p hybridization observed at EB = 5.6 and EB = 6.7 eV is recognized for the first time. The observed restriction of this resonant emission to the clean (1 × 1) surface is readily associated with the contraction of Ti(5-fold)O bond lengths expected for this surface. For the (1 × 2) surface we observe a strong damping of the 4s-related bonding resonances, a decrease in 3d-bonding emission and an increase of the 3d-“non-bonding” emission. These observations are in accordance with a recent structural model for this surface. Finally, the resonant-emission data demonstrate that the surface Ti present at the (1 × 2) surface should be considered as Ti4+ states.

Ultrathin films of Cu on ZnO(112̄0): growth and electronic structure

Abstract The growth of Cu on ZnO(1120) and the related electronic-structure changes were investigated for deposits until 4 nm thickness. The growth at 300 K follows the monolayer-simultaneous-multilayer mode with a premonolayer break at 0.6 ML coverage. A surface-related plasmon excitation was observed by EELS at 6.7 eV. Core-level synchrotron-based photo-emission demonstrates a monotonic decrease in binding energy with increasing deposition thickness, approaching the bulk Cu value. Initial- and final-state considerations suggest that the shift is due to final-state effects, i.e. that Cu is present in the metallic state. By annealing 0.6 nm-deposited surfaces to 875 K, however, an observed shift in the initial 5.7 eV plasmon-energy with d Cu corresponding to a 0.28 e charge transfer, and a missing dangling bond of O, suggest bonding of the small Cu islands to substrate O, leading to fully coordinated O atoms and Cu(I).

The interaction of CO2 with potassium-promoted Cu(100): adsorption, reactions and radiation induced dissociation of CO2

Abstract The coadsorption of CO 2 and K on Cu(110) has been studied by the methods of photoelectron spectroscopy using synchrotron radiation, high resolution electron energy loss spectroscopy (HREELS) and work function measurements (ΔΦ). Both weakly adsorbed molecular carbon dioxide, chemisorbed carbonate and carbon monoxide are found at 107 K. It was found that the presence of free copper sites is necessary for the adsorption of CO and the large amount of physisorbed CO 2 in comparison with the copper surface completely covered with potassium. A coherent picture of the adsorption and reactions of CO 2 with a potassium modified Cu(110) surface is obtained by combination of the above mentioned spectroscopies with earlier thermally programmed desorption measurements and binding energy determinations of C 1s and O 1s. Irradiation with 125 eV photons causes a transformation of physisorbed CO 2 to chemisorbed CO and O. The kinetics of this photoinduced process was followed via the intensity variations of the O 1s characteristic of physisorbed and reacted CO 2 . The cross-section for the reaction was determined to 3 × 10 7 barn at a photon energy of 125 eV. Both the large cross-section and the photon energy dependence indicate that photon generated secondary electrons play a dominant role in the transformation process.

Photoemission and high resolution electron energy loss spectroscopy study of CO/K/Cu(110)

Adsorption of CO on the Cu(110) surface modified with a low (near the work function minimum) potassium precoverage has been investigated by use of low energy electron diffraction, photoelectron spectroscopy based on synchrotron radiation, work function measurements (ΔΦ), and high resolution electron energy loss spectroscopy (HREELS). During potassium adsorption at 118 K the binding energy (BE) of the K 3p peak shifts −0.3 eV. For a thick potassium overlayer both a surface and a bulk K 3p peak, with a 0.9 eV difference in BE, are observed. The work function results at ΘK∼0.5 ML show after saturation with CO an increase of 1.11 eV. An electrostatic interaction energy between CO and K equal to 0.23 eV is determined. This energy indicates a moderate attractive interaction between CO and K. A CO overlayer on K/Cu(110) shows the 4σ satellite, the 4σ orbital, and the 5σ/1π joint feature. No observable change in the BE separation between the 4σ and the 5σ/1π band (3.1 eV) was found, thereby indicating a negligibl...

Synthesis and characterization of mesoporous ZSM-5 core-shell particles for improved catalytic properties

Abstract HZSM-5 is a unique catalyst for the conversion of methanol, dimethyl ether and other oxygenates into gasoline. During this process, catalyst deactivation by coking requires frequent regeneration and the improvement of catalyst life time is one of the challenges in catalyst development. In this study, a series of mesoporous samples consisting of a ZSM-5 core and a silicalite shell have been synthesized and characterized by XRD, N2-sorption, IR spectroscopy and electron microscopy techniques. Additionally, desilicated conventional and mesoporous ZSM-5-type samples were investigated. All samples were tested in the MTG reaction, and the results showed that both the shell-coated and the desilicated zeolites are significantly more resistant to coke formation. These results are ascribed to the effect of the removal of structural defects rather than to an improvement of the diffusion properties due to the formation of mesopores.

Synthesis of formate on K-modified Cu(110) based on coadsorption of H and CO2

The reactions between CO2 and H on a Cu(110) surface modified with K in the submonolayer region are studied with photoelectron spectroscopy and high resolution electron energy loss spectroscopy. It is demonstrated that formate can be synthesized by admitting CO2 to a KCu(110) interface with preadsorbed atomic hydrogen. The adsorption sequence of CO2 and H is decisive for the formate yield.

Adsorption of H2S on InP(001) studied by photoemission spectroscopy

Abstract Adsorption of H 2 S on InP(001) at 140 K has been studied by valence-band and core-level synchrotron-radiation-induced photoemission spectroscopy (SRPES). Peaks were found in the SRPES spectra after the H 2 S exposure at E F -6.7, -9.4, and -12.4 eV upon adsorption of H 2 S. They correspond to the 2b 1 , 5a 1 , and 2b 1 orbitals of H 2 S gas, considering the intensity ratios and energy separations. This indicates that H 2 S adsorbs molecularly on the surface at 140 K. We find two kinds of chemical state for sulfur: S2p at 162.0 and 160.8 eV for H 2 S strong interaction with surface In atoms and physisorption, respectively. This is consistent with the changes in S2p and In4d spectra upon adsorption of H 2 S. Phosphorus atoms on the surface are scarcely interacting with H 2 O.

Change in adsorption bond length with coverage for KAl(111)

Abstract The local bond geometry of K adsorbed on Al(111) at low temperature has been studied by photoelectron diffraction (PED) as a function of K coverage. It is found that K atoms occupy on-top sites in the coverage range 0.05-0.4 monolayer and that the KAl bond length increases by 0.17 A over this coverage range. The reliability of this result is supported by PED studies of the (√3 × √3)R30° structures formed by adsorption of one-third monolayer Na and K at 300 K, and K at 150 K, which give results in quantitative agreement with previous structure determinations by SEXAFS and LEED.