F.P. Netzer

CO adsorption on Pd(1 1 1): a high-resolution core level photoemission and electron energy loss spectroscopy study

By combining high-resolution X-ray photoelectron and electron energy loss spectroscopies a comprehensive analysis of the adsorption of CO on Pd(1 1 1) at 300 K has been performed. The characteristic fingerprints for various CO‐ Pd(1 1 1) bonding configurations have been identified from the decomposition analysis of the adsorbate C 1s and the substrate Pd 3d5=2 core-level photoemission spectra obtained after CO adsorption at 120 K. The cO4 2U structure at 0.5 monolayer (ML) and theO2 2U-3CO structure at 0.75 ML formed at low temperature have been used for calibration purposes. The core-level results are consistent with CO adsorbing in a mixture of fcc and hcp threefold hollow sites in the cO4 2U structure and of hollow and on-top sites in theO2 2U structure, as reported in the literature. For CO adsorption at 300 K, a diAerent site occupation is evidenced by the presence of two components in the C 1s and Pd 3d5=2 core-level and C‐O stretching vibration lineshapes. At coverages up to 0.1 ML only fcc threefold hollow sites in a O AAA

Growth and thermal properties of ultrathin cerium oxide layers on Rh(1 1 1)

Abstract Ultrathin layers of cerium oxide have been deposited on a Rh(1xa01xa01) surface and their growth morphology, structure, and thermal stability have been investigated by LEED, STM, XPS, and valence band resonant photoemission. STM and LEED indicate that the ceria grows epitaxially in form of ordered CeO 2 islands at elevated substrate temperature (250–300 °C), with (1xa01xa01) faces parallel and orientationally aligned to the main azimuthal directions of the substrate. The ultrathin ceria films contain significant amounts of reduced Ce 3+ species, which appear to be located predominantly at the ceria–Rh interface. For thicker films (>6 equivalent monolayers) stoichiometric CeO 2 is detected in XPS. Vacuum annealing produces morphologically well-defined hexagonal islands, accompanied by partial reduction and the formation of oxygen vacancies at the ceria surface. The thermal stability and the degree of reduction is a function of the oxide layer thickness, with thinner layers being thermally less stable. At temperatures >800 °C, the ceria decomposes and Ce–Rh alloy phases are identified.

The structure of the indium-Si(111) (7 × 3) monolayer surface

Abstract The structure of the ordered indium overlayer which forms at around one monolayer (ML) coverage at elevated temperature on Si(111)7 × 7 surfaces has been investigated by scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS) and low energy electron diffraction (LEED). The surface is characterised by a | 2 −1 1 2 | unit cell, designated in the following as ( 7 × 3 ), but two different local atomic configurations are observed to coexist at the same sample surface in atomically resolved STM images: a quasi-hexagonal arrangement with a local coverage of exactly 1 ML, and a quasi-rectangular arrangement with a local coverage of 1.2 ML. Both surface structures are metallic and reveal a very similar electronic structure in the STS spectra. The structures are discussed in terms of a close-packed In(001)-type overlayer and a straindashinduced distortion of a pseudomorphic overlayer.

Adsorption and reaction of CO on a ceria–Rh(1 1 1) “inverse model catalyst” surface

Abstract The adsorption of CO and the reaction of CO with preadsorbed oxygen at room temperature has been studied on the Rh(1xa01xa01)2xa0×xa01-O surface and on ceria–Rh(1xa01xa01) “inverse model catalyst” surfaces using C 1s and O 1s core level and valence band photoelectron spectroscopy with synchrotron radiation. The adsorption of CO on the oxygen-precovered (2xa0×xa01)-O surface was found to proceed slower than on the clean Rh(1xa01xa01) surface, because of a kinetic limitation which is introduced by the Oxa0+xa0CO clean-off reaction. The latter removes adsorbed oxygen as CO 2 at 300–320 K, which desorbs into the gas phase. On the ceria–Rh(1xa01xa01) surfaces the Oxa0+xa0CO oxidation reaction is much faster than on the Rh(1xa01xa01)2xa0×xa01-O surface suggesting a catalytically active role of the ceria–Rh interface. The XPS spectra indicate a preferential occupation of hollow-type CO adsorption sites on the ceria–Rh(1xa01xa01) surface, which might be located at the oxide–metal interface. No CO dissociation has been detected on the Rh(1xa01xa01) supported ceria inverse catalyst surfaces. On a CeRh 3 alloy surface, prepared by thermal decomposition of the ceria in ultrahigh vacuum, the hollow-type CO adsorption sites are energetically favoured suppressing the on-top sites almost completely, but the global adsorption energy is lower than on Rh(1xa01xa01) as indicated by the reduced CO saturation coverage.

Vibrationally resolved C Is photoemission from CO absorbed on Rh(1 1 1) : The investigation of a new chemically shifted C Is component

Abstract High-resolution core-level photoemission at an energy resolution better than the intrinsic width of the C 1s level has been used to study the CO/Rh(1xa01xa01) overlayer system. C 1s spectra have been measured in a large CO coverage range; from low coverages, where the CO molecules only adsorb in on-top positions, to saturation coverage, where CO molecules occupy both on-top and threefold hollow sites. Fine structure components due to vibrational excitation of the C–O stretch mode are clearly resolved in the emission peak from each site. The vibrational splittings and intensity ratios are found to be different for the different adsorption sites. A third C 1s component, which has not been resolved earlier, is found at intermediate CO coverages. Some possible explanations for the origin of this extra component are discussed. In the light of its binding energy and vibrational energy in photoemission, together with supporting evidence from high-resolution electron energy loss spectroscopy data and photoelectron diffraction, this is assigned to bridging CO species at intermediate coverages.

Adsorption of glycine on a NiAl(1 1 0) alloy surface

The adsorption and desorption of glycine (NH2CH2COOH), vacuum deposited on a NiAl(1 1 0) surface, were investigated by means of Auger electron spectroscopy (AES), low energy electron diffraction (LEED), temperature-programmed desorption, work function (Δφ) measurements, and ultraviolet photoelectron spectroscopy (UPS). At 120 K, glycine adsorbs molecularly forming mono- and multilayers predominantly in the zwitterionic state, as evidenced by the UPS results. In contrast, the adsorption at room temperature (310 K) is mainly dissociative in the early stages of exposure, while molecular adsorption occurs only near saturation coverage. There is evidence that this molecularly adsorbed species is in the anionic form (NH2CH2COO−). Analysis of AES data reveals that upon adsorption glycine attacks the aluminium sites on the surface. On heating part of the monolayer adsorbed at 120 K is converted to the anionic form and at higher temperatures dissociates further before desorption. The temperature-induced dissociation of glycine (<400 K) leads to a series of similar reaction products irrespective of the initial adsorption step at 120 K or at 310 K, leaving finally oxygen, carbon and nitrogen at the surface. AES and LEED measurements indicate that oxygen interacts strongly with the Al component of the surface forming an “oxide”-like Al–O layer.

The growth of indium on the H-terminated Si(111)1 × 1 surface

Abstract The growth morphology and the chemical interactions of thin In layers on an H-terminated Si(111)1 × 1 surface, prepared by a wet chemical procedure, have been investigated by scanning tunneling microscopy (STM) and synchrotron radiation based photoemission. The interactions between the In overlayer and the Hue5f8Si(1 × 1) surface are weak as evidenced by valence band and core level photoemission spectra. As on the clean Si(111)7 × 7 surface an island growth mode of In is observed on the Hue5f8Si(1 × 1) surface, but the island structures display significant differences in shape and growth behaviour between the two surfaces, indicating a modified In growth kinetics mediated by the H adlayer. This surfactant-type role of H is discussed. Annealing of the room temperature deposited In layers results in remarkable coverage-dependent morphology changes which signal the predominance of kinetic effects in the room temperature growth. For higher In coverages (>25 monolayers) well-ordered single crystalline In islands are formed at room temperature as indicated by atomically resolved STM images and angle resolved photoemission measurements.

Growth of ordered bithiophene layers on the p(2 · 1)O reconstructed Cu(1 1 0) surface

Abstract Submonolayer to multilayer coverages of bithiophene on the clean Cu(1xa01xa00) and the Cu(1xa01xa00)–p(2xa0×xa01)O reconstructed surface have been investigated comparatively with thermal desorption spectroscopy, angle resolved ultraviolet photoemission spectroscopy (ARUPS) and near edge X-ray absorption fine structure spectroscopy. On the clean Cu(1xa01xa00) substrate the bithiophene monolayer is strongly bound via the π-orbitals and flat lying. After the completion of the wetting layer, further growth occurs in islands with no evidence for a preferred molecular orientation or order. In contrast, on the oxygen reconstructed surface the wetting layer formed has a weaker interaction with the substrate and in this monolayer the molecular plane is tilted with respect to the surface. For the multilayer film on the Cu(1xa01xa00)–p(2xa0×xa01)O reconstructed surface the ARUPS displays a distinct azimuthal anisotropy. This is suggested to arise from a template effect of the reconstruction.

Chemical reactivity of the V–Pd(1 1 1) subsurface alloy: adsorption of CO

Abstract The reactivity of a V–Pd(1xa01xa01)(√3×√3) R 30° subsurface alloy has been investigated, using the adsorption of CO as a test case, by photoelectron spectroscopy with synchrotron radiation of the valence and core states and by kinetic analysis of temperature-programmed thermal desorption spectra. The overall conclusion from this study is that the chemical reactivity towards CO is lowered on the subsurface alloy surface as compared to unmodified Pd(1xa01xa01). The photoemission data indicate that the room-temperature CO saturation coverage is reduced from 0.5 monolayer (ML) on the clean Pd(1xa01xa01) surface to ∼0.28–0.30 ML on the alloy surface, in excellent agreement with the thermal desorption results. This is explained in terms of a reduced CO adsorption energy, as revealed by the kinetic analysis of the thermal desorption spectra. The balance between threefold hollow and bridge CO adsorption sites at saturation coverage is changed in favour of the preferential occupation of hollow sites on the alloy surface. The observed modifications of the electronic structure of valence states upon alloying and on CO adsorption are well reproduced by a recent ab initio DFT study of CO on V–Pd alloys by Hirschl and Hafner [Surf. Sci. 498 (2002) 37].

The aluminium-alcohol interface: Methanol on clean Al(111) surface

Abstract The adsorption and decomposition pathways of methanol on Al(1xa01xa01) at 90 K and 300 K have been studied using UPS, work function and thermal desorption spectroscopy measurements over a wide temperature range. Dissociative adsorption of methanol occurs at 90 K for low coverage ( Θ 3 O ads. and H and the chemisorbed intact methanol were found as the dominant adsorbate species at Θ ≅1 monolayer (1.4 L exposure) coverage. The relative energy shift of the O–H orbital (7a ′ ) to higher binding energy indicates a strong hydrogen bonding in the multilayer. As a stable reaction intermediate, the surface methoxy species was formed on heating the multilayer to ∼170 K. This methoxy overlayer decomposed at high temperature, desorbed as CH 4 and H 2 , and left only O on the surface. A methoxy species was observed for 300 K adsorption which has different band energy and shape compare to simple methoxy overlayer observed at 170 K. The work function of aluminium decreased largely due to the adsorption of methanol.