M. Sock

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

The metal–insulator transition in V2O3(0001) thin films: surface termination effects

Epitaxially grown V2O3(0001) thin films have been prepared with different surface terminations, as evidenced by atomically resolved scanning tunnelling microscopy and high-resolution electron energy loss spectroscopy (HREELS) phonon spectra. The spectral changes observed in valence band photoemission spectra and HREELS on cooling the V2O3 samples from 300 to 100 K have been associated with the metal–insulator transition (MIT) in the bulk of the V2O3 film. The reconstructed surface regions per se do not display the MIT, but affect the MIT signature observed with surface sensitive techniques, depending on the thickness of the reconstructions. Whereas the thermodynamically stable (1 × 1) vanadyl V = O surface termination allows the observation in photoemission and HREELS of a clear signature of the MIT, the latter is screened on a surface formed by V = O defect structures. Doping of the surface with small amounts of adsorbed water restores reversibly the MIT spectral fingerprints. These observations are discussed in terms of the different geometrical and electronic structures of the different surface terminations.

Identification of new adsorption sites of H and D on rhodium(100)

Exposure of Rh(100) to hydrogen (deuterium) in atomic form leads to the population of adsorption sites, not attainable with molecular species. Quantitative thermal desorption spectroscopy (TDS), high resolution electron energy loss spectroscopy (HREELS), and density functional theory (DFT) calculations have been applied to investigate these new adsorption sites. In addition to the fourfold hollow sites (1 ML), which can be populated by dissociative adsorption, occupation of subsurface sites and the population of additional surface sites (for deuterium) have been observed (maximum coverage 3.4 ML). In TDS individual adsorption states show up in the form of three different peaks: Recombination of H (D) atoms from hollow sites around 300 K, desorption of subsurface species between 150–200 K, and recombinative desorption via a molecular precursor at about 120 K (for deuterium only). The exposure of the Rh(100) surface to atomic H (D) leads to a pronounced roughening of the surface, as evidenced in the HREELS ...

Adsorption and reaction of CO on vanadium oxide–Pd(111) “inverse” model catalysts: an HREELS study

The room temperature adsorption and reaction of CO on Pd(111) surfaces decorated with submonolayer coverages of vanadium oxide – so-called “inverse” model catalysts – have been studied by high-resolution electron energy loss spectroscopy (HREELS) and X-ray photoelectron spectroscopy (XPS). The HREELS surface phonon spectra of the V oxide phases have been measured and used to monitor the changes in the oxide as a result of the interaction with CO. The intramolecular C–O stretching frequency of CO adsorbed on the V-oxide/Pd(111) surfaces displays two vibrational loss components as a function of CO coverage as it has been observed on the clean Pd(111) surface. The relative intensities of the two vibrational features as a function of V oxide coverage however suggest that the balance of CO adsorption sites is modified as compared to clean Pd(111) by the presence of the V oxide–Pd phase boundary. Preferential population of high coordination adsorption sites by CO in the vicinity of the oxide–metal interface is proposed. The analysis of the V oxide phonon spectra indicates that adsorbed CO partially reduces the V oxide at the boundaries of the oxide islands to the Pd metal. The reduction of V oxide by CO is dependent on the oxygen content of the V oxide phase. The reduction of V oxide is confirmed by the XPS V 2p core level shifts.